Keywords: Ultrasonic Spray Pyrolysis, Semiconductors, Zinc Oxide, Zinc Magnesium Oxide, Copper Oxide, Preparation, Thin Film, Structural Properties, Optical properties, Electrical properties, All-Oxide Solar Cell, Photovoltaics atmospheric atomic layer deposition. ALD: atomic layer deposition. AFM: atomic force microscopy. ALD: atomic layer deposition aluminum-doped) zinc oxide (ZnAlO) concentrator photovoltaics. CVD: chemical vapor deposition. ECD: electrochemical deposition. FF: fill factor. FTIR: Fourier transform infrared spectroscopy. FTO: fluorine-doped tin oxide. ISC: short-circuit current. ITO: indium tin oxide short-circuit current density. MOCVD: metal organic chemical vapor deposition. MOVPE: metal organic vapor phase epitaxy. PCE: power conversion efficiency. PLD: pulsed laser deposition. PV: photovoltaic. RF sputtering: radio-frequency sputtering. RMS: root mean square. RPE-CVD: remote-plasma-enhanced chemical vapor deposition. RTA: rapid thermal annealing. SEM: scanning electron microscopy. SLG: soda lime glass ultrasonic spray pyrolysis vacuum arc plasma evaporation van der Pauw open-circuit voltage. XRD: X-ray diffraction. XRF: X-ray fluorescence

In this research work, the results on preparation of environmentally compatible, earth-abundant metal oxide thin films using the technique of ultrasonic spray pyrolysis are presented. This technique is a promising technique for preparing low-cost, high-efficiency photovoltaic devices.

Three essential materials are developed for the purpose of realization efficiency of an "all-oxide" solar cell device: zinc oxide (ZnO) as window layer; zinc magnesium oxide (ZnMgO) as a buffer layer and cuprous oxide (Cu2O) used as an absorber layer. Comprehensive design of experiments was set up for the preparation of each material to understand the effect of a particular spray preparation parameter on the properties of the prepared films. Optical, structural, morphological, and electrical characterizations were used to investigate the film properties. The results are discussed and analyzed thoroughly. We were able to successfully demonstrate the spray preparation parameters that needs to be fixed for preparing a high-quality film. Highly transparent ZnO was prepared in wide range of thickness with high crystalline qualities with specific preparation temperature with a precise control on the concentration of the precursors. ZnMgO was prepared by varying the molar compositions of the magnesium precursor in the precursor solution. Up to nearly 30 % of Mg, the ZnMgO films exhibited single crystalline phase with high transparencies as evidenced from the structural, chemical, and optical characterizations. The magnesium miscibility limit (nearly 30 % in our case) is higher than usually obtained in non-vacuum solution-based methods. High-absorbing Cu2O preparation was optimized with effective control on the preparation temperature and the concentration of a new reducing agent: D-sorbitol. To expand the horizon of efficiency of our preparation process, two more materials, ZnAlO and ZnAlMgO, were prepared. It was found with low molar concentrations of aluminum the optical, electrical, and structural properties of the ZnAlO films could be modulated for use as window cum top-contact layer in "all-oxide" solar cells. The fixed low molar concentrations of aluminum were used in conjunction with varying magnesium (up to 7 mol%) to prepare ZnAlMgO films. The bandgap energies and the electrical properties of the films were modulated with the co-incorporation so that they can be integrated as window/topcontact/buffer layers in "all-oxide" solar cells. Additionally, simulations performed using Silvaco Atlas® and Solis also demonstrates the applicability of these films for "all-oxide" solar cells.

Résumé

Dans ce travail de recherche, sont présentés les résultats de l'élaboration de couches minces d'oxydes à base d'éléments abondants par la technique de spray pyrolyse ultrasonique. Cette technique est prometteuse pour élaborer des couches minces à bas coût. Trois matériaux constituant les briques de base de la cellule solaire « tout oxyde » ont été développés : l'oxyde de zinc (ZnO) comme couche fenêtre ; l'oxyde de zinc et de magnésium (ZnMgO) comme couche tampon et l'oxyde de cuivre (Cu2O) comme couche absorbante. Un plan d'expériences précis a été mis en place pour chaque type d'élaboration afin de comprendre l'effet des paramètres d'élaboration sur les propriétés des films.

Des caractérisations optiques, structurelles, morphologiques et électriques ont été utilisées pour étudier les propriétés du film et les résultats discutés et analysés en détail. Nous avons pu démontrer que l'approche utilisée permet d'obtenir des films de haute qualité avec un procédé maîtrisé. Ainsi, des films minces de ZnO avec une transparence élevée dans une large gamme d'épaisseurs ont été obtenus avec une température d'élaboration spécifique et un contrôle précis de la concentration des précurseurs. Le ZnMgO a été élaboré avec près de 30 % de miscibilité de Mg. Les films de ZnMgO présentaient une phase monocristalline avec une transparence élevée, comme montré par les caractérisations structurales et optiques. La limite de miscibilité du magnésium (près de 30 % dans notre cas) est supérieure à celle habituellement obtenue avec les méthodes d'élaboration sans vide. L'élaboration de l'absorbeur Cu2O a été optimisée avec un contrôle efficace de la température d'élaboration et de la concentration d'un nouvel agent réducteur, le D-sorbitol. Pour élargir les perspectives de ce travail, deux autres matériaux, ZnAlO et ZnAlMgO, ont été élaborés. Nous avons ainsi mis en évidence le fait qu'avec de faibles concentrations d'aluminium, les propriétés optiques, électriques et structurales des films de ZnAlO pouvaient être modulées pour une utilisation en tant que couche fenêtre ou TCO dans les cellules solaires « tout-oxyde ». L'incorporation du magnésium (jusqu'à 7 %) a permis d'élaborer des films de ZnAlMgO avec énergies de gap et des propriétés électriques modulables permettant de les intégrer en tant que couches fenêtre, couche tampon, ou TCO dans la cellule solaire « tout-oxyde ». De plus, les simulations réalisées à l'aide des logiciels Silvaco Atlas® et Solis démontrent également le potentiel de ces films pour les cellules solaires « tout-oxyde ».

Mots-clés : Spray pyrolyse ultrasonique, Semi-conducteurs, Oxyde de zinc, Oxyde de zinc et de magnésium, Oxyde de cuivre, Élaboration, Couche mince, Propriétés structurales, Propriétés optiques, Propriétés électriques, Cellule solaire « tout-oxyde », Photovoltaïque.

Résumé étendu

Elaboration par spray pyrolyse ultrasonique et étude de couches minces à base d'oxydes d'éléments abondants pour le photovoltaïque

Dans la filière des couches minces, qui constitue l'une des technologies les plus importantes pour le photovoltaïque, l'objectif est de développer des procédés de fabrication permettant un compromis entre un rendement de conversion élevé et un coût faible. Une famille de matériaux utilisés dans cette filière et qui peuvent répondre à cet exigeant compromis sont les oxydes d'éléments abondants qui peuvent permettre de fabriquer une cellule solaire « tout-oxyde ».

Dans ce cadre, l'objectif de cette thèse était d'élaborer des couches minces à base d'oxydes d'éléments abondants en utilisant la technique polyvalente et peu coûteuse du spray pyrolyse ultrasonique (USP = Ultrasonic Spray Pyrolysis). Les procédés développés avec cette technique de croissance peuvent être facilement transférés à l'industrie pour l'élaboration sur de grandes surfaces. Les matériaux qui font l'objet de ce travail de recherche, les oxydes métalliques, sont une famille de semi-conducteurs à très fort potentiel pour le photovoltaïque : ils sont abondants dans la croûte terrestre, non toxiques et peuvent être élaborés aisément. La structure entière de la cellule solaire constituée de la couche de contact supérieure transparente, de la couche-fenêtre, de la couche tampon et de la couche absorbante, peut être fabriquée avec des oxydes métalliques en utilisant cette technique. Ce type de procédé de fabrication utilisant une seule méthode d'élaboration pour l'ensemble de la cellule solaire n'a pas été mis au point à ce jour.

Il s'agit dans ce travail de recherche d'optimiser les paramètres d'élaboration des couches minces, et en particulier l'optimisation de la solution précurseur, de la température de dépôt et du scan, pour obtenir des films minces d'oxyde métallique de haute qualité, homogènes et ayant les propriétés optoélectroniques requises pour l'application photovoltaïque. Ce travail d'optimisation est fait grâce notamment à l'utilisation de nombreuses techniques de caractérisation des propriétés morphologiques, optiques, structurales et électriques.

Nous avons démarré le travail de recherche en élaboration des oxydes à partir de rien, avec l'installation du nouvel équipement USP au laboratoire en juin 2019, quelques mois avant le début de la thèse. Tout était à faire, et les procédés devaient être mis au point et optimisés. Le travail de recherche a commencé par l'optimisation des solutions précurseurs et la conception et le choix des matériaux pour le porte-substrat thermique qui a un impact important sur la qualité des matériaux élaborés. Tous les films ont été élaborés sur du verre sodé (SLG = Soda Lime Glass).

Pour chaque élaboration, pour un matériau donné, les paramètres ont été variés, et les propriétés morphologiques, optiques et structurales des films minces ont été étudiées en détail. L'analyse des résultats a permis de comprendre l'effet des paramètres d'élaboration, et les résultats ainsi obtenus ont été comparés à ceux des films minces optimaux obtenus par d'autres techniques d'élaboration.

Les étapes expérimentales ont été rigoureusement suivies pour définir un ensemble de paramètres optimisés pour chaque type d'oxyde métallique, avec trois étapes essentielles : (i) la mise au point du procédé d'élaboration par USP ; (ii) la caractérisation morphologique, optique, structurale et électrique des films élaborés ; (iii) l'optimisation des couches pour préparer la réalisation d'un dispositif de cellule solaire « tout-oxyde ».

Ce travail est présenté dans ce manuscrit en 7 chapitres résumés ci-dessous.

Le chapitre 1 est une introduction générale sur le photovoltaïque et les oxydes semi-conducteurs.

Le développement du photovoltaïque et son importance dans la demande énergétique moderne sont présentés. Le principe physique de fonctionnement des cellules solaires ainsi que les différents types de cellules (classés selon les avancées technologiques majeures) sont également abordés. Dans ce contexte, dans la deuxième partie du chapitre, les bases des semi-conducteurs à oxyde métallique sont abordées ainsi que la revue de la littérature des types de cellules solaires « tout-oxyde » en mettant l'accent sur les oxydes métalliques. Les détails sur les hétérostructures Zn(Al)O/Cu2O les plus recherchées pour les cellules solaires « tout-oxyde » sont discutés. En outre, la simulation numérique effectuée pour étudier les performances des cellules solaires « tout-oxyde » ainsi que le choix approprié des matériaux sont présentés. Dans la dernière partie, les objectifs et les défis du travail de thèse sont présentés ainsi que le plan du manuscrit.

Le chapitre 2 est consacré aux principes de base du spray pyrolyse ultrasonique (USP) qui est utilisé pour l'élaboration des couches fenêtre, tampon et absorbante. Une présentation détaillée des paramètres d'élaboration USP est effectuée. Cela inclue la température de dépôt, la pression de la buse, la distance buse-substrat, le flux, la vitesse de scan, le nombre de scans, le temps d'arrêt, la puissance des ultrasons, et les propriétés de la solution de précurseurs (pH et concentrations).

L'effet de ces paramètres est étudié grâce aux techniques de caractérisation des propriétés morphologiques, optiques, structurales et électriques des couches minces qui sont présentées dans ce chapitre.

Le chapitre 3 concerne le développement et l'optimisation du ZnO utilisé comme couche fenêtre.

Les propriétés du ZnO sont brièvement discutées. L'élaboration de ZnO avec une qualité optimale est discutée en détail. L'optimisation des paramètres d'élaboration est effectuée et leurs effets sur les propriétés morphologiques, optiques, structurales et électriques des films sont analysés. L'ensemble du processus pour élaborer des films minces de ZnO de qualité optimale était difficile et long. Nous avons dû établir le plan d'expériences basé sur une vaste revue de l'état de l'art. La configuration du système d'élaboration par spray pyrolyse était la première tâche importante car tous les paramètres, y compris le support de substrat et le substrat lui-même, peuvent avoir un impact considérable sur les propriétés du film. La hauteur de la buse du spray par rapport au substrat et la vitesse de la buse ont été initialement calibrées et réglées en fonction des limites de sécurité du système. La durée de l'élaboration devait être fixée très soigneusement pour toutes les croissances. Au contact des gouttelettes atomisées, la température du substrat chute, et la durée de chaque cycle de spray et le temps d'arrêt entre deux cycles successifs permet au substrat à atteindre rapidement la température optimale. Pour vérifier la répétabilité et la fiabilité de notre procédé, chaque élaboration avec un ensemble donné de paramètres a été faite au moins trois fois. Les caractérisations optiques, morphologiques et structurales détaillées dans ce chapitre et leur analyse approfondie nous ont aidées à comprendre l'effet des paramètres sur la qualité du film. L'un des principaux objectifs de notre travail d'optimisation était de réduire drastiquement la formation d'hydroxydes durant l'élaboration, qui dégraderaient la qualité des films minces. Même de légers changements dans les conditions d'élaboration peuvent conduire à la formation d'hydroxydes, et c'est pourquoi un contrôle maximal de ces conditions d'élaboration est nécessaire. La compréhension du rôle des paramètres d'élaboration dans leur ensemble était importante car nous voulions développer des couches minces de ZnO avec des propriétés optoélectroniques compatibles avec leur utilisation comme couche-fenêtre dans les cellules solaires « tout oxyde ».

Ainsi ce travail a permis de développer des films avec une transparence très élevée dans le visible, avec une morphologie maîtrisée. La qualité cristalline mise en évidence par les mesures de diffraction de rayons X et spectroscopie Raman ont prouvé l'efficacité de notre procédé d'élaboration. Les paramètres d'élaboration optimaux dans ce procédé sont :

(i)
La hauteur du de la buse du spray : 10 cm.

(ii) Vitesse de buse : 70 mm/seconde.

(iii) Temps d'arrêt entre deux cycles : 1 seconde.

(iv) Pression : 13,8 kPa.

Ces paramètres optimisés permettent de préparer des couches minces de ZnO qui remplissent les principaux critères d'utilisation comme couche fenêtre dans les cellules solaires « tout-oxyde » : (i) vitesse de croissance rapide ; (ii) transparence élevée dans une large gamme d'épaisseurs allant de 100 nm à 800 nm ; (iii) orientation cristallographique selon l'axe c avec une grande taille de grain et des défauts minimisés ; (iv) une résistivité électrique relativement faible.

Le chapitre 4 porte sur le développement et l'optimisation de la couche tampon. Le choix de Zn1-xMgxO pour la couche tampon est justifié en détail dans ce chapitre, avec notamment la possibilité de moduler les propriétés optoélectroniques pour les adapter à l'absorbeur.

L'optimisation des paramètres d'élaboration ainsi que l'effet de l'ajout du magnésium sur les qualités du film sont détaillés.

Dans ce travail, nous avons développé un procédé d'élaboration de ZnMgO basé sur le spray pyrolyse ultrasonique en utilisant des solutions aqueuses dans la plage de composition en Mg allant jusqu'à bande interdite modulable de 3.26 eV à 3.65 eV ; (iv) diminution drastique de la concentration de porteurs avec l'augmentation de la teneur en Mg en raison de la modulation de la bande interdite et de l'inhibition des site interstitiels de Zn à l'origine des donneurs peu profonds.

Le chapitre 5 concerne l'élaboration de la couche absorbante, l'oxyde de cuivre, élaborée et Le chapitre 7 est le chapitre de conclusion. Il met l'accent sur l'utilisation du spray pyrolyse ultrasonique pour l'élaboration de films minces d'oxydes métalliques de haute qualité. Tous les résultats importants obtenus sont résumés dans ce chapitre et les principales perspectives de ce travail de recherche sont présentées.

Pursuing the PhD at the LMOPS laboratory of the Université de Lorraine for 3 years has been an absolute privilege with lots of great learning experiences. I am immensely thankful to all the people for their co-operation, support and assistance which immensely contributed towards the success of this work.

In the first place, I would like to deeply thank my thesis director Prof. Sidi OULD SAAD HAMADY for always being there for me. His motivation, guidance, support and knowledge has helped in many different ways and has greatly contributed towards the successful completion of the thesis. He always showed immense faith in my abilities and his ideas, discussions, explanations, patience and encouragements throughout the duration of the thesis work needs to be cherished. The unparallel enthusiasm, positivity and energy helped me to better construct my research.

My sincere thanks go to my thesis co-director, Prof. Nicolas FRESSENGEAS, for his constant support and help. His knowledge, critical feedback, explanations and numerous discussions contributed greatly towards this thesis work. His help towards administrative matters also helped me a lot.

I am really very thankful to Dr. Christyves CHEVALLIER, for always being there for me. Christyves has helped me in all possible situations. From helping me to find an apartment, to speak in French to different persons on my behalf or in experimental and research work, Christyves has done it all.

I owe a lot to him for a smooth stay in the beautiful city of Metz. Our friendly discussions, the fun during working together, nice dinners at French restaurants and car drives to Nancy while listening to Bollywood songs are some happy memories that will stay with me for a long long time.

Thanks a lot, to Stéphanie VERGERIO, for her pleasant behaviour and for always happily helping me in all the administrative tasks. Thanks a lot also to Queny KIEFFER and Maryne CROUZET for their assistance in laboratory technical matters, ordering of items and for chemical assistance. All of your help has contributed greatly towards my research work.

Merci Beaucoup Jean-Claude. Jean-Claude PETIT is a person from whom I have learnt positivity.

Jean-Claude is not only my colleague, I consider him as one of my best friends. I can discuss anything and everything with him. Whatever, French I speak and understand is because of you. Jean-Claude is my coffee partner. Starting the day at the lab, with some friendly discussions always helped me to start the work on a positive note. Your help towards my experiments and your immense knowledge in the technical aspects has helped my research in a great way. I will miss you. I am also thankful to Dr. Brigitte VIGOLO-GREFFIER and Prof. Aotmane EN-NACIRI for agreeing to be the members of "Comité de suivi" for my thesis. Insightful suggestions from both of them, help and support has motivated me a lot to continue my research in a positive way.

Thanks to

Thanks to Stéphane MARCHAL for always his friendly behavior, help and support.

I also want to thank Dr. Pascal BOULET and Dr. Olivier PERROUD for their help with XRD measurements, Dr. Julien GUYON for his help with the SEM, Dr. Thomas GRIES and Dr.

Montassar BILLEH BOUZOURÂA for their help with AFM.

I am immensely thankful to Mr. Laurent SCHMIDT for always his help and support. He has always helped me a lot towards the visa procedures with detailed information. Also, I would like to thank Thank you so much, my friend Rohit GUPTA for the constant encouragement, long friendly chats, and motivation. Much thanks go out to my friend, philosopher and guide Dr. Aruanva PRADHAN (Arunava da, as I call him) for his constant support, motivation and guidance.

I am really lucky to have friends like Debopam BASU and Soumya BOSE. They have supported me and cared for me whole-heartedly from overseas. I have received impeccable support and motivation from them during the thesis period.

A very special thanks and appreciation to my parents and my sister for their overwhelming and unending support that grew even stronger during this thesis period. Knowing the fact that I am being loved from far across was a vital motivation for the successful completion of the thesis. No words are enough to express my thanks to them. The love and support of my father, Amit Kumar BOSE, my mother Krishna BOSE and my sister Arpita BOSE is something that always kept me going. I miss my grandfather Late Sri Hemendra Kumar BOSE, I know that I am loved from high up in the skies.

Finally, I would like to thank each and every one for their support and help. My sincere apologies to all the people whom I could not mention in this small space. All of your support and assistance has been instrumental in the successes I have recorded thus far. I am forever grateful to all.

Thank you so much! Sourav Dedicated to my grandfather, my parents and my sister "Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world."

Albert Einstein

"The sun, --the bright sun, that brings back, not light alone, but new life, and hope, and freshness to man--burst upon the crowded city in clear and radiant glory. Through costly-coloured glass and paper-mended window, through cathedral dome and rotten crevice, it shed its equal ray." Charles Dickens, Oliver Twist

List of Figures

List of Tables

General Introduction to Photovoltaics

The study of photovoltaics deals with the direct conversion of solar radiation to electricity. French physicist Alexandre Edmond Becquerel was the first who evidenced the photovoltaic effect in 1839 [1,2]. Few years later in 1873, Willoughby Smith found the photovoltaic effect in selenium [3]. In

Photovoltaic Technology for Green Energy

"Our dependence on fossil fuels amounts to global pyromania, and the only fire extinguisher we have at our disposal is renewable energy." -Hermann Scheer, German statesman and entrepreneur.

In the past few decades, the world has witnessed the importance of energy usage in terms of human civilization. With the rapid population increase, the demand for energy is rising steeply. By the year 2050, the UN expects the world population to reach 9.3 billion [14]. The huge technological and economic developments, specifically in the developing countries, have also contributed to the enormous increase in energy demand. According to the recent statistics from International Energy Agency (IEA) it is estimated that from 2010 t0 2040 there can be another 0ne-third increase in the global energy use where countries like China and India could account for almost 50 % of the increased growth in demand [15,16]. The current energy supply will be insufficient towards the requirement if the demand continues in this rapid increasing trend.

Due to this huge demand of energy the production of non-renewable resources has increased rapidly. To satisfy the demand, the use of fossil fuels also soared up, although the usage cost is high too. Among the primary energy sources present, globally fossil fuels accounts for 74 % of total energy used [17]. It is an undisputed fact that the rampant use of the fossil fuels leads to the enormous quantities of greenhouse gases being released to the atmosphere which directly contributes to the rise of global temperature thereby contributing immensely to the earth's climate change. The shrinkage of the Arctic ice and the increase in sea levels are the direct consequences of global warming. Submergence of coastal and low-lying regions and salt-water intrusion to groundwater are inevitable due to rise of sea-levels [18][19][20][21]. So, to preserve the earth's ecosystem, the demand for a sustainable energy supply that serves our energy demands is ever so high.

Sustainable energy technologies such as hydropower, wind power, geothermal energy, solar energy etc. are some of the prominent options which can fulfill the energy demands better than fossil fuels.

Nuclear energy too can be an option but in case of an accident its consequences could be dangerous as seen in cases of Chernobyl in 1986 and Fukushima ten years ago [22,23]. Biomass is another potent option for fuel source, but its extensive use without proper planning and management could lead to deforestation which in turn may result in the imbalance of the ecosystem [24,25]. Renewable energy generated from sun, wind and water are not only sustainable but also safe. But none of them can solely satisfy the criteria of meeting the global energy demands [26,27]. A recent review study from Lai et al. [28] and an extensive study by Sharifzadeh et al. [29], incorporates the concept of machine learning to the different renewable energy concepts and shows that both wind and solar energies would go a long way in satisfying the global energy demand but with solar energy taking a slender lead in this matter. So, we focus our attention to the details of solar PV energy production.

The earth receives about 173000 terawatts (trillions of watts) of solar energy over a span of 1 year which is approximately 10000 times the energy consumed worldwide [30,31].

Solar energy technology which incorporates photovoltaic (PV) solar cells works by directly converting the solar radiation into electricity. The worldwide PV electricity share is 3.2 % [32].

Based on the report from IRENA, it is predicted that the global share of solar PV power generation would reach up to 13 % by 2030 and to 25 % by 2050, thanks to the supporting policies and reductions in production costs [33]. Alongside, the decrease in the costs, in the last decade, the installation of PV systems has increased from 14 % to 18 % globally [33]. Thus, the price of electricity has also decreased considerably in most regions in the past decade [34,35]. Electricity generation from solar photovoltaics has been one of the cheapest among all the possible sources.

Continuing in this trend, PV technology will not only be a key contributor to electricity generation but also a major contributor to all the segments of the global energy system [36]. The global economy will also be benefitted by the fact that solar PV cost is expected to dramatically decrease in the coming decades [33].

Majority of the solar cells which are manufactured today globally, comprises of crystalline silicon as the base material. Silicon based solar cells can be called as the first-generation solar cells. The efficiency record for a silicon solar cell is 26.7 % in laboratory [37][38][39].

Solar cells based on thin film are also being fabricated. These solar cells have several advantages over traditional solar cells [40][41][42][43]. Recent research on solar cells is also focussed on developing advanced materials and processes to ramp up the conversion efficiencies along with reduction of production costs [44,45]. In the next section we will discuss about the basic working principles of solar cells. In semiconductors, Fermi energy lies in the forbidden energy zone and is related to the carrier concentration and can be controlled e.g., by doping [46,52,53]. The positioning of the valence and conduction bands along with the Fermi energy level will help us to understand the differences among insulations, semiconductors, and conductors (Figure 1.4). In insulators, the forbidden energy gap is too high (typically bandgap greater than 3 eV) so no free electrons are available in the conduction band and thus no conductivity. In semiconductors, the conductivity is between that of a conductor and insulator (0.5 to 3 eV typically), depending upon the temperature. At very low temperatures, semiconductors are insulators too, but with increasing temperature, electrons are excited to be promoted to the conduction band. Thus, conductivity increases. In conductors (for e.g., metals) there is no forbidden band, and the conductivity is very high in a wide range of temperature [52,53].

Solar Cells

In a pure semiconductor material, the average number of free electrons and holes at a particular temperature is known as the intrinsic carrier concentration ni [52,54,55]. The index "i" means intrinsic and indicates an undoped semiconductor. It is determined from the following equation:

Equation (1.1):

𝑛 𝑖 = √𝑁 𝐶 𝑁 𝑉 𝑒𝑥𝑝 (- 𝐸 𝐺 2 𝑘 𝑇 )
Where 𝑁 𝐶 , 𝑁 𝑉 , k, EG and T stands for density of states respectively in the conduction and in the valence band; Boltzmann constant, Bandgap energy and temperature in Kelvin respectively. In a semiconductor, electron can absorb a photon and be promoted to from the valence band to the conduction band, leaving a hole in the valence band. In n-doped semiconductors, the concentration of electrons is higher than the concentration of holes, and the Fermi level EF is located near the conduction band. Similarly, for p-doped semiconductors EF is located near the valence band since the concentration of holes is higher (Figure 1.5). More details about this phenomenon can be found in the literatures [46,52,[54][55][56][57].

When a p-doped and a n-doped semiconductors are in contact, a diffusion phenomenon occurs to form the depletion region and induces the internal electric field in the p-n junction, shown in the Under illumination, the absorption occurs for photons with energy higher than the bandgap. The additional energy of the electrons over the band gap is lost through thermalization loss. The wavelength at which the photons are just absorbed is the bandgap wavelength (λG). The solar spectrum which is above λG cannot be used for providing electrical energy and hence termed as transmission losses. A balance between the absorption while minimizing the thermalization losses defines the theoretical limit for power conversion efficiency (PCE). The theoretical limit of PCE has been calculated by the Shockley-Queisser model based on certain assumptions and considering an ideal p-n diode model estimated the optimal band gap for photovoltaics to be in the range of 1.1 eV to 1.4 eV [52,[57][58][59][60][61] as illustrated in the Figure 1.9. There are other factors too other than band gap energy that determines the PCE limit for various solar cell materials.

Figure 1.9: Solar spectrum outside (AM0) and inside the atmosphere (AM1.5G). The Shockley-Queisser limit zone corresponding to the optimal bandgap at around 1.3 eV is marked by orange lines [52]. An open-source implementation of the Shockley-Queisser model written in Python can be found here: https://github.com/sidihamady/Shockley-Queisser.

Figure 1.10:

The bandgap wavelengths for some standard semiconductors with respect to the standard AM1.5G solar spectrum [57].

In general, the effect of light absorption in a semiconductor can be described by the following relation:

Equation (1.3): 𝜙 (𝑥) = 𝜙 0 𝑒 -𝛼𝑥
Where 𝜙 is the light irradiance at a certain depth; 𝜙 0 is the irradiance at x=0, x is the position in the absorber and α is the absorption coefficient.

Solar cells absorber materials can have different bandgaps with different absorption coefficients.

For indirect bandgap material like silicon, typically few dozen microns are required for light absorption and practically few hundred microns are required for the final solar module. Thin films based on direct bandgap materials required much lesser thickness of absorbers (less than few microns). They can be eco-friendly too with industrial compatible fabrication methods to reduce the overall cost of electricity production. Based, on the different absorber materials and different junction types of solar cells technologies can be categorized into different generations which is broadly discussed in the next section.

Overview of Different Technologies of Solar Cells

Solar cell technologies can be classified based on the diverse materials used to build up the device and their production types. Broadly, the solar cell technologies can be categorized into three types:

(i) First Generation with crystalline silicon, (ii) Second Generation using thin films and (iii) Third Generation with heterojunctions and new concepts.

First Generation Solar Cell: Crystalline Silicon

The first generation of solar cells consists of crystalline silicon as the absorber material. To create a pn-junction silicon is often doped with p-type boron or n-type phosphorus. Now, silicon solar cells are a major player in the worldwide solar cell market. With very pure silicon qualities, silicon solar cells can produce very high conversion efficiencies considering a pn junction. But for high energy consuming processes for pure silicon production, the cost of solar cells is high too. Based on differences in crystallization, silicon solar cells can be of two types: monocrystalline solar cell and multi-crystalline (polycrystalline) solar cell. For monocrystalline solar cells, the entire wafer consists of a single crystal whereas, for multi-crystalline solar cells, the wafers consist of many fragments of crystal grains. Generally, the efficiency of monocrystalline solar cells is higher than multi-crystalline solar cells [62], but their production cost is higher due to wafer cutting process.

Kaneka Corporation using the real laboratory standards were able to achieve 26.7 % conversion efficiency for silicon solar cells. However, some of the most common drawbacks of silicon solar cells include the high bulk thickness of silicon absorber. Silicon being an indirect bandgap semiconductor, needs high thickness for light absorption. Hence, Auger charge-carrier recombination and bulk nonradiative recombination are dominant and causes loss of charge-carriers [37,64,65].

Second Generation Solar Cell: Thin Films

The second generation of solar cells are mostly known as thin film solar cells. The absorber materials for these types of solar cells are only few micrometres thick. Typical absorber materials for thin film solar cells are amorphous silicon (a-Si), copper indium selenide (CIS), copper indium gallium sulfide/selenide (CIGS) [66][67][68][69], copper zinc tin sulfide/selenide (CZTS), cadmium telluride (CdTe) and gallium arsenide (GaAs). All these absorbers are direct band gap materials unlike silicon; hence the absorber thickness is considerably lower compared to silicon wafer absorbers. So, the production costs can also be considerably lower for thin film solar cells.

Figure 1.12: Some illustrations of thin film solar cells technologies [70].

Mostly the thin film solar cells are fabricated on glass substrates, but flexible substrates are also used. Fabrication on flexible substrates allows thin films solar cells to be more lightweight, costeffective, and usable on various types of surfaces and additionally with attractive looks [66,71].

Typically, thin film solar cells achieve efficiencies of 7-14 %, but with recent high-end research the efficiencies have shot up to 23 % approx. [72][73][74]. CdTe solar cells based on single junctions reaches efficiencies up to 22 % but with major drawbacks including the non-abundancy of Te and the toxicity of Cd [75,76]. GaAs solar cell [79,80].

CIGS technology also has potential to reach high efficiencies, but most CIGS technologies use CdS used as a buffer layer which is a major source of toxicity [81]. Recently, Nakamura et al. [82] were able to fabricate Cd-free CIGS solar cells by a complicated process with double buffer layers to achieve record efficiency of 23 %. Recent research also puts focus on CZTS solar cells, which could be a possible replacement of CIGS solar cells due to the abundancy of zinc (as a replacement of the rare indium), the high absorption coefficient and the bandgap perfectly matching the Shockley-Queisser optimal one. CZTS solar cells have reached a maximum efficiency of 12.6 % using nonvacuum hydrazine-based solution process strategies. But one major drawback is the use of CdS as buffer layer which contributes to toxicity [83][84][85]. Amorphous Si (a-Si) solar cells can reach conversion efficiencies up to 10.2 % [86] but compared to crystalline solar cells or other thin film technologies the lifetime of the cells is much shorter in addition to the lower efficiency. This is due to the excess of recombination centres present in a-Si [87]. Single junction GaAs solar cells has achieved 29.1 % conversion efficiency [88] but with a high production cost and some toxicity [89].

The structure of the different second-generation solar cell types along with the solar cells fabricated on flexible substrates are illustrated in the Figures 1.12 & 1.13 respectively.

Third Generation Solar Cell: Multijunction, Polymers, New Concepts

Third generation solar cells are being developed using polymers, dye sensitized materials, nanocrystal and multijunction solar cells [90][91][92][93][START_REF] Sharma | Dye-Sensitized Solar Cells: Fundamentals and Current Status[END_REF][START_REF] Sokolský | Dye-Sensitized Solar Cells: Materials and Processes[END_REF][START_REF] Upadhyaya | Recent progress and the status of dyesensitised solar cell (DSSC) technology with state-of-the-art conversion efficiencies[END_REF][START_REF] Vatansever | Alternative Resources for Renewable Energy: Piezoelectric and Photovoltaic Smart Structures[END_REF][START_REF] Burschka | Sequential deposition as a route to high-performance perovskite-sensitized solar cells[END_REF].

Polymer solar cells reach a record power conversion efficiency (PCE) of 16.05 % [91]. The polymer technology suffers from two drawbacks: the poor absorption of the solar spectrum and the very low carrier mobility inducing a diffusion length as low as 10 nm [92]. Dye sensitized solar cells have a record PCE of 15 % with a structure with a perovskite [START_REF] Burschka | Sequential deposition as a route to high-performance perovskite-sensitized solar cells[END_REF].

Another promising technology being recently developed is the concentrator photovoltaics (CPV) for high efficiency and small area multijunction (mainly III-V materials and Ge) solar cells [START_REF] Philipps | Current Status of Concentrator Photovoltaic (CPV) Technology[END_REF][START_REF] Shockley | Detailed Balance Limit of Efficiency of p-n Junction Solar Cells[END_REF] with a concentration as high as 1000× corresponding to a solar density power of 100000 mW/cm 2 or 100 W/cm 2 . The fabrication technique and the needed optics make the production cost of these solar cells very high. With control on the growth parameters lattice matched devices based on GaInP/GaAs/GaInAs materials are fabricated [START_REF] Conibeer | Third-generation photovoltaics[END_REF][START_REF] Takamoto | InGaP/GaAs-based multijunction solar cells[END_REF][START_REF] Geisz | High-efficiency GaInP∕GaAs∕InGaAs triple-junction solar cells grown inverted with a metamorphic bottom junction[END_REF][START_REF] Yamaguchi | Multi-junction III-V solar cells: current status and future potential[END_REF][START_REF] Bett | III-V compounds for solar cell applications[END_REF]. With nearly constant lattice parameter of ~3.2 Å InN/GaN/AlN based devices are also being investigated [START_REF] Trybus | InN: A material with photovoltaic promise and challenges[END_REF]. Highest conversion efficiency of 40.7 % (above 40 % for the first time) under 240 suns was possible to achieve by a GaInP/GaInAs/Ge metamorphic device (with strained lattice growth and flexible bandgap design) [START_REF] King | 40% efficient metamorphic GaInP∕GaInAs∕Ge multijunction solar cells[END_REF]. For these solar cells, as already stated, the main disadvantage is the very high cost of production of the epitaxial layers. Crystal imperfections and unwanted impurities severely affects the conversion efficiencies; hence, low-cost production methods cannot be used. Compared to silicon these materials are mechanically weaker. So, although the III-V tandem devices are suitable for space applications, but for terrestrial applications the high cost remains a matter of concern [START_REF] Miles | Inorganic photovoltaic cells[END_REF][START_REF] Horowitz | A Techno-Economic Analysis and Cost Reduction Roadmap for III-V Solar Cells[END_REF][START_REF] Rühle | All-Oxide Photovoltaics[END_REF].

Another emerging technology is that of photovoltaics based on perovskites [START_REF] Moreira | Structural and optical properties of CaTiO3 perovskite-based materials obtained by microwave-assisted hydrothermal synthesis: An experimental and theoretical insight[END_REF][START_REF] Mao | Synthesis of classes of ternary metal oxide nanostructures[END_REF][START_REF] Dogan | Focus on properties and applications of perovskites[END_REF]. Based on the atomic/molecular arrangement in the structure, perovskite materials can have some interesting properties such as, ferroelectricity, superconductivity, ferromagnetism, piezoelectricity, etc. [START_REF] Dogan | Focus on properties and applications of perovskites[END_REF][START_REF] Assirey | Perovskite synthesis, properties and their related biochemical and industrial application[END_REF]. A PCE of 10.9 % was achieved by Lee et al., by using super-structured organometal halide perovskites [START_REF] Lee | Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites[END_REF]. Even higher efficiency of 15.4 % was achieved by using a physical vapour deposited perovskite of the structure CH3NH3PbX3, with compact TiO2 as an electron selective layer [START_REF] Liu | Efficient planar heterojunction perovskite solar cells by vapour deposition[END_REF]. However, perovskite solar cells face serious issues of stability, and, to a lesser extent, toxicity of lead used in the high-performance cells [START_REF] Ke | Unleaded" Perovskites: Status Quo and Future Prospects of Tin-Based Perovskite Solar Cells[END_REF][START_REF] Zhao | Stability Issues on Perovskite Solar Cells[END_REF][START_REF] Wang | Stability of perovskite solar cells[END_REF][START_REF] Rong | Challenges for commercializing perovskite solar cells[END_REF].

Next Generation Photovoltaics: Focus on "All-Oxide" Structures Based on Earth-abundant Semiconductors

As discussed in detail in the Section 1.1.1, we have seen an exponential demand and growth in photovoltaic (solar cell) technology in the last two decades to fulfil the global energy demand. But there are some challenges that needs to be addressed for full-scale implementation of this technology. To this day, the cost of construction of photovoltaic modules remains very high and most technologies use either toxic or scarce elements in fabricating the solar cells. Some solar cells even require very high energy consuming and expensive techniques for their fabrication. To meet the energy demands in a sustainable and cost-effective way, the implementation of photovoltaic technology demands further price reductions. Upscaling of the technology could be made possible with the use of earth-abundant, non-toxic, easily available, and cheap materials with low deposition and fabrication costs. Metal oxides are the materials that most suitably fulfils these criteria and are suitable candidates to bring the production costs down thus allowing for upscaling of the photovoltaic technology with high conversion efficiency in ambient conditions. The typical structure of an all-oxide solar cell, as shown in the

Spinel Oxide Solar Cells

Spinel oxide materials possessing the stoichiometry AB2X4, are also potential materials for all-oxide solar cells due to their light absorbing capacity. In the crystallite, X are anions, mostly chalcogens.

A and B are cations which occupies most or all the octahedral and tetrahedral sites in the lattice [START_REF] Alizadeh | Dye sensitized solar cells go beyond using perovskite and spinel inorganic materials: A review[END_REF][START_REF] Yan | Effect of Spinel Inversion on (CoxFe1-x)3O4 All-Oxide Solar Cell Performance[END_REF][START_REF] Wei | Electronic Structure and Stability of Spinel Oxides[END_REF]. With an optimal bandgap of around 1.5 eV, spinel oxide based solar cells could reach up maximum conversion efficiency theoretically [START_REF] Shockley | Detailed Balance Limit of Efficiency of p-n Junction Solar Cells[END_REF][START_REF] Wei | Electronic Structure and Stability of Spinel Oxides[END_REF]. Using (CoxFe1-x)3O4 as a spinel oxide absorber in a multi-layered geometry, a better photovoltaic performance was achieved with Jsc of 53 mA/cm 2 and Voc of 534 mV. The Co/(Co+Fe) composition ratio was maintained at approximately 55 % [START_REF] Yan | Effect of Spinel Inversion on (CoxFe1-x)3O4 All-Oxide Solar Cell Performance[END_REF]. The structural stability of spinel oxides should be studied extensively as the distribution of cations and anions could seriously alter the material properties [START_REF] Wei | Electronic Structure and Stability of Spinel Oxides[END_REF][START_REF] Julien | Comparative Issues of Cathode Materials for Li-Ion Batteries[END_REF].

"All-Oxide" Structures using Metal Oxides

Using earth-abundant elements, oxides have a large oxygen electronegativity and thus can create stable compounds. Oxides can possess varied conductivity and can be insulators, semiconductors, or even conductors and can be used in photovoltaic devices in different ways [START_REF] Tsuda | Electronic conduction in oxides[END_REF][START_REF] Pérez-Tomás | Metal Oxides in Photovoltaics: All-Oxide, Ferroic, and Perovskite Solar Cells[END_REF]. The focus in this section is to summarize the basic properties of oxides for solar cells, to discuss about "alloxide" solar cells using Cu2O as an absorber.

ZnO and Related Materials

Zinc oxide has an optical bandgap of around 3.3 eV and is electrically conductive due to its nonintentional doping. It is very transparent in the visible region of the spectrum and can be used as a transparent conducting contact (TCO for transparent conducting oxide) [START_REF] Granqvist | Transparent conductors as solar energy materials: A panoramic review[END_REF][START_REF] Fortunato | Transparent Conducting Oxides for Photovoltaics[END_REF]. Most often, in CIGS solar cells a stack of ZnO and ZnAlO layers are used as electron conducting layers for the front contacts [72,[START_REF] Wallin | World-record Cu(In,Ga)Se2 -based thin-film submodule with 17.4% efficiency[END_REF]. In polymer based solar cells, for achieving a selective contact, ZnO has been used [START_REF] Bouclé | Hybrid polymer-metal oxide thin films for photovoltaic applications[END_REF][START_REF] Ravirajan | Hybrid Polymer/Zinc Oxide Photovoltaic Devices with Vertically Oriented ZnO Nanorods and an Amphiphilic Molecular Interface Layer[END_REF]. Beyond ZnO, other wide bandgap oxides, such as ITO (Indium Tin Oxide), TiO2, NiO, V2O5, MoO3 and WO3 are commonly used in solar cells [START_REF] Meyer | Transition Metal Oxides for Organic Electronics: Energetics, Device Physics and Applications[END_REF][START_REF] Ratcliff | Selective Interlayers and Contacts in Organic Photovoltaic Cells[END_REF][START_REF] Singh | Solid state physics[END_REF][START_REF] Hutson | Hall Effect Studies of Doped Zinc Oxide Single Crystals[END_REF][START_REF] Orton | The Hall effect in polycrystalline and powdered semiconductors[END_REF].

P-type Oxide Absorbers

P-type oxides forms with n-type transparent wide bandgap oxide the p-n junction with the built-in electric field necessary to separate the photogenerated electron-hole pairs [START_REF] Wooten | Optical Properties of Solids[END_REF].

It is an important criterion for all-oxide solar cells to have direct bandgap semiconductors for absorbers with an absorption coefficient higher than 10 4 cm -1 [START_REF] Rühle | All-Oxide Photovoltaics[END_REF][START_REF] Rühle | CHAPTER 8. All-oxide Photovoltaics[END_REF].

Cu2O as an Absorber in "All-Oxide" Solar Cells

Cu2O is among the most frequently investigated light absorber material for all-oxide photovoltaics with a bandgap near 2.2 eV and a theoretical photovoltaic efficiency of 20 % under AM1 solar illumination [START_REF] Shockley | Detailed Balance Limit of Efficiency of p-n Junction Solar Cells[END_REF][START_REF] Rühle | All-Oxide Photovoltaics[END_REF][START_REF] Rühle | Tabulated values of the Shockley-Queisser limit for single junction solar cells[END_REF]. In addition, Cu2O has relatively high hole mobility and high absorption coefficient and can be used with n-type wide oxides such as, ZnO, ZnAlO, Ga2O3 and TiO2 [START_REF] Minami | Heterojunction solar cell with 6% efficiency based on an n-type aluminum-gallium-oxide thin film and p-type sodium-doped Cu2O sheet[END_REF][START_REF] Minami | Photovoltaic Properties in Al-doped ZnO/non-doped Zn1-XMgXO/Cu2O Heterojunction Solar Cells[END_REF][START_REF] Ievskaya | Fabrication of ZnO/Cu2O heterojunctions in atmospheric conditions: Improved interface quality and solar cell performance[END_REF][START_REF] Pavan | TiO2/Cu2O all-oxide heterojunction solar cells produced by spray pyrolysis[END_REF].

In tandem solar cells, Cu2O along with ZnO has been used as a sub cell [149] and with perovskite solar cells [START_REF] Zuo | Solution-Processed Cu2O and CuO as Hole Transport Materials for Efficient Perovskite Solar Cells[END_REF].

For Cu2O based all-oxide devices, the efficiency values remain far below the theoretical value of Cu2O is prepared using thermal oxidation of Cu unless otherwise stated. CVD: chemical vapor deposition; ECD: electrochemical deposition.

Heterojunctions Based on ZnO/Cu2O Structure

ZnO/Cu2O heterojunctions have been widely studied towards their potential application in low-cost displays and photovoltaic devices. Cu2O exhibits a bandgap value of about 2.2 eV, which makes it suitable to be used in combination with ZnO for applications mentioned above. A compromise between transparency and electrical performances is often needed depending on the type of application of the ZnO/Cu2O heterojunction [START_REF] Nandy | A Review on Cu2O and CuI-Based p-Type Semiconducting Transparent Oxide Materials: Promising Candidates for New Generation Oxide Based Electronics[END_REF][START_REF] Traverse | Emergence of highly transparent photovoltaics for distributed applications[END_REF].

ZnO/Cu2O heterojunctions have been successfully used for photodetectors with nanostructured architectures [START_REF] Deo | Strong photoresponse in a flip-chip nanowire p-Cu2O/n-ZnO junction[END_REF]. A fast response self-powered photo-detector device was designed using ZnO/Cu2O core shell nanowires [START_REF] Ghamgosar | ZnO-Cu2O core-shell nanowires as stable and fast response photodetectors[END_REF]. Although charge collection remained a matter of concern, but reduction of the optical depth by nanowire architecture improved the solar cell performance.

In this thesis work, we will present the results of preparation of thin films of metal oxides using the very low-cost, versatile, low energy consuming and non-vacuum technique of Ultrasonic Spray Pyrolysis (USP). Achieving the optimum quality window, buffer and absorber layers are the main objectives of the thesis. Material and electrical characterizations are performed to demonstrate the material quality towards their application in "all-oxide" solar cells.

Before proceeding towards the material preparation, we did a detailed study using numerical simulations to understand the choice of the different layers, their growth by USP and impacts on solar cell performances. The main findings from this first published numerical study of the ZnAlO/Zn1-xGexO/Cu2O heterostructure are discussed in the following sub-section.

Numerical Investigations of "All-Oxide" Solar Cell Performances

To establish the potential of using metal oxide materials for future photovoltaic technologies, several studies have been performed in the past two decades. Among several materials, Cu2O being the mostly researched material for the absorber layer. Studies on optimization of "all-oxide" solar cells are being performed to carefully select the window, buffer, and the absorber materials. The low electron affinity (χ = 2.1 eV) of Cu2O creates a conduction band offset with the buffer layer, mostly

ZnO (χ = 4.4 eV). Thus, to decrease the band offset, a layer of zinc germanium oxide with low electron affinity was introduced experimentally by Minami et al. [START_REF] Minami | Efficiency enhancement using a Zn1-xGexO thin film as an n-type window layer in Cu2O-based heterojunction solar cells[END_REF]. In the first part, we performed numerical simulations based on the ZnAlO/Zn1-xGexO/Cu2O heterojunction. Physical parameter values were carefully chosen from various literatures and the simulation results are compared with the solar cells obtained by Minami et al. [START_REF] Minami | Efficiency enhancement using a Zn1-xGexO thin film as an n-type window layer in Cu2O-based heterojunction solar cells[END_REF]. For the extraction of the solar cell figures of merit, the modelling is performed by using Silvaco Atlas® simulator. To account for unwanted phases of copper oxide, at the interface of Cu2O with the buffer layer an interface and defect layer is introduced in agreement with previous study [START_REF] Takiguchi | Device simulation of cuprous oxide heterojunction solar cells[END_REF]. The electrical parameters for Cu2O are chosen from [START_REF] Minami | Efficiency enhancement using a Zn1-xGexO thin film as an n-type window layer in Cu2O-based heterojunction solar cells[END_REF] with Hall mobility value of 100 cm 2 /Vs and hole concentration of the order of 1o 15 cm -3 . The optical parameters are chosen from [START_REF] Malerba | Absorption coefficient of bulk and thin film Cu2O[END_REF]. The electron affinity values can vary with respect to the experimental growth conditions. We chose a value of χ = 3.2 eV in accordance with experimental measurements of the literature [START_REF] Takiguchi | Device simulation of cuprous oxide heterojunction solar cells[END_REF]. For the top window layer, ZnAlO, the doping concentration (7.64×10 20 cm -3 ) and electron mobility (30 cm 2 /Vs) are chosen from [START_REF] Minami | Efficiency enhancement using a Zn1-xGexO thin film as an n-type window layer in Cu2O-based heterojunction solar cells[END_REF][START_REF] Minami | Impurity-doped ZnO Thin Films Prepared by Physical Deposition Methods Appropriate for Transparent Electrode Applications in Thin-film Solar Cells[END_REF] for a PLD deposition process. The optical parameters are selected from [START_REF] Postava | Doping effects on optical properties of epitaxial ZnO layers determined by spectroscopic ellipsometry[END_REF][START_REF] Elanzeery | Refractive index extraction and thickness optimization of Cu2ZnSnSe4 thin film solar cells[END_REF]. The experimental data, carrier lifetimes and defect concentrations are chosen in order of nanoseconds and 10 18 cm -3 respectively [START_REF] Lettieri | Recombination dynamics of deep defect states in zinc oxide nanowires[END_REF][START_REF] Kubota | Recombination dynamics of excitons in Mg0.11Zn0.89O alloy films grown using the high-temperature-annealed self-buffer layer by laser-assisted molecular-beam epitaxy[END_REF]. As, for the buffer layer, Zn1-xGexO, experimental data is not available, so an estimation of bandgap energy, electron affinity and permittivity are done through linear interpolation, details of which can be found in our research article [START_REF] Chevallier | Numerical investigations of the impact of buffer germanium composition and low cost fabrication of Cu2O on AZO/ZnGeO/Cu2O solar cell performances[END_REF]. The estimated values of bandgap energy, electron affinity and permittivity for the compound Zn0.38Ge0.62O, are 4.97 eV, 3.11 eV and 6.18 respectively. The other parameter details for all the layers can be found in the Table 1 of our article [START_REF] Chevallier | Numerical investigations of the impact of buffer germanium composition and low cost fabrication of Cu2O on AZO/ZnGeO/Cu2O solar cell performances[END_REF]. In the simulation model, the solar cell performances for different Ge compositions in the buffer layer are evaluated and compared with the results from Minami et al. [START_REF] Minami | Efficiency enhancement using a Zn1-xGexO thin film as an n-type window layer in Cu2O-based heterojunction solar cells[END_REF]. Without the interface and defect layers, a maximum PCE of about 12 % was predicted (red curve in Figure 1. 16 with Ge composition x = 0.4). However, the experimental optimum (black dots in 2 in our article [START_REF] Chevallier | Numerical investigations of the impact of buffer germanium composition and low cost fabrication of Cu2O on AZO/ZnGeO/Cu2O solar cell performances[END_REF].

With the understanding of the performance of "all-oxide" solar cells from the simulation mentioned above, it is now important to assess the performance of "all-oxide" solar cells when the layers are grown by the method of ultrasonic spray pyrolysis (USP). As the photovoltaic performance is poor (PCE below 1 % [START_REF] Pavan | TiO2/Cu2O all-oxide heterojunction solar cells produced by spray pyrolysis[END_REF][START_REF] David Prabu | Effect of spray pressure on optical, electrical and solar cell efficiency of novel Cu2O thin films[END_REF]) when metal oxides are grown by USP, it is important to investigate the reasons for such low performance. One of the key reasons is the thickness reduction of the absorber layer (from 200 µm down to less than 1 µm) for USP process. The effect of such thickness reductions on the solar cell performances were investigated in our article [START_REF] Chevallier | Numerical investigations of the impact of buffer germanium composition and low cost fabrication of Cu2O on AZO/ZnGeO/Cu2O solar cell performances[END_REF].

A constant decrease in PCE (from 6.6 % to 3.65 %) is observed with the thickness reduction due to the loss in JSC (shown in Figure 1.17). Nevertheless, a PCE of 4.2 % is achieved for a 1 µm thick absorber layer. This value is although higher than most observed values for devices grown by USP (below 1 %) [START_REF] Pavan | TiO2/Cu2O all-oxide heterojunction solar cells produced by spray pyrolysis[END_REF][START_REF] David Prabu | Effect of spray pressure on optical, electrical and solar cell efficiency of novel Cu2O thin films[END_REF], which suggests that there are other factors involved for solution-based processed for the loss of PCE. For ZnAlO grown by USP [START_REF] Ravichandran | Properties of Sprayed Aluminum-Doped Zinc Oxide Films-A Review[END_REF], the electrical properties are nearly like that grown by standard process of PLD. But, for Cu2O, the properties of films grown by USP differs a lot from films grown by thermal oxidation. The electrical properties can even vary a lot depending upon the spray system, doping and other deposition conditions. The Hall mobilities can vary from 0.45 to 22 cm 2 /Vs while carrier concentration varies from 2.68×10 15 cm -3 to 2.1×10 17 cm -3 [START_REF] Pavan | TiO2/Cu2O all-oxide heterojunction solar cells produced by spray pyrolysis[END_REF][START_REF] David Prabu | Effect of spray pressure on optical, electrical and solar cell efficiency of novel Cu2O thin films[END_REF][START_REF] Santhosh Kumar | Improving the conductivity of cuprous oxide thin film by doping Calcium via feasible nebulizer spray technique for solar cell (FTO/ZnO/Ca-Cu2O)[END_REF][START_REF] Kardarian | Effect of Mg doping on Cu2O thin films and their behavior on the TiO2 /Cu2O heterojunction solar cells[END_REF]. To begin the evaluation, a typical hole mobility value of 5 cm 2 /Vs was chosen along with a doping concentration of 10 16 cm -3 . Details of other values can be looked up on our article [START_REF] Chevallier | Numerical investigations of the impact of buffer germanium composition and low cost fabrication of Cu2O on AZO/ZnGeO/Cu2O solar cell performances[END_REF]. With these values for Cu2O, a PCE value of 3.6 % was obtained, which is not far below than previously obtained by us for a 1 µm thick Cu2O. So, this does not account for all other loss factors which takes the experimental value to below 1 %. In the next part, some more factors for the loss analysis are investigated.

Figure 1.17: Variation of the solar cell performances with the absorber thickness [START_REF] Chevallier | Numerical investigations of the impact of buffer germanium composition and low cost fabrication of Cu2O on AZO/ZnGeO/Cu2O solar cell performances[END_REF].

At first, the doping concentration in the absorber is varied in the range from 10 18 cm -3 to a very minimal 10 12 cm -3 . All other parameters as mentioned above are kept constant. Increasing the doping concentration beyond 10 16 cm -3 reduces the VOC and JSC and the PCE as well. For lower doping concentration the effects are minimal. With higher doping, the decrease in the space charge region decreases the carrier collection efficiency while for lower doping, the space charge region (maximum 0.974 µm) is limited by the absorber thickness itself (1 µm). The effects of the variation of the absorber doping concentration are shown in the Figure 1.18. Regarding the introduction of the defects of the absorber in the simulation model, it is seen that the JSC is greatly impacted (reduced) when the defect concentration is increased beyond the doping concentration of 10 16 cm -3 .

Even the PCE rapidly drops below 1 % (shown in Figure 1. 19). Thus, it can be said that high defect concentration is a crucial factor for Cu2O grown by USP. This factor was also hypothesized by the authors in [START_REF] Pavan | TiO2/Cu2O all-oxide heterojunction solar cells produced by spray pyrolysis[END_REF] as they found a very low JSC of 0.4 mA/cm 2 .

Another important factor considered for our simulations is the carrier mobility. Mobility for Cu2O could vary from 200 cm 2 /Vs (for thermal oxidation) down to 1 cm 2 /Vs (for USP). For our simulations, the hole mobility is varied between 0. From the analysis above, we have understood the limiting factors for the performance of "all-oxide" solar cells. In this section we will discuss about the guidelines for the fabrication of "all-oxide" solar cells with USP. To improve the photovoltaic performance, we first estimate a 2 µm thick absorber with hole and electron mobility values of 25 cm 2 /Vs and 50 cm 2 /Vs, thereby maintaining the electron to hole ratio to 2. We keep some flexibility to select these values for USP, as these can be obtained for growth by sputtering [188]. We can further increase the absorber thickness beyond 2 µm but keeping the polycrystalline nature of the layer in mind, it would be very challenging to obtain such thickness by USP. Also, regarding the Ge composition in the buffer layer, the optimum might also depend on the optimal absorber layer parameters. The solar cell performance by varying the absorber doping and the Ge composition and keeping the other parameters fixed are shown in the With 2 µm absorber, a doping concentration of 10 15 cm -3 can be used with Ge composition of x = 0.50 for the buffer. With these, the solar cells can reach a PCE of 4.98 %, which is relatively close to the experimental maximum 6.68 % [START_REF] Minami | Efficiency enhancement using a Zn1-xGexO thin film as an n-type window layer in Cu2O-based heterojunction solar cells[END_REF]. Another slight modification to the simulation model is done without considering the interface defect layers at the buffer-absorber heterojunction. A doping concentration of 10 14 cm -3 is chosen for the absorber with the Ge composition at x = 0.45, thus yielding a PCE of 9.57 %. We further assume that with an overall improvement of the "all-oxide" fabrication by USP like introduction of back surface field and anti-reflection coatings it is further possible to enhance the PCE.

Objectives and Challenges of the Present work

Based on the variety of applications of metal oxide thin films/layers, in this thesis the main objective was to prepare such thin films/layers by using the versatile and low-cost technique of Ultrasonic Spray Pyrolysis (USP). USP is being used for preparation purpose to reduce the production cost thus making it economically feasible. The USP can be easily integrated for industry standards for preparation on large areas. Metal oxides are a family of materials which have very high potential for photovoltaics: they are abundant in the earth's crust, are non-toxic and can be procured easily, as detailed in section 1.3. Moreover, preparation of metal oxides is compatible with low-cost techniques such as USP, thereby enabling the design and fabrication of an "all-oxide" solar cell. The entire solar cell structure consisting of transparent top contact layer, window layer, buffer layer and the absorber layer can be fabricated with metal oxides using the technique of USP. This type of fabrication using only one preparation equipment has not been realized till date.

The plan of the research work is to optimize the spray parameters, including the precursor solution optimization to obtain high-quality, homogenous, and stoichiometric metal oxide thin films applicable for solar cells and possible other optoelectronic applications. The thin-film processing technique thus forms the heart of this research work. Keeping the functionality in mind, each layer thus prepared are characterized in terms of their morphological, optical, structural, and electrical properties.

The versatility of the USP technique lies in the fact that, spray parameters can be tailored and carefully controlled for each individual layer with respect to their chemical functionality. This fact also adds to the challenges to obtain the high-quality films.

In our research work, we began the process of preparation from scratch with the USP equipment installed in the laboratory in June 2019, a few months before the start of the thesis. The entire process at the beginning was non-optimized. The research began with the optimization of the precursor solutions and the design and material choice for the substrate holder. These are very sensitive towards the prepared material quality. All the films were prepared on 1 mm thick soda lime glass (SLG).

The range of spray deposition parameters are manifold. The parameters include: the substrate temperature; substrate position; shaping air pressure of spray nozzle; solution flow rate; vertical 29 distance between the substrate and the spray nozzle; nozzle speed; nozzle power; number of spray scans (spray cycles) and dwell time (a pause or stopping time between each spray scan). Each of these spray parameters are intertwined.

For each spray preparation, for a particular film, the parameters were varied, and the resulting film was studied in terms of the morphological, optical, and structural characteristics. The analysis of the results helped to understand the effect of the spray parameters on the prepared film. The results thus obtained were compared to the best quality films obtained by other common preparation techniques with an aim to reach optimal quality for the films prepared by USP. Thus, a round-robin experimental steps were followed for each film and then a set of optimized spray parameters were finalized for a particular film.

Preparation of ternary and quaternary materials are not so common using USP. In our research we will develop both ternary and quaternary thin film with optimum quality to integrate them for usage in "all-oxide" solar cells. The preparation of quaternary thin films using USP will be quite unique as in most cases quaternary thin films are prepared using very expensive vacuum-based techniques like sputtering and pulsed laser deposition (PLD).

Keeping all the above-mentioned factors and challenges in mind, the thesis work will be based on three pillars: (i) the optimization of the ultrasonic spray preparation process; (ii) the morphological, optical, structural, and electrical characterization of the prepared films; (iii) towards realization of an "all-oxide" solar cell device.

As mentioned above, the preparation of all the films towards the realization of "all-oxide" solar cell device using only the cost-effective and versatile process of USP is unique. The expertise thus developed from this research work can be valued not only in academics but also in industry.

Thesis Organization

Overall, the research work in this thesis deals with the optimization of spray preparation parameters to produce thin films of earth-abundant and non-toxic metal oxides with optimal quality to be applicable for "all-oxide" solar cell devices. The manuscript is organized in total 7 chapters.

Chapter 1 is a general introduction about photovoltaics and oxide semiconductors. The growth of photovoltaics and its importance in the modern-day energy demand scenario are discussed. The basics of solar cells along with the different types of solar cells (classified according to generations) are also discussed. In this context, in the second part of the chapter, the basics of metal oxide semiconductors are discussed along with the literature review of the types of "all-oxide" solar cells with focus on the metal oxides. The details about the mostly researched ZnO-Cu2O heterostructures for "all-oxide" solar cells are discussed. Also, simulation performed for performance "all-oxide" solar cells along with the suitable choice of materials are discussed. In the last part, the objectives and challenges for the thesis work are mentioned along with the detailed literature survey.

Chapter 2 is dedicated to the basic principles of ultrasonic spray pyrolysis (USP). USP is employed for the preparation of the window, buffer, and absorber layers. The fundamental techniques of the characterization tools used for the morphological, optical, structural, and electrical characterizations are also discussed. Chapter 5 is about the preparation of the absorber layer. Cuprous oxide (Cu2O) is the absorber layer prepared with keeping few challenges in mind, the foremost one being the control of the spray parameters to keep the "cuprous" phase (Cu2O) intact. It has been seen that Cu2O to quickly change to cupric phase (CuO) which can be detrimental to solar cell performances. The spray parameters are optimized to have the cuprous phase after preparation. Also, the role of a new reducing agent, D-sorbitol is analyzed in detail.

Chapter 6 focus on the possibilities to tune the optoelectronic properties of oxide alloys to pave the way to the fabrication of the entire "all-oxide" solar cell. In the first step, aluminium is doped with ZnO to prepare a top contact layer of ZnAlO with optimized spray parameters. In the second step, for the first time using USP, a quaternary film of Zinc Magnesium Aluminium Oxide (Zn1-x-yMgxAlyO) is prepared. Zn1-x-yMgxAlyO can be of multi-purpose use, it can be designed to be used as a top contact/window/buffer layer. Lastly, an "all-oxide" device was modelled using the Solis simulation software.

Chapter 7 is the concluding chapter. It emphasizes the use of USP for preparing high-quality metal oxide thin films. All the important results obtained are summarized in this chapter. An outlook is written mentioning about the prospects of the research work.

Introduction to Spray Pyrolysis

Material preparation systems comprising of solution-based techniques are becoming more and more attractive for material production related industries due to their simplicity combined with high throughputs with reduced costs. Solution-based techniques include spray pyrolysis, spray painting, sol-gel, spin coating, spray casting, slot-die coating, doctor-blading, knife-over-edge coating, screen printing, pad printing, offset printing, flexographic printing, gravure printing and others [1]. Spray pyrolysis techniques gained a lot of popularity during the last decade due to their flexibility to deposit a wide variety of thin film materials. Thin films metal oxides (both binary and ternary), metallic spinel oxides, chalcogenides (binary, ternary, and quaternary), kesterites (quaternary) and superconducting oxides among others [2]. Not only the process of spray pyrolysis is simple, but it is also attractive due to its cost-effectiveness. Many studies have been done on spray pyrolysis towards the preparation of thin films but Chamberlin et al. [3] first reported a CdS thin film prepared by spray pyrolysis.

Despite being a cost-effective and simple process, spray pyrolysis has another major advantage due to its tuneable parameters. These parameters in turn makes the spray process widely flexible for preparation purpose without requiring high quality targets or substrates. Even the temperature range of 100 °C to 600 °C (depends on the specific tool set-up) is ample. The deposited rate can be controlled by adjusting the flow rate and the speed of the nozzle, but with a limit on the thickness of the films. Control of the film is important as with higher thickness there is a tendency for the film quality to deteriorate, especially without control and optimization of the spray parameters. So, spray conditions need to suitably be tailored based on the type of the substrate and the thickness of film desired. Basically, with lower financial investments needed and subsequently with lower production costs spray pyrolysis is an interesting thin film deposition technique. Nonetheless, there are some drawbacks such as, the grain sizes in the films are relatively small [4,5], presence of unwanted impurity phases in the films resulting from incomplete pyrolysis which are hard to remove [5][6][7] and often films have high surface roughness [5,8].

In the first half of the chapter different aspects of spray pyrolysis for preparation of thin films are discussed. Types of spray pyrolysis with a focus on ultrasonic spray pyrolysis (USP) is discussed with description of the actual spray system used. All the spray parameters involved in the preparation process are also discussed. The physical processes involved in the USP technique are discussed in detail. The characterization techniques used to investigate the prepared thin films are detailed in the last part of the chapter.

The deposition technique using spray can be known by various names: chemical spray [9], spray coating [9], spray [9,10], spray pyrolysis [11] and combinations of all these. Generally, the different names are given based on the source of the precursors and the processes involved in the formation of the films. More specific names are assigned to this deposition technique based on the methods of atomization. Each atomization technique is unique in terms of the control of the sizes of the generated droplets and their distribution and decomposition across the pre-heated substrates.

Depending on the atomization technique, the types can be named as: corona spray pyrolysis [12],

electrostatic spray pyrolysis [13,14], nebulized spray pyrolysis [15], ultrasonic spray pyrolysis [4,16] and pneumatic spray pyrolysis [17]. The spray systems can be further classified in two more categories according to the relative direction of the droplets: horizontal and vertical configurations [11]. The horizontal configuration permits a separation of the droplets/particles according to their size, while in the vertical configuration the droplets/particles possess a certain velocity provided by the carrier gas and thus undergo a free fall motion [11,18]. Spray systems can even be names on the process of substrate heating. When the hot walls of the vertical or horizontal reactor heats up the substrate, then the system is called hot-wall spray. If the heating of the substrate happens by the direct contact with the heat source (hot-plate or tin-bath) then the spray system is called cool-wall spray.

Research has been carried on for decades to understand and explain all the physical and chemical processes that leads to the formation of films by spray pyrolysis; but the understanding is not yet complete. To understand the underlying physics of the droplets transport process there are both empirical [19] and theoretical [20] models. Considering some factors such as the extremely large number of atomized droplets in a spray process with high and varying velocity, very differing droplet sizes along with the change in time due to coalescence and evaporation, the temperature gradient, the regions with gas between the neighbouring droplets etc., are typically very complex to understand. Depending solely on computational resources are insufficient. All the dynamical and thermal interactions coupled with the large system of equations cannot be solved by only computation. This is the reason why the theoretical explanation of the spray deposition become extremely important. The theoretical explanations consider ideal bounded conditions to simplify the hypothesis thus revealing the level of complexity of the spray process. In this thesis an attempt has been done to explain the parameters leading to the formation of high-quality metal oxide thin films suitable to photovoltaics.

understanding in detail the plethora of spray parameters from the perspective of film quality. They mentioned about the importance of the process optimization for each change of spray parameter for each different kind of film with different material. According to Perednis et al. [19], different processes for spray preparation of a material, occur either sequentially or simultaneously.

Understanding of each of the process is essential to improve the film quality.

From the studies by Perednis et al. [19], Filipovic et al. [20] and others, a general agreement can be reached regarding the steps followed in the deposition and growth of thin films by spray pyrolysis.

The steps are mostly deduced from experimental observations following a logic sequence. Following the logic, the entire process can be broadly classified into three main steps:

(i) Spray of the atomized precursor solution.

(ii) Transport of the aerosol (resultant droplets).

(iii) Pyrolysis (decomposition) of the atomized precursor solution droplets.

Step (i): Spray of the atomized precursor solution:

In the first step, droplets are generated from the precursor solution by the process of atomization and then forms a spray. Hence, the spray in this step can be explained as: formation of very tiny droplets by breaking up a liquid (precursor solution). The droplets are then blown or forced through the air. This forms a classical definition of spray [21]. A typical cross-section of a spray generation system is shown in the The spray pattern is entirely dependent on the nozzle as different nozzles generates different droplet sizes with differing velocity, spray shape and coverage area.

For our preparations we have implemented ultrasonic nozzles in which the high frequency sound waves are converted into mechanical energy which is then transferred into the liquid (precursor solution). With the transfer of the mechanical energy to the liquid, a standing wave is created and as the liquid exits the surface of the nozzle it gets atomized, i.e., it gets broken into fine mist of uniform micron sized droplets. Unlike pressure-based nozzles, in ultrasonic nozzles the liquid flows through the nozzle center (with a large orifice) without pressure and the atomization happens only due to the ultrasonic vibrations in the nozzle. The median droplet size depends on the specific resonant frequency of the nozzle. The mist of droplets thus created (also known as aerosol) are guided towards the substrate by a low velocity spray shaping air (compressed carrier gas). The shaping air controls the shape/pattern of the spray which can be fine lines, conical patterns or wide flat fan shaped [2,22,23].

Step (ii): Transport of the aerosol (resultant droplets):

This step concerns the transport of the generated droplets. During the transport, the droplets will experience physico-chemical changes. The changes are dependent on the location, velocity, and size of the generated atomized droplets and on the temperature of the substrate. As the droplets travel, four forces can act upon them simultaneously: gravitational, electrical, Stokes and thermophoretic [24].

As the droplets travel their size will be reduced. It happens as the solvent undergoes evaporation and the rate of which also determines the droplet trajectory. The evaporation rate is higher when the temperature of the substrate is higher than the boiling point of the solvent. The unevaporated droplets will experience a gravitational force pulling them downward vertically towards the substrate [19]. In the titanium nozzles, a generator is vibrated at ultrasonic frequencies resulting in smaller droplet sizes with a force that is much higher than the gravitational force [2,24].

The thermophoretic force acts mostly near the substrate surface. It acts as a retarding force by pushing the droplets away from the substrate's hot surface and ultimately decreasing their velocity.

The important factors that determine the thermophoretic force are heat conductivity and the heat absorption rate of the droplets in addition to the size and viscosity of the droplets. The magnitude of the temperature gradient is also directly proportional to the force. When the droplets are approaching the preheated substrate, they acquire thermal energy from the hot air atmosphere in the direction of motion. On touching the substrate, the hotter side of the droplets gets rebounded with a higher kinetic energy than compared to the cooler side. This is a forced convection mechanism [19,20,24].

The Stokes forces are dominant in Step (ii) and they are caused as friction happens between the droplets and the air molecules. So, the Stokes force can be defined as a drag force that the droplets experience against the air molecules. The magnitude of the Stokes force is directly dependent upon the size of the droplets, the viscosity of the air molecules (air vapour) and the relative velocity between the air molecules and the droplets [20,24].

A schematic of the process of transport of aerosols is shown in the Step (iii): Pyrolysis (decomposition) of the atomized precursor solution droplets: Process 1: If all the solvent does not vaporize due to the temperature not being high enough, then unevaporated solvent droplet splatters on the substrate. After the droplet is on the substrate, then the vaporization starts leaving a dry precipitate and then the decomposition starts [19,25].

Process 2:

The size of the droplet gets reduced with an increase in the temperature. The solvent gradually evaporates which leads to not only the reduction in size but also a concentration gradient within the droplet. The precursor tends to precipitate on the droplet surface when the concentration exceeds the solubility limit of the solvent. Eventually, a dry precipitate reaches the substrate which leads to the formation of unwanted particles in the sprayed films. The particles are not desired as they tend to remain on the substrate surface while adding to the roughness and loss of texture of the thin films [19,[25][26][27].

Process 3: When the hot atmosphere of the spray system causes the droplets to evaporate then a precipitate will form. A sublimation (vaporization) process takes place when the precipitate is in the proximity of the hot substrate. The vapor thus generated, diffuses to the substrate top and undergoes a complete heterogeneous reaction. This results in high quality depositing of a film with a high sticking probability. This process is comparable to a chemical vapor deposition (CVD) process [19,[25][26][27][28].

Process 4: If a droplet is too far from the substrate or if the droplet size is too small then it quickly forms a precipitate. Precipitate of the droplet can even form when the droplet is closer to the substrate if the substrate temperature is too high. When the precipitate is in the proximity of the hot substrate, vaporization happens. In this vapor phase a chemical reaction starts which is homogenous in nature. The reaction leads to the formation of crystallites by condensation. So, the crystallites get a form of powder precipitate and ultimately this powder without any further reaction reaches the substrate surface and sticks to it [19,24,25,27].

The condition for an ideal deposition is that the solvent gets completely removed (vaporized) because of the temperature as the droplet approaches the substrate surface (similar situation to process 3). However, the atomization of the precursor solution results in droplets of differing sizes, so all the four processes mentioned above occurs simultaneously. When the droplets are larger, they tend to carry most of the solvent salts and for these droplets the substrate temperature needs to be more optimized. For these droplets the powder formation during transportation is improbable. If the droplet size distribution is homogenous, it will be more compatible to have a deposition as in process 3. When the larger droplets will reach the substrate surface they may splash, stick, rebound or spread on the surface. Usually, dense films or rings form when the droplets spread on the surface.

Rings are formed when the droplet velocity is high. Due to the thermophoretic force, the smaller droplets tend to rebound. If they are too fast in the movement, they might splash otherwise they can stick to the surface. The droplets which spread fast leads to high viscosity and dense films with the necessary heat from the substrate so that the solvent evaporates, and the salt completely decomposes. Without proper decomposition, even for viscous films cracks can happen due to stress build up [19,20,24,29]. The different types of droplet impact on heated substrates are shown in the The four processes explained above is according to the hypothesis proposed by Perednis et al. [19],

Viguie et al. [25] with some modifications. Also, research by Rajan et al. [23], Cloupeau et al. [26],

Sears et al. [28] and Albin et al. [27] have been considered while describing the four processes. The complete process of deposition can be schematically designed from the hypothesis of Viguie et al.

Experimental System Setup

Like typical ultrasonic spray systems, our spray pyrolysis equipment consists of the following arrangements: ultrasonic spray nozzle, precursor solution, substrate heater with a temperature controller, liquid flow meter, air pressure controller and others [29]. Parameters can be changed for each material deposition process. The flexibility to change the parameters allows to determine the required conditions to deposit a film of high quality. The set-up of the entire spray system along with the characteristics of the precursor solution will have a significant impact on the preparation of the film. For preparation of different materials, always the parameters are adapted to have a highquality film of that material. A schematic of the spray system setup used in this thesis is shown in the Figure 2.6. The gas pressure control and the liquid flow meter are not shown for simplicity.

The precursor solution container and the central command computer (microcontroller system) is outside of the closable stainless-steel chamber. All other equipment is inside the chamber. The gases generated due to the pyrolysis are extracted from the chamber using a suction pump. So, all the films are prepared under controlled atmosphere.

Figure 2.9:

The evolution of median drop sizes with different ultrasonic frequencies for IPA [22].

In the thesis, for the preparations of the different films we have used two different types of ultrasonic nozzles: Accumist and Impact. Both operate at 120 kHz frequency. For the Accumist nozzle, the droplets first converge at a focal point and then diverges out. This spray shape with an 55 adjustable focus region allows complete control on the spray width. The spray envelope is very narrow and hourglass-shaped. By adjusting the nozzle-to-substrate height, the width of the spray shape can be controlled. Typically, the width varies between 2 and 25 mm. For the Impact nozzle, the atomized spray droplets are entrained in the air stream at the nozzle's atomizing surface creates a fan-shaped spray pattern. The velocity of the air stream can be controlled to have low or high impacts of the atomized spray droplets onto the substrate. This versatile system can generate spray patterns up to 15 cm depending on the height between the nozzle and the substrate. The Impact nozzle is mostly required for wide area applications. For the air/gas inflow to the nozzle to have the desired spray shape is controlled by a pressure flow regulator. The movement of the spray nozzles according to the desired spray pattern can be controlled using specific machine-oriented programs.

The spray systems employ robust ball-screw slides driven by brushless DC servo motors for this The temperature of the heating plate (above which the substrate holder is placed) is measured by a K-type thermocouple and monitored using a PID controller. The thermal inertia of the heater and the dynamic temperature control helps in minimum temperature decrease at the start of the spray and during the entire preparation process. The heating plate was always heated to the specific desired temperature prior to the start of the preparation. During the preparation, the temperature was measured precisely using a N-type thermocouple which was placed in contact with the glass substrate. To protect the heating plate from the chemicals, a substrate holder of 3 mm thickness was placed above covering it. Copper and ceramic substrate holders were used so that the heat distribution to the glass substrate was uniform. 1 mm thick Soda lime glass (SLG) substrates were used for all our preparations as mostly in research cases thin films solar cells are fabricated on SLG substrates. SLG substrates are cheap, hard, chemically stable, easily transmits visible light and with a very high melting point of ≈ 1000 °C.

Spray Parameters

To successfully prepare a thin film a precise control on all the spray parameters is necessary. The optimization of these parameters plays a key role in the quality of the prepared film. All these parameters work in tandem to produce a high-quality film. Higher shaping air pressure brings the focal point down and closer to the substrate which gives incomplete pyrolysis. The pressure needs to be such that the flow of the droplets is complete along with the reaction. For Impact nozzle, the conical spray shape and hence the spray width will be directly related to the shaping air pressure. It needs to be adjusted with respect to the nozzlesubstrate height for a complete pyrolysis reaction to happen.

(iii) Nozzle-substrate height: The height between the nozzle and the substrate such that the droplets can reach the substrate surface without any over-spray. The height must be adjusted in conjunction to the shaping air pressure. The height also determines the area coated. With a high deposition rate, if the height is too small then the coated area is also small. Sufficient height must be allowed for the droplets to converge and then to diverge (for Accumist nozzle) to be deposited on the substrate with the complete pyrolysis.

(iv) Solution flow rate: The solution flow rate (in mL/min) determines the rate at which the precursor solution from the syringe is fed to the nozzle at a constant atomization rate. A high flow rate results in overspray while lower rate results in under-spray. With lower rate, the droplets even before reaching the surface get vaporized and the shaping air pressure drives out the particles before reaching the substrate. With a higher rate, higher number of droplets reaches the substrate without undergoing pyrolysis reaction resulting in over-spray and formation of agglomerates. This also gives rise to pores in the film. The flow rate is also directly linked (proportional) to the thickness of the film but with excessive rates films result to rougher morphologies with pores in the films as observed by Chen et al. [35]. Films with moderate flow rates gives more uniform morphology, contains less agglomerates and are also compact and dense.

(v) Nozzle speed: During the spray the nozzle speed (mm/sec) becomes an important factor. When the spray is happening, the nozzle is equipped to move along x and y axes respectively. The nozzle speed is inversely proportional to the thickness of the film. With very low nozzle speeds, the nozzle scans the substrate very slowly at the fixed flow rate, thus more material is deposited. Also, at this condition, the film experiences the substrate temperature for a longer time which builds up stresses in the film leading to cracks. Too high nozzle speeds are not suitable for the spray system, as the system experiences a vibration due to the fast mechanical movement of the motor connected to the nozzle. This vibration leads to poor quality films and at these high speeds the films are extremely thin with high roughness. So, based on the material to be prepared and on the type of nozzle being used an intermediate speed should be maintained.

(vi) Dwell time: The dwell time is the stopping time (in seconds) between each spray cycle (spray scan). During the to-and-fro movement of the nozzle during preparation, the nozzle can be set to stop for a few seconds. This time is necessary for the droplets to crystalize towards film formation.

The remaining solvent is usually vaporized in this time. But the time should not be long enough otherwise the already hot substrate will dry out the film rapidly leading to stress in the films. This duration depending on the material to be prepared should be set between 0.5 s to 1 s.

(vii) Nozzle power: For achieving best atomization, the nozzle power should be set to 1 watt above the stall point. Stall point is a critical power level below which the atomization process ceases. Below the stall point there is only cavitation where the liquid only wets out without forming atomized droplets. Setting the nozzle power 1 Watt above the stall point produces perfect atomization of the solution without any clogging or wetting. Too high power above the stall point can also cause cavitation. So, the stall point should be checked for each type of precursor solution and then the nozzle power should be set 1 watt above that value. The nozzle power is directly linked to the viscosity, concentration, and surface tension of the precursor solution.

(viii) Spray cycles: The number of spray cycles should be adjusted in conjunction with the other parameters as mentioned above. With more spray cycles we will achieve a film of higher thickness.

The number of cycles should be set according to the desired thickness. However, it must be noted that too high thickness (beyond 1 µm) may lead to high stress and delamination in films. With very high thickness the heat distribution in the film microlayers is not even which probably can lead to delamination.

From the references [36,37], a relation has been deduced about the expected solid film thickness for each spray cycle. The relation is given below:

Equation (2.1): 𝑡 = 𝑞 • 𝑥 0 𝑤 𝑎 • 𝜇 𝑛
Where, t is the film thickness, x0 is the mass fraction of the solute, q is the flow rate, wa is the diameter of the spray and µn is the nozzle speed.

(ix) Spray position:

The spray position should be adjusted according to the size of the substrate with reference to the pattern of the spray. For our experiments, a rectangular shaped glass substrate was used with a to-and-fro movement of the nozzle. So, the length of the spray cycle should cover the entire length of the substrate so that a uniform film is obtained covering the entire substrate.

(x) Precursor solution concentration: the main requirement of a precursor is to be able to form the oxide at a given substrate temperature. The main used precursors are based on acetate salts, or chlorides, or nitrates. Keeping in mind the drawbacks of chlorides and nitrates (toxicity, corrosive, impurities) only acetate-based salts are used for all our preparations. Also, most of the acetate-based salts are easily soluble in water without requiring any alcohols. Alcohols have toxic effects, evaporate quickly and are also flammable at high temperatures. So, the use of alcohols to dissolve precursor salts are avoided in our preparations. The solubility, viscosity, use of additives to reduce surface tension are important factors to consider when preparing the precursor solution. Solubility in relation to the concentration is important. It allows to control the concentration of the precursor to be used. For our preparations, de-ionized water is preferred as a solvent as it has high vapor pressure. The concentration (in mol/L) refers to the proportion of the salts dissolved in the solvent.

The concentration should be carefully chosen such that it maintains the solution stability over time (does not hydrolyze, does not form segregated phases, does not conglomerate or precipitate) and at the end of the pyrolysis it produces the required stoichiometry in the prepared films. The number of spray cycles needed to reach the desired film thickness will also depend on the solution concentration.

The selection of the precursor solution and its optimization must be done before the optimization of all the other spray parameters as detailed above. To conclude about the precursor solutions, it can be rightly said that the solvent, the concentration of the precursor and the solution flux in varying degrees will have major impact on the film formation characteristics. The precursors along with their concentration finally selected to prepare the thin films are explained in the corresponding individual chapters dedicated for specific films.

Summary

In summary we can say that to obtain a film of optimal qualities many parameters have to be considered with a precise control over them. With a specific set of parameters, the films need to be prepared a few times in order not only to understand the film formation better but also to check for the reproducibility. In this thesis, the films are prepared by repeating the process a few times. To establish a co-relation between the system-related and the solution-related parameters Figure 2.11 is drawn schematically. The parameters can be co-related with two main characteristics of the spray prepared high-quality films: the thickness and the composition. These parameters will have an influence on the final device characteristics. However, the final film properties will also depend on some post preparation conditions such as rapid thermal annealing which will be discussed in the experimental chapters of this thesis.

Characterization of the Spray Prepared Thin Films

All the different types of thin films prepared in this thesis work are characterized in terms of their thickness, morphology, optical, structural, and electrical properties. In the following sub-sections, all the tools used for the characterizations are discussed along with the analytical methods.

Thickness and Morphological Measurements

Mechanical and optical profilometers were used to measure the thickness and the roughness of the prepared thin films and their morphology was analyzed by AFM and SEM.

Mechanical and Optical Profilometers

A mechanical profilometer is an equipment that uses as a diamond tip to scan through the surface of a sample by taking each position its vertical coordinate. The profilometer can be used to measure step height and the roughness of the surface profiles. It can also be used to measure stress of thin films deposited on a wafer. The profile which is obtained is a convolution between the real profile and the stylus shape. The minimum width of a laterally measured point is the maximum hole depth that can be measured. Therefore, it is very difficult and often inaccurate to measure hole depths if the lateral dimensions are too short [38]. With the help of digital acquisition, the trace of the surface profile is obtained. Even for thin films this method can be applied to precisely measure thickness down to few nanometres. The roughness of the surface profiles gives additional information about the topography the films.

The system possesses some inherent advantages such as, it is independent of the optical or electrical properties of the material of the film. It can ignore the effects of the pollution as it does not require a vacuum environment or vacuum chamber for its operation. However, we must pay attention to some of its disadvantages. The pressure exerted by the stylus on the sample surface can easily scratch the sample. The force which the stylus exerts on the sample must be calibrated according to nature of the specific sample to avoid any damage to the stylus tip and as well the sample surface.

On the contrary, the sample surface also must be solid enough for the tip so that it can measure the profile without sinking too much into the sample.

In our study, we have used a Dektak XT profilometer from Bruker [39] with a 2 µm tip (picture in Figure 2.12). The analysis of the profiles was done using the Vision64 software which is equipped with a camera along with a pointer for scaling, plotting and analysis tools and therefore allows a quick measurement and an easy qualitative analysis. Optical profilometry measurements are done using Filmetrics Profilm3D optical profilometer [40] based on the principle of vertical scanning interferometry (VSI). Surface topographies can be quantified with a vertical resolution of ≈0.1 nm and a lateral resolution of around 500 nm. The topography of the sample surface including step heights from the interference signal was analyzed by Profilm software. Even the roughness parameters such as arithmetic roughness, total roughness, and the root mean square (RMS) roughness can be (roughly) estimated.

Atomic Force Microscope (AFM)

AFM is used to analyzed the surface morphology of almost any material [43,44,45]. A tip with a nanometric radius of curvature scans the sample surface and the analysis of the deflection of a laser beam on the tip cantilever makes it possible to reproduce the surface topography. The tip can be in contact with the surface (contact mode) or near the surface (non-contact and tapping mode), depending on the material.

In this thesis, Nano-Observer atomic force microscope developed by CSInstruments [46] and available at Institute Jean Lamour Campus Artem was used in tapping mode with an oscillation frequency of 323 kHz with a scan rate of 0.75 Hz. Also, Park NX10 atomic force microscope [47] developed by Park Systems and available at LCP-A2MC Laboratory was used in non-contact mode with an oscillation frequency of 336 kHz with a scan rate of 0.74 Hz.

Scanning Electron Microscopy (SEM)

SEM can be a complementary technique to AFM. With this technique, we can obtain high resolution surface images and their analysis can make it possible to study the phases (and possibly the chemical composition) and the surface morphology. In addition, we can measure the film thickness and analyze the growth of the thin film with a cross-sectional study [48,49].

In this work, to understand the morphology and the growth of thin films, top-view and cross-section scanning electron micrographs were acquired. A FIB-SEM system of the Auriga series from Zeiss [50] with an In-lens detector available at LEM3 Laboratory has been used to acquire the micrographs at an acceleration voltage of 2o kV. The In-lens has been used as it has a very resolution for surface imaging in SE imaging. It is located just above the objective lens and can directly detect the low energy secondary electrons in the beam path [51,52].

Optical Measurements

A UV-VIS-NIR spectrophotometer has been used to measure the transmittance of the prepared thin films. The optical bandgap energy of the thin films was calculated using the Tauc model. Urbach energies of the thin films were also studied to estimate the crystalline lattice disorders.

UV-VIS-NIR spectroscopy is mainly used to characterize the optical transmission of a thin film to determine the optical bandgap, the refractive index, the absorption coefficient and to investigate the sub-bandgap defects. The film is shined with a monochromatic light and the intensity of the transmitted light is measured. The transmittance of the film is defined as the ratio of the transmitted to the incident light intensities. In our work, we have used the glass substrate as a reference and therefore the measured transmittance directly gives the transmittance of the film deposited on the glass. In our setup, the effects of the film-substrate interface (including reflection) are neglected. In this way we found that the film transmittance measurements are very accurate. The principle of UV-VIS-NIR spectroscopy is shown schematically in the In the transmittance spectra is often seen the appearance of a modulation of intensity. This is seen in the form of maxima and minima appearing in the spectra. The analysis of these spectra makes it possible to determine the bandgap and the modulation can give an estimation of the thickness of the thin film [53,54]. To determine the absorption coefficient (α) the standard Lambert-Beer's law [55] is used:

Equation (2.2): 𝛼 = 1 𝑡 × ln ( 1 -𝑅 𝑇 )
Where, T is the transmittance, R the reflectance, and t is the thickness of the film.

For semiconductor materials having a direct bandgap, the optical bandgap (Eg) can be determined from the Tauc plot [56]. In the Tauc method, the relation between the absorption coefficient and the direct bandgap Eg can be written as:

Equation (2.
3):

(𝛼 ℎν) 1/𝑛 = 𝐵 × (ℎν -𝐸 𝑔 )
Where B is a constant and ℎν the photon energy. n = 2 for an indirect bandgap such as CuO. For Cu2O, the direct forbidden process is considered to affect the absorption and n is taken to be equal to 3/2 [57]. For ZnO and related alloys n = 1/2 as usually done for direct bandgap materials.

The absorption coefficient from the Equation (2.2) is also used to estimate the Urbach energy of thin films. Urbach energy is related to the crystalline lattice disorder in the thin films and the values of the Urbach energy of the thin films was obtained from the inverse of the slope of ln(α) versus photon energy (hν) [58]. It is expressed as:

Equation (2.4): 𝛼 = 𝛼 0 𝑒𝑥𝑝 ( ℎ𝜈 𝐸 𝑢 )
Where Eu is the Urbach energy and α0 is a constant.

The determination of the optical bandgap energy and the Urbach energy is shown in the

Structural and Chemical Analysis

The crystalline structure of the prepared thin films is studied by X-ray diffraction (XRD) and by Raman spectroscopy. The chemical analysis of the films has been done by using X-ray fluorescence (XRF) and Fourier Transform Infrared Spectroscopy (FTIR).

X-Ray Diffraction (XRD)

X-ray diffraction is the standard characterization technique used to investigate the structural properties of thin film materials and to determine the crystal type, the lattice parameters, the preferred orientation, the composition, the crystallites size, the defects properties, etc. Generally, XRD setups use wavelengths between of 0.1 and 0.2 nm. The most used wavelength in our topic is 0.15406 nm (CuK𝛼). The basic principle is to measure the diffraction pattern of the thin film and to analyze it, using simulations and crystallographic databases, to identify and study the crystalline phases [59]. Using the classical Bragg's law [60], one can extract the interatomic distance of a particular plane dhkl as a function of the wavelength of the incident radiation (𝜆) and the position of the diffraction peaks (𝜃) [59,60]:

Equation (2.5): 2 𝑑 ℎ𝑘𝑙 𝑠𝑖𝑛(𝜃 ℎ𝑘𝑙 ) = 𝑚 λ
Where m is the order of diffraction.

All the measurements done in this work used the Bragg Brentano Ɵ-2Ɵ configuration, as shown in the Figure 2.17. in which the sample is rotated by an angle of Ɵ and the detector is rotated by an angle of 2Ɵ in order to detect only the crystallographic planes that are parallel to the sample surface.

In our polycrystalline samples, peaks corresponding to different the crystallographic planes are detected.

Figure 2.17: Schematic of the Bragg-Brentano configuration used in XRD. Modified from [61].

In our polycrystalline films, the size of the crystallites can be estimated by using the Scherrer's equation [62]:

Equation (2.6): 𝐷 ℎ𝑘𝑙 = 𝐾 λ 𝛽 cos(𝜃)
Where, Dhkl is the average size (in nm) of crystallite in a direction of plane (hkl), 𝜆 is the X-ray wavelength (in nm), K is a dimensionless shape factor with value of 0.94, 𝛽 is the full width at half maximum (FWHM) of the diffraction peak (radian), after correction for instrumental broadening, and 𝜃 is the Bragg diffraction angle. It should be noted that K can have different values depending on the particle geometry. The Scherrer's equation is limited by the fact it supposes the crystallites to have spherical shape with diameters relatively small, usually less than 100 nm. The average diameter is usually determined by measuring the width of the diffraction peak with the highest intensity in the diffractogram.

In our work, we have used a modified Scherrer method developed by Monshi et al. [63]. In the modified method all the identified crystallite peaks are used to estimate the crystallite size by linearly fitting ln(β) against ln(1/cos(θ)) and taking the value of D from its intercept (shown in the In addition, we have also estimated the texture factor for our thin films. The texture factor is important for polycrystalline films to determine the preferred orientation in the film [64,65]. The texture factor is given by [66]: In this thesis, Bragg-Brentano configuration was used for collecting the X-rays diffractograms. Two Bruker D8 Advanced diffractometers with CuKα1 radiation (λ = 0.15406 nm) available at IJL laboratory and LEM3 laboratory respectively were used. The XRD reference for determining the crystalline structure of the prepared thin films was determined using the JCPDS data provided by Bruker and the RRUFF project database [67].

Equation (2.

Raman Spectroscopy

Raman spectroscopy is another technique used for the determination of the crystal structure of materials. The Raman spectra are different for each material as the spectra mainly depend upon the atomic bonds in the material and the mass of the neighbouring atoms. So, it can be said that for known materials the Raman spectra are unique fingerprints towards material characteristics but for unknown complex materials suitable references should be required [68][69][70][71][72].

Raman spectrometry was performed at 100x confocal magnification using a green laser with an excitation wavelength of 532 nm under z(y-)𝑧̅ configuration to focus the laser light onto the thin film. LabRAM Aramis system from Horiba Jobin-Yvon (Spectroscopies platform of LMOPS laboratory) was used for all the Raman measurements (Figure 2.

19).

Figure 2.19: LabRAM Aramis system from Horiba Jobin-Yvon available at LMOPS laboratory, Spectroscopies platform.

X-Ray Fluorescence (XRF)

When a material is bombarded with high-energy X-rays or gamma rays then there is an emission of characteristic "secondary" (fluorescent) X-rays from the material. This emission can be defined as the X-ray fluorescence (XRF) and this phenomenon can be used widely for the elemental analysis.

When the electrons in the material make transitions between the atomic energy levels then only the characteristic X-rays of the element are emitted. If an electron makes a transition from a level with energy E1 to another level with energy E2, then the emitted X-ray has an energy E = E1 -E2. Each element possesses a unique set of atomic energy levels, so based on the element's characteristic it emits a unique set of X-rays [73][74][75]. X-ray fluorescence system from Horiba Jobin-Yvon (model XGT 7000) [76] available at LMOPS laboratory, Spectroscopies platform, with a controlled X-ray beam diameter ranging from 10 µm to 100 µm was used for the elemental analysis of our prepared thin films on glass substrates.

The XRF technique has some advantages such as: with calibrations the analysis is accurate, it is a fast process with each measurement taking about 10 minutes, non-expensive process and the process is non-destructive but with a limit of acceptable detection range.

Fourier Transform Infrared Spectroscopy (FTIR)

Infrared (IR) spectroscopy is method by which the structure of molecules can be determined using infrared light. The principle of this technique is that it records the changes in the vibrational energy of the molecule. At first, the sample (thin film in our case), absorbs the IR radiation. On absorption, an alteration of the dipole moment of the molecular bond happens and so the vibrational energy level changes from ground state to excited state and this change (gap) in turn determines the absorption frequency. For each molecule type the IR spectrum in unique as each molecule has unique atomic bonds. This property of the molecules makes the qualitative and quantitative analysis of various types of samples possible. In the IR spectra, the unique peak frequency determines the presence of the different functional groups and the quantity of the presence of the groups is determined from the intensity of the peaks. A major advantage of IR spectroscopy is that it can analyze solid, liquid and even gas samples [77][78][79].

The Fourier Transform Infrared (FTIR) spectrometers belongs to the third generation of IR spectrometers. In the FTIR systems instead of a monochromator, a Michelson interferometer is used.

In the spectrum, energy of all the wavelengths versus time are represented. The Fourier transform algorithm is used to interpret the data by translating the spectrum in a way that correlates the signal intensities to the wavelengths [77][78][79].

In our measurements on thin films attenuated total reflection (ATR) mode [80] was used. All the measurements were done using a commercial Agilent FTIR 680 spectrometer available at LMOPS laboratory, Spectroscopies platform. A reference spectrum before each acquisition was acquired to determine the absorption spectra in ambient conditions. To improve the signal-to-noise ratio, each measurement in continuous mode was averaged over 200 scans. With a spectral resolution of 4 cm -1 , all the spectra were acquired in the range of 4000-600 cm -1 .

Electrical Measurements

The resistivity of the prepared thin films is measured using four-point probe method using a manual probe station. The van der Pauw technique is used to measure the resistivity and a Hall Effect setup is used to measure the carrier concentration. Equipment available at LMOPS laboratory, Carel platform, were used, including Keithley source and measure units (SMU) models 2602, 2636B and 236 and a home-made Hall setup with a permanent magnet at 0.55 T.

Prior to the electrical measurements, some of the films were annealed using a rapid thermal annealing AS-Micro system from Annealsys (picture in Figure 2.20). For some films, using a specific mask, metal contacts were deposited by thermal vacuum evaporation technique prior to the

Four-Point Probe (4PP) Method

To measure the thin film resistivity, the standard four-point probe (4PP) method was used. Four

Van der Pauw Method

In this method, as with the four-point method, the resistivity of the sample is analyzed by applying a current to the layer and measuring the voltage. However, this technique allows a more accurate determination of the resistivity of the material. The van der Pauw configuration shown in the Equation (2.9):

exp (-𝜋 𝑅 1 𝑅 𝑆 ) + exp (-𝜋 𝑅 2 𝑅 𝑆 ) = 1
The numerical solution of this equation provides the resistance RS and therefore the resistivity:

ρ = 𝑅 𝑆 t

Hall Effect Method

In addition to the van der Pauw (vdP) measurements, Hall Effect measurements were conducted to determine the charge carrier concentration and combined with vdP, the mobility in the sample in the configuration shown in Figure 2.24. The Hall Effect is based on the accumulation of charges in a medium under the effect of the magnetic Lorentz force FL. This force can be expressed as follows:

Equation (2.10):

𝐹 𝐿 = 𝑞 𝑣 𝐵
Where q is the elementary charge (1.602 × 10 -19 C), 𝑣 is the charge velocity and B is the magnetic field.

The Lorentz force driver movement of the charges creates an electric field and therefore a voltage 𝑉 𝐻 in transverse direction to the carrier's flow. This voltage which can be represented as follows:

Equation (2.11):

𝑉 𝐻 = 𝐼 𝐵 𝑛 𝑞 𝑡
Where, VH is the Hall voltage, I is the current in the longitudinal direction, 𝑡 is the film thickness, 𝑛 the charge carrier concentration and q is the electron charge. For different types of charge carriers VH has different polarity. So, this technique is useful for the determination of the type of conductivity (p or n) for a semiconductor as given by: Equation (2.12): The material is of n-type if (in the used convention):

𝑉 𝐻 = - 𝐼 𝐵 𝑛 𝑞 𝑡 < 0

Equation (2.13):

The material is of p-type if (in the used convention):

𝑉 𝐻 = 𝐼 𝐵 𝑝 𝑞 𝑡 > 0
The mobility is given by the following relationship: A detailed description of the van der Pauw / Hall Effect technique can be found in [81].

Equation (2.

Introduction to Zinc Oxide (ZnO)

ZnO is an inorganic II-VI binary semiconductor compound. It is mostly available as a white powder in the Earth's crust as mineral zincite. The commercially used ZnO as an additive is produced synthetically. ZnO is a compound semiconductor with an ionicity that falls in between covalent and ionic semiconductors. ZnO crystal structures can have three forms as schematically shown in Figure 3.1: rock salt (B1, relatively rare, obtained at high pressure), zinc blende (B3) and wurtzite (B4). The most thermodynamically stable phase is wurtzite even at room temperatures while the zinc blende structure can only be stabilized by growing them on cubic substrates.

The hexagonal lattice in ZnO is defined by two interconnecting Zn 2+ and O 2-sublattices, with each Zn 2+ ion surrounded by a tetrahedron of O 2-ions, and vice versa. The tetrahedral coordination thus results in a non-central symmetric structure, which leads to significant piezoelectric, pyroelectric, and spontaneous polarization effects, all of which are important in crystal growth, etching, and defect generation [1,2]. This coordination is typically of sp 3 covalent bonding nature. The materials in the ZnO group also tend to possess substantial ionic character that results in increasing the bandgap beyond the values generally expected from the covalent bonding. In the hexagonal wurtzite structure of ZnO that interests us, the c-axis, which is parallel to z (0001), is a polar hexagonal axis. The zinc atoms are almost in a hexagonal close packing configuration.

Typically, each sublattice in the wurtzite structure has four atoms per unit cell, and each atom of one kind (group II) is surrounded by four atoms of the other kind (group VI), or vice versa, at the tetrahedron's borders with a ratio of the lattice parameters (c/a) of about 1.66 [4].

Properties of ZnO

For applications in semiconductor devices, the properties of ZnO can be classified into three different types: optical, chemical, and electrical.

Optical Properties

(i) Bandgap: ZnO exhibits a large bandgap value of about 3.44 eV at low temperatures, whereas at room temperatures, it is close to 3.37 eV, corresponding to the emission in the UV region [5]. In comparison, wurtzite gallium nitride (GaN) can have values of 3.44 eV to 3.5 eV [6] very close to those of ZnO. The bandgap of ZnO is perfectly suited to applications in UV optoelectronics, such as light-emitting diodes, laser diodes, solar cells (as a window layer) and "solar-blind" photodetectors [3,[7][8][9][10]. Due to the favourable optical properties ZnO can be used in different forms such as platelets [11], nanowires [12], nanocrystals [13], thin-films [14] and even in p-n homojunctions [15][16][17].

(ii) Binding energy: In comparison to GaN, the free-exciton binding energy of ZnO is quite large.

For ZnO it is 60 meV while GaN has a value of 25 meV [6,11,14]. Because of the high exciton binding energy, efficient excitonic emission in ZnO can be maintained at room temperature and above [11].

The strong exciton binding energy makes ZnO a potential material for optical devices based on excitonic effects, as the oscillator strength of excitons is often significantly higher than that of direct electron-hole transitions in direct gap semiconductors [18].

(iii) Luminescence: ZnO is a suitable material for phosphor applications because of its intense luminosity in the green-white region of the spectrum. The induced emission spectrum exhibits a broad peak at 495 nm [half-width of 0.4 eV] [19]. Because of its n-type conductivity, ZnO is suitable for use in vacuum fluorescent displays and field emission displays. The origin of the luminescence center and its mechanism is unknown, and it is commonly attributed to oxygen vacancies or zinc interstitials despite the lack of evidence [19]. It's been proposed that green luminescence is most likely caused by zinc vacancies.

Chemical Properties

(i) Wet chemical etching: The capacity to use "low-temperature wet chemical etching" in semiconductor device fabrication methods is extremely beneficial. ZnO thin films have been reported to be etched using acidic, alkaline, and combination solutions. The ability to manufacture, develop, and integrate electrical and optoelectronic devices using low-temperature chemical etching gives a lot of flexibility [20][21][22].

(ii) Thermal conductivity: This feature makes ZnO a useful additive (for example, ZnO is added to rubber to improve tyre thermal conductivity). This also makes ZnO a more appealing substrate for homoepitaxy or heteroepitaxy (for example, growth of GaN, which has a comparable lattice constant) [23,24]. Excellent thermal conductivity corresponds to high heat removal efficiency while the device is in use.

(iii) Radiation hardness: For applications at high altitudes or in space, radiation hardness is critical. ZnO has been found to have a very high radiation hardness, much higher than GaN, the cause of which is yet unknown [25,26].

(iv) Surface conductivity: The conductivity of ZnO thin films is highly dependent on the surface's exposure to various gases. Due to its great sensitivity to trimethylamine contained in the odour, it can be employed as a "cheap smell sensor" capable of detecting the freshness of foods and drinks [27]. Sensor action mechanisms are poorly understood. In vacuum-annealed single crystals, recent tests have revealed the presence of a surface electron accumulation layer that disappears when exposed to ambient air. This layer could also play a function in sensor action [28][29][30]. It's been claimed that the presence of this conducting surface channel is linked to some perplexing typeconversion effects found when attempting to generate p-type ZnO.

Electrical Properties

Because of two factors, much research has been conducted on the electrical characteristics of ZnO films to better understand the conduction behaviour. For practical TCOs with rather large carrier concentrations, carrier concentration (n) and mobility (µ) cannot be separately increased. Ionized impurity scattering, or the Coulomb interactions between electrons and dopants, limits carrier transport at high electron densities. Higher levels of doping restrict carrier mobility to the degree where conductivity cannot be enhanced.

Even in the absence of intentional doping, mostly ZnO has an n-type character. Although native defects such as oxygen vacancies and zinc interstitials are frequently considered to be the cause, an alternate explanation based on theoretical calculations has been proposed: unintentional substitutional hydrogen impurities are to be responsible [31].

By replacing Zn with group-III elements Al, Ga, and In, controllable n-type doping can be easily achieved; then, according to most reports about ZnO films deposited using various methods, resistivity and mobility were, to a large extent, independent of the deposition method and limited to about 2×10 -4 Ω×cm and 50 cm 2 /Vs, respectively [32,33]. Furthermore, it is difficult to fabricate p-type ZnO due to the limited solubility of p-type dopants and their compensation by abundant ntype impurities. This fact is analogous to materials like GaN and ZnSe. Due to inhomogeneity, it is often difficult to measure p-type behaviour in an "intrinsically" doped n-type material. High levels of p-type conductivity are dubious and needs to be experimentally verified. Additionally, it reduces optical transmission at the near-infrared edge. The resistivity approaches a lower limit as the dopant concentration rises, beyond which it cannot decrease [34].

Applications of ZnO in Solar Cells

The recent years witnessed massive research and development towards transparent oxides for solar cells. These oxides, used as transparent conducting materials (TCM, in our case: transparent conducting oxides (TCO)), as absorbers or window layers, allow for the development of solar cells with performance characteristics that are suitable for large-scale terrestrial applications. Because of the following advantages, TCO are very successful in solar cell applications: (i) the conducting transparent layer allows direct transmission of solar radiation to the absorber with little or no attenuation; (ii) the p-n junction formation temperature is lower and thus it is easier to fabricate and (iii) can act as both a low-resistance junction contact and an anti-reflective layer. ZnO as a material satisfies all these criteria. Aranovich et al. [35] studied the electrical and photovoltaic properties of a heterojunction fabricated from spray pyrolyzed ZnO films on single crystal p-type CdTe. The optimum cell yielded an efficiency of 8.8 %. A CuInS2/ZnO heterojunction solar cell fabricated by Tiwari et al. [36] using spray pyrolysis produced 2 % conversion efficiency. The ZnO conductivity increased post annealing thus improving the junction behaviour. Tomar et al. [37] prepared a heterojunction ZnO/CuInSe2 solar cell using chemical spray pyrolysis with more than 2 % efficiency. Kao et al. [38] used sol-gel spin coating technique to fabricate a ZnO nano-crystalline thin film on fluorine doped tin oxide (FTO) coated glass substrate for dye sensitized solar cells. Using pre-annealed ZnO at 300 °C, 2.5 % conversion efficiency was possible. For CIGS solar cells, Campa et al. [39] reported that ZnO placed between the CIGS and molybdenum back contact shows increased reflectance thereby enhancing the JSC. Rau et al. [40] shows that in CIGS solar cells, intrinsic ZnO layer prevents electrical inhomogeneities from dominating the device's open circuit voltage. ZnO has been used extensively for heterojunctions with Cu2O absorber in "all-oxide" solar cells. The interested readers can look into the Sections 1.3.2.3 and 1.3.2. 4 of Chapter 1 where it is discussed thoroughly.

Preparation of ZnO by Ultrasonic Spray Pyrolysis

The preparation of ZnO films was done using an ExactaCoat system from Sono-Tek. For the ultrasonic atomization of the precursor solution an Accumist nozzle operating at 120 kHz was employed. A detailed description of the ultrasonic spray pyrolysis process is included in Chapter 2.

Soda lime glass (SLG) slides (76×26×1 mm 3 ) were used as substrates. Prior to the deposition, the soda lime glass was cleaned ultrasonically with solutions such as acetone, isopropanol and distilled water for 15, 10 and 5 minutes respectively to remove any organic residues. A 3 mm thick and uniform copper plate was used as a substrate holder on top the heating block of the spray system to protect it and to supply maximum heat to the glass substrate. The temperature on the heating plate was regulated by an in-built PID temperature controller. The temperatures of the substrate holder and the glass substrate were measured using N-type thermocouples. The substrate was pre-heated to the desired spray temperature. A suction pump was operated constantly during the spray preparation process continuously to extract the gaseous by-products and vapours of the solvent.

The set temperature of the heating plate in the spray system was kept constant during the entire spray preparation process. The height between the spray nozzle and the glass substrate was fixed to 10 cm. Air as carrier gas was used to maintain the shaping air pressure of 13.8 kPa to the Accumist nozzle so that the resulting droplets (aerosols) from the nozzle is guided with a good control and uniformity towards the heated substrate. Zinc acetate dihydrate, Zn(Ac)2.2H2O was used as the source precursor salt dissolved in deionized water. For simplicity henceforth, in this thesis we will always refer it as only Zn(Ac)2. Always 50 mL of precursor solution was prepared before the start of the spray. Acetic acid was added to the solution of Zn(Ac)2 to reduce the formation of zinc hydroxide, Zn(OH)2. Zinc ions could actually get precipitated in the pyrolysis process on the form of a milky textured Zn(OH)2. Thus, the acetic acid quantity added to the precursor solution becomes an important study point in this chapter.

Chemicals Used and Chemical Reactions Involved for ZnO Preparation

All the chemicals used for the preparation of ZnO thin films are listed in the The pH of the precursor solution is monitored using acetic acid. Zn 2+ is transformed into complexes, Zn(Ac) + and Zn(Ac)2, depending on the pH of the solution. The concentration of these species is significant in the development of the film. In order to achieve film deposition, it is preferable to have neutral zinc acetate [written as Zn(CH3COO)2 or Zn(Ac)2] in solution. Addition of acetic acid, keeps the solution pH in the range of 4-5, which actually makes Zn(Ac)2 the dominating species in the solution and thereby the film deposition process in enhanced. Possible pyrolytic reactions taking place in the formation of ZnO thin films are discussed by Paraguay et al. [41] and shown in our own work [42]

Adsorbed on surface

In our study [42], as we already pre-heat the substrate to a very high temperature in ambient air conditions and with Zn(CH3COO)2 as the dominating complex we can write the pyrolytic reaction as:

𝑍𝑛(𝐶𝐻 3 𝐶𝑂𝑂) 2 +4𝑂 2 𝐻𝑒𝑎𝑡 → 𝑍𝑛𝑂 (𝑓𝑖𝑙𝑚) + 4𝐶𝑂 2 (𝐺𝑎𝑠) + 3𝐻 2 𝑂 (𝑉𝑎𝑝𝑜𝑢𝑟) ↑
The formation of zinc hydroxide can be written as:

𝑍𝑛(𝐶𝐻 3 𝐶𝑂𝑂) 2 +2𝐻 2 𝑂 𝐻𝑒𝑎𝑡 → 4𝑍𝑛(𝑂𝐻) 2 + 2𝐶𝐻 3 𝐶𝑂𝑂𝐻 (𝐺𝑎𝑠) ↑
Generally, the droplets that are larger than 50 µm in diameter reacts with the Zn 2+ ions and the atmospheric water vapour present inside the spray chamber to form the hydroxide. Adding acetic acid in the precursor solution allows the ionization of zinc acetate. A subsequent reaction facilitates the production of a zinc-based complex, ZnCH3COO + and thereby preventing the formation of Zn(OH)2. The reactions are written below: In our study, the pH of the solution along with the variation of the spray parameters along with their subsequent effect on the film quality was studied in detail with the help of a design of experiments as discussed in the next sub-section.

𝐶𝐻 3 𝐶𝑂𝑂𝐻 𝑦𝑖𝑒𝑙𝑑𝑠 ↔ 𝐶𝐻 3 𝐶𝑂𝑂 -+ 𝐻 +
The effects of the Zn(Ac)2 precursor solution concentration on the ZnO film quality have been discussed later in this chapter. The quantity of the Zn(Ac)2 powder required in the solution to reach a particular solution concentration is calculated using the relation:

Equation (3.2): 𝑚 = 𝐶 × 𝑉 × 𝑀
Where, m is the mass (in g) of Zn(Ac)2 powder; C is the ultimate solution concentration in mol/L; V is the solution volume in litres (L) and M is the molar mass of Zn(Ac)2 (in g/mol).

So, for example, mass of Zn(Ac)2 needed for preparing a 50 mL solution with concentration of 0.10 mol/L is: m = 0.10 × 0.05 × 219.49 = 1.09 g.

Design of Experiment

Many parameters are involved in the spray pyrolysis preparation process for depositing high-quality thin films, as detailed in Chapter 2. Variation in each parameter impacts the resulting film quality. This is the reason to set up the design of experiments, so that we can understand and analyze the nature and degree of impact of the parameters towards the prepared film quality. The list below shows the variation of spray parameters along with range of their variation.

1.

The hot plate set temperature, varied in the range from 350 °C to 450 °C.

2.

The spray flow rate, which was varied in the range from 0.3 to 0.7 mL/min.

3.

A precursor solution concentration ranging from 0.03 to 0.10 mol/L.

4.

The pH of the precursor solution was adjusted between 4 and 5.

5.

The number of spray cycles, which ranged from 10 to 60. The duration of each preparation is determined by this parameter.

When one spray parameter was varied, all the other parameter remained fixed to a particular value.

This helped in understanding the effect of the variation of the parameter on the film's quality. Some parameters such as, the height between the spray nozzle and the substrate and the scan speed movement of the spray nozzle were fixed before according to the system's safety measures to achieve high transparency thin films with a fast growth rate across a wide thickness range. The repeatability of the ultrasonic spray pyrolysis method was rigorously verified by preparing each sample at least three times with the parameters listed above. For better understanding, the design of experiment to select the optimal ultrasonic spray pyrolysis parameters is shown in the

Results and Discussions on the Preparation of ZnO

The design of experiments scheme mentioned above was used for the preparation of ZnO thin films.

Various characterization techniques were used to understand the variation of the spray parameters on the growth, morphological, optical, and structural characteristics of the ZnO thin films.

Growth of the ZnO Thin Films

The thickness of the spray zone was systematically measured at three sites by the mechanical Dektak XT profilometer and then averaged. Figure 3.2 depicts the relationship between ZnO film thickness and the number of spray cycles. The growth of the ZnO films is shown to be linear in relation to the number of spray cycles. The fitted line does not cross the origin, indicating that there is an incubation period when grains coalesce and grow to form the ZnO thin film. In spray pyrolysis and sol-gel procedures, this nucleation/coalescence duration is commonly reported [43][44][45].

By preparing many samples (up to four) in the same process and by regularly preparing new samples with the same previously employed settings, the procedure's reproducibility and stability were tested. The growth rate was around 120 nm per minute, or 14 nm every spray cycle. We set the number of spray cycles to 20 based on this calibration, which corresponds to a ZnO film thickness of roughly 200 nm. With thicknesses of the ZnO films, ranging from around 100 nm to almost 800 nm, a growth rate of roughly 120 nm/min was achieved from our spray process.

The growth rate achieved in our spray preparation process was higher than reported values in known studies. Kenanakis et al. [46] reports a growth rate of 1 nm/min, Mata et al. [47] reaches up to 42 nm/min, Ergin et al. [48] could reach 55 nm/min, and Ynineb et al. [49] recently reported a 90 growth rate of 80 nm/min. We were able to sustain this high growth rate of 120 nm/min while also maintaining a fast nozzle scan speed of 70 mm/sec with good reproducibility. The set substrate temperature was 450 °C for the depositions with a solution concentration of 0.10 mol/L and pH 4.

Flow rate was constant at 0.5 mL/min.

Optical Properties of the ZnO Thin Films

To investigate the optical characteristics of the ZnO thin films, the transmission of the films was assessed in relation to deposition conditions. The experimental details of the technique are given in A large decrease in transmission at the high flow rate is mostly due to the fact that, at this high flow rate, the atomization of the precursor solution is incomplete, which in turn, results to larger droplets being deposited on the substrate. These large droplets do not contribute to the pyrolysis reaction and generally forms Zn(OH)2, thereby reducing the film transparency. The loss of the transparency for the highest flow rate could be explained from another perspective: in this case the main cause is the oxygen-zinc stoichiometry, as with the high flow rate, the atomic ratio of Zn/O is relatively high.

The high Zn/O ratio makes the reaction of pyrolysis not only incomplete but also ambiguous which directly impacts the growth mechanism of the ZnO films and presents a disrupted surface morphology. Similar evidence with the high Zn/O ratio can also be seen in ZnO films prepared by sputtering [51][52][53]. We need to maintain the flow rate to an optimum value of the flow rate with respect to the shaping air pressure so that a complete pyrolysis takes place. With very low rates the process results in under-spray as the major portion of droplets vaporize even before reaching the hot substrate surface. For this reason, the prepared films are too thin with high surface roughness, so although we achieve high transparency with a low flow rate it is advisable to use the moderate flow rate of 0.5 mol/L to have nearly similar transparency (as shown in the Table 3.3) with good film qualities. Now, with the substrate temperature and flow rate values fixed to 450 °C and 0.5 mol/L respectively, we studied the effect of the variation of the precursor solution concentration on the film transparency. As usual, the number of spray cycles, the solution pH and the dwell times were kept 93 same as before. The concentration was varied from 0.03 mol/L to 0.10 mol/L. But, in this case, we do not see major variations in the transparency of the films (cf. Figure 3.5) as the concentration changes are not major themselves and the pyrolysis reaction is complete in all cases resulting in the desired stoichiometry of the ZnO films. All the films exhibited high average transmission of more than 90 %. But, to choose the exact value of the solution concentration we will show and discuss in the subsequent sections the structural analysis of the films with varying concentrations. From the figure above, we see that the transparency of the films remains almost similar with the increase in thickness of the prepared films. This result suggests the effective control of the spray parameters in our preparation process, and we see in the films uniform and smooth surface without any porosity or any grain pockets (also confirmed later by the AFM and SEM measurements). This is the reason that the transparency remains almost similar even with increasing thickness. To establish the success of our spray process, we compare our results with those of the state-of-the-art results which are detailed in the transmission) with a thickness of 30 nm by using custom made spray pyrolysis and CVD systems for the preparation [63]. Using ultrasonic spray pyrolysis 90 % transmission in ZnO was achieved by Mokhtari et al. [64] for use as a window layer in solar cell applications. It is also shown that an anneal process after the deposition can further increase the transparency beyond 90 % [65]. But with an effective control on the spray parameters, we were able to prepare conformal ZnO films with high precision, such that even without a post-annealing treatment on the ZnO films we were able to achieve transparencies as high as 95 %. All the mentioned spray parameters work in unison to provide excellent atomization of the precursor solution to achieve highly conformal, uniform, compact and dense ZnO films.

Overall, by studying the optical characteristics of the ZnO thin films with respect to the variation of the spray preparation parameters, we can say that that the films prepared with the optimized set of parameters (mentioned in Table 3.6) can be applied as window layers for "all-oxide" solar cells. The slight improvement (widening) in the bandgap energy of the films prepared at higher temperature can be explained by the Burstein-Moss effect [72], in which case, with an increase in the carrier concentration, the Fermi level merges into the conduction band. Also, due to high level of crystallinity, as shown later in the XRD measurements, the Urbach energies are the lowest, as at high temperature the concentration of the oxygen vacancies is low as also shown by Othmane et al. [73]. At a higher temperature, the Zn-O-Zn bonds gets weaker causing the loss of oxygen atoms. In turn, the O 1s binding energy in lowered with lowering of Urbach energies. Hence, the band-tail width is lowered with less disorders in the films, which also causes the increase in the optical bandgap energy.

Morphological Properties of the ZnO Thin Films

To follow-up the optical properties, morphological analysis of the films was performed with the help of atomic force microscopy (AFM). The measurements were done in the tapping (resonant) mode with an oscillation frequency of 323 kHz with a scan rate of 0.75 Hz. The images thus obtained are shown in the Figure 3. 8 (a-d). For all the preparations, the precursor solution concentration and flow rate were maintained at 0.1 mol/L and 0.5 mL/min. For these reasons, the RMS roughness are higher than the films prepared at 450 °C with pH of 4. This result is consistent with the optical studies which also show similar effects towards the film's optical quality. Prasada Rao et al. reported similar temperature effects in their AFM analysis [61]. The RMS roughness of ZnO thin films grown by ultrasonic spray pyrolysis at 450 °C from a pH 4 solution compares favourably to RMS roughness values achieved by more complex techniques such as ion-beam or RF magnetron sputtering [74,75].

To support the results from AFM analysis, SEM micrographs were also captured for the ZnO films.

The SEM micrographs are shown in the From the morphological analysis, we can surely conclude that preparations done at higher substrate temperature of 450 °C and with a precursor solution pH 4 is a much better choice for the preparation of high-quality ZnO films.

Chemical Properties of the ZnO Thin Films

A chemical analysis was performed on the ZnO films to identify the presence of the hydroxides (O-H bands). With Fourier Transform Infrared Spectroscopy (FTIR) analysis we could detect the presence of the O-H bands. Our analysis was performed in accordance with some previous studies [76,77]. The bands corresponding to the oscillating vibrations O-H (≈ 3400 cm -1 ), symmetric and asymmetric valence bands C-H (3000 cm -1 and 1500 cm -1 ), stretching vibrations Zn-OH (≈ 1400 cm -1 ) and Zn-O (≈ 900 cm -1 ) are visible in the spectra illustrated in

Structural and Electrical Properties of the ZnO Thin Films

X-ray diffraction (XRD) was used to examine the structural properties of the sprayed ZnO films, to determine the deposited material structure and to assess the crystalline quality of the films. Figures 3.11 and 3.12 illustrates the X-ray diffractograms of the spray prepared ZnO films. The hexagonal wurtzite phase with space group P63mc features four ZnO peaks in the ( 100), ( 002), [START_REF] Mcconnell | Concentrated Photovoltaics[END_REF], and [START_REF] King | 40% efficient metamorphic GaInP∕GaInAs∕Ge multijunction solar cells[END_REF] planes, according to the diffractograms. The Lorentzian structure of X-ray diffractograms is commonly attributed to size-induced peak widening, whereas strain-induced broadening is better characterized by Gaussian or Voigt functions [78]. The shape of the X-ray peak is affected by other factors like as stoichiometry and defects. A least squares fitting procedure was utilized to precisely calculate peak position and full width at half maximum [79,80]. The detailed procedure is given in the Section 2.7.3.1 of Chapter 2. The crystallite dimensions, as calculated using the modified Scherrer method [82], range from 15 to 39 nm, with the maximum obtained for the film prepared at substrate temperature of 450 °C and 0.05 mol/L precursor solution concentration. This crystallite dimension (39 nm) is higher than those obtained previously in similar deposition conditions by de Godoy et al. [83], and comparable to or better than similar thin films described in the literature, as shown in Table 3.5. The obtained crystallite dimension agrees with the values obtained from the AFM analysis.

From the diffractograms in the Figure 3.11, it is seen that for films prepared at a deposition temperature of 450 °C and precursor solution concentrations of 0.03 mol/L and 0.05 mol/L, the high value of the texture factors, F002 indicates that growth proceeds preferentially with the (002) orientation plane parallel to the substrate in these conditions.

These findings are consistent with prior findings [57,83], which imply that ZnO films with c-axis orientation have the lowest surface energy, uniformly aligned crystallites, and few defects and trapping sites. The c-axis orientation is mostly dominating because of a faster growth rate along this direction during the spray preparation. With a lower concentration of precursor solution, the Zn atoms present in the solution easily finds an O atom to get bonded. The availability of an O atom generally does not depend on the precursor solution concentration. The Zn atoms gets significant mean free path to get bonded to an O atom and thus the crystallization gets optimized in the c-axis direction. While contrarily, when the concentration of the precursor solution is higher, then the abundance of the Zn atoms favours a random orientation as can be usually seen in the case of a powder diffraction pattern. This effect with the change in concentrations is also seen in [84].

In the diffractograms of the Figure 3.12, the F002 values are close to unity or lower than unity. The films in this case, are prepared at lower deposition temperatures of 350 °C and 400 °C, but with precursor solution concentration of 0.10 mol/L. The growth pattern of ZnO indicates that the growth has lost its (002) preferential orientation and the film now has a preferential a-axis orientation along the (100) plane. To support the findings from the XRD measurements and complete the structural analysis, Raman For ZnO films the E2 modes are directly linked to the quality of the crystal and for textured polycrystalline films the E2 modes will be dominant. We generally do not see any E1(TO) mode as it only prominent for films grown at lower substrate temperatures of around 150 °C. We also do not see any A1(LO) modes, as they are prominent at very high substrate temperatures of 600 °C [87].

The dominant peak at a Raman shift of 438 cm -1 is recognized as the E2 High ZnO Raman phonon vibration of the wurtzite structure in all samples obtained by fitting the spectra. In addition, for the lowest deposition temperature of 350 °C, another broad peak develops at a Raman shift around 350 cm -1 , but, due to its position and width, which are too different from the known values for this ZnO mode, this peak could not be identified as the (E2 High -E2 Low ) ZnO mode, which is commonly observed in ZnO [88]. The peak presents a relatively broad nature which could be co-related to change of textures as observed in the XRD diffractograms and could induced by defects in ZnO partially due to an incomplete spray pyrolytic reaction. The E2 High peak full width at half maximum is roughly 8 cm -1 at low deposition temperature, which is similar to the values commonly reported for ZnO thin films deposited by sputtering [89]. According to [90], the dominance of the E2 High mode in our thin films without the appearance of the A1(LO) and E1(LO) modes suggest very good crystallinity with very less formation of zinc interstitial defect states and oxygen vacancies.

The electrical characteristics of very transparent ZnO thin films were investigated using the four probes technique. An annealing treatment has been performed on the samples, using a rapid thermal which acts as a shallow donor in ZnO with a low formation energy both in Zn-rich and O-rich preparation conditions [91]. This origin of the n-doping in ZnO related to Zni is far more probable than oxygen vacancy (VO) as it is supported by both theoretical studies (using the local density approximation) [91] and experimental results [92] evidencing the fact that Zni, acting as a shallow 106 donor in ZnO, is predicted to be far more abundant than other defects, including VO. It can be noted that the resistivity values measured in this work are comparable to results recently obtained in identical conditions for non-intentionally doped ZnO produced by spray pyrolysis [93].

Challenges Faced Towards the Preparation of ZnO

The entire process to successfully prepare ZnO thin films of optimum quality was not only challenging but also time consuming. We had to make the design of experiments based on an extensive literature survey. Setting up the spray system was the first important task as all parameters including the substrate holder and the substrate itself can impact the film properties a lot. After a lot of tries, a scheme was found using a copper substrate holder with the substrate on it to ensure a uniform heat distribution. The substrate to spray nozzle height and the nozzle speed were initially calibrated and set according to the safely limitations of the spray system. The dwell time had to be fixed for all the preparations very carefully. With the contact of the atomized droplets the substrate temperature can fall up to 50 °C, the dwell time which is a stopping time between the spray cycles helps the substrate to quickly reach the set temperature again. The pre-heated copper plate with a high heat conductivity also helps in this purpose. For the design of experiments, to check the effectivity of our process each preparation of each film with a fixed set of parameters were repeated at least 3 times. The detailed optical, morphological, and structural characterizations (in sequence) and their in-depth analysis helped us to understand the effect of the parameters on the film quality. The analysis of the characterizations with one set of spray parameters helped us to proceed with the next preparation process. One of the major targets of our work was to prevent the formation of the hydroxides. The formation of hydroxides was detrimental in terms of the quality of the thin films as we have seen. Even slight changes in the spray conditions can lead to the formation of hydroxides, so utmost control on the spray parameters is needed to avoid the formation of hydroxides. The understanding of the control of the spray parameters as a whole were important as we wanted to prepare the ZnO thin films with high precision so that they could be utilized as a window layer for "all-oxide" solar cells. So, uniform, and compact ZnO films with a smooth surface along with high transmission and high crystalline quality is crucial.

Chapter Conclusions

To conclude, we can rightly mention that through the detailed design of experiment we have successfully prepared ZnO thin films of optimum quality to be used as a window layer for "all-oxide" solar cells. The films with the optimized set of spray parameters were highly transparent with excellent morphology. The crystalline qualities as evidenced from the XRD and Raman measurements proved the success of our preparation process.

Fixed spray parameters based on system safety requirements set prior to creating the design of

Introduction to Zinc-Magnesium Oxide (ZnMgO)

Zinc-magnesium oxide (ZnMgO/Zn1-xMgxO) is obtained by incorporating magnesium in zinc oxide (ZnO) to form a broad bandgap semiconductor with wurtzite structure and a higher conduction band energy level than ZnO [1,2]. As detailed in Chapter 2, ZnO can possess three types of crystal structures: zinc blende, wurtzite and rocksalt [3]. MgO can have only a stable rocksalt structure like NaCl with a lattice constant of 4.215 Å. The crystal structure can be described as a face-centred cubic (fcc) lattice of O 2-ions with Mg 2+ ions filling all octahedral positions or vice versa. Because the tetrahedral ionic radius of Mg 2+ (0.57 Å) is similar to that of Zn 2+ (0.60 Å), Mg atoms are expected to replace a considerable portion of Zn atoms [4]. The MgO crystal structure is shown in the Although the specific values of the Mg content boundaries that separate these three zones vary depending on deposition technique and conditions, the overall phase evolution trend from hexagonal to cubic with increasing Mg concentration remains consistent. When the Mg percentage in the films is less than 35 %, the alloy has a wurtzite structure, but when the Mg content is greater than 50 %, the alloy has a rocksalt structure. Between these two points, a mixed phase zone exists.

Since 1997, thin films of ZnMgO have been formed under vacuum, originally by pulsed-laser deposition onto sapphire substrates at 600 °C [8,9]. Molecular beam epitaxy (MBE) was used to deposit heterojunctions between ZnO and Zn1-xMgxO, which were then annealed at 1,000°C and showed no Mg diffusion across the interface with x ≤ 0.15 [11], despite an anticipated solubility limit of x = 0.04 [5]. ZnMgO metastable phases allow for an adjustable bandgap between 3.4 and 4.1 eV with a stable solid solution up to x = 0.4 [12,13].

ZnMgO depositions by means of aqueous solution has been achieved through ultrasonic spray pyrolysis (USP) [14][15][16] and sol-gel techniques [17]. Mostly nanoparticle formations [14,17] have been reported but recent works also showed formation of thin films by USP [15,16].

Some research groups tried to deposit ZnMgO at very low temperatures of 50 °C [18] and 70 °C

Understanding the Energy Band Structure of ZnMgO

Many studies have indicated that the relationship between the bandgap and Mg content is nearly linear [8,20,21]. Wenckstern et al. [22] compiled experimental values for the basic bandgap energy Eg from the absorption edges from over 30 research articles. They discovered that Vegard's law [23,24], which includes a bowing parameter, can accurately represent the composition evolution of the energy of the basic absorption edge as shown below:

Equation (4.1):

𝑬 𝒈 (𝒁𝒏 𝟏-𝒙 𝑴𝒈 𝒙 𝑶) = 𝟑. 𝟑𝟑𝟗 + 𝟏. 𝟐𝟒𝒙 + 𝟏. 𝟖𝟗𝒙 𝟐 (For wurtzite phase)

Equation (4.2):

𝑬 𝒈 (𝒁𝒏 𝟏-𝒙 𝑴𝒈 𝒙 𝑶) = 𝟑. 𝟔𝟕𝟗 + 𝟏. 𝟓𝟏𝟔𝒙 + 𝟐. 𝟒𝟖𝒙 𝟐 (For cubic rocksalt phase)

The Bloch theorem, which states that the bond strength determines the band gap, can qualitatively explain this characteristic. A greater bandgap is expected because Mg-O has a stronger bond than Zn-O. The Molecular Orbital Theory can be used to make similar conclusions. Zhang et al. [25] investigated the electron exchange correlation in ZnMgO utilizing a first-principles total energy investigation using the generalized gradient approximation (GGA). They concluded that increasing

Properties of ZnMgO

For applications in semiconductor-based devices, the properties of ZnMgO can be classified into three different types: thermal, optical, and electrical.

Thermal Stability

According to the solubility limits, incorporating 30 atomic % Mg in wurtzite-ZnMgO is a metastable state. Because thermal stability is critical for device applications, Ohtomo et al. [11]

Optical Properties

ZnMgO has different optical characteristics than ZnO. The reasons are as follows: First, due to a greater bandgap (than ZnO), the absorption in ZnMgO thin films is at a lower wavelength. ZnMgO is thus more UV-transparent than ZnO [8,20,21]. This shift of the absorption edge towards the lower wavelengths in the UV region is due to the Burstein-Moss shift [28]. Second, because the plasma wavelength of ZnMgO shifts to a greater wavelength, it is more IR transparent [29]. This is primarily due to the doping effect and the large effective mass of ZnMgO [30]. Thirdly, as explained by Lu et al. [31], in the presence of free carriers, the effective mass of electrons increases with the Mg content. In ZnMgO, this phenomenon has a big impact on electron conduction. With same quantity of free carriers in ZnO and ZnMgO, the bandgap widening effect in ZnO is slightly higher than ZnMgO.

Electrical Properties

The major causes that affect the electrical properties of ZnMgO can be classified into two groups.

The first category is linked to changes in crystal quality (extended defects), while the second is linked to changes in quasi-bulk characteristics due to higher Mg incorporation in the alloy. The first is not intrinsic to ZnMgO and can be addressed by enhancing crystal quality, whereas the second is directly tied to ZnMgO's material qualities.

While it is relatively simple to partially substitute Mg atoms for Zn atoms in the ZnO wurtzite structure to increase the bandgap and elevate the conduction band edge of ZnO [20,21,32], it has been widely discovered that the highest obtained free carrier concentration (n) and mobility (µ) achieved in Zn1-xMgxO (x > 0.2) rapidly decreases as a function of Mg [29,30,[33][34][35][36].

There is currently a lack of clarity on the mechanisms that cause the inferior electrical characteristics of wide-bandgap ZnMgO. Various theories based on their observations have been given in previous publications. The decrease in carrier concentration has been attributed to both a decrease of active donors due to composition enrichment [30] and an increase in donor activation energy, which can be induced by either a greater effective mass [30,31,33] or a change in the band structure [37].

Strong alloy disorder scattering [31,34], a larger effective mass [30,33,35], enhanced impurity scattering [30], and/or stronger grain boundary scattering due to fewer carriers have all been proposed as reasons for the reduced mobility with increasing Mg content in ZnMgO. As plausible as several of these theories are, none of them have been thoroughly investigated.

Furthermore, the impact of intrinsic defects such as zinc vacancy (VZn) on the electrical characteristics of ZnMgO has not been discussed in detail in prior research. Recent studies have found that intrinsic acceptors, particularly VZn and its complexes, have a significant impact on the electrical characteristics of ZnO [38][39][40][41]. However, there is no equivalent explanation about ZnMgO, including if its weaker electrical properties are owing to a stronger compensation.

Applications of ZnMgO

For solar cells, ZnMgO is mostly used as buffer layers (placed in between the window and the absorber layers). Unintentionally, ZnO is always an n-type material. As a result, ZnO and n-doped equivalents are good choices for p-n junction buffer layers. The conduction band maximum (CBM) of the p-type absorber should be aligned with an ideal n-type buffer layer. The difference in CBM is measured as the band offset. An optimal conduction band-offset (CBO) should be "flat" and not create a "cliff" or "spike" cases and must vary in accordance with the absorber/buffer-layer combination [42]. The possible band-alignments are shown in the ZnMgO buffer layer increased [43]. Using a ZnS/ZnMgO buffer layer combination in CIGS solar cells yielded an efficiency of 18 % for solar cells under laboratory conditions while for mini modules the efficiency was 15.2 % [44]. ZnMgO was deposited by RF magnetron sputtering. For ALD deposited ZnMgO, the efficiency of CIGS solar cells reached 16 % [45]. Zn1-xMgxO with specific stoichiometries has been used in "all-oxide" solar cells with Cu2O as the absorber layer. The properties of these solar cells have been discussed in detail in the Section 1.3.2 of Chapter 1.

Beyond the photovoltaic application that interests us in this research work, ZnMgO is also an interesting candidate for ultraviolet (UV) photodetectors. Because of their well-developed technology, silicon photodiodes have dominated the market so far. Because Si's band gap is only 1.1 eV, UV detection takes additional filters to prevent the long-wavelength response. Due to the reflection of these filters, the photoresponsivity is poor. Due to these concerns, the band gap adjustable ZnMgO is being investigated as a possible substitute [46]. ZnMgO can be employed for deep UV detection because its absorption edge is abrupt and can be adjusted to the deep UV region.

Fabrication of a metal-semiconductor-metal (MSM) device with a peak responsivity of 270 nm was done by Liu et al. [47].

123

ZnMgO layers can also be used to make thin-film transistors (TFTs). TFTs are utilized in a variety of applications, including liquid crystal displays (LCDs) and organic light-emitting displays (OLEDs).

The hexagonal-ZnMgO can be employed as a channel, whereas the cubic-ZnMgO can be used as a gate insulator [48].

Biosensing is another conceivable application for ZnMgO because it is a non-toxic compound. An ion-sensitive ZnO/ZnMgO heterojunction field-effect transistor (HFET) was fabricated by [49] using an amine-modified gate electrode.

Fabricating a laser with a certain wavelength is difficult. Such applications are made possible through band gap engineering. Stimulated emission from ZnMgO alloy nanowires at ambient temperature was reported by [50]. This behaviour can be employed in applications involving lightemitting devices.

Preparation of ZnMgO by Ultrasonic Spray Pyrolysis

The preparation of ZnMgO films was done using an ExactaCoat system from Sono-Tek. Accumist nozzle operating at 120 kHz. Details about the USP system are given in the Section 2. 4 and Mg(Ac)2 only. Always 50 mL of precursor solution was prepared before the start of the spray.

Acetic acid was added to the precursor solution to reduce the formation and precipitation of hydroxides.

Chemicals Used and Chemical Reactions Involved for ZnMgO Preparation

All the chemicals used for the preparation of ZnMgO thin films are listed in the Table 4.1. The pH of the precursor solution was regulated using the acetic acid. The pH of the solution can also play a role in the film morphology which can vary from nanorods to compact films. We monitored the pH value in collaboration with other spray parameters to have dense, uniform, and compact films. Generally, the acetic acid can reduce the formation of the volatile Zn and Mg species in the solution, thereby reducing the chances of them further reacting to form respective hydroxides.

Further, the acetic acid addition helps to have a lower surface tension of the water-based precursor solution so that during the atomization uniform sized droplets are generated from the spray nozzle.

But, when compared to the effect of the Mg concentration in the solution, the effect of acetic acid concentration on the morphology of the spray prepared films is less pronounced.

In our published study [51], as we already pre-heat the substrate to a very high temperature of 470 °C in ambient air conditions and with Zn(CH3COO)2 and Mg(CH3COO)2 as the dominating complexes we can write the pyrolytic reaction as:

𝑍𝑛(𝐶𝐻 3 𝐶𝑂𝑂) 2 +𝑀𝑔(𝐶𝐻 3 𝐶𝑂𝑂) 2 + 15 2 𝑂 2 𝐻𝑒𝑎𝑡 → 𝑍𝑛𝑀𝑔𝑂 (𝑓𝑖𝑙𝑚) + 8𝐶𝑂 2 (𝐺𝑎𝑠) + 6𝐻 2 𝑂 (𝑉𝑎𝑝𝑜𝑢𝑟) ↑
Adding acetic acid to the precursor solution allows the ionization of zinc acetate and magnesium acetate. Subsequent reactions tentatively facilitate production of zinc and magnesium-based complexes, ZnCH3COO + and MgCH3COO + , thereby preventing the formation of hydroxide complexes. The probable reactions are written below:

𝐶𝐻 3 𝐶𝑂𝑂𝐻 𝑦𝑖𝑒𝑙𝑑𝑠 ↔ 𝐶𝐻 3 𝐶𝑂𝑂 -+ 𝐻 + 𝑍𝑛 2+ + 𝐶𝐻 3 𝐶𝑂𝑂 -𝑦𝑖𝑒𝑙𝑑𝑠 ↔ 𝑍𝑛𝐶𝐻 3 𝐶𝑂𝑂 + 𝑀𝑔 2+ + 𝐶𝐻 3 𝐶𝑂𝑂 -𝑦𝑖𝑒𝑙𝑑𝑠 ↔ 𝑀𝑔𝐶𝐻 3 𝐶𝑂𝑂 +
If some precursor solution droplets are larger than 50 µm in diameter, then there is a possibility that they will not undergo a complete pyrolysis reaction thus forming hydroxides by reacting with the atmospheric water vapour present inside the spray chamber.

The effects of the increase of the Mg(Ac)2 concentration while keeping the Zn(Ac)2 concentration fixed in the precursor solution on the ZnMgO film quality have been discussed later in this chapter.

The calculation procedure to select the concentrations of Zn(Ac

𝐶 𝑀𝑔(𝐴𝑐) 2 = ( 𝑥 1 -𝑥 ) × 𝐶 𝑍𝑛(𝐴𝑐) 2
So, for x =0.3:

𝐶 𝑀𝑔(𝐴𝑐) 2 = ( 0.3 1-0.3
) × 0.05 = 0.0214 mol/L Therefore, the quantity of Mg(Ac)2 powder needed corresponding to the concentration of 0.0214 mol/L is given by the equation below:

Equation (4.5):

𝑚 𝑀𝑔(𝐴𝑐) 2 = 𝐶 × 𝑉 × 𝑀

Where, m is the mass (in g) of Mg(Ac)2 powder; C is the desired concentration of Mg(Ac)2 in mol/L; V is the solution volume in litres (L) and M is the molar mass of Mg(Ac)2 (in g/mol). So, 𝑚 𝑀𝑔(𝐴𝑐) 2 = 0.0214 (mol/L) × 0.1 (L) × 214.45 (g/mol) = 0.4589 g ≈ 0.46 g. Now, we need to prepare the solution S3. It should be noted that S3 will be the final solution that will be used for the actual preparation. For preparing the solution S3, we need to mix different proportions of S1 and S2. Basically we, will collect a certain volume of S2 based on the desired Mg content % value and we will dilute it by adding S1. The final volume "V3" of S3 will be 50 mL. Volume of S1 needed is "V1". The volume "V2" of S2 to be collected is given by: Equation (4.6):

𝑉 2 = 𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑀𝑔(𝐴𝑐) 2 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑀𝑔(𝐴𝑐) 2 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 × 𝑉 3
The different volumes of V1 and V2 are given in the

Design of Experiment

Many parameters are involved in the spray pyrolysis preparation process for depositing high-quality thin films. Variation in each parameter impacts the resulting film quality. This is the reason to set up the design of experiments, so that we can understand and analyze the nature and degree of impact of the parameters towards the prepared film quality. Using a precise design of experiment, a thorough investigation of the morphological, structural, and optical properties was carried out to identify the set of parameters that result in smooth, dense, wurtzite single phase, and highly transparent Zn1-xMgxO thin films. The nozzle speed and the spray flow rates were pre-fixed to 70 mm/sec and 0.5 mL/min based on the safety requirements of the spray system.

The list below shows the variation of other spray parameters along with range of their variation.

1.

The zinc acetate [Zn(Ac)2] molar concentration in the precursor solution varied in the range from 0.03 mol/L to 0.10 mol/L.

2.

The number of spray cycles, which ranged from 10 to 100. The duration of each preparation is determined by this parameter.

Magnesium

3.

The dwell times between each spray cycle varied between 0.5 s to 2 s.

4.

The set temperature was varied between 400 °C to 480 °C.

5.

The relative shaping air pressure was varied between 6.8 kPa to 20.6 kPa.

6.

The height between nozzle to substrate was varied between 90 mm to 110 mm.

When one spray parameter was varied, all the other parameter remained fixed to a particular value.

This helped in understanding the effect of the variation of the parameter on the film's quality. The repeatability of the ultrasonic spray pyrolysis method was constantly monitored by preparing each sample at least three times with the parameters listed above. For better understanding, the design of experiment to select the optimal ultrasonic spray pyrolysis parameters is shown below in the

Results and Discussions on the Preparation of ZnMgO

The optimal value of the spray parameters from the design of experiments scheme mentioned above has been used for the preparation of ZnMgO thin films. Various characterization techniques were used to understand the effect of the Mg incorporation (from 0 to 70 mol%) in the precursor solution on the quality of the prepared ZnMgO thin films.

Chemical and Structural Properties of the ZnMgO Thin Films

In the first step of the analysis of the thin films, we try to understand the miscibility limit of the magnesium incorporation. As mentioned before, the magnesium composition in the precursor solution is varied from 0 to 70 molar %. With X-ray fluorescence (XRF) analysis the solid composition in the prepared films can be analyzed. At a scale of 50 µm, these measurements revealed no substantial fluctuation on the sample surface, indicating a uniform composition in the film.

Furthermore, the measured values of the films prepared under the identical conditions showed no significant variance.

There are two regimes in the variation of solid composition with solution molar composition. A linear variation is observed for molar compositions less than 30 mol%, with Mg incorporation efficiency in the solid film of roughly 80 %. The loss of magnesium can be explained by the fact that the magnesium precursor is less dense (1.454 g/cm 3 versus 1.735 g/cm 3 ) and more volatile than the zinc precursor, as seen in [15]. As a result, a fraction of the magnesium does not assist to the pyrolysis reaction that allows the ZnMgO solid film to develop. The variation of the magnesium molar composition in the solid films with reference to the molar composition in the solution is shown in the The Zn1-xMgxO thin films exhibit a single hexagonal wurtzite crystal structure with peaks for the (100), (002), ( 101), ( 102), ( 110), ( 103) and ( 112) planes of the space group P63mc, identified by using the ZnO powder diffractogram (RRUFF ID: R060027.1) [52] as a reference for magnesium compositions less than 30 mol%. The MgO cubic phase (periclase) emerges for magnesium compositions ranging from 30 to 70 mol%, with defined peaks for the (200) and ( 220 From the XRF and the XRD measurements it can be confirmed that, the phase segregation in the films happens from Mg composition of 30 mol%. This value of 30 mol% of Mg from which phase segregation occurs is one of the highest ones, when compared to other literatures, where Zn1-xMgxO thin films are prepared using ultrasonic spray pyrolysis [15,16,53] or by sol-gel techniques [54]. This value achieved in our work is much higher when it is compared to other solution-based preparation technique such as chemical bath deposition (CBD) as the segregation happens in this process below 5 mol% of Mg [15].

The incorporation of Mg into ZnO to form compact and homogeneous films is presumably facilitated in our process by a combination of the constant pH of 4, which reduces the surface tension of the precursor solution [15,16,55], the higher preparation temperature, which aids the formation of thin compact films, and the lower number of spray cycles with the fast deposition rate. These findings show that the solid solubility limit of Mg in ZnO, and therefore the increase in Mg incorporation efficiency, is substantially influenced by a combination of solution qualities and preparation factors that favour a complete pyrolysis reaction.

To confirm the wurtzite phase of the Zn1-xMgxO thin films, all the films from Mg molar composition 0 % to 30 %, were analyzed using Raman line-scan measurements. The scans were performed using a step of 1 µm. The scans revealed no substantial spatial variation, indicating the prepared thin films' homogeneity at this scale of 1 µm, as demonstrated by the XRF measurements at a larger scale of 50 With a Raman shift of 110 cm -1 and 439 cm -1 , the E2 Low and E2 High peaks of the Zn1-xMgxO wurtzite structure are found. The Raman spectrum for the SLG substrate shows two large peaks at 550 cm -1 and 1100 cm -1 which could be erroneously assigned as the A1 LO and 2A1 LO modes for ZnO based compounds prepared on SLG substrates. The width and relative intensity of E2 Low and E2 High peaks are almost constant with respect to the Mg composition, considering the resolution of about 1 cm -1 .

Due to phase separation, from Mg 30 mol% and at even higher compositions, as evidenced from the XRD diffractograms, the films consist of a mixture of Zn1-xMgxO wurtzite and MgO cubic crystal phases. The Raman spectra for these higher Mg composition films (not shown in the shows that the E2 Low and E2 High peaks exhibit a substantial decline in intensity and in broadening and become nearly invisible and are almost like the SLG substrate spectra.

Morphological Properties of the ZnMgO Thin Films

The evolution of the surface morphology of Zn1-xMgxO thin films as a function of magnesium composition was investigated with the help of atomic force microscopy (AFM). The measurements were done in the non-contact mode with an oscillation frequency of 336 kHz with a scan rate of 0.74

Hz. Figure 4.9 displays AFM images with a 5×5 µm scan area and magnesium compositions ranging from 0 to 70 mol%. For magnesium compositions ranging from 0 to 70 mol%, the surface root mean square (RMS) roughness varies from around 3.8 nm to 21.6 nm. As indicated by XRD and XRF measurements, this variance is mostly due to two factors: first, alloy disorder in the Zn1-xMgxO material, and secondly, the phase separation that occurs for high magnesium composition (from 30 mol%). The maximum 135 RMS value of nearly 22 nm is consistent with values reported in the literature for Zn1-xMgxO films prepared on SLG substrates by sputtering [56] or by spray pyrolysis processes [24].

To support the results from AFM analysis, SEM micrographs were also captured for the Zn1-xMgxO films. The SEM micrographs are shown in the Figure 4.10 and reveal polycrystalline nature of the films with no porosity. The surfaces are smooth when the magnesium content is low. With the incorporation of magnesium, the roughness of the surfaces increases with some grain facets becoming more prominent. This is also seen in the AFM images. At the higher magnesium content, the uniform morphology and grain distribution is completely lost. This change in the surface morphologies with the very high magnesium contact (above 30 mol%) can be attributed to the change of phase as also seen in the X-ray diffractograms.

Optical Properties of the ZnMgO Thin Films

To investigate the optical characteristics of the Zn1-xMgxO thin films, the transmission of the films was assessed in relation to the magnesium incorporation effect. Also, the bandgap and Urbach energies of the films were studied in response to the increase of the Mg content. The thickness of the prepared films was systematically measured at three sites of the spray zone by a mechanical DekTak profilometer and then averaged. To verify the consistency of the films, the measurements were repeated three times by an optical profilometer at the spray zone of the films. Both the profilometers have a vertical resolution of 0.1 nm and the thickness measurements of the films were found to be very consistent with narrow range of differences (± 10 nm). The transparency of all the Zn1-xMgxO thin films prepared with 20 spray cycles along with the average thickness measured in the spray zone are shown in the Table 4.8.

As seen from the Table 4.8, there was no discernible loss in transparency as the Mg content increased. This is also confirmed by the Raman studies, which show that increasing magnesium composition has no significant effect on film crystalline quality. The transmission spectra of the films are shown in the Figure 4.13. All the films exhibit high transparencies, but exceptionally high transparencies noted for the films prepared with Mg 30 % and higher. This can be explained by the phase change and the dominance of the MgO phase which typically exhibits much higher bandgap energies than wurtzite ZnO. no considerable bowing, this linear variation is identical to that obtained for Zn1-xMgxO thin films developed via sputtering [47,57]. This linear trend is compatible with XRF, XRD and the Raman studies, all of which confirms that a single-phase wurtzite structure is formed when the films are prepared with Mg molar compositions below 30 %. In general, when the Zn 2+ gets substituted by Mg 2+ ions in some of the sites of Zn 2+ , there are no free carriers produced. The Zn 4s and O 2p electron states are present in the bottom of the conduction band, so for ZnO thin films the 4s state of the Zn atoms is dominant [58,59]. But with increasing Mg in the Zn1-xMgxO alloy, a dominant state of Mg 3s is present in the conduction band where the minima occur. This state has higher energy than the 4s state. So, a broadening of the bandgap happens with Mg incorporation [58][59][60][61]. So, there is a change in the electron state due to the directional migration of Mg. This effect has repercussions on the Fermi Level, which in effect modifies the bandgap [56,60].

We also calculated the Urbach energy or the "band-tail" to estimate the alloy disorders for the Zn1-xMgxO thin films. It is to be noted that for Zn1-xMgxO films prepared by ultrasonic spray pyrolysis process, the Urbach energy calculations are done for the very first time. The Urbach energies are also calculated to understand the increase of density of defects in the films with the increase in the magnesium content, which was evidenced from AFM images and the reduction in the dimensions of the crystallites from the XRD measurements. The Urbach energies are calculated from the absorption of the thin films in the wavelength regime 300 nm to 400 nm as shown in the transmission spectra of From the analysis, we can estimate that the obtained Urbach energy value for the films with low magnesium compositions, is equivalent to the usual reported values for ZnO grown by pulsed lased deposition (PLD) [62]. The Urbach energy increases considerably as the crystalline lattice alloy disorder grows due to magnesium incorporation, as expected.

Electrical Properties of the ZnMgO Thin Films

The electrical properties of the Zn1-xMgxO thin films were analyzed in terms of the changes to the resistivity, carrier concentration and mobility with the increase of the magnesium molar composition. After preparation, but prior to the electrical measurements, all the films were annealed using a rapid thermal annealing (RTA) system at a temperature of 300 °C for 30 minutes using the procedure detailed in the Section 3.5.5 of Chapter 3. The annealing was performed to increase the grain size, activate the shallow donors and to improve the overall crystal quality [63]. From the resistivity measurements, it is seen that when the magnesium molar composition increases from 0 % to 7 %, the resistivity of the films increases dramatically from 0.121 Ω×cm to 166 Ω×cm. The huge increase in the resistivity is collaterally supported by a decrease of the carrier concentration from about 2×10 19 cm -3 to 5×10 15 cm -3 . The carrier mobility increases from approximately 2 cm 2 /Vs to 6 cm 2 /Vs from Mg 0 mol% to Mg 7 mol%. Compared to the changes in the resistivity and the carrier concentrations, the change in the carrier mobility was nominal with the increase of magnesium. If The prepared Zn1-xMgxO thin films become semi-insulating at magnesium concentrations greater than 7 mol%. Considering the equipment experimental limit and the semi-insulating nature of the thin film with this magnesium content, the resistivity can only be approximated at a value greater than 10 5 Ω×cm for magnesium content of 10 mol%. For Zn1-xMgxO thin films prepared by ultrasonic spray pyrolysis, it is for the first time, that this significant modifications in the electrical transport characteristics are being examined for. The changes can be attributed to two mechanisms: first, the bandgap increase as determined by transmission spectroscopy in conjunction with an increase in the activation energy of the background dopants, and second, a compensating mechanism as detailed below.

Interstitial Zn, which works as a shallow donor in ZnO with a low formation energy in both Zn-rich and O-rich environments, is commonly attributed to background n-type doping in ZnO [64].

Furthermore, magnesium incorporation may result in a reduction in the development of these interstitial Zn sites, lowering the free carrier concentration as seen before (Figure 4.17). Similar type of compensation mechanism has been observed by Li et al. [65] for Zn1-xMgxO films prepared by RF magnetron sputtering. With the increase in the Mg content in the films, a similar high decrease in the carrier concentration has been observed. According to Li et al., this type of decrease happens because of the inhibition of interstitial Zn (Zni) by magnesium. This mechanism, which is based on the notion that the n-doping source in ZnO comes from Zni, is significantly more likely than compensation of oxygen vacancy (VO), which is another commonly mentioned source of ZnO intrinsic doping. Zni behaves as a shallow donor in ZnO, which is projected to be significantly more prevalent than other defects, including VO, according to theoretical calculations using the local density approximation [64] and experimental findings [66].

Thus, the Zn1-xMgxO material conductivity, which can be regulated by adding magnesium, allows the material to be tuned for certain optoelectronic applications, such as ultraviolet detectors and solar cells. In the case of the former, a Mg-controlled reduction in inadvertent doping reduces the dark current, potentially increasing the final device's responsivity. For the latter, it allows for p-doping in ZnMgO, which could be important in the development of heterojunction solar cells, which is incredibly difficult with these materials.

Challenges Faced Towards the Preparation of ZnMgO

The entire process to successfully prepare ZnMgO thin films using only water-based precursor solution was quite challenging. Most literatures on ultrasonic spray pyrolysis prepared ZnMgO thin films report using toxic and inflammable solvents such as methanol and ethanol to dissolve the precursor salts. The disadvantages of using only water-based solution comes from the fact that water possess a high boiling point, strong hydrogen bonds and high surface tension. These properties are unfavourable for the spray process. Adding acetic acid in a controlled way aids in the reductions of the boiling point and the surface tension, which thereby aids in dissolving the precursor salts and in the atomization of the solution during the spray process. Also, Mg incorporation limit in the water-based precursor solutions at preparation temperatures below 400 °C is only 10 %. But in our process, it was possible to incorporate Mg to nearly 30 % molar compositions using precise control on the spray preparation parameters. The ceramic plate was used to provide a uniform heat distribution to the SLG substrate and the dwell time between each spray cycle was very carefully set with a duration such that the temperature quickly rises to the desired value and helps in completion of the pyrolysis reaction. This process must be in unison with the nozzle power, the shaping air pressure, the nozzle speed, and the nozzle to substrate height.

Controlling and understanding the effects of these parameters operation in unison requires utmost precision and care. Adjusting the concentration of the Zn(Ac)2 and the growth rate of the films is also very important as variations in these parameters can easily change the nature of the prepared thin films (from compact films to nanorods). From our previous experience with the ZnO thin films preparation in Chapter 3, we have maintained a constant Zn(Ac)2 molarity of 0.05 mol/L with a growth rate of approximately [120 ± 20] nm/min for the preparation of all the ZnMgO thin films with different magnesium compositions. These parameters values help us to have a uniform and compact films as evidenced from the AFM images and SEM images. The understanding of the effects of the preparation parameters were important as a whole as we wanted to prepare the ZnMgO thin films with high precision so that they could be utilized as a buffer layer for "all-oxide" solar cells.

Chapter Conclusions

We would like to conclude this chapter by stating the reasons for choosing to prepare the ZnMgO material with different magnesium contents. In the Section 1.3.2.5 of Chapter 1 we showed through numerical simulations that zinc germanium oxide (Zn1-xGexO) is a suitable non-toxic candidate for being employed as a buffer layer for "all-oxide" solar cells for enhancing the efficiency of the solar cells. But, in reality we were unable to find a suitable precursor salt that can be added to Zn(Ac)2 for the pyrolysis process which does not require a heavy modification of our machine and/or the use of toxic solvents. We tried to prepare Zn1-xGexO by using 99.99 % pure germanium oxide (GeO2) mesh powder from Alfa Aesar, but the powder showed very poor solubility in water-based solution.

Heating and stirring the solution for more than ten minutes improved the solubility but when the solution cooled down for the spray process, the solubility was again lost. Spray process was carried out using this mixture of GeO2 mesh powder and Zn(Ac)2 but it did not produce the film with the desired characteristics as no possible pyrolysis reaction took place. So, keeping in mind the factor earth-abundancy of magnesium over germanium and the easy availability of the suitable magnesium precursor salt which does not require the use of toxic solvents, we shifted our focus towards the preparation of Zn1-xMgxO thin films. The earth-abundancy of different materials are shown in the Modified from [67].

In this work, we designed a ZnMgO preparation method based on ultrasonic spray pyrolysis using water-based solutions in the Mg content range of up to 70 mol%. For magnesium compositions starting at 30 mol%, phase segregation was observed. All the films had a high transmittance of about 90 % in the visible range, with very little fluctuation in thickness, demonstrating the quality of the control on the spray preparation parameters. The variations in bandgap and Urbach energy with increase in magnesium composition was examined and connected to structural characteristics of the prepared films. The structural characteristics allowed the wurtzite structure to be identified as the single phase in all films with a Mg composition less than 30 mol%, while phase separation was observed at this value. When the Mg content was increased from 0 to 7 mol%, the electrical characteristics showed a significant drop in the n-type carrier concentration. When the Mg content exceeds 7 mol%, the thin films become semi-insulating. The electrical results imply that, in addition to the rise in bandgap, a compensation process involving defects generated in the alloy plays a key role in the drop in carrier concentration. The mobility increases slightly as the magnesium content rises, indicating that the crystal quality of the films does not deteriorate much when the magnesium content rises in the range studied. The high transparency of the films in the visible region of the spectrum along with the modulations of the bandgap energies and the conductivities can have interesting applications for the fabrication of solar cells, ultraviolet photodetectors, and other optoelectronic applications.

To summarize: the preparation by ultrasonic spray pyrolysis of ZnMgO with high magnesium content was developed and a combined structural, optical, and electrical investigations were conducted with a combination of optoelectronic properties that can be exploited for the optoelectronic devices. The main achievements are: (i) a miscibility limit of magnesium in zinc oxide

Introduction to Cuprous Oxide (Cu 2 O)

Cuprous oxide (Cu2O) is a stable binary copper compound with a lattice constant of 4.269 Å and crystallizes in a cubic structure (cuprite, space group Pn-3m). 4 copper atoms and 2 oxygen atoms make up the unit cell. As seen in Figure 5.1, the copper atoms (blue balls) create a face-centeredcubic sublattice, while the two oxygen atoms (red balls) form a body-centered-cubic sublattice. Cu2O is a direct band gap semiconductor with a significant exciton binding energy of 150 meV [3].

At room temperature, the Cu2O optical bandgap values ranging from 2.4 to 2.7 eV have been reported experimentally [4][5][6][7][8][9] as also shown in the present thesis.

Cu2O films possess intrinsic p-type conductivity probably due to: (i) Cu vacancies (VCu) as theoretically predicted [11] and experimentally evidenced with measurements, using deep-level transient spectroscopy, of transition levels at 0.2 and 0.5 eV above the valence band maximum [12];

(ii) extrinsic shallow acceptors related to nitrogen or hydrogen complex with Cu [13,14]. These defects properties strongly depend on the preparation technique, resulting in a great disparity between the resistivity values measured with respect to the methods and the conditions of deposition. For instance, very high resistivity values ranging from 10 2 -10 4 Ω×cm and 10 4 -10 6

Ω×cm were reported for the films prepared by grown by thermal oxidation [15] and electrodeposition [16], corresponding to a very low hole carrier concentration at around 10 14 cm -3 and a mobility of about 1 cm 2 /Vs. Cu2O prepared by sputtering or ALD have usually lower resistivity [8,[17][18][19][20][21][22][23][24] but comparable or even higher mobility than the electrodeposited films.

Cu2O has a direct band gap with a very high absorption coefficient of about 3×10 5 cm -1 at a wavelength around 400 nm [25][26][27][28] and even more than 7×10 5 cm -1 in the ultraviolet spectral region. The refractive index of Cu2O at room temperature is about 2.7 in the transparent spectral region [29][30][31][32].

One can note that the Cu2O optoelectronic properties depend strongly on the crystallinity and phases present in the thin films. Indeed, Cu2O is unstable in air at room temperature and can oxidize to CuO [33][34][35][36][37][38][39]. However, the kinetics of the transformation in CuO is so slow at room temperature that Cu2O can be considered stable for practically all applications.

Cu 2 O as Solar Cell Absorber Material

Cu2O is a promising material for solar cells due to its non-toxic and abundant nature and its very high absorption coefficient. It also has a relatively decent mobility at ambient temperature and a relatively long minority carrier diffusion length if it is close to a single crystal [40]. Cu2O has a direct band gap of about 2.1 eV (depends strongly on the preparation process) and is intrinsically p-type due to Cu vacancies in the lattice (as explained above in the Section 5.1) [41]. All of this adds up to a theoretical conversion efficiency (Shockley-Queisser efficiency limit, cf. Section 1.1.

of

Chapter 1) of 20 %, which is quite high and indicates that Cu2O should be investigated further for solar cell applications [17].

However, no Cu2O solar cells have yet been manufactured that live up to the theoretical predictions. This is due to the fact that manufacturing can be difficult due to the presence of cupric oxide (CuO).

This results in issues such as Schottky junctions and lattice defects, which reduce mobility and conductivity while also causing non-radiative recombination [42]. Furthermore, the Cu2O films produced thus far have a low photoexcited carrier collection probability and considerable surface recombination [17]. Olson et al. [43] published a comprehensive study of Cu2O solar cells, highlighting the difficulties that need to be solved to make commercial production of Cu2O practical.

Rakhshani et al. [44] examined the current methods for preparing Cu2O which are to be used as absorbers in the solar cells. Most preparation techniques required high temperature oxidation of copper of more than 1000 °C [45], therefore this study investigated ways that required far less temperatures, even less than 300 °C. Cu2O preparation at low temperatures has gotten a lot of attention since then. The readers are requested to please look into the Section 1.3.2 of Chapter 1 to find the details of the use of Cu2O as absorber layers in "all-oxide" solar cells.

Preparation of Cu 2 O by Ultrasonic Spray Pyrolysis

The preparation of Cu2O films was done using our ExactaCoat system from Sono-Tek presented in detail in the Sections 2.4 & 2.5 of Chapter 2. The temperature on the heating plate was controlled by an in-built PID temperature controller and measured with thermocouple A. The temperatures of the substrate holder and the glass substrate were measured using a N-type thermocouple (thermocouple B). The schematic arrangement of the spray preparation system is shown in the Copper acetate monohydrate, Cu(CH3COO)2 • H2O was used the source precursor salt dissolved in deionized water. For simplicity henceforth, in this thesis we will always refer it as only Cu(Ac)2.

Always 50 mL of precursor solution was prepared before the start of the spray. D-sorbitol was used as a reducing agent and mixed in the Cu(Ac)2 solution. D-sorbitol helps in the non-formation of the detrimental CuO phase. Acetic acid was also added to the precursor solution to reduce the formation of copper hydroxide, Cu(OH)2. Usually, the dissolution of Cu(Ac)2, in deionized water yields a pH value of about 5.5. At this value, Cu(OH)2 starts to get precipitated [46][47][48]. Cu 2+ ions and other copper-hydro-acetate complexes become highly soluble when acetic acid is added to bring down the pH value below 3 [49][50][51].

of Cu 2+ ions. Compared to D-glucose, glycerol is more volatile. D-sorbitol possess a much less complex thermal decomposition when compared to both glycerol and D-glucose. The thermal decomposition of D-sorbitol starts with the formation of sorbitan which gets entirely converted to volatile and harmless by-products such as H2 and CO2 [58,[61][62][63].

Kosugi et al. [52] also suggested a two-step Cu2O film formation process involving OH-group as shown below:

Step 1:

𝐶𝑢 2+ + 𝑅-𝐶𝐻 2 -𝑂𝐻 + 1 2 𝑂 2 → 𝐶𝑢 0 + 𝑅 -𝐶𝐻𝑂 + 𝐻 2 𝑂
Step 2:

2𝐶𝑢 0 + 1 2 𝑂 2 → 𝐶𝑢 2 𝑂 𝑓𝑖𝑙𝑚
If the deposition temperature is below 200 °C, then Cu will only be partially oxidised to produce a mixed Cu/Cu2O film. Satta et al. [55] proposes a one-step process of Cu2O film formation by reduction of already formed CuO film with utilization of a primary OH-group. The reaction can be described as written below:

2𝐶𝑢𝑂 + 𝑅 -𝐶𝐻 2 𝑂𝐻 → 𝐶𝑢 2 𝑂 (𝑓𝑖𝑙𝑚) + 𝐻 2 𝑂 + 𝑅 -𝐶𝐻𝑂
For all the reaction schemes, the secondary OH-groups can undergo further reactions to form ketones, while the formed aldehydes can be oxidised to carboxylic acids [55]. The further oxidation of the Cu2O film will be limited kinetically due to the presence of a very thin (≈ 1 𝑛𝑚) surface oxide film of CuO. This film acts as a barrier for the oxygen diffusion which prevents the further oxidation of the Cu2O film [64,65]. The ultra-thin CuO film will always be present above a Cu2O film even under ambient conditions but as it is too thin, it will not be detected by characterization technique such as XRD [64][65][66][67]. In our published study [67], D-sorbitol is chosen as the reducing agent. The effect of the concentration of the D-sorbitol in the precursor solution on the quality of the Cu2O film is studied and discussed in detail in the later sections of this chapter.

Setting the Deposition Temperature

When compared to other metal oxides such as zinc oxide or titanium dioxide, temperature control is very important for spray pyrolysis of copper oxide because the thermal degradation of copper acetate precursor can produce a combination of Cu, CuO, or Cu2O phases [68]. The preparation temperature is usually not well defined in the spray pyrolysis related literatures. It's critical, especially for multi-phase materials, to identify the preparation temperature exactly, which could be the set temperature, the substrate temperature, or another observed or estimated temperature connected to the equipment and process. Using the approach below, we precisely define and examine the preparation temperature for the first time. During the preparation process, the temperature profile on the substrate surface is measured every second. around 300 nm, as measured by mechanical and optical profilometry with same vertical resolutions of 0.1 nm. The thin film is formed by spray scanning the nozzle back and forth across the substrate surface 75 times for a total of 720 seconds. After each scan, the nozzle comes to a halt for 0.5 seconds before going back in the opposite direction, resulting in a total cycle time of 1 second. This period is called the dwell time. The time interval between two sprays over the same zone, and thus the preparation temperature, is affected by the dwell time in the spray pyrolysis process. Even though the hot plate is kept at a constant temperature, the preparation temperature changes substantially due to the aerosol stream that reaches the substrate and cools it down. The distinction between the set temperature and the preparation temperature is critical for understanding the spray pyrolysis process, and it has been monitored precisely in real-time in this study using two thermocouples A and B (as shown in the Figure 5.

2).

For all the preparations, the concentration of Cu(Ac)2 powder in the precursor solution was 0.025 mol/L. So, the quantity of the Cu(Ac)2 powder required in the solution to reach this concentration is calculated using the relation:

Equation (5.1):

𝑚 𝐶𝑢 = 𝐶 × 𝑉 × 𝑀
Where, m is the mass (in g) of Cu(Ac)2 powder; C is the concentration in mol/L; V is the solution volume in litres (L) and M is the molar mass of Cu(Ac)2 (in g/mol).

So, mass of Cu(Ac)2 needed for preparing a 100 mL solution is: mCu = 0.025 × 0.1 × 199.65 = 0.499 g ≈ 0.5 g. Now, the quantity of the D-sorbitol powder required in the solution to reach a particular solution concentration is calculated using the relation:

Equation (5.2):

𝑚 𝐷-𝑠𝑜𝑟𝑏𝑖𝑡𝑜𝑙 = 𝐶 × 𝑉 × 𝑀 Where, mD-sorbitol is the mass (in g) of D-sorbitol powder; C is the concentration in mol/L; V is the solution volume in litres (L) and M is the molar mass of D-sorbitol (in g/mol). So, for example, mass of D-sorbitol needed for preparing a 100 mL solution with concentration of 0.05 mol/L is: m = 0.05 × 0.1 × 182.17 = 0.91 g. At first, the required quantity of Cu(Ac)2 powder is dissolved in 50 mL deionized water. Then 30 mL more deionized water is added. After this the required quantity of D-sorbitol powder is added and dissolved in 80 mL of the solution. The solution is shaken gently to dissolve the added powders. The pH of the solution was measured which was approximately always 5.5. The target is to bring down the pH to below 3. For all our preparations we have maintained a constant pH of 2.8 to make the Cu 2+ ions and other copper-hydro-acetate complexes completely soluble. Approximately 7 mL of acetic acid was then added to the 80 mL of the solution to bring the pH value to 2.8 and then again 13 ml deionized water was added to make the final solution of volume 100 mL. For the first preparation study, when only the set temperature was changed, the D-sorbitol concentration in the solution was maintained always to 0.05 mol/L. For the second preparation study, the concentration of D-sorbitol was varied from 0.0000 mol/L to 0.0750 mol/L. Different solutions with increasing D-sorbitol concentrations in steps of 0.0125 mol/L were prepared, with the powder quantities calculated using the Equation 5.2.

Results and Discussions on the Preparation of Cu 2 O

The regulation of the phase, as mentioned earlier, is the main issue for copper oxide optimization.

The ultrasonic spray pyrolysis technique allows for fine-tuning of the preparation parameters involved in phase formation, particularly the preparation temperature and precursor solution concentration. The influence of these factors is investigated here utilizing the new sorbitol reducing agent, first by adjusting the preparation temperature, and then by varying the sorbitol concentration. Using the conditions described in Chapter 2, X-ray diffraction, micro-Raman spectroscopy, and UV-Vis transmission are used to analyze the structural and optical properties of the developed thin films respectively. The van der Pauw and Hall Effect techniques were used to determine the electrical characteristics.

Effect of the Preparation Temperatures on the Structural and Optical Properties of Cu2O

When compared to other metal oxides, such as zinc oxide or titanium dioxide, the control on the preparation temperature is very important for spray pyrolysis of copper oxide because the thermal degradation of the copper acetate precursor can generate a combination of Cu, CuO, or Cu2O phases Where I(hkl) is the integrated peak intensity corresponding to the (hkl) diffraction plane. This relation gives only a qualitative estimation of the CuO/Cu2O phase ratio in the prepared thin films. The Xray diffraction profiles were fitted using a pseudo-Voigt analysis procedure to extract the above parameters [69]. Figure 5.5 also depicts the crystallite size determined using the procedure from Monshi et al. [70] as detailed in the Chapter 2. The Cu2O crystallite size seen at lower temperature (230 °C) is around 20 nm, which is close to the values often achieved for sprayed Cu2O thin films using glucose, such as in [71], and greater than the value acquired for the first and only work employing D-sorbitol [59]. For comparable preparation temperatures, grain sizes in sputtered polycrystalline Cu2O thin films can reach up to roughly 200 nm [17]. The crystallite size changes somewhat (up to 30 nm) as the temperature rises. Due to grain coalescence, such an increase in preparation temperature is commonly observed [17]. Since the CuO phase arises and becomes to dominate in our case, the transition cannot be described just by temperature-related grain coalescence. A limiting factor for the Cu2O crystallite size increase could be the formation of CuO at the grain boundaries [72].

The observed phase transition between Cu2O and CuO employing D-sorbitol as a novel reducing agent follows a similar pattern to that seen using glucose as a reducing agent [52]. The copper acetate precursor solution decomposes into CuO in ambient air without the use of a reducing agent such as glucose or, in our instance, D-sorbitol.

The formation of Cu2O using spray pyrolysis preparation method can happen mostly by two chemical paths which are already detailed in the Section 5.3.1.1 of this chapter. The oxidation rate of Cu2O into CuO increases as the temperature rises, limiting the influence of the D-sorbitol reducing agent and explaining the evolution of the X-ray diffraction peaks. This agrees with the temperature range reported in [52] and the temperature profile recorded directly on the substrate as shown in the Figure 5.3, indicating the temperature at which the pyrolytic reaction takes place. Furthermore, in ambient air, the formed thin films can be oxidized to generate CuO [73].

Micro-Raman measurements were taken on the produced copper oxide thin films to complete the structural study. Table 5.3 lists the primary Cu2O and CuO Raman peaks that have been identified in the literature, along with their values and symmetry.

Cu2O is a distinctive semiconductor from the standpoint of Raman spectroscopy because the theoretically predicted active Raman mode (T2g) is rarely observed, whereas two peaks related to defects and breaking the selection rules, T1u and 2Eu, are consistently observed and dominate the Raman spectra [74][75][76][77]. These peaks are due to point defects related to the p-type doping of Cu2O.

Table 5.3: Cu2O, CuO, and Cu4O3 Raman peaks. Peak position is shown as a single value or a range of values, according to experimental or calculated data that has been published in the literature [74][75][76][77]. The corresponding Raman modes assigned to the peak are also shown. The peaks marked with an asterisk and bold are the most intense ones that were observed. Only the T2g peak in Cu2O is anticipated to be Raman active theoretically [75], however the two peaks T1u and 2Eu are systematically seen and dominate the Raman spectra. These peaks are linked to point defects related to the p-type doping of Cu2O [76]. In our thin films, the Cu4O3 phase Raman peaks were not detected.

The Raman peaks associated with the Cu4O3 copper oxide paramelaconite phase were absent from our thin films. The CuO phase is not detectable in the sample prepared at 230 °C, as seen from the line-scans. This result agrees very well with X-ray diffraction measurements that reveal no CuO signature al the lowest preparation temperature. Even while the signal from Cu2O is still prominent, the CuO Raman peaks are seen when the preparation temperature is raised to 240 °C. The CuO phase becomes totally dominant as the preparation temperature rises. In fact, because the response from the sample surface dominates the Raman signal, this result simply reveals that the thin films are covered by CuO, leaving no assumptions regarding the composition of the bulk and grains. properties and to establish the relationship between these properties and the thin film structural properties. The transmission spectra are plotted with respect to the preparation temperatures in preparation techniques such sputtering [78], and the value obtained using D-sorbitol as a reducing agent in ultrasonic spray pyrolysis [59]. The first difference in bandgap values when compared to sputtering is typical and here too low to be meaningful, while the second difference could be attributable to the impact of the CuO phase, still present in the films, and the bandgap extraction procedure (Section 2.7.2 of Chapter 2 gives details about the Tauc model). Figure 5.9 shows an indirect bandgap value for CuO that is consistent with values commonly published [79].

Film

Effect of the D-Sorbitol Reducing Agent on the Structural and Optical Properties of Cu2O

To understand in-depth the effect of D-sorbitol, which is a novel reducing agent, we set the preparation temperature to 240 °C and the copper acetate concentration to 0.025 mol/L, then adjusted the D-sorbitol concentration from 0 to 0.0750 mol/L in steps of 0.0125 mol/L. By consistently preparing a set of solutions with the same concentration and frequently preparing samples in the same conditions, the repeatability of the preparation process was tested. The Raman spectra for D-sorbitol concentration are shown in Figure 5.11. The Raman spectra of all samples prepared without D-sorbitol (concentration 0 mol/L) reveal just the CuO phase peaks, as expected when employing a precursor solution without a reducing agent. In addition, another broad peak was found at roughly 1100 cm -1 for these samples without the reducing agent, which was tentatively assigned to multiphonon scattering or defects generated by the incomplete pyrolytic reaction.

The Raman results correlate well with XRD measurements shown in

Electrical and Morphological Properties of the Prepared Cu2O Films

The objective of the electrical study was to check for the ohmicity of the gold contacts on the Cu2O prepared samples. 200 nm of gold was deposited on the prepared samples. The samples with the low preparation temperatures (230 °C and 240 °C) were chosen as in these samples the Cu2O phase is dominant. The Cu2O was films considered for the studies in this section is prepared with a pH of The same samples (a & c) were annealed and then measured again to check if there is any difference in ohmicity after the annealing. The annealing was done only after the gold contact deposition. The annealing for all the samples was performed in an oven for 15 minutes in a controlled atmosphere at 250 °C. As an example, the I-V characteristics before and after annealing are shown in the The electrical properties of the prepared Cu2O thin films using this ultrasonic spray pyrolysis were tested, and a p-type conductivity was observed for the first time with the D-sorbitol reducing agent, with a carrier concentration of around 6×10 16 cm -3 , a mobility of about 7 cm 2 /Vs, and resistivity of 15 Ω×cm. The electrical properties were improved for the Cu2O thin film of sample (a) which was prepared at 230 °C. This is because there is no formation of CuO phase, and the thin film is composed entirely of the Cu2O phase. Carrier concentration of around 5×10 16 cm -3 , a mobility of about 12 cm 2 /Vs, and a lower resistivity of 10 Ω×cm is measured for this sample. The mobility is higher than that of spray pyrolysis [71] previously and comparable to that of thin films formed by sputtering [17]. This first electrical evaluation paves the way for a further in-depth examination of the transport properties and their improvement for optoelectronic applications using ultrasonic spray pyrolysis prepared Cu2O thin films.

The copper oxide thin films prepared at 230 °C and 240 °C are checked for their surface morphologies by means of SEM. High magnification was used to visualize closely the distribution of the grains, their shapes, and sizes. The images were taken with a beam voltage at 2.0 kV and using an InLens detector.

For the sample prepared at 230 °C, we observe several things. First, the surface presents some moderate roughness in agreement with the previous observations of a non-uniform surface for e.g.,

Challenges Faced Towards the Preparation of Cu 2 O

The most important factor in the preparation of Cu2O is the preparation temperature. We did an indepth analysis to find the relation between the set and the actual spray preparation temperatures.

The temperature plays an important role in controlling the rate of oxidation and the efficiency of the reducing agent. At higher temperatures, the efficiency of the reducing agent is diminished which paves the way for the formation of unwanted copper phases such as cupric oxide (CuO). By chance if CuO especially in nanoparticle condition is formed it can be toxic too [80]. The major disadvantage that CuO can cause is that introduce deep level trap states at the interface of the heterojunctions which will act as recombination centres for the charge carrier. This can impact greatly in the performance of solar cells and so CuO formation is highly undesirable. We tried to prepare the films at the lowest possible temperature so that the phase of CuO can be avoided in the films. It must also be taken care that the temperature should be such that the pyrolysis reaction is complete. Incomplete pyrolysis can deposit unwanted by-products such as sugar and sugar-based alcohols. D-sorbitol is chosen for this reason as it can decompose at low temperatures and can only give rise to non-harmful by-products such as CO2 and H2. But control on the concentration of D-sorbitol in conjunction with the concentration of copper acetate and the pH in the precursor solution is very important. Once these are controlled, the other spray parameters need to be varied and then fixed to certain values so that a complete pyrolysis takes place to form thin films with only Cu2O phase.

The analysis through the design of experiments helped us to successfully prepare the films with the desired phase.

Chapter Conclusions

A detailed structural and optical investigation was carried out to evaluate the phase transition in copper oxide thin films prepared by ultrasonic spray pyrolysis with D-sorbitol, a new precursor solution reducing agent. Regarding the carefully determined preparation temperature, the study reveals the best range in which the single Cu2O phase can be formed in the same way as with glucose, but with fewer carbon-related defects caused by pyrolytic reaction by-products. Indeed, the research of D-sorbitol concentration demonstrates that the optimal features were attained without the use of excessive D-sorbitol in comparison to copper acetate, as is common when utilizing glucose as the reducing agent. Using a lower concentration of reducing agent results in fewer pyrolysis secondary products, decreasing the defects, and improving electrical characteristics for optoelectronic applications.

To summarize: the ultrasonic spray pyrolysis preparation of copper oxide thin films was developed and optimized using a new reducing agent, D-sorbitol. A comprehensive optical and structural study was performed. The optimization of the preparation temperature using low concentration reducing agent was conducted to obtain single-phase cuprous oxide Cu2O with good optical and structural properties.

The development of ternary and quaternary semiconductor alloys is crucial for optoelectronic applications. For the photovoltaic application, these alloys can have versatile functions. They can be used as TCO as well as window layer, buffer layer or even, in some cases, as absorber. The alloys we develop are all oxides based on abundant and non-toxic elements. Beyond the "all-oxide" solar cell application, these materials can have other uses in optoelectronics and as an active layer in ultraviolet photodetectors with a modulated spectral range. The results of ternary and quaternary preparation and the study of their morphological, structural, optical, and electrical properties are the subject of this chapter. A brief numerical simulation study and a presentation of the perspectives of this work conclude the chapter.

Introduction to Zinc-Aluminum Oxide (ZnAlO)

Pure ZnO thin films are not resistant to corrosive environments, for example, the adsorption of O2 reduces the electrical conductivity of the film and affects the surface morphology. Incorporating various elements in ZnO with in the hopes of making the ZnO system resistant to such changes has been researched [1,2], resulting in an interesting family of materials based on ZnO. Aluminum is a group 13 element, which can easily substitute Zn in the crystal lattice and such incorporation can lead to high carrier concentrations, >10 20 cm -3 [3][4][5][6].

The structural, electrical, and optoelectronic characteristics of ZnO due to Al incorporation have been studied using a quantum chemical approach, which explains the increase in n-type electrical conductivity [7]. The material should have a uniform and homogenous distribution of dopant ions in the correct regions, up to a limiting concentration, to achieve optimal performance.

In terms of donor behaviour from Al, [5] [8]. The question of whether the conditions for maximum Al incorporation are the same as those required to make highly orientated thin films [9] and/or whether any of these results in optimal TCO properties raises an interesting subject.

Preparation of Ternary ZnAlO by Ultrasonic Spray Pyrolysis

The preparation of ZnAlO films was done on soda lime glass using an ExactaCoat system from Sono-Tek with an Impact nozzle operating at 120 kHz. The experimental details are given in the

Chemicals Used for ZnAlO Preparation

All the chemicals used for the preparation of ZnAlO thin films are listed in the Table 6.1.

Table 6.1: Chemicals with high purity used for spray pyrolysis deposition of ZnAlO.

The pH of the precursor solution is monitored using the acetic acid. The pH of the solution can also play a role in the film morphology: nanorods to compact films. We monitored the pH value in collaboration with other spray parameters to have dense, uniform, and compact films. Generally, the acetic acid can reduce the formation of the volatile Zn and Al species in the solution, thereby reducing the chances of them further reacting to form respective hydroxides. Further, the acetic acid addition helps to have a lower surface tension of the water-based precursor solution so that during the atomization uniform sized droplets are generated from the spray nozzle. But, when compared to the effect of the Al concentration in the solution, the effect of acetic acid concentration on the morphology of the spray prepared films is less pronounced.

The effects of the increase of the Al(acac)3 concentration while keeping the Zn(Ac)2 concentration fixed in the precursor solution on the ZnAlO film quality have been discussed later in this chapter.

The calculation procedure to select the concentrations of Zn(Ac)2 and Al(acac)3 along with the Although, in all the previous preparations we have not used ethanol, in this case we had to use, to dissolve the Al(acac)3 soluble. But, as soon as it is mixed with water-based solution the solubility is hindered. So, keeping the poor solubility factor of Al(acac)3 in mind we have only used up to molar concentration of 5 % Al(acac)3. Also, most importantly we see that in the final solutions S3 the volume percentages of ethanol can be only in the range from 1 % to 3 %. This low volume percentage will not be toxic for the pyrolysis process. Based on the experimental details mentioned in the Chapter 2 we have also added acetic acid to each of the precursor solution and maintained a value of pH of 4. Having the same pH for all the solutions helps to analyze the films better. The other advantages of acetic acid are mentioned in the previous chapters.

Design of Experiment

Many parameters are involved in the spray pyrolysis preparation process for depositing high-quality thin films as detailed in the Section 2.5 of Chapter 2. Variation in each parameter impacts the resulting film quality. Using a precise design of experiment, a thorough investigation of the structural and optical properties was carried out to identify the set of parameters that result in high quality and transparent Zn1-yAlyO thin films. The nozzle speed and the spray flow rates were prefixed to 70 mm/sec and 0.5 mL/min based on the safety requirements of the spray system.

The list below shows the variation of other spray parameters along with range of their variation.

1.

The zinc acetate [Zn(Ac)2] molar concentration in the precursor solution varied in the range from 0.03 mol/L to 0.10 mol/L.

2.

The Aluminum acetylacetonate [Al(acac)3] molar concentration in the precursor solution varied in the range from 0.0005 mol/L to 0.0055 mol/L.

3.

The number of spray cycles, which ranged from 10 to 100. The duration of each preparation is determined by this parameter.

4.

The dwell times between each spray cycle varied between 0.5 s to 2 s.

5.

The set preparation temperature was varied between 400 °C to 480 °C.

6.

The relative shaping air pressure was varied between 6.8 kPa to 27.6 kPa.

7.

The height between nozzle to substrate was varied between 90 mm to 110 mm.

When one spray parameter was varied, all the other parameter remained fixed to a particular value and the repeatability of the preparation process was constantly monitored by preparing each sample at least three times with the parameters listed above. The used design of experiment to select the optimal ultrasonic spray pyrolysis parameters is shown below in the Table 6.4.

Results and Discussions on ZnAlO Preparation

The optimal value of the spray parameters from the design of experiments scheme mentioned above has been used for the preparation of ZnAlO thin films. Various characterization techniques were used to understand the effect of the Al incorporation in the precursor solution on the quality of the prepared ZnAlO thin films.

Optical Properties of the ZnAlO Thin Films

In this section we try to analyze the optical properties of the ZnAlO thin films with respect to the increase of the Al molar concentration. At first the transmission of the films was assessed in relation to the aluminum incorporation effect as shown in the Figure 6.1. Also, the bandgap and Urbach energies of the films were studied in response to the increase of the Al molar content. The thickness of the prepared films was systematically measured at three sites of the spray zone by the mechanical DekTak profilometer and then averaged to give about 200 nm thickness. To verify consistency of the films, the measurements were repeated by an optical profilometer at the spray zone of the films.

Both the profilometers have a vertical resolution of 0.1 nm and the thickness measurements of the films were found to be very consistent with narrow range of differences (± 10 nm).

The average transmission of the ZnAlO thin films in the wavelength range 500-800 nm is shown in the The variation of the bandgap energies as a function of increasing aluminum molar composition is not fully linear, although an increasing trend is noticed. For a pure ZnO, the bandgap value is 3.267 eV while for the Zn0.95Mg0.05O film (with Al mol 5%) the value increases to 3.292 eV. At small concentrations of Al, the aluminum atoms most probably substitute the zinc atoms. Since the zinc atoms are divalent and the Al atoms are trivalent, there will be an increase in carrier concentration in corresponding to the blue shift of the bandgap absorption. The blue shift can be tentatively attributed to the Burstein-Moss effect [10]. This increase in the Al atom concentration shifts the Fermi level towards the conduction band. High possibility is that the conduction band minima will occur at the 3s and 3p state of Al which can be dominant over the Zn 4s state [11].

We also calculated the Urbach energy or the "band-tail" to estimate the alloy disorders and increase of density of defects in the films with the increase in the aluminum content for the Zn1-yAlyO thin films. For Zn1-yAlyO films prepared by ultrasonic spray pyrolysis process, the Urbach energy calculations are done for the very first time. The Urbach energies are calculated from the absorption of the thin films in the sub-bandgap spectral region as shown in the From the analysis, we can estimate that the obtained Urbach energy value for the films with low aluminum compositions, is equivalent to the usual reported values for ZnO grown by pulsed lased deposition (PLD) [12]. The Urbach energy increases considerably as the crystalline lattice alloy disorder grows due to aluminum incorporation, as expected.

Structural Properties of the ZnAlO Thin Films

To analyze the structural properties of the Zn1-yAlyO thin films, X-ray diffraction (XRD) measurements were done on the films after preparation. The diffractograms are shown in the for the (100), (002), ( 101), ( 102), ( 110), ( 103) and ( 112) planes of the space group P63mc, identified by using the ZnO powder diffractogram (RRUFF ID: R060027.1) [13] as a reference for aluminum compositions up to 5 mol%. From the diffractograms we could not identify any phase related to formation of Al2O3. At these low aluminum molar concentrations, no additional phase occurs and the original wurtzite phase of ZnO is retained.

Resistivity Measurements of the ZnAlO Thin Films

Prior to the resistivity using the four-point probe technique, the films were annealed using the same conditions mentioned in the Section 4.6.4 of Chapter 4 for the ZnMgO films. The difference is that in this chapter the films are prepared with Impact nozzle instead of Accumist nozzle. The resistivity changes with respect to the increase in the Al molar concentration is shown in the Figure 6.12. ZnO resistivity is found to be approx. 0.12 Ω×cm. The resistivity goes down to 0.08 Ω×cm and 0.09 Ω×cm as the Al molar concentration increases up to 1 mol% and 2 mol% respectively. For the ZnAlO film with Al molar composition 5 mol% the resistivity is increased to 0.6 Ω×cm. The initial decrease in the resistivity can be attributed to the increase of the free carrier concentration. The concentration of the free carriers is increased mostly due to the substitution of Zn 2+ cation sites and in some interstitial positions by the donor Al 3+ ions [17]. From molar content of 3 mol%, the resistivity values go above the one measured for ZnO. We have seen that with the increase in the molar Al content, the lattice disorder in increases and the crystallize size decreases. This can hinder the mobility of the carrier. Even with the Al increase, some intrinsic defects are produced in the films which acts as carrier traps rather than being electron donors [18,19]. Some non-conducting clusters of Al2O3 can form which causes resistivity increase but they were not evidenced from the XRD measurements. In the next section we will discuss about the preparation of a quaternary material Zinc-Aluminum-Magnesium-Oxide (ZnMgAlO). The aim to prepare the material is to modulate the bandgap as well as the conductivity. As we have seen, with increase of aluminum molar composition the increase of defects, loss of transparency and increase of resistivity happens, so for the preparation of the ZnMgAlO we limit ourselves to aluminum molar composition of 0 %, 1 % and 2 % with varying magnesium composition.

Introduction to Zinc Aluminum Magnesium Oxide (ZnMgAlO)

Zn1-y-xMgxAlyO is a quaternary material with a relatively small content of Al and Mg which can be used as a transparent conducting oxide (TCO) in solar cells and display devices but also as an active layer in optoelectronic devices [20,21,22]. Our idea is to prepare a suitable material with sufficient transparency and conductivity so that it can be applied as a TCO for applications in "all-oxide" solar cells but, beyond that, as an active layer in optoelectronic devices such as ultraviolet detectors.

Indeed, a material whose bandgap can be modulated so that it can either be used as a window or buffer layer with suitable band alignments for heterojunctions, or as an absorber in detectors with a modulable spectral response. Conduction band electrons in wider-bandgap layers transfer through heterojunction interfaces to intrinsic ZnO layers in these devices. Mg incorporation can broaden the bandgap of ZnO as shown in this work and detailed in Chapter 4. Due to the similar valence states of Mg 2+ and Zn 2+ , Mg 2+ substitution cannot generate any free carriers. To make a high-conducting ZnO with a wide bandgap and to improve device performance, Al and Mg need to be co-incorporated to increase carrier concentration and widen the bandgap, respectively [23]. In [24] it was first shown that it was possible to make single crystalline ZnMgAlO films with deep UV bandgaps and lattice constants that match those of the ZnO and/or GaN substrates. This was achievable because the lattice constant of Al2O3 is higher than that of MgO, which is lower than that of ZnO. The ZnMgAlO films were suggested as a suitable candidate for high-quality deep-UV optoelectronic devices because they might reduce residual strain [25].

Another factor to keep in mind is the use of abundant and non-toxic materials for preparation of the quaternary ZnMgAlO. So far, this material has been developed using processes like RF magnetron sputtering [24][25][26][27][28], pulsed laser deposition (PLD) [29] and as nanoparticles by sol-gel technique [30]. But we used for the first-time ultrasonic spray pyrolysis to prepare quaternary ZnMgAlO keeping in mind the non-toxicity factor as much as possible.

Preparation of Quaternary ZnMgAlO by Ultrasonic Spray Pyrolysis

The preparation of ZnMgAlO films is being done for the first time using the process of ultrasonic spray pyrolysis based on the procedure explained above at the start of the Section 6. only. Always 50 mL of precursor solution was prepared before the start of the spray. Acetic acid was added to the precursor solution to reduce the formation and precipitation of hydroxides as mentioned earlier.

Chemicals Used for ZnMgAlO Preparation

All the chemicals used for the preparation of ZnMgAlO thin films are listed in the Table 6.5. The final volume "V4" of S4 will be 50 mL. Volume of S1 needed is "V1".

The volume "V2" of S2 to be collected is given by:

Design of Experiment

The nozzle speed and the spray flow rates were pre-fixed to 70 mm/sec and 0.5 mL/min based on the safety requirements of the spray system. The list below shows the variation of other spray parameters along with range of their variation, and the Table 6.7 summarized the used design of experiment.

1. The zinc acetate (Zn(Ac)2) molar concentration in the precursor solution varied in the range from 0.03 mol/L to 0.10 mol/L.

2.

The magnesium acetate tetrahydrate (Mg(Ac)2 • 4H2O) molar concentration in the precursor solution varied in the range from 0.0015 mol/L to 0.0214 mol/L.

3.

The aluminum acetylacetonate (Al(acac)3) molar concentration in the precursor solution varied in the range from 0.0005 mol/L to 0.0026 mol/L.

4.

The number of spray cycles, which ranged from 10 to 100. The duration of each preparation is determined by this parameter.

5.

The dwell times between each spray cycle varied between 0.5 s to 2 s.

6.

The hot plate set temperature was varied between 400 °C to 480 °C.

7.

The relative shaping air pressure was varied between 6.8 kPa to 27.6 kPa.

8.

The height between nozzle to substrate was varied between 90 mm to 110 mm.

Results and Discussions on the Preparation of ZnMgAlO

The optimal value of the spray parameters from the design of experiments scheme mentioned above has been used for the preparation of ZnMgAlO thin films. Various characterization techniques were used to understand the effect of the Al and Mg incorporation in the precursor solution on the quality of the prepared ZnMgAlO thin films. In all the films prepared using the precursor solutions, the Mg molar concentrations were varied from 0 % to 7 % by keeping Al molar concentrations fixed to 0 %, 1 % and 2 % respectively. The details are shown in the Table 6.6 along with the stoichiometry of the prepared films.

Optical and Morphological Properties of the ZnMgAlO Thin Films

In this section we try to analyze the optical properties of the ZnMgAlO thin films with respect to the increase of the Al and Mg molar concentration. The thickness of the prepared films was measured within a variation of ± 10 nm as done for the other metal oxides films. The average transmission of the films in the wavelength range 500-800 nm is shown in the From the analysis, we can estimate that the obtained Urbach energy value for the films without aluminum compositions, the Urbach energy values remains almost constant. This is contrary to our results obtained for the ZnMgO films, where the Urbach energy increased with increase in the Mg content. So, we can say that with an efficient control on the spray parameters with the Impact nozzle does not sufficiently introduce lattice alloy disorders in the films. Even when the Al is introduced, there is not enough lattice alloy disorder in the films. Although the Al molar composition is too low, but still without much disorder the films show high optical quality with the increase in composition.

As stated in the Section 2. The morphological analysis of the thin films was done by SEM. For the films without any Mg, for Al molar compositions 1 % and 2 %, the surfaces reveal polycrystalline nature with some roughness.

Some small clusters can even be seen on the sample with Al mol 2 %. The surfaces tend to get better and smoother with the incorporation of Magnesium. Big clusters are not visible in the micrographs.

This can be related to the less lattice disorders with the increase in the Mg content in the films as seen from the Urbach analysis. Although, some small agglomerations could be noticed for the film with Al mol 2 % with 7 mol % of Mg. All the films showed high quality with uniform grain distribution and without any voids. These morphological results when compared to films developed by RF magnetron sputtering [24,27] are quite similar. In correspondence to the texture analysis of the films, we also estimated the crystallite sizes with respect to the different peaks along with the increase in Mg molar composition. The

Electrical Properties of the ZnMgAlO Thin Films

The electrical properties of the Zn1-y-xMgxAlyO thin films were analyzed in terms of the changes to the resistivity, carrier concentration and mobility with the increase of the magnesium molar composition along with the fixed aluminum compositions. After preparation, and prior to the electrical measurements, all the films were annealed using the rapid thermal annealing (RTA) system at a temperature of 300 °C for 30 minutes. The annealing was performed to increase the Interstitial Zn acts as n-type dopants, but Mg incorporation inhibits these sites causing the decrease of free carriers and increasing the resistivity [29]. But interestingly when Al is introduced in the films, the resistivity still increases but, when for the higher Mg contents (5 and 7 mol%), with Al, the resistivities are lower. Magnesium induces a bandgap increase and inhibits shallow donors.

Aluminum introduces free carriers to some extent by replacing the Zn 2+ sites with donor Al 3+ ions which keeps the resistivity lower. But it can also induce a decrease in the formation of interstitial Zn sites with the change in the stoichiometry, and its activation energy can increase due to the Mg incorporation. There is therefore a complex compensation mechanism between the effect of Al and the effect of Mg. For this reason, correspondingly, the carrier concentration also remains low, for the films with Al. The rate of decrease of the carrier concentration become slower, at the higher Mg contents (Figure 6.26). Contrarily, the carrier mobilities for the films with Al, always remains high (Figure 6.27).

Thus, the Zn1-y-xMgxAlyO material conductivity, which can be regulated by adding magnesium with fixed aluminum contents, allows the material to be tuned for certain optoelectronic applications, such as ultraviolet detectors and solar cells. For "all-oxide" solar cells, with modulated bandgap in conjunction with the conductivity it can even be used as top window or buffer layers.

Challenges Faced Towards the Preparation of the Materials

The entire process to successfully prepare the ternary ZnAlO and the quaternary ZnMgAlO thin films by using ultrasonic spray pyrolysis (USP) was quite challenging. For ZnAlO the most challenging task was to optimize the precursor solution. The precursor raw material Al(acac)3 was insoluble in water, so we had to use ethanol to dissolve it first. Then it was mixed with water-based solution for having the ultimate solution for the spray. For this reason, high Al molar compositions were not used. The design of experiments was done carefully to set up the preparations. The set spray temperature was varied to check for the completeness of the pyrolysis reaction to have uniform films with high transparency. The preparation was repeated few times with the same parameters to check for the consistency of the process.

The task to prepare the quaternary ZnMgAlO was far more challenging as the preparation for this material was being done for the first time using USP. Also, as mentioned earlier, handling the Al precursor to first dissolve it and then mix it with water-based solution was complex. The entire process of the preparation of the precursor solutions was done with extreme care and precision.

Controlling the stoichiometry in the precursor solution with specific molar compositions of both Al and Mg had to be done very precisely. As this material was completely new for the preparation process, we had a certain degree of freedom to have the design of experiments. Lots of parameters were varied by keeping the others fixed to have in-depth idea about the effect of the spray parameters on the quality of the prepared films. In this case too, keeping the same parameters the preparations were repeated a few times to check the consistency. Our idea was to have high transparency in the films along with enough conductivity and good crystalline quality, so both the Al and Mg molar compositions were varied in conjunction with the other spray parameters to have these qualities in the prepared quaternary films.

Numerical Simulation using Solis

To initiate a study related to the perspectives of the use of the materials developed in this work (ZnO, ZnMgO, ZnAlO, Cu2O and lastly the quaternary ZnMgAlO), the Solis 1D semiconductor device simulator [35] was used. Solis solves the drift-diffusion transport equations considering a large variety of physical models, recombination processes (Shockley-Read-Hall nonradiative, radiative and Auger), position-dependent physical parameters (e.g., for graded devices), nonideal ohmic and Schottky contacts, spontaneous and piezoelectric polarizations (in wurtzite materials such as ZnO

Chapter Conclusions

In this chapter we design the experiments for the preparation of both ternary (ZnAlO) and quaternary (ZnMgAlO) materials. ZnAlO was primarily prepared to be used as TCO by enhancing the conductivity with Al incorporation. With low Al molar compositions (1 and 2 mol%) the conductivities were found to be better than only ZnO, however the conductivities decreased beyond 2 mol%. Also, due to extremely poor solubility limit of the Al precursor, the transparency of the films decreases when prepared with high Al molar composition. Even there is a decline in the crystal qualities and conductivity for the films prepared using Al molar compositions beyond 2 %. These results were very useful when Al and Mg were co-incorporated to prepare the ZnMgAlO. When the Mg molar compositions were varied the Al molar compositions were kept only at 1 mol% and 2 mol%. By this we have seen that the transparencies were much better (above 90 %) with higher bandgap energies.

Even the crystallinity of the films was better with less disorder than with only Al. With the different Al and Mg compositions, the wurtzite structure of the crystals was retained. These results demonstrate the quality of the control on the spray preparation parameters. The electrical properties were better with Al and Mg co-incorporation especially for the cases when the Mg

CHAPTER 7: Conclusions and Future Work

Ultrasonic spray pyrolysis (USP) preparation technique has been used in this thesis work to prepare all the thin films based on earth-abundant and easy-procurable metal oxides. The purpose of the preparation of metal oxides thin films was to make them suitable for use in "all-oxide" solar cells.

The goal of the thesis work was to prepare the metal oxides with optimum quality using only the technique of USP and incorporating non-toxic raw materials. The research followed a unique process where all the materials necessary for use in "all-oxide" solar cells are developed using a single equipment. The materials are prepared in such a process such that they can be easily upgraded to industrial scale for large area depositions. In general, the preparation process using USP is flexible, quick, and easily adaptable which makes it a very promising technique for preparation of metal oxide thin films as it can lower the fabrication costs to a great extent.

At the beginning of the thesis, we discussed about the basics of photovoltaics and oxide-based semiconductors. The advantages and disadvantages of the different categories of solar cells are discussed. In this context, the necessity for developing metal oxide based solar cells were discussed.

The typical heterojunctions based on ZnO-Cu2O along with their functionalities were discussed. The various methods to develop such heterojunctions along with the typical power conversion efficiencies achieved by these devices were enlisted. Numerical simulations were performed using Silvaco Atlas® software to understand in-depth about the type and quality of materials needed to fabricate an "all-oxide" solar cell device. By default, Cu2O was chosen as the absorber layer. ZnAlO was used as the window/top contact layer. A ternary material, zinc germanium oxide (ZnGeO) was chosen as the buffer layer. The implications of the content of germanium in the ternary on the performance of solar cells are discussed and analyzed in detail. With a certain percentage of germanium doping the "all-oxide" solar cells can reach conversion efficiencies up to 9.57 %. This also must be in conjunction with certain thickness and quality of absorber and the window layers.

The impact of the interface defect layers is also studied to reach high conversion efficiencies. This study helped us to understand the role of each layer along with fair amount of idea about the qualities of each layer necessary to develop "all-oxide" solar cells.

Prior to the preparations, the idea was to understand in detail the ultrasonic spray process with the involved spray parameters and their possible effect on the prepared films. The spray process and the parameters are discussed in detail in the chapter 2 of the thesis. The ultrasonic spray pyrolysis energies of the films increase monotonously with the increase in magnesium content in the films.

All the films with the different compositions of magnesium (0 mol% to 70 mol%) showed high transparency. The measurement of the electrical properties was limited to films with magnesium 7 mol%, beyond which the films were semi-insulating. With the high transparency of the films, with good crystalline properties the films with specific magnesium compositions could be used as buffer layers for "all-oxide" solar cells.

Cuprous oxide (Cu2O) was next prepared as the possibility to be used as absorber layers in "all-oxide" solar cells. The temperature plays a huge role in the preparations to maintain the Cu2O phase in the films. We have seen from the structural analysis of the films that beyond 240 °C preparation temperature (390 °C set temperature) the films have only the CuO phase. So, control on the temperature in conjunction with the other spray preparation are important. Also, another important aspect is the use of the reducing agent with certain concentrations. Whatever the preparation temperature is, without using the reducing agent which is D-sorbitol in our case we will not be able to control the formation of the "cupric" phase. A typical concentration of D-sorbitol (0.05 mol/L) which is easily dissolved in the precursor solution can be used at the low preparation temperatures to prepare Cu2O. The optical analysis revealed films with only the cuprous phase (Cu2O) with high absorption in the visible spectrum region. The current-voltage measurements showed an ohmic nature of the Cu2O films with gold contacts on top. These characteristics makes the Cu2O films suitable candidate for absorber layers in "all-oxide" solar cells.

To broaden the horizon of the preparation process using USP, two more materials, one ternary and the other a quaternary were prepared. These two materials were prepared keeping in mind their applicability for implementation in "all-oxide" solar cells. The ternary zinc aluminum oxide (ZnAlO)

was prepared to be used a window cum top contact layer with good conductivities. The quaternary zinc aluminum magnesium oxide (ZnAlMgO) was prepared to be used window or top contact or as buffer layers for "all-oxide" solar cells. For preparing ZnAlO, low molar compositions of Al (0 % to 5 %) were used in the precursor solution due to extremely low solubility limit of the Al precursor.

XRD analysis showed the wurtzite nature for all the films with the different Al compositions. On the contrary, the optical properties especially the transparency of the films degraded with the increase of the Al content in the films. Even the films exhibited higher resistivity for the higher Al content films. From the perspective of ZnAlO being used as the top contact layers, it is essential to maintain very low molar concentrations of Al precursor (up to 2 mol%) in the precursor solution for successful high-quality film preparations. The quaternary ZnAlMgO was prepared with magnesium molar composition varying from (0 % to 7 %) but the Al molar concentration were always fixed to 1 and 2 mol%. The texture quality of the films was better with the higher Mg contents with no impact on the lattice disorders as revealed from the structural (XRD measurements) and optical analysis (Urbach energy estimations). The films were highly transparent (more than 90 %) with higher bandgap energies compared to only ZnO. Compared to films with only Mg, the films prepared with co-doped Mg and Al, showed improved electrical properties with higher Mg compositions. So, with the high optical and structural properties the ZnAlMgO films can be used as either top contact, window, or buffer layers for "all-oxide' solar cells. Simulation performed using Solis device simulator showed a conversion efficiency of about 8 % using the "all-oxide" solar cell structure:

ZnAlO/ZnO/ZnAlMgO/Cu2O/ITO substrate. The results are comparable with the results obtained by using Silvaco Atlas® with ZnGeO as the buffer layer.

The preparation of all the above-mentioned films opens the pathway for the actual fabrication of an "all-oxide" solar cell device. But the fabrication won't be an easy task to achieve. We have prepared all the films on soda lime glass substrates. For the device fabrication, the films need to be prepared on different substrates and on top of other prepared films. So, the spray parameters that we have used for a particular film preparation may be not as effective in these cases. Suitable adjustment and modifications in the spray parameters and even in the spray process should be essential for preparing the layers. The spray preparation temperature would be particularly crucial in this aspect.

We have seen that different set preparation temperatures are necessary for preparation of the different films with optimum quality. So, for e.g., if ZnO requiring a high set preparation temperature is prepared on top of Cu2O that demands a low preparation temperature, very easily an interface defect layer (CuO in all possibilities) can be formed. So, the control on the spray parameters to have a control on the interfaces will be important. Using different sample holders with different levels of heat conductivities can be tried.

The spray process along the parameters needs to be adjusted keeping the desired structure of the "all-oxide' solar cells. The structure can be either a substrate based (light from the front/top) or superstrate based (light from the back/rear). So, the sequence of the preparation of the films would be different and hence the preparation process needs to be suitably adjusted.

Another idea is to try to prepare other materials as buffer layers. The buffer layers are essential to have a better heterojunction quality. Gallium oxide (Ga2O3) is one such material that can be prepared to be used as buffer layers in "all-oxide" solar cells. Even we tried to prepare Ga2O3, but we needed

  optimisée pour obtenir uniquement la phase Cu2O. En effet, le Cu2O s'oxyde rapidement en CuO, diminuant ainsi les performances des cellules solaires. Le rôle d'un nouvel agent réducteur, le Dsorbitol est analysé en détail. Une étude structurale et optique détaillée a été réalisée pour évaluer la transition de phase dans des films minces d'oxyde de cuivre préparés par pyrolyse ultrasonique avec du D-sorbitol, avec un contrôle précis et poussée de la température d'élaboration. L'étude a permis de déterminer la plage de température dans laquelle seule la phase Cu2O peut être formée, mais avec moins de défauts qu'avec l'agent réducteur classique, le glucose. En effet, l'étude en fonction de la concentration de D-sorbitol démontre que les caractéristiques optimales ont été atteintes sans l'utilisation excessive de D-sorbitol par rapport à l'acétate de cuivre, comme cela est courant lors de l'utilisation du glucose comme agent réducteur. L'utilisation d'une concentration plus faible d'agent réducteur entraîne moins de produits secondaires de pyrolyse, une diminution des défauts et une amélioration des caractéristiques électriques et optiques pour les applications optoélectroniques. En résumé, l'élaboration par pyrolyse ultrasonique de couches minces d'oxyde de cuivre a été développée et optimisée à l'aide d'un nouvel agent réducteur, le D-sorbitol. Une étude optique et structurale complète a été réalisée. L'optimisation de la température d'élaboration à l'aide d'agent réducteur à faible concentration a été menée pour obtenir des films avec uniquement la phase Cu2O ayant de bonnes propriétés optiques et structurales. Le chapitre 6 aborde les possibilités d'ajuster les propriétés optoélectroniques des alliages d'oxydes métalliques pour ouvrir la voie à la fabrication de l'ensemble de la cellule solaire « tout-oxyde ». Dans la première étape, l'alliage ZnAlO est élaboré pour une utilisation en tant que couche de contact supérieure (TCO pour « Transparent Conducting Oxide ») avec des paramètres de croissance optimisés. Dans la deuxième étape, et pour la première fois en utilisant le spray pyrolyse ultrasonique, un quaternaire d'oxyde d'aluminium, de magnésium et de zinc (Zn1-x-yMgxAlyO) a été élaboré. Le Zn1-x-yMgxAlyO est polyvalent et peut être utilisé comme contact supérieur, couchefenêtre ou encore comme couche-tampon. Enfin, un dispositif « tout oxyde » a été modélisé à l'aide du logiciel de simulation Solis pour montrer le potentiel de ces matériaux pour la réalisation du composant photovoltaïque. Comme indiqué plus haut, le ZnAlO a été élaboré principalement pour être utilisé comme TCO en améliorant la conductivité avec l'incorporation d'Al. Avec des compositions molaires faibles en Al (1 et 2 %), la conductivité s'est révélée meilleure que celle du ZnO, mais cette conductivité a diminué au-delà de 2 % d'Al. De plus, en raison de la limite de solubilité extrêmement faible du précurseur d'aluminium, la transparence des films diminue lorsqu'ils sont élaborés avec une composition d'Al élevée. Ces résultats ont été très utiles dans le développement du quaternaire ZnMgAlO, avec la co-incorporation d'Al et de Mg. Ainsi la transparence des films de ZnMgAlO était optimale (supérieure à 90 %) avec une largeur de la bande interdite plus élevée. De façon corrélée, la qualité cristalline des films était meilleure qu'avec l'incorporation d'Al seul. Avec les différentes compositions Al et Mg, la structure wurtzite des cristaux a été conservée. Les propriétés électriques sont améliorées avec la co-incorporation d'Al et de Mg, en particulier pour les cas où les compositions de Mg sont élevées, notamment avec une mobilité plus élevée. La transparence élevée des films dans la région visible du spectre ainsi que la modulation de la largeur de la bande interdite et de la conductivité sont importantes pour des applications en tant que TCO, couche-fenêtres ou couche-tampon dans des cellules solaires « tout oxyde ». Le quaternaire ZnMgAlO peut aussi être utilisé dans les photodétecteurs ultraviolets et autres composants optoélectroniques.

  Prof. David HORWAT and Prof. Jean-François PIERSON for being actively involved in the research work. Their discussions, knowledge and support has contributed immensely towards the quality improvement of the thesis. I would like to sincerely thank Dr. Thomas KAUFFMANN for always his help and discussions on Raman spectroscopy, X-ray fluorescence and FTIR. I would also sincerely like to thank Prof. Patrice BOURSON, Prof. Germano MONTEMEZZANI, Prof. Thierry AUBERT, Dr. David CHAPRON, Dr. Virginie CODA-BOUCHOT, Prof. Alain MAILLARD, Prof. Michel AILLERIE, Dr. Zhixue ZHENG and Dr. Laurent WEISS for always their support and friendly discussions.

Mme. Nawal AQAIDI

  for her help towards administrative matters. I am thankful to Mme. Christine SARTORI, Prof. Dominique DALOZ and Mr. Stéphane LEBRETON for their help in matters related to ADUM, the doctoral studies and university registration procedures. I am also exceptionally thankful to my friends from LMOPS and from Université de Lorraine: Ahmad Sauffi BIN YUSOF (also from Universiti Sains Malaysia), Rokaya EL OTMANI (University of Casablanca -Morocco), Dr. Christy FADEL, Sadaqat ALI, Muhammad Waqar IQBAL, Dr. Yaya DOUMBIA, Rodolph ISKANDAR, Christelle HABIS, Carmen MOYA LOPEZ PELAEZ, Lucas OLIVERIO, Jordan MAUFAY, Nawal FERDI, Xin WANG, Dr. Rim ALRIFAI, Dr. Bachirou BOGNO, Dr. Luis Antonio García GUTIÉRREZ, David Javier Rincón ADARME, Dr. Dieudonné KIDMO KAOGA, Surya Nilamegam KUMARAN, Mohammad Iktiham Bin TAHER, Mohammed El ISMAILI, Dr. Ali ZAITER and Dr. Montassar BILLEH BOUZOURÂA for their tremendous support, understanding and encouragement throughout the thesis period.

Figure 1 . 1 : 2 Figure 1 . 2 : 3 Figure 1 . 3 : 5 Figure 1 . 4 : 6 Figure 1 . 5 : 7 Figure 1 . 6 : 7 Figure 1 . 7 : 8 Figure 1 . 8 : 8 Figure 1 . 9 : 9 Figure 1 . 10 : 10 Figure 1 . 11 : 11 Figure 1 . 12 : 12 Figure 1 . 13 : 13 Figure 1 . 14 : 15 Figure 1 . 15 : 18 Figure 1 . 16 : 23 Figure 1 . 17 : 24 Figure 1 . 18 : 25 Figure 1 . 19 : 25 Figure 1 . 20 : 26 Figure 1 . 21 : 26 Figure 2 . 1 : 46 Figure 2 . 2 : 47 Figure 2 . 3 : 49 Figure 2 . 4 : 51 Figure 2 . 5 : 51 Figure 2 . 6 : 52 Figure 2 . 7 : 53 Figure 2 . 8 : 54 Figure 2 . 9 : 54 Figure 2 . 10 : 55 Figure 2 . 11 : 60 Figure 2 . 12 : 62 Figure 2 . 13 : 64 Figure 2 . 14 : 64 Figure 2 . 15 : 65 Figure 2 . 16 : 66 Figure 2 . 17 : 67 Figure 2 . 18 : 68 Figure 2 . 19 : 69 Figure 2 . 20 : 71 Figure 2 . 21 : 72 Figure 2 . 22 : 72 Figure 2 . 23 : 73 Figure 2 . 24 : 74 Figure 3 . 1 : 82 Figure 3 . 2 : 90 Figure 3 . 3 : 91 Figure 3 . 4 : 92 Figure 3 . 5 :

 112123135146157167178188199110101111111212113131141511518116231172411825119251202612126214622472349245125512652275328542954210552116021262213642146421565216662176721868219692207122172222722237322474318232903391349235 Figure 1.1: The Einstein's article on the photovoltaic effect published in "Annalen der Physik" in 1905 ............................................................................................................................................... 2 Figure 1.2: Gerald Pearson, Daryl Chapin and Calvin Fuller (from left to right), the inventors of the first silicon solar cell in 1954 ................................................................................... 3 Figure 1.3: Schematic of a typical solar cell structure .................................................................... 5 Figure 1.4: Energy band diagram of the three types of material considering the electric conductivity ...................................................................................................................................................... 6 Figure 1.5: Schematic energy-band diagram showing the positioning of Fermi energy levels for n type, p type and intrinsic semiconductors .................................................................... 7 Figure 1.6: Schematic diagram of p-n junction ................................................................................. 7 Figure 1.7: Diffusion potential VD of a p-n junction ......................................................................... 8 Figure 1.8: Energy diagram of a p-n junction under illumination showing the electronhole photogeneration, the deep trap assisted recombination, the drift and diffusion of photocarriers ................................................................................................................................................... 8 Figure 1.9: Solar spectrum outside (AM0) and inside the atmosphere (AM1.5G) .............. 9 Figure 1.10: The bandgap wavelengths for some standard semiconductors with respect to the standard AM1.5G solar spectrum ............................................................................................. 10 Figure 1.11: Three types of silicon wafers for solar cells ........................................................... 11 Figure 1.12: Some illustrations of thin film solar cells technologies...................................... 12 Figure 1.13: Some illustrations of thin film solar cells technologies...................................... 13 Figure 1.14: Schematic representation of "all-oxide" solar cell .............................................. 15 Figure 1.15: Power conversion efficiency by year of publication of oxide solar cells using Cu2O as absorber .......................................................................................................................................... 18 Figure 1.16: Photovoltaic characteristics simulated for ZnAlO/Zn1-xGexO/Cu2O solar cell with respect to the germanium composition ...................................................................................... 23 Figure 1.17: Variation of the solar cell performances with the absorber thickness ........ 24 Figure 1.18: Variation of the solar cell performances with the doping concentration ... 25 Figure 1.19: Variation of the solar cell performances with the defect concentration ..... 25 Figure 1.20: Variation of the solar cell performances with the carrier mobility ............... 26 Figure 1.21: Variation of the solar cell performances with the germanium composition in Zn1-xGexO, the doping concentration in the absorber and the density of defects in the interfacial layer .............................................................................................................................................. 26Figure 2.1: Ultrasonic nozzle cross-section showing the generation of spray ................... 46 Figure 2.2: Schematic representation of the complete spray pyrolysis process ............... 47Figure 2.3: A view of the transport of the aerosols during the spray operation................ 49Figure 2.4: Schematic representation of the impact of the droplets on heated surface of

Figure 4 . 1 : 116 Figure 4 . 2 : 117 Figure 4 . 3 : 119 Figure 4 . 4 : 119 Figure 4 . 5 : 122 Figure 4 . 6 : 130 Figure 4 . 7 : 131 Figure 4 . 8 : 133 Figure 4 . 9 : 134 Figure 4 . 10 :

 411164211743119441194512246130471314813349134410 Figure 4.1: Rocksalt crystal structure of MgO .............................................................................. 116Figure 4.2: The bandgap energy of Zn1-xMgxO as a function of Mg concentration ......... 117Figure 4.3: Band alignment at the CdS/ZnO and CdS/Zn0.85Mg0.15O interfaces .............. 119Figure 4.4: Work function versus Mg content for ZnMgO films coated on Indium Tin Oxide .............................................................................................................................................................. 119Figure 4.5: Possible pn-heterojunction conduction band alignments ................................ 122 Figure 4.6: Relation between the magnesium composition in Zn1-xMgxO solid films and the magnesium composition in the precursor solution. ............................................................ 130Figure 4.7: XRD diffraction patterns for Zn1-xMgxO solid films with respect to the magnesium molar composition ........................................................................................................... 131 Figure 4.8: Raman spectra for Zn1-xMgxO thin films with different Mg composition .... 133 Figure 4.9: AFM images of Zn1-xMgxO thin films with Mg molar compositions ............... 134Figure 4.10: SEM micrographs of Zn1-xMgxO thin films with Mg molar compositions . 135

Figure 4 . 11 : 137 Figure 4 . 12 : 138 Figure 4 . 13 : 138 Figure 4 . 14 : 139 Figure 4 . 15 :

 411137412138413138414139415 Figure 4.11: Transmission spectra of Zn1-xMgxO thin films as a function of magnesium molar composition .................................................................................................................................... 137Figure 4.12: Estimation of the bandgap energies of Zn1-xMgxO thin films as a function of magnesium molar composition using the Tauc plot ................................................................... 138Figure 4.13: Variation of the bandgap energies of Zn1-xMgxO thin films as a function of magnesium molar composition ........................................................................................................... 138Figure 4.14: The Urbach energies of Zn1-xMgxO thin films as a function of magnesium molar composition .................................................................................................................................... 139Figure 4.15: Variation of Urbach energies of Zn1-xMgxO thin films as a function of magnesium molar composition ........................................................................................................... 140

Figure 5 . 6 : 164 Figure 5 . 7 : 165 Figure 5 . 8 : 165 Figure 5 . 9 : 166 Figure 5 . 10 : 166 Figure 5 . 11 : 167 Figure 5 . 12 : 168 Figure 5 . 13 : 169 Figure 5 . 14 : 169 Figure 5 . 15 : 170 Figure 5 . 16 : 171 Figure 5 . 17 : 171 Figure 5 . 18 : 172 Figure 5 . 19 : 172 Figure 5 . 20 : 173 Figure 5 . 21 : 173 Figure 5 . 22 : 174 Figure 5 . 23 : 175 Figure 5 . 24 : 175 Figure 5 . 25 : 176 Figure 6 . 1 :Figure 6 . 2 :Figure 6 . 3 :Figure 6 . 4 :Figure 6 . 5 :Figure 6 . 6 :Figure 6 . 7 :Figure 6 . 8 :Figure 6 . 9 :Figure 6 . 10 :Figure 6 . 11 :Figure 6 . 12 :Figure 6 . 13 :Figure 6 . 14 :Figure 6 . 15 :

 56164571655816559166510166511167512168513169514169515170516171517171518172519172520173521173522174523175524175525176616263646566676869610611612613614615 Figure 5.6: Raman line-scan spectra of copper oxide thin films with respect to different preparation temperatures ..................................................................................................................... 164 Figure 5.7: Transmission spectra for copper oxide thin films with respect to the preparation temperature ....................................................................................................................... 165 Figure 5.8: Tauc plots of the transmission spectra with respect to temperatures. ....... 165 Figure 5.9: Tauc plot for the CuO phase .......................................................................................... 166 Figure 5.10: Absorption coefficient and the Urbach energies for Cu2O and CuO thin films .......................................................................................................................................................................... 166 Figure 5.11: Raman spectra of copper oxide thin films with respect to the concentration of D-sorbitol reducing agent ................................................................................................................. 167 Figure 5.12: XRD diffractograms of copper oxide thin films with respect to the D-sorbitol concentration ............................................................................................................................................... 168 Figure 5.13: Transmission spectra for copper oxide thin films with respect to the Dsorbitol concentration ............................................................................................................................. 169 Figure 5.14: Tauc plots with respect to the sorbitol concentration for the Cu2O thin films with respect to the D-sorbitol concentration ................................................................................. 169 Figure 5.15: Bandgap and Urbach energy variation for Cu2O thin films with respect to the D-sorbitol concentration ........................................................................................................................ 170 Figure 5.16: Gold contact on different copper oxide samples ............................................... 171 Figure 5.17: The current-voltage characteristic between different contacts of gold on copper oxide thin film prepared at 230 °C (sample a) ............................................................... 171 Figure 5.18: The current-voltage characteristic between different contacts of gold on copper oxide thin film prepared at 230 °C (sample b) ............................................................... 172

Figure 6 . 23 :Figure 6 . 24 :Figure 6 . 25 :Figure 6 . 26 :Figure 6 . 27 :Figure 6 . 28 :Figure 6 . 29 :

 623624625626627628629 Figure 6.23: Lattice parameter estimations for Zn1-y-xMgxAlyO thin films with respect to the Mg molar compositions ................................................................................................................... 213 Figure 6.24: Raman spectra for Zn1-y-xMgxAlyO thin films with respect to Mg molar composition and fixed Al molar compositions .............................................................................. 214 Figure 6.25: Variation of resistivity for Zn1-y-xMgxAlyO thin films with respect to Mg molar composition ................................................................................................................................................. 215 Figure 6.26: Variation of carrier concentration for Zn1-y-xMgxAlyO thin films with respect to Mg molar composition ....................................................................................................................... 215 Figure 6.27: Variation of mobility for Zn1-y-xMgxAlyO thin films with respect to Mg molar composition ................................................................................................................................................. 216 Figure 6.28: The simulated "all-oxide" solar cell structure .................................................... 218 Figure 6.29: Simulated current-voltage characteristic under AM1.5 illumination of the ZnO/ZnMgAlO/Cu2O solar cell ............................................................................................................. 219

1. 3 . 2 . 4 21 1. 4 28 1. 5 30 1. 6

 324214285306 Heterojunctions Based on ZnO/Cu2O Structure ...................................................................................... 20 1.3.2.5 Numerical Investigations of "All-Oxide" Solar Cell Performances .................................................. Objectives and Challenges of the Present work ...................................................... Thesis Organization .......................................................................................................... References ............................................................................................................................ 32

March 1905 ,

 1905 photovoltaic technology and the physics of the solar cells are discussed respectively.

Figure 1 . 1 :

 11 Figure 1.1: The Einstein's article on the photovoltaic effect published in "Annalen der Physik" in 1905. Right: Einstein in the same year, 1905 [12].

Figure 1 . 2 :

 12 Figure 1.2: Gerald Pearson, Daryl Chapin and Calvin Fuller (from left to right), the inventors of the first silicon solar cell in 1954 [13].

A

  solar cell is usually based on a p-n semiconductor junction device converting the incident sunlight into electric current. A typical p-n junction solar cell structure is shown in Figure 1.3. To understand in depth about semiconductor properties literatures [46-53] can be looked upon.

Figure 1 . 3 :

 13 Figure 1.3: Schematic of a typical solar cell structure.

Figure 1 . 4 :

 14 Figure 1.4: Energy band diagram of the three types of material considering the electric conductivity.In a semiconductor, electron can absorb a photon and be promoted to from the valence band to the conduction band, leaving a hole in the valence band.

Figure 1 . 5 :

 15 Figure 1.5: Schematic energy-band diagram showing the positioning of Fermi energy levels for n type, p type and intrinsic semiconductors.

Figure 1 . 6 .

 16 Figure 1.6. The corresponding potential q×VD that builds up in the junction can be written as: Equation (1.2):

Figure 1 . 6 :

 16 Figure 1.6: Schematic diagram of p-n junction.

Figure 1 . 7 :

 17 Figure 1.7: Diffusion potential VD of a p-n junction [52].

Figure 1 . 8 :

 18 Figure 1.8: Energy diagram of a p-n junction under illumination showing the electron-hole photogeneration, the deep trap assisted recombination, the drift and diffusion of photocarriers [57].

Figure 1 . 11 :

 111 Figure 1.11: Three types of silicon wafers for solar cells: (a) 156-mm monocrystalline solar wafer and cell; (b) 156-mm multi-crystalline solar wafer and cell; and (c) 280-Wsolar cell module (from multi-crystalline wafers) [63].

Figure 1 . 13 :

 113 Figure 1.13: Some illustrations of thin film solar cells technologies: (a) CZTS solar cell [77,78], (b) GaAs solar cell [79,80].

Figure 1 . 14 ,

 114 consists of a TCO coated glass substrate, a ptype absorber layer, a buffer layer, a window layer, and a top contact that can be a TCO or an interdigitated metal contact. In the next sections the different solar cell technologies based on oxide semiconductors are presented.

Figure 1 . 14 :

 114 Figure 1.14: Schematic representation of "all-oxide" solar cell.

Figure 1 . 15 :

 115 Figure 1.15: Power conversion efficiency by year of publication of oxide solar cells using Cu2O as absorber. The corresponding references are shown with markers indicating the preparation technique conditions (vacuum or non-vacuum based).

Figure 1 . 16 )

 116 was achieved for a Ge composition of x = 0.62 with PCE value at 6.68 %. Including interface layer with high density of defects makes the model more accurate and the simulated characteristics more coherent with the experimental results. The simulated PCE value reaches the experimental value of 6 % when x is between 0.5 and 0.65. The simulation results start to diverge when x is increased beyond 0.65, which might be due to the fact that the electrical properties of the buffer layer start to degrade with Ge increase (strong resistivity increase) and these are not included in the simulation model. The solar cell model thus developed with the simulations matches closely with the experimental values which can found in Table

Figure 1 . 16 :

 116 Figure 1.16: Photovoltaic characteristics simulated for ZnAlO/Zn1-xGexO/Cu2O solar cell with respect to the germanium composition. Also given the conduction band offset χZnGeO -χCu2O [182]. The line curves show the simulation results with (blue) and without (red) the defect. The experimental data taken from [154] are also show (black dots).

1 cm 2 /

 2 Vs and 200 cm 2 /Vs while keeping the electron to hole mobility ratio to 2. The change of mobility does not show a pronounced effect on the solar cell performances. Even for very low mobility value of 1 cm 2 /Vs the PCE is 2.4 %. The mobility variation effects are shown in the Figure 1.20. From the analysis with all these parameters it is proven that the high defect density of Cu2O (both bulk and interfaces) combined with low thickness are the most detrimental factors towards low performance of the solar cells. Very importantly Plankensteiner et al. shows in their research [187] that impurities are incorporated in the absorbers due to incomplete pyrolysis of agents present in the precursor solution.

Figure 1 . 18 :

 118 Figure 1.18: Variation of the solar cell performances with the doping concentration [182].

Figure 1 . 19 :

 119 Figure 1.19: Variation of the solar cell performances with the defect concentration [182].

Figure 1 . 20 :

 120 Figure 1.20: Variation of the solar cell performances with the carrier mobility [182].

Figure 1 . 21 :

 121 Figure 1.21: Variation of the solar cell performances with the germanium composition in Zn1-xGexO, the doping concentration in the absorber and the density of defects in the interfacial layer [182].

Figure 1

 1 Figure 1.21.

Chapter 3 Chapter 4

 34 is about the development and optimization of ZnO used as the window layer. The material properties of ZnO are discussed briefly. Preparation of ZnO with optimal quality is discussed in detail. The optimization of the spray parameters and their subsequent effect on the film quality are discussed. The morphological, optical, and structural characterizations are discussed with reference to the spray parameters. The electrical characterizations are done to analyze the conductive properties of the films. is about the development and optimization of the buffer layer. The choice of Zn1-xMgxO for the buffer layer is explained with brief literature review. The optimization of spray parameters along with the effect of addition of Mg on the film properties.

Figure 2 . 1 .

 21 The entire process of spray pyrolysis is schematically shown in the Figure 2.2.

Figure 2 . 1 :

 21 Figure 2.1: Ultrasonic nozzle cross-section showing the generation of spray [22].

Figure 2 . 2 :

 22 Figure 2.2: Schematic representation of the complete spray pyrolysis process. Concept adapted and modified from [19].

Figure 2

 2

Figure 2 . 3 :

 23 Figure 2.3: A view of the transport of the aerosols during the spray operation [19].

From the Figure 2 . 2 ,

 22 we see that Step (iii) commence even before the Step (ii) is over. Step (ii) is generally complete before reaching the heated substrate. The Step (iii) begins when the solvent is evaporated. In this step it is expected that due to the action of the heat the droplets will undergo transformation. For each individual droplet, the starting point of Step (iii) will be dependent upon few factors: size of the droplet, the position (proximity) of the droplet with the substrate and the substrate temperature. To precisely define the Step (iii), we can think of four different types in which the atomized droplets might interact with the heated substrate.

Figure 2

 2 Figure 2.4.

Figure 2 . 4 :

 24 Figure 2.4: Schematic representation of the impact of the droplets on heated surface of substrate. Designed and modified from reports of[19,20,24].

[ 25 ]

 25 . The four processes have been named by them as A, B, C and D, but with our modifications we named the four processes as 1, 2, 3 and 4.

Figure 2 . 5 :

 25 Figure 2.5: The modifications in the droplet size during the transport is shown with the increase of temperature and time. Designed and modified from the reports of [19,20,24-29].

Figure 2 . 6 :

 26 Figure 2.6: Simplified schematic representation of the ultrasonic spray system used in this thesis.

Figure 2 . 7 :

 27 Figure 2.7: ExactaCoat ultrasonic spray system from Sono-Tek (available at LMOPS laboratory) used in this thesis.

Figure 2 . 8 :

 28 Figure 2.8: Nozzle droplet formation [31].

  purpose. Typical structures of Accumist and Impact nozzles along with their spray patterns are shown in the Figure 2.10.

Figure 2 . 10 :

 210 Figure 2.10: Ultrasonic spray pyrolysis nozzle with its spray shape. Accumist: Left (top & bottom); Impact: Right (top & bottom) [22,32-34].

Figure 2 . 11 :

 211 Figure 2.11: Schematic representation of the spray preparation parameters and their relationship with the composition and the thickness of the prepared films. Designed based on the reports of [19,22,24-27].

Figure 2 . 12 :

 212 Figure 2.12: Dektak XT mechanical profilometer from Bruker available at LMOPS laboratory.

Figure 2 .

 2 13. UV-VIS-NIR Perkin-Elmer Lambda 900 spectrophotometer used for our measurements is shown in the Figure 2.14 along with the inset showing the placement positions of the glass reference and the actual sample.

Figure 2 . 13 :

 213 Figure 2.13: Principle of UV-VIS-NIR spectroscopy is shown schematically.

Figure 2 . 14 :

 214 Figure 2.14: UV-VIS-NIR Perkin-Elmer Lambda 900 spectrophotometer available at LMOPS laboratory.

Figure 2 . 15 :

 215 Figure 2.15: Transmittance spectra of a ZnO thin film with glass substrate as reference.

Figures 2 .

 2 16 (a & b) respectively.It should be noted that the Urbach energy is a qualitative indicator and does not allow to quantify the disorder in the material, and, moreover, it is not sensitive to any type of disorder. It should therefore be used as one indicator among others.

Figure 2 .

 2 Figure 2.16: (a) Bandgap calculation of a ZnO thin film using Tauc plot method; (b) Urbach energy calculation of a ZnO thin film.

Figure 2 .

 2 Figure 2.12).

Figure 2 . 18 :

 218 Figure 2.18: Modified Scherrer equation plot by Monshi et al. [63].

  integrated peak intensity relatively to the reference one and N is the total number of peaks.

  measurements. An example of the used shadow mask and the contact evaporation setup are shown in Figure 2.21.

Figure 2 . 20 :

 220 Figure 2.20: Rapid thermal annealing (RTA) system available at LMOPS laboratory.

Figure 2 .

 2 Figure 2.21: (a) Example of an aluminum mask used for contact deposition; (b) Thermal vacuum evaporation system available at LMOPS Laboratory.

  contacts A, B, C and D are uniformly aligned in the middle of the sample. The current is injected between the external contacts A and D, and the voltage is measured between internal contacts B and C. The semiconductor thin film resistivity ρ is determined as follows: is the semiconductor resistance and t the film thickness. The actual probe station used for the measurements in shown in the Figure 2.22.

Figure 2 . 22 :

 222 Figure 2.22: Probe station used for 4PP measurements available at LMOPS Laboratory, Carel platform.

Figure 2 .

 2 23 requires four small ohmic contacts placed at the corners of a square on the top of the sample. The extraction of the sheet resistance RS is based on the measurement of two different characteristic resistances: R1 measured by applying the current between two contacts (C and D) and measuring the resulting voltage between the other two contacts (A and B); and R2 measured by applying the current between D and A and measuring the voltage between B and C.

Figure 2 . 23 :

 223 Figure 2.23: Representation of the van der Pauw measurements.

Figure 2 . 24 :

 224 Figure 2.24: Schematic of Hall-effect measurements.

CHAPTER 3 : 3 . 2 3 . 3

 33233 Preparation of Zinc Oxide: Optimization for Use as a Window Layer 3.1 Introduction to Zinc Oxide (ZnO) ................................................................................. Properties of ZnO ............................................................................................................... 3.2.1 Optical Properties ..................................................................................................................................... 3.2.2 Chemical Properties................................................................................................................................. 3.2.3 Electrical Properties ................................................................................................................................ Applications of ZnO in Solar Cells................................................................................. 3.4 Preparation of ZnO by Ultrasonic Spray Pyrolysis ................................................. 3.4.1 Chemicals Used and Chemical Reactions Involved for ZnO Preparation ........................... 3.4.2 Design of Experiment ..............................................................................................................................3.5 Results and Discussions on thePreparation of ZnO .............................................. 3.5.1 Growth of the ZnO Thin Films ............................................................................................................. 3.5.2 Optical Properties of the ZnO Thin Films........................................................................................ 3.5.3 Morphological Properties of the ZnO Thin Films ........................................................................ 3.5.4 Chemical Properties of the ZnO Thin Films ................................................................................. 3.5.5 Structural and Electrical Properties of the ZnO Thin Films .................................................. 3.6 Challenges Faced Towards the Preparation of ZnO ............................................ 3.7 Chapter Conclusions ...................................................................................................... 3.8 References .........................................................................................................................

Figure 3 . 1 :

 31 Figure 3.1: ZnO Crystal structures (a) cubic rocksalt, (b) cubic zinc blende and (c) Hexagonal wurtzite. Zn atoms are shown as small black spheres and the O atoms are shown as large white spheres [3].

Figure 3 . 2 :

 32 Figure 3.2: ZnO thickness variation with the number of spray cycles.

Section 2 . 7 . 2 of Chapter 2 .

 2722 In the first step, the set substrate temperature was varied from 350 °C to 450 °C (in three different experimental steps of 50 °C each). The spray parameters which are kept fixed for these preparations are: number of spray cycles: 20, solution flow rate: 0.5 mL/min; precursor solution concentration and pH: 0.1 mol/L and 4 respectively; dwell time: 1 sec for each spray cycle. The resulting transmission spectra's is shown in the Figure 3.3.

Figure 3 . 3 :

 33 Figure 3.3: Transmission spectra for ZnO films prepared at different temperatures.

Figure 3 . 4 :

 34 Figure 3.4: Transmission spectra for ZnO films prepared at different flow rates.

Figure 3 . 5 :

 35 Figure 3.5: Transmission spectra for ZnO films prepared at different solution concentrations.

Figure 3 . 6 : 7 . 2 of Chapter 2 . 3 ,

 367223 Figure 3.6: Transmission spectra for ZnO films prepared with different pH of the precursor solutions.

Figure 3 . 7 :

 37 Figure 3.7: Transmission spectra for ZnO films prepared with different spray cycles (a) and Tauc plots for 20 and 50 cycles (b).

Figure 3 . 8 :Figure 3 . 3 )

 3833 Figure 3.8: AFM images of ZnO films prepared with pH 4 at substrate temperatures (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) with pH 5 at substrate temperature 450 °C.

Figure 3 .

 3 9 (a-d). Similar to the AFM micrograph, we can see the polycrystalline nature of the ZnO film with a smooth and uniform surface (Figure3.9c) with no porosity for the preparation done at 450°C. Preparations done at lower temperature (Figure3.9 a, b) shows surface roughness with the presence of agglomerates proving the fact of incomplete pyrolysis at lower substrate temperatures. A rough surface morphology is also evidenced for the film prepared with pH of 5 (Figure 3.9d).

Figure 3 . 9 :

 39 Figure 3.9: SEM micrographs for ZnO films prepared with pH 4 at substrate temperatures (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) with pH 5 at substrate temperature 450 °C.

Figure 3 .

 3 10 (a & b). The presence of O-H bands is more prominent for ZnO sprayed at lower substrate temperatures and with a precursor solution of pH 5, as expected. This behaviour is strongly connected with the production of agglomerates observed with AFM and SEM studies and the resulting decrease of the transmission and it could be the result of hydroxide formation coupled with an incomplete spray pyrolytic reaction. It is consistent with the findings of Edinger et al.[54] and Li et al.[55], which imply that acetic acid in the precursor solution aids in the reduction of Zn(OH)2 precipitation. The reduction mechanism of Zn(OH)2 precipitation by acetic acid is shown in the Section 3.4.1.

Figure 3 . 10 :

 310 Figure 3.10: FTIR spectra for ZnO prepared with pH 4 (a) at different preparation temperatures and (b) with precursor solution pH 4 and 5 at substrate temperature 450 °C.

Figure 3 . 11 :

 311 Figure 3.11: X-Ray diffractograms for ZnO thin films with different precursor solution concentrations but with fixed pH 4. The preparations done at substrate temperature 450 •C with 20 spray cycles at a spray flow rate of 0.5 mL/min.

Figure 3 . 12 :

 312 Figure 3.12: X-Ray diffractograms for ZnO thin films prepared at different substrate temperatures. The preparations done with a precursor solution concentration of 0.1 mol/L with pH 4. Number of spray cycles were always 20 with a spray flow rate of 0.5 mL/min.

  spectroscopy measurements were performed on our prepared ZnO thin films. The Raman spectra of the ZnO films prepared with different substrate temperatures of 350 °C, 400 °C and 450 °C are shown in the Figure 3.13. All the spectra are shown after a Lorentzian fit of the obtained peaks.

Figure 3 . 13 :

 313 Figure 3.13: ZnO Lorentzian fitted Raman spectra.

annealing system (shown in Section 2 . 7 . 4 , chapter 2 )

 2742 to obtain semiconducting films since all deposited films were highly resistive before annealing. The resistivity variation with respect to the annealing temperature is shown in the Figure 3.15 while the voltage-current characteristics in the four-point probe configuration are shown in the Figure 3.14. The optimum annealing temperature obtained is 300 °C. This temperature makes it possible both to obtain a low resistivity and to remain below the preparation temperature.After the post-annealing at 300 °C for 30 minutes, samples deposited at 450 °C with an optimum solution concentration of 0.05 mol/L, spray flow rate of 0.5 mL/min, number of cycles of 50, and pH of 4 were evaluated. The average resistivity was found to be 1.5 Ω×cm. The relatively low resistivity due to the background n-type doping in ZnO is usually attributed to interstitial Zn (Zni)

Figure 3 . 14 :

 314 Figure 3.14: Voltage-current characteristics of ZnO thin films in the four-point probe (4PP) configuration for the measurement of resistivity. The 4PP technique is presented in section 2.7.4.1 of chapter 2. IAD (in µA) is the current passed through the outer two probes (A & D) and the VBC (in V) is the voltage between the inner two probes (B & C).

Figure 3 . 15 :

 315 Figure 3.15: Resistivity of ZnO thin films with respect to the annealing temperature.

Figure 4 . 1 .

 41 Figure 4.1. The solubility of Mg in the ZnO wurtzite structure, on the other hand, has been acontentious issue. The solid solution limit of MgO in wurtzite ZnO is less than 4 % at 1800 °C, according to the MgO-ZnO binary system phase diagram described by Segnit and Holland[5], and this limit is further decreased at lower temperatures. Bergstein et al.[6] later used ultra-fine MgO source powder to produce a small portion of the (Zn,Mg)O phase diagram to clarify the solubility limit of MgO . The solubility limits for samples sintered at 1500 °C, 1400 °C, and 1200 °C are approximately 18, 17 and 15 atomic %.

Figure 4 . 1 :

 41 Figure 4.1: Rocksalt crystal structure of MgO [7].

[ 19 ]

 19 . Miyazaki et al.[18] used an aqueous solution consisting of zinc acetate (Zn(CH3COO)2), magnesium acetate (Mg(CH3COO)2) and ammonia (NH3), with a pH of 8-9 to deposit thin films. At this lower tempearuture, they found no evidence of Mg in the ZnO films. Gayen et al.[19] were able to produce Zn1-xMgxO nanofibers with 0 ≤ x ≤ 0.2. They used an aqueous solution of zinc nitrate (Zn(NO3)2), magnesium nitrate (Mg(NO3)2) and sodium hydroxide (NaOH), with a pH > 13.

Figure 4 . 2 :

 42 Figure 4.2: The bandgap energy of Zn1-xMgxO as a function of Mg concentration is summarized. Ohtomo et al. [8] (circle) provided Mg-poor wurtzite structure data; Choopun et al. [20] provided Mg-rich rocksalt structure data (triangle and square).

  Mg concentration causes bandgap widening in ZnMgO via lowering the Zn 4s states. The energy difference between the conduction band edge (ECB) and the valence band edge (EVB) of ZnMgO widens as the band gap increases with increasing Mg concentration. The displacement of the band edge relative to the vacuum energy level has been investigated through experiments. To study the relative band alignment between ZnO and ZnMgO, Rao et al. [26] used photoelectron spectroscopy to investigate the electronic characteristics of CdS/ZnO and CdS/Zn0.85Mg0.15O interfaces. The valence band edge EVB and vacuum level EVAC of ZnO and Zn0.85Mg0.15O remained unchanged when compared to CdS (shown in Figure 4.3). The variation in band gap between ZnO and Zn0.85Mg0.15O could thus be ascribed entirely to a shift in the conduction band edge ECB. Olson et al. [27] measured the work function of ZnMgO films (shown in Figure 4.4) and came to the same conclusion. With increasing Mg concentration from 0 to 25 atomic % of Mg, the absolute value of the work function of ZnMgO decreases. They assumed the Fermi energy of the layers they found was close to ECB since they were n-type semiconductors. However, due to the influence of other parameters such as carrier concentration and surface states, it should be emphasized that the ECB could not be retrieved clearly from work function measurement. Nonetheless, the lower work function supports the general pattern of the conduction band edge moving upward to vacuum as Mg concentration increases.

Figure 4 . 3 :

 43 Figure 4.3: Band alignment at the CdS/ZnO and CdS/Zn0.85Mg0.15O interfaces as determined by Rao et al. [26]. All measurements are in eV.

Figure 4 . 4 :

 44 Figure 4.4: Work function versus Mg content for ZnMgO films coated on Indium Tin Oxide (ITO) as measured by Olson et al. [27].

Figure 4 . 5 .

 45

Figure 4 . 5 :

 45 Figure 4.5: Possible pn-heterojunction conduction band alignments [42].

  )2 and Mg(Ac)2 along with the quantities of Zn(Ac)2 and Mg(Ac)2 powder needed is discussed below.Based on the preparation experience of ZnO, as detailed in the Section 3.4.1 of Chapter 3, we select for Zn(Ac)2 always a concentration of 0.05 mol/L. The chemical protocol for preparation of ZnMgO is based on 3 types of solutions: Solution 1 (S1), Solution 2 (S2) and solution 3 (S3). The final alloy of ZnMgO film will be of the stoichiometry Zn1-xMgxO, with x being the Mg content in mol%.The solution S1 of 100 mL is only a solution of Zn(Ac)2 which is dissolved only in deionized water. . The quantity of Zn(Ac)2 powder needed is given by the equation below:Equation (4.3): 𝑚 𝑍𝑛(𝐴𝑐) 2 = 𝐶 × 𝑉 × 𝑀Where, m is the mass (in g) of Zn(Ac)2 powder; C is the Zn(Ac)2 solution concentration in mol/L; V is the solution volume in litres (L) and M is the molar mass of Zn(Ac)2 (in g/mol). So, 𝑚 𝑍𝑛(𝐴𝑐) 2 = 0.05 (mol/L) × 0.1 (L) × 219.49 (g/mol) = 1.09 g ≈ 1.1 g.The solution S2 of 100 mL is a solution of both Zn(Ac)2 and Mg(Ac)2. Both the precursor salts are dissolved only in deionized water. The mass of Zn(Ac)2 remains the same as in S1, 1.1 g. Initially, we choose x = 0.3, corresponding to Mg doping of 30 mol%. This value is taken as a base or reference for preparing solutions with other Mg doping mol%. The value corresponding to 30 mol% is selected after an extensive literature survey about the best values Mg concentrations required for ultrasonic spray pyrolysis process for preparation of ZnMgO films and keeping in mind the low solubility limit of Mg(Ac)2 in only water-based solutions. The concentration for 30 % Mg doping with a corresponding constant concentration of Zn(Ac)2 is given by: Equation (4.4):

Finally, the 100

 100 mL solution S2 consists of 1.1 g of Zn(Ac)2 and 0.46 g of Mg(Ac)2 dissolved completely in deionized water. Now let us assume that CMg(Ac)2 = 0.0214 mol/L is the initial concentration. Using the Equation 4.5, we can calculate the different values of concentration corresponding to different Mg content %. The values are shown in the

Figure 4 . 6

 46 and the values are shown in the Table 4.7.

Figure 4 . 6 :

 46 Figure 4.6: Relation between the magnesium composition in Zn1-xMgxO solid films and the magnesium composition in the precursor solution. The dotted line depicts the extrapolated linear fit from the first region (with single-phase Zn1-xMgxO) to illustrate the phase separation impact for compositions below 30 mol%.

  findings from the XRF measurements, X-ray diffraction (XRD) measurements were done on the Zn1-xMgxO solid films. The diffractograms are shown in the Figure 4.7.

Figure 4 . 7 :

 47 Figure 4.7: XRD diffraction patterns for Zn1-xMgxO solid films with respect to the magnesium molar composition.

7 . 3 . 1 & 3 . 5 . 5 of

 731355 ) planes, allocated using the standard JCPDS card (No: 78-0430), showing the development of several phases (hexagonal and cubic). Due to the increased disorder and phase segregation in the Zn1-xMgxO thin films, the crystallite size for the wurtzite phase decreases approximately linearly from 28 nm down to 12 nm when the magnesium composition increases from 0 mol % up to 70 mol %, as determined using the procedure presented in the Sections 2.Chapters 2 & 3 respectively.

µm.

  The Raman modes of ZnO and Zn1-xMgxO anticipated for the wurtzite structure (C6v symmetry) are classified using the irreducible representation A1+2B1+E1+2E2, with four Raman active modes: one A1, one doubly degenerate E1, and two doubly degenerate E2. Both the B1 branches are IR and Raman inactive. The E2 High and E2 Low mode peaks were systematically observed for all samples with magnesium content less than 30 mol%, and no other mode related to a different phase was detected.This demonstrates the single-phase wurtzite structure of the prepared Zn1-xMgxO films as already evidenced from the XRD and XRF measurements. The Raman spectrum corresponding to all the Zn1-xMgxO thin films prepared on soda lime glass (SLG) substrates are shown in the Figure 4.8.

Figure 4 . 8 :

 48 Figure 4.8: Raman spectra for Zn1-xMgxO thin films with different Mg composition. The Raman spectra of the soda-lime glass substrate is also presented as a reference for appropriately assigning the deposited Zn1-xMgxO Raman peaks.

Figure 4

 4

Figure 4 . 9 :

 49 Figure 4.9: AFM images of Zn1-xMgxO thin films with Mg molar compositions. 5 µm × 5 µm is the scan area. To better display the morphology and avoid saturating the images, the vertical scale has been changed differently for the images.

Figure 4 . 10 :

 410 Figure 4.10: SEM micrographs of Zn1-xMgxO thin films with Mg molar compositions of (a) 0 mol%; (b) 3 mol%; (c) 5 mol%; (d) 10 mol%; (e) 15 mol%; (f) 25 mol%; (g) 30 mol%; (h) 50 mol% and (i) 70 mol%.

Figure 4 . 11 :Figure 4 . 13 .

 411413 Figure 4.11: Transmission spectra of Zn1-xMgxO thin films as a function of magnesium molar composition. The spectrum of the SLG substrate is also shown, with nearly constant transmission at 100 %. A zoom in the absorption edge spectral region is shown in the inset plot.

Figure 4 . 12 :

 412 Figure 4.12: Estimation of the bandgap energies of Zn1-xMgxO thin films as a function of magnesium molar composition using the Tauc plot.

Figure 4 . 13 :

 413 Figure 4.13: Variation of the bandgap energies of Zn1-xMgxO thin films as a function of magnesium molar composition. The bandgap energies were extracted using the Tauc model.

Figure 4 . 11 .

 411 The Urbach energy calculations are shown in the Figure 4.14 and the variations of the Urbach energies with respect to the magnesium molar compositions are shown in the Figure 4.15.

Figure 4 . 14 :

 414 Figure 4.14: The Urbach energies of Zn1-xMgxO thin films as a function of magnesium molar composition.

Figure 4 . 15 :

 415 Figure 4.15: Variation of Urbach energies of Zn1-xMgxO thin films as a function of magnesium molar composition.

  carefully observed, the gain in Urbach energy with Mg concentration in the films previously showed appears to have no effect on the diffusion and mobility of free carriers. The variations in the electrical properties of the Zn1-xMgxO thin films with the increase of the magnesium molar composition are shown in the Figures 4.16, 4.17 & 4.18 respectively.

Figure 4 . 16 :

 416 Figure 4.16: Variation of resistivity of Zn1-xMgxO thin films as a function of magnesium molar composition.

Figure 4 . 17 :

 417 Figure 4.17: Variation of carrier concentration of Zn1-xMgxO thin films as a function of magnesium molar composition.

Figure 4 . 18 :

 418 Figure 4.18: Variation of carrier mobility of Zn1-xMgxO thin films as a function of magnesium molar composition.

Figure 4

 4 Figure 4.19.

Figure 4 . 19 :

 419 Figure 4.19: Chemical element abundance in the Earth's upper crust as a function of atomic number is shown. The elements that are circled with red are relevant materials for thin film technologies. Modified from [67].

of 30 %

 30 ; (ii) transparency in the visible region above 90 %; (iii) modulated bandgap from 3.26 eV to 3.65 eV; (iv) dramatic decrease in the carrier concentration with Mg due to the bandgap modulation and the inhibition of Zn interstitials related shallow donors.

Figure 5 . 1

 51 Figure 5.1 also shows that each oxygen atom is 4-fold coordinated with oxygen atoms, and each copper atom is linearly coordinated with the two oxygen atoms [1]. The typical bond lengths are: Cu-Cu: 3.019 Å ; Cu-O: 1.848 Å; O-O: 3.68 Å [2].

Figure 5 . 1 :

 51 Figure 5.1: Cu2O crystal structure (a) and a two-dimensional projection of 64 Cu2O unit cells (b). The oxygen outward diffusion pathway is shown schematically with yellow arrows. Cu atoms are represented as blue balls, while oxygen atoms are represented by red balls [1].

Figure 5

 5 Figure 5.2.

Figure 5 . 2 :

 52 Figure 5.2: Schematic view of the ultrasonic spray pyrolysis system used to prepare Cu2O. The thermocouple A is used to measure the hot plate set temperature, whereas the thermocouple B is used to monitor the preparation temperature directly on the substrate.

Figure 5 . 3

 53 shows the temperature profile for a given temperature of 390 °C, for example. The scanning of the nozzle across the surface begins at time t = 0 s as shown in the figure. The temperature at the surface is measured at an average of 321 °C, which is 69 °C lower than the set temperature, before the preparation begins. The comparatively low heat conductivity of the glass-ceramic holder and the soda lime glass substrate itself (≈ 1 W/m/K) causes the offset between the substrate temperature and the set temperature before the start of the spray.

Figure 5 . 3 :

 53 Figure 5.3: Change in substrate temperature during the preparation of Cu2O at 390 °C set temperature. The zero time corresponds to the beginning of spray, which takes 720 seconds to complete. The inset plot depicts two spray cycles, with one half of each cycle having a higher temperature than the other due to the thermocouple's off-center position in the nozzle path.

[

  68]. X-ray diffractograms of four copper oxide samples are shown in the Figure 5.4. The preparation temperatures were the ones mentioned in the Table 5.2. The concentration of reducing agent D-sorbitol in the precursor solution is kept constant at 0.05 mol/L. At 36.69°, 42.53° and 61.85°, respectively, the major cubic Cu2O peaks ((111), (200), and (220) are identified. The measured (111), (111), and (112) peaks at 35.66°, 38.79° and 44.19°, respectively, are the signatures of the monoclinic CuO phase at the highest temperatures.

Figure 5 . 4 : 5 . 5 .

 5455 Figure 5.4: XRD diffractograms of copper oxide thin films with different preparation temperatures. The red lines represent the diffractogram fitting using a pseudo-Voigt peak function.

Figure 5 . 5 :

 55 Figure 5.5: The crystallite size and CuO/Cu2O phase integrated peak intensity ratio with respect to the preparation temperature.

Figure 5 . 6 (

 56 a, b, c & d) show Raman line-scans of samples prepared at temperatures of 230 °C, 240 °C, 260 °C, and 273 °C, respectively, with a lateral resolution of 1 µm.

Figure 5 . 7 .

 57 Figure 5.7. The absorption edge of Cu2O is apparent at temperatures of 230 °C and 240 °C, while another absorption edge (due to formation of CuO) appeared at temperatures of 260 °C and 273 °C, keeping consistency with the previously reported XRD and Raman analysis. The use of the Tauc model yields the bandgap values illustrated in Figure 5.8, which range between 2.38 eV and 2.34 eV with increase in the preparation temperature.

Figure 5 . 6 :

 56 Figure 5.6: Raman line-scan spectra of copper oxide thin films with respect to different preparation temperatures: (a): 230 °C ; (b): 240 °C ; (c): 260 °C ; (d): 273 °C. All measurements were systematically replicated in different spots on the sample surface, with a lateral resolution of 1 µm (for a total scan width of 7 µm).

Figure 5 . 7 :

 57 Figure 5.7: Transmission spectra for copper oxide thin films with respect to the preparation temperature.

Figure 5 . 8 :

 58 Figure 5.8: Tauc plots of the transmission spectra with respect to temperatures.

Figure 5 . 10 shows

 510 the extracted Urbach energy, which increases noticeably as the preparation temperature rises, which is compatible with Raman and X-ray diffraction studies demonstrating an increase in the proportion of CuO phase.

Figure 5 . 9 :

 59 Figure 5.9: Tauc plot for the CuO phase for preparation temperatures of 260 °C and 273 °C, are plotted. The indirect bandgap model was utilized to compute the bandgap energy for the CuO phase, while the direct model was employed for the Cu2O phase.

Figure 5 . 10 :

 510 Figure 5.10: Absorption coefficient and the Urbach energies for Cu2O and CuO thin films.

Figure 5 .

 5 11 for the same copper oxide films prepared with different D-sorbitol concentrations. For the sample prepared without D-sorbitol, the peaks of the monoclinic CuO phase are identified. With the addition of D-sorbitol, only the Cu2O cubic phase is detected, as already evidenced in the Raman spectra.

Figure 5 . 11 :

 511 Figure 5.11: Raman spectra of copper oxide thin films with respect to the concentration of D-sorbitol reducing agent. The preparation temperature is set at 240 °C (corresponding to a set temperature of 390 °C). M is the net concentration which can also be represented as mol/L. The peak fitting using the Lorentzian function is represented by black lines.

Figure 5 . 12 :

 512 Figure 5.12: XRD diffractograms of copper oxide thin films with respect to the D-sorbitol concentration. Black lines represent the diffractogram fitting using a pseudo-Voigt peak function.

Figure 5 . 13 :

 513 Figure 5.13: Transmission spectra for copper oxide thin films with respect to the D-sorbitol concentration.

Figure 5 . 14 :

 514 Figure 5.14: Tauc plots with respect to the sorbitol concentration for the Cu2O thin films with respect to the D-sorbitol concentration.

Figure 5 . 15 :

 515 Figure 5.15: Bandgap and Urbach energy variation for Cu2O thin films with respect to the D-sorbitol concentration.

2. 8 5 . 16 .

 8516 and a D-sorbitol concentration of 0.05 mol/L in the precursor solution. The chosen samples with the gold contacts are shown in the Figure The samples (a) and (b) are prepared at 230 °C, while (c) prepared at 240 °C. The corresponding I-V curves are shown in the Figures 5.17

Figure 5 . 16 :

 516 Figure 5.16: Gold contact on different copper oxide samples. Samples (a) and (b) are prepared at 230 °C, while (c) prepared at 240 °C.

, 5 . 18 & 5 . 19 .

 518519 Figures 5.17, 5.18 & 5.19. This shows that gold is well suited to make ohmic contacts on Cu2Oand that the interface does not impact negatively on the electrical properties of the Au/Cu2O contact.

Figure 5 . 17 :

 517 Figure 5.17: The current-voltage characteristic between different contacts of gold on copper oxide: Sample (a) prepared at 230 °C. The current maximum scale is at 1.00 × 10 -5 A.

Figure 5 . 18 :

 518 Figure 5.18: The current-voltage characteristic between different contacts of gold on copper oxide: Sample (b) prepared at 230 °C. The current maximum scale is at 8 × 10 -6 A.

Figure 5 . 19 :

 519 Figure 5.19: The current-voltage characteristic between different contacts of gold on copper oxide: Sample (c) prepared at 240 °C. The current maximum scale is at 5 × 10 -6 A.

Figures 5 .

 5 Figures 5.20 & 5.21 respectively.

Figure 5 . 20 :

 520 Figure 5.20: The current-voltage characteristic before annealing between different contacts of gold on copper oxide for sample (a) prepared at 230 °C. The current maximum scale is at 4 × 10 -6 A.

Figure 5 . 21 :

 521 Figure 5.21: The current-voltage characteristic after annealing between different contacts of gold on copper oxide for sample (c) prepared at 240 °C. The current maximum scale is at 4 × 10 -6 A.

Figure 5 . 22 :

 522 Figure 5.22: The electrical parameters of copper oxide using gold contacts measured using the van der Pauw / Hall Effect method: Sample (c) prepared at 240 °C.

  in sputtering. Second, the sample is substantially uniform: we do not observe any difference in the morphology with respect to the position. The top surface image is shown in the Figure 5.23. A crosssection of the film is also analyzed in the Figure 5.24 with a much larger view of the film surface.

Figure 5 . 23 :

 523 Figure 5.23: SEM surface image of Cu2O thin film prepared at 230 °C.

Figure 5 . 24 :

 524 Figure 5.24: A lateral cum cross-sectional view taken using SEM for the same Cu2O thin film prepared at 230 °C. The Cu2O film thickness (marked by the white arrow) is about 300 nm.

Figure 5 . 25 :

 525 Figure 5.25: SEM surface image of Cu2O thin film prepared at 240 °C.

Section 2 .4 and 2 . 5 of

 225 Chapter 2 and in the Section 3.4 of Chapter 4. Zinc acetate dihydrate [Zn(Ac)2•2H2O] and Aluminum acetylacetonate (C15H21AlO6) were used as the source precursor raw materials dissolved in deionized water and ethanol respectively. For simplicity henceforth, in this chapter we will always refer the precursor materials as Zn(Ac)2 and Al(acac)3 only. Always 50 mL of precursor solution was prepared before the start of the spray. Acetic acid was added to the precursor solution to reduce the formation and precipitation of hydroxides.

Figure 6 . 2 .Figure 6 . 3 .

 6263 It is seen from the Figures 6.1 & 6.2 that as the molar composition of aluminum increases the transparency decreases. This is mostly related to the poor solubility of the Al(acac)3 precursor. The estimation of the bandgap energies of the films are done using the Tauc model and shown in the Figure 6.4 shows the variation of the bandgap with respect to the Al molar composition.

Figure 6 . 1 :

 61 Figure 6.1: Transmission spectra of Zn1-yAlyO thin films as a function of Al molar composition. The spectrum of the SLG substrate is also displayed, with nearly constant transmission at 100 %. A zoom in the absorption edge spectral region is shown in the inset plot.

Figure 6 . 2 :

 62 Figure 6.2: Average transmission of Zn1-yAlyO thin films as a function of Al molar composition in the wavelength region 500 nm to 800 nm.

Figure 6 . 3 :

 63 Figure 6.3: Estimation of the bandgap energies of Zn1-yAlyO thin films as a function of Al molar composition using the Tauc model.

Figure 6 . 4 :

 64 Figure 6.4: Variation of the bandgap energies of Zn1-yAlyO thin films as a function of Al molar composition extracted using the Tauc model as shown in Figure 6.3.

Figure 6 . 5 .Figure 6

 656 Figure 6.6.

Figure 6 . 5 :

 65 Figure 6.5: Estimation of the Urbach energies of Zn1-yAlyO thin films as a function of Al molar composition.

Figure 6 . 6 :

 66 Figure 6.6: Variation of Urbach energies of Zn1-yAlyO thin films as a function of Al molar composition.

Figure 6 . 7 .

 67 Figure 6.7. The Zn1-yAlyO thin films exhibit a single hexagonal wurtzite crystal structure with peaks

Figure 6 . 7 : 2 . 7 . 3 . 1 & 3 . 5 . 5 of 5 of 3 )

 67273135553 Figure 6.7: XRD diffraction patterns for Zn1-yAlyO solid thin films with respect to the Al molar composition.

Figure 6 . 8 :

 68 Figure 6.8: Crystallite size estimation for Zn1-yAlyO solid thin films with respect to the Al molar composition.

Figure 6 . 9 :

 69 Figure 6.9: Variation of lattice parameter "a" for Zn1-yAlyO solid thin films with respect to the Al molar composition.

Figure 6 . 11 :

 611 Figure 6.11: Texture factor estimation of different peaks for Zn1-yAlyO solid thin films with respect to the Al molar composition using the method of Lotgering.

Figure 6 . 12 :

 612 Figure 6.12: Resistivity of Zn1-yAlyO solid thin films with respect to the Al molar composition.

  )2 and Al(acac)3 concentration while keeping the Zn(Ac)2 concentration fixed in the precursor solution on the ZnAlO film quality have been discussed in this chapter. The calculation procedure to select the concentrations of Zn(Ac)2, Mg(Ac)2 and Al(acac)3 along with the quantities of Zn(Ac)2, Mg(Ac)2 and Al(acac)3 powder needed is discussed below. Based on the preparation experience of ZnO, as detailed in the Section 3.4.1 of Chapter 3, we select for Zn(Ac)2 always a concentration of 0.05 mol/L. The chemical protocol for preparation of ZnMgAlO is based on 4 types of solutions: Solution 1 (S1), Solution 2 (S2), Solution 3 (S3) and Solution 4 (S4). The final alloy of ZnMgAlO film will be of the stoichiometry Zn1-y-xMgxAlyO. x and y are the Mg and Al contents in mol%. The solution S1 of 100 mL is only a solution of Zn(Ac)2 which is dissolved only in deionized water. The details of S1 can be found in the Section 6.2.1 above. The solution S2 of 100 mL is a solution of Zn(Ac)2 and Mg(Ac)2. The preparation of S2 is exactly same the preparation of S2 in Chapter 4 (described in detail in the Section 4.5.1).At first Zn(Ac)2 is dissolved only in 100 mL deionized water with same concentration as in S1. Then Mg(Ac)2 powder corresponding to Mg content of 30 mol% (x = 0.3) is added to this solution and dissolved. This makes the concentration of Mg(Ac)2 is S2 as 0.0214 mol/L. Then, solution S3 is prepared. The preparation of S3 is exactly same as the preparation of S2 as described above in details in the Section 6.2.1. Al(acac)3 is dissolved separately in 50 mL ethanol, as Al(acac)3 is insoluble is water. The concentration for 30 % Al content is given by the Equation 6.2 and Equation 6.3 gives the required quantity of the Al(acac)3 powder needed for the required concentration of 0.0214 mol/L. The 50 mL solution of Al(acac)3 in ethanol is added to the 50 mL solution of Zn(Ac)2. So, final volume of S3 is 100 mL. The solution S3 then is put to ultrasound bath for 15 mins for complete homogenization. Now, we need to prepare the solution S4. It should be noted that S4 will be the final solution that will be used for the actual preparation. For preparing the solution S4, we need to mix different proportions of S1, S2 and S3. Basically we, will collect a certain volume of S2 and S3 based on the desired Mg and Al composition values and we will dilute it by adding S1.

  At first the transmission of the films was assessed in relation to the magnesium incorporation effect by keeping the aluminum concentrations fixed. Using the same process, the bandgap and Urbach energies of the prepared films were studied. The transmission spectra of the Zn1-y-xMgxAlyO thin films with aluminum molar compositions 0 %, 1 % and 2 % and with varying magnesium molar compositions are shown in the Figure6.13 (a, b & c) respectively.

Figure 6 . 3 of Chapter 4 .

 634 Figure 6.14 (a, b & c). It is seen from the Figures 6.13 & 6.15 that without aluminum, when only magnesium composition is increased, the transparency is always more than 90 %. This is in accordance with the optical results of ZnMgO films prepared by Accumist nozzle in the Section 4.6.3 of Chapter 4. As aluminum is introduced the transparency falls due to the poor solubility of the aluminum precursor. With aluminum 1 mol%, the average transmission in the films is approx.

Figure 6 . 13 :

 613 Figure 6.13: Transmission spectra of Zn1-y-xMgxAlyO thin films as a function of Mg molar composition with fixed Al molar compositions of 0 % (a), 1 % (b) and 2 % (c). The spectrum of a SLG substrates is also shown, with nearly constant transmission at 100 %. A zoom in the absorption edge spectral region is shown in the inset plots of each figure.

Figure 6 . 14 :

 614 Figure 6.14: Average transmission of Zn1-y-xMgxAlyO thin films as a function of Mg molar composition with fixed Al molar compositions of 0 % (a), 1 % (b) and 2 % (c) in the wavelength region 500 nm to 800 nm.

Figure 6 . 15 :

 615 Figure 6.15: Estimation of the bandgap energies of Zn1-y-xMgxAlyO thin films as a function of Mg molar composition with fixed Al molar compositions of 0 % (a), 1 % (b) and 2 % (c).

Figure 6 . 17 :

 617 Figure 6.17: Estimation of the Urbach energies of Zn1-y-xMgxAlyO thin films as a function of Mg molar composition with fixed Al molar compositions of 0 % (a), 1 % (b) and 2 % (c).

7 . 2 . 1 of 2 ,

 7212 Chapter Urbach energy is only a qualitative indicator and does not quantify the disorder in the material, nor is it sensitive to any kind of disorder. It is one indicator among others and should be correlated to other characterization results.

Figure 6 . 18 :

 618 Figure 6.18: Variation of the Urbach energies of Zn1-y-xMgxAlyO thin films as a function of Mg molar composition with fixed Al molar compositions of 0 % (a), 1 % (b) and 2 % (c).

Figure 6 . 19 :

 619 Figure 6.19: SEM micrographs of Zn1-y-xMgxAlyO thin films with different Al and Mg molar compositions.

Figure 6 . 20 :

 620 Figure 6.20: XRD diffraction patterns for Zn1-y-xMgxAlyO solid thin films with respect to the Mg and Al molar compositions.

Figure 6 . 21 :

 621 Figure 6.21: Texture factor estimation of different peaks for Zn1-y-xMgxAlyO thin films with respect to the Mg molar compositions and fixed Al molar compositions using the method of Lotgering factor.

Figure 6 .

 6 22 (a, b) shows the estimated values of the crystallite sizes. The values estimated with the (002) peaks, corresponding the wurtzite phase always shows higher values which agree with our previously prepared ZnO and ZnMgO films. The procedure for the estimation of the crystallite sizes is explained in the Section 2.7.3.1 of Chapter 2.

Figure 6 . 22 :

 622 Figure 6.22: Crystallite size estimations of different peaks for Zn1-y-xMgxAlyO thin films with respect to the Mg molar compositions and fixed Al molar compositions of 1 % (a) and 2 % (b).

Figure 6 . 24 :Chapter 4 .

 6244 Figure 6.24: Raman spectra for Zn1-y-xMgxAlyO thin films with respect to Mg molar composition and fixed Al molar compositions. The Raman spectra of the soda-lime glass substrate is also presented as a reference for appropriately assigning the deposited Zn1-y-xMgxAlyO Raman peaks.

Figure 6 . 27 :

 627 Figure 6.27: Variation of mobility for Zn1-y-xMgxAlyO thin films with respect to Mg molar composition and fixed Al molar compositions of 0 %, 1 % and 2 %.

Figure 6 . 29 :

 629 Figure 6.29: Simulated current-voltage characteristic under AM1.5 illumination of the ZnO/ZnMgAlO/Cu2O solar cell structure shown in the Figure 6.29.

  Transmission spectra for ZnO films prepared at different solution concentrations............................................................................................................................................... 93 Transmission spectra for ZnO films prepared with different pH of the precursor solutions ..................................................................................................................................... 94 Ray diffractograms for ZnO thin films with different precursor solution concentrations............................................................................................................................................ 101 Figure 3.12: X-Ray diffractograms for ZnO thin films prepared at different substrate temperatures .............................................................................................................................................. 102 Figure 3.13: ZnO Lorentzian fitted Raman spectra .................................................................... 104

	Figure 3.6:

Figure 3.7: Transmission spectra for ZnO films prepared with different spray cycles ... 95 Figure 3.8: AFM images of ZnO films .................................................................................................. 98 Figure 3.9: SEM micrographs for ZnO ................................................................................................ 99 Figure 3.10: FTIR spectra for ZnO..................................................................................................... 100 Figure 3.11: X-Figure 3.14: Voltage-current characteristics of ZnO thin films in the four-point probe (4PP) configuration for the measurement of resistivity............................................................ 106 Figure 3.15: Resistivity of ZnO thin films with respect to the annealing temperature 106

  Variation of resistivity of Zn1-xMgxO thin films as a function of magnesium molar composition .................................................................................................................................... 141 Variation of carrier concentration of Zn1-xMgxO thin films as a function of magnesium molar composition ........................................................................................................... 141 Cu2O crystal structure ..................................................................................................... 152 Schematic view of the ultrasonic spray pyrolysis system used to prepare Cu2O .......................................................................................................................................................................... 154 Change in substrate temperature during the preparation of Cu2O at 390 °C set temperature ......................................................................................................................................... 157

	Figure 4.17: Figure 5.2: Figure 5.3:

energies of Zn1-xMgxO thin films as a function of magnesium molar composition ........................................................................................................... 140

Figure 4.16: Figure 4.18: Variation of carrier mobility of Zn1-xMgxO thin films as a function of magnesium molar composition ........................................................................................................... 142

Figure 4.19: Chemical element abundance in the Earth's upper crust as a function of atomic number ........................................................................................................................................... 144 Figure 5.1: Figure 5.4: XRD diffractograms of copper oxide thin films with different preparation temperatures .............................................................................................................................................. 161 Figure 5.5: The crystallite size and CuO/Cu2O phase integrated peak intensity ratio with respect to the preparation temperature .......................................................................................... 162

  of Mg molar composition ....................................................................................................................... 206 Variation of the bandgap energies of Zn1-y-xMgxAlyO thin films as a function of Mg molar composition ....................................................................................................................... 207 Estimation of the Urbach energies of Zn1-y-xMgxAlyO thin films as a function of Mg molar composition ....................................................................................................................... 208 Variation of the Urbach energies of Zn1-y-xMgxAlyO thin films as a function of Mg molar composition ............................................................................................................................. 209 Figure 6.19: SEM micrographs of of Zn1-y-xMgxAlyO thin films with different Al and Mg molar compositions ................................................................................................................................... 210 Figure 6.20: XRD diffraction patterns for Zn1-y-xMgxAlyO solid thin films with respect to the Mg and Al molar compositions ..................................................................................................... 211

	Figure 6.16: Figure 6.17: Figure 6.18:

Figure 6.21: Texture factor estimation of different peaks for Zn1-y-xMgxAlyO thin films with respect to the Mg molar compositions ................................................................................... 212 Figure 6.22: Crystallite size estimations of different peaks for Zn1-y-xMgxAlyO thin films with respect to the Mg molar compositions ................................................................................... 212

Table 1 . 1 :

 11 Cu2O solar cells state-of-the-art with increasing power conversion efficiency (PCE) values ................................................................................................................................................... 19

Table 3 . 1 :

 31 Chemicals with high purity used for spray pyrolysis deposition of ZnO ......... 87

Table 3 . 2 :

 32 Design of experiment to select the optimal spray parameters for ZnO preparation ..................................................................................................................................................... 89

Table 3 . 3 :

 33 Spray pyrolysis preparation parameters of ZnO with 20 spray cycles ............ 95

Table 3 . 4 :

 34 Spray pyrolysis preparation parameters of ZnO with varying spray cycles ..[START_REF] Vatansever | Alternative Resources for Renewable Energy: Piezoelectric and Photovoltaic Smart Structures[END_REF]

Table 3 . 5 :

 35 Structural parameters of ZnO extracted from XRD measurements in this work and a comparison with the state-of-the-art. ................................................................................... 103

Table 3 . 6 :

 36 Optimized spray preparation parameters along with their values ................ 108

Table 4 .

 4

1: Chemicals with high purity used for spray pyrolysis deposition of

ZnMgO 123

Table 4 . 2

 42

: Calculation for different concentrations of Mg(Ac)2 ............................................. 126

Table 4 . 3

 43

: Calculation for volumes to prepare the final precursor solution .................... 127 Table 4.4: Calculation for quantities of Zn(Ac)2 and Mg(Ac)2 powders needed to prepare precursor solutions .................................................................................................................................. 127 Table 4.5: Volume percentage of acetic acid in the precursor solutions ........................... 128 Table 4.6: Design of experiment to select the optimal spray parameters for ZnO preparation .................................................................................................................................................. 129 Table 4.7: Calibrated X-ray fluorescence system used to determine the magnesium molar composition ................................................................................................................................................. 131

Table 4.8: Transmission and thickness estimations of the Zn1-xMgxO solid films .......... 136 Table 5.1: Chemicals with high purity used for spray pyrolysis deposition of Cu2O .... 155

Table 5 .2: Correlation

 5

between the set temperature obtained with thermocouple A and the actual temperature measured directly on the substrate with thermocouple B .......

[START_REF] Tanaka | Electrical and optical properties of TCO-Cu2O heterojunction devices[END_REF]

Table 5 .3: Cu2O

 5

, CuO, and Cu4O3 Raman peaks ............................................................................ 163

Table 6 . 1 :

 61 Chemicals with high purity used for spray pyrolysis deposition of ZnAlO ..[START_REF] Santhosh Kumar | Improving the conductivity of cuprous oxide thin film by doping Calcium via feasible nebulizer spray technique for solar cell (FTO/ZnO/Ca-Cu2O)[END_REF]

Table 6 . 2

 62

: Calculation for different concentrations of Al(acac)3 ........................................... 187

Table 6 . 3

 63

: Calculation for volumes to prepare the final precursor solution .................... 187

Table 6 . 4 :

 64 Design of experiment to select the optimal spray parameters for ZnAlO .... 189

Table 6 . 5

 65

: Chemicals with high purity used for spray pyrolysis deposition of ZnMgAlO .......................................................................................................................................................................... 199

Table 6 . 6 :

 66 Calculation for volumes to prepare the final precursor solution for the preparation of ZnMgAlO ......................................................................................................................... 201

Table 6 .

 6

7: Design of experiment to select the optimal spray parameters for ZnMgAlO .......................................................................................................................................................................... 202 CHAPTER 1:

Earth-Abundant Oxide Semiconductors for Photovoltaics: Current State and Challenges 1.1 General Introduction to Photovoltaics

  .1.1 Photovoltaic Technology for Green Energy ...................................................................................... 3 1.1.2 Solar Cells ........................................................................................................................................................ 5 .2.1 First Generation Solar Cell: Crystalline Silicon ............................................................................. 11

......................................................................... 2 11.2 Overview of Different Technologies of Solar Cells ................................................. 10 11.2.2 Second Generation Solar Cell: Thin Films ....................................................................................... 12 1.2.3 Third Generation Solar Cell: Multijunction, Polymers, New Concepts ............................... 14 1.3 Next Generation Photovoltaics: Focus on "All-Oxide" Structures Based on Earth-abundant Semiconductors......................................................................................... 15 1.3.1 Spinel Oxide Solar Cells .......................................................................................................................... 16 1.3.2 "All-Oxide" Structures using Metal Oxides ..................................................................................... 16 1.3.2.1 ZnO and Related Materials ................................................................................................................................ 16 1.3.2.2 P-type Oxide Absorbers ...................................................................................................................................... 16 1.3.2.3 Cu2O as an Absorber in "All-Oxide" Solar Cells ........................................................................................ 17

Table 1 .1: Cu2O

 1 solar cells state-of-the-art with increasing power conversion efficiency (PCE) values.

	Solar Cell Structure		VOC (V)	JSC (mA/cm 2 )	FF (%)	PCE (%) Ref.
	Vacuum Based Methods
	ZnAlO/Cu2O			0.4	7.1	40	1.28	[158]
	ZnAlO /Cu2O			0.4	6.91	50	1.39	[159]
	ZnAlO:Ga/Cu2O			0.41	6.94	53	1.52	[159]
	ITO/ZnO/Cu2O			0.59	6.78	50	2	[151]
	ZnO/Cu2O	by CVD	0.55	11.42	49.8	3.17	[160]
	ZnAlO/ZnO/Cu2O		0.69	10.1	55	3.83	[161]
	ZnAlO/Ga2O3/Cu2O	by CVD	1.2	7.37	44.7	3.97	[157]
	ZnAlO/ZnO/Cu2O		0.72	9.67	58	4.12	[152]
	ZnAlO/Zn0.91Mg0.09O/Cu2O	0.85	8	60	4.31	[146]
	ZnAlO/Ga2O3/Cu2O		0.8	9.99	67	5.38	[156]
	ZnAlO/Al0.025Ga0.975O/Cu2O:Na	0.84	10.95	66	6.1	[145]

ZnAlO/(Al0.025Ga0.975)2O3/Cu2O:Na

		0.84	10.8	69	6.25	[153]
	ZnAlO/Zn0.38Ge0.62O/Cu2O:Na	1.1	11.1	62	8.1	[154]
	Non-Vacuum Based Methods

ZnO/Zn0.88Mg0.12O/Cu2O by ECD 0

				.34	3.76	52.7	0.67	[155]
	ZnO/Cu2O/Cu2O	by ECD	0.32	6.32	45	0.9	[162]
	ZnO/Cu2O	by ECD	0.59	7.1	42	1.28	[163]
	ZnO/Cu2O	by ECD	0.54	4.47	59	1.43	[164]
	ITO/ZnO/Cu2O			0.49	7.5	40	1.46	[147]
	ITO/Zn0.79Mg0.21O/Cu2O	0.65	6.9	49.2	2.2	[147]

  This parameter is important for preparation. Different substrate temperatures produce different film morphologies. Usually, when the temperature is below 200 °C, the deposited droplets are rich in solvent. A thin wet layer is thus present on top of the film during deposition. If the drying is too fast, then it adds to stresses in the film and causes cracking. At high temperatures of more than 350 °C, the droplets are devoid of the solvent and almost dry. These discrete particles are deposited on the substrate which might add to the roughness. It must be kept in mind that, for the pyrolysis reaction to be complete and to avoid formation any unnecessary Adjusting the shaping air pressure is important to have a smooth thin film. The range of air pressure in the system is 0.5 to 3.5 psi (35 to 245 mbar). Using too high pressure, results in a high velocity spray that results in overspray (incomplete pyrolysis) and loss of material. The spray shapes for the two nozzle types are shown in the Figure2.10. For Accumist nozzle, the uniformly distributed droplets first merge at a focal point and then diverges uniformly.

The parameters that we optimized for our preparations are as follows: Substrate temperature; shaping air pressure; nozzle-substrate height; solution flow rate; nozzle speed; dwell time; nozzle power; spray cycles; spray position; substrate choice; precursor solution concentration.

(i) Substrate Temperature:

byproducts certain temperature needs to be maintained throughout the preparation process. For different material preparations the temperature of the substrate needs to be adjusted.

(ii) Shaping air pressure:

Table 3 .1.

 3

Table 3 . 1 :

 31 Chemicals with high purity used for spray pyrolysis deposition of ZnO.

Table 3 .2.Table 3 . 2 :

 332 Design of experiment to select the optimal spray parameters for ZnO preparation.

	Preparation Parameter	Range
	Preparation temperature	[ 350 °C ; 450 °C ]
	Spray flow rate	[ 0.3 ; 0.7 ]
	Precursor solution concentration	[ 0.03 ; 0.10 ]
	pH of precursor solution	[ 4 ; 5 ]
	Number of spray cycles	[ 10 ; 60 ]

Table 3 . 3 :

 33 Spray pyrolysis preparation parameters of ZnO with 20 spray cycles. Measured optical parameters values are provided for all the films.

Table 3 .4.

 3 the state-of-the-art values for ZnO thin films prepared with different processes. The values of Urbach energies for the films with the best spray parameters lies between 63-78 meV (shown in

	Temperature (°C)	Precursor Solution Concentration (mol/L)	Precursor Solution pH	Solution Flow Rate (mL/min)	Average Transmission (%) in [450-800] nm range	Bandgap (eV) ± 0.01 eV	Urbach Energy (meV) ± 5 meV
	350	0.10	4	0.5	68.3	3.26	96
	400	0.10	4	0.5	79.9	3.28	84
	450	0.10	4	0.5	93.5	3.26	63
	450	0.10	4	0.3	95.4	3.28	69
	450	0.10	4	0.7	80.0	3.29	67
	450	0.03	4	0.5	91.5	3.28	77
	450	0.05	4	0.5	90.0	3.26	78
	450	0.10	5	0.5	80.3	3.26

It is observed that the transparency of the films remains in the range of 90-95 % in the wavelength regime of 400-850 nm. The thickness of the ZnO films varies from ≈ 200 nm (for 20 spray cycles) up to ≈ 640 nm (for 50 spray cycles). As seen from the Table

3

.4, our values can well be compared with

Table 3 . 4 :

 34 Spray pyrolysis preparation parameters of ZnO with varying spray cycles. Measured optical parameters values are provided for all the films along with comparison of the state-of-theart values.

	Preparation Technique on Different Substrates along with References	Deposition Temperature (°C)	Number of Spray Cycles	Film Thickness (nm) ± 10 nm	Average Transmission (%)	Bandgap (eV) ± 0.01 eV	Urbach Energy (meV) ± 5 meV
	Our work [42]	450	20	196	93.5	3.28	63
	Our work [42]	450	35	508	92.4	3.25	72
	Our work [42]	450	50	640	90.6	3.26	61
	USP: ZnO/Glass [60]	400	N.M.	N.M.	86	3.25	N.M.
	USP: ZnO/Glass [57]	400	N.M.	N.M.	84; 90	3.24	N.M.
	USP: ZnO/Glass [66]	365 ; 465	N.M.	N.M.	83; 91	3.35 ; 3.40	N.M.
	USP: ZnO/Glass [61]	350 ; 450	N.M.	N.M.	85	3.29 ; 3.31	N.M.
	EB:
	ZnO/Sapphire	600	N.A.	N.M.	N.M.	N.M.	74 ; 99
	[67]
	SC: ZnO/Glass [68]	450 (Anneal 1 hour)	N.A.	N.M.	84	3.24 ; 3.33	N.M.
	RFS: ZnO/Si [69]	RT	N.A.	N.M.	80 ; 92	3.24 ; 3.29	N.M.
	PLD: ZnO/Glass [70]	550	N.A.	N.M.	N.M.	3.34	N.M.
	PLD: ZnO/Glass [71]	300	N.A.	N.M.	70	3.83	120

USP: Ultrasonic spray pyrolysis; EB: e-beam; SC: Spin-coating; RFS: Radio-frequency sputtering; PLD: Pulsed laser deposition; RT: room temperature; N.M.: Not mentioned in literature; N.A.: Not applicable

Table 3 . 5 :

 35 Structural parameters of ZnO extracted from XRD measurements in this work and a comparison with the state-of-the-art.

	Preparation Technique on Different Substrates along with References	Deposition Temperature (°C)	Precursor Concentration (mol/L)	Texture Factor F002	Crystallite Size (nm) ± 5 nm	Lattice Parameters c;a (nm) ± 0.00010 nm
	Our work [42]	350	0.10	0.18	16	0.52434 ; 0.32532
	Our work [42]	400	0.10	0.45	15	0.52227 ; 0.32513
	Our work [42]	450	0.10	0.83	21	0.51955 ; 0.32379
	Our work [42]	450	0.05	2.48	39	0.51905 ; 0.32417
	Our work [42]	450	0.03	1.96	35	0.51896 ; 0.32418
	ZnO powder [81]	N.A.	N.A.	1.00	N.A.	0.52074 ; 0.32503
	USP: ZnO/Glass [60]	400	N.M.	N.M.	25	0.51980 ; 0.31830
	USP: ZnO/Glass [83]	300 (cycles)	0.03 ; 0.10	160 ; 2.60	12 ; 17	N.M.
	USP: ZnO/Glass [66]	365 ; 465	0.03 ; 0.20	N.M.	2 ; 4	N.M.

USP: Ultrasonic spray pyrolysis; SC: Spin-coating; RFS: Radio-frequency sputtering; PLD: Pulsed laser deposition; RT: room temperature; N.M.: Not mentioned in literature; N.A.: Not applicable.

  examined the thermal stability of supersaturated Zn1-xMgxO thin films and ZnMgO/ZnO bilayers. Laser molecular beam epitaxy (LMBE) was used to deposit the Zn1-xMgxO thin films on sapphire at 550 °C. The asdeposited films were annealed at temperatures from 700 °C to 1000 °C. MgO segregation occurred at 850 °C in films with Mg concentration of more than 15 atomic %. However, even at annealing temperatures as high as 1000 °C, films with a Mg concentration of 15 atomic % or less showed no MgO segregation. Furthermore, Mg thermal diffusion across the ZnMgO/ZnO interface was only

seen after annealing at temperatures exceeding 700 °C. As a result, Ohtomo et al.

[11]

determined that MgO has a thermodynamic solubility of around 15 atomic % in wurtzite ZnMgO epitaxial films, and ZnO-based alloy films and heterointerfaces are stable enough to fabricate highly crystalline heterostructures at high temperatures.

Table 4 . 1 :

 41 Chemicals with high purity used for spray pyrolysis deposition of ZnMgO.

	Name	Formula	Supplier
	Zinc acetate dihydrate	Zn(CH3COO)2 • 2H2O	Merck 108802
	Magnesium acetate tetrahydrate	Mg(CH3COO)2 • 4H2O	SigmaAldrich M5661
	Acetic Acid	CH3COOH	Alfa Aesar 10994-AE
	Deionized water	H2O	Avantor-VWR

Table 4 .2.Table 4 . 2 :

 442 Calculation for different concentrations of Mg(Ac)2.

Table 4 .3.

 4

	Mg content values (mol%)	CZn(Ac)2 (mol/L)	CMg(Ac)2 (mol/L)
	0	0.05	0.0000
	3	0.05	0.0015
	5	0.05	0.0026
	7	0.05	0.0037
	10	0.05	0.0056
	13	0.05	0.0074
	15	0.05	0.0088
	20	0.05	0.0125
	25	0.05	0.0167
	30	0.05	0.0214
	50	0.05	0.0500
	70	0.05	0.1167

Table 4 . 3 :

 43 Calculation for volumes of S1 and S2 to prepare the final precursor solution S3.

For Mg doping beyond 30 %, the dominant salt in the precursor solution is Mg(Ac)2. The concentration values of Mg(Ac)2 for 50 % and 70 % doping are shown in the Table 4.2. Final precursor solutions of volume 50 mL each prepared separately for the preparations of Mg 50 % and 70 % are named as S4 and S5 respectively. The quantities of Zn(Ac)2 and Mg(Ac)2 powders are calculated using the Equations (4.3 & 4.5) respectively and shown in the Table 4.4.

Table 4 . 4 :

 44 Calculation for quantities of Zn(Ac)2 and Mg(Ac)2 powders needed to prepare precursor solutions S4 and S5. Based on the experimental details mentioned in the Chapter 2 we have added acetic acid to each of the precursor solution and maintained a value of pH of 4. Having the same pH for all the solutions helps to analyze the films better. The other advantages of acetic acid are mentioned in the beginning of this section. The volume percentages of acetic acid in all the precursor solutions are mentioned in the

Table 4 .5.

 4

	Mg content values (mol%)	CMg(Ac)2 (mol/L)	Final stoichiometry of ZnMgO	Volume V2 of S2 (mL)	Volume V1 of S1 (mL)
	0	0.0000	ZnO	0	50
	3	0.0015	Zn0.97Mg0.03O	3.6	46.4
	5	0.0026	Zn0.95Mg0.05O	6.074	43.926
	7	0.0037	Zn0.93Mg0.07O	8.65	41.35
	10	0.0056	Zn0.90Mg0.10O	13.084	36.916
	13	0.0074	Zn0.87Mg0.13O	17.29	32.71
	15	0.0088	Zn0.85Mg0.15O	20.560	29.44
	20	0.0125	Zn0.80Mg0.20O	29.205	20.795
	25	0.0167	Zn0.75Mg0.25O	39.019	10.981
	30	0.0214	Zn0.70Mg0.30O	50	0
	Mg

content values (mol%) CMg(Ac)2 (mol/L) Final stoichiometry of ZnMgO Precursor solution Mass of Mg(Ac)2 (g) Mass of Zn(Ac)2 (g)

	50	0.0500	Zn0.50Mg0.50O	S4	0.536	0.55
	70	0.1167	Zn0.30Mg0.70O	S5	1.251	0.55

Table 4 . 5 :

 45 Volume percentage of acetic acid in the precursor solutions.

Table 4 .6.Table 4 . 6 :

 446 Design of experiment to select the optimal spray parameters for ZnO preparation.

	Preparation Parameter	Range	Optimal value selected after experiments
	Zn(Ac)2 molar concentration	[ 0.03 mol/L ; 0.1 mol/L ]	0.05 mol/L
	Number of spray cycles	[ 10 ; 100 ]	20
	Dwell times between each spray cycle	[ 0.5 s ; 2 s ]	0.5 s
	Set temperature	[ 400 °C ; 480 °C ]	470 °C
	Relative shaping air pressure	[ 6.8 kPa ; 20.6 kPa ]	14.8 kPa
	Height between nozzle to substrate	[ 90 mm ; 110 mm ]	100 mm

Table 4 . 7 :

 47 Calibrated X-ray fluorescence system used to determine the magnesium molar composition in the precursor solution (mol%) and the corresponding composition in the Zn1-xMgxO solid films (sol%).

	Magnesium composition in the precursor solution (mol%)	Magnesium acetate concentration in the precursor solution (mol/L)	Magnesium composition in the solid film (sol%)
	0	0.0000	0.00
	3	0.0015	2.52
	5	0.0026	4.36
	7	0.0037	5.56
	10	0.0056	7.91
	13	0.0074	10.32
	15	0.0088	11.02
	20	0.0125	16.46
	25	0.0167	19.85
	30	0.0214	23.82
	50	0.0500	31.22

Table 4 . 8 :

 48 Transmission and thickness estimations of the Zn1-xMgxO solid films with respect to the increase of Mg molar composition (mol%).

	Magnesium

composition in the precursor solution (mol%) Average Transmission (%) in the wavelength range 450 nm to 850 nm Zn1-xMgxO film thickness [nm; ±10 nm]

Raman Shift (cm -1 ) Mode

	Transmittance spectroscopy was used to investigate the effect of preparation temperature on optical
	Cu2O	[140,160] ; 220 ; [500,550]	T1u*; 2Eu*; T2g
	CuO	[282,303] ; [330,350] ; [616,636]	Ag*; Bg*; Bg
	Cu4O3	318 ; 510 ; 541	Eg; Eg; A1g*

  explains the rise in conductivity of ZnO when Al is incorporated. Because Al has one more valence electron than Zn, substituting Al for Zn or occupying the interstitial spaces with Al increases the concentration of charge carriers in the material. In the tetrahedral and octahedral configurations, the modest difference in electronegativity values of Zn (1.65) and Al (1.61), as well as the smaller ionic radii of Al (0.530, 0.675 Å) when compared to Zn (0.60, 0.710 Å), favour Al incorporation in ZnO. Half of the tetrahedral holes and all the octahedral holes in the hexagonal close packed lattice of the ZnO wurtzite structure are unfilled, allowing for more potential dopant sites. Although the Al 3+ ion is smaller than the Zn 2+ ion and might fit in either hole, geometry preference criteria suggest that Al would choose the octahedral hole

Table 6 . 4 :

 64 Design of experiment to select the optimal spray parameters for ZnAlO preparation.

			Optimal value
	Preparation Parameter	Range	selected from
			experiments
	Zn(Ac)2 molar concentration	[ 0.03 mol/L ; 0.1 mol/L ]	0.05 mol/L
	Aluminum acetylacetonate concentration	[ 0.0005 mol/L ; 0.0055 mol/L ]	0.0005 mol/L to 0.0026 mol/L
	Number of spray cycles	[ 10 ; 100 ]	20
	Dwell times between each spray cycle	[ 0.5 s ; 2 s ]	0.5 s
	Set preparation temperature	[ 400 °C ; 480 °C ]	470 °C
	Relative shaping air pressure	[ 6.8 kPa ; 27.6 kPa ]	20.6 kPa
	Height between nozzle to substrate	[ 90 mm ; 110 mm ]	100 mm

Table 6 . 5 :

 65 Chemicals with high purity used for spray pyrolysis deposition of ZnMgAlO.

	Name	Formula	Supplier
	Zinc acetate dihydrate	Zn(CH3COO)2 • 2H2O	Merck 108802
	Aluminum acetylacetonate	C15H21AlO6	Alfa Aesar 44538
	Magnesium acetate tetrahydrate	Mg(CH3COO)2 • 4H2O	Sigma Aldrich M5661
	Acetic Acid	CH3COOH	Alfa Aesar 10994-AE
	Ethanol anhydrous	CH3CH2OH	Merck-Sigma Aldrich 186414
	Deionized water	H2O	Avantor-VWR

Table 6 . 7 :

 67 Design of experiment to select the optimal spray parameters for ZnMgAlO preparation.

			Optimal value
	Preparation Parameter	Range	selected from
			experiments
	Zn(Ac)2 molar concentration	[ 0.03 mol/L ; 0.1 mol/L ]	0.05 mol/L
	Magnesium acetate tetrahydrate concentration	[ 0.0015 mol/L ; 0.0214 mol/L ]	0.0015 mol/L to 0.0037 mol/L
	Aluminum acetylacetonate concentration	[ 0.0005 mol/L ; 0.0026 mol/L ]	0.0005 mol/L & 0.0010 mol/L
	Number of spray cycles	[ 10 ; 100 ]	20
	Dwell times between each spray cycle	[ 0.5 s ; 2 s ]	0.5 s
	Set preparation temperature	[ 400 °C ; 480 °C ]	470 °C
	Relative shaping air pressure	[6.8 kPa ; 27.6 kPa]	20.6 kPa
	Height between nozzle to substrate	[ 90 mm ; 110 mm ]	100 mm

%. Pour les compositions de magnésium à partir de 30 %, une ségrégation de phase a été observée. Tous les films avaient une transmittance élevée d'environ 90 % dans le domaine visible, et variant très peu avec l'épaisseur, démontrant la maîtrise du procédé d'élaboration. Les variations de la largeur de la bande interdite et de l'énergie d'Urbach avec l'augmentation de la composition en magnésium ont été étudiées et corrélées aux caractéristiques structurales des films minces. Les caractéristiques structurales ont permis d'identifier la structure wurtzite comme la seule phase présente dans tous les films avec une composition en Mg inférieure à 30 %, tandis qu'une séparation de phase a été observée au-delà de cette composition. Lorsque la composition en Mg a été augmentée de 0 à 7 % en moles, les caractérisations électriques ont montré une chute significative de la concentration des porteurs. Lorsque la teneur en Mg dépasse 7 %, les films minces deviennent semiisolants. Les résultats électriques impliquent qu'en plus de l'augmentation de la largeur de la bande interdite, un processus de compensation impliquant des défauts dans l'alliage joue un rôle clé dans la diminution de la concentration de porteurs. La mobilité augmente légèrement lorsque la teneur en magnésium augmente, indiquant que la qualité cristalline des films ne se détériore pas beaucoup lorsque la teneur en magnésium augmente dans la gamme étudiée. La grande transparence des films dans la région visible du spectre ainsi que la modulation de la largeur de la bande interdite et de la conductivité peuvent avoir des applications intéressantes pour la fabrication de cellules solaires, de photodétecteurs ultraviolets et d'autres composants optoélectroniques.En résumé, l'élaboration par pyrolyse ultrasonique de ZnMgO à haute teneur en magnésium a été développée et des études structurales, optiques et électriques ont été menées avec une combinaison de propriétés optoélectroniques pouvant être exploitées pour les composants optoélectroniques. Les principales réalisations sont : (i) une limite de miscibilité du magnésium dans l'oxyde de zinc de

Acknowledgements

108 experiments are listed as:

(i)

Substrate to spray nozzle height: 1o cm.

(ii)

Nozzle speed: 70 mm/sec.

(iii)

Dwell time: 1 sec for each spray cycle.

(iv) Shaping air pressure: 13.8 kPa.

The values of the optimized spray parameters that can be implemented for preparation of ZnO to be used as window layer for "all-oxide" solar cells are mentioned in the Table 3.6 below.

Table 3.6: Optimized spray preparation parameters along with their values. All the chemicals used for the preparation of Cu2O thin films are listed in the Table 5.1.

Spray Preparation Parameters

Table 5.1: Chemicals with high purity used for spray pyrolysis deposition of Cu2O.

Among the spray preparation parameters, the deposition temperature plays a key role to prevent the formation of unwanted copper phases such as CuO. As suggested by Kosugi et al. [52] the preparation of Cu2O by spray pyrolysis follows a two-step process. Firstly, the Cu 2+ ions in the solution are reduced by the reducing agent (glucose or D-sorbitol) to form elemental copper. The elemental copper gets adsorbed to the surface by forming clusters. These clusters in turn, depending on the temperature and the precursor solution composition gets oxidised to form Cu2O film. The film formation process is shown below:

𝑂 2 𝐶𝑙𝑢𝑠𝑡𝑒𝑟 → 𝐶𝑢 2 𝑂 𝑓𝑖𝑙𝑚 (ii)

An alternate process for Cu2O film formation could be over the reduction of an already formed CuO film by a suitable reducing agent as mentioned in the literatures [53][54][55]. All the pathways for Cu2O formation are substantially influenced by the deposition temperature. The influence of the temperature is discussed in the Section 5.3.1.2.

Choice of Reducing Agents

Depending on the pH value, the dissolution of copper acetate in a water-based precursor solution gives either Cu 2+ ions or Cu 2+ complexes with acetate and water ligands. So, in order to produce a Cu2O film, the Cu 2+ ions must be reduced to their +1 oxidation state [49,56,57]. D-glucose is commonly used a reducing agent because of the presence of the terminal aldehyde and hydroxyl groups which favours reduction. But sugar and sugar-based alcohols are very low-volatile substances [58], whose by-products after the pyrolysis remains in the deposited film and reduces the film quality [58,59]. D-glucose possess strong O-H bonds whose decomposition at high temperature pyrolytic reactions produces hydroxymethylfurfurol, which tends to be present in the Cu2O film, thus declining its quality [60,61]. The effect of the increase of the set and preparation temperatures on the quality of the Cu2O film is studied and discussed in detail in the later sections of this chapter.

Design of Experiment Along with the Chemical Protocol

Many parameters are involved in the spray pyrolysis preparation process for depositing high-quality thin films as detailed in the Section 2.5 of Chapter 2. Variation in each parameter impacts the resulting film quality. This is the reason to set up the design of experiments, so that we can

understand and analyze the nature and degree of impact of the parameters towards the prepared film quality. The preparation temperature, the number of spray scans, the nozzle speed, the nozzleto-substrate distance, the air flow pressure, the flow rate, and the precursor solution and reducing agent concentrations all play a role in the Cu2O preparation process. The impact of the preparation temperature and the reducing agent concentration is the focus of this research. The effect of the other parameters was researched and, following a thorough investigation, they were set to their optimal values as follows:

1) The number of spray cycles has been set at 75 scans.

2) The spray flow rate was set to 0.5 mL/min.

3) The nozzle speed was set to 70 mm/s.

4)

The height between the spray nozzle and the substrate is set to 10 cm.

5)

The concentration of copper acetate in the precursor solution is set to 0.025 mol/L.

6)

The acetic acid volume percent of about 7 % corresponding to a pH of 2.80.

7)

The shaping air pressure fixed at 13.8 kPa.

8)

The dwell time set to 1 sec between each spray cycle.

The following are the preparation parameters examined in this research:

1. The set temperature of the hot plate, which can range from 375 °C to 435 °C.

2.

The reducing agent concentration, ranging from 0 mol/L to 0.075 mol/L. So, for y =0.3:

Therefore, the quantity of Al(acac)3 powder needed corresponding to the concentration of 0.0214 mol/L is given by the equation below:

Equation (6.3):

𝑚 𝐴𝑙(𝑎𝑐𝑎𝑐) 3 = 𝐶 × 𝑉 × 𝑀 where, mAl(acac)3 is the mass (in g) of Al(acac)3 powder; C is the desired concentration of Al(acac)3 in mol/L; V is the solution volume in litres (L) and M is the molar mass of Al(acac)3 (in g/mol). So, 𝑚 𝐴𝑙(𝑎𝑐𝑎𝑐) 3 = 0.0214 (mol/L) × 0.1 (L) × 324.30 (g/mol) = 0.694 g.

Finally, the 100 mL solution S2 is prepared by: dissolving 1.1 g of Zn(Ac)2 in 50 mL deionized water first and then dissolving 0.694 g of Al(acac)3 in ethanol and then mixing these two solutions together. For complete homogenization, the solution S2 after the mixing is put to ultrasound bath for 15 minutes. Now let us assume that C Al(acac) 3 = 0.0214 mol/L is the initial concentration. Using the Equation 6.2, we can calculate the different values of concentration corresponding to different Al content %.

The values are shown in the Table 6.2. Table 6.2: Calculation for different concentrations of Al(acac)3. Now, we need to prepare the solution S3. It should be noted that S3 will be the final solution that will be used for the actual preparation. For preparing the solution S3, we need to mix different proportions of S1 and S2. Basically we, will collect a certain volume of S2 based on the desired Al content % value and we will dilute it by adding S1. The final volume "V3" of S3 will be 50 mL. Volume of S1 needed is "V1". The volume "V2" of S2 to be collected is given by: Equation (6.4):

The different volumes of V1 and V2 are given in the Table 6.3. Where 𝐿𝐹 002 is the Lotgering factor for this peak (002); 𝑃 002 = 𝐼 002 / ∑ 𝐼 ℎ𝑘𝑙 is the considered (002) peak integrated intensity divided by the integrated peak intensities of all the peaks for the thin film;

𝑃 002 = 𝐼 002 / ∑ 𝐼 ℎ𝑘𝑙 ; And, similarly, 𝑃 002 0 = 𝐼 002 0 / ∑ 𝐼 ℎ𝑘𝑙 0 (002) is peak integrated intensity divided by the integrated peak intensities of all the peaks for the powder used as the reference. For other peaks, the formula is similar by replacing (002) by any other peak indices.

For a randomly oriented film 𝐿𝐹 002 = 0. If a preferred orientation exists, the Lotgering factor of the corresponding peak will increase 𝐿𝐹 002 > 0 and the other Lotgering factors will decrease below zero. For a perfect monocrystalline thin film, the Lotgering factor will be equal to the unity for the peak corresponding to the unique orientation.

The volume "V3" of S3 to be collected is given by: Equation (6.7):

The different volumes of V1, V2 and V3 are given in the Table 6.6. Also, noticed is that the lattice parameters a and c do not change much with the increase in Mg for both the molar compositions of Al.

The variation in the lattice parameters corresponding to (002) peak is shown in the Figure 6.

(a to d).

The obtained values can be compared to the values obtained by USP for ZnO in our work [14] (detailed in Table 3.5 of Chapter 3). For the lattice parameters our results disagree with coatings developed by Tsai et al. [26] using RF magnetron sputtering. In our case, we do not have too much of shrinkage in the lattice parameters as obtained by Tsai et al. This could be due to less disorders in our films with effective control on the spray preparation parameters. The considering of realistic simulation parameters makes it possible to find similar photovoltaic efficiency than the experimental record and theoretical one found by using ZnGeO as a buffer layer as detailed in [36] and in the Section 1.3.2.5 of Chapter 1, which opens very interesting perspectives. A comprehensive numerical study with respect to the parameters of the materials will provide useful guideline for the development for the first time of the complete "all-oxide" solar cell with the ultrasonic spray pyrolysis technique, using the materials prepared in this work. The high transparency of the films in the visible region of the spectrum along with the modulations of the bandgap energies and the conductivities can be interesting for application as TCO's, window or buffer layers in "all-oxide" solar cells. The ZnMgAlO quaternary can even have useful applications in ultraviolet photodetectors and other optoelectronic devices.

for preparation process was classified by the generation and transport of the atomized droplets and ultimately getting deposited on a heated substrate with a pyrolysis chemical reaction. The spray parameters were optimized to work in tandem to produce high-quality films. The parameters that we optimized for our preparations are as follows: Substrate temperature; shaping air pressure; nozzle-substrate height; solution flow rate; nozzle speed; dwell time; nozzle power; spray cycles; spray position; substrate choice; precursor solution concentration. With in-depth knowledge about these parameters, we proceeded towards the preparation of the specific films. For the preparations of all the films, specific design of experiments was set up to understand the role of a specific parameter on the quality of the prepared films.

We started with preparation of zinc oxide (ZnO). As zinc oxide properties can be tuned with respect to the preparation conditions, it has the potential to be used as a window or top contact layer. We ZnO thereby proving its effectiveness for a conductive window layer for applications in "all-oxide" solar cells.

Next, we moved on to prepare zinc magnesium oxide (ZnMgO). The molar composition of magnesium precursor was varied from 0 % to 70 %. This range is the most as of now compared to other literatures. Mostly, low magnesium incorporation in zinc oxide is favoured due to the low solubility limit. For, our case it was even more challenging as we used only water-based precursor solution for the preparations. So, precise control on the spray preparation parameters was of utmost importance. In this preparation also high set temperature of 470 °C was used with acetic acid in the precursor solution. From the chemical and structural analysis of the films we found the miscibility limit of magnesium in zinc oxide to be nearly 30 mol%. This range is quite high compared to other solution-based preparation processes. The XRD analysis showed that till nearly 30 mol% of magnesium incorporation the wurtzite crystalline structure of the films is retained. The bandgap more time to control the precursor solution properties and the suitable adjustment of the spray preparation parameters to be able to deposit Ga2O3 with optimum quality. The Ga2O3films that we prepared were mostly amorphous. With better control on the preparation process, Ga2O3 films can be prepared by USP.

Another aspect that can be looked upon is the uniformity of the deposited films. In our case, we have prepared the films using a linear to and from movement of the spray nozzle. The films were very uniform at the spray zone. But for large area depositions, uniformity of the films can be an issue. In this case, different schematic for the movement of the spray nozzle during the actual spray process can be tried. The thickness of the films would not be the same, but with control on the spray flow rate, the nozzle speed, the preparation temperature, and the control on the precursor solution concentration can make the large area films uniform. The resistivity of the films can also vary with the uniformity. Highly uniform films are desired to have large-area controlled resistivity in the films.

All our films were prepared keeping in mind their applicability for "all-oxide" solar cells. But the metal oxide thin films can be used for other opto-electronic applications. The preparations can be upgraded according to the quality of the films that needs to be achieved for a particular application.

Doping and co-doping can play a big role in this matter. A thorough understanding is required about the chemistry behind the process. Then only the spray preparation parameters need to be adjusted for preparation of high-quality films.

At the end we can say that the USP process to prepare metal oxide thin films has high potential in laboratory-based research and in the industry. This process can prepare high-quality films for different optoelectronic applications with much less time and energy consumption compared to other vacuum-based techniques.

CV

R E S E A R C H L I N K S

List of Publications

Articles

Abstract

The results on preparation of environmentally compatible, earth-abundant metal oxide thin films using the technique of ultrasonic spray pyrolysis are presented. Three essential materials are developed for the purpose of realization of an "all-oxide" solar cell device: Zinc oxide (ZnO) as window layer; zinc magnesium oxide (ZnMgO) as a buffer layer and cuprous oxide (Cu2O) used as an absorber layer. Comprehensive design of experiments was set up for the preparation of each material. Highly transparent ZnO was prepared in wide range of thickness with high crystalline qualities with specific preparation temperature with a precise control on the concentration of the precursors. ZnMgO was prepared by varying the molar compositions of the magnesium precursor in the precursor solution. Up to nearly 30 % of Mg, the ZnMgO films exhibited single crystalline phase with high transparencies. High-absorbing Cu2O preparation was optimized with effective control on the preparation temperature and the concentration of a new reducing agent (D-sorbitol). To expand the horizon of efficiency of our preparation process, two more materials, ZnAlO and ZnAlMgO, were prepared. It was found that the optical, electrical, and structural properties of the ZnAlO films could be modulated for use in "all-oxide" solar cells by varying magnesium (up to 7 mol%) and aluminum (up to 2 %). The bandgap energies and the electrical properties of the films were modulated with the co-doping so that they can be integrated as window/top-contact/buffer layers in "all-oxide" solar cells. Additionally, simulations performed using Silvaco Atlas® and Solis also demonstrates the applicability of these films for "all-oxide" solar cells.

Keywords: Ultrasonic Spray Pyrolysis, Semiconductors, Zinc Oxide, Zinc Magnesium Oxide, Copper Oxide, Preparation, Thin Film, Structural Properties, Optical properties, Electrical properties, All-Oxide Solar Cell, Photovoltaics.

Résumé

Dans ce travail de recherche, sont présentés les résultats de l'élaboration de couches minces d'oxydes à base d'éléments abondants par la technique de spray pyrolyse ultrasonique. Trois matériaux constituant les briques de base de la cellule solaire « tout oxyde » ont été développés : l'oxyde de zinc (ZnO) comme couche fenêtre ; l'oxyde de zinc et de magnésium (ZnMgO) comme couche tampon et l'oxyde de cuivre (Cu2O) comme couche absorbante. Un plan d'expériences précis a été mis en place pour chaque type d'élaboration afin de comprendre l'effet des paramètres d'élaboration sur les propriétés des films. Des caractérisations optiques, structurelles, morphologiques et électriques ont été utilisées pour étudier les propriétés du film et les résultats discutés et analysés en détail. Des films minces de ZnO avec une transparence élevée dans une large gamme d'épaisseurs ont été obtenus. Le ZnMgO a été élaboré avec une phase monocristalline et une transparence élevée. La limite de miscibilité du magnésium (près de 30 % dans notre cas) est supérieure à celle habituellement obtenue avec les méthodes d'élaboration sans vide. L'élaboration de l'absorbeur Cu2O a été optimisée avec un contrôle efficace de la température d'élaboration et de la concentration d'un nouvel agent réducteur (D-sorbitol). Pour élargir les perspectives de ce travail, deux autres matériaux, ZnAlO et ZnAlMgO, ont été élaborés. Nous avons ainsi mis en évidence le fait qu'avec de faibles concentrations d'aluminium (jusqu'à 2 %) et de magnésium (jusqu'à 7 %), les propriétés optiques, électriques et structurales des films de ZnAlO pouvaient être modulées pour une utilisation en tant que couche fenêtre ou TCO dans les cellules solaires « tout-oxyde ». De plus, les simulations réalisées à l'aide des logiciels Silvaco Atlas® et Solis démontrent également le potentiel de ces films pour les cellules solaires « tout-oxyde ».

Mots-clés : Spray pyrolyse ultrasonique, Semi-conducteurs, Oxyde de zinc, Oxyde de zinc et de magnésium, Oxyde de cuivre, Élaboration, Couche mince, Propriétés structurales, Propriétés optiques, Propriétés électriques, Cellule solaire « tout-oxyde », Photovoltaïque.