granting me the opportunity to pursue my Ph.D. in his esteemed research group. Throughout the entire research process, his guidance, patience, and unwavering support have been invaluable. I am deeply grateful for the freedom and flexibility that he afforded me to undertake various research tasks and activities, which allowed me to become proficient in different fabrication and characterization systems and techniques. I would also like to extend my heartfelt thanks to my cosupervisor, Damien Aureu, for his invaluable assistance in the characterization, analysis, and interpretation of XPS data. The expertise and insights of my supervisors have played a crucial role in shaping the direction and quality of my work.

I wish to express my gratitude to the members of the jury for their invaluable contributions, feedback, and constructive criticism during my defense. The fruitful discussions that we had enabled me to gain fresh perspectives and insights that enhanced the quality of my research and writing.

I would like to express my sincere gratitude to the various research groups and entities whose steadfast support and assistance made it possible for me to successfully complete my Ph.D. thesis.

Firstly, I extend my heartfelt thanks to the MOPGA team for their constant help and support in the laboratory, as well as for the invaluable scientific discussions we had during our weekly meetings.

Beyond scientific research, I am immensely grateful to have them not only as colleagues but also as friends, and for the unforgettable moments we shared both inside and outside the IPVF building.

I am also grateful to the team at ILV, including Solène Bechu, Muriel Bouttemy, Pia Dally, Mirella Al Katrib, and Mathieu Frégnaux, for imparting their extensive knowledge and expertise on photoemission spectroscopy techniques. I feel exceptionally lucky to have worked with Mathieu, whose responsiveness, helpfulness, and knowledge were invaluable to my work over the past three years.

Furthermore, I extend my appreciation to the team at the Bessy II synchrotron center, particularly Roberto Félix, Elif Hüsam, Regan G. Wilks, and Marcus Bär, for their kind cooperation during HAXPES measurements.

I am also immensely grateful to Professor Hans-Gerd Boyen and Derese Desta for their assistance during my unique opportunity to visit the laboratory of Professor Boyen at Hasselt University and perform lab-based HAXPES measurements.

One of the crucial parts of my thesis involved preparing thin films by the ALD process, and the reliable and reproducible ALD procedure was crucial for my Ph.D. research. I am deeply grateful III for the efforts and support of the ALD group at IPVF, particularly Nathanaelle Schneider and Damien Coutancier, who ensured that I never had any concerns in this regard. I am also thankful to the outstanding team at LIPCM, École Polytechnique, especially Denis and Haeyeon, for their assistance in co-evaporation of perovskite. I would like to express my gratitude to the administrative and management team of CNRS at IPVF for their invaluable support during my Ph.D. studies. I extend my sincere appreciation to Claire Vialette for her excellent administrative assistance and Jean-Francois Guillemoles for his understanding guidance as the director of CNRS UMR 9006.

I extend my thanks to all my colleagues at IPVF from different entities for their help, kindness, and warm smiles. I will always cherish the incredible moments I spent with them.

I would also like to acknowledge all of my colleagues and friends at IPVF who shared wonderful moments with me from the beginning of my Ph.D. until the end. Thanks to Celia (and your car  ), Davide R (Enter & Submit guy) , Nitin, Claire, Pilar, Vincent (Coauteur du résumé en français), Marie (Éditeur du résumé en français), Alex Py, Stefania, Ashish, Alexandre C, Alexandre B ( Merci pour AFM, MEB et tous les gros mots ), Cécilia, Davide C (leadman), Van son (Prof), Amelle, Stefanie, Karim, Salim, Guillaume, Arthur, and others who contributed to making my journey enjoyable and memorable.

Moreover, I am grateful to my friends outside of work, including Miguel, Ali, Julie, Pardis, Amir, Rawad, Martina, Mar, Kathy, Daniel, Yacine, Laura, Jasone, Daniel V, Saeed, Shahin, Samereh, Mohsen, Ghazale, Charlos, Maryam, Saman, and others. At this moment, I would like to express my gratitude to my family, whose constant support has served as a significant driving force throughout my life, both present and future. In order to convey my appreciation to them effectively and completely, I have chosen to compose the following paragraph in Persian. A big thank to Zhila, with whom I have shared the last 12 years. Throughout our journey together, we have encountered numerous challenges, yet also shared many memorable moments. Zhila, thank you for being an unwavering source of love and inspiration in my life. 1.13: XPS spectra of N 1s core level before and after ALD-SnO2 film deposition on bare FA0.83Cs0.17Pb(I0.83Br0.17)3 (a) LiF/C60 covered FA0.83Cs0.17Pb(I0.83Br0.17)3 (b) [START_REF] Palmstrom | Interfacial Effects of Tin Oxide Atomic Layer Deposition in Metal Halide Perovskite Photovoltaics[END_REF] ........ 3) the standard of life, based on the gross national income per capita [1]. According to United Nations Conference on Trade and Development (UNCTAD, 2017), there is a tight correlation between HDI and energy accessibility. Access to reliable, affordable, and modern energy can improve life quality and facilitates and accelerates development in countries by promoting job opportunities, economy, and social services and impeding poverty [2]. The accessibility of energy cannot be achieved before guaranteeing three elements: energy security, energy equity, and environmental sustainability [3].

بابا و مامان

Table of Contents

However, the abovementioned elements could be affected, threaten, and damaged by various issues. Geopolitics plays a key controller role in the energy sector. One example of the influence of geopolitical issues has been recently experienced in Europe. Approximately 70% of final energy consumption in Europe is supplied by fossil fuels including 43% oil and 22% gas. Around 40% of the imported gas (90% of the whole consumption) comes from Russia. However, Russia's invasion of Ukraine in February 2022 has caused uncertainty about energy supplies along with an increase in energy prices. As a consequence, the inflation rate in Europe hits its highest value in the past quarter century. This issue not only affects Europe itself but also leads to a global cost-of-living crisis [4]. Additionally, the energy sector is undoubtedly the most dominant contributor to greenhouse gas emissions and therefore global climate change. This sector accounts for more than 73% of global greenhouse emissions ( Renewable energies are what could securitize energy security, energy equity, and environmental sustainability. Renewable energies due to their availability, diverse sources (from solar, geothermal, wind, and hydroelectric power) [START_REF] International | Renewable Energy to Support Energy Security[END_REF], affordability, reliability, and carbon-zero emissivity can play a game-changer role in a war fighting against universal poverty, inequality, and climate apocalypse.

Among various types of renewable energies, solar energy presents a steady, abundant, and worldwide accessible source which can be converted into thermal and electrical energy. According shown in Figure i. 3 [6]. However, despite their promising aspects regarding efficiency and fabrication cost, some barriers such as scale-up manufacturing and particularly long-term stability have obstructed their trajectory toward commercialization [START_REF] Rong | Challenges for commercializing perovskite solar cells[END_REF]. To this end, researchers in this field are immensely executing different plans and solutions to address the aforementioned issues.

Regarding instability in MHP solar cells performance, one of the main sources is attributed to the chemically active charge transport layers and their interfaces with the perovskite absorber layer.

Noteworthy, designing and optimizing interfaces could affect and enhance both efficiency and stability of the solar cells [START_REF] Rong | Challenges for commercializing perovskite solar cells[END_REF]. Therefore, studying, understanding, and improving interfacial properties in MHP solar cells is an imperative action in order to accomplish the commercialization target [START_REF] Schulz | Interface Design for Metal Halide Perovskite[END_REF]- [START_REF] Shao | The Role of the Interfaces in Perovskite Solar Cells[END_REF].  Chapter 2 describes the materials and the fabrication methods that we employed to fabricate the samples and solar cell devices. Additionally, the techniques and setups we used to characterize thin films and solar cells are explained in this chapter.

The Purpose and structure of this work

 Chapter 3 focuses on the surface modification of nickel oxide and its adjacent interface with the perovskite absorber layer in an inverted p-i-n structure. In this chapter, three different phosphonic acid derivative molecules to passivate the nickel oxide hole transport layer are presented. Subsequently, their optical, morphological, chemical, and electronic properties are studied. Also, the influence of these interlayers on the photovoltaic parameters of fabricated solar cells is reported.

 Chapter 4 studies the interface between single-walled carbon nanotubes and perovskite absorber layer in the conventional n-i-p structure of MHP solar cells with a focus on the hard

x-ray photoelectron spectroscopy characterization method. In this chapter, using this technique we see how growing carbon nanotube on a perovskite absorber layer impacts the chemical and electronic properties of perovskite.

 Chapter 5 presents the detrimental sources that lie in the interface between perovskite and its top electron transport layer i.e. tin oxide grown by atomic layer deposition. In this chapter, we aim to understand why growing tin oxide on top of the MHP layer with and without using a protective interlayer in p-i-n configuration was not sufficient to get reasonable efficiency from produced solar cells. Again in this chapter hard x-ray photoelectron spectroscopy is employed as the core characterization technique to probe the interfaces.

Introduction

Metal halide perovskite materials, from their first emergence in the photovoltaic community, have passed a meandering road until now. MHP, for the first time, was integrated as an absorber layer into a liquid dye-sensitized solar, showing an efficiency of 3.8% [START_REF] Kojima | Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic[END_REF]. These solar cells with their low stability due to using liquid electrolyte hole transport material did not highly attract the attention of the photovoltaic community [START_REF] Perovski | Perovskites-Based Solar Cells : A Review of Recent Progress , Materials and Processing Methods[END_REF]. The first solid-state perovskite-based solar cell was presented in 2012 yielding a power conversion efficiency (PCE) of 9.7% [START_REF] Kim | All-Solid-State Submicron Thin Film[END_REF]. Replacing liquid electrolyte with a solid-based material boosted the efficiency and stability of the devices and triggered rapid progress of this technology over the last decade [START_REF] Guo | The high open-circuit voltage of perovskite solar cells: a review[END_REF]. The current record PCE of perovskite solar cells is achieved in 2021 showing an efficiency of 25.6% with excellent operational stability of 450h [START_REF] Jeong | Pseudo-halide anion engineering for[END_REF].

In the following sections, metal halide perovskite materials and their properties are described.

Then, the working principle of MHP solar cells is described. Subsequently, we explain why the interfaces are strategic regions to improve the performance of MHP solar cells. Lastly, we introduce the interface systems that are studied in this Ph.D. research work with a look at their background.

Metal Halide Perovskites

Crystal structure

The term perovskite denotes the crystal structure of an oxide mineral composed of calcium titanate, with the chemical formula CaTiO3. In the general case of this ABX3 structure, A is a monovalent cation, B is a divalent metal cation, and X could be oxygen, carbon, nitrogen, or a halogen anion. corners of X site. In MHP, methylammonium (MA +) , formamidinium (FA +), and Cesium (Cs + ) are large cations that mainly occupy the A site. B site accommodates metal cations such as lead (Pb 2+ ) and tin (Sn 2+ ). I -, Br -, or Cl -are the halide anions filling the X site [START_REF] Chilvery | A perspective on the recent progress in solution-processed methods for highly efficient perovskite solar cells[END_REF]. [START_REF] Green | The emergence of perovskite solar cells[END_REF] 1.

Band structure and Compositional features

In MHP as a semiconductor, the energy band gap (Eg), which is defined as the energy difference between the valence band maximum (VBM) and conduction band minimum (CBM), is a key parameter determining the optoelectronic properties of MHP. Eg can be changed by altering the composition of the MHP [START_REF] Kim | High-E ffi ciency Perovskite Solar Cells[END_REF]. VBM and CBM are formed by different contributions of atomic orbitals of MHP components. In the case of VBM, the atomic orbital of 5p in X anion and 6s in B cation are the determinative orbitals. CBM position is mainly derived by 6p orbitals of B cation and other orbitals [START_REF] Brivio | Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers[END_REF]. Figure 1.2 : shows the VBM and CBM formation of MAPbI3. As it is illustrated, Pb 6s and I 5p atomic orbitals define the VBM, and Pb 6p and I 5p from CBM [START_REF] Umebayashi | Electronic structures of lead iodide based low-dimensional crystals[END_REF].

Atomic orbitals of A cation do not directly influence the band structure of MHP since its electronic contribution is positioned too far from VBM. However, the size of A cation and how it fits and moves in the crystal structure of MHP can cause changing in B-X bonding and crystal structure, hence material properties [START_REF] Even | Importance of Spin -Orbit Coupling in Hybrid Organic/Inorganic Perovskites for Photovoltaic Applications[END_REF].

Therefore, one can tune the properties of MHP by altering or mixing the components of A, B, and X sites. As we discussed previously, the electronic properties of MHP are highly dependent on the components fill B and X sites. As an example, MAPbX3, employing iodide, bromide, and chloride [START_REF] Bojar | Heterojunctions and silicon/perovskite tandem solar cells[END_REF] anions in X sites gives the Eg of 1.6 eV, 2.3 eV, and 3.1 eV, respectively. Additionally, mixing these anions in different ratios provides the possibility of fine-tuning the Eg [START_REF] Manuscript | [END_REF]. A decrease in the size of A cation results in a rise in the Eg. In the case of MAPbI3 with an Eg of 1.6 eV, substituting MA + with larger FA + and smaller Cs + cations leads to a change to a lower value of 1.48 eV and a higher value of 1.73 eV, respectively [START_REF] Lu | iScience Compositional and Interface Engineering of Organic-Inorganic Lead Halide Perovskite Solar Cells[END_REF]. In the case of B sites, Pb is widely used due to its good performance and high stability. However, its toxic nature is a serious issue that pushes the researcher to replace it with other materials such as Sn. Again in the case of MAPbI3, replacing Pb with Sn can reduce the band gap to 1.3 eV, but lead-containing MHPs suffer an instability problem because of tin oxidation from Sn 2+ to Sn 4+ [START_REF] Prasanna | operational stability[END_REF].

Alongside compositional tuning, the level of intrinsic and extrinsic defects can impact the electronic properties of MHP which will be discussed in the next section.

Defects in MHP

Defects in MHP can be categorized based on their formation. In this manner, point defects formed around a single spot in the lattice including atomic vacancies, interstitials, anti-site substitutions, and as well as multidimensional defects including dislocations, grains boundaries, and precipitates are the various defects that make disorder in the lattice. In addition, multidimensional defects can appear in the shape of line defects, planar defects, and bulk defects. and grey circles) [START_REF] Pratheek | Materials Science for Energy Technologies Recent progress on defect passivation in perovskites for solar cell application[END_REF] interstitial space in the lattice. Anti-site substitution occurs when two constituent atoms switch their position in the lattice. If the impurities exist in the perovskite, then they can occupy an atom position or an interstitial position which is known as substitutional impurity and interstitial impurity. Line defects are defined by a misalignment in the lattice. Grain boundary defects appear when various crystalline orientations exist in the lattice and the meeting border of their planes creates grain boundaries. Defects from participates form when the proportion of impurities is high enough that they can create their crystal structure which appears as small particles [START_REF] Bera | Materials Advances Review of defect engineering in perovskites for photovoltaic application[END_REF], [START_REF] Pratheek | Materials Science for Energy Technologies Recent progress on defect passivation in perovskites for solar cell application[END_REF]. the ETL layer which is formed on the TCO substrate, and HTL is grown on top of the perovskite absorber layer. In p-i-n or inverted architecture, HTL formed on TCO, is prior exposed to sunlight radiation compare to ETL which is located between the MHP layer and metal contact [START_REF] Habibi | Progress in emerging solution-processed thin fi lm solar cells -Part II : Perovskite solar cells[END_REF]. During the photovoltaic process in MHP solar cells, the irradiated light is absorbed in MHP layer leading to charge carries creation. The photo-generated electrons and holes move toward charge transport layers. ETL functions to collect electron carriers and block holes while HTL collects holes and blocks electrons. The collected electrons and holes finally move toward the cathode and anode electrodes, respectively, and generate photocurrent [START_REF] Huang | Towards Simplifying the Device Structure of High-Performance Perovskite Solar Cells[END_REF]. The energy band alignment of an MHP solar cell and charge carrier extraction is depicted in Figure 1.4(c).

Metal Halide Perovskite

The materials serving as CTLs should possess some specific features to have better functionality.

The first characteristic which is particularly important for the CTL facing the incoming light is to have a high optical transmittance. Equipping to a high energy band gap, CTL absorbs less light and lets a large portion transmits through it and reached to absorber layer. Another key feature is the high electrical conductivity of CTL that accelerates charge carrier' transfer from MHP to the corresponding electrodes. Suitable energy band alignment with MHP layer in a way that facilitates the collection of the desired carrier and blocks the other one, should be met by CTLs [START_REF] Chen | Materials and Methods for Interface Engineering toward Stable and E ffi cient Perovskite Solar Cells[END_REF].

Recombination Mechanism in MHP Solar Cells

Theoretical studies show that the maximum PCE of MHP solar cells is around 31%. While experimental results show that the short circuit current of MHP solar cells has reached its maximum theoretical quantity, losses in their open circuit voltage, fill factor, and PCE are still observed. These losses are in direct connection with the recombination rate of phot-generated charge carriers [START_REF] Nh | Nano Energy Role of surface recombination in perovskite solar cells at the interface[END_REF].

Recombination of charge carriers in MHP solar cells can be classified in three main mechanisms:

(1) Shunt recombination (2) Bulk recombination (3) interfacial recombination. However, later on, by the progress achieved in growing a uniform pin-hole-free MHP layer, this recombination has become a negligible loss center in the current MHP solar cell technology.

Bulk recombination includes both band-to-band radiative recombination and non-radiative recombination happening in the bulk of the MHP absorber layer, shown in Figure 1.5 by numbers

(3) and (4), respectively. Non-radiative recombination is governed by the density of the trap states in the MHP film. The number of traps is in close correlation with the quality of the perovskite crystal lattice. The level of defects such as vacancies, interstitials, grain boundary defects, etc.

have a direct impact on this process [START_REF] Boix | Organic -Inorganic Perovskite Solar Cells Working Principles of Perovskite Solar Cells[END_REF]. Studies have shown that the energy formation of the common lattice defects is relatively low resulting in shallow traps. Shallow traps located very close to the valance or conduction band have a minimal impact on recombination and act mostly as doping states [START_REF] Yin | Superior Photovoltaic Properties of Lead Halide Perovskites: Insights from First-Principles Theory[END_REF]. In addition, the quality of the MHP absorber has reached such a level that the photo-generated career lifetime surpasses the average diffusion time toward MHP/CTLs interfaces. Thus the recombination in the bulk of MHP is considered a minor source of losses [START_REF] Schulz | Interface Design for Metal Halide Perovskite Solar Cells[END_REF].

Interfacial recombination arises from motion and accumulation of ionic vacancy in the interface of the MHP absorber layer and CTLs. This phenomenon leads to interface polarization which in turn causes reverse injection of charge carriers and thus their recombination [START_REF] Boix | Organic -Inorganic Perovskite Solar Cells Working Principles of Perovskite Solar Cells[END_REF], [START_REF] Calado | Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis[END_REF], [START_REF] Gottesman | Dynamic Phenomena at Perovskite/Electron-Selective Contact Interface as Interpreted from Photovoltage Decays[END_REF].

Interfacial recombination is presented by numbers ( 5) and (6) in Figure 1.5. This recombination 4) is non-radiative trap-assisted recombination ( 5) & ( 6) are interfacial recombination path ways ( 7) is shunt recombination. Adapted from [START_REF] Boix | Organic -Inorganic Perovskite Solar Cells Working Principles of Perovskite Solar Cells[END_REF] is likely the main performance controller of photovoltaic devices [START_REF] Boix | Organic -Inorganic Perovskite Solar Cells Working Principles of Perovskite Solar Cells[END_REF]. Furthermore, the arrangement and perfection of the crystalline lattice structure of MHP can be more disrupted at their interface with charge-extracting layers. This interruption and its consequent defects formation lead to an increase in the recombination rate at corresponding interfaces [START_REF] Schulz | Interface Design for Metal Halide Perovskite Solar Cells[END_REF].

Energy Level Alignment and band bending at Interfaces

The current is generated in MHP by extracting the photo-generated charge carries to the external circuit. However, before the extraction, the charge carrier should be perfectly collected from the MHP absorber in its joint regions with ETL and HTL. An excellent energy level alignment (ELA) between perovskite and charge collective layers is required to guarantee a suitable transfer. This alignment should ideally be in a way that electrons and holes travel simply and without the presence of any energy barriers to the CBM of ETL and VBM of HTL, respectively.

Simultaneously, the undesired charge carrier transfers to each CTL (hole in ETL and electron in HTL) should be blocked in the interfaces. Thus, an excellent ELA in the interfaces and particularly in CTLs/MHP interfaces could promise high carrier collectively, low recombination rate, and improved device performance. It should be mentioned that ETL and HTL terms are described as the highest occupied molecular level (HOMO) and lowest unoccupied molecular level (LUMO) in organic materials [START_REF] Wang | Energy Level Alignment at Interfaces in Metal Halide Perovskite Solar Cells[END_REF]. The possible ELA for metal/organic and organic/organic interfaces are depicted in Figure 1.6. In the metal/organic material interface when the vacuum level aligns, the energy offset at the interface is simply extracted by comparing the work function (WF) of each element. It is demonstrated in panel (i) of Figure 1.6(b). However, the formation of dipole regions at the interface does not allow vacuum-level alignment always happens. The dipole region is caused by charge transfer or physical/chemical interaction at the interface. This process results in a shift in vacuum energy level, as shown in panels (ii) and (iii) of Figure 1.6(b), and consequently changes the energy offset between the HOMO of organic compounds and the fermi level of metal. In turn, a downward or upward band bending of energy levels in the organic layer may be formed at the interface, illustrated in panels (iv) and (v) of Figure 1.6(b), respectively. A similar process could happen when two organic layers come into contact with each other. Depending on the characteristic of each material and the interaction that occurs at their interface, various scenarios such as vacuum level alignment, charge transfer, dipole formation, and bend bending could occur at the interface [START_REF] Wang | Energy Level Alignment at Interfaces in Metal Halide Perovskite Solar Cells[END_REF], [START_REF] Zu | Surface State Density Determines the Energy Level Alignment at Hybrid Perovskite / Electron Acceptors Interfaces[END_REF].

Considering various organic and inorganic ETLs and HTLs in contact with metal organicinorganic halide perovskite materials, the complexity of interfaces in MHP solar cells is perceived and the importance of their study is deduced. Along with this complexity, various formation methods of layers and their sequences, different substrates, and diverse MHP compositional properties could alter ELA. Additionally, in some cases, different research groups reported different results on ELA even though the materials and their fabrication process were similar [START_REF] Shao | The Role of the Interfaces in Perovskite Solar Cells[END_REF],

[39], [START_REF] Li | Recent Progress of Critical Interface Engineering for Highly Efficient and Stable Perovskite Solar Cells[END_REF], [START_REF] Béchu | Photoemission Spectroscopy Characterization of Halide Perovskites[END_REF].

The electronic and chemical properties of materials including VBM/HOMO, CBM/LUMO, charge transfer, interfacial dipoles, band bending after interface formation, etc. could be determined by different techniques of photoemission spectroscopy such as ultraviolet photoelectron spectroscopy (UPS), inverse photoelectron spectroscopy (IPES), X-ray photoelectron spectroscopy (XPS) and so on [START_REF] Teeter | Photoelectron Spectroscopy Methods in Solar Cell Research[END_REF].

Photoemission Spectroscopy (PES)

Photoemission spectroscopy (PES) is a non-destructive technique to investigate the chemical and electronic properties of the surface and near-surface region of conductive and semi-conductive materials. This technique is based on the photoelectric effect and in general consists of three steps.

In the first step, the high-energy photons emitted from various sources such as X-ray tube, a gas discharge lamp, a synchrotron-based source, etc. irradiate the sample and excited the electrons in the surface and subsurface region. Then the excited electron from core levels or the valence band of the samples moves toward the surface. Various scattering could occur for the excited electrons and make them lose some part of their initial energy and transfer it to the other electron around them which is called secondary electron scattering. This phenomenon appears in the background contribution of the captured photoelectron curve. As the last step, those electrons having enough energy to leave the material through its surface and jump into an ultrahigh vacuum are detected by the spectrometer. The probing depth of PES depends on the photon energy employed to excite the photoelectrons and it could differ from a few nanometers in the lab-scale setups to higher than 30 nm in the synchrotron-based facilities. The kinetic energy with which most of the electrons are emitted from the material corresponds to the binding energy by which they were bound in their initial state in the material. The value of binding energy and the number of emitted photoelectrons provide the information by which we can determine the chemical and electronic features of the target samples.

While the samples and the spectrometer are at the same fermi level by grounding them to the same reference, the measured kinetic energy of an excited electron is mathematically calculated by Eq.

(1.1)

𝐸 𝐾 = ℎ𝜈 -𝐸 𝐵 -𝑊𝐹 𝑠𝑝 Eq. (1.1)
Where 𝐸 𝐾 is the measured kinetic energy of the detected photoelectron, ℎ𝜈 is the excitation photon energy, 𝐸 𝐵 is the binding energy that electron had in the material, and 𝑊𝐹 𝑠𝑝 is the work function of the spectrophotometer (Figure 1.7) [START_REF] Béchu | Photoemission Spectroscopy Characterization of Halide Perovskites[END_REF]. The work function of the spectrometer can be determined using a reference sample, commonly the surface of metallic gold is utilized. Thus we can easily determine the binding energy of the photoelectron by using its kinetic energy and used photon energy values [START_REF] Doh | Applications of Synchrotron-Based X-Ray Photoelectron Spectroscopy in the Characterization of Nanomaterials[END_REF].

The probe depth is a key point in the PES technique, its estimated value strongly depends on the escaped length of the excited photoelectrons. This value is related to the inelastic mean free path (IMFP) of the photo-emitted electron in the solid. IMFP defines the average length a photo-excited electron can pass in a solid before transferring its energy to other electrons around them described as inelastic scattering. The IMFP could be calculated using TPP-2M developed by Tanuma et al. [START_REF] Tanuma | Calculations of electron inelastic mean free paths. IX. Data for 41 elemental solids over the 50 eV to 30 keV range[END_REF]. The IMFP value depends on the excitation energy and thus the final kinetic energy of the electron. Therefore different PES techniques that are categorized based on their photon energy and their objective, have various IMFP ranges as illustrated as an "universal curve" in Figure 1.8 : The actual probing depth is estimated as three times the IMFP, taking into account that more than 60% of the entire signal is coming from a depth equal to one IMFP. Among the PES techniques, soft X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy are widely used in the field of perovskite solar cells. However, in recent years, researchers have also employed hard X-ray photoelectron spectroscopy in order to probe deeper into the perovskite material. [START_REF] Tanuma | Calculations of electron inelastic mean free paths. IX. Data for 41 elemental solids over the 50 eV to 30 keV range[END_REF] During my thesis ultraviolet photoelectron spectroscopy, soft and hard X-ray photoelectron spectroscopy have been used and are described in this section.

Lab-based Ultraviolet Photoelectron Spectroscopy (UPS)

Ultraviolet photoelectron spectroscopy (UPS) is the most surface-sensitive photoemission method that is mainly used to determine the value of work function and valence band maximum. In this method, the samples are irradiated by ultraviolet photons often generated by the discharge lamp of He I or He II with the excitation energy of 21.2 or 40.8 eV respectively [START_REF] Béchu | Photoemission Spectroscopy Characterization of Halide Perovskites[END_REF].

A visualized photoelectron distribution curve from collecting and detecting UV-excited photoelectrons is used to extract the data. Importantly, the acquired data should be calibrated using a clean metal surface (gold is usually selected). The Fermi edge of the metal serves as the reference point for the binding energy scale to find the valence band onset as well as the work function value.

In order to determine the band edge, a linear fitting to the edge and the background of the spectrum is used to find the valence band maximum (VBM) as well as the work function. However, some level of inaccuracy in mainly VBM determination could happen due to the low density of states at the leading edge. In this regard, a semi-logarithmic plot could help to raise the accuracy of the VBM position. Nevertheless, the best method appears to be the use of both linear and semilogarithmic plots to compare the value to each other as well as to the literature [START_REF] Robert | Practices in Materials Characterization and Calculations Version Terms of Use Perovskite-inspired photovoltaic materials : Toward best practices in materials characterization and calculations[END_REF].

Thus, in order to find out the WF and VBM, we first position the fermi edge using a gold spectrum.

Then a linear fitting is performed on the secondary electron cut-off and the obtained value is subtracted from photon energy. We used both linear and semi-logarithmic plots and after comparing the result we deduce that the semi-logarithmic plot gives us a more reliable value.

UPS analysis in chapter 3 is performed in a Thermofisher Scientific Escalab 250 Xi setup. The samples are irradiated by a helium-gas discharge lamp with an excitation energy of 21.22 eV (He I). The data are recorded with a step size of 0.05 eV and a pass energy of 1.00-2.95 eV. The energy scale is calibrated with a sputtered gold assuming its fermi level edge at 0 eV. The other spectra are therefore shifted accordingly.

Soft X-ray Photoelectron Spectroscopy (XPS)

Another type of PES technique is soft X-ray photoelectron spectroscopy which is widely known as X-ray photoelectron spectroscopy or XPS. XPS is the most common technique to characterize the chemical and electronic properties of matter (often solid surfaces). In the standard lab XPS technique, the surface of the material is exposed to an X-ray beam typically originating from either Al Kα or Mg Kα radiation source with maximum excitation energy of 1486.6 eV and 1256.6 eV respectively. An illustration of an XPS measurement setup is shown in Figure 1.9. XPS instrument consists of an X-ray source, extraction lenses, substrate stage, and detector that all are placed in an ultra-high vacuum chamber [START_REF] Sci | Introduction to x-ray photoelectron spectroscopy Introduction to x-ray photoelectron spectroscopy[END_REF].

In the registered spectrum, one or various peaks, depending on the binding energy of their core levels as well as XPS excitation energy, could attribute to each element presenting on the surface of the material. Except for Helium (He) and Hydrogen (H) which do not have core electrons, all the elements existing in the periodic table of the elements can be traced and identified by this technique. Each core level of each element has a binding energy according to which each photo peak in XPS spectra could be assigned. Furthermore, all the orbital levels, except "s" orbitals, are composed of a doublet peak with a spin-orbital splitting in which two possible states (1/2 and 3/2 in "p" orbital as an example) have a specific spin-orbital splitting (𐊅). Also, XPS photo peaks are utilized for quantitative analysis of the surface. In other words, the relative intensities of photoelectron peaks are related to the elemental composition of the surface of matter [START_REF] Béchu | Photoemission Spectroscopy Characterization of Halide Perovskites[END_REF]. Importantly, we should know that the intensity of each core level is a function of the interaction probability between the surface and light. This probability which defines as the photoionization cross-section is described by Fermi's golden rule shown in Equation )

𝜎 ∝ |⟨𝛹 𝑓 |𝛥| 𝛹 𝑖 ⟩| 2 Eq. (1.2)
Where 𝛹 𝑓 and 𝛹 𝑖 are respectively the final and initial states and 𝛥 is an operator that defines the transition between two states. σ for each core level highly depends on the excitation energy used in the analyzer [START_REF] Doh | Applications of Synchrotron-Based X-Ray Photoelectron Spectroscopy in the Characterization of Nanomaterials[END_REF]. Generally, different parameters affect the measured intensity of the excited photoelectrons. These factors can be mathematically described by the following equation:

𝐼 ∝ 𝐼 𝑝ℎ 𝜌 𝜎(ℎ𝜐, 𝜃) ∫ 𝑒 -𝑧 λ ⁄ ∞ 0 𝑑𝑧 Eq. (1.3)
Where I and 𝐼 𝑝ℎ are respectively the intensity of the core level and X-ray, 𝜌 is the molar concentration of species, 𝜎 is the photoionization cross-section described in Eq. (1.2) and is a function of photon energy and the emission angle, z is the travel length of the electron in the material and 𝜆 stands for inelastic mean free path (IMFP) of the electrons.

For the quantification of XPS data, first, background subtracting should be applied to the raw spectral intensity. In subsequence, the intensity should be corrected to some parameters such as IMFP, and photoionization cross-section σ of each core level. Transmission function value which is a function of the analyzer and it differs for different modes of the analyzer is another parameter that should be taken into account.

Other important criteria in XPS measurement affecting the precision of the measurements and accuracy of assessment are the energy and spatial resolution of the analyzer. An energy resolution as low as 0.3 eV could be obtained using a monochromatic source which allows to perform accurate analysis of chemical and elemental properties. Regarding the spot size of the X-ray beam, the laboratory-based XPS provides spots from tens to hundreds of microns.

The XPS characterization of samples was performed by a Thermofisher Scientific Nexsa spectrometer (installed at Institut Lavoisier de Versailles (ILV) of Versailles Saint-Quentin-en-Yvelines University) with a monochromatic Al-Kα X-ray source (hν = 1486.6 eV) with a spot size of 400 µm under ultrahigh vacuum. The photoelectrons are detected using a constant energy analyzer mode with a pass energy of 20 eV and XPS spectra were recorded with an energy resolution of 0.1 eV. The samples were transferred from the glovebox at IPVF to ILV in a sealed transport tube under N2. Then the samples were mounted on the substrate holder of XPS setup for a short period to minimize their exposure to the air.

Hard X-ray Photoelectron Spectroscopy (HAXPES)

The maximum probing depth in Soft-XPS is approximately 10 nm from the surface. If we want to go beyond this value and get information from a deeper region, higher excitation energy should be employed. HAXPES benefit from excitation energy higher than 2000 eV, which decreases the contribution of surface contamination and provides more intense signals coming from the deeper region of the target sample. In this technique, thanks to the ability to vary the excitation energy level, we can study chemical and elemental distribution in the surface and sub-surface including bulk layers and interfaces of the samples. This advantage has great importance in MHP materials to see any probable deviation from the expected stoichiometry ratio which in turn allows to detection of the source of defects and malfunction effects [START_REF] Béchu | Photoemission Spectroscopy Characterization of Halide Perovskites[END_REF]. In addition, this approach due to higher excitation energy can excite deeper core levels which have higher binding energies. This could provide useful information in particular for the core level signals including satellite features [START_REF] Regoutz | A novel laboratory-based hard X-ray photoelectron spectroscopy system A novel laboratory-based hard X-ray photoelectron spectroscopy system[END_REF].

Generally, there are two types of HAXPES instruments depending on the radiation source i.e.

synchrotron-based and laboratory scale. In the next two sections, we will explain these two systems.

Laboratory-based HAXPES

The working principle of laboratory-based HAXPES is very similar to laboratory-based XPS.

However, in HAXPES systems, instead of using Al Kα or Mg Kα X-ray sources, Cr Kα or Ga Kα sources with excitation energy of 5420 eV and 9250 eV are mainly used. Figure 1.10 compares IMFP and probing depth of Si 2p for different lab-based soft and hard X-ray photoelectron spectroscopy. We can see in Figure 1.10 that by using lab-based HAXPES the information depth increases drastically in comparison to lab-based (soft) XPS [START_REF] Regoutz | A novel laboratory-based hard X-ray photoelectron spectroscopy system A novel laboratory-based hard X-ray photoelectron spectroscopy system[END_REF].

As a collaborative work with Hasselt university in Belgium, a commercial laboratory-based HAXPES system installed (manufacturer Scienta Omicron) with a liquid metal hard X-ray source (Ga-kα, photon energy 9251.3 eV) is utilized to characterize some samples in chapter 3. This system is coupled with a soft X-ray source (Al-kα) that makes it possible to run both laboratorybased XPS and HAXPES in one setup. X-ray sources have individual monochromators, allowing photon energy spreading of 0.45 eV and 0.25 eV (FWHM) for the hard and the soft X-rays. Au 4f7/2 core level peak position at 84.0 eV is used to calibrate the recorded energy scale. The samples for this measurement were moved under N2 in a sealed container from a glovebox at IPVF to the glovebox connected to the HAXPES setup at Hasselt university.

Synchrotron-based HAXPES

In a synchrotron (Figure 1.11), in order to produce an X-ray beam, electrons shot from an electron gun are accelerated first in a linear accelerator (LINAC) and then in the second accelerator called Booster. In the next step, they are injected into a storage ring, where they get energies from 500

MeV to 8 GeV depending on the size of the ring. As the charged particles are bent around the ring (by magnetic devices), energy is dissipated and electromagnetic radiation is emitted from infrared to X-rays. This radiation is highly intense, highly focused, and strongly polarized. Part of this radiation can emerge with energies from 0.1 keV (soft X-rays) to 100 keV (hard X-rays), which is well suited to study different depths of solid materials. The resulting X-rays are each directed toward a beamline, in our case HAXPES beamline, next to the accelerator to use for measurements [START_REF] Chloé | Development of coevaporated perovskite thin films for tandem solar cells applications[END_REF].

Interfaces in This Report

In this section, a background on the topics studied in this report is presented.

Nickel oxide Interface with MHP Top Layer

Among all inorganic hole transport materials that can be implemented in MHP solar cells, in particular p-i-n configuration, NiOx is widely known in the perovskite community and therefore broadly studied. The maximum power conversion efficiency of NiOx-based solar cells has exceeded 22% [START_REF] Isikgor | Molecular engineering of contact interfaces for high-performance perovskite solar cells[END_REF]. This excellent electrical performance besides other decent properties such as a band gap larger than 3.6 eV with a favorable ionization potential (IP) of 5 eV -5.4 eV, high transmittance in the near ultraviolet and visible wavelength, outstanding thermal, chemical, and light stability present NiOx as a promising material for MHP technology. Low-cost, lowtemperature, reproducible, and scalable fabrication processes are the other merits of Nickel oxide.

Numerous approaches have been employed to deposit the thin film of NiOx such as Sol-gel, combustion, spin coating, electrodeposition, pulsed laser deposition, sputtering process, atomic layer deposition, and so on [START_REF] Di Girolamo | Progress, highlights and perspectives on NiO in perovskite photovoltaics[END_REF], [START_REF] Yin | Nickel Oxide as Efficient Hole Transport Materials for Perovskite Solar Cells[END_REF].

Despite all these advantages, NiOx suffers low intrinsic conductivity, considerable defect states in the surface, and interfaces with perovskite and non-optimized energy level alignment in the corresponding interface. These drawbacks influence drastically the open circuit voltage, short [START_REF] Chloé | Development of coevaporated perovskite thin films for tandem solar cells applications[END_REF] circuit current, and fill factor of MHP solar cells. Therefore, it appears crucial to deal with these shortcomings to boost further its performance and expand its application. The doping of the NiOx layer and the surface modification by organic and inorganic molecules are two general approaches to tackle these problems [START_REF] Yin | Nickel Oxide as Efficient Hole Transport Materials for Perovskite Solar Cells[END_REF], [START_REF] Ma | Nickel oxide for inverted structure perovskite solar cells[END_REF].

NiOx/MHP interface in perovskite solar cells is the site where the hole charge carriers are extracted.

However, this process could be interrupted by charge recombination arising from interfacial defects, unoptimized band alignment, and the presence of reactive sites. This issue could be mitigated by surface modification of NiOx and interface engineering using small molecules, amines, excess organic cations, etc. [START_REF] Yin | Nickel Oxide as Efficient Hole Transport Materials for Perovskite Solar Cells[END_REF], [START_REF] Zhumagali | Linked Nickel Oxide/Perovskite Interface Passivation for High-Performance Textured Monolithic Tandem Solar Cells[END_REF].

Boyd et al. [START_REF] Boyd | Overcoming Redox Reactions at Perovskite-Nickel Oxide Interfaces to Boost Voltages in Perovskite Solar Cells[END_REF] used the excess of A-site cations to modify the interface of NiOx(as-deposited and

UV+Ozone treated) and perovskite with the formula of Cs0.25FA0.75Pb(Br0.2I0.8)3. They reported that the NiOx surface reacts with perovskite. Under-coordinated Ni ≥3+ sites on nickel oxide surface reduce to Ni ≥2+ by concurrently oxidizing the I -species and deprotonating the amine precursors (HA + ). The oxidation of I -forms I 3 -which is equivalent to volatile I2 gas and could leave the interface. Consequently, the interface with lead-halide (PbI2-xBrx) surplus and A-site cation deficiency hinders the hole harvesting and increases the interfacial recombination. The corresponding chemical reactions are demonstrated below:

𝐻𝐴𝑃𝑏𝐼 3 ↔ 𝑃𝑏𝐼 2 + 𝐻𝐴 + + 𝐼 - 𝐻𝐴 + ↔ 𝐴 (𝑔) + 𝐻 + 𝐼 -↔ 1 2 𝐼 2 (𝑔) + 𝑒 - 𝑁𝑖 ≥3+ 𝑂 𝑥 + 𝑒 -+ 𝐻 + ↔ 𝑁𝑖 ≥2+ 𝑂 𝑥 𝐻 𝐻𝐴𝑃𝑏𝐼 3 + 𝑁𝑖 ≥3+ 𝑂 𝑥 ↔ 𝑁𝑖 ≥2+ 𝑂 𝑥 𝐻 + 𝑃𝑏𝐼 2 + 𝐴 (𝑔) + 1 2 𝐼 2 (𝑔)
According to them, any interruption in reduction or oxidation results in cutting off the entire reaction. To surpass the process and improve the solar cell performance, they added an excess value of FAI cation into the perovskite precursors solution.

Furthermore, they reported that NiOx UV+Ozone post-treatment presents a higher absorption due to a rise in the sub-band-gap absorption without observing any change in its optical band gap.

Simultaneously, an increase in the work function and ionization energy is found coming from the formation of mid-gap states. UV+Ozone treatment increases the density of under-coordinated

Ni ≥3+ . This leads to an increase in the density of holes and thus a decrease in the occupied density of states at the valence band. Higher number of Ni ≥3+ sites make the surface of NiOx more reactive.

In consequence, a higher concentration of detrimental PbI2-xBrx species is found after perovskite deposition.

Small molecules are extensively utilized to modify the surface of NiOx. The structure of small molecules also addressed as self-assembled monolayers consist of three main parts i.e. the head groups, the spacer, and the tail group. The head group governs the strength of the anchoring to the surface of the substrate. Carboxylic or phosphonic acids are two common head groups. The spacer is mainly a linear aliphatic chain (-CH2-)n and links the head group to the tail group. It influences the organization of the molecules on the surface. The tail group controls the functionality of the final film. Based on its nature (organic/inorganic, electron donator/acceptor, etc.), it can influence the surface features such as wettability, optical properties, energy level offset at the interface, etc [START_REF] Sun | NiOx-Seeded Self-Assembled Monolayers as Highly Hole-Selective Passivating Contacts for Efficient Inverted Perovskite Solar Cells[END_REF].

Considering a large variety of small molecules, three phosphonic acid derivative small molecules based on the 2PACz functional group are chosen and utilized in this work with the aim of modifying the interface of NiOx/MHP. Therefore, with a focus on these types of small molecules, a background of using phosphonic-acid derivative 2PACz-based small molecules is provided in the following.

In The lower performance of the solar cells made on bare NiOx is attributed to the presence of defects on its surface that can react with perovskite and make a detrimental interlayer between NiOx and MHP. Based on XPS measurement, they observed a shift toward higher binding energy for O 1s

and Ni 2P attributed to the dipoles introduced by MeO-2PACz. Further, it is reported that MeO-2PACz molecules are chemisorbed on NiOx via a tridentate bond as shown in Figure 1.12. Also, they argued that small molecules might have a multilayer arrangement on the surface and bond to each other via hydrogen bonds.

In another study, MeO-2PACz is employed as an interlayer between sputtered NiOx and MAPbI3.

Alghamdi et al. [START_REF] Alghamdi | Surface Passivation of Sputtered NiO x Using a SAM Interface Layer to Enhance the Performance of Perovskite Solar Cells[END_REF] did not observe any noticeable change in the optical and morphological properties of nickel oxide after the deposition of the interlayer. Larger grains size and better crystallinity (no PbI2 formation) are found in NiOx/MeO-2PACz samples. In addition, using photothermal deflection spectroscopy a decrease in the level of the defects for the modified samples is observed. In the end, they reported higher device parameters i.e. Jsc, VOC, FF, and PCE for the

MHP Interface with Tin Oxide Top Layer

The stability of the MHP layer, besides its quality, extremely depends on the interfaces formed when it is sandwiched between charge transport materials. Inorganic materials, in particular metal oxide materials, because of better thermal, chemical, and mechanical properties are generally preferred to be used as CTL. However, the charge carrier transport and electrical surface passivation at the interface between inorganic materials and MHP layers are more interrupted in comparison to the organic/MHP interface. This issue becomes worse when the inorganic material is formed directly on top of MHP and it could hinder the stability of the MHP layer through heat and chemical reaction exposure [START_REF] Deng | Modification Engineering in SnO 2 Electron Transport Layer toward Perovskite Solar Cells : Efficiency and Stability[END_REF].

Among inorganic charge transport materials, tin oxide due to its wide bandgap, high optical transmission, good carrier mobility, well-conduction band alignment with perovskites, and high chemical stability has been demonstrated as a perfect ETL for MHP solar cells. Formation of SnO2

does not demand a high temperature that makes it more attractive for industrial fabrication [START_REF] Altinkaya | Tin Oxide Electron-Selective Layers for Efficient, Stable, and Scalable Perovskite Solar Cells[END_REF], [START_REF] Wali | Tin oxide as an emerging electron transport medium in perovskite solar cells[END_REF]. SnO2 could be integrated into both n-i-p and p-i-n solar cells configuration and be deposited utilizing various deposition methods such as sol-gel process, chemical bath deposition, atomic layer deposition (ALD), physical vapor deposition, and so on [START_REF] Uddin | Progress and Challenges of SnO2 Electron Transport Layer for Perovskite Solar Cells: A Critical Review[END_REF].

Among various techniques to grow this material, atomic layer deposition (ALD) is more popular because it is an industry-compatible, thickness-controllable, and low-process technique fabrication method. Although the integration of SnO2 into the n-i-p configuration in direct contact with MHP has demonstrated decent performance, its employment in p-i-n architecture still suffers malfunctioning. The detrimental chemical reaction between ALD precursors and MHP is the source of this problem which could be addressed by adding an organic interlayer such as C60 or PCBM between the SnO2 and MHP layers. However, removing organic material and depositing SnO2 directly on MHP will open a new door beyond where there are technical, scientific, and economical interests for the researchers and industry. For this purpose, understanding the interface formation process at MHP/SnO2 with and without the presence of an organic layer is the key to unlock this door [START_REF] Hultqvist | SnO x Atomic Layer Deposition on Bare Perovskite An Investigation of Initial Growth Dynamics[END_REF]. In this regard, several works have been previously performed with a focus on using the photoemission electron spectroscopy technique to study the interface of MHP/SnO2.

In 2018. Palmstrom et al. [START_REF] Palmstrom | Interfacial Effects of Tin Oxide Atomic Layer Deposition in Metal Halide Perovskite Photovoltaics[END_REF] study the interface between FA0.83Cs0.17Pb(I0.83Br0.17)3 and ALD-SnO2 with and without LiF(1 nm)/C60 (10 nm) interlayer. The ALD-SnO2 process is performed using tetrakis(dimethylamino)tin(IV) (TDMASn) and water at 100 °C and 150 °C. The perovskite layer with and without LiF/C60 capping layer showed thermal stability after ALD process at 100 °C. Whereas, the formation of PbI2 is observed after the ALD-SnO2 process at 150 °C for both perovskites with and without interlayer and is mainly attributed to the effect of TDMASn.

According to the XPS measurement, they reported no change in cesium and iodine composition with and without interlayers. However, as shown in Figure 1.13, a loss of nitrogen contribution is observed. They attributed this loss to the removal of organic formamidinium cation due to an interaction between TDMASn and perovskite at elevated ALD temperatures [START_REF] Palmstrom | Interfacial Effects of Tin Oxide Atomic Layer Deposition in Metal Halide Perovskite Photovoltaics[END_REF]. (a) (b) (c)

MHP Interface with Carbone Nanotubes

Carbone nanotubes (CNTs), thanks to their excellent properties such as excellent charge transport features and chemical inertness, are applied in various industrial and scientific fields. CNTs depending on their synthesis method and the shape of the tubes are categorized as single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes [START_REF] Hussain | Functional materials , device architecture , and flexibility of perovskite solar cell[END_REF].

Single-walled carbon nanotubes (SWCNTs) are hollow tubes made from wrapped graphene sheets. Depending on how the graphene sheet is rolled, SWCNTs could act either as metal or semiconductors [START_REF] Mceuen | Single-Walled Carbon Nanotube Electronics[END_REF] (see Figure 1.16). Thanks to their unique optoelectronic properties,

semiconductor SWCNTs show a strong potential to be integrated into various architectures of perovskite solar cells [START_REF] Habisreutinger | Carbon Nanotubes in Perovskite Solar Cells[END_REF]. The high chemical and mechanical stability of this material introduce them as an interesting material in electronic technology in which the long-life device has great importance. SWCNTs with high charge carrier mobility improve carrier collection which in turn, enhances halide perovskite solar cell performance. In addition, their high optical transmission in In another work, the MHP solar cells utilizing SWCNT as an interlayer between the absorber layer and spiro-OMeTAD presented a PCE improvement from 17.4% to 20%. This better performance is the result of a reduction in interfacial recombination via the faster hole extraction ability of SWCNTs [START_REF] Blackburn | Rapid Charge-Transfer Cascade through[END_REF]. Additionally, employing SWCNT as a counter electrode without an HTL layer directly atop of perovskite absorber yield a maximum PCE of 11% in the n-i-p structure [START_REF] Manuscript | [END_REF].

Overview

In this chapter MHP material and its crystal, band, and compositional structures are described. The working principle of MHP solar cells is explained. The interfaces of MHP/CTLs are introduced as one of the main detrimental sources against device performance. We explained that the perfection and periodicity of the MHP crystalline lattice as well as the energy level alignment in these interfaces could be interrupted. The surface of metal oxide layers and their adjacent interfaces with MHP are demonstrated as defective sites. Integration of interlayers is an effective method to mitigate and modify their imperfection. The basics of photoemission spectroscopy in sealed by PMMA [START_REF] Habisreutinger | Carbon Nanotube/Polymer Composites as a Highly Stable Hole Collection Layer in Perovskite Solar Cells[END_REF] general and UPS, XPS, and HAXPES, in particular, are explained. In the end, a review of the previous studies relevant to the interface systems investigated in this report is provided.

 Thermal co-evaporation of MAPbI3

In collaborative work with Laboratoire de Physique des Interfaces et des Couches Minces

(LPICM) at École Polytechnique and in particular in chapter 3, we employed a thermal coevaporation system to grow thick (450 nm) and thin (10 nm & 20 nm) film of MAPbI3 following two targets of making solar cells and study the interface between NiOx and MAPbI3. To do so, we perform the co-evaporation in a system from RIBER company equipped with three sources, and a single Quartz Crystal Microbalance (QCM) sensor is used. After loading samples in the vacuum chamber, we launch the primary and turbo pump to obtain a vacuum level of around 2 × 10 -6 𝑚𝑏𝑎𝑟. Then we switch on the heating sources containing PbI2 and MAI precursors and raise their temperature to 298-300 °C and 176-180 °C, respectively. When an overall deposition rate of 1 Å/s achieved, we open the shutter, and the evaporation starts. It should be mentioned that from starting point of precursors sublime until reaching to desired deposition rate the pressure of the vacuum chamber increase to the magnitude of 5 -6 × 10 -5 𝑚𝑏𝑎𝑟. We keep this value of pressure as well as deposition rate constant over the deposition process by controlling the temperature of each source until the target thickness is grown. No post-annealing process is done on the MAPbI3 layer.

 Spin coating of MAPbI3 and MAPbBr3

In chapter 4 in the frame of collaborative work, MAPbI3 and MAPbBr3 are prepared at National Renewable Energy Laboratory (NREL) in the U.S. To prepare MAPbI3 and MAPbBr3 solutions, respectively MAI: PbI2 (molar ratio of 1:1) and MABr: PbBr2 (molar ratio of 1:1) are dissolved in γ-Butyrolactone (GBL)/DMSO (7:3 v:v). Perovskite solution is spin-coated on FTO/TiO2 substrate at 4500 rpm for 50 seconds. Toluene as an anti-solvent is dripped on the samples. Then the samples are moved to a hotplate to be annealed at 85 °C for 10 minutes.

Electron Transport Layer (ETL)

Titanium Dioxide (TiO2)

Among different n-type electron transport layers integrated into the perovskite with different architectures, TiO2 is the most commonly used material which is mainly employed in n-i-p MHP solar cells. Forming a TiO2 layer with high crystallinity is the key factor to reach high electron mobility and low density of trap state and consequently remarkable electrical conductivity.

Nevertheless, a remaining basic drawback is that it requires a high-temperature time-consuming process of 400-500 °C. Reducing the stability of perovskite solar cells under UV illumination is the other disadvantage of this layer. This issue could be addressed by using different interfacial layers [START_REF] Valadi | Metal oxide electron transport materials for perovskite solar cells: a review[END_REF], [START_REF] Wang | Metal oxide charge transport layers in perovskite solar cells-optimising low temperature processing and improving the interfaces towards low temperature processed, efficient and stable devices[END_REF]. Spin-coating, spray-pyrolysis, atomic layer deposition, thermal oxidation, sputtering, chemical vapor deposition, etc. are techniques to deposit TiO2 [START_REF] Huang | The Investigation for Coating Method of Titanium Dioxide Layer in Perovskite Solar Cells[END_REF].

In chapter 4 of the current thesis dissertation, a compact layer of TiO2, as ETL, is deposited by spray pyrolysis method using 0.2 M titanium diisopropoxide bis(acetylacetonate) in 1-butanol solution at 450 °C. This process is done at NREL as a part of a collaborative project with IPVF and HZB.

Tin Oxide (SnO2)

Tin oxide as an inorganic electron transport layer is previously introduced in chapter 1. We explained that ALD technique is commonly used to form SnO2. However, there are still some challenges regrading growing this material in p-i-n structure that needs to be tackled.

In chapter 5, we grow SnO2 as ETL in p-i-n MHP solar cells using the thermal atomic layer deposition (ALD) method (see 2.1.1). To do so, we used TDMASn (Tetrakis(dimethylamido)tin(IV)) and H2O2 precursors in a Beneq TFS-200 ALD reactor at a deposition temperature of 100°C and pressure of 1-3 mbar.

Zinc oxide Nanoparticles (ZnO)

Zinc oxide is another ETL which represents a high-potential alternative for TiO2 thanks to its wide band gap and excellent electron mobility. ZnO can be integrated into both p-i-n and n-i-p configurations. ZnO nanoparticles liberated from the sintering process can be deposited by spin or spray. This makes them attractive ETL for flexible solar cells as well as large-scale fabrications [START_REF] Dehghan | Deposition of zinc oxide as an electron transport layer in planar perovskite solar cells by spray and SILAR methods comparable with spin coating[END_REF].

We purchase ZnO nanoparticle ink from Sigma-Aldrich and utilize it in chapter 3 to fabricate p-in MHP solar cells. 40 µL ZnO ink is dynamically deposited on PCBM layer by spin coating method at 4000 rpm for 30 seconds with the acceleration rate of 1000 rpm/sec followed by 10 min annealing at 100 °C to remove residual solvent species. During deposition, the ink was stirred continuously.

PCBM

Phenyl-C61-butyric acid methyl ester (PCBM) is a soluble form of fullerenes that are used widely as organic ETLs in p-i-n MHP solar cells. Belonging to electron acceptor groups (Lewis acid), PCBM is a suitable ETL material as well as a passivation layer for perovskite absorber film by interacting with the under-coordinated halides which in turn could hinder the halide-ion migration in perovskite [START_REF] Said | Recent Progress in Organic Electron Transport Materials in Inverted Perovskite Solar Cells[END_REF].

In our research study, PCBM is employed to have both passivation electron layer roles coupled with ZnO nano particles in chapter 3 and SnO2 in chapter 5. To form PCBM layer in chapter 3, 20 mg PCBM is dissolved in 1 mL of anhydrous Chlorobenzene. The prepared solution is stirred for 3-4 hours. Then 30 minutes before starting PCBM deposition, the solution is heated up at 60 °C and kept at this temperature during the deposition. In the next step, 40 µL is spin-coated at 2000 rpm for 40 secs on the perovskite film. In the end, the samples are moved to the hot plate to be annealed at 60 °C for 10 minutes. In chapter 5, the solution with the same concentration is prepared.

However, it does not warm up before deposition. In addition, the samples are annealed at 100 °C.

The spin coating speed and acceleration rate remain unchanged.

Hole Transport Layer (HTL)

Nickel Oxide (NiOx)

NiOx is the most known p-type metal oxide material broadly used in p-i-n MHP solar cells. The properties, cons and pros, and its fabrication technique were explained previously in chapter 1.

Regarding its industry-compatible fabrication techniques, NiOx is employed as HTL layer in the perovskite solar cell being developed at IPVF. In this study work, with the aim of addressing its drawbacks such as defect states on its surface and energy level alignment in its interface with MHP, surface modifier molecules are employed.

In the current Ph.D. study, we employed the ALD technique to grow 15 nm of NiOx on Glass/FTO substrates. After that, the Glass/FTO substrates are cleaned in an ultrasonic bath in three steps: in RBS (2%) for 30 minutes, in Acetone for 20 minutes and finally in isopropanol for 20 minutes.

The substrates are then dried by an N2 flow gun. Prior to the ALD process, the samples undergo UV+Ozone treatment for 20 minutes to get rid of any residual organic compounds. In the next step, the substrates are moved to Beneq TFS-200 ALD reactor. The deposition is performed at a temperature of 160 °C and a pressure of 1.2-1.5 mbar. Ni(amd)2 (full name: Bis(N,N'-di-tbutylacetamidinato)nickel(II)) and H2O are used as precursors. To deposit 15 nm NiOx, 250 cycles with the sequence of Ni(amd)2 (Pulse)/N2 (purge)/H2O (Pulse)/ N2 (purge) in a pulse and purging time of 1.9s/5s/0.8s/5s are performed.

Single-Walled Carbon Nanotubes (SWCNTs)

SWCNTs are introduced in chapter 1. This material is used as HTL in n-i-p structure in chapter 4.

The SWCNT film is prepared by ultrasonic spray deposition at NREL. This process is done in different steps. First SWCNTs are dispersed in 2.5mg/ml polyfluorene Bis-Pyridine (PFO-BPy) in toluene. Then the excess polymer is removed by centrifuging the solution for 20 h at 24 100 rpm and 0 °C. This process is repeated until a solution with the mass ratio of 1:1 (SWCNT:PFO-BPy) is obtained. Subsequently, 10 nm and 20 nm thickness of SCWNT are formed via spray pyrolysis technique on the substrates.

Phosphonic Acid Derivatives Small Molecules

The small molecules are introduced in chapter 1. In light of those explanations, we decided to

Fabrication Techniques

Atomic Layer Deposition (ALD)

Atomic layer deposition (ALD) is an industry-compatible deposition method that is widely used to grow thin films particularly metal oxides materials such as TiO2, SnO2, and NiOx. The major characteristic of this technique is the self-limiting surface reaction as a result of controlled alternating of precursor and co-reactant. In general, ALD process is composed of four steps, see

Erreur ! Source du renvoi introuvable.:

1) Exposing the precursor in order to adhesion its components to the substrate and form the chemical bonds between them

2) Purging the excess precursor as well as the byproduct left from the first step

3) Injecting the co-reactant (mainly oxidants or other reagents) into the chamber in order to expose it to an as-deposited precursor and form the reaction Beneq TFS-200 ALD reactor is used to deposit NiOx and SnO2 in chapters 3 and 5, respectively.

Spin Coating

Spin-coating is the most common and simplest technique to grow thin films in lab-scale fabrications.

In this method, shown in Figure 2.3, a certain amount of a solution is statically or dynamically dropped on a substrate that is stationed on the stage of the spin coater system. Then, spinning the substrate at an applied speed during chosen time spreads the solution on the substrate. The thickness of the final film strongly depends on the speed of rotation and the nature of the substrate. This technique is suitable for the research and development process.

Thermal Co-evaporation of Perovskite

Among all the existing methods to form a perovskite absorber layer, thermal co-evaporation is presented as a game changer in the industrial fabrication of MHP solar cells due to its high process and thickness controllability and decent reproducibility. The films produced by this approach are quite smooth, compact and free from any pin-holes [START_REF] Momblona | Co-evaporation as an optimal technique towards compact methylammonium bismuth iodide layers[END_REF]. In this technique, different precursors of perovskite are loaded in various crucibles and are simultaneously heated up in a high vacuum chamber [START_REF] Vaynzof | The Future of Perovskite Photovoltaics-Thermal Evaporation or Solution Processing?[END_REF] (see Figure 2.4). As soon as the temperature arrives at the sublime point of each precursor, evaporation starts. After reaching the desired deposition rate, which has a significant impact on the stoichiometry and morphology of the final perovskite film, the covering shutter between crucibles and substrates gets open and perovskite film with a desired thickness is formed.

As a part of collaboration work with LPICM, as discussed in section 2.1. MAPbI3 film in chapter 3 is vacuum deposited by using the co-evaporator system of RIBER company.

Characterization Techniques

Ultraviolet-Visible Spectroscopy (UV-Vis)

Ultraviolet-Visible (UV-Vis) spectroscopy is a straightforward analytical technique to measure the absorbance, transmittance, and reflectance of matter as a function of wavelength in the range of 200-1100 nm. Knowing the range of UV (100-380 nm) and visible (380-800 nm) spectra, a big part of spectrophotometers covers wavelengths higher than 800 nm (infrared spectrum) as well [85]. This technique functions based on the attenuation of electromagnetic light when passes through an absorbing matter. The intensity of the reflected or transmitted ray from the substance is collected by a detector. The detector compares this intensity with the one of incident light and the output of the instrument is the value of this ratio in different wavelengths [START_REF] Passos | Detection in UV-visible spectrophotometry : Detectors , detection systems , and detection strategies[END_REF].

During this work, we utilized Agilent Cary 5000 spectrophotometer to measure the transmission and reflection in the range of 350 m -800 nm. Subsequently, the absorption is calculated using the following equation:

𝐴(𝜆) = 100 -𝑅(𝜆) -𝑇(𝜆) (2.1)
The setup equips two different light sources: a deuterium arc UV source up to 300 nm and a tungsten halogen source for a higher wavelength than 300. The samples are measured by exposing them to the light from the top side.

X-ray Diffraction (XRD)

X-ray diffraction (XRD) is an essential technique to study the crystal structure of materials. This approach is based on the periodic atomic arrangement. In this instrument, the collimated monochromatic X-rays emitted from a cathode tube hits the samples and consequently, the constructive interference from the scattered beam is collected by a detector. The constructive interference is produced only in the case that the interaction between the incident beam and lattice planes of matter obeys Bragg's law [START_REF] Gumustas | Effect of Polymer-Based Nanoparticles on the Assay of Antimicrobial Drug Delivery Systems[END_REF][88]:

𝑛𝜆 = 2𝑑𝑠𝑖𝑛𝜃 Eq. (2.2)
Where 𝜆 is the wavelength of the X-ray, d is the interplanar spacing causing diffraction, and 𝜃 is the diffraction angle of the scattered beam.

The output of the instrument which is known as a diffractogram provides the information in two

x-axes and a y-axis representing Bragg angle (2θ) and scattered intensity, respectively. Various parameters and information such as average grain size, crystallinity orientation, and phases [START_REF] Bunaciu | X-Ray Diffraction: Instrumentation and Applications[END_REF].

We used the Panalytical analyzer for XRD characterizations and the Highscore software package to treat the data and remove the background.

Atomic Force Microscopy (AFM)

AFM is a powerful technique to probe and acquire topographical properties such as roughness of the surface in the range of nanometers. AFM system is composed of four main pieces: (1) A probe in the shape of a cantilever on which a tip is mounted (2) A laser source (3) a quadrant photodetector (4) a scanner (see Figure 2.7). For the measurement, the laser beam is focused on [START_REF] Ermrich | XRD for the analyst[END_REF] the back side of the cantilever. While the tip is scanning the surface of the sample, the reflected light from the cantilever is collected by the photodetector. In general, there are three differed modes of AFM technique: in-contact, noncontact and intermittent. In the case of an in-contact tip with the surface, called contact mode, the static bending of the cantilever changes. In the case of a vibrating tip at a short distance from the surface (noncontact mode), the topography of the sample is achieved by the changes in the resonance frequency of the cantilever. In the intermittent mode, also called tapping mode, a small piezo is commonly used to oscillate the cantilever at or close to its resonance frequency. This oscillation usually has an amplitude from several nm to 200 nm. In this mode, the cantilever is driven by a signal at constant frequency and amplitude. This keeps the amplitude of the cantilever oscillation unchanged as long as no interaction occurs with the surface.

As the tip gets closer to the surface of the sample, the interaction of forces existing on the cantilever varies causing the amplitude of the cantilever's oscillation changes. The amplitude value is utilized to adjust the height of the cantilever above the surface. In this way, as the cantilever scan the surface, a set amplitude of cantilever oscillation is maintained by controlling the height. [START_REF] Geisse | AFM and combined optical techniques[END_REF], [START_REF] Abou-Ras | Physics of Solar Cells Thin Film Solar Cells Handbook of Photovoltaic Science and Engineering[END_REF].

We used an Agilent Technologies 5600LS intermittent mode AFM system to measure the roughness of the samples, with an ACL tip form AppNano.

Scanning Electron Microscopes (SEM)

SEM is a vastly used advanced characterization technique to study the morphological and compositional features of the samples by providing high-resolution magnified images. In this technique, a low-energy electron beam emitted from an electron gun moves toward the specimen.

The beam gets focused by passing through a series of lenses and apertures under vacuum [START_REF] Abou-Ras | Physics of Solar Cells Thin Film Solar Cells Handbook of Photovoltaic Science and Engineering[END_REF] conditions. The interaction between the irradiated beam and the atoms in the target surface generates signals in the form of secondary, backscattered, and diffracted backscattered electrons.

The secondary electrons provide more information from the morphological properties of the surface while the backscattered electrons are quite sensitive to the difference in atomic number and can contrast the composition of the surface. A detector creates and displays a high-resolution image on the computer by collecting these signals from electrons [START_REF] Akhtar | Scanning Electron Microscopy : Principle and Applications in Nanomaterials Characterization[END_REF]. A schematic skim of an SEM system is shown in Figure 2.8.

The SEM images in this work are obtained using Zeiss Merlin VP compact 35 field emission gun (FEG) scanning electron microscope.

Kelvin Probe (KP)

The main application of this technique is to measure the work function (WF) of a surface and its changes under illumination called surface photovoltage (SPV).

On the surface of a semiconductor, defect states that are called surface states exist. These defects states usually get partially or entirely charged by free carriers coming from the bulk. Consequently, the concentration of the free carriers near the surface is different from the bulk counterpart. This leads to appear a space charge region near the surface. This space charge region has an electric field. Depending on the sign of the charge in the space the energy bands are bent upward or downward. Therefore, the surface potential 𝑉 𝑠 is always different from the potential of bulk. Under illumination, the excess charge carriers are generated in the surface charge region or at a diffusion length from it. These carriers depending on their sign are driven either toward the surface or the bulk. In this way, a considerable amount of charge carriers is exchanged or redistributed between or within the bulk and the surface. The difference in the charge distribution near the surface changes the electrical potential distribution and thus the surface potential. This change in the surface potential is SPV.

SPV quantity (𝛿𝑉 𝑠 ) is realized by finding the difference between measured surface potential under dark and light conditions (𝑉 𝑠 and 𝑉 𝑠 * respectively) using EquationEq. (2.3) Where WFs and WFm are the work function of the surface and metal, respectively [START_REF] Donchev | Surface photovoltage spectroscopy of semiconductor materials for optoelectronic applications Surface photovoltage spectroscopy of semiconductor materials for optoelectronic applications[END_REF].

𝛿𝑉 𝑠 = 𝑉 𝑠 * -𝑉 𝑠 Eq. (2.3)
We used this technique in chapter 3 to measure the work function and surface photo voltage of samples. In this order, a Kelvin probe system (Besocke Delta Phi: Kelvin Control 07 and Kelvin Probe S) is employed to measure the WF and SPV. The samples are illuminated by a 530 nm LED from Thorlabs (M530L2) using a focused beam. The beam intensity is around 30mW/cm 2 . The contact potential difference is measured by the KP system. This value is the difference between the work function of the tip and the sample. In order to find the WF of the tip, an evaporated gold sample with a work function of 4.8 is used as a reference sample. CPD variation over time is measured using a multimeter (Keithley 2635Bé) which is connected to the CPD output of the KP setup.

Hyper-Spectral Imaging of Photoluminescence

Photoluminescence (PL) spectroscopy is a contactless optoelectronic technique that is broadly used to probe charge carriers' recombination and transport properties. In this technique, the active layer is illuminated and excited by photons coming from a light source and having energy larger than the band gap of the film. The absorption of the photons generates electron-hole pairs. The excited electrons and holes will relax to the conduction band edge and valence band edge, respectively, losing part of their energy as heat. The electrons and holes are then recombined radiatively and in turn, photons will be emitted. Recording theses photons by a detector generates PL spectroscopy. The position of the PL peak is indicative of the band-gap and its intensity represents the radiative charge carrier recombination in the film and carrier collectivity properties at contacts. Additionally, the presence of trap states as well as the charge transfer process in the bulk and interlayers and charge carrier lifetime can be probed by different types of PL spectroscopy techniques [START_REF] Abdi-Jalebi | Optical absorption and photoluminescence spectroscopy[END_REF].

Hyperspectral imaging is one of the derivatives of PL spectroscopy that provide an image from the PL illumination of the specimen in a range of excitation wavelengths. This method benefits from uniform illumination allowing the study of the sample under a state close to the real operating condition [START_REF] El-Hajje | Quantification of spatial inhomogeneity in perovskite solar cells by hyperspectral luminescence imaging[END_REF]. To run this measurement, we used in this work a setup composed of a hyperspectral imager (Grand-EOS from Photon etc.) and an sCMOS camera (Orca-Flash 4.0 from Hamamatsu) that allows capturing large-size images (160 mm × 160 mm) in the wavelength range of 400-900 nm and a spectral resolution of 2 nm.

I-V Measurements

Current-voltage (I-V) measurement is a widely known technique to measure the electrical properties of a solar cell under simulated natural sunlight illumination. A solar simulator is used to produce light at different intensities. This measurement is performed by applying voltage in forward and reverse bias and the flowing current in the solar cell is measured. Additionally, a solar cell behaves like a diode under dark conditions. Subsequently, the recorded voltage and current curve (I and V) is fitted with a single-diode model according to Eq. (2.5).

𝐼 = 𝐼 0 [exp ( 𝑉 + 𝐼𝑅 𝑠 𝑛𝑉 𝑇 ) -1] - 𝑉 + 𝐼𝑅 𝑠 𝑅 𝑆𝐻 Eq. (2.5)
Where n is the diode ideality factor (unit less value with 1-2 range for a single junction solar cell), 𝐼 0 is the reverse saturated current of the diode, 𝑉 𝑇 is the thermal voltage equal to 𝑘𝑇 𝑐 𝑞 ⁄ (with 𝑘 as Boltzmann's constant and q as the elementary charge) 𝑅 𝑆𝐻 and 𝑅 𝑆 are the shunt and series resistance. In Figure 2.10, we see an illustration including the I-V curve (red line) and power curve (blue line)

and their parameters. Having these parameters, one can calculate the power conversion efficiency of a solar cell using Equation Eq. ( 2 Concerning I-V measurements in this work, solar cells are illuminated through the glass side using an AAA sun simulator of Oriel Sol3A, with an aperture area of 0.09 cm 2 . A bias of -0.1 V to 1.1

V is forwardly and reversely applied to the devices using a digital source meter (Keithley 2400)

External Quantum efficiency (EQE)

The external quantum efficiency is a technique to see solar cell characteristics in their wavelength range. EQE defines as the ratio of the collected charge carriers by a solar to the total of incident photons on the solar cell. In an ideal solar cell, the EQE value is 100% in all the wavelengths lower than the band gap edge. However, there are different barriers to overcome to achieve this ideality, shown in Figure 2.11, among which the recombination effect in the bulk of absorber and corresponding interfaces plays a major role. The recombination in the front interface to the light source impacts the charge carrier collection in the blue range while recombination in the back interface hinders the charge carrier collectivity in the red light region.

Short-circuit current value (𝐽 𝑠𝑐 ) can be extracted from the EQE spectrum following Equation Eq.

(2.7):

𝐽 𝑠𝑐 = -𝑞 ∫ 𝐸𝑄𝐸(𝜆)𝜙(𝜆)𝑑𝜆 ∞ 0 Eq. (2.7)
Where q is the elementary charge, 𝜙(𝜆) is the density of the incident photon spectra density.

To perform this measurement, we used an Oriel IQE200 system coupled with a source meter (Keithley 2400) and a 300W xenon lamp. The perovskite solar cells are measured in the range 300 nm to 850 nm. The short circuit current is calculated using the EQE spectra.

Overview

In this chapter, we discussed the different materials we employed in the Ph.D. work to fabricate samples and devices. We also introduced three foremost deposition methods (ALD, spin coating, and thermal co-evaporation) we utilized to grow thin films at IPVF as well as in collaboration with

Introduction

In chapter 1, we explained that the nickel oxide hole transport layer despite its favorable features such as high band gap, transparency, and stability, possesses some non-ideal material properties.

The presence of defect states at the surface and potentially mismatch in the energy level alignment with adjacent perovskite films are two main detrimental issues that require a dedicated investigation. Generally, these issues can be addressed by using surface modifier molecules atop of NiOx layer. As it is previously explained in chapter 1, employing interlayers between NiOx and MHP layers has improved the solar cell performance by passivating the defect states and optimizing the energy level alignment at the corresponding interface [START_REF] Phung | Enhanced Self-Assembled Monolayer Surface Coverage by ALD NiO in pi -n Perovskite Solar Cells[END_REF], [START_REF] Zhu | Nano Energy Inverted planar heterojunction perovskite solar cells with high ultraviolet stability[END_REF], [START_REF] Li | Flexible all-perovskite tandem solar cells approaching 25% efficiency with molecule-bridged hole-selective contact[END_REF], [96]. Another important topic in the NiOx domain is the effect of UV+Ozone post-treatment on film properties and also on device performance. As we discussed earlier UV+O3 treatment induces the surface reactivity of NiOx and could have detrimental effects on device performance. However, its impact on the properties of the perovskite absorber layer and the performance of the cells has not been entirely investigated.

Regarding these issues, in the first section of this chapter, we aim to understand more about the surface of as-deposited ALD-NiOx and the influence of UV+O3 post-treatment. To this end, their optical, chemical, and electronic properties are studied and compared. It should be mentioned that the term "NiOx" will be used in this chapter to address and describe "as-deposited NiOx" for the sake of simplicity. In the next step, three

Optical Properties

The optical properties of the NiOx layers with and without UV+O3 treatment are investigated using UV-vis spectroscopy. Transmission and reflection measurements are performed by illuminating the samples from the NiOx side.

Chemical and Electronic Properties

To further expand our understanding of the ALD-NiOx films and the effect of the UV+O3 treatment on the surface properties, I performed a set of XPS and UPS measurements. These measurements help us to evaluate the chemical changes of the surface environment as well as to determine the surface energetics and energy level diagram of the samples. To explain these observations, first, we should take a look at the reaction that occurs in the UV-O3 process. During this treatment, O3 molecules are combined by a photo-reaction process with the H2O originating from moisture in the air and/or remaining ALD precursors, resulting in the formation of hydroxide functional groups on the surface [START_REF] Noonuruk | Ozone-Induced Optical Density Change of NiO Thin Films and Their Applicability as Neutral Optical Density Filter[END_REF]:

𝑂 3 (𝐻 2 𝑂) + ℎ𝜈 →2OH+O2
Subsequently, Ni 2+ from NiO species react with the O2 that results in Ni 3+ and O -formation [START_REF] Carley | Defects in Oxide Overlayers at Nickel Single-Crystal Surfaces[END_REF]:

𝑁𝑖 2+ (𝑠) + 1 2 𝑂 2 (𝑔) → 𝑁𝑖 3+ (𝑠) + 𝑂 -(𝑠)
Therefore, our observation from the O 1s and Ni 2p3/2 signals can be explained based on the products of the above reactions. The higher contribution of hydroxide-containing chemical states in Ni 2p3/2 after UV+O treatment could be attributed to the formation of Ni 3+ species which can result in an increase in the concentration of NiOOH. Higher intensity of "OH -and Ovacancies" photopeak is due to higher hydroxide-based nickel species as well as the O -species that are located in the vicinity of nickel vacancies. Also, researchers have shown that UV+O3 increases the density of defects, i.e. Ni vacancies on the NiOx surface [START_REF] Boyd | Overcoming Redox Reactions at Perovskite-Nickel Oxide Interfaces to Boost Voltages in Perovskite Solar Cells[END_REF], [START_REF] Sun | A low-temperature-annealed and UV+Ozone-enhanced combustion derived nickel oxide hole injection layer for flexible quantum dot light-emitting diodes[END_REF] which is in agreement with the rise of O -contribution.

Furthermore, the rise in photo peak of physisorbed species might also be attributed to the adsorption of the H2O and O2 molecules [START_REF] Noonuruk | Ozone-Induced Optical Density Change of NiO Thin Films and Their Applicability as Neutral Optical Density Filter[END_REF]. Using the latter, the conduction band minimum is not measured directly but its position is instead projected using VBM and Eg quantities. This procedure results in a good representation of the single-particle or transport gap of the NiOx semiconductor film which is almost the same as the optical gap since, unlike in organic materials or materials with quantum confinement (e.g. carbon nanotubes), the exciton binding energies are negligible.

In

Surface Modification of ALD-NiOx by Phosphonic-acid Derivative Molecules

In this section, I employ three small organic molecules of the phosphonic acid derivatives with the aim of modifying the surface of the as-deposited and UV+O3-treated nickel oxide films. This analysis comprises the characterization of the NiOx materials properties assessed for the eight samples using AFM, UV-vis spectroscopy, UPS, XPS, HAXPES, AFM, and KP techniques.

Morphological Properties

In order to study whether the deposition of molecular surface modifiers changes the morphological properties of the NiOx layer, AFM characterization is performed. NiOx which is a negligible deviation value. This signifies that the anchoring of these molecules does not make a significant change in the surface topography of NiOx. This also suggests that the deposition of the molecular layer happens in a conformal manner without major regions of aggregates. However, in order to confirm this observation quantitative measurements on a larger scale would need to be performed.

Optical Properties

In the next step, I investigated the impact of the grafted surface modifiers on the optical properties of NiOx and NiOx(UV+O3) layers.

The transmission and reflection spectra of the eight various layers are given in Figure 3.6(a). For the UV-vis measurements, samples are illuminated via the front side, i.e. the light hits the sample surface first. The comparison of the reflectance of the layers shows a slight increase after the anchoring of the organic molecules. In addition, in the magnified inset, we also detect a shift, which further attests to the successful deposition of a molecular layer that modulates optical interference. Regarding the transmittance spectra, the samples without UV+O3 post-treatment demonstrate a higher degree of light transmission in comparison to the samples after UV+O3 treatment. In this regard, the bare NiOx, NiOx/M2P, NiOx/2P and NiOx/B2P respectively present almost 5%, 3%, 3% and 1% higher transmission over the entire wavelength range of 300 nm-800 nm compared to the UV+O3 treated ones. Aside from this expected behavior, an interesting trend is found when comparing the transmission of UV+O3-treated samples with and without top layers. Despite the addition of another layer within the whole stack of samples by anchoring the small molecules, we detected a higher transmission. This behavior can be explained by the modification that the molecules induce on the UV+O3 treated NiOx and the fact that the deposited layers are ultrathin on the order of only several monolayers.

The Tauc plots of all eight samples are drawn in

Chemical and Electronic Properties

To gain more insight into the mechanism of growing the thin layers of small molecules on NiOx and NiOx(UV+O3) surfaces the following section discusses the XPS, UPS, and KP measurements of the samples. At the same time, these results describe the chemical and electronic properties of The normalized C 1s XPS spectra of surfaces grafted with organic small molecules, shown in Figure 3.9 (c) & (d), contain some information that is not simply observable in the as-acquired spectra. The main C 1s peak of B2P and 2P, in both cases on NiOx and on NiOx(UV+O3), are positioned at a binding energy that is 150-200 meV lower with respect to their M2P counterpart.

This shift is related to the different electronegativity of the molecules particularly in their tail group. Electron-donating methoxy group in M2P [START_REF] Mo | Calculation of Intrinsic Group Electronegativity , Hardness , and Softness[END_REF], [START_REF] Satoh | electron-withdrawing groups in acidic aqueous solutions[END_REF] and electron-withdrawing bromine in B2P could modify the electronic delocalization in the carbazole ring [START_REF] El Hassani | DFT Theoretical Study of 5 -( 4 -R -Phenyl )-1 H -tetrazole ( R = H ; OCH 3 ; CH 3 ; Cl ) as Corrosion Inhibitors for Mild Steel in Hydrochloric Acid[END_REF] [109] and in turn, affect the kinetic energy of the electrons in the carbazole ring.

Other information that can be extracted from normalized spectra is related to the C-Br bond in Br-2PACz. Regarding the molecular structure of Br-2Pacz and 2PACz, the only difference is the presence of two bromine elements bonding to the carbazole rings. Therefore, this C-Br contribution should be observed in C 1s spectra. To show this contribution the 2P spectra are subtracted from B2P one and the resulting curve is illustrated in Figure 3. After the discussion of the chemical changes to the NiOx surfaces with the surface modifiers, we now turn to the assessment of the surface energetics and electronic properties. However, the increase of WF on NiOx(UV+O3) after 2P and B2P formation is not as significant as the as-deposited NiOx, with an increase of 0.25 eV and 0.43 eV to 5.04 eV and 5.22 eV, respectively. In contrast to M2P, the deposition of 2P and B2P on NiOx and NiOx(UV+O3) leads to an increase in IE with the values of 5.95 eV(NiOx/2P), 5.84 eV (NiOx(UV+O3) /2P), 6.19 eV (NiOx/B2P)and 6.15 eV (NiOx(UV+O3) /B2P). The decrease in the IE value of M2P and the increase in the one of 2P follow the similar behavior that was previously reported by Li et al [START_REF] Li | hole-selective contact[END_REF].

The dipole moment effect of Br-2PACz could lead to an unsuitable electrostatic potential on the surface of NiOx and thus high IE value that results in an unfavorable band alignment [START_REF] Li | hole-selective contact[END_REF]. The However, as a first assumption, one can guess that M2P creates a negative total dipole moment on both NiOx and NiOx(UV+O3) surfaces which leads to a lower IE. In the case of 2P and B2P, this parameter could be positive with a higher value for B2P.

To study how the UV+O3 process and the deposition of the small organic molecules affect the surface potential of nickel oxide, we performed further measurements in the collaboration with

Davide Regaldo. Figure 3.12 shows the contact potential difference (CPD) evolution over time.

According to the results, the CPD of UV+O3 treated NiOx changes quite drastically and rapidly over time compared to the as-deposited NiOx. It signifies that the surface potential of nickel oxide becomes unstable after UV+O3 treatment. This suggests that chemical changes induced by UV+O3

are not stable on bare NiOx(UV+O3) likely because the water and oxygen species adsorbed during UV+Ozone treatment are desorbed over time [START_REF] Noonuruk | Ozone-Induced Optical Density Change of NiO Thin Films and Their Applicability as Neutral Optical Density Filter[END_REF]. However, grafting of 2PACz-based molecules stabilizes the chemical environment of the NiOx(UV+O3) surface and thus the CPD value does not vary notably over time. The CPD of the as-deposited NiOx surface does not present any significant change before and after organic molecule grafting with a minor evolution over time.

Study the Interface of ALD-NiOx/MAPbI3 with and without Interlayers

In this section, MAPbI3 film is deposited by co-evaporation on eight different HTL substrates and the properties of the MHP layers are studied and reported. In addition, solar cell devices are fabricated based on all eight various substrates and their performances are investigated and compared.

Structural and Morphological Properties

First, we assess the structural parameters and hence film formation of the MAPbI3 layers on the various substrates. The crystal structures of the co-evaporated MAPbI3 films with a nominal

Optical Properties

In the next steps to assess the optoelectronic properties of the UV-vis spectroscopy is performed to study the optical properties of the 450 nm thick MAPbI3 films co-evaporated on the eight different substrates. The reflectance and transmittance spectra are depicted in Figure 3.16 :(a) and (b), respectively. The measurements are performed by irradiating the samples from the perovskite side.

Since the light is passing through the MAPbI3 from its surface, which means that the major part of the spectrum in the lower wavelength range (300 nm-550 nm), and therefore higher photon energies, is absorbed in the MHP regions close to the surface. Hence, the impact of the substrate and grafted molecules as well as their corresponding interference are mostly observable in the higher wavelength, and hence lower photon energy range. In Figure 3. substrates in the range of 500 nm -800 nm. Moreover, all the UV+O3 treated substrates demonstrate a slightly higher transmittance in comparison to the pristine counterparts. This shows the importance of the process during which the interface is formed and in consequence could affect and alter the entire stack properties, here optical features in particular. In the next step we will hence analyze the chemical properties and their effect on the surface energetics of the MAPbI3 films on the various substrates in the early stages of growth.

Chemical and Electronic Properties of thin MAPbI3 layer on the NiOx surfaces

This section presents the chemical and electronic characteristics of the NiOx/MAPbI3 interface studied by XPS. For this, ultrathin layers (10 nm and 20 nm) of MAPbI3 are grown by co-evaporation on eight different substrates to track the evolution of the core levels related to the elemental components of MAPbI3. For the sake of a better overview and as a quick reference the peak position, FWHM, and the stoichiometric ratios obtained from peak fitting of Pb 4f7/2, C 1s, N 1s and I 3d5/2 core levels are tabulated in Table 3.1.

First, we investigate the growth of the thin MAPbI3 layer on the bare NiOx surfaces in Figure 3.17 We find no noticeable deviation in the elemental distribution of Pb, I, and N in 10 nm and 20 nm of MHP deposited on the bare untreated NiOx. Only a small deficiency of the A-site cation MA of 1% occurs in the 10 nm MHP compared to 20 nm, which is negligible in the accuracy limits of the measurement. However, an approximately 10 % lower concentration of N and thus A-site cation is found in the 10 nm MAPbI3 film compared to the 20 nm film on UV+O3 treated NiOx. The larger amount of Pb and I is consistent with the earlier finding from the XRD measurements that indicated the formation of a PbI2 rich phase at the interface. This observation is in agreement with the results previously reported by Boyd et al. for a different perovskite.. [START_REF] Boyd | Overcoming Redox Reactions at Perovskite-Nickel Oxide Interfaces to Boost Voltages in Perovskite Solar Cells[END_REF] In the next stage, we study the impact of the organic molecule interlayers on the electronic and chemical properties of MAPbI3. Figure 3.18 presents the Pb 4f7/2 and I 3d5/2 spectra of 10 nm and To mark the differences and the role of the organic molecules as interlayers as well as their impact on the core level spectra of the MAPbI3 components, the spectra of MAPbI3 component with 10 nm and 20 nm thickness are compared. As all core levels show the same shift in binding energy, we compare only the I 3d5/2 spectra in this section in decreasing with increasing MAPbI3 thickness in the case of M2P and B2P, the same is not true for the 2P interlayer, where the ratio remains the same. This phenomenon could be attributed to either less homogenous perovskite layer or slower growth of MAPbI3 on 2P at the very beginning of the evaporation process.

We generally observe a shift to higher binding energy for the MAPbI3 core levels of the film grown on the surface-modified NiOx(UV+O3) surface when compared to the perovskite film grown on the molecular layer on top of the untreated NiOx surface (50 meV for 2P, 150 meV for B2P and 200 meV for M2P). Apparently, the defective NiOx surface underneath the film of the 2PACz derivates leads to a slight modulation of the energy level alignment between molecular film and the MAPbI3 layer compared to the pristine NiOx layer. This effect is further exacerbated for the band bending induced in the MAPbI3 films. If we compare the core level positions of the perovskite as we increase the thickness from 10 nm to 20 nm, we observe a shift by 300 meV, 600 meV, and 400 meV to higher binding energies, for the M2P, 2P and B2P films on top of NiOx(UV+O3), respectively. We again attribute these shifts in binding energy of the MAPbI3 elemental components to downward band bending in MHP film induced by the interlayers. The higher magnitude of the peak shifts in the samples with NiOx(UV+O3) substrates compared to asdeposited NiOx is hence similar to the stronger peak shift to higher binding energy observed in the MAPbI3 deposited on the NiOx(UV+O3) without organic interlayer when compared to the MAPbI3 layer deposited on top of as-deposited NiOx film.

Again, a closer at the elemental distribution is performed to evaluate these peak shifts in the light of potential chemical changes at the interface (Table 3.

1). Opposite to what we observed for the

MAPbI3 deposited directly on nickel oxide substrates, we find a higher concentration of the A-site cation compared to Pb and I species in the presence of the interlayers in the first 10 nm of MHP deposition. This indicates that the small molecules mitigate the detrimental reaction caused by 𝑁𝑖 ≥3+ at the interface between the nickel oxide and perovskite layers. The surplus value of A-site

Electrical Features of Devices

To see how the organic molecule interlayers, influence the photovoltaic parameters, solar cells with the eight different substrate configurations are fabricated and characterized. The schematic illustration of the full stack of solar cell devices is shown in Figure 3.23. To produce the devices, after evaporation of 450 nm MAPbI3, PCBM and ZnO layers are spin-coated as described in chapter 2. To finalize the fabrication process, 100 nm silver with an active layer of 3×3 mm 2 is evaporated on the ZnO layer. I-V and EQE measurements are carried out to characterize the solar cell for 3-4 devices for each configuration for statistics. this behavior originates from the non-ideal band alignment between the absorber layer and the HTL. From the energetics, depicted in Figure 3.22 we observe that the ionization energy of the HTL (6.2 eV) is much higher than the one of the MAPbI3 layer (5.7 eV). In the hypothetical case of vacuum level alignment, this would place the VBM of the HTL below the VBM of the MHP and hence lead to a significant barrier. In the devices, this results in lower currents and s-shaped J-V curves, which yield a low fill factor. In the case of the absence of vacuum level alignment, the two layer would build up a significant interface dipole, which pushes the VBM of the MAPbI3 significantly below its equilibrium position, i.e. p-dope the layer in the vicinity of the hole contact.

As we increase the MAPbI3 layer thickness the system relaxes to its intrinsic position leading to an effective downwards band bending towards the interface with the B2P HTL, which is detrimental to carrier transport towards the hole collecting contact. Furthermore, scattered electrical parameters are observed in the case of M2P and 2P deposited on bare NiOx. However, the electrical parameters of the devices based on the NiO(UV+O3)/2P and NiO(UV+O3)/M2P

HTLs, present a narrow range of variation for the device statistics along with an overall enhancement of the performance. The pristine NiOx(UV+O3) cells exhibits an average VOC, JSC, FF, and PCE of 0.68 V, 12.80 mA/cm 2 , 41%, and 3.86%, respectively. The 2P interlayer boosts these parameters to 1.04 V, 18.80 mA/cm 2 , 45%, and 9%, respectively. The devices with NiO(UV+O3)/M2P demonstrate the best average performance of VOC:1.02 V, JSC:20.5 mA/cm 2 , FF: 54.8%, and PCE: 11.6%. The champion cell present a performance of VOC:1.07 V, JSC:21.73 mA/cm 2 , FF: 62.9%, and PCE: 14.6%. The devices fabricated on bare NiOx(as-deposited) and NiOx(UV+O3) have a noticeably low Jsc in comparison to M2P-modified ones. This suggests that the M2P interlayers facilitate the extraction of holes at the interface. Additional information can be deduced from this figure by comparing the tangent of the gray and black J-V curves at the VOC point. This tangent is indicative of the series resistance of the cell and is perpendicular to the x-axis in an ideal solar cell [START_REF] Park | An electrical model with junction temperature for light-emitting diodes and the impact on conversion efficiency[END_REF]. We see that the NiOx(UV+O3) curve has a lower slope than NiOx(as-deposited) one, which suggests that the series resistance is higher in this device. This effect is also mirrored at the Jsc point. In addition, a lower VOC value is obtained for this sample as well. These observations arise from the detrimental effect of UV+Ozone post-treatment that causes an increase in defect densities on the NiOx surface that can lead to higher non-radiative recombination rates and hence a lower open circuit voltage as well as a reduction of p-doping of the NiOx, leading to the higher resistance. The electrical measurements are presented in subsection 3.5.4.

Structural and Morphological Properties

In order to study the effect of interlayers on the crystallinity of 2C, XRD characterization is performed on the samples and the acquired patterns are illustrated in Figure 3.27. According to which shows a slightly smoother surface with a value of 4 nm. This is generally in agreement with the previous observation that MHP grown on M2P grafted substrates has the smallest grain size.

Optical Properties

Transmittance and reflectance spectra of the 2C-coated substrates are acquired using UV-vis spectroscopy. The samples are illuminated from the MHP surface. In general, the transmission and reflection of all samples do not change significantly in the wavelength range of 300 nm -800 nm.

We solely notice a small difference in the range between 750 nm and 800 nm attributed to the interference of the substrates at the interface of the NiOx and MHP layers. In this wavelength range, the samples with the M2P-grafted substrates follow a slightly different trend compared to the other substrates. An increase in reflectance going along with a lower transmittance is observed in M2P samples and this behavior is more dominant in NiOx(UV+O3)/M2P samples. This observation besides the UV-vis measurements of the substrates without the 2C top layer, suggests a different interface formation at the NiOx/MHP interface with the presence of the MeO-2PACz molecule. In Figure 3.32 the average PL intensity spectra obtained from hyperspectral images are illustrated.

In both panels, we observe that the samples with 2P interlayer have the highest PL intensity. This might be an indication that the 2P molecules passivate more efficiently the interface between nickel oxide and MHP and therefore decrease the non-radiative recombination. According to the last value of the surface potential in the dark, i.e. after 600s. However, we observe a higher value of the SPV of -0.17 V in the case of M2P and 2P interlayers. According to a study by Kronik et al. [START_REF] Kronik | Surface photovoltage phenomena : theory , experiment , and applications[END_REF] the SPV is not sensitive solely to the surface but buried interfaces can play a role as well. Also, the SPV is indicative of the redistribution of the charge carrier and thus the VOC [START_REF] Kronik | Surface photovoltage phenomena : theory , experiment , and applications[END_REF],

[117]. Therefore, the higher SPV for M2P and 2P suggests a higher open circuit voltage. This

Electrical Features of Devices

In this section, we discuss the solar cell parameter of the devices in the

J-V and EQE Measurements

The photovoltaic performance, assessed by the J-V measurement are illustrated and summarized in Figure 3.36 and Table A.2(in the appendix).

When we compare the devices produced on as-deposited NiOx and NiOx(UV+O3) without the interlayers, the latter yields a lower JSC (approximately 10%) in both forward and reverse bias. We discussed previously in section 3.2.1. that the UV+Ozone treatment reduces the transmission in the NiOx layer (5% compared to reference untreated NiOx over the range of 300 nm -800 nm) and as result, the number of photons arriving at the MHP layer decreases. Consequently, the lower charge carrier generation rate results in lower JSC. Additionally, the less p-type characteristic of NiOx(UV-O3) could make a barrier for an efficient charge extraction. Comparing the Voc of the cells, contrary to PL spectra that we found lower illumination for NiOx(UV+O3), a larger average value of ~1 V is measured in the UV+O3 treated cells compared to ~0.91 V for pristine NiOx. To this point, this discrepancy remains unanswered and would require further study to reveal the origin of this behavior. Given the less homogeneous MHP film formation on the as-deposited NiOx, we can envision that the cause of the lower Voc could be due to statistical variations, which also explains the larger spread in these values. Furthermore, the average FF of UV+O3 treated devices in forward bias is 10% higher than that of reverse bias, but in the case of as-deposited NiOx, only a 5% difference is observed. This observation shows a more significant hysteresis in the UV+O3 post-treated device that is attributed to the more reactive and defective interface of NiOx/MHP. Despite the different average values of Jsc, Voc, and FF, both configurations yield almost equivalent PCE average values. Approximately,11.7% and 11.4% in forward bias and 12.7% and 12.9% in reverse bias are obtained for NiOx(as-deposited) and NiOx(UV+O3), respectively.

After employing the organic molecules as interlayers in the device structure, we find a different trend. Similar to the case of the MAPbI3-based devices, the overall performance of the devices with B2P is poor compared to the other device configurations. This indicates that NiOx/B2P HTL despite its high ionization energy, which could offer better electron-blocking ability, has a detrimental effect likely due to the barrier for hole extraction. Note that the reduction of the barrier height compared to the MAPbI3-based solar cells, leads to a less significant reduction in the device performance here. Note, that this issue could be further amplified by other effects aside from the non-ideal band alignment between HTL and MHP layer impacts both Voc and Jsc.

Anchoring the 2P and M2P molecules on the NiOx(UV+O3) surface goes along with a minimal improvement in the PCE of the device. The 2P interlayer leads to an increase of the average PCE to ~13.1% and ~14.2% in forward and reverse bias, respectively, while the M2P interlayer leads to a PCE of ~12.6% and ~13.9%, respectively. The higher PCE in these configurations is accompanied by an improvement in the JSC. Improvement in optical transmission over NiOx(UV+O3) after grafting the small molecules, allows more light to reach the absorber layer. In addition, the more efficient hole collection due to the higher conductivity of the HTL and possibly more suitable band alignment could be further reasons for the improved performance. Moreover, the less prominent hysteresis in the devices with interlayer compared to the devices without interlayer is a sign of less non-radiative recombination at the interface thank to defect passivation.

However, regarding the VOC parameters, we found the results obtained for the 2P and M2P

interlayer to be in conflict with the data from the Hyperspectral PL measurements. Generally, we would expect higher VOC values for the devices with the interlayer due to less non-radiative recombination, particularly for the case of the 2P interlayer, which leads to the strongest quasi fermi level splitting in the absolute PL measurements, but we rather see a minimal decrease in the average Voc of the interlayer devices compared to the device on the bare UV+O3 treated NiOx.

However, this difference is not statistically relevant and individual cells, such as the champion devices, reach a similar Voc.

By integrating the 2P interlayer between the as-deposited NiOx and MHP layers, we observe an overall improvement in the JSC value in comparison to the bare NiOx reference architecture. Also, the data are distributed in a narrow range with only small deviations. The FF values also exhibit only a small spread, but the overall average PCE is reduced compared to the reference cells without 2P interlayer. This reduction in fill factor can be attributed to the series resistance as we see in Figure 3.37 that the curve is significantly tilted away from a perpendicular position at the VOC point. Therefore, the higher JSC could be attributed to a good band alignment at the interface and thus more efficient hole extraction. Nevertheless, the more dense and thick layer of 2P which was an observation of XPS data (Figure 3.8) could explain the increase in series resistance and hence a drop in FF. Regarding the VOC, although it shows a higher average value, the data is dispersed in a range from ~0.78 V to ~1.02 V. This date dispersion is reflected also in the PCE value and makes this configuration less reproducible. This places the average PCE of this device configuration slightly below the one of the reference devices and hence makes the 2P molecule not an ideal candidate to modify the interface between as-deposited NiOx and double-cation mixed halide MHP.

Devices based on grafted M2P on as-deposited NiOx represent the best and most reproducible cells.

This interface modifier boosts all the photovoltaic parameters showing average values of VOC:0.99 V, JSC:22 mA/cm 2 , FF: 72%, and PCE: 16.1% in reverse bias compared to the reference cells having average values of VOC:0.91 V, JSC:21.2 mA/cm 2 , FF: 65%, and PCE: 12.7%. If we discard one outlier device with the lowest performance whose parameters lie far away from the majority of the data, the average parameters rise to VOC:1.04 V, JSC:22 mA/cm 2 , FF: 79%, and PCE: 18%. We should mention that the higher VOC of the devices is not in agreement with our observation of Hyperspectral imaging PL. There is the possibility that the sample utilized for PL measurement suffers from the partial coverage of the M2P layer or disrupted MHP formation, which will require further local probes in future measurements. The improvement of the device via the MeO-2PACz interlayer can be broken down into the following components. First, the M2P layer passivates the adjacent interface with the MHP layer similar to the case we reported for the MAPbI3-based device in the previous section. Second, the comparably low ionization energy of the M2P leads to favorable energy level alignment with the MHP layer suppressing the formation of any barrier for hole extraction (as would be the case for the B2P molecule) and therefore promoting hole extraction. and Br-2PACz interlayer between MHP and NiOx(as-deposited or UV+O3) layers Figure 3.37 shows the J-V curve of the champion cell with NiOx(as-deposited)/MeO-2PACz hole transport layer as well as the best cells with NiOx(as-deposited)/MeO-2PACz and bare NiOx(asdeposited) hole transport layer. We observe a lower hysteresis in the cell with the modified NiOx surface. This indicated that the M2P successfully passivated the NiOx/MHP interface which reduces the hysteresis by hindering non-radiative charge recombination and probable ion migration at the MHP layer [START_REF] Singh | Origin of Hysteresis in Perovskite Solar Cells[END_REF]. While the reference cell presents the photovoltaic parameters of VOC:1.02 V, JSC:21.84 mA/cm 2 , FF: 72.7%, and PCE: 16.14% in reverse bias, the M2P interlayer improves all the parameters to VOC:1.05 V, JSC:22.3 mA/cm 2 , FF: 79.7%, and PCE: 18.6%. This is the record efficiency that has been obtained so far for the p-i-n perovskite solar cells at IPVF and this configuration is now utilized as the baseline to produce inverted cells, modules, and tandem solar cells in our lab.

The EQE spectra of the best-performing cells of each configuration are demonstrated in Figure 3.38. NiOx(UV+O3) besides two devices with B2P interlayer show a lower overall EQE which is consistent with significantly lower JSC obtained in J-V measurements. Also, the samples on asdeposited NiOx with the M2P and 2P interlayers present the highest EQE which again is in line with our observation for JSC from J-V characterization. The spectrum of NiOx(UV+O3)/2P contains interesting information which could be a confirmation of our hypothesis about the disruption of the MHP/PCBM interface. We see that in the wavelength between 600 nm-800 nm the EQE spectrum drops gently as an indication of a destructive interface. to study the effect of UV+Ozone post-treatment. Also, the solar cells with NiOx(asdeposited)/M2P and NiOx(UV+O3)/M2P are characterized by this method to evaluate the impact of MeO-2PACz interlayer on the stability of devices under 1 sun illumination.

Indoor accelerated aging is conducted for 260 hours under LED solar simulator whose spectrum matches the AM1.5G (including UV portion) in a climate chamber kept under the N2 atmosphere.

Maximum power point tracking (MPPT) is conducted all along the aging process. In addition, J-V characterizations were recorded every 20 min.

Figure 3.39 shows the normalized maximum power point tracking over 260 hours. The measured power values are normalized to the third measured value. In Figure 3.39(a), we see that in bare NiOx(as-deposited) and NiOx(UV+O3) the power starts from a lower value and then increases gently until it gets stabilized at its maximum. This effect is attributed to the fact that under light illumination the photogenerated electrons can passivate the defects at interfaces and bulk MHP as well as ion migration [START_REF] Zhao | Revealing Underlying Processes Involved in Light Soaking Effects and Hysteresis Phenomena in Perovskite Solar Cells[END_REF]. However, this process is quite fast in devices with M2P interlayer indicating that the defect states are highly passivated by MeO-2PACz molecules. Furthermore, both devices of NiOx(UV+O3) are not yielding any power output after almost 25 and 80 hours. suggests that the 2P molecules form a more densely packed layer on nickel oxide substrate due to the size and structure of the molecule. We observe that different electronegativity of the electrondonating methoxy group in M2P has an impact on the C1s spectrum and placed it in higher binding energy than 2P and B2P. We found that the 2PACz-based molecules push the VBM of the NiOx layer closer to Ef which makes the layer more p-type. Further, while M2P decreases the ionization energy of nickel oxide substrate, 2P, and B2P increase this value, more significantly in the case of the B2P molecule. This effect is linked to the different molecular dipole moment values that each molecule creates on the surface.

In the third part of the chapter, we discussed that the presence of 2PACz-based interlayers mitigates the formation of PbI2 in the bulk of evaporated MAPbI3, according to XRD patterns.

However, no significant change in the morphological and topographical features is observed.

Optical measurements present an obvious difference in the growth of MAPbI3 on each molecule.

In XPS measurement of the thin film of MHP, we observed a downward band bending within the MAPbI3 layer as we move away from the interface, in the samples of NiOx(UV+O3) with and without interlayer as well as NiOx(as-deposited)/M2P and NiOx(as-deposited)/B2P. However, we find no peak shifts for the MHP thin-film growth on top of the bare NiOx(as-deposited) and NiOx(as-deposited)/2P samples. Additionally, we found the formation of metallic nickel after the evaporation of MAPbI3 on bare NiOx(as-deposited) and NiOx(UV+O3). This process is suppressed by the integration of the molecular interlayer. Also, we observed a downward band bending in the NiOx layer of all samples towards the interface with MAPbI3. Furthermore, we detect a deficiency of the A-site cation in the MHP at 10 nm compared to 20 nm in the bare UV+O3 treated NiOx.

However, the integration of the small molecules suppresses the occurrence of this deficiency.

Regarding the device performance, we reported that utilizing 2P and M2P as an interlayer between NiOx(UV+O3) and MAPbI3 could improve the efficiency of the cells. Nevertheless, they do not provide reproducible results on as-deposited NiOx. The M2P surface modifier on NiOx(UV+O3) has provided higher improvement in the performance with a champion cell showing VOC:1.07 V, JSC:21.73 mA/cm 2 , FF: 62.9%, and PCE: 14.6%. Solar cells fabricated with the B2P interlayer show poor performance parameters due to the large offset of IE between the B2P-modified NiOx surfaces and MAPbI3 layer and thus the formation of a significant barrier for hole extraction.

In the last part of the chapter, M2P, 2P, and B2P are employed as interlayers between NiOx and FA 0.7 Cs 0.3 Pb(I 0.9 Br 0.1 ) 3 films. Our study shows that these interlayers do not change the topography and crystallinity of MHP. Hyperspectral PL measurements suggest that UV+O3

treatment has a detrimental effect on the optical properties of the material, as collectively the UV+O3 treated samples show a lower PL intensity. Also, 2P on both pristine and UV+Ozonetreated NiOx has higher PL illumination. Concerning the performance of the solar cell and in the case of NiOx(UV+O3), 2P and M2P improve the average overall PCE of the devices. Again, B2P devices produce the worst devices due to unfavorable band alignment in terms of a large valence band offset due to the high ionization energy of the B2P/NiOx surfaces. However, the best and most reproducible result is obtained with an M2P surface modifier on as-deposited NiOx with a champion cell showing VOC:1.05 V, JSC:22.3 mA/cm 2 , FF: 79.7%, and PCE: 18.6%.

All in all, we conclude that MeO-2PACz and 2PACz can work as passivating interlayers between NiOx and MHP. However, their functionality highly depends on the treatment of NiOx as well as on the composition and deposition method of the MHP overlayer.

SWCNTs in MHP solar cells

Thanks to their unique optoelectronic properties, single-walled carbon nanotubes (SWCNTs) show a high potential to be integrated into various architectures of perovskite solar cells [START_REF] Habisreutinger | Carbon Nanotubes in Perovskite Solar Cells[END_REF], [START_REF] Habisreutinger | Carbon Nanotube/Polymer Composites as a Highly Stable Hole Collection Layer in Perovskite Solar Cells[END_REF], [START_REF] Li | Laminated Carbon Nanotube Networks for Metal Electrode-Free Efficient Perovskite Solar Cells[END_REF] . The high chemical and mechanical stability introduce them as an interesting material in electronic technology in which the long-life device has great importance. SWCNTs with high charge carrier mobility improve carrier collection which in turn, enhances halide perovskite solar cell performance. In addition, their high optical transmission in the visible range makes them excellent candidates for the charge transport layer in PSCs. Tunable electronic properties, via changing the design of the tubes and the doping characteristics, make them suitable for both electron and hole transport layers [START_REF] Menon | Reversible Charge Transfer with Single-Walled Carbon Nanotubes Upon Harvesting the Low Energy Part of the Solar Spectrum[END_REF][75] [START_REF] Ihly | Environmental Science Efficient charge extraction and slow recombination in organicinorganic perovskites capped with semiconducting single-walled carbon nanotubes †[END_REF].

In chapter 1, we discussed the integration of the SWCNTs layer into perovskite has presented an improvement in the performance of the cell, Additionally, they offer stability enhancement in comparison to the organic charge transport layer. However, their interfacial properties and electronic structure are not fully discovered and understood. In order to probe and understand the features of SWCNTs integrated into perovskite solar cell stack, various studies are carried out.

Previously, Schulz et al. [START_REF] Schulz | Charge Transfer Dynamics between Carbon Nanotubes and Hybrid Organic Metal Halide Perovskite Films[END_REF]studied the electronic structure at the interface between SWCNTs and MAPbI3 by XPS. They reported that SWCNTs deposition forms interfacial dipoles which in turn makes SWCNTs layer n-type and causes an electron donating process from MAPbI3, more precisely from methylammonium cation, to SWCNTs film. This phenomenon forces the perovskite layer to have a downward band bending in the adjacent interface. In this chapter, following up on the abovementioned study, a study is conducted in collaboration with the National Renewable Energy Laboratory (NREL) in the U.S. and Helmholtz-Zentrum Berlin (HZB) in Germany. This work aims to understand how the growth of SWCNTs on two different MHP materials i.e. MAPbI3 and MAPbBr3 influences the electronic properties and elemental distribution of MHP components in the surface/interface and subsurface/sub-interface regions. Firstly, the samples are fabricated at NREL. To do so, a compact layer of TiO2 is deposited via the spray pyrolysis technique. In the following, perovskite precursors are prepared by spin coating technique as explained in chapter 2. SWCNTs wrapped with the high band gap polymer polyfluorene Bis-Pyridine (PFO-BPy) are deposited on MHP films in 10 nm and 20 nm thickness via spray pyrolysis technique.

The prepared samples were shipped under inert atmosphere in a sealed transfer package to BESSY II (Helmholtz Zentrum Berlin, Germany). HAXPES measurements were carried out at the HIKE end-station of the KMC-1 beamline, generally under three various excitation photon energy i.e. 2003 eV (2 keV), 6009 eV (6 keV) and 8012 eV (8 keV). The 8 keV excitation photon energy was mainly used for I 3d and Br 3d core levels to evaluate any potential band bending effects that could range deeper into the material. Furthermore, for these scans, the X-rays have been further attenuated to carefully examine the effect of degradation on the band bending at the sample surface. Aluminum and beryllium filters are employed to attenuate the X-ray beams. In the following, I will focus on our observations for this set of HAXPES measurements.

Data analysis is performed with two software packages i.e. Igor Pro and CasaXPS. The scale energy of raw data measured under different excitation energies is corrected in Igor Pro by first converting their kinetic energy to binding energy. The Fermi edge is calibrated using the Au 4f7/2 signal, centered at 84 eV binding energy, and which is acquired in between every two complete sets of sample measurements, i.e. approximately twice per day, on a sputtered clean gold foil.

Subsequently, the intensity of the recorded photoelectron spectra is normalized to the beam current, which is measured in an ionization chamber, and the number of sweeps to obtain the accurate intensity in counts per second for the quantitative analysis. The data are then transferred to CasaXPS for peak fitting and quantitative analysis. Peak fitting itself is performed by applying constraints on the FWHM, the magnitude of the spin-orbital splitting, and the corresponding area ratio, for the core levels with doublet peaks of their two states (Pb 4f, I 3d, Br 3d, and Au 4f).

Finally, for the quantification assessments, the raw area quantities obtained from CasaXPS are normalized with respect to the IMFP, photoionization cross-section, and transmission factor.

We begin with the assessment of the band bending in the pure perovskite layers. Figure 4.2(a)

shows the spectra of the I 3d5/2 core levels of the samples with MAPbI3 as the absorber layer. The spectra are first recorded using the more strongly attenuating Al-filtered X-ray. Noteworthy, in order to minimize the exposure time of the samples to the incoming X-ray beam, the spectra are acquired over a shorter scan range covering only the I 3d5/2 region, which however is sufficient to project a shift of the Fermi level in the gap. A shift of 270 meV, 190 meV, and 180 meV to lower binding energy is observed in the bare MAPbI3, MAPbI3/SWCNTs(10 nm), and MAPbI3/SWCNTs(20 nm), respectively, while increasing the excitation energy from 2 keV to 8 keV.

The samples are also characterized under Be-filtered X-ray in a longer kinetic energy window covering both I 3d5/2 and I 3d3/2 for 2 keV and 6 keV and a short window (I 3d5/2) under 8 keV.

According to the spectra, shown in However, it seems that using the beryllium filter with the increased X-ray intensity on the sample compared to the previous initial measurements under the Al filter damages the perovskite film creating defect states in the surface of MAPbI3.Even if these damages do not necessarily show a changed in surface composition in XPS, the defect states can have significant electronic effects.

In turn, the Fermi level is pinned to the conduction band at the surface/interface of the perovskite film [START_REF] Zhang | Band Bending in Semiconductors : Chemical and Physical Consequences at Surfaces and Interfaces[END_REF] , which therefore reduces band bending measured in the samples with Be-filter at higher beam intensity and after longer exposure times. Of note, while EF appears to be pinned at the surface and in the sub-surface region down to approximately 25 nm, as suggested by the 2 keV and 6 keV data, the 8 keV scans reveal that in the Fermi level in the bulk is not yet pinned.

However, the initial magnitude of band bending from bulk to the surface has been reduced by approximately 100 meV in all cases, i.e. with and without SWCNT overlayer. this shows that using a highly reduced Al-filtered X-ray beam could not provide precise information on Br 3d core level of perovskite film in the buried layers. In order to have a better understanding of deeper regions in the sample and probable shifts in the peak positions and consequently band bending, we considered the use of a beam with higher beam flux.

Electronic properties in dark and light

In However, in the sample with 10 nm SWCNT on MAPbI3, we found a 100 meV shift to lower binding energy, from dark to illuminated condition which could be explained by an upward band bending of I 3d core level toward interface when 10 nm SWCNT are deposited on the perovskite layer and the sample is exposed to light. This effect also seems to appear for the sample with the 20 nm SWCNTs top layer, but to a lesser extent. Note that this shift is barely within the resolution limit of the experiment.

The spectra of Br 3d core level of the MAPbBr3 with and without SWCNT cap layer under dark and light conditions are depicted in Figure 4.4(b) . No significant peak shift is found in any samples by changing the measurement condition from spectrum acquisition in the dark to spectrum acquisition under illumination.

Impact of SWCNTs growth and its thickness on electronic properties

After this initial assessment of potential measurement artifacts in the first set of rapid scans, we will now closer investigate how depositing SWCNTs film and increasing its thickness on two different metal halide perovskite layers will influence the measured spectra of all relevant component core levels. These observations will help us to have a deeper understanding of the behavior and response of the perovskite layer to the SWCNTs growth at the interface and subinterface regions. and I 3d5/2 core levels, respectively. In the C 1s and N 1s spectra, we see the appearance of a second peak, centered at ~285 eV and ~399.9 eV respectively, after the deposition of the 10 nm and 20 nm thick SWCNT film. The second C 1s peak is easily attributed to graphitic carbon in the SWCNTs.

The second contribution to the N 1s signal originates from the wrapping PFO-BPy polymer which is utilized for SWCNT deposition. Note, that the carbon in the wrapping polymer also contributes to the C 1s signal, leading to a rise of a shoulder at around 286 eV. We observe a slight shift of this new C 1s contribution to lower binding energies of EB = 0.1 eV when increasing the SWCNT overlayer thickness from 10 nm to 20 nm. This shift indicates slight downward band bending in the SWCNTs layer towards the interface with the MHP film as previously reported in Schulz et.

al. [START_REF] Schulz | Charge Transfer Dynamics between Carbon Nanotubes and Hybrid Organic Metal Halide Perovskite Films[END_REF] A similar peak shift to lower binding energy is observed in PFO-BPy-related N 1s spectra.

Growing 10 nm and 20 nm thick SWCNT does not impact the peak position of MA-related N 1s spectra centered at ~402.5 eV.

Comparing Pb 4f and I 3d core levels under 6 keV excitation energy, which means that there is no band bending as we measure the surface vs the sub-surface region.

Note that this appears to be in direct contrast to the earlier results presented in Figure 4.2, where band bending was observed particularly for the pristine MAPbI3 surface. The data, here taken with only the beryllium filter for attenuation and hence at high beam flux and longer integration times, suggests that the initial band bending is substituted by Fermi level pinning to the conduction band as high concentration of donor defect states are created under X-ray irradiation. For the N 1s spectra of the samples with the SWCNT, we use the fact that the signal excited at 6 keV corresponds to collecting information from deeper layers, whereas the signal at 2 keV stems from the surface. Also, no shift is observed under 6 keV.

In the samples with MAPbBr3 absorber layer, we find a different behavior for the evolution of the perovskite component core levels with respect to the SWCNT overlayer growth. We see the appearance of a new peak in bare perovskite spectra at around 136.6 eV, which we attribute to the formation of metallic lead (Pb 0 ). However, no trace of this peak is found in the samples with SWCNTs capping. The conversion of Pb 2+ to Pb 0 is likely due to the X-ray irradiation, [START_REF] Kerner | Electrochemical and Thermal Etching of Indium Tin Oxide by Solid-State Hybrid Organic-Inorganic Perovskites[END_REF], and covering the perovskite layer by SWCNT capping layer prevents this degradation of perovskite to metallic lead. Comparing the Pb 0 /Pb 2+ ratio in the surface and subsurface regions, 0.08 and 0.05 respectively, we see a higher amount of metallic lead in the surface regions.

Moreover, Pb 4f, Br 3d and N 1s peak positions show a shift of 100-200 meV to higher binding energies after growing 10 nm and 20 nm SWCNT capping layer on the plain MAPbBr3. This phenomenon is seen under both 2 keV and 6 keV excitation energies. The shift is more significant at 2 keV in the topmost regions. One could explain that formation of Pb 0 in the bare MAPbBr3

layer pinned the fermi level to the conduction band and the lower binding energy in the samples with SWCNT over layer could not be considered as a band bending.

In addition, we observe the C 1s spectrum shifts slightly to lower binding energy by increasing the thickness of SWCNTs from 10 nm to 20 nm. This effect is more noticeable at 2 keV. This observation is similar to the case of MAPbI3 and again suggests a downward band bending towards the interface in the SWCNTs layer.

Potential band bending and electronic properties in different depth

In order to more closely evaluate the observation of potential band bending at the pristine perovskite surfaces as well as for the perovskite/SWCNT interface systems, it becomes more convenient to plot the data presented in us to the explanation that the deposition of SWCNT on MAPbI3 does not alter the composition of lead and iodine-containing species and also might not make any band bending at the interface.

Furthermore, as discussed before in section 4.2.1, exposing MAPbI3 film with and without SWCNTs layer to Be filter-attenuated beam might cause some minor initial degradation in the layer, i.e. a finite amount of donor defect generation by X-ray in perovskite absorber layer resulting in pinning of the Fermi level to the conduction band. Consequently, we are not able to re-measure the previously assessed band bending at the MAPbI3 surface with this method despite the reduced beam intensity.

In the N 1s spectra of plain MAPbI3, only one peak at around 402.5 eV is observable under each excitation energy. This peak is attributed to the MA component of the perovskite absorber.

Comparing 2 keV and 6 keV, we observe a peak shift of 250 meV to higher binding energy (~402.50 eV to 402.75 eV) from the surface toward the bulk area. After growing SWCNT on top of MAPbI3, the second peak of N 1s core level appears at lower binding energy than the MArelated one, which originates from the wrapping PFO-BPy polymer used to deposit the SWCNT.

Both PFO-BPy and methylammonium (MA) related N 1s peaks shifted to higher binding energy by increasing the excitation energy. Due to the low signal-to-noise value of the MA-related N 1s peak for the sample with the 20 nm SWCNT layer, we cannot make a concrete statement about the exact peak position and any related peak shift. Given the C 1s core level spectra, a similar shift to higher binding energy from 2 keV to 6 keV can be seen in all the samples. At first glance, one could be tempted to associate these peak shifts in the C 1s and N 1s core levels from 2 keV to 6 keV with the formation of new species as their binding energy differs from one another.

However, it is critically important to reconsider the magnitude of any potential core level shifts for changed photon energies, and hence changed kinetic energies of the corresponding photoelectrons, after correcting the energies for the recoil effect. This effect is a direct consequence of the conservation law of momentum and is more pronounced in materials composed of light atoms such as carbon, nitrogen, and oxygen [START_REF] Takata | Recoil effects of photoelectrons in a solid[END_REF]. Based on this effect, if a photoelectron with a mass of m and kinetic energy of Ek is emitted from an atom i with mass Mi, then we can estimate the recoil energy Er,i as [START_REF] Taguchi | Hard X-ray photoelectron spectroscopy: A few recent applications[END_REF]:

∆𝐸 𝑟,𝑖 = 𝐸 𝑘 ( 𝑚 𝑀 𝑖 )
Eq. (4.1)

According to our calculations, the recoil energy for the C and N atoms at 2 keV is around 80 meV and 60 meV, respectively. Under 6 keV, the energy increases significantly to around 260 meV for C and 220 meV for N. Carbon due to its lower atomic presents higher recoil energy than nitrogen.

The difference between these two energies for each element i.e. 180 meV for carbon and 160 meV for nitrogen is the impact of recoil effect should be roughly seen between 2 keV and 6 keV. the perovskite, we observe that this core level exhibits a shift by around 100 meV to higher binding energy under 6 keV excitation energy compared to 2 keV.

Concerning the Br 3d signal, we see that the spectra recorded under 6 keV are shifted by 150 meV towards higher binding energy for the sample with 20 nm SWCNTs overlayer compared to the recorded data under 2 keV, which is in line with the peak shift of Pb 4f. Comparing spectra under 2 keV and 6 keV, no noticeable change in peak position is found for plain MAPbBr3 and 10 nm SWCNTs.

Comparing 2 keV and 6 keV spectra of the N 1s in all three samples with MAPbBr3 as absorber layer with and without SWCNTs layer, we observe peak shifts of at least 100 meV to higher binding energy for methylammonium and PFO-BPy components as we increase the excitation energy. The entire MA-related peak position shifts of plain MAPbBr3 and with 10 nm SWCNT, as well as some portion of the overall peak shift of the samples with 20 nm SWCNT capping layer are mostly related to the recoil effect. The remainder of the shift could be attributed to a different type of binding energy shift of this element in the surface and subsurface regions as already observed for the other elements of the perovskite layer, i.e. in the Pb 4f and I 3d core levels.

These changes in peak position to higher binding energies for all perovskite components after 20 nm SWCNT on MAPbBr3, suggest an upward band bending of approximately 100 meV in the perovskite layer and downward band bending in the SWCNT film toward the interface.

Compositional gradient of samples buried and topmost regions

The photoelectron peak intensity of a core level is proportional to the concentration of the corresponding element in the probed sample volume, and therefore core-level spectra can be used for quantitative analysis as well. In order to do so, the probability for each element to produce a photoelectron needs to be taken into account, by using measured, empirical or theoretically calculated photoionization cross-sections (σ), which are also called atomic sensitivity factors (ASF). σ strongly depends on the element, the specific core-level shell, and the excitation photon energy [START_REF] Doh | Applications of Synchrotron-Based X-Ray Photoelectron Spectroscopy in the Characterization of Nanomaterials[END_REF] .

In Figure 4.8, the compositional gradient of the perovskite in the surface and sub-surface region for the two perovskite systems with and without SWCNTs overlayer are illustrated. In order to do the compositional quantitative assessment of the samples, the intensity Ii for the core level of element i is corrected to the number of sweeps (NS), beam current (I0), inelastic mean free path (IMFP) [START_REF] Shinotsuka | Calculations of electron inelastic mean free paths. X. Data for 41 elemental solids over the 50eV to 200keV range with the relativistic full Penn algorithm[END_REF], σ and transmission factor (TF) for each element using Eq. (4.2) to get the corrected intensity I * i. σ and TF are calculated by the KMC-1 beamline scientists at Bessy II.

𝐼 𝑖 * = 𝐼 𝑖 𝑁𝑆 × I 0 × σ × 𝐼𝑀𝐹𝑃 × 𝑇𝐹 Eq. (4.2)
The concentration of each element using the corrected intensity is subtracted from the concentration ci,exp of the expected stoichiometric ratio to find the deviation ∆𝑐 𝑖 , Eq. ( 4.3).

∆𝑐 𝑖 = ( 𝐼 𝑖 * ∑ 𝐼 𝑖 * -𝑐 𝑖,𝑒𝑥𝑝 )

Eq. ( 4.3)

In Figure 4.8(a), we observe a deficiency of nitrogen in all samples with MAPbI3 absorber. This deficiency becomes even more dominant in the topmost layer of the perovskite film. In addition, depositing and increasing SWCNTs film leads to a larger deficiency of MA-related N, particularly in the interface area. We found the accumulation of lead and iodine in the surface and sub-surface levels of all samples with MAPbI3. Although a higher surplus of Pb is observed in the buried layers than the surface for the bare MAPbI3, SWCNTs growth changes this distribution by resulting in a Iodine showed a stronger accumulation in the topmost layers in all samples compared to the buried regions after SWCNT overlayer deposition and increasing its thickness. Therefore, in the samples with SWCNTs capping layer, we observe a high level of Pb and I surplus. This effect is more significant in the topmost regions. . This indicates that SWCNT deposition cause PbI2 compound accumulation at the interface compare to the bulk of perovskite in MAPbI3. we found lead accumulation in all samples. A higher positive deviation from the expected value was found in the buried layers than topmost layers. SWCNTs deposition is accompanied by a rise in the surplus of lead, which is more prominent at the interface than in the sub-interface region.

Furthermore, metallic lead has a higher concentration at the surface of the plain perovskite (+7%) compared to the deeper layers (+5%). However, after growing SWCNTs, no trace of metallic lead is found in the perovskite layer. Additionally, we observed a Br-deficiency in bare MAPbBr3 film which is even more pronounced in the sub-surface region. Growing SWCNT raises bromine concentration in the interface regions leading to a positive deviation after the deposition of 20 nm SWCNT. However, SWCNT growth does not noticeably change bromine distribution in the buried layers, showing almost an equal deficiency ratio in all the samples.

Conclusion

In this chapter, we employed the HAXPES technique to study how incremental deposition of the SWCNTs layer changes the interfacial properties of MAPbI3 and MAPbBr3. By short-time exposure measurements of I 3d5/2 core level and using Al filter which highly attenuates the X-ray beam, we discovered a downward band bending in the MAPbI3 layer toward interface with and without the presence of SWCNTs overlayer. However, the deposition of SWCNTs reduces the energy value of band bending from 300 meV to 200 meV. This downward band bending goes along with downward band bending toward interface in the SWCNTs layer. Short scan measurements of Br 3d using Be filter in the samples with MAPbBr3 absorber layer do not reveal any band bending in the perovskite layer, likely due to low ionization cross-section and thus low signal-to-noise ratio. However, by increasing the beam flux using Be filter, a small upward band bending in the MAPbBr3 is found in the samples with 20 nm thick SWCNTs layer. Similar to MAPbI3 samples, we track a small downward band bending in the SWCNTs layer on top of MAPbBr3.

According to the stoichiometric assessment, an accumulation of lead halide species is found in all the samples with MAPbI3 as well as in MAPbBr3 with 20 nm thick SWCNTs layer. The lead halide surplus, halide accumulation in particular, could be the source of the band bending we observed in these samples but with different effects in MAPbI3 and MAPbBr3

ALD-SnO2 in MHP Solar Sells

SnO2 exhibits high thermal and chemical stability, as well as higher transparency, compared to TiO2 films. It is therefore known as an excellent ETL in perovskite solar cells. This inorganic material generally assumes favorable energy level alignment with perovskite that in turn facilitates electron extraction from the perovskite absorber layer. Among all the techniques to deposit metal oxide layers, atomic layer deposition (ALD) due to its potential for large-scale fabricating as well as low-temperature process and high homogeneity outcome is considered a suitable and favorite method in photovoltaic technology [START_REF] Green | The emergence of perovskite solar cells[END_REF][START_REF] Kim | High-E ffi ciency Perovskite Solar Cells[END_REF]. Although ALD-SnO2 is nowadays widely employed in n-i-p structures showing efficiency higher than 18% [START_REF] Correa Baena | Highly efficient planar perovskite solar cells through band alignment engineering[END_REF], its deposition directly on the perovskite layer in inverted p-i-n architecture has not reached high-performance levels, presenting power conversion efficiency between 0.5% and 8.2% [START_REF] Bracesco | The chemistry and energetics of the interface between metal halide perovskite and atomic layer deposited metal oxides[END_REF]. This deficiency highly originates from the interference of ALD precursors and their reactions with perovskite components. This issue could be potentially addressed by interface engineering and adding an organic molecule, i.e. PCBM and C60, as an interlayer between perovskite and SnO2. However, understanding the source of detrimental interfaces of ALD-SnO2 and bare perovskite and the impact of interlayers on boosting interfacial properties could lead us to deposit this inorganic layer directly on the perovskite layer.

For this purpose, our group started to study and adopt the deposition of SnO2 by ALD technique to our p-i-n solar cell with and without interlayer. We follow two objectives in this approach, firstly to develop an industry-compatible fabrication method and secondly to study the corresponding interfaces by photoelectron spectroscopy techniques and compare them with the already reported results. For this work, Nitin Mallik, a Ph.D. student at IPVF, as a part of his thesis has been particularly working on the ALD technique in order to develop and optimize SnO2 deposition and integrate this layer into p-i-n perovskite solar cells. Investigating the process occurring in the interface is always one of the most prior steps that should be done in developing a new material adjoining to other layers. This helps to better understand the process, its effect and plan wisely the next steps. In this regard, we carry out a synchrotron-based HAXPES study on ALD-SnO2 deposited on bare PCBM-protected MHP layer. I, as a part of this project team and in line with the objective of my thesis, processed and analyzed the HAXPES data during my Ph.D.

thesis.

In this work, for samples fabrication, NiOx films are prepared in two steps. The first 20 nm of Ni is deposited by thermal evaporation at a deposition rate of 1 Å/s on glass/FTO substrate followed by a 10-minute UV+Ozone treatment. Subsequently, the samples are moved to an oven and annealed at 300 °C for 3 hours for oxidizing Nickel. Then, 1.2M double-cation mix-halide perovskite, FA 0.7 Cs 0.3 Pb(I 0.9 Br 0.1 ) 3 , and 40 nm PCBM (20mg in 1 ml chlorobenzene) are spincoated on the NiOx layer. In the next step, the SnO2 electron transport layer is grown via a thermal ALD process using two precursors i.e. TDMASn (Tetrakis(dimethylamido)tin(IV)) and H2O2 at an operating temperature of 100 °C and chamber pressure at 1 mbar (see

Structural and Optical Properties by XRD and UV-vis Spectroscopy

Before starting the discussion about HAXPES results, in this section, we see how ALD-SnO2 impacts the structural and optical properties of MHP with and without PCBM interlayer. that this MHP is quite stable at 100 °C, the formation of this peak is attributed to perovskite absorber layer degradation after the ALD process due to its chemical reaction with ALD precursors. Even using a protective PCBM interlayer does not perfectly prevent this degradation.

The impact of PbI2 formation is also observable in the absorbance spectra of the samples as shown in Figure 5.2(b). Having in mind the fact that PbI2 has an absorption onset at around 515 nm with absorption rise toward UV-region [START_REF] Manuscript | Materials Chemistry A[END_REF], We see that while the samples with ALD-SnO2 present higher absorption in the wavelength range of 400 nm and 550 nm, their absorption becomes lower in the wavelength higher than 550 nm.

Electronic Properties of MHP/ALD-SnO2 Interface by HAXPES

In this section, we discuss the electronic properties of the MHP samples before and after ALD-SnO2 using HAXPES data.

Data treatment of HAXPES measurement is done using two software packages i.e Igor pro and CasaXPS. We used Igor pro software to correct the scale energy and intensity of collected photopeaks. The analyzer of all the photoemission spectroscopy setups records the kinetic energy of the photoelectrons. Therefore, it is necessary to convert them to binding energy by subtracting their value from excitation energy. In the next step, the scale energy should be adjusted using Au 4f7/2 BE position. Au 4f spectra are collected from a gold film during sample characterization.

Assuming the peak position of Au 4f7/2 at 84 eV and finding the deviation from this value, all the recorded spectra are shifted accordingly. In the following, we should correct the intensity. For doing so, each spectrum is divided by the number of sweeps and beam current. The corrected data are then transferred to casaXPS for peak fitting. For peak fitting of the element's orbital levels of p, d, and f having doublet peaks three constraints including the area ratio, FWHM equality, and orbital splitting value of their states are applied. Before visualization data, all the peaks are normalized to the intensity of I 3d5/2. For the quantification assessment, the raw area of each component extracted from casaXPS is divided by IMFP (taken from [START_REF] Shinotsuka | Calculations of electron inelastic mean free paths. X. Data for 41 elemental solids over the 50eV to 200keV range with the relativistic full Penn algorithm[END_REF]) and photoionization cross-section (σ), and transmission factor (calculated and provided by the scientists of KMC-1 beamline of Bessy II).

The HAXPES spectra of the Cs 3d5/2, N 1s, Pb 4f, I 3d, Sn 3d5/2, and O 1s core level regions from the samples with bare MHP and after ALD SnO2 deposition are depicted in Figure 5. respectively. These peak shifts originate from the detrimental ALD process which consequently leads to defect formation at the interface and changes the band alignment in the surface layer.

Notably, this coincides with the formation of PbI2 species at the interface which is in agreement with our observation from the XRD measurements. We also monitored and compared the ratio of iodine to lead at the surface and subsurface of the perovskite layer before and after the ALD process. The quantification analysis shows that before SnO2 deposition, the I/Pb ratio near the MHP surface is 2.7 i.e. almost equal to the expected stoichiometry value followed by a minor negative deviation in the subsurface regions. However, we observed a severe iodine deficiency after the ALD process. This deficiency is more dominant in the topmost regions in which the I/Pb ratio drops to 0.7 almost one-fourth of its stoichiometry ratio. In addition, we observe around -36% deviation (the ratio of 1.72) from the expected stoichiometry value in the buried layers of MHP with SnO2 top layer measured under 6 keV excitation energy. Halide deficiency forms deeplevel defects in the metal halide perovskite solar cells. In consequence, this provokes interfacial charge recombination which would impede the carrier extraction and influences perovskite solar cells performance. In addition, the migration of the other anions and cations is another consequence of halide vacancies which results in irreversible chemical degradation of MHP that is a road blocker for the long-term stability of perovskite solar cells [START_REF] Wu | Reducing Surface Halide De fi ciency for E ffi cient and Stable Iodide-Based Perovskite Solar Cells[END_REF].

Additionally, in the Pb 4f core-level spectra for MHP/SnO2 sample, a new peak appears at 138.9 eV and 139.2 eV (under 2 keV and 6 keV respectively) which corresponds to the Sn 4s core level from the SnO2 film; however, it is not clear due to the adjacent Pb 4f core level fitting. This shift in peak position might be attributed to a band bending in the SnO2 film. To confirm this hypothesis Sn 3d and O 1s should be checked.

In the N 1s spectra shown in Figure 5.3 (c), we observe a new nitrogen species contribution at 398.5 eV for the sample with ALD-SnO2 under 2 keV excitation energy. However, this peak is not found under 6 keV excitation energy. This finding suggests that the formation of the new species is mostly confined to the interface region. The formation of a new chemical state of the nitrogen species at the perovskite/SnO2 interface and not in the buried layers of MHP film, shows the occurrence of an unwanted interfacial chemical reaction, as has been suggested in a study done by Hultqvist et al. [START_REF] Hultqvist | SnO x Atomic Layer Deposition on Bare Perovskite An Investigation of Initial Growth Dynamics[END_REF]. Here, we tentatively attribute this species to sym-triazine i.e. the Ncontaining heterocycle 1,2,5-triazine, which is formed due to formamidinium (-FA-) components of MHP [START_REF] Gunasekar | A Covalent Triazine Framework, Functionalized with Ir/N-Heterocyclic Carbene Sites, for the Efficient Hydrogenation of CO2 to Formate[END_REF], [START_REF] Thampy | Bulk and interfacial decomposition of formamidinium iodide (HC(NH2)2I) in contact with metal oxide[END_REF]. Furthermore, the N 1s peak centered at around 400 eV is linked to the FA species in MHP shifts by approximately 340-390 meV to lower binding energy after SnO2 deposition, in agreement with the I 3d and Pb 4f core level shifts. In addition, in Figure 5.3 (d), we see that the Cs 3d5/2 photopeak obtained under 2 keV and 6 keV, shows approximately 450 meV shifts to lower binding energies after the ALD process. This observation is in line with the peak shifts we observe in Pb 4f, I 3d, and N 1s core levels.

Collectively Pb 4f, I 3d, N 1s, Cs 3d5/2 core level shifts to lower binding energies after ALD-SnO2 deposition providing evidence of an upward band bending in the MHP films towards the interface with metal oxide overlayer. If we consider an approximate flat band condition for MHP prior to SnO2 deposition, the ALD process makes an upward band bending in MHP which in turn forms a detrimental electron transport barrier that hinders perovskite solar cell performance.

In Figure 5.3 (e) & (f) Sn 3d5/2 and O 1s of SnO2 components are illustrated. Concerning Sn 3d5/2 core level measured under 2 keV and 6 keV excitation energies, a 100 meV shift to higher binding energy from the surface to subsurface regions is observed with the peaks centered at 486.7 eV and 486.8 eV for 2 keV and 6 keV excitation energy, respectively. These peaks are attributed to the Sn 4+ state. In acquired O 1s spectra of bare MHP under 2 keV excitation energy, we see the presence of a peak arising from adventitious species. However, no trace of this peak is found in the 6 keV signal which collects information from subsurface regions from bare MHP film.

After performing the ALD deposition of SnO2 on MHP film, we see the presence of two O 1s peaks using 2 keV and 6 keV photon energies. The O 1s peak located at 530.5 eV under 2 keV corresponds to the SnO2 contribution while the same peak slightly shifted to 530.7 eV binding energy under 6 keV. The second contribution of O 1s spectra under 2 keV and 6 keV positioned respectively at 531.9 eV and 532.1 eV is attributed to the (Sn) hydroxide species [START_REF] Bracesco | The chemistry and energetics of the interface between metal halide perovskite and atomic layer deposited metal oxides[END_REF]. Both Sn 3d

and O 1s exhibit a peak shift to higher binding energy indicating a downward band bending toward interface in the SnO2 film itself. A slightly higher binding shift of O1s compared to that of Sn 3d (approximately 100 meV) is observed. This difference could arise from the recoil effect that is explained in section 4.2.4. of chapter 4.

All in all, the presence of residual byproducts such as hydroxides in the ALD-SnO2 film and detrimental effects originating from the reaction between MHP and precursors of the ALD process are the main barriers to the stability of MHP film. Noteworthy that the unwanted residual byproducts are commonly found in the low-temperature ALD process and they can be removed by a post-annealing treatment of 150-200 °C. However, the double cation mixed halide perovskite we developed at IPVF does not withstand this range of temperature.

Electronic Properties of PCBM /ALD-SnO2 Interface by HAXPES

In this section, a buffer layer of PCBM is integrated between ALD-SnO2 and MHP layers and the interfacial properties are studied using the HAXPES technique. Here, as demonstrated in Figure 5.4(a), although we see the Pb 4f signature under 2 keV photon energy, before the ALD process, no trace of this core level is found after 5 nm SnO2 deposition again under 2 keV. Additionally, same as the samples studied in the previous section, we found a new peak at binding energy higher than 138. All in all, we observe unwanted chemical reactions at the interface in both cases that could negatively affect the electronic interface properties.

135

Conclusion and Perspectives

This doctoral dissertation focused on the study of different interface systems between absorber and charge transport layers in metal halide perovskite solar cells. Interfaces in MHP solar cells are known as detrimental sources against the efficiency and stability of devices. Therefore, investigating interfacial features helps the perovskite community to understand better the interfacial properties and optimize them. With this target, we tried to shed light on three various interfaces. The main results and observations of each system are presented in the following. Also, some suggestions for future research work are given.

Interface Modification of ALD-NiOx/MHP using small organic molecules

The defective and reactive surface of the nickel oxide hole transport layer besides their nonoptimized energy level alignment with the MHP layer can limit the performance of perovskite solar cells. To tackle this problem, we employed three small organic molecules as the interlayer between NiOx and MHP (evaporated MAPbI3 and solution-processed FA 0.7 Cs 0.3 Pb(I 0.9 Br 0.1 ) 3 ).

In addition, another crucial topic that we studied is the impact of UV+Ozone post-treatment on the properties of NiOx film and consequently on the performance of the NiOx-based solar cells.

Our findings demonstrate that UV+Ozone post-treatment reduces the optical transmission of the NiOx layer. Additionally, XPS analysis showed that UV+Ozone treatment can induce an increase in the NiOOH ( Ni ≥3+ ) that makes the surface more reactive. In addition, lab-HAXPES measurements helped us to conclude that the UV+O3 process induces changes in the surface and topmost regions of the NiOx and not the bulk buried regions. According to UPS analysis, we reported a higher WF quantity of the UV+O3 treated NiOx compare to bare NiOx. This observation is linked to the formation of the dipole moment of NiOOH species. Additionally, we found that the UV+Ozone process moves the VBM of NiOx away from the Fermi level. This NiOx(UV+O3) layer less p-type, which is consistent with the generation of defects.

The properties of the NiOx layer after grafting three different types of organic molecules, i.e. PACz derivates, are studied. We observed that grafting MeO-2PACz, 2PACz, and Br-2PACz organic molecules on the NiOx surface, partially recovers the lower optical transmission of NiOx(UV+O3) by modification in the chemical properties of NiOx surface. According to XPS data, we observed that the small organic molecules reduce the high concentration of hydroxide-containing species in NiOx(UV+O3) by reconversion to stoichiometric NiO but also through the formation of Ni-O-P bonds. In addition, the XPS analysis showed that a more packed layer of 2P is found in comparison to M2P and B2P likely due to their smaller molecular structure. We reported that all three molecules move the VBM closer to the Fermi level in both two cases of as-deposited and UV+Ozone treated that makes the layer more p-type. We found that the different electronegativity of the electron-donating methoxy group in M2P affects the C1s spectrum and moves it to higher binding energy than 2P and B2P. Also, we found that the small organic molecules influence the ionization energy of the surface differently. While M2P reduces IE value, 2P and B2P increase it, with a higher IE quantity for B2P. This is attributed to different molecular dipole moments values that each molecule creates on the surface Based on XRD pattern, we found that the integration of 2PACz-based interlayers between NiOx and MHP mitigates the formation of PbI2 in the bulk of the perovskite layer. Further, we did not observe any significant change in the morphological and topographical properties. An obvious difference in optical measurements in the growth of MAPbI3 on each molecule is found. According to the XPS analysis, we observed a downward band bending within the MAPbI3 layer as we move away from the interface in the samples of NiOx(UV+O3). A similar observation is observed in the samples with the presence of organic interlayers except for the case of NiOx(asdeposited)/2P/MAPbI3. In addition, we found a deficiency of A-site cations at 10 nm thick MHP compared to the 20 nm thick counterpart deposited on NiOx(UV+O3). However, this effect is suppressed by the integration of the small molecules.

Regarding the device performance, we observe an enhancement in the overall performance of NiOx(UV+O3)-based solar cells utilizing M2P and 2P interlayers. However, they do not provide reproducible results on as-deposited NiOx. We obtained the champion cell performance of VOC:1.07 V, JSC:21.73 mA/cm 2 , FF: 62.9%, and PCE: 14.6% utilizing M2P interlayer between NiOx(UV+O3) and MAPbI3. Solar cells based on B2P interlayer present poor performance parameters due to the large offset of IE between the B2P-modified NiOx surfaces and MAPbI3 layer and thus the formation of a significant barrier for hole extraction.

We observed that utilizing 2PACz-based interlayers does not change the topography and crystallinity of FA 0.7 Cs 0.3 Pb(I 0.9 Br 0.1 ) 3 . Hyperspectral PL measurements suggest that UV+O3

treatment has a detrimental effect on the optical properties of the material, as collectively the UV+O3 treated samples show a lower PL intensity. Also, the samples with 2P interlayer show higher PL illumination. Regrading the performance of the solar cell, 2P and M2P improve the average overall PCE of the devices based on NiOx(as-deposited). Again using B2P devices results in the worst performances due to unfavorable band alignment in terms of a large valence band offset due to the high ionization energy of the B2P/NiOx surfaces. However, M2P surface modifier provides the best and most reproducible results on as-deposited NiOx with a champion cell showing VOC:1.05 V, JSC:22.3 mA/cm 2 , FF: 79.7%, and PCE: 18.6%.

To conclude we found that MeO-2PACz and 2PACz can function as passivating interlayers between NiOx and MHP. However, their functionality highly depends on the treatment of NiOx as well as on the composition and deposition method of the MHP overlayer.

For future research work, one important topic could be to determine the VBM, WF, and Eg and build a full band energy diagram of both two MHP materials deposited on eight different substrates. This would help us to understand better how energy level alignment at the interface is linked to the improvement or deterioration of device performance. The other step could be to find the source of inconsistency between PL and device performance for this one need to redo the PL measurement on a set of data from each configuration. Regarding the MAPbI3-based solar cells, more devices should be fabricated to validate the results and tendencies we obtained in this dissertation. In a broader scope, one interesting research topic could be grafting a combination of three organic molecules as interlayers. For example, Br-2PACz could likely offer a high degree of electron-blocking properties, and MeO-2PACz has presented good passivation ability, thus integrating an interlayer of their mixture might result in better device performance. In addition, similar to the XPS study done on MAPbI3, could be performed on FA 0.7 Cs 0.3 Pb(I 0.9 Br 0.1 ) 3 as well as the other MHP compositions to study how the band energy of MHP would be formed and evolved on different substrates.

Impact of carbon nanotubes growth on the interfacial composition and energetics of organic metal halide perovskites

Synchrotron-based HAXPES characterizations are carried out to investigate the interfacial properties between SWCNTs and MHP layers. 10 nm and 20 nm thick SWCNTs layers are deposited on MAPbI3 and MAPbBr3 perovskites film. Our findings show a downward band bending in the MAPbI3 with and without the presence of SWCNTs capping layer. The band bending is stronger in the case of bare MAPbI3. The results are obtained by measuring I 3d5/2 core level with the highly attenuated X-ray beam using an Al filter. However, using the X-ray beam with the same flux intensity does not reveal any band bending in the MAPbBr3 MHP film, likely due to low ionization cross-section and thus low signal-to-noise ratio. Employing the more intense Be filter-attenuated beam, we found an upward band bending in all the core level photopeaks of MAPbBr3 with a 20 nm thick SWCNTs overlayer. In addition, we monitor a downward band bending in the SWCNTs layer deposited on both MAPbI3 and MAPbBr3. The band bending in the MHP layer could be linked to the accumulation of lead halide species in the samples. The surplus of Pb and I could make a downward band bending in MAPbI3, but the accumulation of Pb and Br in the topmost regions of MAPbBr3 with 20 nm thick SWCNTs top layer likely has a reverse effect and induce an upward band bending in the MHP layer.

These findings have raised our understanding of SWCNTs and their interaction with MHP. For future works, it would be intriguing to complete the architecture with a top contact layer and fabricate the complete solar cells. Bringing the photovoltaic performance and our results together will provide us with deeper insight into SWCNTs functionality as HTL. In addition, one of the limitations for us in this work was that we had access only to the HAXPES data. Therefore, the next step could be to produce similar samples and perform some characterizations such as XRD and PL.

Synchrotron-based HAXPES Study of growing ALD-SnO2 on perovskite layer

We employed the synchrotron-based HAXPES technique to investigate the interface between These findings lead our group to optimize the PCBM deposition procedure and enhance the PCE of devices from almost 1% to 10%.

Looking ahead, finding a solution to reduce the interfacial reactions and limit the surface damage of MHP caused by the ALD process is crucial. From a fabrication point of view, one pathway is to mitigate the halide deficiency in the surface of MHP. For this, one can use the excess MHP precursors i.e. FAI, PbI2, CsI, PbBr2, etc. in the perovskite solution. The success of this approach could result in depositing SnO2 by the ALD method directly on the MHP layer which could facilitate the industrialization of this technology. Also, the integration of small molecules to create hydrophobic protective interlayers against the detrimental H2O and H2O2 ALD precursors is another interesting topic to pursue. Concerning the characterization, a combination of the HAXPES technique (different photon energy X-ray beams) and XPS and UPS would help to further understand the evolution of electronic and chemical properties at the interface of MHP/SnO2.

In the end, we can conclude that every individual interface systems depend on the type of layers (ETLs, absorber layers, HTLs, and interlayers), their deposition method, pre/post-treatment, the sequence of deposition, and other factors that could form differently which is required to be investigated and discovered. Abstract: Energy security, energy equity, and environmental sustainability are three important parameters to enhance life quality. Renewable energies are adapted to provide and guarantee these factors. Among various forms of renewable energy, solar energy is a steady and limitless source of energy that is globally available and accessible. Solar photovoltaic (PV) is an advanced technology that converts solar energy directly into electricity. Solar cells based on metal halide perovskite photoabsorbers represent an emerging PV technology showing a rapid enhancement in performance during the last decade. However, their efficiency and stability are limited by some phenomena such as non-radiative recombination of charge carriers and imperfection of crystalline lattice. These issues are more dominant at interfaces when the perovskite absorber layer is in immediate contact with charge transport layers made from different functional materials. Therefore, studying and understanding the interfaces is mandatory to push forward their efficiency and stability. This dissertation focuses on investigating the formation of interfaces between perovskite and charge transport layers in conventional and inverted solar cell configurations. Nondestructive surface-sensitive photoemission spectroscopy techniques i.e. XPS, UPS, and HAXPES are employed as the main characteristic methods to probe the interfaces.

The first part of the thesis is focused on the investigation of the nickel oxide hole transport layer surface and its adjacent interface with the absorber layer are known as detrimental sources in the inverted solar cell architecture. With the aim of modifying its interface and improving solar cell performance, three small organic molecules are employed as interlayers. Their effect on the optical, morphological, structural, chemical, and electronic properties of the thin films as well as the electrical properties of devices are reported. Subsequently, two additional alternative interface systems with a layer deposited on top of the perovskite are studied. Thin films of single-walled carbon nanotubes are formed on two various perovskite materials in conventional solar cell architecture. The formation of interfaces as well as the compositional gradients in the topmost and buried regions of perovskite are investigated. Finally, the interface of perovskite and tin oxide is studied. Deposition of tin oxide by the ALD technique directly on bare perovskite is found to be damaging to the perovskite layer underneath. To better understand the source of this process the perovskite/tin oxide interfacial features with and with the presence of PCBM interlayer are particularly investigated. The obtained results will help to expand our knowledge of interfaces and thus to enhance the efficiency and stability of perovskite solar cells.

Chapter 1 :Chapter 2 :

 12 Photoemission Spectroscopy (PES) ............................................................................................... 1.4.1.Lab-based Ultraviolet Photoelectron Spectroscopy (UPS) .............................................. 1.4.2.Soft X-ray Photoelectron Spectroscopy (XPS) ................................................................ 1.4.3.Hard X-ray Photoelectron Spectroscopy (HAXPES) ...................................................... 1.4.3.1.Laboratory-based HAXPES ....................................................................................... 1.4.3.2.Synchrotron-based HAXPES ..................................................................................... Interfaces in This Report .................................................................................................................. 1.5.1.Nickel oxide Interface with MHP Top Layer .................................................................. 1.5.2.MHP Interface with Tin Oxide Top Layer....................................................................... 1.5.3.MHP Interface with Carbone Nanotubes ......................................................................... Overview ............................................................................................................................................

Figure i.

  Figure i.1:Global greenhouse gas emission by sector [data of 2016] (a) Global energy consumption

1 : 1 Figure i. 2 : 2 Figure i. 3 : 3 Figure 1 . 1 : 7 Figure 1 . 2 : 8 Figure 1 . 3 : 9 Figure 1 10 Figure 1 . 5 : 12 Figure 1 . 6 : 13 Figure 1 . 7 : 16 Figure 1 . 8 : 16 Figure 1 . 9 : 18 XIFigure 1 . 10 :

 11223311712813911015121613171618161918110 Figure i.1:Global greenhouse gas emission by sector [data of 2016] (a) Global energy consumption per source [data of 2020](b) [3] .......................................................................................... 1

Figure 1 . 11 :

 111 Figure 1.11 : Structure of a synchrotron facility[49] .........................................................................

Figure 1 . 12 :

 112 Figure 1.12 : The schematic diagram of tridentate bonding between MeO-2PACz and NiO ...........

Figure

  Figure 1.13: XPS spectra of N 1s core level before and after ALD-SnO2 film deposition on bare

Figure 1 . 14 :

 114 Figure 1.14 : XPS spectra of I 3d5/2(a)Br 3d(b) Pb 4f(c) core levels of perovskite/ALD-SnO2[67] .

Figure 1 . 15 :

 115 Figure 1.15 : XPS and HAXPES spectra N 1s, Pb 4f, Br 3d, I 4d, O 1s and Sn 3d core levels of perovskite with and without ALD-SnO2 top layer [65] ....................................................

Figure 1 . 16 :

 116 Figure 1.16 : Schematic of graphene sheet(a) and rolled graphene sheet forming single-walled carbon nanotube(b) [137]Depending on the wrapping direction of SWCNTs, they could be metallic or semiconducting(c) [68] ...................................................................................

Figure 1 . 17 :

 117 Figure 1.17 : Schematic of a n-i-p MHP solar cell with a P3HT-wrapped SWCNT HTL sealed by PMMA[72] ........................................................................................................................

Figure 2 . 1 :

 21 Figure 2.1 : Chemical structure of the molecules (a) MeO-2PACz (b) 2PACz (c) Br-2PACz .........

Figure 2 . 2 :

 22 Figure 2.2: Schematic diagram of ALD process (e.g., Al2O3), each four-step growth is considered as one cycle [81] ....................................................................................................................

Figure 2 . 3 :

 23 Figure 2.3 : Illustration of a spin-coating system [138] .....................................................................

Figure 2 . 4 :

 24 Figure 2.4 : Schematic view of a vacuum thermal co-evaporation system. Adapted from [139] ......

Figure 2 . 5 :

 25 Figure 2.5: Schematic diagram of the UV-Vis spectrophotometer[140] ...........................................

Figure 2 . 6 :

 26 Figure 2.6: Schematic diagram of X-ray diffraction system (a) Bragg's law condition for diffraction from planes (b) [88]...........................................................................................................

Figure 2 . 7 :

 27 Figure 2.7: Schematic of working principle of an AFM system[91] .................................................

Figure 2 . 8 :

 28 Figure 2.8: Schematic skim of a SEM machine[141] ........................................................................

Figure 2 . 9 :

 29 Figure 2.9 : Illustration of energy band showing surface potential by Vs under dark (solid lines) and illumination (dashed line) and SPV by δVs ......................................................................

Figure 2 .

 2 Figure 2.10 : Current-voltage curve for a solar cell[142] ..................................................................

Figure 2 .Figure 3 . 1 : 52 Figure 3 . 2 : 54 Figure 3 . 3 : 55 Figure 3 . 4 : 57 Figure 3 . 5 : 58 Figure 3 59 Figure 3 . 7 : 61 Figure 3 . 8 : 63 Figure 3 . 9 : 64 Figure 3 . 10 : 66 Figure 3 . 11 : 68 Figure 3 . 12 : 69 Figure 3 . 13 : 70 Figure 3 . 14 : 71 Figure 3 . 15 : 71 Figure 3 . 16 : 72 Figure 3 . 17 : 74 Figure 3 . 18 : 75 Figure 3 . 19 : 77 Figure 3 . 20 : 78 Figure 3 . 79 Figure 3 . 21 : 80 Figure 3 . 22 : 81 Figure 3 . 23 : 82 XIVFigure 3 . 84 Figure 3 . 25 : 85 Figure 3 . 26 : 87 Figure 3 . 27 : 87 Figure 3 . 28 : 88 Figure 3 . 29 : 89 Figure 3 . 30 : 90 Figure 3 . 31 : 91 Figure 3 . 32 : 92 Figure 3 . 33 : 93 Figure 3 . 34 : 93 Figure 3 . 35 : 97 Figure 3 . 98 XVFigure 3 . 37 : 99 Figure 3 . 38 : 100 Figure 3 .Figure 4 . 1 : 108 Figure 4 . 2 : 110 Figure 4 . 3 :Figure 4 . 4 :Figure 4 . 5 : 113 Figure 4 115 Figure 4 . 7 :Figure 4 . 8 : 119 Figure 5 . 1 : 125 XVIFigure 5 . 2 : 126 Figure 5 . 3 : 129 Figure 5 . 4 :Figure A . 2 :

 23152325433553457355835937613863396431066311683126931370314713157131672317743187531977320783793218032281323823843258532687327873288832989330903319133292333933349333597398337993381003411084211043444511341154748119511255212653129542 Figure 2.11: A diagram representation of EQE measurement of a solar cell[142] ............................

Figure A. 7 :

 7 Figure A.7 : UPS spectra of MAPbI3 and FA0.7Cs0.3Pb(I0.9Br0.1)3 to determine work function(a)valence band maximum(b) Tauc plot to find energy band gap (c) ................

Figure A. 8 :

 8 Figure A.8 : SEM images of FTO/NiO/MHP/PCBM .....................................................................

  Figure i.1(a)). The biggest portion of consumed energy is

Figure i. 1 :

 1 Figure i.1:Global greenhouse gas emission by sector [data of 2016] (a) Global energy consumption per source [data of 2020](b)[3]

  Figure i.2 : Global cumulative installed and annual addition solar PV from 2009 to 2021[START_REF] Members | Renewables[END_REF]

Following this general

  Photoluminescence, I-V, etc. photoelectron spectroscopy is particularly used as the master technique to probe the interfacial physico-chemical characteristics between charge transport and MHP perovskite absorber layers. The fabrication and characterization processes performed in this

Figure i. 3 :

 3 Figure i.3 :The trend of MHP solar cells efficiency from 2013 to 2022 with a record efficiency of 25.7% [6]

Figure 1 .Figure 1 . 1 :

 111 Figure 1.1 illustrates an octahedral crystal structure of perovskite. In an ideal cubic perovskite structure, A is in cubo-octahedral site, and B occupies the center of the octahedral composed of six

Figure 1 . 2 :

 12 Figure 1.2 : Schematic illustration of VBM and CBM formation by atomic orbitals[START_REF] Bojar | Heterojunctions and silicon/perovskite tandem solar cells[END_REF]

Figure 1 .

 1 Figure 1.3 presents how different defects can cause disorder in an ideal lattice of MHP. Atomic vacancy happens when an atom leaves its position in the lattice which could lead to deep-level trap regions, and thus impede charge carrier mobility and reduce diffusion length. Interstitial defects can be a subsequent effect of atomic vacancies when the dislocated atom occupies an

Figure 1 . 3 :

 13 Figure 1.3 : Schematic illustration of an ideal lattice of ABX3 and the possible defects could occur in its lattice ( Green, black and blue circles shows A, B and X sites ,respectively. Impurities are shown in Red

Solar Cells 1 . 3 . 1 .

 131 Architecture and Working PrincipleThe MHP perovskite solar cells are composed of five main elements: (1) Transport conductive oxide (TCO) electrode layer such as FTO and ITO (2) Metallic conductive electrode layer such Au, Ag, Cu (3) Hole transport layer (HTL) such as Spiro-OmeTAD, NiO, CuO, CuI, Cu2O, PTAA (4) Electron transport layer (ETL) such as TiO2, ZnO, SnO2, SiO2 (5) Metal halide organicinorganic perovskite absorber layer. In a solar cell, the perovskite absorber layer is sandwiched between two charge transport layers (CTL) i.e. ETL and HTL[START_REF] Hussain | Functional materials , device architecture , and flexibility of perovskite solar cell[END_REF]. Depending on which charge transport layer is exposed to the sunlight, perovskite solar architecture is classified into n-i-p (conventional) and p-i-n (inverted) configurations. Figure 1.4(a) & (b) shows the schematic illustration of perovskite solar architectures. In the n-i-p structure, the sunlight first passes through

Figure 1 .

 1 Figure 1.4: n-i-p or conventional configuration of MHP solar cells (a) : p-i-n or inverted configuration of MHP solar cells (b)Energy level alignment and working principle of MHP solar cell (c) (Red circles are electrons and grey circles are holes)

Figure 1 . 5 demonstrates

 15 the charge transport and recombination path ways in MHP solar cells. The electrical parameters in a solar cell are controlled by a kinetic competition that occurs between charge carrier extraction to the external circuit and carrier recombination.Shunt recombination is caused by direct electrical contact between ETL and HTL. This recombination is indicated by number (7) in Figure 1.5. When MHP solar cell was in the beginning steps of their progress, recombination through shunt pathways was considered one of the main source of malfunctioning and low efficiency. Non-complete coverage of perovskite film and hence the presence of pin-holes was the main reason for the charge loss by shunt recombination.

Figure 1 . 5 :

 15 Figure 1.5: Possible charge Transfer and recombination processes in MHP solar cell: (1) & (2) are charge transfer path ways (3) is radiative recombination (4) is non-radiative trap-assisted recombination (5) & (6) are

Figure 1 . 6 :

 16 Figure 1.6 : Schematic view of energy levels of metal and organic material before contact (a) different cases of energy level diagrams of metal/organic interface after contact(b). c) Energy levels of organic and organic materials before contact (c) Various cases of energy level diagrams of organic/organic interface after contact (d) in subpanels of (b) & (d): vacuum level alignment(i) dipole layer formation(ii and iii) occurrence of band bending(iv and v)[START_REF] Wang | Energy Level Alignment at Interfaces in Metal Halide Perovskite Solar Cells[END_REF]

Figure 1 . 7 :Figure 1 . 8 :

 1718 Figure 1.7: Schematic of energy level alignment of a sample and spectrophotometer under photoemission spectroscopy[START_REF] Doh | Applications of Synchrotron-Based X-Ray Photoelectron Spectroscopy in the Characterization of Nanomaterials[END_REF]

Figure 1 . 9 :

 19 Figure1.9 : Schematic illustration of an XPS instrument[START_REF] Sci | Introduction to x-ray photoelectron spectroscopy Introduction to x-ray photoelectron spectroscopy[END_REF]

Figure 1 . 10 :

 110 Figure 1.10: Diagram of relative photoelectron intensity (P) respect to the probing depth (d) for Si 2p. IMFP for different radiation sources are listed in the legend[START_REF] Regoutz | A novel laboratory-based hard X-ray photoelectron spectroscopy system A novel laboratory-based hard X-ray photoelectron spectroscopy system[END_REF]

Figure 1 . 11 :

 111 Figure 1.11 : Structure of a synchrotron facility[START_REF] Chloé | Development of coevaporated perovskite thin films for tandem solar cells applications[END_REF]

  2022 Phung et al.[57] utilized MeO-2PACz with the chemical name of [2-(3,6-dimethoxy-9Hcarbazol-9-yl)ethyl]phosphonic acid as a surface modifier for NiOx deposited by ALD. A triple cation MHP of Cs0.05FA0.92MA0.03Pb(I0.83Br0.17)3 is employed as an absorber layer in a solar cell with ITO/NiO/(MeO-2PACz)/MHO/C60/ BCP/Cu configuration. They suggested that MeO-2PACz is grafted to the hydroxyl groups of the surface as shown below: An enhancement in VOC, FF, and PCE of the devices was observed after growing MeO-2PACz between NiOx and MHP accompanied by a dispersion in a narrower data range for MeO-2PACz.

  modified samples in comparison to the bare NiOx. EQE and IQE measurements reveal that the generation and collection of charge carriers are more efficient after the integration of MeO-2PACz interlayer into the device architecture. Moreover, a more suitable energy level alignment at the HTL/perovskite interface is achieved using MeO-2PACz that facilitates the hole extraction.In 2022, Zhul et al.[START_REF] Zhu | Nano Energy Inverted planar heterojunction perovskite solar cells with high ultraviolet stability[END_REF] studied the impact of MeO-2PACz and 2PACz ([2-(9H-Carbazol-9-yl)ethyl] phosphonic acid) interlayers to enhance the performance and stability of inverted perovskite with the configuration of FTO/NiOx/interlayer/ FA0.79MA0.16Cs0.05Pb(I0.9Br0.1)3 /LiF/C60/BCP/Cu. Both interlayers improved the performance of devices. The champion solar cell based on 2PACz interlayer demonstrates higher PCE and VOC from 1.05 V and 19.9% to 1.13 V and 22.2% compared to pristine NiOx/MHP device. The enhancement in VOC is attributed to more efficient charge extraction due to the passivation of interfacial defects. The stability of the different samples is examined under UV with a photon flux 10 times higher than AM 1.5G sunlight level. The samples with both interlayers are more stable under UV light than the reference sample and no formation of lead halide species is found. They suggested that UV light causes shallow traps at the interface of NiOx and MHP. The efficiency of samples with 2PACz drops to 90% of its initial PCE after 400

Figure 1 . 12 :

 112 Figure1.12 : The schematic diagram of tridentate bonding between MeO-2PACz and NiO[START_REF] Sun | NiOx-Seeded Self-Assembled Monolayers as Highly Hole-Selective Passivating Contacts for Efficient Inverted Perovskite Solar Cells[END_REF]

Figure 1 .

 1 Figure 1.13: XPS spectra of N 1s core level before and after ALD-SnO2 film deposition on bare FA0.83Cs0.17Pb(I0.83Br0.17)3 (a) LiF/C60 covered FA0.83Cs0.17Pb(I0.83Br0.17)3 (b)[START_REF] Palmstrom | Interfacial Effects of Tin Oxide Atomic Layer Deposition in Metal Halide Perovskite Photovoltaics[END_REF]

Figure 1 . 14 :

 114 Figure 1.14 : XPS spectra of I 3d5/2(a)Br 3d(b) Pb 4f(c) core levels of perovskite/ALD-SnO2[START_REF] Sci | The chemistry and energetics of the interface between metal halide perovskite and atomic layer deposited metal oxides The chemistry and energetics of the interface between metal halide perovskite and atomic layer deposited metal oxides[END_REF]

Figure 1 . 15 :

 115 Figure 1.15 : XPS and HAXPES spectra N 1s, Pb 4f, Br 3d, I 4d, O 1s and Sn 3d core levels of perovskite withand without ALD-SnO2 top layer[START_REF] Hultqvist | SnO x Atomic Layer Deposition on Bare Perovskite An Investigation of Initial Growth Dynamics[END_REF]

Figure 1 . 16 :

 116 Figure 1.16 : Schematic of graphene sheet(a) and rolled graphene sheet forming single-walled carbon nanotube(b)[137]Depending on the wrapping direction of SWCNTs, they could be metallic or semiconducting(c)[START_REF] Mceuen | Single-Walled Carbon Nanotube Electronics[END_REF]

Figure 1 . 17 :

 117 Figure 1.17 : Schematic of a n-i-p MHP solar cell with a P3HT-wrapped SWCNT HTL

Materials 2 . 1 . 1 .

 211 Metal Halide Perovskite (MHP) Absorber LayerIn the previous chapter, we exhaustively discussed the general compositional, structural, and electronic properties of halide perovskite materials. In this section, we talk about the different MHP materials used in this work. Generally, three various types of MHP materials are used in this report i.e. double-cation mixed-halide perovskite (FA 0.7 Cs 0.3 Pb(I 0.9 Br 0.1 ) 3 ), methyl ammonium lead iodide (MAPbI3) and methylammonium lead bromide (MAPbBr3). The thin films of these MHP are prepared either by wet process deposition (spin coating) or dry process (thermal coevaporation). In the following, the MHP fabrication processes followed in this work are described separately. Spin coating of 𝐅𝐀 𝟎.𝟕 𝐂𝐬 𝟎.𝟑 𝐏𝐛(𝐈 𝟎.𝟗 𝐁𝐫 𝟎.𝟏 ) 𝟑 This type of MHP is used in chapters 3 and 5. First, the solution with 1.2M concentration is prepared by dissolving PbI2, PbBr2, FAI, and CsI in Dimethylformamide (DMF)/ dimethyl sulfoxide (DMSO) (4:1 v:v) corresponding to the stoichiometric ratio mentioned here. To fabricate the films by spin-coating technique, 40 μL of perovskite precursor solution was spun at 1000 rpm for 10 seconds and 6000 rpm for 30 seconds, and 250 μL of ethyl acetate was dripped in a continuous stream within 15 seconds remaining in the second spin cycle, following by film annealing at 100 °C for 30 min.

  select three phosphonic acid-based molecules with identical head groups and spacer and different functional groups with the aim of modifying and passivating the surface of NiOx in chapter 3. Br-2PACz (B2P), 2PACz (2P), MeO-2PACz (M2P) are the molecules we employed and studied as surface modifiers. The chemical structures of the three molecules are shown in Figure 2.1 . As depicted, the three molecules are composed of a phosphonic acid head group (red highlighted) and an alkyl chain group (blue highlighted). Also, in the functional group, all of them consist of a carbazole organic structure. Nevertheless, the functional group of each molecule (green highlighted) has different type terminations with an organic component (OCH3) for MeO-2PACz, no component attached to the carbazole group in 2-PACz, and an inorganic termination of two bromine bonds to the carbazole group in Br-2PACz. In chapter 3, these three molecules are used to form an interlayer between NiOx and perovskite layers with the objective of interface passivation. Solutions of each molecule are prepared with a concentration of 2 mmol/l by adding the powders in anhydrous Ethanol in the glovebox. Then the

Figure 2 . 1 :

 21 Figure 2.1 : Chemical structure of the molecules (a) MeO-2PACz (b) 2PACz (c) Br-2PACz

4 )

 4 Another purging step to evacuate the byproducts, unreacted and undesirable compounds in the vapor phase from the chamber Controllability of ALD process through layer-by-layer growth and the possibility to alternate the number of cycles equip this technique with some excellent features such as high-conformality, precise thickness control, and low-temperature deposition[START_REF] Sci | Review Article : Atomic layer deposition for oxide semiconductor thin film transistors : Advances in research and development[END_REF][START_REF] Xing | Atomic Layer Deposition of Metal Oxides in Perovskite Solar Cells : Present and Future[END_REF].

Figure 2 . 2 :

 22 Figure 2.2: Schematic diagram of ALD process (e.g.,Al2O3), each four-step growth is considered as one cycle[START_REF] Sci | Review Article : Atomic layer deposition for oxide semiconductor thin film transistors : Advances in research and development[END_REF]

Figure 2 . 3 :

 23 Figure 2.3 : Illustration of a spin-coating system [138]

Figure 2 . 4 :

 24 Figure 2.4 : Schematic view of a vacuum thermal co-evaporation system. Adapted from[START_REF]greffage de M2P, 2P et B2P grâce à la modification des propriétés chimiques de la surface. Les mesures XPS montrent que la déposition de molécules sur les couches NiOx traitées par UV+Ozone réduit la concentration en hydroxydes[END_REF]

Figure 2 . 5 :

 25 Figure 2.5: Schematic diagram of the UV-Vis spectrophotometer[140]

Figure 2 .

 2 Figure 2.6: Schematic diagram of X-ray diffraction system (a) Bragg's law condition for diffraction from planes (b)[START_REF] Ermrich | XRD for the analyst[END_REF]

Figure 2 . 7 :

 27 Figure 2.7: Schematic of working principle of an AFM system[START_REF] Abou-Ras | Physics of Solar Cells Thin Film Solar Cells Handbook of Photovoltaic Science and Engineering[END_REF]

Figure 2 . 8 :

 28 Figure 2.8: Schematic skim of a SEM machine[141]

Figure 2 .

 2 Figure 2.9 depicts the band bending of the energy levels at the surface under dark and illumination. Considering that the energy is equal to -𝑒𝑉 (𝑒 is absolute number of the electron charges ), the potential profile can be described

Figure 2 . 9 :

 29 Figure 2.9 : Illustration of energy band showing surface potential 𝑏𝑦 𝑉 𝑠 under dark (solid lines) and illumination (dashed line) and SPV by 𝛿𝑉 𝑠

Figure 2 .

 2 Figure 2.10 : Current-voltage curve for a solar cell[142]

  LPICM and NREL. The techniques to study the optical, morphological, structural, chemical and electronic characteristics of thin films as well as the electrical properties of solar cells are described. The characterization processes are done in four different research centers i.e. IPVF, ILV, Hasselt university, and Bessy II synchrotron center.

Figure 2 .

 2 Figure 2.11: A diagram representation of EQE measurement of a solar cell[142]

3 Figure 3 . 1 :

 331 Figure 3.1 : Substrates configurations used in theis work of as-deposited and post treated NiOx with and without top layers of MeO-2PACz, 2PACz and Br-2PACz

Figure 3 .

 3 Figure 3.1 shows all substrate configurations used in this chapter. As-deposited and UV+O3 post-treated NiOx films are illustrated by gray (a) and black (b). To introduce the samples with the same molecule capping layer, similar color ranges are used. In this order orange, green, and blue are used to present the samples with M2P (c & d), 2P (e & f), and B2P (g & h). Additionally, darker color is used to contrast the UV+O3 treated samples with as-deposited ones. These color codes are kept throughout the entire chapter for a clear assignment of the results across the various measurement techniques and completed layer stacks.

Figure 3 .

 3 Figure 3.2(a) shows the transmission and reflection spectra of both samples in the range of 300-800 nm. The reflectance of NiOx after UV+O3 treatment exhibit almost no changes compared to NiOx. However, a drop of 5% in the sub-band-gap transmission (360 nm-800 nm) is observed for the UV+O3 treated sample. Regarding the absence of changes in the reflectance, a decrease in the transmittance goes normally along with an increase in the absorption of the UV+O3 treated film.

Figure 3 .

 3 Figure 3.2(b) illustrates the Tauc plot extracted using absorption spectra which itself is achieved by subtracting transmittance and reflectance from 100% incoming light. Using the Tauc plot, the optical band gap of the films can be determined. In this figure, we observe that despite the changein the transmission and absorption of NiOx film after the UV+O3 process, the value of the optical band gap remains constant at 3.64 eV. In addition, the appearance of a shoulder is found in the sub-gap region, marked by a red circle in the figure. According to Boyd et al.[START_REF] Boyd | Overcoming Redox Reactions at Perovskite-Nickel Oxide Interfaces to Boost Voltages in Perovskite Solar Cells[END_REF] this bump and therefore increase in absorption is attributed to an increase in Ni ≥3+ species after UV+O3 process.It is reported that Ni ≥3+ species include γ-NiOOH or β-NiOOH that have alternating Ni 3+ and Ni 4+ cations[START_REF] Li | Mosaic Texture and Double c-Axis Periodicity of β-NiOOH: Insights from First-Principles and Genetic Algorithm Calculations[END_REF].

Figure 3 . 2 :Figure 3 . 3 (

 3233 Figure 3.2 : Transmission and reflection spectra of NiOx and NiOx(UV+O3)(a) Tauc plot of NiOx and NiOx(UV+O3)

Figure 3 . 3 :

 33 Figure 3.3 : O 1s(a) Ni 2p3/2(b) XPS spectra of NiOx and NiOx(UV+O3)

  UPS measurements are performed to determine the NiOx energy level diagram and study how UV-O3 post-treatment impacts its VBM and WF. Figure 3.4 illustrates the secondary electron cutoff for the work function determination (left panel), UPS spectra of the valence band region (middle panel), and energy level diagram of NiOx and NiOx(UV+O3) samples. The VBM values are extracted from UPS measurements using an intersection between the background baseline and a linear fit to the leading edge of the low-binding energy onset of the main spectra. The energy level diagrams are then constructed from the WF and VBM values measured by UPS and the optical band gap Eg extracted from the UV-vis spectroscopy measurements shown in section 3.2.1.

Figure 3 .

 3 4 (a), we observe that the WF of NiOx increased from 4.57 eV to 4.79 eV after the UV-O3 process. In addition, while the VBM lies at 0.81 eV below EF, the UV+O3 post-treatment induces a shift of the VBM away from EF to 0.98 eV binding energy. The increase in WF is attributed to the formation of the dipole on the surface. The increase in the concentration of NiOOH and Ni(OH)2 on the surface after UV+O3 is the reason for the dipole formation[103]-[105]. However, NiOOH with a higher dipole moment of 1.09 Debye compare to Ni(OH)2 with a dipole moment of 0.19 Debye has a higher impact in enlarging of work function [106]. The shift of VBM further from EF signifies that the hole concentration of NiOx(UV+O3) is reduced and in turn, the surface of the film becomes less p-type and therefore less conductive.

Figure 3 . 4 :

 34 Figure 3.4 : UPS spectra of NiOx and NiOx(UV+O3) to determine work function(a)valence band maximum(b) energy level diagram of NiOx and NiOx(UV+O3) layers (c)

Figure 3 .

 3 5 contains the topographical images and the root means square (RMS) roughness values of bare NiOx as well as with three different top layers i.e. MeO-2PACz, 2PACz, and Br-2PACz. As we can see, the RMS values of the three 2PACz-based SAMs show a deviation of less than 1 nm from the one of bare

Figure 3 . 5 :

 35 Figure 3.5 : The topography AFM images of the surface of the bare NiOx(a) NiOx/MeO-2PACz (b) NiOx/ 2PACz (c) NiOx/Br-2PACz(d)

Figure 3 .Figure 3 . 6 :

 336 Figure 3.6 : Transmission and reflection spectra of NiOx and NiOx(UV+O3) with and without 2PACz-based top layers (a) Tauc plot of NiOx and NiOx(UV+O3) with and without 2PACz-based top layers (b)

  the surface-modified NiOx samples. For the sake of clarity in presenting the XPS characterization, samples made on as-deposited NiOx and UV+O3 post-treated substrates are shown separately next to each other. In this way, we more clearly capture the effect of the molecular grafting on the two inherently different NiOx surfaces (see section 3.2) The normalized XPS spectra of Ni 2p3/2 and O 1s core levels are depicted in Figure 3.7. Comparing the pristine NiOx and NiOx(UV+O3) to those with molecular overlayer in Figure 3.7(a) and (b), we see an upward shift of the background in the samples with the top layers. This difference might be explained by the fact that photo-excited electrons are further attenuated as they traverse the organic layer on top before they reach the analyzer. Aside from this change, we do not observe any changes to the main Ni 2p3/2 peak at 853 eV and the first satellite peak at 855 eV for the molecular films grown on top of the as-deposited NiOx film (Figure 3.7(a)). This suggests that grafting the phosphonic acids on top of the NiOx film do not significantly change the chemical coordination of the Ni species, i.e. do not cause the creation of defects. The Ni 2p3/2 spectra of the UV+O3 post-treated samples in Figure 3.7(b), contain other important information regarding the change made by small molecules on the surface. As we discussed in section 3.2.2., the UV+O3process causes an increase in hydroxide-containing species on the nickel oxide surface. This effect clearly shows up in the Ni 2p3/2 signal, for which the intensity of the peak positioned at around 855.5 eV increases. However, we now observe that after grafting of the molecule, the effect is reduced as the intensity of this peak decreases. This suggests a decrease in the concentration of nickel oxyhydroxide species. Notably, even with the grafted molecules on top the surfacemodified NiOx(UV+O3) films still exhibit more defective sites compared to any of the untreated NiOx films. For a more detailed picture of the interface chemistry affecting the NiOx surface, we turn to the analysis of the oxygen core levels.The normalized O 1s spectra of the samples on the untreated NiOx substrates, illustrated in Figure3.7(c), show that deposition of the organic molecules increases the peak intensity of "OH -and Ovacancies" contribution with respect to "O 2-" contribution. These new oxygen species can be attributed to the anchored phosphonic acid which also comprises a hydroxide functional group. In the case of MeO-2PACz we observe a further significant increase of the O 1s signal at 532.5 eV which is attributed to the methoxy end group. Nevertheless, in Figure 3.7(d) we find the inverse trend for the molecule-grafted NiOx(UV+O3). This trend is in agreement with the one in the Ni 2p3/2 spectra and hence suggests that the growth of organic molecules indeed changes the proportion of NiOOH and Ni(OH)2 species on the NiOx(UV+O3) surface likely by stripping of OH -groups in favor of the O 2-coordinated oxygen in NiO. With the exception of the O 2- contribution, the peak components and amplitude ratios in the O 1s region of the phosphonic acid-

Figure 3 . 7 :

 37 Figure 3.7 : Normalized XPS spectra of Ni 2p3/2 of the bare and molecules-grafted NiOx (a) Ni 2p3/2 of the bare and molecules-grafted NiOx(UV+O3) (b) O 1s of the bare and SAMs-grafted NiOx (c) O 1s of the bare and molecules-grafted NiOx(UV+O3) (d)

Figure 3 . 8 :Figure 3 . 9 :

 3839 Figure 3.8 : XPS spectra of Ni 2p3/2 of the bare and molecules-grafted NiOx (a) Ni 2p3/2 of the bare and molecules -grafted NiOx(UV+O3) (b) O 1s of the bare and molecules -grafted NiOx (c) O 1s of the bare and molecules -grafted NiOx(UV+O3) (d)

  9 (c) & (d). A peak in the positive direction in the range of 284 eV-285 eV appeared which originates from the C-Br bond. It should be mentioned that in the Br 2p spectra shown in Figure A.1(c) in the appendix, the Br trace is also found in the sample with a B2P top layer. Besides the proofs that we observed in Ni 2P3/2, O 1s, and C 1s core levels regarding the successful grafting process of small molecules, the P 2P and N 1s spectra of all the samples shown in Figure A.1(a) &(b) in the appendix verify the presence of the molecules on the nickel oxide surface. To understand more about the mechanism of UV+O3 treatment as well as 2PACz-based molecules grafting on the chemical properties of bulk and the surface of NiOx layer, lab-based HAXPES measurement with a photon energy of 9251.3 eV is performed in a collaboration with Hasselt University in Belgium. The setup in Hasselt is equipped with both, an Al Kα and a Ga Kα source, and therefore allows regular XPS as well as HAXPES measurements on NiOx and NiOx(UV+O3)/MeO-2PACz samples in the same analyzer spot. The normalized spectra of the Ni 2p and O 1s region are depicted in Figure 3.10. First, comparing the XPS spectra of Ni 2p3/2, shown in Figure3.10(a), we find the same trend observed and discussed previously. The UV+O3 posttreatment and also the grafting of the M2P molecules cause an increase in the contribution of hydroxide-containing nickel species. However, the HAXPES spectra of the two samples are almost identical. This finding proves two points: First without the UV+O3 treatment, the XPS and HAXPES data of the NiOx films are very comparable, suggesting a low density of hydroxide species and oxygen vacancy defects at the surface of the ALD-produced films. Only a marginally higher contribution of NiOOH and Ni(OH)2 is found on the surface of NiOx in comparison to the bulk. Second, while the XPS data indicates a change in the chemical properties of the topmost regions after the UV+O3 treatment and molecular grafting reproducing the data shown in Figure3.7b, the HAXPES measurements reveal that the bulk remains unaffected. In fact, the HAXPES signal of both samples are almost identical meaning that even after the UV+O3 treatment and M2P grafting the NiO bulk exhibit as low a ratio of hydroxide species and oxygen vacancy defects comparable to the pristine NiO film.Normalized HAXPES spectra of O 1s core levels in Figure3.10(b) also present similarly larger intensities of "OH -and Ovacancies" and "physisorbed species" contributions in NiOx(UV+O3)/M2P sample. However similar to Ni 2p3/2 spectra, almost no changes are found in the HAXPES spectra of the two samples. This again explains that both processes of UV+O3 and deposition of surface modifiers truly alter only the surface properties of the samples and not the bulk regions.

Figure 3 .

 3 11(a) & (b) show the UPS measurements to determine the WF and VBM of all layers discussed in this section. As explained previously, WF and VBM values are extracted from the intersection point of the baseline and linear fitting of secondary electron cut-off and valence band region, respectively. Moreover, in collaboration work with Davide Regaldo,graduate student at IPVF, the WF values are also measured by the KP technique. The obtained WF values from KP and UPS are shown and compared in Figure A.2 in the appendix. We achieve a good overall agreement between the two values measured by the techniques. The energy diagram, in Figure 3.11(c), is prepared

Figure 3 . 10 :

 310 Figure 3.10 : Normalized XPS and HAXPES spectra of Ni 2p3/2 (a) O 1s (b) core levels of NiOx and NiOx(UV+O3)/MeO-2PACz

  non-similar tendency in WF and IE of three molecules on two different substrates is attributed to the role of the dipoles that each molecule creates on the surface. Based on DFT calculations the molecular dipole moment values of 0.2 D, 2 D, and 4.98 D are reported for M2P, 2P, and B2P[111]. These values are calculated for gas-phase molecules without phosphonic acid (PA) anchoring group. However, the overall dipole moment of a system by considering other parameters such as the dipole moment of the substrate and charge exchange due to the chemical bond resulting from the grafting could change. As an example, Lin et al. calculated dipole moments of three molecules on ITO substrate and they reported -0.94 D to -1.57 D for M2P, 1.53 D for 2P, and 3.56 D for B2P[111]. Considering the NiOx substrate and the role of dipoles induced by UV+O3 post-treatment, a DFT modeling is required to calculate the individual dipole moment of each system.

Figure 3 . 11 :

 311 Figure 3.11 : UPS spectra of bare and molecule grafted NiOx and NiOx(UV+O3) to determine work function(a)valence band maximum(b) energy level diagram of of bare and molecule grafted NiOx and NiOx(UV+O3) (c)

Figure 3 . 12 :Figure 3 . 13 :

 312313 Figure 3.12 : CPD evolution of the bare and molecule grafted NiOx and NiOx(UV+O3)

Figure 3 . 14 :Figure 3 . 15 :

 314315 Figure 3.14 : Top view SEM image of the MAPbI3 (450 nm thickness) on bare and molecule grafted NiOx and NiOx(UV+O3)

  Figure 3.16 : Reflection (a) and transmission (b) spectra of MAPbI3(450 nm thickness) deposited on bare and molecule grafted NiOx and NiOx(UV+O3)

  , where I show the normalized Pb 4f7/2, C 1s, N 1s and I 3d5/2 core levels spectra of 10 nm and 20 nm of MAPbI3 grown on as-deposited and UV+O3 treated NiOx films. The peaks are normalized to their maximum intensity. Comparing the binding energy position of all the core levels of 10 nm and 20 nm MHP layer formed on the bare untreated NiOx, we observe a minor change as a function of MAPbI3 layer thickness. By taking into account the energy resolution of XPS measurement i.e. 100 meV the binding energy shifts are negligible. This observation is an indication that no band bending occurs at the immediate interface of NiOx and MAPbI3. Nevertheless, when the MAPbI3 layer is deposited on UV+O3-treated NiOx, shifts of approximately 150 meV-200 meV are found in all the core levels by changing the thickness of the MHP. The binding energy positions move towards higher values for increasing MHP thickness from 10 nm to 20 nm suggesting a downward band bending as we move away from the interface towards the MAPbI3 surface. Moreover, comparing the core level position of 10 nm MAPbI3 on as-deposited and post-treated NiOx, no change in peak position is observed indicating that the nature of the nickel oxide surface does not influence the peak position of MAPbI3 at the very immediate interface regions. An observation that should be pointed out is the difference in the second peak of C 1s spectra between two substrates. This peak centered at around 285 eV is attributed to the physisorbed species on the surface of NiOx. The photo-reaction process during UV+O3 post-treatment increases the concentration of this contribution as described in section 3.2. The stoichiometric analysis is carried out by peak fitting of the Pb, I, and N spectra. Due to the complexity of the C 1s peak given the potential contaminants, we refrain from providing an affirmative analysis of this core level. Alternatively, to provide the concentration of MA we consider the amount of N which has the number of atoms as C in MAPbI3. The elemental distribution of the 10 nm and 20 nm thick MAPbI3 layers are extracted and tabulated in Table 3.1.

Figure 3 . 17 :Figure 3 . 18 :

 317318 Figure 3.17: XPS spectr for the core levels of Pb 4f7/2 (a) C 1s (b) N 1s (c) I 3d5/2, (d) of 10 nm and 20 nm MAPbI3 on pristine and UV+Ozone treated NiOx

Figure 3 .

 3 19, while the corresponding the Pb 4f7/2, N 1s, and C 1s data can be found in the appendix in Figure A.4-Figure A.6.The I 3d5/2 signal for MAPbI3 deposited on M2P and B2P-modified NiOx exhibits a shift of 250 meV to a higher binding energy, when the thickness of the MHP layer is increased from 10 nm to 20 nm. Thus, considering the flat band in MAPbI3 grown directly on as-deposited NiOx, as discussed previously, M2P and B2P interlayers induce a downward band bending within the MAPbI3 layer as we move away from the interface, which is a generally more favorable alignment improving hole extraction. However, in the case of the 2P interlayer, we did not monitor any change in the binding energy of the photo peaks coming from the 10 nm and 20 nm perovskite layers. The C 1s spectra shown in Figure A.6 in the appendix could explain this. Although the ratio of C 1s signal originating from the organic HTL to the C 1s signal from the MA species is

Figure 3 . 19 : 2 I 2 Figure 3 . 20 :

 31922320 Figure 3.19 : Normalized XPS spectra of I 3d5/2 of 10 nm and 20 nm MAPbI3 deposited on : as-deposited NiOx with MeO-2PACz interlayer (a) as-deposited NiOx with 2PACz interlayer (b & d) as-deposited NiOx with Br-2PACz interlayer (c) UV+Ozone treated NiOx with MeO-2PACz interlayer (d) UV+Ozone treated NiOx with 2PACz interlayer (e) UV+Ozone treated NiOx with Br-2PACz interlayer(f)

Figure 3 . 22 :

 322 Figure 3.22: Energy level diagram of of bare and molecule grafted NiOx and NiOx(UV+O3) and MAPbI3 deposited on as-deposited NiOx

Figure 3 . 23 :

 323 Figure 3.23 : Schematic of the p-i-n MAPbI3-based solar cells with MeO-2PACz, 2PACz and Br-2PACz interlayer between MHP and NiOx(as-deposited or UV+O3) layers

Figure 3 . 24 :

 324 Figure 3.24 : Photovoltaic parameters of MAPbI3-based solar cells with MeO-2PACz, 2PACz and Br-2PACz interlayer between MHP and NiOx(as-deposited or UV+O3) layers

Figure 3 .

 3 Figure 3.25 shows the J-V curve for the best devices with NiOx(as-deposited) and NiOx(UV+O3) HTL as well as M2P-modified counterparts. Although the devices with NiOx(as-deposited)/M2P do not show a high degree of consistency, one device with this HTL alongside the reproducible devices with NiOx(UV+O3)/M2P HTL demonstrates the highest PCE of 14.6% in the reverse bias.

Figure 3 .

 3 Figure 3.25 : J-V measurements for the non-modified and M2P grafted devices based on MAPbI3 photoabsorber (a) for the 2P grafted and B2P grafted devices based on MAPbI3 photoabsorber(b) Photovoltaeic parameters of the solar cells

Figure 3 . 26 :

 326 Figure 3.26 : EQE curve representing MAPbI3-based devices with five different HTL

Figure 3 .

 3 Figure 3.27(a) MHP films show almost similar crystal structures on the different substrates.Additionally and in contrast to the evaporated MAPbI3 films, no formation of PbI2 is found in the bulk of the MHP layers.

Figure 3 .Figure 3 . 28 :

 3328 Figure 3.27 : XRD pattern of double-cation mixed-halide perovskite films on the bare and molecule grafted NiOx and NiOx(UV+O3) in the range of 2θ between 10° and 45° (a)the magnified pattern between 11.5° and 15° to see the possible formation of PbI2 (b)

Figure 3 . 29 :Figure 3 . 30 :

 329330 Figure 3.29: The topography AFM images of the surface of the double-cation mixed halide perovskite on bare and molecule grafted NiOx and NiOx(UV+O3)

Figure 3 .

 3 Figure 3.32(a), the PL spectrum of MHP on NiOx/M2P shows a lower intensity than NiOx reference samples which might be linked to the smaller grain domain size seen in NiOx/M2P/MHP sample, discussed in the previous section. In Figure 3.32(b), the intensity of MHP formed on UV+O3 treated NiOx does not change significantly after the integration of M2P and B2P interlayers.

Figure 3 . 31 :Figure 3 . 32 :

 331332 Figure 3.31 : Hyperspectral images at 760nm for double-cation mixed halide perovskite on bare NiOx (a) MeO-2PACz surface-modified NiOx(b) 2PACz surface-modified NiOx(b) Br-2PACz grafted NiOx(b)

Figure 3 . 33 :

 333 Figure 3.33 : Energy level diagram of of bare and molecule grafted NiOx and NiOx(UV+O3) and double-cation mixed halide perovskite deposited on UV+Ozone treated NiOx

FTO

  /NiOx/(Interlayer)/FA0.7Cs0.3Pb(I0.9Br0.1)3/PCBM/ZnO/Ag configuration. PCBM layers are grown by spin-coating on the MHP followed by ZnO nanoparticles deposition from spin-coating. The device fabrication is finalized by evaporation of a 100 nm silver top contact with an active area of 3×3 mm 2 in the final step. The schematic figure of the devices is shown in Figure 3.35. . I then determined the electrical properties using J-V, EQE, and photostability tests.

Figure 3 . 35 :

 335 Figure 3.35 : Scheme of the cell based on double-cation mixed-halide perovskite with MeO-2PACz, 2PACz and Br-2PACz interlayers between MHP and NiOx(as-deposited or UV+O3) layers

Figure 3 . 36 :

 336 Figure 3.36 : Photovoltaic parameters of FA0.7Cs0.3Pb(I0.9Br0.1)3-based solar cells with MeO-2PACz, 2PACz

Figure 3 .

 3 Figure 3.37 : J-V measurements for the best cells with HTL of NiOx(asdeposited), NiOx(as-deposited )/2P and NiOx(as-deposited )/M2P HTLs

Figure 3 .Figure 3 . 39 :Figure 3 .Conclusion

 33393 Figure 3.38 : EQE curve representative of FA0.7Cs0.3Pb(I0.9Br0.1)3-based devices with eight different HTL

Contents 4 . 1 . 108 4. 2 . 1 . 108 4. 2 . 2 . 115 4. 3 . 118 4. 4 .

 41108211082211531184 SWCNTs in MHP solar cells............................................................................................................... 106 4.2. Electronic properties at Perovskite/SWCNTs interface ...................................................................... Impact of attenuated intensity beam on perovskite films .......................................................... Electronic properties in dark and light ....................................................................................... 111 4.2.3. Impact of SWCNTs growth and its thickness on electronic properties ..................................... 112 4.2.4. Potential band bending and electronic properties in different depth ......................................... Compositional gradient of samples buried and topmost regions ......................................................... Conclusion ........................................................................................................................................... 120

  Figure 4.2(b), we do not observe any peak shift in all the samples by raising excitation energy from 2 keV to 6 keV. However, then by increasing the excitation energy to 8 keV, we find a larger shift of 130-200 meV peak shift to the higher binding energy. Considering all the observations, one could explain that the initial measurements performed with the highly attenuated X-ray beam reduced using the aluminum filter provide the initial state of the MAPbI3 perovskite layer without significant beam damage. Particularly the short exposure time and the observed peak positions shift indicate band bending in the MAPbI3 absorber layer by 300 meV without the SWCNT layer and 200 meV with the SWCNTs layer on top .

  Similar measurements are performed on the Br 3d core level of MAPbBr3. The short-range binding energy spectra obtained by Al-filter attenuated beam are shown in Figure 4.3(a). Contrary to I 3d core level of MAPbI3, no noticeable change in the peak position of Br 3d of MAPbBr3 samples is found due to the low photoionization cross-section and hence low signal-to-noise ratio. Likely,

Figure 4 . 2 :

 42 Figure 4.2 : HAXPES spectra curve fitted for the I 3d5/2 of MAPbI3 core level measured under attenuated X-ray beam using the aluminum filter(a) and the beryllium filter(b)

Figure 4 .

 4 4(a), the I 3d5/2 core level spectra of MAPbI3 film with and without SWCNTs top layer acquired in dark and light are presented. The light condition is provided using an external halogen lamp. Comparing the peak positions of bare MAPbI3 and the sample with 20 nm SWCNTs film, we do not observe any change in peak position comparing measurements performed in the dark and measurements performed under external illumination, with peaks centered at around 619.5 eV.

Figure 4 . 3 :

 43 Figure 4.3 : HAXPES spectra curve fitted for the Br 3d of MAPbBr3 core level measured under attenuated Xray beam using the aluminum filter(a) and the beryllium filter(b)

Figure 4 .

 4 5(a) depicts the spectra of the principal core levels composing MAPbI3

Figure 4 . 4 :

 44 Figure 4.4 : HAXPES spectra curve fitted for (a) the I 3d5/2 of MAPbI3 core level under light and dark conditions (b) the Br 3d of MAPbBr3 core level under light and dark conditions

  Figure 4.5(b), the corresponding peaks are positioned at the same binding energies of 2 keV without presenting any

Figure 4 . 5 :

 45 Figure 4.5 : HAXPES spectra curve fitted for the main core levels, i.e., of Pb 4f, Br 3d. C 1s and N 1s of MAPbI3 before and after SWCNT deposition (10 nm and 20 nm) measured under (a) 2 keV and (b) 6 keV excitation energy

  Figure 4.6(a) and

Figure 4 .

 4 Figure 4.6(b) show the spectra acquired under 2 keV and 6 keV excitation energies, respectively.

Figure 4 .

 4 5 and Figure 4.6 as a direct comparison of the signal obtained for the 2 keV and 6 keV excitation energies for each sample. Pb 4f, I 3d, C 1s and N 1s core levels of MAPbI3 layer without capping layer and also with 10 nm and 20 nm SWCNT top layers measured under 2 keV and 6 keV excitation energies are compared in Figure 4.7(a).Peak positions and FWHMs of fitted peaks for mentioned core levels except for C 1s, due to the complexity of C 1s, are depicted in Table4.1. All the spectra in this section are measured under the beryllium filter-attenuated X-ray beam and for Pb 4f and I 3d core levels the data is acquired in the large window kinetic energy to cover both spin-orbit split orbitals i.e. Pb 4f7/2 and Pb 4f5/2 in the case of Pb and I 3d5/2 and I 3d3/2 in the case of I. Bare MAPbI3 does not show any shift in Pb 4f5/2 and I 3d5/2 core levels peak from 2 keV to 6 keV. The samples with SWCNTs(10nm) and SWCNTs(20nm) present the same manner and the core level peak positions near the surface and buried layer (corresponding to 2 keV and 6 keV, respectively) remain unchanged. This could lead

Figure 4 .

 4 Figure 4.6 : HAXPES spectra curve fitted for the main core levels, i.e., of Pb 4f, Br 3d. C 1s and N 1s of MAPbBr3 before and after SWCNT deposition (10 nm and 20 nm) measured under (a) 2 keV and (b) 6 keV excitation energy

Figure 4 .

 4 Figure 4.7 (b) presents the spectra of Pb 4f, Br 3d, C 1s and N 1s core levels originating from MAPbBr3 with and without SWCNTs overlayers measured under a Be filter-attenuated X-ray beam.

Figure 4 . 7 :

 47 Figure 4.7 : HAXPES spectra curve fitted for the main core levels, i.e. (a) Pb 4f, I 3d. C 1s and N 1s of MAPbI3 (b) Pb 4f, Br 3d. C 1s and N 1s of MAPbBr3 with and without SWCNTs overlayers under 2 keV and 6 keV excitation energy.

Figure 4 . 8 :

 48 Figure 4.8 : Schematic illustration summarizing the compositional gradient in various depths of (a) MAPbI3 and (b) MAPbBr3 with and without SWCNTs overlayers

Figure 4 .

 4 Figure 4.8(b) shows the obtained quantitative analysis for the MAPbBr3 layer. Contrary to bare MAPbI3, we observe a surplus of nitrogen in the surface and subsurface of the bare, more dominant in the near-surface region, Growing SWCNTs on top of perovskite is accompanied by a drop in the nitrogen concentration, particularly in the topmost layers, which eventually results in a nitrogen deficiency of 5% in the sample with the 20 nm SWCNTs. Similar to MAPbI3 samples,

Figure 5 .

 5 photoelectron spectroscopy techniques, a very thin layer of SnO2 (5 nm) was formed by performing ALD in 30 cycles. The schematic of the samples with and without PCBM buffer layer is illustrated in Figure5.1 (b).

Figure 5 .

 5 2 (a) illustrates the XRD patterns of bare perovskite film, with ALD-SnO2, PCBM, and PCBM/ALD-SnO2 overlayers. Comparing the pattern of all samples, we see that a low-intensity PbI2 peak at 12.5° appeared in the samples with the ALD-SnO2 layer on top. Considering the fact

Figure 5 . 1 :

 51 Figure 5.1 : Schematic of the ALD process with Tetrakis(dimethylamido)tin(IV) (TDMASn) and H2O2 precursors to form SnO2 layers.[Reprinted from Nitin Mallik's thesis with permission] (a) Schematic of the p-i-n MHP samples configuration measured under 2 keV and 6 keV excitation energies (b)

Figure 5 . 2 :

 52 Figure 5.2 : (a) XRD pattern of double cation MHP films with and without PCBM/ALD-SnO2 top layers (b) UV-Vis absorption spectra of samples with and without (PCBM)/ALD-SnO2 top layers. [Reprinted from Nitin Mallik's thesis with permission]

  3. The binding energy values of fitted core levels of all samples are listed in Table 5.1 The Pb 4f and I 3d core level signals (Figure 5.3 (a) & (b)) of pristine MHP show a minor shift in peak position(20- 100 meV) between the data acquired for photon energies of 2 keV and 6 keV. Considering the resolution of the HAXPES spectra this order of magnitude for peak shift cannot be considered as a shift in their actual binding energy. However, the deposition of SnO2 results in a bigger shift of the lead and iodine core levels by approximately 210 and 290-320 meV to lower binding energies,

Figure 5 . 3 :

 53 Figure 5.3 : HAXPES spectra curve fitted to the core levels of (a) Pb 4f (b) I 3d (c) N 1s (d) Cs 3d5/2, (e) Sn 3d5/2, (f) O 1s of double cation MHP without (blue spectra) and with (green spectra) ALD SnO2 top layers in FTO/NiOx/MHP recorded with 2 and 6 keV excitation energy (g) Schematics of band bending at the interface of MHP/ALD-SnO2

Figure 5 .

 5 Figure 5.4 illustrates the HAXPES results of the samples with PCBM deposited on MHP film with and without the ALD-SnO2 buffer layer. We deposit a 40nm thick PCBM interlayer in this experiment to protect the MHP from the detrimental interfacial reaction at initial exposure to the ALD precursors. The procedure for PCBM growth follows the recipe according to which the PCBM solution did not warm up before deposition and the samples are annealed at 100 °C for 10 minutes after PCBM deposition.

  photon energy. After the ALD process, the I 3d core level, show a shift of almost 390 and 330 meV to higher binding energy for the 2 keV and 6 keV excited spectra, respectively. The peak shift observed could be linked to both ion migration and undesired ALD reactions. Additionally, the Cs 3d core level signals, Figure5.4 (c), are found in the sample with PCBM top layer under

Figure 5 . 4 :

 54 Figure 5.4 : HAXPES detail spectra for the core levels of (a) Pb 4f (b) I 3d (c) Cs 3d5/2 (d) Sn 3d5/2 and (e) O 1s of double cation MHP and SnO2 top layers for the samples without (yellow spectra) and with (red spectra) ALD-SnO2 top layer in FTO/NiOx/MHP/PCBM structure measured using 2 and 6 keV excitation energy

FA 0. 7

 7 Cs 0.3 Pb(I 0.9 Br 0.1 ) 3 and ALD-SnO2 layers. The chemical and electronic properties of the MHP/SnO2 interface with and without the PCBM organic buffer layer were investigated and reported. Our study presents that deposition of the SnO2 ETL layer directly on the MHP causes the formation of new chemical species such as azine and lead dihalides. The presence of residual byproducts combined with the ALD-induced degradation at the MHP/metal oxide interface hinders the development of more stable MHP solar cells. Additionally, we reported deposition of SnO2 by the ALD process induces an upward band bending which forms an electron transport barrier detrimental to PSCs performance. After the ALD process, we observed a halide deficiency in the topmost regions of MHP. Halide deficiency forms deep-level defects in the MHP solar cells and causes interfacial charge recombination. This could interrupt an efficient carrier extraction and influences the performance of perovskite solar cells.Moreover, the interface study of MHP/ALD-SnO2 with the presence of a 40 nm PCBM buffer layer revealed that the PCBM interlayer does not fully protect the MHP layer. Two possible sources are attributed to this issue. Ion migration of MHP-segregated compounds into the PCBM layer or the presence of pinholes in the surface of PCBM are two possible detrimental sources.

Figure A. 3 :Figure A. 4 :Figure A. 5 :FigureFigure A. 7 :

 3457 Figure A.3 : Normalized XPS spectra of C 1s and N 1s of : 10 nm MAPbI3 deposited on the bare and molecules-grafted NiOx (a & c) 10 nm MAPbI3 deposited on the bare and molecules -grafted NiOx(UV+O3) (b & d) 20 nm MAPbI3 deposited on the bare and molecules-grafted as-deposited NiOx (e & g) 20 nm MAPbI3 deposited on the bare and molecules -grafted NiOx(UV+O3)) (f & h)

  .6) 𝑃 𝑚 is the maximum electrical power produced by a solar cell  𝑃 𝑖𝑛𝑐 is the incident power to the solar cell  𝑉 𝑚𝑝 and 𝐼 𝑚𝑝 are voltage and current at maximum power respectively  𝐹𝐹 stands for fill factor which is equal to

	𝑃𝐶𝐸 =	𝑃 𝑚 𝑃 𝑖𝑛𝑐	=	𝑉 𝑚𝑝 𝐼 𝑚𝑝 𝑃 𝑖𝑛𝑐	=	𝑃 𝑖𝑛𝑐 𝐹𝐹 × 𝑉 𝑂𝐶 × 𝐽 𝑆𝐶	Eq. (2.6)
	Where:
	 𝑉 𝑚𝑝× 𝐼 𝑚𝑝
						𝑉 𝑂𝐶 ×𝐽 𝑆𝐶

 𝑉 𝑂𝐶 and 𝐼 𝑆𝐶 are open-circuit voltage and short-circuit current respectively

  In this chapter, the surface of the NiOx layer deposited by ALD is studied. Three different phosphonic acid derivatives are used as a molecular layer to passivate and modify the surface of NiOx. To evaluate the impact of these surface modifiers on solar cell performance, devices based on co-evaporated MAPbI3 and solution-processed double-cation mixed-halide perovskite are fabricated. The surface of the NiOx layer and its adjacent interface with perovskite are characterized. Subsequently, the performance of the solar cell fabricated using two MHP materials is reported. .5.4.1. J-V and EQE Measurements.................................................................................................. 3.5. Study the Interface of ALD-NiOx/ FA0.7Cs0.3Pb(I0.9Br0.1)3with and without Interlayers ....................... 3.5.1. Structural and Morphological Properties ..................................................................................... 3.5.2. Optical Properties ........................................................................................................................ 3.5.3. Electronic Properties .................................................................................................................... 3.5.4. Electrical Features of Devices ..................................................................................................... 3.5.4.1. J-V and EQE Measurements..................................................................................................

	3. Chapter 3
	Interface Modification of ALD-NiO x /MHP using small
	organic molecules

Summary: Contents 3.1. Introduction ........................................................................................................................................... 3.2. The Effect of UV+Ozone Post Treatment on ALD-NiOx..................................................................... 3.2.1. Optical Properties ........................................................................................................................ 3.2.2. Chemical and Electronic Properties ............................................................................................. 3.3. Surface Modification of ALD-NiOx by Phosphonic-acid Derivative Molecules .................................. 3.3.1. Morphological Properties ............................................................................................................ 3.3.2. Optical Properties ........................................................................................................................ 3.3.3. Chemical and Electronic Properties ............................................................................................. 3.4. Study the Interface of ALD-NiOx/MAPbI3 with and without Interlayers ............................................. 3.4.1. Structural and Morphological Properties ..................................................................................... 3.4.2. Optical Properties ........................................................................................................................ 3.4.3. Chemical and Electronic Properties ............................................................................................. 3.4.4. Electrical Features of Devices ..................................................................................................... 33.5.4.2. Photo-Stability Test ............................................................................................................... 3.6. Conclusion ...........................................................................................................................................

NiO x NiO x (UV-O 3 ) NiO x /M2P NiO x (UV-O 3 )/M2P NiO x /2P NiO x (UV-O 3 )/2P NiO x /B2P NiO x (UV-O )/B2P 10 20 30 40 50 60 70 FF (%) Fill Factor

		26							Short Circuit Current
		24
		22
	J sc (mA/cm 2 )	6 8 10 12 14 16 18 20
		4
		2
		0
		-2	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev
			NiO x		NiO x (UV-O 3 )	NiO x /M2P	NiO x (UV-O 3 )/M2P	NiO x /2P	NiO x (UV-O 3 )/2P	NiO x /B2P	NiO x (UV-O )/B2P
			Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev
			Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev

NiO x NiO x (UV-O 3 ) NiO x /M2P NiO x (UV-O 3 )/M2P NiO x /2P NiO x (UV-O 3 )/2P NiO x /B2P NiO x (UV-O 3 )/B2P FF (%) Fill Factor

								Short	Circuit	Current
		24
		22
		20
	(mA/cm 2 )	10 12 14 16 18
	J sc	8
		6
		4
		2
		0
		Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd		Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev
		NiO x		NiO x (UV-O 3 )	NiO x /M2P	NiO x (UV-O 3 )/M2P	NiO x /2P	NiO x (UV-O 3 )/2P	NiO x /B2P	NiO x (UV-O 3 )/B2P
		1.2						Open Circuit Voltage
		1.1
		1.0
		0.9
		0.8
	0.4 0.5 V oc 0.6 0.7 (V)
		0.3
		0.2
		0.1
		0.0
		Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev
		NiO x	NiO x (UV-O 3 )	NiO x /M2P	NiO x (UV-O 3 )/M2P	NiO x /2P	NiO x (UV-O 3 )/2P	NiO x /B2P	NiO x (UV-O 3 )/B2P
		Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev
		Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev	Fwd	Rev

Table 3 . 1

 31 In this chapter, we report on the electronic and chemical properties of the interface between MHP materials, MAPbI3 and MAPbBr3, and SWCNTs overlayers. Synchrotron-based HAXPES technique is employed to characterize the samples and estimate the energy level alignment. Our results show generally different electronic and chemical characteristics forMAPbI3 and MAPbBr3 before and after SWCNTs film growth. We observe selective core-level peak shifts which can be attributed to the different origins. Furthermore, the compositional study shows that the distribution of elements is different between the buried and topmost perovskite layers for MAPbI3 and MAPbBr3 upon deposition of the SWCNTs, respectively.

	on various substrates	oncentration	(10 nm -20 nm)	1%	-1%	-3%	5%	-11%	8%	-7%	8%	-4%	-4%	5%	-2%	-1%	1%	0%	-2%	5%	-9%	-1%	2%	0%	-6%	8%	-6%
	of 10 nm and 20 nmMAPbI3 deposited	stoichiometry FWHM Position	0.50 1.15 619.16	0.19 1.21 402.31	0.13 0.86 138.33	0.51 1.1 619.19	0.18 1.22 402.54	0.13 0.87 138.49	0.49 1.09 618.94	0.20 1.21 402.12	0.12 0.83 138.11	0.50 1.1 619.17	0.19 1.22 402.34	0.12 0.84 138.35	0.48 1.1 618.74	0.20 1.22 401.89	0.12 0.84 137.91	0.50 1.11 619.27	0.19 1.22 402.45	0.12 0.84 138.46	0.49 1.11 619.03	0.20 1.24 402.24	0.11 0.84 138.24	0.50 1.1 619.33	0.19 1.23 402.55	0.12 0.84 138.54
	: Binding energy, FWHM, stoichiometry and concentration difference values of XPS spectra curve fitted	Core MAPbI3 FWHM Level Position Core Level MAPbI3 Thickness Sample Identifier stoichiometry Thickness	I 3d5/2 0.51 1.09 619.05 I 3d5/2	N 1s 0.19 1.23 402.28 N 1s NiOx/MAPbI3	Pb 4f7/2 0.12 0.83 138.26 Pb 4f7/2	I 3d5/2 0.53 1.11 619.03 I 3d5/2	N 1s 0.16 1.21 402.31 N 1s NiOx(UV+O3)/MAPbI3	Pb 4f7/2 0.14 0.87 138.3 Pb 4f7/2	I 3d5/2 0.46 1.1 618.71 I 3d5/2	N 1s 0.21 1.29 401.77 N 1s NiOx/M2P/MAPbI3	Pb 4f7/2 0.11 0.88 137.86 Pb 4f7/2	I 3d5/2 0.48 1.09 618.9 I 3d5/2	N 1s 20 Pb 4f7/2 0.20 0.12 1.27 0.85 402 138.05 N 1s Pb 4f7/2 10 NiOx(UV+O3)/M2P/MAPbI3	I 3d5/2 nm N 1s 0.47 0.21 1.1 1.27 618.73 401.88 I 3d5/2 N 1s NiOx/2P/MAPbI3 nm	Pb 4f7/2 0.11 0.86 137.88 Pb 4f7/2	I 3d5/2 0.49 1.11 618.69 I 3d5/2	N 1s 0.20 1.25 401.95 N 1s NiOx(UV+O3)/2P/MAPbI3	Pb 4f7/2 0.11 0.86 137.89 Pb 4f7/2	I 3d5/2 0.48 1.1 618.81 I 3d5/2	N 1s 0.20 1.24 401.98 N 1s NiOx/ B2P/ MAPbI3	Pb 4f7/2 0.11 0.85 137.98 Pb 4f7/2	I 3d5/2 0.48 1.1 618.94 I 3d5/2	N 1s 0.20 1.34 402.11 N 1s NiOx(UV+O3)/ B2P/ MAPbI3	0.11 Pb 4f7/2 138.11 0.87 Pb 4f7/2

Summary:

Table 4 . 1

 41

	acquired under 2
	SWCNTs
	and
	(MAPbI3, MAPbBr3)	light conditions
	of perovskite	and dark and
	FWHM values of HAXPES spectra curve fitted	energies using aluminum and beryllium filters
	Binding energy and	and 8 keV excitation
	:	keV
		6
		keV,

Acknowledgments

Firstly, I would like to express my sincere appreciation to my thesis director, Philip Schulz, for

List of Tables Table 3.

Electronic properties at Perovskite/SWCNTs interface

In this section, the electronic features at the corresponding interface under different conditions, various beam intensities and SWCNTs film thicknesses are probed and elucidated. All the spectra are calibrated against Au 4f7/2 at 84 eV. Table 4.1 contains the binding energy values and FWHM of all the peaks fitted and reported in this chapter.

Impact of attenuated intensity beam on perovskite films

In this section, two main objectives are concurrently investigated. The effect of using different filters with different levels of beam attenuation is discussed. Additionally, the behavior of I 3d3/2 and Br 3d core levels under various excitation energy attenuated by two different filters are studied.

To do so, the perovskite layers with and without 10 nm and 20 nm thick SWCNT layers on top are measured under 2 keV, 6 keV and 8 keV. The X-ray beam is attenuated by employing aluminum and beryllium filters before hitting the sample, which reduces the beam flux to a factor of

Synchrotron-based HAXPES Study of growing ALD-SnO on perovskite layer

Summary: In this chapter, we report a synchrotron-based Hard X-ray photoelectron spectroscopy (HAXPES) interface investigation between metal halide perovskite (MHP) and ALD-SnO2 with and without PCBM interlayer. The objective of this chapter is to investigate and understand why the p-i-n solar cells fabricated with ALD-SnO2 as ETL with and without a protective PCBM layer did not show acceptable performance. We found an upward bending at the MHP/ALD-SnO2 interface as well as ions migration into PCBM film at the interface of MHP/PCBM

MHP/PCBM/SnO