Marcin William Greenbank

Damien Thuau

Pierre Henrie

Thérèse Gorisse

Georgio Mattana

Yu-Tang Tsai

Yan-Fang Chen

Afshin Zamani

Mike Zhang

Sibi Sutty

Yoshitaka Kajiyama

Anne Bouchauty

Tyler

Baolin Thomas

Alex Cunha

Marie Asano

Edgar Cao

Mathilde Champeau

Camillle Legros

C B Chlorobenzene

Mass 1.5 ATR Atenuated Total Reflection BHJ Bulk Heterojunction C 60 Buckminster fullerene

They were all unique

Titre : Étude sur les additifs solvants introduits pour augmenter le rendement de cellules solaires organiques : leurs sélections et leurs effets Résumé : Les cellules solaires organiques à hétérojonction en volume (BHJ en anglais) font l'objet d'un grand intérêt car représentent une source d'énergie bon marché et renouvelable. Cependant, à cause des rendements généralement bas, ce type de cellule peine à intégrer le marché. Afin d'en augmenter le rendement, contrôler la morphologie des semi-conducteurs dans la BHJ représente un élément clé. Dans ce contexte, il apparaît, dans la littérature, que les additifs solvant permettent de contrôler cette morphologie et d'augmenter les rendements. Cette thèse a pour but de fournir une étude complète sur l'utilisation des additifs. Le couple de semi-conducteurs étudié est le poly(3-hexylthiophene) (P3HT)/ [START_REF] Aïch | Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent[END_REF][START_REF] Aïch | Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent[END_REF]-phényl-C 61 -butanoate de méthyle (PC 61 BM). Une première partie présente une méthode développée pour guider la sélection d'additifs parmi une liste de solvants. Cette méthode emploie les paramètres de solubilité de Hansen des semi-conducteurs. Elle est appliquée au système P3HT/PC 61 BM et résulte en l'identification de trois nouveaux additifs performants. Ensuite, des caractérisations structurales, électriques et optiques sont menées sur la BHJ et permettent d'identifier les effets des additifs. Les effets de ces additifs se révèlent être différents en fonction de l'architecture des dispositifs. L'origine de telles différences est corrélée aux variations de mobilités des porteurs de charge causées par les additifs. Des tests de photo-stabilité ont été menés et montrent que les additifs sont capables d'augmenter la stabilité des cellules solaires. L'origine de telles améliorations est étudiée. Enfin, l'étude est étendue à deux autres nouveaux polymères semi-conducteurs.

Mots clés : dispositif photovoltaïque, matériaux semi-conducteurs, polymère Title : Studies on processing additives introduced to increase the efficiency of organic solar cells: selection and mechanistic effects Abstract : Polymeric bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted significant interest as a low cost and renewable technology to harvest solar energy. However, their generally low efficiencies are a barrier for their movement into commercial applications. Controlling the BHJ morphology is a key step in the pursuit of higher OSC efficiencies. Processing additives have emerged as effective components for optimizing the BHJ morphology. This thesis provides a comprehensive study on the introduction of additives in the formulation of semiconductors. The semiconductor system studied is based on poly(3-hexylthiophene) (P3HT) and [START_REF] Aïch | Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent[END_REF][START_REF] Aïch | Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent[END_REF]-phenyl-C61-butyric acid methyl ester (PC 61 BM). First, a method was developed to guide the selection of additives from a large range of solvents. This method employs the Hansen solubility parameters of the semiconductors and was successfully applied to the P3HT/PC 61 BMsystem. It resulted in the identification of three new efficient additives. Next, the mechanistic role of additives in influencing the BHJ morphology is investigated by performing structural, electrical and optical characterizations. Also, the effect of additives on OSC performance was found to depend on the type of the OSC architecture. Such differences were correlated to the variations in charge carrier mobilities caused by the additive. Furthermore, photo-stability tests, performed on different types of OSCs, showed that processing additives can improve the photo-stability. The origin of such improvement is investigated. Finally, the scope of this study is extended to two other donor semiconducting polymers.

Résumé en français:

Les cellules solaires organiques à hétérojonction en volume (BHJ en anglais) font l'objet d'un intérêt grandissant car elles représentent une technologie bon marché pour extraire l'énergie solaire. Cependant, à cause des rendements généralement bas, cette génération de cellules solaires peine à intégrer le marché.

Afin d'augmenter le rendement de ce type de dispositif, plusieurs routes de recherche sont ont été explorées dans la littérature, parmi lesquelles figurent la synthèse de nouveaux matériaux semi-conducteurs, l'optimisation de l'architecture des dispositifs photovoltaïques et le contrôle de la morphologie des semi-conducteurs dans la BHJ. Comme la BHJ est au coeur du mécanisme de conversion de photons en porteurs de charge, le contrôle de sa morphologie a un impact considérable sur le rendement du dispositif photovoltaïque.

Dans la littérature, l'introduction d'additifs dans la formulation de semi-conducteurs organiques apparaît comme un élément clé pour contrôler -et par conséquent pour optimiser -la morphologie de la BHJ [START_REF] Peet | Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols[END_REF][START_REF] Peet | Method for increasing the photoconductive response in conjugated polymer/fullerene composites[END_REF]. Ainsi, de nombreux exemples ont montré que l'introduction d'additif peut augmenter de manière significative le rendement des cellules solaires organiques [START_REF] Liao | Additives for morphology control in high-efficiency organic solar cells[END_REF][START_REF] Liu | The influence of additive property on performance of organic bulk heterojunction solar cells[END_REF]. Malgré l'utilisation généralisée des additifs solvant, leurs effets sur la couche active ne sont pas entièrement compris ni maîtrisés et, par conséquent, leurs sélections font généralement l'objet d'expériences de type essai et erreur. Ce type de démarche et la méconnaissance des effets des additifs représentent un obstacle pour l'exploitation complète du potentiel des additifs solvant.

Cette thèse a pour but de constituer une étude complète sur l'utilisation des additifs: leurs sélections, leurs effets sur la BHJ, leurs effets sur les performances du dispositif et leurs effets sur la stabilité des dispositifs sont les thématiques abordées. Le couple de semi-conducteurs majoritairement étudié dans cette thèse est le système poly(3-hexylthiophene) (P3HT)/ [START_REF] Aïch | Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent[END_REF][START_REF] Aïch | Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent[END_REF]-phényl-C 61 -butanoate de méthyle (PC 61 BM).

Une première partie présente une méthode développée afin de guider la sélection des additifs. L'originalité de cette méthode réside dans l'utilisation de la théorie des paramètres de solubilité de Hansen pour étudier les interactions entre solvants et matériaux semi-conducteurs. Cette théorie est généralement sollicitée dans v l'industrie de la formulation ou de la peinture, son utilisation dans le domaine de l'électronique organique est, à ce jour, très limitée. Les paramètres de La détermination de ces paramètres s'est effectuée par la réalisation de tests de solubilité. Ensuite, une étude bibliographique a permis d'identifier des additifs efficaces existants. Leurs propriétés physico-chimiques et leurs paramètres de Hansen sont mis en corrélation avec ceux du P3HT et du PC 61 BM. Enfin, les résultats de ces études ont été analysés afin de déterminer une liste de critères (ou règles de sélection) que des additifs solvant doivent satisfaire afin d'augmenter le rendement de cellules solaires.

Les règles de sélection, ainsi déterminées, sont appliquées à une liste de 723 solvants afin d'identifier de nouveaux additifs. Parmi cette liste, trois solvants remplissent les critères tout en n'étant pas cancérigènes (pour des raisons de sécurité) : le phtalate de diméthyle, l'o-acétylcitrate de tributyl et le 1-cyclohexyl-2-pyrrolidinone. Afin de vérifier la validité des règles de sélection, les trois additifs sont introduits dans des formulations de P3HT et de PC 61 BM destinées à être intégrées dans des dispositifs. Les résultats montrent que les trois additifs sont efficaces et augmentent le rendement de cellules solaires. La plus performante des formulations est obtenue avec le phtalate de diméthyle: un rendement de 3.2 % est obtenu (sans recuit thermique), ce qui représente une augmentation de 113 % par rapport à une cellule solaire de référence. L'o-acétylcitrate de tributyl et le 1-cyclohexyl-2-pyrrolidinone démontrent des augmentations de rendement de 67 % et 93 % respectivement. Ces résultats montrent que les additifs ainsi sélectionnés sont performants et que la méthode développée est efficace pour identifier des additifs solvant.

Le chapitre suivant aborde le sujet des effets des additifs solvant sur la morphologie de la couche active dans le but d'expliquer les augmentations de rendement vi obtenu. Différents types de caractérisation sur couches minces sont effectués : de la diffraction des rayons X (DRX), de la spectroscopie UV-visible et de la microscopie à force atomique. Aussi, des études sur la physique des semi-conducteurs et des dispositifs sont conduites par des mesures de mobilité de ix At the end of my engineering school, I had the opportunity to perform this PhD in the field of organic photovoltaics as part of an international program between Canada and France. Back then, I was very excited because I pictured this as a major opportunity in my professional and personal life. Professional, because I was passionate about organic electronics and this PhD was the opportunity to strengthen my knowledge and to contribute scientifically to this innovative field. Personal, because the development of renewable energy is of major importance to me and I was also looking forward to live and work in Canada. Four years later, I am glad to observe that my expectations have been successfully fulfilled and that I gained valuable experiences that were beyond my expectations. I grew up and I learn a lot. This would not have been possible without the large body of people I was lucky and honored to work with and to be friend with.

These are the people I want to thank here.

First of all, I would like to thank all my supervisors for giving me the great opportunity to perform this PhD and for their guidance throughout these years. I am highly grateful to Bertrand Pavageau for giving me the opportunity to perform this PhD at LOF and for his constant support and his never fading enthusiasm towards my research and my ideas. I would like to address my sincere gratitude to Pr. Laurent Servant for his contribution and his advice in my research but also when I had administrative issues.

His availability and his concern for my PhD helped me to carry out my research in good conditions. I am truly grateful to Pr. Hany Aziz for welcoming me in his group. His guidance and his trust in me pushed me to take risks and to do my best. Particularly, during the writing of my thesis, his patience and his advices highly contributed to its quality. To him, I want to express my respect and my sincere gratitude. Sincere thanks go to Dr. Dario Bassani for his support and for fruitful discussions about my research and my manuscripts.

I would like to address my sincere gratitude to Dr. Guillaume Wantz and Dr.

Lionel Hirsch for welcoming me in the Laboratoire d'intégration du matériau aux systèmes (IMS) which is a scientifically great and friendly environment. Special thanks to Guillaume for the discussions; for valuing my work and for always pushing me to be more ambitious.

I would like to address my sincere thanks to Pr. Thomas Heiser from the University of Strasbourg, Pr. Alejandro Briseno from the University of Massachusetts and Pr.

x Siva Sivoththaman from the University of Waterloo for accepting to be part of my committee, for reviewing my thesis and for providing valuable comments.

I would like to acknowledge Solvay, the Natural Sciences and Engineering Research Council of Canada as well as the Waterloo Institute for Nanotechnology for funding my PhD research.

As my thesis was performed in a co-tutelle program, I had the pleasure of being part of four different laboratories. In France, I started my work at LOF Solvay. I thank the director at the time, Patrick Maestro for welcoming me at LOF. I thank Simon Rousseau and Samantha Armisen for sharing their valuable expertise and for their help in the determination of the Hansen Solubility Parameters. I would like to thank all the LOF team for welcoming me and more particularly few buddies for their supports and their friendships: Vincent Mansard (now lost somewhere in the west coast),

Julie Angly (for her enthusiasm, her optimism and for the tea), Cyril Vidaillac (for all the valuable conversations), Hongyu (for always being smiley, strong and therefore inspiring), Julien Jolly, Ana Maldonado, Marta Romano, Rawad Tadmouri and many more.

In Introduction to organic solar cells

General introduction

The industrial revolution represented a serious transition in humanity's standard of life leading to significant advances that contributed to higher life expectancy, quality of life and comfort. These developments, along with a growing population, caused exponential growth in energy demand over the past years.

To date, energy demand has mostly been met by burning fossil fuels (oil, gas and coal). However, such processes are not environmentally sustainable due to the emission of green house gases in Earth's atmosphere, contributing to global warming. Additionally, as fossil fuel is a non-infinite source of energy, the resource depletion causes serious societal and political issues. Alternatives must be developed to satisfy the energy demand in a sustainable way.

In this context, solar energy offers many advantages as a renewable source of energy. One of them is the large amount of energy provided by solar irradiation on Earth: estimations -considering realistic irradiated surface and conversion yield -showed that solar energy could provide up to two times the world's energy demand [START_REF] Darling | The case for organic photovoltaics[END_REF]. Additionally, the worldwide availability of solar energy could benefit remote regions of the world by means of decentralized production of energy.

Solar radiation is converted into electricity by means of photovoltaic cells, also called solar cells. They have been fabricated from a wide range of materials as shown in the National Renewable Energy Laboratory chart (Figure 1.1). To date, the most prevalent types of photovoltaic panels use crystalline silicon in the monocrystalline or polycrystalline form. At the laboratory scale, conversion efficiencies reach 25% [START_REF] Green | Solar cell ef fi ciency tables ( version 43 )[END_REF], while in commercial products, the efficiencies tend to be around 20% [START_REF] Krauter | Solar Electric Power Generation[END_REF]. Crystalline silicon solar cells dominate in terms of efficiency, but suffer from high fabrication costs and restrictive mechanical and physical properties, such as weight and fragility. Therefore, they have not been able to fully compete with fossil fuels. In the pursuit of further lowering module cost and encouraging wide-spread application, other technologies were developed. Thin film technologies (indicated by data in green in Figure 1.1) are a good alternative as they require less material and lower fabrication costs. Organic photovoltaics (depicted in red in Figure 1.1) are also an interesting and exciting route for the prospect of extremely low cost solar panels with additional features such as low weight and flexibility. efficiency of 0.95% [START_REF] Tang | Two-layer organic photovoltaic cell[END_REF]. In 1995, solution processed organic photovoltaics were fabricated for the first time from a blend of polymer and oligomer [START_REF] Yu | Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions[END_REF]. Until the early 2000s, the maximum power conversion efficiencies of organic photovoltaics remained around 3% [START_REF] Coakley | Conjugated Polymer Photovoltaic Cells[END_REF][START_REF] Rispens | Influence of the solvent on the crystal structure of PCBM and the efficiency of MDMO-PPV:PCBM 'plastic' solar cells[END_REF][START_REF] Shaheen | 2.5% Efficient Organic Plastic Solar Cells[END_REF]. For a long time, these low efficiencies were seen as a significant barrier to commercialization. However, much research was dedicated to improve the fundamental and technical knowledge in the field, which led to significant improvements in performance. Today, organic photovoltaics demonstrate their potential in becoming a market reality by exhibiting efficiencies greater than 10%. Heliatek, a spin-off from a University of Dresden laboratory in Germany is currently holding the power conversion efficiency record of 12% with a multi-junction OSC. Efficiencies approaching 10% were obtained by several groups with single-junction devices [START_REF] Guo | Polymer solar cells with enhanced fill factors[END_REF][START_REF] He | Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure[END_REF][START_REF] Nguyen | Semi-crystalline photovoltaic polymers with efficiency exceeding 9% in a 300 nm thick conventional single-cell device[END_REF]. From a technological point of view, the physico-chemical properties of organic semiconductors open up a new route in the development of photovoltaic devices. One of the key properties is the ability to dissolve organic semiconductors in solvents, which enables

OSCs to be fabricated using solution processes similar to those used in the printing industry [START_REF] Alstrup | Ultra fast and parsimonious materials screening for polymer solar cells using differentially pumped slot-die coating[END_REF][START_REF] Böttiger | High-throughput roll-to-roll X-ray characterization of polymer solar cell active layers[END_REF][START_REF] Byun | Bladed organic photovoltaic cells[END_REF][START_REF] Chang | Polymer solar cell by blade coating[END_REF][START_REF] Eom | High efficiency polymer solar cells via sequential inkjet-printing of PEDOT:PSS and P3HT:PCBM inks with additives[END_REF][START_REF] Forrest | The path to ubiquitous and low-cost organic electronic appliances on plastic[END_REF][START_REF] Krebs | Fabrication and processing of polymer solar cells: A review of printing and coating techniques[END_REF][START_REF] Krebs | Product integration of compact roll-to-roll processed polymer solar cell modules: methods and manufacture using flexographic printing, slot-die coating and rotary screen printing[END_REF][START_REF] Myers | Organic Semiconductors and their Applications in Photovoltaic Devices[END_REF]. Such processes mean easier fabrication of large area and low cost photovoltaics and consequently, a number of innovative devices can be considered, including flexible, transparent, and light weight devices as depicted in Figure 1.2 [START_REF] Chen | Visibly transparent polymer solar cells produced by solution processing[END_REF][START_REF] Hübler | Printed Paper Photovoltaic Cells[END_REF][START_REF] Sø Ndergaard | Roll-to-Roll fabrication of large area functional organic materials[END_REF]. In addition, the fabrication of OSCs has a lower energy payback time in the long term and a smaller environmental impact than other technologies [START_REF] Darling | The case for organic photovoltaics[END_REF]. Overall, they represent a promising technology for low cost, accessible and sustainable energy. To become a competitive technology, OSCs need to improve in three fronts: efficiency, lifetime and fabrication cost. Research, with the goal of improving in these areas, has been focused on the synthesis of organic semiconductors, their processing and the development of smart device architectures. All three topics contribute to the overall performance of the final device (Figure 1.3). The past decade was marked by a tremendous increase in types of organic semiconductors [START_REF] Cheng | Synthesis of conjugated polymers for organic solar cell applications[END_REF][START_REF] Kroon | Small Bandgap Polymers for Organic Solar Cells[END_REF], which indubitably contributed to the current success of organic photovoltaics. However, the structural properties of materials alone are not sufficient to produce high efficiency devices. The arrangement, e.g. the morphology, of the materials is of crucial importance in defining their performance in devices. High efficiency OSCs require a thorough control of the semiconductor morphology to best assess their potential in a device configuration.

In this context, this thesis addresses the interplay between processing method and semiconductor morphology and subsequently photovoltaic performance. The effects of processing are assessed by means of semiconductor formulation and introduction of processing additive in the formulation.

The first chapter of this thesis provides an introduction to fundamental principles of organic semiconductors and to the field of organic photovoltaics. Chapter 2 presents a detailed literature review on the current state of knowledge on formulation strategies to control active layer morphology. Emphasis is placed on the system based on poly(3-hexylthiophene) (P3HT) and [START_REF] Aïch | Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent[END_REF][START_REF] Aïch | Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent[END_REF]-phenyl-C61-butyric acid methyl ester (PC 61 BM) as it is the main model system in this thesis. The objectives of this thesis are described at the end of Chapter 2.

Chapter 3 details the experimental and analytical methods used throughout the thesis.

Chapter 4 reports a novel method for identifying efficient processing additives to be introduced in the formulation of organic semiconductors. Details about the selection method and results confirming the model are presented.

Chapter 5 focuses on the mechanistic effects of processing additives on the morphology and efficiency. Results of structural, optical and electrical characterizations are reported and a mechanism describing the effects of processing additives on the morphology is proposed.

Chapter 6 presents results on the influence of device architecture on the performance of additive-treated OSCs.

The effects of processing conditions on device stability is addressed in Chapter 7.

Results of stability tests performed on OSCs processed under different conditions are reported. Also, investigation on the origin of device degradation is presented.

Chapter 8 reports on the investigation of formulation strategies to increase the efficiency of two polymers, other than the model polymer P3HT discussed in the previous chapters.

Chapter 9 summarizes the results and provides suggestions for future work.

Functioning of solar cells

Before introducing the concept of OSCs, this section discusses the electrical functioning of solar cells and introduces the parameters that characterize their performance.

The simplified structure of a solar cell consists of two electrodes separated by a photo-active layer which contains a p-n junction. A solar cell without any irradiance (dark characteristic) behaves as a diode as shown in Figure 1.4. When a photon hits the active layer, a bond state of an electron and a vacant site of an electron called a hole is formed. The separation of the bound state, also referred to as exciton, generates charge carriers that can travel across the active layer to be collected by the electrodes. The extraction of these photo-generated charge carriers by an external load constitutes the photo-current (J ph ). Under illumination, the diode characteristic shifts by the amount of the photo-current.

-0 . 4 -0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 resistance and the shunt resistor models any leakage current through the device.

-1 2 -1 0 -8 -6 -4 -2 0 2 4 6 V O C J m J S C J P H V O C Current density [mA/cm²] V o l t a g e [ V ] D A R K I L L U M I N A T I O N J S C V m
The total current density J flowing through the circuit can be described by the following relationship:

J = J D (V ) + V -JR serie R shunt -J ph (1.1)
Where R s is the series resistance, R sh the shunt resistance and J D the current characteristic for a diode.

The electrical parameters that define the efficiency of a solar cell are extracted from the current density-voltage (J-V ) curves measured under light illumination.

Devices are characterized by the short-circuit current (J sc ), the open-circuit voltage (V oc ) and the fill factor (FF). The V oc and the J sc are indicated in the typical J-V curve depicted in Figure 1.4. The J sc is defined as the current at which the applied voltage is equal to 0 V. This parameter represents the number of charge carriers that are photo-generated and collected at the electrodes at short-circuit condition. The V oc is defined as the voltage at which the current density is 0 mA.cm -2 . The FF defines the shape of the J-V curve and is defined as:

F F = J m .V m J sc .V oc (1.2)
where J m and V m are respectively the current density and the voltage at the point of the maximum output power, as represented in Figure 1.4. Finally, the power conversion efficiency (PCE) of a solar cell can be calculated as follows:

P CE = V oc .J sc .F F P in (1.3)
where P in is the input power density.

Solar cells are typically characterized under 100 mW.cm -2 light of the Air Mass 1.5 Global (AM 1.5 G) solar spectrum. The AM 1.5 G spectrum is presented in Within a π-conjugated system, the overlap of the wave functions of the p z orbitals of the carbon atom (and also of other atoms such as nitrogen, oxygen, sulfur) results in the formation of the π-band (forming the highest occupied molecular orbital (HOMO)) and the π*-band (forming the lowest unoccupied molecular orbital (LUMO)). Across the conjugated system, the π-electrons are delocalized.

The difference between the HOMO and the LUMO levels defines the bandgap of the semiconductor. The gap narrows down as the conjugation length of the polymer chain increases. In molecular solids, the degree of intermolecular ordering has a large impact on the energetic landscape in these materials and thus affects the band gap. Overall, increased conjugation length and intermolecular ordering cause a greater degree of electron delocalization while short conjugation length and intermolecular disorder localize electrons.

The specific electronic structure of organic semiconductors leads to major differences between the properties of organic and conventional semiconductors. An understanding of these differences, described in brief in the following sections, is essential to provide insights into understanding the design requirements for OSC active layers.

Charge carrier generation in organic semiconductors

Free charge carriers in semiconductors are generated by light, chemical doping, or injection of charge from an electrode. The generation of charge carriers by light occurs when a photon with an energy equal to or higher than the band gap hits the semiconductor, resulting in the photo-generation of an exciton. To generate free charge carriers, the bound electron-hole pair in the exciton must be separated. In a traditional inorganic semiconductor, the large dielectric constant (around 10 [START_REF] Deibel | Organic Bulk-Heterojunction Solar Cells[END_REF]) reduces the Coulomb interaction that binds the hole and the electron. This results in a large-sized type of exciton called a Wannier-Mott exciton characterized by an exciton binding energy in the range of ∼ 0.01 eV.

This type of exciton can be dissociated by thermal energy at room temperature as the binding energy is lower than the thermal energy (0.025 eV). In contrast, in organic semiconductors, the dielectric constant is much smaller (around 3 [START_REF] Deibel | Organic Bulk-Heterojunction Solar Cells[END_REF]) causing a strong Coulombic interaction between the hole and the electron. This results in a small-sized exciton with a high binding energy (from hundreds of meV to 1.5 eV [START_REF] Alvarado | Direct Determination of the Exciton Binding Energy of Conjugated Polymers Using a Scanning Tunneling Microscope[END_REF][START_REF] Hill | Charge-separation energy in films of p -conjugated organic molecules[END_REF][START_REF] Kersting | Ultrafast Field-Induced Dissociation of Excitons in Conjugated Polymers[END_REF]) referred to as a Frenkel exciton. This high binding energy prevents the exciton from being dissociated by thermal energy at ambient temperature. One alternative for exciton dissociation is to introduce a second semiconductor with a different electron affinity. In organic semiconductors, the charge separation is generally a two-step process, where excitons are first separated into less strongly bound electron-hole pairs which can subsequently be dissociated.

Charge transport in organic semiconductors

In crystalline inorganic semiconductors with a 3D crystal lattice, atoms are held together by strong covalent bonds. The strong interatomic interaction leads to the formation of a conduction band and a valence band in which the free charge carriers are highly delocalized and can travel freely throughout the bands [START_REF] Spanggaard | A brief history of the development of organic and polymeric photovoltaics[END_REF].

In contrast, in disordered organic semiconductors, such as π-conjugated polymers, charge carriers are localized on individual molecules because of poor intermolecular coupling. The charge transport is generally described by the variablerange hopping model [START_REF] Klauk | Organic thin-film transistors[END_REF][START_REF] Stallinga | Electrical Characterization of Organic Electronic Materials and Devices[END_REF] which describes systems with mobilities around or below 10 -2 cm 2 .V -1 .s -1 [START_REF] Klauk | Organic thin-film transistors[END_REF]. The mobility measures the ease with which the charge carriers move in the system and corresponds to the drift velocity of the charge carrier in an electrical field per unit of electrical field as described in the following equation [START_REF] Brabec | Organic Photovoltaics Concepts and Realization[END_REF]:

µ = eD K b T (1.4)
Where D is the diffusion coefficient, e the charge, K b the Boltzman constant and T the temperature.

In π-conjugated polymers, the hopping transport is highly influenced by the conjugation length and the degree of intermolecular order [START_REF] Brabec | Organic Photovoltaics Concepts and Realization[END_REF]. The mobility of charge carriers in organic semiconductors thus appears to be dependent not only on the chemical structure of the semiconductors but also, and just as importantly, on their intermolecular arrangements. Generally, for semiconducting polymers processed from solution, mobilities in the range of 10 -3 -10 -6 cm 2 .V -1 .s -1 are obtained [START_REF] Klauk | Organic thin-film transistors[END_REF]. A control over molecular organization can improve the mobility [START_REF] Sun | Record high electron mobility of 6.3 cm2 V-1 s-1 achieved for polymer semiconductors using a new building block[END_REF]. In the case of highly-ordered organic molecular crystals, charge carriers are sufficiently delocalized for band transport to occur [START_REF] Myers | Organic Semiconductors and their Applications in Photovoltaic Devices[END_REF].

Organic solar cells 1.4.1 Active layer architectures

As previously mentioned, thermal energy is not sufficient to dissociate Frenkel type excitons photo-generated in organic semiconductors. Instead, a junction must be created at which it is energetically favourable for an exciton to dissociate.

The first type of OSCs consisted of single layer of semiconductor that formed a rectifying junction with one of the electrodes [START_REF] Antoniadis | Photovoltaic and photoconductive properties of aluminum/poly(p-phenylene vinylene) interfaces[END_REF][START_REF] Chamberlain | Photovoltaic properties of merocyanine solid-state photocells[END_REF]. In this architecture, because of the high exciton binding energy and the low exciton diffusion length

(1 -10 nm) in organic semiconductors [START_REF] Halls | Exciton diffusion and dissociation in a poly(p-phenylenevinylene)/C60 heterojunction photovoltaic cell[END_REF][START_REF] Markov | Dynamics of exciton diffusion in poly(p-phenylene vinylene)/fullerene heterostructures[END_REF][START_REF] Stubinger | Exciton diffusion and optical interference in organic donor-acceptor photovoltaic cells[END_REF], the charge generation only occurs near the interface between the electrode and the active layer. This resulted in poor photo-generation of charge carriers and high series resistance as one of the charge carriers has to travel across the entire active layer to be collected. These issues were later addressed using a heterojunction between an electron donating material (D) and an electron accepting material (A) to form a so-called bi-layer.

In a D/A couple, the A is the material with the largest electron affinity and the D the material with the lowest ionization potential. The energy offset at the D/A interface helps to dissociate excitons. The first heterojunction device, developed in 1986, consisted of a bi-layer of an A (tetracarboxylic derivative) and a D (copper phthalocyanine) material. These bi-layer OSCs demonstrated efficiencies approaching one percent (0.95%) [START_REF] Tang | Two-layer organic photovoltaic cell[END_REF]. However, the photo-generation of charges was still limited to occur at one single interface and excitons formed further away from the D/A interface do not dissociate into free charge carriers, therefore recombine, and thus do not contribute to photo-current. C 60 and its derivatives are the most widely used A small molecules. C 60 is strongly electronegative and can accept up to six electrons. Additionally, it exhibits a small reorganization energy from the ground state to the radical anion upon electron transfer which enables fast photoinduced electron transfer and slows down charge recombination compared to other types of A such as quinone derivatives [START_REF] Guldi | Fullerenes: three dimensional electron acceptor materials[END_REF][START_REF] Imahori | Fullerenes as Novel Acceptors in Photosynthetic Electron Transfer[END_REF]. As the solubility of C 60 is low in common organic solvents [START_REF] Semenov | Solubility of Light Fullerenes in Organic Solvents[END_REF], Examples are shown in Figure 1.9. To form a BHJ, the D polymer and the A small molecule are generally mixed together in a solvent and the resulting blend is coated on a substrate. The drying process involves various phenomena including self-organization of the materials and phase separation between D and A. The resulting phase separation and degree of order in the BHJ govern the conversion from light absorption to charge collection and are therefore crucial to control. It is noteworthy to point here that few studies in the literature focused on the formation of BHJs with fixed morphology by the mean of various methods such as the use of D or A nanoparticles [START_REF] Gärtner | Eco-Friendly Fabrication of 4% Efficient Organic Solar Cells from Surfactant-Free P3HT:ICBA Nanoparticle Dispersions[END_REF] or nanowires. [START_REF] Xin | Highly Efficient Solar Cells Based on Poly ( 3-butylthiophene ) Nanowires[END_REF][START_REF] Xin | Bulk Heterojunction Solar Cells from Poly(3-butylthiophene)/Fullerene Blends: In Situ Self-Assembly of Nanowires, Morphology, Charge Transport, and Photovoltaic Properties[END_REF] In these approaches, the domain sizes in the BHJ are mainly determined by the size of the initial objects and are less dependent on the conditions of the drying process. This in turn enables a large degree of control in the BHJ morphology under the condition that the sizes of the initial objects are fully controlled.

soluble

Functioning of BHJ-OSCs

Within the interpenetrated network formed by the D and A, the conversion from light to electrical current occurs according to the following steps: The microstructure of the BHJ, e.g. the morphology, affects the efficiency of the different mechanisms described above [START_REF] Brabec | Influence of blend microstructure on bulk heterojunction organic photovoltaic performance[END_REF]. The morphology of a BHJ is defined by (1) the phase separation between A and D and (2) the molecular organization.

For high performing OSCs, both charge generation and charge collection need to be considered. These two processes are in competition: charge generation requires small D/A domains to provide a large amount of D/A interface, while charge collection requires large pathways that subsequently limit the amount of D/A interfaces. A compromise is obtained when D and A domain sizes are in the range of the exciton diffusion length. The exciton diffusion length, which is in the range of 1 to 10 nm [START_REF] Halls | Exciton diffusion and dissociation in a poly(p-phenylenevinylene)/C60 heterojunction photovoltaic cell[END_REF][START_REF] Markov | Dynamics of exciton diffusion in poly(p-phenylene vinylene)/fullerene heterostructures[END_REF][START_REF] Stubinger | Exciton diffusion and optical interference in organic donor-acceptor photovoltaic cells[END_REF] implies that D and A domain sizes of about 10 nm offer a decent compromise. [START_REF] Günes | Conjugated polymer-based organic solar cells[END_REF].

In addition to phase separation, the molecular organization within D or A domains is important. In conjugated polymers, the interlayer spacing between polymer chains, the degree of ordering and the orientation of the chains have an influence on charge carrier mobility. In OSCs, charge carrier mobility is closely related to recombination processes [START_REF] Oklobia | Correlation between charge carriers mobility and nanomorphology in a blend of P3HT/PCBM bulk heterojunction solar cell: Impact on recombination mechanisms[END_REF][START_REF] Park | Enhanced device performance of organic solar cells via reduction of the crystallinity in the donor polymer[END_REF]. Studies revealed that charge carrier mobility value along with a balance between electron and hole mobility contribute in improving the performance of OSCs [START_REF] Abbas | Balanced charge carrier mobilities in bulk heterojunction organic solar cells[END_REF][START_REF] Chen | Improving the efficiency of an organic solar cell by a polymer additive to optimize the charge carriers mobility[END_REF][START_REF] Kotlarski | Role of Balanced Charge Carrier Transport in Low Band Gap polymer : fullerene Bulk Heterojunction Solar Cells[END_REF][START_REF] Lee | Conversion efficiency improvement mechanisms of polymer solar cells by balance electron-hole mobility using blended P3HT:PCBM:pentacene active layer[END_REF][START_REF] Oklobia | Correlation between charge carriers mobility and nanomorphology in a blend of P3HT/PCBM bulk heterojunction solar cell: Impact on recombination mechanisms[END_REF].

Factors influencing BHJ-OSC efficiency

The previous sections describe the mechanisms related to the functioning of BHJ-OSC and the importance of the BHJ morphology. This section addresses the parameters characteristic of OSCs.

-Jsc The J sc is given by the amount of charge carriers that are collected at the electrodes. This parameter is therefore depends on the amount of photons absorbed and to the loss mechanisms that prevent charge collection. The morphology of the BHJ has a great impact on the amount of photo-generated charges, due to the amount of D/A interface, and on the collection of charges by the presence of D or A pathways. Recent findings have highlighted the important role of the intermixed phase in the BHJ in exciton quenching and charge generation processes [START_REF] Bartelt | The Importance of Fullerene Percolation in the Mixed Regions of Polymer-Fullerene Bulk Heterojunction Solar Cells[END_REF][START_REF] Chen | Hierarchical Nanomorphologies Promote Exciton Dissociation in polymer/fullerene bulk heterojunction solar cells[END_REF][START_REF] Shoaee | Charge Photogeneration for a Series of Thiazolo-Thiazole Donor Polymers Blended with the Fullerene Electron Acceptors PCBM and ICBA[END_REF][START_REF] Treat | Polymer-fullerene miscibility: a metric for screening new materials for high-performance organic solar cells[END_REF][START_REF] Westacott | On the role of intermixed phases in organic photovoltaic blends[END_REF]. In this intermixed phase, the D and the A are molecularly mixed and form amorphous domains. Studies have shown that it is crucial for these intermixed phases to coexist with the relatively pure D and A domains that stabilize charge separation and enable transport to the electrodes.

-Voc

Several works have ascribed the dependence of the V oc to the spectral position of the charge transfer state [START_REF] Kumar | Recent progress and future aspects of organic solar cells[END_REF][START_REF] Loi | Charge Transfer Excitons in Bulk Heterojunctions of a Polyfluorene Copolymer and a Fullerene Derivative[END_REF][START_REF] Vandewal | On the origin of the open-circuit voltage of polymer-fullerene solar cells[END_REF]. The energy of the charge transfer state originates primarily from the energy difference between the HOMO of the D and the LUMO of the A (∆E DA ). ∆E DA determines the theoretical maximum value of V oc [START_REF] Brabec | Origin of the Open Circuit Voltage of Plastic Solar Cells[END_REF][START_REF] Scharber | Design Rules for Donors in Bulk-Heterojunction Solar Cells-Towards 10 % Energy-Conversion Efficiency[END_REF][START_REF] Widmer | Open-Circuit Voltage and Effective Gap of Organic Solar Cells[END_REF]. Experimentally, the V oc is observed to be lower than the value set by ∆E DA because of several loss mechanisms at the contacts or in the active layer [START_REF] Vandewal | On the origin of the open-circuit voltage of polymer-fullerene solar cells[END_REF][START_REF] Yamanari | Origin of the open-circuit voltage of organic thin-film solar cells based on conjugated polymers[END_REF]. In their early work, Scharber et al [START_REF] Scharber | Design Rules for Donors in Bulk-Heterojunction Solar Cells-Towards 10 % Energy-Conversion Efficiency[END_REF]. investigated the relationship between the energy levels of the D/A blends and the V oc in 26 BHJ-OSCs. They found that the V oc of the OSCs could be estimated by:

V oc = 1 e (|E Donor HOM O| -|E P CBM LU M O|) -0.3 (1.5)
where e is the elementary charge and the value of 0.3 an empirical factor accounting for losses in OSCs.

Regarding the loss at the contacts, the properties of the HTL (energy levels and conductivity) were shown to have an influence on the built-in voltage in the device and subsequently on the V oc [START_REF] Uhrich | Origin of open circuit voltage in planar and bulk heterojunction organic thin-film photovoltaics depending on doped transport layers[END_REF]. In the case of ohmic contacts, the loss in V oc at the contact was attributed to diffusion of charge carriers into the active layer at the interface with the metal electrodes resulting in a reduction of the voltage at which the flat band conditions are reached [START_REF] Veldman | The Energy of Charge-Transfer States in Electron Donor-Acceptor Blends: Insight into the Energy Losses in Organic Solar Cells[END_REF]. Regarding the active layer, several loss mechanisms that affect the charge transfer band position were identified. The charge transfer band was found to be dependent on the electrostatic environment, which can be altered by the concentration of PC 61 BM nanocrystals because of their high dielectric permittivity [START_REF] Loi | Charge Transfer Excitons in Bulk Heterojunctions of a Polyfluorene Copolymer and a Fullerene Derivative[END_REF][START_REF] Piersimoni | Influence of Fullerene Ordering on the Energy of the Charge-Transfer State and Open-Circuit Voltage in Polymer Fullerene Solar Cells[END_REF]. Also, the level of polymer aggregation affects the position of the charge transfer state because of its dependence on intermolecular interactions [START_REF] Perez | Molecular and Morphological Influences on the Open Circuit[END_REF]. Strong intermolecular interactions were shown to result in low V oc whereas low intermolecular interactionsobtained in the case of amorphous polymers, for example -lead to higher V oc .

-FF The origins of low FF generally arise from space-charge build up. The spacecharge regions can arise in the vicinity of the active layer and the contacts, in the case of poor charge extraction at the electrodes. In this context, the nature of the charge transport layers between the active layer and the electrodes have the important role of preventing charge accumulation [START_REF] Park | Optimization of polymer photovoltaic cells with bulk heterojunction layers hundreds of nanometers thick: modifying the morphology and cathode interface[END_REF]. Efficient contacts and interlayers facilitate the extraction of charges and reduce bimolecular recombination in the active layer. Space-charge build up can also be observed in the case of increasing light intensity [START_REF] Kumar | Recent progress and future aspects of organic solar cells[END_REF][START_REF] Wu | Origin of the fill factor loss in bulk-heterojunction organic solar cells[END_REF], or unbalanced charge carrier mobility [START_REF] Abbas | Balanced charge carrier mobilities in bulk heterojunction organic solar cells[END_REF][START_REF] Kroon | Small Bandgap Polymers for Organic Solar Cells[END_REF]. The value of the FF is generally found to be dependent on the values of R s and the R sh measured in OSCs [START_REF] Kim | Effective variables to control the fill factor of organic photovoltaic cells[END_REF][START_REF] Perez | Molecular and Morphological Influences on the Open Circuit[END_REF]. This chapter provided an overview of the mechanisms behind OSCs and more specifically of BHJ-OSCs. The next chapter focuses specifically on the formation of that morphology and strategies to control and optimize it.

Chapter 2

Formulation strategies for controlling BHJ morphology

This chapter reviews the benefits and the limitations of three main strategies that can control the morphology of BHJ-active layers: the solvent choice, the use of post-processing steps and the use of processing additives. The objectives of the thesis are presented at the end of the chapter.

Role of solvent

The BHJ morphology is strongly dependent on thermodynamic and kinetic aspects involved during spin-casting and drying of the material formulation to form the thin film [START_REF] Mens | Influence of the processing solvent on the photoactive layer nanomorphology of P3HT/PC60BM solar cells[END_REF][START_REF] Yang | Crystalline Organization of a Methanofullerene as Used for Plastic Solar Cell Applications[END_REF]. The thermodynamics of the system is dictated by the intrinsic properties of the D and A materials (their tendency to crystallize, their interactions with each other and their miscibility) and by the properties of the solvent. The kinetics of drying primarily depend on the boiling point and the vapor pressure of the solvent. The effects of the solubility properties and the boiling point of solvents are reviewed below.

-Solubility properties D and A solubility properties in solvent were found to affect the aggregation of the fullerene derivatives in the D/A films. Table 2.1 displays the solubility limits of PC 61 BM in some conventional solvents including chloroform (CF), chlorobenzene (CB), o-dichlorobenzene (ODCB) and 1,2,4-trichlorobenzene (TCB). The study by Brabec et al. on a blend of MDMO-PPV and PC 61 BM was among the first to bring some insights into the relationship between solvent quality and the morphology of a BHJ [START_REF] Shaheen | 2.5% Efficient Organic Plastic Solar Cells[END_REF]. In the D/A system based on MDMO-PPV/PC 61 BM, the size of PC 61 BM aggregates were found to be dependent on the solubility limit of PC 61 BM in the solvent [START_REF] Duong | Molecular solubility and hansen solubility parameters for the analysis of phase separation in bulk heterojunctions[END_REF][START_REF] Hoppe | Efficiency limiting morphological factors of MDMO-PPV:PCBM plastic solar cells[END_REF][START_REF] Hoppe | Nanoscale Morphology of Conjugated Polymer/Fullerene-Based Bulk-Heterojunction Solar Cells[END_REF]. When poor solvents are used (such as pyridine and toluene), the BHJ exhibits large size aggregates of PC 61 BM in the order of hundreds of nanometers. In the films spin-cast from good solvents, such as CB or ODCB, the large features are absent. Figure 2.1 shows atomic force microscopy (AFM) images that clearly depicts the dependence of PC 61 BM aggregate sizes on solvent quality. This morphology is attributed to the fact that in good solvents, PC 61 BM molecules remain finely dispersed and no over-sized aggregates are formed. Smooth films of MDMO-PPV/PC 61 BM exhibited the highest PCEs, in the order of 2.5%. and < 50 nm for pyridine). In stark contrast to the fused nanoclusters, porous networks of MDMO-PPV polymers (average pore size in these films is calculated to be 190 6 32 nm) and micron-sized aggregates are observed in the annealed films deposited from CS2. This is consistent with the film optical data and suggests that the PC 61 BM domains in the underlying film are quite mobile and can quickly diffuse out of the internal structure to form larger aggregates. The diffusion of PC 61 BM is consistent with reports on the diffusion of PC 61 BM through other conjugated polymers; however, our data suggest that the diffusivity of PC 61 BM may be affected by the solvent used to process the film. 24 Longer annealing time would lead to larger aggregates. 45 The morphologies of MEH-PPV:PC 61 BM (Supporting Information Fig. S5) are in striking contrast to those of MDMO-PPV, even though there is little structural difference between the two polymers (3,7 dimethyloctyl vs. 2-ethylhexyl solubilizing groups), HSP values or the optical properties of the films. Similar results were reported with blends of PCDTBT/PC 61 BM [START_REF] Beaupré | PCDTBT: en route for low cost plastic solar cells[END_REF][START_REF] Park | Bulk heterojunction solar cells with internal quantum efficiency approaching 100 %[END_REF][START_REF] Shin | Effects of Organic Solvents for Composite Active Layer of PCDTBT / PC 71 BM on Characteristics of Organic Solar Cell Devices[END_REF] and P3HT/PC 61 BM [START_REF] Ruderer | Solvent-Induced Morphology in Polymer-Based Systems for Organic Photovoltaics[END_REF][START_REF] Troshin | Material Solubility-Photovoltaic Performance Relationship in the Design of Novel Fullerene Derivatives for Bulk Heterojunction Solar Cells[END_REF]. In a BHJ of P3HT/PC 61 BM, it is well known that thermal annealing causes PC 61 BM to crystallize and to form microcrystals [START_REF] Chang | Effect of Trace Solvent on the Morphology of P3HT:PCBM Bulk Heterojunction Solar Cells[END_REF][START_REF] Motaung | Comparative study: the effect of annealing conditions on the properties of P3HT:PCBM blends[END_REF]. Several studies revealed that thermally annealed BHJs presented the largest PC 61 BM microcrystals when spin-cast from poor solvents of PC 61 BM [START_REF] Ruderer | Solvent-Induced Morphology in Polymer-Based Systems for Organic Photovoltaics[END_REF][START_REF] Troshin | Material Solubility-Photovoltaic Performance Relationship in the Design of Novel Fullerene Derivatives for Bulk Heterojunction Solar Cells[END_REF].

Clearly, when PC 61 BM is blended with P3HT or MDMO-PPV, the crystallization of PC 61 BM molecules is governed by its solubility limit. When comparing P3HT

and MDMO-PPV, it is worthy to note that the crystallization of PC 61 BM seems to depend more on solvent quality when MDMO-PPV is the D. This divergent behavior was attributed to the different strength of the interactions between the polymer and the fullerene components [START_REF] Schmidt-Hansberg | Moving through the phase diagram: morphology formation in solution cast polymer-fullerene blend films for organic solar cells[END_REF]. Overall, the aggregation of PC 61 BM is shown to be affected by the D-A interactions and to be dependent on the type of solvent used.

The effect of the solvent quality on the polymeric component is another important aspect to consider. A common method to investigate the effects of solvent quality on the behavior of P3HT chains is to add a poor solvent for the polymer (e.g., acetone or hexane) to the host solvent in order to reduce the solubility of the polymer in the solvent system. Keum et al. studied the structural evolution, in solution, of P3HT chains as a function of the solvent quality, using UV-Vis absorption spectroscopy and small-angle neutron scattering [START_REF] Keum | Solvent quality-induced nucleation and growth of parallelepiped nanorods in dilute poly(3-hexylthiophene) (P3HT) solution and the impact on the crystalline morphology of solution-cast thin film[END_REF]. They showed that the introduction of a poor solvent caused the P3HT chains to aggregate and to grow into nanorods in order to reduce the unfavorable solvent-polymer interactions. As a consequence of the preformed aggregates in solution, the crystallinity of P3HT in the dry film greatly improved [START_REF] Scharsich | Control of aggregate formation in poly(3-hexylthiophene) by solvent, molecular weight, and synthetic method[END_REF]. Chang et al. used grazing incidence x-ray diffraction (GIXD) to show that the introduction of acetone in the solvent system increased the crystallinity of P3HT (Figure 2.2) causing as much as a 4-fold increase in mobility [START_REF] Chang | Solvent Based Hydrogen Bonding : Impact on Poly (3-hexylthiophene) Nanoscale Morphology and Charge[END_REF]. Overall, poor solvent quality affects the behavior of the polymer by introducing a driving force for the polymer to aggregate in order to reach a thermodynamically favorable state. -Solvent's boiling point

The organization of the material and the transition from the liquid to the solid state can be affected by the drying kinetics. Using highly volatile solvents, the kinetics of evaporation can be much higher than the kinetics of crystallization. In such cases, the resulting morphology is far from that observed under equilibrium conditions [START_REF] Nilsson | Morphology and Phase Segregation of Spin-Casted Films of Polyfluorene / PCBM Blends[END_REF]. The effects of the drying kinetics on the amount of polymer chain aggregation have been widely investigated using various deposition techniques [START_REF] Yang | Crystalline Organization of a Methanofullerene as Used for Plastic Solar Cell Applications[END_REF] and solvents with different boiling points. When P3HT/PC 61 BM films were spin-cast from different solvents, it was generally observed that a high boiling point solvent leads to better efficiency [START_REF] Dang | Polymeric solar cells based on P3HT:PCBM: Role of the casting solvent[END_REF]. Ruderer et al. studied the morphology of P3HT/PC 61 BM films spin-cast from four different solvents (CF, CB, toluene and xylene) [START_REF] Ruderer | Solvent-Induced Morphology in Polymer-Based Systems for Organic Photovoltaics[END_REF]. Using grazing incidence wide angle x-ray scattering, they showed that the crystallite sizes of P3HT increased with increasing boiling point of the solvent. Similarly, Verploegen et al. showed that films spincast from CB gave larger crystal sizes than films spin-cast from CF [START_REF] Verploegen | Manipulating the Morphology of P3HT-PCBM Bulk Heterojunction Blends with Solvent Vapor Annealing[END_REF]. This was attributed to the fact that a high boiling point solvent resulted in slow drying that provided time for the self-assembly of polymer chains during solvent evaporation. These results indicate that spin-casting from high boiling point solvents allows the P3HT component to arrange in a lower free-energy state compared with thin films spin-cast from a low boiling point solvent. This phenomenon is also observed with several other D polymers such as PCPDTBT and PTB7 [START_REF] Fischer | Influence of Processing Solvents on Optical Properties and Morphology of a Semicrystalline Low Bandgap Polymer in the Neutral and Charged States[END_REF][START_REF] Guo | Influence of Solvent and Solvent Additive on the Morphology of PTB7 films probed via X-ray scattering[END_REF].

The examples above show that the choice of solvents has a large impact on the morphology of the BHJ. During solvent evaporation, the drying kinetics and thermodynamics are in competition. For P3HT/PC 61 BM blends, high boiling point solvents are preferred to provide time for the polymer chains to self-assemble.

An appropriate choice of solvent can, therefore, optimize the morphology of a BHJ. However, there is a only a limited number of solvents that can solubilize both D and A components. In the best solvents, as cast P3HT/PC 61 BM-OSCs still exhibit low PCEs generally reported to be below 1.5% [START_REF] Chambon | MoO3 Thickness, Thermal Annealing and Solvent Annealing Effects on Inverted and Direct Polymer Photovoltaic Solar Cells[END_REF][START_REF] Xu | High efficient inverted polymer solar cells with different annealing treatment[END_REF]. The poor performance is due to fast solvent evaporation caused by the use of spin-coating (or any other deposition technique). The fast solvent evaporation prevents the materials from aggregating and forming proper phase separated domains. As a result, P3HT and PC 61 BM form an intimately mixed network lacking of well defined domains. This, in turn, primarly limits charge collection and results in low J sc and FF. The application of post-processing steps were demonstrated to enable further optimization of the BHJ morphology.

Effects of post-processing steps

Thermal treatment -also called thermal annealing -on P3HT/PC 61 BM-films was shown to significantly increase the degree of crystallinity of P3HT. and the appearance of two vibronic peaks around 555 nm and 605 nm [START_REF] Zhokhavets | Relation between absorption and crystallinity of poly(3-hexylthiophene)/fullerene films for plastic solar cells[END_REF]. In the XRD pattern in Figure 2.3b, the diffraction peak at 5.5°corresponds to the edge-on orientation of the polymer chains [START_REF] Zhokhavets | Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties[END_REF]. The appearance of the diffraction peak is a signature of increased crystallinity.

The increased degree of P3HT crystallinity is attributed to the fact that the polymer becomes mobile upon annealing and can self-assemble into crystalline domains. This leads to increased ordering between polymer chains. In parallel to the increased ordering of polymer chains, PC 61 BM molecules diffuse out of the increase resulting in PCEs of up to 5% [START_REF] Chirvase | Influence of nanomorphology on the photovoltaic action of polymer-fullerene composites[END_REF][START_REF] Li | High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends[END_REF][START_REF] Ma | Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology[END_REF][START_REF] Padinger | Effects of Postproduction Treatment on Plastic Solar Cells[END_REF][START_REF] Zhokhavets | Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties[END_REF].

A slow drying approach -or solvent annealing -was also used to control the morphology of P3HT/PC 61 BM-films. This approach generally involves spin-casting the D/A for a short period of time so that the resulting film still contains solvents;

the film can then be placed in a solvent saturated environment. In the saturated environment, the solvent evaporation is slowed compared with evaporation during conventional spin-casting. Similarly to what is observed in thermal annealing, the degree of P3HT crystallinity increases and high efficiencies are obtained upon solvent annealing [START_REF] Dang | :PCBM, Best Seller in Polymer Photovoltaic Research[END_REF][START_REF] Hegde | The impact of controlled solvent exposure on the morphology, structure and function of bulk heterojunction solar cells[END_REF][START_REF] Li | Solvent Annealing" Effect in Polymer Solar Cells Based on Poly(3-hexylthiophene) and Methanofullerenes[END_REF]. This approach can be interpreted as an extension of the effects of high boiling point solvents described in section 2.1: during solvent annealing, the polymer chains are given more time to aggregate which results in an overall increase in domain sizes and degree of crystallinity [START_REF] Park | Effect of Annealing Solvent Solubility on the Performance of Poly(3-hexylthiophene)/Methanofullerene Solar Cells[END_REF][START_REF] Verploegen | Manipulating the Morphology of P3HT-PCBM Bulk Heterojunction Blends with Solvent Vapor Annealing[END_REF].

Post-processing steps can increase the efficiency of P3HT/PC 61 BM-based OSCs.

From a fundamental point of view, these approaches also provide insight into the relationship between active layer morphology and photovoltaic performance. In practice, however, thermal annealing is hardly compatible with flexible substrates that require low temperature processing conditions; overall the post-processing steps are difficult to implement in a large area high throughput line of fabrication without impacting the final cost of OSCs. Additionally, these approaches are inefficient for some low band gap polymers [START_REF] Chu | Morphology control in polycarbazole based bulk heterojunction solar cells and its impact on device performance[END_REF][START_REF] Park | Bulk heterojunction solar cells with internal quantum efficiency approaching 100 %[END_REF]. These limitations motivate the need of a different strategy to control the BHJ morphology -specifically the use of processing additives in D/A formulations. proposed that processing additives should possess: [START_REF] Yao | Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells[END_REF] (i) a lower vapor pressure than the host solvent and

Use of processing additives in BHJ-OSCs

(ii) a lower solubility limit towards the fullerene derivative acceptor than that of the host solvent.

They based their selection rules on the P3HT/PC 61 BM system and suggested that during spin-casting, the processing additive and the host solvent evaporate with different rates because of the difference in vapor pressure. The host solvent, with a higher vapor pressure, would evaporate more quickly causing an increase in the additive concentration in the solvent mixture. As a consequence, the solvent quality towards PC 61 BM would decrease, leading to early formation and precipitation of clusters. This mechanism is illustrated in In the same year, Lee et al. proposed a different set of criteria for selecting processing additives, claiming that an ideal processing additive should have [START_REF] Lee | Processing additives for improved efficiency from bulk heterojunction solar cells[END_REF]:

(i) a good solubility towards the fullerene derivative acceptor, (ii) a poor solubility towards the polymer and

(iii) a boiling point higher than that of the host solvent.

The mechanism proposed here is based on the fact that processing additives are good solvents for the fullerene derivatives and are poor solvents for the D (such as P3HT or PCDTBT). During the transition from the liquid to the solid states, the high boiling point of the solvent means that the fullerene derivatives remain longer in the solution state; this enables control of the phase separation between the polymer and the small molecule. Such a mechanism is depicted in Figure 2.5.

This mechanism is therefore based on the differential solubility of the additive towards the polymer and the fullerene derivative whereas the first mechanism accounts for the poorer solubility of the acceptor derivative in the additive compared with the host solvent. So far, however, no clear selection rules for selecting processing additives have emerged. Experimentally, other solvents were found to increase the PCE of OSCs: alkanedithiol with various alkane chain lengths [START_REF] Ouyang | High-performance polymer photovoltaic cells with thick P3HT:PCBM films prepared by a quick drying process[END_REF][START_REF] Peet | Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols[END_REF][START_REF] Salim | Solvent additives and their effects on blend morphologies of bulk heterojunctions[END_REF], 1,8-diiodooctane (DIO) [START_REF] Lee | Processing additives for improved efficiency from bulk heterojunction solar cells[END_REF][START_REF] Liu | Improvement of polycarbazole-based organic bulk-heterojunction solar cells using 1,8-diiodooctane[END_REF], 1-chloronaphthalene (CN) [START_REF] Aïch | Control of the active layer nanomorphology by using co-additives towards high-performance bulk heterojunction solar cells[END_REF][START_REF] An | Solvent Additive to Achieve Highly Ordered Nanostructural Semicrystalline DPP Copolymers: Toward a High Charge Carrier Mobility[END_REF][START_REF] Jo | Bulk heterojunction solar cells based on a low-bandgap carbazole-diketopyrrolopyrrole copolymer[END_REF][START_REF] Moon | Effect of processing additive on the nanomorphology of a bulk heterojunction material[END_REF], nitrobenzene [START_REF] Chu | High-efficiency inverted solar cells based on a low bandgap polymer with excellent air stability[END_REF],

1-methyl-2-pyrrolidinone (NMP) [START_REF] Yao | Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells[END_REF] and 1,8-dichlorooctane [START_REF] Gu | Multi-Length-Scale Morphologies in PCPDTBT/PCBM Bulk-Heterojunction Solar Cells[END_REF][START_REF] Lee | Processing additives for improved efficiency from bulk heterojunction solar cells[END_REF] among others [START_REF] Liao | Additives for morphology control in high-efficiency organic solar cells[END_REF][START_REF] Liu | The influence of additive property on performance of organic bulk heterojunction solar cells[END_REF]. Figure 2.6 depicts the chemical structures of some of the processing additives found in literature.

For the D/A system based on P3HT/PC 61 BM, introducing nitrobenzene or alkanedithiols in the D/A solutions increased the PCEs up to 4% [START_REF] Moule | Controlling Morphology in Polymer-Fullerene Mixtures[END_REF][START_REF] Ouyang | High-performance polymer photovoltaic cells with thick P3HT:PCBM films prepared by a quick drying process[END_REF]. These efficiencies are in the range of those normally obtained with post-processing steps [START_REF] Chen | Fast-Grown Interpenetrating Network in Poly(3-hexylthiophene): Methanofullerenes Solar Cells Processed with Additive[END_REF][START_REF] Ouyang | High-performance polymer photovoltaic cells with thick P3HT:PCBM films prepared by a quick drying process[END_REF]. High efficiency P3HT/PC 61 BM-based OSCs can therefore be fabricated without the use of post-processing steps, which is particularly appealing for the prospect of low fabrication cost and large area OSCs [START_REF] Eom | High efficiency polymer solar cells via sequential inkjet-printing of PEDOT:PSS and P3HT:PCBM inks with additives[END_REF]. Literature shows that processing additives can increase the efficiency of OSCs in a wide range of D/A systems by optimizing the active layer morphology [START_REF] Graham | Improved Performance of Molecular Bulk-Heterojunction Photovoltaic Cells through Predictable Selection of Solvent Additives[END_REF][START_REF] Liao | Additives for morphology control in high-efficiency organic solar cells[END_REF][START_REF] Perez | Solvent additive effects on small molecule crystallization in bulk heterojunction solar cells probed during spin casting[END_REF][START_REF] Wang | The role of additive in diketopyrrolopyrrole-based small molecular bulk heterojunction solar cells[END_REF]. Table 2.2 shows examples of some of these D/A systems along with the PCE values before and after introducing the processing additives. Based on these examples, it is clear that the use of processing additives represents a widely applicable and efficient approach to optimize a large range of D/A systems.

Context and objectives of the thesis

High-efficiency OSCs require a controlled and optimized BHJ morphology. Solvent choice in the D/A formulation is of major importance, but has a limited impact on the morphology due to the limited number of good solvents available for organic semiconductors. Also, upon spin-casting, the drying kinetics often dominate over the thermodynamic aspects of the D/A system, which leads to poor phase separation and subsequently low PCE in the case of P3HT/PC 61 BM.

Post-processing steps were successful in optimizing P3HT/PC 61 BM-based OSCs, but were not efficient in optimizing several systems based on other low band gap polymers. Additionally, post-processing steps are not ideal when thinking towards flexible and low-cost OSCs.

The use of processing additives appears to be a key approach to optimize D/A morphology: it is widely applicable and compatible with large area and high throughput fabrication of OSCs. While many studies have reported the success of processing additives in increasing the PCE of OSCs, fundamental aspects surrounding their use have not been addressed as successfully.

So far, there has been no systematic approach to identify suitable processing additives for a given D/A system. Very few studies have addressed the issue of how to identify or select processing additives from the vast array of available 1 The acronyms used for the D and A materials are defined in the Abbreviations section 2 No exact PCE was reported solvents. Consequently, their selection is generally the result of trial and error experiments.

Comparing the different studies in the literature also shows that there is no clear consensus on the mechanistic role of processing additives in influencing and controlling the BHJ morphology. Their effects on D/A phase separation have been the subject of several studies. The results of such studies, summarized in Table 2.2, show that the effects of processing additives on phase separation vary with the type of D/A blend under investigation: in some cases they increase the phase separation while in other cases, they decrease it. In the case of D/A blends based on P3HT, the literature reports contradictory results regarding the effects of additives on phase separation.

Another major aspect not considered in the literature is the effect of additives on the long-term stability of OSCs. As OSC stability is equally important as the PCE, investigating their effects on stability is of major importance.

An in-depth understanding of the effects of processing additives is required to enable and accelerate the development of efficient D/A formulations. This thesis aims to carry out a comprehensive investigation of processing additives, addressing the following objectives:

1. To establish a predictive method for identifying processing additives for D/A systems with certain given properties.

2. To elucidate the mechanistic role of processing additives in influencing and controlling the BHJ morphology and subsequently, the OSC performance.

3. To determine the impacts of the processing additive approach on the stability of OSCs.

In pursuit of these objectives, the study will primarily focus on the archetypical P3HT/PC 61 BM system. The last chapter of this thesis deals with the effects of processing additives in systems using other types of D polymers.

Chapter 3

Experimental details

Fabrication and characterization of OSCs

This PhD thesis being done in the context of a co-tutelle program between the University of Waterloo and the University of Bordeaux, the experimental details differ from experiment to experiment. Experimental details are described according to the laboratory where the studies were conducted.

Experiments reported in Chapters 4, 6 and 8 were performed at the University of Bordeaux laboratory (UB Lab).

Experiments reported in Chapters 5 and 7 were performed at the University of Waterloo Laboratory (UW Lab).

Materials and substrates

The P3HT and PC 61 BM were purchased from Solaris Chem Inc. and used as received. The P3HT has a molecular weight of 53 kDa and a polydispersity index of 1.49. The purity of PC 61 BM is 99.74%.

The PCDTBT is purchased from Solaris Chem Inc., its molecular weight is 85 kDa.

The PDQT was synthesized in UW Lab according to the literature [START_REF] Li | Annealing-Free High-Mobility Diketopyrrolopyrrole-Quaterthiophene Copolymer for Solution-Processed Organic Thin Film Transistors[END_REF]. The average molecular weight is 21100 and the polydispersity index 2.72.

The solvents: 1,2-dichlorobenzene, chlorobenzene were purchased from Sigma Aldrich and used as received. The processing additives: dimethyl phthalate, 1,8octanedithiol, 1,8-diiodooctane, 1-cyclohexyl-2-pyrrolidinone, tributyl-o-acetylcitrate were purchased from Sigma Aldrich and used as received. 1-chloronaphthalene was purchased from TCI Chemical N.V. and used as received.

Zinc acetate dihydrate was purchased from Sigma Aldrich with a purity above 99.9%.

At UB Lab, the ITO coated substrates used for device fabrication were purchased from Kintec. The resistance of the ITO layer is 10 Ω/square. The shadow masks used for device fabrication enable the fabrication of four OSCs, with a surface area of 8.6 mm 2 , on each substrate.

At UW Lab, the ITO coated substrates were purchased from Luminescence Technology Corporation. The resistance of the ITO layer is 15 Ω/square. The shadow masks used for device fabrication enable the fabrication of three OSCs, with surface areas of 10 mm 2 , 12.5 mm 2 and 17.5 mm 2 , on each substrate.

Active layer formulation

The organic semiconductors are weighed with the desired proportion in vials.

Solvent mixtures are introduced to the organic semiconductors to solubilize them.

The preparation of solvent mixtures differs as a function of the proportion of processing additive in the host solvent.

For mixtures with a processing additive concentration below 0.8 vol%, the solvent mixture is prepared separately by blending and stirring the additive and the host solvent for 1 -2 hours at ambient temperature. The prepared solvent mixture is further introduced in the vial containing the organic semiconducting materials.

For solvent mixtures with a processing additive concentration above 0.8 vol%, the solvent and the processing additive are introduced directly with the desired concentration in the vial containing the organic semiconductors.

The final mixture containing the solvent mixture and the organic semiconductors is first stirred for 15 -20 minutes at 90 °C in order to ensure an efficient dissolution of the polymer and to avoid large aggregates. The mixture is then stirred at 50

°C. For P3HT/PC 61 BM solutions, the mixtures are found to be stable up to at least 6 weeks. Therefore P3HT/PC 61 BM solutions were kept and re-used several times. For PCDTBT/PC 61 BM and PDQT/PC 61 BM solutions, the mixtures were stirred for a maximum period of 12 hours prior to use.

Substrate cleaning

The ITO coated substrates were systematically cleaned in an ultrasonic bath of acetone, ethanol and isopropanol for 15 minutes each. The substrates were then dried using a nitrogen gun and were treated in a UV-ozone oven (in UB Lab) or were treated with O 2 Plasma (in UW Lab). ) and silver (70 nm) were thermally evaporated under a secondary vacuum (10 -6 mbar) onto the active layer through a shadow mask.

J -V characteristics measurements of OSCs

At UB Lab, J -V analysis was conducted using a K.H.S. SolarCelltest-575 solar simulator with AM1.5G filters set at 100 mW.cm -2 with a calibrated radiometer (IL 1400BL). A LabVIEW-controlled Keithley 2400 SMU enabled the measurement of J -V curves.

At UW Lab, the J -V analysis was conducted on ABET technologies Sun 2000

Solar Simulator with AM1.5G filters set at 100 mW.cm -2 . A LabVIEW-controlled Keithley 2400 SMU enabled the measurement of J -V curves.

J sc , V oc , FF and the PCE were extracted from the Labview softwares used for the J -V characterizations. The shunt resistance (R sh ) and the series resistance (R s ) can also be extracted from the J-V characteristics.

Three regions of the J-V characteritsics can be defined as a function of the value of the applied voltage:

-At low voltage, the J-V characteristic is primarily determined by R sh which can be determined from the slope of the J-V characteristic at V = 0 V:

1 R sh = ( dJ dV ) V =0 (3.1)
-At intermediate voltage, the diode parameter dominates.

-At higher voltage, the J-V characteristic is primarily determined by R s .

There are several methods for determining and fitting R s . One of the simplest is to determine the slope of the J-V curve at high voltage. In the literature, the slope is found to be determined at different voltage values [START_REF] Chu | Highly efficient polycarbazole-based organic photovoltaic devices Highly efficient polycarbazole-based organic photovoltaic devices[END_REF][START_REF] Jin | Thickness dependence and solution-degradation effect in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester based solar cells[END_REF][START_REF] Treat | Polymer-fullerene miscibility: a metric for screening new materials for high-performance organic solar cells[END_REF][START_REF] Wakim | Highly efficient organic solar cells based on a poly(2,7-carbazole) derivative[END_REF]. It is however common to determine the series resistance at V oc in order to be close to the operating conditions: [START_REF] Keshtov | New conjugated alternating benzodithiophene-containing copolymers with different acceptor units: synthesis and photovoltaic application[END_REF][START_REF] Kouijzer | Predicting morphologies of solution processed polymer:fullerene blends[END_REF] 1

R s = ( dJ dV ) V =Voc (3.2)

Mobility measurements

In this thesis, the measurements of charge carrier mobilities are determined by two methods: the determination of charge carrier mobility in an organic thin film transistor (OTFT) configuration and in a single diode configuration.

Mobility measurement in OTFT configuration

An OTFT is a three-terminal device with a gate, a source and a drain. The current flowing between source and drain (I DS ) can be controlled by the applied gate voltage across a thin dielectric film. By applying a gate source voltage (V GS ) across the dielectric, a channel of charge carriers is induced in the semiconductor layer at the interface with the dielectric. This channel allows the drain current to flow through the semiconductor when another voltage (the drain source voltage V DS ) is applied between the drain and the source. If V GS is positive, negative charges will be attracted to the interface between the semiconductor and the dielectric, the channel is called n-type channel. On the contrary, when V GS is negative, positive charges are induced at the interface between semiconductor and dielectric and the channel is a p-type channel. Electron mobility can be extracted from the n-type OTFT, while hole mobility can be extracted from the p-type OTFT.

The OTFT architecture used in this work is bottom gate, top contact as presented in Figure 3 A layer of poly(1-vinyl-1,2,4-triazole) (PVT) is used as a dielectric. The PVT solution is prepared by dissolving PVT in ultra-pure water at a concentration of 7 weight%. The solution is filtered and spin-cast at 700 rpm for 60 seconds. The substrates are further dried at 85 °C for 120 min under a primary vacuum. The organic semiconductor materials to study are deposited on top of the dielectric.

The deposition of the metals is performed by thermal evaporation. For p-type OTFTs, the electrode is gold. For n-type OTFTs, the electrode is aluminum.

The electrical characterizations are performed in a glovebox environment using a probe station (SUSS Microtec). Micro probes are used to take the contacts.

A Labview controlled Keithley 4200 SMU was used for the acquisition of the electrical characteristics. A V DS of +5 V is applied for n-type OTFTs and a

V DS of -5 V is applied for p-type OTFTs.
The mobility is extracted from the saturation regime where

|V DS | > |V GS -V th | > 0.
In the saturation regime, the current is given by:

I DS = µC i W 2L (V GS -V th ) 2 (3.3)
and the carrier field effect mobility is given by:

µ sat = 2L C i W ( ∂ √ I DS ∂V GS ) 2 (3.4)
where V th is the threshold voltage, C i the gate dielectric capacitance per unit area (=14.7 nF/cm 2 ), µ sat the mobility, W the channel width and L the channel length.

V th can be extracted from measurements in the saturation region by plotting √ I DS versus V GS and extrapolating to I DS = 0 [START_REF] Wong | Flexible Electronics : Materials and Applications Electronic Materials : Science and Technology[END_REF].

Mobility measurement in single diode configuration

In the field of organic semiconductors, the extraction of the mobility using the space charge limited current (SCLC) has emerged as a common method to extract the charge mobility of materials [START_REF] Kuik | Th Anniversary Article: Charge Transport and Recombination in Polymer Light-Emitting Diodes[END_REF]. The SCLC method is applied for single charge carrier devices, so called hole only devices or electron only devices. The example of hole only devices is taken to describe the electrical characteristics of a single charge carrier diode. In hole only devices, holes are injected in abundance by one of the electrode. These charge carriers are not compensated by an equal density of electrons and build up in the semiconductor: a positive space charge is formed in the semiconductor. Further increase of the voltage leads to the spacecharge limited current, which is the maximum current that the semiconductor can sustain. Above a certain voltage, the current shows a quadratic behavior and the current density J can be characterized by the Child's law (also called Mott-Gurney law):

J = 9 8 r 0 µ V 2 L 3 (3.5)
where 0 is the permittivity of the vacuum, r the dielectric constant of the polymer (assumed to be 3, which is a commonly used value for conjugated polymers), µ the mobility, V the voltage drop across the device and L the thickness of the material under study.

The previous equation assumes that the mobility is field independent. In order to consider a mobility that is field dependent, a modified equation is preferred (also called Murgatroyd equation, [START_REF] Malliaras | Electrical characteristics and efficiency of single-layer organic light-emitting diodes[END_REF]) and J can be described by:

J = 9 8 0 µ 0 V 2 L 3 exp(0.89 V E 0 L ) (3.6)
where µ 0 is the zero-field mobility and E 0 the characteristic field.

Single charge carrier devices are fabricated and further electrically characterized in order to extract the dark J -V curves. By fitting the J -V curves with the equations 3.5 or 5.2, the mobility can be extracted. In the early years, the Child's law was used to determine the mobility of single layers of materials such as (phenylene vinylene) derivatives [START_REF] Blom | Electron and hole transport in poly(p-phenylene vinylene) devices[END_REF][START_REF] Malliaras | Electrical characteristics and efficiency of single-layer organic light-emitting diodes[END_REF][START_REF] Martens | Comparative study of hole transport in poly(p-phenylene vinylene)derivatives[END_REF], P3HT [START_REF] Tanase | Unification of the Hole Transport in Polymeric Field-Effect Transistors and Light-Emitting Diodes[END_REF] and PC 61 BM [START_REF] Mihailetchi | Electron Transport in a Methanofullerene[END_REF].

Later, the use of the SCLC method has been broadened to the determination of single charge carrier mobility of a material blend in a BHJ configuration [START_REF] Melzer | Hole Transport in Poly ( phenylene vinylene )/ Methanofullerene Bulk-Heterojunction Solar Cells[END_REF].

In order to assess the mobility of a single charge carrier of a material in a BHJ configuration, the other carrier has to be blocked. For example, for the study of UV-Vis spectra are recorded using a SAFAS UVMC 2 spectrophotometer at UB Lab and a Shimadzu UV-2501PC UV-Vis spectrophotometer at UW Lab.

X-ray diffraction measurements

The diffraction patterns obtained from the PDQT study were recorded at UB Lab using a Goniometer Pranatycal X'pert Pro.

The diffraction patterns obtained from the P3HT/PC 61 BM study were recorded at UW Lab using a Bruker D8 advance diffractometer. The XRD measurements were performed in a reflection mode using Cu Kα1 radiation at 1.5406 Å.

The average size L of crystallites can be determined with the full width at half maximum of the diffraction peak using the Scherrer equation:

L = Kλ ∆cos(θ) (3.7)
where K is the Scherrer's constant which depends on the crystallite shape and size distribution [START_REF] Langford | Seherrer after Sixty Years: A Survey and Some New Results in the Determination of Crystallite Size[END_REF].

Atomic force microscopy

AFM images of PDQT were performed in UB Lab using a Veeco Dimension 3100.

AFM images of P3HT/PC 61 BM-films were performed in UW Lab using a Dimension 3100 Scanning Probe Microscope.

Infrared absorption spectroscopy

PM-IRRAS spectra were recorded on a ThermoNicolet Nexus 670 FTIR spectrometer at a resolution of 4 cm -1 , by coadding several blocks of 1500 scans (30 minutes acquisition time). Experiments were performed at an incidence angle of 75 °using an external homemade goniometer reflection attachment, adding a ZnSe photoelastic modulator (PEM, Hinds Instruments, type III) after the polarizer [START_REF] Buffeteau | Polarization Modulation FT-IR Spectroscopy of Surfaces and Ultra-thin Films : Experimental Procedure and Quantitative Analysis[END_REF].

The ATR spectra of the additives were recorded with a ThermoNicolet Nexus 670 FTIR spectrometer equipped with a liquid nitrogen cooled narrow-band mercury cadmium telluride (MCT) detector using a Silver-Gate (diamond crystal) ATR accessory (Specac). Each spectrum was obtained from the acquisition of 100 scans at a resolution of 4 cm -1 .

Photo-stability tests

Photo-degradation tests were carried out with white light provided by a 300 W halogen lamp. The distance between the lamp and the device was adjusted so that the light intensity was 100 mW.cm -2 . The temperature is kept below 32 °C throughout the experiment using a fan to cool down the OSCs subject to light irradiation. The temperature of the OSCs were monitored with a k-type thermocouple and an Omega panel monitor. All OSCs were kept in inert conditions during light-irradiation and were placed in air during the measurements of the J-V characteristics.

This work has been published in AdvenMat, 4 (3) 1-9, 2013. It has been adapted

with permission from the publisher under license number 3479061354313.

As noted in Chapter 2, processing additives have been widely used for increasing the efficiency of OSCs. However, very few studies have addressed the critical question of how to identify or select processing additives from the vast array of available solvents. In this chapter, guidelines for the selection of processing additives are developed for P3HT/PC 61 BM-based OSCs.

First, the properties of existing processing additives reported in the literature are analyzed. The novelty of the developed approach is to use the theory of the Hansen solubility parameters to analyze these processing additives. This theory is commonly used in the coating and chemical formulation industries to determine the solubility of compounds. Nevertheless, its use in the area of BHJ started only very recently and has so far been limited to the identification host solvents [START_REF] Park | High performance bulk-heterojunction organic solar cells fabricated with non-halogenated solvent processing[END_REF][START_REF] Walker | A Systematic Approach to Solvent Selection Based on Cohesive Energy Densities in a Molecular Bulk Heterojunction System[END_REF], or to correlate the host solvent solubility properties with the active layer morphology [START_REF] Duong | Molecular solubility and hansen solubility parameters for the analysis of phase separation in bulk heterojunctions[END_REF] and with the performance of OSCs [START_REF] Machui | Determination of the P3HT:PCBM solubility parameters via a binary solvent gradient method: Impact of solubility on the photovoltaic performance[END_REF]. Here, the

Hansen solubility parameters are used to define the interactions between solvent, processing additives and active materials.

The Hansen solubility parameters as a method for selecting processing additives

The solubility parameter (δ), first used by Hildebrand and Scott, was originally defined as the square root of the cohesive energy density (E C ) over the molar volume of the pure solvent (V):

δ = E c V (4.1)
The Hansen theory is based on substituting the total cohesive energy by three components; E D , E P and E H which describe the energy involved in three principal types of interactions, respectively: (i) dispersion interactions, (ii) permanent dipolar -permanent dipolar molecular interactions and (iii) hydrogen bonding interactions [START_REF] Hansen | Solubility Parameters: A User's Handbook, Second Edition, taylor and[END_REF]. The parameter δ can therefore be similarly substituted by three components that describe these three interactions:

E C = E D + E P + E H (4.2)
δ 2 = δ 2 D + δ 2 P + δ 2 H (4.3)
Where δ D , δ P and δ H are the Hansen solubility parameters (HSPs). Graphically, every chemical compound can be represented by its position in a 3D space, the Hansen solubility space, with coordinates defined by the three solubility parameters. The solubility of a solute in a solvent is predicted from similarities in their interactions. Such similarity is quantified by the distance R A between the HSPs of the solvent (δ D1 , δ P 1 and δ H1 ) and the HSPs of the solute (δ D2 , δ P 2 and δ H2 ).

The distance R A between them is calculated using the following equation:

R 2 A = 4(δ D1 -δ D2 ) 2 + (δ P 1 -δ P 2 ) 2 + (δ H1 -δ H2 ) 2 (4.4)
In addition to δ D2 , δ P 2 and δ H2 , a solute requires a boundary of solubility to define and differentiate between "sufficient" and "non-sufficient" interactions from a solubility standpoint. Therefore a solute is described as a sphere in the Hansen solubility space, the HSPs are the centre of the sphere and R O is the radius representing the boundary of solubility. The interactions between a solvent and a solute are considered to be strong only if the distance R A is smaller than the radius of the sphere R O . The relative energy difference (RED) can be used as a numerical parameter to compare R A and R O , and is defined as:

RED = R A /R O (4.5)
If the RED is higher than 1, the solvent is outside the solubility sphere of the solute and can be expected to be a bad solvent. On the other hand, if the RED is between 0 and 1, the solvent is inside the solubility sphere of the solute and is expected to be a good solvent. The HSPs of a wide range of solvents can be found in reference textbooks [START_REF] Hansen | Solubility Parameters: A User's Handbook, Second Edition, taylor and[END_REF]. They have been calculated using either equations of state derived from statistical thermodynamics or using the group contribution method. The group contribution method predicts the HSPs of a compound by adding the solubility parameters of all the contributing chemical groups or atoms. For a new solute, the HSPs can be also determined experimentally by performing solubility tests. The solute is mixed with a wide range of solvents with different known HSPs. The quality of interactions is scored for each solutesolvent combination. The results are recorded on the Hansen solubility space and are fitted to a sphere with all the good solvents inside the sphere and the poor solvents outside. The HSPs of the compound result from the fitting of the sphere. Figure 4.1 depicts the process of sphere fitting using the software HSPiP 3rd edition.

Good solvents

Poor solvents assessed by visual inspection and assigned a score of 1 for good solubility or 0 for poor solubility, depending on whether or not a solid residue can still be detected after the stirring step. These results are used to fit the solubility sphere of the material where all the good solvents (the ones scored 1) are inside the sphere and the bad solvents (the ones scored 0) are outside the sphere. As the data fitting may contain false negative and false positive errors (i.e bad solvents inside the solubility sphere and good solvents outside the solubility sphere, respectively), the fitting accuracy needs to be estimated. This is usually estimated using a quality of fit function of the form:

Fitted solubility sphere

DAT A F IT = (A 1 * A 2 * . . . * A n ) 1/n (4.6)
where n is the number of solvents used for the solubility tests and A i is given by:

A i = e -(Error distance i ) (4.7)
The Error distance i represents the distance of the false positive and false negative solvents from the sphere boundary. The DATA FIT reaches 1 for an ideal fit without any erroneous solvents. The results of the fitting are summarized in

HSPs of commonly used processing additives

Next, the solubility properties of some processing additives reported to improve the efficiency for P3HT/PC 61 BM-based OSCs in the literature are examined:

ODT, 1,6-hexanedithiol, 1,12-dodecanedithiol, di(ethylene glycol)-diethyl ether (DEGDE), n-methyl-2-pyrrolidinone (NMP) and 4-bromoanisole. With the exception of 4-bromoanisole, which HSPs need to be calculated using the group contribution method, the HSPs of all these processing additives can be obtained from the database supplied by Hansen et al. [START_REF] Hansen | Solubility Parameters: A User's Handbook, Second Edition, taylor and[END_REF]. After determining the HSPs of the processing additives and the active materials, the RED values are calculated using the equations described above. The results are shown in

Identification of novel processing additives

Based on the RED values above, the criteria that need to be satisfied by a processing additive for P3HT/PC 61 BM-based OSCs are defined as follows: RED with PC 61 BM < 0.9, RED with P3HT > 1.0 and boiling point 100 °C higher than that of the host solvent. The RED with PC 61 BM is set to < 0.9 instead of < 1 in order to ensure selecting solvents in which PC 61 BM solubility is sufficiently high. The third criterion is based on the general requirement that the boiling point of the processing additive must be higher than that of the host solvent, For each processing additive, the spectra show an increase in the main P3HT absorption peak (at 520 nm) and the appearance of vibronic bands at 550 nm and 600 nm. The appearance of these bands is indicative of an increase in P3HT crystallinity [START_REF] Zhokhavets | Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties[END_REF]. Similar bands are known to appear when P3HT/PC 61 BM films are treated by thermal annealing, or solvent annealing. These results suggest that the three processing additives increase P3HT crystallinity and may, therefore, increase the efficiency of OSCs. [START_REF] Chambon | MoO3 Thickness, Thermal Annealing and Solvent Annealing Effects on Inverted and Direct Polymer Photovoltaic Solar Cells[END_REF][START_REF] Xu | High efficient inverted polymer solar cells with different annealing treatment[END_REF]. The main parameter responsible for the increase in PCE is the J sc which is found to increase significantly on increasing the concentrations for C-PYR, TRIB and ODT. The increase in PCE is, on the other hand, somewhat limited by the negative impact of the processing additives on the V oc which is observed to decrease as the concentration of the processing additive increases. This change in V oc is commonly observed for OSCs undergoing solvent annealing or thermal annealing processes [START_REF] Li | Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene)[END_REF][START_REF] Li | Solvent Annealing" Effect in Polymer Solar Cells Based on Poly(3-hexylthiophene) and Methanofullerenes[END_REF]. Chen et al.

showed that the introduction of ODT in a P3HT/PC 61 BM blend increased the crystallinity of P3HT and caused the interlayer spacing between P3HT chains to be shorter [START_REF] Chen | Fast-Grown Interpenetrating Network in Poly(3-hexylthiophene): Methanofullerenes Solar Cells Processed with Additive[END_REF]. The decrease in V oc observed with ODT and the other processing additives may be attributed to the increase in P3HT crystallinity, together with the reduction of the interlayer spacing of P3HT which can induce a reduction of the band gap [START_REF] Chen | Regiocontrolled Synthesis of poly(3-alkylthiophenes) mediated by Rieke zinc: their characterization and solid-state properties[END_REF]. As a result, the difference between the LUMO of PC 61 BM and the HOMO of P3HT becomes somewhat smaller, causing a decrease in V oc [START_REF] Scharber | Design Rules for Donors in Bulk-Heterojunction Solar Cells-Towards 10 % Energy-Conversion Efficiency[END_REF].

Clearly, all three additives produce similar trends in terms of their effects on OSC parameters; not unlike those produced by ODT and all lead to significant improvements in the PCE. The results therefore show that the use of HSPs and the specific selection criteria that were defined are indeed successful in screening and identifying processing additives that are capable of improving the performance of BHJ-OSCs.

Conclusion

In conclusion, the HSPs are shown to be efficient in guiding the selection of processing additives for P3HT/PC 61 BM-OSCs. Using selection criteria based on the HSPs, three novel processing additives for the P3HT/PC 61 BM system have been identified: C-PYR, DPH and TRIB. Through the developed approach, the number of suitable processing additives that can be investigated could be significantly increased. Also, it provides a methodological approach based on the solubility parameters of the active materials and the solvents for selecting processing additives.

This chapter focuses on elucidating the mechanistic effects of processing additives on the formation of the BHJ. To verify the universal aspect of the study, three processing additives of significantly different molecular structures are investigated: C-PYR, DPH and ODT. Optical, structural and electrical characterizations are conducted to understand the effects of processing additives on the formation of the BHJ during spin-casting. Using the results of these characterizations, a general mechanism that explains the role of additive in optimizing the BHJ morphology is proposed.

Effects of processing additives on the efficiency of OSCs

First, the effects of processing additive concentration on the electrical parameters of the OSCs are studied. A series of OSCs utilizing formulations that contain various concentrations of the three processing additives (from 0 vol% to 4 vol%)

in ODCB is fabricated. The J -V characteristics of the OSCs are measured using a solar simulator with AM1.5G filters set at 100 mW.cm -2 . in the case of DPH: from an optimum J sc of 8.5 mA.cm -2 , the J sc drops to 7.2 mA.cm -2 when the additive concentration reaches 4 vol%.

Overall, the value of the PCE reflects the evolution of the FF. The maximum PCEs are 2.9% for both C-PYR and DPH and 3.0% for ODT.

Characterizations of films prepared from processing additive 5.2.1 UV-Vis absorption spectroscopy

In order to gain insights into the effects of processing additives on the crystallinity of P3HT, UV-Vis absorption spectroscopy was carried out on a series of P3HT/PC 61 BM-films spin-cast from a solution with varying additive concentrations. The UV-Vis spectra of films prepared with processing additives are presented in Figure 5.2. In these spectra, the absorption band of P3HT increases on increasing processing additive concentration and the vibronic bands of P3HT at 556 nm and 605 nm become more intense. These changes in P3HT absorption spectra suggest that the introduction of processing additive in the solvent mixture increases the crystallinity of P3HT in the BHJ, which is in agreement with the V oc trends observed in Figure 5.1 .

XRD measurements

In order to further investigate the effects of the processing additives on the crystallinity of P3HT, X-ray diffraction (XRD) measurements were conducted on P3HT/PC 61 BM films. C o u n t s ( a . u . ) Where L is the average crystallite size of P3HT (1 0 0) plane, λ is the wavelength of the X-ray (λ = 0.154 nm), ∆ is the full width at half maximum (FWHM) of the diffraction peak, θ is the diffraction angle and K the Scherrer's constant. Using this equation, the FWHM can be correlated to the size of P3HT crystallites.

2 θ (°) 0 v o l % 0 . 4 v o l % 0 . 8 v o l % 1 . 6 v o l % 2 . 4 v o l % 3 . 2 v o l % 4 v o l % c )
Table 5.1 displays the values of the FWHM for all the films. As shown in Table 5.1, the values of the FWHM of the diffraction peak at 5.5°increase with increasing concentration of processing additives. According to Equation 5.1, the domain size is inversely proportional to the FWHM. Therefore, the increase in FWHM suggests that the introduction of processing additives leads to smaller domains of P3HT crystallites. The average crystallites sizes can be assessed using equation 5.1, providing that the value of K is known which value depends on the crystallite shape and the crystallite-size distribution [START_REF] Langford | Seherrer after Sixty Years: A Survey and Some New Results in the Determination of Crystallite Size[END_REF]. The determination of K in a BHJ configuration is beyond the scope of this work but an approximation of the crystallite sizes can nevertheless be assessed. Figure 5.4 depicts the evolution of crystallite sizes as calculated with equation 5.1 using 0.9

for the value of K (considering crystalline domains as being spherical). The results clearly show that P3HT crystallites decrease with increasing concentration of processing additives. As a comparison, XRD measurements were also conducted on a film of P3HT/PC 61 BM which was thermally annealed at 110 °C for 15 min (Figure 5.3d). In this case, the diffraction peak of P3HT becomes narrower (FWHM=0.32°) which implies that the P3HT domain sizes increase after thermal annealing. This shows that the BHJ morphology of a thermally annealed P3HT/PC 61 BM film is fundamentally different from that obtained with processing additives, despite the fact these two methods increase the PCE of OSCs in a similar manner. When a thermal annealing is performed, the domain sizes of P3HT crystallites increase whereas when processing additives are used, the domains sizes of P3HT crystallites decrease.

The previous results in UV-Vis absorption spectroscopy and the trend in V oc suggest that processing additives increase crystallinity of P3HT in the BHJ. The increased crystallinity can be due to two factors: either an increasing crystallite domain sizes and/or an increasing number of crystallite domains. Since the XRD measurements show that the size of the crystallite domains decreases, it is clear that the introduction of processing additives contribute to generate more crystallites of P3HT and smaller in size.

The question that is naturally arising now is whether the processing additives have a direct effect on P3HT crystallinity or they have an effect on PC 61 BM and the observed crystallinity change in P3HT is a consequence of it. To address this question, XRD measurements were performed on films of sole P3HT spin-cast from solutions with various concentrations of processing additive, in this case DPH. The XRD patterns of P3HT-films, depicted in Figure 5.5, show that the introduction of DPH decreases the intensity of the diffraction peak of P3HT, similar to that observed in P3HT/PC 61 BM films. This result indicates that processing additives have a direct effect on the crystallinity of P3HT by generating more numerous and smaller crystallites. In order to determine how this structuring reflects on the transport of holes, the hole mobility across the BHJ is measured.

Hole mobility measurements

The hole mobility across the BHJ can be determined by using the standard Space Charge Limited Current (SCLC) method [START_REF] Dante | Three-Dimensional Nanoscale Organization of Highly Efficient Low Band-Gap Conjugated Polymer Bulk Heterojunction Solar Cells[END_REF][START_REF] Graham | Improved Performance of Molecular Bulk-Heterojunction Photovoltaic Cells through Predictable Selection of Solvent Additives[END_REF][START_REF] Li | Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene)[END_REF]. Single carrier devices, in this case hole-only devices, are fabricated using the following structure: Above a certain voltage, the transport of holes in this device architecture becomes limited by the space charge that is formed. In this regime, the hole mobility can be extracted from the fitting of the J -V characteristics in the dark using the following equation:

J = 9 8 0 µ 0 V 2 L 3 exp(0.89 V E 0 L ) (5.2)
where 0 is the permittivity of the vacuum, r is the dielectric constant of the polymer (assumed to be 3, which is a commonly used value for conjugated polymers), V is the voltage drop across the device, L the thickness of the material under study and where µ 0 is the mobility.

Hole-only devices are fabricated from active layers with various compositions. For each type of active layers, the thicknesses are measured using a profilometer. The thickness of the active layer devoid of processing additive is 101 nm and increases up to 121 nm, 135 nm and 113 nm for active layers prepared with 4 vol% of C-PYR, DPH and ODT respectively. To measure the J -V characteristics, a positive bias, defined as the ITO being at a more positive potential relative to the metal electrode, was used. Under such a bias, holes flow across the P3HT/PC 61 BM layer and are collected by the MoO 3 /Ag electrode. Therefore, the direction of hole transport in these hole-only devices is the same as in the operating mode of inverted OSCs devices. Figure 5.7 shows plots depicting J.L 3 as a function of the applied voltage from a series of hole-only devices fabricated from different types and concentrations of additive. In order to extract the mobility, the J -V characteristics are fitted with Equation 5.2 for voltages ranging from around 2 to 5 V. The starting voltage is found to be appropriate for fitting around 2 V and slightly varies as a function of the composition of the active layer. The average hole mobility in a BHJ without processing additive is 1.4x10 -4 cm 2 .V -1 .s -1 . For the three processing additives, two regimes appear: the hole mobility increases initially with increasing concentration of the processing additive, reaching a certain peak value and then starts to decrease upon increasing concentration. The optimum concentration at which this peak value is reached is 0.8 vol% for C-PYR and DPH, at which the hole mobility reaches 1.5x10 -4 cm 2 .V -1 .s -1 and 2.1x10 -4 cm 2 .V -1 .s -1 respectively. For ODT, the optimum concentration is 1.6 vol% and leads to a hole mobility of 2.4x10 -4 cm 2 .V -1 .s -1 . The two regimes can be explained by the structural modification in the BHJ caused by processing additives. As suggested earlier from the UV-Vis absorption spectra and the XRD measurements above, processing additives increase the overall crystallinity of P3HT and decrease the crystallite sizes. The initial increase in hole mobility likely reflects the increase in P3HT crystallinity in the BHJ. But alongside with the increased crystallinity, the crystallites get smaller which contributes to reduced sizes of hole transport pathways and the appearance of grain boundaries. The shorter pathways together with the grain boundaries are likely responsible for the decrease in hole mobility at high concentrations of processing additive.

0 1 2 3 4 5 1 E -9 1 E -8 1 E -7 1 E -6 a ) J . L 3 [ A . c m ] V 1 . 6 v o l % 2 . 4 v o l % 3 . 2 v o l % 4 v o l % C -P Y R c o n c e n t r a t i o n : 0 v o l % 0 . 4 v o l % 0 . 8 v o l % 0 1 2 3 4 5 1 E -9 1 E -8 1 E -7 1 E -6 b ) J . L 3 [ A . c m ] V 1 . 6 v o l % 2 . 4 v o l % 3 . 2 v o l % 4 v o l % D P H c o n c e n t r a t i o n : 0 v o l % 0 . 4 v o l % 0 . 8 v o l % 0 1 2 3 4 5 1 E -9 1 E -8 1 E -7 1 E -6 c ) J . L 3 [ A . c m ] V 1 . 6 v o l % 2 . 4 v o l % 3 . 2 v o l % 4 v o l % O D T c o n c e n t r a t i o n : 0 v o l % 0 . 4 v o l % 0 . 8 v o l % 2 3 4 5 1 E -7 1 E -6 C -P Y R c o n c e n t r a t i o n : 2 . 4 v o l % 3 . 2 v o l % 4 v o l % J . L 3 [ A . c m ] V 0 v o l % 0 . 8 v o l % 1 . 6 v o l % d ) 2 3 4 5 1 E -7 1 E -6 e ) 1 . 6 v o l % 2 . 4 v o l % 3 . 2 v o l % 4 v o l % J . L 3 [ A . c m ] V D P H c o n c e n t r a t i o n : 0 v o l % 0 . 4 v o l % 0 . 8 v o l % 2 3 4 5 1 E -7 1 E -6 f ) J . L 3 [ A . c m ] V 0 v o l % 0 . 4 v o l % 0 . 8 v o l % 1 . 6 v o l % 2 . 4 v o l % 3 . 2 v o l % 4 v o l % 1 . 6 v o l % 2 . 4 v o l % 3 . 2 v o l % 4 v o l % O D T c o n c e n t r a t i o n : 0 v o l % 0 . 4 v o l % 0 . 8 v o l %
Figure 5.9 displays the evolution of hole mobility as a function of additive concentration along with the FFs of the corresponding OSCs. The graphs show that the variation in hole mobility appears to correlate with the change in FF.

For C-PYR and DPH, both the FF and the hole mobility reach their maximum values for an additive concentration of 0.8 vol%. For ODT, the maximums of FF and hole mobility are obtained for a concentration of 1.6 vol%. This suggests that the evolution of FF upon the introduction of processing additive arises from changes in the hole mobility and thus, from the morphological changes caused by the processing additive.

Mechanistic effects of processing additives on the selfassembly of P3HT

The optical and structural characterizations showed that processing additives have a significant effect on the BHJ morphology. The introduction of processing additives leads to a morphology which is different from that obtained by means of thermal annealing, as revealed by the XRD measurements. With processing additives, the crystallites of P3HT become smaller and more numerous. Such morphological modifications naturally have an effect on the mobility and the electrical performance. In the literature, a number of mechanisms have been proposed for describing the effects of processing additives. For example, Lee et al. suggested that processing additives lead to three separate phases during the process of liquid-liquid phase separation step of the drying process: a fullereneadditive phase, a polymer aggregate phase and a polymer-fullerene phase. As the processing additive has a higher boiling point, the fullerene remains in solution longer which affects the phase separation [START_REF] Lee | Processing additives for improved efficiency from bulk heterojunction solar cells[END_REF]. Chen et al. proposed that processing additives provide a driving force for the aggregation of polymer chains [START_REF] Chen | Fast-Grown Interpenetrating Network in Poly(3-hexylthiophene): Methanofullerenes Solar Cells Processed with Additive[END_REF]. However, neither mechanism adequately explains the evolution of the structure of P3HT crystallites and its dependence on the concentration of processing additive. In this section, a mechanism based on the HSPs of the solvent mixture is proposed.

During the process of solvent evaporation, the higher boiling point of the pro- In all cases, the RED values between PC 61 BM and the solvent mixture remain below 1 despite the increasing concentration of additive. Therefore, the solvent mixture remains a good solvent for PC 61 BM for any concentration of additive.

The evolution of the RED values between P3HT and the solvent mixtures exhibits a different behavior. For the initial concentration (from 0 to 4 vol%), the RED between P3HT and the solvent mixture is < 1, meaning that P3HT is soluble in the initial solution mixture. As the effective concentration of the additive increases, the RED also increases, eventually reaching the threshold concentration at which the RED becomes 1. The threshold concentrations for C-PYR and ODT are 49 vol% and 44 vol% respectively, whereas for DPH, it is only 18 vol%.

When the concentration of additive has reached the threshold concentration, the solvent mixture becomes a poor solvent for P3HT while remaining a good solvent for PC 61 BM. The change in solvent quality causes the transition of P3HT from the liquid state to the solid state by the formation of nuclei of P3HT. Without processing additive, the transition from the liquid state to the solid state happens when the solubility limit of P3HT is reached [START_REF] Schmidt-Hansberg | Moving through the phase diagram: morphology formation in solution cast polymer-fullerene blend films for organic solar cells[END_REF] and with processing additive, the transition is shifted forward.

The XRD data showed that the size of the P3HT crystallite domains is dependent on the initial concentration of the processing additive. This effect can also be understood using the RED versus processing additive concentration plots. For a formulation with a given concentration of processing additive, a certain volume of the host solvent has to evaporate before reaching the threshold concentration.

When the initial concentration of the processing additive is high, a small amount of the host solvent has to evaporate before the threshold concentration is reached.

Therefore, the P3HT nucleates in a medium that is fairly dilute. In this case, the P3HT chains are more spread apart and are also given less time to assemble before the nucleation process starts. As a result, the crystallites of P3HT are expected to be more numerous and smaller. On the other hand, when the initial concentration of the processing additive is low, a larger amount of the host solvent needs to evaporate before the threshold concentration is reached. The P3HT chains nucleate in a more concentrated medium and are given more time to aggregate prior to the transition from the liquid to the solid states. Consequently, the crystallites are larger than in the first case. The starting concentration therefore plays a crucial role in the size and the amount of crystallites in the final BHJ morphology. Figure 5.11 illustrates the BHJ morphologies obtained from D/A formulations containing different starting concentrations of processing additive.

Without additive Low concentration High concentration

Figure 5.11: Schemes representing the BHJ morphology spin-cast from formulations with various concentration of additive. The crystallites of P3HT become more numerous and smaller as the starting concentration or processing additive increases.

Conclusions

Correlating structural and optical characterizations revealed the mechanistic role of processing additives in improving the BHJ morphology, hence attaining higher OSC efficiency. The characterizations were performed on BHJs with varying concentration of processing additives, investigating, in this pursuit, three processing additives with different chemical structures in order to determine and verify the universality of the behavior.

The evolution of the V oc and of the UV-Vis absorption spectra showed that processing additives increased the overall crystallinity of P3HT in the BHJ whereas XRD patterns revealed that the increased crystallinity is associated with a decrease in the polymer crystallite sizes. These results suggest that processing additives leads to the formation of BHJ with more numerous but smaller polymer crystallites. Studies of charge carrier mobility revealed that this modification in crystallinity caused the mobility to increase at first but ultimately to decrease because of factors accompanying the crystallinity such as the appearance of grain boundaries.

In order to determine how such a morphology can be induced, the role of processing additives during solvent evaporation is identified by studying the evolution The previous chapters depicted OSCs that were fabricated exclusively in inverted configurations. Here, the effect of device architecture on the performance of OSCs prepared with additive is studied. The initial motivation behind this study is to determine whether processing additives cause a vertical phase separation of D and A in the BHJ.

Vertical phase separation appears to be an important parameter for the efficiency of OSCs [START_REF] Zhang | Vertical phase separation in bulk heterojunction solar cells formed by in situ polymerization of fulleride[END_REF], but currently there is no clear consensus on this subject [START_REF] Kim | Electrical Performance of Organic Solar Cells with Additive-Assisted Vertical Phase Separation in the Photoactive Layer[END_REF][START_REF] Parnell | Depletion of PCBM at the cathode interface in P3HT/PCBM thin films as quantified via neutron reflectivity measurements[END_REF][START_REF] Pavlopoulou | Phase separation-driven stratification in conventional and inverted P3HT:PCBM organic solar cells[END_REF]. Vertical phase separation was shown to be dictated by several parameters such as the surface energy of the layer beneath the active layer [START_REF] Campoy-Quiles | Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends[END_REF][START_REF] Germack | Interfacial Segregation in Polymer/Fullerene Blend Films for Photovoltaic Devices[END_REF], the application of a thermal annealing treatment [START_REF] Van-Bavel | Three-dimensional nanoscale organization of bulk heterojunction polymer solar cells[END_REF][START_REF] Xue | Vertical Stratification and Interfacial Structure in P3HT:PCBM Organic Solar Cells[END_REF] and the spin-casting conditions [START_REF] Campoy-Quiles | Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends[END_REF]. Overall, the literature suggests that processing conditions affect the vertical phase separation in BHJ and it is therefore natural to verify whether processing additives also play a role in phase separation. In case of vertical phase separation induced by additives, the performance of additive-treated OSCs would be sensitive to the device architecture. Comparing the photovoltaic parameters of OSCs fabricated in inverted and in conventional architectures represents therefore an indirect approach to verify whether such vertical phase separation occurs in the active layer.

The studies presented here are carried out with two processing additives: DPH and ODT.

6.2 Electrical performance of OSCs using different archi- The V oc and the R sh decrease with increasing concentration of additive independent of the type of architecture used. Apart from V oc and R sh , the other electrical parameters demonstrate significant differences as a function of the type of device architecture. The most significant difference arises from the FF: in conventional OSCs, the FF decreases significantly upon the introduction of the additive while in inverted OSCs, not much change is observed. When OSCs are prepared with 2.4 vol% of ODT, the FF of conventional OSCs drops to 0.31 causing the PCE to decrease to 1.23 % (less than half the PCE of OSC without additive). Significant differences are also observed from the J -V characteristics of OSCs (Figure 6.3).

The J -V characteristics under illumination of conventional OSCs prepared with 2.4 vol% of ODT demonstrate a S-shape behavior which is generally associated with charge accumulation [START_REF] Qi | Fill factor in organic solar cells[END_REF][START_REF] Wang | Charge accumulation induced S-shape J-V curves in bilayer heterojunction organic solar cells[END_REF]. This can be induced by several parameters such as interfacial barriers [START_REF] Cheun | Electrical and Optical Properties of ZnO Processed by Atomic Layer Deposition in Inverted Polymer Solar Cells[END_REF][START_REF] Wang | Charge accumulation induced S-shape J-V curves in bilayer heterojunction organic solar cells[END_REF] and unbalanced charge carrier mobilities [START_REF] Qi | Fill factor in organic solar cells[END_REF]. In contrast, the J -V characteristics of inverted OSCs do not exhibit this behavior. Clearly, the processing additives give different effects in the different architectures. While ODT is highly beneficial for the performance of inverted OSCs, it demonstrates negative impact on that of conventional OSCs. In order to verify that these trends are not limited to the specific use of ODT only, similar studies are carried out with DPH. The electrical performance shows a similar trend as previously observed with ODT. V oc and R sh decrease with increasing concentration of DPH regardless of the architecture. The FF and the J sc exhibit poor values at high DPH concentration in conventional OSCs while in inverted OSCs, the FF remains relatively unchanged and the J sc is increased. In the case of DPH, the differences between conventional and inverted OSCs appear to be more pronounced than in the ODT case. Conventional OSCs prepared with 2.4 vol% of DPH exhibit a particularly low efficiency with a FF of 0.23 and an average PCE of 0.12%. This low performance is expected from the non-diode behavior observed in its J -V characteristics depicted in Figure 6.4. As the proportions of D and A in the layer are expected to influence the charge carrier mobility, the hypothesis of vertical phase separation is verified by means of hole and electron mobility measurements using an organic thin film transistor (OTFT) configuration. Such technique appears to be suitable for this study as it selectively measures the mobility at the lower part of the active layer. In the saturation regime, the current is given by:

I DS = µ sat C i W 2L (V GS -V th ) 2 (6.1)
and the carrier field effect mobility is given by:

µ sat = 2L C i W ( ∂ √ I DS ∂V GS ) 2 (6.2)
where V th is the threshold voltage, C i the gate dielectric capacitance per unit area, µ sat the mobility, W the channel width and L the channel length. In p-type OTFTs, the transfer characteristics show that higher I DS is obtained in films treated with ODT. In contrast, for the n-type OTFTs, no significant change is observed from the transfer characteristics, except those prepared with 1.6 vol% of ODT.

The mobilities are then calculated for different values of V GS using Equation 6.2.

The extracted hole and electron mobilities are plotted as a function of V GS -V th in Figure 6.8a. For p-type OTFTs prepared from 0 and 0.4 vol% of ODT, the I DS in the transfer characteristics is so low that the calculated hole mobilities appear to be in the noise margin and can therefore not be considered reliable.

For ODT concentrations ≥ 1.6 vol%, the hole mobility increases significantly. In order to evaluate clearly the dependence of electron and hole mobilities on ODT concentration, they are extracted and compared at V GS -V th values of + (or -) 2

V for electron (or hole) mobility. The mobility values are depicted as a function of ODT concentration in Figure 6.8b. Table 6.1 shows the corresponding mobility values as well as the ratio of hole mobility (µ h + ) over electron mobility (µ e -). Table 6.1: Mobility of holes and electrons as a function of the concentration of ODT.

0 v o l % 0 . 4 v o l % 1 . 6 v o l % 2 . 4 v o l % M o b i l i t y ( c m 2 . V -1 . s -1 ) V g -V t h ( V ) (a

ODT ratio N-TYPE P-TYPE

µ h + /µ e - [vol%] *10 -4 [cm 2 .V -1 .s -1 ] *10 -4 [cm 2 .V -1 .s -1 ] 0 2.

Mobility measurements of films prepared from DPH

Similarly to ODT, n-type and p-type OTFTs are fabricated with active layers prepared with concentrations of DPH ranging from 0 to 2.4 vol%. Figure 6.9 depicts the transfer characteristics of the n-type and p-type OTFTs. The transfer characteristics of p-type OTFTs, depicted in Figure 6.9a, also demonstrate higher

I DS upon increasing DPH concentration.
Next, the OTFT mobilities are calculated at different values of V GS using Equation 6.2. Figure 6.8a depicts the hole and electron mobilities as a function of V GS -V th for OTFTs prepared from various concentrations of DPH and Figure 6.8b

shows the evolution of the electron and hole mobility extracted at V GS -V th values of +2 V and -2 V respectively. Similarly to the case of ODT, the hole mobility increases with increasing concentration of DPH. It is found here that the electron mobility decreases upon the introduction of DPH. The ratios µ h + /µ e -, presented in Table 6.2, are shown to be < 1 in OTFTs prepared with DPH concentrations ≤ 0.8 vol%. This ratio increases and is > 2 for OTFTs prepared with DPH concentrations of 1.6 and 2.4 vol%. This indicates that the introduction of DPH also cause the hole mobility to surpass that of electrons, similar to that observed with ODT.

-3 0 -2 5 -2 0 -1 5 -1 0 - 5 0 -2 4 -2 2 -2 0 -1 8 -1 6 -1 4 -1 2 -1 0 -8 -6 -4 -2 0
C o n c e n t r a t i o n o f D P H : C o n c e n t r a t i o n o f D P H : -

0 v o l % 0 . 4 v o l % 0 . 8 v o l % 1 . 6 v o l % 2 . 4 v o l % I d s ( n A ) V g ( V ) (a)
0 v o l % 0 . 4 v o l % 0 . 8 v o l % 1 . 6 v o l % 2 . 4 v o l % I d s ( n A ) V g ( V ) (b)
4 -3 -2 -1 0 1 2 3 4 0 . 0 2 . 0 x 1 0 -4 4 . 0 x 1 0 -4 6 . 0 x 1 0 -4 0 v o l % 0 . 4 v o l % 0 . 8 v o l % 1 . 6 v o l % 2 . 4 v o l % C o n c e n t r a t i o n o f D P H : M o b i l i t y ( c m 2 . V -1 . s -1 ) V g -V t h ( V ) (a

Origins of additive dependence on OSCs architecture

In order to explain the higher performance obtained from an additive-treated OSC in an inverted architecture compared to a conventional one, the question of the vertical phase separation with a depleted polymer layer at the bottom was addressed. The results show that OTFTs prepared with processing additives exhibit an enhanced hole mobility compared the OTFTs without the additive.

This result suggests that processing additives do not lead to any vertical phase separation that decreases the D concentration in the lower part of the film. In view of the results obtained in OTFT-mobility measurements, it is concluded that the difference between conventional and inverted architectures do not arise from vertical phase separation. This observation is in disagreement with a recent report by Wang et al. where several techniques (transmission electron microtomography, X-ray photoemission spectroscopy and dynamic secondary ion mass spectroscopy) were used to suggest that the introduction of ODT caused the formation of a P3HT-depleted layer at the bottom of the BHJ [START_REF] Wang | Preparation and characterization of MoO3 hole-injection layer for organic solar cell fabrication and optimization[END_REF]. However, the OTFT-mobility measurements conducted here show that, even in the hypothetical case of vertical phase separation caused by additives, the electron and hole mobilities are not impacted by it.

Ruling out vertical phase separation effects, the difference between conventional and inverted OSCs may instead be explained in terms of the location where the excitons are formed. In both architectures, the light is received by the OSC through the ITO substrate. Consequently, photon absorption is expected to occur predominantly in the bottom part of the active layer. As a result, charge carriers are also predominantly generated near the bottom. In a conventional architecture, the electrons are collected by the top electrode. In this case, the photo-generated electrons have to traverse across the whole thickness of the organic layer to be collected by the top electrode. In contrast, holes travel a much shorter distance to reach the hole collecting electrode. Therefore, in this configuration, a high electron mobility is more favorable to account for the long distance that the electrons need to travel. In other words, a ratio µ h + /µ e -< 1 is preferred in this configuration. Such a mobility ratio is obtained in films treated with low concentrations of additive. This, in turn, could explain the superior performance of conventional OSCs prepared with low concentrations of additive as depicted in Figure 6.2 and Figure 6.5. An explanatory scheme supporting this proposition is depicted in Figure 6.11.

-12 -8 -4 0 4 8 12 0.0

Vg-Vth (V) In contrast, in an inverted architecture, most of the photo-generated holes are required to travel a longer distance relative to the electrons. In this configuration, a ratio µ h + /µ e -> 1 can be expected to be more favorable for a more balanced collection. As the introduction of processing additive increases the hole mobility, the inverted architecture is favorable here. An explanatory scheme supporting this proposition is depicted in Figure 6.12.

 h+ <<  e- LiF/

LiF/Al

Pedot:PSS/ITO

Vg-Vth (V)

 h+ >>  e- Figure 6.12: A schematic description of hole and electron transport in OSC prepared with 2.4 vol% of additive and subsequent preferential architecture. OSCs with 2.4 vol% depict the case where hole mobility is higher than electron mobility.

To conclude, additive-treated OSCs were shown to perform better in inverted OSCs compared to conventional OSCs. The results of OTFT mobility measurements confirmed that vertical phase separation is not responsible for the differences obtained in conventional and inverted OSCs. Instead, the increasing hole mobility with additive concentration indicates that the difference arises from the location where the excitons are formed. Additionally, this analysis indicates that the ratio of electron and hole mobilities appear to play a crucial role in the choice of OSC architecture.

In this chapter, the question of the effect of the active layer processing conditions on the stability of OSCs is addressed. Commercial application of OSCs is only viable when stability requirements are met. Therefore the research on OSCs cannot be only focused on higher efficiencies, but also on the stability. Much research has focused on several stability related topics that include the identification of degradation factors and mechanisms, and routes for stability improvement. Therefore, before addressing the question of the effect of processing additives on the OSC stability, it is prudent to provide first a brief background on the stability issues of OSCs.

7.1 Background on the stability issues of OSCs

Stability of organic semiconductors

Organic semiconducting materials are known for their relatively poor stability to ambient conditions. Pure P3HT was shown to be susceptible to photo-bleaching in the presence of oxygen in the solution state [START_REF] Abdou | Mechanisms of photodegradation of poly(3-alkylthiophenes) in solution[END_REF], as well as in the solid state [START_REF] Hintz | Photodegradation of P3HT A Systematic Study of Environmental Factors[END_REF][START_REF] Manceau | Involvement of Singlet Oxygen in the Solid-State Photochemistry of P3HT[END_REF][START_REF] Manceau | The mechanism of photo-and thermooxidation of poly(3-hexylthiophene) (P3HT) reconsidered[END_REF]. In the solid state, the degradation occurs by radical oxidation that first occurs at the side chains and further leads to the degradation of the polythiophene ring causing a loss of π-conjugation. Such degradation appears in the form of a decrease in the polymer optical absorption in the UV-Vis absorption spectrum.

The photo-oxidation is accelerated in the presence of humidity, light and high temperature [START_REF] Hintz | Photodegradation of P3HT A Systematic Study of Environmental Factors[END_REF]. Aside from the light-induced changes in the presence of oxygen, P3HT is also subject to degradation under light irradiation in the absence of oxygen, a phenomenon called photolysis [START_REF] Manceau | Effects of long-term UV-visible light irradiation in the absence of oxygen on P3HT and P3HT:PCBM blend[END_REF]. It is noteworthy to point out that the degradation rate caused by photolysis is much slower. Manceau et al.

showed that light irradiation of P3HT in inert atmosphere caused its absorbance to decrease by 20% within 10 000 hours, whereas the same happens in only 20 hours in the presence of oxygen [START_REF] Manceau | Effects of long-term UV-visible light irradiation in the absence of oxygen on P3HT and P3HT:PCBM blend[END_REF].

Despite the severe degradation behavior observed in neat materials, blends of D/A generally exhibit less degradation. The introduction of fullerene derivatives was shown to slow down the rate of photo-oxidation and photolysis of πconjugated polymers [START_REF] Chambon | Durability of MDMO-PPV and MDMO-PPV: PCBM blends under illumination in the absence of oxygen[END_REF][START_REF] Neugebauer | Stability and photodegradation mechanisms of conjugated polymer/fullerene plastic solar cells[END_REF][START_REF] Sarkas | Enhanced Photooxidative Stability of Conjugated Polymers via C 60 Doping[END_REF]. The comparison demonstrates that the introduction of PC 61 BM reduces significantly the photo-bleaching of P3HT in the presence of oxygen and suppresses it almost completely in the absence of oxygen [START_REF] Rivaton | Light-induced degradation of the active layer of polymer-based solar cells[END_REF]. In inert atmosphere, the optical properties of P3HT were shown to be stable for periods as long as 5000 hours. The enhanced photo-stability of blends was attributed to radical scavenging property of PC 61 BM and its ability to quench the P3HT singlet state [START_REF] Manceau | Effects of long-term UV-visible light irradiation in the absence of oxygen on P3HT and P3HT:PCBM blend[END_REF].

Thermal stability represents a severe issue in the stability of D/A blends, mainly due to the tendency of PC 61 BM to form micrometer size crystals. Such crystals can be observed in optical microscopy, as depicted in the Figure 7.2 which shows a thermally annealed P3HT/PC 61 BM films exhibiting PC 61 BM crystals. with permission of Wiley.

Stability of OSCs

The factors that cause degradation at the material level such as oxygen and light irradiation subsequently cause significant failure at the device level. Additionally, the OSCs under usage conditions can be subject to several other degradation mechanisms such as electrical stress and mechanical stress [START_REF] Brabec | Polymer-fullerene bulk-heterojunction solar cells[END_REF]. The mechanisms of degradation can be categorized in three types: [START_REF] Grossiord | Degradation mechanisms in organic photovoltaic devices[END_REF] 1. Degradation of D and A materials, 2. Morphological changes in the active layer, 3. Interfacial degradation.

Oxygen was shown to play a key role in degradation mechanisms (1) and (3).

The effects of oxygen on mechanism (1) were discussed previously. Additionally, oxygen induces several types of interfacial degradation mechanism especially in conventional OSCs [START_REF] Krebs | Lifetimes of organic photovoltaics: photochemistry, atmosphere effects and barrier layers in ITO-MEHPPV:PCBM-aluminium devices[END_REF][START_REF] Wang | Degradation mechanism of organic solar cells with aluminum cathode[END_REF]. Inverted OSCs in which holes are collected by the top electrode are more stable, owing in part, to the higher work function of the top electrode [START_REF] Cros | Definition of encapsulation barrier requirements: A method applied to organic solar cells[END_REF][START_REF] Lloyd | Impact of contact evolution on the shelf life of organic solar cells[END_REF]. Nevertheless, even in inverted OSCs, oxygen remains one the major factors of degradation because of the severe damages caused to the active layer [START_REF] Seemann | Influence of oxygen on semi-transparent organic solar cells with gas permeable electrodes[END_REF]. In order to prevent deterioration from oxygen and humidity, much work has been done on the use of protection barriers and as a result, OSCs protected with barriers could demonstrate stabilities of several years [START_REF] Hauch | Flexible organic P3HT:PCBM bulk-heterojunction modules with more than 1 year outdoor lifetime[END_REF][START_REF] Krebs | Encapsulation of polymer photovoltaic prototypes[END_REF][START_REF] Krebs | Analysis of the Failure Mechanism for a Stable Organic Photovoltaic During 10 000 h of Testing[END_REF]. To conclude, the issue of oxygen can then (at least partially) be solved by choosing an appropriate device architecture as well as using barriers to encapsulate OSCs.

In the absence of oxygen, light irradiation only does not play a major role on mechanisms 1 and 2. However, several studies did reveal the dependence of the photo-stability of OSCs on the types of interlayers suggesting the presence of photo-degradation at the interfaces (mechanism 3) [START_REF] Voroshazi | Long-term operational lifetime and degradation analysis of P3HT:PCBM photovoltaic cells[END_REF][START_REF] Williams | The Photo-Stability of Polymer Solar Cells: Contact Photo-Degradation and the Benefits of Interfacial Layers[END_REF].

As reported in this brief literature review, each component of an OSC is subject to degradation: the electrodes, the interface between active layer and the contacts, and the organic semiconducting materials. These components have to be chosen carefully in order to limit OSCs degradation. On the other hand, the dependence of OSCs stability on the active layer morphology -and subsequently the processing conditions -has not yet been addressed adequately.

In this chapter, the photo-stability of OSCs that are thermally annealed is compared to the photo-stability of OSCs fabricated using additives. Stability tests under light irradiation in air and in inert atmosphere are performed and compared. In order to limit thermal degradation of OSCs, the temperature of the substrates is kept below 32°C during light irradiation.

Traces of processing additives in the active layer

The boiling points of the processing additives are relatively high (C-PYR: 306

°C, DPH: 284 °C, ODT: 270 °C and TRIB: 388 °C). Therefore, traces of them can be expected to be present in the active layer and to subsequently alter the stability of OSCs.

In order to detect the presence of processing additive in the active layer, infrared (IR) absorption spectra of the active layers are characterized with Polarization Modulation-Infrared Reflection Absorption Spectroscopy (PM-IRRAS).

The PM-IRRAS spectrum is compared to that of the pure additive determined by Attenuated Total Reflectance (ATR) Spectroscopy to detect whether peaks related to the additive are present in the PM-IRRAS spectrum. For each additive, two sets of P3HT/PC 61 BM films are investigated: one set consists of as cast-films and a second set consists of films that were placed under vacuum at a pressure of ∼ 5x10 -6 mbar, which reproduces the vacuum step caused by thermal evaporation of top contacts.

PM-IRRAS spectra of C-PYR-treated active layers

As depicted in the ATR spectrum of C-PYR in Figure 7.4a, C-PYR exhibits an intense absorption peak at 1683 cm -1 that accounts for the C=O stretching mode.

Figure 7.4b shows the PM-IRRAS spectra of P3HT/PC 61 BM films, prepared with and without C-PYR, in the 1600 -1900 cm -1 region. The peak at 1683 cm -1 is absent in the control active layer but is detected in the films prepared with C-PYR. This suggests that, after spin-casting, residual C-PYR is present in the active layer. When the film is placed under vacuum, the intensity of the absorption peak at 1683 cm -1 decreases and is almost not detectable anymore.

Therefore, the vacuum is effective in reducing the amount of C-PYR in the active layer.

PM-IRRAS spectra of DPH-treated active layers

The ATR spectrum of DPH (Figure 7.5a) presents no absorption peak that can be clearly discriminated from the PM-IRRAS absorption spectra of P3HT/PC 61 BM films (Figure 7.5b). However, the absorption peak corresponding to the C=O in DPH appears at lower wavelength compared to the C=O peak in P3HT/PC 61 BM films. This difference causes a broadening of the peak in DPH-treated active layers (Figure 7.5b). In the spectrum of the control active layer, the FWHM of this peak is 17.1 cm -1 and it increases up to 19.6 cm -1 for DPH-treated films. This suggests that there is a contribution of the C=O from DPH that is remaining in the active layer. The vacuum step reduces the FWHM to 17.7 cm -1 . This shows that the vacuum step is effective in removing DPH, but traces of it are still detectable in the active layer.

PM-IRRAS spectra of TRIB-treated active layers

As depicted in the ATR spectrum of TRIB in Figure 7.6a, the additive exhibits an intense absorption peak at 1740 cm -1 , which remains visible in the PM-IRRAS spectra of TRIB-treated films (Figure 7.6b). In the active layer prepared with TRIB, the intensity of the peak at 1740 cm -1 is four times more intense than in a control film. The vacuum step reduces the intensity to 41% of the original signal before the vacuum step. Other peaks characteristic for the ATR spectrum of TRIB (at 959 cm -1 and 1076 cm -1 ) appear in films prepared with TRIB. The intensity of these peaks decreases after the vacuum step. To conclude, TRIB remains in P3HT/PC 61 BM film after spin-casting and after the vacuum step.

PM-IRRAS spectra of ODT-treated active layers

The peak at 2560 cm -1 in the ATR spectrum of ODT, attributed to S-H, is absent in the PM-IRRAS spectra of ODT-treated films shown in Figure 7.7. Overall, as cast-films and films placed under vacuum prepared from ODT show no significant difference compared to films prepared without additive. This observation suggest that the amount of residual ODT in P3HT/PC 61 BM-films is negligeable. To conclude, the vacuum step was shown to be effective in reducing the amount of additive in as-cast films. Traces of DPH and TRIB are detected in the active layer even after the vacuum step while C-PYR and ODT are not. The OSCs are prepared and exposed to light irradiation in air. OSCs containing processing additives are exposed to light irradiation for a period of 252 hours while thermally annealed OSCs are exposed for only 90 hours because of the low performance obtained after this period. The J -V curves were measured at several intervals within the duration of the light irradiation. Table 7. For all OSCs, the FF is the parameter that is the most impacted by the degradation. The decrease in FF is likely to be linked to the significant increase in R s upon light irradiation. The increase in R s suggests either an increase in the internal resistance of the active layer or an increase in the resistance at the contacts.

The photo-stability of OSCs differs significantly as a function of the active layer processing conditions. The thermally annealed OSC is the least photo-stable with a PCE of 1.8% of the initial value after 90 hours of light irradiation. On the other hand, ODT-treated OSCs present the best photo-stability with a PCE remaining at 36.2% of the initial value after 292 hours of light irradiation. The most striking difference between thermally annealed and ODT-treated OSC is the

Photo-stability tests on OSCs in inert atmosphere

OSCs prepared in conditions identical to those previously described are fabricated and irradiated for a period of 393 hours in inert atmosphere. The J -V curves were measured at several intervals within the duration of the light irradiation.

Table 7.3 reports the normalized photovoltaic parameters of OSCs after 393 hours of light irradiation and Figure 7.9 depicts the evolution of the photovoltaic parameters as a function of the time of light irradiation.

The results depicted in Figure 7.9 show that OSCs photo-stability in inert atmosphere is significantly higher than that in air. This is expected as the inert atmosphere substantially reduces degradation due to photo-oxidation. Interestingly, the dependence of the photo-stability on the processing conditions follows the same trend as observed in air: thermally annealed OSCs exhibit the poorest photo-stability whereas ODT-treated OSCs exhibit the highest photostability. OSCs with C-PYR and DPH exhibit intermediate stabilities. Similarly, the V oc is the most discriminant parameter: the thermally annealed OSC has its V oc reduced to 79.2% after 393 hours of light irradiation, whereas the V oc of ODT-treated OSC remains almost unchanged (less than 5 % decrease from the initial value). Along the same line, the decrease in FF is the most significant in thermally-annealed OSC, whereas irradiated ODT-treated OSC demonstrate a FF maintained at 94.4% of the initial value after the total irradiation time. To conclude, in air and in inert atmosphere, processing conditions have an influence on the photo-stability of OSCs. It is noteworthy to remind here that DPH-treated active layers were shown to retain some additive. However the photo-stability tests show that DPH-treated OSCs have a higher stability than thermally annealed OSCs. This suggests that the remaining additive does not play a major role in OSC photo-degradation.

The following sections aim at identifying the origins of the photo-stability dependence on the processing conditions. In air, the intensity of the absorption band of P3HT decreases after light irradiation for all the types of active layers, indicating that photo-oxidation of P3HT occurs in all cases. The loss in optical density at 601 nm is calculated and recorded in Table 7.4.

The decrease in optical density of ODT-treated active layers appears to be in the same range as that of C-PYR and DPH-treated active layers. The higher photo-stability of ODT-treated OSCs seems therefore to be independent of the optical density losses associated with photo-bleaching.

The UV-Vis absorption spectra of light-irradiated active layers in inert atmosphere are depicted in Figure 7.11. In inert atmosphere, the absorption spectra do not exhibit significant change after 60 hours of light irradiation indicating that the π-conjugated system as well as the chemical structure of P3HT are not affected by light irradiation. This confirms that OSC photo-degradation in inert atmosphere does not arise from chemical photo-degradation of P3HT.

The results in UV-Vis absorption spectroscopy do not explain the dependence of OSC photo-stability on processing conditions. Besides, the presence of traces of additive does not explain it neither. These two results suggest that the major origin of the photo-degradation is not the active layer. Therefore, the degradation at the interfaces (between the active layer and the top electrode or the bottom electrode) are investigated. As the trends in photo-stability are similar in air and in inert atmosphere, the following studies are carried out in inert atmosphere only.

Photo-stability study on the bottom interface

Photo-stability tests are carried out on OSCs containing a buffer interlayer which makes the bottom interface independent from the active layer processing conditions. The objective here is to verify if the photo-stability dependence on the processing conditions remains identical after the insertion of the buffer interlayer.

To this end, a layer of C 60 is introduced as an interlayer between ZnO and the active layer as depicted in Figure 7.12. The energy level diagram, shown in Figure 7.13, suggests that a C 60 interlayer introduces a small energy barrier (0.4 eV) to the transport of electrons from the active layer to the ZnO layer. Therefore, the photovoltaic performance of OSCs with a C 60 interlayer needs to be optimized. The photovoltaic parameters of OSCs subjected to photo-degradation in the Interestingly, the OSCs subjected to light irradiation without top electrode appear to be more stable than the OSCs irradiated with the top electrode. In the photo-degradation tests on complete OSCs, the V oc is the parameter that discriminates the most the different types of processing conditions, whereas in the tests on incomplete OSCs, the V oc remains relatively similar for all the types of active layers. Two conclusions can be drawn:

1. The higher photo-stability of incomplete OSCs versus complete OSCs suggests that the presence of a top electrode plays a role in the photo-degradation.

2. The fact that the photo-stability of incomplete OSCs is less dependent on the processing conditions than in the case of complete OSCs suggests that the processing conditions affect the photo-stability at the top interface.

Therefore, further investigations are performed on the photo-stability of the top interface.

Effects of light irradiation on the adhesion of the top electrode

Wang et al. demonstrated that light irradiation of an organic/metal stack caused deterioration at the interface by decreasing metal-organic bond density in organic light emitting diodes [START_REF] Wang | Photochemical deterioration of the organic/metal contacts in organic optoelectronic devices[END_REF]. This photochemical deterioration at the interface caused a decrease in interfacial adhesion between the metal and the organic layer.

In order to investigate the degradation in OSCs at the top electrode, interfacial adhesion measurements are carried out on OSC stacks before and after light irradiation. The adhesion energy at the interface between the active layer and the top contact is measured using a four-point bending test configuration [START_REF] Barakat | The effects of low temperature and pressure on the fracture behaviors of organosilicate thin films[END_REF][START_REF] Wang | Photochemical deterioration of the organic/metal contacts in organic optoelectronic devices[END_REF].

During the adhesion experiment, a load is applied on the two sides of the sample under study. The load as a function of the displacement is recorded and a typical displacement plot is shown in Figure 7.17 to cause delamination at the interface between two layers. Generally, the load first increases linearly with the displacement, which is characteristic for an elastic deformation. If a weak interface is present in the stack, a crack initiated by a pre-notch propagates along the interface. Such propagation causes a release of energy which can be related to the interfacial adhesion strength, G c expressed as:

G c = 21(1 -ν 2 )P 2 l 2 16Eb 2 h 3 (7.1)
With ν the Poisson's ratio of the glass substrate, E is the elastic modulus of the substrate, P the total force exerted onto the sample, b, h and l geometrical characteristics of the sample and the setup.

The OSC stack configuration is prepared on a rectangular glass substrate (1 cm x 5 cm) in order to be suitable for the four point bending test (Figure 7.17).

A second glass substrate is glued on top of the stack and a pre-notch is made on it. During the adhesion measurement test, equal loads are applied on both side of the sample. The loads deform the sample at a constant velocity of 0.25 µm.s -1 while the load as a function of the sample displacement is recorded. The adhesion measurement tests are performed on the two OSCs that exhibit the most differences in photo-stability: the thermally annealed and ODT-treated OSCs. Figure 7.18 shows the load versus displacement plots obtained. The two samples demonstrate the same evolution: first, the load increases linearly with the displacement and when the displacement reaches 6 to 7 mm, a slight load drop is observed, followed by a linear increase in the load. Ultimately, for higher displacement values, the samples are broken. In case of complete delamination, a complete load drop would be observed (as depicted in Figure 7.17).

In Figure 7.18, the plots show that no total delamination occurs. Instead, a small load drop is observed, suggesting that a crack propagates only slightly in the interface without fully delaminating it. At higher load values, the cracks continue to penetrate vertically throughout the stack and ultimately break the entire sample. As no total delamination occurs, the interface adhesion strength cannot be calculated using Equation 7.1. Instead, the load loss during the partial delamination (∆N) is determined (∆N is depicted in Figure 7.18). This load loss is related to the energy released during the partial delamination and therefore describes how far the delamination propagates at the interface. Several observations and conclusions can be deduced form the results:

1. Regarding only fresh samples, the load loss in ODT-treated samples (0.73 N) is higher than the load loss in thermally annealed samples (0.16 N). This indicates that the interfacial adhesion is dependent on the processing conditions of the active layer. The AFM images of P3HT/PC 61 BM-films treated with ODT and thermal annealed are recorded and depicted in Figure 7.19. The roughness measurement show that film treated with ODT presents a higher roughness than thermally annealed films. The difference in surface topography are likely the cause of the differences observed in the adhesion tests.

2. The load loss in photo-degraded OSCs is larger than that observed in fresh OSCs indicating that the interfacial delamination is more severe in photo-degraded OSCs. This, in turn, strongly suggests that the light irradiation affects the OSCs at the top interface by weakening the interfacial adhesion.

3. The percentage of load loss increase in thermally annealed samples is calculated to be 76.8%, whereas in ODT-samples, the increase is only 46.0%. This suggests that the interfacial adhesion is more severely deteriorated in the case of thermally annealed samples.

Conclusions

In this chapter, the effects of thermal annealing and the use of additive on the stability of OSCs are investigated. The main finding of this chapter is the OSC photo-stability dependence on the active layer processing conditions, a result which is valid for photo-degradation in both inert atmosphere as well as in air.

The best photo-stability is obtained with ODT-treated OSCs, followed by C- annealed OSCs. This is consistent with the fact that the photo-stability of ODTtreated OSCs is better than thermally annealed OSCs.

Overall, processing additives are not detrimental for the photo-stability of OSCs compared to thermally annealed OSCs. In the specific case of ODT, the photostability is even improved. This shows that next to improving the efficiency, additives can also be employed to improve the photo-stability. The DPP-based copolymer investigated in this work contains a quaterthiophene group as the electron donor to form the PDQT copolymer depicted in Figure 8.2.

As suggested in

This copolymer was shown to form films with a high degree of crystallinity due to strong intermolecular π-π stacking. As a result, PDQT was shown to exhibit high hole mobility in OTFT configuration with mobilities up to 0.97 cm 2 .V -1 .s -1 [START_REF] Li | 4-dione-co-thieno[3,2-b]thiophene): a high performance polymer semiconductor for both organic thin film transistors and organic photovoltaics[END_REF][START_REF] Li | Annealing-Free High-Mobility Diketopyrrolopyrrole-Quaterthiophene Copolymer for Solution-Processed Organic Thin Film Transistors[END_REF]. Another interesting feature of PDQT is its large absorption spectrum which The HSPs of PDQT have been determined experimentally by performing solubility tests. The HSPs resulting from the fitting are presented in Table 8.1 and the graphical representation of the PDQT solubility sphere is depicted in Figure 8.5. Using these HSPs, the RED values between PDQT -CB and PDQT -ODCB are calculated and found to be respectively 0.56 and 0.90. These numbers suggest that PDQT is soluble in both solvents. It was shown in the literature, that PDQT-based OSCs can be fabricated with thick active layers (up to ∼ 800 a processing additive in PDQT/PC 61 BM -OSCs. DIO is also studied as a processing additive because several studies reported on its success in increasing the efficiency of DPP-based copolymer OSCs [START_REF] Li | Effect of the fibrillar microstructure on the efficiency of high molecular weight diketopyrrolopyrrole-based polymer solar cells[END_REF][START_REF] Son | Effect of asymmetric solubility of diketopyrrolopyrrole-based polymers and PC71BMs in a binary solvent system on the performance of bulk heterojunction solar cells[END_REF]. The solubility properties of DIO are similar to the ones of ODT in the sense that PDQT has a lower solubility in DIO than in halogenated solvents [START_REF] Son | Effect of asymmetric solubility of diketopyrrolopyrrole-based polymers and PC71BMs in a binary solvent system on the performance of bulk heterojunction solar cells[END_REF].

OSCs are fabricated utilizing formulations that contain various concentrations of ODT or DIO. The formulations are prepared from a D/A ratio of 1/3 and a total solid content of 30 mg.mL -1 . Figure 8.8 shows the photovoltaic parameters (V oc , J sc , FF and PCE) of the OSCs with the different processing additives and their corresponding concentrations. The best values for each system are recorded in Table 8.4. Next, the chemical structure of the polymer side chains also needs consideration.

Generally, long side chains are desirable for increasing the solubility of polymers but besides their effects on the solubility, the side chains were also shown to affect the domain sizes of polymer in films. A study from Li et al. on DPP-based polymers showed that increasing side chains length increases the diameter of the D fibrils which was detrimental for the performance. The efficiency of OSCs was best for polymer with small side chain lengths [START_REF] Li | Effect of the fibrillar microstructure on the efficiency of high molecular weight diketopyrrolopyrrole-based polymer solar cells[END_REF]. PDQT possesses large substituents (Figure 8.2) wich are consequently also expected to contribute in its aggregation behavior.

Further investigation is required to identify and to understand the influence of the polymer backbone and the more general polymer properties such as the molecular weight and the side chain length on the morphology. The effects of additives on the morphology of diverse D/A system needs to consider the above parameters. Among all the derivatives, PCDTBT represents a good candidate as a D material in OSCs [START_REF] Blouin | A Low-Bandgap Poly(2,7-Carbazole) Derivative for Use in High-Performance Solar Cells[END_REF][START_REF] Chu | Bulk Heterojunction Solar Cells Using Thieno[3,4-c]pyrrole-4,6-dione and Dithieno[3,2-b:20,30-d]silole Copolymer with a Power Conversion Efficiency of 7.3%[END_REF][START_REF] Park | Bulk heterojunction solar cells with internal quantum efficiency approaching 100 %[END_REF][START_REF] Sun | Efficient, air-stable bulk heterojunction polymer solar cells using MoO(x) as the anode interfacial layer[END_REF][START_REF] Wang | Enhanced Power Conversion Efficiency in PCDTBT/PC70BM Bulk Heterojunction Photovoltaic Devices with Embedded Silver Nanoparticle Clusters[END_REF]. Its chemical structure is presented in One of the interesting features of PCDTBT is the low lying HOMO level (5.5

eV) which is suitable to obtain OSCs with high V oc [START_REF] Park | Bulk heterojunction solar cells with internal quantum efficiency approaching 100 %[END_REF][START_REF] Wakim | Highly efficient organic solar cells based on a poly(2,7-carbazole) derivative[END_REF]. Using Scharber's equation for the estimation of V oc (Equation 1.5) [START_REF] Scharber | Design Rules for Donors in Bulk-Heterojunction Solar Cells-Towards 10 % Energy-Conversion Efficiency[END_REF], a V oc of 0.9 V is expected.

The energy diagram of PCDTBT/PC 61 BM-based OSC is depicted in Figure 8.11. Unlike P3HT and PDQT, PCDTBT is an amorphous polymer which was reported to highly mix with PC 61 BM resulting in poor D/A phase separation [START_REF] Chen | The Role of Fullerene Mixing Behavior in the Performance of Organic Photovoltaics: PCBM in Low-Bandgap Polymers[END_REF][START_REF] Wang | Correlating Structure with Function in Thermally Annealed PCDTBT:PC70BM Photovoltaic Blends[END_REF]. Such morphology prevents PC 61 BM to form A domains with sizes that are efficient for charge transport to the electrode. As a result, the efficiency of OSCs is limited by the charge transport. In order to overcome these issues, the active layer generally needs to fulfill two requirements. First, PCDTBT/PC 61 BM active layers are required to be prepared with a high proportion of A in order to exhibit larger A domains. In the literature, efficiencies of 6% to 7% were obtained from formulations containing a D/A ratio between 1/2 and 1/4 [START_REF] Liu | High-efficiency inverted polymer solar cells with transparent and work-function tunable MoO(3)-Al composite film as cathode buffer layer[END_REF][START_REF] Moon | Nanomorphology of PCDTBT:PC70BM Bulk Heterojunction Solar Cells[END_REF][START_REF] Sun | Efficient, air-stable bulk heterojunction polymer solar cells using MoO(x) as the anode interfacial layer[END_REF]. The other requirement for high efficiency PCDTBT based OSCs is a thin active layer, generally ∼ 70 -100 nm [START_REF] Beaupré | PCDTBT: en route for low cost plastic solar cells[END_REF]. Thicker films exhibit inefficient charge collection and therefore low FF and J sc [START_REF] Moon | Nanomorphology of PCDTBT:PC70BM Bulk Heterojunction Solar Cells[END_REF]. However, the D semiconductor is the main absorbing material. Therefore, such active layers (thin and with a high proportion of A) optimize charge collection at the expense of light absorption. Optimization of the morphology can help improving charge collection without sacrificing light absorption. To this end, an investigation on formulation strategies is carried out.

Solubility properties of PCDTBT

The HSPs of PCDTBT are determined experimentally by performing solubility tests. The HSP values resulting from the fitting are presented in Table 8.5 and the graphical representation of the PCDTBT solubility sphere is depicted in Figure 8.12.

In Figure 8.12, the solubility sphere of PCDTBT visually appears to be relatively close to the one of PC 61 BM suggesting a certain affinity between them. The RED between PCDTBT and PC 61 BM can be calculated in order to estimate their affinity. Graham et al. used this method to predict the extent of phase separation between small molecules [START_REF] Graham | Improved Performance of Molecular Bulk-Heterojunction Photovoltaic Cells through Predictable Selection of Solvent Additives[END_REF]. Here, the RED between PCDTBT and PC 61 BM is calculated to be 0. In Figure 8.15, the UV-visible absorption spectra of films prepared with and without DIO show that the introduction of DIO leads to a red-shift of PCDTBT absorption peak. The wavelength of PCDTBT absorption maximum is 568 nm in films without DIO and increases to 582 nm in films prepared with 5 vol% of DIO. Similarly, ODT causes a red-shift of the absorption peak (Table 8.7). This red-shift is similar to the effects of DIO or ODT on other D polymers and is attributed to an increased degree of π-conjugation [START_REF] Agostinelli | The role of alkane dithiols in controlling polymer crystallization in small band gap polymer:Fullerene solar cells[END_REF][START_REF] Gu | Multi-Length-Scale Morphologies in PCPDTBT/PCBM Bulk-Heterojunction Solar Cells[END_REF][START_REF] Imamura | Effect of additives on the structure, nanomorphology and efficiency of PCPDTBT: PC71BM solar cells[END_REF]. The UV-Vis absorption spectra therefore suggest that DIO and ODT are effective in increasing the degree of ordering within PCDTBT chains. Table 8.8 displays the average electron mobility measured from 2 to 4 OTFTs at V GS -V th = 4 V. Regarding the effects of processing additives on the electron mobility, the results

show that increasing proportion of A in the D/A blend increases the mobility. This result is expected since a high proportion of A leads to an increased amount of A pathways. For a D/A ratio of 1/2, the electron mobility increases significantly with the introduction of C 60 : the ternary blend exhibits an electron mobility six times higher than that of a blend without C 60 (Table 8.8). Such an increase in electron mobility shows that the introduction of C 60 has an effect on the aggregation of PC 61 BM. Interestingly, the mobility of the ternary blend in a 1/2 ratio is also much higher than that of PCDTBT/PC 61 BM in a 1/4 ratio (which contains more PC 61 BM). This suggests that the introduction of C 60 results in a particular electron mobility that cannot be obtained in binary blend, even when fabricated with a high proportion of A. mg.mL -1 [START_REF] Islamova | Solubility of C60 fullerene in o-dichlorobenzene-tetrachloromethane mixtures[END_REF][START_REF] Semenov | Solubility of Light Fullerenes in Organic Solvents[END_REF]) which is likely to cause coarse aggregates. The V oc is highest in OSCs with only PC 61 BM as the A component (0.90 V for the 1/2 ratio and 0.91

Series of

V for the 1/3 ratio) and decreases with increasing fraction of C 60 . At a fraction x of 1, the V oc values drop to 0.62 V and 0.79 V for OSCs with D/A ratios of 1/2 and 1/3 respectively. The variation in V oc with the composition of the ternary blends has been the subject of investigation in the literature and remains under debate.

Street et al. argued that a ternary blend can be described with an alloy model of D and A with a V oc depending on the average HOMO LUMO energy levels [START_REF] Street | Origin of the tunable open-circuit voltage in ternary blend bulk heterojunction organic solar cells[END_REF].

Such model fails to describe the present ternary blend because it predicts an increase in V oc with the introduction of C 60 due to its higher lying LUMO level.

As previously introduced in Section 1.4.4 (General factors influencing BHJ-OSC efficiency), presence of fullerene nanocrystals can shift the energy of the charge transfer state. Such shift is reported to decrease the V oc of OSCs [START_REF] Piersimoni | Influence of Fullerene Ordering on the Energy of the Charge-Transfer State and Open-Circuit Voltage in Polymer Fullerene Solar Cells[END_REF]. Therefore, the decrease in V oc observed in this experiment suggest that the introduction of

Conclusions

Polymer chain ordering was controlled using processing additives such as DIO and ODT. However, the introduction of these processing additives is inefficient in increasing the performance of PCDTBT/PC 61 BM -OSCs and even decreases it.

The use of a ternary blend based on PCDTBT/PC 61 BM (1-x) /C 60(x) appears to alter the aggregation of PC 61 BM and to cause a disruption of PCDTBT polymer chains. Measurements of photovoltaic parameters of ternary blend-OSCs show that the PCE is relatively independent on the C 60 fraction as long as the fraction is ≤ 0.7. This result is relevant for the prospect of low cost OSCs because PC 61 BM is a relatively expensive material (∼ 10 times more expensive than C 60 [2]). Around 10 to 30% of the total fabrication costs of organic solar modules is attributed to the organic semiconductors [START_REF] Azzopardi | Economic assessment of solar electricity production from organic-based photovoltaic modules in a domestic environment[END_REF][START_REF] Mulligan | A projection of commercial-scale organic photovoltaic module costs[END_REF], therefore the replacement of a fraction of PC 61 BM by C 60 would reduce module costs. Also, the introduction of C 60 was shown to prevent the aggregation of PC 61 BM upon thermal annealing.

As a result, the OSCs fabricated from ternary blends demonstrate significant thermal stability compared to the binary blend.

Conclusions

This thesis provides a comprehensive study on the introduction of processing additives used to increase the performance of solution-processed BHJ-OSCs.

First, a systematic method for the selection of processing additives was developed.

This method uses the Hansen theory to describe the solubility properties of the organic semiconductors under study and to determine numerical figures of merit that can be used for selecting suitable processing additives. This method was successfully applied to the P3HT/PC 61 BM-system. Three novel processing additives, that result in up to a two-fold increase in the PCE, were identified.

The mechanistic role of processing additives in improving the BHJ morphology was elucidated by correlating structural and optical characterizations. Studies of the trends in V oc and UV-visible absorption spectra showed that processing additives increased the overall crystallinity in P3HT in the BHJ whereas XRD patterns revealed that the increased crystallinity is associated with a decrease in the polymer crystallite sizes. These results suggest that processing additives lead to the formation of a BHJ with more numerous but smaller polymer crystallites.

The photovoltaic performances of additive treated-OSCs were shown to significantly depend on the device architecture. These differences were attributed to variations in hole and electron mobilities induced by the introduction of additives. As the excitons are predominantly generated near the bottom of the active layer, holes and electrons are required to travel different distances before their collection by the respective electrodes. As a result, the photovoltaic performance of OSCs is dependent on the ratio between electron and hole mobilities. Since the mobilities are influenced by the processing additives, the device architecture must be considered when additives are used.

As OSC stability is equally important as the PCE, the effects of processing additives on OSC stability were studied by means of photo-stability measurements.

Compared to the commonly used thermal annealing technique, the results show that the use of additives improves the photo-stability of OSCs. The best results are obtained with ODT-treated OSCs. Several photo-stability studies, which include adhesion measurements, show that light irradiation causes interfacial degradation. Interestingly, adhesion measurements show that ODT-treated OSCs suffer less from interfacial photo-degradation than thermally annealed OSCs, which could explain the improved life time.

Besides P3HT, two other polymers were studied: the semi-crystalline PDQT and the amorphous PCDTBT. In the case of PDQT, ODT and DIO are successful in increasing the PCE of OSCs, indicating that the additive selection rules identified for P3HT/PC 61 BM also apply to this case. On the other hand, the additive approach appeared to be unsuccessful for PCDTBT-based OSCs, most likely due to the amorphous character of PCDTBT, which does not require control of the crystallinity as is required for than semi-crystalline polymers. These results suggest that the selection rules designed for P3HT/PC 61 BM systems apply for semi-crystalline polymers but not for amorphous polymers.

Future work

Chapter 8 showed that the use of conventional processing additives failed to improve the performance of PCDTBT-based OSCs. Although additives increased the PCE of PDQT-based OSCs, they appeared to have a different effect on the BHJ morphology than they have on P3HT-based OSCs. Overall, the effectiveness of additives is found to strongly depend on the type of polymer. Therefore, the additive selection method developed in this thesis based on the P3HT/PC 61 BM system needs to be extended to a more general perspective. Not only the solubility properties, but several other aspects of the D polymers need to be considered, namely the tendency to crystallize, the molecular weight, the nature of the side chains and the miscibility with A materials. A next step would be to identify how processing additives affect the morphology of BHJ-OSCs as a function of the properties of the D polymers. This is an essential step towards a more general applicability of processing additives and a streamlined way to formulate D/A blends for high efficiency OSCs.

The photo-stability tests suggest that additives can be employed to perform a double function: to increase the efficiency and to increase the photo-stability of OSCs. To fully exploit this double function, the specific case of ODT-treated OSCs needs to be understood due to their particularly high photo-stability. Two possible hypotheses about the role of additive in this regard arise: either the increase in interfacial adhesion is due to morphological changes in the active layer or it is due to the presence of additive. Although the PM-IRRAS spectra gave no trace of ODT in the BHJ, remaining traces of ODT may be present in quantities that are below the detection limit of this technique. ODT contains two thiol groups which are known for acting as scavengers for free radicals [START_REF] Takashima | Reactivity toward oxygen radicals and antioxidant action of thiol compounds[END_REF],

which can be generated during photo-degradation. Therefore, the presence of these scavenger groups may prevent free radical reactions that cause degradation.

  Keywords : photovoltaic device, semiconducting materials, polymer Unités de recherche Institut des Sciences Moléculaires (ISM) -UMR 5255 351 cours de la libération 33405 Talence cedex Laboratoire du Futur (Solvay -LOF) -UMR 5258 178, avenue du Dr Schweitzer F-33608 Pessac University of Waterloo Department of Electrical and Computer Engineering 200 University Ave. ON N2L 3G1, Canada

  Hansen sont trois paramètres qui décrivent les interactions majeures entre matériaux organiques : les interactions hydrogènes, les interactions dipôle-dipôle et les interactions de London. Comparer les paramètres de Hansen entre un matériau et un solvant donne des informations sur les interactions entre eux et de déterminer si elles sont favorables à la solubilisation ou non. Les paramètres de Hansen sont ici sollicités pour étudier les interactions entre les matériaux semi-conducteurs organiques (P3HT et PC 61 BM) et des additifs ou des solvants. La mise en place de notre méthode s'est articulée autour de trois axes. Tout d'abord, les paramètres de Hansen du P3HT et du PC 61 BM ont été expérimentalement déterminés.

  porteurs de charges et par des analyses électriques des dispositifs photovoltaïques. Les caractérisations sont menées sur films sans additif et avec additif en concentration variant de 0 à 4 vol % dans la formulation initiale. Afin de vérifier que les effets observés sont universels aux additifs solvant en général et non pas spécifiques à un type d'additif en particulier, les études sont conduites sur trois additifs avec des structures chimiques différentes. L'analyse des performances photovoltaïques montre une diminution de la tension à circuit ouvert (Vco) avec l'introduction d'additif. Cette diminution de la Vco est attribuée à une augmentation de la cristallinité du P3HT qui s'accompagne d'une diminution de sa bande interdite. La diminution de la Vco suggère donc une augmentation de la cristallinité du P3HT, résultat confirmé par les mesures de spectres de spectroscopie UV-visible qui montrent un déplacement du pic d'absorption du P3HT vers de haute longueur d'onde avec l'ajout d'additif. D'autre part, les mesures des DRX montrent que la taille des cristallites de P3HT diminue avec l'introduction d'additif. La mise en corrélation de ces résultats suggère que l'introduction d'additif provoque la formation de cristallites de P3HT plus petits en taille mais plus nombreux de façon à ce que la cristallinité totale, dans la BHJ, est plus importante que dans un système sans additif. Ces résultats sont cohérents avec l'évolution de la mobilité des trous mesurée dans la couche active et permettent d'expliquer l'évolution des performances photovoltaïques. Afin de comprendre comment l'introduction d'additif induit une telle morphologie, les paramètres de solubilité de Hansen sont sollicités afin de suivre l'évolution des interactions entre le système solvant/additif et les semi-conducteurs organiques pendant le séchage de la formulation. L'étude de telles interactions a permis de prédire, qualitativement, à quel moment les cristallites de P3HT sont formés durant le séchage et quelles en sont les tailles en fonction de la concentration d'additif dans la formulation initiale. Au terme de ce chapitre, un mécanisme expliquant les effets des additifs est proposé. Dans le chapitre 6, l'effet de l'architecture des cellules solaires sur le mode de fonctionnement des additifs est étudié. Les cellules solaires peuvent être fabriquées vii suivant deux architectures: une architecture dite conventionnelle et une architecture inversée. La différence entre les deux architectures réside dans l'endroit où sont collectés les porteurs de charge: dans une architecture conventionnelle, les électrons sont collectés par l'électrode du haut et les trous par l'électrode du bas. Comme l'indique son nom, l'endroit de la collection des porteurs de charge est inversée dans une architecture inversée. Nous avons montré, expérimentalement, que ces deux architectures ne sont pas équivalentes lorsque des additifs sont introduits dans la formulation de semi-conducteurs: les additifs augmentent le rendement photovoltaïque dans une architecture inversée mais le diminuent dans une architecture conventionnelle. Les effets des additifs diffèrent donc en fonction de l'architecture employée. Afin de comprendre la provenance de telles différences, les mobilités des porteurs de charge sont mesurées à l'aide de transistor à effet de champ organique. La mobilité de saturation est mesurée et montre que la mobilité des trous augmente avec l'introduction d'additif tandis que celle des électrons diminue. Cette différence de mobilité est mise en corrélation avec la distance que les porteurs de charges ont à parcourir en fonction du type d'architecture. En effet, quelque soit l'architecture utilisée, des simulations numériques montrent que les excitons sont majoritairement formés près de l'électrode transparente, c'est à dire dans la partie inférieure de la couche active. Cela cause une différence dans les distances que les électrons et les distances que les trous doivent parcourir avant d'être collectés. La mise en corrélation des variations de mobilité avec les distances à parcourir des porteurs de charge explique les différences observées dans les performances photovoltaïques en fonction du type d'architecture. Le chapitre suivant se concentre sur les effets des additifs sur la stabilité des cellules solaires. La stabilité des cellules solaires organiques est une thématique de recherche majeure et il est crucial de s'assurer que les additifs n'impactent pas négativement celle-ci. Des études de spectrométrie IR réflexion absorption par modulation de la polarisation révèlent des traces d'additif dans la couche active après la formation de celle-ci. Afin de déterminer si ces traces d'additif affectent la stabilité des cellules solaires, des tests de photo-stabilité sont menés sur des dispositifs avec ou sans additif. Trois types d'additifs sont analysés. Les tests de photo-stabilité consistent en l'illumination continue, sous intensité lumineuse de 100 mW.cm -2 , de cellules solaires. Différentes conditions expérimentales ont été mises en place. Les résultats montrent que l'introduction d'additif ne dégrade pas la stabilité des dispositifs en comparaison avec un dispositif recuit thermiquement avec un rendement initial équivalent. Au contraire, l'introduction d'un type d'additif spécifique contribue même à augmenter la photo-stabilité des dispositifs. Deux conclusions sont tirées de ces expériences : 1. L'introduction d'additif dans la formulation de semi-conducteur ainsi que les traces d'additifs ne dégradent pas la photo-stabilité des dispositifs et 2. L'introduction de l'additif spécifique permet d'améliorer considérablement la photo-stabilité des dispositifs. Afin de comprendre l'origine d'une telle amélioration, différentes caractérisations ont été menées. Les résultats suggèrent que la photo-dégradation affecte primordialement l'interface entre la couche active et l'électrode métallique. Des tests d'adhésion mécanique ont été effectués afin de mesurer l'énergie d'adhésion entre l'électrode métallique et la couche active. Les résultats montrent que l'irradiation continue provoque une diminution de l'énergie d'adhésion confirmant qu'une photo-dégradation opère à cette interface. Par ailleurs, les mesures d'adhésion montrent que, dans le cas où l'additif spécifique est utilisé, le phénomène de dégradation interfacial est ralenti, ce qui explique l'amélioration de la photo-stabilité des dispositifs contenant cet additif. Dans le dernier chapitre, les études sur les additifs sont étendues à différents types de polymères semi-conducteurs: un polymère amorphe et un polymère hautement cristallin. Les différences du mode d'action des additifs en fonction des propriétés des polymères sont étudiées.

  France, I worked in parallel in the Institut des Sciences Moléculaires (ISM). In the group of Nano-structures Organiques, I thank Dr. Debdas Ray and Dr. Chih-Kai Liang who helped with the synthesis of PC 61 BM. In the group of Spectroscopie Moléculaire, I thank Colette Belin for helping me with the AFM imaging, Thierry Buffeteau and Gwenaelle Lebourdon for their valuable help in the PM-IRRAS experiments on active layer thin films. Still in France, I had the chance to be part of IMS. I would like to thank Dr. Mamatimin Abbas for fruitful discussions and for helping me with transistor fabrication and measurement (despite my stubbornness). I also thank Dr. Sylvain Chambon for always being available for scientific discussions. I truly appreciated his help and his interest in my research. I also want to thank all the members or ELORGA: Pr. Laurence Vignau (for being the first one to trigger my interest in organic electronic), Pr. Pascal Tardy and all the students who brought a enjoyable atmosphere in the lab: Dargie Daribew (my friend), Elodie Destouesse (for her great help when I first arrived in IMS), Maxime Lebail, Lionel Derue, Frédéric Guillain, Gildas Laurans (pour les caramels au beurre salé et ta bonne humeur au bad entre autres), Léo Peres (malgré tes blagues malheureusement sans fin, merci pour ton soutien), Yolande Murat (la plus gentille en 2015 j'espère), Geoffroy Houin (mon List of Figures 1.1 Photovoltaic device efficiency within different families of semiconductors. Reproduced from reference [1]. . . . . . . . . . . . . . . . 1.2 Photographs of (a) an OSC printed on paper, reproduced from reference [87] with permission of Wiley and (b) a transparent OSC, reprinted with permission from [34]. Copyright (2012) American Chemical Society . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Building blocks for device fabrication and the three major axes of performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Example of a typical solar cell characteristic under light illumination and under dark condition. . . . . . . . . . . . . . . . . . . . . 1.5 Equivalent circuit model for a solar cell. . . . . . . . . . . . . . . 1.6 AirMass 1.5 Global solar spectrum. . . . . . . . . . . . . . . . . . 1.7 Chemical structures of: (a) ethylene, (c) polyacetylene and the overlapping of orbitals in (b) ethylene and (d) polyacetylene. . . . 1.8 Simplified schematic of (a) a bi-layer active layer and (b) a BHJactive layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Top row: examples of A small molecules and bottom row: example of D polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.10 Energy diagram in BHJ-OSCs. The figure depicts a situation where the light is absorbed by the D. The photogeneration of the exciton is followed by an electron charge transfer from the LUMO of the D to the LUMO of the A. . . . . . . . . . . . . . . . . . . .

  3.1 P-type bottom-gate, top-contact OTFT device architecture from a side-view (left) and from a top-view (right). L represents the channel length and W the channel width. . . . . . . . . . . . . . . 3.2 Hole only device structure. . . . . . . . . . . . . . . . . . . . . . . 4.1 HSPs diagrams showing the good and the poor solvents resulting from solubility tests and the fitted solubility sphere of the compound under study. . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Solubility spheres of P3HT and PC 61 BM. . . . . . . . . . . . . . . 4.3 Molecular structures of: (a) C-PYR, (b) DPH and (c) TRIB. . . . 4.4 Positions of C-PYR, DPH and TRIB in the Hansen solubility space with respect to the solubility spheres of P3HT and PC 61 BM. 4.5 UV-Vis absorption spectra of solutions of P3HT in ODCB, C-PYR and TRIB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Calibration curves obtained from dissolving PC 61 BM in CB with different concentrations. . . . . . . . . . . . . . . . . . . . . . . . 4.7 Solid state UV-Vis absorption spectra for P3HT/PC 61 BM blends with varying concentration of processing additives: a) C-PYR, b) DPH and c) TRIB. . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8 Photovoltaic parameters of OSCs with varying concentration of processing additives: (a) V oc , (b) FF, (c) J sc and (d) PCE. . . . . 5.1 Electrical parameters of OSCs with different concentrations for the three processing additives: a) V oc , b) FF, c) J sc and d) PCE. . 5.2 Solid state UV-Vis absorption spectra for P3HT/PC 61 BM blends with various concentration of a) C-PYR, b) DPH and c) ODT. . . 5.3 XRD patterns of P3HT/PC 61 BM fims spin-cast from solutions containing 1.6 vol% of a) C-PYR, b) DPH, c) ODT and d) thermallyannealed film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Average size of P3HT crystallites as a function of processing additive concentration. . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 XRD patterns of P3HT films spin-cast from solutions containing various concentrations of DPH. . . . . . . . . . . . . . . . . . . . 5.6 (a) Device structure of hole-only device and (b) corresponding energy diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 J.L 3 as a function of applied voltage for hole-only devices prepared from: a) and d) C-PYR, b) and e) DPH, and c) and f) ODT. Figures a -c ) depict plots for an applied voltage ranging from 0 to 5 V and figures d -f) depict zoomed-in plots in the region 2 to 5 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Calculated SCLC hole mobility as a function of the concentration of processing additive. . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Evolution of hole mobility and FF as a function of the concentration of processing additive in OSCs prepared with (a) C-PYR, (b) DPH and (c) ODT. . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Evolution of the RED with P3HT (in black) and with PC 61 BM (in red) as a function of the effective concentration of a) C-PYR, b) DPH and c) ODT. The dotted line at RED of 1 depicts the theoretical RED threshold value separating the regions where the compound is non-soluble and soluble. . . . . . . . . . . . . . . . . 5.11 Schemes representing the BHJ morphology spin-cast from formulations with various concentration of additive. The crystallites of P3HT become more numerous and smaller as the starting concentration or processing additive increases. . . . . . . . . . . . . . . . 6.1 OSC architecture: (a) inverted and (b) conventional. . . . . . . . 6.2 Effects of ODT on the electrical parameters in a conventional and in an inverted architecture of OSCs: (a) V oc , (b) J sc , (c) FF, (d) PCE, (e) R s and (f) R sh . . . . . . . . . . . . . . . . . . . . . . . . 6.3 J -V curves of OSCs with various concentrations of ODT under dark conditions (a) in a conventional configuration and (c) in an inverted configuration and J -V curves under illumination (b) in a conventional configuration and (d) in an inverted configuration. 6.4 J -V curves of OSCs with various concentrations of DPH under dark conditions (a) in a conventional configuration and (c) in an inverted configuration and J -V curves under illumination (b) in a conventional configuration and (d) in an inverted configuration. 6.5 Effects of DPH on the electrical parameters in a conventional and in an inverted architecture of OSCs: (a) V oc , (b) J sc , (c) FF, (d) PCE, (e) R s and (f) R sh . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Different scenarios of vertical phase separation in BHJ: (a) no vertical phase separation -homogeneous active layer, (b) BHJ with a P3HT-enriched top and a PC 61 BM-enriched bottom and (c) BHJ with a P3HT-enriched bottom and a PC 61 BM-enriched top. . . . 6.7 Transfer characteristics of OTFTs prepared with various concentrations of ODT in: (a) p-type OTFTs and (b) n-type OTFTs. . . 6.8 (a) Mobility values of OTFTs prepared with various concentrations of ODT as a function of V GS -V th : the left side of the graph depicts hole mobility and the right side the electron mobility. (b) Electron and hole mobilities at V GS -V th of +2 V and -2 V respectively, as a function of ODT concentration. . . . . . . . . . . . . . 6.9 Transfer characteristics of OTFT prepared with various concentrations of DPH in: (a) p-type OTFTs and (b) n-type OTFTs. . . 6.10 (a) Mobility values of OTFTs prepared with various concentrations of DPH as a function of V GS -V th : the left side of the graph depicts hole mobility and the right side the electron mobility. (b) Electron and hole mobilities at V GS -V th of +2 V and -2 V respectively, as a function of DPH concentration. . . . . . . . . . . . . . 6.11 A schematic description of hole and electron transport in OSC without processing additive and subsequent preferential architecture. 6.12 A schematic description of hole and electron transport in OSC prepared with 2.4 vol% of additive and subsequent preferential architecture. OSCs with 2.4 vol% depict the case where hole mobility is higher than electron mobility. . . . . . . . . . . . . . . . . 7.1 (a) Normalized UV-Vis absorption (at 500 nm) of MDMO-PPV ( ), MDMO-PPV/PC 61 BM ( ) and normalized UV-Vis absorption (at 520 nm) of P3HT (•) and P3HT/PC 61 BM ( ) samples during photo-oxidation. (b) Normalized UV-Vis absorption of MDMO-PPV/PC 61 BM (at 500 nm ( )) and P3HT/PC 61 BM (at 520 nm ( )) samples during photolysis. Reproduced from reference [180] with permission of Elsivier. . . . . . . . . . . . . . . . . 7.2 Optical microscopy images of P3HT/PC 61 BM layers before and after being thermally annealed at 150 o C for 5 hours or 24 hours. Reproduced from reference [222] with permission of Wiley. . . . . 7.3 Scheme of the procedure for the detection of processing additive in P3HT/PC 61 BM films. . . . . . . . . . . . . . . . . . . . . . . . 7.4 (a) ATR spectrum of C-PYR, (b) Zoomed-in PM-IRRAS spectra of P3HT/PC 61 BM films with and without C-PYR (region 1900 -1600 cm -1 ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 (a) ATR spectrum of DPH, (b) Zoomed-in PM-IRRAS spectra of P3HT/PC 61 BM films with and without DPH (region 1820 -1700 cm -1 ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 (a) ATR spectrum of TRIB, (b) Zoomed-in spectra of the region 1800 -1700 cm -1 of PM-IRRAS spectra of P3HT/PC 61 BM films with and without TRIB. . . . . . . . . . . . . . . . . . . . . . . . 7.7 (a) ATR spectrum of ODT, (b) PM-IRRAS spectra of P3HT/PC 61 BM films with and without ODT. . . . . . . . . . . . . . . . . . . . . 7.8 Normalized electrical parameters of OSCs subjected to light irradiation in air: (a) J sc , (b) V oc , (c) FF, (d) PCE, (e) R s and (f) R sh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.9 Normalized electrical parameters of OSCs subjected to light irradiation in inert atmosphere: (a) J sc , (b) V oc , (c) FF, (d) PCE, (e) R s and (f) R sh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.10 UV-Vis absorption of P3HT/PC 61 BM films before and after light irradiation for 60 hours in air for (a) Thermally annealed, (b) C-PYR, (c) DPH and (d) ODT-treated active layers. . . . . . . . . . 7.11 UV-Vis absorption spectra of P3HT/PC 61 BM films before and after light irradiation for 60 hours in inert atmosphere for (a) Thermally annealed, (b) C-PYR, (c) DPH and (d) ODT-treated active layers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.12 Scheme of photo-stability tests on OSC with a C 60 buffer interlayer.103 7.13 Energy levels of OSCs containing C 60 as an interlayer between ZnO and the active layer. . . . . . . . . . . . . . . . . . . . . . . 7.14 Effects of C 60 thickness on the PCE of OSCs. . . . . . . . . . . . 7.15 Evolution of normalized (a) V oc and (b) PCE as a function of irradiation time for OSCs without (solid line) and with (dashed line) a C 60 interlayer. . . . . . . . . . . . . . . . . . . . . . . . . . 7.16 Comparison of the normalized (a) V oc and (b) PCE after 40 hours of light irradiation on OSCs without top electrode or with top electrode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.17 (a) Schematic drawing of a four-point bend adhesion sample stack. A load P/2 is applied on each side of the sample. (b) A typical load versus displacement characteristic. Figure (b) is reproduced from reference [220] with permission of AIL Publishing LLC. . . . 7.18 Load versus displacement characteristics for fresh and irradiated samples for (a) samples with ODT and (b) thermally annealed samples. On (a) the measured force loss (∆N) is showed. The plots of the irradiated samples were manually vertically down shifted for the clarity of the Figures. . . . . . . . . . . . . . . . . . . . . . . 7.19 AFM topography (1 µm x 1 µm) images of active layers films: (a) thermally annealed and (b) ODT-treated. . . . . . . . . . . . . .

PCDTBT Poly[N- 9 '

 9 -heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-ben--zothiadiazole)] PCPDTBT Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethyl-hexyl)-4H-cyclopenta[2,1--b:3,4-b0]dithiophene-2,6-diyl]] PCE Power Conversion Efficiency PDQT Diketopyrrolopyrrole (DPP) with β-unsubstituted quaterthiophene (QT) PDTSTPD Poly(4,4'-bis(2-ethylhexyl)-dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-((5-octyl--thieno[3,4-c]pyrrole-4,6-}dione)-1,3-diyl)] Pedot-PSS Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) PM-IRRAS Polarization Modulated-Infra Red Reflection Absorption Spectroscopy PTB7 Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b9]dithiophene-2

Figure 1 . 1 :

 11 Figure 1.1: Photovoltaic device efficiency within different families of semiconductors. Reproduced from reference [1].

Figure 1 . 2 :

 12 Figure 1.2: Photographs of (a) an OSC printed on paper, reproduced from reference [87] with permission of Wiley and (b) a transparent OSC, reprinted with permission from [34]. Copyright (2012) American Chemical Society

Figure 1 . 3 :

 13 Figure 1.3: Building blocks for device fabrication and the three major axes of performance.

Figure 1 . 4 :Figure 1 . 5 :

 1415 Figure 1.4: Example of a typical solar cell characteristic under light illumination and under dark condition.

Figure 1 . 6 , 5 SFigure 1 . 6 :

 16516 Figure 1.6, it corresponds to the solar spectrum through atmosphere, 48.2°from zenith.

Figure 1 . 7 :

 17 Figure 1.7: Chemical structures of: (a) ethylene, (c) polyacetylene and the overlapping of orbitals in (b) ethylene and (d) polyacetylene.

A

  significant improvement with respect to bi-layer devices is the Bulk Heterojunction (BHJ) concept. A BHJ type of active layer consists of an intimate mixture of a D and an A. Such mixtures result in many D/A interfaces throughout the entire active layer and thereby provide a compromise between the amount of interface for exciton separation and D/A pathways for hole/electron transport.

Figure 1 .Figure 1 . 8 :

 118 Figure 1.8 depicts the schematic of a bi-layer and a BHJ type of active layer. In 1992, Sariciftci et al. revealed a fast photoinduced charge transfer between

Figure 1 . 9 :

 19 Figure 1.9: Top row: examples of A small molecules and bottom row: example of D polymers.

( i )Figure 1 . 10 :

 i110 photo-generated within the exciton diffusion length from a D/A interface can contribute to the generation of free charge carriers. Excitons that are formed within a larger distance from this interface undergo geminate recombination and are lost. At the interface, charge transfer from the D to the A is possible if the energy offset at the interface is sufficiently large. Charge transfer occurs if the LUMO level of the A lies between the HOMO-LUMO levels of the D as shown in Figure1.10. The energy offset between the LUMO of the D and the LUMO of the A has to be greater than 0.3 eV for charge transfer to occur[START_REF] Servaites | Organic solar cells: A new look at traditional models[END_REF]. At the interface, the exciton dissociates to form the charge transfer state corresponding to the e -/h + pair state in which the electron is located on a neighbor molecule of the molecule on which the hole is located. The dissociation of the charge transfer state generates a free electron and a free hole which are then transported through A or D pathways to their respective electrode under the influence of an internal electric field.

FIGURE 2

 2 FIGURE 2 AFM topography images (2 lm  2 lm) of as-cast and annealed films of MDMO-PPV:PC61BM spin-casted from chlorobenzene (a, b), CS2 (c, d), CHCl3 (e, f), pyridine (g, h), trichloroethylene (i, j), toluene (k, l), and 1-methylpyrrole (m, n).

Figure 2 .

 2 Figure 2.1: AFM topography images (2 µm x 2 µm) of as-cast and annealed films of MDMO-PPV-PC 61 BM spin-cast from (a, b) chlorobenzene (CB),(c, d) carbon disulfide, (e, f) chloroform (CF), (g, h) pyridine, (i, j) trichloroethylene, (k, l) toluene and (m, n) 1-methylpyrrole. Reproduced from reference[START_REF] Duong | Molecular solubility and hansen solubility parameters for the analysis of phase separation in bulk heterojunctions[END_REF] with permission of Wiley.

Figure 2 . 2 :

 22 Figure 2.2: (a) GIXD profiles of P3HT films spin-cast from P3HT/CF solutions containing a range of added acetone; (b) 2θ angle (left axis) of (100) peak and corresponding layer spacing (right axis) as a function of the additional acetone volume ratio. Reprinted with permission from[START_REF] Chang | Solvent Based Hydrogen Bonding : Impact on Poly (3-hexylthiophene) Nanoscale Morphology and Charge[END_REF]. Copyright (2013) American Chemical Society.

Figure 2 . 3 shows

 23 the effects of thermal annealing on P3HT/PC 61 BM films observed by UV-Vis absorption spectroscopy and by XRD measurements. UV-Vis absorption spectroscopy represents a useful technique to detect the degree of crystallinity of P3HT because increased crystallinity induces a red-shift in the absorption peak

Figure 2 . 3 :

 23 Figure 2.3: Effects of thermal annealing on the crystallinity of P3HT depicted by (a) an appearance of P3HT vibronic bands in UV-Vis absorption spectra (b) an increase in diffraction peak of P3HT in XRD pattern. Figures (a) and (b) are respectively reproduced from references[START_REF] Li | Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene)[END_REF] with permission of AIP Publishing LLC and[START_REF] Zhokhavets | Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties[END_REF] with permission of Elsivier.

A

  processing additive refers to a solvent which is introduced in small proportion (generally a few volume %) into the host solvent used to solubilize the active materials. In 2006 -2007, Peet et al. showed that introducing 1,8-octanedithiol (ODT) in a D/A solution blend increased the photocurrent and the PCE of OSCs using P3HT or PCPDTBT as D materials [169, 170]. For PCPDTBT/PC 70 BMbased OSCs, the PCE was increased from 2.8% to 5.5%. This pioneering work represented an exciting discovery as processing additives were seen as a new method for improving the photovoltaic performance of OSCs in addition to the conventional post-processing steps. Further work led to two competing theories regarding the specific properties of what makes a solvent an efficient processing additive. In 2008, Yao et al.

  Figure 2.4. With a smaller amount of PC 61 BM in the solution, the P3HT chains would be able to self-organize more easily and form more crystalline domains at the origin of the improved active layer morphology.

Figure 2 . 4 :

 24 Figure 2.4: Proposed model during spin-coating process. Black wire: P3HT polymer chain; big black dots: PC 61 BM; blue dots: ODCB molecules; and red dots: ODT molecules. (a-c) correspond to three stages in the spin-coating process when ODCB is the sole solvent;(d-f) correspond to three stages in the spin-casting process when ODT is added in ODCB. Reproduced from reference[START_REF] Yao | Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells[END_REF] with permission of Wiley.

Figure 2 . 5 :

 25 Figure 2.5: Schematic of the role of the processing additive in the self-assembly of BHJ blend materials. Reprinted with permission from [114]. Copyright (2008) American Chemical Society.

Figure 2 . 6 :

 26 Figure 2.6: Chemical structures of processing additives.

Figure 3

 3 Figure 3.1: P-type bottom-gate, top-contact OTFT device architecture from a sideview (left) and from a top-view (right). L represents the channel length and W the channel width.

Figure 3 . 2 :

 32 Figure 3.2: Hole only device structure.

Figure 4 . 1 :

 41 Figure 4.1: HSPs diagrams showing the good and the poor solvents resulting from solubility tests and the fitted solubility sphere of the compound under study.

Figure 4 . 2 :

 42 Figure 4.2: Solubility spheres of P3HT and PC 61 BM.

  which in this case is ODCB (boiling point = 180 °C). A processing additive with a boiling point 100 °C higher provides a sufficient latitude for differential evaporation of the two solvents. Additionally, for safety concerns, only solvents that are not carcinogenic are considered. Applying the above selection criteria on an array of 723 solvents with known HSPs that are present in the Hansen software database (HSPiP 3rd edition), three solvents were shown to satisfy the requirements. These are 1-cyclohexyl-2-pyrrolidinone (C-PYR), dimethyl phthalate (DPH) and tributyl o-acetylcitrate (TRIB). The molecular structures of the selected solvents are presented in Figure 4.3. As shown in Table 4.3 , they all have REDs with PC 61 BM < 0.9 and REDs with P3HT > 1.0. Graphically, these three solvents are positioned inside the solubility sphere of PC 61 BM and outside the sphere of P3HT, as depicted in Figure 4.4.

Figure 4 . 3 :

 43 Figure 4.3: Molecular structures of: (a) C-PYR, (b) DPH and (c) TRIB.

Figure 4 . 4 :

 44 Figure 4.4: Positions of C-PYR, DPH and TRIB in the Hansen solubility space with respect to the solubility spheres of P3HT and PC 61 BM.

Figure 4 . 5 :Figure 4 . 6 :

 4546 Figure 4.5: UV-Vis absorption spectra of solutions of P3HT in ODCB, C-PYR and TRIB.

Figure 4 . 7 : 2 ]Figure 4 . 8 :

 47248 Figure 4.7: Solid state UV-Vis absorption spectra for P3HT/PC 61 BM blends with varying concentration of processing additives: a) C-PYR, b) DPH and c) TRIB.

Figure 5 .Figure 5 . 1 :

 551 photovoltaic parameters V oc , FF, J sc and PCE of the OSCs with the different processing additives and their corresponding concentrations. The data in the figures represent the average values obtained from six individual OSCs for each system.

Figure 5 . 2 :

 52 Figure 5.2: Solid state UV-Vis absorption spectra for P3HT/PC 61 BM blends with various concentration of a) C-PYR, b) DPH and c) ODT.

Figure 5 .

 5 3 depicts the XRD patterns of P3HT/PC 61 BM films with different concentrations of the three processing additives.The diffraction peak at 5.5°corresponds to the edge-on orientation of the polymer chains[START_REF] Zhokhavets | Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties[END_REF]. The intensity of the diffraction peak at 5.5°decreases when processing additives are introduced in the solution blend. This result is consistent with earlier observations by Guo et al. who showed that introducing processing additives into a blend of P3HT/indene-C70 bisadduct (IC 70 BA) leads to a decrease in the P3HT diffraction peak[START_REF] Guo | High efficiency polymer solar cells based on poly(3-hexylthiophene)/indene-C70 bisadduct with solvent additive[END_REF]. They attributed this result to a decrease in phase separation. The average domain sizes of P3HT can be estimated using the Scherrer equation: c o n c e n t r a t i o n :

C o n t r o l T h e r m a l l y a n n e a l e dFigure 5 . 3 :

 53 Figure 5.3: XRD patterns of P3HT/PC 61 BM fims spin-cast from solutions containing 1.6 vol% of a) C-PYR, b) DPH, c) ODT and d) thermally-annealed film.

Figure 5 . 4 :

 54 Figure 5.4: Average size of P3HT crystallites as a function of processing additive concentration.

  c o n c e n t r a t i o n :

Figure 5 . 5 :

 55 Figure 5.5: XRD patterns of P3HT films spin-cast from solutions containing various concentrations of DPH.

ITO/MoO 3 /Figure 5 . 6 :

 356 Figure 5.6: (a) Device structure of hole-only device and (b) corresponding energy diagram.

Figure 5 . 7 :

 57 Figure 5.7: J.L 3 as a function of applied voltage for hole-only devices prepared from: a) and d) C-PYR, b) and e) DPH, and c) and f) ODT. Figuresa -c) depict plots for an applied voltage ranging from 0 to 5 V and figures d -f) depict zoomed-in plots in the region 2 to 5 V.

Figure 5 .

 5 Figure 5.8 shows the evolution of the hole mobility as a function of the concentration of processing additive. The data in the figures represent the average values obtained from measurements on two to four individual hole-only devices for each material system.

Figure 5 . 8 :

 58 Figure 5.8: Calculated SCLC hole mobility as a function of the concentration of processing additive.

Figure 5 . 9 :

 59 Figure 5.9: Evolution of hole mobility and FF as a function of the concentration of processing additive in OSCs prepared with (a) C-PYR, (b) DPH and (c) ODT.

  cessing additive causes it to evaporate more slowly than the host solvent and hence the effective concentration of processing additive gradually increases in the solvent mixture. Consequently, the solvent quality towards P3HT changes with time. In order to follow the evolution of the solvent quality towards P3HT, the RED values between the solvent mixture and the polymer are measured. The HSPs of the solvent mixture for various concentration of processing additives are calculated by adding the contributions of each of the component (the processing additive and the ODCB), as described in the following equation: δD mixture = φ ODCB .δD ODCB + φ additive .δD additive δP mixture = φ ODCB .δP ODCB + φ additive .δP additive δH mixture = φ ODCB .δH ODCB + φ additive .δH additive (5.3) where: δD mixture , δP mixture and δH mixture are the HSPs of the solvent mixture, δD ODCB , δP ODCB and δH ODCB are the HSPs of ODCB, δD additive , δP additive and δH additive are the HSPs of the additive, φ ODCB and φ additive are the volume fraction of respectively ODCB and the additive in the solvent mixture. The plots showing the evolution of the RED of P3HT and PC 61 BM as a function of the effective concentration of the processing additive during film forming are displayed in Figure 5.10. In Figure 5.10, the dotted line at RED of 1 represents the threshold value separating the regions where the compound is soluble (RED < 1) and non soluble (RED > 1) in the solvent mixture.

Figure 5 . 10 :

 510 Figure 5.10: Evolution of the RED with P3HT (in black) and with PC 61 BM (in red) as a function of the effective concentration of a) C-PYR, b) DPH and c) ODT. The dotted line at RED of 1 depicts the theoretical RED threshold value separating the regions where the compound is non-soluble and soluble.

Figure 6 . 1 :

 61 Figure 6.1: OSC architecture: (a) inverted and (b) conventional.

tectures 6 . 2 . 1

 621 OSCs with ODTOSCs prepared with various concentrations of ODT (0 to 2.4 vol%) are fabricated in an inverted and in a conventional architecture. The J -V characteristics of the OSCs are measured and Figure6.2 shows the photovoltaic parameters (V oc , J sc , FF, PCE, R s and R sh ) of the OSCs as a function of additive concentration. The data in the figures represents the average values obtained from four to eight OSCs for each system.

Figure 6 . 2 :Figure 6 . 3 :

 6263 Figure 6.2: Effects of ODT on the electrical parameters in a conventional and in an inverted architecture of OSCs: (a) V oc , (b) J sc , (c) FF, (d) PCE, (e) R s and (f) R sh .

Figure 6 . 4 :Figure 6 . 5 :Figure 6 . 6 :

 646566 Figure 6.4: J -V curves of OSCs with various concentrations of DPH under dark conditions (a) in a conventional configuration and (c) in an inverted configuration and J -V curves under illumination (b) in a conventional configuration and (d) in an inverted configuration.

6. 3

 3 Measurements of charge carrier mobility using OTFT configuration 6.3.1 Fabrication of OTFTs The electron and hole mobilities are respectively measured using n-type and p-type OTFTs. In n-type OTFTs, a positive gate-source voltage leads to the formation of a negative channel at the interface between the semiconductor and the dielectric, where electrons can be injected by the source and collected by the drain. In p-type OTFTs, a negative gate-source voltage is applied to form a positive channel at the interface where hole transport can occur. Source and drain consist of aluminum as a low work function metal for n-type OTFTs, and of gold as a high work function metal for p-type OTFTs. The transfer characteristic (which is the drain-source current (I DS ) as a function of the gate-source voltage (V GS )) is measured at a drain-source voltage (V DS ) of +5 V for n-type OTFTs and -5 V for p-type OTFTs. The mobility is extracted from the saturation regime where |V DS | > |V GS -V th | > 0.

6. 3 . 2 Figure 6 . 7 :

 3267 Figure 6.7: Transfer characteristics of OTFTs prepared with various concentrations of ODT in: (a) p-type OTFTs and (b) n-type OTFTs.

Figure 6 .

 6 Figure 6.8b clearly shows that hole mobility increases with increasing concentration of ODT while electron mobilities remain relatively unchanged upon the introduction of ODT. Films prepared with concentrations of ODT ≤ 1.6 vol% demonstrate a µ h + /µ e -ratio < 1 indicating higher electron mobility. However, OTFTs with an ODT concentration of 2.4 vol%, exhibit a µ h + /µ e -ratio of 7.05 indicating that additives cause the hole mobility to surpass that of electrons.

  c e n t r a t i o n o f O D T :

)Figure 6 . 8 :

 68 Figure 6.8: (a) Mobility values of OTFTs prepared with various concentrations of ODT as a function of V GS -V th : the left side of the graph depicts hole mobility and the right side the electron mobility. (b) Electron and hole mobilities at V GS -V th of +2 V and -2 V respectively, as a function of ODT concentration.

Figure 6 . 9 :

 69 Figure 6.9: Transfer characteristics of OTFT prepared with various concentrations of DPH in: (a) p-type OTFTs and (b) n-type OTFTs.

)Figure 6 . 10 :

 610 Figure 6.10: (a) Mobility values of OTFTs prepared with various concentrations of DPH as a function of V GS -V th : the left side of the graph depicts hole mobility and the right side the electron mobility. (b) Electron and hole mobilities at V GS -V th of +2 V and -2 V respectively, as a function of DPH concentration.

Figure 6 .

 6 Figure 6.11: A schematic description of hole and electron transport in OSC without processing additive and subsequent preferential architecture.

Figure 7 .

 7 1 compares the UV-Vis absorption spectra of neat polymer and of polymer/PC 61 BM blends under light irradiation in the presence or absence of oxygen.

Figure 7

 7 Figure 7.1: (a) Normalized UV-Vis absorption (at 500 nm) of MDMO-PPV ( ), MDMO-PPV/PC 61 BM ( ) and normalized UV-Vis absorption (at 520 nm) of P3HT (•) and P3HT/PC 61 BM ( ) samples during photo-oxidation. (b) Normalized UV-Vis absorption of MDMO-PPV/PC 61 BM (at 500 nm ( )) and P3HT/PC 61 BM (at 520 nm ( )) samples during photolysis. Reproduced from reference [180] with permission of Elsivier.

Figure 7 . 2 :

 72 Figure 7.2: Optical microscopy images of P3HT/PC 61 BM layers before and after being thermally annealed at 150 o C for 5 hours or 24 hours. Reproduced from reference[START_REF] Wantz | Stabilizing polymer-based bulk heterojunction solar cells via crosslinking[END_REF] with permission of Wiley.

Figure 7 .

 7 3 depicts a scheme explaining the tests.The films of P3HT/PC 61 BM are spin-cast from solutions containing 1.6 vol% of processing additives on ITO substrates coated with ZnO. A P3HT/PC 61 BM-film spin-cast without processing additive serves as a control film.

Figure 7 . 3 :

 73 Figure 7.3: Scheme of the procedure for the detection of processing additive in P3HT/PC 61 BM films.

Figure 7 . 4 :

 74 Figure 7.4: (a) ATR spectrum of C-PYR, (b) Zoomed-in PM-IRRAS spectra of P3HT/PC 61 BM films with and without C-PYR (region 1900 -1600 cm -1 ).

Figure 7 . 5 :

 75 Figure 7.5: (a) ATR spectrum of DPH, (b) Zoomed-in PM-IRRAS spectra of P3HT/PC 61 BM films with and without DPH (region 1820 -1700 cm -1 ).

Figure 7 . 6 :

 76 Figure 7.6: (a) ATR spectrum of TRIB, (b) Zoomed-in spectra of the region 1800 -1700 cm -1 of PM-IRRAS spectra of P3HT/PC 61 BM films with and without TRIB.

Figure 7 . 7 :

 77 Figure 7.7: (a) ATR spectrum of ODT, (b) PM-IRRAS spectra of P3HT/PC 61 BM films with and without ODT.

  2 indicates the normalized electrical parameters of OSCs after light irradiation and Figure 7.8 depicts the evolution of the photovoltaic parameters as a function of irradiation time. The normalized parameters correspond to the percentage of initial value (at t=0) measured before the prolonged light exposure.

  m a l i z e d v a l u e o f J s c [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] T h e r m a l a n n e a l i n g C m a l i z e d v a l u e o f V o c [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] m a l i z e d v a l u e o f F F [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] m a l i z e d v a l u e o f P C E [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] m a l i z e d v a l u e o f R s e r i e [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] m a l i z e d v a l u e o f R s h u n t [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ]

Figure 7 . 8 :

 78 Figure 7.8: Normalized electrical parameters of OSCs subjected to light irradiation in air: (a) J sc , (b) V oc , (c) FF, (d) PCE, (e) R s and (f) R sh .

  m a l i z e d v a l u e o f J s c [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] T h e r m a l a n n e a l i n g C m a l i z e d v a l u e o f V o c [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] m a l i z e d v a l u e o f F F [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] m a l i z e d v a l u e o f P C E [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] m a l i z e d v a l u e o f R s e r i e [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ] m a l i z e d v a l u e o f R s h u n t [ % ] T i m e o f d e g r e d a t i o n ( a i r ) [ h o u r s ]

Figure 7 . 9 :

 79 Figure 7.9: Normalized electrical parameters of OSCs subjected to light irradiation in inert atmosphere: (a) J sc , (b) V oc , (c) FF, (d) PCE, (e) R s and (f) R sh .

Figure 7 . 10 :

 710 Figure 7.10: UV-Vis absorption of P3HT/PC 61 BM films before and after light irradiation for 60 hours in air for (a) Thermally annealed, (b) C-PYR, (c) DPH and (d) ODT-treated active layers.

Figure 7 .

 7 Figure 7.11: UV-Vis absorption spectra of P3HT/PC 61 BM films before and after light irradiation for 60 hours in inert atmosphere for (a) Thermally annealed, (b) C-PYR, (c) DPH and (d) ODT-treated active layers.

Figure 7 . 12 :

 712 Figure 7.12: Scheme of photo-stability tests on OSC with a C 60 buffer interlayer.

Figure 7 . 13 :

 713 Figure 7.13: Energy levels of OSCs containing C 60 as an interlayer between ZnO and the active layer.

Figure 7 . 14 :

 714 Figure 7.14: Effects of C 60 thickness on the PCE of OSCs.

Figure 7 . 15 :

 715 Figure 7.15: Evolution of normalized (a) V oc and (b) PCE as a function of irradiation time for OSCs without (solid line) and with (dashed line) a C 60 interlayer.

  complete or incomplete way are compared to identify the role of the top electrode in photo-degradation. The normalized photovoltaic performances are compared after 40 hours of light irradiation (for the tests performed on complete OSCs, the electrical performances were determined by the extrapolation of the trends in Figure 7.9 at 40 hours). The normalized V oc and PCE in the two experiments are shown as a function of the processing conditions in Figure 7.16.

Figure 7 . 16 :

 716 Figure 7.16: Comparison of the normalized (a) V oc and (b) PCE after 40 hours of light irradiation on OSCs without top electrode or with top electrode.

Figure 7 .

 7 Figure 7.17: (a) Schematic drawing of a four-point bend adhesion sample stack. A load P/2 is applied on each side of the sample. (b) A typical load versus displacement characteristic. Figure (b) is reproduced from reference [220] with permission of AIL Publishing LLC.

4 FFigure 7 . 18 :

 4718 Figure 7.18: Load versus displacement characteristics for fresh and irradiated samples for (a) samples with ODT and (b) thermally annealed samples. On (a) the measured force loss (∆N) is showed. The plots of the irradiated samples were manually vertically down shifted for the clarity of the Figures.

Figure 7 . 19 :

 719 Figure 7.19: AFM topography (1 µm x 1 µm) images of active layers films: (a) thermally annealed and (b) ODT-treated.

  photo-degradation occurs primarily at the top interface. By means of adhesion measurements it is confirmed that light irradiation causes interfacial degradation which results in a weaker adhesion of the top electrode upon light irradiation. These results suggest that the increased in R s observed in the photo-stability tests (in Sections 7.3.2 and 7.3.1) is due to increased contact resistance at the top electrode, which in turn decrease the V oc by affecting the built-in voltage in the device. Additionally, adhesion measurements show that ODT-treated OSCs are in a lower extend subjected to interfacial photo-degradation than thermally

Chapter 2 ,

 2 the selection and the effects of processing additives depend on the type of D/A systems. In order to broaden the scope of the study on processing additive, other types of D semiconductors are investigated. P3HT can be classified as a low mobility semi-crystallline polymer. In this chapter, two other families of D are studied: a high mobility semi-crystalline polymer (a 1,4-diketopyrrolo[3,4-c]pyrrole derivative with a quaterthiophene substituent (PDQT)) and an amorphous polymer (poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole (PCDTBT)).

8. 1

 1 Studies on PDQT8.1.1 Introduction to DPP based copolymersIn the past few years, 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) has attracted considerable attention as an electron acceptor building block in conjugated copolymers for applications in OTFTs[START_REF] Li | A stable solution-processed polymer semiconductor with record high-mobility for printed transistors[END_REF] and OSCs[START_REF] Hendriks | High-molecular-weight regular alternating diketopyrrolopyrrole-based terpolymers for efficient organic solar cells[END_REF][START_REF] Hendriks | Homocoupling defects in diketopyrrolopyrrole-based copolymers and their effect on photovoltaic performance[END_REF][START_REF] Hendriks | Band Gap Control in Diketopyrrolopyrrole-Based Polymer Solar Cells Using Electron Donating Side Chains[END_REF][START_REF] Li | Wide band gap diketopyrrolopyrrole-based conjugated polymers incorporating biphenyl units applied in polymer solar cells[END_REF][START_REF] Li | Effect of the fibrillar microstructure on the efficiency of high molecular weight diketopyrrolopyrrole-based polymer solar cells[END_REF][START_REF] Li | High mobility diketopyrrolopyrrole (DPP)-based organic semiconductor materials for organic thin film transistors and photovoltaics[END_REF]. The general chemical structure of DPP-based copolymers, depicted in Figure8.1, contains an alternation of electron donating and DPP building blocks.

Figure 8 . 1 :

 81 Figure 8.1: On the left: DPP-based conjugated polymers where R is a substituent, Donor 1 and Donor 2 are electron donating building blocks. On the right, examples of electron donating building blocks are depicted. Adapted from reference [121] with permission of the Royal Society of Chemistry .

Figure 8 . 4 :

 84 Figure 8.4: Energy levels of PDQT/PC 61 BM -OSC in an inverted architecture.

Figure 8 . 5 :

 85 Figure 8.5: Solubility spheres of PC 61 BM and PDQT.

Figure 8 . 6 :

 86 Figure 8.6: XRD patterns of: (a) pure PDQT and (b) PDQT/PC 61 BM blends in various D/A ratios.

Figure 8 . 7 :

 87 Figure 8.7: AFM images of PDQT/ PC 61 BM in various D/A ratios: (a-d) phase images, (e-h) topography images: (a) and (e) ratio pure PDQT, (b) and (f) ratio 2/1, (c) and (g) ratio 1/2, (d) and (h) ratio 1/4.

  c e n t r a t i o n o f a d d i t i v e [ v o l % ] c e n t r a t i o n o f a d d i t i v e [ v o l % ] c e n t r a t i o n o f a d d i t i v e [ v o l % ] c e n t r a t i o n o f a d d i t i v e [ v o l % ]

Figure 8 . 8 :Figure 8 . 9 :

 8889 Figure 8.8: Electrical performance of PDQT/PC 61 BM-OSCs as a function of additive concentration: (a) J sc , (b) V oc , (c) FF and (d) PCE.

8. 2

 2 Studies on PCDTBT 8.2.1 Introduction to carbazole based copolymers Poly(2,7-carbazole) derivatives represent an efficient family of copolymers for BHJ-OSCs [18, 19]. The carbazole is an electron rich moiety that consists of two six-membered benzene rings fused on either side of a five-membered nitrogencontaining ring (See Figure 8.10a).

Figure 8 . 10 :

 810 Figure 8.10: Chemical structures of: (a) a carbazole unit and (B) PCDTBT.

Figure 8

 8 Figure 8.10b.

Figure 8 . 11 :

 811 Figure 8.11: Energy levels of PCDTBT/PC 61 BM-OSCs in an inverted architecture.

Figure 8 . 13 :

 813 Figure 8.13: Photovoltaic parameters of OSCs as a function of the thickness for various D/A ratios: (a) J sc , (b) V oc , (c) FF and (d) PCE.

Figure 8 . 14 :

 814 Figure 8.14: Hansen solubility spheres of PCDTBT and PC 61 BM and the following solvents: DIO (black), ODT (yellow).

  Figure 8.15: Solid-state UV-Vis absorption spectra of PCDTBT/PC 61 BM films (ratio 1/3) prepared without or with DIO.

  The effects of DIO and ODT on the photovoltaic performance of OSCs are investigated by preparing OSCs from solutions containing DIO or ODT in various concentrations (from 0.5 vol% to 2 vol%). The results show that the introduction of these solvents decreases the performance of OSCs. This suggests that increased order within the polymer chains does not improve the photovoltaic performance of PCDTBT/PC 61 BM -based OSCs.Next to formulations with DIO and ODT, several other solvent formulations are tested. OSCs are fabricated using the widely used 1-chloronaphthalene as a processing additive, xylene as a good solvent for both semiconductors, NMP as a good solvent for PC 61 BM and poor solvent for PCDTBT. None of these formulations were successful in improving the photovoltaic performance of PCDTBT/PC 61 BM -OSCs. It is now noteworthy to point out that all these approaches are generally used to enhance the aggregation or the crystallinity of D polymers. The fact that all of these approaches are unsuccessful suggests that increasing the ordering of PCDTBT chains is inefficient in increasing the PCE of PCDTBT/PC 61 BM -OSCs. Next, studies on the aggregation of PC 61 BM are carried out. 8.2.4 PCDTBT/PC 61 BM/C 60 ternary blend. This next study focuses on the effect of C 60 as a nucleating agent for PC 61 BM in the PCDTBT/PC 61 BM system. Because of the low solubility of C 60 in ODCB [90, 189], C 60 is expected to undergo the transition from the liquid state to the solid state at an earlier stage than PC 61 BM during the process of film formation. The introduction of C 60 is therefore expected to alter the aggregation of PC 61 BM. Systems based on a ternary blend of PC 61 BM and C 60 as the acceptor components, and PCDTBT as the donor are studied. The resulting active layers are of the type: PCDTBT/PC 61 BM (1-x) /C 60(x) with x the fraction of C 60 varying from 0 to 1. The UV-Vis spectra of ternary blends with C 60 fractions of 0, 0.4 and 0.7 are depicted in Figure 8.16. The UV-Vis spectra show that the introduction of C 60 causes a blue-shift in the absorption peak of PCDTBT. The wavelength of PCDTBT absorption maximum is 564 nm in films without C 60 and decreases to 558 nm in PCDTBT/PC 61 BM (0.3) /C 60(0.7) films. In contrast to the red-shift caused by increased ordering within polymer chains, the blue-shift can be interpreted as a reduction of polymer chain ordering. Next, the electron mobility in a ternary blend is measured and compared to the electron mobilities of PCDTBT/PC 61 BM in various D/A ratios. The electron mobilities are measured in OTFT configurations for PCDTBT/PC 61 BM systems with D/A ratios of 1/4, 1/3 and 1/2 and for the ternary blend PCDTBT/PC 61 BM (0.6) /C 60(0.4) at a D/A ratio of 1/2.

Figure 8 .

 8 17a depicts the transfer characteristics of the resulting n-type OTFTs. The OTFT mobilities are calculated at different values of gate voltage and Figure 8.17b depicts the mobility as a function of V GS -V th .

7 FFigure 8 . 16 :

 7816 Figure 8.16: Solid state UV-Vis spectra of PCDTBT/PC 61 BM (1-x) /C 60(x) films with various fraction of C 60 .

Figure 8 .

 8 Figure 8.17: (a) Transfer characteristics and (b) Mobility as a function of V GS -V th for various D/A ratios and for PCDTBT/PC 61 BM (0.6) /C 60(0.4) .

  OSCs are prepared from ternary blends: PCDTBT/PC 61 BM (1-x) /C 60(x) , with a D/A ratio kept constant and x the fraction of C 60 varied from 0 to 1. The energy levels of such ternary-blend OSCs are displayed in Figure 8.18. Two D/A ratios are studied: 1/3 and 1/2.

60 Figure 8 . 18 :

 60818 Figure 8.18: Energy levels of PCDTBT/PC 61 BM/C 60 -OSCs in an inverted architecture.

Figure 8 .F r a c t i o n o f C 6 0 Figure 8 . 19 :

 80819 Figure 8.19 shows the photovoltaic parameters (V oc , J sc , FF and PCE) of ternary OSCs as a function of the fraction of C 60 .

C

  60 increases the amount of fullerene nanocrystals (PC 61 BM or C 60 or a mixture of both). The increase in fullerene aggregation caused by the introduction of C 60 is consistent with the trend in electron mobility observed from the OTFT mobility measurements, along with the disruption of polymer chain ordering suggested by the blue-shift in the UV-Vis absorption spectra. Such morphological changes caused by C 60 do not affect the PCE. The effects of C 60 introduction on OSC-stability is then studied. As presented in the literature section on OSC stability in chapter 7-section 7.1, PC 61 BM is subject to severe thermal degradation. Upon prolonged thermal annealing, PC 61 BM molecules can form micrometer size aggregates which are profoundly detrimental for the performance of OSCs. In PCDTBT/PC 61 BM films, Derue et al. showed that thermal annealing above 160 °C causes the formation of micrometer sizes PC 61 BM [56]. In the literature, several studies were carried out to improve the thermal stability of PC 61 BM [222]. Among these, few studies reported on the beneficial effect of the introduction of C 60 on the thermal stability of D/A blends based on P3HT/PC 61 BM [178], or poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt thiophene-2,5diyl](TQ1)/PC 61 BM [126].For the study on OSC thermal stability, three types of ternary blend OSCs are fabricated with a D/A ratio kept constant at 1/2 and with a C 60 fraction, x, of 0, 0.4 and 0.7. The ZnO layer and the active layer are deposited following the procedure used for the fabrication of inverted OSCs. After the deposition of the active layer, the samples are thermally annealed on a hot plate set at 160 °C for two or four hours. The top electrodes (MoO 3 /Ag) are evaporated on top of the thermally annealed samples. The J -V characteristics of these OSCs are measured and the photovoltaic performances are normalized with respect to the parameters of fresh OSCs. Figure8.20 shows the evolution of normalized PCE as a function of the annealing time. The normalized photovoltaic performances of the OSCs after two hours of thermal treatment are listed in

  m a l i z e d P C E [ % ] Duration of thermal annealing at 160°C [hours]Fraction of C 60 :

Figure 8 . 20 :Figure 8 . 21 :

 820821 Figure 8.20: Normalized PCE as a function of the time of annealing treatment at 160 o C.

Figure 8 . 22 :

 822 Figure 8.22: Microscopic images of PCDTBT/PC 61 BM (1-x) /C 60(x) films thermally treated at 160 o C for 2 hours, 4 hours and 8 hours (magnification 50).

  One idea to test the radical scavenging function of ODT would be to introduce another thiol containing molecule in the active layer instead of ODT and to test the OSC photo-stability.Another stability-related subject raised by this thesis concerns the introduction of C 60 in the active layer. The ternary blend based on PCDTBT/PC 61 BM (x) /C 60(1-x) was shown to exhibit a higher thermal stability compared with the binary system based on PCDTBT/PC 61 BM. Replacing a fraction of PC 61 BM by C 60 is attractive as a method to increase thermal stability in addition to economic benefits. To fully exploit the beneficial properties of C 60 as an acceptor, a binary system based on PCDTBT/C 60 would be of great interest. However, when a pure PCDTBT/C 60 system was made, it exhibited a low PCE, likely due to morphological issues in the BHJ. Processing additives that can help to improve the morphology of the acceptor in the BHJ should be therefore investigated. To date, processing additives are generally investigated for their effects on the donor polymer. Further work should consider the effects on the acceptor in order to optimize systems such as PCDTBT/C 60 .

  Schematic of the role of the processing additive in the self-assembly of BHJ blend materials. Reprinted with permission from[START_REF] Lee | Processing additives for improved efficiency from bulk heterojunction solar cells[END_REF]. Copyright (2008) American Chemical Society. . . . . . . . . . . . 2.6 Chemical structures of processing additives. . . . . . . . . . . . .

	2.3 Effects of thermal annealing on the crystallinity of P3HT depicted
	by (a) an appearance of P3HT vibronic bands in UV-Vis absorp-
	tion spectra (b) an increase in diffraction peak of P3HT in XRD
	pattern. Figures (a) and (b) are respectively reproduced from ref-
	erences [116] with permission of AIP Publishing LLC and [238]
	with permission of Elsivier. . . . . . . . . . . . . . . . . . . . . . .
	2.4 Proposed model during spin-coating process. Black wire: P3HT
	polymer chain; big black dots: PC 61 BM; blue dots: ODCB molecules;
	and red dots: ODT molecules. (a-c) correspond to three stages in
	the spin-coating process when ODCB is the sole solvent; (d-f) cor-
	respond to three stages in the spin-casting process when ODT is
	added in ODCB. Reproduced from reference [234] with permission
	of Wiley.
	. .
	2.2 (a) GIXD profiles of P3HT films spin-cast from P3HT/CF solu-
	tions containing a range of added acetone; (b) 2θ angle (left axis)
	of (100) peak and corresponding layer spacing (right axis) as a
	function of the additional acetone volume ratio. Reprinted with
	permission from [32]. Copyright (2013) American Chemical Society.

2.1 AFM topography images (2 µm x 2 µm) of as-cast and annealed films of MDMO-PPV-PC 61 BM spin-cast from (a, b) chlorobenzene (CB), (c, d) carbon disulfide, (e, f) chloroform (CF), (g, h) pyridine, (i, j) trichloroethylene, (k, l) toluene and (m, n) 1methylpyrrole. Reproduced from reference [57] with permission of Wiley. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5

  Chemical structure of PDQT. . . . . . . . . . . . . . . . . . . . . 8.3 Solid-state UV-Vis spectrum of PDQT/PC 61 BM in a 1/3 ratio. . 8.4 Energy levels of PDQT/PC 61 BM -OSC in an inverted architecture.116 8.5 Solubility spheres of PC 61 BM and PDQT. . . . . . . . . . . . . . 8.6 XRD patterns of: (a) pure PDQT and (b) PDQT/PC 61 BM blends in various D/A ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8 Electrical performance of PDQT/PC 61 BM-OSCs as a function of additive concentration: (a) J sc , (b) V oc , (c) FF and (d) PCE. . . . 8.9 AFM images of PDQT/PC 61 BM in a 1/3 ratio prepared with var-.18 Energy levels of PCDTBT/PC 61 BM/C 60 -OSCs in an inverted architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.19 Electrical parameters of OSCs with various fraction of C 60 : (a) V oc , (b) J sc , (c) FF and (d) PCE. . . . . . . . . . . . . . . . . . . 8.20 Normalized PCE as a function of the time of annealing treatment at 160 o C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.21 Microscopic images of PCDTBT/PC 61 BM (1-x) /C 60(x) films thermally treated at 160 o C for 2 hours, 4 hours and 8 hours (magnification 20). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.22 Microscopic images of PCDTBT/PC 61 BM (1-x) /C 60(x) films thermally treated at 160 .1 HOMO LUMO levels of acceptor materials. . . . . . . . . . . . . . 2.1 Boiling points and solubility limits of PC 61 BM in various solvents. Normalized electrical performance after light irradiation in air: 252 hours of light irradiation for OSCs with processing additives, 90 hours for thermally annealed OSCs. . . . . . . . . . . . . . . . 7.3 Normalized photovoltaic parameters of OSCs after 393 hours of light irradiation in inert atmosphere. . . . . . . . . . . . . . . . . 7.4 Decrease in optical density measured at 601 nm. . . . . . . . . . . 7.5 Normalized photovoltaic performance of C-PYR, DPH and ODT-Force loss in the adhesion measurement test of thermally annealed samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7 Force loss in the adhesion measurement test of ODT-treated samples.110 7.8 Roughness of active layers with different processing additives. . . 8.1 HSPs of PDQT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Photovoltaic parameters of OSCs with various ratios of PDQT/PC 61 BM.117 8.3 Diffraction peaks and domain sizes of PDQT/PC 61 BM with various D/A ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Electrical performance of PDQT/PC 61 BM-OSCs with and without additive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 HSPs of PCDTBT. . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Properties of PCDTBT/PC 61 BM active layers investigated. . . . . 8.7 Absorption maximum of PCDTBT/PC 61 BM films with or without processing additive. . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8 Average electron mobility measured in a saturation regime at V GS -V

	IC 70 BA	Indene-C 70 bisadduct
	ITO		Indium Tin Oxide
	J		Current density
	J -V		Current density -Voltage
	8.7 AFM images of PDQT/ PC 61 BM in various D/A ratios: (a-d) J sc Short-Circuit current density
	LUMO	phase images, (e-h) topography images: (a) and (e) ratio pure PDQT, (b) and (f) ratio 2/1, (c) and (g) ratio 1/2, (d) and (h) Lowest Unoccupied Molecular Orbital
	Donor Molybdenum oxide n-methyl-2-pyrrolidinone O-Dichlorobenzene 1,8-octanedithiol Organic Solar Cell Organic Thin Film Transistor Poly-3-hexylthiophene ratio 1/4D MoO 3 NMP ODCB ODT OSC OTFT P3HT PC 61 BM [6,6]-phenyl-C60-butyric acid methyl ester
	DDP		1,4-diketopyrrolo[3,4-c]pyrrole
	DEGDE	di(ethylene glycol)-diethyl ether
	DPH		Dimethyl Phthalate
	DPPBT	Poly(diketopyrrolopyrrole-quaterthiophene
	DPPT		Poly(diketopyrrolopyrrole-terthiophene)
	ETL		Electron Transport Layer
	FF		Fill Factor
	FWHM	Full Width at Half Maximum
	GIXD		Grazing Incidence X-ray Diffraction
	I DS		Drain source current
	HOMO		Highest Occupied Molecular Orbital
	HSP		Hansen Solubility Parameters
	HTL		Hole Transport Layer
	ICBA		Indene-C 60 bisadduct
			xxiv

8.1 On the left: DPP-based conjugated polymers where R is a substituent, Donor 1 and Donor 2 are electron donating building blocks. On the right, examples of electron donating building blocks are depicted. Adapted from reference [121] with permission of the Royal Society of Chemistry . . . . . . . . . . . . . . . 8.2 ious concentrations of additive: (a-c) topography images, (d-f) phase images: (a) and (d) no additive (b) and (e) 5 vol%, (c) and (f) 11 vol%. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.10 Chemical structures of: (a) a carbazole unit and (B) PCDTBT. . 8.11 Energy levels of PCDTBT/PC 61 BM-OSCs in an inverted architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.12 Solubility spheres of PCDTBT (in blue) and PC 61 BM (in red) in the Hansen solubility space. . . . . . . . . . . . . . . . . . . . . . 8.13 Photovoltaic parameters of OSCs as a function of the thickness for various D/A ratios: (a) J sc , (b) V oc , (c) FF and (d) PCE. . . . 8.14 Hansen solubility spheres of PCDTBT and PC 61 BM and the following solvents: DIO (black), ODT (yellow). . . . . . . . . . . . . 8.15 Solid-state UV-Vis absorption spectra of PCDTBT/PC 61 BM films (ratio 1/3) prepared without or with DIO. . . . . . . . . . . . . . 8.16 Solid state UV-Vis spectra of PCDTBT/PC 61 BM (1-x) /C 60(x) films with various fraction of C 60 . . . . . . . . . . . . . . . . . . . . . . 8.17 (a) Transfer characteristics and (b) Mobility as a function of V GS -V th for various D/A ratios and for PCDTBT/PC 61 BM (0.6) /C 60(0.4) . 8o C for 2 hours, 4 hours and 8 hours (magnification 50). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi List of Tables 12.2 Effects of processing additives on the PCE of several types of D/A OSCs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 HSPs of P3HT and PC 61 BM. . . . . . . . . . . . . . . . . . . . . 4.2 Characteristics of processing additives: HSPs, RED with P3HT and PC 61 BM, reported PCEs. . . . . . . . . . . . . . . . . . . . . 4.3 Characteristics of processing additives: HSPs, REDs with P3HT and PC 61 BM and boiling points. . . . . . . . . . . . . . . . . . . . 4.4 Solubility of P3HT and PC 61 BM. . . . . . . . . . . . . . . . . . . 4.5 Photovoltaic properties of OSCs. For each processing additive, the table shows the photovoltaic properties of OSCs containing the processing additive at a concentration giving the best PCE. . 5.1 Values of the FWHM of P3HT diffraction peak at 5.5°for different active layer compositions. . . . . . . . . . . . . . . . . . . . . . . 6.1 Mobility of holes and electrons as a function of the concentration of ODT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Mobility of holes and electrons as a function of the concentration of DPH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Initial electrical performances of OSCs with different types of active layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 treated OSCs and thermally annealed OSCs subjected to light irradiation in inert atmosphere for 40 hours. . . . . . . . . . . . . xxii 7.6 th = 4 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9 Normalized photovoltaic parameters of ternary blend OSCs after two hours of thermal treatment at 160 °C. . . . . . . . . . . . . .

  derivatives such as PC 61 BM are preferred for solution processed appli-

	cations. Their HOMO LUMO energy levels are shown in Table 1.1. PC 61 BM
	and other fullerene derivatives such as [6,6]-phenyl-C71-butyric acid methyl es-
	ter (PC 71 BM) and indene-C 60 bisadduct (ICBA) are the best performing A small
	molecules to date. Alternatives have been investigated but have not yet been
	able to compete with them [174, 177]. On the other hand, D polymers have been
	the subject of much research resulting in a large panel of π-conjugated polymers.

Table 2 .

 2 1: Boiling points and solubility limits of PC 61 BM in various solvents.

	Solvents	Boiling point	Solubility limit	References
			of PC 61 BM
	CF	61

°C 25 -26 mg.mL -1 [32, 51, 57, 181] CB 132 °C 25 -59.5 mg.mL -1 [32, 51, 57, 181] ODCB 183 °C 42 -107 mg.mL -1 [57, 162] TCB 214 °C 81.4 mg.mL -1 [162] Toluene 112 °C 9 -15.6 mg.mL -1 [32, 51, 57, 181] Xylene 138 °C 5 -22.1 mg.mL -1 [32, 51, 57, 181] Mesitylene 163 °C 47 -48.1 mg.mL -1 [32, 51] Carbon Disulfide 46 °C 207 mg.mL -1 [57]

TABLE 2

 2 Summarized Hansen Solubility Parameters for Various Organic Semiconductors

		dD (MPa ½ )	dP (MPa ½ )	dH (MPa ½ )	R0	RED (PC61BM)
	Small molecules
	PC61BM	19.89 6 0.34	5.68 6 1.03	3.64 6 0.92	6.6	0.00
	PC71BM	20.16 6 0.28	5.37 6 0.80	4.49 6 0.57	7.0	0.15
	DPP(TBFu)2	19.33 6 0.05	4.78 6 0.50	6.26 6 0.48	5.1	0.59
	F8-NODIPS	18.48 6 0.28	2.62 6 0.56	3.24 6 0.91	8.1	0.52
	DPP(PhTT)2	19.64 6 0.32	3.54 6 0.56	6.12 6 0.65	-	-
	Polymers
	MDMO-PPV	19.06	5.62	5.28	5.5	0.42
	MEH-PPV	19.06	5.38	5.44	6.0	0.41
	P3HT	18.56	2.88	3.19	3.6	1.08
	PFO	18.55	2.8	4.51	4.1	0.98

Table 2 .

 2 2: Effects of processing additives on the PCE of several types of D/A OSCs.

	D/A systems 1	Processing	PCE w/o	PCE with	Phase	Ref.
		additive	additive [%] additive [%] separation
		ODT	0.6	2.6	increase	[37]
		1,6-hexanedithiol	0.46	3.16	increase	[182]
	P3HT/PC61BM	Nitrobenzene	1.2	3.94	-	[149]
		Dodecanedithiol	1.74	4.03	-	[159]
		NMP	0.29	∼1.5 2	-	[234]
	P3HT/IC70BA	DIO	1.15	2.97	decrease	[70]
		ODT		3.09	decrease
	PCDTBT/PC71BM	DIO	4.89	5.91	increase	[131]
		Alkanedithiol	2.8	5.5	increase	[35]
		DIO	2.6	4.5	increase	[8]
	PCPDTBT/PC71BM	ODT	3.35	4.5	increase	[114]
		DIO	3.35	5.12	increase	[114]
		1,8-dibromooctane	3.35	4.66	increase	[114]
		DIO		4.62	increase
	PCPDTBT/PC61BM	ODT	1.68	3.87	increase	[66]
		1,8-dichlorooctane		3.45	increase
	PTB7/PC71BM	DIO	3.92	7.4	decrease	[124]
	DPPT/PC61BM	DIO	0.6	3.4	decrease	[127]
	DPPBT/PC61BM	DIO	1.0	5.2	decrease	[127]
	PDTSTPD/PC61BM	DIO	1.0	7.3	decrease [6, 46]

Table 4 .

 4 1.Figure 4.2 depicts the solubility spheres of P3HT and PC 61 BM.

Table 4 .

 4 1: HSPs of P3HT and PC 61 BM.

	Materials		HSP[MPA 1/2 ]	Fit	Errors
		δD	δP	δH	R o	False negative False positive
	P3HT	19.05 3.3 2.8 3.9 0.95 a	2/64 b	1/64 c
	PC 61 BM	20.02 5.2 5.88 8.4 0.97 a	2/65 b	0/65 c

a As determined by the HSPiP software using Equation 4.6.

b Number of bad solvents inside the solubility sphere.

c Number of good solvents outside the solubility sphere.

Table 4 . 2 .

 42 For 4-bromoanisole, ODT, hexanedithiol and dodecanethiol, the RED values are found to be < 1 with PC 61 BM and > 1 with P3HT. This is in agreement with the conclusions of Lee et al. that a processing additive should be a good solvent of PC 61 BM and a poor solvent of P3HT. In contrast, NMP and DEGDE exhibit RED values with PC 61 BM that are close to 1. The solubility of PC 61 BM in NMP and DEGDE is reported to be low (∼0.3 mg.mL -1[START_REF] Yao | Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells[END_REF]) which may explain their higher RED values. It is also noteworthy to point that DEGDE and NMP seem less effective in improving the efficiency of OSCs in comparison to the other processing additives (PCE: 1.5 % in case of NMP or DEGDE versus 2.6 -4.0% with the other processing additives)[START_REF] Liu | Solvent Additive Control of Morphology and Crystallization in Semiconducting Polymer Blends[END_REF][START_REF] Ouyang | High-performance polymer photovoltaic cells with thick P3HT:PCBM films prepared by a quick drying process[END_REF][START_REF] Salim | Solvent additives and their effects on blend morphologies of bulk heterojunctions[END_REF]. The observation that can be made from this study is that efficient processing additives possess RED values with PC 61 BM clearly below 1 and RED values with P3HT above 1. This indicates the effectiveness of using the HSPs as a tool to predict novel processing additives for P3HT/PC 61 BM-based OSCs.

Table 4 .

 4 2: Characteristics of processing additives: HSPs, RED with P3HT and PC 61 BM, reported PCEs.

	Materials	HSPs [MPa 1/2 ]		RED with	PCEs [%]
		δD	δP	δH	R o	P3HT PC 61 BM	without	with
								additive	additive
	P3HT	19.05	3.3	2.8	3.9	ND	ND
	PC 61 BM	20.02	5.2	5.88	8.4	ND	ND
	DEGDE	15.8	4.7	4.4		1.75	1.02	0.29%	5 times
									higher a) [27]
	NMP	18	12.3	7.2		2.62	0.99	0.29%	5 times
									higher a) [27]
	4-Bromoanisole	19.8	7.7	7		1.61	0.33	1.65%	2.60%[32]
	ODT	17.4	7.3	5.2		1.47	0.68	0.46%	3.12%[19]
	Hexanedithiol	17.4	7.6	6.4		1.67	0.69	0.46%	3.16%[19]
	Dodecanethiol	17.3	5.7	4.1		1.14	0.68	1.74%	4.03%[26]

a) No exact PCE is reported

Table 4 .

 4 3: Characteristics of processing additives: HSPs, REDs with P3HT and PC 61 BM and boiling points.

		Solvents	HSPs [MPa 1/2 ]	RED with	Boiling point
			δD	δP	δH	P3HT PC 61 BM
		C-PYR	18.2 6.8	6.5	1.38	0.48	306 o C
		DPH	18.6 10.8	4.9	2.01	0.76	284 o C
		TRIB	16.7 2.5	7.4	1.70	0.87	388 o C
	N	O
	(a)

Table 4 .

 4 4: Solubility of P3HT and PC 61 BM.The test results in Table4.4 confirmed the solubility of PC 61 BM and the poor solubility of P3HT in the three solvents. Also, it is noteworthy to mention that the values of the solubility limits appear to be consistent with the trend in the RED values reported in Table4.3: C-PYR, which exhibited the smallest RED, with PC 61 BM appears to be the best solvent while TRIB, which exhibited the highest RED with PC 61 BM, demonstrates the lowest solubility. The RED values appear to be efficient in predicting the solubility of organic semiconductors in solvents. Next, to investigate the influence of C-PYR, DPH and TRIB as processing additives on P3HT/PC 61 BM-BHJ morphology, UV-Vis absorption measurements were carried out on a series of P3HT/PC 61 BM films. The films were fabricated by spin-casting solutions of P3HT/PC 61 BM in ODCB that contain a processing additive in one of the following concentrations (by volume): 0.4 vol%, 0.8 vol%, 1.2 vol%and 1.6 vol%. The UV-Vis absorption spectra obtained are displayed in Figure 4.7.

	Solvents	Solubility limits [mg.mL -1 ]
		P3HT	PC 61 BM
	C-PYR	<0.002	31.4
	DPH	<0.002	2.2
	TRIB	<0.002	0.4

Table 4 .

 4 5: Photovoltaic properties of OSCs. For each processing additive, the table shows the photovoltaic properties of OSCs containing the processing additive at a concentration giving the best PCE.

	Processing additive Concentration	Voc [V]	FF	Jsc [mA.cm -2 ] PCE [%]
	No additive		0.63 ±0.01 0.55 ±0.02	4.4 ±0.4	1.5 ±0.1
	C-PYR	1.6 vol%	0.50 ±0.01 0.53 ±0.01	11.0 ±0.7	2.9 ±0.2
	DPH	0.8 vol%	0.53 ±0.01 0.57 ±0.02	10.4 ±0.5	3.2 ±0.2
	TRIB	1.6 vol%	0.54 ±0.01 0.56 ±0.03	8.2 ±0.6	2.5 ±0.1
	ODT	1.6 vol%	0.51 ±0.00 0.50 ±0.02	9.9 ±0.3	2.5 ±0.2
	DPH and C-PYR increase the PCE by respectively 113% and 93% in compar-
	ison to control devices devoid of processing additives and any post processing
	step, resulting in a PCE of 3.2% in case of DPH (at a concentration of 0.8 vol%)
	and 2.9% in case of C-PYR (at a concentration of 1.6 vol%) (Table 4.5). At a
	concentration of 1.6 vol%, the OSCs with ODT and TRIB exhibit efficiencies of

2.5% corresponding to an increase of 67%. The PCE of a control OSC is 1.5% and is in par with what is generally obtained for OSCs fabricated without any thermal or solvent annealing

Table 5

 5

	.1: Values of the FWHM of P3HT diffraction peak at 5.5°for different active
			layer compositions.
	Type of				FWHM (°)
	Processing	0 vol % 0.4 vol% 0.8 vol% 1.6 vol% 2.4 vol% 3.2 vol% 4 vol%
	C-PYR	0.49	0.42	0.49	0.61	0.71	0.79	0.81
	DPH	0.49	0.51	0.58	0.61	0.77	0.65	0.71
	ODT	0.49	0.51	0.6	0.63	0.64	0.72	0.71
	Thermally annealed	0.32

Table 6 .

 6 2: Mobility of holes and electrons as a function of the concentration of DPH.

	DPH ratio	N-TYPE	P-TYPE	µ h + /µ e -
	[vol%]	*10 -4 [cm 2 .V -1 .s -1 ] *10 -4 [cm 2 .V -1 .s -1 ]
	0	2.51 ±0.12	∼0	-
	0.4	0.95 ±0.19	∼0	-
	0.8	0.40 ±0.00	0.10 ±0.03	0.25
	1.6	0.35 ±0.12	0.93 ±0.04	2.63
	2.4	0.23 ±0.02	2.04 ±0.17	9.02

Table 7 .

 7 1: Initial electrical performances of OSCs with different types of active layer.

	Initial	Thermal	C-PYR	DPH	ODT
	values	annealing
	J sc	9.2	9.2	8.7	8.9
	[mA.cm -2 ]	±0.6	±0.4	±0.7	±0.6
	V oc	0.56	0.53	0.52	0.50
	[V]	±0.03	±0.02	±0.01	±0.02
	FF	0.58	0.62	0.66	0.59
		±0.03	±0.06	±0.02	±0.03
	PCE	2.6	3.0	3.0	2.6
	[%]	±0.4	±0.4	±0.3	±0.2
	R s	4.4	2.5	1.4	3.9
	[Ohm.cm 2 ]	±2.3	±1.7	±0.4	±1.3
	R sh	8008	8460	8356	7192
	[Ohm.cm 2 ]	±1771	±2268	±1680	±1987

Table 7 .

 7 2: Normalized electrical performance after light irradiation in air: 252 hours of light irradiation for OSCs with processing additives, 90 hours for thermally annealed OSCs.V oc . For thermally annealed OSCs, the V oc decreases significantly with increasing irradiation time, whereas ODT-treated OSCs demonstrate less than 15% loss inVoc after the total irradiation time. OSCs with C-PYR and DPH-treated active layers present intermediate stabilities as depicted in Figure 7.8.

	Normalized values [%]	Thermal annealing	C-PYR	DPH	ODT
	J sc	49.1	63.2	59.4	85.7
	V oc	16.7	55.3	48.3	81.5
	FF	22.3	33.2	13.0	51.8
	PCE	1.8	11.8	3.7	36.2
	R s	153.8	1168.8	2642.7	297.3
	R sh	6.4	34.1	60.7	77.0

Table 7 .

 7 3: Normalized photovoltaic parameters of OSCs after 393 hours of light irradiation in inert atmosphere.Overall, ODT-treated OSCs are the most photo-stable OSCs with a final PCE of 71.2% of the initial value while the PCE of thermally annealed OSCs is reduced to 32.0% of the initial value.

	Normalized value [%]	Thermal annealing	C-PYR	DPH	ODT
	J sc	75.4	66.8	67.9	78.9
	V oc	79.2	91.6	78.3	95.6
	FF	52.9	73.7	67.2	94.4
	PCE	32.0	44.9	36.0	71.2
	R s	128.2	145.1	402.2	119.1
	R sh	33.9	52.8	50.0	77.3

Table 7 .

 7 4: Decrease in optical density measured at 601 nm.

	Types of active layers Loss in optical density [%]
	Thermally annealed	11.0
	C-PYR	4.8
	DPH	7.0
	ODT	6.4

Table 7 .

 7 5: Normalized photovoltaic performance of C-PYR, DPH and ODT-treated OSCs and thermally annealed OSCs subjected to light irradiation in inert atmosphere for 40 hours.

			Normalized parameters [%]
	Types of OSCs	Jsc	Voc	FF	PCE	Rs	R sh
	Thermally annealed	86.3	93.3	88.6	82.8	164.4 111.1
	C-PYR	93.4	95.7	92.8	83.0	202.1	48.3
	DPH	100.8	92.3	83.6	78.5	333.6	58.2
	ODT	109.0 101.1 94.5 104.4 153.0	71.3

Table 7

 7

	.6 and

Table 7 .

 7 [START_REF] Aïch | Control of the active layer nanomorphology by using co-additives towards high-performance bulk heterojunction solar cells[END_REF] show the load loss values before and after degradation for thermally annealed and ODT-treated OSCs respectively. The values represent the average of two samples (except for the case of irradiated OSCs with ODT).

Table 7 .

 7 6: Force loss in the adhesion measurement test of thermally annealed samples.

			∆N [N]
		Sample 1 Sample 2 Average
	Fresh -TA	0.18	0.13	0.16
	Irradiated -TA	0.33	0.23	0.28
	Percentage increase		76.8 %

Table 7 .

 7 7: Force loss in the adhesion measurement test of ODT-treated samples.

			∆N [N]
		Sample 1 Sample 2 Average
	Fresh -ODT	0.86	0.59	0.73
	Irradiated -ODT	1.06	-	1.06 a
	Percentage increase		46.0 %

a The average takes into consideration the result of Sample 1 only.

Table 7 . 8

 78

		Thermally annealed 1.6 vol% of ODT
	Roughness [nm]	1.4	7.9

: Roughness of active layers with different processing additives.

Table 8 .

 8

	PDQT	18.28	4.11	3.07	3.2	0.860

1: HSPs of PDQT. δD [MPa 1/2 ] δP [MPa 1/2 ] δH [MPa 1/2 ] R O [MPa 1/2 ] FIT

Table 8 .

 8 3: Diffraction peaks and domain sizes of PDQT/PC 61 BM with various D/A ratios.Following the same approach as the one applied to the P3HT/PC 61 BM system, processing additives are selected under the conditions that they are good solvents for PC 61 BM and poor solvents for PDQT. Using the HSPs as a numerical tool to determine such solvents, ODT was found to have RED values with PDQT and PC 61 BM of respectively 1.32 and 0.68. Therefore, ODT is investigated as

	D/A system	2θ [°] Domain sizes [nm]
	Pure PDQT	4.47	14
	Ratio 1/1	4.45	13
	Ratio 1/2	4.45	14
	Ratio 1/3	4.53	13
	Ratio 1/4	4.39	14
	and PC 61 BM crystallites. The increase in PC 61 BM proportion also leads to a
	gradual increase in roughness: the roughness increases from 1.8 nm for pure
	PDQT to 4.0 nm in blends of PDQT/PC 61 BM prepared in a 1/4 ratio. The
	fact that the films present such a heterogeneous distribution of crystallite sizes
	suggests that PDQT and PC 61 BM aggregate independently, explaining the non
	dependence of PDQT crystallite sizes on the proportion of A.
	8.1.3 Effects of processing additives

Table 8 .

 8 4: Electrical performance of PDQT/PC 61 BM-OSCs with and without additive. ± 0.00 0.60 ± 0.02 2.1 ± 0.0 11.7 ± 0.3 1040 ± 241 5 vol% of ODT and DIO increase the PCE by respectively 26% and 46% in comparison to control devices devoid of processing additives, resulting in a PCE of 1.85% in case of ODT and 2.14% in the case of DIO. The main parameter responsible for the increase in PCE is the J sc , similarly to the case of P3HT/PC 61 BM -OSCs. The J sc increases with additive concentrations up to 5 vol% but decreases at higher concentrations. In contrast to the case of P3HT/PC 61 BM, no significant change in V oc is observed (except for OSCs fabricated with 11 vol% of ODT). This suggests that processing additives do not lead to significant change in the overall crystallinity, certainly because PDQT is already highly crystalline in PDQT/PC 61 BM-films devoid of processing additives.

	Type of	Jsc	Voc	FF	PCE	Rs	R sh
	additive	[mA.cm -2 ]	[V]		[%]	[Ohm.cm 2 ] [Ohm.cm 2 ]
	No additive	4.2 ± 0.3	0.58 ± 0.00 0.62 ± 0.00 1.5 ± 0.1 12.2 ± 0.3	905 ± 43
	ODT -5 vol%	5.8 ± 0.1	0.62 ± 0.00 0.52 ± 0.01 1.9 ± 0.1 10.1 ± 0.5	604 ± 49
	DIO -5 vol%	6.2 ± 0.2	0.58

Table 8 .

 8 5: HSPs of PCDTBT.

		δD	δP	δH	R O	FIT	RED
		[MPa 1/2 ] [MPa 1/2 ] [MPa 1/2 ] [MPa 1/2 ]		(with PC 61 BM)
	PCDTBT	19.10	3.50	5.19	5.90	0.98	0.30

Figure 8.12: Solubility spheres of PCDTBT (in blue) and PC 61 BM (in red) in the Hansen solubility space.

  30 MPa 1/2 . For comparison, PC 61 BM has a RED of 0.50 MPa 1/2 with P3HT and a RED of 0.55 MPa 1/2 with PDQT. These results suggest that PC 61 BM has a higher affinity with PCDTBT than with P3HT or PDQT, which is in line with the highly mixed interpenetrated network formed between PCDTBT and PC 61 BM.

Table 8 .

 8 7: Absorption maximum of PCDTBT/PC 61 BM films with or without processing additive.

		Control	DIO		ODT
		0	1 vol% 5 vol% 1 vol% 5 vol%
	Absorption maximum [nm]	568	576	582	572	578

Table 8 .

 8 8: Average electron mobility measured in a saturation regime at V GS -V th = 4 V.

	Formulations	Electron mobility
		x10 -3 [cm 2 .V -1 .s -1 ]
	Ratio 1/4	1.56 ± 0.29
	Ratio 1/3	1.13 ±0.09
	Ratio 1/2	0.48 ±0.08
	Ratio 1/2 in the ternary blend	4.96 ±0.45

Table 8

 8 .9.After two hours of thermal treatment, the PCE of the binary blend PCDTBT/PC 61 BM drops to 51.2% of the initial value. Ternary blend OSCs demonstrate a superior thermal stability with PCEs maintained at 83.0% and 94.2% of the initial PCEs

Table 8 .

 8 9: Normalized photovoltaic parameters of ternary blend OSCs after two hours of thermal treatment at 160 °C.

	C 60	Normalized parameters [%]
	fraction	J sc	V oc	FF	PCE
	0	76.9	89.3 74.5	51.2
	0.4	101.1 92.3 88.9	83.0
	0.7	112.9 94.7 88.1	94.2

Acknowledgements

Abbreviations

Chapter 4

Determination of selection rules for processing additives Chapter 5 The influence of processing additives on the formation of the bulk heterojunction Chapter 6 Effect of device architecture on OSCs prepared with additives Chapter 7 The effects of processing additives on the stability of OSCs The HOMO LUMO energy levels of PDQT are estimated to be respectively and 5.3 eV and 4.0 eV respectively [START_REF] Murphy | Master thesis: Influence of High Mobility Polymer Semiconductors in Organic Photovoltaics[END_REF]. Such a LUMO energy level leads to an energy difference of 0.3 eV with the LUMO of PC 61 BM and is therefore suitable for efficient charge transfer. The energy diagram of PDQT/PC 61 BM-based OSC is depicted in Figure 8.4.

Performance of OSCs based on PDQT/PC 61 BM

Prior to the fabrication of OSCs, the HSPs of PDQT are determined in order to verify its solubility in conventional solvents such as CB or ODCB. nm) [START_REF] Murphy | Organic photovoltaics with thick active layers (-800nm) using a high mobility polymer donor[END_REF]. As such active layers are usually obtained from low boiling point solvents, CB is chosen as the host solvent due to its low boiling point (51 °C below that of ODCB) in order to fabricate thick active layer OSC.

PDQT and PC 61 BM formulations are prepared with a total solid content of 30 mg.mL -1 in CB with various D/A ratio (from 1/1 to 1/4) and spin-cast at a spin speed of 800 rpm to form the active layer. Table 8.2 shows the photovoltaic performance of inverted OSCs prepared from various D/A ratios. OSCs with a D/A ratio of 3/1 exhibit a low PCE of 0.24%, mostly due to low J sc . This low J sc is likely to be due to charge transport issues also evident from the high R s . Overall, the PCEs of OSCs are similar for each D/A ratio, with the exception of the