Les composés organiques à base d'hétérocycles pyraniques peuvent être utilisé dans les cellules photovoltaïques ou dans des matériaux du domaine des télécommunications grâce à leurs propriétés en optique non linéaire. Plusieurs de ces composés ont été synthétisés, analysés et testés dans des cellules solaires à colorants. Des rendements photovoltaïques ont pu ainsi être obtenus et des modifications sur ses composés ont été effectuées afin d'obtenir une meilleure conversions de l'énergie solaire en courant électrique. / Organic compounds with pyranic heterocycles could be used in solar cells or in materials for telecommunications due tot their nonlinear optic properties. Several compounds synthesized were tested in dyes solar cells. The efficiencies of these dyes were obtained and the effect of structural modifications were studied with the goal to obtain a better efficiency.

The importance of renewable energy sources is becoming clearer and clearer as unsustainable energy sources are running out and global warming is getting worse. Energy derived from sunlight is already commonly used, but more energy can be produced from sunlight when solar cells become cheaper. Organic solar cells use organic compounds as semiconductors which can be prepared from solutions, resulting in lower production costs. However, these semiconductors;Poly(3-hexylthiophene) and [6,6]-Phenyl-C61-butric acid methyl ester, are commonly dissolved inhalogenated and aromatic solvents. These solvents have toxic properties, which is why alternative solvents should be identified. Potential solvents were predicted by using the Hansen solubilityparameters and thin films were prepared by spin-coating the solutions. The thin films were evaluated with absorption spectroscopy, the fluorescence spectroscopy and atomic force microscopy. Since the alternative solvents had lower solubilites than the commonly used solvents, i.e., chlorobenzene, ortho-dichlorobenzene and chloroform, the absorption of the films with new solvents were lower as well. Using tetrahydrofuran resulted in the highest absorption of the used solvents, while xylene hada better film morphology. Increasing the absorption was attempted by spin-coating multiple thin films on top each other or by using a lower rotational speed. Spin-coating multiple films had nouniform effect on the absorption, while lowering the rotational speed increased the absorption, but not enough to equal the original absorption. Contrasting the results of the absorption spectra, the morphology improved when multiple layers were used while the film with the lower rotational speed’s morphology got worse. In conclusion, tetrahydrofuran and xylene are the best alternative solvents and using multiple layers as well as decreasing the rotational speed show improvements oneither the morphology or the absorption.

Le développement de la technologie des cellules photovoltaïques organiques nécessite des compétences diverses liées à l’ingénierie moléculaire, à l’ingénierie des interfaces, au contrôle et à la caractérisation de la morphologie des films, à l’optimisation de la structure du dispositif et à la compréhension de la photo-physique des matériaux utilisés. Dans ce contexte, le travail présenté dans cette thèse contribue à la compréhension des propriétés photo-physiques des matériaux organiques π-conjugués et propose des outils de caractérisations optiques pour le suivi de la morphologie de ces matériaux. Dans un premier temps, une méthodologie rigoureuse de détermination des indices optiques des films organiques par ellipsométrie a été proposée. Les modèles utilisés en ellipsométrie ont ainsi été choisis en tenant compte des propriétés physiques des matériaux organiques π-conjugués ce qui a permis de remonter à la structure électronique de dérivés de fullèrenes (PC60BM et PC70BM). Dans un second temps, nous avons associé des données ellipsométriques à des mesures complémentaires d’absorbance et de photoluminescence dans le cas de deux copolymères (PTB7 et PTB7-Th) en films et en solutions afin d’isoler les interactions inter et intra-chaînes. Nous avons démontré que la photo-physique de ces copolymères diffère de celle du P3HT. Nous avons montré que même en solution dans le chlorobenzène, le PTB7 et le PTB7-Th s'agrègent fortement. Ces agrégats, de type H, se cassent plus facilement dans les solutions de chlorobenzène à base de PTB7-Th que dans celles à base de PTB7. / The development of organic photovoltaic cell technology requires various skills related to the molecular engineering, interface engineering, controlling and characterizing the morphology of the films, device structure optimization and understanding of photophysics of the materials. In this context, the work presented in this thesis contributes to the understanding of the photophysical properties of π-conjugated organic materials and propose optical characterizations tools for probing the morphology of these materials. First, a rigorous methodology for determining refractive indices of organic films by ellipsometry has been proposed. The models used in ellipsometry have been chosen by taking into account the physical properties of π-conjugated organic materials which allow the determination of the electronic structure of fullerene derivatives (PC60BM and PC70BM). Secondly, we associated ellipsometric data to complementary measurements of absorbance and photoluminescence in the case of two copolymers (PTB7 and PTB7-Th) in films and solutions in order to isolate inter and intra-chain interactions. We have demonstrated that the photophysics of these copolymers differs from the P3HT. We have shown that even in solution in chlorobenzene, the PTB7 PTB7-Th aggregate strongly. These aggregates, H-type, break more easily in the chlorobenzene solutions based of PTB7-Th as in those based on PTB7.

>Magister Scientiae - MSc / In a quest for low operational and maintenance cost solar cell devices, organic photovoltaics remain a potential source of energy worthy to be explored. In order to generate cost- effective electricity from solar energy, either the efficiency of the solar cells must be improved or alternatively the manufacturing cost must be lowered. The power conversion efficiency (PCE) of organic photovoltaics is influenced by the choice of electron acceptor material, the structure of the polymer, the morphology of the film, the interfaces between the layers and the ratio between the electron acceptor material and the polymer. Nevertheless, efficiency is still limited compared to conventional silicon based PV cells due to low mobility of charge carriers with a short exciton diffusion length in the active layer. Currently, hybrid solar cells have been considered as one of the most promising concepts to address the limited efficiency of organic solar cells. Therefore in this thesis ZnO nanoparticles were synthesized using hydrothermal assisted method. These nanoparticles were incorporated in the poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and used as additional acceptors of electrons released from the polymer donor material, with the anticipation to increase the electron mobility, and ultimately the PCE. The thermo-gravimetric analyses revealed improved thermal stability of P3HT upon incorporating ZnO in the polymer matrix. X-ray diffraction analyses revealed that the diffraction peaks shift to higher angles when incorporating the ZnO in the P3HT:PCBM surface and this is consistent with the Raman observation. The photovoltaic properties demonstrated that the addition of ZnO nanoparticles in P3HT:PCBM bulk-heterojunction increases PCE from a baseline of ∼1.0 % in the P3HT:PCBM system to 1.7% in the P3HT:PCBM:ZnO ternary system. The enhanced PCE was due to improved absorption as compared to its counterparts. Upon increasing the addition of ZnO nanoparticles in the P3HT:PCBM matrix, the PCE decreases, due to a large phase separation between the polymer, PCBM and ZnO induced by ZnO agglomerations which resulted in increased surface roughness of the active layer. These findings signify that incorporation of ZnO nanostructures in the P3HT:PCBM polymer matrix facilitates the electron transport in the photoactive layer which results to improved efficiency.

Le kësterite Cu 2 ZnSnS 4 (CZTS) est un semi-conducteur de type p composé d'éléments abondants et non toxiques. Ces atouts, en plus d’un gap direct, compris entre 1,45 et 1,5 eV, en font un excellent candidat pour remplacer les matériaux Cu(In,Ga)Se 2 et CdTe utilisés dans les photopiles en couches minces. Il a cependant été mis en évidence que les performances des photopiles utilisant CZTS comme absorbeur souffrent de la présence de phases secondaires dans les couches minces. Ainsi le travail présenté dans cette thèse décrit le dépôt de couches minces de CZTS par un procédé simple et à faible coût appelé Close SpacedVapor Transport(CSVT). Son objectif essentiel est de réaliser un composé CZTS dépourvu de toute phase secondaire en vue d’améliorer les rendements de conversion des cellules photovoltaïques à base de CZTS. Pour cela, le matériau massif a d’abord été synthétisé sous forme de lingot par le refroidissement lent et programmé d’un bain fondu obtenu à partir d'éléments purs. Les caractérisations réalisées sur le massif montrent qu’il s’agit d’un composé monophasé Cu 2 ZnSnS 4 , de composition quasi-stœchiométrique, dans la structure kësterite. Le lingot broyé et mis sous forme de pastille, est utilisé par la suite comme source à évaporer dans un réacteurCSVT utilisant de l’iode comme agent de transport, pour la formation de couches minces CZTS. L'optimisation des paramètres clés de dépôt des couches minces que sont la température du substrat et la pression d'iode a été effectuée. Les résultats d'analyses menées sur les couches de CZTS déposées à des températures de substrat comprises entre 460 à 500 °C, sous des pressions d'iode de 2 Kpa à 4 kPa, ont révélés d’excellentes propriétés physico-chimiques. / The kësterite Cu 2 ZnSnS 4 (CZTS) is a p-type semiconductor material made from abundant and nontoxic chemical elements. These advantages in addition to a direct band gap, with energy between 1.45eV and 1.5 eV, make it an excellent candidate for replacement of Cu(In, Ga)Se 2 and CdTe absorber layers currently used in thin film solar cells. It has although been highlighted that photovoltaic devices based on thin CZTS absorber layers are highly suffering from the presence of secondary phases in the thin films. So the work presented in this thesis describes thin CZTS layers deposition by a simple and low-cost process called Close Spaced Vapor Transport (CSVT). Its main objective is to realize a CZTS compound free of any secondary phase with the aim of improving conversion efficiencies of CZTS thin films based photovoltaic solar cells. For this purpose, the bulk CZTS material was first synthesized in the form of ingot by a slow cooling of a molten stoichiometric mixture of pure elements. Characterizations realized on this bulk material showed that it relates to a single phase, quasi-stoichiometric Cu 2 ZnSnS 4 compound in the kësterite structure. The ingot was milled into powders and pressed to give 1 mm thick pellets. These pellets were therefore used as evaporating sources in a CSVT reactor with iodine as transport agent, for the thin CZTS layers deposition. Optimizations of the key deposition parameters that are substrate temperature and iodine pressure were performed. The Results of the investigations conducted on the CZTS layersdeposited at substrate temperature in the range 460 °C-500 °C, under iodine pressure in the order of 2 kPa to 4 kPa, revealed excellent physico-chemical properties.

No description available.

Dans cette thèse, la croissance des couches minces de Cu(In,Ga)Se2 (CIGS) a été optimisée et étudiée systématiquement. Une étude de calibration de la température du substrat à l'aide d'une caméra infrarouge a été effectuée. La mise au point et l'optimisation d'un procédé en 3 étapes sur un nouveau réacteur de co-évaporation a permis la réalisation de cellules solaires avec un rendement de 16,7 % sans couche antireflet. La clé de ce développement a été le contrôle du gradient de Ga. Les inhomogénéités ont été caractérisées par une nouvelle approche basée sur le décapage chimique de l'absorbeur. Des caractérisations ex situ à différentes étapes de la croissance ont révélé l'importance des phases intermédiaires sur les mécanismes de croissance, le gradient de composition en profondeur et la morphologie des couches. L'interface absorbeur/couche tampon a été étudiée en variant la composition en surface du CIGS pour des couches tampons de CdS et Zn(S,O). Il a été montré qu'une adaptation de la composition en surface est favorable pour le remplacement de la couche tampon de CdS par Zn(S,O). Des rendements équivalents ont été obtenus pour ces deux matériaux si ils sont combinés avec la composition da Ga optimale correspondante. Des mesures courant-tension à basse température indiquent une position de la bande de condition plus basse que celle trouvée dans la littérature. Pour une optimisation ultérieure de nos cellules solaires vers et au-delà de 20 % de rendement, trois axes sont proposées : L'optimisation de la finalisation de l'absorbeur, la réduction de l'absorption par la couche tampon et l'incorporation de potassium ayant des effets positifs sur les propriétés du CIGS. / In this thesis the growth of Cu(In,Ga)Se2 (CIGS) thin films by co-evaporation has been optimised and studied systematically. Being a key parameter, the substrate temperature has been calibrated with an infrared camera. The set-up and optimisation of a three-stage process at a new co-evaporation reactor has led to cell efficiencies up to 16.7 % without anti-reflection coating. The key for this achievement was the control of the Ga gradient. In depth inhomogeneities have been characterised by a novel method based on chemical etching of the absorber layer. Break-off experiments during the 3-stage process unveiled the importance of precursor and intermediate phases on growth mechanisms, in-depth compositional gradients and film morphology. The absorber/buffer layer interface has been investigated by varying the CIGS surface composition for solar cells both with a CdS and a Zn(O,S)-based buffer layer. It has been shown that an adaptation of the CIGS surface composition is beneficial for the replacement of the CdS by a Zn(O,S) buffer layer. Equivalent efficiencies can be achieved with the two buffer layers if each of them is combined with the corresponding optimal interface Ga composition. Low temperature current-voltage measurements indicate a lower conduction band offset at the CIGS/Zn(O,S) buffer layer as reported in the literature. For the further optimisation of our CIGS devices towards 20 % and beyond three routes are proposed: the optimisation of the absorber layer deposition finalisation, the reduction of detrimental absorption in the buffer layer (larger band gap or thinner buffer) and the incorporation of potassium which has beneficial effects on CIGS.

M.Sc. (Physics) / Amorhous silicon is one of the most promising materials for large area solar cells for terestrial photovoltaic applications. Unfortunately these cells suffer from two serious problems: the efficiencies drop when laboratory processes are scaled up and the cells degrade after some exposure to sunlight. The exact causes of these two problems are still unknown. In this project some aspects of the above two problems where investigated. The drop in efficiency due to scaling up of laboratory processes can be ascribed to macroscopic inhomogeneities in the film. An investigation was done by changing the chamber geometry and gas flow pattern to establish empirical conditions to obtain films with maximum macroscopic homogeneity. It was found that a uniform electric field above the substrate was the most important factor determining the macroscopic homogeneity of the film. The hydronamic gas flow pattern was of secondary importance. Some techniques to obtain a uniform electric field has been devised. The photo-degradation was investigated by illuminating films of o-Si.H with simulated sunlight for different lenghts of time. The change in the electrical and optical properties of the intrinsic films were determined as function of total photon flux. No change in the optical properties could be detected. The effect of the photo-degradation manifests itself in a drop in the the dark conductivity and photoconductivity. The observed phenomena is explained in terms of photo-induced deep levels in the gap. The Fermi level shifts to the middle of the gap due to these defect states, causing a drop in the free carrier concentration and conductivity. These defect levels increase the absorptiom coefficient in the long wavelength region, but they also decrease the lifetime of the photo-generated carriers. The photo-induced defects were investigated with the CPM-technique. A large part of this project involved the construction and commissioning of the CPM-apparatus. It was found that the light introduced two types of defects at energies 0.5 eV and 0.75 eV below the conduction band edge. The concentration of the defects increases with illumination, but saturates after about 24 hours of illumination. The defects could almost completely be annealed at ISOaC. The photo-degradation of o-Si.H solar cells is ascribed to the reduction in the carrier lifetimes of photo-generated carriers due to recombination at these defect centers.

Ce travail de thèse a été consacré à l’étude de la stabilité dans différents milieux des matériaux constitutifs de la couche active des cellules solaires organiques. Les deux objectifs principaux étaient premièrement d’approfondir la compréhension sur certains mécanismes complexes intervenant dans la couche active, et deuxièmement d’étudier les processus de dégradation intervenant spécifiquement aux interfaces de la couche active au sein de la cellule. La première partie de ce mémoire est consacrée à l’étude de la dégradation photochimique et thermique de la couche active des cellules solaires en faisant varier le matériau polymère qui la constitue. La deuxième partie est dédiée au rôle d’un troisième composant de la couche active que peut être la présence d’additifs résiduels provenant de la mise en forme, ou d’un additif stabilisant ajouté à dessein. La troisième partie est consacrée au processus de délamination susceptible d’intervenir à l’interface couche active / couche transporteuse de trous. Enfin, une dernière partie concerne l’étude de l’alignement des niveaux énergétiques entre la molécule de C60 et divers substrats transporteurs d’électrons. L’influence de la lumière et de la température sur les propriétés d’interface couche active / couche transporteuse d’électrons est également reportée. / This word was devoted to the stability in various conditions of materials used in the active layer of organic solar cells. The main goals of this work were first to provide deeper understanding about complex mechanisms occurring in the active layer and second to investigate interfacial degradation pathways involving the active layer. A first part was dedicated to the photo and thermal stability of the polymer blend materials which constitute the active layer of the solar cells. A second section focused on the role of the third component of the active layer which can be undesired residual additives coming from the processing or the desired insertion of a stabilizer additive. A third part concerned the delamination issue which takes place at the active layer / hole transporting layer interface. Finally, a last section was devoted to the energy level alignment between the C60 molecules and various electron transporting substrates. The photo and thermal stability of the active layer / electron transporting layer interface was also studied in this section.

Dye sensitized solar cells (DSSCs) are an attractive way to convert light into electricity. Its development requires a detailed understanding and kinetic optimization of various electronic processes, especially those occurring at the mesoporous TiO2/dye/electrolyte interface. This dissertation work is focused on the exploration of the various electronic processes at the sensitized-electrode/electrolyte interface by using various electrochemical and photochemical methods. Firstly, an alternative redox couple—TEMPO/TEMPO·+ with a relatively high positive redox potential—is explored, aiming to reduce the energy loss during the dye regeneration process. Despite the fast dye regeneration, the charge recombination between the electrons in the conduction band of mesoporous TiO2 and the oxidized redox species is found to be the limiting factor of the device. Further, a more efficient tandem-electrolyte system is developed, leading to DSSCs with the power conversion efficiency of 10.5 % and 11.7 % at 1 sun and 0.5-sun illumination, respectively. An electron-transfer cascade process during dye regeneration by the redox mediators is discovered to be beneficial. Further stability studies on the device suggest the crucial role of TiO2/dye/electrolyte interfaces in the long-term stability of cobalt bipyridyl electrolyte-based DSSCs. On the fundamental level, the local electric field and Stark effects at the TiO2/dye/electrolyte interface are investigated in various aspects—including the charge compensation mechanism, the factors affecting the electric field strength, as well as its impact on charge transfer kinetics. These results give further insights about the TiO2/dye/electrolyte interface, and contribute to the further development and understanding of DSSCs.