In the recent years area organic photovoltaics is generating a lot of interests because whole process of synthesis and fabrication is less energy intensive process as well as it is cost effective compared to conventional inorganic Si based photovoltaic technology. This work mainly deals with the fabrication and optimization of device fabrication conditions for organic photovoltaic materials. In first part of the work, the solar cell fabrication conditions were optimized for the commonly used system P3HT – PCBM. The fabricated device was optimized for the solvents used for the active material, concentration of the active material solution, donor- acceptor ratio of the active material, annealing conditions of the active layer and the metal evaporation conditions for the cathode. All the optimization procedures were carried out in controlled atmosphere to minimize the environmental effect inference during fabrication of the solar cell devices. All the characterization was carried out at ambient conditions. The efficiency of the solar cell was improved from 0.009% to 6.2%. the environmental stability of the fabricated devices were carried out after encapsulating it with epoxy based resin in both ambient conditions as well as extreme conditions like 85% RH at 25°C inside the humidity chamber. It was observed that both the data matches well with each other indicating proper encapsulation required to safe guard the device for the better performance over the period of time. Second part of this work mainly deals with understanding the structure property relationship of thiophene based donor- acceptor- donor molecule 2,5-dithienyl-3,4-(1,8-naphthylene) cyclopentadienone (DTCPA), which is highly crystalline, low band gap organic molecule which absorbs over entire visible region of the solar spectra. DTCPA crystals of various morphologies were prepared by various recrystallization routes. It was observed that macro scale morphology of these crystals differs from each other. Also depending on the method of recrystallization sizes of the crystals also varies. All the recrystallized DTCPA shows strong orientation toward (001) direction. However, it was observed that lattice parameters of these crystals slightly differ from each other owing to the recrystallization methodology. These variations in crystal parameters are more than 0.02 which is significant. It was also observed that the crystallite sizes depend on the recrystallization routes. Slow evaporation of concentrated solution (SEC) grown crystals has the larger crystallite size of 170nm. It was observed that absorption range of these crystals slightly differ from each other owing to the change in the crystallite sizes and crystal parameters. Third part of this work deals with the fabrication and optimization of thermal evaporation process of DTCPA for photovoltaic applications. DTCPA is stable at higher temperatures as well as has sharp melting point which make it ideal candidate for thermal evaporation. In this work films of DTCPA were fabricated for various evaporation rates by thermal evaporation technique. Chemical integrity of the molecules upon evaporation is found to be intact as observed from FTIR spectroscopy. XRD shows that at lower (25 W/m2) as well as higher (40 W/m2) films are oriented to (001), (400) as well as (311) directions, at 30 W/m2 and 35 W/m2 there is a strong orientation towards (311) and (001) directions respectively. Photo luminescence studies indicate that there is strong 410 nm emission for films deposited at the power of 25 W/m2 and 40 W/m2. Microscopic studies confirm that morphology is dependent on the deposition rates as it changes with the change in deposition rate. This in turn reflects in the device characteristics of these films. It was observed that films deposited at high deposition rates show better device characteristics with high VOC and current density values. All these device fabrication and characterizations were carried out in ambient conditions. Fourth part of this work deals with P3HT - DTCPA composites which exhibit wide range of light absorption. It was observed that DTCPA act as nucleating centers for the P3HT molecules and increases crystallinity in the composite. Furthermore, DTCPA helps in exciton separation because of donor and acceptor moieties present in the molecule. It also helps in charge transportation because of its crystalline nature and further it induces molecular ordering in the P3HT matrix. The band diagram of P3HT- DTCPA suggests that the band edges of both materials are ideal for charge separation. In addition, crystalline nature of the DTCPA molecule helps in effective charge transportation. J-V characteristics shows that there is large built in potential in the devices from these blends leading to large Voc. Composites with lower DTCPA loadings show higher efficiency than with higher loadings. These devices were prepared in ambient conditions and needs to be optimized for obtaining better device properties. In the fifth part of the work two types of system were studied to understand the band edge matching on the photovoltaic properties, carbazole based copolymers and DTCPA based copolymers. In the case of carbazole based copolymers it was observed that by copolymerizing carbazole with thiophene based derivatives lowers the band gap and modifies the HOMO and LUMO levels for better suit for the photovoltaic device fabrication. It was observed that that is two orders of improvements in the efficiency by co polymerizing carbazole with benzothiodizole as improves the JSC and VOC. Also the copolymerization of carbazole with both benzothiodiazole and bithiophene results in better light harvesting as the optical band gap was lowered. In the case of DTCPA copolymers with DTBT and DHTBT as both are random copolymers the solubility was low as well as their HOMO band edge was mismatched with the PEDOT: PSS which is a hole transport layer. However, the alternate polymerization of DTCPA with DTBT improved the band edge matching and also the solubility. As a result there was tenfold improvement in the charge collection and hence the efficiency was improved from 0.02% to 2.4%. Many of the conducting polymers have good material property but poor filmability. In the sixth part of this work deals with fabrication of device quality films by alternate deposition technique like pulsed laser deposition. Two types of system were studied in this work (i) polypyrrole- MWCNT nanocomposites and (ii) Poly DTCPA polymer. In both the cases it was observed that chemical integrity of the polymer retained during ablation. PolyDTCPA films were fabricated by pulsed laser deposition by both IR (Nd-YAG) and UV (KrF) laser source. Morphological studies indicate that IR laser ablated films were particulate in nature whereas UV laser ablated films were grown as continuous layers as polyDTCPA absorbs better in UV region. As a result the IV characteristics indicate that IR laser ablated films are resistive in nature and UV laser ablated films are good rectifiers indicating the suitability of the process for fabrication of device quality films.

The present thesis deals with a theoretical study of electronic structures in -conjugated molecular materials with focus on their application in organic elec-tronics. We also discuss a modified and efficient symmetrized DMRG algorithm for studying excited states in these systems. In recent times, organic conjugated systems have emerged as potential candidates in a wide range of fascinating fields by virtue of their tunable electronic properties, easy processability and low cost. Tunability in the electronic and optical properties primarily are centered on the or-dering and nature of the low-lying excited states. Probing these important excited states also demands development of efficient and adaptable techniques. Chapter 1 provides a basic overview of conjugated organic polymers which have been utilized over decades in diverse fields as in organic light emitting diodes (OLED), organic solar cells (OSC) and non-linear optical (NLO) devices. These systems also contribute significantly to theoretical understanding as they pro vide important insights of one and quasi-one dimensional systems. In this chapter, we have given basic description of the electronic processes in OLED and OSC along with a brief theoretical description of -conjugated organic systems. Chapter 2 gives an account of the numerical techniques which are necessary for the study of low-dimensional strongly correlated systems like -conjugated sys-tems. For this purpose, effective low-energy model Hamiltonians viz. Huckel,¨ Hubbard and Pariser-Parr-Pople Hamiltonians are discussed. Exact diagonalization technique within the diagrammatic valence bond (DVB) basis and density matrix renormalization group (DMRG) technique are discussed in details. We have also given brief accounts of the methods employed to study real-time dynamics. A short description of different computational techniques for the study of NLO properties in -conjugated systems is also provided. Engineering the position of the lowest triplet state (T1) relative to the first excited singlet state (S1) is of great importance in improving the efficiencies of organic light emitting diodes and organic photovoltaic cells. In chapter 3, we have carried out model exact calculations of substituted polyene chains to understand the fac-tors that affect the energy gap between S1 and T1. The factors studied are backbone dimerization, different donor-acceptor substitutions and twisted backbone geome-try. The largest system studied is an eighteen carbon polyene which spans a Hilbert space of about 991 million in the triplet subspace. We show that for reverse inter-system crossing (RISC) process, the best choice involves substituting all carbon sites on one half of the polyene with donors and the other half with acceptors. Singlet fission (SF) is a potential pathway for significant enhancement of efficiency in OSC. In chapter 4, we study singlet fission in a pair of polyene molecules in two different stacking arrangements employing exact many-body wave packet dy-namics. In the non-interacting model, SF is absent. The individual molecules are treated within Hubbard and Pariser-Parr-Pople (PPP) models and the interac-tion between them involves transfer terms, intersite electron repulsions and site-charge—bond-charge repulsion terms. Initial wave packet is construc ted from ex-cited singlet state of one molecule and ground state of the other. Time develop-ment of this wave packet under the influence of intermolecular interactions is fol-lowed within the Schrodinger¨ picture by an efficient predictor-corrector scheme. In unsubstituted Hubbard and PPP chains, 21A state leads to significant SF yield while the 11B state gives negligible fission yield. On substitution by donor-acceptor groups of moderate strength, the lowest excited state will have sufficient 2 1A char-acter and hence gives significant SF yield. Because of rapid internal c onversion, the nature of the lowest excited singlet will determine the SF contribution to OSC effi - ciency. Furthermore, we find the fission yield depends considerably on th e stacking arrangement of the polyene molecules. In chapter 5, we have given an account of a new modified algorithm for symmetry adaptation within symmetrized density matrix renormalization group (SDMRG) technique. SDMRG technique has been an efficient method for studying low-lying eigenstates in one and quasi-one dimensional electronic systems. However, SDMRG method until now, had bottlenecks involving construction of linearly in-dependent symmetry adapted basis states as the symmetry matrices in the DMRG basis were not sparse. Our modified algorithm overcomes this bottleneck. T he new method incorporates end-to-end interchange symmetry (C2), electron-hole symmetry (J) and parity or spin-flip symmetry (P) in these calculations. The one-to-one correspondence between direct-product basis states in the DMRG Hilbert space for these symmetry operations renders the symmetry matrices in the new ba-sis with maximum sparseness, just one non-zero matrix element per row. Using methods similar to those employed in exact diagonalization technique for Pariser-Parr-Pople (PPP) models, developed in the eighties, it is possible to construct or-thogonal SDMRG basis states while bypassing the slow step of Gram-Schmidt orthonormalization procedure. The method together with the PPP model which incorporates long-range electronic correlations is employed to study the correlated excited states of 1,12-benzoperylene. In chapter 6, we have studied the correlated excited states of coronene and ova-lene within Pariser-Parr-Pople (PPP) model employing symmetry adapted density matrix renormalization group technique. These polynuclear aromatic hydrocar-bons can be considered as graphene nanoflakes and study of their ele ctronic struc-tures will shed light on the electron correlation effects in these finite-size gr aphene analogues. The electron correlation effect usually diminishes on going from one-dimensional to higher-dimensional systems, yet, it is significant within these fin ite-size graphene derivatives where it depends on the molecular topology. We have characterized these low-lying energy states by calculating bond orders, spin den-sities in the lowest triplet state and two-photon absorption cross-sections for low-lying two-photon states. vi

La lumière du soleil est un bon candidat comme source propre et abondante d'énergie renouvelable. Cette source d'énergie écocompatible peut être exploitée pour répondre aux besoins croissants en énergie du monde. Plusieurs générations de cellules photovoltaïques ont été utilisées pour convertir directement la lumière solaire en énergie électrique. La troisième génération de type multijonction des cellules photovoltaïques est caractérisée par un niveau d'efficacité plus élevé que celui de tous les autres types de cellules photovoltaïques. Des dispositifs optiques, tels que des concentrateurs optiques, des séparateurs optiques et des dispositifs optiques réalisant simultanément la séparation du spectre et la concentration du faisceau ont été utilisés dans des systèmes de cellules solaires. Récemment, les Eléments Optiques Diffractifs (EOD) font l'objet d'un intérêt soutenu en vue de leur utilisation dans la conception de systèmes optiques appliqués aux cellules photovoltaïques. Cette thèse est consacrée à la conception d'un EOD qui peut réaliser simultanément la séparation du spectre et la concentration du faisceau pour des cellules photovoltaïques de type multijonction latéral ou similaire. Les EOD qui ont été conçus ont une structure sous-longueur d'onde et fonctionnent en espace lointain pour implanter la double fonction séparation du spectre et concentration du faisceau. Pour cette raison, des outils de simulation ont été développés pour simuler le comportement du champ magnétique à l'intérieur de l'EOD à structure sous-longueur d'onde. De plus, un propagateur hybride rigoureux a aussi été développé, il est basé sur les deux théories de la diffraction, à savoir la théorie scalaire et la théorie rigoureuse. La méthode FDTD (Finite Difference Time Domain) ou méthode de différences finies dans le domaine temporel a été utilisée pour modéliser la propagation du champ magnétique en champ proche c'est-à-dire à l'intérieur et autour de l'EOD. La méthode ASM (Angular Spectrum Method) ou méthode à spectre angulaire a été utilisée pour modéliser de façon rigoureuse la propagation libre en champ lointain. Deux EOD différents ont été développés permettant d'implanter les fonctions souhaitées (séparation du spectre et concentration du faisceau) ; il s'agit d'une part d'un composant diffractif intitulé G-Fresnel (Grating and Fresnel lens) qui combine un réseau avec une lentille de Fresnel et d'autre part d'une lentille hors-axe. Les composants proposés réalisent la séparation du spectre en deux bandes pour une plage visible-proche infrarouge du spectre solaire. Ces deux bandes peuvent être absorbées et converties en énergie électrique par deux cellules photovoltaïques différentes et disposées latéralement par rapport à l'axe du système. Ces dispositifs permettent d'obtenir un faible facteur de concentration et une efficacité de diffraction théorique d'environ 70 % pour les deux bandes séparées. Grâce à une distance de focalisation faible, ces composants peuvent être intégrés dans des systèmes compacts de cellules solaires. La validation expérimentale du prototype fabriqué montre une bonne correspondance entre les performances expérimentales et le modèle théorique / Sunlight represents a good candidate for an abundant and clean source of renewable energy. This environmentally friendly energy source can be exploited to provide an answer to the increasing requirement of energy from the world. Several generations of photovoltaic cells have been successively used to convert sunlight directly into electrical energy. Third generation multijunction PV cells are characterized by the highest level of efficiency between all types of PV cells. Optical devices have been used in solar cell systems such as optical concentrators, optical splitters, and hybrid optical devices that achieve Spectrum Splitting and Beam Concentration (SSBC) simultaneously. Recently, diffractive optical elements (DOE’s) have attracted more attention for their smart use it in the design of optical devices for PV cells applications.This thesis was allocated to design a DOE that can achieve the SSBC functions for the benefit of the lateral multijunction PV cells or similar. The desired design DOE's have a subwavelength structure and operate in the far field to implement the target functions (i.e. SSBC). Therefore, some modelling tools have been developed which can be used to simulate the electromagnetic field behavior inside a specific DOE structure, in the range of subwavelength features. Furthermore, a rigorous hybrid propagator is developed that is based on both major diffraction theories (i.e. rigorous and scalar diffraction theory). The FDTD method was used to model the propagation of the electromagnetic field in the near field, i.e. inside and around a DOE, and the ASM method was used to model rigorously propagation in the free space far field.The proposed device required to implement the intended functions is based on two different DOE’s components; a G-Fresnel (i.e. Grating and Fresnel lens), and an off-axis lens. The proposed devices achieve the spectrum splitting for a Vis-NIR range of the solar spectrum into two bands. These two bands can be absorbed and converted into electrical energy by two different PV cells, which are laterally arranged. These devices are able to implement a low concentration factor of “concentrator PV cell systems”. These devices also allow achieving theoretically around 70 % of optical diffraction efficiency for the both separated bands. The impact distance is very small for the devices proposed, which allows the possibility to integrate these devices into compact solar cell systems. The experimental validation of the fabricated prototype appears to provide a good matching of the experimental performance with the theoretical model.

The main goal of this thesis was to simulate a solar cell with the symbolic manipulation tool Maple and discuss the strength and weaknesses of using Maple instead of the already known simulation program PC1D. This was done mainly by solving the three essential differential equations governing the current density and excess electron and hole densities in the solar cell. This could be done easily by using known simplifications especially the low injection assumption. However it was also a success without using this particular simplification but the solutions had to be achieved using a numerical method instead of direct methods. The results were confirmed by setting up the same solar cell with PC1D. The conclusion is that Maple gives the user increased freedom when setting up the solar cell, however PC1D is easier to use if this freedom is not needed. At the end of this thesis a brief introduction is also made on the possibility of using Maple with a tandem cell setup instead of single junction.

Ce document présente notre travail sur la modélisation en formalisme des fonctions de Green (abrégé formalisme de Green) du transport quantique et de l’intéraction éléctron-photon dans une cellule photovoltaïque composée d’une boîte quantique connectée à deux nanofils semi-infinis, La simulation numérique a été réalisée sur le cluster de calculs MERLIN (IM2NP). Nous présentons le formalisme de Green en général puis appliqué à cellule. Le fonctionnement général de la cellule est déduit de son diagramme de bande qui comporte des contacts sélectifs. Ensuite, nous présentons les résultats obtenus dans l’approximation de bande plate, qui simplifie le contact aux nanofils. Ceux-ci mettent en lumière des effets intriqués du couplage tunnel (couplage entre la boîte et les nanofils) et du couplage optique (couplage avec la lumière). Nous présentons ensuite un calcul analytique effectué dans le régime de fort couplage tunnel et qui explique le fonctionnement contre-intuitif du couplage tunnel dans ce régime. Nous observons également une transition dans le processus de production du courant entre le régime de fort couplage tunnel et le régime de fort couplage optique. Ensuite, nous sortons de l’approximation de bande plate et découvrons que les effets contre-intuitifs sont toujours valides, même si le modèle analytique lui ne l’est plus. Nous présentons le nouvel effet induit par la nouvelle forme du couplage aux réservoirs hors de l’approximation de bande plate: la courbe courant-tension présente une conductance de shunt négative. Cela n’a jamais été observé dans une cellule photovoltaïque auparavant. Enfin, nous présentons une réalisation possible de notre cellule. / This document present our work on the modeling of quantum transport coupled to electron-photon interaction in a solar cell composed of one quantum dot connected to two semi-infinite quantum wires. The proposed cell based on a dot in a wire, is a concept imagined in order to investigate quantum effects inside 1D structures in contact with 0D ones. The numerical simulation powered on the Merlin cluster (IM2NP) relies on Green’s function formalism. The philosophy of Green’s function formalism is introduced and then applied to the photovoltaic cell. An overview of the functioning of the cell is given. Results on the cell are presented in the wide band limit (approximation that simplifies the contact to wires). We observe an interlinked impact of the tunneling coupling (dot-wires coupling) and the optical coupling (to light) on the current. In the strong tunneling regime, an increase of the tunneling coupling decreases the current and similarly in the strong optical coupling regime, an increase of the optical coupling decreases the current. We investigate the counter-intuitive impact of the tunneling coupling in the strong tunneling regime through analytical calculations, considering only the first loop of the numerical code instead of the whole self-consistent process. We observe a transition in the current creation process while switching from the strong tunneling regime to the strong optical coupling regime. Results on the cell beyond the wide band limit approximation are presented in which the system exhibits another atypical response to illumination: I-V curve exhibits a negative shunt conductance! Finally, a realization proposal for the concept cell is described.

L’optimisation de la morphologie de la couche active est primordiale pour l’augmentation des rendements des cellules solaires photovoltaïques organiques. Nous avons montré l’influence du ratio de matériaux donneur (P3HT) et accepteur (PCBM) d’électrons ainsi que de la masse molaire du P3HT sur la morphologie de la couche active. Afin de contrôler la séparation de phases entre les matériaux donneur et accepteur d’électrons, il est possible d’utiliser des copolymères à blocs afin d’aider la compatibilisation entre le P3HT et le PCBM. Nous avons choisi de synthétiser des copolymères à blocs P3HT-b-polystyrène et des P3HT-b-polyisoprène présentant une certaine compatibilité avec les matériaux de la partie active. L’ajout optimisé de P3HT-b-polyisoprène permet une augmentation de 30% des rendements et de 90% de durée de vie des cellules solaires. / Active layer morphology optimization is fundamental to achieve high efficiency in organic photovoltaic solar cells. We showed the influence of the donor (P3HT) and acceptor (PCBM) material ratio and the impact of the P3HT molecular weight on the active layer morphology. We demonstrated the possibility of using well-designed block copolymers to help P3HT and PCBM compatibilization and to control their phase separation. We chose to synthesize P3HT-b-polystyrene and P3HT-b-polyisoprene for which each block is compatible with the active materials. Optimal addition of P3HT-b-polyisoprene enables to get a 30%-improved efficiency and a 90%-enhanced lifetime of the solar cells.

This dissertation deals with two major topics that involve spectroscopic studies of (a) divalent group 10 metals and (b) silver(I)-phosphine complexes. The scope of the work involved the delineation of the electronic structure of these complexes in different environments and their use in electronic devices. The first topic is a look at the luminescence of tetrahedral silver(I)-phosphine complexes. Broad unstructured emissions with large Stokes shifts were found for these complexes. Computational analysis of the singlet and triplet state geometries suggests that this emission is due to a Jahn-Teller type distortion. The second topic represents the major thrust of this research, which is an investigation into the electronic structure of M(diimine)X2 (M= Pt(II), Pd(II), or Ni(II); X = dichloro, or dithiolate ligands) complexes and their interactions with an electron acceptor or Lewis acid. Chapter 3 assesses the use of some of these complexes in dye sensitized solar cells (DSSCs); it is shown that these complexes may lead to a viable alternative to the more expensive ruthenium-based dyes that are being implemented now. Chapter 4 is an investigation into donor/acceptor pairs involving this class of complexes, which serves as a feasibility test for the use of these complexes in organic photo-voltaics (OPVs) and thin-film field-effect transistors (OTFTs). The mixing of a donor Pt molecule with an electron deficient nitrofluorenone gives rise to new absorption bands in the NIR region. Computational studies of one of the solids suggest that these complexes may have metallic behavior. Chapter 5 demonstrates association in solution, previously unobserved, for Pt(diimine)Cl2 complexes. This chapter is an investigation into the effects of the association mode for this class of complexes on the absorption and emission properties. One of the complexes was used as the emitter in organic light emitting diodes (OLEDs). The results of this study show that these complexes have tunable absorption and emission energies that are concentration dependant. The concentration dependence of the absorption and emission energies is utilized in the OLED device where association enhances the performance.

Cette thèse de doctorat traite de la mise en œuvre du procédé de soulèvement épitaxial (ou ELO pour epitaxial lift-off) à partir d'un substrat d'InP permettant le détachement des couches actives et le recyclage du substrat afin de rendre économiquement compétitive la fabrication de cellules solaires multi-jonctions pour le photovoltaïque concentré. Ce procédé, qui consiste à sous-graver sélectivement une couche sacrificielle comprise entre le substrat et les couches actives, est bien connu et maîtrisé sur un substrat de GaAs avec l'utilisation d'une couche sacrificielle d'AlAs d'épaisseur voisine de 5 nm, ce qui n'est pas possible sur un substrat d'InP en raison du fort désaccord de maille cristalline existant entre l'AlAs et l'InP. Pour l'adapter à un substrat d'InP, le développement d'une couche sacrificielle spécifique basée sur un super-réseau AlAs/InAlAs a été réalisé, ce qui permet de contourner les problématiques liées au désaccord de maille et à la croissance de matériaux contraints. Après optimisation des conditions de croissance de ce super-réseau, les épaisseurs atteintes et donc les vitesses de sous-gravure obtenues en utilisant ce type de couche sacrificielle ont satisfait aux exigences du procédé ELO. Ensuite, le report et le soulèvement de structures actives de cellules solaires InGaAs en couches minces cristallines ont été développés. Les cellules solaires ainsi fabriquées ont montré des performances semblables à celles réalisées par épitaxie standard sur un substrat d'InP, voire meilleures sous concentration en raison d'effets de confinement optique. Finalement, le recyclage du substrat d'InP réalisé avec un procédé utilisant seulement deux étapes de nettoyage par voies chimiques humides, a permis de produire des surfaces d'InP de qualité suffisante pour réaliser une reprise d'épitaxie satisfaisante. / This PhD thesis deals with the implementation of the epitaxial lift-off (ELO) process from an InP substrate allowing the detachment of active layers and the substrate recycling. The final target is to realize multi-junction solar cells in an economically competitive way for concentrated photovoltaic. The ELO process consists in the under-etching of a sacrificial layer inserted between the substrate and the active layers. It is well known and mastered on a GaAs substrate with the use of a sacrificial layer of AlAs with a thickness of about 5 nm. Such a layer is not usable on an InP substrate due to the high lattice mismatch between AlAs and InP. In order to adapt the ELO process to an InP substrate, this work aimed to develop a specific sacrificial layer based on an AlAs/InAlAs superlattice. Thus, it is possible to circumvent problems related to the lattice mismatch and to the strained layer growth. After optimization of growth conditions of this superlattice, using this type of sacrificial layer, we achieve a sufficient thickness and therefore a sufficient under-etching rate in order to meet the requirements of the ELO process. Then, the transfer and lift-off of thin crystalline film based InGaAs solar cells have been developed. This kind of solar cells showed performances similar to those obtained with a standard epitaxial growth on an InP substrate, or even better under concentration due to optical confinement effects. Finally, the recycling of the InP substrate carried out by a process using only two wet chemical cleaning steps made it possible to produce InP surfaces of sufficient quality to achieve a promising second epitaxial growth.

L'objectif de cette thèse est de réaliser l'intégration monolithique de nanofils (NFs) à base de l’alliage Al0.2Ga0.8As sur des substrats de Si par épitaxie par jets moléculaires via la méthode vapeur-liquide-solide (VLS) auto-assistée et de développer une cellule solaire tandem (TSC) à base de ces NFs.Pour atteindre cet objectif, nous avons tout d'abord étudié la croissance de NFs GaAs, étape clé pour le développement des NFs p-GaAs/p.i.n-Al0.2Ga 0.8As coeur/coquille, qui devraient constituer la cellule supérieure de la TSC. Nous avons montré, en particulier, l'influence de l'angle d'incidence du flux de Ga sur la cinétique de croissance des NFs GaAs. Un modèle théorique et des simulations numériques ont été réalisées pour expliquer ces résultats expérimentaux.Nous avons ensuite utilisé le savoir-faire acquis pour faire croître des NFs p-GaAs/p.i.n-Al0,2Ga0,8As coeur/coquille sur des substrats de Si prêts pour l'emploi. Les caractérisations EBIC réalisées sur ces NFs ont montré qu'ils sont des candidats potentiels pour la réalisation d’une cellule photovoltaïque. Nous avons ensuite fait croître ces NFs sur des substrats de Si patternés afin d'obtenir des réseaux réguliers de ces NFs. Nous avons développé un protocole, basé sur un pré-traitement thermique, qui permet d'obtenir des rendements élevés de NFs verticaux (80-90 %) sur une surface patternée de 0,9 x 0,9 mm2.Enfin, nous avons consacré une partie de notre travail à définir le procédé de fabrication optimal pour la TSC, en concentrant notre attention sur le développement de la jonction tunnel de la TSC, l'encapsulation des NFs et le contact électrique supérieur du réseau de NFs. / The objective of this thesis is to achieve monolithical integration of Al0.2Ga0.8As-based nanowires (NWs) on Si substrates by molecular beam epitaxy via the self-assisted vapour-liquid-solid (VLS) method and develop a NWs-based tandem solar cell (TSC).In order to fulfil this purpose, we firstly focused our attention on the growth of GaAs NWs this being a key-step for the development of p-GaAs/p.i.n-Al0.2Ga0.8As core/shell NWs, which are expected to constitute the top cell of the TSC. We have shown, in particular, the influence of the incidence angle of the Ga flux on the GaAs NW growth kinetic. A theoretical model and numerical simulations were performed to explain these experimental results.Subsequently, we employed the skills acquired to grow p-GaAs/p.i.n-Al0.2Ga0.8As core/shell NWs on epi-ready Si substrates. EBIC characterizations performed on these NWs have shown that they are potential building blocks for a photovoltaic cell. We then committed to growing them on patterned Si substrates so as to obtain regular arrays of NWs. We have developed a protocol, based on a thermal pre-treatment, which allows obtaining high vertical yields of such NWs (80-90 %) on patterned Si substrates (on a surface of 0.9 x 0.9 mm2).Finally, we dedicated part of our work to define the optimal fabrication process for the TSC, focusing our attention to the development of the TSC tunnel junction, the NW encapsulation and the top contacting of the NWs.

Tesis por compendio / [ES] La creciente urbanización e industrialización de las sociedades en las últimas décadas han provocado una alta demanda de energía. No obstante, mientras no se desarrollen fuentes de energía renovables que se constituyan como un reemplazo plausible de las actuales, la liberación de gases de efecto invernadero y sus consecuencias sobre el cambio climático difícilmente serán atajadas. Por ello, se está llevando a cabo una intensa búsqueda de energías renovables para un futuro inmediato. En los últimos años, la perovskita ha alcanzado una gran popularidad, centrando numerosos esfuerzos de investigación debido a sus propiedades ópticas y eléctricas únicas. Como por ejemplo su alto coeficiente de absorción y su alta movilidad de carga. Esta tesis comprende seis artículos científicos en torno a las perovskitas. Por un lado, 3 trabajos estudian los fenómenos ópticos que ocurren en el seno del material, con la finalidad de lograr una mejor comprensión de éstos. Puesto que el conocimiento de estos fenómenos ópticos a nivel individual (cristales aislados) permite modular y adaptar su síntesis y morfología para aplicaciones determinadas. Por otro lado, las perovskitas han sido implementadas en distintos dispositivos para su aplicación en tres campos: fotocatálisis, sensores y fotovoltaica. Por primera vez, se ha logrado utilizar este material para la obtención de hidrógeno llevando a cabo la reacción en fase vapor. Asimismo, diversas composiciones de perovskita se han usado para decorar grafeno y detectar niveles traza de Compuestos Orgánicos Volátiles (COV) como benceno y tolueno. Y finalmente se ha desarrollado una estrategia para insertar en la estructura de la perovskita moléculas orgánicas, de forma que se permite una ampliación de la fotorespuesta en el espectro visible. Por lo tanto, las perovskitas se han implementado exitosamente en aplicaciones de diversa índole, constituyéndose como un material prometedor y fácilmente adaptable a los distintos requisitos de cada campo de estudio. / [CAT] La creixent urbanització i industrialització de les societats durant les últimes dècades han provocat una alta demanda d'energia. No obstant això, fins que no es desenvolupen fonts d'energia renovables que puguen ser un reemplaçament plausible de les actuals, l'alliberament de gasos d'efecte hivernacle i les seues conseqüències sobre el canvi climàtic seran difícilment aturades. Per tant, s'està duent a terme una intensa cerca d'energies renovables per a un futur immediat. En els últims anys, la perovskita ha aconseguit una gran popularitat, centrant nombrosos esforços de recerca a causa de les seues propietats òptiques i elèctriques úniques. Per exemple, el seu al coeficient d'absorció i la seua alta mobilitat de càrrega. Aquesta tesi reuneix sis articles científics al voltant de les perovskites. Per una banda, 3 treballs estudien els fenòmens òptics que ocorren en el material, amb la finalitat d'assolir una major comprensió d'aquests. Donat que el coneixement d'aquests fenòmens òptics a nivell individual (cristalls aïllats) permeten modular i adaptar la seua síntesi i morfologia per a determinades aplicacions. Per altra banda, les perovskites han sigut implementats en diferents dispositius per a la seua aplicació en tres camps: fotocatàlisi, sensors i fotovoltaica. Per primera vegada, s'hi ha aconseguit utilitzar aquest material per a l'obtenció d'hidrogen duent a terme la reacció en fase vapor. Així mateix, diverses composicions de perovskita s'han utilitzat per a decorar grafè i detectar nivells traça de Compostos Orgànics Volàtils (COV) com benzè i toluè. I finalment s'ha desenvolupat una estratègia per a inserir en l'estructura de la perovskita molècules orgàniques, de forma que es permet una ampliació de la fotoresposta en l'espectre del visible. Per tant, les perovskites s'han implementat exitosament en aplicacions de diversa índole, constituint-se com un material prometedor i fàcilment adaptable als diferents requisits de cada camp d'estudi. / [EN] During the last decades, the growing urbanization and industrialization result in a significant need for energy. However, since feasible renewable energy sources should be further developed to replace the current energy source, the release of greenhouse gases and their climate change consequences are difficult to overcome. For that reason, the development of renewable energy sources has been attracting growing research efforts. Recently, perovskites gathered great interest owing to their outstanding optical and electrical properties. For instance, their high absorption coefficient and superior charge mobility. This thesis comprises six scientific articles about perovskites. On one hand, 3 works study the optical phenomena that occur within the material in order to achieve a better understanding. The deep knowledge of these optical phenomena's at the individual level (isolated crystals) enable the modulation and tuning of their synthesis and morphology to match specific applications. On another hand, perovskites have been implemented in several devices for their application in three research fields: photocatalysis, sensors, and photovoltaic. For the first time, this nanomaterial was successfully employed for obtaining hydrogen carrying out the reaction in the vapor phase. Likewise, several perovskite compositions have been used for decorating graphene and detect trace levels of Volatile Organic Compounds (VOC) as benzene and toluene. And finally, it has been developed a strategy to insert organic molecules in the perovskite structure, resulting in an enhanced photoresponse in the visible range. Therefore, perovskites have been successfully implemented in several applications, becoming a promising material and easily adaptable to the different requirements needed in each field of study. / Financial support from the Spanish Ministry of Economy and Competitiveness (Severo Ochoa, SEV-2016-0683), Intramural CSIC project 201680I006, and Fundación Ramón Areces (XVII Concurso Nacional para la adjudicación de Ayudas a la Investigación en Ciencias de la Vida y de la Materia) is gratefully acknowledged. This research was financially supported by the Spanish Ministry of Economy and Competitiveness (Mineco) of Spain (TQ2011-26455)2, MAT2015-69669-P, and regional government grant PrometeoII/2017/026. This work was supported by Spanish ministry of Economy, Industry and Competitiveness (MINECO) through the projects TEC2015-74405-JIN, MAT2015-69669-P as well as the regional projects of both Provincia Autonoma di Trento (PAT) of Italy, through the call Grandi Progetti 2012: SIQURO and the Comunidad Valenciana of Spain project PrometeoII/2014/026. This work was supported in part by MICINN and FEDER via grants no. RTI2018-101580- B-I00, by AGAUR under grant. 2017SGR418. S. R.-B. thanks the Research Executive Agency (REA) and the European Commission for the funding received under the Marie Skłodowska Curie actions (H2020-MSCA-IF-2015/Grant agreement number 709023/ZESMO). R. G. A. acknowledges the FPI scholarship from MINECO MAT2015-69669-P. P. A. acknowledges the financial support from the Spanish Government through ‘Severo Ochoa” (SEV-2016-0683, MINECO) and PGC2018-099744-B-I00 (MCIU/AEI/FEDER) / García Aboal, R. (2021). Propiedades optoelectrónicas en perovskitas halogenadas y su aplicación en energía y sensores [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/175357 / TESIS / Compendio