Le but de ce travail est d'étudier et de développer des voies pour améliorer l'efficacité des cellules solaires à base de Kesterite. La première partie de ce manuscrit traite du développement d’un procédé de base : le mécanisme de formation de l’absorbeur est étudié en fonction des conditions de croissance du composé Cu2ZnSnS4 (CZTS à base de soufre pur) et Cu2ZnSnSe4 (CZTSe à base de sélénium pur). Un procédé séquentiel en deux étapes a été utilisé pour synthétiser l’absorbeur en Kesterite. La première étape est un dépôt par pulvérisation cathodique des précurseurs métalliques (Cu, Zn et Sn élémentaires) et la deuxième étape consiste en un recuit des précurseurs sous atmosphère de sélénium (pour le CZTSe dans un réacteur semi-ouvert) ou de soufre (pour le CZTS dans un réacteur ouvert). Différentes optimisations du procédé sont réalisées pour améliorer la microstructure et les performances des dispositifs. Dans le cas du dispositif à base de CZTSe, le meilleur rendement de conversion photovoltaïque obtenu est de 7,6% en utilisant un profil de température en deux étapes et un suscepteur fermé. Pour les cellules solaires à base de CZTS, la meilleure performance obtenue est de 5,9% grâce à l’optimisation de la température et de la pression partielle ensoufre : Les performances des dispositifs augmentent avec la pression partielle en soufre.L’incorporation de Na (Sodium) et de Sb (Antimoine) dans les absorbeurs Kesterite en pur soufre a été testée comme la première stratégie pour améliorer les performances des dispositifs à base de CZTS. L'incorporation de Sb n‘entraîne pas d'amélioration en termes de propriétés des matériaux ou des dispositifs, tandis que le co-dopage avec Na et Sb a montré une morphologie améliorée des absorbeurs. Cependant, cette amélioration n’est suivie d’aucun effet sur les propriétés photovoltaïques du dispositif. L’incorporation de Sb n’est donc pas bénéfique pour la cellule solaire à base de CZTS. D'autre part, la contamination intentionnelle avec du Na s'est avérée bénéfique pour les cellules solaires, particulièrement pour la tension en circuit ouvert. Par conséquent, l’efficacité des dispositifs avec une teneur en Na optimisée est doublée (> 4,5 %) par rapport à celle des échantillons de référence sans Na.La seconde étude pour améliorer les performances des cellules solaires à base de Kesterite concerne l’introduction de gradients de chalcogènes (S/Se) dans l’épaisseur de l’absorbeur. Le but est d’obtenir des gradients de bande interdite afin d’augmenter la longueur de collection des porteurs et de diminuer les phénomènes de recombinaison. Dans ce but, deux procédés sont développés pour réaliser des gradients simples (en face avant ou en face arrière de l’absorbeur). Ces procédés consistent en des recuits successifs (sulfurisation/sélénisation) d’empilements de précurseurs. Pour obtenir un gradient en face avant, un recuit de sulfurisation à différentes températures et durées est appliqué après un recuit de sélénisation standard. Une température plus importante entraîne un gradient plus marqué. Une couche de défaut à base de soufre pur est également formée au cours de ce processus, qui peut être éliminée à l'aide d'une gravure au HCl. Le rendement de conversion photovoltaïque le plus élevé obtenu à l’aide de ce procédé est de 3,5%. Pour obtenir un gradient en face arrière, un recuit de sulfurisation à différentes températures avant un recuit de sélénisation standard a été utilisé. A faible température de sulfurisation, des absorbeurs avec une bonne morphologie ont été obtenus mais sans gradient de composition en chalcogène tandis que l’utilisation de températures de sulfurisation plus importantes ont entraîné l’apparition de gradients de composition mais ont détérioré la morphologie des absorbeurs. Ainsi, les voies et limites pour réaliser des absorbeurs de Kesterite à gradient de bande interdite sont proposées. / The goal of this work is to study and to develop routes toward efficiency improvement of Kesterite based solar cells. The first part of the manuscript deals with the development of a baseline process: formation mechanism of the absorber is studied according to the growth condition for both Cu2ZnSnS4 (pure sulfur absorber CZTS) and Cu2ZnSnSe4 (pure selenium absorber CZTSe) compounds. Two-step sequential process is used for synthesizing Kesterite material. The first step consists in the sputtering deposition of pure metallic precursors (elemental Cu, Zn, and Sn) and the second step consists in the annealing of precursors under selenium (for CZTSe in a semi-open reactor) or sulfur (for CZTS in an open reactor). In the case of CZTSe based solar cell, a maximum power conversion efficiency of 7.6% has been obtained using a two-step temperature profile and a closed susceptor. The best performance for a CZTS based device is 5.9%, this result has been obtained by optimizing the process temperature and sulfur vapor pressure: the higher sulfur vapor pressure the better device performance.Incorporation of Na (Sodium) and Sb (Antimony) in the pure sulfur Kesterite absorber has been tested as a first strategy to enhance performances of CZTS devices. Incorporation of Sb does not show any improvement in terms of material or device properties, whereas improved morphology is obtained by co-doping with Na and Sb. However, this improvement is not related to any effect on device properties. Thus, using Sb proved to be not beneficial for the CZTS-based solar cell. On the other hand, intentional contamination with Na is found to be beneficial particularly in terms of open circuit voltage. As a result, the device power conversion efficiency with optimized Na content is doubled (> 4.5%) compared to the reference sample without Na.The second study to increase efficiencies in Kesterite solar cells deals with the introduction of chalcogen (S/Se) gradients as the function of depth in the absorber. The aim is to obtain bandgap gradients in order to increase carrier collection length as well as decrease carrier recombination. For this purpose, two processes are developed to realize only simple grading (front or back surface gradients) which consist of sequential annealing stages (sulfurization/selenization) of precursor stacks. To obtain a front surface gradient, a sulfurization step at various temperatures and for different duration has been tested after a standard selenization process. A higher sulfurization temperature shows a higher degree of grading. A pure sulfur-based defect layer is also formed during this process, which can be removed using an HCl etching. A maximum efficiency of 3.5% is achieved with a CZTS-based device using this synthesis process. To realize back grading, variable temperature sulfurization annealing prior to a standard selenization process has been used. At a low temperature of sulfurization, good absorber morphologies are obtained but without the evidence of chalcogen gradient while using higher sulfurization temperature leads to graded absorbers but with poor morphology. Thus, the routes and limitations to realize kesterite absorber with gradient are proposed.

Technologies based on alternative and sustainable energy sources present a vital solution in the present and for the future. These technologies are strongly driven by the increased global energy demand and need to reduce environmental issues created by fossil fuel. Solar energy is an abundant, clean and free-access resource, but it requires harvesting and storage for a sustainable future. Direct conversion and storage of solar energy using heterogeneous photocatalysts have been identified as parts of a promising paradigm for generating green fuels from sunlight and water. This thesis focused on developing semiconductor absorbers in a visible light region for photocatalytic hydrogen production reaction. In addition, theoretical studies are combined with experimental results for a deep understanding of the intrinsic optoelectronic properties of the obtained materials. The study presents a novel family of oxysulfide BiAgOS, produced by applying a full substitution strategy of Cu by Ag in BiCuOS. I was interested to address how the total substitution of Cu by Ag in a BiCuOS system affects its crystal structure, optical and electronic properties using experimental characterizations and theoretical calculations. Single-phase bismuth silver oxysulfide BiAgOS was prepared via a hydrothermal method. Rietveld refinement of the powder confirmed that BiAgOS is an isostructural BiCuOS. The diffraction peak positions of BiAgOS, relative to those of BiCuOS, were shifted toward lower angles, indicating an increase in the cell parameters. BiCuOS and BiAgOS were found to have indirect bandgaps of 1.1 and 1.5 eV, respectively. The difference in the bandgap results from the difference in the valence band compositions. The hybrid level of the S and Ag orbitals in BiAgOS is located at a more positive potential than that of S and Cu, leading to a widened bandgap. Both materials possess high dielectric constants and low electron and hole effective masses, making them interesting for photoconversion applications. BiAgOS has a potential for photocatalytic hydrogen evolution reaction in the presence of sacrificial reagents; however, it is inactive toward water oxidation. BiCuOS and BiAgOS can be considered interesting starting compositions for the development of new semiconductors for PV or Z-scheme photocatalytic applications. The second study investigates the synthesis and characterization of NaBiS2, this contains Bi3+, which belongs to the p-block electronic configuration Bi3+ 6s26p0, and NaLaS2, which contains La3+ with electronic configuration 6s05d0. Solid-state reactions from oxide precursor starting materials were applied for synthesis the materials. The sulfurization process was conducted by pressurizing a saturated vapor of CS2. The obtained black material of NaBiS2 has an indirect transition with high absorption coefficients in the visible region of the spectrum and the absorption edge is determined at 1.21 eV. However, NaBiS2 did not show photocatalytic activity toward hydrogen production. NaLaS2 is characterized by an indirect transition with a bandgap in the UV region at 3.15 eV and can drive the photocatalytic hydrogen evolution reaction in Na2S/Na2SO3 solution. Utilizing the solid solution NaLa1-xBixS2 strategy, the absorption properties and band edge position for photocatalytic hydrogen evolution reaction were optimized. The results indicated that the bismuth content is critical parameter for maintaining the photocatalytic activity. The incorporation of low Bi content up to 6% in NaLaS2 leads to extending the photon absorption from the UV to the visible region and enhancing the photocatalytic activity of hydrogen production. In contrast, all the solid solutions that have Bi content of more than 12% present absorption edges close to that of pure NaBiS2, and they are inactive for photocatalytic hydrogen production. Combining the experimental measurements with density functional theory calculations, such behavior can be explained by the degree of overlapping of Bi and La states on the conduction band minimum (CBM). Finally, self-assembly of Bi2S3 nanorods were grown on FG or FTO substrates. Bi2S3 thin films were prepared by sulfurization of Bi metal layer using the hydrothermal method. The results show that Bi2S3 has absorption up to 1.3 eV and has a moderate absorption coefficient in the visible region. The ultraviolet photoelectron spectroscopy and photoelectron spectroscopy in air results showed that the conduction band minimum of Bi2S3 is located slightly above the hydrogen redox potential. However, Pt/Bi2S3 did not evolve a detectable amount of hydrogen, suggesting the presence of surface states that can hinder the hydrogen reduction reaction.

No description available.

The ability to fabricate semiconductor wafers with spatially selected regions of different bandgap material is required for the fabrication of monolithic photonic integrated circuits (PIC's). Although this subject has been studied for three decades and many semiconductor engineering approaches have been proposed, the problem of achieving reproducible results has constantly challenged scientists and engineers. This concerns not only the techniques relaying on multiple sequential epitaxial growth and selective area epitaxy, but also the conventional quantum well intermixing (QWI) technique that has been investigated as a post-growth approach for bandgap engineering. Among different QWI techniques, those based on the use of different lasers appear to be attractive in the context of high-precision and the potential for cost-effective bandgap engineering. For instance, a tightly focused beam of the infrared (IR) laser could be used for the annealing of small regions of a semiconductor wafer comprising different quantum well (QW) or quantum dot (QD) microstructures. The precision of such an approach in delivering wafers with well defined regions of different bandgap material will depend on the ability to control the laser-induced temperature, dynamics of the heating-cooling process and the ability to take advantage of the bandgap engineering diagnostics. In the frame of this thesis, I have investigated IR laser-induced QWI processes in QW wafers comprising GaAs/A1GaAs and InP/InGaAsP microstructures and in InAs QD microstructures grown on InP substrates. For that purpose, I have designed and set up a 2-laser system for selective area rapid thermal annealing (Laser-RTA) of semiconductor wafers. The advantage of such an approach is that it allows carrying out annealing with heating-cooling rates unattainable with conventional RTA techniques, while a tightly focused beam of one of the IR lasers is used for `spot annealing'. These features have enabled me to introduce a new method for iterative bandgap engineering at selected areas (IBESA) of semiconductor wafers. The method proves the ability to deliver both GaAs and InP based QW/QD wafers with regions of different bandgap energy controlled to better than « 1nm of the spectral emission wavelength. The IBESA technique could be used for tuning the optical characteristics of particular regions of a QW wafer prepared for the fabrication of a PIC. Also, this approach has the potential for tuning the emission wavelength of individual QDs in wafers designed, e.g., for the fabrication of single photon emitters. In the 1st Chapter of the thesis, I provide a short review of the literature on QWI techniques and I introduce the Laser - RTA method. The 2nd Chapter is devoted to the description of the fundamental processes related to the absorption of laser light in semiconductors. I also discuss the results of the finite element method applied for modeling and semi-quantitative description of the Laser - RTA process. Details of the experimental setup and developed procedures are provided in the 3rd Chapter. The results concerning direct bandgap engineering and iterative bandgap engineering are discussed in the 4th and 5th Chapters, respectively.

Ce travail de thèse porte sur la diffusion des porteurs de charge en excès dans deux semiconducteurs à large bande interdite: l’alliage ZnMgO et le diamant 13C. Il est basé sur l’étude d’hétérostructures ZnMgO/ZnO/ZnMgO et 13C/12C/13C à puits de collecte ZnO ou 12C. Sur leurs sections transverses et avec la résolution nanométrique en excitation par cathodoluminescence (CL), nous avons étudié l’évolution de l’intensité de l’émission issue du puits en ZnO ou 12C en fonction de la distance entre l’impact de l’excitation et le puits. Cela nous a permis de mesurer directement les longueurs de diffusion effectives dans ZnMgO et le diamant.Dans ZnMgO, la valeur de 55 nm à 300 K, mesurée sur section transverse clivée, est proche de celle du matériau massif. Elle correspond à une diffusion mixte excitons/porteurs libres. Avec l’utilisation de lames minces érodées par faisceau d’ions, une diminution de a été observée jusqu’à 8 nm dans les parties les plus fines. Cet effet est attribué aux recombinaisons non radiatives de surface. Les lames minces apparaissent alors d’un grand intérêt pour améliorer la résolution spatiale des images CL.Dans le diamant, la diffusion excitonique à basse température montre une faible dépendance de avec l’énergie incidente des électrons. Cela indique que ≈ 15 µm à 20 K dans le diamant massif 13C. Une diminution de jusqu’à 3,3 µm à 118 K est observée en fonction de la température.Enfin, nous avons mis en évidence la formation de polyexcitons dans le diamant en augmentant la densité des paires électron-trou, soit par la puissance d’excitation, soit par le confinement spatial des excitons dans des puits de diamant 12C de faible épaisseurs. / This work focuses on the determination of the carrier diffusion length in two wide bandgap semiconductors: the ternary alloy ZnMgO and diamond. This determination has been achieved by using of ZnMgO/ZnO/ZnMgO and 13C/12C/13C heterostructures containing ZnO or 12C collecting wells. Their transverse section was scanned by CL spectroscopy with a nanometer scale resolution in excitation. The effective excess carrier diffusion length is deduced from the evolution of the well emission intensity with the distance between the excitation impact and the well.In ZnMgO, the value at 300 K is 55 nm, obtained from a cleaved cross section. It is close to the bulk material diffusion and is attributed to a mixed diffusion of excitons/free carriers. A decrease of down to 8 nm is observed in the thinnest portions of cross sections shaped by focused ion beam (FIB). This effect is attributed to non-radiative surface recombinations. These thin slabs appear of great interest to enhance the spatial resolution of CL images.In diamond, the exciton diffusion at 20 K exhibits a slight dependence on the incident electron energy. This indicates that the exciton diffusion length is around 15 µm in 13C bulk diamond. The values decrease down to 3.3 µm at 118 K.Finally, we highlighted the formation of polyexcitons in diamond by increasing the electron-hole pairs density either by the excitation power, or by the spatial confinement of excitons in thin 12C wells.

No description available.

No description available.

[pt] Heteroestruturas sensíveis à luz solar possuem grande potencial em diferentes aplicações direcionadas a um futuro limpo e sustentável, como a fotoprodução de hidrogênio (H2) e a remediação ambiental. No contexto da fotocatálise, o dióxido de titânio (TiO2) desempenha um papel crucial devido à sua ampla gama de aplicações, excelente estabilidade química, baixa toxicidade e custo relativamente baixo. No entanto, o TiO2 puro possui algumas desvantagens, como uma alta taxa de recombinação e baixa sensibilidade à luz solar, o que limita sua eficiência em aplicações fotocatalíticas. Portanto, o desenvolvimento contínuo de novos materiais com o objetivo de superar essas desvantagens é obrigatório. Entre as abordagens reportadas para superar as deficiências do TiO2 puro está a formação de heterojunções com outros semicondutores, melhorando a separação de cargas e, portanto, a eficiência fotocatalítica. Óxidos de níquel e óxidos de cobre são relatados como alternativas promissoras para a formação de heterojunções com TiO2, melhorando a transferência de carga e aumentando a absorção de luz no espectro visível do TiO2 puro. Esta tese apresenta diferentes estudos voltados para a síntese e caracterização de novos nanomateriais heteroestruturados eficientes para geração fotocatalítica de hidrogênio e degradação de poluentes perigosos. No primeiro estudo, foi relatada uma heterojunção p-n de NiO/TiO2 obtida via mecanoquímica, que apresentou uma taxa elevada de fotoprodução pelo sol de H2 em comparação com o TiO2 puro (8.85 mmol h-1 g-1 vs. 0.73 mmol h-1 g-1). Em todos os casos, a adição de NiO suportado em TiO2 reduziu a taxa de recombinação e aumentou a absorção de luz visível. Estudos de TEM, XPS e XAS demonstraram que uma dispersão homogênea e uma configuração de spin favorável dos pequenos aglomerados de NiO suportados em TiO2 foram responsáveis pela eficiência superior exibida pela amostra preparada via mecanoquímica, denominada NiO/P90- BM. Notavelmente, testes de ciclagem, de longo prazo e de envelhecimento mostraram que o fotocatalisador relatado é eficiente após vários ciclos, para uso prolongado e após longos períodos de armazenamento. Além disso, foram realizados estudos combinando EPR e a técnica de captura de spin para aprofundar na produção de superóxido e hidroxila pelas heterojunções de NiO/TiO2. Esses estudos forneceram insights sobre a aplicação potencial das heterojunções de NiO/TiO2 para a degradação fotocatalítica de poluentes gasosos e aquosos. Os resultados de EPR lançaram luz sobre a amostra de NiO/P90-BM como a mais eficiente na fotogeração de ROS, revelando que a síntese mecanoquímica resultou em uma arquitetura mais eficiente para a geração de radicais superóxido e hidroxila. Finalmente, foi relatada uma rota simples de química branda para preparar uma heteroestrutura de nanocubos de óxido cúprico (Cu2O NCs) e TiO2, denominada Cu2O NCs/TiO2, como um adsorvente eficiente para a tetraciclina (TC), que é um antibiótico de amplo espectro. FTIR e TGA foram realizados antes e após o processo de adsorção para demonstrar a adsorção de TC pela heteroestrutura Cu2O NCs/TiO2. Além disso, foram realizados testes com irradiação de luz visível para distinguir entre os processos de remoção por adsorção e fotocatalítica. Além disso, foram realizadas medições de EPR usando captura de spin para investigar a fotoprodução de ROS. Curiosamente, não houve fotoprodução de ROS detectável pela heteroestrutura Cu2O NCs/TiO2, demonstrando que a remoção de TC é exclusivamente devido à adsorção. Estes resultados contribuem para esclarecer uma discrepância na literatura quanto à atividade fotocatalítica dos Cu2O NCs sob luz visível. Coletivamente, esta pesquisa avançou o entendimento dos mecanismos fotocatalíticos e relatou novos nanomateriais heteroestruturados, destacando seu potencial para aplicações sustentáveis em diversos contextos relacionados ao meio ambiente e transição energética. / [en] Solar light-responsive heterostructures hold great potential in different applications toward a clean and sustainable future, such as hydrogen (H2) photoproduction and environmental remediation. In the context of photocatalysis, titanium dioxide (TiO2) plays a crucial role due to its wide range of applications, excellent chemical stability, low toxicity, and relatively low cost. However, neat TiO2 has some shortfalls, such as a high recombination rate and low sensitivity to solar light, which limits its efficiency in photocatalytic applications in general. Therefore, the continuous development of new materials aimed at improving these limitations is mandatory. Among the approaches to overcome the neat TiO2 shortfalls is the formation of heterojunctions with suitable semiconductors, improving charge separation and, therefore, photocatalytic efficiency. Nickel oxides and copper oxides are reported as promising alternatives for forming heterojunctions with TiO2, enhancing the charge transfer and broadening the light absorption in the visible spectrum. This thesis presents different studies aimed at the synthesis and characterization of new efficient heterostructured nanomaterials for photocatalytic hydrogen generation and hazardous pollutants abatement. In the first study, a NiO/TiO2 p-n heterojunction obtained via mechanochemistry was reported, which exhibited an improved solar-driven H2 photoproduction rate compared to neat TiO2 (8.85 mmol h-1g-1vs. 0.73 mmol h-1g-1). In all cases, the addition of NiO supported on TiO2 reduced the recombination rate and enhanced the visible light absorption. TEM, XPS, and XAS studies demonstrated that a homogenous dispersion and a favorable spin configuration of NiO clusters supported on TiO2 were responsible for the superior efficiency exhibited by the sample prepared via mechanochemistry, labeled as NiO/P90-BM. Noticeably, cycling, long-term, and aging tests have shown that the reported photocatalyst is efficient after several cycles, prolonged use, and after long periods of storage. Furthermore, studies combining EPR and the spin trapping technique were carried out to delve into the production of superoxide and hydroxyl by NiO/TiO2 heterojunctions. These studies provided insights into the potential application of the NiO/TiO2 heterojunctions for the photocatalytic degradation of gaseous and aqueous pollutants. The EPR results shed light on the NiO/P90-BM sample as the most efficient in ROS photogeneration, revealing that mechanochemical synthesis resulted in a more efficient architecture for generating superoxide and hydroxyl radicals. Besides, a simple soft chemistry route was reported to prepare a heterostructure of cuprous oxide nanocubes (Cu2O NCs) and TiO2, labeled as Cu2O NCs/TiO2, as an efficient adsorbent for tetracycline (TC), which is a broad-spectrum antibiotic. FTIR and TGA were carried out before and after the adsorption process to demonstrate the adsorption of TC by the Cu2O NCs/TiO2 heterostructure. Additionally, tests with visible light irradiation were performed to distinguish between adsorption and photocatalytic removal processes. In addition, EPR measurements were also carried out using spin trapping to investigate the ROS photoproduction. Interestingly, there was no detectable ROS photoproduction by the Cu2O NCs/TiO2 heterostructure, demonstrating that TC removal is solely due to adsorption. These results contribute to clarifying a discrepancy in the literature regarding the photocatalytic activity of Cu2O NCs under visible light. Collectively, this research has advanced the understanding of photocatalytic mechanisms and reported new heterostructured nanomaterials, while highlighting their potential for sustainable applications in diverse environmental and energy transition related contexts.