Cu2ZnSnS4 (CZTS) is a promising absorber material for thin-film solar cells since it contains no rare or toxic elements, has a high absorption coefficient and a near ideal bandgap energy. It does, however, present some challenges due to the limited single-phase region of the desired kesterite phase and its instability towards decomposition. Sputtering of CZTS from quaternary, compound targets using RF magnetron sputtering is known. In this thesis work CZTS absorbers were made using pulsed DC magnetron sputtering on stainless steel substrates. The effects of varying substrate temperature and adding seed layers to promote grain growth were investigated, as well as the effects of a rapid thermal anneal in a S-rich atmosphere. Film compositions determined by X-ray fluorescence were found to be inside or close to the kesterite single-phase region in the phase diagram, but were generally too Cu-rich and Zn-poor to yield good results. The kesterite phase was confirmed with X-ray crystallography and Raman spectroscopy, indicating that it is possible to sputter CZTS from a single target with a high deposition rate. It was found that Cu2S seed layers could induce a significant increase in grain size, and preliminary experiments showed no evidence of the seed layer remaining after deposition of the absorber. Higher substrate temperatures also lead to increased grain size, but excessive heating caused the decomposition of the CZTS. Annealing induced grain growth, relaxed internal stress in the material and improved the electrical properties of the CZTS films, primarily by the removal of shunts.

Cu2ZnSnS4 (CZTS), being related to the highly successful Cu(In,Ga)(S,Se)2, and CuInS2 materials, is a promising candidate for thin film photovoltaic devices. It has the advantage that it contains no rare or expensive elements, and therefore has cost-reduction potential for commercial systems. A two-stage process for fabrication of CZTS films is presented, which consists of sequential electrodeposition of Cu, Sn and Zn layers followed by a heat treatment in the presence of S vapour (‘sulfurisation’). Electrodeposition conditions are developed to give uniform Cu|Sn|Cu|Zn precursors of controlled morphology and composition, by the use of a rotating disc electrode system. Precursors are converted to CZTS by sulfurisation in the presence of elemental S, using a rapid thermal processing system (RTP). The sulfurisation reaction is studied by XRD and Raman spectroscopy as a function of temperature and at short time intervals, and a sequence of reactions is derived for the formation of CZTS. It is shown that the sulfurisation reaction occurs within minutes above 500°C. A model is presented for film formation when rapid heating rates are employed. The effects of sulfurisation time, background pressure and precursor composition on the morphological and structural properties of the CZTS films are investigated. Observations of grain size changes and compositional modification are made and explained in terms of the likely secondary phases present. The opto-electronic properties of the CZTS films are measured using a photoelectrochemical technique. Changes in the external quantum efficiency and band gap are studied as a function of sulfurisation parameters and precursor composition. After crystallisation of the CZTS phase during sulfurisation, the photocurrent obtained from the films continued to rise upon heating in the absence of S, which is explained by changes in acceptor concentration. Large shifts in the band gap are seen, and some proposals are made to explain the behaviour. The observations are discussed in the context of the particular compositions and sulfurisation conditions routinely used in the CZTS literature, and recommendations are made for further work.

Dans le domaine du photovoltaïque, le composé Cu2ZnSnS4 (CZTS) serait une solution alternative aux composés classiques en couches minces qui sont à base d'éléments chers ou toxiques. Mise à part un gap de 1.5 eV et un coefficient d'absorption de 10-4 cm-1, il est constitué d'éléments bénins et abondants, ce qui réduira le coût de revient de la cellule finale. Il connaît un intérêt particulier et bien qu'il ait atteint un rendement de 12.6%, il demeure méconnu quant à l'effet de ses propriétés intrinsèques sur ses performances photovoltaïques. En raison de la volatilité du soufre, des déviations à la stoechiométrie peuvent être enregistrées rendant la synthèse d'un monophasé très difficile. Les phases secondaires sont difficilement inévitables, elles constituent une barrière à la formation de la phase CZTS, ce qui rend difficile la fixation du gap et augmente le taux de recombinaison des porteurs de charges. Dans le cadre de cette thèse, une série de composés CZTS a été synthétisée par réaction à l'état solide et liquide avec des excès en soufre pour compenser les pertes liées à la décomposition chimique et l'évolution de la composition dans le diagramme de phase Cu-Zn-Sn-S. L'effet du souffre sur la cristallinité, la pureté et l'ordre dans la maille a été mis en évidence. Le domaine monophasé a été déterminé et il a été montré qu'il est possible d'obtenir des composés de grande pureté. La morphologie par microscopie optique a révélé des polycristaux granulaires avec rejet des phases secondaires dans les joints de grains. Le composé Cu2ZnGeS4 (CZGS) pourrait trouver des applications dans le photovoltaïque et l'optoélectronique. L'ajout de l'étain pourrait sensiblement améliorer la cinétique réactionnelle et la cristallinité d'où l'intérêt d'étudier le composé Cu2ZnGexSn(1-x)S4 ( x=0 à 1). L'analyse cristallographique par DRX a montré une transition structurale d'une kesterite pour CZTS vers une orthorhombique pour CZGS. Le composé Cu2Zn(Ge,Sn)S4 est une solution solide avec gap de miscibilité entre 0 et 20% de germanium / In the photovoltaic field, Cu2ZnSnS4 (CZTS) compound is an alternative solution to substitute solar thin film based on toxically and expensive conventional materials. The gap of this material is around 1.5eV and absorption coefficient 10-4 cm-1, in addition this material is composed of abundant and harmless elements which will strongly decrease the price of the final cell. This material present a particular interest and in spite of the efficiency which reached 12.6%, till now this material is not well known especially the effect of its intrinsic properties on its photovoltaic performances. Because of the sulfur volatility, it is difficult to prepare single phase compound. Also, it is difficult to surmount the formation of secondary phases which are a barrier to CZTS complete reaction allowing difficulties to fix the gap and increase the recombination of carrier. In the frame of this PhD thesis, a serial of CZTS compounds has been synthetized from solid and liquid state using an excess of sulfur to compensate its volatility and the composition change in the Cu-Zn-Sn-S equilibrium diagram. We have determined the monophased field and we have shown that it is possible to obtain a compound with high purity. By optical microscopy we have observed a granular morphology composed of polycrystalline grains and the secondary phases were rejected in the grains boundary. The Cu2ZnGeS4 (CZGS) compound can be used for photovoltaic and optoelectronic applications. The addition of tin can be a good way to improve the kinetic reaction and the crystallinity of this materials, So, it is interesting to study Cu2ZnGexSn(1-x)S4 ( x=0 to 1) compound . By X ray diffraction we have shown a structure transition from Kesterite (CZTS) to orthorhombic (CZGS). The Cu2Zn(Ge,Sn)S4 compound is a solid solution with a gap miscibility between 0 and 20% of germanium

New thin-film solar cell materials and a greater understanding of their properties are needed to meet the urgent demand for sustainable, lower-cost and scalable photovoltaics. Computational techniques have been used to investigate Cu2ZnSnS4, CuSbS2 and CuBiS2 , which are potential absorber layer materials in thin-film photovoltaics. Their low cost, low toxicity and their constituent’s relative abundance make them suitable replacements for current thin-film absorbers, which are CdTe or Cu(In, Ga)(S, Se)2 based systems. Firstly, we have used hybrid Density Functional Theory (DFT) calculations to study CuSbS2 and CuBiS2. We calculate band gaps of 1.69 eV and 1.55 eV respectively, placing CuBiS2 within the optimal range for a viable absorber material. The density of states for both these materials indicate that formation of electron hole charge carriers will occur in the Cu d10 band. Consequently, photoexcitation leads to the oxidation of Cu(I). Secondly, we have derived interatomic potentials which describe the complex structure of Cu2ZnSnS4 accurately. We find that the Cu/Zn antisite defect represents the lowest energy form of intrinsic defect disorder. For these antisite defects, we find a preference for small neutral defect clusters, which suggests a degree of self-passivation exists. Investigations of Cu-ion transport find VCu migration is possible via a vacancy hopping mechanism. There are pathways which can be connected to give 3D long-range diffusion. Investigations of the Cu/Zn site disorder in Cu2ZnSnS4 find that configurations which are kesterite-like will dominate synthetic samples. However, perfectly ordered kesterite will not be formed due to entropic effects. The simulations indicate the stannite and stannite-like polymorphs are less favourable, and can only account for ≈2.5% of a sample. Investigations of the surfaces of Cu2ZnSnS4, suggest that the vast majority of the low index surfaces are dipolar and that only the (1 1 2), (0 1 0) and (1 0 1) surfaces have low surface energies.

O objetivo deste estudo foi a síntese de Cu2ZnSnS4 (CZTS) por electrospinning, sem a etapa de sulfurização e com tratamento térmico em atmosfera ambiente. A solução precursora consistiu na dissolução dos sais dos metais de interesse: tiourea como fonte de enxofre e polivinil butiral (PVB) em etanol como polímero condutor. Esta solução foi submetida ao processo de electrospinning com uma tensão elétrica de 16 kV e uma distância entre coletor e capilar de 120 mm e fluxo de 3 mL/h. O efeito da temperatura de tratamento térmico sobre a formação do CZTS foi investigada. Para isso, o material resultante do processo de electrospinning foi tratado termicamente em diferentes temperaturas: 150°C durante 72h, 150°C durante 24h mais 300°C durante 24h, 300°C durante 48h, 400°C durante 1h, 450°C durante 1h, 500°C durante 1h e 550°C durante 1h. A taxa de aquecimento foi de 0,5°C/mim. A Influência do agente complexante dietanolamina (DEA) sobre as propriedades do material formado, quando adicionado à dolução precursora, foi também investigada. As amostras resultantes destes tratamentos térmicos foram caracterizadas através de análises térmicas (ATG), difração de raios X (DRX), espectroscopia Raman, microscopia eletrônica de varredura (MEV/EDS), microscopia eletrônica de transmissão (MET) e, por medidas ópticas (UV-Vis) e espectroscopia de reflectância difusa (ERD). Foram obtidos filmes homogêneos e compactos com uma espessura de aproximadamente 10 nm. Os resultados indicaram que foi obtida fase CZTS a 400°C, com as razões S/(Cu+Zn+Sn) = 1,1, Cu/(Zn+Sn) = 0,8 e Zn/Sn = 1,26. No entanto, a presença de algumas fases secundárias elevou o band gap para aproximadamente 2,2 eV. Além disso, foi verificado que a adição de DEA na solução precursora não aumenta a qualidade do CZTS formado por electrospinning. / The aim of this work was to study the production of CZTS by electrospinning method without sulfurization process and in-air heat treatment. The precursor solution was prepared by dissolving metal salts, tiourea as a sulfur source and PVB as conductive polymer. The resulting solution was electrospun onto a cylindrical target with an electric voltage of 16 kV at a 120 mm distance and flow rate of 3 mL/h. The effect of in-air heat treatment in the phase formation and morphology of electrospun CZTS fibers were investigated by the following conditions, 150°C for 72h, 150°C for 24h plus 300°C for 24h, 300°C for 48h, 400°C for 1h, 450°C for 1h, 500°C for 1h e 550°C for 1h, with a used a heating rate of 0.5°C.min-1 The Influence of the addition of complexing agent diethanolamine (DEA) on the properties of the final material was investigated. The samples were characterized by thermal analysis (TGA), R-X diffraction, scanning electron microscopy (SEM/EDS), transmission electron microscopy and optical measurements (UV-Vis). The results show the obtainment of a well crystallized CZTS phase with the heat treatment of 400°C with ratios S/(Cu+Zn+Sn) =1.1, Cu/(Zn+Sn) = 0.8 e Zn/Sn = 1.26. Homogeneous and compact films with the morphology of 10 nm spheres were found in this study. However, the presence of some secondary phases increases the band gap to approximately, 2.2 eV. Furthermore, it was found that the addition of DEA in the precursor solution does not increase the quality of CZTS formed by electrospinning.

O objetivo deste estudo foi a síntese de Cu2ZnSnS4 (CZTS) por electrospinning, sem a etapa de sulfurização e com tratamento térmico em atmosfera ambiente. A solução precursora consistiu na dissolução dos sais dos metais de interesse: tiourea como fonte de enxofre e polivinil butiral (PVB) em etanol como polímero condutor. Esta solução foi submetida ao processo de electrospinning com uma tensão elétrica de 16 kV e uma distância entre coletor e capilar de 120 mm e fluxo de 3 mL/h. O efeito da temperatura de tratamento térmico sobre a formação do CZTS foi investigada. Para isso, o material resultante do processo de electrospinning foi tratado termicamente em diferentes temperaturas: 150°C durante 72h, 150°C durante 24h mais 300°C durante 24h, 300°C durante 48h, 400°C durante 1h, 450°C durante 1h, 500°C durante 1h e 550°C durante 1h. A taxa de aquecimento foi de 0,5°C/mim. A Influência do agente complexante dietanolamina (DEA) sobre as propriedades do material formado, quando adicionado à dolução precursora, foi também investigada. As amostras resultantes destes tratamentos térmicos foram caracterizadas através de análises térmicas (ATG), difração de raios X (DRX), espectroscopia Raman, microscopia eletrônica de varredura (MEV/EDS), microscopia eletrônica de transmissão (MET) e, por medidas ópticas (UV-Vis) e espectroscopia de reflectância difusa (ERD). Foram obtidos filmes homogêneos e compactos com uma espessura de aproximadamente 10 nm. Os resultados indicaram que foi obtida fase CZTS a 400°C, com as razões S/(Cu+Zn+Sn) = 1,1, Cu/(Zn+Sn) = 0,8 e Zn/Sn = 1,26. No entanto, a presença de algumas fases secundárias elevou o band gap para aproximadamente 2,2 eV. Além disso, foi verificado que a adição de DEA na solução precursora não aumenta a qualidade do CZTS formado por electrospinning. / The aim of this work was to study the production of CZTS by electrospinning method without sulfurization process and in-air heat treatment. The precursor solution was prepared by dissolving metal salts, tiourea as a sulfur source and PVB as conductive polymer. The resulting solution was electrospun onto a cylindrical target with an electric voltage of 16 kV at a 120 mm distance and flow rate of 3 mL/h. The effect of in-air heat treatment in the phase formation and morphology of electrospun CZTS fibers were investigated by the following conditions, 150°C for 72h, 150°C for 24h plus 300°C for 24h, 300°C for 48h, 400°C for 1h, 450°C for 1h, 500°C for 1h e 550°C for 1h, with a used a heating rate of 0.5°C.min-1 The Influence of the addition of complexing agent diethanolamine (DEA) on the properties of the final material was investigated. The samples were characterized by thermal analysis (TGA), R-X diffraction, scanning electron microscopy (SEM/EDS), transmission electron microscopy and optical measurements (UV-Vis). The results show the obtainment of a well crystallized CZTS phase with the heat treatment of 400°C with ratios S/(Cu+Zn+Sn) =1.1, Cu/(Zn+Sn) = 0.8 e Zn/Sn = 1.26. Homogeneous and compact films with the morphology of 10 nm spheres were found in this study. However, the presence of some secondary phases increases the band gap to approximately, 2.2 eV. Furthermore, it was found that the addition of DEA in the precursor solution does not increase the quality of CZTS formed by electrospinning.

O objetivo deste estudo foi a síntese de Cu2ZnSnS4 (CZTS) por electrospinning, sem a etapa de sulfurização e com tratamento térmico em atmosfera ambiente. A solução precursora consistiu na dissolução dos sais dos metais de interesse: tiourea como fonte de enxofre e polivinil butiral (PVB) em etanol como polímero condutor. Esta solução foi submetida ao processo de electrospinning com uma tensão elétrica de 16 kV e uma distância entre coletor e capilar de 120 mm e fluxo de 3 mL/h. O efeito da temperatura de tratamento térmico sobre a formação do CZTS foi investigada. Para isso, o material resultante do processo de electrospinning foi tratado termicamente em diferentes temperaturas: 150°C durante 72h, 150°C durante 24h mais 300°C durante 24h, 300°C durante 48h, 400°C durante 1h, 450°C durante 1h, 500°C durante 1h e 550°C durante 1h. A taxa de aquecimento foi de 0,5°C/mim. A Influência do agente complexante dietanolamina (DEA) sobre as propriedades do material formado, quando adicionado à dolução precursora, foi também investigada. As amostras resultantes destes tratamentos térmicos foram caracterizadas através de análises térmicas (ATG), difração de raios X (DRX), espectroscopia Raman, microscopia eletrônica de varredura (MEV/EDS), microscopia eletrônica de transmissão (MET) e, por medidas ópticas (UV-Vis) e espectroscopia de reflectância difusa (ERD). Foram obtidos filmes homogêneos e compactos com uma espessura de aproximadamente 10 nm. Os resultados indicaram que foi obtida fase CZTS a 400°C, com as razões S/(Cu+Zn+Sn) = 1,1, Cu/(Zn+Sn) = 0,8 e Zn/Sn = 1,26. No entanto, a presença de algumas fases secundárias elevou o band gap para aproximadamente 2,2 eV. Além disso, foi verificado que a adição de DEA na solução precursora não aumenta a qualidade do CZTS formado por electrospinning. / The aim of this work was to study the production of CZTS by electrospinning method without sulfurization process and in-air heat treatment. The precursor solution was prepared by dissolving metal salts, tiourea as a sulfur source and PVB as conductive polymer. The resulting solution was electrospun onto a cylindrical target with an electric voltage of 16 kV at a 120 mm distance and flow rate of 3 mL/h. The effect of in-air heat treatment in the phase formation and morphology of electrospun CZTS fibers were investigated by the following conditions, 150°C for 72h, 150°C for 24h plus 300°C for 24h, 300°C for 48h, 400°C for 1h, 450°C for 1h, 500°C for 1h e 550°C for 1h, with a used a heating rate of 0.5°C.min-1 The Influence of the addition of complexing agent diethanolamine (DEA) on the properties of the final material was investigated. The samples were characterized by thermal analysis (TGA), R-X diffraction, scanning electron microscopy (SEM/EDS), transmission electron microscopy and optical measurements (UV-Vis). The results show the obtainment of a well crystallized CZTS phase with the heat treatment of 400°C with ratios S/(Cu+Zn+Sn) =1.1, Cu/(Zn+Sn) = 0.8 e Zn/Sn = 1.26. Homogeneous and compact films with the morphology of 10 nm spheres were found in this study. However, the presence of some secondary phases increases the band gap to approximately, 2.2 eV. Furthermore, it was found that the addition of DEA in the precursor solution does not increase the quality of CZTS formed by electrospinning.

Les cellules solaires nanostructurées ont été développées pour réduire le coût du photovoltaïque et le rendre compétitif aux autres sources d’énergies. Dans ce but deux cellules solaires ont été étudié durant la thèse: la cellule « eta » (Extremely Thin Absorber) et la cellule hybride à polymères. Dans un premier temps, des couches 2D et nanofils de ZnO ont été réalisés par voie électrochimique sur des substrats verre/TCO (oxyde transparent et conducteur). Il est montré que la température du bain, la densité de charge et la concentration de l’électrolyte support (KCl) infleuncent la morphologie, composition, cristallisation et propriétés optiques des couches. Les films déposés à 0,1 M KCl et à T ≥ 50°C, présente de bonnes propriétés physico-chimiques. La couche 2D est ensuite utilisée pour la croissance des nanofils de ZnO et leurs dimensions sont ajustées avec la moprhologie et l’épaisseur de cette couche. L’électrolyte support et la densité de charge permettent également de contrôler les dimensions des nanofils. Dans un deuxième temps, les nanofils de ZnO ont été photo-sensibilisés par deux types d’absorbeurs : CuInS2 (CIS) et Cu2ZnSnS4 (CZTS). Ils ont été réalisés par différentes méthodes : SILAR (Successive Ion Layer Adsorption and Reaction), électrodépôt et dépôt de nanoparticules pré-synthétisées (pour CIS). Les films préparés par voie SILAR sont très uniformes autour des nanofils. Tandis que ceux réalisés par électrodépôt sont moins homogènes mais de très bonnes qualités cristallines. Grâce à la fonctionnalisation des nanofils, une couche de nanoparticules de CuInS2 très uniforme est déposée. Les cellules « eta » réalisées avec ces structures cœur/coquille montrent un effet photovoltaïque. Les films de ZnO électrodéposés ont été intégrés dans des cellules solaires hybrides à polymères sur substrats verres et plastiques. Ces cellules ont montré de bons rendements et une haute stabilité. / Nanostructured solar cells have been proposed as a solution for photovoltaic cost reduction and to rival the cost of grid-powered electricity. Regarding this challenge, two kinds of solar cells have been studied within the PhD thesis: the Extremely Thin Absorber Solar cells (eta) and the polymer hybrid solar cell. First, we are reporting on the electrochemical deposition of ZnO 2D layers and nanowires on glass substrates covered with TCO (Transparent Conducting Oxide). It is shown that the bath temperature and the supporting electrolyte concentration (KCl) play an important role on the ZnO layer morphology, composition, crystallization and optical properties. The film deposited from 0.1 M KCl and T ≥ 50°C exhibit very good optical and structural properties. These 2D layers are used for consequent ZnO nanowires electrodeposition and their dimensions could be tailored by the seed layer morphology and thickness. The supporting electrolyte concentration and the passed charge density could be additionally used to control their dimensions. Then, the ZnO nanowires have been photosensitized with two absorbers: CuInS2 (CIS) and Cu2ZnSnS4 (CZTS). These materials are prepared by: Successive Ion Layer Adsorption and Reaction (SILAR), electrodeposition and nanoparticules deposition (for CIS). The SILAR films are very uniform around the nanowires. The layers prepared by electrodeposition are less uniform but exhibit very good structural properties. Uniform thin film of CuInS2 nanoparticules are deposited onto functionalized ZnO nanowires. The eta solar cells fabricated with these core/shell nanostructures have shown a photovoltaic effect. The ZnO thin films have been integrated in hybrid solar cells on flexible and rigid substrates. These cells show good power conversion efficiency and a high stability.

Structure, composition, and optical properties of colloidal mercaptoacetate-stabilized Cu2ZnSnS4 (CZTS) nanocrystal inks produced by a “green” method directly in aqueous solutions were characterized. A size-selective precipitation procedure using 2-propanol as a non-solvent allows separating a series of fractions of CZTS nanocrystals with an average size (bandgap) varying from 3 nm (1.72 eV) to 2 nm (2.04 eV). The size-selected CZTS nanocrystals revealed also phonon confinement, with the main phonon mode frequency varying by about 4 cm−1 between 2 nm and 3 nm NCs.

Cu2ZnSnS4 (CZTS) is a thin film solar cell material that only contains abundant elements and for which promising conversion efficiencies of 9.2 % have been shown. In this thesis composition measurements and reactive sputtering of precursors for CZTS films have been studied. These precursors can be annealed to create high quality CZTS films. Accurate control and measurement of composition are important for the synthesis process. The composition of a reference sample was determined using Rutherford backscattering spectroscopy. This sample was thereafter used to find the composition of unknown samples with x-ray fluorescence measurements. Pros and cons with this approach were discussed. The reactive sputtering process, and the resulting thin films, from a CuSn- and a Zn-target sputtered in H2S-atmosphere were investigated and described. A process curve of the system was presented and the influence of sputtering pressure and substrate temperature were examined. The pressures tested had little influence on the film properties but the substrate temperature affected both composition and morphology, giving less Zn, Sn and S and a more oriented film with increasingly facetted surface for higher temperatures. The precursors produced with this method are suggested to have a disordered phase with randomized cations, giving a CZTS-like response from Raman spectroscopy but a ZnS-pattern from x-ray diffraction measurements. The films have an excellent homogeneity and it is possible to achieve stoichiometric sulfur content. The complete steps from precursors, to annealed films, to finished solar cells were investigated for three controlled compositions and three substrate temperatures. The films sputtered at room temperature cracked when annealed and thus gave shunted solar cells. For the samples sputtered at higher temperatures the trend was an increased grain size for higher copper content and increased temperature. However, no connection between this and the electrical properties of the solar cells could be found.