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.

L'objectif principal de cette thèse est dirigé vers l'établissement et l'explication des relations entre les conditions de synthèse des couches minces de CZTSSe, ses propriétés physiques et les performances des dispositifs photovoltaïques. Pour faire face à cette tâche la première approche était de comprendre le mécanisme de formation de la matière par rapport aux conditions de croissance du matériau. Le CZTSSe est synthétisé par un processus de sélénisation en deux étapes, où une première étape de dépôt par PVD de précurseurs est nécessaire, suivie d'une seconde étape de recuit sous atmosphère de sélénium. Différents ordres d'empilement de précurseurs ont été étudiés afin de comprendre la séquence de réactions qui, à partir de leur dépôt, conduise à la couche finale de CZTSSe. Cette étude, fait en plusieurs étapes, a nécessité de un effort important sur la caractérisation du matériau à chaque étape de la synthèse. Le résultat a montré que dans le cas du procédé en deux étapes, le matériau final est indépendant du dépôt de précurseurs. Les possibles implications bénéfiques en raison de l'incorporation de sodium dans le CZTSSe sont également décrites. Cette étude est réalisée en synthétisant la couche de CZTSSe sur différents substrats contenant diffèrent taux de sodium: de cette manière, pendant la synthèse, le sodium migre de substrats vers l'absorbeur. Après quantification du Na dans le CZTSSe juste après la croissance, le matériau est caractérise afin d'évaluer sa qualité. Ensuite il est employé dans une cellule solaire complète pour vérifier ses propriétés photovoltaïques. Les résultats ont montré que, comme pour la technologie CIGS, le sodium est bénéfique pour le CZTSSe, permettant l'augmentation de la tension à circuit ouvert et le rendement de cellule. Le molybdène est le contact arrière le plus utilisé pour les cellules solaires à base CZTSSe. Cependant, il a été suggéré récemment que le Mo n'est pas stable à l'interface avec le CZTSSe. En outre, à ma connaissance, aucune étude expérimentale n'a été effectuée à ce jour pour tester si les cellules solaires construites sur un autre contact arrière pourraient présenter de meilleures propriétés photovoltaïques. A cet effet, divers métaux (Au, W, Pd, Pt et Ni) sont déposées sur le dessus de Mo et testés comme contacts arrières dans les cellules solaire à base de CZTSSe. Il est démontré qu'il est possible synthétiser de films minces de CZTSSe de qualité quand le tungstène, l'or et le platine sont employé comme contacts arrière. Il est démontré que les contacts en W et Au permettent d'augmenter le courant photogénéré, mais aussi que le Mo reste le meilleur contact arrière en termes d'efficacité de conversion. Les effets de la variation du rapport [S]/([S]+[Se]) sur les performances des cellules solaires à base CZTSSe ont été étudiés. Cette étude a été faite par simulations des cellules solaires à base de CZTSSe, où le taux de chalcogènes dans l'absorbeur est varié, avec l'objective de trouver la composition optimale de l'absorbeur. Deux types d'approche différente ont été étudiés: la variation linéaire du rapport des chalcogènes, et une variation parabolique. Les simulations conduisent à un rendement de 16,5% (avec une tension en circuit ouvert de 0,56 V, courant de court-circuit de 37,0 mA/cm2 et un facteur de forme de 79,0%) lorsque la teneur en soufre est diminué linéairement à partir du contact arrière en direction de la couche tampon. Sur la base de ces résultats, nous proposons que l'ingénierie de bande interdite sur la base de la variation du taux [S]/([S]+[Se]) dans l'absorbeur est un outil puissant qui permet d'augmenter les performances des cellules solaires à base CZTSSe sans changer la qualité de l'absorbeur en lui-même. / The main objective of this PhD thesis was directed toward establishing and explaining the relationships between synthesis conditions of CZTSSe, its physical properties and performance of photovoltaic devices. To tackle on this task the first approach was to understand the formation mechanism of the material in relation to the growth conditions. CZTSSe is synthesized by two-step selenization process, where a first step of precursor deposition by PVD is required, followed by a second step of annealing. Different precursor stacking orders have been studied in order to understand the sequence of reactions that, starting from their deposition, lead to the final CZTSSe layer. This study made step-by-step has required a strong effort on the material characterization at each step of the synthesis. The result demonstrated that in the case of two-step process, the final material is independent of the precursor deposition. The possible beneficial involvements due to incorporation of sodium in CZTSSe are also disclosed. This study is carried out by synthesizing CZTSSe on different sodium-containing substrates: in this way sodium migrates from the substrates to the absorber. After quantification of Na in CZTSSe right after growth, the latter is characterized to evaluate its quality and employed in a full solar cell to check on its photovoltaic properties. Results demonstrated that, as for CIGS technology, sodium is beneficial for CZTSSe allowing increasing the open circuit voltage and efficiency. Molybdenum is the most used back contact in CZTSSe based solar cells. However, it has been suggested recently that Mo is not stable at the interface with CZTSSe. In addition, to the best of our knowledge, no experimental study has been carried out so far to test whether solar cells built on another back contact could exhibit better photovoltaic properties. For this purpose, various metals (Au, W, Pd, Pt, and Ni) are deposited on top of Mo, and it is demonstrated that it is possible to synthesize device-quality CZTSSe thin films on W, Au, and Pt back contacts. It is shown that that W and Au back contacts allow enhancing the photogenerated current, but that Mo remains the best back contact in terms of power conversion efficiency. The effects of [S]/([S]+[Se]) ratio tuning on CZTSSe based solar cell performances have been studied by solar cell capacitance simulator (SCAPS) to find out the optimum absorber composition. Two different kind of approach have been studied: linear variation of the chalcogens ratio, and a parabolic variation. The simulations lead to an efficiency of 16.5% (with open-circuit voltage of 0.56 V, short-circuit current of 37.0 mA/cm2 and fill factor of 79.0%) when the sulfur content is linearly decreased from the back contact towards the buffer layer. Based on these results, we propose that bandgap engineering based on the control of [S]/([S]+[Se]) ratio in the absorber is a powerful tool which allows increasing the performances of CZTSSe based solar cells without changing the absorber material quality.

<div>Because of the limited reserve of fossil fuels and issues brought up by their combustion, the demand on renewable energy is considerably increasing. Solar energy is one of the most promising renewable energy sources considering the large amount of solar irradiation received by Earth and solar cell is such a device that allows us to directly convert sunlight directly into electricity. In this thesis, kesterite (I2-II-IV-IV4) system is the main focus as the light absorber material in thin film solar cells.</div><div><br></div><div>Cu2ZnSn(S,Se)4 (CZTSSe) has been first studied intensively. However, due to the band tailing resulting from Cu-Zn anti-site defects, further improvement on power conversion efficiency of this material has been hindered. Substituting Cu with Ag is expected to solve this problem by decreasing this defect density as a result of the high formation energy of Ag-Zn antisite defects. Herein, different concentrations of Ag are used to substitute Cu in the kesterite system through a nanoparticle-ink route for the fabrication of light absorber thin films. For Ag-alloying concentration less than 50%, it suggests that the Ag can induce inhomogeneity as well as secondary phase formation during nanoparticle formation. Moreover, Ag alloying is shown to enlarge the grain size and reduce film roughness after selenization, which are beneficial for the optoelectronic properties and device performance.</div><div><br></div><div>Additionally, the synthesis process for kesterite Ag2ZnSnS4 nanoparticles is explored. AZTS nanoparticles are achieved by solvent-thermal reaction. The reaction pathway during reaction is investigated by different material characterization methods to shed light on the Ag-based nanoparticle synthesis. The final nanoparticles obtained have high crystallinity and homogeneous composition, demonstrating great potential as light absorber materials. Also, the sulfide nanoparticles are converted into selenide thin films in Se vapor at elevated temperature (selenization). The selenization conditions, including temperature, heating ramp and selenization time, are optimized for the pure phase kesterite AZTSe thin films with large and dense grains. The optoelectronic properties are explored on these films and an initial research already demonstrates a 0.35% power conversion efficiency as the first solution processed AZTSe device.</div><div><br></div><div>In summary, multiple material characterization techniques are utilized to understand the microstructure evolution, phase transformation, and composition change for solution-processed nanoparticles and their resulting thin films. The material characteristics, process methods and film optoelectronic properties are associated for the future analysis and development of kesterite thin films for photovoltaic applications.</div><div><br></div>

>Magister Scientiae - MSc / Tellurium is a member of the chalcogen group in the periodic table and is known to be a better semiconductor material when compared to sulfur and selenium. By introducing tellurium into the kesterite structure there would be an improvement in the semiconducting property of the kesterite material. This research focused on incorporating tellurium into kesterite structure in order to reduce its band-gap thereby improving its light absorption and ultimately lead to a more efficient photovoltaic effect. For a typical synthesis, kesterite nanoparticles were synthesized by anion hot injection process which involved the injection of the anion precursor comprising of sulfur, selenium and tellurium in diethylene glycol into a solution containing the cation precursor which are copper (II) chloride, Zinc chloride and tin (II) chloride which are dissolved in diethylene glycol. The synthesized nanoparticles were copper zinc tin sulfide (CZTS), copper zinc tin sulfide selenide telluride (CZTSSeTe) and copper zinc tin sulfide telluride (CZTSTe). Morphological characterization of the synthesized nanoparticles was carried out by high-resolution scanning electron microscopy (HRSEM) and high-resolution transmission electron microscopy (HRTEM) to obtain the shape of the surface and internal structure of the nanoparticles respectively. The micrograph obtained from HRSEM shows that all synthesized nanoparticles had a flower-like surface appearance which is a common morphology obtained for non-vacuum synthesized kesterite nanoparticles. The micrograph obtained from TEM showed that all nanoparticles were agglomerated and had a black surface covering which attributable to the solvent used during synthesis, washing and centrifugation. The internal structure of the synthesized nanoparticles was obtained through small angle x-ray scattering (SAXS) plot of the shapes. The shape obtained for the nanoparticles were core shell hollow sphere for CZTS, core shell dumb-bell for CZTSSeTe and solid sphere for CZTSTe.

Copper zinc tin selenide (Cu2ZnSnSe4 or CZTSe) is a quaternary compound semiconductor material that has attained more and more attention for thin film photovoltaic applications. CZTSe is only comprised of abundant and non-toxic elements. People have concerns about availability and cost of indium from CIGS and tellurium from CdTe, also about cadmium’s toxicity. These concerns have promoted CZTSe as an alternative thin film solar cell material. The major issues about CZTSe absorber fabrication are: tin loss during selenization process and existence of secondary phases. Recent improvements of CZTSe absorber have increased the efficiency of CZTSe thin film solar cell to 9.7% in laboratory, and this was accomplished by using H2Se as selenium source in a “two-stage” process. [1] However “one-stage” vacuum co-evaporation technique is still the most popular technique for CZTSe thin-film solar cells fabrication. In this research, Cu2ZnSnSe4 thin-film solar cells have been fabricated by using a two-step rapid thermal selenization process. The first step selenization is operated at 375℃, a relatively low annealing temperature, which helps avoiding the most common issue of tin loss. The second step selenization is carried out at a higher annealing temperature, 400℃ to 500℃, at where the formation of CZTSe quaternary compound can be completed, and fewer secondary phases remain in the CZTSe absorber bulk. A specially designed metallic precursor stacks deposition order has been developed to inhibit tin loss and zinc loss during selenization. Vacuum co-evaporation technique is not feasible to mass production, due to facility difficulty and bad uniformity. And H2Se is toxic and dangerous. We have developed these metallic precursor stacks vacuum deposition process and two-step selenium vapor selenization process. We believe this technique is more suitable for potential mass production in future. The properties of CZTSe thin-films and the performance of CZTSe thin-film solar cells have been characterized using techniques, including J-V, Raman spectroscopy, spectral response, and SEM/EDS. The best performance CZTSe thin-film solar cell that have been accomplished, has an open circuit voltage of 0.42 volt, shirt circuit current densities of 14.5 mA/cm2, fill factor of 47%, and efficiency of 2.86%.

Halbleitermaterialien sind in den elektrischen Anwendungen der heutigen Zeit unerlässlich geworden. In dieser Arbeit wird der Fokus auf die Untersuchung der elektrischen und thermischen Eigenschaften von zwei Halbleiterklassen gelegt. Zum einen wird mit -Ga2O3 ein Mitglied der Klasse der transparenten leitfähigen Oxide untersucht.Hier wurden die elektrischen Eigenschaften von dünnen Schichten (Dicke von 28nm-225nm) und Volumenkristallen temperaturabhängig untersucht.Dabei zeigt sich bei Volumenkristallen und mindestens 150nm dicken Schichten eine Steigerung der elektrischen Leitfähigkeit bis 100K durch die Streuung von Elektronen an Störstellen und bei Temperaturen über 100K wieder ein Abfall der elektrischen Leitfähigkeit durch Elektron-Phonon-Wechselwirkung. Die Untersuchung der thermische Leitfähigkeit von beta-Ga2O3 zeigt ein anisotropes Verhalten mit minimalen Werten in [100]-Richtung und maximalen Werten in [010]-Richtung. Die Temperaturabhängigkeit der thermischen Eigenschaften zeigt eine Verringerung der thermischen Leitfähigkeit und der thermischen Diffusivität mit steigender Temperatur. Eine zweite untersuchte Materialklasse ist die der Kesterite. Zu dieser Kristallstruktur wurden zwei Elementkonfigurationen untersucht, Kupfer-Zink-Zinn-Sulfid und Kupfer- Zink-Zinn-Selenid. Der Transport bei Raumtemperatur und darunter findet über verschiedene Tunnelprozesse lokalisierter Ladungsträger statt. Zusätzlich wird auf die Veränderung der elektrischen Eigenschaften durch die Kristallinität und Komposition eingegangen. Die thermischen Eigenschaften zeigen analog zum beta-Ga2O3 eine Dominanz der Phonon-Phonon-Umklapp-Streuung bei hohen Temperaturen, während bei niedrigen Temperaturen Streuung an Störstellen und Grenzflächen vorherrscht. Methodisch zeigt diese Arbeit unterschiedlichste Messmethoden zur Charakterisierung der elektrischen und thermischen Eigenschaften, welche die Standardmethoden sowohl nutzen, als auch sinnvoll erweitern. / Semiconductors are essential for electronic applications nowadays. Here, the electrical and thermal properties of two semiconductor classes with huge application potential are investigated. As a transparent conducting oxide beta-Ga2O3 is investigated. In this work, the temperature dependent electrical properties were investigated for bulk materials and thin films. An increase in the electrical conductivity until 100K is found through electron-impurity-scattering and a decrease at higher temperatures through electron-phonon-scattering for for films with a thickness of at least 150nm. The investigation of the thermal properties of -Ga2O3 show an anisotropy for the different crystal orientations with minimal primary axis values for the [100]-direction and maximal values for the [010]-direction. The temperature-dependence of the thermal properties shows a decease in conductivity and diffusivity for increasing temperature. For temperatures over 150K phonon-phonon-Umklapp-scattering can explain the measured values. For low temperatures phonon-impurity scattering is most likely the dominant scattering mechanism. A second investigated material class are kesterites. For this crystal structure two configurations were investigated, copper-zinc-tin-sulfide and copper-zinc-tin-selenide. The electrical properties show semiconducting characteristics with p-type conduction. The transport processes are defined through localised thermal activated tunneling within the band gap. Other reductions of the mobility are found by the crystalinity and the composition of the materials. The thermal properties show dominant phonon-phonon- Umklapp-scattering at higher temperatures and phonon-impurity-scattering for lower temperatures in a similar way as in beta-Ga2O3. This work shows new implemented measurement methods for investigating electrical and thermal properties as extentions to common methods.

Dans un contexte général d’augmentation de la demande énergétique et de préoccupation croissante face au réchauffement climatique et à la limitation des ressources naturelles, l’utilisation d’énergie solaire devrait augmenter. L’avenir des différentes technologies photovoltaïques dépend évidemment de leur rendement de conversion photovoltaïque et de leur coût mais aussi de la disponibilité des ressources. Les couches minces de kesterite, Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) ou Cu2ZnSn(S,Se)4 (CZTSSe), composées d’éléments abondants dans la croûte terrestre se positionnent en candidat prometteur pour la conversion d’énergie solaire à grande échelle.Dans cette thèse, l’électro-dépôt, un procédé compatible avec des exigences industrielles de production, est utilisé pour déposer un précurseur de cuivre, étain et zinc sur des substrats de 15 × 15 cm2, de composition et épaissseur contrôlables. Ce précurseur est ensuite converti en semiconducteur par traitement thermique en présence de soufre ou de sélénium. Les couches ainsi formées de Cu-Zn-Sn-S ou Cu-Zn-Sn-Se, doivent être uniformes et présenter les propriétés appropriées (phases, composition, morphologie) pour la fabrication de cellules solaires à haut rendement. Le procédé de fabrication de la cellule solaire complète, notamment les étapes qui interviennent dans la formation de la jonction p-n (décapage chimique et dépôt de couche tampon) est également optimisé pour maximiser les rendements. A l’issue de ces optimisations, un rendement de 9.1% est obtenu pour une cellule solaire CZTSe, un nouveau record pour les cellules solaires à base de kesterite fabriquées par électro-dépôt. / Facing growing energy demand and increasing concerns about climate change and finite energy sources, solar energy use should increase. The future of the different photovoltaic technologies obviously depends on their power conversion efficiency and cost (summarized by the ratio cost per watt), but also on the elements availability. Thin films of earth-abundant kesterite, Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) or Cu2ZnSn(S,Se)4 (CZTSSe), which can be manufactured with low-cost processes, are promising candidates for solar energy conversion at large scale.In this thesis, a copper tin and zinc precursor of controllable composition and thickness is electrodeposited on 15 × 15 cm2 substrates. Electrodeposition is a process compatible with high throughput low-cost and safety industry requirements. The precursor is converted into a semiconductor by thermal treatments in presence of sulfur or selenium. The resulting Cu-Zn-Sn-S or Cu-Zn-Sn-Se layers should be uniform and have adequate properties (phases, composition and morphology) to produce high efficient solar cells. Full device processing, including the pn junction formation steps (wet chemical etching and buffer layer deposition) is also investigated in order to maximize device efficiency. The best CZTSe solar cell exhibits a 9.1% powerconversion efficiency, setting a new record for kesterite solar cells produced by electrodeposition.

Ce travail de thèse porte sur la modélisation et la simulation numérique de cellules solaires à base de kësterite (CZTSé, CZTS) dans le but d’étudier leurs mécanismes physiques et d’améliorer la conception de ces dispositifs. Les kësterites sont une classe de matériaux que l’on peut déposer en couches minces et qui sont constitués d’éléments abondants sur Terre et donc à faible coût. Deux modèles numériques pour les cellules solaires CZTSe et CZTS sont proposés. Des simulations 1D et 2D sont réalisées: le logiciel SCAPS est utilisé pour étudier l’impact des couches de molybdène et de MoSe2, présents au contact arrière des cellules solaires CZTSe. Nous étudions également les propriétés idéales de couches d’interface alternatives qui pourraient remplacer le MoSe2 pour améliorer les performances des cellules solaires. La méthode des matrices de transfert (TMM) et le logiciel SCAPS sont utilisés conjointement pour effectuer des simulations optoélectroniques dans le but d’optimiser l’épaisseur du buffer (CdS) et le TCO (Transparent Conductive Oxide) afin de maximiser le courant de court-circuit (JSC ) des cellules solaires. Enfin Silvaco est utilisé pour réaliser des simulations 2D des joints de grains (GBs) du CZTSe présents à l’intérieur des absorbeurs polycristallins de la kësterite. Pour ce faire, des caractérisations KPFM sont effectuées dans le but de trouver des corrélations possibles entre les pertes de rendement et l'activité électrique des GBs. / This thesis deals with modeling and simulations of kesterite solar cells with the aim of studying their physical mechanisms and improving the design of the devices. Synthetic kesterites are thin film materials made of cheap/earth-abundant elements. Two numerical models for a Cu2ZnSnSe4 (CZTSe) and a Cu2ZnSnS4 (CZTS) solar cell are proposed. The provided values of the material parameters, for all the layers of the solar cell, are obtained either from comparisons/analysis of data found in literature or, in some cases, from direct measurements. 1D and 2D simulations are performed: the software SCAPS is used to study the impact of the Molybdenum and the MoSe2 layers, present at the back contact of CZTSe solar cells. We investigate also the ideal properties of alternative interfacial layers that could replace the MoSe2 layer to improve the device performances. The transfer matrix method (TMM) and SCAPS are employed together to perform optoelectronic simulations with the aim of optimizing the thickness of the buffer (CdS) and the window (ITO) layers in order to maximize the short circuit current (JSC ) of the device. Finally Silvaco is used to perform 2D simulations of the CZTSe grain boundaries (GBs) present inside the polycrystalline kesterite absorbers. For the latter work, experimental Kelvin probe force microscopy (KPFM) characterizations are performed in order to find possible correlations between the performance losses and the electrical activity of the GBs.

Because of the anticipated high demand for Indium, ongoing growth of CIGS technology may be limited. Kesterite materials, which replace In with a Zn/Sn couple, are thought to be a solution to this issue. However, efficiencies are still below the 10% level, and these materials are proving to be complex. Even determination of the bandgap is not settled because of the occurrence of secondary phases. We use a film growth process, 2SSS, which we believe helps control the formation of secondary phases. Under the right growth conditions we find 1/1 Zn/Sn ratios and XRD signatures for Cu2ZnSnSe4 with no evidence of secondary phases. The optical absorption profile of our films is also a good match to the CIS profile even for films annealed at 500° C. We see no evidence of phase separation. The effect of intentional variation of the Zn/Sn ratio on material and device properties is also presented.