Optical and Morphological Characterization of Copper Zinc Tin Sulfide (CZTS) Nano-crystal Thin Film Solar Cell

Abstract

Development of economical and highly proficient solar cells is crucial to gratify the rising global call. Solar energy generation is a technologically feasible method, though primarily it’s expensive. Copperzinc-tin-sulfide

(CZTS) is another alternative compound that fascinates all soalr cell material researchers. To date, the best CZTS solar cell converts only 11% (for CZTSe, it’s 12.6% for ACZTSSe, 13.8%.) of light into electricity, paralleled to above 27.6% for single crystal silicon concentrator type solar cells. As the theoretical efficiency of CZTS is about 30%, therefore, it can be said that CZTS solar cells are still in a premature enquiry stage where much of the area is yet to be explored. The rapid advancements in the efficiency of CZTS solar cells are accredited to a trial-and-error methodology to module assembly. This has led to a fundamental knowledge gap of CZTS. This thesis aimed to reduce this knowledge gap to its best. Throughout the study, the ability to regulate the composition, crystallinity and stoichiometry of CZTS thin films has been demonstrated and preliminary efforts in pre and post annealing treatment towards crystalline films are promising indeed. In the context of this project, tunable band gap absorber material, specifically, Cu 2 ZnSnS 4 (CZTS) is fabricated by three different methods to find out better morphology, composition, stoichimetry and other opto electronic properties for photovoltaic application. Kesterite CZTS is collection of plentiful fundamentals and non-toxic substantial, with necessary attributes for photovoltaic (PV) uses, for example, high absorption coefficient ~10 -4 cm -1 and a band gap energy (E g ) adjacent to 1.5 eV. The effect of various deposition techniques and annealing parameters on film growth was explored using optical, morphological, and structural material characterizations along with composition analyses. The sputtered kesterite which reveals best quality thin film for this study, must be annealed to produce device-grade films. Moreover, sputter system provides the facilities achieving a desired thickness of the film. The prepared film is undergoing post-annealing treatment with different temperatures (250-560° C) and pressure (150- 450 Torr). XRD pattern shows preferential characteristics peak along (112), (220), and (312) and phase purity is inveterate by Raman studies. The granules are compact, and as the annealing temperature rises, agglomeration increases, boosting the absorption coefficient. The band gap energy (E g ) differs between 1.47-1.51 eV which is compatible with optimum values. Sputtered films have uniform surface topography and thickness. As a result, the film has significantly better covering than the sol deposited film. RMS roughness increases with annealing heat and base pressure. The chemical composition of the fabricated sample shows good atomic stoichiometry of the film. The elemental composition is observed as Cu and S enriched Zn and Sn deficient without sulfurization. SnS loss occurs at temperatures of above 500 °C which results cavities on film surface and affects the stoichiometry of the film. The current study shows that sputter deposition may be used to produce CZTS thin films on soda lime glass (SLG) with molybdenum (Mo) back contact for prospective solar cell applications. In addition, impurity doped ZnO ( ZnO: Al or AZO), which has been examined for this thesis, is currently the best indium-free contender for an alternative TCO compound. In particular, the study is concentrated on the consequence of depositions factors and techniques with extensive analysis of AZO fabrication to find an equally transparent, efficient, cheap, more readily available, and electrically conductive alternate to Indium. The findings of the research work encompasses a range of significant contributions, from discovering empirical evidence to optimizing different key parameters for CZTS and AZO thin films, and finally, developing a complete and optimized SLG/ Mo/CZTS/CdS/iZnO/AZO solar cell fabrication process. Moreover, A comprehensive theoretical model of CZTS solar

cell

has been developed and anlyzed. From the Simulations, the maximum PCE is shown as 23.74% with V oc = 1.62 V and J sc = 28.37 mA/cm 2 for 3000 nm CZTS absorber layer. The research not only advances the understanding of solar cell technology but also has practical implications for sustainable energy solutions in the future.