Effect of heating rate and precursor composition on secondary phase formation during Cu2ZnSnS4 thin film growth and its properties

Abstract

Cu2ZnSnS4 (CZTS) as an absorber material has proven its potential for high efficiency solar cells. However, commercial viability for low cost photovoltaic depends on improving its efficiency beyond the existing values which require an understanding of secondary phase formation. In this work, CZTS layer of up to one micron thickness is prepared by sulfurization (annealing in the sulfur environment) of a precursor prepared using magnetron co-sputtering of Cu, Zn and Sn on soda lime glass. Precursor composition during co-sputtering and heating rate of the sulfurization process were varied to study its effect on secondary phase formation during CZTS growth. Characterization techniques like X-ray diffraction, Raman spectroscopy and UV-Vis spectroscopy were used to detect the phases formed in the film. Scanning electron microscopy and energy dispersive x-ray spectroscopy were performed to estimate the elemental composition and its distribution in the film. The studies and the results obtained therein bring out the impact of heating rate on the grain size. Simultaneously the secondary phase formation is found to be affected more by precursor composition than the heating rate. Raman measurements and bandgap estimation from UV-visible absorption spectra indicated the formation of ZnS in the film. The study showed the possibility to completely avoid the formation of both copper sulfide (CuS, Cu2S, etc.) as well as ZnS secondary phases under certain combination of process parameters.