Investigation of Zn/Sn ratio for improving the material quality of CZTS thin films with the reduction of Cu2-xS secondary phase

Abstract

In this study, a sputtering system was adopted for the preparation of Cu2ZnSnS4 (CZTS) thin films with high material quality. The system precisely controlled the elemental ratio in a copper-zinc-tin (CZT) precursor for contributing to the grain growth of CZTS crystallites, and reduced the detrimental formation of the Cu2-xS secondary phase during the high-temperature sulfurization process.The CZT precursor was prepared in a layer sequence of Zn/Sn/Cu/Zn, as the Cu-capping layer could protect the underlying Zn/Sn bilayer from volatilization during the high-temperature sulfurization. The CZT precursors were subjected to the sulfurization process at varied temperatures ranging from 480 to 560 °C for 20 min to form the CZTS thin film. In this study, the Cu/(Zn + Sn) and Zn/Sn ratios in the CZTS film were studied by energy dispersive X-ray spectroscopy measurements, and both ratios revealed elevations with an increasing sulfurization temperature, owing to the thermal volatilization of Zn and Sn. Thus, the formation of the undesirable Cu2-xS secondary phase was enhanced, and the surface roughness and density of voids on the CZTS thin films were increased. For a reduced Zn/Sn ratio with a reduced sulfurization temperature, the improved CZTS thin-film quality was demonstrated by the increased X-ray diffraction (XRD) and Raman intensity, along with the reduction of the XRD linewidth of the (112) CZTS crystal phase. A CZTS thin film with a sulfurization condition of 500 °C for 20 min showed a low Zn/Sn ratio (at 1.1), along with the features of a dense and smooth thin-film surface. Moreover, a reduced photoluminescence spectra full width at half maximum (FWHM) value of 92.7 meV was observed.In addition, for further improving the grain growth of CZTS, a pre-annealing process of a CZT precursor was conducted at a temperature of 350 °C for 30 min. An enhancement of the CZTS crystallite size with the stoichiometric formation of a CZTS phase and a reduced Cu2-XS phase was observed, indicating the superior material quality of the CZTS thin films.