Synthesis, Processing, and Fundamental Phase Formation Study of CZTS Films for Solar Cell Applications

Abstract

Copper zinc tin sulfide (Cu2ZnSnS4 or CZTS) kesterite compound has attracted much attention in the last years as a new abundant, low cost, and environmentally benign material with desirable optoelectronic properties for Photovoltaic (PV) thin film solar cell applications. Among various synthesis routes for CZTS thin films, sol-gel processing is one of the most attractive routes to obtain CZTS films with superior quality and low cost. In this study, sol-gel sulfurization process parameters for CZTS thin films were systematically investigated to identify the proper process window. In addition, temperature dependent Raman spectroscopy was employed to monitor the CZTS sulfurization process in real time and gain fundamental information about the phase formation and degradation mechanisms of CZTS under the relevant processing conditions. It was found that CZTS thin films with different Cu stoichiometry can be prepared using parts-per-million (ppm) level of hydrogen sulfide (H2S) gas as opposed to high percentage level of H2S (e.g., ≥ 5%) in all previous studies. Samples sulfurized at lower temperatures of ~350°C and 125°C revealed the formation of CZTS phase as confirmed by XRD, Raman micro-spectroscopy, and sheet resistance measurement. Local EDS analysis indicates that CZTS films prepared at those low temperatures have a near-stoichiometric composition and are sometimes accompanied by the formation of Cu2-xS phase(s). Also, stoichiometric and Cu-rich precursor solutions tend to yield CZTS samples with better crystallinity and superior optical properties compared with the Cu-deficient solution. Moreover, in situ Raman monitoring of phase formation of CZTS material was carried out from room temperature up to 350°C in a 100 ppm H2S+4%H2+N2 gas mixture. The results showed that CZTS phase formed in about 30 min via a direct reaction between the metal oxide precursor film and the H2S-H2 gas mixture at an intermediate temperature of 350°C and remained stable upon extended exposure. In comparison, at a lower temperature (170°C), the oxide precursor film had to be reduced first (e.g., in 4% H2/N2 forming gas) and then the CZTS phase emerged. However, continued sulfurization at a lower temperature (e.g., 170°C) led to the disintegration of CZTS and the formation of CuS impurity, which remains stable upon cooling the sample down to room temperature. Furthermore, results of in situ Raman monitoring of CZTS films in an oxygen-rich atmosphere at elevated temperatures up to 600°C suggested that CZTS oxidizes first at ~400°C to form tin oxide (SnO2) and binary sulfides of mainly copper sulfide (Cu2-xS) and zinc