Unveiling the Phase Evolution of Sol–Gel Sulfurized Cu2ZnSnS4 Thin Films in ppm-Level H2S: From Binary Sulfides to Quaternary Cu-Zn-Sn-S System

Abstract

Understanding the reaction process governing the formation of sol–gel sulfurized copper zinc tin sulfide (Cu2ZnSnS4, CZTS) films and the growth of other secondary sulfide phases is crucial to process optimization and solar cell device performance. In the current study, sulfurization of the relevant single Cu oxide, single Sn oxide, and Cu-Sn oxide films related to the Cu-Zn-Sn-S system was carried out in 100 ppm H2S + 4%H2 + 96%N2 (by volume) at low to intermediate temperatures. At intermediate temperature of 350°C, sulfurization of Cu oxide, Sn oxide, and Cu-Sn oxide precursors showed no evidence for the formation of any binary or ternary sulfide phase(s) as observed during the in situ Raman monitoring experiments. These observations were also confirmed by subsequent x-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) analyses, while the stoichiometric Cu-Zn-Sn oxide film underwent a direct reaction between the oxides and H2S to form CZTS without binary or ternary impurities. In comparison, at lower temperature of 170°C, sulfurization of the single Cu oxide, Sn oxide precursors in 100 ppm H2S + 4%H2 + 96%N2 led to the formation of simple sulfides such as Cu2−xS (with x close to 1) and SnS2. As a result, despite sulfurization of sol–gel-based stoichiometric Cu-Zn-Sn oxide film at 170°C in the same gas mixture leading to CZTS formation, extended exposure to 100 ppm H2S leads to over-sulfurization and formation of detrimental impurities, especially CuS. The implications of the results with respect to the understanding and optimization of the phase formation process for CZTS light absorber material are discussed, and the direction for future research is suggested.