Imaging and phase identification of Cu₂ZnSnS₄ thin films using confocal Raman spectroscopy

Copper zinc tin sulfide (Cu₂ZnSnS₄ or CZTS) is a potential candidate for next generation thin film solar cells because it contains abundant and nontoxic elements and exhibits high light absorption. Thin films of CZTS are typically synthesized by sulfidizing a stack of zinc, copper, and tin films. In addition to CZTS, a variety of binary and ternary metal sulfides can form and distinguishing among phases with similar crystal structure can be difficult. Herein, the authors show that confocal Raman spectroscopy and imaging can distinguish between CZTS and the other binary and ternary sulfides. Specifically, Raman spectroscopy was used to detect and distinguish between CZTS (338 cm^-1), Cu₂SnS₃ (298 cm^-1), and Cu₄SnS₄ (318 cm^-1) phases through their characteristic scattering peaks. Confocal Raman spectroscopy was then used to image the distribution of coexisting phases and is demonstrated to be a useful tool for examining the heterogeneity of CZTS films. The authors show that, during sulfidation of a zinc/copper/tin film stack, ternary sulfides of copper and tin, such as Cu₂SnS₃ form first and are then converted to CZTS. The reason for formation of Cu₂SnS₃ as an intermediary to CZTS is the strong tendency of copper and tin to form intermetallic alloys upon evaporation. These alloys sulfidize and form copper tin sulfides first, and then eventually convert to CZTS in the presence of zinc. As a consequence, films sulfidized for 8 h at 400C contain both CZTS and Cu₂SnS₃, whereas films sulfidized at 500C contain nearly phase-pure CZTS. In addition, using Cu K radiation, the authors identify three CZTS X-ray diffraction peaks at 37.1 [(202)], 38 [(211)], and 44.9 [(105) and (213)], which are absent in ZnS and very weak in Cu₂SnS₃.