In this study, we produced thin-film solar cells using co-evaporated Ge-Sn-S thin film as the light-absorbing layer. The thin films were prepared at different concentrations of Ge and substrate temperatures. We characterized the solar cells and compared their physical properties with that of an SnS thin film fabricated using only Sn and S. The GexSn1-xS (x = 0.27) thin film solar cell exhibited the best performance, with short circuit current density Jsc = 0.66 mA/cm2, curve factor FF = 0.324, power conversion efficiency PCE = 0.036%, and open circuit voltage Voc = 0.169 V. The band gap of the GexSn1-xS (x = 0.27) thin film estimated by extrapolating the absorption edge of the external quantum efficiency was 1.57 eV, which is larger than that of the SnS thin film. This suggests that Sn (in SnS) is partially replaced by Ge to form a solid solution, thus widening the band gap.

Cu2SnS3 (CTS) obtained by depositing Au on an Sn/Cu metal stacked precursor fabricated by electron beam deposition and sulfurization was investigated. In thin films obtained by sulfurization at 560 °C of the precursor with SLG/Mo/Sn/Cu/Au/NaF structures fabricated on Soda lime glass substrates containing alkali metals, a significant increase in the CTS grain size was observed in the Au deposition thickness range of 525 nm. By contrast, no crystal growth was observed in thin films with a precursor without an NaF layer fabricated using alkali-free glass (EAGLE XG), regardless of the thickness of the Au-deposited film. Therefore, appropriate amounts of Au and Na promote the crystal growth of CTS. In addition, at the sulfurization temperature of 570 °C, the crystal grains were larger than those of the thin film fabricated at 560 °C. In the fabricated CTS thin-film solar cells, with sulfurization temperature of 570 °C and Au deposition thickness of 10 nm, open circuit voltage of 0.261 V, short circuit current density of 25.4 mA/cm2, fill factor of 0.425, and power conversion efficiency of 2.82% were obtained.