Cu₂(Sn1−xGex)S₃ solar cells prepared via co‐evaporation and annealing in germanium sulfide and sulfur vapor

Cu 2 Sn 1 −x Ge x S 3 (CTGS) thin films were prepared by co‐evaporation of Cu, Sn, and S to form Cu 2 SnS 3 (CTS) precursors, which were then annealed at 570 °C in an atmosphere composed of N 2 , GeS 2 , and S vapor. The films were then used to fabricate photovoltaic cells with the structure glass/Mo/CTGS/CdS/ZnO:Al/Al. A cell with a film composition of Cu 2 (Sn 0.86 Ge 0.14 )S 3 fabricated from slightly Cu‐rich CTS (Cu/Sn = 2.07) exhibited a conversion efficiency of 3.4% and an open‐circuit voltage of 0.29 V. The band gap based on the external quantum efficiency was estimated to be approximately 1.0 eV. The open‐circuit voltage was found to be larger than that for a CTS thin film solar cell and increased with band gap energy.