Cu 2 Sn 1- x Ge x S 3 ( x = 0.17) Thin-Film Solar Cells with High Conversion Efficiency of 6.0%

We have fabricated Cu 2 Sn 1- x Ge x S 3 thin-film solar cells by cosputtering deposition of Cu and Sn followed by sulfurization in S and GeS 2 vapors. The conversion efficiency was significantly improved to be as high as 6.0% compared with the values of Cu 2 SnS 3 solar cells similarly fabricated. Scanning electron microscopy observation revealed that alloying with Ge accelerated the grain growth during the sulfurization process, contributing to the improvement in the conversion efficiency. The bandgap energy of Cu 2 Sn 0.83 Ge 0.17 S 3 was about 1.0 eV, which is suitable for bottom cells used in double-junction solar cells.