Boosting the efficiency of GeSe solar cells by low-temperature treatment of p-n junction

Germanium monoselenide (GeSe) is an emerging promising photovoltaic absorber material due to its attractive optoelectronic properties as well as non-toxic and earth-abundant constitutes. However, all previously reported GeSe solar cells rely on a superstrate configuration coupled with a CdS buffer layer, and suffer from unsatisfactory performance. Here we demonstrate that this low efficiency arises from the inevitable high-temperature treatment of p-n junction in superstrate configuration. This results in the diffusion of Cd atoms from CdS layer into GeSe film that introduces detrimental deep trap states inside the bandgap of GeSe (∼0.34 eV below conduction band minimum). We adopt therefore a substrate configuration that enables the deposition of CdS atop pre-deposited polycrystalline GeSe film at room temperature, avoiding the Cd diffusion. By optimizing the annealing temperature of complete devices via a high-throughput screening method, the resulting substrate solar cells annealed at 150°C achieve an efficiency of 3.1%, two times that of the best previously reported superstrate GeSe results.