Efficiency improvement of flexible Sb2Se3 solar cells with non-toxic buffer layer via interface engineering

Thin film Sb2Se3 solar cells have gained a worldwide attention as a highly potential Earth-abundant photovoltaic technology. While Sb2Se3 solar cells are typically fabricated in superstrate structure, substrate configuration devices offer versatility in substrate material choice particularly important for flexible devices. However, all substrate type Sb2Se3 solar cells use CdS as a buffer layer synthesized by chemical bath deposition method, during which Sb2Se3 surface is degraded limiting solar cell performance. Additionally, contamination layer has been reported to exist on Sb2Se3 surface after deposition under low vacuum conditions. In this study, we demonstrate a facile strategy to improve Sb2Se3 solar cell power conversion efficiency (PCE) by replacing CdS with In2S3 and applying a two-step treatment involving chemical etching and subsequent annealing. We show that a combined treatment which first removes the contamination layer and then passivates interface defects improves PCE more than three-fold, from 1.6% to 5.35%. Effective passivation of interface defects is ascribed to the formation of thin crystalline Sb2O3 layer on Sb2Se3 absorber surface after the treatment. In conclusion, optimized two-step treatment offers a simple and effective way to improve PCE and replacement of toxic CdS with In2S3 demonstrates new device design options towards flexible and non-toxic Sb2Se3 solar cells.