Improved Open‐Circuit Voltage of Sb2Se3 Thin‐Film Solar Cells Via Interfacial Sulfur Diffusion‐Induced Gradient Bandgap Engineering

The performance of thermally deposited Sb 2 Se 3 solar cells are severely limited by various bulk and interfacial recombination, instigating a large open‐circuit voltage ( V OC ) deficit. Ternary Sb 2 (S,Se) 3 is considered as a remedy, however, it is also subjected to a dilemma that improvement in V OC will be escorted by J SC loss due to the shrinkage of light harvest. Thus, a gradient of S/Se across the film is a prerequisite to avoid this detrimental compromise. Herein, the incorporation of S in the Sb 2 Se 3 absorber layer evaporated from a CdS buffer layer during vapor transport deposition (VTD) process, and its further self‐activated diffusion at the interface upon ambient storage is explored. For the gradient indium tin oxide (ITO)/CdS/Sb 2 (S,Se) 3 /Sb 2 Se 3 /Au solar cell, the large bandgap Sb 2 (S,Se) 3 at the heterojunction side contributes to high V OC , while the narrow bandgap Sb 2 Se 3 at the top side confirms high J SC . Sulfur diffusion at the CdS/Sb 2 Se 3 interface also improves the junction quality with an enlarged V bi , reduced interfacial defects and recombination loss, thus improving V OC from 393 to 430 mV. Such V OC represents the highest value for that of thermally deposited Sb 2 Se 3 solar cells. The champion device also delivers an interesting efficiency of 7.49%. This research provides substantial guidance in exploring efficient approaches to improve the performance of Sb 2 Se 3 solar cells.