Revisiting the Photoelectric Conversion Mechanism in Hydrothermally Deposited Sb₂(S,Se)₃ Solar Cells

The photoelectric conversion mechanism is the core foundation of photovoltaic devices, determining their ability to convert sunlight into electricity. For hydrothermally deposited Sb₂(S,Se)₃ solar cells, it is conventionally recognized that gradient-bandgap Sb₂(S,Se)₃ was thought to form a heterojunction with flat-bandgap CdS for carrier separation/transport. In this study, we uncover a previously unidentified photoelectric conversion mechanism. Annealing is shown to drive selenium diffusion into CdS, resulting in the formation of gradient bandgap Cd(S,Se). The Cd(S,Se) homojunction-based solar cell unexpectedly delivers a short-circuit current density of 1.43 mA cm^-2. This reveals that the electron-hole separation is not exclusively occurred at the Sb₂(S,Se)₃/Cd(S,Se) interface, the Sb₂(S,Se)₃ and Cd(S,Se) form a "V"-shaped bandgap mixed absorber layer concurrently separates charge carriers with the remaining Cd(S,Se). Additionally, the Na₂S₄O₆ additive significantly improves the heterojunction performance. These findings reshape the understanding of photoelectric conversion mechanism and guide band alignment design in the hydrothermally deposited Sb₂(S,Se)₃ solar cells.