Revisiting the Photoelectric Conversion Mechanism in Hydrothermally Deposited Sb ₂ (S,Se) ₃ Solar Cells

ABSTRACT The photoelectric conversion mechanism is the core foundation of photovoltaic devices, determining their ability to convert sunlight into electricity. For hydrothermally deposited Sb ₂ (S,Se) ₃ solar cells, it is conventionally recognized that gradient‐bandgap Sb ₂ (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 Sb ₂ (S,Se) ₃ /Cd(S,Se) interface, the Sb ₂ (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 Na ₂ S ₄ O ₆ additive significantly improves the heterojunction performance. These findings reshape the understanding of photoelectric conversion mechanism and guide band alignment design in the hydrothermally deposited Sb ₂ (S,Se) ₃ solar cells.