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.