Structure–Property–Performance Relationships in a Low Band‐Gap AgBiS2 Photoanodic Material

AgBiS2 is a narrow-band-gap, water-stable semiconductor with strong visible-light absorption, making it a promising absorber material in devices for solar-driven photoelectrocatalysis. Despite extensive studies on its photovoltaic performance, the role of cation disorder in governing PEC activity remains unexplored. Here, we investigate this effect on solvothermally synthesized AgBiS2 nanoparticles in photoanode thin films fabricated using ultrasonic spray coating. Mild thermal annealing partially homogenizes the cation distribution, as evidenced by lattice contraction in X-ray diffraction and absorption analyses, while spectroscopic characterization reveals subtle band-structure tuning toward a slightly n-type behavior. The annealed electrodes exhibit higher photocurrents and increased donor density. Impedance spectroscopy reveals improved hole flux to the semiconductor/electrolyte interface, while transient absorption spectroscopy identifies sub-bandgap trap-mediated recombination as the primary performance bottleneck. In addition, Operando X-ray absorption measurements show that annealing stabilizes the material under bias by suppressing ion migration. Together, these results reveal influences of thermal treatment on the structure and charge-carrier dynamics in AgBiS2 photoanodes and support its implementation as low band gap material in electrode architectures. For the first time, we also demonstrate their optimal use with fast redox mediators for selective photooxidation and sustainable solar energy conversion.