Conduction Band Tuning by Controlled Alloying of Fe into Cs₂AgBiBr₆ Double Perovskite Powders

Halide double perovskite semiconductors such as Cs₂AgBiBr₆ are widely investigated as a more stable, less toxic alternative to lead-halide perovskites in light conversion applications including photovoltaics and photoredox catalysis. However, the relatively large and indirect bandgap of Cs₂AgBiBr₆ limits efficient sunlight absorption. Here, it is shown that controlled replacement of Bi³⁺ with Fe³⁺ via mechanochemical synthesis results in a remarkable tunable absorption onset between 2.1 and ≈1 eV. First-principles density functional theory (DFT) calculations suggest that this bandgap reduction originates primarily from a lowering of the conduction band upon the introduction of Fe³⁺, and predict a direct bandgap when >50% of Bi³⁺ is replaced with Fe³⁺. The tunability of the conduction band energy is found and reflected in the photoredox activity of these semiconductors. These findings open new avenues for enhancing the sunlight absorption of double perovskite semiconductors and for harnessing their full potential in sustainable energy applications.