Lead‐Free Halide Double Perovskite Cs$_{2}$AgBiBr$_{6}$ with Decreased Band Gap

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs$_{2}$AgBiBr$_{6}$, shows attractive optical and electronic features, making it promising for high‐efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal‐engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs$_{2}$AgBiBr$_{6}$ under ambient conditions. The band‐gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first‐principles calculations indicate that enhanced Ag–Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band‐gap narrowing effect. This work provides new insights for achieving lead‐free double perovskites with suitable band gaps for optoelectronic applications