Lead‐Free Halide Double Perovskite Cs2AgBiBr6 with Decreased Band Gap

Abstract

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs2AgBiBr6, shows attractive optical and electronic features, making it promising for high‐efficiency optoelectronic devices. However, the large bandgap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal‐engineering strategy to significantly decrease the bandgap by ~0.26 eV, reaching the smallest reported bandgap of 1.72 eV for Cs2AgBiBr6 at ambient conditions. The bandgap 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 bandgap, providing a possible explanation of the observed bandgap narrowing effect. This work provides new insights for achieving lead‐free double perovskites with suitable bandgaps for optoelectronic applications.