Wave Function Localization Reduces the Bandgap of Disordered Double Perovskite Cs2AgBiBr6

Double perovskite Cs₂AgBiBr₆ is a promising alternative to lead-based perovskites with excellent stability and attractive optoelectronic properties. However, a relatively large bandgap severely limits its performance in many applications such as solar cells and photodetectors. It has been reported that a random distribution of Ag and Bi atoms in Cs₂AgBiBr₆ effectively reduces its bandgap without introducing dopants or impurities, while the mechanism remains unclear. Here, using density functional theory calculations, we demonstrate that the Ag-Bi disorder in Cs₂AgBiBr₆ generates localized electronic states as band edges to regulate the bandgap. The disordered structures segregate Ag and Bi atoms in the lattice, and the formed homoatomic clusters lead to wave function localization. Moreover, the bandgap decrease exhibits a non-monotonic dependence on the degree of disorder. Our results are comparable with experimental observations and provide crucial insights into understanding the order-disorder transition in double perovskites.