Tuning the Structural and Optoelectronic Properties of Cs2AgBiBr6 Double‐Perovskite Single Crystals through Alkali‐Metal Substitution

Abstract Lead‐free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs 2 AgBiBr 6 has emerged as a promising material, with suboptimal photon‐to‐charge carrier conversion efficiency, yet well suited for high‐energy photon‐detection applications. Here, the optoelectronic and structural properties of pure Cs 2 AgBiBr 6 and alkali‐metal‐substituted (Cs 1− x Y x ) 2 AgBiBr 6 (Y: Rb + , K + , Na + ; x = 0.02) single crystals are investigated. Strikingly, alkali‐substitution entails a tunability to the material system in its response to X‐rays and structural properties that is most strongly revealed in Rb‐substituted compounds whose X‐ray sensitivity outperforms other double‐perovskite‐based devices reported. While the fundamental nature and magnitude of the bandgap remains unchanged, the alkali‐substituted materials exhibit a threefold boost in their fundamental carrier recombination lifetime at room temperature. Moreover, an enhanced electron–acoustic phonon scattering is found compared to Cs 2 AgBiBr 6 . The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.