Tuning the Structural and Optoelectronic Properties of Cs2 AgBiBr6 Double-Perovskite Single Crystals through Alkali-Metal Substitution.

Lead-free double perovskites have great potential as stable and nontoxic optoelectronic materials. Recently, Cs2 AgBiBr6 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 Cs2 AgBiBr6 and alkali-metal-substituted (Cs1-x Yx )2 AgBiBr6 (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 Cs2 AgBiBr6 . The study thus paves the way for employing cation substitution to tune the properties of double perovskites toward a new material platform for optoelectronics.