Tuning Self‐Trapped Exciton States via Trivalent‐Metal Alloying in Lead‐Free 2D Double‐Perovskites

Abstract Strong carrier‐phonon coupling‐induced self‐trapped excitons (STEs) limit the application of 2D double‐perovskites in optoelectronic devices because the separation and collection of the highly localized STEs are challenging. In this study, antimony (Sb) is incorporated into (BA) 4 AgBiBr 8 (BA = CH 3 (CH 2 ) 3 NH 3 + ) to form (BA) 4 AgSb x Bi 1‐ x Br 8 (0 ≤ x ≤ 1) trivalent‐metal alloyed 2D double‐perovskites. It is found that introducing Sb into the (BA) 4 AgBiBr 8 2D double‐perovskite will decrease the carrier‐phonon coupling strength and change STEs from “dark” states to “bright” states at room temperature. Combining detailed spectroscopic analysis with density functional theory calculation, it is concluded that STEs localized at [SbBr 6 ] octahedron in (BA) 4 AgSb x Bi 1‐ x Br 8 give the broadband bright emission. The “bright” STEs with moderate carrier‐phonon coupling opens the possibility to efficiently separate and collect charge‐carriers in (BA) 4 AgSbBr 8 2D double‐perovskites. Photodetectors based on (BA) 4 AgSbBr 8 crystals are further fabricated. The photodetectors exhibit high responsivity up to 1368 A W −1 and an external quantum efficiency of 4.7 × 10 5 . These figures are over three times higher than that in (BA) 4 AgBiBr 8 ‐based photodetectors. This study provides a unique strategy for developing high‐performance optoelectronic devices based on STEs.