Tuning Emission and Electron-Phonon Coupling in Lead-Free Halide Double Perovskite Cs2AgBiCl6 under Pressure

Lead-free halide double perovskites have been proposed as candidates to replace Pb-halide perovskites in photovoltaic and optoelectronic applications due to their enhanced stability and nontoxicity. However, the limited understanding of the fundamental properties of halide double perovskites represents a hurdle to further improvement of their device performance. Our experimental studies demonstrate that the broad emission of Cs2AgBiCl6 with a large Stokes shift stems primarily from exciton self-trapping owing to strong electron-phonon coupling. An unusual blue shift of the emission accompanied by a red shift of the absorption edge occurred due to the reduced lattice relaxation energy upon lattice compression in the cubic phase. Electron-phonon coupling reduction is critical to the enhancement of photoluminescence intensity and tuning emission range in Cs2AgBiCl6 under high pressure. The structure-property relationships illuminated by our work can provide the basis for improving the performance of halide double perovskites.