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.