Vacancy‐Ordered Double Perovskite Rb₂ZrCl_6−xBr_x: Facile Synthesis and Insight into Efficient Intrinsic Self‐Trapped Emission

Abstract Lead‐free vacancy‐ordered double perovskites with formula of A₂M(IV)X₆ have become promising alternatives to lead halide perovskites. Herein, novel direct bandgap lead‐free vacancy‐ordered double perovskites Rb₂ZrCl_6−xBr_x (0 ≤ x ≤ 2) with highest photoluminescence quantum yield (PLQY) of 90% are synthesized by using a facile wet grinding approach. Experimental and theoretical analyses demonstrate that the bright emissions originate from the self‐trapped excitons (STEs). Additionally, thermally activated delayed fluorescence (TADF) is observed in these perovskites, confirmed by temperature‐dependent steady‐state PL spectra and time‐resolved PL spectra measurements. The competitive equilibrium relationship is demonstrated further between the singlet STEs and the triplet STEs of Rb₂ZrCl_6−xBr_x interrelated to temperature. Meanwhile, satisfying the Vegard's law, the optical bandgap of Rb₂ZrCl_6−xBr_x decreases linearly from 3.895 to 3.778 eV with x increasing. Furthermore, density functional theory (DFT) calculations prove the highly localized electronic states generated in flat electronic bands of Rb₂ZrCl_6−xBr_x, as evidence by the presence of self‐trapped state in [ZrCl_6−xBr_x]²⁻ octahedra, and the energy level of the halogen 3p orbitals is the key factor that dictates their bandgaps.