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

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.