Understanding luminescent properties in zero-dimensional halide perovskites

Zero-dimensional (0D) halide perovskites exhibit strong exciton self-trapping, which may lead to efficient exciton emission. First-principles calculations on hybrid halide perovskites (C₄N₂H₁₄X)₄SnX₆ (X = Br, I) and inorganic Cs₄PbBr₆ show large exciton binding energies and exothermic exciton trapping at deep halogen vacancy levels. The calculated excitation and emission energies of excitons are in good agreement with experimental values. Suppressing exciton migration and the subsequent energy loss at defects are critical for efficient luminescence. Although the thermally-activated exciton hopping should be limited, fast exciton migration may occur through the resonant exchange of the excitation energy. To prevent this, large molecular cations may be incorporated in 0D halide perovskites; the key is to suppress the wavefunction overlap between luminescent centers and to increase the Stokes shift to prevent the spectral overlap between excitation and emission. Our results explain the high photoluminescence quantum efficiency observed in (C₄N₂H₁₄X)₄SnX₆ and the severe thermal quenching of luminescence in Cs₄PbBr₆. The green luminescence in Cs₄PbBr₆ is likely the result of exciton emission from CsPbBr₃ inclusions within the bulk of Cs₄PbBr₆.