Unraveling Mechanism of Photoluminescence in Hybrid Indium Halide Materials

ABSTRACT Indium‐based hybrid halides are promising lead‐free luminescent materials, yet the design principles for these closed‐shell ns ⁰ systems remain underdeveloped compared to traditional ns ² analogues. Here, we synthesized and characterized the (Pr ₂ NH ₂ ) ₃ In(Cl _1‐x Br _x ) ₆ (0 ≤ x ≤ 1) solid‐solution series to unravel their halide‐dependent effects. Combined spectroscopic and DFT analyses confirm that photoluminescence originates from ligand‐to‐metal charge transfer (LMCT) excitation followed by radiative recombination from self‐trapped excitons (STEs). The systematic substitution of halides induces a non‐monotonic red shift in emission energy that plateaus beyond x ≥ 0.3, while the photoluminescence quantum yield (PLQY) increases monotonically with bromide content, reaching up to 70% for (Pr ₂ NH ₂ ) ₃ In(Cl _0.2 Br _0.8 ) ₆ . These trends are rationalized by the electronic isolation of [InHal ₆ ] ³⁻ units and increased lattice softness in bromide‐rich compositions, which facilitates radiative STE formation. Our work establishes a coherent mechanistic foundation for indium halide hybrids, demonstrating how halide identity and electronic isolation dictate radiative processes in efficient lead‐free emitters.