From nonemission to nearly unity quantum yield: Breaking parity-forbidden transitions in rubidium indium chloride through 5s² lone pair Sb-doping

Halide perovskites doped with ns² metal ions have stimulated widespread research owing to their efficient and stable self-trapped emissions. Here we observe the strictly forbidden transition ³P₀→¹S₀ is broken and afford nearly unity triplet orange self-trapped excitons (O-STEs) emission, through 5 s² lone pair Sb cation doping in host Rb₂InCl₅·H₂O single crystal. In particular, additional triplet blue self-trapped excitons (B-STEs) emission centered at 475 nm, stemming from partially allowed ³P₁→¹S₀, is also observed. Spectroscopic characterization along with literature analysis demonstrates that the behavior of forbidden transition breaking originates from the mixing of ³P₀ and ³P₁ states. First-principles density-functional theory (DFT) calculations as well confirm the involvement of atomic orbitals in bandgap construction, as evidence of 5s5p-5 s² transition, affording the STEs emission. Combining the efficient, stable broad emission Rb₂In_{96.39 %}Cl₅·H₂O: 3.61 % Sb³⁺ crystal with Cs₃Cu₂Cl₅ and Cs₃Cu₂I₅ phosphors, a lead-free white light-emitting diode with a high color rendering index of 93.4 has been achieved. Furthermore, the unique sensitive and steep temperature dependence photoluminescence (PL) lifetimes may prompt this type of self-trapped zero-dimensional (0D) perovskites to great potential in the field of thermometry application.