Unraveling the triplet excited-state dynamics of Bi³⁺ in vacancy-ordered double perovskite Cs₂SnCl₆ nanocrystals

Luminescent metal halides doped with ns²-metal ions such as 6s²-metal Bi³⁺ have aroused reviving interest owing to their outstanding optical properties; however, the origin of the photoluminescence (PL) remains controversial and unclear. Herein, we report a strategy for the controlled synthesis of Bi³⁺-doped vacancy-ordered double perovskite Cs₂SnCl₆ nanocrystals (NCs) and unravel the triplet excited-state dynamics of Bi³⁺ through temperature-dependent PL and ultrafast femtosecond transient absorption spectroscopies. Owing to the aliovalent Bi³⁺ doping in the spatially confined zero-dimensional (0D) structure of Cs₂SnCl₆, Bi³⁺ ions experience an enhancive Jahn-Teller distortion in the excited state, which results in intense broadband blue PL originating from the inter-configurational ³P_0,1 → ¹S₀ transitions of Bi³⁺ at 450 nm, with a large Stokes shift and a quantum yield of 35.2%. Specifically, an unusual thermal-enhanced Jahn-Teller splitting of the excitation band and a remarkable transition of the PL lifetime from ms at 10 K to μs at 300 K were observed, as solid evidence for the isolated Bi³⁺ emission. These findings clarify the controversy about the PL origin in ns²-metal ion-doped lead-free luminescent metal halides, thereby paving the way for exploring their optoelectronic applications.