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 3P0,1 → 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.