Regulating broadband near-infrared luminescence and thermal stability of Cs2AgBiCl6 double perovskite via alkali metal ions doping

Lead-free double perovskites have attracted much attention for their unique optical properties, though achieving efficient broadband near-infrared (NIR) emission with them remains a challenge. In this work, we successfully synthesized a new microcrystalline structure of lead-free double perovskites Cs₂Ag_1-xR_xBiCl₆ (R = Na, K, Rb) using a simple chemical reflux method, achieving NIR emission. Notably, Cs₂Ag_0.7K_0.3BiCl₆ demonstrates a broad emission spectrum spanning from 500 to 1200 nm with a significant half-peak width of 300 nm under 365 nm excitation. This is attributed to the doping of alkali metal ions can break the inherent parity-forbidden transition and induce strong exciton-phonon coupling through lattice distortion. Temperature-dependent photoluminescence spectroscopy and density functional theory calculations analysis reveal that alkali metal ions can enhance the thermal stability of self-trapped exciton emission by reducing exciton-phonon coupling strength. Finally, we demonstrate the promising applications of Cs₂Ag_0.7K_0.3BiCl₆ crystals with broad emission bands for NIR night vision and information encryption. This provides new insights into the realization of near-infrared emission and thermal stability regulation.