Improved Optical Properties of Lead‐Free Double Perovskite Cs2AgBiBr6 Nanocrystals via Na Ions Doping

Abstract Lead‐free double perovskite nanocrystals (NCs) are a promising alternative to lead‐based perovskite NCs for optoelectronic materials due to their nontoxic and good moist‐thermal stability. However, most of the lead‐free double perovskite NCs exhibit unsatisfying photoluminescence quantum yield (PLQY); the metal ions doping is an effective method to improve the optical properties. Here, monodisperse, homogeneous cubic morphology of Na + ‐doped Cs 2 AgBiBr 6 NCs is prepared via a colloidal synthesis strategy. When Na + is doped into Cs 2 AgBiBr 6 NCs, the inversion‐symmetry‐induced parity‐forbidden transition is broken. So, the wave function parity of the self‐trapped excitons at Ag atoms is changed, which allows the radiative transition to occur. This leads to a higher PLQY for Na + ‐doped Cs 2 AgBiBr 6 NCs (9.08%) than that for Cs 2 AgBiBr 6 NCs (0.40%), and the exciton lifetime is extended from 1.87 ns for Cs 2 AgBiBr 6 NCs to 17.15 ns for Na + ‐doped Cs 2 AgBiBr 6 NCs. The Na + doping elevates the longitudinal optical phonon energy and exciton binding energy, which results in weaker nonradiative transitions during exciton recombination and improved NC thermal stability. So, the Na + ‐doped Cs 2 AgBiBr 6 NCs maybe possess potentially valuable applications in optoelectronic devices.