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 Cs₂AgBiBr₆ NCs is prepared via a colloidal synthesis strategy. When Na⁺ is doped into Cs₂AgBiBr₆ 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 Cs₂AgBiBr₆ NCs (9.08%) than that for Cs₂AgBiBr₆ NCs (0.40%), and the exciton lifetime is extended from 1.87 ns for Cs₂AgBiBr₆ NCs to 17.15 ns for Na⁺‐doped Cs₂AgBiBr₆ 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 Cs₂AgBiBr₆ NCs maybe possess potentially valuable applications in optoelectronic devices.