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

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.