Incorporation sodium ions into monodisperse lead-free double perovskite Cs2AgBiCl6 nanocrystals to improve optical properties

Lead-free double perovskite nanocrystals(NCs) have emerged as a promising candidate in the optical field, owing to their non-toxic, good moist heat and chemical stability. However, their poor optical properties limited their application. To improve the optical properties of lead-free double perovskite NCs, metal ion doping or alloying had been suggested as a promising strategy. Here, we prepared monodisperse,uniformly sized, cubic morphology of Cs2AgBiCl6NCs with different Na+incorporation amounts via a simple hot-injection method. The Na+incorporation broke the parity-forbidden transition by reducing the inversion symmetry of the electron wave function at the Ag site, which changed the parity of the self-trapped exciton wave function and thus allowed radiative recombination. As a result, the photoluminescence quantum yield(PLQY) of Na+-alloyed Cs2AgBiCl6NCs(12.1%) was higher than that of Cs2AgBiCl6NCs(2.4%), and the exciton lifetime of Na+-alloyed Cs2AgBiCl6NCs increased to 36.98 ns from 17.58 ns for Cs2AgBiCl6NCs. By adjusting the amount of Na+incorporation, the band gap of Cs2AgBiCl6NCs can be significantly tuned from ～2.90 eV to ～3.50 eV. Furthermore, the temperature-dependent photoluminescence spectra indicated that the Na+-alloyed Cs2AgBiCl6NCs possessed higher longitudinal optical phonon energy and exciton binding energy compared to Cs2AgBiCl6NCs. This suggested that there were strong exciton-phonon interactions during exciton recombination, a reduced probability of non-radiative processes, and excellent thermal stability. It offers a promising strategy for improving the optical properties of lead-free double perovskite NCs, and have the potential to replace traditional lead halide perovskite NCs in future optoelectronic applications.