Improved optical properties of lead-free double perovskite Cs2NaBiCl6 nanocrystal via K ions doping

Double perovskite has shown great potential to solve the toxicity and instability problems of lead halide perovskite in practical applications. However, most lead-free double perovskite nanocrystals have low photoluminescence quantum yield (PLQY). Metal ion doping is usually an effective method to improve the optical properties of lead-free double perovskite nanocrystals (NCs). Here, we have prepared monodisperse Cs2NaBiCl6 NCs with uniform morphology and good crystallinity, and a series of Cs2NaBiCl6 NCs with different K+ doping amounts were prepared by colloid synthesis strategy. As the amount of potassium ion doping increased, the absorption spectra of the perovskites shifted towards the red, and the bandgap decreased from 3.56 eV to 3.50 eV. At the same time, the introduction of K+ improved the optical properties of Cs2NaBiCl6 NCs. The PLQY of Cs2Na0.62K0.38BiCl6 NCs reached 7.06 %, which was 4.38 times higher than that of Cs2NaBiCl6 NCs. The lifetime of exciton was increased from 3.42 ns to 12.02 ns by 3.5 times. Thermogravimetric analysis showed that Cs2Na0.62K0.38BiCl6 NCs had better thermal stability than Cs2NaBiCl6 NCs. Temperature-dependent PL spectra showed that Cs2Na0.62K0.38BiCl6 had higher exciton binding energy and longitudinal optical phonon energy than Cs2NaBiCl6 NCs.