Investigation of bandgap and optical properties of Ag+/Al3+ co-alloyed lead-free double perovskite Cs2NaBiCl6

Abstract

Lead-free bismuth metal halide perovskites have caused widespread concerns due to their low toxicity and intrinsic thermodynamic stability. In this work, a series of Cs₂Na_1-xAg_xBiCl₆ and Al³⁺-alloyed Cs₂(Na/Ag)BiCl₆ perovskites were designed and prepared by a single-step solvothermal method. The results showed that Ag⁺ could effectively enhance the luminescence performance increasing the photoluminescence quantum yield (PLQY) from 0.46 % to 25.11 %, and adjust the bandgap from 3.06 eV(x = 0) to 2.53 eV(x = 0.75). Al³⁺-alloyed Cs₂(Na/Ag)BiCl₆ perovskites further raised the PLQY up to 31.39 %, which was attributed to that Al³⁺ could increase the self-trapped excitons (STEs) density and radiative transition rate of Cs₂(Na/Ag)BiCl₆, determined from density of states analysis, photoluminescence (PL)/photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL) spectra. The bandgap trend of Ag⁺ and Al³⁺ replacing Na⁺ and Bi³⁺ was also explained by density functional theory (DFT) calculations. The stability experiment results showed that the prepared perovskite could maintain its structure at 673 K and keep the original PL intensity after exposure to ambient air for 30 days, which indicated the sample had excellent structural stability and luminescence stability.