Bright orange-yellow emission from lead-free double perovskite single crystals with an indirect band gap

Abstract

As a novel class of luminescent material, double perovskite still encounters the problem of low luminescence efficiency. Previous reports suggested that manipulating the band gap from indirect to direct, or breaking the inversion-symmetry-induced parity-forbidden transition, can lead to a dramatic increase in emission. Our study designs a series of Cs₂AgBi_1-xIn_xCl₆ double perovskite single crystals. Notably, we observe over one hundredfold luminescence enhancement in Cs₂AgBi_0.2In_0.8Cl₆ with an indirect band gap. The absorption and photoluminescence spectra suggest that an appropriate Bi/In ratio opens radiative transition channels. Furthermore, the density functional theory calculation verifies that the significant luminescence improvement is attributed to the increase in transition matrix elements of the optimal Cs₂AgBi_0.2In_0.8Cl₆, which promotes radiative recombination of photo-induced charge carriers. These carries relax to the self-trapped exciton state, and result in strong emission. Our study demonstrates that in addition to band gap type and transition parity, the transition matrix element also accounts for the photoelectric performance of double perovskite, which should be taken into consideration when designing lead-free double perovskites for optoelectronic applications.