Breaking Forbidden Transitions for Emission of Self-Trapped Excitons in Two Dimensional (F2CHCH2NH3)2CdBr4 Perovskite through Pb Alloying.

In this work, a new two-dimensional Cd-based (F2CHCH2NH3)2CdBr4 perovskite (Cd-P) with indirect bandgap and a direct Pb-based (F2CHCH2NH3)2PbBr4 (Pb-P) are successfully synthesized with isostructural features. Compared to the blueish white light emission of Pb-P, almost no white light can be observed for Cd-P due to the forbidden transition of self-trapped exciton (STE) emission. Interestingly, the white light emission of CdxPb1-x-P (x represents the feed ratio of Cd) is significantly improved with the photoluminescence (PL) quantum yield (QY) raising from <1% to 32.5% by alloying these two isostructural perovskites, which is attributed to the breaking of selection rules for forbidden transitions of STEs with Jahn-Teller like octahedral distortion, as suggested by the results from density functional theory (DFT) calculations and time-resolved spectroscopies. This study demonstrates the intriguing effect of alloying on activating STE emission as an effective approach to control and enhance the optical properties of metal halide perovskites.