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 Cd_xPb_1–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.