Pb alloying enables efficient broadband emission of two dimensional [NH3(CH2)4NH3]CdBr4

Two dimensional (2D) hybrid metal halide perovskites (MHPs) with broadband emission have attracted enormous attention, which is highly promising for single-component solid state lighting. However, it remains a challenge to unveil the underlying origin of broadband emission and achieve high quantum yield (QY). Herein, 2D single-layered [NH 3 (CH 2 ) 4 NH 3 ]CdBr 4 (Cd–P) has been successfully prepared. By partially replacing Cd 2+ with Pb 2+ , the energy band of Cd–P shows a transformation from indirect to direct, supported by the density functional theory (DFT) calculations. Benefiting from the change of energy band, alloyed Cd x Pb 1-x -P shows an intensive broadband emission spanning the whole visible range with a maximum of photoluminescence (PL) QY of 58%. Temperature-dependent and time-resolved PL uncover that the broadband emission origins from the coupling between free excitons (FEs) and [CdBr 6 ] 4- octahedra , instead of [PbBr 6 ] 4- . These findings provide a deep insight into the origin of STEs emission and an effective approach to promote the broadband emission for Cd-based MHPs.