Effects of Quantum and Dielectric Confinement on the Emission of Cs-Pb-Br Composites

The halide perovskite CsPbBr3 belongs to the Cs-Pb-Br material system, which features two additional thermodynamically stable ternary phases, Cs4PbBr6 and CsPb2Br5. The coexistence of these phases and their reportedly similar photoluminescence (PL) have resulted in a debate on the nature of the emission in these systems. Herein, optical and microscopic characterizations are combined with an effective mass, correlated electron-hole model of excitons in confined systems, to investigate the emission properties of the ternary phases in the Cs-Pb-Br system. It is found that all Cs-Pb-Br phases exhibit green emission and the non-perovskite phases exhibit PL quantum yields orders of magnitude larger than CsPbBr3. In particular, blue- and red-shifted emission for the Cs- and Pb-rich phases, respectively, are measured, stemming from embedded CsPbBr3 nanocrystals (NCs). This model reveals that the difference in emission shift is caused by the combined effects of NC size and different band mismatch. Furthermore, the importance of including the dielectric mismatch in the calculation of the emission energy for Cs-Pb-Br composites is demonstrated. The results explain the reportedly limited blue shift in CsPbBr3@Cs4PbBr6 composites and rationalize some of its differences with CsPb2Br5.