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

The halide perovskite CsPbBr$_3$ belongs to the Cs-Pb-Br material system, which features two additional thermodynamically stable ternary phases, Cs$_4$PbBr$_6$ and CsPb$_2$Br$_5$. The coexistence of these phases and their reportedly similar photoluminescence have resulted in a debate on the nature of the emission in these systems. Here, we combine optical and microscopic characterization 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. We find that all Cs-Pb-Br phases exhibit green emission and the non-perovskite phases exhibit photoluminescence quantum yields orders of magnitude larger than CsPbBr$_3$. In particular, we measure blue- and red-shifted emission for the Cs- and Pb-rich phases, respectively, stemming from embedded CsPbBr$_3$ nanocrystals. Our model reveals that the difference in emission shift is caused by the combined effects of nanocrystal size and different band mismatch. Furthermore, we demonstrate the importance of including the dielectric mismatch in the calculation of the emission energy for Cs-Pb-Br composites. Our results explain the reportedly limited blue shift in CsPbBr$_3$@Cs$_4$PbBr$_6$ composites and rationalize some of its differences with CsPb$_2$Br$_5$.