Size-dependent dark exciton properties in cesium lead halide perovskite quantum dots

The fine structure of the band edge exciton and the dark exciton photoluminescence (PL) are topics of significant interest in the research of semiconducting metal halide perovskite nanocrystals, with several conflicting reports on the level ordering of the bright and dark states and the accessibility of the emitting dark states. Recently, we observed the intense dark exciton PL in strongly confined CsPbBr₃ nanocrystals at cryogenic temperatures, in contrast to weakly confined nanocrystals lacking dark exciton PL, which was explained by the confinement enhanced bright-dark exciton splitting. In this work, we investigated the size-dependence of the dark exciton photoluminescence properties in CsPbBr₃ and CsPbI₃ quantum dots in the strongly confined regime, showing the clear role of confinement in determining the bright-dark energy splitting (ΔE_BD) and the dark exciton lifetime (τ_D). We observe the increase in both ΔE_BD and τ_D with increasing quantum confinement in CsPbBr₃ and CsPbI₃ QDs, consistent with the earlier predictions on the size-dependence of ΔE_BD and τ_D. Our results show that quantum confinement plays a crucial role in determining the accessibility to the dark exciton PL and its characteristics in metal halide perovskite nanocrystals.