Effect of dimensionality on the optical absorption properties of CsPbI₃ perovskite nanocrystals

The bandgaps of CsPbI₃ perovskite nanocrystals are measured by absorption spectroscopy at cryogenic temperatures. Anomalous bandgap shifts are observed in CsPbI₃ nanocubes and nanoplatelets, which are modeled accurately by bandgap renormalization due to lattice vibrational modes. We find that decreasing dimensionality of the CsPbI₃ lattice in nanoplatelets greatly reduces electron-phonon coupling, and dominant out-of-plane quantum confinement results in a homogeneously broadened absorption line shape down to cryogenic temperatures. An absorption tail forms at low-temperatures in CsPbI₃ nanocubes, which we attribute to shallow defect states positioned near the valence band edge.