From Cubic to Hexagonal: Electronic Trends across Metal Halide Perovskite Polytypes

Polytypes formed during the growth of metal halide perovskitescan give rise to the formation of face-sharing sequences in corner-sharingoctahedral networks. Here, the electronic influences of such structures,including the 6H and 12R phases, are found to be correlated with thefraction and stacking sequence of the face-sharing layers. The bandgaps of polytypes feature a characteristic evolution from indirectto direct from pure hexagonal (2H) to cubic (3C) phases. Rather thanarising from orbital mixing at the atomic level, a large band gapbowing of 1.96 eV in the CsPbI3 family was attributedto the long-range electronic interaction between octahedral buildingblocks. While retaining a high carrier velocity (& SIM;2 x10(5) m s(-1)), Fermi surfaceanalysis further revealed a decrease of dimensionality from 3D to2D in frequently observed polytypes, indicating a carrier blockingand anisotropic transport effect of hexagonal impurity phases, withconsequences for their applications in solar cells and other optoelectronicdevices.