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

Polytypes formed during the growth of metal halide perovskites can give rise to the formation of face-sharing sequences in corner-sharing octahedral networks. Here, the electronic influences of such structures, including the 6H and 12R phases, are found to be correlated with the fraction and stacking sequence of the face-sharing layers. The band gaps of polytypes feature a characteristic evolution from indirect to direct from pure hexagonal (2H) to cubic (3C) phases. Rather than arising from orbital mixing at the atomic level, a large band gap bowing of 1.96 eV in the CsPbI₃ family was attributed to the long-range electronic interaction between octahedral building blocks. While retaining a high carrier velocity (∼2 × 10⁵ m s^-1), Fermi surface analysis further revealed a decrease of dimensionality from 3D to 2D in frequently observed polytypes, indicating a carrier blocking and anisotropic transport effect of hexagonal impurity phases, with consequences for their applications in solar cells and other optoelectronic devices.