Inversion symmetry in methylammonium lead iodided single crystals probed by second harmonic optical anisotropy

Single crystals of the methylammonium lead iodide perovskite (CH₃NH₃PbI₃) are promising candidates for solar cells due to their long (tens of microns) photo-carrier diffusion lengths.^[1] It has been proposed that such long diffusion lengths are a consequence of bulk static Rashba fields resulting from an absence of bulk inversion symmetry at room temperature.^[2] Here we directly probe the point group symmetries of CH₃NH₃PbI₃ using high sensitivity optical second harmonic rotational anisotropy measurements. We show that contrary to previous proposals, the bulk crystallographic structure of tetragonal CH₃NH₃PbI₃ at room temperature is centrosymmetric with a space group of I4/mcm. These results are consistent with our density functional theory calculations, which establish the absence of a large bulk Rashba effect and show that incorrect structural relaxations are responsible for the previously predicted large Rashba effect.

[1] S. D. Stranks et al. Science 342, 341-344 (2013).
[2] Y. Rakita et al. Proc. Natl. Ac. Sci. USA 114, 5504-5512 (2017).