Temperature-dependent band structure evolution determined by surface geometry in organic halide perovskite single crystals

Organic halide perovskites have attracted much attention due to their potential applications in optoelectronic devices. Since the generally higher flexibility compared to their inorganic counterparts, their structures are prone to be more sensitive toward external effects, where the fundamental understanding of their band structure evolutions is still inconclusive. In this study, different electronic structure evolutions of perovskite single crystals are found via angle-resolved photoelectron spectroscopy: (i) Unchanged top valence band (VB) dispersions under different temperatures can be found in the $\mathrm{C}{\mathrm{H}}₃\mathrm{N}{\mathrm{H}}₃\mathrm{Pb}{\mathrm{I}}₃$. (ii) Phase transitions induced the evolution of top VB dispersions, and even a top VB splitting with Rashba effects can be observed in the $\mathrm{C}{\mathrm{H}}₃\mathrm{N}{\mathrm{H}}₃\mathrm{Pb}{\mathrm{Br}}₃$. Combined with low-energy electron diffraction, metastable atom electron spectroscopy, and density functional theory calculation, we confirm that different band structure evolutions observed in these two perovskite single crystals originated from the cleaved top surface layers, where the different surface geometries with $\mathrm{C}{\mathrm{H}}₃\mathrm{N}{{\mathrm{H}}₃}⁺\text{\ensuremath{-}}\mathrm{I}$ in $\mathrm{C}{\mathrm{H}}₃\mathrm{N}{\mathrm{H}}₃\mathrm{Pb}{\mathrm{I}}₃$ and Pb-Br in $\mathrm{C}{\mathrm{H}}₃\mathrm{N}{\mathrm{H}}₃\mathrm{Pb}{\mathrm{Br}}₃$ are responsible for finding band dispersion change and appearance of the Rashba-type splitting. Such findings suggest that the top surface layer in organic halide perovskites should be carefully considered to create functional interfaces for developing perovskite devices.