Ligang Yuan

A successful observation of the top valence band structure of CH₃NH₃PbI₃ single crystals by using angle‐resolved ultraviolet photoelectron spectroscopy on the cleaved crystal surfaces is reported. Top valence band shows the energy dispersion width of 0.55 eV to the ΓM direction and 0.26 eV to the ΓX direction for the cubic structure.

Solar cells incorporating organic–inorganic perovskites, especially methylammonium lead iodide (CH₃NH₃PbI₃), have recently shown remarkable performances and therefore attracted wide interest. For understanding the origin of the high performance, the effective charge carrier masses of CH₃NH₃PbI₃ are critical. However, reliable experimental data on its electronic band structure, which determines the effective mass, is yet to be provided. Here, the electronic structure of CH₃NH₃PbI₃ single crystals is studied by using angle‐resolved photoelectron spectroscopy on cleaved crystal surfaces after characterizing the surface structure by low‐energy electron diffraction. Coexisting cubic and tetragonal phases of CH₃NH₃PbI₃ are found in diffraction patterns. Moreover, a clear band dispersion of the top valence band is observed along directions parallel to different high‐symmetry points of the cubic structure, in consistence with theoretical calculations. Based on these values, the effective hole mass is then estimated to be 0.24(±0.10)m ₀ around the M point and 0.35(±0.15)m ₀ around the X point, which are significantly lower than in organic semiconductors. These results reveal the physical origin of the high performance of solar cells incorporating perovskite materials compared to pure organic semiconductors.