Band Dispersion and Hole Effective Mass of Methylammonium Lead Iodide Perovskite

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.