Band Dispersion and Hole Effective Mass of Methylammonium Lead Iodide Perovskite

Abstract

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)m0 around the M point and 0.35(±0.15)m0 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.

Photoelectron spectroscopy on single crystals of organic semiconductors: experimental electronic band structure for optoelectronic properties

“…Despite such difficulties, several research groups have challenged the ARUPS measurements on the single crystal halide perovskites. [196][197][198][199][200] We herein digest the cases of the single crystal methylammonium lead triiodide (CH 3 NH 3 PbI 3 ; MAPI). † † † Lee et al reported the ARUPS results on MAPI single crystal samples obtained under various excitation photon energies in a range of 74-200 eV.…”

“…In contrast, Yang and coworkers succeeded in resolving the E-K 8 dispersion of the valence bands on MAPI single crystal samples by ARUPS under the excitation by He-Ia line (21.218 eV) and synchrotron radiation as well. 199 The crystal samples were cut along a specific crystal axis by using a knife blade in atmosphere and were immediately introduced into a ultra-high vacuum system. MCP-LEED observations confirmed that the surface lattice was predominantly the (001) face of the cubic MAPI at 350 K and 300 K, while slight incorporation of the tetragonal phase, which became intense on decreasing the temperature to 250 K, was also detected.…”

“…209 Afterward, Zu et al also demonstrated the ARUPS analyses on the single crystal MAPI. 200 Their experimental conditions were similar to those of Yang et al; 199 an only apparent difference is that they cleaved the MAPI single crystal surface in a N 2 -filled glovebox, instead of in air, and the ARUPS measurements were conducted on the samples without air exposure. In this work, the ARUPS results taken along the G-% X and G-% M directions were interpreted to mainly represent the valence band dispersion in the X-M and X-R directions, respectively, as shown in Fig.…”

Ferroelectric Properties of Perovskite Thin Films and Their Implications for Solar Energy Conversion

“…For solar cell applications, only the tetragonal phase of MAPbI 3 at room temperature and the cubic high-temperature phase above 327 K are relevant. [32] With the mandatory requirements of non-centrosymmetry and piezoelectricity of the crystal structure being fulfilled, the question remains if any experimental evidence of ferroelectricity in MAPbI 3 in its tetragonal phase at room temperature exists. The MAPbI 3 unit cell comprises two perovskite cubes with slightly twisted PbI 6 octahedrons (purple).…”

Ferroelectricity and stability measurements in perovskite solar cells

“…If aging experiments are carried out at elevated temperatures beyond the Curie temperature T C , the perovskite will furthermore undergo a (gradual) phase transition from the ferroelectric tetragonal phase to the paraelectric (non-ferroelectric) cubic phase [21], where the distinct ferroelectric domains vanish [22]. Common standards for accelerated aging, such as the IEC 61215, use temperatures of 65°C or 85°C, both of which are beyond the T C =55°C of MAPbI 3 [23].…”