DFT study on surface and interface states of tetragonal CH3NH3PbI3 for understanding interfacial charge transfer

Jun Haruyama^a, Liyuan Han^a^,^b, Yoshitaka Tateyama^a^,^b^,^c, Keitaro Sodeyama^a^,^c

^aNational Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305, Japan

^bJapan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan

^cKyoto University, Japan, Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Yoshitaka Tateyama, presentation 066
Publication date: 5th February 2015

Many characters of bulk CH₃NH₃PbI₃ (MAPbI₃) such as small effective masses of the photo-excited carriers have been elucidated so far, whereas little is known about surface and interface properties of MAPbI₃ yet due to the difficulty in the observations. Here we investigated surfaces of tetragonal MAPbI₃ toward organic hole transport materials (HTMs) and the interfaces with model oxide anodes (ex. TiO₂ surface), by density functional theory (DFT) based supercell calculations with PBE and rev-vdW-DF2 functionals. Regarding the surface stability, we found that a vacant termination is more stable than flat termination on all probable surfaces, under the thermodynamic equilibrium conditions of bulk MAPbI₃. More interestingly, both terminations can coexist especially on the more probable (110) and (001) surfaces. The electronic states of the stable vacant and PbI₂-rich flat terminations on these two faces effectively maintain the characteristics of bulk MAPbI₃ without midgap levels. This indicates less carrier trap sites on the surfaces, which is likely to contribute to the long carrier diffusion length observed in the perovskite solar cells (PSCs). Furthermore, the shallow surface states appearing on the (110) and (001) flat terminations can facilitate efficient hole transfer to the neighboring HTMs. With these stable surfaces exposing cutting planes of PbI₆ octahedrons, we also examined the relaxed structures and electronic states of the interfaces with anatase TiO₂ and alpha Al₂O₃, as model mesoporous oxides. For initial structures of the geometry optimizations, we chose the configurations with smaller Pb-O and I-Ti distances. Note that the present interfaces do not involve MA at the interfaces and keep almost stoichiometric conditions. It is then found that the atoms on the MAPbI₃ interfaces slightly deform to fit the oxide materials. However, the associated midgap levels do not appear and simple band alignments of the two semiconductors are observed. As expected, the conduction band minimum (CBM) of TiO₂ is lower than the MAPbI₃ CBM, while Al₂O₃ has higher CBM. In our interfacial conditions, there are still sufficient band gaps, consistent with the band diagram proposed so far. These aspects obtained here will provide basis of fundamental understanding and design of the interfaces for improvement of the PSC performance.
(a) Surface structures and the hole states of stable vacant (left) and PbI2-rich flat (right) terminations. (b) Schematic picture of the interface between PbI2-rich flat termination of MAPbI3 and anatase TiO2.
Haruyama, J.; Sodeyama, K.; Han L.; Tateyama, Y. Termination Dependence of Tetragonal CH3NH3PbI3 Surfaces for Perovskite Solar cells. J. Phys. Chem. Lett. 2014, 5, 2903-2909.

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