First-Principles Study of Surface Properties and Ion Diffusions in Lead Iodide Perovskite
Jun Haruyama a, Liyuan Han a, Keitaro Sodeyama a b c, Yoshitaka Tateyama a c
a National Institute for Materials Science, Japan, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan
b PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, 333-0012, Japan
c Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Poster 050
Publication date: 7th November 2016

Lead iodide perovskite materials, e.g. CH3NH3PbI3 (MAPbI3), have a greater advantage for the photovoltaic applications because of their extremely long photocarrier diffusion lengths (μm scale), which can be regarded as a result of shallow defect levels in MAPbI3. [1] In a similar way, the energy levels of surface states are of great account as well. Therefore, we calculated the structural stability and electronic states on the simple surfaces of tetragonal MAPbI3 by use of first-principles calculations. We found that there are two major phases on all of the four surface facets. They can be classified as vacant- and flat- type terminations, and both terminations can coexist especially on the probable (110) and (001) surfaces. These surfaces can contribute to long carrier lifetimes in MAPbI3 because they have no midgap surface states. [2,3]

Despite the outstanding properties of the perovskite, the dependence of current−voltage (J−V) curves on the voltage scan direction and speed makes it difficult to evaluate power conversion efficiencies. A recent experiment showed that the rate-dependent J−V curves are related to built-up potentials induced by ion displacement in perovskite photo-absorbing layers. [4] In this respect, we calculated activation barriers of ion (or vacancy) migrations in tetragonal MAPbI3 and trigonal (NH2)2CHPbI3 (FAPbI3) by first-principles calculations. The migrations of I anions in both perovskites show low barriers of 0.3 to 0.45 eV, which values indicate these perovskites are potential ion conductors. Furthermore, MA+ and FA+ cations have rather low barriers c.a. 0.6 eV, namely molecular cations can also migrate. Based on the dilute diffusion theory, we can expect that small vacancy concentrations suppress these ion conductions. [5]

[1] W. -J. Yin, T. Shi, and Y. Yan, Appl. Phys. Lett. 104, 063903 (2014).

[2] J. Haruyama, K. Sodeyama, L. Han, and Y. Tateyama, J. Phys. Chem. Lett., 5, 2903 (2014).

[3] J. Haruyama, K. Sodeyama, L. Han, and Y. Tateyama, Acc. Chem. Res., 49, 554 (2016).

[4] W. Tress, N. Marinova, T. Moehl, S. M. Zakeeruddin, M. K. Nazeeruddin, and M. Grätzel, Energy Environ. Sci. 8, 995 (2015).

[5] J. Haruyama, K. Sodeyama, L. Han, and Y. Tateyama, J. Am. Chem. Soc., 137, 10048 (2015).

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