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Collin Stecker1 Kexi Liu2 Jeremy Hieulle1 Zhenyu Liu2 Robin Ohmann1 Luis Ono1 Guofeng Wang2 Yabing Qi1

1, Okinawa Institute of Science and Technology, Okinawa, , Japan
2, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States

Understanding defects in perovskite films is key to improving device performance and stability. It has been hypothesized that vacancy defects enable ion migration1, which has been implicated to cause current-voltage hysteresis and long-term material degradation. Additionally, defects at the surface of the perovskite film may affect the interfaces in a device, and interface engineering is seen as an important avenue for improving device performance. Scanning tunneling microscopy (STM) offers the ability to probe the surface of OHPs with atomic resolution, including resolving individual vacancy defects.2-4 Scanning the same area multiple times allows for observation of dynamic events. Here, multiple types of defects were resolved and dynamic ion migration to and from the surface was imaged at the atomic scale. DFT calculations indicate vacancy defects are MABr vacancies and that vacancy defects at the surface of the film change the local work function, which has important implications for energy level alignment and charge transfer between layers in a photovoltaic device.

References
1 J. Azpiroz, E. Mosconi, J. Bisquert and F. De Angelis, Eng. Environ. Sci. 8, 2118 (2015).
2 L. She, M. Liu, D. Zhong, ACS Nano. 10, 1126 (2016).
2 R. Ohmann, L.K. Ono, H.S. Kim, H. Lin, M.V. Lee, Y. Li, N.G. Park, Y.B. Qi, J. Am. Chem. Soc. 137, 16049 (2015).
4 Y. Liu, K. Palotas, X. Yuan, T. Hou, H. Lin, Y. Li and S.T. Lee, ACS Nano. 11 2060 (2017).

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