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Environmental Degradation of MAPbBr Single Crystal: More Unstable than Expected Congcong Wang, Benjamin R. Ecker, Haotong Wei, Jinsong Huang, and Yongli Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12740 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018
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The Journal of Physical Chemistry
Environmental Degradation of MAPbBr3 Single Crystal: More Unstable than Expected Congcong Wang1, Benjamin R. Ecker1, Haotong Wei2, Jinsong Huang2, and Yongli Gao1* 1
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA.
2
Department of Applied Physical Sciences, University of North Carolina at Chapel Hill, Chapel
Hill, NC 27599, USA. *
[email protected] Abstract Organic-inorganic halide perovskites have emerged as a promising semiconductor family because of their remarkable performance in optoelectronic devices. On the other hand, the stability of perovskites remains a critical issue. In this work, we report a quantitative and systematic investigation of in-situ cleaved MAPbBr3 single crystal degradation processes in Xray, N2, O2 and H2O environments. The high-quality crystals were monitored by high resolution X-ray photoelectron spectroscopy with careful control of the exposure time and pressure. The detailed electronic structure and compositional changes of the crystal were tracked throughout the different exposures, and these studies provided insights into the various degradation mechanisms. We identified that ~10% of the surface MAPbBr3 degraded into metallic lead under X-ray, while N2 could protect the sample from the X-ray degradation for 9 hours under the same condition. Other measurements showed that while the crystal was not sensitive to pure O2, it was susceptible to H2O exposure and a reaction threshold of ~108 Langmuir was found. Below the threshold, H2O only acted as an n-type dopant; above it, the crystal began to decompose. These observations highlight possible future directions to improve the material stability by environmental control.
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Introduction Hybrid organometal halide perovskites have attracted considerable attention in the past few years, with remarkable power conversion efficiency (PCE) rising from 3.8% to over 22%.1-8 In particular, methylammonium lead halide perovskites, namely, CH3NH3PbX3 (MAPbX3), have unique properties, such as long diffusion length, low carrier effective masses, high absorption coefficients, and low-cost fabrication processes.9-13 These characteristics have expanded the applications of MAPbX3 beyond photovoltaic devices to light emitting diodes and photo detectors.14-16 However, the stability of perovskites remains a critical issue, and it can impede the future developments and applications of perovskite based devices. The degradations have been intensively investigated at the device level,17-22 but few measurements have been made at the surface analytical level to understand the intrinsic properties and degradation processes of the materials.6,
11, 23-25
As a result, the mechanisms of the degradation are still under debate. A
complete understanding of the degradation processes is necessary for widespread photovoltaic and other potential applications of the perovskites. A number of studies have been made to determine the factors that induce the degradation of perovskites. Grätzel and co-workers found that the device should be fabricated under controlled atmospheric conditions with a humidity of
