Degradation of Co-Evaporated Perovskite Thin Films
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Degradation of Co-Evaporated Perovskite Thin Films Congcong Wang,1 Youzhen Li,2 Xuemei Xu,2 Benjamin Ecker1, Chenggong Wang,1 Yongli Gao1,2* 1 Department of Physics and Astronomy, University of Rochester, Rochester, NY, 14627ˈUSA 2 School of Physics and Electronics, Central South University, Changsha, Hunan, 410083, P. R. China ABSTRACT Methylammonium lead halide perovskites have been developed as highly promising materials to fabricate efficient solar cells in the past few years. We have investigated degradation of co-evaporated CH3NH3PbI3 films in ambient air, oxygen and water respectively using x-ray photoelectron spectroscopy (XPS), small angle x-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The CH3NH3PbI3 film has an excellent atomic ratio and crystallinity. XPS results indicate that the film is not sensitive to oxygen and dry air, while ambient and water exposures achieve similar effects. XRD further indicates a structural conversion to PbI2 and a drastic morphology change from smooth to rough is revealed by AFM and SEM. The experiment indicated that H2O plays a dominated role in the degradation of CH3NH3PbI3 films. The degradation can be characterized by almost complete removal of N, substantial reduction of I, residual of PbI2, C, O, and I compounds on the surface. INTRODUCTION Organometal trihalide perovskites, as a new generation of photovoltaic materials, has a high power conversion efficiency (PCE). Methylammonium lead halide perovskites (CH3NH3PbX3, X=Cl, Br, I) have a wide absorption range, high charge-carrier mobility, and low cost.[1-7] The first solid-state perovskite solar cell with a PCE of 9.7% was reported in 2012 by H. S. Kim and co-workers[8]. The PCE was quickly improved to 15.4%[9], 19.3% [10] and 20.1%.[11] Despite the rapid progress in efficiency, perovskite solar cells suffers significant degradation in humid air with prolonged exposure.[12] Grätzel et al. reported perovskite solar cell could be fabricated under controlled conditions with a humidity
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