Moderate Humidity Delays Electron-Hole Recombination in Hybrid Organic-Inorganic Perovskites: Time-Domain Ab Initio Simulations Rationalize Experiments Run Long,* ,†,‡ Weihai Fang, † and Oleg. V. Prezhdo* ,§ † College of Chemistry, Key Laboratory of Theoretical & Computational Photochemistry of Ministry of Education, Beijing Normal University, Beijing 100875, P. R. China ‡ School of Physics, Complex & Adaptive Systems Lab, University College Dublin, Belﬁeld, Dublin 4, Ireland § Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States ABSTRACT: Experiments show both positive and negative changes in performance of hybrid organic-inorganic perovskite solar cells upon exposure to moisture. Ab initio nonadiabatic molecular dynamics reveals the inﬂuence of humidity on nonradiative electron-hole recombination. In small amounts, water molecules perturb perovskite surface and localize photoexcited electron close to the surface. Importantly, deep electron traps are avoided. The electron-hole overlap decreases, and the excited state lifetime increases. In large amounts, water forms stable hydrogen-bonded networks, has a higher barrier to enter perovskite, and produces little impact on charge localization. At the same time, by contributing high frequency polar vibrations, water molecules increase nonadiabatic coupling and accelerate recombination. In general, short coherence between electron and hole beneﬁts photovoltaic response of the perovskites. The calculated recombination time scales show excellent agreement with experiment. The time-domain atomistic simulations reveal the microscopic eﬀects of humidity on perovskite excited-state lifetimes and rationalize the conﬂicting experimental observations. O rganic-inorganic hybrid perovskite solar cells have attracted enormous attention because of high and rapidly growing power conversion eﬃciencies. 1-8 The eﬃciency of methylammonimum lead iodine CH 3 NH 3 PbI 3 (MAPbI 3 ) solar cells has risen from 3.8% 9 to the Shockley-Queisser limit achieved via recycling of photons created during electron-hole recombination. 10 The signiﬁcant improvement of power conversion eﬃciencies stems from the excellent electronic and optical properties of perovskites, such as high absorption cross sections, 11 long charge carrier lifetimes, 12,13 and high photoluminescence (PL) yields. 14-16 Despite this remarkable progress, instability of MAPbI 3 ﬁlms in humid environments remains a major obstacle to real applications. 3 It has been hypothesized that the presence of moisture during ﬁlm fabrication may aﬀect the size of grains in the ﬁlms. 12,17-19 The eﬀect of humidity can be either positive or negative, having a strong inﬂuence on the excited electron lifetime, as reported by diﬀerent experiments. 17,20-23 On the one hand, some measurements demonstrate that humidity increases the excited- state lifetime of MAPbI 3 and improves the solar cell performance. 17 On the other hand, some experiments show that moisture accelerates the electron-hole recombination in MAPbI 3 and degrades the solar cell quality. 21, 22 Slow nonradiative electron-hole recombination is needed to achieve high photovoltaic device eﬃciencies, since the recombination constitutes a major pathway for energy and current losses. Snaith and coauthors have shown that MAPbI 3 ﬁlms annealed in air have a lower trap density, higher PL quantum yield, and longer PL lifetime compared to ﬁlms annealed in nitrogen. 17 Similarly, the same group has demonstrated that MAPbI 3 ﬁlms fabricated in a humid environment have better optoelectronic properties than ﬁlms fabricated under dry conditions. 17 Zhou et al. have reported that exposure to a low level (<30%) of relative humidity during solar cell fabrication has boosted the power conversion eﬃciency to 19.3%. 5 The improvement of the photovoltaic cell performance has been attributed to moisture facilitating formation of large grain ﬁlms and reducing electron-hole recombination. This suggestion is in agreement with our previous calculations demonstrating that grain boundaries accelerate nonradiative electron-hole recombination. 24 However, long annealing of MAPbI 3 ﬁlms in air increases the recombination. 17 Kamat et al. have shown that the performance of perovskite solar cells fabricated under humidity greater than 50% has degraded notably, compared with those made under dry conditions. 21 An additional hole transport layer covering perovskite materials can protect against humidity, reduce electron-hole recombination, and increase device lifetimes. 3,20,22 These diverse observations indicate that moisture can have both positive and negative eﬀects on the excited-state lifetime, depending on the humidity level. The presence of water aﬀects not only grain size of ﬁlms, but also the electronic structure. Ab initio molecular dynamics (MD) simulations and static calculations have been performed Received: June 26, 2016 Accepted: August 2, 2016 Published: August 2, 2016 Letter pubs.acs.org/JPCL © 2016 American Chemical Society 3215 DOI: 10.1021/acs.jpclett.6b01412 J. Phys. Chem. Lett. 2016, 7, 3215-3222 Downloaded via UNIV OF SOUTHERN CALIFORNIA on November 7, 2019 at 23:09:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.