Article pubs.acs.org/JPCC
Ab Initio Study of Interaction of Water, Hydroxyl Radicals, and Hydroxide Ions with CH3NH3PbI3 and CH3NH3PbBr3 Surfaces Linghai Zhang and Patrick H.-L. Sit* School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong S.A.R
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S Supporting Information *
ABSTRACT: Although there have been tremendous breakthroughs in perovskite solar cells over the past few years, degradation of perovskite has been a huge problem. Recently, a number of experimental studies have demonstrated that organic−inorganic halide perovskite materials are sensitive to humid air, and several degradation mechanisms have been proposed. However, the decomposition process of perovskites is only partially known and controversial. In this paper, we theoretically study the structures of the tetragonal CH3NH3PbI3 and CH3NH3PbBr3 (110) surfaces and the degradation mechanism using density functional theory calculations both with and without the van der Waals correction. The computed results indicate that the CH3NH3+ (MA) cations preferentially orient with the NH3 group pointing into the surface. This allows the formation of more hydrogen···halide hydrogen bonds between the MA cations and the halides. Moreover, the interactions of water molecules, hydroxyl radicals, and hydroxide ions with the perovskite surfaces are investigated. It has been suggested that the deprotonation of the MA cations followed by the desorption of the CH3NH2 molecules is a key step in the degradation mechanism. We found that the hydroxyl radicals and hydroxide ions facilitate this desorption process while water molecules have little effect on it. These present findings are pertinent to revealing the decomposition mechanisms of perovskite materials.
1. INTRODUCTION Perovskite solar cells have increasingly attracted scholarly attention over the past few years because of advantages like their superb photovoltaic performance, simple fabrication procedures, sustainability, low price of raw materials, and long electron−hole diffusion lengths.1−6 The world has witnessed the emergence and development of this novel kind of solar cells that is based on metal-halide perovskites solar materials, and the power conversion efficiencies (PCEs) of perovskite solar cells have jumped from ∼4% to ∼20% in just 5 years.7−10 The editors of Nature and Science highlighted perovskite-sensitized solar cell technology as one of the Top 10 Breakthroughs in 2013.11 In perovskite solar cells, inorganic−organic metal halide perovskite materials are used as light harvesters, mounted on mesoporous TiO2 or other oxide scaffolds. The metal halide perovskite compounds are crystalline materials that possess the AMX3 (A = CH3NH3, (NH2)2CH, etc.; M = Sn, Ge, Pb, etc.; X = halide) perovskite structure.3 The crystal structures of MAPbX3 (MAPbX3 = CH3NH3PbX3) are temperature-dependent. For example, MAPbI3, the most widely studied compound in this family, has a cubic perovskite structure in hightemperature conditions. As the temperature decreases, the system undergoes a cubic-to-tetragonal structural transition at ∼327 K. 12 The tetragonal MAPbI 3 converts into the orthorhombic phase at temperatures below ∼162 K.12 Similar © 2015 American Chemical Society
transitions occur for the bromide counterpart, MAPbBr3. The critical temperatures for the cubic-to-tetragonal and tetragonalto-orthorhombic transitions are ∼237 K and ∼145 K, respectively.13 Compared to MAPbI3, MAPbBr3 was experimentally found to be more stable against degradation in the presence of moisture.14 One of the main problems faced by the current perovskite solar cell systems is their stability and short lifetime as these materials degrade readily in the atmosphere (moist air) under sunlight.15−22 Grätzel et al. suggested that the fabrication of these solar cell devices should be carried out under controlled atmospheric conditions and with a humidity of
