First-principles Study on Water Dissociation in Grain Boundary of MAPbI 3 Perovskite
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.329
First-principles Study on Water Dissociation in Grain Boundary of MAPbI3 Perovskite M.A.A. Asad1, K. Sato1 and K. Tsuruta1 1
Dept. of Electrical and Electronic Eng., Okayama University, Okayama 700-8530, Japan
ABSTRACT
Using first-principles calculation, we investigate water-dissociation dynamics in a 65 tilt grain boundary (GB) of Methyl-Ammonium Lead Triiodide (MAPbI3) perovskite. We find that the water dissociation process undergoes with two-step reaction at the GB: one of H ions of a water molecule that segregates into the GB is dissociated, migrates along the GB, and is attracted by an N atom in the MAPbI3, following the H-ion release from the ammonium. The process thereby generates OH ion and, in turn, leads to possible initiation of the degradation for crystallinity in the perovskite.
INTRODUCTION Solar cells based on Methyl-Ammonium Lead Triiodide (CH3NH3PbI3 or MAPbI3) perovskite have gained much attention due to their remarkable progress in photovoltaic efficiency during recent years [1]. It has been, however, hampered to put them on the market due to their stability under exposure to moisture [2] which is one of the major obstacles toward outdoor use of photovoltaic devices. A comprehensive study on degradation mechanism initiated with water molecules is thus essential for practical realization of MAPbI3 based solar cells. Previous studies have observed that the MAPbI3 may readily hydrolyze in the presence of water due to deprotonation of CH3NH3 by H2O, resulting in degradation products such as CH3NH2, HI and PbI2[3-6] The decomposition reaction is considered to be undertaken uniformly by a strong interaction between H2O and a N-H bond in MAPbI3 [7,8]. However, this microscopic process has not been confirmed experimentally. On the other hand, the mechanism proposed by Xin Guo [9] for zirconia ceramics has revealed that the degradation is accomplished by the formation of OH ions which cause the subsequent annihilation of oxygen vacancies along the surface and/or the GBs interior. Since many of experimental sample for MAPbI3 study have been a
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form of the poly-crystalline film, it can be assumed that similar degradation processes to the zirconia degradation are undertaken as grain boundaries (GBs) and/or surfaces of MAPbI3 are the weakest points where the degradation would start first. Unveiling the detailed mechanism of the OH formation and migration may thus be essential to identify an initiation of the degradation process in the perovskite. The migration of ions can also provide explicit explanations for the performance of photovoltaic device such as ionic conduction, hysteresis and field switchable photovoltaic effect [10]. In this work, first-principles calculation ba
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