Ab Initio Study of the Degradation Mechanisms of Organic-Inorganic Metal Halide Perovskites

Abstract

Perovskite solar cells have attracted great attention in the past few years due to their many advantages like superb photovoltaic and electronic properties, simple fabrication procedures and low cost of raw materials. However, numerous studies have demonstrated that the perovskite materials degrade rapidly under humid air and sunlight, which greatly hinders perovskite solar cells from practical application. The microscopic understanding of the degradation mechanism for perovskite materials has been limited. In particular, the reaction intermediates, the reaction pathways under different conditions, the interactions between the perovskite and humid air, as well as the role of excess electrons and holes in the degradation process remain unclear. In-depth knowledge of the details of the reaction of the perovskite is essential for the study and design of efficient and robust photovoltaic materials.

In this thesis, systematic ab initio study was carried out to investigate the perovskite degradation mechanism under different conditions in the presence of different molecular species. We first carried out simulations on CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ (110) perovskite surfaces in the presence of water, hydroxyl radicals and hydroxide ions. The interactions between the perovskite surfaces and water, hydroxyl radicals as well as hydroxide ions were studied in detail. Ab initio molecular dynamics simulations were then carried out to study the dynamics of the interaction of water molecules, hydroxyl radicals and hydroxide ions with the MAPbI₃ surface. The infiltration pathways of water molecules, hydroxyl radicals and hydroxide ions into the CH₃NH₃PbI₃ surface were also studied. These static and dynamic simulations showed that the hydrogen atoms (or protons) of the organic cation in perovskite surfaces spontaneously transfers to the hydroxyl radicals (or hydroxide ions). It is followed by the resulting organic molecule diffusing out of the perovskite surface. Moreover, we studied the interactions between the CH₃NH₃PbI₃ (110) surface and O₂ molecules in the presence of mobile excess electrons. The results showed that molecular O₂ only weakly interacts with the perovskite surface, while the presence of excess electrons accelerates the degradation process. In particular, a superoxide, which is formed from the reaction between a molecular oxygen and an excess electron, react readily with the perovskite surface by forming a Pb-O covalent bond with a surface Pb. Further reactions lead to the disintegration of the local Pb-I octahedral structure. We also identified a pathway for the formation of the PbO local structure and demonstrated the key roles of the mobile excess electrons and oxygens in CH₃NH₃PbI₃ degradation. Finally, it was recently found experimentally that iodine molecules, as the degradation products, has a detrimental effects on the perovskite stability once they are formed. To gain microscopic insights into such effects, the reactions between the CH₃NH₃PbI₃ (110) surfaces and I₂ molecules as well as I· radicals were investigated. We also further studied the properties of photo-generated electrons and holes on the CH₃NH₃PbI₃ surfaces, and their roles in the degradation process. We identified the mechanism of the formation of a surface I₃⁻ ion from the surface-trapped holes. This provided key understanding of photo-induced CH₃NH₃PbI₃ degradation in a vacuum.

Overall, this work contributes to providing comprehensive understanding of the poor stability of the organic–inorganic metal halide perovskites materials. This is crucial for deriving effective strategies in the design of more robust materials for the next-generation photovoltaic systems.

Date of Award	20 Jul 2018
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Patrick SIT (Supervisor)