title: Atomistic Origins of Surface Defects in CH3NH3PbBr3Perovskite and Their Electronic Structures.
abstract: The inherent instability of CH3NH3PbX3remains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CH3NH3PbX3have been successfully characterized by using density functional theory (DFT) calculations together with the high-resolution scanning tunneling microscopy (STM) results. The two coexisting phases of the perovskite surfaces have been ascribed to the alternate orientation of the methylammonium (MA) cations. Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various experimental methods. As such, these defects are expected to be intrinsic to the perovskite crystals and may play an important role in the structural decomposition of perovskite materials. Understanding the nature of such defects should provide some useful information toward understanding the instability of perovskite materials. Thus, we investigate the chemical identity of the surface defects systematically with first-principles density functional theory calculations and STM simulations. The calculated STM images of the Br and Br-MA vacancies are both in good agreement with the experimental measurements. In vacuum conditions, the formation energy of Br-MA is 0.43 eV less than the Br vacancy. In the presence of solvation effects, however, the formation energy of a Br vacancy becomes 0.42 eV lower than the Br-MA vacancy. In addition, at the vacancy sites, the adsorption energies of water, oxygen, and acetonitrile molecules are significantly higher than those on the pristine surfaces. This clearly demonstrated that the structural decomposition of perovskites are much easier to start from these vacancy sites than the pristine surfaces. Combining DFT calculations and STM simulations, this work reveals the chemical identities of the intrinsic defects in the CH3NH3PbX3perovskite crystals and their effects on the stability of perovskite materials.
authors.affiliation: Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University , 199 Ren'ai Road, Suzhou 215123, P. R. China.
authors.forename: Yunxia
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Department of Theoretical Physics, Budapest University of Technology and Economics , Budafoki út 8, H-1111 Budapest, Hungary.|Center for Computational Materials Science, Department of Complex Physical Systems, Institute of Physics, Slovak Academy of Sciences , SK-84511 Bratislava, Slovakia.
authors.forename: Krisztian
authors.initials: K
authors.lastname: Palotas
authors.affiliation: Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University , 199 Ren'ai Road, Suzhou 215123, P. R. China.
authors.forename: Xiao
authors.initials: X
authors.lastname: Yuan
authors.affiliation: Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University , 199 Ren'ai Road, Suzhou 215123, P. R. China.
authors.forename: Tingjun
authors.initials: T
authors.lastname: Hou
authors.affiliation: Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University , 199 Ren'ai Road, Suzhou 215123, P. R. China.
authors.forename: Haiping
authors.initials: H
authors.lastname: Lin
authors.affiliation: Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University , 199 Ren'ai Road, Suzhou 215123, P. R. China.
authors.forename: Youyong
authors.identifier: 0000-0002-5248-2756
authors.initials: Y
authors.lastname: Li
authors.affiliation: Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University , 199 Ren'ai Road, Suzhou 215123, P. R. China.
authors.forename: Shuit-Tong
authors.initials: ST
authors.lastname: Lee
keywords: CH3NH3PbBr3; STM simulations; density functional theory; surface defects