Atomistic Origins of Surface Defects in CH₃NH₃PbBr₃ Perovskite and Their Electronic Structures

Abstract

The inherent instability of CHNHPbX remains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CHNHPbX have 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 CHNHPbX perovskite crystals and their effects on the stability of perovskite materials.

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“…and Cai et al . provided the STM images on vacuum co‐evaporated MAPbI 3 polycrystalline films (Figure d,e) in which similar dark protrusions were observed most likely corresponding to the I and/or I‐MA vacancies …”

“…It is likely that local properties on a much smaller scale, at the micrometer or even nanometer scale, are the key to settling many existing controversial issues in perovskite materials and solar cells . Since 2014, scanning tunneling microscopy and spectroscopy (STM/STS) have been used to investigate and correlate the structural and electronic properties at the atomic level . STM and STS can be used to visualize in real space defects at the atomic level and characterize their electronic properties on the perovskite surfaces (Figure f and Figure d,e).…”

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“…Searching more immobilized and robust ligands or protective shells is imperative to gain solid‐state perovskite films with high quantum yields. Liu et al revealed that the pristine surface was robust, but if the defects occurred, e.g., vacancy, the perovskites were much easier to decompose starting from these defect points . The weak interaction between ligands and nanocrystals is easy to cause the formation of defects during film fabrication.…”

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“…However, this raises the question of: how these defects will affect the physical properties of perovskite crystals. Liu et al revealed the influence of defects in the crystal, as discussed in Figure 10 [60]. Vacancy sites, V Br , V MA , V Br-Pb , and V Br-MA , can be formed easily when water molecules are adsorbed on the (010) surface of the CH 3 NH 3 PbBr 3 perovskite crystal because water reduces the formation energy of vacancy sites.…”