Abstract
Surface ligand treatment of organic metal halide perovskites can improve material and device stability, alter interfacial energetics, and increase perovskite solar cell (PSC) performance. Unlike in traditional inorganic semiconductors such as silicon, surface ligands often penetrate into the perovskite crystal and result in the formation of reduced dimensionality structures (where the 3D perovskite framework is broken into 2D sheets of varying thickness). Such ligand penetration can have a large impact on the surface energetics and material stability; thus, it is important to understand how surface ligand structure impacts ligand penetration. Using angle-dependent x-ray photoelectron spectroscopy (XPS) and x-ray diffraction we show that surface ligands with ammonium and phosphonic acid binding groups often penetrate into MAPbI3. This ligand penetration can significantly influence surface energetics, as probed with ultraviolet and inverse photoelectron spectroscopy measurements. The observed changes in surface energetics upon surface ligand treatment have a large impact on the PSC performance, with better photovoltaic performance arising for ligands that result in more favorable energy landscapes for electron transfer from MAPbI3 to the electron transport layer, C60. For a series of ammonium containing ligands, we find that both the cross-sectional area and extent of ligand fluorination impact the penetration of the surface ligand into the perovskite. Furthermore, surface ligands with larger cross-sectional areas result in improved MAPbI3 stability.