Unraveling the Origin of Mid-Gap Photoemission in Lead Halide Perovskites

Despite being solution-processed, the defect tolerant property of perovskites has allowed them to compete in the same league as silicon solar cells in terms of power conversion efficiency. However, even though often touted as one of its main strengths, proper identification and management of its many defects are crucial to the achievement of highly performant and stable photovoltaic devices. Among the various techniques used to investigate the presence of defects in perovskites, ultraviolet photoelectron spectroscopy and microscopy provides the most direct method to probe the presence, spatial distributions, and trapping dynamics of hole traps in perovskite thin films. These traps are often characterized by the presence of occupied mid-gap states in the photoemission spectra of perovskites that stretches from the valence band up to the Fermi level. Thus far, these mid-gap states have been attributed to a combination of defects such as PbI₂ defects, grain boundary defects, and polytype defects. Here, we investigate the diffraction patterns and photoemission spectra of a variety of perovskite thin films using spectroscopic low energy and photoemission electron microscope. By changing the stoichiometry of the perovskite films, we observe a positive correlation between the presence of excess PbI₂ and the photoemission intensity of these mid-gap states. In contrast, these mid-gap photoemission is strangely absent in Sn-based perovskites. Using microprobe low energy electron diffraction, we found that the presence of PbI₂ diffraction has a negative correlation with the presence of the mid-gap photoemission. Instead, upon laser irradiation, the disappearance of the PbI₂ diffraction coincides with the rise of the mid-gap photoemission. Based on these measurements, we thus reach a conclusion that the origin of these mid-gap photoemission spectra is in fact the spectra of metallic Pb, a decomposition by-product of PbI₂. The absence of these mid-gap photoemission in Sn-based perovskites is thus due to the propensity of Sn²⁺ to form various Sn⁴⁺ compounds such as SnI₄ and SnO₂ rather than decompose to metallic Sn.