Structural and Electronic Properties of Lead Halide Perovskite Thin Film Surfaces

Although often quoted as ’defect-tolerant’, understanding and control of the many defects in the polycrystalline perovskite thin films are crucial for the achievement of stable and highly performant photovoltaic devices. Photoemission from within the perovskite band gap has often been attributed to the presence of various deep electronic defects on the perovskite surfaces. As such, ultraviolet photoelectron spectroscopy and microscopy are routinely used to directly probe the presence, spatial distribution, and trapping dynamics of these defects. However, the relatively high photoemission intensity and the broad spectral distribution of these mid-gap photoemissive states seem to suggest the presence of a hidden contributor in addition to potential localized electronic states within the semiconductor bandgap.

Here, we exploit a multimodal approach that is capable of measuring locally, at sub-micrometer scale, the surface crystalline and electronic properties of individual perovskite grains. For the first time the crystalline structure of the perovskite surface is reported and by correlating the crystalline and electronic properties of various perovskite compositions, we reveal the chemical nature of mid-gap photo-emissive states.