Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO20)
DOI: https://doi.org/10.29363/nanoge.nipho.2020.065
Publication date: 25th November 2019
Surface defect states have been repeatedly identified as a limiting factor for luminescence and photovoltaic device efficiencies in metal halide perovskite materials. [1-3]
Using a state-of-the-art photoemission electron microscopy (PEEM) [4,5] setup, we map locally the distribution of surface defect states on triple cation, mixed halide perovskite films ((CsFAMA)Pb(I0.83Br0.17)3) with 30 nm spatial resolution. Significant photoemission arises from sub-bandgap states at discrete locations which act as hole traps. The formation of these defects is linked to grain-to-grain variation in composition and structure. Confocal photoluminescence maps show a significant quenching of photoluminescence intensity at the locations of surface defects located with PEEM.
Previously, we found that light and atmospheric treatments [6] can improve luminescence yields of perovskite films by the passivation of defect sites, thereby reducing trap densities. Here, we utilise light treatments in a variety of atmospheric conditions as a lever to control surface trap distribution. With PEEM, we observe the creation of nanoscale defect states during in-situ illumination of the perovskite, in ultra-high vacuum conditions. Conversely, illumination in an oxygen-rich environment leads to a tuneable suppression of the photoemission from defect-rich sites. Crucially, we determine that the local change in defect density is non-uniform, and in fact depends strongly on the initial presence of defects. We show that the photoluminescence heterogeneity previously reported for perovskite films is inherently linked to the distribution of these nanoscale defects and can be similarly controlled.
In-situ nanoscopic x-ray diffraction (nXRD) and ex-situ scanning electron diffraction (SED) measurements reveal that complex structural reorganization occurs on the nanoscale following illumination with light. This study unveils crucial details about the mechanisms which govern defect creation and annihilation in the presence of light.
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