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Tremendous progress in employing metal halide perovskites (MHPs) in a variety of applications, especially in photovoltaics, has been made in the past decade. To unlock the full potential of MHP materials in optoelectronic devices, an improved understanding of the electronic energy level alignment at perovskite-based interfaces is required. This particularly pertains to such interfaces under device operation conditions, e.g. under illumination with visible light such as in a solar cell. Herein, it is revealed that the energy level alignment at the buried interface between a double cation lead halide perovskite film and charge-selective organic transport layers changes upon white light illumination. This is found from photoemission experiments performed with the samples in dark and under illumination, and the interfacial energy level shift is reversible. The underlying mechanism is attributed to the accumulation of one charge carrier type within the perovskite film at the interface under illumination, as a result of the charge-selective nature of the organic layer. The fact that the interfacial energy level alignment at MHP-based junctions under illumination can differ from that in dark is to be taken into account to fully rationalize device characteristics., Peer Reviewed

Vapor deposition processes, well established in the industry, are considered a cost‐effective manufacturing route for large‐area depositions of lead halide perovskites. However, the vapor deposition of perovskites is constrained by its low reproducibility, whereby in particular the evaporation of methylammonium iodide (MAI) is frequently reported as extremely challenging. Herein, the influence of impurities on the evaporation dynamics of MAI is analyzed. It is proved that MAI heated in a high vacuum, depending on the purity, either undergoes a sublimation with a subsequent desublimation or a chemical decomposition with a following reverse reaction, leading to solid MAI in both cases. In situ mass spectrometry enables us to show that low purity MAI containing water and methylammonium phosphite impurities mainly sublimes, which results in a directional and by crystal microbalances traceable evaporation. In contrast, high purity MAI sublimes at higher temperatures and tends to form decomposition products such as methylamine and hydrogen iodide, which omnidirectional diffuse and can react with lead iodide to perovskite. The findings reveal the reason for the reported challenges, serve as a guideline for future material selections, and can enhance the reproducibility of the evaporation process to achieve a more controlled solar cell fabrication., Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347, Helmholtz Association within the HySPRINT Innovation Lab Project, HyPerCells Joint Graduate School, Peer Reviewed