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Summary The most important obstacle to widespread use of perovskite solar cells is their poor stability under operational stressors. Here, we systematically monitor the evolution of the photovoltaic performance of perovskite solar cells based on formamidinium-cesium lead iodide (FA0.9Cs0.1PbI3) for 600 h, under a series of controlled operational stressors. Although these devices exhibit reasonable thermal stability, their stability under illumination or stabilized power output (SPO) is far from commercial demands. Synchrotron-based nanoprobe X-ray fluorescence and X-ray-beam-induced current measurements reveal that current-blocking Cs-rich phases segregate during stress tests. The decrease in performance is in line with the increasing density of the Cs-rich clusters in area upon illumination. Theoretical calculations indicate that light-generated carriers provide the thermodynamic driving force for that phase segregation. Our findings correlate device performance to microscopic behavior and atomistic mechanisms and shed light on inhibiting the cation-dependent phase segregation during device operation.

Summary Perovskite solar cells (PSCs) are one of the emerging solar cell technologies with high conversion efficiency. Several deposition methods had been applied for preparing their hole transport layer (HTL). However, there are few direct evidences to demonstrate whether HTL and its interfaces in PSCs have been influenced by the deposition methods. In this study, the 3D morphology of PSCs has been reconstructed by focused ion beam-scanning electron microscopy from the PSCs in which HTLs are deposited by different methods. The compositional distribution of HTLs is unveiled as well. All these associated layers and interfaces display obvious morphological and compositional differences that are attributed to the HTL components’ solubility differences in the precursor solvent. Our investigation demonstrates the PSCs that HTL fabricated by dynamic spin-coating method have higher efficiency, better film uniformity, and less interfacial roughness than the static spin-coating-based devices.