How Humidity and Light Exposure Change the Photophysics of Metal Halide Perovskite Solar Cells

Metal halide perovskites exhibit outstanding optical and electronic properties, but are very sensitive to humidity and light-soaking. In this work, the photophysics of perovskites that have been exposed to such conditions are studied and, in this context, the impact of excess lead iodide (PbI2) is revealed. For exposed samples, the formation of subbandgap states and increased trap-assisted recombination is observed, using highly sensitive absorption and time-resolved photoluminescence (TRPL) measurements, respectively. It appears that such exposure primarily affects the perovskite surface. Consequently, on n–i–p device level, the spiro-OMeTAD/perovskite interface is more rapidly affected than its buried electron-collecting interface. Moreover, both stoichiometric and nonstoichiometric MAPbI3-based solar cells show reduced device performance mainly due to voltage losses. Overall, this study brings forward key points to consider in engineering perovskite solar cells with improved performance and material stability.