Rationalizing the effect of overstoichiometric PbI2 on the stability of perovskite solar cells in the context of precursor solution formulation

In this work, we demonstrate the impact of solution formulation on the photostability of MAPbI3 thin films and solar cells containing overstoichiometric PbI2. Our results revealed that the addition of NMP co-solvent improved the photostability of thin films while excess PbI2 content is present in the absorber layer. Later, by employing NMP co-solvent into precursor in combination with overstoichiometric PbI2, we obtained photostable solar cell devices up to 1500 h. Our finding suggests that the addition of NMP forms an adduct with overstoichiometric PbI2 that further slows down the formation of metallic lead and therefore improves the photostability of MAPbI3.