Deeper Insight into the Role of Organic Ammonium Cations in Reducing Surface Defects of the Perovskite Film.

Organic ammonium salts (OASs) have been widely used to passivate perovskite defects. The passivation mechanism is usually attributed to coordination of OASs with unpaired lead or halide ions, yet ignoring their interaction with excess PbI 2 on the perovskite film. Herein, we demonstrate that OASs not only passivate defects by themselves, but also redistribute excess aggregated PbI 2 into a discontinuous layer, augmenting its passivation effect. Moreover, alkyl OAS is more powerful to disperse PbI 2 than a F-containing one, leading to better passivation and device efficiency because F atoms restrict the intercalation of OAS into PbI 2 layers. Inspired by this mechanism, exfoliated PbI 2 nanosheets are adopted to provide better dispersity of PbI 2 , further boosting the efficiency to 23.14 %. Our finding offers a distinctive understanding of the role of OASs in reducing perovskite defects, and a route to choosing an OAS passivator by considering substitution effects rather than by trial and error.