Surface Passivation and Energetic Modification Suppress Nonradiative Recombination in Perovskite Solar Cells.

Surface passivation via post-treatment is an important strategy for improving power conversion efficiency and operational stability of perovskite solar cells. However, so far the interaction mechanisms between passivating additive and perovskite are not well understood. Here, we report the atomic-scale interaction of surface passivating additive 2,2-difluoroethylammonium bromine (2FEABr) on the MAPbI. It is found that the bulky 2FEA cations tend to distribute at film surface, while the Br anions diffuse from surface into bulk. A combination of F, Pb, and H solid-state NMR further reveal the Br anions' partial substitution for the I sites, the restricted motion of partial MA cations, and the firmed perovskite lattices, which would improve charge transport and stability of the perovskite films. Optical spectroscopy and ultraviolet photoelectron spectroscopy demonstrate that the 2FEABr induced surface passivation and energetic modification suppress the nonradiative recombination loss. These findings enable the efficiency of the p-i-n structured PSC significantly increasing from 19.44 to 21.06%, accompanied by excellent stability. Our work further establishes more knowledge link between passivating additive and PSC performance.