Surface Defect Passivation and Energy Level Alignment Engineering with a Fluorine-Substituted Hole Transport Material for Efficient Perovskite Solar Cells

The surface and boundary defects present in the perovskite film are reported to be nonradiative recombination and degradation centers, restricting further improvement of the power conversion efficiency (PCE) and long-term stability of perovskite solar cells. To address this problem, herein, we introduce a fluorine-substituted small molecular material 2FBTA-1 as a bifunctional buffer layer to efficiently passivate the surface defects of perovskite and tune the energy level alignment between the perovskite/2,2',7,7'-tetrakis(N, N-di-(p-methoxyphenyl)amino)-9,9'-spirobifluorene (Spiro-OMeTAD) interface. X-ray photoelectron spectroscopy shows that with the insertion of 2FBTA-1 between perovskite and Spiro-OMeTAD, the metallic Pb-0 defects and uncoordinated Pb2+ defects are well restricted. Consequently, the average PCE is distinctly improved from 18.4 +/- 0.51 to 20.3 +/- 0.40%. Moreover, the long-term stability of unencapsulated devices with 2FBTA-1 treatment under ambient conditions (relative humidity 40-60%) is effectively enhanced, retaining 87% of the initial efficiency after storage for 500 h.