Tuning Perovskite Surface Polarity via Dipole Moment Engineering for Efficient Hole-Transport-Layer-Free Sn–Pb Mixed-Perovskite Solar Cells

Post-treatment has been recognized as one of the effective methods for passivating the underlying defects in perovskite solar cells (PSCs), but little attention has been paid to how to pick suitable passivation agents with diverse isomers for efficient PSCs, particularly for the tin-lead (Sn-Pb) mixed PSCs. Here, we introduce the dependence of the power conversion efficiency (PCE) on a dipole moment for surface passivator screening, in which we chose three trifluoromethyl-phenylethylamine hydroiodide (CF₃-PEAI) isomers as surface-treatment materials for hole-transport-layer-free (HTL-free) Sn-Pb mixed PSCs. The different positions of the -CF₃ group for the CF₃-PEAI isomer result in different dipole moments, which influences the interaction between CF₃-PEAI and lead iodide. The para position CF₃ with the highest dipole moment exhibits a higher PCE than the ortho-position with a lower dipole moment, which is attributed to the large dipole moment on the surface that could tune the surface polarity from p-type to n-type, facilitating electron charge transport in the HTL-free Sn-Pb mixed PSCs. An ultrathin 2D layer is formed on the perovskite surface to passivate the surface defects, which is responsible for the enhancement of the PCE and stability of the PSCs. As a result, the open-circuit voltage (V_OC) of the device is improved from 0.775 to 0.824 V, yielding a champion PCE of 20.17%, which is one of the highest PCEs among the reported HTL-free Sn-Pb mixed PSCs. The device also shows improved stability with remaining 75% of its initial PCEs after storage in N₂ for 700 h.