Tuning Perovskite\nSurface Polarity via Dipole Moment\nEngineering for Efficient Hole-Transport-Layer-Free Sn–Pb Mixed-Perovskite\nSolar Cells

Abstract

Post-treatment has been recognized as one of the effective\nmethods\nfor passivating the underlying defects in perovskite solar cells (PSCs),\nbut little attention has been paid to how to pick suitable passivation\nagents with diverse isomers for efficient PSCs, particularly for the\ntin–lead (Sn–Pb) mixed PSCs. Here, we introduce the\ndependence of the power conversion efficiency (PCE) on a dipole moment\nfor surface passivator screening, in which we chose three trifluoromethyl-phenylethylamine\nhydroiodide (CF₃-PEAI) isomers as surface-treatment materials\nfor hole-transport-layer-free (HTL-free) Sn–Pb mixed PSCs.\nThe different positions of the −CF₃ group for the\nCF₃-PEAI isomer result in different dipole moments, which\ninfluences the interaction between CF₃-PEAI and lead iodide.\nThe para position CF₃ with the highest dipole moment exhibits\na higher PCE than the ortho-position with a lower dipole moment, which\nis attributed to the large dipole moment on the surface that could\ntune the surface polarity from p-type to n-type, facilitating electron\ncharge transport in the HTL-free Sn–Pb mixed PSCs. An ultrathin\n2D layer is formed on the perovskite surface to passivate the surface\ndefects, which is responsible for the enhancement of the PCE and stability\nof the PSCs. As a result, the open-circuit voltage (<i>V</i>_OC) of the device is improved from 0.775 to 0.824 V, yielding\na champion PCE of 20.17%, which is one of the highest PCEs among the\nreported HTL-free Sn–Pb mixed PSCs. The device also shows improved\nstability with remaining 75% of its initial PCEs after storage in\nN₂ for 700 h.