Tuning interfacial energetics with surface ligands to enhance perovskite solar cell performance

Surface ligands are often used to improve perovskite solar cells (PSCs). Here, a series of surface ligands with varying dipole moments are applied at the methylammonium lead iodide (MAPbI₃)/C₆₀ interface to vary the energy-level alignment. All investigated surface ligands improve PSC performance, with 4-fluorophenethylammonium iodide (FPEAI) displaying the greatest improvement. Transient absorption and reflectance measurements show similar recombination dynamics in all ligand-treated MAPbI₃/C₆₀ bilayer films. Transient photovoltage measurements also show similar recombination lifetimes at similar charge-carrier densities; however, differential capacitance measurements indicate that FPEAI shifts the differential capacitance curve to higher voltages. In situ ultraviolet photoemission spectroscopy measurements show that the interfacial energy gap at the MAPbI₃/C₆₀ interface increases from 1.19 eV with phenethylammonium iodide (PEAI) to 1.50 eV with FPEAI, corresponding to the shift of the differential capacitance curve. This increased interfacial energy gap is responsible for the increased open-circuit voltage (V_OC) and should be considered in surface ligand selection.