N‑Doped Zwitterionic Fullerenes as Interlayers in Organic and Perovskite Photovoltaic Devices

The efficient operation of polymer- and perovskite-based photovoltaic devices depends on selective charge extraction layers that are placed between the active layer and electrodes. Herein, we demonstrate that integration of a tetra-<i>n</i>-butyl ammonium iodide-doped zwitterionic fulleropyrrolidine derivative, C₆₀-SB, as a cathode modification interlayer significantly improves the photovoltaic device performance. Compared to the intrinsic (undoped) zwitterionic material, which is an efficient interlayer itself, the doped interlayers further improve average power conversion efficiencies from 8.37% to 9.68% in polymer-based devices and from 12.53% to 15.31% in perovskite-based devices. Ultraviolet photoelectron spectroscopy revealed that doping increases the interfacial dipole at the C₆₀-SB/Ag interface, i.e., reduces the effective work function of the resultant composite cathode. This effect originates from the population of negative polaron states in C₆₀-SB by extrinsic charges that prevent directional charge transfer from Ag to the integer charge-transfer states in C₆₀-SB, pinning the Fermi level at higher energy. The reduced resistivity of the doped interlayer, as measured by impedance spectroscopy, enables efficient device operation with a broad range of interlayer thicknesses, thus simplifying the solution-based device fabrication process