Doping efficiency and energy-level scheme in C₆₀F ₄₈-doped zinc-tetraphenylporphyrin films

High resolution synchrotron-based core level spectroscopy was used to examine the energy level alignment at the interface of zinc-tetraphenylporphyrin films doped by the surface acceptor C₆₀F₄₈. Two distinct fluorofullerene charge states were identified, corresponding to ionized and neutral molecules, and their relative concentration as a function of coverage was used to evaluate the probability of occupation of the acceptor lowest unoccupied molecular orbital (LUMO). From an initial acceptor energy of -0.25 eV, the C₆₀F₄₈ LUMO shifts upwards with coverage due to a doping-induced interfacial dipole potential, and stabilization of the LUMO at an energy 0.45 eV above the Fermi energy was obtained. While the energy difference upon saturation is consistent with the results obtained for other donor-acceptor systems that have been interpreted as Fermi level pinning, the present work shows that the energy offset is a direct consequence of the interplay between Fermi-Dirac statistics in combination with the interfacial dipole potential.