Impact of Molecular Dipole Moments on Fermi Level Pinning in Thin Films

A series of merocyanine dyes with a wide range of molecular dipole moments were deposited on metal oxides covering a wide work function (Phi) range (2.3 to 7.0 eV), and the energy level alignment at these interfaces was studied with photoelectron spectroscopy. We find that a preferential orientation of the merocyanines and their dipoles in the monolayer systematically lowers Phi of the oxides such that Fermi-level (E-F) pinning at the highest occupied molecular level of the merocyanines only occurs for very high Phi oxides (>= 6 eV). Correspondingly, pinning at the electron affinity level can readily be achieved also with moderate oxide Phi, e.g., for indium tin oxide, and electron transfer between the merocyanines to these oxides can proceed readily. Noteworthy, the E-F-pinning behavior and the associated Phi values seem independent of the molecular dipole moment magnitude, most likely due to the self-limiting effect of Phi as soon as the pinning regime is reached.