Metal screening effect on energy levels at metal/organic interface: Precise determination of screening energy using photoelectron and inverse-photoelectron spectroscopies

Abstract

The energy level alignment at the metal/organic interface greatly affects the charge injection/extraction efficiency of electrodes in organic semiconductor devices. The charge carrier in the vicinity of the metal surface should be stabilized by the screening effect of the metal surface, thereby resulting in the narrowing of the energy gap (highest occupied molecular orbital (HOMO)--lowest unoccupied molecular orbital (LUMO) gap) and modifying the energy level alignment at the metal/organic interface. However, this metal screening effect has not been fully clarified because there has been no precise way to quantitatively evaluate the experimental values. In this study, we employed ultraviolet photoelectron spectroscopy (UPS), metastable atom electron spectroscopy (MAES), and low-energy inverse photoelectron spectroscopy (LEIPS) to examine the quasi--layer-by-layer grown film of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on the atomically flat metal surface of Ag(111) and Au(111). By applying a procedure we established recently, we precisely evaluated the energy of the metal screening effect. The results demonstrate that the screening effect of the metal surface modifies the charge injection/collection barrier at the metal/organic interface by as much as 0.25 eV. We found that the PTCDA thickness dependent screening effect is precisely reproduced by the image charge model.