Metal-Diffusion-Induced Interface Dipole: Correlating Metal Oxide–Organic Chemical Interaction and Interface Electronic States

Abstract

The effects of metal oxide diffusion on the interface dipole (ID) energy at a metal oxide (SnO2)/organic semiconductor (copper phthalocyanine, CuPc) interface were studied. In situ synchrotron radiation photoelectron spectroscopy and ultraviolet photoemission spectroscopy studies showed that the ID energy for SnO2-on-CuPc (−0.65 eV) was higher by 0.15 eV than that of CuPc-on-SnO2 (−0.50 eV). When SnO2 deposited on a CuPc layer, hot Sn atoms release enough condensation energy to disrupt the weakly bonded CuPc and diffuse through the surface. The diffused Sn atoms made a chemical reaction with nitrogen atoms in CuPc molecules and made organo-metallic compounds, Sn2CuPc, resulting in the generation of gap states at the former lowest unoccupied molecular orbital. This observation explains why the ID and hole injection barrier at SnO2-on-CuPc are larger than those at the CuPc-on-SnO2 interface. Organic light-emitting diodes with a SnO2-on-CuPc interface showed a lower luminous efficiency (2.63 cd/A) than that of the device with the CuPc-on-SnO2 interface (5.26 cd/A), and this result indicates that ID tuning at SnO2–CuPc interfaces by adjusting the metal diffusion can be readily applicable.

Interface Dipoles for Tuning Energy Level Alignment in Organic Thin Film Devices

“…Ve ry often, the reaction has reductive or oxidative character and therefore gives rise to a surface dipole contribution. [110][111][112] If ions are present in the add-layer, similar phenomenon occurs as with an electrolyte at the electrodeliquid interface (build-up of a Helmholtz layer). [113,114] Ve ry often, various surface interaction effects cooperate and contribute to the resulting energy level shifts observed.…”