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 wealdy 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.