Interface formation and properties of α-NPD thermally deposited on CVD diamond films

We have investigated the interface formation between hydrogen- or oxygen-terminated homoepitaxial (1 0 0) chemical-vapor-deposited (CVD) diamond film and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (α-NPD) using X-ray photoelectron spectroscopy (XPS) to confirm both possibilities of CVD diamond as hole transport layer and transparent anode electrode for α-NPD as a possible organic light-emitting diode (OLED) material. In the case of α-NPD deposited on the H-terminated diamond substrates, the peak positions of both C 1s and N 1s spectra shifted to their lower binding energy sides, clearly differing from those in case of α-NPD deposited on a Si substrate where no remarkable peak shift was observed. This suggests that some electrons were transferred from the H-terminated CVD diamond to the deposited α-NPD. In the case of O-terminated diamond, however, completely different variations were found for the measured XPS peaks. No thickness-dependent peak shift was observed while the relative energy position of the α-NPD C 1s to the diamond C 1s was higher by 1.7 eV. The possible origin of the interface interaction is discussed in relation to band bending effect, change in the surface dipole layer between the H- and O-terminated diamond and change in energy levels of the α-NPD molecular orbitals after the interface formation.