Interfacial charge transfer and hole injection in a-NPD organic overlayer–CVD diamond substrate system

We have investigated interfacial charge transfers and hole injection characteristics between oxygen-terminated boron-doped homoepitaxial (1 0 0) chemical-vapor-deposited (CVD) diamond films and thermally deposited N,N0-diphenyl-N,N0-bis(1- naphthyl)-1,10biphenyl-4,400diamine (a-NPD) mainly using X-ray photoelectron spectroscopy (XPS), Hall effect measurements and current–voltage (J–V) characteristic measurements. On one hand, no chemical shift was observed for C 1s and N 1s XPS spectra taken from the a-NPD overlayer–CVD diamond substrate system. On the other hand, the low-temperature hole concentration of the diamond substrate decreased by 10–17% after the a-NPD depositions, verifying that small but substantial amounts of holes were transferred from the diamond substrates to the a-NPD films. However, compared with those obtained from the specimens before the a-NPD depositions, the sheet electrical conductivities of the specimens after the a-NPD depositions increased at T 230 K while decreasing at T 200 K. In addition, J–V data measured from layered a-NPD– diamond hetero-structures show evidence that the junction worked as a diode. These facts suggest that the a-NPD–CVD diamond system may be useful for a transparent anode electrode with a hole transport layer appropriate for organic light emitting diodes.