Vibrational and photoemission study of the interface between phenyl diamine and indium tin oxide

High-resolution electron-energy-loss spectroscopy (HREELS) and ultraviolet photoemission (UPS) measurements of the interface of phenyl diamine (NPB) and indium tin oxide (ITO) were performed. The HREELS spectrum of the oxygen-plasma-treated ITO was characterized by dipolar-active-phonon modes at 71 and 134 meV. Upon deposition of NPB on ITO, the HREELS loss peaks of ITO gradually diminished, while the NPB-derived loss peaks appeared. The negligible energy shifts of the NPB peaks with NPB thickness suggested a rather weak interaction between NPB and ITO. The remarkable intensity change of the loss band at 71–65 meV gave evidence for a strong vibrational coupling between the ITO phonon mode at 71 meV and the NPB-derived vibrational mode at 65 meV. Annealing of the NPB/ITO interface with 20 Å of NPB resulted in the decoupling of the two vibrational modes, presumably due to crystallization of the NPB overlayer. UPS measurements showed a sharp decrease in work function at the initial NPB deposition, which is attributable to a dipole layer formed at the interface.