Photoemission study of the interface between phenyl diamine and treated indium–tin–oxide

The interface formation between indium–tin–oxide (ITO) having various work functions and ${\mathrm{N,N}}^{\prime }-\mathrm{bis-(1-naphthyl)}{\mathrm{-N,N}}^{\prime }-{\mathrm{diphenyl-1,1}}^{\mathrm{\prime }}{\mathrm{-biphenyl-4,4}}^{\mathrm{\prime }}\mathrm{-diamine}$ (NPB) was investigated using x-ray and ultraviolet photoelectron spectroscopy. Phosphoric acid and tetrabutylammonium hydroxide were used to modify the ITO work function. The energy difference between the highest occupied molecular orbital (HOMO) position of NPB and the ITO Fermi level can be varied substantially by surface treatment. The work function of acid-treated ITO dramatically decreases after deposition of a thin NPB layer. We attribute this to the reaction between the NPB nitrogen and the proton of the adsorbed dipole layer. No significant reaction was observed for NPB deposited on standard ITO.