Interface formation between NPB and processed indium tin oxide

We have investigated the interface formation between ITO and N, N′-bis-(1-naphthyl)- N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), an organic materials often used as hole transport layer in organic light-emitting devices (OLED), by using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and atomic force microscopy (AFM). Acid or base treatment of indium tin oxide (ITO) surfaces can significantly alter the surface work function which, in the case of acid treatment, points to an improved energy level alignment with NPB and, therefore, enhanced hole injection efficiency. We found no significant reactions nor level bending for NPB deposited on standard ITO. In contrast, for acid-treated ITO, reaction of NPB nitrogen with the proton of the dipole layer on the ITO surface is observed. At low NPB coverages, AFM images reveal uniform island growth of NPB on ITO. Further deposition leads to a more complete covering of the ITO surface by NPB layer, corresponding to a laminar growth mode.