Interface formation between NPB and processed indium tin oxide

We have investigated the interface formation between ITO and N,N'-bis-(1-na phthyl) -N,N'-diphenyl- 1,1 '-biphenyl-4,4'-diamine (NPB), an organic mater ials often used as hole transport layer in organic light-emitting devices ( OLED), by using X-ray and ultraviolet photoelectron spectroscopy (XPS and U PS) and atomic force microscopy (AFM?). Acid or base treatment of indium ti n oxide (ITO) surfaces can significantly alter the surface work function wh ich, in the case of acid treatment, points to an improved energy level alig nment with NPB and, therefore, enhanced hole injection efficiency. We found no significant reactions nor level bending for NPB deposited on standard I TO. In contrast, for acid-heated ITO, reaction of NPB nitrogen with the pro ton of the dipole layer on the ITO surface is observed. At low NPB coverage s, AFM images reveal uniform island growth of NPB on ITO. Further depositio n leads to a more complete covering of the ITO surface by NPB layer, corres ponding to a laminar growth mode. (C) 2000 Published by Elsevier Science S. A. AU rights reserved.