Influence of copper phthalocynanine on the charge injection and growth modes for organic light emitting diodes

The stability and efficiency for organic light emitting diodes (OLEDs) based on tris-8-(hydroxyquinoline) aluminum and N,N'-bis-(l-naphthyl)-N,N'-diphenyl1-1, 1-biphenyl1-4,4'-diamine (NPB) are enhanced with the use of a copper phthalocynanine (CuPc) layer between the indium tin oxide (ITO) anode and the NPB hole transport layer. In the following, we have investigated two fundamental properties of the CuPc buffer that contribute to improved stability. Using time of flight and dark current as a function of applied field, we show that the CuPc interlayer influences the hole injection from the ITO to the NPB transport layer. Because NPB is trap free, a direct and self-consistent measure of the hole injection efficiency can be determined from the measured drift mobility and dark current at the same applied field. By this means we have been able to demonstrate that the hole transport from the ITO anode to the NPB is injection limited and the hole injection efficiency is further reduced with a 15.0 nm CuPc interlayer. Using atomic force microscopy, we have determined that NPB organic molecules grow in island-like modes on ITO compared to a layer by layer growth for the CuPc organic molecules. With CuPc as the substrate, NPB also grows in apparent layers. The combined electrical and structural characteristics of the CuPc layer are important to the overall performance of OLEDs. (C) 2000 American Vacuum Society. [S0734-2101(00)11004-8].