The role of copper phthalocyanine for charge injection into organic light emitting devices

We investigate the charge injection efficiency of plasma treated indium. ti n oxide (ITO) anodes into copper phthalocyanine (CuPc) in single-layer diod es fabricated under inert conditions. Using electroabsorption and Kelvin p' robe surface potential measurements, we demonstrate that the effective ITO work function is pinned at the energy level of the highest occupied molecul ar orbital of CuPc. We ascribe this effect to oxygen doping from the ITO el ectrode. Such doping results in high-efficiency hole injection from ITO as inferred from the current-voltage characteristics. We find evidence for a t ime-dependent modification of the device characteristics particularly in re verse bias that we attribute to oxygen diffusion from ITO into bulk CuPc. O xygen plasma treatment of ITO produces an oxide surface that is stable with respect to oxygen diffusion. (C) 2001 Elsevier Science B.V. All rights res erved.