Dependence of the hole-injection barrier on the hole conductor in organic light emitting diodes based on composites

Organic light-emitting diodes based on composites using different hole cond uctors were fabricated and characterized. In these devices the current flux is limited by the injection of holes into the semi-conducting polymer laye r through tunneling. The data were evaluated using the common Fowler-Nordhe im formalism. The barrier height was found to depend linearly on the oxidat ion potential of the hole conductor, but unexpectedly the slope is only 0.5 5 +/- 0.1. This result is explained by the non-polar nature of the internal interface between ITO and the conductive layer.