Abstract
A comprehensive numerical model is established for the electrical processes in a sandwich organic semiconductor device with high carrier injection barrier. The model prediction agrees in quantitative details with the experiments where high bandgap polyfluorene between poly(3,4-ethylenedioxythiophene)-poly(styrenesulfo- nate)(PEDOT:PSS) anode and metal cathode is taken as the example. The charge injection at the anode interface with 0.8 eV energy is dominated by the hopping among the gap states of the semiconductor caused by disorder. The Ohmic behavior at low voltage is demonstrated to be not due to the background doping but the filaments formed by conductive clusters. In bipolar devices with low work function cathode it is shown that near the anode the electron traps significantly enhance hole injection through Fowler-Nordheim tunneling, resulting in rapid increase of the hole carrier and current in comparison with the hole-only devices. In addition to current, carrier density is independently probed by electroluminescence-induced infrared absorption at various temperature and agree well with the model. It is found that temperature dependence of the carrier mobility is reduced at high carrier density.