High electric field effect on hopping conduction in molecularly doped polymer systems

Time of flight mobility measurements were carried out in thin films of soli d solution of amorphous polycarbonate molecularly doped with N-isopropylcar bazole by using electron beam bombardment. The dependence of the hole drift mobility (mu) with the applied electric field was analysed in the scope of both, the Poole-Frenkel emission model and a density of state model which does not invoke the existence of charged traps. The experimental results we re in just agreement with the Poole-Frenkel model through the whole range o f the electric field used during the experiment; however, the agreement wit h the density of state model was evident only at fields greater than 0.8 MV /cm. To justify this behaviour, an additional consideration in the Poole-Fr enkel model was proposed which allowed to conclude, that the circumstantial agreement with the density of state model at the highest range of electric field results from the fact that the traps become inoperative at this rang e of field, and not because the traps were really absent. (C) 1999 Elsevier Science Ltd. All rights reserved.