Charge transport in poly(p-phenylene vinylene) light-emitting diodes

Since the discovery of electroluminescence in conjugated polymers it has been recognized that charge transport is a key ingredient for the efficiency of the polymer light-emitting diodes (PLEDs). This review focuses on the charge transport properties of these materials. From temperature dependent current density-voltage characteristics it has been obtained that the hole transport in poly(dialkoxy-p-phenylene vinylene) (PPV) is governed by a combination of space-charge effects and a field- and temperature-dependent mobility. The origin of the hole mobility, which seems to be generic for a large class of disordered materials, arises from hopping in a system with both energetic and structural disorder. The response time of PPV-based PLEDs is governed by the dispersive transport of holes towards the cathode. Based on the results of the electron- and hole-transport a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. The unbalanced electron and hole transport gives rise to a bias dependent efficiency. By comparison with experiment it is found that the bimolecular recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency of current into light of a PLED is temperature independent. The understanding of the device operation of PLEDs indicates directions for further improvement of the performance. (C) 2000 Elsevier Science S.A. All rights reserved.