Electrical characterization of polymer light-emitting diodes

This paper presents a device model for the current and light generation of polymer light-emitting diodes (PLEDs). The model is based on experiments carried out on poly(dialkoxy-p-phenylene vinylene) (PPV) devices. It is demonstrated that PLED´s are fundamentally different as compared to conventional inorganic LEDs. The hole conduction in PPV is space-charge limited with a low-field mobility of only 5×10^-11 m²/Vs, which originates from the localized nature of the charge carriers. Furthermore, the hole mobility is highly dependent on the electric field and the temperature. The electron conduction in PPV is strongly reduced by the presence of traps. Combining 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. It is calculated that 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 toward each other. This is in contrast to conventional semiconductors, in which the bimolecular recombination is governed by the joint density-of-states of electrons and holes and is not limited by charge transport. 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 PLED´s indicates directions for further improvement of the performance