ELECTRICAL CHARACTERIZATION OF POLYMER LIGHT-EMITTING-DIODES

This paper presents a device model for the current and light generatio n of polymer light-emitting diodes (PLED's), The model is based on exp eriments carried out on poly(dialkoxy-p-phenylene vinylene) (PPV) devi ces. It is demonstrated that PLED's are fundamentally different as com pared to conventional inorganic LED's, The hole conduction in PPV is s pace-charge limited with a low-field mobility of only 5 x 10(-11) m(2) /Vs, which originates from the localized nature of the charge carriers , Furthermore, the hole mobility is highly dependent on the electric f ield and the temperature. The electron conduction in PPV is strongly r educed by the presence of traps. Combining the results of the electron -and hole transport a device model for PLED's is proposed in which the light generation is due to bimolecular recombination between the inje cted electrons and holes. It is calculated that the unbalanced electro n and hole transport gives rise to a bias dependent efficiency. By com parison with experiment it is found that the bimolecular recombination process Is of the Langevin-type, in which the rate-limiting step is t he diffusion of electrons and holes toward each other. This is in cont rast to conventional semiconductors, in which the bimolecular recombin ation is governed by the joint density-of-states of electrons and hole s and is not limited by charge transport, The occurrence of Langevin r ecombination explains why the conversion efficiency of current into li ght of a PLED is temperature independent. The understanding of the dev ice operation of PLED's indicates directions for further improvement o f the performance.