Device physics of polymer light-emitting diodes

Abstract

This article reviews 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. The transport properties of holes in PPV have been investigated with indium tin oxide (ITO)/PPV/Au hole-only devices. The hole current is dominated by bulk conduction properties of the PPV, in contrast to previous reports. As the hole current is space-charge limited, the hole mobility as a function of electric field E and temperature T can be directly determined. The hole mobility exhibits a field dependence ln(μ) ∼ ✓E as also has been observed from time-of-flight experiments in many molecularly doped polymers and amorphous glasses. For the zero-field hole mobility an activation energy of 0.48 eV is obtained. The electron conduction in PPV has been studied by using Ca/PPV/Ca electron-only devices. It appears that the electron current is strongly reduced by the presence of traps with a total density of 1018 cm−3. Combining the results of electron- and hole-only devices 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 recombination process in PPV is for 95% nonradiative. Furthermore, the experiments reveal that the bimolecular recombination process is thermally activated with an identical activation energy as measured for the charge carrier mobility. This demonstrates that the 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 (photon/carrier) of a PLED is temperature independent. © 1998 John Wiley & Sons, Ltd.