DEVICE PHYSICS OF POLYMER LIGHT-EMITTING-DIODES

This article reviews a device model for the current and light generati on of polymer light-emitting diodes (PLEDs). The model is based on exp eriments carried out on poly(dialkoxy-p-phenylene vinylene) (PPV) devi ces. The transport properties of holes in PPV have been investigated w ith 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 hol e mobility as a function of electric field E and temperature T carl be directly determined. The hole ;mobility exhibits a field dependence l n(mu) similar to root 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 ob tained. The electron conduction in PPV has been studied by using Ca/PP V/Ca electron-only devices. If appears that the electron current is st rongly reduced by the presence of traps with a total density of 10(18) cm(-3). Combining the results of electron- and hole-only devices a de vice 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 give s rise to a bias-dependent efficiency. By comparison with experiment i t is found that the recombination process in PPV is for 95% nonradiati ve. Furthermore, the experiments reveal that the bimolecular recombina tion process is thermally activated with an identical activation energ y as measured for the charge carrier mobility. This demonstrates that the recombination process is of the Langevin-type, in which the rate-l imiting step is the diffusion of electrons and holes towards each othe r. The occurrence of Langevin recombination explains why the conversio n efficiency (photon/carrier) of a PLED is temperature independent. (C ) 1998 John Wiley & Sons, Ltd.