Temperature-dependent single carrier device model for polymeric light emitting diodes

We present an investigation of the temperature dependence of the current-voltage characteristics of a single carrier poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) polymer light-emitting diode (LED) with indium tin oxide (ITO) and aluminium electrodes. We find that although injection effects are the dominant current Limiting mechanisms, bulk transport effects also play a significant role due to the low charge carrier mobility. Tunnelling theory cannot account for the observations made and we find that the tunnelling barrier predicted by a simple Fowler-Nordheim analysis increases by more than a factor of two from 0.2 to 0.45 eV as the LED is cooled from room temperature to 13 K. We use a device model combining the interfacial currents of thermionic emission, tunnelling and interface recombination, with drift currents to describe successfully the measured current-voltage characteristics over a temperature range from 300 K to 100 K using the barrier height to injection and the field-dependent mobility as fitting parameters. Good quality fits are obtained over a wide range of temperatures. The held-dependent mobility parameters are found to have a temperature dependence characteristic of hopping in disordered solids with an activation energy of 0.75 eV for the zero field mobility. The barrier to injection for holes from the ITO anode is found to be lowered from 0.56 eV at 300 K to 0.41 eV at 100 K, which can in part be accounted for by the reduction of the band gap of MEH-PPV with decreasing temperature. The temperature dependence of the parameters is greater than values reported elsewhere, suggesting that approximating the thermally-activated, microscopic injection processes by a thermionic emission term overestimates the activation energy for charge carrier injection. We looked at the effect of photo-oxidation on device characteristics as a further test of the model and found that these modified characteristics are well described by a substantial reduction in mobility at constant barrier height.