Temperature-dependent device model for polymer light-emitting diodes: significance of barrier height

We present a temperature-dependent single carrier device model for polymer light-emitting diodes (LEDs). The model includes both the injection of charge carriers over a barrier and the transport of charges across the device. To test the model, the temperature dependence of an LED based on the conjugated polymer poly[2-methoxy, 5-(2′ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) in the configuration ITO/MEH-PPV/Al is studied. Good agreement with experiment is found using a strongly field- and temperature-dependent mobility. Current–voltage characteristics were fitted using three parameters, the barrier to injection, the zero field mobility and the field dependence of the mobility. The curves were fitted independently of each other over a temperature range from 100 to 300 K. The resulting mobility parameters have an activation energy type form and are found to vary with temperature according to previously reported results. The barrier height to injection is found to decrease strongly between 300 and 220 K, but decreases more slowly below 220 K. This reduction with temperature is correlated with the red-shift of the absorption peak of MEH-PPV. In both cases, a change of slope is observed around 220 K. However, the rate of change of the position of the absorption peak with temperature is found to be half of the change in barrier height.