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Device model for polymer light-emitting diodes: effect of temperature and oxidation

We present a temperature-dependent single carrier device model for polymer light-emitting diodes. The model includes both the injection of charge carriers over a barrier and the transport of charge 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) with indium tin oxide (ITO) and aluminum contacts was studied. Good agreement with experiment is found using a strongly field and temperature dependent mobility. Current-voltage characteristics were fitted over a temperature range from 100 K to 300 K using three parameters: the barrier to injection, the zero-field mobility, and the field dependence of the mobility. 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 K and 220 K, but decreases more slowly below 220 K. This reduction with temperature is found to relate to the red-shift of the absorption peak of MEH-PPV. The model is used to fit current-voltage characteristics of aged devices. The effect of photo-oxidation is well described by the model through a reduction of mobility at constant barrier height, giving insight into the effects of the creation of charge trapping carbonyl groups in the bulk polymer on injection and transport.