We investigate the magnetic field effects in thin-film diodes made of the conducting polymer poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) as a function of temperature and electrical current. Magnetoresistance of these devices can be measured to high precision on two distinct magnetic field scales: <3 mT, where a pronounced nonmonotonic magnetoresistance response can be resolved, owing to weak hyperfine coupling, and at intermediate magnetic fields, ranging between 3 and 10 mT, where strong monotonic magnetoresistance is seen. The detailed examination of the magnetoresistance effects in both regimes allows one to scrutinize the accuracy of the underlying models for the behavior of these kinds of materials.
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.