Lifetime study of polymer light-emitting electrochemical cells

The lifetime and degradation characteristics of frozen-junction polymer light-emitting electrochemical cells have been studied. Two seemingly minor factors, the end group of the same luminescent polymer and the type of substrate on which the device is constructed, have shown profound effects on the stress behaviors of these devices operated at a nominal operating temperature of 200K. In devices made on glass substrates, an anomalous large increase in light intensity and driving voltage has been observed under constant current stress measurement. This has been attributed to the relaxation of the electrochemical doping, which leads to reduced photoluminescence quenching and increased resistance in the polymer film. The doping relaxation is accelerated by the poor thermal conductivity of glass substrate, which causes significant temperature rise in the device due to self-heating. Evidence also suggests that doping relaxation can even occur at below the glass transition temperature of the polymer electrolyte. All devices are shown to be short lived when operated at or above 250K due to fast doping relaxation. However, devices made on sapphire substrates based on 3,5-dimethyl phenyl-terminated poly[5-(2′- ethylhexyloxy)-2-methoxy-1, 4-phenylene vinylene] are remarkably stable when operated at 200K, with an estimated half-life of 700h even when stressed at a very high current density of 1A∕cm2.