Effect of electrical annealing on the luminous efficiency of thermally annealed polymer light-emitting diodes

Both high brightness and high efficiency of polymer light-emitting diodes can be obtained by electrical annealing after an appropriate thermal treatment. Light-emitting diodes made of poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with an Al electrode were used for this study. Thermal annealing after Al deposition dramatically enhanced the luminous efficiency of electroluminescent (EL) devices about 230 times at 350 mA/cm2, and the maximum external quantum efficiency (QE) also approached 1.43% photons/electrons. With the bias voltage annealing after thermal annealing, not only the onset voltage of the device was reduced but also the efficiency was further enhanced. This enhanced luminance and efficiency is attributed to the dipoles and accumulated space charges of polar ionic impurities within the MEH-PPV that induce more electronic charges for bipolar recombination. We obtained an external QE of 1.96% photons/electrons after 170 °C thermal annealing and 15 V bias annealing.