Improvement of EL efficiency in polymer light-emitting diodes by heat treatments

Abstract

Light-emitting diodes (LEDs) based on conjugated polymers have attracted much attention because of their potential application to flat, large area displays which can be operated at low driving voltage. To make more efficient electroluminescent(EL) device, it is necessary to enhance the injection efficiency of the negative carriers. Heat treatment was performed to enhance the efficiency of the EL devices. The performance and efficiency based on Poly[2-methoxy-5-(2``-ethly-hexyloxy)-1,4-phenylene vinylene](MEH-PPV) has been studied. Several different annealing treatments have been tried to improve the EL efficiency by altering both polymer packing and polymer/Al interface. Only baked EL device under $T_g$ follows the two exponential behaviors because of the impurity semiconducting phase that is originated from pores, disorders within the polymer, and interfacial impurities. The I-V characteristics in semi-log plot have the characteristic of stepwise two exponential increases. Even after long time baking under $T_g$, the transitional behavior between turn-on and operating voltage still exists, and an anomalous two-stage increase in high field in case of linear plot was not changed, either. Pre-deposition annealing, however, can change the two-stage behavior in linear plot. The main emission is altered from the one from the unannealed device. Pre-deposition annealing reveals the characteristics of red-shifted spectrum by the growth of an shoulder peak and its screening main peak characteristic. After the pre-deposition annealing, the UV, PL, and EL spectrums are broadened and the band gap gets narrower. The emitted light output after annealing outlives at high voltage in comparison with the unannealed. This annealing, however, can not change the transition region in semi-log plot. From these results, it is said that pre-deposition annealing mainly changes the high voltage characteristics in linear plot, which may be resulted from the bulk morphological change in...