Improvement of white organic light emitting diode performances by an annealing process

White organic light emitting diode (OLED) devices with the structure [TO/PHF:rubrene/Al, in which PHF (poly (9,9-di-n-hexylfluorenyl-2,7-diyl)) is used as blue light emitting host and rubrene (5,6,11,12-tetraphenylnapthacene) as an orange dye dopant, have been fabricated. Indium tin oxide (]TO) coated-glass and aluminium were used as anode and cathode, respectively. The devices were fabricated with various rubrene-dopant to obtain a white light emission. The CLED device that composed of several concentrations of rubrene-doped PHF film was prepared in this study. It was found that the concentration of rubrene in the PHF-rubrene thin film matrix plays a key role in producing the white color emission. In a typical result, the device composed of 0.06 wt.% rubrene-dopant produced the white light emission with the Commission Internationale de L'Eclairage (ClE) coordinate of (0.30,0.33). The turn-on voltage and the brightness were found to be as low as 14.0 V and as high as 6540 cd/m(2), respectively. The annealing technique at relatively low temperature (50 degrees C, 100 degrees C, and 150 degrees C) was then used to optimize the performance of the device. In a typical result, the turn-on voltage of the device could be successfully reduced and the brightness could be increased using the annealing technique. At an optimum condition, for example, annealed at 150 degrees C, the turn-on voltage as low as 8.0 V and the brightness as high as 9040 cd/m(2) were obtained. The mechanism for the improvement of the device performance upon annealing will be discussed. (C) 2009 Elsevier B.V. All rights reserved.