High-efficiency White Phosphorescent Organic Light-Emitting Diodes

High-efficiency white organic light-emitting diodes (WOLEDs) based on phosphorescent dye bis[2-(4-tert- butylphenyl)benzothiazolato-N,C2′]iridium (acetylacetonate) [(t-bt)2Ir(acac)] phosphor doped 4,4′-bis(carbazol-9-yl) biphenyl (CBP) as a yellow emitting layer were fabricated. The structure of the devices is indium-tin oxide (ITO)/N,N′-bis-(1-naphthyl)- N,N′-biphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/CBP:(t-bt)2Ir(acac)/NPB/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/8- hydroxyquinoline aluminum (Alq3)/Mg:Ag, and the second NPB layer from the anode was used as a blue fluorescent emitter and the first NPB layer was used as a hole transporting layer, the BCP and Alq3 were used as hole blocking and electron transporting layers, respectively. The results showed that the turn-on voltage of the device was 3 V with a maximum luminance of 15460 cd·m−2 at 16.5 V and a maximum luminance efficiency of 7.5 lm·W−1 at 4 V, respectively. The Commission Internationale de l′Eclairage (CIE) coordinates located at (0.33, 0.32) remained stable at the driving voltage higher than 8 V, which is almost consistent with the optimum white CIE of (0.33, 0.33). The purity of white light could be adjusted by doping concentration. The effects of charge carrier trapping and energy transfer on the device performance were also discussed.