Efficient solution-processed phosphor-sensitized single-emitting-layer white organic light-emitting devices: fabrication, characteristics, and transient analysis of energy transfer

We present high-efficiency solution-processed WOLEDs with good color stability and high color rendering index using a phosphor-sensitized system. The devices with a single active layer in which the blue phosphorescent dye FIrpic as emitter and sensitizer, and the fluorescencent dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as orange emitter, show a maximum luminance efficiency of 16.4 cd A(-1) and a maximum luminance of 24300 cd m(-2). By adding a hole-blocking layer, a substantially enhanced efficiency of 26.9 cd A(-1) can be achieved. The origin and mechanism of the efficient white emission are also discussed by using the transient photoluminescence and electroluminescence techniques. Transient analyses indicate that white emission is from the residual blue phosphorescence of the sensitizer, the sensitized orange-red fluorescence due to triplet-to-singlet energy transfer, and the instantaneous fluorescence resulting due to singlet-singlet transfer and direct exciton formation caused by charge trapping in DCJTB molecules. It is also demonstrated that the direct triplet exciton formation on DCJTB due to charge trapping are the primary loss process. For the first time, the energy transfer efficiency of the incomplete energy transfer from FIrpic to DCJTB, and the ratios of instantaneous to sensitized fluorescence emissions are investigated.