Hybrid white organic light-emitting diodes with a double light-emitting layer structure for high color-rendering index

High-efficiency hybrid white organic white light-emitting diodes (WOLEDs) with a double light-emitting layer structure for high color-rendering index (CRI) have been fabricated. The double light-emitting layer structure is composed of a red phosphorescent layer and a blue-green layer of a blue light-emitting fluorescent host doped with a green phosphor separated by a bipolar spacer. The simultaneous emission of a blue light from the fluorescent host and a green light from the phosphor was realized by controlling the doping concentration of the green phosphor in the fluorescent host. The resulting hybrid WOLEDs achieved a CRI of 90 and kept rather stable spectral emission with Commission Internationale de L'Eclairage coordinates of (0.42, 0.44) independent of driving voltages. Furthermore, the hybrid WOLEDs also exhibited a high efficiency that the maximum current efficiency, external quantum efficiency and power efficiency reached 29.4 cd/A, 13.8% and 34.2 lm/W, and still remained at 25.4 cd/A, 11.9% and 23.0 lm/W at 1000 cd/m 2, respectively. Mechanism of the two emission layers and role of the bipolar spacer were systematically studied and we have attributed the significant improvement in device performances to the effective and stable control of exciton recombination in the emissive regions by the bipolar functionality of the used spacer. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759045