We report an improved bottom-emission two-stacked white OLED structure consisting of one unit emitting fluorescent blue light and the other unit emitting phosphorescent yellow in one-host and two-dopant (R+G) system. By searching EL condition optimizing both phosphorescent green and red dopants within just one lightemitting layer, we can get WOLEDs suitable for a display application with very high current and power efficiencies, and excellent color characteristics. Our device exhibits high external quantum efficiency of 28.8%, current efficiency of 56.5 cd/A, and power efficiency of 30 lm/W at 5.9 V and 1000 nit where color point is (0.368, 0.385) in x-y color space (CIE 1931). Halflifetime (LT 50 ) of the device at 1000 nit is estimated more than 31,000 hrs and color variation by aging is suppressed within less than 0.02 in u '-v'color space (CIE 1976). Color filter developed for our WOLED enables color gamut to become wider than the 100% NTSC in uniform color space (u', v'). IntroductionWhite organic light emitting diode (WOLED) has been attracting researchers' interests in view point of lighting sources which are eco-friend since no mercury is involved, and in view point of a strong candidate for OLED display for large-sized substrate being larger than Gen. 4 (730 x 920 mm 2 ) with a combination of color filter (CF).[1,2] Though WOLED+CF patterning method needs no fine metal mask (FMM), it has been challenged due to its low efficiency,[3] since WOLED has low current efficiency corresponding average value of each efficiency of RGB monocolor OLED, and since CF of each subpixel absorbs about 2/3 of the white light. To overcome the issue, two methods were proposed, i.e., one is 4-subpixel color filter of RGBW,[2] and the other is tandem structure in which a charge generation layer (CGL) connects two independent OLED units. [4,5] There have been many studies about the development of high efficiency WOLEDs. In the late-news paper at SID'08 [6], Liao et al.reported their tandem WOLEDs with 56 cd/A, 30 lm/W, CIE (0.34, 0.40), and 5.9 V at 1000 nit by "combining fluorescent and phosphorescent emission". In order to increase the efficiency, they applied an orange dopant into a light-emitting layer, instead of a deep red dopant, but they have showed broad electroluminescent (EL) spectrum in the phosphorescent emitting portion of the white spectrum. In this work, we focus on improving efficiency and color characteristics of the phosphorescent emitting portion which is strongly affected by energy transfer from phosphorescent host to red dopant via green dopant. We demonstrate two-stacked WOLEDs in which a fluorescent blue stack and a phosphorescent stack are connected by the charge generation layer. The latter stack whose emission layer was co-deposited with phosphorescent red and green dopants was optimized by controlling doping ratio of the two dopants. And then, we find it is possible to get relatively sharp emission peaks in red, green, and blue wavelength ranges for display application, while still maintaining the hi...

We report 1-stacked and 2-stacked white OLED (WOLED) device with higher color temperature and 20.7-inch white and black medical display. Fabricating WOLED with two emitting layers of fluorescent blue dopant and phosphorescent dopant(s), we achieved luminous efficiency of 15.4 cd/A and CIE coordinates of (0.275, 0.303) for 1-stacked WOLED, and luminous efficiency of 54.1cd/A and CIE coordinates of (0.327 0.321) for 2-stacked WOLED at 10 mA/cm 2 . The 20.7-inch WOLED proto type shows higher ambient contrast ratio, l ower power consumption compared to conventional LCD, which is critical feature for supporting accurate medical diagnosis.