Study On High Color Rendering Index White Organic Light-emitting Diodes

Since the first reports of white-emitting devices by the group of Kido, white organiclight-emitting devices (WOLEDs) have attracted considerable attention due to their potentialapplications in fat panel displays. OLEDs possess many advantages such as ultra-thin, lightweight, and environmental friendliness. As a result, OLEDs are considered as one of thecandidates for next generation planar lighting source. In recent years, phosphorescentmaterials are widely used due to the potential for achieving100%internal quantum efficiency.However, compared with fluorescent WOLEDs, phosphorescent WOLEDs typically haveshorter operational lifetime. Basically, the shorter operational lifetime of blue phosphorsworsens the color stability of all phosphorescent WOLEDs. In addition, thetriplet–triplet annihilation (TTA) or triplet–polaron annihilation (TPA) phenomenon alsoworsens the stability of phosphorescent devices.We have demonstrated high color rendering index (CRI) WOLEDs using a multilayerstructure based on all fluorescent emitters. The CRI of white light can be easily tuned byadjusting the thickness of the blue emitting layer (B-EML). In particular, the device with8-nm-thick B-EML obtains very high CRI values (91~92) over a wide range of luminance(4000-23000cd/m2). Furthermore, the Commission Internationale De L’Eclairagecoordinates (CIE) of the device is rather stable and the variation is only (±0.005,±0.003) over4000–23000cd/m2. The values are very close to white light equivalent energy point of (0.333,0.333).In2000, M.A.Baldo et al. reported the phosphor-sensitized fluorescence mechanism.The host molecular energy can, in theory, all be transfered to the fluorescent guest molecularaccording to this mechanism. We report high CRI WOLEDs based on blue fluorescent andyellow phosphor-sensitized-fluorescent emitters. By introducing an interlayer between thesetwo emitting layers (EMLs), performances of the devices are greatly improved. The WOLED withtheinterlayerexhibitshigherefficiencyandbrightnessthanthedevicewithoutit,andshowsacriashighas90withciecoordinateof(0.32,0.32)at100cd/m2.moreover,byoptimizingthedopingconcentrationofthereddye,anefficientwoledwiththemaximumcurrentefficiencyof11.00cd/aandthemaximumluminanceof13330cd/m2isobtained.furthermore,allwoledssatisfythecrineedforfuturesolid-statelightingapplications.