Structure Design And Performance Study Of Fluorescent/Phosphorescent White Organic Light-emitting Device With High Color Stability

White organic light-emitting devices （WOLEDs） have attractedconsiderable attentions in the past two decades owing to thier unique merits,including large-scale fabrication, flexible, transparent, light and thin, forfull-color displays and solid-state lighting sources. Up to now, tremendousprogress has been made. In terms of device efficiency and life, WOLEDs havemet the commercial requirements, and the development of WOLEDs is shiftingfrom the research stage in the laboratory to the applications stage in commercialproduction. However, for WOLEDs, there are still some issues which need to befurther investigated and improved.For example, OLED materials still suffer from high cost，and the absenceof blue phosphorescent emitters with a long operational lifetime is still abottleneck for the development of all phosphorescent WOLEDs. In addition,even in applications for display and lighting products, WOLEDs are alsorequired to show a good color stability. However, most WOLEDs reported inliteratures show brightness-dependent color-shifts that significantly exceed the tolerable margin. Based on above issues, in our paper, we mainly focus on thecolor stability of multiple emissive layers WOLEDs for application, as well ascost, color quality, and color balance, to carry out a series of basic and appliedresearch. The main research contents include the following four aspects:1．In order to reduce the cost of WOLEDs，we detailedly investigated thephotophysical and thermal properties of Levofloxacin（LOFX）, which is lowcost. The results indicate that LOFX can be used in OLEDs. Then, a series ofWOLEDs were fabricated by combining a452nm blue emitting layer with1wt%LOFX doped in4,4’-bis（carbazol-9-yl）biphenyl（CBP） and a584nmorange emitting layer with0.8wt%4-（dicyanomethylene）-2-tert-butyl-6-（1,1,7,7-tetramethyljulolidin-4-yl-vinyl）-4H-pyran（DCJTB） doped in CBP, which areseparated by a20nm thick buffer layer of2,2’,2‖-（benzene-1,3,5-triyl）-tri(1-phenyl-1H-benzimidazole（TPBi）. When the thickness of blue emitting layer is30nm and the thickness of orange emitting layer is10nm, WOLED shows amaximum luninance of3869cd/m2and a maximum current efficiency of1.79cd/A. When voltage is14V, this WOLED also exhibits a color rendering index（CRI） of84.5and Commission Internationale de1’Eclairage （CIE） coordinatesof （0.33,0.32）, which is close to ideal white emission CIE （0.33,0.33）.2．The development of phosphorescent WOLEDs is severely limited by theabsence of efficient blue phosphorescent emitters with a long operationalstability, and most WOLEDs reported in literatures still suffer from a poor colorstability. Here, we have employed Bis（2-（3-trifluoromethyl-4-fluorophenyl） -4-methylquinolyl）（acetylacetonate）iridium（III）（DDBICZ）（a new type of bluefluorescent material） as blue emitter and triplet host combined with symmetriclight-emitting layers（S-EMLs） to structure two-and three-color S-EMLsfluorescent-phosphorescent WOLEDs. Two-and three-color WOLEDs allexhibit high color stability and CRI. For two-color S-EMLs WOLED, as voltageincreases from5to10V, CIE coordinates, correlated color temperature （CCT）and CRI only change from （0.35,0.36）,4833K and79to （0.34,0.35）,5191Kand81. For three-color S-EMLs WOLED, as the brightness increases from100to10000cd/m2, CIE coordinates and CCT also only change from （0.40,0.41）and3790K to （0.40,0.42） and3875K, and CRI is maintained at85⁸6. Inaddition, two-and three-color S-EMLs WOLEDs also all reveal a high currentefficiency,24.4cd/A for two-color WOLED and25.9cd/A for three-colorWOLED. The improvement of color stability is due to the fact that S-EMLseffectively offset the changes of emission intensity from different EMLs causedby the shift of carrier recombination zone with the increase of voltage.3．In order to improve the color stability of WOLEDs, further, we haveemployed four adequate complementary phosphor emitters （blue, green, yellow,and red） and two host and transport materials（CBP and TPBi） to configure asymmetrical doped emissive layers WOLED, in which four emitters are in turnincorporated in CBP and TPBi from high to low energy level from CBP/TPBiinterface. This WOLED has only one organic/organic heterojunction interfaceand shows extremely high chromatic stability with a slight CIE coordinates shift at a wide rang of voltage. As the voltage increases from5to9V, WOLEDmaintains CIE coordinates of （0.384,0.467）^0.387,0.469 and high CRI of85⁸6with ideal CCT of4407⁴292K. Furthermore, this white device alsoexhibits a maximum current efficiency of43.6cd/A and a maximum powerefficiency of31.1lm/W, which is1.43and1.50times, respectively, those of thereference device with four emitters only being doped in single CBP host. Theimprovement of color stability is attributed to the fact that symmetrical dopedemissive layer effectively suppresses the shift of carrier recombination zone asthe increase of voltage, and the improvement of efficiency is attributed to theeffective utilization of excitons.4．WOLED with the structure of ITO/NPB/EML（blue）/CBP/EML（red）/CBP/EML（blue）/TPBi/LiF/Al was selected as reference. Then, the referencedevice was modified by nanoplatforms patterning structure at the TPBi/LiF orNPB/EML interface. The results indicate that WOLED with nanoplatforms atTPBi/LiF interface exhibits an enhanced current efficiency relative to referencedevice, and the enhancement factor is1.52, which is due to improved lightextraction efficiency. When nanopillars are located at the NPB/EML interface,WOLED shows more significantly improved current efficiency relative toreference device, and the enhancement factor is1.80. Here, the improvement incurrent efficiency is attributed to increased light extraction efficiency andbroadened carrier recombination zone. In addition, it is also found that WOLEDwith nanopillars located at the NPB/EML interface exhibits remarkably strengthened red light intensity in electroluminescence spectrum and good colorbalance, in which the intensity ratio of red-light to blue-light （Ired/Iblue） is0.68,higher than that of reference WOLED （Ired/Iblue=0.5）. The improvement in colorbalance is attributed to the nanoplatforms located at NPB/EML interface, inwhich nanoplatforms as hole transporting channels lead holes to reach red EMLmore easily, inducing strengthened red-light intensity.