The Research Of Fluorescent-Phosphorescent Hybrid White Organic Light-Emitting Diodes

Organic Light-Emitting Diodes (OLEDs) have been considered as nextgeneration full color displays and solid-state lighting sources, because of their highluminous efficiency, low driving voltage, fast response, wide visual angle, andflexible characteristics, etc. There still are some issues which need to be investigatedand improved in OLED. For the efficiency, the blue OLEDs are not as good as theirgreen and red counterparts, the phosphorescent OLEDs have serious efficiencyroll-off at high luminance. For the quality of lighting sources, the color renderingindex (CRI) of OLEDs needs to be improved further. In this thesis, based on theissues mentioned above, we develop fundamental and applicable research combiningOLEDs theory, electroluminescent mechanism and process. Some results have beenobtained as follows:(1) The efficiency and lifetime of blue-light-emitting devices are usually not asgood as those of their green and red counterparts. Additionally, it is difficult togenerate high-performance pure/deep blue emission devices. These two reasonsindicate that the blue devices become the biggest impediment for the development ofwhite OLEDs. First, we assess a novel fluorescence dye named6,12-bis{[N-(3,4-dimethylphenyl)-N-(2,4,5-trimethylphenyl)]amino}chrysene(BmPAC) with emission peak of452nm. We fabricate blue OLED using BmPAC asemitter, the highest luminescence of12500cd/m~2is obtained, and the maximumcurrent efficiency reaches4.89cd/A at current density of23mA/cm~2. The emissionpeak of blue OLED is located at456nm, with CIE coordinates of (0.16,0.17). Thenwe combine orange dye with BmPAC to fabricate white OLED, with the emittinglayer (EML) structure of orange EML-interlayer-blue EML. The turn-on voltage forwhite OLED is4V, the maximum current efficiency and power efficiency are36.11cd/A and22.78lm/W at1300cd/m~2, and the current efficiency rolls off to34.59cd/A and35.61cd/A at100cd/m~2and1000cd/m~2, respectively. The spectra which are warm white lights, are almost stable with the increase of luminance. We only finda slight decrease of blue emission with the increasing brightness, and the highestCRI of45is obtained. We change the EML structure to be blue EML-interlayer-orange EML-interlayer-blue EML to improve the performance of white OLED. Theturn-on voltage is reduced to2.5V, the maximum current efficiency and powerefficiency are enhanced to40.9cd/A and49.4lm/W, respectively. The currentefficiency is37.3cd/A at100cd/m~2, or33.1cd/A at1000cd/m~2. Meanwhile a higherCRI of53is realized. We attribute the improvement to better balance of hole andelectron charges by using double blue EMLs. Finally, we demonstrate efficient blueorganic light-emitting diode with the EML structure of1,4-bis[N-(1-naphthyl)-N’-phenylamino]-4,4’-diamine/9,10-di(2-naphthyl)anthracene(ADN):1-4-di-[4-(N,N-di-phenyl)amino]styryl-benzene (DSA-ph). Improvedefficiencies and longer operational lifetime are obtained by co-doping astyrylamine-based deep blue dopant BD-3(0.1wt%) into the emitting layer of ADNdoped with DSA-ph compared to the case of non-co-doping. This is due to theimproved charge balance and expansion of exciton recombination zone. The bettercharge balance is obtained by reducing the electron mobility of ADN which is higherthan the hole mobility in the case of non-co-doping.(2) Utilization of phosphorescent emitters may solve the problem of achievingefficient emissions. A large number of triplet excitons formed at high luminance cancause serious triplet-triplet annihilation, which results in roll-off phenomenon inphosphorescent OLEDs. Mixed host EML, double EMLs, and interlayer betweentwo adjacent EMLs structures are employed here to improve the roll-off problem,respectively. We employ Ir(ppy)3as green emitter for phosphorescent OLEDs. Formixed host EML structure device, the maximum current efficiency of26.23cd/A andthe maximum power efficiency of31.67lm/W are obtained respectively. For doubleEMLs structure device, the maximum current efficiency of29.14cd/A and themaximum power efficiency of22.29lm/W are obtained respectively. For interlayer between two adjacent EMLs structure device, the maximum current efficiency of50.38cd/A and the maximum power efficiency of29.29lm/W are obtainedrespectively. Compared with the other two methods, using interlayer can result in thehighest efficiency and slightest roll-off. Employing mixed host EML structure couldimprove the transport of carriers into EML and expand the recombination zone. Thenwe report successful fabrication of efficient and high-color-rendering phosphorescentwhite OLED by employing ultra-thin red emitter-spacer layer-ultra-thin greenemitter architecture. By fine-tuning the thicknesses of emitting layers and spacerlayer, we obtain white OLED with a high CRI of80and high luminance efficiencyof29.2cd/A which only rolls off to22.8cd/A at1000cd/m~2. A typical WOLED forcomparison exhibits a CRI below40and maximum luminance efficiency of15.8cd/A which decreases to11.4cd/A at1000cd/m~2. We attribute the improvement tothe island-like morphology generated by deposition of ultra-thin film. This allowssufficient space for exciton formation and possibility of minimizing triplet-tripletannihilation.(3) High CRI is crucial for white OLED which can be applied in thehigh-quality lighting sources. We develop three types of white OLEDs with two-,three-and four-peak electroluminescence spectra. For two-peak OLED, a luminanceefficiency of34.59cd/A at100cd/m~2with a CRI of45and almost stable whiteemission with CIE coordinates around (0.47,0.42) are obtained. For three-peakOLED, highest CRI of88and correlated color temperature (CCT) below2800K atvarious brightness and a maximum luminance efficiency of27.29cd/A are realized.For four-peak OLED, color-temperature tunable white emission in the luminancerange of100cd/m~2to5000cd/m~2is obtained, the spectra turns from cold whiteemission to warm white emission with the change of luminance. Additionally, asunlight-like spectrum with a highest CRI of89is realized at5000cd/m~2. Wedemonstrate that to increase the number of emission wavebands could expand thecoverage area of visible light for the spectra of OLEDs, which results in higher CRI. Especially, we focus on the role of orange component on how to enhance the qualityof lighting source.