Study On White Organic Light-Emitting Diodes Based On Blue And Yellow Emission

Organic light-emitting devices(OLEDs), also called organic light-emitting diodes, have the merits of richness in natural resources, capability of realizing flexible devices, multi-function and so on, due to the utilization of organic materials. Rescently, researchers in both academy and industy have been tackling the technical problems in OLEDs, making them able to achieve high efficiency, low fabrication cost and large-area fabrication. Meanwhile, OLEDs have already been widely used in information display industry nowadays. In addition, OLED can be fabricated into white OLEDs(WOLEDs) due to their surface-emitting feature, which can be used as solid-state lighting sources and backlights for liquid crystal displays. WOLEDs can be realized either from complementary colors(blue+yellow) or from three primary colors(red+green+blue). However, compared to other light sources, low efficiency, low brightness, short lifetime, low yield rate and high cost remain in OLEDs. Therefore, basic scientific research needs to be carried out in aspects of materials, device structures and mechanisms to overcome these problems. In this dissertation, aiming at solving the above problems in complementary color-based WOLEDs, blue OLEDs with fluorescent, phosphorescent and thermally activated delayed fluorescent emitters(TADF) were fabricated, and the optimized blue OLEDs were achieved. Meawhile, yellow OLEDs device performance were optimized in aspects of novel host materials and novel device configurations, and the emission mechanisms were analyzed and discussed. Finally, WOLEDs with a blue phosphorescent emitter and a yellow phosphorescent emitter were fabricated and optimized, and effect of single and dual electron transporting layers on the device performance of the WOLEDs was studied. In addition, a yellow emission TADF emitter and a novel exciton adjusting layer structure were applied to an organic integrated device with ultraviolet photodetective and electroluminescent properties, aiming to resolve the low integration level, high cost and poor performance problems in organic photonic devices. The contents are shown as follows:1. Blue OLEDs with host-guest emissive layer structure were fabricated using fluorescent emitter 4,4’-bis(2,2’-diphenylyinyl)-1,1’-biphenyl(DPVBi), phosphorescent emitters bis[(4,6-difluorophenyl)-pyridinato-N,C2’](picolinate) iridium(III)(FIrpic) and bis(2,4-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate iridium(III)(FIr6), TADF emitter 4,5-di(9H-carbazol-9-yl)phthalonitrile(2Cz PN), and N,N’-dicarbazolyl-3,5-benzene(m CP) was used as the host. The luminance, efficiency, emission mechanisms were discussed. The results showed that the FIrpic-based OLED achieved a maximum luminance, a maximum current efficiency and a maximum power efficiency of 22680 cd/m2, 9.72 cd/A and 5.42 lm/W, respectively, owing to the utilization of 100% excitons for emission and efficient energy transfer. The performance of the FIrpic-based OLED was the best among the four kinds of OLEDs.2. Yellow OLEDs were fabricated using a charge-transfer-featured material with triplet-triplet annihilation(TTA) property, 6-{3,5-bis-[9-(4-t-butylphenyl)-9H-carbazol-3-yl]-phenoxy}-2-(4-t-butylphenyl)-benzo[de]isoquinoline-1,3-dione(Cz Ph ONI), as the host. Two fluorescent and two phosphorescent emitters were used as guests. According to the photoluminescent quantum yields, it was found that the external quantum efficiencies of both fluorescent and phosphorescent OLEDs were exceeding their theoretical limits. Based on the analysis of electroluminescent characteristics, the high device performance of fluorescent and phosphorescent OLEDs was attributed to both efficient energy transfer and triplet energy up-conversion, while direct exciton formation was also involved in phosphorescent OLEDs. In addition, the host film possessed high thermal and morphological stabilities due to the attachment of steric bulks on host molecule, resulting in the high doping concentration for both fluorescent and phosphorescent dyes. The maximum luminance, maximum current efficiency and maximum external quantum efficieny are 19510 cd/m2, 15.0 cd/A and 5.0% for rubrene device, 41710 cd/m2, 44.2 cd/A and 16.5% for(tbt)2Ir(acac) device, 5350 cd/m2, 6.2 cd/A and 3.4% for DCJTB device, 12990 cd/m2, 10.5 cd/A and 12.6% for Ir(piq)3 device, respectively.3. The effect of exciplex and non-exciplex forming interfaces consisting of an ultrathin rubrene emissive layer on the performance of organic light-emitting diodes was studied. An ultrathin rubrene layer was inserted into the two kinds of interfaces and the devices were optimized in terms of the film thickness of rubrene. The exciplex is well-known as a charge transfer state formed between electron donating and electron-accepting molecules, so it can realize the TADF process. The results showed that energy transfer dominated the exciplex-type device, while direct charge trapping followed by exciton formation was the main emission process of the non-exciplex-type device. The exciplex-type device achieved a maximum luminance, a maximum current efficiency and a maximum power efficieny of 18311 cd/m2, 16.6 cd/A and 12.7 lm/W, while these of non-exciplex-type device were 11860 cd/m2, 17.3 cd/A and 8.1 lm/W.4. WOLEDs were fabricated using FIrpic doped m CP as blue emissive layer and(tbt)2Ir(acac) as ultrathin emissive layer. The device performance was optimized by tailoring the doping concentration of FIrpic and the thickness of(tbt)2Ir(acac). The results showed that the optimal device was achieved at 9 wt.% FIrpic ratio and 1 nm thick(tbt)2Ir(acac). The turn-on voltage was 2.6 V, the maximum luminance was 70520 cd/m2, the maximum current and power efficiency were 33.3 cd/A and 25.6 lm/W, the color coordinates was(0.364, 0.417). The effect of varying electron transporting layers on device performance was then studied, and the effect of electron mobility, energy level and triplet energy on the efficiency and spectra was discussed. The results showed that stable white emission spectra was achieved by using TPBi as the electron transporting layer, and high efficiency was obtained by using 4,7-diphenyl-1,10-phenanthroline(Bphen) as the electron transporting layer. Subsequently, color stability was studied by using a dual-electron-transporting-layer structure. By changing the thicknesses of TPBi and Bphen, electron transport was manipulated. When the thicknesses of TPBi and Bphen were both 20 nm, the WOLED showed the best spectra stability, with a slight color coordinates shift of(-0.003, 0.007) at a voltage range from 7 to 11 V.5. A high performance organic integrated device with ultraviolet photodetective and electroluminescent properties was obtained by using a TADF emitter, and an exciton adjusting layer was delicately designed to influence charge carrier transport. As a result, the bi-functional device exhibited a high detectivity of 1.4?1012 Jones under 350 nm UV light. Under positive bias, the integrated device yielded yellow emission with a maximum luminance, current efficiency and power efficiency of 26370 cd/m2, 8.2 cd/A and 4.9 lm/W, respectively.In summary, this work paves the way to develop high performance WOLEDs based on complementary colors. Meanwhile, the development of organic integrated device provides guidance to low-cost large-scale fabrication of organic photonic devices, and this integrated device have great potential to be widely applied to portable and wearable electronic products in the future.