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Study On Phosphorescent White Organic Light-emitting Devices With High Performance

Posted on:2017-03-06Degree:DoctorType:Dissertation
Country:ChinaCandidate:J ZhaoFull Text:PDF
GTID:1108330482979897Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Organic light-emitting devices(OLEDs) have advantages of lightweight, low driving voltage, wide view angle, high brightness, high efficiency, wide temperature range, and so on. Especially, the OLEDs demonstrate unique ability to realize flexible and transparent display. Among of which, white OLEDs have beening attracted worldwide attention in both academic and industry, because they show huge potential applications in display, lighting and backlight source for liquid-crystal display. In white OLEDs, the organic emitter materials are one of the most important components. However, the organic emitter materials tend to aggregate and quench emission, leading to low emission efficiency. To overcome this problem, the emitters are usually doped into a host material, forming a host-guest doping structure. This method is complicated, because it is difficult to control the doping ratio of the host and guest, the repeatability and the yield is low, thus it is not favorable to realize large-area OLEDs. In this work, a series of white OLEDs were fabricated by employing ultrathin non-doped yellow phosphorescent bis[2-(4-tertbutylphenyl) benzothiazolato-N,C2’] iridium(acetylacetonate), namely(tbt)2Ir(acac) emitting layer(EML), which was able to simplify the device structure and fabrication process. Study has been mainly performed on structral design, device optimization and theoretical analysis, and the high-performance white OLEDs were achieved. Solution-processed OLEDs were also fabricated to investigate the spectra variations of blue phosphorescent bis[(4, 6-difluorophenyl)-pyridinato-N,C2’](picolinate) iridium(III)(FIrpic). The main studies were divided into four parts as follows:1. White OLEDs were constructed by using the ultrathin non-doped(tbt)2Ir(acac) phosphor EML(also called Delta EML), and the influence factors were studied through a series of work on device optimization. Firstly, the doping concentration of FIrpic and thickness of the ultrathin(tbt)2Ir(acac) were investigated, which were optimized to be 8% and 1 nm, respectively. Secondly, the host of blue EML was changed, and the non-doped and doped yellow EMLs were compared, based on four white OLEDs with structures of mCP:FIrpic/(tbt)2Ir(acac), mCP:FIrpic/mCP:(tbt)2Ir(acac),(tbt)2Ir(acac)/UGH2:FIrpic and UGH2:(tbt)2Ir(acac)/UGH2:FIrpic. Compared to the devices with doped yellow EML, the devices with non-doped yellow EML, which were based on mCP:FIrpic/(tbt)2Ir(acac) and(tbt)2Ir(acac)/UGH2:FIrpic structures, achieved the best performance with a maximum luminance of 41,790 cd/m2 and 24,700 cd/m2, the highest current efficiency of 58.8 cd/A and 65.3 cd/A, and the highest external quantum efficiency of 18.77% and 19.04%, respectively, together with stable white light emission. Thirdly, the relative position of the blue and yellow EMLs was changed, and the results demonstrated that the white OLEDs based on(tbt)2Ir(acac)/mCP:FIrpic and UGH2:FIrpic/(tbt)2Ir(acac) showed blue-dominate emission, indicating that the EMLs’ positon played great influence on the color emission, mainly due to different carrier transporting property of the host materials.2. By employing double ultrathin non-doped(tbt)2Ir(acac) EMLs, white OLEDs with a configuration of TAPC/(tbt)2Ir(acac)/mCP:FIrpic/(tbt)2Ir(acac)/Bphen were fabricated. The results showed that the(tbt)2Ir(acac) EML adjacent to the anode mainly made contribution to charge carrier trapping, while the other(tbt)2Ir(acac) EML next to the cathode primarily contributed to yellow emission. A 5-nm layer of gold was introduced at the anode side to achieve stable white emission. Furthermore, multiple-quantum-well OLEDs with ultrathin non-doped yellow and blue EMLs were fabricated, while device optimization was realized by changing the potential barrier layers, along with its thickness. The results indicated that white OLEDs were realized by using 6-nm mCP and 6-nm TPBI as potential barrier layers, which could effectively balance the intensity of yellow and blue emission. Moreover, white OLEDs with three ultrathin non-doped red, green and blue EMLs were fabricated and suggested that, the ultrathin non-doped EMLs played important role on charge carrier transport and adjustment.3. WOLEDs with two doped emitting layers were realized, where two kinds of material with different carrier transporting property, namely TAPC and TPBI, were separately used as the host of both FIrpic and(tbt)2Ir(acac) EMLs. Different spacer materials were utilized to optimize device performance, and the influence of the spacer materials on device performance was also investigated. In case of the TAPC host, Bphen was the best spacer compared to TPBI and 3TPYMB spacers, and the highest current efficiency of 11.3 cd/A and stable Commission Internationale de l’Eclairage(CIE) coordinates of(0.394, 0.435), were obtained. In case of the TPBI host, compared to TAPC, mCP or TCTA spacer, the OLED using CBP as spacer achieved the highest current efficiency of 18.1 cd/A and relatively stable CIE coordinates of(0.284, 0.333). It was inferred that the carrier mobility, triplet energy, the energy band of the spacer material played importance effect on device performance. The optimized performance from the white OLEDs using Bphen and CBP spacers was attributing to balanced carrier, broadened exciton recombination region and improved energy transfer between the blue and yellow emitting layers. Moreover, it was found by using phosphorescent sensitizer, the triplet excitons from the EML tended to diffuse into the adjacent organic layers if there was no energy barrier. Therefore, an exciton blocking layer could be introduced to confine excitons in the EML, which greatly improved the device efficiency.4. Solution-processed blue and white OLEDs were studied, where electroluminescence(EL) spectra variation of FIrpic was observed, and the reasons were explored. In the solution-processed blue OLEDs, as the doping concentration of FIrpic was higher than 20% and high polar solvent was utilized, the EL spectral variation of FIrpic showed that its shoulder intensity gradually enhanced and exceeded the peak intensity. Theoretical mechanism suggested that the spectra variation was related to the solvation effect induced by strong intermolecular interaction between the FIrpic and solvent molecules, which affected the energy difference between the excited state and the ground state of FIrpic. Solution-processed white OLEDs were also fabricated, and it was found that the spectra variation of FIrpic had no dependence on the thickness of the EML, which further confirmed the solvation effect on the spectra variation of FIrpic. Meanwhile, the solution-processed white OLEDs achieved good optical and electrical device performance.In summary, the ultrathin non-doped emitting-layers of one layer, two layer, and mulilayer had been demonstrated to realize high performance white OLEDs, leading to simple device structure and simplified fabricatin process. For solution-processed white OLEDs, the reason for spectra variation of blue phosphoresecent FIrpic was investigated, which was expected to develop novel emitters with high efficiency and facilitate their applications in solution-processable OLEDs.
Keywords/Search Tags:organic light-emitting device, ultrathin non-doped layer, yellow phosphorescent emitter, blue phosphorescent emitter, solvation effect
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