Improvement of the device performance of phosphorescence organic light-emitting devices and physical characteristics for solution-processed fabrication
Date Issued
2014
Date
2014
Author(s)
Wang, Po-Chuan
Abstract
In this study, the small molecule of organic materials would be applied to solution process for phosphorescent organic light-emitting device (PHOLED). The blue PHOLED of two structures, extremely simplified structure (single layer) and multilayer structure, would be optimized for the base of white PHOLED.
Our thesis is separated into three parts. In the front section of first part, we first demonstrate that how to optimize the extremely simplified structures step by step. In the latter section of first part, the device performance could be further improved by increasing the electron injection ability to reach more charge balance. The small molecule materials of Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) that we choose is widely used as host material of blue phosphorescent system due to quite fast hole mobility, suitable energy bandgap, and high triplet state energy and the popular blue phosphorescent dye Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) would be used in our research. For the extremely simplified structure, the electron transport material 2,2''-(1,3-Phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7) would be mixed into the emissive layer and then the excitons will be greatly increased due to improving electron injection and transport to reach more charge balance. In addition, the recombination zone will be kept far away from the cathode to prevent the excitons quenching effect. Extremely simplified structure OLED having high efficiency is the objective of solution processed OLED and the device structure is ITO/PEDOT:PSS/EML/CsF/Al. For our experiment, the maximum efficiency in our blue PHOLED is achieved 17 cd/A and 9 lm/W. In the latter section of first part, we modify electron injection at the interface of cathode to investigate the interface of physical mechanism. For most of organic materials, the hole mobility is usually quite higher than electron mobility, and it would result in the device having series charge imbalance, especially occurring solution process. To solve such a problem, we utilize alkali metal Barium (Ba) in replace of CsF at the interface of cathode. The metal Ba has high work function and strong activity, and hence it easily cases oxidation-reduction reaction with other materials. This is why we use Ba to improve the electron injection of the device. The experiment results are also proved our concention and the device performance is enhanced again in comparison of CsF. Consequently, the maximum luminous and power efficiency of 18 cd/A (increasing 8.5%) and 11.5 lm/W (27.8%) are achieved.
In the second part of the research with including three topics, the double layer structure, multilayer structure and co-host system are investigated for achieving higher efficiency PHOLED. In order to improve the PHOLED performance further, the double layer structure PHOLED would be used to replace extremely simplified structure. Therefore, the electron transport layer are separated from emissive layer to substitute for mixing OXD-7 into the emissive layer and the electron transport material TmPyPb, which contains higher electron mobility and T1, are evaporated after emissive layer. By re-tuning the FIrpic doping ratio and the device structure, we could optimize the device performance due to better charge balance and optimal micro-cavity effect. No matter what single layer or double layer structures that we developed, our blue PHOLEDs performances are greatly higher than other groups. The maximum luminous of 30 cd/A and power efficiency of 18.45 lm/W are achieved and 76.4% enhancement is attained in comparison with single layered PHOLED. Moreover, we also devise triple layer structure and co-host system OLED even though it does not achieve the optimal performance. We will work on these topics in the future for pursuing higher PHOLED efficiency.
In the third part of the research, we fabricated white PHOLED of single layer and double layer structure according to the high efficiency bases of blue PHOLED, that we devised in the previous part. The red phosphorescent dye Tris(2-phenylquinoline)iridium(III) (Ir(2-phq)3) would be used in our experiment. By tuning the relative ratios of red and blue dopant materials, we could optimize the white PHOLED. Therefore, the maximum luminous efficiency of 12.5 cd/A, power efficiency of 4.7 lm/W, and maximum external quantum efficiency (EQE) of 2.2%, CIE coordinate of (0.32, 0.41) are achieved for the single layered structure of white PHOLED and the maximum luminous efficiency of 30 cd/A, power efficiency of 23 lm/W, and EQE of 11.6%, and CIE coordinate of (0.30, 0.42) are achieved for the double layered structure of white PHOLED.
Subjects
磷光有機電致發光二極體
濕式製程
白光有機電致發光二極體
Type
thesis
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