申 眞旭

English

博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))othe

申, 眞旭

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Studies on Graphene Electrodes for OrganicLight-emitting Diodes Application著者 申 眞旭number 62学位授与機関 Tohoku University学位授与番号 工博第5477号URL http://hdl.handle.net/10097/00124915シン   ジンウク 氏 名          申    眞旭  Jin-Wook Shin 授 与 学 位          博士（工学） 学位授与年月日          平成３０年３月２７日 学位授与の根拠法規 学位規則第４条第１項 研究科，専攻の名称 東北大学大学院工学研究科（博士課程）電子工学専攻 学 位 論 文 題 目          Studies on Graphene Electrodes for Organic  Light-emitting Diodes Application 有機発光ダイオード用グラフェン電極の研究 指 導 教 員 東北大学教授 末光 眞希 論 文 審 査 委 員          主査 東北大学教授   末光 眞希 東北大学教授  佐藤 茂雄   東北大学教授    金子  俊郎 論 文 内 容 要 旨          Graphene has been intensively investigated as a material for components used in optoelectronic applications such as transparent electrodes, sensors, solar cells, and transistors, due to its outstanding transport and optical properties centered on its ultrahigh electron mobility and optical transparency. Its high mechanical compliance further opens applications for flexible devices. A number of research projects have focused on the development of graphene as a flexible transparent electrode material for OLED technologies to replace the brittle indium tin oxide (ITO) used in conventional electrodes. Despite these efforts, various challenges still remain to be overcome in the use of graphene in flexible OLEDs. The major issues are the degradation in efficiency, caused by reduced light extraction and substantial light absorption within graphene, and the difficulty in achieving residue free graphene film pixel arrays with sufficient geometrical precision in large scale. This study aims to solve these two problems.  To overcome the efficiency issue, an internal random scattering layer has been introduced to extract the light produced in the organic layer. Also, the number of layers in the graphene electrode has been reduced to minimize the light absorption within the graphene layers as shown in Fig. 1. Electrically, the graphene-OLEDs with and without the scattering layer exhibited similar current density (J)–voltage (V) characteristics with low leakage current levels and on/off current ratios higher than 108 (Fig 2(a) and (b)). This high electrical stability is ascribed to the low surface roughness of the scattering layer (Ra ~0.4 nm) after the planarization process (Fig 1(c)). Optically, the introduction of the scattering layer between the substrate and the single layer-graphene anode was found to enhance the external quantum efficiency (EQE) and luminous efficacy (LE) by more than 50% (Fig. 2(c)). This improvement is due to the scattering effect and the reduction in the optical absorption of the graphene anode in OLED. As a result, we succeeded in the ? 431 ?fabrication of graphene-OLEDs having efficiencies comparable to those of conventional OLEDs with oxide anodes. Furthermore, the angular electroluminance (EL) spectrum variations were stabilized, which is difficult to achieve using the conventional OLEDs with microcavity designs.  To overcome the patterning issue, a new method, the liquid bridge treatment, has been developed to improve the adhesion of graphene on the substrate. The weak adhesion of graphene against the supporting layers was found to impede the formation of accurate graphene pattern by causing damages to graphene during the photolithographic patterning process. Using the new method, the effective adhesion energy of the graphene film was improved by a factor of around two as compared to that of pristine graphene film as shown in Fig. 3. By combining the photolithographic patterning with this liquid bridge treatment, an accurate patterning with sufficient dimensional precision and without surface contaminants has been realized. Also, the OLED using the graphene electrode patterned with this method exhibited a stable electrical and optical performance (Fig. 3(a) and (b)). A two-color graphene-OLED panel has been successfully demonstrated on a glass substrate (Fig. 3(c) and a flexible graphene-OLED panel on a polyimide film as well. Graphene pixel was defined by the new patterning process, which yielded the pixel size of 170  250 m2 for both of the devices.  We believe that our result is an important precursor toward graphene-OLED applications. Our results strongly suggest direct applications of graphene not only in conventional displays but also in flexible and/or stretchable displays which require thin and highly flexible electrodes.              ? 432 ? Fig.1 Schematic of graphene-OLED with the scattering layer (a), SEM image of the nanostructure without (b) and with (c) the planarization layer.   Fig.2 Comparison of electrical, optical and efficiency characteristics of graphene-OLED with and without the scattering layer; J-V characteristics and EL spectra [(a) and (b)], and EQE (c). The number of graphene layers was varied as one, two, and four.   Fig.3 Surface morphology of graphene film before (a) and after (b) the liquid bridge treatment. Its effective adhesion energy (histogram) and the average surface roughness (diamonds) are shown in (c).  ? 433 ? Fig.4 (a) Actual image of the fabricated backplane OLED with graphene film pixels. (b) A SEM image of the graphene film pixel including the electrical pad, via hole and addressing metal line. (c) Two-color OLED with pixelated graphene films as the transparent electrode.                     ? 434 ?? 435 ?

Studies on Graphene Electrodes for Organic Light-emitting Diodes Application

https://tohoku.repo.nii.ac.jp/?action=repository_action_common_download&item_id=127509&item_no=1&attribute_id=18&file_no=1

有機発光ダイオード用グラフェン電極の研究

Abstract

Similar works

Full text

Available Versions

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ