Polyimide as a Plastic Substrate for the Flexible Organic Electroluminescent Device
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Polyimide as a Plastic Substrate for the Flexible Organic Electroluminescent Device Jin-Woo Park, Youngkyoo Kim1, Hyuntaek Lim2, Cheol Min Bae, Il Kim, Shinji Ando3, and Chang-Sik Ha* Department of Polymer Science and Engineering, Pusan National University, Busan 609 -735, Korea. 1 EXSS Group, Department of Physics, The Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BW, UK 2 Device Team, NESS Display Co., Ltd., Yongin P.O.BOX 25, Kyoung Gi-Do, 449-860, Korea 3 Department of Organic and Polymer Materials, Tokyo Institute of Technology, Ookayama, Meguro-Ku, Tokyo 152-8552, Japan ABSTRACT In this work, transparent flexible substrates based on polyimides(PI) with indium tin oxide (ITO) thin films for organic electroluminescent devices have been prepared. 2,2’-bis-(3,4dicarboxyphenyl) hexafluoropropanedianhydride (6FDA) and 2,2’-bis-(trifluoromethyl)-4,4’diaminobiphenyl (TFDB) PI films were used for transparent flexible substrates. ITO thin films were prepared at two substrate deposition temperatures of 25˚C and 150˚C with a typical radiofrequency (r.f.) planar magnetron sputtering system. The sheet resistance and the optical transmission properties of the fluorinated PI substrate were comparable with those for the slide glass substrate. The substrate properties are better when the deposition temperature is higher. SiO2 coating onto the fluorinated PI substrate by sol-gel reaction was also attempted to improve the optical transmission of the ITO/PI substrate. It was found that the coating of SiO2 affected on the morphology of the PI substrates.
INTRODUCTION In recent years, interests of transparent flexible plastic substrates with a transparent conducting oxide (TCO) layer such as ITO have been increased in the fields of portable devices, roll-up displays, or conformable displays. So far, the use of the flexible substrates for organic electroluminescent devices (OELDs) has been restricted mainly to the polyester films due to their excellent transparent property. However, the plastic substrates till now can be severely damaged at high temperature during deposition process, mainly due to their intrinsic low thermal and mechanical properties compared with other inorganic or metal substrates. Consequently, reduced electrical resistivity and long-term stability of the ITO surface cannot be achieved and maintained, which is directly related to the performance of applied devices. In this sense, wholly aromatic polyimides (PI’s) are one of the best candidates for the substrate because of their excellent thermal stability and good mechanical, optical, electrical, chemical properties [1]. However, most aromatic PI’s are highly colored polymers with colors ranging from yellow to brown, as well as poor processability and low solubility. Structural modification of PI’s, therefore, has been attempted including the incorporation of flexible bridging units [2], bulky substituent groups [3], and aliphatic moieties [4] into the PI backbone. In this work, colorless fluorinated polyimide substrate wa
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