Synthesis of TPD-Containing Polymer Thin Films by Physical Vapor Deposition
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Synthesis of TPD-containing Polymer Thin Films by Physical Vapor Deposition Hiroaki Usui, Terufumi Yoshioka, Takahiro Katayama, Kuniaki Tanaka and Hisaya Sato1 Department of Organic and Polymer Materials Chemistry, 1 Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, JAPAN ABSTRACT Polymer thin films having tetraphenyl diaminobiphenyl (TPD) side chain were prepared by evaporating its acrylate monomer by the ionization-assisted deposition (IAD) method. IR absorption and GPC analyses showed that the electron irradiation to the evaporated material works efficiently to initiate the polymerization reaction on the substrate surface, while the simple evaporation produced films mainly consisting of the monomers. The reaction was enhanced by increasing the substrate temperature. However, the surface roughness increased with increasing substrate temperature. Post annealing can be an alternative method to enhance the polymer yield. Light emitting diodes were prepared by stacking the TPD polymer layer, Alq3 emissive layer, and Al electrode on ITO-coated glass. The IAD polymerization can be a useful method for preparing polymer thin films for organic optoelectronic devices. INTRODUCTION The research on organic light emitting diodes (OLEDs) has recently made rapid advancement toward the practical application. From the standpoint of the material, there are two approaches, i.e., low-molecular materials and polymers. The low-molecular materials are usually deposited by vacuum evaporation, while the polymers by wet processes such as spin coating and dipping. Although the latter methods have higher throughput and cost performance, the vacuum evaporation technique has advantages in low inclusion of impurities, uniform film deposition down to nanometer thickness range, and formation of multilayered microstructure. On the other hand, polymers have higher thermal stability and can be effective for improving the device lifetime, which is one of the key issues in the practical use of OLEDs. With this respect, vacuum deposition of polymer thin films can combine the advantages originating to the physical vapor deposition and polymer material. It has been reported that some acrylate compounds can be polymerized upon evaporation with the aid of thermal or UV irradiation [1, 2]. The authors have used the ionization-assisted deposition (IAD) method for the deposition of acrylate polymer films [3, 4]. With the IAD method, the evaporated material is exposed to electron bombardment before reaching to the substrate. This process generates activated species that can initiate the polymerization. Unlike the plasma polymerization, the film formation proceeds in high vacuum without causing complicated collision processes in the volume of vacuum chamber. The polymerization reaction proceeds on the substrate surface, and can be easily controlled by the electronic manipulation of deposition condition. Therefore, the IAD method can be effective for the preparation of
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