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I3.7.1

Orientation of Organic Semiconductor Films on Photoreactive Polyimide Films and its Influence on Field-Effect Transistor Characteristics Hiroshi Kikuchi, Yuichiro Uchida1, Yoshihide Fujisaki, Hiroto Sato, Hideo Fujikake, Taiichiro Kurita, Kuniharu Takizawa1 and Fumio Sato NHK Science & Technical Research Laboratories, Setagaya-ku, Tokyo 157-8510, Japan 1 Department of Applied Physics, Seikei University, Musashino-shi, Tokyo 180-8633, Japan ABSTRACT In this study, we have investigated the effect of surface treatment on the orientation and mobility of pentacene by using a photoreactive polyimide film to modify the gate-insulator surfaces of organic field effect transistors (OFETs). Surface modification includes a photoreactive polyimide film, presenting a passivated interface on which the semiconductor can grow. This polyimide film can control of the orientation of semiconductor by using linearly polarized deep UV (LPDUV) irradiation. Fabricated OFETs include stacked structures of Ta2O5 as the gate insulators and the photoreactive polyimide. Most of the characteristic parameters of the OFETs, such as carrier mobility and on/off current ration, have been improved by using the photo-alignment treatment achieved with LPDUV irradiation. INTRODUCTION Recently, organic field effect transistors (OFETs) have received considerable interest. OFETs offer several advantages compared to amorphous silicon transistors, including flexibility and low-cost. It is highly anticipated that applications of OFETs will include flexible active matrix displays, integrated circuits, radio frequency tags, smart cards, etc. The performance of OFETs has been improved through the use of new organic semiconductors and the optimization of the device fabrication methods. The dielectric materials used as insulator layers in OFETs are now being addressed since the characteristics of OFETs are significantly affected by these dielectric materials and their interfacial properties [1]. To achieve a high mobility at a low operating voltage, gate insulators with high dielectric constants have been employed. Moreover, an important criterion in the fabrication of OFETs is the control of the orientation of the organic semiconductor molecules. Functionalizing the dielectric surface using a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) [2-4], hexamethyldisilazane (HMDS) [4], or poly α–methylstyrene (AMS) [5] has been widely used to increase order in the organic thin film. In this study, we have investigated the effect of a new surface treatment on the orientation and mobility of pentacene by using a photoreactive polyimide film to modify the gate-insulator surfaces in organic FETs. We fabricated pentacene FETs with tantalum oxide (Ta2O5) as the gate insulator by using this photo-alignment treatment. EXPERIMENTAL Polyimide film, based on a cyclobutane tetracarboxylic dianhydride (CBDA) component [6], was used for the modification in order to induce orientation effects with organic semiconductor molecules. This film was prepared by imidiza