In recent years, organic thin-film transistors (OTFTs) have attracted great attention, and their performance has continually improved. OTFTs have many advantages in terms of a low-cost, low-temperature process, compatibility with flexible substrates, and a large variety of composite materials. In comparison to transistors made from conventional inorganic semiconductors such as amorphous silicon, the stability and uniformity of OTFTs require however improvements. The investigation of initial stages of crystal growth of organic semiconductors is important to solve the problems. Distyryl-oligothiophene, one of the representative organic semiconductors, has been used in OTFTs as a promising material. We have so far clarified that some kinds of distyryl-oligothiophenes show the marvelous stability in OTFT [1,2]. To reveal the cause of the stability in OTFTs based on distyryl-oligothiophene, investigation of in-plane structure of the thin films could be helpful. In this study, in-plane structures of thin films based of oligothiophenes on SiO2 substrates were investigated by grazing incidence X-ray diffractmetry (GIXD). The effects of film thickness and substrate temperatures on the in-plane structure were examined by GIXD. Distyryl-oligothiophene was vacuum deposited onto SiO2 substrates under 1 × 10-4Pa. The substrates were maintained at a temperature between 20°C and 100°C. The deposition rate was set at 0.1Å/s. The mean film thicknesses were between 10Å and 1000Å. Characterization of the films were done by using x-ray diffractometers (Rigaku Co., ATX-G and synchrotron radiation at the BL13XU ATX-GSOR in SPring-8) which were specially designed for characterization of thin films. Both in-plane and out-of-plane diffractions could be measured, because the goniometer has not only conventional θ/2θ axes but also in-plane φ/2θχ axes. As the result, in-plane diffraction patterns were obtained from the ultra-thin films, and the lattice spacing obviously changed with increasing film thickness. Therefore, it was considered that the unique in-plane structures at the interface of the thin films could contribute to the remarkable stability in OTFT performance.

[1] C. Videlot-Ackermann, J. Ackermann, H. Brisset, K. Kawamura, N. Yoshimoto, P. Raynal, A. El Kassmi, F. Fages, J. Am. Chem. Soc. 127 (2005) 16346.

[2] C. Videlot-Ackermann, J. Ackermann, H. Brisset, K. Kawamura, N.Yoshimoto, P. Raynal, A. El Kassmi, and F. Fages ,Organic Electronics, 7 (2006) 465.

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