In-Plane Molecular Alignment in Thin Films of Pentacene Grown by Solution Casting and Performance of Thin Film Transisto
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1003-O01-09
In-Plane Molecular Alignment in Thin Films of Pentacene Grown by Solution Casting and Performance of Thin Film Transistors Takashi Minakata and Yutaka Nastume Central R&D Laboratories, Asahi-KASEI Corporation, 2-1 Samejima, Fuji, Shizuoka, 416-8501, Japan ABSTRACT We have fabricated solution-processed thin films of pentacene by casting solution on a substrate and vaporizing solvent. The films with large oriented platelet domains were obtained by directionally grown condition. Molecular alignment in the directionally grown grains has been studied by several kinds of structural analysis. Oriented domains with the width of several hundreds microns and the length in an order of cm of the films were confirmed by polarized microscopy. In-plane crystalline structure of the domain has been studied by grazing incidence X-ray diffraction (GIXD) and strong anisotropy of in-plane crystalline structure was confirmed. Crystalline growth direction of the film was determined to be b-axis from both transmission electron diffraction and GIXD. Thin films transistors (TFTs) with directionally oriented domains of the films were fabricated on electrode patterned substrate. The observed maximum carrier mobility of 2.7 cm2/Vs was comparable to that of single crystal, which indicated that the quality of the film was almost identical with the single crystal. Correlation between FET performance and growth direction was studied and preferred performance of TFTs with the film grown perpendicularly to the channel was observed. INTRODUCTION Thin film preparation of organic semiconductor via solution process attracts significant attention since it is essential to fabricate large-area and low-cost electronic devices, such as thin film transistors for display, identification tags, and sensors [1-3]. We have reported the direct formation method for pentacene thin-films with solution casting [4]. In this method, no particular precursor molecule(s) have been used during the process and thin films of pentacene can be fabricated directly using solution of pentacene. This method can be applied to other crystalline organic materials, such as oligoacenes, oligothiophenes and several poly-aromatichydrocarbons [5]. Recently, solution-processing of crystalline organic semiconductors have been studied actively and good performance of TFTs with solution-processed films have been reported [6]. Crystalline growth of rubrene by controlling preparation conditions from the phase diagram of rubrene and solvents and high carrier mobility of solution-processed transistors has been reported [7]. The solution-processed pentacene thin-films were formed with platelet crystalline domains and showed higher crystalline structure than that of sublimed films with granular domains, since stronger and narrower X-ray diffraction peaks were observed in the solutionprocessed films. TFTs formed with the solution-processed film exhibited good transistor performance with the carrier mobility above 1cm2/Vs and relatively small threshold voltage. Stable performance of the TFT
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