Low-temperature processable inherently photosensitive polyimide gate dielectric for organic thin-film transistors: Synth
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Low-temperature processable inherently photosensitive polyimide was prepared from a dianhydride, 3,3⬘,4,4⬘-benzophenone tetracarboxylic dianhydride, and aromatic diamines, 4,4⬘-diamino-3,3⬘dimethyl-diphenylmethane, through a polycondensation reaction, followed by a chemical imidization method. The photosensitive polyimide cured at 180 °C is used as a gate dielectric to fabricate flexible organic thin-film transistors with pentacene as an active semiconductor on polyethersulfone substrate. With the inherently photosensitive polyimide, the access to the gate electrode could be created easily without complicated and expensive lithographic techniques. A field effect carrier mobility of 0.007 cm2/V s was obtained for the pentacene organic thin-film transistors (OTFTs) with the photo-patterned polyimide as a gate dielectric. More detailed analysis for the pentacene OTFTs will be given with electrical properties of the thin polyimide film. Low-temperature processability and patternability of the polyimide give us more freedom to choose plastic substrates in OTFTs and facilitate the realization of low-cost organic electronics.
I. INTRODUCTION
Active organic-based thin-film transistors have attracted much attention due to their potential application to low-cost, large-area flexible displays and low-end electronics.1 The two key materials of organic thin film transistors (OTFTs) are an active organic/polymeric semiconductor and a gate dielectric. Much research has been done on active semiconductors such as pentacene2 and poly(3-hexylthiophene),3 and the performance of pentacene-based OTFTs has already reached status comparable to that of hydrogenated amorphous silicon transistors (a-Si:H TFT).4,5 However, those OTFTs comparable to a-Si:H TFT in performance were mainly fabricated on high-quality inorganic gate insulators. 6–8 Therefore, to fully take advantage of the OTFTs, much research needs to be focused on a high-performance polymeric gate dielectric compatible with plastic substrates. In the past, several polymeric gate dielectics such as poly(vinylphenol) (PVP),9–11 poly(methlymethacrylate) (PMMA),12 polyvinylalcohol (PVA),13,14 benzocyclobutene (BCB),15 and polyimide (PI)15,16 were
Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2005.0121 J. Mater. Res., Vol. 20, No. 4, Apr 2005
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investigated. Most recently, Kato et al. reported pentacene OTFTs with polyimide as a gate insulator.16 However, few of these materials have met the basic requirements for a polymer gate dielectric, such as relatively high-dielectric constant, heat and chemical resistance, pinhole-free thin film formability, and compatability with organic/polymeric semiconductors. Furthermore, since OTFTs are intended for low-cost large area flexible applications, gate dielectrics should be patternable for the creation of access to a gate electrode, and their processing temperature should be low enough so they do not deform commerci
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