High Mobility and Liquid Phase Processable Organic Semiconductors
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ABSTRACT New thiophene oligomers and fused ring compounds have been designed and synthesized with the aim of maximizing semiconductor mobility in thin film transistors (TFTs) while allowing for liquid phase processability. Thiophene hexamers with alkyl side chains of various lengths, some with an ether oxygen embedded in the chain, were synthesized, as were derivatives of the novel heterocycle anthradithiophene. Mobilities of the vaporphase-deposited films ranged from 0.01 cm 2/Vs for the hexamers to 0.15 cm 2/Vs for dihexylanthradithiophene. The latter is the highest mobility yet reported for a polycrystalline film. Cast films of some of these compounds from solution gave mobilities within factors of 2-10 of the corresponding values from gas-phase-deposited films.
INTRODUCTION TFTs where all of the materials utilized, including the se'miconductor, are organic or polymeric are of interest for use in low-level memories and logic circuits, and as drivers for flexible displays.' A wide variety of materials have been screened as organic TFT semiconductors, primarily by groups at CNRS-Thiais, Philips-Eindhoven, IBM-New York, Penn State, and Bell Laboratories. The original reports of TFTs employing organic molecular solids as semiconductors were from the Gamier group at CNRS, where the linear hexamer of thiophene was viewed as promising' and the xc,(o-dihexyl derivative was considered as a superior alternative.3 Further structural improvements in these solids arose from the use of fused ring systems, culminating in the optimization of the mobility of pentacene to a value above I cm 2/Vs.4 It has become apparent that the principal advantages of organic TFTs will lie in the inexpensive options available for their processing, rather than in any performance advantage over silicon. To fully utilize the least expensive fabrication methods, such as printing and casting of features, the semiconductors must be deposited from solution to produce films that retain high mobilities as well as robustness and reproducibility. Regioregular polythiophene has been demonstrated to form a high-mobility spun film useful in TFTs.5 Pentacene has been deposited via a precursor route as a solution, followed by thermal conversion in situ.6 Besides a preliminary report of dihexylquaterthiophene forming high-mobility films from the condensed phase7 , there have been no accounts, particularly of high-melting compounds, so utilized in TFTs.
425 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
This paper describes new synthetic chemistry leading to high-melting organic semiconductors that may be deposited from solution as part of a TFT fabrication process. The value of the side chains in maximizing the solubility and preserving the mobility that could otherwise be obtained from these compounds by gas-phase deposition is discussed. Especially dramatic is the use of the hexyl substituent to solubilize the highly infusible anthradithiophene ring system, investigated for the first time as a part of this work. SYNTHESES Hexameric th
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