Facile Deposition Processes for Semiconducting Molecular Solids
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PROCESSES
FOR
SEMICONDUCTING
Authors: H.E. Katz, W. Li, A.J. Lovinger, V.R. Raju, J.’A. King, Y.-Y. Lin, A. Dodabalapur, Z. Bao, and J. Rogers Bell Laboratories-Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 07974 ABSTRACT Films of end-substituted dihexyl-α-sexithiophene and –quinquethiophene were cast from solutions in aromatic solvents onto SiO 2 and polyimide dielectrics at moderately elevated temperatures and reduced pressure. X-ray diffraction showed perpendicular orientation for most samples, while electron and optical microscopy revealed considerable variations in grain sizes, spacings, and uniformity depending on deposition conditions. For favorable morphologies, thin film transistor (TFT) mobilities were as high as those typically obtained from vacuum-deposited films, in the range of 0.01-0.2 cm2 /Vs, with on/off ratios >1000 in accumulation mode. INTRODUCTION The most important advantage offered by organic semiconductor-based circuits is the opportunity to employ less costly fabrication processes relative to Si-based analogues. “Organic electronics” technology is therefore contemplated for applications where the high device density and speed of Si integrated circuits are not essential, and where a particularly facile process could enable the application of transistor circuitry to new situations, such as smart cards, identification tags, and simple flexible displays.1 Such steps as casting, spraying, and printing2 are especially preferred in the process because they can be performed as part of a reelto-reel sequence. It would be particularly attractive to minimize or eliminate material deposition steps requiring high vacuum from any organic electronics fabrication scheme, in order to increase throughput and decrease capital equipment costs. The organic semiconductors that display the best mobility, on/off ratio, uniformity over large areas, and reproducibility, such as pentacene 3 , thiophene oligomers and fused rings4 , phthalocyanines5 , and C60 6 , have all been deposited by vacuum sublimation. Mostly preliminary results have been reported for the solution-phase deposition of molecular solids as TFT semiconductors7-9 , with one more elaborate study involving spin-coating under very specific conditions.10 Greater progress has been demonstrated with polymeric semiconductor solutions, especially regioregular poly(hexylthiophene) (mobilities 0.01-0.1 cm2 /Vs)11 , and lower-mobility poly(thienylenevinylene) prepared with an in situ conversion step, from which complex circuits were fabricated.12 Large-area coverage and uniformity are easier to achieve with polymer solutions, although the spin-coating and unusual
temperatures that are sometimes required are not ideal in a reel-to-reel process. However, it is difficult to purify polymers to the level required for high mobilities and on/off ratios, and batch-to-batch variability has been observed. Smaller molecules, on the other hand, can be purified to the level where on/off ratios above one million can be obtained in accumulation mode, without
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