Pentacene transistors with polymer gate dielectrics on metallized optical fibers
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It is difficult to deposit a very thin polymer layer onto a fiber-shaped substrate from solution because the high interfacial energy can lead to dewetting. This difficulty presents itself when attempting to apply a gate dielectric to conductive fiber substrates during the fabrication of fiber transistors for use in applications such as “electrotextiles” and optical switches. In this article, we present a dip coating process that applies a gate dielectric to metal-coated optical fibers with high uniformity and reproducibility, resulting in pentacene field-effect transistors with excellent transistor characteristics including mobilities up to 0.4 cm2/Vs and on/off ratios up to 7000. In one case, a memory effect was demonstrated. Several gate dielectrics were successfully applied to the optical fibers, suggesting a baseline set of suitable materials for this purpose. A thorough study of the dip coating conditions is presented, including proposed explanations of the effects of different coating procedures and solution physical properties. Finally, alternative architectures that would provide much higher W/L ratios and on-currents are described.
I. INTRODUCTION
One of the major advantages projected for organic and printed electronic technologies is the ability to form transistors on highly curved substrates. While organic transistors and related devices are being studied at a rapidly expanding rate,1,2 virtually all of the demonstrations have been on planar substrates. There are at least two classes of applications, however, which may call for transistors to be fabricated on fiber-shaped substrates. One is a proposed circuit technology broadly known as “electrotextiles,”3,4 where sensing, switching, and signal processing components will be embedded in a fabric of woven fibers. Ideally, some or all of the contacts will be through overlying perpendicular fibers placed by the weaving process. Amorphous silicon thin film transistors have been fabricated on Kapton fibers.5 More closely related to this work, pentacene transistors have been deposited on wire substrates using inorganic, vapor-deposited dielectrics6 with mobilities up to 0.05 cm2/Vs. Another possible application of on-fiber transistors is in the more intimate integration of electronic control circuitry with in-fiber photonic switches and tuners. A wide
variety of schemes are being proposed for the alteration of fiber transmission properties, especially through gratings, using electrically controlled claddings.7 These claddings may comprise microfluidic channels,8,9 liquid crystals,10,11 or resistive metal films.12,13 A transistor controller for these electrical components can be used for stabilization via adaptive circuitry or for remote access with a reduction in wire bonding. A significant challenge for on-fiber electronics is to fashion a gate dielectric using a facile process that results in a film with sufficient integrity to prevent undue gate current leakage while retaining sufficient capacitance for transistor switching at accessible electric fields.
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