A low-temperature process for fabricating flexible, ohmic contacts for use in organic thin-film transistors (OTFTs) has been developed. Typical drainsource contact materials used previously for OTFTs include (1) vacuum deposited noble metal contacts and (2) solution deposited intrinsically conducting molecular or polymeric contacts. Both of these approaches, however, have serious drawbacks.

Use of vacuum-deposited noble metal contacts (such as gold or platinum) obviates one of the main benefits of organic electronics, which is low-cost processing based on solution or printing techniques. First, it requires the use of vacuum-deposition techniques (such as sputtering or evaporation) instead of the less expensive solution-based processes such as spin coating, casting, or printing. Second, the use of gold or platinum for coating large area devices is potentially expensive (both from a standpoint of materials and processing equipment). Again, this approach runs counter to the perceived low cost benefit of organic electronics.

Furthermore, adhesion of gold to many organic materials is very poor. Some recent work has been carried out regarding intrinsically conducting molecularor polymeric-based contacts such as polyaniline and TTF-TCNQ. Unfortunately, these materials tend to exhibit high resistivities and poor overall performance, are prone to reaction with the surrounding environment, and are potentially unstable with time.

To achieve an ohmic contact to the organic semiconductor, the work function of the contact should be well matched to that of the semiconductor. Due to the similar chemical nature of the graphite filler to the conjugated poly(3-hexylthiophene) (P3HT) polymer, it was surmised that a carbon paste may possess a similar work function and therefore behave as suitable ohmic contact in this application.

Figure 1. An Organic Field-Effect Transistor was fabricated in an inexpensive process, mostly at room temperature, with brief heating at 100 °C.

To demonstrate the effectiveness of this approach, bottom contact thin-film transistors were fabricated (Fig. 1). A highly doped silicon wafer was used as the substrate, with a thermally grown 300-nm oxide gate dielectric layer. In this case, a 5-mil (127-µm) thick laser-cut stainless-steel stencil was used to pattern the contacts.

The carbon-based conductor used was a paste comprising a stable, flexible polymer binder and a conducting graphite/carbon-based filler. The paste was stencil-printed through the apertures using a metal squeegee, and the contacts were cured at 100 °C on a hot plate for 30 minutes. The P3HT material was then drop cast on the surface of the substrate, over the printed contacts. Contact was made to the cleaved wafer to form the gate. Contact was made to the drain and source carbon contacts through probes connected to micromanipulators. Device measurements were conducted on an HP 4145B semiconductor parameter analyzer, with the drain-source voltage varying from 0 to –100 V, and the gate bias varying from 0 to –100 V in –10 V steps.

Figure 2. These Current-Versus-Voltage Curves, obtained from measurements on a device like that of Figure 1, are characteristic of a field-effect transistor.

As seen in Fig. 2, very good transistor curves are obtained from the devices, indicating clear field-effect phenomena, which demonstrates the effectiveness of the carbon-based contacts. In this case, a device with a channel length of 500 µm and a channel width of 5,000 µm was measured (multiple devices were characterized to verify repeatability of the data). Other device geometries are possible, as printed feature sizes down to 37 µm have been demonstrated using state of the art thick-film stencil/screen printing techniques.

Enhancement of the drain-source current is clearly seen as a function of increasing gate bias in Fig. 2. In this case, an Ion/Ioff ratio of 44 was determined at Vds = -100 V, with Vg = 0 (Ioff) and Vg = -100 V (Ion). A carrier mobility of µ ≈ 0.007 cm2/V-s was also estimated from the data.

This work was done by Erik Brandon of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free online at www.techbriefs.com/tsp under the Semiconductors & ICs category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive Pasadena,
CA 91109-8099
(818) 354-2240
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Refer to NPO-40168, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Flexible, Carbon-Based Ohmic Contacts for Organic Transistors

(reference NPO-40168) is currently available for download from the TSP library.

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This article first appeared in the November, 2005 issue of NASA Tech Briefs Magazine.

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