These contacts are printed using an inexpensive, low temperature process.
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.
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.
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.
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Refer to NPO-40168, volume and number of this NASA Tech Briefs issue, and the page number.
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