Continued from Transparent Electrode Materials Replacing ITO
Many engineers in the field agree that of the various new materials, conductive polymers have shown extremely rapid improvement in characteristics such as transmittance and sheet resistance recently. Especially outstanding is poly(3,4-ethylene dioxythiophene), or PEDOT.
Sanyo Electric Co Ltd of Japan and a research group under Prof Takakazu Yamamoto of Tokyo Institute of Technology of Japan prototyped a high-conductivity PEDOT-based conductive polymer in March 2009, achieving a conductivity of 1200S/cm or higher. The material was used to fabricate a transparent conductive film 120nm thick, which showed a sheet resistance of about 68Ohm/square. Transmittance at 550nm was only about 75%, however, so the invention has not yet reached the commercial level.
In May 2009, a group led by Hu Yan, University of Yamanashi of Japan, Research Interdisciplinary Graduate School of Medicine & Engineering prototyped a PEDOT-based conductive polymer combining both transparency and conductivity.
For a sheet resistance of 244Ohm/square, reasonable for touch panel applications, total light transmittance was about 89%, on a par with ITO film.
Improvement in the
characteristics of conductive polymers is leading to more and more
actual uses in electronic device prototypes. Bridgestone, for example,
announced the use of a PEDOT-based conductive polymer in an e-paper
prototype in June 2009 (Fig A1). The firm tried the material because it
could be printed, and out of various materials with good bending
characteristics offered (according to a source at the firm) "A
reasonably good combination of high transmittance and low sheet
resistance."
The sheet resistance of the conductive polymer adopted by Bridgestone was 300Ohm/square to 400Ohm/square, with a total light transmittance of about 86%. A source at the firm explained it operated "satisfactorily" when used with the company's "electronic liquid powder" e-paper. Current only flows when the display is changed, making high conductivity unnecessary, the firm commented. The prototyped e-paper has a resolution of 82dpi, which the company says is about the same as the e-paper now in volume production for price-tag applications.
Bridgestone believes that conductive polymer characteristics can be further improved and that "It should be possible to achieve the same sheet resistance of a few Ohm/square that ITO offers." They have already got a conductive polymer with a sheet resistance of 200Ohm/square for a transmittance of about 86%.
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