Bridgestone Corp has come up with a material that could replace ITO used to manufacture transparent electrodes for the company's electronic paper (e-paper).
The company made this announcement in a symposium at SID Display Week 2009.
This speech was marked as "Withdrawn" in the program handed out to the participants, indicating that the speaker could not come from Japan to make the speech. However, the speech was delivered through the use of PowerPoint and an audio system.
Bridgestone has already developed its own e-paper technology, but the fragility of ITO was a problem in realizing flexible e-paper. To solve the problem, the company focused on an organic material known as PEDOT/PSS (poly (3,4-ethylenedioxythiophene)/ poly (4-styrenesulfonate)).
The conductivity of PEDOT/PSS was low at about 1S/cm in the early 1990s, when the material was first developed. But it was rapidly enhanced, and some of the materials now have a conductivity exceeding 1,000S/cm and are attracting much attention (See related article 1, 2, 3).
In respect to optical transmittance, which generally trades off conductivity, "ITO is superior in consideration of all visible lights, but PEDOT/PSS is advantageous in some of the wavelengths," Bridgestone said.
The optical transmittance of PEDOT/PSS materials that are used for transparent electrodes is about 86% when the sheet resistance is 300 to 400Ω/〈. The company considers that the 86% transmittance is high enough for use in e-paper.
A transparent electrode made by coating ITO on PET has an optical transmittance of 87.5% in the 550nm wavelength light when the sheet resistance is 200Ω/〈.
The important point in using PEDOT/PSS for e-paper is the method of forming matrix electrode patterns. This time, Bridgestone employed the laser ablation method. Specifically, an excimer laser was scanned over a photomask to form the electrode pattern. With this method, a productivity equivalent to that of ITO can be realized, according to the company.
Also, Bridgestone compared an e-paper made using a PEDOT/PSS transparent electrode with one incorporating an ITO transparent electrode. As a result, the reflectance of the former e-paper was only 1% less than that of the latter e-paper when they display white color or black color. And there was not a big difference in contrast ratio durability between the two.
PEDOT/PSS is far superior to ITO in terms of resistance to breakage and is lower in cost, according to Bridgestone. For these reasons, the company reached the conclusion that "PEDOT/PSS is suitable for use as a transparent electrode for e-paper."
Thus far, PEDOT/PSS has been evaluated mainly by material manufacturers and research institutes, and it was unclear whether it is applicable to actual products. This time, Bridgestone endorsed PEDOT/PSS through its announcement that the material can be used for e-paper. And it is expected that the replacement of ITO with EDOT/PSS will be accelerated in the future.
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