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Organic Material Offers Fast Response

Photonics Spectra
Mar 2000
Aaron J. Hand

Because of their low cost and good manufacturability, organic photoconductors play an increasing role in xerographic applications and as the carrier transport layer in electroluminescent displays. But the photoelectric properties of organic materials -- even molecularly doped polymers -- are less than ideal.

Researchers at the Tokyo Institute of Technology have developed a semiconductor material that offers several advantages over standard organic photoconductors, including a high mobility that is independent of temperature or electric field levels.

The researchers are calling their material a self-organizing molecular semiconductor. Although it could be referred to as a liquid crystalline photoconductor or photoconductive liquid crystal, the properties are far from those of conventional photoconductors or liquid crystal materials, pointed out Jun-ichi Hanna, who has led this research in the university's Imaging Science and Engineering Laboratory. It exhibits self-organizing molecular alignment akin to molecular crystals, and liquidlike fluidity that makes it useful for large-area electronic devices.

Compared with organic materials or molecularly doped polymers (right), self-organizing molecular semiconductors (center) exhibit alignment close to that of molecular crystals (left), but maintain a liquidlike fluidity. Their good photoelectric properties make them suitable for several current and future applications.

Conventional organic semiconductors have poor photoelectric properties because of disordered molecular hopping sites, giving them a mobility of 10–5 to 10–6 cm2/Vs. The self-organized molecular hopping sites of the new material are closer together (about 4 Å apart compared with >8 Å in amorphous materials), raising mobility to better than 10–3 cm2/Vs and even 10–2 cm2/Vs in some phases. But the self-organizing molecular semiconductors also have anisotropic functionality, unlike amorphous materials.

Working up to 1000 times faster than conventional organic photoconductors, the new material could be used to make faster devices. The Japanese researchers have demonstrated its feasibility in high-speed photosensors, xerographic photoreceptors and electroluminescent diodes. But they are also working to exploit the liquid crystal properties of self-organizing molecular semiconductors in new application areas.

Dai Nippon Printing Co. Ltd. in Tokyo is interested in the material's capabilities. Hiroki Maeda, of the company's Central Research Institute, has been collaborating with Hanna, studying the materi-al's basic physical properties. Although Dai Nippon has not declared a clear business target, Maeda said, it is confident that the material has the potential to expand the application of organic electronic devices.

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