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Something New Under the Sun?

Photovoltaics: Transparent solar cells could find killer app in consumer goods
September 1, 2000

Imagine a smart credit card that not only stores electronic money and records your transactions but also has its own energy source. Or a sun roof that delivers electricity to your car battery. Imagine each powered by flexible, ultrathin, see-through solar panels.

These scenarios may not be far off, thanks to a photovoltaic cell production process unveiled by Toshiba scientists in May at the 16th European Photovoltaic Solar Energy Conference and Exhibition in Glasgow, Scotland. The Toshiba design is an improvement to the Graetzel cell, a new type of solar panel that relies on titanium dioxide nanocrystals coated with a dye. When struck by light, the dye “injects” energized electrons into the semiconducting titanium, which generates electrical power. Graetzel cells’ advantages over conventional silicon solar panels include transparency, low materials costs and the ability to operate efficiently under cloudy skies.

Earlier Graetzel designs, however, mostly relied on a liquid electrolyte to replenish the dye with electrons; this proved impractical because of the risk of leakage. Toshiba is the first to succeed in encapsulating liquid electrolyte in a durable solid-a “cross-linked” gel that can withstand temperatures of up to 120 C.

Shuzi Hayase, chief research scientist at Toshiba’s Power Supply Materials & Devices Laboratory in Kawasaki, says the cells achieve a respectable 7.3 percent solar-energy conversion efficiency and should be easy to manufacture. “We do not need expensive production lines and sophisticated vacuum systems currently employed in the manufacture of silicon-based cells. The new cells could be manufactured by [silk-screen] printing technologies.”

According to Michael Graetzel at the Swiss Federal Institute of Technology, who invented the basic cell design in the early 1990s, the Toshiba development is “a very important step forward” towards simplifying dye-based cell production. During the next decade, he says, the technology should find its first uses in low-power applications such as sun-powered timepieces (watch manufacturer Swatch already has a prototype) and price scanners. At least seven companies in Japan, Europe and Australia are also developing improved Graetzel cells that may ultimately grace cellular phones, laptop computers and windows in energy-efficient homes.

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