Harnessing the sun's energy through transparent photovoltaics.

• Author: Jean, Grace V.
• Position: INSIDE SCIENCE + TECHNOLOGY

TSUKUBA, Japan -- Researchers here have developed a small transparent solar cell prototype that may' one day capture sunlight streaming in through a window and produce enough electricity to power homes and office buildings.

The coin-size photovoltaic unit uses transparent oxide semiconductors to convert solar energy into electricity. Unlike conventional solar cells that absorb visible and infrared light to generate power, this cell allows visible and infrared light to pass through it while absorbing ultraviolet radiation and converting it into electricity.

"The visible light from the sun is essential. On the other hand, the UV light is often unnecessary for us, even harmful to the human body. So I tried to utilize the UV light by converting it into electricity," says Kazuhiko Tonooka, the project's lead scientist at the Nanoelectronics Research Institute.

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Silicon solar cells are typically used in photovoltaic power generation devices. They produce electricity by absorbing all visible light, so they are often opaque and require large surface areas, such as rooftops, to soak tip enough sunlight to produce adequate amounts of power.

In Japan, where most city residences are small apartments, the sheer size of conventional solar cells prohibits their practical use as alternative energy sources. That is why scientists here are investigating the possibility of developing these transparent solar cells into larger sheets to replace conventional glass windows, says Kozo Uto, director of the international affairs department at the National Institute of Advanced Industrial Science and Technology--Japan's largest public research institute.

"We can utilize UV and visible light from the sun at the same time by using one device," says Tonooka.

If the device is incorporated into windows, it could potentially shut out the sun's harmful UV rays and generate electricity for an entire household, Uto says. In addition, the solar cell could help control the infrared light that passes through the window to help heat or cool the home.

But there are a number of hurdles to overcome. The U.S. quarter-size prototype on display here generates only 67 millivolts of energy. Such voltage is adequate for powering small electronics, such as cell phones or laptop computers, but not entire homes.

"The efficiency of our transparent cell is very small. It is the weak point of our device," says Tonooka.

Another concern is that the UV light radiating from...