Harnessing the Sun’s Energy Through Transparent Photovoltaics

By Grace V. Jean

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.

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 up 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 the sun is not as abundant as the energy emanated in the visible and infrared spectrums. “I believe that the full use of solar energy is necessary,” says Tonooka.

The present technology on transparent oxide semiconductors is inadequate to produce transparent solar cells for practical use, he adds. Improvements to the deposition technology and increases in the power generation efficiency of transparent semiconductors are necessary if the research is to continue further and develop into larger photovoltaic devices, such as solar sheets.

In the meantime, Tonooka is applying his research to another solar cell project that will reduce the cooling load of buildings by reflecting the infrared radiation of the sun. This type of technology seems to be more practical, he says.

To gain efficiency in converting sunlight into electricity, researchers are looking to plants and trees for inspiration. The chlorophyll found in leaves absorbs the sun’s rays and transforms the light into energy for the plant through a process called photosynthesis. Scientists working in photovoltaics have mimicked nature’s process by using dyes to capture particular wavelengths of light.

Recently, the researchers here have developed a new solar energy device that combines the structures of two types of dye-sensitized solar cells. By sandwiching the two cells together, light photons are trapped inside the device, which increases the electric current it can produce. This tandem solar cell has an energy conversion efficiency of 11 percent — more than any other similar device to date, AIST officials say.

The upper cell consists of a red sensitizing dye that absorbs visible light. A transparent titanium oxide electrode enhances the conversion of the energy to a higher voltage of electricity. The lower cell contains a black sensitizing dye to absorb the infrared light that passes through the upper cell. Though it generates a higher current than the upper cell, the lower cell produces electricity at a lower voltage through its stacked titanium oxide semiconductor films.

Researchers plan to simplify the production process and improve the shape of the solar cell. They also hope to develop a dye that can convert longer wavelengths of light with greater efficiency.

Topics: Energy, Alternative Energy, Power Sources, Science and Engineering Technology

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