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Solar Cell Doubles as Battery

A practical solar energy system usually includes solar cells that convert light to electricity and batteries that store the energy for later use.

Scientists from Toin University of Yokohama in Japan have designed a single, compact device that can both convert solar energy to electricity and store the electricity. "We succeeded in incorporating both photovoltaic and storage functions in a single cell with a thin, sandwich-type structure," said Tsutomu Miyasaka, a researcher at the University.

The researchers' photocapacitor is also efficient at capturing energy from weak light sources like sunlight on cloudy or rainy days and indoor lighting.

The light-driven, self-charging capacitor could eventually be used to power portable electronic devices like phones, cameras, and PDAs, said Miyasaka. "Users can just bring the device anywhere and expose it to indoor and outdoor ambient light whether they need power or not [then] release the stored electricity anytime they want," he said.

Solar cells convert light to electricity by absorbing photons and using their energy to move electrons. There are two basic types of solar cells. Conventional cells are solid-state devices usually made from silicon. It is also possible to capture the energy from photons using dye molecules.

The researchers' device is an electro-chemical cell made up of a pair of electrodes sandwiching a liquid electrolyte. The electrolyte contains a high concentration of ions, or atoms that carry a charge because they have gained or lost an electron. The electrodes are glass plates with metal coatings on the inside surfaces. The top electrode sports a film of titanium dioxide semiconductor nanoparticles that has pores 15 to 30 nanometers in diameter and contains ruthenium dye molecules. Both electrodes have porous inner layers of carbon particles that are about 5,000 nanometers in diameter, which is about the size of a red blood cell. The carbon layers encase the electrolyte.

Dye-based solar cells use dye molecules to absorb photons, which causes negatively-charge electrons and positively-charged holes to separate in the semiconductor layer. The researchers' photocapacitor transfers these charges to the carbon layers.

The electrons travel toward the bottom electrode, where they accumulate on the carbon surface near the electrolyte. A chemical reaction that restores the electrical balance of the dye also makes holes accumulate on the carbon surface of the top electrode. "Electrons and holes generated by light-excited organic dye can be directly accumulated on the large surface area of the carbon layer," said Miyasaka.

There are three types of silicon solar cells: those made from pure silicon, which are the most expensive and most efficient, those made from amorphous silicon, which are fairly cheap and one-quarter to one-half as efficient, and those made from polysilicon, which lie between pure silicon and amorphous silicon in expense and efficiency.

Silicon solar cells reflect rather than absorb light that hits the silicon surface at angles greater than 40 degrees. Although the researchers' device is less efficient in direct sunlight than silicon, it absorbs light that hits the surface at a much broader angle, making it able to absorb diffuse light. This allows it to harvest photons in the morning, in the evening evening, on cloudy days, and from indoor lighting.

"While the experimentally measured highest efficiency is higher for silicon-based cells, the... practical efficiency of the cell [is]comparable with the amorphous silicon cell or surpass it," Miyasaka.

The cells can also be connected to form larger, more powerful cells. Conventional capacitors that are charged using electricity can produce a voltage that is no greater than the input, or charging voltage, of one of the cells in a connected series. In contrast, the photocapacitor, like conventional batteries, can produce voltage equivalent to the collective input of photocapacitors connected in series. The researchers' prototype produces 0.7 volts. Connecting 18 cells would yield 12 volts, which is the output of a car battery, said Miyasaka.

The thickness of the photocapacitor depends on the thickness of the electrodes, and could be made narrower than one millimeter, said Miyasaka.

The device could be used in practical applications in two years, said Miyasaka. The researchers are working on boosting the cell's capacity and making a flexible, lightweight plastic version of the device, he said.

Miyasaka's research colleague was Takurou N. Murakami. The work appeared in the October 25, 2004 issue of Applied Physics Letters. The research was funded by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).

Technology Research News

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