Saturday, January 12, 2008

A New Type of Solar Cell

Scientists in Japan have made the first device that can convert solar energy into electricity and then store the resulting electric charge. The “photo-capacitor” designed by Tsutoma Miyasaka and Takuro Murakami at Toin University in Yokohama could be used to power mobile phones and other hand-held devices (Appl. Phys. Lett. 85 3932).
Conventional solar cells need a secondary device, such as a battery, to store the electrical power
generated from light. The photo-capacitor combines the photo-electric and storage functions in a single structure.

The Japanese device consists of two electrodes a light-aborting photo electrode made of semi
conducting titanium dioxide and a counter electrode made of platinum coated
glass separated by a resin film. Both electrodes include a porous layer of activated carbon that has a large surface area. All three layers filled with an ionic solution and form a capacitor that has a light collection area of 0.64 square centimeters.

Photoreceptor dye molecules collect on the surface of the titanium dioxide layer. When exposed to light, electrons from the dye molecules are transferred to the conducting band in the titanium dioxide layer, thus producing current. They then transfer to the activated carbon layer at the counter electrode via an external circuit.

Conversely, the positively charged holes left behind re-transferred to the carbon layer at the photo electrode.The accumulation of positive and negative charges at different carbon therefore allows the device to store energy or charge like a capacitor. The energy can be released by simply discharging the device. “The photo-capacitor is twice as efficient traditional silicon-based solar cells in utilizing weak light,”

Miyasaka told “Physics Web.” “This means that it can utilize indirect sunlight, for example on cloudy or rainy days, and even indoor light. Moreover, it can release electrical energy anytime, even in the dark.”

Miyasaka says that the next goal is to increase the charging voltage and the charge discharge capacity to a practically and industrially useful level for applications.

Dye-adsorbed titanium dioxide (10 microns thick) and activated carbon (150 to 200 microns thick) forms a hetero-junction with the interface (which is doped by an intermediated layer of a hole trapping compound lithium iodide). The layers are impregnated with ionic electrolyte solution.

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