Solar power doubles in the land of the rising sun

Bring Me Sunshine

A quantum dot is a semiconductor of around tens of nanometers. Since electrons whiz through these nanostructure, it's hoped they could form the basis of super-fast quantum computers.

Mr Okada has other ideas. He wants to convert this light into electricity. Using a series quantum dots from an indium-arsenide compound semiconductor material, he stacked a number them to form a photovoltaic cell measuring several millimeters in size. He found this structure generates electricity with a conversion efficiency of 7%.

It's not a new idea. A european scientist mooted it back in 1990. But Okada's genius has taken it to a practical level of efficiency.

Quantum dots will generate electricity in response to different wavelengths of light, ranging from infrared to ultraviolet. It all depends on their size. By stacking quantum dots of different sizes, Okada utilized a wide range of wavelengths for energy-conversion efficiency.

In principle, quantum dot solar cells can operate with an efficiency of as high as 63%. In comparison, the best silicon-based solar cells now operate with efficiencies at the 20% level, and even with improvements they hit their theoretical upper limit at the 30% level.

So generating electricity from solar power is expensive. This might be the solution. The efficiency of quantum-dot solar cells could save the planet?

WHAT NEXT?

For Okada, the next step is to bolster the robustness of the solar cells and improve their conversion efficiency. Toward that end, he hopes to collaborate with a maker of electric machinery with a powerful stable of semiconductor technologies. If all goes according to plan, quantum-dot solar cells could reach the commercially practical level around the year 2020.

Solar cells in general are expected to play an important role in society's efforts to reduce greenhouse gases, and Japan is technologically adept in this field. Japanese companies have gained strength through repetitive competition to improve the conversion efficiency of their solar cells, even by a fraction of a percentage.

The leader in the field of silicon-based solar cells is Sanyo. Sanyo has developed a practical-sized device (i.e., one measuring at least 100 square cm) with a world-leading 22% conversion efficiency. The company expects to have commercial versions of this solar cell available by fiscal 2010.

Next are the copper-indium-selenide solar cells, which can be processed into thin shapes and -- since they do not contain silicon -- do not face the supply problems of their silicon-based siblings. Honda began selling these kinds of solar cells in June.

Another possible solution is the dye-sensitized solar cell. These can be made into supple films for attachment to rounded surfaces and are less expensive to make than silicon solar cells.

One other type of solar cell now making waves is the multi-junction solar cell, made from numerous layers of compound semiconductor materials. Sharp has developed a solar cell of this type that boasts 37% efficiency and plans to sell products in Europe this year.

The global market for solar cells is expanding at a rate of 50% each year. But the main problem is the high cost of power generation. A thermal power plant can generate electricity at a cost of 7 yen a kilowatt, while the cost for solar cells is 46 yen. If solar cells are to help Japan reach its target of halving greenhouse gas emissions by 2050, the cost will need to drop down in line with thermal power plants. "What is needed is to break the silicon barrier and attain an efficiency of better than 40% with solar cells," said the Agency for Natural Resources and Energy. Okada believes that quantum dot solar cells could be the answer.

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