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Solar Energy from Cheap Materials - 3 Innovations

From the Economist

Solar Energy from Cheap Materials - Innovation 1

Kwanghee Lee of Pusan National University, in South Korea, and Alan Heeger of the University of California, Santa Barbara, work on solar cells made of electrically conductive plastics. (Indeed, Dr Heeger won a Nobel prize for discovering that some plastics can be made to conduct electricity.) They found that by adding titanium oxide to such a cell and then baking it in an oven, they could increase the efficiency with which it converted solar energy into electricity.

The trick is to put the titanium oxide in as a layer between the part of the cell where the electrons are liberated and the part where they are collected for dispatch into the wider world. This makes the electrically conductive plastic more sensitive to light at wavelengths where sunlight is more intense. Pop the resulting sandwich in the oven for a few minutes at 150°C and the plastic layer becomes crystalline. This improves the efficiency of the process, because the electrons find it easier to move through crystalline structures.

The technique used by Dr Lee and Dr Heeger boosts the efficiency of plastic cells to 5.6%. That is still poor compared with silicon, but it is a big improvement on what was previously possible. Dr Lee concedes that there is still a long way to go, but says that even an efficiency of 7% would bring plastic cells into competition with their silicon cousins, given how cheap they are to manufacture.

Solar Energy from Cheap Materials - Innovation 2

A second approach, taken by Michael Grätzel of the Swiss Federal Institute of Technology, is to copy nature. Plants absorb solar energy during photosynthesis. They use it to split water into hydrogen ions, electrons and oxygen. The electrons released by this reaction are taken up by carrier molecules and then passed along a chain of such molecules before being used to power the chemical reactions that ultimately make sugar.

Dye-sensitised solar cells seek to mimic this assembly line. The dye acts like chlorophyll, the pigment that makes plants green and that is responsible for absorbing sunlight and liberating electrons. The electrons are passed via a semiconductor to an electrode, through which they leave the cell. By using a dye called phthalocyanine, which absorbs not only visible light but also infra-red wavelengths, Dr Grätzel has been able to raise the efficiency of the process to 11%. That, he says, should be enough to make dye-sensitised cells competitive with silicon.

Solar Energy from Cheap Materials - Innovation 3

The third technique, being developed by Prashant Kamat of the University of Notre Dame, Indiana, and his colleagues, uses that fashionable scientific tool, the carbon nanotube. This is a cylinder composed solely of carbon atoms, and one of its properties is good electrical conductivity. In effect, nanotubes act as wires a few billionths of a metre in diameter.

Dr Kamat and his team covered the surface of an experimental cell made of cadmium sulphide, zinc oxide and titanium dioxide with nanotubes, so that the tubes stuck up from the surface like hairs. The tubes then eased the passage of the liberated electrons from the cell to the electrode that collected them. Using this technique doubled the efficiency of Dr Kamat's cell from 5% to 10% at ultraviolet wavelengths and he reckons it would create similar increases in efficiency in both plastic and dye-based cells.

Such a boost would take novel solar cells closer to becoming a commercial reality. And that would be a very good thing. Production of solar cells has increased by 32% a year, on average, for the past decade and jumped by 45% in 2005. That sounds impressive, but it has been achieved largely by subsidies from the governments of Germany, Japan and California. Only in places unconnected to an electricity grid, such as much of rural Africa and rural Asia, are solar cells truly commercially viable. But if the price were to come down because efficient cells could be made from cheap materials, that could change quickly. The rest of the world would then be able to join the poor of Africa and the rich of California, and generate solar power for itself.

Our Comments: We believe the second innovation would succeed... we imitate nature...