A Way to Double the Efficiency of
SUNLIGHT is free, but that is no reason to waste it. Yet even the best silicon solar cells -- by far the most common sort -- convert only a quarter of the light that falls on them. Silicon has the merit of being cheap: manufacturing improvements have brought its price to a point where it is snapping at the heels of fossil fuels. But many scientists would like to replace it with something fundamentally better.
John Rogers, of the University of Illinois, Urbana-Champaign, is one. The cells he has devised (and which are being made, packaged into panels and deployed in pilot projects by Semprius, a firm based in North Carolina) are indeed better. By themselves, he told this year's meeting of the American Association for the Advancement of Science, they convert 42.5% of sunlight. Even when surrounded by the paraphernalia of a panel they manage 35%. Suitably tweaked, Dr Rogers reckons, their efficiency could rise to 50%. Their secret is that they are actually not one cell, but four, stacked one on top of another.
Solar cells are made of semiconductors, and every type of semiconductor has a property called a band gap that is different from that of other semiconductors. The band gap defines the longest wavelength of light a semiconductor can absorb (it is transparent to longer wavelengths). It also fixes the maximum amount of energy that can be captured from photons of shorter wavelength. The result is that long-wavelength photons are lost and short-wave ones incompletely utilised.
Dr Rogers gets round this by using a different material for each layer of the stack. He chooses his materials so that the bottom of the band gap of the top layer matches the top of the band gap of the one underneath, and so on down the stack. Each layer thus chops off part of the spectrum, converts it efficiently into electrical energy and passes the rest on.
The problem is that the materials needed to make these semiconductors (including arsenic, gallium and indium) are costly. But Dr Rogers has devised a way to overcome this. Normal solar-cell modules are completely covered by semiconductor, but in his only 0.1% of the surface is so covered. The semiconducting stacks, each half a millimetre square, are scattered over that surface as a matrix of dots, meaning that a panel with an area of 125 square metres has half a million of them. Each stack then has a pair of cheap glass lenses mounted over it. These focus the sun's light onto the stack, meaning that all incident light meets a semiconductor.
The semiconductor stacks themselves are printed onto a cell one layer at a time by a rubber stamp, which picks them up from a crystal wafer of the appropriate material. This wafer has been grown as a series of layers, separated by a substance which can easily be dissolved away. By scoring a chequerboard of cuts through the layers to create squares of the correct size, and then dissolving the filler, layer after layer of semiconductor squares are created, which the rubber stamp peels away and places on the cell. Repeat the process with the other three semiconductors, and package the whole thing with electrical connectors and a transparent protective coat, and -- presto! -- you have a highly efficient solar panel.
Semprius's panels are now being tested at 14 sites around the world. How much they will cost to make when manufacturing is running at full tilt is not yet clear, but Dr Rogers said that Siemens, a big German firm which is one of Semprius's investors, reckons that they have the potential to produce cheaper electricity than coal-fired generators can. Solar energy obviously cannot replace fossil fuels completely until the problem of banking some of what is collected during the day, for use at night, is solved. But at this sort of cost it can make a useful (and unsubsidised) contribution.
The new panels have aesthetic advantages, too. The 99.9% of them not covered by stacks can be used for art. Seen from the sun's point of view (ie, straight ahead), they appear black because the lenses are focused on the stacks, which absorb all the light falling on them. Viewed obliquely, however, their foci are on other parts of the panel. The result, as the picture shows, can be quite pleasing -- and certainly prettier than a coal-fired power station.
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