Cost-efficient and energy-efficient solar cells could be right around the corner. These so-called “third-generation” photovoltaics begin to emerge nowadays and dismiss a theory stated in 1961 by William Shockley and Hans Queisser, who showed that there is a theoretical 31 percent limitation in what regards the maximum efficiency for a single solar cell.
Bruce Parkinson and Justin Sambur from the University of Wyoming in Laramie, joined by Thomas Novet from Voxtel in Beaverton, OR, used quantum dots to enhance the possibilities of a solar cell in specific conditions, namely when incoming photons have more energy than to displace only one electron off its atom. So far, no matter how strong the light was, it had only been able to free one electron, with the rest being mostly lost as heat.
The theory that the scientists put into practice has the power to eventually double the harvested current and to increase the efficiency of solar cells proportionally.
They coated a smooth titanium dioxide electrode (also used in dye-sensitized solar cells) with a single layer of lead-sulphide quantum dots. They chose the quantum dots so that they should absorb 0.85 and 1.39 electronvolts (eV) to free up one electron.
When they illuminated the device with reddish light, in which every photon carries 1 to 2 eV, 70 to 80 percent of the relatively small number of photons absorbed released an electron. When they switched to a more energetic area of the spectrum, in blue light, the photons carried over 2.4 eV, which was 2.7 times more than they needed to free up an electron.
At this point, their theory had been transformed into practice: the displaced electrons had been twice the number of photons absorbed, which confirms the presumption that some of them had indeed freed up two electrons at once.
The new discovery could be very well applied to dye-sensitized (aka Graetzel) solar cells, because they are super-cheap and not so efficient these days (up to 11 percent). Replacing the organic dyes with quantum dots would bring the cells to a whopping 15 percent, which would be satisfactory for their price.
Only the issue posed by lead remains in my mind as an ecological threat. Materials others than lead, though, can be discovered and used in the near future.
[via new scientist]
