To use current silicon technology at the thickness (200 microns) that it is employed in the present generation of solar cells is not feasible on the grand scale. The main problem is how quickly high grade silicon wafers can be fabricated and I calculated previously that we would need around 100 times the present production capacity to be installed over 20 years to make a real dent in the anticipated shortfall of future world electricity production. Thin-film cells, which use perhaps 100 times less materials, and in more accessible amorphous forms rather than crystalline wafers, would represent a considerable advantage in terms of raw material requirements, but this technology needs further refinement.
In contrast to such devices based on "mineral", inorganic, semiconductor materials, there is the possibility of organic cells, which instead are made from carbon molecules. A conventional photovoltaic (PV) cell consists of a silicon wafer of thickness 200 microns (one fifth of a millimeter), to be compared with a human hair which is around 70 microns thick. This is treated with other materials to form a double-layer structure which is known in electronics as a p-n junction. Photons of light are absorbed by the silicon causing a flow of electrons and a hence a small electric current. In an organic cell, the double layer is made from two ultra-thin (100 nanometer or 0.1 micron) films of organic conducting polymers, embossed onto glass.
A prototype organic cell has been developed by Neil Cavendish at Cambridge University, and one about the size of your hand can produce enough electricity to run a pocket calculator. Most standard solar cells operate with a light-to-electricity conversion efficiency of around 10 - 15%, but organic cells have proved so far much less efficient, perhaps only 3 - 4%. However, they are much cheaper to produce. Indeed, Paul O'Brien at Manchester University thinks that solar cells need be no more expensive to make than high-performance self-cleaning glass. He said: "We're very interested in solar cells where we take an organic layer that's printable or sprayable containing an inorganic mineral like lead sulphide which will actually do the photon capture."
Indeed, lead sulphide can be fabricated into minute "nanorods", perhaps 100 nanometers in length and 20 nanometres in cross section, and can be dispersed in the semiconducting polymer, hence releasing electrons within the material, which can then conduct them carrying an electric current. All researchers in the field stress the need to move away from carbon-based fossil fuels in order to mitigate climate change, or in my view, more urgently to use less of the cheap oil and gas that we are running short of. In principle, cheap solar cells could be incorporated into the walls and roofs of buildings in the form of building integrated photovoltaics (BIPV), as a means to bear the burden of costs yet further. O'Brien reckons that the new solar cell technology might cost as little as one hundredth as much as silicon cells, and that will surely provide an incentive for further development.
Nonetheless, the clock is ticking away toward the Oil Dearth Era, and any such technologies need to be installed quickly, if we are to avoid a massive energy crunch, especially if electricity is implemented in various strategies to keep transportation running. There is also the issue of other materials such as platinum, which will be needed on a massive scale to underpin such innovations - another potential bottleneck in the shifting schemes of potential "solutions" to the impending and unavoidable energy crisis.
"How solar power could become organic - and cheap," By Michael Pollitt, The Independent, 29-11-07.