David Faiman & Eugene Katz
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One of the more remarkable discoveries of the late 20th century was that nature builds nanostructures in the form of geodesic domes. The prototype molecule of such materials, consisting of 60 carbon atoms arranged in a pattern resembling a soccer ball, was aptly named buckminsterfullerene. However, it soon became apparent that “buckyballs” were only the first of a huge family of so-called fullerenes having a rich variety of shapes and sizes. |
Because it turned out to be so simple to synthesize, the original C60 fullerene received much attention from scientists in many fields of expertise. Our interest, as solar cell specialists was sparked both by its optical properties and by the fact that the material can be synthesized from graphite and purified using nothing but heat–that is without chemicals. The optical properties of C60 have much in common with a theoretically ideal (albeit environmentally questionable) photovoltaic (PV) material, cadmium telluride. This fact, together with the exciting possibility that it may be possible to generate and purify C60 using merely an intense beam of sunlight, suggest that buckyballs might be the ideal material for solar cells.
There has been some controversy in the scientific literature regarding the optical properties of C60. Could one actually make a solar cell from C60, or only a detector of short-wave ultraviolet light?
We resolved that question by actually making a C60 solar cell. We grew a small crystal of C60, coated one side of it with silver, and exposed it to light. It produced an electric current! We then studied its optical properties in a more thorough and scientific manner and were able to understand many of the reasons leading to the controversy in the first place.
But the path to carbon solar cells is still long because buckminsterfullerene is not a conventional semiconductor. Instead, it combines within a single material the properties of both semiconductors and molecular crystals, which necessitates careful studies of all of its physical and chemical properties. One of the problems that we have yet to solve is how to “dope” C60. (Doping is the addition of trace amounts of foreign atoms to a pure semiconductor to achieve the desired electrical properties.) This will be necessary if we are to produce a high-efficiency carbon cell that could compete in performance with silicon cells. While we have a number of ideas that are still being tested, we have still not produced a carbon solar cell with any significant degree of efficiency.
However, a group of European researchers centered in Austria has recently achieved an efficiency of 3% in a fullerene solar cell. Their approach is different from ours. Instead of trying to produce a so-called pn junction cell, as is done in the case of conventional silicon PV cells, the European group bonds buckyballs to a polymer chain, producing, in effect, a plastic solar cell.
Now 3% might not seem very impressive compared to more than 10% efficiency obtained from garden-variety crystalline silicon cells. However, this number must be seen in a correct context. First of all, the earliest silicon cells did not achieve 3% efficiency until long after the PV properties of silicon had been realized. Secondly, 3% already comes quite close to the efficiency of many low-cost amorphous silicon solar cells that are widely marketed today.
Perhaps most exciting is the prospect that plastic solar cells, whatever their ultimate efficiency may be, suggest totally new usage paradigms. The European group talks about “throw-away” solar cells that would be manufactured in huge rolls like today’s polyethylene sheeting. Such material could be made into disposable clothing or other temporary covers, and used to generate power for all kinds of purposes that would not even be considered today because of the high cost of solar cells.
David Faiman is a professor of physics at Ben-Gurion University, and director of Israel’s National Solar Energy Center at Sede Boqer in the Negev Desert. He can be contacted at faiman@bgumail.bgu.ac.il
Eugene Katz is a senior researcher at Ben-Gurion University’s Jacob Blaustein Institute for Desert Research, Sede Boqer.
Image Prof. Sariciftci, University of Linz, Austria.
Reposted from Buckminster Fuller Institute