Broadly speaking, there are two types of photovoltaic materials: traditional inorganic solar cells, which are stiff (sometimes to the point of being brittle) and often expensive to make, but have decent efficiency of around 25-35% (with the potential for up to 50-60% with current research); and organic polymer solar cells, which are flexible (sometimes to the point of being able to be sprayed or painted on a surface) and relatively inexpensive to produce, but tend to have relatively short lifespans (generally no more than a couple of years, and sometimes far worse) and very low efficiency of around 3-5%. Ilan Gur, working at the Lawrence Berkeley National Laboratory, may well have found a best-of-both-worlds solution: nanocrystal solar cells.
In the current Science magazine, Gur (a UC Berkeley doctoral candidate) and his research group report on the development of ultra-thin inorganic photovoltaic semiconductors using nano-scale crystals. The material can be cast from solution, like organic photovoltaics, meaning the nanocrystal solar cells are far less costly to make than traditional silicon cells. Unlike the organic pv materials, however, the nanocrystal solar cells respond to a wide range of light frequencies, and can last for years. In fact, aging seems to increase the performance of the nanocrystal cells, rather than degrade it:
In this paper, the researchers describe a technique whereby rod-shaped nanometer-sized crystals of two semiconductors, cadmium-selenide (CdSe) and cadmium-telluride (CdTe), were synthesized separately and then dissolved in solution and spin-cast onto a conductive glass substrate. The resulting films, which were about 1,000 times thinner than a human hair, displayed efficiencies for converting sunlight to electricity of about 3 percent. This is comparable to the conversion efficiencies of the best organic solar cells, but still substantially lower than conventional silicon solar cell thin films.
Ah, yes. The drawback, at least for now, is that the efficiency of the nanocrystal solar is no better than that of polymer organic photovoltaics. While this is disappointing, Gur indicates that increasing the efficiency of the material is a primary future goal. And the value of stable thin-film solar cells shouldn't be discounted; as one of the likely applications for this kind of material is as a coating for building and vehicle surfaces, they'll need to be as weather-resistant as possible, and not require frequent "touch-ups."
For now, the nanocrystal solar material goes on the pile of potentially-transformative breakthroughs: keep an eye on them, but don't expect to see them at Costco any time soon.
