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Using nanotechnology to separate electrons and holes in thin-film solar cells

A nanostructure called a “gyroid” self-assembled from diblock copolymers provides a scaffold with the proper dimensions so that when light knocks electrons loose from a dye molecule, the electrons and the holes left behind can be separated to obtain an electric current, providing the basis for a more efficient, inexpensive nanotech solar cell. PhysOrg.com points us to this Cornell University news article by Bill Steele, “Nanomanufactured polymer film could lead to lower-cost solar cells“:

You never know where basic research may lead. For decades materials scientists have been experimenting with a corkscrew-like polymer structure called a gyroid. Now an international team of researchers has shown that the gyroid structure can be used to “self-assemble” a low-cost photovoltaic cell.

The idea could lead to more economical solar collectors and more efficient fuel cells.

The prototype is a variation on the Graetzel solar cell, which uses an organic dye sandwiched between two conductors. Forming the conductors into an interlocking corkscrew allows current to be transported out efficiently. The team’s first cell, made in a thin film 400 nanometers thick, has a conversion efficiency ranging from 0.7 to 1.7 percent — low compared with silicon-based photocells, but “amazing” for such a thin film, said Ulrich Wiesner, the Spencer T. Olin Professor of Materials Science and Engineering at Cornell.

“The next step is to make it thicker” so more of the light falling on it can be captured, he said. “We hope that we will eventually get efficiencies that rival silicon-based devices.” Currently available silicon-based solar cells convert about 15 percent of the energy of the light falling on them to electricity, although some new designs promise higher efficiencies.

The work by Wiesner and scientists at the Universities of Cambridge and Oxford in Britain, the Freiburg Institute for Advanced Studies in Germany, Institute Curie in France and the University of Minnesota, Minneapolis, is described in the online version of the journal Nano Letters [abstract] ….

…the researchers assembled a copolymer gyroid film, then dissolved away just the corkscrew part of the structure, leaving a corkscrew-shaped mold that they filled with titanium oxide. Heating then burned off the other polymer part and crystallized the titanium oxide into a solid structure that conducts electrons. This was coated with a light-sensitive dye, and finally the space around it was filled with a material that conducts “holes” (positive charges).

When light strikes the dye it knocks loose electrons, which flow into the titanium oxide framework, while the holes left behind flow into the other conductor. Electrodes above and below the film carry off the resulting current.

The secret of a solar cell, Wiesner explained, is that the electron-hole pairs must find the interface between the two conductors within 10 nanometers (about the width of 30 atoms) so they can separate and flow away, or they will recombine.

“This is why block copolymers are exciting,” Wiesner said, “because that is the characteristic length scale of separation of the two blocks.”

—Jim

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