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SELF-ASSEMBLING SOLAR CELLS
Crystalline dye and liquid crystal self-organize into photovoltaic thin film
Through combinations of unique compounds and cleverly designed structures, photovoltaic devices made of organic materials are beginning to approach the efficiencies of inorganic solar cells. Now, researchers in England and Germany are reporting a significant advance in constructing organic devices: Their blend of a crystalline dye and a liquid crystal self-assembles into a highly efficient photovoltaic thin film [Science, 293, 1119 (2001)].
"We've taken a set of materials and combined them so that you can carry out simple, energy-efficient solution processing to create an elegant structure," says lead author J. Devin MacKenzie, a visiting scientist at the University of Cambridge's Cavendish Laboratory. In contrast, current vacuum-processing techniques for making inorganic solar cells use huge amounts of energy, while the most promising organic devices to date require multiple fabrication steps.
"An appreciable number of recent publications have shown that organic and polymer solar cells are possibly on the verge of a breakthrough in terms of efficiencies," notes René A. J. Janssen, professor at Eindhoven University of Technology's Laboratory of Macromolecular & Organic Chemistry. He expects the new results to stimulate further research in the use of well-defined organic molecules for solar cell applications.
Successful charge separation and transport in an organic solar cell require materials with complementary electron affinities. MacKenzie, Cambridge graduate student Lukas Schmidt-Mende, and coworkers have been investigating some new hexabenzocoronene compounds synthesized by Andreas Fechtenkötter, Klaus Müllen, and their colleagues at the Max Planck Institute for Polymer Research.
These disk-shaped molecules are liquid crystals at room temperature, stacking up into columns that efficiently conduct charge along their length. To partner with the liquid crystal, the researchers chose a perylene dye that is known to be a good electron acceptor and transport material.
The team combines the materials in a unique process that yields the active layer of a solar cell in a single step. A solution of the two molecules in chloroform is evenly distributed over a solid substrate by spinning. As the solvent evaporates, the perylene crystallizes on top of a liquid-crystalline layer that assembles itself on top of the substrate. The extensive interfacial contact between the organic layers allows efficient separation of the charges induced by light striking the surface. Photodiodes made from the films show external quantum efficiencies of 34%--that is, for every 100 photons that go in, 34 electrons come out.
The development is "a significant step because it paves the way to exploit the superior properties of well-defined molecules with easy processing," Janssen says. "It is too early to tell whether this will result in practical or commercial solar cells, because processing on a large scale, stability, and availability and cost of materials are important factors that have to be addressed."
The team already is checking out variations of the liquid crystals, MacKenzie tells C&EN. "So far, we've only scratched the surface of this simple way to make complicated structures," he says.
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Copyright © 2001 American Chemical Society