New method lights a path for solar cells

Using a technique in which chemical ingredients assemble themselves, a research team has developed a potentially inexpensive way of making solar cells. So far, high cost has hampered the large-scale use of solar energy systems.

A new organic film for making solar cells. Atomic-force microscopy shows it at two magnifications. Science

Solar cells, also known as photovoltaic cells, transform photons into electric current without producing pollution. Commercially available solar cells already convert sunlight efficiently enough for certain applications, such as satellites, notes J. Devin MacKenzie of the University of Cambridge in England.

The widespread use of solar power has been elusive because it can be difficult and costly to manufacture the commercial photovoltaic cells, which are made of inorganic crystals such as silicon, MacKenzie says.

Some researchers have tried to circumvent this problem by creating photovoltaic cells from organic components that they can process as easily as plastics. Some of these cells remain in various stages of development, but none are close to becoming commercially successful.

In the Aug. 10 Science, MacKenzie and his colleagues report a development that they say could develop into a particularly simple way of making organic solar cells. Their new cell–which they claim is nearly as efficient as the best organic solar cells already made–contains a liquid crystal called peri-hexabenzocoronene and an organic dye called perylene. When poured onto a flat surface, the ingredients arrange themselves into a two-layered film with an internal structure that promotes electron flow.

This simple self-assembly offers a potential new route toward low-cost, mass production of organic solar cells, comments Jenny Nelson of Imperial College in London. Other organic solar cells currently operate a bit more efficiently, she says, but “potentially, [the new technique] could lead to better performance.”

“This is a beautiful example of combining molecular engineering with nanoscale self-assembly,” adds Alan J. Heeger of the University of California, Santa Barbara and a recent Nobel laureate for his work on conductive polymers (SN: 10/14/00, p. 247). “They have made an important step forward toward the goal of low-cost, high-efficiency, organic photovoltaic cells.”

In general, organic cells offer certain benefits over inorganic ones, Nelson says. For example, organic cells might be made in different colors and be flexible enough to use on window blinds, walls, and other materials in buildings. This provides design options that could lower the cost of using the cells.

High hurdles remain, however. For one, the efficiency of organic solar cells doesn’t approach that of the harder-to-make inorganic ones. The best organic cells convert a little more than 2 percent of sunlight into electric current, while commercial inorganic cells reach efficiencies of 20 percent, Nelson says.

It remains to be seen whether the new technique, others in development, or a completely unforeseen one will eventually win out, says Nelson.

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