Sustainable Energy

Printable Solar-Cell Material Reaches a Milestone

A performance boost for “small-molecule” solar cells could make the materials more practical.

A new way of manufacturing printable organic solar cells could eventually lead to new kinds of low-cost, cheap, and flexible solar panels.

The work is being led by Alan Heeger and Guillermo Bazan, both professors of chemistry at the University of California, Santa Barbara. Heeger shared the Nobel Prize in Chemistry in 2000 for developing the kind of conductive polymers that are already used to make plastic solar cells and organic light-emitting diodes.

Polymer solar cells are inefficient compared to silicon solar cells, but they are much cheaper to make. Organic materials—whether made of polymers or so-called “small molecules,” which are organic compounds with a low molecular weight—can be made into inks and printed over large areas. They’re also lightweight and flexible, which makes them promising for applications like rooftop installations or solar-cell patches for charging portable electronics.

Using a new small molecule designed by Bazan, Heeger built a solar cell that converts 6.7 percent of the light energy that strikes it into electricity. Bazan expects to reach 9 percent efficiency within a year. Although efficiencies in lab tests tend to be much greater than those in a manufactured cell, this would put these materials on par with the best polymer solar cells on the market.

Until now, most efforts to improve the performance and cost of organic solar cells have focused on developing new polymer materials.

Bazan used a combination of theory and trial-and-error to develop the new small molecule material. He started by optimizing its electrical properties, so that the molecule would be able to support the high current and voltage needed to get power out of a solar cell. It’s especially tricky to create a small-molecule material that makes a good film; while polymers are long and get tangled into a stable film, small molecules don’t tend to make the kind of planar films needed to make a layer in a solar cell.

After a lot of tinkering on the lab bench, Bazan’s group came up with a suitable small molecule and process for making a high-efficiency small-molecule solar cell. The work is described in the journal Nature Materials. Bazan expects to further improve the performance by tailoring the design of the materials.

Heeger says he had not taken small-molecule solar materials seriously in the past because the performance was dismal. “Other people have utilized small molecules, but the performance was far below that of polymers,” he says.

Still, it may be hard for organic solar cells to become real contenders in the energy market, especially when silicon cells are getting cheaper. “The performance and lifetimes are not there yet,” says Yang Yang, professor of materials science and engineering at the University of California, Los Angeles. Yang is working on polymer solar materials at his company, Solarmer, as well as small-molecule solar cells in his academic lab; his goal is 15 percent efficiency in a lab-made cell. But Yang says the Santa Barbara work is an important demonstration of the potential of small-molecule solar.

As for how these materials will fare in the market, Heeger says, “it’s too soon to know,” but he believes that the efficiencies have reached a respectable level and that these solar materials show promise. “Now we should take them seriously,” he says.

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