Carbon-based materials such as Heeger’s polymers could steal markets away from conventional semiconductors because they can be applied in even thinner layers and, in theory at least, could lead to simpler and less expensive manufacturing processes. For example, they can be dissolved to produce a photovoltaic ink, which an ink-jet printer could squirt in a thin film on a variety of surfaces.
The underlying technology has been around for years: researchers at Eastman Kodak created the first organic solar cells during the energy crisis of the 1970s. Kodak was already churning out vast quantities of photographic film containing light-absorbing organic dyes and figured it could adapt those dyes to capture energy instead of images. The project fell to Ching Tang, a physicist fresh out of graduate school. Tang struggled for four years and nearly gave up before a breakthrough in 1979, when he borrowed a set of organic pigments developed for other purposes by Kodak’s chemists and layered them to mimic the arrangement of electron-shuttling semiconductors in conventional photovoltaics. While these first organic solar cells could convert only one percent of the sun’s energy into electricity, they showed promise for improvement.
But Tang kept his success quiet until 1987, because Kodak was close to developing other commercial uses for the proprietary pigments and forbade him from publishing his results. By the time he did, the energy crisis had passed and Kodak was onto a seemingly more enticing opportunity: using Tang’s layered structure to turn similar pigments into organic light-emitting diodes for flat-panel computer displays (see “A Bright Future for Displays,” TR April 2001). For over a decade, Kodak and its competitors nurtured this technology, which is now poised to take a piece of the $25 billion-per-year flat-panel market, while organic solar cells were forgotten.
With yet another energy crisis looming, however, organic solar cells are enjoying a renaissance. After two decades stuck at Tang’s one percent power output, researchers are succeeding in pushing the boundaries of organic solar cells’ performance, and investment in the field is soaring. Recently, research groups produced solar cells that can convert two to 4.5 percent of the energy in sunlight to electricity. They are bullish about matching the power of the low-end thin-film photovoltaics in as little as three years. “We’re creeping up on amorphous silicon and there’s no reason to believe that we couldn’t do as well as crystalline silicon,” says Princeton’s Forrest.
Uniax, a company cofounded by Alan Heeger and acquired by DuPont two years ago, is taking a somewhat different tack, developing solar cells using polymer blends. And rather than immediately adapting the polymers to make solar cells for powering entire homes, Uniax is first testing the materials as photodetectors in imaging devices like scanners and digital video cameras. Photodetectors consist of arrays of millions of tiny solar cells; the cells reconstruct images by creating electrical currents proportional to the intensity of light shining on them. Each cell represents one pixel of information. Heeger says Uniax has already developed polymer-based photodetectors that rival the sensitivity of commercial photodetectors, which employ standard semiconductors. And unlike conventional devices, plastic photodetectors can be built to larger scales, say for flexible sensors that capture images from sheets of paper without scanning, or still larger detectors for rapid medical imaging.
Manufacturing the plastic solar cells could be relatively quick and easy. Heeger envisions using ink-jet printers to spray a series of films on a surface-the organic semiconductors, electrodes and protective coatings-to fashion a photodetector. And fabricating large devices for power generation could be even simpler, since the light-absorbing films do not need to be divided into pixels. “Making large areas of a thin film from these organic semiconductors in solution is straightforward. That’s why it’s attractive,” says Heeger.
First-generation organic solar cells could begin to enter the market in the next five to 10 years through applications like Heeger’s photodetectors, or alternatively through ultra-low-margin products such as solar-powered musical cards and other disposable electronics. Then they could tackle small electronic devices, such as solar-powered calculators and toys.
But to ready organic solar cells for the rooftop, developers must overcome the material’s ultimate weakness: its fragility. The light-sensitive organic molecules under development for use in photovoltaics break down when exposed to oxygen. Will they ever be ready to bake under the sun day in, day out for several decades and still generate electricity? Kodak’s Tang says the question reminds him of his own early doubts about organic light-emitting diodes. He says he wondered 15 years ago why anyone would bother trying to make a device out of this highly unstable material. Today he marvels at the colorful organic displays entering the market.
What Tang couldn’t see 15 years ago was that solar cells could be encapsulated with a polymer similar to Teflon that is all but impervious to the elements and provides a hermetic seal for the fragile organics. Encapsulated organic solar cells could already provide the several thousand hours of working life required of a solar-powered calculator or digital video camera, and the materials’ utility could be extended to the hundreds of thousands of working hours required for providing buildings with electrical power.
Kodak is no longer pursuing the idea of organic solar cells, but Tang dreams of returning to research that could help make them a reality. While beautiful displays may be more lucrative in the short term, Tang says the challenge of replacing fossil fuels is more pressing. “People can do without television,” says Tang, “but you cannot do without energy.”
Playing the Sun
A sampling of companies developing new thin-film and organic solar cells.
Company Location Technology Materials BP Solar Linthicum, MD Thin film Amorphous silicon, cadmium telluride Energy Photovoltaics Princeton, NJ Thin film Amorphous silicon, copper indium gallium diselenide Siemens Solar Munich, Germany Thin film Copper indium diselenide Cambridge Display Technologies Cambridge, England Organic Pigments and organic liquid crystals DuPont Displays’ Uniax Santa Barbara, CA Organic Polymers Global Photonic Energy Ewing, NJ Organic Pigments and fullerenes Quantum Solar
Energy Linz Linz, Austria Organic Polymers and fullerenes