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Reaching for Rays

Scientists work toward a solar-based energy system

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7:19pm, May 21, 2007
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In the bright blue skies that he enjoys from his academic perch in southern California, Nathan S. Lewis sees the answer to the world's energy needs. "The sun is the champion of all energy sources," says Lewis, a chemist at the California Institute of Technology in Pasadena. "More energy from the sun hits the Earth in 1 hour than all of the energy consumed by humans in an entire year."

Lewis and other scientists consider the sun's rays the optimal means of satisfying the planet's substantial—and ever-growing—energy habit. In 2001, the world consumed energy at an average rate of more than 13 trillion watts (terawatts, TW), according to the Department of Energy. Taking into account population increases, worldwide economic growth, and conservation and energy-efficiency measures, some researchers predict that the global energy-consumption rate will double by 2050 and triple by the end of the century.

Of that 2001 energy consumption, 86 percent derived from coal, oil, and natural gas. However, evidence tying global warming to the carbon dioxide that these fossil fuels pump into the atmosphere continues to grow (SN: 2/10/07, p. 83). "As we go hunting around for how to replace fossil fuels, solar is the one place where we can see a truly abundant and renewable resource," says A. Paul Alivisatos, a materials scientist at Lawrence Berkeley (Calif.) National Laboratory and the University of California, Berkeley.

Among non-fossil fuel choices, the sun offers by far the deepest energy reserves. To achieve energy generation of 10 TW through nuclear power, a 1-gigawatt electric-power plant fueled by nuclear fission would need to be built every one and a half days for the next 45 years, says Lewis. The remaining exploitable hydroelectric resources around the world could contribute less than 0.5 TW, according to the United Nations. And the Intergovernmental Panel on Climate Change estimates that the total amount of extractable wind power available worldwide is 2 to 4 TW.

The sun, however, showers Earth with energy at a rate of 120,000 TW, notes Lewis. Added together, the other energy sources "aren't even close to the amount of energy the sun gives," he says.

But the planet is taking advantage of only a tiny slice of the sun's largesse. Less than 0.1 percent of the world's electricity came from the sun in 2001, according to the Department of Energy. A major issue is cost. For current silicon-based solar cells, the price of electricity must be around 30 cents per kilowatt-hour to make up for the cost of the installed system, notes Lewis. This can't compete with fossil fuel–derived electricity, which now costs less than 4 cents per kilowatt-hour, he says.

Moreover, "the sun has this nasty habit of going out locally every night," Lewis continues. "Unless you can find a way to cost-effectively store massive amounts of energy, then the sun could only be a peak supplement on a sunny day."

A completely solar-based system would not only create electricity for immediate use but also turn some of the sun's energy into fuel that would power homes or vehicles when the sun isn't shining. Realizing this vision will require breakthroughs in chemistry, physics, materials science, and engineering, scientists say. Some researchers are focusing on solar capture and its conversion to electricity. Others are examining strategies to store that energy in the chemical bonds of hydrogen gas that can later generate electricity in a fuel cell.

If the work succeeds, rolls of electricity-producing solar cells—processed like newsprint—could span rooftops and deserts, while other solar devices churn out hydrogen gas. "Every roof looks at the heavens," says Stephen R. Forrest, an electrical engineer at the University of Michigan in Ann Arbor. "So why can't it be generating energy?"

Creating a buzz

Solar cells already adorn some rooftops. The majority of these panels are made of silicon doped with two materials that create an electric field in the cell. When light strikes the cell, its energy frees electrons within the silicon. Driven by the electric field, the electrons travel to an electrode and thence into an electrical circuit. These cells convert 10 to 20 percent of the solar energy striking them into electricity.

Although the cost of purifying silicon has decreased over the years, scientists don't expect traditional silicon-based solar cells to become competitive with fossil fuels for electricity production.

So, researchers are looking for new solar cell technologies that combine high performance with low cost. "To really impact the [energy] problem, we have to come up with something that scales to big areas," Alivisatos says.

Some groups believe that the answer lies in solar cells composed of organic materials, nanomaterials, or both. Researchers have been developing organic solar cells in the laboratory for the past 20 years. Some of the earliest prototypes have been improved during this time, leading to devices that can convert up to 5 percent of light to electricity. Groups also continue to introduce new models in the quest for the right combination of materials, efficiency, and cost.

A common organic-based approach combines two materials in a film, explains Sean E. Shaheen, a physicist at the National Renewable Energy Laboratory in Golden, Colo. When light hits the cell, the sun's energy creates an exciton—an electron paired with its positively charged counterpart, a hole—in one of the materials, called the donor. But to become electricity, the electron needs to separate from the hole. This split occurs when the electron moves from the donor into the other material, called the acceptor. Freed electrons travel through the acceptor to one electrode, while holes travel to another electrode. Those electrodes act as the poles of a battery do and can power an electric circuit.

Excitons can travel about 5 or 6 nanometers before they decay, so a donor-acceptor boundary should be available every 10 nm, says Shaheen. At the same time, the donor-acceptor mixture must have clear pathways to the electrodes, so that charges don't become trapped in islands of material.

Some of the recent research in organic solar cell technology focuses on ways to introduce a more orderly structure into the system. Forrest and his group make solar cells in which buckyballs—nanoscale cages of carbon—act as the acceptor and an organic material called copper phthalocyanine is the donor. In their latest version of these cells, the researchers formed the two materials into a crystallike network, which improved the mobility of the charges, they reported in March at the American Physical Society meeting in Denver.

A new approach, reported by Alivisatos and his colleagues in the February Nano Letters, combines hyperbranched nanocrystals of cadmium selenide with a polymer. The nanocrystals have diameters of 100 to 200 nm, says team member Neil A. Fromer. This matches the thickness of the composite film. Therefore, an electron released in the nanocrystal has a clear path to its electrode at the film's surface, says Fromer.

The structure of these composites is largely determined by the shapes and sizes of the nanocrystals, which the researchers can control. That makes the devices tolerant of slight variations that occur during mixing, which leads to better reproducibility for solar cells of this type, notes Fromer.

While this is an "exciting time" in solar cell research, says Alivisatos, he cautions that the latest technology remains very much laboratory based. "Nobody yet has achieved the kind of performance that's ultimately needed," he says.

"Our efficiency is too low," agrees Shaheen. Before organic solar cells can be ready for large-scale development, they'll need to demonstrate efficiencies of 10 percent or more, scientists say.

But the potential for low-cost manufacturing will also determine the future of this technology. That's "the promise of the organics," Shaheen says. It may be possible for an assembly process to print hundreds to thousands of square meters of these solar cells per day onto sheets of plastic. "Then, ideally, you have this big roll of solar cells," Shaheen says. "You unroll it, anchor it, plug it in, and you're ready to go."

Holding on to helios

To bask in the sun's energy at night or on a cloudy day requires storage. Current methods to store energy, such as batteries, aren't yet capable of storing the amounts necessary, says Daniel G. Nocera, a chemist at the Massachusetts Institute of Technology (MIT). "You don't want big, heavy batteries that are 10 times the weight of your car," he says.

Another solution—one that offers a much higher energy density, says Nocera—relies on chemical bonds. Plants store the sun's energy this way. A crucial step in photosynthesis is the splitting of water into hydrogen and oxygen. Plants release the oxygen and ultimately store the hydrogen in sugars.

Nocera and other chemists are working to better understand how nature splits water so that they can "build something artificial outside of the leaf," he says. By developing hydrogen- and oxygen-producing catalysts, chemists could use the sun's energy to break the bonds in water.

Engineers have already developed fuel cells that combine hydrogen and oxygen to create electricity. "What we are really interested in doing is making a fuel cell that runs in the reverse direction," says Christopher C. Cummins, an inorganic chemist at MIT.

Among the challenges is that water is a very stable molecule. Chemists know a great deal about reactions that, thermodynamically speaking, move downhill, releasing heat or some other form of energy as they proceed, says Cummins. But splitting water consumes energy. There is a dearth of chemical know-how about such uphill reactions, he notes.

To establish principles critical to water-splitting, Nocera's group has been working with a complex molecule incorporating ruthenium. This compound catalyzes the production of oxygen from water. With the mechanism in place, chemists could try making catalysts with more-abundant metals, such as iron or manganese, Nocera says.

Cummins' team has begun work on oxygen-producing catalysts that contain manganese or cobalt. The researchers are designing a new type of architecture in their catalysts to foster the formation of oxygen-oxygen bonds.

To complete the splitting of water, however, a second catalyst must jump-start a reaction that forms hydrogen gas. Jonas C. Peters of the California Institute of Technology and his colleagues will soon publish results on a hydrogen-producing catalyst that contains cobalt.

Ultimately, researchers would like to pour their newfound knowledge of water splitting into a device. Although this stage of the project would require some engineering and materials science expertise, the basics are as follows. The device would have two catalyst-containing segments—one to produce oxygen, the other to produce hydrogen. A barrier between the two would capture sunlight, much as a solar cell does, to power the reaction.

Working out the science behind artificial photosynthesis "is a hard problem—we shouldn't expect to be running a car on this soon," says Peters. But the good news, he adds, is that "we already know it's chemically doable, because plants do it."

Solar support

As researchers learn to better tap into the sun's rays, solar energy stands to become an important resource for the planet. Whether sunlight becomes the sole source of sustainable energy or works in concert with wind and other renewables, scientists are optimistic that the planet can break its dependence on fossil fuels.

"The research community really wants to work on this problem," says Alivisatos. "If you talk to young students about this, their eyes light up."

Some researchers point out, however, that the funding doesn't match the urgency of the energy situation. "It's incredible how slow we've been as a nation to actually start pumping the kinds of resources toward this problem that are commensurate with the problem," says Peters.

"We should treat energy in research like we treat health," says Lewis. "It's as great a challenge as curing cancer, except that in 20 years, if we don't cure cancer, the world will be the same. If we don't develop ways to provide people with clean, cheap energy, we absolutely know that we will have emitted so much carbon dioxide that the world isn't going to be the same."


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