BERKELEY, Calif., Nov. 19 (UPI) -- A chance discovery by an international team of researchers could increase dramatically the efficiency of the photovoltaic cells that change sunlight into electricity, thereby bringing solar power closer to a much larger base of consumers, scientists said Tuesday.
For 30 years, science has attempted to overcome a basic deficiency of the cells, said Wladek Walukiewicz, a researcher at the Materials Sciences Division of Lawrence Berkeley National Laboratory in Berkeley, Calif. Existing photovoltaic materials can absorb only narrow bands of light frequencies -- they waste the remainder of the energy as heat.
Attempts by researchers to combine different materials to increase their efficiency have been hampered by chemical and mechanical incompatibilities.
The MSD team, working with colleagues at Cornell University in Ithaca, N.Y., and Japan's Ritsumeikan University in Kyoto, had been investigating indium nitride, a material similar to what is used in current solar cells. However, their studies had a different -- and opposite -- purpose: they were looking for substances that would convert electric current directly into light in the blue spectrum.
Decades-old work from Australia indicated that indium nitride might do the trick, Walukiewicz said, but the researchers could not generate blue light at the expected energy levels.
Further examination led them to discover that indium nitride had a much lower than expected "band gap." That is, it required very little energy to move electrons around in the material -- to create an electric current.
Because indium nitride is so similar to gallium -- the primary photoelectric material -- the team began mixing the two materials in increasing proportions. They found that the mixture can produce electricity from almost the entire spectrum of visible light, Walukiewicz said.
A solar cell lattice based on indium nitride theoretically could transform 50 percent of absorbed light into electricity, Walukiewicz said, compared with 30 percent by today's best cells. A solar cell using multiple layers could approach 70 percent -- considered the absolute upper limit of the photovoltaic process, he said.
Solar cells with such high efficiencies could begin to compete more effectively for a bigger share of the electricity generation market, thereby reducing dependency on fossil fuels and nuclear power, and helping to ease the greenhouse gas emissions that could be contributing to global climate change.
However, before indium nitride cells become a commercial reality, researchers have one primary challenge to solve, Walukiewicz said.
To generate electric current, photovoltaic devices must produce both electrons and "holes." The electrons carry the current, but the spaces are what they fill in their orbits around an atomic nucleus, he said. Indium nitride does not produce the holes easily. But other semiconductor materials have presented similar issues, Walukiewicz said, so it is only a matter of time before indium nitride's challenge is solved.
The material's photovoltaic potential might be even greater when it comes to powering spacecraft, he added. Current solar arrays can be damaged by the high-energy particles produced by the sun, he said, but a combination of gallium and indium nitride might be more resilient.
Other experts are not so sure.
A gallium-indium nitride combination would generate more defects in the material, so considerations of temperature and gravity extremes come into play, said Paul Ostdiek, technology manager in the Space Engineering and Technology Branch of Johns Hopkins University's Applied Physics Laboratory in Baltimore. On the other hand, even if the discovery can only boost efficiency to 40 percent, it is well worth pursuing, he told UPI via e-mail.
"Such an improvement may extend the range of solar-electric-propulsion efficacy (for) exploring the solar system," Ostdiek said. "Military spacecraft would also benefit from having more power in a smaller, stealthier package."
The initial higher costs associated with creating sufficient quantities of the material will delay earthbound applications for the discovery, Walukiewicz told UPI. The vastly improved efficiency, however, could lower the long-term costs of generating power via sunlight, he said.
The discovery should hasten the day when solar power plays a central role in safely meeting the world's energy needs, said Rob Sargent, a senior energy policy analyst with the State Public Interest Research Groups in Boston.
"This is the kind of research we need to be encouraging," Sargent told UPI. "If we spent the kind of money on this that we have on nuclear power, we'd already be at the point of having electricity too cheap to meter."
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(Reported by Scott R. Burnell, UPI Science Correspondent, in Washington)
