Solar Cell Items of Interest - KeelyNet 12/22/01
Hydrogen & Current from Solar Cells
Here's a recipe for a cleaner, healthier planet: Take some water, add solar energy, extract hydrogen, and use it to power fuel cells for running cars and other machines. Then, collect their water emissions and start the procedure again. ...processes that use sunlight to extract hydrogen remain costly and inefficient, fossil fuels still supply the hydrogen in most fuel cells.
Hoping to break the fossil fuel habit, a team of Israeli, German, and Japanese scientists has created a device that boosts the efficiency of solar-powered hydrogen extraction by 50 percent. The group placed a photovoltaic cell on top of two flat, finger-long electrodes.
The combination "is very efficient in converting solar energy [into an electric current] but also provides nearly the ideal voltage for splitting water" into hydrogen and oxygen, says team leader Stuart Licht of the Technion in Haifa, Israel. A water molecule splits, or undergoes electrolysis, at only 1.23 volts. Licht and his colleagues describe their device in the Sept. 14 Journal of Physical Chemistry B.
The gadget converts sunlight to an electrolysis current with 18.3 percent efficiency. In turn, the current creates hydrogen gas as it passes through acidic water. The device is "showing the pathway towards higher efficiencies for direct solar-to-hydrogen production," comments John A. Turner of the National Renewable Energy Laboratory (NREL) in Golden, Colo.
The newly achieved efficiency may already be high enough for commercial hydrogen generators to be feasible. "That still needs to be figured out," Turner says. In 1998, he and Oscar Khaselev, then also of NREL, demonstrated a novel apparatus for solar-to-hydrogen conversion (SN: 4/18/98, p. 246). To achieve unprecedented efficiency, the device used multiple layers of semiconductor materials.
The researchers arranged the layers to form two active regions, or junctions, that would absorb solar photons that dislodge electrons. Some of the less energetic photons weren't captured in the first junction but passed to the second, where they generated more current. The design gained an energy advantage by combining solar electricity and water splitting into one unit.
Their cell's 12.4 percent efficiencynearly twice that of any previous solar-to-hydrogen devicehas held as the record until now. Licht and his colleagues have improved upon that pioneering effort in several crucial ways. In one sense, the NREL device was all wet: It had to be completely immersed in water to operate. That feature forced the researchers to select semiconductors that wouldn't break down in solution.
By keeping their stack of semiconductor layers high and dry, Licht and his group were free to optimize them for both converting sunlight to electricity and water splitting. Their design permits a low electrolysis current, which also reduces energy waste. Licht and his coworkers say that besides besting the solar-to-hydrogen conversion record, their work opens the way to efficiencies not considered possible before.
Using measured photoelectric efficiencies of seven semiconductor combinations not yet tested in hydrogen generation, they predict maximum solar-to-hydrogen conversion efficiencies of up to 31 percent. Thermodynamics theory says the maximum could range above 40 percent for a two-junction converter, but no one has previously predicted better than 24 percent performance for practical devices, Turner says. Experimentally achieving the new prediction "would be an accomplishment indeed!" he adds.
Chemical Engineering - Plastic Power
Polymers take a step forward as photovoltaic cells and lasers For nearly 20 years, scientists have expected great things from semiconducting polymers--chimerical chemicals that can be as pliable as plastic wrap and as conductive as copper wiring. Indeed, these organic compounds have conjured dreams of novel optoelectronic devices, ranging from transparent transistors to flexible light-emitting diodes.
Few of these ideas have made it out of the laboratory. But in the past year, researchers have added two promising candidates to the wish list: solar cells and solid-state lasers. The lasting appeal of these materials--also called synthetic metals--is that they are more durable and less expensive than their inorganic doubles. Furthermore, they are easy to make.
Like all plastics, they are long, carbon-based chains strung from simple repeating units called monomers. To make them conductive, they need only be doped with atoms that donate negative or positive charges to each unit. These charges clear a path through the chain for traveling currents. Scientists at Advanced Research Development in Athol, Mass., have made plastic solar cells using two different polymers, polyvinyl alcohol (PVA) and polyacetylene (PA).
Films of this copolymer, patented as Lumeloid, polarize light and, in theory at least, change nearly three quarters of it into electricity--a remarkable gain over the 20 percent maximum conversion rate predicted for present-day photovoltaic cells. Lumeloid also promises to be cheaper and safer. Alvin M. Marks, inventor and company president, estimates that whereas solar cells now cost some $3 to $4 per watt of electricity produced, Lumeloid will not exceed 50 cents. The process by which these films work resembles photosynthesis, Marks explains.
Plants rely on diode structures in their leaves, called diads, that act as positive and negative terminals and channel electrons energized by sunlight. Similarly, Lumeloid contains molecular diads. Electrodes extract current from the film's surface. To go the next step, Marks is developing a complementary polymer capable of storing electricity.
"If photovoltaics are going to be competitive, they must work day and night," he adds. His two-film package, to be sold in a roll like tinfoil, would allow just that. Plastics that swap electricity for laser light are less well developed, but progress is coming fast. Only four years ago Daniel Moses of the University of California at Santa Barbara announced that semiconducting polymers in a dilute solution could produce laser light, characterized by a coherent beam of photons emitted at a single wavelength.
This past July, at a conference in Snowbird, Utah, three research teams presented results showing that newer polymer solids could do the same. "I'm a physicist. I can't do anything with my hands," says Z. Valy Vardeny of the University of Utah, who chaired the meeting. "But the chemists who have created these new materials are geniuses." Earlier generations of semiconducting polymers could not lase for two main reasons.
First, when bombarded with electricity or photons, they would convert most of that energy into heat instead of light--a problem called poor luminescence efficiency. Second, the films usually absorbed the photons that were produced, rather than emitting them, so that the polymers lacked optical gain--a measure of a laser medium's ability to snowball photons into an intense pulse.
Because the newer materials have fewer impurities, they offer much higher luminescence efficiencies and show greater lasing potential, Vardeny states. In the Japanese Journal of Applied Physics, his group described a derivative of poly (p-phenylenevinylene), or PPV, with a luminescence efficiency of 25 percent. The red light was composed of photons having the same wavelength, but it did not travel in a single beam.
In Nature, another group from the Snowbird meeting offered a way around this shortcoming. Richard H. Friend and his colleagues at the University of Cambridge placed a PPV film inside a device called a microcavity. Mirrors in the structure bounced the emitted light back and forth, amplifying it into a focused laser beam. The third group from Snowbird, led by Alan J. Heeger of U.C.S.B., tested more than a dozen polymers and blends as well.
Their results, which appeared in the September 27 issue of Science, show that these materials can emit laserlike light across the full visible spectrum--even in such rare laser hues as blue and green. In place of a microcavity, Heeger set up his samples so that the surrounding air confined the emitted photons to the polymer, where they could stimulate further emissions.
"We wanted to show that a whole class of materials do this and that they definitely provide optical gain," Heeger says. The challenge now will be finding a way to power these polymers electrically. All three groups energized their samples using another laser, but practical devices will need to run off current delivered from electrodes. It is no small problem.
Vardeny notes that electrical charges generate destructive levels of heat and that electrodes can react chemically with the film, lowering the polymer's luminescence efficiency. "It's going to be hard," Heeger concurs, "but I'm optimistic."
Personal power: Solar Utility in a Backpack
Toby Kinkaid, founder and CEO of Solardyne Corporation, developed solar power to become portable with this new power pack. The U.S. Army is testing one at its South Pole station. Greenpeace International has ordered some for use in India. A dive boat operator is using one to run compressors and lights in the Caribbean.
A solar power unit that can be carried in a backpack was created by a Portland, Oregon, inventor and released just three months ago. It is already making its way around the world. The Solar Power Pack contains a folding monocrystalline solar panel, battery, controller, plugs, cords and light. It weighs only 24 pounds but provides users with 120 watt-hours of power a day. The unit can power AC and DC electronics up to 300 watts. It can be used in recreational vehicles as well as for field research, emergency home power, disaster relief and international aid.
After charging for six hours with the unit's solar photovoltaic panel, the Solar Power Pack can run a laptop computer for three hours or its own high-efficiency light for 14 hours. "The Solar Power Pack is a personal solar power utility designed to be operated and transported by a single person," said Toby Kinkaid, founder and CEO of Solardyne Corporation, a developer and on-line retailer of renewable energy technology and high efficiency appliances.
An international traveller, Kinkaid came up with the idea for the solar backpack when he ran out of camera batteries while exploring Malaysia and the Maldives. People in the Maldives, an island nation in the Indian Ocean, use noisy diesel generators for power, said Kinkaid. Getting fuel there is difficult, as it is in all remote areas. The Solar Power Pack weighs only 24 pounds but provides users with 120 watt-hours of power a day.
Kinkaid studied physics in university and has been running a solar laboratory for 18 years. To reduce the cost of expensive solar electric cells, he developed a process that intensifies sunlight before it is converted into electricity. His Mariposa solar module uses reflectors to concentrate twice the amount of solar energy onto half the number of the solar cells.
"Reflectors cost $1 per square foot, he says. Solar cells cost $30 per square foot. The result is a solar pack that sells for $549. The solar panel is designed to last 20 years. The battery lasts for 600 charge cycles, which equals about two years if the system is used daily. Once spent, the battery can be replaced and recycled. Kinkaid discovered that he could rely on his invention when the battery in his old BMW car ran out of juice. "The solar pack unit was not even fully charged," he said, "but I put on the cable and jump-started my car."
Solardyne has sold four dozen solar backpacks to date. At its South Pole station, the Army is testing one as the six months of annual sunlight come to an end. Greenpeace ordered 20 units for use in India. Solardyne has added a converter for Indian power that runs at a higher voltage than power in the United States. The package includes a water sterilizer powered by the solar pack that decontaminates tap water using ultraviolet rays.
"We are particularly excited about the prospect of humanitarian organizations using the Solar Power Pack for their relief efforts," Kinkaid said. "Imagine the difference these groups can make in people's lives by taking a portable source of ready power to Third World nations."
Solar Cell Paint
University of California, Berkeley, chemists have found a way to make cheap plastic solar cells flexible enough to paint onto any surface and potentially able to provide electricity for wearable electronics or other low-power devices. The group's first crude solar cells have achieved efficiencies of 1.7 percent, far less than the 10 percent efficiencies of today's standard commercial photovoltaics.
The best solar cells, which are very expensive semiconductor laminates, convert, at most, 35 percent of the sun's energy into electricity. "Our efficiency is not good enough yet by about a factor of 10, but this technology has the potential to do a lot better," said A. Paul Alivisatos, professor of chemistry at UC Berkeley and a member of the Materials Science Division of Lawrence Berkeley National Laboratory. "There is a pretty clear path for us to take to make this perform much better."
Alivisatos and his co-authors, graduate student Wendy U. Huynh and post-doctoral fellow Janke J. Dittmer, report their development in today's issue of Science. "The beauty of this is that you could put solar cells directly on plastic, which has unlimited flexibility," Dittmer said. "This opens up all sorts of new applications, like putting solar cells on clothing to power LEDs, radios or small computer processors."
The solar cell they have created is actually a hybrid, comprised of tiny nanorods dispersed in an organic polymer or plastic. A layer only 200 nanometers thick is sandwiched between electrodes, and can produce, at present, about 0.7 volts. The electrode layers and nanorod/polymer layers could be applied in separate coats, making production fairly easy. And unlike today's semiconductor-based photovoltaic devices, plastic solar cells can be manufactured in solution in a beaker without the need for clean rooms or vacuum chambers.
"Today's high-efficiency solar cells require very sophisticated processing inside a clean room and complex engineering to make the semiconductor sandwiches," Alivisatos said. "And because they are baked inside a vacuum chamber, they have to be made relatively small." The team's process for making hybrid plastic solar cells involves none of this. "We use a much dirtier process that makes it cheap," Huynh said.
The technology takes advantage of recent advances in nanotechnology, specifically the production of nanocrystals and nanorods pioneered by Alivisatos and his laboratory colleagues. These are chemically pure clusters of from 100 to 100,000 atoms with dimensions on the order of a nanometer, or a billionth of a meter. Because of their small size, they exhibit unusual and interesting properties governed by quantum mechanics, such as the absorption of different colors of light depending upon their size.
It was only two years ago that a UC Berkeley team led by Alivisatos found a way to make nanorods of a reliable size out of cadmium selenide, a semiconducting material. Conventional semiconductor solar cells are made of polycrystalline silicon or, in the case of the highest efficiency ones, crystalline gallium arsenide. Huynh and Dittmer manufactured nanorods in a beaker containing cadmium selenide, aiming for rods of a diameter -- 7 nanometers -- to absorb as much sunlight as possible. They also aimed for nanorods as long as possible -- in this case, 60 nanometers.
They then mixed the nanorods with a plastic semiconductor, called P3HT - poly-(3-hexylthiophene) -- and coated a transparent electrode with the mixture. The thickness, 200 nanometers -- a thousandth the thickness of a human hair -- is a factor of 10 less than the micron-thickness of semiconductor solar cells. An aluminum coating acting as the back electrode completed the device.
The nanorods act like wires. When they absorb light of a specific wavelength, they generate an electron plus an electron hole -- a vacancy in the crystal that moves around just like an electron. The electron travels the length of the rod until it is collected by the aluminum electrode. The hole is transferred to the plastic, which is known as a hole-carrier, and conveyed to the electrode, creating a current. P3HT and similar plastic semiconductors currently are a hot area of research in solar cell technology, but by themselves these plastics are lucky to achieve light-conversion efficiencies of several percent.
"All solar cells using plastic semiconductors have been stuck at two percent efficiency, but we have that much at the beginning of our research," Huynh said. "I think we can do so much better than plastic electronics." "The advantage of hybrid materials consisting of inorganic semiconductors and organic polymers is that potentially you get the best of both worlds," Dittmer added.
"Inorganic semiconductors offer excellent, well-established electronic properties and they are very well suited as solar cell materials. Polymers offer the advantage of solution processing at room temperature, which is cheaper and allows for using fully flexible substrates, such as plastics." Visiting scientist Keith Barnham, professor of physics at Imperial College, London, and an expert on high-efficiency solar cells, agreed. "This is exciting, cheap technology if they can get the efficiency up to 10 percent, which I think they will, in time," Barnham said.
"Paul's approach is a very promising way to get around the problem of the efficiency of plastic solar cells." Some of the obvious improvements include better light collection and concentration, which already are employed in commercial solar cells. But Alivisatos and his colleagues hope to make significant improvements in the plastic/nanorod mix, too, ideally packing the nanorods closer together, perpendicular to the electrodes, using minimal polymer, or even none -- the nanorods would transfer their electrons more directly to the electrode.
In their first-generation solar cells, the nanorods are jumbled up in the polymer, leading to losses of current via electron-hole recombination and thus lower efficiency. They also hope to tune the nanorods to absorb different colors to span the spectrum of sunlight. An eventual solar cell might have three layers, each made of nanorods that absorb at different wavelengths. "For this to really find widespread use, we will have to get up to around 10 percent efficiency," Alivisatos said. "But we think it's very doable."
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