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Berkeley - 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 the March 29 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."The work was funded by the National Renewable Energy Laboratory and the Department of Energy.
One day last July, Ted Sargent was typing away in his office at the University of Toronto when a graduate student rushed in. His excited visitor explained that he had just shone infrared light -- invisible to the human eye -- onto a tiny sample of a special material Sargent and his researchers had developed, and the sample actually converted the light into energy. Always the skeptic, Sargent asked, "Did you turn the [overhead] lights off?"Soon, however, it became clear that this research group had stumbled onto something big. Sargent and his team describe their discovery -- the world's first plastic solar cell able to absorb infrared light -- in the February issue of the prestigious industry journal Nature Materials. Their little sample could bring about a sea change in the energy industry, perhaps making solar energy so cheap that it becomes a viable alternative to fossil fuels.Solar cells in commercial production today are expensive, around $6 per watt. To understand what that means, consider this: If you install $600 worth of solar cells, you can power a typical light bulb for 25 years, figures Ron Pernick, co-founder of renewable-energy consultancy Clean Edge in San Francisco. That's about twice the cost of coal-based electricity.Through various technological improvements, solar-cell prices have typically fallen by 5% to 6% a year -- but no more, because cells are manufactured through complex processes similar to those employed for making PC processors and memory cards.NARROWING THE GAP. To bridge that price gap, scientists have long attempted to develop so-called plastic solar cells. Essentially, they're a thin film that can be manufactured through a much cheaper process, one analogous to a newspaper printing press. They can be flexible and light. Plastic solar cells can also, potentially, be simply sprayed onto any surface -- and, voila! -- that wall, roof, or consumer electronics case becomes a solar-energy collector. Goodbye, ugly solar-panel roofs. Goodbye, lead storage batteries. Welcome, walls, cars, MP3 players, even shirts doubling as electricity generators.A person could, potentially, unfurl a roll of such plastic solar cells in a field and create a huge solar farm in a matter of minutes, says Sargent. The beauty of plastic solar cells is that they do away with the costly installation required for traditional, heavy solar panels.On the downside, today's plastic solar cells are highly inefficient. They only convert about 6% of the sunlight that hits them into energy. Standard solar cells can have 30%-plus efficiency. That's why no company produces plastic solar cells today, though one, startup Konarka in Lowell, Mass., plans to begin selling its cells for use as a supplemental energy sources for consumer electronics later this year, says Daniel McGahn, the outfit's executive vice-president and chief marketing officer. Konarka is mum on its product's features, but McGahn admits that even under optimal conditions, the cells are only 7% efficient.Plastic solar cells are also terribly expensive. They can cost 10 times more than the traditional, semiconductor solar cells.EXCITED ELECTRONS. Sargent's discovery could drastically increase plastic solar cells' light absorption -- and tip the cost-benefit scale in favor of the cells. A plastic solar cell that captures both visible and infrared light might be able to reach 30% efficiency, figures Peter Peumans, an organic-electronics expert at Stanford University.The main principle of Sargent's solar-cell operation is nothing out of the ordinary. Light hits the cell's material, which absorbs a portion of its energy. The energy knocks tiny electrons that are part of the material loose, and they start flowing in a certain direction. That creates an electrical current.Sargent's knowhow is in the material from which electrons are generated. Semiconductor material used in regular solar cells requires particularly intense solar power, found in visible sunlight but not infrared light, for electrons to be knocked out of place.But electrons in Sargent's material "get excited" under the influence of infrared light. The material is made with so-called quantum dots. These are particles of semiconductor material so small they're invisible to the human eye. To those, Sargent attaches nanosize organic molecules sometimes found in skin moisturizers. They're about 100,00 times smaller than the diameter of a human hair. This unique combination is superresponsive to infrared light.LONG ROAD. The work is far from done, of course. Sargent's still-unnamed material will have to be improved before it's used in commercial products. So far, it can convert only a very small amount of infrared light into energy -- about 1,000 times less than what's needed for commercial use. "We have a hint of a solution, maybe," says Stanford's Peumans.What's more, materials containing organic molecules decompose when heated. So, theoretically, such organic-based plastic solar cells will have a life span that's a lot shorter than today's mainstream solar cells, which are guaranteed to function for more than 25 years. Still, Sargent says his material has withstood being heated to 200 degrees Celsius (392 degrees Fahrenheit) without disintegrating. Plus, a dirt-cheap plastic solar cell that can last for, say, three to five years, will find its uses -- particularly in consumer-electronics devices, which typically arn't designed to last longer, anyway.It will probably take Sargent and the industry up to 10 years to get this technology to become a significant commercial product. But many venture capitalists and solar-cell companies believe it's worth the wait. "I view this work to be groundbreaking," says Josh Wolfe, managing partner at New York-based venture-capital firm Lux Partners. "There's an opportunity for a disruptive breakthrough technology with major social implications."Indeed, with its potential to be used in power-generating garments, the day may not be that far off when the term "power suit" takes on a whole new meaning.
Plastic solar cell efficiency breaks record at WFU nanotechnology centerApril 18, 2007The global search for a sustainable energy supply is making significant strides at Wake Forest University as researchers at the university’s Center for Nanotechnology and Molecular Materials have announced that they have pushed the efficiency of plastic solar cells to more than 6 percent.In a paper to be published in an upcoming issue of the journal Applied Physics Letters, Wake Forest researchers describe how they have achieved record efficiency for organic or flexible, plastic solar cells by creating “nano-filaments” within light absorbing plastic, similar to the veins in tree leaves. This allows for the use of thicker absorbing layers in the devices, which capture more of the sun’s light.Efficient plastic solar cells are extremely desirable because they are inexpensive and light weight, especially in comparison to traditional silicon solar panels. Traditional solar panels are heavy and bulky and convert about 12 percent of the light that hits them to useful electrical power. Researchers have worked for years to create flexible, or “conformal,” organic solar cells that can be wrapped around surfaces, rolled up or even painted onto structures.Three percent was the highest efficiency ever achieved for plastic solar cells until 2005 when David Carroll, director of the Wake Forest nanotechnology center, and his research group announced they had come close to reaching 5 percent efficiency. Now, a little more than a year later, Carroll said his group has surpassed the 6 percent mark."Within only two years we have more than doubled the 3 percent mark,” Carroll said. “I fully expect to see higher numbers within the next two years, which may make plastic devices the photovoltaic of choice.”In order to be considered a viable technology for commercial use, solar cells must be able to convert about 8 percent of the energy in sunlight to electricity. Wake Forest researchers hope to reach 10 percent in the next year, said Carroll, who is also associate professor of physics at Wake Forest.Because they are flexible and easy to work with, plastic solar cells could be used as a replacement for roof tiling or home siding products or incorporated into traditional building facades. These energy harvesting devices could also be placed on automobiles. Since plastic solar cells are much lighter than the silicon solar panels structures do not have to be reinforced to support additional weight.A large part of Carroll’s research is funded by the United States Air Force, which is interested in the potential uses of more efficient, light-weight solar cells for satellites and spacecraft. Other members of Carroll’s research team include Jiwen Liu and Manoj Namboothiry, postdoctoral associates at Wake Forest’s nanotechnology center, and Kyungkon Kim, a postdoctoral researcher at the center, who has moved to the Materials Science & Technology Division at the Korea Institute of Science and Technology in Seoul.