Berkeley Scientists Synthesize Cheap, Easy-to-Make
Ultra-thin Photovoltaic Films

SPX 24oct05

Berkeley CA — Imagine a future in which the rooftops of residential homes and commercial buildings can be laminated with inexpensive, ultra-thin films of nano-sized semiconductors that will efficiently convert sunlight into electrical power and provide virtually all of our electricity needs.

This future is a step closer to being realized, thanks to a scientific milestone achieved at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab).

Researchers with Berkeley Lab and the University of California, Berkeley, have developed the first ultra-thin solar cells comprised entirely of inorganic nanocrystals and spin-cast from solution.

These dual nanocrystal solar cells are as cheap and easy to make as solar cells made from organic polymers and offer the added advantage of being stable in air because they contain no organic materials.

Ilan Gur, a researcher in Berkeley Lab's Materials Sciences Division, is the principal author of a paper in Science that describes an inexpensive process for mass-producing solar cell thin films from semiconductors.

"Our colloidal inorganic nanocrystals share all of the primary advantages of organics — scalable and controlled synthesis, an ability to be processed in solution, and a decreased sensitivity to substitutional doping — while retaining the broadband absorption and superior transport properties of traditional photovoltaic semiconductors," said Ilan Gur, a researcher in Berkeley Lab's Materials Sciences Division and fourth-year graduate student in UC Berkeley's Department of Materials Science and Engineering.

Gur is the principal author of a paper appearing in the October 21 issue of the journal Science that announces this new development. He is a doctoral candidate in the research group of Paul Alivisatos, director of Berkeley Lab's Materials Sciences Division, and the Chancellor's Professor of Chemistry and Materials Science at UC Berkeley.

Alivisatos is a leading authority on nanocrystals and a co-author of the Science paper. Other co-authors are Berkeley Lab's Neil A. Fromer and UC Berkeley's Michael Geier.

In this paper, the researchers describe a technique whereby rod-shaped nanometer-sized crystals of two semiconductors, cadmium-selenide (CdSe) and cadmium-telluride (CdTe), were synthesized separately and then dissolved in solution and spin-cast onto a conductive glass substrate.

The resulting films, which were about 1,000 times thinner than a human hair, displayed efficiencies for converting sunlight to electricity of about 3 percent. This is comparable to the conversion efficiencies of the best organic solar cells, but still substantially lower than conventional silicon solar cell thin films.

"We obviously still have a long way to go in terms of energy conversion efficiency," said Gur, "but our dual nanocrystal solar cells are ultra-thin and solution-processed, which means they retain the cost-reduction potential that has made organic cells so attractive vis-à-vis their conventional semiconductor counterparts."

As every consumer in this country is painfully aware, the costs of fossil fuels are rising. From escalating prices at gas pumps, to melting polar ice caps, the message is loud and clear: Alternative energy sources must be found. Solar energy is in many ways an ideal choice.

As a source it is plentiful — the sun shines approximately 1,000 watts of energy per square meter of the planet's surface every day — and would last the lifetime of our planet. It would add no pollutants to the atmosphere, contribute nothing to global climate change, and is free. The cost comes in when solar energy is converted to electrical power.

Most commercial solar cells today are made from silicon. Like many conventional semiconductors, silicon offers excellent, well-established electronic properties. However, the use of silicon or other conventional semiconductors in photovoltaic devices has to date been limited by the high cost of production — even the fabrication of the simplest semiconductor cell is a complex process that has to take place under exactly controlled conditions, such as high vacuum and temperatures between 400 and 1,400 degrees Celsius.

This image, which was produced through scanning electron microscopy, shows a typical spin-cast film of nanocrystal solar cells that is homogeneous and defect free. The film edge of this 100 nanometer film is shown for contrast with the silicon substrate.

When it was discovered, back in 1977, that a certain group of "conjugated" organic polymers could be made to conduct electricity, there was immediate interest in using these materials in photovoltaic devices. While it was shown that plastic solar cells could be made in bulk quantities for a few cents each, the efficiency by which these devices converted light into electricity has always been poor compared to the power conversion efficiencies of cells made from semiconductors.

In 2002, Alivisatos and members of his research group announced a breakthrough in which they were able to fashion hybrid solar cells out of organic polymers and CdSe. While these hybrids offer some of the best features of semiconductor and plastic solar cells, they remain sensitive to air because they contain organics.

"A solar cell that relies exclusively on colloidal nanocrystals has been anticipated theoretically in recent years," said Alivisatos. "We've now demonstrated such a device and have presented a mechanism for its operation."

Unlike conventional semiconductor solar cells, in which an electrical current flows between layers of n-type and p-type semiconductor films, with these new inorganic nanocrystal solar cells, current flows due to a pair of molecules that serve as donors and receptors of electrical charges, also known as a donor-acceptor heterojunction. This is the same mechanism by which current flows in plastic solar cells.

"Because our inorganic nanocrystal solar cells appear to work primarily based on the donor-acceptor heterojunction model that is typical of organic systems, they help us to better understand the specific material properties needed to make such devices," said Gur. "This work also elucidates some key similarities between polymer and nanocrystal films."

The CdSe and CdTe films are electrical insulators in the dark but when exposed to sunlight undergo a dramatic rise in electrical conductivity, as much as three orders of magnitude. Sintering the nanocrystals was found to significantly enhance the performance of these films.

Unlike plastic solar cells, whose performance deteriorates over time, aging seems to improve the performance of these inorganic nanocrystal solar cells.

"The next step is for us to better characterize and further develop our prototypical system, as there is still a great deal we don't fully understand," said Gur. "After that, we have a lot of directions that we'd like to pursue, such as introducing variations in the system architecture and our choice of semiconductor materials."

According to the Energy Foundation, if the available residential and commercial rooftops in this country were to be coated with solar cell thin films, they could furnish an estimated 710,000 megawatts of electricity across the United States, which is more than three-quarters of all the electricity that this country is currently able to generate.

Because of its favorable sunlight levels, California is considered a prime candidate for this technology.

source: http://www.spacedaily.com/news/solarcell-05i.html 24oct2005

UCLA Engineers Pioneer Affordable Solar Energy Cells Made Of Plastic 

SPX 11oct05

Los Angeles CA (SPX) Oct 11, 2005 With oil and gas prices in the United States hovering at an all-time high, interest in renewable energy alternatives is again heating up. Researchers at the UCLA Henry Samueli School of Engineering and Applied Science hope to meet the growing demand with a new and more affordable way to harness the sun's rays: using solar cell panels made out of everyday plastics. In research published today in Nature Materials magazine, UCLA engineering professor Yang Yang, postdoctoral researcher Gang Li and graduate student Vishal Shrotriya showcase their work on an innovative new plastic (or polymer) solar cell they hope eventually can be produced at a mere 10 percent to 20 percent of the current cost of traditional cells, making the technology more widely available.

"Solar energy is a clean alternative energy source. It's clear, given the current energy crisis, that we need to embrace new sources of renewable energy that are good for our planet. I believe very strongly in using technology to provide affordable options that all consumers can put into practice," Yang said.

The price for quality traditional solar modules typically is around three to four times more expensive than fossil fuel. While prices have dropped since the early 1980s, the solar module itself still represents nearly half of the total installed cost of a traditional solar energy system.

Currently, nearly 90 percent of solar cells in the world are made from a refined, highly purified form of silicon — the same material used in manufacturing integrated circuits and computer chips. High demand from the computer industry has sharply reduced the availability of quality silicon, resulting in prohibitively high costs that rule out solar energy as an option for the average consumer.

Made of a single layer of plastic sandwiched between two conductive electrodes, UCLA's solar cell is easy to mass-produce and costs much less to make — roughly one-third of the cost of traditional silicon solar technology. The polymers used in its construction are commercially available in such large quantities that Yang hopes cost-conscious consumers worldwide will quickly adopt the technology.

Independent tests on the UCLA solar cell already have received high marks. The nation's only authoritative certification organization for solar technology, the National Renewable Energy Laboratory (NREL), located in Golden, Colo., has helped the UCLA team ensure the accuracy of their efficiency numbers. The efficiency of the cell is the percentage of energy the solar cell gathers from the total amount of energy, or sunshine, that actually hits it.

According to Yang, the 4.4 percent efficiency achieved by UCLA is the highest number yet published for plastic solar cells.

"As in any research, achieving precise efficiency benchmarks is a critical step," Yang said. "Particularly in this kind of research, where reported efficiency numbers can vary so widely, we're grateful to the NREL for assisting us in confirming the accuracy of our work."

Given the strides the team already has made with the technology, Yang calculates he will be able to double the efficiency percentage in a very short period of time. The target for polymer solar cell performance is ultimately about 15 percent to 20 percent efficiency, with a 15–20 year lifespan. Large-sized silicon modules with the same lifespan typically have a 14 percent to 18 percent efficiency rating.

The plastic solar cell is still a few years away from being available to consumers, but the UCLA team is working diligently to get it to market.

"We hope that ultimately solar energy can be extensively used in the commercial sector as well as the private sector. Imagine solar cells installed in cars to absorb solar energy to replace the traditional use of diesel and gas. People will vie to park their cars on the top level of parking garages so their cars can be charged under sunlight. Using the same principle, cell phones can also be charged by solar energy," Yang said. "There are such a wide variety of applications."

source: http://www.spacedaily.com/news/solarcell-05h.html 24oct2005

Cost Of Photovoltaic Concentrators Falling Fast

SPX 19jul05

"Concentrating solar electric power is on the cusp of delivering on its promise of low-cost, reliable, solar-generated electricity at a cost that is competitive with mainstream electric generation systems," said Vahan Garboushian, president of Amonix, Inc. of Torrance, CA.

Golden CO — Solar concentrators using highly efficient photovoltaic solar cells will reduce the cost of electricity from sunlight to competitive levels soon, attendees were told at a recent international conference on the subject. Herb Hayden of Arizona Public Service (APS) and Robert McConnell and Martha Symko-Davies of the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) organized the conference held May 1-5 in Scottsdale, Ariz.

"Concentrating solar electric power is on the cusp of delivering on its promise of low-cost, reliable, solar-generated electricity at a cost that is competitive with mainstream electric generation systems," said Vahan Garboushian, president of Amonix, Inc. of Torrance, Calif. "With the advent of multijunction solar cells, PV concentrator power generation at $3 per watt is imminent in the coming few years," he added.

We have seen steady progress in photovoltaic concentrator technology. We are working with advanced multijunction PV cells that are approaching 38% efficiency, and even higher is possible over time. Our goal is to install PV concentrator systems at $3 per watt, which can happen soon at production rates of 10 megawatts per year. Once that happens, higher volumes are readily achieved," Hayden, Solar Program Coordinator at APS, said.

Growth in the photovoltaic (PV) concentrator business was reflected in the conference attendance, three times that of the 2003 version. This rapid growth was attributed to recent PV concentrator installations and sales forecasts along with excitement created by new solar cell efficiencies approaching 40%.

At the conference, NREL announced a new record efficiency of 37.9 percent at 10 suns, a measure of concentrated sunlight. Soon thereafter Boeing-Spectrolab, under contract to NREL and the Department of Energy, surpassed the NREL record with 39.0 percent at 236 suns announced at the European photovoltaic conference in Barcelona, Spain. The efficiency of a solar cell is the percentage of the sun's energy the device converts to electricity.

Photovoltaic (PV) concentrator units are much different than the flat photovoltaic modules sold around the world; almost 1,200 megawatts of flat PV modules were sold last year. PV concentrators come in larger module sizes, typically 20 kilowatts to 35 kilowatts each, they track the sun during the day and they are more suitable for large utility installations.

Another highlight of the conference was the announcement by Amonix Inc. of a joint venture with Spain's Guascor which will build a 10-megawatt per year assembly plant in Spain by the end of 2005. Amonix also plans to install 3 megawatts of PV concentrator systems in the southwestern U.S. while Guascor plans to install 10 megawatts of concentrator PV systems in Spain in 2006.

Solar Systems of Australia announced plans to install more than 5 megawatts of PV concentrator systems in 2006. "Solar Systems' experience gained from installing and operating reliable PV concentrator systems over the last decade combined with its strong relationship with Spectrolab Inc., a leading manufacturer of multijunction solar cells, is poised to make a major step towards being a mainstream power producer," said Dave Holland, CEO of Solar Systems Australia.

"The new solar cell technology from Spectrolab will enable us to upgrade our systems from 24 kilowatts to 35 kilowatts, a 46 percent increase in output," he added.

The ultra-high efficiency solar cell technology, initially discovered at NREL and successfully developed for space satellites in the 1990s by Boeing-Spectrolab Inc., in Sylmar, Calif., proves to be enabling for low-cost terrestrial SEC systems.

"Today, we are capitalizing on the major investments made by the space satellite industry and reducing the cost of the semiconductor solar cell by two to three orders of magnitude by operating the cells under high sun concentrations, typically 300 to 1000 times.

"Boeing-Spectrolab and NREL have demonstrated over 37 percent efficient concentrator solar cells and field testing of Spectrolab's cells for over one year with no degradation promise a bright future.

"We expect concentrator solar cell performance to reach or exceed 40 percent by 2006 and anticipate continued enhancement in performance and reliability," said Dr. Nasser Karam, vice president of Advanced Technology Products at Spectrolab Inc.

"We are working closely with PV concentrator manufacturers to ensure their success and expedient deployment of the multijunction PV concentrator cells" said Dr. Raed Sherif, director of PV concentrator products, at Spectrolab.

The U.S. Department of Energy, through NREL and its High Performance Photovoltaic Project, funds many of the U.S. research efforts reported at the conference.

source: http://www.spacedaily.com/news/solarcell-05f.html 24oct2005