Paul Preuss paul_preuss@lbl.gov, Berkeley Lab News Center, http://newscenter.lbl.gov/feature-stories/2011/01/24/practical-full-spectrum/ January 24, 2011

ScienceDaily (Jan. 26, 2011) - Solar cells are made from semiconductors whose ability to respond to light is determined by their band gaps (energy gaps). Different colors have different energies, and no single semiconductor has a band gap that can respond to sunlight's full range, from low-energy infrared through visible light to high-energy ultraviolet.

The new design promises highly efficient solar cells that are practical to produce. The results are reported in a recent issue of Physical Review Letters, available online to subscribers.

How to make a full-spectrum solar cell

"Since no one material is sensitive to all wavelengths, the underlying principle of a successful full-spectrum solar cell is to combine different semiconductors with different energy gaps," says Walukiewicz.

One way to combine different band gaps is to stack layers of different semiconductors and wire them in series. This is the principle of current high-efficiency solar cell technology that uses three different semiconductor alloys with different energy gaps. In 2002, Walukiewicz and Kin Man Yu of Berkeley Lab's MSD found that by adjusting the amounts of indium and gallium in the same alloy, indium gallium nitride, each different mixture in effect became a different kind of semiconductor that responded to different wavelengths. By stacking several of the crystalline layers, all closely matched but with different indium content, they made a photovoltaic device that was sensitive to the full solar spectrum.

However, says Walukiewicz, "Even when the different layers are well matched, these structures are still complex -- and so is the process of manufacturing them. Another way to make a full-spectrum cell is to make a single alloy with more than one band gap."

In 2004 Walukiewicz and Yu made an alloy of highly mismatched semiconductors based on a common alloy, zinc (plus manganese) and tellurium. By doping this alloy with oxygen, they added a third distinct energy band between the existing two -- thus creating three different band gaps that spanned the solar spectrum. Unfortunately, says Walukiewicz, "to manufacture this alloy is complex and time-consuming, and these solar cells are also expensive to produce in quantity."

The new solar cell material from Walukiewicz and Yu and their colleagues in Berkeley Lab's MSD and RoseStreet Labs Energy, working with Sumika Electronics Materials in Phoenix, Arizona, is another multiband semiconductor made from a highly mismatched alloy. In this case the alloy is gallium arsenide nitride, similar in composition to one of the most familiar semiconductors, gallium arsenide. By replacing some of the arsenic atoms with nitrogen, a third, intermediate energy band is created. The good news is that the alloy can be made by metalorganic chemical vapor deposition (MOCVD), one of the most common methods of fabricating compound semiconductors.

How band gaps work

Band gaps arise because semiconductors are insulators at a temperature of absolute zero but inch closer to conductivity as they warm up. To conduct electricity, some of the electrons normally bound to atoms (those in the valence band) must gain enough energy to flow freely -- that is, move into the conduction band. The band gap is the energy needed to do this.

When an electron moves into the conduction band it leaves behind a "hole" in the valence band, which also carries charge, just as the electrons in the conduction band; holes are positive instead of negative.

A large band gap means high energy, and thus a wide-band-gap material responds only to the more energetic segments of the solar spectrum, such as ultraviolet light. By introducing a third band, intermediate between the valence band and the conduction band, the same basic semiconductor can respond to lower and middle-energy wavelengths as well.

This is because, in a multiband semiconductor, there is a narrow band gap that responds to low energies between the valence band and the intermediate band. Between the intermediate band and the conduction band is another relatively narrow band gap, one that responds to intermediate energies. And finally, the original wide band gap is still there to take care of high energies.

"The major issue in creating a full-spectrum solar cell is finding the right material," says Kin Man Yu. "The challenge is to balance the proper composition with the proper doping."

In solar cells made of some highly mismatched alloys, a third band of electronic states can be created inside the band gap of the host material by replacing atoms of one component with a small amount of oxygen or nitrogen. In so -- called II-VI semiconductors (which combine elements from these two groups of Mendeleev's original periodic table), replacing some group VI atoms with oxygen produces an intermediate band whose width and location can be controlled by varying the amount of oxygen. Walukiewicz and Yu's original multiband solar cell was a II-VI compound that replaced group VI tellurium atoms with oxygen atoms. Their current solar cell material is a III-V alloy. The intermediate third band is made by replacing some of the group V component's atoms -- arsenic, in this case -- with nitrogen atoms.

Finding the right combination of alloys, and determining the right doping levels to put an intermediate band right where it's needed, is mostly based on theory, using the band anticrossing model developed at Berkeley Lab over the past 10 years.

"We knew that two-percent nitrogen ought to do the job," says Yu. "We knew where the intermediate band ought to be and what to expect. The challenge was designing the actual device."

Passing the test

Using their new multiband material as the core of a test cell, the researchers illuminated it with the full spectrum of sunlight to measure how much current was produced by different colors of light. The key to making a multiband cell work is to make sure the intermediate band is isolated from the contacts where current is collected.

"The intermediate band must absorb light, but it acts only as a stepping stone and must not be allowed to conduct charge, or else it basically shorts out the device," Walukiewicz explains.

The test device had negatively doped semiconductor contacts on the substrate to collect electrons from the conduction band, and positively doped semiconductor contacts on the surface to collect holes from the valence band. Current from the intermediate band was blocked by additional layers on top and bottom.

For comparison purposes, the researchers built a cell that was almost identical but not blocked at the bottom, allowing current to flow directly from the intermediate band to the substrate.

The results of the test showed that light penetrating the blocked device efficiently yielded current from all three energy bands -- valence to intermediate, intermediate to conduction, and valence to conduction -- and responded strongly to all parts of the spectrum, from infrared with an energy of about 1.1 electron volts (1.1 eV), to over 3.2 eV, well into the ultraviolet.

By comparison, the unblocked device responded well only in the near infrared, declining sharply in the visible part of the spectrum and missing the highest-energy sunlight. Because it was unblocked, the intermediate band had essentially usurped the conduction band, intercepting low-energy electrons from the valence band and shuttling them directly to the contact layer.

Further support for the success of the multiband device and its method of operation came from tests "in reverse" -- operating the device as a light emitting diode (LED). At low voltage, the device emitted four peaks in the infrared and visible light regions of the spectrum. Primarily intended as a solar cell material, this performance as an LED may suggest additional possibilities for gallium arsenide nitride, since it is a dilute nitride very similar to the dilute nitride, indium gallium arsenide nitride, used in commercial "vertical cavity surface-emitting lasers" (VCSELs), which have found wide use because of their many advantages over other semiconductor lasers.

With the new, multiband photovoltaic device based on gallium arsenide nitride, the research team has demonstrated a simple solar cell that responds to virtually the entire solar spectrum -- and can readily be made using one of the semiconductor industry's most common manufacturing techniques. The results promise highly efficient solar cells that are practical to produce.

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Subject: Energy Invention Media stories that have been overlooked - sent in by Elliott Maynard, IRI Advisory Board

Dear friends,
The exciting, clean energy inventions reported in the major media articles below could easily transform our world within just a few years. These amazing inventions are ultra green and renewable. Concise summaries of each fascinating media article are given, with links provided to the full original article.
Big oil companies and the power elite don't want us to know about these incredible energy breakthroughs. They will not likely support inventions which could seriously erode their huge profits and power. A personal friend of mine who is a genius inventor came up with an amazing energy invention a few years ago. He had a $7 million company, and I personally witnessed how he was attacked and shut down by powerful interests.
None of these amazing inventions are receiving the front page coverage they deserve. Yet when you consider them all together, it is clear that given the necessary funding, one or more of these energy inventions could literally transform our world. What we need now is for the public and politicians to be informed so that pressure will build to support these exciting projects. Please help to spread the word. For how to do this, and for other great links on this topic, see the " What you can do" section below these articles. Thanks for caring.

With very best wishes,
Fred Burks for PEERS and the WantToKnow.info Team
Former language interpreter for Presidents Bush and Clinton

**************

The Bloom Box: An Energy Breakthrough?

2010-02-18, CBS 60 Minutes

http://www.cbsnews.com/stories/2010/02/18/60minutes/main6221135.shtml

In the world of energy, the Holy Grail is a power source that's inexpensive and clean, with no emissions. Over 100 start-ups in Silicon Valley are working on it. One of them, Bloom Energy, is about to make public its invention: a little power-plant-in-a-box they want to put literally in your backyard. You'll generate your own electricity with the box and it'll be wireless. The idea is to one day replace the big power plants and transmission line grid. K.R. Sridhar ... says he knows it works because he originally invented a similar device for NASA. He really is a rocket scientist. He invented a new kind of fuel cell, which is like a very skinny battery that always runs. Sridhar feeds oxygen to it on one side, and fuel on the other. The two combine within the cell to create a chemical reaction that produces electricity. There's no need for burning or combustion, and no need for power lines from an outside source. "It's cheaper than the grid, it's cleaner than the grid." Twenty large, well-known companies have quietly bought and are testing Bloom boxes. The first customer was Google. Four units have been powering a Google datacenter for 18 months. They use natural gas, but half as much as would be required for a traditional power plant. John Donahoe, eBay's CEO, says its five boxes were installed nine months ago and have already saved the company more than $100,000 in electricity costs. eBay's boxes run on bio-gas made from landfill waste, so they're carbon neutral. "In five to ten years, we would like to be in every home." [Sridhar] said a unit should cost an average person less than $3,000.

2,757.1 MPG Achieved at 2009 Shell Eco-marathon Americas
2009-04-19, CNBC News
http://classic.cnbc.com/id/30287740

Distance, not speed, was the goal this weekend on the track at the 2009 Shell Eco-marathon Americas(R), a challenge for students to design, build and test fuel-efficient vehicles that travel the farthest distance using the least amount of fuel. This year, more than 500 students from North and South America were on hand to stretch the boundaries of fuel efficiency. So who came out on top? The student team from Laval University, with an astonishing 2,757.1 miles per gallon, equivalent to 1,172.2 kilometres per liter, won the grand prize in the "Prototype" category. And in the "UrbanConcept" category - new to the Americas event this year - the team from Mater Dei High School took the grand prize by achieving 433.3 mpg, equivalent to 184.2 km/l. With 44 participating teams at track competition was steep. This year's challenge brought together a number of returning teams determined to beat the 2,843 mpg (1,208 km/l) record set by Mater Dei High School (Evansville, Ind.) in 2008, combined with a number of new teams adding fresh innovation and vehicle designs to the competition. "The Shell Eco-marathon is a platform for students to let their imaginations run wild," said Mark Singer, global project manager for the Shell Eco-marathon. "By encouraging these students to build vehicles with greater energy efficiency, we hope this will help inspire others; and together we can find solutions that will help meet the global energy challenge."
Note: If the above links fails, this article is also available on the Shell website at this link. Why so little media attention to this most exciting race for top gas mileage? And if high school students can build a car that gets over 400 mpg, what's up with Detroit? Could big business be suppressing, or at the very least ignoring these inspiring inventions some of which use clean, green, and renewable energy sources?

The Prophet of Garbage
Popular Science - March 2007 Issue
http://www.popsci.com/popsci/science/873aae7bf86c0110vgnvcm1000004eecbccdrcrd.

The Plasma Converter can consume nearly any type of waste from dirty diapers to chemical weapons by annihilating toxic materials in a process called plasma gasification. A 650-volt current passing between two electrodes rips electrons from the air, converting the gas into plasma. The plasma arc is so powerful, it disintegrates trash into its constituent elements by tearing apart molecular bonds. The system is capable of breaking down pretty much anything except nuclear waste. The only by-products are an obsidian-like glass [and] a mixture of primarily hydrogen and carbon monoxide that can be converted into a variety of marketable fuels, including ethanol, natural gas and hydrogen. Perhaps the most amazing part of the process is that its self-sustaining. Once the cycle is under way, the 2,200°F syngas is fed into a cooling system, generating steam that drives turbines to produce electricity. About two thirds of the power is siphoned off to run the converter; the rest can be used on-site for heating or electricity, or sold back to the utility grid. Even a blackout would not stop the operation of the facility. New York City is already paying an astronomical $90 a ton to get rid of its trash. According to Startech, a few 2,000-ton-per-day plasma-gasification plants could do it for $36. Sell the syngas and surplus electricity, and you actualually net $15 a ton. But the decision-making bureaucracy can be slow, and it is hamstrung by the politically well-connected waste-disposal industry. Startech isnt the only company using plasma to turn was waste into a source of clean energy. A handful of start-ups Geoplasma, Recovered Energy, PyroGenesis, EnviroArc and Plasco Energy among others have entered the market.
Note: Why isn't this amazing, proven machine and technology making front page headlines? Read the exciting full article to find how it is already being used. For why you don't know about it, click here.

Researcher sets saltwater on fire
2007-11-14, CNN
http://edition.cnn.com/2007/TECH/science/11/14/saltwater.fire/

Last winter, inventor John Kanzius was already attempting one seemingly impossible feat -- building a machine to cure cancer with radio waves -- when his device inadvertently succeeded in another: He made saltwater catch fire. TV footage of his bizarre discovery has been burning up the blogosphere ever since, drawing crackpots and Ph.D.s alike into a raging debate. Can water burn? And if so, what good can come of it? Some people gush over the invention's potential for desalinization or cheap energy. Briny seawater, after all, sloshes over most of the planet's surface, and harnessing its heat energy could power all sorts of things. Skeptics say Kanzius's radio generator is sucking up far more energy than it's creating, making it a carnival trick at best. For now, Kanzius is tuning out the hubbub. Diagnosed with leukemia in 2002, he began building his radio-wave blaster the next year, soon after a relapse. If he could seed a person's cancerous cells with nanoscopic metal particles and blast them with radio waves, perhaps he could kill off the cancer while sparing healthy tissue. The saltwater phenomenon happened by accident when an assistant was bombarding a saline-filled test tube with radio waves and bumped the tube, causing a small flash. Curious, Kanzius struck a match. "The water lit like a propane flame," he recalls. "People said, 'It's a crock. Look for hidden electrodes in the water,' " says Penn State University materials scientist Rustum Roy, who visited [Kanzius] in his lab in August after seeing the feat on Google Video. A demo made Roy a believer. "This is discovery science in the best tradition," he says. Meanwhile, researchers at MD Anderson Cancer Center in Houston and the University of Pittsburgh Medical Center have made progress using Kanzius's technology to fight cancer in animals. They published their findings last month in the journal Cancer.

BlackLight's physics-defying promise: Cheap power from water
2008-07-02, CNN Money
http://money.cnn.com/2008/07/01/smallbusiness/blacklight.fsb/index.htm

Imagine being able to convert water into a boundless source of cheap energy. That's what BlackLight Power, a 25-employee firm in Cranbury, N.J., says it can do. The only problem: Most scientists say that company's technology violates the basic laws of physics. Such skepticism doesn't daunt Dr. Randell Mills, a Harvard-trained physician and founder of BlackLight, who recently claimed that he has created a working fuel cell using the world's most pervasive element: the hydrogen found in water. Mills says he has a market-ready product: a fuel cell that produces a chemical reaction to alter hydrogen atoms. The fuel cell releases heat that turns water into steam, which drives electric turbines. The working models in his lab generate 50 kilowatts of electricity - enough to power six or seven houses. But these, Mills says, can be scaled [up] to drive a large, electric power plant. The inventor claims this electricity will cost less than 2 cents per kilowatt-hour, which compares to a national average of 8.9 cents. Mills developed the patented cocktail that enables the reaction - a solid fuel made of hydrogen and a sodium hydride catalyst - only a year ago. (He recently posted instructions on the company's Web site). Now that the device is ready for commercialization, he says, BlackLight is negotiating with several utilities and architecture and engineering firms. The business, Mills says, has attracted $60 million in funding from wealthy individuals, investment firms ... and it is no longer seeking money. BlackLight's board of directors reads like a Who's Who of finance and energy leaders.

Cold Fusion Is Hot Again
2009-04-19, CBS News
http://www.cbsnews.com/stories/2009/04/17/60minutes/main4952167.shtml

Twenty years ago it appeared, for a moment, that all our energy problems could be solved. It was the announcement of cold fusion - nuclear energy like that which powers the sun - but at room temperature on a table top. It promised to be cheap, limitless and clean. Cold fusion would end our dependence on the Middle East and stop those greenhouse gases blamed for global warming. It would change everything. But then, just as quickly as it was announced, it was discredited. So thoroughly, that cold fusion became a catch phrase for junk science. Well, a funny thing happened on the way to oblivion - for many scientists today, cold fusion is hot again. "We can yield the power of nuclear physics on a tabletop. The potential is unlimited. That is the most powerful energy source known to man," researcher Michael McKubre told 60 Minutes correspondent Scott Pelley. McKubre says he has seen that energy more than 50 times in cold fusion experiments he's doing at SRI International, a respected California lab that does extensive work for the government. McKubre is an electro-chemist who imagines, in 20 years, the creation of a clean nuclear battery. "For example, a laptop would come pre-charged with all of the energy that you would ever intend to use. You're now decoupled from your charger and the wall socket," he explained. The same would go for cars. "The potential is for an energy source that would run your car for three, four years, for example. And you'd take it in for service every four years and they'd give you a new power supply," McKubre told Pelley.

Fusion's Ups and Downs
2010-03-23, MSNBC
http://cosmiclog.msnbc.msn.com/archive/2010/03/23/2237165.aspx

This week, scientists gathered at the American Chemical Society's spring meeting in San Francisco to turn the spotlight on a highly unorthodox path: the effect known as cold fusion. This year's session featured nearly 50 presentations - including reports on batteries and bacteria that appear to exhibit the cold-fusion effect. Back in 1989, cold fusion was heralded as a simple, inexpensive way to get a power-generating fusion reaction on a desktop. But when the experimental results couldn't be reproduced, the researchers were driven into obscurity [and] the term "cold fusion" became synonymous with quackery. Chemists, however, have kept up their interest in the effect. Rick Nebel [has headed] up a handful of researchers following the less-traveled path to fusion at EMC2 Fusion Development Corp. EMC2 recently created a buzz in the fusion underground by reporting on its Web site that it successfully completed a series of experiments to "validate and extend" earlier results. The company is now using a $7.9 million contract from the U.S. Navy to build a bigger test machine. Nebel and his colleagues are now seeking contributions to fund the development of what they say would be a 100-megawatt fusion plant - a "Phase 3" effort projected to cost $200 million and take four years. "Successful Phase 3 marks the end of fossil fuels," the Web site proclaims.

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