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A new approach to making photovoltaics based on patterned III-V nanopillars has been unveiled by researchers at the University of California at Los Angeles and Sandia National Laboratories. The devices made have high surface-to-volume ratios that allow for greater absorption of sunlight and the diameter, pitch and height of the nanopillars can all be separately optimized â so maximizing the optical absorption over a broad range of wavelengths.
“The reported efficiency in our devices is the highest for bottom-up gallium arsenide nanopillar solar cells to date,” team member Giacomo Mariani of UCLA told nanotechweb.org. “The work is also a significant step towards device reproducibility and controllability compared with traditional techniques that lead to random nanowire growth.”
Nanostructured solar cells show much promise thanks to light-trapping effects that dramatically reduce the amount of photons reflected from a device. This ultimately enhances optical absorption. In recent years, researchers have studied structures such as nanodomes, nanocones, nanoparticles and nanowires as possible candidates for improving performance in solar cells. The high surface-to-volume ratio of these materials also increases the all-important photoactive junction area so that more photons are harnessed, something that leads to enhanced power-conversion efficiency.
Nanopillars for next-generation solar cells
Nanopillars densely packed nanoscale arrays of electro-optically active semiconductors could be used to make a next generation of relatively cheap and scalable solar cells, but these materials have been hampered by efficiency issues. Another problem is that growing such structures normally requires a metal catalyst but this technique produces randomly located nanopillars. The metal catalyst can also contaminate the pillars and increase leakage currents in finished devices.
The new method, developed by Diana Huffaker and colleagues, relies on a lithographically defined substrate for selective area epitaxy and the mask used is pre-defined to fix nanopillar diameter and pitch. What is more, it provides a way to make large-area nanopillar arrays.
The researchers grow their nanopillars in a metal-organic chemical vapour deposition reactor that allows both axial (core) and lateral (shell) nanopillar growth to be controlled at will. No metal catalyst is required, which means high crystal quality. Indeed, p-n junctions made from the nanopillars have a low leakage current of around just 236 nA at 1 V and the power conversion efficiency of the material is as high as 2.54%.
The team now plans to port the III-V devices to silicon substrates, because silicon is a much more cost-effective platform than the gallium arsenide used in this work. It is also looking at other materials as potential substrates. “For example, the pillars can be embedded in flexible polymers and peeled off from the growth platform to realize a flexible solar cell with the high efficiency of III-V materials,” said Mariani.
“We are just beginning to develop this new class of GaAs device,” he added. “Hetero-epitaxy on silicon will certainly lead to higher efficiency, low-cost solar cells that might even lend themselves to being mass produced.”
The work was published in Nano Letters.
Belle Dum© is a contributing editor at www.nanotechweb.org
Jim Sims, from Molycorp, says China is starting to export fewer rare earth elements than previously
Wars have been fought over oil and water. But are the future global tensions going to be over access to Scandium, Neodymium or Dysprosium?
Or could conflicts be fought over any other of the 17 rare earth elements, which, week by week, are becoming more and more important in developing the latest high-tech products?
Tucked onto the periodic table of the elements, in a little section once ignored by chemistry teachers, rare earths are now everywhere.
They are in your iPod or tablet computer, are vital for the red colour in your TV screen whatever make you have and allow your headphones to be small enough to fit into your ears.
As China’s exports are being restricted, we are looking at outright shortages of rare earths, probably this year and next.
Jim Sims Molycorp representative
They are in hybrid cars – both in the batteries and the fuel – and in new generation wind turbines, missile defence systems, solar panels and even F-16 fighter jets.
At the moment China provides 97% of the world’s rare earth elements, which is making America nervous from both an economic and a security perspective.
Their price has gone up 1000% in just a year, which is making mining them in the US worthwhile once again.
‘Rare earth shortages’
A deep hole in the ground high up in the Mojave Desert is America’s only rare earths mine, and the race is on to dig out the supply to match the demand as only a few places in the world have enough reserves to make mining them practical.
“The world – America, Britain, everyone – relies on what China exports to meet their needs,” says Jim Sims from Molycorp, the company running California’s Mountain Pass mine.
“As China’s exports are being restricted, we are looking at outright shortages of rare earths, probably this year and next,” he adds.
America’s only rare earths mine is located in the Mojave Desert in the US south-west
So the huge diggers and trucks moving vast volumes of rocks around, the daily explosive charges blasting the mountainside apart, are harvesting one of the world’s biggest deposits.
The mine closed down 10 years ago when a flood of cheap Chinese rare earth elements made profits hard to maintain.
Until just a few weeks ago, Molycorp was asking for the US government’s help to cover costs of digging these elements out, separating them off and moulding them into metal alloys.
But the price has gone up so rapidly, rare earths is suddenly looking like a good business.
Last year China’s exports of rare earth elements to Japan were interrupted during a political row over territorial waters, which sent shudders around the world.
“We should be worried when any country completely dominates any raw material supplies,” says Christine Parthemore, from the Center for New American Security in Washington DC.
“I don’t think China is uniquely at fault in this situation, but they are using the political leverage that’s derived from cornering the market they have as any country would.
“I’m sure America would do the same,” he adds.
The creation of permanent magnets, a key component in so many green technologies, is one of the key uses of rare earths.
They make the new generation of wind turbines more efficient and reliable. But there are such an increasing variety of uses for these elements, down to glass polishing, that there aren’t enough of the raw materials to go around.
The speed of China’s growth means the country is consuming more of its own rare earths, which has led to a drop in the amount available for export.
“It is a security issue strictly in the sense that these minerals are used in critical military components for their properties, which we don’t currently have substitutes for,” says Christine Parthemore.
“If the prices go way up or there are actual supply shortages, it can drive prices up over the long term on military procurement – or it can mean there are parts that we can’t manufacture here in the United States anymore.”
It increases the need for an industry to extract the ore and process the materials.
“The elements are all mixed together in the ore we mine,” Jim Sims says.
“We turn them into a liquid, and let these elements settle out into oxides which are like powders,” he adds.
Inside a warehouse at the mine are dozens of huge white sacks, each weighing a metric tonne and each worth $200,000 (£125,700).
“Those powders then get turned into metals as magnets or used in their oxide forms for a variety of uses in a variety of different substances,” Mr Sims says.
As new uses are found for materials like rare earth elements, there will be more competition, and access to them may change the shape of global politics.
By Alastair Leithead BBC News, Mojave Desert, US July 12, 2011
There were a number of reports over the weekend, about a group of Japanese researchers who say that they have found significant quantities of rare-earth elements (REEs) at multiple sites on the seabed of the Pacific Ocean. In a paper published in Nature Geoscience on July 3, 2011, lead author Yasushiro Kato and his colleagues shared the extensive work that was undertaken, to obtain and to analyze 2,037 samples from 78 different sites across the Pacific Ocean.
Reuters, the BBC, Nikkei and others reported that there is an estimated 100 billion tonnes of rare earths in these deposits. Which is rather interesting, because the scientists themselves made no such claim in their paper.
What they do report, are two regions of the sea bed with so-called REE-rich muds:
- one in the eastern South Pacific containing 0.1-0.22% total REEs (including 0.02-0.04% heavy REEs), in layers 10 to 40 meters thick;
- one in the central North Pacific, containing 0.04-0.1% total REEs (including 0.007-0.02% heavy REEs), in layers 30 to greater than 70 meters thick.
The authors compare these muds to the ion-absorption-type clays found in China, which are presently the world’s primary source of heavy REEs. They comment that the mud in the eastern South Pacific has heavy REE content that is nearly twice as abundant as in the Chinese deposits. Of course, those Chinese deposits are not sitting under great water depths (mostly 4,000-5,000 meters) and below the surface of the sea floor. It is because they are readily accessible and processable, that the Chinese ion-absorption deposits are exploited, despite their very low concentrations of REEs (heavy or otherwise).
Doing a couple of rough calculations, the authors estimate that a 10 meter-thick bed of mud in the eastern South Pacific, with an area of 1 square kilometer, could yield approximately 9,000 tonnes of rare earths. They also estimate that a 70 meter-thick bed of mud in the central North Pacific, with an area of 1 square kilometer, could yield approximately 25,000 tonnes of rare earths. These numbers are not too shabby (if we again forget about the 2.5-3 miles of water sat above them, and their remote location from any significant land masses). As I’ve said elsewhere, I can’t see these deposits ever being commercially exploited, but the empirical work done by the Japanese researchers which is presented in this paper, is impressive.
What the authors do NOT estimate, is a size of the total mineral resource, and wisely so. While they mention that the thick distributions of mud at numerous sites might mean that the REEs on the sea floor could exceed the world’s current land reserves of [110 million tonnes], they acknowledge the considerable challenges and significant variability present on the seafloor, and thus state that âresource estimates for large regions cannot be made until more detailed data are available for areas lacking cores.
Perhaps the lead author later just threw out a wild-ass, ridiculous guess at the size of the deposits, in response to a reporter’s question. But if he did not, and if the scientists themselves are not making the claim that there are an estimated 100 billion tonnes of rare-earth deposits, as reported by Reuters, Nikkei, and the BBC just who IS making this claim? Who has inserted these comments into this story, and fed them to the mainstream media, and why might they have done that? Can we find clues in the current pricing turmoil, worries about supply from China, and the increasing politicization of the rare-earths story?
I leave those questions as an exercise for the reader to ponder!
Gareth Hatch on July 4, 2011