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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
LCD screens continue to drive demand
31 Jan 2011 16:21 | by Matthew Finnegan in London | posted in Business
Indium market will see demand rise 16 percent until 2013 -
The market for indium is expected to see an annual average increase of 16 percent up until 2013, following a good 2010.
The growth will be supported by the continued demand for LCD screens, while applications for Photovoltaic solar cells are expected to contribute on a smaller scale.
Indium, a key raw material in indium tin oxide (ITO), is widely used in liquid crystal displays and touchscreen technologies.
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With the increasing proliferation of devices using the material it is expected that growth will continue, with demand particularly strong in the television and computer monitor markets.
Like many markets the indium industry is seeing demand recover from a downturn in 2009, and is now seeing focus switch to the supply side, according to Roskill Information Services. Production capacity is currently significantly above current output, so it will be possible to increase production at existing facilities.
China, where roughly half the global output is centered, may have more capacity than previously thought – with scope to increase production.
It is not thought that the use of ITO in the production of PV panels is expected to contribute hugely to the expected indium demand growth, due to supposed doubts over growth rates for the solar panels.
Of course it is noted that the PV market is a newer and faster growing market, with solar applications for indium increasing by 40 percent per year, though from a smaller base rate.
Following many fluctuations in the past years, indium prices surged upward in early 2010, buoyed by purchases from Japanese producers of LCDs who appeared to have completed a long period of de-stocking and were beginning to rebuild inventories.
With indium demand expected to increase strongly over the next two years it is thought that the main problem will be for production capacity to continue to keep up with demand.
If this is the case it is expected that indium prices could reach around $850/kg by 2013, though it is thought that in the longer term, as supply begins to meet demand, prices are likely to stabilise.
There is some controversy over the amount of indium availability in the world. According to Indium Corporation the figure stands at 26,000 megatonnes in the “western world”, while China and Russia account for 23,000 megatonnes of the world’s reserves.
To illustrate how widespread it claims indium is, Indium Corporation reckons the raw material is more prevalent in the earth than silver.