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Nanopillars could spell cheaper, more efficient solar cells
News and opinions
379
Nanopillars could spell cheaper, more efficient solar cells Cordelia Sealy Arrays of CdS nanoscale pillars could spell cheaper and more efficient solar cells, according to researchers Ali Javey and colleagues at Lawrence Berkeley National Laboratory and the University of California at Berkeley [Z. Fan et al., Nature Materials (2009), doi:10.1038/nmat2493]. ‘‘Single-crystalline semiconductors offer a lot of promise, but standard ways of making them aren’t economical,’’ explains Javey. Instead, Javey and his colleagues have used a templateassisted vapor—solid—liquid (VLS) process to fabricate large-scale arrays of highly ordered, single crystalline nanopillars on an Al substrate. Highly periodic anodic alumina membranes (AMMs) form the template, which can be controllably etched away after nanopillar growth. Unlike conventional two-dimensional solar cells, the nanopillar array offers a much larger surface area for collecting light. The researchers’ simulations indicate that the structures should improve the collection and separation of photocarriers, boosting the overall energy conversion efficiency of devices. Test devices based on the nanopillar arrays achieve an efficiency of 6%, which — although lower than the 10—18% typically reached by commercial solar cells — is a significant improvement on most previously reported photovoltaic devices based on nanostructured materials. The researchers are confident that the efficiency can be improved beyond 6% through further optimization of the device and materials. One area where the researchers are already focusing their attention is the top Cu/Al electrical contact, which currently produces a 50% efficiency loss because of its low transparency. An advantage of the approach is that the nanopillar arrays can be embedded in other flexible materials (Fig. 1), such as polydimethylsiloxane (PDMS). The PDMS-embedded nanopillar solar cells can endure repeated bending without degrading their structure or their performance. ‘‘The process could potentially be made compatible with a roll-to-roll fabrication scheme, making the process very cost effective,’’ says Javey. The three-dimensional configuration of the nanopillar arrays relaxes the material quality and purity requirements for solar cells and reduces the amount of active semiconductor material needed in a device. The researchers do admit, however, that there is significant work to be done before these potential material cost savings can be realized. The savings could also be partially offset by the device fabrication costs, including the alumina anodization and the top contact formation. Ulrich Gösele of the Max Planck Institute of Microstructure Physics in Germany agrees that the costs associated with the approach need to be addressed. However, he says
Figure 1 A fully fabricated flexible nanopillar solar cell. (Credit: Zhiyong Fan.)
that it does give ‘‘credible hope that nanopillar-based solar cells in the long run will have a good chance of getting better than thin-film or bulk material solar cells’’. ‘‘The capability of fabricating ordered arrays of singlecrystalline, vertically aligned nanorods without the use of epitaxy or a single crystalline substrate is a major advance,’’ he told Nano Today. Cordelia Sealy has many years’ experience as a scientific journalist and editor in areas spanning nanotechnology, materials science and engineering, physics and chemistry. She has served as Editor of Materials Today and Nano Today, and more latterly as Managing Editor of both titles. She has also worked in academic publishing as a books acquisitions editor and in business-to-business publishing as a journalist on European Semiconductor. She has a First in Physical Sciences (BSc) from University College London and a DPhil in materials science from the University of Oxford, and is a Member of the Institute of Physics. Cordelia is currently a freelance science writer for her own company, Oxford Science Writing, and News and Opinions Editor for Nano Today. E-mail address: [email protected] 1748-0132/$ — see front matter doi: 10.1016/j.nantod.2009.08.004
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Nanopillars could spell cheaper, more efficient solar cells Cordelia Sealy Arrays of CdS nanoscale pillars could spell cheaper and more efficient solar cells, according to researchers Ali Javey and colleagues at Lawrence Berkeley National Laboratory and the University of California at Berkeley [Z. Fan et al., Nature Materials (2009), doi:10.1038/nmat2493]. ‘‘Single-crystalline semiconductors offer a lot of promise, but standard ways of making them aren’t economical,’’ explains Javey. Instead, Javey and his colleagues have used a templateassisted vapor—solid—liquid (VLS) process to fabricate large-scale arrays of highly ordered, single crystalline nanopillars on an Al substrate. Highly periodic anodic alumina membranes (AMMs) form the template, which can be controllably etched away after nanopillar growth. Unlike conventional two-dimensional solar cells, the nanopillar array offers a much larger surface area for collecting light. The researchers’ simulations indicate that the structures should improve the collection and separation of photocarriers, boosting the overall energy conversion efficiency of devices. Test devices based on the nanopillar arrays achieve an efficiency of 6%, which — although lower than the 10—18% typically reached by commercial solar cells — is a significant improvement on most previously reported photovoltaic devices based on nanostructured materials. The researchers are confident that the efficiency can be improved beyond 6% through further optimization of the device and materials. One area where the researchers are already focusing their attention is the top Cu/Al electrical contact, which currently produces a 50% efficiency loss because of its low transparency. An advantage of the approach is that the nanopillar arrays can be embedded in other flexible materials (Fig. 1), such as polydimethylsiloxane (PDMS). The PDMS-embedded nanopillar solar cells can endure repeated bending without degrading their structure or their performance. ‘‘The process could potentially be made compatible with a roll-to-roll fabrication scheme, making the process very cost effective,’’ says Javey. The three-dimensional configuration of the nanopillar arrays relaxes the material quality and purity requirements for solar cells and reduces the amount of active semiconductor material needed in a device. The researchers do admit, however, that there is significant work to be done before these potential material cost savings can be realized. The savings could also be partially offset by the device fabrication costs, including the alumina anodization and the top contact formation. Ulrich Gösele of the Max Planck Institute of Microstructure Physics in Germany agrees that the costs associated with the approach need to be addressed. However, he says
Figure 1 A fully fabricated flexible nanopillar solar cell. (Credit: Zhiyong Fan.)
that it does give ‘‘credible hope that nanopillar-based solar cells in the long run will have a good chance of getting better than thin-film or bulk material solar cells’’. ‘‘The capability of fabricating ordered arrays of singlecrystalline, vertically aligned nanorods without the use of epitaxy or a single crystalline substrate is a major advance,’’ he told Nano Today. Cordelia Sealy has many years’ experience as a scientific journalist and editor in areas spanning nanotechnology, materials science and engineering, physics and chemistry. She has served as Editor of Materials Today and Nano Today, and more latterly as Managing Editor of both titles. She has also worked in academic publishing as a books acquisitions editor and in business-to-business publishing as a journalist on European Semiconductor. She has a First in Physical Sciences (BSc) from University College London and a DPhil in materials science from the University of Oxford, and is a Member of the Institute of Physics. Cordelia is currently a freelance science writer for her own company, Oxford Science Writing, and News and Opinions Editor for Nano Today. E-mail address: [email protected] 1748-0132/$ — see front matter doi: 10.1016/j.nantod.2009.08.004