Nano Focus: Stretchable gold conductor grows its own wires
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Nano Focus
trical back contact by dc sputtering and layers of MoO3 (150 nm) and Te (50 nm) by vacuum evaporation. Next, they used high-vacuum evaporation to deposit 4–6 μm of CdTe, followed by a standard recrystallization step. They then deposited a carefully controlled layer of Cu through high-vacuum evaporation followed by annealing at 400°C to promote diffusion into the CdTe. The cells were completed with a CdS layer and an i-ZnO/ZnO:Al bilayer front contact. Using secondary ion mass spectroscopy, the researchers found that the CdTe layer was successfully doped with Cu. For the optimal Cu concentra-
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tion (equivalent to a submonolayer of approximately 1 Å thickness), the glasssubstrate cells displayed efficiencies of up to 13.6%, while the Mo-foil-substrate and steel-foil-substrate cells achieved 11.5% and 10.9%, respectively. These results suggest that roll-to-roll manufacturing of CdTe solar cells with efficiencies approaching those of conventional CdTe cell configurations may be possible, offering significant manufacturing cost reductions and potentially positioning CdTe as an even more notable competitor to silicon-based solar cells. Colin McCormick
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Stretchable gold conductor grows its own wires
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etworks of spherical nanoparticles embedded in elastic materials may make the best stretchy conductors yet, engineering researchers at the University of Michigan and Korea Basic Science Institute have discovered. Flexible electronics have a wide variety of possibilities, from bendable displays and batteries to medical implants that move with the body. “Essentially the new nanoparticle materials behave as elastic metals,” said lead researcher Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering. Finding good conductors that still work when pulled to twice their length is a tall order—researchers have tried wires in zigzag or spring-like patterns, liquid metals, and nanowire networks. The team was surprised that spherical gold nanoparticles embedded in polyurethane could outcompete the best of these in their stretchability and concentration of electrons. “We found that nanoparticles aligned into chain form when stretching. That can make excellent conducting pathways,” said U-Mich. graduate student Yoonseob Kim, first author of the study published in the July 17 online edition of Nature (DOI: 10.1038/nature12401). To find out what happened as the material was stretched, the team took
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(a) An electron microscope image of the gold nanoparticles in a relaxed sample of the layer-by-layer material. The nanoparticles are dispersed. (b) A similar sample stretched to a little over twice its original length, at the same magnification. The nanoparticles form a distinct network. Reproduced with permission from Nature 500 (2013), DOI: 10.1038/nature12401. © 2013 Macmillan Publishers Ltd.
state-of-the-art electron microscope images of the materials at various tensions. The nanoparticles started out dispersed, but under strain they could filter through the minuscule gaps in the polyureth
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