'Nanowelding' silver mesh

Engineers fabricating solar cells have been searching for ways to make better electrodes that conduct electricity, while also allowing light through. However, the transparent materials typically used in solar cells, such as zinc oxide, are expensive and do not conduct electrical current as well as metals such as silver.

Mark Brongersma, a materials scientist at California's Stanford University and colleagues decided to make electrodes from silver nanowires of 30–80 nanometres in diameter, describing their technique in the journal Nature Materials. The nanowires were arranged in a chicken-like mesh to allow light to pass through into the cell, explains Brongersma. The challenge, however, was to make good electrical contacts at the points where the nanowires crossed each other. "If you try and heat the wires on a hot plate to weld them together, the metal wires break-up."

The team — which includes Fakhruddin Mahmood, an electrical engineer at both Stanford and the King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia — helped devise an alternative 'nano-welding' method. They shone the light of a tungsten halogen lamp onto the wires. The light's electromagnetic field induced currents in the wires, with most of the oscillating electrical charge being concentrated at the points where the wires touch. This contact generates heat only where wires cross, fusing the silver wires together without disrupting the structure of the greater nanowire mesh. "It's like using a magnifying glass to focus the heat just where you need it," says Brongersma.

The team has already built a 5 centimeters squared piece of mesh, and now plan on using their welding equipment to make nanowire meshes of around 30 centimeters squared — the size of scale needed in commercial solar cells.

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