Nick Rosen | |
Caltech's brightest minds

Nanotechnology has come to the rescue of the solar industry — and not a moment too soon.

Solar cells have long been a favorite of people trying to reduce or eliminate their energy bills. Yet throughout its history, the relatively expensive up-front costs of obtaining photovoltaic cells has deterred many from investing in the technology. Now scientists from the California Institute of Technology (Caltech) have developed a new solar cell that will cost a tiny fraction of current prices. The researchers hope that their breakthrough will change the economics of solar forever.
The new cells consist of tiny silicon wires that measure a mere 1 micron in diameter. These wires are embedded into plastic plates where they convert light into electricity at an exceptional rate of efficiency. Any light that is leftover bounces around inside the wire matrix until it finds another wire that can absorb it. When all is said and done, nearly all the light is captured and converted into electricity.
Less silicon
The beauty of the new cell technology is that only two percent of the cell is composed of semiconductors, which are the most expensive component. The other 98 percent is made from inexpensive (albeit oil-based) plastic, which translates into significantly lower prices for consumers as opposed to existing solar cell technologies.
That low price is in inverse proportion to the rate at which the cells convert sunlight to electrical power.  The tips of the wires on a conventional panel cover only a few per cent of the cell’s sun-facing surface, so much of the light hitting the cell passes through unabsorbed.
In February this year, Harry Atwater and his colleagues (pictured above) at the California Institute of Technology in Pasadena reported a solution to this problem. They used microscale silicon rods slightly thicker than nanowires, and poured a polymer containing light-reflecting nanoparticles into the spaces between them. The polymer scatters unabsorbed light back onto the rods and this, combined with a silver reflecting layer at the bottom of the device, allows the cells to absorb up to 85 per cent of incoming light. Still, losses – chiefly from imperfections in the crystal structure of the microrods – drive the overall efficiency below the 20 per cent achieved by the best crystalline silicon cells (Nature Materials , vol 9, p 239).
So why the fuss, if these devices are no more efficient than what went before? The key is that although these cells are merely as efficient as conventional devices, they use only about a hundredth of the material. What’s more, they are highly flexible: grown on a bed of silicon, Atwater’s microrod arrays can simply be peeled off and stuck pretty much wherever you want. “They could even be integrated into buildings, as components that match the shape of roof tiles,” says Atwater. He has started up a company, Alta Devices, to do just that, and has recently received research funding from the US Department of Energy.
Though roughly the same thickness as conventional solar cells, the new cells contain far less silicon. The team is currently working on expanding the voltage capacity and size of the cells in order to manufacture large, flexible sheets that can be manufactured inexpensively using “roll-to-roll” fabrication equipment.
The team has yet to release any side-by-side comparisons of the new cells versus the old cells, but as the technology is refined, researchers will likely conduct the necessary experiments to gain this data and make it available to the public.

Solar cell breakthrough
Scientists from the California Institute of Technology (CIT) have developed a new solar cell that will cost a tiny fraction of current prices.

Solar cells have long been a favorite of people trying to reduce or eliminate their energy bills. Yet throughout its history, the relatively expensive up-front costs of obtaining photovoltaic cells has deterred many from investing in the technology. The researchers hope that their breakthrough new technology will change that.
The new cells consist of tiny silicon wires that measure a mere 1 micron in diameter. These wires are embedded into plastic plates where they convert light into electricity at an exceptional rate of efficiency. Any light that is leftover bounces around inside the wire matrix until it finds another wire that can absorb it. When all is said and done, nearly all the light is captured and converted into electricity.
The beauty of the new cell technology is that only two percent of the cell is composed of semiconductors, which are the most expensive component. The other 98 percent is made from inexpensive plastic, which translates into significantly lower prices for consumers as opposed to existing solar cell technologies.
That low price is in inverse proportion to the rate at which the cells convert sunlight to electrical power.  The tips of the wires on a conventional panel cover only a few per cent of the cell’s sun-facing surface, so much of the light hitting the cell passes through unabsorbed.
In February this year, Harry Atwater and his colleagues at the California Institute of Technology in Pasadena reported a solution to this problem. They used microscale silicon rods slightly thicker than nanowires, and poured a polymer containing light-reflecting nanoparticles into the spaces between them. The polymer scatters unabsorbed light back onto the rods and this, combined with a silver reflecting layer at the bottom of the device, allows the cells to absorb up to 85 per cent of incoming light. Still, losses – chiefly from imperfections in the crystal structure of the microrods – drive the overall efficiency below the 20 per cent achieved by the best crystalline silicon cells (Nature Materials , vol 9, p 239).
So why the fuss, if these devices are no more efficient than what went before? The key is that although these cells are merely as efficient as conventional devices, they use only about a hundredth of the material. What’s more, they are highly flexible: grown on a bed of silicon, Atwater’s microrod arrays can simply be peeled off and stuck pretty much wherever you want. “They could even be integrated into buildings, as components that match the shape of roof tiles,” says Atwater. He has started up a company, Alta Devices, to do just that, and has recently received research funding from the US Department of Energy.
Though roughly the same thickness as conventional solar cells, the new cells contain far less silicon. The team is currently working on expanding the voltage capacity and size of the cells in order to manufacture large, flexible sheets that can be manufactured inexpensively using “roll-to-roll” fabrication equipment.
The team has yet to release any side-by-side comparisons of the new cells versus the old cells, but as the technology is refined, researchers will likely conduct the necessary experiments to gain this data and make it available to the public.

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