Scientists from the California Institute of Technology (Caltech) have developed a new solar cell that they hope will cost a tiny fraction of current production. The new cells consist of tiny silicon wires that measure a mere 1-micron in diameter. These wires are embedded lengthwise and perpendicular 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, thus nearly all the light is captured and converted into electricity.
Professor Harry Atwater at his namesake research group at Caltech explains the new solar material made of tiny silicon wires could “dramatically reduce the cost of making a silicon solar cell. Instead of the expensive process of making a wafer and slicing it up with a saw, throwing away two thirds of it,” says Atwater, “We grow the material and literally peel it off. The plastic sheet is peeled off like scotch tape off a tape dispenser.”
The savings in the new cell technology is that only 2% of the cell is composed of semiconductors – the most expensive component. The other 98% is made from inexpensive plastic, which should translate into significantly lower prices for consumers compared to existing solar cell technologies. That lower price is in inverse proportion to the rate at which the cells convert sunlight to electrical power.
Professor Atwater and his colleagues used microscale silicon wires (microwires) 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. But losses mostly from imperfections in the crystal structure of the microwires drive the overall efficiency below the 20 per cent achieved by the best crystalline silicon cells.
The point is that while these cells are merely as efficient as very good photovoltaic panels, they use only about a hundredth of the material. Also the new design is highly flexible: built on a bed of silicon, Atwater’s micrwire 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.
At about the same thickness as conventional photovoltaic cells, Atwater’s 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 Caltech team had Dave Bullock from Wired.com in for a tour. The article is short but the photos quite good. Over the course of nine photos one comes away with a good idea of the production process. It’s worth the click and a look. The full paper from Nature Materials Letters is available in a pdf download.
There are two primary types of photovoltaic cells. The first is a solid silicon-based PV cell that is very efficient, but also expensive to make and relatively fragile. The second is a thin film cell, which is relatively cheap to make but not as efficient. The Caltech group’s new microwire material potentially bridges that gap, creating a photovoltaic cell that should be low cost to manufacture, but which is close to the efficiency of traditional silicon-based solar panels and perhaps not so easily broken.
This is just round one in the research. Intuition suggests that much of the regular silicon research might well transfer to Atwater’s concept. We’ll see – this idea is worth watching and it seems Atwater’s team is looking into it as well.