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Scientists in California have come up with a radical new approach to flexible solar-cell technology that apparently makes them both more effective and cheaper.
The California Institute of Technology (Caltech) team is using arrays of long, thin silicon wires embedded in a polymer substrate.
The result is a cell that enhances the absorption of sunlight and efficiently converts its photons into electrons, and this using only a fraction of the expensive semiconductor materials required by conventional solar cells.
The light-trapping limit of a material refers to how much sunlight it is able to absorb. The silicon-wire arrays absorb up to 96% of incident sunlight at a single wavelength and 85% of total collectible sunlight. This surpasses previous optical microstructures developed to trap light.
The silicon-wire arrays created at Caltech are able to convert 90-100% of the photons they absorb into electrons. In technical terms, the wires have a near-perfect internal “quantum efficiency”.
Basically, high absorption plus good conversion makes for a high-quality solar cell.
The key to the success of these solar cells is their silicon wires, each of which, in its own right, is a high-efficiency, high-quality solar cell. When brought together in an array, however, they are even more effective because they interact to increase the cell’s ability to absorb light. Light comes into each wire and a portion is absorbed and another portion scatters. The collective scattering interactions between the wires makes the array very absorbing. This effect occurs despite the sparseness of the wires in the array – they cover only 2-10% of the cell’s surface area.
When the scientists first considered silicon-wire-array solar cells, they assumed that sunlight would be wasted on the space between wires. Indeed, the initial plan was to “grow” the wires as close together as possible.
But when they started quantifying their absorption, they realised that more light could be absorbed than predicted by the wire-packing fraction alone. By developing light-trapping techniques for relatively sparse wire arrays, not only did they achieve suitable absorption, they also demonstrated effective optical concentration – an exciting prospect for further enhancing the efficiency of silicon-wire-array solar cells. Each wire measures 30-100 microns in length and only one micron in diameter. The entire thickness of the array is the length of a wire. But in terms of area or volume, just 2% of it is silicon and 98% is polymer. In other words, while these arrays have the thickness of a conventional crystalline solar cell, their volume is equivalent to that of a two-micron-thick film.
Since the silicon material is an expensive component of a conventional solar cell, a cell that requires just one-fiftieth of the amount of this semiconductor will be much cheaper to produce. The composite nature of these solar cells means they are also flexible.
This is a valuable property as it means flexible, thin films can be manufactured in a roll-to-roll process, an inherently lower-cost process than one that involves brittle wafers such as those used to make conventional solar cells. It also means that such cells can be applied to a surface like wallpaper.
Supporters of this Caltech work include UK petroleum giant BP, which is also a leading manufacturer of solar cells.