Scientists at the California Institute of Technology have developed a new type of solar cell that comprise of arrays of thin silicon wires embedded in polymer substrate. The unique optical interactions between these wires provide the cells with an enhanced light absorption capability and improved internal quantum efficiency over conventional solar cells. These new cells are much cheaper to produce on account of the very low amounts of silicon needed to build them. The superior structural flexibility possessed by the silicon wire array solar cells is expected to further reduce their production cost since they can be produced using a lower-cost process.
by Naimish Upadhyay, Green Economy Post
By employing long, thin silicon wires embedded in a polymer substrate, California Institute of Technology (Caltech) scientists have developed a new type of inexpensive solar cell that not only absorb more sunlight but is also more efficient at converting it into electrical power.
Scientists at Caltech’s Resnick Institute formed an array of very thin silicon wires, each measuring between 30 and 100 microns in length and only 1 micron in diameter. While each wire was known to act independently as a high-efficiency, high-quality solar cell, the researchers found that the efficiency of light absorption increased by bringing them together in an array.
When light falls on the silicon surface, only a portion of it gets absorbed and another portion scatters. By placing the wires next to each other, however, the collective scattering interactions between them result in an enhanced absorption – up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight. These new solar cells thus surpass the conventional light-trapping limit for absorbing materials.
The scattering interaction effect occurs despite the sparseness of the wires in the array – they cover only between 2 and 10 percent of the cell’s surface area. When the researchers first began constructing on silicon wire-array solar cells, they assumed that sunlight would be wasted on the space between wires. But while quantifying the absorption, they realized that even relatively sparse wire arrays are able to produce effective optical concentration, thus enhancing the cell efficiency.
Caltech solar arrays also show improved performance in converting the absorbed sunlight into electrical power. According to reports, between 90 and 100 percent of the photons absorbed by the silicon wires are converted into electrons – in technical terms, near-perfect internal quantum efficiency. It is this combination of high absorption and good conversion that makes the new solar cells high-quality.
Another breakthrough achieved in this research is that in terms of area or volume, just 2 percent of the array is silicon, and the rest (98 percent) 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 also means that they are flexible. Given their flexibility, the thin films can be manufactured in a roll-to-roll process – an inherently lower-cost process than one that involves brittle wafers, like those used to make conventional solar cells.
The Caltech team is now working to increase the operating voltage and the overall size of their creation in order to scale them up to the size of conventional solar cells.
The scientific team is comprised of Harry Atwater, Director of Caltech’s Resnick Intitute, Nathan Lewis, professor of Chemistry at Caltech, and graduate student Michael Kelzenberg. In addition, the co-authors of this research are postdoctoral scholars Shannon Boettcher and Joshua Spurgeon; undergraduate student Jan Petykiewicz; and graduate students Daniel Turner-Evans, Morgan Putnam, Emily Warren, and Ryan Briggs.
This research has been published in the paper “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications,” in the February 14, 2010 advance online edition of the journal Nature Materials.
Diagram Credit: Caltech/Michael Kelzenberg
