Solar Cells Constructed with Silicon Nanowires
Michael A. Greenwood
Although most photovoltaic cells are constructed from traditional crystalline silicon, researchers are seeking alternatives to power a generation of solar technology with improved efficiency and lower costs.
One method being considered involves using silicon nanowires as the building blocks for solar cells. The rod-shaped wires offer several advantages, including improved charge transport in comparison with other nanostructures and the potential to provide performance equal to, or even better than, that of crystalline silicon, while potentially costing substantially less.
A scanning electron micrograph shows a typical silicon nanowire array on stainless steel foil. The insets show a cross-sectional view of the device (top left) and an individual wire highly magnified (top right). Reprinted with permission of Applied Physics Letters.
Researchers from General Electric Global Research Center in Niskayuna, N.Y., and from GE Energy Solar Technologies in Newark, Del., led by Loucas Tsakalakos, report that they have fabricated all-inorganic, large-area solar cells with silicon nanowires.
The array was created by depositing a 100-nm-thick Ta2N film onto a stainless steel foil substrate, followed by the deposition of a 50-Å-thick Au film. Chemical vapor deposition was used to grow p-type Si nanowires with a diameter of 109 ±30 nm and a length of ∼16 μm. A layer of n-type amorphous silicon was added to create the photoactive p-n junction.
During experiments with the nanowire-based cells, the researchers found that their level of optical reflectance was lower than that of traditional solar cells by one to two orders of magnitude over a spectrum ranging from 300 to 1100 nm. This was a result of the improved index matching of the nanowires with air compared with solid films, although the team’s previous work also has shown enhanced effective absorption (because of light trapping) in nanowire arrays.
The nanowire device exhibited spectrally broad external quantum efficiency, with a maximum value of ∼12 percent occurring at 690 nm, and its architecture generated a current density of ∼1.6 mA/cm2 for 1.8-cm2 cells. Although Tsakalakos described this level as promising, he said that, with further research, he would like to achieve current density levels as high as 25 mA/cm2, as well as to improve further the open-circuit voltage to greater than 600 mV and to increase the fill factor to greater than 0.6.
The researchers said their calculations show that silicon nanowire-based solar cells have the potential to provide power conversion efficiencies of 15 to 18 percent under 1 sun illumination, depending on the size and the quality of the nanowires. That level is comparable to that being achieved now with bulk silicon solar cells, but the nanowire design has the potential advantage of being significantly less expensive to mass-produce.
The research team is continuing to work on improving the efficiency of its nanowire cells by further engineering the nanowire geometry, by improving the quality of the p-n junction and by reducing contact resistance.
Applied Physics Letters, Dec. 3, 2007, Vol. 91, 233117.
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