Silicon Nanowires Show Promise for Solar Applications

Michael A. Greenwood

As the search continues for efficient and inexpensive materials to harvest solar energy, investigators at MIT in Cambridge have found that silicon nanowire arrays may offer some advantages over the more established thin-film technology.

Researchers studied the optical absorption abilities of silicon nanowire arrays. Courtesy of Lu Hu.

Researchers Lu Hu and Gang Chen report that, based on numerical simulation, nanowire arrays in some configurations have a much lower reflectance compared with thin films, meaning that more incoming light is harnessed and converted into usable energy. They also found that, at higher optical energy levels, the nanowires absorb more light than thin films.

The team calculated optical absorption in crystalline silicon nanowires with diameters between 50 and 80 nm, with lengths of 1.16, 2.33 and 4.66 μm and with a periodicity of the square lattice of 100 nm. The frequency range was from 1.1 to 4 eV. A silicon thin film with a thickness of 2.33 μm was used as a reference.

The researchers found that, at lower photon energy levels, all three nanowire lengths exhibited very little optical absorption. The thin film, meanwhile, had relatively high absorption at lower energy levels.

The absorption of nanowires of three lengths (1.16, 2.33 and 4.66 μm) was lower than that of thin film when photon energy was low. The absorption of the nanowires increased as photon energy increased. The nanowires had a diameter of 50 nm and a periodicity of 100 nm. Reprinted with permission of Nano Letters.

However, as the energy frequency increased, the optical absorption of the nanowires rose sharply and surpassed the absorption level of the thin film at ~2.5 eV. The absorption of the nanowires continued to climb, until each of the lengths reached a plateau at ~3 eV. The absorption of the thin film, meanwhile, declined gradually after being surpassed by the absorption of the nanowires. The absorption of the longest wire was slightly higher than that of the other two until hitting the plateau point shared by all three nanowire lengths.

Another potential advantage of the nanowires compared with thin films is that their small level of reflection is achieved without specially designed antireflection coatings. On the other hand, one weakness identified with the nanowires is their poor absorption at low energy levels. The researchers said that this likely could be overcome with light-trapping techniques or with longer silicon nanowires.

Chen said that the research is in a very preliminary stage and that it is unlikely nanowires will be used in photovoltaic devices anytime soon.

Nano Letters, ASAP Edition, Oct. 10, 2007, doi:10.1021/nl071018b.