One-dimensional nanowires made of semiconductors like indium phosphide, gallium phosphide, and silicon are promising nanomaterials for next-generation inorganic solar-cell devices. Understanding their optical response is extremely important for achieving increased performance in solar-energy collection. Researchers at the FOM-Institute AMOLF (Amsterdam, The Netherlands) and Philips Research (Eindhoven, The Netherlands) have investigated strong light scattering in layers of nanowires, showing that trapping of light by multiple scattering is important in the design of nanowire devices.1,2
The investigators demonstrate that the sensitive balance between absorption and light scattering leads to large variations in the diffuse reflection of light from the nanowires. By proper design, multiple scattering can be used both to significantly reduce reflective losses with respect to a homogeneous film and to maximize absorption in a thin layer by folding of light paths in a random walk. The researchers demonstrate that light scattering produces high absorption in a 2-micrometer-thick layer of indium phosphide nanowires, making this an extremely relevant material for solar cells. For silicon nanowires, the absorption length of several micrometers compares less favorably to the nanowire scattering mean free path, which leads to much higher reflective losses in the visible range of the spectrum.
In their more-recent publication, the researchers have demonstrated that nanowires actually can be grown to form one of the most strongly scattering materials available today. Next to its technological relevance, this property opens exciting new prospects in fundamental research on random lasers and Anderson localization of light. By matching the nanowire diameter to the optical wavelength, light can be trapped for several periods inside the nanowire, leading to a resonant enhancement of its scattering efficiency. The high tunability of nanowire properties and alignment, and the general applicability to groups III-V, II-VI, and IV semiconductors, open up new possibilities for harvesting of the solar spectrum.
This work is part of an industrial partnership program between Philips and FOM.
1. O. L. Muskens et al., Nano Lett. 8, 2638:2642 (2008).
2. O. L. Muskens et al., Nano Lett. ASAP (2009).