June 22, 2012
Nanotech Jump For Solar Cells
Solar power cells are limited by the wavelengths of light they can absorb from the sun. Ideally a solar cell would absorb every wavelength, but to date the goal has not been reached. Silicon, today's photovoltaic industry standard, is limited in the wavelength range it can 'see' and absorb.
A possible solution has been found in indium gallium nitride. Using this material, scientists can tune the material's response so it collects solar energy from a variety of wavelengths. The more variations designed into the system, the more of the solar spectrum can be absorbed, leading to increased solar cell efficiencies.
But there is a problem: Indium gallium nitride, part of a family of materials called III-nitrides, is typically grown on thin films of gallium nitride. Because gallium nitride atomic layers have different crystal lattice spacings from indium gallium nitride atomic layers, the mismatch leads to structural strain that limits both the layer thickness and percentage of indium that can be added. Thus, increasing the percentage of indium added broadens the solar spectrum that can be collected, but reduces the material's ability to tolerate the strain.
Jonathan Wierer Jr. and George Wang of Sandia National Laboratories reported in the journal Nanotechnology that if the indium mixture is grown on a phalanx of nanowires rather than on a flat surface, the small surface areas of the nanowires allow the indium shell layer to partially "relax" along each wire, easing strain. This relaxation allowed the team to create a nanowire solar cell with indium percentages of roughly 33 percent, higher than any other reported attempt.
The results, says Wierer, although modest, represent a promising path forward for III-nitride solar cell research. The nano-architecture not only enables higher indium proportion in the InGaN layers but also increased absorption via light scattering in the faceted InGaN canopy layer, as well as air voids that guide light within the nanowire array.