Solar Power Possibilities Prevail

Thursday, June 21, 2012 @ 06:06 PM gHale

The sun has plenty of energy to give, why not use as much of that power as possible – at least that is the thinking behind the new materials for a photovoltaic system.

Indium gallium nitride can be a valuable future material for photovoltaic systems, researchers said.

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Changing the concentration of indium allows researchers to 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 the system can absorb, which can lead to increased solar cell efficiencies. There are limitations in silicon, today’s photovoltaic industry standard, as to how far the wavelength range can “see” and absorb.

As it always goes with solar energy, there is a problem: Indium gallium nitride, part of a family of materials called III-nitrides, typically grows 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 the layer thickness and percentage of indium that can add in. Thus, increasing the percentage of indium broadens the solar spectrum that a system can collect, but it reduces the material’s ability to tolerate the strain.

But there is now a new twist. If researchers grow indium mixture 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, said Sandia National Laboratories scientists Jonathan Wierer Jr. and George Wang in a paper on the subject. This relaxation allowed the team to create a nanowire solar cell with indium percentages of roughly 33 percent, higher than any other reported attempt at creating III-nitride solar cells.

This initial attempt also lowered the absorption base energy from 2.4eV to 2.1 eV, the lowest of any III-nitride solar cell to date, and made a wider range of wavelengths available for power conversion. Power conversion efficiencies were low — only 0.3 percent compared to a standard commercial cell that hums along at about 15 percent — but the demonstration took place on imperfect nanowire-array templates. Refinements should lead to higher efficiencies and even lower energies.

Several unique techniques helped create the III-nitride nanowire array solar cell. A top-down fabrication process created the nanowire array by masking a gallium nitride (GaN) layer with a colloidal silica mask, followed by dry and wet etching. The resulting array consisted of nanowires with vertical sidewalls and of uniform height.

Next, shell layers containing the higher indium percentage of indium gallium nitride (InGaN) formed on the GaN nanowire template via metal organic chemical vapor deposition. Lastly, they were able to grow In0.02Ga0.98N in such a way that caused the nanowires to coalescence. This process produced a canopy layer at the top, facilitating simple planar processing and making the technology manufacturable.

The results, although modest, represent a promising path forward for III-nitride solar cell research, Wierer said. 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.

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