Growing an indium mixture on a clump of nanowires rather than on a flat surface boosts the percentage of the energy-absorbing material, potentially broadening the solar spectrum that photovoltaic systems can collect. Indium gallium nitride, part of a family of materials called III-nitrides, is typically grown on thin films of gallium nitride. Gallium nitride atomic layers have different crystal lattice spacings from indium gallium nitride atomic layers, so the mismatch causes a structural strain that limits both the percentage of indium that can be added and the layer thickness. Although increasing the indium percentage broadens the solar spectrum that can be collected, it reduces the material’s ability to tolerate the strain. Scientists at Sandia National Laboratories grew an indium mixture on the nanowires and found that their small surface areas enabled the indium shell layer to partially relax along each wire, thus easing strain. The relaxation yielded nanowire solar cells with indium percentages of about 33 percent, higher than any other reported attempt at creating III-nitride solar cells. Cross-sectional images of the indium gallium nitride nanowire solar cell. (Image: Sandia National Laboratories) The group’s initial attempt also lowered the absorption base energy from 2.4 to 2.1 eV, the lowest of any III-nitride solar cell to date, and made a broader range of wavelengths available for power conversion. Power conversion efficiencies were low — only 0.3 percent compared with a standard commercial cell that works at 15 percent — but the demonstration took place on imperfect nanowire-array templates. Refinements should yield higher efficiencies at even lower energies. Although modest, the results represent a promising path forward for III-nitride solar cell research, said Jonathan Wierer Jr., a Sandia scientist. A top-down fabrication process was used to create the nanowire array, which masked a gallium nitride (GaN) layer with a colloidal silica mask, followed by dry and wet etching. The array that resulted consisted of nanowires with vertical sidewalls and of uniform height. Next, shell layers that contained the higher indium percentage of indium gallium nitride (InGaN) were formed on the GaN nanowire template using a metallorganic chemical vapor deposition method (MOCVD). In the final stage, the scientists grew In0.02Ga0.98N in a way that caused the nanowires to coalesce. The process produced a canopy layer at the top, facilitating simple planar processing and making the technology manufacturable. The nanoarchitecture of the device not only enables higher indium proportion in the InGaN layers, but also increases absorption via light scattering in the faceted InGaN canopy layer and air voids that guide light within the nanowire array. The study appeared in Nanotechnology. For more information, visit: www.sandia.gov
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