Nanowire substrate allows researchers to tune InGaN's response for flexible energy absorption
June 19, 2012 // Julien Happich
To convert as many of the sun's wavelengths as possible, researchers aim at changing the concentration of indium in photovoltaic materials such as indium gallium nitride (InGaN) 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 can be absorbed. Though 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.
Sandia National Laboratories scientists Jonathan Wierer Jr. and George Wang 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 at creating III-nitride solar cells.
Sandia nanowire template permits flexible energy absorption. Source: Sandia Laboratory
This initial attempt also lowered the absorption base energy from 2.4eV to 2.1 eV, claimed to be the lowest of any III-nitride solar cell to date, and made a wider range of wavelengths available for power conversion. Several unique techniques were used to create the III-nitride nanowire array solar cell. A top-down fabrication process was used to create 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) were formed on the GaN nanowire template via metal organic chemical vapor deposition. Lastly, In0.02Ga0.98N was grown, 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 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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