22 June 2012

Sandia’s InGaN nanowire template permits flexible solar energy absorption

Researchers in solar energy want to convert as many of the sun’s wavelengths as possible to achieve maximum efficiency. They hence see indium gallium nitride as a valuable future material for photovoltaic systems. Changing the concentration of indium allows tuning of 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 efficiency. In contrast, the photovoltaic industry’s incumbent material (silicon) is limited in the wavelength range that it can ‘see’ and absorb.

Image: Cross-sectional images of InGaN nanowire solar cell. (Courtesy of Sandia National Laboratories.)

However, indium gallium nitride (InGaN) is typically grown on thin films of gallium nitride (GaN). Because GaN layers have different crystal lattice spacings from InGaN layers, the mismatch leads to structural strain that limits both the layer thickness and the percentage of indium that can be added. Thus, increasing the percentage of indium that is added broadens the solar spectrum that can be collected, but reduces the material’s ability to tolerate the strain.

Recently, funded by the US Department of Energy (DOE) Office of Science through the Solid State Lighting Science Energy Frontier Research Center (SSLS EFRC) and by Sandia National Laboratories’ Laboratory Directed Research and Development program, Sandia scientists including Jonathan Wierer Jr and George Wang have reported that, if the indium mixture is grown on a phalanx of nanowires rather than on a flat surface, then the small surface areas of the nanowires allow the indium shell layer to partially ‘relax’ along each wire, easing strain (Jonathan J Wierer Jr et al, ‘III-nitride core-shell nanowire arrayed solar cells’, 2012 Nanotechnology 23 194007). This relaxation allowed the team to create a nanowire solar cell with indium percentages of about 33%, 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% compared to about 15% for a standard commercial cell — but the demonstration took place on imperfect nanowire-array templates. Refinements should lead to higher efficiencies and even lower energies.

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 GaN layer with a colloidal silica mask, followed by dry and wet etching. The resulting array consisted of nanowires with vertical sidewalls and uniform height.

Next, shell layers containing the higher indium percentage of InGaN were formed on the GaN nanowire template via metal-organic chemical vapor deposition (MOCVD). 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.

Although modest, the results represent a promising path forward for III-nitride solar cell research, says Wierer. The nano-architecture not only enables a 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.  

Tags: InGaN nanowire solar cell MOCVD

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