Aug 7, 2012
Sandia team demonstrates III-nitride nanowire array solar cells
Indium gallium nitride (InGaN) semiconductors are often described as a potential next-generation photovoltaic material. The energy absorption of the alloy can be tuned across the entire solar spectrum by varying its composition, which could lead to devices with higher conversion efficiencies. Unfortunately, there are some fabrication issues to contend with. In practice, InxGa1–xN is grown as a thin film on thin films of GaN. The mismatch of atomic lattices caused by the presence of indium induces structural strain between layers and limits the composition of indium (x) to low percentages. Under these conditions, InxGa1–xN is only able to absorb at the highest energies of the solar spectrum, giving solar cells with low power-conversion efficiencies. To overcome strain limitations, a team of scientists at Sandia National Laboratories has grown InxGa1–xN-based shell layers on a GaN nanowire array rather than on a flat thin film. The small surface area of the nanowires allows the shell layer to partially relax along each wire to accommodate increased indium content.
The group employed several unique techniques to create the III-nitride nanowire array solar cell. First, top-down fabrication was used to produce the nanowire array by masking an n-type GaN layer with colloidal silica and then dry and wet etching. The resulting array consisted of nanowires with vertical sidewalls and of uniform height. Next, shell layers were formed on the GaN nanowire template via metal organic chemical vapor deposition. The InGaN/GaN multiple quantum well (MQW) absorption layer achieved indium percentages of 33%. Last, a p-type In0.02Ga0.98N layer was grown. Here, the close proximity of the nanowires allowed the material to coalescence, which produced a canopy over the top of the structure and facilitated simple planar processing.
Foundation for future work
Thanks to the approach, the team was able to lower the energy band gap of the solar cells from 2.4 to 2.1 eV, the lowest of any III-nitride solar cell to date. In future work, the scientists plan to refine the nanowire templates, which should lead to higher efficiencies and even lower energies.
The results, although modest, represent a promising path forwards for InGaN solar-cell research. The nano-architecture not only enables InGaN layers of higher indium composition, but also facilitates increased absorption via light scattering in the faceted p-InGaN canopy layer and light guiding via air voids within the nanowire array.
The researchers presented their results in the journal of Nanotechnology.
About the author
The research was funded by Sandia’s Department of Energy, Basic Energy Sciences Solid State Lighting Science Energy Frontier Research Center, and the Laboratory Directed Research and Development programme. The III-nitride nanowire solar-cell programme is led by Jonathan J Wierer, Jr and George T Wang. Sandia National Laboratories is a multi-programme laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.