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.