Nanowires could be used to make solar cells that are more flexible and cheaper than devices made from conventional III-V semiconductors. The nanostructures are efficient absorbers of light and can act as “antennas”, harvesting much more light thanks to plasmonic resonant modes that couple strongly to the electromagnetic radiation. “One consequence of this strong absorption on nanowires is that we observe high light absorption efficiencies even though only a small part of the device surface is covered by the nanomaterials,” explained team member Magnus Borgström of Lund University.

The researchers grew indium phosphide (InP) nanowires using a vapour solid growth technique, where metal particles help growth in regions underneath the particles themselves. During growth, the particles flow on top of the growing wires. The team, which includes scientists from Solid State Physics in Lund, Fraunhofer ISE in Freiburg, the University of Kassel, and the start-up company Solvoltaics AB, also in Lund, chose InP because it has a direct bandgap of 1.34 eV, which means that it can absorb light over a broad range of solar spectrum wavelengths.

Challenges and triumphs

“Our biggest challenge in the process was to grow the nanowires with the correct doping profile and without any parasitic radial overgrowth (which InP is prone to and which leads to short circuiting), while making large-area devices (necessary for solar-cell applications),” Borgström told “Our nanowires needed to be uniform, having a certain diameter and length in a certain pitch. From our first working p-n InP junctions, it has taken us four years to reach this result, thanks to a technique we developed involving in situ hydrochloric acid, which impedes parasitic radial overgrowth of the nanowires.”

The highest cell efficiency ever reported for InP is 22% and it remains to be seen if we can break this record using smaller amounts of the nanowire material, adds Borgström. “We believe that the road ahead for solar-cell application involves multi-junction technology with nanowires for which the record is 44% in thin films.”

The current work is reported in Science.