Nanowires make good solar cells

physicsworld.com
21 Jan 2013

Researchers in Sweden and Germany say they have made an important breakthrough in the development of highly efficient solar cells based on nanowires. They have shown that cells made from tiny wires of the semiconductor indium phosphide (InP) have efficiencies as high as 13.8% while covering only about 12% of the surface of a device. While an efficiency of 13.8% is not as good as the best commercial silicon devices, the team believes that it could be improved significantly by further research.

Nanowires-tiny semiconductor wires with a thickness of just a few hundred nanometres or less-show great potential for making solar cells that are more flexible, lightweight and cheaper than conventional planar devices. Nanostructures such as wires are efficient absorbers of light and can act as "antennas", harvesting much more light than a device with a planar surface. This is thanks to collective oscillations of charge carriers-called plasmons-that interact strongly with light. "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's surface is covered by the nanomaterials", explains team member Magnus Borgstroem of Lund University.

Millions of wires
The devices made by the team measured about a square millimetre and each contains about four million InP nanowires. The researchers grew their nanowires using an established technique called "vapour solid growth". "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", Borgstroem explains.

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, also in Lund, chose InP because it has a direct band gap of 1.34 eV, which means that it can absorb light over a range of solar-spectrum wavelengths.

In this latest work, the team was able to identify the ideal diameter of the nanowires-which turned out to be about 180 nm. "The right size is essential for the nanowires to absorb as many photons as possible. If they are just a few tenths of a nanometre too small, their function is significantly impaired", says Borgstroem.

Proof of principle
The cells made by the team have efficiencies as high as 13.8%-which is promising but still significantly less than the best commercial silicon devices, which operate at 15 22%. "Our findings are the first to show that it really is possible to use nanowires to manufacture solar cells", says Borgstroem. However, the highest efficiency ever reported for a conventional InP solar cell is 22% and the team admits that it still remains to be seen whether that record can be broken using smaller amounts of nanowire material.

The researchers also believe that the way forward for nanowire-based solar cells is the multi-junction approach, which boosts efficiency by using several different structures tuned to different wavelengths of light from the Sun. "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", says Borgstroem.

The work is reported in Science.