A fundamental limit to the efficiency of a solar cell is the band gap of the semiconductor from which it is made. The band gap is the energy difference between the conduction and valence bands in a semiconductor. It is difficult to find a single semiconducting material that can match the broad range of energies found in solar radiation. Light below the band gap of the semiconductor is not absorbed, while light above the band gap is absorbed but the excess is lost as heat.

Higher efficiencies can be achieved by using stacks of different semiconductor materials - the higher gap materials convert the most energetic photons into electrical current, leaving the lower energy photons to pass through to the lower gap materials. The highest efficiency observed to date with these types of systems is about 30%. These systems have a maximum theoretical efficiency of 70% when many different semiconductor layers are stacked on top of each other, but problems related to mismatches between the layers can destroy the optical properties of the device.

Now Wladek Walukiewicz and co-workers have measured the optical properties of pure indium nitride and a wide range of alloys made of indium, gallium and nitrogen. They find that the band gap can vary between 0.7 and 3.4 eV, which covers the entire solar spectrum.

The researchers believe their results to be more reliable than previous results because they used higher quality samples grown with epitaxial techniques. Although these samples were grown on lattice-mismatched substrates, they still have strong optical properties. Indium nitride materials seem to be able to accommodate large lattice mismatches without much effect on their optoelectronic properties.

The team also found that alloys made from indium, aluminium and nitrogen had an even wider range of band gaps - from 0.7 to 6.2 eV. This should allow nitride-based alloys to be used in a range of optoelectronic applications from the near infrared to the far ultraviolet.