FEATURE PLASMONICS
CHANGE SOLAR WINDOF
Light harvesting is set to reap the rewards of many years of research. Rob Coppinger talks to the scientists about the new dawn for our energy future
Solar power is plentiful and promises endless amounts of energy from our solar system’s Sun that should burn for many more billions of years, but the road to this future of power through light harvesting that is too cheap to meter is proving harder than anyone expected. ‘It’s increasingly more complicated than you imagine; the big news story ten years ago was probably hopelessly naïve in terms of what plasmonics could achieve. Implementation inside a real device is very problematic. We think we know what we need to do to make the best devices, but it’s hard to integrate it in an actual device process,’ says Professor Darren Bagnall, head of the Nano Research Group at the University of Southampton. Plasmons can be described as an oscillation of free electron density with respect to the fixed positive ions in a metal. In this case a metal nanoparticle. ‘We started working on it eight or nine years ago.’ Bagnall continues, ‘Ten years ago there was a lot of excitement around the use of metal nano particles to enhance the efficiency of solar cells. People have learnt a lot and looked at all sorts of structures and ways you can use plasmons. People at various institutes have shown the enhancements in the
8 PHOTOVOLTAICS 2012
performance of certain types of solar cell, and solar cell designs.’
The idea is that a metal nanoparticle interacts with a photon and this creates a strong field in the nanoparticle that prolongs the plasmon state giving the cell a better chance of absorbing the photon. With this improved photon absorbed the nanoparticle will still re-emit the photon, but this emission is scattered and the way in which this happens is viewed as improving the light trapping properties of the cell. This is because the photons are scattered sidewards. This has the advantage of lengthening the absorption period, giving the cell more time to trap the light. Finally, solar cells need to be anti- reflective so they are simply not bouncing back the photons. The prolonged presence of the photon, because of the plasmon interaction and the sideward scattering, helps to prevent reflection. ‘The light trapping benefit for a solar cell is that where it is hard to absorb the near infrared you ought to be able to get an efficiency enhancement,’ explains Bagnall. However, one drawback of this is that there is always some intrinsic absorption, also called parasitic absorption. This means about 10 per
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