cent of the photons inside the nanoparticles will be lost to the nanoparticles and that is deemed by the scientists to be the smallest loss possible. To limit that loss and ensuring that the enhanced field in and around the nanoparticle is achieved with organic solar cells, the nanoparticles need to be in direct contact with the solar cell’s semiconductor material. However, for inorganic silicon solar cells it is a more serious problem because, Bagnall explains, ‘the metal will cause a recombination increase and you will end up losing more carriers than you generated.’ The solution is to put the metal nanoparticles on the back of the solar cell. With this approach the ideal solar cell model is to have a normal antireflective coating, through which most of the first pass of the light works normally, and then the infrared light that has gone through the cell is scattered by the nanoparticles. Because the plasmons


are then at the back of the cell their photon capture is prolonged, so the quantum efficiency in the infrared is improved in combination with the scattering. ‘We can use less material, so a 10μm piece of silicon instead of 200μm piece of silicon to make the technology cheaper. That is our line of inquiry to try and design this scattering back-reflector that will work in the infrared where we minimise the absorption and maximise the lateral scattering,’ says Bagnall. For now the plasmons can scatter 60 per cent of the long wavelength light, but absorb less than 5 per cent. To improve this performance Bagnall’s team has a good idea about the shape, size and position and type of design of the metal nanoparticles and where they need to be in the overall structure. Primarily at the bottom of the cell, how many nanometres they are away from the back-reflector and the semiconductor is a critical factor. To manufacture this


We can use less


material, so a 10μm piece of silicon instead of 200μm piece


➤ PHOTOVOLTAICS 2012 9


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