FEATURE PLASMONICS
Honda’s concept, the EV-N, has solar cells on its roof
➤ layer of well-designed, precisely positioned nanoparticles, electron beam lithography could be used to produce a proof of principle demonstration. The proof of principle work will see lithographic tools used to create the nanoparticles for Bagnall’s team to evaluate. Self-organised metal films are preferred for cheap mass manufacture and that will be the next stage, a self-organised way of making the particles in solution and then spin them on a substrate. With an organic solar cell the nanoparticles can be embedded in with the solution and can help the scattering and enhanced absorption. ‘There has been a lot of hype over the years and we’ve plodded on to find real answers. So far I haven’t seen anyone show a better solar
cell; as far as I know no-one has come up with a solar cell that uses plasmonics that is ready for commercialisation,’ Bagnall explains. ‘Inkjet printing might be a cheap way, or nano imprint lithography, while annealing of a thin film gives you particles, but none of these things are quite as controlled. You create a wide variety of shapes and sizes that broaden the features and you don’t have control over the tricky problems.’ Another method where inkjet printing is applicable is quantum dots. ‘One can envisage larger volume deposition processes from solution or various printing techniques, for example organic LEDs: it’s been shown you can deposit and pattern those by inkjet printing,’ says University of St Andrews physics and
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astronomy department professor, Ifor Samuel. His research includes organic solar cells. ‘For solar cells you might use a different coating or printing technology, because you don’t have to pattern them quite so accurately, but you still might use inkjet printing, which we hope would reduce the cost and energy of manufacture.’ Samuel is working as part of an international Anglo-US team through an Engineering and Physical Sciences Research Council grant that includes the University of Glasgow. His US counterparts are the US government’s National Science Foundation and the University of Massachusetts at Amherst. ‘The idea is to tune the properties of the material to make better solution-processed solar cells. Processed for solution, the organic semiconductor and colloidal quantum dots could be deposited from solution, because it is a very simple process. It’s very suitable for large area and low cost,’ Samuel explains. The reason the team is looking at quantum dots is that their absorption can be tuned by changing their size. This means their absorption can be complementary to the absorption of the organic materials used in the solar cell. Those other materials include flavins, which are naturally occurring pigments. ‘We are working on flavins as a new class of acceptor and there we’re aiming to tune energy levels, which should make the cells more efficient by increasing their open circuit voltage. Typically we’re trying to get the absorption to be where we want it, and where we want it depends on the solar spectrum, because we want to absorb sunlight,’ says Samuel.
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