Many engineers and scientists are familiar with the "Shockley-Queisser" limit for the efficiency of solar cells, and many aspire to surpass it. There is a less familiar, but perhaps more important, limit for real materials, which is the maximum extent to which sunlight can be trapped in a layer of definite thickness. Light-trapping substantially improves the efficiencies for thin-film silicon solar cells, and is valuable for all technologies. The "classical" limit is an enhancement of the absorptance of the cell by 4n2 over its value for a single pass of a beam through the film, where n is the film's index of refraction.
In this seminar, I'll explain this classical limit, and then describe some proposals for surpassing it. Imposing periodic gratings onto the cell may crack the limit, as may using hybrid plasmonic-photonic excitations. I'll summarize our own experiments and those of other groups that use varying nanostructures to implement these strategies. To date, the classical limit towers above actual light-trapping results for thin film solar cells. We now think that supraclassical light-trapping will need to be enabled by improvements in the conducting oxides used in the cells, and I'll present experiments and simulations that indicate how this can be done.