The radiation efficiency of nano-antennas is a key parameter in the emerging field of IR and optical energy harvesting. This parameter is the first factor in the total efficiency product by which nano-antennas are able to convert incident light into useful energy. This efficiency is investigated in terms of the metal used as conductor and the dimensions of the nano-antenna. The results set upper bounds for any possible process transforming light into electrical energy. These upper bounds are the equivalent of the theoretical upper bounds for the efficiency of conventional solar cells. Silver shows the highest efficiencies, both in free space and on top of a glass (SiO2) substrate, with radiation efficiencies near or slightly above 90%, and a total solar power harvesting efficiency of about 60–70%. This is considerably higher than conventional solar cells. It is found that fine-tuning of the dipole dimensions is crucial to optimize the efficiency.
► Upper bounds are derived for the efficiency of solar energy harvesting with nano-antennas. ► These upper bounds show what actually can be expected from this emerging research field. ► It is shown that the performance of metals can be totally different. ► Silver is found to be the best in this study. ► Fine tuning of the nano-antennas is crucial.
In this study upper bounds are derived for the efficiencies by which energy can be harvested from the sun using nano-antenna technology. To this goal, the parameter “total harvesting efficiency” is introduced. Both dipoles in free space and on a glass substrate are considered. For silver nano-dipoles, a maximum of about 60–70% is found. It is an open question whether it is possible to construct alloys with even lower losses at plasmonic frequencies, and thus higher efficiencies. A simple approximating formula is derived to assess the intrinsic harvesting capabilities of a material. Several challenges remain. Silver is more susceptible to oxidation, which can completely destroy its superiority, and which is one of the reasons why gold is so popular in this field. This can be solved by embedding it within the glass substrate, which requires to develop alternative fabrication processes and technology. The problem can also be solved by using aluminum, which has a transparent oxide, but a bit lower total efficiency of around 50%.
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