Upper bounds for the solar energy harvesting efficiency of nano-antennas

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. (C) 2012 Elsevier Ltd. All rights reserved.