Optimal solar energy harvesting efficiency of nano-rectenna systems

Recently, upper bounds have been derived for the efficiency of nantenna(=nano antenna) systems for solar energy harvesting. The maximum upper bounds were found to be in the order of 60–70% for dipoles made of silver. These upper bounds are determined solely by the losses in the nantenna. In this paper, the second crucial factor in the efficiency product for a real nano-rectenna topology is studied: the matching efficiency due to the unavoidable mismatch between nantenna and rectifier impedance. Since suitable rectifiers do not exist yet, and since it would be totally unfeasible to optimize the nantenna – rectifier system based on experiments, for obvious reasons of cost per fabricated sample, an optimization technique is used based on full-wave simulations to assess the efficiency that can be reached when the two impedances are optimally compatible. To this goal, first a comprehensive numerical study is made of the impedance of nano dipoles made from three different metals and deposited on a glass substrate. Then the rectifier impedance is determined for which the matching efficiency is maximum. Two different rectifier impedance models are involved. They are (1) the polynomial (0th, 1st, and 2nd order) model and (2) the equivalent circuit (EC) model for the non-packaged case. The result is that a maximum matching efficiency of about 97% can be reached for Al dipoles, while a maximum efficiency product of upper bound and matching efficiency of about 54% can be reached, in this case for Ag dipoles. These values are reached for a 2nd order polynomial model. Important is also that the results for the EC model are almost identical to the results of the 0th order polynomial model. Finally, the maximum power delivered by a single linearly polarized dipole is shown to be in the order of 5–10 pW.

► Nano-rectennas for solar energy harvesting are considered.
► The matching between nano-antenna and rectifier affects the harvesting efficiency.
► This matching is studied and optimal rectifier impedances are derived.