Operating regimes in an optical rectenna (Vol. 48 No. 2)
Optical rectennas: where Maxwell meets Einstein
Conventional semiconductor solar cells convert the solar spectrum to dc electricity, relying on the photoelectric effect. Their ultimate efficiency is limited to 44% because the entire photon spectrum is used at a voltage equal to the semiconductor bandgap. An unconventional approach is to use optical rectennas, nanoantennas with high-speed diodes. In this work we show how to break the efficiency limit using optical rectennas.
Microwave rectennas are described by classical electromagnetics and have been used for rectifying microwaves with power conversion efficiencies greater than 80%. However, the interaction of high-speed diodes with light is different than with microwaves. Instead, an optical rectenna can operate in one of three different regimes: quantum, transition, and classical.
The quantum regime occurs for weak optical intensities and is subject to the 44% limit because each incoming photon is used to produce an electron at the rectenna operating voltage, as in conventional solar cells. Classical operation occurs when the intensity is strong and the photon energy is low. Here, electrons absorb multiple photons to produce current at higher voltages, as in classical rectennas, resulting in higher solar energy conversion efficiencies that ideally can exceed 80%.
S. Joshi and G. Moddel, Optical rectenna operation: where Maxwell meets Einstein, J. Phys. D: Appl. Phys. 49, 265602 (2016)