Efficiency limits of rectenna solar cells: Theory of broadband photon-assisted tunneling

Because rectifiers can convert a wide range of frequencies to dc it was thought that rectenna solar cells–antennas coupled to ultra-high speed diodes–could efficiently harvest the entire solar spectrum and exceed the Shockley-Queisser limit. We show that there are efficiency limits to broadband optical conversion and provide a quantitative analysis using the theory of photon-assisted tunneling. This quantum-based approach differs from classical rectification in lower frequency rectennas. The conversion efficiency approaches 100% for monochromatic illumination. For broadband illumination at terrestrial solar intensities, the diode operating voltage plays the role that bandgap plays in conventional solar cells.