Designing Nano-loop antenna arrays for light-trapping in solar cells

Many types of wavelength-scale optical structures have been investigated for light trapping in solar cells. Nano-loops have not yet been studied on solar cells, even though they play a central role in arrays for meta-materials in the microwave (MW) region. In this paper, we use standard antenna theory to provide a rigorous analysis of closed circular metallic loops as antennas in the infrared (IR) and optical region (OR), the regions of solar activity. We provide an exact impedance model for closed loops and an approximate RLC model from which we determine key design factors (resonances, quality factors and radiation efficiencies). Using numerical simulations, we find that these results extend to hexagons and to squares. The principle differences between loops in the radio frequency region (RF) and in the IR/OR are due to dispersion in the loop material. This causes a scaling such that resonances eventually reach saturation; that is, closed loops made of the noble metals will not have their first fundamental resonance at frequencies above the IR. Closed loops, though, do have strong higher harmonic resonances with quality factors on the order of 2 to 5, and these can appear in the OR depending on the loop circumference. Such higher order resonances may be promising for light trapping in solar cells.