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

Abstract

Many types of wavelength-scale optical structures have been investigated for light trapping in solar cells. Nanoloops 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.

Optimization of Far-Field Radiation From Impedance-Loaded Nanoloops Accelerated by an Exact Analytical Formulation

“…Plugging (12) into (22) and using the modal admittances results in a more convenient form for the loss resistance:…”

“…It has been shown through full-wave simulation that impedance loading can be used to achieve a uniform traveling-wave current distribution [19] or an omni-directional left-handed circularly polarized radiation pattern [20]. Optical nanoloops are extremely promising with a wide variety of applications including sensing [21] and light-trapping in solar cells [22]. Due to the complexity of the integrals that must be solved, fully analytical expressions for the radiation properties of impedance-loaded loops valid from the RF to optical regimes have not been developed.…”

Closed-Form Expressions for the Radiation Properties of Nanoloops in the Terahertz, Infrared and Optical Regimes

“…The knowledge of the surface currents given by (10) and (11) enables the derivation of expressions for the far-zone electromagnetic fields and, consequently, the associated far-zone antenna parameters. Hence, the far-zone electric field may be expressed in spherical coordinates (θ , ϕ) as [16], [17]…”

“…where J m is the derivative (with respect to the argument) of the Bessel function of order m, and the modal currents I m are derived from (10) and (11) as…”

“…In comparison to the large body of literature devoted to linear dipole-type nanowire antennas [7]- [8], much less work has been done to understand the radiation properties of nanoloop antennas. These structures are extremely promising for their potential applications in sensing [9], spectroscopy [10], and light-trapping in solar cells [11]. A few nanoloop structures have been designed using the finite-difference time-domain [12] and the finite integration technique [13].…”