Thermodynamic limit to photonic-plasmonic light-trapping in thin films on metals

Abstract

We calculate the maximum optical absorptance enhancements in thin semiconductor films on metals due to structures that diffuse light and couple it to surface plasmon polaritons. The calculations can be used to estimate plasmonic effects on light-trapping in solar cells. The calculations are based on the statistical distribution of energy in the electromagnetic modes of the structure, which include surface plasmon polariton modes at the metal interface as well as the trapped waveguide modes in the film. The enhancement has the form 4n 2 þ nk=h (n -film refractive index, k -optical wavelength, h -film thickness), which is an increase beyond the non-plasmonic "classical" enhancement 4n 2 . Larger resonant enhancements occur for wavelengths near the surface plasmon frequency; these add up to 2 mA=cm 2 to the photocurrent of a solar cell based on a 500 nm film of crystalline silicon. We also calculated the effects of plasmon dissipation in the metal. Dissipation rates typical of silver reverse the resonant enhancement effect for silicon, but a non-resonant enhancement remains.

Light trapping beyond the 4n2 limit in thin waveguides

“…Despite the early work suggesting that the absorption enhancement in thin slabs is less than 4n 2 , recent work has shown that the enhancement may in fact exceed this limit through the use of photonic crystals, 5 plasmonic waveguides, 6 and high index claddings, 7,8 which all elevate the local density of optical states to achieve enhancment. 9 Here we revisit the case of a thin waveguide and determine that the 4n 2 limit can be exceeded for a number of structures that support large propagation constants and/or slow modal group velocities.…”

Upper Bound on the Modal Material Loss Rate in Plasmonic and Metamaterial Systems

“…The calculation of the modal material loss rate has also been used to understand the effect of plasmonic loss in solar cell light trapping schemes [19]. However, only a few recent papers have attempted to understand the general behavior of loss in plasmonic systems from a purely analytic perspective.…”

Comparison of optical properties of periodic photonic–plasmonic and randomly textured back reflectors for nc-Si solar cells

“…Propagating plasmons are not expected in random structures which may instead support localized plasmons [11][12][13] at the location of the nanoparticles. These plasmons can lead to optical losses, unless localized plasmons re-radiate into photon modes [14].…”