Omnidirectional and broadband absorption enhancement from trapezoidal Mie resonators in semiconductor metasurfaces

TL;DR

This paper presents a broadband, angle-insensitive metasurface design with trapezoidal Mie resonators that significantly enhances light absorption in ultrathin silicon solar cells, demonstrated through theoretical and experimental methods.

Contribution

It introduces a novel trapezoidal Mie resonator metasurface design that improves broadband light absorption in ultrathin silicon films, surpassing traditional planar structures.

Findings

Achieved a fourfold increase in photocurrent density in 210 nm silicon films.

Demonstrated broadband, angle-insensitive absorption across visible and near-IR spectra.

Validated the design through numerical simulations and experimental measurements.

Abstract

Light trapping in planar ultrathin-film solar cells is limited due to a small number of optical modes available in the thin-film slab. A nanostructured thin-film design could surpass this limit by providing broadband increase in the local density of states in a subwavelength volume and maintaining efficient coupling of light. Here we report a broadband metasurface design, enabling efficient and broadband absorption enhancement by direct coupling of incoming light to resonant modes of subwavelength-scale Mie nanoresonators defined in the thin-film active layer. Absorption was investigated both theoretically and experimentally in prototypes consisting of lithographically patterned, two-dimensional periodic arrays of silicon nanoresonators on silica substrates. A crossed trapezoid resonator shape of rectangular cross section is used to excite broadband Mie resonances across the visible and near-IR spectra. Our numerical simulations, optical absorption measurements and photocurrent spectral response measurements demonstrate that crossed trapezoidal Mie resonant structures enable angle-insensitive, broadband absorption. A short circuit current density of 12.0 mA/cm2 is achieved in 210 nm thick patterned Si films, yielding a 4-fold increase compared to planar films of the same thickness. It is suggested that silicon metasurfaces with Mie resonator arrays can provide useful insights to guide future ultrathin-film solar cell designs incorporating nanostructured thin active layers.