9 Aperiodic Order in Nanoplasmonics
367
9.3.5 Applications to Thin-Film Solar Cell Enhancement
The solar cell market is predominately based on crystalline silicon (c-Si) wafers with
absorbing layer thickness in between 100 and 300 μm to guarantee complete light
absorption and effective carrier collection. However, the increasing materials costs
related to the fabrication of highly pure c-Si and the ever-growing efficiency demands
of the solar industry have motivated novel approaches to increase light absorption
in cells with reduced active thickness. In particular, promising approaches consist in
the engineering of thin-films non-crystalline (amorphous or poly-crystalline Si) and
nanocrystalline Si (Si-ncs) structures [132, 133].
Amorphous and nanocrystalline materials can be fabricated with strongly reduced
thermal budgets, costs and with much larger volumes compared to traditional Si
wafers. However, their shorter diffusion lengths (limited by defects and grain boundaries) restrict the active cell thicknesses to approximately a few hundreds of nanometers, severely decreasing the probability of photon absorption. This has recently
spurred the search for advanced photon recycling and light-trapping schemes capable
of increasing the optical paths of photons, and therefore the absorption probability, in
ultra-thin film Si solar cells (<200 nm-thick) [134–138]. Recent studies have shown
that metal nanostructures can lead to effective light trapping into thin-film solar cells
improving the overall efficiency due to the enhancement of optical cross sections
associated to the excitation of Localized Surface Plasmon modes (LSPs) [139–142].
One commonly utilized geometry consists of the fabrication of metal-dielectric
nanoparticles on the front/bottom surface of the absorbing cell structure. When the
nanoparticles shapes are correctly designed, incident light is preferentially scattered
into the thin-film absorbing Si layer over an increased angular range, effectively
enhancing the material absorption [143–146].
Plasmon-enhanced light absorption in thin-film Si solar cells has been demonstrated using periodic arrays of gold (Au) or silver (Ag) nanoparticles, which give
rise to best enhancement in the spectral regions where evanescent diffraction grating orders spectrally overlap the broader LSP resonances characteristic of metallic
nanoparticles. However, polarization sensitivity and the narrow frequency range for
effective photonic–plasmonic coupling in periodic grating structures inherently limit
these approaches. In order to broaden the spectral region of enhancement, it is crucial
to engineer aperiodic nanoparticle arrays with a higher density of spatial frequencies without resorting to uncontrollable random systems, which have only limited
engineering appeal. To overcome these limitations, recent studies have proposed
to utilize plasmonic arrays with aperiodic quasicrystal structures, such as Penrose
lattices, which exhibit non-crystallographic rotational symmetries [138, 147]. Such
arrays, by virtue of their higher degree of rotational symmetry as compared to traditional periodic structures, give rise to enhanced scattering along multiple directions
and over a broader wavelength range.
Our group has recently introduced plasmonic aperiodic spiral arrays as a viable
strategy to engineer wide-angle light scattering for broadband and polarization insensitive absorption enhancement in thin-film Si solar cells [67].
367
9.3.5 Applications to Thin-Film Solar Cell Enhancement
The solar cell market is predominately based on crystalline silicon (c-Si) wafers with
absorbing layer thickness in between 100 and 300 μm to guarantee complete light
absorption and effective carrier collection. However, the increasing materials costs
related to the fabrication of highly pure c-Si and the ever-growing efficiency demands
of the solar industry have motivated novel approaches to increase light absorption
in cells with reduced active thickness. In particular, promising approaches consist in
the engineering of thin-films non-crystalline (amorphous or poly-crystalline Si) and
nanocrystalline Si (Si-ncs) structures [132, 133].
Amorphous and nanocrystalline materials can be fabricated with strongly reduced
thermal budgets, costs and with much larger volumes compared to traditional Si
wafers. However, their shorter diffusion lengths (limited by defects and grain boundaries) restrict the active cell thicknesses to approximately a few hundreds of nanometers, severely decreasing the probability of photon absorption. This has recently
spurred the search for advanced photon recycling and light-trapping schemes capable
of increasing the optical paths of photons, and therefore the absorption probability, in
ultra-thin film Si solar cells (<200 nm-thick) [134–138]. Recent studies have shown
that metal nanostructures can lead to effective light trapping into thin-film solar cells
improving the overall efficiency due to the enhancement of optical cross sections
associated to the excitation of Localized Surface Plasmon modes (LSPs) [139–142].
One commonly utilized geometry consists of the fabrication of metal-dielectric
nanoparticles on the front/bottom surface of the absorbing cell structure. When the
nanoparticles shapes are correctly designed, incident light is preferentially scattered
into the thin-film absorbing Si layer over an increased angular range, effectively
enhancing the material absorption [143–146].
Plasmon-enhanced light absorption in thin-film Si solar cells has been demonstrated using periodic arrays of gold (Au) or silver (Ag) nanoparticles, which give
rise to best enhancement in the spectral regions where evanescent diffraction grating orders spectrally overlap the broader LSP resonances characteristic of metallic
nanoparticles. However, polarization sensitivity and the narrow frequency range for
effective photonic–plasmonic coupling in periodic grating structures inherently limit
these approaches. In order to broaden the spectral region of enhancement, it is crucial
to engineer aperiodic nanoparticle arrays with a higher density of spatial frequencies without resorting to uncontrollable random systems, which have only limited
engineering appeal. To overcome these limitations, recent studies have proposed
to utilize plasmonic arrays with aperiodic quasicrystal structures, such as Penrose
lattices, which exhibit non-crystallographic rotational symmetries [138, 147]. Such
arrays, by virtue of their higher degree of rotational symmetry as compared to traditional periodic structures, give rise to enhanced scattering along multiple directions
and over a broader wavelength range.
Our group has recently introduced plasmonic aperiodic spiral arrays as a viable
strategy to engineer wide-angle light scattering for broadband and polarization insensitive absorption enhancement in thin-film Si solar cells [67].