Research ArticleNANOFABRICATION

Bioinspired phase-separated disordered nanostructures for thin photovoltaic absorbers

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Science Advances  19 Oct 2017:
Vol. 3, no. 10, e1700232
DOI: 10.1126/sciadv.1700232
  • Fig. 1 Structure and absorption spectra of the P. aristolochiae butterfly wing scales.

    (A) Image of a P. aristolochiae butterfly. (B) Microscopic image of the matt black region. A gradient of blackness is observed along each scale. (C) Cross-sectional SEM image of a matt black scale. (D) SEM images of a scale from the matt black region reveal that the air-filling fraction at the apex is higher (by about +59%) than at the base. The inset in the lower left corner shows the 2D Fourier power spectrum of the corresponding nanohole array. (E) Absorption spectra measured in the apex and base of a single scale from the matt black region of P. aristolochiae. The scale apex of a single scale exhibits higher absorption than the base.

  • Fig. 2 3D FEM optical simulations of the micro- and nanostructures of the P. aristolochiae butterfly.

    (A) 3D model of the butterfly scale and corresponding parameters extracted from SEM images. The dimensions of this model are as follows: a = 2.0 μm, b = 1.2 μm, c = 0.8 μm, d = 1.2 μm, e = 0.22 μm, f = 5.0 μm, and g = 1.5 μm. (B) Comparison of experimental and simulated absorption spectra under normal incidence. The experimental data are measured with an integrating sphere in the matt black region (solid line). The dashed line corresponds to the simulated 3D model with an air-filling fraction of 59%. For comparison, we also constructed an unpatterned slab model by squeezing the 3D original model into a slab with the same bottom area and volume (long dashed line). (C) Normalized electric field intensity distribution calculated for normal incidence and wavelengths of 350, 550, 850, and 1050 nm. (D) Influence of different structural components of the scale architecture of P. aristolochiae on the absorption properties. The schematics represent different 3D models. The corresponding simulated absorption spectra are shown in the plot on the right. The nanohole array is found to be the primary contributing element for the absorption in the UV-vis–NIR region.

  • Fig. 3 Simulated optical properties of thin and patterned absorbers made of a-Si:H with increasing disorder magnitudes.

    (A) Schematic view of the four considered geometries: “unpatterned” bare slab, ordered periodically arranged holes with a single diameter, perturbed structure of holes with periodic arrangement but varying diameters, and correlated structure with disordered hole positions and varying diameters. The ordered structure has a hole diameter of 240 nm and a period of 300 nm. The ff of all patterned configurations is set to 50.26%. The insets in the lower right corners show the 2D Fourier power spectra of the corresponding structures. (B) A Gaussian distribution with a mean hole diameter of 240 nm and a variance of 20 nm is introduced into the ordered configuration to investigate the effect of size dispersion. (C) Absorption spectra of the four simulated geometries under normal incidence for unpolarized light [(TE + TM)/2]. The IA of each geometry is reported in the legend. (D) IA of the three patterned geometries versus the AOI. The resulting IA varies from 67 to 45% for angles up to 80°.

  • Fig. 4 Fabrication of the bioinspired patterned thin PV absorbers.

    (A) Schematic view of the three main fabrication steps including the spin coating of a blend solution of poly(methyl methacrylate) (PMMA) and polystyrene (PS) in methyl ethyl ketone (MEK) on a thin a-Si:H layer deposited on a glass substrate, followed by a selective development of the PS, and finally the transfer of the pattern into a-Si:H by dry etching (RIE). (B) Image of a patterned 130-nm-thin a-Si:H layer on a glass substrate with completely etched disordered nanoholes (right) demonstrates the high omnidirectional absorption properties with respect to the unpatterned sample (left). All images are taken under diffusive white light with observation angles of 30° (top) and 80° (bottom). (C) SEM top-view image of the nanostructured a-Si:H thin film shows the distribution of nanoholes with both size and position disorders. The ring-shaped pattern in the 2D Fourier power spectrum corresponding to this SEM image (inset) confirms the correlated disordered nature of the nanoholes introduced in the thin absorber. (D) Statistical analysis of the nanohole diameters of this sample. The histogram depicting the distribution of nanohole diameters can be approximated by a Gaussian profile with a mean diameter of 238 ± 105 nm.

  • Fig. 5 Characterization of the bioinspired patterned thin PV absorbers.

    (A) 3D AFM image of the bioinspired a-Si:H thin film. 2D surface profile shows a short-range ordered hole distribution that is used for bioinspired solar cell simulation (fig. S7). 1D line profile shows uniform surface patterning with height (etching) profile. (B) Impact of the etched disordered nanoholes on the absorption spectrum measured at normal AOI with unpolarized light. (C) Angular dependence of the IA of the patterned and unpatterned samples. The relative increase in IA raises up to +200% for an AOI of 50°.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/10/e1700232/DC1

    fig. S1. Matt black and dull black scales of the P. aristolochiae butterfly wings.

    fig. S2. Optical indices of the materials considered for the simulations.

    fig. S3. Simulated absorption spectra of different perturbed patterned PV absorbers.

    fig. S4. Polarization and angle-resolved simulated absorption spectra of the different patterned thin PV absorbers considered in Fig. 3.

    fig. S5. Air-filling fraction influence on absorption properties of PV absorber patterned with correlated disorder design.

    fig. S6. Comparison of simulated absorption spectra of unpatterned, ordered, and bioinspired nanohole patterned solar cells.

    fig. S7. Simulated absorption spectrum of a 100-nm-thick a-Si:H film patterned with exact high absorbing nanohole geometry of P. aristolochiae butterfly wings.

    fig. S8. Simulated absorption spectra of the P. aristolochiae butterfly wings obtained with a nanohole array made of the “actual” and correlated arrangement.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Matt black and dull black scales of the P. aristolochiae butterfly wings.
    • fig. S2. Optical indices of the materials considered for the simulations.
    • fig. S3. Simulated absorption spectra of different perturbed patterned PV absorbers.
    • fig. S4. Polarization and angle-resolved simulated absorption spectra of the different patterned thin PV absorbers considered in Fig. 3.
    • fig. S5. Air-filling fraction influence on absorption properties of PV absorber patterned with correlated disorder design.
    • fig. S6. Comparison of simulated absorption spectra of unpatterned, ordered, and bioinspired nanohole patterned solar cells.
    • fig. S7. Simulated absorption spectrum of a 100-nm-thick a-Si:H film patterned with exact high absorbing nanohole geometry of P. aristolochiae butterfly wings.
    • fig. S8. Simulated absorption spectra of the P. aristolochiae butterfly wings obtained with a nanohole array made of the “actual” and correlated arrangement.

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