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Optimization of generalized dielectric nanostructures for enhanced light trapping in thin-film photovoltaics via boosting the local density of optical states

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Abstract

Recent work has shown that using a high-index cladding atop a lower-index photovoltaic absorber enables absorption of light beyond the ergodic (4n2) limit. In this paper, we propose a generalized optimization method for deriving optimal geometries that allow for such enhancement. Specifically, we adapted the direct-binary-search algorithm to optimize a complex 2-D multi-layer structure with the explicit goal of increasing photocurrent. We show that such an optimization results in enhancing the local density of optical states in an ultra-thin absorber, which forms a slot-waveguide geometry in the presence of a higher-index overcladding. Numerical simulations confirmed optical absorption approaching 100% and absorption-enhancement beyond the ergodic (4n2) limit for specific spectral bands of interest. Our method provides a direct, intuitive and computationally scalable approach for designing light-trapping nanostructures.

© 2013 Optical Society of America

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Figures (6)

Fig. 1
Fig. 1 Schematic of the simulation model for optimization. (a) The photovoltaic device with light trapping nanostructures defined by 22 geometric parameters. Ha is kept constant during optimization. (b) Reference device with the same volume of absorbing material as that of the structured design in (a). The angle of incidence is defined by θ. The TE and TM polarizations are defined as shown.
Fig. 2
Fig. 2 Flow chart of the direct-binary-search-based algorithm adopted for optimizing nanophotonic light trapping structures via boosting LDOS.
Fig. 3
Fig. 3 Results of the four optimized light trapping designs. In each figure, top insets: schematics of the optimized structures labelled with dimensions, bottom insets: schematics of the active regions in detail labelled with other smaller dimensions.
Fig. 4
Fig. 4 Analysis of the optimized designs. Four device geometries are again shown in (c), (f), (i) and (l) corresponding to reference absorber thicknesses of 33nm, 42nm, 68nm and 88nm, respectively. The corresponding absorbance spectra are plotted in (a), (d), (g) and (j), respectively. The corresponding current-density-enhancement spectra are plotted in (b), (e), (h) and (k), respectively. The insets in these figures showcase current-density and current-density enhancement as a function of incident angle, θ. The corresponding normalized intensity distributions at wavelengths of maximum current-density enhancement are shown in (c), (f), (i) and (l), respectively.
Fig. 5
Fig. 5 Impact of the refractive index of the cladding layer. Optimized design with (a) high-index cladding (taken from Figs. 3(b) and 4(f)) and (c) with low-index cladding (taken from Ref [26].). The corresponding absorbance spectra for (b) the high-index-cladding design and (d) the low-index-cladding design. (e) Current-density-enhancement spectra of the optimized designs and those corresponding to the ergodic limit and the LDOS limit.
Fig. 6
Fig. 6 Results of the light trapping designs with triangular scattering elements. Enhancement spectra of different angles of incidence for both rectangular and triangular scatterers of designs 1-4 are shown in (a), (c), (e) and (g) (insets: schematics of the structures). The angular analyses of Jsc enhancements of designs 1–4 with both rectangular and triangular structures are given in (b), (d), (f) and (h).

Tables (1)

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Table 1 Ranges and unit perturbations of geometric parameters for optimization

Equations (5)

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A( λ )= 1 Λ active 1 2 ωε''(λ) | E( x,y,λ ) | 2 dxdy P inc (λ) .
J sc = λ min λ max 1 Λ ( qλ hc active 1 2 ωε''(λ) | E( x,y,λ ) | 2 dxdy ) IQE(λ)dλ,
E(λ)= A( λ ) A ref ( λ ) = active | E( x,y,λ ) | 2 dxdy active | E ref ( x,y,λ ) | 2 dxdy = U(λ) U ref (λ) .
E Ergodic =4 n L 2 ,
E LDOS =4 n H 2 ,
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