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Imperfectly geometric shapes of nanograting structures as solar absorbers with superior performance for solar cells

Nghia Nguyen-Huu, Michael Cada, and Jaromír Pištora

Open Access Open Access

Abstract

The expectation of perfectly geometric shapes of subwavelength grating (SWG) structures such as smoothness of sidewalls and sharp corners and nonexistence of grating defects is not realistic due to micro/nanofabrication processes. This work numerically investigates optical properties of an optimal solar absorber comprising a single-layered silicon (Si) SWG deposited on a finite Si substrate, with a careful consideration given to effects of various types of its imperfect geometry. The absorptance spectra of the solar absorber with different geometric shapes, namely, the grating with attached nanometer-sized features at the top and bottom of sidewalls and periodic defects within four and ten grating periods are investigated comprehensively. It is found that the grating with attached features at the bottom absorbs more energy than both the one at the top and the perfect grating. In addition, it is shown that the grating with defects in each fourth period exhibits the highest average absorptance (91%) compared with that of the grating having defects in each tenth period (89%), the grating with attached features (89%), and the perfect one (86%). Moreover, the results indicate that the absorptance spectrum of the imperfect structures is insensitive to angles of incidence. Furthermore, the absorptance enhancement is clearly demonstrated by computing magnetic field, energy density, and Poynting vector distributions. The results presented in this study prove that imperfect geometries of the nanograting structure display a higher absorptance than the perfect one, and provide such a practical guideline for nanofabrication capabilities necessary to be considered by structure designers.

© 2014 Optical Society of America

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Figures (11)
Fig. 1
Fig. 1 (a) Schematic diagram of the grating structure of which geometry is defined by the grating period Λ, the grating thickness d, and the filling factor f. The transverse magnetic wave (H) (parallel to the grating grooves or y-axis) is incident on the grating with a wavevector k and an incident angle θ; (b) Optical constants of the Si material used in this study include the extinction index κ and the refractive index n [31]

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Fig. 2
Fig. 2 a) Absorptance spectra of SWG Si grating and plain Si for TM waves at normal incidence and b) Contour plot of the absorptance as a function of the wavelength and angle of incidence for TM waves.

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Fig. 3
Fig. 3 Schematic illustration of imperfect gratings with the grating period Λ = 130 nm, the grating thickness d = 90 nm, the filling factor f = 0.5, and the square feature size h = 10 nm. The coordinate system used is the same as that in Fig. 1.

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Fig. 4
Fig. 4 a) Absorptance spectra of SWG structures at normal incidence and b) Absorptance peaks of the perfect absorber with different angles of incidence calculated using the RCWA and the FEM-based COMSOL Multiphysics.

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Fig. 5
Fig. 5 Near-field patterns of the SWG structures at normal incidence including the optimized grating, GII, and GIV at peaks A (at λ = 405 nm), AII (at λ = 410 nm), and AIV (at λ = 380 nm). Note that the top of Fig. 5 represents magnetic field distributions, and the bottom of Fig. 5 shows energy density and the Poynting vector in one period.

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Fig. 6
Fig. 6 Near-field patterns of the SWG structures at normal incidence including the optimized grating, GII, and GIV at dips B (at λ = 440 nm), BII (at λ = 450 nm), and BIV (at λ = 430 nm). Note that the top of Fig. 6 represents magnetic field distributions, and the bottom of Fig. 6 shows energy density and the Poynting vector in one period.

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Fig. 7
Fig. 7 Near-field patterns of the SWG structures at normal incidence including the optimized grating, GII, and GIV at peaks C, CII, and CIV at the same wavelength of λ = 530 nm. Note that the top of Fig. 7 represents magnetic field distributions, and the bottom of Fig. 7 shows energy density and the Poynting vector in one period.

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Fig. 8
Fig. 8 Schematic illustration of imperfect gratings with periodic defects within a*Λ periods. Note that geometry parameters are the same as the ones for the optimized grating including the grating period Λ = 130 nm, the grating thickness d = 90 nm, and the filling factor f = 0.5; the coordinate system is the same as that in Fig. 1; and the constant a is the number of the grating periods considered in the simulation defined as a = 4 and 10.

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Fig. 9
Fig. 9 Absorptance spectra of imperfectly periodic structures at TM normal incidence: a) in the fourth periods and b) in the tenth periods. Absorptance spectra at zero order diffraction are also calculated for comparison between the fourth and the tenth periods with one period using the RCWA.

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Fig. 10
Fig. 10 Near-field patterns of GVII with defects at normal incidence in the fourth periods at peaks A4VI and C4VII and the tenth periods at peaks A10VI and C10VII at wavelengths of 400 nm and 600 nm, respectively. Note that the top of Fig. 10 represents magnetic field distributions, and the bottom of Fig. 10 shows energy density and Poynting vectors.

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Fig. 11
Fig. 11 Contour plots of the absorptance as a function of the wavelength and angle of incidence for TM waves of imperfect structures exhibiting efficient performance: a) GIV and b) GVII with periodic defects in the fourth periods.

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