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Strong broadband absorption in GaAs nanocone and nanowire arrays for solar cells

Baomin Wang, Erica Stevens, and Paul W. Leu

Open Access Open Access

Abstract

We studied the influence of geometric parameters on the optical absorption of gallium arsenide (GaAs) nanocone and nanowire arrays via finite difference time domain simulations. We optimized the structural parameters of the nanocone and nanowire arrays to maximize the ultimate efficiency across a range of lengths from 100 to 1000 nm. Nanocone arrays were found to have improved solar absorption, short-circuit current density, and ultimate efficiencies over nanowire arrays for a wide range of lengths. Detailed simulations reveal that nanocones have superior absorption due to reduced reflection from their smaller tip and reduced transmission from their larger base. Breaking the vertical mirror symmetry of nanowires results in a broader absorption spectrum such that overall efficiencies are enhanced for nanocones. We also evaluated the electric field intensity, carrier generation and angle-dependent optical properties of nanocones and nanowires. The carrier generation in nanocone arrays occurs away from the surface and is more uniform over the entire structure, which should result in less recombination losses than in nanowire arrays.

© 2014 Optical Society of America

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Figures (7)
Fig. 1
Fig. 1 (a) Schematic of the GaAs nanocone array structure. (b) The parameters for the array are the length L, the period a, the top diameter dtop, and the bottom diameter dbot.

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Fig. 2
Fig. 2 Plots of the (a) real n and (b) imaginary part k of the GaAs refractive index. Values from Palik Volume 1 [20] and FDTD fitted values are shown.

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Fig. 3
Fig. 3 (a) Optimal ultimate efficiency of GaAs nanowires and nanocones as compared with thin film and ideal single pass thin film. The short-circuit (SC) current density is shown on the right y-axis. (b) The ultimate efficiency enhancement from GaAs nanowires and nanocones compared to ideal single pass thin film.

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Fig. 4
Fig. 4 Optimal parameters for GaAs (a) nanowire and (b) nanocone for each length L, respectively. The volume filling factor of the optimal structure is shown on the right y-axis of both plots.

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Fig. 5
Fig. 5 Optical properties of three different GaAs nanostructures: nanowire (NW) arrays with d = 200 nm and d = 520 nm and nanocone (NC) arrays with dtop = 200 nm and dbot = 600 nm. a = 600nm in all three systems. (a), (b), and (c) show the reflectance, transmittance, and absorption spectra respectively. The absorption spectrum of the ideal single pass thin film is also plot in (c). The irradiance of the Air Mass 1.5 global solar spectrum is shown in right y-axis of (c).

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Fig. 6
Fig. 6 The (a) electric field intensity |E(r)|2 and (b) solar-spectrum-weighted generation rate G(r) for three representative GaAs nanowires and nanocones. From left to right, they are nanowire arrays with d = 520 nm, nanowire arrays with d = 200 nm, and nanocone arrays with dtop = 200 nm and dbot = 600 nm. a = 600 nm in all three systems.

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Fig. 7
Fig. 7 Relationship between absorption and zenith angle θ of optimal nanowire and nan-cone arrays for TM and TE illumination.

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Tables (1)

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Table 1 Absorption in different wavelength regimes. a = 600 nm in all three systems. The infrared and total solar regions are calculated for those portions that are above the GaAs band gap (E > 1.43 eV).

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Equations (4)

Equations on this page are rendered with MathJax. Learn more.

η = ∫ E g ∞ I ( E ) A ( E ) E g E d E ∫ 0 ∞ I ( E ) d E
J s c = q ∫ E g ∞ I ( E ) E A ( E ) d E .
A ( E ) = 1 − exp [ − α ( E ) L ]
G ( r , E ) = ε i ( E ) | E ( r , E ) | 2 2 ℏ
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