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Light trapping in thin-film solar cells with randomly rough and hybrid textures

Piotr Kowalczewski, Marco Liscidini, and Lucio Claudio Andreani

Open Access Open Access

Abstract

We study light-trapping in thin-film silicon solar cells with rough interfaces. We consider solar cells made of different materials (c-Si and μc-Si) to investigate the role of size and nature (direct/indirect) of the energy band gap in light trapping. By means of rigorous calculations we demonstrate that the Lambertian Limit of absorption can be obtained in a structure with an optimized rough interface. We gain insight into the light trapping mechanisms by analysing the optical properties of rough interfaces in terms of Angular Intensity Distribution (AID) and haze. Finally, we show the benefits of merging ordered and disordered photonic structures for light trapping by studying a hybrid interface, which is a combination of a rough interface and a diffraction grating. This approach gives a significant absorption enhancement for a roughness with a modest size of spatial features, assuring good electrical properties of the interface. All the structures presented in this work are compatible with present-day technologies, giving recent progress in fabrication of thin monocrystalline silicon films and nanoimprint lithography.

© 2013 Optical Society of America

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Figures (9)
Fig. 1
Fig. 1 Structures under consideration: (a) thin-film silicon solar cell with the randomly rough interface, described by the RMS of height σ and the lateral correlation length lc; (b) thin-film silicon solar cell with the hybrid interface, being a combination of a rough interface and a diffraction grating. The grating has period a, width of the etched region b, and etching depth h.

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Fig. 2
Fig. 2 Short-circuit current density as a function of lateral correlation length lc and RMS deviation of height σ, for a 1μm thick μc-Si solar cell with rough interface. Each point is calculated as an average of 10 surface realizations.

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Fig. 3
Fig. 3 Short-circuit current density as a function of RMS deviation of height σ, for 1μm thick c-Si and μc-Si solar cells with rough interfaces. Lateral correlation length is equal to lc = 160nm.

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Fig. 4
Fig. 4 Absorption (left) and spectral contribution to the short-circuit current density (right) in 1μm-thick c-Si and μc-Si solar cells, at three different values of RMS deviation of height σ. Lateral correlation length is equal to lc = 160nm.

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Fig. 5
Fig. 5 Left: absorption in c-Si (a) and μc-Si (b) solar cells with the optimized rough ARC/Si interface (σ = 300nm, lc = 160nm) and an absorbing layer of 1μm, along with the corresponding Lambertian Limit. Both silver and perfect back reflectors (BR) were considered. Right: absorption in c-Si solar cells with rough ARC/Si interface (σ = 300nm, lc = 160nm), a perfect back reflector (BR), and an absorbing layer of thickness d = 2μm (c), 5μm (d), and 10μm (e), along with the corresponding Lambertian Limit.

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Fig. 6
Fig. 6 Haze of transmitted light as a function of energy for increasing RMS deviation of height σ, calculated for the rough interface sketched in the inset. Both anti-reflection coating (ARC) and c-Si layer are assumed to be semi-infinite. Each point is taken as an average of 500 rough surface realizations. The lateral correlation length is equal to lc = 160nm.

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Fig. 7
Fig. 7 AID of transmitted light as a function of energy for increasing RMS deviation of height σ, calculated for the rough ARC/c-Si interface. The AID is taken as an average of 500 rough surface realizations. The black rectangles denote the specular part of transmitted light (i.e., the light propagating within the cone between −1.5° and 1.5°). The lateral correlation length is equal to lc = 160nm.

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Fig. 8
Fig. 8 AID of transmitted light at (a) E = 1.24eV and (b) E = 2.86eV, calculated at three values of RMS deviation of height σ. The results are compared with the cosine distribution corresponding to the Lambertian Limit. Lateral correlation length is equal to lc = 160nm.

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Fig. 9
Fig. 9 Absorption in 1μm thick c-Si solar cells with (a) the rough interface with a modest spatial features (σ = 80nm, lc = 60nm), (b) the optimal diffraction grating (a = 600nm, b = 180nm, h = 240nm), and (c) the hybrid interface. The structures are sketched on the right. The results are compared with the corresponding Lambertian Limit of absorption.

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Tables (2)

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Table 1 Short-circuit current densities calculated for 1μm thick c-Si and μc-Si solar cells, with either silver or perfect back reflectors (BR), together with the values corresponding to the Lambertian Limit. The parameters of the rough interface are: σ = 300nm, lc = 160nm.

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Table 2 Short-circuit current density JSC and its relative enhancement for a 1μm thick c-Si solar cell with the optimal diffraction grating (a = 600nm, b = 180nm, h = 240nm), the rough interface with a modest spatial features (σ = 80nm, lc = 60nm), the hybrid interface, and the optimized rough interface (σ = 300nm, lc = 160nm), compared with the JSC corresponding to the Lambertian Limit. Relative enhancement of the JSC was calculated with respect to the structure with a flat ARC/Si interface.

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Equations (4)

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J SC = q ∫ b S ( E ) A ( E ) d E ,
A T = ( 1 − R ext ) ( 1 − T + T − ) 1 − R f T + T − ,
T + T − = ∫ − π / 2 π / 2 e − 2 α d / cos θ cos ( θ ) d θ ∫ − π / 2 π / 2 cos ( θ ) d θ ,
R f = 1 − 1 / n .
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