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The versatile designs and optimizations for cylindrical TiO2-based scatterers for solar cell anti-reflection coatings

Albert Lin, Yan-Kai Zhong, and Sze-Ming Fu

Open Access Open Access

Abstract

The anti-reflection coating(ARC) based on dielectric nano-particles has been recently proposed as a new way to achieve the low reflectance required for solar cell front surfaces. In this scenario, the Mie modes associated with the dielectric nano-particles are utilized to facilitate photon forward scattering. In this work, versatile designs together with systematically optimized geometry are examined, for the ARCs based on dielectric scatterers. It is found that the Si3N4-TiO2 or SiO2-TiO2 stack is capable of providing low reflectance while maintaining a flat and passivated ARC-semiconductor interface which can be beneficial for reduced interface recombination and prevent VOC degradation associated with topography on the active materials. It is also confirmed that the plasmonic nano-particles placed at the front side of solar cells is not a preferred scheme, even with thorough geometrical optimization. At the ultimate design based on mixed graded index(GI) Mie-scattering, the averaged reflectance can be as low as 0.25%. Such a low reflectance is currently only achievable by ultra-long silicon nano-tips, but silicon nano-tips introduce severe surface recombination. On the other hand, the mixed GI Mie design preserves a flat and passivated ARC-silicon interface, with total thickness reduced to 279.8nm, much thinner than 1.6μm for silicon nanotips. In addition, the light trapping capability of mixed GI Mie design is much better than silicon nanotips. In fact, when compared to the state-of-art TiO2 light trapping anti-reflection coating, the mixed GI Mie design provides same light trapping capability while providing much lower reflectance.

© 2013 Optical Society of America

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Figures (9)
Fig. 1
Fig. 1 The cross-section and the topview of the cylindrical dielectric scatterer anti-reflection coating. The geometrical parameters under global optimization is labeled in the cross sectional view. (Left) Cylindrical TiO2 scatterers with Si3N4 wrapping. (Right) Cylindrical TiO2 scatterers with SiO2 wrapping.

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Fig. 2
Fig. 2 (Left) the spectral reflectance for TiO2-Si3N4 and TiO2-SiO2 dielectric scatterer ARC. (Right) the corresponding field profiles Ey at y = 0 for λ = 400nm and λ = 800nm.

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Fig. 3
Fig. 3 The cross-section and the topview of the cylindrical surface plasmonic (SP) assisted dielectric scatterer anti-reflection coating. The geometrical parameters under global optimization is labeled in the cross sectional view.

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Fig. 4
Fig. 4 (Left) the spectral transmittance(T), reflectance(R), and metal absorbance(Abs) for the surface plasmonic assisted dielectric scatterer anti-reflection coating. (Right) the corresponding field profiles Ey at y = 0, for λ = 400nm,λ = 600nm, λ = 800nm, and λ = 1000nm.

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Fig. 5
Fig. 5 The cross-section and the topview of the cylindrical mixed graded index(GI) Mie scattering anti-reflection coating. The geometrical parameters under global optimization is labeled in the cross sectional view. This structure will be abbreviated as mixed GI Mie ARC below.

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Fig. 6
Fig. 6 Comparison of mixed graded index Mie scattering anti-reflection coating with ultra-low reflectance silicon nano-tip ARC in reference [7] and the state-of-art light trapping anti-reflection TiO2 coating in [5]. The dimension for TiO2 nanotip ARC [5] is P = 0.6μm, the dimension for the silicon nanotip ARC [7] is L = 1.6μm and P = 0.2μm, and the dimension for the planar multi-layer ARC is from [6].

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Fig. 7
Fig. 7 (Left) the simulation structure for calculating the reflectance of anti-reflection coatings for Fig. 2, Fig. 4, Fig. 6, and Fig. 9 and the Ravg in Table 1. (Right) the simulation structure for calculating the integrated absorbance weight by AM1.5 spectrum, AInt, in Table 1.

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Fig. 8
Fig. 8 The cross-sectional and topview of the sidewall-free (SWF) Mie scattering anti-reflection coating and the geometrical parameters under global optimization.

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Fig. 9
Fig. 9 The spectral reflectance of the sidewall-free cylindrical Mie scattering ARC at its optimal geometry.

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Tables (1)

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Table 1 Comparison of different anti-reflection coatings at their respective optimized geometry

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Equations (2)

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

A(λ)= 1 2 ∫ V ω ε 0 ε ″ (λ) | E ⇀ ( r ⇀ ) | 2 dv 1 2 ∫ S Re{ E ⇀ ( r ⇀ )× H ⇀ * ( r ⇀ ) }⋅d s ⇀ .
A Int = V Si, Ref V Si ∫ λ hc Ω(λ)A(λ)dλ ∫ λ hc Ω(λ)dλ .
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