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Nanophotonic light trapping in 3-dimensional thin-film silicon architectures

Daniel Lockau, Tobias Sontheimer, Christiane Becker, Eveline Rudigier-Voigt, Frank Schmidt, and Bernd Rech

Open Access Open Access

Abstract

Emerging low cost and large area periodic texturing methods promote the fabrication of complex absorber structures for thin film silicon solar cells. We present a comprehensive numerical analysis of a 2μm square periodic polycrystalline silicon absorber architecture designed in our laboratories. Simulations are performed on the basis of a precise finite element reconstruction of the experimentally realized silicon structure. In contrast to many other publications, superstrate light trapping effects are included in our model. Excellent agreement to measured absorptance spectra is obtained. For the inclusion of the absorber into a standard single junction cell layout, we show that light trapping close to the Yablonovitch limit can be realized, but is usually strongly damped by parasitic absorption.

© 2012 Optical Society of America

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Figures (6)
Fig. 1
Fig. 1 (a) SEM image of the 2.4μm thick silicon layer after e-beam evaporation on a periodic substrate (large). The inset depicts a similar sample after the etching of amorphous regions. (b) Cross-sectional SEM images of the silicon layer before and after the etching process. Cone opening angle: θ ≈ 20°. Distance between the inner and outer groove surfaces: d ≈ 0.34μm. (c) Cross-section and perspective view of the computational model of the unit cell. Letters “a” and “c” denote amorphous and crystalline regions of the unetched absorber, “s” marks the textured solgel substrate and “e” an extruded intermediate layer. (d) Real part of the refractive index and absorption coefficients of silicon, ZnO:Al and ZrO2. ZrO2is assumed to be non-absorptive. (e) Diagram of the simulated solar cell, which is illuminated through its deposition substrate.

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Fig. 2
Fig. 2 Comparison of experimental absorptance (red full line without markers) and simulated absorptance, including the approximated superstrate light trapping contribution. The region of highest difference between experiment and simulation is shaded in gray and related to defect absorption. The superstrate light trapping contribution is included for additional reference.

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Fig. 3
Fig. 3 Transmittance through the glass superstrate and a textured ZnO:Al layer into a silicon half space. The 2μm periodic solgel texture shown in Fig. 1(c) was applied to the interface, with a vertically extruded 300 nm thick ZnO:Al layer. Box plots depict mean (crosses), median, first / third quartile and whiskers show extreme values. Left subplot: complete scaling of the geometry, including TCO layer thickness; the inset depicts the wavelength resolved transmittance at 2μm domain period. Right subplot: height scaling of the 2μm periodic texture at constant TCO layer thickness.

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Fig. 4
Fig. 4 (a) Electrical vertical device layout and the simplified optical layout used for simulation. (b) Simulated generation rate in silicon and losses in other absorbing media for a conformal back reflector layout, depicted as configuration “B” in Fig. 5(a). The generation rate and front TCO a well as transmittance losses of the reflector-less, but otherwise identical layout are included as broken red lines. A white line depicts the superstrate light trapping contribution to silicon absorptance. (c) Absorptance difference of the silicon absorptance from (b) to the reflector-less case and the case of a crystalline absorber without etching grooves.

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Fig. 5
Fig. 5 (a) Center cross sections of the different reflector designs. (b) Average absorptance of lossy materials for the three layouts shown in (a), in the wavelength region between 600 nm and 1100 nm. Percentage numbers denote silicon absorptance. Slim bars depict symmetrically split low and high wavelength contributions of silicon absorptance. (c) Difference of absorptance proportions between the back reflector layouts “B” and “C” with individually non-absorptive TCO layers to the corresponding reference results in diagram (b).

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Fig. 6
Fig. 6 Average light path enhancement factors for wavelengths between 1000 nm and 1100 nm for the reflector designs “B”, “C” and varying domain periods. Simulations of design “B” are with absorbing TCO. Simulations of design “C” are with non-absorbing back TCO and non-absorbing back as well as front TCO.

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Equations (1)

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LPEF ( λ ) = − ln ( 1 − A silicon ( λ ) / I ( λ ) ) α silicon ( λ ) d ,
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