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Combined front and back diffraction gratings for broad band light trapping in thin film solar cell

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Abstract

In this paper, we present the integration of combined front and back 1D and 2D diffraction gratings with different periods, within thin film photovoltaic solar cells based on crystalline silicon layers. The grating structures have been designed considering both the need for incident light absorption enhancement and the technological feasibility. Long wavelength absorption is increased thanks to the long period (750 nm) back grating, while the incident light reflection is reduced by using a short period (250 nm) front grating. The simulated short circuit current in a solar cell combining a front and a back grating structures with a 1.2 µm thick c-Si layer, together with the back electrode and TCO layers, is increased up to 30.3 mA/cm2, compared to 18.4 mA/cm2 for a reference stack, as simulated using the AM1.5G solar spectrum intensity distribution from 300 nm to 1100 nm, and under normal incidence.

©2012 Optical Society of America

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Figures (9)

Fig. 1
Fig. 1 Schematic views of the investigated complete (a) unpatterned stack (reference) and (b) patterned stack with the front and back 1D diffraction grating with different periods.
Fig. 2
Fig. 2 Schematic view of the back grating structure (a); influence of the grating depth (b) and period (c) on the light intensity distribution on the diffracted orders, for a TM polarization.
Fig. 3
Fig. 3 Schematic view of the front grating (a), and reflected intensity versus the wavelength, for various grating periods and considering a TM polarization, and a 50% ff; the reference corresponds to a planar Si surface covered by a 80nm thick ITO layer (b).
Fig. 4
Fig. 4 Absorption spectra of the c-Si layer for the front grating, the back grating and the double grating optimized structures, for (a) TM and (b) TE polarized incident light, (c) with a close-up of the spectra for TE polarized light between 800 and 1000 nm.
Fig. 5
Fig. 5 |E|2 field maps, for TE polarized incident light in the double grating structures (peaks 1-2), in the front grating structures (peaks 3-4), in the back grating structures (peaks 5-6), and in the planar device (peaks 7-8) of Fig. 4(c).
Fig. 6
Fig. 6 Schematic view of the double side structure (a), the top view of the patterned structure with 1D (b) and 2D (c) diffraction gratings.
Fig. 7
Fig. 7 Absorption spectra for the flat unpatterned reference, compared to the 1D double grating structure, considering the average of spectra corresponding to TE and TM polarized incident light, and the 2D double grating structure (a), and short circuit current density for all the structures investigated, with a total c-Si layer thickness of 1.2 µm (b). As a comparison, a full absorption of the incident light would lead to a 43.5 mA/cm2 current density (dotted line).
Fig. 8
Fig. 8 Schematic cross section view of the optimized front and back gratings solar cell stack (a), and photocurrent versus the c-Si layer thickness, for a TM polarized incident light (b).
Fig. 9
Fig. 9 Refractive index n and extinction coefficient k used for optical simulations, for ITO (measured by ellipsometry), c-Si (measured by ellipsometry) (k = 0 is considered for λ>1µm), ZnO [24] and Ag [24].

Equations (2)

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J sc = e hc λ 1 λ 2 λA(λ) dI dλ dλ
sin θ m +sin θ i =+mλ/ Λ b n d
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