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Dual gratings for enhanced light trapping in thin-film solar cells by a layer-transfer technique

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Abstract

Thin film solar cells benefit significantly from the enhanced light trapping offered by photonic nanostructures. The thin film is typically patterned on one side only due to technological constraints. The ability to independently pattern both sides of the thin film increases the degrees of freedom available to the designer, as different functions can be combined, such as the reduction of surface reflection and the excitation of quasiguided modes for enhanced light absorption. Here, we demonstrate a technique based on simple layer transfer that allows us to independently pattern both sides of the thin film leading to enhanced light trapping. We used a 400 nm thin film of amorphous hydrogenated silicon and two simple 2D gratings for this proof-of-principle demonstration. Since the technique imposes no restrictions on the design parameters, any type of structure can be made.

©2013 Optical Society of America

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Figures (5)

Fig. 1
Fig. 1 Scanning electron microscopy (SEM) images of the 2D dual interface grating (a) The bottom grating is a photonic lattice of SiO2 holes in a-Si:H with a period of 300 nm. The measured hole diameter and nominal etching depth are 240 nm and 60 nm, respectively. (b) The hole diameter of the top grating is set to 300 nm, which is also the lattice period, such that the air holes touch each other. The top grating therefore resembles a chessboard lattice [24], i.e. star-like columns of a-Si:H in air. The nominal etch depth is 80 nm.
Fig. 2
Fig. 2 (a) Microscope image (top view) of the fabricated 2D dual interface grating in a 400 nm thick a-Si:H slab on glass. The top and bottom grating are independently patterned by e-beam lithography. The two gratings can be clearly identified, as they have been off-set to facilitate individual characterisation. (b) Cross-sectional scheme of the fabricated structure.
Fig. 3
Fig. 3 Measured absorption characteristics of the unpatterned a-Si:H film deposited by plasma enhanced chemical vapor deposition (PECVD). The 400 nm thin film is characterised with a white light laser source and an integrating sphere.
Fig. 4
Fig. 4 Measured absorption characteristics of the patterned a-Si:H film. The measurements are taken with a white light laser source and an integrating sphere (solid line) and afterwards compared to numerical results (dashed line). (a) The measured spectrum for Fig. 1(a) is in excellent agreement with the simulated spectrum, while for the top grating of Fig. 1(b) we observe disagreement in the short-wavelength range. We think this discrepancy is due to the lattice structure used in the simulations, since the top grating is not perfectly described by either a lattice of circular holes or a chessboard lattice. (b) The dual grating shows a very good level of agreement between measured and simulated spectrum.
Fig. 5
Fig. 5 Real and imaginary parts of the dielectric function of the a-Si:H layer determined by spectroscopic ellipsometry.

Tables (1)

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Table 1 Measured absorption and absorption enhancement of a 400 nm thick a-Si:H slab on glass following e-beam patterning. The measurements were integrated over the wavelength range from 600 nm to 750 nm. For comparison the calculated results of a structure consisting of a Ag/Al BR, the a-Si:H slab and a 70 nm Si3N4 ARC are also included (last column). The calculated absorption of the unpatterned film without BR and ARC is 16% instead of the measured 15%, which explains the lower enhancement of the simulated structure.

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