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Improving thin-film crystalline silicon solar cell efficiencies with photonic crystals

Peter Bermel, Chiyan Luo, Lirong Zeng, Lionel C. Kimerling, and John D. Joannopoulos

Open Access Open Access

Abstract

Most photovoltaic (solar) cells are made from crystalline silicon (c-Si), which has an indirect band gap. This gives rise to weak absorption of one-third of usable solar photons. Therefore, improved light trapping schemes are needed, particularly for c-Si thin film solar cells. Here, a photonic crystal-based light-trapping approach is analyzed and compared to previous approaches. For a solar cell made of a 2 µm thin film of c-Si and a 6 bilayer distributed Bragg reflector (DBR) in the back, power generation can be enhanced by a relative amount of 24.0% by adding a 1D grating, 26.3% by replacing the DBR with a six-period triangular photonic crystal made of air holes in silicon, 31.3% by a DBR plus 2D grating, and 26.5% by replacing it with an eight-period inverse opal photonic crystal.

©2007 Optical Society of America

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Figures (9)
Fig. 1.
Fig. 1. (a) conventional solar cell design using traditional geometric optics concepts of reflection and refraction to trap light [5]; (b) novel solar cell design using wave optics (photonic crystals) to trap light with higher efficiency [6].

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Fig. 2.
Fig. 2. Illustration of two metallic solar cell designs: (a) a common design with a perfect metal backing and no front surface texturing, which displays only spectral reflection; (b) a metal with periodic grating on the back [21–25]. Crystalline silicon is in green, metal in grey, and air is transparent.

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Fig. 3.
Fig. 3. (a) Absorption versus wavelength for a plain 2 µm c-Si thin film with a perfect metal back reflector, compared to the same structure with a metal grating of period p=255 nm and etch depth 67 nm (b) Absorption peak wavelength as a function of peak number — note that peak spacing increases with peak number, implying diffraction is strongest right at the diffraction threshold (here, 920 nm).

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Fig. 4.
Fig. 4. Absorption vs. wavelength for three 2 µm-thick Si cell designs: no back reflector, perfect metal back reflector (Fig. 2(a)), and perfect metal grating with a 2D-periodic “checkerboard” pattern of period 255 nm in both lateral directions and an etch depth of 67 nm (Fig. 2(b)). Note that the narrow peaks seen in Fig. 3 are smoothed out with a moving average that preserves the area under the curve.

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Fig. 5.
Fig. 5. Illustration of three solar cell designs: (a) a simple design with a distributed Bragg reflector (DBR), which displays only spectral reflection [13–16] (b) a DBR plus a periodically etched grating, displaying spectral reflection and diffraction [13–16], and (c) a photonic crystal consisting of a triangular lattice of air holes, displaying simultaneous reflection, diffraction, and refraction from the photonic crystal layer (based on Ref. 6). Crystalline silicon is in green, low dielectric in yellow, and air is transparent.

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Fig. 6.
Fig. 6. Illustration of the enhancement of the absorption spectrum created when introducing a 1D grating into a DBR (a=165) with period 310 nm and etch depth 67 nm, quantified as the quotient of the absorption with the grating with the absorption without it. Note that the narrow peaks seen in Fig. 3 are smoothed out with a moving average that preserves the area under the curve.

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Fig. 7.
Fig. 7. Efficiency of power generation versus py/px for the geometry described in the text: a 4-bilayer DBR with a 2D “checkerboard” pattern etch (t=2µm, a=165 nm, e=67 nm). Three different values of px are used; as predicted, smaller values of px see peak efficiencies at higher values of py/px .

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Fig. 8.
Fig. 8. Bandstructures of two photonic crystal structures made of circular air holes in a high index medium (n=3.5) and radius r=0.375a, arranged in (a) a square lattice and (b) a triangular lattice. Note that the triangular lattice provides a larger gap between TE modes, which also results in flatter bands.

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Fig. 9.
Fig. 9. Absorption vs. wavelength at normal incidence for four 2 µm-thick Si cell designs with continuous symmetry in at least one dimension: no back reflector, plain DBR, DBR plus 1D-periodic grating, and finally, a 2D photonic crystal of air holes in silicon. The last two designs consist of six complete layers. The DBR plus grating and the photonic crystal-based design yield the highest efficiencies, and have very similar magnitudes.

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Tables (1)

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Table 1. Percentage efficiency of various solar cell designs as a function of the number of periods in the z-direction for a plain distributed Bragg reflector (DBR), a DBR with 1D and 2D etched gratings (based on Fig. 5(b)), a triangular photonic crystal of air holes in silicon (based on Fig 5(c)), a woodpile of air trenches in silicon (based on Ref. 43), and an inverse silicon opal (based on Ref. 44).

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Equations (1)

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J ( V ) = ∫ 0 ∞ d λ [ e λ h c d I d λ A ( λ ) ] − e ( n 2 + 1 ) E g 2 k T 4 π 2 h ̅ 3 c 2 exp ( e V − E g k T ) ,
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