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Energy Express

  • Editor: Christian Seassal
  • Vol. 20, Iss. S6 — Nov. 5, 2012
  • pp: A997–A1004
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Design of input couplers for efficient silicon thin film solar absorbers

Sun-Kyung Kim, Kyung-Deok Song, and Hong-Gyu Park  »View Author Affiliations


Optics Express, Vol. 20, Issue S6, pp. A997-A1004 (2012)
http://dx.doi.org/10.1364/OE.20.00A997


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Abstract

We investigated light absorption in various Si thin film solar absorbers and designed efficient input couplers using finite-difference time-domain simulation. In the simulation, a dielectric coating on Si thin film led to enhanced light absorption at near-ultraviolet to blue wavelengths, while the absorption peaks at longer wavelengths were nearly preserved. In a 300-nm-thick Si film with a 60-nm-thick Si3N4 top-coated layer, current density was augmented by ~35% compared to a bare Si film. For broadband absorption, we introduced two-dimensional square-lattice periodic patterns consisting of low-index dielectric materials, SiO2 or Si3N4, or high-index material, Si. The periodic pattern exhibited tunable and pronounced absorption peaks that are indentified as horizontally-propagating waveguide modes. The high absorption peaks were significantly amplified with increasing refractive index of the dielectric pattern. For a Si-patterned structure with a pitch size of 400 nm and a pattern depth of 80 nm, current density was achieved up to 17.0 mA/cm2, which is enhanced by a factor of 2.1 compared to the current density of bare Si film. Deep understanding of the light absorption in optical cavities with wavelength-scale thickness will be useful in the design of efficient thin film solar absorbers as well as novel nanophotonic elements.

© 2012 OSA

1. Introduction

Over the past decade, photovoltaic solar cells have been considered as promising alternative sources of electricity [1

1. Renewables 2011 Global Status Report (Renewable Energy Policy Network, 2011).

]. These electric devices harness pollution-free solar energy, but occupy only a small portion of the world energy market, which is currently dominated by fossil fuel and nuclear energy [1

1. Renewables 2011 Global Status Report (Renewable Energy Policy Network, 2011).

3

3. L. Fraas and L. Partain, Solar Cells and their Applications 2nd ed. (Wiley Series in Microwave and Optical Engineering, 2010), Chap. 2.

]. Both the power conversion efficiency and correlated efficiency-to-cost metric must be further increased [2

2. D. M. Powell, M. T. Winkler, H. J. Choi, C. B. Simmons, D. B. Needleman, and T. Buonassisi, “Crystalline silicon photovoltaics: a cost analysis framework for determining technology pathways to reach baseload electricity costs,” Energy Environ. Sci. 5(3), 5874–5883 (2012). [CrossRef]

,3

3. L. Fraas and L. Partain, Solar Cells and their Applications 2nd ed. (Wiley Series in Microwave and Optical Engineering, 2010), Chap. 2.

]. The efficiency or efficiency-to-cost of solar cells was degraded partly due to carrier recombination determined by material defects and incomplete light absorption determined by reflection or transmission loss [4

4. A. Polman and H. A. Atwater, “Photonic design principles for ultrahigh-efficiency photovoltaics,” Nat. Mater. 11(3), 174–177 (2012). [CrossRef] [PubMed]

]. Although the power conversion efficiency can be improved in wafer-based solar cells by diverse technologies that include purification of the wafer [5

5. R. H. Hopkins and A. Rohatgi, “Impurity effects in silicon for high efficiency solar cells,” J. Cryst. Growth 75(1), 67–79 (1986). [CrossRef]

] together with anti-reflection coating [6

6. S. Chhajed, M. F. Schubert, J. K. Kim, and E. F. Schubert, “Nanostructured multilayer graded-index antireflection coating for Si solar cells with broadband and omnidirectional characteristics,” Appl. Phys. Lett. 93(25), 251108 (2008). [CrossRef]

,7

7. J. Ko, D. Gong, K. Pillai, K.-S. Lee, M. Ju, P. Choi, K.-R. Kim, J. Yi, and B. Choi, “Double layer SiNx:H films for passivation and anti-reflection coating of c-Si solar cells,” Thin Solid Films 519(20), 6887–6891 (2011). [CrossRef]

] and surface texturing [8

8. X. Meng, V. Depauw, G. Gomard, O. El Daif, C. Trompoukis, E. Drouard, C. Jamois, A. Fave, F. Dross, I. Gordon, and C. Seassal, “Design, fabrication and optical characterization of photonic crystal assisted thin film monocrystalline-silicon solar cells,” Opt. Express 20(S4Suppl 4), A465–A475 (2012). [CrossRef] [PubMed]

15

15. J. S. Li, H. Y. Yu, Y. L. Li, F. Wang, M. F. Yang, and S. M. Wong, “Low aspect-ratio hemispherical nanopit surface texturing for enhancing light absorption in crystalline Si thin film-based solar cells,” Appl. Phys. Lett. 98(2), 021905 (2011). [CrossRef]

], the cost relative to efficiency remains a nontrivial issue.

2. Anti-reflection coating on Si thin film

To quantify the dielectric coating effect in a Si thin film structure, we calculated current density while varying the thickness (t) of Si3N4 (n = 2.0) layer (Fig. 1(D)). The current density was calculated by integrating the absorption spectrum multiplied by the solar spectrum over λ = 280-1000 nm [22

22. T. J. Kempa, J. F. Cahoon, S.-K. Kim, R. W. Day, D. C. Bell, H.-G. Park, and C. M. Lieber, “Coaxial multishell nanowires with high-quality electronic interfaces and tunable optical cavities for ultrathin photovoltaics,” Proc. Natl. Acad. Sci. U.S.A. 109(5), 1407–1412 (2012). [CrossRef] [PubMed]

,23

23. S.-K. Kim, R. W. Day, J. F. Cahoon, T. J. Kempa, K.-D. Song, H.-G. Park, and C. M. Lieber, “Tuning light absorption in core/shell silicon nanowire photovoltaic devices through morphological design,” Nano Lett. 12(9), 4971–4976 (2012). [CrossRef] [PubMed]

]. For simplicity, we assumed that the internal quantum efficiency is unity. The result shows that current density is augmented by up to 35% at t = 60 nm. This thickness, which provides the largest current density, agrees well with that obtained in the quarter-wavelength interference condition, if the central wavelength of incident light is ~450 nm. Consequently, the dielectric coating effect in a thin film solar absorber results in increased absorption at shorter wavelengths, and thus an efficient input coupler for broadband light absorption will be needed to further enhance the current density.

3. Two-dimensional SiO2 or Si3N4 periodic patterns on Si absorber

As another way of enhancing light absorption, a periodic pattern can be used in solar absorbers. We introduced a 2-D square-lattice periodic pattern on a Si thin film structure (Fig. 2(A)
Fig. 2 (A) Schematic illustration of a Si thin film with a 2-D patterned structure on the top. (B) Absorption spectra of a 300-nm-thick Si thin film absorber with a patterned SiO2 (solid blue) or Si3N4 layers (solid pink). The thickness of both dielectric layers is 200 nm and the pitch size of the pattern is 400 nm. (C) Absorption mode profiles at λ = 460, 505, 575, 700 nm (left to right) in the absorption spectrum of the Si3N4-patterend Si film structure. (D) Absorption mode profiles at λ = 525, 665, 825, 950 nm (left to right) in the absorption spectrum of the Si3N4-patterened Si film structure.
). In the simulation model, the 200-nm-thick SiO2 (n = 1.45) or Si3N4 (n = 2.0) pillars with square cross-section were formed periodically on top of a 300-nm-thick Si thin film. For both dielectric materials, we assumed that the refractive indices are constant and the absorption coefficients are zero over the range of wavelengths [20

20. D. R. Lide, CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data (CRC Press, 2008).

]. Figure 2(B) shows the absorption spectra of a Si film with patterned SiO2 (solid blue, Fig. 2(B)) or Si3N4 (solid pink, Fig. 2(B)) structures, where the pitch size of the pattern (a) and the diameter of the pattern were set to 400 nm and 200 nm, respectively. The simulation result highlights several key features to explain how a periodic pattern enhances light absorption. First, these structures with periodic patterns support several new absorption peaks, as compared to a planar structure shown in Fig. 1(C). The new peaks have relatively sharp bandwidths and high amplitudes. Second, the new absorption peaks have much higher amplitudes in the structure with the Si3N4 pattern. Third, the spectral features of the original peaks obey the same trend as those shown in a thin film with the top dielectric coating. The amplitudes of the peaks were enhanced at short wavelengths, while only marginally changed at long wavelengths. To understand the origin of these new absorption peaks in the dielectric-patterned structures, we obtained representative absorption mode profiles at several resonant wavelengths. Comparison of the mode profiles from the original absorption peaks (Fig. 2(C)) to those from the new absorption peaks (Fig. 2(D)) reveals important features. The original peaks correspond to 1-D Fabry-Perot modes with progressively increasing number of anti-nodes, similarly to the peaks that appeared in a thin film cavity without pattern as shown in the inset of Fig. 1(B). On the other hand, the new resonant peaks have additional anti-nodes along the horizontal direction. The wavelength of new peaks is red-shifted compared to the original peak with the same number of anti-nodes along the vertical direction. For example, the wavelength of the new peak with three anti-nodes is λ = 825 nm while that of the original peak is λ = 700 nm. We note that the spectral density of the new peaks is higher than that of the original peak since the peaks with more horizontal anti-nodes are also excited in the spectrum.

In general, a periodic pattern generates in-plane Bloch momentum that can be supplied to the initial momentum of photons at specific wavelengths [24

24. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. U.S.A. 107(41), 17491–17496 (2010). [CrossRef] [PubMed]

]. In the case of a thin film structure, normally incident photons can be converted into horizontally-propagating photons under an appropriate diffraction condition. The new absorption peaks, identified as waveguide modes, are generated due to the diffraction of the periodic pattern. Therefore, the absorption mode profiles with the transverse anti-nodes show a manifestation of the additional in-plane Bloch momentum. Each new absorption peak is assigned to a waveguide mode with a different diffraction order. Apparently, the horizontal propagation of the waveguide modes increases the photon life time in a thin film structure such that sharp and high-amplitude peaks emerge in the spectrum [23

23. S.-K. Kim, R. W. Day, J. F. Cahoon, T. J. Kempa, K.-D. Song, H.-G. Park, and C. M. Lieber, “Tuning light absorption in core/shell silicon nanowire photovoltaic devices through morphological design,” Nano Lett. 12(9), 4971–4976 (2012). [CrossRef] [PubMed]

]. Since the diffraction is enhanced by larger contrast of the refractive index (n) in the dielectric pattern [25

25. S.-K. Kim, H. K. Cho, D. K. Bae, J. S. Lee, H.-G. Park, and Y.-H. Lee, “Efficient GaN slab vertical light-emitting diode covered with a patterned high-index layer,” Appl. Phys. Lett. 92(24), 241118 (2008). [CrossRef]

], the amplitudes of the absorption peaks in the Si film structure with Si3N4 (n = 2.0) pattern are larger than those in the structure with the SiO2 (n = 1.45) pattern. On the other hand, for the original peaks resulting from normal cavity modes, the dielectric pattern is considered as a single layer with an average refractive index determined by the filling-factor of the pattern.

Next, we calculated the current density of the Si3N4-patterend Si film structure over a wide range of pitch sizes (Fig. 3(B)). The result shows that current density plateaus at a = 600 nm with an enhancement factor of 1.68 or 1.24 as compared to a bare Si film or the optimized Si3N4-coated Si film structure as shown in Fig. 1(D), respectively. The optimum pitch size of ~600 nm agrees well with the values shown in previously reported works [24

24. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. U.S.A. 107(41), 17491–17496 (2010). [CrossRef] [PubMed]

]. For a high-refractive-index thin film structure with a dielectric pattern, light absorption is maximized at a pitch size similar to the wavelength of incident light [24

24. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. U.S.A. 107(41), 17491–17496 (2010). [CrossRef] [PubMed]

]. If a<<λ, there are only a few channels to meet the diffraction condition such that the number of waveguide modes excited by the periodic pattern is reduced. On the other hand, if a>>λ, the magnitude of Bloch momentum is too small, and thus only higher-order waveguide modes can be diffracted efficiently. Because the solar irradiance has a relatively high amplitude at λ = 450-700 nm, the optimum pitch size for the maximum light absorption lies in the middle of this wavelength range. Together, it is important to choose an appropriate pitch size of a dielectric pattern to achieve the maximum power conversion efficiency of a solar absorber.

4. Two-dimensional periodic pattern in Si absorber

We found that the amplitude of waveguide modes was highly enhanced with increasing the refractive index of a dielectric pattern. In this section, we introduce a 2-D square-lattice periodic pattern to a Si thin film absorber (Fig. 4(A)
Fig. 4 (A) Schematic illustration of a Si-patterned thin film absorber. (B) Absorption spectra of a Si-patterned thin film absorber with a pattern depth of 20 nm (solid black) and 80 nm (solid purple). The total thickness of the Si film is 300 nm and the pitch size of the pattern is 400 nm. Right, absorption mode profiles of the peaks at λ = 670 nm (top), 740 nm (bottom), marked by * in the absorption spectra of the structure with a pattern depth of 20 nm and 80 nm, respectively. (C) Calculated current densities of the Si-patterned thin film absorbers with a Si thickness of 300 nm (solid blue) and 600 nm (solid red) as a function of Si pattern depth. (D) Absorption spectra of the Si-patterned thin film absorber with (solid orange) and without (solid purple) a conformal 20-nm-thick Si3N4 coating layer. The pattern depth in Si is 100 nm. Inset, schematic of the calculated structure.
). The absorption spectra calculated at different Si pattern depths (h) show that the Si-patterned structures excite several new absorption peaks (left, Fig. 4(B)). Significantly, the spectrum at h = 80 nm (solid purple, Fig. 4(B)) has higher amplitude than the spectrum at h = 20 nm (solid black, Fig. 4(B)), while both spectral densities are nearly the same. The mode profiles of the new absorption peaks show that the peaks are also assigned to the waveguide modes with anti-nodes in the horizontal direction (right, Fig. 4(B)). For λ<450 nm, the anti-reflection effect allows for the structure with h = 80 nm to yield higher absorption efficiency than the structure with h = 20 nm, because the Si pattern with larger depth can satisfy a quarter-wavelength interference condition. On the other hand, for λ>450 nm, because of the amplified waveguide modes, the structure with h = 80 nm shows greatly enhanced light absorption. We calculated the current density of the Si-patterned structure with varying h, and as a result, the maximum current density was 17.0 mA/cm2 at h = 80 nm (solid blue, Fig. 4(C)). This current density was augmented by a factor of 2.1 compared to a bare Si film structure as shown in Fig. 1(D). The current density is reduced for larger Si pattern depths, which implies that the diffraction strength is saturated at a pattern depth beyond the point at which the loss of absorbing volume becomes dominant.

5. Conclusion

In this study, we designed several anti-reflection structures in a Si thin film solar absorber. The dielectric coating layer with a quarter-wavelength thickness enhanced light absorption effectively near blue wavelengths, while the absorption modes at longer wavelengths were slightly red-shifted, showing a small change in amplitude. The 2-D periodic pattern composed of a dielectric material led to a conventional anti-reflection effect as well as excitation of new waveguide modes, and resulted in broadband light absorption. In a Si thin film absorber with patterned low-index material, current density increased with increasing refractive index of the patterned material. We calculated a maximum current density of 17.0 mA/cm2 in the Si-patterned structure with a total thickness of 300 nm. A tailored design (e.g. filling-factor, lattice type, pattern morphology) of a dielectric pattern can further enhance current density [27

27. J. Zhu, C.-M. Hsu, Z. Yu, S. Fan, and Y. Cui, “Nanodome solar cells with efficient light management and self-cleaning,” Nano Lett. 10(6), 1979–1984 (2010). [CrossRef] [PubMed]

]. We believe that our deep understanding of light absorption interplaying with optical resonances can be utilized to design diverse multifunctional photonic devices, including semiconductor solar absorbers.

Acknowledgments

S.-K.K and K.-D.S. contributed equally to this work. We thank R. W. Day for helpful discussions. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (Grant No. 2012-0000242).

References and links

1.

Renewables 2011 Global Status Report (Renewable Energy Policy Network, 2011).

2.

D. M. Powell, M. T. Winkler, H. J. Choi, C. B. Simmons, D. B. Needleman, and T. Buonassisi, “Crystalline silicon photovoltaics: a cost analysis framework for determining technology pathways to reach baseload electricity costs,” Energy Environ. Sci. 5(3), 5874–5883 (2012). [CrossRef]

3.

L. Fraas and L. Partain, Solar Cells and their Applications 2nd ed. (Wiley Series in Microwave and Optical Engineering, 2010), Chap. 2.

4.

A. Polman and H. A. Atwater, “Photonic design principles for ultrahigh-efficiency photovoltaics,” Nat. Mater. 11(3), 174–177 (2012). [CrossRef] [PubMed]

5.

R. H. Hopkins and A. Rohatgi, “Impurity effects in silicon for high efficiency solar cells,” J. Cryst. Growth 75(1), 67–79 (1986). [CrossRef]

6.

S. Chhajed, M. F. Schubert, J. K. Kim, and E. F. Schubert, “Nanostructured multilayer graded-index antireflection coating for Si solar cells with broadband and omnidirectional characteristics,” Appl. Phys. Lett. 93(25), 251108 (2008). [CrossRef]

7.

J. Ko, D. Gong, K. Pillai, K.-S. Lee, M. Ju, P. Choi, K.-R. Kim, J. Yi, and B. Choi, “Double layer SiNx:H films for passivation and anti-reflection coating of c-Si solar cells,” Thin Solid Films 519(20), 6887–6891 (2011). [CrossRef]

8.

X. Meng, V. Depauw, G. Gomard, O. El Daif, C. Trompoukis, E. Drouard, C. Jamois, A. Fave, F. Dross, I. Gordon, and C. Seassal, “Design, fabrication and optical characterization of photonic crystal assisted thin film monocrystalline-silicon solar cells,” Opt. Express 20(S4Suppl 4), A465–A475 (2012). [CrossRef] [PubMed]

9.

X. Meng, E. Drouard, G. Gomard, R. Peretti, A. Fave, and C. Seassal, “Combined front and back diffraction gratings for broad band light trapping in thin film solar cell,” Opt. Express 20(S5Suppl 5), A560–A571 (2012). [CrossRef] [PubMed]

10.

J. Grandidier, D. M. Callahan, J. N. Munday, and H. A. Atwater, “Light absorption enhancement in thin-film solar cells using whispering gallery modes in dielectric Nanospheres,” Adv. Mater. (Deerfield Beach Fla.) 23(10), 1272–1276 (2011). [CrossRef] [PubMed]

11.

L. Li, K.-Q. Peng, B. Hu, X. Wang, Y. Hu, X.-L. Wu, and S.-T. Lee, “Broadband optical absorption enhancement in silicon nanofunnel arrays for photovoltaic applications,” Appl. Phys. Lett. 100(22), 223902 (2012). [CrossRef]

12.

P. Spinelli, M. A. Verschuuren, and A. Polman, “Broadband omnidirectional antireflection coating based on subwavelength surface Mie resonators,” Nat Commun 3(692), 692 (2012). [CrossRef] [PubMed]

13.

R. A. Pala, J. White, E. Barnard, J. Liu, and M. L. Brongersma, “Design of plasmonic thin-film solar cells with broadband absorption enhancements,” Adv. Mater. (Deerfield Beach Fla.) 21(34), 3504–3509 (2009). [CrossRef]

14.

F. Wang, H. Yu, J. Li, S. Wong, X. W. Sun, X. Wang, and H. Zheng, “Design guideline of high efficiency crystalline Si thin film solar cell with nanohole array textured surface,” J. Appl. Phys. 109(8), 084306 (2011). [CrossRef]

15.

J. S. Li, H. Y. Yu, Y. L. Li, F. Wang, M. F. Yang, and S. M. Wong, “Low aspect-ratio hemispherical nanopit surface texturing for enhancing light absorption in crystalline Si thin film-based solar cells,” Appl. Phys. Lett. 98(2), 021905 (2011). [CrossRef]

16.

N. S. Lewis, “Toward cost-effective solar energy use,” Science 315(5813), 798–801 (2007). [CrossRef] [PubMed]

17.

I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf, C. L. Perkins, B. To, and R. Noufi, “19·9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81·2% fill factor,” Prog. Photovolt. Res. Appl. 16(3), 235–239 (2008). [CrossRef]

18.

S.-K. Kim, H.-S. Ee, K.-D. Song, and H.-G. Park, “Design of out-coupling structures with metal-dielectric surface relief,” Opt. Express 20(15), 17230–17236 (2012). [CrossRef]

19.

A. Vial and T. Laroche, “Comparison of gold and silver dispersion laws suitable for FDTD simulations,” Appl. Phys. B 93(1), 139–143 (2008). [CrossRef]

20.

D. R. Lide, CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data (CRC Press, 2008).

21.

Y. Yu, V. E. Ferry, A. P. Alivisatos, and L. Cao, “Dielectric core-shell optical antennas for strong solar absorption enhancement,” Nano Lett. 12(7), 3674–3681 (2012). [CrossRef] [PubMed]

22.

T. J. Kempa, J. F. Cahoon, S.-K. Kim, R. W. Day, D. C. Bell, H.-G. Park, and C. M. Lieber, “Coaxial multishell nanowires with high-quality electronic interfaces and tunable optical cavities for ultrathin photovoltaics,” Proc. Natl. Acad. Sci. U.S.A. 109(5), 1407–1412 (2012). [CrossRef] [PubMed]

23.

S.-K. Kim, R. W. Day, J. F. Cahoon, T. J. Kempa, K.-D. Song, H.-G. Park, and C. M. Lieber, “Tuning light absorption in core/shell silicon nanowire photovoltaic devices through morphological design,” Nano Lett. 12(9), 4971–4976 (2012). [CrossRef] [PubMed]

24.

Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. U.S.A. 107(41), 17491–17496 (2010). [CrossRef] [PubMed]

25.

S.-K. Kim, H. K. Cho, D. K. Bae, J. S. Lee, H.-G. Park, and Y.-H. Lee, “Efficient GaN slab vertical light-emitting diode covered with a patterned high-index layer,” Appl. Phys. Lett. 92(24), 241118 (2008). [CrossRef]

26.

J. N. Munday and H. A. Atwater, “Large integrated absorption enhancement in plasmonic solar cells by combining metallic gratings and antireflection coatings,” Nano Lett. 11(6), 2195–2201 (2011). [CrossRef] [PubMed]

27.

J. Zhu, C.-M. Hsu, Z. Yu, S. Fan, and Y. Cui, “Nanodome solar cells with efficient light management and self-cleaning,” Nano Lett. 10(6), 1979–1984 (2010). [CrossRef] [PubMed]

OCIS Codes
(050.0050) Diffraction and gratings : Diffraction and gratings
(350.6050) Other areas of optics : Solar energy
(070.7345) Fourier optics and signal processing : Wave propagation

ToC Category:
Photovoltaics

History
Original Manuscript: September 17, 2012
Revised Manuscript: October 21, 2012
Manuscript Accepted: October 23, 2012
Published: November 1, 2012

Citation
Sun-Kyung Kim, Kyung-Deok Song, and Hong-Gyu Park, "Design of input couplers for efficient silicon thin film solar absorbers," Opt. Express 20, A997-A1004 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-S6-A997


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References

  1. Renewables 2011 Global Status Report (Renewable Energy Policy Network, 2011).
  2. D. M. Powell, M. T. Winkler, H. J. Choi, C. B. Simmons, D. B. Needleman, and T. Buonassisi, “Crystalline silicon photovoltaics: a cost analysis framework for determining technology pathways to reach baseload electricity costs,” Energy Environ. Sci.5(3), 5874–5883 (2012). [CrossRef]
  3. L. Fraas and L. Partain, Solar Cells and their Applications 2nd ed. (Wiley Series in Microwave and Optical Engineering, 2010), Chap. 2.
  4. A. Polman and H. A. Atwater, “Photonic design principles for ultrahigh-efficiency photovoltaics,” Nat. Mater.11(3), 174–177 (2012). [CrossRef] [PubMed]
  5. R. H. Hopkins and A. Rohatgi, “Impurity effects in silicon for high efficiency solar cells,” J. Cryst. Growth75(1), 67–79 (1986). [CrossRef]
  6. S. Chhajed, M. F. Schubert, J. K. Kim, and E. F. Schubert, “Nanostructured multilayer graded-index antireflection coating for Si solar cells with broadband and omnidirectional characteristics,” Appl. Phys. Lett.93(25), 251108 (2008). [CrossRef]
  7. J. Ko, D. Gong, K. Pillai, K.-S. Lee, M. Ju, P. Choi, K.-R. Kim, J. Yi, and B. Choi, “Double layer SiNx:H films for passivation and anti-reflection coating of c-Si solar cells,” Thin Solid Films519(20), 6887–6891 (2011). [CrossRef]
  8. X. Meng, V. Depauw, G. Gomard, O. El Daif, C. Trompoukis, E. Drouard, C. Jamois, A. Fave, F. Dross, I. Gordon, and C. Seassal, “Design, fabrication and optical characterization of photonic crystal assisted thin film monocrystalline-silicon solar cells,” Opt. Express20(S4Suppl 4), A465–A475 (2012). [CrossRef] [PubMed]
  9. X. Meng, E. Drouard, G. Gomard, R. Peretti, A. Fave, and C. Seassal, “Combined front and back diffraction gratings for broad band light trapping in thin film solar cell,” Opt. Express20(S5Suppl 5), A560–A571 (2012). [CrossRef] [PubMed]
  10. J. Grandidier, D. M. Callahan, J. N. Munday, and H. A. Atwater, “Light absorption enhancement in thin-film solar cells using whispering gallery modes in dielectric Nanospheres,” Adv. Mater. (Deerfield Beach Fla.)23(10), 1272–1276 (2011). [CrossRef] [PubMed]
  11. L. Li, K.-Q. Peng, B. Hu, X. Wang, Y. Hu, X.-L. Wu, and S.-T. Lee, “Broadband optical absorption enhancement in silicon nanofunnel arrays for photovoltaic applications,” Appl. Phys. Lett.100(22), 223902 (2012). [CrossRef]
  12. P. Spinelli, M. A. Verschuuren, and A. Polman, “Broadband omnidirectional antireflection coating based on subwavelength surface Mie resonators,” Nat Commun3(692), 692 (2012). [CrossRef] [PubMed]
  13. R. A. Pala, J. White, E. Barnard, J. Liu, and M. L. Brongersma, “Design of plasmonic thin-film solar cells with broadband absorption enhancements,” Adv. Mater. (Deerfield Beach Fla.)21(34), 3504–3509 (2009). [CrossRef]
  14. F. Wang, H. Yu, J. Li, S. Wong, X. W. Sun, X. Wang, and H. Zheng, “Design guideline of high efficiency crystalline Si thin film solar cell with nanohole array textured surface,” J. Appl. Phys.109(8), 084306 (2011). [CrossRef]
  15. J. S. Li, H. Y. Yu, Y. L. Li, F. Wang, M. F. Yang, and S. M. Wong, “Low aspect-ratio hemispherical nanopit surface texturing for enhancing light absorption in crystalline Si thin film-based solar cells,” Appl. Phys. Lett.98(2), 021905 (2011). [CrossRef]
  16. N. S. Lewis, “Toward cost-effective solar energy use,” Science315(5813), 798–801 (2007). [CrossRef] [PubMed]
  17. I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf, C. L. Perkins, B. To, and R. Noufi, “19·9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81·2% fill factor,” Prog. Photovolt. Res. Appl.16(3), 235–239 (2008). [CrossRef]
  18. S.-K. Kim, H.-S. Ee, K.-D. Song, and H.-G. Park, “Design of out-coupling structures with metal-dielectric surface relief,” Opt. Express20(15), 17230–17236 (2012). [CrossRef]
  19. A. Vial and T. Laroche, “Comparison of gold and silver dispersion laws suitable for FDTD simulations,” Appl. Phys. B93(1), 139–143 (2008). [CrossRef]
  20. D. R. Lide, CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data (CRC Press, 2008).
  21. Y. Yu, V. E. Ferry, A. P. Alivisatos, and L. Cao, “Dielectric core-shell optical antennas for strong solar absorption enhancement,” Nano Lett.12(7), 3674–3681 (2012). [CrossRef] [PubMed]
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