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Energy Express

  • Editor: Bernard Kippelen
  • Vol. 19, Iss. S5 — Sep. 12, 2011
  • pp: A1051–A1056
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Broadband antireflection and absorption enhancement by forming nano-patterned Si structures for solar cells

Y. Liu, S.H. Sun, J. Xu, L. Zhao, H.C. Sun, J. Li, W. W. Mu, L. Xu, and K. J. Chen  »View Author Affiliations


Optics Express, Vol. 19, Issue S5, pp. A1051-A1056 (2011)
http://dx.doi.org/10.1364/OE.19.0A1051


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Abstract

In this letter, we report the antireflection and light absorption enhancement by forming sub-wavelength nano-patterned Si structures via nano-sphere lithography technique. It is found that the surface reflection can be significantly suppressed in a wide spectral range (400-1000 nm) and the weighted mean reflection is less than 5%. Meanwhile, the broad band optical absorption enhancement is achieved consequently. Heterojunction solar cells are prepared by depositing ultrathin amorphous Si film on the nano-patterned Si structures, the short circuit current density increases to 37.2 mA/cm2 and the power conversion efficiency is obviously improved compared to the reference cell on flat Si substrate.

© 2011 OSA

1. Introduction

In this letter, we report the formation of nano-patterned Si structures by using polystyrene nanosphere lithography method. The reflection can be suppressed to less than 10% and the optical absorption exceeds 90% in a wide spectral range (400-1000 nm) for nano-patterned samples. Moreover, by depositing amorphous Si (a-Si) on the nano-patterned Si substrate to obtain a-Si/c-Si heterojunction solar cells, the short circuit current and power conversion efficiency are obviously increased compared to that of reference sample on flat Si wafer.

2. Experimental

3. Results and discussion

The antireflection effect of the nano-patterned substrates was characterized by the calibrated process over a broad wavelength range (400-1000 nm). It is found that the measured reflection spectra are almost the same whether using the integrating sphere or not, which indicates that the antireflection is angle-independent for our samples. Figure 2
Fig. 2 Reflection spectra for the front surface of flat substrate and nano-patterned substrate after 12 min etching.
shows the reflection spectra for the front surface of nano-patterned substrate after 12 min etching. The result for flat Si wafer is also presented for comparison. The polished surface of flat Si exhibits the high reflection (>40%) in the measurement range. The reflection is obviously suppressed for nano-patterned substrates as shown in Fig. 2. The reflection is reduced to less than 10% in the whole measurement range after 12 min etching and it is further reduced to 5% in the visible light region (400-800nm) after 16min etching [12

12. Y. Liu, J. Xu, H. Sun, S. Sun, W. Xu, L. Xu, and K. Chen, “Depth-dependent anti-reflection and enhancement of luminescence from Si quantum dots-based multilayer on nano-patterned Si substrates,” Opt. Express 19(4), 3347–3352 (2011). [CrossRef] [PubMed]

]. The mean reflection can be calculated by weighting the reflection ratio of the substrate to the incident AM1.5 light in the 400-1000 nm spectral range. As shown in Fig. 3
Fig. 3 Weighted mean reflection versus the etching time.
, the weighted mean reflection is gradually reduced by increasing the etching time and it can be less than 5% after 12 min etching.

The absorption spectra of nano-patterned (12 min etching) and flat Si wafer are can be calculated based on the reflection and transmission measurements. As given in Fig. 4
Fig. 4 Absorption spectra of nano-patterned (12 min etching) and flat Si wafer.
, it is found that the absorption of nano-patterned Si substrate is significantly enhanced in a wide spectral range (400-1000 nm) which covers the most of the useful spectrum for Si-based solar cells. The absorption of nano-patterned sample is above 90% while that of the flat one is less than 60%. The mean absorption is also weighted with AM1.5 solar spectrum. It is shown that the weighted absorption is about 56% for flat Si substrate but increase to 92% for 12 min etched nano-patterned substrate.

Recently, several reports have been published on the suppression of light reflection by forming textured surface [7

7. J. Li, H. Y. Yu, Y. Li, F. Wang, M. 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]

,15

15. S. A. Boden and D. M. Bagnall, “Tunable reflection minima of nanostructured antireflective surfaces,” Appl. Phys. Lett. 93(13), 133108 (2008). [CrossRef]

]. Our results suggest that the nano-patterned Si structures formed by the present approach exhibit the good antireflection and absorption enhancement characterization because it provides grading refraction index at the air/Si interface [12

12. Y. Liu, J. Xu, H. Sun, S. Sun, W. Xu, L. Xu, and K. Chen, “Depth-dependent anti-reflection and enhancement of luminescence from Si quantum dots-based multilayer on nano-patterned Si substrates,” Opt. Express 19(4), 3347–3352 (2011). [CrossRef] [PubMed]

]. Meanwhile, the optical absorption can be enhanced by using sub-wavelength nano-structures. Since the feature size of surface pattern is smaller than wavelength, the incident electromagnetic wave can be coupled with the whole surface sub-wavelength structures which can trap the light to enhance the light harvesting in a wide spectral range [6

6. S. Chattopadhyay, Y. F. Huang, Y. J. Jen, A. Ganguly, K. H. Chen, and L. C. Chen, “Anti-reflecting and photonic nanostructures,” Mater. Sci. Eng. Rep. 69(1-3), 1–35 (2010). [CrossRef]

,7

7. J. Li, H. Y. Yu, Y. Li, F. Wang, M. 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

16. J. S. Li, H. Y. Yu, S. M. Wong, G. Zhang, X. W. Sun, P. G. Lo, and D. L. Kwong, “Si nanopillar array optimization on Si thin □lms for solar energy harvesting,” Appl. Phys. Lett. 95(3), 033102 (2009). [CrossRef]

]. Usually, the antireflection behaviors were achieved by introducing the Si nanostructures with high aspect-ratio. More recently, J. Li et al. discussed the light-trapping capability of nanostructures with low aspect ratio theoretically [7

7. J. Li, H. Y. Yu, Y. Li, F. Wang, M. 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]

]. Their study indicates that the reflection can still be suppressed especially in the high energy region and light absorption can be significantly enhanced even with low aspect-ratio surface nano-texturing. In our case, the aspect-ratio is not so high according to the TEM observation which is about 0.67. However, the good antireflection and absorption enhancement properties reflect their possible application in the future solar cells without introducing a high level of surface defects and low carrier collection efficiency as in the high aspect-ratio structures [7

7. J. Li, H. Y. Yu, Y. Li, F. Wang, M. 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]

,17

17. V. Sivakov, G. Andrä, A. Gawlik, A. Berger, J. Plentz, F. Falk, and S. H. Christiansen, “Silicon nanowire-based solar cells on glass: synthesis, optical properties, and cell parameters,” Nano Lett. 9(4), 1549–1554 (2009). [CrossRef] [PubMed]

].

The antireflection and optical absorption enhancement should improve the power conversion efficiency of solar cells. We fabricated the a-Si/c-Si heterojunction solar cells by successively depositing amorphous Si and n-type amorphous Si thin films on the nano-patterned Si substrate. The reference cell on flat Si substrate was also prepared under the same conditions for comparison. After film deposition, ITO and Al were evaporated on the front and back side as electrodes. The electrical properties of cells are characterized by current density (J) – voltage (V) measurements under a solar simulation with AM1.5 global spectrum (100 mW/cm2). The nano-patterned cell has the power conversion of 8.3%, which is significantly improved compared with the flat one. In order to further investigate the contribution of nano-patterned structures on the improvement of the cell performance, the external quantum efficiency (EQE) was measured for nano-patterned and flat cells and the results are given in Fig. 5
Fig. 5 External quantum efficiency of heterojunction solar cells on flat and nano-patterned substrates, respectively.
. It is shown that the EQE for nano-patterned cells is higher than that of flat one in almost the whole spectra range, especially in the visible light range. As a consequence, the short circuit current density Jsc, calculated based on EQE results, is obviously increased from 22.2 mA/cm2 for flat one to 32.1 mA/cm2 for nano-patterned one, which is close to the best result reported on the heterojunction solar cell by using p-type c-Si wafer [18

18. Q. Wang, “High-efficiency hydrogenated amorphous/crystalline Si heterojunction solar cells,” Philos. Mag. 89(28), 2587–2598 (2009). [CrossRef]

,19

19. M. Taguchi, E. Maruyama, and M. Tanaka, “Temperature Dependence of Amorphous/Crystalline Silicon Heterojunction Solar Cells,” Jpn. J. Appl. Phys. 47(2), 814–818 (2008). [CrossRef]

]. The improvement of EQE and short-circuit current density clearly proves that more incident photons are absorbed by the Si materials and generated carriers are efficiently collected due to the nano-patterned structures.

It is worth pointing out that the cell fabrication conditions in the present work are not optimized, the cell performance, especially the open circuit voltage and fill factor, can be further improved by optimization of the deposition parameters of the i- and n-layers and the electrode contacts to reduce the series resistance and interface states.

4. Conclusion

In summary, the sub-wavelength nano-patterned Si structures are fabricated by using nano-sphere lithography technique which exhibits the good antireflection and optical absorption enhancement characteristics in a wide spectral range. The mean reflection can be reduced to less than 5% and the absorption can exceed 90%. The initial results of a-Si/c-Si heterojunction solar cell demonstrate that optical improvement by using nano-patterned Si substrate results in the significant increase of EQE and the short circuit current due to the enhanced light absorption and the current collection.

Acknowledgements

This work was supported by NSFC (Nos. 61036001, 10874070), “973” project (2007CB613401), NSF of Jiangsu Province (BK2010010) and the Fundamental Research Funds for the Central Universities (1112021001).

References and links

1.

J. Zhu and Y. Cui, “Photovoltaics: More solar cells for less,” Nat. Mater. 9(3), 183–184 (2010). [CrossRef] [PubMed]

2.

G. Yue, L. Sivec, J. M. Owens, B. Yan, J. Yang, and S. Guha, “Optimization of back reflector for high efficiency hydrogenated nanocrystalline silicon solar cells” Appl. Phys. Lett. 95(26), 263501 (2009). [CrossRef]

3.

J. Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S. Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel re〉ection,” Nat. Photonics 1, 176 (2007).

4.

S. Koynov, M. S. Brandt, and M. Stutzmann, “Black nonreflecting silicon surfaces for solar cells,” Appl. Phys. Lett. 88(20), 203107 (2006). [CrossRef]

5.

V. V. Iyengar, B. K. Nayak, and M. C. Gupta, “Optical properties of silicon light trapping structures for photovoltaics,” Sol. Energy Mater. Sol. Cells 94(12), 2251–2257 (2010). [CrossRef]

6.

S. Chattopadhyay, Y. F. Huang, Y. J. Jen, A. Ganguly, K. H. Chen, and L. C. Chen, “Anti-reflecting and photonic nanostructures,” Mater. Sci. Eng. Rep. 69(1-3), 1–35 (2010). [CrossRef]

7.

J. Li, H. Y. Yu, Y. Li, F. Wang, M. 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]

8.

S. J. Jang, Y. M. Song, J. S. Yu, C. I. Yeo, and Y. T. Lee, “Antireflective properties of porous Si nanocolumnar structures with graded refractive index layers,” Opt. Lett. 36(2), 253–255 (2011). [CrossRef] [PubMed]

9.

M. D. Kelzenberg, S. W. Boettcher, J. A. Petykiewicz, D. B. Turner-Evans, M. C. Putnam, E. L. Warren, J. M. Spurgeon, R. M. Briggs, N. S. Lewis, and H. A. Atwater, “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications,” Nat. Mater. 9(3), 239–244 (2010). [CrossRef] [PubMed]

10.

J. Zhu, Z. Yu, G. F. Burkhard, C. M. Hsu, S. T. Connor, Y. Xu, Q. Wang, M. McGehee, S. Fan, and Y. Cui, “Optical absorption enhancement in amorphous silicon nanowire and nanocone arrays,” Nano Lett. 9(1), 279–282 (2009). [CrossRef] [PubMed]

11.

Y. Wang, J. Rybezynski, D. Z. Wang, and Z. F. Ren, “Large-scale triangular lattice arrays of sub-micron island by microsphere self-assembly,” Nanotechnology 16(6), 819–822 (2005). [CrossRef]

12.

Y. Liu, J. Xu, H. Sun, S. Sun, W. Xu, L. Xu, and K. Chen, “Depth-dependent anti-reflection and enhancement of luminescence from Si quantum dots-based multilayer on nano-patterned Si substrates,” Opt. Express 19(4), 3347–3352 (2011). [CrossRef] [PubMed]

13.

D. Chen, Y. Liu, J. Xu, D. Wei, H. Sun, L. Xu, T. Wang, W. Li, and K. Chen, “Improved emission efficiency of electroluminescent device containing nc-Si/SiO(2) multilayers by using nano-patterned substrate,” Opt. Express 18(2), 917–922 (2010). [CrossRef] [PubMed]

14.

Q. Wang, S. Ward, L. Gedvilas, B. Keyes, E. Sanchez, and S. Wang, “Conformal thin-film silicon nitride deposited by hot-wire chemical vapor deposition,” Appl. Phys. Lett. 84(3), 338 (2004). [CrossRef]

15.

S. A. Boden and D. M. Bagnall, “Tunable reflection minima of nanostructured antireflective surfaces,” Appl. Phys. Lett. 93(13), 133108 (2008). [CrossRef]

16.

J. S. Li, H. Y. Yu, S. M. Wong, G. Zhang, X. W. Sun, P. G. Lo, and D. L. Kwong, “Si nanopillar array optimization on Si thin □lms for solar energy harvesting,” Appl. Phys. Lett. 95(3), 033102 (2009). [CrossRef]

17.

V. Sivakov, G. Andrä, A. Gawlik, A. Berger, J. Plentz, F. Falk, and S. H. Christiansen, “Silicon nanowire-based solar cells on glass: synthesis, optical properties, and cell parameters,” Nano Lett. 9(4), 1549–1554 (2009). [CrossRef] [PubMed]

18.

Q. Wang, “High-efficiency hydrogenated amorphous/crystalline Si heterojunction solar cells,” Philos. Mag. 89(28), 2587–2598 (2009). [CrossRef]

19.

M. Taguchi, E. Maruyama, and M. Tanaka, “Temperature Dependence of Amorphous/Crystalline Silicon Heterojunction Solar Cells,” Jpn. J. Appl. Phys. 47(2), 814–818 (2008). [CrossRef]

OCIS Codes
(160.4236) Materials : Nanomaterials

ToC Category:
Photovoltaics

History
Original Manuscript: April 6, 2011
Revised Manuscript: June 13, 2011
Manuscript Accepted: June 19, 2011
Published: July 15, 2011

Citation
Y. Liu, S.H. Sun, J. Xu, L. Zhao, H.C. Sun, J. Li, W. W. Mu, L. Xu, and K. J. Chen, "Broadband antireflection and absorption enhancement by forming nano-patterned Si structures for solar cells," Opt. Express 19, A1051-A1056 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-S5-A1051


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References

  1. J. Zhu and Y. Cui, “Photovoltaics: More solar cells for less,” Nat. Mater. 9(3), 183–184 (2010). [CrossRef] [PubMed]
  2. G. Yue, L. Sivec, J. M. Owens, B. Yan, J. Yang, and S. Guha, “Optimization of back reflector for high efficiency hydrogenated nanocrystalline silicon solar cells” Appl. Phys. Lett. 95(26), 263501 (2009). [CrossRef]
  3. J. Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S. Y. Lin, W. Liu, and J. A. Smart, “Optical thin-film materials with low refractive index for broadband elimination of Fresnel re〉ection,” Nat. Photonics 1, 176 (2007).
  4. S. Koynov, M. S. Brandt, and M. Stutzmann, “Black nonreflecting silicon surfaces for solar cells,” Appl. Phys. Lett. 88(20), 203107 (2006). [CrossRef]
  5. V. V. Iyengar, B. K. Nayak, and M. C. Gupta, “Optical properties of silicon light trapping structures for photovoltaics,” Sol. Energy Mater. Sol. Cells 94(12), 2251–2257 (2010). [CrossRef]
  6. S. Chattopadhyay, Y. F. Huang, Y. J. Jen, A. Ganguly, K. H. Chen, and L. C. Chen, “Anti-reflecting and photonic nanostructures,” Mater. Sci. Eng. Rep. 69(1-3), 1–35 (2010). [CrossRef]
  7. J. Li, H. Y. Yu, Y. Li, F. Wang, M. 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]
  8. S. J. Jang, Y. M. Song, J. S. Yu, C. I. Yeo, and Y. T. Lee, “Antireflective properties of porous Si nanocolumnar structures with graded refractive index layers,” Opt. Lett. 36(2), 253–255 (2011). [CrossRef] [PubMed]
  9. M. D. Kelzenberg, S. W. Boettcher, J. A. Petykiewicz, D. B. Turner-Evans, M. C. Putnam, E. L. Warren, J. M. Spurgeon, R. M. Briggs, N. S. Lewis, and H. A. Atwater, “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications,” Nat. Mater. 9(3), 239–244 (2010). [CrossRef] [PubMed]
  10. J. Zhu, Z. Yu, G. F. Burkhard, C. M. Hsu, S. T. Connor, Y. Xu, Q. Wang, M. McGehee, S. Fan, and Y. Cui, “Optical absorption enhancement in amorphous silicon nanowire and nanocone arrays,” Nano Lett. 9(1), 279–282 (2009). [CrossRef] [PubMed]
  11. Y. Wang, J. Rybezynski, D. Z. Wang, and Z. F. Ren, “Large-scale triangular lattice arrays of sub-micron island by microsphere self-assembly,” Nanotechnology 16(6), 819–822 (2005). [CrossRef]
  12. Y. Liu, J. Xu, H. Sun, S. Sun, W. Xu, L. Xu, and K. Chen, “Depth-dependent anti-reflection and enhancement of luminescence from Si quantum dots-based multilayer on nano-patterned Si substrates,” Opt. Express 19(4), 3347–3352 (2011). [CrossRef] [PubMed]
  13. D. Chen, Y. Liu, J. Xu, D. Wei, H. Sun, L. Xu, T. Wang, W. Li, and K. Chen, “Improved emission efficiency of electroluminescent device containing nc-Si/SiO(2) multilayers by using nano-patterned substrate,” Opt. Express 18(2), 917–922 (2010). [CrossRef] [PubMed]
  14. Q. Wang, S. Ward, L. Gedvilas, B. Keyes, E. Sanchez, and S. Wang, “Conformal thin-film silicon nitride deposited by hot-wire chemical vapor deposition,” Appl. Phys. Lett. 84(3), 338 (2004). [CrossRef]
  15. S. A. Boden and D. M. Bagnall, “Tunable reflection minima of nanostructured antireflective surfaces,” Appl. Phys. Lett. 93(13), 133108 (2008). [CrossRef]
  16. J. S. Li, H. Y. Yu, S. M. Wong, G. Zhang, X. W. Sun, P. G. Lo, and D. L. Kwong, “Si nanopillar array optimization on Si thin □lms for solar energy harvesting,” Appl. Phys. Lett. 95(3), 033102 (2009). [CrossRef]
  17. V. Sivakov, G. Andrä, A. Gawlik, A. Berger, J. Plentz, F. Falk, and S. H. Christiansen, “Silicon nanowire-based solar cells on glass: synthesis, optical properties, and cell parameters,” Nano Lett. 9(4), 1549–1554 (2009). [CrossRef] [PubMed]
  18. Q. Wang, “High-efficiency hydrogenated amorphous/crystalline Si heterojunction solar cells,” Philos. Mag. 89(28), 2587–2598 (2009). [CrossRef]
  19. M. Taguchi, E. Maruyama, and M. Tanaka, “Temperature Dependence of Amorphous/Crystalline Silicon Heterojunction Solar Cells,” Jpn. J. Appl. Phys. 47(2), 814–818 (2008). [CrossRef]

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