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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 5 — Feb. 27, 2012
  • pp: 5061–5068
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Influence of the light trapping induced by surface plasmons and antireflection film in crystalline silicon solar cells

Rui Xu, Xiaodong Wang, Liang Song, Wen Liu, An Ji, Fuhua Yang, and Jinmin Li  »View Author Affiliations


Optics Express, Vol. 20, Issue 5, pp. 5061-5068 (2012)
http://dx.doi.org/10.1364/OE.20.005061


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Abstract

In this paper, silicon solar cells with Ag nanoparticles deposited on a SiO2 spacer were studied concentrating on the influence of the surface plasmon and the antireflection film. We experimentally found that the photocurrent conversion efficiency of the solar cell decorated by random arrays of self-assembled Ag nanoparticles increases firstly and decreases afterwards with increasing spacer thickness. Further investigations on the external quantum efficiency (EQE) illustrated this trend more clearly. It was also found that the effect of the surface plasmon on light absorption dominates over that of the antireflection film at the resonance wavelength which is an important factor determining the light trapping. Moreover, surface plasmon is determined by both the Si substrate and the SiO2 spacer. For self-assembled Ag particles on the surface of the solar cells in our experiments, appropriate spacer thickness (9-35 nm) could broaden the plasmon resonance, narrow the photocurrent suppression range, weaken the suppression amplitude and strengthen the gain at the resonance wavelength, while still providing antireflection effect.

© 2012 OSA

1. Introduction

Photovoltaic solar cells are best known as an important technology for the future energy production [1

1. T. Dittrich, A. Belaidi, and A. Ennaoui, “Concepts of inorganic solid-state nanostructured solar cells,” Sol. Energy Mater. Sol. Cells 95(6), 1527–1536 (2011). [CrossRef]

8

8. Y. A. Chang, H. C. Kuo, T. C. Lu, F. Lai, S. Y. Kuo, L. W. Laih, L. H. Laih, and S. C. Wang, “Efficiency improvement of single-junction In0.5Ga0.5P solar cell with compositional grading p-emitter/window capping configuration,” Jpn. J. Appl. Phys. 49(12), 122301 (2010). [CrossRef]

]. To make power from photovoltaics competitive with fossil-fuel technologies, it is desirable to reduce the cost and increase the photocurrent conversion efficiency of solar cells. Usually a dielectric layer is deposited on the solar cell surface as a passivation layer to reduce the surface recombination, as well as an antireflection film to reduce the surface reflection. Silicon dioxide (SiO2), silicon nitride (Si3N4), and titanium dioxide (TiO2) are commonly used in photovoltaic cell fabrication with refractive indices of 1.5, 2.0, and 2.5, respectively. Another important way to enhance the solar cell absorption is the surface plasmon effect. In recent years, metal nanoparticles via surface plasmon have been widely utilized to enhance light absorption in solar cells because of the unique characters such as large normalized scattering cross section and adjustable resonance characteristics by changing nanoparticle geometries, etc [9

9. U. Guler and R. Turan, “Effect of particle properties and light polarization on the plasmonic resonances in metallic nanoparticles,” Opt. Express 18(16), 17322–17338 (2010). [CrossRef] [PubMed]

12

12. Z. Ouyang, S. Pillai, F. Beck, O. Kunz, S. Varlamov, K. R. Catchpole, P. Campbell, and M. A. Green, “Effective light trapping in polycrystalline silicon thin-film solar cells by means of rear localized surface plasmons,” Appl. Phys. Lett. 96(26), 261109 (2010). [CrossRef]

]. Considering the above two approaches, the antireflection effect is more pronounced for a thick dielectric layer, while the increased overlap of the surface plasmon with the low-index dielectric layer causes a blue shift in resonance, which is not beneficial for the absorbance of near-bandgap light. So there is a dilemma where the antireflection effect is negative correlation with the light trapping induced by surface plasmons.

In this article, we investigated the effect of the antireflection film and the surface plasmon on the photocurrent conversion efficiency of silicon solar cells. Because of the dynamic depolarization and radiation damping for large particles and more absorption for small particles, we made use of self-assembled nanoparticles with an appropriate size range (10-140 nm) [13

13. R. Xu, X. D. Wang, W. Liu, X. N. Xu, Y. Q. Li, A. Ji, and F. H. Yang, “Dielectric layer dependent surface plasmon effect of metallic nanoparticles on silicon substrate,” Chin. Phys. B (submitted).

,14

14. H. Mertens, A. F. Koenderink, and A. Polman, “Plasmon-enhanced luminescence near noble-metal nanospheres: Comparison of exact theory and an improved Gersten and Nitzan model,” Phys. Rev. B 76(11), 115123 (2007). [CrossRef]

]. The effect of spacer thickness in the range of 0-100 nm was studied systematically, in particular for the thin ones where the characteristics of surface plasmon vary significantly. We found that there are optimum thicknesses (9-35 nm)of the spacer for the maximum conversion efficiency, increased by 38.3% compared to the nanoparticle-decorated cells without the spacer.

2. Experiments

As a demonstration, crystalline solar cells were employed here for their easy availability. Figure 1
Fig. 1 Schematic diagram of the experimental design.
shows a schematic diagram of the silicon solar cell geometry. The test p+-n-n+ cells are realized by applying the ion implantation method on a 4-in n-type silicon wafer with a thickness of 380 μm and the thickness of the p+ emitter layer is about 400 nm. Then the aluminum electrodes are fabricated on both sides of the device.

After these processes, the wafer was cut into many single solar cells and eight of them were selected as the experimental samples. The SiO2 layers with varying thicknesses (3, 9, 15, 35, 50, 75, 100 nm) were sputtered on seven of the samples using an Oxford Optofab3000 ion beam sputter. Deposition was performed at a pressure around 10−4 torr. The refractive index of the sputtered oxide was characterized by a Cary 5E spectrophotometer around 1.50. Meanwhile, a sample without SiO2 layer is remained for comparison. Then the 12-nm silver layer was deposited on all of the samples using a Denton Explorer 5E electron beam evaporator, followed by a rapid thermal annealing in N2 ambient. The size of the resulted nanoparticles was controlled by the annealing conditions such as the temperature and the time. The current-voltage (I-V) and external quantum efficiency (EQE) measurements were performed on the solar cell samples before and after the formation of nanoparticle arrays on oxide with different thicknesses.

Figure 2(a)
Fig. 2 (a) Scanning electron microscopy (SEM) of Ag nanoparticles on Silicon substrates coated with the SiO2 layers with different thicknesses. (b) The corresponding distribution of particle sizes. The numbers at the top right corner indicate the thicknesses of the SiO2 layers.
shows SEM images of Ag particles on the silicon wafers, separated from the substrate by SiO2 layer with different thicknesses (the pictures for other thicknesses show the same situation). These SEM images are analyzed using the software Image J [15], as shown in Fig. 2(b). The diameter of the particle is taken to be equivalent to the longest distance between any two points along the particle boundary. Figure 2(a) shows that the quasi-circular Ag islands have the surface coverage of approximately 30%, irrespective of the oxide thickness. Particle size distribution is quite broad, with particle diameter in the range from 10 to 140 nm. Moreover, more than 78% of the particles have diameters between 20 and 100 nm.

Figure 3
Fig. 3 Photovoltaic I-V curves for the samples after Ag particles deposition under one-sun illumination (AM1.5, 100 mWcm−2) using a solar simulator. The numbers at the down left corner indicate the thicknesses of SiO2 layers.
shows the I-V characteristics of solar cells obtained under the illumination from a 100 Wcm−2, AM1.5 spectrum by an Oriel solar simulator (Model 94041A). All solar cells are coated by Ag nanoparticles and SiO2 films with different thicknesses. Details of the I-V parameters are summarized in Table 1

Table 1. Photovoltaic performance of the solar cells coated by different thickness of oxide after Ag deposition

table-icon
View This Table
. The efficiency increases firstly with increasing spacer thickness, from 4.3% for the solar cell with no oxide to 6.1% for 35 nm SiO2 layer. Afterwards, the efficiency decreases and achieves the minimum 4.9% for the 100 nm thick spacer. Figure 3 and Table 1 also show that the open circuit voltage Voc of all samples is almost equal, which has a margin of error of plus or minus 3.3 percent. The fill factor (FF) also changes little, with a margin of error of plus or minus 3.9 percent. While the short circuit current density Isc varies significantly with increasing spacer thickness. The largest photocurrent, enhanced by 41.7% compared to the case of no spacer, was achieved for the sample with a 35-nm-thick SiO2. The maximum Isc lead to the best performance in the power conversion efficiency η = FF Isc Voc/Pin, where Isc, and Pin are the short circuit current and input power, respectively. According to the description above, Isc is a determining factor in improving the photocurrent conversion efficiency and depends obviously on the SiO2 layer thickness.

3. EQE analysis

Figure 4
Fig. 4 The external quantum efficiency (EQE) of the Si solar cells with Ag nanoparticles, separated from the Si substrate with different thickness of SiO2 layer. The numbers at the bottom shows the oxide thicknesses.
shows the EQE of the solar cells measured for further investigation on the light trapping provided by nanoparticles placed atop SiO2 layers with different thicknesses. The extra phase lag between the light reflected from the front and rear surfaces of the spacer is more pronounced at short wavelengths where the path length in the spacer layer is significant relative to the light wavelength, consistent with the EQE peaks at the short wavelength for 75 nm and 100 nm SiO2 layer. As can be seen in Fig. 4, the curves are broader for the thin SiO2 layer, which can be ascribed to that the surface plasmon under the influence the SiO2 layer and the Si substrate has a wider resonance. A local maximum in EQE peak, of up to 83.9%, is observed for the 35 nm SiO2. An interesting observation is that the EQE peak is linearly red shifted roughly with the oxide thickness, under the influence of the antireflection effect.

To investigate the effect of surface plasmons for different thickness of the SiO2 layer, the EQE spread of solar cells coated by different thickness of SiO2 layer after and before Ag particles deposition is shown in Fig. 6
Fig. 6 Measured EQE as a function of wavelength for cells after Ag deposition, shown as difference relative to reference EQE of the same cells prior to the nanoparticles deposition.
. It must be mentioned that the EQE of the bare solar cell is higher than the same cell after Ag deposition in the entire wavelength range, which can be attributed to two mechanisms, the first is that the recombination of electron-hole pairs at the metal-semiconductor interface is rapid, especially for the cell without SiO2 passivation layer; the second is that the destructive interference between incident and scattered light in the Si substrate occurs at wavelengths below resonance which localizes in the long wavelength range. Figure 6 shows that the EQE changes obviously with the SiO2 layer thickness due to the presence of the surface plasmon, except at the longer wavelengths where the EQE spread is almost 0, indicating that the effect of Ag nanoparticles is negligibly small. In other word, the surface plasmon determines EQE in the range where the resonance is strong, demonstrating the resonance wavelength is an important factor in determining the light coupling. Figure 6 also shows that there is a crossover point at which the EQE spread is equal to 0. In Mie scattering, there is a phase mismatch between the scattered and the incident light, which can varies with wavelength and lead to the constructive or destructive interference. Especially, this phase mismatch is amplified near the plasmon resonance, corresponding to the experimental peaks and dips. Figure 6 also shows that there tends to be a reduction in crossover wavelength for the thin oxide, e.g. from 580 nm for 3 nm SiO2 layer to 440 nm for 9 nm SiO2 layer, whereas a linear increase for the thick ones, demonstrating the tunability of the EQE of nanoparticle-decorated cells by the spacer thickness that are typical for solar cell manufacturing. The dip wavelength correlated with the plasmon resonance is blue shifted as the SiO2 layer is very thin, and show little change as the oxide thickness is beyond 9 nm, which can be ascribed to that the surface plasmon depends on the SiO2 layer and the Si substrate for the thin spacer, while the effect of the SiO2 layer dominates over that of the Si substrate as the spacer is thick enough. Moreover, the dip value increases with the thickness for the thin spacer, and decreases subsequently as the oxide thickness is beyond 9 nm. The largest reduction in EQE after the Ag nanoparticle deposition, of up to 46.1%,is observed at a wavelength of 490 nm for the sample with 75-nm-thick oxide. It may be noted that the peak is also clearly red shifted, as the thickness is beyond 9 nm. In conclusion, the light trapping provided by the random sized particles on the thin SiO2 layer can overcome the suppression of photocurrent as a result of high gain, little suppression and blue shifted crossover wavelength.

4. Optical simulations

5. Conclusions

In summary, we investigated the influence of the antirefelction film and the surface plasmon on the silicon solar cell performance, and discovered that the effect of the surface plasmon on light absorption dominates over that of the antireflection film at resonance, so the resonance wavelength is an important factor in determining the light trapping. For self-assembled Ag particles on the surface of solar cells in our experiments, appropriate spacer thickness (9-35 nm) were desirable to broaden the plasmon resonance, narrow photocurrent suppression range, weaken the suppression amplitude and strengthen the gain at the resonance wavelength, while still providing antireflection effect. Our results show that there is a trade-off between the benefit of the antireflection effect and the influence of the surface plasmon.

Acknowledgments

The authors greatly acknowledge the support from the National Basic Research Program of China (973 Program) under grant number 2010CB933800 and the National Natural Science Foundation of China under grant number 61076077.

References and links

1.

T. Dittrich, A. Belaidi, and A. Ennaoui, “Concepts of inorganic solid-state nanostructured solar cells,” Sol. Energy Mater. Sol. Cells 95(6), 1527–1536 (2011). [CrossRef]

2.

T. H. Chang, P. H. Wu, S. H. Chen, C. H. Chan, C. C. Lee, C. C. Chen, and Y. K. Su, “Efficiency enhancement in GaAs solar cells using self-assembled microspheres,” Opt. Express 17(8), 6519–6524 (2009). [CrossRef] [PubMed]

3.

R. Dewan, M. Marinkovic, R. Noriega, S. Phadke, A. Salleo, and D. Knipp, “Light trapping in thin-film silicon solar cells with submicron surface texture,” Opt. Express 17(25), 23058–23065 (2009). [CrossRef] [PubMed]

4.

J. Schaffner, M. Motzko, A. Tueschen, A. Swirschuk, H. J. Schimper, A. Klein, T. Modes, O. Zywitzki, and W. Jaegermann, “12% efficient CdTe/CdS thin film solar cells deposited by low-temperature close space sublimation,” J. Appl. Phys. 110(6), 064508 (2011). [CrossRef]

5.

W. Liu, X. D. Wang, Y. Q. Li, Z. X. Geng, F. H. Yang, and J. M. Li, “Surface plasmon enhanced GaAs thin film solar cells,” Sol. Energy Mater. Sol. Cells 95(2), 693–698 (2011). [CrossRef]

6.

M. Lira-Cantu, A. Chafiq, J. Faissat, I. Gonzalez-Valls, and Y. Yu, “Oxide/polymer interfaces for hybrid and organic solar cells: Anatase vs. Rutile TiO2,” Sol. Energy Mater. Sol. Cells 95(5), 1362–1374 (2011). [CrossRef]

7.

M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. Yang, “Nanowire dye-sensitized solar cells,” Nat. Mater. 4(6), 455–459 (2005). [CrossRef] [PubMed]

8.

Y. A. Chang, H. C. Kuo, T. C. Lu, F. Lai, S. Y. Kuo, L. W. Laih, L. H. Laih, and S. C. Wang, “Efficiency improvement of single-junction In0.5Ga0.5P solar cell with compositional grading p-emitter/window capping configuration,” Jpn. J. Appl. Phys. 49(12), 122301 (2010). [CrossRef]

9.

U. Guler and R. Turan, “Effect of particle properties and light polarization on the plasmonic resonances in metallic nanoparticles,” Opt. Express 18(16), 17322–17338 (2010). [CrossRef] [PubMed]

10.

H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

11.

K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, “The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment,” J. Phys. Chem. 107(3), 668–677 (2003). [CrossRef]

12.

Z. Ouyang, S. Pillai, F. Beck, O. Kunz, S. Varlamov, K. R. Catchpole, P. Campbell, and M. A. Green, “Effective light trapping in polycrystalline silicon thin-film solar cells by means of rear localized surface plasmons,” Appl. Phys. Lett. 96(26), 261109 (2010). [CrossRef]

13.

R. Xu, X. D. Wang, W. Liu, X. N. Xu, Y. Q. Li, A. Ji, and F. H. Yang, “Dielectric layer dependent surface plasmon effect of metallic nanoparticles on silicon substrate,” Chin. Phys. B (submitted).

14.

H. Mertens, A. F. Koenderink, and A. Polman, “Plasmon-enhanced luminescence near noble-metal nanospheres: Comparison of exact theory and an improved Gersten and Nitzan model,” Phys. Rev. B 76(11), 115123 (2007). [CrossRef]

15.

http://rsb.info.nih.gov/ij/.

16.

http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.html.

17.

S. Pillai, F. J. Beck, K. R. Catchpole, Z. Ouyang, and M. A. Green, “The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions,” J. Appl. Phys. 109(7), 073105 (2011). [CrossRef]

18.

N. D. Arora, S. G. Chamberlain, and D. J. Roulston, “Diffusion length determination in p-n junction diodes and solar cells,” Appl. Phys. Lett. 37(3), 325 (1980). [CrossRef]

OCIS Codes
(240.0310) Optics at surfaces : Thin films
(240.6680) Optics at surfaces : Surface plasmons
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Solar Energy

History
Original Manuscript: November 10, 2011
Revised Manuscript: February 3, 2012
Manuscript Accepted: February 9, 2012
Published: February 15, 2012

Citation
Rui Xu, Xiaodong Wang, Liang Song, Wen Liu, An Ji, Fuhua Yang, and Jinmin Li, "Influence of the light trapping induced by surface plasmons and antireflection film in crystalline silicon solar cells," Opt. Express 20, 5061-5068 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-5-5061


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References

  1. T. Dittrich, A. Belaidi, and A. Ennaoui, “Concepts of inorganic solid-state nanostructured solar cells,” Sol. Energy Mater. Sol. Cells 95(6), 1527–1536 (2011). [CrossRef]
  2. T. H. Chang, P. H. Wu, S. H. Chen, C. H. Chan, C. C. Lee, C. C. Chen, and Y. K. Su, “Efficiency enhancement in GaAs solar cells using self-assembled microspheres,” Opt. Express 17(8), 6519–6524 (2009). [CrossRef] [PubMed]
  3. R. Dewan, M. Marinkovic, R. Noriega, S. Phadke, A. Salleo, and D. Knipp, “Light trapping in thin-film silicon solar cells with submicron surface texture,” Opt. Express 17(25), 23058–23065 (2009). [CrossRef] [PubMed]
  4. J. Schaffner, M. Motzko, A. Tueschen, A. Swirschuk, H. J. Schimper, A. Klein, T. Modes, O. Zywitzki, and W. Jaegermann, “12% efficient CdTe/CdS thin film solar cells deposited by low-temperature close space sublimation,” J. Appl. Phys. 110(6), 064508 (2011). [CrossRef]
  5. W. Liu, X. D. Wang, Y. Q. Li, Z. X. Geng, F. H. Yang, and J. M. Li, “Surface plasmon enhanced GaAs thin film solar cells,” Sol. Energy Mater. Sol. Cells 95(2), 693–698 (2011). [CrossRef]
  6. M. Lira-Cantu, A. Chafiq, J. Faissat, I. Gonzalez-Valls, and Y. Yu, “Oxide/polymer interfaces for hybrid and organic solar cells: Anatase vs. Rutile TiO2,” Sol. Energy Mater. Sol. Cells 95(5), 1362–1374 (2011). [CrossRef]
  7. M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. Yang, “Nanowire dye-sensitized solar cells,” Nat. Mater. 4(6), 455–459 (2005). [CrossRef] [PubMed]
  8. Y. A. Chang, H. C. Kuo, T. C. Lu, F. Lai, S. Y. Kuo, L. W. Laih, L. H. Laih, and S. C. Wang, “Efficiency improvement of single-junction In0.5Ga0.5P solar cell with compositional grading p-emitter/window capping configuration,” Jpn. J. Appl. Phys. 49(12), 122301 (2010). [CrossRef]
  9. U. Guler and R. Turan, “Effect of particle properties and light polarization on the plasmonic resonances in metallic nanoparticles,” Opt. Express 18(16), 17322–17338 (2010). [CrossRef] [PubMed]
  10. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]
  11. K. L. Kelly, E. Coronado, L. L. Zhao, and G. C. Schatz, “The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment,” J. Phys. Chem. 107(3), 668–677 (2003). [CrossRef]
  12. Z. Ouyang, S. Pillai, F. Beck, O. Kunz, S. Varlamov, K. R. Catchpole, P. Campbell, and M. A. Green, “Effective light trapping in polycrystalline silicon thin-film solar cells by means of rear localized surface plasmons,” Appl. Phys. Lett. 96(26), 261109 (2010). [CrossRef]
  13. R. Xu, X. D. Wang, W. Liu, X. N. Xu, Y. Q. Li, A. Ji, and F. H. Yang, “Dielectric layer dependent surface plasmon effect of metallic nanoparticles on silicon substrate,” Chin. Phys. B (submitted).
  14. H. Mertens, A. F. Koenderink, and A. Polman, “Plasmon-enhanced luminescence near noble-metal nanospheres: Comparison of exact theory and an improved Gersten and Nitzan model,” Phys. Rev. B 76(11), 115123 (2007). [CrossRef]
  15. http://rsb.info.nih.gov/ij/ .
  16. http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.html .
  17. S. Pillai, F. J. Beck, K. R. Catchpole, Z. Ouyang, and M. A. Green, “The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions,” J. Appl. Phys. 109(7), 073105 (2011). [CrossRef]
  18. N. D. Arora, S. G. Chamberlain, and D. J. Roulston, “Diffusion length determination in p-n junction diodes and solar cells,” Appl. Phys. Lett. 37(3), 325 (1980). [CrossRef]

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