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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 23 — Nov. 7, 2011
  • pp: 23078–23084
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Highly-ordered vertical Si nanowire/nanowall decorated solar cells

Jian Wang, Zhenhua Li, Navab Singh, and Sungjoo Lee  »View Author Affiliations


Optics Express, Vol. 19, Issue 23, pp. 23078-23084 (2011)
http://dx.doi.org/10.1364/OE.19.023078


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Abstract

Highly-ordered vertical nanowire and nanowall arrays are studied on Si solar cell surface. The nanowall textured solar cell is found to be more effective in reducing the overall optical reflectance, resulting in higher short circuit current (Jsc = 24.9 mA/cm2) over nanowire structured (Jsc = 23.3 mA/cm2) and planar (Jsc = 17.5 mA/cm2) solar cells. The extracted energy conversion efficiency (η) from planar solar cell is 7.1%, while nanowire/nanowall cells show efficiency of 8.2% and 6.3%, respectively. If corrected with series resistance (Rs), nanowall solar cell exhibits the highest η of 9.8% in this experiment. A careful study of the series resistance from different types of the nanostructures is also presented.

© 2011 OSA

1. Introduction

Solar cells with surface decorated by nanostructures are being considered as one of the most promising candidates for low-cost high-efficiency photovoltaic device due to their improved light trapping behavior [1

1. L. Hu and G. Chen, “Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications,” Nano Lett. 7(11), 3249–3252 (2007). [CrossRef] [PubMed]

4

4. J. S. Li, H. Y. Yu, S. M. Wong, G. Zhang, G. Q. Lo, and D. L. Kwong, “Surface nanostructure optimization for solar energy harvesting in Si thin film based solar cells,” in 2009 IEEE International Electron Devices Meeting (IEDM) (IEEE, 2009), pp. 1–4.

] and promise for material cost reduction [5

5. L. Tsakalakos, J. Balch, J. Fronheiser, B. A. Korevaar, O. Sulima, and J. Rand, “Silicon nanowire solar cells,” Appl. Phys. Lett. 91(23), 233117 (2007). [CrossRef]

,6

6. B. Tian, X. Zheng, T. J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang, and C. M. Lieber, “Coaxial silicon nanowires as solar cells and nanoelectronic power sources,” Nature 449(7164), 885–889 (2007). [CrossRef] [PubMed]

]. As a result, solar cells based on nanowires are intensively investigated [7

7. K. Peng, Y. Xu, Y. Wu, Y. Yan, S. T. Lee, and J. Zhu, “Aligned single-crystalline Si nanowire arrays for photovoltaic applications,” Small 1(11), 1062–1067 (2005). [CrossRef] [PubMed]

,8

8. S. M. Wong, H. Y. Yu, J. S. Li, G. Zhang, G. Q. Lo, and D. L. Kwong, “Design high-efficiency Si nanopillar-array-textured thin-film solar cell,” IEEE Electron Device Lett. 31(4), 335–337 (2010). [CrossRef]

].

In this letter, we report our recent findings on solar cells decorated with two kinds of nanostructures, the nanowire and the nanowall. The nanowall array was demonstrated for its stronger physical strength and effective light trapping property. Comparisons between the nanowire and nanowall were made in terms of the spectrum reflectivity, the short circuit current (Jsc) (which indicates overall sunlight absorption and carrier collection) and energy conversion efficiency (η). Specifically, η of ~9.8% is deduced from the nanowall solar cell. Furthermore, a detailed study of the nanowire/nanowall solar cells’ series resistance is also presented for the first time. It is shown that a more conformal metal deposition approach is much desirable for a practical nanostructure-decorated solar cell.

2. Experimental details

Standard bulk Si wafer which is typical for CMOS fabrications was used for the proof-of-concept-demonstration solar cell fabrication. The schematic of the solar cell device is illustrated in Fig. 1 (a)
Fig. 1 (a) 3D schematic of the Si nanowire/nanowall solar cell featuring buried pn junction, and back surface field. (b) SEM image of Si nanowire array. (c) SEM image of Si nanowall array. The NWires/NWalls align well with each other leading to effective light trapping. nanowall array. (d) TEM image of Si nanowires showing nanowire length of 1.3 µm and diameter of 100 nm. (e) HRTEM image of Si Nanowire cross-section, showing smooth Si nanowire side wall after RIE. Single-crystalline lattice is also visible.
. The process started on p-type (100) single-crystalline Silicon wafers with resistivity in the range of 6-9 Ω-cm. BF2/1e15 cm−2/20 keV was implanted onto the wafer backside for effective back-surface field formation. Dopant activation was done by 1000 °C/5 sec rapid thermal anneal. After cleaning, the wafer front-side was patterned using standard KrF deep ultraviolet (DUV) lithography to create pillar and line/space patterns over large area (12 mm × 12 mm). Reactive ion etching was used to transfer the patterns into the silicon. After pattern transfer, we obtained 1.3-µm-tall nanowire array with wire diameter of 100 nm at a pitch of 400 nm (Fig. 1 (b)). The nanowall’s width/height was 100 nm/1.3 µm at a pitch of 360 nm (Fig. 1 (c)). The selection of the pitches was based on minimum resolution criteria as smaller pitch is more effective in suppressing reflections as confirmed by RSoft simulations. Pillar patterns used to define wires have poorer resolution than line/space patterns required for wall due to 2D diffraction and mechanical weakness in the photo resist. Therefore the wire pitch is larger than wall pitch. The TEM image of the Si nanowires is Fig. 1 (d). Figure 1 (e) is the zoom-in image of the Si surface showing smooth profile.

After nanowire/nanowall etch, the under-surface p-n junction was formed by tilted 20 keV/4e15 cm−2 phosphorus implant. Tilt in the implant was to ensure the formation of junction below the wire/wall. Finally, Ti/Cu metal was sputtered on both sides of the wafer. Though in current process DUV lithography was used, a low cost fabrication technique can easily be adopted for the formation of nanowire and nanowall structures by different means, such as self-assembly [7

7. K. Peng, Y. Xu, Y. Wu, Y. Yan, S. T. Lee, and J. Zhu, “Aligned single-crystalline Si nanowire arrays for photovoltaic applications,” Small 1(11), 1062–1067 (2005). [CrossRef] [PubMed]

] or nano-imprint [9

9. L. Guo, “Recent progress in nanoimprint technology and its applications,” J. Phys. D Appl. Phys. 37(11), R123–R141 (2004). [CrossRef]

].

3. Results and discussion

Figure 2
Fig. 2 Reflectance spectra of Si planar surface, nanowires and nanowalls. The solar power spectrum under AM1.5 condition is also shown.
shows the reflectance (R) spectrum of the samples measured by Shimadzu UV-3600 UV-VIS-NIR spectrophotometer. The red line is the reflectance spectrum measured on reference Si planar surface sample, which fits well with the standard data up to wavelength of ~1000nm. At the wavelength range beyond 1000nm, the measured reflectance rises due to transparency of the Silicon at near-infra-red (NIR) range and the reflectance of the materials at the back of the sample. In comparison, the surface textured by the nanowire/nanowall nanostructure shows largely suppressed reflectance, in agreement with earlier findings using nanowire decoration [2

2. L. Tsakalakos, J. Balch, J. Fronheiser, M. Y. Shih, S. F. LeBoeuf, M. Pietrzykowski, P. J. Codella, B. A. Korevaar, O. Sulima, J. Rand, A. Davuluru, and U. Rapol, “Strong broadband optical absorption in silicon nanowire films,” J. Nanophotonics 1(1), 013552 (2007). [CrossRef]

,5

5. L. Tsakalakos, J. Balch, J. Fronheiser, B. A. Korevaar, O. Sulima, and J. Rand, “Silicon nanowire solar cells,” Appl. Phys. Lett. 91(23), 233117 (2007). [CrossRef]

,7

7. K. Peng, Y. Xu, Y. Wu, Y. Yan, S. T. Lee, and J. Zhu, “Aligned single-crystalline Si nanowire arrays for photovoltaic applications,” Small 1(11), 1062–1067 (2005). [CrossRef] [PubMed]

].

The minima of nanowire’s reflectance at ~550nm is due to the interference between light and nanostructure. The nanowall array sample shows even lower reflectance with respect to nanowire sample. It is well known that in diffraction grating-like structures, the zero-order transmission is inversely proportional to the pattern’s filling ratio [10

10. M. Born, E. Wolf, and A. Bhatia, Principles of Optics (Pergamon, Oxford, 1975).

]. In our case for nanowire, the filling ratio is ~4.9% and the nanowall pattern is ~28.6%. So much lower zero-order transmission from nanowall pattern is expected, since there is less light which transmits directly through the nanowall array and bounces back from the highly reflective bulk Si surface underneath. Therefore in nanowall array, higher percentage of the incoming light is diffracted to tilted propagation path where stronger interaction with the nanostructure takes place, leading to more absorption/trapping and less reflection [1

1. L. Hu and G. Chen, “Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications,” Nano Lett. 7(11), 3249–3252 (2007). [CrossRef] [PubMed]

].

The overall reflectance of each sample was evaluated by the weighted average reflection (RW) [11

11. B. S. Richards, “Single-material TiO2 double-layer antireflection coatings,” Sol. Energy Mater. Sol. Cells 79(3), 369–390 (2003). [CrossRef]

] over the interested wavelength range from 300 nm to 1200 nm. The RW of planar Si surface is estimated to be ~31.6%. On the other hand, nanowire textured surface is able to reduce the RW to 14.5% due to the interaction of nanoscaled structure with light. Nanowall surface further reduces the RW to 6.1%, showing more than 50% reduction in total solar power as compared to nanowire. Worth noticing is that the RW of 6.1% from nanowall samples is lower than RW=8.6% for a typical commercial TiO2 single-layer antireflection (SLAR) coating [11

11. B. S. Richards, “Single-material TiO2 double-layer antireflection coatings,” Sol. Energy Mater. Sol. Cells 79(3), 369–390 (2003). [CrossRef]

]. In comparison, optimized double-layer antireflection (DLAR) coatings on non-encapsulated silicon flat surface have RW in the rage of 1.5–2.5% [12

12. J. Zhao and M. A. Green, “Optimized antireflection coatings for high-efficiency silicon solar cells,” IEEE Trans. Electron. Dev. 38(8), 1925–1934 (1991). [CrossRef]

].

The electrical performances of these nanostructure solar cells were also characterized by quantum efficiency measurement. The external quantum efficiency (EQE) spectra of the samples are shown in Fig. 3 (a)
Fig. 3 (a) External quantum efficiency (EQE) and (b) internal quantum efficiency (IQE) of the solar cell samples.
. Figure 3 (b) depicts the internal quantum efficiency (IQE) which is calculated from IEQ=EQE/(1-R). In Fig. 3 (b), one can see that the carrier collection efficiencies of the difference samples are close to each other, despite of the fact that there are RIE-formed nanostructures in the Si surface for nanowire, nanowall samples.

Then the samples were placed under AM1.5G, 100 mW/cm2 illumination using “SOLAR LIGHT” 16S-150-007 solar simulator. The measurement temperature was maintained at 25 °C. The I-V characteristics for planar, wire decorated and wall decorated devices, are plotted in Fig. 4
Fig. 4 IV characteristics of solar cells under standard AM1.5 illumination for planar, nanowire, nanowall solar cells. Jsc(nanowall) > Jsc(nanowire) > Jsc(planar) is due to stronger light trapping and absorption.
. As seen from the figure, the planar surface solar cell shows the lowest Jsc of ~17.5 mA/cm2 as a result of high reflection from the surface. In comparison, nanowire and nanowall surface-textured solar cells demonstrate much enhanced Jsc of 23.3 mA/cm2 and 24.9 mA/cm2, respectively, which is consistent with their strong light trapping effect presented in Fig. 2.

As a closer examination, the percentage of the light absorption is also estimated. This is done by subtracting the weighted reflected light from 100%. Also since silicon is almost transparent at the wavelength larger than 1000nm, we also ignored the absorption at that wavelength range. Following the detailed equation,
Absorption%=1-λminλSiR(λ)Nph(λ)d(λ)λminλmaxNph(λ)d(λ)-λSiλmaxNph(λ)d(λ)λminλmaxNph(λ)d(λ)
(1)
Where R(λ) is the wavelength dependent reflection, Nph is the photon flux of the solar spectrum, λmin = 300nm is the start of the measurement range, λmax = 1200nm is the end of the measurement range and λSi = 1000nm is the wavelength beyond which silicon has ignorable light absorption, the absorbed light is calculated to be α(planar):α(nanowire):α(nanowall) = 55.7%:70.7%:76.2% = 1:1.27:1.37, where α is the total absorbed light percentage. On the other hand, Jsc(planar): Jsc(nanowire): Jsc(nanowall) = 17.5:23.3:24.9 = 1:1.33:1.42. Therefore, the ratio of the electrical current between the samples agrees with the optical spectrum data. This result also verifies that the carrier collection efficiencies of the difference samples are similar to each other.

The performances of the fabricated solar cells are summarized in Table 1

Table 1. Summary of fabricated Si solar cells with planar, nanowire-textured and nanowall-textured surface

table-icon
View This Table
. Comparing to the η of 7.1% attained by the planar solar cell, high η of 8.2% achieved for the nanowire solar cell is an indication of the essential role of the nanostructure light trapping layer. Despite good Jsc, for the nanowall devices, the overall efficiency is 6.3%. It is mainly limited by relatively low fill factor (FF) of 47.9%.

To investigate the cause for the degraded FF, multiple light intensity IV characteristics was recorded and analyzed for all three types of devices following [13

13. D. Schroder and D. Meier, “Solar cell contact resistance—a review,” IEEE Trans. Electron. Dev. 31(5), 637–647 (1984). [CrossRef]

]; shown in Fig. 5
Fig. 5 Multiple intensity IV characteristics of the planar surface, nanowire and nanowall solar cell. The illuminations are of arbitrary intensities.
is the illustration of such for the solar cells. To extract the series resistance (Rs) of the device, one point is picked up on each curve at Jsc-∆I, where ∆I is a small arbitrary number. The slope of the line connecting all such points provides Rs, which is ~1.37 Ω-cm2 for the planar solar cell as estimated in Fig. 5. Using the same method, Rs of 3.10 Ωcm2 and 7.86 Ωcm2 were extracted from the typical nanowire and nanowall solar cells, respectively.

The high value of Rs in our devices is due to the non-optimized metallization process. It is well known that metal sputter process is incapable of filling deep gap as in our case where the nanostructures are with high aspect ratio. This is especially true for the nanowall structure, which have an aspect ratio of ~5:1. Therefore, the contact between metal and Si is degraded and the Rs from the nanowall cells is the highest among the different samples. However, it should be noted that the Rs can readily be lowered to a typical value of less than 1 Ωcm2 [13

13. D. Schroder and D. Meier, “Solar cell contact resistance—a review,” IEEE Trans. Electron. Dev. 31(5), 637–647 (1984). [CrossRef]

], upon process optimization by more conformal metallization method such as electroplating or atomic layer deposition. Assuming the impact of Rs is eliminated, high η of 9.5% and 9.8% can be obtained from the nanowire and nanowall devices, respectively (Table 1).

4. Conclusion

In Conclusion, we have studied highly-ordered vertical nanowire/nanowall arrays on Si solar cell surface. While the nanowire-array-based solar cells show reflectivity of ~13% and energy conversion efficiency (Rs corrected) of 9.5%, the nanowall array textured solar cell demonstrates much reduced reflectivity of 6% and a corrected η of 9.8%. Our results indicate that the nanowall array is a promising candidate for future photovoltaic applications with further improvements through sidewall junction.

References and links

1.

L. Hu and G. Chen, “Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications,” Nano Lett. 7(11), 3249–3252 (2007). [CrossRef] [PubMed]

2.

L. Tsakalakos, J. Balch, J. Fronheiser, M. Y. Shih, S. F. LeBoeuf, M. Pietrzykowski, P. J. Codella, B. A. Korevaar, O. Sulima, J. Rand, A. Davuluru, and U. Rapol, “Strong broadband optical absorption in silicon nanowire films,” J. Nanophotonics 1(1), 013552 (2007). [CrossRef]

3.

R. A. Street, P. Qi, R. Lujan, and W. S. Wong, “Reflectivity of disordered silicon nanowires,” Appl. Phys. Lett. 93(16), 163109 (2008). [CrossRef]

4.

J. S. Li, H. Y. Yu, S. M. Wong, G. Zhang, G. Q. Lo, and D. L. Kwong, “Surface nanostructure optimization for solar energy harvesting in Si thin film based solar cells,” in 2009 IEEE International Electron Devices Meeting (IEDM) (IEEE, 2009), pp. 1–4.

5.

L. Tsakalakos, J. Balch, J. Fronheiser, B. A. Korevaar, O. Sulima, and J. Rand, “Silicon nanowire solar cells,” Appl. Phys. Lett. 91(23), 233117 (2007). [CrossRef]

6.

B. Tian, X. Zheng, T. J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang, and C. M. Lieber, “Coaxial silicon nanowires as solar cells and nanoelectronic power sources,” Nature 449(7164), 885–889 (2007). [CrossRef] [PubMed]

7.

K. Peng, Y. Xu, Y. Wu, Y. Yan, S. T. Lee, and J. Zhu, “Aligned single-crystalline Si nanowire arrays for photovoltaic applications,” Small 1(11), 1062–1067 (2005). [CrossRef] [PubMed]

8.

S. M. Wong, H. Y. Yu, J. S. Li, G. Zhang, G. Q. Lo, and D. L. Kwong, “Design high-efficiency Si nanopillar-array-textured thin-film solar cell,” IEEE Electron Device Lett. 31(4), 335–337 (2010). [CrossRef]

9.

L. Guo, “Recent progress in nanoimprint technology and its applications,” J. Phys. D Appl. Phys. 37(11), R123–R141 (2004). [CrossRef]

10.

M. Born, E. Wolf, and A. Bhatia, Principles of Optics (Pergamon, Oxford, 1975).

11.

B. S. Richards, “Single-material TiO2 double-layer antireflection coatings,” Sol. Energy Mater. Sol. Cells 79(3), 369–390 (2003). [CrossRef]

12.

J. Zhao and M. A. Green, “Optimized antireflection coatings for high-efficiency silicon solar cells,” IEEE Trans. Electron. Dev. 38(8), 1925–1934 (1991). [CrossRef]

13.

D. Schroder and D. Meier, “Solar cell contact resistance—a review,” IEEE Trans. Electron. Dev. 31(5), 637–647 (1984). [CrossRef]

OCIS Codes
(250.0250) Optoelectronics : Optoelectronics
(350.6050) Other areas of optics : Solar energy

ToC Category:
Solar Energy

History
Original Manuscript: July 5, 2011
Revised Manuscript: August 21, 2011
Manuscript Accepted: September 20, 2011
Published: October 28, 2011

Citation
Jian Wang, Zhenhua Li, Navab Singh, and Sungjoo Lee, "Highly-ordered vertical Si nanowire/nanowall decorated solar cells," Opt. Express 19, 23078-23084 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-23-23078


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References

  1. L. Hu and G. Chen, “Analysis of optical absorption in silicon nanowire arrays for photovoltaic applications,” Nano Lett.7(11), 3249–3252 (2007). [CrossRef] [PubMed]
  2. L. Tsakalakos, J. Balch, J. Fronheiser, M. Y. Shih, S. F. LeBoeuf, M. Pietrzykowski, P. J. Codella, B. A. Korevaar, O. Sulima, J. Rand, A. Davuluru, and U. Rapol, “Strong broadband optical absorption in silicon nanowire films,” J. Nanophotonics1(1), 013552 (2007). [CrossRef]
  3. R. A. Street, P. Qi, R. Lujan, and W. S. Wong, “Reflectivity of disordered silicon nanowires,” Appl. Phys. Lett.93(16), 163109 (2008). [CrossRef]
  4. J. S. Li, H. Y. Yu, S. M. Wong, G. Zhang, G. Q. Lo, and D. L. Kwong, “Surface nanostructure optimization for solar energy harvesting in Si thin film based solar cells,” in 2009 IEEE International Electron Devices Meeting (IEDM) (IEEE, 2009), pp. 1–4.
  5. L. Tsakalakos, J. Balch, J. Fronheiser, B. A. Korevaar, O. Sulima, and J. Rand, “Silicon nanowire solar cells,” Appl. Phys. Lett.91(23), 233117 (2007). [CrossRef]
  6. B. Tian, X. Zheng, T. J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang, and C. M. Lieber, “Coaxial silicon nanowires as solar cells and nanoelectronic power sources,” Nature449(7164), 885–889 (2007). [CrossRef] [PubMed]
  7. K. Peng, Y. Xu, Y. Wu, Y. Yan, S. T. Lee, and J. Zhu, “Aligned single-crystalline Si nanowire arrays for photovoltaic applications,” Small1(11), 1062–1067 (2005). [CrossRef] [PubMed]
  8. S. M. Wong, H. Y. Yu, J. S. Li, G. Zhang, G. Q. Lo, and D. L. Kwong, “Design high-efficiency Si nanopillar-array-textured thin-film solar cell,” IEEE Electron Device Lett.31(4), 335–337 (2010). [CrossRef]
  9. L. Guo, “Recent progress in nanoimprint technology and its applications,” J. Phys. D Appl. Phys.37(11), R123–R141 (2004). [CrossRef]
  10. M. Born, E. Wolf, and A. Bhatia, Principles of Optics (Pergamon, Oxford, 1975).
  11. B. S. Richards, “Single-material TiO2 double-layer antireflection coatings,” Sol. Energy Mater. Sol. Cells79(3), 369–390 (2003). [CrossRef]
  12. J. Zhao and M. A. Green, “Optimized antireflection coatings for high-efficiency silicon solar cells,” IEEE Trans. Electron. Dev.38(8), 1925–1934 (1991). [CrossRef]
  13. D. Schroder and D. Meier, “Solar cell contact resistance—a review,” IEEE Trans. Electron. Dev.31(5), 637–647 (1984). [CrossRef]

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