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  • Editor: Christian Seassal
  • Vol. 22, Iss. S2 — Mar. 10, 2014
  • pp: A328–A334
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Electrochemically synthesized broadband antireflective and hydrophobic GaOOH nanopillars for III-V InGaP/GaAs/Ge triple-junction solar cell applications

Jung Woo Leem, Hee Kwan Lee, Dong-Hwan Jun, Jonggon Heo, Won-Kyu Park, Jin-Hong Park, and Jae Su Yu  »View Author Affiliations


Optics Express, Vol. 22, Issue S2, pp. A328-A334 (2014)
http://dx.doi.org/10.1364/OE.22.00A328


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Abstract

We report the efficiency enhancement of III-V InGaP/GaAs/ Ge triple-junction (TJ) solar cells using a novel structure, i.e., vertically-oriented gallium oxide hydroxide (GaOOH) nanopillars (NPs), as an antireflection coating. The optical reflectance properties of rhombus-shaped GaOOH NPs, which were synthesized by a simple, low-cost, and large-scalable electrochemical deposition method, were investigated, together with a theoretical analysis using the rigorous coupled-wave analysis method. For the GaOOH NPs, the solar weighted reflectance of ~8.5% was obtained over a wide wavelength range of 300-1800 nm and their surfaces exhibited a high water contact angle of ~130° (i.e., hydrophobicity). To simply demonstrate the feasibility of device applications, the GaOOH NPs were incorporated into a test-grown InGaP/GaAs/Ge TJ solar cell structure. For the InGaP/GaAs/Ge TJ solar cell with broadband antireflective GaOOH NPs, the conversion efficiency (η) of ~16.47% was obtained, indicating an increased efficiency by 3.47% compared to the bare solar cell (i.e., η~13%).

© 2014 Optical Society of America

1. Introduction

Recently, there have been many studies on the growth of various antireflective nanostructures such as nanowires and nanorods using zinc oxides, indium tin oxides, gallium oxide hydroxides [9

9. Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, “ZnO nanostructures as efficient antireflection layers in solar cells,” Nano Lett. 8(5), 1501–1505 (2008). [CrossRef] [PubMed]

11

11. H. K. Lee, M. S. Kim, and J. S. Yu, “Enhanced light extraction of GaN-based Green light-emitting diodes with GaOOH rods,” IEEE Photon. Technol. Lett. 24(4), 285–287 (2012). [CrossRef]

]. Particularly, the gallium oxide hydroxide (GaOOH) with a wide bandgap of about 4.75 eV exhibits a high transparency in the wide wavelength region, a high thermal stability, and a relatively lower refractive index than 1.7-1.9 of gallium oxide (Ga2O3) [12

12. M. Sun, D. Li, W. Zhang, X. Fu, Y. Shao, W. Li, G. Xiao, and Y. He, “Rapid microwave hydrothermal synthesis of GaOOH nanorods with photocatalytic activity toward aromatic compounds,” Nanotechnology 21(35), 355601 (2010). [CrossRef] [PubMed]

,13

13. S. Yan, L. Wan, Z. Li, Y. Zhou, and Z. Zou, “Synthesis of a mesoporous single crystal Ga2O3 nanoplate with improved photoluminescence and high sensitivity in detecting CO,” Chem. Commun. (Camb.) 46(34), 6388–6390 (2010). [CrossRef] [PubMed]

]. Furthermore, GaOOH nanopillars (NPs) can be easily synthesized by a simple, cost-effective, controllable, rapid, and low-temperature electrochemical deposition (ED) method over a larger area [14

14. H. K. Lee, D. H. Joo, M. S. Kim, and J. S. Yu, “Improved light extraction of InGaN/GaN blue LEDs by GaOOH NRAs using a thin ATO seed layer,” Nanoscale Res. Lett. 7(1), 458 (2012). [CrossRef] [PubMed]

]. Although GaOOH NPs have been reported for the enhancement of light extraction efficiency in light-emitting diodes [11

11. H. K. Lee, M. S. Kim, and J. S. Yu, “Enhanced light extraction of GaN-based Green light-emitting diodes with GaOOH rods,” IEEE Photon. Technol. Lett. 24(4), 285–287 (2012). [CrossRef]

,14

14. H. K. Lee, D. H. Joo, M. S. Kim, and J. S. Yu, “Improved light extraction of InGaN/GaN blue LEDs by GaOOH NRAs using a thin ATO seed layer,” Nanoscale Res. Lett. 7(1), 458 (2012). [CrossRef] [PubMed]

], there is no work on the solar cells as an ARC. Meanwhile, a hydrophobic surface, which can self-clean dust and other contaminants on the surface of devices, is commonly needed for solar cell applications [15

15. 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]

]. Thus, it is very meaningful to analyze the surface reflection and wettability properties of the GaOOH NPs. In this work, we demonstrate a useful approach to enhance the efficiency of the III-V InGaP/GaAs/Ge triple-junction (TJ) solar cells with antireflective GaOOH NPs. The structural, optical, and surface wetting properties of GaOOH NPs, which were grown by the ED method, were investigated. For the analysis of optical reflection, theoretical calculations were also performed by the rigorous coupled-wave analysis (RCWA) method. For device application feasibility, the device characteristics of a test-grown InGaP/GaAs/Ge TJ solar cell with AR GaOOH NPs were studied in comparison with the bare solar cell structure without an ARC.

2. Experimental details

3. Results and discussion

Fig. 2 (a) Calculated Ravg and SWR of ATO/Ge as a function of ATO film thickness and (b) top- and side-view SEM images of the GaOOH NPs grown on the ATO/Ge substrate. The n and k of ATO and Ge used in this calculation are shown in the inset of (a).
Figure 2(a) shows the calculated average reflectance (Ravg) and solar weighted reflectance (SWR) of ATO film on the Ge substrate (i.e., ATO/Ge) as a function of ATO film thickness. The refractive index (n) and extinction coefficient (k) of ATO and Ge used in this calculation are also shown in the inset of Fig. 2(a). The ATO film as a seed layer allows for the growth of vertically-oriented GaOOH NPs. However, the reflection is strongly dependent on the thickness of the film. Thus, the desirable thickness of the ATO film should be chosen to efficiently suppress the surface reflection of solar cells. For the theoretical analysis of optical reflectance, the RCWA simulations were performed using a commercial software (DiffractMOD 3.1, Rsoft Design Group). We assumed that the incident light enters from air into the structure at normal incidence. The Ravg value is decreased from 40.9 to 21.3% with increasing the thickness of ATO film from 10 to 100 nm. These values are much lower than that (i.e., Ravg~42.3%) of Ge substrate due to the graded refractive index profile between air (nair = 1) and the Ge (nGe~4.7) via the ATO (nATO~1.96). For solar cell applications, it is necessary to investigate the SWR [16

16. H. Sai, Y. Kanamori, K. Arafune, Y. Ohshita, and M. Yamaguchi, “Light trapping effect of submicron surface textures in crystalline Si solar cells,” Prog. Photovolt. Res. Appl. 15(5), 415–423 (2007). [CrossRef]

], which is the ratio of the usable photons reflected to the total usable photons, of ATO film. The SWR can be evaluated by normalizing the reflectance and the AM1.5g spectrum integrated over a wavelength range of 300-1800 nm. For the ATO film, the estimated SWR value was decreased from 42.9% at 10 nm of film thickness to 19.4% at 70 nm, and then was increased to 21.7% at 100 nm. It is noted that the optical absorption of film is strongly dependent on the film thickness. Therefore, we chose the ATO film thickness of 60 nm, which is about λ/4 thickness at λ~500 nm (near the maximum intensity of the solar spectrum), (i.e., SWR = 20.2%) to suppress the surface reflection and to reduce the optical absorption in the ATO film as much as possible for the efficient antireflection and seed layer. Figure 2(b) shows the top- and side-view SEM images of the GaOOH NPs grown on the ATO/Ge substrate. The GaOOH NPs with a closely rhombus-shaped structure were vertically and uniformly formed on the ATO/Ge by the ED method. Clearly, the ATO film as a seed layer helps the GaOOH NPs with high density to grow along the vertical direction. This can be explained by the fact that the heterogeneous nucleation is enhanced by the ATO under a cathodic voltage in the ED process [14

14. H. K. Lee, D. H. Joo, M. S. Kim, and J. S. Yu, “Improved light extraction of InGaN/GaN blue LEDs by GaOOH NRAs using a thin ATO seed layer,” Nanoscale Res. Lett. 7(1), 458 (2012). [CrossRef] [PubMed]

]. The average height and lateral length of the grown GaOOH NPs were approximately 1 μm ± 250 nm and 500 nm ± 150 nm, respectively.

Fig. 3 (a) Measured reflectance spectra of Ge substrate, ATO/Ge and GaOOH NPs/ATO/Ge and solar spectral irradiance of AM1.5g and (b) calculated reflectance spectra and electric field intensity distributions at λ = 800 nm of (i) Ge substrate, (ii) ATO/Ge, and (iii) GaOOH NPs/ATO/Ge. For comparison, the measured reflectance spectrum of a typical Al2O3/TiO2 DLARC is also shown in (a).
Figure 3(a) shows the measured reflectance spectra of Ge substrate, ATO/Ge, and GaOOH NPs/ATO/Ge and solar spectral irradiance of AM1.5g. For comparison, the measured reflectance spectrum of a typical Al2O3/TiO2 double-layer ARC (DLARC) with λ/4 thicknesses of ~70/55 nm at λ = 500 nm (i.e., nAl2O3 = 1.77, nTiO2 = 2.3) is also shown in Fig. 3(a). For the ATO (60 nm)/Ge, the reflectance was lower than that of the Ge substrate over a wide wavelength range of 300-1800 nm, especially at wavelengths of around 500 nm which are the highest region of AM1.5g solar spectrum. This is ascribed to the graded refractive index profile between air and the Ge substrate via the ATO as wells as the λ/4 thickness (i.e., ~60 nm) of ATO with a refractive index of ~1.96 at λ = 500 nm. The incorporation of vertically-oriented GaOOH NPs into the ATO/Ge led to much lower reflectance compared to the ATO/Ge and Ge substrate at wavelengths of 300-1800 nm though it was slightly higher than that of ATO/Ge at wavelengths of 440-550 nm, exhibiting the SWR value of ~8.5% which is lower than those of the other samples (i.e., SWR~19.3% for ATO/Ge, SWR~43.5% for Ge substrate, and SWR~11.6% for Al2O3/TiO2 DLARC).

To explore the influence of the geometry of GaOOH NPs on the antireflective characteristics, a theoretical optical analysis was carried out using the RCWA simulation. For simplicity, we designed the model of periodic square-shaped GaOOH NPs with an area of 500 × 500 nm2 and height of 1 μm on ATO (60 nm)/Ge (175 μm). The period between GaOOH NPs with a two-dimensional six-fold hexagonal symmetry pattern was kept at 1.1 μm. For the GaOOH, the refractive index was assumed to be 1.75 and the extinction coefficient was not considered at the whole wavelength range. Figure 3(b) shows the calculated reflectance spectra and electric field intensity distributions at λ = 800 nm of (i) Ge substrate, (ii) ATO/Ge, and (iii) GaOOH NPs/ATO/Ge. For the calculated results, the overall trend appears to be similar with the measured data. As expected, the GaOOH NPs on ATO/Ge exhibit a relatively lower reflectivity over a wide wavelength region of 300-1800 nm except for wavelengths of 450-600 nm compared to the other structures. This results from the effective medium effect [17

17. D. E. Aspnes, J. B. Theeten, and F. Hottier, “Investigation of effective-medium models of microscopic surface roughness by spectroscopic ellipsometry,” Phys. Rev. B 20(8), 3292–3302 (1979). [CrossRef]

]. The GaOOH pillars with nanoscale dimensions can be regarded as an effective single layer, thus creating the gradient effective refractive index profile between air (nair = 1) and the Ge (nGe~4.7) via the GaOOH NPs (nGaOOH~1.75)/ATO (nATO~1.96). The surface reflection is also suppressed by the destructive interference between the waves with different phases which are reflected at different positions (or depths) from the surface of ATO by the GaOOH NPs. Moreover, the GaOOH NPs help the incident light to propagate across the interfaces of air/ATO and ATO/Ge as well as to diffuse within the Ge substrate by increasing the light scattering at the surface. Thus, the relatively strong electric field intensity transmitted through the GaOOH NPs is observed within the Ge substrate compared to the other structures, as illustrated in Fig. 3(b).

In order to investigate the effect of the antireflective GaOOH NPs on the efficiency of actual solar cells, the GaOOH NPs were synthesized on a test-grown InGaP/GaAs/Ge TJ solar cell structure.
Fig. 5 (a) Low- and high-magnification SEM images of GaOOH NPs grown on the InGaP/GaAs/Ge TJ solar cell with the ATO seed layer and (b) measured J-V characteristics on the test-grown III-V InGaP/GaAs/Ge TJ solar cells with ATO film, GaOOH NPs/ATO seed layer, and Al2O3/TiO2 double-layer as an ARC. For a reference, the measured J-V curve of solar cell with the bare surface is also shown in (b). The device characteristics of the corresponding solar cells are summarized in the inset of (b).
Figure 5 shows the (a) low- and high-magnification SEM images of GaOOH NPs grown on the InGaP/GaAs/Ge TJ solar cell with the ATO seed layer and (b) measured J-V characteristics on the test-grown III-V InGaP/GaAs/Ge TJ solar cells integrated with ATO film and GaOOH NPs/ATO structures as an ARC. For a reference, the J-V curve of solar cell with the bare surface is also shown in Fig. 5(b). As shown in Fig. 5(a), the vertically-oriented GaOOH NPs were relatively well grown on ATO seed layer in the InGaP/GaAs/Ge TJ solar cell. The measured device characteristics (i.e., open circuit voltage, Voc; short circuit current density, Jsc; fill factor, FF; conversion efficiency, η) of the corresponding solar cells are summarized in the inset of Fig. 5(b). For the solar cell with the ATO film as a seed layer, the η was increased to 15.3% compared to the bare solar cell (i.e., η = 13%) mainly due to the increase of Jsc from 7.98 to 9.35 mA/cm2 rather than other solar cell characteristics (e.g., Voc and FF). By growing the GaOOH NPs on the ATO seed layer, the higher η value of 16.47% was achieved, exhibiting a further enhanced Jsc value of 10.01 mA/cm2. These Jsc and η values are also slightly higher than those (i.e., Jsc = 9.9 mA/cm2 and η = 16.34%) of the test-grown III-V InGaP/GaAs/Ge TJ solar cell employed with a typical Al2O3/TiO2 DLARC while there are almost no variations on the Voc and FF. This is due to the graded effective refractive index profile between air and the window layer of solar cell via the GaOOH NPs/ATO seed layer and the destructive interference of waves reflected at different positions (or depths) from the surface due to the GaOOH NPs. Besides, the GaOOH NPs help the incident light to propagate across the interfaces of air/ATO/solar cell and to spread within the solar cell by increasing the light scattering on the surface. This phenomenon can effectively trap the light in the active medium of solar cell, which leads to the efficiency enhancement.

4. Conclusion

Vertically-oriented GaOOH NPs were synthesized on a test-grown III-V InGaP/GaAs/Ge TJ solar cells as an ARC using the ATO seed layer by the ED method. Their antireflective characteristics over a wide wavelength range as well as wetting behavior were investigated, together with the theoretical analysis using the RCWA method. The GaOOH NPs/ATO/Ge structure exhibited much lower reflectivity than that of Ge substrate over a wide wavelength region of 300-1800 nm, indicating a lower SWR value of ~8.5% (i.e., SWR~43.5% for Ge substrate). Also, the hydrophobic surface with a water contact angle of ~130° was formed. For the InGaP/GaAs/Ge TJ solar cell structure integrated with broadband antireflective GaOOH NPs, the η value of ~16.47% was obtained under AM1.5g illumination, indicating an efficiency improvement by ~3.47% compared to the bare solar cell (i.e., η~13%). These results can give a deep understanding of the vertically-oriented GaOOH NPs, which can be easily fabricated by a simple and cost-effective ED method, with broadband antireflective surface as well as self-cleaning function for high-efficiency III-V compound material-based multi-junction solar cell applications.

Acknowledgments

The work was supported by the International Collaborative R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Knowledge Economy (No. 20118520010030-11-2-100).

References and links

1.

M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cell efficiency tables (version 41),” Prog. Photovolt. Res. Appl. 21(1), 1–11 (2013). [CrossRef]

2.

J. Zhao, A. Wang, M. A. Green, and F. Ferrazza, “19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon solar cells,” Appl. Phys. Lett. 73(14), 1991–1993 (1998). [CrossRef]

3.

D. J. Aiken, “High performance anti-reflection coatings for broadband multi-junction solar cells,” Sol. Energy Mater. Sol. Cells 64(4), 393–404 (2000). [CrossRef]

4.

D. Bouhafs, A. Moussi, A. Chikouche, and J. M. Ruiz, “Design and simulation of antireflection coating systems for optoelectronic devices: Application to silicon solar cells,” Sol. Energy Mater. Sol. Cells 52(1–2), 79–93 (1998). [CrossRef]

5.

X. Yan, D. J. Poxson, J. Cho, R. E. Welser, A. K. Sood, J. K. Kim, and E. F. Schubert, “Enhanced omnidirectional photovoltaic performance of solar cells using multiple-discrete-layer tailored- and low-refractive index anti-reflection coatings,” Adv. Funct. Mater. 23(5), 583–590 (2013). [CrossRef]

6.

H. Sai, H. Fujii, K. Arafune, Y. Ohshita, Y. Kanamori, H. Yugami, and M. Yamaguchi, “Wide-angle antireflection effect of subwavelength structures for solar cells,” Jpn. J. Appl. Phys. 46(6A), 3333–3336 (2007). [CrossRef]

7.

C. H. Chang, J. A. Dominguez-Caballero, H. J. Choi, and G. Barbastathis, “Nanostructured gradient-index antireflection diffractive optics,” Opt. Lett. 36(12), 2354–2356 (2011). [CrossRef] [PubMed]

8.

J. W. Leem, Y. M. Song, and J. S. Yu, “Biomimetic artificial Si compound eye surface structures with broadband and wide-angle antireflection properties for Si-based optoelectronic applications,” Nanoscale 5(21), 10455–10460 (2013). [CrossRef] [PubMed]

9.

Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, “ZnO nanostructures as efficient antireflection layers in solar cells,” Nano Lett. 8(5), 1501–1505 (2008). [CrossRef] [PubMed]

10.

P. Yu, C. H. Chang, C. H. Chiu, C. S. Yang, J. C. Yu, H. C. Kuo, S. H. Hsu, and Y. C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflection indium tin oxide nanocolumns,” Adv. Mater. 21(16), 1618–1621 (2009). [CrossRef]

11.

H. K. Lee, M. S. Kim, and J. S. Yu, “Enhanced light extraction of GaN-based Green light-emitting diodes with GaOOH rods,” IEEE Photon. Technol. Lett. 24(4), 285–287 (2012). [CrossRef]

12.

M. Sun, D. Li, W. Zhang, X. Fu, Y. Shao, W. Li, G. Xiao, and Y. He, “Rapid microwave hydrothermal synthesis of GaOOH nanorods with photocatalytic activity toward aromatic compounds,” Nanotechnology 21(35), 355601 (2010). [CrossRef] [PubMed]

13.

S. Yan, L. Wan, Z. Li, Y. Zhou, and Z. Zou, “Synthesis of a mesoporous single crystal Ga2O3 nanoplate with improved photoluminescence and high sensitivity in detecting CO,” Chem. Commun. (Camb.) 46(34), 6388–6390 (2010). [CrossRef] [PubMed]

14.

H. K. Lee, D. H. Joo, M. S. Kim, and J. S. Yu, “Improved light extraction of InGaN/GaN blue LEDs by GaOOH NRAs using a thin ATO seed layer,” Nanoscale Res. Lett. 7(1), 458 (2012). [CrossRef] [PubMed]

15.

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]

16.

H. Sai, Y. Kanamori, K. Arafune, Y. Ohshita, and M. Yamaguchi, “Light trapping effect of submicron surface textures in crystalline Si solar cells,” Prog. Photovolt. Res. Appl. 15(5), 415–423 (2007). [CrossRef]

17.

D. E. Aspnes, J. B. Theeten, and F. Hottier, “Investigation of effective-medium models of microscopic surface roughness by spectroscopic ellipsometry,” Phys. Rev. B 20(8), 3292–3302 (1979). [CrossRef]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(310.1210) Thin films : Antireflection coatings
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Photovoltaics

History
Original Manuscript: August 14, 2013
Revised Manuscript: November 14, 2013
Manuscript Accepted: February 3, 2014
Published: February 13, 2014

Citation
Jung Woo Leem, Hee Kwan Lee, Dong-Hwan Jun, Jonggon Heo, Won-Kyu Park, Jin-Hong Park, and Jae Su Yu, "Electrochemically synthesized broadband antireflective and hydrophobic GaOOH nanopillars for III-V InGaP/GaAs/Ge triple-junction solar cell applications," Opt. Express 22, A328-A334 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-S2-A328


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References

  1. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cell efficiency tables (version 41),” Prog. Photovolt. Res. Appl.21(1), 1–11 (2013). [CrossRef]
  2. J. Zhao, A. Wang, M. A. Green, and F. Ferrazza, “19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon solar cells,” Appl. Phys. Lett.73(14), 1991–1993 (1998). [CrossRef]
  3. D. J. Aiken, “High performance anti-reflection coatings for broadband multi-junction solar cells,” Sol. Energy Mater. Sol. Cells64(4), 393–404 (2000). [CrossRef]
  4. D. Bouhafs, A. Moussi, A. Chikouche, and J. M. Ruiz, “Design and simulation of antireflection coating systems for optoelectronic devices: Application to silicon solar cells,” Sol. Energy Mater. Sol. Cells52(1–2), 79–93 (1998). [CrossRef]
  5. X. Yan, D. J. Poxson, J. Cho, R. E. Welser, A. K. Sood, J. K. Kim, and E. F. Schubert, “Enhanced omnidirectional photovoltaic performance of solar cells using multiple-discrete-layer tailored- and low-refractive index anti-reflection coatings,” Adv. Funct. Mater.23(5), 583–590 (2013). [CrossRef]
  6. H. Sai, H. Fujii, K. Arafune, Y. Ohshita, Y. Kanamori, H. Yugami, and M. Yamaguchi, “Wide-angle antireflection effect of subwavelength structures for solar cells,” Jpn. J. Appl. Phys.46(6A), 3333–3336 (2007). [CrossRef]
  7. C. H. Chang, J. A. Dominguez-Caballero, H. J. Choi, and G. Barbastathis, “Nanostructured gradient-index antireflection diffractive optics,” Opt. Lett.36(12), 2354–2356 (2011). [CrossRef] [PubMed]
  8. J. W. Leem, Y. M. Song, and J. S. Yu, “Biomimetic artificial Si compound eye surface structures with broadband and wide-angle antireflection properties for Si-based optoelectronic applications,” Nanoscale5(21), 10455–10460 (2013). [CrossRef] [PubMed]
  9. Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, “ZnO nanostructures as efficient antireflection layers in solar cells,” Nano Lett.8(5), 1501–1505 (2008). [CrossRef] [PubMed]
  10. P. Yu, C. H. Chang, C. H. Chiu, C. S. Yang, J. C. Yu, H. C. Kuo, S. H. Hsu, and Y. C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflection indium tin oxide nanocolumns,” Adv. Mater.21(16), 1618–1621 (2009). [CrossRef]
  11. H. K. Lee, M. S. Kim, and J. S. Yu, “Enhanced light extraction of GaN-based Green light-emitting diodes with GaOOH rods,” IEEE Photon. Technol. Lett.24(4), 285–287 (2012). [CrossRef]
  12. M. Sun, D. Li, W. Zhang, X. Fu, Y. Shao, W. Li, G. Xiao, and Y. He, “Rapid microwave hydrothermal synthesis of GaOOH nanorods with photocatalytic activity toward aromatic compounds,” Nanotechnology21(35), 355601 (2010). [CrossRef] [PubMed]
  13. S. Yan, L. Wan, Z. Li, Y. Zhou, and Z. Zou, “Synthesis of a mesoporous single crystal Ga2O3 nanoplate with improved photoluminescence and high sensitivity in detecting CO,” Chem. Commun. (Camb.)46(34), 6388–6390 (2010). [CrossRef] [PubMed]
  14. H. K. Lee, D. H. Joo, M. S. Kim, and J. S. Yu, “Improved light extraction of InGaN/GaN blue LEDs by GaOOH NRAs using a thin ATO seed layer,” Nanoscale Res. Lett.7(1), 458 (2012). [CrossRef] [PubMed]
  15. 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]
  16. H. Sai, Y. Kanamori, K. Arafune, Y. Ohshita, and M. Yamaguchi, “Light trapping effect of submicron surface textures in crystalline Si solar cells,” Prog. Photovolt. Res. Appl.15(5), 415–423 (2007). [CrossRef]
  17. D. E. Aspnes, J. B. Theeten, and F. Hottier, “Investigation of effective-medium models of microscopic surface roughness by spectroscopic ellipsometry,” Phys. Rev. B20(8), 3292–3302 (1979). [CrossRef]

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