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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 4 — Feb. 19, 2007
  • pp: 1907–1912
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High solar absorption of a multilayered thin film structure

Xiao-Fan Li, Yue-Rui Chen, Jian Miao, Peng Zhou, Yu-Xiang Zheng, Liang-Yao Chen, and Young-Pak Lee  »View Author Affiliations


Optics Express, Vol. 15, Issue 4, pp. 1907-1912 (2007)
http://dx.doi.org/10.1364/OE.15.001907


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Abstract

We report a structure with 4 thin film layers composed of pure metal and dielectric materials and prepared by sputtering. The reflectance and transmittance are lower than 5% with the absorption to be achieved higher than 95% in the 400–1000nm wavelength region as match to the solar radiance spectrum. The thermal emittance of the structure is in the range of 0.063–0.10 through data analysis. The good reproducibility and stability of spectral data associated with the deposition process imply the advantage of the solar energy absorber which is cost-effective in application.

© 2007 Optical Society of America

1. Introduction

The global energy shortage has put great pressure on human being to look for the new energy resource. The solar energy is a kind of unique clean and sustainable energy resource as compared with other conventional ones such as the coal, oil, natural gas and so on [1

1. D. Behrman, Solar Energy (Little, Brown & Company Limited, 1976).

]. Photon-electrical and photon-thermal conversions are two most feasible approaches in application of the solar energy [2

2. B. O. Seraphin, Solar Energy Conversion: Solid-State Physics Aspects, B. O. Seraphin, ed., Topics in Applied Physics (Springer, 1979) Vol. 31.

]. By converting photons into electrons, the solar cell has the advantage of easier application in many fields, but has the limit due to its narrower absorption spectrum window with relatively high material and production cost. For the device using the photon-thermal conversion, the optimal structure can be designed to achieve both of high solar absorption α in the visible and near infrared region with respect to the solar radiance spectra and the low thermal emittance ε in the far infrared region. These are two main physical criteria used to characterize the high photo-thermal conversion efficiency η [3

3. D. M. Trotter Jr. and A. J. Sievers, “Spectral selectivity of high-temperature solar absorbers,” Appl. Opt. 19,711–728 (1980). [CrossRef] [PubMed]

].

Different approaches have been tried to realize high and broadband solar absorption in the solar-thermal conversion by using the material with proper structures like cermet [4

4. Q. C. Zhang, “Recent progress in high-temperature solar selective coatings,” Sol. Energ. Mat. Sol. C. 62,63–74 (2000). [CrossRef]

, 5

5. S. X. Zhao and Ewa Wäckelgård, “Optimization of solar absorbing three-layer coatings,” Sol. Energ. Mat. Sol. C. 90,243–261 (2006). [CrossRef]

] and black components [6

6. L. L. Ma, Y. C. Zhou, N. Jiang, X. Lu, J. Shao, W. Lu, J. Ge, X. M. Ding, and X. Y. Hou, “Wide-band “black silicon” based on porous silicon,” Appl. Phys. Lett. 88,171907 (2006). [CrossRef]

, 7

7. K. D. Lee, W. C. Jung, and J. H. Kim, “Thermal degradation of black chrome coatings,” Sol. Energ. Mat. Sol. C. 63,125–137 (2000). [CrossRef]

] realized all through graded-index film [8

8. I. T. Ritchie and B. Window, “Applications of thin graded-index films to solar absorbers,” Appl. Optics 16,1438–1443 (1977). [CrossRef]

] and metal-based periodic microstructures [9

9. S. Maruyama, T. Kashiwa, H. Yugami, and M. Esashi, “Thermal radiation from two-dimensionally confined modes in microcavities,” Appl. Phys. Lett. 79,1393–1395 (2001). [CrossRef]

,10

10. H. Sai, H. Yugami, Y. Kanamori, and K. Hane, “Solar selective absorbers based on two-dimensional W surface gratings with submicron periods for high-temperature photothermal conversion,” Sol. Energ. Mat. Sol. C. 79,35–49 (2003). [CrossRef]

], and so on. Some of the structures show low thermal emittance to be practically put into application, but encounter difficulties to precisely and consistently control the composition of the material in the gas flow and chemical reaction process to produce the high quality graded-index structures.

2. Structure design

Fig. 1. Sketch of the 4-layer (dielectric/metal/dielectric/metal) structure of solar-thermal conversion structure on the Si substrate.

In terms of the structure and optical constants of the materials [11

11. D. W. Lynch and W. R. Hunter, Handbook of Optical Constants of Solids (Academic, 1998).

], the optical transfer-matrix method was used to simulate the optical properties of the device by calculating the reflectance, transmittance and absorptance of the structure in advance. The detailed parameters used in the simulation are as follows: the top layer SiO2 (90∼110nm) is to have the function of protection and antireflection; the second Ti metal layer (10∼30nm) is to play the key role of high solar energy absorption; the third one is SiO2 layer (80∼120nm), which is to perform the phase and amplitude matching function; the bottom Al (or Ag) layer (≥100nm) is to provide high optical reflection and almost zero transmission in the 400–1000nm wavelength region. The simulated results at normal incidence are shown in Fig. 2. It can be seen that over 95% of optical absorption can be reached in the 450-1000nm region ( > 90% in 420–1200nm region) as match to the solar radiance spectrum. The steep decrease of the absorption initiated at the wavelength λ ≈350nm (about 3.5eV) is due to the mismatching of the optical constants of layered materials in the region.

Fig. 2. Simulated data with respect to transmittance, reflectance and absorptance of the 4-layer structure [SiO2(105nm)/Ti(15nm)/SiO2(95nm)/Al(100nm)] on the Si substrate at normal incidence as compared with the solar irradiance spectrum [12].

3. Experiment

The multilayered thin film sample was prepared by sputtering in the Leybold LAB600SP chamber in room temperature. The metal and dielectric films with the target purity of 99.99% were deposited by DC and RF magnetron on the Si substrate, respectively with a background pressure of 6.0×10-6 mbar. The thickness of the film is controlled by deposition time according to the sputtering rate which was determined by the power and flux of Argon gas flow rate as calibrated by the ellipsometer and Kosaka Surfcorder ET300 in advance.

Although Cr, W and other metal materials can be chosen as the energy absorption layer, we find that it will be more efficient for Ti to perform the high and uniform solar energy absorption as match to the solar radiance spectrum. Although either Al or Ag can be the candidate of the optical refraction layer, Al is superior with higher reflection extending to the ultraviolet region and costs less in production. The pure dielectric SiO2 layer is transparent and stable in the environment with the optical refractive index less variable in the 200–1000nm wavelength region, and then it is suitable to use as the protection and interference layer for the device.

The spectra of the reflectance R [R=(Rs+Rp)/2, where Rs and Rp is the polarized reflectance normal and parallel to the incidence plane, respectively] and transmittance T (T ≈ 0) were measured by a variable- incidence-angle spectrometer in the 300–800nm wavelength region with the incidence angle changing from 35° to 80°. In terms of the spectral data of T and R, the absorptance A was reduced according to the conversation of energy A =1-R-T in the data analysis afterwards.

4. Results and discussions

We have measured the dependence of incident angle from 35° to 80° with a step of 5° for a typical layered structure [SiO2(105nm)/Ti(15nm)/SiO2(95nm)/Al(≥100nm)]. The measured and simulated spectra of absorption are shown in Figs. 3 and 4, respectively. In general the higher absorptance can be achieved at the small incidence angle condition; it decreases with the increasing of the incident angle. Taking the absorption at 35° as an example, the measured spectrum is in close agreement with the simulated one to show high absorptance over 90% in the 350–800nm region. Especially, the highest absorption efficiency of more than 99% has been reached in the deep blue region. Although the absorptance decreases with the incidence angle, experimental data show that it still has the value higher than 50% almost in the entire spectral region even at higher incidence angle of 80°.

Fig. 3. The measured spectra of absorptance changing with the incident angle for the layered structure of SiO2(105nm)/Ti(15nm)/SiO2(95nm)/Al( > 100nm).
Fig. 4. The simulated spectra of absorptance changing with the incident angle based on the layered structure given in Fig. 3.

In the study of the Al-based multilayer structure, we also tried to change the thickness of the Ti layer and found that the absorptance is not particularly sensitive to the slight variation of the Ti layer thickness in the spectral region. The results are shown in Fig. 5.

Fig. 5. The measured spectra of absorptance changing with the thickness of the Ti layer under the condition in which the thicknesses of the SiO2 and Al layers are fixed and other parameters are the same as that given in Fig. 3.

According to Planck’s black-body radiation E(T, λ), the thermal emittance ε can be calculated based on the spectra of reflectance R(θ, λ), especially in the long wavelength region by using the equation [3

3. D. M. Trotter Jr. and A. J. Sievers, “Spectral selectivity of high-temperature solar absorbers,” Appl. Opt. 19,711–728 (1980). [CrossRef] [PubMed]

]:

εθT=0dλETλ[1Rθλ]0dλETλ
(1)

We calculated the reflectance of the structure in the infrared region (1-32μm) by using the transfer-matrix method at normal incidence condition to obtain the thermal emittance which is 0.063 at 600K with a comparison to that at different incident angles as shown in the inset of Fig. 6. We also measured the reflectance R in the 1-5 μm infrared region at the incidence angle of 35° with the result given in Fig. 6 to show a good agreement with the simulated one which indicates a very low thermal emittance for the solar-to-thermal convert structure of the device.

Fig. 6. The comparison of reflectance spectra between measured and simulated one in the 1-5 μm infrared region for the structure given in Fig. 3. The inset shows the calculated emittance with respect to different incident angles.

5. Conclusion

We have designed and achieved a multilayer structure which has high solar absorption efficiency (>95%) in the 400–1000nm wavelength region and low thermal emittance (ε=0.063 at 600K and normal incidence condition). The materials used in application are cost-effective with relatively simple procedure for production and quality control to show the potential application of the structure in the solar energy field.

Acknowledgments

This work was supported by the NSF projects of China with the contract numbers: #60327002 and #60478019 and by KOSEF through q-Psi.

References and links

1.

D. Behrman, Solar Energy (Little, Brown & Company Limited, 1976).

2.

B. O. Seraphin, Solar Energy Conversion: Solid-State Physics Aspects, B. O. Seraphin, ed., Topics in Applied Physics (Springer, 1979) Vol. 31.

3.

D. M. Trotter Jr. and A. J. Sievers, “Spectral selectivity of high-temperature solar absorbers,” Appl. Opt. 19,711–728 (1980). [CrossRef] [PubMed]

4.

Q. C. Zhang, “Recent progress in high-temperature solar selective coatings,” Sol. Energ. Mat. Sol. C. 62,63–74 (2000). [CrossRef]

5.

S. X. Zhao and Ewa Wäckelgård, “Optimization of solar absorbing three-layer coatings,” Sol. Energ. Mat. Sol. C. 90,243–261 (2006). [CrossRef]

6.

L. L. Ma, Y. C. Zhou, N. Jiang, X. Lu, J. Shao, W. Lu, J. Ge, X. M. Ding, and X. Y. Hou, “Wide-band “black silicon” based on porous silicon,” Appl. Phys. Lett. 88,171907 (2006). [CrossRef]

7.

K. D. Lee, W. C. Jung, and J. H. Kim, “Thermal degradation of black chrome coatings,” Sol. Energ. Mat. Sol. C. 63,125–137 (2000). [CrossRef]

8.

I. T. Ritchie and B. Window, “Applications of thin graded-index films to solar absorbers,” Appl. Optics 16,1438–1443 (1977). [CrossRef]

9.

S. Maruyama, T. Kashiwa, H. Yugami, and M. Esashi, “Thermal radiation from two-dimensionally confined modes in microcavities,” Appl. Phys. Lett. 79,1393–1395 (2001). [CrossRef]

10.

H. Sai, H. Yugami, Y. Kanamori, and K. Hane, “Solar selective absorbers based on two-dimensional W surface gratings with submicron periods for high-temperature photothermal conversion,” Sol. Energ. Mat. Sol. C. 79,35–49 (2003). [CrossRef]

11.

D. W. Lynch and W. R. Hunter, Handbook of Optical Constants of Solids (Academic, 1998).

12.

P. H. Moon, The Scientific Basis of Illuminating Engineering (McGraw-Hill book company, inc., 1936).

OCIS Codes
(160.6840) Materials : Thermo-optical materials
(220.4830) Optical design and fabrication : Systems design
(230.4170) Optical devices : Multilayers
(310.6860) Thin films : Thin films, optical properties

ToC Category:
Thin Films

History
Original Manuscript: January 4, 2007
Revised Manuscript: January 31, 2007
Manuscript Accepted: February 7, 2007
Published: February 19, 2007

Citation
Xiao-Fan Li, Yue-Rui Chen, Jian Miao, Peng Zhou, Yu-Xiang Zheng, Liang-Yao Chen, and Young-Pak Lee, "High solar absorption of a multilayered thin film structure," Opt. Express 15, 1907-1912 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-4-1907


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References

  1. D. Behrman, Solar Energy (Little, Brown & Company Limited, 1976).
  2. B. O. Seraphin, Solar Energy Conversion: Solid-State Physics Aspects, B. O. Seraphin, ed., Topics in Applied Physics (Springer, 1979) Vol. 31.
  3. D. M. Trotter, Jr. and A. J. Sievers, "Spectral selectivity of high-temperature solar absorbers," Appl. Opt. 19, 711-728 (1980). [CrossRef] [PubMed]
  4. Q. C. Zhang, "Recent progress in high-temperature solar selective coatings," Sol. Energ. Mater. Sol. Cells 62, 63-74 (2000). [CrossRef]
  5. S. X. Zhao and Ewa Wäckelgård, "Optimization of solar absorbing three-layer coatings," Sol. Energ. Mat. Sol. C. 90, 243-261 (2006). [CrossRef]
  6. L. L. Ma, Y. C. Zhou, N. Jiang, X. Lu, J. Shao, W. Lu, J. Ge, X. M. Ding and X. Y. Hou, "Wide-band "black silicon" based on porous silicon," Appl. Phys. Lett. 88, 171907 (2006). [CrossRef]
  7. K. D. Lee, W. C. Jung and J. H. Kim, "Thermal degradation of black chrome coatings," Sol. Energ. Mater. Sol. Cells 63, 125-137 (2000). [CrossRef]
  8. I. T. Ritchie and B. Window, "Applications of thin graded-index films to solar absorbers," Appl. Opt. 16, 1438-1443 (1977). [CrossRef]
  9. S. Maruyama, T. Kashiwa, H. Yugami and M. Esashi, "Thermal radiation from two-dimensionally confined modes in microcavities," Appl. Phys. Lett. 79, 1393-1395 (2001). [CrossRef]
  10. H. Sai, H. Yugami, Y. Kanamori and K. Hane, "Solar selective absorbers based on two-dimensional W surface gratings with submicron periods for high-temperature photothermal conversion," Sol. Energ. Mater. Sol. Cells 79, 35-49 (2003). [CrossRef]
  11. D. W. Lynch and W. R. Hunter, Handbook of Optical Constants of Solids (Academic, 1998).
  12. P. H. Moon, The Scientific Basis of Illuminating Engineering (McGraw-Hill Inc., 1936).

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