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Solar selective absorbers are the core part for solar thermal technologies such as solar water heaters, concentrated solar power, solar thermoelectric generators and solar thermophotovoltaics. Colorful solar selective absorber can provide new freedom and flexibility beyond energy performance, which will lead to wider utilization of solar technologies. In this work, we present a monolithic integration of colored solar absorber array with different colors on a single substrate based on a multilayered structure of Cu/TiNxOy/TiO2/Si3N4/SiO2. A colored solar absorber array with 16 color units is demonstrated experimentally by using combinatorial deposition technique via changing the thickness of SiO2 layer. The solar absorptivity and thermal emissivity of all the color units is higher than 92% and lower than 5.5%, respectively. The colored solar selective absorber array can have colorful appearance and designable patterns while keeping high energy performance at the same time. It is a new candidate for a number of solar applications, especially for architecture integration and military camouflage.
© 2015 Optical Society of America
1. Introduction
Solar selective absorber possesses the characteristic of high absorptivity for solar radiation and low emissivity for thermal infrared radiation, which is the core part of solar thermal technologies, such as solar water heater, concentrated solar power (CSP), solar thermoelectric generators (STEGs) and solar thermophotovoltaics (STPV) [1–5]. It is an effective and low-cost method for energy utilization to integrate these solar applications into buildings. For architectural integration solar applications, colorful appearance should be taken into account beyond the energy performance based on aesthetic considerations [6–10]. For other solar applications such as military application, the appearance with colorful pattern plays a critical role in camouflage [11, 12]. Therefore, colorful solar selective absorber can provide new freedom and flexibility, which will lead to wider usage of solar technologies.
However, it is very difficult to obtain color appearance other than conventional black or dark blue while keeping high performance at the same time. Several approaches have been suggested to realize colored absorbers, including silicon nanostructures [13], metallic nanoshells [14], spectrally selective colored paints [15–17] and colored solar selective absorbing coatings [18, 19]. The silicon nanostructures can produce a wide spectrum of colors due to the strong resonant light scattering properties of nanostructures. The metallic nanoshells can produce tunable color because its resonance frequencies can be tuned by varying nanoparticle geometry. However, neither of the two kinds of colored absorbers has high absorption in the whole solar radiation spectrum and low thermal emissivity in infrared wavelengths, which is an essential characteristic for solar thermal applications. The spectrally selective colored paints can provide different colored absorbers and easy to be used. But they suffer from higher thermal emissivity (>20%) and poor energy performance since they are comparatively thick (>1 μm) [15–17]. The colored solar selective absorbing coatings are formed by absorbing layer itself or two absorbing layers with different component plus anti-reflection (AR) layer, or metal and dielectric multilayers. However, their color appearance range and energy performance still can't be optimized simultaneously [18, 19]. Additionally, their limited colors are generated from different material and different number of layers, which increases the complexity of manufacture procedure and cost.
We promoted a colored solar absorber structure which can generate almost all the colors while keep very high performance and be suitable for many kinds of substrates including flexible substrate in previous work [20]. However, one absorber has only a monotonous color in all previous works. Colorful solar absorber with different colors or patterns on a single substrate still has not been developed and reported up to now, which is a new way and choice for a number of solar applications, especially for architecture integration and military camouflage. In this work, we present a monolithic integrated colored solar selective absorber array based on a multilayered structure of Cu/TiNxOy/TiO2/Si3N4/SiO2. A colored solar selective absorber array with 16 colors has been demonstrated by using combinatorial deposition technique.
2. Structure and model
An infrared (IR) reflector/absorption layer/dielectric stack structure has been utilized in our previous work, demonstrating that it is an effective way to produce a variety of colors and high energy performance by introducing dielectric stack onto absorption layer [20]. The color can be tuned by the thicknesses of each layer while keeping high solar absorptivity and low infrared emissivity. The adjustable thickness range of each layer is wide to have high absorptivity of the structure. However, one absorber has only a monotonous color. How to realize colorful solar absorber with different colors or patterns on a single substrate then? We proposed a novel monolithic integration of colored solar selective absorber array based on the above multilayered structure by integrating different colored absorbers with different thickness of certain layer(s) on a single substrate, as shown in Fig. 1(a). The color and pattern can be designed and tuned arbitrarily and independently by changing the thickness and shape of each absorber unit. In this work, Cu/TiNxOy/TiO2/Si3N4/SiO2 mutilayer is proposed to implement this structure both in theory and experiment.
Fig. 1 (a) Schematic diagram of monolithic integrated colored solar selective absorber array with structure of IR reflector/absorption layer /dielectric stack. (b) Schematic diagram of monolithic integrated colored solar selective absorber array with different thickness of SiO2.
Colored solar selective absorber array can be obtained by producing the dielectric stack array with different thicknesses. The thickness of the TiO2/Si3N4/SiO2 three-layered dielectric stack can be changed by each layer alone, two or three layers simultaneously. First, we demonstrate colored solar selective absorber array by changing the layer’s thickness of SiO2. Other layers' thicknesses were optimized to appropriate values as initialization parameters to make a good balance between the color appearance and energy performance. They are 70 nm, 16 nm and 46 nm for TiNxOy, TiO2 and Si3N4 respectively. The thickness of SiO2 was adjusted from 60 nm to 180 nm with an interval of 8 nm on different areas of a single substrate, forming a 4 × 4 colors solar selective absorber array. The schematic diagram of monolithic integrated colored solar selective absorber array with different thickness of SiO2 is shown in Fig. 1(b).
In order to ensure the experimental results agree with the theoretical designed ones, the experimentally determined optical constants of each layer were obtained. Cu, TiNxOy, TiO2, Si3N4 and SiO2 single-layer film was deposited respectively on K9 glass and Si substrates, then the optical constants of each layer were derived from the measured transmittance, reflectance spectra and ellipsometric spectroscopy. The detailed derivation process of optical constants was presented in our previous article [20,21].
For solar selective absorption, the absorber should have high solar absorbance (α) in solar radiation region and low thermal emissivity (ε) in thermal infrared region at the same time. To obtain such spectrally selective property, a suitable complex refractive index should be obtained first. TiNxOy films exhibit excellent spectrally selective properties and can be used as solar selective absorbing coatings when deposited on highly infrared reflective metal substrates such as Cu or Al [21]. The complex refractive index in the wavelength range 0.3-2.5 μm of Cu and TiNxOy films used in the structure is shown in Fig. 2(a). The Cu film with thickness thicker than the skin depth can supply low thermal emissivity. It shows that the dispersion of refractive index n (solid line) and extinction coefficient κ (dotted line) of TiNxOy film change sharply at visible wavelengths. The n value shows a minimum at 0.581 μm and thereafter it increases with wavelength but the increase tendency is more and more slow. The κ value also shows a minimum but at 0.418 μm and thereafter it increases with wavelength sharply and then tends to a constant about 1.5 after 1.0 μm. This high dispersion comes from the intraband and interband transitions of TiNxOy material [21], which is very useful for composing the colors and chosen as solar absorption layer.
Fig. 2 (a) Refractive index n and extinction coefficient κ of the TiNxOy, Cu films. (b) Absorption coefficient of the TiNxOy. (c) Refractive index n and extinction coefficient κ of TiO2, Si3N4 and SiO2 dielectric films..
To better understand the absorption properties of TiNxOy film, the absorption coefficient was calculated as shown in Fig. 2(b). The absorption coefficient β is related to the extinction coefficient κ as follows
The TiNxOy film shows high absorption coefficient in near-ultraviolet region (3.1 × 105 cm−1 at 300 nm). There exists a minimum value of β at 463 nm in the visible light region, which is the reason why most of the traditional TiNxOy based absorbing coatings are blue. Since 463 nm, the absorption coefficient increases with wavelength sharply and reaches to a maximum value about 2.01 × 105 cm−1 at 800 nm, then it decreases with the wavelength and the value at 2.5 μm is only about one-third of the maximum value at 800 nm, it will further decrease as the increase of wavelength. These results indicate that TiNxOy film is high absorption in solar radiation region and transparent in thermal infrared region, which shows excellent properties of spectrum absorbing selectively.
With the help of the dielectric stack, some narrow interference peaks in visible region can be produced, therefore a variety of colors can be obtained. In addition, TiO2, Si3N4 and SiO2 are chosen as the dielectric stack layers with gradient refractive index to decrease the reflection loss of solar radiation except for the interference peaks in visible region. The optical constants of TiO2, Si3N4 and SiO2 dielectric films are shown in Fig. 2(c). The refractive index of TiO2, Si3N4 and SiO2 decreases from 3.0 to 2.3, 2.0 to 1.8 and 1.50 to 1.43 with increase of wavelength, respectively. The extinction coefficient of Si3N4 and SiO2 film is close to 0, except for TiO2 at near ultraviolet due to the interband transition. The function of dielectric stack is demonstrated in our previous paper in detail [20].
Since the size of the structure in x, y plane is far larger than the wavelength concerned, the optical property of the structure was simulated by the optical transmission matrix method (TMM). Both in the numerical simulation and experiment, the thickness of Cu was set as 150 nm, which was thicker than the skin depth so that the transmissivity could be regard as 0. The thickness of TiNxOy, TiO2, and Si3N4 were set as 65 nm, 16 nm and 40 nm, respectively. The thickness of SiO2 were adjusted independently from 40 nm to 180 nm with an interval of 8 nm while the other layers remained unchanged.
3. Experimental details
In the experiment, a combinatorial deposition technique was employed to fabricate the colored solar selective absorber array on a single substrate, which has been successfully developed for high-efficiency fabrication of integrated filter array [22–24]. In the aid of masks and selective deposition in different areas, an absorber array integrated with 2N elements needs only N times of deposition processes. Figure 3(a) shows the diagram of fabrication procedure by the combinatorial deposition technique. Two different thickness areas (colors) form when a thin film with thickness of h was selectively deposited in the aid of mask 1. Four different thickness areas (colors) form when another thin film with thickness of 2h was selectively deposited in the aid of mask 2. Similarly, 2N different thickness areas (colors) need only N times of combinatorial deposition. The shape and size of each unit depend on the masks used, which can be square, rectangular, triangular, circular or other shapes with size from several micrometers to meters. The pattern can be designed according to practical demands. It is very flexible and efficient.
Fig. 3 (a) Schematic diagram of the fabrication procedure for colored solar selective absorber array by using the combinatorial deposition technique. (b) Photo of the fabricated 4 × 4 colored solar selective absorber array on a 60 mm × 60 mm glass substrate (taken in direct sunlight).
A monolithic integrated colored solar selective absorber array with different thickness of SiO2 has been fabricated for demonstration. Firstly, the multilayered structure of Cu/TiNxOy/TiO2/Si3N4/SiO2 with thickness of 150 nm, 70 nm, 16 nm, 46 nm and 60 nm is deposited onto substrate sequentially. Then, the combinatorial deposition processes have been run 4 times to deposit extra SiO2 iteratively, with thicknesses of 8 nm, 16 nm, 32 nm and 64 nm, respectively. All films were prepared by magnetron sputtering with a homemade multi-targets magnetron sputtering system. The detailed sputtering parameters were given in our previous article [21]. A 4 × 4 colored solar selective absorber array was fabricated on a 60 × 60 mm glass substrate (Represented by U1, U2, ..., and U16 with the increase of SiO2 thickness). The photo of the patterned colorful sample is exhibited in Fig. 3(b), showing a pattern of mosaic. It can also be designed and fabricated in other patterns or forms according to practical demands, such as circular or triangular units.
Reflection (R) and transmission (T) spectra of all samples in the visible (VIS) and near infrared (NIR) regions (0.3-2.5 μm) were measured by Perkin Elmer Lamda 950 UV/VIS/NIR spectrometer equipped with an integration sphere. Infrared spectra of all samples were obtained with Bruker IFS 125HR Fourier transform infrared (FTIR) spectrophotometer in the range of 2.5-25 μm (4000-400 cm−1). The film thickness was measured by Dektak 8 Stylus Profiler.
The solar energy photothermal conversion performance of solar selective absorbing coatings is usually characterized by two main parameters: solar absorptivity (α) and thermal emissivity (ε). Solar absorptivity was calculated in the range of 0.3-2.5 μm, which covers almost all of the solar radiation energy at AM1.5. Thermal emissivity was calculated in the range of 2.5-25 μm at temperature of 373 K (100 °C). They are defined as the formulas mentioned in our previous article [21].
The color appearance of the colored absorbing coatings is described quantitatively in the International Commission on Illumination (CIE) 1931 XYZ color spaces [25]. The chromaticity coordinates were pointed on the chromaticity diagram, which represents all chromaticity visible to the average person. The gamut of all visible chromaticity on the CIE plot is a horseshoe-shaped figure shown in color. The curved edge of the gamut is called the spectral locus and corresponds to monochromatic light, while light with a flat power spectrum corresponds to the point (x,y) = (1/3,1/3). The lightness of color is described quantitatively by L* (ranges between 0 and 100, L* = 0 yields black and L* = 100 indicates white), which is defined in CIE 1976 (L* a* b*), intending to mimic the nonlinear response of the eye [26]. D65 CIE standard illuminant, corresponding roughly to the standard daylight illuminant, was used in the simulations and experiments.
4. Results and discussion
4.1 Colored solar selective absorber array with different thickness of SiO2
The color characteristic of the colored solar selective absorber array was obtained based on the simulated and measured reflection spectra in visible range, which are shown in Figs. 4(a) and 4(b). The simulated and measured color coordinates of the colored solar selective absorber array are marked on the chromaticity diagram as shown in Figs. 4(c) and 4(d), respectively. The results show that the colored solar selective absorber array provides a wide color gamut, covering from violet to blue, green, yellow and red. A comparison between the experimental and theoretical results shows they are in good agreement. The corresponding color lightness is presented in Fig. 4(e), which shows considerable color lightness ranges from 11 to 41. The experimental and theoretical results are in reasonable agreement. All these results demonstrate that the structure of Cu/TiNxOy/TiO2/Si3N4/SiO2 is capable of producing plenty of changeable and controllable colors.
Fig. 4 (a) The simulated and (b) measured reflection spectra in visible and near infrared of the 16 elements in the colored solar selective absorber array. (c) The simulated and (d) measured chromaticity coordinates of the colored solar selective absorber array pointed on the chromaticity diagram. (e) The simulated and measured color lightness of colored solar selective absorber array with different thickness of SiO2.
Essentially, the color of the absorber is determined by the reflectance spectrum in visible wavelength range (0.38-0.78 μm). As shown in the Figs. 2(a) and 2(b) and analysised in section 2, the TiNxOy layer on Cu film exhibits strong absorption in visible region. However, it will produce a broad reflection peak because the TiNxOy film exists a minimum value of absorption coefficient at 463 nm, which has been demonstrated in pur previous paper [20]. It is known that a dielectric stack with suitable thickness can produce interference peaks in visible region. When the TiO2/Si3N4/SiO2 dielectric stack is combined with the Cu/TiNxOy system, some narrower peaks will appear in visible region and those peaks can shift easily with the thickness of each layer, which gives a simple way to obtain different colors [20].
From the simulated and measured reflection spectra in Figs. 4(a) and 4(b), it is obvious that there exist two reflection peaks in visible region for each colored solar selective absorber unit, one of which covers from 400 nm to 600 nm while another covers from 600 nm to 800 nm. Both of the two peaks are caused by a combination of interference and absorption in the multilayer structure. The intensity of the two reflection peaks fluctuates around 10%, producing considerable color lightness. It also shows that the intensity of the peak at short wavelength decreases while the peak at long wavelength increases with the thickness of SiO2, which is because the antireflective effect shifts toward longer wavelength with the increase of SiO2 thickness. In addition, the position of the two reflection peaks shifts toward longer wavelength with the increase of SiO2 thickness due to the increase of total optical thickness. The dominant wavelength was tuned from 420 nm to 768 nm, covering most of the visible region. What’s more, the two peaks appeared in the visible region caused blend colors which expanded the range of colors. All these result in a wide color gamut.
The simulated and measured reflection spectra of the 16 units in the colored solar selective absorber array were shown in Figs. 5(a) and 5(b). All these absorbers show low reflectivity (corresponding to high absorptivity) for wavelengths shorter than 1.5 μm while a steep reflection edge appears longer than 1.5 μm and the reflectivity reaches nearly 100% at middle infrared, showing an excellent spectral selectivity. The absorption peak between 9 μm to 10 μm is due to the molecular vibrations absorption of Si3N4 and SiO2 [27]. The energy performance was evaluated by solar absorptance and thermal emissivity. The solar absorptance of the 16 colored solar selective absorbers deduced from the measured reflection spectra are presented in Fig. 5(c) together with the simulated solar absorptivity. There exists a maximum absorptivity when the thickness of SiO2 is 76 nm and the maximum value is up to 95.4%. All the absorbers can absorb more than 92% of the solar radiation. It demonstrates that the experimental results are in good accordance with the theoretical simulation. The thermal emissivity (at 100 °C) of the sample was also deduced from the measured reflection spectra, as shown in Fig. 5(d). The results indicate that the thermal emissivity of all the absorbers increases from 3.3% to 5.5% with the increase thickness of SiO2.
Fig. 5 (a) The simulated and (b) measured reflection spectra of the 16 elements in the colored solar selective absorber array. (c) The simulated and measured solar absorptivity of the colored solar selective absorber array. (d) The measured thermal emissivity of colored solar selective absorber array with different thickness of SiO2.
4.2 Colored solar selective absorber array with different thickness combinations of Si3N4 and SiO2.
The color of the absorbers can also be tuned by two or more layers' thicknesses of the multilayered structure together. Here, we give another example to obtain the colored solar selective absorber array by combining different thicknesses of Si3N4 and SiO2. The thickness of TiNxOy and TiO2 were set as 70 nm and 16 nm, respectively. The combination of four thicknesses of film Si3N4 and four thicknesses of film SiO2 will generate 16 colors too. For instance, four thicknesses of 30 nm, 40 nm, 50 nm and 60 nm for Si3N4 and four thicknesses of 40 nm, 80 nm, 120 nm and 160 nm for SiO2 are combined to form 16 different colors solar selective absorber units as shown in Fig. 6. Other thickness combination also works to form other colorful patterned absorber array easily. The shape and size of the colored absorber units can be designed arbitrarily. Therefore, the pattern and colors of such a multilayered absorber structure can be designed intentionally according to practical demands. It can provide new freedom and flexibility for widening the usage of solar technology.
Fig. 6 Schematic diagram of monolithic integrated colored solar selective absorber array with different thickness combinations of Si3N4 and SiO2.
The calculated reflection spectra of the 16 color units were shown in Fig. 7(a). For the same thickness of SiO2, the reflection peaks shift towards the longer wavelength side with the increase of Si3N4 thickness. In the same way, for the same thickness of Si3N4, the reflection peaks shift to longer wavelength with the increase of SiO2 thickness. All the 16 color units show low reflectivity at NIR for wavelengths shorter than 1.5 μm while a steep reflection edge appears longer than 1.5 μm and the reflectivity reaches more than 55% at 2.5 μm, showing an excellent spectral selectivity. The corresponding solar absorptivity, chromaticity coordinates and color lightness of the 16 color units were shown in Figs. 7(b)-7(d), respectively. The solar absorptivity of most absorber units is higher than 92%. The results show that they can also provide a wide color gamut and considerable color brightness while keeping excellent energy performance. Such a structure can also be fabricated by the combinatorial deposition technique efficiently.
Fig. 7 (a) The simulated (range from 0.3 μm to 2.5 μm) reflection spectra of the 16 absorbers with different thicknesses combinations of Si3N4 and SiO2. The corresponding (b) solar absorptivity, (c) chromaticity coordinates and (d) color lightness of the 16 absorbers.
4. Conclusion
In conclusion, we demonstrate a monolithic integrated colored solar selective absorber array based on the structure of infrared reflector/absorption layer/multilayer dielectric stack. Cu/TiNxOy/TiO2/Si3N4/SiO2 multilayered absorber has been chosen as a promising example. Different color absorbers can be integrated on a single substrate by adjusting the thickness of one layer along or more layers together of the multilayered structure, which can form a pattern of mosaic or more complex patterns. A 4 × 4 colored solar selective absorber array on a 60 × 60 mm glass substrate has been fabricated successfully by using the combinatorial deposition technique which is a very high efficiency and flexible approach for fabricating such structures. Colored solar selective absorber array with more colors, other unit shapes or patterns can also be designed and fabricated by using this technique. Both the energy performance and color appearance of the colored solar selective absorber array were investigated theoretically and experimentally. The designed and experimental data are in good agreements with each other. The color of the absorber array covers a very wide color gamut, ranging from violet to blue, green, yellow and red. The solar absorptivity of all the color units is larger than 92% and thermal emissivity (at 100 °C) maintains low than 5.5%, indicating excellent solar absorbing selectivity. There exists a maximum absorptivity of 95.4% when the thickness of SiO2 is 76 nm. The thermal emissivity of the absorber increases only from 3.3% to 5.5% with increasing the thickness of SiO2. In summary, the patterned colorful solar selective absorber can have colorful appearance and flexible patterns while keeping high energy performance. It provides new freedom and flexibility for extending the utilization of solar technology, especially for architecture integration and military camouflage.
Acknowledgment
This work was partially supported by the Shanghai Science and Technology Foundations (13JC1405902 and 15dz2282100), National Natural Science Foundation of China (NSFC) (61223006), Youth Innovation Promotion Association of the Chinese Academy of Sciences (CAS) (2012189).
rences and links
1. D. Kraemer, B. Poudel, H.-P. Feng, J. C. Caylor, B. Yu, X. Yan, Y. Ma, X. Wang, D. Wang, A. Muto, K. McEnaney, M. Chiesa, Z. Ren, and G. Chen, “High-performance flat-panel solar thermoelectric generators with high thermal concentration,” Nat. Mater. 10(7), 532–538 (2011). [CrossRef] [PubMed]
2. A. Lenert, D. M. Bierman, Y. Nam, W. R. Chan, I. Celanović, M. Soljačić, and E. N. Wang, “A nanophotonic solar thermophotovoltaic device,” Nat. Nanotechnol. 9(2), 126–130 (2014). [CrossRef] [PubMed]
3. X.-F. Li, Y.-R. Chen, J. Miao, P. Zhou, Y.-X. Zheng, L.-Y. Chen, and Y. P. Lee, “High solar absorption of a multilayered thin film structure,” Opt. Express 15(4), 1907–1912 (2007). [CrossRef] [PubMed]
4. N. P. Sergeant, M. Agrawal, and P. Peumans, “High performance solar-selective absorbers using coated sub-wavelength gratings,” Opt. Express 18(6), 5525–5540 (2010). [CrossRef] [PubMed]
5. H. Deng, Z. Li, L. Stan, D. Rosenmann, D. Czaplewski, J. Gao, and X. Yang, “Broadband perfect absorber based on one ultrathin layer of refractory metal,” Opt. Lett. 40(11), 2592–2595 (2015). [CrossRef] [PubMed]
6. A. G. Hestnes, “Building integration of solar energy system,” Sol. Energy 67(4-6), 181–187 (1999). [CrossRef]
7. T. Matuska and B. Sourek, “Facade solar collectors,” Sol. Energy 80(11), 1443–1452 (2006). [CrossRef]
8. Y. Tripanagnostopoulos, M. Souliotis, and T. H. Nousia, “Solar collectors with colored absorbers,” Sol. Energy 68(4), 343–356 (2000). [CrossRef]
9. T. N. Anderson, M. Duke, and J. K. Carson, “The effect of colour on the thermal performance of building integrated solar collectors,” Sol. Energy Mater. Sol. Cells 94(2), 350–354 (2010). [CrossRef]
10. B. P. Jelle, C. Breivik, and H. D. Røkenes, “Building integrated photovoltaic products: A state-of-the-art review and future research opportunities,” Sol. Energy Mater. Sol. Cells 100, 69–96 (2012). [CrossRef]
11. K. M. Trautz, P. P. Jenkins, R. J. Walters, D. Scheiman, R. Hoheisel, R. Tatavarti, R. Chan, H. Miyamoto, J. G. J. Adams, V. C. Elarde, and J. Grimsley, “Mobile solar power,” IEEE J. Photovolt. 3(1), 535–541 (2013). [CrossRef]
12. M. P. Lumb, W. Yoon, C. G. Bailey, D. Scheiman, J. G. Tischler, and R. J. Walters, “Modeling and analysis of high-performance, multicolored anti-reflection coatings for solar cells,” Opt. Express 21(S4Suppl 4), A585–A594 (2013). [CrossRef] [PubMed]
13. L. Cao, P. Fan, E. S. Barnard, A. M. Brown, and M. L. Brongersma, “Tuning the color of silicon nanostructures,” Nano Lett. 10(7), 2649–2654 (2010). [CrossRef] [PubMed]
14. R. Bardhan, N. K. Grady, T. Ali, and N. J. Halas, “Metallic nanoshells with semiconductor cores: optical characteristics modified by core medium properties,” ACS Nano 4(10), 6169–6179 (2010). [CrossRef] [PubMed]
15. Z. C. Orela, M. K. Gundea, and M. G. Hutchins, “Spectrally selective solar absorbers in different non-black colours,” Sol. Energy Mater. Sol. Cells 85, 41–50 (2005).
16. B. Orela, H. Spreizera, L. Slemenik Persea, M. Fira, A. Surca Vuk, D. Merlini, M. Vodlanb, and M. Kohl, “Silicone-based thickness insensitive spectrally selective (TISS) paints as selective paint coatings for coloured solar absorbers (Part I),” Sol. Energy Mater. Sol. Cells 91(2-3), 93–107 (2007). [CrossRef]
17. B. Orel, H. Spreizer, A. Surca Vuk, M. Fir, D. Merlini, M. Vodlan, and M. Kohl, “Selective paint coatings for colored solar absorbers: polyurethane thickness insensitive spectrally selective (TISS) paints (Part II),” Sol. Energy Mater. Sol. Cells 91(2-3), 108–119 (2007). [CrossRef]
18. D. Zhu and S. Zhao, “Chromaticity and optical properties of colored and black solar-thermal absorbing coatings,” Sol. Energy Mater. Sol. Cells 94(10), 1630–1635 (2010). [CrossRef]
19. Y. Wu, W. Zheng, L. Lin, Y. Qu, and F. Lai, “Colored solar selective absorbing coatings with metal Ti and dielectric AlN multilayer structure,” Sol. Energy Mater. Sol. Cells 115, 145–150 (2013). [CrossRef]
20. F. Chen, S.-W. Wang, X. Liu, R. Ji, L. Yu, X. Chen, and W. Lu, “High-performance colored selective absorbers for architecture integratable solar applications,” J. Mater. Chem. A Mater. Energy Sustain. 3(14), 7353–7360 (2015). [CrossRef]
21. F. Chen, S.-W. Wang, L. Yu, X. Chen, and W. Lu, “Control of optical properties of TiNxOy films and application for high performance solar selective absorbing coatings,” Opt. Mater. Express 4(9), 1833–1847 (2014). [CrossRef]
22. S.-W. Wang, X. Chen, W. Lu, L. Wang, Y. Wu, and Z. Wang, “Integrated optical filter arrays fabricated by using the combinatorial etching technique,” Opt. Lett. 31(3), 332–334 (2006). [CrossRef] [PubMed]
23. S.-W. Wang, M. Li, C. Xia, H. Wang, X. Chen, and W. Lu, “128 Channels of integrated filter array rapidly fabricated by using the combinatorial deposition technique,” Appl. Phys. B 88(2), 281–284 (2007). [CrossRef]
24. S.-W. Wang, C. Xia, X. Chen, W. Lu, M. Li, H. Wang, W. Zheng, and T. Zhang, “Concept of a high-resolution miniature spectrometer using an integrated filter array,” Opt. Lett. 32(6), 632–634 (2007). [CrossRef] [PubMed]
25. T. Smith and J. Guild, “The C.I.E. colorimetric standards and their use,” Trans. Opt. Soc. 33(3), 73–134 (1931). [CrossRef]
26. F. W. Billmeyer and M. Saltzman, Principles of Color Technology, 2nd ed. (John Wiley, 1981).
27. S. M. Hu, “Infrared absorption spectra of SiO2 precipitates of various shapes in silicon: calculated and experimental,” J. Appl. Phys. 51(11), 5945 (1980). [CrossRef]
