In this work, a feasible way for perfect absorption in the whole solar radiance range is numerically demonstrated via the multiple resonances in a 600-nm-thick refractory prismoid. Under the standard AM 1.5 illumination, the measured solar energy absorption efficiency reaches 99.66% in the wavelength range from 280 nm to 4000 nm, which indicates only a rather small part of solar light (0.34%) escaped. The record harvesting efficiency directly results from the near-unity absorption for the multi-layer refractory resonators, which can simultaneously benefit from the multi-resonant behaviors of the structure and the broadband resonant modes by the material intrinsic features. The absorption including the intensity and frequency range can be adjusted via the structural features. These findings can hold wide applications in solar energy related optoelectronics such as the thermal-photovoltaics, photo-thermal technology, semiconductor assisted photo-detection, ideal thermal emitters, etc.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Solar energy is a key solution for the crisis on the energy and environment. The purpose for achieving high light harvesting efficiency has long been pursued. Right now, it is even more significant for us to solve the crisis on the environmental pollution and depletion of fossil energy [1,2]. The solar energy occurs every second and the power is much higher than the total consumption of the human activity in the whole earth. Sun light can be trapped and transferred into other kinds of energy such as the electricity  thermo-photovoltaics , and the solar stream power . As for these applications, the key point is to improve the absorption efficiency in the solar radiation spectral range. In 2008, perfect electromagnetic wave absorber in the microwave region has been demonstrated via the metal-insulator-metal metamaterials, which introduced strong plasmonic resonances . Metallic micro-cavities have also been used to excite the surface plasmons for near-unity light absorption in the visible range . Since then, the potential applications in the solar energy absorption via these new approaches have attracted the scientists to improve the techniques step by step. The first effort needs to do is to increase the absorption bandwidth. During the investigations in these years, a typical way to broaden the absorption window was to introduce multiple resonances in the system [8,9]. The multiple resonators  or the sub-resonators  can produce different resonant absorption peaks, related to the structural features such as the size and geometry characteristics, etc. For instance, two- or five-band perfect absorption has been achieved in the metamaterials formed by an unit cell with several sub-resonators with different sizes [12,13]. The effort for artificial design with the peaks occurred at the close frequency range is important for us to achieve a really broadened absorption band. For instance, a super-absorber was achieved in the visible range with the spectral absorption up to 83%  based on the trapezoid-like metal resonator system, which can support several different absorption peaks in the spectrum and therefore lead to the broadband absorption. Discrete metallic nanoparticles with a large size dispersion have also been used to form the black gold absorbers due to the broadband high absorption covering the whole visible light range [15,16]. Based on the tens of paired metal-insulator layers with different patch sizes, ultra-broadband infrared absorption was realized due to the cooperative effects by the tens of resonances [17,18], which was overlapped each other in the wide spectral range. Meanwhile, the sharp metallic cone structures have been demonstrated to efficiently trap light in a wide frequency range .
Recently, other materials have been developed to form the broadband absorption platform. The refractory materials with high absorption loss in a wide frequency range have developed to achieve plasmonic resonances . Broadband light absorption with the frequency range covering the visible light was realized in the refractory metals [21–26] and titanium nitride (TiN) materials [27–29], which confirmed the feasible way to realize the black absorber via using the special material feature. For instance, a broadband absorber was experimentally demonstrated by a thin TiN metasurface layer , showing a near-unity absorption from 400 nm to 800 nm. Moreover, the absorber was observed to be highly stable in structural geometry when a thermal treatment with the temperature of 800 °C and high laser intensity of 6.67 W/cm2 were used to evaluate the structure. It should be noted that the noble metals such as the Au resonator based absorbers were observed to be easily melted under a moderate thermal treatment [31,32]. Therefore, the refractory material based absorbers are with the combination advantageous including the broadband absorption and highly thermal stability, which can promote the applications for solar thermal operation related technologies such as the solar thermo-photovoltaics [33,34]. In our previous report, we combined the TiN and TiO2 resonators to form a bi-resonator system and achieved an ultra-broadband absorption from the visible to the near-infrared range . Metals with high imaginary part  have also been used for the broadband absorbers  based on their strong plasmonic responses. An ultra-broadband absorption was achieved via using a periodic array of tungsten cylinders metamaterial with the absorption range covered from 200 nm to 900 nm . Plasmonic TiN has been demonstrated to interact with the TiO2, which indicated a much higher efficiency generation of hot electrons due to the broadband absorption efficiency in the wavelength range of 500-1200 nm . In addition, refractory metals such as tungsten, nickel and titanium have been developed for broadband absorption. W resonators based absorber was observed to produce a broadband absorption from 300 nm to 1777 nm via using the multiple plasmon resonances by the grating and multilayer structure [40–42]. Based on the refractory metals such as the Ni, Ti, and W, absorbers with the absorption region from 300 nm to 1500 nm were achieved in the metal-insulator-metal structures , which could support much broader absorption bandwidth than that of the noble meals including the Ag, Au, Cu based absorbers. Tungsten/germanium anisotropic nano-cones have also been used for broadband absorption but with a larger height up to several micrometers . As for the wide applications in solar energy related optoelectronics such as the photovoltaics , hot-carrier generation technology , and thermal photovoltaics , the full-spectrum solar energy near-unity absorption is desirable. However, although there were different kinds of reports on the achieving of broadband absorption via these newly emerged materials, it is still unable to realize the exactly black solar absorber to efficiently trapping the light in the whole solar wavelength range.
In this work, we propose and demonstrate a powerful platform to achieve solar energy black absorber via a simple, thin film refractory prismoid, which can produce an absorption efficiency of 99.66% in the solar irradiation full-spectrum wavelength range from 280 nm to 4000 nm in a 600-nm-thick layered structure. Geometry-size related multi-resonant response and the intrinsic broadband resonant mode cooperatively contribute to the full-spectrum absorption. The absorption properties can be widely tuned via the structural parameters and can also even be maintained under a larger polarization and incident angle range even if it is a symmetry-breaking system.
In detail, there are at least three kinds of new insights and findings contributed to the knowledge for the solar absorbers related science and technologies by the investigations in this work. The first is the feasible method for achieving solar full-spectrum perfect absorption via the refractory metals. Herein, the use of refractory metals rather than the dielectrics or noble metals can hold the unique features including the excellent stability under high-temperature situations and the intrinsic broadband absorption response. The solar thermo-photovoltaic models have reported the high efficiency up to 85% by the theoretical studies and attracted great attentions in these years . Nevertheless, the ultra-high operational temperatures are needed for efficient thermal energy conversion, which inevitably hampered the progress in this field since the conventional materials such as the noble metals can be thermally melted [49,50]. The second is the powerful way for realizing multi-resonant plasmonic absorption in the ultra-thin film structure. The near-perfect solar absorption in the whole irradiation region and the deep sub-wavelength structural size (∼ λ/6) are the both desirable factors for applications, indicating the greatly improvement in comparison with that of the systems formed by tens of paired patches [17,18]. The third is the novel design for the cooperative operation of refractory metals and the semiconductors with different energy gap ranges, which can introduce the new insights on the fully using of solar energy for optoelectronic devices including the transformation of solar energy to thermal electricity  and stream generation , and also the hot-electron excitation [53,54], solar photocatalysis  and chemical energy , etc. Moreover, the refractory metals especially for the Ti are with the properties of high biocompatibility, which could pave approaches for solar irradiation assisted photothermal therapy at facile temperature .
2. Structure and model
As depicted in Fig. 1, the solar absorber includes a Ti/SiO2 layered prismoid-like system on an opaque Ti film substrate. The prismoid consists of seven paired Ti/SiO2 layers. The height (h) and width (w) of the prismoid are with the equal value of 600 nm. The film thickness of the Ti and SiO2 layer is 40 nm. The opaque Ti film substrate is with the thickness (t) of 400 nm, which can extremely cancel the light transmission. The electromagnetic resonances and absorption performance are studied by finite-difference time-domain method . Periodic boundary conditions are used in the y-direction. Perfectly matched layers are used in the x and z-directions. The mesh size is down to 2.5 nm. Experimental data of the dielectric permittivity is used for the Ti in the simulation . In this model, the prismoid Ti/SiO2 layered metasurface can introduce multiple plasmon resonances by the Ti nano-patches and the cavity films between the paired multi-layer structure. As the colored distribution patterns shown in the schematic, different resonant modes can be supported under the solar irradiation, which therefore leads to the super-absorption for the solar energy. Experimental realization of this absorber platform can be achieved via the steps similar to the report in the previous investigations for the tapered hyperbolic metamateiral array , where the film deposition and the etching process with the designed mask were both used.
In this work, the spectral absorption absorption A(ω) is under the definition
The R(ω) and T(ω) are respectively represented to the spectral reflection and transmission for the system under illumination. Owing to the using of opaque metal substrate, the light transmission is completely canceled, which leads to a spectral T(ω) = 0. As a result, the absorption A(ω) can be obtained with 1 - R(ω).
For the solar absorption simulation, the spectral absorption efficiency of the absorber ηA can be estimated via according to the Eq. (2), where the IAM1.5(ω) is the incident solar power of the Air Mass 1.5 Global spectrum . The spectral range can be broadened from the UV to the infrared range. The minimal (λMin) and maximal (λMax) wavelengths can also be artificially tuned in the wide range, such as 280 nm and 4000 nm, respectively.
3. Solar black absorber
As shown in Fig. 2(a), light reflection, transmission and absorption are calculated for the Ti/SiO2 layered prismoid-like substrate under normal incidence. It is observed that light reflection is strongly inhibited in a wide wavelength range. Thereby, an ultra-broadband absorption window is achieved. For instance, in the ultra-wide wavelength range from UV to visible and infrared range (0.28-3.54 µm), the minimal absorption A is exceeding 95%, suggesting the bandwidth up to 3.26 µm. It should be noted that the absorption window covering the main sunlight energy range (0.28-2.50 µm) is supported in this thin-film (∼ λ/6) substrate. The absorption performance including the efficiency and the bandwidth are both with great advantages in comparison with the plasmonic absorbers such as the tungsten nanohole array , periodic array of bismuth telluride pyramidal nanostructures , and the refractory resonant structures [63,64].
To well-understand the optical response for the structure, the resonant field distributions at the absorption peaks (λ1-λ4) are shown in Figs. 2(b)–2(e). For instance, at the shorter wavelength range (λ1 = 0.447 µm), the electric field is mainly confined at the top area of the prismoid, suggesting the resonant absorption occurred at this smaller Ti/SiO2 patch particles. With the wavelength increasing from λ2 to λ4, the resonant field distributed area is moving from the top to the bottom positions. These features confirm that the structural size dispersion in the prismoid structure produces the multiple resonant modes, which then lead to the formation of the flat and high absorption spectrum in the ultra-broadband wavelength range.
Figure 3(a) shows the solar absorption for the absorber under the standard sun radiation AM 1.5 source. In the full-spectrum range (0.28-4.00 µm) of the sunlight, the absorption spectral curve shows a nearly perfect absorption. For the sunlight, the solar energy is distributed in the ultra-broadband wavelength range. The previous reports were mainly focused on the achieving of high absorption in the shorter wavelength range such as the visible range [12,15,19,27]. Nevertheless, for a perfect solar absorber, it is necessary to absorb sun energy in the whole range. Based on this absorber platform, the solar energy is extremely absorbed in the full spectrum. The absorption efficiency ηA reaches 99.66%. That is, only 0.34% solar energy is missed for the sun power. In contrast, light absorption with A>87% was achieved in a broadband (450–850 nm) spectrum in a well-defined geometry in two-dimensional arrays of sharp convex gold grooves . A broad absorption over the whole visible range of 400–800 nm was obtained by a refractory Titanium nitride (TiN) metamaterial absorber . Absorption above 90% was achieved over the wavelength range of 0.40–1.40 µm based on one ultrathin layer of the refractory metal chromium . Broadband absorption with the wavelength range from 300 nm to 1500 nm was achieved via using the plasmonic resonances of the refractory metals . These results suggest the achievement of the extremely black solar energy absorber via this prismoid structure.
In addition, the solar absorber is formed via the refractory metal, suggesting the high tolerance of temperature. For instance, the melting point for Ti is up to 1940 K. The W is also with a high melting point of 3500 K. Thereby, the absorber is with wide applications in the high-intensity operation process such as the laser nonlinear optics. Moreover, based on the Kirchhoff's law, the spectral absorption is equal to the emission. Blackbody or the full-spectrum absorber, the so-called black absorber can also produce the radiation or the thermal emitting. For instance, an enhanced solar thermal conversion was achieved in the wavelength range from 540 nm to 900 nm via using the metal-dielectric core-shell nanoparticles . Infrared selective emitters in the near-infrared region were also obtained in the nanostructured thin films . A tungsten binary pyramid grating was used to significantly increase the emittance, which introduced nearly perfect emitting in the wavelength region from 0.6 to 1.72 µm . Figure 3(b) shows the spectral emittance for the black solar absorber via the thermal treatment under a temperature of 1900 K. The ideal blackbody emission under this temperature is also shown. It is observed that the proposed emitter can reproduce the thermal emitting very well to that of the ideal model.
4. Absorption properties and manipulation features
As shown in Fig. 4(a), solar absorbers formed by other refractory materials such as the Cr, Ni, TiN, V and W can still produce extremely high absorption in the wide wavelength range. For instance, the TiN based absorber shows the spectral absorption efficiency up to 97.1% (90.1%) in the wavelength range of 0.28-4.00 µm (0.28-5.00 µm), suggesting a much higher performance for the efficiency and spectral bandwidth in comparison with that of the previous reports [27,30,40]. Nevertheless, the absorption performance is much weakened when the conventional noble metals such as the Au, Ag and Al are used to replace the Ti in the absorber. Figure 4(b) shows the absorption responses for these plasmonic metals based absorbers. It is observed that multiple sharp and narrowband absorption peaks are excited in the shorter wavelength range while the relatively low absorption occurs in the longer wavelength range. That is, the refractory metals based system can achieve much broader absorption than that of the conventional metals. This mainly results from the distinct plasmonic resonant behaviors for them. The conventional metals such as the noble metals can support strong surface plasmon resonances in the visible and near-infrared range. The refractory metals can also support plasmon resonances but with broadband modes due to their intrinsic high imaginary parts in the wide frequency region [36,37]. Figure 4(c) shows the comparison results for these two kinds of metals with the average absorption efficiency in the wide frequency range. For the refractory metals based system, the minimal average absorption in the wavelength range (0.28-4.00 µm) is even above 93%. The maximal value reaches 98% for the Cr based absorber. In the larger wavelength range (0.28-5.00 µm), the maximal/minimal absorption efficiency is 94% (Ti) and 77% (W). The wider wavelength range up to 5.00 µm used in the study is to show the potential applications for the infrared devices operated in the longer wavelength based on this absorber. Nevertheless, for the Au, Ag and Al based systems, the maximal absorption efficiency in the wavelength range (0.28-4.00 µm) is 57%, which is much smaller than that of the Cr based system. For the Au based absorber, the average absorption efficiency is just 34%, suggesting a very high loss for solar absorption. The relatively low absorption efficiency for the noble metals based system mainly results from the intrinsic narrowband resonant absorption by the plasmonic oscillations [7,12]. As a result, only narrowband absorption peaks can be excited in the spectral range.
Following, the absorption evolution for the system under different structural parameters is studied. Figure 5(a) shows the absorption evolution for the Ti/SiO2 prismoid under a tuning of the number of the paired layer. With increasing the number of the paired layers for the Ti/SiO2 prismoid, the absorption is strongly enhanced. This is the main result of the multiple resonant absorption bands for the multi-layer structure. As the results of the average absorption efficiency shown in Fig. 5(b), the absorption is increased from 42% to 89% with the number of the layer from 0 to 3 in the full solar energy range (0.28-4.00 µm). Then, a slight absorption increase of ∼9% is achieved with the number of the layer increased from 3 to 7. In particular, there is only 0.2% improvement for the absorption when the number is increase from 6 to 7. That is, the optimal layer number is just about 7, suggesting a much smaller number of the layers and much thinner of the structure for the ultra-broadband absorbers than the others [17,18].
Absorption for the Ti/SiO2 prismoid under oblique illumination is shown in Fig. 6. The spectral absorption maintains with high efficiency in the whole range when the incident angle is increased from 0° to 70°. Overall, the absorption efficiency for the system under the large incident angle (60°) still remains the near-perfect absorption with the A ∼ 90%. For instance, the A is even still up to 95% for the absorber in the solar irradiation range when a 45° incident angle is used. In the angle range of 0°-30°, the spectral solar absorption efficiency is exceeding 97%. The absorption efficiency becomes to be weakened when the much larger angle is used. For instance, the A is down to 78.6% when a 70° oblique angle is used. These features directly confirm the angle-insensitive solar absorption and suggest the high tolerance of the solar absorber for oblique sunlight absorption [21,27,69]. The strongly confined resonant modes by the Ti/SiO2 patches and cavities are the main contributions for this angle-insensitive absorption.
For this grating-like prismoid structure, the polarization states can introduce effect onto the resonant behaviors. As the results shown in Fig. 7, under normal incidence, the spectral average absorption is reduced from 98.1% to 66.8% when the polarization angle is changed from 0° (electric field along the y-direction) to 90° (electric field along the x-direction), suggesting a remarkable reduction for the absorption under the different polarization states. Moreover, under the oblique excitation with the incident angle of 40°, the average absorption is reduced from 91% to 51.5% when the polarization angle is changed from 0° to 90°, showing a ∼ 40% decrease. These features both confirm the polarization dependent absorption for this system and suggest alternative way to manipulate the absorption efficiency.
5. Solar energy absorption for the platform formed with semiconductors
Finally, the absorption properties for the platform formed with different semiconductors are investigated. Figure 8(a) shows absorption comparison for the absorber formed by the Ti/SiO2 or the Ti/semiconductors. As for the optoelectronic devices, the active materials such as the semiconductors should be included. In this absorber platform, we use semiconductors to replace the silica buffer layer. Moreover, different semiconductors with different bandgaps are utilized to intercalate within the gap area between the resonant Ti patches. The Ti-Semiconductor composite absorbers are formed to act as the photo-electrical models for potential applications. As shown in Fig. 8(a), the absorbers consisting of different semiconductors can still produce strong light absorption in the ultra-broadband wavelength range. Figure 8(b) shows the plotted average absorption for these absorbers under different wavelength ranges. The absorption efficiency reaches ∼95.6% for the Ti(TiO2-CdS-ZnSe-AgGaSe2) absorber. The other Ti(TiO2-CdS-ZnSe-Ge) absorber also presents a high absorption of 94.4% in the solar radiation range. In comparison with that of the Ti/SiO2 absorber, the absorption efficiency for the Ti-semiconductor composite system shows a slight reduction. The reduction absorption of about ∼3% mainly results from the relatively high reflection by the high-index dielectrics. Nevertheless, it still should be noted that near-perfect absorption is obtained for the semiconductors based absorber in the whole solar energy wavelength range. The semiconductors based absorbers with near-unity absorption would introduce a new powerful way to solar energy related optoelectronic devices.
In conclusion, we have proposed and demonstrated an extremely black solar absorber. In the solar energy wavelength range (0.28-4.00 µm), the average absorption efficiency reaches 99.66% in the 600-nm-thick Ti/SiO2 prismoid under the standard AM 1.5 illumination. Moreover, the excellent absorption efficiency is maintained well with only a slight reduction of 3% when the incident angle is up to 45°, suggesting the angle-insensitive absorption. Furthermore, the high efficiency is only with a fluctuation of 7% with the polarization angle changed from −30° to 30°. The absorption efficiency reaches ∼95% for the Ti/Semiconductors absorber in the solar radiation range. These findings can hold wide applications in solar energy related optoelectronics.
National Natural Science Foundation of China (11564017, 11664015, 11804134, 51761015, 62065007); Natural Science Foundation of Jiangxi Province (2018ACB21005, 20182BCB22002, 20181BAB201015, 20202BAB201009).
The authors declare that they have no competing interests.
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