Scalable selective absorber with quasiperiodic nanostructure for low-grade solar energy harvesting

As the current energy crisis compels humanity to search for alternatives to fossil energy industry,¹ realizing clean and permanent energy harvesting has become a long-standing developing strategy. Solar energy has been considered as one of the most ideal available resources because of the stability of optical field, enabling broad applications in solar photovoltaic,^2,3 solar-thermal,^4,5 and solar-chemical approaches.⁶ Thereinto, the solar-thermal technology with a minimum carbon footprint can convert photons into thermal energy directly, which has been proposed as an efficient solar-trapping component that can be integrated in solar-thermal structural engineering for desalination^7–9 and energy generation.^7,10–12 The most vital demand in the technology is attributable to the efficient net heat accumulation for highly effective solar-thermal conversion under a low solar radiation flux.^9,13,14 Under this case, a subwavelength plasmonic nanostructure^15–17 and carbon-based polymeric materials^10,18,19 have been widely exploited to achieve efficient broadband sunlight absorption and rapid heat relaxation. However, non-negligible heat dissipation always remains in the non-equilibrium thermalization and can even exceed 60% when the blackbody absorber reaches 100 °C.^14,20,21 Thus, restraining the excessive thermal radiation while maintaining a high absorbance is a critical consideration. To address this drawback, some efforts have focused on periodic plasmonic structures with selective spectral absorption, which is beneficial to limit the absorption in the desired cutoff wavelength range compared to most disordered organic polymers^8,22,23 and has been gradually explored for solar vapor generation and water heating with commerciality.^24,25 Nevertheless, the advanced strategies with a direct spectral selectivity are always restricted by unscalable costly nano-engineering,^9,23,26 which limits the future widespread application. Therefore, developing a highly versatile strategy for theoretically guiding designs of selective solar-thermal devices and economical large-scale popularization is of great significance in fundamental research and industrial implementation.

Herein, we propose a selective absorber with a quasiperiodic nanostructure for scalable solar-thermal engineering, which is verified to have a significant solar absorption and passive suppression of thermal radiation, possessing an ideal solar-thermal conversion. We investigate the hybrid interactions in the nanostructure and the energy evolution during the non-equilibrium thermalization, finding that the far-field thermal radiation can be suppressed by the arranged density of the plasmonic nanostructure due to near-field in-phase interactions and negative-temperature-correlativity, which induces a selective spectral absorption in the quasiperiodic nanostructure. By abandoning the cumbersome and costly nano-fabrication, the scalable solar absorber can be fabricated via the cost-effective surface self-assembly of quasiperiodic densely arranged Fe₃O₄ nano-particles, with a high absorbance of >94% and passive suppression of thermal emissivity of <0.2. For practicality, by synergizing material properties, thermal management, and environmental effect, a flexible planar solar thermoelectric harvester is demonstrated under natural sunlight (AM1.5G), reaching a sustaining open-circuit output of >20 mV/cm². At lower ambient temperatures, the performance can be prominently increased, over 20%.

In nature, as a low-grade heat source, solar radiation density absorption localizes mainly from ultraviolet (UV) to near-infrared (NIR). The solar-thermal devices always emit energy by lossy thermal radiation across mid-IR range²⁷ (MIR, 8–14 μm) to atmosphere while obtaining solar energy across UV–NIR [shown in Fig. 1(a)]. The efficiency²⁸ of solar-thermal η_OT is always affected by solar absorbance and thermal losses during the thermalization as $ηOT=IIsolar=∫dλα(λ)I(λ)∫dλI(λ)−εeffT∫λ1λ1Ibλ,Tdλ$⁠. Here, α_(λ) and I_(λ) represent the monochromatic absorbance of solar absorbers and the monochromatic solar radiation intensity, respectively, ɛ_eff(T) is the equivalent thermal emissivity of the absorbers, and $∫λ1λ1Ibλ,Tdλ≈σ(Tamb+ΔT)4$ depicts the radiative intensity of a blackbody. As the solar thermal conversion reaches equilibrium state, I_solar is constant. The temperature-rise of solar-thermal devices ΔT is written as

As one can see from Eq. (1), the high absorbance and low emissivity can increase the values of ΔT. Therefore, a significant temperature rise can be generated quickly by a solar absorber with significant selective absorption and low thermal emissivity, which can provide the possibility to enhance the practicality of solar energy harvesting. Meanwhile, a decline in the ambient temperature T_amb can also increase the solar-thermal performance, which is beneficial for application in cool outdoor environments. In this work, we proposed a selective absorber composed of densely arranged Fe₃O₄ nano-particles coated on the graphite–metal substrate, as illustrated in Fig. 1(b). The nanostructure is patterned in a quasi-C6-symmetry and possesses a superior structural stability in 2D configuration, whose natural quasiperiodic structure provides the possibility of scalable selective solar-thermal conversion. Based on the highly versatile strategy, we promote a scalable guideline for exploring a photon–heat–electron conversion by integrating a flexible planar solar thermoelectric harvester for low-grade solar energy, which is expected to provide a platform for the next-generation, high-performance, economical, and practical solar co-harvesting systems.

Under the weak electromagnetic force, the value of the equilibrium separation of nano-particles fluctuates around the particle diameter. Therefore, the group of plasmonic nanostructure can approximate the face-centered cubic lattice with structural stability at mesoscopic scale.²⁹ Dependent on the reduction in periodicity, we approximated an equivalent lattice-spacing C6 rotational symmetric unit-cell including six coupled adjacent and one central plasmonic nanostructure. When the nanostructure is excited with optical illumination, forward scattering is usually enhanced in the Mie-mode, which induces a strong plasmonic resonance.^30,31 In a non-Hermitian system, the non-equilibrium thermalization drives ultrafast heat evolution for heating up along with thermal radiation. As shown in Fig. 2(a), the adjacent plasmonic nanostructure in the same phase couples in near-field region and modulate far-field thermal emission around the resonant frequency. As described in quasinormal mode theory,³² the reduced radiation flux from nano-resonators is the sum of the contributions from thermal emissions at multiple resonant modes.³³ According to the hybrid nanostructure in this work, the normalized thermally excited radiation spectra to far-field at multiple predominant resonant modes from a single nanostructure is plotted in Fig. 2(b). Multiple narrow-band peaks appear in the visible regime, which indicates that the hybrid nanostructure has broadband radiation/absorption based on Kirchhoff’s law. In contrast, a single predominant resonant mode appears in the MIR regime, which benefits the modulation of the far-field thermal radiation.

First, we investigated the optical absorption of the densely arranged plasmonic nanostructure in visible regime, which is preset to separate a distance of 30 nm. In the visible regime, the electric field is nearly bounded around a graphite layer and Fe₃O₄ nano-particles and generates sharp thermalization for increasing the temperature of the absorber [Figs. S2.1(a) and S2.1(b)]. To maximize the energy absorption in terms of energy balanced equation, a genetic algorithm is adopted to optimize the parameters of graphite layer thickness and particle size. Considering the preparation process of Fe₃O₄ nano-particles and graphite layer, the particle radius and layer thickness are difficult to be over the limits, 280 and 50 nm, respectively. In this case, the optimized particle size and the graphite layer thickness are r = 260 nm and d = 45 nm, respectively, where the solar absorption efficiency can be raised to 96.5%, as shown in Fig. 2(c). Furthermore, Fig. 2(d) simulates the average absorbance spectra across UV–NIR spectrum 0.3–2 μm and MIR regime 8–14 μm, respectively. The absorbance of different polarized light both maintains high efficiency in the UV–NIR regime and low efficiency in the MIR regime, which indicates that the absorber can obtain selective spectral absorbance. In addition, compared to scattered optical energy, over 94% energy can be converted to Ohmic losses inside the plasmonic nanostructure, beneficial for local heating and solar-thermal conversion [Fig. S2.1(d)].

As for the lossy thermal radiation of the absorber, coupled near-field interactions between the adjacent plasmonic nanostructures inside the periodic array become a major factor. By means of the predominant resonant mode in the infrared range in Fig. 2(e), we calculated the spectral energy flux of far-field thermal emission varying with the arranged density of the plasmonic nanostructure in Fig. 2(f) (for details, see the supplementary material S1). As one can see, the intensity of far-field thermal emission can be negatively correlated with the arranged density of the nano-particles by means of the enhanced coupled near-field interaction. The scale effect of the plasmonic nanostructure not only weakens the narrow band thermal emission at the predominant resonant frequency but also induces the departure of the resonant mode from the atmospheric window. Furthermore, the absorber often stably operates below 100 °C under one sunlight illumination and the emission spectrum is mainly located in the MIR regime, which overlaps the atmospheric window region (8–14 µm). According to Wien’s law, the thermal emission spectrum of the blackbody will blueshift to that of the densely arranged plasmonic nanostructure as the temperature rises; therefore, the far-field equivalent thermal emissivity will be weakened reversely, following³⁴ 

Therefore, we could calculate the equivalent thermal emissivity (8–14 µm) of the absorber under varying temperatures, as shown in Fig. 2(g). One can see that the equivalent far-field thermal emissivity declines with an increase in the heating temperature and maintains below 0.2, which displays the ideal passive suppression of thermal radiation. Furthermore, we calculated the temperature rise and total solar-thermal efficiency with varying ambient temperatures in Figs. S2.2(b) and S2.2(c), and the time-evolution of the average temperature rise with a low temperature (273 K) and room temperature (293 K) is plotted in Fig. 2(h). The selective absorber can sustain a high thermal response before the thermal equilibrium, and the average temperatures rise rapidly to the steady state at ∼120 s, which indicates the fast thermalization of the plasmonic nanostructure under illumination. In the lower ambient temperature 273 K, the absorber can generate a higher temperature rise of ΔT_H ≈ 80 K, compared to that in the room temperature 293 K (ΔT_H ≈ 70 K). As a result, the absorber features fast thermalization performance and significant temperature rise under natural solar illumination and displays low-temperature enhanced conversion performance.

For demonstration, we developed a scalable fabrication of the selective hybrid solar absorber [Fig. 3(a)]. The absorber was fabricated via the formation of graphite–copper substrate and the self-assembly of the monolayer Fe₃O₄ nano-particles film, by means of a simple thermal evaporation and the Langmuir–Blodgett technique.^35,36 Figure 3(b) shows the photograph of the samples and SEM images of the surface of the absorber. The Fe₃O₄ nano-particles are mostly self-patterned in the densely arranged structure (quasi-C6-symmetry), coating the graphite–copper substrate. Figure 3(c) displays the experimental absorbance spectrum, which has a good agreement with the simulated results, especially in the visible light region. The selective solar absorber approaches an efficient absorbance of >94%, which demonstrates a high-efficiency solar-thermal conversion. In contrast, the significantly reduced absorption in the MIR regime shows the selective spectral absorption performance. Although the unflatness of the commercial substrate causes quasiperiodicity of Fe₃O₄ nano-particles array, the absorber still maintains an excellent solar absorption capacity, making it a cost-effective attraction. As for thermal radiation, we focused on the equivalent emissivity of the solar absorber in MIR range (8–14 µm). The experimental results of the equivalent emissivity (8–14 µm) are shown in Fig. 3(d). The equivalent emissivity declines gradually as the temperature increases and always maintains an extremely low value of ∼0.2 at the steady-state working temperature, which approximates the theoretical analysis. Figure 3(e) presents the solar-thermal performance of the selective absorber under the solar simulator illumination (AM1.5G). After 120 s of illumination, the temperature rise of the sample increases to ∼60 °C in the room temperature (293 K) environment (T = 80 °C). In contrast, the temperature rise of the absorber increases significantly to ∼73 °C in a lower temperature (273 K) environment (T = 73 °C). The results demonstrate that the absorber possesses an ideal selective solar absorption and a superior low-temperature enhanced solar-thermal conversion performance under one sunlight illumination. In the future, the preparation of substrates with a flatter surface is expected to further improve the quality and performance of the absorber by means of more precise methods, such as electrochemical technology.

Due to the planar thermalization, the strategy possesses promising advantages to develop a flexible energy harvester for low-grade solar energy. Figure 4(a) illustrates a flexible planar solar thermoelectric harvester integrated by the selective absorber and an in-plane thermoelectric chip. The simple pattern-cascaded configuration supports the scalable fabrication through a single- or multi-layer circuit package and possesses a lower electronic cost. Dependent on the remarkable temperature rise of the absorber, a more significant thermoelectromotive force can be generated according to the supplementary material Eq. (14). In particular, the thermoelectric configuration and the inner parasitic heat flow are considered in the design to improve the thermoelectric conversion performance. As shown in Figs. 4(b) and 4(c), we calculated the effect of conical angle θ and height–length ratio ξ on the thermoelectric conversion performance by modeling thermoelectric transport under spatial transformation, respectively (for details, see the supplementary material S4). As one can see, the in-plane conical thermoelectric configuration could achieve structural scalability and internal constrictive resistance, enabling a more significant thermoelectromotive force across a compact structure. The average V_oc values curve of a 12-np-pair pattern-cascaded thermoelectric chip is simulated in Fig. S4(e). After a period of 45 s approximately, the stable open-circuit voltage (V_oc) distributions at 273 and 293 K in a heat open boundary condition are shown in Figs. 4(d) and 4(e), where the device demonstrates a superior V_oc (33 mV) in the room-temperature and (40 mV) in a low-temperature, respectively. The harvesting performances of the flexible planar solar thermoelectric harvester were characterized by using the measurement system shown in Fig. 4(f). We simulated one solar illumination and tested the currents and voltages across the output electrodes under two practical cases: a low-temperature environment and a room-temperature environment (273 and 293 K, respectively), which were simulated by an incubator (the two cases can be maintained for long periods). As shown in Fig. 4(g), the open-circuit voltage per unit illuminated area V_oc raises quickly after 1 min illumination without a heat sink, reaching 20 mV/cm² at T = 293 K and 24 mV/cm² at T = 273 K respectively. Moreover, the overall output power per unit illuminated area W of the solar thermoelectric harvester was calculated by multiplying the voltage and the current. The W increased to ∼18 nW/cm² at the room temperature (293 K) and ∼24 nW/cm² at a lower temperature (273 K) without a load, respectively, which demonstrates enhanced harvesting at a lower-temperature environment, over 20%. In more general practical applications, we believe that a higher efficiency output will be achieved by impedance matching.

It should be acknowledged that a restriction of heat exchange and thermoelectric conversion is non-negligible between the solar absorber and the thermoelectric chip, which could be solved by developing an insulating package with a better heat transfer in the future. Nevertheless, the design concept of the solar-heat-driven thermoelectric harvester still demonstrates a remarkable low-grade solar energy harvesting, opening up a promising strategy for scalable applications in the field of photovoltaic-thermoelectric solar co-harvesting.

In this work, we develop a quasiperiodic scalable selective absorber with high-performance solar-thermal conversion for low-grade solar energy. Due to the scale effect of the densely arranged plasmonic nanostructure, the intensity of far-field thermal emission can be suppressed, which addresses the ideal selective spectral absorption and negative-temperature-correlated thermal radiation. By abandoning the cumbersome and costly nano-fabrication, a selective hybrid solar absorber is fabricated by the economical widespread surface self-assembly of quasiperiodic densely arranged Fe₃O₄ nano-particles on the graphite–metal substrate, leading to a significant solar absorbance of >94% and passive suppression of thermal emissivity of <0.2. By means of the strategy, a flexible planar solar thermoelectric harvester is fabricated for natural sunlight, demonstrating a sustaining open-circuit voltage of >20 mV/cm² under AM1.5G, without a heat sink. This highly versatile strategy offers a promising opportunity to explore energy evolution in fundamental research and industrial implementation and encourages future development in the next-generation widespread solar co-harvesting architecture.

The proposed selective solar absorber was prepared by a two-step process: the fabrication of the graphite–copper substrate and the monolayer self-assembly of Fe₃O₄ nanoparticles. Pieces of 50 µm copper foil were first treated with dilute hydrochloric acid to remove the oxidation film and cleaned in acetone and deionized water. For enhancing the interface bonding of graphite–copper layers, the top surface of copper was modified in a plasma cleaner. After that, a 45 nm thin-film of graphite was deposited on the copper foil by a thermal evaporator. Methanol (Aladdin) was then used to modify the graphite layer’s hydrophilic property. Next, the Fe₃O₄ NPs (Aladdin) were separated via washing and centrifugal separation. Methanol and chloroform were added at 1:1, to prepare a colloidal dispersion, followed by 3 min ultrasonic dispersion. The self-assembly of Fe₃O₄ NPs can be performed via the Langmuir–Blodgett film deposition technique, carrying out the formation of the surface particle film and ensuring the homogeneity of the monolayer, at the air/water interface. The obtained graphite–copper substrate was placed below the liquid level at 10° angle, pumping the base liquid and vertically lifting slowly for NPs deposition, with a large number of Fe₃O₄ NPs self-assembled in a C6 pattern coated on the substrate.

Top-view profiles of the absorber samples were observed using a field emission SEM (Thermofisher, Scios 2). The spectral reflectance (0.3–2 and 8–14 µm) of the sample was measured using a UV–visible–NIR spectrophotometer (Shimadzu, UV-3600Plus) with an integrating sphere. The absorption (A) was calculated from the measured reflectance (R) using the equation A = 1 − R. The temperature-varying efficient thermal emission of the samples over 8–14 µm was measured by an IR hemispherical emissometer. To demonstrate the solar-thermal performance, a solar simulator with an AM1.5G filter is used as the light source (SanYou, SS-50), with an incident flux measurement by a calibrated power meter. The temperature data were collected with a chip thermal sensor (OMRON).

The thermoelectric generators (inner diameter: 5 mm; external diameter: 10 mm) were deposited on a flexible polyimide substrate (75 µm) by a thermal evaporator. The polyimide-substrate was cleaned thoroughly by acetone and deionized water for 10 min in an ultrasonic bath. In order to maximize the thermoelectromotive force, the thermoelectric element cross-sectional area ratio has to be matched to the material properties.³⁷ Herein, the conical angle ratio can become $θnθp=AnAp=σpκpσnκn$⁠, with 9° for the p-type material and 10° for the n-type material. The target materials of p-type and n-type for evaporation were Bi_0.5Sb_1.5Te₃ and Bi₂Te_2.7Se_0.3, respectively, and were coated on the PI-substrate for 3 h, which can be connected in a series circuit by Au thin-film electrodes deposited by a thermal evaporator using Au target (99.999%). The thermoelectric capabilities of the thermoelectric generators (conductivity, Seebeck coefficient, and power factor) were measured by a ZEM3 system (ULVAC) (see the supplementary material Table).

The solar absorber was integrated with the thermoelectric generators for the flexible planar solar thermoelectric harvester in the same plane by silicone grease. The solar simulator with an AM1.5G filter was used as the light source for demonstrating the natural sunlight harvesting (SanYou, SS-50). We simulated two cases for a low-temperature environment and room-temperature environment (273 and 293 K) by an incubator and simultaneously measured both the current and the V_ocacross the electrodes at the output port of the solar thermoelectric harvester. The overall output power of the solar thermoelectric harvesting was calculated by multiplying the voltage and the current.

See supplementary material for the complete solar-thermal conversion performance and thermoelectric application of the studied absorber.

Y.L. acknowledges the support from Key Research and Development Program of the Ministry of Science and Technology (Grant No. 2022YFA1405200), the National Natural Science Foundation of China (Grant Nos. 92163123 and 52250191), and the Fundamental Research Funds for the Central Universities (Grant No. 2021FZZX001-19). C.-W.Q. acknowledges financial support from the Ministry of Education, Singapore (Grant No. A-0005143-01-00). T.L. acknowledges the support from the National Natural Science Foundation of China (Grant No. 52175009), Natural Science Foundation of Chongqing (Grant No. CSTB2022NSCQ-MSX0507), Natural Science Foundation of Heilongjiang Province (Grant No. YQ2022E022), and Fundamental Research Funds for Central Universities. W.L. acknowledges the support from the National Natural Science Foundation of China (Grant Nos. 62134009 and 62121005). Z.X. acknowledges the China Scholarship Council (CSC) for financial support.

The authors have no conflicts to disclose.

Z.X. and Y.L. contributed equally to this work.

Zifu Xu: Data curation (equal); Investigation (lead); Software (lead); Validation (lead); Visualization (lead); Writing – original draft (lead). Ying Li: Conceptualization (equal); Funding acquisition (equal); Project administration (lead); Resources (equal); Supervision (equal); Writing – original draft (supporting); Writing – review & editing (lead). Gang Gao: Investigation (supporting); Resources (equal); Validation (supporting). Fei Xie: Data curation (supporting); Investigation (supporting); Resources (supporting). Ran Ju: Investigation (supporting); Software (supporting); Validation (supporting). Shimin Yu: Data curation (supporting); Investigation (supporting); Resources (supporting). Kaipeng Liu: Investigation (supporting); Software (supporting). Jiaxin Li: Investigation (supporting); Software (supporting). Wuyi Wang: Formal analysis (supporting); Supervision (supporting). Wei Li: Conceptualization (supporting); Funding acquisition (equal); Methodology (supporting); Resources (supporting); Writing – review & editing (supporting). Tianlong Li: Funding acquisition (equal); Project administration (equal); Supervision (equal); Writing – review & editing (supporting). Cheng-Wei Qiu: Conceptualization (equal); Formal analysis (equal); Project administration (supporting); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding authors upon reasonable request.