CN104553221B - High-performance optical spectral selectivity inhales ripple element and solar thermal photovoltaic system - Google Patents

Abstract

The invention discloses a high-performance spectrally selective wave-absorbing element and a solar thermal photovoltaic system. The third thin film of low refractive index medium; the first thin film of low refractive index medium is uniformly covered on the metal substrate, on which a high refractive index semiconductor nano-square array arranged in a checkerboard pattern is constructed, and low refractive index semiconductor nano-square arrays are filled between the high refractive index semiconductor nano-square arrays. The second medium thin film covers the third thin film of low refractive index medium on top of it. The invention obtains good spectral selectivity by designing the structure of the dielectric thin film on the metal surface; by selecting different metals, semiconductors, and dielectric materials, and (or) changing the structural parameters, flexible cut-off wavelength regulation and spectral selectivity are realized; The invention is also applicable to high-temperature-resistant metals, semiconductors, and dielectric materials, and can be widely used in solar thermal photovoltaic systems.

Description

High-performance Spectral Selective Absorbing Components and Solar Thermal Photovoltaic System

technical field

The invention relates to the field of solar energy technology and application, in particular to a spectrally selective wave-absorbing element applicable to solar thermal photovoltaic systems.

Background technique

As the clean energy with the largest reserves in the world, solar energy has attracted widespread attention worldwide. How to effectively and efficiently utilize solar energy directly affects the sustainable development of human beings. The traditional solar energy industry is generally based on solar photovoltaic technology, which uses semiconductor diodes to convert incident sunlight into electrical energy that can be directly used by humans. However, this technology does not have a high utilization rate of solar energy, which is limited by Schockley-Queisser (SQ): incident photons with energy lower than the semiconductor bandgap width cannot be absorbed by the semiconductor; while high-energy incident photons higher than the semiconductor bandgap width Part of the energy will be dissipated in thermal relaxation. Even under ideal conditions, the highest all-focus conversion efficiency of single-junction solar cells (with a band gap of 1.1 eV) is only 41% without considering non-radiative losses. In order to break through the SQ limitation, it is proposed to place a selective absorbing-radiation unit in front of the photovoltaic cell, and use the selective absorbing element to absorb sunlight to increase the temperature of the selective radiation element connected to it (1000-2000 K), and the rearward The rear cell selectively radiates photons that match the forbidden band width of the cell, and the rear cell can achieve the highest conversion efficiency at this time. The system is called a solar thermal photovoltaic system, and its theoretical limit efficiency can reach 85%, which is much higher than the value given by the SQ limit. Among them, the selective absorbing element determines the maximum temperature that the pre-selective absorbing-radiation unit can reach and the radiation photon energy of the selective radiating element, which is the core device of the system. According to Kirchhoff's law, the absorption efficiency of an object is equal to the radiation efficiency under the condition of thermal equilibrium. Therefore, in order to maintain a constant high temperature state, the absorption spectrum of the selective absorbing element must have good spectral selectivity, requiring it to have as high absorption as possible in the visible to near-infrared band covered by sunlight, and as high as possible in the long-wave band. Possibly low absorption (ie very low radiation losses). In addition, the high temperature working environment requires that the selective absorbing components must be made of high temperature resistant materials.

There have been many reports on the research of absorbing components, among which, the use of artificial electromagnetic medium has greatly broadened the design ideas of absorbers. Landy et al. first proposed a single-wavelength perfect absorbing element based on an artificial electromagnetic medium (N. I. Landy, et al, Phys. Rev. Lett. 100, 207402, 2008.). Through electrical, magnetic resonance and impedance matching, the incident light of a specific wavelength can be perfectly absorbed by the artificial electromagnetic medium. Changing the shape and size of the unit structure can flexibly adjust its resonance wavelength, that is, the absorption peak wavelength. Reasonable placement of resonant cavities with different sizes and different resonance wavelengths can expand the range of its absorption spectrum to a certain extent (Y. Q. Ye, et al, J. Opt. Soc. Am. B 27,498, 2010.). We have also proposed a slit waveguide grating structure based on multiple optical effects to achieve broadband absorption at 300-1400 nm (F. Zhang, et al, Progress In Electromagnetics Research 134, 95(2013).). In order to further expand its working bandwidth, Søndergaard et al. proposed a non-resonant broadband absorbing element based on the slit surface plasmon nano-focusing effect (T. Søndergaard, et al, Nat. Commun. 3, 969, 2012.). The design of jagged artificial electromagnetic dielectric structure based on slow light effect can realize ultra-broadband absorption from visible light to near infrared to mid infrared (F. Ding, et al, Laser Photon. Rev. 8, 946 (2014).). However, the above absorbing components cannot meet the needs of solar thermal photovoltaic systems, and they do not have good spectral selectivity: too narrow absorption spectrum can lead to low absorption efficiency of sunlight, and too wide absorption spectrum can cause absorption The thermal radiation loss of the element in the long-wave band. Moreover, the materials used in the above absorbing components cannot meet the high temperature requirements of the solar thermal photovoltaic system. “V.Rinnerbauer, et al, Adv. Energy Mater. 4, 1400334 (2014).” and “Y. Nam, et al, Sol. Energy Mater. Sol. Cells 122, 287, 2014.” Both reported selective absorbing elements based on tungsten or tantalum photonic crystal structures, although they solved the high-temperature problem, the spectral selectivity of photonic crystal structures was not outstanding.

Contents of the invention

The object of the present invention is to provide a high-performance spectrally selective wave-absorbing element for a solar thermo-photovoltaic system aiming at the deficiencies of the prior art.

A high-performance spectrally selective wave-absorbing element, which includes a metal substrate, a first film of a low-refractive-index medium, a high-refractive-index semiconductor nano-square array, a second film of a low-refractive-index medium, and a third film of a low-refractive-index medium; The first thin film of medium with low refractive index is uniformly covered on the metal substrate, and the first thin film of low refractive index medium is constructed with high-refractive index semiconductor nano-square arrays arranged periodically in a checkerboard pattern, and the low-refractive index medium is exposed between the arrays of high-refractive index semiconductor nano-squares. The concave area of the first film is filled with the second film of low-refractive index medium, and the top of the high-refractive index semiconductor nano-square array and the filling area are covered with the third film of low-refractive index medium.

The refractive indices of the first low-refractive-index medium film, the second low-refractive-index medium film, and the third low-refractive-index medium film are all lower than that of the high-refractive index semiconductor nano-square array.

The first low-refractive-index medium film, the second low-refractive-index medium film, and the third low-refractive-index medium film are made of different dielectric materials or the same material.

The thickness of the metal substrate is greater than the attenuation length of the incident light therein, and it is used as a metal reflector to modulate the long-wavelength incident light other than the cut-off wavelength, and also serves as a heat conductor to transfer heat to the selective radiation element connected thereto.

The high-refractive-index semiconductor nano-square array is constructed on the first thin film of low-refractive-index medium to selectively absorb incident sunlight, and the spectral selectivity of the structure can be regulated by changing the size and material of the nano-square array.

The second low-refractive-index medium thin film and the low-refractive-index medium second thin film are used to protect the high-refractive index semiconductor nano-square array while reducing surface reflection.

A solar thermal photovoltaic system adopting the high-performance spectrally selective wave-absorbing element.

The beneficial effects of the present invention include:

(1) The present invention only realizes the flexible control of the solar light absorption spectral range of the device through the structural design of the dielectric thin film on the metal surface, and then obtains good spectral selectivity. The micro-nano processing of the metal substrate is avoided, so the preparation method is simple. The high-refractive-index semiconductor nanocubes in the device structure can be prepared by electron beam exposure technology, and nanoimprint technology can also be used to realize large-area and low-cost processing.

(2) The structural design of the present invention is very flexible. By selecting different metals, semiconductors, and dielectric materials, and/or changing structural parameters, flexible cut-off wavelength regulation and spectral selectivity can be realized.

(3) The structural design of the present invention is suitable for high-temperature-resistant metals, semiconductors, and dielectric materials. If the material is properly selected, the invention will have good high temperature stability and can be widely used in solar thermal photovoltaic systems.

Description of drawings

Figure 1 is a schematic diagram of the three-dimensional structure of a high-performance spectrally selective wave-absorbing element (2×2 units);

Figure 2 is a schematic structural diagram of a high-performance selective absorbing element (top view; 2×2 units);

Figure 3 is a cross-sectional view of a high-performance selective absorbing unit (2×2 unit), which is taken along the dotted line in Figure 2;

Figure 4 shows the absorption spectrum (solid line) of the selective absorbing element under the condition of normal incidence of plane waves obtained by numerical simulation, and the energy intensity of the solar spectrum (dashed line), λ _c = 1.2 μm is the cut-off wavelength;

Figure 5 shows the reflection spectrum of the selective absorbing element: numerical simulation results (dotted line); experimental measurement results (solid line);

In the figure, a metal substrate 1 , a first low-refractive-index medium film 2 , a high-refractive-index semiconductor nano-square array 3 , a low-refractive-index medium second film 4 , and a low-refractive-index medium third film 5 .

detailed description

The present invention will be further described below in conjunction with drawings and embodiments.

As shown in Figures 1, 2, and 3, a high-performance spectrally selective wave-absorbing element includes a metal substrate 1, a first thin film 2 of a low-refractive-index medium, a high-refractive-index semiconductor nano-square array 3, and a second low-refractive-index medium. Thin film 4, the third thin film 5 of low-refractive-index medium; the first thin-film 2 of low-refractive-index medium is evenly covered on the metal substrate 1, and an array of high-refractive-index semiconductor nano-squares arranged periodically in a checkerboard pattern is constructed on the first low-refractive index medium film 2 3. For selectively absorbing incident sunlight, the recessed area of the first film 2 of the low-refractive-index medium is exposed between the high-refractive-index semiconductor nano-square arrays 3 and filled with the second film 4 of the low-refractive-index medium, and the high-refractive index semiconductor nano-square Both the top of the array 3 and the filling area are covered with the third thin film 5 of the low-refractive index medium, so as to protect the high-refractive index semiconductor nano-square array 3 and reduce surface reflection.

Example 1

The metal substrate 1 is set to be high-temperature-resistant tantalum, which is thick enough that no light can pass through; the first thin film 2 of the low-refractive index medium is silicon dioxide, and the thickness is 100 nm; the high-refractive index semiconductor nano-square array 3 is a high-absorbing material Germanium, the height of the germanium square is 180 nm, and the side length is 270 nm; the second thin film 4 of the low-refractive index medium and the third thin film 5 of the low-refractive index medium are still set as silicon dioxide, with a thickness of 270 nm, filled in the high-refractive index semiconductor The gaps between the nanowire square array 3 and the top are covered. The materials involved in this embodiment are all high-temperature materials, which can withstand high temperatures of about 1000 K. Numerical simulation results show that the selective absorbing element of the present invention has an average absorption efficiency as high as 95% in the band within the cut-off wavelength of 1.2 μm, just covering the main peak of the solar spectrum; Reduced to about 0.1, it has good spectral selectivity, as shown in Figure 4. Define the quality factor FoM as FoM=η _t · η _c , wherein, η _t represents the quality of its spectral selectivity, η _c represents the height of the absorptivity within the cut-off wavelength λ _c , both expressions are as follows:

Among them, α(λ) represents the absorption spectrum of the selective wave-absorbing element, and I _AM1.5 (λ) represents the AM1.5 solar spectrum. For the structural design of this embodiment, its FoM is as high as 0.91, much higher than the FoM of the photonic crystal structure.

The selective wave-absorbing element of the present invention can be prepared by the following steps: sputtering metal tantalum with a thickness of about 200 nm on any planar substrate, as the metal substrate of the selective wave-absorbing element of the present invention, its thickness is sufficient to prevent light transmission; On the metal tantalum substrate, 100 nm thick silicon dioxide was sequentially sputtered as the first film of low refractive index medium and 180 nm thick germanium was used to prepare high refractive index semiconductor nano-square arrays; 270 nm thick germanium film was spin-coated Negative photoresist MAN2403, and use electron beam exposure technology (or nanoimprinting technology to achieve large-area, low-cost nanofabrication) on the photoresist to prepare a checkerboard-like periodic array of square array patterns, the side length of the square unit is 270 nm; use the photoresist pattern as a mask, use the inductively coupled plasma etching method to dry-etch germanium, and remove the MAN2403, so as to prepare a checkerboard periodic array of germanium squares; on this basis, sputter 270 nm nm silicon dioxide film fills the gaps between the germanium squares and covers them on top. Figure 5 shows the experimentally measured reflectance spectrum (solid line) and the numerically simulated reflectance spectrum (dotted line) of this sample. It can be seen from this figure that the experimental results of this sample are in good agreement with the numerical simulation results.

Example 2

The metal substrate 1 is set to be high-temperature-resistant tungsten, and its thickness is thick enough so that no light can pass through; the first thin film 2 of the low-refractive index medium is silicon dioxide; and side lengths are on the order of sub-micron; the second low-refractive-index medium film 4 and the third low-refractive-index medium film 5 are made of silicon dioxide, filling the gaps in the high-refractive index semiconductor nanowire square array 3 and covering the top thereof . The materials involved in this embodiment are all high-temperature materials, which can withstand high temperatures of about 1000 K.

Example 3

The metal substrate 1 is set to be high-temperature-resistant tantalum, and its thickness is thick enough so that no light can pass through; the first thin film 2 of the low-refractive index medium is silicon dioxide; and side lengths are on the order of submicron; the second film 4 of the low-refractive-index medium and the third film 5 of the low-refractive-index medium are made of low-refractive-index alumina, which fills the gaps in the square array of high-refractive-index semiconductor nanowires 3 and covers the its top. The materials involved in this embodiment are all high-temperature materials, which can withstand high temperatures of about 1000 K.

Example 4

The metal substrate 1 is set to be high-temperature-resistant tungsten, and its thickness is thick enough so that no light can pass through; the first thin film 2 of the low-refractive index medium is silicon dioxide; and side lengths are on the order of submicron; the second film 4 of the low-refractive-index medium and the third film 5 of the low-refractive-index medium are made of low-refractive-index alumina, which fills the gaps in the square array of high-refractive-index semiconductor nanowires 3 and covers the its top. The materials involved in this embodiment are all high-temperature materials, which can withstand high temperatures of about 1000 K.

By selecting different materials and setting different structural dimensions, the absorption spectrum of the selective wave-absorbing element of the present invention can be adjusted to different cut-off wavelengths, that is, its spectral selectivity can be flexibly adjusted. Therefore, those skilled in the art can make targeted modifications and improvements on the basis of the present invention.

Claims (7)

1. a high-performance optical spectral selectivity inhales ripple element, it is characterised in that include metallic substrates (1), low refractive index dielectric the One thin film (2), high-index semiconductor nano square array (3), low refractive index dielectric the second thin film (4), low refractive index dielectric 3rd thin film (5)；Low refractive index dielectric the first film (2) uniform fold is in metallic substrates (1), and low refractive index dielectric first is thin Upper high-index semiconductor nano square array (3) building checkerboard periodic arrangement of film (2), in high-index semiconductor nanometer Low refractive index dielectric the second thin film is filled in the depressed area exposing low refractive index dielectric the first film (2) between square array (3) (4), high-index semiconductor nano square array (3) top and fill area all cover low refractive index dielectric the 3rd thin film (5)；

Described high-index semiconductor uses germanium, silicon.

High-performance optical spectral selectivity the most according to claim 1 inhales ripple element, it is characterised in that described low-refraction is situated between Matter the first film (2), low refractive index dielectric the second thin film (4), the refractive index of low refractive index dielectric the 3rd thin film (5) are below height The refractive index of refractive index semiconductor nano square array (3).

High-performance optical spectral selectivity the most according to claim 1 inhales ripple element, it is characterised in that described low-refraction is situated between Matter the first film (2), low refractive index dielectric the second thin film (4), low refractive index dielectric the 3rd thin film (5) are different materials or of the same race Material.

4. a kind of high-performance optical spectral selectivity as claimed in claim 1 inhales ripple element, it is characterised in that described Metal Substrate The end, thickness is more than incident illumination attenuation length wherein, both incident to the long-wave band beyond cutoff wavelength as metallic mirror Light is modulated, and the conductor as heat transfers heat to the selective radiation element being attached thereto again.

5. a kind of high-performance optical spectral selectivity as claimed in claim 1 inhales ripple element, it is characterised in that described high index of refraction Semiconductor nano square array (3) is implemented on low refractive index dielectric the first film (2), for the selective absorbing incidence sun Light, changes size and material, the spectral selection of this structure of controllable of nano square array.

6. a kind of high-performance optical spectral selectivity as claimed in claim 1 inhales ripple element, it is characterised in that described low-refraction Medium the second thin film (4), low refractive index dielectric the 3rd thin film (5) are used for protecting high-index semiconductor nano square array, with Time reduce surface reflection.

7. one kind uses the solar thermal photovoltaic system that high-performance optical spectral selectivity according to claim 1 inhales ripple element.