Colorless unidirectional diffraction type solar concentrator and preparation method and application thereof

Abstract

The application discloses a colorless unidirectional diffraction type solar concentrator and a preparation method and application thereof. The colorless unidirectional diffraction type solar concentrator comprises a transparent flat plate substrate, a light orientation layer and a multilayer cholesteric liquid crystal film are attached to the side of the transparent flat plate substrate in sequence, the multilayer cholesteric liquid crystal film comprises a plurality of cholesteric liquid crystal layers, and adjacent cholesteric liquid crystal layers have different pitches; and a coupling light solar photovoltaic cell is arranged on the side of the transparent flat plate substrate. The preparation method comprises the following steps: coating a photosensitive alignment layer on the transparent flat plate substrate and obtaining a sub-micron period light orientation layer through exposure; spin coating or dip coating a cholesteric liquid crystal mixture with different pitches on the surface of the light orientation layer or the cholesteric liquid crystal layer and solidifying to form a multilayer cholesteric liquid crystal film; and arranging a solar photovoltaic cell on the side of the transparent flat plate substrate. The application can realize high-efficiency energy collection of the solar concentrator, is compatible with existing glass, and realizes unidirectional coupling.

Description

Technical Field

[0001] This invention pertains to solar concentrators, their manufacturing methods and applications, specifically a colorless unidirectional diffraction type solar concentrator, its preparation method and application. Background Technology

[0002] With the rapid population growth and accelerated urbanization in modern society, more and more towers and supertall buildings are emerging in cities. The dramatic increase in population density promotes production and cooperation efficiency, but it is also accompanied by increasingly severe resource and energy consumption. Thermal and nuclear power plants face the risks of environmental pollution and nuclear waste leakage, while renewable energy sources such as photovoltaics, wind power, and hydropower require significant space and proximity to their respective energy sources. Furthermore, centralized power supply faces the problem of high transmission losses. Therefore, novel building-integrated photovoltaic (BIPV) technology, which integrates solar energy capture into building facades, has attracted widespread attention for net-zero energy buildings. Glass windows are widely used in modern buildings to provide comfortable living and working environments. They are typically installed on sun-drenched buildings to optimize natural lighting and heating. Combining photovoltaic technology with architectural glass offers a promising strategy for green building and a sustainable society.

[0003] Existing photovoltaic technologies, including amorphous silicon cells, organic photovoltaics, gallium arsenide, dye-sensitized, and perovskite solar cells, are hampered by their opacity and brittleness, hindering their replacement of architectural glass. To address these issues, solar concentrators have been developed to laterally concentrate solar energy, which is then captured by photovoltaic cells fixed to the sides of architectural glass. Researchers have developed various solar concentrators based on transparent glass, such as luminescent solar concentrators (LSCs) and scattering solar concentrators (SSCs). These devices typically guide some light into the glass by embedding fluorescent materials or scattering particles within it, allowing it to be collected by photovoltaic cells at the edges. However, these technologies have the following main drawbacks: (1) The fluorescence and scattering mechanisms are isotropic, which means that only a portion of the high incident angle light is captured by the waveguide, resulting in low light concentration efficiency; (2) The fluorescence material has a narrow absorption and emission band, and the light-transmitting area has color in the glass; (3) The scattering medium causes the glass to fog up, affecting the visual aesthetics; (4) The device needs to be embedded inside the glass, which is incompatible with existing building glass, and the manufacturing process is complex and costly.

[0004] Therefore, there is an urgent need for a solar concentrating solution that can achieve efficient energy harvesting while taking into account aesthetics and compatibility. Summary of the Invention

[0005] Purpose of the invention: In order to overcome the shortcomings of the existing technology, the purpose of this invention is to provide a colorless unidirectional diffraction solar concentrator with high light transmittance, high transmittance, high color rendering index and wide viewing angle. Another purpose of this invention is to provide a method for preparing a colorless unidirectional diffraction solar concentrator that is easy to integrate. Yet another purpose of this invention is to provide an application of a colorless unidirectional diffraction solar concentrator on the exterior surface of a building's transparent window.

[0006] Technical solution: The present invention discloses a colorless and unidirectional solar concentrator (CUSC), comprising a transparent flat substrate, wherein a light alignment layer and a multilayer cholesteric liquid crystal film (CLC) are sequentially attached to the side of the transparent flat substrate, the multilayer cholesteric liquid crystal film comprising a plurality of cholesteric liquid crystal layers, adjacent cholesteric liquid crystal layers having different pitches; and a solar photovoltaic cell for receiving and converting coupled light propagating along the total internal reflection inside the transparent flat substrate is provided on the side of the transparent flat substrate.

[0007] Furthermore, the tilt angle between the tilted helical axis of the cholesteric liquid crystal layer and the transparent flat substrate is 10° to 40°.

[0008] Furthermore, the cholesteric liquid crystal layer is made of polymeric chiral liquid crystal (CLC) material. The concentration of chiral agent doped in cholesteric liquid crystal layers with different pitches varies, ranging from 1 to 3 wt%. To cover a broad solar spectrum, the concentration or amount of chiral agent in each CLC layer is adjusted to achieve its Bragg reflection band (λ). min –λ max This covers different bands of the target wavelength. For example, one layer is designed to reflect red light, another green light, and a third blue light. By stacking their individual reflection bands, continuous broadband reflection in the visible light range can be achieved. The total reflection bandwidth after stacking is approximately between the maximum and minimum values ​​of the reflection bands of each layer. In a typical embodiment, a reflection band covering 400–750 nm can be obtained by preparing three or more layers of material, thus enabling the concentrator to have a coupling response in the visible light region. It is worth noting that, due to the strong robustness of the intermolecular interaction forces, multilayer CLC structures with different pitches can be prepared by multiple spin-coating and curing processes to ensure smooth interfaces between layers and reduce the impact of defects on reflection characteristics. Through the design of multilayer cholesteric liquid crystal thin films, the reflection bandwidth can be significantly expanded, solving the problem that the reflection bandwidth of a single-layer CLC is limited by (Δλ=(n e –n o This addresses the issue of P, thereby achieving broadband focusing within the visible light range.

[0009] Furthermore, the chiral agent is any one of R5011, S5011, R811, and S811.

[0010] Furthermore, the pitch of the cholesteric liquid crystal layer is 0–500 nm.

[0011] Furthermore, the average transmittance of visible light transmitted through the solar concentrator is ≥60%.

[0012] The present invention discloses a method for preparing a colorless unidirectional diffraction solar concentrator, comprising the following steps:

[0013] Step 1: Coating a photosensitive alignment layer on a transparent flat substrate and obtaining a photo-alignment layer with submicron-level lateral periodic arrangement through exposure;

[0014] Step 2: Spin-coat or dip-coat multiple layers of cholesteric liquid crystal mixture with different pitches onto the surface of the photo-alignment layer or cholesteric liquid crystal layer obtained in Step 1 and cure them to form a multilayer cholesteric liquid crystal film.

[0015] Step 3: Install solar photovoltaic cells on the side of the transparent flat substrate.

[0016] Further, in step one, the exposure employs a dual-beam interferometry method, and the lateral grating period of the cholesteric liquid crystal layer varies within the range of 400–800 nm according to the designed wavelength. This submicron-level lateral period Λ determines the diffraction angle. According to the Bragg diffraction formula, satisfying a specific incident wavelength λ and period Λ allows the incident light to be diffracted into the waveguide at the designed exit angle (relative to the incident normal); there is no diffraction in other directions. Through geometric analysis, Λ is designed to couple the incident light in the central visible light band at the critical angle and achieve total internal reflection in the substrate, thus realizing unidirectional transmission. The lateral periodic structure also has no effect on polarization, mainly providing coupling directionality: unlike random orientation coupling (scattering light in all directions), the periodic ordered structure allows light to propagate mainly in one direction, reducing reverse leakage and back loss, and improving coupling efficiency. In addition, by controlling the size of the period, the range of incident angles of the coupled light can be adjusted, making the system compatible with sunlight at different incident angles. For example, if Λ is designed so that the principal diffraction angle is approximately θ... c (Within a specific range), it can be ensured that parallel light passing through the periodic structure propagates along the substrate plane.

[0017] Furthermore, in step two, the multilayer cholesteric liquid crystal film produces Bragg reflection of circularly polarized light with the same chirality as its helical direction, while allowing circularly polarized light with the opposite chirality to pass through.

[0018] The present invention relates to the application of a colorless unidirectional diffraction solar concentrator on the exterior surface of a building's transparent window.

[0019] Working Principle: This method utilizes the Bragg reflection property of cholesteric liquid crystals for circularly polarized light of the same chirality to achieve selective diffraction coupling of the circularly polarized component in incident natural light. Specifically, a multilayer right-handed (or left-handed) cholesteric liquid crystal is designed, with each layer arranged in a tilted spiral along its thickness direction (perpendicular to the substrate). Circularly polarized light with the same chirality as the spiral direction of the cholesteric phase structure will undergo Bragg diffraction and couple into the glass waveguide; while the opposite chirality component will be transmitted (not diffracted). Sunlight itself is unpolarized and can be equivalently decomposed into left-handed and right-handed circularly polarized light. This scheme is equivalent to retaining only one polarization for coupling, while the other polarization is not trapped by the concentrator, thus ensuring the device's "color" neutrality and high transmission rate. This polarization-selective coupling mechanism draws inspiration from the polarization volume grating (PVG) structure, providing a theoretical basis for realizing unidirectional waveguide transmission. When coupling is achieved, the incident preferentially coupled polarized light enters the glass substrate after diffraction and undergoes total internal reflection (TIR), propagating along the edge of the substrate to the strip photovoltaic cell; during this process, due to the fixed direction of multiple total internal reflections, unidirectional coupling is achieved.

[0020] The pitch set {P1...P5} of the CLC was numerically optimized based on the modified Bragg equation to ensure gapless spectral coverage of the entire visible light range (400-750 nm), thus maintaining color neutrality. By adjusting the Δn value and pitch P of different CLC layers, the smoothness and integration of the reflection edges can also be optimized to form a wide and flat overall reflection band.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following significant features:

[0022] 1. It can achieve efficient energy harvesting of solar concentrators while also taking into account aesthetics and compatibility. It is compatible with existing building glass, simple and convenient to manufacture, low cost, and achieves unidirectional coupling;

[0023] 2. By setting up a multilayer cholesteric liquid crystal film, the concentrator can have a coupling response in the visible light region, significantly expanding the reflection bandwidth;

[0024] 3. Cholesteric liquid crystal layers with different pitches were prepared by multiple spin-coating and curing processes to ensure smooth interfaces between layers and reduce the impact of defects on reflective properties.

[0025] 4. Colorless unidirectional diffraction solar concentrators can be widely used in modern buildings and mobile equipment, overcoming the problems of color distortion and low efficiency in existing solar concentrating applications. Attached Figure Description

[0026] Figure 1This is a schematic diagram of the structure of the multilayer cholesteric liquid crystal film 2 of the present invention;

[0027] Figure 2 This is a flowchart illustrating the fabrication process of the colorless unidirectional diffraction solar concentrator of the present invention;

[0028] Figure 3 This is a schematic diagram of the preparation of the multilayer cholesteric liquid crystal film 2 of the present invention, wherein a is a schematic diagram of the preparation of the cholesteric liquid crystal layer 21, and b is a light path diagram used for exposure.

[0029] Figure 4 This is a polarization separation diagram of the transmission and reflection modes of the multilayer cholesteric liquid crystal film 2 of the present invention;

[0030] Figure 5 This is a schematic diagram of the equivalent sunlight of orthogonally polarized light according to the present invention;

[0031] Figure 6 This is a broadband transmittance and reflectance of the CUSC of the present invention, and a solar spectrum diagram.

[0032] Figure 7 These are perspective views of the CUSC and morphological characterization images of multilayer broadband CLC of the present invention, wherein a is a perspective photograph of the CUSC, b is a polarized light micrograph of multilayer CLC with a 460 nm grating lateral period, and c is a cross-sectional SEM image of a multilayer CLC film with a thickness of 7.5 μm.

[0033] Figure 8 This is a graph showing the optical performance data of the CUSC of this invention, where a represents the relationship between the reflection band and the incident angle θ, and b represents the CIELAB color space coordinates corresponding to different incident angles;

[0034] Figure 9 This is a diagram showing the experimental results of the unidirectional waveguide of the CUSC of this invention;

[0035] Figure 10 This is a simulation diagram of the circular polarization separation of the CUSC of this invention;

[0036] Figure 11 The diffraction angle of this invention A graph showing the variation of incident angle (θ) and wavelength (λ);

[0037] Figure 12 This is a unidirectional waveguide diagram of the CUSC of this invention;

[0038] Figure 13 This is a physical illustration of an application example of the present invention;

[0039] Figure 14 This is the external quantum efficiency (EQE) spectrum of the CUSC-PV of this invention;

[0040] Figure 15This is a graph showing the PCE variation over time and the corresponding light intensity of the CUSC-PV of this invention;

[0041] Figure 16 This is a test diagram of the long-term stability of the CUSC-PV white light irradiation of this invention;

[0042] Figure 17 This invention compares the key performance indicators of SSC, LSC and CUSC;

[0043] Figure 18 This is a comparison chart of the CUSC-PV performance of bare glass and coated single-layer CLC with multilayer broadband CLC according to the present invention. Detailed Implementation

[0044] Example 1

[0045] like Figure 1 A colorless, unidirectional diffractive solar concentrator compatible with window glass includes a multilayer cholesteric liquid crystal film 2 with directional light concentration and high transmittance characteristics, enabling building-integrated photovoltaics (BIPV). The core structure of the multilayer cholesteric liquid crystal film 2 includes a submicron grating and cholesteric liquid crystal layers 21 with uniformly gradient helical spacing; adjacent cholesteric liquid crystal layers 21 have different pitches. Through an inclined Bragg plane, this structure can transmit left-circularly polarized (LCP) light and perform total internal reflection of right-circularly polarized (RCP) light through Bragg reflection, thus achieving efficient light energy collection and transmission. A transparent flat substrate 1 uses ordinary architectural glass F (such as commonly used float glass or laminated glass) as the concentrator's substrate, exhibiting high transmittance of visible light and being colorless and transparent. The surface of the transparent flat substrate 1 has submicron-level laterally periodically arranged light alignment layers 4. A coupled-light solar photovoltaic cell 3 is mounted on any edge (or multiple edges) of a transparent flat substrate 1, in close contact with the side of the substrate, to receive light coupled from a multilayer cholesteric liquid crystal film 2 and propagated by total internal reflection within the substrate. The cell can be a conventional monocrystalline silicon cell or a thin-film cell, selected according to project requirements.

[0046] Sunlight enters from the outside of the transparent flat substrate 1, passes through the multilayer cholesteric liquid crystal film 2 and the transparent flat substrate 1, and reaches the interior of the transparent flat substrate 1, where it is collected at the edge. Since the multilayer cholesteric liquid crystal film 2 only diffracts light of the same chiral polarization, most of the light enters the transparent flat substrate 1 and propagates along the plane of the transparent flat substrate 1 in a single direction until it reaches the coupled light solar photovoltaic cell 3, realizing photoelectric conversion.

[0047] The cholesteric liquid crystal layer 21 has a diffraction grating structure formed by laterally periodically distributed liquid crystal orientation within the thin film plane. The grating period Λ allows incident circularly polarized light within the designed wavelength range to be diffracted and coupled into the interior of the transparent flat substrate 1 in a predetermined direction. The tilted helical axis of each cholesteric liquid crystal layer 21 has a tilt angle of 10°-40° with the substrate surface. The multilayer cholesteric liquid crystal layer 21 generates Bragg reflection for circularly polarized light with the same chirality as its helical direction, while remaining transparent to circularly polarized light with the opposite chirality. The cholesteric liquid crystal layer 21 is a polymeric cholesteric liquid crystal material with different pitches. After curing, this material has a fixed pitch and reflection band, and different layers of CLC achieve different reflection wavelengths by doping with different concentrations of chiral agents. The chiral agents include R5011, S5011, R811, and S811, with concentrations ranging from 1wt% to 3wt%. The pitch of the CLC is between 200 and 500 nm, and the Bragg reflection wavelength bandwidth is between 400 and 750 nm. The CLC mixture consists of reactive mesocrystalline RM257, chiral dopant R5011 (NCLCP, China), photoinitiator Omnirad 651, and surfactant Zonyl 8857A, dissolved in propylene glycol methyl ether acetate (PGMEA). The helical twisting force of R5011 in RM257 is 108 μm. -1 (At room temperature). The helical pitch of CLC was controlled by adjusting the concentration of R5011 in the precursor.

[0048] The multilayer cholesteric liquid crystal film 2 of this embodiment covers a spectral range that includes the visible light band, and the average visible transmittance (AVT) of the visible light transmitted through the condenser is not less than 60%, ensuring the quality of indoor lighting.

[0049] like Figures 2-3 The method for preparing the colorless unidirectional diffraction solar concentrator in this embodiment includes the following steps:

[0050] Step 1: A photosensitive alignment layer (such as SD1 and BY) is coated onto a clean, transparent flat substrate 1, and the photoalignment material is fixed by heat drying or baking. A periodic alignment pattern is formed on the photoalignment layer 4 using ultraviolet interference light or a photolithographic mask. After exposure, a development / curing process is performed to maintain the periodic pattern. A first right-handed cholesteric liquid crystal mixture is prepared. The photoaligning agent, sulfonyl azo dye SD1, is dissolved in N,N-dimethylformamide at a concentration of 0.3 wt%. The CLC mixture consists of reactive mesocrystalline RM257, chiral dopant R5011, photoinitiator Omnirad 651, and surfactant Zonyl 8857A, dissolved in propylene glycol methyl ether acetate (PGMEA). The mixture is spin-coated onto the alignment layer and uniformly distributed, followed by ultraviolet curing to obtain the first polymer-stabilized cholesteric liquid crystal layer 21. Its pitch is adjusted by the chiral agent ratio, and the reflection center can be confirmed by measurement after curing. The helical torsion force of R5011 in RM257 is 108 μm. -1 (At room temperature). The helical pitch of CLC is controlled by adjusting the concentration of R5011 in the precursor. The UV curing optical path is as follows: Figure 3 As shown in b, this is the existing dual-beam circularly polarized holographic exposure optical path.

[0051] Step two: A second, third, and subsequent cholesteric liquid crystal layers 21 are deposited on the surface of the first cholesteric liquid crystal layer 21 obtained in step one. Inter-layer transfer relies on intermolecular forces, and each layer is cured. After completion, the multilayer cholesteric liquid crystal film 2 is firmly attached to the transparent flat substrate 1, forming a complete structure. A mixture of cholesteric liquid crystals with different pitches is cured to form a multilayer cholesteric liquid crystal film 2. The tilted helical axis of the cholesteric molecules in the cholesteric liquid crystal layer 21 has a certain angle with the normal direction of the film surface. Different cholesteric concentrations are used in each CLC layer, resulting in different pitches, and the reflective bandwidth of each layer complements each other. Each CLC film is approximately a few micrometers thick and can be formed by spin coating or flow coating. Patterning is performed on the alignment layer using dual-beam polarized ultraviolet interference, and co-curing is achieved using a photoinitiator to form a periodic alignment direction. Each cholesteric liquid crystal layer 21 has the same or different grating period Λ, depending on the coupling angle design; the grating period Λ is between 400 and 800 nm.

[0052] Step 3: Fix solar photovoltaic cells 3 to the edge of the transparent flat substrate 1. The area can be selected as needed, and connect the electrodes. After integration, it can be made into a window-integrated module or an attached light-concentrating film.

[0053] I. Polarization-selective waveguide coupling mechanism:

[0054] The cholesteric liquid crystal layer 21 used is made of a right-handed helical structure, diffracting only right-handed circularly polarized (RCP) light and transmitting left-handed circularly polarized (LCP) light. Incident natural light can be considered a mixture of RCP and LCP, therefore only half the light intensity (the RCP portion) is actually coupled. Figure 1 The RCP component of the incident light is diffracted into the transparent flat substrate 1 by the periodically structured cholesteric liquid crystal layer (CLC) 21 according to the Bragg condition. The diffraction angle is designed to satisfy the total internal reflection condition at the interface between the transparent flat substrate 1 and the air, thus allowing it to propagate within the transparent flat substrate 1. The LCP light, however, passes directly through without participating in coupling. The CLC structure achieves "diffraction with a circularly polarized state of the same chirality as the CLC helical twist, while allowing opposite polarization components to pass through." Figure 4 As shown, the coupled light undergoes total internal reflection inside the transparent flat substrate 1, and its polarization state may be reversed during propagation. However, since the light no longer diffracts back when it encounters the CLC coupling layer again (or is kept in a transmission state by setting a half-wave plate), the light is ultimately output only from the edge of the transparent flat substrate 1 to the externally coupled solar photovoltaic cell 3. This mechanism achieves unidirectional light input and output (unidirectional coupling) and requires only photovoltaic devices to be installed on the output side. Compared with traditional visible light scattering couplers, it avoids outward reflection losses and improves efficiency.

[0055] II. Design and Performance of Multilayer Cholesteric Liquid Crystal Thin Films

[0056] Embodiment 1 of the present invention employs a five-layer right-handed CLC film (cholesterol phase liquid crystal layer 21), specifically,

[0057] The sun is Earth's primary energy source, supporting human existence and development. Electromagnetic waves are the main carriers of solar energy, propagating at the speed of light. The transverse wave nature of light induces polarization-dependent interactions with spatial periodic structures. Sunlight is inherently unpolarized; it can be equivalently decomposed into left- and right-circularly polarized light, such as... Figure 5 CLC is a one-dimensional chiral photonic crystal. It selectively reflects circularly polarized light with the same chirality as its light source, while allowing the remaining light to pass through directly. The photonic band is represented by a modified Bragg equation.

[0058] Δλ=(n eff -n o )·P cos(θ+α),

[0059] Where Δλ is the Bragg wavelength; n eff and n oθ and α are the effective refractive index and ordinary refractive index of the liquid crystal, respectively; P is the pitch of the CLC (0–2π); θ and α are the angle of incident light and the tilt angle of the Bragg plane of the CLC, respectively. By stacking a series of CLC layers with different pitches, the photonic band can be effectively extended to the entire visible spectrum. To ensure seamless coverage in the 400–750 nm range, the pitch of the five stacked CLC layers was numerically optimized to achieve continuous overlap of the reflection bands. The center reflection wavelengths of the five CLC layers are 420–470 nm, 470–530 nm, 530–580 nm, 580–640 nm, and 640–750 nm, respectively. The spacing set {P1…P5} was numerically optimized according to the modified Bragg equation to ensure gapless spectral coverage of the entire visible light range (400–750 nm), thus maintaining color neutrality. After the CLC film is coated onto architectural glass, a solar concentrator is formed, which can be seamlessly integrated into a building-integrated photovoltaic (BIPV) system, such as… Figure 6 In this design, when illuminated by AM1.5G light, selected circularly polarized light is reflected within the photonic band and transmitted to the glass edge via total internal reflection (TIR). Light escaping from the edge is captured by silicon photovoltaic (Si-PV) cells mounted on the glass edge. This design provides a novel integrated photovoltaic system for green buildings.

[0060] III. Lateral Grating Period Control

[0061] The periodic structure of the concentrator is fabricated using photo-controlled alignment technology. A photosensitive alignment layer (such as an azobenzene photo-aligning material) is coated on a transparent flat substrate 1. Then, an interference pattern is formed using a coherently polarized UV beam to transfer the preferred period Λ onto the alignment layer. Subsequently, CLC liquid crystal is coated onto the alignment layer according to the multilayer CLC material fabrication process of Example 1 and cured. All CLC layers can share the same grating period, or different periods can be formed through multiple exposures (or rotating the sample for exposure) in different steps. Through experimental optimization, Λ is selected to satisfy the required coupling angle (e.g., Λ ≈ several hundred nanometers, corresponding to a diffraction angle that couples light at an incident angle of ~45°). Figure 1 The diagram illustrates the LC orientation of different regions in the alignment layer after exposure, along with the corresponding spatial periodic structure. When visible light is incident, the diffraction efficiency is highest only along the designed direction, "hitting" the light into the transparent flat substrate 1. Compared to random isotropic coupling, the periodic grating significantly enhances the coupling efficiency in a specific direction and gives the coupled beam good directionality.

[0062] CLC film coated onto commercial building glass (10×10cm) 2 ),like Figure 7, and gratings are fabricated in five 1-inch circular regions. Due to the diffraction of light, the transmittance of these regions is lower than that of the regions without the covered gratings. Notably, due to their broadband diffraction characteristics, these regions are colorless and free of turbidity, making the green plants behind clearly visible. The transmittance at incident angles θ in the range of -60° ≤ θ ≤ 60° (400–800 nm) was further characterized, as Figure 8 a. For vertically incident white light, the average visible transmittance (AVT) reaches 64.2%, while when θ varies from -60° to 60°, the AVT gradually decreases from 90.7% to 60.5%. The color shift at different θ values is caused by the change in the photon reflection band related to the incident angle. Figure 8 b shows the CIELAB color space coordinates (a*, b*) at different incident angles, where the luminance L * is fixed at 90. It can be seen that for most incident angles, the coordinates are within the reasonable color range of architectural glass (-15 < a* < 15 and -15 < b* < 15). By stacking more layers with different pitches to further expand the photon band, the performance can be improved. Therefore, the proposed diffractive solar concentrator is suitable for highly transparent and wide-angle broadband colorless photovoltaic integrated architectural glass.

[0063] IV. Unidirectional waveguide performance

[0064] Experiments and simulation of the unidirectional waveguide performance of the concentrator. When vertically incident collimated white light irradiates the CUSC, the light is asymmetrically diffracted to the -x side edge of the architectural glass. In addition, due to the diffraction of the short-period gratings, the light is spatially dispersed. The unidirectional waveguide characteristics are verified by Figure 9 the top-view image on the bottom left. In addition, the logo of Nanjing University is clearly observed due to the high colorless transmittance of the CUSC, as Figure 9 . The CUSC is modeled using finite-difference time-domain (FDTD) simulation. As shown by a series of parallel tilted Bragg planes calculated from the configuration of the multi-layer CLC thin films, the tilt angle α varies from 21° on the side close to the glass to 30° on the other side. The propagation of the diffracted light follows the following diffraction equation, as Figure 10 :

[0065]

[0066] where, n and n gHere, θ and φ are the refractive indices of air and glass, respectively; λ is the incident wavelength; θ and φ are the angles of incident light in air and diffracted light in glass, respectively (+ / - indicate the wave vector along the x / -x direction). Multilayer CLCs exhibit broadband circularly polarized selective Bragg reflection; only right-handed circularly polarized light within the photonic band is reflected, while light satisfying the TIR condition propagates unidirectionally within the glass waveguide. Notably, light of different wavelengths is diffracted by CLC layers with different pitches. The Bragg plane defines the diffraction angle, and the confinement of light is ultimately determined by the TIR condition, which is crucial for waveguide propagation.

[0067] Further simulations were performed on the incident angle θ-related diffraction of CUSC, such as... Figure 11 As shown, in -60°≤θ<θ t Within the range, where θ t =sin-1(n) g -λ / Λ) and n g =1.52, the CLC thin film acts as a waveplate rather than a grating; therefore, the incident light depends on n g sinφ = sinθ is reflected by the mirror. As a result, the polarization state changes with the film thickness. When θ t When ≤θ≤60°, the CLC thin film functions as a polarization volume grating with a tilted Bragg plane. Results show that circularly polarized selective reflection satisfies Equation 2. Only when |φ|>φ c The TIR condition is satisfied only when = sin⁻¹(1 / ng). In other words, it is only satisfied when = sin⁻¹(1 / ng). Figure 11 The light in the red area propagates in the waveguide and is eventually trapped at the edge of the glass, such as Figure 12 All simulation results are consistent with... Figure 11 The results are consistent. Wide-angle broadband TIR ensures high efficiency of the concentrator in practical applications.

[0068] Application Example 1

[0069] The concentrator of Example 1 is attached or coated onto the building glass. Ensure the periodic grating is aligned horizontally with the direction of illumination; the optimal orientation can be adjusted according to the solar path. After passing through the multilayer cholesteric liquid crystal film 2, the incident light is guided to the edge of the transparent flat substrate 1 via the unidirectional coupling mechanism described above. The window edges not covered by the coupled-light solar photovoltaic cell 3 should be blackened or mirrored to prevent leakage or reflection of uncollected light. The coupled-light solar photovoltaic cell 3 converts the coupled light into current, which is then led outwards via internal wires for grid connection or storage. Due to the wide spectral range and high unidirectional efficiency of this device, significant power generation can be achieved on the side of building windows on sunny days in actual tests, with a significantly improved conversion efficiency compared to conventional transparent solar panels of the same area. Compared to traditional colored glass photovoltaic modules, this technology enables "white walls during the day, power generation during the day." Furthermore, the passive design eliminates the need for a large-area tracking system, allowing for efficient light collection and power generation using large-area windows.

[0070] A Si-PV cell (0.5 × 4 cm) 2 It is installed at the light emission edge to form a CUSC-PV device (4×4×0.5cm). 3 This device, with an active area 1 inch in diameter, can be considered a simple prototype of a CUSC-PV window. Excess battery area is marked with black tape to prevent direct sunlight from reaching the photovoltaic cells. The device operated a 10mW fan under sunlight at 1 PM on July 1, 2024. The fan immediately stopped when the sunlight was blocked. Figure 13 Considering the active region is merely a 1-inch circular CLC grating, this vividly reveals the high power conversion efficiency of CUSC-PV cells. We systematically characterized the performance of the CUSC-PV device under AM 1.5G illumination using a solar simulator. The optical efficiency (η) and power conversion efficiency (PCE) were calculated using the following formulas:

[0071]

[0072] Among them, I SC and I SC A' and A' are the short-circuit currents of the CUSC-PV and PV cells respectively under AM 1.5G illumination; A and A' are the areas of the CUSC and its light escape edge respectively; J SC =I SC / A is the current density; V OC It is the open-circuit voltage; FF is the fill factor of CUSC-PV; P in This refers to the irradiation intensity. The experimental measurement was J = 7.0 ± 0.2 mA·cm. -2 V OC =0.65±0.01V, FF=80±1%. Therefore, η=18.1±0.1%, PCE=3.7±0.1%. The measured average external quantum efficiency (EQE) is consistent with the photonic band structure of CUSC-PV, such as... Figure 14 We tested the PCE (Power Consumption Equation) of CUSC-PV cells outdoors between 8:00 AM and 5:00 PM on July 1, 2024, at corresponding light intensities. The PCE varied from 3.1% at 82 klux to a maximum of 3.7% at 129 klux, and then to 2.7% at 69 klux. Figure 15 The value remained high throughout the day. Long-term stability was also verified. After 1,500 hours of white LED irradiation under ambient conditions (25℃, 60% humidity), the PCE peak value remained at 95.4%. Figure 16 .

[0073] Comparative Example

[0074] The comparative examples have the same structure as Example 1, except that they contain only a single-layer CLC (cholesterol liquid crystal layer 21) or a disordered grating.

[0075] Under real-world conditions, sunlight passes through the architectural glass first, effectively blocking harmful ultraviolet light and protecting the CLC film from aging and yellowing. CLC can be coated onto a polymer film and attached to the interior side of the architectural glass to provide mechanical protection, thus ensuring long-term durability. Figure 17 A radar chart comparing the performance metrics of SSC, LSC, and CUSC is presented. Scattering solar concentrators (SSC); Luminescent solar concentrators (LSC). Clearly, CUSC outperforms the other two in all metrics. Due to its unique colorless unidirectional waveguide properties, CUSC exhibits excellent color rendering index (CRI = 91.3) and clarity, as well as higher AVT (64.2%) and concentration ratio. Multilayer CLCs can be easily coated or transferred onto glass, and only the light escape edge needs to be modified to install photovoltaic cells (reducing the number of photovoltaic cells by 75%). These advantages enable CUSC to be seamlessly integrated with existing building windows, resulting in significant economic efficiency.

[0076] Comparative analysis of the embodiments, as follows Figure 18 Multiple comparative studies were conducted to verify the technological advantages. Figure 18 In the middle: (1) Comparison between single-layer CLC and multi-layer broadband CLC: Single-layer CLC can only couple to one band, and its focusing efficiency and color retention are limited due to the thickness and pitch of the filter. In contrast, the multi-layer design used in this invention can couple multiple spectral bands simultaneously, and the overall reflection bandwidth is greatly increased. For example, experimental results show that the average reflectivity of the multi-layer structure in the visible light region is significantly higher than that of the single-layer structure, and its current density (J) SC (2) Comparison of random direction coupling (bare glass, no CLC on the surface) and unidirectional waveguide coupling: Without periodic modulation, the CLC layer may produce bidirectional or diffuse coupling, and a large amount of light energy is coupled or reflected back, resulting in low efficiency. After adopting the periodic grating of Embodiment 1 of the present invention, the light is only "locked" and coupled towards the edge of the glass, thereby focusing the light in one direction. Experiments and simulations show that the application of the grating structure increases the intensity of light coupled along the waveguide direction, while significantly reducing the intensity of back leakage light. In summary, the above comparative embodiments clearly show that the scheme of multilayer broadband CLC combined with grating coupling is superior to the traditional single-layer or non-directional coupling scheme in terms of visible light utilization, color authenticity (color uniformity and transparency), thermal stability and other indicators.

[0077] In summary, through detailed design and fabrication schemes, this invention verifies the effectiveness of the proposed polarization-selective waveguide coupling and broadband multilayer CLC structure, and successfully realizes a practically applicable colorless unidirectional diffraction solar concentrator.

[0078] Application Example 2

[0079] Intelligent window glass integration solution: The concentrator obtained in Example 1 is directly attached to or sandwiched inside a double-glazed window, enabling the window itself to function as a lighting facility with high visible light transmittance, while also guiding light to the photovoltaic cells at the window edge for collection, meeting building energy consumption requirements. Since the CLC film itself is nearly colorless and transparent, and is designed to ensure high average visible light transmittance (AVT>60%), the indoor lighting quality remains unchanged. By controlling the reflection bandwidth of each layer, indoor lighting needs can be considered, for example, only reflecting invisible ultraviolet and infrared light for power generation, while retaining more mid-visible light illumination.

[0080] Application Example 3

[0081] Mobile / Foldable Photovoltaic Products: The concentrator obtained in Example 1 can be made into a flexible thin film for use in outdoor awnings, tents, solar backpacks, or foldable portable photovoltaic devices. In mobile scenarios, the structure of this invention is lightweight and thin, and can be folded and carried; when in use, it can be attached to a transparent / semi-transparent support material to collect light energy, ensuring good light transmittance and improving the energy density of portable photovoltaic modules.

[0082] Application Example 4

[0083] Photovoltaic Applications in Agricultural Greenhouses: Agricultural greenhouses require sunlight of specific wavelengths to meet the needs of plant growth. The multi-layer CLC concentrator of this invention can be designed to couple and reflect only wavelengths with lower secondary plant requirements (such as some infrared light), while allowing the visible light needed by plants to pass directly through, thus achieving both shading and power generation. Its colorless properties ensure that the light quality inside the greenhouse remains almost constant, avoiding the adverse effects of traditional colored shading materials on plant growth; while the photovoltaic units installed at the edges can utilize the separated light energy to generate electricity, improving the overall efficiency of the greenhouse. For agricultural greenhouse applications, it can be selectively coated on the top or side walls, utilizing the light-guiding properties within the thickness of the transparent flat substrate 1 to provide power to the thin-film batteries placed at the edges, while having minimal impact on the light environment inside the greenhouse.

[0084] Example 2

[0085] To optimize coupling performance, a phase retardation film (such as a half-wave plate) can be introduced at an appropriate position above or below the CLC layer in Example 1 to adjust the polarization state. The presence of this layer can compensate for polarization conversion when the incident light direction is close to the Bragg angle.

Claims

1. A colorless unidirectional diffraction type solar concentrator, characterized in that: The system includes a transparent flat substrate (1), on which a light alignment layer (4) and a multilayer cholesteric liquid crystal film (2) are sequentially attached. The multilayer cholesteric liquid crystal film (2) includes a plurality of cholesteric liquid crystal layers (21), and adjacent cholesteric liquid crystal layers (21) have different pitches. A solar photovoltaic cell (3) for receiving and converting coupled light propagating along the interior of the transparent flat substrate (1) by total internal reflection is provided on the side of the transparent flat substrate (1). The light alignment layer (4) has a submicron-level lateral periodic arrangement.

2. The colorless unidirectional diffraction solar concentrator according to claim 1, characterized in that: The tilt angle between the tilted helical axis of the cholesteric liquid crystal layer (21) and the transparent flat substrate (1) is 10°~40°.

3. A colorless unidirectional diffraction solar concentrator according to claim 1, characterized in that: The cholesteric liquid crystal layer (21) is made of polymer chiral liquid crystal material. The concentration of chiral agent doped in cholesteric liquid crystal layers (21) with different pitches is different, and the concentration of chiral agent is 1~3wt%.

4. A colorless unidirectional diffraction solar concentrator according to claim 3, characterized in that: The chiral agent is any one of R5011, S5011, R811, and S811.

5. A colorless unidirectional diffraction solar concentrator according to claim 1, characterized in that: The pitch of the cholesteric liquid crystal layer (21) is 0~500 nm.

6. A colorless unidirectional diffraction solar concentrator according to claim 1, characterized in that: The solar concentrator has an average transmittance of ≥60% for visible light.

7. A method for preparing a colorless unidirectional diffraction solar concentrator according to any one of claims 1 to 6, characterized in that, Includes the following steps: Step 1: A photosensitive alignment layer is coated on a transparent flat substrate (1) and a photo-alignment layer (4) with a submicron-level lateral periodic arrangement is obtained by exposure. Step 2: Spin-coat or dip-coat multiple layers of cholesteric liquid crystal mixture with different pitches onto the surface of the photo-alignment layer (4) or cholesteric liquid crystal layer (21) obtained in Step 1 and cure them to form a multilayer cholesteric liquid crystal film (2). Step 3: Solar photovoltaic cells (3) are installed on the side of the transparent flat substrate (1).

8. A method for preparing a colorless unidirectional diffraction solar concentrator according to claim 7, characterized in that: In step one, the exposure is performed using a dual-beam interference method, and the grating period of the cholesteric liquid crystal layer (21) varies in the range of 400~800 nm according to the designed wavelength.

9. The method for preparing a colorless unidirectional diffraction solar concentrator according to claim 7, characterized in that: In step two, the multilayer cholesteric liquid crystal film (2) produces Bragg reflection of circularly polarized light with the same chirality as its helical direction, and transmits circularly polarized light with the opposite chirality.

10. The application of a colorless unidirectional diffraction solar concentrator according to claim 1 on the exterior surface of a building's transparent window.

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