PV modules with cholesteric liquid crystal reflectors

Abstract

The present invention relates to a photovoltaic module comprising a photovoltaic cell and a decorative layer on top of the photovoltaic cell, the decorative layer comprising one or more cholesteric liquid crystal reflectors, the one or more cholesteric liquid crystal reflectors having a curved interface. A method for manufacturing a photovoltaic module is also disclosed.

Description

PV MODULES WITH CHOLESTERIC LIQUID CRYSTAL REFLECTORSField of the Invention[oooi] The invention generally relates to a photovoltaic module with cholesteric liquid crystal reflectors, and a method for manufacturing the same.Acknowledgment

[0002] The project leading to this application has received funding from the University of Luxembourg, IAS - Audacity project, SolarZukunft.Background of the Invention

[0003] Climate change requires that we transition from a world powered by carbon emitting fossil fuel to one where we harvest energy from renewable sources without emission of greenhouse gases. The sixth assessment report of the Intergovernmental Panel on Climate Change makes clear that wind and solar energy technologies offer the biggest potential to reduce carbon emissions by far and, encouragingly, they are also the cheapest. Two studies of how our planet could run on 100% renewable energy found for Belgium, Luxembourg, and the Netherlands (exemplary countries with high population density and high energy demand) that between 2% and 6% of their respective land surface would need to be covered with photovoltaic (PV) modules in addition to wind turbines. To put this in perspective, the average percentage of man-made surface of these countries is 11%. This additional large area of modules would lead to cityscapes and some part of the landscapes appearing monotonously black in color, if conventional PV technology were used. Furthermore, although solar energy is generally perceived positively, large PV module installations have received negative feedback from the public. One way to overcome these potential drawbacks and improve public acceptance is to color the PV modules.

[0004] To date, colored PV modules have been developed mainly within the scope of building integrated photovoltaic (BIPV) applications. These are PV modules that have a dual function as energy generator and building faqade element. Since buildings account for 40% of final energy use, it makes great sense to also use the faqades for generating energy, and if BIPV can be realized such that it gives a significant contribution to the energy supply, it would reduce the amount of utility-scale PV that will have to be placed in non-urbanized spaces. Colored BIPV improves the possibility of fitting modules into their local environment in a visually pleasing way, thereby enabling more of the building’s visible surface to be used to generate energy. To allow any faqade section exposed to sunlight to be used it is not enough to make monocolored panels with a limited palette, however, because in many cases truly aesthetic integration requires arbitrary colors as well as patterning.

[0005] Of course, the additional coloring of the PV module should not inhibit an effective generation of electrical power. In fact, there is a trade-off between coloring a PV module and its ability to generate power, and this greatly depends on how the color is generated; any effect other than reflection of the desired color, such as absorption or indiscriminate scattering, must be avoided to minimize negative impact on PV performance.

[0006] Coloring a PV module may be achieved by locally applying a layer comprising pigments in inks. This may even be applied by liquid printing methods so as to create patterns or pictures with arbitrary colors. But this coloring comes at the cost of much greater reduction of up to 50% in PV performance, since the pigments absorb and scatter light that could have been used by the solar cells. However, such a solution offers a coloring of the PV module that is independent of the viewing angle.

[0007] Coloring a PV module may also be achieved by structuring a glass layer of the PV module and applying a thin multi-layer stack tuned for providing coloring by light interferences. Specifically, the multi-layer structures are designed to have regular changes in refractive index on the scale of light wavelengths, causing a Bragg like reflection of light within a specific color band. This allows the rest of the solar radiation to transmit through the underlying solar cells, hence minimizing impact on power conversion efficiency. Typically, these multi-layers are grown by vacuum deposition systems over large areas, the size of the module, to create permanent color whilst keeping a power output of over 90% of the original uncolored module. The disadvantages are (i) that it is challenging to pattern the solar cell in order to go beyond the mono-color PV category, (ii) that only spectral colors can be generated, thus no white or other non-spectral colors like brown or pink, and (hi) that the color is viewing and illumination angle-dependent, such that the actual color of the PV panel appears different e.g. if mounted on a vertical wall or inclined roof, and foreither case the color will also change between a sunny day with directed illumination compared to a cloudy day with diffuse light. In summary, such solutions offer a coloring of the PV module that is viewing angle-dependent and that has much higher manufacturing costs.General Description

[0008] A first aspect of the present invention relates to a photovoltaic module comprising a photovoltaic cell and a decorative layer on top of the photovoltaic cell. The decorative layer comprises (or consists of) one or more cholesteric liquid crystal reflectors. The one or more cholesteric liquid crystal reflectors have a curved interface (i.e. at least one curved interface).

[0009] As used herein, a “decorative layer” designates a decorative motif. The decorative layer is exposed in the sense that it is visually inspectable by the naked eye under ambient light, when looking at the photovoltaic module.

[0010] As used herein, a “liquid crystal” is a state of matter which has properties between those of conventional liquids and those of solid crystals. In other words, a liquid crystal can flow like a liquid but has some degree of ordering in the arrangement of its molecules.

[0011] As used herein, a “cholesteric liquid crystal” (also called chiral nematic liquid crystal) is a liquid crystal that exhibits a helical twisting of its molecules along an axis perpendicular to the preferred orientation of the molecules. In cholesteric liquid crystals, the helical modulation of the refractive index (due to the preferential molecular alignment direction rotating in the helical structure) gives rise to a selective (Bragg) reflection of light in a narrow wavelength band, the central wavelength of which, hereinafter referred to as “peak wavelength” for the sake of simplicity, is determined by the pitch of the helix, the average refractive index of the liquid crystal, the refractive index of the surrounding medium (for air this is very close to unity) and the angle of the light incidence. In particular, a retroreflection corresponds to incident and reflected light directions that are antiparallel. In this particular case, the light incidence is along the cholesteric helix. The retroreflection peak wavelength is thus the peak wavelength at an angle of incidence of zero. The average refractive index of the liquid crystal is the average of the ordinary and extraordinary refractive indices of the same liquid crystal in a state where the helixhas been unwound (for instance by an electric field), which in turn can be calculated from the average dielectric constant (E. Priestley. “Introduction to Liquid Crystals” (Ed: E. Priestley), Springer, Berlin, 1975, 203-218). The reflected light is circularly polarized with the same handedness as the cholesteric helix (left or right).

[0012] “Cholesteric liquid crystal reflectors” are objects based on cholesteric liquid crystals providing the selective (Bragg) reflection as disclosed above.

[0013] It should be noted that in the present document a “solar cell” is equivalent to a “photovoltaic cell” or a “PV cell”. They are thus used interchangeably.

[0014] As is customary in the art, the top (or front) side of a PV module is the side that is facing the environment (e.g. the sky) and where incident light enters the photovoltaic module, in use. Conversely, the back side of a photovoltaic panel is the side that is opposite to the front side and generally facing the wall or roof surface. Throughout the document, every reference to a direction is with respect to this convention.

[0015] It will be appreciated that a photovoltaic module according to the first aspect of the present invention allows for suppressing, or at least mitigating, viewing angle dependence of the Bragg reflections by having one or more cholesteric liquid crystal reflectors with a curved interface. Such a photovoltaic module will thus have decorative features that are less sensitive to variations in the viewing and illumination angles than for flat Bragg reflectors.

[0016] The one or more cholesteric liquid crystal reflectors may be polymerized, here implying the formation of a crosslinked network. Polymerized cholesteric liquid crystal reflectors are in a solid state, and retain the properties of the cholesteric liquid crystals with respect to light reflection as described in the preceding paragraphs since the helix remains and is now fixed (or frozen) due to the polymerization.

[0017] In a particularly preferred embodiment, the one or more cholesteric liquid crystal reflectors have a shape, wherein the shape is selected from: a hemispherical shape, a spherical shape and a semicylindrical shape. A semicylindrical shape offers reduced viewing and illumination angle dependence of Bragg reflections for rotations about a first axis, but not about a second axis that is perpendicular to the first axis. A hemispherical shape offers reduced viewing andillumination angle dependence of Bragg reflections for rotations about both perpendicular axes. In embodiments, the shape of the one or more cholesteric liquid crystal reflectors may be a convex shape. In embodiments, the shape of the cholesteric liquid crystal reflectors may be a convex polyhedron, such as an , a regular dodecahedron, a geodesic polyhedron.

[0018] In an embodiment, the one or more cholesteric liquid crystal reflectors are in direct contact with the photovoltaic cell.

[0019] In an embodiment, the decorative layer comprises (or consists of) a substrate having a top surface and a bottom surface, wherein the cholesteric liquid crystal reflectors are located on at least one of the top surface and the bottom surface of the substrate. In other words, the decorative layer comprises (or consists of) a substrate with cholesteric liquid crystal reflectors.

[0020] The substrate may be a glass layer, a polymeric sheet (e.g. poly(ethylene terephthalate) (PET), Ethylene Tetrafluoroethylene (ETFE), Fluoroethylene propylene (FEP), Polyimide (PI) etc.) or an encapsulation layer, (e.g a polymeric encapsulation layer such as a layer comprising or consisting of elastomers (e.g. poly-ethylene-vinyl-acetate (EVA), Polyisobutylene (PIB) and silicones), thermoplastics (e.g. polyvinyl butyral (PVB) and ionomers), thermoplastic elastomers (thermoplastic silicone elastomers (TPSE), thermoplastic polyolefins (TPO) and polyolefin elastomers (POE)) and multilayered encapsulant consisting of EVA-POE-EVA (EPE).

[0021] In a preferred embodiment, the substrate is a structured substrate comprising one or more indentations (i.e. depressions). The one or more cholesteric liquid crystal reflectors may be located in the one or more indentations. The one or more indentations may have a shape, wherein the shape is selected from: a hemispherical shape and a semicylindrical shape. In embodiments, the shape of the indentation may be a negative of a convex polyhedron, such as an hexagonal pyramid, a regular dodecahedron, a geodesic polyhedron, so that the cholesteric liquid crystal reflectors filling the indentations may have a convex polyhedron shape, such as an hexagonal pyramid, a regular dodecahedron, a geodesic polyhedron.

[0022] The indentations may be arranged as an array tiling the photovoltaic cell, preferably the tiling is a periodic tiling or a non-periodic tiling.

[0023] In embodiments, the periodic tiling has a period comprised in the range from 0.1 to 50 mm, preferably in the range from 5 mm to 25 mm, even more preferably in the range from 10 mm to 15 mm.

[0024] The one or more cholesteric liquid crystal reflectors of the decorative layer may comprise a first set of cholesteric liquid crystal reflectors having a first retroreflection peak wavelength and a second set of cholesteric liquid crystal reflectors having a second retroreflection peak wavelength, the first and second retroreflection peak wavelengths being different.

[0025] The one or more cholesteric liquid crystal reflectors of the decorative layer may comprise a third set of cholesteric liquid crystal reflectors having a third retroreflection peak wavelength, the third retroreflection peak wavelength being different from the first and second retroreflection peak wavelengths.

[0026] The first set of cholesteric liquid crystal reflectors and the second set of cholesteric liquid crystal reflectors may be arranged so that the decorative layer has a(n apparent) color that is non-spectral to the (naked) human eye. As used herein, non-spectral colors are colors that are not in the spectrum of visible light. They can be obtained by a mixture of spectral wavelengths.

[0027] A second aspect of the present invention relates to a method for manufacturing a photovoltaic module, preferably according to the first aspect of the present invention. The method comprises providing a photovoltaic cell and applying a cholesteric liquid crystal mixture so as to provide a decorative layer, on top of the photovoltaic cell, with one or more cholesteric liquid crystal reflectors having a curved interface (i.e. at least one curved interface).

[0028] The method may further comprise polymerizing the applied cholesteric liquid crystal mixture so as to provide the decorative layer with one or more polymerized cholesteric liquid crystal reflectors having the curved interface.

[0029] In an embodiment, the application of the cholesteric liquid crystal mixture is carried out so that the cholesteric liquid crystal reflectors have a shape, the shape being selected from: a hemispherical shape, a spherical shape and a semicylindrical shape. In embodiments, the shape of the cholesteric liquid crystalreflectors may be a convex polyhedron, such as an hexagonal pyramid, a regular dodecahedron, a geodesic polyhedron.

[0030] In an embodiment, the application is carried out so that the cholesteric liquid crystal mixture and the one or more cholesteric liquid crystal reflectors are in direct contact with the photovoltaic cell.

[0031] In an embodiment, the substrate has a top surface and a bottom surface, wherein the cholesteric liquid crystal mixture is applied on at least one of the top surface and the bottom surface of the substrate.

[0032] The substrate may be a glass layer or an encapsulation layer.

[0033] The substrate may be a structured substrate comprising one or more indentations, the application of the cholesteric liquid crystal mixture being carried out so as to at least partially fill the one or more indentations thereby providing cholesteric liquid crystal reflectors in the one or more indentations.

[0034] In an embodiment, the one or more indentations have a shape, wherein the shape is selected from: a hemispherical shape and a semicylindrical shape.

[0035] The method may further comprise applying a second cholesteric liquid crystal mixture so as to provide a decorative layer on top of the photovoltaic cell with one or more second cholesteric liquid crystal reflectors having a curved interface, wherein the cholesteric liquid crystal reflectors have a retroreflection peak wavelength and the second cholesteric liquid crystal reflectors have a second retroreflection peak wavelength, the first and second retroreflection peak wavelengths being different.

[0036] The method may further comprise applying a third cholesteric liquid crystal mixture so as to provide a decorative layer on top of the photovoltaic cell with one or more third cholesteric liquid crystal reflectors having a curved interface, the third cholesteric liquid crystal reflectors having a third retroreflection peak wavelength, the third retroreflection peak wavelength being different from the first and second retroreflection peak wavelengths.

[0037] In an embodiment, the first cholesteric liquid crystal reflectors and the second cholesteric liquid crystal reflectors are arranged so that the decorative layer has a(n apparent) color that is non-spectral to the (naked) human eye.

[0038] The application of the cholesteric liquid crystal mixture may be carried out by at least one of drop on demand techniques such as inkjet printing, possibly inkjet digital printing, or micro-dispensing; or with a doctor blade, bar, slot-die coating, gravure printing, screen printing, spray coating, meniscus and dip coating.

[0039] In embodiments according to the first aspect or the second aspect of the invention, the decorative layer may be a monolithic decorative layer.

[0040] In the present document, the verb “to comprise” and the expression “to be comprised of” are used as open transitional phrases meaning “to include” or “to consist at least of’. Unless otherwise implied by context, the use of singular word form is intended to encompass the plural, except when the cardinal number “one” is used: “one” herein means “exactly one”. Ordinal numbers (“first”, “second”, etc.) are used herein to differentiate between different instances of a generic object; no particular order, importance or hierarchy is intended to be implied by the use of these expressions. Furthermore, when plural instances of an object are referred to by ordinal numbers, this does not necessarily mean that no other instances of that object are present (unless this follows clearly from the context). When reference is made to “an embodiment”, “one embodiment”, “embodiments”, etc., this means that these embodiments may be combined with one another. Furthermore, the features of those embodiments can be used in the combination explicitly presented but also that the features can be combined across embodiments without departing from the invention, unless it follows from context that features cannot be combined.

[0041] As used herein, the expression “at least one of’ followed by a list of items means any one of the listed items or any combination of any plural items from the list. The conjunction “and” possibly preceding an item from the list (e.g., typically, the last item) shall not be construed as an exclusion of combinations that do not include the item(s) preceded by “and” or that do not include all items from the list.Brief Description of the Drawings

[0042] The accompanying drawings illustrate several aspects of the present invention and, together with the detailed description, serve to explain the principles thereof. In the drawings:Fig. 1: depicts several embodiments of photovoltaic modules according to the present invention;Fig. 2: shows the chemical structure of the components used for the cholesteric liquid crystal mixture, according to an embodiment of the present invention;Fig. 3: shows red, green and blue inkjet-printed cholesteric liquid crystal reflector droplets on CIGSe solar cells, under diffuse light illumination;Fig. 4: shows a textured substrate comprising semi cylindrical indentations on a surface of the substrate, according to an embodiment of the present invention;Fig. 5: shows a textured substrate comprising semicylindrical indentations on both top and bottom surfaces of the substrate, according to an embodiment of the present invention;Fig. 6: shows a textured substrate comprising hemispherical indentations on a surface of the substrate, according to an embodiment of the present invention; andFig. 7: a) is an image showing a PDMS substrate comprising hemispherical indentations filled with red cholesteric crystal reflectors at 3 different viewing angles (o°, 30° and 450) under diffuse illumination, b) shows the reflectance as a function of the wavelength for the PDMS encapsulation layer of a) for three different viewing angles: o°, 30°and 450. The image and spectra were illuminated by indoor diffuse ambient light.

[0043] The reader’s attention is drawn to the fact that the drawings are not to scale. Furthermore, for the sake of clarity, proportions between height, length and / or width, even the texture of a layer, may not have been represented correctly.Detailed Description of Preferred Embodiments of the Invention

[0044] Fig. 1 depicts several embodiments of photovoltaic modules 10a, 10b, 10c, lod according to the present invention. Photovoltaic modules 10a, 10b, 10c, lod differ in the arrangement of layers. A photovoltaic module according to the present invention, wherein the specific arrangement of layers is immaterial, is collectively referred to with reference sign “10”.

[0045] The photovoltaic module 10a comprises, from top to bottom, photovoltaic cell 12, a decorative layer 14 on top of the photovoltaic cell 12, and a glass layer 16. The decorative layer 14 comprises an encapsulation layer 18 on top of cholesteric liquid crystal reflectors 20, the encapsulation layer 18 being on top of the cholesteric liquid crystal reflectors 20.

[0046] The photovoltaic module 10b is identical to the photovoltaic module 10a except in that the cholesteric liquid crystal reflectors 20 are on top of the encapsulation layer 18.

[0047] In photovoltaic module 10c, the decorative layer 14 comprises two encapsulation layers 18, 18’ sandwiching the cholesteric liquid crystal reflectors 20.

[0048] Photovoltaic module lod comprises from bottom to top, a photovoltaic cell 12, an encapsulation layer 18, a glass layer 16, a decorative layer 14 consisting of cholesteric liquid crystal reflectors 20 and a protective layer 22. Of course, in other embodiments, the decorative layer 14 may comprise one or more encapsulation layers as shown in the previous embodiments. The protective layer 22 may comprise or consist of a glass layer or a polymer sheet. This embodiment is particularly advantageous for retrofitting a standard photovoltaic module, thereby providing a decorative layer 18 as disclosed herein to the standard photovoltaic module.

[0049] Other layers such as an anti-reflection layer comprising for example MgF2, i-ZnO, AI2O3, Si(O,N), Si02layers may be present in the photovoltaic module 10 (not depicted) in any position above the photovoltaic cell 12.

[0050] The glass layer 16 and / or the protective layer 22 may be transparent or at least translucent, in particular within visible (400 nm to 800 nm), near infrared (800 nm to 2500 nm) and / or near ultraviolet (200 nm to 400 nm) wavelength ranges.

[0051] In embodiments, the cholesteric liquid crystal reflectors 20 may be polymerized.

[0052] In all embodiments, cholesteric liquid crystal reflectors 20 have a curved interface (i.e. a non-zero curvature). It is worthwhile noting that the figures may represent the liquid crystal reflectors 20 as a flat layer for the sake of simplicity and clarity. In particularly preferred embodiments, cholesteric liquid crystal reflectors 20 have a hemispherical shape, a spherical shape or a semi cylindrical shape.

[0053] The photovoltaic module 10 may comprise a decorative layer having cholesteric liquid crystal reflectors exhibiting different retroreflection peak wavelengths. In other words, the decorative layer may reflect different colors. For instance, a first set of cholesteric liquid crystal reflectors may have a first retroreflection peak wavelength and a second set of cholesteric liquid crystal reflectors may have a second retroreflection peak wavelength. The first and second retroreflection peak wavelengths are different. In a preferred embodiment, the photovoltaic module 10 comprises even a third set of cholesteric liquid crystal reflectors having a third retroreflection peak wavelength, the third retroreflection peak wavelength being different from the first and second retroreflection peak wavelengths. The first set may be tuned to have a retroreflection peak wavelength in the red, the second set may be tuned to have a retroreflection peak wavelength in the green, and the third set may may be tuned to have a retroreflection peak wavelength blue. This is particularly advantageous since the three sets of cholesteric liquid crystal reflectors provide primary red, green and blue colors.

[0054] When at least two sets of cholesteric liquid crystal reflectors are part of the decorative layer, the respective cholesteric liquid crystal reflectors may be arranged so that the decorative layer has a color that is non-spectral to the (naked) human eye. In an embodiment, the cholesteric liquid crystal reflectors of different sets may be packed closely together so as to achieved said non-spectral to the (naked) human eye.

[0055] In general, the cholesteric liquid crystal reflectors 20 are applied (directly) on a substrate which, depending on the embodiments, may be the photovoltaic cell 12, the encapsulation layer 18, the glass layer 16 or the protectivelayer 22. The cholesteric liquid crystal reflectors 20 maybe applied on a top surface or a bottom surface of the aforementioned layers.

[0056] The photovoltaic cell 12 may be based on the following technologies: a silicon based solar cell, a tandem or multijunction solar cell, a III-V based solar cell, a thin film solar cell, or a quantum dot solar cell. The silicon based solar cell may be mono- or multi-crystalline (e.g. heterostructures (HIT or HJT), IBC, PERC, TOPCON). The tandem or multijunction solar cell may be a Si / Perovskite solar cell, a CIGS(e) / Perovskite solar cell, a Si / CIGS solar cell, a Perovskite / Perovskite solar cell, or a CIGS / CIGS solar cell. The III-V based solar cell maybe a GaAs solar cell or a GalnP solar cell. The thin film solar cell may be a Perovskite solar cell, Cadmium telluride (CdTe) solar cell, Silver copper indium gallium sulfur selenide ((Ag,Cu)(In,Ga)(S,Se)2where at least one of each element in each bracket is required, commonly referred to in the literature as ACIGSSe or simply CIGS) solar cell, Copper zinc tin sulfide selenide (CZTSSe) solar cell, Organic photovoltaic (OPV) solar cell, Dye-sensitized solar cell, Amorphous silicon (a-Si) solar cell. In a preferred embodiment, the photovoltaic cell 12 is an inorganic thin film Cu(In,Ga)Se2(CIGSe) based solar cell. Of course, the photovoltaic module 10 may comprise a plurality of photovoltaic cells.

[0057] In essence, the method for manufacturing a photovoltaic module comprises providing a photovoltaic cell and applying a cholesteric liquid crystal mixture so as to provide a decorative layer on top of the photovoltaic cell with one or more cholesteric liquid crystal reflectors having a curved interface. The cholesteric liquid crystal mixture maybe polymerized after application.

[0058] As already disclosed, the application is carried out on a substrate which may be any of: the photovoltaic cell 12, the encapsulation layer 18, the glass layer 16 and the protective layer 22.

[0059] In an embodiment, the application is carried out by inkjet printing.

[0060] A first step comprises the preparation of the cholesteric liquid crystal mixture. For example, the mixture may comprise i,4-bis-[4-(3-acryloyloxypropyloxy) benzoyloxy]-2-methylbenzent (ST03021, SYNTHON GmbH), 6-(4-cyano-biphenyl-4'-yloxy) hexyl acrylate (ST03474, SYNTHON GmbH), 4[4[6-Acryloxyhex-i-yl)oxyphenyl]carboxy-benzonitrile (ST02670,SYNTHON GmbH), 1,6-Hexanediol diacrylate (HDDA Sigma Aldrich) as diacrylate crosslinker, S5011 (HCCH) as chiral dopant and IRG651 (Merck) as photoinitiator. Depending on the desired color, the ratio between the components may vary. Fig. 2 shows the chemical structure of the components. Examples of mixtures providing red, green and blue color, respectively, are provided below.ST03021 STO3474 ST02670 S5011 HDDA IRG651 Red 43-7% 28.6% 19.8% 1.9% 5% 1% Green 1043-5% 28.5% 19.7% 2.3% / 5% 1 / 0 Blue 1043-4% 28.4% 19.6% 2.6% / 5% 1 / 0Table 1: Composition of mixtures for providing red, green and blue cholesteric liquid crystal reflectors. The color is the retroreflection color of flat film under diffuse light illumination.

[0061] Other compositions are of course possible.

[0062] If needed, a solvent is added for adapting the viscosity of the mixtures for enabling the use of commercial inkjet heads and nozzles. Particularly preferred solvents are those which evaporate at room temperature after deposition (i.e. for inkjet printing). The solvent maybe amyl acetate, PGME and / or PGMEA.

[0063] Specifically, the components shown in Fig. 2 are weighed separately with specific ratios as disclosed in Table. 1. These ratios are calculated from the Schroder-van Laar equation to formulate eutectic mixtures. The ratios are also tuned depending on which color is desired, since the color is directly affected by the chiral dopant (S5011) concentration. All components are then put together in a vial and kept at 9O°C for one hour, while being stirred with a magnetic bar. After cooling down the mixture, the solvent is added so as to provide a composition (ink) that may be applied by inkjet printing. The mixture with the solvent is then stirred with a magnetic bar and heated to 90 °C until the mixture is homogeneous. After cooling down, the ink is ready to be loaded in the printer’s cartridges and, in turn, deposited on the substrate.

[0064] Fig. 3a) shows red 24, green 26 and blue 28 inkjet-printed cholesteric liquid crystal reflector droplets on CIGSe solar cells under diffuse light illumination. The droplets are approximately hemispherical. Fig. 3b) demonstrates a mitigatedblue shift of the colors of the decorative layer comprising the curved reflectors, when compared to an equivalent decorative layer being a thin film comprising reflectors without curved interface (surface).

[0065] The droplets may be polymerized, preferably before assembling the decorative layer to the other layers of the module (e.g. the glass layer).

[0066] In an embodiment, the application is carried out on an encapsulation layer (or possibly on a glass layer) that is textured. The texture exhibits valleys (indentations) and hills, wherein the cholesteric liquid crystal mixture fills (at least partially) one or more of the valleys (indentations). The application may be carried out by a doctor blade, or any other way disclosed in the present document. In embodiments, the cholesteric liquid crystal mixture may coat the valleys. The thickness of the coating may be comprised in the range from 1 pm to 5 mm, preferably from 5 pm to 100 pm. In other words, the coating forms a (thin) film on the surface of the valleys. The cholesteric liquid crystal mixture may be polymerized after being applied on the encapsulation layer, and possibly before assembling the decorative layer to the other layers of the module.

[0067] For example, the compositions of the mixtures for obtaining blue, green or red retroreflection color for cholesteric liquid crystal reflectors under diffuse light illumination may be the following:ST03021 ST03866 R5011 HDDA Irg65i Color(wt.%) (wt.%) (wt.%) (wt.%) Blue 36.1 54-25 2.65 5 2 Green 36.3 54-45 2.25 5 2 Red 36.5 54-62 1.88 5 2Table 2: CLC mixture composition

[0068] i,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (ST03021 - SYNTHON GmbH) is a bifunctional polymerizable mesogen, 4-Methoxybenzoic acid 4-(6-acryloyloxy-hexyloxy)phenyl ester (ST03866 -SYNTHON GmbH) is a monofunctional polymerizable monomer, 1,6-Hexanediol diacrylate (HDDA Sigma Aldrich) as diacrylate crosslinker, R5011 is a right-handedchiral dopant (HCCH, China) and 2,2 Dimethoxy-2-phenylacetophenone (IRG651 -Sigma Aldrich) is a photo initiator.

[0069] Of course, other mixtures are possible. Another particularly advantageous example of mixture is discussed at the end of the description.

[0070] The final reflectance of the photovoltaic module may be tuned, e.g., by adapting the geometry of the texture, in particular by adapting the size of the valleys, and / or the distance between the valleys. The final reflectance may also be tuned by adapting the optical path of the incoming light within the cholesteric liquid crystal reflectors. For example, increasing the optical path of the incoming light within the cholesteric liquid crystal reflectors will (in general) increase the final reflectance.

[0071] According to an embodiment, the texture is an array 30 of (regularly spaced) semicylindrical indentations, as shown in Fig. 4. The indentations are provided on a top surface of the encapsulation layer. In another embodiment, the indentations maybe provided on a bottom surface of the encapsulation layer. As the cholesteric crystal mixture is applied on the surface, it will conform to the indentations and thus take a semicylindrical shape. Each indentation may be filled with a mixture providing the same color. Each of the indentations may be filled with any one of the prepared mixtures, thereby providing a multi color photovoltaic module. Of course, as indicated above, depending on the arrangement of indentations, domains of the photovoltaic module may appear as having a nonspectral color to the (naked) human eye. In an embodiment, the indentations may be provided on the bottom surface of the glass layer.

[0072] According to an embodiment, both surfaces of the encapsulation layer may be textured. An example of such a configuration is shown in Fig. 5, wherein both the top and bottom surfaces of the encapsulation layer are textured with array 30 of (regularly spaced) semicylindrical indentations. In the example of Fig. 5, the top surface comprises red cholesteric liquid crystal reflectors and the bottom surface comprises green cholesteric liquid crystal reflectors. Humans looking at a photovoltaic module comprising said encapsulation layer would see a photovoltaic module having an olive color. This embodiment simplifies the provision of photovoltaic modules having non spectral colors, and also decreases the costs of producing said photovoltaic modules having non spectral colors, with respect to the previous embodiment.

[0073] Semicylindrical indentations allow for suppressing, or at least mitigating, the viewing and illumination angle dependence of the reflected colors about one rotation axis, i.e. the axis that is perpendicular to the longitudinal axis of the semicylindrical indentations.

[0074] According to a preferred embodiment, the texture comprises an array 30 of (regularly spaced) hemispherical indentations 32, as depicted in Fig. 6. The indentations can individually be (at least partially) filled with any of the mixtures. In other words, this configuration provides a pixelated decorative layer, wherein the color of each pixel maybe individually chosen (e.g. red, green or blue). Such configuration provides a great flexibility and allows for creating (virtually) any decorative pattern.

[0075] Due to their symmetry, the hemispherical indentations allow for suppressing, or at least greatly mitigating, the viewing and illumination angle dependence of the reflected colors of the reflectors for rotation about any axis.

[0076] The indentations may have a diameter comprised in the range from 0.1 mm to 50 mm, preferably from 0.3 mm to 20 mm, even more preferably from 0.4 mm to 15 mm, most preferably from 0.5 mm to 3 mm, thereby allowing for the provision of cholesteric crystal reflectors having a diameter comprised in the same range.

[0077] In the present embodiment, the top surface of the encapsulant is textured. Of course, in alternative, the bottom surface may be textured in the same way. In an embodiment, both surfaces may be textured with hemispherical indentations.

[0078] A polymerization of the applied mixture may be carried out. The polymerization maybe carried out in a N2atmosphere.

[0079] In particular, a PDMS mold having an array of hemispherical indentations (diameter 0.5 mm) has been manufactured by polymer 3D printing.

[0080] Fig. 7a) shows a PDMS layer comprising said hemispherical indentations filled with red retroreflection cholesteric liquid crystal reflectors at 3 different viewing angles (o°, 30° and 450) under diffuse light illumination. The appearance -Fig. 7a)- was observed a DSLR camera (Canon EOS 100D). Fig. 7b) shows the reflectance under diffuse illumination as a function of the wavelength forthe PDMS filled with CLC of Fig. 7a) for three different viewing angles: o°, 30° and 450. The reflection spectra -Fig. 7b)- were measured by using unpolarized white illumination and an Avantes AvaSpec-2048 spectrophotometer. As can be readily seen in Fig. 7b), the blue shift of the reflectance peak around 700 nm is greatly mitigated so that the red appearance remains stable even when the viewing angles increases.

[0081] In the following an alternative and particularly advantageous mixture composition is proposed, which offers the advantage of being able to tune the retroreflection peak wavelength of the reflectors by adapting the temperature at which the mixture is polymerized after application on the substrate. The retroreflection color changes continuously from blue at high temperature to red at low temperature, allowing any spectral color to be selected by choosing the temperature.

[0082] In a preferred embodiment, the CLC mixture comprises the reactive mesogen i,4-Bis[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (ST00975, SYNTHON GmbH), the chiral dopant (3R,3aS,6aS)-Hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-(4-((4- (acryloyloxy)butoxy)carbonyloxy)benzoyloxy)benzoate) (ST06287, SYNTHON GmbH), the benzoic acid derivatives 4-(3-Acryloyloxyn-props-i-yloxy)benzoic acid (ST02453, SYNTHON GmbH), 4-(5-Acryloxypentyl-i-oxy)benzoic acid (ST02454, SYNTHON GmbH) and 4-(6-Acryloyloxy-n-hex-i-yloxy)benzoic acid (ST00902, SYNTHON GmbH) and the reactive mesogen 4-Methoxybenzoic acid 4-(6-acryloyl oxyhexyloxy) phenyl ester (ST06477, SYNTHON GmbH), and the photoinitiator 2,2 Dimethoxy-2-phenylacetophenone (Irg65i, Sigma Aldrich). The relative mass ratios (in wt.%) are shown in Table. 3 below.ST00975 ST06477 ST02453 ST02454 ST00902 ST06287 IRG651 13 38 6.45 17.2 19-35 5 1Table 3: CLC mixture composition in wt. %.

[0083] The CLC mixture of Table 3 is magnetically stirred at 8o°C (above the clearing point of the mixture) for around 5 h to ensure that all the components are homogeneously mixed.

[0084] The mixture is dripped on the photovoltaic cells at 66°C, then covered with glass and left to align for 5 minutes. A pre-designed photomask is then placed on the glass surface and a handheld UV-LED system (30W IP66 Onforuled, China) is used for photopolymerizing the blue retroreflection color. After 10 s UV exposure, the sample is cooled down to 5O°C at 5°C / min. After stabilizing the temperature for 5 minutes, the first photomask is replaced by a second photomask, and then a photopolymerization is again carried out for 10 s to cure the green retroreflection color. Finally, the photomask is removed, and the sample is cooled down to io°C for another photopolymerization for 10 s for curing the red retroreflection color. The PDMS layer shown in Fig. 7a) was manufactured with this method and mixture, but only with the step for having a red retroreflection color was carried out.

[0085] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.

Claims

Claims1. A photovoltaic module comprising:a photovoltaic cell;a decorative layer on top of the photovoltaic cell, the decorative layer comprising one or more cholesteric liquid crystal reflectors, the one or more cholesteric liquid crystal reflectors having a curved interface.

2. The photovoltaic module according to claim 1, wherein the one or more cholesteric liquid crystal reflectors are polymerized.

3. The photovoltaic module according to any one of claims 1 to 2, wherein the one or more cholesteric liquid crystal reflectors have a shape, wherein the shape is selected from: a hemispherical shape, a spherical shape and a semicylindrical shape.

4. The photovoltaic module according to any one of claims 1 to 3, wherein the one or more cholesteric liquid crystal reflectors are in direct contact with the photovoltaic cell.

5. The photovoltaic module according to any one of claims 1 to 3, wherein the decorative layer comprises a substrate having a top surface and a bottom surface, wherein the cholesteric liquid crystal reflectors are located on at least one of the top surface and the bottom surface of the substrate.

6. The photovoltaic module according to claim 5, wherein the substrate is a glass layer, a polymeric sheet or an encapsulation layer.

7. The photovoltaic module according to any one of claims 5 to 6, wherein the substrate is a structured substrate comprising one or more indentations, the one or more cholesteric liquid crystal reflectors being located in the one or more indentations.

8. The photovoltaic module according to claim 7, wherein the one or more indentations have a shape, wherein the shape is selected from: a hemispherical shape and a semicylindrical shape.

9. The photovoltaic module according to any one of claims 7 to 8, wherein the substrate comprises a plurality of indentations arranged as an array tiling thephotovoltaic cell, preferably the tiling is a periodic tiling or a non-periodic tiling.

10. The photovoltaic module according to any one of claims 1 to 9, wherein the one or more cholesteric liquid crystal reflectors of the decorative layer comprise a first set of cholesteric liquid crystal reflectors having a first retroreflection peak wavelength and a second set of cholesteric liquid crystal reflectors having a second retroreflection peak wavelength, the first and second retroreflection peak wavelengths being different.

11. The photovoltaic module according to claim 10, wherein the one or more cholesteric liquid crystal reflectors of the decorative layer comprise a third set of cholesteric liquid crystal reflectors having a third retroreflection peak wavelength, the third retroreflection peak wavelength being different from the first and second retroreflection peak wavelengths.

12. The photovoltaic module according to any one of claims 10 to 11, wherein the first set of cholesteric liquid crystal reflectors and the second set of cholesteric liquid crystal reflectors are arranged so that the decorative layer has a color that is non-spectral to the (naked) human eye.

13. A method for manufacturing a photovoltaic module comprising:providing a photovoltaic cell;applying a cholesteric liquid crystal mixture so as to provide a decorative layer on top of the photovoltaic cell with one or more cholesteric liquid crystal reflectors having a curved interface.

14. The method according to claim 13, further comprising polymerizing the applied cholesteric liquid crystal mixture so as to provide the decorative layer with one or more polymerized cholesteric liquid crystal reflectors having the curved interface.

15. The method according to any one of claims 13 to 14, wherein the application of the cholesteric liquid crystal mixture is carried out so that the cholesteric liquid crystal reflectors have a shape, the shape being selected from: a hemispherical shape, a spherical shape and a semicylindrical shape.

16. The method according to any one of claims 13 to 15, wherein the application is carried out so that the cholesteric liquid crystal mixture and the one or morecholesteric liquid crystal reflectors are in direct contact with the photovoltaic cell.

17. The method according to any one of claims 13 to 15, wherein the substrate has a top surface and a bottom surface, wherein the cholesteric liquid crystal mixture is applied on at least one of the top surface and the bottom surface of the substrate.

18. The method according to claim 17, wherein the substrate is a glass layer or an encapsulation layer.

19. The method according to any one of claims 17 to 18, wherein the substrate is a structured substrate comprising one or more indentations, the application of the cholesteric liquid crystal mixture being carried out so as to at least partially fill the one or more indentations thereby providing cholesteric liquid crystal reflectors in the one or more indentations.

20. The method according to claim 19, wherein the one or more indentations have a shape, wherein the shape is selected from: a hemispherical shape and a semicylindrical shape.

21. The method according to any one of claims 13 to 20, further comprising applying a second cholesteric liquid crystal mixture so as to provide a decorative layer on top of the photovoltaic cell with one or more second cholesteric liquid crystal reflectors having a curved interface, wherein the cholesteric liquid crystal reflectors have a retroreflection peak wavelength and the second cholesteric liquid crystal reflectors have a second retroreflection peak wavelength, the first and second retroreflection peak wavelengths being different.

22. The method according to claim 21, further comprising applying a third cholesteric liquid crystal mixture so as to provide a decorative layer on top of the photovoltaic cell with one or more third cholesteric liquid crystal reflectors having a curved interface, the third cholesteric liquid crystal reflectors having a third retroreflection peak wavelength, the third retroreflection peak wavelength being different from the first and second retroreflection peak wavelengths.

23. The method according to any one of claims 21 to 22, wherein the first cholesteric liquid crystal reflectors and the second cholesteric liquid crystal reflectors are arranged so that the decorative layer has a color that is nonspectral to the (naked) human eye.

24. The method according to any one of claims 13 to 23, wherein the application of the cholesteric liquid crystal mixture is carried out by at least one of inkjet printing, micro-dispensing or with a doctor blade, bar, slot-die coating, gravure printing, screen printing, spray coating, meniscus and dip coating.

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