LU507448B1

LU507448B1 - Photovoltaic module comprising polymerized cholesteric liquid crystals and method for manufacturing the same

Info

Publication number: LU507448B1
Application number: LU507448A
Authority: LU
Inventors: Phillip J Dale; Jan LAGERWALL; Yansong Zhang; Hasan Arif Yetkin; Hakam Agha
Original assignee: Univ Luxembourg
Priority date: 2024-06-11
Filing date: 2024-06-11
Publication date: 2025-12-11
Also published as: WO2025256799A1

Abstract

An aspect of the present invention pertains to a photovoltaic (PV) module. The PV module comprises a photovoltaic cell, a (reflective) decorative layer on top of the photovoltaic cell and a protective layer on top of the decorative layer. The decorative layer comprises (or consists of) polymerized cholesteric liquid crystals (CLCs), the polymerized cholesteric liquid crystals comprising a first set of polymerized cholesteric liquid crystals having a first retroreflection peak wavelength and a second set of polymerized cholesteric liquid crystals having a second retroreflection peak wavelength, the first and second retroreflection peak wavelengths being different, the decorative layer comprising a first domain and a second domain, the first domain comprising (or consisting of) the first set of polymerized cholesteric liquid crystals and the second domain comprising (or consisting of) the second set of polymerized cholesteric liquid crystals. Other aspects of the invention relate to a surface comprising said PV module as well as to a method for manufacturing said PV module.

Description

1 LU507448

DESCRIPTION

PHOTOVOLTAIC MODULE COMPRISING POLYMERIZED

CHOLESTERIC LIQUID CRYSTALS AND METHOD FOR

MANUFACTURING THE SAME

Field of the Invention

[0001] The invention generally relates to a photovoltaic module and to a method for manufacturing a photovoltaic module.

Acknowledgment

[0002] The project leading to this application has received funding from the

University of Luxembourg.

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

2 LU507448 modules that have a dual function as energy generator and building façade element.

Since buildings account for 40% of final energy use, it makes great sense to also use the façades 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 façade 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.

General Description

[0007] À first aspect of the present invention pertains to a photovoltaic (PV) module. The PV module comprises a photovoltaic cell, a (reflective) decorative layer on top of the photovoltaic cell and a protective layer on top of the decorative layer.

The decorative layer comprises (or consists of) polymerized cholesteric liquid crystals (CLCs), the polymerized cholesteric liquid crystals comprising a first set of cholesteric liquid crystals having a first retroreflection peak wavelength and a second set of cholesteric liquid crystals having a second retroreflection peak wavelength, the first and second retroreflection peak wavelengths being different, the (area of the) decorative layer comprising a first domain and a second domain, the first domain comprising (or consisting of) the first set of cholesteric liquid

3 LU507448 crystals and the second domain comprising (or consisting of) the second set of cholesteric liquid crystals.

[0008] 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.

[0009] 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 one experienced by light reflected by the helix (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).

[00010] As used herein “polymerized cholesteric liquid crystals” is matter whichis derived from cholesteric liquid crystals by polymerization. This matter may be in a glassy state. The polymerized cholesteric liquid crystals retain the properties of the cholesteric liquid crystals with respect to light reflection as described in the preceding paragraph since the helix remains and is now fixed (or frozen) due to the polymerization. In particular, as will be seen in the following, the pitch of the helix does not vary with respect to temperature.

[00011] It should be noted that in the present document a “solar cell” is equivalent to a “PV cell”. The two are thus used interchangeably.

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[00012] It will be appreciated that a PV module according to the first aspect of the invention allows for providing PV modules reflecting non-spectral colors which can be the combination of the two colors reflected by the respective first and second domains formed by the polymerized CLCs of the decorative layer, possibly so as to camouflage the PV module in its surrounding. This allows for aesthetically integrating the PV modules into e.g. building facades as well as roofs in a way that is fully acceptable by the public. The PV module according to the first aspect of the invention also allows for minimizing the impact of the decorative layer on the solar cell performance, in comparison to conventional decorative technologies e.g. pigments. Indeed, a key difference from other decorative technologies is that the chirality of CLC helix selects only one handedness of circular polarization for reflection: a right-handed CLC helix reflects all right-handed polarized light within the reflection band, but the left-handed light is passed through. This means that

CLC-generated color can have exceptionally small impact on the performance of PV modules, since all light outside the reflection band and half of the light within the reflection band is transmitted, since the incoming light is unpolarized.

[00013] The polymerized cholesteric liquid crystals comprise (is derived from) at least one of: a chiral dopant, a (blend of) reactive mesogen(s), a photo-initiator, a catalyst, and a radical scavenger. The chiral dopant may be polymerizable or not.

The polymerized cholesteric liquid crystals may also comprise (be derived from) one or more benzoic acid derivatives.

[00014] In an embodiment, the decorative layer has a thickness comprised in the range from 1 um to 100 um, preferably in the range from 3 um to 50 um, more preferably in the range from 7 um to 20 um, even more preferably in the range from the 10 um to 16 um.

[00015] The PV module may have a relative power conversion efficiency of at least 50%, preferably of at least 60%, even more preferably of at least 80%, most preferably of at least 90%, the relative power conversion efficiency being computed as a ratio between the power conversion efficiency of said photovoltaic module and the power conversion efficiency of the photovoltaic module without the decorative layer. The relative power conversion efficiency thus measures the impact of the decorative layer on the incident light to electrical output power conversion efficiency of the PV module.

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[00016] The polymerized cholesteric liquid crystals may comprise a third set of polymerized cholesteric liquid crystals having a third retroreflection peak wavelength, the third retroreflection peak wavelength being different from the first and second retroreflection peak wavelengths, the decorative layer comprising a third domain, the third domain comprising (or consisting of) the third set of cholesteric liquid crystals.

[00017] In an embodiment, at least one of said domains has a surface comprised in the range from 0.01 mm? to 100 mm?2, preferably in the range from 0.5 mm? to 5 mm2, more preferably in the range from 1 mm? to 3 mm2. In another embodiment, at least one of said domains may have a surface comprised in the range from 60 mm? to 90 mm?2, preferably, in the range from 70 mm? to 80 mm2.

Preferably, all domains of the decorative layer have a surface comprised in the above ranges.

[00018] The PV module may comprise a plurality of photovoltaic cells, the decorative layer covering the plurality of photovoltaic cells.

[00019] The PV cell may be 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.

[00020] In an embodiment, the PV cell is curved, preferably with a radius of curvature comprised in the range from 1 cm to 100 cm, preferably in the range from 5cm to 50 cm, even more preferably in the range from 10 cm to 30 cm.

[00021] Preferably, the photovoltaic cell is flexible.

[00022] The domains of the decorative layer may be arranged so as to provide at least one of: an image, a design, a symbol and a graphic security feature.

[00023] Preferably, the domains of the decorative layer are arranged so that the decorative layer has a color that is non-spectral to the (naked) human eye.

[00024] In an embodiment, at least one of the domains of the decorative layer has a rectangular shape, preferably a square shape. Other shapes are of course possible, such as triangular, hexagonal or circular shapes. Preferably, all the domains have the same shape. In other embodiments, the domains may have different shapes.

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[00025] In an embodiment, the domains are tiling the photovoltaic cell, preferably the tiling is a periodic tiling or a non-periodic tiling.

[00026] The protective layer may be a protective glass layer. The protective layer is preferably 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 (300 nm to 400 nm) wavelength range.

[00027] A second aspect of the present invention relates to a surface, e.g. a roof or a wall, having the photovoltaic module according to the first aspect of the invention affixed thereon, wherein the domains of the decorative layer are arranged so that the photovoltaic module is camouflaged in the surface to the (naked) human eye.

[00028] A third aspect of the present invention relates to a method for manufacturing a PV module (possibly according to the first aspect of the invention).

The method comprises applying a decorative layer and a protective layer, the decorative layer being on the photovoltaic cell and the protective layer being on the decorative layer. The application comprises either providing a cholesteric liquid crystal mixture on the photovoltaic cell, or on the protective layer, and arranging the protective layer on top of the photovoltaic cell, so that the cholesteric liquid crystal mixture is sandwiched between the photovoltaic cell and the protective layer; or arranging the protective layer on top of the photovoltaic cell leaving a gap therebetween and providing a cholesteric liquid crystal mixture between the protective layer and the photovoltaic cell, so that the cholesteric liquid crystal mixture (at least partially) fills the gap and so that the cholesteric liquid crystal mixture is sandwiched between the photovoltaic cell and the protective layer. In addition, the application comprises heating or cooling the cholesteric liquid crystal mixture at a first temperature so that the cholesteric liquid crystal mixture has a first retroreflection peak wavelength. Additional steps comprise selectively polymerizing the cholesteric liquid crystal mixture so as to provide the decorative layer with a first domain comprising (or consisting of) polymerized cholesteric liquid crystals having the first retroreflection peak wavelength; heating or cooling the cholesteric liquid crystal mixture at a second temperature so that the cholesteric liquid crystal mixture has a second retroreflection peak wavelength, wherein the second temperature is different from the first temperature; and selectively polymerizing the cholesteric

7 LU507448 liquid crystal mixture so as to provide the decorative layer with a second domain comprising (or consisting of) polymerized cholesteric liquid crystals having the second retroreflection peak wavelength.

[00029] It will be appreciated that the method for manufacturing the PV module according to third aspect of the invention allows for providing the PV module with virtually any desired spectral color for the domains. Indeed, a specific color is obtained by adequately tuning the temperature of the cholesteric liquid crystal mixture at which selective polymerization is carried out.

[00030] It will be appreciated that an encapsulation layer may be applied on the photovoltaic cell and/or on the protective layer before providing the cholesteric liquid crystal mixture on the photovoltaic cell, or on the protective layer, respectively. In those cases, the encapsulation layer is sandwiched between the cholesteric liquid crystal mixture and the photovoltaic cell and/or sandwiched between the cholesteric liquid crystal mixture the protective layer.

[00031] The cholesteric liquid crystal mixture may comprise at least one of: a chiral dopant, a (blend of) reactive mesogen(s), a photo-initiator, a catalyst, and a radical scavenger. The chiral dopant may be polymerizable or not. The cholesteric liquid crystal mixture may also comprise one or more benzoic acid derivatives.

[00032] The cholesteric liquid crystal mixture may comprise a photo-initiator, wherein the selective polymerization is a selective photo-polymerization and wherein the selective photo-polymerization is preferably carried out using a photomask or a digital light-projector.

[00033] In an embodiment, the decorative layer has a thickness comprised in the range from 1 um to 100 um, preferably in the range from 3 um to 50 um, more preferably in the range from 7 um to 20 um, even more preferably in the range from the 10 um to 16 um.

[00034] In an embodiment, the application further comprises heating or cooling the cholesteric liquid crystal mixture at a third temperature so that the cholesteric liquid crystals in the cholesteric liquid crystal mixture have a third retroreflection peak wavelength, wherein the third temperature is different from the first and second temperatures, and also further comprises selectively polymerizing the cholesteric liquid crystal mixture so as to provide the decorative layer with a

8 LU507448 third domain comprising (or consisting of) polymerized cholesteric liquid crystals having the third retroreflection peak wavelength.

[00035] The method may comprise provisioning a plurality of photovoltaic cells, the application being carried out so that the decorative layer covers the plurality of photovoltaic cells.

[00036] In an embodiment, the selective polymerization is carried out so that at least one of said domains has a surface in the range from 0.01 mm? to 100 mm?2, preferably in the range from 0.5 mm? to 5 mm2, more preferably in the range from 1 mm? to 3 mm2. In another embodiment, at least one of said domains may have a surface comprised in the range from 60 mm? to 90 mm?2, preferably, in the range from 70 mm? to 80 mm2. Preferably, all domains of the decorative layer have a surface comprised in the above ranges.

[00037] The selective polymerization may be carried out so that the domains of the decorative layer are arranged so as to provide at least one of: an image, a design, a symbol and a graphic security feature.

[00038] The selective polymerization may be carried out so that the domains of the decorative layer are arranged so that the decorative color has a color that is non-spectral to the human eye.

[00039] The selective polymerization may be carried out so that at least one of the domains of the decorative layer has a rectangular shape, preferably a square shape. Other shapes are of course possible, such as triangular, hexagonal or circular shapes. Preferably, all the domains have the same shape. In other embodiments, the domains may have different shapes.

[00040] The selective polymerization may be carried out so that the domains are tiling the photovoltaic cell, preferably the tiling is a periodic tiling or a non- periodic tiling.

[00041] The PV cell being 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.

[00042] 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

9 LU507448 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.

Brief Description of the Drawings

[00043] 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: shows the different layers of a PV module according to an embodiment of the present invention;

Fig. 2: shows three reflection spectra (dashed lines) and external quantum efficiency (EQE) spectra (continuous lines) of each colored solar module (red, green, blue, and a reference without color) along with the representation of the

AM1.5G photon flux plotted against the right axis;

Fig. 3: shows a PV module according to an embodiment of the present invention comprising a decorative layer representing a Christmas tree;

Fig. 4: shows a method for manufacturing a PV module according to an embodiment of the present invention, macroscopic photograph of orange-green- blue pixelated device with respective POM close-up images as well as a graph showing the current density as a function of the voltage and a further graph showing a comparison of external quantum efficiencies for a device with and without decorative layer;

Fig. 5: shows the temperature dependence of an unpolymerized CLC mixture;

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Fig. 6: shows a sequence of photographs of a 7.5 cm x 7.5 cm black glass representing a PV module according to the present invention;

Fig. 7: shows a CIE 1931 2° chromaticity diagram of the PV module of Fig. 4b;

Fig. 8: shows the different layers of a PV module according to an embodiment of the present invention;

Fig. 9: shows the different layers of a PV module according to an embodiment of the present invention;

Fig. 10: shows the different layers of a PV module according to an embodiment of the present invention; and

Fig. 11: shows an existing PV module that has been upgraded to include a CLC based decorative layer and a protective layer, according to an embodiment of the present invention.

[00044] 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 may not have been represented correctly.

Detailed Description of Preferred Embodiments of the Invention

[00045] Fig.1 depicts a PV module 10 according to an embodiment of the present invention. PV module 10 comprises back PV (glass) layer 12, a plurality of

PV cells 14 on top of the back PV (glass) layer 12, a decorative layer 16 on top of PV cells 14 and a (front) protective layer 18 on top of PV cells 14. The protective layer 18 may comprise or consist of a glass layer or a polymer sheet. Other layers such as

CdS, i-ZnO and ZnO:Al may be present (not depicted) between the decorative layer 16 and the protective layer 18.

[00046] Asis 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 20 enters the 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.

[00047] PV cells 14 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

11 LU507448 multicrystalline (e.g. heterostructures (HIT or HJT), IBC, PERC, TOP-CON). 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 may be 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). where 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, PV cells 14 are inorganic thin film Cu(In,Ga)Se. (CIGSe) based solar cells. The results discussed below are based on such inorganic thin film

CIGS based solar cells.

[00048] The protective layer 18 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 range.

[00049] The decorative layer 16 comprises (or consists of) polymerized cholesteric liquid crystals. The decorative layer 16 has a thickness of 15 um, however a thickness comprised in the range from 1 um to 100 um, preferably in the range from 3 um to 50 um, more preferably in the range from 7 um to 20 um, even more preferably in the range from the 10 um to 16 um, is also contemplated.

[00050] In order to understand the effect of the decorative layer 16 consisting of polymerized cholesteric liquid crystals, external quantum efficiency (EQE) measurements were performed on PV modules that comprise a decorative layer 16 having ared, a green or a blue color, i.e. polymerized cholesteric liquid crystals have a retroreflection peak wavelength causing red, green or blue reflections, and PV modules without the decorative layer 16. The decorative layer 16 is between the PV cells 14 that comprise Mo/CIGS/CdS, i-ZnO and ZnO:Al layers, in this order from bottom to top, and the protective layer 18. Fig. 2 shows reflection spectrum (dashed) and EQE spectrum of each colored solar module (red, green, blue, as the respective retroreflection color) along with the representation of the AM1.5G photon flux (grey shaded area) plotted against the right axis. Also depicted in black is a reference EQE spectrum provided by a PV module 10 without the decorative layer 16.

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[00051] Comparison between wavelength dependent external quantum efficiency (EQE) measurements allows for assessing the effect of the decorative layer on the current collecting ability of the PV cells. A reference PV module is provided by an uncoated PV module (i.e. identical but without decorative layer 16). As shown in Fig. 2, the reference exhibits a typical EQE spectrum for CIGS PV modules with an EQE of greater than 80% above the band gap, observed here around 1100 nm. At wavelengths shorter than 530 nm the EQE decreases down to 10% due to the parasitic absorption by the CdS / i-ZnO / ZnO:Al layers. Since the decorative layer 16 allows all photons except those that are reflected by the polymerized CLCs (i.e., those with wavelengths within the reflection band and the same circular polarization handedness as the CLC helix) to pass through to the solar cells to generate charge carriers, it is expected to observe similar EQE spectra for the PV modules with the decorative layer 16 and for the reference PV module, except for a dip due to the reflection band. This is indeed what is observed, with the red, green, and blue PV modules all showing distinct dips in the EQE. The lack of any other dips confirms the absence of parasitic absorption or scattering at wavelengths outside of the reflection band. Surprisingly, outside of the CLC selective reflection dip in the EQE spectrum, the PV modules with the decorative layer show a slightly higher response than the PV modules not having the decorative layer. This is most likely due to better refractive index matching between the layers on top of the solar cells.

[00052] With reference to Table 1, the PV modules all show almost the same open circuit voltage (Voc) and fill factor (FF) as compared to the reference (without decorative layer 16), demonstrating that the decorative layer 16 comprising polymerized CLCs has no detrimental electrical effect on the underlying solar cell.

Only the Jsc values of the PV modules with the decorative layer 16 are slightly reduced, in order of blue, green, then red, which corresponds to the width of the reflection band of the CLCs and the fact that the number of photons from the sun is the least in the blue part and the most in the red part of the solar spectrum. With reference to Table 2, the relative power conversion efficiencies (PCEs) of the blue, green, and red solar cells are 96.3, 94.5 and 93.1%, respectively, which is better than commercially available modules such as HeliArtec ceramic inks products (Gttps://www.hetartec.com/) which show relative PCEs of the blue, green, and red solar cells that are 42.5, 32.5 and 40.0%, respectively.

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EE RN id (mV) (mA-cm=) (6) (6)

Table 1: PV performances of PV modules with and without decorative layer comprising, open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE).

Reever (%) crc | HeliArtec

Red | 931 | 40 _

Table 2: Comparison of relative power conversion efficiencies compared to undecorated reference (without decorative layer 16). CLC refers to cholesteric liquid crystal decorative layers, which are compared against commercial modules from Heliartec, with color codes RAL3028 (red), RAL6026 (green), RAL5015 (blue)

[00053] The relative PCE is a ratio between the power conversion efficiency of the PV module comprising the decorative layer and the power conversion efficiency of the same photovoltaic module without the decorative layer. In this way, a comparison can be made as to the effect of the decorative layer on the power conversion efficiency of the PV module.

[00054] For reference, the compositions of the mixtures for obtaining the blue, green or red solar cells are the following: (wt.%) (wt. %) (wt. %) (wt. %)

14 LU507448 mue [6 [es fe wae las we

Table 3: CLC mixture composition

[00055] 1,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (STo3021 — SYNTHON GmbH) is a bifunctional polymerizable mesogen, 4’-hex-5- enyloxy-biphenyl-4-carbonitrile (ST04477 — SYNTHON GmbH) is a monofunctional polymerizable monomer, R5011 is a right-handed chiral dopant (HCCH, China) and 2,2 Dimethoxy-2-phenylacetophenone (IRG651 — Sigma

Aldrich) is a photoinitiator.

[00056] An aspect of the present invention proposes to include at least two colors within the same decorative layer so as to provide a multi-colored decorative layer 16. In particular, the present invention proposes a (single) decorative layer comprising a first set of polymerized cholesteric liquid crystals having a first retroreflection peak wavelength and a second set of polymerized cholesteric liquid crystals having a second retroreflection peak wavelength, the first and second retroreflection peak wavelengths being different, the decorative layer comprising a first domain and a second domain, the first domain comprising the first set of polymerized cholesteric liquid crystals and the second domain comprising the second set of polymerized cholesteric liquid crystals. The different domains are not shown in Fig. 1 for the sake of clarity. However, Fig. 3 shows a top view of a PV module 10 having a decorative layer 16 comprising polymerized CLCs having different colors. The different domains (having different colors) represent, in combination, a Christmas tree. As can be readily appreciated, any number of domains can be arranged on PV cells 14.

[00057] Of course, the domains may have any shape, as deemed necessary for achieving a particular decoration of the solar cell, for example an image, a design, a symbol and a graphic security feature. The domains may have a rectangular shape, preferably a square shape. Examples of square domains are shown in Fig. 4(c).

Other shapes are of course possible, such as triangular, hexagonal or circular shapes.

Preferably, all the domains have the same shape. In other embodiments, the domains may have different shapes.

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[00058] The domains may or may not tile the photovoltaic cells. The tiling may be a periodic tiling or a non-periodic tiling.

[00059] In a preferred embodiment, the domains are arranged so as to provide colored pixels (asin e.g. Fig.4(b) or Fig 6(a)V which will be discussed at a later stage) tiling the solar cells. The color of the pixels may be, e.g., red, green or blue.

[00060] Atleast one (preferably all) of said domains (e.g. pixels) has a surface comprised in the range from 0.01 mm? to 100 mm?2, preferably in the range from 0.5 mm? to 5 mm2, more preferably in the range from ı mm? to 3mm2. In embodiments, the surface may be comprised in the range from 60 mm? to 90 mm?2, preferably, in the range from 70 mm? to 80 mm2. This is particularly adapted for larger viewing distances. Preferably, all domains of the decorative layer have a surface comprised in the above ranges.

[00061] In order to go beyond spectral colors, the domains may be arranged so that their surface is small enough not to be discriminated by the human eye at normal viewing distance. In effect, the human eye will therefore see the blend of the spectral colors of the individual neighboring domains.

[00062] The photovoltaic cell (as well as the PV module) may be curved, preferably with a radius of curvature comprised in the range from 1 cm to 100 cm, preferably in the range from 5 cm to 50 cm, even more preferably in the range from 10cm to 30 cm. The photovoltaic cell (as well as the PV module) may be flexible.

[00063] With reference to Fig.4, an embodiment of the method for manufacturing the PV module is described. The CLC mixture comprises a chiral dopant, a blend of reactive mesogens, a photo-initiator. The CLC mixture also comprises a plurality of benzoic acid derivatives.

[00064] Examples of photo-initiators are e.g. compounds with trade names

IRG651, IRG369, IRG819, IRG2022, IRG184.

[00065] Examples of reactive mesogens may be 4-hex-5-enyloxy-biphenyl-4- carbonitrile (ST04477, SYNTHON GmbH), 6-(4-cyano-biphenyl-4'-yloxy) hexyl acrylate (ST03474, SYNTHON GmbH), 1,4-bis-[4-(3-acryloyloxypropyloxy) benzoyloxy]-2-methylbenzent (ST03021, SYNTHON GmbH), 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester (ST06477, SYNTHON GmbH), 1,4-

Bis[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene (ST00975, SYNTHON

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GmbH). Other examples may include acrylic acid 2-(4'-cyano-biphenyl-4- yloxy)ethyl ester, 4[4[6-Acryloxyhex-1-yl)oxyphenyl]carboxybenzonitrile, 4-

Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester, 4-(6-Acryloxy-hex-1- yl-oxy)phenyl 4-(hexyloxy)benzoate, 4-(6-(acryloyloxy)hexyloxy)phenyl 4'-(4- pentylcyclohexyl)biphenyl-4-carboxylate.

[00066] The CLC mixture may comprise one or more chiral dopants such as R- or S-2-Octyl 4-[4-(Hexyloxy)Benzoyloxy|Benzoate (R- or S-811), R5011 or S5011,

CB15. Examples of polymerizable chiral dopants are: (S)-6-(4'-cyanobiphenyl-4- yloxy)-4-methylhexyl acrylate, or 4-(3-Acryloyloxypropyloxy)-benzoesure 2- methyl-1, 4-phenylester.

[00067] In a preferred embodiment, the CLC mixture comprises the reactive mesogen 1,4-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-1-yloxy)benzoic acid (STo2453, SYNTHON GmbH), 4-(5-Acryloxypentyl-1-oxy)benzoic acid (ST02454,

SYNTHON GmbH) and 4-(6-Acryloyloxy-n-hex-1-yloxy)benzoic acid (STo0902,

SYNTHON GmbH) and the reactive mesogen 4-Methoxybenzoic acid 4-(6- acryloyloxyhexyloxy) phenyl ester (ST06477, SYNTHON GmbH), and the photoinitiator 2,2 Dimethoxy-2-phenylacetophenone (Irg651, Sigma Aldrich). The relative mass ratios (in wt.%) are shown in Table. 4 below.

STo0975 | ST06477 | STo2453 | ST02454 STo6287 | IRG651

Table 4: CLC mixture composition in wt. %.

[00068] The CLC mixture of Table 4 is magnetically stirred at 80°C (above the clearing point of the mixture) for around 5 h to ensure that all the components are homogeneously mixed.

[00069] The solar cells 14 are then coated with the CLC mixture, which has the property of changing color across the visible spectrum as a function of the temperature. For example, the reflection color of the unpolymerized CLC mixture as defined in Table 3, shifts continuously from blue to red on cooling (see Fig. 5), from

17 LU507448 a retroreflection peak wavelength of 461nm at 66°C to a retroreflection peak wavelength of 685 nm at 10°C. Specifically, Fig. 5 shows the dependence of the retroreflection color as a function of the temperature for the unpolymerized CLC mixture. In particular (a) shows reflection POM images, (b) shows reflection spectra of polymerizable temperature-sensitive CLC mixture and (c) shows the color coordinates (black dots) of the mixture in the CIE 1931 2° chromaticity diagram obtained according to the reflection spectra. The scale bar indicates 200 um.

[00070] The solar cell 14 coated with the unpolymerized CLC mixture is heated to 66°C, covered with a protective glass layer (see Fig. 4(a)T.) and annealed for 5 min. At this temperature, the solar cell is UV-irradiated (for 10 s with an intensity of 30 W/cm?) with a handheld UV-LED system (30W IP66 Onforuled, China) through a photomask designed with approximately 1 mm? transparent squares for all pixels that should be blue (retroreflection peak at 447 nm -see Fig. 4(a)T,). After that, the solar cell is cooled down to 50°C at 5°C/min, a temperature at which the remaining unpolymerized CLC regions show a green retroreflection (retroreflection peak at 500 nm -see Fig. 4(a)T.). After annealing for 5 min, the first photomask is replaced by another (second) photomask, and the solar cell 14 with the unpolymerized CLC mixture is again UV-irradiated during 10 s at an intensity of 30 W/cmz. The second photomask is transparent at points where green domains are desired, opaque in other areas. Finally, in the same way, the solar cell is cooled down to 10°C at 5°C/min, a temperature at which the remaining unpolymerized CLC regions show orange retroreflections (as a result of the prior local photopolymerization, the retroreflection peak when using this mixture and this procedure is somewhat shorter than for the pristine CLC mixture at the same temperature, now at 585nm -see Fig. 4(a)T;), and polymerized without a photomask, since all domains which should not be red/orange have already been polymerized. In Fig. 4(a), P,, Pa and P; indicate the different pitches of the CLCs that have just been polymerized. It is worthwhile mentioning that the pitch does not change with temperature for the polymerized CLC domains. Fig. 4(b) shows the resulting pixelated solar cell. Fig. 4(c) shows a POM close-up image of the pattern (scale bar: 400 um) and Fig. 4(d)i-iii show each individual color (scale bar: 100 um).

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[00071] It is worthwhile noting that other means for selectively polymerizing the CLC mixture (i.e. other than a photomask) are also contemplated. For example, a digital light-projector may be used. This allows for concentrating the UV radiation specifically on the zones where polymerization is intended. In such a case, a photomask is not necessary since selectivity is already provided by the possibility to tune the UV beam size (dimensions and shape) and location.

[00072] The EQE spectra of the PV module with and without the pixelated

CLCs are shown in Fig. 4(e). For the pixelated PV module, an average of five measurements conducted in regions of different coloring is shown since the light beam size for determining the EQE is slightly larger than that of a pixel. As expected, the EQE spectra are identical outside of the visible light region for both PV modules with or without the pixelated CLCs, and show losses due to the reflection of the CLCs across all retroreflection wavelengths. The loss across all visible wavelengths is expected since the reflection spectra are rather broad for each of the different colors (see Fig. 5b).

[00073] Based on the EQE spectra, the Jsc is extracted to correct the respective

JV-curves of the solar cells (see Fig. 4(f)), as if they were fully covered with the CLC pixels. As previously found for the uniformly colored solar cells, VOC and FF remain almost the same within errors when comparing PV modules with and without the pixelated CLCs (see Fig. 4(e)). The JSC drops from 32.1 to 28.5 mAcm-2, leading the pixelated PV module having a relative PCE of 89% compared to before coating.

Based on the sum-up of the reflection spectra for each individual pixel, it is obvious that the perceived color from a distance moves towards the white part of the CIE diagram. It is thus appropriate to compare relative efficiencies of the pixelated CLC

PV module with a commercially available PV module with the nearest color. The nearest color is a window grey (RAL 7040) PV module from HeliArtec. See Fig. 7 showing color coordinates of the three polymerized pixels, the sum-up result (black star) and commercially available grey (RAL 7040) PV module from HeliArtec (grey dot) in the CIE 1931 2° chromaticity diagram obtained according to the reflection spectra (apart from the RAL 7040). The module from HeliArtec shows a relative

PCE of 60%, significantly lower than that of the present PV module while offering similar color.

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[00074] With reference to Fig.6, a 7.5 cm x 7.5 cm substrate mimicking a PV module was photographed in different environments and under varying lighting conditions so as to illustrate the advantages of the present invention. The substrate is placed on a wooden door and illuminated by indoor diffuse ambient light (top row), an outdoor wooden wall illuminated by indirect sunlight on a sunny day (middle row) or a silver-colored metal panel on a door outside, also illuminated by indirect sunlight on a sunny day. For each situation, panels i to v are obtained with stepwise reduced photography distance, as indicated in the top row. The sample is photographed at five different distances, approaching the camera stepwise from 6 mto0.2mtothe sample. The backgrounds have been chosen based on two criteria: (1) their colors are non-spectral (and the two wooden surfaces are, additionally, textured), hence no conventional uniformly colored CLC surface can be hidden on them by the principle of camouflage, and (2) their colors are close to the effective color of our pixelated CLC array when seen over large distance under the lighting conditions of each photography series. Indeed, the sample is almost impossible to detect over distances greater than 2m (i-iii), especially on the two wooden backgrounds where the effective color is particularly suitable for providing camouflage. Against the silvery background of the metal plate the sample is detectable, but it is still well enough camouflaged that one would not notice it if not looking for it.

[00075] Therefore, the PV module 10 may be designed so that, when affixed to a surface (e.g. a roof, a wall), the photovoltaic module is camouflaged in the surface to the (naked) human eye.

[00076] Fig. 8 depicts an embodiment for the PV module 10. In addition to the layers defined for the embodiment depicted in Fig. 1, an encapsulation layer 22 may be added between the protective layer 18 and the decorative layer 16. Such an encapsulation layer 22 may comprise, or may consist of, POE, EVA or EPE. The embodiment depicted in Fig. 9 differs from the embodiment depicted in Fig. 8 in that the encapsulation layer 22 is placed between the decorative layer 16 and the PV cells 14. Of course, as depicted in Fig. 10, in an embodiment, the decorative layer 16 may be sandwiched between two encapsulation layers 22.

[00077] It is worthwhile noting that the decorative layer 16 may be applied on the back surface of the protective layer 18, top or back surface of the encapsulation

20 LU507448 layer 22, instead of the front surface of the PV cells 14, as in the embodiment depicted in Fig. 1.

[00078] Fig. 11 depicts an existing PV module 24 that has been provided with a decorative layer 16 and a protective layer 18. Encapsulation layers 22 are also provided on the top and bottom surface of the decorative layer 16. Any of the encapsulation layers 22 may be omitted. It will be appreciated that the present invention also teaches a method for providing a CLC based decorative layer to existing a PV module 24 so as to provide an upgraded PV module 10. In particular, a method for retrofitting existing PV modules is contemplated. The method may comprise applying a stack comprising protective layer 18 and decorative layer 16 (and possibly the encapsulation layers 22) on the existing PV module 24. The method for retrofitting an existing PV module is the same as the method of the third aspect of the present invention (method for manufacturing a PV module), except in that the CLC mixture may be applied on the bottom surface of the protective layer 18, or on the top or bottom surface of the encapsulation layers 22 or even directly on the top surface of the existing PV module 24. The heating and selective polymerization steps are then carried out. After that, the stack comprising protective layer 18 and decorative layer 16 (and possibly the encapsulation layers 22) is affixed to the existing PV module 24. The protective layer 18 is preferably a polymer sheet.

Alternatively, the protective layer 18 may be a glass layer.

[00079] An anti-reflective coating (ARC) may be added to the front or back surface of the protective layer 18, possibly also to the front surface of the PV cells 14.

[00080] 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

21 LU507448 Claims

1. À photovoltaic module comprising: a photovoltaic cell; a decorative layer on top of the photovoltaic cell, comprising polymerized cholesteric liquid crystals, the polymerized cholesteric liquid crystals comprising a first set of polymerized cholesteric liquid crystals having a first retroreflection peak wavelength and a second set of polymerized cholesteric liquid crystals having a second retroreflection peak wavelength, the first and second retroreflection peak wavelengths being different, the decorative layer comprising a first domain and a second domain, the first domain comprising the first set of polymerized cholesteric liquid crystals and the second domain comprising the second set of polymerized cholesteric liquid crystals ; and a protective layer on top of the decorative layer.

2. The photovoltaic module according to claim 1, wherein the polymerized cholesteric liquid crystals comprise at least one of: a chiral dopant, a reactive mesogen, a photo-initiator, a catalyst, and a radical scavenger.

3. The photovoltaic module according to any one of claims 1 to 2, wherein the decorative layer has a thickness comprised in the range from 1 um to 100 um, preferably in the range from 3 um to 50 um, more preferably in the range from 7 um to 20 um, even more preferably in the range from the 10 um to 16 um.

4. The photovoltaic module according to any one of claims 1 to 3, wherein the photovoltaic module has a relative power conversion efficiency of at least 50%, preferably of at least 60%, even more preferably of at least 80%, most preferably of at least 90%, the relative power conversion efficiency being computed as a ratio between the power conversion efficiency of said photovoltaic module and the power conversion efficiency of said photovoltaic module without the decorative layer.

5. The photovoltaic module according to any one of claims 1 to 4, the polymerized cholesteric liquid crystals comprising a third set of polymerized cholesteric liquid crystals having a third retroreflection peak wavelength, the third retroreflection peak wavelength being different from the first and second retroreflection peak wavelengths, the decorative layer comprising a third

22 LU507448 domain, the third domain comprising the third set of polymerized cholesteric liquid crystals.

6. The photovoltaic module according to any one of claims 1 to 5, wherein at least one of said domains has a surface comprised in the range from 0.01 mm? to 100 mm2, preferably in the range from 0.5 mm? to 5 mm?2, more preferably in the range from 1 mm? to 3 mm2.

7. The photovoltaic module according to any one of claims 1 to 6, wherein the module comprises a plurality of photovoltaic cells, the decorative layer covering said plurality of photovoltaic cells.

8. The photovoltaic module according to any one of claims 1 to 7, the photovoltaic cell being 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.

9. The photovoltaic module according to any one of claims 1 to 8, wherein the photovoltaic cell is curved, preferably with a radius of curvature comprised in the range from 1 cm to 100 cm, preferably in the range from 5 em to 50 cm, even more preferably in the range from 10 em to 30 cm.

10. The photovoltaic module according to any one of claims 1 to 9, wherein the photovoltaic cell is flexible.

11. The photovoltaic module according to any one of claims 1 to 10, wherein the domains of the decorative layer are arranged so as to provide at least one of: an image, a design, a symbol and a graphic security feature.

12. The photovoltaic module according to any one of claims 1 to 11, wherein the domains of the decorative layer are arranged so that the decorative layer has a color that is non-spectral to the human eye.

13. The photovoltaic module according to any one of claims 1 to 12, wherein the domains of the decorative layer have a rectangular shape, preferably a square shape.

14. The photovoltaic module according to any one of claims 1 to 13, wherein the domains are tiling the photovoltaic cell, preferably the tiling is a periodic tiling or a non-periodic tiling.

23 LU507448

15. À surface, e.g. a roof, having the photovoltaic module according to any one of claims 1 to 14 affixed thereon, wherein the domains of the decorative layer are arranged so that the photovoltaic module is camouflaged in the surface to the naked human eye.

16. A method for manufacturing a photovoltaic module, comprising: providing photovoltaic cell; applying a decorative layer and a protective layer, the decorative layer being on the photovoltaic cell and the protective layer being on the decorative layer, the application comprising: o either: e providing a cholesteric liquid crystal mixture on the photovoltaic cell or on the protective layer; e arranging the protective layer on top of the photovoltaic cell, so that the cholesteric liquid crystal mixture is sandwiched between the photovoltaic cell and the protective layer; o or: o arranging the protective layer on top of the photovoltaic cell leaving a gap therebetween; o providing a cholesteric liquid crystal mixture between the protective layer and the photovoltaic cell, so that the cholesteric liquid crystal mixture at least partially fills the gap and so that the cholesteric liquid crystal mixture is sandwiched between the photovoltaic cell and the protective layer; o heating or cooling the cholesteric liquid crystal mixture at a first temperature so that the cholesteric liquid crystal mixture has a first retroreflection peak wavelength; o selectively polymerizing the cholesteric liquid crystal mixture so as to provide the decorative layer with a first domain comprising polymerized cholesteric liquid crystals having the first retroreflection peak wavelength; o heating or cooling the cholesteric liquid crystal mixture at a second temperature so that the cholesteric liquid crystal mixture has a second

24 LU507448 retroreflection peak wavelength, wherein the second temperature is different from the first temperature; and o selectively polymerizing the cholesteric liquid crystal mixture so as to provide the decorative layer with a second domain comprising polymerized cholesteric liquid crystals having the second retroreflection peak wavelength.

17. The method for manufacturing a photovoltaic module according to claim 16, wherein the cholesteric liquid crystal mixture comprises at least one of: a chiral dopant, a reactive mesogen, a photo-initiator, a catalyst, and a radical scavenger.

18. The method for manufacturing a photovoltaic module according to claim 17, wherein the cholesteric liquid crystal mixture comprises a photo-initiator, wherein the selective polymerization is a selective photo-polymerization and wherein the selective photo-polymerization is preferably carried out using a photomask or a digital light-projector.

19. The method for manufacturing a photovoltaic module according to any one of claims 16 to 18, wherein the decorative layer has a thickness comprised in the range from 1 um to 100 um, preferably in the range from 3 um to 50 um, more preferably in the range from 7 um to 20 um, even more preferably in the range from the 10 um to 16 um.

20. The method for manufacturing a photovoltaic module according to any one of claims 16 to 19, wherein the application of the decorative layer on the photovoltaic cell and the protective layer on the decorative layer further comprises heating or cooling the cholesteric liquid crystal mixture at a third temperature so that the cholesteric liquid crystals in the cholesteric liquid crystal mixture have a third retroreflection peak wavelength, wherein the third temperature is different from the first and second temperatures, and also further comprises selectively polymerizing the cholesteric liquid crystal mixture so as to provide the decorative layer with a third domain comprising polymerized cholesteric liquid crystals having the third retroreflection peak wavelength.

25 LU507448

21. The method for manufacturing a photovoltaic module according to any one of claims 16 to 20, comprising the provision of a plurality of photovoltaic cells, the application being carried out so that the decorative layer covers said plurality of photovoltaic cells.

22. The method for manufacturing a photovoltaic module according to any one of claims 16 to 21, wherein the selective polymerization is carried out so that at least one of said domains has a surface comprised in the range from 0.01 mm?2 to 100 mm?2, preferably in the range from 0.5 mm? to 5 mm2, more preferably in the range from 1 mm? to 3 mm2.

23. The method for manufacturing a photovoltaic module according to any one of claims 16 to 22, wherein the selective polymerization is carried out so that the domains of the decorative layer are arranged so as to provide at least one of: an image, a design, a symbol and a graphic security feature.

24. The method for manufacturing a photovoltaic module according to any one of claims 16 to 23, wherein the selective polymerization is carried out so that the domains of the decorative layer are arranged so that the decorative color has a color that is non-spectral to the human eye.

25. The method for manufacturing a photovoltaic module according to any one of claims 16 to 24, wherein the selective polymerization is carried out so that at least one of the domains of the decorative layer has a rectangular shape, preferably a square shape.

26. The method for manufacturing a photovoltaic module according to any one of claims 16 to 25, wherein the selective polymerization is carried out so that the domains are tiling the photovoltaic cell, preferably the tiling is a periodic tiling or a non-periodic tiling.

27. The method for manufacturing a photovoltaic module according to any one of claims 16 to 26, the photovoltaic cell being 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.