Low reflection coating

Abstract

A low reflection coating has a water contact angle of at least 90° and a refractive index of less than 1.5 in the visible spectral region. The low reflection coating has a thickness of at least 1 µm. A photovoltaic module comprising a photovoltaic panel having an active surface, whereby light incident on the active surfaces causes the photovoltaic panel to generate an electric current, and such a coating on the active surface is also provided.

Description

[0001] LOW REFLECTION COATING

[0002] The present invention relates to a coating and in particular, but not exclusively, to a low reflection coating for a photovoltaic panel.

[0003] As most countries around the world look to drastically reduce their carbon emissions in line with net-zero commitments and to avoid the worst impacts of climate change, renewable energy technologies are playing an increasingly significant role. Solar photovoltaics (PV) is an important renewable energy technology, having achieved spectacular growth over the last decade, and is now the largest renewable energy technology in terms of installed capacity (excluding hydropower), reaching the significant milestone of ITWp installed capacity in April 2022.

[0004] PV modules experience a range of loss mechanisms when out in the field. Soiling, where dust and dirt accumulate on the module surface, reduces light transmission into the absorber and lowers the power output. Soiling rates vary around the world and are highly dependent on local environmental factors such as rainfall and humidity. In the Middle East and other dry, desert regions the losses can reach as high as 50%. In temperate climates, such as most of Western Europe, losses are around 3%.

[0005] Another significant source of power loss is front-surface reflection, as light reflected at the cover glass / air interface accounts for a loss of typically just over 4% due to a mismatch in refractive index between glass and air. This optical loss translates directly to a loss in photocurrent which reduces the power output from the module accordingly. These reflection losses can be addressed by the use of anti-reflection (AR) coatings, and currently around 90% of commercial PV modules are supplied with an AR coating applied to the cover glass. Often, such AR coatings include a single layer of porous Silicon Dioxide (SiO2).

[0006] To create AR coatings with lower reflection, pores or voids have been introduced into SiO2 to lower the refractive index. Whilst this effectively improves performance these pores result in lesser mechanical strength for the coating, as well as allowing water ingress which can damage the coating. One of the most common cleaning methods for utility scale solar is using a tractormounted rotating brush. This type of cleaning is effective at removing dirt adhered to the module surface but causes significant amounts of damage to the porous silica AR coating.

[0007] There is a need for a low reflection coating which will reduce soiling and make PV modules easier to clean whilst also exhibiting excellent abrasion and resistance to UV and humidity.

[0008] Additionally, multilayer anti-reflection (AR) coatings are commonly used to reduce reflection and glare from spectacle lenses. A thin hydrophobic coating is often added as a topcoat to the AR coating to make the lenses easier to clean. The hydrophobic coating is very thin, typically ~10nm, in order to produce minimal optical interference. Most of these AR and hydrophobic coatings are deposited using a physical vapour deposition (evaporation or sputtering) method in vacuum. The hydrophobic coatings are also sometimes applied using a dip or wipe from solution.

[0009] Recently, a lamination process has been developed for spectacle lenses. A polymer sheet is pre-coated with a multilayer AR coating and hydrophobic coating. The sheet is then cut into small coupons that are larger than or the same size as the lens. The coated sheet is then laminated with the lens usually on the concave (rear) side of the lens, but it can also be applied to the convex (front) side. This process is primarily aimed at optical laboratories that fabricate complex progressive lenses. These lenses are bespoke and involve machining the concave side to a precise prescription. This then leaves the problem of applying the AR / hydrophobic coatings in a small Laboratory environment. This process is costly because the multilayer AR coating must be deposited on large area sheet under vacuum in large expensive equipment. The process is two-stage; depositing the multilayer AR coating and then the hydrophobic coating. In addition, the multilayer AR coating and the hydrophobic coating are relatively thin and easy to scratch and remove by abrasion during routine cleaning.

[0010] As such there is a need for a low reflection coating which will reduce soiling and make glass or polymer surfaces such as optical lenses easier to clean whilst also exhibiting excellent abrasion resistance. According to a first aspect of the invention, there is provided a low reflection coating having a water contact angle of at least 90° and having a refractive index of less than 1.5 in the visible spectral region and a thickness of at least 1 pm.

[0011] The coating has a water contact angle of at least 90°, for example 90° or higher, 95° or higher, 100° or higher, 105° or higher, 110° or higher, 115° or higher, 120° or higher, 125° or higher, 130° or higher, 135° or higher, 140° or higher.

[0012] The coating may have a surface energy of less than 50 mJ / m2, for example 45 mJ / m2, 40 mJ / m2, 35 mJ / m2, 30 mJ / m2, 25 mJ / m2, 20 mJ / m2or lower.

[0013] This coating will also reduce soiling build-up due to its hydrophobic nature and will better withstand cleaning due to its improved mechanical properties.

[0014] The coating has a refractive index in the visible spectral region of less than 1.5, for example 1.45, 1.4, 1.35, 1.3 or lower.

[0015] This will reduce front-surface reflection and promote the passage of visible light, which may usefully be transmitted to a photovoltaic panel for conversion to electrical energy.

[0016] We have appreciated that commonly used porous SiCU coatings can actually increase soiling as a result of their hydrophilic, high-energy surface, with a water contact angle of typically -20°. This increase in soiling can outweigh the benefit provided by the lower reflection, reducing the effectiveness of the AR coating. Furthermore, porous SiC>2 coatings are also vulnerable to degradation caused by UV and humidity exposure.

[0017] The coating may have a thickness of more than 1 pm, for example 5 pm, 10 pm, 15 pm, 20 pm, 30 pm, 40 pm, 50 pm or higher.

[0018] The coating will preferably also exhibit excellent UV and humidity resistance.

[0019] The coating may comprise any suitable material. In a preferred embodiment, the coating comprises a polymer sheet. The coating may be or comprise a UV resistant, low surface energy, optically clear polymer.

[0020] The coating, or the polymer sheet, may comprise a fluoropolymer. The fluoropolymer may comprise a low refractive index, UV resistant fluoropolymer, such as for example ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), ethylene chlorotrifluoroethylene (ECTFE), perfluoroalkoxy alkanes (PFA), Polyvinylidene fluoride (PVDF), or others.

[0021] The coating, or the polymer sheet, may comprise a non-fluorinated polymer. Examples of other UV resistant, low surface energy, optically clear polymers that would be fluorine-free alternatives include Polyethylene (PE), Polypropylene (PP), Polymethyl methacrylate (PMMA), Polydimethylsiloxane (PDMS), among others.

[0022] The highly electronegative fluorine atoms draw electrons towards their nuclei, resulting in a shield of electrons and a shorter bond length. Without wishing to be bound by theory, it is believed that the shield of electrons and short bond length may reduce the number of vulnerable areas for incident high energy UV photons to attack, as well as protecting the central C-C backbone chains from reacting with other chemicals, thereby reducing damage to the coating as a result of weathering.

[0023] The coating may be suitable for application to any glass surface. For example, the coating may be suitable for use with photovoltaic panels or modules, spectacle lenses, shower glass panels and the like.

[0024] The coating is also suitable for application to polymer surfaces, such as for example plastic spectacle lenses having a refractive index of 1.5 and above.

[0025] According to a second aspect of the invention, we provide a low reflection coating comprising a layer of fluoropolymer of more than 1 micrometre thickness.

[0026] The second aspect may have any of the optional features of the first aspect of the invention. According to a third aspect of the invention we provide a photovoltaic module comprising a photovoltaic panel having an active surface whereby light incident on the active surfaces causes the photovoltaic panel to generate an electric current and a coating according to the first aspect on the active surface.

[0027] The module may comprise a cover glass over the active surface, and the coating may be on the cover glass. Alternatively, the module may comprise a glass active surface, and the coating may be on the glass active surface.

[0028] According to a fourth aspect of the invention we provide a method of manufacturing the photovoltaic module of the second aspect comprising applying a coating to a sheet of cover glass for use in a photovoltaic panel.

[0029] The method may comprise bonding the coating to the sheet of cover glass via lamination.

[0030] The method may comprise simply pressing the (generally polymeric) coating and the glass together - holding them together by clamping / an applied force. This forcing together of the layers may or may not lead to bonding between the surfaces. In embodiments with no inter-surface bonding, this clamping / pressing together may allow for simple replacement of the coating in the event of damage, such as wear and tear, to the coating surface (or indeed replacement of the glass). This overlaying of the coating and the sheet of cover glass without bonding may be classed as lamination by some definitions thereof, despite not comprising direct bonding between the layers. In other implementations, the lamination process may bond the surfaces together.

[0031] The lamination may take place at a pressure of at least 0 bar, for example 0 bar, 5 bar, 10 bar, 15 bar, 20 bar, 25 bar, 30 bar, 35 bar, 40 bar or more.

[0032] The lamination may take place at a pressure of up to 100 bar, for example 100 bar, 95 bar, 90 bar, 85 bar, 80 bar, 75 bar, 70 bar, 65 bar, 60 bar or less.

[0033] The lamination may be carried out in multiple stages, each stage at a different pressure. For example, the lamination may take place over multiple stages, incrementally increasing in pressure at each stage. The lamination may take place at or above the melt temperature of the coating. For example, the lamination may take place at a temperature of more than 200°C, for example 210°C, 220°C, 230°C, 240°C, 250°C, 260°C or higher.

[0034] The lamination may take place at or above the glass transition temperature of the coating. For example, the lamination may take place at a temperature of more than -50°C, for example -40°C, 0°C, 40°C, 80°C, 120°C, 160°C or higher.

[0035] For example, the lamination may comprise hot press lamination or vacuum lamination.

[0036] The lamination may be or comprise roll lamination.

[0037] The method may comprise placing the cover glass over the active surface of the photovoltaic panel with the coating in situ. The method may comprise exposing the coating to sunlight so that the photovoltaic panel generates electricity from sunlight which has passed through the coating.

[0038] The method may comprise applying the coating to the sheet of cover glass and compressing the coating and the sheet of cover glass between thermally conductive plates, the plates heated to at or above the melting temperature of the coating.

[0039] The method may also comprise providing a release film or solid block, placed between the coating and the thermally conductive plate to prevent adhesion or transfer of the coating onto the thermally conductive plate. For example, the process may use a PTFE release film or solid block having a sufficiently high melt temperature that it is not affected by the temperatures used in the lamination process.

[0040] The method may optionally further comprise pre-treatment of the cover glass prior to lamination. For example, the pre-treatment may comprise chemical etching or plasma etching. Alternatively, the pre-treatment may comprise application of an optically clear adhesive film to the cover glass prior to lamination.

[0041] The method produces a structure that may exhibit low reflection, hydrophobicity, abrasion resistance, and low roll-off angle. These are all desirable properties for a PV module front sheet to reduce reflection losses and soiling build-up and give an immediate advantage over porous SiC AR coated cover glass.

[0042] There now follows description of embodiments of the invention described by way of example only with reference to the accompanying Figures, in which:

[0043] Figure 1 shows the typical structure of a silicon PV module;

[0044] Figure 2 shows the measured reflectance of a porous SiC AR-coated glass compared to uncoated glass;

[0045] Figure 3 shows abrasion damage on porous SiC AR-coated cover glass resulting from module cleaning;

[0046] Figure 4 shows the water contact angle of porous SiC AR coating;

[0047] Figure 5 shows the water contact angle of PFPE hydrophobic coating;

[0048] Figure 6 shows an example of a method according to the invention;

[0049] Figure 7 shows the water contact angle of ETFE laminate;

[0050] Figure 8 shows the measured reflectance of ETFE laminate after abrasion up to 1,000 cycles; and

[0051] Figure 9 shows the measured reflectance of FEP fluoropolymer laminates compared to uncoated glass.

[0052] Examples

[0053] A typical silicon PV module as shown in Fig. 1 consists of a backsheet (100), an encapsulant layer (110), which is traditionally ethylene vinyl acetate (EVA), although recently other materials such as thermoplastic polyolefins (TPO) and polyvinyl butyral (PVB) have entered the market. The solar cells (120) are positioned on the encapsulant material and are then cushioned by another layer of the encapsulant. Finally, a sheet of toughened 3mm-thick, low iron glass forms the protective outer surface as a cover glass (130).

[0054] Comparative Coatings

[0055] The vast majority of current AR coatings are a single layer of porous SiC , around 125nm thick, and are effective at reducing absolute reflectance losses by 2-3%. The measured reflectance of a porous SiC -coated glass substrate compared to uncoated glass is shown in Fig. 2.

[0056] Weighted average reflectance (WAR) is a commonly-used spectrally-weighted value of reflectance taking into account the variation in photon flux of the AMI .5g solar spectrum, across the measured wavelength range of interest. Using WAR provides a single value of reflectance that is applicable for solar applications, and gives a useful figure with which to compare different coatings (or in this case, compare the laminate including the AR coating to uncoated glass): where is the photon flux of the AM 1.5g spectrum, R is the percent reflectance at a given wavelength, X.

[0057] The weighted average reflectance (WAR) for glass is 8.2% (from both front and back surfaces), reduced to 5.53% by the application of the AR coating. This is a reduction of around 2.7% at the front surface.

[0058] Despite high optical performance, these AR coatings have been shown to be highly vulnerable to abrasion damage, mostly resulting from the regular cleaning processes to remove soiling. To give lower reflection, pores or voids are introduced into the SiCU material to lower the refractive index, and whilst this effectively improves optical performance these pores result in lesser mechanical strength for the coating, as well as allowing water ingress which can damage the coating.

[0059] One of the most common cleaning methods for utility scale solar installations uses a tractor-mounted rotating brush. This type of cleaning is effective at removing dirt adhered to the module surface but causes significant amounts of damage to the AR coating. Fig. 3 shows an SEM image of a piece of AR-coated cover glass after 6 years in a UK solar utility, and most of the coating has been removed from the glass surface as a result of abrasion. This results in a permanent loss of performance from the AR coating, as reapplication is not possible.

[0060] In addition to abrasion damage, chemical thinning and moisture ingress have also been observed in the field. The porous SiCU coating is vulnerable to corrosion, which slowly reduces the thickness, resulting in lesser performance, although this thinning has only been observed in modules that were not cleaned regularly, because in this case the abrasion damage dominates. The porous nature of the coating allows the ingress of water, which can result in mechanical damage after expansion / contraction cycles.

[0061] The porous SiCE AR coatings are hydrophilic with a water contact angle of -20° (Fig. 4; in particular, the water contact angle in this example is 24°) and have been shown to increase soiling rates on PV modules compared to uncoated glass as a result of the higher-energy surface. This increase in soiling can outweigh the benefit provided by the lower reflection, reducing the effectiveness of the AR coating.

[0062] Durable AR coatings with similar optical performance but significantly higher abrasion resistance have been developed using multilayer structures of SiCU and ZrCU, although these coatings are also hydrophilic, with a water contact angle of ~7°, and would potentially increase soiling.

[0063] The increase in soiling resulting from the hydrophilic AR coating surface highlights the need for a hydrophobic, low-energy surface to reduce soiling build-up and make PV modules easier to clean.

[0064] Hydrophobic coatings such as those used in ophthalmic applications (e.g., PFPE), can be effective with contact angles of ~115° (Fig. 5), and have been combined with existing AR coatings without compromising optical properties although they are not sufficiently resistant to UV and abrasion damage for long-term use outdoors in PV applications.

[0065] Studies have shown that these coatings offered very little gain in transmittance, as the coatings were not anti-reflective alongside being anti-soiling. Additionally, despite good initial anti-soiling performance, the contact angle decreased, and roll-off angle increased after UV degradation and abrasion, severely reducing coating performance.

[0066] Inventive Method & Coatings

[0067] An embodiment of the presently described method is shown in figure 6. A hot press 600 is used for lamination in this embodiment.

[0068] The bonding of the coating 614 to the sheet of cover glass 612 is achieved by a multistage hot-press lamination process, detailed in Fig. 6. The process is performed at temperatures in the range of the melt temperature of the desired coating 614, in this case a fluoropolymer (e.g. ETFE) having a melt temperature in the range of around 260-280°. The hot press 600 comprises two plates 602, which may be steel plated and may be polished, which are heated to a desired temperature (e.g. 270°). The upper plate 602 may be moved downwards towards the lower plate 602, as indicated by the arrow, to apply pressure to the layers 610, 612, 614 therebetween.

[0069] The process incorporates a release film 610, which may be a PTFE release film, placed between the fluoropolymer and the thermally conductive plate to prevent sticking of the polymer to the plate 602. A release film 610 is present adjacent to each plate 602 in the example pictured. The PTFE has a sufficiently high melt temperature that it is not affected by the temperatures used in the hot press lamination process.

[0070] The lamination may be carried out in multiple stages, each stage at a different pressure and optionally also at a different temperature. For example, 5 minutes with no applied pressure (0 bar), 15 minutes at 30 bar, 15 minutes at 40 bar, and then 15 minutes at 50 bar. Timings and pressures may be selected as appropriate for the material(s) and temperature(s) chosen.

[0071] The resultant coating combines the mechanical strength of the existing glass cover sheet with the desirable surface and optical properties of a UV resistant fluoropolymer.

[0072] An ETFE laminate according to the invention has a low refractive index (-1.3-1.4), is hydrophobic with a high water contact of -149° (shown in Fig. 7, with the angles measured marked), giving superior hydrophobic properties in comparison with other potential solutions, and is also highly abrasion resistant. Another example tested gave a water contact angle of -131°. The polymer sheets are ~50um thick, compared to ~125nm for porous SiC AR coatings, providing superior mechanical properties such as abrasion resistance whilst maintaining good optical properties.

[0073] An ETFE laminate according to the invention was subjected to 1,000 cycles of abrasion using a brush fixture adapted from a SunBrush, with reflectance curves shown in Fig.

[0074] 8.

[0075] A small increase in reflectance is seen after the first 50 cycles, with no subsequent change after 1,000 cycles. Testing up to 1,000 cycles is designed to simulate a cleaning regime of approximately once a week for 25 years, which is more often than would be expected for large parts of the world, especially milder climates such as Western Europe. Dry, desert areas may see cleaning frequencies as often as once a week, and the laminate would be expected to have good abrasion resistance in this case.

[0076] An FEP laminate according to the invention also has a very high water contact angle of -140°. Both polymers give good hydrophobic performance, along with a low roll off angle below the tilt angle of most PV module installations, which is desirable for the self-cleaning ability of the surface.

[0077] Fig. 9 shows the measured reflectance of FEP polymer laminates compared to uncoated glass. The polymers used have a lower refractive index than glass, which will reduce reflectance when applied to the front surface. The FEP laminate lowers the weighted average reflectance of uncoated glass from 8.19% to 5.93% across 350-1150nm, an absolute reduction of 2.26% at the front surface. Transmission is increased with the FEP laminate.

Claims

CLAIMS1. A low reflection coating having a water contact angle of at least 90° and having a refractive index of less than 1.5 in the visible spectral region and a thickness of at least 1 pm.

2. The coating of claim 1, wherein the coating has a surface energy of less than 50 mJ / m2.

3. The coating of any preceding claim, wherein the coating has a thickness of more than 20 pm, optionally more than 50 pm.

4. The coating of any preceding claim, wherein the coating has a water contact angle of 1 10° or higher.

5. The coating of any preceding claim, wherein the coating comprises a polymer sheet.

6. The coating of any preceding claim, in which the coating comprises a fluoropolymer.

7. The coating of claim 6, wherein the fluoropolymer comprises ethylene tetrafluoroethylene (ETFE), fluorinated ethylene propylene (FEP), ethylene chlorotrifluoroethylene (ECTFE) or perfluoroalkoxy (PFA).

8. The coating of claim 7, wherein the fluoropolymer comprises ethylene tetrafluoroethylene (ETFE) or fluorinated ethylene propylene (FEP).

9. The coating of any preceding claim for an optical application, such as for example a PV panel or spectacle lens.

10. A low reflection coating comprising a layer of fluoropolymer of more than 1 micrometre thickness.1 1. A photovoltaic module comprising a photovoltaic panel having an active surface whereby light incident on the active surfaces causes the photovoltaic panel to generate an electric current and a coating according to any preceding claim on the active surface.

12. The photovoltaic module of claim 11 comprising a cover glass over the active surface, wherein the coating is on the cover glass.

13. The photovoltaic module of claim 11 comprising a glass active surface, wherein the coating is on the glass active surface14. A method of manufacturing the photovoltaic module of any of claims 11 to 13, the method comprising applying a coating to a sheet of cover glass for use in a photovoltaic panel.

15. The method according to claim 14, wherein the method further comprises bonding the coating to the sheet of cover glass via lamination.

16. The method according to claim 15, wherein the lamination takes place at a pressure of between 0 bar and 100 bar.

17. The method according to claim 16, wherein the lamination is carried out in multiple stages, each stage at a different pressure.

18. The method according to claim 17, wherein the lamination takes place over multiple stages, incrementally increasing in pressure at each stage.

19. The method according to any of claims 14 to 18, wherein the lamination takes place at or above the melt temperature of the coating.

20. The method of any of claims 14 to 19, wherein the lamination takes place at a temperature of more than 200°C.

21. The method of any of claims 14 to 20, wherein the lamination comprises hot press lamination, roll lamination, or vacuum lamination.

22. The method of any of claims 14 to 21 further comprising placing the cover glass over the active surface of the photovoltaic panel with the coating in situ, and optionallyexposing the coating to sunlight so that the photovoltaic panel generates electricity from sunlight which has passed through the coating.

23. The method of any of claims 14 to 22 comprising applying the coating to the sheet of cover glass and compressing the coating and the sheet of cover glass between thermally conductive plates, the plates heated to at or above the melting temperature of the coating.

24. The method of claim 23 comprising providing a release film or solid block, placed between the coating and the thermally conductive plate to prevent adhesion or transfer of the coating onto the thermally conductive plate.

25. The method of any of claims 14 to 24 comprising pre-treating the cover glass prior to lamination, wherein optionally the pre-treatment comprises: (i) chemical etching or plasma etching; or(ii) application of an optically clear adhesive film.

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