Uniformly and directionally colored photovoltaic modules

Micro-structures and directional and multi-directional coatings for uniformly colored and directionally colored photovoltaic modules and roof tiles are described. The photovoltaic roof tiles include a glass cover with texture of a micro scale on a first side, and one or more layers of a transparent material adjoining the first side of the textured glass configured to reflect light of a color. A glass cover can have texture on a first side and a layer of sphere shaped metal nanoparticles adjoining the first side of the textured glass cover. Directionally colored solar modules can include a textured glass cover with texture on a first side and a coating layer covering one or more facets of the texture on the first side of the glass cover. The coating layer may be deposited by coating the textured glass cover in one or more directions.

CROSS-REFERENCE TO OTHER APPLICATIONS

This is related to U.S. patent application Ser. No. 15/294,042, entitled “COLORED PHOTOVOLTAIC MODULES” filed Oct. 14, 2016, the disclosure of which is incorporated herein by reference in its entirety for all purposes.

BACKGROUND

Field

This disclosure is generally related to the design of photovoltaic (or “PV”) modules. More specifically, this disclosure is related to the design and manufacture of colored photovoltaic roof tiles.

Related Art

Commercial solar panels are constructed from arrays of photovoltaic (or “PV”) modules. Each PV module, in turn, typically includes a two-dimensional array (e.g., 6×12) of solar cells. Usually, the color appearance of these modules is the natural color of the solar cells embedded in the PV modules, which can be blue, dark-blue, or black. But customers often wish to choose the color appearance of PV modules so that, for example, the modules match the color of the buildings in which they are integrated.

Several existing techniques are available to provide color to PV modules. One technique involves applying tinted glass and/or colored encapsulation sheets. However, these extra structures can absorb a large amount of sunlight, causing significant PV power loss. Moreover, these structures' color appearance may degrade over time.

Another coloration technique involves applying a film over the PV modules or solar cells. However, the color appearance achieved by the coatings on conventional glass typically suffers from flop, or angle-sensitivity, and can also degrade over time under environmental stresses (such as marine weather).

Shading, or absorption of incident sunlight, causes PV power loss, a consequential problem of existing coloring techniques. In addition, colored PV modules manufactured with these techniques, and colored glass more generally, commonly suffer from: sparkle, or glint; flop, or angle-dependent color appearance; and graininess. Note that sparkle refers to glint or localized bright spots. Flop, on the other hand, usually refers to angle-dependent color, i.e. an angular dependence of the peak reflected wavelength. The term flop, or light-dark flop, can also refer to angle-dependent brightness, i.e. an angular dependence of total reflectivity.

SUMMARY

One embodiment described herein provides a photovoltaic roof tile. This photovoltaic roof tile comprises a glass cover having a texture of a micro scale on a first side. Moreover, the photovoltaic roof tile comprises one or more layers of a transparent material configured to reflect light of a first color, wherein the one or more layers of transparent material adjoin the first side of the textured glass cover.

In a variation on this embodiment, the glass cover further has a patterned second texture comprising an array of features on the first side. The array of features includes at least one of: an array of grooves; an array of cones; an array of triangular pyramids; an array of triangular pyramids; an array of square pyramids; and an array of hexagonal pyramids.

In a variation on this embodiment, the transparent material comprises Aluminum Zinc Oxide or a transparent conductive oxide.

In a variation on this embodiment, the transparent material having the refractive index comprises silicon dioxide.

In a variation on this embodiment, the micro texture is chemically etched on the first side of the glass cover.

In a variation on this embodiment, the micro texture is blasted on the first side of the glass cover.

In another aspect of this disclosure, a photovoltaic roof tile is disclosed. This photovoltaic roof tile comprises a glass cover and a layer of sphere shaped metal nanoparticles adjoining a first side of the glass cover.

In a variation on this embodiment, the glass cover has a texture on the first side.

In a variation on this embodiment, the layer of sphere shaped metal nanoparticles comprises an outer layer of antimony and an inner layer of metal. The layer of sphere shaped metal nanoparticles is further annealed on the glass cover.

In a variation on this embodiment, the layer of nanoparticles is preferentially grown, dip-coated, or spin-coated on the first side of the glass cover.

In a variation on this embodiment, the metal nanoparticles comprise silver.

In a variation on this embodiment, the metal nanoparticles comprise gold.

In a variation on this embodiment, the metal nanoparticles comprise aluminum.

In another aspect of this disclosure, a directionally colored photovoltaic roof tile is disclosed. This directionally colored photovoltaic roof tile comprises a textured glass cover having a texture on a first side. The directionally colored photovoltaic roof tile further comprises a coating layer covering one or more facets of the texture on the first side of the glass cover, wherein the coating layer is deposited by coating the textured glass cover in one or more directions.

In a variation on this embodiment, the coating layer comprises metal nanoparticles.

In a variation on this embodiment, the coating layer comprises a transparent thin film configured to reflect a predetermined color based on interference.

In a variation on this embodiment, the deposited coating layer varies in thickness on different facets.

In a variation on this embodiment, the coating layer is configured to provide color for viewers at particular viewing angles, while enhancing optical transparency at other angles.

In a variation on this embodiment, the particular viewing angles comprise glancing angles below 20°.

In a variation on this embodiment, the coating layer is configured to create a variation in color across a glass cover of the photovoltaic roof tile.

DETAILED DESCRIPTION

Overview

Various embodiments disclosed herein provide solutions to manufacturing uniformly and/or directionally colored photovoltaic (PV) modules or roof tiles. Embodiments of the present invention can produce PV roof tiles with a uniform color with little light absorption. As a result, a high proportion of the incident light (of colors other than the module's intended color) is transmitted to the PV cells. To facilitate uniform coloring of PV modules and roof tiles, the inside surface of a top glass cover can be texturized, and a transparent material with a predetermined refractive index or combination of refractive indices can be deposited on the texturized surface. Such a micro-textured or frosted glass cover can display significantly less sparkle, flop, and graininess than conventional glass covers, thus improving color uniformity and appearance. Customizable directional coloring, and intentionally controlled angle-dependent color, can provide further aesthetic options, while still performing efficiently for solar conversion.

It is also possible to produce a layer of sphere-shaped metal nanoparticles on the inside surface of the glass cover. These nanoparticles can produce colors efficiently while absorbing little light.

Another feature described herein is directional coloring of PV modules or roof tiles. The texturized surface of a glass cover can have a color filter layer covering one or more facets of the texture. This color filter can include multiple thin film layers formed using a directional thin film deposition technique, such as chemical or physical vapor deposition (CVD or PVD), e.g. sputtering. The coating layer can be deposited by coating the textured glass surface in one or more directions. Such directional coating can reduce unwanted light absorption by the color filter, while still providing a uniform color appearance to viewers at a certain viewing angle.

PV Roof Tiles and Modules

The disclosed system and methods may be used to provide uniform coloring and/or directional coloring of PV roof tiles and PV modules. Such PV roof tiles provide the functions of conventional roof tiles and of solar cells, while also protecting the solar cells.FIG. 1shows an exemplary configuration of PV roof tiles on a house. PV roof tiles100can be installed on a house like conventional roof tiles or shingles. Particularly, a photovoltaic roof tile can be placed with other tiles in such a way to prevent water from entering the building.

A respective solar cell can include one or more electrodes such as busbars and finger lines, and can connect mechanically and electrically to other cells. Solar cells can be electrically coupled by a tab, via their respective busbars, to create serial or parallel connections. Moreover, electrical connections can be made between two adjacent tiles to interconnect multiple tiles into a module, so that a number of photovoltaic tiles can jointly provide electrical power. A PV module typically includes an array such as 6×12 solar cells.

FIG. 2shows a perspective view of the configuration of a photovoltaic roof tile. In this view, solar cells204and206can be hermetically sealed between top glass cover202and backsheet208, which jointly can protect the solar cells from the weather elements. Tabbing strips212can be in contact with the front-side busbars of solar cell204and extend beyond the left edge of glass202, thereby serving as contact electrodes of a first polarity of the photovoltaic roof tile. Tabbing strips212can also be in contact with the back side of solar cell206, creating a serial connection between solar cell204and solar cell206. Tabbing strips214can be in contact with front-side busbars of solar cell216and extend beyond the right-side edge of glass cover202.

Using long tabbing strips that can cover a substantial portion of a front-side busbar can ensure sufficient electrical contact between the tabbing strip and the busbar, thereby reducing the likelihood of detachment between the tabbing strip and busbar. Furthermore, the four tabbing strips being sealed between the glass cover and backsheet can improve the physical durability of the photovoltaic roof tile.

FIG. 3shows the cross section of an exemplary photovoltaic roof tile. In this example, solar cell306can be encapsulated by top glass cover302and backsheet310. Top encapsulant layer304, which can be based on a polymer such as ethylene vinyl acetate (EVA), can be used to seal between top glass cover302and solar cell306. Similarly, lower encapsulant layer308, which can be based on EVA as well, can be used to seal between solar cell306and backsheet310.

FIG. 4Ashows an exemplary configuration of photovoltaic roof tile with cascaded solar cell strips, according to one embodiment of the present invention. In this example, top glass cover404and a backsheet (not shown) can enclose six solar cell strips402, which can be produced by dividing two solar cells. As shown inFIG. 4B, solar cell strips402can be cascaded by overlapping their respective edges. In particular, each strip can have a busbar on one of its edges on the front side. The back side of the strip can be coated with metal. When two strips are cascaded, the edge busbar of the first strip can be positioned to be in contact with the back-side metal layer of the adjacent strip.

PV roof tiles and modules are described in more detail in Provisional Patent Application No. 62/465,694, entitled “SYSTEM AND METHOD FOR PACKAGING PHOTOVOLTAIC ROOF TILES” filed Mar. 1, 2017, which is incorporated herein by reference. The embodiments disclosed herein can be applied to solar cells, PV roof tiles, and/or PV modules.

Uniformly Colored PV Roof Tiles and Modules

FIGS. 5A and 5Billustrate a comparison of micro-textured uniformly colored glass to conventional glass. As can be seen on the right side ofFIG. 5A, colored glass504typically suffers from: sparkle, or glare; flop, or angle-dependent coloring; and graininess. However, micro-structured glass502with micro-texture can display substantially less of each of these problems. (Note that, in various embodiments, the micro-structured glass102may have surface roughness on the order of 100 nm to 10 μm, with comparable peak-to-peak separation). Likewise, as shown inFIG. 5B, micro-textured or frosted glass506can exhibit little to no sparkle, flop, and graininess, whereas conventional glass508shows significant sparkle and graininess. The micro-textured glass can achieve this feature by introducing roughness in order to randomize the glass-color filter interface, thereby averaging over many incident angles.

Table 1 presents details of the improvements in micro-structured glass in terms of sparkle (or glint), flop (or angle-dependent color appearance), and graininess, compared with conventional glass. These measurements indicate that frosting can reduce light-dark flop (or angle-dependent brightness) by 7-10 times (as measured by the flop index of Table 1), color flop by 5-8 times, and sparkle by at least 30 times.

TABLE 1Details of the improvements in micro-structuredglass compared with conventional glass.CharacteristicFrosted GlassConventional GlassFlop index1.6-2.114.5-17.1Sparkle intensity0.1-0.816-32Sparkle area0-135-49Graininess3.4-3.55.6-6.1

In colored PV modules made with conventional glass, the flop, or angular dependence, of color can be caused by interference between light reflected from different surfaces of the solar module.FIG. 6Aillustrates angular dependence of coloring in photovoltaic roof tiles due to interference effects. As shown, PV module600may include transparent glass cover602. Module600may also contain a color filter604that may comprise one or more layers of optical coating. Color filter604may contain a transparent conductive oxide (TCO) such as Iridium Tin Oxide (ITO) or Aluminum-doped Zinc Oxide (AZO). Color filter604may include a multi-layer stack containing one or more of: a high refraction index (e.g., n=1.7-2.5) material, such as TiO2, Ta2O5, NbO2, ZnO, SnO2, In2O3, Si3N4, and AZO; a low refraction index (e.g., n=1.2-1.5) material, such as SiO2, MgF2; and a metal, such as Ag, Cu, and Au. Color filter604may be deposited by physical vapor deposition (PVD), chemical vapor deposition (CVD), or other deposition methods.

Module600may also contain encapsulant sheet606, which can be polyvinyl butyral (PVB), thermoplastic olefin (TPO), ethylene vinyl acetate (EVA), or N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD). Module600also contains array of solar cells608. In some embodiments, module600contains back encapsulant sheet610and back-side cover layer612positioned on the back side of PV module600, opposite to glass cover602.

In embodiments of the present invention, the color filter produces color by means of wavelength selective reflection, which may exhibit angular dependence caused by interference effects. To produce the desired colors, color filter604may have a thickness in the range of hundreds of nanometers, comparable to visible light wavelengths. As shown, light incident on the solar module could reflect at the interface of glass602and color filter604, or could reflect at the interface of color filter604and encapsulant606. Light traveling along these two different reflecting paths has different optical path lengths, and therefore interferes constructively or destructively depending on this difference.

As shown inFIG. 6A, the path length difference depends on the incident angle. For example, steep angle614, moderate angle616, and shallow angle618produce different path lengths. Since interference depends on both the path difference and the light wavelength, different wavelengths of light have interference maxima at different angles, resulting in color flop (angle-dependence in the color of reflected light).

Various embodiments disclosed herein provide solutions to ameliorate this flop effect using a micro-textured glass cover.FIG. 6Bpresents an exemplary structure of a uniformly colored PV roof tile650with micro-texture and layers of color filter, according to an embodiment of the present invention. As shown, glass cover652may be frosted or micro-textured on its inner surface, i.e. the surface adjoining the layers of color filter654and656. This micro-texture may be frosting, or random surface roughness, such as a series of small bumps or indentations on the surface of the glass. In some embodiments, the micro-texture may be in addition to larger-scale texture on the glass, such as a groove, cone, or pyramid pattern. However, the micro-texture is at a different, typically smaller, scale. By randomizing the surface orientation, the micro-texture can average incoming light over a plurality of incident angles, thereby eliminating or significantly reducing the flop effect, as shown inFIGS. 5A and 5Band Table 1.

In some embodiments, the micro-texture can be chemically etched on the glass. The chemical etching may involve using a paste containing Hydrogen Fluoride (HF). The micro-texture can also be produced by using a process where particles are used to blast on the glass surface.

Note that the frosted glass may face a tradeoff between flop reduction and loss of color brightness or power because the frosting diffuses the light reflected from the PV module. In some embodiments, the micro-texturing may be optimized together with coating thickness for colored solar modules. In addition, the micro-texturing may be combined with larger-scale textures on the glass (e.g. a regular or irregular array or pattern of features) to provide finer control over the directional dependence of color appearance.

Uniformly Colored Solar Module with Nanoparticles

Embodiments of the present invention may also provide a uniformly colored solar module with spherical metal nanoparticles positioned on the inside surface of the glass. Such layers of nanoparticles can produce colors while absorbing little light, particularly when the particles are of sufficiently large radius. Rather than absorbing, the nanoparticles instead transmit or scatter a majority of the incident light, depending on the wavelength. Note that transmission of the light to the PV cells608allows it to be converted to electricity in the cells, and scattering provides color; by contrast, absorption generates waste heat, degrading efficiency. Moreover, the nanoparticles can provide color in a way that minimizes or reduces sparkle or glare. In these embodiments, the metal nanoparticles may replace the physical vapor deposition (PVD) layer of optical coating654. The nanoparticles may also replace the micro-textured or frosted glass, while providing both color selection and uniformity.

FIG. 7Ashows an exemplary structure of a uniformly colored photovoltaic roof tile700with a layer of nanoparticles, according to embodiments of the present invention. Nanoparticle layer704may be positioned on one surface of glass cover702. Glass cover702may also have textured surface706. Typically nanoparticle layer704can be on the textured surface of glass cover702. Note that glass cover702need not have texturized surface706. The metal nanoparticles may include silver (for example, 75 nm diameter spherical Ag particles), gold, aluminum, or other metals. The nanoparticles can be included in another film coating, such as silica, titania, or silicon nitride.

Alternatively, the nanoparticles can be directly encapsulated into the PV module.FIG. 7Bshows an exemplary structure of a uniformly colored photovoltaic roof tile encapsulating a layer of nanoparticles, according to embodiments of the present invention. As shown, nanoparticle layer704may be encapsulated by encapsulant layer708, which can be based on materials such as PVB, TPO, EVA, or TPD.

In some embodiments, the nanoparticle layer may be deposited by dip-coating or spin-coating the nanoparticles onto the surface of glass cover702. The nanoparticle layer may also be deposited by preferentially forming the nanoparticles on one side of the glass.FIG. 8illustrates an exemplary process800for depositing a layer of nanoparticles in a uniformly colored photovoltaic roof tile by annealing, according to embodiments of the present invention. As shown, glass cover802may be covered on one side by an underlayer of antimony804and a layer of metal806. The glass cover assembly may then be annealed, during which the exposure to heat causes the antimony to sublimate. During annealing, the antimony sublimates, leading to a solid state dewetting process where layer808of nanoparticles810is formed on the surface of the glass. The sublimating antimony may carry with it excess portions of metal. Note that nanoparticles810may contain both the antimony and the metal.

FIG. 9shows a block diagram illustrating a method for depositing a layer of nanoparticles in a uniformly colored photovoltaic roof tile, according to embodiments of the present invention. As described above, the process includes depositing a layer of antimony on a first side of the glass cover (operation902), for example by CVD or PVD. Next, a layer of metal is deposited on the antimony layer (operation904). The thickness of the antimony layer and metal layer can be adjusted to control the spacing and density of deposited nanoparticles, and hence the intensity of color of the reflected light. In one embodiment, antimony and metal layers thinner than 20 nm are used to create strongly scattering nanoparticles. Therefore, the layers can be deposited very efficiently by physical vapor deposition (PVD). Aside from PVD, the antimony and metal layers may be deposited by CVD, atomic layer deposition (ALD), or other deposition methods. The glass cover is then annealed to form a layer of sphere shaped metal nanoparticles, which can include metal and antimony (operation906).

FIG. 10shows a plot illustrating rates of scattering and absorption by a layer of 75-nm silver nanosphere particles deposited on glass, for various wavelengths of light. As discussed above, scattering provides color to the solar module, whereas low absorption indicates that the nanoparticles are efficient at transmitting the non-scattered light to the PV cells. As shown inFIG. 10, scattering by the 75-nm silver nanosphere particles peaks at wavelength of approximately 500 nm, corresponding to a blue-green color, with a half width at half maximum (HWHM) of approximately 50 nm. By contrast, absorption is uniformly low throughout the visible wavelength range of 400 to 800 nm.

Directional and Multi-Directional Coating of Textured Solar Modules

Existing solutions for colored PV modules, such as tinted glass and colored encapsulation sheets, may suffer from strong shading, or absorption of incident sunlight by the colored surfaces, causing power loss in the PV modules. In order to manage better such a tradeoff between color and optical transmission, embodiments of the present invention provide partially-coated, textured solar modules, where some texture facets may be coated more than others.

To reduce losses in the color filter, in some embodiments the glass cover may have a textured inner surface. Note that this texture is different from the micro-texture or frosting described earlier, and is typically at a larger scale. This textured surface can be configured to cause a majority of the incident light received by the PV module to reflect at least twice on the textured interface, such that wavelength-selective reflections by the color filter include primarily multiple-reflected light. The textured surface can also be designed to control the amount of reflection loss by increasing or decreasing the number of reflections of the incident light on the textured interface. The textured surface may include an array of features, such as an array of grooves, cones, or pyramids (e.g., triangular pyramids, square pyramids, or hexagonal pyramids). This textured back surface is described further in U.S. patent application Ser. No. 15/294,042, entitled “COLORED PHOTOVOLTAIC MODULES” filed Oct. 14, 2016, which is incorporated herein by reference. As described here, these textured surfaces can be directionally coated.

FIG. 11Ashows an example of directional coating of textured photovoltaic roof tiles, according to embodiments of the present invention. Textured glass1102can be part of a solar module, for example on a residential or commercial roof, or on a vertically-oriented window of a tall building. Therefore, glass1102might typically be viewed only from observers at certain angles. For example, glass1102on a roof would be viewed by viewers at ground level mainly from shallow angles such as 20° or less, as shown.

The textured surface of glass1102can be coated directionally in direction1104. This coating can be a chemical coating, or may include nanoparticles if the coating is followed by a dewetting process, such as the methods described in conjunction withFIGS. 8 and 9. Color can also be created by film interference effects created by transparent films. This directional coating can be accomplished by directional PVD, e.g. from a point source, in order to coat the textured surface selectively. For example, as shown, facet1106can be coated, while facet1108of glass1102can be uncoated. This selective or directional coating can ameliorate optical transmission in the colored PV module, and also save coating material, since the surface of glass1102does not need to be fully coated. Such coating thickness variation can create an aesthetic texture with color variation, or create viewing angle-dependent coloring, while enhancing transmission at other angles.

Coating direction1104can be chosen so that a viewer from shallow angles would see solid color, as reflected from the coated facets1106. In practical usage, glass1102is exposed to air1110, and therefore light reflected from facet1106refracts at the interface1112between glass1102and air1110. The texture of the glass may be chosen to achieve particular angles between the texture facet and interfacing surface1112, for example a groove angle of 38.8° with respect to the normal direction. Moreover, these texture angles can be chosen so light reflected from the coated surface is visible to a low-angle observer. For example, light reflected normally from the coated groove facet1106refracts at glass-air interface1112, so that an observer sees solid color from angles of 20° or less.

Given these relationships between the texture geometry and the expected viewing angles, coating direction1104can be chosen to enhance transmission and save coating material while providing a desired color to the observer. For example, as shown inFIG. 11A, coating direction1104could be chosen at a normal or near-normal direction to facet surface1106, in order to cover facet1106economically while leaving facet1108uncovered. Other directions are also possible, to give different coverage.

FIG. 11Bshows an example of directional coating of textured photovoltaic roof tile facets perpendicular to a glass-air interface, according to embodiments of the present invention. As mentioned above, the textured glass can be designed to cause incident light to reflect multiple times. For example, textured glass1114can have a sawtooth texture. Here coating direction1116cannot be exactly perpendicular to the coated surfaces (since these surfaces would block each other during coating), but still may be chosen, e.g., parallel to the uncoated surfaces. Note that the uncoated surfaces in this example still make an angle of 38.8° with the normal direction to glass-air interface1118, so the observer can still see the desired color from angles of 20° or less. Note also that for glass1102, light incident normal to surface1112would be shaded by the coating material 50% less than for a fully coated surface, since only half the textured surface is coated. For glass1114, however, the coated surfaces are perpendicular to glass-air interface1118, so normally-incident light would be virtually unshaded by the coating. Thus, directional coating can significantly improve optical transmission.

Moreover, directional and multi-directional coating (i.e., coating the textured surface from one or more sources in a plurality of directions) may be used to vary coverage in more complex ways.FIG. 11Cillustrates a variation in coating thickness resulting from directional coating of textured photovoltaic roof tiles, according to embodiments of the present invention. As shown, textured glass1120is coated in direction1122perpendicular to facet1124, but not parallel to any of the facets. The coating thickness on a respective facet generally depends on the angle between the facet and the incident coating flux. In the examples shown inFIGS. 11C and 11D, the coating thickness is indicated in terms of a percentage with respect to the thickness resulting from a coating flux perpendicular to the coated surface, e.g. coating flux1122in the example inFIG. 11C. Thus, facet1124has a 100% coating thickness, while facets1126and1128are coated at 70% thickness.

FIG. 11Dillustrates multi-directional coating of textured photovoltaic roof tiles, according to embodiments of the present invention. Multi-directional coating can further increase the complexity, thickness variation, and coloring of PV modules. For example, glass1130can be coated in two separate directions of varying flux strengths 50% and 200%, as shown. As a result, facet1132is coated with 200% thickness, facet1134with 170% thickness, and facet1136with 50% thickness. While in this example, the two coating directions are depicted as lying completely in the same plane, one coating direction could also be out of the plane of the other direction. (For example, a first coating direction might be normal to the glass surface, while a second coating direction might have a component parallel to the surface). When combined with 3-dimensional texturing, such variations can create even more complex coloring, providing further design options and greater aesthetic appeal.

Embodiments of the present invention provide PV roof tiles with a micro-textured or frosted glass cover, which can display a uniform color with little light absorption and less sparkle, flop, and graininess than conventional colored glass. Customizable directional and multi-directional coating, can provide further aesthetic options, including intentionally angle-dependent color, while still performing efficiently for solar conversion. Finally, the disclosed system and methods can also provide a layer of sphere-shaped metal nanoparticles on the inside surface of the glass cover. These nanoparticles can produce colors efficiently while absorbing little light.