SOLAR MODULES

The present disclosure relates to reflective microstructured films having improved durability that are useful in solar modules.

Background

Renewable energy is energy derived from natural resources that can be replenished, such as sunlight, wind, rain, tides, and geothermal heat. The demand for renewable energy has grown substantially with advances in technology and increases in global population. Although fossil fuels provide for the vast majority of energy consumption today, these fuels are non-renewable. The global dependence on these fossil fuels has not only raised concerns about their depletion but also environmental concerns associated with emissions that result from burning these fuels. As a result of these concerns, countries worldwide have been establishing initiatives to develop both large-scale and small-scale renewable energy resources. One of the promising energy resources today is sunlight. Globally, millions of households currently obtain power from photovoltaic systems. The rising demand for solar power has been accompanied by a rising demand for devices and material capable of fulfilling the requirements for these applications.

Harnessing sunlight may be accomplished by the use of photovoltaic (PV) cells (also referred to as solar cells), which are used for photoelectric conversion (e.g., silicon photovoltaic cells). PV cells are relatively small in size and typically combined into a physically integrated PV module (or solar module) having a correspondingly greater power output. PV modules are generally formed from two or more “strings” of PV cells, with each string consisting of a plurality of PV cells arranged in a row and are typically electrically connected in series using tinned flat copper wires (also known as electrical connectors, tabbing ribbons, or bus wires). These electrical connectors are typically adhered to the PV cells by a soldering process.

PV modules typically further comprise the PV cell(s) surrounded by an encapsulant, such as generally described in U.S. Patent Publication No. 2008/0078445 (Patel et ah), the teachings of which are incorporated herein by reference. In some constructions, the PV module includes encapsulant on both sides of the PV cell(s). A panel of glass (or other suitable polymeric material) is bonded to each of the opposing, front and back sides, respectively, of the encapsulant. The panels are transparent to solar radiation and are typically referred to as the front-side layer and the backside layer (or backsheet). The front-side layer and the backsheet may be made of the same or a different material. The encapsulant is a light-transparent polymer material that encapsulates the PV cells and also is bonded to the front-side layer and the backsheet so as to physically seal off the PV cells. This laminated construction provides mechanical support for the PV cells and also protects them against damage due to environmental factors such as wind, snow and ice. The PV module is typically fit into a metal frame, with a sealant covering the edges of the module engaged by the metal frame. The metal frame protects the edges of the module, provides additional mechanical strength, and facilitates combining it with other modules so as to form a larger array or solar panel that can be mounted to a suitable support that holds the modules together at a desired angle appropriate to maximize reception of solar radiation.

The art of making PV cells and combining them to make laminated modules is exemplified by the following U.S. Pat. Nos.: 4,751,191 (Gonsiorawski et ah); 5,074,920 (Gonsiorawski et ah); 5,118,362 (St. Angelo et ah); 5,178,685 (Borenstein et ah); 5,320,684 (Amick et ah); and 5,478,402 (Hanoka).

With many PV module designs, the tabbing ribbons represent an inactive shading region (i.e., area in which incident light is not absorbed for photovoltaic or photoelectric conversion). The total active surface area (i.e., the total area in which incident light is use for photovoltaic or photoelectric conversion) is thus less than 100% of the original photovoltaic cell area due to the presence of these inactive areas. Consequently, an increase in the number or width of the tabbing ribbons decreases the amount of current that can be generated by the PV module because of the increase in inactive shaded area.

To address the above concerns, US Pat. No. 9,972,734 (Chen et ah), the teachings of which are incorporated herein by reference, discloses a light directing medium, in the form of a strip of microstructured film carrying a light reflective layer, applied over the tabbing ribbons. The light directing medium directs light that would otherwise be incident on an inactive area onto an active area. More particularly, the light directing medium redirects the incident light into angles that totally internally reflect (TIR) from the front-side layer; the TIR light subsequently reflects onto an active PV cell area to produce electricity.

Ideally, light impinging on the LRF in a PV module is discretely reflected at angles larger than the critical angle of the outer surface. This light undergoes TIR to reflect back to the silicon wafers for absorption to produce electricity.

Summary

In one aspect of the invention, a light redirecting film article comprises a light redirecting film having a length, width, and thickness, the length being much greater than the width, comprising a base layer having a first major surface and a second major surface opposite the first major surface. The light redirecting film article also includes an arrangement of a plurality of microstructures projecting from the second major surface of the base layer and a reflective layer disposed on the plurality of microstructures opposite the base layer. The light redirecting film has a width of about 1.5 mm or less. The reflective layer has an optical density of about 4.0 to about 6 0

In another aspect of the invention, the reflective layer has thickness of about 100 nm to about 160 nm.

In another aspect of the invention, a solar cell module comprises the aforementioned light redirecting film article.

Brief Description of the Drawings

FIG. l is a sectional view of a solar cell module that includes a light redirecting film according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a PV cell in accordance with an embodiment of the present invention.

Fig. 3 is a cross sectional view of an exemplary LRF according to an embodiment of the present invention.

Fig. 4 is a graph of an acetic acid solution test for several LRF samples of different ODs.

Detailed Description

The present disclosure is directed to a Light Redirecting Film (LRF) to be utilized in a solar cell module comprising a relatively high optical density (OD) reflective coating disposed on a surface of the micro structured film. As the investigators have observed, a relatively narrow- width of less than 1.5 mm LRF article having a reflective coating, such as aluminum, thinly coated on a microstructured surface may lose reflectivity and become transparent over time, as evidenced in some accelerated aging condition experiments. Acetic acid, which is produced during degradation of ethylene-vinyl acetate copolymer (“EVA”) encapsulates, can dissolve (or combine with creeping humidty into the solar panel to dissolve) the reflective layer of the LRF, thereby reducing its effectiveness. The phenomena can be more aggressive when using narrower width LRF. To improve the resistance to reflective coating dissolution, embodiments of the present invention use relatively high Optical Density (OD) reflective coatings and coatings that are made thicker than conventional reflective coatings to counteract the decay. As shown in FIG. 1, according to one embodiment of the present invention, a solar cell module 150 comprises a plurality of solar cells 110; a light transmitting element 300 disposed on the light receiving side of the plurality of solar cells 110; a front encapsulant layer 400 between the plurality of solar cells 110 and the light transmitting element 300; a plurality of electrical connectors, referred to as tabbing ribbons 120, disposed on the light receiving surfaces of the plurality of solar cells 110 for connecting the plurality of solar cells 110; and a light redirecting film (LRF) 200 disposed upon the on-the-cell portion of at least one of the tabbing ribbon 120.

In addition, the solar cell module further includes a backsheet or backsheet glass 600, as well as a rear encapsulant layer 500 located between the backsheet or backsheet glass 600 and the plurality of solar cells 110.

The light redirecting film 200 comprises an optical structure layer facing the light transmitting element 300, for reflecting light towards the interface between the light transmitting element and the air, and such light is subsequently totally internally reflected back to the surfaces of the solar cells. The thickness of the light redirecting film 200 can be between 20 pm and 115 pm; and the gram weight of the front encapsulant layer 400 can be between 400 g/m ² and 500 g/m ² (wherein the thickness of the front encapsulant layer is between 0.46 mm and 0.6 mm). For narrower width applications, the width of the LRF 200 can be less than or equal to 1.5 mm, for example, from about 0.5 mm to about 1.5 mm, preferably about 0.8 mm to about 1.0 mm.

The surface of the light redirecting film 200 facing the light transmitting element 300 comprises an optical structure that reflects the incident light that would have irradiated the upper surface of the tabbing ribbon 120 but instead is reflected by the light redirecting film 200. The LRF article’s optical structure, described in further detail below with respect to Fig. 2, includes a base layer 134, an ordered arrangement of a plurality of microstructures 136, and a reflective coating 138 disposed over the microstructures opposite the base layer 134. The reflective coating 138, which can comprise a metallic coating such as aluminum, silver, or others listed below, can have an OD of from about 2.0 to about 6.0, preferably from about 3.5 to about 6.0, more preferably from about 4.0 to about 6.0, and even more preferably from about 4.0 to about 5.5. In other embodiments, the thickness of the reflective coating 138 can be greater than about 100 nm. A preferred thickness range is from about 100 nm to about 160 nm.

The plurality of microstructures 136 project from the base layer 134. Preferably, the microstructure layer comprises a plurality of triangular prisms, and the vertex angles of the triangular prisms are within a range between 100° and 140°, preferably within a range between 110° and 130°. Further, each of the microstructures extends (in some embodiments, continuously, and in other embodiments, discontinuously) along the base layer to define a corresponding primary axis. As described herein, when a microstructure continuously extends along the base layer across the width of the light redirecting film to define a corresponding primary axis, the primary axis is defined by the elongated shape of the microstructure (along the peak). In general, the film defines an X-Y plane and the microstructures raise or protrude in the Z direction, out of the X-Y plane. The primary axis of at least one (preferably the majority) of the microstructures is oblique with respect to the longitudinal axis (that is, the primary axis is not parallel to the longitudinal axis of the film). With this construction, the obliquely arranged, reflectorized microstructure(s) will reflect light in a unique manner relative to the longitudinal axis that differs from aligned, unbiased arrangement (that is, an arrangement in which the primary axis of the microstructure is parallel to the longitudinal axis of the film). In some embodiments, a majority or all of the microstructures are arranged such that the corresponding primary axes are all oblique with respect to the longitudinal axis. In other embodiments, the longitudinal axis and the primary axis of at least one of the microstructures, optionally a majority or all of the microstructures, forms a bias angle with respect to the longitudinal axis in the range of 1° - 90°, alternative in the range of 20° - 70°, alternative in the range 70° - 90°. In other embodiments, the light redirecting film article further includes an adhesive layer 125 disposed on the base layer 134 opposite the microstructures 136 and in yet other embodiments the LRF further comprises a liner (not shown) adjacent the adhesive layer 125 as an outermost layer.

After the reflected light reaches the light transmitting element 300, it travels within the light transmitting element 300 to the interface between the light transmitting element 300 and the air. Since the light transmitting element 300 is an optically dense medium and air is an optically thinner medium, light can be totally internally reflected at the interface between the light transmitting element 300 and the air, and travel within the front encapsulant layer 400 until it reaches the solar cell 110. Then the light is converted to electrical energy, thereby increasing the power generating efficiency of the solar cell module through the increase in the utilization efficiency of the light.

The overall cost of the solar cell module can be reduced such as by using a narrower tabbing ribbon. As such, a narrower light redirecting film can also be utilized. As a specific scenario, if the width of the tabbing ribbon 120 is less than or equal to 1.0 mm, then the difference obtained by subtracting the width of the tabbing ribbon 120 where it is located from the width of the light redirecting film 200 is preferably within the range of 0 to 0.2 mm. It is contemplated that tabbing ribbon widths of 0.8 mm can be utilized in accordance with the present invention.

In one embodiment of the present innvention, the width of the light redirecting film is greater than the width of the tabbing ribbon. In order to increase the utilization efficiency of sunlight, the width of the light redirecting film is preferably no less than the width of the tabbing ribbon.

In another embodiment of the present invention, the width of the light redirecting film does not exceed 120% of the width of the tabbing ribbon where it is located.

In one aspect, the LRF article includes a light redirecting film having a width and a length, the length being longer than the width, with the length defining a longitudinal axis.

Fig. 2 shows another embodiment of a solar cell 150’ as a portion of photovoltaic (PV) module according to the present application. Solar cell 150’ includes multiple tabbing ribbons (electrical connectors) 124 disposed on solar cell 122a. In some embodiments, electrical connectors 124 are disposed over the entire length of the solar cell. LRF 200 is disposed over one or more electrical connectors 124. In some embodiments, LRF 200 completely overlaps with electrical connector 124 in order to maximize efficiency of the PV module. In some embodiments, LRF 200 can be provided in the form of a continuous strip which is slightly wider than the electrical connectors 124. In some embodiments, the width of each tabbing ribbon/electrical connector is less than 1.5 mm, such as about 0.9 mm to about 1.0 mm, and the width of each LRF article 120 is less than or equal to 1.5 mm, such as from about 1.0 - 1.2 mm.

In more detail, as shown in Fig. 3, one exemplary LRF article 200 is a multilayer construction comprising a base layer that can comprise a flexible polymeric layer 134 having a first generally flat major surface 135a and a second generally flat major surface 135b. Microstructure or structured surface 136 is adjacent to the first major surface 135a of the flexible polymeric (base) layer 134. In some embodiments, the flexible polymeric layer 134 comprises one of a polyolefin ( e.g ., polyethylene, polypropylene), polyester (e.g, polyethylene terephthalate (PET)), and polyacrylate (e.g, polymethyl(meth)acrylate (PMMA)).

In some embodiments, the microstructure/structured surface 136 comprises one of a thermoplastic polymer and a polymerizable resin. In some embodiments, the structured surface further comprises a reflective coating or layer 138 coated thereon. In some embodiments, the microstructures 136 may comprise a polymeric material. In some embodiments, the polymeric material of the microstructures 136 is the same composition as the base layer 134. In other embodiments, the polymeric material of the microstructures 136 is different from that of the base layer 134. In some embodiments, the base layer 134 material is a polyester and the microstructure 136 material is a poly(meth)acrylate. In other embodiments, the microstructures 136 may also comprise conductive materials that are the same or different than the base layer 134.

The reflective layer 138 can assume various forms appropriate for reflecting light, such as metallic, inorganic materials or organic materials. In some embodiments, the reflective layer 138 is a mirror coating. The reflective layer 138 can provide reflectivity of incident sunlight and thus can prevent some of the incident light from being incident on the polymer materials of the microstructures 136. The reflective layer can be formed by utilizing a sputtering process. The materials for the reflective layer can be selected from a group of metallic materials such as silver, aluminum, platinum, titanium, silver alloys, aluminum alloys, platinum alloys, titanium alloys, and the like. The thickness of the reflective layer can be approximately from 20 nm to 200 nm, and preferably from 100 nm to 160 nm. In this instance, the thickness of the reflective layer is measured as an average thickness across a substantial portion of the microstructures.

When a reflective metallic coating is used for the reflective layer 138, the coating can comprise silver, aluminum, tin, tin alloys, or a combination thereof. In one embodiment, the reflective coating comprises aluminum. The use of a metallic layer may require an additional coating to electrically insulate the light redirecting film article from electrical components in the PV module. Some exemplary inorganic materials include (but are not limited to) oxides (e.g., S1O2, T1O2, AI2O3, Ta2C>5, etc.) and fluorides (e.g., MgF2, LaF3, AIF3, etc.) that can be formed into alternating layers to provide a reflective interference coating suitable for use as a broadband reflector.

In another embodiment, in which the reflective layer 138 is provided as a metallic coating (and optionally with other constructions of the reflective layer 138), the microstructures 136 can be configured such that the corresponding peaks are rounded. Depositing a layer of metal (i.e., the reflective layer 138) on rounded peaks is more straightforward than depositing on sharp peaks. Also, when the peaks are sharp (e.g., come to a point), it can be difficult to adequately cover the sharp peak with a layer of metal. This can, in turn, result in a “pinhole” at the peak where little or no metal is present. These pinholes may not reflect light and, also, may permit passage of sunlight to the polymeric material of the microstructure 136, possibly causing the microstructure 136 to degrade over time. With the optional rounded peak constructions, the peak is more straightforward to coat and the risk of pinholes is reduced or eliminated. Further, rounded peak films can be easy to handle and there are no sharp peaks present that might otherwise be vulnerable to damage during processing, shipping, converting or other handling steps.

A reflective coating or a mirror coating 138 can have several advantages. For example, these coatings can provide reflectivity of incident sunlight and thus can block incident sunlight from being incident on the polymer materials (which can degrade due to UV exposure). As mentioned above, an LRF article having a reflective coating, such as aluminum, thinly coated on a structured surface (e.g., structure 136) may lose reflectivity and become transparent over time, as evidenced in some accelerated aging condition experiments described further below. Acetic acid, which is produced during EVA encapsulate degradation can contribute in dissolving the reflective coating, thereby reducing its effectiveness. The phenomena can be more aggressive when using narrower width LRF.

To improve the resistance to reflective coating dissolution, embodiments of the present invention use relatively high Optical Density (OD) reflective coatings and coatings that are made thicker than conventional reflective coatings to counteract the decay. Such a relatively high OD can be from 4.0 to 6.0. Optical Density values can be obtained using conventional test equipment.

Thicker coatings block more UV light. However, coatings that are too thick may cause increased stress within the coating, resulting in coating cracking. When reflective coatings exceed thicknesses of 200 nm, they can become more columnar in the metal structure. These thicker coatings can lead to faster degradation as cracks can more easily form in the aluminum and the columnar structure is more susceptible to acetic acid attack. As such, a reflective or mirror coating thickness of between about 100 nm to about 160 nm provides sufficient optical durability to withstand degradation due to acetic acid exposure.

In another alternative embodiment, reduction of stray light can be accomplished by the presence of roughness, textures, or other features on the surface microstructures that help diffuse reflected light. In this disclosure, the roughness, textures, or other features can be referred to as “features,” regardless of whether they are depressions (extending below the surface level) or protrusions (rising above the surface level).

Polymerizable resins suitable for forming microstructures 136 include blends of photoinitiator and at least one compound bearing an acrylate group. Preferably, the resin blend contains a monofunctional, a difunctional, or a poly functional compound to ensure formation of a cross-linked polymeric network upon irradiation. Illustrative examples of resins that are capable of being polymerized by a free radical mechanism that can be used herein include acrylic-based resins derived from epoxies, polyesters, polyethers, and urethanes, ethylenically unsaturated compounds, isocyanate derivatives having at least one pendant acrylate group, epoxy resins other than acrylated epoxies, and mixtures and combinations thereof. The term “acrylate” is used herein to encompass both acrylates and methacrylates. U.S. Pat. 4,576,850 (Martens) (incorporated herein in its entirety) discloses examples of crosslinked resins that may be used in forming the structured surface of LRF 200. In some embodiments, the resin is a non- halogenated resin. Some benefits of use of non-halogenated resins include the fact that they are more environmentally friendly and do not corrode metals.

One exemplary method of making PV modules as described herein includes the steps of: providing strings of photovoltaic cells, soldering electrical connectors over the photovoltaic cells, and adhering light directing mediums over the electrical connectors.

During the lamination process of the PV module, it may be important to maintain registration between the tabbing ribbons/electrical connectors and the LRF. In one exemplary method of making a PV module, LRF articles are previously laminated with an adhesive. In some embodiments, the adhesive is a hot-melt adhesive. In some embodiments, the hot-melt adhesive comprises ethylene vinyl acetate polymer (EVA). Other types of suitable hot-melt adhesives include polyolefins. The LRF strips are positioned over the tabbing ribbons/electrical connectors and heat is applied thereto to melt the hot-melt adhesive, effectively bonding the LRF to the electrical connectors. In some embodiments, other layers may be laminated or coated onto the PV module ( e.g ., backsheets, encapsulants, front-side layers) prior to the heating step. The heating step may be carried out using any suitable heating mechanism such as, for example, a heat gun or infrared heater. In some embodiments, the heating mechanism is placed under the laminate construction (e.g., adjacent to the backsheet). In some embodiments, the heating mechanism is placed above the laminate construction (e.g, adjacent to the light directing medium).

In some embodiments, the adhesive is a pressure-sensitive adhesive (PSA). Suitable types of PSAs include, but are not limited to, acrylates, silicones, polyisobutylenes, ureas, and combinations thereof. In some embodiments, the PSA is an acrylic or acrylate PSA. As used herein, the term "acrylic" or "acrylate" includes compounds having at least one of acrylic or methacrylic groups. Useful acrylic PSAs can be made, for example, by combining at least two different monomers (first and second monomers). Exemplary suitable first monomers include 2- methylbutyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, lauryl acrylate, n-decyl acrylate, 4- methyl-2-pentyl acrylate, isoamyl acrylate, sec-butyl acrylate, and isononyl acrylate. Exemplary suitable second monomers include a (meth)acrylic acid (e.g., acrylic acid, methacrylic acid, itaconic acid, maleic acid, and fumaric acid), a (meth)acrylamide (e.g., acrylamide, methacrylamide, N-ethyl acrylamide, N-hydroxy ethyl acrylamide, N-octyl acrylamide, N-t-butyl acrylamide, N,N-dimethyl acrylamide, N,Ndi ethyl acrylamide, and N-ethyl-N-dihydroxyethyl acrylamide), a (meth)acrylate (e.g., 2- hydroxyethyl acrylate or methacrylate, cyclohexyl acrylate, t-butyl acrylate, or isobornyl acrylate), N-vinyl pyrrolidone, N-vinyl caprolactam, an alpha-olefin, a vinyl ether, an allyl ether, a styrenic monomer, or a maleate. Acrylic PSAs may also be made by including cross-linking agents in the formulation. As such, in another embodiment, the adhesive may comprise a cross-linked adhesive, such as a cross-linked EVA adhesive or cross-linked PSA. Cross-linking can be achieved through an e-beam cross-linking process, such as described in US Publ. No. 2018-0013027, incorporated by reference herein in its entirety.

In some embodiments, the adhesive is selectively applied to the tabbing ribbons/electrical connectors, with the width of the LRF being equal to or slightly larger than the width of the tabbing ribbons/electrical connectors. Preferably, the adhesive is transparent. Desired transparency is at least 80% transparency to visible light. In some embodiments, the desired transparency is at least 90% to visible light. In other embodiments, the transparent adhesive is applied over the entire surface of the PV cells (e.g., flood coated). The LRF articles are then carefully positioned over, and in registration with, the tabbing ribbons/electrical connectors. The entire structure is then heated to melt the adhesive and ensure adequate bonding of the LRF to the tabbing ribbon/electrical connectors.

A conventional LRF article was observed to degrade when subjected to accelerated conditions 85C/85RH or higher. The reflective coating utilized in this conventional sample comprises an aluminum (Al) coating that is vapor deposited onto the LRF. The optical density of this conventional coating is the range of 2.3 to 2.5 OD, with a corresponding coating thickness of about 40 nm to 50 nm. These conventional coating thicknesses yield O.D. target values of about 2.5 OD based on reflectivity, aluminum raw material cost, and process speed. Further observations are that the LRF reflection degrades to the point where the reflective aluminum can become transparent in sections of the LRF strip for exposures of lOOOhrs. It is believed that the EVA encapsulants degrade forming acetic acid which attacks the Al and dissolves it away from the LRF microstructured film. In further testing, it was uncovered that pressure cooker test (PCT) 120°C contstant temperature at 100% Relative Humidity at 1.0 atm of pressure also can cause the dissolution of an A1 coating, albeit in a much shorter time period of less than 100 hours versus 1000 hours. Using PCT, it was observed that EVA type can cause differences in the A1 coating dissolution location. It was also observed that wider LRF films (from about 2.0 mm to about 5.0 mm) do not become as transparent over the solar cell area whereas narrower strips (width at 1.0 mm) do become transparent with the same conditions and materials. If tested for a substantially long time (e.g., 200 hours), even 2.0 mm LRF strips will start to show some transparency, whereas the 5 mm wide strips do not exhibit the same transparency. Optical Density of the samples was measured as an average value across the web using a Transmission Densitomer (Ihara T 500).

As observed, sample LRF films slowly turn transparent over the silicon cell and this transparency is substantially uniform across the width of the film. This observation suggests acetic acid dissolves an A1 coating by layer as opposed from the edge inward. This phenomena of attacking the A1 in layers was unexpected because traditional attack from acetic acid is expected to occur from the edge inward due to the humidity creeping inward and accelerating the EVA decay. The decay outside of the solar cell near the edges of the panel show more of an edge inward decay different than the transparency over the silicon cell. The narrow films have much less A1 present to disolve so potentially lower concentrations of acetic acid will be effective to degrade the A1 coating. As the trend to reduce solar cell panel costs continues, utilizing narrower and thinner configurations of LRF articles could result in degradation problems using conventional reflective coating ODs and/or thicknesses.

In an experiment as shown in Table 1, several reflective coating thicknesses and optical densities were measured. Specifically, transmission electron microscope (TEM) analysis of the thickness of the aluminum layer on solar Light Redirecting Film (LRF) at 4 different Optical Density levels was performed. The A1 layer was coated on top of microstructures (prisms) via evaporative coating. Samples were prepared as follows: samples were flat embedded in ScotchCast™ 5 and cut with room temperature ultramicrotomy with a diamond knife over water at a section thickness of 90 nm. TEM used: Hitachi H-9000 TEM at 300kV at l,000x, 20,000x, and 50,000x instrument magnification. Table 1

Table 1 shows in one example, the aluminum thickness of the 5.4 OD sample was 117.8 nm on average across a substantial portion of the microstructures and even higher in the peaks and valleys. Using the four optical density LRF materials from Table 1, solar panels were produced containg LRF strips at 1mm, 2mm, and 5mm wide. Two strips of each width were placed across the solar cell and two cells were made on each panel (12 strips total per panel). The panels were subjected to the PCT and removed at several intervals for to photograph the progress of the LRF. After 100 hrs the 2.2 and 3.5 OD panels showed 30 to 50 percent transparency on the 1 mm wide strips and 10 - 20 percent transparency on the 2mm wide strips. However the 4.7 OD and even more so the 5.4 OD samples retained more aluminum. The 5.4 OD sample had 10-20 percent transparency of the 1mm strips and no transparency on the 2mm strips. The panels were carried further to reach 200 hours of PCT testing and the trend remained the same, proving the higher OD samples continued to retain more aluminum and additionally the wider strips also retained more aluminum.

In another experiment, the same LRF made at different optical densities and different widths were cut into strips and submerged in acetic acid solutions to gauge their ability to withstand aluminum dissolution. Four solutions of acetic acid were prepared at 0.001, 0.005, 0.01, 0.025M concentrations. Strips of LRF at 1.0, 2.0, and 5.0 mm wide were all placed in separate beakers of the four acetic acid solutions. This set of 12 solutions was completed for each of the four different OD values 2.2, 3.5, 4.7, and 5.4. The acetic acid solution with LRF strip inside were exposed for 24 hours before removing the strip. The strips were dried and then evaluated visually for the estimated percent of Aluminum remaining on the strip. The overall trend observed was that the higher OD 5.4 samples retained much more of their reflective capability. See Fig. 4.

In summary, an overall trend was observed that higher OD resulted in better resistance to A1 dissolution for both PCT testing and acetic acid solution testing.

All scientific and technical terms used herein have meanings commonly used in the art unless otherwise specified. The definitions provided herein are to facilitate understanding of certain terms used frequently in this application and are not meant to exclude a reasonable interpretation of those terms in the context of the present disclosure.

Unless otherwise indicated, all numbers in the description and the claims expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviations found in their respective testing measurements.

The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g. a range from 1 to 5 includes, for instance, 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.

As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” encompass embodiments having plural referents, unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.

As used herein, the term “microstructure” means the configuration of elements wherein at least two dimensions of the element are microscopic. The topical and/or cross-sectional view of the element must be microscopic.

As used herein, the term “microscopic” refers to element of small enough dimension so as to require an optic aid to the naked eye when viewed from any plane of view to determine its shape. One criterion is found in Modern Optic Engineering by W. J. Smith, McGraw-Hill, 1966, pages 104-105 whereby visual acuity, “. . . is defined and measured in terms of the angular size of the smallest character that can be recognized.” Normal visual acuity is considered to be when the smallest recognizable letter subtends an angular height of 5 minutes of arc of the retina. At a typical working distance of 250 mm (10 inches), this yields a lateral dimension of 0.36 mm (0.0145 inch) for this object.