WO2013172930A1 - Replaceable solar wavelength - converting film - Google Patents

Replaceable solar wavelength - converting film Download PDF

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Publication number
WO2013172930A1
WO2013172930A1 PCT/US2013/030215 US2013030215W WO2013172930A1 WO 2013172930 A1 WO2013172930 A1 WO 2013172930A1 US 2013030215 W US2013030215 W US 2013030215W WO 2013172930 A1 WO2013172930 A1 WO 2013172930A1
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WO
WIPO (PCT)
Prior art keywords
booster
booster film
solar
downshifting
film structure
Prior art date
Application number
PCT/US2013/030215
Other languages
French (fr)
Inventor
Shijun Jia
Paul Storiz
Sevan Demirdogen
Original Assignee
Illinois Tool Works Inc.
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Publication date
Application filed by Illinois Tool Works Inc. filed Critical Illinois Tool Works Inc.
Publication of WO2013172930A1 publication Critical patent/WO2013172930A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
    • H01L31/055—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means where light is absorbed and re-emitted at a different wavelength by the optical element directly associated or integrated with the PV cell, e.g. by using luminescent material, fluorescent concentrators or up-conversion arrangements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/042—PV modules or arrays of single PV cells
    • H01L31/048—Encapsulation of modules
    • H01L31/0481—Encapsulation of modules characterised by the composition of the encapsulation material
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/52—PV systems with concentrators

Definitions

  • the invention is directed to a new and improved replaceable solar booster film structure that absorbs solar light across a broad range of frequencies, converts the light to a narrower range of frequencies that are useful for photovoltaic solar cells, and transmits the light to the solar cells to enable them to operate more efficiently.
  • Luminescent solar booster films are known in the art, and are disclosed in U.S.
  • Patent Application Publication 2010/0186801 to Boehm et al. published in July 29, 2010. Such films are capable of downshifting the high frequency (typically ultraviolet and/or blue) portion of the spectrum to lower frequency radiation that can be efficiently absorbed by the photovoltaic device.
  • One drawback of known films is that they degrade under sunlight, limiting their useful life to 1-2 years, far less than the 20-30 year lifespan of a typical photovoltaic device.
  • FIG. 1 illustrates a known photovoltaic device 10 including a solar cell 12, an encapsulation material 14 surrounding the solar cell 12, a back sheet 13, a cover glass 18, and a solar booster film 16 between the cover glass 18 and the encapsulation material 14.
  • the solar cell 12 converts electromagnetic radiation passing through the cover glass 18 and booster film 16 into electric energy.
  • the booster film 16 partially absorbs electromagnetic radiation passing through the cover glass 18 and emits it at a longer wavelength to the solar cell 12.
  • the cover glass 18 acts as a filter for some of the ultraviolet radiation in order to prolong the useful life of the booster file 16.
  • the present invention is directed to an improved solar booster film structure that remains durable for a longer period of time and offers a longer useful life.
  • the improved film structure is easily removable from the cover material (typically glass) that covers the photovoltaic cell, and can be easily replaced at necessary intervals. Because of its improved durability, the booster film structure can be placed on the outside of the cover glass, and does not need to be placed underneath the cover glass for protection. When the booster film structure reaches the end of its useful life, as evidenced by photobleaching or discoloration, it can simply be peeled off of the cover glass and replaced with another booster film structure.
  • the booster film structure of the invention includes one or more booster film layers, which collectively include about 95 - 99.9 % by weight of a polymer matrix, about 0.05 - 2.5 % by weight of one or more downshifting additives, and about 0.05 - 2.5 % by weight of film stabilizers selected from the group consisting of ultraviolet absorbers, oxygen scavengers, free radical quenchers, and combinations thereof.
  • the booster film structure may include multiple film layers.
  • the booster film structure may include an upper ultraviolet absorbing layer containing ultraviolet absorbers, for the purpose of absorbing ultraviolet radiation that could degrade or photobleach the downshifting additives and is too low in wavelength (too high in frequency) to be converted by the additives, followed by one or more downshifting booster film layers that include downshifting additives to shift the remainder of the ultraviolet and blue light toward the visible light spectrum used by the photovoltaic cell.
  • an upper ultraviolet absorbing layer containing ultraviolet absorbers for the purpose of absorbing ultraviolet radiation that could degrade or photobleach the downshifting additives and is too low in wavelength (too high in frequency) to be converted by the additives
  • one or more downshifting booster film layers that include downshifting additives to shift the remainder of the ultraviolet and blue light toward the visible light spectrum used by the photovoltaic cell.
  • the booster film structure of the invention also includes a sealing layer for providing a removable and peelable seal to the cover material (typically glass) that covers the photovoltaic solar cell. This enables the booster film structure to be peeled away and replaced when its useful life is exhausted.
  • Fig. 1 is a side sectional view of a photovoltaic device in which a booster film is placed between the cover glass and the solar cell, as is known in the art.
  • Fig. 2 is a representative graph showing the shifting of lower wavelength ultraviolet light into the low end of the visible wavelength spectrum, using a booster film structure of the invention.
  • Fig. 3 is a side sectional view of a booster film structure of the invention.
  • Fig. 4 is a side sectional view of a photovoltaic device in which a booster film structure of the invention is placed in the outside of the cover glass, for easy removal and replacement.
  • Fig. 5 is a graph showing the electrical output of a photovoltaic device with and without a booster film structure of the invention.
  • the invention is a removable, replaceable booster film structure that receives incoming sunlight, downshifts the frequency, and thus lengthens the wavelength of the sunlight, in order to covert a portion of incoming ultraviolet and blue light into higher wavelength energy within the visible spectrum.
  • the booster film structure then emits the visible light toward a photovoltaic solar cell which converts the visible light into electrical energy.
  • Photovoltaic solar cells are made using crystalline or amorphous silicon, cadmium telluride-cadmium sulfide, copper indium gallium selenide, and other known materials. These solar cells convert sunlight into electricity most efficiently when the incoming light has a wavelength of about 500 to about 900 nanometers, which is within the visible range of the spectrum. For the incoming light having a frequency less than about 500 nanometers, down to about 300 nanometers (the high end of the ultraviolet wavelength range), the solar cells become less efficient on a sliding scale. The solar cells are essentially inoperative as to incoming ultraviolet light having a wavelength less than about 300 nanometers.
  • Fig. 2 illustrates how a booster film performs the downshifting of higher frequency (lower wavelength) incoming sunlight.
  • the plot on the left side of the graph shows the energy (in arbitrary units) of incoming sunlight that is absorbed by the booster film.
  • the incoming sunlight has a wavelength range of about 300-500 nanometers, with peak energy around 450 nanometers.
  • the plot on the right side of the graph shows the energy (in arbitrary units) of light that is emitted by the booster film toward the solar cell.
  • the emitted light has a wavelength range of about 450-650 nanometers, with peak energy at about 550 nanometers.
  • the booster film used had a thickness of 100 microns, a polymer matrix of polymethyl methacrylate, and contained 0.2 % by weight of an organic yellow dye available from BASF under the trade name Lumogen ® F Yellow 083.
  • Fig. 3 illustrates one embodiment of a color booster film structure 20 of the invention including, in order from top to bottom, an ultraviolet absorbing layer 22, a first booster film layer 24, a second booster film layer 26, and a sealing layer 28.
  • an ultraviolet absorbing layer 22 As explained above, only one booster film layer and a sealing layer are considered essential to the booster film structure 20. However, in preferred embodiments, advantages may be gained by employing additional layers.
  • Each of the layers 22, 24 and 26 includes a polymer matrix.
  • Suitable polymer matrix materials include without limitation polymethyl methacrylate, polyethylene terephthalate, ethylene tetrafluoroethylene, polycarbonate, aliphatic polyurethane, and combinations thereof.
  • the polymer matrix material should be transparent enough to transport and emit suitable quantities of light to the photovoltaic solar cell. If any of the layers is opaque, or insufficiently transparent, then very little light will be transmitted.
  • the ultraviolet absorbing layer 22 is intended primarily for the absorption of high frequency ultraviolet light having such low wavelengths (below about 350 nanometers) that it cannot be rendered useful for the solar cell by downshifting using the booster film layer or layers 24 and 26.
  • the ultraviolet absorbing layer 22 includes one or more ultraviolet absorbing chemicals (herein “ultraviolet absorbers").
  • Suitable ultraviolet absorbers include without limitation Tinuvin ® 328, which is a hydroxyphenylbenzotriazole available from BASF Chemical Co.; Lowlite ® 22, which is a 2-hydroxy-4-octoxybenzophenone available from Chemtura; Lowlite ® 28, which is a 2-(2'-hydroxy-3', S'-di-t-amylphenyl) benzotriazole; UV-5411, which is a 2-(2H-Benzotriazol-2-yl)-4-(l, 1, 3, 3-tetramethyl butyl) phenol available from Cyasorb-, Chimassorb ® 81, which is a 2-hydroxy-benzophenone available from Cyasorb; oxygen scavengers such as Irganox ® B225, which is a hindered phenol available from BASF Chemical Co.; and free radical quenchers such as DABCO , which is a 1 , 4-diazabicyclo [2,2,2] octane available from
  • the ultraviolet absorbing layer 22 suitably includes about 0.05 - 2.5 % by weight of the one or more ultraviolet absorbers, or about 0.07 - 1.0 % by weight, or about 0.1 - 0.5 % by weight. If the separate ultraviolet absorbing layer 22 is not included in the booster film structure 20, then the one or more ultraviolet absorbers may be included in the same amounts in at least one of the booster film layers 24 and 26.
  • the booster film layers can be present as one layer (24 or 26), two layers (24 and 26), or any number of multiple layers beyond two layers.
  • the multiple booster film layers can be firmly bound together, or can be peelably engaged so that each time an outermost booster film layer sufficiently degrades, it can be stripped away to yield a fresh booster film layer underneath.
  • the combination of booster film layers (e.g. 24 and 26) and ultraviolet absorbing layer 22 cannot be so thick as to prevent sufficient light from passing through the booster film structure 20 to the solar cell.
  • the combination of booster film layer(s) and ultraviolet absorbing layers(s) should have a thickness of about 50-500 microns, suitably about 70-200 microns, or about 90-150 microns.
  • the booster film layers should include about 0.05 - 2.5 % by weight of one or more downshifting additives, suitably about 0.07 - 1.0 % by weight, or about 0.1 - 0.5 % by weight.
  • Suitable downshifting additives include organic luminescent dyes, luminescing metal complexes, microscopic semiconductors, and combinations thereof.
  • Suitable organic luminescent dyes include without limitation Lumogen ® F Orange 240, which is a perylene derivative available from BASF Chemical Co.; Lumogen ® F Violet 570, which is a naphthalimide; China G31712, which is a perylene derivative available from Beijing Wenhaiyang Perylenes Chemistry; other perylene derivatives; QK-S101Y and QK-S108Y, which are fluorescent yellow dyes having the general formula C 3 oH 28 0 4 and C 36 H 45 N0 2 S and are available from Hangzhou BTF Chemical Co., Ltd.
  • Suitable luminescing metal complexes include without limitation tris(2- phenylpyridine-C2, N') iridium III, available from Sunatech Inc.; other iridium, rutheniuin and europium complexes; and combinations thereof.
  • Microscopic semiconductor materials include quantum dots, which are particles of semiconductor material with a size so small that, due to quantum mechanics, the electron energies that can exist within them are limited.
  • Quantum dots can be cadmium-based, such as cadmium-selenium, and may have a cadmium- based core and a shell of a different material such as zinc-sulfide.
  • the first booster film layer 24 can include a luminescent material such as Lumogen ® F Yellow 170, which absorbs longer wavelength light averaging about 500 nanometers
  • the second booster film layer 26 can include a luminescent material such as Lumogen ® F Yellow 083, which absorbs shorter wavelength light averaging about 450 nanometers.
  • the layers are suitably arranged to absorb the longest wavelengths first. This results in the best overall performance.
  • the sealing layer 28 of booster film structure 20 can be formed of any transparent polymer that is capable of providing a peelable seal between the booster film structure and the cover glass of the photovoltaic device.
  • Suitable sealing polymers include without limitation ethylene vinyl acetate having a high vinyl acetate content, suitably 18-35% by weight vinyl acetate, acrylic polymers, silicone rubbers, and combinations thereof.
  • the sealing layer 28 should be as thin as possible, having a thickness of 10-50 microns.
  • the sealing layer 28 may be coextruded with the remaining layers of booster film structure 20, or applied by extrusion coating or any suitable coating process.
  • Fig. 4 illustrates one embodiment of a photovoltaic device 30 that includes the booster film structure 20 as an outer layer peelably sealed to the cover layer 32, which is typically made of glass.
  • the photovoltaic device 30 includes at least the booster film structure 20, the cover glass 32, and the solar cell 36.
  • Optional layers, which are present in some photovoltaic devices, include the top and bottom encapsulation layers 34 which surround the solar cell 36, and the back sheet 38.
  • the booster film structure 20 can be easily peeled away from the cover glass 32 and replaced when its useful life is exhausted.
  • the cover layer 32 can be a solar glass such as borosilicate or low iron- containing soda lime glass. Solar glasses are available form PPG Industrial and other solar glass companies. The solar glass may have a thickness of about 1-5 mm, suitably about 2-4 mm.
  • the optional encapsulation layers 34 can be formed of cross-linked ethylene vinyl acetate or another suitable material.
  • the solar cell 36 can be formed of any conventional material, such as crystalline or amorphous silicon, cadmium telluride-cadmium sulfide, or copper indium gallium selenide.
  • the optimal backsheet 38 should be opaque, and can be formed of a conventional opaque glass or polymer.
  • the primary function of the booster film structure 20 is to increase the power output of the photovoltaic device 30.
  • Fig. 4 is a plot of current (I) in milliamperes, versus voltage for a small cadmium telluride solar cell module having areal dimensions of 6.3 cm X 8 cm, with and without a solar booster film structure 20.
  • the booster film structure 20 includes top and bottom booster film layers of polymethyl methacrylate, loaded with Lumogen F Yellow 170 and Lumogen F Yellow 083 downshifting additives, respectively, at concentrations of 0.1 - 0.3 % by weight. As shown in Fig.
  • the photovoltaic device with the booster film structure enhanced the short circuit current by up to 10.9% (132 milliampheres) compared to the photovoltaic device without the booster film structure (119 milliamperes) without significantly affecting the fill factor and open current voltages.
  • the useful life of the booster film layers 24 and 26 can be maximized by minimizing the photo-degradation of the downshifting additives.
  • One way to accomplish this is to maximize the dispersion of the downshifting additives in the film.
  • the downshifting additive can be added to the monomer or monomers that form the film matrix polymer prior to polymerization.
  • the matrix polymer is polymethyl methacrylate
  • the downshifting additive can be added to the reaction mixture of methyl methacrylate monomer prior to polymerization and mixed to achieve a uniform dispersion with a minimum of aggregates.
  • the resulting mixture is then polymerized and formed into a film. Adding the downshifting polymer to the reaction mixture in this manner reduces photo-degradation of the booster film. It is believed that the improved dispersion retards dimerization which is part of the degradation mechanism.
  • Another way to improve the useful life of the booster film layers 24 and 26 is to limit the presence of oxygen within the structure. This can be accomplished by covering or coextruding the booster film layer with a clear oxygen barrier film layer.
  • Clear barrier film layers that limit the transmission of oxygen can be formed of a high barrier resin such as polyvinylidene dichloride (PVDC) or another barrier resin.
  • Clear barrier film layers can also be formed of a dyad structure of multiple metal oxide and polymer layers.
  • Clear barrier films are commercially available from many suppliers including Terra Barrier, Fuji Film, Toppan Printing, DuPont-Tejin, SC Chemical, DNP, Vitex, 3M Company, General Electric, Sigma Tech, UDC, Toray, Huntsman, Konica Minolta, Mitsubishi Plastics, CPI, Fraunhofer and Amcor Flexibles Singen GmbH.

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  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
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Abstract

A durable, replaceable solar booster film structure is provided which can be peelably adhered to the outside of the cover layer on a photovoltaic device. The solar booster film structure includes at least one booster film layer which includes a polymer matrix and one or more downshifting additives, and a sealing layer. The solar booster film structure can also include one or more ultraviolet absorbing chemicals. When the solar booster film structure reaches the end of its useful life, it can be simply peeled away from the cover layer of the photovoltaic device, and replaced.

Description

REPLACEABLE SOLAR WAVELENGTH - CONVERTING FILM
FIELD OF THE INVENTION
The invention is directed to a new and improved replaceable solar booster film structure that absorbs solar light across a broad range of frequencies, converts the light to a narrower range of frequencies that are useful for photovoltaic solar cells, and transmits the light to the solar cells to enable them to operate more efficiently.
BACKGROUND OF THE INVENTION
Luminescent solar booster films are known in the art, and are disclosed in U.S.
Patent Application Publication 2010/0186801 to Boehm et al., published in July 29, 2010. Such films are capable of downshifting the high frequency (typically ultraviolet and/or blue) portion of the spectrum to lower frequency radiation that can be efficiently absorbed by the photovoltaic device. One drawback of known films is that they degrade under sunlight, limiting their useful life to 1-2 years, far less than the 20-30 year lifespan of a typical photovoltaic device. There is a need for solar booster films and structures that have increased stability and a longer useful life.
One known way to improve the performance of known booster films is to place them underneath the cover glass of the photovoltaic device. Fig. 1 illustrates a known photovoltaic device 10 including a solar cell 12, an encapsulation material 14 surrounding the solar cell 12, a back sheet 13, a cover glass 18, and a solar booster film 16 between the cover glass 18 and the encapsulation material 14. The solar cell 12 converts electromagnetic radiation passing through the cover glass 18 and booster film 16 into electric energy. The booster film 16 partially absorbs electromagnetic radiation passing through the cover glass 18 and emits it at a longer wavelength to the solar cell 12. The cover glass 18 acts as a filter for some of the ultraviolet radiation in order to prolong the useful life of the booster file 16.
While this approach extends the life of the booster film 16, it does not approach the 20-30 year lifespan of the solar cell 12. Also, once the booster film 16 is degraded, it has not been possible to remove the photobleached or discolored booster film 16 from the photovoltaic device 10.
SUMMARY OF THE INVENTION
The present invention is directed to an improved solar booster film structure that remains durable for a longer period of time and offers a longer useful life. The improved film structure is easily removable from the cover material (typically glass) that covers the photovoltaic cell, and can be easily replaced at necessary intervals. Because of its improved durability, the booster film structure can be placed on the outside of the cover glass, and does not need to be placed underneath the cover glass for protection. When the booster film structure reaches the end of its useful life, as evidenced by photobleaching or discoloration, it can simply be peeled off of the cover glass and replaced with another booster film structure.
The booster film structure of the invention includes one or more booster film layers, which collectively include about 95 - 99.9 % by weight of a polymer matrix, about 0.05 - 2.5 % by weight of one or more downshifting additives, and about 0.05 - 2.5 % by weight of film stabilizers selected from the group consisting of ultraviolet absorbers, oxygen scavengers, free radical quenchers, and combinations thereof. The booster film structure may include multiple film layers. For example, the booster film structure may include an upper ultraviolet absorbing layer containing ultraviolet absorbers, for the purpose of absorbing ultraviolet radiation that could degrade or photobleach the downshifting additives and is too low in wavelength (too high in frequency) to be converted by the additives, followed by one or more downshifting booster film layers that include downshifting additives to shift the remainder of the ultraviolet and blue light toward the visible light spectrum used by the photovoltaic cell.
The booster film structure of the invention also includes a sealing layer for providing a removable and peelable seal to the cover material (typically glass) that covers the photovoltaic solar cell. This enables the booster film structure to be peeled away and replaced when its useful life is exhausted.
With the foregoing in mind, it is a feature and advantage of the invention to provide an improved booster film structure with an enhanced durable life.
It is also a feature and advantage of the invention to provide a booster film structure that can be easily removed and replaced when its useful life is exhausted.
The foregoing and other features and advantages will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a side sectional view of a photovoltaic device in which a booster film is placed between the cover glass and the solar cell, as is known in the art.
Fig. 2 is a representative graph showing the shifting of lower wavelength ultraviolet light into the low end of the visible wavelength spectrum, using a booster film structure of the invention.
Fig. 3 is a side sectional view of a booster film structure of the invention.
Fig. 4 is a side sectional view of a photovoltaic device in which a booster film structure of the invention is placed in the outside of the cover glass, for easy removal and replacement.
Fig. 5 is a graph showing the electrical output of a photovoltaic device with and without a booster film structure of the invention. DETAILED DESCRIPTION OF THE INVENTION
The invention is a removable, replaceable booster film structure that receives incoming sunlight, downshifts the frequency, and thus lengthens the wavelength of the sunlight, in order to covert a portion of incoming ultraviolet and blue light into higher wavelength energy within the visible spectrum. The booster film structure then emits the visible light toward a photovoltaic solar cell which converts the visible light into electrical energy.
Photovoltaic solar cells are made using crystalline or amorphous silicon, cadmium telluride-cadmium sulfide, copper indium gallium selenide, and other known materials. These solar cells convert sunlight into electricity most efficiently when the incoming light has a wavelength of about 500 to about 900 nanometers, which is within the visible range of the spectrum. For the incoming light having a frequency less than about 500 nanometers, down to about 300 nanometers (the high end of the ultraviolet wavelength range), the solar cells become less efficient on a sliding scale. The solar cells are essentially inoperative as to incoming ultraviolet light having a wavelength less than about 300 nanometers.
Fig. 2 illustrates how a booster film performs the downshifting of higher frequency (lower wavelength) incoming sunlight. The plot on the left side of the graph shows the energy (in arbitrary units) of incoming sunlight that is absorbed by the booster film. As shown, the incoming sunlight has a wavelength range of about 300-500 nanometers, with peak energy around 450 nanometers. The plot on the right side of the graph shows the energy (in arbitrary units) of light that is emitted by the booster film toward the solar cell. The emitted light has a wavelength range of about 450-650 nanometers, with peak energy at about 550 nanometers. The booster film used had a thickness of 100 microns, a polymer matrix of polymethyl methacrylate, and contained 0.2 % by weight of an organic yellow dye available from BASF under the trade name Lumogen® F Yellow 083.
Fig. 3 illustrates one embodiment of a color booster film structure 20 of the invention including, in order from top to bottom, an ultraviolet absorbing layer 22, a first booster film layer 24, a second booster film layer 26, and a sealing layer 28. As explained above, only one booster film layer and a sealing layer are considered essential to the booster film structure 20. However, in preferred embodiments, advantages may be gained by employing additional layers.
Each of the layers 22, 24 and 26 includes a polymer matrix. Suitable polymer matrix materials include without limitation polymethyl methacrylate, polyethylene terephthalate, ethylene tetrafluoroethylene, polycarbonate, aliphatic polyurethane, and combinations thereof. The polymer matrix material should be transparent enough to transport and emit suitable quantities of light to the photovoltaic solar cell. If any of the layers is opaque, or insufficiently transparent, then very little light will be transmitted.
The ultraviolet absorbing layer 22 is intended primarily for the absorption of high frequency ultraviolet light having such low wavelengths (below about 350 nanometers) that it cannot be rendered useful for the solar cell by downshifting using the booster film layer or layers 24 and 26. The ultraviolet absorbing layer 22 includes one or more ultraviolet absorbing chemicals (herein "ultraviolet absorbers"). Suitable ultraviolet absorbers include without limitation Tinuvin® 328, which is a hydroxyphenylbenzotriazole available from BASF Chemical Co.; Lowlite® 22, which is a 2-hydroxy-4-octoxybenzophenone available from Chemtura; Lowlite® 28, which is a 2-(2'-hydroxy-3', S'-di-t-amylphenyl) benzotriazole; UV-5411, which is a 2-(2H-Benzotriazol-2-yl)-4-(l, 1, 3, 3-tetramethyl butyl) phenol available from Cyasorb-, Chimassorb® 81, which is a 2-hydroxy-benzophenone available from Cyasorb; oxygen scavengers such as Irganox® B225, which is a hindered phenol available from BASF Chemical Co.; and free radical quenchers such as DABCO , which is a 1 , 4-diazabicyclo [2,2,2] octane available from Aldrich, Lowlite® Q84, which is a nickel quencher available from Chemtura, and UV 1084, which is a 2, 2'-thiobis (4-tert-octylphenolato)-n-butylamine nickel (II) available from Cyasorb. Other known ultraviolet absorbers selected from benzophenones, benzotriazoles, oxygen scavengers, free radical quenchers, and combinations thereof, can also be used in the ultraviolet absorbing layer 22.
The ultraviolet absorbing layer 22 suitably includes about 0.05 - 2.5 % by weight of the one or more ultraviolet absorbers, or about 0.07 - 1.0 % by weight, or about 0.1 - 0.5 % by weight. If the separate ultraviolet absorbing layer 22 is not included in the booster film structure 20, then the one or more ultraviolet absorbers may be included in the same amounts in at least one of the booster film layers 24 and 26.
The booster film layers can be present as one layer (24 or 26), two layers (24 and 26), or any number of multiple layers beyond two layers. The multiple booster film layers can be firmly bound together, or can be peelably engaged so that each time an outermost booster film layer sufficiently degrades, it can be stripped away to yield a fresh booster film layer underneath. However, the combination of booster film layers (e.g. 24 and 26) and ultraviolet absorbing layer 22 cannot be so thick as to prevent sufficient light from passing through the booster film structure 20 to the solar cell. The combination of booster film layer(s) and ultraviolet absorbing layers(s) should have a thickness of about 50-500 microns, suitably about 70-200 microns, or about 90-150 microns.
The booster film layers, individually and/or collectively, should include about 0.05 - 2.5 % by weight of one or more downshifting additives, suitably about 0.07 - 1.0 % by weight, or about 0.1 - 0.5 % by weight. Suitable downshifting additives include organic luminescent dyes, luminescing metal complexes, microscopic semiconductors, and combinations thereof. Suitable organic luminescent dyes include without limitation Lumogen® F Orange 240, which is a perylene derivative available from BASF Chemical Co.; Lumogen® F Violet 570, which is a naphthalimide; China G31712, which is a perylene derivative available from Beijing Wenhaiyang Perylenes Chemistry; other perylene derivatives; QK-S101Y and QK-S108Y, which are fluorescent yellow dyes having the general formula C3oH2804 and C36H45N02S and are available from Hangzhou BTF Chemical Co., Ltd. Suitable luminescing metal complexes include without limitation tris(2- phenylpyridine-C2, N') iridium III, available from Sunatech Inc.; other iridium, rutheniuin and europium complexes; and combinations thereof. Microscopic semiconductor materials include quantum dots, which are particles of semiconductor material with a size so small that, due to quantum mechanics, the electron energies that can exist within them are limited. Quantum dots can be cadmium-based, such as cadmium-selenium, and may have a cadmium- based core and a shell of a different material such as zinc-sulfide.
When only one booster film layer is included, it will include all of the downshifting additives selected for the particular application. When more than one booster film layer is included, different downshifting additives may be included in different booster film layers for optimum results. In one embodiment, the first booster film layer 24 can include a luminescent material such as Lumogen® F Yellow 170, which absorbs longer wavelength light averaging about 500 nanometers, and the second booster film layer 26 can include a luminescent material such as Lumogen® F Yellow 083, which absorbs shorter wavelength light averaging about 450 nanometers. When using multiple booster film layers, the layers are suitably arranged to absorb the longest wavelengths first. This results in the best overall performance.
The sealing layer 28 of booster film structure 20 can be formed of any transparent polymer that is capable of providing a peelable seal between the booster film structure and the cover glass of the photovoltaic device. Suitable sealing polymers include without limitation ethylene vinyl acetate having a high vinyl acetate content, suitably 18-35% by weight vinyl acetate, acrylic polymers, silicone rubbers, and combinations thereof. The sealing layer 28 should be as thin as possible, having a thickness of 10-50 microns. The sealing layer 28 may be coextruded with the remaining layers of booster film structure 20, or applied by extrusion coating or any suitable coating process.
Fig. 4 illustrates one embodiment of a photovoltaic device 30 that includes the booster film structure 20 as an outer layer peelably sealed to the cover layer 32, which is typically made of glass. The photovoltaic device 30 includes at least the booster film structure 20, the cover glass 32, and the solar cell 36. Optional layers, which are present in some photovoltaic devices, include the top and bottom encapsulation layers 34 which surround the solar cell 36, and the back sheet 38. As shown in Fig. 4, the booster film structure 20 can be easily peeled away from the cover glass 32 and replaced when its useful life is exhausted.
The cover layer 32 can be a solar glass such as borosilicate or low iron- containing soda lime glass. Solar glasses are available form PPG Industrial and other solar glass companies. The solar glass may have a thickness of about 1-5 mm, suitably about 2-4 mm. The optional encapsulation layers 34 can be formed of cross-linked ethylene vinyl acetate or another suitable material. The solar cell 36 can be formed of any conventional material, such as crystalline or amorphous silicon, cadmium telluride-cadmium sulfide, or copper indium gallium selenide. The optimal backsheet 38 should be opaque, and can be formed of a conventional opaque glass or polymer.
The primary function of the booster film structure 20 is to increase the power output of the photovoltaic device 30. Fig. 4 is a plot of current (I) in milliamperes, versus voltage for a small cadmium telluride solar cell module having areal dimensions of 6.3 cm X 8 cm, with and without a solar booster film structure 20. The booster film structure 20 includes top and bottom booster film layers of polymethyl methacrylate, loaded with Lumogen F Yellow 170 and Lumogen F Yellow 083 downshifting additives, respectively, at concentrations of 0.1 - 0.3 % by weight. As shown in Fig. 4, the photovoltaic device with the booster film structure enhanced the short circuit current by up to 10.9% (132 milliampheres) compared to the photovoltaic device without the booster film structure (119 milliamperes) without significantly affecting the fill factor and open current voltages.
The useful life of the booster film layers 24 and 26 can be maximized by minimizing the photo-degradation of the downshifting additives. One way to accomplish this is to maximize the dispersion of the downshifting additives in the film. To achieve maximum dispersion, the downshifting additive can be added to the monomer or monomers that form the film matrix polymer prior to polymerization. For example, when the matrix polymer is polymethyl methacrylate, the downshifting additive can be added to the reaction mixture of methyl methacrylate monomer prior to polymerization and mixed to achieve a uniform dispersion with a minimum of aggregates. The resulting mixture is then polymerized and formed into a film. Adding the downshifting polymer to the reaction mixture in this manner reduces photo-degradation of the booster film. It is believed that the improved dispersion retards dimerization which is part of the degradation mechanism.
Another way to improve the useful life of the booster film layers 24 and 26 is to limit the presence of oxygen within the structure. This can be accomplished by covering or coextruding the booster film layer with a clear oxygen barrier film layer. Clear barrier film layers that limit the transmission of oxygen can be formed of a high barrier resin such as polyvinylidene dichloride (PVDC) or another barrier resin. Clear barrier film layers can also be formed of a dyad structure of multiple metal oxide and polymer layers. Clear barrier films are commercially available from many suppliers including Terra Barrier, Fuji Film, Toppan Printing, DuPont-Tejin, SC Chemical, DNP, Vitex, 3M Company, General Electric, Sigma Tech, UDC, Toray, Huntsman, Konica Minolta, Mitsubishi Plastics, CPI, Fraunhofer and Amcor Flexibles Singen GmbH.
While the embodiments of the invention described herein are presently preferred, various modification and improvements can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated by the appended claims, and all changes that fall within the meaning and range of equivalents are intended to be embraced therein.

Claims

WE CLAIM:
1. A solar booster film structure, comprising:
one or more booster film layers;
each booster film layer comprising a polymer matrix selected from the group consisting of polymethyl methacrylate, polyethlene terephthalate, ethylene tetrafluoroethylene, polycarbonate, aliphatic polyurethane, and combinations thereof;
each booster film layer further comprising a downshifting additive selected from the group consisting of organic luminescent dyes, luminescent metal complexes, microscopic semiconductors, and combinations thereof; and
a sealing layer adapted for peelable sealing of the solar booster film structure to glass.
2. The solar booster film structure of Claim 1, wherein at least one booster film layer comprises an organic luminescent dye downshifting additive selected from the group consisting of perylene derivatives, naphthalamides, fluorescent yellow dyes having the formula C30H28O4, or C36H45N02S and combinations thereof.
3. The solar booster film structure of Claim 1, wherein at least one booster film layer comprises a luminescent metal complex downshifting additive selected from the group consisting of iridium complexes, ruthenium complexes, europium complexes, and combinations thereof.
4. The solar booster film structure of Claim 1, wherein at least one booster film layer comprises a microscopic semiconductor downshifting additive selected from the group consisting of quantum dots and combinations thereof.
5. The solar booster film structure of Claim 1, comprising first and second booster film layers, wherein the first booster film layer comprises a first downshifting additive and the second booster film layer comprises a second downshifting additive.
6. The solar booster film structure of Claim 5, wherein the first downshifting additive absorbs light having relatively longer wavelengths and the second downshifting additive absorbs light having relatively shorter wavelength.
7. The solar booster film structure of Claim 1, wherein the booster film layer is prepared by dispersing the downshifting additive in a monomer mixture which is then polymerized to form the polymer matrix.
8. The solar booster film of Claim 1, further comprising an oxygen barrier layer covering at least one of the booster film layers.
9. The solar booster film structure of Claim 1, wherein the sealing layer comprises a polymer selected from the group consisting of ethylene vinyl acetates, acrylic polymers, silicone rubbers, and combinations thereof.
10. The solar booster film structure of Claim 1, further comprising an ultraviolet absorbing chemical.
11. The solar booster film structure of Claim 10, wherein the ultraviolet absorbing chemical is present in at least one of the booster film layers.
12. The solar booster film structure of Claim 1, further comprising an ultraviolet absorbing layer, the ultraviolet absorbing layer comprising a polymer matrix and the ultraviolet absorbing chemical.
13. The solar booster film of Claim 10, wherein the ultraviolet absorbing chemical is selected from the group consisting of benzotriazoles, benzophenones, oxygen scavengers, free radical quenchers, and combinations thereof.
14. A solar booster film structure, comprising:
an ultraviolet absorbing layer comprising a polymer matrix and an ultraviolet absorbing chemical;
a first booster film layer comprising a polymer matrix and a first downshifting additive;
a second booster film layer comprising a polymer matrix and a second downshifting additive that is different from the first downshifting additive; and a sealing layer.
15. The solar booster film structure of Claim 14, wherein the ultraviolet absorbing layer comprises:
a polymer matrix selected from the group consisting of polymethyl methacrylate, polyethylene terephthalate, ethylene tetraflouoroethylene, polycarbonate, aliphatic polyurethane, and combinations thereof; and
an ultraviolet absorbing chemical selected from the group consisting of benzotriazoles, benzophenones, oxygen scavengers, free radical quenchers, and combinations thereof.
16. The solar booster film structure of Claim 14, wherein the first booster film layer comprises a polymer matrix selected from the group consisting of polymethyl methacrylate, polyethylene terephthalate, ethylene tetrafluoroethylene, poly carbonate, aliphatic polyurethane, and combinations thereof, and the first downshifting additive is selected from the group consisting of organic luminescent dyes, luminescent metal complexes, microscopic semiconductors, and combinations thereof.
17. The solar booster film structure of Claim 16, wherein the second downshifting additive in the second booster film layer is selected from the group consisting of organic luminescent dyes, luminescent metal complexes, microscopic semiconductors, and combinations thereof.
18. The solar booster film structure of Claim 17, wherein the second booster film layer comprises a polymer matrix selected from the group consisting of polymethyl methacrylate, polyethylene terephthalate, ethylene tetrafluoroethylene, polycarbonate, aliphatic polyurethane, and combinations thereof.
19. The solar booster film structure of Claim 14, wherein the sealing layer comprises a polymer selected from the group consisting of ethylene vinyl acetates, acrylic polymers, silicone rubbers, and combinations thereof.
20. The solar booster film layer of Claim 14, wherein the first booster film layer absorbs light having a relatively longer wavelength and the second booster film layer absorbs light having a relatively shorter wavelength.
21. A photovoltaic device, comprising:
a solar cell;
a glass cover layer having an inner surface facing the solar cell and an outer surface facing away from the solar cell; and
a solar booster film structure comprising one or more booster film layers and a sealing layer, each booster film layer including a polymer matrix and a downshifting additive.
22. The photovoltaic device of Claim 21, wherein each booster film layer includes a polymer matrix selected from the group consisting of polymethyl methacrylate, polyethylene terephthalate, ethylene tetrafluoroethylene, polycarbonate, aliphatic polyurethane, and combinations thereof.
23. The photovoltaic device of Claim 21, wherein each booster film layer comprises a downshifting additive selected from the group consisting of organic luminescent dyes, luminescent metal complexes, microscopic semiconductors, and combinations thereof.
24. The photovoltaic device of Claim 21, wherein the solar booster film structure further comprises one or more ultraviolet absorbing chemicals.
25. The photovoltaic device of Claim 21, wherein the sealing layer comprises a polymer selected from the group consisting of ethylene vinyl acetates, acrylic polymers, and combinations thereof.
PCT/US2013/030215 2012-05-15 2013-03-11 Replaceable solar wavelength - converting film WO2013172930A1 (en)

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