Abstract:
The present invention is a photovoltaic cell having improved conversion properties. The cell includes a cover for a photovoltaic device. In one embodiment, the cover includes a fluorescent material that shifts the wavelength of some of the incident light to be closer to the wavelength that produces the least amount of thermal loading on the photovoltaic device. In another embodiment, the cover includes a fluorescent material between two reflective filters. The cell and cover may either be placed together in a stack or separated from one another.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/813,635 filed Jul. 3, 2006, and U.S. Provisional Application No. 60/821,383 filed Aug. 3, 2006. The entire contents of the above-listed provisional application are hereby incorporated by reference herein and made part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field 
     The present invention generally relates to photovoltaics, and more particularly to a method and system for covering photovoltaic cells. 
     2. Discussion of Related Art 
     The efficiency with which a solar-to-electricity power conversion device can convert sunlight into electricity is determined, in part, by how well the device responds to the spectral characteristics of the solar flux. Photovoltaic devices, for example, are capable of absorbing light, at varying efficiency, over a large part of the solar spectrum. While some of the solar photons are efficiently converted to electricity, other photons impart only some of their energy to electric energy, and others simply heat the photovoltaic device. 
     Photovoltaic devices are capable of converting only a fraction of solar photons into electricity. Thus, for example, silicon photovoltaic cells can convert light from approximately 0.3 μm to approximately 1.1 μm into electron-hole pairs, which may be used to generate electricity. Photons having longer wavelengths have an energy that is less than the band gap energy of silicon photovoltaic cells, and are not absorbed. Photons having a wavelength below the upper limit but that are not converted to electricity are converted to heat, which raises the temperature of the photovoltaic device. In addition, some of the energy of photovoltaically converted photons also appears as heat. Specifically, the difference between the photon energy and the band gap energy is not useful for generating electron-hole pairs and is lost as heat within the photovoltaic device. 
     There are thus many mechanisms affecting photovoltaic conversion and unwanted heating of photovoltaic devices. There is a need in the art for a photovoltaic device that more efficiently converts the solar flux into useable electricity. There is also a need in the art for a photovoltaic device minimizes the amount of heat generated therein. Such a device should be simple and inexpensive, and compatible with current photovoltaic technology. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of prior art by converting the spectral characteristics of the solar flux to a spectra that better matches the conversion capabilities of a photovoltaic cell. 
     In one embodiment, the present invention provides a photovoltaic cell and cover for converting incident radiative flux into electricity including a photovoltaic portion having band gap wavelength and a cell cover substantially covering a light-receiving surface of the photovoltaic portion. The cell cover includes a first layer including a reflective filter and adapted to receive the incident radiative flux, a second layer facing the first layer and including a fluorescent material having pump wavelengths and emission wavelengths, and a third layer between the second layer and the photovoltaic portion, and including a reflective filter. In one embodiment, the first layer and third layer are interference filters. In another embodiment, the first layer and third layer are deposited on opposing sides of the second layer. In yet another embodiment, the second layer includes neodymium. In one embodiment, the first layer transmits light at said pump wavelength and reflects light at said pump wavelength. In one embodiment, the third layer reflects light at said pump wavelength. It is preferred that the emission wavelength is less than or equal to said band gap wavelength. 
     In another embodiment, a cover for receiving an incident radiative flux for a photovoltaic device is provided, where the photovoltaic device has a band gap wavelength and a light-receiving surface for converting incident radiative flux into electricity. The cell cover includes a first layer including a reflective filter and adapted to receive the incident radiative flux, a second layer facing the first layer and including a fluorescent material having pump wavelengths and emission wavelengths, and a third layer between the second layer and the photovoltaic portion, and including a reflective filter. In one embodiment, the first layer and third layer are interference filters. In another embodiment, the first layer and third layer are deposited on opposing sides of the second layer. In yet another embodiment, the second layer includes neodymium. In one embodiment, the first layer transmits light at said pump wavelength and reflects light at said pump wavelength. In one embodiment, the third layer reflects light at said pump wavelength. It is preferred that the emission wavelength is less than or equal to said band gap wavelength. 
     These features together with the various ancillary provisions and features which will become apparent to those skilled in the art from the following detailed description, are attained by the photovoltaic cell cover of the present invention, embodiments thereof being shown with reference to the accompanying drawings, by way of example only, wherein: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1A  is a perspective sectional view of a first embodiment photovoltaic cell and cover; 
         FIG. 1B  is a side sectional view of the embodiment of  FIG. 1A ; 
         FIG. 2  is graph illustrating the spectral characteristics of one embodiment of a photovoltaic cell; 
         FIG. 3  is a sectional view of a second embodiment cover for a photovoltaic cell; 
         FIG. 4  is a graph illustrating the absorption properties of a material of a first embodiment photovoltaic cell cover; 
         FIG. 5  is a third embodiment of a photovoltaic cell and cover; 
         FIG. 6  is a fourth embodiment of a photovoltaic cell and cover; 
         FIG. 7  is a perspective view of a fifth embodiment of a photovoltaic cell and cover showing the cell and cover separated from each other; 
         FIG. 8  is perspective view of the embodiment of  FIG. 7  with the cover placed over the cell; 
         FIG. 9  is a first embodiment of sectional view  9 - 9  of  FIG. 8 ; and 
         FIG. 10  is a second embodiment of sectional view  9 - 9  of  FIG. 8 . 
     
    
    
     Reference symbols are used in the Figures to indicate certain components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1A  is a perspective sectional view of a first embodiment photovoltaic cell and cover  100 . As shown in  FIG. 1A , photovoltaic cell and cover  100  includes a photovoltaic portion  110  and a cell cover  120 . Photovoltaic portion  110  includes a photovoltaic material  111  having a front surface  113  and an opposing back surface  115 . Cell cover  120  substantially covers the photovoltaic portion front surface  113 , and extends from a surface  103  adjacent to photovoltaic portion front surface  113  to a surface  101 . In one embodiment, surfaces  101 ,  103 , and  113  are sufficiently planar and parallel to permit light to pass through surface  101  and into photovoltaic portion  110 . 
     Photovoltaic portion  110  includes, in general, a photovoltaic material  111  that is, but is not limited to: a bulk material including, but not limited to, silicon, germanium, or another bulk semiconducting material formed as a monocrystalline, poly- or multicrystalline, or ribbon structure; thin films including, but not limited to, multi-layered thin-film composites, chalcogenide films of Cu(InxGa1−x)(SexS1−x)2, cadmium telluride, conductive polymers, polymer (or organic) solar cells, gallium arsenide (GaAs), dye-sensitized solar cells, silicon thin-films, including but not limited to, amorphous silicon, photocrystalline silicon or nanocrystalline silicon; or quantum dots. 
     Cell cover  120  extends from a surface  103  adjacent to photovoltaic portion front surface  113  to a surface  101 . Cell cover  120  includes one or more optical materials that transmit, interact and modify spectra of light that passes into the cover and into photovoltaic portion  110 . Cell cover  120  may be formed on photovoltaic portion  110 , as by a semiconductor manufacturing process, such as deposition, may be fixed to the photovoltaic portion, such as with an adhesive, or may be a structure that is separate from and may be placed or held adjacent to or in contact with the photovoltaic portion. In an alternative embodiment, cell cover  120  includes elements that providing spacing between different optical materials or between the optical materials and photovoltaic portion  110 . 
     The operation of photovoltaic cell and cover  100  is illustrated with reference to  FIG. 1B , which shows a side sectional view of the photovoltaic cell and  FIG. 2  which includes graph  200  illustrating the spectral characteristics of one embodiment of the photovoltaic cell.  FIG. 1B  also shows, for illustrative purposes, an incident for receiving a radiant flux F 1 , which may be a solar flux, and a pair of electrical leads  131  and  133  connection photovoltaic portion  110  to an electric load  130 . Graph  200  shows a representative solar spectrum  201  (more specifically, the radiant spectral flux density) at the earth&#39;s surface over the wavelength range of 0.3 μm to 1.5 μm. 
     Graph  200  shows characteristics of photovoltaic portion  110 , as follows. One way of specifying a photovoltaic material is by the material&#39;s band gap energy. Photons having an energy greater than the band gap energy may be absorbed by a photovoltaic material and generate electron-hole pairs useful for producing electric power. The wavelength corresponding to a band gap energy is referred to herein, and without limitation, as the “band gap wavelength.” Graph  200  has a line  203  representing the band gap wavelength of photovoltaic material  111 . For illustrative purposes which are not meant to limit the scope of the present invention, line  203  is at a wavelength of 1.12 μm, which corresponds to that of a silicon photovoltaic device. Graph  200  also shows a range  205 , which extends to from small wavelength up to the band gap wavelength. Photons within range wavelength  205  have energies greater than the band gap energy and thus are absorbed by photovoltaic material  111 . 
     Graph  200  also shows characteristics of cell cover  120 , as follows. Cell cover  120  includes a fluorescent material, as described subsequently in greater detail, that absorbs light at or near a first wavelength or range, indicated by a line  207  on graph  200 , and emits light at a longer wavelength or range of wavelengths, indicated by a line  209  on the graph. The shift in wavelength between an absorbed photon and a emitted photon is indicated by arrow  211 . In the present invention, the materials of cell cover  120  are selected to shift light within range  205  to a wavelength closer to, or in an alternative embodiment, equal to, the band gap wavelength  203  of photovoltaic material  111 . 
     In general, cell cover  120  accepts radiant flux F 1  into front surface  101  includes materials that transmit a spectrally altered radiant flux F 2  away from the cell cover and through back surface  103  into photovoltaic portion  110 . The shift is preferably towards a wavelength which is close to or slightly less than the wavelength corresponding to the band gap of photovoltaic material  111 . In one embodiment, the wavelength is shifted to within 10% of the band gap wavelength. In another embodiment, the wavelength is shifted to within 5% of the band gap wavelength. 
     Only a fraction of the photons within the wavelength range  205  produce electron-hole pairs in photovoltaic material  111 . Further, only a fraction of energy of the photons that produce electron-hole pairs becomes useful electrical energy. The energy of the absorbed photons that do not produce electron-hole pairs, and the energy in excess of the band gap energy of photons that do produce electron-hole pairs generate heat in photovoltaic material  111 . By shifting the wavelength of light impinging photovoltaic material  111  closer to the band gap wavelength of the photovoltaic material, less heat is generated within the photovoltaic material. 
       FIG. 3  depicts a second embodiment cell cover  320 , which may be generally similar to cell cover  120 , except as further detailed below. Where possible, similar elements are identified with identical reference numerals in the depiction of the embodiments of  FIGS. 1A ,  1 B, and  2 . 
     Cell cover  320  comprises three adjacent layers of optical material: a first layer  321  which includes surface  101 , a second layer  323 , and a third layer  325  which includes surface  103 . Further, first layer  321  and third layer  325  are reflective filters and second layer  323  is a fluorescent material. In general cover cell  320  materials are selected to shift at least a portion of the radiant flux toward the wavelength corresponding to the band gap energy of photovoltaic portion  111 . 
     In one embodiment, cell cover  320  is a laser-like structure that accepts an incident flux, such as F 1  as shifts at least a portion of the incident flux towards but not exceeding the band gap wavelength of photovoltaic material  111 . Photovoltaic cell  100  including cell cover  320  will now be described with reference specific materials which are meant to be illustrative and not limiting as to the scope of the invention. 
     In one specific embodiment, photovoltaic portion  111  is a silicon photovoltaic having a band gap energy of 1.11 eV (corresponding to a wavelength of 1.117 μm). Second layer  323  is a slab of a neodynium:glass laser material. One such material is a potassium-barium-aluminum-phosphate based glass, such as LG-750 Phosphate Laser Glass (Schott Glass Technologies, Inc., Duryea, Pa.), having a thickness y of from approximately 3 mm to approximately 25 mm. In one embodiment, layers  321  and  323  are thin interference filters deposited on opposing sides of second layer  323  having negligible thicknesses. In a second embodiment, one or more of layers  321  and  323  are separate interference filters that are adhesively fixed to second layer  323 , or that are held in place with a retaining housing element. 
     The optical properties of cell cover  320  are described with reference to  FIG. 4 , which shows a graph  400  of the transmission curve for LG-750. LG-750 has absorption peaks  401 ,  403 , and  405 , in range of approximately 0.6 μm to approximately 0.80 μm at 0.68 μm (peak  401 ), 0.75 μm (peak  403 ), and 0.80 μm (peak  405 ), as well as other absorption features. LG-750 has an emission peak at 1.0537 μm with a width of 0.026 μm. 
     Layer  321  accepts incident flux, such as a solar flux, and traps light near the absorption features of second layer  323 . In one embodiment, first layer  321  is highly transmissive at the absorption wavelengths of second layer  323  and has transmissivity of greater than or equal to 95% at wavelengths from approximately 0.6 μm to approximately 0.80 μm, and is highly reflective, with a reflectivity greater than or equal to 95%, at a wavelength of 1.06 μm. First layer  321  thus accepts solar flux at pumping wavelengths of second layer  323  and reflects any wavelength shifted light back towards photovoltaic portion  110 . 
     In one embodiment, third layer  325  blocks radiant flux that is not effective at generating a current in photovoltaic material  111 , and thus, for example is highly reflective for wavelengths less than approximately 0.35 μm. In another embodiment, third layer  325  is highly reflective at a wavelength of 1.06 μm, and thus reflects fluorescence back into second layer  323 . 
     Alternative embodiments for second layer  323  are solid fluorescent materials the shift light to wavelengths less than or equal to the photovoltaic material that is being covered. Examples of such materials include laser materials including, but are not limited to, other neodymium glasses, neodymium YAG, neodymium YLF, neodymium doped YVO4, neodymium doped yttrium calcium oxoborate, titanium sapphire, ytterbium YAG, ytterbium doped glass, and promethium  147  doped phosphate glass. Importantly, second layer  323  should absorb light at wavelengths within the incident radiative flux spectra, and should have emission at or slightly less than the band gap wavelength of the photovoltaic material being pumped. 
     In general, some or all of the photovoltaic cell and cover or cover layers may include or be a laminated structure, may include substantially planar portions that are bonded or held together, or may include planar portions that are substantially planar and maintained by spacing elements at a spacing with a gap between layers. Two illustrative embodiments having spaced structures are illustrated in  FIGS. 5 and 6 . 
       FIG. 5  depicts a third embodiment of a photovoltaic cell and cover  500 , which may be generally similar to photovoltaic cell and cover  100  except as further detailed. Where possible, similar elements are identified with identical reference numerals in the depiction of the embodiments of  FIGS. 1A ,  1 B,  2 ,  3 , and  4 . Photovoltaic cell and cover  500  includes a photovoltaic portion  110  and a cover  520 , which includes the optical materials of cell cover  320  and spacing elements  501 . Spacing elements  501  are preferably positioned so as to not block active portions of photovoltaic portion  110 , are sufficiently thick to keep portion  110  and  320  from touching, and can be formed, as non-limiting examples, from a metal, plastic, or ceramic material. In one embodiment, spacing elements  501  are elements that are separate from portion  110  and cover  320 , and that may be adhesively or mechanically fixed, as with screws or other fastening means, to one or both of portion  110  and cover  320 . In another embodiment, spacing elements  501  are formed from raised portions of one or both of portion  110  or cover  320 . 
       FIG. 6  depicts a fourth embodiment of a photovoltaic cell and cover  600 , which may be generally similar to photovoltaic cell and covers  100  or  500 , or cell cover  320 , except as further detailed. Where possible, similar elements are identified with identical reference numerals in the depiction of the embodiments of  FIGS. 1A ,  1 B,  2 ,  3 ,  4  and  5 . 
     Photovoltaic cell and cover  600  includes a photovoltaic portion  110  and a cell cover  620 . Cell cover  620  includes layers  321 ,  323 , and  325  and spacing elements  601 ,  603 , and  605 . Spacing elements  601 ,  603 , and  605  are provided to space layers  321 ,  323 , and  325  apart from each other and from photovoltaic portion  110 . Spacing elements  601 ,  603 , and  605  are preferably positioned so as to not block active portions of photovoltaic portion  110 , are sufficiently thick to keep adjacent ones of portion  110  and layers  321 ,  323 , and  325  from touching, and can be formed, as non-limiting examples, from a metal, plastic, or ceramic material. In one embodiment, one or more of spacing elements  601 ,  603 , and  605  are elements that are separate from portion  110  and layers  321 ,  323 , and  325  and that may be adhesively fixed to one or both of portion  110  and layers  321 ,  323 , and  325 . In another embodiment, one or more of spacing elements  601 ,  603 , and  605  are formed from raised portions of one or both of portion  110  or layers  321 ,  323 , and  325 . 
     Alternative embodiments provides a cell cover  120 ,  320 ,  520 , or  620  as an accessory for commercially available photovoltaic modules. Thus, for example,  FIGS. 7 and 8  are perspective views of a fifth embodiment photovoltaic cell and cover  700  as including a cover  720  to fit over photovoltaic portion  110 , where  FIG. 7  shows the photovoltaic portion and cover separated from each other, and  FIG. 8  shows the cover placed over the photovoltaic portion. Photovoltaic cell and cover  700  may be generally similar to photovoltaic cell and covers  100 ,  500 , or  600  or cell covers  120  or  320 , and include spacing elements as described with reference to those embodiments, except as further detailed. Where possible, similar elements are identified with identical reference numerals in the depiction of the embodiments of  FIGS. 1A ,  1 B,  2 ,  3 ,  4 ,  5  and  6 . 
       FIG. 9  shows is a first embodiment of sectional view  9 - 9  of  FIG. 8 , as including a photovoltaic cell and cover  900 , which includes a cover  920 , that may be generally similar to photovoltaic cell and covers  500  and  700  and covers  520  and  720 , except as further detailed. Where possible, similar elements are identified with identical reference numerals in the depiction of the embodiments of  FIGS. 1-8 . 
     Cover  900  that includes a housing  901  that incorporates spacing elements  501  and the material of cover  320 . Housing  901  is a rigid housing formed, for example but not limited to, one or more pieces of a metal, plastic, or ceramic. Housing  901  also includes a lip  903  that may be sized to fit over a photovoltaic system. Alternatively, screws or other mechanical devices may be used to fix housing  901  or lip  903  to a photovoltaic system, such as photovoltaic portion  110 . 
       FIG. 10  shows a second embodiment of sectional view  9 - 9  of  FIG. 8 , as including a photovoltaic cell and cover  1000 , which includes a cover  1020 , that may be generally similar to photovoltaic cell and covers  600  and  700  and covers  620  and  720 , except as further detailed. Where possible, similar elements are identified with identical reference numerals in the depiction of the embodiments of  FIGS. 1-8 . 
     Cover  1020  includes a housing  1001  that incorporates spacing elements  601 ,  603 , and  605 , and layers  321 ,  323 , and  325 . Housing  1001  is a rigid housing formed, for example but not limited to, one or more pieces of a metal, plastic, or ceramic. Housing  1001  also includes a lip  1003  that may be sized to fit over a photovoltaic system. Alternatively, screws or other mechanical devices may be used to fix housing  1001  or lip  1003  to a photovoltaic system, such as photovoltaic portion  110 . 
     Alternatively in the embodiments of  FIGS. 7-10  there may be no spacing elements to prevent optical materials from contacting photovoltaic portion  110 . Thus, for example, spacing element  501  or  601  is optional. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. 
     Similarly, it should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention.