Abstract:
In various embodiments, photovoltaic modules are hermetically sealed by providing a first glass sheet, a photovoltaic device disposed on the first glass sheet, and a second glass sheet, a gap being defined between the first and second glass sheets, disposing a glass powder within the gap, and heating the powder to seal the glass sheets.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/868,203, filed Aug. 21, 2013, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    In various embodiments, the present invention relates to photovoltaic devices, and in particular to glass-sealed photovoltaic modules. 
       BACKGROUND 
       [0003]    Current state-of-the-art photovoltaic (PV) modules based on silicon (Si) wafer technology employ a front glass/encapsulant/wafer/encapsulant/backsheet structure. Polyvinyl fluoride (e.g., Tedlar® from DuPont) is the most commonly used backsheet in wafer-based Si PV modules. This material is inherently transparent for water vapor and gases—i.e., it does not enable or maintain a hermetic seal. Thin-film PV modules based on amorphous Si (a-Si), CdTe, or copper indium gallium selenide (CuIn x Ga 1-x Se 2  or CIGS) most often utilize a glass-glass architecture with one of the glass sheets either serving as the substrate (CIGS) or superstrate (a-Si, CdTe) onto which the active PV material is deposited directly. The function of the second glass sheet is to serve as either the protective top (CIGS) or back cover (a-Si, CdTe). Both sheets of glass are held together with a single encapsulant layer. For practical purposes the 1.8-4 mm-thick sheets of glass are impermeable to water and gases. However, moisture and gas ingress can occur along the edges of the glass/encapsulant/glass sandwich. Exposure to such moisture can result in issues such as corrosion, decreased minority carrier lifetime, and increased series resistance, deleteriously impacting the performance and lifetime of the PV module. To slow down moisture ingress and gas permeation, butyl-based edge seals are typically applied, as butyl rubber has water vapor transmission rates (WVTR) on the order of 10 −6  g/m 2 -day, orders of magnitude below the WVTR for typical encapsulant materials. 
         [0004]    While the WVTR value for butyl is considered to be adequate for long-term protection, the overall vulnerability of a device to moisture usually derives less from the bulk diffusivity of the butyl than from the durability of the interface between the butyl and the glass. Adhesion promoters to establish coupling of the polymer to the glass are typically added to the butyl, but the resulting bonds tend to be weak and easily damaged by ultraviolet radiation, prolonged exposure to elevated temperatures, and humidity—i.e., conditions in which PV modules are commonly deployed. Stresses present between the two sheets of glass can further strain the glass/butyl interfaces. Hence, even for such edge-sealed glass-glass structures, moisture penetration is a common field failure mode. Thus, there is a need for hermetically sealed modules and techniques that enable environmental protection of thin-film PV devices and which are superior to current butyl-rubber-based solutions. 
       SUMMARY 
       [0005]    Embodiments of the present invention provide PV modules hermetically sealed against environmental contamination via edge seals consisting entirely of glass. While preferred embodiments of the invention utilize “thin-film” PV modules featuring PV devices based on CIGS, a-Si, or CdTe, other embodiments of the invention utilize PV devices based on crystalline Si (e.g., in wafer or ribbon form) or III-V semiconductors such as GaAs or InP (e.g., in wafer form) or solid-state dye-sensitized perovskite material (organic-inorganic hybrid). As utilized herein, “PV devices” are the active PV materials within PV modules, and these typically include or consist essentially of materials forming one or more p-n or p-i-n junctions that each absorb at least a portion of the solar spectrum and convert it into electricity. In preferred embodiments, a powder consisting essentially of or consisting of glass is utilized to seal a PV device fronted and backed by glass sheets, thereby forming a sealed PV module. The glass powder is locally heated to a temperature higher than the melting point, glass transition temperature, and/or softening point of the powder such that the powder softens and/or flows into a unified mass that bonds with the glass sheets to form a hermetic seal. 
         [0006]    Advantageously, embodiments of the present invention increase the active area of the PV module, and hence the power output for a fixed PV module size. Typical safety requirements, e.g., IEC or UL, mandate a minimum insulation distance from the current-carrying live parts inside a PV module to the outside edge of the module—i.e., the clearance and creepage distance. Such “edge delete” losses depend on various aspects such as the total system voltage or micro-environment (e.g., pollution degree and altitude), but are typically at the order of 8-15 mm. As conventional polymeric edge seals can fail over time, the insulation against electrical shock is compromised and, in the case of moisture ingress, current can track over the glass or polymer surface from the module circuit to its edge. In contrast, the glass-based seals in accordance with embodiments of the present invention are truly durable cemented joints retaining their superior electrical insulation properties over time. These seals allow the clearance—i.e., the edge delete—to be smaller, in turn proportionally increasing the power output from the PV module for the same total area. 
         [0007]    In addition, the polymeric encapsulant (filler sheet)—e.g., EVA, PVB, polyolefin, ionomers, TPU—conventionally utilized to bond glass sheets together may be eliminated in accordance with embodiments of the present invention. After the cost of the glass sheets, such filler sheets are typically the most expensive elements of thin-film PV modules. Thus, embodiments of the invention enable significant reductions in material cost. Furthermore, elimination of polymeric filler sheets and the conventional butyl rubber edge seals also enables PV modules in accordance with embodiments of the invention to obtain higher fire ratings. Moreover, elimination of such conventional polymer-based filler sheets and edge seals obviates the need for the slow and capital-intensive lamination step, resulting in additional manufacturing cost savings for PV modules in accordance with embodiments of the present invention. 
         [0008]    PV modules that are hermetically sealed in accordance with embodiments of the present invention exhibit advantageously enhanced durability in climates and/or ambient conditions that have high relative humidity, e.g., coastal or equatorial locations. In addition, since PV modules and devices sealed in accordance with embodiments of the invention do not rely on seals based on glass-polymer adhesion, they are less susceptible than to fatigue under conditions such as UV radiation, elevated temperature, and high humidity or moisture levels. Glass-polymer bonds also tend to be susceptible to failure due to mechanical stresses from, e.g., etch pinch during module lamination. 
         [0009]    In an aspect, embodiments of the invention feature a photovoltaic module that includes, consists essentially of, or consists of a first glass sheet, a photovoltaic device disposed on the first glass sheet, a second glass sheet disposed over and in contact with at least a portion of the photovoltaic device, and a layer of melted glass powder. The first glass sheet and the second glass sheet have a gap therebetween spanned, over only a portion of an area of the gap, by the photovoltaic device, and the layer of melted glass powder seals the gap between the first and second glass sheets at an edge region proximate an edge of at least one of the first or second glass sheets so as to hermetically seal the photovoltaic device. 
         [0010]    Embodiments of the invention may include one or more of the following in any of a variety of different combinations. The photovoltaic device may include, consist essentially of, or consist of an active region including, consisting essentially of, or consisting of one or more p-n or p-i-n junctions. The photovoltaic module may include (i) a first substrate layer disposed between the active region and the first glass sheet and/or (ii) a second substrate layer disposed between the active region and the second glass sheet. The first and/or second substrate layers may each include, consist essentially of, or consist of a metal foil and/or a polymer layer. A conductive bus ribbon may be electrically coupled to the photovoltaic device and may extend out from the first and second glass sheets in contact with the layer of melted glass powder. In a region where the conductive bus ribbon extends out from the first and second glass sheets, the conductive bus ribbon may be (i) disposed in contact with both the first and second glass sheets, (ii) disposed in contact with the first glass sheet and the layer of melted glass powder, but not with the second glass sheet, (iii) disposed in contact with the second glass sheet and the layer of melted glass powder, but not with the first glass sheet, or (iv) disposed in contact with the layer of melted glass powder, but not with the first or second glass sheets. The melted glass powder may include a colorant or other absorber utilized to, e.g., locally increase the absorption of particular wavelengths of light. 
         [0011]    The photovoltaic device may include, consist essentially of, or consist of multiple junctions, each of which is a p-n junction or a p-i-n junction. The photovoltaic device may be a thin-film photovoltaic device that includes, consists essentially of, or consists of amorphous silicon. The photovoltaic device may be a thin-film photovoltaic device that includes, consists essentially of, or consists of CdTe. The photovoltaic device may be a thin-film photovoltaic device that includes, consists essentially of, or consists of chalcopyrite (Cu(In,Ga)(S,Se) 2 ). The photovoltaic device may be a thin-film photovoltaic device that includes, consists essentially of, or consists of kesterite (Cu 2 (Zn,Fe)Sn(S,Se) 4 ). The photovoltaic device may include, consist essentially of, or consist of crystalline silicon and/or GaAs. The photovoltaic device may include, consist essentially of, or consist of solid-state dye-sensitized perovskite material (organic-inorganic hybrid). The composition of the melted glass powder may be substantially the same as the composition of the first and/or second glass sheets. The composition of the melted glass powder may be different from a composition of either of the first or second glass sheets (i.e., different from the compositions of both of the first and second glass sheets). 
         [0012]    In another aspect, embodiments of the invention feature a method of hermetically sealing a photovoltaic module. First, a structure is provided. The structure includes, consists essentially of, or consists of a first glass sheet, a photovoltaic device disposed on the first glass sheet, and a second glass sheet disposed over and in contact with at least a portion of the photovoltaic device, the first glass sheet and the second glass sheet defining a gap therebetween spanned, over only a portion of an area of the gap, by the photovoltaic device. A powder is disposed within the gap at an edge region proximate an edge of at least one of the first or second glass sheets. The powder includes, consists essentially of, or consists of glass. The powder is heated within the gap to seal the first and second glass sheets at the edge region with a layer of melted glass powder. 
         [0013]    Embodiments of the invention may include one or more of the following in any of a variety of different combinations. Heating the powder may include, consist essentially of, or consist of application of laser energy to the powder (e.g., through one or both of the first or second glass sheets). The photovoltaic device may include, consist essentially of, or consist of an active region including, consisting essentially of, or consisting of one or more p-n or p-i-n junctions. The photovoltaic module may include (i) a first substrate layer disposed between the active region and the first glass sheet and/or (ii) a second substrate layer disposed between the active region and the second glass sheet. The first and/or second substrate layers may each include, consist essentially of, or consist of a metal foil and/or a polymer layer. A conductive bus ribbon may be electrically coupled to the photovoltaic device and may extend out from the first and second glass sheets in contact with the layer of melted glass powder (i.e., the powder may be melted at least partially around the bus ribbon in the edge region). In a region where the conductive bus ribbon extends out from the first and second glass sheets, the conductive bus ribbon may be (i) disposed in contact with both the first and second glass sheets, (ii) disposed in contact with the first glass sheet and the layer of melted glass powder, but not with the second glass sheet, (iii) disposed in contact with the second glass sheet and the layer of melted glass powder, but not with the first glass sheet, or (iv) disposed in contact with the layer of melted glass powder, but not with the first or second glass sheets. The powder may include a colorant or other absorber utilized to, e.g., locally increase the absorption of particular wavelengths of light. 
         [0014]    The photovoltaic device may include, consist essentially of, or consist of multiple junctions, each of which is a p-n junction or a p-i-n junction. The photovoltaic device may be a thin-film photovoltaic device that includes, consists essentially of, or consists of amorphous silicon. The photovoltaic device may be a thin-film photovoltaic device that includes, consists essentially of, or consists of CdTe. The photovoltaic device may be a thin-film photovoltaic device that includes, consists essentially of, or consists of chalcopyrite (Cu(In,Ga)(S,Se) 2 ). The photovoltaic device may be a thin-film photovoltaic device that includes, consists essentially of, or consists of kesterite (Cu 2 (Zn,Fe)Sn(S,Se) 4 ). The photovoltaic device may include, consist essentially of, or consist of crystalline silicon and/or GaAs. The photovoltaic device may include, consist essentially of, or consist of solid-state dye-sensitized perovskite material (organic-inorganic hybrid). The composition of the melted glass powder may be substantially the same as the composition of the first and/or second glass sheets. The composition of the melted glass powder may be different from a composition of either of the first or second glass sheets (i.e., different from the compositions of both of the first and second glass sheets). 
         [0015]    These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and may exist in various combinations and permutations. As used herein, the terms “approximately” and “substantially” mean±10%, and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. Nonetheless, such other materials may be present, collectively or individually, in trace amounts. For example, a structure consisting essentially of glass will generally include only glass and only unintentional impurities (which may be metallic or non-metallic) that may be detectable via chemical analysis but do not contribute to function. For example, a powder or seal consisting essentially of glass typically does not incorporate organic fillers, binders, solvents, glass frit, frit material(s), glass solder, and/or melting-point reduction agents such as Pb. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0017]      FIG. 1  is a schematic cross-section of a portion of a photovoltaic module being sealed via application of thermal energy in accordance with various embodiments of the invention; 
           [0018]      FIG. 2  is a schematic plan view of a photovoltaic module in accordance with various embodiments of the invention; and 
           [0019]      FIGS. 3A-3D  are schematic cross-sections of edge portions of sealed photovoltaic modules in accordance with various embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates a portion of a PV module  100  being sealed at an edge region via application of thermal energy in accordance with embodiments of the present invention. As shown, the PV module  100  includes an active PV device  110 , one or more conductive bus ribbons  120 , a lower glass sheet  130 , and an upper glass sheet  140 . While preferred embodiments of the invention utilize thin-film PV modules  100  featuring PV devices  110  based on CIGS, a-Si, or CdTe, other embodiments of the invention utilize PV devices  110  based on crystalline Si (e.g., in wafer or ribbon form) or III-V semiconductors such as GaAs or InP (e.g., in wafer form) or solid-state dye-sensitized perovskite material (organic-inorganic hybrid incorporating a dye such as hybrid perovskite CH 3 NH 3 PbI 3  dye). The one or more bus ribbons  120  are electrically coupled to the PV device  110  and extend beyond the lower glass sheet  130  and upper glass sheet  140  (see  FIG. 2 ), thereby enabling external electrical contact to the PV device  110 . The bus ribbon(s) may include, consist essentially of, or consist of one or more highly electrically conductive metals, e.g., aluminum, copper, or a multilayer stack of one or more aluminum layers and one or more copper layers. In various embodiments, conventional alkali alkaline-earth silicate glasses (i.e., soda lime glass) are utilized as one or both of the lower glass sheet  130  and the upper glass sheet  140 , as such glasses are less expensive than, e.g., alkali-lead silicate glass, alkaline-earth aluminosilicate glass, and borosilicate glass, any of which might also be utilized. In an example, one or both of the lower glass sheet  130  and the upper glass sheet  140  have a composition that is approximately 71% SiO 2 , approximately 15% alkali (e.g., primarily Na 2 O, but may also include K 2 O), approximately 13-16% alkaline earths (e.g., CaO+MgO), approximately 0-2% Al 2 O 3 , and, in some embodiments, BaO. 
         [0021]    As shown in  FIG. 1 , a gap  150  between the glass sheets  130 ,  140  is partially or substantially filled with a glass powder  160  at or near the edge of at least one of the glass sheets  130 ,  140 , and the glass powder  160  is then melted via application of thermal energy  170 . The glass powder  160  may be dispensed within the gap  150  (or on at least one of the glass sheets  130 ,  140  in the vicinity of gap  150  prior to one or both of the glass sheets  130 ,  140  being applied to the PV device  110 ) as a bead. In various embodiments of the invention, the gap  150  between the glass sheets  130 ,  140  has a thickness (i.e., height) between approximately 50 μm and approximately 500 μm, for example, between approximately 50 μm and 100 μm. The glass powder  160  may have a grain size (or range of grain sizes) optimized to allow fast melting/fusing and to provide a geometrically stable powder track (e.g., width and height) during powder dispense. For example, the glass powder  160  may have a grain size between approximately 0.1 μm and approximately 10 μm, or even between approximately 0.1 μm and 1 μm. In preferred embodiments of the invention, the glass powder  160  does not require any preconditioning after it has been dispensed onto one or both of the glass sheets  130 ,  140 , i.e. the upper glass sheet  140  may be placed on top immediately after the glass powder  160  is dispensed, and the sealing step is conducted in one operation. 
         [0022]    The melted glass powder  160  fuses into a solid glass seal filling the gap  150  between the sheets  130 ,  140  and bonding to the sheets  130 ,  140 , thereby forming a hermetic seal that is much more durable than similar seals utilizing polymer-based fills. In addition, the glass powder  160  may consist entirely or essentially of glass, in contrast with glass-frit materials that incorporate organic fillers, binders, solvents, and/or melting-point reduction agents such as Pb. The glass powder may include colorants or other absorbers utilized to locally increase the absorption of particular wavelengths of light. However, in preferred embodiments, the glass powder  160  utilized to seal the gap  150  has the same composition as at least one of the glass sheets  130 ,  140 . The glass powder  160  may include, consist essentially of, or consist of a low-melting glass that fuses with glass sheets  130 ,  140  upon melting. The melting point of the glass powder  160  may be, for example, between approximately 200° C. and approximately 550° C., or even between approximately 200° C. and approximately 400° C. In some embodiments, the glass powder  160  includes, consists essentially of, or consists of a zinc-silicoborate glass and/or a binary or ternary mixture of thallium, arsenic and sulfur. 
         [0023]    In preferred embodiments of the invention, pressure  180  is applied to one or both of the glass sheets  130 ,  140  in order to facilitate seal formation when the glass powder  160  is heated. Moreover, various embodiments utilize laser energy as the thermal energy  170  to heat and melt the glass powder  160  during seal formation. The laser utilized to impart the thermal energy  170  may emit substantially red light. The laser energy  170  may be applied via one or multiple passes along the edges of glass sheets  130 ,  140  by a laser, depending upon how well the beam energy is coupled into the glass powder  160  and how best to minimize any thermally induced stress in the glass sheets  130 ,  140  along the seal. In some embodiments, the laser energy  170  is pulsed in order to prevent excess heating of the PV device  110  and/or other parts of module  100  away from the edge region being sealed. 
         [0024]    In other embodiments, other techniques for localized heating, e.g., inductive heating or application of a torch or other heat source, are used to partially or substantially completely melt the glass powder  160 . While the glass powder  160  is at least partially melted to form the hermetic edge seal, the PV device  110  within the module  100  is preferably not exposed to temperatures sufficiently elevated to damage or degrade the device (via, e.g., interdiffusion, melting, etc.). For example, in various embodiments of the present invention, the localized heating temperature does not exceed 400-500° C. for times of ≦1 minute, does not exceed 300-400° C. for times of ≦1-3 minutes, and/or does not exceed 200-300° C. for times of ≦3-10 minutes. 
         [0025]    As mentioned above, in order to enable electrical contact between the encapsulated PV device  110  and outside electronics and/or systems, one or more conductive bus ribbons  120  may be electrically coupled to the sealed PV device  110  and extend out of the sealed module  100  through the layer of melted glass powder  160 . An example is shown in the plan view of  FIG. 2 , in which upper glass sheet  140  is omitted for clarity. The PV device  110  itself includes or consists essentially of one or more p-n and/or p-i-n junctions (i.e., homojunctions and/or heterojunctions), and may be fabricated from a-Si, CdTe, or a chalcopyrite (Cu(In,Ga)(S,Se) 2 ) such as CIGS or a kesterite (Cu 2 (Zn,Fe)Sn(S,Se) 4 ) such as CZTS (copper zinc tin sulfide). Other embodiments of the invention utilize PV devices  110  based on crystalline Si (e.g., in wafer or ribbon form) or III-V semiconductors such as GaAs or InP (e.g., in wafer form) or solid-state dye-sensitized perovskite material (organic-inorganic hybrid). The junction(s) of PV device  110  may be in direct contact with one or both of the glass sheets  130 ,  140  (if, e.g., the glass sheets  130 ,  140  are utilized as a substrate or superstrate for the PV device  110 ), or the PV device  110  may incorporate a substrate layer (e.g., a foil of a metal or another conductor, or of a polymer such as polyimide) below and/or above the PV device junctions and in contact with one or both of the glass sheets  130 ,  140 . In embodiments in which a substrate layer is disposed above the PV device junctions, the “substrate” is understood to include “superstrate” configurations as they are known in the art. 
         [0026]    As shown in  FIGS. 3A-3D , the conductive bus ribbon  120  and glass-powder seal  160  may have any of several different configurations. For example, in the configuration depicted in  FIG. 3A , the conductive bus ribbon  120  is in contact with both the upper glass sheet  140  and the lower glass sheet  130 , and the glass powder  160  seals the remaining portion of the gap  150  between the glass sheets  130 ,  140  and contacts the bus ribbon  120 . In  FIGS. 3B and 3C , the bus ribbon  120  is in contact with only the lower glass sheet  130  ( FIG. 3B ) or upper glass sheet  140  ( FIG. 3C ), and the glass powder  160  seals the remaining portion of the gap  150  between the glass sheets  130 ,  140  and above or below the bus ribbon  120 . In  FIG. 3D , the bus ribbon  120  extends through the glass-powder seal  160  without contacting either of the glass sheets  130 ,  140 . Depending upon the compositions of the bus ribbon  120  and the glass sheets  130 ,  140 , the configurations of  FIGS. 3B-3D  may result in a metal-glass seal between the bus ribbon  120  and the glass powder  160  upon application of the localized thermal energy  170 . 
         [0027]    During the edge-seal formation, the surfaces of the glass sheets  130 ,  140  to be joined together may be treated (e.g., cleaned to remove bond-impeding contamination or have thin surface layers removed) prior to the application of force  180  and localized heating  170 . The force  180  is typically applied to the surface of at least one of the glass sheets  130 ,  140  until the glass powder  160  has melted, sealed the edge region, and then cooled to form a solid (or at least semi-solid) phase. After the localized heating and seal formation, any localized stress at the sealed edge region may be at least partially reduced via annealing of the sealed module  100  (or at least the sealed edge region) at a moderate temperature (e.g., at a temperature lower than the melting point and/or the softening point of the glass powder  160 ). 
         [0028]    The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.