Patent Application
20110315222
Embodiments described herein relate generally to photovoltaic devices and modules, and more specifically to flexible photovoltaic devices and modules comprising protective films, layers and coatings.
Copper indium diselenide (CuInSe2, or CIS) and its higher band gap variants, such as copper indium gallium diselenide (Cu(In,Ga)Se2, or CIGS), and any of these compounds with sulfur replacing some of the selenium represent a group of materials, referred to as copper indium selenide CIS based alloys, have desirable properties for use as the absorber layer in thin-film solar cells as used in photovoltaic modules. These layers are susceptible to damage from water and/or water vapor.
Photovoltaic (“PV”) modules used in residential structures and roofing materials for generating electricity often require additional protection from environmental damage, such as an ingress of water, that can reduce an active lifetime of the photovoltaic system. Additionally, these modules require protection from hail, rocks, dropped tools or other objects that may impact their surfaces. Thick encapsulating layers or glass have been used to increase the protection to the underlying devices. However, the weight of the thicker glass adds to the shipping cost of the module, and may also limit the placement options for the module. Thicker glass also requires more material which leads to increased cost.
Rigid or flexible sheets of glass may be used to support and/or provide protection to the underlying semiconductor layers. These sheets, however, may themselves be susceptible to cracking when impacted, thereby exposing the semiconductor layers to moisture and other environmental conditions that diminish the lifetime of the cell or completely destroy it. Also, certain impacts may cause cracks that do not extend to the underlying semiconductor layers initially, but may propagate over time, for example during thermal expansion and contraction cycles resulting from change of temperature during the day, or over several months and seasons.
Additionally, flexible glasses are susceptible to weakness from micro scratches produced during processing, and/or abrasion during weathering. These micro scratches and abrasions act as stress concentrators and/or crack initiation sites which may compromise resistance to impact and/or resistance to moisture barrier properties.
Furthermore, plural impacts over a narrow radius can exceed the tensile strength of the glass and cause breakage.
One embodiment provides a photovoltaic device, including at least one photovoltaic cell, a flexible transparent layer formed over the at least one photovoltaic cell, a first encapsulant layer formed over a first major surface of the flexible transparent layer facing the at least one photovoltaic cell, and a second encapsulant layer formed over a second major surface of the flexible transparent layer facing away from the at least one photovoltaic cell. In this embodiment, the second encapsulant layer comprises a shear thickening polymer.
Another embodiment provides a method of making a photovoltaic device, including the steps of providing at least one photovoltaic cell, and forming a flexible transparent layer over the at least one photovoltaic cell. In this embodiment, the flexible transparent layer has a first encapsulant layer over a first major surface facing the at least one photovoltaic cell and a second encapsulant layer over a second major surface facing away from the at least one photovoltaic cell, and the second encapsulant layer comprises a shear thickening polymer.
Another embodiment provides a method of using a photovoltaic device, including operating, transporting or installing the photovoltaic device such that the device experiences an impact on its surface. In this embodiment the photovoltaic device includes at least one photovoltaic cell, a flexible transparent layer formed over the at least one photovoltaic cell, a first encapsulant layer formed over a first major surface of the flexible transparent layer facing the at least one photovoltaic cell, and a second encapsulant layer formed over a second major surface of the flexible transparent layer facing away from the at least one photovoltaic cell, wherein a viscosity of the second encapsulant layer increases upon the impact on its surface. In this embodiment, the second encapsulant layer comprises a shear thickening polymer.
As used herein, the term “module” includes an assembly of at least two, preferably more than two photovoltaic cells, such as 3-10,000 cells, for example. The photovoltaic cells of the module can be photovoltaic cells of any type. Each of the photovoltaic cells of the module can be a CIS based alloy (e.g., CIGS) type photovoltaic cell described above. Preferably, the photovoltaic cells of the module are thin film photovoltaic cells. The thin film photovoltaic cells of the module can be located adjacent to each other such that an interconnect provides electrical connection between them. An exemplary interconnect is described in U.S. patent application Ser. No. 11/451,616 filed on Jun. 13, 2006 and incorporated herein by reference in its entirety.
As used herein, a “shear thickening material” includes polymer materials that exhibit dilatant behavior, which viscously flow at low rates of deformation but, at an elevated rate of deformation, undergoes a substantial change in viscosity, for example an increase in viscosity, with rate of change of deformation. In other words, upon impact in a given area of shear thickening material, the material's viscosity in the impact area increases by at least 50%. For example, the shear thickening polymers include silicone polymers, such as borated silicone polymers. Examples of shear thickening materials and their properties are disclosed in U.S. Pat. No. 7,381,460 which is incorporated herein by reference in its entirety.
The transparent protective barrier 100 is disposed over the photovoltaic cell 10 to provide environmental protection and impact protection to the cell. When the photovoltaic cell is part of a photovoltaic module, the protective barrier can be formed continuously over other photovoltaic cells in the module. The transparent protective barrier 100 preferably comprises a thin, coated flexible glass layer. In some cases, the protective barrier 100 can be a self-supporting, i.e., a free standing glass layer. The self-supporting layer can be in a form of a roll, ribbon, web, foil or a sheet. Any suitable glass material may be used for the glass layer, such as soda lime glass, borosilicate glass, low alkali soda lime glass, etc. The glass layer may be sufficiently thin, such as having a thickness of 50-500 μm, to provide flexibility to the glass layer (e.g., so that the glass layer may be rolled up into a roll).
The protective barrier 100 can include one or more transparent sublayers (not shown in
The photovoltaic device in
Shear Thickening Polymer.
The flexible glass layer 210 can have a first major surface 231 facing the at least one photovoltaic cell and a second major surface 223 facing away from the at least one photovoltaic cell (e.g., toward the Sun). Additionally, the photovoltaic cell 10 can comprise a flexible photovoltaic cell formed on a flexible substrate.
The protective barrier 200 may include one or more transparent sublayers, for example a first encapsulant layer 230, a second shear thickening encapsulant layer 222, and an optional weather barrier 224.
The first encapsulant layer 230 can be formed over the first major surface 231 of the flexible glass layer 210. The first encapsulant layer 230 can comprise a shear thickening polymer or a material other than a shear thickening polymer. The material other than a shear thickening polymer can have a modulus of greater than about 500 MPa at room temperature, between about 100 MPa and 500 MPa at room temperature, or less than about 100 MPa at room temperature, such as 2-50 MPa. For example, the first encapsulant layer 230 can comprise a polymer or glass layer having a modulus of less than 50 MPa at room temperature.
The second encapsulant layer 222 can be formed over the second major surface 223 of the flexible glass layer 210, and can comprise a shear thickening polymer, such as a thermoplastic or thermoset polymer. Shear thickening polymers include silicone polymers, such as borated silicone polymers, such as polyborodimethylsiloxane (PBDMS). In one embodiment, the entire layer 222 may consist essentially of only the shear thickening polymer. In another embodiment, the second encapsulant layer 222 comprises a blend comprising the shear thickening polymer and at least one non-shear thickening polymer material. For example, the shear thickening polymer of the blend may comprise PBDMS and the non-shear thickening polymer of the blend may comprise any suitable polymer used in PV module encapsulation, such as Z68 LLDPE encapsulant, SentryGlas® hard glass polymer (architectural safety glass interlayer made by DuPont), Surlyn® resin (thermoplastic random copolymer poly(ethylene-co-methacrylic acid) (EMAA) where incorporation of methacrylic acid is typically low (<15 mol. %), made by DuPont), Tectosil® silicone based polymer encapsulant for PV modules (made by Wacker Silicones), etc. If desired, the shear thickening polymer may be a reversible shear thickening polymer whose viscosity increases upon impact but then decreases again after the impact. Examples of such polymers include silicone sealants, such as dimethyl, methylhydrogen siloxane containing sealants, including Dow Corning 1700 series sealants.
The dilatant material experiences a substantial increase in viscosity proportional to the rate of deformation during impact. In a non-limiting example the second encapsulant layer 222 is capable of experiencing a change in modulus from below 500 MPa to above 750 MPa at room temperature upon impact having a force of at least 3 Joules on its surface, and/or at least a 50% increase in viscosity at room temperature upon the impact on its surface. In other words, the second encapsulant layer 222 experiences a change from a relatively flexible state to a relatively rigid state upon impact on its surface. This behavior spreads the force of the impact as well as prevents or reduces deformation of the glass layer 210, the PV cells and/or other module components.
The optional weather barrier 224 can be a fluorinated polymer weather barrier (e.g., water and/or oxygen barrier) formed over the second encapsulant layer 222. Examples of the weather barrier 224 comprise weatherable polymers, such as ETFE, FEP, CTFE, etc., or one or more layers of other organic or inorganic, flexible barrier materials formed by atomic layer deposition, sol-gel deposition, etc.
Optionally, the flexible transparent glass layer 210 has one or more additional inorganic, organic, or organic-inorganic hybrid protective layers over the surface 223 of the glass layer that faces the Sun either above or below the shear thickening polymer containing encapsulating layer 222. The optional protective layer(s) may provide one or more of the following advantages: they may fill any existing microcracks and/or prevent formation of new ones, they may prevent water contact and interaction with the glass layer surface or with any defects on the glass layer surface, and/or they may decrease the impulse of impacts and/or increase the impact area when an object (e.g., hail, rocks, tree branches, dropped tools, etc.) impacts the barrier 100.
A method of using the photovoltaic device 1000 shown in
It is to be understood that the present invention is not limited to the embodiments and the examples described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims. For example, as is apparent from the claims and specification, not all method steps need be performed in the exact order illustrated or claimed, but rather in any order that allows the proper formation of the solar cells described herein.
