WO2011112701A1

WO2011112701A1 - Photovoltaic module containing buffer layer

Info

Publication number: WO2011112701A1
Application number: PCT/US2011/027722
Authority: WO
Inventors: Kethinni G. Chittibabu; Richard H. Estes; Russell Gaudiana; Melissa A. Kreger; Eitan Zeira
Original assignee: Konarka Technologies, Inc.
Priority date: 2010-03-09
Filing date: 2011-03-09
Publication date: 2011-09-15
Also published as: JP2013522879A

Abstract

Photovoltaic modules containing a buffer layer, as well as related photovoltaic cells, articles, systems, and methods, are disclosed.

Description

Photovoltaic Module Containing Buffer Layer

CROSS-REFERENCE TO RELATED APPLICATION

Under 35 U.S.C. § 119, this application claims priority to U.S. Provisional Patent Application Serial Number 61/312,015, filed March 9, 2010, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to photovoltaic modules containing a buffer layer, as well as related photovoltaic cells, articles, systems, and methods.

BACKGROUND

Photovoltaic cells are commonly used to transfer energy in the form of light into energy in the form of electricity. A typical photovoltaic cell includes a photoactive material disposed between two electrodes. Generally, light passes through one or both of the electrodes to interact with the photoactive material, thereby generating charge carriers (i.e., electrons and holes).

SUMMARY

This disclosure is based on the unexpected discovery that including a buffer layer containing one or more of certain polymers between a barrier layer and an electrode in a photovoltaic cell can reduce mechanical stress on the electrode and prevent delamination of the barrier layer from the electrode, which results in significant decrease in the performance of the photovoltaic cell. The buffer layer can also serve as a barrier to prevent diffusion of any adhesive between the barrier layer and the electrode, as well as oxygen or moisture in the environment, into the photovoltaic cell through the electrode. As a result, a photovoltaic module containing such a buffer layer can possess improved long term stability and increased useable life.

In one aspect, this disclosure features a photovoltaic module that includes a first photovoltaic cell containing first and second electrodes, and a photoactive layer between the first and second electrodes; a barrier layer supported by the first photovoltaic cell; an adhesion layer containing an adhesive; and a buffer layer containing a polymer selected from the group consisting of a polyurethane, a polyolefin, a polyvinyl butyral, an ionomeric polymer, or a copolymer thereof. The first electrode is between the barrier layer and the photoactive layer. The adhesion layer is between the first electrode and the barrier layer. The buffer layer is between the first electrode and the adhesion layer.

In another aspect, this disclosure features a photovoltaic module that includes a first photovoltaic cell containing first and second electrodes, and a photoactive layer between the first and second electrodes; a barrier layer supported by the first photovoltaic cell; and a buffer layer containing a polymer that includes a polyolefin grafted with an anhydride, an acid, or an acrylate. The first electrode is between the barrier layer and the photoactive layer. The buffer layer is between the first electrode and the barrier layer. The module does not include an additional adhesive layer between the buffer layer and the barrier layer.

Embodiments can include one or more of the following optional features.

The polymer can include a polyolefin grafted with an anhydride, an acid, or an acrylate, a polyethylene wax, a poly(ethylene-co-maleic anhydride), a poly(ethylene-co- methacrylic acid) or a salt thereof, a polyisobutylene, or a polyurethane made from a polyol (e.g., a polyether polyol, a polyester polyol, a neopentyl polyol, and a

cyclopentane polyol) and a diisocyanate (e.g., a toluene diisocyanate).

The polymer can have a glass transition temperature of at least about 80°C and/or at most about 120°C.

The polymer can be from about 70% by weight to about 100% by weight of the buffer layer.

The buffer layer can further include a filler, such as a nanoclay. The nanoclay can include montmorillonite, kaolinite, llite, or chlorite. In some embodiments, the filler can be from about 0%> by weight to about 30%> by weight of the buffer layer.

The buffer layer can cover substantially the entire area between the first electrode and the adhesion layer or the entire area between the first electrode and the barrier layer in the absence of the adhesion layer. The module can include a second photovoltaic cell separated from the first photovoltaic cell by an interconnection area for electrically connecting the first and second photovoltaic cells. In such embodiments, the buffer layer can cover the interconnection area.

The module can include a plurality of photovoltaic cells, each having first and second electrodes, and a photoactive layer between the first and second electrodes. In such embodiments, the buffer layer can be between the first electrode in each photovoltaic cell and the adhesion layer, and the buffer can cover substantially the entire area between the first electrode in each photovoltaic cell and the adhesion layer.

The buffer layer and the adhesion layer or the buffer layer and the barrier layer can have a first bonding strength, and the buffer layer and the first electrode can have a second bonding strength larger than the first bonding strength.

The buffer layer can have a moisture vapor transmission rate of at most about 0.01 g/m²/day.

The buffer layer can have a thickness of from about 25 microns to about 250 microns.

The adhesive can include an epoxy resin.

The barrier layer can include at least two (e.g., at least three) polymer layers and at least two ceramic layers between the at least two polymer layers.

The barrier layer can include a metal foil.

The polyolefm can include a homopolymer or copolymer.

The polyolefm can include a polyethylene, a polypropylene, a poly(ethylene-co- vinyl acetate), or a poly(ethylene-co-acrylate).

The buffer layer can include a low-density polyethylene grafted with an anhydride, a linear low-density polyethylene grafted with an anhydride, a high density polyethylene grafted with an anhydride, a polypropylene grafted with an anhydride, a poly(ethylene-co-acrylate) grated with an anhydride or an acid, and a poly(ethylene-co- vinyl acetate) grafted with an anhydride, an acid, or an acrylate.

Other features and advantages of the disclosure will be apparent from the description, drawings and from the claims. DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of a photovoltaic module containing a photovoltaic cell and a buffer layer.

FIG. 2 is a cross-sectional view of an embodiment of a photovoltaic module containing a plurality of photovoltaic cells and a buffer layer.

FIG. 3 is a cross-sectional view of an embodiment of a photovoltaic module containing a plurality of photovoltaic cells and a buffer layer in which the buffer layer covers the interconnection areas between the photovoltaic cells.

FIG. 4 is a graph showing serial resistance change of various photovoltaic modules during a damp heat ageing process.

FIG. 5 is a graph showing efficiency change of various photovoltaic modules during a damp heat ageing process.

FIG. 6 is a graph showing fill factor change of various photovoltaic modules during a damp heat ageing process.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows a cross-sectional view of a photovoltaic module 100 that includes a photovoltaic cell 101, a buffer layer 170, an optional adhesion layer 180, and a barrier layer 190. Photovoltaic cell 101 includes a substrate 110, an electrode 120, an optional hole blocking layer 130, a photoactive layer 140 (e.g., containing an electron acceptor material and an electron donor material), an optional hole carrier layer 150, and an electrode 160. As shown in FIG. 1, buffer layer 170 covers substantially the entire area between electrode 160 and adhesion layer 180 or between electrode 160 and barrier layer 190 (in the absence of adhesion layer 180).

In general, buffer layer 170 is formed from one or more polymers. Exemplary polymers suitable for use in buffer layer 170 include polyurethanes, polyolefins, polyvinyl butyral, ionomeric polymers, or copolymers thereof.

Exemplary polyurethanes include those made from a diisocyanate (e.g., a toluene diisocyanate) and a polyol (a polyether polyol, a polyester polyol, a neopentyl polyol, and a cyclopentane polyol). Commercial examples of suitable polyurethanes include

L020910 available from Epoxies, Etc. (Cranston, RI) and D9940-0401 available from Epic Resins (Palmyra, WI).

As used here, the term ''polyolefm" refers to a homopolymer or a copolymer made from at least a linear or branched, cyclic or acyclic olefin monomer. Examples of polyolefm homopolymers include polyethylene, polypropylene, polybutene, polypentene, and polymethylpentene. A polyolefm copolymer can be formed from an olefin monomer and one or more comonomers other than an olefin. Exemplary comonomers that can be used to make polyolefm copolymers include vinyl acetate or acrylates (e.g., alkyl acrylate such as methyl acrylate, ethyl acrylate, or butyl acrylate, or alkyl methacrylate such as methyl methacrylate, ethyl methacrylate, or butyl methacrylate). Exemplary polyolefm copolymers include poly(ethylene-co-vinyl acetate)s and poly(ethylene-co-acrylate)s. In some embodiments, exemplary polyethylene homopolymers or copolymers include low- density polyethylene (e.g., having a density from 0.910 to 0.925 g/cm2), linear low- density polyethylene (e.g., having a density from 0.910 to 0.935 g/cm2), and high-density polyethylene (e.g., having a density from 0.935 to 0.970 g/cm2). High-density polyethylene can be produced by copolymerizing ethylene with one or more C4 to C20 a- olefin comonomers. Examples of suitable a-olefm comonomers include 1-butene, 1- pentene, 4-methyl-l-pentene, 1-hexene, 1-octene, 1-decene, and combinations thereof. The high-density polyethylene can include up to 20 mole percent of the above-mentioned a-olefm comonomers.

In some embodiments, polyolefms suitable for buffer layer 170 can include a polyolefm grafted with an anhydride, an acid, or an acrylate, a polyethylene wax, a poly(ethylene-co-maleic anhydride), a poly(ethylene-co-methacrylic acid) or a salt thereof, and a polyisobutylene. The salt of a poly(ethylene-co-methacrylic acid) can include any suitable cations, such as Li⁺, Na⁺, K⁺, Mg²⁺, and Zn²⁺. Commercial examples of poly(ethylene-co-methacrylic acid) or its salt include the SURLYN series of polymers available from E.I. du Pont de Nemours and Company, Inc. (Wilmington, DE).

In some embodiments, polyolefms include polyolefm graft copolymers. For example, polyolefm graft copolymers can include polyolefms grafted with an anhydride, an acid, or an acrylate. Examples of such graft copolymers include a low-density polyethylene grafted with an anhydride, a linear low-density polyethylene grafted with an anhydride, a high density polyethylene grafted with an anhydride, a polypropylene grafted with an anhydride, a poly(ethylene-co-acrylate) grated with an anhydride or an acid, and a poly(ethylene-co-vinyl acetate) grafted with an anhydride, an acid, or an acrylate. Commercial examples of the first polymer include the BYNEL series of polymers available from E.I. du Pont de Nemours and Company, Inc.

Ionomeric polymers that can be used in buffer layer 170 including polymers containing an acid moiety (e.g., a carboxylic acid moiety, a sulfonic acid moiety, or a phosphoric acid moiety). The acidic groups in the ionomeric polymers can be partially or fully converted to salts that include suitable cations, such as Li⁺, Na⁺, K⁺, Mg²⁺, and Zn²⁺. Examples of suitable ionomeric polymers include polymers containing an ethylene copolymer moiety and an acid copolymer moiety. The ethylene copolymer moiety can be formed by copolymerizing ethylene and a monomer selected from the group consisting of vinyl acetate, alkyl acrylate, and alkyl methacrylate. The acid copolymer moiety can be formed by copolymerizing ethylene and a monomer selected from the group consisting of acrylic acid and methacrylic acid. Other examples include ionomeric polyvinyl butyral, such as those described in U.S. Patent No. 4,968,745.

Without wishing to be bound by theory, the inventors have unexpectedly discovered that using one or more of the polymers described above as buffer layer 170 between electrode 160 and adhesion layer 180 or electrode 160 and barrier layer 190 can reduce mechanical stress on electrode 160 and prevent delamination of barrier layer 190 from the electrode 160, which results in significant decrease in the performance of photovoltaic cell 101. The polymers described above can also serve as a barrier to prevent diffusion of the adhesive in adhesion layer 180, as well as oxygen or moisture in the environment, into photovoltaic cell 101 through electrode 160. As a result, photovoltaic module 101 possesses improved long term stability and increased useable life.

In some embodiments, the polymer suitable for use in buffer layer 170 has a sufficiently low glass transition temperature (T_g) such that buffer layer 170 is mechanically softer than barrier layer 190 and sufficiently high T_g such that it possesses a relatively high bonding strength with electrode 160. For example, the polymer can have a glass transition temperature of at most about 120°C (e.g., at most about 1 15°C, at most about 1 10°C, at most about 105°C, or at most about 100°C) and/or at least about 80°C (e.g., at least about 85°C, at least about 90°C, at least about 95°C, or at least about 100°C).

In some embodiments, buffer layer 170 can include at least about 70% (e.g., at least about 75%, at least about 80%>, at least about 85%, or at least about 90%) by weight and/or at most about 100% (e.g., at most about 99%, at most about 95%, at most about 90%), or at most about 85%) by weight of one or more of the polymers described above.

In some embodiments, buffer layer 170 can include a filler, e.g., an inorganic filler such as a nanoclay. A nanoclay can include a clay from the smectite family.

Smectite nanoclays have a unique morphology, featuring one dimension (e.g., thickness) in the nanometer range (e.g., from about 1 nm to about 100 nm). An example of a smectite nanoclay is montmorillonite. Other examples of nanoclays include kaolinite, llite, or chlorite. All of the nanoclays described herein can be obtained from commercial sources (e.g., Nanocor, Inc.). Without wishing to be bound by theory, it is believed that including a filler (such as those described above) in buffer layer 170 can prevent diffusion of oxygen or moisture in the environment into photovoltaic cell 101.

In some embodiments, buffer layer 170 can include at least about 1% (e.g., at least about 5%, at least about 10%, at least about 15%, or at least about 20%) by weight and/or at most about 30% (e.g., at most about 25%, at most about 20%, at most about 15%), or at most about 10%) by weight of the filler described above.

In some embodiments, the filler (e.g., a nanoclay) is baked at a sufficiently low temperature prior to being added to buffer layer 170. For example, the filler can be baked at a temperature of at most about 70°C (e.g., at most about 65°C, at most about 60°C, at most about 55°C, or at most about 50°C) and/or at least about 30°C (e.g., at least about 35°C, at least about 40°C, at least about 45°C, or at least about 50°C). Without wishing to be bound by theory, it is believed that baking the filler at the temperature described above can reduce or eliminate agglomerates of nanoclay particles, which can increase haze and reduce light transmission into photovoltaic cell 101. In some embodiments, buffer layer 170 and adhesion layer 180 or buffer layer 170 and barrier layer 190 (in the absence of adhesion layer 180) can have a first bonding strength, and buffer layer 170 and electrode 160 can have a second bonding strength larger than the first bonding strength. The bond strength mentioned herein is measured by a T-Peel test according to ASTM D 1876-01. Without wishing to be bound by theory, it is believed that such a buffer layer can reduce or prevent delamination of barrier layer 190 from electrode 160.

In general, buffer layer 170 has a sufficiently low moisture vapor transmission rate (MVTR) to prevent diffusion of moisture or oxygen in the environment into photovoltaic cell 101. The MVTR mentioned herein is measured according to ASTM E- 96. For example, buffer layer 170 can have a MVTR at most about 0.01 g/m²/day (e.g., at most about 0.005 g/m²/day, at most about 0.001 g/m²/day, at most about 0.0005 g/m²/day, or at most about 0.0001 g/m²/day).

In some embodiments, buffer layer 170 can have a thickness of at most about 250 microns (e.g., at most about 225 microns, at most about 200 microns, at most about 175 microns, or at most about 150 microns) and/or at least about 25 microns (e.g., at least about 50 microns, at least about 75 microns, at least about 100 microns, or at least about 125 microns).

In general, buffer layer 170 can be attached to electrode 160 by the methods known in the art or the method described herein. For example, buffer layer 170 can be a pre-formed film including one or more of the polymers described above and optionally a filler (e.g., a nanoclay). The film can be formed by a known method, e.g., film extrusion. After the film is formed, it can be pressed against electrode 160 such that it is adhered to electrode 160.

In some embodiments, buffer layer 170 can be formed on electrode 160 by an extrusion coating process. For example, when buffer layer 170 includes a SURLYN polymer, the SURLYN polymer can be melted and introduced into an extruder. The molten polymer can then be discharged from the extruder and coated onto electrode 160. In such embodiments, a filler (e.g., a nanoclay) can optionally be mixed with the molten polymer in the extruder to form a mixture, which can then be discharged from the extruder and coated onto electrode 160.

In some embodiments, buffer layer 170 can be formed on electrode 160 by a coating process other than an extrusion coating process, such as solution coating, ink jet printing, spin coating, dip coating, knife coating, bar coating, spray coating, roller coating, slot coating, gravure coating, flexographic printing, or screen printing. For example, buffer layer 170 can be formed by (1) mixing a polymer (e.g., a polyurethane) and optionally a filler (e.g., a nanoclay) with a solvent (e.g., water or an organic solvent) to form a solution or dispersion, (2) coating the solution or dispersion onto electrode 160, and (3) drying the coated solution or dispersion.

In some embodiments, buffer layer 170 can be formed using one of the coating processes described in the preceding paragraph without using a solvent. For example, when buffer layer 170 includes a polyurethane, it can be formed by (1) mixing a diisocyanate monomer with a polyol monomer or prepolymer in the absence of a solvent to form a mixture, (2) coating the mixture onto electrode 160, and (3) polymerizing the monomers in the mixture (e.g., by heating) to form buffer layer 170. In certain embodiments, a solvent can be added during the mixing step to facilitate the mixing of the diisocyanate monomer and the polyol monomer or prepolymer.

In some embodiment, a photovoltaic module described herein includes a plurality of photovoltaic cells and a buffer layer cover substantially the entire area between an electrode in each photovoltaic cell and the adhesion layer or the barrier layer. FIG. 2 shows a cross-sectional view of a photovoltaic module 200 that includes a plurality of photovoltaic cells 201, a buffer layer 270, an optional adhesion layer 280, and a barrier layer 290. Each photovoltaic cell 201 includes an electrode 220, an optional hole blocking layer 230, a photoactive layer 240, an optional hole carrier layer 250, and an electrode 260. Photovoltaic cells 201 share a common substrate 210 and are separated from each other by interconnection areas 202, which can be used for electrical connection between the photovoltaic cells. As shown in FIG. 2, buffer layer 270 covers substantially the entire area between electrode 260 in each photovoltaic cell 201 and adhesion layer 280 or between electrode 260 and barrier layer 290 (in the absence of adhesion layer 280). In addition, buffer layer 270 covers interconnection areas 202.

In some embodiment, a photovoltaic module described herein includes a plurality of photovoltaic cell and a buffer layer that covers substantially the interconnection areas between the photovoltaic cells only. FIG. 3 shows a cross-sectional view of a

photovoltaic module 300 that includes a plurality of photovoltaic cells 301, a buffer layer 370, an optional adhesion layer 380, and a barrier layer 390. Each photovoltaic cell 301 includes an electrode 320, an optional hole blocking layer 330, a photoactive layer 340, an optional hole carrier layer 350, and an electrode 360. Photovoltaic cells 301 share a common substrate 310 and are separated from each other by interconnection areas 302, which can be used for electrical connection between the photovoltaic cells. As shown in FIG. 3, buffer layer 370 covers substantially interconnection areas 302 only.

In general, optional adhesion layer 180 includes an adhesive to attach barrier layer 190 to photovoltaic cell 101. An exemplary adhesive is an epoxy resin. Adhesion layer 180 can be made by methods known in the art. In some embodiments, adhesion layer 180 can be formed by applying a layer containing an epoxide monomer and a comonomer (e.g., a polyamine) onto a surface of barrier layer 190 and then cure the monomers by heat. In some embodiments, adhesion layer 180 can be formed by applying a layer containing an epoxide monomer and an initiator (e.g., an acid or a base) onto a surface of barrier layer 190 and then cure the monomers by UV through a ring opening

polymerization reaction. The article thus formed can then be attached to buffer layer 170 to form photovoltaic module 100.

In some embodiment, adhesion layer 180 can be omitted from photovoltaic module 100. For example, when buffer layer 170 is formed from a polyolefm grafted with an anhydride, an acid, or an acrylate (such as those described above), adhesion layer 180 can be omitted as such a polyolefm can form strong adhesion bonding to both barrier layer 190 and electrode 160. In some embodiments, the polyolefm can be formed on buffer layer 170 by melting the polyolefm and extruding the hot melt onto the buffer layer. In general, barrier layer 190 is made from materials that can sufficiently prevent diffusion of oxygen or moisture in the environment into photovoltaic cell 101. In some embodiments, barrier layer 190 has a multilayer structure and includes a composite containing at least two (e.g., at least three) polymer layers (e.g., each containing a polyester such as a polyethylene terephthalate or a polyethylene naphthalate) and at least two ceramic layers (e.g., each containing silica or alumina) between the at least two polymer layers. In such embodiments, barrier layer 190 can have three polymer layers and two ceramic layers in the following order: a polymer layer, a ceramic layer, a polymer layer, a ceramic layer, and a polymer layer.

In some embodiments, barrier layer 190 is made from a transparent material (e.g., the multilayer structure described in the preceding paragraph). As referred to herein, a transparent material is a material which, at the thickness used in a photovoltaic cell 101, transmits at least about 60% (e.g., at least about 70%, at least about 75%, at least about 80%), at least about 85%) of incident light at a wavelength or a range of wavelengths used during operation of the photovoltaic cell.

In certain embodiments, barrier layer 190 can include a metal foil. Without wishing to be bound by theory, it is believed that a metal foil is generally a better barrier material than a composite material formed from polymers and ceramics. On the other hand, as a metal foil is generally less transparent than a composite material formed from polymers and ceramics, it is typically used only on one side of photovoltaic module 100 (e.g., the bottom side of photovoltaic module 100 which typically cannot be reached by incident light).

Turning to photovoltaic cell 101, substrate 110 can generally be formed of a transparent material. Exemplary materials from which substrate 110 can be formed include polyethylene terephthalates, polyimides, polyethylene naphthalates, polymeric hydrocarbons, cellulosic polymers, polycarbonates, polyamides, polyethers, and polyether ketones. In certain embodiments, the polymer can be a fluorinated polymer. In some embodiments, combinations of polymeric materials are used. In certain

embodiments, different regions of substrate 110 can be formed of different materials. In some embodiments, substrate can be formed of a non-transparent material, such as a metal foil.

In general, substrate 110 can be flexible, semi-rigid or rigid (e.g., glass). In some embodiments, substrate 110 has a flexural modulus of less than about 5,000 megaPascals (e.g., less than about 1,000 megaPascals or less than about 500 megaPascals). In certain embodiments, different regions of substrate 110 can be flexible, semi-rigid, or inflexible (e.g., one or more regions flexible and one or more different regions semi-rigid, one or more regions flexible and one or more different regions inflexible).

Typically, substrate 110 is at least about one micron (e.g., at least about five microns or at least about 10 microns) thick and/or at most about 1,000 microns (e.g., at most about 500 microns thick, at most about 300 microns thick, at most about 200 microns thick, at most about 100 microns, or at most about 50 microns) thick.

Generally, substrate 110 can be colored or non-colored. In some embodiments, one or more portions of substrate 110 is/are colored while one or more different portions of substrate 110 is/are non-colored.

Substrate 110 can have one planar surface (e.g., the surface on which light impinges), two planar surfaces (e.g., the surface on which light impinges and the opposite surface), or no planar surfaces. A non-planar surface of substrate 110 can, for example, be curved or stepped. In some embodiments, a non-planar surface of substrate 110 is patterned (e.g., having patterned steps to form a Fresnel lens, a lenticular lens or a lenticular prism).

In some embodiment, substrate 110 can be the same as barrier layer 190, which can be attached to electrode 120 by an adhesive (such as that used in adhesion layer 180). In such embodiment, the material used in buffer layer 170 can also be included between electrode 120 and the adhesive. In some embodiments, the adhesive can be omitted when buffer layer includes a material (e.g., a polyolefm grafted with an anhydride, an acid, or an acrylate) that can form strong adhesion bonding with both substrate 110 and electrode 120.

Electrode 120 is generally formed of an electrically conductive material.

Exemplary electrically conductive materials include electrically conductive metals, electrically conductive alloys, electrically conductive polymers, and electrically conductive metal oxides. Exemplary electrically conductive metals include gold, silver, copper, aluminum, nickel, palladium, platinum, and titanium. Exemplary electrically conductive alloys include stainless steel (e.g., 332 stainless steel, 316 stainless steel), alloys of gold, alloys of silver, alloys of copper, alloys of aluminum, alloys of nickel, alloys of palladium, alloys of platinum, and alloys of titanium. Exemplary electrically conducting polymers include polythiophenes (e.g., doped poly(3,4- ethylenedioxythiophene) (doped PEDOT)), polyanilines (e.g., doped polyanilines), polypyrroles (e.g., doped polypyrroles). Exemplary electrically conducting metal oxides include indium tin oxide, fluorinated tin oxide, tin oxide and zinc oxide. In some embodiments, combinations of electrically conductive materials are used.

In some embodiments, electrode 120 can include a mesh electrode. Examples of mesh electrodes are described in co-pending U.S. Patent Application Publication Nos. 20040187911 and 20060090791.

In some embodiments, a combination of the materials described above can be used to form electrode 120.

Optionally, photovoltaic cell 101 can include a hole blocking layer 130. The hole blocking layer is generally formed of a material that, at the thickness used in photovoltaic cell 101, transports electrons to electrode 120 and substantially blocks the transport of holes to electrode 120. Examples of materials from which the hole blocking layer can be formed include LiF, metal oxides (e.g., zinc oxide, titanium oxide), and amines (e.g., primary, secondary, or tertiary amines). Examples of amines suitable for use in a hole blocking layer have been described, for example, in co-pending U.S. Application Publication No. 2008-0264488.

Without wishing to be bound by theory, it is believed that when photovoltaic cell 101 includes a hole blocking layer made of amines, the hole blocking layer can facilitate the formation of ohmic contact between photoactive layer 140 and electrode 120 without being exposed to UV light, thereby reducing damage to photovoltaic cell 101 resulted from UV exposure. Typically, hole blocking layer 130 is at least about 0.02 micron (e.g., at least about 0.03 micron, at least about 0.04 micron, or at least about 0.05 micron) thick and/or at most about 0.5 micron (e.g., at most about 0.4 micron, at most about 0.3 micron, at most about 0.2 micron, or at most about 0.1 micron) thick.

Photoactive layer 140 is generally formed of an electron acceptor material (e.g., an organic electron acceptor material) and an electron donor material (e.g., an organic electron donor material).

In some embodiments, the electron donor material can include a photoactive polymer (e.g., a conjugated polymer) containing one or more of the following monomer repeat units: a benzodithiophene moiety, a cyclopentadithiazole moiety, a

benzothiadiazole moiety, a thiadiazoloquinoxaline moiety, a benzoisothiazole moiety, a benzothiazole moiety, a dithienopyrrole moiety, a dibenzosilole moiety, a

thienothiophene moiety, a carbazole moiety, a dithienothiophene moiety, a

tetrahydroisoindole moiety, a fluorene moiety, a silole moiety, a cyclopentadithiophene moiety, a thiazole moiety, a selenophene moiety, a thiazolothiazole moiety, a

naphthothiadiazole moiety, a thienopyrazine moiety, a silacyclopentadithiophene moiety, a thiophene moiety, an oxazole moiety, an imidazole moiety, a pyrimidine moiety, a benzoxazole moiety, a benzimidazole moiety, a quinoxaline moiety, a pyridopyrazine moiety, a pyrazinopyridazine moiety, a pyrazinoquinoxaline moiety, a

thiadiazolopyridine moiety, a thiadiazolopyridazine moiety, a benzooxadiazole moiety, an oxadiazolopyridine moiety, an oxadiazolopyridazine moiety, a benzoselenadiazole moiety, a benzobisoxazole moiety, a thienothiadiazole moiety, a thienopyrroledione moiety, or a tetrazine moiety. The moieties mentioned above can be substituted by one or more halo (e.g., F, CI, Br, or I), C1-C24 alkyl, C1-C24 alkoxy, aryl, heteroaryl, C3-C24 cycloalkyl, C₃-C₂₄ heterocycloalkyl, COR, COOR, CO-N(RR'), CN, or S0₃R, in which each of R and R', independently, is H, C1-C24 alkyl, aryl, heteroaryl, C₃-C24 cycloalkyl, or C₃-C24 heterocycloalkyl. The photoactive polymer can be a homopolymer or a copolymer containing two or more of the above monomer repeat units.

In some embodiments, electron acceptor materials of photoactive layer 140 can include fullerenes. In some embodiments, photoactive layer 140 can include one or more unsubstituted fullerenes and/or one or more substituted fullerenes. Examples of unsubstituted fullerenes include C₆o, C₇₀, C₇₆, C₇₈, C₈₂, C₈₄, and C₉₂. Examples of substituted fullerenes include fullerene substituted with phenyl-butyric acid methyl esters (PCBMs, such as C61-PCBM and C71-PCBM) or fullerenes substituted with Ci-C₂₀ alkoxy optionally further substituted with Ci-C₂o alkoxy and/or halo (e.g.,

(OCH₂CH₂)20CH3 or OCH₂CF₂OCF₂CF₂OCF₃). Without wishing to be bound by theory, it is believed that fullerenes substituted with long-chain alkoxy groups (e.g., oligomeric ethylene oxides) or fluorinated alkoxy groups have improved solubility in organic solvents and can form a photoactive layer with improved morphology. Other examples of fullerenes have been described in, e.g., commonly-owned co-pending U.S.

Application Publication No. 2005-0279399. In some embodiments, the electron acceptor material can include one or more of the photoactive polymers described in the preceding paragraph. In certain embodiments, a combination of electron acceptor materials (e.g., a fullerene and a photoactive polymer described above) can be used in photoactive layer 140.

Other examples of electron donor materials and electron accepting materials have been described in, e.g., commonly-owned U.S. Patents Nos. 7,772,485, and 7,781,673, U.S. Application Publication Nos. 2007-0017571, 2007-0020526, 2008-0087324, 2008- 0121281, and 2010-0032018, and PCT Application No PCT/US2011/020227.

In general, the thickness of photoactive layer 140 should be sufficiently large to absorb incident light to generate electrons and holes, and sufficiently small to allow transport of the generated electrons or holes to a neighboring layer. Typically, the thickness of photoactive layer 140 is at least about 50 microns (e.g., at least about 75 microns, at least about 100 microns, at least about 125 microns, at least about 150 microns, or at least about 200 microns) and/or at most about 300 microns (e.g., at most about 250 microns, at most about 200 microns, or at most about 150 micron).

Optionally, photovoltaic cell 101 can include a hole carrier layer 150. Hole carrier layer 150 is generally formed of a material that, at the thickness used in photovoltaic cell 101, transports holes to electrode 160 and substantially blocks the transport of electrons to electrode 160. Examples of materials from which layer 150 can be formed include polythiophenes (e.g., PEDOT), polyanilines, polycarbazoles, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes,

polythienylenevinylenes, polyisothianaphthanenes, and copolymers thereof. In some embodiments, hole carrier layer 150 can include a dopant used in combination with one of the just-mentioned materials. Examples of dopants include poly(styrene-sulfonate)s, polymeric sulfonic acids, or fluorinated polymers (e.g., fluorinated ion exchange polymers).

In some embodiments, the materials that can be used to form hole carrier layer 150 include metal oxides, such as titanium oxides, zinc oxides, tungsten oxides, molybdenum oxides, copper oxides, strontium copper oxides, or strontium titanium oxides. The metal oxides can be either undoped or doped with a dopant. Examples of dopants for metal oxides include salts or acids of fluoride, chloride, bromide, and iodide.

In some embodiments, the materials that can be used to form hole carrier layer 150 include carbon allotropes (e.g., carbon nanotubes). The carbon allotropes can be embedded in a polymer binder.

In some embodiments, the hole carrier materials can be in the form of nanoparticles. The nanoparticles can have any suitable shape, such as a spherical, cylindrical, or rod-like shape.

In some embodiments, hole carrier layer 150 can include combinations of hole carrier materials described above.

In general, the thickness of hole carrier layer 150 (i.e., the distance between the surface of hole carrier layer 150 in contact with photoactive layer 140 and the surface of electrode 160 in contact with hole carrier layer 150) can be varied as desired. Typically, the thickness of hole carrier layer 150 is at least about 0.01 micron (e.g., at least about 0.05 micron, at least about 0.1 micron, at least about 0.2 micron, at least about 0.3 micron, or at least about 0.5 micron) and/or at most about five microns (e.g., at most about three microns, at most about two microns, or at most about one micron). In some embodiments, the thickness of hole carrier layer 150 is from about 0.01 micron to about 0.5 micron. Electrode 160 is generally formed of an electrically conductive material, such as one or more of the electrically conductive materials described above with respect to electrode 120. In some embodiments, electrode 160 is formed of a combination of electrically conductive materials. In certain embodiments, electrode 160 can be formed of a mesh electrode. In other embodiments, electrode 160 can be formed from metal nanoparticles (e.g., silver nanorods) dispersed in a polymer matrix, such as that disclosed in commonly-owned co-pending U.S. Application Publication No 2008-0236657.

In general, the methods of preparing each layer in photovoltaic cell 101 described above can vary as desired. In some embodiments, a layer can be prepared by a liquid- based coating process. In certain embodiments, a layer can be prepared via a gas phase- based coating process, such as chemical or physical vapor deposition processes.

The term "liquid-based coating process" mentioned herein refers to a process that uses a liquid-based coating composition. Examples of the liquid-based coating composition include solutions, dispersions, or suspensions. The liquid-based coating process can be carried out by using at least one of the following processes: solution coating, ink jet printing, spin coating, dip coating, knife coating, bar coating, spray coating, roller coating, slot coating, gravure coating, flexographic printing, or screen printing. Examples of liquid-based coating processes have been described in, for example, commonly-owned co-pending U.S. Application Publication No. 2008-0006324.

In some embodiments, when a layer includes inorganic semiconductor nanoparticles, the liquid-based coating process can be carried out by (1) mixing the nanoparticles with a solvent (e.g., an aqueous solvent or an anhydrous alcohol) to form a dispersion, (2) coating the dispersion onto a substrate, and (3) drying the coated dispersion. In certain embodiments, a liquid-based coating process for preparing a layer containing inorganic metal oxide nanoparticles can be carried out by (1) dispersing a precursor (e.g., a titanium salt) in a suitable solvent (e.g., an anhydrous alcohol) to form a dispersion, (2) coating the dispersion on a substrate, (3) hydrolyzing the dispersion to form an inorganic semiconductor nanoparticles layer (e.g., a titanium oxide nanoparticles layer), and (4) drying the inorganic semiconductor material layer. In certain

embodiments, the liquid-based coating process can be carried out by a sol-gel process (e.g., by forming metal oxide nanoparticles as a sol-gel in a dispersion before coating the dispersion on a substrate).

In general, the liquid-based coating process used to prepare a layer containing an organic semiconductor material can be the same as or different from that used to prepare a layer containing an inorganic semiconductor material. In some embodiments, to prepare a layer includes an organic semiconductor material, the liquid-based coating process can be carried out by mixing the organic semiconductor material with a solvent (e.g., an organic solvent) to form a solution or a dispersion, coating the solution or dispersion on a substrate, and drying the coated solution or dispersion.

In some embodiments, photovoltaic cell 101 can be prepared in a continuous manufacturing process, such as a roll-to-roll process, thereby significantly reducing the manufacturing cost. Examples of roll-to-roll processes have been described in, for example, commonly-owned co-pending U.S. Application Publication No. 2005-0263179.

In general, during use, incident light can impinge on the surface of substrate 110, and passes through substrate 110, electrode 120, and optional hole blocking layer 130. The light then interacts with photoactive layer 140, causing electrons to be transferred from the electron donor material (e.g., a conjugated polymer) to the electron acceptor material (e.g., a substituted fullerene). The electron acceptor material then transmits the electrons through optional hole blocking layer 130 to electrode 120, and the electron donor material transfers holes through hole carrier layer 150 to electrode 160. Electrodes 120 and 160 are in electrical connection via an external load so that electrons pass from electrode 120 through the load to electrode 160. When barrier layer 190, adhesion layer 180, and buffer layer 170 are transparent, incident light can also pass through these layers to interact with photoactive layer 140 to generate electrons and holes.

While certain embodiments have been disclosed, other embodiments are also possible.

In some embodiments, photovoltaic cell 101 can be a tandem photovoltaic cell, such as those described in commonly-owned co-pending U.S. Application Publication Nos. 2007-0246094, 2007-0181179, 2007-0272296, and 2009-0211633. In some embodiments, photovoltaic cell 101 includes a cathode as a bottom electrode and an anode as a top electrode. In some embodiments, photovoltaic cell 101 can include an anode as a bottom electrode and a cathode as a top electrode.

In some embodiments, photovoltaic cell 101 can include the layers shown in FIG. 1 in a reverse order. In other words, photovoltaic cell 101 can include these layers from the bottom to the top in the following sequence: a barrier layer 190, an adhesion layer 180, a buffer layer 170, an electrode 160, an optional hole carrier layer 150, a photoactive layer 140, an optional hole blocking layer 130, an electrode 120, and a substrate 110.

In some embodiments, photovoltaic module 100 includes a plurality of photovoltaic cells 101 that are electrically connected in series. In some embodiments, photovoltaic module 100 includes a plurality of photovoltaic cells 101 that are electrically connected in parallel. In certain embodiments, some photovoltaic cells 101 in

photovoltaic module 100 are electrically connected in series, and some photovoltaic cells 101 in photovoltaic module 100 are electrically connected in parallel.

While organic photovoltaic cells have been described, other photovoltaic cells can also be included in photovoltaic module 100 described herein. Examples of such photovoltaic cells include dye sensitized photovoltaic cells and inorganic photoactive cells with a photoactive material formed of amorphous silicon, cadmium selenide, cadmium telluride, copper indium selenide, and copper indium gallium selenide. In some embodiments, a hybrid photovoltaic cell can be included in photovoltaic module 100 described herein.

While photovoltaic cells have been described above, in some embodiments, the buffer layer described herein can be used in other devices and systems. For example, the buffer layer can be used in suitable organic semiconductive devices, such as field effect transistors, photodetectors (e.g., IR detectors), photovoltaic detectors, imaging devices (e.g., RGB imaging devices for cameras or medical imaging systems), light emitting diodes (LEDs) (e.g., organic LEDs (OLEDs) or IR or near IR LEDs), lasing devices, conversion layers (e.g., layers that convert visible emission into IR emission), amplifiers and emitters for telecommunication (e.g., dopants for fibers), storage elements (e.g., holographic storage elements), and electrochromic devices (e.g., electrochromic displays).

The contents of all publications cited herein (e.g., patents, patent application publications, and articles) are hereby incorporated by reference in their entirety.

The following example is illustrative and not intended to be limiting.

Example

The following six photovoltaic modules were prepared: (1) a photovoltaic module containing poly(3-hexylthiophene) as a photoactive polymer with no buffer layer ("OPV Gen I with No Buffer"), (2) a photovoltaic module containing poly(3-hexylthiophene) as a photoactive polymer with a buffer layer containing SURLYN 1702 ("OPV Gen I with SURLYN 1702 Buffer"), (3) a photovoltaic module containing poly(3-hexylthiophene) as a photoactive polymer with a buffer layer containing a polyurethane ("OPV Gen I with Coatable Buffer"), (4) a photovoltaic module containing a polysilacyclopentadithiophene copolymer as a photoactive polymer with no buffer layer ("OPV Gen II with No

Buffer"), (5) a photovoltaic module containing a polysilacyclopentadithiophene copolymer as a photoactive polymer with a buffer layer containing SURLYN 1702 ("OPV Gen II with SURLYN 1702 Buffer"), and (6) a photovoltaic module containing a polysilacyclopentadithiophene copolymer as a photoactive polymer with a buffer layer containing a polyurethane ("OPV Gen II with Coatable Buffer").

Each module contained 10 photovoltaic cells that were interconnected by using a continuous coating process. The photovoltaic cells were prepared by the methods described in Example 6 of commonly-owned co-pending U.S. Application Publication No. 2008-0087324. The modules were packaged as follows.

Each of modules (1) and (4) was prepared by first attaching interconnected photovoltaic cells with copper tape on both electrode terminals. Both sides of the interconnected photovoltaic cells were then coated with 10-20 microns of a curable epoxide-containing material and laminated with a transparent barrier film. The article thus formed was subsequently cured at 70-100°C for 5 minutes to form module (1) or (4). Each of modules (2) and (5) was prepared by first attaching interconnected photovoltaic cells with copper tape on both electrode terminals. Both sides of the interconnected photovoltaic cells were then hot laminated with 2-10 mil of a SURLYN 1702 film by using a thermal laminator. The SURLYN 1702 film was subsequently coated with 10-20 microns of a curable epoxide-containing material and laminated with a transparent barrier film. The article thus formed was then cured at 70-100°C for 5 minutes to form module (2) or (5).

Each of modules (3) and (6) was prepared by first attaching interconnected photovoltaic cells with copper tape on both electrode terminals. Both sides of the interconnected photovoltaic cells were then coated with a mixture containing a diisocyanate and a polyol and cured at 70°C to form a buffer layer. The buffer layer was subsequently coated with 10-20 microns of a curable epoxide-containing material and laminated with a transparent barrier film. The article thus formed was then cured at 70- 100°C for 5 minutes to form module (3) or (6).

Modules (l)-(6) were tested for their stability in a damp heating oven at 65°C and 85% relative humidity (RH). The performance of the modules were measured using a solar simulator under AM 1.5 conditions. During testing, the modules were periodically removed from the oven, measured for their performance, and placed back in the oven. The test results were shown in FIGs. 4-6.

As shown in FIGs. 4-6, the serial resistance, efficiency, and fill factor of modules (1) and (4) significantly deteriorated at 65°C and 85% RH after about 100 days.

Unexpectedly, the serial resistance, efficiency, and fill factor of modules (2), (3), (5), and (6) remained relatively constant at 65°C and 85% RH even after more than 400 days.

Other embodiments are in the claims.

Claims

WHAT IS CLAIMED IS:

1. A module, comprising:

a first photovoltaic cell comprising first and second electrodes, and a photoactive layer between the first and second electrodes;

a barrier layer supported by the first photovoltaic cell, the first electrode being between the barrier layer and the photoactive layer;

an adhesion layer comprising an adhesive, the adhesion layer being between the first electrode and the barrier layer; and

a buffer layer comprising a polymer selected from the group consisting of a polyurethane, a polyolefin, a polyvinyl butyral, an ionomeric polymer, or a copolymer thereof, the buffer layer being between the first electrode and the adhesion layer.

2. The module of claim 1, wherein the polymer comprises a polyolefin grafted with an anhydride, an acid, or an acrylate, a polyethylene wax, a poly(ethylene- co-maleic anhydride), a poly(ethylene-co-methacrylic acid) or a salt thereof, a polyisobutylene, or a polyurethane made from a polyol and a diisocyanate.

3. The module of claim 1, wherein the polymer comprises a poly(ethylene- co-methacrylic acid) or a salt thereof, or a polyurethane made from a toluene diisocyanate and a polyol selected from the group consisting of a polyether polyol, a polyester polyol, a neopentyl polyol, and a cyclopentane polyol.

4. The module of claim 1, wherein the polymer has a glass transition temperature of at most about 120°C.

5. The module of claim 1, wherein the polymer has a glass transition temperature of at least about 80°C.

6. The module of claim 1 , wherein the polymer is from about 70% by weight to about 100% by weight of the buffer layer.

7. The module of claim 1, wherein the buffer layer further comprises a filler.

8. The module of claim 7, wherein the filler comprises a nanoclay.

9. The module of claim 8, wherein the nanoclay comprises montmorillonite, kaolinite, llite, or chlorite.

10. The module of claim 7, wherein the filler is from about 0% by weight to about 30%) by weight of the buffer layer.

11. The module of claim 1 , wherein the buffer layer covers substantially the entire area between the first electrode and the adhesion layer.

12. The module of claim 1, wherein the module comprises a second photovoltaic cell separated from the first photovoltaic cell by an interconnection area for electrically connecting the first and second photovoltaic cells, and the buffer layer covers the interconnection area.

13. The module of claim 1 , wherein the module comprises a plurality of photovoltaic cells; each photovoltaic cell has first and second electrodes, and a photoactive layer between the first and second electrodes; the buffer layer is between the first electrode in each photovoltaic cell and the adhesion layer; and the buffer covers substantially the entire area between the first electrode in each photovoltaic cell and the adhesion layer.

14. The module of claim 1 , wherein the buffer layer has a moisture vapor transmission rate of at most about 0.01 g/m²/day.

15. The module of claim 1, wherein the buffer layer has a thickness of from about 25 microns to about 250 microns.

16. The module of claim 1, wherein the adhesive comprises an epoxy resin.

17. The module of claim 1, wherein the barrier layer comprises at least two polymer layers and at least two ceramic layers between the at least two polymer layers.

18. The module of claim 1, wherein the barrier layer comprises a metal foil.

19. A module, comprising:

a barrier layer supported by the first photovoltaic cell, the first electrode being between the barrier layer and the photoactive layer; and

a buffer layer comprising a polymer that comprises a polyolefm grafted with an anhydride, an acid, or an acrylate, the buffer layer being between the first electrode and the barrier layer;

wherein the module does not include an additional adhesive layer between the buffer layer and the barrier layer.

20. The module of claim 19, wherein the polyolefm comprises a

homopolymer or copolymer.

21. The module of claim 19, wherein the polyolefm comprises a polyethylene, a polypropylene, a poly(ethylene-co-vinyl acetate), or a poly(ethylene-co-acrylate).

22. The module of claim 19, wherein the buffer layer comprises a low-density polyethylene grafted with an anhydride, a linear low-density polyethylene grafted with an anhydride, a high density polyethylene grafted with an anhydride, a polypropylene grafted with an anhydride, a poly(ethylene-co-acrylate) grated with an anhydride or an acid, and a poly(ethylene-co-vinyl acetate) grafted with an anhydride, an acid, or an acrylate.

23. The module of claim 19, wherein the polymer has a glass transition temperature of at most about 120°C.

24. The module of claim 19, wherein the polymer has a glass transition temperature of at least about 80°C.

25. The module of claim 19, wherein the polymer is from about 70% by weight to about 100% by weight of the buffer layer.

26. The module of claim 19, wherein the buffer layer further comprises a filler.

27. The module of claim 26, wherein the filler comprises a nanoclay.

28. The module of claim 27, wherein the nanoclay comprises montmorillonite, kaolinite, llite, or chlorite.

29. The module of claim 26, wherein the filler is from about 1% by weight to about 30%) by weight of the buffer layer.

30. The module of claim 19, wherein the buffer layer covers substantially the entire area between the first electrode and the barrier layer.

31. The module of claim 19, wherein the module comprises a second photovoltaic cell separated from the first photovoltaic cell by an interconnection area for electrically connecting the first and second photovoltaic cells, and the buffer layer covers the interconnection area.

32. The module of claim 19, wherein the module comprises a plurality of photovoltaic cells; each photovoltaic cell has first and second electrodes, and a photoactive layer between the first and second electrodes; the buffer layer is between the first electrode in each photovoltaic cell and the barrier layer; and the buffer covers substantially the entire area between the first electrode in each photovoltaic cell and the barrier layer.

33. The module of claim 19, wherein the buffer layer has a moisture vapor transmission rate of at most about 0.01 g/m²/day.

34. The module of claim 19, wherein the buffer layer has a thickness of from about 25 microns to about 250 microns.

35. The module of claim 19, wherein the barrier layer comprises at least two polymer layers and at least two ceramic layers between the at least two polymer layers.

36. The module of claim 19, wherein the barrier layer comprises a metal foil.