WO2010138414A1

WO2010138414A1 - Reflective multilayer electrode

Info

Publication number: WO2010138414A1
Application number: PCT/US2010/035761
Authority: WO
Inventors: Jens Hauch; Christoph Brabec
Original assignee: Konarka Technologies, Inc.
Priority date: 2009-05-27
Filing date: 2010-05-21
Publication date: 2010-12-02

Abstract

Articles having a reflective multilayer electrode, as well as related photovoltaic cells, systems, components, and methods, are disclosed.

Description

Reflective Multilayer Electrode

CROSS REFERENCE TO RELATEDAPPLICATION

Pursuant to 35 U.S. C. § 119(e), this application claims priority to U.S. Provisional Application Serial No. 61/181,457, filed May 27, 2009, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to articles having a reflective multilayer electrode, as well as related photovoltaic cells, systems, components, and methods.

BACKGROUND Photovoltaic cells are commonly used to transfer energy in the form of light into energy in the form of electricity. Atypical 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. As a result, the ability of one or both of the electrodes to transmit light (e.g., light at one or more wavelengths absorbed by a photoactive material) can limit the overall efficiency of a photovoltaic cell. In many photovoltaic cells, a film of semiconductive material (e.g., indium tin oxide) is used to form the electrode(s) through which light passes because although the semiconductive material can have a lower electrical conductivity than electrically conductive materials, the semiconductive material can transmit more light than many electrically conductive materials.

SUMMARY

In one aspect, the disclosure features an article that includes a first electrode, a second electrode including first, second, and third layers, and a photoactive layer between the first electrode and the first layer. The first layer includes a metal oxide. The second layer is between the first and third layers, includes a metal, and has a thickness of at least about 20 nm. The third layer includes a metal oxide or a metal. The article is configured as a photovoltaic cell. In another aspect, the disclosure features an article that includes a first electrode, a second electrode including first, second, and third layers, and a photoactive layer between the first electrode and the first layer. The first layer includes a metal oxide. The second layer is between the first and third layers, is non-transparent, and includes a metal. The third layer includes a metal oxide or a metal. The article is configured as a photovoltaic cell.

In another aspect, the disclosure features an article that includes a first electrode, a second electrode including first, second, and third layers, and a photoactive layer between the first electrode and the first layer. The first layer includes a metal oxide. The second layer is between the first and third layers and has a reflectance of at least about 80% at a wavelength between about 400-850 nm. The third layer includes a metal oxide or a metal. The article is configured as a photovoltaic cell.

In another aspect, the disclosure features an article that includes a first layer including a metal oxide, a second layer including a metal and having a thickness of at least about 20 nm, and a third layer including a metal oxide or a metal. The second layer is between the first and third layers. The article is configured as an electrode.

In another aspect, the disclosure features an article that includes a first layer including a metal oxide, a non-transparent second layer including a metal, and a third layer including a metal oxide or a metal. The second layer is between the first and third layers. The article is configured as an electrode.

In still another aspect, the disclosure features an article that includes a first layer including a metal oxide, a second layer having a reflectance of at least about 80% at a wavelength between about 400-850 nm, and a third layer including a metal oxide or a metal. The second layer is between the first and third layers. The article is configured as an electrode.

Embodiments can include one or more of the following features.

In some embodiments, the second layer has a thickness of at least about 40 nm.

In some embodiments, the second layer includes a metal, such as silver, aluminum, gold, titanium, or a mixture or alloy thereof. In some embodiments, the second layer includes two layers, one layer including a first metal and the other layer including a second metal different from the first metal. In some embodiments, the second layer is non-transparent. In some embodiments, the first layer has a thickness of at least about 8 nm or at most about 50 nm.

In some embodiments, the first or third layer includes doped or undoped tin oxide, doped or undoped zinc oxide, doped or undoped titanium oxide, or a mixture thereof. For example, the first layer can include indium tin oxide, aluminum zinc oxide, zinc tin oxide, indium zinc oxide, titanium oxide doped with carbon or niobium, or a mixture thereof.

In some embodiments, the third layer includes a metal, such as titanium, nickel, chromium, or a mixture or alloy thereof.

In some embodiments, the third layer has a thickness of at least about 5 nm. In some embodiments, the photoactive layer includes an electron donor material and electron acceptor material.

In some embodiments, the articles described above further include a hole carrier between the first electrode and the photoactive layer.

In some embodiments, the articles described above further include a hole blocking layer between the first layer and the photoactive layer.

In some embodiments, the articles described above further include a substrate, the second electrode being between the substrate and the first electrode. In some embodiments, the articles described above further include an encapsulation, the first electrode between the encapsulation and the second electrode. Embodiments can include one or more of the following advantages. Without wishing to be bound by theory, it is believed that a bottom electrode having a highly reflective layer can reflect incident light not absorbed by a photoactive layer back to the photoactive layer, thereby increasing absorption of incident light by this layer and the efficiency of a photovoltaic cell.

Without wishing to be bound by theory, it is believed that including a semiconductive metal oxide layer between a reflective layer in a bottom electrode and a photoactive layer can improve the stability of the bottom electrode by preventing the material (e.g., a metal) used to form the reflective layer from diffusing into the photoactive layer (which could potentially cause short circuit in a photovoltaic cell). Without wishing to be bound by theory, it is believed that including a metal or metal oxide layer between a reflective layer in a bottom electrode and a substrate can improve the stability of the bottom electrode by preventing the material (e.g., a metal) in the reflective layer from oxidation by oxygen or water diffused into a photovoltaic cell through the substrate (which could potentially result in a loss in adhesion between the reflective layer and the substrate and therefore failure of the photovoltaic cell).

Without wishing to be bound by theory, it is believed that a multilayer electrode described above is advantageously used in a bottom electrode, rather than a top electrode, in a photovoltaic cell as using such a multilayer structure in a top electrode could damage to a polymeric layer (e.g., a photoactive layer or a hole carrier layer) beneath the top electrode..

Other features and advantages of the embodiments will be apparent from the description, drawings, and claims.

DESCRIPTION OF DRAWINGS FIG. 1 is a cross-sectional view of an embodiment of a photovoltaic cell;

FIG. 2 is a schematic of a system containing multiple photovoltaic cells electrically connected in series; and

FIG. 3 is a schematic of a system containing multiple photovoltaic cells electrically connected in parallel. Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows a cross-sectional view of a photovoltaic cell 100 that includes a substrate 110, a bottom 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, a top electrode 160, and an optional encapsulation 170. Bottom electrode 120 includes a first layer 126, a second layer 124, and a third layer 122.

In some embodiments, during use, light impinges on the surface of encapsulation 170, and passes through encapsulation 170, top electrode 160, and hole carrier layer 150. The light then interacts with photoactive layer 140, causing electrons to be transferred from the electron donor material in layer 140 to the electron acceptor material in layer 140. The electron acceptor material then transmits the electrons through hole blocking layer 130 to bottom electrode 120, and the electron donor material transfers holes through hole carrier layer 150 to top electrode 160. Electrodes 120 and 160 are in electrical connection via an external load so that electrons pass from electrode 120, through the load, and to electrode 160.

In some embodiments, bottom electrode 120 is an electrode formed on substrate 110 before the other electrode (i.e., top electrode 160) is formed and is closer in distance to substrate 110 than the other electrode. In some embodiments, bottom electrode 120 can include three layers, i.e., first layer 126, second layer 124, and third layer 122. In general, second layer 124 can be used as a reflecting layer. For example, second layer 124 can be made from a highly reflective material, such as a highly reflective metal. Examples of reflective metals that can be used to form second layer 124 include silver, aluminum, gold, titanium, or a mixture or alloy thereof. In some embodiments, the second layer has a reflectance of at least about 80% (e.g., at least about 85%, at least about 90%, at least about 95%, at least about 98%, or at least about 99%) at a wavelength between about 400-850 nm (e.g., between about 400-700 nm). During use of photovoltaic cell 100, incident light passes through top electrode 160 and reaches photoactive layer 140. Without wishing to be bound by theory, it is believed that a bottom electrode having a highly reflective second layer 124 can reflect incident light not absorbed by photoactive layer 140 back to that layer, thereby increasing absorption of incident light by layer 140 and the efficiency of photovoltaic cell 100.

When second layer 124 is used as a reflecting layer, layer 124 is generally non- transparent. As used herein, a non-transparent layer is a layer which, at the thickness used in a photovoltaic cell 100, transmits at most about 10% (e.g., at most about 5%, at most about 3%, at most about 1%, or at most 0.5%) of incident light at a wavelength or a range of wavelengths (e.g., between about 400-850 nm) used during operation of the photovoltaic cell. In such embodiments, second layer 124 typically has a sufficiently large thickness so as to make this layer non-transparent. For example, a non-transparent second layer 124 typically has a thickness of at least about 20 nm (e.g., at least about 30 nm, at least about 40 nm or at least about 50 nm). In some embodiments, the thickness of second layer 124 is not overly large so as to avoid waste of material and to reduce costs. For example, second layer 124 can have a thickness of at most about 500 nm (e.g., at most about 300 nm, at most about 100 nm, or at most about 50 nm).

As a layer in an electrode, second layer 124 is generally formed of an electrically conductive material, such as a metal or alloy described above. In some embodiments, second layer 124 can include two layers (not shown in FIG. 1), one layer including a first metal and the other layer including a second metal different from the first metal. For example, second layer 124 can include a silver layer adjacent to first layer 126 and an aluminum layer 122 adjacent to third layer 122. It is well known that silver, although in large part more reflective than aluminum in the visible light spectrum, is more expensive. In addition, aluminum has similar electrical conductivity to that of silver. Thus, replacing the portion of silver facing away from incident light in second layer 124 can maintain the reflectivity and electrical conductivity of layer 124 at a lower cost.

First layer 126 is generally formed of a semiconductive material such as a semiconductive metal oxide. Examples of suitable oxides include doped or undoped tin oxide, doped or undoped zinc oxide, doped or undoped titanium oxide, or a mixture thereof. For example, first layer 126 can be formed of indium tin oxide (i.e., tin oxide doped with indium), aluminum zinc oxide (i.e., zinc oxide doped with aluminum), zinc tin oxide (i.e., tin oxide doped with zinc), indium zinc oxide (i.e., zinc oxide doped with indium), titanium oxide doped with carbon or niobium, or a mixture thereof. Without wishing to be bound by theory, it is believed that including a semiconductive metal oxide as first layer 126 between second layer 124 and photoactive layer 140 can improve the stability of bottom electrode 120 by preventing the material (e.g., a metal) used to form second layer 124 from diffusing into photoactive layer 140 (which could potentially cause short circuit in photovoltaic cell 100). In addition, without wishing to be bound by theory, it is believed that including a semiconductive metal oxide as first layer 126 can establish a suitable charge carrier selective contact between photoactive layer 140 and second layer 124, thereby facilitating charge carrier (e.g., electrons or holes) transfer between these two layers. In some embodiments, first layer 126 selectively transfers one type of charge carriers (e.g., either electrons or holes). In such embodiments, first layer 126 can serve as a hole blocking layer or a hole carrier layer. In FIG. 1 , when first layer 126 serves as a hole blocking layer, optional hole blocking layer 130 can be omitted. Notwithstanding the foregoing, in some embodiments, first layer 126 can be formed of a metal, such as a metal described herein.

In general, first layer 126 can have any suitable thickness as desired. In some embodiments, first layer 126 has a sufficiently large thickness so it completely covers second layer 124 to prevent the material (e.g., a metal) used in second layer 124 from diffusing into photoactive layer 140. For example, first layer 126 can have a thickness of at least about 8 nm (e.g., at least about 15 nm, at least about 20 nm, at least about 40 nm or at least about 50 nm). In some embodiments, the thickness of first layer 126 is not overly large so as to minimize absorption of incident light and/or maintain flexibility. For example, first layer 126 can have a thickness of at most about 100 nm (e.g., at most about 80 nm, at most about 60 nm, or at most about 40 nm).

In some embodiments, first layer 126 has an electrical conductivity of at least about 1 x 10⁴ S/m (e.g., at least about 2 x 10⁴ S/m, at least about 5 x 10⁴ S/m, and at least about 1 x 10⁵ S/m).

In some embodiment, first layer 126 is a transparent layer. As used herein, a transparent layer is a layer which, at the thickness used in a photovoltaic cell 100, transmits at least about 70% (e.g., at least about 75%, at least about 80%, at least about 85%, or at least about 90%) of incident light at a wavelength or a range of wavelengths (e.g., between about 400-850 nm) used during operation of the photovoltaic cell.

In some embodiment, first layer 126 can be configured as a reflecting layer, e.g., by incorporating a reflective material into the layer. For example, first layer 126 can incorporate a material reflecting light that is not reflective at the surface of second layer 124, thereby increasing incident light absorption by photoactive layer 140 and the efficiency of photovoltaic cell 100.

Third layer 122 is generally formed of a semiconductive material (e.g., a semiconductive metal oxide) or an electrically conductive material (e.g., a metal). Examples of suitable oxides include doped or undoped tin oxide, doped or undoped zinc oxide, doped or undoped titanium oxide, or a mixture thereof. For example, third layer 122 can be formed of indium tin oxide (i.e., tin oxide doped with indium), aluminum zinc oxide (i.e., zinc oxide doped with aluminum), zinc tin oxide (i.e., tin oxide doped with zinc), indium zinc oxide (i.e., zinc oxide doped with indium), titanium oxide doped with carbon or niobium, or a mixture thereof. Example of suitable metals include titanium, nickel, chromium, or a mixture or alloy thereof (e.g., an alloy of nickel and chromium). Without wishing to be bound by theory, it is believed that including third layer 122 between second layer 124 and substrate 110 can (1) improve adhesion between second layer 124 and substrate 100 and (2) improve the stability of bottom electrode 120 by preventing the material (e.g., a metal) in second layer 124 from oxidation by oxygen or water diffused into photovoltaic cell 100 through substrate 110 (which could potentially result in a loss of electrical conductivity in second layer 124). In addition, without wishing to be bound by theory, it is believed that using a metal or a metal alloy in third layer 122 can provide better electrically conductivity to electrode 120 at a lower cost comparing to using a semiconductive metal oxide in third layer 122.

In some embodiments, third layer 122 can include an adhesion promoter, such as titanium or an alloy of nickel and chromium. Such an adhesion promoter can improve adhesion between second layer 124 and substrate 110, thereby minimizing failure of photovoltaic cell 100.

In general, third layer 122 can have any suitable thickness as desired. In some embodiments, third layer 122 has a sufficiently large thickness to prevent oxidation of second layer 124. For example, third layer 122 can have a thickness of at least about 15 nm (e.g., at least about 20 nm, at least about 40 nm or at least about 50 nm). In some embodiments, the thickness of third layer 122 is not overly large so as to avoid waste of material and to reduce costs. For example, third layer 122 can have a thickness of at most about 500 nm (e.g., at most about 300 nm, at most about 100 nm, or at most about 50 nm). Third layer 122 can be transparent, semi-transparent, or non-transparent. In some embodiment, third layer 122 can be configured as a reflecting layer, e.g., by incorporating a reflective material into the layer. For example, first layer 126 can incorporate a material reflecting light that passes through second layer 124, thereby increasing incident light absorption by photoactive layer 140 and the efficiency of photovoltaic cell 100. Without wishing to be bound by theory, it is believed that a multilayer electrode described above (such as electrode 120) is advantageously used in a bottom electrode, rather than a top electrode, in a photovoltaic cell. When a multilayer electrode described above is used as a top electrode, the layer (which corresponds to first layer 126) contacting photoactive layer 140 or hole carrier layer 150 includes a semiconductive metal oxide. Manufacturing of this semiconductive metal oxide (e.g., by sputtering) could damage the photoactive layer 140 or hole carrier layer 150 (which typically includes a photoactive material or a polymer). By contrast, when such a multilayer electrode is used as a bottom electrode, the layer (i.e., first layer 126) contacting photoactive layer 140 or hole blocking layer 130 is disposed on second layer 124 (which typically is made of a metal), which could withstand the conditions used to manufacture this layer.

In some embodiments, bottom electrode 120 can include one or more layers in addition to first layer 126, second layer 124, and third layer 122. Each of these additional layers can be formed of a metal, a metal oxide, or a mixture thereof. Examples of metals of metal oxides suitable for these additional layers can be those described herein. Turning to other components in photovoltaic cell 100, substrate 110 is generally a base (e.g., a pre-formed sheet of material) on which the remaining components in cell 100 are built. Substrate 110 can be formed of a transparent material, a semi-transparent material, or a non-transparent material. Exemplary materials from which substrate 110 can be formed include glass, 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 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. 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, 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, 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).

Optionally, photovoltaic cell 100 can include a hole blocking layer 130. The hole blocking layer is generally formed of a material that, at the thickness used in a photovoltaic cell (e.g., cell 100), 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, doped or undoped 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 commonly-owned co-pending U.S. Application Publication No. 2008- 0264488, the entire contents of which are hereby incorporated by reference.

Typically, hole blocking layer 130 is at least 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. In some embodiments, hole blocking layer 130 can be a non-porous layer. In such embodiments, hole blocking layer 130 can be a compact layer with a small thickness (e.g., less than about 0.1 microns).

Photoactive layer 140 generally contains 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 or acceptor materials can include one or more polymers (e.g., homopolymers or copolymers). A polymer mentioned herein includes at least two identical or different monomer repeat units (e.g., at least 5 monomer repeat units, at least 10 monomer repeat units, at least 50 monomer repeat units, at least 100 monomer repeat units, or at least 500 monomer repeat units). Ahomopolymer mentioned herein refers to a polymer that includes only one type of monomer repeat units. A copolymer mentioned herein refers to a polymer that includes at least two co- monomer repeat units with different chemical structures. The polymers can be photovoltaically active. Examples of electron acceptor materials include fullerenes, inorganic nanoparticles, oxadiazoles, discotic liquid crystals, carbon nanorods, inorganic nanorods, polymers containing moieties capable of accepting electrons or forming stable anions (e.g., polymers containing CN groups, polymers containing CF3 groups), or combinations thereof. In some embodiments, electron acceptor materials of photoactive layer 140 can include fullerenes having 50 to 250 carbon atoms. 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 Ceo, C70, C₇₆, C₇8, Cs₂, Cs4, and C92. Examples of substituted fullerenes include PCBMs (e.g., [6,6]-phenyl C61 -butyric acid methyl ester (Cβi-PCBM) or [6,6]-phenyl C71 -butyric acid methyl ester (C₇₁- PCBM)) or fullerenes substituted with C1-C20 alkoxy optionally further substituted with Ci-C₂₀ alkoxy and/or halo (e.g., (OCH₂CH₂)2θCH₃ 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. In some embodiments, the electron acceptor material can include one or more of the polymers described herein. In certain embodiments, a combination of electron acceptor materials can be used in photoactive layer 140.

Examples of electron donor materials include conjugated polymers, such as polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, polycyclopentadithiophenes, polysilacyclopentadithiophenes, polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles, polybenzothiadiazoles, poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxalines, polybenzoisothiazoles, polybenzothiazoles, polythienothiophenes, poly(thienothiophene oxide)s, polydithienothiophenes, poly(dithienothiophene oxide)s, polytetrahydroisoindoles, and copolymers thereof. In some embodiments, the electron donor material can be polythiophenes (e.g., poly(3-hexylthiophene) (P3HT)), polycyclopentadithiophenes, and copolymers thereof. In certain embodiments, a combination of electron donor materials can be used in photoactive layer 140.

In some embodiments, electron donor or acceptor materials can include a polymer having one or more of the following comonomer repeat units: a silacyclopentadithiophene moiety of formula (1), a cyclopentadithiophene moiety of formula (2), a benzothiadiazole moiety of formula (3), a thiadiazoloquinoxaline moiety of formula (4), a cyclopentadithiophene dioxide moiety of formula (5), a cyclopentadithiophene monoxide moiety of formula (6), a benzoisothiazole moiety of formula (7), a benzothiazole moiety of formula (8), a thiophene dioxide moiety of formula (9), a cyclopentadithiophene dioxide moiety of formula (10), a cyclopentadithiophene tetraoxide moiety of formula (11), a thienothiophene moiety of formula (12), a thienothiophene tetraoxide moiety of formula (13), a dithienothiophene moiety of formula (14), a dithienothiophene dioxide moiety of formula (15), a dithienothiophene tetraoxide moiety of formula (16), a tetrahydroisoindole moiety of formula (17), a thienothiophene dioxide moiety of formula (18), a dithienothiophene dioxide moiety of formula (19), a fluorene moiety of formula (20), a silole moiety of formula (21), a fluorenone moiety of formula (22), a thiazole moiety of formula (23), a selenophene moiety of formula (24), a thiazolothiazole moiety of formula (25), a cyclopentadithiazole moiety of formula (26), a naphthothiadiazole moiety of formula (27), a thienopyrazine moiety of formula (28), an oxazole moiety of formula (29), an imidazole moiety of formula (30), a pyrimidine moiety of formula (31), a benzoxazole moiety of formula (32), a benzimidazole moiety of formula (33), or a benzooxadiazole moiety of formula (34):

In the above formulas, each of X and Y, independently, can be CH₂, O, or S; each of Ri, R₂, R₃, R4, R5, and R₆, independently, can be H, C1-C20 alkyl, C1-C20 alkoxy, C3-C20 cycloalkyl, C₁-C₂₀ heterocycloalkyl, aryl, heteroaryl, halo, CN, OR, C(O)R, C(O)OR, or SO₂R, in which R is H, C₁-C₂0 alkyl, C₁-C₂0 alkoxy, aryl, heteroaryl, C3-C₂0 cycloalkyl, or Ci -C₂0 heterocycloalkyl; and each of R₇ and Rs, independently, can be H, C₁-C₂0 alkyl, C₁-C₂0 alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₁-C₂₀ heterocycloalkyl.

An alkyl can be saturated or unsaturated and branched or straight chained. A C₁- C₂0 alkyl contains 1 to 20 carbon atoms (e.g., one, two, three, four, five, six, seven, eight, nine, 10, 1 1, 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examples of alkyl moieties include -CH₃, -CH₂-, -CH₂=CH₂-, -CH₂-CH=CH₂, and branched -C₃H₇. An alkoxy can be branched or straight chained and saturated or unsaturated. An Ci-C_2O alkoxy contains an oxygen radical and 1 to 20 carbon atoms (e.g., one, two, three, four, five, six, seven, eight, nine, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examples of alkoxy moieties include -OCH₃ and -OCH=CH-CH₃. A cycloalkyl can be either saturated or unsaturated. A C₃-C_2O cycloalkyl contains 3 to 20 carbon atoms (e.g., three, four, five, six, seven, eight, nine, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examples of cycloalkyl moieities include cyclohexyl and cyclohexen-3- yl. Aheterocycloalkyl can also be either saturated or unsaturated. A Ci-C_2O heterocycloalkyl contains at least one ring heteroatom (e.g., O, N, and S) and 1 to 20 carbon atoms (e.g., one, two, three, four, five, six, seven, eight, nine, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, and 20 carbon atoms). Examples of heterocycloalkyl moieties include 4-tetrahydropyranyl and 4-pyranyl. An aryl can contain one or more aromatic rings. Examples of aryl moieties include phenyl, phenylene, naphthyl, naphthylene, pyrenyl, anthryl, and phenanthryl. A heteroaryl can contain one or more aromatic rings, at least one of which contains at least one ring heteroatom (e.g., O, N, and S). Examples of heteroaryl moieties include furyl, furylene, fluorenyl, pyrrolyl, thienyl, oxazolyl, imidazolyl, thiazolyl, pyridyl, pyrimidinyl, quinazolinyl, quinolyl, isoquinolyl, and indolyl. Alkyl, alkoxy, cycloalkyl, heterocycloalkyl, aryl, and heteroaryl mentioned herein include both substituted and unsubstituted moieties, unless specified otherwise. Examples of substituents on cycloalkyl, heterocycloalkyl, aryl, and heteroaryl include Ci- C₂o alkyl, C₃-C_2O cycloalkyl, Ci-C_2O alkoxy, aryl, aryloxy, heteroaryl, heteroaryloxy, amino, Ci-Cio alkylamino, Ci-C_2O dialkylamino, arylamino, diarylamino, hydroxyl, halogen, thio, Ci-Cio alkylthio, arylthio, Ci-Cio alkylsulfonyl, arylsulfonyl, cyano, nitro, acyl, acyloxy, carboxyl, and carboxylic ester. Examples of substituents on alkyl include all of the above-recited substituents except Ci-C_2O alkyl. Cycloalkyl, heterocycloalkyl, aryl, and heteroaryl also include fused groups.

Without wishing to be bound by theory, it is believed that incorporating a silacyclopentadithiophene moiety of formula (1) into a photoactive polymer could significantly improve the solubility and processibility of the polymer and the morphology of a photoactive layer prepared from such a polymer, thereby increasing the efficiency of a photovoltaic cell. Further, without wishing to be bound theory, it is believed that incorporating a silacyclopentadithiophene moiety into a photoactive polymer can shift the absorption wavelength of the polymer toward the red and near IR portion (e.g., 650 - 900 nm) of the electromagnetic spectrum, which is not accessible by most other polymers. When such a co-polymer is incorporated into a photovoltaic cell, it enables the cell to absorb the light in this region of the spectrum, thereby increasing the current and efficiency of the cell. For example, replacing a photoactive polymer having co-monomer repeat units of formulas (2) and (3) with a photoactive polymer having co-monomer repeat units of formulas (1), (2), and (3) can increase the efficiency of a photovoltaic cell from about 3% to about 5% under the AM 1.5 conditions.

In general, the polymer that can be used as an electron donor or acceptor material can include two or more types of comonomer repeat units. In some embodiments, the molar ratio of the two different types of comonomer repeat units is at least about 1 : 1 (e.g., at least about 2:1, at least about 3:1, or at least 4:1) and/or at most about 10:1 (e.g., at most about 5 : 1 , at most about 4: 1 , at most about 3 : 1 , or at most about 2:1).

In some embodiments, the polymer described above can include a silacyclopentadithiophene moiety of formula (1), a cyclopentadithiophene moiety of formula (2), and/or a benzothiadiazole moiety of formula (3). An exemplary polymer that can be used in the photoactive layer 140 is

, in which each of m and n, independently, is an integer greater than 1 (e.g., 2, 3, 5, 10, 20, 50, or 100). This polymer can have superior processibility (e.g., in a solution coating process) and can be used to prepare a photovoltaic cell having an efficiency at least about 5% under AM 1.5 conditions. Other exemplary photoactive polymer that can be used in layer 140 include

, in which n can be an integer greater than 1.

Without wishing to be bound by theory, it is believed that a photovoltaic cell having a photoactive polymer described above can have a high efficiency. In some embodiments, such a photovoltaic cell can have an efficiency of at least about 4% (e.g., at least about 5% or at least about 6%) under AM 1.5 conditions. Further, without wishing to be bound by theory, it is believed that other advantages of the polymers described above include suitable band gap (e.g., 1.4-1.6 eV) that can improve photocurrent and cell voltage, high positive charge mobility (e.g., 10^" to 10^" cm /Vs) that can facilitate charge separation in photoactive layer 140, and high solubility in an organic solvent that can improve film forming ability and processibility. In some embodiments, the polymers can be optically non-scattering.

The polymers described above can be prepared by methods known in the art. For example, a copolymer can be prepared by a cross-coupling reaction between one or more comonomers containing two organometallic groups (e.g., alkylstannyl groups, Grignard groups, or alkylzinc groups) and one or more comonomers containing two halo groups (e.g., Cl, Br, or I) in the presence of a transition metal catalyst. As another example, a copolymer can be prepared by a cross-coupling reaction between one or more comonomers containing two borate groups and one or more comonomers containing two halo groups (e.g., Cl, Br, or I) in the presence of a transition metal catalyst. Other methods that can be used to prepare the copolymers described above including Suzuki coupling reactions, Negishi coupling reactions, Kumada coupling reactions, and Stille coupling reactions, all of which are well known in the art. The comonomers can be prepared by the methods know in the art, such as those described in U.S. Patent Application Serial No. 11/486,536, Coppo et al., Macromolecules 2003, 36, 2705-2711 and Kurt et al., J. Heterocycl. Chem. 1970, 6, 629, the contents of which are hereby incorporated by reference. The comonomers can contain a non-aromatic double bond and one or more asymmetric centers. Thus, they can occur as racemates and racemic mixtures, single enantiomers, individual diastereomers, diastereomeric mixtures, and cis- or trans- isomeric forms. All such isomeric forms are contemplated.

Other examples of photoactive polymers have been described in commonly- owned co-pending U.S. Application Nos. 2007-0014939, 2007-0158620, 2007-0017571, 2007-0020526, 2008-0087324, and 2008-0121281, and U.S. Provisional Application No. 61/086,977, the entire contents of which are herein incorporated by reference. Generally, photoactive layer 240 is sufficiently thick to be relatively efficient at absorbing photons impinging thereon to form corresponding electrons and holes, and sufficiently thin to be relatively efficient at transporting the holes and electrons. In certain embodiments, photoactive layer 240 is at least 0.05 micron (e.g., at least about 0.1 micron, at least about 0.2 micron, or at least about 0.3 micron) thick and/or at most about one micron (e.g., at most about 0.5 micron or at most about 0.4 micron) thick. In some embodiments, photoactive layer 240 is from about 0.1 micron to about 0.2 micron thick. Optionally, photovoltaic cell 100 can include a hole carrier layer 150 to facilitate charge transfer and charge transport. Hole carrier layer 150 is generally formed of a material that, at the thickness used in photovoltaic cell 100, 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 semiconductive polymers, such as polythiophenes (e.g., poly(3,4-ethylenedioxythiophene) (PEDOT) doped with a poly(styrene-sulfonate) (PSS)), polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, and copolymers thereof. Examples of commercially available semiconductive polymers include H.C. Starck BAYTRON^® family of polymers (e.g., PEDOT) and the Air Products^® HIL family of polymers.

In some embodiments, hole carrier layer 150 can include a dopant used in combination with a semiconductive polymer. Examples of dopants include poly(styrene- sulfonate)s, polymeric sulfonic acids, or fiuorinated 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 includes 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, hole carrier layer 150 can include combinations of hole carrier materials described above. 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 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 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.

In general, top electrode 160 is an electrode formed onto the substrate after the bottom electrode. Electrode 160 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)), 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 160 can be substantially transparent. In some embodiments, electrode 160 can be formed of a grid or mesh electrode. Examples of grid or mesh electrodes are described in co-pending U.S. Patent Application Publication Nos. 2004-0187911, 2005-0067007, 2006-0090791, 2007-0131277, 2007- 0193621, 2008-0236657, and the entire contents of which are hereby incorporated by reference. In some embodiments, electrode 160 is in the form of a single layer (e.g., containing one or more of the above-noted electrically conductive materials). In certain embodiments, electrode 160 is formed of multiple layers (e.g., containing one or more of the above-noted electrically conductive materials). In general, bottom electrode 120 (including layers 122, 124, and 126), hole blocking layer 130, photoactive layer 140, hole carrier layer 150, and top electrode 160 described above can be prepared by a gas-based coating process or a liquid-based coating process.

The term "gas-based coating process" mentioned herein refers to a process that forms a coating from a gaseous composition. An examples of a gas-based coating processes is a vapor deposition process, such as a chemical vapor deposition process (e.g., chemical sputtering) or physical vapor deposition process (e.g., physical sputtering). A metal layer (e.g., a silver layer) or a metal oxide layer (e.g., an indium tin oxide layer) can be prepared by a vapor deposition process. 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 can be a solution, a dispersion, or a suspension. The concentration of a coating material in a liquid-based coating composition can generally be adjusted as desired. In some embodiments, the concentration can be adjusted to achieve a desired viscosity of the coating composition or a desired thickness of the coating.

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. Without wishing to bound by theory, it is believed that the liquid-based coating process can be readily used in a continuous manufacturing process, such as a roll-to-roll process, thereby significantly reducing the cost of preparing a photovoltaic cell. Examples of roll-to-roll processes have been described in, for example, commonly-owned co-pending U.S. Application Publication No. 2005-0263179, the contents of which are hereby incorporated by reference. In some embodiments, when a layer (e.g., one of layers 120-160) 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 organic solvent such as 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 photoactive layer, (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.

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, when a layer (e.g., one of layers 120-160) 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. For example, an organic photoactive layer can be prepared by mixing an electron donor material (e.g., P3HT) and an electron acceptor material (e.g., C61 -PCBM) in a suitable solvent (e.g., xylene) to form a dispersion, coating the dispersion onto a substrate, and drying the coated dispersion.

The liquid-based coating process mentioned herein can be carried out at an elevated temperature (e.g., at least about 50⁰C, at least about 100⁰C, at least about 200⁰C, or at least about 300⁰C). The temperature can be adjusted depending on various factors, such as the coating process and the coating composition used. For example, when preparing a layer containing inorganic nanoparticles, the nanoparticles can be sintered at a high temperature (e.g., at least about 300⁰C) to form interconnected nanoparticles. On the other hand, when a polymeric linking agent (e.g., poly(n-butyl titanate)) is added to the inorganic nanoparticles, the sintering process can be carried out at a lower temperature (e.g., below about 300⁰C). Optionally, photovoltaic cell 100 can include an encapsulation 170. In some embodiments, the encapsulation 170 can be formed by laminating a barrier film onto top electrode 160. The barrier film can be formed of a material the same as or different from that used in substrate 110. For example, encapsulation 170 can be formed of one or more suitable polymers, such as those used to form substrate 110 described above. In some embodiments, an additional encapsulation can be laminated onto substrate 110. In certain embodiments, encapsulation 170 can be disposed on top electrode 160 by physical vapor deposition (e.g., sputtering) or chemical vapor deposition (e.g., atomic layer deposition).

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

In some embodiments, photovoltaic cell 100 includes a cathode as a bottom electrode and an anode as a top electrode. In some embodiments, photovoltaic cell 100 can also include an anode as a bottom electrode and a cathode as a top electrode.

In some embodiments, photovoltaic cell 100 can include a hole blocking layer and a hole carrier shown in FIG. 1 in a reverse order. In other words, photovoltaic cell 100 can include its layers from the bottom to the top in the following sequence: a substrate 110, a bottom electrode 120 (e.g., including third layers 122, second layer 124, and first layer 126), an optional hole carrier layer 130, a photoactive layer 140, an optional hole blocking layer 150, a top electrode 160, and a substrate 170. In some embodiments, the multilayer electrodes described above can be used in a system in which two photovoltaic cells share a common electrode. Such a system is also known as a tandem photovoltaic cell. Examples of tandem photovoltaic cells are discussed in commonly-owned co-pending U.S. Patent Application Publication No. 2007- 0181179, 2007-0246094, and 2007-0272296 and U.S. Patent Application Serial No. 12/389,041, the contents of which are hereby incorporated by reference.

In some embodiments, multiple photovoltaic cells can be electrically connected to form a photovoltaic system. As an example, FIG. 2 is a schematic of a photovoltaic system 200 having a module 210 containing photovoltaic cells 220. Cells 220 are electrically connected in series, and system 200 is electrically connected to a load 230. As another example, FIG. 3 is a schematic of a photovoltaic system 300 having a module 310 that contains photovoltaic cells 320. Cells 320 are electrically connected in parallel, and system 300 is electrically connected to a load 330. In some embodiments, some (e.g., all) of the photovoltaic cells in a photovoltaic system can have one or more common substrates. In certain embodiments, some photovoltaic cells in a photovoltaic system are electrically connected in series, and some of the photovoltaic cells in the photovoltaic system are electrically connected in parallel.

While organic photovoltaic cells have been described, other photovoltaic cells can also be integrated with one of the multilayer electrodes described herein. Examples of such photovoltaic cells include dye sensitized photovoltaic cells, and inorganic photoactive cells with an 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 integrated with one of the multilayer electrodes described herein.

While photovoltaic cells have been described above, in some embodiments, the multilayer electrodes described herein can be used in other devices and systems. For example, the polymers 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 following example is illustrative and not intended to be limiting.

Example

A photovoltaic cell (i.e., photovoltaic cell (I)) containing a reflective bottom electrode was prepared as follows: A PET substrate coated with a first ITO layer having a thickness of about 30 nm, a silver layer having a thickness of about 14 nm, and a second ITO layer of about 30 nm was formed by sputtering each layer on the PET substrate. The article thus framed was cleaned by sonicating in acetone and isopropanol for 10 minutes, and then treated with UV/ozone for 10 minutes. A hole blocking layer was then blade coated onto the second ITO layer. A semiconductor blend of P3HT and Cβi-PCBM in a mixture of tetralene and xylene was blade coated onto the hole carrier layer at a speed of 40 mm/s at 65⁰C and was dried to form a photoactive layer. A hole carrier layer containing a doped thiophene was then coated onto the photoactive layer. The article was then annealed in a glove box at 140⁰C for 5 minutes. A layer of silver nanoparticles was blade-coated onto the hole blocking layer at a speed of 10 mm/s at 65°C, followed by annealing in a glove box at 120⁰C for 5 minutes. A photovoltaic cell was formed by evaporating a silver grid with a grid opening of 2 mm onto the silver nanoparticle layer. The photovoltaic cell thus formed containing the following components: PET/ITO (30 nm)-Ag (14 nm)-IT0 (30 nm)/hole blocking layer/P3HT:PCBM/hole carrier layer/Ag nanoparticles/ Ag grid.

Another photovoltaic cell (i.e., photovoltaic cell (2)) having the following components were prepared in a manner similar to that described above: PET/ITO (30 nm)-Ag (80 nm)-IT0 (30 nm)/hole blocking layer/P3HT:PCBM/hole carrier layer/Ag nanoparticles/ Ag grid.

The performance of the photovoltaic cells prepared above was measured under the AM 1.5 conditions. The results showed that photovoltaic cell (1) exhibited an efficiency of about 1.75%, a V_oc of about 0.48 V, a J_sc of about 8.2 mA/cm², and a fill factor of about 40 and photovoltaic cell (2) exhibited an efficiency of about 1.4%, a V_oc of about 0.44 V, a J_sc of about 10.2 mA/cm², and a fill factor of about 33. Unexpectedly, photovoltaic cell (2) exhibited a larger J_sc than photovoltaic cell (1). Other embodiments are in the claims.

Claims

WHAT IS CLAIMED IS:

1. An article, comprising: a first electrode; a second electrode comprising first, second, and third layers, the second layer between the first and third layers; and a photoactive layer between the first electrode and the first layer; wherein the first layer comprises a metal oxide, the second layer comprises a metal and having a thickness of at least about 20 nm, the third layer comprises a metal oxide or a metal, and the article is configured as a photovoltaic cell.

2. The article of claim 1 , wherein the second layer has a thickness of at least about 30 nm.

3. The article of claim 1, wherein the second layer comprises silver, aluminum, gold, titanium, or a mixture or alloy thereof.

4. The article of claim 1, wherein the second layer comprises two layers, one layer comprising a first metal and the other layer comprising a second metal different from the first metal.

5. The article of claim 1, wherein the second layer is non-transparent.

6. The article of claim 1, wherein the first layer comprises doped or undoped tin oxide, doped or undoped zinc oxide, doped or undoped titanium oxide, or a mixture thereof.

7. The article of claim 6, wherein the first layer comprises indium tin oxide, aluminum zinc oxide, zinc tin oxide, indium zinc oxide, titanium oxide doped with carbon or niobium, or a mixture thereof.

8. The article of claim 1 , wherein the first layer has a thickness of at least about 8 nm.

9. The article of claim 1 , wherein the first layer has a thickness of at most about 50 nm.

10. The article of claim 1, wherein the third layer comprises a metal oxide, the metal oxide comprising doped or undoped tin oxide, doped or undoped zinc oxide, doped or undoped titanium oxide, or a mixture thereof

11. The article of claim 10, wherein the metal oxide comprising indium tin oxide, aluminum zinc oxide, zinc tin oxide, indium zinc oxide, titanium oxide doped with carbon or niobium, or a mixture thereof.

12. The article of claim 1 , wherein the third layer comprises a metal, the metal in the third layer comprising titanium, nickel, chromium, or a mixture or alloy thereof.

13. The article of claim 1 , wherein the third layer has a thickness of at least about 5 nm.

14. The article of claim 1, wherein the photoactive layer comprises an electron donor material and electron acceptor material.

15. The article of claim 1 , further comprising a hole carrier between the first electrode and the photoactive layer.

16. The article of claim 1, further comprising a hole blocking layer between the first layer and the photoactive layer.

17. The article of claim 1, further comprising a substrate, the second electrode being between the substrate and the first electrode.

18. The article of claim 17, further comprising an encapsulation, the first electrode between the encapsulation and the second electrode.

19. An article, comprising: a first electrode; a second electrode comprising first, second, and third layers, the second layer between the first and third layers; and a photoactive layer between the first electrode and the first layer; wherein the first layer comprises a metal oxide, the second layer is non- transparent and comprises a metal, the third layer comprises a metal oxide or a metal, and the article is configured as a photovoltaic cell.

20. The article of claim 19, wherein the second layer has a thickness of at least about 30 nm.

21. The article of claim 19, wherein the second layer comprises silver, aluminum, gold, titanium, or a mixture or alloy thereof.

22. The article of claim 19, further comprising a substrate, the second electrode being between the substrate and the first electrode.

23. The article of claim 22, further comprising an encapsulation, the first electrode between the encapsulation and the second electrode.

24. An article, comprising: a first electrode; a second electrode comprising first, second, and third layers, the second layer between the first and third layers; and a photoactive layer between the first electrode and the first layer; wherein the first layer comprises a metal oxide, the second layer having a reflectance of at least about 80% at a wavelength between about 400-850 nm, the third layer comprises a metal oxide or a metal, and the article is configured as a photovoltaic cell.

25. The article of claim 24, wherein the second layer comprises a metal.

26. The article of claim 25, wherein the second layer comprises silver, aluminum, gold, titanium, or a mixture or alloy thereof.

27. The article of claim 24, wherein the second layer has a thickness of at least about 20 nm.

28. The article of claim 24, wherein the second layer is non-transparent.

29. The article of claim 24, further comprising a substrate, the second electrode being between the substrate and the first electrode.

30. The article of claim 29, further comprising an encapsulation, the first electrode between the encapsulation and the second electrode.

31. An article, comprising: a first layer comprising a metal oxide; a second layer comprising a metal and having a thickness of at least about 20 nm; and a third layer comprising a metal oxide or a metal; wherein the second layer is between the first and third layers and the article is configured as an electrode.

32. The article of claim 31 , wherein the second layer has a thickness of at least about 30 nm.

33. The article of claim 31 , wherein the second layer comprises silver, aluminum, gold, titanium, or a mixture or alloy thereof.

34. The article of claim 31 , wherein the second layer comprises two layers, one layer comprising a first metal and the other layer comprising a second metal different from the first metal.

35. The article of claim 31 , wherein the second layer is non-transparent.

36. The article of claim 31 , wherein the first layer comprises doped or undoped tin oxide, doped or undoped zinc oxide, doped or undoped titanium oxide, or a mixture thereof.

37. The article of claim 36, wherein the first layer comprises indium tin oxide, aluminum zinc oxide, zinc tin oxide, indium zinc oxide, titanium oxide doped with carbon or niobium, or a mixture thereof.

38. The article of claim 31 , wherein the first layer has a thickness of at least about 8 nm.

39. The article of claim 31 , wherein the first layer has a thickness of at most about 50 nm.

40. The article of claim 31 , wherein the third layer comprises a metal oxide, the metal oxide comprising doped or undoped tin oxide, doped or undoped zinc oxide, doped or undoped titanium oxide, or a mixture thereof

41. The article of claim 40, wherein the metal oxide comprising indium tin oxide, aluminum zinc oxide, zinc tin oxide, indium zinc oxide, titanium oxide doped with carbon or niobium, or a mixture thereof.

42. The article of claim 31 , wherein the third layer comprises a metal, the metal in the third layer comprising titanium, nickel, chromium, or a mixture or alloy thereof.

43. The article of claim 31 , wherein the third layer has a thickness of at least about 5 nm.

44. An article, comprising: a first layer comprising a metal oxide; a non-transparent second layer comprising a metal; and a third layer comprising a metal oxide or a metal; wherein the second layer is between the first and third layers and the article is configured as an electrode.

45. The article of claim 44, wherein the second layer has a thickness of at least about 30 nm.

46. The article of claim 45, wherein the second layer comprises silver, aluminum, gold, titanium, or a mixture or alloy thereof.

47. An article, comprising: a first layer comprising a metal oxide; a second layer having a reflectance of at least about 80% at a wavelength between about 400-850 nm; and a third layer comprising a metal oxide or a metal; wherein the second layer is between the first and third layers and the article is configured as an electrode.

48. The article of claim 47, wherein the second layer comprises a metal.

49. The article of claim 48, wherein the second layer comprises silver, aluminum, gold, titanium, or a mixture or alloy thereof.

50. The article of claim 47, wherein the second layer has a thickness of at least about 20 nm.

51. The article of claim 47, wherein the second layer is non-transparent.