WO2009009634A1

WO2009009634A1 - Photovoltaic cells with a diffractive layer

Info

Publication number: WO2009009634A1
Application number: PCT/US2008/069591
Authority: WO
Inventors: Christoph Brabec; Gilles Dennler; Karen Forberich; Markus Scharber; Kurt Hingerl
Original assignee: Konarka Technologies, Inc.
Priority date: 2007-07-11
Filing date: 2008-07-10
Publication date: 2009-01-15

Abstract

Photovoltaic cells with a diffractive layer, as well as related components, systems, and methods, are disclosed.

Description

Photovoltaic Cells With a Diffractive Layer

CROSS REFERENCE TO RELATED APPLICATION

Pursuant to 35 U. S. C. § 119(e), this application claims priority to U.S. Provisional Application Serial No. 60/949,110, filed July 11, 2007, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to photovoltaic cells with a diffraction layer, as well as related components, systems, and methods.

BACKGROUND

Photovoltaic cells arc 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. 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 semi conductive 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, this invention features an article that includes first and second electrodes, a photoactive layer between the first and second electrodes, and an optically diffractive layer. The photoactive layer includes an organic electron donor material and an organic electron acceptor material. The first electrode is between the optically diffractive layer and the photoactive layer. The article is configured as a photovoltaic cell.

In another aspect, this invention features an article that includes first and second electrodes, a photoactive layer between the first and second electrodes, and an optically diffractive layer between the first and second electrodes. The photoactive layer includes an organic electron donor material and an organic electron acceptor material. The article is configured as a photovoltaic cell.

In another aspect, this invention features an article that includes first and second electrodes, a photoactive layer between the first and second electrodes, a hole carrier layer, and a substrate supporting the first electrode, the second electrode, the photoactive layer, and the hole carrier layer. The hole carrier layer is between the photoactive layer and the first electrode. At least one of the first electrode, the second electrode, the photoactive layer, the hole carrier layer, and the substrate is configured as an optically diffractive layer. The article is configured as a photovoltaic cell. In another aspect, this invention features an article that includes first and second electrodes, a photoactive layer between the first and second electrodes, a hole carrier layer, a hole blocking layer, and a substrate supporting the first electrode, the second electrode, the photoactive layer, the hole carrier layer, and the hole blocking layer. The hole carrier layer is between the photoactive layer and the first electrode and the hole blocking layer is between the photoactive layer and the second electrode. At least one of the first electrode, the second electrode, the photoactive layer, the hole carrier layer, the hole blocking layer, and the substrate is configured as an optically diffractive layer. The article is configured as a photovoltaic cell.

In another aspect, this invention features an article that includes first and second electrodes, first photoactive layer between the first and second electrodes, second photoactive layer between the first photoactive layer and the second electrode, and an optically diffractive layer. The first electrode is between the optically diffractive layer and the first photoactive layer. The article is configured as a photovoltaic cell.

In another aspect, this invention features an article that includes an optically diffractive layer. The article is configured as a tandem photovoltaic cell. In another aspect, this invention features an article that includes a moth-eye layer. The article is configured as a photovoltaic cell or a tandem photovoltaic cell. Embodiments can include one or more of the following features. In some embodiments, the optically diffractive layer includes a plurality of one- dimensional periodic features, two-dimensional periodic features, or three-dimensional periodic features.

In some embodiments, the optically diffractive layer includes a plurality of spaced apart features. In certain embodiments, the features are periodically arranged along at least one direction. In some embodiments, the features have a period of at most about 2 μm or at least about 50 nm along the at least one direction.

In some embodiments, the features have an average height of at most about 2,000 nm (e.g., at most about 250 nm) or at least about 10 nm.

In some embodiments, the optically diffractive layer includes a polymer. In certain embodiments, the polymer includes polyethylene terephthalate. In certain embodiments, the optically diffractive layer includes a UV-curable polymer.

In some embodiments, the optically diffractive layer includes a filler. In certain embodiments, the filler includes a dye or a metal complex. In some embodiments, the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is either longer or shorter than the first wavelength.

In some embodiments, the optically diffractive layer includes a moth-eye layer. In some embodiments, the photoactive layer is configured as the optically diffractive layer. In some embodiments, one of the first and second electrodes is configured as the optically diffractive layer.

In some embodiments, the hole carrier layer is configured as the optically diffractive layer.

In some embodiments, the hole blocking layer is configured as the optically diffractive layer.

In some embodiments, the substrate is configured as the optically diffractive layer. In some embodiments, the article is configured as a tandem photovoltaic cell.

In some embodiments, the moth-eye layer includes a filler. In certain embodiments, the filler includes a dye or a metal complex. In some embodiments, the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is either larger or smaller than the first wavelength.

Embodiments can provide one or more of the following advantages.

The optically diffractive layer described above can enhance the light path in a photovoltaic cell, thereby increasing the light absorption and efficiency of the photovoltaic cell.

In some embodiments, different regions in the optically diffractive layer described above can have different periods. Without wishing to be bound by theory, it is believed that different periods can diffract light of different wavelengths so that light of a broader range of wavelength can be absorbed by photovoltaic cell. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will be apparent from the description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG l(a) is a cross-sectional view of an embodiment of a photovoltaic cell. FIG l(b) is a cross-sectional view of another embodiment of a photovoltaic cell.

FIG 2 is a cross-sectional view of a third embodiment of a photovoltaic cell.

FIG 3 is a cross-sectional view of an embodiment of a tandem photovoltaic cell.

FIG 4 is a cross-sectional view of another embodiment of a tandem photovoltaic cell. FIG 5 is a schematic of a system containing multiple photovoltaic cells electrically connected in series.

FIG 6 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

FlG. l(a) shows a cross-sectional view of a photovoltaic cell 100 that includes a diffractive layer 105, a substrate 110, an electrode 120, a hole carrier layer 130, a photoactive layer 140 (e.g., containing an organic electron acceptor material and an organic electron donor material), a hole blocking layer 150, an electrode 160, and a substrate 170. Optically diffractive layer 105 can include a substrate 106 and a layer 103. Layer 103 can include a plurality of spaced apart features 101 and portions 102 between features 101.

In general, during use, light can impinge on the surface of substrate 170, and pass through electrode 160 and hole blocking layer 150. The light then interacts with photoactive layer 140, causing electrons to be transferred from the electron donor material (e.g., poly(3-hexylthiophene) (P3HT)) to the electron acceptor material (e.g., C61-phenyl-butyric acid methyl ester (PCBM)). The electron acceptor material then transmits the electrons through hole blocking layer 150 to electrode 160, and the electron donor material transfers holes through hole carrier layer 130 to electrode 120. Electrodes 160 and 120 are in electrical connection via an external load so that electrons pass from electrode 160, through the load, and to electrode 120. The light not absorbed by photoactive layer 140 passes through hole carrier layer 130, electrode 120, and substrate 110, and diffracted at optically diffractive layer 105. The diffracted light can then be reflected back into photovoltaic cell 100 (e.g., at substrate 106) at various angles. The degrees of these angles depend on various factors, such as the height, period, or refractive index of the features in layer 105 or the wavelength of the incidental light. Without wishing to be bound by theory, it is believed that the diffracted light is generally reflected back to photovoltaic cell 100 at angles smaller than 90 degree and therefore can create a longer light path in photoactive layer 140. As such, optically diffractive layer 105 can improve the light absorption and efficiency of the photovoltaic cell.

Features 101 generally can be periodically arranged along at least one direction (e.g., at least two directions or at least three directions). For example, layer 103 can include a plurality of one-dimensional periodic features, two-dimensional periodic features (e.g., slits), or three-dimensional periodic features (e.g., features in a moth-eye structure). In some embodiments, layer 103 includes a plurality of non-periodic features. In some embodiments, certain regions of layer 103 can include periodic features and certain other regions can include non-periodic features.

In general, features 101 have a diffractive index different from that of portions 102 to induce diffraction. In some embodiments, features 101 have a diffractive index of at least about 1.0 (e.g., at least about 1.2, at least about 1 A, or at least about 1.6) or at most about 10 (e.g., at most about 6, at most about 4, or at most about 2). In some embodiments, portions 102 have a diffractive index of at least about 1.0 (e.g., at least about 1.2, at least about 1.4, or at least about 1.6) or at most about 10 (e.g., at most about 6, at most about 4, or at most about 2). When optically diffractive layer 105 includes a plurality of periodic features (e.g., features 101), the period of the features can typically be designed as desired. In some embodiments, the features have a period of at most about 2 μm (e.g., at most about 1 μm, at most about 500 nm, at most about 100 nm, or at most about 50 nm) or at least about 50 nm (e.g., at least about 100 nm, at least about 500 nm, at least about 1 μm, or at least about 2 μm) along the direction in which the features are periodically arranged. In some embodiments, different regions may have a different period. Without wishing to be bound by theory, it is believed that different periods can diffract light of different wavelengths so that light of a broader range of wavelength can be absorbed by photovoltaic cell 100. In some embodiments, features 101 have an average height of at most about

2,000 nm (e.g., at most about 1 ,000 nm, at most about 500 nm, at most about 250 nm, at most about 200 nm, at most about 150 nm, or at most about 100 nm) or at least about 10 nm (e.g., at least about 50 nm, at least about 100 nm, at least about 200 nm, or at least about 400 nm). In some embodiments, features 101 are formed of a polymer, such as polyethylene terephthalate. In certain embodiments, the features are formed of a UV- curable polymer (e.g., a UV-curable polyurethane). In some embodiments, substrate 106 are formed of the same material as that used to form features 101. In some embodiments, portions 102 are formed of the same material as that used to form substrate 110. In some embodiments, optically diffractive layer 105 includes a filler. Generally, the material used to form the filler can vary as desired. In some embodiments, the filler can increase the Young's Modulus of optically diffractive layer 105. An exemplary material that can be used to prepare such a filler includes SiO₂. In some embodiments, the filler can serve as a gas barrier. For example, the filler can include a getter material that reacts with a gas to reduce its diffusion into photovoltaic cell 100. Exemplary materials that can be used to prepare such a getter material include CaO or CaO₂. In some embodiments, the filler can include an UV-absorbing material. Examples of UV- absorbing materials include dyes, such as rhodamine, coumarin, or a metal complex (e.g., a Europium complex, an Iridium complex, or a Nickel complex). In some embodiment, the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength. The second wavelength can be longer or shorter than the first wavelength. In certain embodiments, light having the second wavelength is not absorbed by the filler or optically diffractive layer 105. In some embodiments, the filler is formed of an up- converting material or a down-converting material, such as a metal complex (e.g., a Europium complex, an Iridium complex, or a Nickel complex). In some embodiments, the filler is in the form of nanoparticles. In some embodiment, the filler is formed of a transparent material.

In some embodiments, optically diffractive layer 105 can also function as an anti- reflective layer. In some embodiments, optically diffractive layer 105 is a moth-eye layer. Without wishing to be bound by theory, it is believed that the reflection of an air- glass interface at normal incidence can be reduced to as low as 0.5% by using a moth-eye layer. A moth-eye film can be prepared by nano-imprinting a UV-curable lacquer onto a transparent polymeric film. The moth-eye film can then be attached to photovoltaic cell by using an adhesive to form optically diffractive layer 105. Without wishing to be bound by theory, it is believed that a moth-eye film can be prepared on a large scale by interference lithography and subsequent replication, and therefore are suitable for large- scale industrial production.

In some embodiments, optically diffractive layer 105 can be prepared by attaching a pre- formed optically diffractive film to photovoltaic cell 100 using an adhesive. In some embodiments, the adhesive has a suitable refraction index to allow reduction of optical reflection between optically diffractive layer 105 and substrate 110. In some embodiments, photovoltaic cell 100 can include more than one optically diffractive layer (e.g., two, three, four, five, six, or seven optically diffractive layers).

FIG l(b) shows a cross-sectional view of a photovoltaic cell 100 similar to that shown in FIG. l(b) except that diffractive layer 105 includes periodically arranged non- transparent portions 101 and opening portions 102 along one direction. In general non- transparent portions 101 can be formed of the same materials, or can have the same characteristics, as those shown in FIG l(a).

Turning to other components of photovoltaic cell 100, substrate 110 is generally formed of a transparent material. As referred to herein, a transparent material is a material that, at the thickness used in a photovoltaic cell 100, 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. 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 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 5,00 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).

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., poly(3,4-ethelynedioxythiophene) (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 commonly owned co-pending U.S. Patent Application Publication Nos. 20040187911 and 20060090791, the contents of which are hereby incorporated by reference.

Hole carrier layer 130 is generally formed of a material that, at the thickness used in photovoltaic cell 100, transports holes to electrode 120 and substantially blocks the transport of electrons to electrode 120. Examples of materials from which layer 130 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 130 can include combinations of hole carrier materials. In some embodiments, hole carrier layer 130 can include a dopant used in combination with a hole carrier material. Examples of dopants include poly(styrene- sulfonate)s (PSSs), polymeric sulfonic acids, or fluorinated polymers (e.g., fluorinated ion exchange polymers). In general, the thickness of hole carrier layer 130 (i.e., the distance between the surface of hole carrier layer 130 in contact with photoactive layer 140 and the surface of electrode 120 in contact with hole carrier layer 130) can be varied as desired. Typically, the thickness of hole carrier layer 130 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 130 is from about 0.01 micron to about 0.5 micron.

In some embodiments, photoactive layer 140 contains an electron acceptor material (e.g., an organic electron acceptor material) and an electron donor material (e.g., an organic electron donor material).

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 or polymers containing CF₃ groups), and combinations thereof. In some embodiments, a combination of electron acceptor materials can be used in photoactive layer 140.

In general, the term "fullerene" includes both unsubstituted (e.g., C₆o) or substituted fullerenes (e.g., PCBM). Examples of fullerenes are described in, for example, commonly-owned co-pending U.S. Patent Application Serial No. 11/141,979, the contents of which are hereby incorporated by reference.

Examples of electron donor materials include conjugated polymers, such as polythiophenes, polyanilines, polycarbazoles, 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)), polycyclopentadithiophenes, and copolymers thereof. In certain embodiments, a combination of electron donor materials can be used in photoactive layer 140.

In some embodiments, the electron donor materials or the electron acceptor materials can include a polymer having a first comonomer repeat unit and a second comonomer repeat unit different from the first comonomer repeat unit. The first comonomer repeat unit can include a cyclopentadithiophene moiety, a silacyclopentadithiophene moiety, a cyclopentadithiazole moiety, a thiazolothiazole moiety, a thiazole moiety, a benzothiadiazole moiety, a thiophene oxide moiety, a cyclopentadithiophene oxide moiety, a polythiadiazoloquinoxaline moiety, a benzoisothiazole moiety, a benzothiazole moiety, a thienothiophene moiety, a thienothiophene oxide moiety, a dithienothiophene moiety, a dithienothiophene oxide moiety, or a tetrahydroisoindoles moiety.

In some embodiments, the first comonomer repeat unit includes a cyclopentadithiophene moiety. In some embodiments, the cyclopentadithiophene moiety is substituted with at least one substituent selected from the group consisting of C₁-C₂₀ alkyl, Q -C₂₀ alkoxy, C₃-C₂₀ CyClOaIlCyI, C_1-C₂₀ heterocycloalkyl, aryl, heteroaryl, halo, CN, OR, C(O)R, C(O)OR, and SO₂R; R being H, C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or Ci -C₂₀ heterocycloalkyl. For example, the cyclopentadithiophene moiety can be substituted with hexyl, 2-ethylhexyl, or 3,7- dimethyloctyl. In certain embodiments, the cyclopentadithiophene moiety is substituted at 4-position. In some embodiments, the first comonomer repeat unit can include a cyclopentadithiophene moiety of formula (1):

In formula (1), each of Ri, R₂, R₃, or R₄, independently, is H, C_1-C₂₀ alkyl, C_1-C₂₀ alkoxy, C₃-C₂₀ cycloalkyl, C₁-C₂₀ heterocycloalkyl, aryl, heteroaryl, halo, CN, OR, C(O)R, C(O)OR, or SO₂R; R being H, C₁-C₂₀ alkyl, C_1-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₁-C₂₀ heterocycloalkyl. For example, each of Rj and R₂, independently, can be hexyl, 2-ethylhexyl, or 3,7-dimethyloctyl.

The second comonomer repeat unit can include a benzothiadiazole moiety, a thiadiazoloquinoxaline moiety, a cyclopentadithiophene oxide moiety, a benzoisothiazole moiety, a benzothiazole moiety, a thiophene oxide moiety, a thienothiophene moiety, a thienothiophene oxide moiety, a dithienothiophene moiety, a dithienothiophene oxide moiety, a tetrahydroisoindole moiety, a fluorene moiety, a silole moiety, a cyclopentadithiophene moiety, a fluorenone moiety, a thiazole moiety, a selenophene moiety, a thiazolothiazole moiety, a cyclopentadithiazole moiety, a naphthothiadiazole moiety, a thienopyrazinc moiety, a silacyclopentadithiophene moiety, an oxazole moiety, an imidazole moiety, a pyrimidine moiety, a benzoxazole moiety, or a benzimidazole moiety. In some embodiments, the second comonomer repeat unit is a 3,4-benzo-1,2,5- thiadiazole moiety.

In some embodiments, the second comonomer repeat unit can include a benzothiadiazole moiety of formula (2), a thiadiazoloquinoxaline moiety of formula (3), a cyclopentadithiophene dioxide moiety of formula (4), a cyclopentadithiophene monoxide moiety of formula (5), a benzoisothiazole moiety of formula (6), a benzothiazole moiety of formula (7), a thiophene dioxide moiety of formula (8), a cyclopentadithiophene dioxide moiety of formula (9), a cyclopentadithiophene tetraoxide moiety of formula (10), a thienothiophene moiety of formula (11), a thienothiophene tetraoxide moiety of formula (12), a dithienothiophene moiety of formula (13), a dithienothiophene dioxide moiety of formula (14), a dithienothiophene tetraoxide moiety of formula (15), a tetrahydroisoindole moiety of formula (16), a thienothiophene dioxide moiety of formula (17), a dithienothiophene dioxide moiety of formula (18), a fluorene moiety of formula (19), a silole moiety of formula (20), a cyclopentadithiophene 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), a silacyclopentadithiophene moiety of formula (29), an oxazolc moiety of formula (30), an imidazole moiety of formula (31), a pyrimidine moiety of formula (32), a benzoxazole moiety of formula (33), or a benzimidazole moiety of formula (34):

In the above formulas, each of X and Y, independently, is CH₂, O, or S; each of R₅ and R₆, independently, is 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 C₁-C₂₀ heterocycloalkyl; and each of R₇ and R₈, independently, is H, C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, or C₃-C₂₀ heterocycloalkyl. In some embodiments, the second comonomer repeat unit includes a benzothiadiazole moiety of formula (2), in which each of R₅ and R₆ is H.

The second comonomer repeat unit can include at least three thiophene moieties. In some embodiments, at least one of the thiophene moieties is substituted with at least one substituent selected from the group consisting of C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C₃-C₂₀ cycloalkyl, and C₃-C₂oheterocycloalkyl. In certain embodiments, the second comonomer repeat unit includes five thiophene moieties.

The polymer can further include a third comonomer repeat unit that contains a thiophene moiety or a fluorene moiety. In some embodiments, the thiophene or fluorene moiety is substituted with at least one substituent selected from the group consisting of C₁-C₂₀ alkyl, C₁-C₂₀ alkoxy, aryl, heteroaryl, C3-C20 cycloalkyl, and C3-C20 heterocycloalkyl.

In some embodiments, the polymer can be formed by any combination of the first, second, and third comonomer repeat units. In certain embodiments, the polymer can be a homopolymer containing any of the first, second, and third comonomer repeat units.

In some embodiments, the polymer can be

or

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

The monomers for preparing the polymers mentioned herein may 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.

The polymers described above can be prepared by methods known in the art, such as those described in commonly owned co-pending U.S. Application No 1 1/601,374, the contents of which arc hereby incorporated by reference. For example, a copolymer can be prepared by a cross-coupling reaction between one or more comonomers containing two alkylstannyl groups and one or more comonomers containing two halo groups 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 in the presence of a transition metal catalyst. The comonomers can be prepared by the methods know in the art, such as those described in U.S. Patent Application Serial No. 1 1/486,536, Coppo ct al., Mcicromolecules 2003, 36, 2705-271 1, and Kurt et al., J. Heterocycl. Chem. 1970, 6, 629, the contents of which are hereby incorporated by reference.

Without wishing to be bound by theory, it is believed that an advantage of the polymers described above is that their absorption wavelengths shift toward the red and near IR regions (e.g., 650 - 800 nm) of the electromagnetic spectrum, which is not accessible by most other conventional polymers. When such a polymer is incorporated into a photovoltaic cell together with a conventional polymer, it enables the cell to absorb the light in this region of the spectrum, thereby increasing the current and efficiency of the cell. In some embodiments, photoactive layer 140 can contain an inorganic semiconductor material. In some embodiments, the inorganic semiconductor material includes group IV semiconductor materials, group III-V semiconductor materials, group H-VI semiconductor materials, chalcogen semiconductor materials, and semiconductor metal oxides. Examples of group IV semiconductor materials include amorphous silicon, crystalline silicon (e.g., microcrystalline silicon or polycrystalline silicon), and germanium. Examples of group IH-V semiconductor materials include gallium arsenide and indium phosphide. Examples of group II- VI semiconductor materials include cadmium selenide and cadmium telluride. Examples of chalcogen semiconductor materials include copper indium selenide (CIS) and copper indium gallium selenide (CIGS). Examples of semiconductor metal oxides include copper oxides, titanium oxides, zinc oxides, tungsten oxides, molybdenum oxides, strontium copper oxides, or strontium titanium oxides. In certain embodiments, the bandgap of the semiconductor can be adjusted via doping. In some embodiments, the inorganic semiconductor material can include inorganic nanoparticles. Generally, photoactive layer 140 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 140 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) thick and/or at most about one micron (e.g., at most about 0.5 micron, at most about 0.4 micron) thick. In some embodiments, photoactive layer 140 is from about 0.1 micron to about 0.2 micron thick.

In general, photovoltaic cell 100 can include an optional hole blocking layer 150. Hole blocking layer 150 is generally formed of a material that, at the thickness used in photovoltaic cell 100, transports electrons to electrode 160 and substantially blocks the transport of holes to electrode 160. Examples of materials from which the hole blocking layer can be formed include LiF and metal oxides (e.g., zinc oxide, titanium oxide). In some embodiments, hole blocking layer 150 is formed of an electron donating compound, such as a nitrogen-containing compound, a phosphorus-containing compound, and/or a sulfur-containing compound. Examples of such electron donating compounds are described in commonly owned co-pending U.S. Provisional Application Serial No. 60/926,459, the contents of which are hereby incorporated by reference. In some embodiments, photovoltaic cell 100 includes a layer containing an electron donating compound (e.g., a nitrogen-containing compound) in addition to hole blocking layer 150. In certain embodiments, this additional layer is disposed between photoactive layer 140 and hole blocking layer 150.

Typically, hole blocking layer 150 is at least 0.02 micron (e.g., at least about 0.03 micron, at least about 0.04 micron, 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, at most about 0.1 micron) thick. Without wishing to be bound by theory, it is believed that when hole blocking layer 150 is formed of metal oxides (such as zinc oxide or titanium oxide), an additional layer containing an electron donating compound (e.g., a nitrogen-containing compound) between photoactive layer 140 and hole blocking layer 150 can facilitate the formation of ohmic contact between the metal oxide and photoactive layer 140 without UV light exposure, thereby reducing damage to photovoltaic cell 100 resulted from such exposure. In some embodiments, hole blocking layer 150 can be omitted from photovoltaic cell 100.

Electrode 160 is generally formed of an electrically conductive material, such as one or more of the electrically conductive materials described above. 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.

Substrate 170 can be identical to or different from substrate 110. In some embodiments, substrate 170 can be formed of one or more suitable polymers, such as those described above. In general, each of hole carrier layer 130, photoactive layer 140, hole blocking layer 150, and additional layer containing an electron donating compound described above can be prepared by a liquid-based coating 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 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., layer 130, 140, or 150) includes inorganic semiconductor nanoparticles, the liquid-based coating process can be carried out by (1) mixing the nanoparticles (e.g., CIS or CIGS 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 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., layer 130, 140, or 150) 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., 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 can be carried out at an elevated temperature (e.g., at least about 50°C, at least about 100°C, at least about 200°C, or at least about 300°C). 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 300°C) 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 300°C).

FlG. 2 shows a cross-sectional view of a photovoltaic cell 200 that includes a substrate 210, an electrode 220, a hole carrier layer 230, a photoactive layer 240, a hole blocking layer 250, an electrode 260, and a substrate 270. Photoactive layer 240 is configured to form an optically diffractive layer (e.g., a layer containing a plurality of periodically arranged slits). In some embodiment, the optically diffractive layer in photoactive layer 240 can be formed of the same material, or have the same characteristics, as optically diffractive layer 105 described above. In some embodiments, photoactive layer 240, which also functions as an optically diffractive layer, can be formed by a hot-stamping method as follows. After photoactive materials (e.g., P3HT and PCBM) are applied onto hole carrier layer 230, they can be heated to a temperature (e.g., a glass transition temperature) at which they are at least partially melt. A die (e.g., a hot stamping die) having a diffractive pattern machined into its surface can then be brought into contact with the photoactive materials to form a diffractivc pattern. After the photoactive materials are cooled, the die can be removed to form a fixed diffractive pattern in the photoactive materials.

In some embodiments, photoactive layer 240 can be formed by a nano-imprinting lithographic method as follows. After photoactive materials (e.g., P3HT and PCBM) are applied onto hole carrier layer 230, a mask having a diffractive pattern is brought into contact with the photoactive materials while they are still wet or partially melt to form a diffractive pattern. The photoactive materials can then be cross-linked by exposing to UV light. The die can then be removed to form a fixed diffractive pattern in the photoactive materials. In some embodiments, photoactive layer 240 having an optically diffractive layer can be formed by a dry-etching method as follows. After photoactive materials (e.g., P3HT and PCBM) are applied onto hole carrier layer 230 and dried, a mask having a diffractive pattern is brought into contact with the photoactive materials. The exposed photoactive materials can then be partially etched to form a diffractive pattern. While FlG 2 shows that photoactive layer 240 has an optically diffractive layer, in some embodiments, other layers in photovoltaic cell 200 can also be configured to form an optically diffractive layer. For example, each of substrate 210, electrode 220, hole carrier layer 230, hole blocking layer 250, electrode 260, and substrate 270 can be configured to form an optically diffractive layer. In some embodiments, photovoltaic cell 200 can include more than one optically diffractive layer (e.g., two, three, four, five, six, or seven optically diffractive layers). In some embodiments, hole blocking layer 250 can be omitted from photovoltaic cell 200.

FIG 3 shows a tandem photovoltaic cell 300 having two semi-cells 302 and 304. Semi-cell 302 includes an electrode 320, a hole carrier layer 330, a first photoactive layer 340, and a recombination layer 342. Semi-cell 304 includes a recombination layer 342, a second photoactive layer 344, a hole blocking layer 350, and an electrode 360. An optically diffractive layer 305 is attached to a substrate 310, which supports semi-cells 302 and 302. Optically diffractive layer 305 includes a substrate 306 and a layer 303. Layer 303 can include a plurality of spaced apart features 301 and portions 302 between features 301. An external load is connected to photovoltaic cell 300 via electrodes 320 and 360. Depending on the production process and the desired device architecture, the current flow in a semi-cell can be reversed by changing the electron/hole conductivity of a certain layer (e.g., changing hole carrier layer 230 to a hole blocking layer). By doing so, a tandem cell can be designed such that the semi-cells in the tandem cells can be electrically interconnected either in series or in parallel. In general, optically diffractive layer 305 can be formed of the same materials, or have the same physical characteristics (e.g., the same thickness or electron injection properties), as noted above regarding optically diffractive layer 105.

A recombination layer refers to a layer in a tandem cell where the electrons generated from a first semi-cell recombine with the holes generated from a second semi- cell. Recombination layer 342 typically includes a p-type semiconductor material and an n-type semiconductor material. In general, n-type semiconductor materials selectively transport electrons and p-type semiconductor materials selectively transport holes. As a result, electrons generated from the first semi-cell recombine with holes generated from the second semi-cell at the interface of the n-type and p-type semiconductor materials. In some embodiments, the p-type semiconductor material includes a polymer and/or a metal oxide. Examples p-type semiconductor polymers include polythiophenes (e.g., PEDOT), polyanilines, polycarbazoles, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, polycyclopentadithiophenes, polysilacyclopentadithiophenes, polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles, polybenzothiadiazoles, poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxaline, polybenzoisofhiazole, polybenzothiazole, polythienothiophene, poly(thienothiophene oxide), polydithienothiophene, poly(dithienothiophene oxide)s, polytetrahydroisoindoles, and copolymers thereof. The metal oxide can be an intrinsic p-type semiconductor (e.g., copper oxides, strontium copper oxides, or strontium titanium oxides) or a metal oxide that forms a p-type semiconductor after doping with a dopant (e.g., p-doped zinc oxides or p-doped titanium oxides). Examples of dopants includes salts or acids of fluoride, chloride, bromide, and iodide. In some embodiments, the metal oxide can be used in the form of nanoparticles. In some embodiments, the n-type semiconductor material includes a metal oxide, such as a titanium oxide, a zinc oxide, a tungsten oxide, a molybdenum oxide, and a combination thereof. The metal oxide can be used in the form of nanoparticles. In some embodiments, the n-type semiconductor material includes a material selected from the group consisting of fullerenes, inorganic nanoparticles, oxadiazoles, discotic liquid crystals, carbon nanorods, inorganic nanorods, polymers containing CN groups, polymers containing CF₃ groups, and combinations thereof.

In some embodiments, the p-type and n-type semiconductor materials are blended into one layer. In certain embodiments, the recombination layer includes two layers, one layer including the p-type semiconductor material and the other layer including the n-type semiconductor material. In such embodiments, recombination layer 342 can also include a layer of mixed n-type and p-type semiconductor material at the interface of the two layers.

In some embodiments, recombination layer 342 includes at least about 30 wt% (e.g., at least about 40 wt% or at least about 50 wt%) and/or at most about 70 wt% (e.g., at most about 60 wt% or at most about 50 wt%) of the p-type semiconductor material. In some embodiments, recombination layer 342 includes at least about 30 wt% (e.g., at least about 40 wt% or at least about 50 wt%) and/or at most about 70 wt% (e.g., at most about 60 wt% or at most about 50 wt%) of the n-type semiconductor material.

Recombination layer 342 generally has a sufficient thickness so that the layers underneath are protected from any solvent applied onto recombination layer 342. In some embodiments, recombination layer 342 can have a thickness at least about 10 ran (e.g., at least about 20 nm, at least about 50 nm, or at least about 100 nm) and/or at most about 500 nm (e.g., at most about 200 nm, at most about 150 nm, or at most about 100 nm).

In general, recombination layer 342 is substantially transparent. For example, at the thickness used in a tandem photovoltaic cell 300, recombination layer 342 can transmit 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., from about 350 nm to about 1,000 nm) used during operation of the photovoltaic cell. Recombination layer 342 generally has a sufficiently low surface resistivity. In some embodiments, recombination layer 342 has a resistivity of at most about 1 x 10⁶ ohm/square (e.g., at most about 5 x 10⁵ ohm/square, at most about 2 x 10⁵ ohm/square, or at most about 1 x 10⁵ ohm/square).

Without wishing to be bound by theory, it is believed that recombination layer 342 can be considered as a common electrode between two semi-cells (e.g., one including cathode 320, hole carrier layer 330, photoactive layer 340, and recombination layer 342, and the other include recombination layer 342, photoactive layer 344, hole blocking layer 350, and anode 360) in photovoltaic cell 300. In some embodiments, recombination layer 342 can include an electrically conductive mesh material, such as those described above. An electrically conductive mesh material can provide a selective contact of the same polarity (cither p-type or n-type) to the semi-cells and provide a highly conductive but transparent layer to transport electrons to a load.

In some embodiments, recombination layer 342 can be prepared by applying a blend of an n-type semiconductor material and a p-type semiconductor material on a photoactive layer. For example, an n-type semiconductor and a p-type semiconductor can be first dispersed and/or dissolved in a solvent together to form a dispersion or a solution, which can then be coated on a photoactive layer to form a recombination layer.

In some embodiments, a two-layer recombination layer can be prepared by applying a layer of an n-type semiconductor material and a layer of a p-type semiconductor material separately. For example, when titanium oxide nanoparticles arc used as an n-type semiconductor material, a layer of titanium oxide nanoparticles can be formed by (1) dispersing a precursor (e.g., a titanium salt) in a solvent (e.g., an anhydrous alcohol) to form a dispersion, (2) coating the dispersion on a photoactive layer, (3) hydrolyzing the dispersion to form a titanium oxide layer, and (4) drying the titanium oxide layer. As another example, when a polymer (e.g., PEDOT) is used a p-typc semiconductor, a polymer layer can be formed by first dissolving the polymer in a solvent (e.g., an anhydrous alcohol) to form a solution and then coating the solution on a photoactive layer.

Other components in tandem cell 300 can be identical to the corresponding components described with respect to photovoltaic cell 100 In some embodiments, the semi-cells in a tandem cell are electrically interconnected in series. When connected in series, in general, the layers can be in the order shown in FIG. 3. In certain embodiments, the semi-cells in a tandem cell are electrically interconnected in parallel. When interconnected in parallel, a tandem cell having two semi-cells can include the following layers: a first cathode, a first hole carrier layer, a first photoactive layer, a first hole blocking layer (which can serve as an anode), a second hole blocking layer (which can serve as an anode), a second photoactive layer, a second hole carrier layer, and a second cathode. In such embodiments, the first and second hole blocking layers correspond to the recombination layer and can be either two separate layers or one single layer. In case the conductivity of the first and second hole blocking layer is not sufficient, an additional layer (e.g., an electrically conductive mesh layer) providing the required conductivity may be inserted.

In some embodiments, a tandem cell can include more than two semi-cells (e.g., three, four, five, six, seven, eight, nine, ten or more semi-cells). In certain embodiments, some semi-cells can be electrically interconnected in series and some semi-cells can be electrically interconnected in parallel. FIG. 4 shows a tandem photovoltaic cell 400 having two semi-cells 402 and 404.

Semi-cell 402 includes an electrode 420, a hole carrier layer 430, a first photoactive layer 440, and a recombination layer 442. Semi-cell 404 includes a recombination layer 442, a second photoactive layer 444, a hole blocking layer 450, and an electrode 460. First photoactive layer 440 is configured to form an optically diffractive layer (e.g., a moth-eye layer). In some embodiment, the optically diffractive layer in first photoactive layer 440 can be formed of the same material, or have the same characteristics, as optically diffractive layer 240 described above. In some embodiments, the optically diffractive layer in first photoactive layer 440 can be prepared by the same methods as those used to prepare optically diffractive layer 240. While certain embodiments have been disclosed, other embodiments are also possible.

In some embodiments, photovoltaic cell 100 can include a cathode as a bottom electrode (e.g., electrode 120) and an anode as a top electrode (e.g., electrode 160). In certain 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 the layers shown in FIG 1 in a reverse order. In other words, photovoltaic cell 100 can include these layers from the bottom to the top in the following sequence: a substrate 170, an electrode 160, a hole blocking layer 150, a photoactive layer 140, a hole carrier layer 130, an electrode 120, a substrate 110, and an optically diffractive layer 105. In some embodiments, photovoltaic cell 100 can include the layers shown in FlG 1 except optically diffractive layer 105 in a reverse order. In other words, photovoltaic cell 100 can include these layers from the bottom to the top in the following sequence: an optically diffractive layer 105, substrate 170, an electrode 160, a hole blocking layer 150, a photoactive layer 140, a hole carrier layer 130, an electrode 120, and a substrate 110.

In some embodiments, the hole carrier layer shown in FIGs. 1-4 can be replaced with a hole blocking layer and the hole blocking layer shown in these figures can be replaced with a hole carrier layer.

In some embodiments, multiple photovoltaic cells can be electrically connected to form a photovoltaic system. As an example, FIG. 5 is a schematic of a photovoltaic system 500 having a module 510 containing photovoltaic cells 520. Cells 520 are electrically connected in series, and system 500 is electrically connected to a load 530. As another example, FIG 6 is a schematic of a photovoltaic system 600 having a module 610 that contains photovoltaic cells 620. Cells 620 are electrically connected in parallel, and system 600 is electrically connected to a load 630. 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 photovoltaic cells have been described above, in some embodiments, the optically diffractive layer described herein can be used in other devices and systems. For example, the optically diffractive 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 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: Fabrication of a Photovoltaic Cell Having a Diffraction Layer

A photovoltaic cell with a diffraction layer and a photovoltaic cell without such a layer were fabricated as follows: An indium tin oxide coated glass substrate was first coated with a thin layer of PEDOT: PSS by doctor blading and dried to form a hole carrier layer. A solution of P3HT:PCBM was then deposited on the hole carrier layer by the same technique and dried to form a photoactive layer. A polymeric stamp was pressed against the photoactive layer with a pressure of 50 Newtons for 7 minutes while the device was heated to 100°C to form two dimensional diffraction features with a period of about 460 nm within the photoactive layer. Finally, 100 nm of aluminum was thermally evaporated onto the photoactive layer through a shadow mask to form the top electrode of the first photovoltaic cell. A second photovoltaic cell was fabricated in the same manner as that described above except that the photoactive layer was not treated to form diffraction features.

The cross section of the first photovoltaic cell thus formed was observed under a scanning electron microscope. The results showed that two-dimensional features were embossed in the photoactive layer and covered by the top electrode.

External quantum efficiency (EQE) of the first and second photovoltaic cells were measured. The results showed that the first photovoltaic cell exhibited enhanced EQE compared to that of the second photovoltaic cell. Other embodiments are in the claims.

Claims

WHAT IS CLAIMED IS:

1. An article, comprising: first and second electrodes; a photoactive layer between the first and second electrodes, the photoactive layer comprising an organic electron donor material and an organic electron acceptor material; and an optically diffiractive layer, the first electrode being between the optically diffractive layer and the photoactive layer; wherein the article is configured as a photovoltaic cell.

2. The article of claim 1, wherein the optically diffractive layer comprises a plurality of one-dimensional periodic features, two-dimensional periodic features, or three-dimensional periodic features.

3. The article of claim 1, wherein the optically diffractive layer comprises a plurality of spaced apart features.

4. The article of claim 3, wherein the features are periodically arranged along at least one direction.

5. The article of claim 4, wherein the features have a period of at most about 2 μm along the at least one direction.

6. The article of claim 4, wherein the features have a period of at least about 50 nm along the at least one direction.

7. The article of claim 4, wherein the features have an average height of at most about 2,000 nm.

8. The article of claim 4, wherein the features have an average height of at most about 250 ran.

9. The article of claim 4, wherein the features have an average height of at least about 10 nm.

10. The article of claim 1, wherein the optically diffractive layer comprises a polymer.

1 1. The article of claim 10, wherein the polymer comprises polyethylene terephthalate.

12. The article of claim 10, wherein the optically diffractive layer comprises a UV-curable polymer.

13. The article of claim 1, wherein the optically diffractive layer comprises a filler.

14. The article of claim 13, wherein the filler comprises a dye or a metal complex.

15. The article of claim 13, wherein the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is longer than the first wavelength.

16. The article of claim 13, wherein the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is shorter than the first wavelength.

17. The article of claim 1, wherein the optically diffractive layer comprises a moth-eye layer.

18. An article, comprising: first and second electrodes; a photoactive layer between the first and second electrodes, the photoactive layer comprising an organic electron donor material and an organic electron acceptor material; and an optically diffractive layer between the first and second electrodes; wherein the article is configured as a photovoltaic cell.

19. An article, comprising: first and second electrodes; a photoactive layer between the first and second electrodes; a hole carrier layer, the hole carrier layer being between the photoactive layer and the first electrode; and a substrate supporting the first electrode, the second electrode, the photoactive layer, and the hole carrier layer; wherein at least one of the first electrode, the second electrode, the photoactive layer, the hole carrier layer, and the substrate is configured as an optically diffractive layer, and the article is configured as a photovoltaic cell.

20. The article of claim 19, wherein the photoactive layer is configured as the optically diffractive layer.

21. The article of claim 19, wherein one of the first and second electrodes is configured as the optically diffractive layer.

22. The article of claim 19, wherein the hole carrier layer is configured as the optically diffractive layer.

23. The article of claim 19, wherein the substrate is configured as the optically diffractive layer.

24. The article of claim 19, wherein the optically diffractive layer comprises a plurality of one-dimensional periodic features, two-dimensional periodic features, or three-dimensional periodic features.

25. The article of claim 19, wherein the optically diffractive layer comprises a plurality of spaced apart features.

26. The article of claim 25, wherein the features are periodically arranged along at least one direction.

27. The article of claim 26, wherein the features have a period of at most about 2 μm along the at least one direction.

28. The article of claim 26, wherein the features have a period of at least about 50 nm along the at least one direction.

29. The article of claim 26, wherein the features have an average height of at most about 2,000 nm.

30. The article of claim 26, wherein the features have an average height of at most about 250 nm.

31. The article of claim 26, wherein the features have an average height of at least about 10 nm.

32. The article of claim 19, wherein the optically diffractive layer comprises a moth-eye layer.

33. The article of claim 32, wherein the optically diffractive layer comprises a filler.

34. The article of claim 33, wherein the filler comprises a dye or a metal complex.

35. The article of claim 33, wherein the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is longer than the first wavelength.

36. The article of claim 33, wherein the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is shorter than the first wavelength.

37. An article, comprising: first and second electrodes; a photoactive layer between the first and second electrodes; a hole carrier layer, the hole carrier layer being between the photoactive layer and the first electrode; a hole blocking layer, the hole blocking layer being between the photoactive layer and the second electrode; and a substrate supporting the first electrode, the second electrode, the photoactive layer, the hole carrier layer, and the hole blocking layer; wherein at least one of the first electrode, the second electrode, the photoactive layer, the hole carrier layer, the hole blocking layer, and the substrate is configured as an optically diffractive layer, and the article is configured as a photovoltaic cell.

38. An article, comprising: first and second electrodes; first photoactive layer between the first and second electrodes; second photoactive layer between the first photoactive layer and the second electrode; and an optically diffractive layer, the first electrode being between the optically diffractive layer and the first photoactive layer; wherein the article is configured as a photovoltaic cell.

39. The article of claim 38, wherein the article is configured as a tandem photovoltaic cell.

40. The article of claim 38, wherein the optically diffractive layer comprises a moth-eye layer.

41. An article comprising an optically diffractive layer, wherein the article is configured as a tandem photovoltaic cell.

42. The article of claim 41, wherein the article comprises a photoactive layer and the photoactive layer is configured as the optically diffractive layer.

43. The article of claim 41, wherein the article comprises first and second electrodes and at least one of the first and second electrodes is configured as the optically diffractive layer.

44. The article of claim 41, wherein the article comprises a hole carrier layer and the hole carrier layer is configured as the optically diffractive layer.

45. The article of claim 41 , wherein the article comprises a hole blocking layer and the hole blocking layer is configured as the optically diffractive layer.

46. The article of claim 41 , wherein the article comprises a substrate supporting the photovoltaic cell and the substrate is configured as the optically diffractive layer.

47. An article comprising a moth-eye layer, wherein the article is configured as a photovoltaic cell.

48. The article of claim 47, wherein the moth-eye layer comprises a filler.

49. The article of claim 48, wherein the filler comprises a dye or a metal complex.

50. The article of claim 48, wherein the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is longer than the first wavelength.

51. The article of claim 48, wherein the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is shorter than the first wavelength.

52. An article comprising a moth-eye layer, wherein the article is configured as a tandem photovoltaic cell.

53. The article of claim 52, wherein the moth-eye layer comprises a filler.

54. The article of claim 53, wherein the filler comprises a dye or a metal complex.

55. The article of claim 53, wherein the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is longer than the first wavelength.

56. The article of claim 53, wherein the filler absorbs light at least at a first wavelength and emits light at least at a second wavelength, in which the second wavelength is shorter than the first wavelength.