WO2011160021A2

WO2011160021A2 - Fullerene derivatives

Info

Publication number: WO2011160021A2
Application number: PCT/US2011/040884
Authority: WO
Inventors: Kap-Soo Cheon; David Waller
Original assignee: Konarka Technologies, Inc.
Priority date: 2010-06-17
Filing date: 2011-06-17
Publication date: 2011-12-22
Also published as: WO2011160021A3

Abstract

Fullerene derivatives, as well as related compositions, photovoltaic cells, systems, and methods, are disclosed.

Description

Fullerene Derivatives

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Serial No.

61/355,751 , filed June 17, 2010, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to fullerene derivatives, as well as related compositions, photovoltaic cells, systems, and methods.

BACKGROUND

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

SUMMARY

This disclosure is based on the unexpected discovery that a fullerene derivative containing a pendant tetramethoxyphenyl group possesses a higher lowest unoccupied molecular orbital (LUMO) than a conventional fullerene (e.g., PCBM), while still maintaining a relative high electron mobility. As a result, such a fullerene derivative can be used to prepare a photovoltaic cell with an improved open circuit voltage (V_oc) and therefore improved energy conversion efficiency.

In one aspect, this disclosure features a fullerene derivative that includes a fullerene group and a pendant group bonded to the fullerene. The pendant group includes a phenyl group substituted with four substituents. Each substituent is, independently, a Ci-Cio alkoxy group. In another aspect, this disclosure features an article that includes first and second electrodes, and a photoactive layer between the first and second electrodes. The photoactive layer includes the fullerene derivative described above. The article is configured as a photovoltaic cell.

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

In some embodiments, each substituent is, independen -C4 alkoxy group

(e.g., methoxy). For example, the pendant group can include

In some embodiments, the pendant group further includes an ester group (e.g.,

COOCH₃). The ester group and the phenyl group can be spaced apart by at least two

(e.g., at least four) carbon atoms.

In some embodiments, the fullerene group includes from 50 to 250 carbon atoms.

For example, the fullerene group can be formed from C₆o, C70, C76, C78, Cg₂, Cg₄, or Cg₂.

In some embodiments, the fullerene derivative includes a C₆i- tetramethoxyphenyl-butyric acid methyl ester or a C7i-tetramethoxyphenyl-butyric acid methyl ester.

In some embodiments, the fullerene derivative includes two fullerene groups bonded to the pendant group. In such embodiments, the pendant group can include two phenyl groups, in which each phenyl group is substituted with four substituents and each substituent is, independently, a C1-C10 alkoxy group.

In some embodiments, the photoactive layer can further include an electron donor material. For example, the electron donor material can include a polythiophene, a polyaniline, a polycarbazole, a polyvinylcarbazole, a polyphenylene, a

polyphenylvinylene, a polysilane, a polythienylenevinylene, a polyisothianaphthanene, a polycyclopentadithiophene, a polysilacyclopentadithiophene, a polycyclopentadithiazole, a polythiazolothiazole, a polythiazole, a polybenzothiadiazole, a poly(thiophene oxide), a poly(cyclopentadithiophene oxide), a polythiadiazoloquinoxaline, a

polybenzoisothiazole, a polybenzothiazole, a polythienothiophene, a poly(thienothiophene oxide), a polydithienothiophene, a poly(dithienothiophene oxide), a polyfluorene, a polytetrahydroisoindole, or a copolymer thereof.

Other features and advantages of the fullerene derivatives and photovoltaic cells described in this disclosure 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. 2a is an embodiment of a fullerene derivative.

FIG. 2b is another embodiment of a fullerene derivative.

FIG. 3 is a schematic of a system containing multiple photovoltaic cells electrically connected in series.

FIG. 4 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 an exemplary photovoltaic cell 100 that includes a substrate 110, an electrode 120, an optional hole blocking layer 130, a photoactive layer 140 (e.g., containing an electron acceptor material and an electron donor material), an optional hole carrier layer 150, an electrode 160, and a substrate 170.

In general, during use, light can impinge on the surface of substrate 110, and passes through substrate 110, electrode 120, and optional hole blocking layer 130. The light then interacts with photoactive layer 140, causing electrons to be transferred from the electron donor material (e.g., a conjugated polymer) to the electron acceptor material (e.g., a substituted fullerene). The electron acceptor material then transmits the electrons through optional hole blocking layer 130 to electrode 120, and the electron donor material transfers holes through optional hole carrier layer 150 to electrode 160.

Electrodes 120 and 160 are in electrical connection via an external load so that electrons pass from electrode 120 through the load to electrode 160. In some embodiments, photoactive layer 140 includes an electron donor material (e.g., an organic electron donor material) and an electron acceptor material (e.g., an organic electron acceptor material).

The electron acceptor materials in photoactive layer 140 can include fullerenes. In some embodiments, photoactive layer 140 can include a fuUerene derivative containing at least one fuUerene group (e.g., at least two fuUerene groups) and a pendant group that is bonded to the at least one fuUerene group. An example of such a fuUerene derivative is shown in FIG. 2a.

Referring to FIG. 2a, a fuUerene derivative 200 includes a fuUerene group 202 and a pendant group 204. In general, the pendant group includes a phenyl group substituted with four substituents, each of which is, independently, a Ci-Cio alkoxy group (e.g., a Ci- C₄ alkoxy group). For example, each substituent can be a methoxy group. In the embodiment shown in FIG. 2a, the pendant group includes a 2,3,4,6-tetramethoxyphenyl

In some embodiments, the pendant group can include a

2,3,4,5-tetramethoxyphenyl group or a 2,3,5,6-tetramethoxyphenyl group.

Without wishing to be bound by theory, it is believed that a fuUerene derivative containing a pendant tetramethoxyphenyl group possesses an unexpectedly higher LUMO than a conventional fuUerene (e.g., a phenyl-C 61 -butyric acid methyl ester (PCBM-C60) or a phenyl-C 61 -butyric acid methyl ester (PCBM-C70)), while still maintaining a relative high electron mobility. As a result, such a fuUerene derivative can be used to prepare a photovoltaic cell with an improved open circuit voltage (V_oc) and therefore improved energy conversion efficiency.

Without wishing to be bound by theory, it is believed that a fuUerene derivative containing a pendant tetramethoxyphenyl group possesses an unexpectedly higher LUMO than a fuUerene derivative containing a pendant trimethoxyphenyl group (i.e., a phenyl group substituted with three methoxy groups) or a fuUerene derivative containing a pendant pentamethoxyphenyl group (i.e., a phenyl group substituted with five methoxy groups), while exhibiting electron mobility similar to these two fuUerene derivatives. As a result, a photovoltaic cell containing such a fuUerene derivative exhibits an improved open circuit voltage and therefore improved energy conversion efficiency.

Without wishing to be bound by theory, it is believed that, although a fuUerene derivative containing a pendant tetramethoxyphenyl group possesses a LUMO similar to that of a PCBM containing two pendant phenylbutyric acid methyl ester groups (i.e., bis- PCBM; see, e.g., Yun et al, J. Mater. Chem. 2010, 20, 7710-7714), the former fuUerene derivative possesses higher electron mobility than the latter fuUerene derivative. More specifically, without wishing to be bound by theory, it is believed that the random distribution of the two pendant groups on the latter fuUerene derivative leads to poor packing association between molecules and therefore results in lower electron mobility.

Typically, the pendant group can further include an ester group (e.g., COOCH₃). In some embodiments, the ester group and the phenyl group are spaced apart by at least two (e.g., at least three or four) carbon atoms in the pendant group.

In general, the fuUerene used to form the fuUerene group disclosed herein can include from 50 to 250 carbon atoms. Exemplary fullerenes include C₆o, C70, C76, C78, Cg2, Cg4, and C92. In the embodiment shown in FIG. 2a, the fuUerene group is formed from C₆o-

In some embodiments, the fuUerene derivative disclosed herein can be a tetramethoxyphenyl-C 61 -butyric acid methyl ester (i.e., the fuUerene derivative shown in FIG. 2a; also referred herein as tetramethoxy-PCBM-C60) or a tetramethoxyphenyl-C71- butyric acid methyl ester (also referred herein as tetramethoxy-PCBM-C70).

In some embodiments, the fuUerene derivative can include two fuUerene groups bonded to the pendant group. In such embodiments, the pendant group can include two phenyl groups, in which each phenyl group is substituted with four substituents and each substituent is, independently, a C1-C10 alkoxy group (e.g., a C1-C4 alkoxy group). For example, each substituent can be a methoxy group.

An example of a fuUerene derivative containing two fuUerene groups is shown in FIG. 2b. Referring to FIG. 2b, a fuUerene derivative 201 includes two fuUerene groups 202 and a pendant group 204. Pendant group 204 includes two 2,3,4,6- tetramethoxyphenyl groups bonded with two ester groups (i.e., -C(O)O-) via alkylene groups. In some embodiments, one of the ester groups and one of the phenyl groups are spaced apart by at least two (e.g., at least three or four) carbon atoms in the pendant group.

In some embodiments, the fuUerene derivative can include two identical fuUerene groups (e.g., both formed from C₆o or C₇₀) or include two different fullerenes (e.g., one formed from C₆o and one formed from C₇₀). In some embodiments, the fuUerene derivative can include two identical tetramethoxy substituted phenyl groups (e.g., two 2,3,4,6-tetramethoxyphenyl groups) or two different tetramethoxy substituted phenyl groups (e.g., one 2,3,4,6-tetramethoxyphenyl group and one 2,3,5,6-tetramethoxyphenyl group).

In some embodiments, the fuUerene derivative described herein can have a LUMO value of at least about -3.65 eV (e.g., at least about -3.63 eV, at least about -3.61 eV, at least about -3.60 eV, at least about -3.58 eV, or at least about -3.55 eV) as determined by cyclic voltammetry by using the method described in Example 5.

In some embodiments, the fuUerene derivative described herein can have an electron mobility value of at least about 1 x 10^"3 cm²/V s (e.g., at least about 2x 10^"3 cm²/Vs, at least about 3x 10^"3 cm²/Vs, at least about 4x 10^"3 cm²/Vs, at least about 5x 10^"3

2 3 2 2 2

cm /V s, at least about 7x 10^" cm /Vs, at least about 1 x 10^" cm /Vs) as determined by using the method described in Example 5.

In some embodiments, photoactive layer 140 can include a combination of the fuUerene derivatives described herein (e.g., a mixture of tetramethoxy-PCBM-C60 and tetramethoxy-PCBM-C70). In some embodiments, photoactive layer 140 can include one of the fuUerene derivatives described herein and a fuUerene known in the art, such as an unsubstituted fuUerene (e.g., C₆o, C₇o, C₇₆, C₇g, Cg₂, Cg₄, and Cg₂) or a substituted fuUerene (e.g., PCBM-C60 or PCBM-C70).

The fuUerene derivatives described herein can be made by methods known in the art. As an example, a fuUerene derivative containing one fuUerene group can be prepared by the following general procedure: A tetramethoxybenzene (e.g., 1,2,3,5- tetramethoxybenzene) can first react with a compound containing both an acyl chloride group and an ester group (e.g., methyl 5-chloro-5-oxopentanoate) to form a first intermediate compound in which a phenyl group is directly bonded to a carbonyl group. The first intermediate compound can then react with p-tosylhydrazide to form a second intermediate compound, in which the carbonyl group directly bonded to the phenyl group is converted to a p-tosylhydrazone group. The second intermediate compound can then react with a fuUerene (e.g., C₆o or C₇₀) to form the desired compound (e.g., tetramethoxy- PCBM-C60). As another example, a fuUerene derivative containing two fuUerene groups can be prepared by the following general procedure: The ester group in the just- mentioned fuUerene derivative (e.g., tetramethoxy-PCBM-C60) can first be converted to an acyl chloride group. The compound thus formed can then react with a linking compound (e.g., an ethylene glycol) in a 2: 1 molar ratio to form a fuUerene derivative containing two fuUerene groups (e.g., tetramethoxy-PCBM-C60 dimer). Detailed procedures of preparing exemplary fuUerene derivatives are described in Examples 1-4.

In general, the electron donor material in photoactive layer 140 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). A homopolymer mentioned herein refers to a polymer that includes monomer repeat units with the same chemical structure. A copolymer mentioned herein refers to a polymer that includes at least two (e.g., three or four) monomer repeat units with different chemical structures. In general, the polymers suitable for use as an electron donor material are photovoltaically active.

In some embodiments, the electron donor material can include a polythiophene, a polyaniline, a polycarbazole, a polyvinylcarbazole, a polyphenylene, a

Examples of other polymers suitable for use in photoactive layer 140 have been described in, e.g., U.S. Patent Nos. 7,781,673 and 7,772,485, PCT Application No.

PCT/US2011/020227, and U.S. Application Publication Nos. 2010-0224252, 2010- 0032018, 2008-0121281, 2008-0087324, 2007-0020526, and 2007-0017571.

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 which, 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 500 megaPascals). In certain embodiments, different regions of substrate 110 can be flexible, semi-rigid, or inflexible (e.g., one or more regions flexible and one or more different regions semi-rigid, one or more regions flexible and one or more different regions inflexible).

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

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

Electrode 120 is generally formed of an electrically conductive material.

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

In some embodiments, electrode 120 can include a mesh electrode. Examples of mesh electrodes are described in co-pending U.S. Patent Application Publication Nos. 2004-0187911 and 2006-0090791.

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

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 photovoltaic 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, metal oxides (e.g., zinc oxide, titanium oxide), and amines (e.g., primary, secondary, or tertiary amines). Examples of amines suitable for use in a hole blocking layer have been described, for example, in co-pending U.S. Application

Publication No. 2008-0264488.

Without wishing to be bound by theory, it is believed that when photovoltaic cell 100 includes a hole blocking layer made of amines, the hole blocking layer can facilitate the formation of ohmic contact between photoactive layer 140 and electrode 120 without being exposed to UV light, thereby reducing damage to photovoltaic cell 100 resulted from UV exposure.

Typically, hole blocking layer 130 is at least about 0.02 micron (e.g., at least about 0.03 micron, at least about 0.04 micron, or at least about 0.05 micron) thick and/or at most about 0.5 micron (e.g., at most about 0.4 micron, at most about 0.3 micron, at most about 0.2 micron, or at most about 0.1 micron) thick.

Optionally, photovoltaic cell 100 can include a hole carrier layer 150. Hole carrier layer 150 is generally formed of a material that, at the thickness used in photovoltaic cell 100, transports holes to electrode 160 and substantially blocks the transport of electrons to electrode 160. 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 150 can include a dopant used in combination with one of the just-mentioned materials. Examples of dopants include poly(styrene-sulfonate)s, polymeric sulfonic acids, or fluorinated polymers (e.g., fluorinated ion exchange polymers).

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

In some embodiments, the materials that can be used to form hole carrier layer 150 include carbon allotropes (e.g., carbon nanotubes). The carbon allotropes can be embedded in a polymer binder. In some embodiments, the hole carrier materials can be in the form of nanoparticles. The nanoparticles can have any suitable shape, such as a spherical, cylindrical, or rod-like shape.

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

In general, the thickness of hole carrier layer 150 (i.e., the distance between the surface of hole carrier layer 150 in contact with photoactive layer 140 and the surface of electrode 160 in contact with hole carrier layer 150) can be varied as desired. Typically, the thickness of hole carrier layer 150 is at least about 0.01 micron (e.g., at least about 0.05 micron, at least about 0.1 micron, at least about 0.2 micron, at least about 0.3 micron, or at least about 0.5 micron) and/or at most about five microns (e.g., at most about three microns, at most about two microns, or at most about one micron). In some embodiments, the thickness of hole carrier layer 150 is from about 0.01 micron to about 0.5 micron.

Electrode 160 is generally formed of an electrically conductive material, such as one or more of the electrically conductive materials described above with respect to electrode 120. In some embodiments, electrode 160 is formed of a combination of electrically conductive materials. In certain embodiments, electrode 160 can be formed of a mesh electrode.

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 the polymers used in substrate 110 described above.

In general, the methods of preparing each layer in photovoltaic cell 100 can vary as desired. In some embodiments, a layer can be prepared by a liquid-based coating process. In certain embodiments, a layer can be prepared via a gas phase-based coating process, such as chemical or physical vapor deposition processes.

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

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

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

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

In some embodiments, photovoltaic cell 100 can be prepared in a continuous manufacturing process, such as a roll-to-roll process, thereby significantly reducing the manufacturing cost. Examples of roll-to-roll processes have been described in, for example, commonly-owned co-pending U.S. Application Publication No. 2005-0263179.

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 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, an optional hole carrier layer 150, a photoactive layer 140, an optional hole blocking layer 130, an electrode 120, and a substrate 110.

In some embodiments, one of substrates 110 and 170 can be transparent. In other embodiments, both of substrates 110 and 170 can be transparent.

In some embodiments, the fullerene derivatives described above can be used as an electron acceptor material in a system in which two photovoltaic cells share a common electrode. Such a system is also known as tandem photovoltaic cell. Exemplary tandem photovoltaic cells have been described in, e.g., co-pending U.S. Application Publication No. 2009-0211633, 2007-0181179, 2007-0246094, or 2007-0272296.

In some embodiments, multiple photovoltaic cells can be electrically connected to form a photovoltaic system. As an example, FIG. 3 is a schematic of a photovoltaic system 300 having a module 310 containing a plurality of photovoltaic cells 320. Cells 320 are electrically connected in series, and system 300 is electrically connected to a load 330. As another example, FIG. 4 is a schematic of a photovoltaic system 400 having a module 410 that contains a plurality of photovoltaic cells 420. Cells 420 are electrically connected in parallel, and system 400 is electrically connected to a load 430. In some embodiments, some (e.g., all) of the photovoltaic cells in a photovoltaic system can be disposed on 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 fullerene derivatives described herein. Examples of such photovoltaic cells include dye sensitized photovoltaic cells and inorganic photoactive cells with a photoactive material formed of amorphous silicon, cadmium selenide, cadmium telluride, copper indium selenide, and copper indium gallium selenide. In some embodiments, a hybrid photovoltaic cell can be integrated with one of the fullerene derivatives described herein.

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

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

The following examples are illustrative and not intended to be limiting.

Example 1 : Synthesis of PCBM-C60 substituted with four methoxy groups

(Tetramethoxy-PCBM-C60)

Tetramethoxy-PCBM-C60

5-(2,3,4,6-tetramethoxyphenyl)-5-oxo-pentanoic acid methyl ester (1): To a suspension of A1C1₃ (4.1 g; 30.8 mmol) in dichloromethane (100 ml) on an ice-bath was added 1,2,3,5-tetramethoxybenzene (4.0 g; 20. 2 mmol) at once. After the mixture was stirred for 5 - 10 minutes in a cooling bath, methyl 5-chloro-5-oxopentanoate (3.3 g, 20.2 mmol) was added at once. After the cooling bath was removed, the reaction mixture was stirred at 40°C for 20 hours. The reaction mixture was poured onto the iced water. After the mixture was extracted from dichloromethane, the organic layer was washed with water and brine, dried over MgS0₄, and concentrated in vacuo. The crude product was purified by flash chromatography (eluent: 1.5% methanol in dichloromethane) to give compound 1. Yield: 3.2 g (49.1%). 1H NMR (CDC1₃): 2.0 (2H, qt), 2.4 (2H, t), 2.8 (2H, t), 3.7 (3H, s), 3.80 (3H, s), 3.82 (3H, s), 3.90 (3H, s), 3.91 (3H, s), 6.3 (1H, s).

5-(2,3,4,6-tetramethoxyphenyl)-5-(p-tosylhydrazono)-pentanoic acid methyl ester (2): Compound 1 (3.2 g, 9.82 mmol) and p-tosylhydrazide (2. 19 g, 11.8 mmol) were dissolved in methanol (70 ml). The mixture was refluxed overnight. After the mixture was cooled to room temperature, it was concentrated in vacuo. The crude product was recrystallized from methanol to give compound 2 as a white solid. Yield: 3.88 g (80%). 1H NMR (CDC1₃): 1.7 (2H, qt), 2.2 (2H, t), 2.4-2.5 (5H, m), 3.6 (3H, s),

3.64 (3H, s), 3.66 (3H, s), 3.80 (3H, s), 3.90 (3H, s), 6.25 (1H, s), 7.28-7.34 (4H, m), 7.85 (2H, d).

Tetramethoxy-PCBM-C60: Compound 2 (1.03 g, 2.08 mmol) was dissolved in pyridine (20 ml). Sodium methoxide (0.16 g, 2.89 mmol) was then added to the above solution. After the mixture was stirred at room temperature for 20 - 30 minutes, a solution of C₆o (1.0 g, 1.39 mmol) dissolved in o-dichlorobenzene (80 ml) was added to the mixture via a dropping funnel for 30 minutes. After the addition, the mixture was stirred at 70°C overnight. The reaction mixture was concentrated to 20 - 30 ml, which was loaded onto a column for purification (eluent: toluene). After the purified product was re-dissolved in 50 ml o-dichlorobenzene, the mixture was stirred at 180°C overnight. The mixture was concentrated to 10-20 mL, which was poured onto methanol (200 ml) to form a precipitate. The precipitate was filtered to give tetramethoxy-PCBM-C60 as a brown solid. Yield: 0.57g (40%). 1H NMR (CDC1₃): 2.25 (2H, t), 2.60 (2H, t), 2.9 (2H, m), 3.7 (3H, s), 3.85 (3H, s), 3.90 (3H, s), 4.0 (3H, s), 4.2 (3H, s), 6.40 (1H, s). Example 2: Synthesis of PCBM-C70 substituted with four methoxy groups (tetramethoxy-PCBM-C70)

Tetramethoxy-PCBM-C70 was prepared by using the method described in Example 1 except that C₆o was replaced with C70.

Example 3 : Synthesis of tetramethoxy-PCBM-C60 dimer

Tetramethoxy-PCBM-C60 Tetramethoxy-PCBM-C60-OH Tetramethoxy-PCBM-C60-Cl

Tetramethoxy-PCBM-C60-OH: Tetramethoxy-PCBM-C60 (1.0 g, 0.97 mmol) was dissolved in o-dichlorobenzene (80 ml). Acetic acid (100 ml) and cone. HC1 (40 ml) were added to the above solution. The reaction mixture was refluxed at 150°C overnight and then concentrated in vacuum. After the residue thus obtained was suspended in methanol, the mixture was centrifuged and dried to give Tetramethoxy-PCBM-C60-OH as a brown solid (0.94 g, 95% yield).

Tetramethoxy-PCBM-C60-Cl: Tetramthoxy-PCBM-C60-OH (0.94 g, 0.93 mmol) was dispersed in dry carbon disulfide (300 ml). Thionyl chloride (80 ml) was added to the above mixture at room temperature. The mixture thus obtained was refluxed for 20 hours and was concentrated in vacuum. The residue was suspended in t- butylmethyl ether, filtered, and dried under vacuum to give 0.9 g (94% yield) of tetramethoxy-PCBM-C60-Cl.

Tetramethoxy-PCBM-C60 dimer: Tetramethoxy-PCBM-C60-Cl (0.9 g, 0.87 mmol) was dispersed in dry toluene (150 ml). Dry pyridine (10 ml) and ethylene glycol (27.0 mg, 0.44 mmol) were then added to the above mixture. The reaction mixture was stirred at room temperature for 20 hours. After the solvents were removed in vacuum, the residue was purified by flash chromatography (eluent: toluene) to give tetramethoxy- PCBM-C60 dimer as a brown solid (0.30 g).

Example 4: Synthesis of tetramethoxy-PCBM-C70 dimer

Tetramethoxy-PCBM-C70 dimer was prepared by using the method described in Example 3 except that the starting material, tetramethoxy-PCBM-C60, was replaced with tetramethoxy-PCBM-C70.

Example 5 : Comparisons of physical properties of various fullerene derivatives

The following six fullerene derivatives were prepared and test for their LUMO and electron mobility properties.

Trimethoxy-PCBM-C60 Tetramethoxy-PCBM-C60 Pentamethoxy-PCBM-C60 The LUMO values of the above fullerene derivatives were measured by cyclic voltammetry using the method described in Kooistra et al, Organic Letters 2007, 9(4), 551-554. In particular, ferrocene was used as an external standard, whose vacuum level was taken as -4.8 eV. The working electrode was a platinum disk, the counter electrode was a Pt wire, and the reference electrode was Ag/AgN0₃ (0.01M)/0.1M

nBu₄NPF₆/CH₃CN.

The electron mobility properties of the above fullerene derivatives were measured by using the following procedure: A field effect transistor (FET) substrate was cleaned by successive ultrasonic treatment in acetone and isopropyl alcohol, following by drying with N₂ and vacuum. A commercially available hexamethyldisilazane (HMDS) solution (MicroChem Corporation, Newton, MA) containing about 20% HMDS and 80% acetate and having a purity over 97% was applied on the FET substrate for improving the contact between the FET substrate and the fullerene derivative layer. The spin-speed for the HMDS was 500 RPM for 9 seconds and 1000 RPM for another 40 seconds. The FET device was prepared by spin-coating the fullerene derivative layer at room temperature in a nitrogen glove box. The film was then dried in vacuum for at least 30 minutes before measuring its electron mobility. A set of SUSS PHI 00 Miniature ProbeHead was used in the glove box as source and drain contacts. Agilent 4155C was used to measure the I-V curves from the FET device. The annealing process for the FET device was also completed in the nitrogen glove box on a hotplate.

The test results are summarized in Table 1 below.

Table 1

* The LUMO values shown in this table are values obtained from fullerene derivative solutions. Example 6: Fabrication of photovoltaic cells based on PCBM-C60 and tetramethoxy- PCBM-C60

Photovoltaic cells containing PCBM-C60 and tetramethoxy-PCBM-C60 were prepared as follows: A PEDOT:PSS (1 :5 in isopropyl alcohol) solution was deposited onto an ozone treated ITO glass substrate by blade coating at 65°C at a speed of 5 mm/s with a 500 μιη slit. A photoactive layer solution was blade coated onto this layer at a speed of 20 mm/s at 70°C. The photoactive layer solution included a 1.5 wt% o- dichlorobenzene solution containing polymer 1 shown below and a fullerene derivative (i.e., PCBM-C60 or tetramethoxy-PCBM-C60). The solution was prepared by dissolving polymer 1 and the fullerene derivative in o-dichlorobenzene overnight at 110°C and cooled to 70°C. To this photoactive layer was then deposited a top electrode containing LiF -0.7 nm/Al -100 nm. The device was then annealed for 4 minutes at 140°C.

Polymer 1 has the following chemical structure:

Six photovoltaic cells were prepared using the above procedure: Photovoltaic cell

(1) included a photoactive layer containing PCBM-C60 only and having a thickness of about 720 nm. Photovoltaic cells (2), (3), and (4) included a photoactive layer containing tetramethoxy-PCBM-C60 and polymer 1 in a weight ratio of 1 :2 and having a thickness of about 670 nm, about 800 nm, and about 880 nm, respectively. Photovoltaic cells (5) and (6) included a photoactive layer containing tetramethoxy-PCBM-C60 C60 and polymer 1 in a weight ratio of 1 : 1.5 and having a thickness of about 640 nm and about 800 nm, respectively.

Efficiencies of the photovoltaic cells prepared above were measured by using a 1000 Watt Newport-Oriel AAA certified solar simulator operating at a level of 100 mW/cm². Solar simulator illumination intensity was calibrated using a standard silicon photovoltaic with a protective KG5 filter calibrated at the National Renewable Energy Laboratory (NREL). The test results of the photovoltaic cells above are summarized in Table 2 below.

Table 2

Photovoltaic cells (1), (2), and (5) had photoactive layers with similar thickness.

As shown in Table 2, photovoltaic cells (2) and (5) (which contained tetramethoxy- PCBM-C60) exhibited significantly higher I_sc, V_oc, and efficiency than photovoltaic cell (1) (which contained PCBM-C60).

Other embodiments are in the claims.

Claims

WHAT IS CLAIMED IS:

1. A fuUerene derivative, comprising a fuUerene group and a pendant group bonded to the fuUerene group,

wherein the pendant group comprises a phenyl group substituted with four substituents, and each substituent is, independently, a Ci-Cio alkoxy group.

2. The fuUerene derivative of claim 1, wherein each substituent is, independently, a C₁-C4 alkoxy group.

3. The fuUerene derivative of claim 2, wherein each substituent is methoxy.

4. The fuUerene derivative of claim 3, wherein the pendant group

comprises

5. The fuUerene derivative of any of claims 1-4, wherein the pendant group further comprises an ester group.

6. The fuUerene derivative of claim 5, wherein the ester group comprises COOCH3.

7. The fuUerene derivative of claim 5 or 6, wherein the ester group and the phenyl group are spaced apart by at least two carbon atoms.

8. The fuUerene derivative of claim 5 or 6, wherein the ester group and the phenyl group are spaced apart by at least four carbon atoms.

9. The fullerene derivative of any of claims 1-8, wherein the fullerene group comprises from 50 to 250 carbon atoms.

10. The fullerene derivative of claim 9, wherein the fullerene group is formed from C₆o, C70, C76, C78, C82, Cs4, or C92.

11. The fullerene derivative of any of claims 1-10, wherein the fullerene derivative comprises a tetramethoxyphenyl-C61 -butyric acid methyl ester or a tetramethoxyphenyl-C71 -butyric acid methyl ester.

12. The fullerene derivative of claim 1, wherein the fullerene derivative comprises two fullerene groups bonded to the pendant group.

13. The fullerene derivative of claim 12, wherein the pendant group comprises two phenyl groups, each phenyl group substituted with four substituents and each substituent being, independently, a C₁-C₁₀ alkoxy group.

14. An article, comprising:

first and second electrodes; and

a photoactive layer between the first and second electrodes, the photoactive layer comprising a fullerene derivative that comprises a fullerene group and a pendant group bonded to the fullerene group, the pendant group comprising a phenyl group substituted with four substituents, each substituent being, independently, a C₁-C₁₀ alkoxy group. wherein the article is configured as a photovoltaic cell.

15. The article of claim 14, wherein each substituent is, independently, a Ci- C₄ alkoxy group.

16. The article of claim 15, wherein each substituent is methoxy.

17. The article of claim 16, wherein the pendant group comprises

18. The article of any of claims 14-17, wherein the pendant group further comprises an ester group.

19. The article of claim 18, wherein the ester group comprises COOCH₃.

20. The article of claim 18 or 19, wherein the ester group and the phenyl group are spaced apart by at least two carbon atoms.

21. The article of claim 18 or 19, wherein the ester group and the phenyl group are spaced apart by at least four carbon atoms.

22. The article of any of claims 14-21, wherein the fuUerene group comprises from 50 to 250 carbon atoms.

23. The article of claim 22, wherein the fuUerene group is formed from C₆o, C70, C₇6, C₇8, C82, C84, or C92.

24. The article of any of claims 14-23, wherein the fuUerene derivative comprises a C6i-tetramethoxyphenyl-butyric acid methyl ester or a C₇₁- tetramethoxyphenyl-butyric acid methyl ester.

25. The article of claim 14, wherein the fuUerene derivative comprises two fuUerene groups bonded to the pendant group.

26. The article of claim 26, wherein the pendant group comprises two phenyl groups, each phenyl group substituted with four substituents and each substituent being, independently, a Ci-Cio alkoxy group.

27. The article of any of claims 14-26, wherein the photoactive layer further comprises an electron donor material.

28. The article of claim 27, wherein the electron donor material comprises a polythiophene, a polyaniline, a polycarbazole, a polyvinylcarbazole, a polyphenylene, a polyphenylvinylene, a polysilane, a polythienylenevinylene, a polyisothianaphthanene, a polycyclopentadithiophene, a polysilacyclopentadithiophene, a polycyclopentadithiazole, a polythiazolothiazole, a polythiazole, a polybenzothiadiazole, a poly(thiophene oxide), a poly(cyclopentadithiophene oxide), a polythiadiazoloquinoxaline, a

polybenzoisothiazole, a polybenzothiazole, a polythienothiophene, a

poly(thienothiophene oxide), a polydithienothiophene, a poly(dithienothiophene oxide), a polyfluorene, a polytetrahydroisoindole, or a copolymer thereof.