WO2008051205A2

WO2008051205A2 - Carbon nanotube use in solar cell applications

Info

Publication number: WO2008051205A2
Application number: PCT/US2006/040206
Authority: WO
Inventors: Christopher Weeks; Paul J. Glatkowski
Original assignee: Eikos, Inc.
Priority date: 2005-10-14
Filing date: 2006-10-16
Publication date: 2008-05-02
Also published as: WO2008051205A3

Abstract

A photovoltaic cell having a first electrode, a second electrode, and an active layer wherein carbon nanotubes are present in the active layer. The first electrode is a transparent conductive coating comprising carbon nanotubes consisting of single- walled carbon nanotubes, multi-walled carbon nanotubes, mixtures or combinations thereof. The carbon nanotubes may act as an electron donor or an electron acceptor.

Description

CARBON NANOTUBE USE IN SOLAR CELL APPLICATIONS Background

1. Field of the Invention

This invention is directed to photovoltaic devices containing carbon nanotubes.

2. Description of the Background

Optoelectronic devices are electrical-to-optical or optical-to-electrical transducers, or instruments that use such devices in their operation. Such devices can be light-emitting devices that produce radiation as a result of an applied electric voltage/current or photovoltaic devices that produce an electric voltage/current as a result of applied radiation. Photovoltaic (PV) cells/devices typically employ a substrate or carrier (wafer, film, foil, etc.), a bottom electrode, one or more layers of PV material and a top electrode. Either the bottom electrode or the top electrode may be the anode and the opposite the cathode and vice versa. PV materials and layer structures are, broadly speaking, materials that create a voltage and current between the two electrodes when the PV material/layer structure is exposed to light.

Inorganic materials such as silicon (crystalline, poly-crystalline, amorphous), GaAs, CdTe, CIGS, or nano/meso-porous titania-based dye+liquid electrolyte cells ('Graetzel cell') were generally the materials used for PV layers. More recently, organic materials have been used as PV materials including semiconducting gels, conjugated polymers, molecules, and oligomers. Organic PV materials may also include porous films of sintered particles such as titania particles. These materials may or may not be doped to improve performance (e.g. reduce resistance to improve efficiency). One of the great advantages of organic or partly organic solar cells is that they can be made much thinner than e.g. silicon-based PV cells (few 100 nm as opposed to several micrometers).

PV cells may be optimized for solar-cell applications such as applications in which daylight impinges on the cell and the voltage and current output from the PV cell. In such solar cell applications large currents have to be carried from the PV cell(s) to an outside electrical circuit or device. One of the PV cell surfaces, an incident light surface, has to be at least semi-transparent to collect this outside light but this light also has to penetrate through the electrode on this side. Thus, the PV cell incident-light surface must be transparent to allow radiation for photoelectric effect in the active layer, but also have low resistive properties to collect as much charge as possible from the active layer.

For example, in cases where the light entering a PV cell penetrates into the device through the bottom substrate the top electrode does not normally have to be transparent. In cases where the bottom substrate is opaque the light needs to reach the electronically or optically active layer or layers through the top electrode. Naturally, both the top and bottom electrodes, and the substrate must be at least partly transparent for light to reach the active layer(s)from both sides.

Transparent conducting electrodes (TCEs) have generally been made using a transparent conducting oxide (TCO) such as indium-tin-oxide, ITO, or tin oxide, SnO_x (with or without fluorine doping), Al-doped ZnO_x, etc. Such TCO layers have often been combined with metallic grids of additional lower resistance materials, such as screen-printed metal-particle pastes (for example, silver-paste). Ichinose et al., U.S. Pat. No. 6,472,594, describes coating metal with a conductive adhesive in order to attach the wires to and to make electrical contact with an underlying TCO. Such approaches are still far from optimal as limited light transmission and residual resistances limit device efficiency and manufacturing is costly. Furthermore, such approaches are not compatible with the use of organic PV cells. Ichinose, in particular does not address applications involving organic PV cells.

TCO materials, particularly where they act as anodes to extract and/or inject positive charge carriers, may not form good ohmic or near-ohmic contacts with organic p-type materials such as those employed in organic or partly-organic solar cells. Furthermore, organic or partly organic solar cells are often more sensitive to 'process conditions'. For example, depositing a TCO layer (for example, via the typical sputtering processes or even reactive sputtering processes that create UV and/or plasma conditions) can damage the organic layers such that cells may have electrical shorts. Because organic solar cells tend to be much thinner than silicon-based PV cells, any damage and/or surface modification due to the TCO deposition process can be relatively more relevant and damaging in an organic PV cell. Furthermore, TCO deposition processes typically employ vacuum-coating steps that are difficult and costly, even in a web-based roll-to-roll process.

Conductive polymer films, such as PEDOT, Pani or polypyrrole, are alternatives to TCO electrodes. Such polymer materials are far more suitable for roll- to-roll processing, as they can be solution processed/coated. Furthermore, such conductive polymer materials do not require sputtering or plasma processes to put them on an active layer. Unfortunately, after processing such as coating and drying, such conductive polymer films have sheet resistances significantly higher than TCOs, for example about 200 Ohms/square. Because of this, the resistive power loss would be far too high. Thus, pure conductive polymer transparent electrodes are unacceptable for PV approaches.

The active layer of a cell interacts with radiation and electric current or voltage.

Such an interaction may be a photovoltaic or light-emitting interaction. The active layer may be a multi-layer structure having two or more sub-layers stacked on top of one another. Alternatively, the presence of two or more materials may alternate with the plane of the active layer. In general, the active layer may include one or more semiconductor materials. The active layer may include silicon, which may be doped with p-type and/or n-type dopant. The silicon may be crystalline, poly-crystalline or amorphous. Other materials suitable for the active layer include inorganic semiconductors (crystalline, poly-crystalline or amorphous) such as Cadmium Telluride, Copper Indium Gallium Selenide, etc. The active layer may be a PV-active layer that converts incident radiant energy to electrical energy.

The active layer may also include one or more organic materials (for example molecules, oligomers, polymers, co-polymers, blends) of one or more layers or phase- separated blends. Suitable polymers include conjugated polymers, (for example, semiconductive polymers such as polyphenylvinylene, polythiophene, polyfluorenes, polyparaphenylene and polymers containing C60 or dyes such as perylenes or phthalocyanines) or conductive polymers such as doped PEDOT (Baytron™), polyaniline or polyacetylene. Other suitable organic materials include organic pigments or dyes, azo-dyes having azo chromofores (-N=N-) linking aromatic groups, phthalocyanines including metal-free phthalocyanine; (HPc), perylenes, naphthalocyanines, squaraines, merocyanines and their respective derivatives, poly(silanes), poly(germinates), 2,9-Di(pent-3-yl)-anthra[2,l ,9-def:6,5,10- d'e'f Jdiisoquinoline- 1,3,8,10-tetrone, and 2,9-Bis-( 1 -hexyl-hept- 1 -yl)-anthra[2, 1 ,9- def:6,5,10-d'e'f]diisoquinoline-l,3,8,10-tetrone.

As shown in Pichler, US Patent No. 6,936,761, the active layer may include a nano-architected porous film. For example, the active layer may include a combination of a nanostructured grid of porous inorganic conductive oxide such as, but not limited to, titania (in its various phases) wherein the pores in the nanostructured grid are filled with combinations of pore-filling materials such as conjugated organic materials (molecules, oligomers, polymers, copolymers, blends, spiro-compounds), organo- metallic sensitizing dyes (such as ruthenium complexes), solid or liquid electrolytes and/or ionic or electronic charge transporting compounds, inorganic semiconductive compounds (such as CuOx, CIGS) and the like. The porous conductive oxide and the pore filling material have complementary charge-transfer properties. For example, a negative-charge transporting porous titania grid may be filled with a positive charge transporting material such as CuOx.

The transparent conductive polymer layer may be made from any of a number of conductive polymers. Preferably, the conductive polymer layer is made from conductive polythiophenes, conductive polyanilines or conductive polypyrroles, PSS- doped PEDOT (for example, Baytron™), a derivative of PEDOT, a derivative of polyaniline, a derivative of polypyrrole. The conductive polymer layer is typically between about 50 run and about 5000 nm thick, preferably between about 100 nm and about 1000 nm and more preferably between about 100 nm and about 500 nm thick. Preferably, the conductive polymer material for the conductive polymer layer is solution-processible. Solution processing is advantageous for several reasons, for instance roll-to-roll-coat the solution (or suspension, dispersion, emulsion, etc.) onto a substrate to form the polymer layer.

The TCE may include an optional first layer (for example, a very thin metal layer or transparent conductive oxide) may define the charge injection/extraction efficiency (for example, via its work-function) and the conductive polymer layer may increase the electrical conductivity of the TCE and provide electrical contact with the array metal wires. The first layer may be a transparent conductive oxide (TCO) such as indium-tin-oxide, Al-doped ZnOx, F-doped SnOx, or the like that fills the spaces between the wires. The first layer may also be a very thin layer of a metal or alloy. In general, the thin metal or alloy layer used as the first layer must be thin enough that it is sufficiently transparent, for example, less than about 15 ran thick. The first layer may be a different conductive polymer layer that is different from the transparent conductive polymer layer. Specifically, the first layer could be a conductive polymer layer optimized for charge injection or extraction whereas the transparent conductive polymer layer may be optimized for transparency and conductivity and contact with the wire array.

There are several different configurations for the TCE that incorporate an optional first layer, such as a TCO or thin metal (or metal alloy) layer in electrical contact with the conductive polymer layer and/or wire array. For example, the wire array may be disposed between the first layer and the conductive polymer layer. Alternatively, the TCO or thin metal layer may be disposed between the wire array and the conductive polymer layer. More preferably, the conductive polymer layer may be disposed between the first layer and the wire array. This configuration is especially preferable when using a thin metal or alloy as the first layer.

The first layer may have either a higher or lower work function than the transparent conducting polymer layer depending on the desired application for the TCE. For example, a higher work function for the first layer may be desirable in some applications where the TCE acts as an anode (for example, a positive charge carrier collecting electrode for a PV cell or detector or a positive charge injecting electrode for other devices). In some applications where the TCE acts as a cathode (for example, a negative charge carrier collecting electrode for a PV cell or detector or a negative charge injecting electrode for other devices) a lower work function may be desirable for the first layer.

In an alternative embodiment, the array of wires may be placed between the transparent conductive polymer layer and the active layer. In some situations, direct electrical contact between the wires and the active layer is undesirable. In such cases the under-side of the array of wires may be partially or fully coated to protect against such undesirable electrical contact.

Other variations on the TCE are possible. A TCE may include a layer of short- proofing material disposed between the wires and active layer. The short proofing material may be an insulating layer such as a plastic, a resin such as an epoxy-resin or an inorganic insulator (oxide, nitride, oxy-nitride, etc.) to protect against such undesirable short circuits. The short-proofing material may be pre-deposited on the wire or deposited during the lamination process. The short-proofing material may also be pre-deposited on the active layer before the wire is put down. Portions of the conductive polymer layer may "creep-up" the sides of the wire, e.g., under the influence of surface tension forces, to provide contact between the wire and the conductive polymer layer. The short-proofing material may be deposited in a paste or solution/suspension/dispersion form or be laminated (for example, in the form of a tape).

It is preferred that the metal wires be made from wire material for which an industrially stable supply is available and which may be readily formed into wires or meshes. It is also desirable that the metal composing the metal wires possess a low resistivity, for example, 10 Ohm cm or less, more preferably below 10 ^s Ohm cm. Copper, silver, gold, platinum, aluminum, molybdenum and tungsten are suitable because of their low electric resistance. Of these, copper, silver, and gold are materials more preferred due to their low electric resistance. Alternatively, the wires may be made of aluminum or steel. In particular, Al, Cu, Ag etc have resistivities in the range of around 3x 10^"6 Ohm cm. The wires can be made from alloys of two or more metals. The wires may also be made from a low melting point or reflowable/remeltable low temperature alloy/metal (such as a solder alloy) that can be melted onto or into the underlying conductive polymer layer. The active layer may include an optional first interface layer that contacts the TCE. One function of the first interface layer is to avoid malfunctions such as shorts between the TCE and the nanoscale grid network. The interface layer may also include a metal or alloy layer having a lower work- function than the TCE as described above with respect to the first layer. The first interface layer may also improve mechanical properties such as flexibility. The active layer may also include a second optional interface layer that contacts the base electrode. The second interface layer may inhibit electrical shorts between the base electrode and the network filling material. The second interface layer may optionally include a metal or alloy layer with a lower work-function than the base electrode.

A nanoscale grid network is also available and may contain substantially uniformly distributed, structures roughly 1 nm to 100 ran in diameter and preferably, about 5 nm to about 25 nm in diameter. In general, neighboring structures are between about 1 nm and about 100 nm apart, measured, for example, from nearest edge to nearest edge. Preferably, the pores are between about 5 nm apart and 25 nm apart, edge to edge. By way of example, and without loss of generality, the nanoscale grid network may be made from an electron-accepting material, such as Titania, (TiO₂) zinc oxide

(ZnO₂), zirconium oxide, lanthanum oxide, niobium oxide, aluminum oxide, tungsten oxide, strontium oxide, calcium/titanium oxide, sodium titanate, potassium niobate,

Cadmium Selenide (CdSe), Cadmium Sulfide (CdS), or Cadmium Telluride (CdTe) as well as blends of two or more such materials such as TiO₂/SiO₂ blends/hybrids. In such case, network filling material may be made from a hole accepting material. The first optional interface layer may inhibit or prevent direct contact between the nanoscale grid network and the transparent electrode. The first interface layer may be made from the same material as the network filling material.

The network-filling material fills the spaces between the structures in the nanoscale grid network. The spaces between the structures may be in the form of pores in layer of porous material. Alternatively, spaces between structures may be gaps left behind when pores in a porous material have been filled with a pore-filling material and the porous material etched away leaving behind structures made from the pore-filling material. The second optional interface layer may inhibit or prevent direct contact between the network filling material and the base electrode. The second interface layer may be made from the same material as the nanoscale grid network. Where the nanoscale grid network is an electron accepting material, the network-filling material is a complementary, hole-accepting and hole-transporting, organic semiconducting material. Examples of suitable semiconducting organic materials include those set forth above with respect to the active layer.

Numerous electronic devices require electrical conductors which are optically transparent to visible light. The transparent electrical conductors function by transmitting electrical power to operate user interfaces such as touch screens or to send a signal to a pixel in a liquid crystal display (LCD). Transparent conductors are an essential component in many optoelectronic devices including flat panel displays, touch screens, electroluminescent lamps, solar panels, "smart" windows, and organic light emitting diode (OLED) lighting systems, hi such applications, light must pass through a conductive layer to perform an operation and for the device to function. In addition, transparent patterned conductors are valuable in making biometric identification cards, i.e. Smart cards in which the information is stored in or transferred through the conductive layer. Future electronic devices are limited in function and form by the current materials and processes utilized to create electrically conductive transparent layers.

Today most transparent electrodes are made from transparent conducting oxides, such as Indium Tin Oxide (ITO), having been the preferred choice for four decades. ITO is applied to an optically transparent substrate by vacuum deposition and then patterned using costly photolithographic techniques to remove excess coating and form the wire and electrodes. It is difficult and expensive to scale either of these processes to cover large areas of film. Further, ITO has some rather significant limitations: 1) ITO films are brittle (mechanical reliability concern for flexible applications such as in plastic displays, plastic solar voltaic, and wearable electrical circuitry); 2) ITO circuits are typically formed by vacuum sputtering, followed by photolithographic etching (fabrication cost may be too high for high volume / large area applications).

Efforts have been made to provide transparent electrodes to replace ITO film.

A typical example is a suspension of ITO particles in a polymer binder. However, this ITO filled system cannot match the electrical conductivity of a continuous ITO film. In accordance, transparent conductive polymer materials are being developed. These polymers typically require dopants to impart conductive properties and are applied on a substrate using screen printing or ink jet application techniques. Although still at a development stage and yet to reach the conduction level of an ITO film, the presence of dopants is expected to have an adverse effect on controlling the conductive properties, and may not be compatible with device miniaturization.

Photovoltaics fall into several broad categories: crystalline, thin film, and organic. Each of these categories has different manufacturing techniques and even different fundamental physics for generating power. Each category may also be categorized into several types: crystalline photovoltaics may be mono-crystalline or poly-crystalline; thin film photovoltaics include cadmium telluride and copper indium gallium diselenide; and organic photovoltaics include dye-sensitized, planar, bulk heterojunction, and small molecule.

The typical crystalline photovoltaic is comprised of an aluminum back contact, P-type silicon electron absorber, N-type electron acceptor, and a silicon nitride anti- reflective and passivation layer, with an aluminum grid for front contact. The typical copper indium gallium diselenide photovoltaic, as a thin film example, is comprised of a glass backed substrate, silicon oxide barrier, molybdenum contact, Copper Indium/Gallium [Di]Selenide absorber, and Cadmium Selenide layer, with a Zinc Oxide front contact. Organic photovoltaics, using bulk heterojunction as an example, have a glass substrate, Indium Tin Oxide transparent conductive layer, an electron donor and electron acceptor bulk mixture, lithium fluoride electron blocking layer, with an aluminum back contact.

Therefore, a need exists in the art for an improved transparent conducting electrode that overcomes the above disadvantages and a corresponding method for making it. The present invention satisfies the need for new electrically conductive, optically transparent coatings and films which are more transparent, flexible and conductive while processed using large area patterning and ablative techniques at a low cost.

Summary The present invention overcomes the problems and disadvantages associated with current strategies and designs and provides new tools and methods for a creating a photovoltaic cell containing carbon nanotubes.

One embodiment of the invention is directed to a photovoltaic cell, comprising a first electrode, a second electrode, an active layer formed between the first and second electrodes wherein the active layer comprises an electron acceptor material and an electron donor material, wherein carbon nanotubes are a component. The first electrode may be a transparent conductive coating containing carbon nanotubes. The electrode coating may contain greater than approximately 0.01 wt % carbon nanotubes and up to approximately 60.00 wt % carbon nanotubes. The carbon nanotubes consists of single-walled carbon nanotubes, multi-walled carbon nanotubes, mixtures or combinations thereof.

In another embodiment, the present invention is directed to a second electrode that is a transparent conductive coating containing carbon nanotubes. The electrode coating contains at least approximately 0.01 wt % carbon nanotubes and up to approximately 60.00 wt % carbon nanotubes. The carbon nanotubes are single-walled carbon nanotubes, multi-walled carbon nanotubes, mixtures or combinations thereof.

In another embodiment, the present invention is directed to an active layer containing carbon nanotubes. The active layer containing carbon nanotubes acts as an electron acceptor. The carbon nanotubes have been doped by functionalization to affect their electron acceptance. The active layer containing carbon nanotubes may also as an electron donor. The carbon nanotubes may have been doped by functionalization to affect their electron donation. Additionally, the carbon nanotubes enhance charge collection.

In another embodiment, the present invention is directed to a photovoltaic cell, comprising a first electrode, a second electrode, a photoactive layer between the first and second electrodes, the photoactive layer comprising an electron donor and an electron acceptor comprising carbon nanotubes, a first layer comprising an alkali halogenide between the photoactive layer and the first electrode; and a second layer comprising the alkali halogenide between the photoactive layer and the second electrode. The alkali halogenide comprises lithium fluoride. The first layer has a thickness of at most five nanometers or two nanometers. The electron donor may be comprised of a conjugated polymer and the electron acceptor may be comprised of carbon nanotubes.

Other embodiments and advantages of the invention are set forth in part in the description, which follows, and in part, may be obvious from this description, or may be learned from the practice of the invention.

Description of the Figures Figure 1 : Types of carbon nanotubes formed during synthesis.

Figure 2: Surface topography of CNT coating. Figure 3 : Model of a crystalline photovoltaic device (solar cell) Figure 3b: Crystalline photovoltaic device with carbon nanotubes as contact and anti-reflection film Figure 3c: Crystalline photovoltaic device with n-doped carbon nanotubes in n-semiconductor layer

Figure 3d: Crystalline photovoltaic device with p-doped carbon nanotubes in p-semiconductor layer Figure 3e: Crystalline photovoltaic device with carbon nanotubes deposited within the n-semiconductor layer

Figure 4: Amorphous Silicon Photovoltaic Device Figure 4b: Amorphous Silicon Photovoltaic Device with the transparent conductive layer replaced by carbon nanotubes

Figure 5: Cadmium Telluride Photovoltaic Device

Figure 5b: Cadmium Telluride Photovoltaic Device with carbon nanotubes replacing the conductive layer (ITO/TO/CTO layer)

Figure 6: Copper Indium Gallium DiSelenide Photovoltaic Device Figure 6b: Copper Indium Gallium DiSelenide Photovoltaic Device with carbon nanotubes replacing the transparent conductive layer (ZnO front contact layer) Figure 7: Bulk Heterojunction Organic Photovoltaic Device Figure 7b: Bulk Heterojunction Organic Photovoltaic Device with carbon nanotubes dispersed within the donor/acceptor blend Figure 8: Dye Sensitized Photovoltaic Device

Figure 8b: Dye Sensitized Photovoltaic Device with carbon nanotubes replacing one transparent conductive layer

Figure 9: Small Molecule Organic Photovoltaic Device Figure 9b: Small Molecule Organic Photovoltaic Device with carbon nanotubes dispersed within the working electrode

Description of the Invention

As embodied and broadly described herein, the present invention is directed to photovoltaic cells with carbon nanotube components and methods of operating. Coatings comprising carbon nanotubes such as carbon nanotube-containing films have been previously described (see Glatkowski, U.S. Patent Application No. 10/105,623, which is incorporated herein by reference). For example, such films may have a surface resistance as low as 10² ohms/square and a total light transmittance as high as 95%. The content of the carbon nanotubes in the film may be as high as 50%.

Carbon nanotubes were found from electron microscopic observation by Dr. lijima at Maijo University, Japan in 1991. Since then, carbon nanotubes have received profound studies. Typically, a carbon nanotube is like a hollow cylinder made of a graphite sheet, whose inner diameter ranges from 1 to 20 nm.

Graphite has been known to have a peculiar structure. That is, the covalent bonds between carbon atoms constituting graphite are arranged in an unusual style, so that graphite has a shape of rigid, flat hexagonal sheet. The upper and lower regions of the sheet are filled with dispersed free electrons, which translate in a parallel to the plain of the sheet. Carbon nanotubes are a recently identified carbon form in which a tube consists of a single graphite sheet with helical structure dependant on the arrangement of the graphitic sheet. Electric properties of the carbon nanotube are in functional relation with the helical structure and diameter thereof (Phys. Rev. (1992) B46:1804 and Phys. Rev. Lett. (1992) 68:1579). Thus, an alteration of either helicity or chirality of the carbon nanotube results in a change of motion of the free electrons. Consequently, the free electrons are allowed to move freely as in a metallic material, or must overcome an electronic band gap barrier as in a semiconductive material depending on the structure of the tube.

In addition, any modifications to the carbon atoms forming the sidewalls of these tubes will consequently modify the electrical properties of the tube. Semiconductive carbon nanotubes can be chemically doped with electron donating or electron withdrawing chemicals to produce a tube with metallic-like conduction. Furthermore metallic nanotubes can be transformed into poor conductors by damaging the sidewalls, chemical reactions to the sidewalls, irradiation with electrons or other high energy particles.

It has been surprisingly discovered that carbon nanotubes may provide an alternative to metal oxide coatings on photovoltaic devices. Carbon nanotubes (CNT) themselves are known (R. Saito, G. Dresselhaus, M. S. Dresselhaus, "Physical Properties of Carbon Nanotubes," Imperial College Press, London U.K. 1998, or A. Zettl "Non-Carbon Nanotubes" Advanced Materials, 8, p. 443, 1996). Carbon nanotubes are comprised of straight and/or bent multi-walled nanotubes (MWNT), straight and/or bent double-walled nanotubes (DWNT), and straight and/or bent single- walled nanotubes (SWNT), and combinations and mixtures thereof. CNT may also include various compositions of these nanotube forms and common by-products contained in nanotube preparations such as described in U.S. Patent No. 6,333,016 and WO 01/92381, and various combinations and mixtures thereof. Carbon nanotubes may also be modified chemically to incorporate chemical agents or compounds, or physically to create effective and useful molecular orientations (see U.S. Patent No. 6,265,466), or to adjust the physical structure of the nanotube.

Carbon nanotubes are an allotrope of carbon that is found in both a single- walled carbon nanotube and multi-walled carbon nanotube variety. Carbon Nanotubes are known to exhibit extraordinary strength, heat conductance, and electrical properties. Different carbon nanotube structures are pictured in Figure 1.

In a preferred embodiment, the nanotubes comprise single walled carbon-based SWNT-containing material. SWNTs can be formed by a number of techniques, such as laser ablation of a carbon target, decomposing a hydrocarbon, and setting up an arc between two graphite electrodes. Bethune et al., U.S. Pat. No. 5,424,054, describes a process for producing single-walled carbon nanotubes by contacting carbon vapor with cobalt catalyst. The carbon vapor is produced by electric arc heating of solid carbon, which can be amorphous carbon, graphite, activated or decolorizing carbon or mixtures thereof. Smalley (Guo, T., Nikoleev, P., Thess, A., Colbert, D. T., and Smalley, R. E., Chem. Phys. Lett. 243: 1-12 (1995)) describes a method of producing single-walled carbon nanotubes wherein graphite rods and a transition metal are simultaneously vaporized by a high-temperature laser. Smalley (Thess, A., Lee, R., Nikolaev, P., Dai, H., Petit, P., Robert, J., Xu, C, Lee, Y. H., Kim, S. G., Rinzler, A. G., Colbert, D. T., Scuseria, G. E., Tonarek, D., Fischer, J. E., and Smalley, R. E., Science, 273: 483-487 (1996)) also describes a process for production of single- walled carbon nanotubes in which a graphite rod containing a small amount of transition metal is laser vaporized in an oven at about 1,200^°C. Single- wall nanotubes were reported to be produced in yields of more than 70%. Moy, U.S. Patent No. 6,221,330, discloses methods of producing single-walled carbon nanotubes which employs gaseous carbon feedstocks and unsupported catalysts. Films made of carbon nanotubes are known to have surface resistances as low as 10² ohms/square. Shibuta, U.S. Patent No. 5,853,877, entitled "Method for Disentangling Hollow Carbon Microfibers, Electrically Conductive Transparent Carbon Microfibers Aggregation Film and Coating for Forming Such Film," describes formation of such conductive carbon nanotube films, and Moy, U.S. Patent No. 6,221,330, entitled "Processing for Producing Single Wall Nanotubes Using Unsupported Metal Catalysts," generally describes production of such carbon nanotubes used for forming the conductive films. However, prior art does not teach a method for patterning the film made of carbon nanotubes.

The electrical properties of the SWNT change dramatically as they are functionalized. The untreated SWNT are essentially metallic and their two point resistance (essentially a contact resistance, Bozhko, et al., 1998, Appl. Phys. A, 67:75- 77) measured across 5 mm of the "bucky paper" surface is 10-15 .OMEGA. When fluorinated, the tubes become insulating and the two point resistance exceeds 20 M .OMEGA.m. Margrave, et al., US Patent 6,645,455, describes methods of fluorinating carbon nanotubes. After methylation, the tubes possess a two point resistance of .about.20 k.OMEGA. Pyrolysis of the methylated product brings the resistance down to .about.100 .OMEGA. Incomplete return of the electrical conductivity upon pyrolysis may be due to an increased contact resistance that results from disorder induced into the rope lattice following the sequence of reaction steps. The usefulness of this approach to selectively pattern carbon nanotube coatings is severely limited by the reaction conditions, which limit the ability to selectively control the placement of the patterns and to process coatings on standard substrates such as plastic and glass which will also be modified by the same reaction conditions that cause the fluorination of the nanotubes.

Recently methods of forming patterned carbon nanotube coating on substrates include typical approaches of either creating the pattern by subtracting the excess material from a continuous coating of nanotubes on the substrate or creating the pattern additively by applying the nanotubes directly onto the substrate in the form of the pattern leaving uncoated areas to act as the insulation between the conductive pathways.

Park et al., in US Patent Application 20040265755, discloses a method of making carbon nanotube patterned film or carbon nanotube composite using carbon nanotubes surface-modified with polymerizable moieties. This approach does not lead to electrically conductive coatings with low electrical resistance since all the nanotubes are chemically functionalized on the sidewalls and are dispersed in polymer during deposition which disrupts formation of the nanotube conductive network. In this disclosure the deposited nanotube/polymer layer is later subtracted selectively through photolithography methods.

Lee et al., US Patent Application 20020025374, describes a selective growth method on a substrate to form patterned carbon nanotubes. This is a type of additive approach of growing the nanotubes directly on a surface at high temperatures >500 C. This limits the use of this technology to high temperature substrates and doe not scale easily to allow production of large parts or continuous films. Similarly, Delzeit, US Patent 6,858,197, discloses a patterning method wherein the nanotube are selectively grown on a substrate to form a pattern. This method first patterns a polymer on a surface and then grows the nanotubes on areas where the polymer was not deposited thereby creating a nanotube patterned surface with the unique feature of also providing aligned nanotubes in the pathways. This method also suffers from requiring high temperatures to form the nanotubes and is restricted in the size of the coated substrate which can be processed due to limitation of vacuum chamber size.

In US Patent 6,835,591, Rueckes, et al. discloses nanotube films and articles and methods of making the same. Herein subtractive removal methods of forming conductive patterned films of carbon nanotubes are disclosed. However modification of the nanotubes chemically to switch the electronic state of the nanotubes is not disclosed as a way of form patterns from continuous coatings of nanotubes. Furthermore the subtractive methods describe this disclosure are not reversible and are easily detected by the optical appeared change between regions with nanotubes and regions where the nanotube are removed as in the present invention.

As an alternative to metal oxide coatings is the use of carbon nanotubes (CNT) to form a conductive network, coatings on a surface, or an internal layer. This coating is formed using traditional wet coating processes, such as spraying, dipping, or roll coating which are low cost and can cover large areas. This coating can be patterned during deposition by applying only where needed with a selective process such as inkjet printing, silk screen printing, gravure coating, etc. Through the controlled application of this network of nanotubes by means of printing or spraying, patterned areas can be formed to function as electrodes in devices. The use of printing technology to form these electrodes obviates the need for more expensive process such as vacuum deposition and photolithography typically employed today during the formation of ITO coating.

Alternatively, the coating can be ablated or subtracted from a continuous coating on the surface to form a pattern. For example, laser etching can selectively remove the CNT where not needed to leave a pattern. Numerous other subtractive methods have been employed.

It has been surprisingly discovered that such materials can be formed by a two step method, which results in carbon nanotube film that have a low electrical resistance as well as a high light transmittance. First, a dilute water solution of carbon nanotubes is sprayed on a substrate, and water is evaporated leaving only the consolidated carbon nanotubes on the surface. Then, a resin is applied on the consolidated carbon nanotubes and penetrates into the network of the consolidated carbon nanotubes.

In a preferred embodiment, a photovoltaic cell is created by forming an active layer between a first and a second electrode. Preferably, the active layer comprises an electron acceptor material and an electron donor material. More preferably, the first electrode is a transparent conductive coating containing carbon nanotubes, more preferably, at least 0.01 wt % carbon nanotubes and up to about 60.00 wt % of carbon nanotubes, more preferably between 1.00 wt % to 50.00 wt %, more preferably 5.00 wt % to 40.00 wt % carbon nanotubes, more preferably from 20.00 wt % to 30.00 wt %even more preferably from 50.00 wt % to 60.00 wt %. Preferably, the carbon nanotubes are single walled carbon nanotubes (SWNT) or multi-walled carbon nanotubes (MWNT), mixtures, or some combination thereof.

In a preferred embodiment, the second electrode is a transparent conductive coating containing carbon nanotubes. More preferably, the second electrode is a transparent conductive coating containing carbon nanotubes, more preferably, at least 0.01 wt % carbon nanotubes and up to about 60.00 wt % of carbon nanotubes, more preferably between 1.00 wt % to 50.00 wt %, more preferably 5.00 wt % to 40.00 wt % carbon nanotubes, more preferably from 20.00 wt % to 30.00 wt %even more preferably from 50.00 wt % to 60.00 wt %. Preferably, the carbon nanotubes are single walled carbon nanotubes (SWNT) or multi-walled carbon nanotubes (MWNT), mixtures, or some combination thereof.

In a preferred embodiment, the active layer contains carbon nanotubes, preferably the carbon nanotubes act as an electron acceptor, wherein the carbon nanotubes have been doped by functionalization to affect their electron acceptance.

In another preferred embodiment, the active layer contains carbon nanotubes as an electron donor, wherein the carbon nanotubes have been doped by functionalization to affect their electron donation. In any of the embodiments mentioned herein, the carbon nanotubes preferably enhance charge collection.

In another preferred embodiment, a photovoltaic cell is formed by providing a first electrode, a second electrode and a photoactive layer between the first and second electrodes. Preferably, the photoactive layer comprises an electron donor and an electron acceptor comprising a fullerene component, a first layer comprising an alkali halogenide between the photoactive layer and the first electrode and a second layer comprising the alkali halogenide between the photoactive layer and the second electrode. Preferably, the alkali halogenide comprises lithium fluoride, more preferably the first layer has a thickness of about six nanometers, more preferably the first layer has a thickness of about three nanometers, and even more preferably the first layer has a thickness of about two nanometers. The photovoltaic cell itself may be from 10 nanometers to several hundred nanometers in thickness, preferably from 10 nanometers to 100 nanometers, more preferably from 20 nanometers to 80 nanometers, more preferably from 30 nanometers to 50 nanometers, more preferably from 5 nanometers to 10 nanometers, and even more preferably from 2 nanometers to 5 nanometers.

In another preferred embodiment, a photovoltaic cell is formed by providing a first electrode, a second electrode and a photoactive layer between the first and second electrodes. Within the photoactive layer, carbon nanotubes are aligned orthogonal to the plane of the first and second electrode, and thus along the shortest straight line path between them. The carbon nanotubes act as a wire, helping move charge between the two electrodes.

In preferred embodiments, the electron donor comprises a conjugated polymer and the electron acceptor comprises a fullerene component.

In further embodiments, one or more layers of the photovoltaic cell is flexible.

The substrate may be flexible and or one or more electrode layers may be a mesh electrode. The mesh electrode may be comprised of materials including, but not limited to, metals (such as palladium, platinum, titanium, stainless steels, and alloys thereof) and conductive polymers (such as poly(3,4-ethylene dioxythiophene), polythiopene derivatives and polyaniline as described by Gaudiana et al. in U.S. Patent Application 20030230337.

The following examples illustrate embodiments of the invention, but should not be viewed as limiting the scope of the invention.

Examples

A model crystalline photovoltaic device is depicted in Figure 3 a. The crystalline solar cell is comprised of a rear metal contact, a p-semiconductor layer, an n- semiconductor layer, a p-n-junction, a contact and an anti-reflection film. The thickness of the solar cell is approximately 0.3 mm while the thickness of the n- semiconductor layer is approximately 0.002 mm. Carbon nanotubes may replace or be dispersed in one of the active layers in the film. Preferably, carbon nanotubes either fully or partially replace the contact and anti-reflection film of the model crystalline solar cell shown in Figure 3b. Figure 3c shows the n-semiconductor layer in crystalline photovoltaic devices as pictured in Figure 3a is replaced by n type doped carbon nanotubes. In another embodiment, the p-semiconductor layer in crystalline photovoltaic devices as pictured in Figure 3a is replaced by p type doped carbon nanotubes as shown in Figure 3d.

Figure 3e shows carbon nanotubes are deposited within the n-semiconductor layer in the crystalline photovoltaic device. The CNT act as a charge conductor.

Figure 4a shows a model amorphous silicon photovoltaic device comprising a rear metal contact, an n-layer, an i-layer, a p-layer, and a transparent conductive layer. In another embodiment shown in Figure 4b, carbon nanotubes replace the transparent conductive layer in the amorphous silicon photovoltaic device shown in Figure 4a. Another example of a photovoltaic device is depicted in Figure 5a. This figure shows a cadmium telluride photovoltaic device comprising a contact, a CdTe layer, a CdS layer, an ITO/TO/CTO layer, and a layer of glass. In the embodiment shown in Figure 5b, carbon nanotubes replace the conductive layer (ITO/TO/CTO layer) of Figure 5 a. Figure 6a shows a model copper indium gallium diselenide photovoltaic device comprising a glass substrate, a barrier layer of SiO₄, an MO back contact, an absorber layer, a layer of CdS, and a front contact layer of ZnO. In another embodiment as shown in Figure 6b, carbon nanotubes replace the transparent conductive layer, the ZnO front contact layer, shown in Figure 6a. Figure 7a shows a model bulk heterojunction organic photovoltaic device comprising a glass substrate, a TCO layer, a conducting polymer anode, a donor/acceptor blend layer, an LIF, and an aluminum layer. In another embodiment, carbon nanotubes act as a charge transport in the bulk heterojunction organic photovoltaic device pictured in Figure 7b. Preferably, carbon nanotubes are dispersed within the donor/acceptor blend, more preferably, the carbon nanotubes are within the donor/acceptor blend, acting as an electron acceptor. Figure 8a depicts a dye sensitized photovoltaic device model comprising at least one glass substrate, at least one conducting transparent layer, a Pt catalyst, an electrolyte, a dye monolayer, a titania layer, a counter electrode, and at least one working electrode. In the embodiment shown in Figure 8b, carbon nanotubes replace at least one transparent conductive layer.

A model small molecule organic photovoltaic device is shown in Figure 9a. The device comprises a glass substrate, an ITO layer, a front cell, a back cell, and a layer of Ag. hi a preferred embodiment as shown in Figure 9b, carbon nanotubes are dispersed within the working electrode acting as a charge transport in a dye sensitized photovoltaic device, more preferably, the transparent conductive layer (ITO layer) in a small molecule organic photovoltaic device is replaced by carbon nanotubes.

In another embodiment carbon nanotubes are dispersed within the CuPc :C60 blend in a small molecule organic photovoltaic device. More preferably, carbon nanotubes are placed within the C60 layer acting as an electron acceptor in small molecule organic photovoltaic devices.

Other embodiments and uses of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. All references cited herein, including all publications, U.S. and foreign patents and patent applications, are specifically and entirely incorporated by reference. It is intended that the specification and examples be considered exemplary only with the true scope and spirit of the invention indicated by the following claims.

Claims

1. A photovoltaic cell, comprising: a first electrode; a second electrode; an active layer formed between the first and second electrodes wherein the active layer comprises an electron acceptor material and an electron donor material, wherein carbon nanotubes are a component.

2. The photovoltaic cell of claim 1, wherein the first electrode is a transparent conductive coating containing carbon nanotubes.

3. The photovoltaic cell of claim 2, wherein the electrode coating contains greater than approximately 0.01 wt % carbon nanotubes.

4. The photovoltaic cell of claim 2, wherein the electrode coating contains up to approximately 60.00 wt % carbon nanotubes.

5. The photovoltaic cell of claim 2, wherein the carbon nanotubes consists of single-walled carbon nanotubes, multi-walled carbon nanotubes, mixtures or combinations thereof.

6. The photovoltaic cell of claim 1, wherein the second electrode is a transparent conductive coating containing carbon nanotubes.

7. The photovoltaic cell of claim 6, wherein the electrode coating contains at least approximately 0.01 wt % carbon nanotubes.

8. The photovoltaic cell of claim 6, wherein the electrode coating contains up to approximately 60.00 wt % carbon nanotubes.

. The photovoltaic cell of claim 6, wherein the carbon nanotubes are single- walled carbon nanotubes, multi-walled carbon nanotubes, mixtures or combinations thereof.

10. The photovoltaic cell of claim 1 , wherein the active layer contains carbon nanotubes.

1 1. The photovoltaic cell of claim 10, wherein the active layer contains carbon nanotubes as an electron acceptor.

12. The photovoltaic cell of claim 10, wherein the carbon nanotubes have been doped by functionalization to affect their electron acceptance.

13. The photovoltaic cell of claim 10, wherein the active layer contains carbon nanotubes as an electron donor.

14. The photovoltaic cell of claim 10, wherein the carbon nanotubes have been doped by functionalization to affect their electron donation.

15. The photovoltaic cell of claim 1 , wherein the carbon nanotubes enhance charge collection.

16. A photovoltaic cell, comprising: a first electrode; a second electrode; a photoactive layer between the first and second electrodes, the photoactive layer comprising: an electron donor; and an electron acceptor comprising carbon nanotubes; a first layer comprising an alkali halogenide between the photoactive layer and the first electrode; and a second layer comprising the alkali halogenide between the photoactive layer and the second electrode.

17. The photovoltaic cell of claim 16, wherein the alkali halogenide comprises lithium fluoride.

18. The photovoltaic cell of claim 16, wherein the first layer has a thickness of at most five nanometers.

19. The photovoltaic cell of claim 16, wherein the first layer has a thickness of at most two nanometers.

20. The photovoltaic cell of claim 17, wherein the electron donor comprises a conjugated polymer.

21. The photovoltaic cell of claim 20, wherein the electron acceptor comprises carbon nanotubes.