Patent Application
20080023066
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Certain marks referenced herein may be common law or registered trademarks of third parties affiliated or unaffiliated with the applicant or the assignee. Use of these marks is by way of example and shall not be construed as descriptive or limit the scope of this invention to material associated only with such marks.
The present invention relates in general to solar cells, and more particularly to thin-film solar cells comprising at least one nanostructure-film.
A solar cell is a photoelectric device that converts photons from the sun (solar light) into electricity. Fundamentally, the device needs to photo-generate charge carriers (e.g., electrons and holes) in a photosensitive active layer, and separate the charge carriers to conductive electrode(s) that will transmit the electricity.
Historically, bulk technologies employing crystalline silicon (c-Si) have been used as the light-absorbing semiconductors in most solar cells, despite the fact that c-Si is a poor absorber of light and requires a high material thickness (e.g., hundreds of microns). However, the high cost of c-Si wafers has led the industry to research alternate, and generally less-expensive, solar cell materials.
Specifically, thin film solar cells can be fabricated with relatively inexpensive materials on flexible surfaces. The selected materials are preferably strong light absorbers and need only be about a micron thick, thereby reducing materials costs significantly. Thin film solar cell materials include, but are not limited to, those based on silicon (e.g., amorphous, protocrystalline, nanocrystalline), cadmium telluride (CdTe), copper indium gallium selenide (CIGS), chalcogenide films of copper indium selenide (CIS), gallium arsenide (GaAs), light absorbing dyes, quantum dots, organic semiconductors (e.g., polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine and carbon fullerenes) and other non-silicon semiconductor materials. These materials are generally amenable to large area deposition on rigid (e.g., glass) or flexible (e.g., PET) substrates, with semiconductor junctions formed in different ways, such as a p-i-n device (e.g., with amorphous silicon) or a hetero-junction (e.g., with CdTe and CIS).
Regardless of the thin-film device architecture chosen, an at least semi-transparent conducting layer is generally required to form a front electrical contact of the cell, so as to allow light transmission through to the active layer(s). As used herein, a layer of material or a sequence of several layers of different materials is said to be “transparent” when the layer or layers permit at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers. Similarly, layers that permit some but less than 50% transmission of ambient electromagnetic radiation in relevant wavelengths are said to be “semi-transparent.”
Currently, the most commonly used transparent electrodes are transparent conducting oxides (TCOs), specifically indium-tin-oxide (ITO) on glass. However, ITO can be an inadequate solution for many emerging applications (e.g., non-rigid solar cells due to ITO's brittle nature), and the indium component of ITO is rapidly becoming a scarce commodity. Moreover, deposition of transparent conducting oxides (TCOs) for minimal light loss normally requires a high-temperature sputtering process, which can severely damage underlying active layers.
Consequently, more robust and abundant transparent conductors are required not only for solar cell applications but for optoelectronic applications in general.
The present invention provides an optoelectronic device comprising at least one nanostructure-film.
Nanostructure-films include, but are not limited to, network(s) of nanotubes, nanowires, nanoparticles and/or graphene flakes. Specifically, transparent conducting nanostructure-films composed of randomly distributed single-wall nanotubes (SWNTs) (networks) have been demonstrated as substantially more mechanically robust than ITO. Additionally, SWNTs can be deposited using a variety of low-impact methods (e.g., they can be solution processed) and comprise carbon, which is one of the most abundant elements on Earth. Nanostructure-films according to embodiments of the present invention were demonstrated as having sheet resistances of less than 200 Ω/square with at least 85% optical transmission of 550 nm light.
A solar cell according to an embodiment of the present invention comprises a photosensitive active layer sandwiched between a first electrode and a second electrode, wherein at least one of the first and second electrodes comprises a network of nanostructures (e.g., a nanostructure-film). Active layers compatible with the present invention may include, but are not limited to, those based on silicon (e.g., amorphous, protocrystalline, nanocrystalline), cadmium telluride (CdTe), copper indium gallium selenide (CIGS), chalcogenide films of copper indium selenide (CIS), gallium arsenide (GaAs), light absorbing dyes, quantum dots, organic semiconductors (e.g., polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine and carbon fullerenes (e.g., PCBM)) and other non-silicon semiconductor materials. These materials are generally amenable to large area deposition on rigid (e.g., glass) or flexible (e.g., PET) substrates, with semiconductor junctions formed in different ways, such as a p-i-n device (e.g., with amorphous silicon) or a hetero-junction (e.g., with CdTe and CIS).
A solar cell according to a further embodiment of the present invention may additionally incorporate a different material (e.g., a polymer) that may serve to fill open porosity in the nanostructure (e.g., SWNT) network, encapsulate the network and/or planarize the network (thereby preventing shorting by wayward nanostructures through the active layer of the cell to another electrode). The different material may be mixed with nanostructures prior to deposition (e.g., to form a composite), and/or may be deposited separately (e.g., and allowed to diffuse into the nanostructure network).
The solar cell of the present invention may further comprise an electrode grid that is, for example, superimposed on the nanostructure network. This electrode grid may be composed of a conventional metal and/or may be at least semi-transparent (e.g., composed of nanostructures and/or ITO).
This and the above device architectures may be equally applicable to other optoelectronic devices. Other features and advantages of the invention will be apparent from the accompanying drawings and from the detailed description. One or more of the above-disclosed embodiments, in addition to certain alternatives, are provided in further detail below with reference to the attached figures. The invention is not limited to any particular embodiment disclosed.
The invention is better understood from reading the following detailed description of the preferred embodiments, with reference to the accompanying figures in which:
Features, elements, and aspects of the invention that are referenced by the same numerals in different figures represent the same, equivalent, or similar features, elements, or aspects in accordance with one or more embodiments of the system.
Referring to
A solar cell active layer 120, according to embodiments of the present invention, is preferably a strong light absorber such as, for example, one based on silicon (e.g., amorphous, protocrystalline, nanocrystalline), cadmium telluride (CdTe), copper indium gallium selenide (CIGS), chalcogenide films of copper indium selenide (CIS), gallium arsenide (GaAs), light absorbing dyes, quantum dots, organic semiconductors (e.g., polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine and carbon fullerenes (e.g., PCBM) and other non-silicon semiconductor materials. These materials are generally amenable to large area deposition on rigid (e.g., glass) or flexible (e.g., PET) substrates, with semiconductor junctions formed in different ways, such as a p-i-n device (e.g., with amorphous silicon) or a hetero-junction (e.g., with CdTe and CIS).
The nanostructure-film 110 (also referred to herein as “nanostructure network”) preferably comprises an interconnected network of nanotubes, nanowires, nanoparticles and/or graphene flakes. This nanostructure-film 110 is preferably at least semi-transparent so as to allow light transmission through to the active layer(s), and electrically conductive so as to collect separated charges (e.g., electrons) from the underlying active layer (e.g., as an anode). As used herein, a layer of material or a sequence of several layers of different materials is said to be “transparent” when the layer or layers permit at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers. Similarly, layers which permit some but less than 50% transmission of ambient electromagnetic radiation in relevant wavelengths are said to be “semi-transparent.”
The electrode 130 (e.g., cathode) is also preferably electrically conductive so as to collect separated charges (e.g., electrons) from the active layer. This electrode 130 may also be at least semi-transparent, but needs not be in many devices (e.g., where another device electrode comprises a transparent and conductive nanostructure-film).
Referring to
The polymer 140 may be deposited separately from the nanostructure-film, and/or may be mixed with the nanostructures and deposited as a composite layer. For example, SWNTs can be dispersed in aqueous solution and sonicated for a period of time, then mixed with an aqueous solution containing a polymer. The resulting mixture can then be sonicated and spin-coated onto a substrate, with the resulting film subsequently cured over a hotplate. Additionally or alternatively, a nanostructure network may be first deposited on a substrate, with a conducting polymer solution subsequently deposited onto the nanostructure network and allowed to freely diffuse.
In a preferred embodiment, the nanostructure network comprises substantially SWNTs, and the polymer comprises PEDOT:PSS (i.e., a conducting polymer). Other suitable conducting polymers may include, but are not limited to, ethylenedioxythiophene (EDOT), polyacetylene and poly(para phenylene vinylene) (PPV). Additional layers can be used to optimize parameters such as the work function of the layer (e.g., as a buffer layer). The composite layer may additionally contain conducting nanoparticles to be used as resins for increasing the viscosity of nanostructure solutions. As used herein, “substantially” shall mean that at least 40% of components are of a certain type.
In one experiment, a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) solution deposited onto a nanostructure network comprising single walled carbon nanotubes (SWNTs) reduced electrode sheet resistance by about 20% (e.g., to about 160 Ω/square). Given that the same PEDOT:PSS film (˜95 nm thick) spun on glass (i.e., with no nanostructures) had a sheet resistance of about 15 kΩ/square, the above drop in sheet resistance cannot be attributed merely to parallel conduction. Rather, the reduction in electrode sheet resistance may be attributed to a reduction in sheet resistance between conducting SWNTs (e.g., by filling a plurality of pores in the network) and/or doping of semiconducting SWNTs in the network.
Referring to
Nanostructure network(s) 110 may be deposited on the substrate 150 through a variety of techniques such as, for example, spraying, drop-casting, dip-coating and transfer printing, which are discussed in greater detail below.
Referring to
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A transparent conductor, such as a nanostructure-film 110, that fills gaps in the metal electrode grid can improve device efficiency significantly by allowing separated charges additional collection pathways. As depicted in
The electrode grid may comprise a conventional metal, for example gold. Metal electrode grids can be fabricated using known processes such as standard lithographic techniques, shadow masking, and gold deposition techniques. As used herein, “grid” shall mean a layer having openings (e.g. corrugated, perforated) penetrating through it, and shall include, for example, a framework of crisscrossed and/or parallel bars.
Additionally or alternatively, in a further embodiment of the present invention, the electrode grid is at least semi-transparent, comprising, for example, a patterned nanostructure network(s) (given that a thick nanostructure network can have metallic properties) and/or ITO. For example, such a device may comprise a thin SWNT network superimposed on a thick SWNT network, wherein the latter network acts as an electrode grid.
Various methods for fabricating and depositing nanostructure networks are described in PCT application US/2005/047315 entitled “Components and Devices Formed Using Nanoscale Materials and Methods of Production,” which is herein incorporated in its entirety by reference.
In an additional embodiment of the present invention, a nanostructure network solar cell fabricated according to the method described by M. Rowell, et al., Appl. Phys. Lett 88, 233506 (2006) can be improved by incorporating the electrode architecture of the present invention.
Referring to
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To fabricate this exemplary sample, water soluable P3 arc-discharged nanotube powder from Carbon Solutions, Inc. was first dispersed in distilled oxide (DI) water by bath sonication with 100 W for 2 hours. Nanotube solution and PEDOT:PSS (Baytron F. HC) in water were then mixed together in different proportions, and the resulting mixture was subsequently bath-sonicated for 1 hour. The mixture was then spin-coated onto a pre-cleaned glass slide at a speed of 1000 rpm, and cured over a hotplate at 120 degrees for 18 minutes. The transmittance and sheet resistance of the deposited films was measured and plotted in
Referring to
Additionally or alternatively, nanostructures may be deposited using a transfer stamping method. For example, commercially available SWNTs (e.g., produced by arc discharge) may be dissolved in solution with surfactants and then sonicated. The well dispersed and stable solutions may then be vacuum filtered over a porous alumina membrane. Following drying, the SWNT films may be lifted off with a poly(dimethylsiloxane) (PDMS) stamp and transferred to a flexible poly(ethylene terephthalate) (PET) substrate by printing.
This method has the added advantage of allowing deposition of patterned films (e.g., where the PDMS stamp is already patterned). Other compatible patterning methods include, but are not limited to, photolithography/etching and liftoff (e.g., using photoresist or toner).
Referring to
At this point, a polymer may be mixed with the nanostructure solution and deposited as a composite. Additionally or alternatively, the nanostructure solution may be deposited on the substrate 1030 by itself to form a nanostructure network, with a polymer already deposited 1080 on the substrate or subsequently deposited onto the nanostructure network and allowed to freely diffuse. Preferably, even where the polymer is deposited separately from the nanostructure network, it will fill a plurality of pores in the adjacent nanostructure network.
After deposition, solvent may be evaporated from the solution 1040 and/or composite, preferably in a uniform manner using, for example, a flash-drying method (where evaporation begins on one side of the substrate, and sweeps across the substrate in a “drying wave”). Heat can be applied in various manners, e.g., by linear heating bar and/or infrared laser. Additionally, solvent evaporation may be aided by air-flow blow drying.
Where a surfactant is used, the substrate will preferably undergo a subsequent wash to remove surfactant from the dried nanostructure-film on the substrate 1050. Washing may comprise rinsing the film with water and/or methanol, and then drying it with air-flow blow dry or heat.
The composite and/or nanostructure-film may be patterned before (e.g., using PDMS stamp transfer), during (e.g., using a lift-off technique) and/or after (e.g., using photolithography and etching) deposition.
In an exemplary embodiment, a nanostructure solution may be prepared by dispersing water soluble P3 arc-discharged nanotube powder from Carbon Solutions Inc. in DI water by bath sonication with 100 W for 2 hours. A PET substrate with an electrode grid (e.g., a metal electrode grid fabricated using known metal deposition and patterning techniques) formed thereon may be dipped into this solution, such that a 30-nm-thick SWNT network film (T=85%, Rs=200 Ω/square) is formed. This film may be subsequently coated with PEDOT:PSS by spin-casting (e.g., at 1000 rpm) and heating of the substrate (e.g., on a 110° C. hotplate for 20 minutes). Consistent results were obtained when either the PEDOT:PSS solution was applied on the surface and let free to diffuse several minutes before the spin-coating operation in order to fill in open porosity of the SWNT film or when a PEDOT:PSS/isopropanol 1:1 mix was used to improve the wetting.
An active layer may subsequently be deposited over the nanostructure network and/or polymer. Preferably, the active layer is photosensitive and may be based on silicon (e.g., amorphous, protocrystalline, nanocrystalline), cadmium telluride (CdTe), copper indium gallium selenide (CIGS), chalcogenide films of copper indium selenide (CIS), gallium arsenide (GaAs), light absorbing dyes, quantum dots, organic semiconductors (e.g., polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine and carbon fullerenes) and other non-silicon semiconductor materials.
In an exemplary embodiment, an organic active layer may be deposited by transferring the PET substrate coated with the 30-nm-thick SWNT film and a PEDOT:PSS layer with low roughness to an inert glove box where a solution of MDMO-PPV/PCBM in a 1:4 weight ratio or P3HT/PCBM in a 1:0:8 weight ratio (10 mg P3HT/mL) in chlorobenzene was spin-cast at 700 rpm.
In another exemplary embodiment, thin silicon active layers may be deposited over the SWNT film by chemical vapor deposition (CVD). For example, amorphous silicon may be deposited using hot-wire chemical vapor deposition (CVD) (e.g., decomposing silane gas (SiH4) using a radiofrequency discharge in a vacuum chamber) or alternatively may be sputter deposited (e.g., using ZnO/Ag). Nanocrystalline silicon may also be deposited effectively by hot-wire CVD (e.g., using a high hydrogen dilution (H2/SiH4=166), a high gas pressure of 2 Torr, and a high power-density of 1.0 W/cm2 at a low substrate temperature of 70° C). Similarly, protocrystalline silicon may be deposited using photo-assisted CVD (e.g., employing alternate H2 dilution under continuous ultraviolet (UV) light irradiation).
In yet another exemplary embodiment, a cadmium telluride active layer (CdTe) is deposited over the nanostructure-film, possibly with a corresponding cadmium sulphide (CdS) layer, using close-space sublimation (CSS) (e.g., based on the reversible dissociation of the materials at high temperatures: 2CdTe(s)=Cd(g)+Te2(g)). Alternatively, physical vapour deposition (PVD), CVD, chemical bath deposition and/or electrodeposition may be used.
In still another exemplary embodiment, copper-indium-gallium-selinide (CIGS) may be deposited over the SWNT film using a rapid thermal annealing and anodic bonding process. Such thermal annealing processes are also compatible with copper-indium-selinide (CIS) systems, the parent systems for CIGS.
In additional exemplary embodiments, gallium arsenide (GaAs) solar cells may be fabricated from epilayers grown directly on silicon substrates by atmospheric-pressure metal organic chemical vapor deposition (MOCVD); and active layers comprising quantum dots (e.g., suspended in a supporting matrix of conductive polymer or mesoporous metal oxide) may be fabricated by growing nanometer-sized semiconductor materials on various substrates (e.g., using beam epitaxy on a semi-insulating GaAs(100) substrate).
Another electrode layer may be deposited over the active layer. In the present exemplary embodiment, this electrode (e.g., cathode) needs not be transparent, and thus may comprise a conventional metal (e.g., aluminum) deposited using known techniques.
Alternatively, in other embodiments of the present invention this conventional metal electrode may be formed first, with the active layer, optional polymer, nanostructure-film and electrode grid respectively deposited thereon.
The present invention has been described above with reference to preferred features and embodiments. Those skilled in the art will recognize, however, that changes and modifications may be made in these preferred embodiments without departing from the scope of the present invention. These and various other adaptations and combinations of the embodiments disclosed are within the scope of the invention.
This application claims priority to U.S. Provisional Patent Application No. 60/833,846, filed Jul. 28, 2006, and entitled “TRANSPARENT ELECTRODES FORMED OF METAL ELECTRODE GRIDS AND NANOSTRUCTURE NETWORKS,” which is hereby incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60833846 | Jul 2006 | US |
