Patent Application
20100096004
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The owner has no objection to the facsimile reproduction by any one of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyrights whatsoever.
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 a conductive electrode 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 created 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. Such 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 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 which 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 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 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., it 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 300 Ω/square with at least 90% 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) 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 conductive material (e.g., a conducting polymer) that serves to fill open porosity in the nanostructure (e.g., SWNT) network. The different conductive 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).
In an exemplary embodiment, the first electrode may be an anode comprising at least one network of nanostructures. The second electrode may also comprise a nanostructure network(s), but if it is to serve as a cathode a first buffer layer (e.g., a hole blocking layer with a low work function) may be required between the second electrode and the active layer. The solar cell may further comprise a second buffer layer (e.g., an electron blocking layer) between the first electrode and the active layer.
In a yet further embodiment, a solar cell according to the present invention may also comprise a second active layer sandwiched between a third electrode and the first or second electrode. Preferably, at least two of the first, second and third electrodes are at least semi-transparent to allow light transmission through to the multiple active layers. If the third electrode serves as a cathode, it may require a third buffer layer (e.g., a hole blocking layer with a low work function). In this way, transparent solar cells (e.g., thin-film solar cells) with different spectral responses may be stacked together with intervening at least semi-transparent electrodes (e.g., comprising nanostructure networks) to achieve high power conversion efficiency.
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
The active layer 120 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) 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 network 110 may form a nanostructure-film, and may comprise, but is not limited to, nanotubes, nanowires, nanoparticles and/or graphene flakes. The nanostructure network 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., holes) 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 is also preferably electrically conductive so as to collect separated charges (e.g., electrons) from the active layer (e.g., as a cathode). This electrode 130 may also be at least semi-transparent, but needs not be for many applications.
Referring to
The polymer 140 may be deposited separately, and/or may be mixed with 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 conducting polymer. The 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 SWNTs, and the conducting polymer comprises PEDOT:PSS. Other suitable conducting polymers may include, but are not limited to, ethylenedioxythiophene (EDOT), polyacetylene and poly(para phenylene vinylene) (PPV). The combination of different transparent and conducting layers can be used to optimize parameters such as the work function of the layer. The composite layer may additionally contain conducting nanoparticles to be used as resins for increasing the viscosity of nanostructure solutions.
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
Referring to
Referring to
Referring to
Unfortunately, such grids are generally not transparent and the electrodes thereof must typically be spaced relatively far apart to avoid unduly reducing light transmission to the underlying functional layer(s) (reduction is proportional to the fractional area covered by the metals). Consequently, devices (e.g., optoelectronics) in which charges are collected solely by a metal electrode grid(s) are usually quite inefficient, as many separated charges recombine before reaching an electrode.
A transparent conductor, such as a nanoscale network 110, that fills gaps in the metal electrode grid can improve device efficiency 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.
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) and/or ITO.
In a solar cell, incoming light separates charges in a photosensitive active layer 120. However, these separated charges usually cannot travel far within this usually-thin functional layer before recombining.
Electrodes in an electrode grid 510 can gather these separated charges, but only if the charges can reach points in the active layer 120 that contact the electrodes 510. Unfortunately, these electrodes 510 must be spaced relatively far apart in a conventional metal electrode grid, given that the non-transparent electrodes thereof reduce light transmission to the underlying active layer 120 (the reduction in light transmission is proportional to the fractional area covered by the non-transparent electrodes of the metal electrode grid 510). Semi-transparent electrodes can be more closely spaced, however without an additional transparent conductive layer(s), a solar cell having only an electrode grid may nonetheless have low efficiency.
Solar cell efficiency can be increased markedly by employing a nanostructure network(s) 110 as a transparent conductor, in accordance with an embodiment of the present invention. Whether above, below, or deposited within an electrode grid, the nanostructure network 110 provides additional pathways for separated charges to more easily and successfully reach the electrodes of the electrode grid 510.
In an exemplary embodiment, both the nanostructure network 110 and the electrode grid 510 comprise nanostructures, given that a thick nanostructure network can have both metallic properties and low sheet resistance. For example, a device according to the present invention 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 “Components and Devices Formed Using Nanoscale Materials and Methods of Production.”
Similarly, there are a variety of methods for fabricating electrode grids. For example, a patterned SWNT film can be fabricated using standard lithographic techniques. The new electrode architecture can be made by, for example, either depositing nanostructure films on the grid or depositing the grid on nanostructure films.
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
Referring to
Referring to
To fabricate this exemplary sample, water soluable 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. The resulting mixture was subsequently bath-sonicated for 1 hour.
The solution was 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 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
In an exemplary embodiment of the present invention, the nanostructure solution may be prepared by dispersing water soluble arc-discharged nanotube powder from Carbon Solutions Inc. in distilled oxide (DI) water by bath sonication with 100 W for 2 hours. This solution may further be mixed with a different conductive material (e.g., a conductive polymer binder such as PEDOT:PSS) at this stage to form a composite.
The solar cell begins to take shape when the first electrode is deposited and patterned (patterning is optional), for example, on the substrate 1030. The nanostructure-film may be patterned before (e.g., using PDMS stamp transfer), during (e.g., using a lift-off technique) or after (e.g., using photolithography and etching) deposition. The substrate is preferably transparent, but does not need to be for all embodiments and/or applications. The first electrode may be an anode or a cathode, depending, for example, on whether an electrode grid is to be included in the device.
In a first embodiment, wherein the device comprises a single active layer and no electrode grid, the first electrode is an anode comprising a nanostructure network. The nanostructures comprising the network are preferably SWNTs (prepared in step 1010 following (a)), which may be deposited by such methods as those described in connection with
For example, the substrate may be dipped into the nanostructure solution, the former having been either treated or matched with an appropriate surfactant such that a layer of solution coats the substrate surface. The solvent may then be evaporated from the solution, 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 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. Washing may comprise rinsing the film with water, and then drying it with air-flow blow dry or heat. Other rinsing agents can be used, e.g., methanol.
Additionally or alternatively, the first electrode may comprise a composite material of nanostructures and, for example, a conductive polymer binder. Such composites may be sonicated and spin-coated onto a substrate, with the resulting film subsequently cured over a hotplate.
Additionally or alternatively, the first electrode may comprise a composite material of nanostructures and, for example, a conductive polymer binder. Such composites may be sonicated and spin-coated onto a substrate, with the resulting film subsequently cured over a hotplate.
In another example, a nanostructure network is first deposited on a substrate 730, and a conducting polymer solution is then deposited 740 onto the nanostructure network and allowed to freely diffuse. Alternatively, the polymer may be deposited 1040 before the nanostructure network 1030. Preferably, even where the different conducting material is deposited separately, it will fill a plurality of pores in the adjacent nanostructure network.
For example, in one experiment, a 30-nm-thick SWNT network film (T=85%, Rs=200 Ω/square) deposited on a poly(ethylene terephthalate) (PET) flexible substrate was coated with a PEDOT:PSS by spin-casting (e.g., at 1000 rpm) and then heating 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.
Once the first electrode 1030 and optional different conducting material 1040 are deposited on the substrate, an active layer may be deposited onto the first electrode and/or different conducting material. 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 is deposited by transferring a PET substrate coated with a 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 are deposited on a transparent substrate (e.g., glass) 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 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, 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 using a rapid thermal annealing and anodic bonding process. 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).
After the active layer is deposited, another optional conductive material may be deposited 1060. Preferably, this material is deposited when the second electrode comprises a nanostructure network.
Finally, a second electrode may be deposited 1070 to complete the basic device architecture. In said first embodiment, this second electrode (e.g., cathode) needs not be transparent, and thus may comprise a conventional metal (e.g., Aluminum) deposited using known techniques. Alternatively, in later-described embodiments, this second electrode is preferably at least semi-transparent (e.g., comprising a nanostructure network(s) and/or ITO).
In a second embodiment of the present invention, a solar cell again comprises a single active layer, but this time further comprises an electrode grid for increased performance.
In this embodiment, steps 1010 and 1020 remain the same, however the first electrode deposited in step 1030 may be, for example, a non-transparent cathode. Accordingly, the second electrode deposited and patterned in step 1070 is preferably at least semi-transparent and comprises a network of nanostructures.
The electrode grid may be deposited 1080 before, after or concurrently with the second electrode and/or optional other conducting material (e.g., polymer). The electrode grid is preferably at least semi-transparent, and may, for example, comprise nanostructures and/or ITO. For example, a thin network of SWNTs can be deposited on the functional layer, with a thick SWNT grid subsequently deposited and patterned on top using a lithography mask. Preferably, whether deposited before, after or concurrently, the network of nanostructures will fill gaps in the electrode grid (e.g. see
Referring to
To solve this problem, it is widely accepted that multiple solar cells in tandem, covering different spectral regions, must be used. However, it is difficult to realize multiple polymer films in tandem without destroying the underlying polymer layer(s). An alternative approach is to fabricate efficient transparent solar cells (e.g., polymer solar cells) with different spectral responses and stack them together with intervening at least semi-transparent electrodes (that allow light transmission through to underlying active layers).
Nanostructure-films, such as those comprising SWNTs, are typically good transparent electrical conductors with high work functions, making them well-suited for use as an anode 1110. Such films can be used as a cathode 1120 with, for example, the addition of a low work function buffer layer 1150 (e.g., a hole-blocking layer such as copper phthalocyanine (CuPc), cesium carbonate (Cs2CO3) and/or titanium dioxide (TiO2)) between the cathode 1120 on one side, and the active layer 1140 and anode 1110 on the other. Preferably, another buffer layer 1130 (e.g., an electron-blocking layer such as PEDOT:PSS) is included for optimal device efficiency. These buffer layers are preferably at least semi-transparent.
Referring to
Referring to
Referring to
Referring to
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.
