Patent Application
20110203632
The present disclosure relates to photovoltaic device and solar cells, and more particularly to the use of semiconducting nanotube layers within such devices and cells.
Any discussion of the related art throughout this specification should in no way be considered as an admission that such art is widely known or forms part of the common general knowledge in the field.
The most commonly known photovoltaic (PV) devices are formed by placing a layer of n-type crystalline silicon into contact with a layer of p-type crystalline silicon. In practice, this is typically accomplished by diffusing an n-type dopant (such as, but not limited to, phosphorus into one side of a silicon layer previously doped with a p-type dopant (such as, but not limited to, boron) or vice versa. Free electrons within the n-type silicon layer flow into the p-type layer—which possesses a deficiency of free electrons or, taken another way, possesses a plurality of excess holes—until the Fermi levels of the two layers are substantially equal. In this way, a p-n junction—a voltage barrier introduced by the initial electron diffusion—is formed along the interface of the two silicon layers. Within silicon, this voltage barrier is typically around 0.6 to 0.7 Volts.
Within such a PV device, the p-type silicon layer becomes the light absorption material. When a photon impacts the p-type silicon layer—as would be the case if the PV device was placed in direct sunlight—the photon may be absorbed into the layer. A number of factors determine the likelihood of a photon being absorbed by the silicon layer, including the thickness of the p-type silicon layer (the more silicon material a photon travels through, the more likely it will be absorbed) and the energy of the photon itself. Photons with energy levels approximately equal to the band gap energy of crystalline silicon (about 1.1 eV) and higher—or in the case of non-silicon based PV devices, the band gap energy of the material within the light absorption layer—can be absorbed. Photons with energy levels below the band gap energy typically pass directly through the silicon layer without being absorbed. Photons with energy levels significantly higher than the band gap energy may be absorbed, but the excess energy will be converted to heat (and not electricity).
When a photon is absorbed into the p-type silicon layer, the photon's energy is transferred to a valance band electron within the silicon layer. This energy will usually be enough to excite the electron into the conduction band, allowing the electron to move freely through the silicon material. This process is known by those skilled in the art as photoexcitation. When a load is placed across the PV device, these freed electrons will tend to flow from the p-type crystalline silicon layer into the n-type crystalline layer and also through the load under the influence of the voltage generated at the p-n junction. In this way, the energy of the photons striking the PV device (solar energy) is converted into electricity which can either be stored within or used by an attached load.
One significant limitation of crystalline silicon based PV devices is the thickness of the p-type silicon layer. As crystalline silicon is a relatively poor light absorber, the p-type layer within such PV devices usually needs to be relatively thick (in some cases on the order of hundreds of microns thick) in order to realize a PV device with reasonable efficiency. This can significantly increase the cost of a PV device as well as limit the efficiency (freed electrons will have a tendency to fall back into holes left by other electrons when traveling through a thick p-type layer).
Thin-film PV devices attempt to overcome this limitation through the use of less expensive materials which are strong light absorbers and can be disposed over large areas (in some cases on the order of one meter or more). Materials well suited to thin-film PV devices include amorphous silicon, cadmium telluride (CdTe), and copper indium (gallium) diselenide. A thin-film PV device is formed by applying one or more thin layers of a photovoltaic material over a substrate (such as glass) which has been coated with a transparent or partially transparent conducting layer. Depending on the material used, these thin layers can range in thickness from tens of nanometers to tens of microns and are typically formed through chemical vapor deposition (CVD).
Although typically less efficient than traditional crystalline silicon PV devices, lower material cost and ease of fabrication in large scale devices make thin-film PV devices viable solutions within certain applications.
Within all of the previously described types of PV devices, conductive elements are necessary to form electrodes along the top and bottom of the PV device such that a load can be placed across the PV device to either store or use the electrical energy generated within the PV device. The electrode placed over the light absorption layer (usually simply referred to as the “top electrode” by those skilled in the art) must allow photons to pass through into the light absorption layer while maintaining good electrical contact with the PV device such as to maximize the conductivity between the electron donor layer and an attached load. Typically, these functions are both served by selecting a transparent conductive material such as ITO for the top electrode.
U.S. Pat. Nos. 6,835,591, 7,335,395, 7,259,410, 6,924,538, and 7,375,369 disclose approaches for making nanotube films and articles, e.g., nanotube fabrics such as carbon nanotube fabrics and articles made therefrom. The entire contents of each of these patents is incorporated herein by reference.
U.S. Patent Application Publication Nos. 20080299307, 20050058797, 20080012047, 20060183278, 20080251723, and 20080170429 also disclose approaches for making nanotube films and articles, e.g., nanotube fabrics such as carbon nanotube fabrics and articles made therefrom. The entire contents of each of these published patent applications is incorporated herein by reference.
Copending U.S. patent application Ser. No. 12/274,033 filed Nov. 19, 2008, discloses approaches for making nanotube films, e.g., nanotube fabrics, that include nanoscopic particles, and copending U.S. patent application. Ser. No. 12/553,695 filed Jul. 31, 2009, discloses approaches for making nanotube films, e.g., nanotube fabrics, having anisotropy. The entire contents of each of these patent applications is incorporated herein by reference.
The present inventors have identified a need for photovoltaic devices that are less costly and easier to fabricate that conventional silicon-based PV devices. The present inventors have also recognized a desire for PV devices which are physically flexible so as to allow a PV device to conform to a plurality of underlying support structures including, but not limited to, flexible support structures.
The present disclosure relates to photovoltaic (PV) devices which employ layers of semiconducting carbon nanotubes as light absorption materials.
According to one example, a photovoltaic device comprises a first electrode element, a second electrode element; a layer of semiconducting elements disposed between said first and second electrode elements, said layer of semiconducting elements comprising a fabric of semiconducting carbon nanotubes having a first conductivity type, said layer of semiconducting elements having a first side and a second side, and at least one charge-separating junction formed at (e.g., on a surface thereof or within) said layer of semiconducting elements. The first side of said layer of semiconducting elements is electrically coupled to said first electrode element, and the second side of said layer of semiconducting elements is electrically coupled to said second electrode element.
According to another example, a photovoltaic power generating system comprises multiple photovoltaic devices electrically coupled together, and an electrical inverter electrically coupled to an output section of said multiple photovoltaic devices, wherein said inverter receives a DC electric current from said output section and converts the DC electric current to an AC electric current. Each of the multiple photovoltaic devices comprises a first electrode element, a second electrode element; a layer of semiconducting elements disposed between said first and second electrode elements, said layer of semiconducting elements comprising a fabric of semiconducting carbon nanotubes having a first conductivity type, said layer of semiconducting elements having a first side and a second side, and at least one charge-separating junction formed at said layer of semiconducting elements. The first side of said layer of semiconducting elements is electrically coupled to said first electrode element, and the second side of said layer of semiconducting elements is electrically coupled to said second electrode element.
According to another example, a method of fabricating photovoltaic device comprises forming a layer of semiconducting elements on a first electrode element, said layer of semiconducting elements comprising a plurality of carbon nanotubes of a first conductivity type, said layer of semiconducting elements having a first side and a second side, said first side of the layer of semiconducting elements disposed at a surface of said first electrode element, said first side of said layer of semiconducting elements being electrically coupled to said first electrode element. The method also comprises forming a second electrode element at said second side of said layer of semiconducting elements, said second side of said layer of semiconducting elements being electrically coupled to said second electrode element, and forming at least one charge-separating junction at said layer of semiconducting elements.
In particular, for example, the present disclosure describes a photovoltaic device comprising a first electrode element, a second electrode element, a first layer of semiconducting elements, and a second layer of semiconducting elements. The first layer of semiconducting elements, which includes a first side and a second side, comprises a plurality of n-type carbon nanotubes. The second layer of semiconducting elements, which also includes a first side and a second side, comprises a plurality of p-type carbon nanotubes. Within this photovoltaic device the first side of the first layer of semiconducting elements is electrically coupled to the first electrode element, and the first side of the second layer of semiconducting elements is electrically coupled to the second electrode element. Further, the second side of the first layer of semiconducting elements is electrically coupled to the second side of the second layer of semiconducting elements, forming a p-n junction interface between the first layer and the second layer.
The present disclosure also describes a photovoltaic device comprising a first electrode element, a second electrode element, and a layer of semiconducting elements. The layer of semiconducting elements, which includes a first side and a second side, comprises a first plurality of n-type carbon nanotubes and a second plurality of p-type carbon nanotubes. Within this photovoltaic device the first side of the layer of semiconducting elements is electrically coupled to said first electrode element and the second side of the layer of semiconducting elements is electrically coupled to the second electrode element.
The present disclosure also describes a photovoltaic device comprising a first electrode element, a second electrode element, and a layer of semiconducting elements. The second electrode element is comprised of a metal, e.g., a low work function metal, and the layer of semiconducting elements, which includes a first side and a second side, comprises a plurality of semiconducting carbon nanotubes. Within this photovoltaic device the first side of the layer of semiconducting elements is electrically coupled to the first electrode element, and the second side of the layer of semiconducting elements is electrically coupled to the second electrode element, forming a Schottky barrier between the layer of semiconducting elements and the second electrode element.
The present disclosure also describes a photovoltaic device comprising a first electrode element, a second electrode element, a layer of semiconducting elements, and a layer of insulating material. The second electrode element is comprised of a metal, e.g., a low work function metal. The layer of semiconducting elements, which includes a first side and a second side, comprises a plurality of semiconducting carbon nanotubes. The layer of insulating material includes a first side and a second side. Within this photovoltaic device the first side of the layer of semiconducting elements is electrically coupled to the first electrode element, and the first side of the layer of insulating material is electrically coupled to the second electrode element. The second side of the layer of semiconducting elements is electrically coupled to the second side of said layer of insulating material. Further, at least a portion of the layer of insulating material is positioned within a Schottky barrier formed between the layer of semiconducting elements and the second electrode element.
The present disclosure also describes a photovoltaic device comprising a first electrode element, a second electrode element, and a composite active layer. The composite active layer, which includes a first side and a second side, comprises a first plurality of semiconducting nanotube elements and a second plurality of silicon particles. Within this photovoltaic device the first side of the composite active layer is electrically coupled to the first electrode element, and the second side of the composite active layer is electrically coupled to the second electrode element.
Within one aspect of the present disclosure a layer of p-type semiconducting nanotubes are disposed adjacent to a layer of n-type semiconducting nanotubes to realize a p-n junction photovoltaic device.
Within another aspect of the present disclosure a mixed layer of p-type and n-type semiconducting nanotubes is formed to realize a bulk hetero junction photovoltaic device.
Within another aspect of the present disclosure a layer of semiconducting nanotubes is disposed adjacent to a metal, e.g., a low work function metal electrode to realize a Schottky barrier photovoltaic device.
Within another aspect of the present disclosure a layer of insulating material is disposed between a layer of semiconducting nanotubes and a layer of metal, e.g., a low work function metal to realize a metal-insulator-semiconductor (MIS) photovoltaic device.
Within another aspect of the present disclosure photosensitive particles are used within a layer of semiconducting nanotubes to improve the light absorption rate of the layer.
Within another aspect of the present disclosure a layer of metallic nanotubes are used to provide a flexible electrode element for a photovoltaic device.
Within another aspect of the present disclosure a layer of metallic nanotubes are used to provide a substantially transparent electrode element for a photovoltaic device.
Other features and advantages of the present disclosure will become apparent from the following description of the disclosure which is provided below in relation to the accompanying drawings.
The present disclosure teaches a plurality of photovoltaic (PV) devices which use layers of semiconducting carbon nanotubes as the light absorption layer.
In some embodiments of the present disclosure a p-n junction PV device is realized by forming a layer of p-type semiconducting carbon nanotubes adjacent to a layer of n-type semiconducting carbon nanotubes, creating a p-n junction across the interface of the two layers. Within such embodiments, the p-type carbon nanotube layer acts as a light absorption material, releasing valance electrons into the conduction band of the nanotube structures when photons are absorbed. In some aspects of these embodiments, these p-type carbon nanotube layers include photosensitive particles such as, but not limited to, photosensitive dyes such as ruthenium-polypyridine and quantum dots made from III-V compounds such as GaAs, GaSb, and InP or II-VI compounds such as CdS, CdSe, ZnS, and ZnSe. Such nanoparticles can be dispersed in the nanotube layers (e.g., by spin coating or spray coating a mixture carbon nanotubes and photosensitive particles onto an electrode) to increase the rate of photon absorption.
In other embodiments of the present disclosure a bulk hetero junction PV device is realized by forming a single mixed layer of n-type and p-type semiconducting carbon nanotubes. Within such embodiments, p-type and n-type carbon nanotube elements are mixed together on a nanometer scale allowing for carrier diffusion of photoexcited electrons through the nanotube layer. In some aspects of these embodiments, this mixed p-type and n-type carbon nanotube layer is infused with photosensitive particles (such as, but not limited to, photosensitive dyes and quantum dots as described above) to increase the rate of photon absorption.
In other embodiments of the present disclosure a PV device is realized by forming a layer of p-type semiconducting carbon nanotubes adjacent to a metal, e.g., a low work function metal layer (such as, but not limited to, calcium, potassium, manganese, silver, aluminum, zinc, titanium, and iron). A Schottky barrier is created along the interface between the p-type nanotube layer and the metal layer, allowing the transport of minority carriers across the interface. Within such embodiments, the p-type carbon nanotube layer acts as a light absorption material, releasing valance electrons into the conduction band of the nanotube structures when photons are absorbed. In some aspects of these embodiments, these p-type carbon nanotube layers include photosensitive particles (such as, but not limited to, photosensitive dyes and quantum dots as described above) to increase the rate of photon absorption.
In other embodiments of the present disclosure a metal-insulator-semiconductor (MIS) PV device is constructed by forming a thin barrier layer of insulating material between a layer of p-type carbon nanotubes and a metal, e.g., a low work function metal layer (such as, but not limited to, calcium, potassium, manganese, silver, aluminum, zinc, titanium, and iron). Within such embodiments, the insulating barrier layer enhances the transport of photo generated charge carriers (photoexcited electrons within the p-type nanotube layer) through the PV device.
Within the embodiments of the present disclosure, PV devices comprise one or more nanotube layers which are formed over or adjacent to other material layers. The formation of such nanotube layers is taught in several of the incorporated references. For example, U.S. Pat. No. 7,335,395 to Ward et al., incorporated herein by reference in its entirety, teaches a plurality of methods for forming nanotube layers and films on a substrate element using preformed nanotubes. The methods include, but are not limited to, spin coating (wherein a solution of nanotubes is deposited on a substrate which is then spun to evenly distribute the solution across the surface of the substrate), spray coating (wherein a plurality of nanotube are suspended within an aerosol solution which is then disbursed over a substrate) and roll-to-roll coating (or roll coating, for brevity) such as Gravure coating (wherein an engraved roller with a surface spinning in a coating bath picks up the coating solution in the engraved dots or lines of the roller, and where the coating is then deposited onto a substrate as it passes between the engraved roller and a pressure roller). Further, U.S. Pat. No. 7,375,369 to Sen et al., incorporated herein by reference in its entirety, teaches solvents that are well suited for suspending nanotubes and for forming a nanotube layer over a substrate element via a spin coating process. For example, such solvents include but are not limited to ethyl lactate, dimethyl sulfoxide (DMSO), monomethyl ether, 4-methyl-2 pentanone, N-methylpyrrolidone (NMP), t-butyl alcohol, methoxy propanol, propylene glycol, ethylene glycol, gamma butyrolactone, benzyl benzoate, salicyladehyde, tetramethyl ammonium hydroxide and esters of alpha-hydroxy carboxylic acids. Such solvents can disperse the nanotubes to form a stable composition without the addition of surfactants or other surface-active agents.
Depending on their physical structure, individual carbon nanotubes can be highly conductive or semiconducting (such that they behave like silicon). The conductivity of an individual carbon nanotube is determined by the orientation of the hexagonal rings around the wall of the nanotube. This orientation is referred to as the chirality (or twist) of the nanotube by those skilled in the art and can be quantified as the angle between the hexagonal pattern of the individual carbon rings making up the wall of the nanotube and the axis of the nanotube itself. Within a typical distribution of fabricated carbon nanotubes, for example, roughly one third will be conducting (often simply referred to as metallic nanotubes) and two thirds will be semiconducting.
In some applications it is desirable to form a layer of carbon nanotube elements that includes substantially only semiconducting carbon nanotubes. For example, U.S. Published Patent. Application No. 20060183278 to Bertin et al., incorporated herein by reference in its entirety, teaches the construction of a FET device which includes a layer of semiconducting carbon nanotubes as the channel element. U.S. 20060183278 teaches a method of “burning off” the metallic nanotubes within a deposited nanotube layer during the fabrication of a FET device. A nanotube layer is deposited over a substrate and thermally isolated from the underlying substrate (in at least one embodiment, by forming a gap within the substrate beneath the nanotube layer). An electrical current is then passed through the nanotube layer. With no structure available to dissipate the heat generated, the thermally isolated metallic nanotubes within the nanotube layer are burnt off, leaving behind a nanotube layer comprised of substantially only semiconducting nanotubes.
Further, as the need for semiconducting carbon nanotubes increases, additional techniques are being developed within the industry to manufacture supplies of semiconducting only carbon nanotubes. Such techniques include methods to sort metallic carbon nanotubes from semiconducting nanotubes, as well as methods for fabricating carbon nanotubes such that the percentage of metallic nanotubes produced is much smaller than the percentage of semiconducting nanotubes produced. As these techniques continue to develop, supplies of semiconducting only carbon nanotubes are expected to become more readily available. Current separation techniques of metallic SWNTs and MWNTs impurities from s-SWNT result in s-SWNT concentrations in the range of greater than 80%, but less than 100%, with some metallic CNTs remaining. Examples of separation techniques in use are metallic burnoff, dielectrophoresis (e.g., AC dielectrophoresis and agarose gel electrophoresis), amine extraction, polymer wrapping, selective oxidation, CNT functionalization, and density-gradient ultracentrifugation.
Typically, semiconducting carbon nanotubes are formed as p-type semiconducting elements. However semiconducting carbon nanotubes can be converted to n-type semiconducting elements through a plurality of methods well known to those skilled in the art. These include, but are not limited to, high temperature thermal anneal processes and doping a layer of p-type carbon nanotubes with other materials such as nitrogen or potassium.
The layer 140 of semiconducting elements can comprise a fabric of semiconducting carbon nanotubes (CNTs) having a first conductivity type, which in this example is provided by a layer 135 of p-type carbon nanotubes (comprised of a plurality of individual p-type carbon nanotubes 135a). In this example, the layer 140 of semiconducting elements further comprises a plurality of semiconducting nanostructures, provided in this example by a layer 130 of n-type (second conductivity type) nanotubes (comprised of a plurality of individual n-type carbon nanotubes 130a), which can also be in the form of a fabric of nanotubes. Of course the first and second conductivity types could be reversed such that layer 130 was p-type and layer 135 was n-type. A nanostructure as referred to herein for the purposes of the present disclosure refers a structure having at least one dimension ranging in size from 1 nm to 100 nanometers. As shown in
A fabric of semiconducting carbon nanotubes as referred to herein for the present disclosure comprises a layer of multiple, interconnected carbon nanotubes. A fabric of nanotubes (or nanofabric), in the present disclosure, e.g., a non-woven CNT fabric, may, for example, have a structure of multiple entangled nanotubes that are irregularly arranged relative to one another. Alternatively, or in addition, for example, the fabric of nanotubes for the present disclosure may possess some degree of positional regularity of the nanotubes, e.g., some degree of parallelism along their long axes. The fabrics of nanotubes retain desirable physical properties of the nanotubes from which they are formed. The fabric preferably has a sufficient amount of nanotubes in contact so that at least one electrically semi-conductive pathway exists from a given point within the fabric to another point within the fabric. Individual nanotubes may typically have a diameter of about 1-2 nm and may have lengths ranging from a few microns to about 200 microns, for example. The nanotubes may curve and occasionally cross one another. Gaps in the fabric, i.e., between nanotubes either laterally or vertically, may exist. Such fabrics may comprise single-walled nanotubes, multi-walled nanotubes, or both. The fabric may have small areas of discontinuity with no tubes present. The fabric may be prepared as an individual layer or as multiple fabric layers, one formed upon another. The thickness of the fabric can be chosen as thin as substantially a monolayer of nanotubes or can be chosen much thicker, e.g., tens of nanometers to hundreds of microns in thickness. The porosity of the fabrics can be tuned to generate low density fabrics with high porosity or high density fabrics with low porosity. The porosity and thickness can be chosen as desired depending upon the application at hand. Such fabrics can be prepared by growing nanotubes using chemical vapor deposition (CVD) processes in conjunction with various catalysts, for example. Other methods for generating such fabrics may involve using spin-coating techniques and spray-coating techniques with preformed nanotubes suspended in a suitable solvent. Nanoparticles of other materials can be mixed with suspensions of nanotubes in such solvents and deposited by spin coating and spray coating to form fabrics with nanoparticles dispersed among the nanotubes. Such exemplary methods are described in more detail in the related art cited in the Background section of this disclosure.
The device 100 also includes a charge-separating junction 150 formed at the layer 140 of semiconducting elements. In particular, in this example, the charge-separating junction 150 is a p-n junction formed at the interface of the two semiconductor carbon nanotube layers (130 and 135). A charge-separating junction as referred to herein means a junction between two materials for which a difference in electrical characteristics, e.g., work functions or Fermi energies, causes a separation of charge, i.e., electrons and holes, at the junction. A p-n junction formed at an interface between a p-type semiconductor and an n-type semiconductor is one type of charge-separating junction. As discussed elsewhere herein in connection with other exemplary embodiments, a Schottky barrier formed at an interface of a metal and a semiconductor is another type of charge-separating junction. Because of the charge separation, a potential barrier or junction voltage is created at the junction. At the interface of the two semiconductor layers (130 and 135) the p-n junction 150 rectifies current flowing through the PV device 100.
The layer of semiconducting elements 140 has first side (at the upper surface of electrode 110) and a second side (at the lower surface of electrode 120). The first side of the layer 140 of semiconducting elements is electrically coupled to said first electrode element 110, and second side of the layer 140 of semiconducting elements is electrically coupled to the second electrode element 120. It will be appreciated that for elements to be electrically coupled for the present disclosure does not require that they be in direct electrical contact.
The first electrode element 110 can be formed of a metal such as a high work function metal such as, but not limited to, copper, gold, nickel, platinum, and ITO so as to make an ohmic contact or near-ohmic contact with the n-type carbon nanotube layer 130. The second electrode element 120 can also be formed of a metal such as a high work function metal such as, but not limited to, copper, gold, nickel, platinum, or formed from a conductive oxide such as, but not limited to, indium tin oxide (ITO) so as to form an ohmic or near-ohmic contact with the p-type carbon nanotube layer 135. Suitable electrode materials (e.g., metals) for forming ohmic or near-ohmic contacts can be selected by one skilled in the art by trial and error testing, for example, by measuring the electrical properties of the contact, e.g., resistance, to identify electrode materials that provide the desired contact properties. If the second electrode element 120 is formed from a non-transparent, electrically conductive material, e.g., a high work function metal, it can be formed in such a way as to allow light to penetrate through to the p-type carbon nanotube layer 135 such as, by depositing the second electrode material in long narrow strips or in a grid with gaps therebetween to permit light to pass through and impinge upon the carbon nanotube layer 135. The first electrode 110 can be supported by any suitable substrate (as can the other exemplary embodiments disclosed herein), if desired, such as a glass, polymer or semiconductor substrate, e.g., the first electrode 110 can be deposited on such a substrate by physical vapor deposition or chemical vapor deposition, for instance. The thickness of the first electrode element 110 may range from 1-100 microns, for instance. The thickness of the second electrode element 120 may also range from 1-100 microns, for example.
Photons 190 passing through the transparent protective layer 160 and the second electrode element 120 will reach the p-type carbon nanotube layer 135 and, in some cases, be absorbed. This absorption will cause a photoexcitation event, freeing a valance electron up into the conduction band where it can move freely through the nanotube layers 135 and 130. By placing a load or an electronic storage element (e.g., battery) across the first and second electrode elements (110 and 120), freed electrons within the p-type carbon nanotube layer 135 will tend to flow from the p-type carbon nanotube layer 135 into the n-type carbon nanotube layer 130, such that an electrical current is generated to the load or electronic storage element between the first and second electrode elements 110 and 120. In this way, solar energy (the impact of the photon 190) is converted into electrical current which can be used or stored by a circuit element placed across the first and second electrode elements 110 and 120.
Exemplary solvents for spinning, spraying, dipping or drawing may include, for example, dimethylformamide, n-methyl pyrollidinone, n-methyl formamide, orthodichlorobenzene, paradichlorobenzene, 1,2, dichloroethane, alcohols, and water with appropriate surfactants, such as, for example, sodium dodecylsulfate or TRITON X-100. Other exemplary solvents may include ethyl lactate, dimethyl sulfoxide (DMSO), monomethyl ether, 4-methyl-2 pentanone, N-methylpyrrolidone (NMP), t-butyl alcohol, methoxy propanol, propylene glycol, ethylene glycol, gamma butyrolactone, benzyl benzoate, salicyladehyde, tetramethyl ammonium hydroxide and esters of alpha-hydroxy carboxylic acids. The addition of surfactants or other surface-active agents may be used but is not required. The nanotube concentration and deposition parameters, such as surface functionalization, spin-coating speed, temperature, pH, and time, can be adjusted to control deposition of monolayers or multilayers of nanotubes as desired.
The first electrode element 110 itself can be formed of a metal such as a high work function metal such as, but not limited to, copper, gold, nickel, platinum, and ITO so as to make an ohmic or near-ohmic contact with the n-type carbon nanotube layer 130. The first electrode element can be self-supporting, e.g., a layer of conductive polymer material, or it can be deposited, e.g., by physical vapor deposition or chemical vapor deposition, onto any suitable substrate, such as a glass, polymer or semiconductor substrate.
The method also comprises at step S30 forming a second electrode element 120 at a second (upper) side of the layer 140 of semiconducting elements. The second electrode element 120 can be formed from a transparent, electrically conductive material (such as, but not limited to, ITO) or formed from other electrode materials such as high work function metals such as, but not limited to, copper, gold, nickel, platinum in such a way as to allow light to penetrate through to the p-type carbon nanotube layer 135 (such as, by depositing the second electrode material in long narrow strips or in a grid with gaps therebetween to permit light to pass through and impinge upon the carbon nanotube layer 135). The first electrode 110 can be supported by any suitable substrate (as can the other exemplary embodiments disclosed herein), if desired, such as a glass, polymer or semiconductor substrate, e.g., the first electrode 110 can be deposited on such a substrate by physical vapor deposition or chemical vapor deposition, for instance. The first side of the layer 140 of semiconducting elements is electrically coupled to said first electrode element 110, and second side of the layer 140 of semiconducting elements is electrically coupled to the second electrode element 120.
The method also comprises forming at least one charge-separating junction at the layer of semiconducting elements, which in this example is provided by p-n junction 150 formed at step S20. The charge separating junction could be formed at an interface of the layer of semiconducting elements or within the layer of semiconducting elements as will be apparent from the various embodiments disclosed herein. The method comprises at step S40 forming a protective layer 160 at an upper surface of the second electrode element 120.
Referring now to
The structure and operation of the p-n junction PV device 200 depicted in
The photosensitive particles 235b within p-type carbon nanotube layer 235 can increase the likelihood that a photon 290 passing through the layer 235 will be absorbed. In some embodiments, such particles 235b can be mixed with the individual p-type nanotube elements 235a prior to the formation of the p-type nanotube layer 235 and suspending in suitable solvents such as described above for deposition by spin coating, spray coating, dipping, drawing or roll coating the electrode element 210 with any previously deposited nanotube layers through a liquid of nanotubes and particles suspended in the solvent. Each of the layers 230 and 235 may be formed in one operation, or they may be formed in multiple operations wherein each of the layers 230 and 235 is formed of multiple sub-layers (e.g., layer 230 and layer 235 could be formed by several spinning, spraying, dipping or drawing operations, each of which forms a sub-layer). Such a process—that is forming a composite nanotube layer comprised of individual nanotube elements and nanoscopic particles—is described in U.S. patent application Ser. No. 12/274,033 to Ghenciu et al. These photo sensitive particles 240b can include, but are not limited to, photo sensitive dyes and quantum dots such as described above.
First electrode element 310—formed of a metal such as a high work function metal such as, but not limited to, copper, gold, nickel, and platinum—makes an ohmic or near-ohmic contact with a first side of the mixed nanotube layer 340. A second electrode element 320—also formed of a high work function metal or formed from a conductive oxide such as, but not limited to, indium tin oxide (ITO)—forms an ohmic or near-ohmic contact with a second side of mixed nanotube layer 340. This second electrode element 320 can be formed from a transparent material (such as, but not limited to, ITO) or formed in such a way as to allow light to penetrate through to mixed nanotube layer 340 (such as, by depositing the second electrode material in long narrow strips or in a grid). A transparent protective layer 360 is applied over the second electrode element 320.
Photons 390 passing through the transparent protective layer 360 and the second electrode element 320 will reach the mixed nanotube layer 340 and, in some cases, be absorbed by a p-type carbon nanotube element 340a. This absorption will cause a photoexcitation event, freeing a valance electron up into the conduction band where it can move freely through the mixed nanotube layer 340. By placing a load or an electronic storage element across the first and second electrode elements (310 and 320), freed electrons within mixed nanotube layer 340 will tend to flow through the mixed nanotube layer 340. In this way, solar energy (the impact of the photon 390) is converted into electrical current which can be used or stored by a circuit element placed across the first and second electrode elements 310 and 320.
Referring now to
The structure and operation of the bulk hetero-junction PV device 400 depicted in
The photosensitive particles 440c within mixed nanotube layer 440 increase the likelihood that a photon 490 passing through the layer 440 will be absorbed. In some embodiments, such particles 440c can be mixed with the individual p-type nanotube elements 440a and the individual n-type nanotube elements 440b prior to the formation of the mixed nanotube layer 440. The mixed nanotube layer 440 can be formed by methods already explained elsewhere herein, e.g., by suspending both p-type and n-type carbon nanotubes in a suitable solvent and depositing the mixture of p-type and n-type carbon nanotubes by spin coating, spray coating, dipping, drawing or roll coating., e.g., such as described in U.S. patent application Ser. No. 12/274,033 to Ghenciu et al. These photosensitive particles 440c can include, but are not limited to, photosensitive dyes and quantum dots such as described above.
A second electrode element 520—comprised of a metal such as a low work function metal (such as, but not limited to, calcium, potassium, manganese, silver, aluminum, zinc, titanium, and iron)—can be deposited using any suitable deposition method such as described elsewhere herein and forms a Schottky barrier 550 at the interface between the semiconducting nanotube layer 540 and the second electrode element 520. As with the p-n junction of the PV devices (100 and 200) depicted in
In this embodiment of the present disclosure, second electrode element 520 is formed in such a way as to allow light to penetrate through to the semiconducting nanotube layer 540 (such as, by depositing the second electrode material in long narrow strips or in a grid). A transparent protective layer 560 is applied over the second electrode element 520.
Photons 590 passing through the transparent protective layer 560 and the second electrode element 520 will reach the semiconducting nanotube layer 540 and, in some cases, be absorbed. This absorption will cause a photoexcitation event, freeing a valance electron up into the conduction band where it can move freely through the nanotube layer 540. By placing a load or an electronic storage element across the first and second electrode elements (510 and 520), freed electrons within the semiconducting nanotube layer 540 will tend to flow across the Schottky barrier 550 and into second electrode element 520. In this way, solar energy (the impact of the photon 590) is converted into electrical current which can be used or stored by a circuit element placed across the first and second electrode elements 510 and 520.
Referring now to
The structure and operation of the Schottky barrier PV device 600 depicted in
The photosensitive particles 640b within semiconducting nanotube layer 640 increase the likelihood that a photon 690 passing through the layer 640 will be absorbed. In some embodiments, such particles 640b can be mixed with the individual nanotube elements 640a prior to the formation of the semiconducting nanotube layer 640. Layer 640 can be formed by methods already explained elsewhere herein, e.g., by suspending both p-type and n-type carbon nanotubes in a suitable solvent and depositing the mixture of p-type and n-type carbon nanotubes by spin coating, spray coating, dipping, drawing, or roll coating e.g., such as described in U.S. patent application Ser. No. 12/274,033 to Ghenciu et al. These photosensitive particles 640b can include, but are not limited to, photosensitive dyes and quantum dots such as described above.
While Schottky barrier PV devices—such as the devices (500 and 600) depicted in FIGS. 5 and 6—can provide effective voltage generation in many applications, such devices can be limited, in some cases, by the low open circuit voltage and dark current typical of the Schottky barrier structure. To address this issue, a modified Schottky barrier PV device can be provided that utilizes a metal-insulator-semiconductor (MIS) three layer structure. A very thin layer of insulating material is placed between a layer of semiconducting material and a layer of metal such as a low work function metal. A Schottky barrier is formed across the layer of insulating material. This layer of insulating material allows minority carriers (electrons for p-type semiconducting material and holes for n-type semiconducting material) to pass through into the metal layer while preventing majority carriers (holes for p-type semiconducting material and electrons for n-type semiconducting material) from passing through. In this way, dark current—essentially the reverse bias leakage current introduced by the Schottky barrier—is significantly reduced, allowing for a more efficient PV device.
For example,
A thin layer of insulating material 780—such as, but not limited to, silicon dioxide (SiO2), titanium dioxide (TiO2), gallium trioxide (Ga2O3), tantalum pentoxide (Ta2O5), aluminum oxide (Al2O3), and diamond—is deposited over the semiconducting nanotube layer 740 using any suitable deposition method. Layer 780 may be deposited by physical vapor deposition such as sputtering and electron-beam evaporation and may range in thickness from a few monolayers to several nanometers, e.g., 1, 2, 3 nanometers, for instance, so as to be thin enough to permit electrons to tunnel through that layer. A second electrode element 720—comprised of a metal such as a low work function metal (such as, but not limited to, calcium, potassium, manganese, silver, aluminum, zinc, titanium, and iron)—is deposited over the layer of insulating material 780. A Schottky barrier 750 is formed across the insulating material layer 780 between the semiconducting nanotube layer 740 and the second electrode element 720.
As within the Schottky barrier PV devices (500 and 600) depicted in
In this embodiment of the present disclosure, second electrode element 720 is formed in such a way as to allow light to penetrate through to the semiconducting nanotube layer 740 (such as, by depositing the second electrode material in long narrow strips or in a grid). A transparent protective layer 760 is applied over the second electrode element 720.
Photons 790 passing through the transparent protective layer 760 and the second electrode element 720 will reach the semiconducting nanotube layer 740 and, in some cases, be absorbed. This absorption will cause a photoexcitation event, freeing a valance electron up into the conduction band where it can move freely through the nanotube layer 740. By placing a load or an electronic storage element across the first and second electrode elements (710 and 720), freed electrons within the semiconducting nanotube layer 740 will tend to flow across the Schottky barrier 750 and into second electrode element 720. In this way, solar energy (the impact of the photon 790) is converted into electrical current which can be used or stored by a circuit element placed across the first and second electrode elements 710 and 720.
A consideration associated with the design and fabrication of MIS PV devices is forming very thin and uniform layers of insulating material between the semiconducting layer and metal layer. Within traditional silicon based MIS PV devices, such insulating layers tend to be highly brittle (limiting the durability of such a device) and include a number of pin hole defects (small gaps within the insulating layer which significantly limit the effectiveness of the insulating layer). However, by using a layer of semiconducting nanotubes as the semiconducting layer within a MIS PV device, a number of techniques for reliably depositing thin, uniform, and robust insulating layers are available.
The structure and operation of the MIS PV device 800 depicted in
The structure and operation of the MIS PV device 900 depicted in
The structure and operation of the MIS PV device 1000 depicted in
Nonconductive nanoparticles can be realized from a plurality of materials including, but not limited to, silicon dioxide, aluminum oxide, titanium oxide, silicon nitride, and aluminum nitride.
Further, nonconductive nanoparticles can be adhered to a nanotube element via a plurality of methods, including, but not limited to:
The structure and operation of the MIS PV device 1100 depicted in
Within all such aspects, electron donor elements (the p-type elements) and electron acceptor elements (the n-type elements) are mixed together on a nanometer scale. That is, both types of semiconducting elements are separated by only nanometers within the composite active layer 1240. In this way, the composite active layer 1240 provides very efficient carrier diffusion of photoexcited electrons (freed when photons 1290 are absorbed by p-type elements within composite active layer) through the PV device 1200.
First electrode element 1210—formed of a metal such as a high work function metal such as, but not limited to, copper, gold, nickel, and platinum—makes an ohmic or near-ohmic contact with a first side of composite active layer 1240. A second electrode element 1220—also formed of a metal such as a high work function metal or formed from a conductive oxide such as, but not limited to, indium tin oxide (ITO)—forms an ohmic or near-ohmic contact with a second side of composite active layer 1240. This second electrode element 1220 can be formed from a transparent material (such as, but not limited to, ITO) or formed in such a way as to allow light to penetrate through to mixed nanotube layer 1240 (such as, by depositing the second electrode material in long narrow strips or in a grid). A transparent protective layer 1260 is applied over the second electrode element 1220.
Photons 1290 passing through the transparent protective layer 1260 and the second electrode element 1220 will reach the composite active layer 1240 and, in some cases, be absorbed by a p-type element within the composite active layer 1240. This absorption will cause a photoexcitation event, freeing a valance electron up into the conduction band where it can move freely through the composite active layer 1240. By placing a load or an electronic storage element across the first and second electrode elements (1210 and 1220), freed electrons within composite active layer 1240 will tend to flow through the composite active layer 1240. In this way, solar energy (the impact of the photon 1290) is converted into electrical current which can be used or stored by a circuit element placed across the first and second electrode elements 1210 and 1220.
The composite active layer 1240 may additionally comprise a plurality of photosensitive particles such as photosensitive dye particles or quantum dots as disclosed elsewhere herein in addition to the carbon nanotube elements and semiconductor nanostructures referred to above.
The PV devices described within the present disclosure all make use of electrode elements to make ohmic connections with semiconducting nanotube layers. While metallic layers such as high work function metallic layers are well suited to form these electrode elements, in some applications it is desirable to use layers of conductive nanotube elements (metallic nanotubes) to realize these electrode elements.
For example, nanotube layers are highly conformal when applied over an underlying material layer. In some aspects of the present disclosure, metallic nanotube layers could be used to realize PV devices of complex geometry including, but not limited to, curved, angled, and multi-faced PV devices. Also, in some cases metallic nanotubes can be formed into very thin layers which are both substantially transparent and highly conductive. For example, U.S. patent application Ser. No. 12/553,695 to Sen et al., incorporated herein by reference in its entirety, teaches the formation of thin, anisotropic nanotube fabric layers which would be well suited for forming transparent electrode elements within many of the PV devices taught in the present disclosure (the second electrode element 120 within the PV device 100 of
Further, nanotube layers are substantially flexible. A PV device comprising metallic nanotube layer electrode elements and semiconducting nanotube layer PV material can be used to form large, flexible PV devices. For example, such PV devices could be constructed over a large flexible substrate (such as, but not limited to, a plastic sheet) such as to realize a large, conformal PV device.
Further, while the different PV devices of the present disclosure are all depicted with transparent protective layers (layer 160 in
Each of the multiple photovoltaic devices 1410 can be formed according to the examples disclosed herein. As explained previously herein, such PV devices comprise a first electrode element, a second electrode element, and a layer of semiconducting elements disposed between the first and second electrode elements. The layer of semiconducting elements comprises a fabric of semiconducting carbon nanotubes having a first conductivity type such as explained previously herein, and at least one charge-separating junction is formed at the layer of semiconducting elements. The layer of semiconducting elements of each device 1410 has a first side and a second side, wherein the first side of the layer of semiconducting elements is electrically coupled to said first electrode element, and wherein the second side of said layer of semiconducting elements is electrically coupled to said second electrode element.
A flexible PV device such as illustrated in
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore the present invention not be limited by the specific disclosure herein.
