US RE39967 E1
A solid state photovoltaic device is formed on a substrate and includes a photoactive channel layer interposed between a pair of electrodes. The photoactive channel layer includes a first material which absorbs light and operates as a hole carrier. Within the first material are nanoparticles of a second material which operate as electron carriers. The nanoparticles are distributed within the photoactive channel layer such that, predominantly, the charge path between the two electrodes at any given location includes only a single nanocrystal. Because a majority of electrons are channeled to the electrodes via single nanocrystal conductive paths, the resulting architecture is referred to as a channel architecture.
1. A photoelectric device, comprising:
a first electrode formed on said substrate;
a second electrode spaced apart from and being in opposing relationship to the first electrode; and
a photoactive channel layer interposed between the first electrode and the second electrode, said the photoactive channel layer including a hole carrying material with and semiconductor particulate material including nanoparticles having an average diameter therein, said hole carrying material having a thickness in the range of one to two times the average diameter of the nanoparticles; and , wherein an average distribution of the semiconductor nanoparticles within the photoactive channel layer is such that predominantly, only a single nanoparticle is interposed between the first electrode and the second electrode along any straight-line path extending between oppositely-situated locations of the first and second electrodes
a second electrode; said photoactive channel layer being interposed between said first and second electrodes .
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12. A photoelectric device comprising
a first electrode formed on said substrate;
a second electrode spaced apart from and being in opposing relationship to the first electrode;
a first photoactive channel layer interposed between the first electrode and the second electrode, the photoactive channel layer including a hole carrying material and semiconductor nanoparticles, wherein an average distribution of the semiconductor nanoparticles within the first photoactive channel layer is such that predominantly, only a single nanoparticle is interposed between the first electrode and the second electrode along any straight-line path extending between oppositely-situated locations of the first and second electrodes;
a second electrode, said first photoactive channel layer being interposed between said first and second electrodes;
an electrical insulating layer, said insulating layer being formed on said second electrode;
a third electrode disposed over said insulating layer;
a second photoactive channel layer; and
a fourth electrode, said second photoactive channel layer being interposed between said third and fourth electrodes.
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The present application claims priority to U.S. Provisional Application, Ser. No. 60/103,758, entitled Solid-State Photodevice Using a Channel Architecture, which was filed on Oct. 9, 1998.
The present invention relates generally to photoelectric devices and more particularly relates to solid state photoelectric devices which employ a channel architecture.
Photovoltaic cells were first developed in the 1950's as p-n junctions of inorganic materials. A wide variety of cells since then have been fabricated using homojunction, heterojunction and tandem architectures with inorganic materials, most commonly silicon. As solar cells, the devices convert solar radiation (sunlight) directly into direct-current electrical power. However, widespread terrestrial use of the cells has been impeded by the high peak-watt energy cost compared with that derived from oil, natural gas, and coal.
Amorphous silicon has been contemplated as a highly promising alternative to the more expensive crystalline silicon and efforts have been undertaken at constructing cells from this material. However, at present the best silicon photovoltaic cells are about seven times more expensive than conventional energy sources.
An alternative to silicon-based cells was introduced by Graetzel et al. of EPFL-Lausanne, Switzerland. The cell developed by Graetzel et al. is about as efficient as the best amorphous silicon devices; however, these cells employ a liquid electrolyte which requires that the cells be hermetically sealed. In practice, such sealing can be difficult to achieve. If the cells are not properly sealed, the electrolyte can evaporate with a concomitant decrease in efficiency.
The cell introduced by Graetzel et al. is an example of a relatively efficient photovoltaic device which is fairly simple to fabricate using low-cost materials. The operating principle is based on the dye-sensitization of a wide-band gap metal oxide, nanoporous semiconductor layer. In particular, the layer is formed with an interconnected network of nanocrystals of titanium dioxide coated with a single molecular layer of a light absorbing ruthenium-based dye. When the dye layer absorbs light, electrons are transferred to the nanocrystal conduction band. The charge is transported through a number of nanocrystals in the nanocrystal layer (on the order of microns thick) to a transparent, conducting oxide electrode. The circuit is completed with an electrolyte with a rodox couple and a counter electrode impregnated with a platinum catalyst.
Conjugated polymers have been developed which are promising for a variety of electronic device applications, such as FETs, photovoltaic cells, LEDs and lasers. Photovoltaic devices using conjugated polymers blended with C60 have been formed in both a p-type junction architecture, as well as in an interpenetrating network architecture of semiconductor nanocrystals. However, the efficiencies of these devices tend to be much lower than those of the Graetzel device or silicon devices. Because of such inefficiencies, such devices are not currently viable candidates for widespread commercialization.
The photovoltaic devices in the prior art can be grouped into two basic architectures. The conventional photovoltaic devices (inorganic materials such as silicon) and several of the conjugated polymer devices are planar junction devices. In these devices, the free charge carriers, or excitons, created by light absorption diffuse to the junction interface where they are spatially separated, leading to the photovoltaic effect.
The other group embodies an interpenetrating network architecture and includes the Graetzel cell and the conjugated polymer devices blended with C60 and semiconductor nanocrystals. In this type of architecture, elements of one type of material, for example semiconductor nanocrystals or C60 molecules, interpenetrate another material where they are physically and electrically coupled to form a charge-transporting network. The network is necessary in the case of the dye-sensitized nanocrystal device to produce sufficient surface area, and thus dye area, to effect adequate light absorption. In devices formed from conjugated polymers blended with other polymers, nanocrystals or C60 molecules, the structure and operation of the devices is based on an interpenetrating network wherein elements of the latter materials are electrically interconnected and embedded within the polymer material.
Despite the extensive work previously conducted in the field of photovoltaics, there remains a need for a device which exhibits high efficiency, low cost and ease of fabrication which is formed in a fully solid-state embodiment.
An object of the present invention is to provide a solid-state photovoltaic or photoelectric device constructed from relatively low-cost materials which is inexpensive, simple to fabricate and exhibits a high efficiency for converting incident radiation to electric power.
In accordance with a first embodiment, a photoelectric device includes a substrate, a first electrode formed on the substrate, a photoactive channel layer and a second electrode. The photoactive channel layer is interposed between the first and second electrodes. A photoactive channel layer is defined as a layer formed with a first material and a second material wherein an average distribution of the first material within the second material is such that, predominantly, only a single particle of the first material is interposed between the first electrode and the second electrode along any imaginary normal axis extending between the two electrodes.
The photoactive channel layer can include a conjugated polymer material and a semiconductor particulate material. The semiconductor particulate material can take the form of nanoparticles having an average diameter and wherein the conjugated polymer material has a thickness in the range of one to two times the average diameter of the nanoparticles. The nanoparticles can be semiconductor crystals, such as titanium dioxide.
In an alternate embodiment, multiple photoelectric devices can be formed in a stacked configuration on a common substrate. In this case, the devices can be formed such that they are responsive to light in more than one spectral band. In either the single or stacked configuration, the device can be formed such that it is mechanically flexible.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention, in which:
Throughout the figures, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the subject invention will now be described in detail with reference to the figures, it is done so in connection with the illustrative embodiments. It is intended that changes and modifications can be made to the described embodiments without departing from the true scope and spirit of the subject invention as defined by the appended claims.
In the exemplary configuration of
The device illustrated in
The nanoparticles 204 generally take the form of semiconductor crystals, such as titanium dioxide (TiO2) and have a diameter in a range between 30 and 80 nanometers. The thickness of the polymer layer 202 is preferably on the order of between 1 and 1.5 times the average diameter of the nanoparticles 204. However, the layer 202 can be formed with sufficient thickness such that a region of the polymer material exists between the edge of the nanoparticles 204 and the second electrode 108. This polymer region acts as a blocking layer which inhibits bidirectional charge carrier flow, thereby reducing electron-hole recombination at the second electrode 108. Alternatively, the thickness of the polymer can be reduced and an additional blocking layer 302 can be formed between the polymer layer 202 and the second electrode 108, as illustrated in FIG. 3. The use of a separate blocking layer, in combination with a thinner polymer layer, may improve device efficiency as more excitons which form within the polymer layer 202 will be within one diffusion length of a nanocrystal interface 206. However, the addition of a separate blocking layer adds an additional processing step, which will generally increase the device cost.
The individual chains of the polymer forming polymer layer 202 can have a variety of size distributions. The nanoparticles 204 should be somewhat monodisperse, of roughly spherical morphology and exhibit an average diameter from on the order of a few nanometers to hundreds of nanometers. When distributed in the polymer layer 202, the spacing between nanoparticles 204 is preferably on the order of the diffusion length of an exciton in the particular polymer. This is generally in the range of about 10-30 nm, but can vary outside of this range.
The nanoparticles 204 can be prepared as a dispersion in methanol or water. The crystal structure of the nanoparticles 204 may be either anatase, rutile, brookite, some other form known in the field, or some mixture thereof. The soluble precursor form of the PPV, in a roughly 1 weight % preparation in 95:5 methanol:water (by volume) solvent, can be mixed with the nanocrystal sample to yield a mixture with about 65% by weight nanoparticles. The nanocrystal-polymer mixture may require some solvent evaporation to allow for a suitable viscosity for spin-coating. A portion 114 of the TCO layer 104 located in a region under contact 112 can be removed, such as by chemical etching (using HCl and Zn as a catalyst). This can reduce the likelihood of a short circuit between the first electrode 104 and second electrode 108 which can result from pressure exerted by contact 112 through the mechanically soft polymer-nanocrystal photoactive layer 106 when making contact to the device.
The polymer-nanocrystal mixture can be spin-coated on the TCO layer 104 at a suitable speed and duration to establish a desired thickness of the polymer layer 202 and distribution of the nanoparticles 204. For the above-described solution, spin coating at 1000 RPM for about 2 minutes provides suitable results. The polymer-nanocrystal solution should be allowed to dry for several minutes. The precursor-PPV in the film can be converted to the final form, PPV, by thermal heating at 150° C. in a nitrogen environment. The second electrode 108 can be formed over the photoactive layer 106 by vapor deposition of aluminum, or other suitable conductive material.
As illustrated in
In this embodiment, the blocking layer 302, second electrode 108, isolating layer 304 and third electrode layer 306 should be substantially transparent to allow transmission of incident light to the second photoactive layer 308. Preferably, the first photoactive layer 106 is selected to be responsive to a first wavelength of incident light whereas the second photoactive layer 308 is selected to be responsive to a second wavelength of incident light. For example, a PPV-TiO2 combination, which is responsive to light of about 350-500 nm, can be used for the first photoactive channel layer 106 and a MEH-PPV-TiO2 combination, which is responsive to light in the region of about 500-600 nm, can be used to form the second photoactive channel layer 308, where MEH-PPV is poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene).
The device materials and preparation conditions described herein may be widely varied while still achieving a photoelectric device with the desired channel architecture. The material composition, size and shape of the nanoparticles, as well as the type of polymer or the size distribution of the polymers can also be varied. For example, MEH-PPV, MEH-CN-PPV, POPT-poly(octylthiophene) or other such conjugated polymers may be used. Both the polymer and the nanocrystal materials can be doped to improve the charge transport properties. Materials other than conjugated polymers or nanoparticles may be used. For instance, particles of amorphous metal oxides may replace the nanoparticles, a dyestuff may replace the conjugated polymer, or a combination of dyestuffs and polymers can be used with nanoparticles of other suitable particles. In general, many combinations of material components can be used as long as at least two of the components exhibit a mutual, photoinduced charge transfer process and the materials are arranged in the channel architecture, i.e., a predominance of single nanocrystal charge paths between a first and second electrode at any given point.
The size, shape and composition of electrodes 104, 108 can be varied. For example, a wide range of metals including Cu, Al, Ag, Au and others may be used in addition to various TCOs. The electrodes can also be composed of polymeric electrodes such as PEDOT (a highly conductive p-doped polythiophene polymer). Furthermore, the composition of the supporting substrate 102 may be varied. For instance, the substrate layer 102 may be a polymer or polymer foil, such as polyethylene terephthalate, polyvinyl chloride, aramid or polyamide, and also metal foils provided with an insulating top layer, glass or other layer. The substrate 102 is generally transparent or translucent, however, the substrate can be formed from an opaque material if the materials on the opposing side of the photoactive channel layer(s) are formed from a transparent or translucent material.
The electrodes and substrates of the device can be curvilinear or mechanically flexible resulting in a light-weight device suitable for building applications among others. The device can be made integrally or separately (modular or in sections) according to the advantages of each mode of fabrication. The electrode etching can be done in a variety of methods available to a person skilled in the art. The photoactive channel layer 106 can be applied via means other than spin-coating, such as by spin-casting, spraying, chemical vapor deposition, physical vapor deposition, dip-coating, printing, painting, etc. The TCO layer 104 can be formed in a known manner, such as by Metal Organic Chemical Vapor Deposition (MOCVD), sputtering, Atmospheric Pressure Chemical Vapor Deposition (APCVD), PECVD, evaporation, screen printing, sol-gel processing, etc. Examples of materials suitable for use as the TCO layer 104 are indium tin oxide, tin oxide, SnO2:F, etc. For each of the processing steps described herein, a range of processing temperatures pressures, and other process parameters can be used to achieve the desired channel architecture.
The present invention employs a novel channel architecture to achieve a highly efficient, solid state photoelectric device. The channel architecture lends itself to the use of low cost materials and low cost manufacturing steps, which result in a cost-effective end product.
Although the present invention has been described in connection with specific exemplary embodiments, it should be understood that various changes, substitutions and alterations can be made to the disclosed embodiments without departing from the spirit and scope of the invention as set forth in the appended claims.