Patent Application
20080257407
For photoactive devices and systems, the utility of the device or system is generally measured in terms of the efficiency with which it takes the ambient or incident light and converts that light to another form of energy, e.g., electricity, heat or light of different wavelength. The overall conversion efficiency is impacted by a number of factors for any given circumstance, including the amount of ambient light that impacts the photoactive components of the device or system, the amount of light that the photoactive components are able top absorb, the efficiency with which the active component converts absorbed light to such other form of energy, and the ability to extract or transfer that energy to a point at which it can be accessed and exploited. In photoactive devices, the losses at each of these steps substantially affect the overall efficiency of the device. It is the inefficiencies at these steps that can have some of the most substantial impacts on the conversions efficiency, and for example, are among the major stumbling blocks of cost effective solar energy, as the costs associated with producing more efficient devices, and their resulting yields, have not yet moved into the realm of cost effectiveness relative to other forms of energy production, e.g., fossil fuels.
Researchers have explored all aspects of the efficiency and cost equation in efforts to bring the cost of solar energy into line with the cost of other energy forms. For example, conventional photovoltaic cells made using rigid semiconductor substrates have been developed to the point where they are capable of converting greater than 30% of the incident light into electricity. However, the costs for achieving these efficiencies have proven too high for all but the most cost insensitive applications, e.g., space and military applications. At the other end of the equation, researchers have explored methods of producing solar cells from lower cost materials using high volume, low cost manufacturing techniques. For example, composite active layers have been explored that employ nano- or micro-crystal composites as a portion or all of the photoactive component of a photovoltaic device or system. These composites claim the benefit of potentially being processible like thin films or liquids to permit high volume, low cost application, using conventional technologies available in the film processing industries, e.g., roll-to-roll processing and lamination processing.
Such high volume manufacturing could substantially reduce the costs associated with photovoltaic device production relative to conventional semiconductor processes, provided the efficiency of the device is high enough.
While potentially dramatically reducing the costs of manufacturing of photovoltaic devices, these composite technologies have not yet achieved efficiencies necessary to provide a commercially viable approach to solar energy based electricity production. As a result, there exists a substantial need to provide low cost manufacturable photoactive devices with substantially improved conversion efficiencies. The present invention meets these and a variety of other needs by addressing many of the aforementioned inefficiencies.
The present invention generally provides devices, compositions and methods for producing photoactive devices, systems and compositions that have improved conversion efficiencies relative to previously described devices, systems and compositions. This improved efficiency is generally obtained by one or both of improving the efficiency of light absorption into the photoactive component, and improving the efficiency of energy extraction from that active component.
The present invention is directed to improved photoactive devices and components that make up such devices. In particular, the invention relates to improved compositions, architectures and processes whereby one can produce photoactive devices that more efficiently absorb the incident light for ultimate conversion to another energy form, e.g., electricity. Also included are improvements to device architecture that enhance extraction of converted energy from these photoactive devices, to further improve efficiency.
In a first aspect, the invention provides a photoactive device that comprises a photoactive layer sandwiched between the first electrode and a second translucent electrode, wherein the device is configured to provide an elongated light path length for light entering into the photoactive layer through the second transparent electrode. The elongated path length may be provided by providing multiple photoactive layers, or by redirecting or reflecting light back into a single photoactive layer.
In a related aspect, the invention provides a photoactive device that comprises a first transparent electrode, a photoactive composite layer, and a back electrode that has at least a first surface. The photoactive composite layer is deposited upon the first surface of the back electrode, and the transparent electrode layer is provided over the photoactive composite layer. In this aspect of the invention, the first surface of the back electrode comprises a reflective surface to reflect light back into the photoactive layer.
In an alternate aspect, the invention provides a photoactive device that comprises at least first and second discrete photoactive layers sandwiched between a first electrode layer and a second electrode layer. At least a first transparent boundary layer is provided that separates the first photoactive layer from the second photoactive layer, where the boundary layer is substantially discrete from each of the first and second photoactive layers.
In a related aspect, the invention provides a photoactive device that comprises first and second photoactive layers disposed between first and second electrodes, the first and second photoactive layers being separated by a recombination layer. The recombination layer typically comprises a conductive material and is configured to selectively and substantially conduct electrons from but not to the first photoactive layer to but not from the second photoactive sublayer.
Relatedly, the invention provides a photoactive device that comprises a back electrode layer, a transparent top electrode layer, and a plurality of discrete photoactive layers disposed between the back electrode layer and the top electrode layer, wherein each of the plurality of photoactive layers is separated from each other photoactive layer by a charge recombination layer comprised of a material that is different from the photoactive layers.
In addition to the foregoing, the invention also provides a photoactive device that comprises a first electrode layer, a photoactive layer disposed upon the first electrode layer, and a second electrode layer disposed upon the photoactive layer. The photoactive layer comprises at least a first sublayer comprising an electron donor material and substantially no electron acceptor material, a second sublayer disposed upon the first sublayer that comprises a mixture of electron donor material and electron acceptor material; and a third sublayer disposed upon the second sublayer that comprises an electron acceptor material and substantially no electron donor material. Typically at least one of the electron donor and acceptor materials comprises nanoparticles, e.g., nanorods, quantum dots, bucky balls, nanofibers or nanowires.
The invention also provides a transparent photoactive device, comprising first and second electrode layers that are transparent to at least a portion of a visible light spectrum, and a photoactive layer, wherein the photoactive layer comprises a population of nanocrystals as at least a portion of the photoactive layer, and further wherein the photoactive layer is transparent to a portion of a visible light spectrum. This aspect of the invention finds application in, for example, electronic devices with display or viewing windows, where the transparent photoactive device is disposed over the viewing window and is electrically coupled to the electronic device to provide electric power without impeding viewing through the viewing window.
The invention also provides processes for producing the foregoing photoactive devices. For example, in at least one aspect, the invention provides a method of providing a photoactive device that comprises providing a back electrode layer having a first surface, depositing a nanocrystal/first conductive polymer composite layer on the first surface, depositing a transparent electrode layer over the composite layer, wherein the transparent electrode layer comprises a second conductive polymer disposed in a nonaqueous solvent, and evaporating away the nonaqueous solvent to leave a transparent electrode layer.
Nanocomposite photoactive or photovoltaic devices have been previously described in the art. For example, Published U.S. Patent Application No. 20040118448 (incorporated herein by reference in its entirety for all purposes) describes photovoltaic devices that employ a nanocomposite active layer sandwiched between two electrode layers. The active layer includes semiconductor nanocrystals dispersed within a conductive polymer matrix. Together, the nanocrystals and polymer form a diode, where the nanocrystal and polymer posses a type-II energy band gap offset relative to each other. When the nanocrystals are exposed to light, an electron is displaced from its orbital within the nanocrystal, giving rise to an electron-hole pair, collectively referred to as an “exciton” within the nanocrystal. Typically, when the exciton is allowed to recombine within the nanocrystal, it results in a release of the stored energy, e.g., in the form of light. In the nanocomposite photoactive layer, however, the electron and its hole are separated from each other with the hole being conducted away from the nanocrystal by the conductive polymer and the electron being conducted away by the nanocrystal itself. The electron and hole are further extracted from the photoactive by virtue of the opposing electrodes between which the active layer is disposed or sandwiched, which conduct the different carriers depending upon their respective work functions.
The general operation of a nanocomposite photoactive device as described above, is illustrated in
Inefficiencies in the above architecture can arise in a number of areas, including when electrons and holes recombine before they are separated, where light is not completely absorbed by the photoactive layer, e.g., light passes through the active layer prior to impacting and being absorbed by a photoactive component, or where separated charges must travel through multiple hops or phases before traveling to the respective electrode.
In at least a first respect, the present invention is aimed at improving the efficiency with which light is absorbed by a photoactive layer or an overall photoactive device, by increasing the probability that the light will be absorbed by a photoactive component, e.g., a nanocrystal. As a general matter, this is generally accomplished by configuring the device or components of such devices to increase the path length that light entering the photoactive layer will travel, and thereby enhance the probability that such light will be absorbed by the photoactive layer before it exits that layer or is absorbed by, e.g., a back electrode or surface or other non-photoactive component.
a. Redirection of Light to Improve Absorption
In a first aspect, the invention seeks to improve light absorption by redirecting light that passes through the active layer in order to increase the chance that the light will be absorbed by a photoactive component within the active layer. In particular, the present invention provides a reflective surface, and preferably a structured reflective surface upon the back electrode of the photoactive device so that light that passes completely through the active layer is reflected back into that layer for potential absorption by photoactive components. In preferred aspects, the light is reflected back orthogonally to the thickness dimension of the active layer, so as to provide a longer path along which it may be absorbed by a photoactive component, i.e., a photoactive nanocrystal.
While generally described in terms of nanocomposite photoactive layers that comprise a nanocrystal component and a conductive polymer component, e.g., a P3HT polymer, a number of the aspects of the invention, unless specifically required otherwise, may comprise a variety of different configurations of such photoactive layers, including, e.g., active layers that are just comprised of two different types of nanocrystals that possess a type-II energy band gap offset from each other, non-nanocrystal devices, e.g., that include only semiconductive polymers as the photoactive layer, and the like. Such compositions and architectures are generally described in Published U.S. Patent Application No. 20040118448, previously incorporated by reference herein.
In accordance with a first aspect of the invention, this unabsorbed light is sought to be absorbed by redirecting it through the photoactive layer to increase the likelihood that it will be absorbed. In particular, by reflecting the light back into the photoactive layer, one increases the chance that the light will impinge upon and be absorbed by a photoactive component. In merely reflecting the light directly back through the photoactive layer, one would expect to achieve absorption of approximately the same percentage of such light as was absorbed in the first pass. Specifically, if 50% of the light is absorbed on the first pass, approximately 50% of the reflected light should be absorbed on the second pass, for a total absorption of approximately 75% (allowing for losses in reflection, etc.). In order to ensure a higher percentage of absorption of the reflected light, the present invention not only reflects the unabsorbed light back into the photoactive layer, but directs such light at an angle that increases the reflected light's path-length through the photoactive layer so as to increase the likelihood of absorption before such light exits the photoactive layer.
Structuring of the reflective surface can generally take advantage of well known techniques that are used to enhance or redirect reflected light, e.g., as used in reflective surfaces, such as in signage, etc. For example, pyramid shaped or other prismatic surfaces or other structured surfaces are often employed to enhance reflection or redirect such reflection.
b. Multilayered Devices to Improve Absorption
In an alternative approach, a multiple photoactive layer device may be employed to increase the path length of relevant light entering into a given photoactive device, and thereby absorb light that is not absorbed in passing through a single photoactive layer. In particular, multiple photoactive layers are provided stacked upon each other, and separated by transparent electrode layers or charge recombination layers. Light that is not absorbed within the first photoactive layer passes through the separating layer and passes through a second photoactive layer, and optionally, a third, fourth, fifth or more layer.
A schematic illustration of a multilayered photoactive device of the invention is shown in
As noted, each of the photoactive layers is discrete from its neighboring photoactive layer. By “discrete” in this context, is generally meant that each layer is structurally separated from the adjoining layer through a discernible structural boundary or boundary layer (shown in
While described in terms of multiple layers to absorb optimal amounts of light, e.g., minimize light that passes through unabsorbed, it will be understood that the individual transparent device layers, e.g., not stacked into multiple layers, have substantial utility on their own. Specifically, a photoactive layer sandwiched between two transparent electrode layers may be applied on its own, e.g., in situations where light transmission is desired, but where electricity generation would also be of benefit. By way of example, such devices may be applied as layers of architectural glass, or as transparent power generators for use where there is limited space for a conventional, non-transparent photoactive cell. In such cases, one can exploit the photoactive cell as both a transparent barrier, e.g., a glass window, and exploit its power generation capabilities. Relatedly, one may use such transparent photoactive cells where a conventional cell would obscure the underlying component that is powered by that cell, or would otherwise add to the footprint of such a device. In particular, employing a transparent photocell over a visual display or viewing window would allow one to exploit the entire footprint of the display or viewing window for both power generation and viewing. Using conventional photocells, one would require a greater footprint size and would be required to place the photocell adjacent to the viewing area, thus increasing the overall footprint of the device. In a number of cases, this would be particularly preferred for low power display applications, e.g., shelf signage in stores, e.g., supermarkets, that employ small LCD displays, calculators, watches, clocks, handheld computer games, and the like. Likewise, such uses are directly related to architectural glass applications of the photoactive devices of the invention, where the entire surface of a window may also be employed in solar energy conversion. While the inability of the overall device/window may not provide optimal amounts of power generation, e.g., enough power to meet all needs of the building, due to its inability to absorb all incident light, the incremental power generation that was previously wasted provides significant advantages in energy efficient building design.
The transparent photoactive devices described above are generally electrically coupled to the underlying electronic device, or in the case of architectural glass, to a power conversion and storage facility within the building or directly connected to the window or glass component. In the case of electronic devices, and particularly those having electronic displays, the photoactive device is generally electrically connected to the device so as to provide electricity to the underlying display. Such connection may be direct, e.g., through appropriate circuitry connecting it to the display and/or the entire device that the display is applied to, e.g., a calculator, etc., or it may be indirectly connected, e.g., being connected to a battery which stores converted energy for later or unvarying supply of electricity. In either case, such photoactive devices are referred to as being electrically connected to the display.
In the foregoing photoactive devices, it will be appreciated that a substantial amount of light transmission is tolerated, and even desired, so that one can view the underlying display, or see through the window. Titration of the amount or wavelength of passed or transmitted light may depend upon the particular application, the type of light that is sought to be absorbed for the given device, e.g., the predominant light source, etc. In any event, the nanocrystal composites described herein are readily tuned to adjust their absorption spectra by adjusting the size and/or composition of the nanocrystal component of the composite.
In an alternative architecture, the boundary layer may comprise a layer of material that operates as a charge recombination layer for charges separated from the various photoactive layers, but that does not require any electrical connectors, e.g., pin-outs, attached to such intermediate layers. While referred to as a charge recombination layer, such terminology is primarily used to describe an intermediate or middle electrode layer (and both terms may be used interchangeably with the phrase “recombination layer”) that is not separately connected to the external circuit. While it is believed that this layer operates as a layer where charges recombine, the use of the term “recombination layer” should not be construed as binding the presently described invention to any particular theory of operation, and is primarily used for ease of discussion subject to the foregoing. In particular, stacking photoactive layers together, separated by a charge recombination layer, effectively functions in the same manner as batteries arrayed in series, such that it generates effectively the same current level as a single layer but at a higher voltage, thus providing higher power per unit area of photoactive device footprint.
As noted, in this particular aspect, boundary layers 412 comprise a charge recombination layer, e.g., a layer that receives electrons from one photoactive layer and holes from the other, and allows them to recombine. As noted, charge recombination layers are typically configured to selectively accept electrons from one photoactive layer and holes from the other photoactive layer adjoining that recombination layer. As such, recombination layer may be comprised of a variety of different materials, including, e.g., metal layers, i.e., gold, platinum, aluminum, indium-tin-oxide (ITO), etc. or may alternatively be comprised of conductive or semiconductive organic materials, e.g., polymers, carbon based materials, e.g., nanotubes, amorphous graphite, and the like. In many cases, a recombination layer may be comprised of more than one type of material layer. For example, a recombination layer typically includes a highly conductive material, but may also include a blocking layer to selectively block one charge carrier from one of the adjoining photoactive layers. As noted previously, these recombination layers are preferably transparent to allow light not absorbed by one layer top pass to the next layer. As such, in many cases, e.g., in the case of metals or other materials that are not generally transparent in a thicker bulk state, such layers may be provided at thin enough dimensions to remain transparent and/or translucent. Typically, such thin metal coatings are as used in glass coating processes for, e.g., architectural glass.
In operation, electron-hole pairs generated within each photoactive layer (shown by Ø for holes and e- for electrons) are separated within each of the discrete photoactive layers. Electrons are selectively conducted toward one recombination layer while holes are conducted to the other (by virtue of an included blocking layer and an increase in electrons in that particular recombination layer that further attracts holes to the recombination layer). Although referred to herein differently as electrons and holes, it will be understood that such designation generally refers to a directional flow of electrons, e.g., if holes are “conducted from node A to node B, it is the same as electrons being conducted from node B to node A. Likewise, in describing material as a hole conductor, it will be appreciated that such material is also termed an electron donor material, while electron conductors, as used herein, are also termed electron acceptor materials. Electrons and holes recombine within the recombination layer, while some are conducted to the electrodes to generate current, but at a higher voltage than with a single layer.
The various active layers may be comprised of different photoactive components in some cases, whereas in other cases, they may be comprised of the same materials. For example, where one wishes to tailor each layer to absorb different portions of the light spectrum that is incident upon the overall device, one may use materials in each layer that absorb at different wavelengths, thus allowing one to capture a broader portion of the spectrum. Alternatively, where one simply wishes to absorb the same wavelength of light in each layer, but capture more of that light, e.g., that which passes through a preceding layer, then the active photoactive layer components may be uniform among the different layers. Additionally, even where the component materials of different photoactive layers are the same, such layers may differ in their thickness, in their relative concentrations of, e.g., nanocrystal and polymer, and in their electrode make-up. For example, a variety of combinations of electrode metals or other materials, e.g., Al, Ag, Mg, Ca, Au, PEDOT, carbon materials, etc, may be provided in various combinations either between electrodes in a single device, or as an alloy or composite within a given electrode, and such materials may be varied across a multilayered device.
As noted above, the recombination layer is maintained as a discrete layer from the photoactive layers that it bounds. As a result, fabrication of a device including such layers requires the ability to mate discrete layers of different materials together. Such methods may involve lamination processes where different layers exist separately as films that are subsequently laminated together. While potentially useful, the requirements of intimate contact between layers may place a high burden upon such a lamination process. Alternatively, the recombination layer is deposited upon the underlying layer in a solution or otherwise fluid form or using a vapor or gas phase deposition technique. For example, in the case of metal recombination layers, such material may generally be evaporated onto the underlying layer or sputtered onto that underlying layer using well known techniques.
Where recombination layers are comprised of more than one sublayer, e.g., including a blocking layer, it may be that such other layer is deposited upon the first, photoactive layer by a film deposition method similar to that used to deposit the underlying layer, e.g., spin or tape casting methods, screen printing or spreading methods, e.g., using a doctor blade, etc. In many such cases, it is desirable to provide the additional layer without permitting intermixing with the underlying photoactive layer. As such, it may be necessary to provide the second layer in a solvent or matrix that prohibits any excessive resolubilization of or intermixing with the first layer, as well as preventing any other degradation of that underlying layer, e.g., by exposure to harmful solvents such as water, or exposure to oxygen, or the like. In at least a first example, a blocking layer is deposited over a photoactive layer, either as a portion of a recombination or other boundary layer, or as a layer adjacent to an electrode of the device. In depositing this blocking layer, it will be desirable to minimize resolubilization of the underlying active layer.
Additionally, it will be desirable to avoid exposing the underlying photoactive layer to any adverse conditions associated with deposition of the blocking layer material. For example, many conductive polymers, e.g., that are used as matrices for photoactive nanocomposites, are oxygen and/or water sensitive. As such, depositing a water based material or working in an oxygen rich environment can damage the underlying photoactive layer. Previously described blocking layers have used a conductive polymer, poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (“PEDOT-PSS”). Typically, such PEDOT-PSS has only been solubilized in aqueous solutions which could not be prepared or used in the oxygen-free and water-free environments often required for manipulation of the photoactive composites. In particular, contamination of water or oxygen sensitive materials, e.g., the photoactive nanocrystal/P3HT blends. Accordingly, the present invention takes advantage of a discovered ability to solubilize such organic conductive polymers in organic solvents, and particularly, alcohols, such as ethanol and methanol. In addition to being manipulable in water-free and oxygen-free environments, such alcohol-based solutions also benefit from reduced resolubilization of the underlying photoactive layer, as that layer is insoluble in the alcohol.
As noted previously, another method of improving the overall efficiency of photoactive devices of the invention, is to improve the efficiency with which charges are separated within and extracted from the photoactive layer. In particular, charge separation may be enhanced by providing an increased interface region between an electron donor and electron acceptor so as to prevent charge recombination within one or the other. Additionally, once the charges are separated, efficiencies could be improved by providing as direct a path as possible for a given carrier to its electrode while not permitting it to shunt to the other electrode or contact the other charge carrier in transit. The present invention addresses this by providing a photoactive layer that includes at least three sublayers: an electron donor layer, an electron acceptor layer, and a graded or mixed layer between the two.
Generally, at least one of the three sublayers will comprise a nanocrystal component. Further, in most aspects, at least one of the three sublayers will comprise a transparent material to allow for light absorption within the photoactive layer. In many aspects, one of the electron donor or acceptor material layers, e.g., the first and third sublayers, will comprise a bulk material. As used herein, a bulk material refers to a solid, monocrystalline, polycrystalline, or amorphous substrate that is usually nonporous. A variety of different architectures that fits these criteria may be used. A few of these are schematically illustrated in
In a first aspect,
An alternate arrangement/architecture of a multi-sublayered photoactive layer is illustrated in
Still another variation of the devices of the invention is illustrated in
As will be clear upon reading this disclosure, in preferred aspects of the invention, the intermediate sublayer will generally comprise nanocrystals as at least one of the electron donor or acceptor material, and in some cases both the electron donor and electron acceptor components. As such, the material in the layers adjacent to the electrodes will generally be selected from a semiconducting polymer, a nanocrystal material and/or a bulk material.
A. PEDOT from Ethanol/Methanol:
Devices incorporating PEDOT-PSS spun from ETHANOL yielded efficiencies of 3.0% and showed similar performance to regular PEDOT. Similar results have been achieved for devices with PEDOT-PSS spun from METHANOL (3.1%). The latter even exceeded the performance of regular devices (but it is not clear whether this was due to differences in PEDOT only—I would just claim that they are comparable, but not PEDOT spun from Methanol yields better performance).
B. Multilayer Devices
Multilayer devices have been fabricated, comprising two photoactive layers made of CdSe nanocrystal-P3HT blends. The first blend layer (˜40 to 60 nm thick) was deposited onto a transparent ITO/PEDOT substrate and covered with a transparent Al (8 to 12 nm) electrode and in some cases, an additional PEDOT layer, which was spun from Methanol (30 nm thick). A second blend layer (˜60 to 90 nm thick) was spun on top and covered with a 130 nm thick Al electrode. An electrode pattern was designed for the devices that allowed for independent pin-out of the respective electrodes, i.e. pin-out of the bottom cell only, the top cell only, and/or the entire cell. By this, independent measurement and understanding of the contributions of the respective layers was possible.
Devices have been fabricated, and they did show good performance, probably in the region of 2 to 3%, but so far, we cannot give accurate numbers, due to uncertainties in the actual device areas.
Although described in terms of nanocrystal components and bulk components, it will also be appreciated that an entirely polymer based system may be employed as well, e.g., with electron acceptor polymer layers and electron donor polymer layers separated by a mixed polymer layer. Similarly, while described in certain orientations in terms of whether the bulk or crystal layers are electron acceptor or electron donor materials, it will be clear to those of skill in the art that either the acceptor or donor material may exist as a bulk material, a nanocrystal material, a polymer material, or a composite of these.
This application is a divisional of U.S. patent application Ser. No. 11/271,484, filed Nov. 10, 2005, which claims priority to U.S. Provisional Patent Application No. 60/629,095, filed Nov. 17, 2004, which is incorporated in its entirety herein.
Portions of this invention may have been made with United States Government support under National Reconnaissance Office Prime Contract No. NRO-000-01-C-0130. As such, the United States Government may have certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 60629095 | Nov 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11271484 | Nov 2005 | US |
| Child | 12132967 | US |
