Self-cleaning coatings

SELF-CLEANING COATINGS

CROSS REFERENCE TO RELATED APPLICATIONS

0003This Application claims priority to United States Patent Application No. 11/315,523, filed December 21, 2005, which claims priority to United States Provisional Patent Application No. 60/700,982, filed July 19, 2005, each of which is hereby incorporated by reference herein in its entirety.

1. FIELD OF APPLICATION

0005This present application generally relates to apparatus comprising a high albedo layer and a self-cleaning layer overlayed on the high albedo layer. The present application further generally relates to a high albedo compositions having an albedo, where the high albedo compositions comprise a high albedo component intermixed with a self-cleaning component. The present application further generally relates to photovoltaic cell assemblies for converting photovoltaic energy into electrical energy and more particularly to improved photovoltaic cell energy conversion by maintaining the high albedo of the environment surrounding the photovoltaic cells.

2. BACKGROUND OF THE APPLICATION

0007Interest in photovoltaic cells, also known as solar cells, has grown rapidly in the past few decades. Photovoltaic cells comprise semiconductor junctions such as p-n junctions. It is known that light with photon energy greater than the band gap of an absorbing semiconductor layer in a semiconductor junction is absorbed. This absorption causes optical excitation and the release of free electrons and free holes in the semiconductor. Because of the potential difference that exists at a semiconductor junction {e.g., a p-n junction), these released holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. The flow of carriers into the external circuit constitutes a electrical current density, J amp cm^"2, which, under short-circuit conditions, is known as the short-circuit current density, J_sc. At the same time, the separation of the charges (holes and electrons) sets up a potential difference between the two ends of the material, φ , which under open circuit conditions is known as the open-circuit voltage, φ oc- It is desirable to maximize both J_sc and φ oc. For interaction with the solar spectrum, J_scandjz5 oc are optimized when the junction semiconductor absorber has a band gap of about 1.4 electron volts (eV).

0008It is presently common practice to provide an array of photovoltaic cells to generate electrical energy from solar radiation. Many photovoltaic cells are made of silicon. However, cells comprising other materials, e.g., cadmium sulfide and gallium arsenide, have also been developed and tested. Crystalline silicon has traditionally been a favored material since it has a band gap of approximately 1.1 eV and thus favorably responds to the electromagnetic energy of the solar spectrum. However, because of the expense in making crystalline silicon-based cells, thin film photovoltaic cells made of materials other than silicon have been explored and used.

0009Presently, many photovoltaic cells are fabricated as separate physical entities with light gathering surface areas on the order of 4-6 cm² or larger. For this reason, it is standard practice for power generating applications to mount the cells in a flat array on a supporting substrate or panel so that their light gathering surfaces provide an approximation of a single large light gathering surface. Also, since each cell itself generates only a small amount of power, the required voltage and/or current is realized by interconnecting the cells of the array in a series and/or parallel matrix. Any of the solar cell assemblies of the present application can be monfacial or bifacial.

00102.1 Photovoltaic cell mechanism and structure

0011The structure of a conventional prior art photovoltaic cell panel is shown in Figure 1. Because of the large range in the thickness of the different layers, they are depicted schematically. Moreover, Figure 1 is highly schematized so that it will represent the features of both "thick-film" photovoltaic cells and "thin-film" photovoltaic cells. In general, photovoltaic cells that use an indirect band gap material to absorb light are typically configured as "thick-film" photovoltaic cells because a thick film of the absorber layer is required to absorb a sufficient amount of light. Photovoltaic cells that use direct band gap materials to absorb light are typically configured as "thin-film" photovoltaic cells because only a thin layer of the direct band-gap material is need to absorb a sufficient amount of light.

0012The arrows at the top of Figure 1 show the direction of the solar illumination on the cell. Layer (element) 102 is the substrate. Glass or metal is a common substrate. In thin-film photovoltaic cells, substrate 102 can be-a polymer-based backing, metal, or glass. In some instances, there is an encapsulation layer, not shown, coating substrate 102. Layer 104 is the back electrical contact for the photovoltaic cell. It makes ohmic contact with the absorber layer of semiconductor junction 106. Layer 106 is the semiconductor absorber layer. In many but not all cases, it is a p-type semiconductor. Absorber layer 106 is thick enough to absorb light. Layer 108 is the semiconductor junction partner-that completes the formation of a p-n junction, which is a common type of junction found in photovoltaic cells. In a photovoltaic cell based on a p-n junction, when absorber 106 is a/?-type doped material, junction partner 108 is an n- type doped material. Conversely, when layer 106 is an «-type doped material, layer 108 is a p-type doped material. Generally, junction partner 108 is much thinner than absorber 106. For example, in some instances junction partner 108 has a thickness of about 0.05 microns. Junction partner 108 is highly transparent to solar radiation. Junction partner 108 is also known as the window layer, since it lets the light pass down to absorber layer 106.

0013In a typical thick-film photovoltaic cell, layers 106 and 108 can be made from the same semiconductor material but have different carrier types (dopants) and/or carrier concentrations in order to give the two layers their distinct p-typβ and n-type properties. In thin-film photovoltaic cells in which copper-indium-gallium-diselenide (CIGS) is absorber layer 106, the use of CdS to form layer 108 has resulted in high efficiency cells. Other materials that can be used for layer 108 include, but are not limited to, SnO₂, ZnO, ZrO₂ and doped ZnO.

0014Layer 110 is the top transparent electrode, which completes the functioning cell. Layer 110 is used to draw current away from the junction since junction partner 108 is generally too resistive to serve this function. As such, layer 110 should be highly conductive and transparent to light. Layer 110 can in fact be a comb-like structure of metal printed onto layer 108 rather than forming a discrete layer. Layer 110 is typically a transparent conductive oxide (TCO) such as zinc oxide (ZnO), indium-tin-oxide (ITO), or tin oxide (SnO₂). However, even when a TCO layer is present, a bus bar network 114 is typically needed to draw off current since the TCO has too much resistance to efficiently perform this function in larger photovoltaic cells. Network 114 shortens the distance charger carriers must move in the TCO layer in order to reach the metal contact, thereby reducing resistive losses. The metal bus bars, also termed grid lines, can be made of any reasonably conductive metal such as, for example, silver, steel or aluminum. There is design a tradeoff in network 114 between thicker grid lines that are more electrically conductive but block more light, and thin grid lines that are less electrically conductive but block less light. The metal bars are preferably configured in a comb-like arrangement to permit light rays through TCO layer 110. Bus bar network layer 114 and TCO layer 110, combined, act as a single metallurgical unit, functionally interfacing with a first ohmic contact to form a current collection circuit. In United States Patent Number 6,548,751 to Sverdrup et al, which is hereby incorporated by reference herein in its entirety, a combined silver (Ag) bus bar network and indium-tin-oxide layer function as a single, transparent ITO/ Ag layer.

0015Layer 112 is an antireflection (AR) coating, which can allow a significant amount of extra light into the cell. Depending on the intended use of the cell, it might be deposited directly on the top conductor (as illustrated), or on a separate cover glass, or both. Ideally, the AR coating reduces the reflection of the cell to very near zero over the spectral region that photoelectric absorption occurs, and at the same time increases the reflection in the other spectral regions to reduce heating. United States Patent Number 6,107,564 to Aguilera et al, which is hereby incorporated herein by reference in its entirety, describes representative antireflective coatings that are known in the art.

0016Photovoltaic cells typically produce only a small voltage. For example, silicon based photovoltaic cells produce a voltage of about 0.6 volts (V). Thus, photovoltaic cells are interconnected in series or parallel in order to get a reasonable voltage. When connected in series, voltages of individual cells add together while current remains the same. Thus, photovoltaic cells arranged in series reduce the amount of current flow through such cells, compared to analogous photovoltaic cells arrange in parallel, thereby improving efficiency. As illustrated in Figure 1, the arrangement of photovoltaic cells in series is accomplished using interconnects 116. In general, an interconnect 116 places the first electrode of one photovoltaic cell in electrical communication with the counterelectrode of an adjoining photovoltaic cell.

0017As noted above and as illustrated in Figure 1, conventional photovoltaic cells are typically in the form of a plate structure. Although such cells are highly efficient when they are smaller, larger planar photovoltaic cells have reduced efficiency because it is harder to make the semiconductor films that form the junction in such photovoltaic cells uniform. Furthermore, the occurrence of pinholes and similar flaws increase in larger planar photovoltaic cells. These features can cause shunts across the junction. Detailed descriptions of various types of photovoltaic cells within the scope of the present application, as defined by junction type, are in Section 4.4, below.

00182.2 Albedo energy and photovoltaic cells

0019Regardless of their difference in set-up and physical shape, photovoltaic cells function by converting solar radiation into electrical energy. Those skilled in the art will appreciate that the efficiency of a specific type of photovoltaic cell is proportional to the amount of photovoltaic radiation that is absorbed by the cell. Such absorption is influenced by numerous conditions including but not limited to cloud cover, location, geographic latitude, and time of the day. For example, an extremely cloudy day reduces the amount of photovoltaic radiation available for absorption by a photovoltaic cell system. In an effort to absorb more of the available photovoltaic radiation, workers have developed photovoltaic cell architectures such as the bi-facial photovoltaic array that has both a front and a back panel. United States Patent Number 5,990,413 to Ortabasi et al. describes such photovoltaic cell panels. The goal of the bi-facial design is to take advantage of both direct solar radiation and albedo.

0020Albedo is the fraction of incident electromagnetic radiation reflected by a surface. Albedo measures the reflectivity of a surface or body. It is the ratio of electromagnetic radiation reflected to the amount incident upon it. This ratio is a percentage that ranges anywhere from zero percent to one hundred percent. The amount of albedo a material exhibits is a function of its physical properties. Albedo is a product of the reflection of the incident solar radiation by the surroundings of the photovoltaic cells, such as sand, snow, water, rocks, grass, and buildings. Albedo is strongly dependent upon the nature of the environment. For example, albedo due to fresh snow can be higher than seventy percent while albedo due to large bodies of water is less than ten percent. In some instances, enhanced albedo improves the efficiency of existing photovoltaic cell assemblies.

0021Using albedo to improve photovoltaic cell efficiency has been described. For example, United States Patent 5,990,413 to Ortabasi et al. and United States Patent 5,344,496 to Stern et al., each of which is hereby incorporated by reference in its entirety, describe bi-facial and concentrator photovoltaic cell array designs, respectively, that make use of the albedo affect. High albedo materials, however, are often difficult to maintain. The albedo properties of a high albedo material change as the physical properties of the material change. Unfortunately, the albedo properties of material degrade over time. For example, numerous factors, such as bacteria growth, dirt built up, and/or watermarks on the surface of the material can all deleteriously affect the albedo property of such material. Such degradation in the albedo affect of materials adversely affects the efficiency of photovoltaic cell assemblies that make use of the albedo effect to improve efficiency since the efficiency of such systems is a function, in part, of the strength of such albedo effects.

0022In the art, regular cleaning of albedo surfaces has been done in order to maintain the albedo properties of materials in the vicinity of solar cells. However, such regular cleaning of such materials, in order to maintain their albedo properties, is inconvenient, time consuming, and expensive. For example, many conventional photovoltaic cell assemblies have a large number of photovoltaic cell panels arranged into concentrated arrays to optimize their efficiency. Because of this compact arrangement, the albedo surfaces around such photovoltaic cells are difficult to access for cleaning purposes. Furthermore, use of detergents to facilitate cleaning can have adverse effects on albedo surfaces that are sensitive to such chemicals. Reduced albedo effect can result from such cleaning. Moreover, some albedo surface cleaning regimens require the removal of photovoltaic cell panels prior to cleaning. Clearly, such efforts are costly and time consuming.

0023Some approaches to maintaining albedo surfaces involve repainting such surfaces with, for example, a highly reflective white paint. However, such approaches are inconvenient for many of the same reasons described above in relation to cleaning regimens. Such reasons include the difficulty in gaining access to such regions and the manual labor involved in painting. Further, this approach is not advantageous because it typically requires a cleaning prep stage, because of the expense of the paint used, and because, over time, an excessive buildup comprising multiple layers of paint foπn on the albedo surface that need to be stripped away in order to maintain the integrity of such surfaces.

0024Just as cleaning albedo surfaces is a necessary task in order to maintain the albedo properties of such surfaces, cleaning of photovoltaic cell panels themselves is necessary as well. However, such cleaning has the same drawbacks as the cleaning of albedo surfaces. Namely, such cleaning is time consuming, can degrade the material properties of the panels through adverse interaction (chemical, abrasive, or otherwise) with the cleaning agents, and is expensive. However, such cleaning may be necessary since any debris build up on photovoltaic cell panel surfaces adversely affects their efficiency.

0025Given the above background what is needed in the art are improved systems or methods for preserving the high albedo properties of albedo surfaces used in photovoltaic cell configurations. Moreover, what is needed in the art are improved systems and methods for cleaning photovoltaic cell panel surfaces. More generally, what is needed in the art are self-cleaning high albedo surfaces that can be used for a wide range of applications, including for use as decals on pavement and other surfaces, the solar cell industry, as building materials, and a wide range of other industries. 3. BRIEF DESCRIPTION OF THE DRAWINGS

0026FIG.l depicts a photovoltaic cell in accordance with the prior art.

0027FIG. 2A depicts a perspective view of a photovoltaic cell assembly in accordance with one embodiment of the present application.

0028FIG. 2B depicts a cross-sectional view of a photovoltaic cell assembly with portable self-cleaning albedo surfaces in accordance with one embodiment of the present application.

0029FIG. 3 illustrates a bifacial photovoltaic cell assembly in accordance with an embodiment of the present application.

0030FIG. 4 depicts a process for maintaining albedo surfaces and solar cells in accordance with one aspect of the present application.

0031FIG. 5 depicts various junction types in photovoltaic cell systems in accordance with embodiments of the present application.

0032FIG. 6A depicts a self-cleaning mechanism in a photocatalytic protective layer in accordance with an embodiment of the present application.

0033FIG. 6B depicts a hydrophilic mechanism in a photocatalytic self-cleaning protective layer in accordance with an embodiment of the present application.

0034FIG. 7 depicts a self-cleaning mechanism of a non-photocatalytic hydrophobic protective layer in accordance with an embodiment of the present application.

0035FIG. 8 depicts a monolithically integrated solar cell that can be used in the present application.

0036FIG. 9A illustrates a solar cell with tubular casing, in accordance with an embodiment of the present application.

0037Fig. 9B illustrates a cross-sectional view of an elongated solar cell in a transparent tubular casing, in accordance with an embodiment of the present application.

0038Like referenced elements refer to corresponding parts throughout the several views of the drawings.

4. DETAILED DESCRIPTION

0040To address the problems found in the prior art, one aspect of the present application provides self-cleaning high albedo surfaces that can be used for a wide range of applications, including for use as decals on pavement and other surfaces, the solar cell industry, as building materials, and a wide range of other industries.

0041One aspect of the present application provides systems and methods for maintaining the efficiency of photovoltaic cells by use of self-cleaning albedo surfaces and self-cleaning photovoltaic cell panels. In some instances, self-cleaning albedo surfaces and photovoltaic cell panels are prepared by applying one or more layers of self-cleaning material onto the surface of photovoltaic cell panels as well as albedo surfaces. In this way, the drawbacks found in the prior art, namely laborious time consuming cleaning and repainting efforts, are minimized, thereby yielding a more efficient cost effective photovoltaic cell system.

0042One aspect of the application provides a photovoltaic cell system comprising a plurality of photovoltaic cell assemblies. Each assembly in the plurality of photovoltaic cell assemblies comprises a front side. Each of these front sides is configured to receive electromagnetic energy. Each of the photovoltaic assemblies in the plurality of photovoltaic assemblies is configured to convert electromagnetic energy into an electrical current. The photovoltaic cell system further comprises an albedo surface configured to reflect electromagnetic energy on one or more photovoltaic cell assemblies in the plurality of photovoltaic cell assemblies. The albedo surface comprises a high albedo layer and a self-cleaning layer disposed on the high albedo layer.

0043In some embodiments, the self-cleaning layer comprises a photocatalytic material, Ti-hydroxyapetite, anatase TiO₂, brookitein TiO₂ or rutile TiO₂. In some embodiments, the self-cleaning layer comprises between 1 percent TiO₂ and 90 percent TiO₂ by weight. In some embodiments, the self-cleaning layer comprises ZnO, SnO₂, SrTiO₃, WO₃, Bi₂O₃, Fe₂O_3> Teflon, polytetrafluoroethylene or polychlorotrifluoroethylene. In some embodiments, the self-cleaning layer is formed from perotitanium solution or perox- modifϊed anatase sol. In some embodiments, the self-cleaning layer comprises a plurality of nano-particles. Such nano-particles can comprise, for example SiO₂, TiO₂, Al₂O₃, ZrO₂ or SnO₂, silica, or any combination thereof. In some embodiments, the self-cleaning layer comprises (100-X)CaF₂ -(X)TiO₂, where X is the molar percentage of TiO₂ and X is between 1 percent and 50 percent. In some embodiments, the self-cleaning layer comprises a composite of CaF₂ and TiO₂.

0044In some embodiments, the self-cleaning layer comprises a photocatalytic oxide and a resin. In some embodiments, the resin comprises a silicone resin or a fluororesin. Examples of suitable silicone resins include, but are not limited to ethyltrimethoxysilane, methyltriethoxysilane, methyltrichlorosilane, methyltribromosilane, methyltriisopropoxysilane, methyl-tri-t-butoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltrichlorosilane, ethyltribromosilane, ethyltriisopropoxysilane, ethyl-tri-t-butoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, n- propyltrichlorosilane, n-propyltribromosilane, n-propyltriisopropoxysilane, n-propyl-tri-t- butoxysilane, n-hexyltrimethoxysilane, n-hexyltriethoxysilane, n-hexytrichlorosilane, n-hexyltribromosilane, n-hexyltriisopropoxysilane, n-hexyl-tri-t-butoxysilane, n- decyltrimethoxysilane, n-decyltriethoxysilane, n-decyltrichlorosilane, n- decyltribromosilane, n-decyltriisopropoxysilane, n-decyl-tri-t-butoxysilane, n- octatrimethoxysilane, n-octatriethoxysilane, n-octatrichlorosilane, n-octatribromosilane, n-octatriisopropoxysilane, n-octa-tri-t-butoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, phenyltrichlorosilane, phenyltribromosilane, phenyltriisopropoxysilane, phenyl-t-butoxysilane, dimethyldichlorosilane, dimethyldibromosilane, dimethyldimethoxysilane, dimethyldiethoxysilane, diphenyldichlorosilane, diphenyldibromosilane, diphenyldimethoxysilane, diphenyldiethoxysilane, phenylmethyldichlorosilane, phenylmethyldibromosilane, phenylmethyldimethoxysilane, phenylmethyldiethoxysilane, vinyltrichlorosilane, vinyltribromosilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriisopropoxysilane, vinyl-tri-t-butoxysilane, trifluoropropyltrichlorosilane, trifluoropropyltribromosilane, trifluoropropyltrimethoxysilane, trifluoropropyltriethoxysilane, trifluoropropyltriisopropoxysilane, trifluoropropyl-tri-t- butoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropyltriethoxysilane, γ-glycidoxypropyltriisopropoxysilane, γ-glycidoxypropyl-tri-t-butoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ-methacryloxypropylmethyldiethoxysilane, γ -methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, γ-methacryloxypropyltriisopropoxysilane, γ-methacryloxypropyl-tri-t-butoxysilane, γ-aminopropylmethyldimethoxysilane, γ-aminopropylmethyldiethoxysilane, γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-aminopropyltriisopropoxysilane, γ-aminopropyl-tri-t- butoxysilane, γ-mercaptopropylmethyldimethoxysilane, γ- mercaptopropylmethyldiethoxysilane, γ-mercaptopropyltrimethoxysilane, γ- mercaptopropyltriethoxysilane, γ-mercaptopropyltriisopropoxysilane, γ-mercaptopropyl- tri-t-butoxysilane, β-(3,4-poxycyclohexyl)ethyltrimethoxysilane, β-(3,4- epoxycyclohexyl)ethyltriethoxysilane, or any combination thereof. Examples of fluororesin include, but are not limited to, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polychlorotrifluoroethylene, tetrafluoroethylene/hexafluoropropylene copolymer, ethylene/tetrafluoroethylene copolymer, ethylene/chlorotrifluoroethylene copolymer, tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer, perfluorocyclo polymer, vinyl ether/fluoroolefin copolymer, vinyl ester/fluoroolefin copolymer, tetrafluoroethylene/vinyl ether copolymer, chlorotrifluoroethylene/vinyl ether copolymer, urethane-crosslinked tetrafluoroethylene, epoxy-crosslinked tetrafluoroethylene, acryl- crosslinked tetrafluoroethylene, melamine-crosslinked tetrafluoroethylene, or any combination thereof.

0045Another aspect of the present application provides a high albedo composition having an albedo, where the high albedo composition comprises a high albedo component intermixed with a self-cleaning component. Such a high albedo composition can be formed, for example, by mixing the high albedo component with the self-cleaning component. In some embodiments, the albedo of this composition is 80 percent or greater, 85 percent or greater, 90 percent or greater, 95 percent or greater, 98 percent or greater, or 99 percent or greater. In some embodiments, the self-cleaning component is photocatalytic. In some embodiments, the self-cleaning component is Ti-hydroxyapetite, anatase TiO₂, rutile TiO₂ brookitein TiO₂, or between 1 percent and 90 percent TiO₂ by weight. In some embodiments, the self-cleaning component is ZnO, SnO₂, SrTiO₃, WO₃, Bi₂O₃, Fe₂O_3, or a mixture thereof. In some embodiments, the self-cleaning component is formed from perotitanium solution or perox-modified anatase sol. In some embodiments, the self-cleaning component is a plurality of nano-particles (e.g., particles made of SiO₂, TiO₂, Al₂O₃, ZrO₂ or SnO₂, silica, or any combination thereof). In some embodiments, the self-cleaning component is Teflon, polytetrafluoroethylene, polychlorotrifluoroethylene, or a composite of CaF₂ and TiO₂. In some embodiments, the self- cleaning component comprises a photocatalytic oxide and a resin. In some embodiments, the resin comprises a silicone resin or a fluororesin. In some embodiments, the resin comprises a silicone resin, and wherein the silicone resin comprises ethyltrimethoxysilane, methyltriethoxysilane, methyltrichlorosilane, ethyltribromosilane, methyltriisopropoxysilane, methyl-tri-t-butoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltrichlorosilane, ethyltribromosilane, ethyltriisopropoxysilane, ethyl-tri-t-butoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, n- propyltrichlorosilane, n-propyltribromosilane, n-propyltriisopropoxysilane, n-propyl-tri-t- butoxysilane, n-hexyltrimethoxysilane, n-hexyltriethoxysilane, n-hexytrichlorosilane, n- hexyltribromosilane, n-hexyltriisopropoxysilane, n-hexyl-tri-t-butoxysilane, n- decyltrimethoxysilane, n-decyltriethoxysilane, n-decyltrichlorosilane, n- decyltribromosilane, n-decyltriisopropoxysilane, n-decyl-tri-t-butoxysilane, n- octatrimethoxysilane, n-octatriethoxysilane, n-octatrichlorosilane, n-octatribromosilane, n-octatriisopropoxysilane, n-octa-tri-t-butoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, phenyltrichlorosilane, phenyltribromosilane, phenyltriisopropoxysilane, phenyl-t-butoxysilane, dimethyldichlorosilane, dimethyldibromosilane, dimethyldimethoxysilane, dimethyldiethoxysilane, diphenyldichlorosilane, diphenyldibromosilane, diphenyldimethoxysilane, diphenyldiethoxysilane, phenylmethyldichlorosilane, phenylmethyldibromosilane, phenylmethyldimethoxysilane, phenylmethyldiethoxysilane, vinyltrichlorosilane, vinyltribromosilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriisopropoxysilane, vinyl-tri-t-butoxysilane, trifluoropropyltrichlorosilane, trifluoropropyltribromosilane, trifluoropropyltrimethoxysilane, trifluoropropyltriethoxysilane, trifluoropropyltriisopropoxysilane, trifluoropropyl-tri-t- butoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ- glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ- glycidoxypropyltriethoxysilane, γ-glycidoxypropyltriisopropoxysilane, γ- glycidoxypropyl-tri-t-butoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ- methacryloxypropylmethyldiethoxysilane, γ -methacryloxypropyltrimethoxysilane, γ- methacryloxypropyltriethoxysilane, γ-methacryloxypropyltriisopropoxysilane, γ- methacryloxypropyl-tri-t-butoxysilane, γ-aminopropylmethyldimethoxysilane, γ- aminopropylmethyldiethoxysilane, γ-aminopropyltrimethoxysilane, γ- aminopropyltriethoxysilane, γ-aminopropyltriisopropoxysilane, γ-aminopropyl-tri-t- butoxysilane, γ-mercaptopropylmethyldimethoxysilane, γ- mercaptopropylmethyldiethoxysilane, γ-mercaptopropyltrimethoxysilane, γ- mercaptopropyltriethoxysilane, γ-mercaptopropyltriisopropoxysilane, γ-mercaptopropyl- tri-t-butoxysilane, β-(3,4-poxycyclohexyl)ethyltrimethoxysilane, β-(3,4-epoxycyclohexyl)ethyltriethoxysilane, or any combination thereof. In some embodiments, the resin comprises a fluororesin, and wherein the fluororesin comprises polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polychlorotrifluoroethylene, tetrafluoroethylene/hexafluoropropylene copolymer, ethylene/tetrafluoroethylene copolymer, ethylene/chlorotrifluoroethylene copolymer, tetrafluoroethylene/perfiuoroalkyl vinyl ether copolymer, perfluorocyclo polymer, vinyl ether/fluoroolefϊn copolymer, vinyl ester/fluoroolefin copolymer, tetrafluoroethylene / vinyl ether copolymer, chlorotrifluoroethylene/vinyl ether copolymer, urethane- crosslinked tetrafluoroethylene, epoxy-crosslinked tetrafluoroethylene, acryl-crosslinked tetrafluoroethylene, melamine-crosslinked tetrafluoroethylene, or any combination thereof.

0046Another aspect of the application provides a photovoltaic cell system comprising a plurality of photovoltaic cell assemblies. Each assembly in the plurality of photovoltaic cell assemblies comprises a base, a solar cell panel attached to the base, and a self- cleaning albedo device attached to the base. The self-cleaning albedo device comprises a flexible substrate, a high albedo layer, and a self-cleaning layer. The high albedo layer is disposed on the flexible substrate and the self-cleaning layer is disposed on the high albedo layer. In some embodiments, a photovoltaic cell system in accordance with this aspect of the application comprises a retractable roller mounted on the base of an assembly in the plurality of photovoltaic assemblies for retracting the self-cleaning albedo device into a roll. In some embodiments, the self-cleaning albedo device has an open configuration and a closed configuration. The self-cleaning albedo device is rolled up in the retractable roller when the self-cleaning albedo device is in the closed position. The self-cleaning albedo device is extended in a downward gradient away from the base when the self-cleaning albedo device is in the open position.

0047In some embodiments, the self-cleaning layer comprises a photocatalytic material, Ti-hydroxyapetite, anatase TiO₂, rutile TiO₂ or brookitein TiO₂. In some embodiments, the self-cleaning layer comprises between 1 percent TiO₂ and 90 percent TiO₂ by weight. In some embodiments, the self-cleaning layer comprises ZnO, SnO₂, SrTiO₃, WO₃, Bi₂O₃, Fe₂O_3, Teflon, polytetrafluoroethylene or polychlorotrifluoroethylene. In some embodiments, the self-cleaning layer is formed from perotitanium solution or perox- modified anatase sol. In some embodiments, the self-cleaning layer comprises a plurality of nano-particles. Such nano-particles can comprise, for example SiO₂, TiO₂, Al₂O₃, ZrO₂ or SnO₂, silica, or any combination thereof. In some embodiments, the self-cleaning layer comprises (100-X)CaF₂ -(X)TiO₂, where X is the molar percentage of TiO₂ and X is between 1 percent and 50 percent. In some embodiments, the self-cleaning layer comprises a composite Of CaF₂ and TiO₂.

0048Still another aspect of the present application provides an albedo protective composite. The albedo composite comprises a high albedo layer and a self-cleaning layer. In some embodiments in accordance with the present application, the high albedo layer has an albedo value of twenty percent or greater, twenty-five percent or greater, thirty percent or greater, thirty-five percent or greater, forty percent or greater, forty-five percent or greater, fifty percent or greater, or fifty-five percent or greater. In other embodiments in accordance with the present application, the high albedo layer has an albedo of sixty percent or greater, sixty-five percent or greater, seventy percent or greater, seventy-five percent or greater, eighty percent or greater, eighty -five percent or greater, ninety percent or greater or ninety-five percent or greater. The self-cleaning layer can be made of any of the materials described in relation to self-cleaning layers of other aspects of the application described above.

0049One aspect of the present application provides systems and materials to improve photovoltaic cell performance by implementing a self-cleaning function on photovoltaic cells and/or on albedo surfaces in the vicinity of photovoltaic cell assemblies. One aspect of the application provides materials for protecting albedo surfaces that surround photovoltaic cell assemblies, thereby maximizing energy input into the photovoltaic cell assemblies. Another aspect of the application provides materials for self-cleaning photovoltaic cell panels, thereby maintaining their efficiency. A third aspect of the present application provides designs for portable albedo collecting devices that are associated with photovoltaic cell assemblies to optimize energy efficiency.

00504.1 General architecture of one aspect of the application

0051One aspect of the present application provides systems and methods for improving existing photovoltaic cell assemblies that utilize albedo effects thereby improving their performance. As depicted in Fig. 2A, an albedo collector system in accordance with an embodiment of the present application comprises photovoltaic cell panels 210 and panel support systems 216. In some embodiments, photovoltaic cell panels 210 are bifacial. In such optional embodiments, each photovoltaic panel 210 has a back surface 214 in addition to a front surface 212. However, the present application is not limited to bifacial photovoltaic cell panels. Photovoltaic cell panels 210 are generally mounted on a surface 202. Surface 202 is typically not reflective enough to produce a high albedo effect. Therefore, in order to reflect solar light rays 218, surface 202 is covered with a high albedo layer that is comprised of a high albedo layer 206 of material such as white paint, white sand, white gravel, white reflective plastic, or any combination thereof. In some embodiments of the present application high albedo layer 206 has an albedo of seventy percent or more, eight percent or more, ninety percent or more, ninety-five percent or more, or ninety eight percent or more. The drawback with high albedo layer 206, as described in the background section above, is that it requires cleaning in order to maintain its high albedo properties. Advantageously, in the present application, high albedo layer 206 is coated with a self-cleaning layer 208 thereby alleviating the need for cleaning high albedo layer 206. In some embodiments, self-cleaning layer 208 is a single layer. In some embodiments, self-cleaning layer 208 is, in fact, a plurality of layers. In some embodiments the components used to maked self-cleaning layer 208 are intermixed with the components used to make high albedo layer 206 to form a single composite layer that has high albedo properties and self-cleaning properties.

0052In embodiments where self-cleaning layer 208 is in fact a plurality of layers, each such layer can made of the same or a different compound. Compounds that can be used to make self-cleaning layer 208 are described below and include photo and non- photocatalytic materials, both hydrophobic and non-hydrophobic. In some embodiments, the thickness of self-cleaning layer 208 is not greater than 0.2 μm. With such thicknesses, coloring of self-cleaning layer 208 due to the interference of light can be avoided. Moreover, self-cleaning layer 208 is more transparent when the layer is made thinner. Thus, in some embodiments, self-cleaning layer 208 is not greater than 0.1 μm.

0053In some embodiments, an optional reflector wall 204 is used. Like ground 202, optional reflector wall 204 is coated with high albedo layer 206' such as white paint. Advantageously, this high albedo layer 206' is coated with a self-cleaning layer 208' so that high albedo layer 206' does not have to be cleaned on a regular basis. The materials that can be used to form self-cleaning layer 208' are the same as those that can be used to form self-cleaning layer 208. Thus, although not explicitly mentioned below, it will be appreciated that any description of a possible composition of self-cleaning layer 208 is also a description of a possible composition for layer 208'. In some embodiments, self- cleaning layer 208 and/or 208' is one or more layers of a photocatalytic material, such as titanium dioxide or titanium-based nanoparticles, and/or a hydrophobic coating, such as a silicone and/or acrylic resin.

0054As described in the background section, dust or other debris collects on photovoltaic cell panels thereby degrading their efficiency. The present application addresses this problem by providing a self-cleaning layer to such photovoltaic cell panels. Fig. 3 demonstrates this architecture. Fig. 3 illustrates a bi-facial photovoltaic cell 300. Substrate 102, back-electrode 104, absorber 106, window layer 108, transparent conductive layer 110, and optional bus bar network 114 are as described above in conjunction with Fig. 1. The assembly is encapsulated with a material 302, such as EVA, and encased by panels 304. Panels 304 are typically made of glass or a plastic such as Tedlar. Although Fig. 3 is drawn as a bifacial assembly, the application is not so limited. Photovoltaic cells that include just one absorbing face benefit from the advantages of the present application as well. Advantageously, photovoltaic cell panel 300 is coated with a self-cleaning layer 306. In some embodiments, self-cleaning layer 306 is one or more layers of a photocatalytic material, such as titanium dioxide or titanium-based nanoparticles, and/or a non-photocatalytic hydrophobic coating, such as a blend of a silicone and/or acrylic resin. In some embodiments, the thickness of self-cleaning layer 306 is not greater than 0.2 μm. With such a thickness, coloring of self-cleaning layer 306 due to the interference of light can be avoided. Moreover, self-cleaning layer 306 is more transparent when the layer is made thinner. Thus, in some embodiments, self-cleaning layer 306 is not greater than 0.1 μm. In some embodiments, in accordance with Figs. 3, there is, more generally, a semiconductor junction 510 rather than simply an absorber 106 and window layer 108. Semiconductor junctions 510 are disclosed in more detail below.

0055Although the junctions in solar cells in Fig. 3 are illustrated as a planar, the application is not so limited. Within panels 304 and encapsulation layer 302, the solar cell junctions can adopt any known solar cell configuration, including planar (as shown), wire based, or rod based. As such, in some embodiments, panels 304 encase a series of parallel wires, where each wire comprises an elongated back electrode core 104, absorber 106, window layer 108, and transparent conductive layer 110 all circumferentially disposed on the elongated back electrode core 104. In one specific example in accordance with this embodiment, each elongated solar cell in the plurality of elongated solar cells comprises (i) a conductive core configured as a first electrode, (ii) a semiconductor junction circumferentially disposed on the conductive core, and (iii) a transparent conductive oxide layer disposed on the semiconductor junction. Elongated solar cells in the plurality of elongated solar cells are geometrically arranged in a parallel or a near parallel manner thereby forming a planar array having a first face and a second face. The plurality of elongated solar cells is arranged such that one or more elongated solar cells in the plurality of elongated solar cells do not contact adjacent elongated solar cells. The solar cell assembly further comprises a plurality of electrode strips. Each respective electrode strip in the plurality of electrode strips is lengthwise disposed on a first side of a corresponding elongated solar cell in the plurality of elongated solar cells. The first side of the solar cell is part of the first face of the planar array. The solar cell assembly further comprises a transparent electrically insulating substrate (panel 304) that covers all or a portion of the first face of the planar array. A first and second elongated solar cell in the plurality of elongated solar cells are electrically connected in series by an electrical contact that connects the first electrode of the first elongated solar cell to the corresponding electrode strip of the second elongated solar cell. The plurality of elongated solar cells is configured to receive direct light from the first face and the second face of the planar array. In fact, in some embodiments, the monolithically integrated solar cell such as that illustrated in Fig. 8 is used. In Fig. 8, layer 112 is 112 is an antireflective coating that can allow a significant amount of extra light into the cell. Preferably, the antireflective coating reduces the reflection of the cell to very near zero over the spectral region in which photoelectric absorption occurs, and at the same time increases the reflection in the other spectral regions to reduce heating. United States Patent Number 6,107,564 to Aguilera et al., hereby incorporated by reference herein in its entirety, describes representative antireflective coatings that are known in the art.

0056Some embodiments of the present application provide the solar cell depicted in Fig. 8 in which self-cleaning layer 306 (not shown in Fig. 8) is optionally deposited directly on layer 112 of the solar cell. In such embodiments, panel 304 of Fig. 3 is not required. Some embodiments of the present application provide the solar cell depicted in Fig. 8 in which self-cleaning layer 306 (not shown in Fig. 8) is optionally deposited directly on layer 110 of the solar cell and layer 112 is not used. In such embodiments, panel 304 of Fig. 3 is not required. Figure 8 an exemplary embodiment of a solar cell 270 comprising a plurality of photovoltaic units 700. In some embodiments, a solar cell 270 comprises a plurality of photovoltaic units 700 linearly arranged on substrate in a monolithically integrated manner as illustrated in Fig. 8. In some embodiments, the substrate is either (i) tubular shaped or (ii) a rigid solid rod shaped. In some embodiments the substrate is flexible tubular shaped. Examples of such solar cells are found in United States Patent Application Number 11/378,835, filed March 18, 2006, which is hereby incorporated by reference herein in its entirety.

0057Another example of solar cells that may be used rather than the flat planar solar cells are tube in tube solar cells such as that illustrated in Fig. 9 and further disclosed in United States Patent Application No. 11/378,847, filed March 18, 2006, which is hereby incorporated by reference in its entirety. In some embodiments in accordance with, for example Fig. 9, substrate 102 is made of a plastic, metal, metal alloy, or glass. In some embodiments, substrate 102 is cylindrical shaped. In some embodiments, substrate 102 has a hollow core, as illustrated in Figure 9B. In some embodiments, substrate 102 has a solid core. In some embodiments, the shape of substrate 102 is only approximately that of a cylindrical object, meaning that a cross-section taken at a right angle to the long axis of substrate 102 defines an ellipse rather than a circle. As the term is used herein, such approximately shaped objects are still considered cylindrically shaped in the present application. In some embodiments, substrate 102 is made of a urethane polymer, an acrylic polymer, a fluoropolymer, polybenzamidazole, polyimide, polytetrafluoroethylene, polyetheretherketone, polyamide-imide, glass-based phenolic, polystyrene, cross-linked polystyrene, polyester, polycarbonate, polyethylene, polyethylene, acrylonitrile-butadiene-styrene, polytetrafluoro-ethylene, polymethacrylate, nylon 6,6, cellulose acetate butyrate, cellulose acetate, rigid vinyl, plasticized vinyl, or polypropylene. In some embodiments, substrate 102 is made of aluminosilicate glass, borosilicate glass (e.g., Pyrex, Duran, Simax, etc.), dichroic glass, germanium / semiconductor glass, glass ceramic, silicate / fused silica glass, soda lime glass, quartz glass, chalcogenide / sulphide glass, fluoride glass, pyrex glass, a glass-based phenolic, cereated glass, or flint glass. In some embodiments, substrate 102 is a solid cylindrical shape. Such solid cylindrical substrates 102 can be made out of a plastic, glass, metal, or metal alloy.

0058In some embodiments, in accordance with, for example, Fig. 9, Back-electrode 104 is circumferentially disposed on substrate 102. Back-electrode 104 serves as the first electrode in the assembly. In general, back-electrode 104 is made out of any material such that it can support the photovoltaic current generated by solar cell unit 300 with negligible resistive losses. In some embodiments, back-electrode 104 is composed of any conductive material, such as aluminum, molybdenum, tungsten, vanadium, rhodium, niobium, chromium, tantalum, titanium, steel, nickel, platinum, silver, gold, an alloy thereof, or any combination thereof. In some embodiments, back-electrode 104 is composed of any conductive material, such as indium tin oxide, titanium nitride, tin oxide, fluorine doped tin oxide, doped zinc oxide, aluminum doped zinc oxide, gallium doped zinc oxide, boron dope zinc oxide indium-zinc oxide, a metal-carbon black-filled oxide, a graphite-carbon black-filled oxide, a carbon black-carbon black-filled oxide, a superconductive carbon black-filled oxide, an epoxy, a conductive glass, or a conductive plastic. As defined herein, a conductive plastic is one that, through compounding techniques, contains conductive fillers which, in turn, impart their conductive properties to the plastic. In some embodiments, the conductive plasties used in the present application to form back-electrode 404 contain fillers that form sufficient conductive current-carrying paths through the plastic matrix to support the photovoltaic current generated by solar cell unit 300 with negligible resistive losses. The plastic matrix of the conductive plastic is typically insulating, but the composite produced exhibits the conductive properties of the filler.

0059In some embodiment in accordance with, for example, Figure 9, a semiconductor junction 510 is formed around back-electrode 104. Semiconductor junction 510 is any photovoltaic homojunction, heterojunction, heteroface junction, buried homojunction, ap- i-n junction or a tandem junction having an absorber layer that is a direct band-gap absorber {e.g., crystalline silicon) or an indirect band-gap absorber (e.g., amorphous silicon). Such junctions are described in Chapter 1 of Bube, Photovoltaic Materials, 1998, Imperial College Press, London, as well as Lugue and Hegedus, 2003, Handbook of Photovoltaic Science and Engineering, John Wiley & Sons, Ltd., West Sussex, England, each of which is hereby incorporated by reference herein in its entirety. Details of exemplary types of semiconductors junctions 510 in accordance with the present application are disclosed in United States Patent Application 11/378,847, filed March 18, 2006, which is hereby incorporated by reference in its entierty. Junctions 510 can be multij unctions in which light traverses into the core of junction 510 through multiple junctions that, preferably, have successfully smaller band gaps. In some embodiments, semiconductor junction 510 include a copper-indium-gallium-diselenide (CIGS) absorber layer. Typically junction 510 comprises absorber 106 and window layer 108.

0060In some embodiment in accordance with, for example, Figure 9, optionally, there is a thin intrinsic layer (/-layer) 415 circumferentially coating semiconductor junction 510. This /-layer 415 can be formed using any undoped transparent oxide including, but not limited to, zinc oxide, metal oxide, or any transparent material that is highly insulating. In some embodiments, /-layer 415 is highly pure zinc oxide.

0061In some embodiment in accordance with, for example, Figure 9, there is transparent conductive layer 110 is circumferentially disposed on the semiconductor junction layers 510 thereby completing the circuit. As noted above, in some embodiments, a thin /-layer 415 is circumferentially disposed on semiconductor junction 510. In such embodiments, transparent conductive layer 110 is circumferentially disposed on /-layer 415. In some embodiments, transparent conductive layer 110 is made of tin oxide SnO_x (with or without fluorine doping), indium-tin oxide (ITO), doped zinc oxide (e.g., aluminum doped zinc oxide, gallium doped zinc oxide, boron dope zinc oxide), indium-zinc oxide or any combination thereof. In some embodiments, transparent conductive layer 110 is either p-άoped or n- doped. In some embodiments, transparent conductive layer is made of carbon nanotubes. Carbon nanotubes are commercially available, for example from Eikos (Franklin, Massachusetts) and are described in United States Patent 6,988,925, which is hereby incorporated by reference herein in its entirety. For example, in embodiments where the outer semiconductor layer of junction 510 is />-doped, transparent conductive layer 110 can bep-doped. Likewise, in embodiments where the outer semiconductor layer of junction 510 is 77-doped, transparent conductive layer 110 can be π-doped. In general, transparent conductive layer 110 is preferably made of a material that has very low resistance, suitable optical transmission properties {e.g., greater than 90%), and a deposition temperature that will not damage underlying layers of semiconductor junction 510 and/or optional /-layer 415. In some embodiments, transparent conductive layer 110 is an electrically conductive polymer material such as a conductive polytiophene, a conductive polyaniline, a conductive polypyrrole, a PSS-doped PEDOT (e.g., Bayrton), or a derivative of any of the foregoing. In some embodiments, transparent conductive layer 110 comprises more than one layer, including a first layer comprising tin oxide SnO_x (with or without fluorine doping), indium-tin oxide (ITO), indium-zinc oxide, doped zinc oxide (e.g., aluminum doped zinc oxide, gallium doped zinc oxide, boron dope zinc oxide) or a combination thereof and a second layer comprising a conductive polytiophene, a conductive polyaniline, a conductive polypyrrole, a PSS-doped PEDOT (e.g., Bayrton), or a derivative of any of the foregoing. Additional suitable materials that can be used to form transparent conductive layer are disclosed in United States Patent publication 2004/0187917Al to Pichler, which is hereby incorporated by reference herein in its entirety.

0062In some embodiments in accordance with Fig. 9, counter-electrode strips or leads 420 are disposed on transparent conductive layer 110 in order to facilitate electrical current flow. In some embodiments, electrode strips 110 are thin strips of electrically conducting material that run lengthwise along the long axis (cylindrical axis) of the cylindrically shaped solar cell, as depicted in Figure 9A. In some embodiments, optional electrode strips are positioned at spaced or nonspaced intervals on the surface of transparent conductive layer 110. For instance, in Figure 9B, electrode strips 420 run parallel to each other and are spaced out at ninety degree intervals along the cylindrical axis of the solar cell. In some embodiments, electrode strips 420 are spaced out at five degree, ten degree, fifteen degree, twenty degree, thirty degree, forty degree, fifty degree, sixty degree, ninety degree or 180 degree intervals on the surface of transparent conductive layer 412. In some embodiments, there is a single electrode strip 420 on the surface of transparent conductive layer 412. In some embodiments, there is no electrode strip 420 on the surface of transparent conductive layer 412. In some embodiments, there is two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, fifteen or more, or thirty or more electrode strips on transparent conductive layer 412, all running parallel, or near parallel, to each down the long (cylindrical) axis of the solar cell. In some embodiments electrode strips 420 are evenly spaced about the circumference of transparent conductive layer 412, for example, as depicted in Figure 9B. In alternative embodiments, electrode strips 420 are not evenly spaced about the circumference of transparent conductive layer 412. In some embodiments, electrode strips 420 are only on one face of the solar cell. Elements 102, 104, 510, 415 (optional), and 110 of Figure 9B collectively comprise solar cell 402 of Figure 9A. In some embodiments, electrode strips 420 are made of conductive epoxy, conductive ink, copper or an alloy thereof, aluminum or an alloy thereof, nickel or an alloy thereof, silver or an alloy thereof, gold or an alloy thereof, a conductive glue, or a conductive plastic. In some embodiments, there are electrode strips that run along the long (cylindrical) axis of the solar cell and these electrode strips are interconnected to each other by grid lines. These grid lines can be thicker than, thinner than, or the same width as the electrode strips. These grid lines can be made of the same or different electrically material as the electrode strips. In some embodiments, electrode strips 420 are deposited on transparent conductive layer 412 using ink jet printing. Examples of conductive ink that can be used for such strips include, but are not limited to silver loaded or nickel loaded conductive ink. In some embodiments epoxies as well as anisotropic conductive adhesives can be used to construct electrode strips 420. In typical embodiments, such inks or epoxies are thermally cured in order to form electrode strips 420.

0063In some embodiments in accordance with Fig. 9, a filler layer 330 of sealant such as ethyl vinyl acetate (EVA), silicone, silicone gel, epoxy, polydimethyl siloxane (PDMS), RTV silicone rubber, polyvinyl butyral (PVB), thermoplastic polyurethane (TPU), a polycarbonate, an acrylic, a fluoropolymer, and/or a urethane is coated over transparent conductive layer 110 to seal out air and, optionally, to provide complementary fitting to a transparent tubular casing 310. In some embodiments, filler layer 330 is a Q-type silicone, a silsequioxane, a D-type silicon, or an M-type silicon. However, in some embodiments, optional filler layer 330 is not needed even when one or more electrode strips 420 are present.

0064In some embodiments in accordance with Fig. 9, a transparent tubular casing 310 is circumferentially disposed on transparent conductive layer 110 and/or optional filler layer 330. In some embodiments tubular casing 310 is made of plastic or glass. In some embodiments, elongated solar cells 402, after being properly modified for future packaging, are sealed in transparent tubular casing 310. As shown in Figure 9 A, transparent tubular casing 310 fits over the outermost layer of elongated solar cell 402. In some embodiments, elongated solar cell 402 is inside transparent tubular casing 310 such that adjacent elongated solar cells 402 do not form electric contact with each other except at the ends of the solar cells. Methods, such as heat shrinking, injection molding, or vacuum loading, can be used to construct transparent tubular casing 310 such they exclude oxygen and water from the system as well as to provide complementary fitting to the underlying solar cell 402. In some embodiments, transparent tubular casing 310 is made of a urethane polymer, an acrylic polymer, polymethylmethacrylate (PMMA), a fluoropolymer, silicone, poly-dimethyl siloxane (PDMS), silicone gel, epoxy, ethyl vinyl acetate (EVA), perfluoroalkoxy fluorocarbon (PFA), nylon / polyamide, cross-linked polyethylene (PEX), polyolefin, polypropylene (PP), polyethylene terephtalate glycol (PETG), polytetrafluoroethylene (PTFE), thermoplastic copolymer (for example, ETFE^®' which is a derived from the polymerization of ethylene and tetrafluoroethylene: TEFLON^® monomers), polyurethane / urethane, polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), Tygon^®, vinyl, Viton^®, or any combination or variation thereof. In some embodiments, transparent tubular casing 310 comprises a plurality of transparent tubular casing layers. In some embodiments, each transparent tubular casing is composed of a different material. For example, in some embodiments, transparent tubular casing 310 comprises a first transparent tubular casing layer and a second transparent tubular casing layer. Depending on the exact configuration of the solar cell, the first transparent tubular casing layer is disposed on the transparent conductive layer 412, optional filler layer 330 or the water resistant layer. The second transparent tubular casing layer is disposed on the first transparent tubular casing layer. In some embodiments, each transparent tubular casing layer has different properties. In one example, the outer transparent tubular casing layer has excellent UV shielding properties whereas the inner transparent tubular casing layer has good water proofing characteristics. Moreover, the use of multiple transparent tubular casing layers can be used to reduce costs and/or improve the overall properties of transparent tubular casing 310. For example, one transparent tubular casing layer may be made of an expensive material that has a desired physical property. By using one or more additional transparent tubular casing layers, the thickness of the expensive transparent tubular casing layer may be reduced, thereby achieving a savings in material costs. In another example, one transparent tubular casing layer may have excellent optical properties (e.g., index of refraction, etc.) but be very heavy. By using one or more additional transparent tubular casing layers, the thickness of the heavy transparent tubular casing layer may be reduced, thereby reducing the overall weight of transparent tubular casing 310.

0065In some embodiments, one or more layers of water resistant layer are coated over solar cell 402 of Fig. 9 to prevent the damaging effects of water molecules. In some embodiments, this water resistant layer is circumferentially coated onto transparent conductive layer 110 prior to depositing optional filler layer 330 and encasing the solar cell 402 in transparent tubular casing 310. In some embodiments, such water resistant layers are circumferentially coated onto optional filler layer 330 prior to encasing the solar cell 402 in transparent tubular casing 310. In some embodiments, such water resistant layers are circumferentially coated onto transparent tubular casing 310 itself. In embodiments where a water resistant layer is provided to seal molecular water from solar cell 402, it is important that the optical properties of the water resistant layer not interfere with the absorption of incident solar radiation by solar cell 402. In some embodiments, this water resistant layer is made of clear silicone, SiN, SiO_xNy, SiO_x, or Al₂O₃, where x and y are integers. In some embodiments, water resistant layer is made of a Q-type silicone, a silsequioxane, a D-type silicon, or an M-type silicon.

0066In some embodiments, an optional antireflective coating is also circumferentially disposed on transparent tubular casing 310 to maximize solar cell efficiency. In some embodiments, there is a both a water resistant layer and an antireflective coating deposited on transparent tubular casing 310. In some embodiments, a single layer serves the dual purpose of a water resistant layer and an anti-reflective coating. In some embodiments, antireflective coating, made OfMgF₂, silicone nitrate, titanium nitrate, silicon monoxide (SiO), or silicon oxide nitrite. In some embodiments, there is more than one layer of antireflective coating. In some embodiments, there is more than one layer of antireflective coating and each layer is made of the same material. In some embodiments, there is more than one layer of antireflective coating and each layer is made of a different material.

0067In embodiments such as that disclosed in Fig. 9, layers of material are successively circumferentially disposed on a cylindrical substrate 102 in order to form a solar cell. As used herein, the term circumferentially disposed is not intended to imply that each such layer of material is necessarily deposited on an underlying layer. In fact, methods by which such layers are molded or otherwise formed on an underlying layer can be used. Nevertheless, the term circumferentially disposed means that an overlying layer is disposed on an underlying layer such that there is no annular space between the overlying layer and the underlying layer. Furthermore, as used herein, the term circumferentially disposed means that an overlying layer is disposed on at least fifty percent of the perimeter of the underlying layer. Furthermore, as used herein, the term circumferentially disposed means that an overlying layer is disposed along at least half of the length of the underlying layer.

0068In the present application, the term circumferentially sealed is not intended to imply that an overlying layer or structure is necessarily deposited on an underlying layer or structure. In fact, the present application encompasses methods by which such layers or structures (e.g., transparent tubular casing 310) are molded or otherwise formed on an underlying layer or structure. Nevertheless, the term circumferentially sealed means that an overlying layer or structure is disposed on an underlying layer or structure such that there is no annular space between the overlying layer or structure and the underlying layer or structure. Furthermore, as used herein, the term circumferentially sealed means that an overlying layer is disposed on the full perimeter of the underlying layer. In some embodiments, a layer or structure circumferentially seals an underlying layer or structure when it is circumferentially disposed around the full perimeter of the underlying layer or structure and along the full length of the underlying layer or structure. However, the present application contemplates embodiments in which a circumferentially sealing layer or structure does not extend along the full length of an underlying layer or structure.

0069Improving the efficiency of solar cells. The flowchart depicted in Fig. 4 illustrates two general approaches that improve the efficiency of photovoltaic cell assemblies. The first approach, as depicted by steps 421 through 428, aims to improve the amount of light energy delivered to photovoltaic systems. As such, the first approach operates on self-cleaning layer 208 as depicted in Fig. 2A. This approach includes the addition of one or more albedo layers to enhance the albedo effect of surfaces in the vicinity of solar cell panels (424); protecting such albedo layers (426), and addition of a self-cleaning function to such layers (428). Practice of these steps causes additional solar energy to be absorbed by photovoltaic systems in the form of albedo energy. The second approach, as depicted by steps 411 and 412, aims to maintain the general energy conversion efficiency of existing photovoltaic systems (solar cells). This second approach operates directly on solar cell panels such as those disclosed in Fig. 2A. This second approach involves maintaining the energy conversion efficiency of solar cell panels (411) and the addition of a self-cleaning to such solar cell panels (412). Novel systems and methods, for example, as summarized by steps 430, 432, and 434 of Fig. 4, operate on either albedo surfaces, as illustrated in Fig. 2, and solar cells, as illustrated in Fig. 3, to help improve efficiency. Aspects of steps 430, 432, and 434 include adding a photocatalytic coating 610 to solar cells or albedo surfaces and/or a hydrophobic coating (e.g., micro- or nano-structured) to the same.

00704.2 Materials and methods for fabricating a self-cleaning layer on an albedo coating

0071Both photocatalytic and non-photocatalytic materials can be used to realize self-cleaning albedo surfaces. Such materials can be either hydrophobic or hydrophilic. Some specific embodiments of the present application provide a self-cleaning layer 208 comprising a photocatalytic material. As illustrated in Fig. 2, self-cleaning layer 208 is applied over high albedo layer 206 in some embodiments. In other embodiments, one or more self-cleaning components are mixed with one or more high albedo components to form a composite high albedo layer with self-cleaning properties. Some embodiments of the present application provide a self-cleaning layer 208 that comprises a non- photocatalytic hydrophobic material. Furthermore, in instances where the photocatalytic or non-photocatalytic material is transparent, it can be used in layer 306 (Fig. 3) of solar cell assemblies.

00724.2.1 Photocatalytic materials

0073Photocatalytic material is activated by solar radiation in a process termed photocatalysis. Such activation causes the surface to be hydrophilic. Titanium dioxide or titania (e.g., TiO₂) is a known photocatalysts. The preparation of TiO₂ is described, for example, in United States Patent Number 6,699,317, which is hereby incorporated by reference herein in its entirety. TiO₂ exists in at least three natural mineral forms: anatase, rutile or brookitein TiO₂. Among the three natural mineral forms, anatase is the most photoactive. Because of its unique properties, anatase is a highly favored photocatalytic material. In the present application, TiO₂ can be used to form layer 306 (Fig. 3) and/or self-cleaning layer 208 (Fig. 2A and Fig. 2B). In some embodiments of the present application, TiO₂, in the form of anatase, is present in layers 208 and 306.

0074Photocatalytic coatings comprise materials that are light radiation sensitive. One such photocatalytic coating is TiO₂. Although not intending to be limited to any particular theory, the mechanism for a light-activated self-cleaning layer 208 and/or 306 made of a material such as TiO₂ is illustrated in Fig. 6A. The mechanism illustrated in Fig. 6A depicts TiO₂ will be appreciated, although it is not so limited. In fact, the mechanism explains a broad range of photocatalytic materials of which TiO₂ is just one example. In Fig. 6A, the photocatalytic material, TiO₂ for example, is chemically activated by solar light or ultraviolet (UV) radiation. Electrons in TiO₂ are excited by UV radiation. The resulting roaming electrons (e^") leave behind holes (h⁺). Water, dirt, and other forms of debris deposit on the coated surface and either react with the electrons or donate their own electrons to fill the holes. This exchange in electrons generates highly reactive materials such as H₂O₂, OH^", and free oxygen radicals. As a result, through a series of UV radiation catalyzed reactions, the impurity or contaminant in the deposited layer is gradually decomposed.

0075In addition to their photocatalytic properties, materials such as TiO₂ make it so that it is more difficult for water to nucleate and form droplets on surfaces containing such materials. Rather, the water slides over the surface as a thin film taking with it any decomposed organic matter. Although not intending to be limited to any particular theory, it is believed that this hydrophilic property arises through the mechanism depicted in Fig. 6B. The mechanism illustrated in Fig. 6B depicts TiO₂ will be appreciated, although it is not so limited. In fact, the mechanism explains a broad range of photocatalytic materials of which TiO₂ is just one example. Photocatalytic material 610, such as TiO₂, is embedded in resin 620, for example, to form self-cleaning layer 208. When exposed to UV radiation, water molecules on the surface of self-cleaning layer 208 are chemisorbed by both photocatalyst 610 and resin 620. As a result, the surface of self- cleaning layer 208 becomes increasingly hydrophilic. Water spreads over the surface in a thin film instead of forming droplets. The decomposed organic and inorganic debris then gets washed away when exposed to water (e.g., rain). Thus, photocatalytic materials render a surface self-cleaning by chemically breaking up dirt and causing it to wash away

0076Some photocatalytic materials in accordance with the present application are hydrophobic rather than hydrophilic. In some embodiments, the self-cleaning surface has combined hydrophilic and hydrophobic properties. United States Patent Number 6,337,129 to Watanabe et al, which is hereby incorporated by reference herein in its entirety, describes bi-characteristic coating compositions that can be used in layers 306 in accordance with the present application.

0077TiO₂ has a band gap of 3.2 eV, which allows the oxide to absorb light with wavelengths shorter than 385 nm. Solar radiation, however, contains only a very narrow wavelength range of such high energy rays. Systems and methods have been developed to render TiO₂ more photoactive to radiation with longer wavelengths and hence less energy. For example, the photocatalytic property of TiO₂ can be altered by adding dopant, such as metallic ions like Ce. Ce retards the phase transition from reactive anatase to inert rutile so that the photocatalytic coating layer remains more sensitive to UV radiation for a longer time. The Ce-doped TiO₂ nano particles have better band gap and modified solar absorption. Accordingly, in some embodiments of the present application, layers 306 and/or self-cleaning 208 comprise doped TiO₂.

0078In some embodiments, layers 306 and/or self-cleaning layer 208 comprise anatase, TiO₂, rutile TiO₂, brookitein TiO₂, anatase sol, peroxo-modified anatase (Kon Corporation, Kishima-gun, Saga-prefecture Japan), and peroxotitanium acid (Kon Corporation). In some embodiments, layers 306 and/or self-cleaning layer 208 comprise ZnO, SnO₂, SrTiO₃, WO₃, Bi₂O₃, Fe₂O₃, or any combination thereof. Although not intending to be limited to any particular theory, it is believed that, similar to titania, these materials adsorb surface hydroxyl groups (OH^") from oxides, water or oxygen that are exposed to these materials. Thus, incorporation of any combination of these materials into layers 208 and/or 306 provides the advantageous self-cleaning properties similar to the case Of TiO₂. Further, photocatalysts described in United States Patent No. 6,013,372 and WO 97/23572, each of which is hereby incorporated by reference herein in its entirety, can be used in layers 306 and/or 208.

0079In some embodiments in accordance with the present application, inorganic oxides (e.g., 630) are combined with photocatalysts 610 to form self-cleaning layer 208. The inorganic oxides include, but are not limited to silica, alumina, water glass, silicate, aluminosilicate, zirconia, ceria, tin oxide, calcia, magnesia, amorphous titania and other metal oxides. In some embodiments, such materials are hydrophilified by photocatalytic action of the photocatalytic oxide.

0080The present application provides no limits on the amount of photocatalytic materials present in layers 208 and 306. For example, the amount of the photocatalytic material in layer 208 and/or 306 can be between 1 percent and 90 percent by weight, between 20 percent and 70 percent by weight, or between 20 percent and 50 percent by weight. The balance of such layers 208 and 306 comprises optional resin 620 (Fig. 6B), optional inorganic oxides 630 (Fig. 6B) and any other filler materials, at all possible combinations of ratios.

0081There is no requirement that layer 208 and/or 306 be hydrophilic. For example, Ti-hydroxyapatite (TiHAP), obtained, for example, by using titanium to replace calcium in Ca-hydroxyapatite (CaHAP), shares the same photocatalytic properties as TiO₂. TiHAP, however, is different from a composite of TiO₂ and hydroxyapatite. It has a low refractive index and high UV adsorption power, but it does not provide the photo-induced hydrophilicity. See for example, Nonami et al, Mater. Res. Bull, 33, ppl25, 1998, which is hereby incorporated by reference herein in its entirety. Nevertheless, although it is not hydrophilic, Ti-hydroxyapatite (TiHAP) is a suitable material for use in self-cleaning layer 208 and/or 306 in accordance with embodiments of the present application.

0082In some embodiments of the present application, self-cleaning layer 208 and/or 306 has both hydrophilic and hydrophobic properties. For example, as described in United States Patent Number 6,337,129 to Watanabe et at, which is hereby incorporated by reference herein in its entirety, self-cleaning layer 208 comprises a photocatalytic oxide 610 and resin 620. Resin comprises a silicone resin or silica, and a water-repellent fluororesin. In this embodiment, the silicone or silica and the water-repellent fluororesin are present in a microscopically dispersed and exposed state on the surface of self- cleaning layer 208 and/or 306.

0083Embodiments of the present application in which self-cleaning layer 208 comprises a photocatalytic oxide (e.g., 610 of Fig. 6B), supporting resin (e.g., 620 of Fig. 6B), and optional inorganic oxide (e.g., 630 of Fig. 6B) have been described. Exemplary photocatalytic oxides, supporting resins, and optional inorganic oxides of the present application include, but are not limited to, any chemically stable combination of compounds listed in Table 1 below.

0084Table 1 - Exemplary materials for assembling layers 208 and 306

0085Component Exemplary materials

00861. Photocatalytic materials (e.g., 610 of Fig. 6B) photocatalytic oxides anatase TiO₂ rutile TiO₂ brookitein TiO₂

0087ZnO

0088SnO₂

0089SrTiO₃

0090WO₃

0091Bi₂O₃

0092Fe₂O₃ peroxotitanium acid peroxo-modified anatase sol

0093Apatite derivatives Ti-HAP Component Exemplary materials

00942. Resins {e.g., 620 of Fig. 6B) silicone resin or silica methyltrimethoxysilane

0095(for imparting hydrophilic or methyltriethoxysilane hydrophobic properties) methyltrichlorosilane methyltribromosilane methyltriisopropoxysilane methyl-tri-t-butoxysilane ethyltrimethoxysilane ethyltriethoxysilane ethyltrichlorosilane ethyltribromosilane ethyltriisopropoxysilane ethyl-tri-t-butoxysilane n-propyltrimethoxysilane n-propyltriethoxysilane n-propyltrichlorosilane n-propyltribromosilane n-propyltriisopropoxysilane n-propyl-tri-t-butoxysilane n-hexyltrimethoxysilane n-hexyltriethoxysilane n-hexytrichlorosilane n-hexyltribromosilane n-hexyltriisopropoxysilane n-hexyl-tri-t-butoxysilane n-decyltrimethoxysilane n-decyltriethoxysilane n-decyltrichlorosilane n-decyltribromosilane n-decyltriisopropoxysilane n-decyl-tri-t-butoxysilane n-octatrimethoxysilane n-octatriethoxysilane n-octatrichlorosilane n-octatribromosilane n-octatriisopropoxysilane n-octa-tri-t-butoxysilane Component Exemplary materials phenyltrimethoxysilane phenyltriethoxysilane phenyltrichlorosilane phenyltribromosilane phenyltriisopropoxysilane phenyl-t-butoxysilane dimethyldichlorosilane dimethyldibromosilane dimethyldimethoxysilane dimethyldiethoxysilane diphenyldichlorosilane diphenyldibromosilane diphenyldimethoxysilane diphenyldiethoxysilane phenylmethyldichlorosilane phenylmethyldibromosilane phenylmethyldimethoxysilane phenylmethyldiethoxysilane vinyltrichlorosilane vinyltribromosilane vinyltrimethoxysilane vinyltriethoxysilane vinyltriisopropoxysilane vinyl-tri-t-butoxysilane trifluoropropyltrichlorosilane trifluoropropyltribromosilane trifluoropropyltrimethoxysilane trifluoropropyltriethoxysilane trifluoropropyltriisopropoxysilane trifluoropropyl-tri-t-butoxysilane γ-glycidoxypropylmethyldimethoxysilane γ-glycidoxypropylmethyldiethoxysilane γ-glycidoxypropyltrimethoxysilane γ-glycidoxypropyltriethoxysilane γ-glycidoxypropyltriisopropoxysilane γ-glycidoxypropyl-tri-t-butoxysilane γ-methacryloxypropylmethyldimethoxysilane Component Exemplary materials γ-methacryloxypropylmethyldiethoxysilane γ -methacryloxypropyltrimethoxysilane γ-methacryloxypropyltriethoxysilane γ-methacryloxypropyltriisopropoxysilane γ-methacryloxypropyl-tri-t-butoxysilane γ-aminopropylmethyldimethoxysilane γ-aminopropylmethyldiethoxysilane γ-aminopropyltrimethoxysilane γ-aminopropyltriethoxysilane γ-aminopropyltriisopropoxysilane γ-aminopropyl-tri-t-butoxysilane γ-mercaptopropylmethyldimethoxysilane γ-mercaptopropylmethyldiethoxysilane γ-mercaptopropyltrimethoxysilane γ-mercaptopropyltriethoxysilane γ-mercaptopropyltriisopropoxysilane γ-mercaptopropyl-tri-t-butoxysilane β-(3,4-poxycyclohexyl)ethyltrimethoxysilane β-(3,4-epoxycyclohexyl)ethyltriethoxysilane

0096water-repellent fluoro-resin polytetrafluoroethylene polyvinylidene fluoride polyvinyl fluoride polychlorotrifluoroethylene tetrafluoroethylene/hexafluoropropylene copolymer ethylene/tetrafluoroethylene copolymer ethylene/chlorotrifluoroethylene copolymer tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer perfluorocyclo polymer vinyl ether/fluoroolefin copolymer vinyl ester/fluoroolefin copolymer tetrafluoroethylene/vinyl ether copolymer chlorotrifluoroethylene/vinyl ether copolymer urethane-crosslinked tetrafluoroethylene epoxy-crosslinked tetrafluoroethylene acryl-crosslinked tetrafluoroethylene Component Exemplary materials melamine-crosslinked tetrafluoroethylene

00973. Inorganic oxide {e.g., material silica 630 of Fig. 6B) _alumina water glass silicate aluminosilicate zirconia ceria tin oxide calcia magnesia amorphous titania

0098Various techniques and methods are available for coating photocatalytic materials of the present application onto high albedo layer 206 (Fig. 2A) and layer 304 (Fig. 3) in order to form self-cleaning layers 208 and/or 306. Such techniques include, but are not limited to, screen or roller printing, electrostatic glazing, and spraying. In addition, techniques described in Madou, 2002, Fundamentals of Microfabrication, Second Edition, CRC Press, Boca Raton, which is hereby incorporated by reference herein in its entirety, can be used to form self-cleaning layer 208 and/or 306.

0099In some embodiments, a transparent coating is used for layer 306 and/or self-cleaning layer 208. In some embodiments, the photocatalytic coating materials are available as water-based solutions (e.g., Titernal W, Fushimi Co. Mizoguchi, Sanda City, Hyogo Japan) or ethanol-based solutions (e.g., Titernal E, Fushimi Co.). In some embodiments, such solutions are sprayed onto high albedo layer 206 and panel 304 in order to form self-cleaning layers 208 and/or 306.

0100In some embodiments, self-cleaning layer 208 and/or layer 306 is formed using a reverse micelle dip coating method in which a transparent nano-crystalline TiO₂ thin film is deposited. In the reverse micelle method a surfactant (e.g., Triton X-100), a co-surfactant (e.g., hexanol), a continuous phase (e.g., cyclohexane), and titanium isopropoxide are mixed together. The solution is applied to the proper surface (e.g., surface 304), for example, by dip coating. After drying and calcination in air, a multifunctional thin film of nano-crystalline TiO₂ is formed. In some embodiments, application of the micelle solution in order to form self-cleaning layer 208 and/or layer 306 comprises removing excess solution, padding the surface, and heating the surface (e.g., to over 90 °C). See, for example, Daoud et al, 2004, J. Am. Ceram. Soc. 87, p. 953-555, which is hereby incorporated by reference herein in its entirety.

0101In some embodiments of the application, self-cleaning layer 208 and/or layer 306 is formed from a peroxotitanium acid solution. The solution comprises peroxotitanium acid anywhere from 0.3 % by weight up to the maximum allowed by the solubility of peroxotitanium acid in solution. Examples of such a solution include, but are not limited to, PTA-85 and PTA- 170 (Green Millennium, Los Angeles, California), which comprises peroxotitanium acid at 0.85% and 1.7% by weight, respectively.

0102In some embodiments of the application, self-cleaning layer 208 and/or layer 306 is a self-cleaning photocatalytic coating material formed from peroxo-modified anatase sol. The solution comprises from peroxo-modified anatase sol anywhere from 0.3 % by weight up to the maximum allowed by the solubility of peroxo-modified anatase sol in solution. Examples of such a solution include, but are not limited to, peroxo-modified anatase sol (Green Millennium, Los Angeles, California) at 0.85% and 2.2% by weight, respectively.

0103In some embodiments of the application, self-cleaning layer 208 and/or layer 306 is a self-cleaning photocatalytic coating material formed from a mixture of peroxo- modified anatase sol and peroxotitanium acid. The solution comprises peroxo-modified anatase sol and peroxotitanium acid anywhere from 0.3 % by weight up to the maximum allowed by the solubility of these compounds in solution. The mixture of peroxo- modified anatase sol and peroxotitanium acid comprises two components at any ratio. Examples of such a mixture include, but are not limited to TPX-85 and TPX-220 (Green Millennium, Los Angeles, California) which comprises peroxo-modified anatase sol and peroxotitanium acid at 0.85% and 2.2% by weight, respectively.

0104In some embodiments, a hybrid system is used to apply self-cleaning layer 208 and/or layer 306 when such a layer is made from anatase TiO₂. The hybrid system utilizes an inert sol-gel coating and a layer of anatase TiO₂ on top. For example, referring to Fig. 6B, a sol-gel layer 620 acts to adhere nano-sized TiO₂ particles 610. Layer 620 also acts as a barrier to protect high albedo layer 206 from photocatalytic reactions.

0105In some embodiments, a suitable transparent coating of TiO₂ is applied by methods such as painting (e.g., spraying, roller painting, etc., followed by optional heat treatment) to form self-cleaning layer 208 and/or 306. In some embodiments, layer 306 and/or self-cleaning layer 208 comprises TiHAP, and is applied as a spray paint. In some embodiments, layer 306 and 304 is purchased as assembled composite comprising a glass pane (layer 304) that has already been coated with a photocatalytic layer 306. Examples of such assembled composites include, but are not limited to, Activ™ glass from Pilkington Holdings Inc. (Toledo, Ohio).

0106In some embodiments, in order to make layer 208 and/or layer 306, a coating of an amorphous form of titania is deposited and then calcined to thereby transform by phase transition the amorphous titania into crystalline titania {e.g., anatase, rutile or, brookitein). Such a method can be used, for example, where high albedo layer 206 is highly heat resistant {e.g., white gravel, white sand, white cement, etc.). Formation of amorphous titania can be carried out by one of the following methods:

0107(1) Hydrolysis and Dehydration Polymerization of Organic Titanium Compound. To an alkoxide of titanium, such as tetraethoxytitanium, tetraisopropoxytitanium, tetra-n- propoxytitanium, tetrabuthoxytitanium, or tetramethoxytitanium, is added a hydrolysis inhibitor such as hydrochloric acid and ethylamine, the mixture being diluted by alcohol such as ethanol or propanol. While subjected to partial or complete hydrolysis, the mixture is applied to the surface of the substrate by spray coating, flow coating, spin coating, dip coating, roll coating or any other suitable coating method, followed by drying at a temperature ranging from the ambient temperature to 200 ⁰C. Upon drying, hydrolysis of titanium alkoxide will be completed to result in the formation of titanium hydroxide which then undergoes dehydration polymerization whereby a layer of amorphous titania is formed on the surface of the substrate {e.g., on layer 206). In lieu of titanium alkoxide, other organic compounds of titanium such as chelate of titanium or acetate of titanium may be employed.

0108(2) Formation of Amorphous Titania from Inorganic Titanium Compound. An acidic aqueous solution of an inorganic compound of titanium such as TiCl₄ or Ti(SO₄)₂ is applied to the surface of a substrate by spray coating, flow coating, spin coating, dip coating, or roll coating. The substrate is then dried at a temperature of 100-200 ⁰C to subject the inorganic compound of titanium to hydrolysis and dehydration polymerization to form a layer of amorphous titania on the surface of the substrate. Alternatively, amorphous titania may be formed on the surface of the substrate by chemical vapor deposition of TiCl₄.

0109(3) Formation of Amorphous Titania by Sputtering. Amorphous titania can be deposited on the surface of the substrate {e.g., onto layer 206) by bombarding a target of metallic titanium with an electron beam in an oxidizing atmosphere.

0110(4) Calcination Temperature. Calcination of amorphous titania can be carried out at a temperature at least higher than the crystallization temperature of anatase. Upon calcination at a temperature of 400-500 ⁰C or more, amorphous titania can be transformed into the anatase form of titania. Upon calcination at a temperature of 600-700 ⁰C or more, amorphous titania can be transformed into the rutile form of titania.

0111In some embodiments, layer 208 and/or 306 comprises a mixture of titania and silica. In such embodiments, the ratio of silica to the sum of titania and silica (by mole percent) can be 5-90%, preferably 10-70%, more preferably 10-50%. The formation of a layer 208 and/or 306 comprised of silica-blended titania can be carried out by any of the following methods:

0112(I) A suspension containing particles of the anatase form or rutile form of titania and particles of silica is applied to panel 304 and/or high albedo layer 206, followed by sintering at a temperature less than the softening point of panel 304 and or high albedo layer 206.

0113(2) A mixture of a precursor of amorphous silica {e.g., tetraalkoxysilane such as tetraethoxysilane, tetraisopropoxysilane, tetra-n-propoxysilane, tetrabuthoxysilane, and tetramethoxysilane; silanol formed by hydrolysis of tetraalkoxysilane; or polysiloxane having a mean molecular weight of less than 3000) and a crystalline titania sol is applied to panel 304 and/or high albedo layer 206 and is subjected to hydrolysis where desired to form silanol, followed by heating at a temperature higher than about 100 ⁰C to subject the silanol to dehydration polymerization to thereby form a photocatalytic coating wherein titania particles are bound by amorphous silica. In this regard, if dehydration polymerization of silanol is carried out at a temperature higher than about 200 ⁰C, polymerization of silanol is accomplished to a high degree so that the alkali resistance of the photocatalytic coating is enhanced.

0114(3) A suspension comprised of particles of silica dispersed in a solution of a precursor of amorphous titania {e.g., an organic compound of titanium such as alkoxide, chelate or acetate of titanium; or an inorganic compound of titanium such as TiCl₄ and Ti(SO₄)₂) is applied to panel 304 and/or high albedo layer 206 and then the precursor is subjected to hydrolysis and dehydration polymerization at a temperature ranging from the ambient temperature to 200 ⁰C to thereby form a thin film of amorphous titania such that particles of silica are dispersed. Then, the thin film is heated at a temperature higher than the crystallization temperature of titania but lower than the softening point of the substrate to thereby transform amorphous titania into crystalline titania by phase transition.

0115(4) Added to a solution of a precursor of amorphous titania such as an organic compound of titanium such as an alkoxide, chelate or acetate of titanium; or an inorganic compound of titanium such as TiCl₄ or Ti(SO₄)₂) is a precursor of amorphous silica such as a tetraalkoxysilane such as tetraethoxysilane, tetraisopropoxysilane, tetra- n-propoxy-silane, tetrabuthoxysilane, or tetramethoxysilane; a hydrolyzate thereof, e.g., silanol; or a polysiloxane having a mean molecular weight of less than 3000) and the mixture is applied to panel 304 and/or high albedo layer 206. Then, these precursors are subjected to hydrolysis and dehydration polymerization to form a thin film made of a mixture of amorphous titania and amorphous silica. Thereafter, the thin film is heated at a temperature higher than the crystallization temperature of titania but lower than the softening point of panel 304 and/or high albedo layer 206 to thereby transform amorphous titania into crystalline titania by phase transition.

0116In some embodiments, layer 208 and/or layer 306 comprises a mixture of titania and tin oxide. The ratio of tin oxide to the sum of titania and tin oxide can be 1-95% by weight, preferably 1-50% by weight. Formation of a layer 208 and/or layer 306 comprised of tin oxide-blended titania can be carried out by any of the following methods:

0117(1) A suspension containing particles of the anatase form or rutile form of titania and particles of tin oxide is applied to panel 304 and/or high albedo layer 206, followed by sintering at a temperature less than the softening point of panel 304 and/or high albedo layer 206.

0118(2) A suspension comprised of particles of tin oxide dispersed in a solution of a precursor of amorphous titania (e.g., an organic compound of titanium such as alkoxide, chelate or acetate of titanium; or an inorganic compound of titanium such as TiCl₄ or Ti(Sθ₄)₂) is applied to panel 304 and/or high albedo layer 206 and then the precursor is subjected to hydrolysis and dehydration polymerization at a temperature ranging from the ambient temperature to 200 ⁰C to thereby form a thin film of amorphous titania such that particles of tin oxide are dispersed within the film. Then, the thin film is heated at a temperature higher than the crystallization temperature of titania but lower than the softening point of panel 304 and/or high albedo layer 206 to thereby transform amorphous titania into crystalline titania by phase transition.

0119In some embodiments, layer 208 and/or layer 306 is a coating composition wherein particles of a photocatalyst are dispersed in a film forming element of uncured or partially cured silicone (organopolysiloxane) or a precursor thereof. In some embodiments, the coating composition is applied on panel 304 and/or high albedo layer 206 and the film-forming element is then subjected to curing. Upon photoexcitation of the photocatalyst, the organic groups bonded to the silicon atoms of the silicone molecules are substituted with hydroxyl groups under the photocatalytic action of the photocatalyst. This method provides several advantages. Since the photocatalyst- containing silicone paint can be cured at ambient temperature or at a relatively low temperature, this method can be applied to a substrate formed of a non-heat-resistant material such as plastics. The coating composition containing the photocatalyst can be applied whenever desired by way of brush painting, spray coating, roll coating and the like on any existing substrate requiring superhydrophilification of the surface.

0120In some embodiments, layer 208 and/or 306 is doped with a metal such as Ag, Cu and Zn. Doping of the layer with a metal such as Ag, Cu or Zn can be carried out by adding a soluble salt of such metal to a suspension containing particles of the photocatalyst, the resultant solution being used to form layer 208 and/or 306. Alternatively, after forming layer 208 and/or 306, a soluble salt of such metal can be applied thereon and can be subjected to irradiation of light to deposit the metal by photoreduction. While not intending to be limited to any particular theory, is believed that layer 208 and/or 306 doped with a metal such as Ag, Cu or Zn is capable of killing bacteria adhered to the surface. Moreover, it is believed that doped layers 208 and/or 306 inhibit growth of microorganisms such as mold, algae and moss. As a result, albedo surface 206 and/or panel 304 can be maintained clean for a longer period. In some embodiments, layer 208 and/or 306 can additionally be doped with a metal of the platinum group such as Pt, Pd, Rh, Ru or Ir. These metals can be similarly doped into the composition used to make layer 208 and/or 306 by photoreduction deposition or by addition of a soluble salt. While not intending to be limited to any particular theory, it is believed that a layer 208 and/or 306 doped with a metal of the platinum group develops an enhanced photocatalytic redox activity so that decomposition of contaminants adhering on the surface is promoted.

0121In some embodiments, layer 208 and 206 is a single combined layer comprising white paint that is doped with any of the photoactive materials described in this section. Properties of suitable white paints are described in the section below. In one particular embodiment, combined layer 208/206 is one or more layers of white paint that has been doped with TiO₂, anatase TiO₂, rutile TiO₂, brookitein TiO₂, ZnO, SnO₂, SrTiO₃, WO₃, Bi₂O₃, or Fe₂O₃, or any combination of such dopants. In some embodiments, care is taken to ensure that such dopants are in the uppermost part of combined layer 208/206 by adding the dopant particles (e.g., TiO₂) while the paint is still wet.

0122In some embodiments, layer 208 and/or 306 is administered using a hot aerosol layering operation. Such processes have been developed by companies such as Nextrom (Duluth, Georgia and Camarillo, California). The hot aerosol process is based on the combustion of liquid raw materials in an atmospheric flame. As the precursor liquid is atomized, the droplets containing the raw materials and solvent evaporate, and finally condense again to become homogeneous, nano-sized particles, that are deposited on layer 206 and/or 304.

01234.2.2 Non-photocatalytic materials

0124In some embodiments of the present application, self-cleaning layers 208 and 306 are self-cleaned through mechanisms other than a primary photocatalytic mechanism. In other words, in some embodiments, self-cleaning layers 208 and/or 306 have a self- cleaning property through some mechanism independent of a photocatalytic property. This does not necessarily mean that such layers do not include photocatalytic materials and that some self-cleaning functionality is present in such materials. In fact, it is possible that some of the compositions of matter described in this section are photocatalytic. However, in each instance, the compositions in this section provide a self- cleaning functionality by way of a mechanism that is additional to or lieu of a photocatalytic mechanism.

0125In some embodiments, self-cleaning layer 208 and/or layer 306 is a hydrophobic material. Without intending to be limited by any particular theory, when a drop of water touches a solid surface, both the solid surface and the drop are surrounded by air at the same time (Fig 7A). Thus, there are three phases in contact with each other: a solid, a liquid, and a gas. The interactions at this "three-phase boundary" determine the shape of the drop and how well or poorly the liquid spreads over the solid surface, in other words, how well it wets the solid substrate. One way of measuring this is the contact angle θ (Fig 7A). When a drop of water contacts a cleaned glass surface, the contact angle θ is around 15 degrees whereas by comparison, the same drop of water is on a Teflon coated surface forms a contact angle of 109°. This demonstrates that Teflon has desirable hydrophobic properties. However, Teflon absorbs light in the upper visible and UV ranges (Seres et al, 1994, Opt. Eng. 33, p. 3031-3033, which is hereby incorporated by reference herein in its entirety) and in the infrared region between 6 μm to 8 μm (Hollahan et al, 1974, Applied Opt. 13, p. 1844-1849, which is hereby incorporated by reference heien in its entirety). Such absorption compromises the efficiency of solar energy conversion. Therefore, the use of Teflon coating for use as self-cleaning layers 208 and/or 306 may be limited.

0126An alternative to Teflon material is a water repellant coating constructed from a composite material of CaF₂ and TiO₂, as described by Tsai et al, in United States Patent Number 6,383,629, which is hereby incorporated by reference herein in its entirety. Though the composite material of CaF₂ and TiO₂ does not have a contact angle θ that is significantly better than that of Teflon, it does not have the optical limitations of Teflon material. In some embodiments, self-cleaning layer 208 and/or layer 306 is made of polytetrafluoroethylene (PTFE) [-CF₂-CF₂-]_n or polychlorotrifluoroethylene (PCTFE) [- CF₂-CFCl-]_n. In some embodiments, self-cleaning layer 208 comprises a Teflon coating. See, for example, United States Patent No. 4,869,922 to D'Agositno, United States Patent Number 4,410,563 to Richter and Dibble, as well as Hollahan et al, Applied Opt. 13, p. 1844-1849, 1974, each of which is hereby incorporated by reference herein in its entirety. In some embodiments, self-cleaning layer 208 comprises CaF₂ and TiO₂. In some embodiments, self-cleaning layer 208 comprises a material having a chemical formula (100-X)CaF₂ -(X)TiO₂, where X is the molar percentage of TiO₂. For more information on such compounds, see United States Patent No. 6,383,629 to Tsai et al., which is hereby incorporated by reference herein in its entirety. In some embodiments, X is between 1 percent and 50 percent. In some embodiments, X is between 2 percent and 45 percent. In some embodiments, X is between 10 percent and 30 percent.

0127In some embodiments in accordance with the present application, the Lotus-Effect is used to provide desired self-cleaning properties like that found in plant leaves such as the Sacred Water Lilly (Nelumbo nucifera). The Lotus-Effect is based on the interaction between a solid surface (lotus leaf) and its environment (water droplets), and is therefore a surface phenomenon. Electron microscopy study of the lotus leaves revealed that they have "rough" surfaces of small humps and some fronds (e.g., 710 in Fig. 7C). In addition, the surfaces of the lotus leaves are coated by hydrophobic wax- like material (e.g. 720 in Fig. 7C). The combination of these two factors results in a hydrophobic nano-or micro- structured surface (e.g., self-cleaning 208 or layer 306 in Fig. 7B and Fig. 7C). The relationship of micro- or nano- structured "rough" surfaces and hydrophobicity, and the applicability of the "Lotus Effect" on surfaces, is described in United States Patent Number 6,800,354 to Baumann, et al. and by United States Patent Number 6,852,389 to Nun, et al, each of which is hereby incorporated by reference herein in its entirety.

0128In some embodiments, of the present application, self-cleaning layer 208 and/or layer 306 is a rough surface comprising hydrophobic nano- or micro-structured surfaces as illustrated in Fig. 7B. In some embodiments, such structured surfaces cause water to have a contact angle θ that exceeds 160 degree. The size of the surface structures (e.g., 710 in Fig. 7C) ranges from several nanometers up to 50 microns. When combined with hydrophobic coatings (e.g., 720 in Fig. 7C), such "rough" surfaces are effectively hydrophobic. Spherical water droplets roll off such surfaces (e.g., self-cleaning layer 208 or layer 306 in Fig. 7B and 7C). Therefore, they remain dry even when it is raining. In some embodiments the materials used to make surface structures 710 and hydrophobic coatings 720 are transparent. Such materials can therefore be used to form layer 306 on solar cells in addition to forming a self-cleaning layer 208 on high albedo layer 206. Particles 710 can be obtained in commercially available sizes and or by a precipitation processes such as, for example, by a pyrogenic process. Alternatively, particles 710 can be obtained be obtained from a gaseous starting material that is converted into a pulverulent substance.

0129Particles 710 can be any combination of the materials listed in Table 2 below. In some embodiments, particles 710 comprise oxides such as SiO₂, TiO₂, Al₂O₃, ZrO₂ or SnO₂. These oxides are typically pyrogenically prepared oxides. In some embodiments particles 710 are pyrogenic silica with an average primary particle size in the range between about 7 nm and 40 nm. In some embodiments, particles 710 comprise a silicate, a doped silicate, a mineral, a metal oxide, a silica, a polymer, and/or a coated metal powder. In some embodiments, particles 710 comprise fumed silica, precipitated silica, or a pulverulent polymer, e.g., a cryogenically milled or spray-dried polytetrafluoroethylene (PTFE).

0130In some embodiments, self-cleaning layer 208 is Lotusan™ paint (Sto, Germany). The paint can be applied with brush, roller, or proper spray equipment. The Lotusan™ paint comprises 1-5% triethoxy (2, 4, 4-trimethylpentyl) silane, 15-40% Cristobalite (SiO₂), and 10-30% TiO₂, by weight. In some embodiments, self-cleaning layer is a hydrophilic paint that comprises an acrylic resin, an acryl-silicone resin, an aqueous silicone, a block copolymer of silicone resin and acrylic resin, an acryl-styrene resin, an ethylene oxide of sorbitan fatty acid, an ester of sorbitan fatty acid, an acetate of urethane, a cross-linked urethane of polycarbonatediol and/or polyisocyanate, or a cross-linked polymer of alkylester polyacrylate. In some embodiments, self-cleaning layer 208 is a water-repellant paint containing polytetrafluoroethylene (PFTE). In embodiments where layer 208 is a paint, high-albedo layer 206 is optional, particularly in instances where the paint used for layer 208 is a white paint or otherwise has an albedo greater than ninety percent.

0131It is to be noted that although TiO₂ and other photocatalytic oxides are found in various hydrophobic coating materials listed herein, it is not because of their possible photocatalytic properties. Rather, such oxides provide roughness and control the formation of micro- or nano- structured hydrophobic surfaces. Table 2 - Exemplary materials for use in layers 208 and 306 in which photoactivity is not the primary basis for self-cleaning

0132Component Exemplary materials

01331. Teflon Coating polytetrafluoroethylene (PTFE) polychlorotrifluoroethylene (PCTFE)

01342. Composite inorganic coating CaF₂ and TiO₂

01353. Coating based on Lotus Effect 3 a. Structural forming material inorganic substances metal oxides mixed oxides silicates sulfates phosphates borates metal sulfides oxosulfides selenides sulfoselenides metal nitrides oxide nitrides

0136Organic substances nanoscale organic polymeric particles nanoscale organic polymeric particles containing fluorine.

01373b. Hydrophobic coating particles comprising PTFE or any other suitable hydrophobic materials

01384.2.3 Self-cleaning albedo layers

0139In some embodiments, high albedo layer 206 and self-cleaning layer 208 form a single self-cleaning high albedo layer. The self-cleaning property in such a layer can be rendered by either a photocatalytic or hydrophobic mechanism. One embodiment in accordance with this aspect of the application is the application of white Lotusan™ paint as a single combined layer 206/208.

01404.3 High albedo devices

0141Another aspect of the present application provides a self-cleaning albedo device 220 as illustrated, for example, in Fig. 2B. In some embodiments, device 220 is attached to the base of photovoltaic cell supporting system 216. In some embodiments, device 220 comprises a substrate 222 with a high albedo layer 206 disposed thereon. Typically, substrate 222 is made of a highly flexible material that can be rolled or folded up. In some embodiments, a high albedo layer 206, for example, a layer of white paint or any of the materials described for layer 206 in preceding sections, is disposed on substrate 222. In some embodiments, flexible substrate 222 is itself a high albedo material. High albedo layer 206 is not necessary embodiments of device 220 where the substrate 222 has high albedo (e.g. albedo greater than ninety percent). As illustrated in Fig. 2B, self-cleaning layer 208 is disposed on high albedo layer 206 (or directly onto flexible substrate 222 in instances where flexible substrate 222 possesses albedo properties and thus high albedo layer 206 is not present).

0142In some embodiments device 220 comprises a single layer of mylar or any of a number of other low-cost plastics that can be manufactured with high albedo. In such embodiments, flexible substrate 222 is optional and often is not included. Furthermore, because of the low cost the mylar (or other type of low-cost plastic), in some embodiments self-cleaning layer 208 is not present as well.

0143In some embodiments, there is a sloped gradient between respective solar cell assemblies in the manner illustrated in Fig. 2B. Each solar cell assembly includes a respective device 220 as illustrated in Fig. 2B. In some embodiments, device 220 is rolled down the gradient. Thus, any water that lands on device 220 rolls down the gradient rather than collecting on the surface and forming water spots. In some embodiments, device 220 is a folded assembly instead of the roll-up type. Regardless of whether assembly 220 is of the roll-up type or the fold-up type, the assembly 220 will preferably attach to hooks on the bottom of each photovoltaic cell supporting system 216 in order to allow the device 220 to be removed to make room for maintenance.

0144In some embodiments in accordance with the present application, device 220 is a plastic sheet that can be easily rolled or folded away, stored and assembled with any existing photovoltaic cell system. See Section 4.5 for more description of photovoltaic cell systems that can be used in the present application. In some embodiments in accordance with the present application, device 220 is a self-cleaning albedo plastic sheet that can be rolled or folded out and attached to the base 216 of the photovoltaic cell support 216 by a hinge mechanism.

0145In some embodiments in accordance with the present application, substrate 222 is a board that can be easily stored and assembled in any existing photovoltaic cell systems. High albedo layer 206, for example a layer of white paint, is disposed upon substrate 222. A self-cleaning layer 208 is then disposed upon high albedo layer 206.

0146Some embodiments of the present application provide a photovoltaic cell system comprising a plurality of photovoltaic cell assemblies. Each assembly in the plurality of photovoltaic cell assemblies comprises a base, a solar cell panel attached to the base, and a self-cleaning albedo device (e.g., device 220 of Fig. 2B), attached to the base. The self- cleaning albedo device comprises a flexible substrate (e.g., 222 of Fig. 2B), a high albedo layer (e.g., layer 206 of Fig. 2B), and a self-cleaning layer (e.g., layer 208 of Fig. 2B). The high albedo layer is disposed on the flexible substrate and the self-cleaning layer is disposed on the high albedo layer. In some embodiments, the photovoltaic system further comprises a retractable roller mounted on the base for retracting the self-cleaning albedo device into a roll. For example, in some embodiments, the self-cleaning albedo device has an open configuration and a closed configuration. The self-cleaning albedo device is rolled up in the retractable roller when the self-cleaning albedo device is in the closed position. Further, the self-cleaning albedo device is extended in a downward gradient away from the base when the self-cleaning albedo device is in the open position.

01474.4 Exemplary semiconductor junctions

0148Referring to Figure 5 A, in one embodiment, semiconductor junction 510 is a heterojunction between an absorber layer 502, disposed on conductive core 504, and a junction partner layer 504, disposed on absorber layer 502. Layers 502 and 504 are composed of different semiconductors with different band gaps and electron affinities such that junction partner layer 504 has a larger band gap than absorber layer 502. In some embodiments, absorber layer 502 is/»-doped and junction partner layer 504 is n- doped. In such embodiments, TCO layer 512 is n -doped. In alternative embodiments, absorber layer 502 is n-doped and junction partner layer 504 is /'-doped. In such embodiments, TCO layer 512 is p⁺-doped. In some embodiments, the semiconductors listed in Pandey, 1996, Handbook of Semiconductor Electrodeposition, Marcel Dekker Inc., Appendix 5, which is hereby incorporated by reference herein in its entirety, are used to form semiconductor junction 510. 4.4.1 Thin-film semiconductor junctions based on copper indium diselenide and other type I-III-VI materials

0149Continuing to refer to Figure 5 A, in some embodiments, absorber layer 502 is a group 1-IH-VI₂ compound such as copper indium di-selenide (CuInSe₂; also known as CIS). In some embodiments, absorber layer 502 is a group 1-IH-VI₂ ternary compound selected from the group consisting of CdGeAs₂, ZnSnAs₂, CuInTe₂, AgInTe₂, CuInSe₂, CuGaTe₂, ZnGeAs₂, CdSnP₂, AgInSe₂, AgGaTe₂, CuInS₂, CdSiAs₂, ZnSnP₂, CdGeP₂, ZnSnAs₂, CuGaSe₂, AgGaSe₂, AgInS₂, ZnGeP₂, ZnSiAs₂, ZnSiP₂, CdSiP₂, or CuGaS₂ of either the/>-type or the rø-type when such compound is known to exist.

0150In some embodiments, junction partner layer 504 is CdS, ZnS, ZnSe, or CdZnS. In one embodiment, absorber layer 502 is/Mype CIS and junction partner layer 504 is n- type CdS, ZnS, ZnSe, or CdZnS. Such semiconductor junctions 510 are described in Chapter 6 of Bube, 1998, Photovoltaic Materials, Imperial College Press, London, which is hereby incorporated by reference herein in its entirety.

0151In some embodiments, absorber layer 502 is copper-indium-gallium-diselenide (CIGS). In some embodiments, absorber layer 502 is copper-indium-gallium-diselenide (CIGS) and junction partner layer 504 is CdS, ZnS, ZnSe, or CdZnS. In some embodiments, absorber layer 502 isjc-type CIGS and junction partner layer 504 is π-type CdS, ZnS, ZnSe, or CdZnS.

01524.4.2 Semiconductor junctions based on amorphous silicon or polycrystalline silicon

0153In some embodiments, referring to Figure 5B, semiconductor junction 510 comprises amorphous silicon. In some embodiments, this is an nln type heterojunction. For example, in some embodiments, layer 514 comprises SnO₂(Sb), layer 512 comprises undoped amorphous silicon, and layer 510 comprises n+ doped amorphous silicon.

0154In some embodiments, semiconductor junction 510 is ap-i-n type junction. For example, in some embodiments, layer 514 isp⁺ doped amorphous silicon, layer 512 is undoped amorphous silicon, and layer 510 is n⁺ amorphous silicon. Such semiconductor junctions 510 are described in Chapter 3 of Bube, Photovoltaic Materials, 1998, Imperial College Press, London, which is hereby incorporated by reference herein in its entirety.

0155In some embodiments of the present application, semiconductor junction 510 is based upon thin-film polycrystalline. Referring to Figure 5B, in one example in accordance with such embodiments, layer 510 is a;?-doped polycrystalline silicon, layer 512 is depleted polycrystalline silicon and layer 514 is «-doped polycrystalline silicon. Such semiconductor junctions are described in Green, 1995, Silicon Solar Cells: Advanced Principles & Practice, Centre for Photovoltaic Devices and Systems, University of New South Wales, Sydney; and Bube, 1998, Photovoltaic Materials, pp. 57-66, Imperial College Press, London, which is hereby incorporated by reference herein in its entirety.

0156In some embodiments of the present application, semiconductor junctions 510 based upon^-type microcrystalline Si:H and microcrystalline Si:C:H in an amorphous Si:H photovoltaic cell are used. Such semiconductor junctions are described in Bube, 1998, Photovoltaic Materials, pp. 66-67, Imperial College Press, London, and the references cited therein, which is hereby incorporated by reference herein in its entirety.

01574.4.3 Semiconductor junctions based on gallium arsenide and other type III-V materials

0158In some embodiments, semiconductor junctions 510 are based upon gallium arsenide (GaAs) or other III-V materials such as InP, AlSb, and CdTe. GaAs is a direct- band gap material having a band gap of 1.43 eV and can absorb 97% of AMI radiation in a thickness of about two microns. Suitable type III-V junctions that can serve as semiconductor junctions 510 of the present application are described in Chapter 4 of Bube, 1998, Photovoltaic Materials, Imperial College Press, London, which is hereby incorporated by reference herein in its entirety.

0159Furthermore, in some embodiments semiconductor junction 510 is a hybrid multijunction photovoltaic cells such as a GaAs/Si mechanically stacked multijunction as described by Gee and Virshup, 1988, 20^th IEEE Photovoltaic Specialist Conference, IEEE Publishing, New York, p. 754, which is hereby incorporated by reference herein in its entirety, a GaAs/CuInSe₂ MSMJ four-terminal device, consisting of a GaAs thin film top cell and a ZnCdS/CuInSe₂ thin bottom cell described by Stanbery et al, 19^th IEEE Photovoltaic Specialist Conference, IEEE Publishing, New York, p. 280, and Kim et al, 20^th IEEE Photovoltaic Specialist Conference, IEEE Publishing, New York, p. 1487, each of which is hereby incorporated by reference herein in its entirety. Other hybrid multijunction photovoltaic cells are described in Bube, 1998, Photovoltaic Materials, pp. 131-132, Imperial College Press, London, which is hereby incorporated by reference herein in its entirety.

01604.4.4 Semiconductor junctions based on cadmium telluride and other type II- VI materials

0161In some embodiments, semiconductor junctions 510 are based upon II-VI compounds that can be prepared in either the «-type or the/>-type form. Accordingly, in some embodiments, reterring to Figure 5C, semiconductor junction 510 is ajs-w heterojunction in which layers 520 and 540 are any combination set forth in the following table or alloys thereof.

0162Layer 520 Layer 540

0163«-CdSe J^-CdTe

016477-ZnCdS J^-CdTe

0165R-ZnSSe _js-CdTe

0166,P-ZnTe W-CdSe w-CdS P-CdTe w-CdS js-ZnTe p-ZnTs W-CdTe

0167W-ZnSe P-CdTe

0168W-ZnSe js-ZnTe w-ZnS P-CdTe w-ZnS js-ZnTe

0169Methods for manufacturing semiconductor junctions 510 are based upon II-VI compounds are described in Chapter 4 of Bube, 1998, Photovoltaic Materials, Imperial College Press, London, which is hereby incorporated by reference herein in its entirety.

01704.4.5 Semiconductor junctions based on crystalline silicon

0171While semiconductor junctions 510 that are made from thin semiconductor films are preferred, the application is not so limited. In some embodiments semiconductor junctions 510 is based upon crystalline silicon. For example, referring to Figure 5D, in some embodiments, semiconductor junction 510 comprises a layer of js-type crystalline silicon 540 and a layer of w-type crystalline silicon 550. Methods for manufacturing crystalline silicon semiconductor junctions 510 are described in Chapter 2 of Bube, Photovoltaic Materials, 1998, Imperial College Press, London, which is which is hereby incorporated by reference herein in its entirety.

01724.5 Photovoltaic cell types

0173The self-cleaning surfaces of the present application can be used with a wide variety of photovoltaic types including, but not limited to, wire-shaped, tube-shaped and flat panel-shaped. The self-cleaning surfaces of the present application can be used with mono-facial, bi-facial, and multi-facial photovoltaic cells. The self-cleaning surfaces of the present application can be used with moveable as well as stationary photovoltaic cells. The self-cleaning surfaces of the present application can be used with photovoltaic cells having any of the junction types described in Section 4.4. Furthermore, the photovoltaic cells of the present application can be used with photovoltaic cell concentrators.

0174There is no limit to the types and architectures of photovoltaic cells 210 (Fig. 2) that can benefit from the self-cleaning layers 208 and 306 of the present application. Representative photovoltaic cells 210 include, but are not limited to any of the photovoltaic cells described in United States Patent Nos. 6,846,984; 6,830,740; 6,825,409; 6,818,819; 6,809,252; 6,803,515; 6,802,926; 6,787,692; 6,784,358; 6,750,394; 6,746,709; 6,743,974; 6,717,519; 6,696,739; 6,692,981; 6,689,950; 6,683,361; 6,676,459; 6,667,434; 6,664,169; 6,660,930; 6,639,143; 6,638,788; 6,635,817; 6,617,508; 6,613,973; 6,606,830; 6,605,881; 6,593,522; 6,581,589; 6,566,595; 6,566,159; 6,563,289; 6,555,443; 6,552,259; 6,552,258; 6,548,751; 6,545,211; 6,541,696; 6,541,693; 6,540,178; 6,534,704; 6,534,334; 6,525,264; 6,525,262; 6,524,880; 6,521,827; 6,521,826; 6,521,823; 6,521,821; 6,518,944; 6,515,216; 6,512,171; and 6,509,204, each of which is hereby incorporated by reference herein in its entirety.

0175Still another type of photovoltaic cell that can benefit from the self-cleaning layers of the present application are the type of photovoltaic cells found in United States Patent Number 6,762,359 B2 to Asia et ah, which is hereby incorporated by reference herein in its entirety, in which a photovoltaic cell including a/?-type layer and an «-type layer is disclosed. Still another type of photovoltaic cell that can benefit from the self-cleaning layers of the present application is the type of photovoltaic cells found in United States Patent Number 3,976,508 to Mlavsky, which is hereby incorporated by reference herein in its entirety, in which a tubular photovoltaic cell comprising a cylindrical silicon tube of rø-type conductivity that has been subjected to diffusion of boron into its outer surface to form an outers-conductivity type region 4 and thus a p-n junction. Still another type of photovoltaic cell that can benefit from the self-cleaning layers of the present application are the type of photovoltaic cells found in United States Patent No. 3,990,914 to Weinstein and Lee, which is hereby incorporated by reference herein in its entirety, in which another form of tubular photovoltaic cell is disclosed. Still another type of photovoltaic cell that can benefit from the self-cleaning layers of the present application are the type of photovoltaic cells found in Japanese Patent Application Kokai Publication Number S59-125670, Toppan Printing Company, published July 20, 1984 (hereinafter "S59-125670 ), which is hereby incorporated by reference herein in its entirety, in which a rod-shaped photovoltaic cell is disclosed. Yet another type of photovoltaic cell that can benefit from the self-cleaning layers of the present application is the type of photovoltaic cells found in German Unexamined Patent Application DE 43 39 547 Al to Twin Solar- Technik Entwicklungs-GmbH, published May 24, 1995, (hereinafter "Twin Solar"), which is hereby incorporated by reference herein in its entirety, in which a plurality of rod-shaped photovoltaic cells arranged in a parallel manner inside a transparent sheet, which forms the body of the photovoltaic cell.

01764.6 Additional embodiments

0177One aspect of the application provides a photovoltaic cell system comprising a plurality of photovoltaic cell assemblies. Each assembly in the plurality of photovoltaic cell assemblies comprises a front side and a back side. Each of these front sides and back sides is configured to receive electromagnetic energy. Each of the photovoltaic assemblies in the plurality of photovoltaic assemblies is configured to convert electromagnetic energy into an electrical current. Each respective photovoltaic cell assembly in the plurality of photovoltaic cell assemblies further comprises a first self- cleaning layer disposed on the corresponding front side of the respective photovoltaic cell assembly. In some embodiments, each respective photovoltaic cell assembly in the plurality of photovoltaic cell assemblies further comprises a second self-cleaning layer disposed on the corresponding back side of the respective photovoltaic cell assembly. In some embodiments, the first self-cleaning layer and/or the second self-cleaning layer is one or more layers of photocatalytic coating, nano-particles and/or titanium oxide. In some embodiments, the first self-cleaning layer and/or the second self-cleaning layer is a hydrophobic layer.

0178An aspect of the present application provides a photovoltaic cell system, comprising a plurality of photovoltaic cell assemblies, where each assembly in the plurality of photovoltaic cell assemblies comprises a front side, where each such front side is configured to receive electromagnetic energy, and where each such photovoltaic assembly in the plurality of photovoltaic assemblies is configured to convert electromagnetic energy into an electrical current. The photovoltaic cell system further comprises an albedo surface configured to reflect electromagnetic energy on one or more photovoltaic cell assemblies in the plurality of photovoltaic cell assemblies, where the albedo surface comprises a high albedo layer and a self-cleaning layer disposed on the high albedo layer. In some instances, the self-cleaning layer comprises a photocatalytic material, Ti-hydroxyapetite, anatase TiO₂, brookitein TiO₂ rutile TiO₂, between 1 percent TiO₂ and 90 percent TiO₂ by weight, ZnO, SnO₂, SrTiO₃, WO₃, Bi₂O₃, Fe₂O₃, Teflon, polytetrafluoroethylene, or polychlorotrifluoroethylene. In some instances, the self-cleaning layer is formed from perotitanium solution or perox-modified anatase sol. In some instances, the self-cleaning layer comprises a plurality of nano-particles (e.g., nano-particles made of SiO₂, TiO₂, Al₂O₃, ZrO₂ or SnO₂, or silica). In some embodiments, the self-cleaning layer comprises (100-X)CaF₂ -(X)TiO₂, where X is the molar percentage of TiO₂ and X is between 1 percent and 50 percent. In some embodiments, the self-cleaning layer comprises a composite Of CaF₂ and TiO₂. In some embodiments, the self-cleaning layer comprises a photocatalytic oxide and a resin. The resin, can be, for example, a silicone resin or a fluororesin. The silicone resin can comprises, for example, ethyltrimethoxysilane, methyltriethoxysilane, methyltrichlorosilane, methyltribromosilane, methyltriisopropoxysilane, methyl-tri-t- butoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltrichlorosilane, ethyltribromosilane, ethyltriisopropoxysilane, ethyl-tri-t-butoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, n- propyltrichlorosilane, n-propyltribromosilane, n-propyltriisopropoxysilane, n-propyl-tri-t- butoxysilane, n-hexyltrimethoxysilane, n-hexyltriethoxysilane, n-hexytrichlorosilane, n-hexyltribromosilane, n-hexyltriisopropoxysilane, n-hexyl-tri-t-butoxysilane, n- decyltrimethoxysilane, n-decyltriethoxysilane, n-decyltrichlorosilane, n- decyltribromosilane, n-decyltriisopropoxysilane, n-decyl-tri-t-butoxysilane, n- octatrimethoxysilane, n-octatriethoxysilane, n-octatrichlorosilane, n-octatribromosilane, n-octatriisopropoxysilane, n-octa-tri-t-butoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, phenyltrichlorosilane, phenyltribromosilane, phenyltriisopropoxysilane, phenyl-t-butoxysilane, dimethyldichlorosilane, dimethyldibromosilane, dimethyldimethoxysilane, dimethyldiethoxysilane, diphenyldichlorosilane, diphenyldibromosilane, diphenyldimethoxysilane, diphenyldiethoxysilane, phenylmethyldichlorosilane, phenylmethyldibromosilane, phenylmethyldimethoxysilane, phenylmethyldiethoxysilane, vinyltrichlorosilane, vinyltribromosilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriisopropoxysilane, vinyl-tri-t-butoxysilane, trifluoropropyltrichlorosilane, trifluoropropyltribromosilane, trifluoropropyltrimethoxysilane, trifluoropropyltriethoxysilane, trifluoropropyltriisopropoxysilane, trifluoropropyl-tri-t- butoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ- glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ- glycidoxypropyltriethoxysilane, γ-glycidoxypropyltriisopropoxysilane, γ- glycidoxypropyl-tri-t-butoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ- methacryloxypropylmethyldiethoxysilane, γ -methacryloxypropyltrimethoxysilane, γ- methacryloxypropyltriethoxysilane, γ-methacryloxypropyltriisopropoxysilane, γ- methacryloxypropyl-tri-t-butoxysilane, γ-aminopropylmethyldimethoxysilane, γ- aminopropylmethyldiethoxysilane, γ-aminopropyltrimethoxysilane, γ- aminopropyltriethoxysilane, γ-aminopropyltriisopropoxysilane, γ-aminopropyl-tri-t- butoxysilane, γ-mercaptopropylmethyldimethoxysilane, γ- mercaptopropylmethyldiethoxysilane, γ-mercaptopropyltrimethoxysilane, γ- mercaptopropyltriethoxysilane, γ-mercaptopropyltriisopropoxysilane, γ-mercaptopropyl- tri-t-butoxysilane, β-(3 ,4-poxycyclohexyl)ethyltrimethoxysilane, β-(3 ,4- epoxycyclohexyl)ethyltriethoxysilane, or any combination thereof. The fluororesin can comprise, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polychlorotrifluoroethylene, tetrafluoroethylene/hexafluoropropylene copolymer, ethylene/tetrafluoroethylene copolymer, ethylene/chlorotrifluoroethylene copolymer, tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer, perfluorocyclo polymer, vinyl ether/fluoroolefϊn copolymer, vinyl ester/fluoroolefm copolymer, tetrafluoroethylene/vinyl ether copolymer, chlorotrifluoroethylene/vinyl ether copolymer, urethane-crosslinked tetrafluoroethylene, epoxy-crosslinked tetrafluoroethylene, acryl- crosslinked tetrafluoroethylene, melamine-crosslinked tetrafluoroethylene, or any combination thereof. In some embodiments, the high albedo layer is self-cleaning.

0179Another aspect of the present application provides a photovoltaic cell system comprisinga plurality of photovoltaic cell assemblies. Each assembly in the plurality of photovoltaic cell assemblies comprises: a base, a solar cell panel attached to the base, and a self-cleaning albedo device, attached to the base, where the self-cleaning albedo device comprises a flexible substrate, a high albedo layer, and a self-cleaning layer. In such embodiments, the high albedo layer is disposed on the flexible substrate and the self-cleaning layer is disposed on the high albedo layer. In some embodiments, the photovoltaic system further comprises a retractable roller mounted on the base of an assembly in the plurality of photovoltaic cell assemblies, and the retractable roller is configured to retract the self-cleaning albedo device into a roll. In some applications, the self-cleaning albedo device has an open configuration and a closed configuration, and the self-cleaning albedo device is rolled up in the retractable roller when the self-cleaning albedo device is in the closed position. Furthermore, the self-cleaning albedo device is extended in a downward gradient away from the base when the self-cleaning albedo device is in the open position. In some embodiments, the self-cleaning layer comprises a photocatalytic material. In some embodiments, the self-cleaning layer comprises anatase TiO₂, rutile TiO₂, brookitein TiO₂ or mylar. In some embodiments, the self-cleaning layer comprises between 1 percent TiO₂ and 90 percent TiO₂ by weight, ZnO, SnO₂, SrTiO₃, WO₃, Bi₂O₃, or Fe₂O₃. In some embodiments, the self-cleaning layer is formed from perotitanium solution or perox-modified anatase sol. In some embodiments, the self-cleaning layer comprises a plurality of nano-particles (e.g., particles made of SiO₂, TiO₂, Al₂O₃, ZrO₂ or SnO₂, or silica). In some embodiments, the albedo self-cleaning layer comprises polytetrafluoroethylene or polychlorotrifluoroethylene. In some embodiments, the self-cleaning layer comprises (100-X)CaF₂ -(X)TiO₂, where X is the molar percentage OfTiO₂ and X is between 1 percent and 50 percent. In some embodiments, the self-cleaning layer comprises a composite of CaF₂ and TiO₂. In some embodiments, the self-cleaning layer comprises a photocatalytic oxide and a resin. In some embodiments, the resin comprises a silicone resin or a fluororesin. In some embodiments, the silicone resin comprises ethyltrimethoxysilane, methyltriethoxysilane, methyltrichlorosilane, methyltribromosilane, methyltriisopropoxysilane, methyl-tri-t- butoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltrichlorosilane, ethyltribromosilane, ethyltriisopropoxysilane, ethyl-tri-t-butoxysilane, n- propyltrimethoxysilane, n-propyltriethoxysilane, n-propyltrichlorosilane, n- propyltribromosilane, n-propyltriisopropoxysilane, n-propyl-tri-t-butoxysilane, n- hexyltrimethoxysilane, n-hexyltriethoxysilane, n-hexytrichlorosilane, n-hexyltribromosilane, n-hexyltriisopropoxysilane, n-hexyl-tri-t-butoxysilane, n- decyltrimethoxysilane, n-decyltriethoxysilane, n-decyltrichlorosilane, n- decyltribromosilane, n-decyltriisopropoxysilane, n-decyl-tri-t-butoxysilane, n- octatrimethoxysilane, n-octatriethoxysilane, n-octatrichlorosilane, n-octatribromosilane, n-octatriisopropoxysilane, n-octa-tri-t-butoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, phenyltrichlorosilane, phenyltribromosilane, phenyltriisopropoxysilane, phenyl-t-butoxysilane, dimethyldichlorosilane, dimethyldibromosilane, dimethyldimethoxysilane, dimethyldiethoxysilane, diphenyldichlorosilane, diphenyldibromosilane, diphenyldimethoxysilane, diphenyldiethoxysilane, phenylmethyldichlorosilane, phenylmethyldibromosilane, phenylmethyldimethoxysilane, phenylmethyldiethoxysilane, vinyltrichlorosilane, vinyltribromosilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriisopropoxysilane, vinyl-tri-t-butoxysilane, trifluoropropyltrichlorosilane, trifluoropropyltribromosilane, trifluoropropyltrimethoxysilane, trifluoropropyltriethoxysilane, trifluoropropyltriisopropoxysilane, trifluoropropyl-tri-t- butoxysilane, γ-glycidoxypropylmethyldimethoxysilane, γ- glycidoxypropylmethyldiethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ- glycidoxypropyltriethoxysilane, γ-glycidoxypropyltriisopropoxysilane, γ- glycidoxypropyl-tri-t-butoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ- methacryloxypropylmethyldiethoxysilane, γ -methacryloxypropyltrimethoxysilane, γ- methacryloxypropyltriethoxysilane, γ-methacryloxypropyltriisopropoxysilane, γ- methacryloxypropyl-tri-t-butoxysilane, γ-aminopropylmethyldimethoxysilane, γ- aminopropylmethyldiethoxysilane, γ-aminopropyltrimethoxysilane, γ- aminopropyltriethoxysilane, γ-aminopropyltriisopropoxysilane, γ-aminopropyl-tri-t- butoxysilane, γ-mercaptopropylmethyldimethoxysilane, γ- mercaptopropylmethyldiethoxysilane, γ-mercaptopropyltrimethoxysilane, γ- mercaptopropyltriethoxysilane, γ-mercaptopropyltriisopropoxysilane, γ-mercaptopropyl- tri-t-butoxysilane, β-(3 ,4-poxycyclohexyl)ethyltrimethoxysilane, β-(3 ,4- epoxycyclohexyl)ethyltriethoxysilane, or any combination thereof. In some embodiments, the fluororesin comprises polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polychlorotrifluoroethylene, tetrafluoroethylene/hexafluoropropylene copolymer, ethylene/tetrafluoroethylene copolymer, ethylene/chlorotrifluoroethylene copolymer, tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer, perfluorocyclo polymer, vinyl ether/fluoroolefin copolymer, vinyl ester/fluoroolefin copolymer, tetrafluoroethylene/vinyl ether copolymer, chlorotrifluoroethylene/vinyl ether copolymer, urethane-crosslinked tetrafluoroethylene, epoxy-crosslinked tetrafluoroethylene, acryl- crosslinked tetrafluoroethylene, melamine-crosslinked tetrafluoroethylene, or any combination thereof.

0180Yet another aspect of the present application provides is a photovoltaic cell system comprising a plurality of photovoltaic cell assemblies, where each assembly in the plurality of photovoltaic cell assemblies comprises: (i) a base, (ii) a solar cell panel attached to the base, and (iii) an albedo device, attached to the base, where the albedo device comprises a flexible substrate having a high albedo and where the albedo device is capable of hooking to the base of a neighboring photovoltaic cell assembly in the plurality of photovoltaic cell assemblies. In some embodiments, the albedo device has an open configuration and a closed configuration, where the albedo device is rolled up or folded up when the albedo device is in the closed position. Furthermore, the self-cleaning albedo device is extended in a downward gradient away from the base and hooks the base of the neighboring photovoltaic cell assembly when the albedo device is in the open position. In some embodiments, the albedo device is made of plastic or mylar. In some embodiments, a photovoltaic cell assembly in the plurality of photovoltaic cell assemblies comprises a back side, the the back side is configured to receive electromagnetic energy. In some embodiments, the solar cell panel of a photovoltaic cell assembly in the plurality of photovoltaic cell assemblies is mono-facial or bi-facial.

0181Another aspect of the present application provides an apparatus comprising (A) a high albedo layer and (B) a self-cleaning layer overlayed on the high albedo layer. In some embodiments, the apparatus further comprises (C) a solar unit comprising (i) a substrate having a first end and a second end, and (ii) a plurality of photovoltaic cells linearly arranged on the substrate. In such embodiments, the plurality of photovoltaic cells are in optical communication with the high albedo layer. This means that light reflected from the albedo layers can reach the plurality of photovoltaic cells. The plurality of photovoltaic cells comprise a first photovoltaic cell and a second photovoltaic cell. Each photovoltaic cell in the plurality of photovoltaic cells comprises a back-electrode circumferentially disposed on the substrate, a semiconductor junction layer circumferentially disposed on the back-electrode, and a transparent conductive layer circumferentially disposed on the semiconductor junction, such that the transparent conductive layer of the first photovoltaic cell in the plurality of photovoltaic cells is in serial electrical communication with the back-electrode of the second photovoltaic cell in the plurality of photovoltaic cells. Such monolithically integrated photovoltaic cells are disclosed in, for example, United States Patent application 11/378,835, filed March 18, 2006, which is hereby incorporated by reference herein in its entirety. n some embodiments, plurality of photovoltaic cells comprises (i) a first terminal photovoltaic cell at the first end of the substrate, (ii) a second terminal photovoltaic cell at the second end of the substrate, and (iii) at least one intermediate photovoltaic cell between the first teπninal photovoltaic cell and the second photovoltaic cell, such that the transparent conductive layer of each intermediate photovoltaic cell in the at least one intermediate photovoltaic cell is in serial electrical communication with the back-electrode of an adjacent photovoltaic cell in the plurality of photovoltaic cells. In some embodiments, the adjacent photovoltaic cell is the first terminal photovoltaic cell or the second terminal photovoltaic cell. In some embodiments, the adjacent photovoltaic cell is another intermediate photovoltaic cell. In some embodiments the plurality of photovoltaic cells comprises three or more photovoltaic cells, ten or more photovoltaic cells, fifty or more photovoltaic cells, or one hundred or more photovoltaic cells. In some embodiments, the solar cell further comprises a transparent tubular casing that is circumferentially disposed onto the transparent conductive layer of all or a portion of the photovoltaic cells in the plurality of photovoltaic cells. In some embodiments, the transparent tubular casing is made of plastic or glass. In some embodiments, the transparent tubular casing comprises aluminosilicate glass, borosilicate glass, dichroic glass, germanium / semiconductor glass, glass ceramic, silicate / fused silica glass, soda lime glass, quartz glass, chalcogenide / sulphide glass, fluoride glass, flint glass, or cereated glass. In some embodiments, transparent tubular casing comprises a urethane polymer, an acrylic polymer, a fluoropolymer, a silicone, a silicone gel, an epoxy, a polyamide, or a polyolefin. In some embodiments, the transparent tubular casing comprises polymethylmethacrylate (PMMA), poly-dimethyl siloxane (PDMS), ethyl vinyl acetate (EVA), perfluoroalkoxy fluorocarbon (PFA), nylon, cross-linked polyethylene (PEX), polypropylene (PP), polyethylene terephtalate glycol (PETG), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), or polyvinylidene fluoride (PVDF). In some embodiments, the substrate comprises plastic, metal or glass. In some embodiments, the substrate comprises a urethane polymer, an acrylic polymer, a fluoropolymer, polybenzamidazole, polymide, polytetrafluoroethylene, polyetheretherketone, polyamide-imide, glass-based phenolic, polystyrene, cross-linked polystyrene, polyester, polycarbonate, polyethylene, polyethylene, acrylonitrile- butadiene-styrene, polytetrafluoro-ethylene, polymethacrylate, nylon 6,6, cellulose acetate butyrate, cellulose acetate, rigid vinyl, plasticized vinyl, or polypropylene. In some embodiments, the substrate comprises aluminosilicate glass, borosilicate glass, dichroic glass, germanium / semiconductor glass, glass ceramic, silicate / fused silica glass, soda lime glass, quartz glass, chalcogenide / sulphide glass, fluoride glass, a glass-based phenolic, flint glass, or cereated glass. In some embodiments, the substrate is tubular shaped. In some embodiments, a fluid (e.g., air, water, nitrogen, helium, etc.) is passed through the substrate. In some embodiments the substrate comprises a solid rod. In some embodiments, the back-electrode of a photovoltaic cell in the plurality of photovoltaic cells is made of aluminum, molybdenum, tungsten, vanadium, rhodium, niobium, chromium, tantalum, titanium, steel, nickel, platinum, silver, gold, an alloy thereof, or any combination thereof. In some embodiments, the back-electrode of a photovoltaic cell in the plurality of photovoltaic cells is made of indium tin oxide, titanium nitride, tin oxide, fluorine doped tin oxide, doped zinc oxide, aluminum doped zinc oxide, gallium doped zinc oxide, boron dope zinc oxide indium-zinc oxide, a metal-carbon black-filled oxide, a graphite-carbon black-filled oxide, a carbon black-carbon black-filled oxide, a superconductive carbon black-filled oxide, an epoxy, a conductive glass, or a conductive plastic. In some embodiments, the semiconductor junction of a photovoltaic cell in said plurality of photovoltaic cells comprises a homojunction, a heterojunction, a heteroface junction, a buried homojunction, a p-i-n junction, or a tandem junction. In some embodiments, the transparent conductive layer of a photovoltaic cell in the plurality of photovoltaic cells comprises carbon nanotubes, tin oxide, fluorine doped tin oxide, indium-tin oxide (ITO), doped zinc oxide, aluminum doped zinc oxide, gallium doped zinc oxide, boron dope zinc oxide indium-zinc oxide or any combination thereof or any combination thereof.

0182In some embodiments, the semiconductor junction of a photovoltaic cell in the plurality of photovoltaic cells comprises an absorber layer and a junction partner layer, wherein the junction partner layer is circumferentially deposed on the absorber layer. In some embodiments, the absorber layer is copper-indium-gallium-diselenide and said junction partner layer is In₂Se₃, In₂S₃, ZnS, ZnSe, CdInS, CdZnS, ZnIn₂Se₄, Zn_1-xMg_xO, CdS, SnO₂, ZnO, ZrO₂, or doped ZnO. In some embodiments, a photovoltaic cell in the plurality of photovoltaic cells further comprises an intrinsic layer circumferentially disposed on the semiconductor junction of the photovoltaic cell and the transparent conductive layer of the photovoltaic cell is disposed on the intrinsic layer. In some embodiments, the intrinsic layer comprises an undoped transparent oxide {e.g., an undoped zinc oxide). In some embodiments, the solar cell unit further comprises a filler layer that is circumferentially disposed onto the transparent conductive layer of all or a portion of the photovoltaic cells in the plurality of photovoltaic cells and a transparent tubular casing that is circumferentially disposed on the filler layer. In some embodiments, the filler layer comprises ethyl vinyl acetate (EVA), silicone, silicone gel, epoxy, polydimethyl siloxane (PDMS), RTV silicone rubber, polyvinyl butyral (PVB), thermoplastic polyurethane (TPU), a polycarbonate, an acrylic, a fluoropolymer, or a ur ethane.

0183In some embodiments, the solar cell unit further comprises a water resistant layer that is circumferentially disposed onto the transparent conductive layer of all or a portion of the photovoltaic cells in the plurality of photovoltaic cells and a transparent tubular casing that is circumferentially disposed on said water resistant layer. In some embodiments, the water resistant layer comprises clear silicone, SiN, SiO_xNy, SiO_x, or Al₂O₃, where x and y are integers. In some embodiments, a fluorescent material is coated on the water resistant layer. In some embodiments, the solar cell unit further comprises a transparent tubular casing that is circumferentially disposed onto the transparent conductive layer of all or a portion of the photovoltaic cells in said plurality of photovoltaic cells and an antireflective coating circumferentially disposed on the transparent tubular casing. In some embodiments, the antireflective coating comprises MgF₂, silicone nitrate, titanium nitrate, silicon monoxide, or silicone oxide nitrite. In some embodiments, the solar cell unit further comprises an antireflective coating that is circumferentially disposed onto the transparent conductive layer of all or a portion of the photovoltaic cells in the plurality of photovoltaic cells. In some embodiments, the antireflective coating comprises MgF₂, silicone nitrate, titanium nitrate, silicon monoxide, or silicone oxide nitrite. In some embodiments, the solar cell unit has a length that is between 2 centimeters and 300 centimeters, between 2 centimeters and 30 centimeters, or between 30 centimeters and 300 centimeters. Still another aspect of the present application provides a solar cell assembly comprising a plurality of solar cell units, where solar cell units in the plurality of solar cell units are arranged in coplanar rows to form the solar cell assembly.

5. REFERENCES CITED; MODIFICATIONS

0185All references cited herein are incorporated by reference herein in their entirety and for all purposes to the same extent as if each individual publication or patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety for all purposes.

0186Many modifications and variations of this application can be made without departing from its spirit and scope, as will be apparent to those skilled in the art.