Patent Publication Number: US-8525152-B2

Title: Formation of solar cells with conductive barrier layers and foil substrates

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of commonly assigned, U.S. patent application Ser. No. 11/427,328 entitled “Formation of Solar Cells with Conductive Barrier Layers and Foil Substrates” filed Jun. 28, 2006, now U.S. Pat. No. 7,732,229 which is a continuation-in-part of commonly assigned, co-pending U.S. patent application Ser. No. 10/943,685, entitled “Formation of CIGS Absorber Layers on Foil Substrates”, filed Sep. 18, 2004, and also a continuation-in-part of commonly assigned, U.S. patent application Ser. No. 11/207,157, entitled “Series Interconnected Optoelectronic Device Module Assembly”, filed Aug. 16, 2005 now U.S. Pat. No. 7,838,868 and a continuation-in-part of commonly assigned, U.S. patent application Ser. No. 11/039,053, entitled “Optoelectronic Architecture Having Compound Conducting Substrate”, filed Jan. 20, 2005 now U.S. Pat. No. 7,276,724. This application is related to commonly assigned, co-pending U.S. patent application Ser. No. 10/771,250 entitled “Photovoltaic Devices Fabricated from Insulating Template with Conductive Coating” and filed on Feb. 2, 2004. The entire disclosures of the above applications are fully incorporated herein by reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to photovoltaic devices and more specifically to fabrication of absorber layers for photovoltaic devices. 
     BACKGROUND OF THE INVENTION 
     Efficient photovoltaic devices, such as solar cells, have been fabricated using absorber layers made with alloys containing elements of group IB, IIIA and VIA, e.g., alloys of copper with indium and/or gallium or aluminum and selenium and/or sulfur. One common combination of the aforementioned elements is copper-indium-gallium-diselenide (CIGS) and the resulting devices are often referred to as CIGS solar cells. The CIGS absorber layer may be deposited on a substrate. It would be desirable to fabricate such an absorber layer on an aluminum foil substrate because aluminum foil is relatively inexpensive, lightweight, and flexible. Unfortunately, current techniques for depositing CIGS absorber layers are incompatible with the use of aluminum foil as a substrate. 
     Typical deposition techniques include evaporation, sputtering, chemical vapor deposition, and the like. These deposition processes are typically carried out at high temperatures and for extended times. Both factors can result in damage to the substrate upon which deposition is occurring. Such damage can arise directly from changes in the substrate material upon exposure to heat, and/or from undesirable chemical reactions driven by the heat of the deposition process. Thus, very robust substrate materials are typically required for fabrication of CIGS solar cells. These limitations have excluded the use of aluminum and aluminum-foil based foils. 
     An alternative deposition approach is the solution-based printing of the CIGS precursor materials onto a substrate. Examples of solution-based printing techniques are described, e.g., in Published PCT Application WO 2002/084708 and commonly-assigned U.S. patent application Ser. No. 10/782,017, both of which are incorporated herein by reference. Advantages to this deposition approach include both the relatively lower deposition temperature and the rapidity of the deposition process. Both advantages serve to minimize the potential for heat-induced damage of the substrate on which the deposit is being formed. 
     Although solution deposition is a relatively low temperature step in fabrication of CIGS solar cells, it is not the only step. In addition to the deposition, a key step in the fabrication of CIGS solar cells is the selenization and annealing of the CIGS absorber layer. Selenization introduces selenium into the bulk CIG or CI absorber layer, where the element incorporates into the film, while the annealing provides the absorber layer with the proper crystalline structure. In the prior art, selenization and annealing has been performed by heating the substrate in the presence of H 2 Se or Se vapor and keeping this nascent absorber layer at high temperatures for long periods of time. 
     While use of Al as a substrate for solar cell devices would be desirable due to both the low cost and lightweight nature of such a substrate, conventional techniques that effectively anneal the CIGS absorber layer also heat the substrate to high temperatures, resulting in damage to Al substrates. There are several factors that result in Al substrate degradation upon extended exposure to heat and/or selenium-containing compounds for extended times. First, upon extended heating, the discrete layers within a Mo-coated Al substrate can fuse and form an intermetallic back contact for the device, which decreases the intended electronic functionality of the Mo-layer. Second, the interfacial morphology of the Mo layer is altered during heating, which can negatively affect subsequent CIGS grain growth through changes in the nucleation patterns that arise on the Mo layer surface. Third, upon extended heating, Al can migrate into the CIGS absorber layer, disrupting the function of the semiconductor. Fourth, the impurities that are typically present in the Al foil (e.g. Si, Fe, Mn, Ti, Zn, and V) can travel along with mobile Al that diffuses into the solar cell upon extended heating, which can disrupt both the electronic and optoelectronic function of the cell. Fifth, when Se is exposed to Al for relatively long times and at relatively high temperatures, aluminum selenide can form, which is unstable. In moist air the aluminum selenide can react with water vapor to form aluminum oxide and hydrogen selenide. Hydrogen selenide is a highly toxic gas, whose free formation can pose a safety hazard. For all these reasons, high-temperature deposition, annealing, and selenization are therefore impractical for substrates made of aluminum or aluminum alloys. 
     Because of the high-temperature, long-duration deposition and annealing steps, CIGS solar cells cannot be effectively fabricated on aluminum substrates (e.g. flexible foils comprised of Al and/or Al-based alloys) and instead must be fabricated on heavier substrates made of more robust (and more expensive) materials, such as stainless steel, titanium, or molybdenum foils, glass substrates, or metal- or metal-oxide coated glass. Thus, even though CIGS solar cells based on aluminum foils would be more lightweight, flexible, and inexpensive than stainless steel, titanium, or molybdenum foils, glass substrates, or metal- or metal-oxide coated glass substrates, current practice does not permit aluminum foil to be used as a substrate. 
     Thus, there is a need in the art for a method for fabricating solar cells on aluminum substrates. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention address at least some of the drawbacks set forth above. The present invention provides photovoltaic devices that are cost-effectively manufactured in high-throughput manner on foil substrates. The thin, flexible nature of the photovoltaic devices may also allow them to be rolled or folded into a smaller form factor for ease of transportation but also allow for streamlined fabrication. Embodiments of the present invention may also be designed to reduce the amount raw materials used in the manufacturing process. It also should be understood that embodiments of the present invention may be adapted for use with absorber layers of a variety of materials and are not limited to only CIGS absorber layers. At least some of these and other objectives described herein will be met by various embodiments of the present invention. 
     In one embodiment of the present invention, a method of manufacturing photovoltaic devices may be comprised of providing a substrate having at least one electrically conductive metal foil substrate, at least one electrically conductive diffusion barrier layer, and at least one electrically conductive electrode layer above the diffusion barrier layer. Although not limited to the following, the foil substrate may be an aluminum foil substrate. The electrically conductive diffusion barrier layer may prevent chemical interaction between the aluminum foil substrate and the electrode layer. The method may include forming an absorber layer on the substrate. In one embodiment, the absorber layer may be a non-silicon absorber layer. In another embodiment, the absorber layer may be an amorphous silicon (doped or undoped) absorber layer. Optionally, the absorber layer may be based on organic and/or inorganic materials. 
     For any of the embodiments described herein, the following may also apply. The forming step may be comprised of first forming a nascent absorber layer. The nascent absorber layer may be reacted to form a dense film. In some embodiments, the dense film is the absorber layer. In other embodiments, the dense film is process in another step to form the desired absorber layer. The nascent absorber layer may be heated to form a dense film. It should be understood that the diffusion barrier layer inhibits inter-diffusion of aluminum in the foil substrate and metal in the electrode layer during heating. The diffusion barrier layer may include one or more of the following materials: chromium, vanadium, tungsten, glass, and/or nitrides, tantalum nitride, tungsten nitride, titanium nitride, zirconium nitride, hafnium nitride, and silicon nitride, oxides, or carbides. The electrode layer may be comprised of molybdenum. Alternatively, the electrode layer may be comprised of copper, silver, aluminum, and niobium. 
     In another embodiment of the present invention, a photovoltaic device is provided having a substrate comprising of at least one electrically conductive aluminum foil substrate, at least one electrically conductive diffusion barrier layer, and at least one electrically conductive electrode layer above the diffusion barrier layer, wherein the diffusion barrier layer prevents chemical interaction between the aluminum foil substrate and the electrode layer. The device may include an absorber layer formed on the substrate. In one embodiment, the absorber layer may be a non-silicon absorber layer. Optionally, the absorber layer may be based on organic and/or inorganic materials. 
     In yet another embodiment of the present invention, a method for forming an absorber layer of a photovoltaic device comprises providing a substrate having at least one electrically conductive metallized polymer foil substrate, at least one electrically conductive diffusion barrier layer, and at least one electrically conductive back electrode layer above the diffusion barrier layer. The diffusion barrier layer prevents chemical interaction between the metallized polymer foil substrate and the back electrode layer. The method may include forming an absorber layer on the substrate. In one embodiment, the absorber layer may be a non-silicon absorber layer. In another embodiment, the absorber layer may be an amorphous silicon (doped or undoped) absorber layer. Optionally, the absorber layer may be based on organic and/or inorganic materials. The foil substrate may contain a polymer selected from the group of: polyesters, polyethylene naphtalates, polyetherimides, polyethersulfones, polyetheretherketones, polyimides, and/or combinations of the above. The metal used for metallization of the polymer foil substrate may be aluminum or an alloy of aluminum with one or more metals. 
     In a still further embodiment of the present invention, a photovoltaic device is provided comprising of a substrate having at least one electrically conductive aluminum foil substrate, at least one electrically conductive diffusion barrier layer, and at least one electrically conductive back electrode layer above the diffusion barrier layer, wherein the diffusion barrier layer prevents chemical interaction between the aluminum foil substrate and the back electrode layer. The device may include an absorber layer formed on the substrate. In one embodiment, the absorber layer may be a non-silicon absorber layer. In another embodiment, the absorber layer may be an amorphous silicon (doped or undoped) absorber layer. Optionally, the absorber layer may be based on inorganic and/or organic materials. 
     For any of the embodiments described herein, the following may also apply. The absorber layer may include one or more inorganic materials selected from the following: titania (TiO2), nanocrystalline TiO2, zinc oxide (ZnO), copper oxide (CuO or Cu2O or CuxOy), zirconium oxide, lanthanum oxide, niobium oxide, tin oxide, indium oxide, indium tin oxide (ITO), vanadium oxide, molybdenum oxide, tungsten oxide, strontium oxide, calcium/titanium oxide and other oxides, sodium titanate, potassium niobate, cadmium selenide (CdSe), cadmium sulfide (CdS), copper sulfide (Cu2S), cadmium telluride (CdTe), cadmium-tellurium selenide (CdTeSe), copper-indium selenide (CuInSe2), cadmium oxide (CdOx), CuI, CuSCN, a semiconductive material, a group IB element, a group MA element, a group VIA element, or any combination of the above. 
     Optionally, any of the photovoltaic devices disclosed in the present application may include an organic material in the absorber layer. The absorber layer may include one or more organic materials from the following: a conjugated polymer, poly(phenylene) and derivatives thereof, poly(phenylene vinylene) and derivatives thereof (e.g., poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV), poly(para-phenylene vinylene), (PPV)), PPV copolymers, poly(thiophene) and derivatives thereof (e.g., poly(3-octylthiophene-2,5,-diyl), regioregular, poly(3-octylthiophene-2,5,-diyl), regiorandom, Poly(3-hexylthiophene-2,5-diyl), regioregular, poly(3-hexylthiophene-2,5-diyl), regiorandom), poly(thienylenevinylene) and derivatives thereof, and poly(isothianaphthene) and derivatives thereof, 2,2′7,7′ tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene(spiro-Me OTAD), organometallic polymers, polymers containing perylene units, poly(squaraines) and their derivatives, and discotic liquid crystals, organic pigments or dyes, ruthenium-based dye, liquid iodide/triiodide electrolyte, azo-dyes having azo chromofores (—N═N—) linking aromatic groups, phthalocyanines including metal-free phthalocyanine; (HPc), perylenes, perylene derivatives, Copper pthalocyanines (CuPc), Zinc Pthalocyanines (ZnPc), naphthalocyanines, squaraines, merocyanines and their respective derivatives, poly(silanes), poly(germinates), 2,9-Di(pent-3-yl)-anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone, and 2,9-Bis-(1-hexyl-hept-1-yl)-anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone and pentacene, pentacene derivatives and/or pentacene precursors, an N-type ladder polymer such as poly(benzimidazobenzophenanthroline ladder) (BBL), or any combination of the above. 
     For any of the embodiments described herein, the following may also apply. The absorber layer may include one or more materials from the group consisting of: an oligimeric material, micro-crystalline silicon, inorganic nanorods dispersed in an organic matrix, inorganic tetrapods dispersed in an organic matrix, quantum dot materials, ionic conducting polymer gels, sol-gel nanocomposites containing an ionic liquid, ionic conductors, low molecular weight organic hole conductors, C60 and/or other small molecules, or combinations of the above. The absorber layer may be comprised of one or more of the following: a nanostructured layer having an inorganic porous template with pores filled by an organic material (doped or undoped), a polymer/blend cell architecture, a micro-crystalline silicon cell architecture, or combinations of the above. 
     Optionally, a photovoltaic device module may use any of the photovoltaic devices disclosed in the present application in a high efficiency cell configuration described below. The photovoltaic device module may include the photovoltaic device, an insulator layer, and a conductive back plane, wherein the insulator layer is sandwiched between the substrate and the back plane. A transparent conducting layer may be disposed such that the absorber layer is between the substrate and the transparent conducting layer. One or more electrical contacts may be positioned between the transparent conducting layer and the back plane to define a conductive pathway, wherein the electrical contacts are formed through the transparent conducting layer, the absorber layer, the substrate, and the insulating layer. The electrical contacts may be electrically isolated from the absorber layer, the substrate, and the insulating layer. The coupling of the electrical contacts to the back plane allows the back plane to carry electric current. Since the back plane carries electric current from one device module to the next, the pattern of traces on the top side of the device need not contain thick busses, as used in the prior art for this purpose. Instead, the pattern of traces need only provide sufficiently conductive “fingers” to carry current to the electrical contacts. In the absence of busses, a greater portion of the absorber layer is exposed, which enhances efficiency. In addition, a pattern of traces without busses can be more aesthetically pleasing. 
     A further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional schematic diagram illustrating fabrication of an absorber layer according to an embodiment of the present invention. 
         FIG. 2  is a vertical cross-sectional schematic diagram of a portion of an array of optoelectronic devices according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
     Although the following detailed description contains many specific details for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the exemplary embodiments of the invention described below are set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. 
     Embodiments of the present invention allow fabrication of CIGS absorber layers on aluminum foil substrates. According to embodiments of the present invention, a nascent absorber layer containing elements of group IB and IIIA formed on an aluminum substrate by solution deposition may be annealed by rapid heating from an ambient temperature to a plateau temperature range of between about 200° C. and about 600° C. The temperature is maintained in the plateau range for between about 2 minutes and about 30 minutes, and subsequently reduced. Alternatively, the annealing temperature could be modulated to oscillate within a temperature range without being maintained at a particular plateau temperature. 
       FIG. 1  depicts a partially fabricated photovoltaic device  10 , and a rapid heating unit  20  the device generally includes an aluminum foil substrate  12 , an optional base electrode  14 , and a nascent absorber layer  16 . The aluminum foil substrate  12  may be approximately 5 microns to one hundred or more microns thick and of any suitable width and length. The aluminum foil substrate  12  may be made of aluminum or an aluminum-based alloy. Alternatively, the aluminum foil substrate  12  may be made by metallizing a polymer foil substrate, where the polymer is selected from the group of polyesters, polyethylene naphtalates, polyetherimides, polyethersulfones, polyetheretherketones, polyimides, and/or combinations of the above. By way of example, the substrate  12  may be in the form of a long sheet of aluminum foil suitable for processing in a roll-to-roll system. The base electrode  14  is made of an electrically conducive material compatible with processing of the nascent absorber layer  16 . By way of example, the base electrode  14  may be a layer of molybdenum, e.g., about 0.1 to 25 microns thick, and more preferably from about 0.1 to 5 microns thick. The base electrode layer may be deposited by sputtering or evaporation or, alternatively, by chemical vapor deposition (CVD), atomic layer deposition (ALD), sol-gel coating, electroplating and the like. The layer of material such as molybdenum or other material may be applied on one and/or both sides of the aluminum foil substrate. 
     Aluminum and molybdenum can and often do inter-diffuse into one another, with deleterious electronic and/or optoelectronic effects on the device  10 . To inhibit such inter-diffusion, an intermediate, interfacial layer  13  may be incorporated between the aluminum foil substrate  12  and molybdenum base electrode  14 . The interfacial layer may be composed of any of a variety of materials, including but not limited to chromium, vanadium, tungsten, and glass, or compounds such as nitrides (including tantalum nitride, tungsten nitride, titanium nitride, and silicon nitride), oxides, and/or carbides. Optionally, the materials selected for the diffusion barrier layer may be electrically conductive. The thickness of this layer can range from 10 nm to 50 nm, and more preferably from 10 nm to 30 nm. Optionally, the interfacial layer  13  may be between a base electrode  14  on the topside of the foil substrate and/or between a layer on the bottom side of the foil substrate. 
     The nascent absorber layer  16  may include material containing elements of groups IB, IIIA, and (optionally) VIA. Preferably, the absorber layer copper (Cu) is the group IB element, Gallium (Ga) and/or Indium (In) and/or Aluminum may be the group IIIA elements and Selenium (Se) and/or Sulfur (S) as group VIA elements. The group VIA element may be incorporated into the nascent absorber layer  16  when it is initially solution deposited or during subsequent processing to form a final absorber layer from the nascent absorber layer  16 . The nascent absorber layer  16  may be about 1000 nm thick when deposited. Subsequent rapid thermal processing and incorporation of group VIA elements may change the morphology of the resulting absorber layer such that it increases in thickness (e.g., to about twice as much as the nascent layer thickness under some circumstances). 
     Fabrication of the absorber layer on the aluminum foil substrate  12  is relatively straightforward. First, the nascent absorber layer is deposited on the substrate  12  either directly on the aluminum or on an uppermost layer such as the electrode  14 . By way of example, and without loss of generality, the nascent absorber layer may be deposited in the form of a film of a solution-based precursor material containing nanoparticles that include one or more elements of groups IB, IIIA and (optionally) VIA. Examples of such films of such solution-based printing techniques are described e.g., in commonly-assigned U.S. patent application Ser. No. 10/782,017, entitled “SOLUTION-BASED FABRICATION OF PHOTOVOLTAIC CELL” and also in PCT Publication WO 02/084708, entitled “METHOD OF FORMING SEMICONDUCTOR COMPOUND FILM FOR FABRICATION OF ELECTRONIC DEVICE AND FILM PRODUCED BY SAME” the disclosures of both of which are incorporated herein by reference. 
     Alternatively, the nascent absorber layer  16  may be formed by a sequence of atomic layer deposition reactions or any other conventional process normally used for forming such layers. Atomic layer deposition of IB-IIIA-VIA absorber layers is described, e.g., in commonly-assigned, co-pending application Ser. No. 10/943,685, entitled “FORMATION OF CIGS ABSORBER LAYER MATERIALS USING ATOMIC LAYER DEPOSITION AND HIGH THROUGHPUT SURFACE TREATMENT ON COILED FLEXIBLE SUBSTRATES”, which has been incorporated herein by reference above. 
     The nascent absorber layer  16  is then annealed by flash heating it and/or the substrate  12  from an ambient temperature to an average plateau temperature range of between about 200° C. and about 600° C. with the heating unit  20 . The heating unit  20  preferably provides sufficient heat to rapidly raise the temperature of the nascent absorber layer  16  and/or substrate  12  (or a significant portion thereof) e.g., at between about 5 C.°/sec and about 150 C.°/sec. By way of example, the heating unit  20  may include one or more infrared (IR) lamps that provide sufficient radiant heat. By way of example, 8 IR lamps rated at about 500 watts each situated about ⅛″ to about 1″ from the surface of the substrate  12  (4 above and 4 below the substrate, all aimed towards the substrate) can provide sufficient radiant heat to process a substrate area of about 25 cm 2  per hour in a 4″ tube furnace. The lamps may be ramped up in a controlled fashion, e.g., at an average ramp rate of about 10 C.°/sec. Those of skill in the art will be able to devise other types and configurations of heat sources that may be used as the heating unit  20 . For example, in a roll-to-roll manufacturing line, heating and other processing can be carried out by use of IR lamps spaced 1″ apart along the length of the processing region, with IR lamps equally positioned both above and below the substrate, and where both the IR lamps above and below the substrate are aimed towards the substrate. Alternatively, IR lamps could be placed either only above or only below the substrate  12 , and/or in configurations that augment lateral heating from the side of the chamber to the side of the substrate  12 . 
     The absorber layer  16  and/or substrate  12  are maintained in the average plateau temperature range for between about 2 minutes and about 30 minutes. For example, the temperature may be maintained in the desired range by reducing the amount of heat from the heating unit  20  to a suitable level. In the example of IR lamps, the heat may be reduced by simply turning off the lamps. Alternatively, the lamps may be actively cooled. The temperature of the absorber layer  16  and/or substrate  12  is subsequently reduced to a suitable level, e.g., by further reducing or shutting off the supply of heat from the heating unit  20 . 
     In some embodiments of the invention, group VIA elements such as selenium or sulfur may be incorporated into the absorber layer either before or during the annealing stage. Alternatively, two or more discrete or continuous annealing stages can be sequentially carried out, in which group VIA elements such as selenium or sulfur are incorporated in a second or latter stage. For example, the nascent absorber layer  16  may be exposed to H 2 Se gas, H 2 S gas or Se vapor before or during flash heating or rapid thermal processing (RTP). In this embodiment, the relative brevity of exposure allows the aluminum substrate to better withstand the presence of these gases and vapors, especially at high heat levels. 
     Once the nascent absorber layer  16  has been annealed additional layers may be formed to complete the device  10 . For example a window layer is typically used as a junction partner for the absorber layer. By way of example, the junction partner layer may include cadmium sulfide (CdS), zinc sulfide (ZnS), or zinc selenide (ZnSe) or some combination of two or more of these. Layers of these materials may be deposited, e.g., by chemical bath deposition, chemical surface deposition, or spray pyrolysis, to a thickness of about 50 nm to about 100 nm. In addition, a transparent electrode, e.g., a conductive oxide layer, may be formed on the window layer by sputtering, vapor deposition, CVD, ALD, electrochemical atomic layer epitaxy and the like. 
     Embodiments of the present invention overcome the disadvantages associated with the prior art by rapid thermal processing of nascent CIGS absorber layers deposited or otherwise formed on aluminum substrates. Aluminum substrates are much cheaper and more lightweight than conventional substrates. Thus, solar cells based on aluminum substrates can have a lower cost per watt for electricity generated and a far shorter energy payback period when compared to conventional silicon-based solar cells. Furthermore aluminum substrates allow for a flexible form factor that permits both high-throughput roll-to-roll printing during solar cell fabrication and faster and easier installation processes during solar module and system installation. 
     Embodiments of the present invention allow the fabrication of lightweight and inexpensive photovoltaic devices on aluminum substrates. Flash heating/rapid thermal processing of the nascent absorber layer  16  allows for proper annealing and incorporation of group VIA elements without damaging or destroying the aluminum foil substrate  12 . The plateau temperature range is sufficiently below the melting point of aluminum (about 660° C.) to avoid damaging or destroying the aluminum foil substrate. The use of aluminum foil substrates can greatly reduce the materials cost of photovoltaic devices, e.g., solar cells, made on such substrates thereby reducing the cost per watt. Economies of scale may be achieved by processing the aluminum foil substrate in a roll-to-roll fashion, with the various layers of the photovoltaic devices being built up on the substrate as it passes through a series of deposition annealing and other processing stages. 
     Although CIGS solar cells are described for the purposes of example, those of skill in the art will recognize that embodiments of the present invention can be applied to almost any type of solar cell architecture. For example, the layer  16  may be an absorber layer comprised of organic oligomers or polymers (for organic solar cells), bi-layers or interpenetrating layers or inorganic and organic materials (for hybrid organic/inorganic solar cells), dye-sensitized titania nanoparticles in a liquid or gel-based electrolyte (for Graetzel cells in which an optically transparent film comprised of titanium dioxide particles a few nanometers in size is coated with a monolayer of charge transfer dye to sensitize the film for light harvesting), copper-indium-gallium-selenium (for CIGS solar cells), CdSe, CdTe, and/or combinations of the above, where the active materials are present in any of several forms including but not limited to bulk materials, micro-particles, nano-particles, or quantum dots. Additionally, other possible absorber layers may be based on amorphous silicon (doped or undoped), a nanostructured layer having an inorganic porous semiconductor template with pores filled by an organic semiconductor material (see e.g., US Patent Application Publication US 2005-0121068 A1, which is incorporated herein by reference), a polymer/blend cell architecture, organic dyes, and/or C 60  molecules, and/or other small molecules, micro-crystalline silicon cell architecture, randomly placed nanorods and/or tetrapods of inorganic materials dispersed in an organic matrix, quantum dot-based cells, or combinations of the above. Many of these types of cells can be fabricated on flexible substrates. 
     It should be understood that the P-type layer may be either organic or inorganic. Alternatively, the N-type layer may be either organic or inorganic. The possible combinations may result in an inorganic P-type layer with an inorganic N-type layer, an inorganic P-type layer with an organic N-type layer, an organic P-type layer with an inorganic N-type layer, or an organic P-type layer with and organic N-type layer. 
     By way of nonlimiting example, suitable inorganic materials for the P-type and/or N-type layer include metal oxides such as titania (TiO 2 ), zinc oxide (ZnO), copper oxide (CuO or Cu 2 O or Cu x O y ), zirconium oxide, lanthanum oxide, niobium oxide, tin oxide, indium oxide, indium tin oxide (ITO), vanadium oxide, molybdenum oxide, tungsten oxide, strontium oxide, calcium/titanium oxide and other oxides, sodium titanate, potassium niobate, cadmium selenide (CdSe), cadmium sulfide (CdS), copper sulfide (e.g., Cu 2 S), cadmium telluride (CdTe), cadmium-tellurium selenide (CdTeSe), copper-indium selenide (CuInSe 2 ), cadmium oxide (CdO x ) i.e. generally semiconductive materials, as well as blends or alloys of two or more such materials. 
     By way of nonlimiting example, suitable organic materials for the P-type and/or N-type layer include conjugated polymers such as poly(phenylene) and derivatives thereof, poly(phenylene vinylene) and derivatives thereof (e.g., poly(2-methoxy-5-(2-ethyl -hexyloxy)-1,4-phenylene vinylene (MEH-PPV), poly(para-phenylene vinylene), (PPV)), PPV copolymers, poly(thiophene) and derivatives thereof (e.g., poly(3-octylthiophene-2,5,-diyl), regioregular, poly(3-octylthiophene-2,5,-diyl), regiorandom, Poly(3-hexylthiophene-2,5-diyl), regioregular, poly(3-hexylthiophene-2,5-diyl), regiorandom), poly(thienylenevinylene) and derivatives thereof, and poly(isothianaphthene) and derivatives thereof. Other suitable polymers include organometallic polymers, polymers containing perylene units, poly(squaraines) and their derivatives, and discotic liquid crystals. Other suitable organic materials include organic pigments or dyes, azo-dyes having azo chromofores (—N═N—) linking aromatic groups, phthalocyanines including metal-free phthalocyanine; (HPc), perylenes, perylene derivatives, Copper pthalocyanines (CuPc), Zinc Pthalocyanines (ZnPc), naphthalocyanines, squaraines, merocyanines and their respective derivatives, poly(silanes), poly(germinates), 2,9-Di(pent-3-yl)-anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone, and 2,9-Bis-(1-hexyl-hept-1-yl)-anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone and pentacene, pentacene derivatives and/or pentacene precursors, an N-type ladder polymer such as poly(benzimidazobenzophenanthroline ladder) (BBL), or any combination of the above. 
     One organic solar cell suitable for use with the present invention is a Graetzel cell which is comprised of a dye-sensitized, nanocrystalline TiO 2  based solar cell using a liquid electrolyte (O&#39;Regan et al. “A Low-cost, High-efficiency solar cell based on dye-sensitized colloidal TiO2 Films”, Nature, Vol. 353 pp 737-740, 24 Oct., 1991, fully incorporated herein for all purposes). Dye-sensitized solar cells (DSSC) disaggregate the processes of light absorption and charge separation. A monolayer of a Ruthenium-based dye that is chemically adsorbed onto a semiconductor surface absorbs light. After having been excited by a photon, the dye injects an electron into a nano-particulate past composed of a semiconductor, TiO 2  (titania), upon which the electric field inside the titania allows extraction of the electron. In a parallel process, positive charge is transferred from the dye to a liquid iodide/triiodide-based redox mediator, which allows hole transfer through the solution to a counter electrode, after which the redox mediator is returned to its reduced state, closing the circuit. 
     In alternative embodiments, the liquid iodide/triiodide electrolyte may be replaced with a solid-state material that exhibits sufficient hole transport for efficient device function. These approaches include: (1) Application of ionic conducting polymer gels (Wang et al., 2003), (2) sol-gel nanocomposites containing an ionic liquid (Stathatos et al., 2003), (3) ionic conductors (Cao et al., 1995), (4) inorganic p-type semiconductors, such as CuI or CuSCN (Tennakone et al., 1995; O&#39;Regan and Schwartz, 1998; O&#39;Regan et al., 2003), and (5) low molecular weight organic hole conductors such as 2,2′7,7′ tetrakis (N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene(spiro-Me OTAD) (Krüger et al., 2001). 
     High Efficiency Cell Configuration 
     It should be understood that the device manufactured as shown in  FIG. 1  and the above paragraphs may be suitable for use in a high efficiency cell configuration as detailed below in  FIG. 2 .  FIG. 2  illustrates an array  100  of optoelectronic devices according to an embodiment of the present invention. In some embodiments, this may be considered a series interconnection in an array  100  of optoelectronic devices. The array  100  includes a first device module  101  and a second device module  111 . The device modules  101 ,  111  may be photovoltaic devices, such as solar cells, or light-emitting devices, such as light-emitting diodes. In a preferred embodiment, the device modules  101 ,  111  are solar cells. The first and second device modules  101 ,  111  are attached to an insulating carrier substrate  103 , which may be made of a plastic material such as polyethylene terephthalate (PET), e.g., about 50 microns thick. The carrier substrate  103  may, in turn, be attached to a thicker structural membrane  105 , e.g., made of a polymeric roofing membrane material such as thermoplastic polyolefin (TPO) or ethylene propylene diene monomer (EPDM), to facilitate installing the array  100  on an outdoor location such as a roof. 
     By way of nonlimiting example, the device modules  101 ,  111 , which may be about 4 inches in length and 12 inches wide, may be cut from a much longer sheet containing several layers that are laminated together. Each device module  101 ,  111  generally includes a device layer  102 ,  112  in contact with a bottom electrode  104 ,  114  and an insulating layer  106 ,  116  between the bottom electrode  104 ,  114  and a conductive back plane  108 ,  118 . It should be understood that in some embodiments of the present invention, the back plane  108 ,  118  may be described as a backside top electrode  108 ,  118 . The bottom electrodes  104 ,  114 , insulating layers  106 ,  116  and back planes  108 ,  118  for substrates S 1 , S 2  support the device layers  102 ,  112   
     In contrast to prior art cells, where the substrates are formed by depositing thin metal layers on an insulating substrate, embodiments of the present invention utilize substrates S 1 , S 2  based on flexible bulk conducting materials, such as foils. Although bulk materials such as foils are thicker than prior art vacuum deposited metal layers they can also be cheaper, more readily available and easier to work with. Preferably, at least the bottom electrode  104 ,  114  is made of a metal foil, such as aluminum foil. Alternatively, copper, stainless steel, titanium, molybdenum or other suitable metal foils may be used. By way of example, the bottom electrodes  104 ,  114  and back planes  108 ,  118  may be made of aluminum foil about 1 micron to about 200 microns thick, preferably about 25 microns to about 100 microns thick; the insulating layers  106 ,  116  may be made of a plastic foil material, such as polyethylene terephthalate (PET) about 1 micron to about 200 microns thick, preferably about 10 microns to about 50 microns thick. In one embodiment, among others, the bottom electrode  104 , 114 , insulating layer  106 ,  116  and back plane  108 ,  118  are laminated together to form the starting substrates S 1 , S 2 . Although foils may be used for both the bottom electrode  104 ,  114  and the back plane  108 ,  118  it is also possible to use a mesh grid on the back of the insulating layer  106 ,  116  as a back plane. Such a grid may be printed onto the back of the insulating layer  106 ,  116  using a conductive ink or paint. One example, among others, of a suitable conductive paint or ink is Dow Corning® PI-2000 Highly Conductive Silver Ink available from Dow Corning Corporation of Midland Mich. Dow Corning® is a registered trademark of Dow Corning Corporation of Midland Mich. Furthermore, the insulating layer  106 ,  116  may be formed by anodizing a surface of a foil used for the bottom electrode  104 ,  114  or back plane  108 ,  118  or both, or by applying an insulating coating by spraying, coating, or printing techniques known in the art. 
     As seen in  FIG. 2 , the device layers  102 ,  112  generally include an active layer or absorber layer  107  disposed between a transparent conductive layer  109  and the bottom electrode  104 . By way of nonlimiting example, the device layers  102 ,  112  may be about 2 microns thick. At least the first device  101  includes one or more electrical contacts  120  between the transparent conducting layer  109  and the back plane  108 . The electrical contacts  120  are formed through the transparent conducting layer  109 , the active layer  107 , the bottom electrode  104  and the insulating layer  106 . The electrical contacts  120  provide an electrically conductive path between the transparent conducting layer  109  and the back plane  108 . The electrical contacts  120  are electrically isolated from the active layer  107 , the bottom electrode  104  and the insulating layer  106 . 
     To make the electrical interconnection from the transparent conducting layer  109  to the back plane  108 , the electrical contacts  120  may be formed in vias formed through the active layer  107 , the transparent conducting layer  109 , the bottom electrode  104  and the insulating layer  106 . By way of nonlimiting example, each via may be about 0.1 millimeters to about 1.5 millimeters, preferably 0.5 millimeters to about 1 millimeter in diameter. Although not limited to the following, the vias may be formed by punching or by drilling, for example by mechanical, laser or electron beam drilling, or by a combination of these techniques. An insulating material  122  coats sidewalls of the via such that a channel is formed through the insulating material  122  to the back plane  108 . The insulating material  122  may have a thickness between about 1 micron and about 200 microns, preferably between about 10 microns and about 200 microns. Optionally, the insulating material  122  along the side walls of the via may be at least 10 microns thick to ensure complete coverage of the exposed conductive surfaces behind it. It should be understood that, although not limited to the following, the insulating material  122  may be formed by a variety of printing techniques, including for example inkjet printing or dispensing through an annular nozzle. 
     In one embodiment of the present invention, the electrical contact  120  may be comprised of a plug  124  made of an electrically conductive material at least partially fills the channel and makes electrical contact between the transparent conducting layer  109  and the back plane  108 . Although not limited to the following, the electrically conductive material may similarly be printed. A suitable material and method, for example, is inkjet printing of solder (called “solderjet” by Microfab, Inc., Plano, Tex., which sells equipment useful for this purpose). Printing of conductive adhesive materials known in the art for electronics packaging may also be used, provided time is allowed subsequently for removal of solvent which may or may not be present, and curing. The plug  124  may have a diameter between about 5 microns and about 500 microns, preferably between about 25 and about 100 microns. 
     By way of nonlimiting example, in other embodiments, the device layers  102 ,  112  may be about 2 microns thick, the bottom electrodes  104 ,  114  may be made of aluminum foil about 100 microns thick; the insulating layers  106 ,  116  may be made of a plastic material, such as polyethylene terephthalate (PET) about 25 microns thick; and the backside top electrodes  108 ,  118  may be made of aluminum foil about 25 microns thick. The device layers  102 ,  112  may include an active layer  107  disposed between a transparent conductive layer  109  and the bottom electrode  104 . In such an embodiment, at least the first device  101  includes one or more electrical contacts  120  between the transparent conducting layer  109  and the backside top electrode  108 . The electrical contacts  120  are formed through the transparent conducting layer  109 , the active layer  107 , the bottom electrode  104  and the insulating layer  106 . The electrical contacts  120  provide an electrically conductive path between the transparent conducting layer  109  and the backside top electrode  108 . The electrical contacts  120  are electrically isolated from the active layer  107 , the bottom electrode  104  and the insulating layer  106 . 
     The formation of good contacts between the conductive plug  124  and the substrate  108  may be assisted by the use of other interface-forming techniques such as ultrasonic welding. An example of a useful technique is the formation of gold stud-bumps, as described for example by J. Jay Wimer in “3-D Chip Scale with Lead-Free Processes” in Semiconductor International, Oct. 1, 2003, which is incorporated herein by reference. Ordinary solders or conductive inks or adhesives may be printed on top of the stud bump. 
     In forming the vias, it is desirable to avoid making shorting connections between the top electrode  109  and the bottom electrode  104 . Therefore, mechanical cutting techniques such as drilling or punching may be advantageously supplemented by laser ablative removal of a small volume of material near the lip of the via, a few microns deep and a few microns wide. Alternatively, a chemical etching process may be used to remove the transparent conductor over a diameter slightly greater than the via. The etching can be localized, e.g., by printing drops of etchant in the appropriate places using inkjet printing or stencil printing. 
     A further method for avoiding shorts involves deposition of a thin layer of insulating material on top of the active layer  107  prior to deposition of the transparent conducting layer  109 . This insulating layer is preferably several microns thick, and may be in the range of 1 to 100 microns. Since it is deposited only over the area where a via is to be formed (and slightly beyond the borders of the via), its presence does not interfere with the operation of the optoelectronic device. In some embodiments of the present invention, the layer may be similar to structures described in U.S. patent application Ser. No. 10/810,072 to Karl Pichler, filed Mar. 25, 2004, which is hereby incorporated by reference. When a hole is drilled or punched through this structure, there is a layer of insulator between the transparent conducting layer  109  and the bottom electrode  104  which may be relatively thick compared to these layers and to the precision of mechanical cutting processes, so that no short can occur. 
     The material for this insulating layer can be any convenient insulator, preferably one that can be digitally (e.g. inkjet) printed. Thermoplastic polymers such as Nylon PA6 (melting point (m.p.) 223° C.), acetal (m.p. 165° C.), PBT (structurally similar to PET but with a butyl group replacing the ethyl group) (m.p. 217° C.), and polypropylene (m.p. 165° C.), are examples which by no means exhaust the list of useful materials. These materials may also be used for the insulating layer  122 . While inkjet printing is a desirable way to form the insulator islands, other methods of printing or deposition (including conventional photolithography) are also within the scope of the invention. 
     In forming the vias, it is useful to fabricate the optoelectronic device in at least two initially separate elements, with one comprised of the insulating layer  106 , the bottom electrode  104  and the layers  102  above it, and the second comprised of the back plane  108 . These two elements are then laminated together after the vias have been formed through the composite structure  106 / 104 / 102 , but before the vias are filled. After this lamination and via formation, the back plane  108  is laminated to the composite, and the vias are filled as described above. 
     Although jet-printed solders or conductive adhesives comprise useful materials for forming the conductive via plug  124 , it is also possible to form this plug by mechanical means. Thus, for example, a wire of suitable diameter may be placed in the via, forced into contact with the back plane  108 , and cut off at the desired height to form the plug  124 , in a manner analogous to the formation of gold stud bumps. Alternatively a pre-formed pin of this size can be placed into the hole by a robotic arm. Such pins or wires can be held in place, and their electrical connection to the substrate assisted or assured, by the printing of a very thin layer of conductive adhesive prior to placement of the pin. In this way the problem of long drying time for a thick plug of conductive adhesive is eliminated. The pin can have tips or serrations on it which punch slightly into the back plane  108 , further assisting contact. Such pins may be provided with insulation already present, as in the case of insulated wire or coated wire (e.g. by vapor deposition or oxidation). They can be placed in the via before the application of the insulating material, making it easier to introduce this material. 
     Optionally, if the pin is made of a suitably hard metal, and has a slightly tapered tip, it may be used to form the via during the punching step. Instead of using a punch or drill, the pin is inserted into the composite  106 / 104 / 102 , to a depth such that the tip just penetrates the bottom; then when the substrate  108  is laminated to this composite, the tip penetrates slightly into it and forms a good contact. These pins may be injected into the unpunched substrate by, for example, mechanical pressure or air pressure directed through a tube into which the pin just fits. 
     Fabricating the device modules  101 ,  111  on substrates S 1 , S 2  made of relatively thick, highly conductive, flexible bulk conductor bottom electrodes  104 ,  114  and backplanes  108 ,  118  and forming insulated electrical contacts  120  through the transparent conducting layer  109 , the active layer  130 , the bottom electrodes  104 ,  114  and the insulating layer  106 ,  116  allows the device modules  101 ,  111  to be relatively large. Consequently the array  100  can be made of fewer device modules requiring fewer series interconnections compared to prior art arrays. For example, the device modules  101 ,  111  may be between about 1 centimeter and about 30 centimeters long and between about 1 and about 30 centimeters wide. Smaller cells (e.g., less than 1 centimeter long and/or 1 centimeter wide) may also be made as desired. 
     Note that since the back planes  108 ,  118  carry electric current from one device module to the next, the pattern of traces  126  need not contain thick busses, as used in the prior art for this purpose. Instead, the pattern of traces  126  need only provide sufficiently conductive “fingers” to carry current to the contacts  120 . In the absence of busses, a greater portion of the active layers  102 ,  112  is exposed, which enhances efficiency. In addition, a pattern of traces  126  without busses can be more aesthetically pleasing. This type of configuration where the back planes  108 ,  118  carry electric current may be termed an emitter-wrap through configuration. 
     Electrical contact between the back plane  108  of the first device module  101  and the bottom electrode  114  of the second device module  111  may be implemented by cutting back the back plane  118  and insulating layer  116  of the second device module to expose a portion of the bottom electrode  114 . 
     Electrical contact may be made between the back plane  108  of the first device module  101  and the exposed portion of the bottom electrode  114  of the second device module  111  in a number of different ways. For example, as shown in  FIG. 2A , thin conducting layer  128  may be disposed over a portion of the carrier substrate  103  in a pattern that aligns with the notches  117 ,  119 . 
     The thin conducting layer may be, e.g., a conductive (filled) polymer or silver ink. The conducting layer can be extremely thin, e.g., about 1 micron thick. A general criteria for determining the minimum thickness of the thin conducting layer  128  is that the fractional power p=(J/V) ρ (L o   2 /d) dissipated in this layer is about 10 −4  or less, where J is the current density, V is the voltage, L o  is the length of the thin conductive layer  128  (roughly the width of the gap between the first and second device modules) and p and d are respectively the resistivity and the thickness of the thin conductive layer  128 . In that case the loss of power from this source is far less than 1% of the power being generated, and is negligible. By way of numerical example, for many applications (J/V) is roughly 0.06 A/Vcm 2 . If L o =400 microns=0.04 cm then p is approximately equal to 10 −4  (ρ/d). Thus, even if the resistivity ρ is about 10 −5  Ωcm (which is about ten times less than for a good bulk conductor), the criterion can be satisfied with d less than about 1 micron (10 −4  cm) thick. Thus, even a relatively resistive polymer conductor of almost any plausible printable thickness will work. 
     The first device module  101  may be attached to the carrier substrate  103  such that the back plane  108  makes electrical contact with the thin conducting layer  128  while leaving a portion of the thin conducting layer  128  exposed. Electrical contact may then be made between the exposed portion of the thin conducting layer  128  and the exposed portion of the bottom electrode  114  of the second device module  111 . For example, a bump of conductive material  129  (e.g., more conductive adhesive) may be placed on the thin conducting layer  128  at a location aligned with the exposed portion of the bottom electrode  114 . The bump of conductive material  129  is sufficiently tall as to make contact with the exposed portion of the bottom electrode  114  when the second device module  111  is attached to the carrier substrate. The dimensions of the notches  117 ,  119  may be chosen so that there is essentially no possibility that the thin conducting layer  128  will make undesired contact with the back plane  118  of the second device module  111 . For example, the edge of the bottom electrode  114  may be cut back with respect to the insulating layer  116  by an amount of cutback CB 1  of about 400 microns. The back plane  118  may be cut back with respect to the insulating layer  116  by an amount CB 2  that is significantly larger than CB 1 . 
     The device layers  102 ,  112  are preferably of a type that can be manufactured on a large scale, e.g., in a roll-to-roll processing system. There are a large number of different types of device architectures that may be used in the device layers  102 ,  112 . By way of example, and without loss of generality, the inset in  FIG. 2  shows the structure of a CIGS active layer  107  and associated layers in the device layer  102 . By way of example, the active layer  107  may include an absorber layer  130  based on materials containing elements of groups IB, IIIA and VIA. Preferably, the absorber layer  130  includes copper (Cu) as the group IB, Gallium (Ga) and/or Indium (In) and/or Aluminum as group IIIA elements and Selenium (Se) and/or Sulfur (S) as group VIA elements. Examples of such materials (sometimes referred to as CIGS materials) are described in U.S. Pat. No. 6,268,014, issued to Eberspacher et al on Jul. 31, 2001, and US Patent Application Publication No. US 2004-0219730 A1 to Bulent Basol, published Nov. 4, 2004, both of which are incorporated herein by reference. A window layer  132  is typically used as a junction partner between the absorber layer  130  and the transparent conducting layer  109 . By way of example, the window layer  132  may include cadmium sulfide (CdS), zinc sulfide (ZnS), or zinc selenide (ZnSe) or some combination of two or more of these. Layers of these materials may be deposited, e.g., by chemical bath deposition or chemical surface deposition, to a thickness of about 50 nm to about 100 nm. 
     As seen in the inset of  FIG. 2 , a contact layer  134  of a metal different from the bottom electrode may be disposed between the bottom electrode  104  and the absorber layer  130  to inhibit diffusion of metal from the bottom electrode  104 . For example, if the bottom electrode  104  is made of aluminum, the contact layer  134  may be a layer of molybdenum. The contact layer  134  may be an electrically conductive diffusion barrier layer. Optionally, the aluminum foil substrate  104  may be configured in a manner similar to that of  FIG. 1 , wherein at least one electrically conductive diffusion barrier layer  13  and an electrode layer  14  may be formed over the foil substrate  104 . 
     Although CIGS solar cells are described for the purposes of example, those of skill in the art will recognize that embodiments of the series interconnection technique can be applied to almost any type of solar cell architecture. Examples of such solar cells include, but are not limited to: cells based on amorphous silicon, Graetzel cell architecture (in which an optically transparent film comprised of titanium dioxide particles a few nanometers in size is coated with a monolayer of charge transfer dye to sensitize the film for light harvesting), a nanostructured layer having an inorganic porous semiconductor template with pores filled by an organic semiconductor material (see e.g., US Patent Application Publication US 2005-0121068 A1, which is incorporated herein by reference), a polymer/blend cell architecture, organic dyes, and/or C 60  molecules, and/or other small molecules, micro-crystalline silicon cell architecture, randomly placed nanorods and/or tetrapods of inorganic materials dispersed in an organic matrix, quantum dot-based cells, or combinations of the above. Furthermore, embodiments of the series interconnection technique described herein can be used with optoelectronic devices other than solar cells. 
     Alternatively, the optoelectronic devices  101 ,  111  may be light emitting devices, such as organic light emitting diodes (OLEDs). Examples of OLEDs include light-emitting polymer (LEP) based devices. In such a case, the active layer  107  may include a layer of poly(3,4) ethylendioxythiophene:polystyrene sulfonate (PEDOT:PSS), which may be deposited to a thickness of typically between 50 and 200 nm on the bottom electrodes  104 ,  114 , e.g., by web coating or the like, and baked to remove water. PEDOT:PSS is available from Bayer Corporation of Leverkusen, Germany. A polyfluorene based LEP may then be deposited on the PEDOT:PSS layer (e.g., by web coating) to a thickness of about 60-70 nm. Suitable polyfluorene-based LEPs are available from Dow Chemicals Company. 
     The transparent conductive layer  109  may be, e.g., a transparent conductive oxide (TCO) such as zinc oxide (ZnO) or aluminum doped zinc oxide (ZnO:Al), which can be deposited using any of a variety of means including but not limited to sputtering, evaporation, CBD, electroplating, CVD, PVD, ALD, and the like. Alternatively, the transparent conductive layer  109  may include a transparent conductive polymeric layer, e.g. a transparent layer of doped PEDOT (Poly-3,4-Ethylenedioxythiophene), which can be deposited using spin, dip, or spray coating, and the like. PSS:PEDOT is a doped, conducting polymer based on a heterocyclic thiophene ring bridged by a diether. A water dispersion of PEDOT doped with poly(styrenesulfonate) (PSS) is available from H.C. Starck of Newton, Massachussetts under the trade name of Baytron® P. Baytron® is a registered trademark of Bayer Aktiengesellschaft (hereinafter Bayer) of Leverkusen, Germany In addition to its conductive properties, PSS:PEDOT can be used as a planarizing layer, which can improve device performance. A potential disadvantage in the use of PEDOT is the acidic character of typical coatings, which may serve as a source through which the PEDOT may chemically attack, react with, or otherwise degrade the other materials in the solar cell. Removal of acidic components in PEDOT may be carried out by anion exchange procedures. Non-acidic PEDOT can be purchased commercially. Alternatively, similar materials can be purchased from TDA materials of Wheat Ridge, Colo., e.g. Oligotron™ and Aedotron™. 
     The gap between the first device module  101  and the second device module  111  may be filled with a curable polymer, e.g. epoxy or silicone. An optional encapsulant layer (not shown) may cover the array  100  to provide environmental resistance, e.g., protection against exposure to water or air. The encapsulant may also absorb UV-light to protect the underlying layers. Examples of suitable encapsulant materials include one or more layers of fluoropolymers such as THV (e.g. Dyneon&#39;s THV220 fluorinated terpolymer, a fluorothermoplastic polymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride), Tefzel® (DuPont), Tefdel, ethylene vinyl acetate (EVA), thermoplastics, polyimides, polyamides, nanolaminate composites of plastics and glasses (e.g. barrier films such as those described in commonly-assigned, co-pending U.S. Patent Application Publication US 2005-0095422 A1, to Brian Sager and Martin Roscheisen, entitled “INORGANIC/ORGANIC HYBRID NANOLAMINATE BARRIER FILM” which is incorporated herein by reference), and combinations of the above. 
     Additionally, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a thickness range of about 1 nm to about 200 nm should be interpreted to include not only the explicitly recited limits of about 1 nm and about 200 nm, but also to include individual sizes such as but not limited to 2 nm, 3 nm, 4 nm, and sub-ranges such as 10 nm to 50 nm, 20 nm to 100 nm, etc. . . . 
     The publications discussed or cited herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed. All publications mentioned herein are incorporated herein by reference to disclose and describe the structures and/or methods in connection with which the publications are cited. 
     While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.”