WO2011044382A1

WO2011044382A1 - Porous substrates for fabrication of thin film solar cells

Info

Publication number: WO2011044382A1
Application number: PCT/US2010/051854
Authority: WO
Inventors: Kurt H. Weiner; Gaurav Verma; Doron Gal; Scott Andrews
Original assignee: Reel Solar Incorporated
Priority date: 2009-10-07
Filing date: 2010-10-07
Publication date: 2011-04-14

Abstract

Back contacts are formed using substrate fabrication protocols. Absorber layers deposited on porous substrates are exposed to chemical treatment via the porous substrates for modification of a region of the absorber layer proximate the porous substrate to form back (ohmic) contacts between the absorber layer and the porous substrates. Methods for fabrication of photovoltaic modules using continuous porous substrates is described.

Description

POROUS SUBSTRATES FOR FABRICATION OF THIN FILM

SOLAR CELLS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Application, Serial No. 61/249,490, filed October 7, 2009, and U.S. Provisional Application, Serial No. 61/249,932, filed October 8, 2009, the contents of both of which are incorporated herein by reference in their entirety and for all purposes.

FIELD OF INVENTION [0002] The invention relates generally to methods of solar cell fabrication. BACKGROUND

[0003] Solar or photovoltaic cells are devices that convert photons into electricity by the photovoltaic effect. Solar cells are assembled together to make solar panels, solar modules, or photovoltaic arrays. Thin film solar cells are stacked structures, having layers of materials, including photovoltaic materials, typically fabricated as a stack on a superstrate or substrate. The photovoltaic stack may ultimately be sandwiched between a superstrate and a substrate.

[0004] Manufacturing of thin film photovoltaic (PV) devices promises coal- competitive levelized cost of electricity due to the efficient use of photovoltaic materials, process simplicity and high conversion efficiencies that can be recognized using the technology.

SUMMARY

[0005] Back contacts are formed using substrate fabrication protocols. Absorber layers deposited on porous substrates are exposed to chemical treatment via the porous substrates for modification of a region of the absorber layer proximate the porous substrate to form back (ohmic) contacts between the absorber layer and the porous substrates.

[0006] One embodiment is a method of forming a solar cell, including: (a) providing a porous conductive substrate; (b) forming an absorber layer over the porous conductive substrate; (c) exposing the absorber layer to a chemical species via the porous conductive substrate so as to selectively modify a physical and/or chemical property of a region of the absorber layer proximate the porous conductive substrate; and (d) forming an ohmic contact at the interface of the absorber layer and the porous conductive substrate. In one embodiment, the absorber layer is or includes CdTe, and (c) and (d) include, respectively: i) applying a bromine solution to the porous conductive substrate thereby transforming the region into a Te rich region; ii) applying a copper containing solution to the Te rich region via the porous conductive substrate and annealing the stack to form a copper tellurium complex at the interface of the Te rich region and the porous conductive substrate. Porous graphite substrates find particular use in methods of the invention. Photovoltaics formed using methods of the invention are another embodiment.

[0007] Various embodiments include using graphite foils as substrates to fabricate solar cells. In particular methods, continuous graphite foils are used to make photovoltaics in line type manufacturing. One embodiment is a method of forming a plurality of photovoltaic cells, including: (a) forming an absorber layer over a graphite foil, where the graphite foil is substantially continuous and flexible; (b) forming a window layer over the absorber layer; (c) forming a back contact layer at an interface between the graphite foil and the absorber layer; (d) forming a front contact layer over the window layer; and, (e) isolating the plurality of photovoltaic cells from the photovoltaic device thus formed; where at least one of the absorber layer, the window layer, the back contact layer, and the front contact layer is formed by moving the graphite foil through a line type manufacturing system. In certain embodiments, at least one of the absorber layer, the window layer, the back contact layer, and the front contact layer is formed by electrodeposition while moving the graphite foil through an electrodeposition system.

[0008] Particular aspects of the invention are described in more detail below. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a cross-sectional depiction of a solar cell photovoltaic stack structure.

[0010] Figure 2 is a cross-sectional depiction of conventional superstate photovoltaic stack formation.

[0011] Figure 3 is a cross-sectional depiction of a substrate solar cell photovoltaic stack formation.

[0012] Figure 4 is a cross-sectional depiction of a solar cell fabrication scheme in accord with embodiments described herein. [0013] Figure 5 is a process flow describing aspects of a method.

[0014] Figure 6 is a simplified cross-sectional illustration of a conventional electrodeposition apparatus that employs a continuous substrate.

[0015] Figures 7A and 7B are simplified cross-sectional illustrations, side and top view, respectively, of an improved electrodeposition apparatus that employs a continuous substrate.

DETAILED DESCRIPTION

A. Solar Cell Structure

[0016] Figure 1 depicts a simplified diagrammatic cross-sectional view of a typical thin film solar cell, 100. As illustrated, thin film solar cells typically include the following components: back encapsulation, 105, substrate, 110, a back contact layer, 115, an absorber layer, 120, a window layer, 125, a front (or top) contact layer, 130, and front (or top) encapsulation layer, 135.

[0017] Back encapsulation can generally serve to provide encapsulation for the cell and provide mechanical support. Back encapsulation can be made of many different materials that provide sufficient sealing, moisture protection, adequate mechanical support, ease of fabrication, handling and the like. In many thin film solar cell implementations, back encapsulation is formed from glass although other suitable materials may be used.

[0018] A substrate layer can also be used to provide mechanical support for the fabrication of the solar cell. The substrate can also provide electrical connectivity. In many thin film solar cells, the substrate and back encapsulation are the same. Glass plate is commonly used in such instances, although opaque substrates are also commonly used since light need not pass through this side of the photovoltaic stack.

[0019] A back contact layer can be formed from a thin film of material that provides one of the contacts to the solar cell. Typically, the material for the back contact layer is chosen such that the contact resistance for the electrons/holes flowing from/to the absorber layer is minimized. This result can be achieved by fabricating an ohmic or a tunneling back contact layer. This back contact layer can be formed from many different materials depending on the type of thin film solar cell. For example, in copper indium gallium diselenide (CIGS) solar cells, this layer can be molybdenum. In cadmium telluride (CdTe) thin film solar cells, this back contact layer can be made, for example, of nickel or copper or graphite. These materials are merely illustrative examples. That is, the material composition of the back contact layer is dependent on the type of absorber material used in the cell. The thickness of a back contact layer film is typically in the range of a few microns. [0020] The absorber layer is a thin film material that generally absorbs the incident photons (indicated in Figure 1 by the squiggly lines) and converts the photons to electrons. This absorber material is typically semiconducting and can be a p-type or an n-type semiconductor. An absorber layer can be formed from, for example, CIGS, CdTe or amorphous silicon. The thickness of the absorber layer depends on the semiconducting material, and is typically of the order of microns, varying from a few microns to tens of microns.

[0021] A window layer is also typically a thin film of semiconducting material that creates a p-n junction with the absorber layers and, in addition, allows the maximum number of photons in the energy regime of interest to pass through to the absorber layer. The window layer can be an n or p-type semiconductor, depending on the material used for the absorber layer. For example, the window layer can be formed from a cadmium sulphide (CdS) n-type semiconductor for CdTe and CIGS thin film solar cells. The typical thickness of this layer is of the order of hundreds to thousands of angstroms. [0022] A top contact is typically a thin film of material that provides one of the contacts to the solar cell. The top contact is made of a material that is transparent to the photons in the energy regime of interest for the solar cell. This top contact layer is typically a transparent conducting oxide (TCO). For CdTe, CIGS, and amorphous silicon thin film solar cells, the top contact can be formed from, for example, indium tin oxide (ITO), aluminum doped zinc oxide (ZnO) or flourine doped tin oxide (Sn0₂). The top contact layer thickness can be of the order of thousands of angstroms.

[0023] A top encapsulation layer can be used to provide environmental protection and mechanical support to the cell. The top encapsulation is formed from a material that is highly transparent in the photon energy regime of interest. This top encapsulation layer can be formed from, for example, glass. In this respect, a glass encapsulation serves also as a superstate for structural support.

[0024] Thin film solar cells are typically connected in series, in parallel, or both, depending on the needs of the end user, to fabricate a solar module or panel. The solar cells are connected to achieve the desired voltage and current characteristics for the panel. The number of cells connected together to fabricate the panel depends on the open circuit voltage, short circuit current of the cells, and on the desired voltage and current output of the panel. The interconnect scheme can be implemented, for example, by laser scribing for isolation and interconnection during the process of the cell fabrication. Once these panels are made, additional components such as bi-pass diodes, rectifiers, connectors, cables, support structures etc. are attached to the panels to install them in the field to generate electricity. The installations can be, for example, in households, large commercial building installations, large utility scale solar electricity generation farms and in space, for example, to power satellites and space craft. B. Superstrate Solar Cell Fabrication

[0025] Solar cell photovoltaic stacks are conventionally constructed in an order starting from, for example, a top encapsulation layer, a top contact layer, a window layer, an absorber layer, a back contact layer and so on, that is, in an order opposite of the description of the layers with reference to Figure 1. The top encapsulation layer is a superstrate upon which the photovoltaic stack is built. For example the top encapsulation layer can be glass, a superstrate upon which subsequent layers are formed.

[0026] Figure 2 shows a diagrammatic illustration of conventional photovoltaic stack formation, using a superstrate fabrication protocol. This example is described in terms of CdS-based window layers and CdTe-based absorber layers, but solar cells can have a wide variety of window and absorber layer materials so long as they are compatible with one another. The process starts with the top encapsulation layer, and the cell stack is built by subsequent depositions of top contact layer, window layer, absorber layer, etc. Other layers may be formed in addition to the described layers and formation of some of the described layers is optional, depending on the desired cell stack structure.

[0027] Referring again to Figure 2, a top encapsulation layer, 205, and top contact layer, 210, can be initially cleaned, dried, cut to size, and edge seamed. Commonly, the top encapsulation and top contact layer are in the form of a glass superstrate (encapsulation layer) coated with a transparent conductive oxide (top contact). Float glass with transparent conductive oxide coatings, for example indium tin oxide, doped zinc oxide or doped tin oxide, are commercially available from a variety of venders, for example, glasses sold under the trademark TEC Glass™ by Pilkington of Toledo, Ohio, and SUNGATE™ 300 and SUNGATE™ 500 by PPG Industries of Pittsburgh, Pennsylvania. TEC Glass™ is a glass coated with a fluorinated tin oxide conductive layer. A wide variety of solvents, for example deionized water, alcohols, detergents and the like, can be used for cleaning the glass. As well there are many commercially available industrial-scale glass washing apparatus appropriate for cleaning large substrates, for example, Lisec^™ (a trade name for a glass washing apparatus and process available from (LISEC Maschinenbau Gmbh of Seitenstetten, Austria). [0028] Once the TCO coated glass is cleaned, a CdS layer, 215, is then deposited, for example, by using an aqueous solution of, for example, a cadmium salt and elemental sulphur or sulphur containing compound composition. The solution does not have to be aqueous. That is, other solvents, such as dimethylsulfoxide (DMSO), can be used. This deposition can be done using electrodeposition. For electrodeposition, the ITO coated glass can form one of the electrodes. The other electrode can be, for example, made of graphite, and the electrolyte can be, for example, a DMSO solution of a cadmium salt and elemental sulfur. Potential is applied between the electrodes so that CdS is deposited from the solution onto the ITO coated glass substrate. Another method of depositing the CdS layer is chemical deposition, for example via wet chemistry or dry application such as CVD. The CdS deposited is an n-type semiconductor and its thickness is typically between 500 A and 1 μιη. Subsequent to the deposition, the layer can be annealed, for example under an inert atmosphere such as argon, to achieve film densification and grain growth to improve the electrical and mechanical properties of the CdS film.

[0029] For illustration purposes, electrodeposition of CdS is sometimes described herein as being used in the fabrication of window layers for CdTe-based solar cells. However, window layers can include materials other than CdS, and electrodeposition is not the only method of depositing CdS, that is, the invention is not limited to this particular chemistry or application technique.

[0030] Cadmium sulphide (CdS) is an important semiconductor, and finds particular use as a window layer and n-type semiconductor in, for example, cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), cadmium selenium telluride (CdSe_xTei__x), and the like for solar cell manufacturing. In these exemplary solar cells, CdS can serve two purposes, as an n-type semiconductor for forming the p- n junction with the absorber layer and as a window layer to allow photons to pass through to the absorber layer.

[0031] The nanostructure of CdS can be tailored to influence such a window layer's band gap. Band gap has a direct influence on the amount of light the window layer allows to pass through to the absorber layer. For example, a smaller grain size increases the band gap of the semiconductor which allows more light to pass through the window layer. Methods described herein allow for taking full advantage of processing CdS films for smaller grain size and increased band gap.

[0032] A cadmium telluride layer, 220, is then, for example, electrochemically deposited on the CdS/TCO/Glass stack (now a substrate for electrodeposition), for example, from an acidic or basic media containing a cadmium salt and tellurium oxide. In this process, the CdS/TCO/Glass substrate forms one of the electrodes and platinum or other materials can be used as the other electrode. The electrolyte can contain an acidic or basic media, in solvents such as water, DMSO or other solvents, with a cadmium salt and tellurium oxide, for example. Films of thickness ranging from 1 to 10 μιη are typically deposited. Cadmium telluride films may then be annealed at approximately 400 °C in an air or oxygen or CdCl₂ environment so as to improve the electrical properties of the film and also to convert the CdTe film to a p- type semiconductor. While not wishing to be bound by theory, it is believed that these methods optimize grain size and passivate grain boundaries and thereby improve the electrical properties of the films.

[0033] After this CdTe deposition and annealing, a laser scribing process is typically performed to remove CdS and CdTe from specific regions (not shown). In this scribing operation, the laser scribing is utilized such that CdS and CdTe are removed from specific regions of the solar panel. However, the conductive oxide (e.g., Al doped ZnO or ITO) is not removed by the laser scribe. Then a second laser scribing step is performed in which CdS, CdTe and TCO are removed from specified regions.

[0034] A back contact layer, 225, can then be deposited on the CdTe layer, using for example sputtering or electrodeposition. For example, copper, nickel, graphite and/or other metals, alloys and composites can be used for the back contact layer. This back contact fabrication step can be followed by an anneal, for example, at temperatures of between about 150° and about 200 °C to form a conductive (ohmic or tunneling) contact. The back contact layer can cover the CdTe layer and also fill the vias (not shown) created in the CdTe/CdS layer by the laser scribing process. [0035] After back contact layer deposition and annealing, laser scribing can typically be used to remove the back contact layer material from specific areas, but the CdTe layer is not etched away in this process. This removal step can complete the process for isolation and interconnecting the solar cells in series in the solar panel/module.

[0036] After the deposition of the back contact layer, an encapsulation layer, 230, can be applied, for example, using ethylvinyl acetate (EVA). Encapsulation protects the photovoltaic stack. Glass, 235, can be added for further structural support (and protection) of the stack. [0037] The above described superstate fabrication protocol represents a brief outline and many variants of this process can be employed for the fabrication of CdTe thin film solar cells. For other types of thin film solar cells, different chemicals, etc. can be employed. In this description, example process steps have been described for illustrative purposes. Other steps would typically include additional details of the laser scribing and ablation steps employed for the fabrication of the interconnect schemes and cell isolations, multiple clean and drying steps between the different layer depositions and the like. Values for the layer thicknesses, anneal temperatures, chemical composition etc. described herein are merely illustrative, and can vary across a wide range as processes are optimized for many different output variables. C. Substrate Solar Cell Fabrication

[0038] In conventional superstrate fabrication protocols for solar cells, for example as outlined above in relation to Figure 2, the window layer is deposited before the deposition of the absorber layer and formation of the back ohmic contact. The absorber layer typically requires processing, such as a relatively high temperature thermal anneal, to improve its photovoltaic properties. For example, a CdTe layer can be annealed in cadmium chloride to passivate its grain boundaries and to convert it to a p-type semiconductor. Anneals can also be utilized to optimize the grain structure of the absorber layer. These anneals can range from about 200 °C to about 650 °C. The formation of the ohmic back contact layer also requires an optimization process, such as an anneal, for example at temperatures between about 100 °C and about 300 °C. The anneals for improving absorber layer characteristics and for the formation of the back contact negatively impacts the grain structure of the window layer, for example increasing the grain size of the window layer and decreasing the band gap, which decreases the transmission properties of the window layer. Thus in the superstrate integration scheme, the properties of the window layer can not be controlled independently of the process parameters of the absorber layer and the back contact. As a result, superstrate fabrication protocols lead to degradation of the properties of the window layer and thus lower performance solar cells. The anneals for the optimization of the properties of the absorber layer can also significantly degrade the performance fo the back contact and the cell itself for example by diffusing metal species from the back contact into the absorber layer due to the anneal and degrading the performance fo the absorber layer.

[0039] By inverting the photovoltaic stack formation order, that is, using substrate fabrication protocols, independent control of fabrication conditions absorber layer and window layer is achieved however the back contact conditions can not be independently optimized since the back contact thermal budget includes the processing for the optimization of the absorber and the window layer since these latter layers are formed, conventionally, after the back contact. For example, methods of improving the window layer's properties by decoupling the formation parameters of the window layer from those of the absorber layer, are described in U.S. Patent Application, serial number 12/764,812, filed April 21, 2010, naming as inventors Kurt Weiner, et al, and entitled, "Methods for Integrating Quantum Window Structures into Solar Cells", which is incorporated by reference herein for all purposes. The general procedure for substrate photovoltaic stack formation is described below. Techniques for fabricating individual layers for substrate fabrication may be similar to techniques for forming each similar individual layer as described above with reference to the superstrate fabrication.

[0040] Figure 3 illustrates a cross-section, 300, of a photovoltaic stack fabricated according to substrate protocols, that is, starting from a (back) substrate and forming thin films thereon successively to form a photovoltaic stack. Various embodiments include more or less process parameters and/or stack layers; this description is provided for a more thorough understanding of the context of embodiments of the invention.

[0041] Referring to Figure 3, photovoltaic stack 300 is a thin film stack including the following components: substrate, 305, a back contact layer, 310, an absorber layer, 315, a window layer, 320, a top contact layer, 325, and top encapsulation layer, 330. As described above, photovoltaic stacks can also have a back encapsulation layer, which in this example would be below the substrate layer 305. Back encapsulation can provide additional mechanical support and/or protection of the stack from outside contamination, for example, from moisture. In some embodiments, glass serves both as the back substrate and back encapsulation layer.

[0042] Starting with substrate 305, a back contact layer, 310 is applied to substrate 305. Back contact layer 310 is a thin film of material that provides one of the electrical contacts of the solar cell. In one embodiment, the back contact layer includes at least one of molybdenum, nickel, graphite, copper, tin and aluminum. The back contact can also be fabricated by laminating multiple layers for example using copper, graphite and a metal layer such as aluminum, tin and the like. This multiple layered back contact structure allows optimization of contact and series resistance. In one embodiment, the thickness of the back contact layer is between about 0.1 micron and about 10,000 microns, in another embodiment between about 0.1 microns and about 10 microns, in another embodiment between about 0.1 microns and about 1 micron. The back substrate may also serve as a back contact and provide mechanical support, as well as minimum resistance for holes/electrons flowing from/to the absorber layer.

[0043] Next, an absorber layer film, 315, is deposited. An absorber layer can be formed from, for example, CIGS, CdTe or amorphous silicon. In one embodiment, the absorber layer includes CdTe and/or is CdTe. In the embodiment described above, a process is performed that includes changing the grain structure of the absorber layer so as to optimize the properties of the absorber layer. Although the process for optimizing absorber layers may be described in terms of annealing, other processes that affect the grain structure of the absorber layer can alternatively be performed so as to optimize the absorber layer. In one embodiment, this includes annealing the absorber layer.

[0044] When the absorber layer is CdTe, annealing the CdTe layer includes heating at between about 250 °C and about 600 °C for between about 1 minute and about 60 minutes, in another embodiment between about 250 °C and about 450 °C for between about 1 minutes and about 30 minutes, in another embodiment between about 350 °C and about 450 °C for between about 10 minutes and about 20 minutes. In one embodiment, annealing the CdTe layer includes heating in the presence of cadmium chloride. The CdCl₂ can be dissolved in a solvent, for example an alcohol such as methanol, and spray applied to the surface of the CdTe. Annealing can be done under inert atmosphere or in air.

[0045] The processing of the absorber layer may also include converting the absorber layer to a p-type semiconductor and/or passivating the grain boundaries of the absorber layer and/or improving or establishing ohmic contact with the back contact layer. The thickness of the CdTe absorber layer is typically on the order of microns, varying from a few microns to tens of microns. In one embodiment, the thickness of the absorber layer is between about 0.5 micron and about 15 microns, in another embodiment between about 0.5 micron and about 5 microns, in another embodiment between about 0.5 microns and about 2 microns. [0046] After processing the absorber layer for optimal properties, there may be void defects in the absorber layer. Defects in the absorber layer can include fissures or voids that may or may not span the entire thickness of the absorber layer, these defects are termed "voids" from herein. Whether or not a void spans the thickness of the absorber layer initially, it should be repaired because it may later cause a short due to breakdown of any remaining absorber material remaining between the contacts of the photovoltaic stack. That is, even if the void doesn't span the entire thickness of the absorber layer, if the void is not repaired there is more risk of eventual electrical shorts because there is less material between, for example, the back contact and the window layer. Voids in the absorber layer, if present, are repaired. [0047] Referring again to Figure 3, after processing absorber layer 315 for optimal properties and repairing voids in the absorber layer, a window layer, 320, is deposited. The window layer is formed after deposition, optimization (e.g. affecting the grain structure and electrical properties of the absorber layer) and repair of the absorber layer. Window layer 320 is a film of semiconducting material that creates a p-n junction with absorber layer 315 and allows the maximum number of photons in the energy regime of interest to pass through to absorber layer 315. It is desirable to maintain nanostructures in the window layer for optimal properties, as described in U.S. Patent Application, serial number 12/764,812 {supra). In one embodiment, the window layer includes at least one of CdS, ZnSe (zinc selenide), ZnS (zinc sulphide), ZnO (zinc oxide), Cd(OH)SH (cadmium hydroxide sulphide), In(OH)SH (indium hydroxide sulphide), Sn0₂ (tin (II) oxide) and Sn(0₂)S₂ (tin (IV) oxide sulphide). Window layer 320 can be an n- or p-type semiconductor, depending on the material used for the absorber layer. In one embodiment, the window layer is formed from a cadmium sulphide (CdS) which is an n-type semiconductor. Cadmium sulfide films are used, for example, in CdTe and CIGS thin film solar cells. In one embodiment, the thickness of the window layer is between about 50 A and about 2000 A, in another embodiment between about 50 A and about 1000 A, in another embodiment between about 50 A and about 500 A. [0048] In one embodiment, the CdS film is nanostructured. Nanostructured CdS films can be produced, for example, via electrodeposition, wet chemical deposition, dry sputtering and the like. Nanostructured grain sizes make the grains behave as quantum structures. The quantum nature of the grains in the film increases the window layer's band gap, allowing a greater percentage of the incident radiation to pass through to the absorber layer, thereby increasing the efficiency of the solar cell. The thermal budget to which the window layer is subjected is carefully controlled so as to maintain the quantum nature of the grains and to take advantage of the increase in the band gap. In one embodiment, the window layer material includes a grain size of between about 5 A and about 300 A, in another embodiment between about 5 A and about 50 A, in another embodiment between about 5 A and about 10 A. [0049] Referring again to Figure 3, after the window layer is formed, a front contact layer, 325, is formed. Front (or top) contact layer 325 is a thin film of material that provides the other contact for photovoltaic stack 300. As described above, the top contact is made of a material that is transparent to the photons in the energy regime of interest for the ultimate solar cell. In one embodiment, the top contact layer includes at least one of indium tin oxide (ITO), aluminum doped zinc oxide, zinc oxide, fluorinated tin oxide, and/or a grid of metallic lines or a combination of transparent conductive oxide and a grid of metallic lines. The top contact layer thickness can be on the order of thousands of angstroms. In one embodiment, the thickness of the top contact layer is between about 500 A and about 10,000 A, in another embodiment between about 500 A and about 5000 A, in another embodiment between about 500 A and about 3000 A.

[0050] After the front contact layer is formed, an encapsulation layer, 330, is deposited. Encapsulation is used to provide environmental protection and/or further mechanical support to the cell. The top encapsulation is formed from a material that is highly transparent in the photon energy regime of interest. This top encapsulation layer can be formed from, for example, glass or other transparent material such as a polymeric material. In the event the encapsulation layer is not mechanically strong enough to provide support, a (front) suprastrate may optionally be applied. In one embodiment, the front contact and top encapsulation layers are applied in one step, for example, using a glass (the encapsulation layer and supportive superstate) coated with a TCO (the front contact), where the TCO adjoins the window layer. In this embodiment, heating or other processing may be needed to ensure proper bonding and ohmic contact between the TCO and the window layer. In another embodiment, glass alone serves as an encapsulation layer (and front superstate for mechanical support) that is applied over the front contact.

D. Substrate Fabrication where Back Contact is formed after Absorber Layer

[0051] The superstate first integration scheme is most commonly employed for the fabrication of the CdTe thin film solar cells due to the difficulties associated with forming an acceptable ohmic contact between the back contact material and the CdTe (absorber) layer. Since CdTe is a high work function material, it can be challenging to form an ohmic contact to it. Typically, a tunneling junction contact is formed to the CdTe. This contact process typically involves creating a Te rich layer in the CdTe by, for example, a wet etch using chemical solutions which preferentially etches Cd from the CdTe layer. This etching process can, for example, be achieved by using chemical solutions that contain bromine.

[0052] Using superstrate fabrication, after Cd is etched from the CdTe to form a Te rich region, the back contact material is applied. For example, a conductive layer can be deposited over the Te rich region of the CdTe layer and then conditioned to react with the Te rich layer. This conductive layer could, for example, be Cu, which is then conditioned to be reactive with the Te rich region of the CdTe layer by, for example, using a thermal anneal to fabricate a metal Te complex, such as Cu_xTe.

[0053] This methodology for fabricating the contact to the CdTe layer works well in the superstrate integration scheme. However, this contact forming method is often not well suited for substrate fabrication protocols. Typically, Cu and/or other conductive materials diffuse readily into CdTe. Thus, if these conductive materials diffuse to the CdTe/CdS junction, they can significantly degrade the device's photoelectric performance. Also, these types of contacts can be unstable at high processing temperatures. Hence, in substrate (first) fabrication if the back contact is fabricated before the CdTe transformation anneal, the contact will typically degrade during subsequent annealing processes. Also, metallic materials used for back contacts can diffuse into the bulk of the CdTe film, which can also cause degrading of the device performance. Moreover, if the ohmic contact were to be formed after a CdTe transformation and other high temperature anneals, then it would be challenging to form a Te rich layer at the bottom of the CdTe layer.

[0054] These constraints on the fabrication of the contact to the CdTe layer are some of the primary reasons why the CdTe solar cells are predominantly made using superstrate fabrication. However, substrate fabrication can offer significant advantages over superstrate fabrication. Embodiments of the invention address the aforementioned constraints. Substrate fabrication is used and significant enhancements to the CdTe solar cells are achieved. Methods of the invention, which include substrate fabrication, allow fabrication of cells on flexible substrates with increased efficiency by virtue of, for example, retention of nanostructure in the CdS layer thereby enhancing the CdS layer's transmission. Also, embodiments employing substrate fabrication can also allow the use of transparent conductive oxides (such as ZnO), which have better resistance and transmission characteristics than, for example, tin oxide (Sn0₂), which is typically used due to its high temperature stability. In superstrate fabrication the transparent conductive oxide typically must have a high temperature stability because of the high temperature anneal processes, which are subsequent to the deposition of the transparent conductive oxide. However, this constraint can be alleviated in substrate fabrication protocols because the transparent conductive oxide is deposited after high temperature anneals.

[0055] Methods described herein allow formation of the back (ohmic and/or tunneling) contact using substrate fabrication protocols, after the absorber layer is formed and in some embodiments after the absorber layer is optimized. Certain embodiments utilize a porous conductive back substrate layer through which one or more chemical solutions are transported so as to interact with and thereby alter a physical and/or chemical property of only a bottom portion of the absorber layer in order to facilitate formation of an ohmic back contact at the interface of the back contact material and the absorber layer. In one example, this chemical treatment is done after an annealing process, for example, annealing the CdTe absorber layer. Although certain embodiments are described herein in the context of the CdTe thin film solar cell, such techniques may be applied to other types of thin film solar cells fabricated using substrate fabrication.

[0056] In a specific implementation, a porous conductive substrate is utilized as a back substrate. Figure 4 is cross-sectional view of a solar cell, 400, fabricated using a substrate integration scheme with a porous conductive substrate (back), 407. Figure 4 depicts the completed stack 400, as well as various stages of fabrication of cell 400. In one embodiment, the porous conductive substrate includes at least one of graphite, copper and nickel. [0057] In one embodiment, a graphite foil is used as a back substrate. Graphite foils have a high electrical conductivity, can be manufactured cheaply, have a coefficient of thermal expansion that is well matched to that of CdTe, is stable up to very high temperatures, and can be flexible. Also, graphite foils obviate the issue of metal from the back contact diffusing into the absorber layer material. For the purposes of this application, the term "graphite foil" is meant to include foils, cloths, felts and the like. The material may be woven or not and may be reinforced with other materials, for example in one embodiment, the graphite foil is reinforced with porous stainless steel foil. Graphite foil is typically manufactured from high purity, high crystalline natural graphite flakes, which are processed into continuous foil, for example, by an acid and thermal treatment to produce expanded graphite crystals. The expanded graphite crystals are then formed into foil through an extensive calendaring process without any resins or binders.

[0058] Graphite foil is porous and its porosity can be easily controlled (can for example easily range from between about 60% porosity to about 20% porosity). The size of the pores, their density, percentage of the pores that are open (porosity) can easily be controlled by varying parameters in the fabrication process used for the graphite foil. Graphite foils with specified porosity are commercially available from SGL Group of Valencia, CA. The porosity of the foil can be selected based on the specifics of the processes which are employed, such as the rate at which a particular chemical species diffuses through the graphite foil, the amount of chemical species required to be diffused through the graphite foil, the processing temperature, the time for which diffusion is performed, and the like.

[0059] Figure 4 is described in relation to Figure 5, which depicts a process flow, 500, describing aspects of methods described herein. In certain embodiments, the back substrate takes the form of a graphite foil. An absorber layer is first deposited on a porous conductive substrate, see 505. In one embodiment, the absorber layer is CdTe, which can be deposited by various methods, such as electrodeposition, closed space sublimation, vapor transport deposition, and the like. An optimization process may be performed on the absorber layer prior to formation of a window layer thereon so as to optimize the properties of the absorber layer, see 510. For instance, an absorber CdTe layer can be annealed at, for example, approximately 500 °C in air to transform the CdTe layer from such layer's deposited n-type conductivity to p-type conductivity. Subsequent to the anneal, the CdTe layer can, for example, be exposed to CdCl₂ and annealed at 400 °C to passivate the grain boundaries in the film and enhance the CdTe layer's electrical performance. Embodiments include optimization of the absorber layer as described herein. Optimization may also include repairing voids in the absorber layer.

[0060] Although the process for optimizing the absorber layer is described herein as an annealing process, other processes that affect the grain structure of the absorber layer can alternatively be performed so as to optimize the absorber layer. In certain embodiments, the window layer is formed after formation of the absorber layer and after performing any process for optimizing such absorber layer (and that affects the grain structure of the absorber layer), which optimization process would typically adversely affect the nanostructure of the window layer if present during such optimization process.

[0061] Referring again to Figures 4 and 5, after formation and optimization of the absorber layer, an n-type window layer, 420, such as CdS, is deposited, for example using chemical bath deposition, electrodeposition, evaporation and the like, see 515. In other embodiments, the window layer is deposited after formation of the back contact as described below.

[0062] After deposition of the window layer, the absorber layer is chemically treated, via the porous conductive substrate, in order to form and/or enhance ohmic contact between the absorber and the porous conductive substrate, see 520. For example, after the deposition of the CdS window layer, a chemical solution (e.g., a chemical solution containing bromine) for creating a Te rich layer is applied to the back side of a graphite foil (the side opposite to the side on which the CdTe is deposited). In the example depicted in Figure 4, a region of absorber layer 415 proximate the porous conductive substrate 407 is altered, for example a physical and/or chemical property of the region of the absorber layer is changed, in order to facilitate formation of an ohmic back contact at the interface of the back contact material and the absorber layer. Thus absorber layer is subsequently labeled 415a, and the altered portion of the absorber layer is labeled 410 (which will ultimately become the back contact, see below). [0063] In the illustrated example, due to the porosity of the back substrate, for example graphite foil, the chemical solution (e.g., a solution containing bromine) can transport to the conductive substrate foil/absorber interface and diffuse through a bottom portion of the absorber layer. In this example a CdTe absorber layer is exposed to the bromine containing solution proximate the graphite foil to thereby create a Te rich region in the CdTe by preferentially leaching Cd from the CdTe layer without affecting the bulk of the CdTe layer. In this example, the amount of etchant reaching the graphite foil/CdTe interface and, hence, the etch rate of Cd and, hence, the thickness of the Te rich layer can, for example, be controlled by varying the concentration of the active etchant, for example, bromine, in the solution, time duration of the application of the chemical to the back side of the graphite foil, the porosity of the graphite foil, open pore density of the graphite foil, thickness of the graphite foil, and the like. For example, in one embodiment, these factors are controlled as to form a Te rich bottom portion of the interface having a thickness between about 100 A (Angstroms) and about 3000 A. In another embodiment, the interface has a thickness of between about 100 A and about 1000 A, in another embodiment between about 100 A and about 500 A. The thickness of the Te rich layer that is to be fabricated can depend on the particular details of the contact formation process. [0064] After chemical treatment of the absorber layer via the porous conductive substrate, a metallic species is delivered to the altered region via the porous conductive substrate, see 525. For example, after creating the Te rich interface, a metallic species or solution is then transported through the foil so as to deposit over the modified bottom portion of the absorber layer (e.g., the Te rich CdTe layer), and then this deposited metallic species is conditioned, see 530, to react with such modified bottom portion (e.g., Te rich layer) to form the back contact, 410, at the interface. In one embodiment, a copper containing solution for example Cu ions in a copper iodide and methanol solution or copper acetate and methanol solution is applied to the back of the graphite foil and transported to the interface between the graphite foil and the Te rich portion of the CdTe absorber layer. After this copper transporting operation, a thermal anneal can be performed to form the ohmic back contact to the CdTe layer (e.g., by fabricating a metal Te complex, such as Cu_xTe where x can typically range from 1 to 2. One embodiment is a photovoltaic stack including: a graphite foil substrate, a contact layer including a metal Te complex, a CdTe absorber layer, and a CdS window layer. In one embodiment, the characteristics of the individual layers are each, independently, as described herein. [0065] In one embodiment, exposing the absorber layer to a chemical species via the porous conductive substrate so as to selectively modify a physical and/or chemical property of a region of the absorber layer proximate the porous conductive substrate; and forming an ohmic contact at the interface of the absorber layer and the porous conductive substrate are performed prior to formation of a window layer over the absorber layer. In another embodiment, exposing the absorber layer to a chemical species via the porous conductive substrate so as to selectively modify a physical and/or chemical property of a region of the absorber layer proximate the porous conductive substrate; and forming an ohmic contact at the interface of the absorber layer and the porous conductive substrate are performed after formation of a window layer over the absorber layer and prior to formation of a front contact layer over the window layer. In yet another embodiment, exposing the absorber layer to a chemical species via the porous conductive substrate so as to selectively modify a physical and/or chemical property of a region of the absorber layer proximate the porous conductive substrate; and forming an ohmic contact at the interface of the absorber layer and the porous conductive substrate are performed after formation of a window layer over the absorber layer and after formation of a front contact layer over the window layer.

[0066] After conditioning, 530, for example annealing, cell fabrication is continued to completion of the desired stack, see 535. For example, processing of the thin film photovoltaic can continue with the deposition of the TCO, 425, top encapsulation, 430, back encapsulation, 405, interconnect definition, and the like. Additionally, any of the individual fabrication operations described above, for example, with respect to the superstate or conventional substrate schemes (e.g., laser scribing, interconnection, etc.) may be applied to the illustrated porous conductive substrate embodiment. [0067] The processes described above are illustrative and not meant to be limiting. The point at which the process for the fabrication of the back contact can be inserted in the substrate fabrication scheme can be varied depending on the particular optimization constraints of the fabrication process. Depending on the details of the process, embodiments include fabrication of the back contact, for example, prior to the deposition of the CdS layer or after the application of the TCO layer. In one embodiment, no high temperature anneals are performed after formation of the back contact so as to not destabilize the back contact or lead to diffusion of metallic species, such as copper, into the bulk of the CdTe absorber layer.

E. Continuous Graphite Foil as Back Substrate

[0068] In a specific embodiment, a plurality of photovoltaic cells and/or modules are formed by depositing absorber, window, back contact, and front contact layers on a substantially continuous porous conductive substrate (e.g., graphite foil). In one embodiment the porous conductive substrate is also flexible so as to provide a back substrate upon which multiple layers may be fabricated in a line type manufacturing systems that employ rollers over which the substrate must pass during photovoltaic stack formation. For example, multiple photovoltaic cells and/or modules are formed using a graphite foil back substrate that is fed into a continuous line fabrication process. For instance, the graphite foil can take the form of a long, flexible reel (e.g., miles long) that is fed into a processing line. In a specific implementation, a graphite foil back substrate, which is flexible and porous, is moved through one or more electrodeposition systems that each are configured to deposit a specific material over the back substrate via an electrodeposition process. As the back substrate is moved through each electrodeposition system, the back substrate serves as an electrode so that the specific material (e.g., an absorber layer, a window layer, or a front contact layer) is deposited over the back substrate (and any other previously deposited layers). The absorber, window, back contact, and front contact layers, together with the graphite foil, then result in a single photovoltaic device, from which individual photovoltaic cells can be isolated to create, for example, a plurality of photovoltaic modules that each comprise a plurality of cells. The chemistry and method used to make the solar cells on the continuous sheet of graphite foil can be, for example, as described in relation to Figures 4 and 5. [0069] Figure 6 is a simplified cross-sectional illustration of a conventional electrodeposition apparatus, 600, that employs a continuous substrate. So as not to complicate the description, other components of the equipment, such as the electronics for control systems, for applying potentials to the electrodes, chemical handling system for the electrolyte etc., are not shown, but would typically be included in the system. The dimensions of the different components of the system can vary across a large range depending on the application for which the equipment is designed. Also, different electrodeposition systems or modules may be used in a line type manufacturing system so as to each deposit a different material on the foil as it moves through the line. One embodiment is a photovoltaic device fabricated on a graphite foil, where two or more layers of the photovoltaic are sequentially deposited using distinct electrodeposition systems in a line type format.

[0070] As shown, apparatus 600 includes a large tub, 605, which holds the electroplating solution, 610, and through which a porous continuous substrate, 615 is passed. Deposition on the substrate is achieved by application of an electric potential between the substrate, for example conventionally a metal foil such as aluminum, nickel or steel, and a counter electrode, 620. A large tub typically contains the electroplating solution, counter electrode 620, and a mechanism, for example rollers, 625, for moving the substrate through the electroplating zone etc. The moving foil is the substrate on which deposition takes place. This foil also forms one of the electrodes of the electrodeposition system. In embodiments described herein, a graphite foil serves as the substrate.

[0071] Counter electrode 620 is the second electrode in the system and is typically immersed in the electrolytic solution. This counter electrode can be made of a large variety of materials. Typically, the counter electrode is electrically conductive and chemically compatible with the electrolytic solution. For example, the counter electrode may be formed from materials, such as platinum and/or graphite.

[0072] Conventional electrodeposition apparatus that employ a continuous substrate will typically include a mechanism for moving the substrate through the active electroplating zone. This movement mechanism may be in the form of rollers, 625. These rollers are used to move the substrate through the system and the electrolyte and over the counter electrode to enable electrodeposition. In this example, substrate 615 is curled (or bent) as it passes over each roller, in this example the graphite foil substrate is bent four times as it passes into and out of the electrolyte bath and on to further processing steps. If any of these rollers are immersed in the electrolyte or are in contact with it, care is typically taken to ensure that the roller material of construction is chemically compatible with the electroplating solution.

[0073] The composition of the electrolyte depends on the material to be deposited. Examples of electroplating solutions that might be used for fabricating different layers of a solar cell are described above in relation to Figures 1-5. [0074] Many of the advantages of graphite foil discussed above, as well as other advantages, are realized when using graphite foil as a continuous substrate in a substrate fabrication process. Graphite foil is typically inexpensive to manufacture in large rolls, and the foil's properties can easily be modulated. Graphite foil can be flexible and has a thermal expansion coefficient that is well matched to the thermal expansion coefficient of CdTe. Graphite foils are typically chemically resistant, have a high thermal conductivity, and are thermally stable. The surface roughness, porosity, density, thickness, grain structure and other properties of the graphite foil can easily be modulated during its fabrication process. As well, CdTe layers tend to have good adhesion to graphite foil. [0075] The porosity of graphite foil can be exploited to overcome the challenges related to fabrication of the back contact to CdTe in a substrate fabrication scheme. However, the porosity of the graphite foil may create a challenge during electrodeposition. During wet chemical processing (e.g., chemical electrodeposition of CdTe), the electrolyte tends to seep into the graphite foil and slightly change the foil's volume. Hence, as the CdTe layer is being electrodeposited, the surface of the graphite foil on which the electrodeposition is being performed may be physically changing shape. This surface change may cause cracks in the CdTe layer. Cracks can then result in a degradation of the solar cell properties.

[0076] In one embodiment, the graphite foil is treated with a chemical solution to electrodeposition. The chemical solution can be specifically formulated to prevent crack formation in the electrodeposited layer. The treatment may include soaking the graphite foil in a chemical solution prior to the electrodeposition process so that crack formation in the electrodeposited material is minimized. In this soak process, the graphite foil is immersed in an appropriate liquid, for example, de-ionized water or the solution in which the deposition is being performed, for a duration that is sufficient to fully soak the graphite foil to saturation. In one embodiment, the tub used for electrodeposition includes a volume through which the graphite foil is passed prior to electrodeposition. In another embodiment, a separate soaking vessel is used. In another embodiment, the graphite foils is treated by spraying with a chemical solution prior to electrodeposition. Subsequent to treatment, electrodeposition can commence. By soaking the graphite foil to saturation, the foil's volume does not change during electrodeposition, and more uniform (minimally-cracked) films result.

[0077] The duration of treatment, for example pre-soaking, and the liquid to be used for treatment will depend on the properties of the graphite foil (e.g. porosity, density, thickness etc.) and also on the properties of the liquid being used for the electrodeposition. For example, if a CdTe layer is being deposited from an aqueous solution, de-ionized water and/or the electrolyte can be used as a pre-soak solution. In another example, if the deposition is being performed in a DMSO based solution, then the pre-soaking can be done in DMSO or the deposition DMSO based solution. [0078] Graphite foils tend to have high tensile strengths. However, foils can also have low tearing resistance. This feature may become significant when the graphite foil is wet since the foil retains liquid due to its porosity. In one embodiment, a tape is used on the edges of the graphite foil. The tape can be applied to one or both sides of the graphite foil proximate the edge, for example, a single piece of tape can be folded over each edge, two pieces of tape applied to each side on both edges of the graphite foil, or a single piece of tape applied proximate each edge of the graphite foil. In one embodiment, the tape is conductive, such as steel tape (or another type of tear resistant tape) is applied to the foil sides. The tape can provide enhanced tear strength to the graphite foil. In one embodiment, reinforced graphite foil, for example reinforced with stainless steel foil, wire or mesh, is used as the substrate, with or without tape application. [0079] In a continuous flow implementation, the graphite foil typically needs to be moved through the processing line uniformly. One way to accomplish uniform movement can be, for example, via mechanical mechanisms or vacuum grabs at the foil edges so as to uniformly move the foil through the processing line. However, the low tear strength of the foil, especially when it is soaked, can make it difficult to achieve uniform movement. The tape at the foil edges can provide a good place to mechanically, or through vacuum, hold the foil and uniformly move it through the processing line.

[0080] For certain electrodeposition process embodiments, the graphite foil is electrically contacted. In one embodiment, the tape at the edges is electrically conductive (e.g., steel tape) which provides a good point for making electrical contact to the graphite foil. In one embodiment, electrical contact is made via a slipping type contact so that the contact touches the tape on the foil and remains stationary while the graphite foil is moved past the contact. In one embodiment, the slip type contact is constructed to have a length that substantially matches the length of the region in which the electrodeposition is being performed. In this embodiment, the improved mechanical strength that the tape provides reduces the complexity of the mechanism for making contact to the graphite foil.

[0081] In one embodiment, a contact seal is applied at the edges of the graphite foil. In one implementation, where tape is applied to the edges of the foil, the ability to form contact seals to the edges of the graphite foil is enhanced. The strength and other mechanical properties of the tape (e.g. the friction of the tape to the contact sealing material etc.) at the edges of the graphite foil can be optimized independently of the requirements for the depositions that impose constraints on the properties of the graphite foil.

[0082] In one embodiment, contact seals are used at least at an upstream and downstream location so that the graphite foil passing between the seals can have material electrodeposited thereon without passing the graphite foil with the newly deposited film thereon over rollers. Figures 7 A and 7B depict a cross section, side and top view, respectively, of an electrodeposition apparatus, 700. Apparatus 700 is similar to conventional apparatus 600 (Figure 6) but apparatus 700 moves substrate 615 via application of, for example, mechanical mechanisms or vacuum grabs at the foil edges so as to uniformly move the foil through the processing line as described above. Since there are no rollers, the graphite substrate, and thus the films deposited thereon, are not subjected to multiple bends over many rollers which can negatively affect the ultimate performance of the stack. In this example, the graphite foil is drawn between contact seals 705 and 710, at opposing ends of a tub which holds the electrolyte, these seals correspond to an upstream and downstream location, between which electrodeposition can take place. Referring to Figure 7B, in this example, a conductive tape, such as steel tape, 715, is applied to both edges of graphite foil substrate 615. Electrical contact is made to the graphite foil via tape 715 (contacts not depicted). Tape 715 also aids in drawing graphite foil substrate 615 through the plating bath 605 via contact seals 705 and 710. Counter electrode 620 can not be seen in Figure 7B, as the cross section is at the top surface of graphite foil substrate 615.

[0083] In one embodiment, where reinforced graphite foil is used, the reinforcing material may protrude from the sides of the graphite foil and thereby create areas where electrical contact and/or areas where the substrate is held so as to uniformly move it through the process line. For example, a stainless steel reinforcing foil, inside the graphite foil, may protrude from the edges of the graphite foil and be used for electrical contact and/or holding and positioning the graphite foil. These areas of protruding stainless steel or other conductive reinforcing material would be analogous to tape 715 in Figure 7B, except that tape would not be needed. In one embodiment, tape is applied to the protruding reinforcing material, for example, for added reinforcement and/or conductive purposes.

[0084] In one embodiment, two or more such apparatus, analogous to 700, are positioned in series so that the graphite film can be drawn through successive electrodeposition baths in order to deposit the layers of the photovoltaic stack. In one embodiment, the contact seals are soft seals, that is, they are pliable and do not damage the substrate or deposited film, for example a soft blade seal, elastomeric and/or spring actuated seal.

Claims

CLAIMS What is claimed is:

1. A method of forming a solar cell, comprising:

(a) providing a porous conductive substrate;

(b) forming an absorber layer over the porous conductive substrate;

(c) exposing the absorber layer to a chemical species via the porous conductive substrate so as to modify a physical and/or chemical property of a region of the absorber layer proximate the porous conductive substrate; and

(d) forming an ohmic contact at the interface of the absorber layer and the porous conductive substrate.

2. The method of claim 1, wherein the absorber layer comprises CdTe, and (c) and (d) comprise, respectively: i) applying a bromine solution to the porous conductive substrate thereby transforming the region into a Te rich region; ii) applying a copper containing solution to the Te rich region via the porous conductive substrate and annealing the stack to form a copper tellurium complex at the interface of the Te rich region and the porous conductive substrate.

3. The method of claim 1, further comprising forming a window layer after (b) and before (c).

4. The method of claim 1, further comprising encapsulating the window layer.

5. The method of claim 1, wherein the porous conductive substrate comprises a porous graphite material.

6. The method of claim 5, wherein the porous graphite material is a graphite foil.

7. The method of claim 1, further comprising encapsulating the porous conductive substrate.

8. The method of claim 2, wherein (b) comprises an optimization process that alters the grain structure of the absorber layer, passivates the grain boundaries of the absorber layer, and converts the absorber layer to a p-type semiconductor.

9. The method of claim 8, wherein the optimization process comprises an annealing process.

10. The method of claim 1, wherein (c) and (d) are performed prior to formation of a window layer over the absorber layer.

11. The method of claim 1 , wherein (c) and (d) are performed after formation of a window layer over the absorber layer and prior to formation of a front contact layer over the window layer.

12. The method of claim 1, wherein (c) and (d) are performed after formation of a window layer over the absorber layer and after formation of a front contact layer over the window layer.

13. A method of forming a plurality of photovoltaic cells, comprising:

(a) forming an absorber layer over a graphite foil, wherein the graphite foil is substantially continuous and flexible;

(b) forming a window layer over the absorber layer;

(c) forming a back contact layer at an interface between the graphite foil and the absorber layer;

(d) forming a front contact layer over the window layer; and,

(e) isolating said plurality of photovoltaic cells from the photovoltaic device thus formed;

wherein at least one of the absorber layer, the window layer, the back contact layer, and the front contact layer is formed by moving the graphite foil through a line type manufacturing system.

14. The method of claim 13, wherein at least one of the absorber layer, the window layer, the back contact layer, and the front contact layer is formed by electrodeposition while moving the graphite foil through an electrodeposition system.

15. The method of claim 14, further comprising treating the graphite foil with at least one of water and an electrolyte prior to electrodeposition.

16. The method of claim 15, wherein treating the graphite foil with at least one of water and an electrolyte comprises soaking the graphite foil in said at least one of water and the electrolyte.

17. The method of claim 13, wherein a tape is applied along the edges of the graphite foil so as to reduce tearing of the graphite foil as it moves through the line type manufacturing system.

18. The method of claim 17, wherein the graphite foil is moved through the line type manufacturing system via a mechanical and/or a vacuum mechanism, each of which hold and/or move the tape so that the graphite foil is moved uniformly.

19. The method of claim 17, wherein the tape is a conductive tape and an electrical contact is applied to the graphite foil via the conductive tape during at least one electrodeposition process used to form the plurality of photovoltaic cells, the graphite foil serving as an electrode during said at least one electrodeposition process.

20. The method of claim 13, wherein (c) comprises: (i) exposing the absorber layer to a chemical species via the graphite foil so as to modify a physical and/or chemical property of a region of the absorber layer proximate the graphite foil; and

(d) forming an ohmic contact at the interface of the absorber layer and the graphite foil.

21. The method of claim 13, wherein forming the absorber layer comprises moving the graphite foil through an electrodeposition system wherein the graphite foil serves as an electrode and the absorber layer is electrodeposited over the graphite foil.

22. The method of claim 21, further comprising electrodepositmg the window layer over the absorber layer.

23. The method of claim 22, further comprising electrodepositmg the front contact layer over the window layer.

24. The method of claim 23, wherein the absorber layer, the window layer, the back contact layer, and the front contact layer are formed by electrodeposition while moving the graphite foil through an electrodeposition system.