WO2011035234A1

WO2011035234A1 - Method of producing a flexible photovoltaic cell using a flexible polymer-fixture laminate

Info

Publication number: WO2011035234A1
Application number: PCT/US2010/049476
Authority: WO
Inventors: Alvin D. Compaan; Kristopher Wieland; Ryan Zeller
Original assignee: The University Of Toledo
Priority date: 2009-09-18
Filing date: 2010-09-20
Publication date: 2011-03-24

Abstract

A roll-to-roll manufacturing process for producing flexible, thin-film photovoltaic cells includes a flexible manufacturing fixture to transport a superstrate layer between successive processing stations. The processing stations apply functional layers of the photovoltaic cell in succession while the flexible fixture absorbs the loads imparted by the manufacturing process to prevent delamination of the built up cell layers. The superstrate is releasably retained onto the fixture. The fixture is further configured to impart performance characteristics onto the superstrate layer.

Description

TITLE

METHOD OF PRODUCING A FLEXIBLE PHOTOVOLTAIC CELL USING A FLEXIBLE POLYMER-FIXTURE

LAMINATE

Inventors: Alvin D. Compaan, Kristopher Wieland, and Ryan Zeller

CROSS-REFERENCE TO RELATED APPLICATIONS AND STATEMENT REGARDING SPONSORED RESEARCH

[0001] The present invention claims the benefit of the provisional patent application Ser. No. 61/243,894, filed September 18, 2009. This invention was not made with government support and the government has no rights in this invention.

FIELD OF THE INVENTION

[0002] The present invention relates generally to photovoltaic cells and methods for the fabrication thereof. More particularly, the present invention relates to a method of manufacturing a flexible photovoltaic cell having a transparent polymer front layer or superstrate.

BACKGROUND OF THE INVENTION

[0003] There is no admission that the background art disclosed in this section legally constitutes prior art.

[0004] It is well known that solar cells or photovoltaic cells (PV cells) can be used to convert solar energy into electric current. Typical photovoltaic cells include a substrate layer and two ohmic contacts or electrode layers for passing current to an external electrical circuit. The cell also includes an active semiconductor junction, usually comprised of two or three semiconductor layers in series. The two-layer type of semiconductor cell consists of an n-type layer and a p-type layer, and the three-layer type includes an intrinsic (i-type) layer positioned between the n-type layer and the p-type layer for absorption of light radiation. The photovoltaic cells operate by having readily excitable electrons that can be energized by solar energy to higher energy levels, thereby creating positively charged holes and negatively charged electrons at the interface of various semiconductor layers. The creation of these positive and negative charge carriers applies a net voltage across the two electrode layers in the photovoltaic cell, establishing a current of electricity.

[0005] Solar cells or photovoltaic cells are examples of diode structures where, in solar or PV cells, light passes through a transparent electrode layer and energizes an active semiconductor junction. The semiconductor layers of diode structures may be formed from single crystalline materials, amorphous materials, poly crystalline, or protocrystalline materials. Single crystalline layers are often made with a molecular beam epitaxy (MBE) process or other vapor deposition processes such as, for example, metal-organic chemical vapor deposition (MOCVD). The largest area of a substrate, however, that can be practically covered using such processes is on the order of several tens of square centimeters. Thus, the area of coverage is limited by the size of single crystal substrates, which is an impractical size when considering the surface area required for economically practical PV cells.

[0006] Therefore, although single crystal photovoltaic materials can be used to generate conversion efficiencies over 20 percent, they have significant drawbacks because of their high manufactured cost. Accordingly, where PV cells are able to compete with conventional electricity generation by nuclear or fossil fuel, polycrystalline and amorphous materials are viewed as the materials of choice, rather than crystalline materials, for the production of semiconductors and PV cells using such semiconductors.

[0007] Certain characteristics of the transparent electrode layers can impact the ability to pass current to the external electrical circuit in diode structures. Some notable characteristics of these electrode layers are a capability to: 1) conduct electricity to and from the diode structure; and 2) be substantially transparent to certain light wavelengths (typically greater than 400 nm) so that the solar energy can reach the primary

semiconductor layers that form the active semiconductor junction. In many cases, the restriction on the amount of light that can pass through the conductor layer sets a practical limit on the efficiency of the photovoltaic cell. Also, the electrical conductivity of the electrode layer impacts the overall efficiency of a photovoltaic cell.

[0008] One of the polycrystalline or protocrystalline material of choice for a

semiconductor in a photovoltaic cell is a group II-group VI compound, such as cadmium telluride. Cadmium telluride is preferred for thin film photovoltaic applications because its direct band gap of 1.5 electron volts is well-matched to the solar spectrum. Further, cadmium telluride has the ability to be doped in n-type and p-type configurations, which permits formation of a variety of junction structures. In addition, p-type cadmium telluride is also compatible with different n-type semiconductor partners, such as cadmium sulfide, to form heteroj unction PV cells.

[0009] Cadmium telluride PV cells are traditionally built on glass in a superstrate configuration, taking advantage of glass's transparency, mechanical rigidity and the opportunity to form the back contact last. However, glass is heavy and its rigidity and fragility are disadvantages for many applications.

[0010] It would be advantageous to manufacture a flexible diode such as a

photovoltaic (PV) cell that has a superstrate layer with high transparency and low solar radiation absorption and that can be assembled economically and in high volume.

SUMMARY OF THE INVENTION

[0011] In a first aspect, there is provided herein a polymer- fixture laminate system having a flexible manufacturing fixture for use in the construction and assembly of photovoltaic (PV) modules.

[0012] In another aspect, there is provided herein a polymer- flexible manufacturing fixture laminate system for use in manufacturing photovoltaic (PV) modules comprised of a polymer releasably adhered to a metal flexible manufacturing fixture.

[0013] In another aspect, there is provided herein a polymer- flexible manufacturing fixture laminate system for use in manufacturing photovoltaic (PV) modules comprised of a polymer releasably adhered to a metal flexible manufacturing fixture and optionally having a release agent interposed between the polymer and metal flexible manufacturing fixture.

[0014] In another aspect, there is provided herein a monolithically integrated, light weight, flexible photovoltaic (PV) module based on thin-film cadmium telluride made using a polymer-based flexible manufacturing fixture laminate system.

[0015] In another aspect, there is provided herein a PV module made using a polymer- based flexible manufacturing fixture laminate system.

[0016] In another aspect, there is provided herein a PV module that is semi-transparent and suited for use in superstrate applications made using a polymer-based flexible manufacturing fixture laminate system.

[0017] In another aspect, there is provided herein a method for making a PV module having series-connected cells that yield high-voltage PV modules using a polymer- fixture laminate system including a flexible manufacturing fixture.

[0018] In another aspect, there is provided herein a method for making a PV module having series-connected cells that yield high-voltage PV modules using a polymer- fixture laminate system in an online, or roll-to-roll, process.

[0019] In another aspect, there is provided herein a method of formation for a thin- film PV cell having a polymeric superstrate, comprising:

i) forming the superstrate layer on a surface of flexible manufacturing fixture layer;

ii) successively forming a heteroj unction device in a multilayer arrangement on the superstrate layer to form a semi-finished PV cell; and iii) removing the flexible fixture layer from the superstrate layer of the semifinished PV cell.

[0020] In certain embodiments, the superstrate layer comprises a polyimide material. In certain embodiments, the polymeric superstrate is a polyimide material that is applied by a casting process. In certain embodiments, the superstrate layer may be a silicon-based resin material. In certain embodiments, the flexible manufacturing fixture layer is sufficiently flexible such that no microstructural changes are observed in the photovoltaic active layers. In certain embodiments, the fixture layer comprises a stainless steel material. In certain embodiments, a release agent is disposed between the polyimide material and the stainless steel laminate. In certain embodiments, the release agent is a zinc oxide layer applied to the stainless steel laminate.

[0021] In certain embodiments, one or more of the steps i), ii) and iii) are connected in an in-line arrangement. In certain other embodiments, one or more of the steps i), ii) and iii) are adjoining independent steps. Also, in certain embodiments, the step i) is preceded by applying a release agent to the top surface of the fixture layer. In addition, in certain embodiments, the method includes conveying the fixture layer immediately after formation of the superstrate layer thereon to the PV cell formation step ii).

[0022] In another aspect, there is provided herein an apparatus for formation of a thin- film PV cell having a superstrate layer, the apparatus comprising:

an application part in which a superstrate layer is formed on a flexible

manufacturing fixture layer;

a forming part in which a semifinished PV cell is formed on the superstrate layer; and

a detachment part in which the flexible manufacturing fixture layer is removed from the superstrate layer such that the superstrate layer remains with the semi-finished PV cell assembly.

[0023] In another aspect, there is provided herein a method of making a diode structure comprising:

providing a flexible manufacturing fixture;

applying a polymer material onto the flexible manufacturing fixture; and introducing the flexible manufacturing fixture and polymer subassembly into a roll-to-roll manufacturing process.

[0024] In certain embodiments, the diode structure is a thin-film photovoltaic cell.

[0025] In certain embodiments, the diode structure is a thin- film photovoltaic cell having a conversion efficiency greater than about 8 percent.

[0026] In certain embodiments, the diode structure is a thin- film photovoltaic cell having a conversion efficiency greater than about 10 percent.

[0027] In another aspect, there is provided herein a photovoltaic cell according to any of the figures included herein.

[0028] In another aspect, there is provided herein a photovoltaic cell having a polyimide superstrate layer, a TCO/HRT bilayer, an n-type layer, a p-type layer, and a back contact, formed by any of the methods described herein.

[0029] In another aspect, there is provided herein a method of making a photovoltaic cell in a roll-to-roll manufacturing process comprising:

providing a flexible manufacturing fixture that is adapted to transport at least a polymer material through a manufacturing process;

applying a thin film of polymer material onto a surface of the flexible fixture to form a polymer- fixture laminate, the polymer material being selectively releasable from the flexible fixture;

sequentially transferring the polymer-fixture laminate through one or more processing stations for forming one or more layers of a PV cell on the polymer material of the polymer-fixture laminate; and

separating the polymer from the flexible manufacturing fixture, while retaining the layers of the PV cell adhered to the polymer.

[0030] In certain embodiments, at least one processing station includes a sputtering deposition station configured to form a layer of the photovoltaic cell.

[0031] In another aspect, there is provided herein a method of making a photovoltaic cell comprising:

attaching a polymer material onto a flexible fixture to form a polymer-fixture laminate;

sequentially transferring the polymer-fixture laminate through one or more processing stations configured to form at least a semi-finished photovoltaic cell;

sputter forming a first active layer onto the polymer material and transferring the flexible fixture and applied layers to a second sputtering deposition station;

sputter forming a second active layer onto the first active layer;

transferring the polymer-fixture laminate and the applied layers to a back contact assembly station;

laser scribing and electrically connecting a back contact to the second active layer; and

detaching the flexible manufacturing fixture from the polymer-fixture laminate at an interface between the polymer and flexible manufacturing fixture.

[0032] In another aspect, there is provided herein a PV cell that is fabricated onto a polyimide film in a roll-to-roll manufacturing process. The polyimide layer has a thickness that is optimized to provide high transparency and low radiation absorption. The polyimide layer is releasably attached to a flexible manufacturing fixture layer that has sufficient strength to carry the polyimide film through the cell manufacturing process. Additionally, the flexible manufacturing fixture may provide dimensional stability to the polyimide layer and the assembled cell from mechanical and thermal loads imparted by subsequent processes.

[0033] Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Fig. 1 is a schematic illustration of a process for making a photovoltaic cell that can be used for implementing certain embodiments of the present invention.

[0035] Fig. 2 is a schematic illustration of another process for making a photovoltaic cell that can be used for implementing certain embodiments of the present invention.

[0036] Fig. 3A is a schematic illustration of a portion of a process for making a photovoltaic cell that can be used for implementing certain embodiments of the present invention.

[0037] Fig. 3B is a schematic illustration of a portion of another process for making a photovoltaic cell that can be used for implementing certain embodiments of the present invention.

[0038] Fig. 4A is a schematic illustration of a first step in a process for making a photovoltaic cell.

[0039] Fig. 4B is a schematic illustration of a second step in a process for making a photovoltaic cell.

[0040] Fig. 4C is a schematic illustration of a third step in a process for making a photovoltaic cell.

[0041] Fig. 4D is a schematic illustration of a fourth step in a process for making a photovoltaic cell.

[0042] Fig. 5 is a schematic illustration of an embodiment of a flexible manufacturing fixture separation method.

[0043] Fig. 6 is a schematic illustration of another embodiment of a flexible

manufacturing fixture separation method.

[0044] Fig. 7 is a schematic illustration of another embodiment of a flexible

manufacturing fixture separation method.

[0045] Fig. 8 is a perspective view, partially assembled, of an embodiment of a flexible manufacturing fixture and PV cell subassembly.

[0046] Fig. 9 is a schematic illustration of a portion of a monolithic interconnection of a photovoltaic module showing an embodiment of an electron flow path.

[0047] Fig. 10 is a schematic illustration of a monolithic integration suitable for a flexible module on a polyimide film layer.

DETAILED DESCRIPTION OF THE PREFERRED EMB ODIMENT(S)

[0048] In a first aspect of the invention, there is described an apparatus for the manufacture of flexible, thin film photovoltaic cells. The apparatus comprises a flexible manufacturing fixture, a plurality of manufacturing stations, and a removal station. The flexible manufacturing fixture is configured to move between the plurality of

manufacturing stations and releasably retain a superstrate layer. The flexible fixture is also adapted to be reusable in order to receive a second superstrate layer. The plurality of manufacturing stations are configured to form layers of a thin-film photovoltaic cell (PV) on the superstrate layer such that the layers are in communication to form functional PV cells. The plurality of manufacturing stations may include, among other stations, a first station that is configured to form a transparent conductive oxide (TCO) layer onto the superstrate layer. Also, a second station may be configured to form a first active semiconductor layer, and a third station may be configured to form a second active semiconductor layer of the PV cell. The removal station is configured to separate at least a semi-finished photovoltaic cell from the flexible fixture such that the formed

photovoltaic layers maintain their applied relative physical orientations with adjacent layers.

[0049] In another aspect, the manufacturing apparatus may also include a fourth station configured to apply a back contact layer such that the back contact layer and the TCO are in electrical communication with at least one of the first and second active semiconductor layers. In another aspect, the flexible manufacturing fixture is a stainless steel foil that may include a release agent. The removal station includes a mechanical parting structure configured to separate the superstrate layer from the fixture. The mechanical parting structure may be a blade adapted to remove the release agent from at least one of the fixture and the superstrate layer or a vibrating roller configured to cleave a frangible release agent. Alternatively, the parting structure may be a fluid stream. The fluid stream may be configured to remove the release agent by dissolving and flushing the release agent from between the fixture and the superstrate layer. In one aspect, the fluid stream may be a water jet and the release agent may be a salt-based material. The release agent may , alternatively, comprise an oxide or nitride of at least one of Si, Al, Sn, and Zn, the superstrate being a polyimide material. In another aspect, the fluid stream comprises a methanol stream that is reactive with the release agent.

[0050] In another aspect of the invention, the superstrate layer may be a polyimide layer and the manufacturing stations may include an electrostatic generator that is configured to create a residual charge onto the fixture. The superstrate is selectively retained onto the fixture by the residual charge. In yet another aspect, the flexible manufacturing fixture is a stainless steel foil having a surface configured to imprint a textured surface finish onto the superstrate. The textured surface finish of the superstrate is adapted to scatter light admitted into the PV cell such that the PV cell output is greater than a PV cell without the textured surface finish. In certain embodiments, the flexible manufacturing fixture is a continuous belt and in other embodiments, the fixture is a coiled stainless steel foil having a pay-out spool and a take-up spool. Certain embodiments may include the first, second, and third stations configured as sputtering processing stations, along with at least one of a chloride activation station, a laser scribing and ink backfill station, and an encapsulation station.

[0051] According to a second aspect of the invention, there is described herein a flexible, thin- film photovoltaic cell The PV cell comprises a transparent conductive oxide layer, a back contact layer, n-type and p-type active semiconductor layers, and a superstrate layer. The TCO forms a first electrical contact, and the back contact layer forms a second electrical contact. The p-type active semiconductor layer is in contact with the n-type layer such that an electrical potential is formed from exposure to a light source. The superstrate layer is formed from a flexible material that is at least semi-transparent and has a characteristic imparted onto an outermost surface from a flexible manufacturing fixture. The fixture-imparted characteristic effects electrical performance of the n-type and p-type active semiconductor layers. In one embodiment, the superstrate layer has a first index of refraction and a release agent has a second index of refraction, and at least a portion of the superstrate layer includes a residual amount of the release agent. The characteristic imparted onto the outermost surface of the superstrate may be that the index of refraction of the release agent that is closely matched to the index of refraction of the superstrate such that residual release agent left after removal of the superstrate layer from the fixture is configured to enhance or not significantly affect the transmission of light through the PV cell. In another aspect, the superstrate layer has a first index of refraction, the release agent has a second index of refraction, and the outermost layer of the PV cell has a third index of refraction. At least a portion of the superstrate layer includes a residual amount of the release agent that is configured to function as an anti-reflective coating. The index of refraction of the release agent is generally near a geometric mean of the index of refraction of the superstrate layer and the outermost layer. In another aspect, the outermost layer is an encapsulation layer and the release agent is chosen from a plurality of release agents such that the chosen release agent has an index of refraction closest to the encapsulation layer index of refraction. In yet another aspect, the outermost layer is one of air or a vacuum, both air and the vacuum having an index of refraction of 1.00. In another embodiment, the characteristic imparted by the flexible fixture is a textured pattern that is configured to be transferrable from the fixture to the superstrate layer and further configured to promote light scattering to the active semiconductor layers. The textured pattern characteristic may be a geometric shape having a repeated pattern on an outer surface of the superstrate or a random roughness pattern that is transferred onto the superstrate from contact with the fixture, the random roughness being determinable by a surface roughness measurement.

052] In a third aspect of the invention, there is disclosed a method of producing a flexible, thin- film photovoltaic cell. The method comprises the steps of providing a flexible manufacturing fixture, applying a flexible superstrate layer onto the flexible manufacturing fixture, forming subsequent layers of the at least semi-finished photovoltaic cell onto the superstrate layer, and removing the at least semi-finished photovoltaic cell from the flexible fixture. The flexible manufacturing fixture is configured to be moved between a plurality of photovoltaic assembly operations, and is further configured to releasably retain layers of at least a semi-finished photovoltaic cell during assembly. The semi-finished PV cell is removed from the fixture such that the subsequent layers of the cell are configured to function to produce an electrical output. In an embodiment of the method, a release agent is applied onto the flexible manufacturing fixture prior to applying the flexible superstrate layer. In yet another aspect of the invention, the step of forming the subsequent layers of the at least semi-finished photovoltaic cell includes sputtering a transparent conductive oxide layer onto the superstrate layer and sputtering an n-type active semiconductor layer onto the TCO layer and sputtering a p-type active

semiconductor layer onto the n-type semiconductor layer. The step of removing the at least semi-finished photovoltaic cell includes providing a mechanical parting structure configured to cleave the release agent between the superstrate layer and the flexible fixture.

[0053] In yet another embodiment of the method of producing a flexible, thin-film photovoltaic cell, the n-type layer is a CdS active semiconductor layer and the p-type layer is a CdTe active semiconductor layer and a CdCl₂ treatment/annealing process is conducted after the active semiconductor layers are applied. In certain embodiments, the mechanical parting structure is a blade. In other embodiments, a fluid stream is provided that is configured to remove the release agent and cause the at least semifinished photovoltaic cell to separate from the flexible fixture such that the active semiconductor layers are configured to function to produce an electrical output. In another embodiment, the flexible fixture includes a textured surface and the steps of applying the flexible superstrate layer onto the fixture and removing the at least semi-finished PV cell from the fixture provides a mirror-image textured surface onto the outermost surface of the superstrate layer. In certain embodiments, the release agent is provided onto that flexible fixture as a salt and the parting structure is a fluid stream of a water jet. The water jet is directed toward the salt release agent to dissolve and flush the release agent from between the superstrate layer and the flexible fixture. IN other embodiments, the release agent may comprise one of an oxide or nitride coating of one of silicon, aluminum, tin, and zinc.

[0054] In another aspect of the method to produce flexible, thin-film PV cells, the steps of forming the active semiconductor layers include laser scribing the active semiconductor layers by forming an insulating scribe onto the TCO layer after the TCO sputtering step, then forming a via scribe onto the CdS and CdTe layers after the active semiconductor layer sputtering steps, and applying a back contact and then forming a back contact insulating scribe onto the back contact. In certain embodiments, forming the TCO insulating scribe includes backfilling the insulating scribe with an insulating ink and the step of forming the via scribe includes backfilling the scribe with an electrically conductive ink. The method also includes, in certain embodiments, the step of forming electrical output leads onto the TCO and the back contact layers. Forming the electrical contacts and connections is conducted after the step of scribing the back contact. The step of encapsulating the at least semi-finished PV cell includes providing an encapsulant and applying the encapsulant on at least the back contact layer and sides of the semi-finished PV cell.

[0055] Photovoltaic (PV) cells rely on a substantially transparent or translucent front layer to admit solar radiation and to provide protection for the underlying cell layers. The front layer is sometimes described as a superstrate layer if the cell active layers are assembled onto the front layer. If the PV cell layers are assembled onto, for example, a back contact layer, the transparent front layer is applied last. Then, the back contact is termed a substrate layer. For purposes of this application, the terms superstrate and substrate will refer to the layer that is applied onto a flexible manufacturing fixture, as will be described below, and the terms may be used interchangeably. Described herein is an improvement over PV cells that rely on rigid materials as the transparent front layer material.

[0056] In a broad aspect, polymer materials are used as an alternative medium to glass. While certain polymer materials may be less transparent (e.g., some having light transmissive characteristics in the blue and green wavelengths), certain polymer materials have greater flexibility and reduced weight than glass materials. In particular, polymer films, such as for example polyimide films, can be made sufficiently thin which improves the optical transmissibility of light to the PV cell active layers and which reduces material cost. The term polymer, for purposes of this application, will refer to carbon-based polymers such as, for example polyimides, polycarbonates, polyesters, and non-carbon- based materials, such as silicon-based resins.

[0057] In another aspect, there is provided herein a PV cell that is fabricated on a transparent polymer superstrate. In certain embodiments, the PV cell can be fabricated using a magnetron sputter deposition process to form the semiconductor layers.

Improvements to the performance of certain layers, some of which are deposited by magnetron sputtering onto polyimide superstrates or substrates, may be realized over those described in U.S. Patent No. 7,141,863 to Compaan et al. entitled "Method of Making Diode Structures," the disclosure of which is incorporated herein by reference in its entirety. These improvements relate to processing techniques to assemble the stack arrangement, or specific layer composition and orientation that has been developed beyond the disclosure of '863 patent, as described herein.

[0058] Referring now to Fig. 1, there is presented a schematic illustration of an apparatus 10 useful for carrying out a method for producing photovoltaic cells 12, of the type shown in Fig. 4D. It is to be understood that Fig. 1 is being shown for illustrative purposes and the other steps and/or processes can be practiced with the inventive method described herein.

[0059] Fig. 1 illustrates an apparatus 10 for a batch run roll-to-roll process where a flexible manufacturing fixture 14 is supplied on a pay-out spool 15a. In one embodiment, the method includes the use of a roll-to-roll manufacturing process wherein coiled materials may be supplied on spools and drawn into process equipment of the apparatus 10 by handling machinery (not shown). The handling machinery may push, pull, or compress the coiled material in order to transfer it to subsequent processing stations. The coiled materials need to have sufficient strength and flexibility to resist damage from the handling process and adequately support the built-up layers of the PV cell 12.

[0060] The flexible manufacturing fixture 14 is a generally thin, flexible material that is capable of supporting various PV cell layers through the various processing stations as the PV cell is being constructed, as will be further described herein in detail.

[0061] In the embodiment shown in Fig. 1, the flexible manufacturing fixture 14 is fed into the apparatus 10 where a polymer material 20 is applied onto an outer surface 18 of the flexible manufacturing fixture 14 to form a polymer-fixture laminate 22. The polymer material 20 can be applied by various suitable processes, some of which are described herein. Alternatively, the flexible manufacturing fixture 14 may be supplied to the apparatus 10 with the polymer material 20 (and, optionally, any other coatings or release agents) already formed as a sub-assembly in an offline process.

[0062] The flexible manufacturing fixture layer 14 acts as a medium to transfer the applied polymer 20 through the manufacturing process. The flexible manufacturing fixture 14 is configured to withstand the various loads imparted by the manufacturing processes used to form the PV cell 12. The flexible manufacturing fixture 14, however, may be any material having sufficient strength, flexibility, thermal properties (i.e., melting point, thermal expansion, and the like), and dimensional stability (i.e., strain, thermal expansion rate, and the like) to support the polymer material 20 throughout the subsequent cell manufacturing processes. In one embodiment, the flexible manufacturing fixture 14 is a stainless steel laminate. Alternatively, the flexible manufacturing fixture 14 may be made from foil or sheet material of stainless steel, copper, or aluminum. Alternatively, the fixture 14 may be made from resin-impregnated carbon fiber or fiberglass sheet material, or high temperature polymers.

[0063] As the polymer- fixture laminate 22 is moved through various processing stations 40, 50, 60, 70 of the apparatus 10, the PV cell is formed on the polymer material 20 comprising the polymer-fixture laminate 22. The processing stations may include a sputtering station 40 configured to apply a transparent conductive oxide (TCO) layer as a transparent front window/contact, a second station to form an n-type layer, such as a CdS active layer, and a third station to form a p-type active layer such as a CdTe layer. Other stations may also be included that are sufficient to form the completed cell 12 or a semifinished PV cell 30.

[0064] After a desired number of processing steps are completed such that at least the semi-finished PV cell 30 is formed on the polymer-fixture laminate 22, the flexible manufacturing fixture 14 is separated from the polymer-fixture laminate 22. The polymer 20 of the polymer-fixture laminate 22 remains with the semi-finished PV cell 30 such that a mostly-finished PV cell can be formed, either online or offline of the apparatus 10. As schematically illustrated in Fig. 1, in certain embodiments of the roll-to-roll manufacturing process, once the flexible fixture 14 is separated from the polymer-fixture laminate 22, the fixture 14 can be recoiled on a take-up spool 15b for recycling and/or reprocessing.

[0065] The polymer material 20 has desired light transmission characteristics, along with desired flexibility and flexural strain characteristics to function with a flexible thin- film photovoltaic cell 12, such as a Cd-based thin film cell. In certain embodiments, the polymer material 20 comprises a polyimide material. One example of a polyimide is a Kapton® polyimide material made by DuPont. In other embodiments, the polymer material 20 may be comprised of other suitable carbon-based polymer materials, such as polycarbonate. In yet other embodiments, the polymer material 20 may be a non-carbon derived material, such as a silicon-based resin.

[0066] The embodiment of Fig. 2 illustrates a continuous belt, roll-to-roll

manufacturing apparatus 100 where a flexible manufacturing fixture 114, similar to the flexible manufacturing fixture 14 described above, forms a continuous loop. A polymer material 120 may be cast onto the flexible manufacturing fixture 114, either in the direction of or against the force of gravity, or applied as a separate sheet material, thus forming a polymer-fixture laminate 122. The continuous process apparatus 100 may also employ one or more of the cell manufacturing processes described herein. After the semifinished PV cell 130 is formed on the polymer- fixture laminate 122, the flexible manufacturing fixture 114 is separated from the polymer layer 120 of the polymer-fixture laminate 122. The flexible manufacturing fixture 114 may be moved to a cleaning and preparation station 124 to ready portions of the fixture 14 for application of polymer material 20.

[0067] In certain embodiments, at least an outer surface 118 of the flexible

manufacturing fixture 114 can be prepared for application of the polymer material 120. As shown in Fig. 2, the PV cell manufacturing apparatus 100 includes the fixture preparation station 124 configured to prepare the fixture 114 to accept and releasably retain the applied polymer material 120. A cleaning station, similar to the preparation station 124, may also be applied to the batch-run process 10 of Fig. 1. In one embodiment of a preparation operation, an outer surface 118 of the fixture 114 is cleaned (for example, by chemical, mechanical, and/or ultrasonic cleaning) and coated, if desired, with a retention coating or a release agent 320, as shown in Fig. 6. The polymer material 120 is then applied to the surface 118 of the flexible manufacturing fixture 114 to form the polymer-fixture laminate 122 as a generally continuous layer.

[0068] Referring now to Fig. 3A, there is illustrated a schematic view of a processing station, such as processing station 40, in the roll-to-roll manufacturing process for constructing a PV cell or a semi finished PV cell 30. In one embodiment, the processing station 40 is a sputtering process used to build up conductive (i.e., a transparent conductive oxide layer or front contact) and active layers (i.e. p, i, and n layers) of the PV cell. The sputtering process may be, for example, an RF magnetron sputtering process; other processing stations, such as processing stations 50, 60, 70, and 80, may include active layer doping, elevated temperature CdCl₂ annealing, laser scribing, back contact application, and encapsulation, to name a few. Additionally, one or more stations may be differentially pumped vacuum isolation stations. As many processing stations may be disposed in the apparatus 10 or 100 as may be needed to construct at least the semifinished PV cell 30 or 130.

[0069] As the polymer-fixture laminate 22 moves to the sputtering station 40, a transparent conductive oxide (TCO) layer 32 is formed onto the exposed surface of the polymer layer 20 of the polymer-fixture laminate 22. The sputtering process may be conducted using known operational parameters in a conventional manner to apply a sputtered material 210 to form the TCO layer 32. The TCO layer 32 may be formed from one or more of ZnO, ZnS, CdO, Sn0₂:F, In₂0₃:Sn (ITO), and CdSn₂0₄. Additionally, the step of forming the TCO layer 32 may also include forming a highly resistive transparent (HRT) layer. The sputtered material 210 is part of a plasma that impinges the polymer surface to form the TCO layer 32 that is shown in Fig. 4B.

[0070] The polymer-fixture laminate 22 then moves to the next operation at processing station 50. In one embodiment, the processing station 50 may be configured to apply a cadmium sulfide (CdS) layer forming an n-type semiconductor active layer 34 onto the previously deposited TCO layer 32, when constructing an n-p or an n-i-p cell arrangement as shown in Fig. 4B. For example, a partial listing of suitable materials such as ZnSe, ZnS, ZnTe may be used instead of CdS. The station 50 may apply the n-type layer 32 by any suitable process, such as sputtering, plating, sintering, various vapor deposition processes, and the like. The next station 60 may be configured to apply a cadmium telluride (CdTe) layer onto the CdS layer when constructing the n-p or n-i-p cell configurations. Other materials such as, for example, zinc telluride that is doped with nitrogen (ZnTe:N) may be used as an alternative to CdTe. Additionally, other materials and material/dopant combinations may be used. The materials, such as for example ZnTe, may be doped by nitrogen gas during a reactive sputtering process.

[0071] When forming an n-i-p cell, an intrinsic layer processing station may be disposed between station 50 and station 60. In another embodiment, the n-p or n-i-p layers may be formed simultaneously in a single station with suitable dopants added during the processing cycle to form the n-, i-, or p-type layers. Alternatively, the processing stations for the semiconductor active layers may be reversed when forming p-n or p-i-n cell configurations.

[0072] Once two active layers, such as the CdS layer 34 and the CdTe layer 36, have been applied, the polymer-fixture laminate 22 with the deposited layers 32, 34, and 36 may move to a processing station 70 where a process, such as a CdCl₂ treatment and/or annealing, may be conducted to passivate the active layers. Once the CdCl₂ treatment and/or annealing process has finished, the fixture and layers may move to a laser scribing station, a back contact application station 80, or an encapsulation station prior to removal of the cell from the fixture 14. Though the fixture 14, and its attendant layers, have been described as being moved to various stations, the progression of the fixture and layers may be one of a constant speed or the fixture may stop at a station for the processing period. Alternatively, the stations 40, 50, 60, or 70 or others may move with the fixture 14 in order to maintain relative orientation of the processing tools in the stations to specific portions of the cell.

[0073] The embodiment of Fig. 3B is a schematic view of an alternate embodiment of a roll-to-roll processing station, such as processing station 140, similar to processing station 40 described above. The processing station 140, when configured as a sputtering station, includes a heat source 250. The heat source 250 may be used to improve the amorphous or polycrystalline structure, consistency and adhesion of the sputtered layer onto the previously deposited layer.

[0074] Figs. 4A-4D schematically illustrate a general process for constructing a photovoltaic cell using a polymer- fixture laminate, as described herein.

[0075] In a first step shown in Fig. 4A, the polymer material 20 is first cast or otherwise applied onto the flexible manufacturing fixture 14. The polymer casting process is generally characterized by application of the polymer in a fluidic state, such as a liquid or a thixotropic paste, onto the flexible manufacturing fixture 14. For example, referring again to Fig. 1, a knife edge 16, such as for example a doctor blade, can be used to evenly distribute the polymer material 20 over the surface 18 of the flexible manufacturing fixture 14. In one embodiment, the knife edge 16 may be a physical blade or roller device that is spaced apart from the surface of the flexible manufacturing fixture. In another embodiment, the knife edge 16 may be a fluid stream (such as, for example heated air) that is directed across the surface of the polymer material 20. The knife edge 16 is

subsequently drawn (in a squeegee-like manner when using a physical blade), moved, or directed over the polymer material 20 to create a thin film of material. The polymer material 20 may be applied to the surface 18 of the flexible manufacturing fixture 14 by other suitable processes, such as, but not limited to, spraying, co-extruding, or as co-linear sheets of material that are brought together as the materials are paid out.

[0076] Fig. 4B illustrates a second step in the process where layers of the thin-film PV cell, described above, are applied onto the polymer surface of the polymer-fixture laminate 22. In certain embodiments, specific layers of the PV cell may be applied by any suitable process such as, for example, by sputtering to apply the active n- and p- layers and the back contact, or by a collinear extrusion process to apply the back contact or the encapsulation layer. For example, referring again to the embodiment of the method illustrated in Figs. 3 A and 3B, the sputtering source 200 applies certain layers of the PV cell, such as the active layers, against the force of gravity. Such an orientation permits the polymer surface to remain free of dust and other contamination that may fall onto the target surfaces for sputtering. Alternatively, the sputtering process may be conducted in the direction of the force of gravity or at any angles relative thereto, if desired. The process of forming the various active PV layers may be any suitable process.

[0077] As shown in Fig. 4B, the PV cell, or an array of cells, may be constructed by being deposited onto the polymer material 20 of the polymer- fixture laminate 22. In one example, the TCO layer 32 forms the front electrical contact and is formulated to allow light to pass through to the active layers 34, 36 below to release electrons, thus creating a voltage and current flow. In another embodiment, the PV cells 12 may be fabricated using sputtered zinc oxide doped with aluminum as the TCO 32. Other materials may be used in the TCO layer 32 such as, for example, indium tin oxide, cadmium tin oxide, tin oxide, and the like.

[0078] In certain embodiments, a highly resistive transparent (HRT) layer may be applied between the TCO 32 and the first active layer 34 to form a bilayer. The HRT layer made of an undoped ZnO material or AI2O₃ material to provide both an electrical isolation function and a chemical diffusion barrier function. For example, in one embodiment, the TCO/HRT bilayer may use a ZnO:Al/ZnO bilayer where the ZnO:Al portion functions as the TCO layer and the undoped portion of ZnO functions as the HRT layer.

[0079] Next, active layers of CdTe 36 and CdS 34, for example, are deposited onto the TCO 32 to form the p-type and n-type layers. The CdS and CdTe layers 34 and 36 may also be deposited by way of the sputtering process. An intrinsic, or i-type, layer may be deposited between the n- and p- layers. Additionally, multiple sputtering stations can be positioned to create multiple layered or tandem PV cells.

[0080] Other processes and/or fabrication steps may be interposed at appropriate points along the manufacturing line to form the various PV layers. Examples of such steps include: (i) doping of the CdTe layer with a suitable dopant, such as for example copper, (ii) a CdCl₂ treatment, which may be performed at approximately 390°C for a time that ranges from 5 to 30 min, depending on the thickness of the CdTe layer, and (iii) a back contact treatment process involving deposition of 10-50 A Cu layer followed by a 5-30 min anneal at 150°C for in-diffusion of the Cu, the processing parameters of which may also depend on the CdTe thickness. These process steps are provided as illustrative examples and are not intended to be an exhaustive list of PV cell process steps.

Additionally, stations may be positioned at appropriate points along the line for scribing the active layers and applying the back contact, if desired. The scribing process may also be interposed between the various sputtering stations to create series or parallel electrical connections for tandem cell construction, similar to the cell of Fig. 9.

[0081] Referring now to Fig. 4C, an encapsulant or encapsulation layer 39 can be applied to the semi-finished PV cell 30 to protect the PV cell 12 from damage and exposure to weather and the elements. The encapsulant may be any suitable material to seal the PV cell. Non-limiting examples of suitable encapsulant materials include resins, sealants, plastics and/or polymers such as, for example, polyimide, polyvinyl chloride, vinyl ester resin, urethane, phenolic resins, and the like.

[0082] While the step shown in Fig. 4C is shown herein as being before the step shown in Fig. 4D, it is to be understood that, in certain embodiments, the encapsulation process may be conducted subsequent to the step shown in Fig. 4D, as will be further understood after reading the explanation of the step shown in Fig. 4D. Alternatively, the encapsulation and/or back contact may also be applied in an offline process in this step of the process.

[0083] Referring now to Fig. 4D, as the assembly of the active layers of the PV cells is completed, the semi-finished PV cell is removed from the flexible manufacturing fixture. As shown in Fig. 1 and Fig. 2, a separation station 28, 128 or removal point is positioned at or near the end of the roll-to-roll manufacturing line. The separation station 28 removes the finished, or semi-finished, PV cell from the flexible manufacturing fixture.

[0084] Referring now to Figs. 5-7, there are illustrated non-limiting examples of separation mechanisms that may be used at the separation station. In the embodiment of Fig. 5, a mechanical parting operation 300 includes a parting structure 310, shown as a blade, wedge, or other tapered device. In one embodiment, the blade 310 may peel the flexible manufacturing fixture 14 or 114 directly away from the polymer- PV cell structure 30. In this instance, the blade 310 may be positioned between the fixture 14 or 114 and the polymer 20 or 120. Alternatively, the blade 310 may cleave the polymer 20 or 120 such that a residual thickness of polymer material 20 is left on the fixture 14, to be removed later. Such a cleaving operation may provide a smooth surface finish or a washboard surface finish on the outermost surface of the polymer layer 20. The blade 310 may cleave the release agent material 320 allowing the polymer layer to fall away from the flexible manufacturing fixture 14. A cleaning process may be applied to both the polymer layer 20 and the flexible manufacturing fixture 14 if residual release agent 320 remains on either or both surfaces. [0085] In another embodiment, a release agent 320, disposed between the flexible manufacturing fixture 14 and the polymer layer 20. In one embodiment, the release agent 320 may be in the form of a somewhat brittle or otherwise frangible material. In another embodiment, the release agent 520 may have an index of refraction sufficiently close to an index of refraction of the superstrate material 20 such that any residual release agent 320 left after the parting operation does not significantly affect the transmission of light through the PV cell. In yet another embodiment of the PV cell 12, the residual release agent 320 is selected to be an anti-reflection layer having an anti-reflective characteristic. The residual release agent 320 has an index of refraction that ideally is the geometric mean of the index of refraction of layers adjacent to the residual release agent 320. In another variation of this embodiment, the index of refraction of the release agent may be generally near the value of the geometric mean of the adjacent layers such that the residual release agent assumes the characteristics of an anti-reflective coating when the PV cell is assembled. The adjacent layers may be the superstrate layer and an outermost layer of the PV cell 12 that is either an encapsulation layer or the adjacent environment. The adjacent environment can be air or a vacuum, such as in space, each having generally an index of refraction of one (1). The geometric mean may be calculated as the square root of the product of the adjacent refraction indices.

[0086] As shown in Fig. 6, the PV cell may be separated from the flexible

manufacturing fixture 14 by way of mechanical or ultrasonic vibrations generated by a vibratory shaker 350. In one embodiment, the vibratory force may excite the structure at a resonant frequency of the polymer 20 to flexible fixture 14 interface, causing the materials to separate. In another embodiment, the vibrations may cause the frangible release agent 320 to crack or otherwise cleave allowing the layers to separate. The vibratory shaker 350 may be any structure capable of imparting an oscillatory force onto the flexible fixture 14 and PV cell assembly 30 such as, for example, a roller at the separation point in the process. Alternatively, the blade 300 of Fig. 5 may be a vibrating parting blade or may be used in conjunction with the vibratory shaker 350 of Fig. 6.

[0087] Referring now to Fig. 7, an alternative embodiment of a separation station 400 or point may include a focused stream 410 acting at the polyimide to flexible fixture interface. In one embodiment, the focused stream 410 may be a fluid stream of water or air (or any other gas) in the form of a fluid knife. Such a stream may cut or otherwise abrade a portion of the polymer layer or abrade or dissolve and flush the release agent layer. Alternatively, the focused stream 410 may be a solvent such as, for example, methanol that is reactive with a release agent 420 to permit separation at the polymer 20 to flexible fixture 14 interface. Yet another embodiment of a focused stream 410 may be a laser beam, electron beam, or other energy stream capable of localized excitation of the polymer to flexible fixture interface or the release agent layer 420.

[0088] As shown in Fig. 8, there is another embodiment of a flexible manufacturing fixture 514 that may include a coating 540 that facilitates selective retention of a release agent 520 or the polymer material 20. In one embodiment, the release agent 520 may be an oxide or nitride coating of silicon, aluminum, tin, and zinc such as, for example, a ZnO release agent. In another embodiment, the coating 540 may be applied to the fixture 514 and configured to permit retention of the release agent and provide easier cleaning of the fixture 514.

[0089] The coating 540, or the flexible fixture 514 itself, may further have a textured finish, shown generally at 560 applied to the surface. The textured finish 560 is transferable as a mirror-image 565 to the superstrate layer 20 to provide greater collection of solar radiation. The textured finish 560 may be in the form of a patterned shape such as, for example, honeycombs, hexagons, triangles, and the like. The pattern may be a repeated pattern formed on an outer surface of the superstrate 20. Alternatively, the textured finish 560 may be a random surface roughness applied to the fixture 14, for example, as may be determined by an Ra, Rz, and the like measurement characteristic.

[0090] Selection of the appropriate flexible fixture material or structure as the fixturing embodiment may be driven by the specific chemistry of the superstrate 20. Such selection criteria may involve, for example, balancing the glass transition temperature (T_g), the mechanical properties of the polymer material 20 before and after RF sputtering exposure, and the surface texture characteristics of the polymer 20 exhibited after casting, or application as a thin film sheet onto the flexible fixture 14 or 140, and prior to RF sputtering exposure. In balancing the factor of surface texture, greater fine scale roughness may improve adhesion of subsequently deposited layers. However, large scale roughness or large asperities may cause pinholes and shorts within the cell structure thus reducing output performance.

[0091] The release agent 320, 420, or 520 may be disposed between the flexible fixture 14 or 140 and the polymer layer 20, as shown in Figs. 5-8. The release agent may be an adhesive; a salt, for example NaCl or any other salt; or other compound that is soluble in water or a solvent to permit separation of the polyimide from the flexible fixture. The release agent may function as a fixturing material to retain the polyimide film onto the flexible fixture. The release agent may work in conjunction with one or more separation mechanisms to permit removal of the flexible fixture without damage to the polyimide layer or the PV cell generally.

[0092] Referring again to Fig. 1, in certain embodiments, the polymer material may be retained onto the flexible fixture by an electrostatic charge, applied by an electrostatic generator 90, to the flexible fixture. The electrostatic generator 90 may be positioned proximate the flexible fixture 14 to induce a charge potential on the surface. A

downstream electrostatic absorber (not shown) may nullify or otherwise eliminate the charge in order to release the assembled PV cell 30 from the flexible fixture 14. Such a polymer retention method is also applicable to the process shown in Fig. 2.

[0093] In a non- limiting example of a structure of a PV module 600, as shown in Fig.

9, a polymer layer 620 forms a superstrate layer 630. The polymer layer, which may be a polyimide film layer, is shown oriented as a superstrate or first layer 630 of the photovoltaic cell 600. The PV module 600 includes a TCO layer 632, a CdS n-type layer 634, an i-type layer 635, a CdTe layer 636, and a back contact layer 638. The PV module 600 further includes scribes 640 and a bridge 644 that alter the electrical communication pathways with in the module 600. In one embodiment, the scribes 640 and the bridge 644 may be arranged to form a monolithically integrated PV module. The electrical communication is illustrated schematically by arrows 650 showing electron movements within the cell from their release within the active semiconductor layers 634, 635, and 636 to the front and back contact layers 632 and 637, respectively.

[0094] It should be understood that to form a tandem cell, the polymer layer 620 may be positioned at other points on the cell such as, for example, between the back contact 638 and a front layer of an adjoining cell (not shown). In this example, the back contact 638 would be a transparent conducting layer, when configured as the uppermost module of a tandem cell. In an alternative embodiment of the photovoltaic cell, the polymer layer is an electrically conductive polymer layer that forms a back contact of the cell.

[0095] Examples

[0096] The inventors herein have shown that the active semiconductor coatings that form the heterojunction CdS/CdTe show improved performance characteristics when the back contact is formed last. This requires the overall structure to have the superstrate configuration. That is, the cells or modules are oriented, in operation, so that sunlight enters through the superstrate layer, which is transparent or translucent.

[0097] While the traditional choice of a material for the superstrate layer is glass, since the active coatings for the PV cell are usually deposited at temperatures of about 550°C to about 650°C, the inventors show herein that coatings may be deposited at much lower temperature on transparent polymer material, than on glass.

[0098] In contrast, the polymer-based layer described herein provides a light-weight and flexible PV cell. In addition, the low weight and flexibility of such PV cells provides a variety of advantages over the rigid and heavy glass-based modules, while still retaining the performance of the polycrystalline CdS/CdTe PV junction.

[0099] Also, a separable polymer-fixture laminate structure ("laminate") provides a practical solution for implementing high volume photovoltaic production.

[00100] In one embodiment, the laminate is comprised of a thin metal foil flexible fixture and a polyimide polymer layer that are detachably adhered, or laminated, together.

The laminate has release-ability characteristics that allow the metal foil flexible fixture to be removed from the polyimide polymer layer after most of the fabrication of the PV module is completed.

[00101] The use of the polymer-fixture laminate allows for the deposition of PV film layers on large-area polyimide films since the manufacturing of flexible CdTe-based modules can be attainable while the polyimide window layer is still attached to the metal flexible fixture.

[00102] The removal of the metal foil fixture provides a PV cell structure that is at least semitransparent. Combined with the excellent thickness control available through magnetron sputtering, this allows for the production of PV cells that can use much of the availably light but still be sufficiently light transmissive for architectural use.

[00103] Semi-transparent PV module

[00104] In one example, a semitransparent PV module can include an electrically

conducting, but transparent, back contact to the CdTe PV cell. In such an embodiment, the polyimide superstrate and the front contact are also transparent, thus permitting some light to pass through the PV cell, especially for thin layers of CdTe.

[00105] The use of the fixture-polymer laminate allows for the production of a very thin layer of polymer which, in turn, allows for the light transmissiveness of the PV cell. In certain embodiments, PV cells can be fabricated with CdTe layers having a thickness of only about 0.5 μιη that still can operate with 10% efficiency and still transmit about 5% of the light through the entire structure. In other embodiments, PV cells thinner than about 0.5 μιη can transmit more light at some sacrifice of efficiency.

[00106] Monolithic Integrated Modules [00107] The polymer-fixture laminate is useful in the manufacturing of a monolithically integrated flexible module based on thin-film silicon (tf-Si). In the past, individual cell modules were formed from large-area tf-Si multi-layer structures in a separate

manufacturing step that involved the post-manufacturing interconnection of many individual PV cells into higher voltage modules under 100 volts. While this two-step process provides excellent flexibility to design modules most suitable for different applications, the present invention provides further and distinct advantages by allowing for the production of a module which is monolithically-integrated into a high-voltage output above 100 V.

[00108] The polymer-fixture laminate and the processes described herein also provide:

1) improvements to the robustness of manufacturing of CdTe -based PV modules through the use of a metal foil/polymer laminate structure in a roll-to-toll (RTR) process that allows the metal to be removed before module encapsulation; 2) a semitransparent module for window PV; and 3) a RTR production line for light-weight and flexible CdTe-based PV modules and develop detailed production cost models.

[00109] In one method of the present invention, a roll-to-roll (RTR) manufacturing process uses a polyimide layer releasably attached (i.e., temporarily adhered) to a metal foil to provide an improvement to the fabricating process of a transparent conducting oxide(TCO)/CdS/ CdTe/(back contact) cell structure. In certain embodiments, a very long (>lkm) and wide (~lm) laminate can be used to facilitate the high volume production in the RTR process.

[00110] In one embodiment, the PV sub-modules, while attached to the polymer-fixture laminate, are monolithically integrated by using a laser scribing and ink jet backfill process. Such methods can also produce a semi-transparent PV cell array suitable for window applications.

[00111] Handling of Polyimide Materials using Metal Flexible manufacturing fixture [00112] In another embodiment, there is described herein an improved method for handling of the polyimide material during processing. The processing steps include: 1) heat-up in vacuum to the deposition temperature of about 250°C followed by the sputter deposition of ZnO:Al, CdS, and CdTe layers; then 2) activation treatment at about 390° C in dry air plus saturated vapors of CdC^; followed by 3) vacuum deposition of the metal back contact; and, 4) final heat treatment near 150°C in air to achieve good ohmic contact.

[00113] In certain embodiments, the method may further include one or more

appropriate interlayer coatings that are applied to the metal flexible manufacturing fixture. During the PV cell fabrication process, the interlay er coating is between the metal flexible manufacturing fixture and polyimide material. The interlayer can act both as a temporary adherent and as a release agent to facilitate removal of the polyimide layer (and the built- up PC cell structure thereon) from the metal flexible manufacturing fixture without damaging the flexible PV cell structure.

[00114] Also, in certain embodiments, the delaminated coated metal foil fixture is sufficiently undamaged by the delamination step to be recycled and reused in further cycles of the manufacturing process of the PC cells.

[00115] The metal foil fixture can be configured to be compatible with the pay-out, transport, and take-up systems needed for a RTR manufacturing line. In one embodiment, the metal foil material may a stainless steel laminate foil material.

[00116] Example of Fabrication Sequence

[00117] The polymer-fixture laminate (comprised of a polyimide film applied to a stainless steel metal foil) supports the steps in the fabrication sequence of CdS/CdTe PV modules. These steps can include: 1) the deposition at ~250°C of a suitable transparent conducting (oxide) layer (TCO) on the polyimide (in one embodiment the TCO layer is ZnO:Al); 2) a high resistivity buffer layer (also called the high resistivity transparent (HRT) layer); 3) the deposition at ~250°C of the active semiconductor layers of CdS and CdTe; 4) an activation step usually involving temperature near 390°C in the presence of CdCl₂, and 5) finally application of a back contact through a metallization process.

[00118] Following this sequence of cell fabrication steps, the metal lamination layer is removed from the polyimide film without damaging the polyimide or the PV-cell layers. Thus, the fabrication of the complete submodule includes the deposition of all the PV cell layers (e.g., TCO/HRT/CdS/CdTe/back contact) and the cadmium chloride activation step.

[00119] Monolithic integration with Polymer-fixture Laminate in Place

[00120] In yet another embodiment of an RTR manufacturing method, one step in the method can include providing for the monolithic integration of individual PV cell strips into high voltage modules while the polymer-fixture laminate is intact and the

polyimide/cell structure is still attached to the underlying metal foil fixture.

[00121] In this embodiment, the metal foil fixture provides extra support, or stiffening, of the semi-finished PV cell in order to protect the thin film of polyimide that will eventually be the window layer of the finished PC cell. In this embodiments, the presence of the flexible fixture layer in the subsequent process steps acts to assist in accurate focusing and in maintaining dimensional stability. [00122] The module integration steps can be done after all active PV layers are deposited using, for example, a high power, high repetition rate, pulsed laser to cut through the top TCO layer to achieve good electrical isolation of individual cells from the damaged cut edges.

[00123] For example, there is schematically illustrated in Fig. 10, a PV cell 700 having a plurality of scribes configured to provide monolithic integration of a plurality of cell modules into a PV cell having a larger electrical output, either based on current or voltage or both. The PV cell 700 includes a superstrate layer 720, a TCO layer 732 that is shown separated into, for example, three segments 732a, 732b, and 732c. A first active semiconductor layer 734 is illustrated, for example, separated into three segments 734a, 734b, and 734c. A second active semiconductor layer 736 is also illustrated as being divided into three segments 736a, 736b, and 736c. Finally, a back contact layer 738 is also illustrated as being divided into three segments 738a, 738b, and 738c. Finally, an encapsulation layer 739 is formed partially around the PV cell 700. Alternatively, the encapsulation layer may be formed around the entire cell 700. It should be understood that the three segments are shown for illustration purposes and that any number of segments may be formed.

[00124] The monolithic integration of PV cells, such as CdTe-based cell 700, can be done with three sequential scribes: 1) an isolation scribe 750 is done after TCO deposition; then 2) a via (scribe 752) is opened after CdS/CdTe deposition; finally, 3) another isolation scribe 754 is done after back contact metallization. The scribes 750, 752, and 754 create separate cell modules within the larger cell 700 that are electrically interconnected by appropriate applications of conductive and insulating materials such as conductive and insulation inks applied in a backfill process. The insulating scribes 750 are filled with an insulating ink and form the separate TCO /front contacts 734a, b, and c. Similarly, the back contact 738 is scribed into the back contact segments 738a, b, and c by way of the insulating scribes 754 which are similarly configured and filled as the insulating scribes 750. The via scribe 754 electrically connects the TCO contact 732b with the back contact 738a to from a cell module comprising the active layers 734b, 736b such that the electrons released by the active layers flows through the TCO contact to the back contact 738a which is connected to another one of the cell modules, either in parallel or in series to form the monolithically integrated PV cell 700.

[00125] In such flexible cell modules manufacturing process that incorporates the roll- to-roll process described herein, the monolithic integration can be accurately and conveniently done after most or all PV-cell layers have been deposited. This can be done through the use of laser scribing, followed by an insulating backfill of PI and a conducting backfill of P2.

[00126] The presence of a relatively stiff metal backing to the polyimide provides at least several advantages. For example, if the metal flexible fixture is magnetic or magnetized, the fixture can provide a hold-down method to stabilize the coated polyimide during the scribing process.

[00127] The metal fixture can also facilitate the handling and the use of thinner

polyimide films which, in turn, reduces the module materials costs and improves light transmission through the superstrate layer.

[00128] In one embodiment, the scribing of the ZnO:Al layer can be done with a 355 nm wavelength to achieve sufficient optical absorption using, for example, a galvanometer scanned mirror system with this wavelength. Additionally, the galvo-scanned laser system can be used in a process of "edge deletion" in which some of the film deposition is removed from the edges of the module to improve the electrical isolation at the module edges. Edge deletion can improve adhesion and moisture barrier performance of encapsulants or sealants at the module edges.

[00129] Uses in Architectural Windows and/or Vehicle Windows

[00130] PV cells produced by the methods described herein can be made for use in architectural windows where it is desired that such windows be at least translucent, if not transparent. For example, these PC cells are especially appropriate for window applications and are well-suited for urban sites with tall buildings having large amounts of glazing and little unobstructed roof areas. Also, in other embodiments, these PV cells are especially appropriate for applications in vehicles including, but not limited to skylights, side and rear windows, panels in roofs, trunks, hoods, etc.

[00131] In such embodiments, the PV cell comprises a semitransparent, monolithic ally integrated PV module. The PV cell can have a reduced thickness of a CdTe layer (e.g., on the order of approximately to 0.5 μιη) and a transparent back contact structure, such as ZnTe:N, that facilitates, together with the thin CdTe, the fabrication of flexible, semi- transparent PV cells. In certain embodiments, a reactive magnetron sputtering can be used to form these transparent contact structures.

[00132] The flexible, thin-film CdTe-based PV modules on polymer are semi- transparent and may exhibit operational characteristics on the order of -7% conversion efficiency with about 5% transparency and good color balance on polyimide sheet in a roll-to-roll deposition process. In other embodiments, a higher transparency may be achieved with somewhat lower PV efficiency.

[00133] Integration with AC Power Grid

[00134] The PV modules described herein can be monolithically integrated to yield high voltage output (60-240VDC) for ease of installation and for simple inversion to AC power for grid-connected operation.

[00135] Use with DC Power

[00136] The PV modules described herein can be used for powering instruments and appliances that operate on DC power, off-grid. The PV module can be designed in various sizes to yield voltage and power outputs optimized for a variety of applications. For example, one of the advantages of RTR manufacturing with on-line monolithic module integration is that there is great flexibility in changing the module size and voltage to suit different applications. This differs from glass-based modules in which the entire manufacturing line must be designed to handle a given size of glass.

[00137] Versatile Uses - Ground-Mounted Installations or Roof-Mounted Installations

[00138] In addition to the use of the PV modules described herein in larger, ground- mounted installations, the flexibility and light weight advantages are most fully realized in rooftop installations and somewhat similar awning or canopy installations.

[00139] Roof-type Structures

[00140] The flexibility and light weight of the PV modules are especially suited for rooftop PV system. Lightweight and flexible modules have many important performance characteristics that are highly desired by customers. For example, many roof structures cannot support the weight of glass based PV modules. Also, in order to support the installation of glass modules, mechanical racking systems are required. For many of these racking systems, roof penetrations are required to anchor the system to the roof and prevent uplift from wind. Once the roof is penetrated, the manufacturer's warranty is generally void.

[00141] In contrast, the flexibility of the PV modules described herein allows the PV modules to be incorporated into building-integrated roofing solutions in many applications that in the past have not been readily amenable to PV cell solutions.

[00142] For example, the PV modules described herein can be developed for such

diverse applications as large roof area businesses, low roof slope businesses, industrial and institutional buildings, and for easily mounted incorporation with standing-seam metal roofing systems for homes and businesses. [00143] For rooftop applications, the flexible PV modules can be integrated as part of the final laminate on low slope industrial, institutional, and business roofing. In the residential market, standing seam metal roofing is attractive for a product made to adhere to the channels between the seams which would minimize installation costs and require no roof penetrations. Similar to rooftop applications, canopies and awnings offer another attractive opportunity for flexible and light weight PV. An example would be coverings for parking lots that could offer shade while generating electricity for electric vehicle and plug-in hybrid vehicles. In this application, DC power conditioning and regulation can be used to allow DC power to flow directly into the automobile batteries to avoid the losses conventionally incurred with inversion to AC followed by rectification back to DC for battery charging.

[00144] The PV module is transformational as a flexible product with DC power

conversion efficiency of -6% in a format that is readily applied to windows either from the inside or outside. The PV module is intrinsically semitransparent and can be deployed in large office buildings, and can simultaneously reduce solar heating and produce power for interior lighting, electronics, and conditioning of the office space.

[00145] Rooftop PV panels deployed horizontally on buildings are particularly well suited for one- and two-story institutional, manufacturing, and warehousing structures.

[00146] The window PV modules are also suited for plug-and-play connections to

existing building electrical infrastructures with a minimum of modifications. For example, in many installations, there would be no need to build an alternate electrical wiring system to handle the current from the modules. Rather, the AC modules can be plugged into the nearest access point of the 115 V or 220/208 V electrical system. Since there would be no need for mounting hardware and little wiring infrastructure, the balance of system costs for the PV window laminate will be minimal. Rather, the module and inverter would constitute almost the complete system. An additional advantage is that many of the installation could be done by individuals with little extensive technical training or expensive equipment.

[00147] Aerospace and/or Military Applications

[00148] The PV modules are also especially suited for use in products needed in

military rapid deployment situations, e.g., for battery charging, power generating tent fabrics, integrated lighting, etc. In addition, the light-weight and flexible PV can be used in high altitude airships and in space applications due its compact stowage, resilience to vibrations and, for CdTe cells, known resistance to radiation damage. [00149] In addition, the PC modules can be useful for lightweight, flexible, high voltage PV modules, in space and for high-altitude airships (e.g., blimps).

[00150] Efficiency in Use of CdTe Materials

[00151] The PV cell structure (and methods used to produce such PC cells) also has applicability when used in conjunction with a standard metal back contact by allowing for the reduction in the thickness of the CdTe layer, while still maintaining the desired high efficiencies of the PC cell.

[00152] Additional benefits include: a reduction of the manufacturing line length, a reduction of CdCl₂ activation time, and a reduction in the amounts of cadmium and tellurium needed.

[00153] Uses with Existing Electrical Networks

[00154] Additionally, the PV cell structures can be coupled with integrated micro- inverters to produce AC power that can be "plugged in" to the existing electrical networks in most buildings with very few modifications.

[00155] In one embodiment, a reactively sputtered, nitrogen-doped ZnTe can serve as a transparent back contact to CdTe. Additionally, suitable diffusion barriers may be included to control impurity migration, including electro-migration, by controlling the grain morphology through the sputter deposition process conditions.

[00156] Efficiencies in Encapsulation

[00157] In another embodiment, the process of encapsulation can include steps such as "edge deletion," forming buss lines, bypass diodes, and junction boxes, together with a robust module encapsulation process. These steps are compatible with the polymer-fixture lamination process described herein.

[00158] By encapsulating using the polymer-fixture laminate process described herein, the manufacturing process yields complete PV modules that exhibit long-term solar exposure endurance, as well as high voltage isolation and the standard thermal and humidity cycling.

[00159] In other embodiments, such as for other CdTe PV modules, the TCO

conductivity and the back contact conductivity are high enough that no grid lines are necessary; current flows perpendicular to the individual cell strips. However, buss lines may be utilized at the ends of the RTR processed modules to collect the current for the junction box, which brings the current through the encapsulation and out of the panel.

[00160] Also, in certain embodiments the RTR manufacturing line can include stations such as a roll-to-roll coating line with on-line chloride activation, followed by the monolithic (sub)module integration and cutting into module sizes. Also, the RTR manufacturing line can include the process of encapsulating the PV submodule to form a completed PV module.

[00161] While the invention has been described with reference to various and preferred embodiments, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the essential scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof.

[00162] Therefore, it is intended that the invention not be limited to the particular

embodiment disclosed herein contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims. The publication and other material used herein to illuminate the invention or provide additional details respecting the practice of the invention, are incorporated by reference herein.

Claims

CLAIMS What is claimed is:

1. An apparatus for the manufacture of flexible, thin film photovoltaic cells, the apparatus comprising:

a flexible manufacturing fixture that is configured to move between a plurality of manufacturing stations, the flexible manufacturing fixture configured to releasably retain a superstrate layer such that the flexible fixture is adapted to be reusable to receive a second superstrate layer;

the plurality of manufacturing stations being configured to form layers of a thin- film photovoltaic cell (PV) on the superstrate layer such that the layers are in

communication to form functional PV cells, the stations comprising:

a first station configured to form a transparent conductive oxide (TCO) layer onto the superstrate layer;

a second station configured to form a first active semiconductor layer; and a third station configured to form a second active semiconductor layer; and a removal station configured to separate at least a semi-finished photovoltaic cell from the flexible fixture such that the formed photovoltaic layers maintain their applied relative physical orientations with adjacent layers.

2. The apparatus of claim 1 wherein the plurality of manufacturing stations includes a fourth station configured to apply a back contact layer, the back contact layer and the TCO being in electrical communication with at least one of the first and second active semiconductor layers.

3. The apparatus of claim 1 wherein the flexible manufacturing fixture is a stainless steel foil and the removal station includes a mechanical parting structure configured to separate the superstrate layer from the fixture.

4. The apparatus of claim 3 wherein the flexible manufacturing fixture includes a release agent and the mechanical parting structure is a blade adapted to remove the release agent from at least one of the fixture and the superstrate layer.

5. The apparatus of claim 3 wherein the flexible manufacturing fixture includes a frangible release agent and the mechanical parting structure is a vibrating roller configured to cleave the release agent and separate the semi-finished PV cell from the fixture.

6. The apparatus of claim 1 wherein the flexible manufacturing fixture is a stainless steel foil and includes a release agent and the removal station includes a fluid stream configured to remove the release agent and separate the semi-finished PV cell from the fixture.

7. The apparatus of claim 6 wherein the release agent comprises a salt, and the fluid stream comprises a water jet.

8. The apparatus of claim 6 wherein the fluid stream comprises a methanol stream that is reactive with the release agent to dissolve the release agent and separate the semi-finished PV cell from the fixture.

9. The apparatus of claim 1 wherein the superstrate layer is a polyimide layer and wherein an electrostatic generator is configured to create a residual charge onto the fixture such that the superstrate is selectively retained onto the fixture by the residual charge.

10. The apparatus of claim 1 wherein the flexible manufacturing fixture is a stainless steel foil having a surface configured to imprint a textured surface finish onto the superstrate, the textured surface finish of the superstrate being adapted to scatter light admitted into the PV cell such that the PV cell output is greater than a PV cell without the textured surface finish.

11. The apparatus of claim 10 wherein the flexible manufacturing fixture is a continuous belt that includes a salt-based release agent, the superstrate is a polyimide material, and the removal station includes a water stream configured to remove the salt- based release agent.

12. The apparatus of claim 3 wherein the flexible manufacturing fixture is a continuous stainless steel foil belt, and the first, second, and third stations are configured as sputtering processing stations, the apparatus further including at least one of a chloride activation station, a laser scribing and ink backfill station, and an encapsulation station.

13. The apparatus of claim 12 wherein the continuous stainless steel foil belt has a surface configured to imprint a textured surface finish onto the superstrate, the textured surface finish of the superstrate being adapted to scatter light admitted into the PV cell such that the PV cell output is greater than a PV cell without the textured surface finish, the fixture further including a release agent comprising an oxide or nitride of at least one of Si, Al, Sn, and Zn, the superstrate being a polyimide material.

14. The apparatus of claim 3 wherein the flexible manufacturing fixture is a coiled stainless steel foil having a pay-out spool and a take-up spool, and the first, second, and third stations are configured as sputtering processing stations.

15. The apparatus of claim 14 wherein the fixture has a surface configured to imprint a textured surface finish onto the superstrate, the textured surface finish of the superstrate being adapted to scatter light admitted into the PV cell such that the PV cell output is greater than a PV cell without the textured surface finish, the fixture further including a release agent comprising an oxide or nitride of at least one of Si, Al, Sn, and Zn, the superstrate being a polyimide material.

16. A flexible, thin- film photovoltaic cell comprising:

a transparent conductive oxide layer forming a first electrical contact;

a back contact layer forming a second electrical contact;

an n-type active semiconductor layer;

a p-type active semiconductor layer that is in contact with the n-type layer such that an electrical potential is formed from exposure to a light source; and

a superstrate layer formed from a flexible material that is at least semi-transparent, the superstrate layer having a characteristic imparted onto an outermost surface from a flexible manufacturing fixture, the fixture-imparted characteristic effecting electrical performance of the n-type and p-type active semiconductor layers.

17. The photovoltaic cell of claim 16 wherein the superstrate layer has a first index of refraction and a release agent has a second index of refraction, and at least a portion of the superstrate layer includes a residual amount of the release agent, the characteristic being the index of refraction of the release agent that is closely matched to the index of refraction of the superstrate such that residual release agent left after removal of the superstrate layer from the fixture is configured to enhance or not significantly affect the transmission of light through the PV cell.

18. The photovoltaic cell of claim 16 wherein the superstrate layer has a first index of refraction, a release agent has a second index of refraction, and an outermost layer of the PV cell has a third index of refraction, at least a portion of the superstrate layer includes a residual amount of the release agent, the characteristic being the release agent is configured to function as an anti-reflective coating and the index of refraction of the release agent is generally near a geometric mean of the index of refraction of the superstrate layer and the outermost layer.

19. The photovoltaic cell of claim 18 wherein the outermost layer is an encapsulation layer and the release agent is chosen from a plurality of release agents.

20. The photovoltaic cell of claim 18 wherein the outermost layer is one of air or a vacuum, both air and the vacuum having an index of refraction of 100.

21. The photovoltaic cell of claim 16 wherein the characteristic is a textured pattern configured to be transferrable from the fixture to the superstrate layer and further configured to promote light scattering to the active semiconductor layers.

22. The photovoltaic cell of claim 21 wherein the textured pattern characteristic is a geometric shape having a repeated pattern on an outer surface of the superstrate.

23. The photovoltaic cell of claim 21 wherein the textured pattern characteristic is a random roughness pattern that is transferred onto the superstrate from contact with the fixture, the random roughness being determinable by a surface roughness measurement.

24. A method of producing a flexible, thin-film photovoltaic cell comprising: providing a flexible manufacturing fixture configured to be moved between a plurality of photovoltaic assembly operations, the flexible fixture being configured to releasably retain layers of at least a semi-finished photovoltaic cell during assembly; applying a flexible superstrate layer onto the flexible manufacturing fixture; forming subsequent layers of the at least semi-finished photovoltaic cell onto the superstrate layer; and

removing the at least semi-finished photovoltaic cell from the flexible fixture such that the subsequent layers of the at least semi-finished photovoltaic cell are configured to function to produce an electrical output.

25. The method of claim 24 wherein a release agent is applied onto the flexible manufacturing fixture prior to applying the flexible superstrate layer, and

the step of forming the subsequent layers of the at least semi-finished photovoltaic cell includes sputtering a transparent conductive oxide layer onto the superstrate layer and sputtering an n-type active semiconductor layer onto the TCO layer and sputtering a p- type active semiconductor layer onto the n-type semiconductor layer; and

the step of removing the at least semi-finished photovoltaic cell includes providing a mechanical parting structure configured to cleave the release agent between the superstrate layer and the flexible fixture.

26. The method of claim 25 wherein the n-type layer is a CdS active semiconductor layer and the p-type layer is a CdTe active semiconductor layer and a CdCl₂ treatment/annealing process is conducted after the active semiconductor layers are applied, and wherein the mechanical parting structure is a blade.

27. The method of claim 25 wherein the n-type layer is a CdS active semiconductor layer and the p-type layer is a CdTe active semiconductor layer and a CdCl₂ treatment/annealing process is conducted after the active semiconductor layers are applied, and

wherein the flexible fixture includes a textured surface and the steps of applying the flexible superstrate layer onto the fixture and removing the at least semi-finished PV cell from the fixture provides a mirror-image textured surface onto the outermost surface of the superstrate layer.

28. The method of claim 24 wherein a release agent is applied onto the flexible manufacturing fixture prior to applying the flexible superstrate layer, and

the step of removing the at least semi-finished photovoltaic cell includes providing a fluid stream configured to remove the release agent and cause the at least semifinished photovoltaic cell to separate from the flexible fixture such that the active semiconductor layers are configured to function to produce an electrical output.

29. The method of claim 28 wherein the release agent is a salt and the fluid stream is a water jet, and removing the at least semi-finished photovoltaic cell includes directing the water jet toward the salt release agent and dissolving and flushing the release agent from between the superstrate layer and the flexible fixture.

30. The method of claim 24 wherein the step of providing the flexible manufacturing fixture includes applying a release agent comprising one of an oxide or nitride coating of one of silicon, aluminum, tin, and zinc, and

the step of applying the flexible superstrate layer onto the flexible manufacturing fixture includes providing an electrostatic retention charge and retaining the superstrate layer onto the fixture, and

the steps of forming the subsequent layers of the at least semi-finished photovoltaic cell are sputtering a TCO layer, then sputtering a CdS n-type active semiconductor layer, and then sputtering a CdTe p-type active semiconductor layer, and then treating/annealing the CdS and CdTe layer with a CdCl₂ process.

31. The method of claim 30 wherein the steps of forming the active semiconductor layers include laser scribing the active semiconductor layers by forming an insulating scribe onto the TCO layer after the TCO sputtering step, then forming a via scribe onto the CdS and CdTe layers after the active semiconductor layer sputtering steps, and applying a back contact and then forming a back contact insulating scribe onto the back contact.

32. The method of claim 31 wherein the step of forming the TCO insulating scribe includes backfilling the insulating scribe with an insulating ink, the step of forming the via scribe includes backfilling the scribe with an electrically conductive ink.

33. The method of claim 32 wherein a step of forming electrical output leads onto the TCO and the back contact layers is conducted after the step of scribing the back contact and a step of encapsulating the at least semi-finished PV cell includes providing an encapsulant and applying the encapsulant on at least the back contact layer and sides of the semi-finished PV cell.