WO2013082091A2

WO2013082091A2 - Method of forming a photovoltaic cell

Info

Publication number: WO2013082091A2
Application number: PCT/US2012/066765
Authority: WO
Inventors: Lindsey A. Clark; Marty W. Degroot; Melissa A. Mushrush; Beth M. Nichols; Mary Anne Leugers
Original assignee: Dow Global Technologies Llc
Priority date: 2011-11-29
Filing date: 2012-11-28
Publication date: 2013-06-06
Also published as: WO2013082091A3

Abstract

The invention is a method comprising providing a substrate that serves as a backside electrode for a photovoltaic cell, forming a photovoltaic cell on a frontside of the substrate wherein at least one portion of the photovoltaic cell comprises a chalcogen, exposing a backside of the substrate to a plasma etch after the formation of the portion of the photovoltaic cell comprising a chalcogen. The process preferably further includes forming at least one electrical connection to the backside electrode. The invention is also a photovoltaic article comprising a backside electrode, a chalcogen containing photoactive region, and a transparent frontside conductive layer wherein less than 0.1 atom % of chalcogen atoms is found on a backside surface of the backside electrode.

Description

METHOD OF FORMING A PHOTOVOLTAIC CELL

Field of the Invention

[001] This invention relates generally to interconnected photovoltaic cells and modules and particularly to an improved approach to maintaining good interconnect performance throughout useful life of the cells or module.

Introduction

[002] Photovoltaic cells typically comprise a photoactive portion that converts light energy to electricity. On the backside of these cells is found a backside electrode and on the front side another electrical collection system. It is common for these cells to be connected in series by multiple thin wires or ribbons that contact the front side of a first cell and the back side on an adjacent cell. One common interconnect configuration is commonly referred to as 'string & tab' . Typically an electrically conductive adhesive (ECA) or solder is used to attach the interconnect ribbon to the front and back sides of the adjacent cells.

[003] These interconnect configurations are commonly used with both rigid and flexible photovoltaic cells such as copper chalcogenide type cells (e.g. copper indium gallium selenides, copper indium selenides, copper indium gallium sulfides, copper indium sulfides, copper indium gallium selenides sulfides, etc.), amorphous silicon cells, crystalline silicon cells, thin-film III-V cells, organic photovoltaics, nanoparticle photovoltaics, dye sensitized solar cells, and combinations of the like.

[004] US 2009/0266398 teaches cleaning the back surface of a CIGS solar cell by mechanical, chemical or electrochemical techniques with a focus on mechanical means such as brushing or sanding. Other examples of approaches to interconnection include

2009/097161, US 2009/0260675, U.S. 7,732,229, US 2005/0263179, US 2008/0216887, US 2009/00025788, US 2008/0011350, US 7,432,438, US 6,936,761; US 2007/0251570 and US.S. 7,022,910.

[005] The industry continues to examine alternative approaches to improve

interconnection.

Summary of Invention

[006] The present inventors have discovered electrical contacts in the interconnected cell string are susceptible to increased electrical resistance after environmental exposure. Upon further examination, applicants found degradation or corrosion at the back interfaces including the ECA component. Applicants have further discovered that this degradation may be instigated by the presence of chalcogen or chalcogen containing compounds on the surface of the back contact. The term chalcogen, as cited herein, is used as a general term to describe group VI elements such as sulfur, selenium and tellurium. Selenium or sulfur may be present on the backside as a residue from the process of forming a chalcogenide based photoabsorber layer, emitter layer, or other electrically active layers such as window or transparent conductive layers in a photovoltaic cell.

[007] The inventors have further identified a method of removing the selenium containing species from the backside which avoids the negative aspects of mechanical cleaning and chemical cleaning using solutions. The inventive method preferably has at least one of the following advantages: the backside can be cleaned as part of the manufacturing process of the photovoltaic cells while the photovoltaic cells are still on the uncut web or film used for manufacture of the photovoltaic cells, no scrubbing or sanding is needed such that no pressure or contact need be exerted on the front or active side of the coated web, no fluid is used that would contact or damage the front side/active side surface of the coated web. As a further benefit, the applicants have found that photovoltaic cells comprising a chalcogen which have the backside electrode cleaned before formation of the backside interconnection have a lower initial resistance than do similar cells that do not undergo such backside cleaning.

[008] Thus, according to a first embodiment, the invention is a method comprising:

providing a substrate that serves as a backside electrode for a photovoltaic cell, forming a photovoltaic cell on a frontside of the substrate wherein at least one portion of the photovoltaic cell comprises a chalcogen

exposing a backside of the substrate to a plasma etch after the formation of the portion of the photovoltaic cell comprising a chalcogen.

The process preferably further includes forming at least one electrical connection to the backside electrode.

[009] According to a second embodiment the invention is a photovoltaic article comprising a backside electrode, a chalcogen containing photoactive region, and a transparent frontside conductive layer wherein less than 0.1 atom of chalcogen atoms is found on a backside surface of the backside electrode. This article may be and preferably is made using the preceding method. Brief Description Of the Drawings

[0010] Fig. 1 is a schematic showing the frontside view of a representative electrical connection from one photovoltaic cell to an adjacent photovoltaic cell.

[0011] Fig. 2 is a schematic of a cross-section showing a representative electrical connector from one cell to an adjacent cell.

Detailed Description

[0012] Fig . 1 shows a frontside view of an exemplary embodiment of the photovoltaic article of this invention in this case showing two adjacent photovoltaic cells 11. Fig. 2 shows a cross section at the location of one interconnect element. Each cell has a backside electrode 14 and a frontside electrical collection system 12, in this case shown as a series of thin wires, located on a frontside transparent electrical contact 26 (region between the wires 12). According to one approach the frontside electrical collector can extend beyond the edge of the first cell to contact the backside of the second cell. In this approach the electrical collection system also serves as the cell to cell electrical connector. This approach is not shown in Figs. l and 2. Alternatively, as shown in Figs. 1 and 2 a separate electrical connector, or interconnect element, in this case a conductive ribbon 13 is provided to connect the frontside electrical collection system 12 of the first cell to the backside electrode 14 of the second cell. Electrically conductive adhesives 15 and 16 can be used to adhere the ribbon 13 to the front and backsides of the cell. Additional cells may be added to form a string of the length desired; the terminal cells will have electrical leads (not shown) provided to enable connection of the cell into a more complex array (formed by modules made of the subject string) or to a electric service system.

[0013] Typically the backside electrode 14 will comprise a conductive substrate layer of metal foils or films or will be such a foil, film or a metal paste or coating on a non- conductive or conductive substrate. Suitable materials include, but are not limited to metal foils or films of stainless steel, aluminum, titanium or molybdenum. Stainless steel and titanium are preferred. Preferably, the backside electrode structure including the substrate is flexible. The conductive substrate layer can be coated with optional backside electrical contact regions on one or both sides. Such regions may be formed from a wide range of electrically conductive materials, including one or more of Cu, Mo, Ag, Al, Cr, Ni, Ti, Ta, Nb, W combinations of these, and the like. Conductive compositions incorporating Mo may be used in an illustrative embodiment. [0014] The photovoltaic cells 11 used in this invention are photovoltaic cells wherein one or more layers or components include a chalcogen. The photoactive layer is preferably a layer of IB-IIIA-chalcogenide, such as IB-IIIA-selenides, IB-IIIA-sulfides, or IB-IIIA-selenide sulfides. More specific examples include copper indium selenides, copper indium gallium selenides, copper gallium selenides, copper indium sulfides, copper indium gallium sulfides, copper gallium sulfides, copper indium sulfide selenides, copper gallium sulfide selenides, and copper indium gallium sulfide selenides (all of which are referred to herein as CIGSS). These can be represented by the formula CuIn(l-x)GaxSe(2-y)Sy where x is 0 to 1 and y is 0 to 2. The copper indium selenides and copper indium gallium selenides are preferred. This layer may be called the absorber layer. The absorber may be formed by any suitable method using a variety of one or more techniques such as evaporation, sputtering, electrodeposition, spraying, and sintering (Dhere, Solar Energy Materials & Solar Cells 90 (2006) 2181-2190.) One preferred method is co-evaporation of the constituent elements from one or more suitable sources, where the individual constituent elements are thermally evaporated onto a hot surface coincidentally at the same time, sequentially, or a

combination of these to form CIGSS (For example see Mickelsen et al. US 4,335,266, 1982). After deposition, the film(s) may be subjected to one or more further treatments to finalize the absorber properties. Another common route is to deposit any combination of Cu, In, Ga, Se, and/or S onto a substrate by sputtering and then to subject the layer(s) to a thermal process in the presence of a chalcogen-containing atmosphere, thereby completing the photoactive CIGSS layer. (Tarrant, Gay; NREL/SR-520-40242 July 2006).

[0015] CIGSS cells usually include additional electroactive layers such as one or more of emitter (buffer) layers, conductive layers (e.g. transparent conductive layer used on the top side) and the like as is known in the art to be useful in CIGSS based cells are also contemplated herein. The emitter or buffer layer is an w-type semiconductor, such as for example CdS, that forms a pn junction with the absorber. It is known that buffer layers such as CdS layers can be formed on various substrates by chemical bath deposition, physical vapor deposition or sputtering. See e.g. Abou-Ras et al (Thin Solid Films 480-481 (2005) 118-123) and 5,500,055. Cells with no buffer or alternative materials for buffer are also known and can be used in this invention.

[0016] An optional window region, which may be a single layer or formed from multiple sublayers via techniques including sputtering and MOCVD, can help to protect against shunts. This region also may protect the buffer region during subsequent deposition of the transparent conductive layer which serves as part or all of the frontside electrical collection system. The window region may be formed from a wide range of materials and often is formed from a resistive, transparent oxide such as an oxide of Zn, In, Cd, Sn, combinations of these and the like. An exemplary window material is intrinsic ZnO. (Cooray et. al. Solar

Energy Materials and Solar Cells 49 (1997) 291-297) A typical window region may have a thickness in the range from about 1 nm to about 200 nm, preferably about 10 nm to about

150 nm, more preferably about 80 to about 120 nm. The cells will have a topside electrical collection system comprising a front electrode, which serves to collect photogenerated electrons from the photoactive region. The transparent topside electrical contact 26 (also referred to as TCL) is formed over the photoactive region on the light incident surface of the photovoltaic device. The TCL has a thickness in the range from about 10 nm to about 1500 nm, preferably about 100 nm to about 300 nm. The TCL may be a very thin metal film that has transparency to the relevant range of electromagnetic radiation or more commonly is a transparent conductive oxide (TCO). A wide variety of transparent conducting oxides

(TCO) or combinations of these may be used. Examples include fluorine-doped tin oxide, tin oxide, indium oxide, indium tin oxide (ΓΓΟ), aluminum doped zinc oxide (AZO), gallium doped zinc oxide, zinc oxide, combinations of these, and the like (for suvey of TCO matierals, see Gordon, MRS Bull 25 52-57, 2000). In one illustrative embodiment, the TCL region 26 is indium tin oxide. The TCL, which may be a single layer or formed from multiple sublayers, are conveniently formed via sputtering or other suitable deposition technique. (A.F. da Cunha et al. / Journal of Non-Crystalline Solids 352 (2006) 1976-1980)

[0017] In some cases, the transparent conductive layer may not be conductive enough to enable sufficient collection of electrons from the device. Thus, the front electrode region may also comprise an electrical collection system 12 (often a grid or mesh of thin wire lines) formed over the transparent contact layer. The grid region desirably at least includes conductive metals such as nickel, copper, silver, aluminum, tin, and the like and/or combinations thereof. In one illustrative embodiment, the grid comprises silver. Since these materials are not transparent, in one illustrative embodiment they are deposited as a grid or mesh of spaced apart lines so that the grid occupies a relatively small footprint on the surface (e.g., in some embodiments, the grid occupies about 10% or less, even about 5% or less, or even about 2% or less of the total surface area associated with light capture to allow the photoactive materials to be exposed to incident light). The grid region can be formed by one of several known methods including but not limited to screen printing, ink jet printing, electroplating, sputtering, evaporating and the like. Alternatively, the electrical collection system 12 can be applied to the frontside of the cell as a series of thin wires, or a continuous sheet of thin wires, such as a wire mesh.

[0018] The grid electrical collection system may itself extend beyond the top edge of a cell to be contacted with the backside of an adjacent cell thus forming the electrical connector. More preferably, as shown in Figs. 1 and 2 additional interconnect element(s) 13 that can be in the form of one or more thin wires or ribbons or wire mesh are provided. The

interconnect element(s) 13 contact and make electrical connection to one or both of the transparent contact layer 26 and the electrical collection system 12 of the front electrode region of the cell. On the backside of an adjacent cell, the interconnect elements 13 contact and make electrical connection to the backside electrode 14. The interconnect elements are comprised of one or more conductive metals such as Cu, Ag, Sn, Al, Au, Fe:Ni, W, Ti, and the like. In preferred embodiments, the interconnect elements comprise a Cu or Fe:Ni ribbon that is coated with a thin layer of Sn or Sn:Ag. The interconnect elements connect adjacent cells and can also be used to connect the cells to external circuitry such as terminal bars. In an exemplary embodiment, the interconnect elements are connected to terminal bars at the leading and trailing edges of the interconnected cell assembly.

[0019] It is desirable to limit the amount of shading to the photoactive portion caused by interconnect elements and collection system, while providing enough conductive material to provide electrical continuity that maximizes efficiency of the cell. The interconnect elements 13 can be applied directly to the topside of the cell, in electrical connection with the TCL 26 and/or the electrical collection system 12, and the backside electrode of an adjacent cell. Alternatively, an additional conductive material can be used to make electrical connections and to ensure that the interconnect elements are well adhered to one or both of the front electrode and backside electrode of the cells. The conductive material may be the same material for connection of interconnect elements with the front and backside electrodes, or different materials may be used. In preferred embodiments, the conductive material is an electrically conductive adhesive or solder. In exemplary embodiments, conductive material connecting the interconnect elements to both the the front electrode and the backside electrode is an electrically conductive adhesive such as a silver filled epoxy resin.

[0020] The electrically conductive adhesives (ECA) may be any such as are known in the industry. Such ECAs are frequently compositions comprising a thermosetting polymer matrix with electrically conductive filler particles. Such thermosetting polymers include but are not limited to thermoset materials comprising epoxy, cyanate ester, maleimide, phenolic, anhydride, vinyl, allyl or amino functionalities or combinations thereof. The conductive filler particles may be for example silver, gold, copper, nickel, carbon nanotubes, graphite, tin, tin alloys, bismuth or combinations thereof. Epoxy based ECAs with silver particles are preferred.

[0021] In another aspect of the invention, the photovoltaic article or interconnected cell assembly may be connected to terminal bars at one or both ends of the string, or at other regions as necessary to connect the module to external circuitry. In an exemplary

embodiment, an interconnect element is connected from the backside electrode of the first photovoltaic cell to a terminal bar at the leading edge of the interconnected assembly, and an interconnect element is also connected from the topside electrode of the last photovoltaic cell to a terminal bar at the trailing edge of the interconnected assembly. It is contemplated that the connection may be created and/or maintained between the interconnect element and the terminal bar via a wide variety of joining techniques including but not limited to welding, soldering, or electrically conductive adhesive.

[0022] According to the method of this invention, at some time after formation of the absorber layer on the backside electrode/substrate, the exposed side of the backside electrode is cleaned by radio frequency (RF) plasma cleaning. For example, as one preferred embodiment, the sample is introduced into a vacuum chamber comprising an inert gas, such as argon gas. Preferably, the pressure is less than 10 Pa, more preferably less than 5 Pa, and most preferably less than 2 Pa, and preferably at least 0.1 Pa. The sample is oriented such that the backside (non-PV side) of the substrate is facing the RF generator. Power should be selected to be sufficient to remove contaminants but not significantly remove the underlying substrate material. According to one embodiment depending upon the equipment being used, a 100-500 Watt (W) plasma may be used. The sample is exposed to the plasma for several minutes, but this could be longer or shorter depending on the power used and the concentration of chalcogen on the backside. The cleaning step can also be made more rigorous by the addition of oxygen or other reactive gasses to the argon process gas of by heating the substrate.

[0023] It is contemplated that the photovoltaic article may further comprise optional encapsulant layers that may perform several functions. For example, the encapsulant layers may serve as a bonding mechanism, helping hold the adjacent layers of the module together. They should also allow the transmission of a desirous amount and type of light energy to reach the photovoltaic cell (e.g. the photoactive portion). The encapsulant layers may also function to compensate for irregularities in geometry of the adjoining layers or translated through those layers (e.g. thickness changes). They also may serve to allow flexure and movement between layers due to environmental factors (e.g. temperature change, humidity, etc.) and physical movement and bending. In a preferred embodiment, first encapsulant layer may consist essentially of an adhesive film or mesh, but is preferably a thermoplastic material such as EVA (ethylene- vinyl-acetate), thermoplastic polyolefin or similar material. It is contemplated that the encapsulant layers may comprise a single layer or may comprise multiple layers (e.g. a first, second, third, fourth, fifth layer, etc). The preferred thickness of this layer can range from about 0.1mm to 1.0mm, more preferably from about 0.2mm to 0.8mm, and most preferably from about 0.25mm to 0.5mm.

[0024] Additional front and backside barrier layers may also be used. Frontside barriers must be selected from transparent or translucent materials. These materials may be relatively rigid or may be flexible. Glass is highly useful as a frontside environmental barrier to protect the active cell components from moisture, impacts and the like. A backside barrier or backsheet may also be used; it is preferably constructed of a flexible material (e.g. a thin polymeric film, a metal foil, a multi-layer film, or a rubber sheet). In a preferred embodiment, the back sheet material may be moisture impermeable and also range in thickness from about 0.05mm to 10.0 mm, more preferably from about 0.1mm to 4.0mm, and most preferably from about 0.2mm to 0.8mm. Other physical characteristics may include: elongation at break of about 20% or greater (as measured by ASTM D882); tensile strength or about 25MPa or greater (as measured by ASTM D882); and tear strength of about 70kN/m or greater (as measured with the Graves Method). Examples of preferred materials include glass plate, aluminum foil, Tedlar® (a trademark of DuPont) or a combination thereof. A supplemental barrier sheet can be connectively located below the back sheet. The supplemental barrier sheet may act as a barrier, protecting the layers above from environmental conditions and from physical damage that may be caused by any features of the structure to which the PV device is subjected (e.g. for example, irregularities in a roof deck, protruding objects or the like). It is contemplated that this is an optional layer and may not be required. Alternately the protective layer could comprise more rigid materials so as to provide additional roofing function under structural and environmental

(e.g. wind) loadings. Additional rigidity may also be desirable so as to improve the coefficient of thermal expansion of the PV device and maintain the desired dimensions during temperature fluctuations. Examples of protective layer materials for structural properties include polymeric materials such polyolefins, polyester amides, polysulfone, acetel, acrylic, polyvinyl chloride, nylon, polycarbonate, phenolic, polyetheretherketone, polyethylene terephthalate, epoxies, including glass and mineral filled composites or any combination thereof.

[0025] In addition to the encapsulant and barrier materials, a framing material may also be provided to enable attachment of the photovoltaic article to structures such as buildings and to enable and protect electrical interconnects between articles and from articles to other electrical devices. See for example the frame, preferably prepared by injection molding around a laminate structure, to form a building integrated photovoltaic device as shown in WO 2009/137353.

[0026] One advantage of the invention as discovered by the inventors is that the back electrode exhibits lower initial resistance when it is cleaned using the plasma treatment versus the untreated electrode. This can translate directly into improved photovoltaic performance of the cells as a result of lower series resistance and improved cell voltage and current. The inventors also discovered a second advantage of the invention as described because the photovoltaic cells or strings are much more stable when exposed to

environmental weathering conditions such as damp heat conditions specified in IEC 61646 Edition 2.0. During the exposure, the solar cell laminates are positioned vertically on a stainless steel fixture within an oven held at 85°C + 2°C. Module electrical performance is mathematically extracted from a current-voltage (I-V) characteristic curve that is measured prior to environmental exposure and at various intervals using a Spire 4600 class AAA solar simulator under AMI.5, 1000 W/m . During the I-V characteristic measurement, the temperature of the modules is maintained at 25°C. The I-V characteristic measurement apparatus and procedure meet the requirements specified in the IEC 60904 (parts 1 - 10) and 60891 standards. Immediately following this measurement the devices are returned to the ambient heat environment for the next test period. This process is repeated for each time period. For each I-V measurement, electrical contact is established at the electrical connectors attached to the terminal bars at the leading and trailing ends of the electrical assembly. The power is measured by varying the resistive load between open and closed circuit. The maximum power (Pmax) is calculated as the area of the largest rectangle under the current-voltage (I-V) characteristic curve. The series resistance (Rs) is the slope of the line near open-circuit voltage (Voc), not taken from a diode fit.

Example 1

[0027] CIGS thin film solar cells are formed on a stainless steel substrate by a precursor- selenization method as set forth below. Stainless steel coupons (Thyssen-Krupp, 4-mil, SS430, annealed) are cleaned by sonication and rinsing with Liquinox™ surfactant from Alconox Inc. and deionized water and then dried. Samples are cleaned on the intended active side by RF plasma at 300W for 5 min in Ar at 8e^~ mbar (0.8 Pa). The samples are coated with an adhesion layer comprising 200 nm of Nb (200W, 6.6e^" mbar ( 0.66 Pa) Ar) and a back electrode of 350 nm sodium-doped molybdenum (3 atom % Na (Protech); 6e^~ mbar (0.6 Pa)). A precursor layer comprising Cu, In, Ga, and Se is deposited by reactive sputtering of a Cu:In:Ga alloy (target available from Heraeus) in H₂Se gas (4.2% H₂Se flow (5sccm H₂Se and 115sccm Ar); 4e-3mbar (0.4 Pa)).

[0028] The precursor sample is then transferred to a reactor for selenization. The selenization follows a two step thermal profile where the first temperature reaches between 520 and 550°C and the second temperature is about 575°C. All samples were selenized at substantially the same conditions. A CdS buffer layer is then deposited (le^_1mbar (10 Pa) Ar, 160W, ~20nm). Samples are sputtered with an intrinsic zinc oxide window layer (iZnO, l lsccm 1*02 in Ar, 44sccm pure Ar, 2.6e^"3 mbar (0.26 Pa); 100W, 150°C; film: 120nm) and a transparent conductive oxide, aluminum-doped zinc oxide or "AZO" completing the solar cell stack (AZO, 2% Al-doped zinc oxide target, 45sccm pure Ar, 2.6e^" mbar (0.26 Pa) 100W, 150°C; film: 270nm).

[0029] The fabricated sample is printed with Ni-Ag grids and scribed to 0.44cm2 test cells.

Solar conversion efficiencies are then measured on a Spectra-Nova solar cell tester and are summarized in the table below. During the I-V characteristic measurement, the temperature of the cells is -25 °C. The I-V characteristic measurement apparatus and procedure meet the requirements specified in the IEC 60904 (parts 1 - 10) and 60891 standards. For each I-V measurement, electrical contacts are established at the Ni-Ag grid and the back side of the substrate. The power is measured by varying the resistive load between open and closed circuit. The maximum power (Pmax) is calculated as the area of the largest rectangle under the current-voltage (TV) characteristic curve. The series resistance (Rs) is the slope of the line near open-circuit voltage (Voc), not taken from a diode fit. After initial characterization, the samples were then split into two subsets. One subset was sanded on the backside while the second were subjected to a second RF plasma cleaning on the backside (non-active, non-photovoltaic side) of the cell for 5 minutes at 300 Watts at 8e-3 mbar (0.8 Pa) argon. The solar conversion efficiencies were then measured again and are summaried in the table below. Cell efficiency, Voc, and Jsc values of the cells that were plasma cleaned on the backside showed a significant improvement compared to cells that were not cleaned.

Example 2

[0030] CIGS cells of 100 X 210 mm dimension comprising a screen printed grid (electrical collection system) on the topside and a conductive substrate electrode with Mo coated back contact were obtained. An electrically conductive adhesive (EC A), DB1541-S from Engineered Conductive Materials, was applied using a spatula in a straight line through a stencil. The deposited ECA line was approximately 2 mm X 90 mm and was equally applied in three regions on the backside electrode of the cell. The stencil was removed and a Sn:Ag coated metalribbon was placed on top of each of the ECA dispense regions along the length of the ECA such that one end of the ribbon extended beyond the edge of the cell. A small piece of high temperature polyimide tape was applied at the cell edge under the ribbon to avoid contact of the ribbon with the edge of the cell. A second long piece of high temperature polyimide tape was applied was applied along the length of the ribbon to hold the ribbons in place until the ECA was cured. All interconnect ribbons were 2.5 mm wide and had a thickness of approximately 0.1 mm. The cells were then placed between two pieces of tempered glass and placed in a laboratory oven that was pre-heated to 180°C. The cells were left in the oven for 40 minutes to allow a controlled cure for the ECA. After 40 minutes, the samples were removed from the oven and the polyimide tape was removed from each of the ribbons. [0031] For the plasma etched samples, the same process as above described was used to prepare the samples. Before attaching the interconnect elements on the backside electrode, the samples were subjected to an RF plasma cleaning on the backside (non-active, non- photovoltaic side) of the cell for 5 minutes at 300 Watts at 8e-3 mbar comprising argon. Using a stencil an electrically conductive adhesive (EC A), DB1541-S from Engineered Conductive Materials, was equally applied in two regions on the backside electrode of the cell, the spacing between the ribbons was the same as the non-plasma etched samples. The stencil was removed and a ribbon was placed on top of each of the ECA dispense regions. High temperature polyimide tape was used to hold the ribbons in place until the ECA was cured. The two ribbons attached to the backside electrode each had a portion of the ribbon extending beyond the leading edge of the cell. All interconnect ribbons were 2.5 mm wide and had a thickness of approximately 0.1 mm. The cells were then placed between two pieces of tempered glass and placed in a laboratory oven that was pre-heated to 180oC. The cells were left in the oven for 40 minutes to allow a controlled cure for the ECA. After 40 minutes, the samples were removed from the oven and the polyimide tape was removed from each of the ribbons. The solar cells were then placed into environmental exposure chambers for stability testing, such as damp heat conditions specified in IEC 61646 Edition 2.0. The cells subjected to the backside plama etch treatment before attachment of the ribbons showed no discernible increase in electrical resistance measured between adjacent ribbons over 700 hours of observation. In contrast the untreated cells showed an increase in electrical resistance of an average of about 300 to 500 times the initial electrical resistance value when observed at intervals of from about 200 to about 700 hours.

Example 3

[0032] The backside of similar CIGS cells to those in Example 2, untreated and after backside plasma etch treatment of from 1 to 5 minutes are analyzed using energy dispersive x-ray spectroscopy data (EDS) The EDS analysis of the uncleaned sample includes peaks indicative of, selenium (-1.4 keV), iron (-0.7 eV), and oxygen (-0.55 eV). After a 1- minute plasma treatment, the selenium content has been substantially decreased while the other peaks remain of a similar intensity. As the plasma etching treatment is extended to 2, 3, and 4 minutes, the selenium peak decreases to an approximately constant level which is not differentiated from the background signal in the EDS data, indicating that the selenium has been substantially removed from the backside surface.

Claims

WHAT IS CLAIMED IS:

1. A method comprising:

2. The method of claim 1 further comprising forming at least one electrical connection to the backside electrode after the plasma etch.

3. The method of claim 2 further comprising forming an electrically conductive layer on the backside of the substrate prior to forming the at least one electrical connection to the backside electrode after the plasma etch

4. The method of any of the preceding claims wherein the plasma etch comprises radio frequency plasma used at a pressure between 0.1 to 5 Pa of inert gas.

5. The method of any of the preceding claims wherein the photovoltaic cell comprises a copper chalcogenide based absorber, one or more layers comprise a buffer layer adjacent to the absorber and a transparent conductive layer.

6. The method of claim 5 wherein the absorber is formed by sputtering or evaporation.

7. The method of claim 6 wherein the absorber is formed by sputtering or evaporation of a precursor material followed by chalcogenization.

8. The method of any of claims 5-7 wherein the buffer layer is formed by a method

selected from chemical bath deposition, evaporation or sputtering.

9. The method of claim 5-8 wherein the transparent conductive layer is a transparent

conductive oxide layer formed by evaporation or sputtering.

10. A photovoltaic article comprising a first photovoltaic cell having a chalcogen containing photoelectrically active layer located between a topside electrode and a backside electrode, at least one first interconnect element in contact with the frontside electrode at a front surface of the cell and the interconnect element is adhered to the topside electrode of the cell, and at least one second interconnect element adhered to the backside electrode, wherein a surface of the backside electrode opposite from the chalcogen-containing absorber layer is characterized by the presence of less than 0.1 atom % of chalcogen present.

1. The photovoltaic article of claim 10 made by the method of any one of claims 1-9.