TWI506799B - Thin film photovoltaic module with contoured deairing substrate - Google Patents

Description

Thin film photovoltaic module with contoured degassing substrate

The present invention relates to thin film photovoltaic modules, and in particular, the present invention relates to a thin film photovoltaic module in which a polymer layer and a photovoltaic device are incorporated on a suitable thin film photovoltaic substrate.

Two common types of photovoltaic (solar) modules are used today. The first type of photovoltaic module utilizes a semiconductor wafer as a substrate and a second type of photovoltaic module to utilize a semiconductor film deposited on a suitable substrate.

Photovoltaic modules of the semiconductor wafer type typically include crystalline germanium wafers (which are commonly used in a variety of solid state electronic devices, such as computer memory chips and computer processors). While this conventional design is applicable, it is relatively expensive to manufacture and difficult to use in non-standard applications.

Thin film photovoltaics, on the other hand, can incorporate one or more conventional semiconductors, such as amorphous germanium, on a suitable substrate. Unlike wafer applications (wafers are cut from ingots with complex and elaborate manufacturing techniques), thin-film photovoltaic systems use fairly simple deposition techniques such as sputter coating, physical vapor deposition (PVD) or chemistry. Vapor deposition (CVD) is formed.

Although thin-film photovoltaics have become more suitable as a practical photovoltaic option for wafer photovoltaics, there is a need to improve efficiency, durability, and manufacturing costs.

A particularly persistent problem encountered in the manufacture of thin film photovoltaic modules is that it is difficult to obtain an acceptable lamination of a polymer that is typically provided in the form of a sheet when bus bars are present. The bus bar area of the module during manufacturing is not properly degassed resulting in a product that is not available.

Therefore, this technology requires an improved method and structure for producing a thin film photovoltaic module that is easy to manufacture and stable.

The present invention provides a thin film photovoltaic module having a protective substrate (e.g., glass) that has been contoured to define a space that avoids trapping air due to bus bars on the thin film photovoltaic device. The contouring of the protective substrate is advantageous for degassing and lamination of the module as it reduces or eliminates trapped air content during lamination.

The photovoltaic module of the present invention can minimize the loss caused by degassing and associated lamination problems.

The thin film photovoltaic device of the present invention utilizes a protective substrate having a surface that has been modified from a flat state to a contoured surface, wherein the contour is used to direct air away from the intercept point near the protruding bus bar of the underlying photovoltaic device.

A schematic representation of the general configuration of a thin film photovoltaic module is generally shown at 10 in FIG. As shown in FIG. 1, a thin film photovoltaic device 14 is formed on a base substrate 12 (which may be, for example, glass or plastic). A protective substrate 18 is attached to the photovoltaic device 14 having a polymer layer 16. As described in more detail below, the polymer layer 16 can comprise any suitable polymer.

Previous attempts to provide an acceptable sealing of the polymer layer of the photovoltaic module have included the use of polymeric materials having relatively high flow rates, the use of relatively thick polymer sheets, the use of higher lamination pressures and temperatures, and the addition of total Lamination time. However, their solutions have their own shortcomings. The contoured substrate of the present invention overcomes the problem of lamination degassing.

As used herein, "contoured" substrate means that the surface of the substrate defines a patterned recess below the regular surface of the substrate. For a planar substrate (e.g., a flat glass panel), the contouring can include the formation of grooves, channels, pockets, or other predetermined depressions.

As used herein, "orientation depression" is any depression used to direct air around the bus bar during lamination to reduce or avoid the formation of bubbles in the laminate. Orientation depressions as used herein may direct air around the bus bar by passing directly under or over the bus bar or by directing air away from the bus bar and into the space between the bus bars.

The contouring of the present invention is not limited to any particular cross-sectional shape, and any suitable form of assembly that facilitates complete lamination of the module can be employed. Furthermore, the profile may be oriented in any direction suitable for the particular photovoltaic device used to provide directional depressions, and may be formed, for example, in a parallel, diagonal or vertical configuration, and may have the same or different depths on the substrate. And shape.

In various embodiments, the contouring may take the form of one or more grooves formed by all or part of the substrate. In this manner, laminating the thin film photovoltaic module of the present invention can substantially improve outgassing and sealing around the bus bar without the need for a relatively thick polymer layer, relatively long lamination time, or relatively high processing temperatures and pressure.

The contoured protective substrate of the present invention can be formed in any suitable manner. For example, in various embodiments, in addition to other well-known techniques (such as sand blasting and chemical, water or laser etching), in particular, by, for example, grinding with a diamond-coated drill bit or by utilizing a coating A stone or diamond wheel is ground to form a contour.

The profile may form a simple pattern of linear depressions or any suitable patterning of more complex patterns including any desired combination of contours.

The profile can be formed in any desired depth or width depending on the application. In various embodiments, the profile has from 0.0254 to 0.508 millimeters (0.001 to 0.020 inches), from 0.127 to 0.305 millimeters (0.005 to 0.012 inches), from 0.0254 to 0.229 millimeters (0.001 to 0.009 inches), or from 0.0254 to 0.127. Depth of millimeters (0.001 to 0.005 inches). The profile having any depth as just described may have any combination of any of the following widths: 0.1 to 15 mm, 0.2 to 10 mm, or 3 to 6 mm.

In various embodiments of the invention, the percentage of surface area of the substrate side in contact with the already contoured polymeric layer may be from 0.01 to 70%, from 0.025 to 50%, or from 0.1 to 30%. In various embodiments, the percentage of surface area of the substrate side in contact with the already contoured polymer layer can be from 0.5 to 70%, from 1 to 70%, from 3 to 70%, from 5 to 70%, from 10 to 70%, or from 20 to 70. %.

In various embodiments, the amount of contouring is measured as a percentage of the total bus bar length placed on the contour, regardless of whether the contour extends beyond the length of the bus bar. In various embodiments, the portion of the total bus bar length that is placed on the contour is 0.1 to 70%, 0.2 to 50%, or 0.4 to 30% of the total bus bar length. In various embodiments, the portion of the total bus bar length that is placed on the contour is 0.5 to 70%, 1 to 70%, 3 to 70%, 5 to 70%, 10 to 70%, or 20% of the total bus bar length. Up to 70%.

For any given substrate, any combination of profiles can be provided, including profiles with different profiles and depths. A profile can be formed on one or two substrates.

In various embodiments of the invention, the thickness of the polymeric layer used may be less than 2.29 millimeters (0.090 inches), 1.143 millimeters (0.045 inches), 0.762 millimeters (0.030 inches), or 0.381 millimeters (0.015 inches). In other embodiments (and especially in the roll non-autoclave process) polymerizations having a thickness of less than 0.508 mm (0.020 inch) or a thickness between 0.254 and 0.508 mm (0.010 inch and 0.020 inch) may be used. The bulk layer, which is generally not the case for conventional applications where the use of this thin layer will not be successfully laminated.

In other embodiments of the invention, the contoured substrate of the present invention is used in a vacuum degassing process, such as in a laminating process using vacuum autoclaves and vacuum degassing without the use of an autoclave. In these embodiments (unlike the roll embodiment), the air system is radially removed from the laminate from the center point and must therefore be drawn around the different portions of the bus bar.

Base substrate

The base substrate of the present invention (shown as element 12 in Figure 1) can be any suitable substrate on which the photovoltaic device of the present invention can be formed. Examples include, but are not limited to, glass and hard plastic glazing materials that produce "hard" film modules and thin plastic films that produce "flexible" film modules (such as poly(ethylene terephthalate), poly Yttrium imine, fluoropolymer and the like). It is generally preferred that the base substrate can transmit most of the incident radiation in the range of 350 to 1,200 nanometers, but those skilled in the art will appreciate that variations are possible, including changes in the light entering the photovoltaic device through the protective substrate. .

Thin film photovoltaic device

The thin film photovoltaic device of the present invention (shown as element 14 in Figure 1) is formed directly on the base substrate. A typical device fabrication method involves depositing a first conductive layer, etching the first conductive layer, depositing and etching a semiconductor layer, depositing a second conductive layer, etching the second conductive layer, and applying a busbar conductor and a protective layer (depending on the application). An electrically insulating layer may be formed between the first conductive layer and the base substrate on the base substrate. This optional layer can be, for example, a layer of germanium.

Those skilled in the art will appreciate that the foregoing device fabrication methods are merely one known method and are merely one embodiment of the invention. A variety of other types of thin film photovoltaic devices are within the scope of the invention. Examples of methods and apparatus for forming include those described in U.S. Patent Nos. 2003/0180983, 7,074,641, 6,455,347, 6,500,690, 2006/0005874, 2007/0235073, 7,271,333, and 2002/0034645, the entire disclosure of which is incorporated herein by reference. The manner of reference is incorporated herein.

The various components of the thin film photovoltaic device can be formed by any suitable method. In various embodiments, chemical vapor deposition (CVD), physical vapor deposition (PVD), and/or sputtering can be used.

The two conductive layers are used as electrodes to carry current generated by the inserted semiconductor material. One of the electrodes is typically transparent to allow solar radiation to reach the semiconductor material. Of course, both conductors may be transparent, or one of the conductors may be reflective to cause light reflected by the semiconductor material to be reflected back to the semiconductor material. The conductive layer may comprise any suitable conductive oxide material, such as tin oxide or zinc oxide, or if transparency is not critical, such as for "black" electrodes, metal or metal alloy layers may be used (if they contain aluminum or Silver). In other embodiments, a metal oxide layer can be combined with the metal layer to form an electrode, and the metal oxide layer can be doped with boron or aluminum and deposited using low pressure chemical vapor deposition. The thickness of the conductive layer can be, for example, 0.1 to 10 microns.

The photovoltaic cell region of the thin film photovoltaic device can comprise, for example, a hydrogenated amorphous germanium of the conventional PIN or PN structure. The crucible typically has a thickness of up to about 500 nanometers and typically comprises a p-layer having a thickness of from 3 to 25 nanometers, an i-layer of from 20 to 450 nanometers, and an n-layer of from 20 to 40 nanometers. The deposition can be carried out by argon or in a mixture of decane and hydrogen, as described in, for example, U.S. Patent No. 4,064,521.

Alternatively, the semiconductor material may be microcrystalline, cadmium telluride (CdTe or CdS/CdTe), copper indium diselenide (CuInSe ₂ or "CIS", or CdS/CuInSe ₂ ), copper indium gallium selenide (CuInGaSe) ₂ or "CIGS" or other photovoltaic active materials. The photovoltaic device of the present invention may have an additional semiconductor layer or a combination of the foregoing semiconductor types and may be a series, triple junction or hybrid contact structure.

Any of the well-known semiconductor fabrication techniques (including but not limited to: screen printing with a photoresist mask, etching with positive or negative photoresist, mechanical engraving, discharge etching, chemical etching, or laser etching) can be used. The layers are etched to form individual components of the device. Etching of the plurality of layers will typically result in individual photovoltaic cells being formed within the device. Such photovoltaic cells can be electrically connected to one another using bus bars that are inserted or formed at any suitable stage in the manufacturing process.

A protective layer may be formed on the photovoltaic cell, optionally in combination with the polymeric layer and the protective substrate. The protective layer can be, for example, sputtered aluminum.

The photovoltaic cell of the present invention is formed from the electrically interconnected photovoltaic cell formed from the optional insulating layer, the conductive layer, the semiconductor layer and the optional protective layer.

Polymer layer

Any suitable thermoplastic can be used in the polymer layer of the present invention, including poly(vinyl butyral), non-plasticized poly(vinyl butyral), polyurethane, poly(ethylene-co-vinyl acetate) ), thermoplastic polyurethane, polyethylene, polyolefin, poly(vinyl chloride), polyfluorene oxide, poly(ethylene-co-vinyl acrylate), partially neutralized ethylene/(meth)acrylic acid copolymer Ionomer (as purchased from DuPont) ), a polyethylene copolymer, polyethylene modified polyethylene (PETG) or any other suitable polymeric material. In various embodiments, the polymer comprises an ionomer of poly(ethylene-co-vinyl acetate) (EVA) or a partially neutralized ethylene/(meth)acrylic copolymer.

In various embodiments, the poly(vinyl butyral) can have at least 30,000, 40,000, 50,000, 55,000, 60,000, 65,000, 70,000, 120,000, 250,000, or at least 350,000 grams per mole (g/mol or Dalton (Dalton) ))) The molecular weight. A small amount of dialdehyde or trialdehyde may also be added during the acetal step to increase the molecular weight to at least 350 g/mol (see, for example, U.S. Patents 4,902,464, 4,874,814, 4,814,529 and 4,654,179). The term "molecular weight" as used herein means the weight average molecular weight.

The poly(vinyl butyral) layer of the present invention may comprise a low molecular weight epoxy resin additive. Any suitable epoxy resin reagents can be used in the present invention, as is known in the art (see, for example, U.S. Patent Nos. 5,529,848 and 5,529,849).

In various embodiments, it has been found that epoxy resin compositions that can be used as described below are selected from the group consisting of: (a) an epoxy resin comprising predominantly bisphenol-A monomer diglycidyl ether; (b) An epoxy resin comprising a bisphenol-F monomer diglycidyl ether; (c) an epoxy resin mainly comprising bisphenol-A hydrogenated diglycidyl ether; (d) a polyepoxidized phenol novolac; (e) a diepoxide of polyethylene glycol or a polyether known as an epoxy-terminated; and (f) a mixture of any of the foregoing (a) to (e) epoxy resins (see Encyclopedia of Polymer Science and Technology) , Vol. 6, 1967, Interscience Publishers, New York, pp. 209-271).

The epoxy resin reagent can be incorporated into the poly(vinyl butyral) layer in any suitable amount. In various embodiments, the epoxy resin reagent is incorporated at 0.5 to 15 phr, 1 to 10 phr, or 2 to 3 phr (parts per hundred resin). Such amounts can be applied to any of the individual epoxy resin agents listed above, and especially to those of Formula I, and to the total amount of the mixture of epoxy resin agents described herein.

Adhesion control agents (ACA) can also be used in the polymeric layers of the present invention and include those disclosed in U.S. Patent No. 5,728,472. Further, the remaining sodium acetate and/or potassium acetate can be adjusted by changing the amount of the relevant hydroxide in the acid neutralization. In various embodiments, the polymeric layers of the present invention comprise (in addition to sodium acetate and/or potassium acetate) bis(2-ethylbutyrate)magnesium (chemical abstracts number 79992-76-0). The content of the magnesium salt can effectively control the adhesion of the polymer layer.

Poly(vinyl butyral) can be prepared by a known acetal process involving reacting poly(vinyl alcohol) with butyraldehyde in the presence of an acid catalyst, followed by neutralization of the catalyst, separation, stabilization and The resin was dried.

As used herein, "resin" means a poly(vinyl butyral) component that has been removed from the acid catalyzed mixture and subsequently neutralized the polymer precursor. In addition to poly(vinyl butyral), the resin generally has other components such as acetates, salts, and alcohols.

Details of suitable methods for preparing poly(vinyl butyral) resins are known to those skilled in the art (see, for example, U.S. Patent Nos. 2,282,057 and 2,282,026). In one embodiment, the solvent method described in Vin Wings of Vinyl Acetal Polymers, Encyclopedia of Polymer Science & Technology, Third Edition, Volume 8, pages 381 to 399 (2003) can be used. In another embodiment, the hydration process described therein can be used. Poly (vinyl butyral) can be made in various forms (e.g.) Solutia Inc., St. Louis, Missouri to Butvar ^TM resins are commercially available.

The term "molecular weight" as used herein means the weight average molecular weight.

Any suitable plasticizer may be added to the poly(vinyl butyral) resin of the present invention to form the poly(vinyl butyral) layer. The plasticizer used in the poly(vinyl butyral) layer of the present invention may especially include polybasic acids or esters of polyhydric alcohols. Suitable plasticizers include, for example: triethylene glycol di-(2-ethylbutyrate), triethylene glycol di-(2-ethylhexanoate), triethylene glycol diheptanoate , tetraethylene glycol diheptanoate, dihexyl adipate, dioctyl adipate, hexyl cyclohexane adipate, a mixture of heptyl adipate and decyl ester, diisononyl adipate , heptanoate adipate, dibutyl sebacate, polymer plasticizer (such as oil-modified azelaic acid alkyd resin), a mixture of phosphate and adipate (as disclosed And U.S. Patent No. 3,841,890, the disclosure of which is incorporated herein by reference. Other plasticizers which may be used are C ₄ to C ₉ alkyl alcohols and ring C ₄ to C ₁₀ alcohols (as disclosed in U.S. Patent No. 5,013,779) and C ₆ to C ₈ adipates ( A mixed adipate prepared, such as hexyl adipate. In a preferred embodiment, the plasticizer is triethylene glycol di-(2-ethylhexanoate).

In certain embodiments, the plasticizer has a hydrocarbon segment of less than 20, less than 15, less than 12, or less than 10 carbon atoms.

Additives can be incorporated into the poly(vinyl butyral) layer to enhance its performance in the final product. Such additives include, but are not limited to, plasticizers, dyes, pigments, stabilizers (eg, UV stabilizers), antioxidants, flame retardants, other IR absorbers, UV absorbers, anti-allergies as are known in the art. An agglomerating agent, a combination of the above additives, and the like.

An exemplary method of forming a poly(vinyl butyral) layer includes extruding molten poly(vinyl butyral) comprising a resin, a plasticizer, and an additive, and then forcing the melt through a sheet die (eg, having a dimension) a stamper that is substantially larger than the opening of the vertical dimension). Another exemplary method of forming a poly(vinyl butyral) layer includes casting a melt onto a roll from a die, curing the melt, and subsequently removing the solidified melt in the form of a sheet.

As used herein, "melt" means a mixture of resins having a plasticizer and optionally other additives. In each of the embodiments, the surface texture of either or both sides of the layer can be controlled by adjusting the surface of the die opening or by providing texture on the surface of the roll. Other techniques for controlling the texture of the layer include altering the parameters of the materials (e.g., the water content of the resin and/or plasticizer, the melting temperature, the molecular weight distribution of the poly(vinyl butyral), or a combination of the foregoing parameters). Additionally, the layer can be configured to include spacer protrusions defining temporary surface irregularities to facilitate degassing of the layer during lamination, after which the higher temperature and pressure of the lamination process causes the protrusions to melt into the layer In the layer, a smooth finish is produced.

Protective substrate

The protective substrate of the present invention (shown as element 18 in the figures) can be any suitable substrate that can be used to support the module as described above and that can be processed to define a contour of sufficient size. Examples include, but are not limited to, glass and hard plastic. It is generally preferred that the protective substrate can transmit most of the incident radiation in the range of 350 to 1200 nm, but those skilled in the art will appreciate that variations are possible, including all light entering the photovoltaic device through the substrate. The change. In such embodiments, the protective substrate need not be transparent, or in most cases, and may, for example, be a reflective film that prevents light from exiting the photovoltaic module through the protective substrate.

Assembly

The final assembly of the thin film photovoltaic module of the present invention involves depositing a polymer layer to contact a thin film photovoltaic device having a bus bar formed on a base substrate, depositing a protective substrate to contact the polymer layer, and laminating The assembly is formed to form a module.

In a number of embodiments of the invention, conventional autoclave lamination methods are used. In other embodiments, a non-autoclave process, such as a roll or vacuum pack or loop process, is used. In one such method, after assembly, the components are placed in a vacuum bag or ring and degassed under vacuum (eg, from 0.7 to 0.97 atmospheres) for a suitable period of time (eg, 0 to 60 minutes), and The temperature is then raised to complete the module at a temperature (e.g., 70 to 150 °C). Optionally, the module can be hot pressed to complete the module. In a number of preferred non-autoclave embodiments, the polymer moisture content is maintained at a relatively low level, for example from 0.1 to 0.35%.

The photovoltaic module of the present invention provides the advantage of allowing the use of a non-autoclave process with an extremely high product acceptability. One particular method (roller non-autoclave method) is described in U.S. Patent Publication No. 2003/0148114 A1. When a 0.762 mm (30 mil) polymer sheet is used, the non-autoclave photovoltaic module without the contoured glass of the present invention creates a problem of extremely high defect rate. The present invention with a contoured substrate allows for excellent outgassing, resulting in very low defect rates. In various embodiments of the invention, it is possible to use a polymer sheet having a haze of about 0.254 mm (10 mil), for example from 0.203 to 0.381 mm (8 to 15 mil) or from 0.203 to 0.305 mm (8 to 12 mil). The non-autoclave process of the thickness) successfully prepares any of the photovoltaic modules of the invention described herein in high yield. Of course, such non-autoclave technology facilitates lamination of thicker layers.

In addition to application to photovoltaic modules, the contoured glass of the present invention can be effectively used in heated and laminated glass applications with bus bars, such as automotive rear defrosters with integrated gates for defrosting . In applications such as those, the gates of the heating elements are typically connected to the difficulty of lamination, as they are encountered in the fabrication of photovoltaic modules.

The present invention includes a method of fabricating a photovoltaic module comprising the steps of: providing a substrate, forming a photovoltaic device thereon, and laminating the photovoltaic device to a protective profile of the present invention using a polymer layer of the present invention The substrate, wherein the contoured substrate has a profile that provides directional depressions around one or more bus bars.

With the present invention, a thin film photovoltaic module having excellent physical stability and low defect rate processing can be provided.

While the invention has been described with respect to the preferred embodiments of the embodiments of the invention In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention. Therefore, the present invention is not intended to be limited to the specific embodiments disclosed in the preferred embodiments of the invention.

It is also to be understood that any range, value or feature of any single component of the invention may be used interchangeably with the scope, values or features provided by any other component of the invention to form a component having the full An example of defining a value. For example, the range of poly(vinyl butyral) epoxides and the range of plasticizers can be combined to form a variety of arrangements that are within the scope of the invention but are extremely difficult to enumerate.

Any reference figures in the abstract or in any of the claims are for illustrative purposes only and are not to be construed as limiting the invention.

The figures are not drawn to scale unless otherwise noted.

The entire contents of each of the references, including the journal articles, patents, applications, and references cited herein, are hereby incorporated by reference.

10. . . Thin film photovoltaic module

12. . . Base substrate

14. . . Thin film photovoltaic device

16. . . Polymer layer

18. . . Protective substrate

Figure 1 shows a schematic cross section of a thin film photovoltaic module.

10. . . Thin film photovoltaic module

12. . . Base substrate

14. . . Thin film photovoltaic device

16. . . Polymer layer

18. . . Protective substrate

Claims (18)

A thin film photovoltaic module comprising: a base substrate; a thin film photovoltaic device disposed in contact with the base substrate, wherein the photovoltaic device comprises a bus bar, wherein the bus bar protrudes from the surface of the device; a polymeric layer in contact with the photovoltaic device; and a protective substrate disposed in contact with the polymeric layer, wherein the protective substrate is contoured to provide one or more directional depressions around the bus bar, wherein The surface area of 20 to 70% of the side of the protective substrate placed in contact with the polymer layer is contoured. The module of claim 1, wherein the base substrate and the protective substrate comprise glass. The module of claim 1 wherein the polymer layer comprises poly(vinyl butyral). The module of claim 1, wherein the bus bar includes a lateral protrusion and the one or more directional depressions are perpendicular to the lateral protrusion. The module of claim 4, wherein the bus bar includes a lateral projection and the one or more directional depressions are directed to a side of the lateral projection. The module of claim 1, wherein the directional depression has a width of 0.1 to 15 mm. The module of claim 1, wherein the directional depression has a width of 3 to 6 mm. The module of claim 1, wherein the directional depression has a value of 0.0254 to 0.508 The depth of millimeters. The module of claim 1 wherein the directional depression has a depth of from 0.0254 to 0.127 millimeters. A method of manufacturing a thin film photovoltaic module, comprising: providing a base substrate; forming a thin film photovoltaic device on the base substrate, wherein the photovoltaic device comprises a bus bar, wherein the bus bar is surfaced by the device Projecting a polymer layer to contact the photovoltaic device; placing a protective substrate in contact with the polymer layer, wherein the protective substrate is contoured to provide one or more directional depressions around the bus bar, wherein 20 to 70% of the surface area of the side of the protective substrate placed in contact with the polymer layer is contoured; and the base substrate is laminated with the device, the polymer layer and the protective substrate to form the module . The method of claim 10, wherein the base substrate and the protective substrate comprise glass. The method of claim 10, wherein the polymer layer comprises poly(vinyl butyral). The method of claim 10, wherein the bus bar includes a lateral projection and the one or more directional depressions are perpendicular to the lateral projection. The method of claim 13, wherein the bus bar includes a lateral projection and the one or more directional depressions are directed to a side of the lateral projection. The method of claim 10, wherein the directional depression has a thickness of 0.1 to 15 mm width. The method of claim 10, wherein the directional depression has a width of 3 to 6 mm. The method of claim 10, wherein the directional depression has a depth of from 0.0254 to 0.508 mm. The method of claim 10, wherein the directional depression has a depth of from 0.0254 to 0.127 millimeters.