Systems and methods for thermally managing high-temperature processes on temperature sensitive substrates

A method for depositing one or more thin-film layers on a flexible polyimide substrate having opposing front and back outer surfaces includes the following steps: (a) heating the flexible polyimide substrate such that a temperature of the front outer surface of the flexible polyimide substrate is higher than a temperature of the back outer surface of the flexible polyimide substrate, and (b) depositing the one or more thin-film layers on the front outer surface of the flexible polyimide substrate. A deposition zone for executing the method includes (a) one of more physical vapor deposition sources adapted to deposit one or more metallic materials on the front outer surface of the substrate, and (b) one or more radiant zone boundary heaters.

BACKGROUND

Photovoltaic devices generate an electric current in response to incident light. One class of photovoltaic devices commonly used today is based on crystalline silicon solar absorber layers. Crystalline silicon photovoltaic devices include, for example, thick silicon wafers. These wafers are fragile and cannot be bent without risk of damage, and therefore, the wafers must be disposed on a rigid substrate, such as a large-area rigid glass substrate. Although crystalline silicon photovoltaic devices may achieve relatively high efficiencies, they are typically expensive and heavy. Additionally, their inflexibility prohibits their use in non-planar applications, or in applications subject to bending. Furthermore, their weight prohibits some roof top applications.

Accordingly, there is great interest in developing “thin-film” photovoltaic devices, which are potentially thinner, lighter, and cheaper than crystalline silicon photovoltaic devices. Additionally, a thin-film photovoltaic device will typically tolerate at least some flexing, potentially allowing use of a flexible substrate, so that the photovoltaic device is flexible. A flexible photovoltaic device may advantageously conform to non-planar surfaces and/or be used in applications subject to bend flexing.

FIG. 1shows a cross-sectional view of a conventional thin-film photovoltaic device100, which includes a thin-film stack102disposed on a substrate104. Photovoltaic stack102includes a first electrical contact layer106, such as a Molybdenum layer, disposed on a first or front outer surface108of substrate104. A solar absorber layer110is disposed on first electrical contact layer106, and a heterojunction partner layer112is disposed on solar absorber layer110. A second electrical contact layer114, such as a conductive oxide layer, is typically disposed on heterojunction partner layer112. Solar absorber layer110and heterojunction partner layer112collectively form a P-N photovoltaic junction, which generates an electric current in response to incident light. First and second electrical contact layers106,114provide an electrical interface to the photovoltaic junction. Some examples of possible solar absorber layer materials include Selenium-based chalcogenides such as Copper-Indium-DiSelenide (CIS), or an alloy thereof. Some examples of CIS alloys include Copper-Indium-Gallium-DiSelenide (CIGS), Silver-Copper-Indium-Gallium-DiSelenide (AgCIGS), and Copper-Indium-Gallium-Aluminum-DiSelenide (CIGAS). Replacing or alloying selenium in the solar absorber with other group VI elements, such as Sulfur or Tellurium, is also attractive in certain applications. Some examples of possible heterojunction partner layer materials include Cadmium Sulfide, metal oxides, or alloys thereof. Additional layers, such as buffer layers and/or stress relief layers, are often added to photovoltaic device100. For example, a metallic layer, such as a Molybdenum layer118, is sometimes disposed on back outer surface116of substrate104to provide stress relief and dissipate static electricity.

Many photovoltaic devices include a plurality of photovoltaic cells electrically coupled in series and/or parallel to meet an application's voltage and/or current requirements. It is often desirable that the plurality of photovoltaic cells be monolithically integrated on a common substrate. Monolithic integration can enable customization of device output voltage and output current ratings during device design, thereby allowing the device to be tailored to its intended application. Additionally, monolithic integration promotes small device size and pleasing aesthetic properties by reducing pitch between adjacent photovoltaic cells, as well by reducing or eliminating use of discrete bus bars to connect adjacent cells, relative to non-monolithically integrated photovoltaic devices.

Monolithic integration requires, though, that an outer surface of the device substrate, such as outer surface108of substrate104(FIG. 1), be dielectric. Specifically, the surface must be dielectric to allow electrical separation of adjacent photovoltaic cells by cell isolation scribes, which are sometimes referred to as “P1” scribes. If the substrate outer surface were instead conductive, or if the P1scribe penetrated a surface dielectric to a conductive substrate, then adjacent photovoltaic cells would electrically short together, thereby rendering the P1scribes ineffective. Some examples of monolithic integration techniques including use of cell isolation scribes are disclosed in U.S. Patent Application Publication Number 2008/0314439 to Misra, which is incorporated herein by reference.

A dielectric surface may be obtained on a conductive flexible substrate, such as metallic foil, by applying a dielectric coating to the substrate. However, this procedure may be difficult to implement because the dielectric coating must be free of defects, such as pinholes, to prevent photovoltaic cell electrical shorting. Additionally, the dielectric coating is prone to damage during thin-film deposition on the substrate, as well as during the monolithic integration patterning process.

Alternately, a dielectric substrate may be used with monolithic integration. However, few flexible substrate materials are both dielectric and able to withstand high temperature processing associated with thin-film layer deposition. One possible flexible dielectric substrate material is flexible glass. However, flexible glass substrates are still in their developmental stage and are not available for use in high volume production applications. Another flexible dielectric substrate material is polyimide. Thin polyimide substrates are widely available and some formulations can often survive processing temperatures of up to 450 degrees Celsius for short periods of time, such as for 30 minutes or less.

FIG. 2illustrates a prior art CIGS deposition zone200adapted to form a CIGS solar absorber layer on a flexible polyimide substrate202having a thickness230and opposing front and back outer surfaces218,228, respectively. Substrate202has been prepped with an electrical contact layer (not shown) on front outer surface218, where the contact layer is analogous to layer106ofFIG. 1. A stress relief layer (not shown) analogous to layer118ofFIG. 1is optionally disposed on back outer surface228. Deposition zone200includes a plurality of sources that respectively emit metallic plumes212,214,216that are subsequently disposed in front210of substrate202. In this system, three sources204,206, and208dispose metallic elements onto front outer surface218of substrate202; these elements typically include one or more of Copper, Indium, and Gallium. A Selenium manifold220provides Selenium vapor222to deposition zone200in front210of substrate202. Substrate heaters224, which are disposed behind226substrate202, provide radiant heat to back outer surface228of substrate202. A zone enclosure and vacuum pump (not shown) maintain a vacuum in deposition zone200. Copper, Indium, and Gallium emitted from sources204,206,208onto front outer surface218of substrate202collectively react in the presence of Selenium vapor222to form a precursor of, or the entirety of, a CIGS layer on substrate front outer surface218, given a proper thermodynamic environment.

Metallic sources204,206,208provide some heat to front outer surface218. Furthermore, additional heaters (not shown) are sometimes disposed in front210of substrate202to enhance early deposition run stability and to prevent elements from condensing and re-evaporating. These additional heaters, if present, will also somewhat heat front outer surface218. However, substrate heaters224, which heat substrate back outer surface228, provide the majority of thermal energy necessary to enable CIGS deposition. Substrate heaters224may require different set points relative to one another to achieve the desired substrate202temperature, due to different levels of heating provided by sources204,206, and208. As known in the art, CIS/CIGS deposition requires high substrate temperatures, and photovoltaic device efficiency is often dependent on substrate temperature. For example, substrate temperatures in excess of 500 degrees Celsius are typically required to obtain highest efficiency CIS/CIGS photovoltaic devices. Additionally, studies have shown potential advantages of increasing the Se thermal energy during thin-film deposition, such as by using a Se furnace or cracker, to reduce the Se evaporant cluster size during the co-evaporated CIGS process. (See, e.g., M. Kawamura et al., “Cu(InGa)Se2 thin-film solar cells grown with cracked selenium,” Journal of Crystal Growth, vol. 311, Jan. 15, 2009, pp. 753-756). While rigid glass substrates are durable to these temperatures, flexible polyimide substrates degrade at elevated temperatures, thereby limiting CIS/CIGS deposition processes on polyimide substrates. Additionally, polyimide substrates are highly susceptible to absorbing water vapor, which can subsequently be released when heated during CIS/CIGS fabrication. Unless extreme care is taken to ensure that polyimide substrates are properly degassed prior to electrical contact layer deposition, moisture may be trapped under the first electrical contact layer during substrate heating, potentially leading to PV device cracking and blistering. Cracking and blistering may dramatically impair photovoltaic device performance and/or manufacturing yield.

SUMMARY

In an embodiment, a method for depositing one or more thin-film layers on a flexible polyimide substrate having opposing front and back outer surfaces includes the following steps: (a) heating the flexible polyimide substrate such that a temperature of the front outer surface of the flexible polyimide substrate is higher than a temperature of the back outer surface of the flexible polyimide substrate; and (b) depositing the one or more thin-film layers on the front outer surface of the flexible polyimide substrate.

In an embodiment, a deposition zone for depositing a material on a substrate, where the substrate has a first outer surface of depth by width, includes the following: (a) one of more physical vapor deposition sources adapted to deposit one or more metallic materials on the first outer surface of the substrate; and (b) one or more radiant zone boundary heaters. Each radiant zone boundary heater includes an outer heating surface adapted to emit infrared radiation, and at least one outer heating surface is angularly displaced from the first outer surface of the substrate by an angle of less than ninety degrees. Each outer heating surface is in line of sight with at least part of the first outer surface of the substrate.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As discussed above, conventional systems for depositing CIS/CIGS on a polyimide substrate include substrate heaters adapted to radiantly heat the substrate's back outer surface, while deposition equipment deposits CIS/CIGS on the front outer surface. The reasons that conventional systems employ back outer surface heating are twofold: (1) it was conventionally thought that back surface heating was necessary to achieve uniform heating for robust CIS/CIGS deposition, and (2) back surface heating allows substrate heaters to be disposed outside of the paths of CIS/CIGS sources.

However, Applicants' thermal modeling indicates that there is a small, but not insignificant, thermal gradient across the substrate's thickness, during typical CIS/CIGS deposition. For example, heating applied to the back outer surface228side of substrate202leads to a small (e.g., 3-7° C.) thermal gradient across the substrate's thickness230, when a contact layer is disposed on front outer surface218(seeFIG. 2). Thus, substrate front outer surface218is slightly cooler than back outer surface228under these conditions. This thermal gradient can be more or less significant, depending on the quality of the contact between the coating and the substrate. A front-to-back temperature gradient is not a problem with conventional glass substrates or metallic substrates, the latter with presumable little or no gradient, because substrate heaters can be adjusted to achieve sufficient heat on the substrate's front outer surface as needed for reacting high quality CIS/CIGS without damaging the substrate. However for temperature limited substrates such as polyimide, or possibly thin glass, the substrate heaters cannot be adjusted higher than the substrate temperature limit, thereby potentially leading to less heat to the front outer surface for reacting high quality CIS/CIGS. Thus, for a 5° C. thermal gradient across the substrate thickness, applying heat to the substrate front side would result in a 10° C. lower back side temperature to achieve the same front side temperature when compared to back side heating.

Applicants have determined that the thermal gradient during CIS/CIGS deposition can be attributed to a combination of the insulating properties of the polyimide substrate, a heat reflective front side metallic layer, and process driven active radiative heating and cooling in the vacuum chamber. For example,FIG. 3shows a cross sectional view of a polyimide substrate300including a front metallic layer302on its front outer surface304and a back side metallic layer306on its back outer surface308. Heat is transferred to and from substrate300during CIS/CIGS deposition primarily by radiation, since the deposition is performed in a vacuum.

Consider the scenario where back outer surface308is heated by radiation310impinging outer surface322of back side metallic layer306. A portion312of radiation310is reflected away by back side metallic layer306, while a portion314of radiation310is transmitted through back side metallic layer306and is partially absorbed by the substrate300. Due to the relatively low thermal conductivity of substrate300, heat conducts relatively slowly through substrate300, such that the back316of substrate300is warmer than the front318of the substrate, aided by heat radiation away from front contact302to the cooler chamber ambient, despite some incident heat radiation from the hot sources. Additionally, some of radiation314that is not absorbed by substrate300is reflected by front metallic layer302back into substrate300, as indicated by arrow320, which further heats substrate300. Similarly, when no back side metal is used, primary radiation from the back side heaters and reflected radiation from the back of front metallic layer302result in substrate heating. In either case, front outer surface324is radiatively cooled because its view is of mostly cooler surfaces.

Accordingly, Applicants have determined that it is better to heat a polyimide substrate's front outer surface, instead of its back outer surface, when depositing CIS/CIGS on the front outer surface. For example, heating the front outer surface potentially allows heating radiation to be substantially confined to the substrate front side where CIS/CIGS deposition occurs, thereby providing the necessary energy to support CIS/CIGS reaction, without excessively heating the entire substrate as required using conventional techniques. This ability to avoid excessive substrate heating while enhancing the CIS/CIGS reaction promotes device reliability and high manufacturing yield by reducing substrate degradation. Furthermore, heating the substrate's front outer surface may provide sufficient thermal energy to achieve small Se evaporant cluster size during a co-evaporated CIGS deposition process, without requiring use of a Se furnace or cracker.

Applicants have also discovered that polyimide substrate heating can be further minimized during CIS/CIGS deposition on the front outer surface by promoting radiant emission of heat from the back outer surface.FIG. 4shows a cross-sectional view of a polyimide substrate400having opposing front and back outer surfaces402,404. A front metallic layer406, such as a Molybdenum layer, is disposed on front outer surface402. Assume that substrate400is heated by radiation408impinging on front metallic layer406. A portion410of the radiation will flow through front metallic layer406into substrate400, along with some conducted heat flow. It is desirable that heat radiation410exit back outer surface404to prevent the radiation410from being trapped inside substrate400, thereby helping minimize substrate heating. As discussed above, excessive substrate heating can cause a number of undesired effects, and it is therefore desirable to minimize substrate heating.

With respect toFIG. 4, a back metallic layer reflects heat from the substrate and front metallic layer back into the substrate, thereby impeding radiation of heat away from the substrate. Such impediment can be removed by either omitting the back metallic coating, or by using an infrared transparent coating412as a stress matching layer in place of the back metallic coating. Additionally, heat radiance away from the substrate can be promoted by making the transparent coating412a high emittance coating. In some embodiments, the high emittance coating is a dielectric coating to reduce the risk of undesired electrical shorting of photovoltaic cells. Some examples of high emittance coatings include transparent oxides and optically-transparent nitrides, such as Al2O3, SiOx. These materials typically achieve infrared transmission and high emittance, with minimal absorption and low conductivity

FIG. 5illustrates one CIGS deposition zone500adapted to deposit CIGS on a flexible, polyimide substrate502. Deposition zone500has a length504, a height506, and a width (not labeled) perpendicular to the length and height directions. Deposition zone500includes an enclosure508and a vacuum pump510that collectively form a vacuum chamber adapted to maintain a vacuum inside enclosure508. A substrate handling apparatus (not shown) internal and/or external to zone500, such as pay-out and take-up spools and substrate supporting rollers, supports substrate502within zone500such that a front outer surface512of substrate502is substantially disposed in the length by width directions. In some embodiments, the substrate handling apparatus is also capable of translating substrate502through zone500in the length504direction.

Deposition zone500includes a plurality of physical vapor deposition sources adapted to deposit metallic materials; in this embodiment, three sources514,516, and518are illustrated. These sources provide, for example, Copper, Indium, and Gallium, as well as other elements necessary to produce a desired semiconductor compound in zone500. Physical vapor deposition sources514,516,518are disposed in front520of substrate502and are adapted to emit metallic plumes522,524, and526representing at least some of the elements required to produce the desired semiconductor compound. For example, in some embodiments, physical vapor deposition sources514,516, and518dispose Copper, Indium, and Gallium, respectively, on front outer surface512of substrate502. A Selenium manifold528is typically included to provide Selenium vapor530to deposition zone500.

Deposition zone500further includes one or more radiant zone boundary heaters532. In this document, specific instances of an item may be referred to by use of a numeral in parentheses (e.g., heater532(1)) while numerals without parentheses refer to any such item (e.g., heaters532). Each radiant zone boundary heater532has a respective outer heating surface536adapted to emit infrared radiation for radiant heating. Outer heating surfaces536are in line of sight with at least part of substrate outer surface512. In some embodiments, heaters532are electric radiant heaters. Radiant zone boundary heaters532(1)-532(4) are placed such that their respective heating surfaces536are disposed substantially in the length by width directions, such that the heating surfaces have a high view factor with at least part of substrate front outer surface512. In the context of this document, view factor of a first surface to a second surface is the ratio of (i) radiation leaving the first surface which strikes the second surface to (ii) the total radiation leaving the first surface. For example, the view factor of heating surface536(1) to substrate outer surface512is the ratio of (i) infrared radiation leaving surface536(1) and striking substrate surface512to (ii) total infrared radiation leaving heating surface536(1). Radiant zone boundary heaters532(5) and532(6), on the other hand, are placed such that their respective heating surfaces536are angularly displaced from substrate front outer surface512by a respective angle540that is less than ninety degrees, such as forty-five degrees. Such placement of heaters532(5),532(6) results in their respective heating surfaces536having a higher view factor to the substrate outer surface512than would be achieved if the heaters were disposed with their heating surfaces perpendicular to the plane of outer surface512. Disposing heaters such that they have high view factors to the substrate front outer surface512enables direct radiant heating of front outer surface512, such that a temperature of front outer surface512is higher than the temperature of a back outer surface542of substrate502. Heat from physical vapor deposition sources514,516,518will also typically directly heat front outer surface512such that these sources and zone boundary heaters532collectively provide the necessary thermal energy such that Copper, Indium, and Gallium react with Selenium at front outer surface512to form CIGS.

It is anticipated that additional deposition zones will typically be used in conjunction with deposition zone500, where each additional deposition zone deposits one or more additional thin-film layers on substrate502, such that the deposited thin-film layers collectively form a thin-film stack on substrate502. For example, an additional deposition zone may be positioned upstream from zone500to deposit an electrical contact layer on substrate front outer surface512before substrate502enters zone500. As another example, an additional deposition zone may be positioned downstream from zone500to deposit a heterojunction partner layer on the CIGS layer deposited in zone500, such that the CIGS layer and heterojunction partner layer collectively form a P-N photovoltaic junction. Furthermore, multiple instances of deposition zone500are optionally employed to deposit multiple CIGS sublayers on substrate502, such as by using techniques disclosed in U.S. Pat. No. 8,021,905 to Nath et al. Moreover, although deposition zone500is shown having a respective enclosure508and vacuum pump510, in some alternate embodiments, zone500shares an enclosure and/or vacuum pump with one or more other deposition zones.

FIG. 6shows a cross-sectional view of a photovoltaic device600, which is one example of a photovoltaic device including a CIGS layer formed by deposition zone500. Deposition zone500, however, is not limited to forming the CIGS layer of photovoltaic device600. Photovoltaic device600includes a first electrical contact layer602formed on substrate front outer surface512, a CIGS solar absorber layer604formed by deposition zone500on contact layer602, a heterojunction partner layer606formed on solar absorber layer604, and a second electrical contact layer608formed on heterojunction partner layer606. Layers602,604,606,608collectively form a thin-film stack610on front outer surface512. Solar absorber layer604and heterojunction partner layer606collectively form a P-N junction, which generates electron-hole pairs in response to incident light. Photovoltaic device600optionally further includes a high emittance oxide layer612formed on substrate back outer surface542, to facilitate radiation of heat from back outer surface542when forming one or more layers of stack610. Additional layers, such as buffer layers and/or stress relief layers, may be added to photovoltaic device600without departing from the scope hereof.

Although deposition zone500is shown as being adapted to deposit CIGS on substrate502, zone500could be modified to deposit CIS, another alloy of CIS, or even a material other than CIS/CIGS that requires high deposition temperatures on temperature limiting substrates, without departing from the scope hereof. Additionally, although deposition zone500is shown as configured such that only substrate front outer surface512is directly radiantly heated, it should be understood that zone500could be modified to additionally include heaters behind544substrate502which are adapted to radiantly back outer surface542of substrate502. However, if back surface heaters are employed, deposition zone500should be configured so that radiant heat generating elements directly radiantly heating front outer surface512, such as zone boundary heaters532and physical vapor deposition sources514,516,518, provide most of the thermal energy required for CIS/CIGS reaction on substrate502, to realize the advantages associated with front outer surface heating discussed above. In an alternate embodiment, deposition zone500further includes substrate chillers (not shown) disposed behind544substrate502to enhance radiative heat transfer from substrate back outer surface542, thereby enabling a greater front outer surface512temperature, while protecting substrate502from excessive heat buildup.

FIG. 7illustrates a method700for depositing one or more thin-film layers on a flexible polyimide substrate including opposing front and back outer surfaces. Method700begins with step702of heating the front outer surface of the substrate, such that a temperature of the front outer surface is higher than a temperature of the back outer surface. An example of step702is heating front outer surface512of substrate502using zone boundary heaters532, such that front outer surface512is at a higher temperature than back outer surface542(seeFIG. 5). In step704, one or more thin-film layers are deposited on front outer surface512. An example of step704is forming GIGS layer604on front outer surface512of substrate502by reacting Copper, Indium, and Gallium from sources514,516,518, respectively, in the presence of Selenium from manifold528at elevated temperature, to form GIGS layer604on front outer surface512(FIGS. 5 and 6). While steps702and704are shown as being separate steps, it is anticipated that at least parts of these steps will be conducted concurrently in many embodiments.

FIG. 8illustrates another GIGS deposition zone800adapted to deposit GIGS on flexible, polyimide substrate502in a similar fashion to that shown inFIG. 5. Deposition zone800is similar to deposition zone500ofFIG. 5, but with several zone boundary heaters532replaced with segmented zone boundary heaters802. Segmented zone boundary heaters802are aligned in such a way as to enable uniform heating while increasing the view factor towards substrate502, thereby increasing the efficiency by which thermal energy is transferred from heaters802to the substrate, with minimal impact upon the zone length504. Shields804are optionally disposed in spaces between adjacent heaters802to contain selenium vapor530, and/or other deposition elements, in zone800. Shields804are optionally fabricated to accommodate electrical and/or optical pass-throughs to accommodate non-contact means for monitoring CIGS deposition in zone800. Each zone boundary heater802has a respective outer heating surface806adapted to emit infrared radiation for radiant heating. Heaters802are disposed such that each outer heating surface806is in line of sight with at least part of substrate outer surface512. Each heater802is disposed such that its outer heating surface806is angularly displaced from substrate front outer surface512by a respective angle808that is less than ninety degrees, such as forty-five degrees. Such placement of heaters802results in their respective heating surfaces806having a higher view factor to substrate outer surface512than would be achieved if the heaters were disposed with their heating surfaces perpendicular to outer surface512. Only some instances of shields804, outer heating surfaces806, and angles808are labeled inFIG. 8to promote illustrative clarity.

Although the systems and methods disclosed herein are discussed in the context of CIS/CIGS deposition, they could also be applied to high temperature deposition of other materials, such as high temperature deposition of high-speed crystalline thin-film transistors for flexible displays and/or for flexible electronics. Additionally, the systems and methods disclosed herein are not limited to use with polyimide substrates, or even polymers with additives and fillers, but are also applicable to other temperature sensitive substrates with low thermal conductivity, such as thin, flexible glass and possibly insulated metallic foils. These substrates may possess higher temperature limits than polyimide, but can exhibit a combination of sensitivity to temperature and structural integrity that can also benefit from the disclosed systems and methods.

Combinations of Features

Features described above as well as those claimed below may be combined in various ways without departing from the scope hereof. The following examples illustrate some possible combinations:

(A1) A method for depositing one or more thin-film layers on a flexible polyimide substrate having opposing front and back outer surfaces may include the following steps: (a) heating the flexible polyimide substrate such that a temperature of the front outer surface of the flexible polyimide substrate is higher than a temperature of the back outer surface of the flexible polyimide substrate; and (b) depositing the one or more thin-film layers on the front outer surface of the flexible polyimide substrate.

(A2) In the method denoted as (A1), the step of heating may include radiantly heating the front outer surface of the flexible polyimide substrate using one or more radiant heat generating elements, where each of the one or more radiant heat generating elements is at least partially in line of sight with the front outer surface of the flexible polyimide substrate.

(A3) In the method denoted as (A2), the one or more radiant heat generating elements may include electric radiant heaters.

(A4) Any of the methods denoted as (A1) through (A3) may further include maintaining a vacuum around the flexible polyimide substrate during the steps of heating and depositing.

(A5) In any of the methods denoted as (A1) through (A4), the step of depositing may include depositing a material selected from the group consisting of Copper-Indium-DiSelenide and an alloy of Copper-Indium-DiSelenide on the front outer surface of the flexible polyimide substrate.

(A6) In any of the methods denoted as (A1) through (A5), the step of depositing may include reacting at least Copper and/or Indium in the presence of Selenium.

(A7) Any of the methods denoted as (A1) through (A6) may further include depositing an oxide layer on the back outer surface of the flexible polyimide substrate.

(B1) A deposition zone for depositing a material on a substrate, where the substrate has a first outer surface of depth by width, may include the following: (a) one of more physical vapor deposition sources adapted to deposit one or more metallic materials on the first outer surface of the substrate; and (b) one or more radiant zone boundary heaters. Each radiant zone boundary heater may include an outer heating surface adapted to emit infrared radiation, and at least one outer heating surface may be angularly displaced from the first outer surface of the substrate by an angle of less than ninety degrees. Each outer heating surface may be in line of sight with at least part of the first outer surface of the substrate.

(B2) In the deposition zone denoted as (B1), at least one of the one or more radiant zone boundary heaters may include an outer heating surface disposed in the length and width directions.

(B3) In either of the deposition zones denoted as (B1) or (B2), the one or more radiant zone boundary heaters may include an electric radiant heater.

(B4) Any of the deposition zones denoted as (B1) through (B3) may further include an enclosure and a vacuum pump adapted to maintain a vacuum in the deposition zone.

(B5) In any of the deposition zones denoted as (B1) through (B4), the one or more physical vapor deposition sources may be adapted to deposit at least one material selected from the group consisting of Copper, Gallium, and Indium, on the first outer surface of the substrate.

(B6) Any of the deposition zones denoted as (B1) through (B5) may further include a Selenium manifold adapted to provide Selenium vapor to the deposition zone.

(B7) In any of the deposition zones denoted as (B1) through (B6), the one or more physical vapor deposition sources may be adapted to deposit at least Copper, Gallium, and Indium on the first outer surface of the substrate.

(B8) In any of the deposition zones denoted as (B1) through (B7), the one or more radiant zone boundary heaters may include a plurality of radiant heaters separated from each other.

(B9) In the deposition zone denoted as (B8), the plurality of radiant heaters separated from each other may include a first and second radiant heater, and the deposition zone may further include a shield disposed between the first and second radiant heaters, where the shield is adapted to contain one or more deposition elements in the deposition zone.

(B10) In any of the deposition zones denoted as (B1) through (B7), the one or more radiant zone boundary heaters may be a single radiant heater.