Patent Publication Number: US-2013252367-A1

Title: System and process for forming thin film photovoltaic device

Description:
FIELD OF THE INVENTION 
     The subject matter disclosed herein relates generally to methods and systems for forming thin film photovoltaic devices. More particularly, the subject matter disclosed herein relates generally to methods and systems for heat strengthening glass substrates of thin film photovoltaic devices. 
     BACKGROUND OF THE INVENTION 
     Solar energy systems using cadmium telluride (CdTe) photovoltaic (PV) devices, also known as modules, are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner. 
     Certain factors greatly affect the efficiency of CdTe PV modules in terms of cost and power generation capacity. For example, the use of relatively thin glass substrates, which may be top layers, also known as front or sunny-side faces, of the modules during use, can limit the absorption of light energy by the glass substrate in use, allowing more light to reach the PV thin films. Furthermore, thin glass can be less expensive than thick glass. However, the use of relatively thin glass substrates reduces the strength of the glass. As such, the glass may be more susceptible to breakage in use. 
     Thus, a need exists for a PV module having improved strength in its glass substrate and methods of manufacturing the same. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In one embodiment, a process of forming a thin film photovoltaic device is disclosed. The process includes heating a thin film photovoltaic sub-device to an anneal temperature. The thin film photovoltaic sub-device includes a glass substrate and a transparent conductive oxide deposited on the glass substrate. The process further includes quenching the thin film photovoltaic sub-device with a quenching gas to cool the thin film photovoltaic sub-device to a quenched temperature. The quenching gas includes an inert gas. 
     In another embodiment, a system for forming a thin film photovoltaic device is disclosed. The system includes a chamber generally sealed from an external environment. The system further includes a quenching system supplying a quenching gas to the chamber to quench a thin film photovoltaic sub-device supported within the chamber. The quenching system includes a manifold assembly. At least a portion of the manifold assembly is disposed within the chamber and defines at least one outlet for the quenching gas to flow therethrough into the chamber. The quenching gas includes an inert gas. 
     The chamber further includes a quenching system supplying a quenching gas to the chamber to quench a glass substrate supported within the chamber. The quenching system includes a manifold assembly. At least a portion of the manifold assembly is disposed within the chamber and defines at least one outlet for quenching gas to flow therethrough into the chamber. 
     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims, or may be obvious from the description or claims, or may be learned through practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, is set forth in the specification, which makes reference to the appended drawings, in which: 
         FIG. 1  is a plan view of a system for forming a thin film PV device according to one embodiment of the present disclosure; 
         FIG. 2  is a perspective view of a quenching apparatus according to one embodiment of the present disclosure; 
         FIG. 3  is a cross-sectional view of a quenching apparatus according to one embodiment of the present disclosure; 
         FIG. 4  is a top perspective view of a manifold assembly of a quenching apparatus according to one embodiment of the present disclosure; 
         FIG. 5  is a bottom perspective view of a manifold assembly of a quenching apparatus according to one embodiment of the present disclosure; and, 
         FIG. 6  is a cross-sectional view of a CdTe PV module according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention encompass such modifications and variations as come within the scope of the appended claims and their equivalents. 
     In the present disclosure, when a layer is being described as “on” or “over” another layer or substrate, it is to be understood that the layers can either be directly contacting each other or have another layer or feature between the layers, unless otherwise stated. Thus, these terms are simply describing the relative position of the layers to each other and do not necessarily mean “on top of” since the relative position above or below depends upon the orientation of the device to the viewer. 
     Additionally, although the invention is not limited to any particular film thickness, the term “thin” describing any film layers of the photovoltaic device generally refers to the film layer having a thickness less than about 10 micrometers (“microns” or “μm”). 
     It is to be understood that the ranges and limits mentioned herein include all ranges located within the prescribed limits (i.e., subranges). For instance, a range from about 100 to about 200 also includes ranges from 110 to 150, 170 to 190, 153 to 162, and 145.3 to 149.6. Further, a limit of up to about 7 also includes a limit of up to about 5, up to 3, and up to about 4.5, as well as ranges within the limit, such as from about 1 to about 5, and from about 3.2 to about 6.5. 
       FIG. 6  represents an exemplary CdTe PV module  10  that can be made at least in part according to system and method embodiment described herein. The module  10  includes a top sheet of glass as the substrate  12 , which may be a high-transmission glass (e.g., high transmission borosilicate glass), low-iron float glass, or other highly transparent glass material. The glass is generally thick enough (e.g., from about 0.5 mm to about 10 mm thick) to provide support for the subsequent film layers, and is substantially flat to provide a good surface for forming the subsequent film layers. 
     A transparent conductive oxide (TCO) layer  14  is shown on the substrate  12  of the module  10  in  FIG. 6 . The TCO layer  14  allows light to pass through with minimal absorption while also allowing electric current produced by the module  10  to travel sideways to opaque metal conductors (not shown). The TCO layer  14  can have a thickness between about 0.1 μm and about 1 μm, for example from about 0.1 μm to about 0.5 μm, such as from about 0.25 μm to about 0.35 μm. The TCO may in exemplary embodiments comprise or consist of cadmium stannate, cadmium tin oxide, or any other suitable TCO. 
     The TCO layer  14  can be deposited on the substrate  12  by sputtering, chemical vapor deposition, spray pryolysis, or any other suitable deposition method. In particular embodiments, the TCO layer  14  is formed by sputtering (e.g. DC sputtering or RF sputtering) on the substrate  12 . For example, the TCO layer  14  can be deposited using a DC sputtering method by applying a DC current to a metallic source material (e.g., elemental zinc, elemental tin, or a mixture thereof) and sputtering the metallic source material onto the substrate  12  in the presence of an oxidizing atmosphere (e.g., O 2  gas). 
     A resistive transparent buffer (RTB) layer  16  is shown on the TCO layer  14 . This layer  16  is generally more resistive than the TCO layer  14  and can help protect the module  10  from chemical interactions between the TCO layer  14  and the subsequent layers during processing of the module  10 . In certain embodiments, the RTB layer  16  can have a thickness between about 0.075 μm and about 1 μm, for example from about 0.1 μm to about 0.5 μm. In particular embodiments, the RTB layer  16  can have a thickness between about 0.08 μm and about 0.2 μm, for example from about 0.1 μm to about 0.15 μm. In particular embodiments, the RTB layer  16  can include, for instance, a combination of zinc oxide (ZnO) and tin oxide (SnO 2 ), and is referred to as a zinc-tin oxide (“ZTO”) layer  16 . 
     The ZTO layer  16  can be deposited by sputtering, chemical vapor deposition, spraying pryolysis, or any other suitable deposition method. In particular embodiments, the ZTO layer  16  is formed by sputtering (e.g. DC sputtering or RF sputtering) on the TCO layer  14 . For example, the layer  16  can be deposited using a DC sputtering method by applying a DC current to a metallic source material (e.g., elemental zinc, elemental tin, or a mixture thereof) and sputtering the metallic source material onto the TCO layer  14  in the presence of an oxidizing atmosphere (e.g., O 2  gas). 
     A cadmium sulfide (“CdS”) layer  18  is shown on ZTO layer  16  of the module  10  of  FIG. 6 . The CdS layer  18  is a n-type layer that generally includes cadmium sulfide but may also include other materials, such as zinc sulfide, cadmium zinc sulfide, etc., and mixtures thereof, as well as dopants and other impurities. The CdS layer  18  may include oxygen up to about 25% by atomic percentage, for example from about 5% to about 20% by atomic percentage. The CdS layer  18  can have a wide band gap (e.g., from about 2.25 eV to about 2.5 eV, such as about 2.4 eV) in order to allow most radiation energy (e.g., solar radiation) to pass therethrough. As such, the cadmium sulfide layer  18  is considered to be a transparent (or “window”) layer in the module  10 . 
     The CdS layer  18  can be formed by sputtering, chemical vapor deposition, chemical bath deposition, or any other suitable deposition method. In one particular embodiment, the CdS layer  18  is formed by sputtering (e.g., radio frequency (RF) sputtering) onto the RTB layer  16 , and can have a thickness that is less than about 0.1 μm. This decreased thickness of less than about 0.1 μm reduces absorption of radiation energy by the CdS layer  18 , effectively increasing the amount of radiation energy reaching the underlying CdTe layer  20 . 
     A CdTe layer  20  is shown on the CdS layer  18  in the exemplary module  10  of  FIG. 6 . The layer  20  is a p-type layer that generally includes cadmium telluride (CdTe), but may also include other materials. As the p-type layer of the module  10 , the CdTe layer  20  is the photovoltaic layer that interacts with the CdS layer  18  (i.e., the n-type layer) to produce current from the absorption of radiation energy by absorbing the majority of the radiation energy passing into the module  10  due to its high absorption coefficient and creating electron-hole pairs. The CdTe layer  20  can have a bandgap tailored to absorb radiation energy (e.g., from about 1.4 eV to about 1.5 eV, such as about 1.45 eV) to create the maximum number of electron-hole pairs with the highest electrical potential (voltage) upon absorption of the radiation energy. Electrons may travel from the p-type side (i.e., the CdTe layer  20 ) across the junction to the n-type side (i.e., the CdS layer  18 ) and, conversely, holes may pass from the n-type side to the p-type side. Thus, the p-n junction formed between the CdTe layer  18  and the CdTe layer  20  forms a diode in which the charge imbalance leads to the creation of an electric field spanning the p-n junction. Conventional current is allowed to flow in only one direction and separates the light induced electron-hole pairs. 
     The cadmium telluride layer  20  can be formed by any known process, such as vapor transport deposition, chemical vapor deposition (CVD), spray pyrolysis, electro-deposition, sputtering, close-space sublimation (CSS), etc. In particular embodiments, the CdTe layer  20  can have a thickness between about 0.1 μm and about 10 μm, such as from about 1 μm and about 5 μm. 
     A back contact layer  22  generally serves as the back electrical contact, in relation to the opposite TCO layer  14  serving as the front electrical contact. The back contact layer  22  can be formed on, and in one embodiment is in direct contact with, the CdTe layer  20 . The back contact layer  22  is suitably made from one or more highly conductive materials, such as elemental nickel, chromium, copper, tin, aluminum, gold, silver, technetium or alloys or mixtures thereof. Additionally, the back contact layer  22  can be a single layer or can be a plurality of layers. In one particular embodiment, the back contact layer  22  can include graphite, such as a layer of carbon deposited on the p-layer followed by one or more layers of metal, such as the metals described above. The back contact layer  22 , if made of or comprising one or more metals, is suitably applied by a technique such as sputtering or metal evaporation. If it is made from a graphite and polymer blend, or from a carbon paste, the blend or paste is applied to the semiconductor device by any suitable method for spreading the blend or paste, such as screen printing, spraying or by a “doctor” blade. After the application of the graphite blend or carbon paste, the device can be heated to convert the blend or paste into the conductive back contact layer. A carbon layer, if used, can be from about 0.1 μm to about 10 μm in thickness, for example from about 1 μm to about 5 μm. A metal layer of the back contact, if used for or as part of the back contact layer  22 , can be from about 0.1 μm to about 1.5 μm in thickness. 
     In the embodiment of  FIG. 6 , an encapsulating glass  24  is shown on the back contact layer  22 . 
     Other components (not shown) can be included in the exemplary module  10 , such as bus bars, external wiring, laser etches, etc. The module  10  may be divided into a plurality of individual cells that are connected in series in order to achieve a desired voltage, such as through an electrical wiring connection. Each end of the series connected cells can be attached to a suitable conductor, such as a wire or bus bar, to direct the photovoltaically generated current to convenient locations for connection to a device or other system using the generated electric. A convenient means for achieving the series connected cells is to laser scribe the module  10  to divide the device into a series of cells connected by interconnects. Also, electrical wires can be connected to positive and negative terminals of the PV module  10  to provide lead wires to harness electrical current produced by the PV module  10 . 
     Referring now to  FIG. 1 , one embodiment of a system for forming a thin film PV device is illustrated. The system  100  may include, for example, one or more annealing apparatus  102  and one or more quenching apparatus  104 . The annealing apparatus  102  and quenching apparatus  104  may be sequentially positioned within the system  100  to at least partially form a thin film PV device  10 . For example, the annealing apparatus  102  and quenching apparatus  104  may respectively anneal and quench a thin film photovoltaic sub-device during forming of the device  10 . A sub-device according to the present disclosure includes various layers of the device  10  before completion of the device  10 . For example, a sub-device may include a glass substrate  12  and a TCO layer  14  deposited thereon. Deposition of the TCO layer  14  on the glass substrate  12  may occur before annealing of the sub-device by the annealing apparatus  102 . Further, annealing and quenching by the annealing apparatus  102  and quenching apparatus  104  may occur prior to depositing or otherwise forming of any other layers on the sub-device, such as an RTB layer  16 , CdS layer  18 , CdTe layer  20 , back contact layer  22 , or encapsulating glass  24 , as discussed above. Thus, the sub-device may comprise only the TCO layer deposited on the glass substrate  12  during annealing and quenching by the annealing apparatus  102  and quenching apparatus  104 . 
     Through annealing and quenching, strength can be added to the sub-device and glass substrate  12  thereof by creating compressive stresses therein, which can help the sub-device and glass substrate  12  thereof endure thermal stress in normal use (e.g., created through temperature variations when deployed in the field) and during processing thereof. This increased strength can, for example, help reduce the occurrence of panel breakage of the resulting PV device once placed in operation in the field. Thus, thinner glass substrates  12  may be able to be used through adding strength to the glass in production of the PV device. For example, the glass substrate  12  can be a borosilicate glass having a thickness of about 0.5 mm to about 2.5 mm, such as about 0.7 mm to about 1.3 mm. 
     Additionally, quenching may reduce or prevent warping of the glass substrate  12  during the production process. In particular, the speed and uniformity of quenching according to the present disclosure may reduce or prevent warping despite the use of increasingly high annealing temperatures. 
     As shown in  FIG. 1 , during forming of devices  10 , individual sub-device may be moved through a series of stations, including the annealing apparatus  102  and quenching apparatus  104 . The sub-devices as shown are moved through the stations in direction of movement D m . For example, substrate  12  may initially be placed onto a load conveyor  106  and subsequently moved into an entry vacuum lock station  110  that includes a load vacuum chamber  112  and a load buffer chamber  114 . A “rough” (i.e., initial) vacuum pump  116  is in communication with the load vacuum chamber  112  to drawn an initial load pressure, and a “fine” (i.e., final) vacuum pump  118  is in communication with the load buffer chamber  114  to increase the vacuum (i.e. decrease the initial load pressure) in the load buffer chamber  112  to reduce the vacuum pressure within the entry vacuum lock station  110 . Valves  120  (e.g., gate-type slit valves or rotary-type flapper valves) are operably disposed between the load conveyor  106  and the load vacuum chamber  112 , between the load vacuum chamber  112  and the load buffer chamber  114 , and between the load buffer chamber  114  and a neighboring station. These valves  120  are sequentially actuated by a motor or other type of actuating mechanism  122  in order to introduce the substrates  12  into the vacuum station  110  in a step-wise manner without affecting the vacuum within the subsequent station. 
     In operation of the system  10 , an operational vacuum is maintained in the system  10  by way of any combination of rough and/or fine vacuum pumps  124 . In order to introduce a substrate  12  into the load vacuum station  110 , the load vacuum chamber  112  and load buffer chamber  114  are initially vented (with the valve  120  between the two modules in the open position). The valve  120  between the load buffer chamber  114  and the subsequent station is closed. The valve  120  between the load vacuum chamber  112  and load conveyor  106  is opened and a substrate  12  is moved into the load vacuum chamber  112 . At this point, the first valve  120  is shut and the rough vacuum pump  116  then draws an initial vacuum in the load vacuum chamber  112  and load buffer chamber  114 . The substrate  12  is then conveyed into the load buffer chamber  114 , and the valve  120  between the load vacuum chamber  112  and load buffer chamber  114  is closed. The fine vacuum pump  118  then increases the vacuum level in the load buffer chamber  114  to approximately the same vacuum level in the subsequent station. At this point, the valve  120  between the load buffer chamber  114  and subsequent station is opened and the substrate  12  is conveyed into this station. 
     Thus, the substrates  12  are transported into the exemplary system  10  first through the load vacuum chamber  112  that draws a vacuum in the load vacuum chamber  112  to an initial load pressure. For example, the initial load pressure can be less than about 250 mTorr, such as about 1 mTorr to about 100 mTorr. Optionally, a load buffer chamber can reduce the pressure to about 1×10 −7  Torr to about 1×10 −4  Ton, and can then be backfilled with an inert gas (e.g., argon, nitrogen, etc.) in a subsequent chamber within the system  10  (e.g., within a sputtering deposition chamber) to a deposition pressure (e.g., about 10 mTorr to about 100 mTorr). 
     In another embodiment, the apparatus is operated in a purged mode. Instead of pumping out atmosphere at the load station and backfilling to a desired pressure with an inert gas, the chamber is continuously filled at or slightly above atmospheric pressure with an inert gas, which thereby keeps atmospheric gases from entering the chamber. 
     The substrates  12  can then be transported into and through subsequent stations. For example, in some embodiments a substrate  12  may be transferred from the entry vacuum lock station  110  to a deposition station  130 . The deposition station  130  may include one or more deposition chambers  132  for depositing a material on the substrates  12 . The chambers  132  may deposit, for example, a TCO layer  14  on the glass substrate  12 . Deposition in each chamber may be a vapor deposition process, such as sputtering, chemical vapor deposition, physical vapor deposition, or any other suitable vapor deposition or deposition process. 
     As discussed, an operational vacuum may be maintained in the deposition chamber  130  and various stations  132  thereof through use of vacuum pumps  124  and valves  120 . The vacuum pumps  124  and valves  120  may be operated as discussed above to maintain the operational vacuum level during transfer of the substrate  12  to and from the deposition chamber  130  and stations  132  thereof, as well as during treatment thereof. 
     After the deposition of a TCO layer  14  on the glass substrate  12 , the sub-device may be transferred to a heating station  140 , which may include one or more annealing apparatus  102 . An annealing apparatus  102  may include heating chambers  142  for heating the substrate to an anneal temperature. Each annealing apparatus  102  may further include one or more independently controlled heaters  144 . Each heater  144  may be in communication with a heating chamber  142  to heat that chamber  142 . Each heater  144  may, for example, define an individual heat zone. A particular heat zone may include one or more heaters  144 . One or more heat zones may be defined in each chamber  142 . The heating chambers  142  can heat the substrates  14  to an anneal temperature, such as about 600° C. to about 650° C. or any other suitable temperature or range of temperatures that anneal the substrate  14 , in order to anneal the substrate  14 . When more than one heating chamber  142  or heater  144  is utilized, the substrate may be incrementally heated to the anneal temperature by subsequent heating chambers  142  or heaters  144 . Although shown with three heating chambers  142 , any suitable number of heating chambers  142  can be utilized. 
     As discussed, an operational vacuum may be maintained in the annealing apparatus  102  and various stations  142  thereof through use of vacuum pumps  124  and valves  120 . The vacuum pumps  124  and valves  120  may be operated as discussed above to maintain the operational vacuum level during transfer of the substrate  12  to and from the annealing apparatus  102  and the various stations  142 . 
     After annealing of the sub-device and glass substrate  12  thereof, the sub-device may be transferred to a quenching station  150 . The quenching station  150  may include one or more quenching apparatus  104 . A quenching apparatus  104  according to the present disclosure may be generally configured to rapidly cool the sub-device, after annealing, to a quenched temperature. The quenched temperature may be, for example, about 450° C. or less, such as about 425° C. or less, such as about 350° C. or less. Further, such quenching may occur relatively quickly, such as in a quenching time of about 4 seconds to about 30 seconds, such as from about 4 seconds to about 10 seconds. In one particular embodiment, the quenching apparatus  104  is generally configured to cool the sub-device, after annealing, to a quenched temperature of about 350° C. or less, in a quenching time of about 4 seconds to about 7 seconds. Such cooling may lock in compressive stresses within the sub-device, and may further minimize warping thereof. 
     A quenching gas is provided to a quenching apparatus  104  according to the present disclosure to quench the sub-device. The quenching gas can generally comprise an inert gas (e.g., argon, neon, nitrogen, helium, xenon, radon, etc., or mixtures thereof). Nitrogen may be a particularly advantageous quenching gas. In one particular embodiment, the quenching atmosphere is substantially non-reactive with the TCO layer  14  and/or the substrate  12 . For example, the quenching gas can, in one embodiment, consist essentially of an inert gas or mixture of inert gases, and is thus substantially free from oxygen and other reactive gases that could chemically interact with the deposited TCO layer  14  and/or the substrate  12 . As used herein, the term “substantially free” means no more than an insignificant trace amount present and encompasses completely free (e.g., 0 molar % up to 0.01 molar %). 
       FIGS. 2 through 5  illustrate an exemplary quenching apparatus  104 . The quenching apparatus  104  may include a chamber  152  and a quenching system  154 . The chamber  152  is a containment vessel that is generally sealed from the external environment. Access to the interior of the vessel for sub-devices may be through valves  120 . For example, as shown, a valve  120  may be included on opposing ends of the chamber  152 . The valves  120  may each be operated by an actuating mechanism  122 , as discussed above. Further, a chamber  152  for a quenching apparatus  104  may adjoin the prior upstream chamber in the direction of movement D m , such as a heating chamber  142  as shown in  FIG. 1  or additional quenching apparatus  104 . As discussed, an operational vacuum may be maintained in the chamber  152  through use of the valves  120  as well as vacuum pumps  124 . The vacuum pumps  124  and valves  120  may be operated as discussed above to maintain the operational vacuum during transfer of the sub-device to and from the chamber  104  of a quenching apparatus  104 . 
     The quenching system  154  may supply the quenching gas to the chamber  152  to quench sub-devices supported therein. For example, the quenching system  154  may include a manifold assembly  156 . At least a portion of the manifold assembly  156  is disposed within the chamber  152 , and quenching gas is flowed through the manifold assembly  156  to the chamber  152  to quench the sub-device. One or more outlets  158  may be defined in the portion of the manifold assembly  156  disposed in the chamber  152  for the quenching gas to flow through into the chamber  152 . 
     For example, a manifold assembly  156  may include one or more quench tubes  160 . The quench tubes  160  may be disposed within the chamber  152 . As shown, a plurality of quench tubes  160  may be aligned in rows in the chamber  152 , such as in a top row above the sub-device and a bottom row below the sub-device. Further, a longitudinal axis of each tube  160  may extend generally perpendicularly to the direction of movement D m . Outlets  158  may be defined in each quench tube  160 , such that quenching gas may be flowed through each tube  160  and into the chamber  160 . The outlets  158  may, for example, face the sub-device. In exemplary embodiments, the outlets  158  are positioned such that quenching gas flowed from each quench tube  160  is exhausted through the outlet  158  generally perpendicularly to the surface of the sub-device, such that the quenching gas initially impinges on the sub-device. Such impingement may further facilitate the quenching process. 
     The quench tubes  160  may be fed by pipes  162 , which are additionally included in the manifold assembly  156 . The pipes  162  may be in fluid communication with the quench tubes  160 , and may be used to distribute the quenching gas to the quench tubes  160 . An exemplary illustration of the flow of quenching gas through pipes  162  is shown by arrows in  FIG. 4 . At least a portion of the pipes  162  may be disposed within the chamber  152 . The pipes  162  may receive the quenching gas from one or more lead pipes, which may be in fluid communication with the pipes  162  to provide the quenching gas to the chamber  152 . As shown, for example, lead pipes included in the manifold assembly  156  may divide it into two or more zones.  FIGS. 2 through 5  illustrate, for example, a top upstream lead pipe  166 , a top downstream lead pipe  167 , a bottom upstream lead pipe  168 , and a bottom downstream lead pipe  169 . The top upstream lead pipe  166  supplies an upstream portion in the direction of movement D m  of the top row of quench tubes  160 . The top downstream lead pipe  167  supplies a downstream portion in the direction of movement D m  of the top row of quench tubes  160 . The bottom upstream lead pipe  168  supplies an upstream portion in the direction of movement D m  of the bottom row of quench tubes  160 . The bottom downstream lead pipe  169  supplies a downstream portion in the direction of movement D m  of the bottom row of quench tubes  160 . 
     The flow of the quenching gas through the various portions of the manifold assembly  156 , such as through the various quench tubes  160 , can occur simultaneously, in one particular embodiment, in order to inhibit the formation of significant temperature gradients across the surfaces of the sub-device and/or within the thickness of the sub-device during quenching. Alternatively, in another embodiment, the respective flows of the quenching gas through the various portions of the manifold assembly  156 , such as through the respective top quench tubes  172  and bottom quench tubes  174  included in the respective top row and bottom row of quench tubes  160 , can differ to promote such temperature gradients, e.g., to counteract the presence of a thermal expansion mismatch of a given coating and an underlying glass substrate. The flow rate and flow time can additionally be adjusted based on multiple factors, such as the size of the substrate  12 , the amount and speed of cooling desired, etc. 
     In some embodiments, for example, the manifold assembly  156  and quench tubes  160  thereof may be compartmentalized into zones, such as top upstream zone  182 , top downstream zone  184 , bottom upstream zone  186 , and bottom downstream zone  188 . Each zone may be supplied with quenching gas by the respective lead pipe. For example, the top upstream zone  182  of quench tubes  160  may be supplied by the top upstream lead pipe  166 . Further, valves (not shown) can respectively control the flow rate of the quenching gas to each of the zones. As such, the flow rate of the quenching gas at different zones can be independently controlled and adjusted. Such control can help to substantially uniformly cool the surfaces of the sub-device across its entire surface area. In one particular embodiment, a suitable controller  220  (see  FIG. 1 ) can monitor the temperature across the surfaces of the sub-device during quenching through the use of a temperature sensor (not shown) and can then adjust the flow rate accordingly in each zone through adjustment of the valves. 
     In exemplary embodiments, the quenching system  154  is a closed circuit quenching system. In these embodiments, at least a portion of the quenching gas may be recirculated to the chamber  152  after being flowed into the chamber  152  and, after a period of time, flowed out of the chamber  152 . For example, exhaust tubes  190  may be included in the quenching system  154  and in fluid communication with the interior of the chamber  152 . Quenching gas may be flowed from the chamber  152  into the exhaust tubes  190 . In embodiments wherein the quenching system  154  defines a closed circuit, at least a portion of this quenching gas may then be re-circulated to the manifold assembly  156 . Additionally or alternatively, however, a portion of the quenching gas may be dumped from the system  154  through exhaust tubes  190 , or the system  154  may be open circuit and all quenching gas may be dumped. 
     A supply source (not shown) may in some embodiments be provided in the quenching system to provide quenching gas thereto. The supply source may be included in both open circuit and closed circuit systems wherein additional quenching gas may be required during operation. 
     The quenching gas is supplied to the chamber  152  at a quenching temperature below the anneal temperature to quench the sub-device therein to a quenched temperature. The quenching temperature may be, for example, from about 0° C. to about 100° C., such as from about 0° C. to about 80° C. Further, in some embodiments such as when the quenching system  150  is a closed circuit quench system, a heat exchanger  200  may be included. The heat exchanger  200  may cool the quenching gas to the quenching temperature before the gas flows into the chamber  152 . For example, as shown in  FIG. 2 , a heat exchanger  200  may be provided between exhaust tubes  190  and a manifold assembly  156  to cool the quenching gas, and in particular re-circulated quenching gas. The heat exchanger  200  may be any suitable heat exchanger, such as a direct contact, indirect contact, parallel flow, or counter-flow heat exchanger. 
     As mentioned above, sub-devices may be supported in the chamber  152 . In some embodiments, the sub-devices may be supported by an interior surface of the chamber  152  or by a shelf disposed therein. Alternatively, in exemplary embodiments as shown, the sub-devices may be supported by a conveyor  210 , which may additionally move the sub-device into, through, and out of the chamber  152 . The sub-device may thus be movably supported on the conveyor  210 . As shown in  FIG. 1 , for example, system  100  may include a conveyor system configured to move the sub-device into, through, and out of the various stations. In the illustrated embodiment, this conveyor system includes a plurality of individually controlled conveyors  210  as well as the load conveyor  106 , with each of the various chambers in each station including a respective one of the conveyors  210 . It should be appreciated that the type or configuration of the conveyors  210  may vary. In the illustrated embodiment, the conveyors  210  are roller conveyors having rotatably driven rollers that are controlled so as to achieve a desired conveyance rate of the sub-device through the respective chamber and the system  100  overall. 
     As described, each of the various chambers, associated apparatus, and respective conveyors  210  in the system  100  are independently controlled to perform a particular function. For such control, each of the individual chambers and associated apparatus may have an associated independent controller  220  configured therewith to control the individual functions of the respective module. The plurality of controllers  200  may, in turn, be in communication with a central system controller (not shown). The central system controller can monitor and control (via the independent controllers  210 ) the functions of any one of the chambers and associated apparatus, so as to achieve an overall desired heat-up rate, deposition rate, cool-down rate, conveyance rate, and so forth, in processing of the sub-devices through the system  10 . 
     As discussed, the present disclosure is further directed to a process for forming a thin film PV device  10 . The process may include heating a sub-device to an anneal temperature, and quenching the sub-device with a quenching gas to cool the glass substrate  12  to a quenched temperature. The sub-device may include a glass substrate  12  and a TCO layer  14  deposited thereon, and the quenching gas may comprise an inert gas, as discussed above. Annealing may occur, for example, in an annealing apparatus  102 . Quenching may occur in, for example, a quenching apparatus  104 . 
     Further, in some embodiments, the process may include re-circulating the quenching gas, as discussed above, and/or cooling the quenching gas to a quenching temperature, as discussed above. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.