Abstract:
A system for manufacture of I-III-VI-absorber photovoltaic cells involves sequential deposition of films comprising one or more of silver and copper, with one or more of aluminum indium and gallium, and one or more of sulfur, selenium, and tellurium, as compounds in multiple thin sublayers to form a composite absorber layer. In an embodiment, the method is adapted to roll-to-roll processing of photovoltaic cells. In an embodiment, the method is adapted to preparation of a CIGS absorber layer having graded composition through the layer of substitutions such as tellurium near the base contact and silver near the heterojunction partner layer, or through gradations in indium and gallium content. In a particular embodiment, the graded composition is enriched in gallium at a base of the layer, and silver at the top of the layer. In an embodiment, each sublayer is deposited by co-evaporation of copper, indium, gallium, and selenium, which react in-situ to form CIGS. In a particular embodiment, a special selenium or tellurium source, valve and delivery subsystem is made of quartz, graphite, coated graphite, or molybdenum. In a particular embodiment, an ion-beam source module configured for surface smoothing the solar absorber sublayer surface before passing through the final deposition zone.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation in part of U.S. patent application Ser. No. 13/793,975 filed Mar. 11, 2013, which is a divisional of U.S. patent application Ser. No. 12/899,446 filed Oct. 6, 2010, which is a continuation of U.S. patent application Ser. No. 12/896,690 filed Oct. 1, 2010 (now abandoned). This application is related to the material of U.S. patent application Ser. No. 12/771,590 filed Apr. 30, 2010, now U.S. Pat. No. 8,021,905, which is a continuation of U.S. patent application Ser. No. 12/701,449 filed Feb. 5, 2010, which claims benefit of priority to U.S. Provisional Patent Application Ser. No. 61/150,282 filed Feb. 5, 2009. Each of the aforementioned patent applications is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present application relates to methods, system, and apparatus for depositing films of semiconductor and other materials in fabricating semiconductor devices such as photovoltaic devices. 
       BACKGROUND 
       [0003]    A popular thin-film photovoltaic technology is called CIGS, which refers to a photovoltaic device having a p-type semiconductor photon-absorber layer containing at least Copper, Indium, Gallium, and Selenium and capable of generating electron-hole pairs upon absorbing photons. In a typical CIGS photovoltaic cell, a Copper-Indium-Gallium-diSelenide (CIGS) layer operates with a heterojunction partner layer to generate a photocurrent when exposed to light. The photocurrent is produced when minority carriers are attracted from the CIGS layer to the heterojunction partner layer. Additional layers, such as a substrate, top and back contact layers, passivation layers, and metallization, may be present in the cell for structural rigidity, to collect the photocurrent, minimize reflections, and protect the cell. CIGS cells may also be layered with photovoltaic devices of other semiconductor materials into a multijunction, layered, structure. 
         [0004]    CIGS semiconductor thin film can be created by a variety of processes, both in vacuo and ex vacuo in nature. Deposition methods such as sputtering, co-evaporation, and combinations of sputtering and evaporation performed in vacuo have produced CIGS photon absorber layers with high demonstrated performance, but traditional means for fabricating absorber layer are perceived as slow and prone to defects. Both sputtering and evaporation may involve a reactive process to create the CIGS alloy film having desired stoichiometry. Slow fabrication speed can lead to high fabrication cost. Defects in an absorber layer can allow recombination of electron-hole pairs thereby reducing cell efficiency and increasing panel area required for a given electrical output. Further, defects may short-circuit part or all of the photocurrent, impairing function of individual photovoltaic cells and modules made from such cells. Defects therefore reduce manufacturing yield and increase fabrication cost for cells and systems. 
         [0005]    Some methods of creating a CIGS absorber layer deposit CIGS directly. Other methods deposit precursor sublayers, such as layers of copper, layers of indium and gallium, and layers of selenium, that are reacted in-situ to form CIGS. Delivery of either CIGS, or the precursor sublayers, can be performed by a single source, or by a plurality of sources. Existing processes typically require that the cell remain in a deposition zone for a lengthy time to deposit and form an absorber layer of the desired thickness. 
         [0006]    Many defects in CIGS solar-absorber layers initiate at the surface of the underlying contact layer when the elements are initially disposed on the surface; these defects originate at the bottom of the CIGS absorber layer. Defects originating at the bottom of the layer may propagate through the entire layer. Growing CIGS films to the desired thickness without termination can allow these defects to propagate through the thickness of the film; defects extending through the thickness of the film are particularly prone to cause short-circuit defects because later deposited layers may contact layers underlying the CIGS layer. 
         [0007]    Traditional in vacuo processing of semiconductor materials is batch-oriented. Substrates and source materials are placed in a chamber, air in the chamber is pumped out, deposition is performed, air is allowed back into the chamber after deposition is completed, and the substrates are moved to further processing stations or deposition sources in the chamber are replaced in preparation for following steps. In order to reduce cost of photovoltaic cells by increasing the area of cell produced with each pumping cycle of the chamber, there is much interest in roll-to-roll processing. In roll-to-roll processing, substrate of a feed roll is unrolled within the chamber, passed through at least one deposition and reaction zone, and wound onto a take-up roll after passing through the deposition and reaction zone. In roll-to-roll processing, there is economic advantage in maintaining high substrate transport speed through the deposition zone. High substrate speed through a deposition zone while reaching a desired film thickness requires either an extended deposition zone length or a rapid deposition rate of the film. 
         [0008]    Increasing deposition rates of traditional in vacuo CIGS deposition processes typically requires larger size or larger quantity of sources, or both, but the basic sequencing of deposition is typically unchanged and propagation of defects through the entire thickness of the CIGS layer may be enhanced at high deposition rates. Defects propagating through the entire thickness of CIGS that cause the short-circuit defects are particularly critical to large-area CIGS modules formed by monolithic integration. Unlike modules made with discrete cells that are sorted to match performance prior to module integration, a monolithically integrated module is processed from a contiguous section of photovoltaic material, and any defect contained therein can severely affect the performance of that module. 
         [0009]    Aluminum is in the same column of the periodic table as Gallium and Indium, Aluminum therefore has some similar chemical properties to Gallium and Indium and these three elements can be considered as forming a group; these three elements are classed as group IIIB in the periodic table. Similarly, Sulfur and Tellurium are in the same column as Selenium and have some similar chemical properties; these three elements can be considered as forming a group and are classed as group VIB in the periodic table. Silver and Gold are in the same column as Copper, have some similar chemical properties to Copper, and can also be considered as forming a group, these elements are classed as group IB in the periodic table. Group Ib-IIIb-VIb semiconductors as described herein typically have a chalcopyrite crystal structure having two parts VIb atoms for one part group Ib and one part group IIIb. While each element in these groups has some similar chemical properties to the other elements of the group, they also have significant physical, electronic, and chemical differences, which influence the physical, electronic and chemical compounds formed with them. 
         [0010]    CIGS is classified, along with many other materials, as a IB-IIIB-VIB compound semiconductor material because of the periodic table groupings of its constituent elements. 
       SUMMARY 
       [0011]    Materials used in the fabrication of a photovoltaic device having an semiconductor absorber layer are disposed as thin films onto a supporting substrate material. 
         [0012]    The present approach for depositing an absorber layer involves depositing the multiple elements of IB-IIIB-VIB semiconductor in such a way as to create a series of sequential absorber sublayers, each of which is of notably less than the desired total thickness. In an embodiment, the sublayers are of substantially identical composition. In an alternative embodiment, the sublayers have graded composition in one or more of the constituent elements. In another embodiment, the first sublayer is specifically designed for reducing back contact recombination velocity, thereby promoting efficiency. In another embodiment, the last sublayer is specifically designed for reducing defects in the junction region. In an embodiment, the deposition of each sublayer is performed in vacuo, confined to an area referred to as a ‘zone’, using physical vapor deposition sources such as sputter and evaporation sources for each of the four or more elements in a deposition zone associated with deposition of the sublayer. Each deposition zone may also incorporate a reactive or annealing process to create CIGS alloy film having desired stoichiometry. Multiple deposition zones are provided, one deposition zone for each sublayer deposited. In an embodiment, an ion-beam etching zone  218  before the final deposition zone is incorporated to provide ion-beam smoothing to the preceding sublayers. An annealing and a cleaning step  220  may optionally be interposed between the ion-beam etching zone  218  and deposition of the final sublayer of CIGS  212 . 
         [0013]    This approach, as discovered, presents improvement over prior art in five ways. 
         [0014]    First, this approach can accommodate virtually any existing process that creates an IB-IIIB-VIB solar absorber, and is particularly adaptable to roll-to-roll processing. In roll-to-roll processing, the substrate is unrolled from a feed spool, moves through the multiple deposition zones sequentially, and is wound onto a takeup spool. Each zone can be defined by the direct fabrication of the IB-IIIB-VIB material, or as sublayers that are later reacted to form the IB-IIIB-VIB material. 
         [0015]    Second, since each of the deposition zones creates a film with a notably lower thickness than the total desired, the transport speed of the substrate through each zone can be higher than if that zone were required to produce the entire solar absorber film thickness. 
         [0016]    Third, by utilizing several sublayers, the growth of each being terminated prior to reaching the total desired thickness, it is less likely that defects will propagate through the total thickness of the resultant absorber layer, thereby presenting a final absorber film with fewer short-circuit defects. 
         [0017]    Fourth, by utilizing several sublayers, the surface morphology can be adjusted prior to reaching the total desired thickness, enabling the opportunity to anneal defects created by any surface smoothing process, the annealing done immediately after the surface smoothing or as a part of the final sublayer deposition. 
         [0018]    This approach provides the ability to adjust the chemistry in individual absorber sublayers in order to produce an overall absorber layer film with specifically designed properties. These properties may include gradients of the elements through the total thickness of an IB-IIIB-VIB absorber layer and consequential gradients in electrical and physical properties. These properties may include gradients of other alloying elements within a CIGS absorber, such as aluminum substituting for some of the gallium or indium, silver substituted for some or all of the copper, or tellurium or sulfur substituted for some or all of the selenium. These gradients can enhance the conversion efficiency of the photovoltaic device of which the absorber layer is a part. 
         [0019]    There are other compound semiconductor materials, such as III-V, II-VI, II-V, I-VI, and IV-VI, some of which have important uses in industry. The present processing approach may be adaptable to manufacture of films of such materials. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a flowchart of a generic fabrication for a PRIOR ART thin-film photovoltaic (PV) device based on the CIGS absorber technology. 
           [0021]      FIG. 1A  is a cross section of a PRIOR ART photovoltaic cell such as may be made by the process of  FIG. 1 . 
           [0022]      FIG. 2  is a flowchart of a process for creating absorber layers by depositing multiple sublayers in contact with each other. 
           [0023]      FIG. 2A  is a cross section of a photovoltaic cell such as may be made by the improved process of  FIG. 2 . 
           [0024]      FIG. 3A  is a diagram of a machine for carrying out the process of  FIG. 2  on individual substrates. 
           [0025]      FIG. 3B  is a diagram of a machine for carrying out the process of  FIG. 2  on a continuous roll-to-roll substrate. 
           [0026]      FIG. 3C  is a diagram of a segmented machine for carrying out the process of  FIG. 2  on a continuous, flexible, roll-to-roll, substrate. 
           [0027]      FIG. 4  is a diagram of an individual segment of the machine of  FIG. 3C , or of an individual deposition zone of the deposition unit of  FIG. 3B . 
           [0028]      FIG. 5  is a cross section of a photovoltaic cell having graded composition and an absorber layer fabricated from multiple sublayers of materials. 
           [0029]      FIG. 6  is an illustration of a segmented machine for carrying out an embodiment of the process for making graded absorber layers on a continuous, flexible, roll-to-roll, substrate. 
           [0030]      FIG. 7  is an illustration of a photovoltaic device produced by the segmented machine, the device having a solar absorber with five sublayers. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0031]      FIG. 1  illustrates a generic fabrication process for a thin-film photovoltaic (PV) device based on the IB-IIIB-VIB semiconductor CIGS (Copper-Indium-Gallium-Selenide) absorber technology, consisting of a sequence of individual process steps as known in the art. A substrate  150  ( FIG. 1A ) is used as the base for all subsequently-deposited thin films. Such a substrate may be rigid or flexible, may be an insulator or conductor, and may incorporate additional layers already deposited on it. An electrically-conductive or contact layer  152  is deposited in a deposition step  102  onto the substrate material  150  to serve as a back contact to the photovoltaic absorber. The contact layer  152  deposited in step  102  may incorporate metals, semiconductors, or conductive oxides. In one embodiment, the contact layer is molybdenum metal. A p-type, IB-IIIB-VIB compound semiconductor typically containing at least copper, indium, gallium, and selenium, often referred to in the industry as Copper-Indium-Gallium-DiSelenide (CIGS)  154 , is deposited over the contact layer  152  in step  106 . This process may include both nonvacuum and vacuum-based deposition technologies. Known vacuum-based processes for deposition  106  of CIGS include evaporation, plasma-assisted evaporation, sputtering, and reactive sputtering. 
         [0032]    Once CIGS absorber layer  154  is deposited, an n-type heterojunction partner layer  156  is deposited  110  onto the substrate/contact/CIGS stack. This process may be either nonvacuum or vacuum based, and may include Cadmium Sulfide or other suitable semi-conductive-oxide materials. The materials deposited must be suitable to form the desired electrical interface with the CIGS film. An optional intrinsic or semiconducting layer may be deposited over the heterojunction partner layer  156  as a buffer layer, and may include Zinc-Oxide or other suitable semi-conductive-oxide materials. Next, a transparent conductive oxide contact layer  158  is deposited  114  onto the substrate/contact/CIGS/heterojunction partner/buffer layer stack. Again, this process may be either nonvacuum or vacuum based, and may include one or more films of transparent oxides, transparent conductive oxides, and transparent conductive polymers. The contact layer  158  is typically made of a material transparent to at least some wavelengths absorbable by the CIGS layer to facilitate transmission of light to the CIGS layer to facilitate the desired photovoltaic energy conversion process. 
         [0033]    Additional conductive layers such as a top metallization layer  160 , and a passivation layer  162 , or even another photovoltaic cell, may be deposited over the contact layer  158 , and patterning and interconnect steps may also be performed to provide a monolithically integrated device. The result of the process described in  FIG. 1  is a photovoltaic device  148  based on the CIGS absorber layer  154 . 
         [0034]    The order of steps in the process of  FIG. 1  could be varied, such as to form alternate photovoltaic device configurations as known in the art. For example, the process of  FIG. 1  could be inverted such that a transparent contact layer is formed on a transparent substrate, a heterojunction partner layer is formed on the transparent contact layer, a CIGS layer is formed on heterojunction partner layer, and a back-contact layer is formed on the CIGS layer. 
         [0035]      FIG. 2  represents a process  200  for depositing a CIGS absorber layer according to the present invention.  FIG. 2A  is a cross sectional diagram of a CIGS PV cell  248  made according to the process of  FIG. 2 . A substrate  250 /contact metal  252  stack, or metalized substrate, is loaded  202  into a machine and exposed to a vacuum. Alternatively, the substrate may be fabricated of conductive metal and serve as a back contact metal layer. 
         [0036]    The substrate  250 /contact metal  252  stack is fed into an optional zone where an adhesion layer (not shown in  FIG. 2A ) may be deposited  204  on the substrate  250 /contact metal  252  stack. The substrate  250 /contact metal  252  stack is then fed along a substrate path into a first deposition zone where a first sublayer of CIGS  254  is deposited  206  on the substrate  250 /contact metal  252  stack. The substrate  250 /contact metal  252  stack is then fed into a second deposition zone where a second sublayer of CIGS  256  is formed  208  on the substrate  250 /contact metal  252  stack. In an embodiment, an annealing and cleaning step  210  may be interposed between deposition  206  of the first sublayer of CIGS  254  and deposition  208  of the second sublayer of CIGS  256  to provide an optimum opportunity for the second sublayer  256  of CIGS to seed and fill defects in the first sublayer of CIGS  254 . In a particular embodiment, cleaning is performed by a plasma etch station interposed along the substrate path. 
         [0037]    Each sublayer  256 ,  254  of CIGS forms part of an overall absorber layer of CIGS throughout which are generated electron-hole pairs in response to photons of electromagnetic energy of sufficient energy incident thereon. In certain embodiments, each sublayer has a same or an approximately same composition. In other embodiments, the compositions of two or more sublayers differ such that the solar absorber layer has a composition that is graded in at least one elemental concentration from at least one sublayer, such as sublayer  256 , to another sublayer, such as sublayer  254 , of the absorber layer. In some embodiments, each sublayer has a same thickness, while in other embodiments, at least two sublayers have different thicknesses. 
         [0038]    Additional zones for deposition of additional sublayers of CIGS may be provided. In an embodiment, the substrate  250 /contact metal  252  stack is next fed into a third deposition zone where a third sublayer of CIGS  258  is deposited  212  on the substrate  250 /contact metal  252  stack. In an embodiment, an annealing and a cleaning step  214  may be interposed between deposition  208  of the second sublayer of CIGS  256  and deposition  212  of the third sublayer of CIGS  258 . 
         [0039]    In an embodiment, deposition module  490  would be devoid of all elemental sources except for an ion-beam based source or sources (not numbered) directed at glancing incidence to substrate  456 . The ion-beam based source would provide an ion-beam etching option for surface smoothing to the multilayer absorber film prior to the final layer deposited in deposition module  500 . The gas used in the ion-beam may contain elements essential to maintaining the proper interface to the subsequent final layer. Further, applicant has theorized that the final layer of ACIGS deposited in deposition module  500  can provide annealing and healing to the etch induced defects while forming the topmost layer, without too much compromise to the surface roughness. In an alternative embodiment, the ion-beam based source could be provided in an additional module between deposition modules  480  and  490 , or between deposition modules  490  and  500 . 
         [0040]    Additional zones, such as a fourth, fifth, etc. zone for deposition of additional sublayers of CIGS may also be provided in some embodiments. In certain embodiments, each deposited CIGS sublayer, if of sufficient thickness, is stoichometrically complete and would itself be suitable for use as a p-type semiconductor solar absorber layer in a photovoltaic cell without requiring any additional processing of the solar absorber sublayer. For example, in some embodiments, each deposited CIGS sublayer is reacted by heat or light to complete formation of CIGS of the sublayer prior to depositing the next CIGS sublayer such that each CIGS sublayer is effectively a p-type semiconductor solar absorber sublayer capable of generating electron-hole pairs in response to photons of electromagnetic energy of sufficient energy incident thereon. In the context of this disclosure and claims, individually reacted means each solar absorber sublayer is reacted independently, or substantially independently, from each other solar absorber sublayer. 
         [0041]    In alternate embodiments, one or more of the deposited sublayers are formed of a IB-IIIB-VIB semiconductor other than CIGS such as Copper-Indium-DiSelenide (CIS) material, a Silver-Copper-Indium-Gallium-Selenide (ACIGS), a Copper-Aluminum-Indium-Gallium-Selenide (CAIGS), Copper-Indium-Aluminum-Telluride (CIAT) material, or an alloy of a CIS material other than CIGS; such alloy layers may be combined with CIGS sublayers to form a composite absorber layer having a graded composition. 
         [0042]    Once a sufficient total thickness of CIGS or other IB-IIB-VIB material has been deposited and alloyed, the resulting substrate  250 /contact metal  252 /CIGS  254   256   258  stack is passed to further zones for additional processing, or unloaded  216  from the machine for continued processing steps  110  and  114 . For example, once a sufficient total thickness of CIGS has been deposited, heterojunction partner layer  260 , optional buffer layer (not shown), contact layer  262 , top metallization layer  264 , and passivation layer  266  are deposited to form a complete photovoltaic cell. 
         [0043]      FIG. 3A  represents an embodiment of a machine for performing the process of  FIG. 2 . Machine  301  represents an inline approach for depositing CIGS suitable for either metalized rigid or flexible substrate. The vacuum system has three separate areas, an entry loadlock  302 , a process chamber  304  that houses the at least two deposition zones  306 , and an exit loadlock  308 . In order to preserve vacuum in the process chamber  304 , a series of valves are placed in between the entrance loadlock  302  and process chamber  304 , and the process chamber  304  and the exit loadlock  308 , respectively. Systematic operation of the valves allow the material to enter the vacuum process chamber  304 , have CIGS or another material disclosed herein deposited onto it in a plurality of sublayers (e.g.,  254 ,  256 ,  258  of  FIG. 2A ), and exit the chamber via the exit loadlock  308  without losing vacuum. The transportation of substrate through the system is facilitated by a series of transport mechanisms  310 . Additional handling apparatus  312  may be provided to feed substrates into the system and to stack substrates exiting the system. 
         [0044]      FIG. 3B  illustrates another embodiment of a machine for performing the process of  FIG. 2 . Machine  342  represents a roll-to-roll approach, where the substrate  330  is necessarily flexible and is transported in a continuous web from a feed spool  332  to a take-up spool  334  through the multiple deposition zones  336 . In this machine, a substrate  330  coated with the first metallic contact is placed in feed spool  332 , and the substrate/contact is transported around a series of rollers  338  and  340  through the deposition zones  336 . IB-IIIB-VIB material, such as CIGS, is deposited and alloyed in multiple sublayers (e.g.,  254 ,  256 ,  258  of  FIG. 2A ) to the desired total thickness of an absorber layer, and the substrate/contact/absorber layer assembly then exits and is rolled up on a take-up spool  334 . This embodiment typically takes place with the entire absorber layer deposition process occurring in vacuum, and typically in the same chamber. Flexible substrates  330  suitable for use with the machines of  FIG. 3B, 3C, and 6  include polyimide substrates and thin metallic foil substrates such as steel. 
         [0045]    Another embodiment of a machine  360  for performing the process of  FIG. 2  is illustrated in  FIG. 3C . This machine  360  is constructed from several independent specialized segments having couplers such that they may be coupled in series in various combinations and with varied numbers of deposition zones. Each segment has a portion of housing that, when the segments are coupled, forms part of the wall of the vacuum chamber of the machine. Airlock doors may optionally be provided at couplers of the segments such that substrate  362  may be loaded onto a feed spool  364 , or coated substrate may be removed from a take-up spool  366 , without admitting air to the entire machine  360 . At least one, and optionally multiple, segments are equipped with vacuum pumps  368  to create and maintain vacuum in the machine  360 . 
         [0046]    A first segment or feed module  370  of the machine  360  contains the feed spool  364 , and associated rollers  372 , which transport a metalized substrate  362  along a substrate path through machine  360 . An optional loading apparatus (not shown) may be provided for loading substrate  362  into the substrate path. In an embodiment, metalized substrate  362  on feed spool  364  is a flexible substrate  250  with a metal contact layer  252  already deposited upon it. 
         [0047]    A second, optional, segment  374  of the machine may deposit an adhesion layer (not shown) in an adhesion layer deposition zone  375 . Metalized substrate  362  then enters the first  376  of several absorber layer deposition segments  376 ,  378 ,  379 . Each absorber layer deposition segment  376 ,  378 ,  379  has one or more IB-IIIB-VIB absorber-layer deposition zones  380 ,  382 ,  383 . Each deposition zone  380 ,  382 ,  383  has source devices  390 ,  391 ,  392  for providing vapor and/or ions of each of the three, four, or more elements required to form a IB-IIIB-VIB absorber layer; in a particular embodiment source devices are provided for each of the four elements required to form a sublayer of CIGS—Copper, Indium, Gallium, and Selenium. Particular embodiments may have additional vapor and/or ion source devices for one or more of the additional IB-IIIB-VIB elements Silver, Aluminum, Sulfur, and Tellurium. The source devices are arranged such that the vapor and/or ions of the elements deposit upon a surface of the substrate as a compound of these elements. In an embodiment, each deposition zone  380 ,  382 ,  383  also has an energy source, such as an annealing heater  393  to control deposition and complete reacting the deposited material to form a IB-IIIB-VIB absorber sublayer such as a sublayer of CIGS; the first zone  380  forming a first absorber sublayer  254 , the second zone  382  forming a second absorber sublayer  256 , and the third zone  383  forming a third absorber sublayer  258 . 
         [0048]    At an output end of the machine  360 , an output segment or output module  384  contains the take-up spool  366 , and associated rollers  386  and apparatus as required for threading the substrate  362  through the substrate path and onto the take-up spool  366 . 
         [0049]    In alternative embodiments, additional segments or modules having additional deposition zones may be provided between the third zone  383  and the output segment  384 . 
         [0050]      FIG. 4  illustrates a IB-IIIB-VIB absorber sublayer deposition segment  376 ,  378 ,  379  such as may be a component of machine  360 . At each end of this segment  376 , are couplers  402  that permit attachment of multiple segments  376  in series as shown in  FIG. 3C . Baffles may optionally be provided as well such that undeposited vapor from segments of one type, such as adhesion layer deposition segment  374 , does not unduly contaminate layers deposited by segments of another type, such as an absorber sublayer deposition segment  376 . Doors  404  may optionally be provided at segment ends to permit loading or unloading of substrate into the first segment or the output segment without opening the entire machine to air. Within the segment  376  are one or more vapor source units  408  for each of the desired IB-IIIB-VIB absorber-layer sublayer&#39;s constituent elements, such as copper, selenium, indium, and gallium where the IB-IIIB-VIB sublayer is to be CIGS; each vapor source may operate through heating of an appropriate material or through sputtering of an appropriate material. Vapor from the source units collects and reacts to form a deposit on substrate  362  suspended near source units  408  by substrate transport apparatus  414 . 
         [0051]    In an embodiment, one or more sources of additional energy  416  source, such as a plasma energy source, an optical energy source, or a electric heat source, are provided for applying additional energy to the substrate as evaporated material condenses upon it; this helps influence deposit composition and grain formation and facilitates formation of the IB-IIIB-VIB absorber-layer alloy. In an embodiment, heaters  416  apply heat to a reverse side of the substrate  362 . In an alternative embodiment, a further plasma cleaning device may be included in a zone to recondition the underlying surface of contact metallization  252  at defects in the first absorber-layer sublayer  254  and allow improved sealing of these defects by new grain formation at these defects of subsequent deposited absorber-layer sublayers  256 ,  258 . 
         [0052]    Each segment may contain more than one deposition zone, where each zone has vapor source units  408  for each of the three or more elements required to form a desired IB-IIIB-VIB absorber sublayer such as vapor source units for the elements selenium, copper, indium, gallium, silver, tellurium, sulfur, and aluminum, and an electric heater  416 . The source units  408 , substrate transport apparatus  414 , and the additional energy  416  sources may be located in various locations within the machine to optimize material quality, substrate transport efficiency. 
         [0053]    In an alternative embodiment, since selenium vapor spreads rapidly through the segment, a segment has a single vapor source unit  408  for selenium, and two deposition zones each having vapor source units  408  for each of copper, indium, and gallium with an alloying heater  416 . 
         [0054]    Each deposition zone within the deposition zones  306 ,  336  of the machines of  FIGS. 3A and 3B  also has at least one source for each of three to six elements selected from the elements copper, silver, indium, gallium, aluminum, sulfur, tellurium, and selenium as illustrated in  FIG. 4  in order to carry out the process of  FIG. 2 . 
         [0055]    In an embodiment of the machine of  FIG. 3C , a first CIGS deposition segment  376  deposits a first sublayer  254  of CIGS that is somewhat enriched in copper, while later CIGS deposition segments  378  deposit a CIGS sublayer  256  unenriched in copper, and a subsequent CIGS deposition segment  379  may deposit a CIGS sublayer  258  slightly depleted in copper. The relative enrichment or depletion in copper is no more than a few percent—the sublayers  254 ,  256 ,  258  produced have substantially similar composition. This embodiment provides capability of producing a copper concentration that is graded across the total CIGS layer thickness as has been previously shown to enhance operating efficiency of CIGS photovoltaic cells. 
         [0056]    In an embodiment of  FIG. 3C , layer deposition segment  374  deposits  204  ( FIG. 2 ) a very thin, adhesion-enhancing, layer onto the metalized substrate containing primarily indium, gallium or aluminum, and selenium or tellurium. The adhesion-enhancing layer serves also to provide preferential grain growth in the next-deposited sublayer  254  of IB-IIIB-VIB absorber, such as a CIGS sublayer, deposited in the subsequent step  206  carried out by CIGS segment  376 . Alternatively, the adhesion-enhancing layer may serve to enhance adhesion of a back surface field sublayer, or a back contact interface sublayer; such back surface field sublayer will in turn be coated with additional absorber sublayers. 
         [0057]    The machine of  FIG. 3C  may contain two, three, four, or more CIGS segments  376 ,  378 ,  379  and can therefore deposit from 2 to N sublayers deposited in N process steps, where N is an integer, with the resulting sublayer films combining to create the desired total thickness of CIGS film. In an embodiment, all of the absorber-layer sublayers disposed in absorber-layer segments  376 ,  378 ,  379  are predominantly CIGS in composition, and may or may not have different thicknesses. 
         [0058]    An alternative photovoltaic cell configuration is illustrated in the cross section of  FIG. 5 . In this embodiment  420 , a substrate  422  having a back contact layer  424 , such as a layer of molybdenum, is coated with an IB-IIIB-VIB composite absorber layer  426 . Within the absorber layer  426  is a CIAT (copper indium aluminum telluride) sublayer  428  adjacent to the back contact metal  424  blended with an adjacent sublayer  430  of intermediate CIGATS (copper indium gallium aluminum telluride selenide) composition. Sublayer  430  is adjacent to sublayers  432  and  434  of CIGS composition; sublayer  434  is adjacent to a sublayer  436  of AIGS (silver indium gallium arsenide) composition. It has been found that the CIAT sublayer tends to repel minority carriers away from the back contact  424  and towards heterojunction partner layer  438 , while the AIGS sublayer tends to have fewer defects than CIGS thereby forming a more perfect, less recombination, junction between composite absorber layer  426  and heterojunction partner layer  438 . Adjacent to AIGS sublayer  436  of absorber layer  426  is a heterojunction partner layer  438 , typically of cadmium sulfide, then a transparent contact layer  440  of a transparent oxide such as zinc oxide or indium tin oxides, and, covering only parts of the cell, a low-resistance top-contact interconnect  442 . Passivation layer  444  covers all except for a portion of low-resistance top-contact interconnect  442  to provide protection to the device. 
         [0059]    In an alternative embodiment, an absorber layer is produced having graded composition with Aluminum replacing some or all of the stochiometric indium and gallium in some sublayers, and with silver replacing some or all of the copper in some sublayers. 
         [0060]    In order to produce the device of  FIG. 5 , a machine  450  as illustrated in  FIG. 6  is used. The machine of  FIG. 6  resembles that of  FIG. 3C , and may embody physically compatible modules, such as substrate feed module  452 . Feed module  452  has a vacuum pump  453  for evacuating the machine, substrate feed roll  454  and associated handling apparatus that provides a continuous feed of metalized substrate  456  to other modules of the machine. 
         [0061]    Metalized substrate  456  passes from feed module  452  into and through an optional adhesion layer deposition module  460 , a CIAT deposition module  470 , multiple CIGS deposition modules  480 ,  490 , an AIGS deposition module  500 , and a take-up segment or module  510 . Take-up segment or module  510  has a takeup roll  512  and appropriate handling apparatus, vacuum pump  514 , and airlocks for collecting coated substrate  516 , and collecting it on roll  512 . 
         [0062]    As substrate  456  is fed into and through machine  450 , adhesion module  460 , if present, deposits an adhesion layer (not shown) over back-contact metal layer  424  ( FIG. 5 ) from appropriate sources  466  in first deposition zone  462 , the layer may be annealed by heater  464 . 
         [0063]    In an embodiment, a special source and vapor delivery subsystem (not shown) is used to provide the selenium or tellurium vapor. The special source and vapor delivery subsystem is designed to be durable at high temperatures to the corrosive selenium or tellurium vapors. The selenium or tellurium vapors can react with typical stainless steel based CIGS deposition chambers or vapor delivery system, greatly limiting their durability. The degraded stainless steel can release iron, nickel and chromium impurities into the CIGS films, degrading the CIGS electronic quality. Tellurium vapor delivery subsystems and materials face even more extreme conditions than even Selenium vapor delivery subsystems, and require temperatures over 450° C. to prevent condensation thus further increasing its reactivity with typical vacuum chamber materials. Applicant has determined that materials such as quartz, graphite, coated graphite, and molybdenum are preferred materials for the selenium or tellurium source and vapor delivery subsystem. In an embodiment, the selenium or tellurium vapor is contained and directed by quartz tubing with graphite based surround. The graphite functions to contain active electrical based heaters and provide a uniform distribution of heat around the quartz tubing. In an alternative embodiment, the source and vapor delivery subsystem is contained and directed by graphite or coated graphite tubing and machined pathways. 
         [0064]    The uniformity and quality of the CIGS, ACIGS, CAIGS, or CIAT layers benefit from uniform and controlled Se or Te vapor pressure during their deposition. The Applicant has determined that source temperature control is not responsive enough by itself to provide adequate Se or Te vapor pressure control to achieve the preferred film uniformity during deposition with the desired speed at which the substrate or web traverses the deposition zones. Further, Applicant has determined that good Se or Te vapor pressure control can be achieved with a valve (not shown) that is integrated into the Se or Te vapor delivery sub-system. However the corrosive selenium and tellurium vapor at high-temperatures greatly restricts the use and durability of typical high-temperature commercially available valves. Applicant has determined that a suitable durable valve enabling fast pressure control can be achieved with the same preferred materials as the vapor delivery subystems (quartz, graphite, coated graphite, and Molybdenum). Valves made of such materials can be designed to have overlapping openings, where one opening is moved relative to the other by mechanical means. In an example of an embodiment, a quartz plate (not shown) with openings that match the position of openings from an underlying quartz manifold can be moved across the manifold openings/nozzles to vary the degree of which the openings overlap, and thus control the escape of the selenium and tellurium vapor from the manifold. 
         [0065]    The substrate  456  next enters a first absorber sublayer deposition module  470 , having one or more sources for vapor and/or ions of each element in a desired first sublayer  428 , in an embodiment the first sublayer is selected from CIGT, or CIGTS. In an alternative embodiment the first sublayer is CIGS. In some embodiments as many as six vapor or ion sources may be present in sources  472  of the deposition zone  474  of module  470  to permit production of an alloy containing six of the IB-IIIB-VIB elements Cu, Ag, In, Ga, Al, S, Se, Te. Further, one or more heaters  476  may be provided in module  470  for alloying and annealing the deposited sublayer. 
         [0066]    In an alternative embodiment, sources for small ratios of one or more additional elements, including the Group JIB materials Cadmium and Mercury, that can substitute for group IB elements as dopants under some conditions may be provided to allow for fine adjustment of electrical properties. Similarly, in an alternative embodiment, sources for Boron and Thallium may be provided because these elements are group IIIB elements, and a source for trace amounts of Oxygen may be provided because this element has many chemical similarities to sulfur and selenium and can fit into the lattice. Since every IB-IIIB-VIB element other than oxygen can react with oxygen, including other group VI elements, excessive oxygen in the deposition zone must, however, be avoided to prevent formation of oxides instead of the chalcogenide alloy; one of the primary purposes of vacuum pumps  453 ,  514 ,  463  provided on many modules is to reduce and maintain oxygen levels below that of atmospheric air to permit evaporation or ion production of other IB-IIIB-VIB elements without excessive formation of such oxides. In some embodiments, Oxygen may also add to the lattice in a post-deposition air-anneal, where it tends to fill selenium vacancies. 
         [0067]    The substrate  456  then moves through a second, third, and in some embodiments additional (not shown for simplicity) IB-IIIB-VIB absorber sublayer deposition modules  480 ,  490  having one or more sources for vapor and/or ions of each element in a desired second, third, and additional sublayers  430 ,  432 ,  434 . In an embodiment these modules  480 ,  490  deposit CIGS. In some embodiments, as many as six vapor or ion sources may be present in sources  482 ,  492  of the deposition zone  484 ,  494  of module  480 ,  490  to permit production of an alloy containing from three up to six of the IB-IIIB-VIB elements Cu, Ag, In, Ga, Al, S, Se, and Te for each sublayer. Further, one or more heaters  476  may be provided in modules  480 ,  490  for alloying and annealing the deposited sublayer. 
         [0068]    The substrate  456  then moves through a final IB-IIIB-VIB absorber sublayer deposition module  500  having one or more sources for vapor and/or ions of each element in a desired final sublayer  436 , in an embodiment the final sublayer is CIGS and in alternative embodiments AIGS or ACIGS. In some embodiments as many as six vapor or ion sources may be present in sources  502  of the deposition zone  504 , of module  500  to permit production of a sublayer of an alloy containing up to six of the IB-IIIB-VIB elements Cu, Ag, In, Ga, Al, S, Se, and Te, such as Silver-Copper-Indium-Gallium-Aluminum-Selenide (ACIGAS), Silver-Copper-Indium-Gallium-Tellurium-Selenide (ACIGTS), or Silver-Copper-Indium-Gallium-Selenide-Sulfide (ACIGSS). Further, one or more heaters  476 , lasers, or other sources of energy may be provided in module  500  for alloying and annealing the deposited sublayer, or, in an alternative embodiment lacking heaters in one or more other compartments, for alloying and annealing the entire absorber layer. 
         [0069]    The coated substrate  516  then moves into take-up module  510  where it is wound on a roll  512 . In an embodiment, the roll  512  is transported to a subsequent machine or machines for deposition of the heterojunction partner layer  438 , top contact  440 , metallization  442 , and passivation  444 ; in an alternative embodiment, additional modules are provided between final absorber layer module  500  and take-up module  510  to perform one or more of these depositions. In various embodiments, etching or scribing steps, and other steps, may also be performed to segment the photovoltaic device into multiple cells, and to couple those multiple cells into series or series-parallel arrangements. 
         [0070]    Although embodiments described above include moving a substrate through a number of deposition zones (e.g., moving the substrate in steps or moving the substrate continuously), one of ordinary skill will appreciate after reading and comprehending the present application, that the embodiments described herein are not limited to only this configuration. 
         [0071]    For simplicity herein, layers are referred to as specifically CIGS, CIAT, ACIGS, or AIGS layers if they comprise primarily those elements; comprising primarily those elements for this purpose means that there is no more than five percent of other elements. 
         [0072]    The machine illustrated with reference to  FIGS. 3C and 6  is modular, and individual modules may be substituted with modules tailored for producing sublayers of particular Ib-IIIb-VIb semiconductor materials. In a particular embodiment, and with reference to  FIG. 7  to indicate layers and sublayers, a similar machine produces a first absorber sublayer  628  of CIGS enriched in gallium and depleted in indium, this sublayer is laid over a molybdenum back-contact layer  624  on a polyimide substrate  622 . Three additional sublayers  630 ,  632 ,  634  of CIGS are then laid down, with each successive sublayer decreasing in gallium and increasing in indium concentrations with respect to the prior sublayer, such that sublayer  634  has the highest ratio of indium to gallium of all the sublayers. Next, a final Ib-IIIb-VIb sublayer of AIGS  636  (silver-indium-gallium-selenide) is deposited and alloyed, the CIGS  628 ,  630 ,  632 ,  634  and AIGS  636  sublayers forming a p-type solar absorber layer having composition tapering from gallium-rich to indium rich from back contact to the junction, and having a silver-rich junction while much of the bulk absorber has a Ib element of copper. Next, a heterojunction partner layer  638  of cadmium sulfide (CdS) and a buffer layer  639  of zinc oxide are deposited, with an indium-tin-oxide (InSnO) transparent conductor layer  640 , a metallic top contact  642 , and a protective layer  644  are deposited. A cell having this structure was found to have a 10.5% efficiency and effective bandgap of 1.32 electron-volts. The resulting solar absorber layer therefore comprises gallium and indium and has a composition graded such that portions of the absorber layer near the back contact layer are enriched in gallium and depleted in indium relative to portions of the absorber layer near the heterojunction partner layer. The solar absorber layer is also graded in silver content such that portions of the absorber layer near the heterojunction partner layer have substantially higher silver content than portions near the back contact layer. The gradients of silver, gallium, tellurium, or other elements content across the final absorber layer may, however, be somewhat less steep than the concentration differences between sublayers as deposited because a portion of the various Ib-IIIb-VIb elements may blend by diffusion across sublayer boundaries during the anneals performed by the heaters or lasers that serve as annealing energy sources in the machine of  FIGS. 3C and 6 . 
         [0073]    In an alternative embodiment, all absorber sublayers  628 ,  630 ,  632 ,  634  and  636  were constructed of an ACIGS material having a silver to silver plus copper ratio of between 0.4 and 0.8, and in a particular embodiment approximately 0.6. The ACIGS material was, however, graded in gallium and indium content, with the lowest sublayer  628  having a substantially higher ratio of gallium to gallium plus indium than upper sublayer  636 , with intermediate sublayers  630 ,  632 ,  634  having intermediate ratios of gallium to gallium plus indium. Next, a heterojunction partner layer  638  of cadmium sulfide (CdS) and a buffer layer  639  of zinc oxide are deposited, with an indium-tin-oxide (InSnO) transparent conductor layer  640 , a metallic top contact  642 , and a protective layer  644  are deposited. A cell having this structure, with a silver to silver plus copper ratio of 0.6, was found to have a 13.2% efficiency and a bandgap of 1.4 electron volts. 
         [0074]    In an alternative embodiment, first absorber sublayer  628  is constructed of a CIAT (copper-indium-aluminum-telluride) material, or in a variation an ACIAT (silver-copper-indium-aluminum-telluride) material; where aluminum substitutes for some or all of the group-Mb elements gallium and indium of CIGS. Similarly, in this alternative embodiment, tellurium substitutes for some or all of the group-VIb element selenium. In this embodiment, following sublayers  630 ,  632 ,  634  and  636  are constructed of an ACIGS material having a silver to silver plus copper ratio of between 0.4 and 0.8, and in a particular embodiment approximately 0.6; and between which the gallium to gallium plus indium ratio may in some variations also be graded from sublayer to sublayer. In a particular variation of this embodiment, the first sublayer  630  following CIAT sublayer  628  may contain some aluminum as well as indium and gallium to provide a blended gradient of aluminum concentration in those portions of the final absorber layer that are near the back contact  624 . The final absorber layer therefore has a graded composition with substantially higher aluminum and tellurium concentrations in those portions of the absorber layer that lie near back contact layer  624  relative to portions lying near heterojunction partner layer  636 , and the absorber layer has substantially higher selenium and indium concentrations near heterojunction partner layer  638  than near back contact layer  624 . 
         [0075]    In yet another alternative embodiment, first absorber sublayer  628  is constructed of a CIAT (copper-indium-aluminum-telluride) material; where aluminum substitutes for some or all of the group-IIIb elements gallium and indium of CIGS, and tellurium substitutes for some or all of the group-VIb element selenium. In this embodiment, intermediate sublayers  630 ,  632 ,  634  are constructed of a CIGS material, and final sublayer  636  of an ACIGS material having a silver to silver plus copper ratio of between 0.4 and 0.8, and in a particular embodiment approximately 0.6; in a variation of this embodiment the gallium to gallium plus indium ratio may in some variations also be graded from sublayer to sublayer of intermediate sublayers  630 ,  632 ,  634 . In this embodiment, the final absorber layer therefore contains tellurium, with a substantially higher tellurium concentration in portions of the absorber layer that lie near the back contact layer  624  than in portions near the heterojunction layer  638 , and silver, with a substantially higher silver concentration in portions of the absorber layer that lie near the heterojunction layer  638  than in portions near the back contact layer  624 . 
         [0076]    In yet another alternative embodiment, the first sublayer  628  is laid down comprising copper, aluminum, and tellurium, and in variations may also contain small amounts of indium and selenium. Subsequent sublayers  630 ,  632 ,  634  have successively decreasing concentrations of aluminum with increasing concentrations of indium as group IIIb elements, and may comprise selenium as the group VIb element with successively decreasing or zero concentrations tellurium, Similarly, subsequent sublayers  630 ,  632 , and  634  may have some silver as group Ib elements in addition to or in place of the copper. The final sublayer  636  is laid down with primarily silver, indium and selenium, and may in variations contain zero or small concentrations of gallium and copper. With this embodiment, the final annealed absorber layer has group IIIb composition graded from high in aluminum near the back contact layer to a much lower aluminum, and much higher indium, concentration near the heterojunction layer  638 . Similarly, the final annealed absorber layer has group VIb composition graded from high in tellurium near the back contact layer to a substantially lower tellurium, and much higher selenium, concentration near the heterojunction layer  638 . The final annealed absorber layer also has group Ib composition graded from high in copper near the back contact layer to a much lower copper, and much higher silver, concentration near the heterojunction layer  638 . In a particular embodiment, the final annealed absorber layer, if it contains any gallium, contains less than five percent gallium. 
         [0077]    Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall there between.