Abstract:
A method of manufacture of CIGS photovoltaic cells and modules involves sequential deposition of copper indium gallium diselenide compounds in multiple thin sublayers to form a composite CIGS absorber layer of a desirable thickness greater than the thickness of each sublayer. In an embodiment, the method is adapted to roll-to-roll processing of CIGS PV cells. In an embodiment, the method is adapted to preparation of a CIGS absorber layer having graded composition through the layer. In a particular embodiment, the graded composition is enriched in copper at a base of the layer. In an embodiment, each CIGS sublayer is deposited by co-evaporation of copper, indium, gallium, and selenium which react in-situ to form CIGS.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application 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 are incorporated herein by reference. 
    
    
     BACKGROUND 
     A popular thin-film photovoltaic technology is called CIGS, which refers to a p-type semiconductor photon-absorber layer containing at least Copper, Indium, Gallium, and Selenium having the ability to absorb solar radiation and generate electron-hole pairs. In a typical CIGS photovoltaic cell, a Copper-Indium-Gallium-diSelenide layer operates with a heterojunction partner layer to generate a photocurrent when exposed to light. Additional layers, such as a substrate, top and bottom contact layers, passivation layers, and metallization, may be present in the cell for structural rigidity, to collect the photocurrent, and protect the cell. 
     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 the CIGS film 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 a CIGS film can allow recombination of electron-hole pairs thereby reducing cell efficiency. Further, defects may short-circuit part or all of the photocurrent, impairing function of individual cells and monolithically integrated modules having CIGS cells. Other defects can provide a path, or via, to underlying conductive materials, thereby allowing subsequent depositions of electrically conductive material to generate an electrical short. Defects that cause short-circuiting are known herein as short-circuit defects. Defects therefore reduce manufacturing yield and increase fabrication cost. 
     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 sublayers can be performed by a single source, or by a plurality of sources. Existing processes require that the cell remain in a deposition zone for a lengthy time to deposit and form a CIGS layer of the desired thickness. 
     Many defects in CIGS 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 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. 
     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. 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. 
     Increasing deposition rates of the traditional in vacuo CIGS deposition process 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. 
     SUMMARY 
     Materials used in the fabrication of a CIGS photovoltaic device are disposed as thin films onto a substrate material that serves as a structural support for the assorted thin films. 
     The present approach for depositing CIGS involves the deposition of the four elements in such a way as to create a series of sequential CIGS sublayers, each of which is substantially identical in composition but of notably less than the desired total thickness. 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 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. 
     This approach, as discovered, presents improvement over prior art in three ways. 
     First, this approach can accommodate virtually any existing process that creates a CIGS, 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 CIGS material, or as sublayers that are reacted to form the CIGS material. 
     Second, by requiring each of the deposition zones to create a substantially identical CIGS 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 CIGS film thickness. With multiple deposition zones, the transport speed of the web through the zones, resulting in CIGS film of the total desired thickness, is much greater than using a single-zone approach. 
     Third, by utilizing several of CIGS 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 CIGS layer, thereby presenting a final CIGS film with fewer short-circuit defects present. 
     This approach provides the ability to adjust the chemistry in individual CIGS sublayers in order to produce an overall CIGS absorber layer film with desirable properties. These properties may include gradients of the four elements through the total thickness of the CIGS absorber layer. These gradients can enhance the conversion efficiency of the photovoltaic device of which the CIGS absorber layer is a part. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a flowchart of a generic fabrication for a thin-film photovoltaic (PV) device based on the CIGS absorber technology. 
         FIG. 1A  is a cross section of a PV cell such as may be made by the process of  FIG. 1 . 
         FIG. 2  is a flowchart of a process for creating CIGS layers by depositing multiple CIGS sublayers in contact with each other. 
         FIG. 2A  is a cross section of a PV cell such as may be made by the improved process of  FIG. 2 . 
         FIG. 3A  is a diagram of a machine for carrying out the process of  FIG. 2  on individual substrates. 
         FIG. 3B  is a diagram of a machine for carrying out the process of  FIG. 2  on a continuous roll-to-roll substrate. 
         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. 
         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 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  illustrates a generic fabrication process for a thin-film photovoltaic (PV) device based on the CIGS 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 CIGS device. 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 compound semiconductor 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. 
     Once CIGS 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 Selenide or other suitable oxide materials. The materials deposited must be suitable to form the desired electrical interface with the CIGS film. Next, a transparent conductive oxide contact layer  158  is deposited  114  onto the substrate/contact/CIGS/heterojunction partner 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 transparent to facilitate transmission of light to the CIGS layer in order to facilitate the desired photovoltaic energy conversion process. 
     Additional conductive layers such as a top metallization layer  160 , and a passivation layer  162 , or 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 . 
     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 varied 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 another contact layer is formed on the CIGS layer. 
       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. The substrate  250 /contact metal  252  stack is fed into an optional zone where an adhesion layer (not shown) may be deposited  204  on the substrate  250 /contact metal  252  stack. The substrate  250 /contact metal  252  stack is then fed 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 . Each sublayer of CIGS forms part of an overall 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 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 to another sublayer. In some embodiments, each sublayer has a same thickness, while in other embodiments, at least two sublayers have different thicknesses. 
     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 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 . 
     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, 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 individually reacted (e.g., subjected to an energy source such as heat and/or subjected to one or more materials) to complete formation of the sublayer prior to depositing the next CIGS sublayer such that each CIGS sublayer is 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. In alternate embodiments, each deposited sublayer is formed of a Copper-Indium-DiSelenide (CIS) material or an alloy of a CIS material other than CIGS. 
     Once a sufficient total thickness of CIGS has been deposited, 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 , contact layer  262 , top metallization layer  264 , and passivation layer  266  are deposited. 
       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 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. 
       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 . CIGS is deposited in multiple sublayers (e.g.,  254 ,  256 ,  258  of  FIG. 2A ) to the desired total thickness, and the substrate/contact/CIGS assembly then exits and is rolled up on a take-up spool  334 . This embodiment typically takes place with the entire CIGS deposition process occurring in vacuum, and typically in the same chamber. 
     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 together 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 together, 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 . 
     A first segment  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. 
     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 CIGS deposition segments  376 ,  378 ,  379 . Each CIGS deposition segment  376 ,  378 ,  379  has one or more CIGS 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 four elements required to form CIGS—Copper, Indium, Gallium, and Selenium. The source devices are arranged such that the vapor and/or ions of the four elements deposit upon a surface of the substrate as a compound of these four 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 CIGS sublayer; the first zone  380  forming a first CIGS sublayer  254 , the second zone  382  forming a second CIGS sublayer  256 , and the third zone  383  forming a third CIGS sublayer  258 . 
     At an output end of the machine  360 , an output segment  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 . 
     In alternative embodiments, additional segments having additional deposition zones may be provided between the third zone  383  and the output segment  384 . 
       FIG. 4  illustrates a CIGS 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 CIGS 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 copper, selenium, indium, and gallium; 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 . 
     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 CIGS 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 CIGS sublayer  254  and allow improved sealing of these defects by new grain formation at these defects of subsequent deposited CIGS sublayers  256 ,  258 . 
     Each segment may contain more than one deposition zone, where each zone has vapor source units  408  for each of selenium, copper, indium, and gallium, 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. 
     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 . 
     Each CIGS 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 copper, indium, gallium, and selenium as illustrated in  FIG. 4  in order to carry out the process of  FIG. 2 . 
     In an alternative embodiment, apparatus as heretofore discussed provides a CIS alloy instead of CIGS by omitting gallium sources. 
     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. 
     In an embodiment of the machine of  FIG. 3C , layer deposition segment  374  deposits  204  ( FIG. 2 ) a very thin layer onto the metalized substrate containing primarily indium, gallium, and selenium that serves as an adhesion-enhancing layer, and can be used to provide preferential grain growth in the next-deposited sublayer  254  of CIGS film deposited in the subsequent step  206  carried out by segment  376 . 
     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 films combining to create the desired total thickness of CIGS film. All of the CIGS sublayers disposed in CIGS segments  376 ,  378 ,  379  are predominantly CIGS in composition, and may or may not have different thicknesses. 
     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. 
     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.