Abstract:
The present invention provides methods of electroplating a film or films onto a top surface of a continuously moving roll-to-roll sheet. In one aspect, the invention includes continuously electroplating a film onto a conductive surface using an electroplating unit as the roll-to-roll sheet moves therethrough, detecting a thickness of the film electroplated onto a portion of the roll-to-roll sheet and generating a thickness signal corresponding thereto. In this aspect, when continuously electroplating, the thickness of the film is adjusted toward a predetermined thickness value using the thickness signal for a subsequent portion of the roll-to-roll sheet that follows the portion of the roll-to-roll sheet.

Description:
[0001]    This application is related to and claims priority from U.S. Provisional Application No. 60/744,252 filed Apr. 4, 2006 entitled “Composition Control For Photovoltaic Thin Film Manufacturing”, which application is expressly incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to methods and apparatus for preparing thin films of Group IBIIIAVIA compound semiconductor films for radiation detector and photovoltaic applications. 
       BACKGROUND 
       [0003]    Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970&#39;s there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods. 
         [0004]    Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se) 2  or CuIn 1-x Ga x (S y Se 1-y ) k , where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications. 
         [0005]    The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te) 2  thin film solar cell is shown in  FIG. 1 . The device  10  is fabricated on a base which includes a substrate  11  coated with a conductive layer  13 . The substrate  11  may be in various forms and shapes, such as a sheet of glass, a sheet of metal (such as aluminum or stainless steel), an insulating foil or web, or a conductive foil or web. The absorber film  12 , which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te) 2 , is grown over the conductive layer  13 , which is previously deposited on the substrate  11  and which acts as the electrical contact to the device. Various conductive layers comprising Mo, Ta, W, Ti, and stainless steel etc. have been used in the solar cell structure of  FIG. 1 . If the substrate itself is a properly selected conductive material, it is possible not to use a conductive layer  13 , since the substrate  11  may then be used as the ohmic contact to the device. After the absorber film  12  is grown, a transparent layer  14  such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation  15  enters the device through the transparent layer  14 . Metallic grids (not shown) may also be deposited over the transparent layer  14  to reduce the effective series resistance of the device. It should be noted that the structure of  FIG. 1  may also be inverted if substrate is transparent. In that case light enters the device from the substrate side of the solar cell. 
         [0006]    In a thin film solar cell employing a Group IBIIIAVIA compound absorber, the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se) 2  absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In +Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good device performance Cu/(In+Ga) molar ratio is kept at around or below 1.0. As the Ga/(Ga+In) molar ratio increases, on the other hand, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current typically may decrease. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition. It should be noted that although the chemical formula is often written as Cu(In,Ga)(S,Se) 2 , a more accurate formula for the compound is Cu(In,Ga)(S,Se) k , where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se) 2  means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1. 
         [0007]    The first technique used to grow Cu(In,Ga)Se 2  layers was the co-evaporation approach which involves evaporation of Cu, In, Ga and Se from separate evaporation boats onto a heated substrate, as the deposition rate of each component is carefully monitored and controlled. 
         [0008]    Another technique for growing Cu(In,Ga)(S,Se) 2  type compound thin films for solar cell applications is a two-stage process where at least two of the components of the Cu(In,Ga)(S,Se) 2  material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CuInSe 2  growth, thin sub-layers of Cu and In are first deposited on a substrate to form a precursor layer and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere contains sulfur, then a CuIn(S,Se) 2  layer can be grown. Addition of Ga in the precursor layer, i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of a Cu(In,Ga)(S,Se) 2  absorber. Other prior-art techniques include deposition of Cu—Se/In—Se, Cu—Se/Ga—Se, or Cu—Se/In—Se/Ga—Se stacks and their reaction to form the compound. Mixed precursor stacks comprising compound and elemental sub-layers, such as a Cu/In—Se stack or a Cu/In—Se/Ga—Se stack, have also been used, where In—Se and Ga—Se represent selenides of In and Ga, respectively. 
         [0009]    Sputtering and evaporation techniques have been used in prior art approaches to deposit the individual layers or sub-layers containing the Group IB and Group IIIA components of metallic precursor stacks. In the case of CulnSe 2  growth, for example, Cu and In sub-layers were sequentially sputter-deposited from Cu and In targets on a substrate and then the stacked precursor layer or film thus obtained was heated in the presence of gas containing Se at elevated temperatures as described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositing a stacked precursor film comprising a Cu—Ga alloy sub-layer and an In sub-layer to form a Cu—Ga/In stack on a metallic back electrode and then reacting this precursor stack film with one of Se and S to form the compound absorber layer. U.S. Pat. No. 6,092,669 described sputtering-based equipment and method for producing such absorber layers. 
         [0010]    Solar cell structures are typically fabricated on glass or metallic foil substrates. For monolithically integrated modules glass sheets are commonly used, whereas metallic foils are more suitable for fabrication of individual cells that may be later interconnected into modules. It is, of course, possible to deposit an insulating layer over the metallic foil substrate and then process solar cells on the insulating layer. This way, monolithically integrated modules may also be fabricated on metallic foil substrates. 
         [0011]    Irrespective of the specific approach used to grow a Cu(In,Ga)(S,Se) 2  absorber film, two molar ratios mentioned before, i.e. the Cu/(In +Ga) ratio and the Ga/(Ga+In) ratio, should be closely controlled from run to run and on large area substrates. In co-evaporation techniques, this composition control is achieved through in-situ monitoring of the evaporation rates of Cu, In, Ga and Se. In two-stage techniques, which involve deposition of sub-layers to form a precursor film and then reaction of the precursor film to form the compound absorber layer, individual thicknesses of the sub-layers forming the stacked precursor film layer need to be well controlled because they determine the final stoichiometry or composition of the compound layer after the reaction step. If the sub-layers are deposited by vacuum approaches, such as evaporation and sputtering, thicknesses of the sub-layers within the precursor layer (such as thicknesses of Cu and In sub-layers) may be monitored and controlled using in-situ measurement devices such as crystal oscillators, or sensors that sense the material deposition flux. In the prior art electroplating techniques, on the other hand, thickness of individual sub-layers such as the Cu sub-layer, In sub-layer, and/or Ga sub-layer have been controlled through control of the charge passed during deposition of the sub-layer, i.e. through control of the deposition current density and the deposition time. However, during processing, it is possible that deposition rates change. For example, in electrodeposition approach the deposition rates may vary due to reasons such as a change in the electrodeposition efficiency, age of the deposition bath, accumulation of byproducts in the bath, change in organic or inorganic additive concentrations and/or activities etc. in the electrolyte. Since solar cell efficiency is a strong function of the molar ratios of the elements within the deposited precursors, new techniques that assure excellent control of these ratios are needed. 
         [0012]    Deposition or growth of layers forming a thin film solar cell in a roll-to-roll or in-line process is attractive for higher throughput, lower cost and better yield of such approaches. There is still need to develop roll-to-roll or in-line deposition techniques for the growth of Group IBIIIAVIA materials wherein the critical IB/IIIA molar ratio as well as the IIIA material composition are tightly controlled. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention provides methods of electroplating a film or films onto a top surface of a continuously moving roll-to-roll sheet. 
         [0014]    In one aspect, the invention includes continuously electroplating a film onto a conductive surface using an electroplating unit as the roll-to-roll sheet moves therethrough, detecting a thickness of the film electroplated onto a portion of the roll-to-roll sheet and generating a thickness signal corresponding thereto. In this aspect, when continuously electroplating, the thickness of the film is adjusted toward a predetermined thickness value using the thickness signal for a subsequent portion of the roll-to-roll sheet that follows the portion of the roll-to-roll sheet. 
         [0015]    In another aspect there is provided a method of electroplating a stacked layer of a plurality of films in a plurality of sequentially disposed electroplating units onto a continuously moving roll-to-roll sheet. In this aspect, there is included continuously electroplating each one of the films onto a top surface of the roll-to-roll sheet using each of the electroplating units as the roll-to-roll sheet moves therethrough, detecting a thickness of each of the films electroplated onto a portion of the roll-to-roll sheet and generating a thickness signal corresponding thereto. In this aspect, when continuously electroplating the thickness of each of the films is adjusted toward a predetermined thickness value for that film using the thickness signal for that film for a subsequent portion of the roll-to-roll sheet that follows the portion of the roll-to-roll sheet. 
         [0016]    In yet another aspect, there is included continuously electroplating a first film onto a top surface of the continuously moving roll-to-roll sheet; detecting a thickness of the first film and generating a thickness signal corresponding thereto; and continuously electroplating a second film over the first film, wherein a thickness of the electroplated second film is controlled by the thickness signal. 
         [0017]    In a still further aspect, when detecting the thickness of an electroplated film, the thickness is detected at a plurality of locations across a width of the continuously moving roll-to-roll sheet to obtain a corresponding plurality of thickness signals. These plurality of thickness signals are used to adjust a thickness of the same film or a subsequent film, such that the thickness is adjusted taking into consideration the location of the detecting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    These and other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein: 
           [0019]      FIG. 1  is a cross-sectional view of a solar cell employing a Group IBIIIAVIA absorber layer. 
           [0020]      FIG. 2  is a schematic of a roll-to-roll electroplating system to deposit a precursor layer that may be used for the formation of a Group IBIIIAVIA absorber layer. 
           [0021]      FIG. 3A  is a flexible foil base. 
           [0022]      FIG. 3B  is a flexible foil base with an electroplated Cu/Ga/In precursor stack. 
           [0023]      FIG. 4  shows a side view of a roll-to-roll electrodeposition system comprising a Cu-processing-unit, a Ga-processing-unit, and an In-processing-unit with their associated metrology-stations. 
           [0024]      FIG. 5  shows a top view of a Cu-processing-unit and a Cu-metrology-station. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The present invention provides methods and apparatus to control the composition of precursor films or layers, which may be used in the formation of CIGS(S) type absorber layers, through an in-line or roll-to-roll manufacturing approach. The techniques may be used for the deposition of Group IB and Group IIIA materials and optionally Group VIA materials on a base to form precursor layers which are then converted into Group IBIIIAVIA compound layers by annealing at elevated temperatures, preferably in presence of additional Group VIA material(s). Deposition of Group IB, Group IIIA and Group VIA materials may be performed by various techniques such as evaporation, sputtering, spraying. doctor blading, printing, electrodeposition etc., and the processing system may be an in-line-system or a roll-to-roll system wherein layers or sub-layers of Group IB-containing, Group IIIA-containing and Group VIA-containing materials are deposited onto the base consecutively, one after another, at a pre-determined sequence. A preferred method of deposition for the present invention is electrodeposition or electroplating and a preferred processing approach is a roll to roll technique, also referred to as a reel-to-reel technique, as will be discussed in more detail below. 
         [0026]      FIG. 2  shows an exemplary roll-to-roll electroplating system  30  with capability to produce, on a flexible foil base  22 , a precursor stack comprising Cu, and at least one of In and Ga with excellent thickness control and uniformity. The electroplating system  30  comprises a series of process-units,  60 A,  60 B,  60 C . . .  60 N, one or more metrology-stations  70 A,  70 B,  70 C . . .  70 N, a supply spool  20 , a return spool  21  and a mechanism to direct the flexible foil base  22  from the supply spool  20  to the return spool  21  through the series of process-units. The series of process-units may comprise at least one Cu electroplating unit, and at least one of a Ga electroplating unit and at least one In electroplating unit. Each process-unit may additionally comprise an integrated cleaning or a cleaning/drying unit so that as the flexible foil base  22  exits one process unit such as the process-unit  60 A of  FIG. 2 , and enters a metrology-unit such as the metrology unit  70 A, it is clean of any chemical residues that may be introduced in the process-unit  60 A. 
         [0027]    It should be noted that the number and order of the process-units may be changed to obtain various stacks comprising Cu, In and Ga on the base. For example, if there were only three process-units;  60 A,  60 B and  60 C in the system of  FIG. 2 , and if the process-units  60 A,  60 B and  60 C comprise a Cu electroplating unit, an In electroplating unit, and a Ga electroplating unit, respectively, then the system would yield a Cu/In/Ga precursor stack on the flexible foil base  22 . Changing this order and optionally adding other electroplating units, one may obtain stacks such as Cu/Ga/In, In/Cu/Ga, Ga/Cu/In, Cu/Ga/Cu/In, Cu/Ga/Cu/In/Cu, Cu/In/Cu/Ga, Cu/In/Cu/Ga/Cu etc. By adding process-units with ability to electroplate a Group VIA material such as Se, precursor stacks comprising Se may also be obtained. Such stacks include but are not limited to Cu/In/Ga/Se, In/Cu/Ga/Se, Ga/Cu/In/Se, Cu/Ga/Cu/In/Se, Cu/Ga/Cu/In/Cu/Se, Cu/In/Cu/Ga/Se, Cu/In/Cu/Ga/Cu/Se etc. It should be noted that many more iterations of such stacks are possible. We will, however, continue our discussion using a roll-to-roll electroplating system configuration comprising one of each of a Cu electroplating unit, a Ga electroplating unit and an In electroplating unit, with their integrated or associated cleaning units. It should be noted that the system depicted in  FIG. 2  may have other components such as annealing units, conditioning units for cleaning and chemical conditioning the flexible foil base  22  before the first process unit  60 A, etc. Some details on such additional components may be found in U.S. Provisional Appln. 60/862,164 filed Oct. 19, 2006 entitled “Roll-to-Roll Electroplating for Photovoltaic Film Manufacturing.” 
         [0028]    A preferred structure of the flexible foil base  22  supplied by the supply spool  20  is shown in  FIG. 3A . The flexible foil base  22  comprises a flexible foil substrate  45  and a conductive layer  46  or a contact layer deposited on a first surface  45 A of the flexible foil substrate  45 . The flexible foil substrate  45  may be made of any polymeric (such as polyimide) or metallic foil, but preferably it is a metallic foil such as a 20-250 um thick stainless steel foil, Mo foil, Ti foil, Al or Al-alloy foil. Various metallic foil substrates (such as Cu, Ti, Mo, Ni, Al) have previously been identified for CIGS(S) solar cell applications (see for example, B. M. Basol et al., “Status of flexible CIS research at ISET”, NASA Document ID: 19950014096, accession No: 95N-20512, available from NASA Center for AeroSpace Information, and B. M. Basol et al., “Modules and flexible cells of CuInSe 2 ”, Proceedings of the 23 rd  Photovoltaic Specialists Conference, 1993, page 426). The conductive layer  46  may be in the form of a single layer or alternately it may comprise a stack of various sub-layers (not shown). Preferably, the conductive layer comprises at least one diffusion barrier layer that prevents diffusion of impurities from the flexible foil substrate  45  into the layers to be electrodeposited and into the CIGS(S) layer during its formation. Materials of the conductive layer  46  include but are not limited to Ti, Mo, Cr, Ta, W, Ru, Ir, Os, and compounds such as nitrides and oxy-nitrides of these materials. Preferably, the free surface  46 A of the conductive layer  46  comprises at least one of Ru, Ir and Os for better nucleation of the electroplated layers. After electrodepositing the Cu, Ga and In layers, the final structure of the flexible foil base coated by the stacked precursor is shown in  FIG. 3B . As can be seen from this figure, a Cu layer  50  is first electroplated on the free surface  46 A. Then a Ga layer  51  is electrodeposited, followed by an In layer  52 . As explained before, the order and nature of this stack may be changed by changing the deposition sequence and the thickness of each layer is selected to attain a targeted Cu/(Ga+In) molar ratio and a Ga/(Ga+In) molar ratio. 
         [0029]    In this example electrodeposition is carried out on the free surface  46 A of the conductive layer  46 . The back surface  45 B of the flexible foil substrate  45  may preferably be covered with a secondary layer  47  (shown with dotted line) to protect the flexible foil substrate  45  during electrodeposition as well as annealing/reaction steps that will follow to form the CIGS(S) compound, or to avoid buckling of the flexible foil substrate  45 . It is important that the material(s) of the secondary layer  47  be stable in chemistries of the Cu, In and Ga plating baths, i.e. not dissolve into and contaminate such baths, and also be resistant to reaction with Group VIA elements. Materials that can be used in the secondary layer  47  include but are not limited to Ru, Os, Ir, Ta, W etc. The secondary layer  47  has an outer surface  47 A that is exposed to the electroplating electrolytes or baths and other process environment present for the formation of CIGS(S) compound. Although  FIG. 3B  shows that the Cu, Ga and In layers are coated on one side of the flexible foil base  22 , it is possible to deposit such stacks on both sides of a base to fabricate solar cells on both faces. 
         [0030]      FIG. 4  shows in more detail a cross sectional view of an exemplary system  200  that may be used to grow a Cu/Ga/In precursor stack on a base  100  such as a flexible foil base of  FIG. 3A . The system  200  comprises a Cu-processing-unit  130 , a Ga-processing-unit  140 , an In-processing-unit  150 , a Cu-metrology-station  160 , a Ga-metrology-station  170  and an In-metrology-station  180 . The Cu-processing-unit  130  may comprise a Cu electroplating unit  131  and a Cu cleaning unit  132 . The Ga-processing-unit  140  may comprise a Ga electroplating unit  141  and a Ga cleaning unit  142 . The In-processing-unit  150  may comprise an In electroplating unit  151  and an In cleaning unit  152 . 
         [0031]    The Cu electroplating unit  131  comprises a Cu plating bath  134 , which may be circulated between a Cu plating reservoir  135  and a chemical cabinet (not shown) through solution inlet and outlet tubes (not shown) on the Cu plating reservoir  135 . The Cu plating bath  134  or solution or electrolyte may be filtered and replenished during circulation or while in the chemical cabinet. Measurement and control of various bath parameters, such as additive content, Cu content, temperature, pH etc. may be continuously or periodically carried out within the chemical cabinet to assure stability of the Cu deposition process. The exemplary system of  FIG. 4  deposits Cu, Ga and In on the front surface  100 A of the base  100 . The front surface  100 A preferably comprises at least one of Ru, Os and Ir. Electrical connection to the base  100  may be achieved by various means including through rollers, brushes etc. The electrical contacts may be touching the base  100  at, at least part of its back surface  100 B or front surface  100 A. Preferably, contacts to the front surface  100 A are made at the two edges avoiding physical contact with most of the front surface  100 A which may be damaged or contaminated by the electrical contact(s). One preferred method comprises making electrical contact to the back surface  100 B of the base  100  through at least one Cu plating contact roller  136 , if the substrate of the base is conductive. A Cu plating anode  137  is placed in the Cu plating bath  134  and a Cu plating power supply  138  is provided to apply a potential difference between the Cu plating anode  137  and the electrical contact(s) such as the Cu plating contact roller  136 , so that Cu is electroplated on a section of the top surface  100 A that is exposed to the Cu plating bath  134  across from the Cu plating anode  137  as the base  100  is moved from left to right at a predetermined speed. After plating, the section with deposited Cu layer gets cleaned in the Cu cleaning unit  132  through use of water sprays  139  directed on both surfaces of the base  100 . Air knives (not shown) may be used to partially or fully dry the surface of the plated Cu as well as the back side of the base  100  before the plated and cleaned section enters the Cu-metrology-station  160 . A metrology tool  201  such as an XRF head or probe may be provided in the Cu-metrology-station  160 . Similar metrology tools  201  may also be provided in the Ga-metrology-station  170  and In-metrology-station  180 . The metrology tools  201  may be mounted on structures (not shown) and may be moved in x, y or z direction in proximity of the base  100  to collect data about plated material layer amount or thickness, or the plated stack thickness/composition as the base  100  is moved from left to right along the x axis. 
         [0032]    After deposition of the Cu layer, the Cu plated section of the base  100  passes through the Cu cleaning unit and the Cu-metrology-station, and enters the Ga electroplating unit  141  of the Ga-processing-unit  140 . The Ga electroplating unit  141  comprises a Ga plating bath  144 , which may be circulated between a Ga plating reservoir  145  and a chemical cabinet (not shown) through solution inlet and outlet tubes (not shown) on the Ga plating reservoir  145 . The Ga plating bath  144  or solution or electrolyte may be filtered and replenished during circulation or while in the chemical cabinet. Measurement and control of various bath parameters, such as additive content, Ga content, temperature, pH etc. may be continuously or periodically carried out within the chemical cabinet to assure stability of the Ga deposition process. Electrical connection to the base  100  or the already plated Cu layer may be achieved by various means including through rollers, brushes etc. One preferred method comprises making electrical contact to the back surface  100 B of the base  100  through at least one Ga plating contact roller  146 , if the substrate of the base is conductive. A Ga plating anode  147  is placed in the Ga plating bath  144  and a Ga plating power supply  148  is provided to apply a potential difference between the Ga plating anode  147  and the Ga plating contact roller  146 , so that Ga is electroplated on a section of the already Cu plated surface that is exposed to the Ga plating bath  144  across from the Ga plating anode  147  as the base  100  is moved from left to right at a predetermined speed. This way a Cu/Ga stack is formed on the base  100 . After Ga plating, the plated section gets cleaned in the Ga cleaning unit  142  through use of water sprays  139 . Air knives (not shown) may be used to partially or fully dry the surface of the plated Ga as well as the back side of the base  100  before the Ga plated and cleaned section enters the Ga-metrology-station  170 . The Ga layer thickness as well as the underlying Cu layer thickness may be measured in the Ga-metrology-station  170  if a XRF probe or unit is employed. 
         [0033]    After Ga deposition, cleaning and metrology, the base  100  with the electroplated Cu/Ga stack enters the In electroplating unit  151  of the In-processing-unit  150 . The In electroplating unit  151  comprises an In plating bath  154 , which may be circulated between an In plating reservoir  155  and a chemical cabinet (not shown) through solution inlet and outlet tubes (not shown) on the In plating reservoir  155 . The In plating bath  154  or solution or electrolyte may be filtered and replenished during circulation or while in the chemical cabinet. Measurement and control of various bath parameters, such as additive content, In content, temperature, pH etc. may be continuously or periodically carried out within the chemical cabinet to assure stability of the In deposition process. Electrical connection to the base  100  or the already plated Cu/Ga stack may be achieved by various means including through rollers, brushes etc. One preferred method comprises making electrical contact to the back surface  100 B of the base  100  through at least one In plating contact roller  156 , if the substrate of the base is conductive. An In plating anode  157  is placed in the In plating bath  154  and an In plating power supply  158  is provided to apply a potential difference between the In plating anode  157  and the In plating contact roller  156 , so that In is electroplated on a section of the already Cu and Ga plated surface that is exposed to the In plating bath  154  across from the In plating anode  157  as the base  100  is moved from left to right at a predetermined speed. This way a Cu/Ga/In stack is formed on the base  100 . After In plating, the plated section gets cleaned in the In cleaning unit  152  through use of water sprays  139 . Air knives (not shown) may be used to partially or fully dry the surface of the plated In as well as the back side of the base  100  before the In plated and cleaned section enters the In-metrology-station  180 . The In layer thickness as well as the underlying Cu and Ga layer thicknesses may be measured in the In-metrology-station  180  if a XRF probe or unit is employed. 
         [0034]    The continuous reel-to-reel electroplating technique of the present invention has several technical benefits. One of these benefits is the fact that each layer is electroplated on another layer that is freshly deposited and has a chemically active surface. Nucleation on such fresh surfaces is superior to nucleation on surfaces that are passivated as a result of oxidation or exposure to environment for extended periods of time after deposition. Another benefit is good thickness uniformity of the electroplated layer. Since the flexible base or substrate travels in front of the anodes during plating, its uniformity in the direction of travel, i.e. along its length, is excellent and very repeatable throughout the whole base. The uniformity along the width of the flexible base is the only factor to be optimized. One other benefit comes from the fact that the flexible base is always electrically “hot” when it enters a plating zone in a plating unit. Copper, In and Ga are different materials with different surface potentials when placed in plating electrolytes. For example, if Cu will be plated on an already plated Ga layer and if the Ga layer is dipped into the Cu plating electrolyte with no applied voltage, it is possible to have an exchange reaction between Ga surface and the Cu electrolyte. Such an exchange reaction may cause Ga to dissolve into the Cu plating electrolyte and Cu depositing (in an electroless manner) onto the Ga surface. It should be appreciated that such reactions would be detrimental to the control of crucial molar ratios such as the Cu/(Ga+In) ratio since these reactions are not well controlled. If the Ga surface enters into the Cu plating electrolyte with an appropriate cathodic voltage present on Ga, then exchange reaction cannot take place and Cu may be electrodeposited on Ga provided that Cu electrolyte is carefully selected. In the roll-to-roll approach described above, each section of the flexible base enters the plating solutions “hot”, i.e. there is cathodic voltage on the base. Therefore, dissolution of films in various electrolytes is avoided. 
         [0035]    As stated before, more processing-units may be added to the system  200  of  FIG. 4 , to obtain other precursor stacks, and more metrology-stations may be added to measure the thickness of each layer in the stack. For example, a Cu/Ga/In/Se stack may be obtained by adding a Se-processing-unit (not shown) after the In-metrology-station  180 . The Se-processing-unit may comprise a Se electroplating unit and a Se cleaning unit A Se-metrology-station (not shown) may then be added to measure the thickness of the electrodeposited Se film. It should be noted that metrology tools such as XRF (X-ray fluorescence) units have the capability of measuring the compositions or thicknesses of stacked layers comprising discrete films of various materials, as well as layers comprising mixtures or alloys of such materials. For example, these metrology tools can detect the amount of Cu and Ga in a Cu/Ga stacked layer as well as a Cu—Ga alloy layer and in the case of alloy layers these amounts may be represented as effective thicknesses of each material within the alloy layer. Therefore, the present invention may still be used even if electroplated stacks such as Cu/Ga stacks, Cu/Ga/In stacks etc. are heat treated and alloyed before they enter a metrology-station. 
         [0036]    Composition control of a precursor stack comprising Cu, and at least one Group IIIA material such as In and/or Ga may be achieved in various ways using the present invention. Some of these approaches will now be discussed for preparing an exemplary Cu/Ga/In precursor stack with a target Cu/(Ga+In) ratio of 0.9 and target Cu, Ga and In thicknesses of 200 nm, 110 nm and 340 nm, respectively. It should be noted that for solar cells fabricated on CIGS(S) type absorber layers, one of the most important parameters affecting device efficiencies is the Cu/(Ga+In) ratio. Specifically, when this ratio exceeds 1.0 the efficiency value drops drastically because the excess Cu gives rise to phases such as Cu—Se and/or Cu—S, which have low resistivity and may introduce low shunt resistance in the solar cells. It is therefore crucial to assure that the Cu/(Ga+In) molar ratio does not exceed 1.0. It is, however, possible to grow Cu-rich CIGS(S) layers with controlled composition and then chemically etch (for example, using cyanide solutions) the excess Cu—Se and/or Cu—S phases bringing the Cu/(Ga+In) ratio down to around 1.0 before fabricating solar cells on the absorber layers. 
         [0037]    If the Cu/(In +Ga) ratio of the solar cell absorber layer is kept under control, its total thickness may be changed, for example by at least +/−10%, without affecting the device efficiencies. The Ga/(Ga+In) ratio is also important to control, but it may be changed, for example by up to +/−10% without adversely affecting the efficiency. Taking into consideration the points in the above discussion, the present invention can be used in controlling the Cu/(In +Ga) and Ga/(Ga+In) molar ratios to different degrees in the following manner: 
         [0038]    In a first approach, the thickness of the Cu layer electroplated on a segment of the base in the Cu-processing-unit  130  may be measured at the Cu-metrology-station  160 . If this thickness value is outside a range defined by an upper limit and a lower limit, a signal may be sent to the Ga electroplating unit  141  to either increase or decrease the amount of Ga plating with respect to the nominal value that is expected to yield the exemplary target of 110 nm. For the exemplary Cu target thickness of 200 nm the upper and lower limits of the Cu thickness may be 205 nm and 195 mm, respectively. As an example, if the measured Cu thickness is 190 nm, the Ga plating power supply  148  may be sent a signal to reduce the Ga plating current density by about 200÷190=1.05 when the segment enters the Ga electroplating unit  141 . If the measured Cu thickness is 210 nm, the Ga plating current density may be increased by about 210÷200=1.05 times. Again, the increase of the current density may be timed so that the change is made when the segment enters the Ga electroplating unit  141  and Ga electroplating initiates. After Ga electroplating, the plated Ga layer thickness may be measured at the Ga-metrology-station  170 . It should be noted that the metrology tool  201  at the Ga-metrology-station  170  may provide the thickness information about the underlying Cu layer also. Once the actual Ga thickness is measured, or the Cu/Ga molar ratio is measured, a computer or control unit may calculate the required thickness of In to be deposited in the In electroplating unit  150  to achieve the exemplary Cu/(Ga+In) ratio target of 0.9. For this purpose, a signal may be sent to the In plating power supply  158  to increase or decrease the current density that is expected to yield the originally targeted In thickness of 340 nm, so that the In thickness is adjusted to reach or get close to the targeted Cu/(Ga+In) ratio. As can be seen from this example, this approach fundamentally aims to obtain a targeted Cu/(In +Ga) ratio rather than to obtain the targeted thicknesses of Cu, Ga and In layers. The thicknesses of the Cu and Ga layers are actually measured and therefore the possible error in the final Cu/(Ga+In) molar ratio is minimized, the only error being due to the possible inaccuracy of the In layer thickness. This is, of course, better than the case of taking no measurements where inaccuracies in the deposited thickness of each layer may accumulate and cause the Cu/(In +Ga) ratio to miss the targeted value or even exceed 1.0. It should be noted that an In-metrology-station  180  provided after the In-processing-unit  150  may measure the In thickness over the Cu/Ga layers or even the Cu/(In+Ga) ratio to confirm the final composition. There may also be at least one tuning station (not shown) after the In-metrology-station to add more In onto the already plated Cu/Ga/In stack or to etch and remove In from the already plated Cu/Ga/In stack to meet the target Cu/(In +Ga) ratio. This approach of using tuning stations is detailed more in U.S. Provisional Application Ser. No. 60/744,252 filed Apr. 4, 2006 entitled “Composition Control for Photovoltaic Thin Film Manufacturing”. 
         [0039]    In another approach, the Cu-metrology-station  160  may not be used. Ga-metrology-station  170  may measure the Ga and Cu thicknesses within the Cu/Ga stack and send a signal to the In plating power supply  158  to control the deposited In thickness as described above. 
         [0040]    In yet another approach, control of the electroplating power supplies may be carried out in a way to reach the targeted Cu, Ga and In thicknesses as well as the Cu/(In+Ga) ratio. For example, when the thickness of the Cu electroplated segment is measured at the Cu-metrology-station  160 , a signal may be sent to the Cu plating power supply  138  to adjust the Cu plating current density to achieve a targeted Cu thickness. If the measured Cu thickness is larger than the targeted thickness, the current density is reduced. If the measured Cu thickness is smaller than the targeted thickness the current density may be increased. This way, the thickness of the plated Cu is kept near the target value. Similar approach may be used for Ga and In plating to keep the thicknesses of all layers in a stack at or near the targeted values. It should be noted that, this approach does not attempt to correct the thickness of a layer already deposited on a segment of the base. Rather, it aims to control the thickness of the layer that will be deposited on later segments of the base. In this approach metrology or thickness measurement may be carried out continuously or intermittently. 
         [0041]    The film thickness control approaches summarized above may also be mixed in improving the performance of a system. In other words, one approach may be used for Cu thickness control, whereas another one may be employed for In and/or Ga film thickness controls. If a certain step of the process, for example the Cu electroplating step, is very stable and provides repeatable Cu thickness for a pre-determined plating current density and base speed, then a metrology-station may not be used for that step. 
         [0042]    It is possible to employ multiple metrology tools in any of the metrology-stations and to control segmented anodes in electroplating units using such metrology tools. For example,  FIG. 5  shows a top view of a process unit and a metrology-station, such as a Cu-processing-unit  130  comprising a Cu electroplating unit  131  and a Cu cleaning unit  132 ; and a Cu-metrology-station  160 . There may be more than one metrology tool in the Cu-metrology-station  160  as is shown in  FIG. 5 . The first metrology tool  201 A may be placed to measure the Cu thickness in the central region of the base  100  and the data collected by the first metrology tool  201 A may be used to control a first Cu plating power supply  138 A connected to a first Cu plating anode segment  137 A that provides plating current to the central region of the base  100 . Similarly, a second metrology tool  201 B and a third metrology tool  201 C may be placed to measure the Cu thickness in the two edge regions of the base  100  and the data collected by these metrology tools may be used to control a second Cu plating power supply  138 B and a third Cu plating power supply  138 C, connected to a second Cu plating anode segment  137 B and a third Cu plating anode segment  137 C, respectively. This way, the edge to center Cu thickness uniformity may be improved on the base and the target thickness may be achieved over the whole surface of the base. For example, increased plating current density applied by the first Cu plating power supply  138 A would increase the Cu thickness at the central region of the base  100  and increased plating current density provided by the second Cu plating power supply  138 B and/or the third Cu plating power supply  138 B would increase the Cu thickness at the edge regions of the base. It should be noted that the metrology tools  201 A,  201 B,  201 C may be moved in x, y directions. Other motions such as circular etc. movements may also be imparted to any of these metrology tools. Such movements define which regions of the base  100  are included in the collected thickness data. The base  100  moves in the x direction during electroplating and metrology at a predetermined speed which may be in the range of 0.1-5 cm/sec. Therefore, the thickness data collected by moving metrology tool(s) is an average value over several points on the base  100 , unless the metrology tool(s) are moved at the same velocity and direction as the base  100 . 
         [0043]    It should be noted that the measurements in the metrology tools may be made continuously or intermittently, and therefore the control of the various plating power supplies may be done continuously or intermittently, for example every 10 seconds or every 60 seconds etc. Although in these examples the plating current densities were changed to achieve the targeted thicknesses or electroplated stack compositions, it is also possible to change the speed of the base  100  to control a plated material thickness. At a given plating current density and in a fixed length of a plating cell or unit, the thickness of the plated material would be smaller as the speed of the base  100  is increased. In a roll-to-roll process, however, changing the speed of the base  100  would affect the thicknesses of all the layers plated. This needs to be taken into consideration in designing the control approach that employs varying speed as one of the parameters to vary deposited film thickness. In another embodiment, changing the thickness of an electroplated film on a moving substrate may be achieved by changing an effective electroplating length of the plating cell through which the substrate moves for deposition. For example, referring back to  FIG. 4 , the length of the Cu plating cell or the Cu electroplating unit  131  is “L”. A region at the front surface  100 A of the base  100  entering the Cu electroplating unit  131  through an inlet slit  135 A, starts plating when it is exposed to the Cu plating bath  134  and a plating current passes from the Cu plating anode  137  to the region. Deposition on the region continues until it exits the Cu electroplating unit  131  through an exit slit  135 B. Therefore, the thickness of the Cu film deposited on the region may be reduced if an anode shadow piece  137 A is introduced between the Cu plating anode  137  and the front surface  100 A of the base  100 . As shown in  FIG. 4 , depending on the length of the anode shadow piece  137 A the “effective plating length” may be reduced to a new value of “L′”. If the anode shadow piece  137 A is extendable then the amount of shadowing it provides can be varied even during the plating process and therefore the deposited film thickness may be manipulated. For example, a signal coming from a metrology station to reduce the deposited Cu thickness may either cause the Cu deposition current density to be lowered, or the anode shadow piece  137 A to extend, or both. 
         [0044]    In the above discussion we described roll-to-roll systems that electroplate all components of a precursor stack comprising Cu and at least one of Ga and In. This is the preferred method because it avoids multiple handling of the electroplated layers and provides electroplating on freshly deposited, active surfaces. However, the invention may also be applied to systems that electrodeposit individual layers or some of the layers of a given stack. For example, if the desired stack is a Cu/Ga/In stack, a first system may be used to deposit the Cu layer on a base. A second system may then electroplate a Ga layer over the Cu layer and a third system may electroplate an In layer over the Cu/Ga stack. In this case the composition/thickness control may be simpler since both electroplating current density variation and base speed variation may easily be used to control the thickness of a single layer. It is, of course, possible to electroplate two of the layers in a first system and the third layer in a separate system. For example, a first system may deposit the Cu layer and a second system may deposit the Ga and In layers. Alternately, a first system may deposit a Cu/Ga stack and a second system may deposit the top In layer. 
         [0045]    It is also possible that instead of pure elements of Cu, In and Ga, the present invention may electroplate alloys or mixtures in the electroplating units and sense the compositions of these alloys or mixtures at the metrology-stations. Accordingly, precursor stacks comprising alloy layers or sub-layers may be obtained. Such stacks include, but are not limited to Cu—Ga/In, Cu—In/Ga, Cu—Ga/Ga/In, Cu/In—Ga, Cu—In/Ga/Cu etc., where Cu—Ga is an alloy or mixture of Cu and Ga, Cu—In is an alloy or mixture of In and Cu and In—Ga is an alloy and mixture of In and Ga. Although the most common form of a base used in solar cell fabrication is a flexible foil, it is possible to have a base in the form of a wire to manufacture, for example, cylindrical devices. In fact wires may have many different cross sectional shapes. The roll-to-roll processing approaches of the present invention are applicable to processing of any flexible base including those in the form of wires. 
         [0046]    Once the precursor stacks, alloys or mixtures, or the “metal precursor/Group VIA material” stacks (such as a Cu/Ga/In/Se stack) of the present invention are formed, reaction or further reaction of these layers with Group VIA materials may be achieved by various means. For example, these layers may be exposed to Group VIA vapors at elevated temperatures. These techniques are well known in the field and they involve heating the layers to a temperature range of 350-600° C. in the presence of at least one of Se vapors, S vapors, and Te vapors provided by sources such as solid Se, solid S, solid Te, H 2 Se gas, H 2 S gas etc. for periods ranging from 5 minutes to 1 hour. In another embodiment a layer or multi layers of Group VIA materials may be further deposited on the metallic precursor layers and then heated up in a furnace or in a rapid thermal annealing furnace and like. Group VIA materials may be evaporated on, sputtered on or plated on the metallic precursor layers in a separate process unit. Alternately inks comprising Group VIA nano particles may be prepared and these inks may be deposited on the precursor layers or stacks to form a Group VIA material layer comprising Group VIA nano particles. Dipping, spraying, doctor-blading or ink writing techniques may be employed to deposit such layers. Reaction may be carried out at elevated temperatures for times ranging from 1 minute to 60 minutes depending upon the temperature. As a result of reaction, the Group IBIIIAVIA compound is formed. It should be noted that reaction chambers may also be added to the systems of the present invention to carry out the whole process in-line so that the flexible foil base with a fully formed CIGS(S) layer on its surface may be rolled onto a return spool  21  after electroplating and reaction steps. 
         [0047]    Solar cells may be fabricated on the Group IBIIIAVIA compound layers of the present invention using materials and methods well known in the field. For example a thin (&lt;0.1 microns) CdS layer may be deposited on the surface of the compound layer using the chemical dip method. A transparent window of ZnO may be deposited over the CdS layer using MOCVD or sputtering techniques. A metallic finger pattern is optionally deposited over the ZnO to complete the solar cell. 
         [0048]    Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.