Abstract:
The present invention relates to method and apparatus for preparing thin films of materials for various applications including electronic devices such as solar cells. In one aspect, each of the method and apparatus passing an electrical current through at least one of the base or sheet to provide controlled localized heat to the base or sheet, or to layers disposed above the base or sheet. In another aspect, the controlled localized heat is provided in combination with a process environment that can be a non-inert gas that contains an element that will become part of a compound on the base or sheet, or an inert gas that allows for the process environment to provide annealing.

Description:
PRIORITY CLAIM 
     This application claims priority to U.S. Provisional Patent Application No. 60/744,827 filed Apr. 13, 2006 entitled “Method and Apparatus to Form Thin Layers of Materials on a Conductive Surface.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to method and apparatus for preparing thin films of materials for various applications including electronic devices such as solar cells. 
     BACKGROUND 
     Thin films of materials are used for many applications. In some of these applications the films or layers deposited on substrates are not electrically active. For example, hard ceramic coatings such as carbides, nitrides and oxides of various elements are used to provide wear resistance to the substrate that they are deposited on. Some coatings provide color. Some others, such as tribological coatings, reduce the friction coefficient on the surface of the substrate that they are deposited on. In applications where the thin film is electrically active, the electrical activity varies depending on the nature of the layer. For example, metallic layers such as Al, Cu, Ni etc. may act as conductors. Semiconducting layers, on the other hand may be used in thin film device fabrication such as thin film transistors, solar cells, photoconductors, detectors etc. 
     Thin films may be deposited on substrates by a large variety of techniques, including vacuum evaporation, sputtering, chemical vapor deposition, electrodeposition, electroless plating, ink deposition, melt deposition, dipping, spinning etc. The mechanical, structural and/or electrical properties of the deposited films generally vary with deposition conditions including the temperature of the substrate during the film deposition step. Therefore, the substrate temperature needs to be closely controlled during the deposition of the films. 
     In some compound layer growth techniques the film deposition involves more than one step. As will be described further below, in two-stage or multi-stage processes, certain elements of the desired compound may first be deposited on a substrate in the form of a precursor layer and then reacted to form the desired compound film. In these cases, the substrate temperature need to be closely controlled, especially during the reaction step because the properties of the resulting compound layer depend, to a large extent, to details of the reaction process. 
     In processes involving heating the substrate during film deposition, various designs of substrate heaters are employed. Typically the substrate to be coated with the film is placed on or brought to proximity of a substrate heater, which may be, for example a lamp heater or a resistive heater. In applications where deposited layers are reacted to form compounds, reactions may be carried out in ovens, furnaces, rapid thermal process tools etc, where various different temperature control means are utilized. In certain applications, however, very fast response times are required for heating and/or cooling the substrates on which the films are grown. Additionally, it is always attractive to apply the heat mostly to the substrate but not to the environment the substrate is placed in. 
     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. 
     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. 
     One 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 components of the Cu(In,Ga)(S,Se) 2  material are first deposited onto a substrate, and then reacted with each other and/or with a reactive atmosphere in a high temperature annealing process. For example, for CuInSe 2  growth, thin layers of Cu and In are first deposited on a substrate and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere also contains sulfur, then a CuIn(S,Se) 2  layer can be grown. Addition of Ga in the precursor layer allows the growth of a Cu(In,Ga)(S,Se) 2  absorber. 
     There are many other versions of the two-stage process that have been employed by different research groups. For example, stacked layers of sputter deposited (Cu—Ga)/In, and co-evaporated (In—Ga—Se)/(Cu—Se), and (In—Ga—Se)/Cu stacks have all been used as precursor materials which were reacted at high temperatures with S and/or Se to form the final absorber film. Sputtering and evaporation techniques have been used in prior art approaches to deposit the layers containing the Group IB and Group IIIA components of the precursor stacks. In the case of CuInSe 2  growth, for example, Cu and In layers were sequentially sputter-deposited on substrates and then the stacked film was heated in the presence of gas containing Se at elevated temperature, 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 copper-gallium alloy layer and an indium layer on a metallic back electrode layer or a contact layer and then reacting this precursor film with one of selenium and sulfur to form the absorber layer. U.S. Pat. No. 6,092,669 described sputtering-based equipment for producing such absorber layers. Electrodeposition may also be used to deposit the metallic precursors, such as Cu and In layers, on a substrate. 
     Irrespective of the technique used for the deposition of the precursor layers comprising at least one Group IB material and at least one Group IIIA material, the precursor layer need to go through a reaction step to form the Group IBIIIAVIA compound layer. This is achieved heating the substrate on which the precursor is deposited and exposing it to at least one Group VIA material. This reaction step may be carried out in batch furnaces where a large number of substrates are heated up slowly to elevated substrates of 400-550 C in the presence of Se and/or S. This batch process is typically carried out for 20-60 minutes. One other approach utilizes a rapid thermal process where, typically, precursors comprising Group IB, Group IIIA and Group VIA materials are heated up rapidly to initiate and carry out reaction between them forming the Group IBIIIAVIA compound layer. It is reported that ramp rates in such a process is very important and that temperature rise rates of about 10 C/sec is needed to avoid defects in the forming compound layer. 
     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 substrate  11 , such as of a sheet of glass, a sheet of metal, 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 a conductive layer  13  or a contact layer, which is previously deposited on the substrate  11  and which acts as the ohmic contact to the device. Various conductive layers comprising Mo, Ta, W, Ti, 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. The preferred electrical type of the absorber film  12  is p-type, and the preferred electrical type of the transparent layer  14  is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of  FIG. 1  is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te) 2  absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the superstrate side. 
     In any process involving heating a substrate on which a thin film material such as a thin film semiconductor is being formed, heating uniformity, heating rate and sometimes cooling rate are important parameters that influence the properties of the film that is formed on the substrate. There is, therefore, a need to develop novel approaches to closely control such parameters. 
     SUMMARY OF THE INVENTION 
     The present invention relates to method and apparatus for preparing thin films of materials for various applications including electronic devices such as solar cells. 
     In one aspect, each of the method and apparatus passing an electrical current through at least one of the base or sheet to provide controlled localized heat to the base or sheet, or to layers disposed above the base or sheet. 
     In another aspect, the controlled localized heat is provided in combination with a process environment that can be a non-inert gas that contains an element that will become part of a compound on the base or sheet, or an inert gas that allows for the process environment to provide annealing. 
     In a particular aspect, the present invention provides a method of forming a reacted film, the method includes providing a base; depositing on the base a precursor layer including two or more elements; and passing an electrical current through at least one of the base and the precursor layer to provide controlled localized heat to the precursor layer and cause a reaction of the two or more elements of the precursor layer to form the reacted film. 
     In another aspect the present invention provides an apparatus for forming a film on a sheet, the apparatus including a process chamber for exposing the sheet to a process environment that includes at least one constituent element; and an electrical circuit disposed with the process chamber that provides an electrical current to the sheet to cause controlled localized heat to the sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a cross-sectional view of a solar cell employing a Group IBIIIAVIA absorber layer. 
         FIG. 2  shows a stack being heated by passing an electrical current through it. 
         FIG. 2A  shows face-to-face processing in accordance with one embodiment. 
         FIG. 3A  shows a stack being placed in a reactor. 
         FIG. 3B  shows the stack of  FIG. 3A  being heated up in accordance with one embodiment while the reactor is closed. 
         FIG. 4  shows two temperature zones formed on a substrate using four electrical contacts. 
         FIG. 5  shows a film being deposited on a layer out of a solution by selectively heating the layer. 
         FIG. 6  shows a method of heating a substrate as a film is formed on a substrate. 
         FIG. 7  shows a method of heating a conductive precursor layer to form a compound layer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In one embodiment, a film is formed on a substrate comprising a conductive layer or portion. A current is passed through the conductive layer or portion to heat up the conductive layer or portion and therefore the film, which is thermally coupled to it. The substrate may itself be a conductor, in which case there may not be a need for an additional conductive layer or portion. The invention will now be described using an example of forming thin layers of Group IBIIIAVIA semiconducting compounds. Those skilled in the art would recognize that the invention may be used to form a large variety of semiconductor, conductor or insulating layers on various substrates. 
       FIG. 2  shows a stack  25  made of a precursor layer  20  deposited on a base  21 . The precursor layer  20  of this example may comprise Cu, In, and Ga. It may additionally comprise a Group VIA material such as Se and/or S. These materials may be in the form of a mixture such as mixture of Cu, In, and Ga, or a mixture of Cu, In, Ga and Se, or they may be in the form of stacked sub-layer to form the precursor layer  20 . Sub-layers may comprise Cu, In, Ga and Se and may be arranged in different orders to form the stacked precursor layers. Some examples of the stacked precursor structures include Cu/Ga/In, Cu/In/Ga, Cu/Ga/In/Se, CuIn/Ga/Se, Cu—Ga/In/Se, Cu—In/Ga/Se, Cu—Se/In—Se/Ga—Se, and Cu/Ga—Se/In—Se. The base  21  may comprise a substrate  22  and a contact layer  23 . The substrate  22  may be a conductive sheet or foil such as a stainless steel foil, a Mo foil, a Ti foil, an Al foil etc. The contact layer may comprise any proper material that makes good ohmic contact to Group IBIIIAVIA compound layers. These materials include Mo, Ta, W, Hf, Ru etc. The stack  25  may be placed in an enclosure (not shown) and a current, “I”, may be passed through it by touching the stack  25  with electrodes  24 . It should be noted that in this example, all the components of the stack  25 , i.e. the substrate  22 , the contact layer  23  and the precursor layer  20 , may be conductive. Therefore the current may pass through all these components. However, since the substrate  22  is typically much thicker (25-250 microns) than the rest of the layers in the stack  25 , most of the current may pass through the substrate  22 . The typical thickness ranges for the contact layer  23  and the precursor layer  20  are, 0.1-3 microns and 0.2-2 microns, respectively. A current of 1-1000 Amperes (A) or larger may be passed through the stack depending on the lateral resistance of the stack which, in turn, is determined by the thickness, the width and the length of the stack  25  or the substrate  22 . When an electrical current “I” is passed thorough the stack  25  or the substrate  22 , the temperature of the stack  25  rises and a reaction is initiated within the precursor layer  20  between Cu, In, Ga and Se. If there is no Se within the precursor layer  20 , Se or another Group VIA material, such as S and/or Te may be provided to the top surface  26  so that a Group IBIIIAVIA compound layer is formed on the contact layer  23  as a result of this reaction. 
     It should be noted that the present invention uses the stack and/or the base and/or the conductive substrate as a heater. Therefore, the temperature of the film forming over the substrate or the base may be controlled accurately with a fast response time. Temperature control may be achieved by attaching at least one thermocouple to the base (not shown). Temperature rise times of 10-100 C/sec may be readily obtained using this approach depending on the current density used to pass through the stack or the substrate. Such high temperature rise times are difficult to achieve in a uniform manner using standard Rapid Thermal Processing (RTP) approaches employing lamps or heater blocks. Another benefit of the present invention is increased throughput. In a regular RTP approach using heating lamps only one substrate is processed at a time. In  FIG. 2 , for example, a bank of lamps may be placed above the precursor layer  20  and these lamps may shine down onto the top surface  26  heating the precursor layer  20 . Therefore, it is necessary to have the top surface  26  exposed to radiation from the lamps. In the present approach however, face-to-face processing may be carried out as shown in  FIG. 2A , where a first stack  27  and a second stack  28  are placed across from each other forming a gap  29  between them. When current is passed through both stacks using two power supplies (as shown) or a single power supply (not shown), both of the stacks are heated up and the first precursor layer  20 A as well as the second precursor layer  20 B may be reacted at the same time. Such an arrangement doubles the throughput of the process and it forms a uniformly heated gap  29  through which inert or reactive gasses may be passed. For example, if the first precursor layer  20 A and the second precursor layer  20 B comprise Cu, In, Ga and Se, they both would be converted into Cu(In,Ga)Se 2  layers after the heating step which may take the temperature to a range of 200-550 C or higher. In this case an inert gas, vacuum and/or Se vapor may be present in the gap  29 . Having a small gap of 1-5 mm also contains the volatile Se species within the gap and increases Se utilization by keeping Se close to the precursor surface during reaction. If the first precursor layer  20 A and the second precursor layer  20 B comprise Cu, In, and Ga but no Se, both precursor layers could be converted into Cu(In,Ga)Se 2  layers after the heating step if a Se containing gas such as H 2 Se or Se vapor is provided to the gap  29  so that the provided Se may react with the Cu, In and Ga species in the two precursor layers. Having a small gap of 1-5 mm contains the volatile Se species within the gap and increases Se utilization by keeping it close to the precursor surface during reaction. 
     The technique of the present invention applies heat directly where it is needed, i.e. directly to the film that is being formed over the substrate. The invention may also be used in a way that allows control of the cooling rate.  FIGS. 3A and 3B  show an example of such a case. 
       FIG. 3A  shows a reactor  30  comprising an upper block  31  and lower block  32 . The stack  25 , which may be similar to the stack  25  of  FIG. 2 , is placed between the upper block  31  and the lower block  32 . Electrodes  24  with electrical leads  35  attached to terminals of a power source (not shown) are placed such that when the reactor  30  is closed by bringing the upper block  31  and the lower block  32  together (as shown in  FIG. 3B ), they make electrical contact with the stack  25 . Once the electrical contact is established between the stack  25  and the electrodes  24 , a current may be passed through the stack  25  via the electrodes  24 , and the stack temperature may be raised to the desired level. In one embodiment, the lower block  32  may be chilled or it may be kept at a pre-selected temperature. There may be at least one gas inlet  33  and at least one gas outlet  34  provided in the lower block  32 . As the stack  25  is heated by passing the electrical current through it, a gas such as nitrogen may be brought into the lower cavity  36  via the gas inlet  33  and taken out of the cavity via the gas outlet  34  as shown in  FIG. 3B . After the film formation/reaction period the electrical current may be cut off and a thermally conductive gas such as He may be brought into the lower cavity  36  to thermally couple the stack  25  and the lower block  32 , and rapidly cool the stack  25  down to the pre-selected temperature of the lower block  32 . By changing the thermal conductivity of the gas (through changing the composition of the gas) in the lower cavity  36 , and the depth of the lower cavity, one may control the rate of cooling down for the stack. It should be noted that, the lower cavity may be very shallow such as only 10-100 microns deep. Alternately, there may not be a lower cavity and the gas may just lift the stack away from the lower block  32  when passed during process. Also, if very rapid cooling is desired, vacuum may be pulled in the lower cavity to establish physical contact between the lower block  32  and the stack  25  once the heating/reaction step of the process is finished and the electrical current passing through the stack is cut off. There may be an upper cavity  37  formed between the upper block  31  and the stack  25 . The upper block  31  may or may not be heated. There may also be gas inlets and outlets (not shown) bringing reaction gases or inert gasses to the upper cavity  37 . For example, Se and/or S vapors or other gases containing Se and/or S may be brought into the upper cavity  37  at the time the stack is heated up by passing the current through it. Such gases may participate during reaction of the precursor layer to form the compound layer. 
     Various geometric versions of the reactors depicted in  FIGS. 3A and 3B  may be utilized. For example, these reactors may be turned 90 degrees or 180 degrees yielding vertical or horizontal designs. The substrate  22  may not be conductive but the contact layer  23  may be conductive. In this case the current may be passed through the contact layer  23  and the contact layer  23  may be used as a thin film heater. Also face-to-face processing may be carried out in the reactors as described in relation to  FIG. 2A . 
     The present invention may also be used in a continuous, in-line process. For example,  FIG. 4  schematically shows a section of a stack  25  being moved from left to right in the direction of arrow  40 . A first set of electrical contacts  41 A and  41 B, which may be sliding contacts, rollers or brushes, are provided making electrical connection to the stack  25  defining a first zone A. A second set of electrical contacts  42 A and  42 B are provided making electrical connection to the stack  25  defining a second zone B. Power supplies P 1  and P 2  may be connected to the two sets of electrical contacts as shown in the figure. Depending upon how much power is applied by the power supplies P 1  and P 2 , the temperature of the zones A and B may be pre-set or varied. For example, a first current I 1  may be passed through the zone A via electrical contacts  41 A and  41 B, and a second current I 2  may be passed through the zone B via electrical contacts  42 A and  42 B. If I 1 &lt;I 2 , the temperature in zone B of the stack would be higher than the temperature in zone A of the stack. For example, the current in zone A may be adjusted so that the temperature in this zone goes to 200 C, whereas the current in zone B may be selected so that the temperature goes up to 500 C. This way, as the stack  25  is moved in the direction of arrow  40  with respect to the electrical contacts (alternately the contacts may also be moved) the precursor layer of the stack may be put through a first temperature in zone A and then a second temperature in zone B. By using even more contact sets and zones, any temperature profile, from uniform to highly varying, may be obtained to carry out the reaction of the precursor layer. 
     The present invention is especially suited in applications where heat needs to be applied just to the substrate or stack without dispersing it extensively to the environment of a reactor. This way desired reactions may be initiated right on the substrate or stack surface rather than on reactor walls etc. In this case the conductive substrate or the stack is immersed into a reactor environment in a way to expose a face of the stack or the substrate to the reactor environment. The reactor environment may contain various chemicals, gasses, vacuum etc. that may be needed for the formation of the film on the substrate. Heat according to the present invention can be applied solely through the usage of passing the electrical current to heat the precursor layer, or a combination of using electrical current to heat the precursor layer and another heating step, either simultaneous with or after using the electrical current. If the heat generated using the electrical current causes a temperature range of 200-400 C within the precursor, there will likely be the need for a further step of heating. If, however, the electrical current heats the precursor layer to a temperature range of 400-600 C, then, depending on the length of time of the heating, another heating means or another heating step may not be, and preferably is not, needed. 
     An example will now be given where a wet deposition process is used to form a thin layer of CdS on a Group IBIIIAVIA compound layer. 
     In prior art techniques, fabrication of a Group IBIIIAVIA solar cells such as a Cu(In,Ga)Se 2  solar cell involves deposition of a thin CdS layer over the Cu(In,Ga)Se 2  absorber. One method of depositing the CdS layer is the chemical dip method where the Cu(In,Ga)Se 2  surface is dipped in a chemical solution comprising Cd (from salts such as Cd-nitrate, Cd-sulfate, Cd-chloride etc.) and S (from sources such as thiourea), complexing agents (such as ammonia). The solution is typically heated to 80-90 C to initiate the reaction between the Cd and S species in the solution. After exposing the Cu(In,Ga)Se 2  surface to the heated solution for a period of time, such as 5-30 minutes, a thin CdS layer forms on the Cu(In,Ga)Se 2  surface. Details of such a process may be found in the literature (see for example, C. Voss et al., Journal of Electrochemical Society, vol:151, p: C655, 2004). One drawback of this approach is the fact that Cd and S reaction and CdS formation happens everywhere in the system, including in the bulk of the solution and on the walls of the reactor or vessel that holds the solution. This is quite wasteful. 
       FIG. 5  shows an exemplary system  50  employing the teachings of the present invention. The system  50  comprises a container  55  containing a solution  54 . The solution may comprise Cd and S species as mentioned before. The container  55  may have an inlet port  56  and an outlet port  57  through which the solution  54  may be fed into and drawn out of the container  55 . A conductive substrate  51  coated with a layer  52  such as a Cu(In—Ga)Se 2  layer is placed in the container  55  such that a surface  53  of the layer  52  is exposed to the solution  54 . Electrical contacts  58  are provided and connected to a power supply  59 . The temperature of the solution may be kept at a range of 10-60 C that does not allow much reaction between the Cd and S species. A current may then be passed through the electrical contacts  58  and the conductive substrate  51 , heating the conductive substrate  51  and the layer  52  to a pre-selected temperature, such as to 60-90 C. A micro-layer of the solution  54  which is in contact with the surface  53  of the layer  52  gets heated and the Cd and S reaction is initiated within that micro-layer and on the surface  53 , selectively forming a CdS layer on the surface  53 , without wasting the bulk of the solution which may be pumped out through the outlet port  57 , filtered and fed back into the container  55  through the inlet port  56 . 
     It should be noted that the invention is applicable to cases where the container  55  may contain a gas rather than a liquid. For example, the layer  52  on the conductive substrate  51  may be a precursor layer comprising Cu, In and Ga and the container  55  may contain a gas comprising Se, such as H 2 Se gas or Se vapor. When the substrate  51  and the precursor layer is heated by passing current through the substrate  51 , a reaction is initiated between the precursor layer and the gas forming a Cu(In,Ga)Se 2  compound film on the substrate  51 . It should also be noted that the substrate  51  may be flat, coiled or it may be a roll of metallic foil immersed in a gas comprising Se. When current is passed through the whole roll, the whole roll may be heated instantly and uniformly initiating reaction uniformly. If a whole roll of metallic foil substrate, such as stainless steel foil, with a precursor layer deposited on it was placed into a furnace filled with a reactive gas such as a gas comprising Se, heating rate of the roll would be very non-uniform, the outside of the roll heating up first, while the core staying cooler. This would mean reaction with the reactive gas would be very non-uniform. If the whole roll is heated up by passing current through it, i.e. using the substrate roll as the heater, heating uniformity would be much better and this way large area production may be achieved without compromising the quality of the film that is formed on the substrate. 
     So far we have described the invention in terms of forming a layer on a substrate through reaction of precursors or reaction from liquid phase. The invention may also be used for other deposition or film formation methods such as evaporation, sputtering, spray pyrolysis etc. For example,  FIG. 6  shows a conductive substrate  60  being coated with a film  65  on its front surface  63 . The substrate  60  may be moving from left to right in a continuous or step-wise fashion. Electrical contacts touching the back surface  66  of the substrate complete the electrical circuit with a power supply, P, and define a zone, D. As the substrate  60  moves from left to right, the section of the substrate within the zone D gets heated to a temperature of, for example, 50-600 C if the power supply is energized. A material flux  62  may be created by a source  61  and directed to the front surface  63  within the zone D. The source  61  may be a sputtering target, an evaporation source, a spray gun, a doctor blade, gravure printer, ink printing head etc., depending on the deposition approach selected. Material deposits and/or reacts and/or dries on the front surface  63  forming the film  65 . It should be noted that the example above used a conductive substrate. However, the substrate may not be totally conductive but it may have a conductive portion or a conductive layer through which an electrical current may be passed to heat up the front surface  63 . The conductive substrate of the present invention may be a metallic foil such as an Al foil or a stainless steel foil and the film  65  may be a precursor layer comprising at least one of a Group IB material, a Group IIIA material and a Group VIA material, or it may be a Group IBIIIAVIA compound absorber layer that is deposited on the conductive substrate for the purpose of fabricating thin film solar cells. 
     The present invention may also be used in a mode where the current passes primarily through the film being formed. For example, a conductive precursor layer comprising elements A, B and C may be used to form a A-B-C compound or a A-B-C-D compound as will be described.  FIG. 7  shows a substrate  71  with a precursor film  70  on its surface. Precursor  70  is conductive and may comprise elements of A, B and C (such as Cu, In and Ga). The substrate  71  in this case may be insulating or may have high resistivity. There may be other layers such as contact layers etc between the substrate  71  and the precursor film  70  that are not shown for brevity. Contacts  72  electrically couple a power supply, P, to the precursor layer  70  and a current, I, passes through the precursor layer  70 . Passing current heats the precursor and a reaction takes place within the precursor layer. For example Se may be provided to the top surface  73  of the precursor layer and thus a Cu(In,Ga)Se 2  compound layer may be formed through reaction of Cu, In and Ga within the precursor and Se that is provided from outside. It is of course possible that the precursor comprises all the elements required for compound formation. For example, the precursor may comprise elements of A, B, C and D (such as Cu, In, Ga and Se) in the form of a mixture or in the form of thin layers. Through heating all the elements may be reacted to form the A-B-C-D compound (such as a Cu(In,Ga)Se 2  compound). As the compound layer forms, its conductivity gets reduced and this may be used as a self-limiting control if power supply provides a constant voltage. It should be understood that under constant voltage conditions, as a compound layer is formed from a conductive precursor layer, the resistivity may increase and therefore the current passing through the forming compound layer gets reduced, in turn reducing heating of the compound layer upon completion of the reaction. 
     All embodiments above have been described using substrates or stacks that are in the form of flat plates or foils. It should be clear to those experienced in the art that the present invention may be used for various forms and shapes of substrates. For example, Group IBIIIAVIA compound layers may be formed on spherical conductors or conductive wires which may be cylindrical in shape. This way, thin film solar cells may be obtained on cylindrical substrates or wires. During the formation of the Group IBIIIAVIA layers on cylindrical wires the heat may be provided by passing current through the wires in accordance with various embodiments discussed so far. The wires may be wires of stainless steel, Mo, Ti, Al etc. They may be coated with a contact layer such as Mo, W, Ta, Ru etc. A precursor layer comprising at least one of Group IB material, a Group IIIA material and optionally a Group VIA material may be deposited on the contact layer by various methods such as sputtering, evaporation, electrodeposition etc. Electrodeposition is especially attractive because of high material utilization. Precursors comprising Cu, In and Ga may be electrodeposited, for example, on contact layers at high efficiency. Once formed, the precursor layers are reacted in presence of Group VIA materials to form Group IBIIIAVIA layers such as Cu(In,Ga)Se 2  layers on the wires. 
     Once the Group IBIIIAVIA compound layers are formed on substrates of various shapes and forms through using various embodiments of the present invention, solar cells may be fabricated on the 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. 
     Although the present invention is described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.