Patent Publication Number: US-2012031492-A1

Title: Gallium-Containing Transition Metal Thin Film for CIGS Nucleation

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of photovoltaic devices, and more specifically to forming thin-film solar cells by sputter depositing an alkali-containing transition metal electrode. 
     BACKGROUND OF THE INVENTION 
     Copper indium diselenide (CuInSe 2 , or CIS) and its higher band gap variants copper indium gallium diselenide (Cu(In,Ga)Se 2 , or CIGS), copper indium aluminum diselenide (Cu(In,Al)Se 2 ), copper indium gallium aluminum diselenide (Cu(In,Ga,Al)Se 2 ) and any of these compounds with sulfur replacing some of the selenium represent a group of materials, referred to as copper indium selenide CIS based alloys, have desirable properties for use as the absorber layer in thin-film solar cells (i.e., photovoltaic cells). To function as a solar absorber layer, these materials should be p-type semiconductors. This may be accomplished by establishing a slight deficiency in copper, while maintaining a chalcopyrite crystalline structure. In CIGS, gallium usually replaces 20% to 30% of the normal indium content to raise the band gap; however, there are significant and useful variations outside of this range. If gallium is replaced by aluminum, smaller amounts of aluminum are used to achieve the same band gap. 
     SUMMARY OF THE INVENTION 
     One embodiment of this invention provides a solar cell comprising a substrate, a first transition metal layer located over the substrate, the first transition metal layer further comprising an alkali element or an alkali compound, a second transition metal layer located over the first transition metal layer, the second transition metal layer further comprising gallium, at least one p-type semiconductor absorber layer located over the second transition metal layer, wherein the p-type semiconductor absorber layer includes a copper indium selenide (CIS) based alloy material, an n-type semiconductor layer located over the p-type semiconductor absorber layer, and a top electrode located over the n-type semiconductor layer. 
     Another embodiment of the invention provides a solar cell comprising a substrate, a first molybdenum layer located over the substrate, the first molybdenum layer further comprising sodium, a second molybdenum layer located over the first molybdenum layer, the second molybdenum layer further comprising gallium, a copper indium gallium selenide (CIGS) p-type semiconductor absorber layer located over the second molybdenum layer, an n-type semiconductor layer located over the p-type semiconductor absorber layer, and a top electrode located over the n-type semiconductor layer. 
     Another embodiment of the invention provides a method of manufacturing a solar cell comprising providing a substrate, depositing a first transition metal layer over the substrate, the first transition metal layer further comprising an alkali element or an alkali compound, depositing a second transition metal layer over the first transition metal layer, the second transition metal layer further comprising gallium, depositing at least one p-type semiconductor absorber layer over the second transition metal layer, wherein the p-type semiconductor absorber layer includes a copper indium selenide (CIS) based alloy material, depositing an n-type semiconductor layer over the p-type semiconductor absorber layer, and depositing a top electrode over the n-type semiconductor layer. The second transition metal layer permits alkali diffusion from the first transition metal layer into the p-type semiconductor absorber layer during at least one of the steps of depositing the p-type semiconductor absorber layer, depositing the n-type semiconductor layer, or depositing the top electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side cross-sectional view of a CIS based solar cell according to one embodiment of the invention. 
         FIG. 2A  shows a highly simplified schematic diagram of a top view of a sputtering apparatus that can be used to forming a first transition metal layer such as an alkali-containing transition metal layer, for example, a sodium-containing molybdenum film. 
         FIG. 2B  shows a highly simplified schematic diagram of a top view of a sputtering apparatus that can be used to forming a second transition metal layer such as an gallium-containing transition metal layer, for example, a gallium-containing molybdenum film. 
         FIG. 3  shows a highly simplified schematic diagram of a top view of a modular sputtering apparatus that can be used to manufacture the solar cell depicted in  FIG. 1 . 
         FIG. 4  illustrates schematically the use of three sets of dual magnetrons to increase the deposition rate and grade the composition of the CIS layer to vary its band gap. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As grown CIS films are intrinsically p-type. However, it was found that a small amount of sodium dopants in CIS films increases the p-type conductivity of the CIGS film and the open circuit voltage, and in turn, improves the efficiency of the solar cell. For example, Ramanathan (Ramanathan et al., Prog. Photovolt. Res. Appl. 11 (2003) 225, incorporated herein by reference in its entirety) teaches that a solar cell, having an efficiency as high as 19.5%, may be obtained by using a soda-lime glass substrate in combination with depositing a CIS film under a high growth temperature. This method significantly improves the efficiency of a traditional solar cell by diffusing sodium from the glass substrate into the CIS film. However, it is difficult to control the amount of the sodium provided to the CIS film and the speed of the sodium diffusion from a glass substrate. Furthermore, unlike glass substrates, other substrates, such as metal and plastic substrates, do not provide such a readily available supply of sodium. 
     Related studies show that CIGS having a high gallium content has a higher bandgap and an improved adhesion to the Mo bottom electrode (see Dullweber et al., Thin Solid Films 387 (2001) 11-13; Lundberg et al., Thin Solid Films 480-481 (2005) 520-525); and Topic et al., J. Appl. Phys. 79 (1996) 8537-8540). It is also suggested that a gallium gradient at the Mo/CIGS interface can yield to better collection efficiency for long wavelength excitations, indicating passivation of the Mo/CIGS contact. However, in order to obtain a CIGS layer having a higher gallium content and the Mo/CIGS interface than at the top of the CIGS layer, and additional CIG target with higher gallium composition is required, which is not desired in manufacturing of the solar cells. 
     One embodiment of this invention provides a transition metal layer containing gallium between the CIGS layer and the back electrode in a CIGS type solar cell. Specifically, in one embodiment of this invention, a solar cell may comprise a substrate, a first transition metal layer comprising an alkali element or an alkali compound located over the substrate, a second transition metal layer comprising gallium located over the first transition metal layer, at least one p-type semiconductor absorber layer (e.g., a CIGS layer) located over the second transition metal layer, an n-type semiconductor layer located over the p-type semiconductor absorber layer, and a top electrode located over the n-type semiconductor layer. One advantage of the above described configuration is that high gallium content at the bottom electrode/CIGS layer interface results in good CIGS layer adhesion. In other words, disposing the second transition metal layer comprising gallium (e.g., a gallium containing molybdenum layer) between the CIGS layer and the first transition metal layer (e.g., a sodium containing molybdenum layer) improves the adhesion between the CIGS and the first transition metal layer. Another advantage is that the high gallium content may also create a back field, which reduces recombination and improves open circuit voltage and short circuit current. 
     As illustrated in  FIG. 1 , one embodiment of the invention provides a solar cell which contains a substrate  100  and a first transition metal layer  202  containing at least an alkali element or an alkali compound over the substrate  100 , and a second transition metal layer  203  comprising gallium over the first transition metal layer  202 . The solar cell may further comprise an optional alkali diffusion barrier layer  201  located between the substrate  100  and the first transition metal layer  202 . Additional adhesion layer (not shown) may be further disposed between the first transition metal layer  202  and the substrate  100 , for example between the optional alkali diffusion barrier layer  201  and the substrate  100 . In some embodiments, the adhesion layer comprises at least 90 atomic percent chromium, such as 90 to 100 atomic percent Cr. 
     The transition metal of the first transition metal layer  202  may be any suitable transition metal, including but not limited to Mo, W, Ta, V, Ti, Nb, and Zr. The alkali element or alkali compound may comprise one or more of Li, Na, and K. The first transition metal layer  202  may have a thickness of 100 to 500 nm, for example 200 to 400 nm, such as around 300 nm. 
     In some embodiments, the first transition metal layer  202  contains an alkali element or an alkali compound but is substantially free of the lattice distortion element or compound. Alternatively, the first transition metal layer  202  may further comprise a lattice distortion element or a lattice distortion compound. The lattice distortion element or the lattice distortion compound may be any suitable element or compound, for example, oxygen, nitrogen, sulfur, selenium, an oxide, a nitride, a sulfide, a selenide, an organometallic compound (e.g. a metallocene, a metal carbonyl such as tungsten pentacyonyl and tungsten hexacarbonyl, and the like), or combinations thereof. In some embodiments, when the transition metal is molybdenum, the latter distortion element may be oxygen, forming the first transition metal layer  202  of body centered cubic Mo lattice distorted by face centered cubic oxide compositions, such as MoO 2  and MoO 3 . For example, in a non-limiting example, the first transition metal layer  202  may comprise molybdenum containing oxygen and sodium, such as a transition metal layer comprising at least 59 atomic percent molybdenum, 5 to 40 atomic percent oxygen and 0.01 to 1.5 atomic percent sodium. In some embodiments, the first transition metal layer  202  may further contain elements other than molybdenum, oxygen and sodium, such as other materials that are diffused into this layer during deposition, such as indium, copper, selenium and/or barrier layer metals. 
     The second transition metal layer  203  comprises gallium. The transition metal of the second transition metal layer  203  may be same or different from the transition metal of the first transition metal layer  202 . Preferably, they are the same. Similarly, any suitable transition metal, including but not limited to Mo, W, Ta, V, Ti, Nb, and Zr, may be used as the transition metal of the second transition metal layer  203 . For example, in some embodiments, the second transition metal layer  203  comprises molybdenum and gallium. The second transition metal layer  203  may have a thickness of about 20 to 80 nm, such as 30 to 70 nm. 
     In some embodiments, the second transition metal layer  203  further comprises at least one of copper, indium, aluminum, or combinations thereof. For example, in a non-limiting example, the second transition metal layer  203  may contain molybdenum, gallium and copper. For example, a transition metal layer  203  may contain 50 to 90 atomic percent molybdenum, 10 to 50 atomic percent gallium and 0 to 10 atomic percent of at least one of copper, indium and aluminum. For example, layer  203  may contain no copper, no indium, and no aluminum. Alternatively, layer  203  may contain 1-10 atomic percent of Cu, or 1-10 atomic percent In, or 1-10 atomic percent Al, or 1-10 atomic percent of a combination of any two of or all three of Cu, In and Al. 
     The gallium content in the second transition metal layer  203  may be graded or uniformly distributed. For example, in some embodiments, the second transition metal layer  203  may comprise multiple sub-layers, for example 1 to 20 sub-layers such as 1 to 10 sub-layers. Each sub-layer has a different gallium concentration, resulting in a graded gallium concentration profile within the second transition metal layer  203 . Preferably, the higher gallium concentration is located on the upper portion of layer  203  which is located adjacent to the CIGS layer  301 . 
     The optional alkali diffusion barrier layer  201  may comprise any suitable materials. For example, they may be independently selected from a group consisting of Mo, W, Ta, V, Ti, Nb, Zr, Cr, TiN, ZrN, TaN, VN, or combinations thereof. In some embodiments, the alkali diffusion barrier layer  201  comprises at least 90 atomic percent molybdenum. The alkali diffusion barrier layer may have a thickness of about 100 to 400 nm such as 100 to 200 nm. 
     Preferably, the alkali diffusion barrier layer  201  has a greater thickness and a higher density than the second transition metal layer  203 . The higher density and greater thickness of the alkali diffusion barrier layer  201  substantially reduces/prevents alkali diffusion from the first transition metal layer  202  into the substrate  100 . On the other hand, the second transition metal layer  203  has a higher porosity than the alkali diffusion barrier layer  201  and permits alkali diffusion from the first transition metal layer  202  into the p-type semiconductor absorber layer  301 . In these embodiments, alkali may diffuse from the first transition metal layer  202 , through the lower density second transition metal layer  203 , into the at least one p-type semiconductor absorber layer  301  during and/or after the step of depositing the at least one p-type semiconductor absorber layer  301 . 
     In preferred embodiments, the p-type semiconductor absorber layer  301  located over the second transition metal layer  203  may comprise a CIS based alloy material selected from copper indium selenide, copper indium gallium selenide, copper indium aluminum selenide, or combinations thereof. Layer  301  may have a stoichiometric composition having a Group I to Group III to Group VI atomic ratio of about 1:1:2, or a non-stoichiometric composition having an atomic ratio of other than about 1:1:2. Preferably, layer  301  is slightly copper deficient and has a slightly less than one copper atom for each one of Group III atom and each two of Group VI atoms. The step of depositing the at least one p-type semiconductor absorber layer may comprise reactively AC sputtering the semiconductor absorber layer from at least two electrically conductive targets in a sputtering atmosphere that comprises argon gas and a selenium containing gas (e.g. selenium vapor or hydrogen selenide). For example, each of the at least two electrically conductive targets comprises copper, indium and gallium; and the CIS based alloy material comprises copper indium gallium diselenide. 
     Gallium may diffuse from the second transition metal layer  203  to the CIS based alloy (e.g., CIGS) layer  301 . In some embodiments, the p-type semiconductor absorber layer  301  may comprise a first portion adjacent to the second transition metal layer  203  which contains more gallium then a second portion distant from the second transition metal layer  203 . Furthermore, sodium impurities may diffuse from the first transition metal layer  202  to the CIS based alloy layer  301  through the second transition metal layer  203 . In one embodiment, the p-type semiconductor absorber layer  301  may comprise 0.03 to 1.5 atomic percent sodium diffused from the first transition metal layer  202  through the second transition metal layer  203 . In one embodiment, the sodium impurities may concentrate at the grain boundaries of CIS based alloy, and may have a concentration as high as 10 21  to 10 22  atoms/cm 3 . 
     An n-type semiconductor layer  302  may then be deposited over the p-type semiconductor absorber layer  301 . The n-type semiconductor layer  302  may comprise any suitable n-type semiconductor materials, for example, but not limited to ZnS, ZnSe or CdS. 
     A transparent top electrode  400 , is further deposited over the n-type semiconductor layer  302 . The transparent top electrode  400  may comprise multiple transparent conductive layers, for example, but not limited to, an Indium Tin Oxide (ITO) layer  402  located over an optional intrinsic Zinc Oxide or a resistive Aluminum Zinc Oxide (AZO, also referred to as RAZO) layer  401 . Of course, the transparent top electrode  400  may comprise any other suitable materials, for example, doped ZnO or SnO. 
     Optionally, one or more antireflection (AR) films (not shown) may be deposited over the transparent top electrode  400 , to optimize the light absorption in the cell, and/or current collection grid lines may be deposited over the top conducting oxide. 
     A solar cell described above may be fabricated by any suitable methods. In one embodiments, a method of manufacturing such a solar cell comprising providing the substrate  100 , depositing the first transition metal layer  202  comprising an alkali element or an alkali compound over the substrate  100 , depositing the second transition metal layer  203  comprising gallium over the first transition metal layer  202 , depositing the at least one p-type semiconductor absorber layer  301  containing a copper indium selenide (CIS) based alloy material over the second transition metal layer  203 , depositing the n-type semiconductor layer  302  over the p-type semiconductor absorber layer  301 , and depositing the top electrode  400  over the n-type semiconductor layer  302 . Optionally, an adhesion layer may be deposited over the substrate  100  followed by depositing the alkali diffusion barrier layer  201  prior to depositing layer  202 . Preferably, the second transition metal layer  203  permits alkali diffusion from the first transition metal layer  202  into the p-type semiconductor absorber layer  301  during at least one of the steps of depositing the p-type semiconductor absorber layer  301 , depositing the n-type semiconductor layer  302 , or depositing the top electrode  400 . 
     Any desirable method, for example but not limited to MBE, CVD, evaporation, plating, etc., may be used for depositing the above described layers. For example, the layers may be deposited over the substrate by sputtering. In some embodiments, one or more sputtering steps may be reactive sputtering. 
     In a non-limiting example, a sputtering apparatus illustrated in  FIG. 2A  may be used for depositing the first transition metal layer (not shown in  FIG. 2A , and referred to as layer  202  in  FIG. 1 ) over a substrate  100 . Targets comprising an alkali-containing material (e.g., targets  37   a   1  and  37   a   2 ) and targets comprising a transition metal (e.g.,  27   a   1  and  27   a   2 ) are located in a sputtering process module  22   a , such as a vacuum chamber. In a non-limiting example, the transition metal targets  27   a   1  and  27   a   2  are rotating Mo cylinders and are powered by DC power sources  7 , and the alkali-containing targets  37   a   1  and  37   a   2  are planar NaF targets and are powered by RF generators through matching networks. The target types alternate and end with a transition metal target, for example target  27   a   2  as shown in  FIG. 2A . 
     In some embodiments, the step of depositing the first transition metal layer  202  may be conducted in an oxygen and/or nitrogen rich environment, and may comprise DC sputtering the transition metal from the first target and pulsed DC sputtering, AC sputtering, or RF sputtering the alkali compound from the second target. Any suitable variations of the sputtering methods may be used. For example, for electrically insulating second target materials, AC sputtering refers to any variation of AC sputtering methods that may be used to for insulating target sputtering, such as medium frequency AC sputtering or AC pairs sputtering. In one embodiment, the step of depositing the first transition metal layer may comprise DC sputtering a first target comprising a transition metal, such as molybdenum, and pulsed DC sputtering, AC sputtering, or RF sputtering a second target comprising alkali-containing material, such as a sodium-containing material, in an oxygen rich sputtering environment. The sodium-containing material may comprise any material containing sodium, for example alloys or compounds of sodium with one or more of selenium, sulfur, oxygen, nitrogen or barrier metal (such as molybdenum, tungsten, tantalum, vanadium, titanium, niobium or zirconium), such as sodium fluoride, sodium molybdate, sodium fluoride, sodium selenide, sodium hydroxide, sodium oxide, sodium sulfate, sodium tungstate, sodium selenate, sodium selenite, sodium sulfide, sodium sulfite, sodium titanate, sodium metavanadate, sodium orthovanadate, or combinations thereof. Alloys or compounds of lithium and/or potassium may be also used, for example but not limited to alloys or compounds of lithium or potassium with one or more of selenium, sulfur, oxygen, nitrogen, molybdenum, tungsten, tantalum, vanadium, titanium, niobium or zirconium. The transition metal target may comprise a pure metal target, a metal alloy target, a metal oxide target (such as a molybdenum oxide target), etc. If desired, a single sodium containing molybdenum target may be used instead of separate molybdenum and sodium containing targets. The single sodium containing molybdenum target may comprise 0.01 to 5 atomic percent sodium, optionally 5 to 40 atomic percent oxygen, and the rest (e.g., at least 59 atomic percent) molybdenum. 
     The substrate  100  may be a foil web, for example, a metal web substrate, a polymer web substrate, or a polymer coated metal web substrate, and may be continuously passing through the sputtering module  22   a  during the sputtering process, following the direction of the imaginary arrow along the web  100 . Any suitable materials may be used for the foil web. For example, metal (e.g., stainless steel, aluminum, or titanium) or thermally stable polymers (e.g., polyimide or the like) may be used. The foil web  100  may move at a constant or variable rate to enhance intermixing. 
     The second transition metal layer  203  comprising gallium may then be deposited over the first transition metal layer  202 . The transition metal of the second transition metal layer  203  may be same or different from the transition metal of the first transition metal layer  202 . Similarly, any suitable transition metal, for example but not limited to Mo, W, Ta, V, Ti, Nb, and Zr, may be used as the transition metal of the second transition metal layer  203 . For example, in some embodiments, the second transition metal layer  203  comprises molybdenum containing gallium. For example, in a non-limiting example, the second transition metal layer comprises 50 to 90 atomic percent molybdenum and 10 to 50 atomic percent gallium. 
     In some other embodiments, the second transition metal layer  203  further comprises at least one of copper, indium, aluminum, or combinations thereof. For example, in a non-limiting example, the second transition metal layer may be a transition metal layer containing molybdenum, gallium and copper, for example a transition metal layer containing 50 to 90 atomic percent molybdenum, 10 to 50 atomic percent gallium and 0 to 10 atomic percent copper, indium and/or aluminum. 
     The step of depositing the second transition metal layer  203  may comprise sputtering the second transition metal layer  203  from a target comprising a molybdenum gallium copper alloy, a molybdenum gallium indium alloy, a molybdenum gallium aluminum alloy or a molybdenum gallium alloy having about the same composition ranges as those described for layer  203  above. Alternatively, the step of depositing the second transition metal layer  203  may comprise sputtering the second transition metal layer  203  from one or more pairs of targets which include a first target  27   b   1  comprising molybdenum and a second target  47   b   1  comprising a gallium alloy (e.g., copper gallium, aluminum gallium or copper aluminum gallium) or a gallium target (e.g., a gallium reservoir in which gallium is liquid at the sputtering temperature). The first target comprising molybdenum and the second target comprising a gallium alloy, such as copper gallium, may be located in the same vacuum chamber  22   b  of a magnetron sputtering system, as shown in  FIG. 2B , similar to that used for depositing the first transition metal layer  202  shown in  FIG. 2A . The distance between the adjacent targets is small enough such that a sufficient overlap  9  may exist between the alternating molybdenum containing fluxes and copper gallium alloy containing fluxes and thus enhance the intermixing of the molybdenum and the copper gallium material during depositing the second transition metal layer  203  containing gallium and copper. If desired, there may be several gallium containing targets (e.g., targets  47   b   1  and  47   b   2 ) and several transition metal targets, such as molybdenum targets (e.g.,  27   b   1  and  27   b   2 ) located in a sputtering process module  22   b , such as a vacuum chamber. In a non-limiting example, the transition metal targets  27   b   1  and  27   b   2  are rotating Mo cylinders that are powered by DC power sources, and the copper gallium alloy targets  47   b   1  and  47   b   2  are either rotating cylinders that are powered by DC power sources or planar targets that are powered by RF generators through matching networks. The copper gallium target&#39;s copper content should be equal or greater than half the atomic content of gallium (e.g., a Cu:Ga atomic ratio of at least 1:2, such as 1:2 to 2:1) to achieve a desired Cu:Ga ratio in the sputtered layer  203  and to preferably maintain the alloy target in the solid rather than liquid state at the sputtering temperature. Preferably, the target  47   b   1 ,  47   b   2  contains at least 33 atomic percent copper such that the target remains solid at 300° C. or above. 
     Preferably, the gallium content of the second transition metal layer  203  may diffuse from the second transition metal layer  203  into the p-type semiconductor absorber layer  301  during at least one of the steps of depositing the at least one p-type semiconductor absorber layer  301 , depositing the n-type semiconductor layer  302 , depositing the top electrode  400 , and an optional post-deposition annealing process. Similarly, the alkali dopant (e.g., sodium dopant) of the first transition metal layer  202  may diffuse from the first transition metal layer  202  into the p-type semiconductor absorber layer  301  through the second transition metal layer  203  during at least one of the steps of depositing the at least one p-type semiconductor absorber layer  301 , depositing the n-type semiconductor layer  302 , depositing the top electrode  400 , and an optional post-deposition annealing process, such that the p-type semiconductor absorber layer  301  comprises copper indium gallium selenide containing 0.03 to 1.5 atomic percent alkali dopant (e.g., sodium dopant) diffused from the first transition metal layer  202 . The amount of sodium diffused into the at least one p-type semiconductor absorber layer  301  may be tuned by controlling the thickness and/or density of the second transition metal layer  203 , which in turn may be tuned by controlling the sputtering rate and/or sputtering parameters such as sputtering power and pressure in the sputtering chamber. 
     Optionally, an alkali diffusion barrier layer  201  may be deposited over the substrate  100  prior to the step of depositing the first transition metal layer  202 . The alkali diffusion barrier layer  201  comprises at least one of Mo, W, Ta, V, Ti, Nb, or Zr, and may have a thickness of around 100 to 400 nm. In some embodiments, the step of sputtering the alkali diffusion barrier layer  201  occurs at a lower pressure than the step of sputtering the second transition metal layer  203 . The alkali diffusion barrier layer  201  substantially prevents alkali dopants diffusion from the first transition metal layer  202  into the substrate  100 . 
     In some embodiments, the steps of depositing the alkali diffusion barrier layer  201 , depositing the first transition metal layer  202  and depositing the second transition metal layer  203  comprises sputtering the alkali diffusion barrier layer  201 , sputtering the first transition metal layer  202 , and sputtering the second transition metal layer  203  in the same sputtering apparatus. 
     More preferably, the steps of depositing the alkali diffusion barrier layer  201 , depositing the first transition metal layer  202  and depositing the second transition metal layer  203 , depositing the at least one p-type semiconductor absorber layer  301 , depositing the n-type semiconductor layer  302 , and depositing the top electrode  400  comprise sputtering the alkali diffusion barrier layer  201 , the first transition metal layer  202 , the second transition metal layer  203 , the p-type absorber layer  301 , the n-type semiconductor layer  302  and one or more conductive films of the top electrode  400  over the substrate  100  (preferably a web substrate in this embodiment) in corresponding process modules of a plurality of independently isolated, connected process modules without breaking vacuum, while passing the web substrate  100  from an input module to an output module through the plurality of independently isolated, connected process modules such that the web substrate continuously extends from the input module to the output module while passing through the plurality of the independently isolated, connected process modules. Each of the process modules may include one or more sputtering targets for sputtering material over the web substrate  100 . 
     For example, a modular sputtering apparatus for making the solar cell, as illustrated in  FIG. 3  (top view), may be used for depositing the layers. The apparatus is equipped with an input, or load, module  21   a  and a symmetrical output, or unload, module  21   b . Between the input and output modules are process modules  22  (e.g.,  22   a ,  22   b ,  22   c  and  22   d , etc). The number of process modules  22  may be varied to match the requirements of the device that is being produced. Each module has a pumping device  23 , such as vacuum pump, for example a high throughput turbomolecular pump, to provide the required vacuum and to handle the flow of process gases during the sputtering operation. Each module may have a number of pumps placed at other locations selected to provide optimum pumping of process gases. The modules are connected together at slit valves  24 , which contain very narrow low conductance isolation slots to prevent process gases from mixing between modules. These slots may be separately pumped if required to increase the isolation even further. Other module connectors  24  may also be used. Alternatively, a single large chamber may be internally segregated to effectively provide the module regions, if desired. U.S. Published Application No. 2005/0109392 A1 (“Hollars”), filed on Oct. 25, 2004, discloses a vacuum sputtering apparatus having connected modules, and is incorporated herein by reference in its entirety. 
     The web substrate  100  is moved throughout the machine by rollers  28 , or other devices. Additional guide rollers may be used. Rollers shown in  FIG. 3  are schematic and non-limiting examples. Some rollers may be bowed to spread the web, some may move to provide web steering, some may provide web tension feedback to servo controllers, and others may be mere idlers to run the web in desired positions. The input spool  31   a  and optional output spool  31   b  thus are actively driven and controlled by feedback signals to keep the web in constant tension throughout the machine. In addition, the input and output modules may each contain a web splicing region or device  29  where the web  100  can be cut and spliced to a leader or trailer section to facilitate loading and unloading of the roll. In some embodiments, the web  100 , instead of being rolled up onto output spool  31   b , may be sliced into solar modules by the web splicing device  29  in the output module  21   b . In these embodiments, the output spool  31   b  may be omitted. As a non-limiting example, some of the devices/steps may be omitted or replaced by any other suitable devices/steps. For example, bowed rollers and/or steering rollers may be omitted in some embodiments. 
     Heater arrays  30  are placed in locations where necessary to provide web heating depending upon process requirements. These heaters  30  may be a matrix of high temperature quartz lamps laid out across the width of the web. Infrared sensors provide a feedback signal to servo the lamp power and provide uniform heating across the web. In one embodiment, as shown in  FIG. 3 , the heaters are placed on one side of the web  100 , and sputtering targets are placed on the other side of the web  100 . Sputtering targets  27 ,  37  and  47  may be mounted on dual cylindrical rotary magnetron(s), or planar magnetron(s) sputtering sources, or RF sputtering sources. 
     After being pre-cleaned, the web substrate  100  may first pass by heater array  30   f  in module  21   a , which provides at least enough heat to remove surface adsorbed water. Subsequently, the web can pass over roller  32 , which can be a special roller configured as a cylindrical rotary magnetron. This allows the surface of electrically conducting (metallic) webs to be continuously cleaned by DC, AC, or RF sputtering as it passes around the roller/magnetron. The sputtered web material is caught on shield  33 , which is periodically changed. Preferably, another roller/magnetron may be added (not shown) to clean the back surface of the web  100 . Direct sputter cleaning of a web  100  will cause the same electrical bias to be present on the web throughout the machine, which, depending on the particular process involved, might be undesirable in other sections of the machine. The biasing can be avoided by sputter cleaning with linear ion guns instead of magnetrons, or the cleaning could be accomplished in a separate smaller machine prior to loading into this large roll coater. Also, a corona glow discharge treatment could be performed at this position without introducing an electrical bias. 
     Next, the web  100  passes into the process module  22   a  through valve  24 . Following the direction of the imaginary arrows along the web  100 , the full stack of layers may be deposited in one continuous process. The first transition metal layer  202  is then sputtered in the process module  22   a  over the web  100 , as illustrated in  FIG. 3  (and previously in  FIG. 2A ). Optionally, the process module  22   a  may include more than two pairs of targets, each pair of targets comprising a transition metal target  27   a  and an alkali-containing target  37   a , arranged in such a way that the types of targets alternate and the series of targets end with a transition metal target  27   a . The alkali-containing target  37   a  has a composition different from that of the transition metal target  27   a.    
     The web  100  then passes into the process module  22   b  through valve  24 . The second transition metal layer  203  may be sputtered in the process module  22   b  over the web  100 . As illustrated in  FIGS. 2B and 3 , two pairs of targets are used for sputtering the second transition metal layer  203 . Each pair of targets comprising a transition metal target  27   b  (e.g., molybdenum target) and a gallium-containing (e.g., gallium copper alloy) target  47   b . The gallium containing target  47   b  has a composition different from that of the transition metal target  27   b . Alternatively, the process module  22   b  may include only one target (e.g., a molybdenum gallium alloy target or a molybdenum copper gallium alloy target), one pair of transition metal targets  27   b  and a gallium-containing target  47   b , or more than two pairs of transition metal target  27   b  and an gallium-containing target  47   b.    
     The web  100  then passes into the next process module,  22   c , for deposition of the at least one p-type semiconductor absorber layer  301 . In a preferred embodiment shown in  FIG. 3 , the step of depositing the at least one p-type semiconductor absorber layer  301  includes reactively alternating current (AC) magnetron sputtering the semiconductor absorber layer from at least one pair of two conductive targets  27   c   1  and  27   c   2 , in a sputtering atmosphere that comprises argon gas and a selenium-containing gas. In some embodiment, the pair of two conductive targets  27   c   1  and  27   c   2  comprise the same targets. For example, each of the at least two conductive targets  27   c   1  and  27   c   2  comprises copper, indium and gallium, or comprises copper, indium and aluminum. The selenium-containing gas may be hydrogen selenide or selenium vapor. In other embodiments, targets  27   c   1  and  27   c   2  may comprise different materials from each other. The radiation heaters  30  maintain the web at the required process temperature, for example, around 400-800° C., for example around 500-600° C., which is preferable for the CIS based alloy deposition. 
     In some embodiments, at least one p-type semiconductor absorber layer  301  may comprise graded CIS based material. In this embodiment, the process module  22   c  further comprises at least two more pairs of targets ( 227 , and  327 ), as illustrated in  FIG. 4 . The first magnetron pair  127  ( 27   c   1  and  27   c   2 ) are used to sputter a layer of copper indium diselenide while the next two pairs  227 ,  327  of magnetrons targets ( 27   c   3 ,  27   c   4  and  27   c   5 ,  27   c   6 ) sputter deposit layers with increasing amounts of gallium (or aluminum), thus increasing and grading the band gap. The total number of targets pairs may be varied, for example may be 2-10 pairs, such as 3-5 pairs. This will grade the band gap from about 1 eV at the bottom to about 1.3 eV near the top of the layer. Details of depositing the graded CIS material is described in the Hollars published application, which is incorporated herein by reference in its entirety. 
     Optionally, the alkali diffusion barrier layers  201  may be sputtered over the substrate  100  in a process module added between the process modules  21   a  and  22   a . The second transition metal layer  203  may be sputtered over the first transition metal layer  202  in a process module added between the process modules  22   a  and  22   b . Further, one or more process modules (not shown) may be added to deposit additional barrier layers and/or adhesion layer to the stack, if desired. 
     In some embodiments, one or more process modules (not shown) may be further added between the process modules  21   a  and  22   a  to sputter a back side protective layer over the back side of the substrate  100  before the first transition metal layer  202  is deposited on the front side of the substrate. U.S. application Ser. No. 12/379,428 (Attorney Docket No. 075122/0139) titled “Protective Layer for large-scale production of thin-film solar cells” and filed on Feb. 20, 2009, which is hereby incorporated by reference, describes such deposition process. 
     The web  100  may then pass into the process modules  22   d  and process modules (not shown) between  22   d  and  21   b , for depositing the n-type semiconductor layer  302 , and the transparent top electrode  400 , respectively. Any suitable type of sputtering sources may be used, for example, rotating AC magnetrons, RF magnetrons, or planar magnetrons. Extra magnetron stations (not shown), or extra process modules (not shown) could be added for sputtering the optional one or more AR layers. Finally, the web  100  may be passed into output module  21   b , where it is either wound onto the take up spool  31   b , or sliced into solar cells using cutting apparatus  29 . 
     It is to be understood that the present invention is not limited to the embodiment(s) and the example(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims. For example, as is apparent from the claims and specification, not all method steps need be performed in the exact order illustrated or claimed, but rather in any order that allows the proper formation of the solar cells of the present invention.