Abstract:
A thin film solar cell including a Group IBIIIAVIA absorber layer on a defect free base including a stainless steel substrate is provided. The stainless steel substrate of the base is surface treated to remove the surface roughness such as protrusions that cause shunts. Before removing the protrusions, a thin protective ruthenium film is first deposited on the recessed surface portions of the substrate to protect these portions during the following protrusion removal. The protrusions on the surface receives very little or no ruthenium during the deposition. After the ruthenium film is formed, the protrusions are etched and removed by an etchant which only attacks the stainless steel but neutral to the ruthenium film. A contact layer is formed over the ruthenium layer and the exposed portions of the substrate to complete the base.

Description:
This application relates to and claims priority from U.S. Provisional Application No. 60/973,443, entitled “Substrate Planarization for Thin Film Solar Cell Manufacturing,” filed Sep. 18, 2007, which application is expressly incorporated by reference herein. This application is also related to U.S. application Ser. No. 12/233,566 entitled “Substrate Preparation for Thin Film Solar Cell Manufacturing,” filed Sep. 18, 2008. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present inventions relate to thin film solar cell fabrication methods and structures. 
     2. Description of the Related Art 
     Solar cells are photovoltaic (PV) 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 the 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 chalcopyrite 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. 
     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 a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film  12 , which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te) 2 , is grown over a 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) or a grid pattern may also be deposited over the transparent layer  14  to reduce the effective series resistance of the device. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in  FIG. 1 . It should be noted that although the chemical formula for a CIGS(S) layer 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. 
     One way of reducing the cost of thin film photovoltaics is to process thin film CIGS(S) type solar cells on flexible metallic foils so that the depositions of multiple films or layers constituting the solar cell structure, such as the contact layer, the CIGS(S) absorber film, the transparent layer and the metallic grids, may all be performed over the flexible foil substrate in a roll-to-roll fashion. This way a long (such as 500-5000 meters long) foil substrate may be processed in relatively compact process tools to form a roll of solar cells, which may then be cut and used for module fabrication. Choice of the substrate material is very important for thin film solar cells since the layers in these device structures are only 1-5 micrometers (μm) thick and they get affected by the nature of the substrate during and after processing. For example, the typical thicknesses of the contact layers, the CIGS absorber layers and transparent layers are 0.3-1μ, 1-3μ and 0.1-0.5μ, respectively. 
     Stainless steel foils are widely used flexible metallic substrates to manufacture CIGS solar cells. However, one problem associated with the use of stainless steel substrates is the surface roughness of the foil and the defects associated with the surface roughness. Topographical defects such as protrusions, gouges and spikes often appear on stainless steel foils and they can originate from different sources such as foreign particle (such as metal particle or ceramic particle) inclusions in the steel foil, defects in the steel and rolling defects and particles which may be introduced on the steel foil surface during the rolling process for foil formation. Depending on their size, such defects not only reduce the efficiency of the solar cells manufactured on such foil surfaces but may also cause complete electrical shunts which are detrimental to solar cell performance. 
       FIG. 2  illustrates a solar cell device structure  50  built on a defective area of a stainless steel foil substrate  52  to exemplify a plausible mechanism that forms shunts within the solar cell device structure. The surface of stainless steel foil substrate includes exemplary protrusions  54  or spikes which may have a height in the range of 100-2000 nm, or even higher. A contact layer or conductive layer  56  is typically a 200-1000 nm thick film of a refractory metal such as Mo, typically deposited by a physical vapor deposition technique such as sputtering. Therefore, when deposited over the rough surface of the substrate  52 , the contact layer  56  may have discontinuities or defective portions where the underlying substrate is exposed, especially over the locations of the protrusions  54 . When an absorber film  58  is formed over the contact layer  56 , the substrate  52  would be exposed to the absorber film  58  and to the reactive atmosphere that is present during the growth of the absorber film  58  at the locations of the discontinuities. As a result, chemical interaction may take place between the substrate  52  and the constituents of the absorber film  58  at these locations. For example, for stainless steel substrates, iron in the stainless steel may react with the Se in the CIGS film growth environment forming iron selenides in the defect regions. Also iron atoms diffusing from the substrate  52  into the absorber  58  through the discontinuities in the contact layer  56  may poison the regions of the absorber  58  directly above the discontinuities. When the solar cells are completed by deposition of the transparent layer  60 , these poisoned regions cause excessive current leakage or shunts reducing the efficiency of the devices. It should be noted that excessive iron in CIGS material lowers its resistivity and degrades its photovoltaic performance. In an alternate mechanism, the regions where the contact layer  56  is discontinuous may not be coated effectively by the absorber film  58 , giving rise to pinholes  62  that may cause shunts, which are killer defects for solar cells. As can be seen in  FIG. 2 , the transparent layer  60  is in direct contact with the substrate  52 , through the protrusions  54  at the locations of the pinholes  62 . 
     Various surface planarization methods, including electropolishing, do not effectively reduce the surface roughness of such stainless steel foils because etchants also etch the surface portions around the peaks and deepen such portions. As a result of this, the spikes or the peaks tend to stay and in most cases new ones are created as more defects such as inclusions hidden in the foil body get exposed through the etched surface. 
     From the foregoing, the much needed manufacturing process yield improvements require identification and development of metallic foil substrates and the processes that are better suited for large volume roll-to-roll processing and manufacturing of CIGS solar cell and modules. 
     SUMMARY 
     The present inventions relate to thin film CIGS solar cell structures. 
     In one aspect, embodiments of the present invention provides a specific flexible metallic foil substrate that improves device performance and manufacturing process yield. 
     These and other aspects and advantages, among others, are described further hereinbelow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a prior art solar cell structure; 
         FIG. 2  is schematic illustration of a prior art solar cell structure which has been built on a substrate having surface roughness; 
         FIG. 3A  is a schematic illustration of a structure with a substrate having surface roughness including protrusion and recesses, wherein an etch resistive film has been formed over the recesses; 
         FIG. 3B  is a schematic illustration of the structure shown in  FIG. 3A , wherein the protrusions have been removed from the surface of the substrate using an material removal process; 
         FIG. 3C  is a schematic illustration of the structure shown in  FIG. 3B , wherein a contact layer has been deposited over the etch resistive film and the exposed surface portions of the substrate formed during the material removal process; and 
         FIG. 3D  is a schematic illustration of the structure shown in  FIG. 3C , wherein a CIGS absorber layer and a transparent layer have been formed over the contact layer to form a solar cell structure. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments of the present invention provide a flexible foil base that enhances the efficiency and manufacturing yield of flexible II-VI compound semiconductor solar cells, i.e. Group IIBVIA thin film solar cells such as CdTe solar cells and Group IBIIIAVIA thin film solar cells such as CIGS(S) type solar cells. The preferred solar cell structure is a Group IBIIIAVIA thin film solar cell which will be described hereinafter. 
     The base of the thin film solar cell includes a substrate that is surface treated using a process that employs a conductive and permanent protective film to protect the surface of the metallic substrate while the protrusions on the surface removed by a material removal process. Although the embodiments of the invention will be described using a CIGS solar cell as an example, it will be appreciated that the embodiments are applicable to any flexible thin film solar cell employing a Group IIBVIA (such as CdTe) or a Group IBIIIAVIA compound semiconductor absorber film. In a preferred embodiment of the present invention, a flexible foil base may comprise a stainless steel substrate, a protective film formed on the stainless steel substrate, and a defect free continuous back contact layer formed over the protective film. 
     The protective thin film covers and protects the surface portions or recessed portions around the peaks or spikes from an etchant employed during a material removal process. The protrusions are not completely covered with the protective layer and hence are exposed to the etchant which removes them from the surface of the substrate. The protective film is conductive and it is made of a metal or alloy that cannot be etched by or substantially resistant to the etchant used during the removal of the peaks. In the context herein, an etchant may be any wet or dry etching chemical species. The base of the removed protrusions exposes the substrate material through openings within the protective film and thus the surface of the protective film is discontinuous and it partially includes openings through which the substrate portions are exposed. The surface of the protective film and the exposed surface portions of the substrate may be substantially coplanar. The back contact layer is deposited on the protective film and the exposed substrate portions in a substantially planar manner. As opposed to the above described prior art back contact films, the back contact film is a continuous film having no discontinuities or defects and strongly adheres to the protective film as well as the exposed substrate portions. In the following steps, an absorber layer and a transparent layer is formed on the base. 
     Reference will now be made to the drawings wherein like numerals refer to like parts throughout.  FIG. 3A  shows an exemplary substrate  100  such as a steel substrate having a front surface  102  with substantially flat surface portions  104  (also referred to as recessed surface portions) and protrusions  106  or elevated surface portions which will be removed. The protrusions  106  are defective surface regions having mostly peaks or spikes of steel extending outwardly from the front surface. The height of protrusions  106  may be in the range of 100 nm to 5000 nm or even more. In this embodiment, the substrate  100  is stainless steel; however, the same principles may be applied to other metal or alloy foil substrates, such as aluminum foil substrates. A protective film  108  having a top surface  110  is formed over the surface  102 , preferably using a sputter deposition process. The protective film  108  generally covers the recessed portions  104  of the front surface  102 . The protrusions  106  receive little or no protective film material due to their geometry; therefore the protrusions  106  are not coated with the protective film  108  in a continuous manner. Once the protective film  108  coats the recessed portions  104  of the front surface  102 , the protrusions  106  may be seen as extending through the discontinuous regions  109  or openings in the protective film  108 . The protective film  108  is preferably a thin conductive film that cannot be etched by the etchant material of the subsequent planarization step. In the preferred embodiment the protective film  108  comprises one of ruthenium (Ru), iridium (Ir), osmium (Os), tantalum (Ta) and hafnium (Hf). The thickness of the protective film  108  may be in the range of 100-1000 angstrom (Å). It should be noted that some of the preferred materials listed above for the protective film  108  are themselves relatively inert (such as Ru, Ir, Os) or they comprise naturally occurring native oxide layers on their surfaces that render them inert. For example, Ta and Hf films are known to develop a thin surface oxide film once exposed to air. Other materials that develop such oxide layers on their surfaces are also good candidates to be used as the material of the protective film  108 . 
     In the following step, a material removal process employing an etchant is applied to selectively remove the protrusions  106  extending through the openings  109  in the protective film  108 . The etchant material selectively attacks the exposed protrusions  106  but not the protective film  108  and the recessed portions covered by the protective film. Therefore, during the material removal process, the protective film  108  protects the underlying recessed portions  104  of the front surface  102  of the substrate  100 . For example, if the protective film is made of Ru an HCI solution may be used as an etchant for this purpose. The same may be achieved using electropolishing with acidic solutions that can remove the steel but not the protective film  108  covering the recessed portions  104 . The etching step may also be achieved through use of vapors in a dry etch method or through a plasma etching or ion beam milling step. As shown in  FIG. 3B , selective removal of the protrusions  106  forms substrate sections  112  surrounded by the protective film  108 . The substrate sections  112  are portions of the substrate that is exposed through the openings  109  in the protective film  108  after the removal of the protrusions  106 . Surfaces  113  of the substrate sections  112  are preferably substantially coplanar with the top surface  110  of the protective film  108 . Such co-planarity may be achieved by careful timing of the etching step. If the material removal process is a planarization process (such as electropolishing) the surfaces  113  of the substrate sections  112  are substantially planar. 
     As shown in  FIG. 3C , a contact layer  116  is formed on the top surface  110  of the protective layer  108  and the surfaces  113  of the substrate sections  112 . The contact layer  116  is a continuous layer and strongly adheres to the substrate sections  112  and the protective layer  108 . 
     As shown in  FIG. 3D  in the following step, a Group IBIIIAVIA semiconductor absorber layer  118  such as a CIGS layer is formed on the contact layer  116 . The solar cell structure is completed by deposition of a transparent layer  120  including a buffer layer (not shown) and a transparent conductive layer (not shown) over the CIGS absorber layer  118 . The contact layer  116  may be a multi-layer structure itself comprising materials that provide; i) good diffusion barrier action against iron (Fe) diffusion into the CIGS absorber layer, ii) good ohmic contact to the CIGS absorber layer, and, iii) good adhesion to the flexible substrate  100 . Such materials include, but are not limited to tungsten (W), tantalum (Ta), molybdenum (Mo), titanium (Ti), chromium (Cr), ruthenium (Ru) and iridium (Ir), osmium (Os) their alloys and/or nitrides. 
     The CIGS absorber layer  118  may be deposited on the contact layer  116  using various techniques well known in the field. These techniques include evaporation, sputtering, ink deposition, electroplating, two-stage techniques, etc. In electroplating method, first a precursor film including at least copper, indium and gallium is electroplated. The precursor film is then reacted at about 400-600° C. in presence of selenium and/or sulfur (which may be included in the precursor film or may be provided from the reaction environment) to form the CIGS absorber layer. The buffer layer is often a sulfide compound such as cadmium sulfide and indium sulfide. The transparent conductive layer may be a transparent conductive oxide (TCO) such as zinc oxide (ZnO), indium tin oxide (ITO), and indium zinc oxide (IZO). The transparent conductive layer may also be a stacked layer of the TCOs listed above. There may be a grid pattern or finger pattern (not shown) formed over the transparent layer  120 . 
     In roll-to-roll manufacturing of CIGS solar cells, all of the above described process steps may be performed in a roll-to-roll tool fashion. In a roll to roll process, the substrate may be a continuous workpiece that may be supplied from a supply roll. After some of or all of the above described process steps, e.g., forming of the protective layer, removal of the protrusions, forming of the contact layer and the following steps, the processed workpiece is taken up and wrapped around a receiving roll. The absorber layer  118 , the transparent layer  120 , and the finger patterns may also be deposited in a roll-to-roll tool fashion. Although the present inventions are described with respect to certain preferred embodiments, modifications thereto will be apparent to those skilled in the art.