Abstract:
The present invention provides for new ohmic contact materials and diffusion barriers for Group IBIIIAVIA based solar cell structures, which eliminate two way diffusion while preserving the efficient ohmic contacts between the substrate and the absorber layers.

Description:
BACKGROUND 
       [0001]    1. Field of the Inventions 
         [0002]    The present inventions generally relate to apparatus and methods of solar cell design and fabrication and, more particularly, to forming contact layers and diffusion barriers for such solar cells. 
         [0003]    2. Description of the Related Art 
         [0004]    Solar cells are photovoltaic devices that convert sunlight directly into electrical energy. Solar cells can be based on crystalline silicon or thin films of various semiconductor materials, usually deposited on low-cost substrates, such as glass, plastic, or stainless steel. 
         [0005]    Thin film based photovoltaic cells, such as amorphous silicon, cadmium telluride, copper indium diselenide or copper indium gallium diselenide based solar cells, offer improved cost by employing deposition techniques widely used in the thin film industry. Group IBIIIAVIA compound photovoltaic cells including copper indium gallium diselenide (CIGS) based solar cells have demonstrated the greatest potential for high performance, high efficiency, and low cost thin film PV products. 
         [0006]    A conventional Group IBIIIAVIA compound solar cell can be built on a substrate that can be a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. A contact layer such as a molybdenum (Mo) film is deposited on the substrate as the back electrode of the solar cell. An absorber thin film including a material in the family of Cu(In,Ga)(S,Se) 2 , is formed on the conductive Mo film. Although there are other methods, Cu(In,Ga)(S,Se) 2  type compound thin films are typically formed by a two-stage process where the components (components being Cu, In, Ga, Se and S) of the Cu(In,Ga)(S,Se) 2  material are first deposited onto the substrate or the contact layer formed on the substrate as a n absorber precursor, and then reacted with S and/or Se in a high temperature annealing process. 
         [0007]    After the absorber film is formed, a transparent layer, for example, a CdS film, a ZnO film or a CdS/ZnO film-stack is formed on the absorber film. The preferred electrical type of the absorber film is p-type, and the preferred electrical type of the transparent layer is n-type. However, an n-type absorber and a p-type window layer can also be formed. The above described conventional device structure is called substrate-type structure. In the substrate structure light enters the device from the transparent layer side. A so called superstrate-type structure can also be formed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga)(S,Se) 2  absorber film, and finally forming an ohmic contact to the device by a conductive layer. In the superstrate structure light enters the device from the transparent superstrate side. 
         [0008]    In standard CIGS as well as Si and amorphous Si module technologies, the solar cells can be manufactured on flexible conductive substrates such as stainless steel foil substrates. Due to its flexibility, a stainless steel substrate allows low cost roll-to-roll solar cell manufacturing techniques. In such solar cells built on conductive substrates, the transparent layer and the conductive substrate form the opposite poles of the solar cells. Multiple solar cells can be electrically interconnected by stringing or shingling methods that establish electrical connection between the opposite poles of the solar cells. Such interconnected solar cells are then packaged in protective packages to form solar modules or panels. Many modules can also be combined to form large solar panels. The solar modules are constructed using various packaging materials to mechanically support and protect the solar cells in them against mechanical damage. Each module typically includes multiple solar cells which are electrically connected to one another using above mentioned stringing or shingling interconnection methods. 
         [0009]    The conversion efficiency of a thin film solar cell depends on many fundamental factors, such as the bandgap value and electronic and optical quality of the absorber layer, the quality of the window layer, the quality of the rectifying junction, and so on. Since the total thickness of the electrically active layers of the CIGS thin film solar cells is in the range of 0.5-5 micrometers, these devices are highly sensitive to defects. Even the sub-micron size defects may influence their illuminated I-V characteristics. A prior art problem associated with manufacturing CIGS thin film devices on stainless steel substrates, however, is the inadvertent introduction of defects into the device structure during the reaction of the absorber precursor. During this step, the constituents of the stainless steel substrate diffuse towards the absorber layer, and Se from the absorber precursor diffuses towards the stainless steel substrate. Despite the advantages of flexible stainless steel substrates in CIGS solar cell applications, during the high temperature anneal, Se from the absorber precursor can diffuse through the Mo contact layer toward the substrate, corrode the stainless steel and form FeSe compounds on the substrate, which cause electrical shunting defects. Such shunting defects introduce a shunting path, between the substrate and the absorber or the transparent layer through which the electrical current of the device may leak. The shunting defects lower the fill factor, the voltage and the conversion efficiency of the solar cells. Furthermore, the atoms of the stainless steel such as iron (Fe) atoms can diffuse from the substrate into the CIGS absorber by diffusing through the Mo contact layer during the aforementioned high temperature anneal or reaction process that forms the absorber. This unwanted material diffusion into the absorber significantly degrades the performance of the absorber and the resulting device. One prior art process uses a chromium (Cr) layer to prevent Fe diffusion into the absorber; however, Cr forms CrSe with Se from the absorber precursor during the reaction step, and just like FeSe, Cr causes shunting defects. 
       SUMMARY 
       [0010]    The present inventions provide for new ohmic contact materials and diffusion barriers for Group IBIIIAVIA based solar cell structures, which eliminate two way diffusion while preserving the efficient ohmic contacts between the substrate and the absorber layers. 
         [0011]    In one aspect is described a method of manufacturing a Cu(In,Ga) Se 2  thin-film solar cell, comprising: forming an intermediate layer on a surface of a flexible conductive substrate, the flexible conductive substrate including iron (Fe) species, the step of forming the intermediate layer comprising: forming a conductive barrier stack on the surface of the flexible conductive substrate, the conductive barrier stack including a chromium (Cr) film applied to the surface of the flexible conductive substrate and a titanium nitride (TiN) film deposited on the Cr film; depositing a contact layer on the conductive barrier stack; depositing a nucleation layer on the contact layer; depositing an absorber precursor including Cu species, In species, Ga species and Se species over the nucleation layer; applying heat to the absorber precursor to form therefrom an absorber layer including a Cu(In,Ga) Se 2  thin film compound, wherein the step of forming the absorber layer causes some of the Fe species in the flexible substrate to diffuse towards the absorber layer and some of the Se species in the absorber layer to diffuse towards the flexible conductive substrate, and wherein the conductive barrier stack inhibits diffusion of both the iron (Fe) species and the Se species across the conductive barrier stack during the step of applying heat; disposing a transparent conductive layer on the thin film absorber layer, the transparent layer including a buffer layer deposited on the thin film absorber and a transparent conductive oxide layer formed on the buffer layer; forming a top terminal on the transparent conductive layer, thereby resulting in the Cu(In,Ga) Se 2  thin-film solar cell; and wherein after the step of forming, the conductive barrier stack continues to inhibit diffusion of both the iron (Fe) species and the Se species across the conductive barrier stack. 
         [0012]    In another aspect is described a solar cell, comprising: a flexible conductive substrate including Fe species; a thin film absorber layer including Cu species, In species, Ga species and Se species formed over the flexible conductive substrate layer; an intermediate layer disposed between the flexible conductive substrate and the thin film absorber layer, the intermediate layer including: a conductive barrier stack on the surface of the flexible conductive substrate, the conductive barrier stack including a chromium (Cr) film applied to the surface of the flexible conductive substrate and a titanium nitride (TiN) film deposited on the Cr film, wherein the conductive barrier stack inhibits diffusion of both the iron (Fe) species from the flexible conductive substrate across the conductive barrier stack and the selenium Se species from the thin film absorber layer across the conductive barrier stack; a contact layer deposited on the conductive barrier stack; a nucleation layer deposited on the contact layer; a transparent layer deposited on the thin film absorber layer, the transparent layer including a buffer layer deposited on the thin film absorber and a transparent conductive oxide layer formed on the buffer layer; and a top terminal formed on the transparent layer. 
         [0013]    These and other aspects and advantages are described further herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1A  is a schematic side view of an embodiment of a solar cell base of the present invention; 
           [0015]      FIG. 1B  is a schematic side view of solar cell using the solar cell base shown in  FIG. 1A ; 
           [0016]      FIG. 1C  is schematic perspective view of the solar cell shown in  FIG. 1B ; and 
           [0017]      FIG. 2  is a schematic side view of another embodiment of a solar cell base of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The preferred embodiments described herein provide solar cell manufacturing methods and device structures to prevent unwanted material diffusion between a metallic substrate and a Group IBIIIAVIA thin film absorber of a solar cell during the manufacture of the solar cell. In one embodiment, an intermediate layer, including multiple conductive material films, is disposed between a stainless steel substrate and an absorber layer of a solar cell. The absorber layer may be a Cu(In,Ga)(Se,S) 2  or CIGSS compound thin film which is formed by annealing (reacting) an absorber precursor including Cu, In, Ga and Se, and optionally S at temperature range of about 400-600° C. in a reactor. Accordingly, during the reaction, the intermediate layer of the present invention inhibits or minimizes both unwanted diffusion mechanisms, namely, the iron (Fe) diffusion from the stainless-steel substrate to the absorber layer and the selenium (Se) diffusion from the absorber layer to the stainless steel. In one embodiment, a first intermediate layer includes: a diffusion barrier stack deposited on the stainless steel substrate; a contact layer, such as a Mo layer, deposited on the diffusion barrier stack; a nucleation layer, such as a Ru layer, deposited on the contact layer; and a seed layer, such as a Cu layer, deposited on the nucleation layer. The diffusion barrier stack preferably includes a metal barrier layer, such as a Cr layer, deposited on the stainless steel substrate and a metal nitride barrier layer, such as a TiN layer, deposited metal barrier layer. In another embodiment, a second intermediate layer preferably includes: a diffusion barrier stack including a Cr layer deposited on the stainless steel substrate and a first TiN layer deposited on the Cr layer; a first contact layer, such as a first Mo layer, deposited on the diffusion barrier stack; a second TiN layer deposited on the first contact layer; a second contact layer, such as a second Mo layer, deposited on the second TiN layer; a nucleation layer, such as a Ru layer, deposited on the second contact layer; and a seed layer, such as a Cu layer, deposited on the nucleation layer. 
         [0019]      FIG. 1A  shows a base  100  including a flexible substrate  102  having a front surface  104 A and a back surface  104 B, and an intermediate layer  105  disposed on the front surface of the flexible substrate  102 . The intermediate layer  105  preferably includes multiple conductive films. In one embodiment, the intermediate layer  105  includes: a diffusion barrier stack  103 , a contact layer  110 , a nucleation layer  112 , and a seed layer  114 . In one example, the diffusion barrier stack  103  preferably includes, but is not limited to, a metal barrier layer  106  deposited on the stainless steel substrate and a metal-nitride barrier layer  108  deposited on the metal barrier layer  106 . The metal barrier layer  106  may be a Cr layer or may be Cr alloy such as CrMo, or multiple layers of Cr and Cr alloys. The thickness of the metal barrier layer  106  may be in the range of 50-100 nm, preferably 80-100 nm, and more preferably 95-100 nm. The metal nitride barrier layer  108  may include titanium-nitride (TiN), tantalum nitride (TaN), or tungsten nitride (WN), and may be fully or partially made of TiN or a combination of the above nitrides. The thickness of the metal-nitride barrier layer  108  may be in the range of 200-400 nm, preferably 250-350 nm, and more preferably 290-300 nm. The metal barrier layer  106  and metal-nitride barrier layer  108  may be deposited using PVD processes such as reactive sputtering processes in a nitrogen-containing atmosphere. 
         [0020]    The contact layer  110  may be a Mo layer and deposited onto the metal-nitride barrier  108  of the barrier stack  105 . Alternatively, materials such as W, Ta and Ti may also be used as contact layer. The thickness of the contact layer may be in the range of 400-1000 nm, preferably 500-900 nm. 
         [0021]    The nucleation layer  112  may be deposited on the contact layer  110  and may include ruthenium (Ru) or a Ru alloy. The nucleation layer forms an additional diffusion barrier on the contact layer  110  and when formed on the contact layer or in replacement of the contact layer, the nucleation layer  112  increases the chemical inertness and strength of the contact layer  110 , especially when wet techniques such as electrodeposition and electroless deposition are used to form precursor stacks. The nucleation layer  112  also provides better nucleation capability and adhesion to the materials deposited on it. Ru layer may be deposited by techniques such as electroless deposition, electroplating, atomic layer deposition, CVD, MOCVD, and PVD among others. The thickness of the nucleation layer  112  may be in range of 1-300 nm, preferably 5-100 nm. Thin nucleation layers are preferred for cost reduction purposes. The seed layer  114  is a thin copper layer and preferably deposited on the nucleation layer  112 . When depositing an absorber precursor layer through a wet process such as electroplating or electroless plating, a seed layer may be used in place of or on top of the nucleation layer. For example, if a stack including Cu, In, and Ga films is electroplated or electroless plated on the seed layer, the seed layer acts as an adhesion/nucleation layer on which the electroplated metal bonds well. For example, a sputtered, CVD deposited, or ALD deposited Cu film of 2-100 nm thickness acts as an efficient seed layer upon which the precursors comprising at least one of Cu, In and Ga may be deposited with good adhesion and morphology. The seed layer also improves adhesion and uniformity of Cu(In,Ga)(Se) 2  layer or Cu(In,Ga)(S, Se) 2  formed by techniques other that electrodeposition. 
         [0022]    As shown in  FIG. 1B  in side view and in  1 C in perspective view, once the base layer  100  is completed, a front side  120  including an absorber layer, a transparent layer and a conductive grid is formed on the base to complete a solar cell  130 . The absorber layer  116  including a Group IBIIIAVIA compound, such as Cu(In,Ga)(Se) 2  or Cu(In,Ga)(S, Se) 2 , formed on the surface  115  of the seed layer  114 . The absorber layer is preferably formed using a two step process including first depositing a precursor layer having Cu, In, Ga and Se, and optionally S, on a surface  115  of the seed layer  114  (or the base  100 ), and second reacting the precursor layer in a reactor at a temperature range of 300-600° C. in an inert or Se gas and optionally S gas containing atmosphere. Cu, In, Ga and Se may be electroplated to form a precursor stack including one or more films of Cu, In, Ga and Se. Optionally, a stack including Cu, In and Ga films may be first formed by electroplating on the base and then one or more Se films may be vapor deposited on the previously formed stack that includes Cu, In and Ga films. In the next step, the transparent layer  118 , which may include a buffer-layer/TCO (transparent conductive oxide) stack, is formed on the absorber layer  116 . An exemplary buffer material may be a (Cd, Zn)S which is generally electroless deposited on the absorber layer. The TCO layer is deposited on the buffer layer and an exemplary TCO material may be a ZnO layer, an indium tin oxide (ITO) layer or a stack comprising both ZnO and ITO. The conductive grid  122 , including a bulbar  124  and conductive fingers  126 , is disposed on a top surface  120  of the transparent layer  118  to collect the current generated when the light depicted by arrows illuminates a top surface  119  of the transparent layer  118 . 
         [0023]    As mentioned above, during the reaction step or other deposition steps to form above mentioned layers, unwanted diffusion of impurities into various parts of the solar cell may occur. Se diffusion into stainless steel substrate from the absorber and Fe diffusion into the absorber layer from the stainless steel substrate are two of the harmful diffusion mechanisms that the present invention attempts to eliminate. During the reaction of the absorber precursor, any unwanted Se diffusion towards the stainless steel substrate is inhibited, more preferably substantially inhibited such that FeSe defects are minimized, and most preferably eliminated such that such that FeSe defects do not exist due to the diffusion barrier stack disposed between the absorber layer and the substrate. Without the barrier stack, Se forms FeSe with Fe, which cause shunting defects. Further, Fe diffusion towards the absorber layer is again inhibited, more preferably substantially inhibited such that FeSe defects are minimized, and most preferably eliminated such that such that FeSe defects do not exist due to the same diffusion barrier stack disposed between the absorber layer and the substrate. 
         [0024]      FIG. 2  shows an alternative base layer  200  including an intermediate layer  205  formed on a flexible substrate  202 . The flexible substrate  202  includes stainless steel. The intermediate layer  205  preferably includes multiple conductive films. In one embodiment, the intermediate layer  205  includes: a diffusion barrier stack  203  having a metal barrier layer  206  and a first metal-nitride barrier layer  208 A, a first contact layer  210 A, a second metal-nitride barrier  208 B, a second contact layer  210 B, a nucleation layer  212 , and a seed layer  214 . The first metal barrier layer may be a Cr layer, a Cr alloy layer or their multiple layers. The first and second nitride barrier layers may be TiN layers. The first and second contact layers may preferably be Mo layers. The nucleation layer is a Ru layer or a Ru alloy, and the seed layer is a Cu layer. 
         [0025]    In one example, the metal barrier layer  206  deposited on the stainless steel substrate and the first metal-nitride barrier layer  208 A deposited on the metal barrier layer  206 . The first contact layer  210 A is deposited onto the first metal-nitride barrier  208 A. The second metal-nitride barrier layer  208 B is deposited onto the first contact layer  210 A. The second contact layer  210 B is deposited onto the second metal nitride barrier layer  208 B. The nucleation layer  212  may be deposited on the second contact layer  210 B. The seed layer  214  is preferably deposited on the nucleation layer  112 . The additional metal/nitride interfaces improve the barrier properties and further reduce diffusion across the barrier layer. The thicknesses of the second bilayer can be in the same range as the first. Various layers depicted in the drawings are not necessarily drawn to scale. 
         [0026]    Although aspects and advantages of the present inventions are described herein with respect to certain preferred embodiments, modifications of the preferred embodiments will be apparent to those skilled in the art.