Abstract:
A photovoltaic device comprising a substrate which has a porous first surface and a transparent conductive oxide layer located on a second surface opposite the first surface. A method of manufacturing the device is also described.

Description:
TECHNICAL FIELD 
       [0001]    The disclosed embodiments relate generally to a photovoltaic device, and more particularly, to a photovoltaic device with an anti-reflective surface and methods of manufacturing same. 
       BACKGROUND 
       [0002]    A photovoltaic device can have a substrate, such as a glass sheet, upon which various additional layers can be formed depending on the desired properties of the photovoltaic device. Light can pass through the substrate and be absorbed by semiconductor materials within the photovoltaic device to generate electric power. When the light interacts with the surface of the substrate, a portion of the light can be reflected and therefore will not be utilized to generate electric power. 
         [0003]      FIG. 1  shows a cross-sectional view of one example of a photovoltaic (PV) device  1000 , which may be a single photovoltaic cell, or a module containing a plurality of photovoltaic cells. The photovoltaic device  1000  can include a barrier layer  1002 , a transparent conductive oxide (TCO) layer  1003 , a buffer layer  1004 , and a semiconductor layer  1010  formed in a stack on substrate  1001 . Substrate  1001 , which may be glass, can include a surface that is exposed to incident light. The barrier layer  1002 , for example silica, alumina or any suitable barrier material, can be formed on the substrate  1001  and functions as a diffusion barrier for preventing chemical elements in substrate  1001  from diffusing into other portions of the device  1000 . TCO layer  1003  can be formed on the barrier layer  1002 , and acts as a conductor and ohmic contact for carrier transport out of the photovoltaic device. TCO layer  1003  can include any suitable conducting material, such as cadmium stannate, indium tin oxide, or tin oxide. TCO layer  1003  can be annealed to provide improved electrical conductivity. The buffer layer  1004 , which may be any buffer layer known in the art, for example, zinc stannate, can be formed on TCO layer  1003  and provides a smooth surface for formation of one or more semiconductor layers. 
         [0004]    Each layer may in turn include more than one layer. For example, the semiconductor layer  1010  can include a first layer including a semiconductor window layer  1011 , such as a cadmium sulfide layer, formed on the buffer layer  1004  and a second layer including a semiconductor absorber layer  1012 , such as a cadmium telluride or copper indium gallium (di)selenide (CIGS) layer, formed adjacent to the semiconductor window layer  1011 . 
         [0005]    The semiconductor window layer  1011 , which is formed adjacent to the semiconductor absorber layer  1012 , is usually n-doped while the semiconductor absorber layer  1012  is p-doped. The semiconductor absorber layer  1012  has a high photon absorptivity for generating high current and a suitable band gap to provide a good voltage. Photovoltaic device  1000  can also include a conductive back contact layer  1013  adjacent to semiconductor absorber layer  1012 . Multiple photovoltaic cells can be formed on a common substrate  1001  and covered by a back cover  1014  to form a photovoltaic module, as an example of photovoltaic device  1000 . 
         [0006]    Each layer can cover all or a portion of the device and/or all or a portion of the layer immediately below or substrate underlying the layer. For example, a layer can include any amount of any material that contacts all or a portion of a surface. It should be appreciated that photovoltaic device  1000  can be formed by any suitable process. Further, photovoltaic device  1000  can be manufactured in the layer sequence described above or with a different layer sequence. 
         [0007]    The amount of electricity produced by a photovoltaic device, such as the device of  FIG. 1 , is proportional to the amount of light absorbed by the device. Substrate  1001  is often made out of a material, such as glass, that reflects some incident light. The reflected light cannot be absorbed by the photovoltaic device. If less light was reflected, then the photovoltaic device could generate more electricity. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  is a diagram illustrating a photovoltaic device. 
           [0009]      FIG. 2  is a diagram illustrating a substrate with a porous surface. 
           [0010]      FIG. 3  is a diagram illustrating a substrate with anti-reflective coating and a protective layer on top of a TCO layer. 
           [0011]      FIG. 4  is a diagram illustrating an anti-reflective surface-creating process 
           [0012]      FIG. 5  is a diagram illustrating an anti-reflective surface-creating process. 
           [0013]      FIG. 6  is a diagram illustrating an anti-reflective surface-creating process. 
           [0014]      FIG. 7  is a diagram illustrating an anti-reflective surface-creating process. 
           [0015]      FIG. 8  is a flow chart illustrating a process of making an anti-reflective surface. 
           [0016]      FIG. 9  is a flow chart illustrating a process of making an anti-reflective surface. 
           [0017]      FIG. 10  is a diagram illustrating a photovoltaic device. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The amount of light reflected by substrate  1001  can be reduced by an anti-reflective coating on the outer surface of substrate  1001 . The anti-reflective coating can be a multilayer thin film with alternating high refractive index and low refractive index materials, or a single layer of low refractive index relative to glass (the refractive index of glass is n=1.52). An applied anti-reflective coating can include MgF 2  (magnesium fluoride), fluoro-polymers, or a porous film material. 
         [0019]    Anti-reflective coatings are sometimes applied on a substrate using a sol-gel coating process. In such a process solid (nano)particles of a non-reflective material, which collectively are called a precursor, are dispersed in a solution (a sol). The solution is applied onto a surface. There, the (nano)particles agglomerate together to form a continuous three-dimensional network extending throughout the liquid (a gel), which becomes the anti-reflective coating upon being cured. However, using sol-gel technology to apply an anti-reflective coating onto a photovoltaic device  1000  has its challenges. 
         [0020]    Creating an anti-reflective coating from a sol-gel process requires performing a heat treatment to anneal the sol-gel coating. If the substrate  1001  was to be annealed after applying the precursor thereon, it would expose TCO layer  1003  to annealing conditions or to annealing time that could damage or alter its properties. 
         [0021]    On the other hand, if the anti-reflective coating were to be applied before the TCO layer is formed, the anti-reflective coating might not survive the thermal and/or chemical processes to which the TCO layer or the photovoltaic device  1000  might later be exposed as subsequent materials or layers are added. 
         [0022]    According to one disclosed embodiment, an anti-reflective surface is formed on the outer (i.e., sunny side) surface of the substrate. During formation of the anti-reflective surface, the TCO layer  1003 , if present, is not substantially degraded or otherwise altered, allowing for normal subsequent processing steps in forming a finished photovoltaic device  1000  to be used. Once formed, the anti-reflective surface can increase the proportion of incoming light being absorbed by the photovoltaic device, thereby increasing the efficiency of the device. 
         [0023]    Referring to  FIG. 2 , a substrate  10 , which may be a glass sheet, has a porous, anti-reflective surface  11  formed thereon. The substrate still contains a non-porous portion  12 . Note that in  FIG. 2  the TCO layer has not yet been formed on substrate  10  and thus there is no need to be concerned about damaging the TCO layer while forming the anti-reflective surface  11 . 
         [0024]    Anti-reflective surface  11  can be porous with a pore size in the nm- or sub-μm-range (pore size is conventionally defined as the diameter of the largest sphere that may be accommodated within the pore). The porous structure of anti-reflective surface may be skeletonized, wherein the porous structure has walls or columns that provide a rigid scaffold, or skeleton, for the porous structure that allows the pores to retain their size and shape. This porosity can be achieved by etching, among other methods. Anti-reflective surface  11  can have a thickness anywhere between 80-200 nm, with the actual thickness of anti-reflective layer  11  being dependent upon light-transmission efficiency requirements of the photovoltaic device, taking into consideration the precise refractive index of anti-reflective surface  11 . For example, as determined by the structure and composition of anti-reflective surface  11 , a thickness of 120 nm may be suitable. In some embodiments, the size of pores  15  in the anti-reflective surface  11  may be in the range of 5 to 50 nm. 
         [0025]    The porous anti-reflective surface  11  reflects less light than a non-porous surface made of the same material. For example, anti-reflective surface  11  can reflect about 0.5% to about 10%, or about 1% to about 4%, less incident light having a wavelength of about 350 nm to about 1000 nm than the same substrate with a non-porous surface. 
         [0026]    Referring to  FIG. 3 , substrate  10  includes anti-reflective surface  11  which is formed on a sunny side  110  of substrate  10 . TCO layer  13  is on the opposite side from the sunny side.  FIG. 3  also shows an enlarged view of anti-reflective surface  11 , including the pore structure. 
         [0027]    Anti-reflective surface  11  ( FIGS. 2 and 3 ) can acquire its porosity through etching of substrate  10 . An etchant can be applied to a sunny side surface of substrate  10 , which includes a non-porous portion  12 , to form anti-reflective surface  11 . If the etchant is an acidic etchant, then basic (alkaline) chemical groups in anti-reflective surface  11  may be neutralized, leaving anti-reflective surface  11  alkaline depleted. When substrate  10  is glass, an alkaline depleted surface can be an additional benefit because glass with an alkaline depleted surface is known to have increased resistance to erosion. The etchant can be applied either before ( FIG. 2 ) or after ( FIG. 3 ) the substrate is coated on the non-sunny side surface with TCO. Etchants suitable for forming a porous, skeletonized anti-reflective surface  11  can be highly corrosive and can damage TCO layer  13  if they come in contact with TCO layer  13 . Consequently, to preserve the integrity and functionality of the device, when TCO layer  13  is on the substrate  10 , etchants may be prevented from contacting TCO layer  13 . 
         [0028]    As shown in  FIG. 3 , TCO layer  13  can be physically protected by forming a protective layer  14  over it. In some embodiments, TCO layer  13  is sufficiently thin such that the amount of etchant that contacts the sides of TCO layer  13  is insubstantial and does not substantially etch TCO layer  13  or otherwise affect the functionality of a fabricated photovoltaic device. In other embodiments, protective layer  14  can cover both the surface and the sides of TCO layer  13 . 
         [0029]    Protective layer  14  can include an etchant-resistant polymer material, such as polypropylene or polyethylene. When protective layer  14  is formed from such materials, etchants such as aqueous hydrofluoric acid (hydrogen fluoride) or fluorosilicic acid, for example, will not remove protective layer  14 . In this embodiment, when an etchant is applied to substrate  10 , TCO layer  13  will be protected from degradation or alteration. Protective layer  14 , while chemically resistant to the etchant, can be removed, for example by washing it with a solvent that can dissolve it after the etching process has been completed. Such solvents may include organic solvents, such as organic alcohols, ethyl acetate, acetone, methylene chloride, hexanes, diethyl ether, and other solvents known in the art. In some embodiments, protective layer  14  may be omitted if the TCO layer  13  is made of an acid-etchant-resistant oxide such as SnO 2 . 
         [0030]    Referring to  FIG. 4 , etching may occur by spraying the substrate  10  with etchant  300 . The surface of substrate  10  that is in contact with etchant  300  becomes the porous, anti-reflective layer  11 . The portion that does not contact the etchant  300  remains as a non-porous portion  12 . Etchant  300  may be sprayed from a conventional spraying apparatus  400 . 
         [0031]    Although  FIG. 4  illustrates etching of a substrate  10  which does not contain a TCO layer, the technique illustrated in  FIG. 4  can also be applied to a substrate containing a TCO layer on its non-sunny side. 
         [0032]      FIG. 5  shows substrate  10  immersed in an etchant  300  within a container  200 . Substrate  10  has a sunny side surface  110  and a TCO layer  13  formed adjacent to the non-sunny side surface  120 . Prior to etching, a protective layer  14  is formed over TCO layer  13 . Protective layer  14  should completely cover the surface of TCO layer  13  while leaving the sunny side surface of substrate  10  exposed. When protective layer  14  is in place, the sunny side of sheet  10  can be exposed to the etchant without disturbing TCO layer  13 . As a result, anti-reflective surface  11  can be formed by immersing substrate  10  in container  200  containing etchant  300 . Etchant  300  can contact and etch the sunny side of substrate  10 . The porous anti-reflective surface  11  includes a skeletonized configuration. After porous anti-reflective surface  11  is formed, substrate  10  still contains a non-porous portion  12 . Substrate  10  can be allowed to remain in contact with etchant  300  for any suitable duration to allow etching to occur. A plurality of substrates  10  can be processed in a batch in the same container to allow for fast processing throughput. Substrate  10  can be held in container  200 , or can be conveyed through container  200  in an in-process manner. 
         [0033]    As shown in  FIG. 6 , substrate  10  can also be conveyed through etchant  300  by any suitable means including a conveyor or rollers  400 , such that only a surface portion of the sunny side of substrate  10  is in contact with etchant  300 . 
         [0034]    Referring to  FIG. 7 , substrate  10  can also be suspended from an overhead conveyor  500 , which can include one or more substrate  10  securing devices such as one or more suction cups  501 , which suspend a sunny side surface of the substrate  10  in the etchant  300 . In  FIGS. 6 and 7 , if the TCO layer  13  is on the back side of the substrate  10 , as shown, then protective layer  14  may be omitted since only a portion of the sunny side of substrate  10  is exposed to the etchant. However, it may nonetheless be desirable to protect TCO layer  13  from splashing etchant  300  by using protective layer  14 . 
         [0035]    Etchant  300  can be selective, only modifying the sunny side surface  110  without affecting TCO layer  13  on the other side, especially when TCO layer  13  is completely covered by protective layer  14 . In addition, an etchant  300  can be selected which does not etch the material used for TCO layer  13  (such as when the etchant is hydrogen fluoride and the material used for TCO layer is stannous oxide), in which case protective layer  14  is not needed. 
         [0036]    Etchant  300  can include hydrogen fluoride, fluorosilicic acid, or any suitable etching solution. In some embodiments, the etchant  300  can include at least one fluorine-containing compound, such as sodium bifluoride, ammonium bifluoride, or other fluorine-containing etchant which can be used for modifying the glass surface  110 . Substrate outer surface  110  can be first treated with one fluorine-containing etchant to remove the glass skin (a thin film covering the glass), and then treated with another fluorine-containing etchant to form an anti-reflective surface  11 . For removing the glass skin, the concentration of etchant in solution can be, for example, in the range of 0.5% to 50%. If a hydrogen fluoride etchant is used, then concentration of hydrogen fluoride in solution may be from 0.5% to 5%. If a bifluoride etchant is used, then the concentration of bifluoride in solution may be, for example, from 5% to 25%. For removing the glass skin, an exemplary etching duration, regardless of the etchant, may be in the range between 10 sec and 10 min, preferably 1 to 2 min. For creation of the porous, anti-reflective coating  11  a solution of fluorosilicic acid, hydrofluoric acid, or other fluorine-containing acid can be used as the etchant. When the etchant is used in a solution, the concentration of the etchant in the solution may be 5% to 35%, preferably 10% to 20%. Exemplary etching times for creation of anti-reflective surface  11  are 5 to 90 min, preferably 10 to 45 min. 
         [0037]    Referring to  FIG. 8 , a selective anti-reflective surface forming process can include the steps of: (1) preparing the substrate, for example, by forming the substrate to a desired size, and by cleaning the substrate; (2) forming a TCO layer on the non-sunny side of the substrate; (3) transporting the substrate to etchant solution container; (4) etching the surface of the sunny side of the substrate to form an anti-reflective surface; (5) cleaning the substrate to remove etchant and byproducts; and (6) ending the surface process and transporting the glass substrate to the subsequent manufacturing process. The anti-reflective surface forming process can further include forming a protective layer on the TCO layer  13  prior to etching. If a protective layer is used then the protective layer  14  is removed after the process described in steps  4  or  5  of  FIG. 8 . 
         [0038]    Referring to  FIG. 9 , in some embodiments step (2) of forming a TCO layer can be done after step (4) etching the surface of the sunny side of the substrate to form an anti-reflective surface and step (5) of cleaning the glass in which case no protective layer is needed for the TCO layer. 
         [0039]    Referring to  FIG. 10 , a photovoltaic device  1000 , for example as shown in  FIG. 1 , may be formed with an etched anti-reflective surface  11  on the sunny side of substrate  1001 . Additional layers may be formed on the non-sunny side of substrate  1001  as described above with reference to  FIG. 1 . 
         [0040]    Although the embodiments above discuss forming the anti-reflective surface by way of an etchant, other means may be used to form the anti-reflective surface. For example, a porous anti-reflective surface may be formed by using a laser, or by using a suitable mechanical means to create pores. 
         [0041]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, although exemplary photovoltaic devices have been shown and elucidated, the invention can be applied to other devices and technologies. It should also be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of features illustrative of the basic principles of the invention.