Patent Publication Number: US-2010126583-A1

Title: Thin film solar cell and method of manufacturing the same

Description:
This application claims the benefit of Korean Patent Application No. 10-2008-0117588 filed on Nov. 25, 2008 and No. 10-2009-0109860 filed on Nov. 13, 2009, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relate to a thin film solar cell and a method of manufacturing the same. 
     2. Discussion of the Related Art 
     Nowadays, in order to solve the energy problem many are facing, various researches for a fuel that can replace existing fossil fuels have been advanced. Particularly, various researches for using natural and renewable energy such as a wind force, atomic energy, and solar energy to replace petroleum resources, for example, to be exhausted within several decades have been advanced. 
     Because a solar cell uses solar energy, which is a virtually infinite and, environmental-friendly energy source, unlike other energy sources, much research has been performed for the last several decades since a Se solar cell was developed in 1983. A currently commercialized solar cell using a monocrystal bulk silicon is not more widely used due to high production and installation costs. 
     In order to solve such a cost problem, research for a thin film solar cell is actively performed, and a large area solar cell can be manufactured at low cost via a technique for manufacturing a thin film solar cell using amorphous silicon (a-Si:H), and thus, interest has increased in the thin film solar cell using the amorphous silicon (a-Si:H). 
     In general, a thin film solar cell has a form in which a first electrode, an absorption layer, and a second electrode are stacked on a first substrate, and in order to improve the efficiency, an unevenness is formed on a surface of the first electrode. Conventionally, as a method of forming the unevenness on the surface of the first electrode, a chemical etching method using an acid/base solution has been used. 
     However, in order to use the chemical etching method, an etching solution should be changed according to a material of the first electrode that is used, and it is difficult to freely adjust the form of the unevenness. Further, there is a problem of waste processing of a waste acid/base etching solution after use, and thus, which requires an urgent solution. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention are directed to a thin film solar cell and a method of manufacturing the same that can easily form an unevenness, be environmental-friendly, and reduce or prevent an electrical characteristic of a solar cell from being deteriorated. 
     According to an embodiment of the invention, provided is a thin film solar cell including a substrate; a transparent layer positioned on the substrate and comprising a plurality of microlenses; a first electrode positioned on the transparent layer; an absorption layer to generate electron-hole pairs from incident light, and positioned on the first electrode; and a second electrode positioned on the absorption layer. 
     According to an embodiment of the invention, provided is a method of manufacturing a thin film solar cell including coating a resin on a substrate; forming a transparent layer comprising a plurality of microlenses from the coated resin by using a mold; forming a first electrode on the transparent layer; forming an absorption layer which generates electron-hole pairs from incident light on the first electrode; and forming a second electrode on the absorption layer. 
     According to an embodiment of the invention, provided is a thin film solar cell including a substrate; a transparent layer positioned on the substrate and comprising a plurality of periodic protrusions; a first electrode positioned on the transparent layer; an absorption layer to generate electron-hole pairs from incident light, and positioned on the first electrode; and a second electrode positioned on the absorption layer. 
     Other embodiments will be disclosed in the detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompany drawings, which provide a further understanding of the invention, which are incorporated and constitute a part of this specification, illustrate embodiments of the invention, and together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a cross-sectional view illustrating a thin film solar cell according to an embodiment of the invention; 
         FIGS. 2   a - 2   e  are perspective views illustrating various forms of an uneven layer of a thin film solar cell according to an embodiment of the invention; 
         FIG. 3  is a view of a microlens according to an embodiment of the invention; 
         FIG. 4  is a diagram illustrating focusing and scattering of light of a thin film solar cell according to an embodiment of the invention; and 
         FIGS. 5   a  to  5   g  are perspective views illustrating processes of a method of manufacturing a thin film solar cell according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 1  is a cross-sectional view illustrating a thin film solar cell according to an embodiment of the invention. Referring to  FIG. 1 , a thin film solar cell  100  according to an embodiment comprises a substrate  110 , an uneven layer  120  positioned on the substrate  110  and comprising a plurality of protrusions  125 , a first electrode  130  positioned on the uneven layer  120 , an absorption layer  140  positioned on the first electrode  130 , and a second electrode  150  positioned on the absorption layer  140 . 
     The substrate  110  is made of glass or a transparent resin film. The glass uses a flat plate glass having silicon oxide (SiO 2 ), sodium oxide (Na 2 O), and/or calcium oxide (CaO) having high transparency and insulating property as a main component. 
     The uneven layer  120  increases a light trapping effect by reducing or preventing total reflection of incident light and by enlarging light scattering, and thus performs a function of increasing the efficiency of the thin film solar cell  100 . 
     Because the uneven layer  120  should transmit light, the uneven layer  120  is made of a light transparent resin. Here, the light transparent resin is made of an acryl-based monomer and may be formed with one selected from a group consisting of polyethylene terephthalate (PET), polycarbonate (PC), polypropylene (PP), polyethylene (PE), polystyrene (PS), and poly epoxy, but a material of the light transparent resin is not limited thereto. 
     The uneven layer  120  comprises the plurality of protrusions  125 . The plurality of protrusions  125  may be periodically placed on the uneven layer  120 , or may be formed together with the uneven layer  120  in a unitary fashion. The plurality of protrusions  125  may have various shapes, for example, a saw-toothed shape, a convex shape, a columnar shape, a pyramidal shape, a ridge shape, or other shapes. In one embodiment of the invention, the plurality of protrusions is microlenses  125 . The microlens  125  may have a protruded form of an embossed hemispherical shape. 
       FIG. 2  is a perspective view illustrating various forms of an uneven layer of a thin film solar cell according to an embodiment of the invention. Referring to  FIGS. 1 , and  2   a  to  2   e , the microlens  125  can have different diffusion, refraction, and focusing characteristics of light according to a size and density thereof. Accordingly, as shown in  FIGS. 2   a  to  2   c , a lens diameter d of the microlens  125  may be uniform or non-uniform, and a height h of the microlens  125  may also be uniform or non-uniform. 
     That is, as is shown in  FIGS. 2   a  and  2   b , the diameters d and the heights h of a plurality of the microlenses  125  may all be uniform on the uneven layer  120 . Additionally, as shown in  FIG. 2   c , the diameters d and/or the heights h of the plurality of microlenses  125  may be non-uniform. The plurality of non-uniform microlenses may be arranged in periodic order, as shown in  FIG. 2   c , where rows of larger microlenses alternate with rows of smaller microlenses, but the plurality of non-uniform microlenses can also be randomly positioned. The microlens  125  can be regularly arranged and arrangement between central points of the microlens  125  can be formed in a line. 
     However, as shown in  FIG. 2(   d ). in arrangement of the microlens  125 , central points of the microlens  125  can be disposed in an oblique line. Further, as shown in  FIG. 2(   e ), the microlens  125  can be irregularly arranged and central points of the microlens  125  may be randomly arranged 
     Further, the diameter d of the microlens  125  is about 1 to about 10 μm, but is not limited thereto. The height h of the microlens  125  is about ½ or less of a diameter d of the microlens  125 . Further, a gap p between the microlenses  125  is about ¼ or less of the diameter d of the microlens  125 , but is not limited thereto. 
     An occupying area of the microlens  125  is about 50 to about 90% or more of, for example, an entire area of the uneven layer  120 , but is not limited thereto. 
       FIG. 3  is a view of a microlens according to an embodiment of the invention. The microlens  125  has a planar base  121 , and a curved surface  123  over the base  121  that contacts the base  121  at least one point  122 . A tangent line  124  may be defined at the at least one point  122  where the curved surface  123  contacts the base  121 . In this case, an contact angle θ is defined between the base  121  and the tangent line  124  of the curved surface  123  at the at least one point  122 . In embodiments of the invention, the contact angle θ may be about 30° to 90°. One or more of microlenses  125  may have the contact angle θ of about 45° to 60°. 
     As described above, when the microlens  125  is formed in an embossed hemispherical shape, some of light applied from the outside, for example, a lower part of the microlens  125 , is uniformly refracted in entire or all the orientation angles of the hemispherical shape to be transmitted in the microlens  125 . Thereby, some of light applied from a lower part of the microlens  125  is uniformly diffused upward. 
     The first electrode  130  is made of a transparent conductive oxide or a metal. The transparent conductive oxide used may be an indium tin oxide (ITO), a tin oxide (SnO 2 ), a zinc oxide (ZnO), or others. In embodiments of the invention, the transparent conductive oxide is ITO. The metal used may be silver (Ag), aluminum (Al), or others. 
     The first electrode  130  is formed with a single layer made of a transparent conductive oxide or a metal, but is not limited thereto and may be formed with a multiple layer in which two layers or more of a transparent conductive oxide/metal are stacked. 
     The absorption layer  140  is made of amorphous silicon and can have a pin structure. Here, the referred pin structure may be a stacked structure of a p+ type amorphous silicon layer/intrinsic-type amorphous silicon layer/n+ amorphous silicon layer. 
     Here, in the pin structure, when light, such as sun light, is applied, a silicon thin film layer absorbs the light and thus an electron-hole pair is generated. In the pin structure, by a built-in potential generated with a p-type and an n-type, the generated electrons and holes are moved to n-type and p-type semiconductors, respectively, and are used generate a current, for example. 
     In the embodiments of the invention, the absorption layer  140  is shown as only one layer, however the absorption layer  140  has a stacked structure formed with a p+ type amorphous silicon layer/intrinsic-type amorphous silicon layer/n+ amorphous silicon layer to generate electron-hole pairs, and to move the generated electrons and holes. 
     Like the first electrode  130 , the second electrode  150  is made of a transparent conductive oxide or a metal. The transparent conductive oxide used may be indium tin oxide (ITO), tin oxide (SnO 2 ), zinc oxide (ZnO), or others. In embodiments of the invention, the transparent conductive oxide is ITO. The metal used may be silver (Ag), aluminum (Al), or others. 
     The second electrode  150  is formed with a single layer made of a transparent conductive oxide or a metal, but is not limited thereto, and can be stacked with two layers or more of a transparent conductive oxide/metal. 
       FIG. 4  is a diagram illustrating focusing and scattering of light of a thin film solar cell according to an embodiment of the invention. 
     Referring to  FIG. 4 , light applied through the substrate  110  can be simultaneously focused and scattered within a thin film solar cell. 
     In more detail, focused light A among light applied through the substrate  110  is focused through a microlens of the uneven layer  120  and can be focused even in an interface of the first electrode  130 . Therefore, due to a focusing effect of a microlens of the uneven layer  120 , a focal depth of applied light is sustained and thus an effective light transmission effect can be obtained. Further, scattered light B among light applied through the substrate  110  can be scattered while being focused in an interface of a microlens of the uneven layer  120 . Light transmitted the uneven layer  120  is again scattered while being focused in an interface of the first electrode  130  and light transmitted the first electrode  130  can be scattered while being focused again in an interface of the absorption layer  140 . Therefore, due to scattering of applied light by a microlens of the uneven layer  120 , a light path transferred to the absorption layer  140  largely increases, thereby improving electrical efficiency of a thin film solar cell. 
     Hereinafter, a method of manufacturing a thin film solar cell according to an embodiment of the invention will be described. 
       FIGS. 5   a  to  5   g  are perspective views illustrating processes of a method of manufacturing a thin film solar cell according to an embodiment of the invention. 
     Referring to  FIG. 5   a , (a) a resin  215  is coated on a substrate  210 . In this case, the substrate  210  is made of glass or a transparent resin film. The glass can use a flat plate glass having silicon oxide (SiO 2 ), sodium oxide (Na 2 O), and/or calcium oxide (CaO) having high transparency and insulating property as a main component. 
     The resin  215  is formed with an acryl-based monomer, but may be formed with one selected from a group consisting of polyethylene terephthalate (PET), polycarbonate (PC), polypropylene (PP), polyethylene (PE), polystyrene (PS), and poly epoxy. 
     Next, (b) a mold  220  is prepared or positioned on the substrate  210  in which the resin  215  is coated. In the mold  220 , an inverse image of a microlens  225  is engraved. Because the inverse image of the microlens  225  engraved in the mold  220  determines a form of the microlens  225  to be formed in the resin  215 , a diameter d and a height h of the microlens  225 , and a gap p between the microlens  225  should be accurately designed. 
     Next, (c) an uneven layer  230  comprising a plurality of microlenses  225  is formed by being stamped with the mold  220  on the substrate  210  in which the resin  215  is coated. While the resin  215  is being stamped by the mold  220 , ultraviolet (UV) light may be applied to the coated resin to set the microlenses  225 . Then, once the mold  220  is removed, the set resin may be subjected to heat to further harden the microlenses  225 . Here, heat curing is performed for 30 minutes at a temperature of about 230° C. 
     In this time, a lens diameter d of the microlens  225  is uniform or non-uniform, and a height h of the microlens  225  is also uniform or non-uniform. 
     Further, the diameter d of the microlens  225  is about 1 to about 10 μm, but is not limited thereto. The height h of the microlens  225  is about ½ or less of the diameter d of the microlens  225 . Further, a gap p between the microlenses  225  may be about ¼ or less of the diameter d of the microlens  225 , but is not limited thereto. An occupying area of the microlens  225  may be 50 to 90% or more than, for example, of an entire area of the uneven layer  120 , but is not limited thereto. 
     Referring to  FIG. 5   b , a first electrode  240  is formed on the substrate  210  in which the uneven layer  230  is formed. The first electrode  240  is made of a transparent conductive oxide or a metal. The transparent conductive oxide used may be an indium tin oxide (ITO), a tin oxide (SnO 2 ), a zinc oxide (ZnO), or other. In embodiments of the invention, the transparent conductive oxide is ITO. The metal used may be silver (Ag) aluminum (Al), or others. 
     Further, the first electrode  240  is formed with a single layer made of a transparent conductive oxide or a metal, but is not limited thereto and may be formed with a multiple layer in which two layers or more of a transparent conductive oxide/metal are stacked. 
     The first electrode  240  can be formed with chemical vapor deposition (CVD), physical vapor deposition (PVD), an electron beam (E-beam) method, or others. In this case, when the first electrode  240  is deposited on the substrate  210  in which the uneven layer  230  is formed, the first electrode  240  is formed along a step coverage of a microlens shape of the uneven layer  230 , and thus, a microlens shape is displayed on a surface of the first electrode  240 . 
     Therefore, a conventional process of forming an uneven portion in the first electrode using an acid/base etching solution may be omitted. Accordingly, unevenness can be easily formed on the first electrode, and the process is environment-friendly and reduces prevents an electrical characteristic of a solar cell from being deteriorated. 
     Next, referring to  FIG. 5   c , the first electrode  240  is patterned. In this case, as a method of patterning the first electrode  240 , a photoresist method, a sand blast method, and/or a laser scribing method are used. Here, the first electrode  240  can be separated by a first patterned line  245 . 
     Next, referring to  FIG. 5   d , an absorption layer  250  is formed on the first electrode  240  in which the patterning process is terminated. In this case, the absorption layer  250  is made of amorphous silicon and is stacked as a pin structure. Here, the pin structure may be a stacked structure of a p+ type amorphous silicon layer/intrinsic-type amorphous silicon layer/n+ amorphous silicon layer. 
     In the pin structure, when light, such as sun light, is applied, a silicon thin film layer absorbs the light, and thus, an electron-hole pair is generated. In the pin structure, by a built-in potential generated with a p-type and an n-type, the generated electron and hole are moved to n-type and p-type semiconductors, respectively, and are used. 
     In embodiments of the present invention, the absorption layer  250  is shown as only one layer, but the absorption layer  250  can have a structure stacked with a p+ type amorphous silicon layer/intrinsic-type amorphous silicon layer/n+ amorphous silicon layer. 
     In this case, the absorption layer  250  can be formed by sequentially depositing amorphous silicon layers with a plasma enhanced chemical vapor deposition (PECVD) method. 
     Next, referring to  FIG. 5   e , the absorption layer  250  is patterned. In this case, the absorption layer  250 , having an area separated from a first patterning line  245  in which the first electrode  240  is patterned, is patterned. Here, as a method of patterning the absorption layer  250 , a photoresist method, a sand blast method, and/or a laser scribing method are used. Therefore, the absorption layer  250  can be separated by a second patterning line  255 . 
     Next, referring to  FIG. 5   f , a second electrode  260  is formed on the substrate  210  in which a patterning process of the absorption layer  250  is terminated. Like the first electrode  240 , the second electrode  260  is made of a transparent conductive oxide or a metal. The transparent conductive oxide used may be an indium tin oxide (ITO), a tin oxide (SnO 2 ), a zinc oxide (ZnO), or others. In embodiments of the invention, the transparent conductive oxide is ITO. The metal used may be silver (Ag), aluminum (Al), or others. 
     The second electrode  260  is formed with a single layer made of a transparent conductive oxide or a metal, but is not limited thereto and may be stacked with two layers or more of a transparent conductive oxide/metal. 
     In this case, like the first electrode  240 , the second electrode  260  can be formed with chemical vapor deposition (CVD), physical vapor deposition (PVD), and/or an electron beam (E-beam) method. 
     Finally, referring to  FIG. 5   g , for electrical insulation, the absorption layer  250  and the second electrode  260  formed on the substrate  210  are patterned. 
     In this case, by patterning an area separated from the first patterning line  245  and the second patterning line  255 , the area can be electrically insulated by a third patterning line  265 . 
     Therefore, as described above, a thin film solar cell in the present implementation can be manufactured. 
     As described above, in a thin film solar cell and a method of manufacturing the same of this document, by forming an uneven layer using a resin on the first substrate, an uneven structure can be easily formed in the solar cell. 
     Further, because a conventional acid/base etching solution is not used, the method is environment-friendly, and because a surface of the first electrode is not etched, an electrical characteristic of the solar cell can be reduced or prevented from being deteriorated. 
     The foregoing embodiments and advantages are merely examples and are not to be construed as limiting the invention. The present teaching can be readily applied to other types of apparatuses. The description of the foregoing embodiments is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Moreover, unless the term “means” is explicitly recited in a limitation of the claims, such limitation is not intended to be interpreted under 35 USC 112 (6).