Patent Publication Number: US-2012042952-A1

Title: Silicon solar cell comprising a carbon nanotube layer

Description:
CLAIM FOR PRIORITY 
     This application claims priority to Korean Patent Application No. 2009-0038062 filed on Apr. 30, 2009 in the Korean Intellectual Property Office (KIPO), the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     Example embodiments of the present invention relate in general to a silicon solar cell, and more specifically, to a silicon solar cell including a carbon nanotube layer disposed on at least one surface of front and back surfaces of a p-n junction silicon substrate. 
     2. Related Art 
     Much interest has been taken in development of clean alternative energy sources so as to solve environmental issues and problems of high oil prices, and so there has been much research into development of a solar cell (or a photovoltaic cell). The solar cell is a semiconductor device that directly converts solar energy into electrical energy using a photovoltaic effect, and has a basic structure as shown in  FIG. 1   
     Referring to  FIG. 1 , like a diode, a solar cell has a junction structure of a p-type semiconductor  110  and an n-type semiconductor  120 . When light (L) is incident onto the solar cell, electrons having negative (−) charges and holes having positive (+) charges due to emission of electrons are generated by interaction of the incident light with a material constituting a semiconductor of the solar cell, and current flows by movement of the electrons and the holes. This is called a photovoltaic effect. In this case, the electrons and the holes are respectively attracted toward the n-type semiconductor  120  and the p-type semiconductor  110  by an electric field formed around the p-n junction  115 , and travel to electrodes  140  and  130 , which have junctions with the n-type semiconductor  120  and the p-type semiconductor  110 , respectively. Then, current flows through an external circuit  150  connected to the electrodes  140  and  130 . 
     Solar cells driven based on the above-described principle are mainly divided into two categories: a silicon solar cell and a compound semiconductor solar cell. However, the solar cells have many problems regarding poor photoelectric conversion efficiency. Therefore, there has been much research conducted in order to improve the photoelectric conversion efficiency of silicon solar cells. For example, in an article [J. Non-crystal. Solids, 354, 19 (2008) Vacuum, 80, 1090 (2006); Science, 285, 692 (1999)], it was disclosed that the reflexibility is reduced using a concavo-convex structure and an antireflective film, or the recombination of minority carriers is suppressed through changes in heat treatment conditions to improve the efficiency of solar cells. However, the photoelectric conversion efficiency is not significantly improved due to the presence of problems yet to be solved, including electron-hole recombination, recombination rate, contact resistance, and the like. 
     SUMMARY 
     Accordingly, example embodiments of the present invention are provided to substantially obviate one or more problems due to limitations and disadvantages of the related art. 
     Example embodiments of the present invention provide a silicon solar cell whose photoelectric conversion efficiency is improved due to introduction of a carbon nanotube layer. 
     In some example embodiments, a silicon solar cell comprises a p-n junction silicon substrate including a p-type silicon layer and an n-type silicon layer, a carbon nanotube layer disposed on a front or back surface of the silicon substrate, and a first electrode disposed on a surface of the silicon substrate on which the carbon nanotube layer is not disposed. 
     The silicon solar cell may further comprise a second electrode disposed on the carbon nanotube layer. 
     Also, the surface of the silicon substrate on which the carbon nanotube layer is disposed may have a concavo-convex structure. 
     In other example embodiments, a silicon solar cell comprises a p-n junction silicon substrate including a p-type silicon layer and an n-type silicon layer, and carbon nanotube layers disposed on front and back surfaces of the silicon substrate. 
     The silicon solar cell may further comprise an electrode disposed on at least one of the carbon nanotube layers disposed on the front and back surfaces of the silicon substrate. 
     Also, at least one of the front and back surfaces of the silicon substrate may have a concavo-convex structure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Example embodiments of the present invention will become more apparent by describing in detail example embodiments of the present invention with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view illustrating a basic structure of a solar cell. 
         FIG. 2  is a cross-sectional view illustrating a silicon solar cell in which a carbon nanotube layer is disposed on a front surface of a p-n junction silicon substrate. 
         FIG. 3  is a cross-sectional view illustrating a silicon solar cell in which a carbon nanotube layer is disposed on a back surface of a p-n junction silicon substrate. 
         FIG. 4  is a schematic view illustrating a carbon nanotube layer disposed on a surface of a p-n junction silicon substrate having (a) a flat structure or (b) a concavo-convex structure. 
         FIG. 5  is a cross-sectional view illustrating a silicon solar cell in which carbon nanotube layers are disposed on front and back surfaces of a p-n junction silicon substrate. 
         FIG. 6  is a cross-sectional view illustrating a silicon solar cell which does not include a carbon nanotube layer produced in Comparative Example 1. 
         FIG. 7  is a graph illustrating the photoelectric conversion efficiency of the silicon solar cell which does not include the carbon nanotube layer produced in Comparative Example 1. 
         FIGS. 8 to 10  are graphs illustrating the photoelectric conversion efficiencies of silicon solar cells which do not include carbon nanotube layers produced in Preparative Examples 1 to 3, respectively. 
         FIGS. 11 and 12  are cross-sectional views illustrating silicon solar cells produced in Preparative Examples 4 and 5, respectively. 
         FIG. 13  is a cross-sectional view illustrating a silicon solar cell produced in Comparative Example 2. 
         FIG. 14  is a graph illustrating the photoelectric conversion efficiencies of the silicon solar cells produced in Preparative Examples 4 and 5 and Comparative Example 2. 
         FIG. 15  is a cross-sectional view illustrating a silicon solar cell produced in Preparative Example 6. 
         FIG. 16  is a graph illustrating the photoelectric conversion efficiency of the silicon solar cell produced in Preparative Example 6. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Example embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention, however, example embodiments of the present invention may be embodied in many alternate forms and should not be construed as limited to example embodiments of the present invention set forth herein. 
     Accordingly, while the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. Like numbers refer to like elements throughout the description of the figures. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     A silicon solar cell according to example embodiments of the present invention comprises a p-n junction silicon substrate including a p-type silicon layer and an n-type silicon layer, a carbon nanotube layer disposed on a front or back surface of the silicon substrate, a first electrode disposed on a surface of the silicon substrate in which the carbon nanotube layer is not disposed, and a second electrode disposed on the carbon nanotube layer. However, it is possible to omit the second electrode disposed on the carbon nanotube layer. 
     Also, the silicon solar cell may have a concavo-convex structure formed at a surface of the silicon substrate in which the carbon nanotube layer is disposed. 
       FIG. 2  is a cross-sectional view illustrating a silicon solar cell in which a carbon nanotube layer is disposed on a front surface of a p-n junction silicon substrate. 
     Referring to  FIG. 2 , the silicon solar cell comprises a p-n junction silicon substrate  100  including a p-type silicon layer  110  and an n-type silicon layer  120 , a carbon nanotube layer  220  disposed on a front surface  122  (i.e., a surface of the n-type silicon layer) of the silicon substrate  100 , a back electrode  130  disposed on a back surface  112  of the silicon substrate  100 , and a front electrode  140  disposed on the carbon nanotube layer  220 . However, the front electrode  140  may be omitted. 
       FIG. 3  is a cross-sectional view illustrating a silicon solar cell in which a carbon nanotube layer is disposed on a back surface of a p-n junction silicon substrate. 
     Referring to  FIG. 3 , the silicon solar cell comprises a p-n junction silicon substrate  100  including a p-type silicon layer  110  and an n-type silicon layer  120 , a carbon nanotube layer  210  disposed on a back surface  112  (i.e., a surface of the p-type silicon layer) of the silicon substrate  100 , a front electrode  140  disposed on a front surface  122  of the silicon substrate  100 , and a back electrode  130  disposed on the carbon nanotube layer  210 . However, the back electrode  130  may be omitted. 
     As shown in  FIGS. 2 and 3 , the p-n junction silicon substrate  100  has a p-n junction  115  formed between the p-type silicon layer  110  and the n-type silicon layer  120 , and serves to receive light and generate current using a photovoltaic effect. The p-type silicon layer  110  may be a silicon layer doped with Group-III elements such as boron (B), gallium (Ga) and indium (In), and the n-type silicon layer  120  may be a silicon layer doped with Group-V elements such as phosphorus (P), arsenic (As) and antimony (Sb). 
     The p-n junction silicon substrate  100  may be formed by coating an n-type dopant such as a Group-V element on a p-type silicon substrate and diffusing the n-type dopant into the p-type silicon substrate using a heat-treatment process. Meanwhile, the p-n junction silicon substrate  100  may be formed by an ion doping process using plasma, or may be formed by stacking a p-type emitter silicon layer on an n-type silicon substrate. However, the present invention is not limited thereto. 
     There is no particular limit to thicknesses of the carbon nanotube layers  220  and  210  disposed on the front and back surface  122  and  112  of the p-n junction silicon substrate  100 . However, the carbon nanotube layer  220  disposed on the front surface  122  onto which sunlight is incident may be preferably formed at a thickness of approximately 200 nm or less. This is because, when the thickness of the carbon nanotube layer  220  disposed on the front surface  122  of the p-n junction silicon substrate  100  exceeds 200 nm, it is possible to prevent some sunlight from penetrating into the p-n junction silicon substarate  100 . 
     Carbon nanotubes used to form the carbon nanotube layers  220  and  210  are not particularly limited, and may include carbon nanotubes such as a single-walled carbon nanotube and a multiwall carbon nanotube, as known in the art. 
     The carbon nanotube layers  220  and  210  may be formed using a spray method or a paste screen printing method. However, the present invention is not limited thereto. Preferably, the carbon nanotube layers  220  and  210  may be formed using a spray method. The spray method has an advantage in that the carbon nanotube layers  220  and  210  may be easily formed by dispersing carbon nanotubes in a solvent such as ethanol to prepare a dispersion solution, spraying the dispersion solution on the p-n junction silicon substrate  100  and heat-treating the p-n junction silicon substrate  100  as a subsequent process (for example, at a temperature of approximately 80° C.). 
     When the carbon nanotube layers  220  and  210  are formed on the p-n junction silicon substrate  100  as described above, the carbon nanotube layers are formed as a network thin film having in-plane conductivity, and the intensity of an electric field applied to the device may be increased. Therefore, it is possible to reduce an electron-hole recombination level and a recombination rate, thereby improving the photoelectric conversion efficiency. Also, since carbon nanotubes generally have similar properties to the p-type semiconductor, a new p-n junction may be formed when a carbon nanotube layer is disposed on the n-type silicon layer, thereby realizing a multiple exciton generation (MEG) effect. In addition, when a carbon nanotube layer is disposed on the p-type silicon layer, a back surface field (BSF) effect may be achieved by the presence of the carbon nanotube layer itself. Therefore, a BSF-forming procedure (as will be described later) through deposition and high-temperature heat treatment of a back electrode (especially, an Al electrode) may be omitted. 
     Further, a surface of the p-n junction silicon substrate  100  on which the carbon nanotube layers  220  and  210  are disposed may have a concavo-convex structure. 
       FIG. 4  is a schematic view illustrating a carbon nanotube layer disposed on a surface of a p-n junction silicon substrate  100  having (a) a flat structure or (b) a concavo-convex structure. 
     Referring to  FIG. 4 , when a concavo-convex structure (b) is formed on the surface of the p-n junction silicon substrate  100 , an amount of carbon nanotubes  200  arranged substantially vertically with respect to a p-n junction interface in the silicon substrate  100  may be increased in addition to increased light absorptance in a light-receiving surface. In this case, as seen from an equipotential surface (E) shown in  FIG. 4 , the intensity applied to the device may be further increased, which contributes to improving the photoelectric conversion efficiency. A texturing process of forming the concavo-convex structure is not particularly limited, and may be performed through a wet etching method using a base or acid solution, or a dry etching method using plasma. 
     Meanwhile, when the formation of the concavo-convex structure is not performed, the vertical orientation of carbon nanotubes in the carbon nanotube layers  220  and  210  may be enhanced using a tapping or rubbing method. Here, the tapping method is performed by forming the carbon nanotube layers  220  and  210  and attaching and detaching the carbon nanotube layers  220  and  210  with an adhesive tape, and the rubbing method is performed by rubbing the carbon nanotube layers  220  and  210  with a rubbing roller. 
     Referring again to  FIGS. 2 and 3 , the front electrode  140  may contain a metal such as gold (Au), silver (Ag), palladium (Pd)/Ag, or titanium (Ti)/Au. The front electrode  140  may be, for example, formed by coating an electrode-forming paste containing Ag or Au along a predetermined pattern and subjecting a heat-treatment process, or may be formed by depositing a metal such as Ti/Au, which may be used as the front electrode, through a sputtering method using a patterned chrome mask. However, when the carbon nanotube layer  220  is disposed on the front surface  122  of the p-n junction silicon substrate  100 , the front electrode  140  may be omitted. In this case, the carbon nanotube layer  220  having excellent light transmission and electric conductivity may function on behalf of the front electrode  140 . 
     The back electrode  130  may, for example, include at least one metal selected from the group consisting of copper (Cu), tungsten (W), iron (Fe), aluminum (Al), carbon (C), Ag, nickel (Ni), Ti and titanium nitride (TiN). Preferably, an aluminum electrode may be used as the back electrode  130 . This is because a junction with the aluminum electrode is easily formed since the aluminum electrode has excellent conductivity and good affinity to silicon. Also, the aluminum electrode is made of a trivalent element, and may form a P +  layer, that is, a back surface field (BSF, not shown) on a junction surface with the back surface  112  of the silicon substrate  100  using a high-temperature heat treatment process. The back surface field may serve to prevent electrons excited by sunlight from traveling from an inner part to a back surface of the silicon substrate  100  and being recombined with holes, thereby reducing a leakage current and improving the efficiency of solar cells. 
     The back electrode  130  may be formed by coating an electrode-forming paste containing a metal such as Al along a predetermined pattern and subjecting a heat-treatment process. However, when the carbon nanotube layer  210  is formed on the back surface  112  of the silicon substrate  100 , the back electrode  130  may also be omitted. In this case, a BSF effect may be achieved by the presence of the carbon nanotube layer  210  itself, as described above. 
     The silicon solar cell according to other example embodiments of the present invention comprises a p-n junction silicon substrate including a p-type silicon layer and an n-type silicon layer, carbon nanotube layers disposed on front and back surfaces of the silicon substrate, and electrodes disposed on the carbon nanotube layers disposed on the front and back surfaces of the silicon substrate. However, at least one of the electrodes disposed on the carbon nanotube layers may be omitted. 
     Also, the silicon solar cell may have a concavo-convex structure on at least one of the front and back surfaces of the silicon substrate. 
       FIG. 5  is a cross-sectional view illustrating a silicon solar cell in which carbon nanotube layers are disposed on front and back surfaces of a p-n junction silicon substrate. 
     Referring to  FIG. 5 , the silicon solar cell according to these example embodiments comprises a p-n junction silicon substrate  100  including a p-type silicon layer  110  and an n-type silicon layer  120 , carbon nanotube layers  220  and  210  disposed respectively on front and back surfaces  122  and  112  of the silicon substrate  100 , and front and back electrodes  140  and  130  disposed respectively on the carbon nanotube layers  220  and  210 . However, at least one of the front and back electrodes  140  and  130  disposed respectively on the carbon nanotube layers  220  and  210  may be omitted. 
     The constituent and production method of the silicon solar cell as shown in  FIG. 5  are described with reference to  FIGS. 2 and 3 , and are identical to those described above, except that the carbon nanotube layers  220  and  210  are formed on the front and back surfaces  122  and  112  of the silicon substrate  100 . 
     Also, at least one of the front and back surfaces  122  and  112  of the silicon substrate  100  may have a concavo-convex structure, and the concavo-convex structure is recited in the example embodiments described with reference to  FIG. 4 . 
     Hereinafter, preferred examples are provided to promote better understanding of the present invention. However, it should be understood that the following examples of the present invention are given for the purpose of illustration only, not intended to limit the scope of the present invention. 
     Preparative Example 1 
     A commercially available p-type silicon wafer (LG Siltron, Inc., Korea; specific resistance: 1 to 5 Ω·cm, thickness: 500±5 nm, doping density: 4.5×10 16  ions/cm 3 , and orientation: 100) was washed using a standard RCA washing process. The standard RCA washing process includes a Standard Clean-1 (SC1) process and a Standard Clean-2 (SC2) process. First, the SC1 process was performed (in a washing solution (NH 4 OH:H 2 O 2 :DI-water=1:1:5) at 70° C. for 5 minutes) to remove metals and organic remnants on a silicon wafer, and the SC2 process was performed (in a washing solution (HCl:H 2 O 2 :DI-water=1:1:5) at 70° C. for 5 minutes) to remove metals on the silicon wafer. Then, the p-type silicon wafer was subjected to a WET cleaning process, such as SPM (sulfuric acid peroxide mixture or piranha solution) treatment and HF treatment. 
     Next, a POCl 3  diffusion process of forming a p-n junction (at a temperature of 950° C. for a predeposition time of 10 minutes and then 30 additional minutes) was performed (the silicon wafer had a surface resistance changed from 20 to 200 Ω/sq to 40 Ω/sq, and a doping density of approximately 2×10 19  ions/cm 3  after the diffusion process). 
     A solution obtained by dissolving carbon nanotubes in ethanol was sprayed on a front surface (i.e., a surface of the n-type silicon layer) of a p-n junction silicon substrate obtained by forming a p-n junction using the POCl 3  diffusion process, thereby forming a carbon nanotube layer having a thickness of 1 nm to 200 nm Then, a front electrode containing Ti/Au was formed on the carbon nanotube layer, which had been formed on the front surface of the p-n junction silicon substrate, using a sputtering method, such that Ti had a thickness of 50 nm and Au had a thickness of 4 nm. 
     Thereafter, a back electrode containing Al was formed at a thickness of 200 nm on the back surface (i.e., a surface of the p-type silicon layer) of the p-n junction silicon substrate using a sputtering method, and a BSF layer was formed by performing an RTA process (at 850° C.) using a halogen lamp. 
     Preparative Example 2 
     A silicon solar cell was produced in the same manner as in Preparative Example 1, except that a carbon nanotube layer was formed on the back surface (i.e., a surface of the p-type silicon layer) of the p-n junction silicon substrate. 
     Preparative Example 3 
     A silicon solar cell was produced in the same manner as in Preparative Example 1, except that carbon nanotube layers were formed on both surfaces (i.e., front and back surfaces) of the p-n junction silicon substrate. 
     Comparative Example 1 
     A silicon solar cell was produced in the same manner as in Preparative Example 1, except that no carbon nanotube layer was formed ( FIG. 6 ). 
     Efficiencies of the silicon solar cells produced in Preparative Examples 1 to 3, which included the carbon nanotube layers, and the silicon solar cell produced in Comparative Example 1, which included no carbon nanotube layer, are shown in  FIGS. 7 to 10  and listed in the following Table 1. 
     In  FIGS. 7 to 10 , a dark current was obtained by measuring current in the light-free condition, and current on the light was measured at the presence of a light source, which is closest to natural light, using a xenon (Xe) lamp. Here, the light intensity of the xenon lamp was 4 mW/cm 2 , and an air mass (AM) was 1.5. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Comparative 
                 Preparative 
                 Preparative 
                 Preparative 
               
               
                   
                 Example 1 
                 Example 1 
                 Example 2 
                 Example 3 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Voltage (V) 
                 0.46 
                 0.47 
                 0.47 
                 0.47 
               
               
                 Current 
                 35.1 
                 87.1 
                 39.9 
                 109 
               
               
                 Density 
               
               
                 (mA/cm 2 ) 
               
               
                 Fill Factor (FF) 
                 0.35 
                 0.42 
                 0.40 
                 0.45 
               
               
                 Photoelectric 
                 5.25 
                 15.56 
                 8.24 
                 20.53 
               
               
                 Conversion 
               
               
                 Efficiency 
               
               
                 (Eff) (%) 
               
               
                   
               
            
           
         
       
     
     Referring to  FIGS. 7 to 10  and Table 1, it was revealed that the silicon solar cells produced in Preparative Example 1 ( FIG. 8 ), Preparative Example 2 ( FIG. 9 ) and Preparative Example 3 ( FIG. 10 ) had highly improved current densities and photoelectric conversion efficiencies, compared to the silicon solar cell produced in Comparative Example 1 ( FIG. 7 ). 
     Therefore, when the carbon nanotube layer is introduced onto at least one of the front and back surfaces of the p-n junction silicon substrate, it is possible to improve the photoelectric conversion efficiency of the silicon solar cell. In particular, when the carbon nanotube layers are introduced onto both surfaces of the p-n junction silicon substrate (Preparative Example 3), it was revealed that the silicon solar cell had the highest efficiency. 
     Preparative Example 4 
     A p-type silicon wafer (specific resistance: 1 to 10 Ω·cm, thickness: 500 nm, doping density: 2.5×10 15  ions/cm 3 , and orientation: 100) was prepared, and soaked in a 50% HF solution for 10 seconds to remove a natural oxide film on the silicon wafer. An etching solution obtained by mixing a 4M KOH solution with isopropyl alcohol (IPA) at a volume ratio of 14:1 was put into a thermostat. Then, the silicon wafer was added to the etching solution, and subjected to an anisotropic wet etching process at 80° C. for 10 minutes, thereby forming a concavo-convex structure on a surface of the p-type silicon wafer. 
     Next, the POCl 3  diffusion process of forming a p-n junction, the process of forming a carbon nanotube layer on a front surface (i.e., a surface of the n-type silicon layer), which had a concavo-convex structure formed thereof, of the p-n junction silicon substrate, and the process of forming front and back electrodes were performed in the same manner as in Preparative Example 1. 
     Accordingly, a silicon solar cell was produced, which included a carbon nanotube layer formed on the front surface (i.e., a surface of the n-type silicon layer) of the p-n junction silicon substrate having a concavo-convex structure ( FIG. 11 ). 
     Preparative Example 5 
     A silicon solar cell was produced in the same manner as in Preparative Example 4, except that a carbon nanotube layers were formed on the front surface (i.e., a surface of the n-type silicon layer) of the p-n junction silicon substrate having a concavo-convex structure and the back surface (i.e., a surface of the p-type silicon layer) of the p-n junction silicon substrate ( FIG. 12 ). 
     Comparative Example 2 
     A silicon solar cell was produced in the same manner as in Preparative Example 4, except that no carbon nanotube layer was formed ( FIG. 13 ). 
     Efficiencies of the silicon solar cells produced in Preparative Examples 4 and 5, which included the carbon nanotube layer, and the silicon solar cell produced in Comparative Example 2, which included no carbon nanotube layer, are shown in  FIG. 14  and listed in the following Table 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Comparative 
                 Preparative 
                   
               
               
                   
                 Example 2 
                 Example 4 
                 Preparative Example 5 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Voltage (V) 
                 0.48 
                 0.51 
                 0.53 
               
               
                 Current Density 
                 49.01 
                 74.43 
                 105.44 
               
               
                 (mA/cm 2 ) 
               
               
                 Fill Factor (FF) 
                 0.53 
                 0.64 
                 0.72 
               
               
                 Photoelectric 
                 9.29 
                 15.13 
                 22.73 
               
               
                 Conversion 
               
               
                 Efficiency (Eff) (%) 
               
               
                   
               
            
           
         
       
     
     Referring to  FIG. 14  and Table 2, it was revealed that the silicon solar cells produced in Preparative Examples 4 and 5 had highly improved current densities and photoelectric conversion efficiencies, compared to the silicon solar cell produced in Comparative Example 2. 
     Therefore, the light absorptance may be increased by the presence of the concavo-convex structure formed on the surface of the p-n junction silicon substrate, and the photoelectric conversion efficiency may be further improved when the carbon nanotube layer is formed on the concavo-convex structure. 
     Preparative Example 6 
     A silicon solar cell was produced in the same manner as in Preparative Example 5, except that no back electrode was formed ( FIG. 15 ). 
     Efficiency of the silicon solar cell produced in Preparative Example 6, which included a carbon nanotube layer but did not include a back electrode, is shown in  FIG. 16  and listed in the following Table 3. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Preparative Example 6 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Voltage (V) 
                 0.51 
               
               
                   
                 Current Density (mA/cm 2 ) 
                 86.21 
               
               
                   
                 Fill Factor (FF) 
                 0.73 
               
               
                   
                 Photoelectric Conversion 
                 20.45 
               
               
                   
                 Efficiency (Eff) (%) 
               
               
                   
                   
               
            
           
         
       
     
     Referring to  FIG. 16  and Table 3, it was seen that the photoelectric conversion efficiency of the silicon solar cell produced in Preparative Example 6 was close to approximately 90% of the photoelectric conversion efficiency of the silicon solar cell produced in Preparative Example 5. That is, it was revealed that, when the carbon nanotube layer was formed on the back surface (i.e., a surface of the p-type silicon layer) of the p-n junction silicon substrate, the silicon solar cell exhibited high photoelectric conversion efficiency, even when a process of forming a back electrode and a high-temperature heat treatment process for forming a back surface field (i.e., an RTA process) were omitted. It seems that this is because the carbon nanotube layer formed on the back surface serves as the back electrode, and a BSF effect may be achieved by the presence of the carbon nanotube layer itself. 
     Meanwhile, although the cases where the back electrode was omitted have been described in these examples, when the carbon nanotube layer was formed on the front surface (i.e., a surface of the n-type silicon layer) of the p-n junction silicon substrate, the carbon nanotube layer having excellent light transmission and electric conductivity may function on behalf of the front electrode, which makes it possible to omit an additional process of forming a front electrode (i.e., a metal electrode). 
     As described above, according to the example embodiments of the present invention, when a carbon nanotube layer is formed on a surface of the p-n junction silicon substrate, the intensity of an electric field applied to the device can be increased. Therefore, it is possible to reduce an electron-hole recombination level and a recombination rate. Also, since a new p-n junction may be formed when the carbon nanotube layer is disposed on the n-type silicon layer, it is possible to realize a multiple exciton generation (MEG) effect. In addition, when the carbon nanotube layer is disposed on the p-type silicon layer, a BSF effect may be achieved by the presence of the carbon nanotube layer itself. Therefore, the photoelectric conversion efficiency of the silicon solar cell may be improved due to introduction of the carbon nanotube layer. 
     While the example embodiments of the present invention and their advantages have been described in detail, it should be understood that various modifications and variations of the present invention may be made thereto by those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims. 
     
       
         
           
               
             
               
                   
               
               
                 Description of Major Reference Numerals 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 100: p-n junction silicon substrate 
                 110: p-type silicon layer 
               
               
                   
                 112: back surface 
                 115: p-n junction 
               
               
                   
                 120: n-type silicon layer 
                 122: front surface 
               
               
                   
                 125: concavo-convex structure 
                 130: back electrode 
               
               
                   
                 140: front electrode 
                 200: carbon nanotube 
               
               
                   
                 210, 220: carbon nanotube layers