Abstract:
The purpose of the present invention is to provide a solar cell with higher conversion efficiency. 
     The method comprises steps of:
       (a) preparing the solar cell comprising a condensing lens ( 101 ) and a solar cell element ( 102 ), wherein the following inequation set (I) is satisfied:       
 
       d2&lt;d1,d3&lt;d1,1 nanometer≦d2≦4 nanometers,1 nanometer≦d3≦4 nanometers,100 nanometers≦w2,and 100 nanometers≦w3  (I);
       and   (b) irradiating a region S which is included in the surface of the p-type window layer ( 105 ) through the condensing lens ( 101 ) with light in such a manner that the following inequation (II) is satisfied so as to generate a potential difference between the n-side electrode ( 110 ) and the p-side electrode ( 109 ):       
 
       w4≦w1  (II).

Description:
[0001]    This is a continuation of International Application No. PCT/JP2011/005870, with an international filing date of Oct. 20, 2011, which claims priority of Japanese Patent Application No. 2011-065556, filed on Mar. 24, 2011, the contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a solar cell. 
       BACKGROUND ART 
       [0003]      FIG. 7  shows a solar cell disclosed in Patent Literature 1. The conventional solar cell comprises a plurality of photoelectric conversion layers  13 , the photoelectric conversion layer  13  having a solar cell element  11  and a lens L. The solar cell element  11  comprises a p-type GaAs buffer layer  13   a , a p-type InGaP-BSF layer  13   b , a p-type GaAs base layer  13   c , an n-type GaAs emitter layer  13   d , an n-type InGaP window layer  13   e , and an antireflection layer  15 . These layers  13   a  to  15  are stacked on a semiconductor substrate  12  in this order. The solar cell element  11  further comprises a separation grid  16  which separates the photoelectric conversion layer  13 , a contact layer  14  around the detector side of the photoelectric conversion layer  13 , a recoupling prevention layer  17  around the outer circumference of the contact layer  14 , a detector side electrode  18  and a back side electrode  19 . 
         [0004]    Sunlight penetrates the lens L and the antireflection layer  15 , and the n-type InGaP window layer  13   e  is irradiated with the sunlight. This irradiation of the sunlight generates electric power. 
       CITATION LIST 
     Patent Literature 
       [0005]    [Patent Literature 1]
   Japanese Laid-Open Patent Application Publication No. 2008-124381   
 
       Non Patent Literature 
       [0007]    [Non Patent Literature 1]
   Jenny Nelson, “The Physics of Solar Cells”, World Scientific Pub Co Inc.   
 
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0009]    The conventional solar cell has a conversion efficiency of approximately 20%. 
         [0010]    The purpose of the present invention is to provide a solar cell with higher conversion efficiency. 
       Solution to Problems 
       [0011]    The present disclosure provides a method for generating electric power with use of a solar cell, the method comprising steps of: 
         [0012]    (a) preparing the solar cell comprising a condensing lens and a solar cell element, wherein 
         [0013]    the solar cell element comprises an n-type GaAs layer, a p-type GaAs layer, a p-type window layer, an n-side electrode, and a p-side electrode; 
         [0014]    a Z-direction denotes the direction of the normal line of the p-type GaAs layer; 
         [0015]    an X-direction denotes a direction orthogonal to the Z-direction, 
         [0016]    the n-type GaAs layer, the p-type GaAs layer, and the p-type window layer are stacked along the Z-direction; 
         [0017]    the p-type GaAs layer is interposed between the n-type GaAs layer and the p-type window layer along the Z-direction; 
         [0018]    the p-side electrode is electrically connected with the p-type GaAs layer; 
         [0019]    the n-side electrode is electrically connected with the n-type GaAs layer; 
         [0020]    the n-type GaAs layer is divided into a center part, a first peripheral part, and a second peripheral part; 
         [0021]    the center part is interposed between the first peripheral part and the second peripheral part along the X-direction; 
         [0022]    the first peripheral part and the second peripheral part have a shape of a layer, 
         [0023]    the following inequation set (I) is satisfied: 
         [0000]      d2&lt;d1,d3&lt;d1,1 nanometer≦d2≦4 nanometers,1 nanometer≦d3≦4 nanometers,100 nanometers≦w2,and 100 nanometers≦w3  (I);
 
         [0024]    wherein
       d 1  represents a thickness of the center part along the Z-direction;       
 
         [0026]    d 2  represents a thickness of the first peripheral part along the Z-direction; 
         [0027]    d 3  represents a thickness of the second peripheral part along the Z-direction; 
         [0028]    w 2  represents a width of the first peripheral part along the X-direction; and 
         [0029]    w 3  represents a width of the second peripheral part along the X-direction; and 
         [0030]    (b) irradiating a region S which is included in the surface of the p-type window layer through the condensing lens with light in such a manner that the following inequation (II) is satisfied so as to generate a potential difference between the n-side electrode and the p-side electrode: 
         [0000]      w4≦w1  (II);
 
         [0031]    wherein
       w 1  represents a width of the center part along the X-direction;       
 
         [0033]    w 4  represents a width of the region S along the X-direction in the cross-sectional view which includes the Z-direction; and 
         [0034]    the first center part overlaps the region S when seen from the Z-direction. 
       Advantageous Effect of the Invention 
       [0035]    The present invention provides a solar cell with higher conversion efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]      FIG. 1A  shows a cross-sectional view of the solar cell according to the embodiment 1. 
           [0037]      FIG. 1B  shows a cross-sectional view of the solar cell element according to the embodiment 1. 
           [0038]      FIG. 2  shows a cross-sectional exploded view of the solar cell element according to the embodiment 1. 
           [0039]      FIG. 3A  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0040]      FIG. 3B  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0041]      FIG. 3C  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0042]      FIG. 3D  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0043]      FIG. 3E  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0044]      FIG. 3F  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0045]      FIG. 3G  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0046]      FIG. 4  shows a cross-sectional view of the solar cell element according to the embodiment 1. 
           [0047]      FIG. 5  shows a cross-sectional view of the solar cell element according to the comparative example 1. 
           [0048]      FIG. 6A  shows a cross-sectional view of the solar cell element according to the comparative example 4. 
           [0049]      FIG. 6B  shows a cross-sectional view of the solar cell element according to the comparative example 5. 
           [0050]      FIG. 7  shows a cross-sectional view of the conventional solar cell. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0051]    The embodiment of the present invention is described below with reference to drawings. 
       Embodiment 1 
     Step (a) 
       [0052]    In the step (a), a solar cell is prepared. 
         [0053]      FIG. 1A  shows a cross-sectional view of the solar cell according to the embodiment 1. As shown in  FIG. 1A , the solar cell comprises a condensing lens  101  and a solar cell element  102 . 
         [0054]    As shown in  FIG. 1B , the solar cell element  102  comprises an n-type GaAs layer  104 , a p-type GaAs layer  103 , a p-type window layer  105 , an n-side electrode  110 , and a p-side electrode  109 . The n-type GaAs layer  104 , the p-type GaAs layer  103 , and the p-type window layer  105  are stacked. A Z-direction denotes a stacking direction. Along the Z-direction, the p-type GaAs layer  103  is interposed between the n-type GaAs layer  104  and the p-type window layer  105 . 
         [0055]    The p-side electrode  109  is electrically connected with the p-type GaAs layer  103 . The n-side electrode  110  is electrically connected with the n-type GaAs layer  104 . 
         [0056]    It is preferable that an n-type barrier layer  106  and an n-type contact layer  108  are interposed between the n-type GaAs layer  104  and the n-side electrode  110  along the Z-direction. Along the Z-direction, the n-type barrier layer  106  is interposed between the n-type GaAs layer  104  and the n-type contact layer  108 . Along the Z-direction, the n-type contact layer  108  is interposed between the n-type barrier layer  106  and the n-side electrode  110 . 
         [0057]    Along the Z-direction, it is preferable that a p-type contact layer  107  is interposed between the p-type window layer  105  and the p-side electrode  109 . The p-side electrode  109 , the p-type contact layer  107 , the p-type window layer  105 , the p-type GaAs layer  103 , the n-type GaAs layer  104 , the n-type barrier layer  106 , the n-type contact layer  108 , and the n-side electrode  110  are electrically connected in this order. 
         [0058]    As shown in  FIG. 1B , the n-type GaAs layer  104  is divided into a center part  104   a , a first peripheral part  104   b , and a second peripheral part  104   c . The center part  104   a  is interposed between the first peripheral part  104   b  and the second peripheral part  104   c  along an X-direction. The X-direction is orthogonal to the Z-direction. 
         [0059]    As shown in  FIG. 2 , the thickness d 1  of the center part  104   a  is greater than the thickness d 2  of the first peripheral part  104   b  and than the thickness d 3  of the second peripheral part  104   c . When the thickness d 1  is the same as the thickness d 2  and the thickness d 3 , the higher conversion efficiency is not achieved (see the comparative examples 1 and 3, which are described later). 
         [0060]    In the embodiment 1, the thickness d 2  is not less than 1 nanometer and not more than 4 nanometers. When the thickness d 2  is less than 1 nanometer, the higher conversion efficiency is not achieved (see the comparative example 10, which is described later). When the thickness d 2  is more than 4 nanometers, the higher conversion efficiency is not achieved (see the comparative examples 7 to 9, which are described later). Similarly, the thickness d 3  is also not less than 1 nanometer and not more than 4 nanometers. 
         [0061]    The first peripheral part  104   b  has a shape of a layer. As shown in  FIG. 6A  and  FIG. 6B , the first peripheral part  104   b  must not have a shape of a taper. This is because the higher conversion efficiency is not achieved (see the comparative examples 4 and 5, which are described later). Similarly, the second peripheral part  104   c  also has a shape of a layer. 
         [0062]    As shown in  FIG. 2 , the center part  104   a  has a width of w 1 . The first peripheral part  104   b  has a width of w 2 . The second peripheral part  104   c  has a width of w 3 . 
         [0063]    The value of w 2  is 0.1 micrometer or more. When the value of w 2  is less than 0.1 micrometer, the conversion efficiency is decreased. For the same reason, the value of w 3  is 0.1 micrometer or more. See the examples 4 and 5 and the comparative example 10, which are described later. 
         [0064]    Accordingly, the following inequation set (I) is required to be satisfied in the embodiment 1. 
         [0000]      d2&lt;d1,d3&lt;d1,1 nanometer≦d2≦4 nanometers,1 nanometer≦d3≦4 nanometers,100 nanometers≦w2,and 100 nanometers≦w3  (I)
 
         [0065]    As described above, the value of d 1  represents a thickness of the center part  104   a  along the Z-direction. 
         [0066]    The value of d 2  represents a thickness of the first peripheral part  104   b  along the Z-direction. 
         [0067]    The value of d 3  represents a thickness of the second peripheral part  104   c  along the Z-direction. 
         [0068]    The value of w 2  represents a width of the first peripheral part  104   b  along the X-direction. 
         [0069]    The value of w 3  represents a width of the second peripheral part  104   c  along the X-direction. 
         [0070]    The obverse surface of the condensing lens  101  is irradiated with light. This is described in more detail in the step (b), which is described later. Sunlight is preferred. 
         [0071]    The reverse surface of the condensing lens  101  is preferably in contact with the solar cell element  102 . The light is focused onto the p-type window layer  105  by the condensing lens  101 . 
         [0072]    It is preferable that the condensing lens  101  has a diameter of 2 millimeters to 10 millimeters, a thickness of 1 millimeter to 5 millimeters, and a refractive index of 1.1 to 2.0. 
         [0073]    The material of the condensing lens  101  is not limited. An example of the material of the condensing lens  101  is glass or resin. 
         [0074]    The p-type window layer  105  is made of a p-type compound semiconductor having a lattice constant close to that of GaAs and having a wider bandgap than GaAs. An example of the material of the p-type window layer  105  is p-type InGaP or p-type AlGaAs. 
         [0075]    The n-type barrier layer  106  is made of an n-type compound semiconductor having a lattice constant close to that of GaAs and having a wider bandgap than GaAs. An example of the material of the n-type barrier layer  106  is n-type InGaP or n-type AlGaAs. 
         [0076]    The material of the p-type contact layer  107  is not limited, as long as ohmic contacts are formed in the interface with the p-type window layer  105  and in the interface with the p-side electrode  109 . An example of the material of the p-type contact layer  107  is p-type GaAs. 
         [0077]    The material of the n-type contact layer  108  is not limited, as long as ohmic contacts are formed in the interface with the n-type barrier layer  106  and in the interface with the n-side electrode  110 . An example of the material of the n-type contact layer  108  is n-type GaAs. 
         [0078]    As shown in  FIG. 1B , the sides of the layers  103  to  108  are preferably covered with an insulating film  111 . An example of the material of the insulating film  111  is non-doped InGaP, silicon dioxide, or silicon nitride. 
         [0079]    When the insulating film  111  is used, as shown in  FIG. 4 , the insulating film  111  is covered with a metal film  118 . The metal film  118  improves the heat radiation property of the solar cell element  102 . 
         [0080]    It is preferred that the metal film  118  is electrically connected with the p-side electrode  109  and that the metal film  118  and the n-side electrode  110  are exposed on one surface (in  FIG. 4 , the bottom surface). 
         [0081]    (Method for Fabricating Solar Cell Element  102 ) 
         [0082]    A method for fabricating a solar cell element  102  is described below with reference to  FIGS. 3A to 3G . 
         [0083]    First, as shown in  FIG. 3A , a sacrificial layer  114 , the p-type contact layer  107 , the p-type window layer  105 , the p-type GaAs layer  103 , the n-type GaAs layer  104 , the n-type barrier layer  106 , and the n-type contact layer  108  are formed in this order on the surface of a GaAs substrate  113  by a known semiconductor growth method such as a molecular beam epitaxy method or a metal organic chemical vapor deposition method (hereinafter, referred to as an “MOCVD method”). The sacrificial layer  114  has a lattice constant close to that of GaAs. The sacrificial layer  114  is a layer for being etched selectively against GaAs. An example of the material of the sacrificial layer  114  is AlAs or InGaP. 
         [0084]    Next, as shown in  FIG. 3B , a first mask  115  is formed on the n-type contact layer  108 . The n-type contact layer  108 , the n-type barrier layer  106 , the n-type GaAs layer  104 , the p-type GaAs layer  103 , the p-type window layer  105 , and the p-type contact layer  107  are etched by dry-etching with use of the first mask  115 . The width of the first mask  115  is equal to the sum of (w 1 +w 2 +w 3 ) shown in  FIG. 2 . In the dry-etching, a mixed gas of BCl 3  and SF 6  may be used. 
         [0085]    As shown in  FIG. 3C , a second mask  116  is formed on the n-type contact layer  108 . The second mask  116  has a smaller width than the first mask  115 . This width of the second mask  116  is the same as the width of w 1  shown in  FIG. 2 . With use of the second mask  116 , the n-type contact layer  108  and the n-type barrier layer  106  are etched. Furthermore, the upper portion of a peripheral part of the n-type GaAs layer  104  is etched. The etching depth of the n-type GaAs layer  104  is equal to the thickness d 1 -d 3  shown in  FIG. 2 . 
         [0086]    As shown in  FIG. 3D , the second mask  116  is removed. The n-side electrode  110  and the insulating film  111  are formed. An example of forming the n-side electrode  110  is a sputtering method or an electron beam deposition technique. An example of forming the insulating film  111  is a chemical vapor deposition method. 
         [0087]    As shown in  FIG. 3E , a base substrate  117  is fixed to the n-side electrode  110 . The GaAs substrate  113  and the sacrificial layer  114  are removed by etching. An example of the base substrate  117  is a silicon substrate or a glass substrate. A wax or an adhesive sheet may be interposed between the n-side electrode  110  and the base substrate  117  optionally. 
         [0088]    As shown in  FIG. 3F , the p-side electrode  109  is formed on the p-type contact layer  107 . Furthermore, a part of the p-type contact layer  107  which is not in contact with the p-side electrode  109  is removed by etching. An example of forming the p-side electrode  109  is a sputtering method or an electron beam deposition technique. 
         [0089]    Finally, as shown in  FIG. 3G , the base substrate  117  is removed. Thus, the solar cell element  102  is obtained. As shown in  FIG. 1A , the obtained solar cell element  102  is fixed to the condensing lens  101 . Thus, the solar cell is obtained. 
       Step (b) 
       [0090]    In the step (b), the p-type window layer  105  is irradiated with the light through the condensing lens  101  to generate a potential difference between the n-side electrode  110  and the p-side electrode  109 . As shown in  FIG. 2 , a region S of the p-type window layer  105  is irradiated with the light. 
         [0091]    The present inventors discovered that the following inequation set (II) is required to be satisfied in the step (b). 
         [0000]      w4≦w1  (II)
 
         [0092]    As described above, the value of w 1  represents the width of the center part  104   a  along the X-direction. 
         [0093]    The value of w 4  represents a width of the region S along the X-direction. 
         [0094]    When seen along the Z-direction, the center part  104   a  overlaps with the region S. 
         [0095]    In the case where the inequation set (II) is not satisfied, the higher conversion efficiency is not achieved (see the comparative example 4). 
         [0096]    As shown in  FIG. 2 , when the n-type GaAs layer  104  has the same width as the p-type window layer  105 , the width of w 1  is equal to or greater than the width of w 4 . Specifically, if the following equation: (w 1 +w 2 +w 3 )=(w 4 +w 5 +w 6 ) is satisfied, the width of w 5  is equal to or greater than the width of w 2 , and the width of w 6  is equal to or greater than the width of w 3 . Both of w 5  and w 6  correspond to the part which is not irradiated with the light. 
       EXAMPLES 
       [0097]    The present invention is described in more detail by the following examples. 
       Example 1 
       [0098]    In the example 1, the solar cell element  102  shown in  FIG. 2  was fabricated by the method shown in  FIGS. 3A to 3G . 
         [0099]    Table 1 shows the composition and the thickness of each layer in the solar cell element  102  according to the example 1. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Dopant 
                 Thickness 
               
             
          
           
               
                 Layers 
                 Composition 
                 Element 
                 Concentration 
                 (micrometer) 
               
               
                   
               
             
          
           
               
                 n-type contact 
                 GaAs 
                 Te 
                 2.0 × 10 19   
                 0.02 
               
               
                 layer 108 
               
               
                 n-type barrier 
                 InGaP 
                 Si 
                 3.0 × 10 17   
                 0.1 
               
               
                 layer 106 
               
               
                 n-type GaAs 
                 GaAs 
                 Si 
                 1.0 × 10 18   
                 2.5 
               
               
                 layer 104 
               
               
                 p-type GaAs 
                 GaAs 
                 Zn 
                 3.0 × 10 17   
                 0.5 
               
               
                 layer 103 
               
               
                 p-type window 
                 InGaP 
                 Zn 
                 3.0 × 10 17   
                 0.1 
               
               
                 layer 105 
               
               
                 p-type contact 
                 GaAs 
                 Zn 
                 1.0 × 10 19   
                 0.02 
               
               
                 layer 107 
               
               
                 Sacrificial 
                 AlAs 
                 — 
                 — 
                 0.1 
               
               
                 layer 114 
               
               
                 Substrate 113 
                 GaAs 
                 — 
                 — 
                 500 
               
               
                   
               
             
          
         
       
     
         [0100]    In the example 1, d 1  to d 3  and w 1  to w 3  were described as below. 
         [0101]    d 1 : 2.5 micrometers 
         [0102]    d 2 : 4 nanometers 
         [0103]    d 3 : 4 nanometers 
         [0104]    w 1 : 90 micrometers 
         [0105]    w 2 : 5 micrometers 
         [0106]    w 3 : 5 micrometers 
         [0107]    The condensing lens  101  was 4 millimeters square and had a thickness of 3 mm. The condensing lens  101  had a focus spot of 80 micrometers square. 
         [0108]    The solar cell according to the example 1 was fabricated as below. 
         [0109]    First, as shown in  FIG. 3A , the layers  104  to  114  shown in Table 1 were grown on the GaAs substrate  113  by an MOCVD method. 
         [0110]    Next, as shown in  FIG. 3B , a square resist film  115  having 100 micrometers square was formed on the n-type contact layer  108  by photolithography. Using this resist film  115  as a first mask, the n-type contact layer  108 , the n-type barrier layer  106 , the n-type GaAs layer  104 , the p-type GaAs layer  103 , the p-type window layer  105 , and the p-type contact layer  107  were removed by ICP plasma etching with use of a mixed gas of BCl 3  and SF 6 . Thus, a pattern having 100 micrometers square was formed. 
         [0111]    After etching, the first mask was removed with a resist stripper liquid. After removed, a square resist film  116  having 90 micrometers square was formed on the n-type contact layer  108 . The center of the resist film  116  corresponded with the center of the resist film  115 . 
         [0112]    Using this resist film  116  as a second mask, the n-type contact layer  108  and the n-type barrier layer  106  were etched. Furthermore, as shown in  FIG. 3C , almost all of the peripheral part of the n-type GaAs layer  104  was etched in such a manner that the peripheral part of the n-type GaAs layer was left slightly. A mixed solution of phosphoric acid and hydrogen peroxide was used to etch the n-type contact layer  108  and the n-type GaAs layer  104 . Hydrochloric acid was used to etch the n-type barrier layer  106 . 
         [0113]    After etching, the thickness of the remaining peripheral part of the n-type GaAs layer  104  was measured with a transmission electron microscope. The thickness was 4 nanometers. 
         [0114]    The second mask was removed with a detachment liquid. After removed, as shown in  FIG. 3D , a titanium film with a thickness of 50 nanometers and a gold film with a thickness of 250 nanometers were stacked on the n-type contact layer  108  to form the n-side electrode  110  with use of an electron beam deposition device. 
         [0115]    Next, as shown in  FIG. 3D , an insulating film  111  made of SiN with a thickness of 400 nanometers was formed with use of a plasma chemical vapor deposition device. 
         [0116]    Next, wax was applied with a spin coater to the surface where the n-side electrode  110  was formed. After the wax was dried, as shown in FIG.  3 E, the n-side electrode  110  was fixed to the base substrate  117  made of glass. 
         [0117]    After fixed, the GaAs substrate  113  was removed with use of a mixture of citric acid and hydrogen peroxide. Subsequently, the sacrificial layer  114  was removed with use of buffered hydrofluoric acid to expose the p-type contact layer  107 . Thus, the structure shown in  FIG. 3E  was obtained. 
         [0118]    As shown in  FIG. 3F , a titanium film having a thickness of 50 nanometers, a platinum film having a thickness of 150 nanometers, and a gold film having a thickness of 250 nanometers were formed in this order on the p-type contact layer  107  to form the p-side electrode  109  with use of an electron beam deposition device. 
         [0119]    After the p-side electrode  109  was formed, the wax was dissolved with isopropanol to remove the base substrate  117 . Thus, the solar cell element  102  shown in  FIG. 3G  was obtained. 
         [0120]    The obtained solar cell element  102  was attached to the condensing lens  101  in such a manner that the center of the focus position of the condensing lens  101  corresponded with the center of the solar cell element  102 . In this manner, the solar cell according to the example 1 was obtained. 
         [0121]    The solar cell according to the example 1 was irradiated with sunlight under the condition that w 4 =90 micrometers and w 5 =w 6 =5 micrometers. The volt-ampere characteristics of the solar cell according to the example 1 were measured, and the conversion efficiency was calculated. Table 2 shows them with the data of the examples 2 to 8 and the comparative examples 1 to 14. 
         [0122]    The conversion efficiency was calculated according to the following equation (I): 
         [0000]      (Conversion efficiency)=(Maximum output value from the solar cell)/(Energy of the sunlight)  (Equation I)
 
         [0123]    The maximum output value described in the above-mentioned equation (I) denotes the maximum value of the output value defined by the following equation (II): 
         [0000]      (Output value)=(Current density obtained from the solar cell)·(Bias voltage obtained from the solar cell)
 
         [0124]    For more detail, see the pages 11 to 13 disclosed in Non-Patent Literature 1, such as Jenny Nelson, “The Physics of Solar Cells”, World Scientific Pub. Co. Inc. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 w2 
                   
                 d2 
                   
                 w5 
                 Conversion 
               
               
                   
                 w1 
                 (=w3) 
                 d1 
                 (=d3) 
                 w4 
                 (=w6) 
                 Efficiency 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                 90 
                 5 
                 2.5 
                 0.004 
                 90 
                 5 
                 25.14 
               
               
                 Example 2 
                 90 
                 5 
                 2.5 
                 0.002 
                 90 
                 5 
                 25.18 
               
               
                 Example 3 
                 90 
                 5 
                 2.5 
                 0.001 
                 90 
                 5 
                 25.2 
               
               
                 Example 4 
                 99.8 
                 0.1 
                 2.5 
                 0.004 
                 90 
                 5 
                 24.85 
               
               
                 Example 5 
                 99 
                 0.5 
                 2.5 
                 0.004 
                 90 
                 5 
                 24.93 
               
               
                 Example 6 
                 90 
                 5 
                 2.5 
                 0.004 
                 86 
                 7 
                 25.12 
               
               
                 Example 7 
                 80 
                 10 
                 2.5 
                 0.004 
                 80 
                 10 
                 24.72 
               
               
                 Example 8 
                 80 
                 10 
                 2.5 
                 0.004 
                 76 
                 12 
                 24.55 
               
               
                 Comparative 
                 90 
                 5 
                 2.5 
                 2.5 
                 100 
                 0 
                 20.65 
               
               
                 Example 1 
               
               
                 Comparative 
                 90 
                 5 
                 2.5 
                 2.5 
                 90 
                 5 
                 22.86 
               
               
                 Example 2 
               
               
                 Comparative 
                 90 
                 5 
                 2.5 
                 0.004 
                 100 
                 0 
                 19.86 
               
               
                 Example 3 
               
               
                 Comparative 
                 — 
                 — 
                 — 
                 — 
                 90 
                 5 
                 22.18 
               
               
                 Example 4 
               
               
                 Comparative 
                 — 
                 — 
                 — 
                 — 
                 90 
                 5 
                 22.42 
               
               
                 Example 5 
               
               
                 Comparative 
                 90 
                 5 
                 2.5 
                 0.1 
                 90 
                 5 
                 22.89 
               
               
                 Example 6 
               
               
                 Comparative 
                 90 
                 5 
                 2.5 
                 0.01 
                 90 
                 5 
                 23.07 
               
               
                 Example 7 
               
               
                 Comparative 
                 90 
                 5 
                 2.5 
                 0.005 
                 90 
                 5 
                 23.75 
               
               
                 Example 8 
               
               
                 Comparative 
                 90 
                 5 
                 2.5 
                 0 
                 90 
                 5 
                 23.08 
               
               
                 Example 9 
               
               
                 Comparative 
                 99.9 
                 0.05 
                 2.5 
                 0.004 
                 90 
                 5 
                 22.84 
               
               
                 Example 10 
               
               
                 Comparative 
                 90 
                 5 
                 2.5 
                 0.004 
                 98 
                 1 
                 20.98 
               
               
                 Example 11 
               
               
                 Comparative 
                 90 
                 5 
                 2.5 
                 0.004 
                 94 
                 3 
                 23.36 
               
               
                 Example 12 
               
               
                 Comparative 
                 80 
                 10 
                 2.5 
                 0.004 
                 88 
                 6 
                 19.4 
               
               
                 Example 13 
               
               
                 Comparative 
                 80 
                 10 
                 2.5 
                 0.004 
                 84 
                 8 
                 22.47 
               
               
                 Example 14 
               
               
                   
               
             
          
         
       
     
       Example 2 
       [0125]    The experiment identical to that of the example 1 was performed except that d 2 =2 nanometers. 
       Example 3 
       [0126]    The experiment identical to that of the example 1 was performed except that d 2 =1 nanometer. 
       Example 4 
       [0127]    The experiment identical to that of the example 1 was performed except that w 1 =99.8 micrometers and w 2 =w 3 =0.1 micrometer. 
       Example 5 
       [0128]    The experiment identical to that of the example 1 was performed except that w 1 =99 micrometers and w 2 =w 3 =0.5 micrometers. 
       Example 6 
       [0129]    The experiment identical to that of the example 1 was performed except that w 4 =86 micrometers and w 5 =w 6 =7 micrometers. 
       Example 7 
       [0130]    The experiment identical to that of the example 1 was performed except that w 1 =80 micrometers, w 2 =w 3 =10 micrometers, w 4 =80 micrometers, and w 5 =w 6 =10 micrometers. 
       Example 8 
       [0131]    The experiment identical to that of the example 1 was performed except that w 1 =80 micrometers, w 2 =w 3 =10 micrometers, w 4 =76 micrometers, and w 5 =w 6 =12 micrometers. 
       Comparative Example 1 
       [0132]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =2.5 micrometers and w 4 =100 micrometers. 
       Comparative Example 2 
       [0133]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =2.5 micrometers. 
       Comparative Example 3 
       [0134]    The experiment identical to that of the example 1 was performed except that w 4 =100 micrometers. 
       Comparative Example 4 
       [0135]    The experiment identical to that of the example 1 was performed except that the p-type GaAs layer  103  was formed by a wet-etching technique, instead of the ICP plasma etching, which is a dry etching, so as to obtain the solar cell shown in  FIG. 6A . 
       Comparative Example 5 
       [0136]    The experiment identical to that of the example 1 was performed except that the p-type GaAs layer  103  and the n-type GaAs layer  104  were formed by a wet-etching technique to obtain the solar cell shown in  FIG. 6B . 
       Comparative Example 6 
       [0137]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0.1 micrometers. 
       Comparative Example 7 
       [0138]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0.01 micrometers. 
       Comparative Example 8 
       [0139]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0.005 micrometers. 
       Comparative Example 9 
       [0140]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0 micrometers. 
       Comparative Example 10 
       [0141]    The experiment identical to that of the example 1 was performed except that w 1 =99.9 micrometers and w 2 =w 3 =0.05 micrometers. 
       Comparative Example 11 
       [0142]    The experiment identical to that of the example 1 was performed except that w 4 =98 micrometers and w 5 =w 6 =1 micrometer. 
       Comparative Example 12 
       [0143]    The experiment identical to that of the example 1 was performed except that w 4 =94 micrometers and w 5 =w 6 =3 micrometers. 
       Comparative Example 13 
       [0144]    The experiment identical to that of the example 1 was performed except that w 1 =80 micrometers, w 2 =w 3 =10 micrometers, w 4 =88 micrometers, and w 5 =w 6 =6 micrometers. 
       Comparative Example 14 
       [0145]    The experiment identical to that of the example 1 was performed except that w 1 =80 micrometers, w 2 =w 3 =10 micrometers, w 4 =84 micrometers, and w 5 =w 6 =8 micrometers. 
         [0146]    As is clear from Table 2, when the following inequation set: d 2 &lt;d 1 , d 3 &lt;d 1 , 1 nanometer≦d 2 ≦4 nanometers, 1 nanometer≦d 3 ≦4 nanometers, 100 nanometers≦w 2 , 100 nannometers≦w 3 , and w 4 ≦w 1  is satisfied, a high conversion efficiency of 24% or more is achieved. 
         [0147]    The examples 1 to 8 and the comparative examples 1 and 2 show that it is necessary that the following inequation set: d 2 &lt;d 1  and d 3 &lt;d 1  is satisfied. 
         [0148]    The examples 1 to 3 and the comparative examples 6 to 9 show that it is necessary that the following inequation set: 1 nanometer≦d 2 ≦4 nanometers and 1 nanometer≦d 3 ≦4 nanometers is satisfied. 
         [0149]    The examples 4 and 5 and the comparative example 10 show that it is necessary that the following inequation set: 100 nanometers≦w 2  and 100 nannometers≦w 3  is satisfied. 
         [0150]    The examples 1, 6 to 8 and the comparative examples 11 to 14 show that it is necessary that the following inequation: w 4 ≦w 1  is satisfied. 
       INDUSTRIAL APPLICABILITY 
       [0151]    The present invention provides a solar cell with higher conversion efficiency. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           101  lens 
           102  solar cell element 
           103  p-type GaAs layer 
           104  n-type GaAs layer 
           104   a  center part 
           104   b  first peripheral part 
           104   c  second peripheral part 
           105  p-type window layer 
           106  n-type barrier layer 
           107  p-type contact layer 
           108  n-type contact layer 
           109  p-side electrode 
           110  n-side electrode 
           111  insulating film 
           112  sunlight 
           113  substrate 
           114  sacrificial layer 
           115  first mask 
           116  second mask 
           117  base substrate 
           118  metal film