Abstract:
A method for generating electric power including the steps of: (a) preparing a solar cell having a condensing lens and a solar cell element, wherein the solar cell element includes an n-type GaAs layer, a p-type GaAs layer, a quantum tunneling layer, an n-type InGaP layer, a p-type InGaP layer, a p-type window layer, an n-side electrode, and a p-side electrode, and satisfies the following equation (I): d 2 &lt;d 1 , d 3 &lt;d 1 , 1 nanometer≦d 2 ≦4 nanometers, 1 nanometer≦d 3 ≦4 nanometers, d 5 &lt;d 4 , d 6 &lt;d 4 , 1 nanometer≦d 5 ≦5 nanometers, 1 nanometer≦d 6 ≦5 nanometers, 100 nanometers≦w 2 , 100 nanometers≦w 3 , 100 nanometers≦w 4 , and 100 nanometers≦w 5  . . . (I); and (b) irradiating a region S which is included in the surface of the p-type window layer through the condensing lens with light to satisfy the following equation (II) in order to generate a potential difference between the n-side electrode and the p-side electrode: w 6 ≦w 1  . . . (II).

Description:
[0001]    This is a continuation of International Application No. PCT/JP2011/006974, with an international filing date of Dec. 14, 2011, which claims priority of Japanese Patent Application No. 2011-099178, filed on Apr. 27, 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. 6  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. 
         [0005]      FIG. 7  shows a solar cell disclosed in Patent Literature 2. The conventional solar cell comprises a top cell  1  made of InGaP and a bottom cell  2  made of GaAs. The top cell  1  and the bottom cell  2  are joined electrically through a quantum tunneling layer  3 . The top cell  1  absorbs light having a wavelength different from the wavelength of light absorbed by the bottom cell  2  so as to cause the solar cell to generate electric power efficiently. 
       CITATION LIST 
     Patent Literatures 
       [0000]    
       
         [Patent Literature 1] 
         Japanese Laid-Open Patent Application Publication No. 2008-124381 
         [Patent Literature 2] 
         Japanese Laid-Open Patent Application Publication No. H 9-64386 
       
     
       Non Patent Literature 
       [0000]    
       
         [Non Patent Literature 1] 
         Jenny Nelson, “The Physics of Solar Cells”, World Scientific Pub Co Inc. 
       
     
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0012]    According to the experiment performed by the present inventors, the solar cell obtained by combining the solar cell element disclosed in Patent Literature 2 with the lens disclosed in Patent Literature 1 has a conversion efficiency of approximately 25%. 
         [0013]    The purpose of the present invention is to provide a solar cell having higher conversion efficiency. 
       Solution to Problems 
       [0014]    The present disclosure is directed to a method for generating electric power with use of a solar cell, the method comprising steps of; 
         [0015]    (a) preparing the solar cell comprising a condensing lens ( 101 ) and a solar cell element ( 102 ), wherein 
         [0016]    the solar cell element ( 102 ) comprises an n-type GaAs layer ( 104 ), a p-type GaAs layer ( 103 ), a quantum tunneling layer ( 108 ), an n-type InGaP layer ( 106 ), a p-type InGaP layer ( 105 ), a p-type window layer ( 107 ), an n-side electrode ( 114 ), and a p-side electrode ( 115 ); 
         [0017]    a Z-direction denotes the direction of the normal line of the p-type GaAs layer ( 103 ); 
         [0018]    an X-direction denotes a direction orthogonal to the Z-direction, 
         [0019]    the n-type GaAs layer ( 104 ), the p-type GaAs layer ( 103 ), the quantum tunneling layer ( 108 ), the n-type InGaP layer ( 106 ), the p-type InGaP layer ( 105 ), and the p-type window layer ( 107 ) are stacked along the Z-direction in this order; 
         [0020]    the p-type window layer ( 107 ) is made of a p-type compound semiconductor having a wider bandgap than InGaP, 
         [0021]    the n-side electrode ( 114 ) is electrically connected with the n-type GaAs layer ( 104 ); 
         [0022]    the p-side electrode ( 115 ) is electrically connected with the p-type InGaP layer ( 105 ); 
         [0023]    the n-type GaAs layer ( 104 ) is divided into a GaAs center part ( 104   a ), a first GaAs peripheral part ( 104   b ), and a second GaAs peripheral part ( 104   c ); 
         [0024]    the GaAs center part ( 104   a ) is interposed between the first GaAs peripheral part ( 104   b ) and the second GaAs peripheral part ( 104   c ) along the X-direction; 
         [0025]    the first GaAs peripheral part ( 104   b ) and the second GaAs peripheral part ( 104   c ) have a shape of a layer, 
         [0026]    the n-type InGaP layer ( 106 ) is divided into an InGaP center part ( 106   a ), a first InGaP peripheral part ( 106   b ), and a second InGaP peripheral part ( 106   c ); 
         [0027]    the InGaP center part ( 106   a ) is interposed between the first InGaP peripheral part ( 106   b ) and the second InGaP peripheral part ( 106   c ) along the X-direction; 
         [0028]    the first InGaP peripheral part ( 106   b ) and the second InGaP peripheral part ( 106   c ) have a shape of a layer, 
         [0029]    the following inequation set (I) is satisfied: 
         [0000]        d 2 &lt;d 1 ,d 3 &lt;d 1,1 nanometer≦ d 2≦4 nanometers,1 nanometer≦ d 3≦4 nanometers, d 5 &lt;d 4 ,d 6 &lt;d 4,1 nanometer≦ d 5≦5 nanometers,1 nanometer≦ d 6≦5 nanometers,100 nanometers≦ w 2,100 nanometers≦ w 3,100 nanometers≦ w 4 ,and  100 nanometers≦ w 5  (I);
 
         [0030]    wherein d 1  represents a thickness of the GaAs center part ( 104   a ) along the Z-direction; 
         [0031]    d 2  represents a thickness of the first GaAs peripheral part ( 104   b ) along the Z-direction; 
         [0032]    d 3  represents a thickness of the second GaAs peripheral part ( 104   c ) along the Z-direction; 
         [0033]    d 4  represents a thickness of the InGaP center part ( 106   a ) along the Z-direction; 
         [0034]    d 5  represents a thickness of the first InGaP peripheral part ( 106   b ) along the Z-direction; 
         [0035]    d 6  represents a thickness of the second InGaP peripheral part ( 106   c ) along the Z-direction; 
         [0036]    w 2  represents a width of the first GaAs peripheral part ( 104   b ) along the X-direction; 
         [0037]    w 3  represents a width of the second GaAs peripheral part ( 104   c ) along the X-direction; 
         [0038]    w 4  represents a width of the first InGaP peripheral part ( 106   b ) along the X-direction; and 
         [0039]    w 5  represents a width of the second InGaP peripheral part ( 106   c ) along the X-direction; and 
         [0040]    (b) irradiating a region S which is included in the surface of the p-type window layer ( 107 ) 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 ( 114 ) and the p-side electrode ( 115 ): 
         [0000]        w 6 ≦w 1  (II);
 
         [0041]    wherein w 1  represents a width of the GaAs center part ( 104   a ) along the X-direction; 
         [0042]    w 6  represents a width of the region S along the X-direction in the cross-sectional view which includes the Z-direction; and 
         [0043]    the first GaAs center part ( 104   a ) overlaps the region (S) when seen from the Z-direction. 
       Advantageous Effect of the Invention 
       [0044]    The present invention provides a solar cell having higher conversion efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]      FIG. 1A  shows a cross-sectional view of the solar cell according to the embodiment 1. 
           [0046]      FIG. 1B  shows a cross-sectional view of the solar cell element according to the embodiment 1. 
           [0047]      FIG. 2  shows a cross-sectional exploded view of the solar cell element according to the embodiment 1. 
           [0048]      FIG. 3A  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0049]      FIG. 3B  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0050]      FIG. 3C  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0051]      FIG. 3D  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0052]      FIG. 3E  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0053]      FIG. 3F  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0054]      FIG. 3G  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0055]      FIG. 3H  shows a fabricating step of the solar cell element according to the embodiment 1. 
           [0056]      FIG. 4  shows a cross-sectional view of the solar cell according to the embodiment 1. 
           [0057]      FIG. 5  shows a cross-sectional view of the solar cell element according to the comparative example 1. 
           [0058]      FIG. 6  shows a cross-sectional view of the solar cell disclosed in Patent Literature 1. 
           [0059]      FIG. 7  shows a cross-sectional view of the solar cell element disclosed in Patent Literature 2. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0060]    The exemplary embodiment of the present invention is described below with reference to drawings. 
       Embodiment 1 
       [0061]    (Step (a)) 
         [0062]    In the step (a), a solar cell is prepared. 
         [0063]      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 . 
         [0064]    As shown in  FIG. 1B , the solar cell element  102  comprises an n-type GaAs layer  104 , a p-type GaAs layer  103 , a quantum tunneling layer  108 , an n-type InGaP layer  106 , a p-type InGaP layer  105 , a p-type window layer  107 , an n-side electrode  114 , and a p-side electrode  115 . 
         [0065]    The n-type GaAs layer  104  and the p-type GaAs layer  103  are stacked. The n-type InGaP layer  106  and the p-type InGaP layer  105  are stacked. A Z-direction denotes a stacking direction. Along the Z-direction, the quantum tunneling layer  108  is interposed between the p-type GaAs layer  103  and the n-type InGaP layer  106 . 
         [0066]    The p-side electrode  115  is electrically connected with the p-type InGaP layer  105 . The n-side electrode  114  is electrically connected with the n-type GaAs layer  104 . 
         [0067]    It is preferable that a first n-type barrier layer  109  and an n-type contact layer  112  are interposed between the n-type GaAs layer  104  and the n-side electrode  114  along the Z-direction. Along the Z-direction, the first n-type barrier layer  109  is interposed between the n-type GaAs layer  104  and the n-type contact layer  112 . Along the Z-direction, the n-type contact layer  112  is interposed between the first n-type barrier layer  109  and the n-side electrode  114 . 
         [0068]    It is preferable that a p-type barrier layer  110  is interposed between the p-type GaAs layer  103  and the quantum tunneling layer  108  along the Z-direction. Along the Z-direction, a second n-type barrier layer  111  is preferably interposed between the n-type InGaP layer  106  and the quantum tunneling layer  108 . 
         [0069]    Along the Z-direction, it is preferable that a p-type contact layer  113  is interposed between the p-type window layer  107  and the p-side electrode  115 . The p-side electrode  115 , the p-type contact layer  113 , the p-type window layer  107 , the p-type InGaP layer  105 , the n-type InGaP layer  106 , the second n-type barrier layer  111 , the quantum tunneling layer  108 , the p-type barrier layer  110 , the p-type GaAs layer  103 , the n-type GaAs layer  104 , the first n-type barrier layer  109 , the n-type contact layer  112 , and the n-side electrode  114  are electrically connected in series in this order. 
         [0070]    As shown in  FIG. 1B , the n-type GaAs layer  104  is divided into a GaAs center part  104   a , a first GaAs peripheral part  104   b , and a second GaAs peripheral part  104   c . The GaAs center part  104   a  is interposed between the first GaAs peripheral part  104   b  and the second GaAs peripheral part  104   c  along the X-direction. The X-direction is orthogonal to the Z-direction. 
         [0071]    As shown in  FIG. 1B , the n-type InGaP layer  106  is divided into a InGaP center part  106   a , a first InGaP peripheral part  106   b , and a second InGaP peripheral part  106   c . The InGaP center part  106   a  is interposed between the first InGaP peripheral part  106   b  and the second InGaP peripheral part  106   c  along the X-direction. 
         [0072]    As shown in  FIG. 2 , the thickness d 1  of the GaAs center part  104   a  is greater than the thickness d 2  of the first GaAs peripheral part  104   b  and than the thickness d 3  of the second GaAs 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 2, which are described later). 
         [0073]    As shown in  FIG. 2 , the thickness d 4  of the InGaP center part  106   a  is greater than the thickness d 5  of the first InGaP peripheral part  106   b  and than the thickness d 6  of the second InGaP peripheral part  106   c . When the thickness d 4  is the same as the thickness d 5  and the thickness d 6 , the higher conversion efficiency is not achieved (see the comparative examples 1 and 2, which are described later). 
         [0074]    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 7, 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 4 to 6, which are described later). Similarly, the thickness d 3  is also not less than 1 nanometer and not more than 4 nanometers. 
         [0075]    In the embodiment 1, the thickness d 5  is not less than 1 nanometer and not more than 5 nanometers. When the thickness d 5  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 5  is more than 5 nanometers, the higher conversion efficiency is not achieved (see the comparative examples 8 and 9, which are described later). Similarly, the thickness d 6  is also not less than 1 nanometer and not more than 5 nanometers. 
         [0076]    As shown in  FIG. 2 , the GaAs center part  104   a  has a width of w 1 . The first GaAs peripheral part  104   b  has a width of w 2 . The second GaAs peripheral part  104   c  has a width of w 3 . 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. See the comparative example 11, which is described later. For the same reason, the value of w 3  is 0.1 micrometer or more. 
         [0077]    As shown in  FIG. 2 , the first InGaP peripheral part  106   b  has a width of w 4 . The second InGaP peripheral part  106   c  has a width of w 5 . The value of w 4  is 0.1 micrometer or more. When the value of w 4  is less than 0.1 micrometer, the conversion efficiency is decreased. See the comparative example 12, which is described later. For the same reason, the value of w 5  is 0.1 micrometer or more. 
         [0078]    Accordingly, the following inequation set (I) is required to be satisfied in the embodiment 1. 
         [0000]        d 2 &lt;d 1 ,d 3 &lt;d 1,1 nanometer≦ d 2≦4 nanometers,1 nanometer≦ d 3≦4 nanometers, d 5 &lt;d 4 ,d 6 &lt;d 4,1 nanometer≦ d 5≦5 nanometers,1 nanometer≦ d 6≦5 nanometers,100 nanometers≦ w 2,100 nanometers≦ w 3,100 nanometers≦ w 4 , and  100 nanometers≦ w 5  (I)
 
         [0079]    As described above, the value of d 1  represents a thickness of the GaAs center part  104   a  along the Z-direction. 
         [0080]    The value of d 2  represents a thickness of the first GaAs peripheral part  104   b  along the Z-direction. 
         [0081]    The value of d 3  represents a thickness of the second GaAs peripheral part  104   c  along the Z-direction. 
         [0082]    The value of d 4  represents a thickness of the InGaP center part  106   a  along the Z-direction. 
         [0083]    The value of d 5  represents a thickness of the first InGaP peripheral part  106   b  along the Z-direction. 
         [0084]    The value of d 6  represents a thickness of the second InGaP peripheral part  106   c  along the Z-direction. 
         [0085]    The value of w 2  represents a width of the first GaAs peripheral part  104   b  along the X-direction. 
         [0086]    The value of w 3  represents a width of the second GaAs peripheral part  104   c  along the X-direction. 
         [0087]    The value of w 4  represents a width of the first InGaP peripheral part  106   b  along the X-direction. 
         [0088]    The value of w 5  represents a width of the second InGaP peripheral part  106   c  along the X-direction. 
         [0089]    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. 
         [0090]    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  107  by the condensing lens  101 . 
         [0091]    It is preferable that the condensing lens  101  has a diameter of approximately 2 millimeters to 10 millimeters, a thickness of approximately 1 millimeter to 5 millimeters, and a refractive index of approximately 1.1 to 2.0. 
         [0092]    The material of the condensing lens  101  is not limited. An example of the material of the condensing lens  101  is glass or resin. 
         [0093]    The p-type window layer  107  is made of a p-type compound semiconductor having a lattice constant close to that of InGaP and having a wider bandgap than InGaP. An example of the material of the p-type window layer  107  is p-type InAlGaP or p-type InAlAs. 
         [0094]    The first n-type barrier layer  109  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 first n-type barrier layer  109  is n-type InGaP or n-type AlGaAs. 
         [0095]    The second n-type barrier layer  111  is made of an n-type compound semiconductor having a lattice constant close to that of InGaP and having a wider bandgap than InGaP. An example of the material of the second n-type barrier layer  111  is n-type InAlGaP or n-type InAIP. 
         [0096]    The p-type barrier layer  110  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 barrier layer  110  is p-type InGaP or p-type AlGaAs. 
         [0097]    The quantum tunneling layer  108  is composed of a p-type semiconductor layer and an n-type semiconductor layer. A p-n junction is formed between the p-type semiconductor layer and the n-type semiconductor layer. The p-type semiconductor layer is doped at a high concentration. The n-type semiconductor layer is also doped at a high concentration. These p-type and n-type semiconductor layers are stacked. The material of the p-type semiconductor layer and the n-type semiconductor layer has a lattice constant close to that of GaAs and InGaP. More particularly, an example of the material of the p-type semiconductor layer and the n-type semiconductor layer is GaAs, InGaP, or AlGaAs. A preferable thickness of the quantum tunneling layer  108  is not less than 20 nanometers and not more than 40 nanometers. 
         [0098]    The material of the p-type contact layer  113  is not limited, as long as ohmic contacts are formed in the interface with the p-type window layer  107  and in the interface with the p-side electrode  115 . An example of the material of the p-type contact layer  113  is p-type GaAs. 
         [0099]    The material of the n-type contact layer  112  is not limited, as long as ohmic contacts are formed in the interface with the first n-type barrier layer  109  and in the interface with the n-side electrode  114 . An example of the material of the n-type contact layer  112  is n-type GaAs. 
         [0100]    As shown in  FIG. 1B , the sides of the layers  103  to  113  are preferably covered with an insulating film  116 . An example of the material of the insulating film  116  is non-doped InGaP, silicon dioxide, or silicon nitride. 
         [0101]    When the insulating film  116  is used, as shown in  FIG. 4 , the insulating film  116  is covered with a metal film  124 . The metal film  124  improves the heat radiation property of the solar cell element  102 . 
         [0102]    It is preferred that the metal film  124  is electrically connected with the p-side electrode  115  and that the metal film  124  and the n-side electrode  114  are exposed on one surface (in  FIG. 4 , the bottom surface). 
         [0103]    (Method for Fabricating Solar Cell Element  102 ) 
         [0104]    A method for fabricating a solar cell element  102  is described below with reference to  FIGS. 3A to 311 . 
         [0105]    First, as shown in  FIG. 3A , a sacrificial layer  119 , the p-type contact layer  113 , the p-type window layer  107 , the p-type InGaP layer  105 , the n-type InGaP layer  106 , the second n-type barrier layer  111 , the quantum tunneling layer  108 , the p-type barrier layer  110 , the p-type GaAs layer  103 , the n-type GaAs layer  104 , the first n-type barrier layer  109 , and the n-type contact layer  112  are formed in this order on the surface of a GaAs substrate  118  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  119  has a lattice constant close to that of GaAs. The sacrificial layer  119  is a layer for being etched selectively against GaAs. An example of the material of the sacrificial layer  119  is AlAs. 
         [0106]    Next, as shown in  FIG. 3B , a first mask  120  is formed on the n-type contact layer  112 . The first mask  120  has a width equal to the value of w 1  shown in  FIG. 2 . The n-type contact layer  112  and the first n-type barrier layer  109  are etched with use of the first mask  120 . Furthermore, the upper peripheral portion of the n-type GaAs layer  104  is etched. The etching depth of the n-type GaAs layer  104  is equal to the thickness of (d 1 -d 3 ) shown in  FIG. 2 . In the etching, a mixed gas of BC 13  and SF 6  may be used. 
         [0107]    As shown in  FIG. 3C , the first mask  120  is removed and a second mask  121  is formed. The width of the second mask  121  is equal to the sum of (w 1 +w 2 +w 3 ) shown in  FIG. 2 . The n-type GaAs layer  104 , the p-type GaAs layer  103 , the p-type barrier layer  110 , the quantum tunneling layer  108 , and the second n-type barrier layer  111  are etched with use of the second mask  121 . Furthermore, the upper peripheral portion of the n-type InGaP layer  106  is etched. The etching depth of the n-type InGaP layer  106  is equal to the thickness of (d 4 −d 5 ) shown in  FIG. 2 . 
         [0108]    As shown in  FIG. 3D , the second mask  121  is removed and a third mask  122  is formed. The width of the third mask  122  is equal to the sum of (w 1 +w 2 +w 3 +w 4 +w 5 ) shown in  FIG. 2 . The n-type InGaP layer  106 , the p-type InGaP layer  105 , the p-type window layer  107 , and the p-type contact layer  113  are etched with use of the third mask  122 . 
         [0109]    As shown in  FIG. 3E , the third mask  122  is removed. The n-side electrode  114  and the insulating film  116  are formed. An example of forming the n-side electrode  114  is a sputtering method or an electron beam deposition technique. An example of forming the insulating film  116  is a chemical vapor deposition method. 
         [0110]    As shown in  FIG. 3F , a base substrate  123  is fixed to the n-side electrode  114 . The GaAs substrate  118  and the sacrificial layer  119  are removed by etching. An example of the base substrate  123  is a silicon substrate or a glass substrate. A wax or an adhesive sheet may be interposed between the n-side electrode  114  and the base substrate  123  optionally. 
         [0111]    As shown in  FIG. 3G , the p-side electrode  115  is formed on the p-type contact layer  113 . Furthermore, a part of the p-type contact layer  113  which is not in contact with the p-side electrode  115  is removed by etching. An example of forming the p-side electrode  115  is a sputtering method or an electron beam deposition technique. 
         [0112]    Finally, as shown in  FIG. 311 , the base substrate  123  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. 
         [0113]    (Step (b)) 
         [0114]    In the step (b), the p-type window layer  107  is irradiated with the light through the condensing lens  101  to generate a potential difference between the n-side electrode  114  and the p-side electrode  115 . As shown in  FIG. 2 , a region S of the p-type window layer  107  is irradiated with the light. 
         [0115]    The present inventors discovered that the following inequation set (II) is required to be satisfied in the step (b). 
         [0000]        w 6 &lt;w 1  (II)
 
         [0116]    As described above, the value of w 1  represents the width of the GaAs center part  104   a  along the X-direction. 
         [0117]    The value of w 6  represents a width of the region S along the X-direction. 
         [0118]    When seen along the Z-direction, the GaAs center part  104   a  overlaps with the region S. 
         [0119]    In the case where the inequation set (II) is not satisfied, the higher conversion efficiency is not achieved (see the comparative example 3 and the comparative examples 13 to 16). 
         [0120]    As shown in  FIG. 2 , when the equation: (w 1 +w 2 +w 3 +w 4 +w 5 )=(w 6 +w 7 +w 8 ) is satisfied, the width of w 7  is equal to or greater than the width of (w 2 +w 4 ). When the equation: (w 1 +w 2 +w 3 +w 4 +w 5 )=(w 6 +w 7 +w 8 ) is satisfied, the width of w 6  is equal to or greater than the width of (w 3 +w 5 ). Both of w 7  and w 8  correspond to the part which is not irradiated with the light. 
       EXAMPLES 
       [0121]    The present invention is described in more detail by the following examples. 
       Example 1 
       [0122]    In the example 1, the solar cell element  102  shown in  FIG. 2  was fabricated by the method shown in  FIGS. 3A to 311 . 
         [0123]    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 layer 112 
                 GaAs 
                 Te 
                 2.0 × 10 19   
                 0.1 
               
               
                 First n-type barrier layer 109 
                 InGaP 
                 Te 
                 1.0 × 10 19   
                 0.05 
               
               
                 n-type GaAs layer 104 
                 GaAs 
                 Si 
                 1.0 × 10 18   
                 2.4 
               
               
                 p-type GaAs layer 103 
                 GaAs 
                 Zn 
                 3.0 × 10 17   
                 0.2 
               
               
                 p-type barrier layer 110 
                 InGaP 
                 Zn 
                 1.0 × 10 19   
                 0.05 
               
               
                 Quantum tunneling layer 108 
                 GaAs 
                 C 
                 1.0 × 10 20   
                 0.012 
               
               
                   
                 AlGaAs 
                 Te 
                 1.0 × 10 19   
                 0.012 
               
               
                 Second n-type barrier layer 111 
                 InAlP 
                 Te 
                 1.0 × 10 19   
                 0.05 
               
               
                 n-type InGaP layer 106 
                 InGaP 
                 Si 
                 1.0 × 10 18   
                 0.9 
               
               
                 p-type InGaP layer 105 
                 InGaP 
                 Zn 
                 3.0 × 10 17   
                 0.1 
               
               
                 p-type window layer 107 
                 InAlGaP 
                 Zn 
                 1.0 × 10 19   
                 0.05 
               
               
                 p-type contact layer 113 
                 GaAs 
                 C 
                 1.0 × 10 19   
                 0.02 
               
               
                 Sacrificial layer 119 
                 AlAs 
                 C 
                 1.0 × 10 19   
                 0.1 
               
               
                 Substrate 118 
                 GaAs 
                 Zn 
                 2.0 × 10 18   
                 350 
               
               
                   
               
             
          
         
       
     
         [0124]    In the example 1, d 1  to d 6  and w 1  to w 5  were described as below. 
         [0125]    d 1 : 2.4 micrometers 
         [0126]    d 2 : 4 nanometers 
         [0127]    d 3 : 4 nanometers 
         [0128]    d 4 : 0.9 micrometer 
         [0129]    d 5 : 4 nanometers 
         [0130]    d 6 : 4 nanometers 
         [0131]    w 1 : 80 micrometers 
         [0132]    w 2 : 5 micrometers 
         [0133]    w 3 : 5 micrometers 
         [0134]    w 4 : 5 micrometers 
         [0135]    w 5 : 5 micrometers 
         [0136]    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. 
         [0137]    The solar cell according to the example 1 was fabricated as below. 
         [0138]    First, as shown in  FIG. 3A , the layers  104  to  119  shown in Table 1 were grown on the non-doped GaAs substrate  118  by an MOCVD method. 
         [0139]    Next, as shown in  FIG. 3B , a square resist film having 80 micrometers square was formed on the n-type contact layer  112  by photolithography. Using this resist film as a first mask  120 , the n-type contact layer  112  and the first n-type barrier layer  109  were removed by ICP plasma etching with use of a mixed gas of BC 13  and SF 6 . Furthermore, 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  104  was left slightly. 
         [0140]    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. 
         [0141]    The first mask  120  was removed with a detachment liquid. After removed, a square resist film having 90 micrometers square was formed as the second mask  121 . The center of the resist film corresponded with the center of the first mask  120 . 
         [0142]    Using the second mask  121 , the n-type GaAs layer  104 , the p-type GaAs layer  103 , the p-type barrier layer  110 , the quantum tunneling layer  108 , and the second n-type barrier layer  111  were etched. Furthermore, as shown in  FIG. 3C , almost all of the peripheral part of the n-type InGaP layer  106  was etched in such a manner that the peripheral part of the n-type InGaP layer  106  was left slightly. 
         [0143]    After etching, the thickness of the remaining peripheral part of the n-type InGaP layer  106  was measured with a transmission electron microscope. The thickness was 4 nanometers. 
         [0144]    The second mask  121  was removed with a detachment liquid. After removed, a square resist film having 100 micrometers square was formed as the third mask  122 . The center of the resist film corresponded with the center of the first mask  120  and the center of the second mask  121 . 
         [0145]    Using the third mask  122 , as shown in  FIG. 3D , the n-type InGaP layer  106 , the p-type InGaP layer  105 , the p-type window layer  107 , and the p-type contact layer  113  were etched so as to expose the sacrificial layer  119 . 
         [0146]    After etching, the third mask  122  was removed with a resist stripper liquid. After removed, as shown in  FIG. 3E , 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  112  to form the n-side electrode  114  with use of an electron beam deposition device. 
         [0147]    Next, as shown in  FIG. 3E , the insulating film  116  made of SiN with a thickness of 400 nanometers was formed with use of a plasma chemical vapor deposition device. 
         [0148]    Next, wax was applied with a spin coater to the surface where the n-side electrode  114  was formed. After the wax was dried, as shown in  FIG. 3F , the n-side electrode  114  was fixed to the base substrate  123  made of glass. 
         [0149]    After fixed, the GaAs substrate  118  was removed with use of a mixture of citric acid and hydrogen peroxide. Subsequently, the sacrificial layer  119  was removed with use of buffered hydrofluoric acid to expose the p-type contact layer  113 . Thus, the structure shown in  FIG. 3F  was obtained. 
         [0150]    As shown in  FIG. 3G , 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  113  to form the p-side electrode  115  with use of an electron beam deposition device. 
         [0151]    After the p-side electrode  115  was formed, the wax was dissolved with isopropanol to remove the base substrate  123 . Thus, the solar cell element  102  shown in  FIG. 311  was obtained. 
         [0152]    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. 
         [0153]    The solar cell according to the example 1 was irradiated with sunlight under the condition that w 6 =80 micrometers and w 7 =w 8 =10 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 13 and the comparative examples 1 to 16. 
         [0154]    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)
 
         [0155]    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)
 
         [0156]    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 
               
               
                   
                   
               
               
                   
                 d1 
                 d2 (=d3) 
                 d4 
                 d5 (=d6) 
                 w1 
                 w2 (=w3) 
                 w4 (=w5) 
                 w6 
                 w7 (=w8) 
                 Conversion 
               
               
                   
                 [um] 
                 [um] 
                 [um] 
                 [um] 
                 [um] 
                 [um] 
                 [um] 
                 [um] 
                 [um] 
                 Efficiency [%] 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 80 
                 10 
                 28.51 
               
               
                 Example 2 
                 2.4 
                 0.002 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 80 
                 10 
                 28.56 
               
               
                 Example 3 
                 2.4 
                 0.001 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 80 
                 10 
                 28.58 
               
               
                 Example 4 
                 2.4 
                 0.004 
                 0.9 
                 0.005 
                 80 
                 5 
                 5 
                 80 
                 10 
                 28.44 
               
               
                 Example 5 
                 2.4 
                 0.004 
                 0.9 
                 0.002 
                 80 
                 5 
                 5 
                 80 
                 10 
                 28.53 
               
               
                 Example 6 
                 2.4 
                 0.004 
                 0.9 
                 0.001 
                 80 
                 5 
                 5 
                 80 
                 10 
                 28.38 
               
               
                 Example 7 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 89.8 
                 0.1 
                 5 
                 80 
                 10 
                 28.18 
               
               
                 Example 8 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 89 
                 0.5 
                 5 
                 80 
                 10 
                 28.27 
               
               
                 Example 9 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 89.8 
                 5 
                 0.1 
                 80 
                 10 
                 28.14 
               
               
                 Example 10 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 89 
                 5 
                 0.5 
                 80 
                 10 
                 28.16 
               
               
                 Example 11 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 76 
                 12 
                 28.49 
               
               
                 Example 12 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 60 
                 10 
                 10 
                 60 
                 20 
                 28.03 
               
               
                 Example 13 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 60 
                 10 
                 10 
                 56 
                 22 
                 27.84 
               
               
                 Comparative 
                 2.4 
                 2.4 
                 0.9 
                 0.9 
                 80 
                 5 
                 5 
                 100 
                 0 
                 25.12 
               
               
                 Example 1 
               
               
                 Comparative 
                 2.4 
                 2.4 
                 0.9 
                 0.9 
                 80 
                 5 
                 5 
                 80 
                 10 
                 27.11 
               
               
                 Example 2 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 100 
                 0 
                 21.92 
               
               
                 Example 3 
               
               
                 Comparative 
                 2.4 
                 0.1 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 80 
                 10 
                 25.96 
               
               
                 Example 4 
               
               
                 Comparative 
                 2.4 
                 0.01 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 80 
                 10 
                 26.16 
               
               
                 Example 5 
               
               
                 Comparative 
                 2.4 
                 0.005 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 80 
                 10 
                 26.93 
               
               
                 Example 6 
               
               
                 Comparative 
                 2.4 
                 0 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 80 
                 10 
                 26.17 
               
               
                 Example 7 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0.1 
                 80 
                 5 
                 5 
                 80 
                 10 
                 27.29 
               
               
                 Example 8 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0.01 
                 80 
                 5 
                 5 
                 80 
                 10 
                 27.25 
               
               
                 Example 9 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0 
                 80 
                 5 
                 5 
                 80 
                 10 
                 27.27 
               
               
                 Example 10 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 89.9 
                 0.05 
                 5 
                 80 
                 10 
                 25.90 
               
               
                 Example 11 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 89.9 
                 5 
                 0.05 
                 80 
                 10 
                 27.30 
               
               
                 Example 12 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 88 
                 6 
                 23.79 
               
               
                 Example 13 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 80 
                 5 
                 5 
                 84 
                 8 
                 26.49 
               
               
                 Example 14 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 60 
                 10 
                 10 
                 68 
                 16 
                 22.00 
               
               
                 Example 15 
               
               
                 Comparative 
                 2.4 
                 0.004 
                 0.9 
                 0.004 
                 60 
                 10 
                 10 
                 64 
                 18 
                 25.48 
               
               
                 Example 16 
               
               
                   
               
             
          
         
       
     
       Example 2 
       [0157]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =2 nanometers. 
       Example 3 
       [0158]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =1 nanometer. 
       Example 4 
       [0159]    The experiment identical to that of the example 1 was performed except that d 5 =d 6 =5 nanometers. 
       Example 5 
       [0160]    The experiment identical to that of the example 1 was performed except that d 5 =d 6 =2 nanometers. 
       Example 6 
       [0161]    The experiment identical to that of the example 1 was performed except that d 5 =d 6 =1 nanometer. 
       Example 7 
       [0162]    The experiment identical to that of the example 1 was performed except that w 1 =89.8 micrometers and w 2 =w 3 =0.1 micrometer. 
       Example 8 
       [0163]    The experiment identical to that of the example 1 was performed except that w 1 =89 micrometers and w 2 =w 3 =0.5 micrometer. 
       Example 9 
       [0164]    The experiment identical to that of the example 1 was performed except that w 1 =89.8 micrometers and w 4 =w 5 =0.1 micrometer. 
       Example 10 
       [0165]    The experiment identical to that of the example 1 was performed except that w 1 =89 micrometers and w 4 =w 5 =0.5 micrometer. 
       Example 11 
       [0166]    The experiment identical to that of the example 1 was performed except that w 6 =76 micrometers and w 7 =w 8 =12 micrometers. 
       Example 12 
       [0167]    The experiment identical to that of the example 1 was performed except that w 1 =60 micrometers, w 2 =w 3 =w 4 =w 5 =10 micrometers, w 6 =60 micrometers, and w 7 =w 8 =20 micrometers. 
       Example 13 
       [0168]    The experiment identical to that of the example 1 was performed except that w 1 =60 micrometers, w 2 =w 3 =w 4 =w 5 =10 micrometers, w 6 =56 micrometers, and w 7 =w 8 =22 micrometers. 
       Comparative Example 1 
       [0169]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =2.4 micrometers, d 4 =d 5 =0.9 micrometer, and w 6 =100 micrometers. 
       Comparative Example 2 
       [0170]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =2.4 micrometers and d 4 =d 5 =0.9 micrometer. 
       Comparative Example 3 
       [0171]    The experiment identical to that of the example 1 was performed except that w 6 =100 micrometers. 
       Comparative Example 4 
       [0172]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0.1 micrometer. 
       Comparative Example 5 
       [0173]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0.01 micrometer. 
       Comparative Example 6 
       [0174]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0.005 micrometer. 
       Comparative Example 7 
       [0175]    The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0 micrometers. 
       Comparative Example 8 
       [0176]    The experiment identical to that of the example 1 was performed except that d 5 =d 6 =0.1 micrometer. 
       Comparative Example 9 
       [0177]    The experiment identical to that of the example 1 was performed except that d 5 =d 6 =0.01 micrometer. 
       Comparative Example 10 
       [0178]    The experiment identical to that of the example 1 was performed except that d 5 =d 6 =0 micrometers. 
       Comparative Example 11 
       [0179]    The experiment identical to that of the example 1 was performed except that w 1 =89.9 micrometers and w 2 =w 3 =0.05 micrometer. 
       Comparative Example 12 
       [0180]    The experiment identical to that of the example 1 was performed except that w 1 =89.9 micrometers and w 4 =w 5 =0.05 micrometer. 
       Comparative Example 13 
       [0181]    The experiment identical to that of the example 1 was performed except that w 6 =88 micrometers and w 7 =w 8 =6 micrometers. 
       Comparative Example 14 
       [0182]    The experiment identical to that of the example 1 was performed except that w 6 =84 micrometers and w 7 =w 8 =8 micrometers. 
       Comparative Example 15 
       [0183]    The experiment identical to that of the example 1 was performed except that w 1 =60 micrometers, w 2 =w 3 =w 4 =w 5 =10 micrometers, w 6 =68 micrometers, and w 7 =w 8 =16 micrometers. 
       Comparative Example 16 
       [0184]    The experiment identical to that of the example 1 was performed except that w 1 =60 micrometers, w 2 =w 3 =w 4 =w 5 =10 micrometers, w 6 =64 micrometers, and w 7 =w 8 =18 micrometers. 
         [0185]    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, d 5 &lt;d 4 , d 6 &lt;d 4 , 1 nanometer≦d 5 ≦5 nanometers, 1 nanometer≦d 6 ≦5 nanometers, 100 nanometers≦w 2 , 100 nannometers≦w 3 , 100 nannometers≦w 4 , 100 nannometers≦w 5 , and w 6 ≦w 1  is satisfied, a high conversion efficiency of 28% or more is achieved. 
         [0186]    The examples 1 to 13 and the comparative examples 1 and 2 show that it is necessary that the following inequation set: d 2 &lt;d 1 , d 3 &lt;d 1 , d 5 &lt;d 4 , and d 6 &lt;d 4  is satisfied. 
         [0187]    The examples 1 to 3 and the comparative examples 4 to 7 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. 
         [0188]    The examples 1 and 4 to 6 and the comparative examples 8 to 10 show that it is necessary that the following inequation set: 1 nanometer≦d 5 ≦5 nanometers and 1 nanometer≦d 6 ≦5 nanometers is satisfied. 
         [0189]    The examples 7 and 8 and the comparative example 11 show that it is necessary that the following inequation set: 100 nanometers≦w 2  and 100 nannometers≦w 3  is satisfied. 
         [0190]    The examples 9 and 10 and the comparative example 12 show that it is necessary that the following inequation set: 100 nanometers≦w 4  and 100 nannometers≦w 5  is satisfied. 
         [0191]    The examples 1, 11 to 13 and the comparative examples 13 to 16 show that it is necessary that the following inequation: w 6 ≦w 1  is satisfied. 
       INDUSTRIAL APPLICABILITY 
       [0192]    The present invention provides a solar cell with higher conversion efficiency. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  top cell 
           2  bottom cell 
           3  quantum tunneling layer 
           11  solar cell element 
           12  semiconductor substrate 
           13   a  p-type GaAs buffer layer 
           13   b  p-type InGaP-BSF layer 
           13   c  p-type GaAs base layer 
           13   d  n-type GaAs emitter layer 
           13   e  n-type InGaP window layer 
           15  antireflection layer 
           101  lens 
           102  solar cell element 
           103  p-type GaAs layer 
           104  n-type GaAs layer 
           104   a  GaAs center part 
           104   b  first GaAs peripheral part 
           104   c  second GaAs peripheral part 
           105  p-type InGaP layer 
           106  n-type InGaP layer 
           106   a  InGaP center part 
           106   b  first InGaP peripheral part 
           106   c  second InGaP peripheral part 
           107  p-type window layer 
           108  quantum tunneling layer 
           109  first n-type barrier layer 
           110  p-type barrier layer 
           111  second n-type barrier layer 
           112  n-type contact layer 
           113  p-type contact layer 
           114  n-side electrode 
           115  p-side electrode 
           116  insulating film 
           117  sunlight 
           118  GaAs substrate 
           119  sacrificial layer 
           120  first mask 
           121  second mask 
           122  third mask 
           123  base substrate 
           124  metal film