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 , 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:
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 
     The present invention relates to a solar cell. 
     BACKGROUND ART 
       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 . 
     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. 
       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 
     
         
         [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
 
         [Non Patent Literature 1] 
         Jenny Nelson, “The Physics of Solar Cells”, World Scientific Pub Co Inc. 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     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%. 
     The purpose of the present invention is to provide a solar cell having higher conversion efficiency. 
     Solution to Problems 
     The present disclosure is directed to a method for generating electric power with use of a solar cell, the method comprising steps of; 
     (a) preparing the solar cell comprising a condensing lens ( 101 ) and a solar cell element ( 102 ), wherein 
     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 ); 
     a Z-direction denotes the direction of the normal line of the p-type GaAs layer ( 103 ); 
     an X-direction denotes a direction orthogonal to the Z-direction, 
     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; 
     the p-type window layer ( 107 ) is made of a p-type compound semiconductor having a wider bandgap than InGaP, 
     the n-side electrode ( 114 ) is electrically connected with the n-type GaAs layer ( 104 ); 
     the p-side electrode ( 115 ) is electrically connected with the p-type InGaP layer ( 105 ); 
     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 first GaAs peripheral part ( 104   b ) and the second GaAs peripheral part ( 104   c ) have a shape of a layer, 
     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 ); 
     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; 
     the first InGaP peripheral part ( 106   b ) and the second InGaP peripheral part ( 106   c ) have a shape of a layer, 
     the following inequation set (I) is satisfied:
 
 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);
 
     wherein d 1  represents a thickness of the GaAs center part ( 104   a ) along the Z-direction; 
     d 2  represents a thickness of the first GaAs peripheral part ( 104   b ) along the Z-direction; 
     d 3  represents a thickness of the second GaAs peripheral part ( 104   c ) along the Z-direction; 
     d 4  represents a thickness of the InGaP center part ( 106   a ) along the Z-direction; 
     d 5  represents a thickness of the first InGaP peripheral part ( 106   b ) along the Z-direction; 
     d 6  represents a thickness of the second InGaP peripheral part ( 106   c ) along the Z-direction; 
     w 2  represents a width of the first GaAs peripheral part ( 104   b ) along the X-direction; 
     w 3  represents a width of the second GaAs peripheral part ( 104   c ) along the X-direction; 
     w 4  represents a width of the first InGaP peripheral part ( 106   b ) along the X-direction; and 
     w 5  represents a width of the second InGaP peripheral part ( 106   c ) along the X-direction; and 
     (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 ):
 
 w 6 ≦w 1  (II);
 
     wherein w 1  represents a width of the GaAs center part ( 104   a ) along the X-direction; 
     w 6  represents a width of the region S along the X-direction in the cross-sectional view which includes the Z-direction; and 
     the first GaAs center part ( 104   a ) overlaps the region (S) when seen from the Z-direction. 
     Advantageous Effect of the Invention 
     The present invention provides a solar cell having higher conversion efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a cross-sectional view of the solar cell according to the embodiment 1. 
         FIG. 1B  shows a cross-sectional view of the solar cell element according to the embodiment 1. 
         FIG. 2  shows a cross-sectional exploded view of the solar cell element according to the embodiment 1. 
         FIG. 3A  shows a fabricating step of the solar cell element according to the embodiment 1. 
         FIG. 3B  shows a fabricating step of the solar cell element according to the embodiment 1. 
         FIG. 3C  shows a fabricating step of the solar cell element according to the embodiment 1. 
         FIG. 3D  shows a fabricating step of the solar cell element according to the embodiment 1. 
         FIG. 3E  shows a fabricating step of the solar cell element according to the embodiment 1. 
         FIG. 3F  shows a fabricating step of the solar cell element according to the embodiment 1. 
         FIG. 3G  shows a fabricating step of the solar cell element according to the embodiment 1. 
         FIG. 3H  shows a fabricating step of the solar cell element according to the embodiment 1. 
         FIG. 4  shows a cross-sectional view of the solar cell according to the embodiment 1. 
         FIG. 5  shows a cross-sectional view of the solar cell element according to the comparative example 1. 
         FIG. 6  shows a cross-sectional view of the solar cell disclosed in Patent Literature 1. 
         FIG. 7  shows a cross-sectional view of the solar cell element disclosed in Patent Literature 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The exemplary embodiment of the present invention is described below with reference to drawings. 
     Embodiment 1 
     (Step (a)) 
     In the step (a), a solar cell is prepared. 
       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 . 
     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 . 
     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 . 
     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 . 
     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 . 
     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 . 
     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. 
     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. 
     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. 
     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). 
     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). 
     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. 
     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. 
     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. 
     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. 
     Accordingly, the following inequation set (I) is required to be satisfied in the embodiment 1.
 
 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)
 
     As described above, the value of d 1  represents a thickness of the GaAs center part  104   a  along the Z-direction. 
     The value of d 2  represents a thickness of the first GaAs peripheral part  104   b  along the Z-direction. 
     The value of d 3  represents a thickness of the second GaAs peripheral part  104   c  along the Z-direction. 
     The value of d 4  represents a thickness of the InGaP center part  106   a  along the Z-direction. 
     The value of d 5  represents a thickness of the first InGaP peripheral part  106   b  along the Z-direction. 
     The value of d 6  represents a thickness of the second InGaP peripheral part  106   c  along the Z-direction. 
     The value of w 2  represents a width of the first GaAs peripheral part  104   b  along the X-direction. 
     The value of w 3  represents a width of the second GaAs peripheral part  104   c  along the X-direction. 
     The value of w 4  represents a width of the first InGaP peripheral part  106   b  along the X-direction. 
     The value of w 5  represents a width of the second InGaP peripheral part  106   c  along the X-direction. 
     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. 
     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 . 
     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. 
     The material of the condensing lens  101  is not limited. An example of the material of the condensing lens  101  is glass or resin. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 . 
     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). 
     (Method for Fabricating Solar Cell Element  102 ) 
     A method for fabricating a solar cell element  102  is described below with reference to  FIGS. 3A to 311 . 
     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. 
     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. 
     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 . 
     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 . 
     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. 
     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. 
     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. 
     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. 
     (Step (b)) 
     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. 
     The present inventors discovered that the following inequation set (II) is required to be satisfied in the step (b).
 
 w 6 &lt;w 1  (II)
 
     As described above, the value of w 1  represents the width of the GaAs center part  104   a  along the X-direction. 
     The value of w 6  represents a width of the region S along the X-direction. 
     When seen along the Z-direction, the GaAs center part  104   a  overlaps with the region S. 
     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). 
     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 
     The present invention is described in more detail by the following examples. 
     Example 1 
     In the example 1, the solar cell element  102  shown in  FIG. 2  was fabricated by the method shown in  FIGS. 3A to 311 . 
     Table 1 shows the composition and the thickness of each layer in the solar cell element  102  according to the example 1. 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 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 
               
               
                   
               
             
          
         
       
     
     In the example 1, d 1  to d 6  and w 1  to w 5  were described as below. 
     d 1 : 2.4 micrometers 
     d 2 : 4 nanometers 
     d 3 : 4 nanometers 
     d 4 : 0.9 micrometer 
     d 5 : 4 nanometers 
     d 6 : 4 nanometers 
     w 1 : 80 micrometers 
     w 2 : 5 micrometers 
     w 3 : 5 micrometers 
     w 4 : 5 micrometers 
     w 5 : 5 micrometers 
     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. 
     The solar cell according to the example 1 was fabricated as below. 
     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. 
     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. 
     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. 
     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 . 
     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. 
     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. 
     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 . 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     The conversion efficiency was calculated according to the following equation (I):
 
(Conversion efficiency)=(Maximum output value from the solar cell)/(Energy of the sunlight)  (Equation I)
 
     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):
 
(Output value)=(Current density obtained from the solar cell)·(Bias voltage obtained from the solar cell)
 
     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. 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 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 
     The experiment identical to that of the example 1 was performed except that d 2 =d 3 =2 nanometers. 
     Example 3 
     The experiment identical to that of the example 1 was performed except that d 2 =d 3 =1 nanometer. 
     Example 4 
     The experiment identical to that of the example 1 was performed except that d 5 =d 6 =5 nanometers. 
     Example 5 
     The experiment identical to that of the example 1 was performed except that d 5 =d 6 =2 nanometers. 
     Example 6 
     The experiment identical to that of the example 1 was performed except that d 5 =d 6 =1 nanometer. 
     Example 7 
     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 
     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 
     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 
     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 
     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 
     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 
     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 
     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 
     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 
     The experiment identical to that of the example 1 was performed except that w 6 =100 micrometers. 
     Comparative Example 4 
     The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0.1 micrometer. 
     Comparative Example 5 
     The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0.01 micrometer. 
     Comparative Example 6 
     The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0.005 micrometer. 
     Comparative Example 7 
     The experiment identical to that of the example 1 was performed except that d 2 =d 3 =0 micrometers. 
     Comparative Example 8 
     The experiment identical to that of the example 1 was performed except that d 5 =d 6 =0.1 micrometer. 
     Comparative Example 9 
     The experiment identical to that of the example 1 was performed except that d 5 =d 6 =0.01 micrometer. 
     Comparative Example 10 
     The experiment identical to that of the example 1 was performed except that d 5 =d 6 =0 micrometers. 
     Comparative Example 11 
     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 
     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 
     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 
     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 
     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 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 
     The present invention provides a solar cell with higher conversion efficiency. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 REFERENCE SIGNS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   1 
                 top cell 
               
               
                   2 
                 bottom cell 
               
               
                   3 
                 quantum tunneling layer 
               
               
                  11 
                 solar cell element 
               
               
                  12 
                 semiconductor substrate 
               
               
                   13a 
                 p-type GaAs buffer layer 
               
               
                   13b 
                 p-type InGaP-BSF layer 
               
               
                   13c 
                 p-type GaAs base layer 
               
               
                   13d 
                 n-type GaAs emitter layer 
               
               
                   13e 
                 n-type InGaP window layer 
               
               
                  15 
                 antireflection layer 
               
               
                 101 
                 lens 
               
               
                 102 
                 solar cell element 
               
               
                 103 
                 p-type GaAs layer 
               
               
                 104 
                 n-type GaAs layer 
               
               
                  104a 
                 GaAs center part 
               
               
                  104b 
                 first GaAs peripheral part 
               
               
                  104c 
                 second GaAs peripheral part 
               
               
                 105 
                 p-type InGaP layer 
               
               
                 106 
                 n-type InGaP layer 
               
               
                  106a 
                 InGaP center part 
               
               
                  106b 
                 first InGaP peripheral part 
               
               
                  106c 
                 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