Abstract:
The purposes of the present invention are: to eliminate an electrode on a top cell of a multi-junction compound solar cell, said electrode blocking solar light; to provide a multi-junction compound solar cell having a structure that is not easily broken in manufacture steps; and to shorten a manufacture lead time of a multi-junction compound solar battery. A multi-junction compound solar cell has: a multi-junction cell laminate having the top cell and a bottom cell; a transparent electrode, which is disposed on the light incoming surface of the top cell; a lower electrode having potential of the bottom cell; and a side-surface electrode, which is disposed on the side surface of the solar cell with an insulating layer therebetween, and is electrically connected to the transparent electrode. In the multi-junction compound solar cell, the side-surface electrode is led out to the lower electrode.

Description:
TECHNICAL FIELD 
       [0001]    The claimed invention relates to a multi-junction compound solar cell, a multi-junction compound solar battery, and a method for manufacturing the same. 
       BACKGROUND ART 
       [0002]    A multi-junction III-V group compound solar battery has been proposed as a solar battery suitable for a concentrating solar battery, which has the highest efficiency among solar batteries (see Patent Literature (hereinafter abbreviated as PTL) 1, for example). An example of a structure of such a multi-junction III-V group compound solar battery and a manufacturing method thereof will be described.  FIG. 16  is a diagram schematically illustrating a cross-sectional structure of a multi-junction III-V group compound solar battery in the related art. 
         [0003]    In order to obtain the multi-junction III-V group compound solar battery in the related art shown in  FIG. 16 , a cell laminate shown in  FIG. 15  is obtained. In order to obtain the cell laminate shown in  FIG. 15 , first, GaAs substrate  1  is prepared as a base substrate. An AlAs layer (sacrifice layer)  4  is formed on a surface of GaAs substrate  1  by epitaxial growth. The sacrifice layer  4  is a layer to be internally broken in a final process. 
         [0004]    Top cell T including pn junction of InGaP is formed on sacrifice layer  4 . It is necessary to initially form top cell T, instead of bottom cell B, in order to match with a grating constant of the GaAs substrate and to prevent misfit dislocation or defects such as pores from occurring. Top cell T is formed by epitaxial growth of InGaP or the like. The band gap of InGaP that constitutes top cell T is about 1.7 to 2.1 eV. 
         [0005]    Next, middle cell M including pn junction of GaAs is formed on top cell T. Middle cell M is formed by epitaxial growth of GaAs or the like. The band gap of GaAs that constitutes middle cell M is about 1.3 to 1.6 eV. 
         [0006]    Further, bottom cell B including pn junction of InGaAs is formed on middle cell M. Bottom cell B is formed by epitaxial growth of InGaAs or the like. The band gap of InGaAs that constitutes bottom cell B is 1.0 eV or lower. 
         [0007]    In this way, a cell laminate is obtained in which three pn junctions of InGaP, GaAs and InGaAs are connected on GaAs substrate  1  in series. The obtained cell laminate is solar cell C of a three-junction III-V group compound solar battery. 
         [0008]    In a case where solar cell C is used as a solar battery, solar light beams are incident from the side of top cell T and proceed toward bottom cell B (InGaAs). According to this configuration, light of a predetermined wavelength based on each band gaps of top cell T, middle cell M and bottom cell B is absorbed and converted into electric energy. Thus, it is possible to realize a solar battery with high efficiency. 
         [0009]    However, in the cell laminate in the state shown in  FIG. 15 , top cell T, middle cell M and bottom cell B are sequentially laminated on GaAs substrate  1 . Thus, GaAs substrate  1  blocks solar light, and thus, the solar light cannot be incident on top cell T. Thus, it is difficult to use the cell laminate in the state shown in  FIG. 15  as a solar battery. Hence, it is necessary to modify the above-described structure into a structure in which light can be incident from top cell T. 
         [0010]    In order to obtain the structure in which light can be incident from top cell T, rear surface electrode  9  is formed on an overall surface of bottom cell B by plating, in a first process. In a second process, solar cell C and GaAs substrate  1  are separated from each other. The separation is performed using weakness of sacrifice layer  4 . Sacrifice layer  4  that remains on the separated solar cell C is removed by etching using hydrofluoric acid. 
         [0011]    Next, front surface electrode  15  is formed to extract an electric potential from top cell T (see  FIG. 16 ). A metallic laminate of Au—Ge, Ni and Au is formed on an overall surface of an n-type GaAs layer (T 1 ) by plating, and an unnecessary portion of the metallic laminate and the GaAs layer (T 1 ) is removed by etching, to thereby form front surface electrode  15 . 
         [0012]    Through these processes, a multi-junction compound solar battery of a double-sided electrode structure in the related art in which top cell T, middle cell M and bottom cell B are sequentially laminated and rear surface electrode  9  and front surface electrode  15  are provided, as shown in  FIG. 16 , is obtained. 
         [0013]    In addition to the above-described technique, various techniques have been proposed as a technique relating to a multi-junction compound solar battery (for example, see PTLs 2 to 6). 
         [0014]    For example, PTL 2 discloses an extraction electrode structure of a thin solar battery in which a first electrode and a second electrode are electrically connected to each other through a connecting groove provided inside a laminated band. According to this technique, it is possible to reduce the area of an extraction electrode section. However, this electrode structure is provided on the first electrode that extends from a connection terminal end portion of a plurality of solar cells that is connected in series, which does not increase the solar light receiving area of each solar cell. 
         [0015]    For example, PTL 3 discloses a solar battery module including a plurality of solar cells in which a lower electrode (rear surface electrode) of each solar cell (tandem photoelectric conversion cell) and a transparent electrode (light receiving surface electrode) of an adjacent solar cell are electrically joined to each other through a grating electrode. According to this technique, it is possible to join the plurality of solar cells in series by the grating electrode. However, this technique does not increase the solar light receiving area of each solar cell. 
       CITATION LIST 
     Patent Literature 
       [0016]    PTL 1: Japanese Patent No. 4471584 
         [0017]    PTL 2: Japanese Patent Application Laid-Open No. HEI 9-83001 
         [0018]    PTL 3: Japanese Patent Application Laid-Open No. 2006-13403 
         [0019]    PTL 4: Japanese Patent Application Laid-Open No. 2008-34592 
         [0020]    PTL 5: US Patent Application Laid-Open No. 2001-0023962 
         [0021]    PTL 6: US Patent Application Laid-Open No. 2010-0065115 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0022]    As described above, the multi-junction compound solar battery in the related art includes front surface electrode  15  on the surface of top cell T. Since front surface electrode  15  is made of a metallic material such as Au, Ni or Ge that does not transmit solar light, the amount of solar light that is incident on top cell T decreases. Further, other techniques in the related art do not propose a method of increasing the solar light receiving area of a solar cell. 
         [0023]    Due to the double-sided electrode structure of front surface electrode  15  and rear surface electrode  9 , mounting of rear surface electrode  9  should be performed in a die bonding process, and mounting of front surface electrode  15  should be performed in a wire bonding process or a soldering process. That is, in order to achieve electric connection with the outside, two mounting processes of the mounting of rear surface electrode  9  and the mounting of front surface electrode  15  are necessary. As a result, a production lead time is prolonged. 
         [0024]    Further, since the thicknesses of top cell T, middle cell M and bottom cell B that constitute solar cell C are only 5 μm to 20 μm, if stress is applied from the outside, solar cell C is easily damaged. Thus, solar cell C may be damaged due to stress generated by the process of separating solar cell C and GaAs substrate  1  using weakness of sacrifice layer  4 , the die bonding process of rear surface electrode  9 , the wire bonding process or the soldering process of front surface electrode  15 , or the like. 
         [0025]    In order to solve the above problems, an object of the invention is to remove an electrode that blocks solar light on top cell T of a multi-junction compound solar cell, to provide a multi-junction compound solar cell having a structure that is not easily damaged in a production process, and to reduce a production lead time of a multi-junction compound solar battery. 
       Solution to Problem 
       [0026]    In order to achieve the above object, the following configurations of the invention are provided. 
         [0027]    [1] According to a first aspect of the invention, there is provided a multi-junction compound solar cell including: a multi-junction cell laminate that includes a top cell and a bottom cell; a transparent electrode that is disposed on a light incident surface of the top cell; a lower electrode that has an electric potential of the bottom cell; and a side surface electrode that is disposed on a side surface of the cell laminate through an insulating layer and is conducted to the transparent electrode, wherein the side surface electrode is extended to the lower electrode. 
         [0028]    [2] According to a second aspect of the invention, there is provided a multi-junction compound solar battery including: the multi-junction compound solar cell according to [1]; and an external member that is connected to each of the lower electrode and the side surface electrode, wherein conductive members that respectively connect the lower electrode and the side surface electrode with the external member include a stress absorption layer. 
         [0029]    [3] According to a third aspect of the invention, there is provided a multi-junction compound solar battery including: the multi-junction compound solar cell according to [1]; and an external member that is connected to each of the lower electrode and the side surface electrode, wherein connection sections that respectively connect the lower electrode and the side surface electrode with the external member are not overlapped with the cell laminate in a pressing direction for connection of the multi-junction compound solar cell with the external member. 
         [0030]    [4] According to a fourth aspect of the invention, there is provided a method for manufacturing the multi-junction compound solar battery according to [2], including: pressing and joining the lower electrode and the side surface electrode of the multi-junction compound solar cell, and the external member through a conductive member, wherein the shape of a side surface of the conductive member is a tapered shape, and the tapered shape is crushed and deformed by the joining. 
       Advantageous Effects of Invention 
       [0031]    According to the multi-junction compound solar cell of the invention, since an electrode other than the transparent electrode is not provided on a solar light receiving surface, usage efficiency of solar light is enhanced. Further, according to the multi-junction compound solar cell of the invention, since the electrodes (an electrode having an electric potential of the top cell and an electrode having an electric potential of the bottom cell) connected to the outside are extended on one surface, a production process for mounting of an external electrode is performed only once. Thus, a production lead time is reduced. 
         [0032]    Further, according to the invention, in a mounting process of the multi-junction compound solar cell to the external member, by positively deforming the stress relaxation layer disposed between the multi-junction compound solar cell and the external member, stress applied to the solar cell is reduced. Alternatively, by regulating the positional relationship between the solar cell and the electrode connected to the outside, stress applied to the solar cell is reduced. Thus, damage of the solar cell is suppressed. 
         [0033]    Further, by adjusting the relationship between the thickness of the solar cell and the thickness of the electrode connected to the outside, it is possible to suppress damage of the solar cell. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0034]      FIG. 1  is a cross-sectional view schematically illustrating an overall configuration of an example of a multi-junction compound solar battery according to the invention; 
           [0035]      FIG. 2  is a cross-sectional view schematically illustrating a cell laminate in a multi-junction compound solar battery, and a solar light spectrum absorbed by each cell; 
           [0036]      FIG. 3A  is a diagram illustrating a substrate preparation process in manufacturing of a compound solar battery,  FIG. 3B  is a diagram illustrating an epitaxial growth process of a solar cell,  FIG. 3C  is a diagram illustrating a patterning process of a lower contact layer, and  FIG. 3D  is a diagram illustrating a patterning process of a cell laminate; 
           [0037]      FIG. 4A  is a diagram illustrating an electrode formation process in manufacturing of a compound solar battery,  FIG. 4B  is a diagram illustrating an insulating layer formation process, and  FIG. 4C  is a diagram illustrating a window opening process of an insulating layer; 
           [0038]      FIG. 5A  is a diagram illustrating an entire-surface Au/Ti film formation process for electroplating in manufacturing of a compound solar battery,  FIG. 5B  is a diagram illustrating a resist formation process and a side surface electrode formation process by Au plating, and  FIG. 5C  is a diagram illustrating a Ti film formation process for plating protection; 
           [0039]      FIG. 6A  is a diagram illustrating a resist removal process in manufacturing of a compound solar battery, and  FIG. 6B  is a diagram illustrating an Au/Ti film removal process on an insulating layer and a Ti film removal process on an Au plated film; 
           [0040]      FIG. 7A  is a cross-sectional view schematically illustrating an interposer substrate in which a protrusion electrode having a stress absorption layer is formed, and  FIG. 7B  is a diagram illustrating a junction process of an electrode of a solar cell with a protrusion electrode on an interposer substrate; 
           [0041]      FIG. 8A  is a diagram illustrating a positioning process before an electrode of a compound solar cell and a protrusion electrode on an interposer substrate are joined to each other, and  FIG. 8B  is a process diagram illustrating a state where an electrode of a solar cell and a protrusion electrode on an interposer substrate are joined to each other; 
           [0042]      FIG. 9A  is a diagram illustrating, in a state where an electrode of a compound solar cell and a protrusion electrode on an interposer substrate are joined to each other, the positional relationship between the electrode on the solar cell and the protrusion electrode, and  FIG. 9B  is a diagram illustrating the dimension relationship of respective members in a solar cell; 
           [0043]      FIG. 10A  is a top view illustrating, in a state where an electrode of a compound solar cell and a protrusion electrode on an interposer substrate are joined to each other, the positional relationship between the solar cell and the protrusion electrode, and  FIG. 10B  is a side sectional view thereof; 
           [0044]      FIG. 11A  is a diagram illustrating a sealing resin filling process in manufacturing of a compound solar battery, and  FIG. 11B  is a diagram illustrating a formation process of concave portion in a sacrifice layer as a starting point for separating an GaAs substrate by a sacrifice layer; 
           [0045]      FIG. 12A  is a diagram illustrating a separation process of a GaAs substrate from a compound solar cell in manufacturing of a compound solar battery, and  FIG. 12B  is a diagram illustrating an etching removal process of a remaining sacrifice layer by hydrofluoric acid; 
           [0046]      FIG. 13A  is a diagram illustrating a transparent electrode formation process in manufacturing of a compound solar battery, and  FIG. 13B  is a diagram illustrating a division process of a solar cell and an interposer substrate into a regulated size; 
           [0047]      FIG. 14A  is a diagram illustrating a process of irradiating ultraviolet rays (UV) onto an electron sheet and extracting divided solar batteries from the electron sheet by a pickup head, in manufacturing of a compound solar battery, and  FIG. 14B  is a diagram illustrating an individualized package form; 
           [0048]      FIG. 15  is a cross-sectional view schematically illustrating a compound solar battery; and 
           [0049]      FIG. 16  is a cross-sectional view schematically illustrating a compound solar battery in the related art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0050]    Hereinafter, a compound solar battery according to an embodiment of the invention will be described with reference to the accompanying drawings. The same reference numerals are given to substantially the same members in the drawings, and description thereof will be omitted. 
       Overall Configuration of Compound Solar Battery 
       [0051]      FIG. 1  is a cross-sectional view schematically illustrating an overall configuration of a compound solar battery according to an embodiment of the invention. As shown in  FIG. 1 , the compound solar battery according to the embodiment includes 1) multi-junction compound solar cell  10 , 2) interposer substrate  24  that is an external member, and 3) a conductive member that electrically connects compound solar cell  10  and interposer substrate  24 . 
         [0052]    Solar cell  10  of the multi-junction compound solar battery shown in  FIG. 1  includes a cell laminate having a three-layer structure of top cell T, middle cell M and bottom cell B. A PN junction layer is present in each of three layers of the cell laminate. The cell laminate includes upper contact layer  2   a  provided on an upper surface of top cell T, and lower contact layer  2   b  provided on a lower surface of bottom cell B. 
         [0053]    Solar cell  10  includes transparent electrode (ZnO)  12  provided on an upper surface of upper contact layer  2   a  of the cell laminate. Transparent electrode  12  extracts an electric potential of top cell T. Upper electrode  9   b  is connected to transparent electrode  12 . Side surface electrode  16   a  is connected to upper electrode  9   b.  Insulating layer  17  is present between side surface electrode  16   a  and the cell laminate to insulate side surface electrode  16   a  from the cell laminate. Insulating layer  17  is composed of a silicon nitride film or the like. 
         [0054]    On the other band, solar cell  10  includes lower electrode  9   a  provided on a lower surface of lower contact layer  2   b  of the cell laminate. Central electrode  16   b  is provided on a lower surface of lower electrode  9   a.    
         [0055]    Here, it is preferable that a lower surface of side surface electrode  16   a  and a lower surface of central electrode  16   b  are aligned with each other on a broken line LL. When interposer substrate  24  joins with solar cell  10  which will be described later referring to  FIGS. 9A and 9B , pressure is uniformly applied to solar cell  10 , and thus, it is possible to prevent solar cell  10  from being damaged. In this way, side surface electrode  16   a  having an electric potential generated by top cell T and central electrode  16   b  having an electric potential generated by bottom cell B are arranged on the same plane. 
         [0056]    The lower surface of side surface electrode  16   a  and the lower surface of central electrode  16   b  that are arranged on the same plane are electrically connected to element-sided electrodes  25   a  and  25   b  of interposer substrate  24  that is the external member through a conductive member, respectively. Side surface electrode  16   a  and central electrode  16   b  are electrically arranged independently of each other. Similarly, element-sided electrode  25   a  and element-sided electrode  25   b,  through electrode  27   a  and through electrode  27   b,  and external extraction electrode  26   a  and external extraction electrode  26   b  are electrically arranged independently of each other. 
         [0057]    Interposer substrate  24  includes element-sided electrode  25  that is arranged on an upper surface thereof (surface that faces solar cell  10 ), external extraction electrode  26  that is arranged on a lower surface thereof, and through electrode  27  that passes through the inside of interposer substrate  24  to connect element-sided electrode  25  with external extraction electrode  26 . 
         [0058]    The conductive member includes protrusion electrode  23  having stress absorption layer  23   a.  Protrusion electrode  23  is connected to element-sided electrode  25  of interposer substrate  24 . 
         [0059]    A gap between interposer substrate  24  and solar cell  10  is sealed by sealing resin  22  in order to reinforce mechanical strength and to improve chemical resistance. In this way, an overall configuration of a single multi-junction compound solar battery is achieved as a package. 
       Cell Laminate 
       [0060]      FIG. 2  shows a cell laminate of the solar battery shown in  FIG. 1 . As described above, the cell laminate includes upper contact layer  2   a,  top cell T, middle cell M, bottom cell B, and lower contact layer  2   b.  The cell laminate is obtained by forming the respective metallic layers on GaAs substrate  1 . Each metallic layer cab be formed by an epitaxial growth method in a longitudinal MOCVD (Metal Organic Chemical Vapor Deposition) apparatus. 
         [0061]    The epitaxial growth of each metallic layer may be performed by a normal technique. For example, an environment temperature may be set to about 700° C. TMG (trimethylgallium) and AsH3 (arshin) may be used as a material for growth of the GaAs layer. TMI (trimethylindium), TMG and PH3 (phosphine) may be used as a material for growth of an InGaP layer. Further, SiH 4  (monosilane) may be used as an impurity for formation of an n-type GaAs layer, an n-type InGaP layer and an n-type InGaAs layer. On the other hand, DEZn (diethyl zinc) may be used as an impurity for formation of a p-type GaAs layer, a p-type InGaP layer and a p-type InGaAs layer. 
         [0062]    First, an AlAs layer having a thickness of about 100 nm is grown on GaAs substrate  1  as sacrifice layer  4 . Then, an n-type InGaP layer having a thickness of about 0.1 μm is grown as upper contact layer  2   a.    
         [0063]    Next, top cell T is formed. An n-type InAlP layer having a thickness of about 25 nm that is a window, an n-type InGaP layer having a thickness of about 0.1 μm that is an emitter, a p-type InGaP layer having a thickness of about 0.9 μm that is a base, and a p-type InGaP layer having a thickness of about 0.1 μm that is a BSF are respectively formed by the epitaxial growth method. As a result, top cell T having a thickness of about 1 μm is formed. 
         [0064]    After top cell T is formed, a p-type AlGaAs layer having a thickness of about 12 nm and an n-type GaAs layer having a thickness of about 20 nm are grown as tunnel layer  19 . As a result, tunnel layer  19  having a thickness of about 30 nm is formed. 
         [0065]    Then, middle cell M is formed. An n-type InGaP layer having a thickness of about 0.1 μm that is a window, an n-type GaAs layer having a thickness of about 0.1 μm that is an emitter, a p-type GaAs layer having a thickness of about 2.5 μm that is a base, and a p-type InGaP layer having a thickness of about 50 nm that is a BSF are respectively formed by the epitaxial growth method. As a result, middle cell M having a thickness of about 3 μm is formed. 
         [0066]    After middle cell M is formed, a p-type AlGaAs layer having a thickness of about 12 nm and an n-type GaAs layer having a thickness of about 20 nm are grown as tunnel layer  19 . As a result, tunnel layer  19  having a thickness of about 30 nm is formed. 
         [0067]    Next, grid layer  20  is formed. Grid layer  20  suppresses occurrence of dislocation, defects or the like due to mismatch of grating constants. An n-type InGaP layer having a thickness of about 0.25 μm is provided to form eight layers, and grid layer  20  having a thickness of about 2 μm is formed. Further, an n-type InGaP layer having a thickness of about 1 μm is formed as buffer layer  21 . 
         [0068]    Next, bottom cell B is formed. An n-type InGaP layer having a thickness of about 50 nm that is a passivation film, an n-type InGaAs layer having a thickness of about 0.1 μm that is an emitter, a p-type InGaAs layer having a thickness of about 2.9 μm that is a base, and a p-type InGaP layer having a thickness of about 50 nm that is a passivation film are respectively formed by the epitaxial growth method. As a result, bottom cell B having a thickness of about 3 μm is formed. Finally, a p-type InGaAs layer having a thickness of about 0.1 μm is formed as lower contact layer  2   b.    
         [0069]      FIG. 2  shows wavelengths of light absorbed by top cell T, middle cell M and bottom cell B. The band gap of top cell T is 1.87 eV, in which a wavelength capable of being absorbed in a solar light spectrum is in the range of 650 nm or less. The band gap of middle cell M is 1.41 eV, in which a wavelength capable of being absorbed in the solar light spectrum is in the range of 650 nm to 900 nm. The band gap of bottom cell B is 1.0 eV, in which a wavelength capable of being absorbed in the solar light spectrum is in the range of 900 nm to 1200 mn. In this way, by configuring the cell laminate of the solar cell as the three-layer structure of top cell T, middle cell M and bottom cell B, it is possible to effectively use the solar light spectrum. Thus, it is possible to realize a solar battery with high efficiency. 
       Manufacturing Method and Structure of Compound Solar Battery 
       [0070]    The manufacturing flow of the compound solar battery will be described with reference to  FIGS. 3A to 3D ,  FIGS. 4A to 4C ,  FIGS. 5A to 5C ,  FIGS. 6A and 6B , and  FIGS. 7A and 7B . In a process of  FIG. 3A , GaAs substrate  1  (wafer) is prepared. The size of GaAs substrate  1  is a 4-inch diameter and a thickness of 500 μm, for example. Normally, a plurality of solar cells  10  is formed on one GaAs substrate  1 . 
         [0071]    In a process of  FIG. 3B , the cell laminate (see  FIG. 2 ) is formed on GaAs substrate  1 . The cell laminate is obtained by the epitaxial growth of sacrifice layer  4 , upper contact layer  2   a,  top cell T, middle cell M, bottom cell B, lower contact layer  2   b,  and the like. 
         [0072]    In a process of  FIG. 3C , lower contact layer  2   b  having a thickness of about 0.1 μm is patterned in a predetermined size. It is preferable to perform the patterning through a dry etching process. 
         [0073]    In a process of  FIG. 3D , the cell laminate having a thickness of 10 μm is patterned in a predetermined size. It is preferable to perform the patterning through a dry etching process. It is confirmed that as the cell laminate is arranged inside edges of GaAs substrate  1 , loss of carriers generated around a solar battery portion is suppressed and conversion efficiency is improved. In this way, a structure in which the cell laminate is etched in its edge portions may be referred to as a “ledge structure.” As disclosed in “J. Vac. Sci. Technol. B, Vol, 11, No. 1, Jan./Feb. 1993”, “IEICE Technical Report ED2007-217, MW2007-148 (2008-1)” or the like, it is known that loss of carriers easily occurs in an end portion of PN junction. On the other hand, by employing the “ledge structure”, carriers are collected inside the substrate, to thereby suppress carrier loss in the end portion. 
         [0074]    In a process of  FIG. 4A , an Au plating electrode is formed as upper electrode  9   b  and lower electrode  9   a.  First, an Au plated film having a thickness of about 10 μm or less is formed on an overall upper surface of the cell laminate shown in  FIG. 3D  by an electric field plating method. The Au plated film is patterned to form upper electrode  9   b  and lower electrode  9   a.  The patterning may be performed by a photolithography method and a wet etching process. 
         [0075]    In a process of  FIG. 4B , an SiN film is formed as insulating layer  17 . For example, the SiN film is formed on the overall upper surface of the cell laminate using a plasma CVD method, for example. 
         [0076]    In a process of  FIG. 4C , an unnecessary portion of insulating layer  17  is removed to form windows  17   a  and  17   b  of insulating layer  17 . Windows  17   a  and  17   b  of insulating layer  17  expose Au plating surfaces that constitute lower electrode  9   a  and upper electrode  9   b,  respectively. 
         [0077]    In a process of  FIG. 5A , an Au/Ti laminated film is formed on the overall upper surface of the cell laminate obtained in  FIG. 4C  using a metal sputtering method. The Au/Ti laminated film is for a pre-processing film for performing electrolytic Au plating in the next process. 
         [0078]    In a process of  FIG. 5B , resist  18  is formed in a portion where it is not necessary to form an electrolytic Au plated film, and then, the electrolytic Au plated film is formed. Resist  18  is formed by forming a predetermined resist pattern for mesa etching and by etching an unnecessary portion by an alkali aqueous solution or an acid solution through an exposure process. 
         [0079]    Central electrode  16   b  and side surface electrode  16   a  are formed by electrolytic Au plating. The thicknesses of central electrode  16   b  and side surface electrode  16   a  made of the Au plated film can be larger than 10 μm that corresponds to a thickness of the cell laminate of the solar cell, which is about 10 μm to about 50 μm. 
         [0080]    In a process of  FIG. 5C , a Ti film for protection of the Au plated film is formed. The Ti film may be formed by a metal sputtering process, and is formed on the overall upper surface of the laminate obtained in  FIG. 5B . 
         [0081]    In a process of  FIG. 6A , resist  18  is removed. Removal of resist  18  is performed by a wet process. It is possible to remove only resist  18  by etching using an alkali aqueous solution or an acid solution. 
         [0082]    In a process of  FIG. 6B , the Au/Ti film on insulating layer  17  and the Ti film on the Au plated electrode are removed. The removal is performed by a dry edge process. In this way, the outermost surface of the Au plated electrode is provided as a clean surface without organic contamination. 
         [0083]    As shown in  FIG. 6B , a platform of a multi-junction compound solar cell of a single-sided junction is obtained. However, in the multi-junction compound solar cell of the single-sided junction shown in  FIG. 6B , top cell T is disposed on the side of GaAs substrate  1 , and bottom cell B is disposed on the side of central electrode  16   b.  In order to obtain a solar battery using this structure, solar light should be allowed to be input from top cell T. Accordingly, GaAs substrate  1  should be separated without causing damage to solar cell  10 . 
         [0084]    One characteristic of the invention is that GaAs substrate  1  is separated to obtain a solar battery without causing damage to the cell laminate, in spite of a reduced thickness (for example, 10 μm or less) of the cell laminate of solar cell  10 . 
       Interposer Substrate 
       [0085]      FIG. 7A  shows interposer substrate  24 . The size of interposer substrate  24  is 20 mm×20 mm, or a 4-inch diameter (the same as in substrate  1 ). Further, the thickness of interposer substrate  24  is 100 μm. 
         [0086]    Interposer substrate  24  can be composed of silicon, ceramic, glass epoxy, glass or the like, and includes through electrode  27  passing through the inside thereof. Further, interposer substrate  24  includes element-sided electrode  25  on a surface thereof where solar cell  10  is to be arranged, and external extraction electrode  26  on an opposite surface thereof. The outermost surfaces of element-sided electrode  25  and external extraction electrode  26  are covered by an Au film. The Au film is formed by flash Au plating or electrolytic Au plating, and has a maximum thickness of 0.5 μm. 
         [0087]    In a process of  FIG. 7B , interposer substrate  24  (see  FIG. 7A ) and solar cell  10  (see  FIG. 6B ) to which GaAs substrate  1  is attached are joined to each other. Specifically, protrusion electrodes  23  of interposer substrate  24  are joined to central electrode  16   b  and side surface electrode  16   a  of solar cell  10 , respectively. Stress generated in this junction is not to be applied to the cell laminate of the solar cell  10 , which is another characteristic of the invention. As examples of a method of preventing stress from being applied to the cell laminate of solar cell  10 , there are 1) a method of arranging a stress absorption layer on protrusion electrode  23  (see  FIGS. 7A and 7B  and  FIGS. 8A and 8B ), 2) a method of forming protrusion electrode  23  composed of conductive paste or a flexible material, 3) a method of forming central electrode  16   b  and side surface electrode  16   a  composed of a flexible material (see  FIGS. 9A and 9B ), and 4) a method of displacing a connection section of protrusion electrodes  23  with central electrode  16   b  and side surface electrode  16   a  of solar cell  10  from the cell laminate of solar cell  10  (see  FIGS. 10A and 10B ). Hereinafter, the respective methods will be described. 
       Stress Absorption Layer 
       [0088]    As shown in  FIG. 7A , protrusion electrode  23  is arranged on element-sided electrode  25  of interposer substrate  24 . Here, protrusion electrode  23  includes stress absorption layer  23   a  and column portion  23   b.  Stress absorption layer  23   a  is formed in a conical shape so that its side surface has a tapered structure. Column portion  23   b  is formed in a cylindrical shape. Specifically, the side surface of stress absorption layer  23   a  is inclined at 30° to 60° with respect to a vertical direction in  FIG. 7A . Further, the cross-sectional area of column portion  23   b  (cross-sectional area of a surface orthogonal to the vertical direction in  FIG. 7A ) is smaller than the cross-sectional area of stress absorption layer  23   a.    
         [0089]    The material of protrusion electrode  23  is generally Au, but may be a single metal such as Ti, Cu, Al, Sn, Ag, Pd, Bi, Pb, Ni or Cr, or may be a composite metal thereof. Protrusion electrode  23  made of a metallic material may be formed by a technique such as a stud bump method using a wire bonding process. For example, the diameter of column portion  23   b  is set to 20 μm to 50 μm, and the thickness of column portion  23   b  (length in a conducting direction) is set to 6 μm to 10 μm, and the thickness of the stress absorption layer is set to 20 μm or more. 
         [0090]    In this way, protrusion electrode  23  is composed of two conductive members (column portion  23   b  and stress absorption layer  23   a ) having different shapes. Further, the cross-section of stress absorption layer  23   a  connected to solar cell  10  is set to be smaller than the cross-section of column portion  23   b.  Stress absorption layer  23   a  is deformed due to stress applied when solar cell  10  and interposer substrate  24  are joined to each other to relieve stress (see  FIG. 8B ). A conical tip end of stress absorption layer  23   a  after deformation is in the state of being crushed flat (see  FIG. 8B ). 
         [0091]    In a process of  FIG. 8A , protrusion electrodes  23  of interposer substrate  24  are aligned in position with central electrode  16   b  and side surface electrode  16   a  of solar cell  10 . The thickness of the cell laminate of solar cell  10  is thin (for example, 10 μm or less) and weak, and is thus easily damaged. Thus, it is preferable that the thickness of central electrode  16   b  formed in solar cell  10  be set to be larger than the thickness of the cell laminate of solar cell  10  (for example, set to 10 μm or more). Further, it is preferable that the thickness of stress absorption layer  23   a  of protrusion electrode  23  be set to be 20 μm or more. 
         [0092]    In a process of  FIG. 8B , central electrode  16   b  and side surface electrode  16   a  of solar cell  10  are metal-joined to stress absorption layers  23   a  of protrusion electrodes  23  via Au/Au bonding. The metal joining may be performed while applying ultrasonic energy under the temperature condition of 150° C. to 250° C. In the Au/Au metal joining, tapered stress absorption layer  23   a  is deformed and crushed. Stress absorption layer  23   a  may be deformed and crushed by 10 μm or more, which is a thickness of solar battery element  10 . The thickness of the stress absorption layer after deformation is set to 10 μm or less. If the deformable amount of the stress absorption layer is larger than the thickness of solar battery element  10 , excessive stress applied to solar cell  10  is reduced. 
         [0093]    Further, column portion  23   b  and stress absorption layer  23   a  of protrusion electrode  23  may be composed of metals having different Young&#39;s modulus. Specifically, column portion  23   b  is composed of a metal having a high Young&#39;s modulus, and stress absorption layer  23   a  is composed of a metal having a low Young&#39;s modulus. Two metallic materials are selected from Au, Al, Cu, Ag, Sn, Bi or the like, respectively. 
         [0094]    Junctions of central electrode  16   b  and side surface electrode  16   a  of solar cell  10  with protrusion electrodes  23  are performed by ultrasonic metal junction using a heating ultrasonic head, for example. In a case where the ultrasonic metal junction is performed, surfaces of side surface electrode  16   a  and central electrode  16   b  are formed by Au, Al, Cu, Ag, Sn or the like. The ultrasonic metal junction is a junction method of breaking oxide films of the metal surfaces with heating and ultrasonic energy so as to for an alloy layer between metals. 
         [0095]    In this way, by arranging stress absorption layer  23   a  that is in contact with solar cell  10  composed of a metal having a low Young&#39;s modulus, upon preforming junction, the stress absorption layer is easily deformed, and thus, stress is further easily relieved. 
       Protrusion Electrode Formed of Conductive Paste or the Like 
       [0096]    Protrusion electrode  23  that is arranged over interposer substrate  24  may be formed of conductive paste. The conductive paste includes a resin component such as epoxy resin or silicone resin, and a conductive metal such as Ag, Pd, Au, Cu, Al, Ni, Cr or Ti. Protrusion electrode  23  that is composed of the conductive paste may be formed by a coating method or a printing method. Protrusion electrode  23  that is composed of the conductive paste may not include stress absorption layer  23   a,  that is, do not necessarily have a tapered shape. Solar cell  10  is in contact with the conductive paste that constitutes protrusion electrode  23 , and then cures the conductive paste. Thus, excessive stress is not applied to solar cell  10 . 
         [0097]    In order to join central electrode  16   b  and side surface electrode  16   a  of solar cell  10  with the protrusion electrode formed of the conductive paste, central electrode  16   b  and side surface electrode  16   a  of solar cell  10  may be in contact with protrusion electrode  23  to cure the conductive paste contained in protrusion electrode  23 . 
         [0098]    Protrusion electrode  23  may be formed of a flexible material (conductive resin or the like). Protrusion electrode  23  composed of the conductive resin may be formed by dispenser coating or mask printing. It is preferable that the viscosity of the conductive resin be 2000 cps to 500000 cps. The conductive resin is a liquid resin including metallic fillers made of Ag, Pd, Au, Cu or the like. 
         [0099]    If central electrode  16   b  and side surface electrode  16   a  of solar cell  10  are joined to protrusion electrode  23  composed of the flexible material, stress applied to solar cell  10  may be absorbed by protrusion electrode  23 . 
       Central Electrode and Side Surface Electrode Formed of Flexible Material 
       [0100]      FIGS. 9A and 9B  show an example in which central electrode  16   b  and side surface electrode  16   a  of solar cell  10  are formed of a flexible material. As shown in  FIG. 9A , side surface electrode  16   a  and central electrode  16   b  are arranged to be electrically independent of each other. In this case, stress absorption layer  23   a  is not necessary, and protrusion electrode  23  may be formed of a hard material. In this case, protrusion electrodes  23  are not deformed and are inserted into central electrode  16   b  and side surface electrode  16   a.  Thus, it is preferable to increase the thicknesses of central electrode  16   b  and side surface electrode  16   a  of solar cell  10  to prevent solar cell  10  from being damaged. For example, in a case where the thickness of the cell laminate of solar cell  10  is 10 μm, the thicknesses of central electrode  16   b  and side surface electrode  16   a  are set to 10 μm or more, and the amount of insertion of the protrusion electrode is set to 10 μm or less. 
         [0101]    Central electrode  16   b  and side surface electrode  16   a  shown in  FIGS. 8A and 8B  and  FIGS. 9A and 9B , and central electrode  16   b  and side surface electrode  16   a  shown in  FIG. 1  or the like have different dimension relationships or ratios. As described above, this shows that the thicknesses of central electrode  16   b  and side surface electrode  16   a  are set to be thick in view of prevention of damage to solar cell  10 . Accordingly, members having the same reference numerals have the same basic functions. 
       Connection Section of Central Electrodes with Side Surface Electrode and Protrusion Electrode 
       [0102]      FIGS. 10A and 10B  show an example in which the connection positions of central electrode  16   b  and side surface electrode  16   a  of solar cell  10  with protrusion electrodes  23  displace from of the cell laminate of solar cell  10 . That is, the connection positions of central electrode  16   b  and side surface electrode  16   a  with protrusion electrode  23  are not overlapped with the cell laminate in a direction where a force for joining solar cell  10  and interposer substrate  24  is applied. 
         [0103]    The solar cell shown in  FIG. 10B  is different from the solar cell shown in  FIG. 6B  in the structure of central electrode  16   b.  Central electrode  16   b  that is connected to bottom cell B of the solar cell shown in  FIG. 10B  is extended to a peripheral portion from the central portion of solar cell  10 . The peripheral portion of the solar cell is not overlapped with the cell laminate. Central electrode  16   b  that is extended to the peripheral portion is joined to protrusion electrode  23 . Thus, upon junction of solar cell  10  with interposer substrate  24 , a stress is prevented from being applied to the cell laminate of solar cell  10 . 
       Reinforcement Due to Sealing Resin 
       [0104]    As described above, after interposer substrate  24  (see  FIG. 7A ) and solar cell  10  (see  FIG. 6B ) to which GaAs substrate  1  is attached are joined to each other, in a process of  FIG. 11A , a gap between interposer substrate  24  and solar cell  10  is filled with resin. By filling the above-mentioned gap with sealing resin  22 , the strength of the package is retained, and chemical resistance is improved. Normally, sealing resin  22  is an epoxy resin or a silicone resin. 
         [0105]    As described above, the size of GaAs substrate  1  is a 4-inch diameter, and the size of interposer substrate  24  is 20 mm×20 mm or a 4-inch diameter. In a case where the size of interposer substrate  24  is a square of 20 mm×20 mm, a plurality of interposer substrates is mounted on GaAs substrate  1  that is a 4-inch wafer. Sealing resin  22  is flow from a gap between the plurality of interposer substrates to a gap between GaAs substrate  1  and interposer substrate  24  using the capillary phenomenon. As a result the gaps are filled with the sealing resin  22 . 
         [0106]    On the other hand, in a case where interposer substrate  24  is the 4-inch diameter, similarly, the gaps are filled with sealing resin  22  using the capillary phenomenon. In this case, it is preferable to employ sealing resin  22  with a low viscosity. 
         [0107]    After the gap between GaAs substrate  1  and interposer substrate  24  is filled with sealing resin  22 , sealing resin  22  is heated at about 150° C. to 200° C. for about 15 minutes to about 1 hour to be cured. 
       Formation of Sacrifice Layer Concave Portion 
       [0108]    In a process of  FIG. 11B , in order to separate GaAs substrate  1 , sacrifice layer concave portion  4   a  is formed on a side surface of sacrifice layer  4 . Since solar cell  10  is very weak, solar cell  10  may be damaged by stress upon separating GaAs substrate  1 . Thus, sacrifice layer concave portion  4   a  is formed as a starting point for reliably internally breaking sacrifice layer  4 . Sacrifice layer concave portion  4   a  may be formed by mechanically providing a “marking” concave portion, grinding by a blade or grinding by a water jet, for example, to provide the breaking starting point to sacrifice layer  4 . By filling the gap between solar cell  10  and interposer substrate  24  with sealing resin  22 , solar cell  10  is mechanically reinforced, and thus, solar cell  10  is not damaged when sacrifice layer concave portion  4   a  is formed. 
       Separation of GaAs Substrate 
       [0109]    In a process of  FIG. 12A , sacrifice layer  4  is internally broken to separate GaAs substrate  1 . As an example of a method of internally breaking sacrifice layer  4 , various SOI (silicon on insulator) related techniques such as dicing, roller separation, water jet or ultrasonic wave breaking may be used. In this way, GaAs substrate  1  is easily separated. 
         [0110]    Further, since the grating constant of GaAs that constitutes substrate  1  is 5.653 angstrom, and the grating constant of AlAs that constitutes sacrifice layer  4  is 5.661 angstrom, both of them approximately matches with each other. Thus, sacrifice layer  4  forms a stable film, and may be stably internally broken. 
       Etching of Sacrifice Layer 
       [0111]    In a process of  FIG. 12B , sacrifice layer  4  that remains in solar cell  10  is removed by wet etching. The wet etching of sacrifice layer  4  may be performed by bring sacrifice layer  4  in contact with a hydrofluoric acid for two to three minutes to be molten and removed. Since solar cell  10  is protected by sealing resin  22 , it is possible to prevent solar cell  10  from being damaged by the hydrofluoric acid. 
       Formation of Transparent Electrode 
       [0112]    In a process of  FIG. 13A , transparent electrode  12  is formed. Transparent electrode  12  constitutes an incident surface of solar light. Transparent electrode  12  may be a ZnO layer, an ITO layer or the like, and may be formed by a sputtering process. Transparent electrode  12  is arranged on the overall upper surface of solar cell  10 , and electrically connects upper contact layer  2   a  and upper contact electrode  9   b.  By adding Al or Ga to the ZnO layer by 0.1% by mass or more, it is possible to improve conductivity. 
         [0113]    Solar cell  10  obtained in this way does not have an electrode that blocks solar light, on the incidence surface of the solar light. Accordingly, the amount of solar light that is incident on solar cell  10  is increased, and power generation efficiency of solar cell  10  is improved. 
       Individualization 
       [0114]    In a process of  FIG. 13B , a solar battery is individualized. A plurality of solar batteries is arranged on interposer substrate  24 . First, interposer substrate  24  on which the plurality of solar batteries is arranged is attached to electron sheet  29 . Next, using a dicing apparatus that includes dicing blade  28 , the solar batteries with interposer substrate  24  are individualized. In the present embodiment, the solar battery is individualized into the size of 500 μm×500 μm. 
         [0115]    In a process of  FIG. 14A , the individualized solar battery is separated from electron sheet  29 . First, UV light  30  is irradiated onto electron sheet  29  to decrease adhesiveness of an adhesive material that is present on the surface of electron sheet  29 . When the adhesiveness of the adhesive material is reduced, the individualized solar battery is extracted from electron sheet  29  by pickup head  31  of the die bonding apparatus, and is then transferred to a predetermined position. 
       Dimension of Solar Battery 
       [0116]      FIG. 14B  is a diagram illustrating a specific dimension of an individualized multi-junction compound solar battery. Since the cell laminate of solar cell  10  is very thin (10 μm or less), the cell laminate is weak. For this reason, it is necessary to secure mechanical strength by increasing the thickness of interposer substrate  24  to a certain degree, and by filling the gap with sealing resin  22 . Thus, the thickness of interposer substrate  24  is set to 100 μm. As a result, the total thickness of the solar battery becomes 130 μm. 
         [0117]    The appearance size of the solar battery is 500 μm×500 μm, and the appearance size of the cell laminate of solar cell  10  is 470 μm×470 μm. Further, the extension length of side surface electrode  16   a  is 15 μm. That is, the occupied area of solar cell  10  (the appearance size of the cell laminate of solar cell  10  with respect to the appearance size of the solar battery) is 88%. 
         [0118]    Since an electrode other than transparent electrode  12  is not provided on a light receiving surface of solar cell  10 , it is possible to use the overall solar light that is irradiated. 
         [0119]    In the solar battery shown in  FIG. 14B , the appearance size of the cell laminate of solar cell  10  is set to 470 μm×470 μm, but may be enlarged up to 500 μm×485 μm. That is, in a case where side surface electrode  16   a  is arranged only on one side surface among four side surfaces of the cell laminate, it is possible to enlarge the appearance size of the cell laminate up to 500 μm×485 μm. Here, the occupied area of solar battery  10  is 97%. 
         [0120]    The present application claims priority based on Japanese Patent Application No. 2011-113643, filed May 20, 2011, the content of which is incorporated herein by reference. 
       INDUSTRIAL APPLICABILITY 
       [0121]    The multi-junction compound solar battery of the invention may be applied to a concentrating solar battery used on the Earth in addition to existing usage in space. Further, it is possible to remarkably enhance conversion efficiency of solar light compared with a silicon solar cell in the related art. Thus, the multi-junction compound solar battery of the invention is particularly suitable for a large-scale power generation system in an area with a large amount of daylight. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  GaAs substrate 
           2  Contact layer 
           2   a  Upper contact layer 
           2   b  Lower contact layer 
           4  Sacrifice layer 
           4   a  Sacrifice layer concave portion 
           5  AlGaAs layer 
           6  GaAs layer 
           7  InGaAs layer 
           8  GaAs layer 
           9  Rear surface electrode 
           9   a  Lower electrode 
           9   b  Upper electrode 
           10  Solar cell 
           12  Transparent electrode 
           15  Surface electrode 
           16   a  Side surface electrode 
           16   b  Central electrode 
           17  Insulating layer 
           17   a  Window of insulating layer 
           18  Resist 
           19  Tunnel layer 
           20  Grid layer 
           21  Buffer layer 
           22  Sealing resin 
           23  Protrusion electrode 
           23   a  Stress absorption layer 
           23   b  Column portion 
           24  Interposer substrate 
           25  Element-sided electrode 
           26  External extraction electrode 
           27  Through electrode 
           28  Dicing blade 
           29  Electron sheet 
           30  UV irradiation light 
           31  Pickup head 
         T Top cell 
         M Middle cell 
         B Bottom cell 
         C Cell main body 
         T 1  GaAs layer 
         T 2  AlInP layer 
         T 3  InGaP layer 
         T 4  InGaP layer 
         T 5  AlInP layer 
         M 1  AlInP layer 
         M 2  GaAs layer 
         M 3  GaAs layer 
         M 4  InGaP layer 
         B 6  InP layer 
         B 7  InGaAs layer 
         B 8  InGaAs layer 
         B 9  InP layer 
         B 10  GaAs layer