Patent Publication Number: US-2015083192-A1

Title: Solar cell and method for manufacturing same

Description:
TECHNICAL FIELD 
     The present invention relates to a solar cell and a manufacturing method thereof. 
     BACKGROUND ART 
     A multi-junction III-V compound solar cell unit is a solar cell which has the highest efficiency among solar cells and which is suitable for a concentrating solar cell. There are several known types of solar cells having such a multi-junction III-V compound solar cell unit (see, PTL 1 and PTL 2, for example).  FIG. 19  to  FIG. 22  illustrate schematic diagrams of a cross-sectional structure of a conventional solar cell having a multi-junction III-V compound solar cell unit. 
       FIG. 19  illustrates a first example of a conventional solar cell (see PTL 1). Solar cell  100  illustrated in  FIG. 19  has optical component  110  which concentrates sunlight and back sheet  140 . Optical component  110  is comprised of a Cassegrain type glass lens. At part of this glass lens, recess  113  for holding solar cell unit  120  is formed. 
     Back sheet  140  is bonded to optical component  110 . Back sheet  140  is comprised of circuit board  150  and adhesion layer  155 . Circuit board  150  is comprised of insulator  153  and conductor  154 . Solar cell unit  120  is electrically and physically connected to electrode portions  154 A and  154 B of conductor  154  by way of first connection portion  124 A and second connection portion  124 B. 
       FIG. 20  illustrates a second example of a conventional solar cell (see PTL 2). Solar cell  200  illustrated in  FIG. 20  has an optical component  210  which concentrates sunlight and primary mirror  230  which is integrated with optical component  210 . Optical component  210  is comprised of a Cassegrain type glass lens. 
     Primary mirror  230  is comprised of two metal films  231  and  234  arranged across gap  237 . Primary mirror  230  is formed in a bowl shape. A flat portion at the bottom of primary mirror  230  has aperture  239 . Aperture  239  serves as a passage of concentrated sunlight. Solar cell unit  220  for receiving sunlight which has passed through aperture  239  is fixed at an outside of the bottom of primary mirror  230 . One of double-sided electrodes of solar cell unit  220  is connected to a wire using a die bonding method, while the other electrode is connected to the wire using a wire bonding method. 
       FIG. 21  illustrates solid transparent optical panel  300  which is an array of solar cells  200  illustrated in  FIG. 20 . Optical component  210  of solar cell  200  has a hexagonal shape. A plurality of optical components  210  (210-1 to 210-7) are adjacent to each other to form one panel-like array. 
       FIG. 22  illustrates concentrating light energy collecting unit  400 C which is an array of solar cells  200  illustrated in  FIG. 20 . In concentrating light energy collecting unit  400 C, solar cells  200  are connected to each other by metal films 900-11 to 900-87. That is, a p-side electrode of one of two adjacent solar cells  200  is electrically connected to an n-side electrode of the other of two adjacent solar cells  200 . Concentrating light energy collecting unit  400 C is composed of a plurality of solar cells  200  connected in series. Power generated at concentrating light energy collecting unit  400 C is drawn outside through socket connector  420 . 
     Besides the above-described techniques, various techniques are disclosed as a technique relating to a multi-junction compound solar cell (see PTL 3 to PTL 6, for example). For example, PTL 3 discloses an extraction electrode structure of thin-film solar cell in which a first electrode is electrically connected to a second electrode via a conducting groove provided inside of a laminated body. According to this invention, it is possible to reduce an area of the extraction electrode portion. However, this electrode structure is provided on the first electrode which extends from a connection termination portion of a plurality of solar cell units connected in series, and does not provide a surface area improvement for receiving sunlight of each solar cell unit. 
     For example, PTL 4 discloses a solar cell module provided with a plurality of solar cell units, in which a lower electrode (backside electrode) of each solar cell unit (a tandem type photoelectric conversion cell) is electrically connected to a transparent electrode (a light receiving surface electrode) of a solar cell unit adjacent to the solar cell unit via a lattice electrode. According to this invention, it is possible to connect a plurality of solar cell units in series using lattice electrodes. However, this invention cannot provide a surface area improvement for receiving sunlight of each solar cell unit. 
     PTL 5 discloses a solar cell including a condenser lens, a solar cell element and a column-like optical member. Light concentrated by the condenser lens passes through the column-like optical member and is guided to the solar cell element. 
     PTL 6 discloses a solar cell module which is integrated by connecting a plurality of unit cells in series, the unit cells being formed by laminating a thin film silicon photoelectric conversion unit and a compound semiconductor photoelectric conversion unit. 
     CITATION LIST 
     Patent Literature 
     PTL 1 
     US Patent Application Publication No. 2007/0256726 
     PTL 2 
     Japanese Patent Application Laid-Open No. 2006-303494 
     PTL 3 
     Japanese Patent Application Laid-Open No. 2006-13403 
     PTL 4 
     Japanese Patent Application Laid-Open No. 2008-34592 
     PTL 5 
     Japanese Patent Application Laid-Open No. 2009-187971 
     PTL 6 
     WO 210/101030 
     SUMMARY OF INVENTION 
     Technical Problem 
     In a step of bonding solar cell units to a lens having a curved surface shape in the conventional multi-junction compound solar cell, each solar cell unit is individually bonded to the lens one by one. This is, it is impossible to collectively bond a plurality of solar cell units, which results in a long production lead time. 
     Further, a solar cell unit of the conventional multi-junction compound solar cell has a surface electrode formed of a metal material such as Au, Ni and Ge which does not transmit sunlight, on a surface of a top cell. Therefore, the solar cell unit has a reduced amount of sunlight incident thereon, which may lead to decrease in efficiency of power generation from sunlight of the solar cell unit. 
     Still further, in the conventional multi-junction compound solar cell, the condenser lens is provided away from the solar cell unit. It is therefore difficult to dissipate heat of the condenser lens generated by sunlight, which may lead to increase in a risk of deterioration of the condenser lens by heat. Accordingly, it is necessary to use the condenser lens formed of a material having high heat resistance, or it is necessary to provide a heat sink for heat dissipation. 
     Therefore, an object of the present invention is to provide a solar cell which realizes a short production lead time, excels in heat dissipation properties and has high power generation efficiency. 
     Solution to Problem 
     A first aspect of the present invention is directed to a solar cell including a substrate having a plate-like base having heat dissipation properties and a first conductive line and a second conductive line disposed and electrically isolated on the base, a plurality of multi-junction solar cell units each having a lower electrode that is bonded on, and electrically connected to, the first conductive line, a cell laminate including a bottom cell layer disposed on an upper surface of the lower electrode and a top cell layer disposed on an upper surface of the bottom cell layer, a transparent electrode disposed on an upper surface of the top cell layer, and a conductor connecting the transparent electrode to the second conductive line, a glass plate having one face bonded to the transparent electrodes of the plurality of multi-junction solar cell units via an adhesive, and condenser lens disposed on the other face of the glass plate via a transparent adhesive. 
     A second aspect of the present invention is directed to a method for manufacturing a solar cell including providing a substrate having a plate-like base having heat dissipation properties and a first conductive line and a second conductive line disposed and electrically isolated on the base, providing a plurality of multi-junction solar cell units each having a lower electrode, a cell laminate including a bottom cell layer disposed on an upper surface of the lower electrode and a top cell layer disposed on an upper surface of the bottom cell layer, a transparent electrode disposed on an upper surface of the top cell layer, and conductor connecting the transparent electrode to the second conductive line, providing a glass plate, bonding upper surfaces of the transparent electrodes of the plurality of solar cell units to one face of the glass plate to fix the plurality of multi-junction solar cell units to the glass plate, attaching the plurality of multi-junction solar cell units to the substrate so that the lower electrode is electrically connected to the first conductive line and the conductor is electrically connected to the second conductive line, providing a sheet-like condenser lens having a plurality of focal points, and bonding the condenser lens to the other face of the glass plate. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a solar cell which realizes a short production lead time, excels in heat dissipation properties and has high power generation efficiency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional diagram of a solar cell according to an embodiment; 
         FIG. 2  is an enlarged view of the schematic cross-sectional diagram of the solar cell according to the embodiment; 
         FIG. 3  schematically illustrates a configuration of a solar cell unit according to the embodiment; 
         FIG. 4  illustrates a schematic configuration of a cell laminate and an absorption wavelength in each cell layer according to the embodiment; 
         FIGS. 5A ,  5 B,  5 C and  5 D illustrate a step of providing a solar cell unit in a method for manufacturing the solar cell unit according to the embodiment; 
         FIGS. 6A ,  6 B and  6 C illustrate a step of providing a solar cell unit in the method for manufacturing the solar cell unit according to the embodiment; 
         FIGS. 7A ,  7 B and  7 C illustrate a step of providing a solar cell unit in the method for manufacturing the solar cell unit according to the embodiment; 
         FIGS. 8A ,  8 B and  8 C illustrate a step of providing a solar cell unit in the method for manufacturing the solar cell unit according to the embodiment; 
         FIGS. 9A ,  9 B,  9 C and  9 D illustrate a step of providing a solar cell unit in the method for manufacturing a solar cell unit according to the embodiment; 
         FIG. 10  illustrates a step of providing a glass plate in the method for manufacturing a solar cell unit according to the embodiment; 
         FIG. 11  illustrates a step of bonding the solar cell unit to the glass plate in the method for manufacturing the solar cell unit according to the embodiment; 
         FIG. 12  illustrates a step of attaching the solar cell unit to the substrate in the method for manufacturing the solar cell unit according to the embodiment; 
         FIG. 13  illustrates a step of attaching the solar cell unit to the substrate in the method for manufacturing the solar cell unit according to the embodiment; 
         FIG. 14  illustrates a step of attaching the solar cell unit to the substrate in the method for manufacturing the solar cell unit according to the embodiment; 
         FIG. 15  illustrates a step of attaching the solar cell unit to the substrate in the method for manufacturing the solar cell unit according to the embodiment; 
         FIG. 16  illustrates a step of bonding a fly-eye lens to the glass plate in the method for manufacturing the solar cell unit according to the embodiment; 
         FIG. 17  illustrates a step of bonding the fly-eye lens to the glass plate in the method for manufacturing the solar cell unit according to the embodiment; 
         FIG. 18  illustrates a state where the solar cell is place according to the embodiment; 
         FIG. 19  schematically illustrates a configuration of a first example of a conventional solar cell; 
         FIG. 20  schematically illustrates a configuration of a second example of a conventional solar cell structure; 
         FIG. 21  schematically illustrates a configuration of a third example of a conventional solar cell structure; and 
         FIG. 22  schematically illustrates a configuration of a fourth example of a conventional solar cell structure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     While the present invention will be explained using embodiments, the present Invention is not limited to the following embodiments. In the accompanying drawings, the same or similar reference numerals are assigned to the components having the same or similar functions, and their explanation will be omitted. The accompanying drawings schematically illustrate the invention. Therefore, a specific dimension, or the like is not limited by the accompanying drawings. 
     &lt;Solar Cell&gt; 
       FIG. 1  is a schematic cross-sectional diagram of a solar cell according to an embodiment.  FIG. 2  is a partial cross-sectional diagram of the solar cell according to the embodiment. As illustrated in  FIG. 1  and  FIG. 2 , the solar cell according to the embodiment has (1) substrate  24 , (2) a plurality of (two) multi-junction solar cell units  10  attached to substrate  24 , (3) glass plate  34  disposed on transparent electrode  12  of solar cell units  10  via a transparent adhesive, and (4) condenser lens  31  which is disposed on glass plate  34  via transparent adhesive  35 . 
     (1) Substrate 
     As illustrated in  FIG. 2 , substrate  24  has plate-like base  27  having heat dissipation properties, first insulating layer  26  disposed on base  27 , first conductive line  25   a  and second conductive line  25   b  which are disposed on first insulating layer  26  so as to be electrically insulated. The heat dissipation properties of base  27  are expressed by, for example, thermal conductivity. The thermal conductivity of base  27  is preferably 1.0 W/(m·K) or higher, and more preferably, 2.0 W/(m·K) or higher so that heat of a lens can be effectively dissipated. The thermal conductivity of base  27  is preferably, for example, 2 to 8 W/(m·K). 
     Examples of base  27  include a metal plate or a ceramic plate having heat dissipation properties. Specifically, base  27  can be an aluminum base substrate or an iron base substrate. The thickness of base  27  is preferably, for example, 1.0 to 1.5 mm. 
     First conductive line  25   a  and second conductive line  25   b  are electrically Independent of each other. First conductive line  25   a  and second conductive line  25   b  can be formed on base  27  by a normal method for forming a conductive layer such as a metal layer in a desired shape. Each thickness of first conductive line  25   a  and second conductive line  25   b  is preferably 18 to 36 μm from the viewpoint of voltage resistance. 
     First conductive line  25   a  and second conductive line  25   b  are comprised of, for Example, a copper layer having a desired planar shape and an Ni—Au layer which has been subjected to Ni or Au plate processing. The thickness of the copper layer is, for example, 10 to 50 μm. The Ni—Au layer is formed by a flash Au plating method or an electrolytic Au plating method. The thickness of the Ni—Au layer is, for example, 0.5 μm at a maximum. 
     First conductive line  25   a  and second conductive line  25   b  are electrically independent of each other. First conductive line  25   a  is electrically connected to later-described central electrode  16   b  in solar cell unit  10 . Second conductive line  25   b  is electrically connected to later-described side electrode  16   a  in solar cell unit  10 . 
     When base  27  has conductive property, substrate  24  may further have an Insulting layer (hereinafter, also referred as “first insulating layer  26 ”) on a surface of base  27 . First insulating layer  26  may be formed on the entire surface of base  27  or may be formed only around first conductive line  25   a  and second conductive line  25   b  so as to increase heat dissipation properties. First insulating layer  26  can be formed using a normal method for forming a layer having a desired planar shape on a plate-like member. Examples of a material of first insulating layer  26  include epoxy resins, phenol resins, fluorine-based resins, polyimide resins, silicone resins and acrylic resins. If the material of the first insulating layer is a resin material, the thickness of first insulating layer  26  is preferably 15 μm to 300 μm so as to ensure sufficient insulation performance and heat-transfer performance between the above-described conductive lines and base  27 . 
     First insulating layer  26  is formed by applying an insulating layer coating material to base  27 . First insulating layer  26  is formed so as not to be aerated and so as not to cause a defect such as a pinhole defect to maintain electric insulation. 
     (2) Solar Cell Unit 
     As illustrated in  FIG. 2 , solar cell unit  10  has lower electrode  9   a  which is bonded and electrically connected to first conductive line  25   a ; cell stack  50  including bottom cell layer B disposed on an upper surface of lower electrode  9   a,  middle cell layer M disposed on an upper surface of bottom cell layer B, and top cell layer T disposed on an upper surface of middle cell layer M; transparent electrode  12  disposed on an upper surface of top cell layer T; insulating layer  17  disposed on a side surface of cell stack  50 ; and side electrode  16   a  disposed on the side surface of cell stack  50  via insulating layer  17  so as to electrically connect transparent electrode  12  and second conductive line  25   b.    
     Cell stuck  50  may include at least bottom cell layer B and top cell layer T. That is, middle layer M in cell stack  50  may be omitted. Further, solar cell unit  10  may have a conductor for connecting transparent electrode  12  to second conductive line  25   b  in place of side electrode  16   a . This conductor is, for example, a wire for wire bonding. 
     Since use of solar cell unit  10  eliminates the necessity for providing electrodes other than transparent electrode  12  on a sunlight receiving surface, usage efficiency of sunlight is improved. 
     While lower electrode  9   a  is electrically connected to first conductive line  25   a , lower electrode  9   a  may be in contact with first conductive line  25   a  or may be connected to first conductive line  25   a  via a conductive member. Further, while side electrode  16   a  is electrically connected to second conductive line  25   b , side electrode  16   a  may be in contact with second conductive line  25   b  or may be connected to second conductive line  25   b  via a conductive member. 
     Solar cell unit  10  may have additional members within a range in which an effect of the present invention can be provided. For example, solar cell unit  10  may have central electrode  16   b  on a lower surface of lower electrode  9   a  in order to improve electrical contact between lower electrode  9   a  and first conductive line  25   a.    
     Further, solar cell unit  10  may have lower contact layer  2   b  between lower electrode  9   a  and bottom cell layer B in order to improve electrical contact between bottom cell layer B and lower electrode  9   a . Still further, solar cell unit  10  may have upper contact layer  2   a  between top cell layer T and transparent electrode  12  in order to improve electrical contact between top cell layer T and transparent electrode  12 . The material of the contact layers can be appropriately selected according to the materials of top cell layer T and bottom cell layer. 
     Further, solar cell unit  10  may have an Au/Ti laminated film (which is not illustrated) between second insulating layer  17  and side electrode  16   a . Still further, solar cell unit  10  may have upper electrode  9   b  for electrically connecting transparent electrode  12  and side electrode  16   a.    
     Transparent electrode (ZnO)  12  provided on an upper surface of upper contact layer  2   a  of cell stack  50  draws a potential of top cell layer T. Upper electrode  9   b  is connected to transparent electrode  12 . Side electrode  16   a  is connected to upper electrode  9   b . Insulating layer  17  is provided between side electrode  16   a  and the cell stack, which are insulated from each other. Insulating layer  17  is a silicon nitride film, or the like. 
     A lower surface of side electrode  16   a  is preferably positioned below a lower surface of lower electrode  9   a . More preferably, the lower surface of side electrode  16   a  corresponds with a lower surface of central electrode  16   b  on dashed line LL. That is, electrical connection portions with external parts (an electrical connection portion having a potential of a top cell and an electrical connection portion having a potential of a bottom cell) are preferably drawn out on one surface. 
     By this means, when solar cell unit  10  is attached to substrate  24  (see  FIG. 12  and  FIG. 13 ), it is possible to prevent breakage of solar cell unit  10  even if pressure is evenly applied to solar cell unit  10 . This is, when side electrode  16   a  having a potential generated at top cell layer T and central electrode  16   b  having a potential generated at bottom cell layer B are disposed on the same plane, it is possible to attach side electrode  16   a  and central electrode  16   b  with an external electrode at one time in a production step, which can shorten a production lead time. 
     The lower surface of side electrode  16   a  and the lower surface of central electrode  16   b  which are disposed on the same plane are respectively electrically connected to first conductive line  25   a  and second conductive line  25   b  of substrate  24  with or without an interposed conductive member. Side electrode  16   a  and central electrode  16   b  are disposed to be electrically independent of each other. 
     In the solar cell according to the embodiment, electrical connection between lower electrode  9   a  and first conductive line  25   a  and electrical connection between side electrode  16   a  and second conductive line  25   b  are achieved via anisotropic conductive material  36 . Use of anisotropic conductive material  36  enables adhesion and electrical connection between substrate  24  and solar cell unit  10  at the same time and easily. Anisotropic conductive material  36  is, for example, a thermosetting resin film (ACF) in which conductive particles are dispersed and an anisotropic conductive paste (ACP). 
     As illustrated in  FIG. 1  and  FIG. 2 , a gap between substrate  24  and glass plate  34  is preferably sealed with sealing resin  22  so as to improve mechanical strength and chemical resistance, and further to suppress concentration of stress due to heating of lens under insolation. Sealing resin  22  improves structural strength of a structure comprised of substrate  24 , solar cell unit  10  and glass plate  34 . Examples of sealing resin  22  include an epoxy resin, a phenol resin, a fluorine-based resin, a polyimide resin, a silicon resin and an acrylic resin. 
     Lower electrode  9   a  and upper electrode  9   b  are conductive members such as metals. Lower electrode  9   a  and upper electrode  9   b  are, for example, Au plating films each having a thickness of about 10 μm. Central electrode  16   b  and side electrode  16   a  are, for example, Au plating films each having a thickness of about 10 to 50 μm. Central electrode  16   b  and side electrode  16   a  are formed to be ticker than lower electrode  9   a  and upper electrode  9   b . Second insulating layer  17  is, for example, a SiN film haing a thickness of about 1 μm. Transparent electrode  12  is, for example, a ZnO layer having a thickness of about 0.5 μm. The thickness of the Au/Ti laminated film is about 0.5 μm. 
     As illustrated in  FIG. 3 , width A of transparent electrode  12  is, for example, 500 μm. Width B of upper contact layer  2   a  is, for example, 470 μm. Width C of a peripheral portion of transparent electrode  12  is, for example, 15 μm. A width of upper electrode  9   b  disposed at the center of the peripheral portion is, for example, 5 μm. A width of a gap between upper electrode  9   b  and cell stack  50  is, for example, 5 μm. A width between upper electrode  9   b  and an edge of transparent electrode  12  is, for example, 5 μm. The thickness of cell stack  50  is, for example, 10 μm. Thickness D of solar cell unit  10  is, for example 25 μm. 
     As illustrated in  FIG. 4 , cell stack  50  is comprised of upper contact layer  2   a,  top cell layer T, tunnel layer  19   a , middle cell layer M, tunnel layer  19   b , grid layer  20 , buffer layer  21 , bottom cell layer B and lower contact layer  2   b . As described above, middle cell layer M may be omitted. 
     A forbidden bandwidth of top cell layer T is 1.87 eV, and a wavelength which can be absorbed in a sunlight spectrum is in a range of 650 nm or less. A forbidden bandwidth of middle cell layer M is 1.41 eV, and a wavelength which can be absorbed in the sunlight spectrum is in a range from 650 nm to 900 nm. A forbidden bandwidth of bottom cell layer B is 1.0 eV, and a wavelength which can be absorbed in the sunlight spectrum is in a range from 900 nm to 1,200 nm. In this way, by forming the cell stack of the solar cell unit to have a three-layer structure including top cell layer T, middle cell layer M and bottom cell layer B, the sunlight spectrum can be effectively utilized, so that it is possible to realize a high-efficient solar cell. 
     Transparent electrode  12  is formed on top cell layer T of cell stack  50 . Transparent electrode  12  can be formed using a normal method for forming transparent electrode  12  at a desired position. Materials of transparent electrode  12  include, for example, zinc oxide (ZnO), ITO, IZO and a graphene transparent conductive film. 
     Insulating layer  17  (hereinafter, referred to as a “second insulating layer”)in solar cell unit  10  is formed on a side surface of cell stack  50 . Second insulating layer  17  may be formed in a range from the side surface of cell stack  50  to the side surface of lower electrode  9   a . Materials of second insulating layer  17  include, for example, SiN, BN, SiO and the same materials as those of first insulating layer  26 . 
     Side electrode  16   a  is formed on second insulating layer  17  at a lateral side of cell stack  50 . Side electrode  16   a  may be formed away from second insulating layer  17 . Materials of side electrode  16   a  can include those used as materials of lower electrode  9   a . Side electrode  16   a  is preferably formed to reach a lateral side of lower electrode  9   a  (but to be separated from the lower electrode) so as to electrically connect to the conductive lines on the substrate surface more easily. 
     (3) Glass Plate 
     Solar cell unit  10  is bonded to a predetermined position which is a focal point of sunlight in glass plate  34  via a transparent adhesive. In order to ensure that solar cell unit  10  is fixed at the predetermined position, it is preferable to form a “hydrophilic area” where the transparent adhesive can be applied and a “water-repellent area” where the transparent adhesive is repelled on the surface of glass plate  34 , and then, to bond solar cell unit  10  as will be described later. 
     It is preferable to form a polytetrafluoroethylene (PTFE) layer in the “water-repellent area” and modify the surface of the glass plate so that the “hydrophilic area” has a hydroxy group (—OH). The “hydrophilic area and the water-repellent area” may be formed using a photolithography method. For example, the “hydrophilic area and the water-repellent area” can be formed by performing patterning using a photosensitive resist and performing wet etching on the patterned area. 
     Glass plate  34  can be a glass material such as soda-line glass, alkali-borosilicate glass, alkali-free glass, silica glass, low-expansion glass, zero-expansion glass and crystalized glass which are available for solar cells. Further, glass plate  34  can be various tempered glasses such as a glass for TFT, a glass for PDP, a base glass for optical filter, a figured glass and a chemically strengthened glass. 
     (4) Lens 
     Lens  31  is bonded to glass plate  34  via an adhesive. Lens  31  has a focal point. The focal point may be located at any point of cell stack  50  or may be located at an arbitrary position other than cell stack  50 . For example, the focal point may be located on a surface of the transparent electrode or on a surface on a side opposite to the incidence surface of the lens. 
     Lens  31  is normally a plano-convex lens which has a curved light receiving surface. Lens  31  is preferably, a fly-eye lens, which has a plurality of focal points on a side opposite to the light receiving surface. 
     Lens  31  is formed of a transparent material. Examples of the material of lens  31  include a glass and a transparent resin. The transparent resin can be, for example, an acrylic resin, a silicone resin or a polycarbonate resin. The material of lens  31  is preferably an inorganic material such as glass from the viewpoint of heat resistance. Meanwhile, the material of lens  31  is preferably a transparent resin from the viewpoint of reduction in weight. Among the transparent resins, it is preferable to use an acrylic resin from the viewpoint of productivity and economic efficiency. 
     Lens  31  is, for example, a fly-eye lens comprised of a plurality of plano-convex lenses arranged on a plane. Each plano-convex lens preferably has a focal point, for example, on a surface on a side opposite to the incidence surface, which is transparent electrode  12  of solar cell unit  10 . A planar shape of lens  31  is a square of about 50 mm each side. The thickness of lens  31  is, for example, 7 mm. 
     The size of each lens and the number of focal points in lens  31  (fly-eye lens) are set according to a light condensing magnification of each lens. For example, when the light condensing magnification of each lens is 400 times, the size of each lens is a 10 mm square. Therefore, lens  31  has 25 (5×5) lenses. When the light condensing magnification of each lens is 1,000 times, the size of each lens is a 16 mm square. Therefore, lens  31  has 9 (3×3) lenses. 
     The transparent resin contains, for example, an ultraviolet absorbing agent. Therefore, even if lens  31  is place under insolation for a long period of time, the color of lens  31  does not change to yellow, and it is possible to secure transparency. 
     Lens  31  is preferably a lens with a lens shape having a curve or a Fresnel lens, utilizing refraction of light. It is preferable to dispose a plurality of multi-junction solar cell units on a single substrate and employ as lens  31  a fly-eye lens in which focal points are provided at the transparent electrodes of the plurality of multi-junction solar cell units, respectively. 
     Lens  31  preferably has a recess at a part of a boundary region with the transparent adhesive. The recess is preferably provided at a region other than a region where light is transmitted. The recess can trap air bubbles in the transparent adhesive and prevent the air bubbles from flowing into light transmitting portion of the lens. 
     (5) Transparent Adhesive 
     Transparent adhesive  35  is used for adhesion between lens  31  and glass plate  34  and adhesion between glass plate  34  and solar cell unit  10 . Specifically, transparent electrode  12  of solar cell unit  10  is bonded to one face of glass plate  34  using transparent adhesive  35 , and a surface of lens  31  on a side opposite to the light receiving surface is bonded to the other face of glass plate  34 . 
     Transparent adhesive  35  is formed of an epoxy material or silicone material. As transparent adhesive  35 , for example, a two-liquid adhesive is used which includes a base compound comprised of a resin material and curing agent which is comprised of a resin material and which is to be mixed into the base compound, or a resin material which cures by ultraviolet rays is used. 
     (6) Other Points 
     Further, the solar cell according to the embodiment may have a configuration in which a plurality of structures, each of which has been described as a single structure above, are integrated. For example, the solar cell according to the embodiment may also have a configuration in which a plurality of solar cell units  10  are attached to single substrate  24  and fly-eye lens which has focal points respectively at a plurality of transparent electrodes  12  is used as lens  31 . Substrate  24  to which the plurality of solar cell units  10  are attached has first conductive line  25   a  and second conductive line  25   b  at a position where each solar cell unit  10  is disposed. 
     The fly-eye lens can be composed of, for example, an array of frames which is formed by bundling a plurality of cylindrical frame bodies, and plano-convex lenses disposed in the respective frame bodies. Alternatively, the fly-eye lens can be composed of, for example, lenses molded such that a plurality of plano-convex lenses are arranged in parallel. 
     The solar cell according to the embodiment has a side electrode and a base. Heat on a side of the incidence surface (for example, lens) of the solar cell unit is transferred to the base via the side electrode. Since the base has heat dissipation properties, the transferred heat is quickly dissipated to outside. Therefore, the solar cell according to the embodiment has excellent heat dissipation properties. 
     In the solar cell according to the embodiment, a plurality of solar cell units bonded to a flat glass plate with little variation in thickness are attached to the substrate. That is, it is possible to collectively attach a plurality of solar cell units and it is not necessary to attach the solar cell units one by one individually, so that it is possible to shorten a production lead time. 
     Further, the solar cell unit according to the embodiment does not have a surface electrode on a surface of the top cell. Therefore, according to the present invention, it is possible to increase a surface area for receiving sunlight of the solar cell unit. 
     &lt;Method for Manufacturing Solar Cell&gt; 
     A method for manufacturing a solar cell includes (1) providing a substrate, (2) providing a plurality of multi-junction solar cell units, (3) providing a glass plate, (4) bonding the plurality of solar cell units to the glass plate, (5) attaching the plurality of solar cell units bonded to the glass plate to the substrate, (6) providing a sheet-like condenser lens having a plurality of focal points, and (7) bonding the condenser lens to the glass plate. 
     (1) Step of Providing Substrate 
     Substrate  24  has, for example, base  27  and first conductive line  25   a  and second conductive line  25   b  which are disposed on base  27  so as to be electrically independent of each other. Each conductive line can be formed using a normal method for forming a metal layer having a desired planar shape. Further, if base  27  has conductive property, first insulating layer  26  is formed between base  27  and the conductive lines. 
     (2) Step of Providing Solar Cell Unit 
     First, disc-like GaAs substrate  1  (a wafer) illustrated in  FIG. 5A  is provided. GaAs substrate  1  has a size of, for example, a diameter of 4 inches (10.16 cm) and a thickness of 500 μm. Typically, a plurality of solar cell units  10  are formed on one GaAs substrate  1 . 
     Manufacturing of Cell Stack 
     As illustrated in  FIG. 5B , cell stack  50  is formed on GaAs substrate  1  via sacrificial layer  4 . As previously explained using  FIG. 4 , cell stack  50  can be obtained by, for example, forming upper contact layer  2   a , top cell layer T, tunnel layers  19   a  and  19   b , middle cell layer M, grid layer  20 , buffer layer  21 , bottom cell layer B and lower contact layer  2   b  on sacrificial layer  4  through epitaxial growth. The height of obtained cell stack  50  is, for example, 10 μm. Cell stack  50  can be obtained by forming each metal layer on GaAs substrate  1 . Each metal layer is put into a vertical Metal Organic Chemical Vapor Deposition (MOCVD) device and can be formed using an epitaxial growth method. 
     Epitaxial growth of each metal layer is performed using a normal method. For example, the method is performed at an ambient temperature of about 700° C. As materials for causing growth of the GaAs layer, tri-methyl gallium (TMG) and arsine (AsH3) can be used. As materials for causing growth of an InGaP layer, tri-methyl indium (TMI), TMG and phosphine (PH3) can be used. Further, as impurities for forming an n-type GaAs layer, an n-type InGaP layer and an n-type InGaAs layer, monosilane (SiH 4 ) can be used. Meanwhile, as impurities for forming a p-type GaAs layer, a p-type InGaP layer and a p-type InGaAs layer, diethyl zinc (DEZn) can be used. 
     Specifically, cell stack  50  can be manufactured through the following steps. An AlAs layer having a thickness of about 100 nm is epitaxially grown on GaAs substrate  1  as sacrificial layer  4 . Then an n-type InGaP layer having a thickness of about 0.1 μm is grown as upper contact layer  2   a.    
     Subsequently, top cell layer T is formed. An n-type InAlP layer having a thickness of about 25 nm as a window, an n-type InGaP layer having a thickness of about 0.1 μm as an emitter, a p-type InGaP layer having a thickness of about 0.9 μm as a base, and a p-type InGaP layer having a thickness of about 0.1 μm as a BSF are formed using an epitaxial growth method. As a result, top cell layer T having a thickness of about 1 μm is formed. 
     After top cell layer 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. 
     Subsequently, middle cell layer M is formed. an n-type InGaP layer having a thickness of about 0.1 μm as a window; an n-type GaAs layer having a thickness of about 0.1 μm as an emitter, a p-type GaAs layer having a thickness of about 2.5 μm as a base; and a p-type InGaP layer having a thickness of about 50 nm as a BSF are formed using the epitaxial growth method. As a result, middle cell layer M having a thickness of about 3 μm is formed. 
     After middle cell layer 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. 
     Subsequently, grid layer  20  is formed. Grid layer  20  suppresses occurrence of dislocation and missing due to mismatching of a lattice constant. Eight layers of n-type InGaP layers each having a thickness of about 0.25 μm are formed to form grid layer  20  having a thickness of about 2 μm. Further, an n-type InGaP layer having a thickness of about 1 μm is formed as buffer layer  21 . 
     Subsequently, bottom cell layer B is formed. An n-type InGaP layer having a thickness of about 50 nm as a passivation film, an n-type InGaAs layer having a thickness of about 0.1 μm as an emitter, a p-type InGaAs layer having a thickness of about 2.9 μm as a base, and a p-type InGaP layer having a thickness of about 50 nm as a passivation film are formed using the epitaxial growth method. As a result, bottom cell layer 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 grown as lower contact layer  2   b.    
     As illustrated in  FIG. 5C , lower contact layer  2   b  having a thickness of about 0.1 μm is patterned in a predetermined size. Patterning can be performed through dry etching processing. 
     As illustrated in  FIG. 5D , cell stack  50  having a thickness of 10 μm is patterned in a predetermined planar shape. A size of the patterned planar shape (for example, a diameter in the case of a circle, and a length in the case of a rectangle) is, for example, 500 μm. Patterning is preferably performed through dry etching processing. It is confirmed that when cell stack  50  is disposed in an inner side of an outer edge of GaAs substrate  1 , loss of carriers occurring around a solar cell portion can be suppressed and conversion efficiency is improved. A structure as described above in which a cell stack at an edge portion is etched is sometimes referred to as a “Ledge structure”. As described in “J. Vac. Sci. Technol. B, Vol. 11, No. 1, January/February 1993” and “IEICE Technical Report ED2007-217, MW2007-148(2008-1)”, it is known that loss of carriers is likely to occur at an edge of a PN junction. To address this problem, by employing the “Ledge structure”, carriers are concentrated inside the substrate, so that loss of carriers at the edge is suppressed. 
     As illustrated in  FIG. 6A , an Au plating electrode is formed as upper electrode  9   b  and lower electrode  9   a . Specifically, first, an Au plating film having a thickness of about 10 μm or less is formed on the entire surface of an upper portion of cell stack  50  in  FIG. 5D  using an electrolytic plating method. The Au plating film is patterned to form upper electrode  9   b  and lower electrode  9   a . Patterning is performed using a combination of photolithography and wet etching. 
     As illustrated in  FIG. 6B , an SiN film is formed as insulating layer  17 . The SiN film is formed on the entire surface of the upper portion of the cell stack using, for example, a plasma CVD method. 
     As illustrated in  FIG. 6C , an unnecessary portion of insulating layer  17  is eliminated to form windows  17   a  and  17   b  of insulating layer  17 . Through windows  17   a  and  17   b  of insulating layer  17 , Au plated surfaces forming lower electrode  9   a  and upper electrode  9   b  are respectively exposed. 
     As illustrated in  FIG. 7A , an Au/Ti laminated film is formed on the entire surface of the upper portion of the cell stack obtained in  FIG. 6C  using a metal sputtering method. The Au/Ti laminated film becomes a preprocessing film on which Au will be electrolytically plated in the subsequent step. 
     As illustrated in  FIG. 7B , a portion where the electrolytic Au plating film is not required to be formed is coated with resist  18 . For example, the portion where the plating film is not required to be formed is coated with resist  18  for mesa etching through an exposure step and etching using an alkali aqueous solution or an acid solution. Then, the electrolytic Au plating film is formed. 
     Central electrode  16   b  and side electrode  16   a  are formed through electrolytic Au plating. Central electrode  16   b  and side electrode  16   a  formed of the Au plating film are thicker than the cell stack of the solar cell unit which has a thickness of 10 μm, and are formed to have a thickness around 10 to 50 μm. 
     As illustrated in  FIG. 7C , a Ti film for protecting the Au plating is formed. The Ti film may be formed using metal sputtering and is formed on the entire surface of the upper portion of the stack obtained in  FIG. 5B . 
     As illustrated in  FIG. 8A , resist  18  ( FIG. 7C ) is removed. Resist  18  is removed through wet processing. It is possible to remove resist  18  alone through etching using an alkali aqueous solution and an acid solution. 
     As illustrated in  FIG. 8B , the Au/Ti film on insulating layer  17  and the Ti film on the Au plated electrode are removed. These films are removed using a dry edge. In this manner, the surface of the Au plated electrode is formed as a clean surface with no organic contamination. 
     As illustrated in  FIG. 8B , a basic structure of the multi-junction compound solar cell unit which is bonded at one side can be obtained. However, in the multi-junction compound solar cell unit which is bonded at one side illustrated in  FIG. 8B , top cell layer T is located at a side of GaAs substrate  1 , and bottom cell layer B is located at a side of central electrode  16   b . In order to allow sunlight to be incident from top cell layer T, it is necessary to peel GaAs substrate  1  from the basic structure of the solar cell unit illustrated in  FIG. 8B . Further, at the time, solar cell unit  10  should not be damaged. 
     Formation of Recess in Sacrificial Layer 
     When GaAs substrate  1  is peeled, solar cell unit  10  should not be damaged. Therefore, as illustrated in  FIG. 8C , after solar cell unit  10  is inverted upside down so that GaAs substrate  1  side is located upper side in a gravity direction, solar cell unit  10  is disposed on holding plate  29  on which wax  28  is provided. Then, in order to peel GaAs substrate  1 , sacrificial layer recess  4   a  is provided at a side surface of sacrificial layer  4 . Since solar cell unit  10  is extremely fragile, there is a case where solar cell unit  10  is destroyed by stress at the time when GaAs substrate  1  is peeled. Therefore, sacrificial layer recess  4   a  is provided as a starting point for reliably causing internal fracture of sacrificial layer  4 . Sacrificial layer recess  4   a  may be provided by, for example, grinding sacrificial layer  4  using a blade, grinding sacrificial layer  4  using a water jet, or performing mechanical “marking-off”. By sealing a gap between solar cell unit  10  and substrate  24  with sealing resin  22 , solar cell unit  10  is mechanically strengthened. Therefore, solar cell unit  10  is not destroyed when sacrificial layer recess  4   a  is formed. 
     Peeling of GaAs Substrate 
     As illustrated in  FIG. 9A , GaAs substrate  1  is peeled by causing internal fracture of sacrificial layer  4 . Sacrificial layer  4  is internally fractured by utilizing silicon on insulator (SOI) related techniques such as dicing, roller peeling, water jetting and ultrasonic disruption. 
     A lattice constant of GaAs configuring substrate  1  is 5.653 Å, a lattice constant of AlAs configuring sacrificial layer  4  is 5.661 Å, and both are substantially the same. Therefore, sacrificial layer  4  is a stable film and can be stably internally fractured. 
     Etching of Sacrificial Layer 
     As illustrated in  FIG. 9B , sacrificial layer  4  remained in solar cell unit  10  is removed by wet etching. For example, sacrificial layer  4  can be melted and removed by being brought into contact with hydrofluoric acid for 2 to 3 minutes. Since solar cell unit  10  is protected by sealing resin  22 , the hydrofluoric acid does not damage solar cell unit  10 . 
     Formation of Transparent Electrode 
     As illustrated in  FIG. 9C , transparent electrode  12  is formed. Transparent electrode  12  constitutes a sunlight incidence surface. Transparent electrode  12  which is a ZnO layer or an ITO layer, can be formed through a sputtering method. Transparent electrode  12  is formed on the entire surface of an upper portion of solar cell unit  10 , and electrically connects upper contact layer  2   a  and upper electrode  9   b.  It is also possible to improve conductive property by adding 0.1 mass % or more of Al or Ga to the ZnO layer. 
     As illustrated in  FIG. 9D , wax  28  is removed from solar cell unit  10 . Solar cell unit  10  obtained in this manner does not have an electrode which intercepts sunlight, on the sunlight incidence surface. Therefore, the amount of sunlight incident on solar cell unit  10  is increased, and power generation efficiency of solar cell unit  10  is improved. 
     According to this embodiment, although cell stack  50  of solar cell unit  10  is thin (for example, 10 μm or less), it is possible to form a solar cell by peeling GaAs substrate  1  without damaging cell stack  50 . 
     (3) Step of Providing Glass Plate 
     (3-1) Liquid Repellent Treatment and Hydrophilic Treatment on Surface of Glass Plate 
     As illustrated in  FIG. 10 , glass plate  34  is provided. Glass plate  34  is a plane glass or a plane tempered glass having a thickness of 2 to 10 mm. Glass plate  34  may be a glass plate which is used in a typical solar cell. 
     As illustrated in  FIG. 10 , liquid repellent layer  23  is provided in a predetermined region of a back side of glass plate  34 . The predetermined region is a region other than the region where the transparent electrodes of a plurality of (two or more) solar cell units  10  will be bonded (which is also referred to as “focal point  32 ”). Liquid repellent treatment is performed through chemical modification using a silane coupling agent having, for example, a fluorocarbon chain such as CF 3 (CF 2 ) 7 C 2 H 4 SiCl 3 , or a hydrocarbon chain such as CH 3 (CH 2 ) 17 SiCl 3 . Further, a liquid repellent layer of polytetrafluoroethylene (PTFE) may be formed to form liquid repellent layer  23 . 
     As a result, regions where the transparent electrodes of the plurality of (two or more) solar cell units  10  are bonded are relatively lyophilic to a transparent adhesive. Further, it is also possible to apply lyophilic treatment to the regions where the transparent electrodes will be bonded (focal point  32 ) to improve wettability of the transparent adhesive. 
     (3-2) Application of Transparent Adhesive 
     As illustrated in  FIG. 10 , transparent adhesive  35  is applied to focal point  32  having lyophilic property. For example, transparent adhesive  35  is applied using a dispenser with a screw type nozzle. Even if the viscosity of the transparent adhesive changes, a fixed amount of transparent adhesive is applied. While the dispensed amount of the adhesive differs according to an element size, in the present embodiment, 100 to 1,000 nanoliter of the transparent adhesive is applied using resin application head  43 . The transparent adhesive wets and spreads focal point  32  while not being applied to liquid repellent layer  23 . 
     (4) Step of Bonding Solar Cell Unit to Glass Plate 
     As illustrated in  FIG. 11 , transparent electrode  12  of solar cell unit  10  is bonded to focal point  32  of glass plate  34  to which transparent adhesive  35  has been applied, while the position of transparent electrode  12  is adjusted to the position of focal point  32 . 
     Solar cell unit  10  is then and has a thickness of 5 to 50 μm, and includes a compound semiconductor such as GaAs and Ge. Therefore, solar cell unit  10  is extremely fragile. It is therefore necessary to bond solar cell unit  10  to glass plate  34  so as not to put a load on solar cell unit  10 . Solar cell unit  10  is sucked by vacuum over suction hole  42  of mount head  41  having a planar shape and mounted on focal point  32 . A mount load is set at about 10 to 50 gf (9.81×10 −2  to 4.90×10 −1  N). 
     The position of solar cell unit  10  mounted on focal point  32  is adjusted to the position of the lyophilic region (that is, a focal point) by solar cell unit  10  getting wet with transparent adhesive  35 . As one example, if a surface of the transparent electrode of solar cell unit  10  is a square of 800 μm×800 μm, focal point  32  is set to be a square of 900 μm×900 μm, and the other region is set as a liquid repellent region. By this means, the position of solar cell unit  10  is adjusted on a glass surface by balance of surface tension of transparent adhesive  35 , and solar cell unit  10  is disposed within focal point  32 . 
     Solar cell units  10  may be mounted one by one using mount head  41  having a planar shape, or a plurality of solar cell units  10  can be collectively mounted on focal points by disposing a metal mask having through holes corresponding to a plurality of focal points on a glass plate. Further, it is also possible to dispose solar cell units  10  to the focal points by applying a liquid in which solar cell units  10  dispersed to glass plate  34 . 
     After solar cell units  10  are disposed on the focal point, transparent adhesive  35  cures. As transparent adhesive  35 , for example, a two-liquid mixing type room temperature curable resin is used. When the room temperature curable resin is used, for example, if the resin is left at room temperature, the resin starts curing after about 90 minutes and completely cures 24 hours later. Transparent adhesive  35  may be an ultraviolet curable resin. When the ultraviolet curable resin is used, an ultraviolet ray is radiated after the positions of solar cell units  10  are adjusted to the focal points and solar cell units  10  are mounted on the focal points. In this manner, transparent electrodes  12  of solar cell units  10  are appropriately fixed at the focal points of glass plate  34 . 
     Removal of Water Repellent Layer 
     After solar cell units  10  are bonded, water repellent layer  23  on glass plate  34  is removed. The water repellent layer can be removed using, the example, a dry edge. Use of the dry edge makes the surface of glass plate  34  lyophilic. If water repellent layer  23  remains on glass plate  34  to which solar cell units  10  are bonded, wettability with sealing resin  22  (see  FIG. 15 ) is degraded, which causes a peeling failure between glass plate  34  and sealing resin  22  due to a stress by a heat cycle. It is therefore preferable to remove water repellent layer  23  on glass plate  34 . 
     (5) Step of Attaching Solar Cell Unit to Substrate 
     Multi-junction solar cell units  10  bonded to glass plate  34  are attached to substrate  24 . Specifically, lower electrode  9   a  is electrically connected to first conductive line  25   a , and side electrode  16   a  is electrically connected to second conductive line  25   b , thereby multi-junction solar cell units  10  bonded to glass plate  34  being attached to substrate  24 . The position where multi-junction solar cell unit  10  bonded to glass plate  34  is attached to substrate  24  can be confirmed by, for example, an image of a bonding position photographed by a camera. 
     (5-1) Disposition of Anisotropic Conductive Material (ACF) on Substrate 
     Multi-junction solar cell units  10  are preferably attached to substrate  24  using an anisotropic conductive material. Anisotropic conductive material  36  is disposed on substrate  24 . Then, lower electrode  9   a  is connected to first conductive line  25   a  and side electrode  16   a  is connected to second conductive line  25   b  via anisotropic conductive material  36 . Use of the anisotropic conductive material enables easy attachment of multi-junction solar cell unit  10  to substrate  24 . 
     Anisotropic conductive material  36  can be a film-like or a paste-like material. Anisotropic conductive material  36  includes an epoxy resin and conductive particles which are dispersed in the epoxy resin. Anisotropic conductive material  36  is mainly used for, for example, implementing a driver for driving a liquid crystal display. 
     Preferably, film-like anisotropic conductive material  36  has a region larger than a region where solar cell unit  10  is disposed in substrate  24 , and has, for example, a size which is sufficient to enclose the second conductive line. It is necessary for anisotropic conductive material  36  to have a thickness sufficiently larger than a gap between electrodes of solar cell units  10  and conductive lines on substrate  24 . That is, a film of anisotropic conductive material  36  has a thickness larger than the thickness of first conductive line  25   a  and the thickness of second conductive line  25   b . For example, when each thickness of first conductive line  25   a  and second conductive line  25   b  is 35 μm, the thickness of anisotropic conductive material  36  may be 40 to 60 μm. 
     First, first conductive line  25   a  and second conductive line  25   b  of substrate  24  are covered with the anisotropic conductive film. Then, multi-junction solar cell unit  10  bonded to glass plate  34  is thermally pressure-bonded to substrate  24  on which the anisotropic conductive film is disposed for attachment. It is also possible to temporarily fix the anisotropic conductive film on the substrate by applying heat and pressure which are sufficient for the anisotropic conductive film to partly cure, when the conductive lines are covered with the anisotropic conductive film. More specifically, as illustrated in  FIG. 12 , the film of anisotropic conductive material  36  is pasted on substrate  24 . The film of anisotropic conductive material  36  is preferably pasted by applying heat and pressure to the whole of anisotropic conductive material  36  for 5 seconds or less using a plane tool which is heated at 60 to 100° C. from above. At this time, it is preferable to paste anisotropic conductive material  36  to a deposition region under the conditions that an epoxy resin inside anisotropic conductive material  36  does not cause a curing reaction. 
     Temporary Attachment of Solar Cell Unit to Substrate 
     As illustrated in  FIG. 12 , a temporary pressure-bonded article is obtained by adjusting the position of a plurality of (two or more) solar cell units  10  bonded to glass plate  34  to substrate  24  to which anisotropic conductive material  36  is pasted and mounting the plurality of solar cell units  10  to substrate  24 . A fiducial mark marked on glass plate  34  and a fiducial mark on substrate  24  are recognized by a CCD camera, and solar cell units  10  are disposed at predetermined positions of substrate  24  using information of the positions of the fiducial marks. Solar cell units  10  are temporarily pressure-bonded at room temperature and under low load. Therefore, there is no electrical conduction between the electrodes of solar cell units  10  and the conductive lines of substrate  24 . 
     Actual Attachment of Solar Cell Unit to Substrate 
     Next, as illustrated in  FIG. 13 , solar cell units  10  are electrically connected to substrate  24 , and solar cell units  10  are fixed to substrate  24 . Specifically, first, the temporal pressure-bonded article obtained by the temporal pressure-bonding step described above is placed on a metal stage so that substrate  24  is placed downside. Then, film-like protective sheet  39  formed of a polytetrafluoroethylene or polyimide material is disposed from above glass plate  34 . Subsequently, pressure is applied to the temporal pressure-bonded article via protective sheet  39  using metal heating and pressurizing head  40  which is heated at approximately 180 to 220° C. A load per one solar cell unit is set at about 50 to 200 gf (0.49 to 1.96 N), and a pressurizing time is set at 5 to 20 seconds. 
     By this pressurization, the epoxy resin inside anisotropic conductive material  36  melts, and then cures. As a result, central electrode  16   b  of solar cell unit  10  is electrically connected to first conductive line  25   a  of the substrate, and side electrode  16   a  of solar cell unit  10  is electrically connected to second conductive line  25   b  of substrate  24 . The electrical connection is achieved via the conductive particles within anisotropic conductive material  36 . In this manner, solar cell unit  10  is electrically connected to first conductive line  25   a  and second conductive line  25   b , and solar cell unit  10  is physically fixed at the substrate. 
       FIG. 14  illustrates a structure of the solar cell unit after the actual pressure-bonding step. Variation in the thickness of glass plate  34  is small and 10 μm or less. Variation in the thickness of substrate  24  is also small. It is therefore possible to attach a plurality of solar cell units  10  having a thickness of 10 μm or less to substrate  24  collectively and stably. 
     Reinforcement Using Sealing Resin 
     As illustrated in  FIG. 15 , gap  50  between substrate  24  and glass plate  34  in the structure illustrated in  FIG. 14  is sealed with sealing resin  22 . By sealing gap  50  with sealing resin  22 , strength of a package is maintained, and chemical resistance is improved. Sealing resin  22  is generally an epoxy resin or a silicone resin. Examples of sealing resin  22  include two-liquid adhesives each including a base compound and a curing agent, resin materials which cure by irradiation with ultraviolet rays, and resin materials which cure by heating. 
     If solar cell unit  10  is fixed at substrate  24  only with anisotropic conductive material  36 , stress is concentrated on a portion connected with anisotropic conductive material  36  due to a difference between a linear expansion coefficient of glass plate  34  and a linear expansion coefficient of substrate  24 . Sealing resin  22  filling gap  50  between substrate  24  and glass plate  34  can reduce this concentration of the stress. When gap  50  is filled with sealing resin  22 , substrate  24  and glass plate  34  which are fixed via solar cell unit  10  are integrated. 
     Gap  50  between substrate  24  and glass plate  34  is filled with sealing resin  22  generally using a method in which substrate  24  is placed on the metal stage heated at 50 to 80° C. and liquid sealing resin  22  is poured into gap  50  using capillary action. After gap between GaAs substrate  1  and substrate  24  is filled with sealing resin  22 , sealing resin  22  is heated at about 150 to 200° C. for 15 minutes to one hour so that sealing resin  22  cures. 
     It is also possible to use an alternative method in which sealing resin  22  is applied to substrate  24  before the temporary pressure-bonding step, gap  50  is filled with sealing resin  22  by pressure being applied during application of heat and pressure in the actual pressure-bonding step, and sealing resin  22  is made to cure by being heated in the actual pressure-bonding step. According to this method, it is possible to perform electrical connection between solar cell unit  10  and first conductive line  25   a  and second conductive line  25   b , and sealing of the gap with sealing resin  22  at the same time. 
     After gap  50  between substrate  24  and glass plate  34  is filled with sealing resin  22 , sealing resin  22  is heated at a temperature of 80° C. or lower (for example, room temperature (20±15° C.)) to naturally cure. Alternatively, sealing resin  22  is made to cure by being irradiated with ultraviolet rays. 
     (6) Step of Providing Condenser Lens 
     A sheet-like condenser lens having a plurality of focal points is provided. The condenser lens is preferably a fly-eye lens having a plurality of focal points on a surface on an opposite side of a light incidence surface. 
     (7) Step of Bonding Condenser Lens to Glass Plate 
     As illustrated in  FIG. 16 , transparent adhesive  35  is applied to glass plate  34 . Transparent adhesive  35  can be applied using a dispensing method, a printing method, a spin coating method, or the like. Transparent adhesive  35  may be the same as an adhesive for bonding solar cell unit  10  to glass plate  34 . 
     As illustrated in  FIG. 17 , lens  31  is pasted to glass plate  34  using transparent adhesive  35  and fixed. It is also possible to provide recess  31   a  at a part of a boundary region with the transparent adhesive, of lens  31 . Recess  31   a  is preferably provided in a region other than a light transmitting portion. Recess  31   a  traps air bubbles included in the transparent adhesive and prevents the air bubble from flowing into the light transmitting portion of lens  31 . In this manner, it is possible to limit a reduction in efficiency as a solar cell due to reflection of light transmitted through lens  31  by the air bubbles. 
     It is also possible to paste lens  31  and glass plate  34  under reduced pressure or under increased pressure so that air does not remain in transparent adhesive  35 . Further, it is also possible to apply transparent adhesive  35  at a central portion of glass plate  34  and spread transparent adhesive  35  while pressing lens  31  against glass plate  34  so that air does not remain in transparent adhesive  35 . 
     As described above, the method for manufacturing a solar cell according to this embodiment includes a step of pasting a plurality of solar cell units  10  to one surface of glass plate  34  and pasting lens  31  which is a fly-eye lens to the other surface of glass plate  34 . The focal points of the fly-eye lens are respectively set at solar cell units  10 , and are preferably set a transparent electrodes  12 . 
     &lt;State Where Solar Cell is Placed&gt; 
       FIG. 18  illustrates a solar cell according to the embodiment including a plurality of solar cell units  10  pasted to one surface of a glass plate, and fly-eye lens  31  pasted to the other surface of the glass plate. Heat dissipating member  37  is disposed at a lower surface of substrate  24  of the solar cell via heat dissipation resin  44 . 
     When the solar cell of  FIG. 18  is disposed under insolation, sunlight  30  is radiated to lens  31  along an arrow direction. The sunlight incident on lens  31  preferably transmits through transparent adhesive  35  and glass plate  34 , and is concentrated at transparent electrode  12 , and is incident on cell stack  50 . The light which has been transmitted through transparent electrode  12  transmits through top cell layer T, middle cell layer M and bottom cell layer B in the cell stack. The light corresponding to an absorption wavelength of each cell layer in the sunlight is converted into an electromotive force. For example, conversion efficiency in solar cell unit  10  is about 30 to 50%. 
     Typically, there is a risk that lens  31  might be heated by infrared rays included in the sunlight. However, in the solar cell according to the embodiment, heat of lens  31  is quickly transmitted to substrate  24  through glass plate  34 , transparent electrode  12 , upper electrode  9   b  and side electrode  16   a , and dissipated outside from substrate  24 . Therefore, lens  31  is less likely to be heated. 
     Further, in the solar cell according to the embodiment, glass plate  34  and lens  31  are in close contact with solar cell unit  10 . Still further, solar cell unit  10  has side electrode  16   a.  Therefore, heat of lens  31  can be transmitted to substrate  24  through side electrode  16   a.  Since base  27  of substrate  24  has a large surface area and thus heat transmitted to substrate  24  is dissipated outside from base  27 , heat of lens  31  is easily dissipated. It is therefore possible to mold lens  31  with a transparent resin having low heat resistance. 
     Accordingly, with lens  31  formed of a transparent resin, it is possible to reduce a cost of the material of lens  31  compared to case where lens  31  is formed of glass. Further, with lens  31  formed of a transparent resin, it is possible to reduce weight of a solar cell compared to a case where lens  31  is formed of glass. By this means, it is possible to improve, for example, workability for setting the solar cell under insolation. 
     Further, typically, although a stress may be caused within the solar cell by heat from lens  31 , the stress caused in the solar cell according to this embodiment is dispersed by sealing resin  22  filling the gap between substrate  24  and lens  31 . It is therefore possible to suppress breakage of the cell stack and solar cell unit  10  due to concentration of the stress on transparent adhesive  35  or anisotropic conductive material  36 . 
     Since the solar cell according to the embodiment has a solar cell unit which includes a transparent electrode disposed on a light receiving surface, the solar cell unit can efficiently receive sunlight. Further, the solar cell according to the embodiment includes solar cell unit  10  having a cell stack with a laminated structure including three layers of top cell layer T, middle cell layer M and bottom cell layer B. Therefore, it is possible to effectively perform photoelectric conversion of light with various wavelength regions included in the sunlight, so that it is possible to realize a high-efficient solar cell. 
     Further, in the solar cell according to the embodiment, since glass plate  34  and lens  31  is in close contact with solar cell unit  10 , the thickness of the solar cell (a distance from a bottom surface of substrate  24  to the top of lens  31 ) can be designed to be about 20 mm. The thickness of the solar cell according to the embodiment can be set to be approximately 10% of the thickness of a conventional solar cell in which lens  31  is disposed away from solar cell unit  10 . 
     The solar cell according to the embodiment includes solar cell unit  10  in which an electrode having a potential of top cell layer T and an electrode having a potential of bottom cell layer B are both disposed at the side opposite to a sunlight incidence surface. Since solar cell unit  10  can be attached to substrate  24  with one step, it is possible to shorten a production lead time of the solar cell. 
     On the other hand, a solar cell unit in a conventional multi-junction compound solar cell has a double-sided electrode structure having a surface electrode and a backside electrode. Therefore, there is often a case where the backside electrode is attached using a die bonding method, while the surface electrode is attached using a wire bonding method. That is, in the conventional solar cell, in order to realize electrical connection to the outside, it requires two attachment steps for attaching the backside electrode and attaching the surface electrode. As a result, the production lead time becomes long. 
     As described above, in this embodiment, it is possible to easily manufacture a solar cell which has high resistance to temperature cycle, high moisture resistance and high impact resistance, and which is light thin, short and small. Further, since an electrode at a sunlight receiving side is electrically connected to second conductive line  25   b  on substrate  24  which has heat dissipation properties, through a side potion of solar cell unit  10 , it is possible to utilize an electric conducting path as a heat conducting path, so that it is possible to realize high heat dissipation of the solar cell. 
     INDUSTRIAL APPLICABILITY 
     The solar cell of the present invention is suitable for use in various situations including power generation use in space and use as concentrating solar cell on earth. Further, it is possible to dramatically improve conversion efficiency of sunlight compared to conventional silicon solar cell. Therefore, the solar cell of the present invention can be used as a large-scale power generation system in an area with a large amount of solar radiation. 
     REFERENCE SIGN LIST 
       1  GaAs Substrate 
       2   a  Upper contact layer 
       2   b  Lower contact layer 
       4  Sacrificial layer 
       4   a  Sacrificial layer recess 
       9   a  Lower electrode 
       9   b  Upper electrode 
       10 ,  120 ,  220  Solar cell unit 
       12  Transparent electrode 
       15  Surface electrode 
       16   a  Side electrode 
       16   b  Central electrode 
       16   c  Au/Ti laminated film 
       16   d  Ti film 
       17  Second insulating layer 
       17   a ,  17   b  Window of second insulating layer 
       18  Resist 
       19   a,    19   b  Tunnel layer 
       20  Grid layer 
       21  Buffer layer 
       22  Sealing resin 
       23  Water repellent layer 
       24  Substrate 
       25   a  First conductive line 
       25   b  Second conductive line 
       26  First insulating layer 
       27  Base (metal plate) 
       28  Wax 
       29  Holding plate 
       30  Sunlight 
       31  Lens 
       32  Focal point 
       34  Glass plate 
       35  Transparent adhesive 
       36  Anisotropic conductive material 
       37  Heat dissipating member 
       38  Stage 
       39  Protective sheet 
       40  Heating and pressurizing head 
       41  Mount head 
       42  Absorption hole 
       43  Resin application head 
       44  Heat dissipation resin 
       50  Cell stack 
       100 ,  200  Solar cell 
       110  Optical component 
       113  Recess 
       124 A First connection portion 
       124 B Second connection portion 
       140  Back sheet 
       150  Circuit board 
       153  Insulator 
       154  Conductor 
       154 A,  154 B Electrode portion 
       155  Adhesion layer 
       210  Optical component 
       230  Primary mirror 
       231 ,  234  Metal film 
       237  Gap 
       239  Aperture 
       300  Solid transparent optical panel 
       400 C Concentrating light energy collecting unit 
       420  Socket connector 
     A Line enclosing periphery of solar cell unit  10  in solar cell in  FIG. 1   
     B Bottom cell layer 
     M Middle cell layer 
     T Top cell layer