Abstract:
An object of the present invention is to provide a simple process to manufacture a wiring connecting photoelectric cells in a photoelectric conversion device. Another object of this invention is to prevent defective rupture from occurring in the said wiring. The photoelectric conversion device comprises a first and a second photoelectric conversion cells comprising respectively a first and a second single crystal semiconductor layers. First electrodes are provided on the downwards surfaces of the first and second photoelectric conversion cells, and second electrodes are provided on their upwards surfaces. The first and second photoelectric conversion cells are fixed onto a support substrate side by side. The second single crystal semiconductor layer has a through hole which reaches the first electrode. The second electrode of the first photoelectric conversion cell is extended to the through hole to be electrically connected to the first electrode of the second photoelectric conversion cell.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to photoelectric conversion devices which convert light energy into electrical energy using a photovoltaic effect of semiconductor and to a structure in which a plurality of photoelectric conversion cells are connected to each other. 
         [0003]    2. Description of the Related Art 
         [0004]    In a photoelectric conversion device in which a plurality of photoelectric conversion cells formed using a silicon wafer are arranged, photoelectric conversion cells are connected to each other with a strand or a flat wire. That is, an electrode on a light receiving plane which is provided on one surface of a silicon wafer, which is a component of a photoelectric conversion cell, is connected with a wiring component to a rear electrode which is provided on the opposite side of the silicon wafer. 
         [0005]    Although a structure in which a plurality of photoelectric conversion cells are connected with a wiring component seems simple, a variety of ways have been devised to prevent deterioration of adhesion or disconnection defect of the wiring member over time. For example, a module is disclosed in which a wiring component is electrically connected to a photoelectric conversion cell body in a part of the module while a wiring component is mechanically connected to the photoelectric conversion cell body with an adhesive in another part of the module (see Patent Document 1). Further, an invention is disclosed in which the shape of a wiring member is devised in order to reduce a warp in the electric conversion cell and further to improve the reliability after connection (see Patent Document 2). 
       [Patent Document] 
       [0000]    
       
         [Patent Document 1] Japanese Published Patent Application No. 2005-268254 
         [Patent Document 2] Japanese Published Patent Application No. 2005-142282 
       
     
       SUMMARY OF THE INVENTION 
       [0008]    Connection between a plurality of photoelectric conversion cells with wiring components is complicated and a problem of connection failure of the wiring arises. In order to electrically connect the wiring components to the photoelectric conversion cell, use of a conductive material such as solder or conductive paste is needed. However, these conductive materials do not have sufficient adhesion. Therefore, there is a problem in that a connection part of the wiring components comes off from the photoelectric conversion cell. 
         [0009]    Additionally, in order to connect neighboring photoelectric conversion cells with a wiring component in series, it is necessary to connect a light receiving plane of one photoelectric conversion cell and a rear plane of another photoelectric conversion cell, which inevitably causes inconvenience in arranging photoelectric conversion cells on a flat plane. 
         [0010]    An object of the present invention is to simplify a process for manufacturing wirings which connect photoelectric conversion cells in a photoelectric conversion device. In addition, another object of the present invention is to prevent a defective rupture in the wiring connection of the photoelectric conversion cells. 
         [0011]    One embodiment of the present invention is a photoelectric conversion device which includes at least a first photoelectric conversion cell and a second photoelectric conversion cell which are fixed on an upwards surface of a support substrate. The first photoelectric conversion cell includes a first single crystal semiconductor layer, a first electrode on a downwards surface of the first single crystal semiconductor layer which is a surface on the support substrate side, a second electrode provided on an upwards surface of the first single crystal semiconductor layer and a third electrode which is provided on the upwards surface and which is in contact with the first electrode through a through hole penetrating the first single crystal semiconductor layer. The second photoelectric conversion cell includes a second single crystal semiconductor layer, a fourth electrode on a downwards surface of the second single crystal semiconductor layer which is a surface on the support substrate side, a fifth electrode provided on an upwards surface of the second single crystal semiconductor layer, and a sixth electrode which is provided on the upwards surface and which is in contact with the fourth electrode through a through hole penetrating the second single crystal semiconductor layer. The second electrode is extended from the upwards surface of the first single crystal semiconductor layer to be connected to the sixth electrode situated on the upwards surface of the second single crystal semiconductor layer. 
         [0012]    The photoelectric conversion cells are fixed on the support substrate and an opening is formed through a single crystal semiconductor layer of the photoelectric conversion cell, whereby an electrode of the photoelectric conversion cell and a wiring which connects photoelectric conversion cells can be integrated. 
         [0013]    “Single crystals” are crystals whose crystal faces and crystal axes are aligned and whose atoms or molecules are spatially ordered. However, although single crystals are structured by orderly aligned atoms, single crystals do not exclude disorder such as a lattice defect in which the alignment is partially disordered or single crystals may include intended or unintended lattice distortion. 
         [0014]    Note that a “damaged layer” refers to a region and its vicinity in which a single crystal semiconductor substrate is divided into a single crystal semiconductor layer and a separation substrate (a single crystal semiconductor substrate) during a division step. The states of the “damaged layer” vary according to a method used for forming the “damaged layer”. For example, the “damaged layer” indicates a region which is weakened by local distortion of crystal structures. Note that a region between a surface of the single-crystal semiconductor substrate and the “damaged layer” is somewhat weakened in some cases. However, the “damaged layer” in this specification refers to a region and its vicinity at which the single crystal semiconductor substrate is divided later. 
         [0015]    Ordinal numbers such as “first”, “second”, “third”, and “fourth” which are used in description of the invention are given for convenience in order to distinguish elements, and they are not intended to limit the number of elements, the arrangement, nor the order of the steps. 
         [0016]    According to one mode of the present invention, in connecting a plurality of photoelectric conversion cells in series or in parallel over a support substrate, through holes are provided in semiconductor layers, and then wiring to connect photoelectric conversion cells and the electrodes of the photoelectric conversion cells are formed in a same fabrication step, whereby a manufacturing process can be simplified. Additionally, a defective rupture in a wiring which connects photoelectric conversion cells can be prevented. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a plane view of a photoelectric conversion device described in Embodiment 1. 
           [0018]      FIGS. 2A and 2B  are cross-sectional views of the photoelectric conversion device described in Embodiment 1. 
           [0019]      FIGS. 3A and 3B  are cross-sectional views of the photoelectric conversion device described in Embodiment 1. 
           [0020]      FIGS. 4A and 4B  are cross-sectional views of steps of manufacturing a photoelectric conversion device described in Embodiment 2. 
           [0021]      FIGS. 5A to 5C  are cross-sectional views of steps of manufacturing the photoelectric conversion device described in Embodiment 2. 
           [0022]      FIGS. 6A and 6B  are cross-sectional views of a step of manufacturing the photoelectric conversion device described in Embodiment 2. 
           [0023]      FIGS. 7A to 7C  are cross-sectional views of steps of manufacturing a photoelectric conversion device described in Embodiment 3. 
           [0024]      FIGS. 8A and 8B  are cross-sectional views of a step of manufacturing the photoelectric conversion device described in Embodiment 3. 
           [0025]      FIG. 9  is a plane view of a photoelectric conversion device described in Embodiment 4. 
           [0026]      FIGS. 10A and 10B  are cross-sectional views of the photoelectric conversion device described in Embodiment 4. 
           [0027]      FIGS. 11A and 11B  are cross-sectional views of the photoelectric conversion device described in Embodiment 4. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Embodiments of the disclosed invention will be described in detail with reference to the drawings. Note that the disclosed invention is not limited to the following description and it will be readily appreciated by those skilled in the art that modes and details can be modified in various ways without departing from the spirit and the scope of the present invention. Accordingly, the disclosed invention should not be construed as being limited to the description of the embodiments to be given below. 
         [0029]    In embodiments described below, the same reference numerals may be used to denote the same components among different drawings. Note that elements in the accompanying drawings, that is, the thickness and width of layers, regions, and the like, the relative positional relationships between the components, and the like may be exaggerated for the sake of clarity of the description in the embodiments. 
       Embodiment 1 
       [0030]    The photoelectric conversion device according to this embodiment will be described with reference to  FIG. 1 ,  FIGS. 2A and 2B , and  FIGS. 3A and 3B . Here,  FIG. 1  is a plane view of a photoelectric conversion device.  FIGS. 2A and 2B  are cross-sectional views taken along lines A 1 -B 1  and C 1 -D 1  of  FIG. 1 , respectively.  FIGS. 3A and 3B  are cross-sectional views taken along lines G 1 -H 1  and E 1 -F 1  of  FIG. 1 , respectively. This embodiment aims at simplifying a process for manufacturing a wiring connecting photoelectric conversion cells and/or at preventing a defective rupture in the wiring connecting the photoelectric conversion cells. The following description will be made with reference to those drawings. 
         [0031]    A photoelectric conversion device  100  according to this embodiment includes a first photoelectric conversion cell  102  and a second photoelectric conversion cell  103  which are fixed over a support substrate  101 . The support substrate  101  is a substrate with an insulating surface or an insulating substrate. It is for example particularly recommended to use any of a variety of glass substrates of the electronic industry such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass. Further, chemically tempered glass or soda lime glass may be used. 
         [0032]    The first photoelectric conversion cell  102  has a first single crystal semiconductor layer  104  which is provided with a first electrode  106  on the support substrate  101  side and a second electrode  108  on a surface opposite thereto. The first single crystal semiconductor layer  104  has a semiconductor junction such as a p-n junction or a p-i-n junction in order to have a photovoltaic effect. Also in the second photoelectric conversion cell  103 , a second single crystal semiconductor layer  105  is provided with a first electrode  107  and a second electrode  109 . 
         [0033]    The first single crystal semiconductor layer  104  and the second single crystal semiconductor layer  105  are fixed to the support substrate  101  by a bonding layer  112 . The bonding layer  112  is provided between the first electrode  106  and the support substrate  101  and between the first electrode  107  and the support substrate  101 . The bonding layer  112  is formed from a thin film having a flat surface and hydrophilicity. A thin film which can be employed may be a thin film formed from an insulator such as silicon oxide, silicon nitride, aluminium oxide, or aluminium nitride. 
         [0034]    The first single crystal semiconductor layer  104  and the second single crystal semiconductor layer IOS are formed by separating a thin piece from a single crystal semiconductor substrate. For example, the first single crystal semiconductor layer  104  and the second single crystal semiconductor layer IOs are formed by a hydrogen ion implantation separation method in such a manner that hydrogen ions are implanted at high concentration into a single crystal semiconductor substrate at a predetermined depth and then heat treatment is performed to separate a single crystal semiconductor layer of a surface portion. Alternatively, a method may be employed in which single crystal semiconductor is epitaxially grown on porous silicon, and then separated from a porous silicon layer by waterjet cleavage. As a single crystal semiconductor substrate, a single crystal silicon wafer is typically used. The thickness of the first single crystal semiconductor layer  104  and the second single crystal semiconductor layer  105  is 0.1 μm to 10 μm, inclusive, preferably 1 μm to 5 μm, inclusive. Since the single crystal semiconductor layer separated from the single crystal semiconductor substrate is fixed over the support substrate, the single crystal semiconductor layer can be prevented from being broken even with the thickness of 0.1 μm to 10 μm, inclusive. In the case of using a single crystal silicon semiconductor for the single crystal semiconductor layer, the single crystal semiconductor layer needs to have a thickness of the above range to absorb sunlight since single crystal silicon semiconductor has an energy gap of 1.12 eV and is of an indirect transition type. 
         [0035]    The first single crystal semiconductor layer  104  and the second single crystal semiconductor layer  105  which are fixed to the support substrate  101  are covered with a protective layer  111 . A transparent material is preferably used for the protective layer  111 . The transparent material can be an insulating material such as silicon nitride, silicon oxide, aluminium oxide, or aluminium nitride or a conductive oxide material such as indium tin oxide, zinc oxide, or tin oxide. The protective layer  111  is provided to prevent the single crystal semiconductor layers from being directly exposed to air and to prevent entry of contaminants such as metal ions. In the case of providing the protective layer  111  in order to isolate neighboring photoelectric conversion cells as in this embodiment, an insulating material is preferably used as the transparent material. 
         [0036]    The second electrodes  108  and  109  which are provided over the first single crystal semiconductor layer  104  and the second single crystal semiconductor layer  105 , respectively, have a lattice-like shape (or a net-like shape). Openings  113  are provided in the protective layer  111  in accordance with the shapes of the second electrodes. The second electrode  108  and the second electrode  109  are in contact with the first single crystal semiconductor layer  104  and the second single crystal semiconductor layer  105  in the openings  113 , respectively. 
         [0037]    A through hole  114  which penetrates the protective layer  111  and the second single crystal semiconductor layer  105  and exposes the first electrode  107  is provided in order to connect the second electrode  108  and the first electrode  107 . Electrical connection between the second electrode  108  and the first electrode  107  allows the first photoelectric conversion cell  102  and the second photoelectric conversion cell  103  to be connected in series. This connection structure does not employ a conventional wiring component. This connection structure can be made by extending the second electrode  108  over the first single crystal semiconductor layer  104 . 
         [0038]    Note that a second electrode  110  provided for the first single crystal semiconductor layer  104  is connected the first electrode  106  in the through hole  115 . Therefore, the second electrode  110  serves as an electrode which leads the first electrode  106 , which is not exposed on the surface, to the surface of the first single crystal semiconductor layer  104 . 
         [0039]    According to this embodiment, a wiring which electrically connects the first photoelectric conversion cell and the second photoelectric conversion cell is also provided in the same step of forming electrodes of the photoelectric conversion cells, whereby a manufacturing process can be simplified. A defective rupture in the wiring which connects the first photoelectric conversion cell and the second photoelectric conversion cell can be prevented. In other words, after the first photoelectric conversion cell and the second photoelectric conversion cell are fixed over the support substrate, the wiring which connects the two conversion cells is provided over the surface of the support substrate, thus, adhesion of the connection wiring can be enhanced. 
       Embodiment 2 
       [0040]    An example of a method for manufacturing the photoelectric conversion device described in Embodiment 1 will be described in this embodiment. In the following description,  FIGS. 5A to 5C  are cross-sectional views taken along line A 1 -B 1  of  FIG. 1 ; and  FIGS. 6A and 6B  are cross-sectional views taken along lines A 1 -B 1  and C 1 -D 1  of  FIG. 1 , respectively. 
         [0041]    A semiconductor substrate  116  in  FIG. 4A  is single crystal semiconductor and has an approximately rectangular planar shape. The semiconductor substrate  116  is typically single crystal silicon. In addition, a surface of the semiconductor substrate  116  is preferably mirror polished so that the semiconductor substrate  116  can be closely attached to the support substrate with an insulating layer for bonding interposed therebetween. For example, a p-type single crystal silicon substrate with a resistance of about 1 Ωcm to 10 Ωcm is used as the semiconductor substrate  116 . 
         [0042]    A protective film  117  is formed from silicon oxide or silicon nitride. The protective film  117  is formed by a chemical vapor deposition method typified by a plasma CVD method. The semiconductor substrate  116  is preferably provided with the protection film  117  because the planarity of the surface of the semiconductor substrate  116  is lost due to irradiation with ions for forming a damaged layer in the semiconductor substrate  116 . The protective film  117  preferably has a thickness of 50 nm to 200 nm. 
         [0043]    Then, the surface which is provided with the protective film  117  of the semiconductor substrate  116  is irradiated with an ion beam  119  including hydrogen ions to form a damaged layer  118 . Hydrogen cluster ions, for example H 3   +  ions are introduced as the hydrogen ions to form the damaged layer  118  at a predetermined depth from the surface. The depth of the damaged layer  118  is controlled by the acceleration energy of the hydrogen cluster ions. The thickness of the single crystal semiconductor layer to be separated from the semiconductor substrate  116  is determined by the depth of the damaged layer  118 ; therefore, the electric field intensity for accelerating the hydrogen cluster ions is determined in consideration of the thickness of the single crystal semiconductor layer to be separated. The damaged layer  118  is preferably formed at a depth of less than 10 μm, that is, 50 nm or more and less than 10000 nm, preferably 100 nm to 5000 nm from the surface of the semiconductor substrate  116 . 
         [0044]    Hydrogen cluster ions such as H 3   +  ions can be obtained by generating hydrogen plasma from an ion source which generates ions and extracting ions from the hydrogen plasma. The hydrogen plasma also includes ions such as H 2   +  ions and H +  ions, in addition to H 3   +  ions. If the hydrogen plasma is generated when the pressure in the ion source is 1×10 −2  Pa to 5×10 −1  Pa, the rate of H 3   +  ions in the above three kinds of ions can be increased to 70% or higher. 
         [0045]    In  FIG. 4B , the protective film  117  is removed and the first electrode  106  is formed over the semiconductor substrate  116 . The first electrode  106  is preferably formed using a refractory metal. As the refractory metal, a metal material such as titanium, molybdenum, tungsten, tantalum, chromium, or nickel is used. The first electrode  106  may have a structure in which any of those metal materials and a nitride of the metal (a metal nitride) are stacked. In that case, by providing a metal nitride on the semiconductor substrate  116  side, the first electrode  106  can have a better adhesion to the semiconductor substrate  116 . 
         [0046]    The bonding layer  112  is formed over the first electrode  106 . The bonding layer  112  is formed using a thin film formed from silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, or the like. It is necessary that the bonding layer  112  is smooth and has an average roughness Ra of 1 nm or less, preferably 0.5 nm or less. Note that the “average roughness” here refers to an average roughness obtained by three-dimensional expansion of a centerline average roughness which is defined by JIS B0601 (adhering to ISO 4287) so as to be able to be applied to a plane. 
         [0047]    A preferable example of a thin film with such smoothness is a thin film of silicon oxide which is formed using organosilane by a chemical vapor deposition method. A thin film which is formed using organosilane, for example, a silicon oxide film can be used as the bonding layer  112 . As organosilane, a silicon-containing compound such as tetraethoxysilane (TEOS, chemical formula: Si(OC 2 H 5 ) 4 ), tetramethylsilane (TMS, chemical formula: Si(CH 3 ) 4 ), tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC 2 H 5 ) 3 ), or trisdimethylaminosilane (SiH(N(CH 3 ) 2 ) 3 ) is used as a material and a thin film is formed by a plasma CVD method. 
         [0048]    Alternatively, a silicon nitride film which is formed using a silane gas and an ammonia gas by a plasma CVD method can be used as the bonding layer  112 . A thin film of silicon oxynitride or silicon nitride oxide can be obtained by a plasma CVD method using a silane gas, an ammonia gas, and a nitrogen oxide gas. 
         [0049]    Note that the silicon oxynitride film refers to a film which contains more oxygen than nitrogen and contains oxygen, nitrogen, silicon, and hydrogen at concentrations ranging from 55 at. % to 65 at. %, 0.5 at. % to 20 at. %, 25 at. % to 35 at. %, and 0.1 at. % to 10 at. %, respectively (the percentages of oxygen, nitrogen, silicon, and hydrogen fall within the above ranges, when the total of atoms is 100 atomic %. The same applies in this paragraph). Further, a silicon nitride oxide film refers to a film which contains more nitrogen than oxygen and contains oxygen, nitrogen, silicon, and hydrogen at concentrations ranging from 15 at. % to 30 at. %, 20 at. % to 50 at. %, 25 at. % to 35 at. %, and 15 at. % to 25 at. %, respectively. 
         [0050]    In  FIG. 5A , the surface which is provided with the bonding layer  112  of the semiconductor substrate  16  is closely attached to the support substrate  101 . When the support substrate  101  is closely attached to the bonding layer  112 , the semiconductor substrate  116  is fixed over the support substrate  101  by hydrogen bonds or Van der Waals forces. If the surfaces of the support substrate  101  and the bonding layer  112  are hydrophilic, hydroxyl groups and water molecules effectively act to facilitate formation of hydrogen bonds. Further, if heat treatment is performed, water molecules are decomposed to form silanol groups (Si—OH) and hydrogen bonds are further increased. Even further, if heat treatment at a high temperature is performed, hydrogen atoms are released and siloxane bonds (O—Si—O) are formed to form covalent bonds, whereby the attachment strength of the semiconductor substrate  116  and the support substrate  101  is improved. 
         [0051]      FIG. 5B  illustrates a step in which heat treatment is performed and the first single crystal semiconductor layer  104  is separated from the semiconductor substrate  116  using the damaged layer  118 . The heat treatment is performed at temperatures of 400° C. to 700° C. By this heat treatment, minute cavities formed in the damaged layer  118  change in volume and a crack occurs at the level of the damaged layer  118 . Since the bonding layer  112  is bonded to the support substrate  101 , the semiconductor substrate  116  can be separated from the support substrate  101  by this heat treatment while the first single crystal semiconductor layer  104  is left over the support substrate  101 . The thickness of the first single crystal semiconductor layer  104  is 50 nm or more and less than 10000 nm, preferably 100 nm to 5000 nm. The thickness of the first single crystal semiconductor layer  104  can be controlled by the depth of the damaged layer  118 . 
         [0052]    After that, as illustrated in  FIG. 5C , an impurity-containing semiconductor layer  120  which has a conductivity type opposite to that of the semiconductor substrate  116  is formed over the first single crystal semiconductor layer  104 . The impurity-containing semiconductor layer  120  may be formed by adding an impurity element which serves as a donor or an acceptor to the first single crystal semiconductor layer  104  or by depositing a layer containing an impurity element which serves as a donor or an acceptor over the first single crystal semiconductor layer  104 . The protective layer  111  is provided to cover the entire surface of the first single crystal semiconductor layer  104 . 
         [0053]    Then, the protective layer  111  is processed.  FIG. 6A  is a cross-sectional view taken along line A 1 -B 1  of  FIG. 1 . The opening  113  is provided in the protective layer  111 . Further,  FIG. 6B  is a cross-sectional view taken along line C 1 -D 1  of  FIG. 1 . The through hole  115  is formed as well as the opening in the protective layer  111 . The opening  113  in the protective layer  111  and the through hole  115  in the first single crystal semiconductor layer  104  are formed by irradiating the protective layer  111  and the first single crystal semiconductor layer  104  with a laser beam to subject them to a groove processing. Through the laser beam process, a groove with a width of 30 μm to 300 μm can be formed. In addition, even when the support substrate  101  has an increased size, the process can be easily performed. 
         [0054]    As illustrated in  FIGS. 2A and 2B , the second electrode  108  and the second electrode  110  are formed in accordance with the opening  113  and the through hole  115 . The second electrode  108  is in contact with the impurity-containing semiconductor layer  120 . The second electrode  110  is in contact with the first electrode  106  through the through hole  115 . 
         [0055]    Through the above steps, the photoelectric conversion device described in Embodiment 1 can be obtained. According to this embodiment, by utilizing a bonding technique, a single crystal semiconductor layer having a thickness of 10 μm or less can be provided over a support substrate such as a glass substrate at a process temperature of 700° C. or lower. Further, a wiring which electrically connects the first photoelectric conversion cell and the second photoelectric conversion cell is also provided in the same step of forming electrodes of the photoelectric conversion cells, whereby a manufacturing process can be simplified. 
       Embodiment 3 
       [0056]    An example of a method for manufacturing the photoelectric conversion device described in Embodiment 1 will be described in this embodiment. The method in this embodiment is different from the method described in Embodiment 2. In the following description,  FIGS. 8A and 8B  are cross-sectional views taken along lines A 1 -B 1  and C 1 -D 1  of  FIG. 1 , respectively. 
         [0057]      FIG. 7A  illustrates formation of the damaged layer  118 . In this embodiment, the semiconductor substrate  116  provided with a protective layer  121  is irradiated with the ion beam  119  including hydrogen ions, whereby the damaged layer  118  is formed. A silicon nitride film is preferably used as the protective layer  121  in order to suppress surface recombination. 
         [0058]      FIG. 7B  illustrates formation of impurity-containing semiconductor layers  123 . To the impurity-containing semiconductor layers  123 , an impurity element which imparts the same conductivity type as the semiconductor substrate  116  is added in such a manner that the impurity concentration of the impurity-containing semiconductor layers  123  is higher than that of the semiconductor substrate  116 . In that case, openings  122  are formed in the protective layer  121 , and the impurity element is added through the openings with the protective layer  121  serving as a mask. Thus, the impurity-containing semiconductor layers  123  are discretely formed, whereby surface recombination of the protective layer  121  can be suppressed. 
         [0059]    As illustrated in  FIG. 7C , the first electrode  106  and the bonding layer  112  are formed. If the surface of the first electrode  106  is uneven due to the formation of openings in the protective layer  121 , it is preferable to perform polishing treatment to flatten the surface after the first electrode  106  is formed. 
         [0060]    After that, as illustrated in  FIGS. 8A and 8B , the first single crystal semiconductor layer  104  is bonded to the support substrate  101 , then, the protective layer  111 , the second electrode  108 , and the second electrode  110  are formed as in Embodiment 2. 
         [0061]    Through the above steps, the photoelectric conversion device described in Embodiment 1 can be obtained. According to this embodiment, by utilizing a bonding technique, a single crystal semiconductor layer having a thickness of 10 μm or less can be provided over a support substrate such as a glass substrate at a process temperature of 700° C. or lower. Further, a wiring which electrically connects the first photoelectric conversion cell and the second photoelectric conversion cell is also provided in the same step of forming electrodes of the photoelectric conversion cells, whereby a manufacturing process can be simplified. Additionally, according to this embodiment, surface recombination of the single crystal semiconductor layer can be suppressed. 
       Embodiment 4 
       [0062]    The photoelectric conversion device according to this embodiment will be described with reference to  FIG. 9 ,  FIGS. 10A and 10B , and  FIGS. 11A and 11B . Here,  FIG. 9  is a plane view of the photoelectric conversion device.  FIGS. 10A and 10B  are cross-sectional views taken along lines A 2 -B 2  and C 2 -D 2  of  FIG. 9 , respectively.  FIGS. 11A and 11B  are cross-sectional views taken along lines G 2 -H 2  and E 2 -F 2  of  FIG. 9 , respectively. In this embodiment, a photoelectric conversion cell which has the structure in Embodiment 1 and in which the semiconductor layer which conducts photoelectric conversion has stacked two layers will be described. 
         [0063]    A photoelectric conversion device  200  according to this embodiment includes the first photoelectric conversion cell  102  and the second photoelectric conversion cell  103  which are fixed over the support substrate  101 . The first photoelectric conversion cell  102  has a first stacked semiconductor layer  124  in which the first single crystal semiconductor layer  104  and a first non-single-crystal semiconductor layer  129  are stacked. The first single crystal semiconductor layer  104  has the first electrode  106  on the support substrate  101  side. A transparent electrode  131  is provided over the first non-single-crystal semiconductor layer  129 . The transparent electrode  131  is formed from a transparent conductive material such as indium oxide, zinc oxide, or tin oxide. The second electrode  108  is provided over the transparent electrode  131 . The second electrode  108  has a lattice-like shape (or a net-like shape) and is provided to compensate for sheet resistance of the transparent electrode  131 . 
         [0064]    Examples of non-single-crystal semiconductor materials which can be used to form the first non-single-crystal semiconductor layer  129  are amorphous silicon and microcrystal silicon. The first non-single-crystal semiconductor layer  129  has a structure in which a p-type and an n-type semiconductor layers sandwiches a semiconductor layer (an i-type semiconductor layer) having a lower dark conductivity than the p-type and the n-type semiconductor layers. 
         [0065]    In the first stacked semiconductor layer  124  of this embodiment, a diode of the first single crystal semiconductor layer  104  and a diode of the first non-single-crystal semiconductor layer  129  are connected in series. Also in a second stacked semiconductor layer  125 , a diode of the second single crystal semiconductor layer  105  and a diode of the second non-single-crystal semiconductor layer  130  are connected in series. 
         [0066]    In the case where the energy gap of the first non-single-crystal semiconductor layer  129  is 1.75 eV, for example, the thickness of the first non-single-crystal semiconductor layer  129  is 200 nm to 400 nm. In the case where the energy gap of the first single crystal semiconductor layer  104  is 1.12 eV, the thickness of the first single crystal semiconductor layer  104  is 1 μm to 5 μm. In any case, the thickness of the first non-single-crystal semiconductor layer  129  and the first single crystal semiconductor layer  104  are determined so that their photoelectric current can be approximately the same. Thus, conversion efficiency can be maximized. 
         [0067]    The first single crystal semiconductor layer  104  and the second single crystal semiconductor layer  105  are spaced over the support substrate  101  as in Embodiment 1. On the other hand, the first non-single-crystal semiconductor layer  129 , the second non-single-crystal semiconductor layer  130 , and the transparent electrode  131  are formed over the entire surface of the support substrate  101  by a thin film deposition method typified by a plasma CVD method or a sputtering method. Therefore, a separation groove  126  and a separation groove  127  are provided in order to isolate neighboring photoelectric conversion cells. The separation groove  126  penetrates the transparent electrode  131  and the first non-single-crystal semiconductor layer  129  to reach the support substrate  101 . On the other hand, the separation groove  127  is provided to divide the transparent electrode  131  in order to connect the photoelectric conversion cells in series. The separation groove  127  may be formed to go through the second non-single-crystal semiconductor layer  130  but not to pierce the first electrode  107 . An insulating layer  128  is formed to fill the separation groove  126  and the separation groove  127 , which maintains isolation. 
         [0068]    Note that as in Embodiment 1, the second electrode  108  provided over the first single crystal semiconductor layer  104  is connected to the first electrode  107  through the through hole  115 . Thus, the first photoelectric conversion cell  102  and the second photoelectric conversion cell  103  are connected in series. 
         [0069]    According to this embodiment, a wiring which electrically connects the first photoelectric conversion cell and the second photoelectric conversion cell is also provided in the same step of forming electrodes of the photoelectric conversion cells, whereby a manufacturing process can be simplified. A defective rupture in the wiring which connects the first photoelectric conversion cell and the second photoelectric conversion cell can be prevented. 
         [0070]    This application is based on Japanese Patent Application serial No. 2008-229103 filed with Japan Patent Office on Sep. 5, 2008, the entire contents of which are hereby incorporated by reference.