Patent Publication Number: US-2011056552-A1

Title: Solar cell and method for manufacturing the same

Description:
TECHNICAL FIELD  
     The present invention relates to a solar cell including transparent conductive films formed on a photoelectric conversion body and a method for manufacturing the same. 
     BACKGROUND ART  
     Solar cells are expected as a new source of energy, because solar cells are capable of directly converting light, which comes from the sun as a clean, unlimited source of energy, to electricity. 
     A solar cell includes a photoelectric conversion body which has an n-type semiconductor layer and a p-type semiconductor layer and receives light to generate photogenerated carriers. The carriers are extracted from the photoelectric conversion body through electrodes formed on the n- and p-type semiconductor layers. Each electrode to be used has a layered structure of a transparent conductive film and a metallic layer in some cases. In such a case, it is desirable that the transparent conductive film have low electric resistivity. 
     Herein, there is proposed a technique to form a transparent conductive film in an atmosphere containing hydrogen (see Japanese Patent Application Publication No. Hei 2-54755). With this technique, hydrogen atoms introduced into the transparent conductive film terminate dangling bonds of atoms constituting the transparent conductive film. The electric resistivity of the transparent conductive film can be therefore reduced. 
     However, if transparent conductive films are formed on the photoelectric conversion body by using the above technique, the surface characteristics of the photoelectric conversion body could be degraded. Specifically, hydrogen radicals having reducing properties could degrade the surfaces of the n- and p-type semiconductor layers when the transparent conductive films are formed. 
     The present invention was made in the light of the aforementioned problem, and an object of the present invention is to provide a solar cell with the surface characteristics of the photoelectric conversion body prevented from being degraded and a method for manufacturing the same. 
     DISCLOSURE OF THE INVENTION  
     The solar cell according to one characteristic of the present invention, is summarized as comprising a photoelectric conversion body configured to generate photogenerated carriers upon receiving light; a first transparent conductive film formed on a first major surface of the photoelectric conversion body; and a second transparent conductive film formed on a second major surface provided on an side opposite to the first major surface, wherein the first major surface is formed by an n-type semiconductor layer, the second major surface is formed by a p-type semiconductor layer, and a hydrogen atom content of a part of the first transparent conductive film on a side close to the n-type semiconductor layer is lower than a hydrogen atom content of the second transparent conductive film. 
     In the solar cell according to a characteristic of the present invention, the hydrogen atom content of the first transparent conductive film formed on the first major surface on the n-type semiconductor layer side is lower than that of the second transparent conductive film formed on the second major surface. This can reduce the effect of hydrogen radicals on the surface of the n-type semiconductor layer forming the first major surface of the photoelectric conversion body. The degradation of the surface characteristics of the photoelectric conversion body can be therefore prevented. 
     In the one characteristic of the present invention, the hydrogen atom content of a part of the first transparent conductive film on an side opposite to the n-type semiconductor layer may be higher than the hydrogen atom content of the part of the first transparent conductive film on the side close to the n-type semiconductor layer. 
     In the one characteristic of the present invention, the part of the first transparent conductive film on the side close to the n-type semiconductor layer may be made of a different material from a material of the part of the first transparent conductive film on the side opposite to the n-type semiconductor layer. 
     In the one characteristic of the present invention, the photoelectric conversion body is mainly made of single-crystalline silicon and each of the n-type semiconductor layer and the p-type semiconductor layer is mainly made of an amorphous silicon semiconductor. 
     The method for manufacturing a solar cell according to one characteristic of the present invention, is summarized as including a photoelectric conversion body configured to generate photogenerated carriers upon receiving light, the method comprising: a step A of forming a first transparent conductive film in an atmosphere not containing hydrogen on a first major surface of the photoelectric conversion body; and a step B of forming a second transparent conductive film in an atmosphere containing hydrogen on a second major surface provided on an side opposite to the first major surface, wherein the first major surface is formed by an n-type semiconductor layer, the second major surface is formed by a p-type semiconductor layer, and in the step B, the second transparent conductive film is formed while hydrogen atoms are being supplied. 
     In the one characteristic of the present invention, the step B may be performed after the step A is performed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [ FIG. 1 ]  FIG. 1  is a plan view of a solar cell  10  according to a first embodiment of the present invention on a second major surface side. 
       [ FIG. 2 ]  FIG. 2  is a cross-sectional view of the solar cell  10  according to the first embodiment of the present invention. 
       [ FIG. 3 ]  FIG. 3  is a cross-sectional view of a solar cell  10  according to a second embodiment of the present invention. 
       [ FIG. 4 ]  FIG. 4  is a cross-sectional view of a solar cell  20  according to another embodiment. 
       [ FIG. 5 ]  FIG. 5  is a view showing a relationship between thickness of an ITO film formed on an n-type amorphous silicon layer and a fill factor FF. 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION  
     Next, the embodiments of the invention are described with reference to the drawings. In the following description of the drawings, identical or similar reference numerals are assigned to identical or similar components. However, the drawings are schematic, thus it should be noted that the dimensions are not shown to scale. Accordingly, specific dimensions should be recognized in consideration of the following description. Also, there are inevitably included some portions of the drawings between which a dimensional relationship and/or a scale are inconsistent. 
     First Embodiment   
     &lt;Schematic Structure of Solar Cell&gt; 
     The schematic structure of a solar cell  10  according to a first embodiment of the present invention is described below with reference to  FIGS. 1 and 2 .  FIG. 1  is a plan view of the solar cell  10  on a second major surface S 2  side.  FIG. 2  is an A-A cross-sectional view of  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , the solar cell  10  includes a solar cell substrate  1 , fine-line electrodes  2 , and connecting electrodes  3 . 
     As shown in  FIG. 2 , the solar cell substrate  1  includes a photoelectric conversion body  11 , a first transparent conductive film  12 , and a second transparent conductive film  13 . The photoelectric conversion body  11  is configured to generate photogenerated carriers upon receiving light. The solar cell substrate  1  includes a first major surface S 1  (the lower surface in  FIG. 2 ) and a second major surface S 2  (the upper surface in  FIG. 2 ) provided on the opposite side to the first major surface S 1 . The structure of the solar cell substrate  1  is described later. 
     The fine-line electrodes  2  are collecting electrodes configured to collect photogenerated carriers from the photoelectric conversion body  11 . The fine-line electrodes  2  are formed on the second major surface S 2  of the solar cell substrate  1  along a first direction substantially parallel to a side of the solar cell substrate  1  as shown in  FIG. 1 . Some of the plurality of fine-line electrodes  2  are arranged substantially at regular intervals in a second direction substantially orthogonal to the first direction. The other fine-line electrodes  2  (not shown) are formed on the first major surface S 1  of the solar cell substrate  1  in a similar manner to those on the second major surface S 2  of the solar cell substrate  1 . The fine-line electrodes  2  formed on the second major surface S 2  of the solar cell substrate  1  collect carriers from the photoelectric conversion body  11  through the second transparent conductive film  13  later described. The fine-line electrodes  2  formed on the first major surface S 1  of the solar cell substrate  1  collect carriers from the photoelectric conversion body  11  through the first transparent conductive film  12  later described. 
     The fine-line electrodes  2  can be formed, for example, by printing resin-type conductive paste which includes a resin material as a binder and conductive particles such as silver particles as a filler or sintered-type conductive paste containing silver powder, glass frit, organic vehicle, organic solvent, and the like (so-called ceramic paste) by using a printing process. Each fine-line electrode  2  may be formed as a metallic film by deposition. The number and size of the fine-line electrodes  2  can be set to proper values in consideration of the size of the photoelectric conversion body  11  and the like. 
     The connecting electrodes  3  are electrodes connected to wiring materials (not shown) electrically connecting a plurality of the solar cells  10  in series or in parallel. The connecting electrodes  3  are formed on the second major surface S 2  of the solar cell substrate  1  along the second direction as shown in  FIG. 1 . Accordingly, some of the connecting electrodes  3  intersect the plurality of fine-line electrodes  2  and are electrically connected to the plurality of fine-line electrodes  2 . The other connecting electrodes  3  (not shown) are formed on the first major surface S 1  of the solar cell substrate  1  in a similar manner to the first major surface S 2  of the solar cell substrate  1 . The connecting electrodes  3  can be formed by printing or deposition in a similar way to the fine-line electrodes  2 . The number and size of the connecting electrodes  3  can be set to proper values in consideration of the size of the photoelectric conversion body  11  and the like. 
     &lt;Structure of Solar Cell Substrate&gt; 
     Next, a description is given of an example of the structure of the solar cell substrate  1  according to the first embodiment of the present invention with reference to  FIG. 2 . 
     The solar cell substrate  1  includes the photoelectric conversion body  11  and the first and second transparent conductive films  12  and  13  as shown in  FIG. 2 . 
     The photoelectric conversion body  11  includes the first major surface S 1  (the lower surface in  FIG. 2 ) and the second major surface S 2  (the upper surface in  FIG. 2 ) provided on the opposite side to the first major surface S 1 . The photoelectric conversion body  11  receives light through the first or second major surface S 1  or S 2  of the photoelectric conversion body  11  to generate photogenerated carriers. Each of the photogenerated carriers refers to a pair of a hole and an electron generated upon the photoelectric conversion body  11  absorbing solar light. 
     The photoelectric conversion body  11  includes an n-type substrate  11   a,  an i-type semiconductor layer  11   b,  a p-type semiconductor layer  11   c,  an i-type semiconductor layer  11   d,  and an n-type semiconductor layer  11   e  as shown in  FIG. 2 . 
     The n-type substrate  11   a  is a main body of the photoelectric conversion body  11 . For the n-type substrate  11   a,  an n-type single-crystalline silicon substrate can be used, for example. 
     The p-type semiconductor layer  11   c  is formed on the second major surface S 2  side of the n-type substrate  11   a  with the i-type semiconductor layer  11   b  interposed therebetween. On the other hand, the n-type semiconductor layer  11   e  is formed on the first major surface S 1  side of the n-type substrate  11   a  with the i-type semiconductor layer  11   d  interposed therebetween. In other words, the second major surface S 2  of the photoelectric conversion body  11  is formed by the p-type semiconductor layer  11   c,  and the first major surface S 1  of the photoelectric conversion body  11  is formed by the n-type semiconductor layer  11   e.    
     For the i-type semiconductor layer  11   b,  p-type semiconductor layer  11   c,  i-type semiconductor layer  11   d,  and n-type semiconductor layer  11   e,  for example, an i-type amorphous semiconductor layer, a p-type amorphous semiconductor layer, an i-type amorphous semiconductor layer, and a n-type amorphous semiconductor layer can be used, respectively. In such a case, it is preferable that the i-type semiconductor layers  11   b  and  11   d  constituted by i-type amorphous semiconductor layers have such thicknesses that the i-type semiconductor layers  11   b  and  11   d  substantially do not contribute to power generation. For example, the thicknesses of the i-type semiconductor layers  11   b  and  11   d  are preferably in a range from several Å to 250 Å. The amorphous semiconductors can be silicon-based semiconductors such as amorphous silicon, amorphous silicon carbide, amorphous silicon germanium, and microcrystalline silicon. 
     The structure of the photoelectric conversion body  11  is not limited to the aforementioned structure. For example, the photoelectric conversion body  11  does not need to include the i-type semiconductor layers  11   b  and  11   d  and may have another structure. 
     As shown in  FIG. 2 , the first transparent conductive film  12  is formed on the n-type semiconductor layer  11   e  forming the first major surface S 1  of the photoelectric conversion body  11 . The first transparent conductive film  12  has translucency and conductivity. For the first transparent conductive layer  12 , a metal oxide such as indium oxide (In 2 O 3 ), zinc oxide (ZnO), tin oxide (SnO 2 ), or titanium oxide (TiO 2 ) can be used. These metal oxides may be doped with a dopant such as tin (Sn), zinc (Zn), tungsten (W), antimony (Sb), titanium (Ti), cerium (Ce), zirconium (Zr), molybdenum (Mo), aluminum (Al), or gallium (Ga). The concentration of the dopant can be 0 to 20 wt %. 
     As shown in  FIG. 2 , the second transparent conductive film  13  is formed on the p-type semiconductor layer  11   c  forming the second major surface S 2  of the photoelectric conversion body  11 . The second transparent conductive film  13  has translucency and conductivity. Similarly to the first transparent conductive film  12 , for the second transparent conductive film  13 , a metal oxide such as indium oxide (In 2 O 3 ), zinc oxide (ZnO). tin oxide (SnO 2 ), or titanium oxide (TiO 2 ) can be used. These metal oxides may be doped with a dopant such as tin (Sn), zinc (Zn), tungsten (W), antimony (Sb), titanium (Ti), cerium (Ce), zirconium (Zr), molybdenum (Mo), aluminum (Al), or gallium (Ga). The concentration of the dopant can be 0 to 20 wt %. The materials constituting the second transparent conductive film  13  may be either the same as or different from materials constituting the first transparent conductive film  12 . 
     In this embodiment, the hydrogen atom content of the first transparent conductive film  12  is lower than that of the second transparent conductive film  13 . Accordingly, a metal oxide formed in an atmosphere not containing hydrogen can be used for the first transparent conductive film  12  while a metal oxide formed in an atmosphere supplying hydrogen atoms (an atmosphere containing hydrogen) can be used for the second transparent conductive film  13 . 
     &lt;Method for Manufacturing Solar Cell&gt; 
     Next, a method for manufacturing the solar cell  10  according to the first embodiment of the present invention is described. First, the n-type substrate  11   a  is etched to form fine roughness on the first and second major surfaces S 1  and S 2  sides. 
     Next, the i-type and p-type semiconductor layers  11   b  and  11   c  are sequentially stacked on the second major surface S 2  side of the n-type substrate  11   a  using CVD or the like. The i-type and n-type semiconductor layers  11   d  and  11   e  are sequentially stacked on the first major surface S 1  side of the n-type substrate  11   a  in a similar manner. The photoelectric conversion body  11  is thus formed. 
     Next, the first transparent conductive film  12  is formed on the n-type semiconductor layer  11   e  by using a sputtering method or the like in an atmosphere not containing hydrogen such as an Ar and O 2  atmosphere. The atmosphere not containing hydrogen refers to an atmosphere not actively supplying hydrogen atoms. The hydrogen atom contents of the transparent conductive films formed in the atmosphere not containing hydrogen are about 1.0×10 20  to 5.2×10 20  (atoms/cc). 
     Next, by using a sputtering method or the like, the second transparent conductive film  13  is formed on the p-type semiconductor layer  11   c  in an atmosphere containing hydrogen. Specifically, the second transparent conductive film  13  is formed in an Ar, O 2 , and H 2 O atmosphere, an Ar, O 2 , and H 2  atmosphere, or the like where hydrogen atoms are being supplied thereto. The hydrogen atom content of the second transparent conductive film  13  is therefore higher than that of the first transparent conductive film  12 . The solar cell substrate  1  is thus formed. Preferably, the process to form the second transparent conductive film  13  is performed after the process to form the first transparent conductive film  12  is performed. 
     Next, epoxy-based thermosetting-type silver paste is placed on the second major surface S 2  of the solar cell substrate  1  in a predetermined pattern by using a printing process such as screen printing or off-set printing. In a similar manner, epoxy-based thermosetting-type silver paste is placed on the first major surface S 1  of the solar cell  1  in a predetermined pattern. The silver paste is heated under predetermined conditions for volatilization of the solvent and is then further heated for final drying. Herein, the predetermined patterns have the shapes corresponding to the fine-line electrodes  2  extending along the first direction and the connecting electrodes  3  extending along the second direction as shown in  FIG. 1 . The solar cell  10  is thus manufactured. 
     &lt;Operation and Effects&gt; 
     In the solar cell  10  according to the first embodiment of the present invention, the first transparent conductive film  12  is formed on the n-type semiconductor layer  11   e,  which forms the first major surface S 1  of the photoelectric conversion body  11 , in the atmosphere not containing hydrogen. Moreover, the second transparent conductive film  13  is formed on the p-type semiconductor layer  11   c,  which forms the second major surface S 2  of the photoelectric conversion body  11 , in the atmosphere containing hydrogen. The hydrogen atom content of the first transparent conductive film  12  formed on the first major surface S 1  of the photoelectric conversion body  11  is therefore lower than that of the second transparent conductive film  13  formed on the second major surface S 2  of the photoelectric conversion body  11 . 
     The applicant examined the influence of hydrogen radicals on the surfaces of the p-type and n-type semiconductor layer  11   c  and  11   e  in the case where the transparent conductive films were formed in the atmosphere containing hydrogen on the p-type semiconductor layer  11   c  forming the second major surface S 2  of the photoelectric conversion body  11  and on the n-type semiconductor layer  11   e  forming the first major surface S 1  of the photoelectric conversion body  11 . As a result, the applicant found that the surface of the n-type semiconductor layer  11   e  was more likely to be degraded by hydrogen radicals. 
     Accordingly, the first transparent conductive film  12  is formed in the atmosphere not containing hydrogen on the n-type semiconductor layer  11   e,  the surface of which is more likely to be degraded in the atmosphere containing hydrogen, so that the surface of the n-type semiconductor layer  11   e  can be prevented from being degraded by hydrogen radicals. On the other hand, the second transparent conductive film  13  is formed on the p-type semiconductor layer  11   c,  which is less affected by hydrogen radicals in the atmosphere containing hydrogen than the n-type semiconductor layer  11   e,  with hydrogen atoms being actively introduced thereto. This can provide increased translucency and reduced electric resistivity to the second transparent conductive film  13 . As described above, in the solar cell  10  according to the first embodiment of the present invention, it is possible to prevent the degradation of the surface characteristics of the photoelectric conversion body  11 . 
     Moreover, since the second transparent conductive film  13  is formed on the p-type semiconductor layer  11   c  after the first transparent conductive film  12  is formed on the n-type semiconductor layer  11   e,  the surface of the n-type semiconductor layer  11   e  is protected by the first transparent conductive film  12 . This can further reduce the influence of hydrogen radicals on the surface of the n-type semiconductor layer  11   e  when the second transparent conductive film  13  is formed in the atmosphere containing hydrogen. 
     Second Embodiment   
     Hereinbelow, a second embodiment of the present invention is described. The following description is mainly about the difference between the aforementioned first embodiment and the second embodiment. Specifically, in the aforementioned first embodiment, the hydrogen atom content of the first transparent conductive film  12  on the n-type semiconductor layer  11   e  side is substantially the same as that of the first transparent conductive film  12  on the opposite side to the n-type semiconductor layer  11   e.  In contrast, in the second embodiment, the hydrogen atom content of the first transparent conductive film  12  on the n-type semiconductor layer  11   e  side is different from that of the first transparent conductive film  12  on the opposite side to the n-type semiconductor layer  11   e.  The schematic structure of the solar cell  10  according to the second embodiment of the present invention is substantially the same as that of the solar cell  10  according to the first embodiment, and the description thereof is omitted herein. 
     &lt;Structure of Solar Cell&gt; 
     An example of the structure of the solar cell substrate  1  according to the second embodiment of the present invention is described with reference to  FIG. 3 . 
     The solar cell substrate  1  includes the photoelectric conversion body  11 , the first transparent conductive film  12 , and the second transparent conductive film  13  as shown in  FIG. 3 . The structures of the photoelectric conversion body  11  and second transparent conductive film  13  are substantially the same as those of the aforementioned first embodiment, and the description thereof is omitted. 
     The first transparent conductive film  12  includes a third transparent conductive film  12   a  and a fourth transparent conductive film  12   b  as shown in  FIG. 3 . The third transparent conductive film  12   a  is formed on the n-type semiconductor layer  11   e  forming the first major surface S 1  of the photoelectric conversion body  11 . The fourth transparent conductive film  12   b  is formed on the third transparent conductive film  12   a.  The third and fourth transparent conductive films  12   a  and  12   b  include translucency and conductivity. For the third and fourth transparent conductive films  12   a  and  12   b,  a metal oxide such as indium oxide (In 2 O 3 ), zinc oxide (ZnO), tin oxide (SnO 2 ), or titanium oxide (TiO 2 ) can be used. These metal oxides may be doped with a dopant such as tin (Sn), zinc (Zn), tungsten (W), antimony (Sb), titanium (Ti), cerium (Ce), zirconium (Zr), molybdenum (Mo), aluminum (Al), or gallium (Ga). The concentration of the dopant can be 0 to 20 wt %. 
     In this embodiment, the hydrogen atom content of the fourth transparent conductive film  12   b  is higher than that of the third transparent conductive film  12   a.  In other words, the hydrogen atom content of a half of the first transparent conductive film  12  on the opposite side to the n-type semiconductor layer  11   e  is higher than that of the other half on the n-type semiconductor layer  11   e  side. Moreover, the hydrogen atom content of the third transparent conductive film  12   a  is lower than that of the second transparent conductive film  13 . In other words, the hydrogen atom content of the half of the first transparent conductive film  12  on the n-type semiconductor layer  11   e  side is lower than that of the second transparent conductive film  13 . 
     Accordingly, for example, for the third transparent conductive film  12   a,  a metallic oxide formed in the atmosphere not containing hydrogen can be used while a metallic oxide formed in the atmosphere supplying hydrogen atoms (the atmosphere containing hydrogen) in a similar manner to the second transparent conductive film  13  can be used for the fourth transparent conductive film  12   b.  The materials constituting the second, third, and fourth transparent conductive films  13 ,  12   a,  and  12   b  may be the same or different. 
     &lt;Method for Manufacturing Solar Cell&gt; 
     Next, a method for manufacturing the solar cell  10  according to the second embodiment of the present invention is described. 
     First, the photoelectric conversion body  11  is formed in the same way as the aforementioned first embodiment. Next, the third transparent conductive film  12   a  is formed on the n-type semiconductor layer  11   e,  which forms the first major surface S 1  of the photoelectric conversion body  11 , by using a sputtering method or the like in the atmosphere not containing hydrogen such as an Ar and O 2  atmosphere. 
     Next, the second transparent conductive film  13  is formed on the p-type semiconductor layer  11   c  by using a sputtering method or the like in the atmosphere containing hydrogen. Specifically, the second transparent conductive film  13  is formed in an Ar, O 2 , and water vapor atmosphere, an Ar, O 2 , and hydrogen atmosphere, or the like where hydrogen atoms are being supplied. The hydrogen atom content of the second transparent conductive film  13  is therefore made higher than that of the third transparent conductive film  12   a.    
     Next, the fourth transparent conductive film  12   b  is formed on the third transparent conductive film  12   a  by using a sputtering method or the like in the atmosphere containing hydrogen. Specifically, the fourth transparent conductive film  12   b  is formed in an Ar, O 2 , and water vapor atmosphere, an Ar, O 2 , and hydrogen atmosphere, or the like where hydrogen atoms are being supplied. The hydrogen atom content of the fourth transparent conductive film  12   b  is therefore made higher than that of the third transparent conductive film  12   a.  The first transparent conductive film  12  is thus formed. The solar cell substrate  1  is formed in such a manner. 
     Next, the fine-line electrodes  2  and connecting electrodes  3  are formed in the same way as the aforementioned first embodiment. The solar cell  10  is thus formed. 
     &lt;Operation and Effects&gt; 
     In the solar cell  10  according to the second embodiment of the present invention, the first transparent conductive film  12  formed on the n-type semiconductor layer  11   e  includes the third and fourth transparent conductive films  12   a  and  12   b.  The third transparent conductive film  12   a  is formed on the n-type semiconductor layer  11   e  in the atmosphere not containing hydrogen, and the fourth transparent conductive film  12   b  is formed on the third transparent conductive film  12   a  in the atmosphere containing hydrogen. 
     Accordingly, it is possible to increase the translucency of the overall first transparent conductive film  12  and reduce the electrical resistivity thereof without degrading the surface of the n-type semiconductor layer  11   e  in comparison with the case where the hydrogen atom content of the first transparent conductive film  12  on the n-type semiconductor layer  11   e  side is substantially equal to that of the first transparent conductive film  12  on the opposite side to the n-type semiconductor layer  11   e.    
     Other Embodiments  
     The present invention has been described by using the embodiments of the present invention. However, it should not be understood that the description and drawings which constitute part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples, and operation techniques will be easily found by those skilled in the art. 
     For example, in the aforementioned first and second embodiments, the description is given of the case where the present invention is applied to the crystal-type solar cell; however, the present invention may be applied to thin film-type solar cells.  FIG. 4  is a cross-sectional view of a thin film-type solar cell  20 . As shown in  FIG. 4 , the thin film-type solar cell  20  includes a substrate  21  having translucency, a fifth transparent conductive film  22 , a photoelectric conversion body  23  configured to receive light and generate photogenerated carriers, a sixth transparent conductive film  24 , and a rear electrode layer  25 . The photoelectric conversion body  23  has a structure in which a p-type semiconductor layer  23   a,  an i-type semiconductor layer  23   b,  and an n-type semiconductor layer  23   c  are sequentially stacked from the substrate  21  side. In the solar cell  20  having such a structure, similarly to the aforementioned first and second embodiments, the fifth transparent conductive film  22  is formed on the substrate  21  in the atmosphere containing hydrogen, and the sixth transparent conductive film  24  is formed on the n-type semiconductor layer  23   c  in the atmosphere not containing hydrogen. This can prevent degradation of the surface of the n-type semiconductor layer  23   c  (corresponding to “the second surface S 2 ” of the aforementioned embodiments). Moreover, the translucency of the fifth transparent conductive film  22  can be increased, and the electrical resistivity thereof can be reduced. Furthermore, since the p-type semiconductor layer  23   a  is formed after the fifth transparent conductive film  22  is formed, it is possible to prevent degradation of the surface of the p-type semiconductor layer  23   a  on the fifth transparent conductive film  22  side (corresponding to “the first surface S 1 ” of the aforementioned embodiment). The structure of the solar cell  20  is not limited to the structure shown in  FIG. 4  and may be a structure in which the fifth transparent conductive film  22 , photoelectric conversion body  23 , sixth transparent conductive film  24 , rear electrode layer  25 , and substrate  21  are stacked in this order. Moreover, the structure of the photoelectric conversion body  23  is not limited to the structure shown in  FIG. 4  and may be a structure in which the n-type, i-type, and p-type semiconductor layers  23   c,    23   b,  and  23   a  are sequentially stacked from the substrate  21  side or a structure in which the p-type and n-type semiconductor layers  23   a  and  23   c  are sequentially stacked from the substrate  21  side. For the p-type, i-type, and n-type semiconductor layers  23   a,    23   b,  and  23   c,  p-type, i-type, and n-type semiconductor materials of amorphous Si, amorphous SiC, amorphous SiGe, microcrystalline Si, or the like can be used, respectively. 
     The aforementioned first and second embodiments describe about the case where the fine-line electrodes  2  and connecting electrodes  3  having the same shapes as those formed on the second major surface S 2  of the solar cell substrate  1  are formed on the first major surface S 1  of the solar cell substrate  1 , but the present invention is not limited to such a case. For example, the fine-line electrodes  2  may be formed so as to cover substantially all over any one of the first and second major surfaces S 1  and S 2  of the solar cell substrate  1 . 
     The aforementioned first and second embodiments describe the case where the photoelectric conversion body  11  includes the n-type substrate  11   a,  i-type semiconductor layer  11   b,  p-type semiconductor layer  11   c,  i-type semiconductor layer  11   d,  and n-type semiconductor layer  11   e,  but the present invention is not limited to such a case. For example, the photoelectric conversion body  11  may include only the p-type and n-type semiconductor layers  11   c  and  11   e.    
     In the aforementioned second embodiment, the third and fourth transparent conductive films  12   a  and  12   b  having different hydrogen atom contents are formed as the first transparent conductive film  12  so that the hydrogen atom content of the part of the first transparent conductive film  12  on the opposite side to the n-type semiconductor layer  11   e  is higher than that of the part of the first transparent conductive film  12  on the n-type semiconductor layer  11   e  side. However, the present invention is not limited to this. Specifically, the first transparent conductive film  12  composed of a single film may be formed so that the hydrogen atom content thereof increases from the n-type semiconductor layer  11   e  side toward the opposite side to the n-type semiconductor layer  11   e.  The first transparent conductive film  12  having such a structure can be formed, for example, by employing the atmosphere not containing hydrogen as the condition for forming the first transparent conductive film  12  in a first part of the forming process and then increasing the supply amount of hydrogen atoms stepwisely so that the condition for forming the first transparent conductive film  12  can be the atmosphere containing hydrogen in a last part of the forming process. 
     As described above, the present invention naturally includes various embodiments which are not described herein. Accordingly, the technical scope of the present invention should be determined only by the matters to define the invention in the scope of claims regarded as appropriate based on the description. 
     EXAMPLES 
     Hereinbelow, the solar cell according to the present invention is specifically described with examples. However, the present invention is not limited to the following examples and can be implemented with appropriate modification without changing the scope of the present invention. 
     Evaluation of Hydrogen Atom Content  
     First, a comparison was made between hydrogen atom contents of transparent conductive films formed in the atmosphere not containing hydrogen and hydrogen atom contents of transparent conductive films formed in the atmosphere containing hydrogen. Specifically, hydrogen atom contents of test films A and B were measured by SIMS. Herein, the test films A were In 2 O 3  films formed in the atmosphere not containing hydrogen by using a sputtering method, and the test films B were In 2 O 3  formed in the atmosphere containing hydrogen by using a sputtering method. The measurement results of the hydrogen atom contents of the test films are shown in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Hydrogen atom  
               
               
                   
                 Test film 
                 content (atoms/cc) 
               
               
                   
                   
               
             
            
               
                   
                 A(In 2 O 3  films formed in atmosphere  
                 1.0 × 10 20 -5.2 × 10 20   
               
               
                   
                 not containing hydrogen)  
                   
               
               
                   
                 B(In 2 O 3  formed in atmosphere  
                 1.6 × 10 21 -2.1 × 10 21   
               
               
                   
                 containing hydrogen) 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 1, it was found that the hydrogen atom contents of the test films A, which were In 2 O 3  films formed in the atmosphere not containing hydrogen, were about 1.0×10 20  to 5.2×10 20  (atoms/cc) while the hydrogen atom contents of the test films B, which were In 2 O 3  films formed in the atmosphere containing hydrogen to actively incorporate hydrogen atoms, were 1.6×10 21  to 2.1×10 21  (atoms/cc). 
     Evaluation of Output Power Characteristics   
     Next, solar cells according to Examples 1 to 7 and Comparative Example were manufactured in the following manner, and comparisons were made in terms of characteristic values including open-circuit voltage V OC , short-circuit current I SC , fill factor FF, and output power P max . 
     Example 1   
     The solar cell according to Example 1 was manufactured as follows. First, an n-type single-crystalline silicon substrate (n-type substrate  11   a ) with a thickness of 200 μm was anisotropically etched with an alkaline aqueous solution to form fine roughness on the first and second major surfaces. The first and second major surfaces of the n-type substrate  11   a  were cleaned to remove impurities. 
     Next, on the second major surface side of the n-type single-crystalline silicon substrate (n-type substrate  11   a ), an i-type amorphous silicon layer (i-type semiconductor layer  11   b ) and a p-type amorphous silicon layer (p-type semiconductor layer  11   c ) were sequentially stacked by using a plasma CVD method or the like. Next, on the first major surface side of the n-type single-crystalline silicon substrate (n-type substrate  11   a ), an i-type amorphous silicon layer (i-type semiconductor layer  11   d ) and an n-type amorphous silicon layer (n-type semiconductor layer  11   c ) were sequentially stacked. The i-type amorphous silicon layer (i-type semiconductor layer  11   b ), p-type amorphous silicon layer (p-type semiconductor layer  11   c ), i-type amorphous silicon layer (i-type semiconductor layer  11   d ), and n-type amorphous silicon layer (n-type semiconductor layer  11   c ) each have a thickness of 5 mm. The photoelectric conversion body (photoelectric conversion body  11 ) was thus formed. 
     Next, on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ), an IHO film (second transparent film  13 ) was formed by using a sputtering method in an Ar, O 2 , and water vapor atmosphere (the atmosphere containing hydrogen). The IHO film (second transparent conductive film  13 ) was an In 2 O 3  film formed in the atmosphere containing hydrogen to have a hydrogen atom content higher than that of an ITO film (first transparent conductive film  12 ) later described. The IHO film (second transparent conductive film  13 ) had a thickness of 1000 Å and was formed at room temperature. 
     Next, on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ), the ITO film (first transparent conductive film  12 ) was formed by using a sputtering method in an Ar and O 2  atmosphere (atmosphere not containing hydrogen). The ITO film (first transparent conductive film  12 ) had a thickness of 1000 Å and was formed at room temperature. In short, in Example 1, the ITO film (first transparent conductive film  12 ) was formed on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) after the IHO film (second transparent conductive film  13 ) was formed on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ). The solar cell substrate (solar cell substrate  1 ) was thus formed. 
     Next, paste including a conductive filler mainly containing Ag and thermosetting resin was placed in predetermined patterns on the second and first major surfaces of the solar cell substrate (solar cell substrate  1 ) by using a screen printing method and was then annealed at 200° C. Fine-line electrodes (fine-line electrodes  2 ) and connecting electrodes (connecting electrodes  3 ) were thus formed. The solar cell according to Example 1 was formed in such a manner. 
     Example 2   
     The solar cell according to Example 2 was manufactured in the following manner. First the photoelectric conversion body (photoelectric conversion body  11 ) was formed in the same way as Example 1 described above. 
     Next, on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ), the ITO film (first transparent conductive film  12 ) was formed by using a sputtering method in an Ar and O 2  atmosphere (atmosphere not containing hydrogen). The ITO film (first transparent conductive film  12 ) had a thickness of 1000 Å and was formed at room temperature. 
     Next, on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ), an IHO film (second transparent film  13 ) was formed by using a sputtering method in an Ar, O 2 , and water vapor atmosphere (the atmosphere containing hydrogen). The IHO film (second transparent conductive film  13 ) had a thickness of 1000 Å and was formed at room temperature. In short, in Example 2, the IHO film (second transparent conductive film  13 ) was formed on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ) after the ITO film (first transparent conductive film  12 ) was formed on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) in a reverse order to that in Example 1. The solar cell substrate (solar cell substrate  1 ) was thus formed. 
     Next, fine-line electrodes (fine-line electrodes  2 ) and connecting electrodes (connecting electrodes  3 ) were formed in the same way as Example 1 described above. The solar cell according to Example 2 was formed in such a manner. 
     Example 3   
     The solar cell according to Example 3 was manufactured in the following manner. First the photoelectric conversion body (photoelectric conversion body  11 ) was formed in the same way as Example 1 described above. 
     Next, on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ), the ITO film (third transparent conductive film  12   a ) was formed by using a sputtering method in an Ar and O 2  atmosphere (atmosphere not containing hydrogen). The ITO film (third transparent conductive film  12   a ) had a thickness of 500 Å and was formed at room temperature. 
     Next, on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ), an IHO film (second transparent film  13 ) was formed by using a sputtering method in an Ar, O 2 , and water vapor atmosphere (the atmosphere containing hydrogen). The IHO film (second transparent conductive film  13 ) had a thickness of 1000 Å and was formed at room temperature. 
     Next, on the ITO film (third transparent conductive film  12   a ), an IHO film (fourth transparent film  12   b ) was formed by using a sputtering method in an Ar, O 2 , and water vapor atmosphere (the atmosphere containing hydrogen). The IHO film (fourth transparent film  12   b ) was an In 2 O 3  film formed in the atmosphere containing hydrogen to have a hydrogen atom content higher than that of the ITO film (third transparent conductive film  12   a ). The IHO film (fourth transparent film  12   b ) had a thickness of 500 Å and was formed at room temperature. 
     Next, fine-line electrodes (fine-line electrodes  2 ) and connecting electrodes (connecting electrodes  3 ) were formed in the same way as Example 2 described above. The solar cell according to Example 3 was formed in such a manner. 
     Example 4   
     In Example 4, the thickness of the ITO film (third transparent conductive film  12   a ) in Example 3 described above was changed to 200 Å, and the thickness of the IHO film (fourth transparent conductive film  12   b ) was changed to 800 Å. The structures of the solar cells according to Example 4 and Example 3 described above were the same except that the thicknesses of the ITO film (third transparent conductive film  12   a ) and the IHO film (fourth transparent conductive film  12   b ) were changed. 
     Example 5   
     In Example 5, the thickness of the ITO film (third transparent conductive film  12   a ) in Example 3 described above was changed to 150 Å, and the thickness of the IHO film (fourth transparent conductive film  12   b ) was changed to 850 Å. The structures of the solar cells according to Example 5 and Example 3 described above were the same except that the thicknesses of the ITO film (third transparent conductive film  12   a ) and the IHO film (fourth transparent conductive film  12   b ) were changed. 
     Example 6   
     In Example 6, the thickness of the ITO film (third transparent conductive film  12   a ) in Example 3 described above was changed to 100 Å, and the thickness of the IHO film (fourth transparent conductive film  12   b ) was changed to 900 Å. The structures of the solar cells according to Example 6 and Example 3 described above were the same except that the thicknesses of the ITO film (third transparent conductive film  12   a ) and the IHO film (fourth transparent conductive film  12   b ) were changed. 
     Example 7   
     In Example 7, the thickness of the ITO film (third transparent conductive film  12   a ) in Example 3 described above was changed to 75 Å, and the thickness of the IHO film (fourth transparent conductive film  12   b ) was changed to 925 Å. The structures of the solar cells according to Example 7 and Example 3 described above were the same except that the thicknesses of the ITO film (third transparent conductive film  12   a ) and the IHO film (fourth transparent conductive film  12   b ) were changed. 
     Example 8   
     In Example 8, an ITO film (first transparent conductive film  12 ) was formed on an n-type amorphous silicon layer (n-type semiconductor layer  11   e ) of the photoelectric conversion body (photoelectric conversion body  11 ) in the same way as Example 2 described above. The ITO film (first transparent conductive film  12 ) had a thickness of 1000 Å and was formed at room temperature. 
     Next, on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ), an IHTO film (second transparent film  13 ) was formed by using a sputtering method in an Ar, O 2 , and water vapor atmosphere (the atmosphere containing hydrogen). The IHTO film (second transparent conductive film  13 ) was an ITO film formed in the atmosphere containing hydrogen to have a high hydrogen atom content. The IHTO film had a thickness of 1000 Å and was formed at room temperature. The solar cell substrate (solar cell substrate  1 ) was thus formed. 
     Next, fine-line electrodes (fine-line electrodes  2 ) and connecting electrodes (connecting electrodes  3 ) were formed in the same way as Example 2 described above. The solar cell according to Example 8 was thus formed. 
     Example 9   
     In Example 9, the photoelectric conversion body (photoelectric conversion body  11 ) was formed in the same way as Example 2 described above. 
     Next, an IWO film (first transparent conductive film  12 ) was formed on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) by using a sputtering method in the Ar and O 2  atmosphere (atmosphere not containing hydrogen). The IWO film (first transparent conductive film  12 ) was an In 2 O 3  film doped with tungsten (W). The IWO film (first transparent conductive film  12 ) had a thickness of 1000 Å and was formed at room temperature. 
     Next, on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ), an IHWO film (second transparent film  13 ) was formed by using a sputtering method in an Ar, O 2 , and water vapor atmosphere (the atmosphere containing hydrogen). The IHWO film (second transparent conductive film  13 ) was an IWO film formed in the atmosphere containing hydrogen to have a high hydrogen atom content. The IHWO film (second transparent conductive film  13 ) had a thickness of 1000 Å and was formed at room temperature. The solar cell substrate (solar cell substrate  1 ) was thus formed. 
     Next, fine-line electrodes (fine-line electrodes  2 ) and connecting electrodes (connecting electrodes  3 ) were formed in the same way as Example 2 described above. The solar cell according to Example 9 was thus formed. 
     Example 10   
     In Example 10, the photoelectric conversion body (photoelectric conversion body  11 ) was formed in the same way as Example 2 described above. 
     Next, an ICO film (first transparent conductive film  12 ) was formed on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) by using a sputtering method in the Ar and O 2  atmosphere (atmosphere not containing hydrogen). The ICO film (first transparent conductive film  12 ) was an In 2 O 3  film doped with cerium (Ce). The ICO film (first transparent conductive film  12 ) had a thickness of 1000 Å and was formed at room temperature. 
     Next, on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ), an IHCO film (second transparent film  13 ) was formed by using a sputtering method in an Ar, O 2 , and water vapor atmosphere (the atmosphere containing hydrogen). The IHCO film (second transparent conductive film  13 ) was an ICO film formed in the atmosphere containing hydrogen to have a high hydrogen atom content. The IHCO film (second transparent conductive film  13 ) had a thickness of 1000 Å and was formed at room temperature. The solar cell substrate (solar cell substrate  1 ) was thus formed. 
     Next, fine-line electrodes (fine-line electrodes  2 ) and connecting electrodes (connecting electrodes  3 ) were formed in the same way as Example 2 described above. The solar cell according to Example 10 was thus formed. 
     Comparative Example   
     The solar cell according to Comparative Example was manufactured in the following manner. First the photoelectric conversion body (photoelectric conversion body  11 ) was formed in the same way as Example 1 described above. 
     Next, on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ), a first IHO film was formed by using a sputtering method in an Ar, O 2 , and water vapor atmosphere (the atmosphere containing hydrogen). The first IHO film had a thickness of 1000 Å and was formed at room temperature. 
     Next, on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ), a second IHO film (second transparent film  13 ) was formed by using a sputtering method in an Ar, O 2 , and water vapor atmosphere (the atmosphere containing hydrogen). The second IHO film had a thickness of 1000 Å and was formed at room temperature. The solar cell substrate (solar cell substrate  1 ) was thus formed. 
     Next, fine-line electrodes (fine-line electrodes  2 ) and connecting electrodes (connecting electrodes  3 ) were formed in the same way as Example 1 described above. The solar cell according to Comparative Example was formed in such a manner. 
     &lt;Evaluation Results of Output Power Characteristics&gt; 
     The characteristic values including the open-circuit voltage V OC , short-circuit current I SC , fill factor FF, and output power P max  were measured for the solar cells according to Examples 1 to 10 and Comparative Example described above. The results of the measurement are shown in Table 2. Table 2 shows the characteristic values of the solar cells according to Examples 1 to 10 normalized with the characteristic values of the solar cell according to Comparative Example set to 100.  FIG. 5  shows a relationship between the thickness of the ITO film formed on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) and the fill factor FF.  FIG. 5  shows the fill factors FF of the solar cells according to Examples 2 to 7 normalized with the fill factor FF of the solar cell according to Comparative Example set to 100. Moreover, in the solar cell according to Comparative Example, the IHO layer was formed on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ), and the ITO film was not formed thereon. In  FIG. 5 , the ITO film thickness (Å) of the solar cells according to Comparative Example is therefore zero. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Open-circuit  
                 Short-circuit 
                 Fill  
                 Output 
               
               
                   
                   
                 voltage 
                 current 
                 factor  
                 power 
               
               
                   
                   
                 (Voc) 
                 (Isc) 
                 (FF) 
                 (Pmax) 
               
               
                   
                   
               
             
            
               
                   
                 Example 1 
                 100.7 
                 99.8 
                 111.0 
                 111.5 
               
               
                   
                 Example 2 
                 100.9 
                 99.6 
                 113.8 
                 114.4 
               
               
                   
                 Example 3 
                 100.7 
                 99.7 
                 114.1 
                 114.5 
               
               
                   
                 Example 4 
                 100.5 
                 99.5 
                 113.0 
                 113.1 
               
               
                   
                 Example 5 
                 100.6 
                 99.9 
                 113.4 
                 114.0 
               
               
                   
                 Example 6 
                 100.5 
                 100.2  
                 111.2 
                 112.0 
               
               
                   
                 Example 7 
                 100.4 
                 99.4 
                 107.9 
                 107.8 
               
               
                   
                 Example 8 
                 100.7 
                 99.5 
                 117.5 
                 117.7 
               
               
                   
                 Example 9 
                 100.6 
                 99.9 
                 117.0 
                 117.6 
               
               
                   
                 Example 10 
                 100.2 
                 100.3  
                 116.5 
                 117.1 
               
               
                   
                 Comparative 
                 100.0 
                 100.0  
                 100.0 
                 100.0 
               
               
                   
                 Example 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 2, it was found that, in comparison with the solar cell according to Comparative Example, the solar cells according to Examples 1 to 10 were improved in the fill factor FF and therefore improved in the output power Pmax. This is because, in each of Examples 1 to 10, the transparent conductive film was formed in the atmosphere not containing hydrogen on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) forming the first major surface of the photoelectric conversion body (photoelectric conversion body  11 ). The degradation of the surface of the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) was prevented in each of Examples 1 to 10 in comparison with Comparative Example in which the transparent conductive film was formed on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) in the atmosphere containing hydrogen. 
     As shown in Table 2, it was found that the solar cell according to Example 2 was further improved in the fill factor FF and output power Pmax in comparison with the solar cell according to Example 1. This is because the ITO film (first transparent conductive film  12 ) was formed on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) in the atmosphere not containing hydrogen before the IHO film (second transparent conductive film  13 ) was formed on the p-type amorphous silicon layer (p-type semiconductor layer  11   c ) in the atmosphere containing hydrogen. The surface of the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) was therefore protected by the ITO film (first transparent conductive film  12 ). Accordingly, the surface of the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) was less likely to be degraded when the photoelectric conversion body (photoelectric conversion body  11 ) was exposed to the atmosphere containing hydrogen. 
     As shown in Table 2, it was found that the solar cells according to Examples 8 to 10 were further improved in the fill factor FF and output power Pmax in comparison with the solar cell according to Example 2. This is because the IHTO film containing metallic dopant had a contact resistance with the p-type amorphous silicon layer (p-type semiconductor layer  11   c ) lower than that of the IHO film not containing metallic dopant. 
     Moreover, as shown in  FIG. 5 , the fill factor FF of the solar cells increased as the thickness of the ITO film (the first transparent conductive film  12  in Example 2 and the third transparent conductive film  12   a  in Examples 3 to 7) formed on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ) increased from 0 to 150 Å. The fill factor FF of the solar cells remained substantially constant when the thickness of the ITO film was 150 Å or more. Accordingly, it has been found that it is preferable to form an ITO film having a thickness of 150 Å or more on the n-type amorphous silicon layer (n-type semiconductor layer  11   e ). 
     INDUSTRIAL AVAILABILITY  
     According to the present invention, it is possible to provide a solar cell with the surface characteristics of the photoelectric conversion body prevented from being degraded and a method for manufacturing the same. Hence, the present invention is useful in the field of sunlight power generation.