Patent Publication Number: US-2011048533-A1

Title: Solar cell

Description:
This application claims the benefit of Korean Patent Application Nos. 10-2009-0082333 filed on Sep. 2, 2009, and 10-2009-0115050 filed on Nov. 26, 2009, the entire contents of which are incorporated herein by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relate to a solar cell. 
     2. Discussion of the Related Art 
     A solar cell is an element capable of converting light energy into electrical energy, and which includes a p-type semiconductor layer and an n-type semiconductor layer. 
     A general operation of the solar cell is as follows. If light coming from the outside is incident on the solar cell, electron-hole pairs are formed inside a semiconductor layer of the solar cell. The electrons move toward the n-type semiconductor layer and the holes move toward the p-type semiconductor layer by an electric field generated inside the semiconductor layer of the solar cell. Hence, electric power is produced. 
     The solar cell may be mainly classified into a silicon-based solar cell, a compound semiconductor-based solar cell, and an organic-based solar cell depending on a material used. The silicon-based solar cell may be classified into a crystalline silicon (c-Si) solar cell and an amorphous silicon (a-Si) solar cell depending on a phase of a semiconductor. 
     SUMMARY OF THE INVENTION 
     In one aspect, there is a solar cell comprising a substrate, a first electrode on the substrate, a second electrode, and at least one photoelectric transformation unit positioned between the first electrode and the second electrode, the at least one photoelectric transformation unit including a p-type semiconductor layer, an intrinsic (i-type) semiconductor layer, an n-type semiconductor layer, and a buffer layer positioned between the p-type semiconductor layer and the i-type semiconductor layer, a hydrogen content of the buffer layer being more than a hydrogen content of the i-type semiconductor layer. 
     The i-type semiconductor layer may be formed of amorphous silicon. The i-type semiconductor layer may be formed of microcrystalline silicon. 
     A thickness of the buffer layer may be less than a thickness of the i-type semiconductor layer. A thickness of the buffer layer may be less than a thickness of the p-type semiconductor layer. 
     The hydrogen content of the buffer layer may be more than a hydrogen content of the p-type semiconductor layer. 
     The i-type semiconductor layer may be formed of microcrystalline silicon. A crystallinity of the i-type semiconductor layer in a junction position between the buffer layer and the i-type semiconductor layer may be equal to or greater than 50% of a crystallinity of the i-type semiconductor layer in a junction position between the i-type semiconductor layer and the n-type semiconductor layer. 
     The crystallinity of the i-type semiconductor layer in the junction position between the buffer layer and the i-type semiconductor layer may be equal to or greater than 75% of the crystallinity of the i-type semiconductor layer in the junction position between the i-type semiconductor layer and the n-type semiconductor layer. 
     In another aspect, there is a solar cell comprising a substrate, a first electrode on the substrate, a second electrode, a first photoelectric transformation unit positioned between the first electrode and the second electrode, the first photoelectric transformation unit including a first p-type semiconductor layer, a first intrinsic (i-type) semiconductor layer formed of amorphous silicon, a first n-type semiconductor layer, and a first buffer layer positioned between the first p-type semiconductor layer and the first i-type semiconductor layer, a hydrogen content of the first buffer layer being more than a hydrogen content of the first i-type semiconductor layer, and a second photoelectric transformation unit positioned between the first photoelectric transformation unit and the second electrode, the second photoelectric transformation unit including a second p-type semiconductor layer, a second intrinsic (i-type) semiconductor layer formed of microcrystalline silicon, a second n-type semiconductor layer, and a second buffer layer positioned between the second p-type semiconductor layer and the second i-type semiconductor layer, a hydrogen content of the second buffer layer being more than a hydrogen content of the second i-type semiconductor layer. 
     A thickness of the first buffer layer may be less than a thickness of the first i-type semiconductor layer, and a thickness of the second buffer layer may be less than a thickness of the second i-type semiconductor layer. 
     A crystallinity of the second i-type semiconductor layer in a junction position between the second buffer layer and the second i-type semiconductor layer may be equal to or greater than 50% of a crystallinity of the second i-type semiconductor layer in a junction position between the second i-type semiconductor layer and the second n-type semiconductor layer. 
     The crystallinity of the second i-type semiconductor layer in the junction position between the second buffer layer and the second i-type semiconductor layer may be equal to or greater than 75% of the crystallinity of the second i-type semiconductor layer in the junction position between the second i-type semiconductor layer and the second n-type semiconductor layer. 
     A thickness of the second i-type semiconductor layer may be greater than a thickness of the first i-type semiconductor layer, and a thickness of the second buffer layer may be greater than a thickness of the first buffer layer. 
     A thickness of the second i-type semiconductor layer may be greater than a thickness of the first i-type semiconductor layer. The hydrogen content of the first buffer layer may be more than the hydrogen content of the second buffer layer. 
     The hydrogen content of the first buffer layer may be more than a hydrogen content of the first p-type semiconductor layer, and the hydrogen content of the second buffer layer may be less than a hydrogen content of the second p-type semiconductor layer. 
     In another aspect, there is a solar cell comprising a substrate, a first electrode on the substrate, a second electrode, a first photoelectric transformation unit positioned between the first electrode and the second electrode, the first photoelectric transformation unit including a first p-type semiconductor layer, a first intrinsic (i-type) semiconductor layer formed of amorphous silicon, a first n-type semiconductor layer, and a first buffer layer positioned between the first p-type semiconductor layer and the first i-type semiconductor layer, and a second photoelectric transformation unit positioned between the first photoelectric transformation unit and the second electrode, the second photoelectric transformation unit including a second p-type semiconductor layer, a second intrinsic (i-type) semiconductor layer formed of microcrystalline silicon, a second n-type semiconductor layer, and a second buffer layer positioned between the second p-type semiconductor layer and the second i-type semiconductor layer. A difference between a hydrogen content of the second i-type semiconductor layer and a hydrogen content of the second buffer layer is greater than a difference between a hydrogen content of the first i-type semiconductor layer and a hydrogen content of the first buffer layer. 
     The hydrogen content of the first buffer layer may be more than the hydrogen content of the first i-type semiconductor layer, and the hydrogen content of the second buffer layer may be more than the hydrogen content of the second i-type semiconductor layer. 
     In another aspect, there is a solar cell comprising a substrate, a first electrode on the substrate, a second electrode, and at least one photoelectric transformation unit positioned between the first electrode and the second electrode, the at least one photoelectric transformation unit including a p-type semiconductor layer, an intrinsic (i-type) semiconductor layer, and an n-type semiconductor layer, the i-type semiconductor layer including first and second portions each having a different hydrogen content. 
     A hydrogen content of the first portion may be more than a hydrogen content of the second portion. The first portion may be positioned between the second portion and the p-type semiconductor layer. 
     A thickness of the first portion may be less than a thickness of the second portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIGS. 1 to 5  relate to solar cells according to embodiments of the invention; 
         FIGS. 6 to 10  relate to double junction solar cells according to embodiments of the invention; 
         FIGS. 11 and 12  illustrate other structures of solar cells according to embodiments of the invention; 
         FIGS. 13 to 16  illustrate other structures of solar cells according to embodiments of the invention; and 
         FIGS. 17 to 32  relate to examples of doping intrinsic semiconductor layers with impurities according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIGS. 1 to 5  relate to solar cells according to embodiments of the invention. A structure of the solar cell shown in  FIG. 1  may be referred to as a pin structure. 
     As shown in  FIG. 1 , a solar cell  10  according to an embodiment of the invention includes a substrate  100 , a front electrode  110  on the substrate  100 , a rear electrode  140 , and a photoelectric transformation unit  120 . 
     The photoelectric transformation unit  120  is positioned between the front electrode  110  and the rear electrode  140  to produce electric power using light coming from the outside. Further, the photoelectric transformation unit  120  may include an intrinsic (referred to as an i-type) semiconductor layer  122  formed of microcrystalline silicon. 
     The substrate  100  may provide a space for other functional layers. Further, the substrate  100  may be formed of a transparent material, such as glass and plastic, so that light coming from the outside is efficiently incident on the photoelectric transformation unit  120 . 
     The front electrode  110  may be formed of a transparent material with electrical conductivity so as to increase a transmittance of incident light. For example, the front electrode  110  may be formed of a material, having high transmittance and high electrical conductivity, selected from the group consisting of indium tin oxide (ITO), tin-based oxide (for example, SnO 2 ), AgO, ZnO—Ga 2 O 3  (or Al 2 O 3 ), fluorine tin oxide (FTO), and a combination thereof, so that the front electrode  110  transmits most of incident light and a current flows in the front electrode  110 . A specific resistance of the front electrode  110  may be approximately 10 −2  Ω·cm to 10 −11  Ω·cm. 
     The front electrode  110  may be formed on the entire surface of the substrate  100  and may be electrically connected to the photoelectric transformation unit  120 . Hence, the front electrode  110  may collect one (for example, holes) of carriers produced by the incident light to output the holes. 
     A plurality of uneven patterns having an uneven pyramid structure may be formed on an upper surface of the front electrode  110 . In other words, the front electrode  110  may have a textured surface. As above, when the surface of the front electrode  110  is textured, a reflectance of light may be reduced, and an absorptance of light may increase. Hence, the efficiency of the solar cell  10  may be improved. 
     Although  FIG. 1  shows the textured surface of the front electrode  110 , the photoelectric transformation unit  120  may have a textured surface. In the embodiment of the invention, the textured surface of the front electrode  110  is described below for the convenience of explanation. 
     The rear electrode  140  may be formed of metal with a high electrical conductivity so as to increase a recovery efficiency of electric power produced by the photoelectric transformation unit  120 . Further, the rear electrode  140  electrically connected to the photoelectric transformation unit  120  may collect one (for example, electrons) of the carriers produced by incident light to output the electrons. 
     The photoelectric transformation unit  120  may convert light from the outside into electrical energy. The photoelectric transformation unit  120  may be a silicon cell using microcrystalline silicon, for example, hydrogenated microcrystalline silicon (mc-Si:H). The photoelectric transformation unit  120  may include a p-type semiconductor layer  121 , an intrinsic (i-type) semiconductor layer  122 , an n-type semiconductor layer  123 , and a buffer layer  124  that are formed on the front electrode  110  in the order named. 
     The p-type semiconductor layer  121  may be formed using a gas obtained by adding impurities of a group III element, such as boron (B), gallium (Ga), and indium (In), to a raw gas containing Si. 
     The i-type semiconductor layer  122  may reduce recombination of the carriers and may absorb light. The i-type semiconductor layer  122  may absorb incident light to produce carriers such as electrons and holes. The i-type semiconductor layer  122  may be formed of microcrystalline silicon, for example, hydrogenated microcrystalline silicon (mc-Si:H). 
     The n-type semiconductor layer  123  may be formed using a gas obtained by adding impurities of a group V element, such as phosphor (P), arsenic (As), and antimony (Sb), to a raw gas containing Si. 
     The photoelectric transformation unit  120  may be formed using a chemical vapor deposition (CVD) method, such as a plasma enhanced chemical vapor deposition (PECVD) method. 
     In the photoelectric transformation unit  120 , the p-type semiconductor layer  121  and the n-type semiconductor layer  123  may form a p-n junction with the i-type semiconductor layer  122  interposed between the p-type semiconductor layer  121  and the n-type semiconductor layer  123 . In other words, the i-type semiconductor layer  122  is positioned between the p-type semiconductor layer  121  (i.e., a p-type doped layer) and the n-type semiconductor layer  123  (i.e., an n-type doped layer), so as to form the pin structure. 
     In such a structure of the solar cell  10 , when light is incident on the p-type semiconductor layer  121 , a depletion region is formed inside the i-type semiconductor layer  122  because of the p-type semiconductor layer  121  and the n-type semiconductor layer  123  each having a relatively high doping concentration to thereby generate an electric field. Electrons and holes generated in the i-type semiconductor layer  122 , being a light absorbing layer, are separated by a contact potential difference through a photovoltaic effect and move in different directions. For example, the holes move to the front electrode  110  through the p-type semiconductor layer  121 , and the electrons move to the rear electrode  140  through the n-type semiconductor layer  123 . Hence, electric power is produced. 
     The buffer layer  124  is positioned between the i-type semiconductor layer  122  and the p-type semiconductor layer  121 . A hydrogen content of the buffer layer  124  may be more than a hydrogen content of the i-type semiconductor layer  122 . More specifically, the hydrogen content of the i-type semiconductor layer  122  formed of microcrystalline silicon may be approximately 3% to 5%, and the hydrogen content of the buffer layer  124  may be approximately 12% to 30%. As above, when the buffer layer  124  is positioned between the i-type semiconductor layer  122  and the p-type semiconductor layer  121 , crystallinity of the i-type semiconductor layer  122  may be uniform. Hence, the efficiency of the solar cell  10  may be improved. 
       FIG. 2  illustrates crystallinity depending on a depth of the i-type semiconductor layer in a comparative example not including the buffer layer and examples 1 to 5 according to embodiments of the invention. Supposing that a crystallinity of the i-type semiconductor layer at a position P 3  is  100  in  FIG. 2 ,  FIG. 2  illustrates a crystallinity of the i-type semiconductor layer at each of positions P 1  and P 2  in the comparative example and the examples 1 to 5. In  FIG. 2 , the position P 3  indicates a junction position between the i-type semiconductor layer and the n-type semiconductor layer, the position P 2  indicates a middle position of the i-type semiconductor layer, and the position P 1  indicates a junction position between the i-type semiconductor layer and the p-type semiconductor layer. 
     A structure of a solar cell according to the examples 1 to 5 is substantially the same as the structure of the solar cell  10  shown in  FIG. 1 , except the hydrogen content of the buffer layer  124 . More specifically, there is a relatively small difference between the hydrogen content of the buffer layer and the hydrogen content of the i-type semiconductor layer in the examples 3 and 5. Further, there is a relatively large difference between the hydrogen content of the buffer layer and the hydrogen content of the i-type semiconductor layer in the examples 1, 2, and 4. 
     As shown in  FIG. 2 , in the comparative example not including the buffer layer, the crystallinity of the i-type semiconductor layer at the position P 1  is approximately 40% based on the crystallinity of the i-type semiconductor layer at the position P 3 . Further, in the comparative example, the crystallinity of the i-type semiconductor layer at the position P 2  is approximately 98% based on the crystallinity of the i-type semiconductor layer at the position P 3 . 
     In the comparative example, the i-type semiconductor layer has the properties similar to amorphous silicon at an initial formation stage of the i-type semiconductor layer and has the properties similar to microcrystalline silicon when the formation process of the i-type semiconductor layer is almost completed. 
     In other words, in the comparative example, there is a relatively large difference between the crystallinity of the i-type semiconductor layer at the initial formation stage and the crystallinity of the i-type semiconductor layer when the formation process of the i-type semiconductor layer is almost completed. This indicates that it is difficult to achieve crystallization of the i-type semiconductor layer at the initial formation stage. The referred to initial formation stage of the i-type semiconductor layer may be represented by, or correspond to, the position P 1  (or a portion of the i-type semiconductor layer adjacent the position P 1 ), and the referred to completed formation stage of the i-type semiconductor layer may be represented by, or correspond to, the position P 3  (or a portion of the i-type semiconductor layer adjacent the position P 3 ). 
     On the other hand, in the example 1 where the hydrogen content of the buffer layer is relatively high, crystallinity of the i-type semiconductor layer at the position P 1  is approximately 85% based on the crystallinity of the i-type semiconductor layer at the position P 3 . Further, in the example 1, the crystallinity of the i-type semiconductor layer at the position P 2  is approximately 95% based on the crystallinity of the i-type semiconductor layer at the position P 3 . 
     The crystallinity in the example 1 is greater than the crystallinity in the comparative example at the initial formation stage of the i-type semiconductor layer. In other words, this indicates that the i-type semiconductor layer in the example 1 has the properties similar to microcrystalline silicon at the initial formation stage. In the example 1, a large amount of hydrogen contained in the buffer layer may form seeds for crystal growth at the initial formation stage of the i-type semiconductor layer, and thus, a large number of crystals may be formed at the initial formation stage of the i-type semiconductor layer. 
     In the example 2 where the hydrogen content of the buffer layer is relatively high, crystallinity of the i-type semiconductor layer at the position P 1  is approximately 80% based on the crystallinity of the i-type semiconductor layer at the position P 3 . Further, in the example 2, the crystallinity of the i-type semiconductor layer at the position P 2  is approximately 93% based on the crystallinity of the i-type semiconductor layer at the position P 3 . 
     In the example 4 where the hydrogen content of the buffer layer is relatively high, crystallinity of the i-type semiconductor layer at the position P 1  is approximately 75% based on the crystallinity of the i-type semiconductor layer at the position P 3 . Further, in the example 4, the crystallinity of the i-type semiconductor layer at the position P 2  is approximately 95% based on the crystallinity of the i-type semiconductor layer at the position P 3 . 
     The hydrogen content of the buffer layer in the example 1 is more than the hydrogen content of the buffer layer in the examples 2 and 4, and the hydrogen content of the buffer layer in the example 2 is more than the hydrogen content of the buffer layer in the example 4. 
     In the example 3 where the hydrogen content of the buffer layer is relatively low, crystallinity of the i-type semiconductor layer at the position P 1  is approximately 53% based on the crystallinity of the i-type semiconductor layer at the position P 3 . Further, in the example 3, the crystallinity of the i-type semiconductor layer at the position P 2  is approximately 97% based on the crystallinity of the i-type semiconductor layer at the position P 3 . 
     As above, in the example 3 where the hydrogen content of the buffer layer is less than those in the examples 1, 2, and 4, the crystallinity of the i-type semiconductor layer at the initial formation stage is less than those in the examples 1, 2, and 4. However, the crystallinity of the i-type semiconductor layer in the example 3 at the initial formation stage is greater than the crystallinity of the i-type semiconductor layer in the comparative example at the initial formation stage. This indicates that the crystallinity of the i-type semiconductor layer at the initial formation stage can be improved even if a small amount of hydrogen is added to the buffer layer. It is a matter of course that in the example 3, the hydrogen content of the buffer layer is more than the hydrogen content of the i-type semiconductor layer. 
     In the example 5 where the hydrogen content of the buffer layer is relatively low, the crystallinity of the i-type semiconductor layer at the position P 1  is approximately 50% based on the crystallinity of the i-type semiconductor layer at the position P 3 . Further, in the example 3, the crystallinity of the i-type semiconductor layer at the position P 2  is approximately 88% based on the crystallinity of the i-type semiconductor layer at the position P 3 . 
     The hydrogen content of the buffer layer in the example 3 is more than the hydrogen content of the buffer layer in the example 5. 
       FIG. 3  illustrates electrical characteristics of each of a comparative example and an example according to the invention. In  FIG. 3 , the comparative example is substantially the same as the comparative example of  FIG. 2 , and the example is substantially the same as the example 1 of  FIG. 2 . 
     As shown in  FIG. 3 , in the comparative example, a short circuit current Jsc was approximately 12.60 mA/cm 2 , an open circuit voltage Voc was approximately 0.911 V, a fill factor FF was approximately 0.72%, and an efficiency Eff was approximately 8.29%. On the other hand, in the example according to the invention, a short circuit current Jsc was approximately 12.80 mA/cm 2 , an open circuit voltage Voc was approximately 0.943 V, a fill factor FF was approximately 0.73%, and an efficiency Eff was approximately 8.70%. 
     As above, the efficiency Eff in the example according to the invention increased by approximately 4.9% as compared with the comparative example. In the example according to the invention, the crystallinity of the i-type semiconductor layer at an initial formation stage can increase by adding the buffer layer containing hydrogen, and thus, the crystallinity of the i-type semiconductor layer can be uniform. Hence, the efficiency of the solar cell  10  can be improved. 
     As above, as the crystallinity of the i-type semiconductor layer increases in the junction portion between the buffer layer and the i-type semiconductor layer, the efficiency of the solar cell  10  may be improved. Considering the description of  FIGS. 2 and 3  and the efficiency of the solar cell  10 , it may be preferable that the crystallinity of the i-type semiconductor layer in the junction portion between the buffer layer and the i-type semiconductor layer is equal to or greater than 50% or equal to or greater than 75% of the crystallinity of the i-type semiconductor layer in the junction portion between the i-type semiconductor layer and the n-type semiconductor layer. 
     The hydrogen content of the buffer layer is not particularly limited, except that the hydrogen content of the buffer layer is more than the hydrogen content of the i-type semiconductor layer. For example, the hydrogen content of the buffer layer may be substantially equal to or less than hydrogen content of the p-type semiconductor layer. Further, the hydrogen content of the buffer layer may be more than hydrogen content of the p-type semiconductor layer. 
     As described above with reference to  FIG. 2 , because it is difficult to form the crystal growth at the initial formation stage of the i-type semiconductor layer, the crystallinity of the i-type semiconductor layer is low. Accordingly, as the thickness of the i-type semiconductor layer decreases, the crystallinity of the i-type semiconductor layer may be relatively low. Considering the thickness of the i-type semiconductor layer, as the thickness of the i-type semiconductor layer adjacent to the buffer layer decreases, it may be preferable that the hydrogen content of the buffer layer helping the crystal growth of the i-type semiconductor layer increases. As the thickness of the i-type semiconductor layer adjacent to the buffer layer increases, it may be preferable that the hydrogen content of the buffer layer helping the crystal growth of the i-type semiconductor layer decreases. 
     When the hydrogen content of the buffer layer is more than the hydrogen content of the i-type semiconductor layer, it may be preferable that the hydrogen content of the buffer layer may be less than or more than the hydrogen content of the p-type semiconductor layer irrespective of the thickness of the i-type semiconductor layer because the crystallinity of the i-type semiconductor layer may increase. More preferably, the hydrogen content of the buffer layer may be more than the hydrogen content of the p-type semiconductor layer. 
     In  FIG. 2 , in the examples 1, 2, and 4, the hydrogen content of the buffer layer is more than the hydrogen content of the i-type semiconductor layer and the hydrogen content of the p-type semiconductor layer. In the examples 3 and 5, the hydrogen content of the buffer layer is more than the hydrogen content of the i-type semiconductor layer and is less than the hydrogen content of the p-type semiconductor layer. 
     In  FIG. 1 , the buffer layer  124  containing a relatively large amount of hydrogen helps the crystal growth of the i-type semiconductor layer  122  and thus, can allow the crystallinity of the i-type semiconductor layer  122  to be uniform. However, when a thickness t 2  of the buffer layer  124  is excessively large, the buffer layer  124  may block a movement of carriers between the p-type semiconductor layer  121  and the i-type semiconductor layer  122 . Hence, the efficiency of the solar cell  10  maybe reduced. Accordingly, it may be preferable that the thickness t 2  of the buffer layer  124  is sufficiently small. More preferably, the thickness t 2  of the buffer layer  124  may be less than a thickness t 1  of the i-type semiconductor layer  122 . 
     Further, the thickness t 2  of the buffer layer  124  may be substantially equal to or greater than a thickness t 3  of the p-type semiconductor layer  121 . However, it may be preferable that the thickness t 2  of the buffer layer  124  is less than the thickness t 3  of the p-type semiconductor layer  121  so as to increase the efficiency of the solar cell  10 . 
     The buffer layer  124  may include a plurality of sub-buffer layers that are positioned adjacent to each other and each have a different hydrogen content. For example, as shown in  FIG. 4 , the buffer layer  124  may include a first sub-buffer layer  300  adjacent to the p-type semiconductor layer  121  and a second sub-buffer layer  310  adjacent to the i-type semiconductor layer  122 . 
     A hydrogen content of the first sub-buffer layer  300  may be more than a hydrogen content of the second sub-buffer layer  310 , and a hydrogen content of the second sub-buffer layer  310  may be more than a hydrogen content of the first sub-buffer layer  300 . However, it may be preferable that the hydrogen content of the second sub-buffer layer  310  is more than a hydrogen content of the first sub-buffer layer  300 , so that a large number of seeds for helping the crystal growth of the i-type semiconductor layer  122  are formed. The above-described structure of the buffer layer  124  may be achieved by allowing an amount of hydrogen gas injected in a formation process of the first sub-buffer layer  300  to be different from an amount of hydrogen gas injected in a formation process of the second sub-buffer layer  310 . 
       FIG. 5  shows a photoelectric transformation unit  130  using a silicon cell formed of amorphous silicon, for example, hydrogenated amorphous silicon (a-Si:H). In the photoelectric transformation unit  130  of  FIG. 5 , an i-type semiconductor layer  132  may be formed of amorphous silicon, for example, hydrogenated amorphous silicon (a-Si:H). 
     As shown in  FIG. 5 , when the i-type semiconductor layer  132  is formed of amorphous silicon, a buffer layer  134  may be positioned between a p-type semiconductor layer  131  and the i-type semiconductor layer  132 . In this case, the buffer layer  134  makes a structure of the i-type semiconductor layer  132  formed of amorphous silicon closer and thus, can improve the efficiency of the solar cell  10 . 
     The crystallinity of the i-type semiconductor layer  132  formed of amorphous silicon is less than the crystallinity of the i-type semiconductor layer  122  formed of microcrystalline silicon. However, the i-type semiconductor layer  132  includes a small amount of crystal material. Accordingly, when the buffer layer  134  is positioned between the p-type semiconductor layer  131  and the i-type semiconductor layer  132 , the small amount of crystal material of the i-type semiconductor layer  132  may be uniformly distributed. 
     Further, because hydrogen contained in the buffer layer  134  increases a density of a layer formed of amorphous silicon when the i-type semiconductor layer  132  formed of amorphous silicon is grown, a density of the i-type semiconductor layer  132  can be improved. Preferably, a hydrogen content of the i-type semiconductor layer  132  formed of amorphous silicon may be approximately 10% to 11%, and a hydrogen content of the buffer layer  134  may be approximately 10% to 11%. 
     A method of manufacturing the buffer layer is briefly described below. 
     The method of manufacturing the buffer layer may be substantially the same as a method of manufacturing the i-type semiconductor layer, except a difference in an amount of hydrogen gas injected in a manufacturing process. More specifically, an amount of hydrogen gas injected into a chamber in the manufacturing process of the buffer layer is more than an amount of hydrogen gas injected into a chamber in the method of manufacturing the i-type semiconductor layer. Thus, a hydrogen content of the buffer layer is more than a hydrogen content of the i-type semiconductor layer. 
     Considering the above manufacturing method, the hydrogen content of the buffer layer is different from the hydrogen content of the i-type semiconductor layer, but the electrical properties of the buffer layer may be similar to the electrical properties of the i-type semiconductor layer. 
     In other words, the i-type semiconductor layer may include a first portion and a second portion each containing a different hydrogen content. Further, a hydrogen content of the first portion is more than a hydrogen content of the second portion, and the first portion is positioned between the second portion and the p-type semiconductor layer. A thickness of the first portion is less than a thickness of the second portion, and the first portion may be the buffer layer. 
     Another method of manufacturing the buffer layer is briefly described below. 
     Another method of manufacturing the buffer layer may be substantially the same or similar as a method of manufacturing the p-type semiconductor layer except a difference in an amount of hydrogen gas injected in a manufacturing process. More specifically, an amount of hydrogen gas injected into a chamber in the manufacturing process of the buffer layer is more than an amount of hydrogen gas injected into a chamber in the method of manufacturing the p-type semiconductor layer. Thus, a hydrogen content of the buffer layer is more than a hydrogen content of each of the i-type semiconductor layer and the p-type semiconductor layer. 
     Considering the above manufacturing method, the hydrogen content of the buffer layer is different from the hydrogen content of the p-type semiconductor layer, but the electrical properties of the buffer layer may be similar to the electrical properties of the p-type semiconductor layer. 
     In other words, the p-type semiconductor layer may include a third portion and a fourth portion each containing a different hydrogen content. Further, a hydrogen content of the fourth portion is more than a hydrogen content of the third portion, and the fourth portion is positioned between the third portion and the i-type semiconductor layer. A thickness of the fourth portion is greater than a thickness of the third portion, and the fourth portion may be the buffer layer. 
     Further, an amount of impurities of the fourth portion may be different from an amount of impurities of the third portion. Preferably, an amount of p-type impurities of the fourth portion may be less than an amount of p-type impurities of the third portion. In this case, interface characteristic between the p-type semiconductor layer and the i-type semiconductor layer may be improved, and thus, the efficiency of the solar cell may be improved. 
       FIGS. 6 to 10  relate to double junction solar cells according to embodiments of the invention. The solar cells shown in  FIGS. 6 to 10  may be a double junction solar cell or a pin-pin solar cell. A further description of structures and components identical or equivalent to those illustrated in  FIGS. 1 to 5  may be briefly made or may be entirely omitted. 
     As shown in  FIG. 6 , a solar cell  10  according to an embodiment of the invention may include a first photoelectric transformation unit  220  including a first i-type semiconductor layer  222  containing amorphous silicon and a second photoelectric transformation unit  230  including a second i-type semiconductor layer  232  containing microcrystalline silicon. 
     As shown in  FIG. 6 , in the solar cell  10  according to the embodiment of the invention, a first p-type semiconductor layer  221 , a first buffer layer  224 , the first i-type semiconductor layer  222 , a first n-type semiconductor layer  223 , a second p-type semiconductor layer  231 , a second buffer layer  234 , the second i-type semiconductor layer  232 , and a second n-type semiconductor layer  233  are stacked on an light incident surface in the order named. 
     In the first photoelectric transformation unit  220 , all of the first p-type semiconductor layer  221 , the first buffer layer  224 , the first i-type semiconductor layer  222 , and the first n-type semiconductor layer  223  may contain amorphous silicon. In the second photoelectric transformation unit  230 , all of the second p-type semiconductor layer  231 , the second buffer layer  234 , the second i-type semiconductor layer  232 , and the second n-type semiconductor layer  233  may contain microcrystalline silicon. Alternatively, the first n-type semiconductor layer  223  of the first photoelectric transformation unit  220  may contain microcrystalline silicon. 
     The first i-type semiconductor layer  222  may mainly absorb light of a short wavelength band to produce electrons and holes. The second i-type semiconductor layer  232  may mainly absorb light of a long wavelength band to produce electrons and holes. 
     As above, because the double junction solar cell absorbs light of the short wavelength band and the long wavelength band to produce carriers, the efficiency of the double junction solar cell is high. 
     Further, a thickness t 10  of the second i-type semiconductor layer  232  may be greater than a thickness t 20  of the first i-type semiconductor layer  222  so as to sufficiently absorb light of the long wavelength band. 
     The first photoelectric transformation unit  220  may include the first p-type semiconductor layer  221 , the first buffer layer  224 , the first i-type semiconductor layer  222 , and the first n-type semiconductor layer  223 . A hydrogen content of the first buffer layer  224  may be more than a hydrogen content of the first i-type semiconductor layer  222 . The hydrogen content of the first buffer layer  224  may be different from a hydrogen content of the first p-type semiconductor layer  221 . The hydrogen content of the first buffer layer  224 , the hydrogen content of the first i-type semiconductor layer  222 , and the hydrogen content of the first p-type semiconductor layer  221  are below described in detail. 
     Further, it may be preferable that a thickness t 21  of the first buffer layer  224  is less than the thickness t 20  of the first i-type semiconductor layer  222 . 
     The second photoelectric transformation unit  230  may include the second p-type semiconductor layer  231 , the second buffer layer  234 , the second i-type semiconductor layer  232 , and the second n-type semiconductor layer  233 . A hydrogen content of the second buffer layer  234  may be more than a hydrogen content of the second i-type semiconductor layer  232 . The hydrogen content of the second buffer layer  234  may be different from a hydrogen content of the second p-type semiconductor layer  231 . The hydrogen content of the second buffer layer  234 , the hydrogen content of the second i-type semiconductor layer  232 , and the hydrogen content of the second p-type semiconductor layer  231  are below described in detail. 
     In the second photoelectric transformation unit  230 , it may be preferable that crystallinity of the second i-type semiconductor layer  232  in a junction portion between the second buffer layer  234  and the second i-type semiconductor layer  232  is equal to or greater than approximately 50% or equal to or greater than approximately 75% of crystallinity of the second i-type semiconductor layer  232  in a junction portion between the second i-type semiconductor layer  232  and the second n-type semiconductor layer  233 . 
     Further, it may be preferable that a thickness t 11  of the second buffer layer  234  is less than the thickness t 10  of the second i-type semiconductor layer  232 . 
     The thickness t 21  of the first buffer layer  224  may be equal to or different from the thickness t 11  of the second buffer layer  234 . The thickness t 11  of the second buffer layer  234  may be greater than the thickness t 21  of the first buffer layer  224  considering that the thickness t 20  of the first i-type semiconductor layer  222  is less than the thickness t 10  of the second i-type semiconductor layer  232 . 
     The hydrogen content of the first buffer layer  224  may be equal to or different from the hydrogen content of the second buffer layer  234 . 
     Crystallinity of the first i-type semiconductor layer  222  formed of amorphous silicon may be less than crystallinity of the second i-type semiconductor layer  232  formed of microcrystalline silicon. Because hydrogen helps the crystal growth to increase crystallinity, the hydrogen content of the second buffer layer  234  adjacent to the second i-type semiconductor layer  232  may be more than the hydrogen content of the first buffer layer  224  adjacent to the first i-type semiconductor layer  222 . 
     As the thickness of the i-type semiconductor layer decreases, the density and the uniformity of the i-type semiconductor layer may be reduced because of characteristics of the manufacturing process. In this case, hydrogen may increase the density and the uniformity of the i-type semiconductor layer. Considering this, the hydrogen content of the first buffer layer  224  may be more than the hydrogen content of the second buffer layer  234 . Preferably, when the thickness t 20  of the first i-type semiconductor layer  222  is sufficiently less than the thickness t 10  of the second i-type semiconductor layer  232 , the hydrogen content of the first buffer layer  224  may be more than the hydrogen content of the second buffer layer  234 . In this case, the hydrogen content of the first buffer layer  224  may be more than the hydrogen content of the first p-type semiconductor layer  221 , and the hydrogen content of the second buffer layer  234  may be less than the hydrogen content of the second p-type semiconductor layer  231 . 
     The hydrogen content of the first i-type semiconductor layer  222  formed of amorphous silicon may be more than the hydrogen content of the second i-type semiconductor layer  232  formed of microcrystalline silicon. The difference is caused by property difference between amorphous silicon and microcrystalline silicon. 
       FIG. 7  is a graph comparing hydrogen contents of the first i-type semiconductor layer  222 , the second i-type semiconductor layer  232 , the first buffer layer  224 , and the second buffer layer  234 . 
     As shown in  FIG. 7 , the hydrogen content of each of the first and second buffer layers  224  and  234  is a maximum value, the hydrogen content of the second i-type semiconductor layer  232  is a minimum value, and the hydrogen content of the first i-type semiconductor layer  222  is less than the hydrogen contents of the first and second buffer layers  224  and  234  and is more than the hydrogen content of the second i-type semiconductor layer  232 . 
     The hydrogen content of the first buffer layer  224  may be different from the hydrogen content of the second buffer layer  234 . However, because a difference between the hydrogen content of the first buffer layer  224  and the hydrogen content of the second buffer layer  234  is much less than a difference between the hydrogen content of the first i-type semiconductor layer  222  and the hydrogen content of the second i-type semiconductor layer  232 , the hydrogen content of the first buffer layer  224  and the hydrogen content of the second buffer layer  234  are not distinguished in  FIG. 7 . 
     For example, the hydrogen content of the first i-type semiconductor layer  222  formed of amorphous silicon may be approximately 11.4%, and the hydrogen content of the second i-type semiconductor layer  232  formed of microcrystalline silicon may be approximately 4.8%. Thus, the difference may be approximately 7 to 8%. Further, the hydrogen content of the first buffer layer  224  may be approximately 18.7%, and the hydrogen content of the second buffer layer  234  may be approximately 17.9%. Thus, the difference may be relatively small value. As above, the difference between the hydrogen content of the second i-type semiconductor layer  232  and the hydrogen content of the second buffer layer  234  may be greater than the difference between the hydrogen content of the first i-type semiconductor layer  222  and the hydrogen content of the first buffer layer  224 . 
     Because the crystallinity of the second i-type semiconductor layer  232  formed of microcrystalline silicon may be greater than the crystallinity of the first i-type semiconductor layer  222 , a relatively large amount of hydrogen may be contained in the second buffer layer  234  so as to produce a large amount of seeds helping silicon crystal growth. Further, because microcrystalline silicon of  FIG. 7  contains a relatively small amount of hydrogen because of the properties of microcrystalline silicon, a difference between the hydrogen content of the second i-type semiconductor layer  232  and the hydrogen content of the second buffer layer  234  is relatively large. 
     On the other hand, as shown in  FIG. 7 , the hydrogen content of the first i-type semiconductor layer  222  may be more than the hydrogen content of the second i-type semiconductor layer  232  because of the properties of amorphous silicon. Thus, even if the hydrogen content of the first buffer layer  224  increases, the difference between the hydrogen content of the first i-type semiconductor layer  222  and the hydrogen content of the first buffer layer  224  may be less than the difference between the hydrogen content of the second i-type semiconductor layer  232  and the hydrogen content of the second buffer layer  234 , so as to increase the density of the first i-type semiconductor layer  222 . 
     If the difference between the hydrogen content of the first i-type semiconductor layer  222  and the hydrogen content of the first buffer layer  224  is greater than the difference between the hydrogen content of the second i-type semiconductor layer  232  and the hydrogen content of the second buffer layer  234 , the hydrogen content of the first buffer layer  224  may excessively increase. Hence, the efficiency of the solar cell  10  may be reduced. 
     Further, the first photoelectric transformation unit  220  and the second photoelectric transformation unit  230  may respectively include the first buffer layer  224  and the second buffer layer  234  as shown in  FIG. 6 . However, as shown in  FIG. 8 , the first buffer layer  224  may be omitted in the first photoelectric transformation unit  220 . Further, as shown in  FIG. 9 , the second buffer layer  234  may be omitted in the second photoelectric transformation unit  230 . 
       FIG. 10  is a table illustrating electrical characteristics of each of a comparative example and examples A, B, and C. In  FIG. 10 , the comparative example is an example where a buffer layer is not formed in a double junction solar cell, the example A is an example of the solar cell illustrated in  FIG. 9 , the example B is an example of the solar cell illustrated in  FIG. 8 , and the example C is an example of the solar cell illustrated in  FIG. 6 . 
     As shown in  FIG. 10 , in the comparative example, a short circuit current Jsc was approximately 11.06 mA/cm 2 , an open circuit voltage Voc was approximately 1.36 V, a fill factor FF was approximately 0.705%, and an efficiency Eff was approximately 10.60%. 
     On the other hand, in the example A according to the invention, a short circuit current Jsc was approximately 11.31 mA/cm 2 , an open circuit voltage Voc was approximately 1.37 V, a fill factor FF was approximately 0.704%, and an efficiency Eff was approximately 10.93%. As above, the efficiency Eff in the example A according to the invention increased by approximately 3.1% as compared with the comparative example. Because the density of the first i-type semiconductor layer  222  increases by adding the first buffer layer  224  to the first photoelectric transformation unit  220  as shown in  FIG. 9 , the efficiency Eff in the example A was improved. 
     Further, in the example B according to the invention, a short circuit current Jsc was approximately 11.30 mA/cm 2 , an open circuit voltage Voc was approximately 1.35 V, a fill factor FF was approximately 0.720%, and an efficiency Eff was approximately 10.96%. As above, the efficiency Eff in the example B according to the invention increased by approximately 3.4% as compared with the comparative example. In the example B according to the invention, the crystallinity of the second i-type semiconductor layer  232  may increase at an initial formation stage of the second i-type semiconductor layer  232  by adding the second buffer layer  234  to the second photoelectric transformation unit  230 , and thus, the crystallinity of the second i-type semiconductor layer  232  may be uniform. Hence, the efficiency of the solar cell may be improved. 
     Further, in the example C according to the invention, a short circuit current Jsc was approximately 11.34 mA/cm 2 , an open circuit voltage Voc was approximately 1.37 V, a fill factor FF was approximately 0.712%, and an efficiency Eff was approximately 11.07%. As above, the efficiency Eff in the example C according to the invention increased by approximately 4.4% as compared with the comparative example. The efficiency Eff in the example C was further improved by forming the first buffer layer  224  in the first photoelectric transformation unit  220  and forming the second buffer layer  234  in the second photoelectric transformation unit  230  as shown in  FIG. 6 . 
     Further, as shown in  FIG. 4 , at least one of the first buffer layer  224  and the second buffer layer  234  may be positioned adjacent to each other and may include a plurality of sub-buffer layers having each a different hydrogen content. 
     For example, when the thickness t 11  of the second buffer layer  234  is greater than the thickness t 21  of the first buffer layer  224 , it may be preferable that the second buffer layer  234  may include a plurality of sub-buffer layers and the first buffer layer  224  has a single-layered structure as shown in  FIG. 4 . 
       FIGS. 11 and 12  illustrate other structures of solar cells according to embodiments of the invention. A further description of structures and components identical or equivalent to those described above may be briefly made or may be entirely omitted. 
       FIG. 11  illustrates a triple junction solar cell  10  according to an embodiment of the invention. The solar cell shown in  FIG. 11  may have a pin-pin-pin structure. 
     As shown in  FIG. 11 , the solar cell  10  according to the embodiment of the invention may include a first photoelectric transformation unit  420 , a second photoelectric transformation unit  430 , and a third photoelectric transformation unit  440  that are stacked on an light incident surface, i.e., a substrate  100  in the order named. 
     The first photoelectric transformation unit  420  may include a first p-type semiconductor layer  421 , a first buffer layer  424 , a first i-type semiconductor layer  422 , and a first n-type semiconductor layer  423 . A hydrogen content of the first buffer layer  424  may be more than a hydrogen content of the first i-type semiconductor layer  422 . 
     The second photoelectric transformation unit  430  may include a second p-type semiconductor layer  431 , a second buffer layer  434 , a second i-type semiconductor layer  432 , and a second n-type semiconductor layer  433 . A hydrogen content of the second buffer layer  434  may be more than a hydrogen content of the second i-type semiconductor layer  432 . 
     The third photoelectric transformation unit  440  may include a third p-type semiconductor layer  441 , a third buffer layer  444 , a third i-type semiconductor layer  442 , and a third n-type semiconductor layer  443 . A hydrogen content of the third buffer layer  444  may be more than a hydrogen content of the third i-type semiconductor layer  442 . 
     The first photoelectric transformation unit  420  may be an amorphous silicon cell using amorphous silicon (a-Si), for example, hydrogenated amorphous silicon (a-Si:H). The first i-type semiconductor layer  422  of the first photoelectric transformation unit  420  may be formed of hydrogenated amorphous silicon (a-Si:H) and may absorb light of a short wavelength band to produce power. 
     The second photoelectric transformation unit  430  may be an amorphous silicon cell using amorphous silicon (a-Si), for example, hydrogenated amorphous silicon (a-Si:H). The second i-type semiconductor layer  432  of the second photoelectric transformation unit  430  may be formed of hydrogenated amorphous silicon (a-Si:H) and may absorb light of a middle wavelength band between a short wavelength band and a long wavelength band to produce power. 
     The third photoelectric transformation unit  440  may be an amorphous silicon cell using microcrystalline silicon (mc-Si), for example, hydrogenated microcrystalline silicon (mc-Si:H). The third i-type semiconductor layer  442  of the third photoelectric transformation unit  440  may be formed of hydrogenated microcrystalline silicon (mc-Si:H) and may absorb light of a long wavelength band to produce power. 
     A thickness of the third i-type semiconductor layer  442  may be greater than a thickness of the second i-type semiconductor layer  432 , and the thickness of the second i-type semiconductor layer  432  may be greater than a thickness of the first i-type semiconductor layer  422 . 
     As shown in  FIG. 12 , a substrate  1200  may be positioned opposite a light incident surface. More specifically, a second n-type semiconductor layer  233 , a second i-type semiconductor layer  232 , a second buffer layer  234 , a second p-type semiconductor layer  231 , a first n-type semiconductor layer  223 , a first i-type semiconductor layer  222 , a first buffer layer  224 , and a first p-type semiconductor layer  221  may be positioned on the substrate  1200  in the order named. 
     In such a structure shown in  FIG. 12 , because light is incident on the opposite side of the substrate  1200 , i.e., on a front electrode  110 , the substrate  1200  does not need to be transparent. Thus, the substrate  1200  may be formed of metal in addition to glass and plastic. The solar cell  10  having the structure shown in  FIG. 12  may be called an nip-type solar cell. Further, although it is not shown in  FIG. 12 , the solar cell according to the invention may further include a reflective layer capable of reflecting transmitted light. 
     Further, although it is not shown, in case the solar cell according to the invention may include a plurality of photoelectric transformation units (for example, in case of a double junction solar cell or a triple junction solar cell), an interlayer may be positioned between two adjacent photoelectric transformation units. The interlayer may reduce a thickness of the i-type semiconductor layer to improve the efficiency of the stability. 
       FIGS. 13 to 16  illustrate other structures of solar cells according to embodiments of the invention. A further description of structures and components identical or equivalent to those described above may be briefly made or may be entirely omitted. 
     As shown in  FIG. 13 , the solar cell  10  according to the embodiment of the invention may include a first photoelectric transformation unit  220  including a first i-type semiconductor layer  222  containing amorphous silicon and a second photoelectric transformation unit  230  including a second i-type semiconductor layer  235  containing amorphous silicon. In other words, both the first i-type semiconductor layer  222  and the second i-type semiconductor layer  235  may contain amorphous silicon. 
     Alternatively, as shown in  FIG. 14 , the solar cell  10  according to the embodiment of the invention may include a first photoelectric transformation unit  220  including a first i-type semiconductor layer  225  containing microcrystalline silicon and a second photoelectric transformation unit  230  including a second i-type semiconductor layer  232  containing microcrystalline silicon. In other words, both the first i-type semiconductor layer  225  and the second i-type semiconductor layer  232  may contain microcrystalline silicon. 
     Alternatively, as shown in  FIG. 15 , the solar cell  10  according to the embodiment of the invention may include a first photoelectric transformation unit  420 , a second photoelectric transformation unit  430 , and a third photoelectric transformation unit  440 , each of which contains amorphous silicon. In other words, all of a first i-type semiconductor layer  422 , a second i-type semiconductor layer  432 , and a third i-type semiconductor layer  445  may contain amorphous silicon. 
     Alternatively, as shown in  FIG. 16 , the solar cell  10  according to the embodiment of the invention may include a first photoelectric transformation unit  420  formed of amorphous silicon, a second photoelectric transformation unit  430  formed of microcrystalline silicon, and a third photoelectric transformation unit  440  formed of microcrystalline silicon. In other words, a first i-type semiconductor layer  422  may contain amorphous silicon, and a second i-type semiconductor layer  435  and a third i-type semiconductor layer  442  may contain microcrystalline silicon. 
       FIGS. 17 to 32  relate to examples of doping intrinsic semiconductor layers with impurities according to embodiments of the invention. A further description of structures and components identical or equivalent to those described above may be briefly made or may be entirely omitted. 
     As shown in  FIG. 17 , photoelectric transformation units  1200  and  1300  of the solar cell  10  according to the embodiment of the invention may include a first i-type semiconductor layer  1220  doped with at least one of carbon (C) and oxygen (O) as impurities and a second i-type semiconductor layer  1320  doped with germanium (Ge)-doped microcrystalline silicon (mc-Si(Ge)), so as to improve the efficiency of the solar cell  10  by adjusting their band gaps. 
     Preferably, the photoelectric transformation units  1200  and  1300  may include the first photoelectric transformation unit  1200  and the second photoelectric transformation unit  1300 . 
     The first photoelectric transformation unit  1200  may be an amorphous silicon cell using amorphous silicon (a-Si), for example, hydrogenated amorphous silicon (a-Si:H). 
     Because the first i-type semiconductor layer  1220  may be doped with at least one of carbon and oxygen as impurities, a band gap of the first i-type semiconductor layer  1220  may increase by doped carbon and oxygen. Hence, an absorptance of the first i-type semiconductor layer  1220  with respect to light of a short wavelength band may increase, and the efficiency of the solar cell  10  may be improved. 
     In the first photoelectric transformation unit  1200 , at least one of the first p-type semiconductor layer  1210  and the first n-type semiconductor layer  1230  may be doped with at least one of carbon (C) and oxygen (O) under condition that the first i-type semiconductor layer  1220  is doped with at least one of carbon (C) and oxygen (O). Preferably, the first p-type semiconductor layer  1210  may be doped with at least one of carbon (C) and oxygen (O). 
     A first buffer layer  224  may contain at least one of carbon (C) and oxygen (O) considering that the first buffer layer  224  may be formed during a manufacturing process of the first i-type semiconductor layer  1220 . 
     The second photoelectric transformation unit  1300  may be a silicon cell using microcrystalline silicon (mc-Si), for example, hydrogenated microcrystalline silicon (mc-Si:H). 
     The second i-type semiconductor layer  1320  of the second photoelectric transformation unit  1300  may be doped with Ge as impurities. Germanium may reduce a band gap of the second i-type semiconductor layer  1320 . Accordingly, an absorptance of the second i-type semiconductor layer  1320  with respect to the light of a long wavelength band may increase, and thus the efficiency of the solar cell  10  may be improved. 
     For example, a plasma enhanced chemical vapor deposition (PECVD) method may be used to dope the second i-type semiconductor layer  1320  with Ge. In the PECVD method, a very high frequency (VHF), a high frequency (HF), or a radio frequency (RF) may be applied to a chamber filled with Ge gas. 
     In the second photoelectric transformation unit  1300 , at least one of the second p-type semiconductor layer  1310  and the second n-type semiconductor layer  1330  may be doped with Ge under condition that the second i-type semiconductor layer  1320  is doped with Ge. Preferably, both the second p-type semiconductor layer  1310  and the second n-type semiconductor layer  1330  may be doped with Ge. 
     A second buffer layer  234  may contain Ge considering that the second buffer layer  234  may be formed during a manufacturing process of the second i-type semiconductor layer  1320 . 
       FIG. 18  illustrates a type 1 solar cell where first and second i-type semiconductor layers are not doped with carbon (C), oxygen (O), and Ge, and a type 2 solar cell where a first i-type semiconductor layer is doped with at least one of carbon (C) and oxygen (O) as impurities and a second i-type semiconductor layer is doped with Ge. 
     As shown in  FIG. 18 , in the type 1 solar cell, the first i-type semiconductor layer is formed of hydrogenated amorphous silicon (a-Si:H), and the second i-type semiconductor layer is formed of hydrogenated microcrystalline silicon (mc-Si:H). In the type 2 solar cell, as in the embodiment of the invention, the first i-type semiconductor layer is formed of hydrogenated amorphous silicon (a-Si:H(C,O)) doped with at least one of carbon (C) and oxygen (O), and the second i-type semiconductor layer is formed of Ge-doped hydrogenated microcrystalline silicon (mc-Si:H(Ge)). 
     In the type 1 solar cell, a band gap of the first i-type semiconductor layer is 1.75 eV, and a band gap of the second i-type semiconductor layer is 1.1 eV. 
     Further, in the type 1 solar cell, an open circuit voltage Voc is approximately 1 V, a short circuit current Jsc is approximately 1 mA/cm 2 , a fill factor FF is approximately 1, and an efficiency Eff is approximately 1. Characteristics Voc, Jsc, FF, and Eff of the type 1 solar cell are set, so that the type 1 solar cell and the type 2 solar cell are compared with each other. 
     On the other hand, in the type 2 solar cell, a band gap of the first i-type semiconductor layer is 1.9 eV and is greater than that in the type 1 solar cell by approximately 0.15 eV. A band gap of the second i-type semiconductor layer is 0.8 eV and is less than that in the type 1 solar cell by about 0.3 eV. 
     Further, in the type 2 solar cell, an open circuit voltage Voc is approximately 1.05 V, a short circuit current Jsc is approximately 1.1 mA/cm 2 , a fill factor FF is approximately 1.0, and an efficiency Eff is approximately 1.15. 
     As above, in the type 2 solar cell, because the first i-type semiconductor layer is formed of hydrogenated amorphous silicon (a-Si:H(C,O)) doped with at least one of carbon (C) and oxygen (O) and the second i-type semiconductor layer is formed of Ge-doped hydrogenated microcrystalline silicon (mc-Si:H(Ge)), the band gap of the second i-type semiconductor layer while increasing the band gap of the first i-type semiconductor layer may be reduced. Hence, the efficiency of the solar cell  10  may be improved. 
       FIG. 19  is a graph illustrating the efficiency of a solar cell depending on a Ge content of a second i-type semiconductor layer. 
     As shown in  FIG. 19 , when the Ge content of the second i-type semiconductor layer is 0 to 1 atom %, the efficiency of the solar cell is approximately 1.06 to 1.07. In this case, the band gap of the second i-type semiconductor layer may not be sufficiently reduced because of a small amount of Ge. 
     On the other hand, when the Ge content of the second i-type semiconductor layer is 3 to 20 atom %, the efficiency of the solar cell has a sufficiently improved value of approximately 1.12 to 1.15. In this case, the band gap of the second i-type semiconductor layer may be sufficiently reduced because of the sufficient Ge content. Hence, an absorptance of the second i-type semiconductor layer with respect to light of a long wavelength band may increase. 
     On the other hand, when the Ge content of the second i-type semiconductor layer is 25 atom %, the efficiency of the solar cell has a reduced value of approximately 1.05. In this case, a portion of Ge may operate as a defect inside the second i-type semiconductor layer because of an excessively large amount of Ge. As a result, the efficiency of the solar cell may be reduced in spite of an increase in the Ge content. 
     Considering the description of  FIG. 19 , it may be preferable that the Ge content of the second i-type semiconductor layer is approximately 3 to 20 atom %. 
     Strontium (Sr) may replace Ge contained in the second i-type semiconductor layer. In other words, even if the second i-type semiconductor layer is doped with Sr, a band gap of the second i-type semiconductor layer may be sufficiently reduced. 
     As shown in  FIG. 20 , the first i-type semiconductor layer  1220  may be doped with Ge as impurities. In this case, the band gap of the first i-type semiconductor layer  1220  may be reduced because of Ge. Hence, an absorptance of the first i-type semiconductor layer  1220  with respect to light of a long wavelength band may increase, and the efficiency of the solar cell  10  may be improved as compared with a related art solar cell. 
     As shown in  FIG. 20 , the solar cell according to the embodiment of the invention includes an i-type semiconductor layer formed of amorphous silicon (i.e., the first i-type semiconductor layer  1220 ) positioned between the front electrode  110  and the rear electrode  140  and an i-type semiconductor layer formed of microcrystalline silicon (i.e., the second i-type semiconductor layer  1320 ) positioned between the front electrode  110  and the rear electrode  140 . The i-type semiconductor layer formed of amorphous silicon and the i-type semiconductor layer formed of microcrystalline silicon may be doped with the same material, i.e., Ge. 
     In this case, in the first photoelectric transformation unit  1200 , at least one of the first p-type semiconductor layer  1210  and the first n-type semiconductor layer  1230  may be doped with Ge under condition that the first i-type semiconductor layer  1220  is doped with Ge. Preferably, both the first p-type semiconductor layer  1210  and the first n-type semiconductor layer  1230  may be doped with Ge. 
     Further, as shown in  FIG. 21 , both the first photoelectric transformation unit  1200  and the second photoelectric transformation unit  1300  may be an amorphous silicon cell using amorphous silicon (a-Si), for example, hydrogenated amorphous silicon (a-Si:H). In this case, both the first i-type semiconductor layer  1220  and the second i-type semiconductor layer  1321  may contain amorphous silicon and may be doped with Ge. 
     Further, as shown in  FIG. 22 , both the first photoelectric transformation unit  1200  and the second photoelectric transformation unit  1300  may be an amorphous silicon cell using amorphous silicon (a-Si), for example, hydrogenated amorphous silicon (a-Si:H). In this case, the first i-type semiconductor layer  1221  formed of amorphous silicon may be doped with at least one of carbon (C) and oxygen (O), and the second i-type semiconductor layer  1321  formed of amorphous silicon may be doped with Ge. 
     As shown in  FIG. 23 , the solar cell  10  according to the embodiment of the invention includes first, second, and third photoelectric transformation units  6200 ,  6300 , and  6000 . The first photoelectric transformation unit  6200  includes a first i-type semiconductor layer  6221  formed of amorphous silicon (a-Si(C, O)) doped with at least one of carbon (C) and oxygen (O) as impurities. The second photoelectric transformation unit  6300  includes a second i-type semiconductor layer  6321  formed of Ge-doped amorphous silicon (a-Si(Ge)). The third photoelectric transformation unit  6000  includes a third i-type semiconductor layer  6020  formed of Ge-doped microcrystalline silicon (mc-Si(Ge)). 
     The first photoelectric transformation unit  6200  may be an amorphous silicon cell using amorphous silicon (a-Si(C, O)) doped with at least one of carbon (C) and oxygen (O) as impurities, for example, hydrogenated amorphous silicon (a-Si:H(C, O)). 
     The second photoelectric transformation unit  6300  may be an amorphous silicon cell using Ge-doped amorphous silicon (a-Si(Ge)), for example, hydrogenated Ge-doped amorphous silicon (a-Si:H(Ge)). 
     The third photoelectric transformation unit  6000  may be a microcrystalline silicon silicon cell using Ge-doped microcrystalline silicon (mc-Si(Ge)), for example, hydrogenated Ge-doped microcrystalline silicon (mc-Si:H(Ge)). 
     A thickness t 3  of the third i-type semiconductor layer  6020  may be greater than a thickness t 2  of the second i-type semiconductor layer  6320 , and the thickness t 2  of the second i-type semiconductor layer  6320  may be greater than a thickness t 1  of the first i-type semiconductor layer  6220 . 
       FIG. 24  is a table illustrating characteristics of a solar cell depending on a structure of a photoelectric transformation unit. 
     In  FIG. 24 , a type 1 solar cell has a double junction structure including a first i-type semiconductor layer formed of hydrogenated amorphous silicon (a-Si:H) and a second i-type semiconductor layer formed of hydrogenated microcrystalline silicon (mc-Si:H). A type 2 solar cell has a triple junction structure including a first i-type semiconductor layer formed of hydrogenated amorphous silicon (a-Si:H), a second i-type semiconductor layer formed of hydrogenated amorphous silicon (a-Si:H), and a third i-type semiconductor layer formed of hydrogenated microcrystalline silicon (mc-Si:H). A type 3 solar cell has a triple junction structure including a first i-type semiconductor layer formed of hydrogenated amorphous silicon (a-Si:H(C, O)) doped with at least one of carbon (C) and oxygen (O) as impurities, a second i-type semiconductor layer formed of hydrogenated amorphous silicon (a-Si:H), and a third i-type semiconductor layer formed of hydrogenated microcrystalline silicon (mc-Si:H). A type 4 solar cell has a triple junction structure including a first i-type semiconductor layer formed of hydrogenated amorphous silicon (a-Si:H), a second i-type semiconductor layer formed of Ge-doped hydrogenated amorphous silicon (a-Si:H(Ge)), and a third i-type semiconductor layer formed of hydrogenated microcrystalline silicon (mc-Si:H). A type 5 solar cell has a triple junction structure including a first i-type semiconductor layer formed of hydrogenated amorphous silicon (a-Si:H(C, O)) doped with at least one of carbon (C) and oxygen (O) as impurities, a second i-type semiconductor layer formed of Ge-doped hydrogenated amorphous silicon (a-Si:H(Ge)), and a third i-type semiconductor layer formed of hydrogenated microcrystalline silicon (mc-Si:H). A type 6 solar cell, as in the embodiment of the invention, has a triple junction structure including a first i-type semiconductor layer formed of hydrogenated amorphous silicon (a-Si:H(C, O)) doped with at least one of carbon (C) and oxygen (O) as impurities, a second i-type semiconductor layer formed of Ge-doped hydrogenated amorphous silicon (a-Si:H(Ge)), and a third i-type semiconductor layer formed of Ge-doped hydrogenated microcrystalline silicon (mc-Si:H(Ge)). 
     In  FIG. 24 , characteristics Voc, Jsc, FF, and Eff of the type 1 solar cell are set, so that the type 1 solar cell and the types 2 to 6 solar cells are compared with one another. The efficiency Eff of the type 1 solar cell having the double junction structure is 1, and the efficiency Eff of the types 2 to 5 solar cells each having the triple junction structure is approximately 0.65 to 1.18. 
     Further, the efficiency Eff of the type 6 solar cell is approximately 1.24 and is much greater than the efficiency Eff of the types 1 to 5 solar cells. More specifically, in the type 6 solar cell, a band gap of the first i-type semiconductor layer is approximately 1.9 eV, and thus, an absorptance of the first i-type semiconductor layer with respect to light of a short wavelength band may increase. Further, a band gap of the second i-type semiconductor layer is approximately 1.5 eV, and thus, an absorptance of the second i-type semiconductor layer with respect to light of middle wavelength band may increase. Further, a band gap of the third i-type semiconductor layer is approximately 0.8 eV, and thus, an absorptance of the third i-type semiconductor layer with respect to light of a long wavelength band may increase. As a result, the efficiency of the type 6 solar cell may be improved. 
       FIG. 25  is a graph illustrating the efficiency of a solar cell depending on a Ge content of a third i-type semiconductor layer. More specifically,  FIG. 25  is a graph illustrating the efficiency of a solar cell depending on a Ge content of a third i-type semiconductor layer when a Ge content of a second i-type semiconductor layer is approximately 20 atom % and a carbon content of a first i-type semiconductor layer is approximately 20 atom %. 
     As shown in  FIG. 25 , when the Ge content of the third i-type semiconductor layer is 0 to 1 atom %, the efficiency of the solar cell is approximately 1.12. In this case, a band gap of the third i-type semiconductor layer may not be sufficiently reduced because of a small amount of Ge. 
     On the other hand, when the Ge content of the third i-type semiconductor layer is 3 to 20 atom %, the efficiency of the solar cell has a sufficiently improved value of approximately 1.19 to 1.25. In this case, the band gap of the third i-type semiconductor layer may be sufficiently reduced because of the sufficient Ge content. Hence, an absorptance of the third i-type semiconductor layer with respect to light of a long wavelength band may increase. 
     On the other hand, when the Ge content of the third i-type semiconductor layer is 25 atom %, the efficiency of the solar cell has a reduced value of approximately 1.10. In this case, a portion of Ge may operate as a defect inside the third i-type semiconductor layer because of an excessively large amount of Ge. As a result, the efficiency of the solar cell may be reduced in spite of an increase in the Ge content. 
     Considering the description of  FIG. 25 , it may be preferable that the Ge content of the third i-type semiconductor layer is approximately 3 to 20 atom %. 
       FIG. 26  is a graph illustrating the efficiency of a solar cell depending on a Ge content of a second i-type semiconductor layer. More specifically,  FIG. 26  is a graph illustrating the efficiency of a solar cell depending on a Ge content of a second i-type semiconductor layer when a Ge content of a third i-type semiconductor layer is approximately 15 atom % and a carbon content of a first i-type semiconductor layer is approximately 20 atom %. 
     As shown in  FIG. 26 , when the second i-type semiconductor layer is not doped with Ge, the efficiency of the solar cell is approximately 1.14. 
     On the other hand, when the Ge content of the second i-type semiconductor layer is 5 to 30 atom %, the efficiency of the solar cell has a sufficiently improved value of approximately 1.21 to 1.25. In this case, an absorptance of the second i-type semiconductor layer with respect to light of middle wavelength band may increase. 
     On the other hand, when the Ge content of the second i-type semiconductor layer is 35 atom %, the efficiency of the solar cell has a reduced value of approximately 1.12. In this case, a portion of Ge may operate as a defect inside the second i-type semiconductor layer because of an excessively large amount of Ge. As a result, the efficiency of the solar cell may be reduced in spite of an increase in the Ge content. 
     Considering the description of  FIG. 26 , it may be preferable that the Ge content of the second i-type semiconductor layer is approximately 5 to 30 atom %. 
     As above, in the second and third i-type semiconductor layers doped with Ge as impurities, the Ge content of the third i-type semiconductor layer formed of Ge-doped microcrystalline silicon is less than the Ge content of the second i-type semiconductor layer formed of Ge-doped amorphous silicon. This is because a doping degree of microcrystalline silicon is less than a doping degree of amorphous silicon and a defect generation possibility of Ge of microcrystalline silicon is greater than a defect generation possibility of Ge of amorphous silicon. 
     In the embodiment, the Ge content indicates a content per unit volume and thus, may be expressed by a concentration. 
       FIG. 27  is a graph illustrating the efficiency of a solar cell depending on a content of impurities contained in a first i-type semiconductor layer. More specifically,  FIG. 27  is a graph illustrating the efficiency of a solar cell depending on a content of impurities contained in a first i-type semiconductor layer when a Ge content of a third i-type semiconductor layer is approximately 15 atom % and a Ge content of a second i-type semiconductor layer is approximately 20 atom %. Carbon (C) and oxygen (O) may be used as impurities with which the first i-type semiconductor layer is doped.  FIG. 27  illustrates an example where carbon (C) is used as impurities. A result obtained when oxygen (O) is used as impurities may be similar to a result obtained when carbon (C) is used as impurities. 
     As shown in  FIG. 27 , when the first i-type semiconductor layer is not doped with carbon, the efficiency of the solar cell is approximately 1.02. 
     On the other hand, when the carbon content of the first i-type semiconductor layer is 10 to 50 atom %, the efficiency of the solar cell has a sufficiently improved value of approximately 1.18 to 1.25. In this case, an absorptance of the first i-type semiconductor layer with respect to light of a short wavelength band may increase. 
     On the other hand, when the carbon content of the first i-type semiconductor layer is 60 atom %, the efficiency of the solar cell has a reduced value of approximately 1.05. In this case, a portion of carbon may operate as a defect inside the first i-type semiconductor layer because of the excessive carbon content. As a result, the efficiency of the solar cell may be reduced in spite of an increase in the carbon content. 
     Considering the description of  FIG. 27 , it may be preferable that the carbon content of the first i-type semiconductor layer is approximately 10 to 50 atom %. 
     As shown in  FIG. 28 , the solar cell  10  according to the embodiment of the invention may include a first photoelectric transformation unit  6200  including a first i-type semiconductor layer  6221  formed of amorphous silicon (a-Si(C, O)) doped with at least one of carbon (C) and oxygen (O) as impurities, a second photoelectric transformation unit  6300  including a second i-type semiconductor layer  6320  formed of Ge-doped microcrystalline silicon (mc-Si(Ge)), a third photoelectric transformation unit  6000  including a third i-type semiconductor layer  6020  formed of Ge-doped microcrystalline silicon (mc-Si(Ge)). 
     In this case, a thickness t 3  of the third i-type semiconductor layer  6020  may be greater than a thickness t 2  of the second i-type semiconductor layer  6320 , and the thickness t 2  of the second i-type semiconductor layer  6320  may be greater than a thickness t 1  of the first i-type semiconductor layer  6220 . 
     Further, as shown in  FIG. 29 , the solar cell  10  according to the embodiment of the invention may include a first photoelectric transformation unit  6200  including a first i-type semiconductor layer  6221  formed of amorphous silicon (a-Si(C, O)) doped with at least one of carbon (C) and oxygen (O) as impurities, a second photoelectric transformation unit  6300  including a second i-type semiconductor layer  6321  formed of Ge-doped amorphous silicon (a-Si(Ge)), a third photoelectric transformation unit  6000  including a third i-type semiconductor layer  6021  formed of Ge-doped amorphous silicon (a-Si(Ge)). In other words, all of the first, second, and third i-type semiconductor layers  6221 ,  6321 , and  6021  may contain amorphous silicon. 
     Further, as shown in  FIG. 30 , the solar cell  10  according to the embodiment of the invention may include a first photoelectric transformation unit  6200  including a first i-type semiconductor layer  6221  formed of amorphous silicon (a-Si(C, O)) doped with at least one of carbon (C) and oxygen (O) as impurities, a second photoelectric transformation unit  6300  including a second i-type semiconductor layer  6322  formed of microcrystalline silicon (mc-Si), a third photoelectric transformation unit  6000  including a third i-type semiconductor layer  6020  formed of Ge-doped microcrystalline silicon (mc-Si(Ge)). In other words, the third i-type semiconductor layer  6020  formed of microcrystalline silicon may be doped with Ge, and the second i-type semiconductor layer  6322  formed of microcrystalline silicon need not be doped with Ge. 
     In case the solar cell  10  according to the embodiment of the invention is a single junction solar cell, the i-type semiconductor layer may be doped with impurities. 
     For example, as shown in  FIG. 31 , when the solar cell  10  being a single junction solar cell includes an i-type semiconductor layer  125  formed of microcrystalline silicon, the i-type semiconductor layer  125  may be doped with Ge. 
     Alternatively, as shown in  FIG. 32 , when the solar cell  10  being a single junction solar cell includes an i-type semiconductor layer  135  formed of amorphous silicon, the i-type semiconductor layer  135  may be doped with Ge. 
     In embodiments of the invention, although most of the same reference numbers refer to the same elements in general, such is not required, and it is understood that the same reference numbers may be used for similar structures in the solar cells. 
     In embodiments of the invention, reference to front or back, with respect to electrode, a surface of the substrate, or others is not limiting. For example, such a reference is for convenience of description since front or back is easily understood as examples of first or second of the electrode, the surface of the substrate or others. 
     While this invention has been described in connection with what is presently considered to be practical example embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.