Abstract:
An object of the present invention is to provide a photoelectric conversion device having a novel anti-reflection structure. An uneven structure is formed on a surface of a semiconductor by growth of the same or a different kind of semiconductor instead of forming an anti-reflection structure by etching a surface of a semiconductor substrate or a semiconductor film. For example, a semiconductor layer including a plurality of projections is provided on a light incident plane side of a photoelectric conversion device, thereby considerably reducing surface reflection. Such a structure can be formed by a vapor deposition method; therefore, the contamination of the semiconductor is not caused.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a photoelectric conversion device and a method for manufacturing the same. 
         [0003]    2. Description of the Related Art 
         [0004]    Recently, a photoelectric conversion device, which is a power generation means that generates power without carbon dioxide emissions, has attracted attention as a countermeasure against global warming. A solar cell for supplying residential power or the like, which generates power from sunlight outdoors, is known as a typical example thereof. For such a solar cell, a crystalline silicon solar cell using single crystal silicon or polycrystalline silicon is mainly used. 
         [0005]    An uneven structure is provided on a surface of a solar cell using a single crystal silicon substrate or a polycrystalline silicon substrate in order to reduce surface reflection. The uneven structure provided on the surface of the silicon substrate is formed by etching the silicon substrate with an alkaline solution such as NaOH. The etching rate by the alkaline solution varies depending on a crystal plane orientation of silicon. Therefore, when a silicon substrate with a (100) plane is used for example, a pyramidal uneven structure is formed. 
         [0006]    Although the above-described uneven structure can reduce surface reflection of the solar cell, the alkaline solution used for etching causes contamination of the silicon semiconductor. In addition, since etching characteristics considerably vary depending on the concentration or temperature of the alkaline solution, it is difficult to form the uneven structure on the surface of the silicon substrate with high reproducibility. For the difficulty, a combination method of a laser processing technique and chemical etching is disclosed (for example, see Patent Document 1). 
         [0007]    On the other hand, in a solar cell whose photoelectric conversion layer is formed using a semiconductor thin film of silicon or the like, it is difficult to form an uneven structure on a surface of the silicon thin film by etching using the above-described alkaline solution. 
       REFERENCE 
     Patent Document 
       [0000]    
       
         [Patent Document 1] 
         Japanese Published Patent Application No 2003-258285 
       
     
       SUMMARY OF THE INVENTIONS 
       [0010]    In any case, the method in which the silicon substrate itself is etched to form the uneven structure on the surface of the silicon substrate is not favorable because the method has a problem in controllability of the uneven shape and affects the characteristics of the solar cell. In addition, since the alkaline solution and a large amount of water for cleaning are needed for etching of the silicon substrate and it is necessary to pay attention to the contamination of the silicon substrate, the method is also not favorable in terms of productivity. 
         [0011]    Thus, an object of an embodiment of the present invention is to provide a photoelectric conversion device having a novel anti-reflection structure. 
         [0012]    One feature of an embodiment of the present invention is to form an uneven structure on a surface of a semiconductor by growth of the same or a different kind of semiconductor instead of forming an anti-reflection structure by etching a surface of a semiconductor substrate or a semiconductor film. 
         [0013]    For example, a semiconductor layer including a plurality of projections is provided on a light incident plane side of a photoelectric conversion device, thereby considerably reducing surface reflection. Such a structure can be formed by a vapor deposition method; therefore, the contamination of the semiconductor is not caused. 
         [0014]    With the use of a vapor deposition method, a semiconductor layer including a plurality of whiskers can be grown, whereby the anti-reflection structure of the photoelectric conversion device can be formed. 
         [0015]    An embodiment of the present invention is a photoelectric conversion device including a first conductive layer, a plurality of second conductive layers that is provided in contact with the first conductive layer, a first conductivity-type crystalline semiconductor region that is provided over the first conductive layer and the second conductive layer and has an uneven surface by including a plurality of whiskers which is formed using a crystalline semiconductor including an impurity element imparting the first conductivity, and a second-conductivity-type crystalline semiconductor region that covers the uneven surface of the first-conductivity-type crystalline semiconductor region having the uneven surface. The second conductivity type is opposite to the first conductivity type. 
         [0016]    An embodiment of the present invention is a photoelectric conversion device including a first-conductivity-type crystalline semiconductor region and a second-conductivity-type crystalline semiconductor region that are stacked over an electrode. The electrode includes a first conductive layer and a plurality of second conductive layers. The first-conductivity-type crystalline semiconductor region includes a crystalline semiconductor region including an impurity element imparting the first conductivity, and a plurality of whiskers that is provided over the crystalline semiconductor region and includes a crystalline semiconductor including an impurity element imparting the first conductivity type. That is, since the first-conductivity-type crystalline semiconductor region includes the plurality of whiskers, a surface of the second-conductivity-type crystalline semiconductor region is uneven. In addition, an interface between the first-conductivity-type crystalline semiconductor region and the second-conductivity-type crystalline semiconductor region is uneven. 
         [0017]    Note that a crystalline semiconductor region may be provided between the first-conductivity-type crystalline semiconductor region and the second-conductivity-type crystalline semiconductor region, and an interface between the first-conductivity-type crystalline semiconductor region and the crystalline semiconductor region may be uneven. 
         [0018]    In the above photoelectric conversion device, the first-conductivity-type crystalline semiconductor region is one of an n-type semiconductor region and a p-type semiconductor region, and the second-conductivity-type crystalline semiconductor region is the other of the n-type semiconductor region and the p-type semiconductor region. 
         [0019]    An embodiment of the present invention is a photoelectric conversion device including, in addition to the above structure, a third-conductivity-type semiconductor region, an intrinsic semiconductor region, and a fourth-conductivity-type semiconductor region that are stacked over the second-conductivity-type crystalline semiconductor region. Accordingly, a surface of the fourth-conductivity-type semiconductor region is uneven. 
         [0020]    Note that in the above photoelectric conversion device, each of the first-conductivity-type crystalline semiconductor region and the third-conductivity-type semiconductor region is one of an n-type semiconductor region and a p-type semiconductor region, and each of the second-conductivity-type crystalline semiconductor region and the fourth-conductivity-type semiconductor region is the other of the n-type semiconductor region and the p-type semiconductor region. 
         [0021]    Directions of axes of the plurality of whiskers which is provided over the first-conductivity-type crystalline semiconductor region may be the normal direction of the first conductive layer. Alternatively, the directions of axes of the plurality of whiskers which is provided over the first-conductivity-type crystalline semiconductor region may be varied. 
         [0022]    The electrode includes a first conductive layer and a plurality of second conductive layers. The second conductive layer can be formed using a metal element which forms silicide by reacting with silicon. Alternatively, the second conductive layer can be formed with a layered structure of a layer which is formed using a material having high conductivity such as a metal element typified by platinum, aluminum, or copper, and a layer which is formed using a metal element forming silicide by reacting with silicon. 
         [0023]    The electrode may include a mixed layer covering the plurality of second conductive layers. The mixed layer may include silicon and a metal element which forms the second conductive layer. In the case where the second conductive layer is formed using a metal element which forms silicide by reacting with silicon, the mixed layer may be formed of silicide. 
         [0024]    In the photoelectric conversion device, the first-conductivity-type crystalline semiconductor region includes a plurality of whiskers, thereby reducing light reflectance at the surface. In addition, since the photoelectric conversion layer absorbs light incident on the photoelectric conversion layer owing to a light-trapping effect, characteristics of the photoelectric conversion device can be improved. 
         [0025]    An embodiment of the present invention is a method for manufacturing a photoelectric conversion device, including the steps of: forming a second conductive layer over a first conductive layer; over the first conductive layer and the second conductive layer, forming a first-conductivity-type crystalline semiconductor region that includes a crystalline semiconductor region and a plurality of whiskers including a crystalline semiconductor by a low pressure CVD method using a deposition gas containing silicon and a gas imparting the first conductivity type as source gases; and forming a second-conductivity-type crystalline semiconductor region over the first-conductivity-type crystalline semiconductor region by a low pressure CVD method using a deposition gas containing silicon and a gas imparting the second conductivity type as source gases. 
         [0026]    An embodiment of the present invention is a method for manufacturing a photoelectric conversion device, comprising the steps of: forming a second conductive layer over a first conductive layer; over the first conductive layer and the second conductive layer, forming a first-conductivity-type crystalline semiconductor region that includes a crystalline semiconductor region and a plurality of whiskers including a crystalline semiconductor by a low pressure CVD method using a deposition gas containing silicon and a gas imparting the first conductivity type as source gases; and forming a second-conductivity-type crystalline semiconductor region over the first-conductivity-type crystalline semiconductor region by a low pressure CVD method using a deposition gas containing silicon and a gas imparting the second conductivity type as source gases. 
         [0027]    Note that the low pressure CVD method is performed at a temperature of higher than 550° C. In addition, silicon hydride, silicon fluoride, or silicon chloride may be used for the deposition gas containing silicon. In addition, the gas imparting the first conductivity type is one of diborane and phosphine, and the gas imparting the second conductivity type is the other of the diborane and the phosphine. 
         [0028]    By a low pressure CVD method, the first-conductivity-type crystalline semiconductor region which includes the plurality of whiskers can be formed over the second conductive layer which is formed using a metal element forming silicide by reacting with silicon. 
         [0029]    Note that in this specification, an “intrinsic semiconductor” refers to not only a so-called intrinsic semiconductor in which the Fermi level lies in the middle of the band gap, but a semiconductor in which the concentration of an impurity imparting p-type or n-type conductivity is 1×10 20  cm −3  or lower and photoconductivity is 100 times or more as high as the dark conductivity. This intrinsic semiconductor may include an impurity element belonging to Group 13 or Group 15 of the periodic table. Accordingly, if the problems can be solved and the same effect can be used, even the semiconductor having n-type or p-type conductivity can be used instead of the intrinsic semiconductor. Such a substantially intrinsic semiconductor is included in an intrinsic semiconductor in this specification. 
         [0030]    According to an embodiment of the present invention, the surface of the second-conductivity-type crystalline semiconductor region is uneven, whereby the characteristics of the photoelectric conversion device can be improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    In the accompanying drawings: 
           [0032]      FIG. 1  is a top view illustrating a photoelectric conversion device; 
           [0033]      FIG. 2  is a cross-sectional view illustrating a photoelectric conversion device; 
           [0034]      FIG. 3  is a cross-sectional view illustrating a photoelectric conversion device; 
           [0035]      FIG. 4  is a cross-sectional view illustrating a photoelectric conversion device; 
           [0036]      FIGS. 5A to 5C  are cross-sectional views illustrating a method for manufacturing a photoelectric conversion device; 
           [0037]      FIGS. 6A and 6B  are cross-sectional views illustrating a method for manufacturing a photoelectric conversion device; 
           [0038]      FIG. 7  is a cross-sectional view illustrating a photoelectric conversion device; 
           [0039]      FIG. 8  is a cross-sectional view illustrating a photoelectric conversion device; and 
           [0040]      FIG. 9  is a cross-sectional view illustrating a photoelectric conversion device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the invention is not limited to the following description, and it will be easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. Thus, the present invention should not be interpreted as being limited to the following description of the embodiments. In description with reference to the drawings, in some cases, the same reference numerals are used in common for the same portions in different drawings. Further, in some cases, the same hatching patterns are applied to similar parts, and the similar parts are not necessarily designated by reference numerals. 
         [0042]    Note that in each drawing described in this specification, the size, the layer thickness, or the region of each component is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such scales. 
         [0043]    Note that terms such as “first”, “second”, and “third” in this specification are used in order to avoid confusion among components, and the terms do not limit the components numerically. Therefore, for example, the term “first” can be replaced with the term “second”, “third”, or the like as appropriate. 
       Embodiment 1 
       [0044]    In this embodiment, a structure of a photoelectric conversion device which is one embodiment of the present invention is described with reference to  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4 , and  FIGS. 5A to 5C . 
         [0045]      FIG. 1  is a schematic view of a top surface of a photoelectric conversion device. Although not illustrated, a photoelectric conversion layer is formed over an electrode  103  which is formed over a substrate  101 . Further, an auxiliary electrode  115  is formed over the electrode  103  and a grid electrode  117  is formed over a second-conductivity-type crystalline semiconductor region. The auxiliary electrode  115  functions as a terminal for extracting electric energy to the outside. The grid electrode  117  is formed over the second-conductivity-type crystalline semiconductor region to reduce resistance of the second-conductivity-type crystalline semiconductor region. Here, a cross section of a dashed-and-dotted line A-B in  FIG. 1  is described with reference to  FIG. 2 ,  FIG. 3 ,  FIG. 4 ,  FIGS. 5A to 5C , and  FIGS. 6A and 6B . 
         [0046]      FIG. 2  is a schematic view of a photoelectric conversion device including a substrate  101 , an electrode  103 , a first-conductivity-type crystalline semiconductor region  107 , and a second-conductivity-type crystalline semiconductor region  111 . The second conductivity type is opposite to the first conductivity type. The first-conductivity-type crystalline semiconductor region  107  and the second-conductivity-type crystalline semiconductor region  111  function as a photoelectric conversion layer. The first-conductivity-type crystalline semiconductor region  107  has an uneven surface by including a plurality of whiskers which are formed using a crystalline semiconductor including an impurity element imparting first conductivity type. In addition, an insulating layer  113  is formed over the second-conductivity-type crystalline semiconductor region  111 . 
         [0047]    In this embodiment, the first-conductivity-type crystalline semiconductor region  107  includes a crystalline semiconductor region  107   a  including an impurity element imparting the first conductivity type and a group of whiskers including a plurality of whiskers  107   b  which are formed using a crystalline semiconductor including an impurity element imparting the first conductivity type. Further, an interface between the first-conductivity-type crystalline semiconductor region  107  and the second-conductivity-type crystalline semiconductor region  111  is uneven. That is, a surface of the second-conductivity-type crystalline semiconductor region  111  is uneven. 
         [0048]    The position and density of the whiskers  107   b  of the first-conductivity-type crystalline semiconductor region  107  can be controlled by changing the shape and size of a plurality of second conductive layers  105   a  formed over the first conductive layer  104  and the shape and size of a plurality of mixed layers  105   b . That is, by the plurality of second conductive layers  105   a  and the plurality of mixed layers  105   b  which are formed over the first conductive layer  104 , the crystalline semiconductor region  107   a  and the whiskers  107   b  can be formed. Thus, the second conductive layers  105   a  and the mixed layers  105   b  are overlapped with the whiskers  107   b . In this embodiment, one whisker  107   b  overlaps with one mixed layer  105   b.    
         [0049]    In this embodiment, a p-type crystalline semiconductor layer and an n-type crystalline semiconductor layer are used as the first-conductivity-type crystalline semiconductor region  107  and the second-conductivity-type crystalline semiconductor region  111 , respectively; however, the p-type conductivity and the n-type conductivity may be interchanged with each other. 
         [0050]    As the substrate  101 , a glass substrate typified by an aluminosilicate glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, a sapphire glass substrate, and a quartz glass substrate can be used. Alternatively, a substrate in which an insulating film is formed over a metal substrate such as a stainless steel substrate or the like may be used. In this embodiment, a glass substrate is used as the substrate  101 . 
         [0051]    Note that in the electrode  103 , a plurality of second conductive layers  105   a  is formed over the first conductive layer  104  in some cases. Alternatively, the electrode  103  includes, over the first conductive layer  104 , the plurality of second conductive layers  105   a  and the plurality of mixed layers  105   b  formed on surfaces of the second conductive layers  105   a  in some cases. Further alternatively, in the electrode  103 , a plurality of mixed layers  105   b  is formed over the first conductive layer  104  in some cases. 
         [0052]    The first conductive layer  104  functions as an electrode of the photoelectric conversion layer. Thus, it is preferable that the first conductive layer  104  have the size which is adjusted to the size of the cell of the photoelectric conversion device. The first conductive layer  104  is formed using a conductive layer having a reflecting property or a light-transmitting property. 
         [0053]    In the case where external light is incident on the photoelectric conversion device from the insulating layer  113  side, a reflective conductive layer is formed as the first conductive layer  104 , whereby a light-trapping effect in the photoelectric conversion layer can be increased. The reflective conductive layer is preferably formed using a metal element having high conductivity and a reflecting property typified by aluminum, copper, tungsten, an aluminum alloy to which an element which improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, is added, or the like. 
         [0054]    In the case where external light is incident on the photoelectric conversion device from the electrode  103  side, a light-transmitting conductive layer is formed as the first conductive layer  104 , whereby loss of the amount of light incident on the photoelectric conversion layer can be reduced. As the light-transmitting conductive layer, a conductive layer formed using an indium oxide-tin oxide alloy (ITO), zinc oxide (ZnO), tin oxide (SnO 2 ), zinc oxide containing aluminum, or the like is preferably used. 
         [0055]    Note that the first conductive layer  104  may have a foil shape, a plate shape, or a net shape. With such a shape, the first conductive layer  104  can hold its shape by itself, and the substrate  101  is therefore not essential. For this reason, cost can be reduced. In addition, the first conductive layer  104  has a foil shape, whereby a flexible photoelectric conversion device can be manufactured. 
         [0056]    The second conductive layer  105   a  is formed using a metal element which forms silicide by reacting with silicon. Alternatively, a stacked layer structure may be used, which includes a layer formed using a metal element having high conductivity typified by aluminum, copper, an aluminum alloy to which an element which improves heat resistance, such as silicon, titanium, neodymium, scandium, or molybdenum, is added, or the like on the substrate  101  side; and a layer formed using a metal element which forms silicide by reacting with silicon on the first-conductivity-type crystalline semiconductor region  107  side. Examples of the metal element which forms silicide by reacting with silicon include zirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, cobalt, nickel, and the like. 
         [0057]    The second conductive layer  105   a  is preferably formed to a thickness of greater than or equal to 100 nm and less than or equal to 1000 nm. 
         [0058]    The mixed layer  105   b  may be formed using silicon and the metal element which forms the second conductive layer  105   a . Note that in the case where the mixed layer  105   b  is formed using silicon and the metal element which forms the second conductive layer  105   a , active species of the source gas are supplied to a deposition portion depending on heating conditions in forming the first-conductivity-type crystalline semiconductor region by an LPCVD method; therefore, silicon is diffused into the second conductive layer  105   a  and thus the mixed layer  105   b  is formed. 
         [0059]    In the case where the second conductive layer  105   a  is formed using a metal element which forms silicide by reacting with silicon, silicide including the metal element is formed in the mixed layer  105   b ; typically, one or more of zirconium silicide, titanium silicide, hafnium silicide, vanadium silicide, niobium silicide, tantalum silicide, chromium silicide, molybdenum silicide, cobalt silicide, and nickel silicide is/are formed. Alternatively, an alloy layer of silicon and a metal element which forms silicide is formed. 
         [0060]    As illustrated in  FIG. 2 , the second conductive layer  105   a  and the mixed layer  105   b  can have a conical shape such as a circular cone or a pyramid or a polyhedral shape whose top surface has a vertex. Alternatively, as illustrated in  FIG. 3 , a second conductive layer  151   a  and a mixed layer  151   b  can have a columnar-like shape such as a cylinder or a prism, a polyhedral shape whose top surface is flat, or a truncated conical shape such as a circular truncated cone or a truncated pyramid. Note that the second conductive layers  105   a  and  151   a  and the mixed layers  105   b  and  151   b  may have rounded corners in which crests and vertexes are rounded in any of the above shapes. In the case where the mixed layer  105   b  is formed over the second conductive layer  105   a , a stacked-layer structure thereof corresponds to the above structure. 
         [0061]    In this embodiment, a whisker grows based on the second conductive layer  105   a , or the mixed layers  105   b  and  151   b . Therefore, when the width of the cross-sectional shapes of the second conductive layer  105   a  and/or the mixed layer  105   b  and the width of the cross-sectional shapes of the second conductive layer  151   a  and/or the mixed layer  151   b  are narrower than the width of the whisker  107   b , the second conductive layer  105   a  and/or the mixed layer  105   b  and the second conductive layer  151   a  and/or the mixed layer  151   b  are overlapped with one whisker. Note that in the case where the second conductive layer  151   a  and/or the mixed layer  105   b  have/has a conical shape or a polyhedral shape, a whisker grows more easily based on a vertex. 
         [0062]    In the case where the mixed layer  105   b  is provided between the second conductive layer  105   a  and the first-conductivity-type crystalline semiconductor region  107 , resistance at an interface between the second conductive layer  105   a  and the first-conductivity-type crystalline semiconductor region  107  can be reduced; therefore series resistance can be further reduced as compared to the case where the first-conductivity-type crystalline semiconductor region  107  is directly stacked over the second conductive layer  105   a . In addition, the adhesiveness between the second conductive layer  105   a  and the first-conductivity-type crystalline semiconductor region  107  can be increased. As a result, yield of the photoelectric conversion device can be improved. 
         [0063]    The first-conductivity-type crystalline semiconductor region  107  is typically formed using a semiconductor to which an impurity element imparting the first conductivity type is added. Silicon is suitable for the semiconductor material, considering productivity, a price, or the like. When silicon is used as the semiconductor material, phosphorus or arsenic, which imparts n-type conductivity, or boron, which imparts p-type conductivity, is used as the impurity element imparting the first conductivity type. Here, the first-conductivity-type crystalline semiconductor region  107  is formed using a p-type crystalline semiconductor. 
         [0064]    The first-conductivity-type crystalline semiconductor region  107  includes a crystalline semiconductor region  107   a  which includes an impurity element imparting the first conductivity type (hereinafter referred to as the crystalline semiconductor region  107   a ) and a group of whiskers including a plurality of whiskers  107   b  which is provided over the crystalline semiconductor region  107   a  and which is formed using a crystalline semiconductor including an impurity element imparting the first conductivity type (hereinafter referred to as the whiskers  107   b ). Note that the interface between the crystalline semiconductor region  107   a  and the whisker  107   b  is unclear. Therefore, a plane that is in the same level as the bottom of the deepest valley of valleys formed among whiskers  107   b  and is parallel to a surface of the electrode  103  is regarded as the interface between the crystalline semiconductor region  107   a  and the whisker  107   b.    
         [0065]    The crystalline semiconductor region  107   a  covers the second conductive layer  105   a  or the mixed layer  105   b . In addition, the whisker  107   b  is a whisker-like protrusion, and a plurality of protrusions is dispersed. Note that the whisker  107   b  may have a columnar-like shape such as a cylinder or a prism, or a needle-like shape such as a cone or a pyramid. The top of the whisker  107   b  may be rounded. The width of the whisker  107   b  is greater than or equal to 100 nm and less than or equal to 10 μm, preferably greater than or equal to 500 nm and less than or equal to 3 μm. Further, the length in the axis of the whisker  107   b  is greater than or equal to 300 nm and less than or equal to 20 μm, preferably greater than or equal to 500 nm and less than or equal to 15 μm. The photoelectric conversion device in this embodiment includes one or more of the above-described whiskers. 
         [0066]    Note that the length in the axis of the whisker  107   b  is the distance between the top of the whisker  107   b  and the crystalline semiconductor region  107   a  in the axis running through the top of the whisker  107   b  or the center of the top surface of the whisker  107   b . The thickness of the first-conductivity-type crystalline semiconductor region  107  is the sum of the thickness of the crystalline semiconductor region  107   a  and the length of a normal from the top of the whisker  107   b  to the crystalline semiconductor region  107   a  (i.e., the height of the whisker). The width of the whisker  107   b  refers to a length of a longer axis of a transverse cross-sectional shape at the interface between the crystalline semiconductor region  107   a  and the whisker  107   b.    
         [0067]    Note that the direction in which the whisker  107   b  extends from the crystalline semiconductor region  107   a  is referred to as a longitudinal direction. A cross-sectional shape along the longitudinal direction is referred to as a longitudinal cross-sectional shape. In addition, the shape of a plane in which the longitudinal direction is a normal direction is referred to as a transverse cross-sectional shape. 
         [0068]    In  FIG. 2 , the longitudinal directions of the whiskers  107   b  included in the first-conductivity-type crystalline semiconductor region  107  are one direction, e.g., the direction normal to the surface of the electrode  103 . Note that the longitudinal direction of the whisker  107   b  may be substantially the same as the direction normal to the surface of the electrode  103 . In that case, it is preferable that the difference between the angles of the directions be typically within 5°. 
         [0069]    Note that the longitudinal directions of the whiskers  107   b  included in the first-conductivity-type crystalline semiconductor region  107  are one direction, e.g., the direction normal to the surface of the electrode  103  in  FIG. 2 ; however, the longitudinal directions of the whiskers  107   b  may be varied. Typically, the first-conductivity-type crystalline semiconductor region  107  may include a whisker whose longitudinal direction is substantially the same as the normal direction and a whisker whose longitudinal direction is different from the normal direction. 
         [0070]    The second-conductivity-type crystalline semiconductor region  111  is formed using an n-type crystalline semiconductor. Note that semiconductor materials which can be used for the second-conductivity-type crystalline semiconductor region  111  are the same as those for the first-conductivity-type crystalline semiconductor region  107 . 
         [0071]    In this embodiment, in the photoelectric conversion layer, an interface between the first-conductivity-type crystalline semiconductor region  107  and the second-conductivity-type crystalline semiconductor region  111  and the surface of the second-conductivity-type crystalline semiconductor region  111  are uneven. Therefore, reflectance of light incident on the insulating layer  113  can be reduced. Further, the light incident on the photoelectric conversion layer is efficiently absorbed by the photoelectric conversion layer due to a light-trapping effect; thus, the characteristics of the photoelectric conversion device can be improved. In the case where light is incident on the photoelectric conversion layer from the substrate  101  side, the first conductive layer  104  which is part of the electrode  103  may be formed using a light-transmitting conductive layer and a reflective conductive layer may be formed between the second-conductivity-type crystalline semiconductor region  111  and the insulating layer  113 . Since the second-conductivity-type crystalline semiconductor region  111  is uneven, the light-trapping effect of the photoelectric conversion layer is increased and more light is absorbed by the photoelectric conversion layer, whereby the characteristics of the photoelectric conversion device can be improved. 
         [0072]    Note that in  FIG. 2  and  FIG. 3 , a PN junction semiconductor layer in which the first-conductivity-type crystalline semiconductor region  107  and the second-conductivity-type crystalline semiconductor region  111  are in contact with each other is used as the photoelectric conversion layer; however, as illustrated in  FIG. 4 , a PIN junction semiconductor layer which includes a crystalline semiconductor region  109  between a first-conductivity-type crystalline semiconductor region  108  and the second-conductivity-type crystalline semiconductor region  111  may be used as the photoelectric conversion layer. Here, as the crystalline semiconductor region  109 , an intrinsic crystalline semiconductor region is used. 
         [0073]    Note that in this specification, an “intrinsic semiconductor” refers to not only a so-called intrinsic semiconductor in which the Fermi level lies in the middle of the band gap, but a semiconductor in which the concentration of an impurity imparting p-type or n-type conductivity is 1×10 20  cm −3  or lower and the photoconductivity is 100 times or more as high as the dark conductivity. This intrinsic semiconductor includes an impurity element belonging to Group 13 or Group 15 of the periodic table. Such a substantially intrinsic semiconductor is included in the intrinsic semiconductor here. 
         [0074]    Note that the first-conductivity-type crystalline semiconductor region  108  includes a crystalline semiconductor region  108   a  including an impurity element imparting the first conductivity type and a group of whiskers including a plurality of whiskers  108   b  which is provided over the crystalline semiconductor region  108   a  and which is formed using a crystalline semiconductor including an impurity element imparting the first conductivity type, like the first-conductivity-type crystalline semiconductor region  107  illustrated in  FIG. 2 . 
         [0075]    Note that the insulating layer  113  which has an anti-reflection function and a protection function is preferably formed over exposed surfaces of the electrode  103  and the second-conductivity-type crystalline semiconductor region  111 . 
         [0076]    For the insulating layer  113 , a material whose refractive index is between the refractive indices of the second-conductivity-type crystalline semiconductor region  111  and air is used. In addition, a material which transmits light with a predetermined wavelength is used so that incidence of light on the second-conductivity-type crystalline semiconductor region  111  is not interrupted. The use of such a material can prevent reflection at the light incident plane of the second-conductivity-type crystalline semiconductor region. Note that as such a material, silicon nitride, silicon nitride oxide, or magnesium fluoride can be given, for example. 
         [0077]    Although not illustrated, an electrode may be provided over the second-conductivity-type crystalline semiconductor region  111 . The electrode is formed using a light-transmitting conductive layer of an alloy of indium oxide and tin oxide (ITO), zinc oxide (ZnO), tin oxide (SnO 2 ), zinc oxide containing aluminum, or the like. In this embodiment, the light is incident on the second-conductivity-type crystalline semiconductor region  111  side; therefore, the second-conductivity-type crystalline semiconductor region  111  is formed using a light-transmitting conductive layer. 
         [0078]    The auxiliary electrode  115  and the grid electrode  117  illustrated in  FIG. 1  are formed of a layer formed using a metal element such as silver, copper, aluminum, palladium, lead, or tin. The grid electrode  117  is formed to be in contact with the second-conductivity-type crystalline semiconductor region  111 , whereby the resistance loss of the second-conductivity-type crystalline semiconductor region  111  can be reduced, and especially, the electrical characteristics under high illuminance can be enhanced. The grid electrode has a grid pattern (or a comb-like pattern, a comb-like shape, or a comb-tooth-like pattern) in order to increase a light-receiving area of the photoelectric conversion layer. 
         [0079]    Next, a method for manufacturing the photoelectric conversion device illustrated in  FIG. 1  and  FIG. 2  will be described with reference to  FIGS. 5A to 5C  and  FIGS. 6A and 6B . Here, a cross section taken along a dashed-and-dotted line C-D in  FIG. 1  will be described with reference to  FIGS. 5A to 5C  and  FIGS. 6A and 6B . 
         [0080]    As illustrated in  FIG. 5A , the first conductive layer  104  is formed over the substrate  101 . The first conductive layer  104  can be formed by a printing method, a sol-gel method, a coating method, an ink-jet method, a CVD method, a sputtering method, an evaporation method, or the like, as appropriate. Note that, in the case where the first conductive layer  104  has a foil shape, it is not necessary to provide the substrate  101 . Further, roll-to-roll processing can be employed. 
         [0081]    Next, a plurality of second conductive layers  105  is formed over the first conductive layer  104 . The second conductive layer  105  is preferably formed assuming the position of the whisker included in the first-conductivity-type crystalline semiconductor region formed later. 
         [0082]    The second conductive layer  105  is formed over the first conductive layer  104  by an ink-jet method, a nano-imprinting method, or the like. Alternatively, the second conductive layer  105  can be formed in the following manner that a conductive layer is formed over the first conductive layer  104  using a CVD method, a sputtering method, an evaporation method, a sol-gel method, or the like and then, a surface of the conductive layer is exposed to plasma until part of the first conductive layer  104  is exposed. Further alternatively, the second conductive layer  105  can be formed in the following manner that a conductive layer is formed over the first conductive layer  104 , and then, the conductive layer is etched by using a resist mask formed by a photolithography process. Note that in this step, the above conductive layer needs to be formed using a layer formed using a metal element which having an etching selectivity with respect to the first conductive layer  104 . 
         [0083]    Next, as illustrated in  FIG. 5B , a first-conductivity-type crystalline semiconductor region  137  and a second-conductivity-type crystalline semiconductor region  141  are formed by an LPCVD method. Then, a second electrode may be formed. 
         [0084]    The LPCVD method is performed as follows: heating is performed at a temperature of higher than 550° C. and in the range of temperature at which an LPCVD apparatus and the conductive layer  104  can withstand, preferably higher than or equal to 580° C. and lower than 650° C.; at least a deposition gas containing silicon is used as a source gas; and the pressure in a reaction chamber of the LPCVD apparatus is set to higher than or equal to a lower limit at which the pressure can be maintained while the source gas flows and lower than or equal to 200 Pa. Examples of the deposition gas containing silicon include silicon hydride, silicon fluoride, and silicon chloride; typically, SiH 4 , Si 2 H 6 , SiF 4 , SiCl 4 , Si 2 Cl 6 , and the like are given. Note that hydrogen may be introduced into the source gas. 
         [0085]    When the first-conductivity-type crystalline semiconductor region  137  is formed by the LPCVD method, a mixed layer  105   b  is formed between the second conductive layer  105  and the first-conductivity-type crystalline semiconductor region  137  depending on heating conditions. In a step of forming the first-conductivity-type crystalline semiconductor region  137 , active species of the source gas are constantly supplied to a deposition portion, and silicon is diffused from the first-conductivity-type crystalline semiconductor region  137  to the second conductive layer  105 , so that the mixed layer  105   b  is formed. On the other hand, a region into which silicon is not diffused in the second conductive layer  105  becomes the second conductive layer  105   a . For this reason, a low-density region (a sparse region) is not easily formed at an interface between the second conductive layer  105   a  and the first-conductivity-type crystalline semiconductor region  137 . In addition, a plurality of minute second conductive layers  105   a  and a plurality of mixed layers  105   b  are formed over the first conductive layer  104 ; thus, a low-density region (a sparse region) is not easily formed at an interface between the first conductive layer  104  and the first-conductivity-type crystalline semiconductor region  137 . For this reason, the characteristics of the interface between the first conductive layer  104  and the first-conductivity-type crystalline semiconductor region  137  are improved, so that series resistance can be reduced. 
         [0086]    The first-conductivity-type crystalline semiconductor region  137  is formed by an LPCVD method in which diborane and a deposition gas containing silicon are introduced into the reaction chamber of the LPCVD apparatus as a source gas. The thickness of the first-conductivity-type crystalline semiconductor region  137  is greater than or equal to 500 nm and less than or equal to 20 μm. Here, a crystalline silicon layer to which boron is added is formed for the first-conductivity-type crystalline semiconductor region  137 . 
         [0087]    Next, the introduction of diborane into the reaction chamber of the LPCVD apparatus is stopped. Then, the second-conductivity-type crystalline semiconductor region  141  is formed by an LPCVD method in which phosphine or arsine and a deposition gas containing silicon are introduced into the reaction chamber of the LPCVD apparatus as a source gas. The thickness of the second-conductivity-type crystalline semiconductor region  141  is greater than or equal to 5 nm and less than or equal to 500 nm. Here, a crystalline silicon layer to which phosphorus or arsenic is added is formed for the second-conductivity-type crystalline semiconductor region  141 . 
         [0088]    Through the above steps, the photoelectric conversion layer including the first-conductivity-type crystalline semiconductor region  137  and the second-conductivity-type crystalline semiconductor region  141  can be formed. 
         [0089]    A surface of the conductive layer  104  may be cleaned with hydrofluoric acid before the formation of the first-conductivity-type crystalline semiconductor region  137 . This step can enhance the adhesiveness between the electrode  103  and the first-conductivity-type crystalline semiconductor region  137 . 
         [0090]    Further, nitrogen or a rare gas such as helium, neon, argon, or xenon may be added to the source gas of the first-conductivity-type crystalline semiconductor region  137  and the source gas of the second-conductivity-type crystalline semiconductor region  141 . In the case where a rare gas or nitrogen is added to the source gas of the first-conductivity-type crystalline semiconductor region  137  and the source gas of the second-conductivity-type crystalline semiconductor region  141 , the density of whiskers can be increased. 
         [0091]    After the formation of the first-conductivity-type crystalline semiconductor region  137  or the formation of the second-conductivity-type crystalline semiconductor region  141 , in the case where introduction of the source gas into the reaction chamber of the LPCVD apparatus is stopped and the temperature is maintained in a vacuum state (i.e., vacuum heating), the density of whiskers included in the first-conductivity-type crystalline semiconductor region  137  can be increased. 
         [0092]    Next, a mask is formed over the second-conductivity-type crystalline semiconductor region  141 , and then the first-conductivity-type crystalline semiconductor region  137  and the second-conductivity-type crystalline semiconductor region  141  are etched with use of the mask. As a result, the first conductive layer  104  is partly exposed, and the first-conductivity-type crystalline semiconductor region  107  and the second-conductivity-type crystalline semiconductor region  111  can be formed as illustrated in  FIG. 5C . 
         [0093]    Then, as illustrated in  FIG. 6A , an insulating layer  147  is formed over the substrate  101 , the first conductive layer  104 , the first-conductivity-type crystalline semiconductor region  107 , and the second-conductivity-type crystalline semiconductor region  111 . The insulating layer  147  can be formed by a CVD method, a sputtering method, an evaporation method, or the like. 
         [0094]    After that, the insulating layer  147  is partly etched so that part of the first conductive layer  104  and part of the second-conductivity-type crystalline semiconductor region  111  are exposed. Next, as in illustrated  FIG. 6B , the auxiliary electrode  115  connected to the first conductive layer  104  is formed in an exposed portion of the conductive layer  104 , and the grid electrode  117  connected to the second-conductivity-type crystalline semiconductor region  111  is formed in an exposed portion of the second-conductivity-type crystalline semiconductor region  111 . The auxiliary electrode  115  and the grid electrode  117  can be formed by a printing method, a coating method, an ink-jet method, or the like. 
         [0095]    Through the above steps, a photoelectric conversion device with high conversion efficiency can be manufactured. 
       Embodiment 2 
       [0096]    In this embodiment, a photoelectric conversion device in which the size of a second conductive layer and the size of a mixed layer are different as compared to those in Embodiment 1 is described with reference to  FIG. 7  and  FIG. 8 . 
         [0097]    The cross section of the dashed-and-dotted line A-B in  FIG. 1  is described with reference to  FIG. 7  and  FIG. 8 . 
         [0098]      FIG. 7  is a schematic view of a photoelectric conversion device including the substrate  101 , the electrode  103 , a first-conductivity-type crystalline semiconductor region  110 , and a second-conductivity-type crystalline semiconductor region  112 . The second conductivity type is opposite to the first conductivity type. The first-conductivity-type crystalline semiconductor region  110  and the second-conductivity-type crystalline semiconductor region  112  function as a photoelectric conversion layer. 
         [0099]    In this embodiment, the electrode  103  includes the first conductive layer  104 , a plurality of second conductive layers  153   a  formed over the first conductive layer  104 , and a plurality of mixed layers  153   b  covering surfaces of the second conductive layers  153   a . Note that although only one pair of the second conductive layer  153   a  and the mixed layer  153   b  is illustrated in  FIG. 7 , plural pairs thereof are formed in the photoelectric conversion device. 
         [0100]    In addition, the first-conductivity-type crystalline semiconductor region  110  includes a crystalline semiconductor region  110   a  which is formed using a crystalline semiconductor including an impurity element imparting the first conductivity type and a group of whiskers including a plurality of whiskers  110   b  which is formed over the crystalline semiconductor region  110   a  and which is formed using a crystalline semiconductor including an impurity element imparting the first conductivity type. 
         [0101]    In this embodiment, a structure in which the plurality of whiskers  110   b  overlaps with one mixed layer  153   b  is described. 
         [0102]    In this embodiment, when the width of the cross section of the second conductive layer  153   a  and the width of the cross section of the mixed layer  153   b  are 2 times or more, preferably 5 times or more as large as that of the whisker  110   b , the plurality of whiskers  110   b  overlaps with the one mixed layer  153   b.    
         [0103]    Note that the plurality of second conductive layers  153   a  and the plurality of mixed layers  153   b  which are formed over the first conductive layer  104  control the position and density of the whiskers  110   b  of the first-conductivity-type crystalline semiconductor region  110 . In other words, the crystalline semiconductor region  110   a  and the whiskers  110   b  can be formed based on the plurality of second conductive layers  153   a  and the plurality of mixed layers  153   b  which are formed over the first conductive layer  104 . This is because the growth directions of the whiskers  110   b  are different due to a vertex or a plane of the mixed layer  153   b . The directions of the axes of the whiskers  110   b  are varied. 
         [0104]    The second conductive layer  153   a  and the mixed layer  153   b  can have the same cross-sectional shapes as those of the second conductive layer  105   a  and the mixed layer  105   b  in Embodiment 1. For example, in the case where the second conductive layer  153   a  and the mixed layer  153   b  are cones or polyhedrons as illustrated in  FIG. 7 , a vertex is formed in the normal direction of the substrate  101 . Thus, a whisker extending in the normal direction based on the vertex is formed, and a whisker extending in a direction perpendicular to the face of the mixed layer  153   b  is also formed. 
         [0105]    As illustrated in  FIG. 8 , when a second conductive layer  155   a  and a mixed layer  155   b  have a columnar-like shape, a polyhedral shape whose top surface is flat, or a truncated conical shape, a whisker extending in the normal direction based on the vertex is formed, and a whisker extending in a direction perpendicular to a plane of the mixed layer  155   b  is also formed. 
         [0106]    Note that the second conductive layers  153   a  and  155   a  can be formed using the same material and with the same thickness as those of the second conductive layer  105   a  in Embodiment 1. In addition, the mixed layers  153   b  and  155   b  can be formed using the same material and with the same thickness as those of the mixed layer  105   b  in Embodiment 1. 
         [0107]    An interface between the first conductive layer  104  and the first-conductivity-type crystalline semiconductor region  110  is flat. Further, the first-conductivity-type crystalline semiconductor region  110  includes the plurality of whiskers  110   b . Thus, a surface of the first conductive layer  104  in contact with the first-conductivity-type crystalline semiconductor region  110  is flat, and a surface of the second-conductivity-type crystalline semiconductor region  112  is uneven. In addition, an interface between the first-conductivity-type crystalline semiconductor region  110  and the second-conductivity-type crystalline semiconductor region  112  is uneven. 
         [0108]    Note that an interface between the crystalline semiconductor region  110   a  and the whisker  110   b  is unclear. Therefore, a plane that is in the same level as the bottom of the deepest valley of valleys formed among whiskers  110   b  and that is parallel to a surface of the first conductive layer  104  and to a surface of the second conductive layer  153   a  or a surface of the mixed layer  153   b  is regarded as the interface between the crystalline semiconductor region  110   a  and the whisker  110   b.    
         [0109]    The whisker  110   b  has a shape similar to that of the whisker  107   b  in Embodiment 1. 
         [0110]    As described in this embodiment, when the width of the second conductive layer which functions as part of the electrode and that of the mixed layer are greater than that of the whisker, whiskers whose axes directions are varied are formed. Thus, the reflectance of light on the surface of the second-conductivity-type crystalline semiconductor region  112  can be reduced. Further, the light incident on the photoelectric conversion layer is absorbed by the photoelectric conversion layer due to a light-trapping effect; therefore, the characteristics of the photoelectric conversion device can be improved. In the case where light is incident on the photoelectric conversion layer from the substrate  101  side, the first conductive layer  104  which is part of the electrode  103  may be formed using a light-transmitting conductive layer and a reflective conductive layer may be formed between the second-conductivity-type crystalline semiconductor region  112  and the insulating layer  113 . Since the second-conductivity-type crystalline semiconductor region  112  is uneven, the light-trapping effect of the photoelectric conversion layer is increased and more light is absorbed by the photoelectric conversion layer, so that the characteristics of the photoelectric conversion device can be improved. 
       Embodiment 3 
       [0111]    In this embodiment, a method for manufacturing a photoelectric conversion layer which has fewer defects than the photoelectric conversion layer in Embodiment 1 is described. 
         [0112]    After one or more of the first-conductivity-type crystalline semiconductor region  107 , the first-conductivity-type crystalline semiconductor region  108 , the first-conductivity-type crystalline semiconductor region  110 , the crystalline semiconductor region  109 , the second-conductivity-type crystalline semiconductor region  111 , and the second-conductivity-type crystalline semiconductor region  112 , which are described in Embodiments 1 and 2, are formed, the temperature of a reaction chamber in an LPCVD apparatus is set at a temperature of higher than or equal to 400° C. and lower than or equal to 450° C., introduction of a source gas into the LPCVD apparatus is stopped, and hydrogen is introduced. Then, in a hydrogen atmosphere, heat treatment at a temperature of higher than or equal to 400° C. and lower than or equal to 450° C. is performed. In this manner, dangling bonds in one or more of the first-conductivity-type crystalline semiconductor region  107 , the first-conductivity-type crystalline semiconductor region  108 , the first-conductivity-type crystalline semiconductor region  110 , the crystalline semiconductor region  109 , the second-conductivity-type crystalline semiconductor region  111 , and the second-conductivity-type crystalline semiconductor region  112  can be terminated with hydrogen. The heat treatment is also referred to as a hydrogenation treatment. As a result of the heat treatment, defects in one or more of the first-conductivity-type crystalline semiconductor region  107 , the first-conductivity-type crystalline semiconductor region  108 , the first-conductivity-type crystalline semiconductor region  110 , the crystalline semiconductor region  109 , the second-conductivity-type crystalline semiconductor region  111 , and the second-conductivity-type crystalline semiconductor region  112  can be reduced, which leads to less recombination of photoexcited carriers in defects and also leads to an increase in conversion efficiency of the photoelectric conversion device. 
         [0113]    Note that this embodiment can be applied to other embodiments. 
       Embodiment 4 
       [0114]    In this embodiment, the structure of a so-called tandem photoelectric conversion device in which a plurality of photoelectric conversion layers is stacked will be described with reference to  FIG. 9 . Although two photoelectric conversion layers are stacked in this embodiment, three or more photoelectric conversion layers may be stacked. In the following description, the photoelectric conversion layer which is closest to the light incident surface may be referred to as a top cell and the photoelectric conversion layer which is farthest from the light incident surface may be referred to as a bottom cell. 
         [0115]      FIG. 9  illustrates a photoelectric conversion device in which the substrate  101 , the electrode  103 , the photoelectric conversion layer  106  which is the bottom cell, a photoelectric conversion layer  120  which is the top cell, and the insulating layer  113  are stacked. Here, the photoelectric conversion layer  106  includes the first-conductivity-type crystalline semiconductor region  107  and the second-conductivity-type crystalline semiconductor region  111  which are described in Embodiment 1. The photoelectric conversion layer  120  includes a third-conductivity-type semiconductor region  121 , an intrinsic semiconductor region  123 , and a fourth-conductivity-type semiconductor region  125 . The band gap of the photoelectric conversion layer  106  is preferably different from that of the photoelectric conversion layer  120 . Use of semiconductors having different band gaps makes it possible to absorb a wide wavelength range of light; thus, a photoelectric conversion efficiency can be improved. 
         [0116]    For example, a semiconductor with a large band gap can be used for the top cell while a semiconductor with a small band gap can be used for the bottom cell, and needless to say, vice versa. Here, as an example, a structure where a crystalline semiconductor (typically, crystalline silicon) is used in the photoelectric conversion layer  106 , which is the bottom cell, and an amorphous semiconductor (typically, amorphous silicon) is used in the photoelectric conversion layer  120 , which is the top cell, is described. 
         [0117]    Note that although a structure where light is incident on the fourth-conductivity-type semiconductor region  125  is described in this embodiment, one embodiment of the disclosed invention is not limited thereto. Light may be incident on the rear surface of the substrate  101  (the lower surface in the drawing). In this case, the substrate  101  and the first conductive layer  104  each have a light-transmitting property. 
         [0118]    The structures of the substrate  101 , the electrode  103 , the photoelectric conversion layer  106 , and the insulating layer  113  are similar to those in the above embodiments and description thereof is omitted here. 
         [0119]    In the photoelectric conversion layer  120 , which is the top cell, a semiconductor layer including a semiconductor material to which an impurity element imparting a conductivity type is added is typically used as the third-conductivity-type semiconductor region  121  and the fourth-conductivity-type semiconductor region  125 . Details of the semiconductor material and the like are similar to those of the first-conductivity-type crystalline semiconductor region  107  in Embodiment 1. In this embodiment, the case where silicon is used as the semiconductor material, the third conductivity type is p-type, and the fourth conductivity type is n-type is described. In addition, the crystallinity of the semiconductor layer is amorphous. It is needless to say that the third conductivity type may be n-type, the fourth conductivity type may be p-type, and the semiconductor layer may be a crystalline semiconductor. 
         [0120]    For the intrinsic semiconductor region  123 , silicon, silicon carbide, germanium, gallium arsenide, indium phosphide, zinc selenide, gallium nitride, silicon germanium, or the like is used. Alternatively, a semiconductor material including an organic material, a metal oxide semiconductor material, or the like can be used. 
         [0121]    In this embodiment, amorphous silicon is used for the intrinsic semiconductor region  123 . The thickness of the intrinsic semiconductor region  123  is greater than or equal to 50 nm and less than or equal to 1000 nm, preferably greater than or equal to 100 nm and less than or equal to 450 nm. It is needless to say that the intrinsic semiconductor region  123  may be formed using a semiconductor material which is not silicon and has a band gap different from that of the crystalline semiconductor region  109  in the bottom cell. Here, the thickness of the intrinsic semiconductor region  123  is preferably smaller than that of the crystalline semiconductor region  109 . 
         [0122]    A plasma CVD method, an LPCVD method, or the like may be employed for forming the third-conductivity-type semiconductor region  121 , the intrinsic semiconductor region  123 , and the fourth-conductivity-type semiconductor region  125 . In the case of a plasma CVD method, the intrinsic semiconductor region  123  can be formed in such a manner that the pressure in a reaction chamber of a plasma CVD apparatus is typically greater than or equal to 10 Pa and less than or equal to 1332 Pa, hydrogen and a deposition gas containing silicon are introduced as a source gas to the reaction chamber, and high-frequency electric power is supplied to an electrode to cause glow discharge. The third-conductivity-type semiconductor region  121  can be formed using the above source gas to which diborane is added. The third-conductivity-type semiconductor region  121  is formed with a thickness of greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm. The fourth-conductivity-type semiconductor region  125  can be formed using the above source gas to which phosphine or arsine is added. The fourth-conductivity-type semiconductor region  125  is formed with a thickness of greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm. 
         [0123]    Alternatively, the third-conductivity-type semiconductor region  121  may be formed by forming an amorphous silicon layer by a plasma CVD method or an LPCVD method without adding an impurity element imparting a conductivity type and then adding boron by a method such as ion injection. The fourth-conductivity-type semiconductor region  125  may be formed by forming an amorphous silicon layer by a plasma CVD method or an LPCVD method without adding an impurity element imparting a conductivity type and then adding phosphorus or arsenic by a method such as ion injection. 
         [0124]    As described above, by using amorphous silicon for the photoelectric conversion layer  120 , light having a wavelength of less than 800 nm can be effectively absorbed and subjected to photoelectric conversion. Further, by using crystalline silicon for the photoelectric conversion layer  106 , light having a longer wavelength (e.g., a wavelength up to approximately 1200 nm) can be absorbed and subjected to photoelectric conversion. Such a structure (a so-called tandem structure) in which photoelectric conversion layers having different band gaps are stacked can significantly increase a photoelectric conversion efficiency. 
         [0125]    Note that although amorphous silicon having a large band gap is used in the top cell and crystalline silicon having a small band gap is used in the bottom cell in this embodiment, one embodiment of the disclosed invention is not limited thereto. The semiconductor materials having different band gaps can be used in appropriate combination to form the top cell and the bottom cell. The structure of the top cell and the structure of the bottom cell can be replaced with each other to form the photoelectric conversion device. Alternatively, a stacked structure in which three or more photoelectric conversion layers are stacked can be employed. 
         [0126]    With the above structure, the conversion efficiency of a photoelectric conversion device can be increased. 
         [0127]    Note that this embodiment can be applied to other embodiments. 
         [0128]    This application is based on Japanese Patent Application serial no. 2010-139997 filed with Japan Patent Office on Jun. 18, 2010, the entire contents of which are hereby incorporated by reference.