Abstract:
The problem to be solved by the present invention is to provide a solar battery which reduces obstructing circumstances in improvement of photoelectric conversion efficiency. The solar battery of the present invention comprises a semiconductor substrate of a first conductivity type, a first semiconductor layer of a second conductivity type formed along at a light transmitting surface of the semiconductor substrate and collecting photo-generated carriers based on solar beam of middle and long wavelength, and a second semiconductor layer of a second conductivity type formed at a light incident surface of the semiconductor substrate and collecting photo-generated carriers which cannot reach a first semiconductor layer among photo-generated carriers based on the solar beam of middle and long wavelength as well as collecting photo-generated carriers based on solar beam of short wavelength.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a photoelectric conversion device and a solar battery equipped therewith, more particularly to a solar battery and its manufacturing method using a crystal-based semiconductor such as a single crystal silicon and/or a polycrystalline silicon. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventionally, Japanese Patent Application Laid-Open No. 11-224954 discloses some technologies for providing a solar battery with both high efficiency in taking out carriers and improved characteristics. This solar battery takes out minority carriers among photo-generated carriers generated in a crystal-based semiconductor layer by incidence of light through both sides of the crystal-based semiconductor layer. In other words, this solar battery is a solar battery comprising a first semiconductor layer having an opposite conductivity type at a light incident surface side on the crystal-based semiconductor layer having one conductivity type, and comprising a second semiconductor layer having an opposite conductivity type at a light transmitting surface side on the crystal-based semiconductor layer. 
       SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
       [0003]    However, in order to improve the performance of the solar battery, although it is indispensable to reduce recombination of photo-generated carriers and injected carriers, the solar battery disclosed in Japanese Patent Application Laid-Open No. 11-224954 is not taken such correspondence. By this reason, there was a limit in improving the performance of the solar battery disclosed in Japanese Patent Application Laid-Open No. 11-224954. 
         [0004]    Furthermore, for the solar battery disclosed in Japanese Patent Application Laid-Open No. 11-224954, since the semiconductor substrate constituting it is relatively thick, most of the minority carriers cannot reach the first semiconductor layer or the second semiconductor layers. By this reason, consequently, photo-generated carriers recombine themselves and an amount of current taken out from the first semiconductor layer or the second semiconductor layers was a little. Also, there is the problem that the increase in recombination electric current causes a decrease in open circuit voltage. 
         [0005]    Thereby, the problem to be solved by the present invention is to provide a solar battery which reduces obstructing circumstances in improvement of photoelectric conversion efficiency. 
       Means for Solving the Problem 
       [0006]    In order to solve the problem described above, the solar battery of the present invention comprising: 
         [0000]    a semiconductor substrate of a first conductivity type;
 
a first semiconductor layer of a second conductivity type formed at a light transmitting surface of the semiconductor substrate and collecting photo-generated carriers based on solar beam of middle and long wavelength; and
 
a second semiconductor layer of a second conductivity type formed at a light incident surface of the semiconductor substrate and collecting photo-generated carriers which do not reach a first semiconductor layer among photo-generated carriers based on the solar beam of middle and long wavelength as well as collecting photo-generated carriers based on solar beam of short wavelength;
 
wherein an impurity concentration of the second semiconductor layer is larger to almost one digit or more as compared with an impurity concentration of the first semiconductor layer.
 
         [0007]    According to the present invention, since the impurity concentration of the second semiconductor layer is larger to almost one digit or more as compared with that of the first semiconductor layer, it is possible to differentiate about 60 [mV] or more between the built-in potential of the semiconductor substrate and the first semiconductor layer and the built-in potential of the semiconductor substrate and the second semiconductor layer. As a result, it is possible to take out about 90[%] or more of photo-generated carriers from the first semiconductor layer. 
         [0008]    Further, it is preferable that the first semiconductor layer is formed in a manner in contact with overall to a light transmission surface of the semiconductor substrate except for formation locations of a semiconductor layer of a first conductive type connected to electrodes for taking out electric signals based on photo-generated carriers collected at the first semiconductor layer, so that decrease of open circuit voltage at the light transmitting surface of the semiconductor substrate may be prevented. 
         [0009]    Furthermore, when the first semiconductor layer and the second semiconductor layer are manufactured by a same process, since it is possible to simplify a manufacturing process, there is also advantage of being able to reduce a manufacturing cost. 
         [0010]    Moreover, it is desirable that the semiconductor substrate is covered with the first semiconductor layer and the second semiconductor layer except for formation locations of a semiconductor layer of a first conductive type connected to electrodes for taking out electric signals based on carriers collected at the first semiconductor layer. In a side surface of the semiconductor substrate, considerably, since it becomes possible to collect photo-generated carriers that do not reach the first semiconductor layer, recombination of photo-generated carriers may be further prevented. 
         [0011]    Furthermore, a current value outputted from the first semiconductor layer may be larger than a current value outputted from the second semiconductor layer. 
         [0012]    Furthermore, the manufacturing method of the present invention includes: 
         [0000]    a step of forming a first semiconductor layer of a second conductivity type collecting carriers generated based on solar beam of middle and long wavelength at a light transmitting surface of a first conductivity type;
 
a step of forming a second semiconductor layer of a second conductivity type collecting carriers which do not reach a first semiconductor layer among carriers generated based on the solar beam of middle and long wavelength as well as collecting carriers generated based on solar beam of short wavelength at a light incident surface of the semiconductor substrate; and
 
a step of increasing an impurity concentration of the second semiconductor layer almost one digit or more as compared with an impurity concentration of the first semiconductor layer.
 
         [0013]    In accordance with the present invention, since the second semiconductor layer is formed in the semiconductor substrate, it makes possible to collect photo-generated carriers which do not reach the first semiconductor layer at the second semiconductor layer. Accordingly, it becomes possible to prevent the recombination of photo-generated carriers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic perspective view watched from a light incident surface side of a solar battery cell  100  constituting a solar battery of Embodiment 1 of the present invention. 
           [0015]      FIG. 2  is a schematic perspective view watched from a light transmitting surface side being a back surface side of  FIG. 1 . 
           [0016]      FIG. 3  is a sectional view taken along a broken line A in  FIG. 1  and  FIG. 2 . 
           [0017]      FIG. 4  is a manufacturing process diagram of a solar battery cell  100  shown in  FIG. 3 . 
           [0018]      FIG. 5  is a schematic sectional view of the solar battery cell  100  of Embodiment 2 of the present invention. 
       
    
    
     REFERENCE SIGNS LIST 
       [0000]    
       
           31  antireflection film 
           32  oxide film 
           100  solar battery cell 
           101  single crystal N type semiconductor substrate 
           102 A P type first semiconductor layer 
           102 B P type second semiconductor layer 
           143  N+ type semiconductor layer 
           170  bus bar wiring 
           171  first electrode 
           172  second electrode 
           173  side surface electrode 
           174  main bus bar at a light incident surface 
       
     
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
       Embodiment 1 
       [0032]      FIG. 1  is a schematic perspective view watched from a light incident surface side of the solar battery cell  100  constituting the solar battery of Embodiment 1 of the present invention. As shown in  FIG. 1 , the solar battery cell  100  comprises a single crystal N type semiconductor substrate  101 , a bus bar wiring  170 , a main bus bar at a light incident surface  174 , and a side surface electrode  173  described below. 
         [0033]    Further, in  FIG. 1 , a solar beam  130  and a pitch  175  between the bus bar wiring  170  are noted. As an example, the solar battery cell  100  may have a length in the X and Y direction as about 150 [mm]-160 [mm] and a thickness as about 150 [μm]-200 [μm]. 
         [0034]    In the single crystal N type semiconductor substrate  101 , the specific resistance, for example, is 0.1 [Ω·cm]-1 [Ω·cm], the thickness, for example, is 150 [μm]-200 [μm], and, the impurity concentration is 10 17 [cm −3 ]-6×10 16 [cm −3 ]. However, in place of the single crystal N type semiconductor substrate  101 , a polycrystalline N type semiconductor substrate may be used, and, by making in reverse type to a conductivity type of semiconductor described thereafter, a single crystal or polycrystalline P type semiconductor substrate may be used. In the present embodiment, since it is contributing to prevent the recombination of photo-generated carriers by making relatively thin the thickness of the single crystal N type semiconductor substrate  101 , the photoelectric conversion efficiency may be improved. 
         [0035]    The bus bar wiring  170  is a wiring formed on the light incident surface of the single crystal N type semiconductor substrate  101  along the Y direction. In here, for example, although a total of 12 pieces of the bus bar wiring  170  are schematically shown, actually, about 1000 pieces of wirings are formed. However, the number of the bus bar wiring  170  may be more or less than these numbers. 
         [0036]    As an example, the bus bar wiring  170  is set with an electrode length of 75 [mm], an electrode width of 3 [μm], and a thickness of 3 [μm]. In this condition, an aperture ratio of the solar battery cell  100  is about 99[%]. In addition, in the bus bar wiring  170 , it is preferable to select the material of about 2.5×10 −6  [Ω·cm]-3.0×10 −6  [Ω·cm] as the specific resistance. 
         [0037]    As such a material, for example, aluminum, silver or copper may be used. These materials may be used, for example, in form of paste, and, may be used by mixing not only used in alone. The pitch  175  between the bus bar wiring  170  may be about 300 [μm]. 
         [0038]    The main bus bar at a light incident surface  174  is formed along the X direction and is connected with each of the bus bar wiring  170 . The size of the main bus bar at a light incident surface  174 , for example, may be the one with a width of about 150 [mm], a length of about 20 [μm], and a thickness of about 3 [μm]. 
         [0039]    The side surface electrode  173  is formed on the side surface of the single crystal N type semiconductor substrate  101 , and, is connected to the main bus bar at a light incident surface  174  and a second electrode  172  described below ( FIG. 2 ). The size of the side surface electrode  173 , for example, may be adopted the one with a length in the X direction of about 150 [mm], a length in the Z direction orthogonal to the X and Y directions (not shown) of about 150 [μm]-200 [μm], and a thickness of about 3 [μm]. 
         [0040]    Further, in the case of using aluminum as the material of the side surface electrode  173 , as compared to the bus bar wiring  170 , since the wiring resistance of the main bus bar at a light incident surface  174  is negligible, in the case of the size described above, the resistance value of the main bus bar at a light incident surface  174  is about 250 [mΩ]. 
         [0041]      FIG. 2  is a schematic perspective view watched from a light transmitting surface side being a back surface side of  FIG. 1 . In  FIG. 2 , a first electrode  171  and a second electrode  172  described below are shown in addition to the parts shown in  FIG. 1 . 
         [0042]    The first electrode  171  is an electrode which is connected to a N+ type semiconductor layer  143  described below ( FIG. 3 ), and, takes out electric signals based on photo-generated carriers generated by the solar beam  130  through the N+ type semiconductor layer  143 . The first electrode  171 , for example, is constituted of an axis portion at which the central portion extends in the Y-direction, and a plurality of comb-shaped portion extending integrally in the X direction orthogonal to the axis portion. 
         [0043]    Furthermore, the material of the first electrode  171 , but is not limited to, for example, may be constituted of aluminum, and the thickness may be about 10 [μm]. 
         [0044]    The second electrode  172  is an electrode which is connected to a P type first semiconductor layer  102 A described below ( FIG. 3 ), and, takes out electric signals based on photo-generated carriers generated by the solar beam  130  through the P type first semiconductor layer  102 A. 
         [0045]    The second electrode  172 , for example, is formed with a predetermined inter-electrode gap such as 10 [μm] around the first electrode  171 . The second electrode  172 , but is not limited to, for example, may be constituted of aluminum, and the thickness may be about 10 [μm]. 
         [0046]      FIG. 3  is a sectional view taken along a broken line A in  FIG. 1  and  FIG. 2 . In  FIG. 3 , the P type first semiconductor layer  102 A, a P type second semiconductor layer  102 B, a N+ type semiconductor layer  143 , an antireflection film  31 , and a oxide film  32  described below are shown in addition to the parts shown in  FIG. 1  and  FIG. 2 . 
         [0047]    The P type first semiconductor layer  102 A is formed in a region excluding the N+ type semiconductor layer  143  among the light transmitting surface of the single crystal N type semiconductor substrate  101 . The P type first semiconductor layer  102 A, together with the single crystal N type semiconductor substrate  101 , generates photo-generated carriers based on mainly medium and long wavelength beam among the solar beam  130 . In the P type first semiconductor layer  102 A, the sheet resistance, for example, may be 10 [Ω/sq.]-200 [Ω/sq.], and the impurity concentration, for example, may be 10 20 [cm −3 ]-10 18 [cm −3 ]. 
         [0048]    Although the P type first semiconductor layer  102 A may be formed by a manufacturing process described below, in place of this process, by forming a groove in the single crystal N type semiconductor substrate  101 , or, first, providing a semiconductor substrate of a solar battery cell of approximately equal thickness as the thickness of 200 [μm] of the single crystal N type semiconductor substrate  101 , next, making a micro cell structure by sectioning it suitably in X and Y direction, finally, by forming a wall-shaped groove on a side surface between the micro cell, the P type first semiconductor layer  102 A may be formed together with the second electrode  172  in the groove. In such a case, since the area of the second electrode  172  may be made larger, the photoelectric conversion characteristics of the solar battery cell  100  may be further improved. 
         [0049]    The P type second semiconductor layer  102 B is formed in a manner covering the light incident surface and the side surface of the single crystal N type semiconductor substrate  101 . In addition, for the P type second semiconductor layer  102 B, from the viewpoint of recombination prevention of photo-generated carriers, although it is not essential to form in a manner to cover even the side surface of the single crystal N type semiconductor substrate  101  which is a small area, according to the manufacturing process described below, the side surface is also manufactured to cover integrally. 
         [0050]    The P type second semiconductor layer  102 B, together with the single crystal N type semiconductor substrate  101 , generates photo-generated carriers by the solar beam of mainly short wavelength among the solar beam  130 . The P type second semiconductor layer  102 B may be made the conditions that the sheet resistance, for example, is 100 [Ω/sq.], the impurity concentration, for example, is 10 20 [cm −3 ]-10 18 [cm −3 ], and the thickness is able to generate photo-generated carriers based on the solar beam of where the wavelength λ, for example, is 0.45 [μm] or less, further 0.34 [μm] or less among the solar beam  130 . 
         [0051]    Further, the condition that the wavelength λ is 0.45 [μm] or less is the same meaning as the number of photons to generate photo-generated carriers is about 5[%]-10[%] of the total number of photons among the whole of the solar beam  130 , and the condition that the wavelength λ is 0.34 [μm] or less is the same meaning as the number of photons to generate photo-generated carriers is about 0[%] of the total number of photons among the whole of the solar beam  130 . 
         [0052]    Here in, in the present embodiment, for the surface of the single crystal N type semiconductor substrate  101 , except for the formation portion of the N+ type semiconductor layer  143  corresponding to the first electrode  171 , the P type second semiconductor layer  102 B or the P type first semiconductor layer  102 A is formed over the entire surface. 
         [0053]    Then, the built-in potential of PN junction of the light incident surface side based on the single crystal N type semiconductor substrate  101  and the P type second semiconductor layer  102 B is the same as the built-in potential of PN junction of the light transmitting surface side based on the single crystal N type semiconductor substrate  101  and the P type first semiconductor layer  102 A. 
         [0054]    Each built-in potential may be adjusted the impurity concentration of the P type second semiconductor layer  102 B and the P type first semiconductor layer  102 A against the single crystal N type semiconductor substrate  101 . Specifically, since the built-in potential is proportional to the acceptor concentration of the P type second semiconductor layer  102 B and the P type first semiconductor layer  102 A, these concentrations may be the same. 
         [0055]    The N+ type semiconductor layer  143  is provided on the light transmitting side and is a semiconductor layer for taking out electric signals connected to the first electrode  171 . The impurity concentration of the N+ type semiconductor layer  143  may be, for example, 3×10 20 [cm −3 ]-3×10 18 [cm −3 ]. In addition, in  FIG. 3 , although the thing of stripe-shape is shown as the shapes of the P type first semiconductor layer  102 A and the N+ type semiconductor layer  143 , instead of this, by applying a dot-shape for the one of them or a grid-shape, the area ratio of the N+ type semiconductor layer  143  against the P type first semiconductor layer  102 A, for example, may be decreased to about 10[%]. 
         [0056]    The antireflection film  31 , at the light incident surface side of the P type second semiconductor layer  102 B, is formed between the bus bar wiring  170 . The antireflection film  31 , but is not limited to, may be used such as nitride film (SiN). In addition, here in, although the shape of the antireflection film  31  is simplified, actually, for example, it has a texture structure of an inverted pyramid style. 
         [0057]    The oxide film  32 , as known, is a passivation film provided on the light transmitting surface side in order to suppress the recombination current on the light transmitting surface. 
         [0058]      FIG. 4  is a manufacturing process diagram of a solar battery cell  100  shown in  FIG. 3 . First, in order to remove the damage of the single crystal N type semiconductor substrate  101 , both surfaces of the single crystal N type semiconductor substrate  101  are etched about 10 [μm], for example, using hydrofluoric acid. Then, anisotropic etching is performed by such as immersion in alkaline solution (for example, KOH solution) to at least the light incident surface of the single crystal N type semiconductor substrate  101 . Thus, at least at the light incident surface of the single crystal N type semiconductor substrate  101 , the textured structure of an inverted pyramid style (not shown) is formed under the condition, for example, that the base is about 30 [μm] and the height is about 20 [μm] (step S 1 ). 
         [0059]    After that, in order to perform a phosphorus injection on the back surface of the single crystal N type semiconductor substrate  101  by a subsequent process, a phosphorus glass layer  231  is formed by coating a phosphorus glass, for example, to the back surface of the single crystal N type semiconductor substrate  101 . Then, the other portion is removed by photolithography in a manner to leave the necessary portions of the phosphorus glass (step S 2 ) 
         [0060]    And then, the single crystal N type semiconductor substrate  101  is performed a heat treatment at a temperature of about 900° C. under boron atmosphere, for example. The heat treatment time may be the condition that the deposition amount of boron into the single crystal N type semiconductor substrate  101  is about 10 20 [cm −3 ], for example. As a result, boron diffuses into the single crystal N type semiconductor substrate  101  in a manner that the sheet resistance is 100 [Ω/sq.] in the case that the diffusion depth is about 0.1 [μm]. Consequently, the P type first semiconductor layer  102 A and the region of the P-type semiconductor layer  102  to be the P type second semiconductor layer  102 B are formed in the single crystal N type semiconductor substrate  101 , and, the surface of the single crystal N type semiconductor substrate  101  is entirely covered with a boron glass layer  232 . In addition, in  FIG. 3 , for the convenience of description, although it is not shown that the boron glass layer  232  on the surface of the phosphorus glass layer  231  forms a laminated state, actually, the boron glass layer  232  slightly forms a laminated state on the surface of the phosphorus glass layer  231 . Further, by this heat treatment, the phosphorus in the phosphorus glass layer  231  coated in step S 3  diffuses into the single crystal N type semiconductor substrate  101 , and, the N+ type semiconductor layer  143  is formed in the single crystal N type semiconductor substrate  101  (step S 3 ). 
         [0061]    In the next, the single crystal N type semiconductor substrate  101 , for example, under oxidizing atmosphere, is performed a heat treatment under the temperature of about 950[° C.]. The heat treatment time may be a sufficient time for which the boron glass layer  232  is replaced with the oxide film  32  (step S 4 ). 
         [0062]    And then, after removing the oxide film  32  on the light incident surface side of the single crystal N type semiconductor substrate  101 , or, without removing the oxide film  32 , the antireflection film  31  is formed, for example, by low temperature CVD method (step S 5 ). 
         [0063]    And then, in order to form the bus bar wiring  170  on the light incident surface side of the single crystal N type semiconductor substrate  101 , and to form the first electrode  171  and the second electrode  172  on the light transmitting surface side of the single crystal N type semiconductor substrate  101  respectively, the opening  251  is formed at the corresponding position of the antireflection film  31  on the light incident surface side of the single crystal N type semiconductor substrate  101 , and at the corresponding position of the oxide film  32  and the phosphorus glass layer  231  on the light transmitting surface side respectively (step S 6 ). 
         [0064]    After that, for example, a liquid phase aluminum is deposited on the surface of the antireflection film  31  on the light incident surface side of the single crystal N type semiconductor substrate  101 , and on the surfaces of the oxide film  32  and the phosphorus glass layer  231  of the light transmitting surface side, then, in order to diffuse the aluminum also in each of the openings  251 , for example, a heat treatment is performed at a temperature of about 800[° C.]. Thereafter, using a sputtering or vapor deposition method, aluminum is formed with a thickness of about 3 [μm] on the light incident surface, and with a thickness of about 10 [μm] on the light transmitting surface, then, an exposure is performed using such as a photolithographic method. For this exposure, a double-sided exposure of the light incident surface and the light transmitting surface is preferable. Thereafter, the aluminum excluding the portion serving as the bus bar wiring  170 , the first electrode  171  and the second electrode  172  is removed by chemical etching such as wet etching or dry etching using necessary chemicals. However, the bus bar wiring  170 , the first electrode  171  and the second electrode  172  may be formed using a paste method instead of the method described above (step S 7 ). 
         [0065]    And then, a plurality of intermediate products of the solar battery cell  100  manufactured in this way are prepared and they are superimposed. And then, the side surface electrode  173  is formed by such as spraying aluminum towards their side surfaces. 
         [0066]    Further, the intermediate products of the solar battery cell  100  are superimposed with shifting slightly so that the formation surface of the side surface electrode  173  becomes a step shape. Further, by making them tilt rotation, not only the side surface electrode  173 , the main bus bar at a light incident surface  174  may also be formed integrally with this. Furthermore, it enables to connect the main bus bar at a light incident surface  174  and each of the bus bar wiring  170 , and also enables to connect the side surface electrode  173  and the second electrode  172 . 
         [0067]    Indeed, in the formation of the side surface electrode  173 , as known, a metal paste such as an aluminum paste may be adopted, a metal vapor deposition (including sputtering) may be adopted, and a plating method may be adopted. Through the above steps, a plurality of the solar battery cells shown in  FIG. 1  is completed. 
         [0068]    And then, a behavior of the solar battery cell  100  of the present embodiment is described. When the solar beam  130  enters the solar battery cell  100 , the solar beam having the ultraviolet beam region or shorter wavelength which the wavelength λ is 0.45 [μm] or less among the solar beam  130 , that is, when the solar beam having short wavelength reaches the PN junction of the light incident surface of the single crystal N type semiconductor substrate  101  and the P type second semiconductor layer  102 B, the photo-generated carriers based on the solar beam are generated. 
         [0069]    Holes generated by the above drift toward a depletion layer formed in the vicinity of the PN junction of the light incident surface of the single crystal N type semiconductor substrate  101  and the P type second semiconductor layer  102 B, and when they reach the depletion layer, they may be taken out as electric signals from the bus bar wiring  170  and the main bus bar at a light incident surface  174  connected thereto. 
         [0070]    On the other hand, the solar beam having the visible beam region or longer wavelength which the wavelength λ is greater than 0.45 [μm] among the solar beam  130  entered to the solar battery cell  100 , that is, when the solar beam having middle and long wavelength reaches the PN junction of the light transmitting surface of the single crystal N type semiconductor substrate  101  and the P type first semiconductor layer  102 A, the photo-generated carriers based on the solar beam are generated. 
         [0071]    Holes generated by the above, mainly, drift toward a depletion layer formed in the vicinity of the PN junction of the light transmitting surface of the single crystal N type semiconductor substrate  101  and the P type first semiconductor layer  102 A, and when they reach the depletion layer, they may be taken out as electric signals from the second electrode  172 . 
         [0072]    In other words, since the holes generated by the solar beam  130  entering the solar battery cell  100  is only necessary to reach the one of depletion layers formed in the vicinity of each of the PN junction described above, the diffusion length of the holes may be shorter. For this reason, in the present embodiment, as described above, the thickness of the single crystal N type semiconductor substrate  101  may be relatively thin. Then, since the recombination of photo-generated carriers may be reduced by diffusion length of holes is short, the photoelectric conversion efficiency of the solar battery cell  100  is improved. 
         [0073]    Furthermore, since the fact that it is possible to reduce the recombination of photo-generated carriers means to be able to take out the electric signals based on the photo-generated carriers prior to the recombination from the main bus bar at a light incident surface  174 , and, since the electrical signals may be added to the output of the whole of the solar battery cell  100 , the photoelectric conversion efficiency of the whole of the solar battery cell  100  is improved. 
         [0074]    In the next, briefly, the photoelectric conversion efficiency of the solar battery cell  100  of the present embodiment tries to be simulated from a viewpoint of the voltage drop (IR drop). First, assume that the current according to the PN junction of the light incident surface side of the single crystal N type semiconductor substrate  101  and the P type second semiconductor layer  102 B is 200 mA, and the current according to the PN junction of the light transmitting surface side of the single crystal N type semiconductor substrate  101  and the P type first semiconductor layer  102 A is 12 A. In this case, the IR drop in the single crystal N type semiconductor substrate  101  is about 66 [mV] (≈5.4 [mΩ]×12.2[A]). 
         [0075]    And then, since the resistance value of the P type second semiconductor layer  102 B is about 37 [mΩ] in the case of conditions described above, the IR drop in the P type second semiconductor layer  102 B becomes 7.4 [mV] 37.0 [mΩ]×200 [mA]). Similarly, since the resistance value of the bus bar wiring  170  is about 250 [mΩ] in the case of conditions described above, the IR drop in the main bus bar at a light incident surface  174  becomes 50 [mV] (=250 [mΩ]×200 [mA]). 
         [0076]    Then, since the resistance value of the side surface electrode  173  is small, and since the voltage drop in these is negligible small, the sum of the voltage drop which the current based on the short wavelength beam among the solar beam  130  entered on the solar battery cell  100  undergoes during passing through the PN junction of the light incident surface side of the single crystal N type semiconductor substrate  101  and the P type second semiconductor layer  102 B, the bus bar wiring  170 , the main bus bar at a light incident surface  174 , and the side surface electrode  173  is about 126 [mV] (≈66 [mV]+57.4 [mV]). 
         [0077]    Similarly, when looking at the voltage drop of the light transmitting surface side, the IR drop in the P type first semiconductor layer  102 A is about 65 [mV] (≈5.4 [m Q]×12[A]). Therefore, the sum of the voltage drop which the current based on the medium and long wavelength beam among the solar beam  130  entered on the solar battery cell  100  undergoes during passing through the PN junction of the light transmitting surface side of the single crystal N type semiconductor substrate  101  and the P type first semiconductor layer  102 A, and the second electrode  172  is about 131 [mV] 66 [mV]+65 [mV]). 
         [0078]    In this case, when both open circuit voltages in each PN junction of the light incident surface side and the light transmitting surface side are assumed as 750 [mV], the voltages in both PN junctions in the behavior state of the solar battery cell  100  are 624 [mV] (=750 [mV] 126 [mV]) and 619 [mV] (=750 [mV] 131 [mV]) respectively. 
         [0079]    The powers in both PN junction at this time are 0.12[W] 624 [mV]×200 [mA]) and 7.43[W] (≈619 [mV]×12[A]) respectively, these total becomes 7.55[W] (=0.12[W]+7.43[W]). In the case that the conversion of the power per square meter is performed, since this is 378[W], the photoelectric conversion efficiency is about 37.8[%]. 
         [0080]    According to the above simulation, since it becomes possible to collect electrical signals based on the solar beam of medium and long wavelengths by forming a P type first semiconductor layer  102 A, and, since it becomes possible to collect photo-generated carriers which do not reach the P type first semiconductor layer  102 A by forming the P type second semiconductor layer  102 B, the solar battery cells  100  of the present embodiment may prevent the recombination of photo-generated carriers. 
         [0081]    Further, with regard to the bus bar wiring  170 , in the case that the width, the length, the thickness are changed to about 10 [μm], about 10 [μm], about 75 [mm] respectively with regard to the size, and in the case that the aperture ratio of the solar battery cell  100  is changed to about 90[%] by changing the number to 3,000 pieces, and in the case that the other conditions described above are kept the same, the photoelectric conversion efficiency will be improved to about 46.2[%]. 
       Embodiment 2 
       [0082]      FIG. 5  is a schematic sectional view of the solar battery cell  100  of Embodiment 2 of the present invention and corresponds to  FIG. 3 . The solar battery cell  100  shown in  FIG. 5  is of the type that the bus bar wiring  170 , the main bus bar at a light incident surface  174  and the side surface electrode  173  shown in  FIG. 3  are not provided. In  FIG. 5 , the portions similar to the portions shown in  FIG. 3  are denoted by the same reference numerals. 
         [0083]    Since the solar battery cell  100  shown in  FIG. 5  is not provided with such as the bus bar wiring  170 , the sheet resistance value of the P type second semiconductor layer  102 B defines a upper limit value of electrical signals generated in the PN junction of the light incident surface side of the P type second semiconductor layer  102 B and the single crystal N type semiconductor substrate  101 . 
         [0084]    Although the upper limit value is preferable as large value as possible, since the P type second semiconductor layer  102 B is composed of a semiconductor, when the sheet resistance of the P type second semiconductor layer  102 B is set to 1 [Ω/sq.] or above, the electrical signals generated in the PN junction of the light incident surface side of the P type second semiconductor layer  102 B and the single crystal N type semiconductor substrate  101  are always smaller than the electrical signals generated in the PN junction of the light transmitting surface side of the P type first semiconductor layer  102 A and the single crystal N type semiconductor substrate  101  (including zero). 
         [0085]    Then, since there is an upper limit to an electrical signal generated in PN junction of the light incident surface side of the P-type second semiconductor layer  102 B and the single crystal N type semiconductor substrate  101 , more electrical signals are obtained at the PN junction of the light transmitting surface side of the P-type first semiconductor layer  102 A and the single-crystal N type semiconductor substrate  101 , specifically, in the case of solar battery cells manufactured by above and below conditions, 99.9[%] or above of the total generated current may be obtained from the PN junction of the light transmitting surface side of the P-type first semiconductor layer  102 A and the single-crystal N type semiconductor substrate  101 . 
         [0086]    In the present embodiment, the P type second semiconductor layer  102 B, for example, may be formed by an ion implantation method. For the P-type second semiconductor layer  102 B formed on the light incident surface of the single crystal N type semiconductor substrate  101 , boron is selected as impurities, and the diffusion depth may be the same depth as Embodiment 1, for example, about 0.1 [μm]. 
         [0087]    Furthermore, the P-type second semiconductor layer  102 B and the P-type first semiconductor layer  102 A covering the side surfaces of the single crystal N type semiconductor substrate  101 , for example, may be formed by a diffusion method in the liquid phase. For these, aluminum or a multilayer structure of aluminum and boron may be selected as impurities, and each diffusion depth may be set to 0.54 [μm], for example. 
         [0088]    Indeed, as long as the conditions that may collect photo-generated carriers based on photons of majority of the solar light spectrum generated by the PN junction of the light transmitting surface side of the P-type first semiconductor layer  102 A and the single-crystal N type semiconductor substrate  101 , both manufacturing methods of the P-type second semiconductor layer  102 B and the P-type first semiconductor layer  102 A, and, each impurity concentration and each diffusion depth are not limited to those de scribed above. 
         [0089]    Therefore, for example, in place of the ion implantation method, the correspondences such as adopting the doping from the impurity source of solid, liquid or gas, and, selecting each diffusion depth, for example, as 0.2 [μm] as well as selecting only boron as impurities with respect to both of the P-type second semiconductor layer  102 B and the P-type first semiconductor layer  102 A are also possible. 
         [0090]    Further, similarly to Embodiment 1, although it is not essential to form the P-type second semiconductor layer  102 B for covering the side surfaces of the single crystal N type semiconductor substrate  101  among the P-type second semiconductor layer  102 B, there is also advantage of forming it. 
         [0091]    Specifically, different from Embodiment 1, for the solar battery cell  100  of the present embodiment, since the bus bar wiring  170 , the main bus bar at a light incident surface  174  and the side surface electrode  173  are not provided, the current based on the carrier reached at the P-type second semiconductor layer  102 B must be taken out to the outside of the solar battery cell  100 . In this regard, separately, although measures such as performing a positive power supply connection to the P-type second semiconductor layer  102 B may be considered, when the P-type second semiconductor layer  102 B is provided, since this is to function as a path flowing through the P-type second semiconductor layer  102 B, there is an advantage that the positive power supply connection becomes unnecessary. 
         [0092]    Moreover, in the present embodiment, the built-in potential of the PN junction of the light incident surface side of the single crystal N type semiconductor substrate  101  and the P-type second semiconductor layer  102 B is to be higher than the built-in potential of the PN junction of the light transmitting surface side of the single crystal N type semiconductor substrate  101  and the P-type first semiconductor layer  102 A. Alternatively, toward the joining direction from the central portion of the single crystal N type semiconductor substrate  101 , the concentration gradient such as to increase the ion concentration of the single crystal N type semiconductor substrate  101  may be provided. For this purpose, against the manufacturing method described in Embodiment 1, prior to the treatment under boron atmosphere, by deposition of phosphorus or antimony, N layer with a high concentration (for example, 10 17 [cm −3 ]-10 18 [cm −3 ]) than the single crystal N type semiconductor substrate  101  may be formed on the inside of the boron layer of the single crystal N type semiconductor substrate  101 . However, the correspondence of providing a concentration gradient may be employed at the time that both of the built-in potentials are the same in the solar battery cell  100  of Embodiment 1. 
         [0093]    For example, when the acceptor concentration of the P-type second semiconductor layer  102 B is increased about two digits as compared to the acceptor concentration of the P-type first semiconductor layer  102 A, the difference of about 120 [mV] occurs between those built-in potentials. Specifically, the acceptor concentration of the P-type second semiconductor layer  102 B, the acceptor concentration of the P-type first semiconductor layer  102 A, and the donor concentration of the single crystal N type semiconductor substrate  101 , for example, may be set to 10 20 [cm −3 ], 10 18 [cm −3 ], and 10 16 [cm −3 ] respectively. With such a setting, electrons of minority carriers generated in the single crystal N type semiconductor substrate  101  become to flow beyond the barrier from the PN junction of the light transmitting surface of the low barrier. In addition, the impurity concentration in the present specification refers to the average concentration in the semiconductor layer in which impurities are contained. 
         [0094]    Additionally, when the acceptor concentration of the P-type second semiconductor layer  102 B is smaller than the acceptor concentration of the P-type first semiconductor layer  102 A, on the contrary, electrons of minority carriers generated in the single crystal N type semiconductor substrate  101  become to flow beyond the barrier from the PN junction of the light incident surface of the low barrier, when both acceptor concentrations are equal, electrons move toward the PN junction corresponding to wavelength of the light spectrum. 
         [0095]    Although the solar battery cell  100  of the present embodiment is inexpensive manufacturing cost by a simple structure as compared with that of Embodiment 1, the open circuit voltage of the PN junction of the light transmitting surface may be the same value as that of Embodiment 1. As a result, the photoelectric conversion efficiency of the solar battery cell  100  of the present embodiment comes to about 37.8[%]. 
       Embodiment 3 
       [0096]    In the present embodiment, a modified example against the Embodiments 1 and 2 described above is described based on  FIG. 4 . 
         [0097]    In the single crystal N-type semiconductor substrate  101  shown in step S 1 , the specific resistance, for example, is set to 1 [Ω·cm]-10 [Ω·cm], and, the impurity concentration is set to 5×10 15 [cm −3 ]-5×10 14 [cm −3 ]. 
         [0098]    In step S 2 , first, the oxide film, for example, is formed by a heat treatment method on the entire surface of the single crystal N type semiconductor substrate  101 . After that, only the oxide film on the back surface portion of the single crystal N type semiconductor substrate  101  is removed. And, for example, phosphorus glass is coated first on the back surface of the single crystal N type semiconductor substrate  101 , and the phosphorus glass layer  231  is formed. In addition, since boron is also injected in addition to the purpose of injecting phosphorus into the back surface of the single crystal N type semiconductor substrate  101 , for example, the boron glass layer is formed by superposing the phosphorus glass layer  231  by a coating method. 
         [0099]    The heat treatment time performed in step S 3  may be in the conditions where the impurity concentration of boron into the single crystal N-type semiconductor substrate  101 , for example, is about 10 18 [cm −3 ], and the thickness is about 0.5 [μm]. In this case, boron diffuses in a manner that the sheet resistance is 100 [Ω/sq.]. As a result, the region to be the P-type first semiconductor layer  102 A is formed in the single crystal N type semiconductor substrate  101 . In this heat treatment, at the same time, when the deposition amount of phosphorus from the phosphorus glass layer  231 , for example, is 3×10 19 [cm −3 ]-10 18 [cm −3 ], the N+ type semiconductor layer  143  having substantially the same diffusion depth (thickness) as the P-type first semiconductor layer  102 A is formed. 
         [0100]    Further, regard to the formation of the P-type second semiconductor layer  102 B, besides the illustrative example described above, for example, a gas phase diffusion method, a liquid phase diffusion method, an ion implantation method, and a coating diffusion method from boron bromide BBr3 may be used. In addition, in the manufacturing process according to the present modified example, the boron glass layer  232  is not formed in the implementation stage of step S 3 . 
         [0101]    In step S 4 , the oxide films on the light incident surface and the side surface of the single crystal N type semiconductor substrate  101  are removed among the oxide film formed in step S 2 . Then, for example, by vaporizing boron bromide and under oxygen atmosphere, the single crystal N type semiconductor substrate  101  is performed a heat treatment at a temperature of about 950[° C.]. Heat treatment time is set so that the P-type second semiconductor layer is formed with a thickness of about 0.1 [μm] and the impurity concentration is formed with 10 19 [cm −3 ]. In this time, the boron glass layer  232  is formed on the light incident surface and the side surface of the single crystal N type semiconductor substrate  101 . 
         [0102]    In step S 5 , after removing the boron glass layer  232  of the light incident surface side of the single crystal N type semiconductor substrate  101 , or, without removing the boron glass layer  232 , the antireflection film  31  is formed by low temperature CVD method, for example. 
         [0103]    In Step S 6 , at the corresponding position of the antireflection film  31  of the light incident surface side of the single crystal N type semiconductor substrate  101 , and at the corresponding position in the case that the phosphorus glass layer  231  and the boron glass layer  232  of the light transmitting surface side were not removed, the openings  251  are formed respectively. 
         [0104]    In Step S 7 , for example, after depositing aluminum on the surface of the antireflection film  31  of the light incident surface side of the single crystal N type semiconductor substrate  101 , and on the surfaces of the phosphorus glass layer  231  and boron glass layer of the light transmitting surface side, in order to diffuse aluminum to each of the openings  251 , for example, the heat treatment is performed at a temperature of about 400[° C.]. After that, after film formation of aluminum by a sputtering or vapor deposition method with the thickness of about 3 [μm] on the light incident surface, and with the thickness of about 10 [μm] on the light transmitting surface, the exposure is performed using such as a photolithographic method. For this exposure, the double-sided exposure of the light incident surface and the light transmitting surface is preferable. After that, the aluminum except the portion to be the bus bar wiring  170 , the first electrode  171  and the second electrode  172  is removed by chemical etching such as wet etching or dry etching using necessary chemicals. 
         [0105]    When the solar battery cell  100  is manufactured by the conditions described above, the built-in potential of the PN junction of the light incident surface side of the single crystal N type semiconductor substrate  101  and the P-type second semiconductor layer  102 B may be as high as about 60 [mV] than the built-in potential of the PN junction of the light transmitting surface side of the single crystal N type semiconductor substrate  101  and the P-type first semiconductor layer  102 A. 
         [0106]    This is the same meaning that the impurity concentration of the P-type second semiconductor layer  102 B is made larger to almost one digit or more as compared with the impurity concentration of the P-type first semiconductor layer  102 A, specifically, the impurity concentration of the P-type second semiconductor layer  102 B, for example, is 10 20 [cm −3 ]-10 19 [cm −3 ], the impurity concentration of the P-type first semiconductor layer  102 A, for example, is 10 19 [cm −3 ]-10 18 [cm −3 ], and the impurity concentration of the single crystal N-type semiconductor substrate  101 , for example, is 5×10 15 [cm −3 ]-5×10 14 [cm −3 ]. For the P-type first semiconductor layer  102 A, the sheet resistance, for example, is 20 [Ω/sq.]-200 [Ω/sq.]. The thickness, for example, is 0.5 [μm]. 
         [0107]    In the case of such setting, 90[%] of generated carriers in the single crystal N type semiconductor substrate  101  flows beyond the barrier from the PN junction of the light transmitting surface of the low barrier. In such a case, when calculating the photoelectric conversion efficiency of the solar battery cell  100  in the manner described in Embodiment 1, it is improved to 44.9[%]. 
         [0108]    Furthermore, for the condition of each layer described in Embodiment 2, in the case that the impurity concentration of the P-type second semiconductor layer  102 B, for example, is 10 20 [cm −3 ]-10 19 [cm −3 ], the impurity concentration of the P-type first semiconductor layer  102 A, for example, is 10 18 [cm −3 ]-10 17 [cm −3 ], and the impurity concentration of the single crystal N type semiconductor substrate  101 , for example, is 5×10 15 [cm −3 ]-5×10 14 [cm −3 ], about 99[%] of generated carriers in the single crystal N type semiconductor substrate  101  flows beyond the barrier from the PN junction of the light transmitting surface of the low barrier. 
         [0109]    In the case of solar battery cells  100  having such conditions, almost 99[%] of the total generated current may be obtained from the PN junction of the light transmitting surface side of the P-type first semiconductor layer  102 A and the single-crystal N type semiconductor substrate  101 . In addition, the photoelectric conversion efficiency of the solar battery cell  100  in this condition is approximately 36.7[%].