Abstract:
A back contact type solar battery which provides a reduced electric power loss, free positioning of a bus bar, and a simple manufacturing process. The solar battery includes: semiconductor substrate; first conductivity type region formed on back surface side located on the opposite side of acceptance surface side of the semiconductor substrate; second conductivity type region formed on the back surface side of the semiconductor substrate; first conductivity type collecting electrode linearly formed on the first conductivity type region; and second conductivity type collecting electrode linearly formed on the second conductivity type region. The first and second conductivity type regions are alternately arranged. Each of the first and second conductivity type collecting electrodes has discontinuous places. The discontinuous places of each conductivity type are substantially aligned on straight line in arrangement direction in which the first and second conductivity type regions are alternately arranged.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to aback contact type solar battery. 
       BACKGROUND ART 
       [0002]    A conventionally main solar battery is manufactured by, for example, diffusing impurities having the opposite conductivity type to that of a single crystal or polycrystal silicon substrate into an acceptance surface of the silicon substrate to form p-n junction, and forming an electrode on each of the acceptance surface of the silicon substrate and a back surface located on the opposite side of the acceptance surface. 
         [0003]    The main silicon solar battery has the acceptance surface having a front electrode (often including a metal electrode referred to as a bus bar and a finger) formed thereon and made of a metal. The front electrode blocks incident sunlight, which disadvantageously loses the output of the solar battery. 
         [0004]    Then, a so-called back contact type solar battery is developed, which is obtained by forming an electrode only on a back surface of a silicon substrate without forming an electrode on an acceptance surface of the silicon substrate. The back contact type solar battery having the acceptance surface having no electrode formed thereon can take 100% of incident sunlight into the solar battery without having a shadow loss caused by an electrode, which can fundamentally achieve high efficiency. 
         [0005]    When the sunlight is incident on the acceptance surface of the back contact type solar battery, careers generated near the acceptance surface of the silicon substrate reach p-n junction formed on the back surface of the back contact type solar battery. The careers are collected by a p-type finger electrode and an n-type finger electrode, and taken out to the outside. 
         [0006]      FIG. 1  shows an electrode structure of a general solar battery. An electrode includes a finger part  102  and a bus bar part  103 . The finger part  102  is a collecting electrode formed for the purpose of efficiently collecting a photoelectric current generated from the solar battery without causing a resistance loss. The bus bar part  103  collects the current through the finger part  102 , and functions as a ground of a tab line.  FIG. 2  shows a current flow when a tab line is applied to an acceptance surface of the general solar battery cell. As shown in  FIG. 2 , electrons generated within a silicon substrate  201  are collected by a finger  202  adjacent to the electrons. Furthermore, a current flows into a bus bar  203  adjacent to the finger  202 , and is taken out as electric power via a tab line  204 . As described above, the bus bar functions to collect the current collected by the finger, and is desirably connected to as many fingers as possible. As shown in  FIG. 3A , a considerable current flows also into a diffusion layer  304  directly below a finger  301  in parallel to a current  303  of a bus bar  302  from the finger  301 . The current passes through the diffusion layer even if the finger  301  is disconnected in a minimal region as shown in  FIG. 3B , which provides a decrease in a resistance loss between a place in which the current is generated and the bus bar. 
         [0007]    When a finger electrode has a decreased cross-sectional area, the finger electrode has an increased serial resistance, which causes a large output loss. Thereby, the cross-sectional area is designed to be increased. That is, the height or width of the electrode are designed to be increased. However, the former requires a plurality of processes and a protracted treatment, and is further limited. The latter can reduce a serial resistance value, but causes a decrease in the acceptance area and a deterioration in surface passivation, as a result of which the output of the solar battery is decreased in many cases. The present techniques reach a ceiling as the method for providing the high power of the solar battery. 
         [0008]    Examples of the method for decreasing the serial resistance include increasing the cross-sectional area of the finger, and shortening a finger length.  FIG. 4  shows a back surface electrode structure of a general back contact cell. The back contact type solar battery has a longer finger length than that of a general solar battery having each surface having an electrode formed thereon. There is sufficient room for an improvement in the back contact type solar battery. 
         [0009]    A back contact type solar battery is disclosed, in which a bus bar can be formed also within a substrate region by fragmenting a conductivity type region and a conductivity type electrode formed thereon in order to shorten a finger length (Patent Literature 1). 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: JP 2009-025147 W 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0011]    However, in the back contact type solar battery of Patent Literature 1, at least two of the fragmented bus bars are installed near both the ends of the substrate in the longitudinal direction of the bus bar, and only one side of the bus bar installed near the end of the substrate is connected to the finger. Therefore, the number of the fingers which can collect a current for one bus bar is decreased although the back contact type solar battery has a constitution in which a number of bus bars can be provided, which cannot sufficiently take advantage of the bus bars. 
         [0012]    In view of the circumstances, it is an object of the present invention to provide a back contact type solar battery which can improve characteristics, has a flexibility for positioning a bus bar, and can be comparatively easily manufactured. 
       Solution to Problem 
       [0013]    A solar battery according to the present invention includes: a semiconductor substrate; a first conductivity type region formed on a back surface side located on the opposite side of an acceptance surface side of the semiconductor substrate; a second conductivity type region formed on the back surface side of the semiconductor substrate; a first conductivity type collecting electrode substantially linearly formed on the first conductivity type region; and a second conductivity type collecting electrode substantially linearly formed on the second conductivity type region, wherein the first and second conductivity type regions are alternately arranged, each of the first and second conductivity type collecting electrodes has discontinuous places, and the discontinuous places of each conductivity type are substantially aligned on a straight line in an arrangement direction in which the first and second conductivity type regions are alternately arranged. 
         [0014]    A portion adjacent to each of the discontinuous places in each of the first and second conductivity type collecting electrodes may be formed as a small bus bar thicker than the other portion in each of the first and second conductivity type collecting electrodes. The small bus bar in each of the first and second conductivity type collecting electrodes may have a line width of 200 to 2000 μm, and the portion other than the small bus bar in each of the first and second conductivity type collecting electrodes may have a line width of 50 to 500 μm. 
         [0015]    The small bus bar may be provided at a position which is not adjacent to a longitudinal end of each of the first and second conductivity type collecting electrodes in the semiconductor substrate. The discontinuous places of one of the first and second conductivity type collecting electrodes may be adjacent to the small bus bars of the other of the first and second conductivity type collecting electrodes in the arrangement direction. 
         [0016]    The number of the discontinuous places is preferably 1 to 4 in a longitudinal direction of each of the first and second conductivity type collecting electrodes. 
       Advantageous Effects of Invention 
       [0017]    The present invention can provide a back contact type solar battery which can improve characteristics. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a schematic view of an electrode structure of a general solar battery cell. 
           [0019]      FIG. 2  is a schematic view of a current flow when a tab line is applied to an acceptance surface of the general solar battery cell. 
           [0020]      FIG. 3A  is a sectional view of a current flow in the general solar battery cell. 
           [0021]      FIG. 3B  is a sectional view of a current flow in a solar battery cell having a locally disconnected electrode. 
           [0022]      FIG. 4  is a schematic view of a back surface electrode structure of a general back contact type solar battery cell. 
           [0023]      FIG. 5  is an example of a diffusion region and a back surface electrode structure in a back contact type solar battery cell of the present invention. 
           [0024]      FIG. 6  is an example of the back surface electrode structure and a tab line position in the back contact type solar battery cell of the present invention. 
           [0025]      FIG. 7  is an example of a diffusion pattern in the back contact type solar battery cell of the present invention. 
           [0026]      FIG. 8  is an example of a back surface electrode pattern in the back contact type solar battery cell of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    Hereinafter, a back contact type solar battery cell in one embodiment of the present invention will be described with reference to the drawings. The size and shape of each of members are schematically shown in a modification or exaggeration form for the convenience of illustration. 
         [0028]    As shown in  FIG. 5 , in a solar battery of the present invention, on a back surface side of a silicon substrate  501 , a first conductivity type diffusion region  502  and a second conductivity type diffusion region  503  having a higher concentration than that of the substrate are alternately and continuously formed substantially linearly at a predetermined distance. A plurality of linear first conductivity type collecting electrodes  504  or second conductivity type collecting electrodes  505  are discontinuously formed on the first conductivity type diffusion region  502  or the second conductivity type diffusion region  503 . The silicon substrate  501  has a second conductivity type. In the present application, a direction where the first conductivity type diffusion region  502  and the second conductivity type diffusion region  503  linearly extend is referred to as a “longitudinal direction”, and a direction where the first conductivity type diffusion region  502  and the second conductivity type diffusion region  503  are alternately arranged is referred to as an “arrangement direction”. 
         [0029]    As shown in  FIG. 5 , electrode discontinuous places  506  are substantially aligned on a straight line in the arrangement direction, and electrode discontinuous places  507  are substantially aligned on a straight line in the arrangement direction. That is, the longitudinal direction of the first conductivity type collecting electrode  504  or second conductivity type collecting electrode  505  is a horizontal direction in  FIG. 5 . The electrode discontinuous places  506  are substantially aligned on a straight line in a vertical direction in  FIG. 5 . The electrode discontinuous places  507  are substantially aligned on a straight line in the vertical direction in  FIG. 5 . 
         [0030]    One to four discontinuous places  506  are formed between both the ends of a semiconductor substrate in the longitudinal direction in the first conductivity type collecting electrode  504 , and one to four discontinuous places  507  are formed between both the ends of the semiconductor substrate in the longitudinal direction in the second conductivity type collecting electrode  505 . 
         [0031]    The line width of a portion  508  corresponding to a bus bar part in the first conductivity type collecting electrode  504  or the second conductivity type collecting electrode  505  may be more thickly formed than that of the other portion. The portion  508  corresponding to the bus bar part is preferably adjacent to the discontinuous place  506  or  507  having the same conductivity type. Hence, for convenience sake, the portion  508  corresponding to the bus bar part is referred to as a small bus bar. 
         [0032]    Second conductivity type small bus bar portions  508  are preferably provided on the substantially same straight line in the arrangement direction in which the discontinuous places  506  are provided side by side. First conductivity type small bus bar portions  508  are preferably provided on the substantially same straight line in the arrangement direction in which the second discontinuous places  507  are provided side by side. 
         [0033]    It is preferable that the small bus bar  508  has a line width  509  of 200 to 2000 μm, and the collecting electrode has a line width  510  of 50 to 500 μm at a place other than the small bus bar  508 . Herein, the small bus bar  508  preferably has a length  511  of 200 to 5000 μm. Each of the electrode discontinuous places  506  and  507  preferably has a length of 300 to 6000 μm. Each of the electrode discontinuous places  506  and  507  is preferably longer by about 100 to 1000 μm than the small bus bar  508 . 
         [0034]    It is preferable that the first conductivity type diffusion region  502  has a width  512  of 500 to 5000 μm, and the second conductivity type diffusion region  503  has a width  513  of 50 to 1000 μm. It is preferable that the width  512  of the first conductivity type diffusion region  502  is greater than the width  513  of the second conductivity type diffusion region  503 . A distance  514  between the first conductivity type collecting electrode  504  and the second conductivity type collecting electrode  505  is preferably 300 to 5000 μm. 
         [0035]    By the cell structure, as shown in  FIG. 6 , during module production, a first conductivity type tab line  601  is installed on a first conductivity type small bus bar  602  and a discontinuous place  603  of a second conductivity type collecting electrode  608 . A second conductivity type tab line  604  is installed on a second conductivity type small bus bar  605  and a discontinuous place  606  of a first conductivity type collecting electrode  607 . 
         [0036]    The material and shape of each of the tab lines  601  and  604  are not particularly limited as long as the tab lines  601  and  604  are conductive. For example, the tab lines  601  and  604  are preferably made of a strip-shaped conductive material such as a foil- or plate-shaped conductive material. When the tab line is strip-shaped, the tab line preferably has a width of about 200 to 5000 μm and a thickness of about 50 to 500 μm. The width of the tab line is preferably shorter by about 100 to 1000 μm than the length of the discontinuous place. Therefore, a sufficient clearance can be secured between the discontinuous place of the different conductivity type collecting electrode and the tab line provided on the discontinuous place. 
         [0037]    The tab line contains various metals and alloys. For example, the tab line contains metals such as Au, Ag, Cu, Pt, Al, Ni, and Ti, or alloys thereof. Among them, Cu is preferably used. Solder plating made of an alloy containing Sn, Pb, and Cu is preferably applied to the tab line. 
         [0038]    Soldering is used in order to electrically bond the small bus bar  602  or  605  and the tab line  601  or  604  installed thereon. Herein, the term “soldering” refers to applying a flux onto the small bus bar  602  or  605 , installing the tab line thereon, and heating the entire cell to join the small bus bar  602  or  605  and the tab line  601  or  604 . 
         [0039]    The small bus bar  602  or  605  is absent near the end of the substrate in the longitudinal direction, and installed inside the substrate. Herein, the first conductivity type collecting electrode  607  which is present on the same straight line in the longitudinal direction is discontinuous. However, at a discontinuous place  606 , a diffusion layer which is a minimal region is present, which provides a decrease in a resistance loss in the discontinuous place  606 . On the other hand, the second conductivity type tab line  604  is electrically separated without being soldered to the discontinuous place  606 . The second conductivity type collecting electrode  608  which is present on the same straight line in the longitudinal direction is discontinuous. However, at a discontinuous place  603 , a diffusion layer which is a minimal region is present, which provides a decrease in a resistance loss in the discontinuous place  603 . On the other hand, the first conductivity type tab line  601  is electrically separated without being soldered to the discontinuous place  603 . Therefore, in the present application, all the small bus bars  602  or  605  can include a finger on each of both sides, and the number of fingers which can effectively collect a current is increased for one small bus bar  602  or  605 . This can reduce a serial resistance, which allows an improvement in the efficiency of the solar battery. 
         [0040]    Since small bus bar positions are substantially linearly aligned, and the first conductivity type small bus bar and the second conductivity type small bus bar are adjacent to each other, the flux is likely to be applied when the tab line is bonded, which facilitates modular manufacture as compared with a general back contact type solar battery. 
         [0041]    Hereinafter, an example of a method for producing a solar battery of the present invention will be described. However, the present invention is not limited to the solar battery produced by the method. 
         [0042]    A group-III element such as boron or gallium is doped into high purity silicon, and slice damage on a surface of an Ascut single crystal {100} p-type silicon substrate having a specific resistance of 0.1 to 5 Ω·cm is etched by using highly-concentrated alkali such as sodium hydroxide or potassium hydrate having a concentration of 5 to 60 mass %, or a mixed acid containing fluoric acid and nitric acid, or the like. The single crystal silicon substrate may be produced by any of a CZ method and a FZ method. 
         [0043]    Subsequently, the surface of the substrate is subjected to minute concavo-convex formation referred to as texture. The texture is a method effective for reducing the reflectance of the solar battery. The texture is easily produced by immersing the substrate into a heated alkali solution (concentration: 1 to 10 mass %, temperature: 60 to 100° C.) such as sodium hydroxide, potassium hydrate, potassium carbonate, sodium carbonate, or sodium bicarbonate for about 10 to 30 minutes. A predetermined amount of 2-propanol is dissolved in the solution for reaction promotion in many cases. 
         [0044]    After the texture formation, the substrate is washed in an acid aqueous solution such as hydrochloric acid, sulfuric acid, nitric acid, fluoric acid, or a mixed solution thereof. From economic and efficient standpoints, the substrate is preferably washed in the hydrochloric acid. In order to raise a cleaning level, the substrate may be washed with a solution obtained by mixing 0.5 to 5% of hydrogen peroxide with a hydrochloric acid solution, and warming the mixed solution to 60 to 90° C. 
         [0045]    An emitter layer and BSF are formed on the back surface side of the silicon substrate. The back contact type solar battery of the present application requires the formation of p-n junction only in a desired region on the back surface. In order to achieve the formation, it is necessary to form a diffusion layer according to patterning or to form a silicon oxide film or a silicon nitride film or the like as a diffusion mask in a region other than a desired diffusion region before diffusion, to prevent the formation of the p-n junction in the region other than the desired diffusion region. 
         [0046]    As shown in  FIG. 7 , when a p-type silicon substrate  701  is used, for example, the desired diffusion region preferably has a pattern in which an n-type diffusion region  702  and a p-type diffusion region  703  having a higher concentration than that of the substrate are alternately formed in a strip shape. It is preferable that the n-type diffusion region  702  has a width  704  of 500 to 5000 μm, and the p-type diffusion region  703  has a width  705  of 50 to 2000 μm. It is preferable that the width  704  of the n-type diffusion region  702  is greater than the width  705  of the p-type diffusion region  703 . A space  706  between the n-type diffusion region  702  and the p-type diffusion region  703  preferably has a width of 50 to 500 μm. A space  707  between the end of the substrate and the end of the diffusion layer nearest to the end of the substrate preferably has a width of 200 to 2000 μm. 
         [0047]    In a method for forming an oxide film, there can be utilized various methods such as deposition according to plasma CVD and a method for applying a polymer containing silicon, and thereafter curing the polymer by heating. The oxide film used as the diffusion mask desirably has a film thickness of 100 to 200 nm. 
         [0048]    In case example shown herein, a p-type diffusion layer is previously formed. For example, if the plasma CVD is used, a pattern-type plate which can prevent the deposition of an oxide film on a desired p-type diffusion region is placed on a substrate, to form an oxide film in a region other than the desired p-type diffusion region (for example, also an entire acceptance surface side). If the polymer containing silicon is used, the polymer containing silicon is applied to the region other than the desired p-type diffusion region (for example, also the entire acceptance surface side) according to screen printing, and cured by heating with a hot plate or the like, to form an oxide film in the region other than the desired p-type diffusion region. A coating agent containing a dopant is applied to a back surface, and then heat-treated at 900 to 1000° C., to form the p-type diffusion region on the back surface. The applying diffusion may be vapor-phase diffusion using BBr 3 . After the heat treatment, a glass component adhering to the silicon substrate is washed by glass etching or the like. At this time, simultaneously, the oxide film formed in the region other than the p-type diffusion region is also removed. Any p-type dopant is used as the dopant, and boron is particularly preferably used. A sheet resistance is 1 to 100 Ω/□, and preferably 3 to 40 Ω/□. 
         [0049]    Next, an n-type diffused layer is formed. An oxide film is formed on a region other than a desired n-type diffusion region (also the entire acceptance surface side) by the same treatment. A coating agent containing a dopant is applied onto the back surface, and then heat-treated at 900 to 1000° C., to form the n-type diffusion region on the back surface. The applying diffusion may be vapor-phase diffusion using POCl 3 . After the heat treatment, a glass component adhering to the silicon substrate is washed by glass etching or the like. At this time, simultaneously, the oxide film formed in the region other than the n-type diffusion region is also removed. Any n-type dopant is used as the dopant, and phosphorus is particularly preferably used. A sheet resistance is 20 to 200 Ω/□, and preferably 40 to 100 Ω/□. 
         [0050]    Subsequently, in order to reduce dangling bond serving as one of recombination sites, an oxide film is formed on the silicon substrate. The silicon substrate having the diffusion layer formed thereon is treated at 800 to 1000° C. under oxygen environment for 0.5 to 2 hours, to form a silicon oxide film serving as a passivation film. The silicon oxide film preferably has a film thickness of 3 to 30 nm. 
         [0051]    Next, an antireflection film is formed on each of the back and front surfaces. A silicon nitride film which is a dielectric film is deposited on each of the back and front surfaces of the substrate by a plasma CVD method. The silicon nitride film preferably has a film thickness of 70 to 100 nm. As a reactant gas, silane (SiH 4 ) and ammonia (NH 3 ) are used in a mixed state in many cases. Nitrogen can also be used in place of NH 3 . In order to adjust process pressure, dilute the reactant gas, and promote the bulk passivation effect of the substrate when polycrystalline silicon is used for the substrate, hydrogen may be mixed with the reactant gas. Thermal CVD and photo CVD or the like may be used in addition to the method according to plasma, as a method for exciting the reactant gas for CVD. Alternative examples of the antireflection film include silicon oxide, a titanium dioxide film, a zinc oxide film, a tin oxide film, a tantalum oxide film, a niobium oxide film, a magnesium fluoride film, and an aluminum oxide film. Examples of the formation method include a coating method and a vacuum evaporation method in addition to the above. From the economic viewpoint, the silicon nitride film is suitably formed by the plasma CVD method. 
         [0052]    Then, an electrode is formed. For example, a paste primarily containing silver is printed in a predetermined pattern on the p-type diffusion layer and the n-type diffused layer on the back surface of the substrate using a screen printing device or the like, followed by drying. 
         [0053]      FIG. 8  shows an example of aback surface electrode pattern of a back contact type solar battery cell of the present invention. As shown in  FIG. 8 , in the back surface electrode pattern, a plurality of lines corresponding to fingers  801  are substantially provided in parallel. One to four finger discontinuous places  802  are present on the same line in the longitudinal direction, and small bus bars  803  are adjacent to the finger discontinuous places  802 . The finger discontinuous places  802  are substantially aligned on a straight line for every line. It is preferable that the finger  801  has a line width  804  of 50 to 500 μm, and the small bus bar  803  has a line width  805  of 200 to 2000 μm and a length  806  of 200 to 5000 μm. A distance  807  between the fingers  801  is preferably 300 to 5000 μm. 
         [0054]    The paste contains a component for increasing adhesive strength between the silicon substrate and the electrode. The component is referred to as glass frit. Since the kind and amount of the glass frit required for the p-type diffusion layer are different from those required for the n-type diffused layer, the paste may be independently printed on a p-type electrode and an n-type electrode. In this case, for example, it is necessary to independently prepare a pattern when the p-type electrode is printed during first printing and a pattern when the n-type electrode is printed during second printing. Since the small bus bar portion nearly unrequires a glass frit component, the paste may be independently printed on the small bus bar portion and a finger portion. 
         [0055]    After these printings, in a firing furnace, firing is performed at 500 to 900° C. for 1 to 30 minutes, and silver powder is passed through the silicon nitride film (fire through), to provide electrical connection between the electrode and silicon. 
         [0056]    As described above, the present invention has been described using the embodiments, but the technical scope of the present invention is not limited to the range described in the embodiments. It is apparent for a person skilled in the art that various changes or improvements can be added to the embodiments. It is apparent from the description of claims that the technical scope of the present invention may include a form to which the changes or the improvements are added. For example, the p-type substrate is used in the example, but an n-type substrate may be used. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           101 ,  201 ,  305 ,  401 ,  501 : silicon substrate 
           102 ,  202 ,  301 ,  402 ,  801 : finger 
           103 ,  203 ,  302 ,  403 : bus bar 
           204 : tab line 
           205 ,  303 : current 
           304 : diffusion layer directly below finger 
           502 : first conductivity type diffusion region 
           503 : second conductivity type diffusion region 
           504 ,  607 : first conductivity type collecting electrode 
           505 ,  608 : second conductivity type collecting electrode 
           506 ,  606 : discontinuous place of first conductivity type collecting electrode 
           507 ,  603 : discontinuous place of second conductivity type collecting electrode 
           508 ,  803 : small bus bar 
           509 ,  805 : line width of small bus bar 
           510 ,  804 : line width of finger 
           511 ,  806 : length of small bus bar 
           512 : width of first conductivity type diffusion region 
           513 : width of second conductivity type diffusion region 
           514 : distance between first and second conductivity type electrodes 
           601 : first conductivity type tab line 
           602 : first conductivity type small bus bar 
           604 : second conductivity type tab line 
           605 : second conductivity type small bus bar 
           701 : p-type silicon substrate 
           702 : n-type diffusion region 
           703 : p-type diffusion region 
           704 : width of n-type diffusion region 
           705 : width of p-type diffusion region 
           706 : space between n-type and p-type diffusion regions 
           707 : space between end of substrate and end of diffusion layer nearest to end of substrate 
           802 : discontinuous place of finger 
           807 : distance between fingers