Abstract:
A solar cell is discussed. The solar cell includes a silicon substrate; a front passivation layer positioned on a front surface of the silicon substrate; an n-doped layer positioned on the front surface of the silicon substrate; an anti-reflection layer positioned on the n-doped layer; a p-doped region positioned on a rear surface of the silicon substrate; an n-doped region positioned on the rear surface of the silicon substrate and spaced apart from the p-doped region; a rear passivation layer positioned on the rear surface of the silicon substrate, the rear passivation layer including: a first portion positioned between the p-doped region and the silicon substrate; a second portion positioned between the n-doped region and the silicon substrate, the second portion being space apart from the first potion; and a third portion disposed between the first portion and the second portion; a first electrode directly contacted to the p-doped region; and a second electrode directly contacted to the n-doped region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of co-pending U.S. patent application Ser. No. 15/276,884 filed on Sep. 27, 2016, which is a Continuation of U.S. patent application Ser. No. 14/720,527 filed on May 22, 2015 (now U.S. Pat. No. 9,508,875 issued on Nov. 29, 2016), which a Continuation of U.S. patent application Ser. No. 12/876,847 filed on Sep. 7, 2010 (now U.S. Pat. No. 9,064,999 issued on Jun. 23, 2015), which claims the benefit under 35 U.S.C. §119(a) to Korean Patent Application Nos. 10-2010-0075028 filed on Aug. 3, 2010, 10-2010-0043961 filed on May 11, 2010, and 10-2009-0084046 filed on Sep. 7, 2009, all of which are hereby expressly incorporated by reference into the present application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    Embodiments of the invention relate to a solar cell and a method for manufacturing the same. 
         [0004]    Description of the Related Art 
         [0005]    Recently, as existing energy sources such as petroleum and coal are expected to be depleted, interests in alternative energy sources for replacing the existing energy sources are increasing. Among the alternative energy sources, solar cells for generating electric energy from solar energy have been particularly spotlighted. 
         [0006]    A solar cell generally includes semiconductor parts that have different conductive types, such as a p-type and an n-type, and form a p-n junction, and electrodes respectively connected to the semiconductor parts of the different conductive types. 
         [0007]    When light is incident on the solar cell, a plurality of electron-hole pairs are generated in the semiconductor parts. The electron-hole pairs are separated into electrons and holes by the photovoltaic effect. Thus, the separated electrons move to the n-type semiconductor and the separated holes move to the p-type semiconductor, and then the electrons and holes are collected by the electrodes electrically connected to the n-type semiconductor and the p-type semiconductor, respectively. The electrodes are connected to each other using electric wires to thereby obtain electric power. 
       SUMMARY OF THE INVENTION 
       [0008]    In one aspect there is a solar cell including a substrate that contains first impurities of a first conductive type and is formed of a crystalline semiconductor, a first field region that is positioned on an incident surface of the substrate, and contains second impurities of a second conductive type, an emitter region that contains third impurities of a third conductive type, is formed of a non-crystalline semiconductor, and is positioned on a non-incident surface of the substrate opposite the incident surface of the substrate, a first electrode electrically connected to the emitter region, and a second electrode electrically connected to the substrate. 
         [0009]    The solar cell may further include a first passivation layer positioned between the substrate and the first field region. 
         [0010]    A concentration of the second impurities contained in the first field region may be higher than a concentration of the first impurities contained in the substrate. The concentration of the second impurities contained in the first field region may be approximately 1×10 16  atoms/cm 3  to 1×10 21  atoms/cm 3 . 
         [0011]    The first field region may have a thickness of 1 nm to 20 nm, and the first passivation layer may have a thickness of approximately 2 nm to 20 nm. 
         [0012]    A concentration of the second impurities contained in the first field region may vary over a thickness of the first field region. The concentration of the second impurities contained in the first field region may increase from a contact surface between the first passivation layer and the first field region to an upper surface of the first field region. A minimum concentration of the second impurities of the first field region may be approximately 1×10 10  atoms/cm 3 , and a maximum concentration of the second impurities of the first field region may be approximately 1×10 21  atoms/cm 3 . 
         [0013]    The first field region may have a thickness of 3 nm to 30 nm, and the first passivation layer may have a thickness of approximately 1 nm to 10 nm. 
         [0014]    The first field region may be formed of amorphous silicon, amorphous silicon oxide, or amorphous silicon carbide. 
         [0015]    The first passivation layer may be formed of intrinsic amorphous silicon. 
         [0016]    The solar cell may further include a second field region that is positioned in the non-incident surface of the substrate to be separated from the emitter region. The second field region may have the same conductive type as the first field region. 
         [0017]    The first conductive type may be the same as the second conductive type and may be opposite to the third conductive type. 
         [0018]    The solar cell may further include a second passivation layer positioned on the non-incident surface of the substrate. The second passivation layer may be positioned on the substrate between the emitter region and the second field region. The second passivation layer may be positioned under the emitter region and under the second field region. 
         [0019]    In another aspect, there is a solar cell including a substrate of a first conductive type, and is formed of a crystalline semiconductor, a field region of the first conductivity type and is positioned on an incident surface of the substrate, an anti-reflection layer directly positioned on the field region, an emitter region of a second conductive type different from the first conductive type, is formed of a non-crystalline semiconductor, and is positioned on a non-incident surface of the substrate opposite the incident surface of the substrate, a first electrode electrically connected to the emitter region, and a second electrode electrically connected to the substrate. 
         [0020]    A concentration of impurities contained in the field region may be higher than a concentration of impurities contained in the substrate. The concentration of the impurities contained in the field region may be approximately 1×10 16  atoms/cm 3  to 1×10 21  atoms/cm 3 . 
         [0021]    The anti-reflection layer may be formed of at least one of silicon nitride, silicon oxide, and transparent metal oxide. 
         [0022]    The solar cell may further include a passivation layer positioned between the substrate and the field region. 
         [0023]    In another aspect, there is a method for manufacturing a solar cell including forming an emitter region containing first conductivity type impurities on a first surface of a substrate, forming a first field region containing second conductivity type impurities on the first surface of the substrate so that the first field region is separated from the emitter region, forming a passivation layer on a second surface, on which light is incident, opposite the first surface of the substrate, forming a second field region containing the second conductivity type impurities on the passivation layer, and forming a first electrode connected to the emitter region and a second electrode connected to the first field region. 
         [0024]    A conductive type of the first conductivity type impurities may be opposite to a conductive type of the second conductivity type impurities. 
         [0025]    The forming of the second field region may include keeping an injection rate of the second conductivity type impurities injected into a process chamber constant or varying the injection rate of the second conductivity type impurities injected into the process chamber as time has passed. 
         [0026]    The substrate may be formed of a crystalline semiconductor, and the emitter region may be formed of a non-crystalline semiconductor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
           [0028]      FIG. 1  is a partial perspective view of a solar cell according to an example embodiment of the invention; 
           [0029]      FIG. 2  is a cross-sectional view taken along line II-II of  FIG. 1 ; 
           [0030]      FIGS. 3A to 3J  sequentially illustrate each of stages in a method for manufacturing the solar cell shown in  FIG. 1 ; 
           [0031]      FIG. 4  is a partial perspective view of a solar cell according to another example embodiment of the invention; 
           [0032]      FIG. 5  is a cross-sectional view taken along line V-V of  FIG. 4 ; 
           [0033]      FIG. 6  is a partial perspective view of a solar cell according to another example embodiment of the invention; 
           [0034]      FIG. 7  is a cross-sectional view taken along line VII-VII of  FIG. 6 ; and 
           [0035]      FIG. 8  schematically illustrates an energy band gap state of a substrate when a front surface field region is not positioned on the substrate and an energy band gap state of the substrate when the front surface field region is positioned on the substrate. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0036]    The invention will be described more fully hereinafter with reference to the accompanying drawings, in which example embodiments of the inventions are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. 
         [0037]    In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. Further, it will be understood that when an element such as a layer, film, region, or substrate is referred to as being “entirely” on another element, it may be on the entire surface of the other element and may not be on a portion of an edge of the other element. 
         [0038]    Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
         [0039]    A solar cell according to an example embodiment of the invention is described in detail with reference to  FIGS. 1 and 2 . 
         [0040]      FIG. 1  is a partial perspective view of a solar cell according to an example embodiment of the invention.  FIG. 2  is a cross-sectional view taken along line II-II of  FIG. 1 . 
         [0041]    As shown in  FIGS. 1 and 2 , a solar cell  1  according to an example embodiment of the invention includes a substrate  110 , a front passivation layer  191  positioned on an incident surface (hereinafter, referred to as “a front surface”) of the substrate  110  on which light is incident, a front surface field (FSF) region  171  positioned on the front passivation layer  191 , an anti-reflection layer  130  positioned on the FSF region  171 , a back passivation layer  192  positioned on a surface (hereinafter, referred to as “a back surface”) of the substrate  110 , on which the light is not incident, opposite the front surface of the substrate  110 , a plurality of emitter regions  121  positioned on the back passivation layer  192 , a plurality of back surface field (BSF) regions  172  that are positioned on the back passivation layer  192  to be separated from the plurality of emitter regions  121 , and an electrode part  140  including a plurality of first electrodes  141  respectively positioned on the plurality of emitter regions  121  and a plurality of second electrodes  142  respectively positioned on the plurality of BSF regions  172 . 
         [0042]    The substrate  110  is a semiconductor substrate formed of first conductive type silicon, for example, n-type silicon, though not required. Silicon used in the substrate  110  may be crystalline silicon such as single crystal silicon and polycrystalline silicon. When the substrate  110  is of an n-type, the substrate  110  is doped with impurities of a group V element such as phosphor (P), arsenic (As), and antimony (Sb). Alternatively, the substrate  110  may be of a p-type, and/or be formed of another semiconductor materials other than silicon. When the substrate  110  is of the p-type, the substrate  110  is doped with impurities of a group III element such as boron (B), gallium (Ga), and indium (In). 
         [0043]    The front surface of the substrate  110  may be textured to form a textured surface corresponding to an uneven surface or having uneven characteristics. For convenience,  FIG. 1  shows that only an edge portion of the substrate  110  has the textured surface and only edge portions of the front passivation layer  191 , the FSF region  171 , and the anti-reflection layer  130  on the front surface of the substrate  110  have the textured surface. However, the entire front surface of the substrate  110  has really the textured surface, and thus the front passivation layer  191 , the FSF region  171 , and the anti-reflection layer  130  on the front surface of the substrate  110  have the textured surface. 
         [0044]    The back surface as well as the front surface of the substrate  110  may have a textured surface. In this instance, all of the back passivation layer  192 , the plurality of emitter regions  121 , the plurality of BSF regions  172 , and the electrode part  140  positioned on the back surface of the substrate  110  may have an uneven surface. 
         [0045]    The front passivation layer  191  on the front surface of the substrate  110  is formed of intrinsic amorphous silicon (a-Si). The front passivation layer  191  performs a passivation operation that converts a defect, for example, dangling bonds existing on the surface of the substrate  110  and around the surface of the substrate  110  into stable bonds to thereby prevent or reduce a recombination and/or a disappearance of carriers moving to the front surface of the substrate  110  resulting from the defect. Hence, the front passivation layer  191  reduces a loss amount of carriers caused to disappear by the defect on or around the surface of the substrate  110 . 
         [0046]    In general, the defect mostly exists on or around the surface of the substrate  110 . Thus, in the embodiment of the invention, because the front passivation layer  191  directly contacts the surface of the substrate  110 , a passivation effect is further improved. Hence, a loss amount of carriers is further reduced. 
         [0047]    In the embodiment of the invention, the front passivation layer  191  may have a thickness of approximately 1 nm to 20 nm. When the thickness of the front passivation layer  191  is equal to or greater than approximately 1 nm, the passivation operation may be well performed because the front passivation layer  191  is uniformly applied to the front surface of the substrate  110 . When the thickness of the front passivation layer  191  is equal to or less than approximately 20 nm, an amount of light absorbed in the front passivation layer  191  is reduced. Hence, an amount of light incident in the substrate  110  may increase. 
         [0048]    The FSF region  171  on the front passivation layer  191  is an impurity region that is more heavily doped with impurities of the same conductive type (for example, the n-type) as the substrate  110  than the substrate  110 . In the embodiment of the invention, an impurity concentration of the FSF region  171  may be at least approximately 1×10 10  atoms/cm 3  and may be at most approximately 1×10 21  atoms/cm 3 . The FSF region  171  may be formed of amorphous silicon, amorphous silicon oxide (a-SiOx), or amorphous silicon carbide (a-SiC). 
         [0049]    The movement of carriers (for example, holes) to the front surface of the substrate  110  is prevented or reduced by a potential barrier resulting from a difference between impurity concentrations of the substrate  110  and the FSF region  171 . Thus, a front surface field effect is obtained by returning the carriers moving to the front surface of the substrate  110  to the back surface of the substrate  110  by the potential barrier. Hence, an output amount of carriers output to an external device increases, and a recombination and/or a disappearance of carriers at the front surface of the substrate  110  are prevented or reduced. 
         [0050]    In general, energy band gaps of amorphous silicon oxide (a-SiOx) and amorphous silicon carbide (a-SiC) are approximately 2.1 eV and 2.8 eV, respectively. The energy band gaps are wide than amorphous silicon having an energy band gap of approximately 1.7 eV to eV 1.9. Thus, when the FSF region  171  is formed of amorphous silicon oxide (a-SiOx) or amorphous silicon carbide (a-SiC), a waveform region of light absorbed in the FSF region  171  decreases. Hence, an amount of light absorbed in the FSF region  171  decreases, and an amount of light incident on the substrate  110  further increases. 
         [0051]    In the present embodiment, the impurity concentration of the FSF region  171  continuously or discontinuously changes in the range of approximately 1×10 10  atoms/cm 3  to 1×10 21  atoms/cm 3  along (or in) a thickness direction, or is substantially uniform in the range of approximately 1×10 16  atoms/cm 3  to 1×10 21  atoms/cm 3  along (or in) the thickness direction. 
         [0052]    When the impurity concentration of the FSF region  171  changes in the range of approximately 1×10 10  atoms/cm 3  to 1×10 21  atoms/cm 3  along the thickness direction, a portion of the FSF region  171  performs the passivation operation in the same manner as the front passivation layer  191 . In this instance, the impurity concentration of the FSF region  171  changes from an impurity concentration at a portion of the FSF region  171  adjoining the front passivation layer  191  to an impurity concentration at a portion of the FSF region  171  adjoining the anti-reflection layer  130 . 
         [0053]    For example, in the FSF region  171  that is closer to the front passivation layer  191 , the impurity concentration of the FSF region  171  decreases. On the contrary, in the FSF region  171  that is closer to the anti-reflection layer  130 , the impurity concentration of the FSF region  171  increases. A portion of the FSF region  171  positioned at a shortest distance between the portion of the FSF region  171  adjoining the front passivation layer  191  and the surface of the substrate  110 , has a minimum impurity concentration. A portion of the FSF region  171  positioned at a shortest distance between the portion of the FSF region  171  adjoining the anti-reflection layer  130  and the surface of the substrate  110 , has a maximum impurity concentration. In this instance, the two shortest distances are measured from the same portion or position of the substrate  110 . 
         [0054]    Accordingly, the minimum impurity concentration is approximately 1×10 10  atoms/cm 3 , and the maximum impurity concentration is approximately 1×10 21  atoms/cm 3  in the FSF region  171 . 
         [0055]    In this instance, because the FSF region  171  performs the passivation operation as well as a front surface field operation, a thickness of the FSF region  171  has to be greater than a thickness of the FSF region when the FSF region performs only the front surface field operation. Further, a thickness of the front passivation layer  191  may slightly decrease. In this instance, the front passivation layer  191  may have a thickness of approximately 1 nm to 10 nm, and the FSF region  171  may have a thickness of approximately 3 nm to 30 nm. 
         [0056]    When the thickness of the front passivation layer  191  is equal to or greater than approximately 1 nm, the passivation efficiency may further increase because the front passivation layer  191  is uniformly applied to the front surface of the substrate  110 . When the thickness of the front passivation layer  191  is equal to or less than approximately 10 nm, an amount of light incident on the substrate  110  may further increase because the front passivation layer  191  performs the passivation operation without the absorption of light in the front passivation layer  191 . 
         [0057]    When the thickness of the FSF region  171  is equal to or greater than approximately 3 nm, the FSF region  171  may generate a field strength capable of stably performing the front surface field operation even if a portion of the FSF region  171  perform the passivation operation. Further, the FSF region  171  may form an electric field with a normal size irrespective of the presence of the front passivation layer  191 , that is positioned between the substrate  110  and the FSF region  171  and which adversely affects a field strength reacting on (or applied to) the substrate  110 , thereby stably performing the front surface field operation. When the thickness of the FSF region  171  is equal to or less than approximately 30 nm, an amount of light incident on the substrate  110  may further increase because the FSF region  171  performs the front surface field operation without the absorption of light in the FSF region  171 . 
         [0058]    Alternatively, when the FSF region  171  has substantially the uniform impurity concentration, the impurity concentration of the FSF region  171  may be substantially uniform irrespective of changes in the thickness of the FSF region  171 . In this instance, because the FSF region  171  mainly performs the front surface field operation for the front surface field effect as compared with the passivation operation thereof, the FSF region  171  has to have the impurity concentration capable of smoothly performing the front surface field operation using the difference between the impurity concentrations of the substrate  110  and the FSF region  171 . 
         [0059]    Accordingly, when the FSF region  171  mainly performs the front surface field operation, the impurity concentration of the FSF region  171  has to be one that is higher than the impurity concentration of the FSF region  171  when the portion of the FSF region  171  performs the passivation operation. Further, the impurity concentration of the FSF region  171  may be higher than the impurity concentration of the substrate  110 . In the present embodiment, the FSF region  171  has substantially the uniform impurity concentration belonging to the range of approximately 1×10 16  atoms/cm 3  to 1×10 21  atoms/cm 3 . 
         [0060]    When the FSF region  171  mainly performs the front surface field operation as compared with the passivation operation, the thickness of the front passivation layer  191  underlying the FSF region  171  may slightly increase for sake of the stable passivation operation, and the thickness of the FSF region  171  may slightly decrease because the FSF region  171  mainly performs the front surface field operation. Hence, the thickness of the front passivation layer  191  may be approximately 2 nm to 20 nm, and the thickness of the FSF region  171  may be approximately 1 nm to 20 nm. 
         [0061]    When the thickness of the front passivation layer  191  is equal to or greater than approximately 2 nm, the defect existing on or around the surface of the substrate  110  may be completely removed by use of only the front passivation layer  191 . Hence, the front passivation layer  191  may excellently perform the passivation operation. When the thickness of the front passivation layer  191  is equal to or less than approximately 20 nm, an amount of light incident on the substrate  110  may further increase because the front passivation layer  191  performs the passivation operation without the absorption of light in the front passivation layer  191 . 
         [0062]    When the thickness of the FSF region  171  is equal to or greater than approximately 1 nm, the FSF region  171  may form an electric field with the normal size irrespective of the presence of the front passivation layer  191 , that is positioned between the substrate  110  and the FSF region  171 , and which adversely affects the electric field strength reacting on or applied to the substrate  110 , thereby stably performing the front surface field operation. When the thickness of the FSF region  171  is equal to or less than approximately 20 nm, an amount of light incident on the substrate  110  may further increase because the FSF region  171  performs the front surface field operation without the absorption of light in the FSF region  171 . 
         [0063]    The anti-reflection layer  130  on the FSF region  171  reduces a reflectance of light incident on the solar cell  1  and increases selectivity of a predetermined wavelength band, thereby increasing the efficiency of the solar cell  1 . 
         [0064]    The anti-reflection layer  130  may be formed of silicon oxide (SiOx) and/or silicon nitride (SiNx). 
         [0065]    Further, the anti-reflection layer  130  may be formed using transparent metal oxide formed of at least one selected from the group consisting of indium tin oxide (ITO), tin (Sn)-based oxide (for example, SnO 2 ), zinc (Zn)-based oxide (for example, ZnO, ZnO:Al, ZnO:B, and AZO), and a combination thereof. A transparency and insulating characteristic of the transparent metal oxide vary depending on oxygen (O 2 ) content. Namely, as the O 2  content increase, a transparency of the transparent metal oxide increases and a transmittance of light increases. Further, when the O 2  content exceeds a predetermined content, the transparent metal oxide represents non-conductive characteristics because resistivity of the transparent metal oxide increases to an infinite value. Hence, the transparent metal oxide changes to an insulating material. Accordingly, the transparent metal oxide according to the embodiment of the invention is a transparent conductive oxide (TCO) or a transparent insulating oxide (TIO) depending on the O 2  content. 
         [0066]    The transparency of the transparent metal oxide is greater than transparencies of silicon oxide (SiOx) and silicon nitride (SiNx). Thus, when the anti-reflection layer  130  is formed of the transparent metal oxide, an amount of light incident inside the substrate  110  further increases. Hence, the efficiency of the solar cell  1  is further improved. 
         [0067]    In the present embodiment, the anti-reflection layer  130  has a singe-layered structure, but may have a multi-layered structure such as a double-layered structure in other embodiments. The anti-reflection layer  130  may be omitted, if desired. 
         [0068]    The back passivation layer  192  directly positioned on the back surface of the substrate  110  performs the passivation operation in the same manner as the front passivation layer  191 , thereby preventing or reducing a recombination and/or a disappearance of carriers moving to the back surface of the substrate  110 . 
         [0069]    The back passivation layer  192  may be formed of amorphous silicon in the same manner as the front passivation layer  191 . 
         [0070]    The back passivation layer  192  has a thickness to the extent that carriers moving to the back surface of the substrate  110  can pass through the back passivation layer  192  and can move to the BSF regions  172  and the emitter regions  121 . In the present embodiment, the thickness of the back passivation layer  192  may be approximately 1 nm to 10 nm. 
         [0071]    When the thickness of the back passivation layer  192  is equal to or greater than approximately 1 nm, the passivation effect may be further obtained or may be greater because the back passivation layer  192  is uniformly applied to the back surface of the substrate  110 . When the thickness of the back passivation layer  192  is equal to or less than approximately 10 nm, an amount of light passing through the substrate  110  absorbed in the back passivation layer  192  is reduced. Hence, an amount of light again incident inside the substrate  110  may further increase. 
         [0072]    Each of the plurality of BSF regions  172  is an impurity region that is more heavily doped with impurities of the same conductive type as the substrate  110  than the substrate  110 . For example, each BSF region  172  may be an n + -type region. 
         [0073]    The plurality of BSF regions  172  extend parallel to one another on the back passivation layer  192  in a fixed direction to be separated from one another. In the present embodiment, the BSF regions  172  are formed of a non-crystalline semiconductor such as amorphous silicon. 
         [0074]    The BSF regions  172  prevent or reduce the movement of holes to the BSF regions  172  used as a moving path of electrons by a potential barrier resulting from a difference between impurity concentrations of the substrate  110  and the BSF regions  172  in the same manner as the FSF region  171 . Further, the BSF regions  172  facilitate the movement of carriers (for example, electrons) to the BSF regions  172 . Thus, the BSF regions  172  reduce a loss amount of carriers by a recombination and/or a disappearance of electrons and holes in or around the BSF regions  172  or in the electrode part  140  and accelerate the movement of electrons to the BSF regions  172 , thereby increasing an amount of electrons moving to the BSF regions  172 . 
         [0075]    Each BSF region  172  may have a thickness of approximately 10 nm to 25 nm. When the thickness of the BSF region  172  is equal to or greater than approximately 10 nm, the BSF region  172  may form a potential barrier sufficient to prevent or reduce the movement of holes to the BSF region  172  to thereby further reduce a loss amount of carriers. When the thickness of the BSF region  172  is equal to or less than approximately 25 nm, the BSF region  172  further reduces an amount of light absorbed in the BSF region  172  to thereby further increase an amount of light again incident inside the substrate  110 . 
         [0076]    The plurality of emitter regions  121  are positioned on the back surface of the substrate  110  to be separated from the BSF regions  172  and extend parallel to the BSF regions  172 . 
         [0077]    As shown in  FIGS. 1 and 2 , the plurality of emitter regions  121  and the plurality of BSF regions  172  are alternately positioned on the back surface of the substrate  110 . 
         [0078]    Each emitter region  121  is of a second conductive type (for example, a p-type) opposite a conductive type of the substrate  110 . Each emitter region  121  is formed of a semiconductor, for example, amorphous silicon different from the substrate  110 . Thus, the plurality of emitter regions  121  and the substrate  110  form a heterojunction as well as a p-n junction. 
         [0079]    A plurality of electron-hole pairs produced by light incident on the substrate  110  are separated into electrons and holes by a built-in potential difference resulting from the p-n junction between the substrate  110  and the emitter regions  121 . Then, the separated electrons move to the n-type semiconductor, and the separated holes move to the p-type semiconductor. Thus, when the substrate  110  is of the n-type and the emitter regions  121  are of the p-type, the separated holes pass through the back passivation layer  192  and move to the emitter regions  121  and the separated electrons pass through the back passivation layer  192  and move to the BSF regions  172  having the impurity concentration higher than the substrate  110 . 
         [0080]    Because the substrate  110  and each emitter region  121  form the p-n junction, the emitter region  121  may be of the n-type when the substrate  110  is of the p-type unlike the embodiment described above. In this instance, the separated electrons pass through the back passivation layer  192  and move to the emitter regions  121 , and the separated holes pass through the back passivation layer  192  and move to the BSF regions  172 . 
         [0081]    When the plurality of emitter regions  121  are of the p-type, the emitter regions  121  may be doped with impurities of a group III element. On the contrary, when the emitter regions  121  are of the n-type, the emitter regions  121  may be doped with impurities of a group V element. 
         [0082]    The plurality of emitter regions  121  may perform the passivation operation along with the back passivation layer  192 . In this instance, an amount of carriers caused to disappear at the back surface of the substrate  110  by the defect is reduced, and thus the efficiency of the solar cell  11  is improved. 
         [0083]    Each emitter region  121  may have a thickness of 5 nm to 15 nm. When the thickness of the emitter region  121  is equal to or greater than approximately 5 nm, the emitter region  121  may form a good p-n junction. When the thickness of the emitter region  121  is equal to or less than approximately 15 nm, an amount of light absorbed in the emitter region  121  is further reduced. Hence, an amount of light again incident inside the substrate  110  may further increase. 
         [0084]    In the present embodiment, a crystallization phenomenon obtained when the emitter regions  121  and the BSF regions  172  are positioned on the back passivation layer  192  is reduced further than a crystallization phenomenon obtained when the emitter regions  121  and the BSF regions  172  are positioned directly on the substrate  110  formed of a crystalline semiconductor material, because the back passivation layer  192  is formed of intrinsic amorphous silicon (a-Si) in which there is no impurities or scarcely any impurities. Hence, characteristics of the emitter regions  121  and the BSF regions  172  positioned on an amorphous silicon layer (i.e., the back passivation layer  192 ) are improved. 
         [0085]    The plurality of first electrodes  141  on the plurality of emitter regions  121  long extend along the emitter regions  121  and are electrically and physically connected to the emitter regions  121 . Each first electrode  141  collects carriers (for example, holes) moving to the corresponding emitter region  121 . 
         [0086]    The plurality of second electrodes  142  on the plurality of BSF regions  172  long extend along the BSF regions  172  and are electrically and physically connected to the BSF regions  172 . Each second electrode  142  collects carriers (for example, electrons) moving to the corresponding BSF region  172 . 
         [0087]    In  FIGS. 1 and 2 , the first and second electrodes  141  and  142  have the same plane shape or sheet shape as the emitter regions  121  and the BSF regions  172  underlying the first and second electrodes  141  and  142 . However, they may have different plane shapes. As a contact area between the emitter regions  121  and the BSF regions  172  and the respective first and second electrodes  141  and  142  increases, a contact resistance therebetween decreases. Hence, the carrier transfer efficiency of the first and second electrodes  141  and  142  increases. 
         [0088]    The plurality of first and second electrodes  141  and  142  may be formed of at least one conductive material selected from the group consisting of nickel (Ni), copper (Cu), silver (Ag), aluminum (Al), tin (Sn), zinc (Zn), indium (In), titanium (Ti), gold (Au), and a combination thereof. Other conductive materials may be used. As described above, because the plurality of first and second electrodes  141  and  142  are formed of the metal material, the plurality of first and second electrodes  141  and  142  reflect light passing through the substrate  110  onto the substrate  110 . 
         [0089]    The solar cell  1  having the above-described structure is a solar cell in which the plurality of first and second electrodes  141  and  142  are positioned on the back surface of the substrate  110 , on which light is not incident, and the substrate  110  and the plurality of emitter regions  121  are formed of different kinds of semiconductors. An operation of the solar cell  1  is described below. 
         [0090]    When light is irradiated onto the solar cell  1 , sequentially passes through the anti-reflection layer  130 , the FSF region  171 , and the front passivation layer  191 , and is incident on the substrate  110 , a plurality of electron-hole pairs are generated in the substrate  110  by light energy based on the incident light. In this instance, because the front surface of the substrate  110  is the textured surface, a reflectance of light at the front surface of the substrate  110  is reduced. Further, because both a light incident operation and a light reflection operation are performed on the textured surface of the substrate  110 , a light absorption increases and the efficiency of the solar cell  1  is improved. In addition, because a reflection loss of the light incident on the substrate  110  is reduced by the anti-reflection layer  130 , an amount of light incident on the substrate  110  further increases. 
         [0091]    The electron-hole pairs are separated into electrons and holes by the p-n junction of the substrate  110  and the emitter regions  121 , and the separated holes move to the p-type emitter regions  121  and the separated electrons move to the n-type BSF regions  172 . The holes moving to the p-type emitter regions  121  are collected by the first electrodes  141 , and the electrons moving to the n-type BSF regions  172  are collected by the second electrodes  142 . When the first electrodes  141  and the second electrodes  142  are connected to each other using electric wires, current flows therein to thereby enable use of the current for electric power. 
         [0092]    Further, because the passivation layers  192  and  191  are positioned on the front surface as well as the back surface of the substrate  110 , a loss amount of carriers caused to disappear by the defect on and around the front and back surfaces of the substrate  110  is reduced. Hence, the efficiency of the solar cell  1  is improved. In this instance, because the front passivation layer  191  as well as the back passivation layer  192  make direct contact with the surface of the substrate  110  in which the defect is frequently generated, the passivation effect is further improved. 
         [0093]    Further, a loss amount of carriers is further reduced because of the FSF region  171  and the BSF region  172  respectively positioned on the front and back surfaces of the substrate  110 , and thus the efficiency of the solar cell  1  is further improved. 
         [0094]    Next, an energy band gap when the front passivation layer and the FSF region are formed on the front surface of the solar cell and an energy band gap when only the front passivation layer is formed on the front surface of the solar cell are described with reference to  FIG. 8 . 
         [0095]    (a) of  FIG. 8  illustrates an energy band gap between the substrate and the front passivation layer when only the front passivation layer without the FSF region is directly positioned on the substrate. (b) of  FIG. 8  illustrates an energy band gap between the substrate and the front passivation layer and the FSF region when the front passivation layer and the FSF region are sequentially positioned on the substrate. 
         [0096]    For a solar cell for measuring the energy band gap shown in  FIG. 8 , the front passivation layer is formed of intrinsic amorphous silicon (i-a-Si) having a thickness of approximately 20 nm, the FSF region is formed of n-type amorphous silicon (n-a-Si) having a thickness of approximately 10 nm and an impurity concentration of approximately 1×10 20  atoms/cm 3 , and the substrate is formed of n-type polycrystalline silicon (n-c-Si). 
         [0097]    As shown in (a) of  FIG. 8 , when the FSF region is not present, an energy band gap of the substrate (n-c-Si) was inclined. Therefore, holes H having the energy band gap characteristic moving from a low energy level to a high energy level mainly moved to the front passivation layer (i-a-Si), i.e., the front surface of the substrate (n-c-Si), and a loss of holes H occurred between the substrate (n-c-Si) and the front passivation layer (i-a-Si). 
         [0098]    However, as shown in (b) of  FIG. 8 , when the FSF region (n-a-Si) existed, an energy band gap of the substrate (n-c-Si) was almost held in a parallel state because of an electric field formed by the FSF region (n-a-Si). Therefore, a directional behavior of holes H to move to the front surface of the substrate (n-c-Si) is not present. Hence, a loss of holes H generated between the substrate (n-c-Si) and the front passivation layer (i-a-Si) was greatly reduced. 
         [0099]    When the FSF region is not present at the front surface of the substrate, a short-circuit current of the solar cell was approximately 28.47 mA/cm 3 . When the FSF region is present at the front surface of the substrate, a short-circuit current of the solar cell was approximately 35.05 mA/cm 3 . Namely, the short-circuit current of the solar cell increased by approximately 30%. The solar cells illustrated in (a) and (b) of  FIG. 8  were manufactured under the same conditions, except the absence or presence of the FSF region. As above, a loss amount of carriers at the back surface of the substrate is greatly reduced by the operation of the FSF region, and thus the short-circuit current of the solar cell increases. 
         [0100]    A method for manufacturing the solar cell  1  according to the embodiment of the invention is described below with reference to  FIGS. 3A to 3J . 
         [0101]      FIGS. 3A to 3J  sequentially illustrate each of stages in a method for manufacturing the solar cell shown in  FIG. 1 . 
         [0102]    As shown in  FIG. 3A , an etch stop layer  60  formed of silicon oxide (SiOx), etc. is formed on the surface of the substrate  110  formed of n-type single crystal silicon or n-type polycrystalline silicon, for example. 
         [0103]    Next, as shown in  FIG. 3B , the surface (for example, the incident surface) of the substrate  110 , on which the etch stop layer  60  is not formed is etched, while another surface is protected using the etch stop layer  60  as a mask, to form a textured surface having a plurality of uneven portions on the front surface (for example, the incident surface) of the substrate  110 . When the substrate  110  is formed of single crystal silicon, the front surface of the substrate  110  may be textured using a basic solution such as KOH, NaOH, and tetramethylammonium hydroxide (TMAH). On the other hand, when the substrate  110  is formed of polycrystalline silicon, the front surface of the substrate  110  may be textured using an acid solution such as HF and HNO 3 . 
         [0104]    If both the front surface and the back surface of the substrate  110  is to have the textured surface, the front surface and back surface of the substrate  110  may be exposed to an etchant or an etching gas without forming the etch stop layer  60  to form the textured surface on the front surface and the back surface of the substrate  110 . 
         [0105]    Next, as shown in  FIG. 3C , the front passivation layer  191  formed of amorphous silicon is formed on the front surface (i.e., the textured surface) of the substrate  110  using a layer forming method such as a plasma enhanced chemical vapor deposition (PECVD) method. 
         [0106]    Next, as shown in  FIG. 3D , the FSF region  171  formed of amorphous silicon is formed on the front passivation layer  191  using the PECVD method, etc. In this instance, because the FSF region  171  has the same conductive type as the substrate  110 , an impurity doping material (for example, POCl 3  or H 3 PO 4 ) containing impurities of a group V element such as phosphor (P) is injected into a process chamber. Hence, the FSF region  171 , that has the same conductive type as the substrate  110  and has an impurity concentration higher than the substrate  110 , is formed. 
         [0107]    The FSF region  171  thus formed may have the uniform impurity concentration irrespective of its thickness. In this instance, the impurity concentration of the FSF region  171  is approximately 1×10 16  atoms/cm 3  to 1×10 21  atoms/cm 3 . An injection rate of the impurity doping material injected into the process chamber for forming the FSF region  171  is substantially uniform. 
         [0108]    However, alternatively, the impurity concentration of the FSF region  171  thus formed may change continuously or discontinuously over the thickness of the FSF region  171 . In this instance, the impurity concentration of the FSF region  171  changes in the range of approximately 1×10 10  atoms/cm 3  to 1×10 21  atoms/cm 3  over its thickness. With greater thickness of the FSF region  171 , there is an increase of the impurity concentration of the FSF region  171 . An injection rate of the impurity doping material injected into the process chamber for forming the FSF region  171  increases with time. 
         [0109]    Next, as shown in  FIG. 3E , the anti-reflection layer  130  formed of silicon oxide (SiOx) and/or silicon nitride (SiNx) is formed on the FSF region  171  using the PECVD method, etc. 
         [0110]    Next, as shown in  FIG. 3F , the etch stop layer  60  formed on the back surface of the substrate  110  is removed. 
         [0111]    Next, as shown in  FIG. 3G , the back passivation layer  192  is formed on the back surface of the substrate  110  using the PECVD method, etc. The back passivation layer  192  is formed of amorphous silicon, etc. 
         [0112]    Next, as shown in  FIG. 3H , after a mask having a plurality of openings is positioned on the back passivation layer  192 , amorphous silicon is deposited using the PECVD method, etc., to form the plurality of emitter regions  121 . When the plurality of emitter regions  121  are formed, an impurity doping material (for example, B 2 H 6 ) containing impurities of a group III element such as boron (B) is injected into a process chamber. Therefore, the emitter regions  121  contain impurities of a conductivity type opposite the conductivity type of the substrate  110 . Hence, the p-n junction between the substrate  110  and the plurality of emitter regions  121  is formed. Further, because the substrate  110  and the emitter regions  121  are formed of different semiconductor materials, the substrate  110  and the emitter regions  121  form the heterojunction. 
         [0113]    Positions of the plurality of openings correspond to portions of the back passivation layer  192  on which the plurality of emitter regions  121  are formed, and thus the plurality of openings expose the portions of the back passivation layer  192  on which the plurality of emitter regions  121  are formed. 
         [0114]    Next, as shown in  FIG. 3I , after a mask having a plurality of openings is positioned on the back passivation layer  192 , amorphous silicon is deposited using the PECVD method, etc., to form the plurality of BSF regions  172 . Because the plurality of BSF regions  172  are formed by injecting an impurity doping material containing impurities of a group V element such as phosphor (P) into the process chamber, the plurality of BSF regions  172  become impurity regions having an impurity concentration higher than the substrate  110 . 
         [0115]    In this instance, positions of the plurality of openings correspond to portions of the back passivation layer  192  on which the plurality of BSF regions  172  are formed, and thus the plurality of openings expose the portions of the back passivation layer  192  on which the plurality BSF regions  172  are formed. 
         [0116]    The formation order of the emitter regions  121  and the BSF regions  172  may vary. The emitter regions  121  and the BSF regions  172  may be formed using the following methods. For example, after a photosensitive layer is formed and a photomask is positioned on the photosensitive layer, the photosensitive layer of portions, on which the emitter regions  121  and the BSF regions  172  will be formed, is exposed. Hence, a desired portion of the back passivation layer  192  is exposed. As a result, the plurality of emitter regions  121  and the plurality of BSF regions  172  are formed in the exposed portion of the back passivation layer  192 . Further, after an emitter layer, for example, an amorphous silicon layer containing corresponding impurities is formed on the back passivation layer  192 , a portion of the emitter layer is selectively removed to form the plurality of emitter regions  121  on the back passivation layer  192 . Then, after a BSF layer, for example, an amorphous silicon layer containing corresponding impurities is formed on the exposed back passivation layer  192  and the emitter regions  121 , a portion of the BSF layer is selectively removed to form the plurality of BSF regions  172 . As described above, the emitter regions  121  and the BSF regions  172  may be formed on the back passivation layer  192  using the various methods. 
         [0117]    Next, as shown in  FIG. 3J , a conductive layer  40  containing a metal material is formed on the exposed back passivation layer  192 , the plurality of emitter regions  121 , and the plurality of BSF regions  172  using the PECVD method, etc. Then, a portion of the conductive layer  40  is sequentially removed through a wet etching process using a mask to form the electrode part  140  (refer to  FIGS. 1 and 2 ) including the plurality of first and second electrodes  141  and  142 . 
         [0118]    Instead of the deposition process illustrated in  FIG. 3J , a paste containing a metal material may be applied on the plurality of emitter regions  121  and the plurality of BSF regions  172  using a screen printing method, and then dried. Hence, the electrode part  140  including the plurality of first electrodes  141  positioned on the plurality of emitter regions  121  and the plurality of second electrodes  142  positioned on the plurality of BSF regions  172  may be formed. 
         [0119]    Alternatively, all of the layers  191 ,  171 ,  130 ,  192 ,  121 ,  172 ,  141 , and  142  formed on the substrate  110  may be formed using a physical vapor deposition method such as a sputtering method as well as a chemical vapor deposition method such as the PECVD method. 
         [0120]    As above, because the layers formed on the substrate  110  are formed using the chemical vapor deposition method or the physical vapor deposition method performed at a low temperature of approximately 200° C., the solar cell  1  may be easily manufactured, and characteristics of each of the layers  191 ,  171 ,  130 ,  192 ,  121 ,  172 ,  141 , and  142  may be prevented from being degraded at a high temperature. As a result, the efficiency of the solar cell  1 , or manufacture thereof, is improved. 
         [0121]    A solar cell  1   a  according to another example embodiment of the invention is described below with reference to  FIGS. 4 and 5 . 
         [0122]      FIG. 4  is a partial perspective view of a solar cell according to another example embodiment of the invention.  FIG. 5  is a cross-sectional view taken along line V-V of  FIG. 4 . In the following description, structures and components identical or equivalent to those illustrated in  FIGS. 1 and 2  are designated with the same reference numerals, and a further description may be briefly made or may be entirely omitted. 
         [0123]    The solar cell  1   a  shown in  FIGS. 4 and 5  has the structure similar to the solar cell  1  shown in  FIGS. 1 and 2 . 
         [0124]    More specifically, the solar cell  1   a  according to the example embodiment of the invention includes a substrate  110 , a front passivation layer  191 , an FSF region  171 , and an anti-reflection layer  130  that are sequentially positioned on a front surface of the substrate  110 , a back passivation layer  192   a , a plurality of emitter regions  121 , and a plurality of BSF regions  172  that are positioned on a back surface of the substrate  110 , and an electrode part  140  including a plurality of first electrodes  141  respectively positioned on the plurality of emitter regions  121  and a plurality of second electrodes  142  respectively positioned on the plurality of BSF regions  172 . 
         [0125]    Unlike the solar cell  1  shown in  FIGS. 1 and 2 , in the solar cell  1   a  shown in  FIGS. 4 and 5 , the plurality of emitter regions  121  and the plurality of BSF regions  172  are positioned to directly contact the back surface of the substrate  110 . Further, the back passivation layer  192   a  is positioned on the back surface of the substrate  110  excluding a formation portion of the emitter regions  121  and the BSF regions  172  from the back surface of the substrate  110 . In other words, the configuration of the solar cell  1   a  shown in  FIGS. 4 and 5  is substantially the same as the solar cell  1  shown in  FIGS. 1 and 2  except a formation location of the back passivation layer  192   a.    
         [0126]    The back passivation layer  192   a  shown in  FIGS. 4 and 5  has a thickness greater than the back passivation layer  192  shown in  FIGS. 1 and 2 , but is not limited thereto. 
         [0127]    The back passivation layer  192   a  performs a passivation operation on and around the surface of the substrate  110  in the same manner as the back passivation layer  192 , thereby reducing a loss amount of carriers caused to disappear by the defect on or around the surface of the substrate  110 . 
         [0128]    The back passivation layer  192   a  is formed of an insulating material such as silicon oxide (SiOx) and silicon nitride (SiNx). Thus, an electrical interference (for example, carrier movement) between the emitter region  121  and the BSF region  172  adjacent to each other is prevented or reduced by the back passivation layer  192   a , and as a result, a loss amount of carriers is reduced. Further, light passing through the substrate  110  is prevented or reduced from being reflected inside the substrate  110 , and thus an amount of light reflected to the outside is reduced. 
         [0129]    As above, because the plurality of emitter regions  121  and the plurality of BSF regions  172  directly contact the back surface of the substrate  110 , a transfer rate of carriers moving to the emitter regions  121  and the BSF regions  172  increases. Hence, the efficiency of the solar cell  1   a  is further improved. 
         [0130]    A solar cell  1   b  according to another example embodiment of the invention is described below with reference to  FIGS. 6 and 7 . 
         [0131]      FIG. 6  is a partial perspective view of a solar cell according to another example embodiment of the invention.  FIG. 7  is a cross-sectional view taken along line VII-VII of  FIG. 6 . In the following description, structures and components identical or equivalent to those illustrated in  FIGS. 1 and 2  are designated with the same reference numerals, and a further description may be briefly made or may be entirely omitted. 
         [0132]    Similar to the solar cell  1  shown in  FIGS. 1 and 2 , the solar cell  1   b  shown in  FIGS. 6 and 7  includes a substrate  110 , a front passivation layer  191 , an FSF region  171 , and an anti-reflection layer  130  that are sequentially positioned on a front surface of the substrate  110 , a back passivation layer  192   c  positioned on a back surface of the substrate  110 , a plurality of emitter regions  121  and a plurality of BSF regions  172  positioned on the back passivation layer  192   c , and an electrode part  140  including a plurality of first electrodes  141  respectively positioned on the plurality of emitter regions  121  and a plurality of second electrodes  142  respectively positioned on the plurality of BSF regions  172 . 
         [0133]    In the solar cell  1   b  shown in  FIGS. 6 and 7 , unlike the solar cell  1  shown in  FIGS. 1 and 2 , the back passivation layer  192   c  includes a plurality of first back passivation layers  1923  positioned between the substrate  110  and the plurality of emitter regions  121 , and between the substrate  110  and the plurality of BSF regions  172 ; and a plurality of second back passivation layers  1924  positioned in other portions of the back surface of the substrate  110 , i.e., between the plurality of emitter regions  121  and the plurality of BSF regions  172 . 
         [0134]    The first back passivation layers  1923  are formed of a conductive material such as amorphous silicon, and the second back passivation layers  1924  are formed of an insulating material such as silicon oxide (SiOx) and silicon nitride (SiNx). Hence, a conversion rate capable of converting unstable bonds existing at a contact surface between the back passivation layer  192   c  and the substrate  110  into stable bonds is improved. Further, an amount of carriers transferred to the emitter regions  121  or the BSF regions  172  increases because of conductive characteristics of the first back passivation layers  1923 , and thus a carrier transfer rate increases. As a result, the efficiency of the solar cell  1   b  is further improved compared with the solar cell  1 . 
         [0135]    Further, as described above with reference to  FIGS. 4 and 5 , an electrical interference (for example, carrier movement) between the emitter region  121  and the BSF region  172  adjacent to each other is prevented or reduced by the second back passivation layers  1924 , and as a result, the efficiency of the solar cell  1   b  is further improved. 
         [0136]    In the solar cell  1   b  shown in  FIGS. 6 and 7 , thicknesses of the first and second back passivation layers  1923  and  1924  are substantially equal to each other, but may also be different from each other. For example, the thickness of the second back passivation layer  1924  may be greater than the thickness of the first back passivation layer  1923 . In this instance, an electrical interference between the emitter region  121  and the BSF region  172  adjacent to each other is further reduced by the second back passivation layer  1924 . 
         [0137]    Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.