Patent Publication Number: US-2016225935-A1

Title: Solar cell and method for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of co-pending U.S. patent application Ser. No. 12/817,998 filed on Jun. 17, 2010, which claims the benefit under 35 U.S.C. §119(a) to Korean Patent Application No. 10-2009-0054450 filed on Jun. 18, 2009, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate to a solar cell and a method for manufacturing the same. 
     2. Discussion of the Related Art 
     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 generating electric energy from solar energy have been particularly spotlighted. A silicon solar cell generally includes a substrate and an emitter region, each of which is formed of a semiconductor, and a plurality of electrodes respectively formed on the substrate and the emitter region. The semiconductors forming the substrate and the emitter region have different conductive types, such as a p-type and an n-type. A p-n junction is formed at an interface between the substrate and the emitter region. 
     When light is incident on the solar cell, a plurality of electron-hole pairs are generated in the semiconductors. The electron-hole pairs are separated into electrons and holes by the photovoltaic effect. Thus, the separated electrons move to the n-type semiconductor (e.g., the emitter region) and the separated holes move to the p-type semiconductor (e.g., the substrate), The electrons and holes are respectively collected by the electrode electrically connected to the emitter region and the electrode electrically connected to the substrate. The electrodes are connected to one another using electric wires to thereby obtain electric power. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a method for manufacturing a solar cell may include forming an emitter region that forms a p-n junction with a semiconductor substrate of a first conductive type, forming a passivation layer on the semiconductor substrate, forming a dopant layer containing impurities of the first conductive type on the passivation layer, and locally forming a back surface field region at the semiconductor substrate by irradiating laser beams onto the semiconductor substrate to diffuse the impurities of the first conductive type into the semiconductor substrate. 
     According to another aspect, a solar cell may include a semiconductor substrate of a first conductive type, an emitter region containing impurities of a second conductive type opposite to the first conductive type, and being positioned at the semiconductor substrate of a first conductive type, a first electrode connected to the emitter region, a passivation layer positioned on the semiconductor substrate, a dopant layer containing impurities of the first conductive type, and being positioned on the passivation layer, a second electrode positioned on the dopant layer and electrically connected to the semiconductor substrate, and a plurality of back surface field regions positioned at the semiconductor substrate, and connected to the second electrode. 
     According to another aspect, a solar cell may include a semiconductor substrate of a first conductive type; an emitter region containing impurities of a second conductive type opposite to the first conductive type, and being positioned at the semiconductor substrate of a first conductive type; a first electrode connected to the emitter region; a passivation layer positioned on the semiconductor substrate; a second electrode positioned on the semiconductor layer and electrically connected to the semiconductor substrate; a plurality of back surface field regions locally positioned at the semiconductor substrate, and connected to the second electrode; a plurality of mixed portions locally positioned at the plurality of back surface field regions, the plurality of mixed portions containing at least impurities of the same type as the semiconductor substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a partial cross-sectional view of an example of a solar cell according to an embodiment of the present invention; 
         FIGS. 2A to 2H  are sectional views sequentially showing processes for manufacturing the solar cell shown in  FIG. 1 ; 
         FIGS. 3 to 6  are partial cross-sectional views of examples of a solar cell according to an embodiment of the present invention; 
         FIG. 7  is a partial cross-sectional views of an example of a solar cell according to another embodiment of the present invention; 
         FIGS. 8A and 8B  are sectional views showing portions of processes for manufacturing the solar cell shown in  FIG. 7 ; and 
         FIGS. 9 and 10  are partial cross-sectional views of other examples of a solar cell according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     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 only the embodiments set forth herein. 
     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. 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     Next, referring to drawings, solar cells according to embodiments of the present invention will be described in detail. 
     First, solar cells according to an example embodiment of the present invention will be described in reference to  FIGS. 1 to 6 . 
       FIG. 1  is a partial cross-sectional view of an example of a solar cell according to an embodiment of the present invention,  FIGS. 2A to 2H  are sectional views sequentially showing processes for manufacturing the solar cell shown in  FIG. 1 .  FIGS. 3 and 6  are partial cross-sectional views of examples of a solar cell according to an embodiment of the present invention. 
     First, referring to  FIG. 1 , one of various examples of a solar cell according to an embodiment of the present invention will be described. 
     Referring to  FIG. 1 , a solar cell  1  according to an example of an embodiment includes a substrate  100 , an emitter region  102  positioned in (at) a surface (hereinafter, referred to as ‘a front surface’) of the substrate  100  on which light is incident, an anti-reflection layer  104  on the emitter region  102 , a passivation layer  108  positioned on a surface (a rear surface) of the substrate  100 , opposite the front surface of the substrate  100 , on which the light is not incident, a dopant layer  110  positioned on the passivation layer  108  and partially or selectively connected to the substrate  100  through the passivation layer  108 , a plurality of front electrodes (a plurality of first electrodes)  106  connected to the emitter region  102 , a rear electrode (a second electrode)  112  positioned on the dopant layer  110  and electrically connected to the substrate  100 , and a plurality of back surface field (BSF) regions  114  positioned at (in) locations at which the substrate  100  and the rear electrode are electrically connected to each other. The plurality of BSF regions  114  are positioned locally at the substrate  110 . 
     The substrate  100  is a semiconductor substrate formed of first conductive type silicon, for example, p-type silicon, though not required. Examples of silicon include crystalline silicon such as single crystal silicon and polycrystalline silicon. If the substrate  100  is of the p-type, the substrate  100  may contain impurities of a group III element such as boron (B), gallium (Ga), and indium (In). Alternatively, the substrate  100  may be of an n-type. If the substrate  100  is of the n-type, the substrate  100  may contain impurities of a group IV element such as phosphorus (P), arsenic (As), and antimony (Sb). In addition, the substrate  100  may be made of other semiconductor materials instead of silicon. 
     The front surface of the substrate  100  is textured to form a textured surface corresponding to an uneven surface. Hence, a surface area of the substrate  100  increases and a light reflectance of the front surface of the substrate  100  is reduced. 
     The emitter region  102  positioned in (at) the front surface of the substrate  100  is an impurity region with impurities (e.g., n-type impurities) of a second conductive type opposite the first conductive type of the substrate  100 . The emitter region  102  forms a p-n junction with the substrate  100 . 
     By a built-in potential difference generated due to the p-n junction, a plurality of electron-hole pairs, which are generated by incident light onto the semiconductor substrate  100 , are separated into electrons and holes, respectively, and the separated electrons move toward the n-type semiconductor and the separated holes move toward the p-type semiconductor. Thus, when the substrate  100  is of the p-type and the emitter region  102  is of the n-type, the separated holes move toward the substrate  100  and the separated electrons move toward the emitter region  102 . 
     Because the emitter region  102  forms the p-n junction with the substrate  100 , when the substrate  100  is of the n-type, then the emitter region  102  is of the p-type, in contrast to the embodiment discussed above, and the separated electrons move toward the substrate  100  and the separated holes move toward the emitter region  102 . 
     Returning to the embodiment, when the emitter region  102  is of the n-type, the emitter region  102  may be formed by doping the substrate  100  with impurities of the group V element such as P, As, Sb, etc., while when the emitter region  102  is of the p-type, the emitter region  102  may be formed by doping the substrate  100  with impurities of the group III element such as B, Ga, In, etc. 
     In reference to  FIG. 1 , the anti-reflection layer  104  positioned on the emitter region  102  is preferably made of silicon nitride (SiNx) or silicon oxide (SiOx), etc. The anti-reflection layer  104  reduces reflectance of light incident onto the substrate  100 , thereby increasing an amount of the incident light on the substrate  100 . The anti-reflection layer  104  may also perform a passivation function which converts defects, such as dangling bonds, existing around the surfaces of the substrate  100  into stable bonds to thereby prevent or reduce a recombination and/or a disappearance of charges moving to the surfaces of the substrate  100 . 
     In  FIG. 1 , the anti-reflection layer  104  has a single-layered structure, but the anti-reflection layer  104  may have a multi-layered structure such as a double-layered structure of SiNx/SiON or SiNx/SiOx or a triple-layered structure of SiOx/SiNx/SiOx. The anti-reflection layer  104  may be omitted, if desired. 
     The passivation layer  108  positioned on the rear surface of the substrate  100  has a plurality of contact holes  116  for contacting the substrate  100 . 
     The passivation layer  108  may be made of a SiO 2 , SiNx, or SiOxNy, etc. The passivation layer  108  performs the passivation function near the rear surface of the substrate  100  to prevent or reduce a recombination and/or a disappearance of charges, and thereby a BSRV (back surface recombination velocity) of the electrons and the charges is decreased below about 500 cm/sec to improve an efficiency of the solar cell  1 . 
     The dopant layer  110  positioned on the passivation layer  108  is an impurity portion of the same conductive type, for example, a p-type, as the substrate  100 . At this time, the dopant layer  110  contains higher concentration of impurities of the same conductive type than the substrate  100 . In the embodiment, the dopant layer  110  may be formed by using boron (B) as the impurities. 
     The dopant layer  110  is connected to the substrate  100  through the contact holes  116  of the passivation layer  108 . 
     The plurality of the back surface field regions  114  are substantially positioned at (in) the substrate  100  contacting the dopant layer  110  through the contact holes  116  of the passivation layer  108 . 
     The dopant layer  110  also includes a plurality of depressed portions  117 . The formation positions of the depressed portions  117  correspond to the formation positions of the back surface field regions  114 . 
     The plurality of back surface field regions  114  are areas heavily doped by impurities of the same conductive type as the substrate  100 . 
     A potential barrier is formed by an impurity concentration difference between the substrate  100  and the back surface field regions  114 , thereby distributing the movement of charges (for example, electrons) to a rear portion of the substrate  100 . Accordingly, the back surface field regions  114  prevent or reduce the recombination and/or the disappearance of the separated electrons and holes in the rear surface of the substrate  100 . 
     The plurality of back surface field regions  114  may be formed by irradiating laser beams to drive the impurities contained in the dopant layer  110  into the substrate  100 . 
     Thereby, the shapes of the plurality of back surface field regions  114  are apparent in detail in  FIG. 1 , but each of the back surface field regions  114  may be a semicircle, a circular cone, a polygonal cone and a pyramid, etc., in the substrate  100 . 
     In the embodiment, the contact holes  116  may be a plurality of openings formed at the passivation layer  108  and exposing portions of the substrate  100 . 
     In this case, portions of the dopant layer  110  are in contact with the substrate  100  exposed through the contact holes (i.e., openings)  116 . 
     Alternatively, the contact holes  116  may be melted portions generated by the laser beams irradiated for forming the back surface field regions  114 . That is, when radiating the laser beams, portions on which the laser beams are irradiated are heated and thereby are melted. Thereby, the melted portions in the passivation layer  108  are formed as the contact holes  116 , and the impurities contained in the dopant layer  110  are driven into the substrate  100  to thereby form the back surface field regions  114 . 
     In the processes, a mixture of the dopant layer  110  and the passivation layer  108  may be formed (or filled) at each contact hole  116 . Accordingly, since the dopant layer  110  contains impurities of the same type as the semiconductor substrate  100 , at each contact hole, there are mixed portions containing at least a material of the passivation layer  108  and impurities of the same type as the semiconductor substrate  100 . Material of the rear electrode  112  may also be included, as well as the material of the semiconductor substrate  100 . 
     Additionally, in an embodiment of the present invention, when the laser beans are irradiated to form the contact holes  116  and/or the BSF regions  114  (and at the BSF regions  114 ), the underlying layers, such as the dopant layer  110 , the passivation layer  108 , the rear electrode layer  120  and/or portions of the semiconductor substrate  100 , are melted as the contact holes  116  and/or the BSF regions  114  are formed, so that the contact holes  116  and/or the BSF regions  114  may be formed in a form of a crater or a crater-like structure, and the materials of the underlying layers may be moved to peripheral edges of the contact holes  116 . Thus, the mixture of the underlying layers may be formed at a crater edge position of the contact holes  116 . Thus, the BSF regions  114  may be formed at crater edge positions of the contact holes  116 , in addition or alternately to the positions shown in  FIG. 1 , for example. 
     Additionally, in an embodiment of the present invention, when the laser beans are irradiated to form the contact holes  116  and/or the BSF regions  114 , the contact holes  116  may be formed deeply to extend into semiconductor substrate  100 . In such an embodiment, various structures, such as the BSF regions  114 , for example, may become a buried structure in the semiconductor substrate  100 . 
     The plurality of front electrodes  106  are connected to the emitter region  102  through the anti-reflection layer  104 . The plurality of front electrodes  106  are spaced apart from each other and extend in a predetermined direction. 
     The front electrodes  106  collect charges, for example, electrons, moving toward the emitter region  102 . 
     The front electrodes  106  are preferably made of at least one conductive metal material. Examples of the conductive metal material may be at least one 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 metal materials may be used. 
     The rear electrode  112  is substantially positioned on the entire rear surface of the substrate  100  and is electrically connected to the substrate  100 . The rear electrode  112  collects charges, for example, holes, moving toward the substrate  100 . 
     The rear electrode  112  preferably contains Al, but may contain other conductive material. Examples of the conductive material may be at least one selected from the group consisting of Ni, Cu, Ag, Sn, Zn, In, Ti, Au, and a combination thereof. Other conductive metal materials may be used. 
     In the embodiment of the present invention, since the dopant layer  110  of a p-type semiconductor may contain boron (B), each back surface field region  114  may be back surface field region (B—BSF) containing boron (B) and the mixture may be a mixture (B+Al) of boron (B) and aluminum (Al). 
     The solar cell  1  may further include at least one bus bar for the front electrodes  106 . The bus bar is connected to the emitter region  102  and extends in a direction intersecting the front electrodes  106 . The bus bar collects the charges collected by the front electrodes  106  and outputs the collected charges to an external device. 
     An operation of the solar cell  1  of the structure will be described in detail. 
     When light irradiated to the solar cell  1  is incident on the substrate  100  of the semiconductor through the anti-reflection layer  104  and the emitter region  102 , a plurality of electron-hole pairs are generated in the substrate  100  by light energy based on the incident light. 
     Further, because both a light incident operation and a light reflection operation are performed on the textured surface, a light absorptance increases, and thereby the efficiency of the solar cell  1  is improved. 
     In addition, since a reflection loss of light incident onto the substrate  100  is reduced by the anti-reflection layer  104 , an amount of the incident light on the substrate  100  increases. 
     The electron-hole pairs are separated by the p-n junction of the substrate  100  and the emitter region  102 , and the separated electrons move toward the emitter region  102  of the n-type and the separated holes move toward the substrate  100  of the p-type. The electrons that move toward the emitter region  102  are collected by the front electrodes  106  in contact with the emitter portions  102 , while the holes that move toward the substrate  100  move to the dopant layer  110  through the contact holes  116  and are collected by the rear electrode  112  connected to the dopant layer  110 . When the front electrodes  106  and the rear electrode  112  are connected with electric wires, current flows therein to thereby enable use of the current for electric power. 
     At this time, due to an effect by the passivation function of the passivation layer  108 , an amount of charges disappearing near the surface of the substrate  100  is decreased, and due to the plurality of back surface field regions  114 , the recombination of the electrons and holes is reduced, to thereby improve the efficiency of the solar cell  1 . 
     Next, referring to  FIGS. 2A to 2H , a method for manufacturing the solar cell  1  of the above-structure is described. 
       FIGS. 2A to 2H  are sectional views sequentially showing processes for manufacturing the solar cell shown in  FIG. 1 . 
     In a comparative example for manufacturing the solar cell, when forming a plurality of back surface field regions locally and partially dispersed at a rear surface of a crystalline silicon substrate, the back surface field regions were formed through processes removing portions of a passivation layer positioned on the rear surface of the substrate to expose portions of the substrate, diffusing impurities into the exposed substrate through the portions, at which the passivation layer is removed, to form the back surface field regions, and forming a rear electrode electrically connected to the back surface field regions. 
     As shown in  FIG. 2A , a front surface of a p-type semiconductor substrate  100  doped by impurities of a p-type is textured to form a textured surface which is an uneven surface. 
     The textured surface may be formed by a wet etching method, a dry etching method such as a RIE (reaction ion etching) method, or a laser beam irradiation process, etc. 
     In an alternative embodiment, a rear of the substrate  100  may be planarized or textured to increase an amount of light incident onto the substrate  100 . At this time, the rear surface of the substrate  100  may be treated by the wet etching method or the dry etching method. 
     As shown in  FIG. 2B , an emitter region  102  is formed by doping impurities of an n-type into the p-type semiconductor substrate  100 . At this time, a high temperature thermal process may be performed on the substrate  100  in an environment containing a material (for example, PH 3  or POCl 3 ) including an impurity of a group V element such as P, As, and Sb, to diffuse the impurity of the group V element into the substrate  100  and to thereby form an emitter layer  102  on the entire surface of the substrate  100 . 
     Next, referring to  FIG. 2C , a portion of the rear surface of the substrate  100  is removed by the wet etching method, or the dry etching method, etc., to remove the emitter region  102  formed in (at) the rear surface of the substrate  100 . 
     Next, referring to  FIG. 2D , an anti-reflection layer  104  is formed on the emitter region  102  positioned at the front surface of the substrate  100 , and a passivation layer  108  is formed on the rear surface of the substrate  100 . 
     At this time, the passivation layer  108  may be formed by a thermal oxide such as silicon oxide (SiO 2 ) generated by a RTO (rapid thermal oxidation) process, which is performed in a furnace for a RTP (rapid thermal process). Further, the passivation layer  108  may be formed by a sputtering method using silicon oxide (SiO 2 ) as a target or may be formed by CVD (chemical vapor deposition) method. The passivation layer  108  may be made of silicon oxide (SiOx), silicon nitride (SiNx), or silicon oxy nitride (SiOxNy). 
     Next, as shown in  FIG. 2E , a dopant layer  110  is formed on the passivation layer  108 . In this embodiment, since boron (B) is used as impurities for the p-type, the dopant layer  110  is a boron layer containing boron (B). However, the dopant layer  110  may be formed by using other materials. 
     In the embodiment, the dopant layer  110  may formed by a film formation method such as a direct printing method, a spray-doping method, a spin-on doping method, or a past doping method using a screen printing method and by a thermal process at a low temperature. 
     Referring to  FIG. 2F , the dopant layer  110  is laser-patterned by laser beams irradiated on portions of the rear surface of the substrate  100  to form a laser pattern at the dopant layer  110 . At this time, surface portions (that is, the laser pattern) of the dopant layer  110  on which the laser beams are irradiated, are depressed. 
     As described above, when the laser beams are irradiated on the portions of the dopant layer  110 , the portions of the dopant layer  110  are heated by the laser beams. Thereby, the portions of the dopant layer  110  and the portions of the underlying passivation layer  108  are melted, so that a plurality of contact holes  116  are formed in the passivation layer  108 . 
     That is, materials of the dopant layer  110  and the passivation layer  108  are mixed, and thereby the contact holes  116  at which the portions of the dopant layer  110  are electrically connected to the substrate  100  are formed. At this time, a mixture by the mixing of materials of the dopant layer  110  and the passivation layer  108  may be generated (and/or filled) at the contact holes  116 . 
     In addition, when the laser beams are irradiated, the p-type impurities existing in the dopant layer  110  are driven into the substrate  100  through the contact holes  116 . Thereby, a plurality of back surface field regions  114  are formed at portions of the substrate  100  at which are in contact with the contact holes  116 . That is, the plurality of back surface field regions  114  is partially or selectively formed at the substrate  100 . Each of the back surface field regions  114  has an impurity concentration heavily doped than that of the substrate  100 . 
     However, in an alternative embodiment, when the passivation layer  108  includes a plurality of openings at positions corresponding to the formation positions of the back surface field regions  114 , after the formation of the passivation layer  108 , the opening are formed at the corresponding positions of the passivation layer  108  by using laser beams, a photolithography, or an etching paste, etc., a dopant layer  110  is formed on the passivation layer and the substrate  100  exposed through the openings, and then, as described in reference with  FIG. 2F , by irradiating laser beams on portions of the dopant layer  110  to drive the impurities of the dopant layer  110  into the substrate  100 , a plurality of back surface field regions  114  are formed at the substrate  100 . At this time, the irradiation positions of the laser beams may be almost consistent with the positions of the openings formed in the passivation layer  108 . 
     The irradiation conditions (the irradiation characteristics) of the laser beams are specially restricted, but it is preferred that the laser beams are irradiated with energy of conditions which not change the characteristics of the substrate  100  within a very short time. 
     For example, a pulse width of the laser beams may be about 10 femto seconds to about 50 nano seconds. Thereby, when the pulse width of the laser beams is outputted by the femto or nano second basis, the irradiation time of the laser beams is very short and thereby thermal damage of the dopant layer  108  and/or the substrate  100  is prevented or decreased. 
     For driving impurities of a p-type into the substrate to form the back surface field regions at the solar cell, in an comparative example, after patterning an impurity film on the rear surface of the substrate by using a liquid source containing the impurities such as boron (B), a thermal process was performed in the substrate at a high temperature of about 900° C. to 1050° C., to drive impurities into the substrate. Thereby, due to the high temperature thermal for the impurity diffusion, the substrate was deteriorated or had a deterioration possibility. 
     However, in case of the embodiment, the dopant layer  110  is formed as one film by using the film formation process performed at a low temperature, and then the impurities of the dopant layer  110  are doped into the corresponding positions of the substrate  100  by partially or selectively irradiating the laser beams, to form the back surface field regions  114 . Thus, it is unnecessary to perform the high temperature thermal process for the entire substrate for driving the impurities into the substrate. Accordingly, the deterioration of the substrate  100  due to the high temperature thermal process is prevented or reduced. 
     Next, as shown in  FIG. 2Q  a front electrode paste  1060  is printed (or provided) and dried on the anti-reflection layer  104 , and, as shown in  FIG. 2H , a rear electrode paste  1120  is printed (or provided) and dried on the dopant layer  110 . 
     The front electrode paste  1060  is an Ag paste including Ag and the rear electrode paste  1120  is an Al paste including Al or an Al—Ag paste including Al and Ag, though not required, but it is not limited to the pastes  1060  and  1120 . 
     The printing order of the front and rear electrode patterns  1060  and  1120  may be changed, and the pastes  1060  and  1120  may be printed by a screen printing method, etc. 
     Next, a thermal process is performed on the substrate  100  with the front and rear electrode patterns  1060  and  1120  to form a plurality of front electrodes  106  contacting with the emitter region  102  by penetrating the anti-reflection layer  104 , a rear electrode  112  partially or selectively contacting with the substrate  100  at the back surface field regions  114 . At this time, by the thermal process, the front electrodes  106  and the rear electrode  112  are chemically coupled with other layers contacting therewith, to decrease contact resistances. Thereby, the charge movement between the electrodes  106  and  112  and the emitter region  102  and the substrate  100  is improved. 
     Next, an edge isolation process is performed to remove the emitter region  102  positioned on sides of the substrate  100 . Accordingly, a solar cell  1  is completed shown in  FIG. 1 . 
     The performing time of the edge isolation process may be changed. 
     At this time, in an alternative example, the solar cell  1  shown in  FIG. 1  and manufactured by the processes of  FIGS. 2 a    to  2 H has the structure shown in  FIG. 3 . 
     As shown in  FIG. 3 , the solar cell  1  does not include a plurality of depressed portions which are depressed toward the substrate  100 . 
     That is, as already described referring to the  FIG. 2F , in forming the laser pattern by partially irradiating the laser beams on the dopant layer  110 , depressed portions generated by the laser beam irradiation are changed into flat portions since a depressed amount of the dopant layer  110  is small or the depressed portions are planarized during the forming the rear electrode  112  to be flatted. Thereby, a surface of the dopant layer  110  is flatted, to have the flat surface shown in  FIG. 3 . 
     Next, referring to  FIG. 4 , another example of the solar cell according to the embodiment of the present invention will be described. 
     As compared with  FIG. 1 , structural elements having the same functions and structures as those illustrated in  FIG. 1  are designated by the same reference numerals, and a further description may be briefly made or may be entirely omitted. 
     A solar cell  1   a  shown in  FIG. 4  has the similar structure to that of the solar cell  1  of  FIG. 1 . 
     That is, the solar cell  1   a  includes a substrate  100 , an emitter region  102  positioned forming a p-n junction with the substrate  100 , an anti-reflection layer  104  on the emitter region  102 , a plurality of front electrodes  106  connected to the emitter region  102 , a passivation layer  108  positioned on a rear surface of the substrate  100 , a dopant layer  110   a  positioned on the passivation layer  108  and connected to the substrate  100  through a plurality of contact holes  116 , a rear electrode  112   a  positioned on the dopant layer  110   a , and a plurality of back surface field regions  114  positioned between the substrate  100  and the dopant layer  110   a.    
     However, unlike the solar cell  1  of  FIG. 1 , in the solar cell  1   a , a formation position of the dopant layer  110   a  on the passivation layer  108  is different from that of the dopant layer  110  of the solar cell  1 . 
     That is, the dopant layer  110   a  includes a plurality of dopant portions  1110  partially or selectively positioned on the passivation layer  108 , instead positioning on the substantially entire passivation layer  108 , and spaced away from each other. At this time, the formation position of each the dopant portion  1110  corresponds to that of each back surface field region  114 . That is, the plurality of dopant portions  1110  may be locally formed. 
     As described above, the contact holes  116  for the connection of the substrate  100  and the dopant layer  110   a  are a plurality of openings formed in the passivation layer  108  or portions made of (or filled with) a mixture mixed with materials of the passivation layer  108  and the dopant layer  110   a.    
     In addition, since the rear electrode  112   a  is positioned only on the dopant layer  110   a , the rear electrode  112   a  also includes a plurality of rear electrode portions  1112 . Each of the rear electrode portions  1112  is positioned only on each dopant portion  1110 , not positioned on the substantially entire passivation layer  108 . That is, the plurality of rear electrode portions  1112  may be locally formed. 
     Except that after partially or selectively forming the dopant layer  110   a  on the passivation layer  108 , the rear electrode  112   a  having the rear electrode portions  1112  is formed only on the dopant layer  110   a , a method for manufacturing the solar cell  1   a  is the same as the method described in reference to  FIGS. 2A to 2H . Thereby, the detailed method for manufacturing the solar cell  1   a  is omitted. 
     As described in reference to  FIG. 3 , in case of the solar cell  1   a  of  FIG. 4 , when forming the laser pattern by irradiating the laser beams on each of the dopant portions  1110  of the dopant layer  110   a , depressed portions generated by the laser beam irradiation are changed into flat portions when a depressed amount of the dopant portions  1110  is small or the depressed portions are planarized during the forming the rear electrode  1112  to be flatted. Thereby, surfaces of the dopant portions  1110  are flatted, to have the flat surface shown in  FIG. 5 . 
     As described above, since it is unnecessary to perform the high temperature thermal process for forming the back surface field regions  114 , the deterioration of the substrate  100  due to the high temperature thermal process is prevented or reduced. In addition, a formation area of the dopant layer  110   a  and the rear electrode  112   a  is reduced, to decrease a manufacturing cost of the solar cell  1   a.    
     Next, referring to  FIG. 6 , another example of a solar cell according an embodiment of the present invention will be described. 
     As compared to  FIG. 4 , except that a rear electrode  112  is positioned on the substantially entire rear surface of the substrate  100 , a solar cell  1   b  shown in  FIG. 6  has the same structure as the solar cell  1   a  of  FIG. 4 . That is, the rear electrode  112  of the solar cell  1   b  is positioned on a plurality of dopant portions  1110  and exposed portions of the passivation layer  108 . Thereby, since an area of the rear electrode  112  contacting with the dopant portions  1110  increase, a transmission efficiency of the charges is improved. 
     As described, each of the dopant portions  1110  may have a flat surface instead of a depressed surface even though the laser beams are irradiated on surfaces of the dopant portions  1110 . 
     Except that after partially or selectively forming the dopant layer  110   a  on the passivation layer  108 , the rear electrode  112  is formed on the entire rear surface of the substrate  100 , a method for manufacturing the solar cell  1   b  is the same as the method described in reference to  FIGS. 2A to 2H . Thereby, the detailed method for manufacturing the solar cell  1   b  is omitted. 
     Next, referring to  FIGS. 7 to 10 , solar cells according to another embodiment of the present invention will be described. 
     As compared with  FIGS. 1 to 6 , structural elements having the same functions and structures as those illustrated in  FIGS. 1 to 6  are designated by the same reference numerals, and a further description may be briefly made or may be entirely omitted. 
       FIG. 7  is a partial cross-sectional view of an example of a solar cell according to another embodiment of the present invention and  FIGS. 8A and 8B  are sectional views showing portions of processes for manufacturing the solar cell shown in  FIG. 7 .  FIGS. 9  and  10  are partial cross-sectional views of other examples of a solar cell according to another embodiment of the present invention. 
     Similar to the solar cell  1  of  FIG. 1 , a solar cell  11  shown in  FIG. 7  includes a substrate  100 , an emitter region  102  positioned forming a p-n junction with the substrate  100 , an anti-reflection layer  104  on the emitter region  102 , a plurality of front electrodes  106  connected to the emitter region  102 , a passivation layer  108  positioned on a rear surface of the substrate  100 , a dopant layer  110  positioned on the entire surface of passivation layer  108  and connected to the substrate  100  through a plurality of contact holes  116   c , a rear electrode  112   c  positioned on the entire surface of the dopant layer  110 , and a plurality of back surface field regions  114  positioned between the substrate  100  and the dopant layer  110 . 
     However, in the solar cell  11 , the rear electrode  112   c  and the underlying dopant layer  110  have a plurality of depressed portions, and positions of the depressed portions correspond to portions of the back surface field regions  114 . Thereby, a surface of the rear electrodes  112   c  that faces the outside has flat portions and depressed portions. 
     At this time, each contact hole  116   c  may be made of (and/or filled with) a mixture mixed with materials of the dopant layer  110  and the rear electrode  112   c  or a mixture mixed with materials of the passivation layer  108  as well as the dopant layer  110  and the rear electrode  112   c . In the former case, the passivation layer  108  includes a plurality of openings corresponding to formation positions of the back surface field regions  114 , while in the latter case, the passivation layer does not include the openings, but the contact holes are formed by the laser beam irradiation, etc. 
     Thus, the solar cell  11  includes portions positioned in order of the back surface field region  114 , the mixture  116   c , and the rear electrode  112 , and portions positioned in order of the passivation layer  108 , the dopant layer  110 , and the rear electrode  112  from the rear surface of the substrate  100  to the outside. 
     Also, the solar cell  11  may further include portions melted and mixed with a dopant of the dopant layer  110  and silicon (Si) of the substrate  100  at interfaces between the substrate  100  and the back surface field regions  114 . 
     A method for manufacturing the solar cell  11  will be described in reference to  FIGS. 8A and 8B  as well as  FIGS. 2A to 2E and 2G to 2H . 
       FIGS. 8A and 8B  are sectional views showing portions of processes for manufacturing the solar cell shown in  FIG. 7 . 
     As above described referring to  FIGS. 2A to 2E , after texturing of a surface of the substrate  110  to form a textured surface, an emitter region  102  and an anti-reflection layer  104  are sequentially formed on a front surface of the substrate, and a passivation layer  108  and a dopant layer  110  are sequentially formed on a rear surface of the substrate  100 . 
     Next, as described referring to  FIGS. 2G and 2H , a front electrode paste  1060  is partially printed and dried on the anti-reflection layer  104  and a rear electrode paste  1120  is printed and dried on the almost entire surface of the dopant layer  110  ( FIG. 8A ). 
     As shown in  FIG. 8B , a plurality of contact holes  116   c  and a plurality of back surface field regions  114  are formed by irradiating laser beams on portions of the rear electrode paste  1120 . At this time, the contact holes  116   c  and the back surface field regions  114  are formed at portions at which a laser pattern is formed by the laser beam irradiation. Thereby, the portions of the rear electrode paste  1120  on which the laser beams are irradiated have depressed portions, respectively, and portions of the dopant layer  110  underlying the portions of the rear electrode paste  1120  also have depressed portions, respectively. 
     However, as already described, when forming the laser pattern by irradiating the laser beams on the portions of the rear electrode paste  1120 , the depressed portions generated by the laser beam irradiation may be changed into flat portions when a depressed amount of the dopant layer  110  is small or the depressed portions is planarized during the forming the rear electrode  112  to be flatted. Thereby, unlike  FIG. 7 , the rear electrode  112   c  may include the depressed portions, while the dopant layer  110  need not include the depressed portions. 
     Thus, when the laser beams are irradiated on the portions of the rear electrode paste  1120 , heat due to the laser beams is applied to the portions of the rear electrode paste  1120 , and then the portions of the dopant layer  110  and the passivation layer  108  which are positioned under the portions of the rear electrode paste  1120  are melted. Thus, materials of the rear electrode paste  1120 , the dopant layer  110  and the passivation layer  108  are mixed to form a plurality of contact holes  116   c , at which the substrate  100  and the portions of the dopant layer  110  are electrically connected. At this time, each of the contact holes  116   c  is made of (or filled with) a mixture mixed with the materials of the rear electrode paste  1120 , the dopant layer  110 , and the passivation layer  108 . 
     In addition, by the laser beam irradiation, impurities of a p-type that are contained in the dopant layer  110  are driven into the substrate  100  through the contact holes  116   c . Thereby, the back surface field regions  114  are formed at portions of the plurality of contact holes  116   c  contacting with the substrate  100 . The back surface field regions  114  have a concentration higher than that of the substrate  100 . 
     As described above, irradiation positions of the laser beams correspond to the formation positions of the back surface field regions  114 . 
     The irradiation conditions (the irradiation characteristics) of the laser beams are specially restricted. 
     In an alternative embodiment, when the passivation layer  108  includes a plurality of openings at positions corresponding to formation positions of the back surface field regions  114 , after the formation of the passivation layer  108 , the opening are formed at the corresponding positions of the passivation layer  108 , a dopant layer  110  and a rear electrode paste  1120  are formed on the passivation and the substrate  100  exposed through the openings of the passivation layer  108 , and then by irradiating laser beams on portions of the rear electrode paste  1120  to drive the impurities of the dopant layer  110  into the substrate  100 , the plurality of back surface field regions  114  are formed at the substrate  100 . At this time, the irradiation positions of the laser beams may be almost consistent with the position of the openings formed in the passivation layer  108 . In addition, since the dopant layer  110  is connected to the substrate  100  through the openings, each opening functions as a contact hole and each contact hole (each opening) is mainly filled with a mixture mixed with materials of the rear electrode paste  1120  and the dopant layer  110 . 
     Then, as described, when a thermal process is performed on the substrate  100  with the front electrode paste  1060  and the rear electrode paste  1120 , the front electrode paste  1060  penetrates the anti-reflection layer  104  to form a plurality of front electrodes  106  contacting with the emitter region  102  and the rear electrode paste  1120  is formed as a rear electrode  112   c  electrically connected to the substrate  100 . Thereby, the solar cell  11  is completed ( FIG. 7 ). 
     For forming the back surface field regions  114 , since, instead of the processes of the pattering the liquid source containing dopants (impurities) of a desired conductive type on the rear surface of the substrate and the diffusing the dopants into the substrate by the high temperate thermal process, the heating for the back surface field regions  114  is partially performed by radiating the laser beams only on desired portions, the deterioration of the substrate  100  due to the thermal process is prevented or reduced. 
     Referring to  FIG. 9 , another example of another embodiment of the present invention will be described. 
       FIG. 9  is a partial cross-sectional view of another example of a solar cell according to another embodiment of the present invention. 
     As compared to  FIG. 7 , in a solar cell  11   a  of  FIG. 9 , instead of the dopant layer  110  that is positioned on the entire rear surface of the substrate  100 , a dopant layer  110   c  includes a plurality of dopant portions  1111  and the dopant portions  1111  are positioned at positions where a plurality of back surface field regions  114  are positioned. A rear electrode  112   d  also includes a plurality of rear electrode portions  1122  where the back surface field portions  1122  are positioned, respectively. Thereby, in the example, the number of dopant portions  1111  and rear electrode portions  1122  is plural. 
     Similar to  FIG. 7 , in the rear electrode  112   d  and the dopant layer  110   c , surfaces toward the outside include a plurality of depressed portions, respectively, and the dopant layer  110   c  is in contact with the substrate  100  through the contact holes  116   c . As described, each contact hole  116   c  may be made of (and/or filled with) a mixture mixed with materials of the dopant layer  110   c  and the rear electrode  112   d  or a mixture mixed with materials of the passivation layer  108  as well as the dopant layer  110   c  and the rear electrode  112   d.    
     Except for the above-description, the structure of the solar cell  11   a  is the same as that of the solar cell  11  of  FIG. 4 , and thereby the detailed description of the same elements is omitted. 
     Except that after partially or selectively forming the dopant layer  110   c  on the passivation layer  108 , the rear electrode  112   c  is formed only on the dopant layer  110   c , a method for manufacturing the solar cell  11   a  is the same as the method described in reference to  FIGS. 2A to 2H  and  FIGS. 8A and 8B . Thereby, the detailed method for manufacturing the solar cell  11   a  is omitted. 
     As described, each of the dopant portions  1111  of the dopant layer  110   c  has a flat surface without the depressed portion. 
     The solar cell  11   a , since it is unnecessary to perform the high temperature thermal process for forming the back surface field regions  114 , the deterioration of the substrate  100  is prevented or reduced. In addition, a formation area of the dopant layer  110   c  and the rear electrode  112   c  is reduced, to decrease a manufacturing cost of the solar cell  11   a.    
     Next, further another example of the solar cell according to another embodiment of the present invention will be described in reference to  FIG. 10 . 
     As compared to  FIG. 6 , a solar cell  11   b  of  FIG. 10  has the same structure as that of the solar cell  11   b , except that a rear electrode  112   c  includes a plurality of depressed portions. 
     In difference from  FIG. 6 , for manufacturing the solar cell  11   b , as described referring to  FIGS. 8A and 8B , a paste for the rear electrode  112   c  is printed and dried on a rear surface of a substrate  100 , and then laser beams are irradiated on the rear surface of the substrate  100  to form a plurality of contact holes  116   c  and a plurality of back surface field regions  114 . The remaining processes for manufacturing the solar cell  11   b  are equal to the description described in detail referring to  FIG. 6 . 
     While this invention has been described in connection with what is presently considered to be practical example embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.