Patent Publication Number: US-2013244373-A1

Title: Solar cell apparatus and method of fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/381,146, filed Dec. 28, 2011, which is the U.S. national stage application of International Patent Application No. PCT/KR2010/004226, filed Jun. 30, 2010, which claims priority to Korean Patent Application Nos. 10-2009-0059502, filed Jun. 30, 2009, and 10-2009-0059513, filed Jun. 30, 2009, which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments relate to a solar cell apparatus and a method of fabricating the solar cell apparatus. 
     BACKGROUND ART 
     Recent increasing consumption of energy has facilitated development of solar cells capable of converting solar energy into electric energy. 
     Particularly, CuInGaSe (CIGS) solar cells are common, which are pn hetero junction devices having a substrate structure constituted by a glass substrate, a metal back electrode layer, a p-type CIGS light absorption layer, a high resistance buffer layer, and an n-type window layer. 
     In this case, the light absorption layer is patterned to connect electrodes to one another, and the buffer layer and the light absorption layer are patterned to form unit cells. 
     To this end, a vacuum state should be changed to an atmospheric state. In addition, each layer may be ripped off through a mechanical patterning process, or layers adjacent to a pattern may be burred through a laser patterning process, which degrade electrical characteristics of a solar cell. 
     DISCLOSURE OF THE INVENTION 
     Technical Problem 
     Embodiments provide a solar cell apparatus and a method of fabricating the solar cell apparatus, which reduces process time and inhibits deformation of each layer. 
     Technical Solution 
     In one embodiment, a solar cell apparatus includes: a substrate; a back electrode layer on the substrate; a light absorption layer on the back electrode layer; and a front electrode layer on the light absorption layer, wherein the back electrode layer is provided with recesses, and inner surfaces of the back electrode layer defining the recesses are inclined from a top surface of the substrate. 
     In another embodiment, a solar cell apparatus includes: a substrate; a back electrode layer on the substrate; a light absorption layer on the back electrode layer; and a front electrode layer on the light absorption layer, wherein the light absorption layer is provided with recesses, and inner surfaces of the light absorption layer defining the recesses are inclined from a top surface of the substrate. 
     In another embodiment, a method of fabricating a solar cell apparatus includes: forming a back electrode layer on a substrate; forming a light absorption layer on the back electrode layer; and forming a front electrode layer on the light absorption layer, wherein, in the forming of the back electrode layer, the light absorption layer, or the front electrode layer, a mask is disposed on the substrate, and then, a raw material used to form the back electrode layer, the light absorption layer, or the front electrode layer is deposited. 
     Advantageous Effects 
     According to the embodiment, the back electrode layer, the light absorption layer, and the front electrode layer are formed by performing deposition and patterning processes at the same time by means of the mask. Thus, an addition process such as laser patterning or mechanical scribing is unnecessary. 
     Thus, the back electrode layer, the light absorption layer, and the front electrode layer can be formed all in a vacuum, and the process time can be reduced. 
     In addition, when the back electrode layer, the light absorption layer, and the front electrode layer are formed, the mask is used in the patterning process, and thus, the back electrode layer, the light absorption layer, and the front electrode layer can be protected from a mechanical shock. That is, ripping of the back electrode layer, the light absorption layer, and the front electrode layer due to a process such as mechanical scribing can be inhibited. 
     In addition, since the patterning process can be more precisely performed than a mechanical patterning process, the non-active area NAA can be decreased, thereby improving the efficiency of the solar cell apparatus. 
     In addition, since the inner surfaces of the light absorption layer defining the recesses are inclined, the material used to form the front electrode layer can be efficiently deposited on the inner surfaces, thereby forming contact patterns. 
     In addition, the inner surfaces of the light absorption layer defining the recesses may be connected to the top surface of the light absorption layer through a round portion. Thus, breakage between the contact pattern and the front electrode layer can be inhibited. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 to 9  are cross-sectional views illustrating a method of fabricating a solar cell apparatus according to a first embodiment. 
         FIG. 2  is a plan view illustrating a mask for forming a back electrode layer. 
         FIG. 3  is a plan view illustrating a back electrode layer formed using the mask of  FIG. 2 . 
         FIGS. 10 to 17  are cross-sectional views illustrating a method of fabricating a solar cell according to a second embodiment. 
         FIG. 13  is a plan view illustrating a first mask according to the second embodiment. 
         FIG. 17  is a plan view illustrating a second mask according to the second embodiment. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     In the description of embodiments, it will be understood that when a substrate, layer, film, or electrode is referred to as being ‘on’ or ‘under’ another substrate, layer, film, or electrode, the terminology of ‘on’ and ‘under’ includes both the meanings of ‘directly’ and ‘indirectly’. Further, the reference about ‘on’ and ‘under’ each component will be made on the basis of drawings. In addition, the sizes of elements and the relative sizes between elements may be exaggerated for further understanding of the present disclosure. 
       FIGS. 1 and 4  to  9  are cross-sectional views illustrating a method of fabricating a solar cell apparatus according to a first embodiment.  FIG. 2  is a plan view illustrating a mask for forming a back electrode layer.  FIG. 3  is a plan view illustrating a back electrode layer formed using the mask of  FIG. 2 . 
     Referring to  FIGS. 1 and 3 , a substrate  100  is prepared, and a back electrode layer  200  is formed on the substrate  100 . 
     The substrate  100  is formed of glass, or may be formed of ceramic such as alumina, stainless steel, titanium, or polymer. Glass used to form the substrate  100  may be sodalime glass, and polymer used to form the substrate  100  may be polyimide. The substrate  100  may be rigid or flexible. 
     A mask  10  is placed on the substrate  100  to form the back electrode layer  200 . 
     The mask  10  is used to selectively deposit a material on the substrate  100 , thereby forming the back electrode layer  200 . Referring to  FIG. 2 , the mask  10  may include a tetragonal frame  11  and a plurality of metal wires  12  fixed to the tetragonal frame  11 . The metal wires  12  extend in a first direction, and are parallel to one another. Both ends of the metal wires  12  are fixed to the tetragonal frame  11 , and the metal wires  12  are strained. 
     The mask  10  includes a plurality of transmitting areas TA and a plurality of blocking areas BA. The transmitting areas TA are disposed between the metal wires  12 , and the blocking areas BA correspond to the metal wires  12 . 
     That is, a material is deposited on the substrate  100  through the transmitting areas TA of the mask  10  to form the back electrode layer  200 . The blocking areas BA of the mask  10  block the material used to form the back electrode layer  200 . 
     For example, the material used to form the back electrode layer  200  may be a conductor such as a metal. For example, the back electrode layer  200  may be formed through a sputtering process using a molybdenum (Mo) target. 
     Mo has high electrical conductivity, an ohmic contact with a light absorption layer, and high temperature stability in Se atmosphere. 
     The mask  10  is open only in regions corresponding to the back electrode layer  200 . That is, the mask  10  may cover only regions dividing the back electrode layer  200 . 
     Thus, first through recesses  201  are formed in the back electrode layer  200  to correspond to the metal wires  12 . The first through recesses  201  expose the top surface of the substrate  100 . The first through recesses  201  extend in the first direction. The first through recesses  201  divide the back electrode layer  200  into back electrodes  210 . 
     Inner surfaces of the back electrode layer  200  defining the first through recesses  201  are inclined from the top surface of the substrate  100 . An upper width of the first through recesses  201  is greater than a bottom width of the first through recesses  201 . The bottom width of the first through recesses  201  may range from about 50 μm to about 100 μm. 
     The back electrodes  210  have inclined side walls. For example, the back electrodes  210  may have a trapezoidal cross-section. That is, the back electrodes  210  may have an upper width W 1  that is smaller than a lower width W 2  contacting the substrate  100 . 
     An inclination angle of the side walls of the back electrodes  210  may be controlled according to an installation height of the mask  10 . A method of controlling the inclination angle of the side walls of the back electrodes  210  is illustrated in  FIGS. 4 and 5 . 
     Referring to  FIG. 4 , the mask  10  is spaced apart from the substrate  100  by a first distance D 1 , and then, a deposition process is performed with a target  50 . Referring to  FIG. 5 , the mask  10  is spaced apart from the substrate  100  by a second distance D 2 , and then, a deposition process is performed with the target  50 . 
     Since the second distance D 2  is greater than the first distance D 1 , a second angle θ 2  formed using the second distance D 2  is greater than a first angle θ 1  formed using the first distance D 1 . 
     That is, due to shadow effect, the inclination angle of back electrodes  212  formed using the second distance D 2  is greater than the inclination angle of back electrodes  211  formed using the first distance D 1 . 
     Thus, an inclination angle θ between the substrate  100  and the inner surfaces defining the first through recesses  201  can be controlled by controlling the distance between the mask  10  and the substrate  100 . Accordingly, as a distance D between the mask  10  and the substrate  100  increases, the inclination angle θ increases. 
     The angle between the top surface of the substrate  100  and the inner surfaces defining the first through recesses  201 , that is, the inclination angle θ may range from about 91° μm to about 170°. In more detail, the inclination angle θ is an angle between the top surface of the substrate  100  and a substantial extending direction of the inner surfaces defining the first through recesses  201 , and may range from about 100° to about 120°. 
     As described above, the first through recesses  201  expose the top surface of the substrate  100 , and separates the back electrodes  210  from one another. Thus, a non-vacuum type patterning process in which the back electrodes  210  are separated to form the back electrode layer  200  is unnecessary. Accordingly, a stand-by time for performing a non-vacuum type patterning process such as a laser scribing process for separating the back electrodes  210  is unnecessary. 
     In addition, since a light absorption layer forming process can be directly performed in a vacuum, a process time can be reduced. In addition, since a pattern is formed without using laser, burring of the back electrodes  210  can be inhibited. That is, the back electrodes  210  are formed without forcibly removing a portion of the back electrode layer  200 . Accordingly, partial burring of the back electrodes  210 , and degradation of the back electrodes  210  due to a patterning process can be inhibited. As a result, adhering characteristics between the back electrode layer  200  and the substrate  100  are improved. 
     The mask  10  may be formed of a metal and a material used to form the back electrode layer  200 . For example, the metal wires  12  may be formed of molybdenum. 
     Although not shown, the back electrode layer  200  may be constituted by at least one layer. When the back electrode layer  200  is constituted by a plurality of layers, the layers may be formed of different materials. 
     The back electrode layer  200  may be provided in a stripe form as illustrated in  FIG. 3 , or in a matrix form, which correspond to the form of solar cells. 
     However, the form of the back electrode layer  200  is not limited thereto. 
     Referring to  FIG. 6 , a light absorption layer  300  and a buffer layer  400  are formed on the back electrode layer  200 . 
     The light absorption layer  300  includes a Ib-IIb-VIb based compound. In more detail, the light absorption layer  300  includes a copper-indium-gallium-selenide based (Cu(In,Ga)Se2; CIGS based) compound. 
     Alternatively, the light absorption layer  300  may include a copper-indium-selenide based (CuInSe2; CIS based) compound, or a copper-gallium-selenide based (CuGaSe2; CGS based) compound. 
     For example, a CIG based metal precursor film may be formed on the back electrode layer  200  with a copper target, an indium target, and a gallium target to form the light absorption layer  300 . 
     Thereafter, the CIG based metal precursor film reacts with selenium (Se) through a selenization process to form a CIGS based light absorption layer as the light absorption layer  300 . 
     During a process of forming the CIG based metal precursor film and the selenization process, alkali components from the substrate  100  are diffused through the back electrode layer  200  into the CIG based metal precursor film and the light absorption layer  300 . The alkali components improve grain sizes and crystallinity of the light absorption layer  300 . 
     Alternatively, the light absorption layer  300  may be formed from copper (Cu) indium (In) gallium (Ga), and selenide (Se) through co-evaporation. 
     The light absorption layer  300  receives and converts light into electrical energy. The light absorption layer  300  generates photoelectron-motive force through photoelectric effect. 
     A material used to form the light absorption layer  300  fills the first through recesses  201 . 
     The buffer layer  400  may be constituted by at least one layer, which may be formed of one of cadmium sulfide (CdS), ITO, ZnO, i-ZnO, and a combination thereof on the substrate  100  with the light absorption layer  300  formed. 
     The buffer layer  400  is an n-type semiconductor layer, and the light absorption layer  300  is a p-type semiconductor layer. Accordingly, the light absorption layer  300  and the buffer layer  400  form a pn junction. 
     The buffer layer  400  is disposed between the light absorption layer  300  and a front electrode to be formed later. 
     That is, since the light absorption layer  300  is significantly different from the front electrode in lattice constant and energy band gap, the buffer layer  400 , which has a band gap between those of the light absorption layer  300  and the front electrode, may be disposed therebetween to improve bonding efficiency. 
     Although a single layer as the buffer layer  400  is formed on the light absorption layer  300 , the buffer layer  400  may be provided in plurality. 
     Then, referring to  FIG. 7 , second through recesses  310 , which pass through the light absorption layer  300  and the buffer layer  400 , are formed. 
     The second through recesses  310  may be formed using a mechanical method to partially expose the back electrode layer  200 . 
     Then, referring to  FIG. 8 , a transparent conductive material is formed on the buffer layer  400  to form a front electrode layer  500  and a plurality of connecting lines  700 . 
     At this point, the transparent conductive material fills the contact patterns  310 , thereby forming the connecting lines  700 . 
     The back electrode layer  200  is electrically connected to the front electrode layer  500  by the connecting lines  700 . 
     The front electrode layer  500  is formed of an aluminum-doped zinc oxide on the substrate  100  through a sputtering process. 
     Since the front electrode layer  500 , which is a window layer forming a pn junction with the light absorption layer  300 , functions as a transparent electrode on the front surface of a solar cell, the front electrode layer  500  is formed of zinc oxide (ZnO) having high light transmissivity and high electrical conductivity. 
     At this point, the zinc oxide may be doped with aluminum to form an electrode having low resistance. 
     A zinc oxide thin film as the front electrode layer  500  may be deposited using a radio frequency (RF) sputtering method with a ZnO target, using a reactive sputtering method with a Zn target, or using a metal organic chemical vapor deposition method. 
     Alternatively, a double structure that an indium tin oxide (ITO) thin film having excellent electro-optical characteristics is deposited on a zinc oxide thin film may be used. 
     Then, referring to  FIG. 9 , isolation patterns  320 , which pass through the light absorption layer  300 , the buffer layer  400 , and the front electrode layer  500 , are formed. 
     The isolation patterns  320  may be formed using a mechanical method to partially expose the back electrode layer  200 . 
     The isolation patterns  320  may divide the buffer layer  400  and the front electrode layer  500 , and isolate the cells from one another. 
     The front electrode layer  500 , the buffer layer  400 , and the light absorption layer  300  may be disposed in a stripe or matrix form by means of the isolation patterns  320 . 
     The form of the isolation patterns  320  is not limited thereto. 
     The isolation patterns  320  define cells C 1  and C 2 , which include the back electrode layer  200 , the light absorption layer  300 , the buffer layer  400 , and the front electrode layer  500 . 
     The cells C 1  and C 2  may be connected to each other through the connecting lines  700 . 
     That is, a portion of the back electrode layer  200  corresponding to a first cell is electrically connected to a portion of the front electrode layer  500  corresponding to a second cell adjacent to the first cell by the connecting line  700 . 
     As shown in  FIG. 9 , the solar cell apparatus according to the current embodiment includes the substrate  100 , the back electrode layer  200 , the light absorption layer  300 , and the front electrode layer  500 . 
     The back electrode layer  210  may be spaced apart from one another on the substrate  100 . The light absorption layer  300  may partially include the contact patterns  310  on the back electrode layer  200  to connect electrodes, and the isolation patterns  320  for forming unit cells. 
     The front electrode layer  500  is disposed on the light absorption layer  300 , and is divided by the isolation patterns  320 . 
     The front electrode layer  500  fills the contact patterns  310  to electrically connect to the back electrode layer  200 , and the side walls of the back electrodes  210  may be inclined. 
     As described above, since all processes for fabricating the solar cell are performed in a vacuum without a stand-by process for patterning the back electrode layer  200 , the process time can be reduced. 
     In addition, the back electrode layer  200  is formed using the mask  10 , to thereby inhibit burring due to a patterning process using laser. In addition, since a patterning process can be more precisely performed, a non-active area NAA can be decreased, thereby improving optical efficiency of the solar cell apparatus. 
     That is, according to the current embodiment, the back electrodes  210  are separated using the mask  10  in a vacuum. Thus, a non-vacuum type process for patterning the back electrode layer  200  is unnecessary. 
     Accordingly, the process time can be reduced, and the back electrode layer  200  can be protected from a mechanical or thermal shock. 
       FIGS. 10 to 12 , and  14  to  16  are cross-sectional views illustrating a method of fabricating a solar cell according to a second embodiment. The current embodiment may refer to the previous embodiment. That is, the previous embodiment may be substantially coupled to the current embodiment except for modified parts. 
     First, referring to  FIG. 10 , a preliminary back electrode layer  202  is formed on the substrate  100 . 
     A substrate  100  is formed of glass, or may be formed of ceramic such as alumina, stainless steel, titanium, or polymer. Glass used to form the substrate  100  may be sodalime glass, and polymer used to form the substrate  100  may be polyimide. The substrate  100  may be rigid or flexible. 
     The preliminary back electrode layer  202  is formed of a conductor such as a metal. For example, the preliminary back electrode layer  202  may be formed through a sputtering process using a molybdenum (Mo) target. Mo has high electrical conductivity, an ohmic contact with a light absorption layer, and high temperature stability in Se atmosphere. Although not shown, the preliminary back electrode layer  202  may be constituted by at least one layer. When the preliminary back electrode layer  202  is constituted by a plurality of layers, the layers may be formed of different materials. 
     Then, referring to  FIG. 11 , the preliminary back electrode layer  202  is patterned to form a back electrode layer  200 . The back electrode layer  200  may expose the substrate  100  through first through recesses  201 . 
     The back electrode layer  200  may be provided in a stripe form or a matrix form, which corresponds to the form of solar cells. However, the form of the back electrode layer  200  is not limited thereto. 
     The back electrode layer  200  may be formed by depositing molybdenum through a mask  10 , like in the previous embodiment. That is, the back electrode layer  200  may be formed through a mask patterning process, without using a laser patterning process. 
     Thereafter, referring to  FIG. 12 , a light absorption layer  300  and a buffer layer  400  are formed on the back electrode layer  200 . 
     To this end, a first mask  20  is disposed on the back electrode layer  200 . The first mask  20  is used to selectively deposit a material on the back electrode layer  200 , thereby forming the light absorption layer  300 . The first mask  20  is used to selectively deposit a material on the back electrode layer  200 , thereby forming the buffer layer  400 . 
     Referring to  FIG. 13 , the first mask  20  may include a first tetragonal frame  21 , a plurality of first metal wires  22  fixed to the first tetragonal frame  21 , and a plurality of second metal wires  23  fixed to the first tetragonal frame  21 . The first and second metal wires  22  and  23  extend in a first direction, and are parallel to one another. Both ends of the first and second metal wires  22  and  23  are fixed to the first tetragonal frame  21 , and the first and second metal wires  22  and  23  are strained. 
     A plurality of the first metal wires  22  and a plurality of the second metal wires  23  are disposed in pairs. That is, the first metal wires  22  are adjacent to the second metal wires  23 , respectively. 
     The first mask  20  includes a plurality of transmitting areas TA and a plurality of blocking areas BA. The transmitting areas TA are disposed between the first and second metal wires  22  and  23 , and the blocking areas BA correspond to the first and second metal wires  22  and  23 . 
     That is, a material is deposited on the back electrode layer  200  through the transmitting areas TA of the first mask  20  to form the light absorption layer  300 . In addition, a material is deposited on the light absorption layer  300  through the transmitting areas TA of the first mask  20  to form the buffer layer  400 . In addition, the blocking areas BA of the first mask  20  block the materials used to form the light absorption layer  300  and the buffer layer  400 . 
     The light absorption layer  300  includes a Ib-IIb-VIb based compound. In more detail, the light absorption layer  300  includes a copper-indium-gallium-selenide based (Cu(In,Ga)Se2; CIGS based) compound. 
     Alternatively, the light absorption layer  300  may include a copper-indium-selenide based (CuInSe2; CIS based) compound, or a copper-gallium-selenide based (CuGaSe2; CGS based) compound. 
     For example, a CIG based metal precursor film may be formed on the back electrode layer  200  with a copper target, an indium target, and a gallium target to form the light absorption layer  300 . 
     Thereafter, the CIG based metal precursor film reacts with selenium (Se) through a selenization process to form a CIGS based light absorption layer as the light absorption layer  300 . 
     During a process of forming the CIG based metal precursor film and the selenization process, alkali components from the substrate  100  are diffused through the back electrode layer  200  into the CIG based metal precursor film and the light absorption layer  300 . The alkali components improve grain sizes and crystallinity of the light absorption layer  300 . 
     Alternatively, the light absorption layer  300  may be formed from copper (Cu) indium (In) gallium (Ga), and selenide (Se) through co-evaporation. 
     The light absorption layer  300  receives and converts light into electrical energy. The light absorption layer  300  generates photoelectron-motive force through photoelectric effect. 
     The buffer layer  400  may be constituted by at least one layer, which may be formed of one of cadmium sulfide (CdS), ITO, ZnO, i-ZnO, and a combination thereof on the substrate  100  with the light absorption layer  300  formed. 
     The buffer layer  400  is an n-type semiconductor layer, and the light absorption layer  300  is a p-type semiconductor layer. Accordingly, the light absorption layer  300  and the buffer layer  400  form a pn junction. 
     The buffer layer  400  is disposed between the light absorption layer  300  and a front electrode layer  500  to be formed later. That is, since the light absorption layer  300  is significantly different from the front electrode layer  500  in lattice constant and energy band gap, the buffer layer  400 , which has a band gap between those of the light absorption layer  300  and the front electrode layer  500 , may be disposed therebetween to improve bonding efficiency. 
     Although a single layer as the buffer layer  400  is formed on the light absorption layer  300 , the buffer layer  400  may be provided in plurality. 
     At this point, the light absorption layer  300  and the buffer layer  400  are formed using the first mask  20 . Accordingly, second through recesses  310  and third through recesses  320  are formed in the light absorption layer  300  and the buffer layer  400 . The second through recesses  310  and the third through recesses  320  pass through the light absorption layer  300  and the buffer layer  400 . 
     That is, a deposition process such as sputtering through the first mask  20  is performed on the substrate  100  with the back electrode layer  200  formed. Accordingly, the second through recesses  310  are formed in areas corresponding to the first metal wires  22 . In addition, the third through recesses  320  are formed in areas corresponding to the second metal wires  23 . 
     The second and third through recesses  310  and  320  expose the back electrode layer  200 . Inner surfaces of the light absorption layer  300  and the buffer layer  400 , which define the second and third through recesses  310  and  320 , are inclined. Accordingly, an upper width W 3  of the second through recesses  310  may be greater than a lower width W 4  thereof. Also, an upper width of the third through recesses  320  may be greater than a lower width thereof. 
     In a same manner as that of the first embodiment, inclination angles of the inner surfaces defining the second and third through recesses  310  and  320  may be controlled through shadow effect according to an installation height of the first mask  20 . 
     A method of controlling the inclination angle of the inner surfaces defining the second through recesses  310  is illustrated in  FIGS. 14 and 15 . 
     Referring to  FIG. 14 , the first mask  20  is spaced apart from the back electrode layer  200  by a third distance D 3 , and then, a deposition process is performed a the target  50 . Referring to  FIG. 15 , the first mask  10  is spaced apart from the back electrode layer  200  by a fourth distance D 4 , and then, a deposition process is performed with the target  50 . 
     Since the fourth distance D 4  is greater than the third distance D 3 , a fourth angle θ 4  formed using the fourth distance D 4  is greater than a third angle θ 3  formed using the third distance D 3 . 
     That is, due to shadow effect, the inclination angle of the inner surfaces defining the second through recesses  310  formed using the fourth distance D 4  is greater than that of the inner surfaces defining the second through recesses  310  formed using the third distance D 3 . 
     Thus, a contact angle θ between the back electrode layer  200  and connecting lines  700  formed in the second through recesses  310  can be controlled by controlling the distance between the first mask  20  and the back electrode layer  200 . Accordingly, as a distance D between the first mask  20  and the back electrode layer  200  increases, the inclination angle θ may increase. 
     The third angle θ 3  and the fourth angle θ 4  are angles between the top surface of the back electrode layer  200  and the inner surface defining the second through recesses  310 . In more detail, the third angle θ 3  and the fourth angle θ 4  are angles between the top surface of the substrate  100  and the inner surface defining the second through recesses  320 . That is, the top surface of the back electrode layer  200  is substantially parallel to the top surface of the substrate  100 . 
     A first inclination angle (also denoted by θ) between the top surface of the back electrode layer  200  and the inner surface defining the second through recesses  320  may range from about 91° to about 170°. In more detail, the first inclination angle θ may range from about 91° to about 120°. 
     Also, a second inclination angle between the top surface of the back electrode layer  200  and the inner surface defining the third through recesses  320  may range from about 91° to about 170°. In more detail, the second inclination angle may range from about 91° to about 120°. 
     Thereafter, referring to  FIG. 16 , a transparent conductive material is formed on the buffer layer  400  to form the front electrode layer  500  and the connecting lines  700 . 
     The front electrode layer  500  may be formed using a second mask  30 . The second mask  30  is used to selectively deposit a material on the buffer layer  400 , thereby forming the front electrode layer  500 . In more detail, a material is deposited through the second mask  30  on the buffer layer  400 , so that the front electrode layer  500  is formed out of areas corresponding to the third through recesses  320 . 
     That is, the second mask  30  may be placed on the substrate  100  provided with the buffer layer  400 , and then, the front electrode layer  500  may be formed through a deposition process such as sputtering. 
     Referring to  FIG. 17 , the second mask  30  may include a second tetragonal frame  31 , and a plurality of third metal wires  32  fixed to the second tetragonal frame  31 . The third metal wires  32  extend in the first direction, and are parallel to one another. Both ends of the third metal wires  32  are fixed to the second tetragonal frame  31 , and the third metal wires  32  are strained. 
     The second mask  30  includes a plurality of transmitting areas TA and a plurality of blocking areas BA. The transmitting areas TA are disposed between the third metal wires  32 , and the blocking areas BA correspond to the third metal wires  32 . 
     That is, a material is deposited on the buffer layer  400  through the transmitting areas TA of the second mask  30  to form the front electrode layer  500 . The blocking areas BA of the second mask  30  block the material used to form the front electrode layer  500 . 
     The third metal wires  32  correspond to the third through recesses  320 , respectively. That is, the second mask  30  is disposed on the buffer layer  400  such that the third metal wires  32  correspond to the third through recesses  320 . 
     Thus, fourth through recesses  510 , which correspond to the third through recesses  320 , are formed in the front electrode layer  500 . The third through recesses  320  may be connected to the fourth through recesses  510 , and be integrated thereto. 
     Alternatively, although not shown, the third through recesses  320  may be slightly misaligned with the fourth through recesses  510 . 
     Inner surfaces of the front electrode layer  500  defining the fourth through recesses  510  may be inclined from the top surface of the substrate  100 . That is, the inner surfaces defining the fourth through recesses  510  may be inclined from the top surface of the light absorption layer  300 . In addition, the inner surfaces defining the fourth through recesses  510  may be inclined from the top surface of the buffer layer  400 . 
     The fourth through recesses  510  may have a reverse trapezoidal cross-section. That is, an upper width of the fourth through recesses  510  may be greater than a lower width thereof. As described above, a third inclination angle of the fourth through recesses  510  may be controlled by controlling an installation height of the second mask  30 . 
     The third inclination angle, which is an angle between the top surface of the substrate  100  and the inner surface defining the fourth through recesses  510 , may range from about 91° to about 170°. In more detail, the third inclination angle may range from about 91° to about 120°. The top surface of the substrate  100  may be substantially parallel to the top surface of the buffer layer  400 . 
     For example, the front electrode layer  500  may be formed of a transparent conductive material. For example, the transparent conductive material may be an aluminum doped zinc oxide (Al doped ZnO; AZO) or indium tin oxide (ITO). 
     The transparent conductive material is deposited on the buffer layer  400  through the second mask  30 . Accordingly, the transparent conductive material is deposited within the second through recesses  310  to form the connecting lines  700  therein. The back electrode layer  200  is electrically connected to the front electrode layer  500  by the connecting lines  700 . 
     Since the front electrode layer  500 , which is a window layer forming a pn junction with the light absorption layer  300 , functions as a transparent electrode on the front surface of a solar cell, the front electrode layer  500  may be formed of zinc oxide (ZnO) having high light transmissivity and high electrical conductivity. At this point, the zinc oxide may be doped with aluminum to form an electrode having low resistance. 
     A zinc oxide thin film as the front electrode layer  500  may be deposited using a radio frequency (RF) sputtering method with a ZnO target, using a reactive sputtering method with a Zn target, or using a metal organic chemical vapor deposition method. 
     Alternatively, a double structure that an indium tin oxide (ITO) thin film having excellent electro-optical characteristics is deposited on a zinc oxide thin film may be used. 
     The third through recesses  320  are connected to the fourth through recesses  510  to form isolation patterns. Accordingly, the isolation patterns may divide the buffer layer  400  and the front electrode layer  500 , and isolate the cells from one another. 
     The buffer layer  400  and the light absorption layer  300  may be disposed in a stripe or matrix form by means of the isolation patterns. The form of the isolation patterns is not limited thereto. 
     The isolation patterns define cells, which include the back electrode layer  200 , the light absorption layer  300 , the buffer layer  400 , and the front electrode layer  500 . 
     The cells may be connected to each other through the connecting lines  700 . That is, a portion of the back electrode layer  200  corresponding to a first cell is electrically connected to a portion of the front electrode layer  500  corresponding to a second cell adjacent to the first cell by the connecting line  700 . 
     A solar cell apparatus according to the current embodiment includes the substrate  100 , the back electrode layer  200 , the light absorption layer  300 , and the front electrode layer  500 . 
     Back electrodes of the back electrode layer  200  may be spaced apart from one another on the substrate  100 . The light absorption layer  300  may partially include the contact patterns  310  on the back electrode layer  200  to connect electrodes, and the isolation patterns for forming unit cells. 
     The front electrode layer  500  is disposed on the light absorption layer  300 , and is divided by the isolation patterns. 
     The front electrode layer  500  fills the contact patterns  310  to electrically connect to the back electrode layer  200 . 
     The second and third through recesses  310  and  320  may have inclined side walls, and an upper width of the second and third through recesses  310  and  320  may be greater than a lower width thereof. 
     According to the current embodiment, since all processes for fabricating the solar cell are performed in a vacuum without a stand-by process for patterning, the process time can be reduced. 
     When the light absorption layer  300 , the buffer layer  400 , and the front electrode layer  500  are formed, the first and second masks  20  and  30  are used. At this point, the second through recesses  310 , the third through recesses  320 , and the fourth through recesses  510  may be formed through a vacuum deposition process. Accordingly, ripping due to mechanical patterning can be inhibited. 
     Patterning according to the current embodiment is more precise than mechanical patterning. Accordingly, the non-active area NAA can be decreased, and optical efficiency of the solar cell apparatus can be improved. 
     In addition, since the light absorption layer  300  and the buffer layer  400  are formed using the first mask  20 , an additional patterning process and a stand-by process for patterning are unnecessary. 
     Thus, since a front electrode forming process, which is a subsequent process requiring a vacuum state, can be directly performed, the process time can be reduced. In addition, since a pattern is formed without using a mechanical method, ripping of the light absorption layer  300  and the buffer layer  400  can be inhibited. 
     In addition, inner uniformity of the second through recesses  310  is better than in a patterning process using a mechanical method. In addition, the top surface of the buffer layer  400  is not perpendicular to the inner surfaces defining the second through recesses  310 , and is gently connected thereto. For example, the top surface of the buffer layer  400  may be connected through a round portion to the inner surfaces defining the second through recesses  310 . 
     Thus, the transparent conductive material can be efficiently deposited on the inner surfaces defining the second through recesses  310 . Accordingly, coverage of the connecting lines  700  is improved, to thereby reduce sheet resistance thereof. In addition, since the transparent conductive material can be efficiently deposited on the inner surfaces, breakage of the connecting lines  700  can be inhibited. 
     In addition, since the light absorption layer  300  and the buffer layer  400  are formed by performing a patterning process and a deposition process at the same time by means of the first mask  20 , the patterning process is more precise, thereby decreasing the non-active area NAA, and improving the optical efficiency of the solar cell. 
     In addition, since the front electrode layer  500  is formed using the second mask  30 , an additional patterning process and a stand-by process for patterning are unnecessary. 
     Furthermore, since an additional patterning process is unnecessary, the process time can be reduced. In addition, since a pattern is formed without using a mechanical method, ripping of the front electrode layer  500  can be inhibited. 
     The first mask  20  and the second mask  30  may be formed of a material used to form the back electrode layer  200 . For example, the first and second masks  20  and  30  may be formed of molybdenum. 
     Although the first embodiment is separated from the second embodiment herein, they are coupled to each other within the spirit and scope of the present disclosure. 
     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 spirit and 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. 
     INDUSTRIAL APPLICABILITY 
     The solar cell apparatus according to the embodiment is used in the field of photovoltaic power generation.