Patent Publication Number: US-2016240700-A1

Title: Solar Battery

Description:
TECHNICAL FIELD 
     The present disclosure relates to a solar cell. 
     BACKGROUND ART 
     With an increase in interest in environmental issue and the depletion of natural resources, interest in a solar cell as alternative energy that has no environmental problems and high energy efficiency is increasing. Solar cells are classified into a silicon semiconductor solar cell, a compound semiconductor solar cell, a stacked solar cell, or the like, and a solar cell that includes a CIGS light absorbing layer according to the present disclosure belongs to the compound semiconductor solar cell. 
     Since copper indium gallium selenide (CIGS) that is an I-III-VI group compound semiconductor has a direct transition type energy band gap of 1 eV or higher, has the highest light absorption coefficient among semiconductors and is significantly stable electro-optically, it is a significantly ideal material as the light absorbing layer of a solar cell. 
     A CIGS based solar cell is formed in such a manner that a support substrate, a rear electrode layer, a light absorbing layer, a buffer layer, and a front electrode layer are sequentially stacked. 
     In this case, the buffer layer may be formed by two or more layers. That is, a high-resistor buffer layer that has high resistance may be further formed on the buffer layer. Such a high-resistor buffer layer may be formed from zinc oxide (i-ZnO) on which impurities are not doped. 
     However, since the buffer layer and the high-resistor buffer layer are formed by different processes, there is a limitation in that a process time increases when the buffer layers are formed. 
     Thus, there is a need for a buffer layer of a new structure that may form the buffer layers by a single process and replace the high-resistor buffer layer when the buffer layers are formed. 
     DISCLOSURE OF THE INVENTION 
     Technical Problem 
     Embodiments provide a solar cell that has enhanced photoelectric conversion efficiency. 
     Technical Solution 
     In one embodiment, a solar cell includes a support substrate; a rear electrode layer arranged on the support substrate; a light absorbing layer arranged on the rear electrode layer; a buffer layer arranged on the light absorbing layer; and a front electrode layer arranged on the buffer layer, wherein the buffer layer comprises oxygen doped zinc sulfide (Zn (O, S)), and content of sulfur (S) in the buffer layer varies towards the front electrode layer starting from the light absorbing layer. 
     Advantageous Effects 
     A solar cell according to an embodiment includes a first buffer layer and a second buffer layer that are different in the content of sulfur. That is, the first buffer layer that is arranged on a light absorbing layer includes less sulfur than the second buffer layer that is arranged on the first buffer layer. 
     Thus, the second buffer layer may be several hundred times larger than the first buffer layer in specific resistance that depends on the content of sulfur. Thus, the second buffer layer may replace the high-resistor buffer layer typically arranged on a buffer layer. 
     Thus, it is possible to omit the process of forming the high-resistor buffer layer that is arranged by a separate process after the forming of the buffer layer. 
     Also, it is possible to generally decrease the series resistance of a solar cell according to the control of specific resistance in the buffer layer. 
     Thus, a solar cell according to an embodiment may have enhanced process efficiency and generally enhanced photoelectric conversion efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plane view of a solar cell according to an embodiment. 
         FIG. 2  is a cross-sectional view of a solar cell according to an embodiment. 
         FIG. 3  is an enlarged view of the circle A in  FIG. 2 . 
         FIGS. 4 to 10  are diagrams for explaining a method of manufacturing a solar cell according to an embodiment. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     In describing embodiments, the description that layers (films), regions, patterns or structures are formed “over/on” or “under/beneath” layers (films), regions, pads or patterns includes that they are formed directly or through another layer. The phrase over/on or under/beneath each layer is described based on the accompanying drawings. 
     Since the thickness or size of layers (films), regions, patterns or structures in the drawings may vary for the clearness and convenience of description, it does not absolutely reflect its actual size. 
     In the following, an embodiment is described in detail with reference the accompanying drawings. 
     In the following, a solar cell and a manufacturing method thereof according to an embodiment are described in detail with reference to  FIGS. 1 to 10 .  FIG. 1  is a plane view of a solar cell according to an embodiment,  FIG. 2  is a cross-sectional view of a solar cell according to an embodiment,  FIG. 3  is an enlarged view of the circle A in  FIG. 2 , and  FIGS. 4 to 10  are diagrams for explaining a method of manufacturing a solar cell according to an embodiment. 
     Referring to  FIGS. 1 to 3 , a solar cell according to an embodiment includes a support substrate  100 , a rear electrode layer  200 , a light absorbing layer  300 , a buffer layer  400 , a front electrode layer  500 , and a plurality of connections  600 . The support substrate  100  may be an insulator. The support substrate  100  may be a glass substrate, a plastic substrate, or a metal substrate. Specifically, the support substrate  100  may be soda lime glass substrate. The support substrate  100  may be transparent. The support substrate  100  may be rigid or flexible. 
     The rear electrode layer  200  is arranged on the support substrate  100 . The rear electrode layer  200  is a conductive layer. An example of a material used as the rear electrode layer  200  may include metal, such as molybdenum (Mo). 
     Also, the rear electrode layer  200  may include two or more layers. In this case, the layers may be formed from the same metal or from different metal. 
     First through holes TH 1  are formed in the rear electrode layer  200 . The first through holes TH 1  are open regions that expose the top surface of the support substrate  100 . The first through holes TH 1  may have a shape extended in the first direction when viewed from the top. 
     The width of the first through holes TH 1  may be about 80 μm to about 200 μm. 
     The rear electrode layer  200  is divided into a plurality of rear electrodes by the first through holes TH 1 . That is, the rear electrodes are defined by the first through holes TH 1 . 
     The rear electrodes are spaced apart by the first through holes TH 1 . The rear electrodes are arranged in the form of stripe. 
     Alternately, the rear electrodes may be arranged in the form of a matrix. In this case, the first through holes TH 1  may be formed in the form of a grid when viewed form the top. 
     The light absorbing layer  300  is arranged on the rear electrode layer  200 . Also, a material included in the light absorbing layer  300  fills the first through holes TH 1 . 
     The light absorbing layer  300  includes I-III-VI group based compound. For example, the light absorbing layer  300  may have a copper-indium-gallium-selenide (Cu (In, Ga) Se 2 ; CIGS) based crystal structure, copper-indium-selenide or copper-gallium-selenide based crystal structure. 
     In this case, the ratio of copper/III group elements may be about 0.8 to about 0.9, and the ratio of gallium/III group elements may be about 0.38 to about 0.40. 
     The energy band gap of the light absorbing layer  300  may be about 1 eV to about 1.8 eV. 
     The buffer layer  400  is arranged on the light absorbing layer  300 . The buffer layer  400  is in direct contact with the light absorbing layer  300 . 
     The buffer layer  400  may include sulfur (S). Specifically, the buffer layer  400  may include oxygen doped zinc sulfide (Zn (O, S)). 
     The buffer layer  400  may vary in the content of sulfur depending on the position. As an example, the buffer layer  400  may increase in the content of sulfur towards the front electrode layer starting from the light absorbing layer. 
     As shown in  FIG. 3 , the buffer layer  400  may include a first buffer layer  410  and a second buffer layer  420 . Specifically, the buffer layer  400  may include the first buffer layer that is arranged on the light absorbing layer  300 , and the second buffer layer  420  that is arranged on the first buffer layer  410 . 
     The first buffer layer  410  and the second buffer layer  420  may include the same or similar material. As an example, the first buffer layer  410  and the second buffer layer  420  may include oxygen doped zinc sulfide (Zn (O, S)). 
     The first buffer layer  410  and the second buffer layer  420  may have different composition. Specifically, the first buffer layer  410  and the second buffer layer  420  may be different in the content of sulfur that is included in Zn (O, S). 
     Specifically, the second buffer layer  420  may include less sulfur than the first buffer layer  410 . As an example, the first buffer layer  410  may include about 10 wt % to about 15 wt % sulfur in Zn (O, S). Also, the second buffer layer  420  may include about 20 wt % to about 25 wt % sulfur in Zn (O, S). 
     Also, the first buffer layer  410  and the second buffer layer  420  may have different thicknesses. Specifically, the first buffer layer  410  may be formed in a larger thickness than the second buffer layer  420 . As an example, the first buffer layer  410  may be formed in a thickness of about 20 nm to about 30 nm. Also, the second buffer layer  420  may be formed in a thickness of about 10 nm to about 20 nm. Also, the total thickness of the buffer layer  400 , i.e., the first buffer layer  410  and the second buffer layer may be about 30 nm to about 50 nm. 
     In the case where the first buffer layer  410  and the second buffer layer  420  is out of the range of wt % of sulfur and the range of thickness, the difference between their specific resistances may not be equal to or larger than a desired value. Also, the second buffer layer  420  may not properly function as an insulator. 
     The first buffer layer  410  and the second buffer layer  420  may have band gaps of about 2.7 eV to about 2.8 eV. 
     The first buffer layer  410  and the second buffer layer  420  may have different specific resistances. Specifically, the specific resistance of the second buffer layer may be larger than the specific resistance of the first buffer layer. As an example, the specific resistance of the first buffer layer  410  may be smaller than or equal to about 10 −3 Ω. Also, the specific resistance of the second buffer layer  420  may be equal to or larger than about 10 −2 Ω. 
     The specific resistances of the buffer layers may vary according to the content of sulfur in Zn (O, S) that is included in the buffer layers. That is, the specific resistance of the buffer may increase with an increase in the content of sulfur. 
     That is, the second buffer layer may include more sulfur than the first buffer layer and thus the specific resistance of the second buffer layer may be larger than that of the first buffer layer. 
     Especially, the second buffer layer may function as an insulator according to an increase in specific resistance. Thus, it is possible to omit the forming of a high-resistor buffer layer that is typically arranged on a buffer layer. 
     That is, after the forming of the buffer layer, the high-resistor buffer layer that functions as an insulator has been further arranged on the buffer layer, typically. As an example, zinc oxide (i-ZnO) on which impurities are not doped is further formed. 
     However, a solar cell according to an embodiment may increase the content of sulfur in forming the second buffer layer to increase specific resistance so that the second buffer layer may replace the typical high-resistor buffer layer. 
     Thus, since it is possible to omit the process of forming the high-resistor buffer layer, it is possible to enhance process efficiency due to the reduction in process time. 
     Also, a solar cell according to an embodiment may regulate the content of sulfur in forming the buffer layer to form the first buffer layer having less sulfur, i.e., smaller specific resistance and then form the second buffer layer having more sulfur, i.e., larger specific resistance so that it is possible to control specific resistance in the buffer layer. Thus, it is possible to generally decrease the series resistance Rs of a solar cell. 
     Thus, a solar cell according to an embodiment may enhance process efficiency and enhance the efficiency of a solar cell on the whole. 
     Second through holes TH 2  may be formed in the buffer layer  400 . The second through holes TH 2  are open regions that expose the top surface of the support substrate  100  and the top surface of the rear electrode layer  200 . The second through holes TH 2  may have a shape extended in one direction when viewed from the top. The width of the second through holes TH 2  may be about 80 μm to about 200 μm but is not limited thereto. 
     The buffer layer  400  is defined as plurality of buffer layers by the second through holes TH 2 . 
     A front electrode layer  500  is arranged on the buffer layer  400 . More specifically, the front electrode layer  500  is arranged on a third buffer layer  430 . The front electrode layer  500  is transparent, conductive layer. Also, the resistance of the front electrode layer  500  is higher than that of the rear electrode layer  200 . 
     The front electrode layer  500  includes oxide. As an example, a material used as the front electrode layer  500  may include Al doped ZnC (AZO), indium zinc oxide (IZO), indium tin oxide (ITO) or the like. 
     The front electrode layer  500  includes connections  600  that are in the second through holes TH 2 . 
     Third through holes TH 3  are formed in the buffer layer  400  and the front electrode layer  500 . The third through holes TH 3  may pass through a portion or whole of the buffer layer  400  and the front electrode layer  500 . That is, the third through holes TH 3  may expose the top surface of the rear electrode layer  200 . 
     The third through holes TH 3  are formed adjacent to the second through holes TH 2 . More specifically, the third through holes TH 3  are arranged next to the second through holes TH 2 . That is, the third through holes TH 3  are arranged next to the second through holes TH 2  side by side when viewed from the top. The third through holes TH 3  may have a shape extended in the first direction. 
     The third through holes TH 3  pass through the front electrode layer  500 . More specifically, the third through holes TH 3  may pass through the light absorbing layer  300 , the buffer layer  400  and/or the high-resistor buffer partially or wholly. 
     The front electrode layer  500  is divided into a plurality of front electrodes by the third through holes TH 3 . That is, the front electrodes are defined by the third through holes TH 3 . 
     The front electrodes have a shape corresponding to the rear electrodes. That is, the front electrodes are arranged in the form of stripe. Alternately, the front electrodes may be arranged in the form of a matrix. 
     Also, a plurality of solar cells C 1 , C 2 , . . . is defined by the third through holes TH 3 . More specifically, the solar batteries C 1 , C 2 , . . . are defined by the second through holes TH 2  and the third through holes TH 3 . That is, a solar cell according to an embodiment is divided into the solar cells C 1 , C 2 , . . . by the second through holes TH 2  and the third through holes TH 3 . Also, the solar cells C 1 , C 2 , . . . are connected to each other in the second direction that crosses the first direction. That is, a current may flow through the solar cells C 1 , C 2 , . . . in the second direction. 
     That is, a solar cell panel  10  includes the support substrate  100  and the solar cells C 1 , C 2 , . . . . The solar cells C 1 , C 2 , . . . are arranged on the support substrate  100  and spaced apart from one another. Also, the solar cells C 1 , C 2 , . . . are connected to each other in series by the connections  600 . 
     The connections  600  are arranged in the second through holes TH 2 . The connections  600  are extended downwards from the front electrode layer  500  and connected to the rear electrode layer  200 . For example, the connections  600  are extended from the front electrode of a first cell C 1  and connected to the rear electrode a second cell C 2 . 
     Thus, the connections  600  connect adjacent solar cells. More specifically, the connections  600  connect the front electrode and the rear electrode that are included in each of adjacent solar cells. 
     The connections  600  are integrally formed with the front electrode layer  500 . That is, a material used as the connection  600  is the same as a material used as the front electrode layer  500 . 
     As described earlier, a solar cell according to an embodiment includes the first buffer layer and the second buffer layer that are different in the content of sulfur. That is, the first buffer layer that is arranged on the light absorbing layer includes less sulfur than the second buffer layer that is arranged on the first buffer layer. 
     Thus, the second buffer layer may be several hundred times larger than the first buffer layer in specific resistance that depends on the content of sulfur. Thus, the second buffer layer may replace the high-resistor buffer layer typically arranged on the buffer layer. 
     Thus, it is possible to omit the process of forming the high-resistor buffer layer that is arranged by a separate process after the forming of the buffer layer. 
     Also, it is possible to decrease the series resistance of a solar cell on the whole according to the control of specific resistance in the buffer layer. 
     Thus, a solar cell according to an embodiment may have enhanced process efficiency and enhanced photoelectric conversion efficiency on the whole. 
     In the following, a manufacturing method of a solar cell according to an embodiment is described with reference to  FIGS. 4 to 10 .  FIGS. 4 to 10  are diagrams for explaining the manufacturing method of the solar cell according to an embodiment. 
     Firstly, referring to  FIG. 4 , the rear electrode layer  200  is formed on the support substrate  100 . 
     Subsequently, referring to  FIG. 5 , the rear electrode layer  200  is patterned so that the first through holes TH 1  are formed. Thus, a plurality of rear electrodes, a first connection electrode and a second connection electrode are arranged on the support substrate  100 . The rear electrode layer  200  may be patterned by a laser beam. 
     The first through holes TH 1  may expose the top surface of the support substrate  100  and have a width of about 80 μm to about 200 μm. 
     Also, it is possible to arrange an additional layer, such as a diffusion barrier between the support substrate  100  and the rear electrode layer  200 , in which case the third through holes TH 1  expose the top surface of the additional layer. 
     Subsequently, referring to  FIG. 6 , the light absorbing layer  300  is arranged on the rear electrode layer  200 . The light absorbing layer  300  may be formed by a sputtering process or vaporization. 
     For example, vaporizing copper, indium, gallium and selenium simultaneously or separately to form the CIGS based light absorbing layer  300 , and forming the light absorbing layer by a selenization process after forming a metal pre-cursor film are being widely used in order to form the absorbing layer  300 . 
     To describe forming the light absorbing layer by the selenization process after forming a metal pre-cursor film, the metal pre-cursor film is formed on the rear electrode by a sputtering process that uses a copper target, an indium target, and a gallium target. 
     Then, the pre-cursor film is formed as the CIGS based light absorbing layer  300  by a selenization process. 
     Alternatively, the sputtering process and the selenization process that use the copper target, the indium target, and the gallium target may be performed simultaneously. 
     Alternatively, it is possible to the CIS based or CIG based light absorbing layer  300  by a sputtering process and a selenization process that use only the copper target and the indium target or use only the copper target and the gallium target. 
     Subsequently, referring to  FIG. 7 , the buffer layer  400  is formed on the light absorbing layer  300 . The buffer layer  400  may include the first buffer layer  410  and the second buffer layer  420 , and the first buffer layer  410  and the second buffer layer  420  may be sequentially deposited. 
     That is, the first buffer layer  410  may be deposited on the light absorbing layer  300 , and the second buffer layer  420  may be deposited on the first buffer layer  410 . 
     As an example, the first buffer layer  410  and the second buffer layer  420  may be deposited through atomic layer deposition. However, an embodiment is not limited thereto, and the first buffer layer  410  and the second buffer layer  420  may be formed by various methods, such as chemical vapor deposition (CVD) or metal organic chemical vapor deposition (MOCVD). 
     In this case, the first buffer layer  410  and the second buffer layer  420  may be deposited in units of nm. Specifically, the first buffer layer  410  may be deposited in a thickness of about 20 nm to about 30 nm, and the second buffer layer  420  may be deposited in a thickness of about 10 nm to about 20 nm. 
     Subsequently, referring to  FIG. 8 , portions of the light absorbing layer  300  and the buffer layer  400  are removed so that the second through holes TH 2  are formed. 
     The second through holes TH 2  may be formed by a mechanical device, such as a tip, or a laser device. 
     For example, the light absorbing layer  300  and the buffer layer  400  may be patterned by a tip that has a width of about 40 μm to about 180 μm. Also, the second through holes TH 2  may be formed by a laser beam that has a wavelength of about 200 nm to about 600 nm. 
     In this case, the width of the second through holes TH 2  may be about 100 μm to about 200 μm. Also, the second through holes TH 2  may expose a portion of the top surface of the rear electrode layer  200 . 
     Subsequently, referring to  FIG. 9 , a transparent, conductive material is deposited on the buffer layer  400 , i.e., the second buffer layer  420  to form the front electrode layer  500 . 
     The front electrode layer  500  may be formed by the deposition of the transparent, conductive material at oxygen-free atmosphere. More specifically, the front electrode layer  500  may be formed by the deposition of Al doped zinc oxide at inert gas atmosphere that does not include oxygen. 
     The forming of the front electrode layer may be performed by the deposition of zinc oxide Al doped by a deposition method using a ZnO target or a reactive sputtering method using a Zn target as an RF sputtering method. 
     Subsequently, referring to  FIG. 10 , portions of the light absorbing layer  300 , the buffer layer  400 , and the front electrode layer  500  are removed so that the third through holes TH 3  are formed. Thus, the front electrode layer  500  is patterned so that a plurality of front electrodes, a first cell C 1 , a second cell C 2 , and a third cell C 3  are defined. The width of the third through holes TH 3  may be about 80 μm to about 200 μm. 
     The characteristics, structures, and effects described in the above-described embodiments are included in at least one embodiment but are not necessarily limited to one embodiment. Furthermore, the characteristic, structure, and effect illustrated in each embodiment may be combined or modified for other embodiments by a person skilled in the art. Thus, it would be construed that contents related to such a combination and such a variation are included in the scope of embodiments. 
     While embodiments have been mainly described above, they are only examples and do not limit the present disclosure and a person skilled in the art to which the present disclosure pertains could appreciate that it is possible to implement many variations and applications not illustrated above without departing from the essential characteristics of the embodiments. For example, components particularly represented in the embodiments may vary. In addition, the differences related to such variations and applications should be construed as being included in the scope of the present disclosure that the following claims define.