Patent Publication Number: US-2019198707-A1

Title: Method of forming an electrode structure and method of manufacturing a photovoltaic cell using the same

Description:
BACKGROUND 
     1. Field of Disclosure 
     The present invention relates to a method of forming an electrode structure and a method of manufacturing a photovoltaic cell using the same. More specifically, the present invention relates to a method of forming an electrode structure on an amorphous silicon semiconductor layer, and a method of manufacturing a photovoltaic cell using the method of forming an electrode structure. 
     2. Description of Related Technology 
     As existing fossil energy resources such as petroleum and coal have depleted, Fukushima nuclear power plant accident occurred, and global warming problem has become serious, there have been research and development on energy sources that can substitute the fossil energy resources with a safe energy sources and can reduce environmental pollution. Specially, Researchers have focused on solar energy development in which solar light can utilized indefinitely. 
     A photovoltaic cell using solar light is a device that converts light energy into electrical energy by using photovoltaic effect, and silicon solar cell is representative. 
     In general, the silicon solar cell includes p-type and n-type semiconductor layers. Further, the silicon solar cell includes a rear electrode and a front electrode such that electrons and holes generated by light irradiation are to be collected at both electrodes to generate electromotive force. 
     In recent years, studies have been made on an n-type silicon cell having an n-type silicon substrate instead of a conventional p-type silicon as a substrate. In this case, the n-type silicon substrate has advantages such that the degradation due to light irradiation is relatively small and sensitivity to impurities is relatively low, so that the silicon solar cell can be realized with an improved efficiency. 
     The n-type silicon solar cell forms a p-type conductive layer by doping a surface portion of the n-type silicon substrate with a group III element such as born (B), aluminum (Al), or gallium (Ga) to transform the surface portion of the n-type silicon substrate into a p-type semiconductor layer to form a p-n junction. At this case, the p-type semiconductor layer for forming the p-n junction may include an amorphous silicon material. As a result, since the amorphous silicon material has a band gap larger than that of the crystalline silicon material, a passivation property may be excellent and a relatively high open-circuit voltage (V OC ) can be secured. 
     However, when the solar cell includes the amorphous silicon semiconductor layer made of the amorphous silicon material, the amorphous silicon semiconductor layer has a relatively low electrical conductivity, and thus, it should be required to further form a transparent conductive oxide (TCO) layer on the amorphous silicon semiconductor layer. In addition, the amorphous silicon material may have thermal damage due to crystallization in a subsequent firing process for forming a metal electrode using the metal paste. Thus, when the metal electrode is formed on top of the transparent conductive oxide (TCO) layer which is formed on the amorphous silicon semiconductor layer, a low-temperature firing process is required. 
     Further, a low line resistance and a low contact resistance between the transparent conductive oxide (TCO) layer and the metal electrode are also required. Specially while performing the low-temperature firing process, problem may occur that voids are generated at an interface portion between the metal electrode and the transparent conductive oxide layer, which may cause a contact resistance to increase. 
     SUMMARY 
     One object of the present invention is to provide a method of forming an electrode structure including a metal electrode formed on a transparent conductive layer of a transparent conductive oxide, capable of having a relatively low contact resistance and securing an excellent adhesive force, and a high aspect ratio. 
     Another object of the present invention is to provide a method of manufacturing a photovoltaic cell capable of realizing a front electrode having a low contact resistance and a low line resistance by using the above-described electrode structure. 
     According to an example embodiment of the present invention, in a method of forming an electrode structure for a photovoltaic cell, a transparent conductive layer is formed on a semiconductor layer of amorphous silicon material doped with dopants of a first conductive type. Then, a preliminary metal pattern is formed on the transparent conductive layer by performing an ink jet process using glass frit-free nano metal ink. After forming a metal paste layer using a conductive paste through a screen printing process to cover the preliminary metal pattern, the preliminary metal pattern and the metal paste layer are fired to transform the preliminary metal pattern and the metal paste layer into a first and second metal pattern such that the first and the second metal patterns are formed on the transparent metal layer to define a metal electrode. 
     In an example embodiment, performing the ink jet process may include applying glass frit-free nano metal ink onto the transparent conductive layer to form a preliminary nano ink layer on the transparent conductive layer, and removing an organic solvent from the preliminary nano ink layer. 
     In an example embodiment, the conductive paste may include a metal powder, a thermosetting polymer and an organic solvent. 
     In an example embodiment, the metal paste layer may be formed to cover both a side face and an upper face of the preliminary metal pattern. 
     In an example embodiment, firing the preliminary metal pattern and the metal paste layer may include performing a firing process at a temperature of about 150 to about 230° C. 
     According to an example embodiment of the present invention, in a method of manufacturing a photovoltaic cell, an amorphous silicon semiconductor layer is deposited on a crystalline silicon layer to form a p-n junction. After forming a transparent conductive layer on the amorphous silicon semiconductor layer, a preliminary metal pattern is formed on the transparent conductive layer by performing an ink jet process using glass frit-free nano metal ink. Then, a metal paste layer is formed using a conductive paste through a screen printing process to cover the preliminary metal pattern. The preliminary metal pattern and the metal paste layer are fired to transform the preliminary metal pattern and the metal paste layer into a first and a second metal patterns such that the first and the second metal patterns are formed on the transparent metal layer to define a metal electrode. 
     In an example embodiment, performing the ink jet process may include applying glass frit-free nano metal ink onto the transparent conductive layer to form a preliminary nano ink layer on the transparent conductive layer, and removing an organic solvent from the preliminary nano ink layer. 
     In an example embodiment, the conductive paste may include a metal powder, a thermosetting polymer and an organic solvent. 
     In an example embodiment, the metal paste layer may be formed to cover both a side face and an upper face of the preliminary metal pattern. 
     In an example embodiment, firing the preliminary metal pattern and the metal paste layer may include performing a firing process at a temperature of about 150 to about 230° C. 
     According to the embodiments of the present invention, the electrode structure includes the first metal pattern formed by the inkjet printing process and the low-temperature firing process, and the second metal pattern formed through the screen printing process and the low-temperature firing process, thereby securing a relatively low contact resistance and line resistance. Further, excellent adhesion with the transparent conductive film can be ensured. In addition, the electrode structure can be easily formed through the inkjet printing process, the screen printing process, and the firing process. On the other hand, crystallization of the semiconductor layer made of amorphous silicon, which is the underlying film, can be suppressed, while forming the second metal pattern by firing the metal paste layer at a relatively low temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages will become more apparent by describing exemplary embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a flow chart illustrating a method of forming an electrode structure according to an example embodiment of the present invention; 
         FIG. 2  is a cross-sectional view illustrating an electrode structure formed by the method of forming the electrode structure in  FIG. 1 ; 
         FIGS. 3 a , 3 b , 3 c  and 3 d    are the scanning electron microscope images showing a difference in cross-sectional microstructure between the electrodes structures without the first metal pattern (a) &amp; (b) and with the first metal pattern (c) &amp; (d) formed by the method of forming the electrode structure; 
         FIG. 4  is a flow chart illustrating a method of manufacturing a photovoltaic cell according to an example embodiment of the present invention; 
         FIG. 5  is a graph illustrating a contact resistance of an electrode structure in variation of a firing temperature with/without a first metal pattern; and 
         FIG. 6  is a graph illustrating a line resistance of an electrode structure in variation of a firing temperature. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the invention will be explained in detail with reference to the accompanying drawings. While the invention is susceptible to various changes have to be introduced in various forms, and the specific embodiments illustrated in the drawings shall be explained in detail in the text. However, it is disclosed in a particular form of the present invention is not intended to limit, the spirit and technical scope of the present invention includes all modifications, equivalents and substitutes should be understood to include. Accompanying drawings, the dimensions of the structure of the present invention are larger than actual in order to clarity the group shown in the drawings. 
     The terms such as first, second, etc., can be used in describing various elements, but the above elements by the above terms should not be limited. The above terms are one element from the other used only to distinguish. For example, in the present invention without departing from the scope of the first component to the second component may be named similarly, the second component to the first component also can be named. 
     Use of a term in the present application for the purpose of describing particular embodiments may only be used, and is not intended to limit the invention. Yield a clearly different meaning in the context of the expression of the plural, unless expressed and the like. In the present application, “including” or “having” and the like is intended to set forth features, integers, steps, operations, elements, parts or combinations, and to possible specify the presence of one or more other features, integers, steps, operations, elements, parts or combinations of those present in or added is not intended to preclude the possibility. 
     Unless otherwise defined, including technical and scientific terms used herein, all terms are to the present invention is not skilled in the art as commonly understood by one party the same meaning. The commonly used terms such as those defined in advance in the context of the related art having the meanings and shall be construed to have a meaning consistent and, in this application, unless otherwise defined explicitly, ideal or excessively formal meaning to be construed not. 
       FIG. 1  is a flow chart illustrating a method of forming an electrode structure according to an example embodiment of the present invention.  FIG. 2  is a cross-sectional view illustrating an electrode structure formed by the method of forming the electrode structure in  FIG. 1 . 
     Referring to  FIGS. 1 and 2 , according to an example embodiment of the present invention, a transparent conductive layer  110  is formed on a semiconductor layer  105  made of amorphous silicon material doped with a first conductive type dopant (S 110 ). The first conductive type dopants may be n-type or p-type dopants. Accordingly, the semiconductor layer  105  may correspond to an n-type semiconductor layer or a p-type semiconductor layer. 
     The semiconductor layer  105  is formed to have an amorphous silicon layer through a deposition process. Further, it is required to suppress the semiconductor layer  105  from changing from the amorphous structure into a (poly) crystalline structure due to a crystallization of the amorphous structure in a subsequent heat treatment process, for example, a relatively low-temperature firing process. That is, in the heat treatment process, it is required that the property deterioration due to crystallization of the semiconductor layer  105  is suppressed. 
     Here, a transparent conductive layer  110  is formed using indium-tin oxide, aluminum-doped zinc oxide, boron-doped zinc oxide, or the like. By forming the transparent conductive layer  110  on the semiconductor layer  105 , the electrode structure  100  may have an improved electrical conductivity, comparing with an electrode structure without the transparent conductive layer  110 . 
     Next, a preliminary metal pattern  121  is formed on the transparent conductive layer  110  through an inkjet printing process using a glass frit-free nano metal ink (S 120 ). That is, the preliminary metal pattern  121  can be easily formed through the inkjet printing process. In addition, a contact resistance of a metal electrode to the transparent conductive layer  110  can be reduced by applying the preliminary metal pattern  121 . 
     In the inkjet process for forming the preliminary metal pattern  121 , a glass frit-free nano metal ink may include metal nanoparticles and an organic solvent. According to the inkjet printing process using the nano metal ink, a nano metal ink layer (not shown) is formed by applying the glass frit-free nano metal ink onto the transparent conductive layer  110 . Then, the preliminary metal pattern  121  is formed on the transparent conductive layer  110  by removing the organic solvent from the nano metal ink layer through a drying process. 
     In the case of a nano metal ink including glass frit, a relatively high temperature sintering process temperature may be required. That is, a relatively high firing temperature, for example, 300 to 1,000° c., for fluidizing the glass in the nano ink may be necessary. While performing the relatively high temperature sintering process, the problem that the amorphous silicon material contained in the underlying amorphous silicon semiconductor layer is crystallized in the sintering process may occur. However, since the glass frit-free nano ink does not include glass frit, the sintering process may not be carried at a high firing temperature. Thus, the problem that the amorphous silicon material contained in the underlying amorphous silicon semiconductor layer is crystallized in the sintering process may be suppressed. 
     The nano metal ink may include 20 to 40 wt % metal nanoparticles, 0.05 to 1.5 wt % dispersing agent, and the remaining solvent. 
     The metal nanoparticles may include, for example, silver nanoparticles. In addition, the metal nanoparticles may have an average particle size (D 50 ) of 10 to 50 nanometers. As a result, the metal nanopaticles may be effectively sintered in a subsequent low-temperature sintering process, which may be carried out at a temperature of below 230° c. Examples of the dispersing agent include 1-ethylpyrrolidin-2-one, poly (acrylic acid) sodium salt, carboxymethyl cellulose sodium salt, polynaphthalene sulfonate formaldehyde condensate, hexadecylamine and the like. 
     Next, a metal paste layer is formed using a conductive paste to cover the preliminary metal pattern  121  (S 130 ). The conductive paste includes a metal powder, a thermosetting polymer, and an organic solvent. The metal powder includes, for example, silver, aluminum, nickel, copper, tin, or the like. Thus, a second metal pattern  123  to be formed by firing the conductive paste layer can secure an improved electrical conductivity of the electrode structure  100 . 
     Specially, the thermosetting polymer may help to maintain a shape of the second metal pattern formed through a subsequent firing process. Further, the thermosetting polymer may increase an adhesive force between the second metal pattern and the transparent conductive layer after the firing process. 
     The metal paste layer may be formed to cover a side face and an upper face of the preliminary metal pattern  121  as a whole. That is, since the metal paste layer covers the preliminary metal pattern  121  entirely, the second metal pattern  123  which is to be transformed from the metal paste layer is more strongly adhered to the preliminary metal pattern  121 , thereby reducing the contact resistance between the preliminary metal pattern  121  and second metal pattern  123 . 
     Further, the metal paste layer is formed through a screen printing process. Therefore, a patterning process, which might be required for forming the second metal pattern  123 , may be omitted. Further, the second metal pattern  123  transformed from the metal paste layer may have a relatively low line resistance owing to a relatively large aspect ratio thereof. 
     Then, the preliminary metal pattern  121  and the metal paste layer are fired to convert the preliminary metal pattern and the metal paste layer into the first metal pattern  122  and the second metal pattern  123  to form a metal electrode  120  including the first and second metal patterns  122  and  123  on the transparent conductive layer  110  (S 140 ). The firing process for firing the metal paste layer may be performed at a relatively low temperature below 230° C. Thus, the amorphous silicon material contained in the semiconductor layer  105  underneath the transparent conductive layer  110  can be suppressed from crystallizing. As a result, deterioration of electrical characteristics due to crystallization of the semiconductor layer  105  can be suppressed. 
     According to the embodiments of the present invention, the electrode structure  100  includes the preliminary metal pattern  121  formed by the inkjet printing process and the first and second metal patterns  122  and  123  formed by the screen printing process and the low-temperature firing process. Thus, a low contact resistance and a low line resistance can be ensured and excellent adhesion of the metal electrode  120  to the transparent conductive layer  110  can be ensured. In addition, the electrode structure  100  can be easily formed through an inkjet printing process and a screen printing process. On the other hand, crystallization of the semiconductor layer  110  made of amorphous silicon material, which is positioned below the metal paste layer, can be suppressed, while firing the preliminary metal pattern  121  and the metal paste layer at a relatively low temperature for forming the first and second metal patterns  122  and  123  from the preliminary metal pattern  121  and the metal paste layer. 
       FIG. 3  depicts a difference in contact microstructure between the electrodes structures without the first metal pattern (a) &amp; (b) and with the first metal pattern (c) &amp; (d) formed by the method of forming the electrode structure 
     Referring to  FIG. 3 , (a) and (b) show an electrode structure where a second metal pattern was formed directly on an indium-tin oxide (ITO) transparent conductive layer, and a large number of voids existed at an interface between the transparent conductive layer and the second metal pattern. Thus, a contact resistance between the transparent conductive layer and the second metal pattern can be increased. On the other hand, in the case of (c) and (d), the electrode structure having the first and second metal patterns on the ITO transparent conductive layer was formed, and no voids were observed at the interface between the transparent conductive layer and the metal electrode. Thus, the contact resistance between the transparent conductive layer and the electrode can be expected to be reduced. 
       FIG. 4  is a flow chart illustrating a method of manufacturing a photovoltaic cell according to an example embodiment of the present invention. 
     Referring to  FIGS. 2 and 4 , an n-type or p-type crystalline silicon substrate is prepared (S 210 ). 
     Next, an intrinsic amorphous silicon layer and an extrinsic amorphous silicon layer doped with first type dopants are sequentially formed on the crystalline silicon substrate through a deposition process (S 220 ). The first type dopants may correspond to Group III elements when the crystalline silicon substrate is n-type, whereas may correspond to Group V elements when the crystalline silicon substrate is p-type. As a result, the crystalline silicon substrate and the amorphous silicon semiconductor layer form a p-n junction. 
     Then, a transparent conductive layer is formed on the amorphous silicon layer doped with the first-type dopants (S 230 ). 
     The transparent conductive layer may be formed using indium-tin oxide, aluminum-doped zinc oxide, boron-doped zinc oxide, or the like. By forming the transparent conductive layer on the amorphous silicon layer, an electrical conductivity can be improved 
     Next, a preliminary metal pattern  121  is formed on the transparent conductive layer  110  through an inkjet printing process using a glass frit-free nano metal ink (S 240 ). That is, the preliminary metal pattern  121  can be easily formed through the inkjet printing process. In addition, a contact resistance of a metal electrode to the transparent conductive layer  110  can be reduced by applying the preliminary metal pattern  121 . 
     In the inkjet process for forming the preliminary metal pattern  121 , a glass frit-free nano metal ink including metal nanoparticles and an organic solvent may be used. According to the inkjet printing process using the nano ink, a preliminary nano ink layer (not shown) is formed by applying the glass frit-free nano metal ink onto the transparent conductive layer  110 . Then, the preliminary metal pattern  121  is formed on the transparent conductive layer  110  by removing the organic solvent from the preliminary nano ink layer through a drying process. 
     Next, a metal paste layer is formed using a conductive paste to cover the preliminary metal pattern  121  (S 250 ). The conductive paste includes a metal powder, a thermosetting polymer, and an organic solvent. The metal powder may be, for example, silver, aluminum, nickel, copper, tin, or the like. Thus, a second metal pattern to be formed by firing the conductive paste layer can secure an improved electrical conductivity. 
     Specially, the thermosetting polymer may help to maintain a shape of the second metal pattern formed through a subsequent firing process. Further, the thermosetting polymer may increase an adhesive force between the second metal pattern and the transparent conductive layer after the firing process. 
     The metal paste layer may be formed to cover a side face and an upper face of the preliminary metal pattern  121  as a whole. That is, the metal paste layer covers the preliminary metal pattern  121  entirely. 
     Further, the metal paste layer is formed through a screen printing process. Therefore, a patterning process, which might be required for forming the second metal pattern  123 , may be omitted. Further, the second metal pattern  123  transformed from the metal paste layer may have a relatively low line resistance owing to a relatively large aspect ratio thereof. 
     Then, the preliminary metal pattern  121  and the metal paste layer are fired to convert the preliminary metal pattern  121  and the metal paste layer into the first metal pattern  122  and the second metal pattern  123  to form a metal electrode  120  including the first and second metal patterns  122  and  123  on the transparent conductive layer  110  (S 260 ). The firing process for sintering the metal paste layer may be performed at a relatively low temperature below 230° C. Thus, the amorphous silicon material contained in the semiconductor layer  105  underneath the transparent conductive layer  110  can be suppressed from crystallizing. As a result, deterioration of electrical characteristics due to crystallization of the semiconductor layer  105  can be suppressed. 
     Further, a back surface field (BSF) layer and a rear surface electrode are formed on a lower face of the crystalline silicon substrate to manufacture a photovoltaic cell. 
       FIG. 5  is a graph illustrating a contact resistance of an electrode structure in variation of a firing temperature with/without a first metal pattern. 
     Referring to  FIG. 5 , the electrode structure with the first metal pattern using nano ink has a contact resistance value relatively lower than that of the electrode structure without the first metal pattern. Further, the contact resistance value of the electrode structure is abruptly increased at a firing temperature of 140° C. or less. 
       FIG. 6  is a graph illustrating a line resistance of an electrode structure in variation of a firing temperature. 
     Referring to  FIG. 6 , the line resistance value of the electrode structure is abruptly increased at a firing temperature of 140° C. or less. 
     According to example embodiments, the method of forming an electrode structure and the method of manufacturing a photovoltaic cell can be adapted to a method of manufacturing a photovoltaic cell including an amorphous silicon layer. 
     The foregoing is illustrative of the present teachings and is not to be construed as limiting thereof. Although a few exemplary embodiments have been described, those skilled in the art will readily appreciate from the foregoing that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure of invention. Accordingly, all such modifications are intended to be included within the scope of the present teachings. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also functionally equivalent structures.