In the following exemplary embodiments, each component of the present invention will be described in more detail. However, the exemplary embodiments are provided to understand the present invention but are not limited to the following embodiments. Other details not described in this case can be easily inferred by those with ordinary knowledge in the relevant technical field, and are therefore omitted.
 Composition for solar cell electrode
 The composition for solar cell electrodes according to the present invention includes conductive powder (A), glass frit (B), organic vehicle (C) and thixotropic agent (D), and can be screen-printed in a fine line width manner. It is printed on the substrate and has high conversion efficiency.
 Now, each component of the composition for solar cell electrodes according to the present invention will be described in more detail.
 (A) Conductive powder
 For the conductive powder, any organic or inorganic powder with conductivity can be used. Preferably, the conductive powder includes silver (Ag), gold (Au), palladium (Pd), platinum (Pt), copper (Cu), chromium (Cr), cobalt (Co), aluminum (Al), tin ( Sn), lead (Pb), zinc (Zn), iron (Fe), iridium (Ir), osmium (Os), rhodium (Rh), tungsten (W), molybdenum (Mo), nickel (Ni) or indium tin Oxide (ITO) powder. These conductive powders can be used alone or two or more of them can be used in combination. The conductive powder preferably includes silver (Ag) particles, and may further include nickel (Ni), cobalt (Co), iron (Fe), zinc (Zn), or copper (Cu) particles.
 The average particle diameter (D50) of the conductive powder may be about 0.1 μm to about 10 μm. The average particle diameter of the conductive powder is preferably from about 0.2 μm to about 7 μm, more preferably from about 0.5 μm to about 5 μm.
 Based on the total weight of the composition, the amount of conductive powder may be about 50 wt% to about 90 wt%. The amount of conductive powder is preferably about 70 wt% to about 90 wt%. Within this range, the conductive powder can avoid the deterioration of the conversion efficiency caused by the increase in resistance, and can avoid the formation of jelly due to the relative decrease in the amount of organic mediators, while providing the composition with proper dispersibility, fluidity and Printability.
 (B) Frit glass
 The glass frit can form silver grains in the emitter region by etching the anti-reflective layer and melting the silver powder, thereby reducing contact resistance during the baking process of the gel used for the electrode. In addition, during the baking process, the glass frit is used to enhance the adhesion between the conductive powder and the wafer and soften to reduce the baking temperature.
 When the area of the solar cell is increased to improve the efficiency or fill factor of the solar cell, the contact resistance of the solar cell may be increased. Therefore, it is necessary to minimize the series resistance (Rs) and the influence on the pn junction. In addition, as the frequency of using various wafers with different sheet resistances increases, the baking temperature changes in a wide range, and it is expected that the glass frit maintains sufficient thermal stability to cope with a wide range of baking temperatures.
 The glass frit may be at least one of a lead-containing glass frit and a lead-free glass frit, which is generally used in the composition of the solar cell electrode.
 The glass frit may include one metal oxide or a mixture of two or more metal oxides, such as lead oxide, silicon oxide, tellurium oxide, bismuth oxide, zinc oxide, boron oxide, aluminum oxide, tungsten oxide, and combinations thereof . For example, the glass frit may include zinc oxide-silicon oxide (ZnO-SiO 2 ), zinc oxide-boron oxide-silicon oxide (ZnO-B 2 O 3 -SiO 2 ), zinc oxide-boron oxide-silicon oxide-aluminum oxide (ZnO-B 2 O 3 -SiO 2 -Al 2 O 3 ), bismuth oxide-silicon oxide (Bi 2 O 3 -SiO 2 ), bismuth oxide-boron oxide-silicon oxide (Bi 2 O 3 -B 2 O 3 -SiO 2 ), bismuth oxide-boron oxide-silicon oxide-aluminum oxide (Bi 2 O 3 -B 2 O 3 -SiO 2 -Al 2 O 3 ), bismuth oxide-zinc oxide-boron oxide-silicon oxide (Bi 2 O 3 -ZnO-B 2 O 3 -SiO 2 ), bismuth oxide-zinc oxide-boron oxide-silicon oxide-aluminum oxide (Bi 2 O 3 -ZnO-B 2 O 3 -SiO 2 -Al 2 O 3 ) Glass frit and its analogues.
 The glass frit can be prepared from the above-mentioned metal oxides by any conventional method. For example, the aforementioned metal oxides can be mixed in a preset ratio. A ball mill or a planetary mill can be used for mixing. The mixture is melted at about 900°C to about 1300°C, and then quenched to about 25°C. The resulting product is subjected to a pulverization process using a disc mill, a planetary mill, or the like, thereby preparing a glass frit.
 The glass frit may have a spherical shape or an amorphous shape.
 Glass frit can be purchased, or can be selectively melted (for example) silicon dioxide (SiO 2 ), alumina (Al 2 O 3 ), boron oxide (B 2 O 3 ), bismuth oxide (Bi 2 O 3 ), sodium oxide (Na 2 O), zinc oxide (ZnO) and the like are used to prepare glass frit so that the glass frit has the required composition.
 Based on the total weight of the composition, the amount of the glass frit may be about 1 wt% to about 15 wt%, preferably about 2 wt% to about 10 wt%. Within this content range, the glass frit can provide the composition with proper dispersibility, fluidity and printability.
 (C) Organic vehicle
 The organic medium is mechanically mixed with the inorganic components of the composition to give the composition for solar cell electrodes suitable viscosity and rheological characteristics for printing.
 The organic vehicle may be any conventional organic vehicle used in the electrode composition of a solar cell, and may include a binder resin, a solvent, and the like.
 Acrylic resin or cellulose resin can be used as the binder resin. Ethyl cellulose is generally used as the binder resin. In addition, the binder resin may be a blend of ethyl hydroxyethyl cellulose, nitrocellulose, ethyl cellulose and phenol resin, alkyd resin, phenol resin, acrylate resin, two Toluene (xylene) resin, polybutane (polybutane) resin, polyester resin, urea resin, melamine resin, vinyl acetate resin, wood rosin, alcohol polymethacrylate and the like.
 For example, the solvent can be selected from hexane, toluene, ethyl cellulose, cyclohexanone, butyl cellulose, butyl carbitol (diethylene glycol monobutyl ether), dibutyl carbitol (two Ethylene glycol dibutyl ether), butyl carbitol acetate (diethylene glycol monobutyl ether acetate), propylene glycol monomethyl ether, hexanediol, terpineol (terpineol), methyl ethyl ketone, carbitol (benzylalcohol), γ-butyrolactone, ethyl lactate, and combinations thereof.
 The amount of the organic vehicle may be about 3 wt% to about 40 wt% based on the total weight of the composition. Within this range, the organic vehicle can provide the composition with sufficient adhesion strength and excellent printability.
 (D) Thixotropic agent
 According to the present invention, the composition includes a thixotropic agent. The thixotropic agent may include selected from the group consisting of amine compounds, castor oil compounds, carbon black compounds, fatty acid amide compounds, smoked silica compounds, organoclay compounds and nano-grade organic/inorganic particles At least one compound in the group.
 For example, examples of the thixotropic agent may include the amine compound THIXATROL P600 (Elementis); the castor oil compound THIXATROL ST (the Hai Mingsi company); the carbon black compound VULCAN XC72 (Cabot Corporation) (Cabot)); Fatty acid amide compound Flownon (Kyoeisha); Smoked silica compound A200 (Evonik); and Organoclay compound BENTONE SD-3 (Himins ).
 Based on the total weight of the composition, the amount of the thixotropic agent may be about 0.01 wt% to about 2 wt%, preferably about 0.05 wt% to about 1 wt%. Within this range, the thixotropic agent can provide the composition with sufficient adhesion strength and excellent printability.
 (E) Other additives
 According to requirements, the composition can also include conventional additives to increase fluidity, process characteristics and stability. Additives may include dispersants, plasticizers, viscosity stabilizers, anti-foaming agents, colorants, UV stabilizers, antioxidants, coupling agents and the like. These additives can be used alone or as a mixture thereof. Based on the total weight of the composition, the amount of these additives may be 0.1 wt% to 5 wt%, but is not limited thereto.
 The composition for solar cell electrodes according to the present invention is suitable for screen printing, and satisfies the following formulas 1 to 7, wherein the composition can exhibit fine pattern printability and high conversion efficiency.
 [Formula 1]
 3 <5.5
 [Equation 2]
 4 <7
 [Equation 3]
 6 <7.5
 [Equation 4]
 6 <7.5
 [Equation 5]
 1 <2
 [Equation 6]
 0.5 <1.5
 [Equation 7]
 0≤|TI 50-TI 100| <0.8
 In Equations 1 to 7, the thixotropic index (TI) is calculated by substituting the viscosity value measured at 23°C with a No. 14 mandrel using a rotary viscometer at every revolution/min.
 Specifically, in Formula 1 to Formula 4, the thixotropic index (TI) can be defined as the ratio of the viscosity values measured by the rotational viscometer at different revolutions/min. For example, the thixotropic index (TI 10) means the ratio of the viscosity at 1 rpm to the viscosity at 10 rpm measured by a rotary viscometer using a No. 14 mandrel at 23°C, and the thixotropic index (TI 20 ) Means the ratio of the viscosity at 2 rpm to the viscosity at 20 rpm measured by a rotary viscometer using a No. 14 mandrel at 23°C. An example of a rotational viscometer may include HBDV-II+Pro (Brookfield Co., Ltd.).
 In addition, in terms of printability, the viscosity of the composition for solar cell electrodes is preferably about 200 Pa·s (Pa·s) to about 600 Pa·s. Here, the viscosity is measured by rotational viscosity measurement at 23°C and 10 rpm.
 When the composition for solar cell electrodes according to the present invention is printed on a substrate (especially by screen printing), the line width of the printed pattern may be about 75 μm to about 90 μm and the line thickness may be about 15 μm to about 20 μm. In addition, the aspect ratio (line thickness/line width) that the composition can provide is about 0.15 or higher, preferably about 0.15 to about 0.20, more preferably about 0.16 to about 0.18. In this range of aspect ratio, the composition can exhibit excellent printability.
 Solar cell electrode and solar cell including the electrode
 Another aspect of the present invention relates to an electrode formed from the above-mentioned solar cell electrode composition and a solar cell including the electrode. figure 2 A solar cell according to an embodiment of the invention is shown.
 Reference figure 2 , The back electrode 210 and the front electrode 230 can be formed by printing and baking the above composition on a wafer or substrate 100. The wafer or substrate 100 includes a p-type layer (or n-type layer) 101 and an n-type layer (or p Type layer) 102, p-type layer (or n-type layer) 101 and n-type layer (or p-type layer) 102 will serve as emitters. For example, the preliminary process of preparing the back electrode is performed by printing the composition on the back surface of the wafer and drying the printed composition at about 200° C. to about 400° C. for about 10 seconds to about 60 seconds. In addition, the preliminary process of preparing the front electrode can be performed by printing a jelly on the front surface of the wafer and drying the printed composition. Next, the front electrode and the back electrode may be formed by baking the wafer at about 400° C. to about 950° C. (preferably about 750° C. to about 950° C.) for about 30 seconds to about 50 seconds.
 Next, the present invention will be described in more detail with reference to examples. However, it should be noted that these examples are provided for illustration only and should not be considered as limiting the present invention in any way.