Abstract:
The present invention is directed to a silver paste for a Si solar cell comprising a high purity Bi 2 O 3  additive and a solar cell having a silicon wafer with the silver paste on its front-side surface. The resultant cell exhibits improved efficiency.

Description:
[0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 62/000,141 filed May 19, 2014, which is incorporated herein by reference in its entirety and for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to a silver paste for a silicon (Si) solar cell comprising a high purity Bi 2 O 3  additive and a solar cell having a silicon wafer with the silver paste on its front-side surface. The solar cell exhibits improved efficiency resulting from the use of the high purity additive in the paste. 
       BACKGROUND TO THE INVENTION 
       [0003]    Silicon solar cells are extensively used in the rapidly growing photovoltaic (PV) industry. 
         [0004]    Silicon solar cells typically include a silicon wafer with a silver (Ag) paste screen-printed with a pattern on the front-side (facing the sunlight) of the silicon wafer. The silicon wafer also typically has two overlapping layers containing aluminum and silver respectively printed on the opposite (back-side) of the silicon wafer. 
         [0005]    U.S. Pat. No. 5,066,621 and U.S. Pat. No. 5,336,644 are directed to sealing glass compositions containing metal oxides. 
         [0006]    US 2013/0037761 is directed to an electroconductive thick film paste comprising Ag for use in an electrode for a solar cell. 
         [0007]    US 2012/0171810 describes paste compositions for an electrode of a solar cell which contains a conductive powder, an organic vehicle and glass frits. 
         [0008]    US 2012/0138142 is directed to lead-free and cadmium free paste compositions for use on contacts for solar cells. 
         [0009]    US 2010/0294360 and US 2010/0294361 are directed to a process of forming a front-grid electrode on a silicon wafer which printed and dried metal pastes containing glass frits thereon. 
         [0010]    US 2012/0312368 and US 2012/173875 describe an electroconductive thick film paste comprising Ag and Pb free bismuth based oxide both dispersed in an organic medium for the use in the manufacture of semiconductor devices. 
         [0011]    US 2011/0147677 is directed to zinc containing glass compositions for use in conductive pastes for silicon semiconductor devices and photovoltaic cells. 
         [0012]    WO 2012/135551 describes high aspect ratio screen printable thick film paste wax compositions for positioning conductive lines on a solar cell device. 
         [0013]    Finally Journal Article: Development of lead-free silver ink for front contact metallization Author(s): Kalio, A.; Leibinger, M.; Filipovic, A.; Kruger, K.; Glatthaar, M.; Wilde, J. is directed to solar energy materials and solar cells. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention provides a composition for silicon solar cells comprising a silver powder, glass frits, at least one organic resin, at least one solvent and between 0.02 to 1.5 wt % of a Bi 2 O 3  additive wherein the Bi 2 O 3  has an average particle size of between 5 to 9000 nm and wherein the additive contains greater than 98 wt % Bi 2 O 3 . 
         [0015]    The present invention also provides a solar cell comprising a silicon wafer and the composition on the front side surface of the silicon wafer. 
         [0016]    Finally the present invention provides a process for making a solar cell comprising applying a coating of the composition onto the front side surface of a silicon wafer. 
         [0017]    These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the methods and formulations as more fully described below. 
     
    
     DETAILED DESCRIPTION 
       [0018]    It has been found that the use of silver paste compositions with a high purity Bi 2 O 3  additive in a particular weight range and having a particular particle size results in the production of solar cells with a higher cell efficiency and thus greater power output when exposed to sunlight. 
         [0019]    Typically glass frits are added to the silver paste compositions when used in the production of solar cells to etch through the anti-reflective coating (ARC) on the front-side of a silicon wafer. 
         [0020]    Furthermore, the use of glass frits also increases the uniformity of the compositions which potentially increases ribbon adhesion and helps to prevent spotty areas caused by the surge in one metal oxide concentration. 
         [0021]    However, acceptable uniformity can also be achieved by incorporating metal oxides separately as additives directly into the silver paste compositions. 
         [0022]    It has now been found that a high purity Bi 2 O 3  additive imparts particularly advantageous properties to high efficiency front-side silver paste compositions. Furthermore it has also been found that Bi 2 O 3  with a particular particle size, used as an additive separately or as an additive incorporated into a glass frit, imparts higher efficiency into the front-side silver paste compositions. 
         [0023]    By the use of the term “high purity”, it is meant that bismuth oxide is essentially the only metal oxide in the additive and while it is possible to use a bismuth oxide additive in conjunction with other metal oxides, the bismuth additive is incorporated alone and not as a multi-component metal oxide mixture. 
         [0024]    Preferably, the Bi 2 O 3  additive contains greater than 99.0 wt % such as 99.999 wt %. 
         [0025]    Preferably, the composition comprises between 0.02 to 1.5 wt % Bi 2 O 3  additive and more preferably between 0.05 to 0.5 wt % and advantageously between 0.1 to 0.2 wt % based on the total weight of the composition. 
         [0026]    The average particle size of the Bi 2 O 3  additive is preferably between 10 nm to 3000 nm and more preferably between 20 nm to 300 nm. 
         [0027]    Typically the Ag powder has a purity of greater than 99.5% and usually contains impurities such as Zr, Al, Fe, Na, Zn, Pb at advantageously less than 100 ppm. 
         [0028]    Typically the composition contains between 70 to 95 wt % of Ag powder, and more preferably between 80 to 92 wt %. 
         [0029]    The Ag powder(s) may consist of one or more Ag powder(s) preferably with a particle size D50 between 0.1 to 5 μm, more preferably 0.5 to 2 μm. 
         [0030]    Usually when two or more Ag powders are used a higher Ag particle packing density is achieved and the proximity of the Ag particles facilitates Ag sintering and percolation during the firing process. This results in a more connected and condensed electron conduction path which generally improves the solar cell efficiency. 
         [0031]    The Ag powder(s) are not limited in morphology and may be spherical, elliptical, etc. and typically could be thermally sintered to form a conductive network during the solar cell metallization firing step. 
         [0032]    Furthermore the Ag powder(s) may be pre-coated with different surfactants to avoid particle agglomeration and aggregation. The surfactant is advantageously a straight-chain, or branched-chain fatty acid, a fatty acid ester, fatty amide or a mixture thereof. 
         [0033]    Additionally long-chain alcohols may also be used for rheology modification. 
         [0034]    The composition usually comprises between 0.5 to 10 wt % of glass frits. 
         [0035]    The glass frits may be formed from the group consisting of PbO, Al 2 O 3 , SiO 2 , B 2 O 3 , Li 2 O, TiO 2 , ZnO, P 2 O 5 , V 2 O 5 , SrO, CaO, Sb 2 O 3 , SO 2 , As 2 O 3 , Bi 2 O 3 , Tl 2 O 3 , Ga 2 O 3 , MgO, Y 2 O 3 , ZrO 2 , Mn 2 O 5 , CoO, NiO, CuO, SrO, Mo 2 O 3 , RuO 2 , TeO 2 , CdO, In 2 O 3 , SnO 2 , La 2 O 3 , BaO and mixtures thereof. 
         [0036]    Additionally, the composition preferably contains between 0.2 to 2 wt % of organic resin and more preferably between 0.5 to 1.5 wt %. 
         [0037]    Typically the resin is selected from acrylic resin, epoxy resin, phenol resin, alkyd resin, cellulose polymers, polyvinyl alcohol, rosin and mixtures thereof. 
         [0038]    Advantageously the resins should burn off during the firing of the coated silicon wafer such that no residue remains thereon. 
         [0039]    Additionally, the composition preferably contains between 0.2 to 20 wt % of solvent and more preferably between 2 to 8 wt %. 
         [0040]    Typical solvents include texanol, propanol, isopropyl alcohol, ethylene glycol and diethylene glycol derivatives (glycol ether solvents), toluene, xylene, dibutyl carbitol, terpineol and mixtures thereof. 
         [0041]    The solvent is effective for dissolving the resins, rosins, and thixotropic agents and is preferably capable of sustaining paste printing whilst subsequently evaporating thoroughly during the drying step. 
         [0042]    The composition also typically contains an adhesion promoting agent, a thixotropic agent and/or a dispersant. 
         [0043]    Usually the composition contains between 0.1 to 0.7 wt % of an adhesion promoting agent, between 0.01 to 3.0 wt % of a dispersant and between 0.1 to 2.0 and advantageously between 0.5 to 1.5 wt % of a thixotropic agent. 
         [0044]    Typically the thixotropic agent is a cellulose polymer such as ethyl cellulose, hydroxyethyl cellulose, castor oil, hydrogenated castor oil, an amide modified castor oil derivative or a fatty amide. Suitable thixotropic agents include Thixatrol Max, Thixatrol ST and Thixatrol Pro. 
         [0045]    Usually the dispersant is long-chain fatty acid such as stearic acid with functional amine, acid ester or alcohol groups. Suitable dispersants include BYK 108, BYK 111, Solsperse 66000 and Solsperse 27000. 
         [0046]    The composition is usually in the form of paste and preferably has a viscosity of between 50 to 250 Pa·S at 10 recipocal second. 
         [0047]    The present invention also provides a process for making a solar cell which involves applying a coating of the composition onto the front side surface of a silicon wafer. Furthermore the process usually involves applying two overlapping layers containing aluminum and silver respectively to the back side surface of the silicon wafer. The coated silicon wafer is then fired. 
         [0048]    The composition is usually deposited on a silicon wafer by screen/stencil printing. The stroke movement across the screen provides high shear rate to the composition through micro-channels of mesh pattern. The size of micro-channels is preferably 40 to 80 microns for fingers, and preferably 1.0 to 2.0 mm for bus bars. The fingers are preferably narrower in order to leave more open area for sunlight collection whilst the bus bars are preferably dashed rather than continuous due to the cost of Ag. The thickness of the printed finger lines is typically between 10 to 35 microns. Advantageously the higher the printed fingers the better the finger&#39;s conductivity. 
         [0049]    The manufacturing of silicon solar cells typically includes several steps namely; 
         [0000]    1) the transfer of SiO 2  into a Si ingot;
 
2) the transfer of the Si ingot to the Si wafer by sawing, etching, doping, ARC and other surface-treatments;
 
3) screen-printing and drying the back side silver (Ag) paste on the back side of the wafer;
 
4) screen-printing and drying aluminum (Al) paste on the back side of the wafer;
 
5) screen-printing and drying the front side silver (Ag) paste on the front side of the wafer;
 
6) co-firing the coated wafer in a furnace wherein the wafer goes through a temperature curve optimized for the overall efficiency of the device.
 
         [0050]    Thus the Al and Ag metals in the two back side coatings form a physical contact with the Si wafer through penetrating SiO 2  on the back side. Furthermore they also form a contact with each other through the overlapping area. The front side Ag paste penetrates the anti-reflection layer and reaches n-type Si beneath it and a good ohmic contact is formed between Ag lines and the n-Si emmiter during the firing process. The contact resistance between the Ag lines and the emitter for the current flow is preferred to be minimal to maximize the efficiency of the device. In general, a thin layer of glass frits between the emitter and Ag traces is also preferred and results in higher efficiency. 
         [0051]    The present invention has been described in detail, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the art, upon consideration of the present disclosure, may make modifications and/or improvements on this invention that fall within the scope and spirit of the invention. 
         [0052]    The invention is further described by the examples given below. 
       EXAMPLES 
       [0053]    The following examples illustrate specific aspects of the present invention and are not intended to limit the scope thereof in any respect and should not be so construed. 
       Example 1 Procedure for Making Ag Paste 
     Step 1 
       [0054]    The varnish in Table 1 was made by dissolving rosin(s) and thixotropic agent(s) in a solvent (ingredients 1-3). The varnish is a mixture of solvent, thixotropic polymer, resins such as ethylene cellulose, polycarbonate, and rosin such as ester of hydrogenated rosin and hydrogenated castor oil. These can immerse glass frit(s), Ag powders and other solids, and make the paste fluidic enough to be capable of going through stainless-steel-mesh/emulsion channels with 30-100 micron in channel width, 30-55 micron in mesh thickness and 10-30 micron in emulsion thickness, forming paste finger lines on the wafer. However, the varnish preferably allows the printed finger lines to have a thixotropy suitable to minimize the paste from spreading, thus more area is left for capturing sunlight to convert to electricity. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Varnish Formulation for use with Examples A-F and 1-4 
               
             
          
           
               
                   
                 Ingredient 
                 wt. % 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 Ester of Hydrogenated 
                 11.5 
               
               
                   
                   
                 Rosin (Eastman) 
               
               
                   
                 2 
                 Crayvallac Super (Arkema) 
                 14.1 
               
               
                   
                   
                 (thixotropic agent) 
               
               
                   
                 3 
                 Texanol 
                 74.4 
               
               
                   
                 4 
                 Dispersant (Altana BYK) 
                 0.2 
               
               
                   
                   
                 Total 
                 100.0 
               
               
                   
                   
               
             
          
         
       
     
       Step 2 
       [0055]    The dispersant (ingredient 4) is then added into the above mixture and was aggressively mixed until it became uniform. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Front-Side Silver Paste formulations 
               
             
          
           
               
                   
                 Ingredient 
                 wt. % 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 Ag powder (Ames GoldSmith Corp.) 
                 82.2 
               
               
                   
                 2 
                 V2173 (3M Cerodyne Viox Inc.) 
                 2.5 
               
               
                   
                   
                 Glass Frit 
               
               
                   
                 3 
                 ZnO (HorseHead Corp.) Additive 
                 5.5 
               
               
                   
                 4 
                 Table 1 Varnish 
                 7.8 
               
               
                   
                 5 
                 Texanol 
                 2.0 
               
               
                   
                 6 
                 Bismuth Additive (as needed) 
                 see Examples 
               
               
                   
                   
                   
                 A-F &amp; 1-4 
               
               
                   
                   
                 Total 
                 100.0 
               
               
                   
                   
               
             
          
         
       
     
       Step 3 
       [0056]    The mixture from step (2) was aggressively mixed with glass frit(s), solvent and additives, including bismuth additives as needed (Table 2—ingredients 2, 3, 4, 5, 6). The glass frits are commercially available lead borosilicate from 3M Cerodyne Viox Inc. and a typical frit such as V2173, V2172, V0981 may be used alone or as the mixture of in the final paste. 
       Step 4 
       [0057]    Ag powder(s) (ingredient 1) was then added to the step (3) mixture and mixed aggressively with DAC speed mixer from FlackTek Inc. 
       Step 5 
       [0058]    The mixture from step (4) is then triple-roll milled to a preferred grind of 6-9 μm. The preferred viscosity of the resulting pastes at 10/s is 50-250 Pa*s, more preferably 70-150 Pa*s as measured on AR-2000EX rheometer from TA Instruments. 
         [0059]    The three main requirements of the paste are 1) electrical performance, mainly efficiency; 2) green strength (i.e. the lines will hold their integrity and will be resistant to smear during a finger rub test after drying and before firing; 3) ribbon adhesion after firing. 
         [0060]    It has been found that Bi 2 O 3  powder with an average particle size of 10 nm to 3 μm can increase the efficiency of the paste. The following experiments using Bi 2 O 3  of differing particle size show that the above range is the most advantageous for improving silver paste efficiency. 
       Solar Cell Fabrication and Test Performance for Pastes: 
       [0061]    A 5 inch mono-crystalline wafer with an emitter sheet resistance of 80 to 90 Ohm/square are used in this test and 3 steps as described below are used for preparation: 1) 1.0 g of Al paste is screen-printed on the back-side of each Si wafers, it is then dried using BTU International D914 dryer with the setting of belt speed=90 ipm, 310° C. (Zone 1), 290° C. (Zone 2), and 285° C. (Zone 3). The screen used for printing is 325 mesh, 23 micron wire diameter, and 10 micron emulsion, 45 degree bias, the squeegee used is 65-75 shore in hardness; 2) the front-side Ag paste is screen-printed on the front surface of the same wafer and it is dried in the same drying furnace with the setting of belt speed=165 ipm, 340° C. (Zone 1), 370° C. (Zone 2), and 370° C. (Zone 3). The screen used for printing is 325 mesh, 23 micron wire diameter, and 16 micron emulsion, 22.5 degree bias, the squeegee used is 65-75 shore in hardness; 3) the wafers are fired using BTU International PV309 firing furnace with the setting of belt speed belt speed=200 ipm, 850° C. (Zone 1), 790° C. (Zone 2), 790° C. (Zone 3), and 1000° C. (Zone 4). The electrical performance (open-circuit voltage Voc (V), efficiency, fill factor, series resistance and shunt resistance in the dark and under light) is measured using a Solar Simulator/I-V tester from PV Measurements Inc. The illumination of the lamp was calibrated using a sealed calibration cell, and the measured characteristics were adjusted to the standard AM1.5G illumination conditions (1000 mW/cm 2 ). During testing, the cells were positioned on a vacuum chuck located under the lamp and the chuck temperature was maintained at 25° C.+/−1 using a chiller. Both dark and light I-V curves were collected by sweeping voltage between −0.2V and +1.2V and measuring current. Standard solar cell electrical parameters were collected from the instrument including Cell efficiency (%), Series resistance (Rs), Shunt Resistance (Rsh) and Open Circuit Voltage (Voc), short-circuit current (Isc), and short-circuit current density (Jsc). The Cell efficiency 11 is a key parameter in evaluating the performance of a solar cell. The fill factor is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. Graphically, the fill factor is the division of the area of the largest rectangle which could fit between the I-V curve and IN axes by Isc*Voc. The results were obtained using standard computer software available in the industry for measuring electrical parameters of solar cells. 
       Inventive Examples A-F; Comparative Example G 
       [0062]    An example of a control paste (Comparative Example G) is the formulation in Table 2 with no bismuth additive. The Example pastes A-F in Table 3 contain 0.2% Bi 2 O 3  added to the Table 2 formulations with different particle sizes. The pastes were mixed with the DAC mixer vigorously and are designated as the final pastes used in this experiment. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Comparison among different particle sizes of Bi 2 O 3   
               
             
          
           
               
                   
                   
                   
                 Average 
                   
               
               
                   
                   
                   
                 Particle 
               
               
                   
                 Paste 
                 Bi 2 O 3   
                 Size* 
                 Efficiency 
               
               
                   
                   
               
             
          
           
               
                   
                 A 
                 Bi 2 O 3  - 1 
                 38 
                 nm 
                 17.8% 
               
               
                   
                 B 
                 Bi 2 O 3  - 2 
                 ~140 
                 nm 
                 17.7% 
               
               
                   
                 C 
                 Bi 2 O 3  - 3 
                 ~300 
                 nm 
                 15.9% 
               
               
                   
                 D 
                 Bi 2 O 3  - 4 
                 ~300 
                 nm 
                 16.0% 
               
               
                   
                 E 
                 Bi 2 O 3  - 5 
                 ~3 
                 um 
                 10.7% 
               
               
                   
                 F 
                 Bi 2 O 3  - 6 
                 ~20 
                 um 
                 3.7% 
               
               
                   
                 G 
                 without Bi 2 O 3   
                   
                   
                 6.2% 
               
               
                   
                   
                 additive 
               
               
                   
                   
               
               
                   
                 *average particle size is based on manufacturer&#39;s specifications 
               
             
          
         
       
     
         [0063]    Table 1 shows that 1) the Si solar cells efficiency is 6.2% without the Bi 2 O 3  additive in the paste and that after adding Bi 2 O 3 , the greatest efficiency achieved is 17.8%; 2) the solar cell&#39;s efficiency is dramatically altered with different Bi 2 O 3  particle size added into the Ag paste; 3) in general the smaller particle size the better efficiency. In Example F, the relatively large particle size detracts from the efficiency and thus this would not be a preferred material. 
       Inventive Example B; Comparative Examples 1-4 
       [0064]    The same procedure described above in the Inventive Examples A-G for making a paste is used for making the Example 1-4 pastes. Examples 1-4 were prepared by adding the molar concentration equivalent of bismuth additives as shown in Table 4 into the formulation shown in Table 2. These new materials are Bi contained inorganic compounds. 6 inch multi-crystalline wafer with emitter sheet resistance of 75 to 85 Ohm/square are used in this test. Solar cell fabrication and test performance for pastes are similar to those processes described for example A-G. This controlled experiment clearly shows the role played by pure Bi 2 O 3  with the right particle size. The purity is preferably &gt;98%, preferably &gt;99.9%. The right Bi source in the paste can help form a good contact between n-Si layer and Ag, thus a lower series resistance Rs, in turn a significantly higher efficiency. 
         [0065]    Table 4 shows that Example B, which is based on Bi 2 O 3 -2 (see Table 3) of average particle size of 140 nm, imparts far better efficiency and much reduced resistance in comparison with other bismuth additives. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Comparison among different Bi additives 
               
             
          
           
               
                   
                 Electrical Performance 
               
             
          
           
               
                   
                 Chemical 
                 Efficiency 
                 Rs 
                 Voc 
                 Jsc 
                 F.F. 
               
             
          
           
               
                 Paste 
                 structure 
                 chemical name 
                 (%) 
                 (Ohm*cm2) 
                 (mV) 
                 (mA/cm2) 
                 (%) 
               
               
                   
               
             
          
           
               
                 1 
                 Bi 2 S 3   
                 Bismuth(III) 
                 2.00 
                 — 
                 620.4 
                 13.8 
                 23.9 
               
               
                   
                   
                 sulfide 
               
               
                 2 
                 Bi 2 Te 3   
                 Bismuth(III) 
                 8.51 
                 9.91 
                 624.8 
                 33.7 
                 40.1 
               
               
                   
                   
                 telluride 
               
               
                 3 
                 Bi 2 Se 3   
                 Bismuth(III) 
                 8.23 
                 9.55 
                 624.2 
                 33.4 
                 38.9 
               
               
                   
                   
                 selenide 
               
               
                 4 
                 BiPO 4   
                 Bismuth(III) 
                 4.04 
                 — 
                 622.9 
                 22.8 
                 27.6 
               
               
                   
                   
                 phosphate 
               
               
                 B (see 
                 Bi 2 O 3 -2 in 
                 Bismuth(III) 
                 17.70 
                 1.21 
                 632.1 
                 36.7 
                 76.4 
               
               
                 Table 3) 
                 Table 3 
                 Oxide 
               
               
                   
               
             
          
         
       
     
         [0066]    The present invention has been described in detail, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the art, upon consideration of the present disclosure, may make modifications and/or improvements on this invention that fall within the scope and spirit of the invention.