Patent Publication Number: US-2011062030-A1

Title: Electrolyte composition

Description:
CROSS-REFERENCE 
     The subject matter described and claimed herein below is also described in German Patent Application No. 10 2009 029 558.5, filed on Sep. 17, 2009 in Germany. This German Patent Application provides the basis for a claim of priority of invention for the invention described and claimed herein below under 35 U.S.C. 119 (a)-(d). 
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present application relates to an electrolyte composition for the deposition of metals onto substrates. The electrolyte composition at least comprises the metal ions of the metal which should be deposited with this composition. 
     2. The Description of the Related Art 
     The galvanic deposition of metals from electrolyte solutions is known in the art. For this purpose the respective metal must be dissolved in the electrolyte solutions to form its ion and to maintain it in this state. For the latter purpose complexing agents are usually used. An important criterion for suitable complexing agents is the sufficiently high stability of the respectively formed complex. Cyanide has often been used in the prior art as the complexing agent, because it forms extremely stable complexes with numerous metals. Especially silver forms excellent complexes with cyanide and provides good results in a deposition process of that metal on surfaces. However cyanide has the disadvantage of being extremely toxic so that its use as a complexing agent is not desired. 
     Therefore much effort has been made to replace the toxic complexing agent cyanide by other complexing agents. One example of this sort of complexing agent is hydantoin and some of its derivatives. The use of hydantoin as a complexing agent is discussed for example in EP 1 918 426 A2. 
     A further organic complexing agent, which is known in prior art, is succinimide as well as its derivatives. This complexing agent is provided for example in U.S. Pat. No. 4,126,524. 
     Known electrolyte compositions free of cyanide have one crucial disadvantage, namely very low deposition rates of only 0.8 to 1 μm per minute. A low deposition rate must be considered in long-term design of a facility for electrolytic deposition. Not only the acquisition and maintenance costs of the facility will be greatly increased, but also the cost of the electrolyte composition. Therefore because of the high prices of particular metals, such as silver, a high deposition rate becomes increasingly desirable. 
     A suitable electrolyte composition should also have the ability to form complexes with foreign metal ions, which are not intended to be deposited onto the respective substrate, and thus to maintain the stability of the electrolyte composition during the whole deposition process and also thereafter. It is also desirable that the electrolyte composition survives fluctuations of the pH occurring during the galvanization process. Succinimide and hydantoin derivatives do not have these properties. 
     It is particularly desirable to provide electrolyte compositions which are suitable for a deposition of metals, in particular silver, onto solar cells. On the one hand, the galvanic deposition of silver onto solar cells is particularly desirable, because the applied layer has a conductivity which is near the theoretical conductivity, but on the other hand it is technically challenging. Usually, solar cells have a back side of aluminium. Therefore a mild alkaline pH is extremely important, because otherwise the aluminium would be dissolved which is not desired. An ideal pH of 9 to 10 has to be provided during the whole deposition process which requires special electrolyte compositions. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to provide an electrolyte composition which combines a high deposition rate with a high stability with respect to foreign ions and fluctuations of the pH and which at the same time does not have toxicity. In addition, the electrolyte composition should be suitable to be used for deposition of metals, in particular silver, onto solar cells. 
     The aforesaid object is attained according to the invention by the subject matter of the appended patent claims. 
     This object is in particular attained by an electrolyte composition for the deposition of metals onto a substrate, particularly onto a solar cell, which comprises at least one metal ion and which is characterized by the presence of an iminodisuccinate derivative as a complexing agent. In the following, the iminodisuccinate derivative of the present invention is abbreviated as IDSD. 
     The IDSD in the electrolyte composition of the present invention is characterized by the following chemical formula (I): 
     
       
         
         
             
             
         
       
     
     In the above formula each of the R1, R2, R5 and R6 groups is selected from the group consisting of O, S and NH; each of the R3, R4, R7 and R8 groups is selected from the group consisting of OH, NH 2 , SH, O −  and S − , and when at least one of the R3, R4, R7 and R8 groups is an O −  or S −  the IDSD is a salt comprising a sodium and/or potassium cation. The IDSD has the advantage of excellent complexing properties for numerous metals so that the composition according to the present invention is well suited for the deposition of a large number of metals. At the same time, the sensitivity of the electrolyte composition is decreased, since also impurities in the form of foreign ions can form complexes so that in the solution no precipitation takes place. In addition, IDSD is not toxic and is biodegradable. The IDSD of the present invention is preferably used in the form of its alkali metal salts. These are for example the sodium salt or the potassium salt, wherein the sodium salt is a preferred embodiment of the present invention. In particularly preferable embodiments the IDSD is an iminodisuccinate, in particular tetrasodium iminodisuccinate. 
     According to the present invention the IDSD can be contained in the electrolyte composition in a concentration of 50 g/L to 250 g/L, provided that the IDSD is the main complexing agent in the composition. In preferred embodiments the IDSD is the main complexing agent and is present in the electrolyte composition in a concentration of 150 g/L to 225 g/L. 
     The term “main complexing agent” in the sense of the present invention means that the ratio of the complexing agent which is designated as the main complexing agent to each other complexing agent in the electrolyte composition is at least 2:1. Preferably, this ratio is 5:1, further preferably it is 10:1 and most preferably the complexing agent designated as the main complexing agent is the only complexing agent in the electrolyte composition. 
     In alternative embodiments the IDSD is used in combination with another complexing agent. In these embodiments the IDSD is used in a quantitatively smaller proportion than in cases in which the IDSD is the main complexing agent. In embodiments in which the IDSD is not the main complexing agent, but is used in combination with another complexing agent the proportions in which the IDSD is present in the electrolyte composition of the present invention are from 2 g/L to 40 g/L, preferably from 5 g/L and 20 g/L. 
     Under consideration of the above-mentioned amounts the IDSD can substantially increase the current efficiency of the deposition process. 
     A further potential complexing agent, which can advantageously be used in the electrolyte composition, is hydantoin or one or more of its derivatives. The group of the complexing agents “hydantoin or one or more of its derivatives” is abbreviated in the following by the generic name, hydantoin. Thus, the scope of the term “hydantoin” in the following encompasses 1-methyl hydantoin, 1,3-dimethyl hydantoin, 5,5-dimethyl hydantoin and 1-hydroxymethyl-5,5-dimethyl hydantoin. In particularly preferable embodiments besides the IDSD 5,5-dimethyl hydantoin is used. 
     In preferred embodiments the additional complexing agent can be used in the electrolyte composition in a concentration of 50 g/L to 250 g/L. More preferably the concentration is in a range from 100 g/L to 175 g/L. As already mentioned above, hydantoin cannot form complexes with many foreign ions. Here the term “foreign ions” means ions of nickel, chromium and iron. However, when hydantoin is used together with IDSD, an electrolyte solution with a very low sensitivity can be prepared. This combination of complexing agents has the additional advantage that higher amounts of the metal ion can be complexed. For example, the complexing agent IDSD alone can only complex up to 20 g/L of silver, but in combination with 5,5-dimethyl hydantoin up to 60 g/L can be complexed. 
     The electrolyte composition according to the present invention is preferably free of sulfonic acid derivatives. Furthermore, the electrolyte composition according to the present invention is preferably free of cyanides. 
     In preferred embodiments the electrolyte composition according to the present invention comprises a conductivity additive. The conductivity additive improves the conductivity of the electrolyte composition and thus reduces the cell voltage during the galvanization process. This is desirable, because with a better conductivity of the electrolyte solution an improved current efficiency can be achieved. According to the present invention, preferably the conductivity additive is a citrate. This citrate is preferably present in a concentration of 20 g/L to 75 g/L, more preferably 30 g/L to 50 g/L. In this case citrate has not only the function of a conductivity additive, but does also form complexes with calcium and magnesium ions. In addition, the use of citrate has the advantage that the electrolyte solution is not sensitive with respect to the incorporation of acids, since citrate can act as a buffer when fluctuations of pH occur. Therefore, this addition provides an advantage regarding the stability of the composition. A preferred citrate for use in the electrolyte composition of the invention is tripotassium citrate. 
     The electrolyte composition according to the present invention is preferably adjusted to a pH of 8 to 12, more preferably 9.5 to 11. The IDSD can advantageously be used in the aforesaid pH ranges, since the degree of protonation of the complexing groups is sufficiently low. 
     The electrolyte composition according to the present invention is suitable for deposition of different kinds of metals onto numerous substrates. Preferably, the electrolyte composition comprises metal ions, which are ions of silver, calcium, magnesium, iron, chromium, cobalt, nickel, copper, tin and/or aluminium. In particularly preferable embodiments the electrolyte composition comprises silver ions. 
     Preferably, the metal ions are present in the electrolyte composition in a concentration of 10 g/L to 60 g/L, preferably of 15 g/L to 40 g/L. A particularly preferable embodiment relates to an electrolyte composition in which IDSD is the only complexing agent and in which the proportion of metal ions is limited to at most 20 g/L. 
     In preferred embodiments of the present invention the electrolyte composition comprises a wetting agent in addition to the IDSD. The wetting agent increases the wettability of the substrate and thus results in an easier galvanic deposition. It has surprisingly been found that the addition of a wetting agent allows the deposition of glossy metal layers onto the substrate. On the contrary, with the methods and electrolyte compositions of the prior art only lustreless surfaces can be produced. The wetting agents are surface-active substances comprising a hydrophilic and a lipophilic part. The hydrophilic part preferably comprises a polyalkylene oxide chain, in particular a PEG chain. The wetting agent polyethylene glycol octyl (3-sulphopropyl)diether is most preferable. 
     The use of the electrolyte composition for the galvanic deposition of metals onto substrates is a further part of the present invention. Preferably, the substrates are selected from electrically conductive material. In particular, glass, metal, metal alloy and/or semiconductor substrates are preferable. Glass and silicon substrates are particularly preferred, but silicon is the most preferred substrate material. 
     A method for silvering substrates, in particular the above-mentioned substrates is also another aspect of the present invention. The method preferably comprises the steps of placing the substrate in the electrolyte composition according to the present invention and applying a voltage between an anode and a cathode in electrical contact with the electrolyte composition, wherein the cathode is the substrate. 
     Preferably, the current density is about 1.5 A/dm 2 . 
     With an increase of temperature the substrate can be electroplated with an increased current density. Temperatures of 0 to 100° C., particularly of 20 to 70° C. are preferred. 
     Preferably, the electrolyte composition is stirred during the galvanization process, to allow homogenous deposition. 
     Surprisingly it has been found that the electrolyte compositions according to the present invention are suitable to deposit metals even without current. So in particular embodiments of the method according to the present invention no voltage is applied. In this case for example it is possible to apply a metal layer onto an electrically conductive glass or a sheet brass. For such a method the temperature preferably has to be at least 40° C. 
    
    
     EXAMPLES 
     The following specific embodiments of the present invention do not limit the scope thereof, but are for illustration. 
     Comparative Composition 1 
     An electrolyte composition composed of the following components was tested: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Silver as silver methane sulfonate 
                 30 g/L 
               
               
                   
                 Tripotassium citrate monohydrate 
                 40 g/L 
               
               
                   
                 Potassium hydroxide 
                 65 g/L 
               
               
                   
                 5,5-Dimethyl hydantoin 
                 130 g/L  
               
               
                   
                   
               
            
           
         
       
     
     The pH of the composition was 10.3. 
     Exemplary Composition 1 
     An electrolyte composition composed of the following components was tested: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Silver as silver methane sulfonate 
                 30 g/L 
               
               
                   
                 Tripotassium citrate monohydrate 
                 40 g/L 
               
               
                   
                 Potassium hydroxide 
                 65 g/L 
               
               
                   
                 5,5-Dimethyl hydantoin 
                 130 g/L  
               
               
                   
                 Tetrasodium iminodisuccinate 
                 10 g/L 
               
               
                   
                   
               
            
           
         
       
     
     The pH of the composition was 10.3. 
     Test Procedure: 
     The above-mentioned electrolyte according to example 1 of the invention and the electrolyte according to the comparative example were filled into separate beakers, and in each beaker a brass sheet having the dimensions of 10 cm×7 cm was electroplated on one side with non-pulsed direct current for 10 min. Both electrolytes were stirred with a respective stirrer at 400 rpm and the current strength was 0.75 A. Higher current densities could not be achieved with these electrolytes in the beakers. 
     After the galvanization process on the brass sheet, which has been treated with the solution according to the present invention, a weight increase of 498 mg was measured, whereas the weight increase in the case of the comparative electrolyte composition without IDSD was only 333 mg. 
     In a further test the coating of solar cells with the electrolyte compositions of the comparative example and the example 2 according to the present invention was performed. This test was otherwise conducted in the same way as the first test, except that the electrolyte compositions were stirred with respective stirrers at 210 rpm and a current strength of 150 mA was used over a time period of 7.5 min. 
     Exemplary Composition 2 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Silver as silver methane sulfonate 
                 30 g/L 
               
               
                   
                 Tripotassium citrate monohydrate 
                 40 g/L 
               
               
                   
                 Potassium hydroxide 
                 65 g/L 
               
               
                   
                 5,5-Dimethyl hydantoin 
                 130 g/L  
               
               
                   
                 Tetrasodium iminodisuccinate 
                  5 g/L 
               
               
                   
                   
               
            
           
         
       
     
     The pH of the composition was 10.3. 
     The following specific examples for the deposition of silver onto a solar cell were conducted with the same current density and over the same period of time. The solar cell was of the size of 125×125 mm with 6% of conductive area. 
     With the electrolyte composition according to the present invention in the case of a voltage of 0.7 V a weight increase of 353 mg on the solar cell was measured. The current efficiency was 70.5%. The electroplated layer thickness at the contact finger was 11.9 μm. 
     In the case of a voltage of 0.59 V with the electrolyte solution of the comparative example a weight increase of 320 mg on the solar cell was measured. The current efficiency was 63%. The electroplated layer thickness at the contact finger was 9 μm. 
     Thus it has been shown that, when IDSD is included in the electrolyte compositions with otherwise the same conditions, an improvement of electrolyte efficiency can be achieved. The electroplated layer thickness correlates with the galvanization time. As shown, for the same conditions with respect to current density and time according to the present invention a layer thickness which is increased by more than 30% could be achieved. In the case of intended mean layer thicknesses of about 7 to 8 μm, with the use of the electrolyte composition according to the present invention a considerable advantage in time of much more than 30% can be achieved. 
     Exemplary Composition 3 
     An electrolyte composition according to the present invention with IDSD as the only complexing agent comprises the following components: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Tetrasodium iminodisuccinate (Baypure CX100 
                 200 g/L 
               
               
                   
                 from Lanxess) 
               
               
                   
                 Tripotassium citrate monohydrate 
                  40 g/L 
               
               
                   
                 Potassium hydroxide 
                  65 g/L 
               
               
                   
                 Silver as silver methane sulfonate 
                  18 g/L 
               
               
                   
                 Ionogenic wetting agent EA 15-90 Raschig 
                  7 ml/L 
               
               
                   
                 Methane sulfonic acid (for pH adjustment to 10.0) 
                  10 ml/L 
               
               
                   
                   
               
            
           
         
       
     
     Exemplary Composition 4 
     An electrolyte composition according to the present invention with IDSD as the only complexing agent comprises the following components: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Tetrasodium iminodisuccinate (Baypure CX100 
                 200 g/L 
               
               
                   
                 from Lanxess) 
               
               
                   
                 Sodium methane sulfonate 
                  40 g/L 
               
               
                   
                 Potassium hydroxide 
                  65 g/L 
               
               
                   
                 Silver as silver methane sulfonate 
                  18 g/L 
               
               
                   
                 Ionogenic wetting agent EA 15-90 Raschig 
                  7 ml/L 
               
               
                   
                 Methane sulfonic acid (pH adjusted to 10.0) 
                  10 ml/L 
               
               
                   
                   
               
            
           
         
       
     
     Test for Improving of Conductivity with Citrate 
     Comparative Composition 2 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Silver as silver methane sulfonate 
                 30 g/L 
               
               
                   
                 Tripotassium citrate monohydrate 
                  0 g/L 
               
               
                   
                 Potassium hydroxide 
                 65 g/L 
               
               
                   
                 5,5-Dimethyl hydantoin 
                 130 g/L  
               
               
                   
                   
               
            
           
         
       
     
     The pH of the composition was 10.3. 
     Exemplary Composition 5 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Silver as silver methane sulfonate 
                 30 g/L 
               
               
                   
                 Tripotassium citrate monohydrate 
                 20 g/L 
               
               
                   
                 Potassium hydroxide 
                 65 g/L 
               
               
                   
                 5,5-Dimethyl hydantoin 
                 130 g/L  
               
               
                   
                   
               
            
           
         
       
     
     The pH of the composition was 10.3. 
     Results 
     In the case of a process temperature of 40° C., the electrolytes were stirred with a stirrer at 130 rpm and a current density of 0.75 A. With an electrolyte solution containing citrate it was possible to deposit 347 mg of silver onto the sheet brass, whereas in the case of an electrolyte solution without citrate only 154 mg of silver was deposited. The voltage of 2.4 V and 4 V, respectively, is a measure for the conductivity of the composition. 
     While the invention has been illustrated and described as embodied in an electrolyte composition, it is not intended to be limited to the details shown, since various modifications and changes may be made without departing in any way from the spirit of the present invention. 
     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention. 
     What is claimed is new and is set forth in the following appended claims.