Patent Publication Number: US-2007113889-A1

Title: Composition for semiconductor electrode sintered at low temperature and dye-sensitized solar cell comprising the composition

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
      This application claims the benefit of Korean Patent Application No. 10-2005-0112959, filed on Nov. 24, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a dye-sensitized solar cell, and more particularly, to a dye-sensitized solar cell including a semiconductor electrode containing titanium dioxide nanoparticles.  
      2. Description of the Related Art  
      Dye-sensitized solar cells are photoelectrochemical solar cells that were invented by Michael Gratzel et al. in 1991. Since dye-sensitized solar cells are less expensive than other solar cells and have an energy conversion efficiency of about 11%, the dye-sensitized solar cells are expected to be a next generation solar cell that will replace typical silicon solar cells. In general, a dye-sensitized solar cell includes a transparent conductive electrode coated with a nanocrystalline oxide material onto which dye molecules are adsorbed, an opposite electrode coated with metal nanoparticles such as platinum, or carbon, and an iodine based electrolyte for oxidation and reduction reactions.  
      In a typical method of manufacturing a dye-sensitized solar cell using a transparent conductive glass substrate, a nanocrystalline titanium dioxide (TiO 2 ) film is coated on a glass substrate and is subjected to a thermal process at a high temperature of 450° C. or higher to obtain a TiO 2  based electrode. In detail, to obtain a highly viscous coating solution, a colloid solution including TiO 2  nanoparticles is mixed with a high polymer such as carbowax. The coating solution is coated on the glass substrate, and a thermal process is performed thereon at a high temperature of approximately 450° C. to approximately 500° C. under an air or oxygen atmosphere. The thermal process is performed at a high temperature of 450° C. or higher to remove the high polymer through combustion, improve adhesiveness between the nanoparticles and the transparent conductive substrate, and to induce necking or interconnections between the nanoparticles. Therefore, the nanocrystalline TiO 2  film manufactured at 450° C. or higher has good reciprocal interconnections between the nanoparticles, and thus has good photoelectric conversion efficiency.  
      A transparent conductive plastic substrate needs to be used instead of a transparent conductive glass substrate to manufacture flexible dye-sensitized solar cells. TiO 2  electrodes of plastic based dye-sensitized solar cells need to be formed at a certain temperature or lower so as to protect the plastic substrate. For instance, in the case of a polyethylene terephthalate (PET) substrate, the temperature should be lower than about 150° C. A TiO 2  film formed at a low temperature should have good reciprocal interconnectivity. Therefore, a colloid solution including TiO 2  nanoparticles that can be coated at a high temperature and to which high polymers such as carbowax are added cannot be used with the plastic substrate. A TiO 2  based coating solution that can be coated at a low temperature and contains no high polymer needs to be developed to manufacture a TiO 2  film having good reciprocal interconnectivity at a low temperature.  
      According to a conventional method of manufacturing such a low temperature coating solution (i.e., a solution that can be coated at a low temperature), TiO 2  nanoparticles are manufactured by dispersing TiO 2  in water or alcohol. It is often difficult to control the viscosity of the coating solution using this manufacturing approach, and as a result, the coating thickness and other coating conditions cannot be easily controlled. When only water or alcohol is used to disperse the TiO 2 , the reciprocal interconnectivity between TiO 2  particles at a low temperature may not be easily induced. Accordingly, a TiO 2  paste that can be coated at a low temperature needs to be developed.  
     SUMMARY OF THE INVENTION  
      The present invention provides a composition for a semiconductor electrode of a dye-sensitized solar cell that can be sintered at a low temperature by ensuring reciprocal interconnectivity between nanoparticles.  
      The present invention also provides a method of manufacturing a composition for a semiconductor electrode of a dye-sensitized solar cell that can be sintered at a low temperature.  
      The present invention also provides a dye-sensitized solar cell in which damage to a substrate is low and photoelectric conversion efficiency is high by using a composition for a semiconductor electrode that can be sintered at a low temperature.  
      The present invention also provides a method of manufacturing a dye-sensitized solar cell using a composition for a semiconductor electrode that can be sintered at a low temperature.  
      According to an aspect of the present invention, there is provided a composition for a semiconductor electrode of a dye-sensitized solar cell, the composition including: a colloid solution containing a nanocrystalline oxide material; and an aqueous base solution.  
      The nanocrystalline oxide material may be a compound selected from the group consisting of TiO 2 , ZnO, and Nb 2 O 5 . The aqueous base solution may be an aqueous ammonia solution.  
      According to another aspect of the present invention, there is provided a method of manufacturing a composition for a semiconductor electrode of a dye-sensitized solar cell, the method including: preparing a colloid solution containing a nanocrystalline oxide material by causing a hydrothermal reaction between the nanocrystalline oxide material and a solvent; replacing the solvent for the colloid solution with an alcohol through a substitution reaction; and adding an aqueous base solution to the colloid solution obtained through the substitution reaction. The manufactured paste composition can be used for a semiconductor electrode of a dye-sensitized solar cell.  
      According to another aspect of the present invention, there is provided a dye-sensitized solar cell including: a semiconductor electrode obtained by coating a paste composition on a conductive substrate, the paste composition comprising a colloid solution containing a nanocrystalline oxide material and an aqueous base solution; an opposite electrode; and an electrolyte solution interposed between the semiconductor electrode and the opposite electrode.  
      The conductive substrate may be a conductive plastic substrate, a conductive glass substrate, a conductive metal substrate, a semiconductor substrate or a nonconductive substrate. Particularly, the conductive substrate may be a conductive plastic substrate. The semiconductor electrode may further include a layer of dye molecules chemically adsorbed on the paste composition. The layer of dye molecules may include a ruthenium adsorbent. The opposite electrode may be a conductive transparent substrate or a Pt coated transparent substrate. The electrolyte solution may be an iodine based oxidizing and reducing electrolyte.  
      According to another aspect of the present invention, there is provided a method of manufacturing a dye-sensitized solar cell, the method including: coating a composition of a semiconductor electrode on a first conductivity type substrate, wherein the composition comprises a colloid solution containing a nanocrystalline oxide material, and an aqueous base solution; drying the first conductivity type substrate coated with the composition at room temperature to approximately 200° C.; forming a dye molecular layer on the first conductivity type substrate to obtain the semiconductor electrode; coating a conductive material on a second conductivity type substrate to form an opposite electrode; and interposing an electrolyte solution between the semiconductor electrode and the opposite electrode. The dried semiconductor electrode can further be immersed in a TiCl 4  solution and dried again at room temperature to approximately 200° C. The opposite electrode can be obtained by coating Pt on the other conductive substrate.  
      Despite not including binders, the paste composition can allow sintering of nanoparticles at a low temperature. The sintering of the nanoparticles can be reinforced by treating the semiconductor electrode coated with the paste composition with the TiCl 4  solution. As a result, damage to the substrate caused by a high temperature process can be prevented. Using the semiconductor electrode coated with the paste composition that can be sintered at a low temperature, dye-sensitized solar cells having excellent photoelectric conversion efficiency can be manufactured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1A  illustrates a colloid solution including TiO 2  nanoparticles;  
       FIG. 1B  illustrates a paste composition obtained by adding aqueous ammonia to the colloid solution of  FIG. 1A ;  
       FIG. 2  is a graph illustrating the viscosity of a composition for a semiconductor electrode according to an embodiment of the present invention versus the weight ratio of an aqueous ammonia solution with respect to TiO 2  nanoparticles;  
       FIGS. 3A and 3B  are diagrams illustrating the viscosity of the composition for a semiconductor electrode according to an embodiment of the present invention before and after an aqueous ammonia solution is added to a colloid solution including TiO 2  nanoparticles, respectively;  
       FIG. 4  is a simplified diagram illustrating the configuration of a dye-sensitized solar cell according to an embodiment of the present invention;  
       FIG. 5  is a graph illustrating photocurrent versus voltage in a dye-sensitized solar cell according to an embodiment of the present invention;  
       FIG. 6  is a graph illustrating the incident photo-to-current conversion efficiency (IPCE) of a dye-sensitized solar cell according to an embodiment of the present invention; and  
       FIG. 7  is a graph of photocurrent versus voltage in a dye-sensitized solar cell treated with an aqueous TiCl4 solution according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described more fully with reference to the accompanying drawings, in which a composition for a semiconductor electrode of a dye-sensitized solar cell and a dye-sensitized solar cell comprising the same according to exemplary embodiments of the invention are shown. Exemplary products and test results will be described. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.  
      Although a composition according to embodiments of the present invention may include TiO 2  nanoparticles, zinc oxide (ZnO) or diniobium pentaoxide (Nb 2 O 5 ) as a nanocrystalline oxide material, a semiconductor electrode including TiO 2  is exemplified in the embodiment described herein.  
      An aqueous colloid solution including TiO 2  nanoparticles is prepared as follows. Titanium isopropoxide, acetic acid, isopropanol, and water are reacted at approximately 230° C. for approximately 12 hours using a hydrothermal synthesis method well known in the art. Water is separated from the result using a centrifuge, and alcohol is redistributed thereafter. In order for the composition to be coated at a low temperature, the colloid solution includes approximately 5 to 20 wt % of TiO 2  nanoparticles, more particularly, approximately 10 to 15 wt % of TiO 2  nanoparticles.  
      An approximately 10 M aqueous ammonia solution is added dropwise to approximately 10 grams of the colloid solution including approximately 12.5 wt % of the TiO 2  nanoparticles while being stirred with a magnetic stirrer. Particularly, the colloid solution and the aqueous ammonia solution are mixed in a weight ratio of approximately 1:0.1 to 10. The molar concentration of the aqueous ammonia solution may range from approximately 1 M to approximately 10 M. As the aqueous ammonia solution is added to the colloid solution, the colloid solution becomes creamy, and the composition is ready to be used for the semiconductor electrode.  
       FIG. 1A  illustrates a colloid solution including TiO 2  nanoparticles and  FIG. 1B  illustrates a paste composition obtained by adding aqueous ammonia to the colloid solution of  FIG. 1A .  
      Referring to  FIG. 1A , the colloid solution containing nanocrystalline TiO 2  is in a liquid state. On the other hand, when the aqueous ammonia solution is added to the colloid solution, the paste composition, which is highly viscous, is obtained (see  FIG. 1B ).  
       FIG. 2  is a graph of the viscosity of the composition for a semiconductor electrode according to an embodiment of the present invention versus the weight ratio of an aqueous ammonia solution with respect to TiO 2  nanoparticles.  
      Referring to  FIG. 2 , a nanocrystalline TiO 2  containing colloid solution with no aqueous ammonia solution has a low viscosity of approximately 100 cP or lower. The composition for the semiconductor electrode obtained by adding approximately 0.5 wt % of the aqueous ammonia solution based on approximately 100 w % of TiO 2  to the colloid solution has a high viscosity of approximately 30,000 cP. After the addition of 5 wt % of the aqueous ammonia solution to the colloid solution, the composition has a viscosity of approximately 53,000 cP higher than in the case when no aqueous ammonia solution is added. However, the viscosity of the composition decreases when the amount of the aqueous ammonia solution included is greater than 5 wt % based on 100 wt % of the TiO 2  because the amount of distilled water included in the aqueous ammonia increases.  
       FIGS. 3A and 3B  are diagrams illustrating the viscosity of a composition according to an embodiment of the present invention before and after an aqueous ammonia solution is added to a nanocrystalline TiO 2  colloid solution, respectively.  
      Referring to  FIG. 3A , the TiO 2  nanoparticles of the colloid solution having a pH of approximately 1.9 and a viscosity of approximately 100 cP or lower are spaced apart.  
      Referring to  FIG. 3B , when a small amount of the aqueous ammonia solution is added to the colloid solution, a highly viscous paste composition is obtained. The paste composition has a viscosity of approximately 53,000 cP and a pH of approximately 2.2 to 3.6. At this point, some of the TiO 2  nanoparticles are clustered together. In more detail, after the aqueous ammonia solution is added, a surface charge of the TiO 2  nanoparticles, i.e., positive hydrogen ions, decreases due to the neutralization of a base material, and the TiO 2  nanoparticles are flocculated due to an increase in an electrolyte including negative ions obtained from acetic acid and positive ions obtained from ammonium.  
      A method of manufacturing a dye-sensitized solar cell using the above prepared paste composition according to an exemplary embodiment of the present invention will now be described.  
       FIG. 4  is a simplified diagram illustrating the configuration of a dye-sensitized solar cell according to an embodiment of the present invention.  
      Referring to  FIG. 4 , the dye-sensitized solar cell includes a semiconductor electrode  10 , an opposite electrode  20  and an electrolyte solution  30  interposed between the semiconductor electrode and the opposite electrode  20 .  
      The semiconductor electrode  10  is manufactured by coating a paste composition  14  on a transparent conductive substrate  12  using a doctor blade method. Particularly, the paste composition  14  is coated on the transparent conductive substrate  12  such as a transparent conductive plastic substrate or a transparent conductive glass substrate and dried at approximately 150° C. under increasing pressure conditions for approximately 10 to 30 minutes. The transparent conductive substrate  12  coated with the dried TiO 2  is immersed in an approximately 0.01 M to 0.6 M TiCl 4  aqueous solution, more preferably, an approximately 0.1 M to 0.3 M TiCl 4  aqueous solution, for approximately 1 to 10 minutes. The transparent conductive substrate  12  is dried in air and then dried again at approximately 150° C. under increasing pressure conditions for approximately 10 to 60 minutes.  
      The opposite electrode  20  is manufactured by coating another transparent conductive substrate  22  with platinum  24 . The platinum  24  of the opposite electrode  20  is disposed to face the paste composition  14  of the semiconductor electrode  10 . The semiconductor electrode  10  and the opposite electrode  20  are closely adhered with a high polymer layer therebetween. At this time, heat and pressure are applied to the semiconductor and opposite electrodes  10  and  20  to make the high polymer layer adhere strongly to the surfaces of the semiconductor electrode  10  and the opposite electrode  20 . The electrolyte solution  30  is filled into the space between the semiconductor electrode  10  and the opposite electrode  20  via micro-openings  26  formed in the opposite electrode  20 . The electrolyte solution  30  may include an iodine based oxidizing and reducing electrolyte. After the complete filling of the electrolyte solution  30 , thin glass is heated instantaneously to close the micro-openings  26 .  
       FIG. 5  is a graph of photocurrent versus voltage in a dye-sensitized solar cell according to an embodiment of the present invention.  
      A semiconductor electrode of a dye-sensitized solar cell for a test group was manufactured as follows. A composition for the semiconductor electrode was coated on a transparent conductive substrate to a thickness of approximately 4.2 μm and dried. The transparent conductive substrate was then treated with an aqueous solution of TiCl 4  at a low temperature of approximately 150° C.  
      A semiconductor electrode of a dye-sensitized solar cell for a comparison group was manufactured as follows. A paste composition including a high polymer binder containing TiO 2  was coated on a transparent conductive substrate to a thickness of approximately 4.7 μm and thermally treated at approximately 500° C. for approximately 30 minutes.  
      Photocurrent and voltage characteristics of semiconductor electrodes of the test group and the comparison group were evaluated. The evaluation results are shown in  FIG. 5  and Table 1 below. In  FIG. 5 , (c) and (d) represent the comparison group and the test group, respectively. Referring to  FIG. 5 , the test group and the comparison group exhibited similar electric characteristics. Table 1 below shows the details of the electric characteristics. When AM 1.5G-1 solar energy (1,000 Wm −2 ) was applied, the semiconductor electrode of the test group had an energy conversion efficiency of approximately 4.18%, while the semiconductor electrode of the comparison group had an energy conversion efficiency of approximately 4.27%.  
                                   TABLE 1                                               Energy           Density of   Open Circuit       Conversion           Current   Voltage   Charge   Efficiency           (mAcm −2 )   (V)   Coefficient   (%)                                                        Test Group   8.77   0.704   0.676   4.18       Comparison   9.04   0.712   0.663   4.27       Group                  
 
       FIG. 6  is a graph illustrating the incident photo-to-current conversion efficiency (IPCE) of a dye-sensitized solar cell according to an embodiment of the present invention.  
      Particularly,  FIG. 6  illustrates the IPCEs of the semiconductor electrodes of the test group and the comparison group. In  FIG. 6 , (f) and (e) represent the test group and the comparison group, respectively. Referring to  FIG. 6 , the semiconductor electrode which was manufactured at a low temperature of approximately 150° C. or lower had a similar IPCE to the semiconductor electrode which was manufactured via a high temperature process at approximately 500° C. or higher and had good sintering characteristics between the nanoparticles. Based on this result, the dye-sensitized solar cell according to an embodiment of the present invention has excellent energy conversion efficiency despite excluding binders and being manufactured at a low temperature.  
       FIG. 7  is a graph of photocurrent versus voltage in a dye-sensitized solar cell treated with an aqueous solution of TiCl 4  according to an embodiment of the present invention.  
      Particularly, the dye-sensitized solar cell was manufactured as follows. A semiconductor electrode of the dye-sensitized solar cell was formed by coating a paste composition including approximately 20 wt % light scattering TiO 2  particles (anatase type with a crystalline diameter of approximately 400 nm) on a transparent substrate and then treated with an aqueous TiCl 4  solution. The coating was performed at approximately 150° C. or lower to a target thickness of approximately 4.5 μm. Referring to  FIG. 7  and Table 2 below, the dye-sensitized solar cell had a high energy conversion efficiency of approximately 4.8% under AM 1.5G-1 solar energy (1,000 Wm −2 ). Even though the coating was performed at a low temperature of approximately 150° C. or lower, the dye-sensitized solar cell still had the electric characteristic usually obtained through a high temperature process for the following reasons. First, the composition for the semiconductor electrode according to an embodiment of the present invention ensured interconnectivity between the nanoparticies. Second, the chemical post-treatment using the aqueous TiCl 4  solution reinforced the sintering between the nanoparticles. Therefore, according to an embodiment of the present invention, the nanoparticles could be sintered even at a low temperature of approximately 150° C. or lower.  
                                   TABLE 2                                               Energy           Density of   Open Circuit       Conversion           Current   Voltage   Charge   Efficiency           (mAcm −2 )   (V)   Coefficient   (%)                                                        Test Group   10.16   0.689   0.682   4.8                  
 
      As described above, the composition for the semiconductor electrode according to an embodiment of the present invention includes the TiO 2  nanoparticle containing colloid solution and the aqueous ammonia solution. Despite not including binders, the composition for the semiconductor electrode can be sintered at a low temperature.  
      According to exemplary embodiments of the present invention, in the manufacturing method of the composition for the semiconductor device, the colloid solution containing TiO 2  nanoparticles is synthesized and then the aqueous ammonia solution was added thereto. Accordingly, the composition for the semiconductor electrode that can be sintered at a low temperature can be manufactured effectively.  
      According to exemplary embodiments of the present invention, the dye-sensitized solar cell has excellent electrical characteristics based on the aforementioned composition for the semiconductor electrode.  
      Also, according to exemplary embodiments of the present invention, the semiconductor device can be formed by sintering the nanoparticles even if the paste composition is dried at a low temperature. The electrolyte solution was interposed between the semiconductor electrode and the opposite electrode. After the paste composition is coated, the semiconductor electrode is chemically treated with the TiCl 4  solution. This post-treatment can reinforce the sintering of the nanoparticles. Because the composition for the semiconductor electrode can be coated at a low temperature, damage to the substrate that often results from high temperatures is less likely to occur, and the interconnectivity between the nanoparticles can be improved. As a result, the coating thickness and other coating conditions can be controlled easily.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.