Abstract:
A process for preparing a photoanode of dye-sensitized solar cells (DSSCs) is disclosed, which contains nano TiO 2  and functionalized carbon nanomateiral. The process includes reacting a dispersion of functionalized carbon nanomateiral and a TiO 2  precursor in a liquid organic medium under sol-gel conditions to form a carbon nanomaterial/nano TiO 2  composite colloidal solution; mixing with an aqueous polymer solution, and forming a paste suitable for coating by concentrating the resulting mixture; coating the paste on a conductive glass substrate and calcining the coated layer at 300-520° C. in air for 10-60 minutes to obtain a conductive glass plate having a coating of nanocomposite, which can be used to prepare a photoanode of DSSCs by immersing in a dye solution to adsorb a dye thereon.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to preparation of a photoanode for dye-sensitized solar cells, and more particularly to preparation of a photoanode for dye-sensitized solar cells having a composite of carbon nanomaterials/nano semiconductor. 
       BACKGROUND OF THE INVENTION 
       [0002]    The dye-sensitized solar cells (DSSCs) were originally developed by M. Gratzel, hence they are also called the Gratzel cells. Essentially, a DSSC is comprised of a photoanode, electrolyte, and platinum electrodes. The photoanode is basically a transparent and conductive glass substrate formed by coating a layer of ITO or FTO film over the glass substrate, and an insulating surface formed over the conductive glass substrate includes a semiconductor film having titanium dioxide (TiO 2 ) particles, as well as dyes adsorbed onto the semiconductor film. 
         [0003]    The Taiwan Patent 1241029 discloses a DSSC and electrodes thereof, in which the semiconductor nanocrystal film of the photoanode is further comprised of conductive micro-particles such as metal particles and carbon nanomaterials. However, the carbon nanomaterials in the nanocrystal film often form aggregates due to greater surface energy, and the patent does not disclose methods for adding carbon nanomaterials into the nanocrystal film. Therefore, the search for a photoanode for DSSCs that has both high adsorption for dyes and high conductivity continues to this date. 
       SUMMARY OF THE INVENTION 
       [0004]    A primary objective of the invention is to provide a process for preparing a photoanode for dye-sensitized solar cells (DSSCs), wherein carbon nanomaterials are evenly distributed within a nano semiconductor film thereof, which enhances the dye adsorption of the photoanode, thereby elevating the overall efficiency of the DSSCs. 
         [0005]    A further objective of the invention is to provide a photoanode for DSSCs that has both high dye adsorption and high conductivity. 
         [0006]    In order to accomplish the above-mentioned objectives, a process for preparing a nanocomposite photoanode for dye-sensitized solar cells (DSSCs) provided according to the present invention comprises the following steps: 
         [0007]    a) dispersing functionalized carbon nanomaterials in a liquid medium; 
         [0008]    b) dissolving or dispersing a TiO2 precursor in a dispersion obtained in step a), wherein a weight ratio between said TiO2 precursor and said carbon nanomaterials is in the range of 10000:1 to 100:1; 
         [0009]    c) reacting said precursor under hydrothermal conditions or sol-gel conditions so as to form a colloidal solution of carbon nanomaterial/nano TiO2 composite; 
         [0010]    d) heating said carbon nanomaterial/nano TiO2 composite colloidal solution in an autoclave at 140-350° C. for 5-48 hours, so as to result in anatase TiO2 therein; 
         [0011]    e) mixing the colloidal solution having anatase TiO2 obtained in step d) with a polymer solution; 
         [0012]    f) concentrating the resulting mixture of the colloidal solution and the aqueous polymer solution from step e); 
         [0013]    g) coating a concentrated paste obtained in step f) on an insulating surface of a conductive substrate; 
         [0014]    h) calcining the coated layer obtained in step g) at 300-520° C. in air for 10-60 minutes; 
         [0015]    i) immersing the conductive substrate having a coating of carbon nanomaterial/nano TiO2 composite from step h) in a dye solution, such that the dyes are allowed to adsorb onto the coating of carbon nanomaterial/nano TiO2 composite; and 
         [0016]    j) removing said conductive substrate from the dye solution so as to prepare a nanocomposite photoanode for DSSCs. 
         [0017]    Preferably, the functionalized carbon nanomaterials in step a) include acidic groups, hydroxyl groups, or amino groups as functional groups thereof. More preferably, the functionalized carbon nanomaterials in step a) are acidified single-wall carbon nanotubes, acidified double-wall carbon nanotubes, acidified multi-wall carbon nanotubes, acidified carbon nanohoms, or acidified carbon nanocapsules. Most preferably, the functionalized carbon nanomaterials are acidified single-wall, double-wall, or multi-wall carbon nanotubes. 
         [0018]    Preferably, the carbon nanotubes used in the present invention are multi-wall carbon nanotubes having a length of 1-25 μm, a diameter of 1-50 nm, a specific surface area of 150-250 m 2 /g, and an aspect ratio of 20-2500. 
         [0019]    Preferably, the TiO 2  precursor is titanium alkoxide, titanium chloride, titanium oxysulfate, or titanium sulfate. 
         [0020]    Preferably, the precursor is reacted under sol-gel conditions in step c). 
         [0021]    Preferably, the TiO 2  precursor is titanium alkoxide. More preferably, the TiO 2  precursor is titanium tetra-isopropoxide (TTIP) 
         [0022]    Preferably, the liquid medium in step a) is an alcohol, when the precursor is reacted under sol-gel conditions in step c). More preferably, the liquid medium in step a) is isopropyl alcohol, and the isopropyl alcohol has a weight that is 200-1200% the weight of carbon nanomaterials, while the dispersing is carried out using supersonic treatment. 
         [0023]    Preferably, in step b) the dissolving or dispersing of a TiO 2  precursor in a dispersion obtained from step a) is carried out using supersonic treatment. 
         [0024]    Preferably, the step of reacting the precursor under sol-gel conditions comprises adding water into the mixture obtained in step b), and allowing the titanium alkoxide to undergo hydrolytic and condensation reactions. More preferably, the step of reacting the precursor under sol-gel conditions further comprises adding an acid into the mixture undergoing the hydrolytic and condensation reactions. Most preferably, the water added has a weight that is 100-1000% the weight of carbon nanomaterials, and the acid added is of a volume that adjusts pH value of the mixture undergoing the hydrolytic and condensation reactions to 1-5. 
         [0025]    Preferably, the autoclave in step d) is set at 150-300° C., and the heating time is 10-30 hours. 
         [0026]    Preferably, the conductive substrate is an electrically conductive glass plate having an electrically conductive layer on a surface thereof. 
         [0027]    Preferably, the polymer solution in step e) is an aqueous solution of a polymer having a weight average molecular weight of 200-30000 g/mol therein. More preferably, the polymer is polyol, cyclodextrin, or cellulose. 
         [0028]    Preferably, the polymer is polyethylene glycol, polypropylene glycol, or polybutylene glycol. More preferably, the polymer is polyethylene glycol. 
         [0029]    Preferably, in step f); the mixture is concentrated into a paste comprising 100-250 g of a solid content per liter. 
         [0030]    Preferably, the coating in step g) is carried out by using the doctor-blade method. 
         [0031]    As an example, the functionalized carbon nanomaterials in the invention may be added at 0.1-0.5 wt %, and is preferably added at 0.3-0.5 wt %, based on the total weight of overall nano semiconductor composite materials. 
         [0032]    In a preferred embodiment of the present invention, 0.1 wt % of acidified multi-wall carbon nanotubes were used to prepare a nano semiconductor composite photoanode for DSSCs, which has a dye adsorption rate of 9.62×10 −8  mol/cm 2 ; an open-circuit photovoltage (V oc ) of 0.69V; a short-circuit photocurrent density (J sc ) of 7.73 mA/cm −2 , and a fill factor (FF) of 70.12%. The DSSC resulted therefrom has a conversion efficiency of 3.75%. 
         [0033]    In another preferred embodiment of the present invention, 0.5 wt % of acidified multi-wall carbon nanotubes were used to prepare a nano semiconductor composite photoanode for DSSCs, which has a dye adsorption rate of 1.16×10 −7  mol/cm 2 ; a V oc  of 0.74V; a J sc  of 7.91 mA/cm −2 , and a FF of 72.17%. The DSSC resulted therefrom has a conversion efficiency of 4.22%. 
         [0034]    In yet another preferred embodiment of the present invention, 0.3 wt % of acidified multi-wall carbon nanotubes were used to prepare a nano semiconductor composite photoanode for DSSCs, which has a dye adsorption rate of 1.32×10 −7  mol/cm 2 ; a V oc  of 0.72V; a J sc  of 8.82 mA/cm −2 , and a FF of 73.17%. The DSSC resulted therefrom has a conversion efficiency of 4.62%. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0035]    The structure and the technical means adopted by the present invention to achieve the above and other objectives can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying diagrams, wherein: 
           [0036]      FIG. 1  is a graph that shows the surface roughness factor of a carbon nanotubes/TiO 2  photoanode, in which the horizontal axis represents the used amount of carbon nanotubes, and the vertical axis represents the value of the roughness factor. 
           [0037]      FIG. 2  is a graph that shows the spectra of incident photon conversion to charge carrier conversion efficiency (IPCE) of every single wavelength for DSSCs assembled from photoanodes obtained in Control Example and Examples A1-A3. 
           [0038]      FIG. 3  is a graph that shows curves between photocurrent density and voltage (J-V) for DSSCs assembled from photoanodes obtained in Control Example and Examples A1-A3. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0039]    The present invention discloses a process for forming carbon nanomaterials/nano semiconductor composite materials over an insulating surface of conductive glass substrates, which results in high dye adsorption and high conversion efficiency in the resulted DSSCs. Therefore, photoanodes produced by using the process of the invention is highly applicable in the making of DSSCs. 
         [0040]    The present invention can be further understood by referring to the following embodiments; the embodiments are used for explanatory purposes only, and are not to be used to limit the scope of the invention in any ways. 
         [0041]    Materials used in the following Examples and Control Example include: titanium tetra-isopropoxide (TTIP), acidified multi-wall carbon nanotubes, and polyethylene glycol 20000 (PEG 20000). 
         [0042]    Titanium tetra-isopropoxide (TTIP) has a molecular weight of 284.26 g/mole, and can be purchased from Aldrich Co., St. Louis, Mo., U.S.A. 
         [0043]    Multi-wall carbon nanotubes (MWCNTs) are purchased from a Korean company, CNT Co. Ltd., under a code of C tube   100 , which have a length of 1-25 μm, a diameter of 1-50 nm, a specific surface area of 150-250 m 2 /g, and an aspect ratio of 20-2500 m 2 /g. 
         [0044]    Before actual usage, the multi-wall carbon nanotubes had to undergo acid treatment, which was achieved by using the traditional method of acid washing with nitric acid. Firstly, placed 8g of carbon nanotubes and 400 ml of nitric acid in a three-necked flask, followed by acid refluxing at 120° C. for 8 hours. Afterwards, a large amount of purified water was used to terminate the acidification reaction, and then the reacted nanotubes underwent five rounds of washing and vacuum filtration, and dried in an oven at 70° C., thereby resulting in acidified multi-wall carbon nanotubes as required. After the acid treatment, the multi-wall carbon nanotubes were put under the transmission electron microscope and observed, which found that impurities contained in the MWCNTs before treatment had been effectively removed. Moreover, analysis of the acidified MWCNTs using FT-IR spectra revealed that the surfaces of the acidified MWCNTs were grafted with functional groups that included —C═O (1167 cm −1 ) and —COO (1702 cm −1 ) groups, as reported in relevant literature. 
         [0045]    Dyes: the particular dye used had a molecular weight of 1188.55 g/mole, and is manufactured by the company Solaronix SA Co. (Aubonne, Switzerland) under the name of N719. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0046]    PEG 20000: which has a weight average molecular weight of 20000 g/mole, and is manufactured by the company Mallinckrodt Baker, Inc. under the name of U204-07. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0047]    Isopropyl alcohol: it has a molecular weight of 60.1 g/mole and can be purchased from Aldrich Co., St. Louis, Mo., U.S.A. 
         [0048]    Conductive glass plate with transparent conducting oxide (TCO): which is a fluorine-doped tin oxide (FTO) conductive glass plate purchased from the Japanese company Asahi Glass Co. 
       Examples A1-A3 
     The Preparation of Nanocomposite Photoanode 
       [0000]    
       
         1. The acidified multi-wall carbon nanotubes (MWCNTs) were added into 60 ml of anhydrous isopropyl alcohol according to the proportions listed in Table 1. 
       
     
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Example 
                 Weight of MWCNTs added, in grams (wt %)* 
               
               
                   
                   
               
             
             
               
                   
                 A1 
                 0.0064 (0.1%) 
               
               
                   
                 A2 
                 0.0192 (0.3%) 
               
               
                   
                 A3 
                 0.0320 (0.5%) 
               
               
                   
                   
               
               
                   
                 *wt % = Weight of MWCNTs/(Weight of MWCNTs + weight of TiO 2 ) 
               
             
          
         
       
       
         
           
             The acidified MWCNTs were evenly dispersed in anhydrous isopropyl alcohol under supersonic vibration, which was followed by the addition of 23.5 g (80 mmole) of TTIP thereinto, and the mixture was subjected to supersonic vibration for 30 minutes continuously. The evenly dispersed solution of acidified MWCNTs/TTIP/isopropyl alcohol was then slowly poured into a beaker that contained 150 ml of deionized water, and the pH value of the solution was adjusted to 1.8 by using 69% nitric acid, which was followed by heating the solution to 80° C. and keeping it at this temperature for 8 hours. The isopropyl alcohol was allowed to evaporate completely during heating, which resulted in approximately 130 ml of acidified MWCNTs/TiO 2  colloidal solution. 
           
         
         2. Once the reaction was finished, the MWCNTs/TiO 2  colloidal solution was put into an autoclave and heated at 200° C. for 12 hours. After allowing the solution to cool down, a MWCNTs/anatase TiO 2  colloidal solution was obtained. 
         3. Subsequently, 2.644 g of 40 wt % polyethylene glycol 20000 aqueous solution was added into the aforesaid MWCNTs/anatase TiO 2  colloidal solution, and the resulted mixture was vigorously stirred for 1 hour. The above-mentioned step resulted in approximately 130 ml of mixture, which was concentrated into a highly viscous paste comprising around 170 g of nanocomposites in every liter of mixture under reduced pressure. The paste was then coated on an insulating surface of a FTO conductive glass plate by using the doctor-blade method, and the effective area coated thereon was 0.25 cm 2 . The conductive glass plate was left to dry for 1 hour under room temperature, and then subjected to calcination in air at 45° C. for 30 minutes, so as to obtain a photoanode substrate of nano semiconductor composites as required. 
         4. The fabricated photoanode substrate of nano semiconductor composite materials was immersed in a dye solution having 3×10 −4  M of the N719 dye at room temperature for 10-24 hours. Afterwards, said photoanode substrate with dyes adsorbed thereon was kept in ethanol for 6-15 hours in order to desorb excessive dyes. The N719 dye solution was cis-dithiocyanato-N,N′-bis(2,2′-bipyridyl-4-carboxylicacid-4′-tetrabutyl ammonium carboxylate) ruthenium (II) dissolved in a mixed solvent of acetonitrile and t-butyl alcohol at a ratio of 1:1 (V:V). 
       
     
       Control Example 
       [0054]    The aforesaid steps in Examples A1 to A3 were repeated except that the acidified multi-wall carbon nanotubes were not used, and supersonic vibration was not carried out. 
       Dye Adsorption 
     Method for Testing: 
       [0055]    In order to compare dye adsorption and photoanode roughness between the two groups, the fabricated photoanodes were immersed in N719 dye solution having a concentration of 3×10 −4  M for 10-24 hours, and then the photoanodes were immersed in an aqueous KOH solution having a concentration of 0.1 M to remove the dyes. Subsequently, UV-visible spectroscopy (CARY 50 Conc, Varian) was employed to measure absorbance (a) thereof, so as to find out dye adsorption for each photoanode. 
         [0000]        a=ε×b×c    (Formula 1) 
         [0000]    In Formula 1, a represents the ability of a material to absorb a particular wavelength of light; ε represents the molar extinction coefficient (cm 2 /mole); b is the light path (1 cm), while c is the molar concentration of the dye solution. The measured dye adsorption are listed in Table 2. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Samples 
                 Dye Adsorption (mole/cm2) 
               
               
                   
                   
               
             
             
               
                   
                 A1 
                 9.62 × 10 −8   
               
               
                   
                 A2 
                 1.32 × 10 −7   
               
               
                   
                 A3 
                 1.16 × 10 −7   
               
               
                   
                 Control 
                 8.66 × 10 −8   
               
               
                   
                   
               
             
          
         
       
     
         [0056]    Theoretically, a TiO 2  semiconductor layer having rougher surfaces and more pores would have higher dye adsorption. Therefore, changes in the factor of surface roughness may be used to investigate the effects of adding carbon nanotubes on the surface characteristics of TiO 2  photoanodes, as indicated in  FIG. 1 . In  FIG. 1 , the roughness factor for each photoanode was mainly calculated by using the adsorption of the TiO 2  photoanodes for N719 dyes, and based on the fact that a single N719 dye molecule has a surface area of approximately 1.6 nm 2 , and the unit of the roughness factor is the specific surface area of every unit area of semiconductor film (cm 2 /cm 2 ). The formula for calculating the roughness factor is shown in Formula 2: 
         [0000]        D   ad   ×N   A   ×D   A   =R   f    (Formula 2) 
         [0000]    In Formula 2, D ad  represents the dye adsorption in each unit of area (cm 2 ) in moles (1/ε) for a material, N A  is the Avogadro constant, D A  is the area per dye molecule (1.6 nm 2 /dye molecule), and R f  is the value of the roughness factor of the sample. In theory, when dye molecules are adsorbed onto a TiO 2  photoanode, a mono-layer of dyes is formed thereon, thus the dye adsorption of a photoanode can be used to indicate the surface area thereof. If an electrode adsorbs more dye molecules, it suggests that the surface and internal parts of the electrode are rougher and more porous.  FIG. 1  shows that when the addition of carbon nanotubes was increased from 0 wt % to 0.3 wt %, the roughness factor of the electrode increased from 834 to 1267. This demonstrates that the addition of carbon nanotubes increases the roughness factor of the electrode (the total surface area allowing for dye adsorption). 
         [0057]    Moreover, the rougher the surface an electrode has, the more contact there is between the electrode and the electrolyte, and the higher the efficiency of the photo-induced electron transfer process. As a result, the photoelectric conversion efficiency of a solar cell using the electrode can be elevated as well. 
       Incident Photon Conversion to Charge Carrier Conversion Efficiency (IPCE) Method for Testing: 
       [0058]    The photocurrent-voltage was measured by using a potentiostat at AM 1.5, with an illumination of 100 mW/cm 2  (Oriel). Under the illumination of fixed wavelength of light, the monochromatic photocurrent-wavelength measurement was completed by placing an auto filtering monochromator (Sciencetech Model 9030) between the DSSCs and a light source (100 mW/cm 2 ). Photocurrents corresponding to the illumination under different wavelengths of light were recorded individually, and then the spectra of IPCE were obtained by using Formula 3 for relevant calculations. 
         [0000]    
       
         
           
             
               
                 
                   
                     IPCE 
                      
                     
                         
                     
                      
                     
                       ( 
                       % 
                       ) 
                     
                   
                   = 
                   
                     
                       1240 
                        
                       
                         J 
                         sc 
                       
                     
                     
                       λ 
                        
                       
                           
                       
                        
                       
                         P 
                         
                           i 
                            
                           
                               
                           
                            
                           n 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                      
                     
                         
                     
                      
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    In Formula 3, J sc  represents the short-circuit photocurrent density, λ is the incident light wavelength (400-800 nm), whereas P in  is the power of the incident light. 
         [0059]      FIG. 2  is a graph that shows the IPCE spectra of every single wavelength for DSSCs assembled from photoanodes obtained in Control Example and Examples A1-A3.  FIG. 2  shows that when the addition of carbon nanotubes was increased (from 0 to 0.3 wt %), the IPCE power was significantly increased as well, which suggest that: (1) there was an elevation in dye adsorption; (2) the addition of carbon nanotubes led to higher rate of photo-induced electron transfer; (3) the charge recovery efficiency in DSSC components was reduced. 
         [0060]    However, when the addition of carbon nanotubes was made even higher (0.5 wt %), the IPCE power decreased as a result, which was visibly different from the IPCE values obtained from using lower amount of carbon nanotubes (0-0.3 wt %). The carbon nanotubes can usually absorb higher wavelength of lights (&gt;400 nm), while TiO 2  materials can only absorb lower wavelength of lights (&lt;400 nm). Therefore, when a DSSC is illuminated and starts to produce power, the presence of excessive carbon nanotubes can lead to an inhibition of photo-capture capability in the dye molecules, which consequently limits the photocurrent activities in the cell. The aforesaid observation indicates that the optimal amount for the addition of carbon nanotubes should be 0.3 wt % in the photoanode of the DSSC. Consequently, the result suggests that the addition of multi-wall carbon nanotubes during TiO 2  synthesis is an effective way for elevating the photoelectric conversion efficiency in a DSSC. 
       Testing the Performance of the DSSCs 
     Method for Testing: 
       [0061]    The performance tests of the DSSCs were carried out under the illumination of simulated sunlight at AM 1.5. 
         [0062]      FIG. 3  is a graph that shows curves between photocurrent density and voltage (J-V) for DSSCs assembled from photoanodes obtained in Control Example and Examples A1-A3. Table 3 lists the open-circuit photovoltage (V oc ), the short-circuit photocurrent density (J sc ), the fill factor (FF), and the photoelectric conversion efficiency (η) of DSSCs assembled from photoanodes obtained in Control Example and Examples A1-A3. When the addition of carbon nanotubes was increased from 0 to 0.5 wt %, the short-circuit photocurrent density (J sc ) of the cell was increased from 5.97 to 8.82 mA/cm 2  (0-0.3 wt %), but the J sc  was reduced to 7.91 mA/cm 2  when the addition of carbon nanotubes was increased even higher (0.5 wt %). In addition, the V oc  also increased from 0.65V to 0.74V, whereas the FF was maintained at approximately 72%. The photoelectric conversion efficiency (η) of the DSSC initially elevated from 2.87% (Control Example) to approximately 4.62% (A2), which was an increase of roughly 61%; but later declined slightly to 4.22% (A3). 
         [0063]    The changes in all characteristics shown in Table 3 indicated that the addition of multi-wall carbon nanotubes led to fluctuations in the open-circuit photovoltage and the short-circuit photocurrent density of the DSSC, and the fluctuations had obviously affected the overall photoelectric conversion efficiency of the DSSC as a result. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 V oc  (V) 
                 J sc  (mA/cm 2 ) 
                 F.F. (%) 
                 Efficiency (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Control 
                 0.65 
                 5.97 
                 73.21 
                 2.87 
               
               
                 A1 
                 0.69 
                 7.73 
                 70.12 
                 3.75 
               
               
                 A2 
                 0.72 
                 8.82 
                 73.17 
                 4.62 
               
               
                 A3 
                 0.74 
                 7.91 
                 72.17 
                 4.22 
               
               
                   
               
             
          
         
       
     
         [0064]    The present invention has been described with a preferred embodiment thereof and it is understood that many changes and modifications to the described embodiment can be carried out without departing from the scope and the spirit of the invention that is intended to be limited only by the appended claims.