Abstract:
The invention provides a quantum dots-sensitized solar cell and a method of enhancing the optoelectronic performance of a quantum dots-sensitized solar cell using a co-adsorbent, in which a bifunctional molecule is used as the co-adsorbent and is mixed with aqueous quantum dots to form a quantum dots sensitizer, thereby improving the photoelectric conversion efficiency of the solar cell.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a quantum dots-sensitized solar cell (QDSSC) and a method of enhancing the optoelectronic performance of a quantum dots-sensitized solar cell using a co-adsorbent, in which a bifunctional molecule is used as the co-adsorbent and is mixed with aqueous quantum dots to form a quantum dots sensitizer, thereby improving the photoelectric conversion efficiency of the solar cell. 
       BACKGROUND TO THE INVENTION 
       [0002]    As an alternative energy, solar energy has features such as wide distribution and ease to obtain, and its utilization is by a conversion of light energy into electric energy through a solar cell, during which conversion no environmental pollution is caused; therefore, solar energy is a must potential renewable energy to be developed. 
         [0003]    The solar cell can be classified into three types including, in development sequence, silicon solar cells, film solar cells, and dye-sensitized solar cells (DSSCs). Among them, the DSSC as the third generation is operated by capturing incident light through sensitive dyes and converting the energy of photons into electric energy. The DSSC can be made of a variety of materials, and its manufacturing process needs no clean room and thus is simpler than those of other solar cells; therefore, the DSSC has an advantage of reducing manufacturing cost. However, most of highly efficient DSSCs use organic ruthenium complexes as the dye, which is expensive in production cost and cannot be decomposed in the environment. Therefore, in recent years, industries and research units were enthusiastically looking for alternative sensitizers such as, for example, quantum dots, to replace the organic ruthenium complexes. 
         [0004]    On the other hand, the photoelectric conversion efficiency of a solar cell depends on light capture efficiency, electron injection efficiency, electron collection efficiency, etc., of which the electron injection efficiency can be enhanced by using a co-adsorbent to prevent the dye from gathering on the surface of the semiconductor, so as to improve the photoelectric conversion efficiency. For example, CN 103295795 B discloses using organic materials of acetylacetone and its derivatives as the co-adsorbent, which improve the photoelectric conversion efficiency of the DSSC to a certain extent. 
       SUMMARY OF THE INVENTION 
       [0005]    In view that the production cost of the conventional technology in which organic ruthenium complexes are used as the sensitizer is expensive and the optoelectronic performance of the DSSCs using inorganic materials as the sensitizer is too low, the present invention therefore provides a quantum dots-sensitized solar cell, in which quantum dots are used as the sensitizer and a co-adsorbent is used to improve the optoelectronic performance of the solar cell. 
         [0006]    According to one aspect of the present invention, provided is a quantum dots-sensitized solar cell, comprising:
       a photoelectrode, formed on a first substrate and having a quantum dots sensitizer adsorbed thereon;   a back electrode, formed on a second substrate; and   a polysulfide electrolyte, injected between the photoelectrode and the back electrode;   wherein the photoelectrode having the quantum dots sensitizer adsorbed is modified by a co-adsorbent, and the co-adsorbent has a structure of HS—R—COOH or HS—R—OH where R represents a substituted or unsubstituted organic carbon chain having 1 to 10 carbon atoms.       
 
         [0011]    According to another aspect of the present invention, provided is a method of enhancing the optoelectronic performance of a quantum dots-sensitized solar cell using a co-adsorbent, characterized in that a photoelectrode is dipped into a mixed solution of a co-adsorbent and a quantum dots sensitizer to increase the coverage of the quantum dots sensitizer on the photoelectrode and thereby improve the photoelectric conversion efficiency of the quantum dots-sensitized solar cell, wherein the co-adsorbent has a structure of HS—R—COOH or HS—R—OH where R represents a substituted or unsubstituted organic carbon chain having 1 to 10 carbon atoms. 
         [0012]    The substrate of a solar cell should be excellent in light transparency. Generally, there are two types of transparent electrically conductive glass that are for use as the substrate of a solar cell. One is fluorine-doped tin oxide (FTO) transparent electrically conductive glass in which tin oxide (SnO 2 ) is duped with fluorine (CnO 21 F), and the other is indium tin oxide (ITO) transparent electrically conductive glass in which indium oxide (In 2 O 3 ) is doped with SnO 2 . In the present invention, the first substrate and the second substrate each can be either of the aforementioned types of transparent electrically conductive glass, and preferably FTO. 
         [0013]    The photoelectrode is mainly composed of an oxide semiconductor such as TiO 2 , SnO 2 , ZnO, SrTiO 3 . Using different oxide semiconductors as the carrier for adsorbing the sensitizer results in different open-circuit voltages (V oc ). TiO 2  is preferable because of its low cost, ease to obtain, good stability, and good effect. However, the present invention is not limited to using TiO 2 , and the aforementioned oxide semiconductor such as SnO 2 , ZnO, and SrTiO 3  can also be used. 
         [0014]    In order to absorb the solar light energy to excite electrons more efficiently, the oxide semiconductor may adsorb sensitizers of smaller energy gap to broaden the light absorption range and facilitate the excitation of electrons. There are two kinds of sensitizers including organic metal dye sensitizers, of which the most typical one is polypyridyl complexes of ruthenium, and quantum dots sensitizers. The present invention uses quantum dots sensitizers, which can be a semiconductor material selected from the group consisting of CdS, CdSe, CdTe, PbS, PbSe, Ag 2 S, Ag 2 Se, AgS x Se 1-x , CuS, Sb 2 S 3 , Sb 2 Se 3 , CdS x Se 1-x , CdSe x Te 1-x , InP, PbS x Se 1-x , PbSe x Te 1-x , AgInS x Se 1-x , AgInS 2 , AgInSe 2 , AgInTe 2 , CuInS x Se 1-x , CuInS x Te 1-x , CuInS 2 , CuInSe 2 , CuInTe 2 , and CuIn 2 S 3 , and preferably CuInS 2.    
         [0015]    I − /I 3   −  electrolytes are used in most of conventional DSSCs so as to reduce the dye from an oxidation state and transfer charges from the back electrode to the dye through a reduction-oxidation reaction. In the QDSSC of the present invention, a polysulfide (S 2− /Sn 2− ) electrolyte is used. The reduction-oxidation reaction of polysulfide can not only facilitate transferring the holes on the sulfide semiconductor that has absorbed light and been excited, but also allow a higher photocurrent. However, the polysulfide electrolyte will also cause the problem that polysulfide poisons the Pt back electrode, which is usually used in the DSSCs. Therefore, the differences between the QDSSCs according to the present invention and the conventional DSSCs are not only the sensitizer but also the choice of materials of the electrolyte and the back electrode, which are changed in accordance with the sensitizer. 
         [0016]    As to the back electrode, materials such as graphono, carbon nanotube, metal sulfides (such as, for example, PbS, NiS, FeS 2 , CoS, CuS, Cu 2-x S and Cu 2 S), and metal selenides (such as, for example, PbSe, NiSe, FeSe 2 , CoSe, CuSe, Cu 2-x Se and Cu 2 Se) have better charge transfer capability, and namely are good for reduction-oxidation reaction, with respect to the polysulfide solution. In the present invention, a metal sulfide preferably selected from the group consisting of PbS, NiS, CoS, CuS and Cu 2 S is used as the back electrode of the QDSSC together with the polysulfide electrolyte, thereby significantly improving the photoelectric conversion efficiency of the QDSSC. 
         [0017]    There are two methods to sensitize the electrode with the quantum dots, including (i) in situ method, by which the quantum dots are prepared on the surface of the photoelectrode film, and (ii) ex situ method, also called pre-synthesized method, by which colloidal quantum dots (CQDs) are made to adhere to the surface of the electrode. The in situ method further includes chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR), and electrodeposition (ED). 
         [0018]    In addition, leakage current will be generated in the contact interfaces between liquid electrolyte, wide bandgap semiconductor, and quantum dots to lower the conversion efficiency, and therefore a passivation layer having a bandgap wider than that of the quantum dots should be deposited on the quantum dots adsorbed to the wide bandgap semiconductor in order to avoid causing severe leakage current. 
         [0019]    To make the quantum dots adhere to the photoelectrode, the present invention is not limited to using the aforementioned methods, and any method for adsorbing the quantum dots sensitizer to the photoelectrode can be used. Any combination of the aforementioned methods can also be used to adsorb the quantum dots sensitizer and the passivation layer, respectively. 
         [0020]    The co-adsorbent as used in the present invention has a structure of HS—R—COOH or HS—R—OH where R represents a substituted or unsubstituted organic carbon chain having 1 to 10 carbon atoms. Specifically, the co-adsorbent having the structure of HS—R—COOH includes, but is not limited to, thioglycolic acid (TGA), L-Cystine, D-Cystine, DL-Cystine, L-cysteine (Cys), D-cysteine, DL-cysteine, L-homocysteine, N-isobutyryl-L-cysteine, N-carbamoyl-L-cysteine, glutathione (GSH), 2-mercaptopropionic acid (2-MPA) 3-mercaptopropionic acid (3-MPA), 4-mercaptobutyric acid, 6-mercaptohexanoic acid, 8-mercaptooctanoic acid, mercaptosuccinic acid, meso-2,3-dimercaptosuccinic acid, 2-methyl-3-sulfanylpropanoic acid, dihydrolipoic acid, thiolactic acid, methyl thioglycolate, ethyl thioglycolate, methyl 3-mercaptopropionate, and pentaerythritol tetrakis(2-mercaptoacetate); the co-adsorbent having the structure of HS—R—OH includes, but is not limited to, 1,4-dithiothreitol (DTT), L-(-)-dithiothreitol, trans-4,5-dihydroxy-1,2-dithiane, 1-mercapto-2-propanol, 2-mercaptoethanol (ME), 4-mercapto-1-butanol, 3-mercapto-1-propanol, 6-mercapto-1-hexanol, and 8-mercapto-1-octanol. Those co-adsorbents having the structure of HS—R—COOH or HS—R—OH can stabilize the bifunctional molecules on the surfaces of the quantum dots so as not to form disulfide bonds, thereby increasing the coverage of the quantum dots on the photoelectrode. The chemical structures of some bifunctional molecules are shown below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0021]    According to the present invention, a required amount of aqueous quantum dots is synthetized by means of a microwave-assisted method, and after the steps such as purification and drying, the required quantum dots sensitizer is prepared. Subsequently, a photoelectrode is dipped into a solution composed of the sensitizer and a co-adsorbent for a period of time, and a layer of passivation layer is deposited, thereby obtaining a photoelectrode with the quantum dots sensitizer adsorbed. The photoelectrode with the quantum dots sensitizer adsorbed is then combined with a back electrode so that the quantum dots-sensitized solar cell according to the present invention is obtained. 
         [0022]    The present invention will be further described by referring to the preferred embodiments below, which are, however, not intended to restrict the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1( a )  is a TEM (Transmission Electron Microscope) diagram of the aqueous CuInS 2  quantum dots,  FIG. 1( b )  is an XRD (X-ray Diffraction) diagram of the aqueous CuInS 2  quantum dots, and  FIG. 1( c )  is an EDS (Energy Dispersive Spectrometer) diagram of the aqueous CuInS 2  quantum dots. 
           [0024]      FIG. 2  is schematic diagram showing assembly of the components of a solar cell. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]    (Synthesis of the aqueous CuInS 2  quantum dots) 
         [0026]    A solution was prepared by adding 0.213 ml of CuCl 2  solution, 0.553 ml of InCl 3  solution, and 0.25 ml of sodium citrate (SC) solution (all prepared in advance) into a microwave reaction vial G30, being stirred until its color turned blue, adding 1000 ml of L-cysteine (Cys) precursor solution so that the color became transparent from blue, adding 17.48 ml of deionized water, and fast adding Na2S with stirring so that the color became yellow from transparent, wherein Cu:In:SC:Cys:S is 1:4:16:7.2:6.5. 
         [0027]    The resultant solution was placed in a microwave assisting device Microwave 300 at a standard mode, and a microwave reaction was carried out at 180° C. for 15 minutes. The cooling temperature was set to 55° C., and the pressure during the reaction was about 10.5-11 Bar. The color of the solution turned deep brown from yellow. After reaction, the solution was mixed with 2-propanol, and then centrifuged to collect a precipitate. The precipitate was placed in an oven at 40° C. for 16-18 hours, and the dried substance in deep brown color was the aqueous CuInS 2  quantum dots as synthetized. 
         [0028]      FIG. 1( a )  shows the lattice structure of the aqueous CuInS 2  quantum dots, obtained from an image analysis conducted with a high resolution transmission electron microscope.  FIG. 1( b )  shows the XRD signal of the aqueous CuInS 2  quantum dots. In comparison with CuInS 2  with tetragonal structure (JCPDS-15-0681), the XRD signal of the aqueous CuInS 2  quantum dots quite correspond to that of CuInS 2  with tetragonal structure which shows three main peaks (112), (220), and (312) at 2θ=28.2°, 46.8°, and 55.3°, respectively, and it was thus confirmed that the synthetized material was CuInS 2  quantum dots.  FIG. 1(C)  is an EDS diagram of the aqueous CuInS 2  quantum dots, which shows that there are element signals of Cu, In, S, and Au. Because the EDS element analysis was conducted by dropping the aqueous CuInS 2  quantum dots on a gold specimen after dissolving, an Au signal appeared in the EDS result. 
         [0029]    (Adsorption of the aqueous CuInS 2  quantum dots) 
         [0030]    The dried aqueous CuIn 3   2  quantum dots were dissolved in water, and then different kinds of co-absorbents were added in different concentrations so as to prepare aqueous CuInS 2  quantum dots solutions containing the kinds and concentrations of co-absorbents listed in the Examples below. Subsequently, a photoelectrode in which a TiO 2  film was formed on an FTO electrically conductive glass was immersed in each of the aforementioned aqueous CuInS 2  quantum dots solutions at 40° C. for 24 hours, and then the photoelectrode was taken out, washed with methanol, and dried. Subsequently, a ZnS passivation layer was deposited thereon with the SILAR method. Finally, a TiO 2  photoelectrode having CuInS 2  quantum dots adsorbed was thus obtained. 
         [0031]    The obtained TiO 2  photoelectrode having CuInS 2  quantum dots adsorbed was combined with the Cu 2 S back electrode, which was prepared with a spin coating method, and the polysulfide electrolyte, which was prepared in advance by the following steps of weighting 4.3232 g of Na 2 S, 0.1491 g of KCl, and 0.6401 g of S powder, dissolving these solid solutes in 7 ml of water, followed by adding 3 ml of methanol, and then adding 29.5 mg of GuSCN into 5 ml of the aforementioned mixed solution. The assembly of the solar cell is shown in  FIG. 2 . 
         [0032]    Table 1 is the analysis result of the optoelectronic performance of the QDSSCs produced by using different concentrations of DTT as the co-adsorbent, in which J sc  represents a short-circuit current density (where the short-circuit current is a current generated upon irradiation) and is defined as a current density measured when the applied voltage is zero, V oc  represents an open-circuit voltage and is defined as a voltage applied when the measured current density is zero, FF (fill factor) is defined as an actual largest output power divided by a target output power (J sc ×V oc ), which is a dimensionless value, and can be used as an index for indicating the difference between the actual solar cell and the ideal solar cell, as a solar cell is closer to an ideal solar cell if the FF value is closer to 1, and η represents the photoelectric conversion efficiency of a solar cell and is defined as a ratio of the largest output power to the power of an incident light. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Efficiency of QDSSCs with different DTT concentrations 
               
             
          
           
               
                   
                   
                 J sc   
                 V oc   
                 FF 
                 η 
               
               
                   
                 DTT Conc. 
                 (mA/cm 2 ) 
                 (mV) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 Compar. 
                     0M 
                 0.282 
                 290 
                 46.8 
                 0.038 
               
               
                 Ex. 1 
                   
                   
                   
                   
                   
               
               
                 Compar. 
                 0.1M 
                 0.296 
                 240 
                 50.3 
                 0.036 
               
               
                 Ex. 2 
                   
                   
                   
                   
                   
               
               
                 Ex. 1 
                 0.5M 
                 2.046 
                 478 
                 54.5 
                 0.533 
               
               
                 Ex. 2 
                 4.0M 
                 7.207 
                 592 
                 60.6 
                 2.587 
               
               
                   
               
             
          
         
       
     
         [0033]    It can he learned from the Comparative Examples 1 and 2 that the η values were merely 0.038% and 0.036%, respectively, for the aqueous CuInS 2  quantum dots dissolved in pure water and low concentration 0.1 M of DTT. When the DTT concentration was increased to 0.5 M, all data of the device increased greatly, indicating that a high concentration disulfide bond reagent can stabilize the bifunctional molecule on the surfaces of the aqueous CuInS 2  quantum dots so that the carboxylic group of the bifunctional molecule can successfully bond to TiO 2 , thereby greatly increasing the coverage and providing more excited electrons to increase J sc . V oc  depends on the Fermi energy level of TiO 2  and the potential difference of oxidation-reduction pairs in the electrolyte. The increase in the coverage of the aqueous CuInS 2  quantum dots implies that more electrons are injected into TiO 2  so that the Fermi energy level of TiO 2  moves toward a negative potential, thereby increasing the potential difference with respect to the oxidation-reduction pairs of the electrolyte and increasing V oc . In the Example 2, the DTT concentration was further increased to 4.0 M, and therefore J sc  increased greatly to 7.207 mA/cm 2 , V oc  increased to 592 mV, FF increased to 60.6%, and η even increased from 0.533% to 2.587%, indicating that the reduction conducted in high concentration may provid the wide bandgap oxide semiconductor with a high load of aqueous CuInS 2  quantum dots. 
         [0034]    Table 2 is the analysis result of the optoelectronic performance of the QDSSCs produced by using different bifunctional molecules as the co-adsorbent. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Efficiency of QDSSCs with 0.5M of 
               
               
                 different bifunctional molecules 
               
             
          
           
               
                   
                   
                 J sc   
                 V oc   
                 FF 
                 η 
               
               
                   
                 co-adsorbent 
                 (mA/cm 2 ) 
                 (mV) 
                 (%) 
                 (%) 
               
               
                   
               
             
          
           
               
                   
                 Ex. 1 
                 DTT 
                 2.046 
                 478 
                 54.5 
                 0.533 
               
               
                   
                 Ex. 3 
                 TGA 
                 12.820 
                 640 
                 54.1 
                 4.438 
               
               
                   
                 Ex. 4 
                 Cys 
                 4.462 
                 544 
                 56.8 
                 1.379 
               
               
                   
                 Ex. 5 
                 GSH 
                 6.930 
                 628 
                 56.4 
                 2.455 
               
               
                   
               
             
          
         
       
     
         [0035]    It can be learned from Table 2 that the photoelectric conversion efficiencies of the solar cells of the Examples 3 to 5, in which TGA, Cys, and GSH, respectively, were used as the co-adsorbent, all increased greatly, in comparison with the Example 1, in which DTT was used as the co-adsorbent. TGA, Cys, and GSH each have carboxylic groups in their molecular structures, which may be a main reason why the coverage increased greatly. Among them, TGA as the co-adsorbent has the best result for photoelectric conversion efficiency, followed by GSH, and the last is Cys, and this may be because TGA&#39;s molecular structure and thus steric effect are smaller so that the aqueous CuInS 2  quantum dots could be successfully adsorbed to the surface of TiO 2  and the photoelectric conversion efficiency greatly increased to 4.438%. GSH has two carboxylic groups in its molecular structure, which can provide a higher capability of bonding to TiO 2 . However, the said molecular structure and thus the steric effect are larger, so that its photoelectric conversion efficiency of 2.455% was worse than that of TGA. The steric effect of the molecular structure of Cys is between those of TGA and GSH, but its photoelectric conversion efficiency is worse than those of TGA and GSH. It is inferred that Cys is not higher in capability of reducing disulfide bonds than GSH, and therefore Cys is worse in efficiency than GSH even if its steric effect is smaller. 
         [0036]    Table 3 is the analysis result of the optoelectronic performance of the QDSSCs produced by using different concentrations of TGA as the co-adsorbent. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Efficiency of QDSSCs with different TGA concentrations 
               
             
          
           
               
                   
                   
                 J sc   
                 V oc   
                 FF 
                 η 
               
               
                   
                 TGA Conc. 
                 (mA/cm 2 ) 
                 (mV) 
                 (%) 
                 (%) 
               
               
                   
               
             
          
           
               
                 Ex. 6 
                 0.1M 
                 9.230 
                 616 
                 56.7 
                 3.225 
               
               
                 Ex. 3 
                 0.5M 
                 12.820 
                 640 
                 54.1 
                 4.438 
               
               
                 Ex. 7 
                 1.0M 
                 14.015 
                 642 
                 51.8 
                 4.661 
               
               
                 Ex. 8 
                 2.0M 
                 14.415 
                 642 
                 52.1 
                 4.821 
               
               
                 Ex. 9 
                 4.0M 
                 14.837 
                 630 
                 52.6 
                 4.920 
               
               
                 Ex. 10 
                 6.0M 
                 13.705 
                 650 
                 50.9 
                 4.534 
               
               
                   
               
             
          
         
       
     
         [0037]    According to the result of Table 2, TGA was the best co-absorbent for photoelectric conversion efficiency and thus was used in concentrations of 0.1 M to 6.0 M in the Examples 6 to 10, respectively. It can be learned from Table 3 that the photoelectric conversion efficiency increased as the TGA concentration was increased from 0.1 M to 4.0 M. The best result was the Example 9 with 4.0 M TGA used and the photoelectric conversion efficiency was 4.920%. 
         [0038]    Table 4 is the analysis result of the optoelectronic performance of the QDSSCs produced by using 4.0 M TGA as the co-adsorbent and immersing the photoelectrode for different periods of time. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Efficiency of QDSSCs with immersion in 
               
               
                 0.4M TGA for different periods of time. 
               
             
          
           
               
                   
                 Immersion 
                 J sc   
                 V oc   
                 FF 
                 η 
               
               
                   
                 Time 
                 (mA/cm 2 ) 
                 (mV) 
                 (%) 
                 (%) 
               
               
                   
               
             
          
           
               
                   
                 Ex. 11 
                 0.5 
                 hr 
                 7.634 
                 626 
                 61.2 
                 2.926 
               
               
                   
                 Ex. 12 
                 1 
                 hr 
                 9.225 
                 630 
                 57.7 
                 3.352 
               
               
                   
                 Ex. 13 
                 3 
                 hr 
                 12.133 
                 630 
                 57.0 
                 4.360 
               
               
                   
                 Ex. 14 
                 6 
                 hr 
                 12.998 
                 630 
                 54.5 
                 4.461 
               
               
                   
                 Ex. 15 
                 12 
                 hr 
                 13.691 
                 624 
                 54.0 
                 4.615 
               
               
                   
                 Ex. 9 
                 24 
                 hr 
                 14.837 
                 630 
                 52.6 
                 4.920 
               
               
                   
               
             
          
         
       
     
         [0039]    According to the result of Table 3, 0.4 M TGA exhibited the best result for photoelectric conversion efficiency among different concentrations of TGA, and thus was used as the co-adsorbent in the Examples 11 to 15 to determine the influence of immersion time on the photoelectric conversion efficiency of the solar cell. It can be learned from Table 4 that J sc  increased from 7.634 mA/cm 2  to 12.133 mA/cm 2  and the photoelectric conversion efficiency increased from 2.926% to 4.360% when the immersion time was increased from 0.5 hours to 3 hours, while J sc  increased from 12.133 mA/cm 2  to 14.837 mA/cm 2  and the photoelectric conversion efficiency increased merely from 4.360% to 4.920% when the immersion time was increased from 3 hours to 24 hours. In other words, taking the immersion time of 3 hours as a cut-off point, the photoelectric conversion efficiency rose rapidly before 3 hours and tended to rise gently after 3 hours. Also, it can be learned from the results of all immersion time conditions that as the immersion time was increased, FF decreased from 61.2% at 0.5 hours to 52.6% at 24 hours, indicating that the coverage of the quantum dots increases as the immersion time increases, but too many quantum dots will cause a continuous increase in internal impedance and thus a reduction in FF, thereby slowing down the rising of the photoelectric conversion efficiency. 
         [0040]    The TiO 2  photoelectrode has to be immersed in a solution composed of both TGA co-adsorbent and aqueous CuInS 2  quantum dots so as to have a better photoelectric conversion efficiency. In the Comparative Examples 3 to 5, the optoelectronic performance of the QDSSCs produced from aqueous CuInS 2  quantum dots with the TGA co-adsorbent added in different sequences was analyzed. 
         [0041]    In Table 5, the Comparative Example 3 was to immerse the TiO 2  photoelectrode in the TGA co-adsorbent for 24 hours and then in the aqueous CuInS 2  quantum dots for 24 hours; the Comparative Example 4 was to immerse the TiO 2  photoelectrode in the aqueous CuInS 2  quantum dots for 24 hours and then in the TGA co-adsorbent for 24 hours; the Comparative Example 5 was to immerse the TiO 2  photoelectrode only in the TGA co-adsorbent for 24 hours without being immersed in any aqueous quantum dots. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Efficiency of QDSSCs with different immersion sequences 
               
             
          
           
               
                   
                 J sc   
                 V oc   
                 FF 
                 η 
               
               
                   
                 (mA/cm 2 ) 
                 (mV) 
                 (%) 
                 (%) 
               
               
                   
               
             
          
           
               
                   
                 Compar. 
                 0.597 
                 354 
                 53.8 
                 0.114 
               
               
                   
                 Ex. 3 
                   
                   
                   
                   
               
               
                   
                 Compar. 
                 1.411 
                 460 
                 55.2 
                 0.358 
               
               
                   
                 Ex. 4 
                   
                   
                   
                   
               
               
                   
                 Compar. 
                 0.495 
                 410 
                 38.1 
                 0.077 
               
               
                   
                 Ex. 5 
                   
                   
                   
                   
               
               
                   
                 Ex. 3 
                 12.820 
                 640 
                 54.1 
                 4.438 
               
               
                   
               
             
          
         
       
     
         [0042]    It can be learned from Table 5 that the photoelectric conversion efficiencies of the Comparative Examples 3 and 4 were far lower than that of the Example 3. On the other hand, the photoelectric conversion efficiency of the Comparative Example 5, in which the TiO 2  photoelectrode was immersed only in the TGA co-adsorbent for 24 hours without being immersed in any aqueous quantum dots, is even merely 0.077%. It is thus demonstrated that a mixed solution composed of both TGA co-adsorbent and aqueous CuInS 2  quantum dots results in a better photoelectric conversion efficiency. 
         [0043]    Table 6 is the analysis result of the optoelectronic performance of the QDSSCs produced by using aqueous CdSe, CdSe x Te 1-x , AgInSe 2 , and AgInS 2  quantum dots. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Efficiency of aqueous CdSe, CdSe x Te 1−x , AqInSe 2 , and AqInS 2   
               
               
                 QDSSCs with the TGA co-adsorbent 
               
             
          
           
               
                   
                   
                 co- 
                 J sc   
                 V oc   
                 FF 
                   
               
               
                   
                 QD 
                 adsorbent 
                 (mA/cm 2 ) 
                 (mV) 
                 (%) 
                 η (%) 
               
               
                   
               
             
          
           
               
                 Compar. 
                 CuInS 2   
                 — 
                 0.282 
                 282 
                 46.8 
                 0.038 
               
               
                 Ex. 6 
                   
                   
                   
                   
                   
                   
               
               
                 Ex. 16 
                 CuInS 2   
                 4M TGA 
                 14.478 
                 630 
                 53.1 
                 4.864 
               
               
                 Compar. 
                 AgInS 2   
                 — 
                 0.178 
                 212 
                 47.5 
                 0.018 
               
               
                 Ex. 7 
                   
                   
                   
                   
                   
                   
               
               
                 Ex. 17 
                 AqInS 2   
                 4M TGA 
                 6.518 
                 382 
                 64.0 
                 1.594 
               
               
                 Compar. 
                 CdSe 
                 — 
                 0.385 
                 264 
                 45.7 
                 0.046 
               
               
                 Ex. 8 
                   
                   
                   
                   
                   
                   
               
               
                 Ex. 18 
                 CdSe 
                 4M TGA 
                 4.759 
                 552 
                 53.8 
                 1.413 
               
               
                 Compar. 
                 CdSe x Te 1−x   
                 — 
                 0.640 
                 384 
                 46.0 
                 0.113 
               
               
                 Ex. 9 
                   
                   
                   
                   
                   
                   
               
               
                 Ex. 19 
                 CdSe x Te 1−x   
                 4M TGA 
                 14.38 
                 630 
                 50.5 
                 4.578 
               
               
                 Compar. 
                 AgInSe 2   
                 — 
                 1.538 
                 492 
                 50.4 
                 0.381 
               
               
                 Ex. 10 
                   
                   
                   
                   
                   
                   
               
               
                 Ex. 20 
                 AgInSe 2   
                 4M TGA 
                 16.39 
                 610 
                 54.1 
                 5.411 
               
               
                   
               
             
          
         
       
     
         [0044]    It can be learned from Table 6 that the photoelectric conversion efficiencies of the solar cells using the aqueous CdSe x Te 1-x  and AgInSe 2  quantum dots and the TGA co-adsorbent also increased notably. Also, the photoelectric conversion efficiencies of the AgInS 2  and CdSe QDSSCs increased from 0.018% and 0.046% to 1.594% and 1.413%, respectively. Although their improvements are not as obvious as that of CuInS 2 , yet it can still be demonstrated that the TGA co-adsorbent can be used with various aqueous quantum dots to improve the coverage. 
         [0045]    Table 7 further provides the analysis result of the optoelectronic performance of the QDSSCs produced by immersing the TiO 2  photoelectrode in different concentrations of GSH, 3-MPA, and Cys co-adsorbents for 24 hours, which shows that different kinds of co-adsorbents all can improve the efficiencies of the QDSSCs by increasing their concentrations. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Efficiency of the QDSSCs with different 
               
               
                 concentrations of GSH, 3-MPA, and Cys 
               
             
          
           
               
                   
                   
                 J sc   
                 V oc   
                 FF 
                 η 
               
               
                   
                 co-adsorbent 
                 (mA/cm 2 ) 
                 (mV) 
                 (%) 
                 (%) 
               
               
                   
               
             
          
           
               
                 Compar. 
                 0.1M 
                 GSH 
                 0.108 
                 490 
                 41.2 
                 0.136 
               
               
                 Ex. 11 
                   
                   
                   
                   
                   
                   
               
               
                 Ex. 5 
                 0.5M 
                 GSH 
                 6.930 
                 628 
                 56.4 
                 2.455 
               
               
                 Ex. 21 
                 1.0M 
                 GSH 
                 12.234 
                 676 
                 51.7 
                 4.273 
               
               
                 Ex. 22 
                 2.0M 
                 GSH 
                 11.862 
                 660 
                 53.5 
                 4.185 
               
               
                 Compar. 
                 0.1M 
                 3-MPA 
                 0.878 
                 496 
                 58.8 
                 0.256 
               
               
                 Ex. 12 
                   
                   
                   
                   
                   
                   
               
               
                 Ex. 23 
                 0.5M 
                 3-MPA 
                 2.620 
                 574 
                 56.0 
                 0.842 
               
               
                 Ex. 24 
                 1.0M 
                 3-MPA 
                 5.648 
                 600 
                 58.2 
                 1.973 
               
               
                 Ex. 25 
                 2.0M 
                 3-MPA 
                 10.596 
                 624 
                 53.5 
                 3.536 
               
               
                 Ex. 26 
                 4.0M 
                 3-MPA 
                 11.693 
                 632 
                 51.2 
                 3.787 
               
               
                 Compar. 
                 0.1M 
                 Cys 
                 0.707 
                 412 
                 48.0 
                 0.140 
               
               
                 Ex. 13 
                   
                   
                   
                   
                   
                   
               
               
                 Ex. 4 
                 0.5M 
                 Cys 
                 4.462 
                 544 
                 56.8 
                 1.379 
               
               
                 Ex. 27 
                 1.0M 
                 Cys 
                 8.174 
                 616 
                 57.4 
                 2.888 
               
               
                 Ex. 28 
                 2.0M 
                 Cys 
                 9.403 
                 618 
                 53.1 
                 3.084 
               
               
                 Ex. 29 
                 4.0M 
                 Cys 
                 9.539 
                 626 
                 52.8 
                 3.154