JP6180243B2 - Method for producing photoelectrode for quantum dot-sensitized solar cell - Google Patents

Description

  The present invention relates to a method for producing a photoelectrode for a quantum dot-sensitized solar cell and a quantum dot-sensitized solar cell.

  As a next-generation solar cell, a quantum dot-sensitized solar cell, which has a theoretically high photoelectric conversion efficiency as compared with a currently mainstream silicon crystal solar cell, is promising. A quantum dot-sensitized solar cell is formed by making a photoelectrode and a counter electrode face each other via an electrolyte layer, and the photoelectrode forms a transparent electrode layer on the surface of the substrate, and a photoelectric conversion material on the surface of the transparent electrode layer. It is produced by forming a semiconductor layer and adsorbing quantum dots as a sensitizer on the surface of the semiconductor layer.

  Here, as a method for adsorbing quantum dots, a method in which a substrate on which a semiconductor layer is formed is immersed in a dispersion liquid in which quantum dots are dispersed is generally known. The quantum dots are coated with a surfactant having a long-chain alkyl group in order to enhance the dispersibility and are dispersed in a solvent. For this reason, a ligand having a long-chain alkyl group inevitably adheres to the surface of the quantum dot adsorbed on the semiconductor layer. Since the long chain alkyl group inhibits the injection of electrons from the electrolyte layer to the quantum dot, when applied as a photoelectrode with the long chain alkyl group attached, the photoelectric conversion efficiency of the quantum dot sensitized solar cell is reduced. . Therefore, it is necessary to remove the long-chain alkyl group at the production stage of the photoelectrode.

  For example, Non-Patent Documents 1 and 2 propose methods for removing long-chain alkyl groups from quantum dots. In the thing of the said nonpatent literature 1, by circulating a quantum dot for 24 hours in a pyridine solution, the ligand which has a long-chain alkyl group is substituted with a pyridine, Then, a quantum dot is made to adsorb | suck to a semiconductor layer. Yes. Moreover, in the thing of the said nonpatent literature 2, after producing the quantum dot thin film, after substituting the ligand which has the long-chain alkyl group terminated by the carboxyl group in the ammonium sulfide solution with sulfur, quantum Dots are adsorbed on the semiconductor layer.

  However, since the quantum dots after ligand substitution are poorly dispersible and easily aggregate, it is difficult to efficiently adsorb the quantum dots to the semiconductor layer, resulting in a decrease in the productivity of the photoelectrode. there were. Especially in the thing of the said nonpatent literature 1, since it is necessary to perform a recirculation for a long time, the fall of productivity was remarkable.

  Further, it has been found that when the long-chain alkyl group is removed by the above-described conventional method, the amount of electrons injected from the quantum dots into the semiconductor layer increases, but reverse electron transfer from the quantum dots to the electrolyte layer occurs. This reverse electron transfer was not manifested when a long-chain alkyl group was present, so it is considered that the long-chain alkyl group played a role in preventing backflow of electrons.

  Therefore, the present inventors have conducted extensive research, and after adsorbing quantum dots to the semiconductor layer, the ligand substitution of the quantum dots is performed, and the quantum dots and the semiconductor layer after ligand substitution are covered with a buffer layer. Thus, when applied to a quantum dot-sensitized solar cell, a photoelectrode capable of increasing the amount of electrons injected from the quantum dot to the semiconductor layer and suppressing the reverse electron transfer from the quantum dot to the electrolyte layer can be produced with high productivity. And gained knowledge.

Hyo Joong Lee, 8 others, “CdSe Quantum Dot-Sensitized Solar Cells Exeeding Efficiency 1% at Full-Sun Intensity”, J. Phys. Chem., Vol. 112, No. 30, 11600-11608, 2008 Haitao Zhang, 6 others, “Surfactant Ligand Removal and Rational Fabrication of Inorganically Connected Quantum Dots”, American Chemical Society, Nano letters Vol. 11, 5356-5361, 2011

  In view of the above points, the present invention can remove long-chain alkyl groups that inevitably adhere to quantum dots in a short time, and reverse electrons from quantum dots when applied to quantum dot-sensitized solar cells. It is an object of the present invention to provide a method for producing a photoelectrode of a quantum dot-sensitized solar cell capable of producing a photoelectrode capable of suppressing movement with high productivity.

  In order to solve the above problems, the present invention provides a method for forming a transparent electrode layer on a surface of a substrate in a method for producing a photoelectrode of a quantum dot-sensitized solar cell that is disposed to face a counter electrode via an electrolyte layer. And a step of forming a semiconductor layer that performs photoelectric conversion on the surface of the transparent electrode layer, a step of adsorbing the quantum dots on the surface of the semiconductor layer by immersing the substrate in a dispersion in which the quantum dots are dispersed, A step of substituting sulfur with ligands inevitably attached to the quantum dots when the quantum dots are adsorbed by immersing the substrate in a solution containing sulfide ions, and the surfaces of the quantum dots and the semiconductor layer Forming a buffer layer for preventing electrons from flowing into the electrolyte layer.

  In the present invention, the electrolyte layer includes not only the electrolyte solution but also a compound semiconductor or an organic semiconductor. Further, the semiconductor layer includes not only those composed of metal oxide particles but also those composed of porous metal oxide films.

  According to the present invention, after adsorbing the quantum dots to the semiconductor layer, the ligand having a long chain alkyl group inevitably attached to the quantum dots is replaced with sulfur, so that the semiconductor layer as in the above-described conventional example Since the quantum dots do not aggregate before adsorbing to the semiconductor layer, the ligand substitution of the quantum dots into the semiconductor layer can be performed in a short time. Therefore, the photoelectrode for quantum dot sensitized solar cells can be produced with high productivity. Furthermore, when the photoelectrode of the present invention is applied to a quantum dot-sensitized solar cell by covering the surface of the quantum dot and semiconductor layer after ligand substitution with a buffer layer, electrons once injected into the quantum dot Can be prevented from flowing back to the electrolyte layer. For this reason, coupled with the fact that the amount of electrons injected from the quantum dots into the semiconductor layer can be increased by the ligand substitution, the conversion efficiency of the quantum dot-sensitized solar cell can be increased.

  In the present invention, the solution containing sulfide ions is preferably an ammonium sulfide solution.

  In the present invention, the buffer layer is preferably made of a II-VI group semiconductor.

The schematic sectional drawing of a quantum dot sensitized solar cell provided with the photoelectrode produced by the production method of the photoelectrode of this invention. (A)-(c) is sectional drawing explaining the preparation methods of the photoelectrode of embodiment of this invention. The IR spectrum which shows the experimental result of this invention. The graph which shows the experimental result of this invention.

  Referring to FIG. 1, SC is a quantum dot sensitized solar cell (hereinafter referred to as “solar cell”), and solar cell SC is a photoelectrode (light) formed on the surface of a substrate (base) 1 made of glass or the like. A negative electrode) 2, a frame-shaped seal member 3 formed on the outer peripheral portion of the surface of the substrate 1, a counter electrode (photo positive electrode) 4 disposed opposite to the photoelectrode 2 via the seal member 3, and these photoelectrodes 2 And an electrolyte layer 5 formed in a space defined by the counter electrode 4 and the seal member 3.

  Referring also to FIG. 2, the photoelectrode 2 includes a transparent electrode layer 21 made of a material such as ITO or FTO, and a semiconductor layer 22 made of metal oxide particles such as titanium oxide formed on the surface of the transparent electrode layer 21. The quantum dots 23 adsorbed on the surface of the semiconductor layer 22 and the buffer layer 24 that covers the surfaces of the semiconductor layer 22 and the quantum dots 23 and prevents the inflow (reverse flow) of electrons to the electrolyte layer 5 are provided.

  As the counter electrode 4, one having a known structure can be used. For example, a substrate made of glass, a film, or the like, and a thin film made of platinum, carbon, brass or the like formed on the substrate by a method such as vapor deposition What is comprised by these can be used. As the electrolyte layer 5, an electrolytic solution such as a sodium polysulfide solution, a compound semiconductor such as copper iodide, or an organic semiconductor hole transport layer such as a polythiophene analog can be used. Hereinafter, a method for producing the photoelectrode 2 will be described.

  As shown in FIG. 2A, first, the transparent electrode layer 21 is formed on the surface of the substrate 1 with a thickness of, for example, 0.5 to 1 μm. As a material for the transparent electrode layer 21, it is preferable to use ITO, FTO, or the like. As a method for forming the transparent electrode layer 21, a sputtering method, a CVD method, or the like can be used. Since the formation conditions of the transparent electrode layer 2 can use known ones, detailed description is omitted here.

  Next, the semiconductor layer 22 is formed on the surface of the transparent electrode layer 21. The semiconductor layer 22 can be composed of, for example, fine particles of metal oxide such as titanium oxide, zinc oxide, tin oxide, niobium oxide, or a porous film. As a method for forming the semiconductor layer 22, a method in which a metal oxide paste is applied by a squeegee method or the like and baked can be used. Since the formation conditions of the semiconductor layer 22 can be known, detailed description is omitted here.

Next, the quantum dots 23 are adsorbed on the surface of the semiconductor layer 22 by immersing the substrate 1 on which the semiconductor layer 22 has been formed in a dispersion of quantum dots synthesized by the colloid method. For example, a quantum dot made of CuInSe ₂ can be used as the quantum dot 23, and copper (I) iodide, indium (III) iodide, selenourea, or the like can be used as the raw material. The dispersion contains a surfactant (dispersion medium) having a long-chain alkyl group that enhances the dispersibility of the quantum dots. Examples of the surfactant include dodecanethiol, oleylamine, and tri-n-octylphosphine. Can be used. At this time, a ligand having a long-chain alkyl group inevitably adheres to the quantum dots 23 adsorbed on the semiconductor layer 22 due to the surfactant. In this case, the immersion time can be set within a range of 10 minutes to 60 minutes, for example.

  Next, the substrate 1 on which the quantum dots 23 have been adsorbed is immersed in a solution containing sulfide ions. As a result, as shown in FIG. 2B, the ligand having the long chain alkyl group attached to the quantum dot 23 is replaced with sulfur, and the long chain alkyl group is removed from the quantum dot 23. At this time, the treatment temperature (temperature of the solution) is preferably set to 50 ° C. or higher and lower than the boiling point of the solvent, and can be set to 60 ° C., for example. If it is less than 50 degreeC, the ligand which has a long-chain alkyl group cannot be removed efficiently. As the solution containing sulfide ions, an ammonium sulfide solution, a sodium sulfide solution, a sodium hydrogen sulfide solution, or the like can be used. As the solvent, a polar solvent such as methanol or acetonitrile can be used.

Finally, as shown in FIG. 2C, the buffer layer 24 is formed on the surfaces of the semiconductor layer 22 and the quantum dots 23. As the buffer layer 24, a II-VI group semiconductor can be used. For example, a ZnSe layer can be used, and a known ion layer adsorption reaction method (SILAR method) can be used as a manufacturing method thereof. In this case, for example, a 0.1M Zn (NO ₃ ) ₂ methanol solution and a methanol solution containing 0.1M selenide ions may be alternately immersed. One immersion time can be set in the range of 0.5 to 5 minutes, and the number of immersions can be set in the range of 1 to 4 times.

  As described above, in the present embodiment, since the quantum dots 23 are substituted after the quantum dots 23 are adsorbed to the semiconductor layer 22, aggregation of the quantum dots 23 in the dispersion can be prevented. Moreover, it is not necessary to carry out reflux for a long time as in the conventional example. Therefore, the photoelectrode for quantum dot sensitized solar cells can be produced with high productivity. Further, by covering the surfaces of the quantum dots 23 and the semiconductor layer 22 with the buffer layer 24, when the photoelectrode of the present invention is applied to a quantum dot-sensitized solar cell, the electrons once injected into the quantum dots 23 are electrolytes. Since it is possible to prevent reverse electron transfer that flows back to the layer 5, the conversion efficiency of the solar cell can be increased in combination with the ligand substitution that can increase the amount of electrons injected into the semiconductor layer 22. .

The present inventors conducted experiments to confirm the effects of the present invention. First, a transparent electrode layer 21 made of FTO with a thickness of about 1 μm is formed on the surface of the substrate 1 by sputtering, and a titanium oxide paste is applied to the surface of the transparent electrode layer 21 by a squeegee method and baked at 450 ° C. for 30 minutes for oxidation A semiconductor layer 22 made of titanium was formed with a thickness of about 6 μm. The substrate 1 on which the semiconductor layer 22 had been formed was immersed in a dispersion of quantum dots having an average particle diameter of 5 nm made of CuInSe ₂ synthesized by a colloidal method for 10 minutes to adsorb the quantum dots 23 to the semiconductor layer 22. . By immersing the quantum dot 23 adsorbed in a 0.1 M ammonium sulfide methanol solution at 60 ° C. for 20 minutes, the ligand having a long chain alkyl group adsorbed on the surface of the quantum dot 23 was substituted with sulfur. Finally, by alternately immersing twice in a 0.1M Zn (NO ₃ ) ₂ methanol solution and a methanol solution containing 0.1M selenide ions (one immersion time is 1 minute) (ion layer adsorption reaction) Method), a buffer layer 24 made of ZnSe was formed on the surfaces of the semiconductor layer 22 and the quantum dots 23 to obtain a photoelectrode. Transmission IR measurement was performed before and after the ligand substitution, and the results are shown in FIG. According to this, it was confirmed that the absorption peak due to C—H observed before the ligand substitution disappeared after the ligand exchange, and the long-chain alkyl group was removed.

A quantum dot-sensitized solar cell was produced using the photoelectrode obtained above (invention). As the counter electrode 4, a commercially available brass plate immersed in concentrated hydrochloric acid at 70 ° C. for 15 minutes was used, and as the electrolyte layer (electrolytic solution) 5, a sodium polysulfide methanol solution was used. For comparison with the inventive product, a quantum dot-sensitized solar cell was prepared using a photoelectrode prepared without performing ligand substitution (comparative product). FIG. 4 shows IV curves obtained for the inventive product and the comparative product, respectively. As shown in Table 1 below, the current density Jsc of the comparative product is 3.3 (mA / cm ² ), the open circuit voltage Voc is 0.55 (V), the fill factor FF is 0.51, and the conversion efficiency PCE is 0. The current density Jsc of the invention was 5.3 (mA / cm ² ), the open circuit voltage Voc was 0.51 (V), the fill factor FF was 0.45, and the conversion efficiency was 0.93 (%) The PCE was 1.22 (%), and it was confirmed that the invention product with ligand substitution had excellent conversion efficiency PCE of about 1.3 times that of the comparative product without ligand substitution. It was. From this, it was found that the amount of electrons injected from the quantum dots 23 to the semiconductor layer 22 is increased by ligand substitution, and the buffer layer 24 can prevent backflow of electrons.

  The present invention is not limited to the above embodiment. For example, in the above-described embodiment and experiment, the case where the transparent electrode layer 21 is formed by the sputtering method and the semiconductor layer 22 is formed by the squeegee method and baking has been described, but the transparent electrode layer 21 and the semiconductor layer 22 are formed by other methods. May be formed.

SC ... Quantum dot sensitized solar cell, 1 ... substrate, 2 ... photoelectrode, 4 ... counter electrode, 5 ... electrolyte layer, 21 ... transparent electrode layer, 22 ... semiconductor layer, 23 ... quantum dot, 24 ... buffer layer.

Claims (3)

In the method for producing a photoelectrode of a quantum dot-sensitized solar cell that is disposed to face the counter electrode via an electrolyte layer,
Forming a transparent electrode layer on the surface of the substrate;
Forming a semiconductor layer that performs photoelectric conversion on the surface of the transparent electrode layer;
A step of adsorbing quantum dots on the surface of the semiconductor layer by immersing the substrate in a dispersion in which quantum dots are dispersed;
Immersing the substrate in a solution containing sulfide ions to replace the ligand inevitably attached to the quantum dots with sulfur when adsorbing the quantum dots; and
Forming a buffer layer for preventing electrons from flowing into the electrolyte layer on the surface of the quantum dot and the semiconductor layer. A method for producing a photoelectrode for a quantum dot-sensitized solar cell, comprising:   The method for producing a photoelectrode for a quantum dot-sensitized solar cell according to claim 1, wherein the solution containing sulfide ions is an ammonium sulfide solution. The method for producing a photoelectrode for a quantum dot-sensitized solar cell according to claim 1 or 2, wherein the buffer layer is made of a II-VI group semiconductor .

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