JP2008016369A - Quantum dot sensitized solar cell - Google Patents

Abstract

A semiconductor quantum dot-sensitized solar cell having excellent light resistance and extremely high current conversion efficiency is provided.
A metal oxide n-type semiconductor film such as titanium oxide formed on a transparent electrode side, a semiconductor quantum dot having a diameter of 6 nm or less, such as CdS or PbS, carried on the n-type semiconductor film, and a polysulfide An inorganic liquid electrolyte such as an aqueous solution and a counter electrode are provided.
[Selection] Figure 1

Description

  The present invention relates to a solar cell sensitized by quantum dots.

Dye-sensitized solar cells are attracting attention as next-generation batteries because they can be manufactured at a lower cost than silicon solar cells.
On the other hand, it is known that semiconductor quantum dots have excellent light absorption properties and exhibit a high sensitizing action over dye sensitizers in a three-electrode cell (Non-Patent Document 1). A battery using a solid electrolyte in which quantum dots are supported on a transparent electrode side semiconductor film has also been proposed (Patent Document 1).
JP2002-111031 (Kamat et al, JACS 128, 2385 (2006))

However, when a liquid is used as an electrolyte, quantum dots that are oxidized by absorbing light react with a solvent such as water and decompose in that state, and thus cannot be put into practical use as a solar cell. In Patent Document 1, a solid electrolyte is used. Even in the embodiment disclosed therein, the current photoconversion rate (IPCE) of incident photons is as low as 25% at the maximum, and the electrolyte is an organic substance, so that it is light resistant. It is scarce.
Therefore, an object of the present invention is to provide a solar cell that is excellent in light resistance and has a remarkably high current conversion rate.

In order to solve the problem, the quantum dot-sensitized solar cell of the present invention is
A metal oxide n-type semiconductor film formed on the transparent electrode side, a semiconductor quantum dot supported on the n-type semiconductor film, an inorganic liquid electrolyte, and a counter electrode are provided.
In the solar cell of the present invention, the semiconductor quantum dots are supported on the semiconductor film and the electrolyte is liquid, so that the electrolyte penetrates to every corner of the semiconductor film supporting the quantum dots. As a result, it has high current conversion efficiency. And since electrolyte is inorganic type, it does not deteriorate easily under light irradiation.
A preferable combination of quantum dots and an electrolyte is such that the semiconductor to be the quantum dots is a compound semiconductor such as PbS or CdS, and the inorganic liquid electrolyte is an aqueous solution of a polysulfide such as Na ₂ S _X / Na ₂ S. is there. According to this combination, the polysulfide can reduce the compound semiconductor component much faster than the rate at which the compound semiconductor reacts with the water in the electrolyte and decomposes, so that the quantum dots can exist stably. . Instead of polysulfide, the inorganic liquid electrolyte may include iodine ions and those containing negative ions having a redox potential between the redox potential of iodine ions and the valence band upper end of the quantum dots. Similarly, quantum dots can exist stably. This is because when iodine ions are used alone, the decomposition rate of the compound semiconductor is usually higher than the reduction rate, but when the negative ions coexist, the electrons from the iodine ions to the compound semiconductor components are bridged.

  As described above, it has excellent light resistance, extremely high conversion efficiency, and also has the following effects. Since the sensitizer is a semiconductor quantum dot, the band gap can be changed by controlling the particle size, and since it has a high absorption coefficient for all light having energy higher than the band gap, It is easier to set the absorption region than the dye. That is, it is possible to design a more efficient solar cell. Furthermore, since the synthesis of semiconductor quantum dots on an n-type semiconductor is very simple and the raw materials are inexpensive, the manufacturing cost of solar cells is low.

As a metal oxide that becomes an n-type semiconductor film, the lower end of the conduction band is on the negative side of the lower end of the conduction band of titanium oxide, and the upper end of the valence band is on the positive side of the redox potential of the electrolyte. sufficient if, for example _{TiO 2, Ta 2 O 5,} Nb 2 O 5, SnO 2, ZnO, ZrO ₂ or the like can be mentioned. Preferred is TiO _2. As the semiconductor to be a quantum dot, the above-mentioned PbS and CdS are preferably mentioned, and CdTe, CdSe, Sb ₂ S ₃ , Bi ₂ S ₃ , Ag ₂ S, Ag ₂ Se, Ag ₂ Te, Au ₂ S, Au ₂ Se, Au ₂ Te, Cu ₂ S, Cu ₂ Se, Cu ₂ Te, Fe ₂ S, Fe ₂ Se, Fe ₂ Te, PbSe, PbTe, In ₂ S ₃ , SnS, CuInS ₂ , CuInSe ₂ , CuInTe ₂ A compound semiconductor such as The quantum dot is usually a particle having a diameter of 10 nm or less, preferably 6 nm or less, but is not particularly limited as long as it produces a quantum size effect.

Example 1
A glass substrate (A110U80 (U film) manufactured by Asahi Glass Co., Ltd.) on which a transparent conductive film made of fluorine-doped SnO ₂ was formed was washed, and a TiO ₂ paste (average particle size 15 nm, BET surface area 165 m ² / g, anatase type) was printed on a 150 mesh screen to a thickness of 6-7 nm. After baking at 500 ° C., an aqueous TiCl ₄ solution (concentration 50 mM) was applied. Next, the operation of immersing the substrate with the titanium oxide film in a Cd (ClO ₄ ) ₂ aqueous solution (concentration 0.1M) for 1 minute and then immersing the substrate in a Na ₂ S aqueous solution (concentration 0.1M) for 1 minute is repeated a predetermined number of times. Thus, a semiconductor quantum dot-sensitized titanium oxide electrode (hereinafter, “titanium oxide electrode”) was prepared. The baked portion of the TiO ₂ paste, which was initially colorless, turned yellow, and the color became darker each time the dipping operation was repeated. When the colored portion was analyzed by the X-ray diffraction method, it was confirmed that CdS was supported on any of the titanium oxide electrodes, and the CdS peak became higher as the number of immersions increased. That is, when the baking part before immersion was observed with SEM, only particles of 10 nm or more (derived from titanium oxide) were observed, but those immersed for 5 times were 2-5 nm on such relatively large particles. Particles were attached.

A counter electrode on which a Na ₂ S _X / Na ₂ S aqueous solution (concentration 0.05 M / 0.1 M), a frame-like PET spacer, and a platinum thin film as an electrolyte was deposited was prepared. Then, a solar cell was manufactured by placing a spacer on the TiO ₂ printed surface of the titanium oxide electrode, allowing the electrolytic solution to permeate the titanium oxide electrode, and superimposing a counter electrode thereon. FIG. 1 shows the result of measuring the absorption spectrum of this solar cell using an Xe lamp in-plane uniform light irradiation device manufactured by Spectrometer Co., Ltd. The numbers in the figure are the number of immersions. Moreover, the result of having measured the current voltage characteristic and IPCE of what repeated the said immersion operation 5 times is shown in FIG.2 and FIG.3, respectively.

  As seen in FIG. 1, the absorption edge moved to the long wavelength side as the number of immersions increased. Therefore, it was found that the size of the CdS quantum dots increases with an increase in the number of immersions, and the band gap decreases. Further, as shown in FIG. 2, it showed excellent operating characteristics, and as shown in FIG. 3, the IPCE reached 52% at the maximum.

-Example 2-
The same conditions as in Example 1 except that the composition of the electrolyte was changed to NaSCN (0.7 M) + LiI (0.7 M) + I ₂ (0.05 M), and the number of immersions was set to 5 to obtain CdS particles having a diameter of 2 to 5 nm. A solar cell was manufactured. For comparison, an electrolyte solution having a composition of LiI (0.7 M) + I ₂ (0.05 M) was also produced in the same manner. The results of measuring the IPCE of these solar cells are shown in FIG. As seen in FIG. 4, the iodine electrolyte alone showed only photoelectric conversion by TiO ₂ light absorption (˜400 nm), but coexistence with NaSCN showed photoelectric conversion by CdS light absorption (˜570 nm). Thus, while the CdS is photodissolution with light irradiation iodine electrolyte alone and NaSCN coexist SCN ^- since it is on the positive side than, CdS and SCN ^{- -} oxidation potential of I and between SCN ^- and I ^- electron transfer reaction between is estimated to proceed efficiently.

Example 3
FIG. 5 shows the result of measuring the IPCE by optimizing the composition of the Na ₂ S _X / Na ₂ S aqueous solution as the electrolytic solution in Example 1. IPCE reached a maximum of 68%.

Example 4
Except that Pb (ClO ₄ ) ₂ aqueous solution (0.1 M) was used instead of the Cd (ClO ₄ ) ₂ aqueous solution in Example 1, the dipping operation was repeated three times to obtain PbS particles having a diameter of 2 to 4 nm. FIG. 6 shows the result of manufacturing a solar cell under the same conditions as in Example 1 and measuring the absorption spectrum. As in Example 1, the absorption edge moved to the long wavelength side as the number of immersions increased, and the absorption edge reached 3030 nm after the immersion operation was repeated 10 times. Therefore, it was found that light in a considerably wide wavelength range can be converted into current.

  Further, when the IPCE was measured by optimizing the electrolyte composition, the maximum reached 42% as shown in FIG. In the figure, the value of the inverted triangle is a transcription of the result of FIG. For comparison, a solar cell was produced under the same conditions as in Example 1 except that it was immersed in an alcohol solution of a dye sensitizer N719 composed of a known ruthenium complex instead of repeating the dipping operation, and the IPCE was measured. The results are also shown in FIG. From this figure, it is recognized that a battery carrying PbS quantum dots may have an energy conversion efficiency comparable to that of an N719-sensitized solar cell in which an energy conversion efficiency of 10% is confirmed.

It is a graph which shows the change of the absorption spectrum by the frequency | count of immersion in CdS quantum dot raw material aqueous solution. 4 is a graph showing current-voltage characteristics of the solar cell of Example 1. 4 is a graph showing IPCE of the solar cell of Example 1. 6 is a graph showing IPCE of the solar cell of Example 2. 10 is a graph showing IPCE of the solar cell of Example 3. It is a graph which shows the change of the absorption spectrum by the frequency | count of immersion in PbS quantum dot raw material aqueous solution. It is a graph which shows IPCE of the solar cell of Example 4.

Claims (6)

  Quantum dot enhancement comprising: a metal oxide n-type semiconductor film formed on the transparent electrode side; a semiconductor quantum dot carried on the n-type semiconductor film; an inorganic liquid electrolyte; and a counter electrode Sensitive solar cell.   The battery according to claim 1, wherein the semiconductor serving as the quantum dot is one or more selected from PbS and CdS.   The battery according to claim 1, wherein the quantum dots are particles having a diameter of 6 nm or less.   The battery according to claim 1, wherein the inorganic liquid electrolyte is a polysulfide aqueous solution. The battery according to claim 4, wherein the polysulfide aqueous solution is a Na ₂ S _X / Na ₂ S mixed solution.   The inorganic liquid electrolyte contains negative ions having an oxidation-reduction potential between iodine ions and an oxidation-reduction potential of iodine ions and an upper end of the valence band of the quantum dots. battery.

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