JP5158761B2 - Dye-sensitized semiconductor electrode and photoelectric conversion device using the same - Google Patents

Description

  The present invention relates to a semiconductor electrode for photoelectric conversion, and more particularly to a semiconductor electrode and a photoelectric conversion device for efficiently sensitizing a dye that can be used up to a long wavelength.

  The dye-sensitized photovoltaic cell invented by Gretzell et al. (Patent Document 1) does not use expensive raw materials, is easy to manufacture, and does not require a large amount of energy, so that it can be put to practical use as a low-cost photovoltaic cell. Expected and researched energetically at home and abroad.

  A conventional dye-sensitized photocell has a semiconductor electrode (working electrode) made of a dye-containing semiconductor layer, a counter electrode made of a conductor such as platinum or carbon, and an iodide sandwiched between the working electrode and the counter electrode as main components. And an electrolyte layer. This semiconductor electrode is generally manufactured as follows.

  The substrate is made of conductive glass in which a conductive material such as tin oxide doped with fluorine (hereinafter referred to as FTO) or indium-tin oxide is coated on a glass substrate, and porous semiconductor particles (for example, titanium oxide) are formed thereon. A thin film is formed. Thereafter, the glass substrate on which the semiconductor thin film is fixed is immersed in a dye solution and dried, so that a semiconductor electrode (anode electrode) having the dye adsorbed on the surface of the semiconductor particles is completed. Next, a photoelectric conversion device is completed by facing a counter electrode (cathode electrode) coated with a catalyst such as platinum or carbon, sandwiching the electrolytic solution therebetween, and sealing the whole.

  By the way, in order to improve the performance of such a photoelectric conversion device, it is necessary to improve the short circuit photocurrent (Isc), the open circuit voltage (Voc), and the form factor (ff). In order to improve Isc, in order to broaden the absorption wavelength region of the solar spectrum, it is necessary to use dyes that have extended absorption to long wavelengths such as infrared rays, but infrared absorption dyes have low excitation levels (electron potentials). Therefore, it is often impossible to sensitize with a titanium oxide electrode. Therefore, a semiconductor having a low conduction band level such as tin oxide is generally used (Patent Document 2).

  On the other hand, tungsten oxide is also known to be a semiconductor having a low conduction band level and can be used as a semiconductor electrode of a photoelectric conversion element as well as a semiconductor compound such as titanium oxide, zinc oxide, and niobium oxide ( Patent Document 3, Non-Patent Document 1).

However, in these references, titanium oxide is the only semiconductor disclosed with high-efficiency data at a practical level, and carboxylic acid (acidic) anchors such as ruthenium complexes (N3 and N719) are used as sensitizing dyes. There were only examples of use of dye molecules having groups.
As described above, various semiconductor electrodes have been proposed that broaden the absorption wavelength region of the solar spectrum and are highly sensitized by long-wavelength light such as infrared rays and are extremely useful as photoelectric conversion devices. It wasn't.

On the other hand, since sensitizing dyes and semiconductors are usually studied separately, TiO ₂ semiconductors are the standard for sensitizing dye research, and carboxylic acid (acidic) anchors such as ruthenium complexes (N3 and N719) are used for semiconductor research. A dye molecule having a group is a standard. However, the relationship between the dye and the semiconductor is complicated, such as the adsorption characteristics and the correlation between the conduction band level and the dye excitation level (LUMO). Specific combinations are important, but these points have not been fully studied so far.

Japanese Patent No. 2664194 JP 2002-100418 A JP 2005-243393 A Chem. Mater., Vol. 10, pp.3825-3832, 1998

  The present invention has been made in view of the above circumstances, and broadens the absorption wavelength region of the solar spectrum and is highly sensitized by long-wavelength light such as infrared rays to improve short-circuit photocurrent and energy efficiency. An object of the present invention is to provide a novel semiconductor electrode in which a semiconductor and a dye are selectively combined, which is extremely useful as a photoelectric conversion device.

As a result of intensive studies to solve the above problems, the present inventors have found that tungsten oxide and molybdenum oxide are dyes having an acidic group such as carboxylic acid that is generally used as an anchor group (N3, N719, black Die etc.) is hardly adsorbed, but strongly adsorbs dyes with basic groups as anchor groups, and tungsten oxide and molybdenum oxide have lower conduction band than TiO ₂ and higher open circuit voltage than SnO ₂ The inventors have found that the above problem can be solved by combining these requirements, and have completed the present invention.
That is, according to this application, the following invention is provided.
<1> In the dye-sensitized porous semiconductor electrode, the semiconductor is a compound containing (1) tungsten oxide, (2) molybdenum oxide, or (3) at least one element of tungsten or molybdenum, Is a dye containing a basic anchor group, A dye-sensitized semiconductor electrode.
<2> The semiconductor electrode according to <1>, wherein the basic anchor group is an amino group or a pyridine group.
<3> The semiconductor electrode according to <1> or <2>, wherein the light absorption wavelength of the dye is 800 nm or more.
<4> The semiconductor electrode according to any one of <1> to <3>, wherein the excitation level of the dye is lower than the TiO ₂ conduction band.
<5> A photoelectric conversion device using the semiconductor electrode according to any one of <1> to <4>.
<6> A tandem photoelectric conversion device using the semiconductor electrode according to any one of <1> to <5> in a subsequent stage of the tandem photoelectric conversion device.

  Since the dye-sensitized semiconductor electrode of the present invention broadens the absorption wavelength region of the solar spectrum and is highly sensitized by long-wavelength light such as infrared rays, it can improve short-circuit photocurrent and energy efficiency. It is extremely useful as a photoelectric conversion device.

  The dye-sensitized porous semiconductor electrode of the present invention has, as a semiconductor, (1) tungsten oxide, (2) molybdenum oxide, or (3) a compound containing at least one element of tungsten and molybdenum as a dye. And a dye containing a basic anchor group.

  Examples of the semiconductor that satisfies the above requirements include tungsten oxide, molybdenum oxide, or a compound containing at least one element of tungsten and molybdenum. In this case, the conductivity may be slightly improved by doping or substituting WOx, a foreign metal, or an anion (N, C, S) having oxygen defects. Molybdenum compounds that are similar to tungsten and have similar characteristics can also be used as semiconductors.

Specific examples of the semiconductor used in the present invention include WO _x (X ≦ 3), MoO _x (X ≦ 3), WyMo _1-y O _x (x ≦ 3, y ≦ 1) Bi ₂ WO ₆ , BiMoO ₆ Etc. are exemplified.

A typical example of the tungsten oxide WOx (X ≦ 3) is tungsten oxide (WO ₃ ), which will be described in detail below as an example. WO ₃ preferably has a small particle size, a large surface area, and high crystallinity. Specifically, the particle size is 5 to 500 nm, preferably 7 to 200 nm. The surface area is 3 to 120 m ² / g, preferably 5 to 80 m ² / g. As a method of increasing the surface area, there is a method of making a WO ₃ mesoporous structure using a polymer template (J. Mater. Chem. 11 (2001) 92, J. Alloys. Comp., 396 (2005) 295, etc.). Since it is known, you may refer to this.

According to the study by the present inventors, even when tungsten oxide or molybdenum oxide is immersed in a solution of a dye having an acidic group (carboxylic acid) (N3, N719, black dye, etc.), these dyes are almost adsorbed. In addition, it was found that even if it was adsorbed, it immediately desorbed and its sensitizing action hardly functioned.
In other words, most of the dyes common in dye-sensitized solar cells have acidic groups such as carboxylic acid groups, phosphoric acid groups, and sulfuric acid groups as anchor groups. It was found that it does not adsorb on the surface of molybdenum oxide. Furthermore, even if a specific dye is adsorbed to tungsten oxide or molybdenum oxide, it actually contributes to the sensitization reaction, and further, a tin oxide semiconductor generally known as having a low conduction band level It is unclear whether a more efficient electrode can be obtained, and studies on this point have been awaited.

In recent years, tandem cells have been studied for improving the performance of photoelectric conversion devices, and it is essential to use dyes that absorb long-wavelength light such as infrared rays in the subsequent stage. However, long-wavelength absorption dyes generally have the disadvantage that they cannot be sensitized by TiO _{2 which} has a low excitation level and a high conduction band level, and SnO ₂ has a low conduction band level but leakage current is low. There was a disadvantage that the open circuit voltage was low because of its large size.
As a result of further research to solve these problems all at once, the present inventors have used the above-mentioned unique compound as a semiconductor and a basic anchor group as a dye in a dye-sensitized porous semiconductor electrode. It was found that a semiconductor electrode having a conduction band lower than that of TiO ₂ , an open-circuit voltage higher than that of SnO ₂ , and capable of strongly adsorbing the dye and having excellent sensitization can be obtained by using a dye having the above.

  Next, the dye used in the present invention will be described. In the present invention, as described above, it is necessary to use a dye containing a basic anchor group. This is because these dyes are easily adsorbed to the acid sites of the semiconductor or negatively charged portions on the surface.

Here, the anchor group refers to a “site that binds and adsorbs to the semiconductor surface in the constituent molecules of the dye”. Specifically, it includes 5-membered rings containing N such as amino group (—NR ₂ ), protonated amino group (——NR ₃ +), pyridine group, pyridine ring substituent, cyano group, oxazolinyidene ring, etc. Examples include groups.
In the formula, R is H, an alkyl group, a phenyl group containing an aromatic ring, a substituent containing N, S, O, a halogen element, or the like. When there are a plurality of R, they may or may not be the same. Regarding the size of R, smaller substituents are desirable, and H is the best.
N719 dye has a pyridine ring coordinated to Ru metal, but this part is inside the dye, so it does not function as an anchor. In the present invention, the anchor group needs to have a three-dimensional structure capable of adsorbing to the semiconductor surface.
The anchor group does not need to be conjugated with the chromophore (chromophore) of the dye. However, in consideration of the electron transfer from the chromophore to the semiconductor, it is preferably conjugated or not conjugated with the chromophore as much as possible. An anchor base structure that shortens the distance of the semiconductor is desirable. Specifically, the direction of the anchor group is preferably a short anchor group such as —NH 2 or a conformation in which the chromophore surface is close to the semiconductor surface. Considering strong adsorption characteristics, it is desirable that the number of anchor groups is not only one but more.

  The dye used in the present invention can be selected from conventionally known dyes. That is, conventionally known metal complex dyes (Ru metal complexes, Os metal complexes, Fe metal complexes, Pt metal complexes) or organic dyes (methine dyes, mercurom dyes, sachinten dyes, porphyrin dyes, phthalocyanine dyes) Azo dyes, coumarin dyes, and other organic dyes) may be selected from dyes having a basic anchor group.

Typical examples of such dyes include those having the structures shown below.

  In addition to the above, rhodamine derivatives, safranine derivatives, methylene blue derivatives, thionine derivatives, proflavine derivatives, basic dye groups (basic red 5 derivative, basic orange 14 derivative, basic blue 3 derivative, etc.) are also specific examples of organic dyes. Take as an example.

  The dye used in the present invention needs to satisfy the above-described requirements, but the excitation level (LUMO) is preferably higher than the conduction band level of the semiconductor. However, when LUMO shifts lower due to adsorption, LUMO may be slightly lower than the conduction band level of the semiconductor. It is desirable that the HOMO of the dye is more positive than the redox level used.

Further, the dye used in the present invention preferably has a longer absorption wavelength region, and it is particularly desirable that the dye has strong absorption in the vicinity of 800 to 1000 nm. In order for both LUMO and HOMO to be at the optimum level for the sensitization reaction, the absorption edge is desirably 1200 nm or less. Further, in the case of a dye having a LUMO that is too high and absorbs a long wavelength, it is difficult to satisfy the condition that it is more positive than the redox level used by the HOMO of the dye. Therefore, it is sufficient that the LUMO is as high as necessary for the semiconductor of the present invention, and it is not necessary to be too high. The standard is that the LUMO of the dye is lower than the TiO ₂ conduction band level.

For the production of the porous semiconductor electrode according to the present invention, for example, a method for producing a porous electrode of a dye-sensitized solar cell using TiO ₂ can be applied, but the method is not particularly limited thereto.
Specifically, the above-mentioned WO ₃ fine particles and a compound containing tungsten, for example, a precursor solution such as H ₂ WO ₄ or WCl ₆ are used to form an electrode substrate by spin coating, dip coating, bar coating, screen printing, etc. Form a film. In this case, an organic substance such as polyethylene glycol or Triton X may be added to control the porosity or improve the contact between the particles. By baking this, a semiconductor thin film in which WO ₃ fine particles are bonded to each other can be produced. Moreover, a pressurizing method or a method of forming a film by applying a high voltage can also be used.

In general, the dye is adsorbed on the porous semiconductor electrode by immersing the porous semiconductor electrode in an aqueous solution or organic solvent solution in which the dye is dissolved for a long time, whereby the anchor group is chemically adsorbed on the semiconductor surface. In order to promote chemisorption, it is also effective to heat or combine decompression and pressurization. Coadsorbents can also be placed in the adsorption solution to control dye association.
The electrode substrate can be used if it has conductivity. For example, conductive glass, conductive plastic, metal and the like.

  This electrode can be used for various photoelectric conversion devices by combining with a counter electrode or a reference electrode. In particular, it is effective when used for a photoelectric conversion device having a tandem structure. In the case of the tandem type, most of the light on the short wavelength side is absorbed by the front electrode, and the light on the long wavelength side that has passed through it is absorbed by the back electrode. An electrode combining a semiconductor having a low conduction band and a long wavelength absorption dye according to the present invention is most effective in the latter stage electrode of the tandem structure, and utilizes photons that could not be used in the former stage electrode to the maximum extent. Can do.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Reference example 1
Each powder of WO ₃ powder (same as Example 1 described later) and MoO ₃ powder (manufactured by Wako) was deposited on conductive glass in the same manner as Example 1 described later. The TiO ₂ film was formed in the same manner as in Comparative Example 1.
Dye B (Basic Fuchsin, Basic Red 9) having the following structure as a dye having a basic anchor group and ruthenium complex dye (N719) as a dye having a carboxylic acid anchor group were put in an ethanol solution to adsorb the dye. Thereafter, it was washed with ethanol and dried. The adsorption characteristics of dyes having various anchor groups on these dye adsorbing semiconductor films were evaluated by the reflection spectrum method using an integrating sphere. The results are shown in Table 1 below. A film having a smaller reflectance (R%) has a higher light absorption and a higher amount of dye adsorption. A large amount of dye adsorption indicates that the adsorption force of the adsorption anchor group is large.

From Table 1, in the case of WO _3, the N719 dye having an acidic carboxylic acid anchor has a reflectance close to the number without the dye and hardly adsorbs, but the B dye having a basic amino group anchor has a high reflectance. It can be seen that the dye is adsorbed more than the N719 dye. The same was true for MoO ₃ .
On the other hand, in the case of TiO _2, the B dye with a basic amino group anchor has a reflectance close to the number without the dye and hardly adsorbs, but the N719 dye with an acidic carboxylic acid anchor has a greatly reduced reflectance. It can be seen that the dye is adsorbed more than the B dye.

この電極をエタノールで洗浄して物理吸着している過剰な色素を除去し、室温で乾燥させ、色素吸着WO₃電極とした。次に、溶媒がアセトニトリルでヨウ化リチウム０．１Ｍ、ヨウ素０．０５Ｍ、ヨウ化ジメチルプロピルイミダゾリウム０．６２Ｍを溶解した電解質溶液を調製した（電解液ELS1）。この電解液にさらにｔ−ブチルピリジンを濃度０．５Ｍになるように添加し溶解したものを電解液ELA1とした。
この電解質溶液を、上述の増感色素吸着WO₃半導体電極と白金対極との間に60mmのスぺーサーをつけてはさんで、クリップで止めた。得られた光電変換素子に、ソーラーシュミレーター（AM-1.5、JIS-A）を光源として強度１００ｍＷ／ｃｍ²の光を照射した。半導体自身の光励起による光電流の影響を除外するため、カットフィルター(L-48)を付けて450nm以上の光を照射した。結果を表２に示す。同じ電解液同士で比較して、後述の比較例１，２のSnO₂やTiO₂に比べて短絡電流Iscや太陽エネルギー変換効率(hsun)が高いことが確認できた。開放電圧（Voc）はSnO₂より高かった。 Example 1
WO ₃ fine particles were prepared by a thermal decomposition method of a peroxide of tungstic acid (H ₂ WO ₄ manufactured by Wako). That is, 2 g of tungstic acid was slowly dissolved in 40 ml of hydrogen peroxide (30% aqueous solution) with stirring at 100 rpm. When the obtained milky white transparent solution is slowly heated in a drier set at 100 ° C. and water and hydrogen peroxide are evaporated, a white solid is deposited. This was baked in an electric furnace at 400 ° C. for 0.5 hours to prepare WO ₃ fine particles. The surface area of the WO3 fine particles was 33 m ² / g.
Next, 40 mg of the WO ₃ fine particle powder obtained above and 10 μL of acetylacetone are well kneaded in an agate mortar, and 200 μL of ethanol is added and kneaded further to make a slurry. A mask film with a 5 mm square window is pasted on conductive glass (F-SnO ₂ , 10Ω / sq), and this WO ₃ slurry is thinly formed by a doctor blade method to form a film. After drying, the mask film was removed and air baked at 500 ° C. for 1 hour to obtain a WO ₃ electrode.
Next, the following dye A (Cresyl Violet 670, 5,9- Diaminobenzophenoxazonium Perchlorate,) of the blue-violet ethanol solution (10 mg / 50 ml) for 24 hours immersed the WO3 electrodes dye was adsorbed on the WO _3. The structure of Dye A is shown below. There are —NH ₂ groups or ═NH ₂ + groups, which are considered to be adsorbed at this portion.

This electrode was washed with ethanol to remove excess dye that was physically adsorbed, and dried at room temperature to obtain a dye-adsorbed WO ₃ electrode. Next, an electrolyte solution was prepared by dissolving 0.1M lithium iodide, 0.05M iodine, and 0.62M dimethylpropylimidazolium iodide with an acetonitrile solvent (electrolytic solution ELS1). Electrolytic solution ELA1 was obtained by further adding t-butylpyridine to this electrolytic solution to a concentration of 0.5 M and dissolving it.
The electrolyte solution was clamped with a spacer of 60 mm between the sensitizing dye adsorbing WO ₃ semiconductor electrode and the platinum counter electrode. The obtained photoelectric conversion element was irradiated with light having an intensity of 100 mW / cm ^{2 using} a solar simulator (AM-1.5, JIS-A) as a light source. In order to exclude the influence of the photocurrent due to the photoexcitation of the semiconductor itself, a cut filter (L-48) was attached and irradiated with light of 450 nm or more. The results are shown in Table 2. Compared with the same electrolyte solution, it was confirmed that the short-circuit current Isc and the solar energy conversion efficiency (hsun) were higher than those of SnO ₂ and TiO ₂ of Comparative Examples 1 and 2 described later. Open-circuit voltage (Voc) was higher than SnO _2.

Example 2
In Example 1, the same experiment as in Example 1 was performed except that the dye A was replaced with the red dye B (Basic Fuchsin, Basic Red 9). The results are shown in Table 3. As described above, the structure of the dye B has an —NH ₂ group or ═NH ₂ + group, and is considered to be adsorbed at this portion. Compared with the same electrolyte solution, it was confirmed that the short-circuit current Isc and the solar energy conversion efficiency (hsun) were higher than those of SnO ₂ and TiO ₂ of Comparative Examples 3 and 4 described later.

Example 3
In Example 1, the dye A was converted to an orange nium complex dye C (self-prepared according to the following literature: E. Ishow, A. Gourdon, J.-P. Launay, Chem. Commun., 1998, 1909-1910). An experiment similar to that of Example 1 was performed except that it was replaced. The results are shown in Table 4. The structure of the dye C is shown below, and there is an —NH ₂ group, which is considered to be adsorbed at this portion. Compared with the same electrolyte solution, it was confirmed that the short circuit current Isc and the solar energy conversion efficiency (hsun) were higher than SnO ₂ of Comparative Example 5 described later.

Comparative Examples 1 and 3
In Examples 1 and 2 , the same experiment as in Examples 1 and 2 was performed except that TiO ₂ was used instead of WO ₃ .
Using TiO ₂ fine particles (self-prepared, anatase type), a TiO ₂ paste containing 30% by weight of TiO ₂ fine particles was prepared. Here, TiO ₂ fine particles are 30% by weight, α-terpineol (manufactured by Kanto Chemical Co., Inc.) is 65% by weight, and ethyl cellulose (manufactured by Kanto Chemical Co., Inc.) is 5% by weight.
Then, each was weighed and mixed, and then kneaded for a predetermined time using three rolls to obtain a TiO ₂ paste.
On the conductive glass substrate, the TiO ₂ paste obtained above was applied by screen printing. Thereafter, baking was performed at 500 ° C. for 60 minutes in the air to obtain a TiO ₂ electrode. Tables 2 and 3 show the photoelectric conversion characteristics of the electrode on which the dye was adsorbed . The solar energy conversion efficiency was lower than that of the WO ₃ electrode. Especially the short circuit current was small. This is presumably because the conduction band level of TiO ₂ was higher than that of WO ₃ and it was difficult to inject electrons from the dye.

Comparative Examples 2, 4, and 5
In Examples 1 to 3 , experiments similar to those in Examples 1 to 3 were performed except that SnO ₂ was used instead of WO ₃ . SnO ₂ fine particles were manufactured by Kanto Chemical. The fine particles were pasted, printed and fired in the same manner as in Comparative Example 1 to obtain a SnO ₂ electrode. Tables 2 to 4 show the photoelectric conversion characteristics of the electrode on which the dye was adsorbed. The solar energy conversion efficiency was lower than that of the WO ₃ electrode. In particular, the open circuit voltage was low. The conduction band level of SnO ₂ is said to be in a position similar to that of WO ₃ , but the reason why the open circuit voltage was low is considered to be that SnO ₂ had a larger leakage current than WO ₃ .

Claims (6)

A dye-sensitized semiconductor electrode, wherein the semiconductor is tungsten oxide and the dye is a dye containing a basic anchor group.   The semiconductor electrode according to claim 1, wherein the basic anchor group is an amino group or a pyridine group.   The semiconductor electrode according to claim 1 or 2, wherein the light absorption wavelength of the dye is 800 nm or more. 4. The semiconductor electrode according to claim 1, wherein an excitation level of the dye is lower than that of the TiO ₂ conduction band. 5.   The photoelectric conversion device using the semiconductor electrode of any one of Claim 1 to 4.   A tandem photoelectric conversion device using the semiconductor electrode according to any one of claims 1 to 5 in a subsequent stage of the tandem photoelectric conversion device.