JP4802478B2 - Dye-sensitized solar cell - Google Patents

Description

The present invention belongs to an electrochemical cell using an ionic liquid crystal electrolyte, and is used for a photoelectrochemical cell such as a dye-sensitized solar cell .

  Since many imidazolium iodides are room temperature molten salts (ionic liquids), they function alone as an electrolyte without using a volatile solvent. Since there is no vapor pressure and it is non-volatile, there is no fear of depletion and it is expected to be used for a dye-sensitized solar cell (Patent Document 1). Although the technique described in Patent Document 1 widely defines groups bonded to the heterocyclic ring of imidazolium, in any case, it is essential that the electrolyte is in a liquid state at room temperature or has a melting point lower than room temperature. It is said. This is presumably because the ionic conductivity decreases with increasing viscosity of the electrolyte. Therefore, among various imidazolium salts, the one used in the examples of Patent Document 1 is 1-hexyl-3- which is said to have the highest photoelectric conversion efficiency (Non-Patent Documents 2 and 3). Only methyl imidazolium iodide (hereinafter referred to as “6MImI”).

In addition, a liquid crystalline compound composed of a 5-membered or 6-membered aromatic heterocyclic cation containing nitrogen as a ring constituent element and an anion such as iodide ion has high charge transport performance in a liquid crystal state with high viscosity. A photoelectrochemical cell using this as an electrolyte has also been proposed (Patent Document 2).
WO95 / 18456 JP 2000-319260 A J. Mater. Chem, 8, 2627-2636 (1998) Chem. Commun., 2002, 374 J. Phys. Chem. B, 107, 4374 (2003)

However, since the electrolytic solution described in Patent Document 1 is in a liquid state at room temperature, there is still a risk of leakage, and durability is still insufficient.
On the other hand, although the technique described in Patent Document 2 is proposed as an improvement of the technique described in Patent Document 1, it is an example (in Patent Document 2, 6MImI is not an example but a comparative example). The conversion efficiency (sample number 118 in the same document) when using an imidazolium salt is inferior to 6MImI (sample number 102).
Therefore, an object of the present invention is to provide an electrochemical cell using an electrolyte which is completely different from the conventional flow and can achieve both high viscosity and improvement in charge transport characteristics.

［式（１）中、Ｒ１はＣ１２以上の直鎖アルキル基、Ｒ３はＲ１よりも炭素数の少ない直鎖アルキル基、Ｒ２、Ｒ４及びＲ５はそれぞれ水素原子である。］
前記イオン性液晶電解質中の電気伝導は、電荷担体であるアニオンの拡散、並びにアニオン間（例えばＩ₃ ^-とＩ^-との間）の結合交換による電子ホッピングの二つの機構に担われる。そして、この発明の電池では電解質がスメクティック相を示すイオン性液晶であるから、層間に電荷担体が局在しており、ホッピング伝導が促進される。しかも液晶層が前記のように配向しているので、電極の間隔方向とホッピング方向が一致する。よって、粘度が高くても電荷輸送特性に優れる。 In order to solve the above problems, the dye-sensitized solar cell of the present invention is
A pair of electrodes;
Comprising at least one layer composed of an ionic electrolyte including one or two or more imidazolium iodide liquid crystals represented by the following general formula (1) disposed between the electrodes;
The ionic electrolyte layer is oriented so that its main surface is parallel to or less than 45 degrees with respect to the interval direction of the pair of electrodes.

[In Formula (1), R1 is a linear alkyl group having 12 or more carbon atoms , R3 is a linear alkyl group having fewer carbon atoms than R1, and R2, R4, and R5 are each a hydrogen atom. ]
Electrical conductivity in the ionic liquid electrolyte, the diffusion of the anion is a charge carrier, as well as between the anion (e.g. I ₃ ^- and between ^- and I) are borne on two mechanisms of electron hopping by binding exchange. In the battery of the present invention, since the electrolyte is an ionic liquid crystal exhibiting a smectic phase, charge carriers are localized between the layers, and hopping conduction is promoted. In addition, since the liquid crystal layer is oriented as described above, the electrode spacing direction matches the hopping direction. Therefore, even if the viscosity is high, the charge transport property is excellent.

This is because the imidazolium iodide of the general formula (1) can be easily synthesized from commercially available imidazole, and since the charge carrier is an iodine anion, it has excellent oxidation resistance and is stably present as a charge carrier.
More preferred is when R3 is a C1-3 straight chain alkyl group . This is because the larger the carbon number difference between R1 and R3 and the more asymmetric and planar the whole, the less steric hindrance and the easier the liquid crystallinity.
In the battery of the present invention, the electrolyte is not limited as long as the above conditions are satisfied, and the liquid crystal electrolyte may be mixed with a liquid electrolyte such as an ionic liquid.
Further, it is preferable that a gelling agent is mixed with the smectic liquid crystal electrolyte between the electrodes since the liquid crystallinity of the electrolyte is improved and the conductivity is improved accordingly.

It has excellent charge transport properties regardless of high viscosity, so it is applied to dye-sensitized solar cells with excellent conductivity, low internal resistance, excellent durability, and excellent conversion efficiency. .

The imidazolium salt contained in the electrolyte of the dye-sensitized solar cell of the present invention starts from a commercially available imidazole represented by the following chemical formula (2) , and when no substituent is bonded to nitrogen, the imidazolium salt is obtained by two-step alkylation. When a substituent is bonded to one nitrogen, it can be synthesized only by the second-stage alkylation. The first-stage alkylation is performed by mixing equimolar amounts of imidazole and alkyl halide and heating.
[Examples of imidazole]

[Synthetic reaction]

Example 1
[Synthesis of electrolyte]
In a dry box in a nitrogen stream, methylimidazole and alkyl iodide (alkyl group having 11 or 12 carbon atoms) were mixed in equimolar amounts and heated at 100 ° C. for 7 days. After being allowed to cool, it is dissolved in acetonitrile and recrystallized, whereby 1-dodecyl-3-methylimidazolium iodide (hereinafter, referred to as “12MImI”) and 1-undecyl-3-methylimidazolium iodide (hereinafter, referred to as “12decyl”). , Referred to as “11MImI”). The chemical structure of the product was confirmed to be the target imidazolium salt by ¹ H NMR at 400 MHz as follows.

¹ H NMR (CDCl ₃ δ / ppm)
11MimI
0.87 (3H, t, -N (CH ₂ ) ₁₀ CH ₃ ), 1.2-1.31 (16H, m, -NCH ₂ (CH ₂ ) ₉ CH ₃ ), 1.9 (2H, m, -NCH ₂ CH ₂ (CH ₂ ) ₈ CH ₃ ), 4.1 (3H, s, -NCH ₃ ), 4.31 (2H, t, -NCH ₂ (CH ₂ ) ₉ CH ₃ ), 7.2 (1H, s, H (5)), 7.3 ( 1H, s, H (4)), 10.2 (1H, s, H (2))
12MImI
0.87 (3H, t, -N (CH ₂ ) ₁₁ CH ₃ ), 1.2-1.31 (18H, m, -NCH ₂ (CH ₂ ) ₁₀ CH ₃ ), 1.9 (2H, m, -NCH ₂ CH ₂ (CH _{2) 9 CH 3), 4.1} (3H, s, -NCH 3), 4.31 (2H, t, -NCH 2 (CH 2) 10 CH 3), 7.2 (1H, s, H (5)), 7.3 ( 1H, s, H (4)), 10.2 (1H, s, H (2))

The differential scanning calorimetry (DSC) of the imidazolium salt was measured using DSC-60 manufactured by Shimadzu Corporation according to JIS K7121. Further, using a polarizing microscope (Olympus DP70), 12 MImI alone and a mixture of 12 MImI and iodine were observed at 40 ° C. (liquid crystal temperature). The DSC measurement result is shown in FIG.
As seen in FIG. 1, a phase transition peak was observed at 80 ° C. for 12 MImI, but not for 11 MImI. When 0.65M iodine was added to the 12MImI, the transition temperature decreased to 45 ° C. as shown in FIG. Further, when observed with a polarizing microscope, a focal conic fan structure peculiar to the smectic liquid crystal phase (SmA) (Maruzen Co., Ltd., published in 2000, “Liquid Crystal Handbook”, page 125) was observed in 12 MImI imaging. Accordingly, it was found that 12 MImI exhibits a smectic liquid crystal phase (SmA) at 27-80 ° C.

[Preparation of dye-sensitized solar cell]
In a plastic cylindrical container with an inner volume of 50 ml coated with Teflon (registered trademark) on the inner surface, 6 g of titanium dioxide (DegussaP-25 manufactured by Nippon Aerosil Co., Ltd.), 7 ml of water, 0.2 ml of acetylacetone (Wako Pure Chemical Industries) as a dispersant, 70 g of zirconia balls having a diameter of 3 mm were added and mixed for 2.5 hours with a paint shaker (manufactured by Red Devil). The mixture, excluding the zirconia balls, was transferred to a beaker. The average particle diameter of titanium dioxide was 2.5 μm.
The above mixture was applied to the conductive surface of a conductive glass plate (made by Nippon Sheet Glass) coated with tin oxide to which fluorine was added, and then heated at 450 ° C. for 30 minutes. A glass plate is taken out and immersed in a 3 × 10 ⁻⁴ M acetonitrile / tert-butanol mixed solution of the dye Z907 specified by the following structural formula for 16 hours, followed by washing with acetonitrile and drying, whereby a titanium dioxide electrode is obtained. Created.

  A mixture of 12MImI and 0.65M iodine synthesized as described above was applied to a titanium dioxide electrode, and a platinum-deposited glass plate having the same area was overlaid so that the deposition surface was opposed to the titanium dioxide electrode. This element was heated at 60 ° C. for 10 minutes to allow the high-viscosity ionic liquid crystal electrolyte to penetrate into the pores of the porous titanium dioxide electrode. This completed a dye-sensitized solar cell in which the titanium dioxide electrode, the dye layer, the electrolyte, and the platinum layer of the platinum-deposited glass plate were laminated in order from the conductive surface of the conductive glass plate. For comparison, a dye-sensitized solar cell for comparison was prepared under the same conditions except that 11 MImI was used instead of 12 MImI.

[IV measurement]
The dye-sensitized solar cell was irradiated with pseudo-sunlight (JIS C 8933 1.0 sun) having an intensity of 100 mW / cm ² that passed through the AM1.5 filter, and the current voltage (IV) was measured at 40 ° C. A measurement result is shown in FIG. 3 with the value measured without light irradiation.
As shown in FIG. 3, the short-circuit current value of 12 MImI, which is a liquid crystal electrolyte, is about 1.2 times that of 11 MImI, which is a liquid electrolyte, and the conversion efficiency is improved by about 1.2 times.

[CV measurement]
A microelectrode having a silver wire as a reference electrode, a platinum wire as a counter electrode, and a platinum wire having a diameter of 12 μm is filled with a mixture of 12 MImI and iodine (0.65 M) or a mixture of 11 MImI and iodine (0.65 M) as an electrolyte. Then, a cyclic voltammogram was measured at 50 ° C. The starting potential was 800 mV, the folding potential was 0 mV, and the scanning speed was 1 mV / s. Measured twice per electrolytes, both times the steady response observed in CV curves of each of the electrolyte, the reduction reaction of I ₃ therefrom ^{_{(I 3 - + 2e = 3I}} -) was determined limit current I _lim of. I _lim = 4nFDCr (n: number of electrons, r: working electrode radius, C: I ₃ ^- concentration) based on, to calculate the diffusion coefficient. The calculation results are shown in Table 1.

表１に見られるように、液晶電解質である１２ＭＩｍＩの限界電流値が液体電解質である１１ＭＩｍＩのそれの１．３倍程度であった。

As seen in Table 1, the limit current value of 12 MImI, which is a liquid crystal electrolyte, was about 1.3 times that of 11 MImI, which is a liquid electrolyte.

[Viscosity measurement]
Viscosity of electrolytes obtained by adding 0.65M iodine to each of 12MImI, 11MImI, and 6MImI when the shear rate is changed from 4.5 mm / sec to 90 mm / sec using a TV20 type viscosity meter manufactured by Toki Sangyo Co., Ltd. Was measured. The measurement results are shown in FIG.
As shown in FIG. 4, the viscosity of each electrolyte was almost constant at a shear rate of 11.25 to 45 mm / sec, and 12MImI showed a viscosity value about 2.5 times 11MImI.

[Battery electrode arrangement and electrolyte conductivity]
A pair of comb-shaped Pt electrodes is formed on a glass substrate so that the comb teeth mesh with each other and the opposing conductor spacing (d in FIG. 6) is 5 μm, and an electrolyte in which 0.65 M of iodine is added to 12 MImI is applied. CellA. Separately, a glass substrate and an ITO glass substrate, each having a Pt electrode formed on the entire main surface, are arranged via a frame made of an insulating material having a thickness of 300 μm so that the conductive surfaces face each other. The above electrolyte was filled into the space surrounded by, and cell B was obtained.

When conoscope observation was performed on the cell A from the upper side in the thickness direction of the glass substrate with a polarizing microscope, an isoger (cross image) was observed. Therefore, it was recognized that 12 MImI was oriented perpendicular to the glass substrate, that is, the molecular axis of 12 MImI molecules was perpendicular to the glass substrate.
Next, cell A and cell B were put into a thermostat, voltage was applied to each, and conductivity at various temperatures from 30 ° C. to 60 ° C. was measured with a 1260 impedance analyzer manufactured by Toyo Technica. The measurement results are shown in FIG.
As shown in FIG. 5, the conductivity of all cells decreased from the high temperature side to the low temperature side, but the conductivity of cell A once increased at 45 ° C., which is the liquid-liquid crystal phase transition temperature.

-Example 2-
In the dye-sensitized solar cell of Example 1, a battery was prepared under the same conditions as in Example 1 except for the following four conditions, and the IV characteristics were measured. The first condition different from Example 1 is that an equimolar mixture of 12MImI and 1-tetradecyl-3-methylimidazolium bromide (hereinafter referred to as “14MImBr”) was used instead of 12MImI. The second different condition is that a dye N719 having the following chemical structural formula was used in place of the dye Z907. Third different conditions, a pseudo solar light of intensity 100 mW / cm ² which has passed through the AM1.5 filter (JIS C8933 1.0sun), further through the ND filter reduces the intensity to an intensity 70 mW / cm ² It was irradiated with light. The fourth different condition is that the measurement temperature is 60 ° C. For comparison, the measurement was performed under the same conditions as the equimolar mixture except that 6MImI was used instead of the equimolar mixture.

表２に見られるように、イオン性液体６ＭＩｍＩよりもイオン性液晶１２ＭＩｍＩとイオン性液晶１４ＭＩｍＢｒを混合した電解質の方が光電変換効率に優れていた。 Table 2 shows the photoelectric conversion efficiency (EFF), short-circuit current density (Jsc), open circuit voltage (Voc), and form factor (FF) obtained from the IV characteristics.

As can be seen from Table 2, the electrolyte obtained by mixing the ionic liquid crystal 12MImI and the ionic liquid crystal 14MImBr was superior in photoelectric conversion efficiency to the ionic liquid 6MImI.

-Example 3-
In the dye-sensitized solar cell of Example 1, a battery was prepared under the same conditions as in Example 1 except for the following three conditions, and the IV characteristics were measured. The first condition different from that of Example 1 is that an equimolar mixture of 12MImI and ionic liquid 6MImTFSI having the following structural formula was used instead of 12MImI. The second different condition is that the dye N719 was used in the same manner as in Example 2 instead of the dye Z907. The third different condition is that the measurement temperature is 25 ° C. For comparison, measurement was performed under the same conditions as the above equimolar mixture except that 6 MImI was used instead of 12 MImI in the above equimolar mixture.

表３に見られるように、イオン性液体同士の混合物よりもイオン性液体にイオン性液晶を加えた混合物の方が光電変換効率に優れていた。 Table 3 shows the photoelectric conversion efficiency (EFF), short-circuit current density (Jsc), open-circuit voltage (Voc), and form factor (FF) obtained from the IV characteristics.

As seen in Table 3, the mixture obtained by adding ionic liquid crystal to the ionic liquid was superior in photoelectric conversion efficiency to the mixture of the ionic liquids.

Example 4
In the dye-sensitized solar cell of Example 1, a battery was prepared under the same conditions as in Example 1 except for the following five conditions, and the IV characteristics were measured. The first condition different from that of Example 1 is that 1-hexadecyl-3-methylimidazolium iodide (hereinafter referred to as “16MImI”) or 69-215 ° C. which exhibits a liquid crystal phase at 48-187 ° C. instead of 12 MImI. 1-octadecyl-3-methylimidazolium iodide (hereinafter referred to as “18MImI”) which exhibits a liquid crystal phase. The second different condition is that the above dye N719 was used instead of the dye Z907. The third different condition is that the ionic liquid crystal electrolyte was applied and then heated to 80 ° C. The fourth different condition is that pseudo-sunlight (JIS C 8933 1.0 sun) having an intensity of 100 mW / cm ² that has passed through an AM1.5 filter, and further passing through an ND filter to have an intensity of 70 mW / cm ^2. This is the irradiation of reduced light. The fifth different condition is that the measurement temperature is 70 ° C or 80 ° C.
Table 4 shows the photoelectric conversion efficiency (EFF), short-circuit current density (Jsc), open circuit voltage (Voc), and form factor (FF) obtained from the IV characteristics.

-Example 5-
A gelling agent concentration of 5 g / L, 10 g / L, 20 g / L, 40 g / L, and 80 g / L is obtained by adding a gelling agent represented by the following chemical formula to the mixture of 12MImI and 0.65M iodine synthesized in Example 1. Was added as follows.

After the addition, liquid crystal gel was obtained by stirring at 80 ° C. And about the liquid crystal gel prepared by each gelatinizer density | concentration, conductivity was measured on the same conditions as Example 1 except having made measurement temperature constant 40 degreeC. For comparison, a gelling agent was added at a concentration of 5 g / L or 80 g / L to a mixture of 11 MImI and 0.65 M iodine instead of the above mixture, and the conductivity was measured. Each measurement result is shown in FIG.
As shown in FIG. 7, the conductivity was improved at 12 MImI at 5-20 g / L with no gelling agent, whereas the conductivity was not added at 11 MImI regardless of the gelling agent concentration. It was the same level as the one.
Next, DSC was measured for liquid crystal gels having gelling agent concentrations of 5 g / L and 20 g / L. As a result, the liquid-to-liquid crystal transition temperature decreased by about 5 ° C. compared to the gelling agent-free liquid crystal gel and the concentration was 5 g / L. It was clearly confirmed that the liquid crystal gel of L (hereinafter referred to as “12MImI-5 g / L”) has a sharper transition peak than the liquid crystal without the addition of a gelling agent.
Further, when a conoscope of 12 MImI and 12 MImI-5 g / L without adding a gelling agent was compared and observed at 40 ° C., it was found that the cross shape of 12 MImI-5 g / L was clearer than that of 12 MImI alone. It was.

Next, a dye-sensitized solar cell was manufactured. The manufacturing conditions in Example 1 were that instead of applying a mixture of 12 MImI and 0.65 M iodine, a mixture of 12 MImI-10 g / L and 0.65 M iodine was applied to titanium dioxide, and 10 ° C. at 80 ° C. after the application. The same as Example 1 except that it was heated for a minute. And the artificial sunlight was irradiated on the same conditions as Example 1, and the current voltage was measured. For comparison, a battery was similarly manufactured for 11 MImI-10 g / L, and the current voltage was measured. The measurement results are shown in FIGS. 9 and 8 together with the measurement results of 12 MImI and 11 MImI in Example 1, respectively.
In the case of 11MImI which is a liquid electrolyte as shown in FIG. 8, the characteristics did not change even when the gelling agent was added, but in the case of 12MImI which is a liquid crystal electrolyte as shown in FIG. By adding, the current value increased.

It is a graph which shows the DSC measurement result of an imidazolium iodide. It is a graph which shows the DSC measurement result of the mixture of an imidazolium iodide and an iodine. It is a graph which shows the current-voltage characteristic of the dye-sensitized solar cell of an Example. It is a graph which shows the result of having measured the viscosity of electrolyte. It is a graph which shows the result of having measured the ionic conductivity of the electrolyte in various temperature. 1 is a plan view showing a battery of Example 1. FIG. It is a graph which shows the relationship between a gelatinizer density | concentration and the conductivity of electrolyte. It is a graph which shows the current-voltage characteristic of 11MImI and 11MImI-10g / L. It is a graph which shows the current voltage characteristic of 12MImI and 12MImI-10g / L.

Claims (3)

A pair of electrodes;
Comprising at least one layer composed of an ionic electrolyte including one or two or more imidazolium iodide liquid crystals represented by the following general formula (1) disposed between the electrodes;
The dye-sensitized solar cell, wherein the ionic electrolyte layer is oriented so that a main surface thereof is parallel to or less than 45 degrees with respect to a distance direction of the pair of electrodes.

[In Formula (1), R1 is a linear alkyl group having 12 or more carbon atoms , R3 is a linear alkyl group having fewer carbon atoms than R1, and R2, R4, and R5 are each a hydrogen atom. ] The dye-sensitized solar cell according to claim 1, wherein R 3 is a C 1-3 straight chain alkyl group . The dye-sensitized solar cell according to claim 1 or 2 , wherein a gelling agent is mixed with an ionic electrolyte containing the imidazolium iodide liquid crystal between the electrodes.

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