JP2009016304A - Binder for dye-sensitized solar cell, paste, white insulation film for dye-sensitized solar cell, and dye-sensitized solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a binder capable of preventing a reflection rate from reducing and improving film strength by restraining dyeing of a reflection layer with a colorant when specially forming the reflection layer of a dye-sensitized solar cell, a paste including the same binder, a white insulation film capable of using as a reflection layer obtained by sintering the paste, and a dye-sensitized solar cell having the same reflection layer. <P>SOLUTION: The binder having a silica polymer wherein at least a part of a surface functional group is substituted with an alkyl group is provided, and the conductive paste having the binder and inorganic particles is manufactured. Then, a white insulation film and the reflection layer of the dye-sensitized solar cell are formed by applying and sintering the paste. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a dye-sensitized solar cell binder, a paste produced using this binder, a white insulating film for dye-sensitized solar cells produced from this paste, and a reflection of the white insulating film. The present invention relates to a dye-sensitized solar cell provided as a layer.

  Conventionally, a semiconductor layer (photoelectric conversion layer) made of titanium oxide or the like carrying a dye is provided between a transparent electrode and a counter electrode having a conductive layer carrying a catalyst, and the periphery is sealed with a resin or the like. There is known a dye-sensitized solar cell configured by sealing with a stopping material (for example, JP-A-2005-228594). In addition, a dye-sensitized type in which a semiconductor layer (photoelectric conversion layer), which is also made of titanium oxide or the like, which carries a dye, is sequentially laminated on a transparent electrode, an insulating reflective layer, a catalyst layer, and a conductive layer. (For example, Unexamined-Japanese-Patent No. 2003-142171) is known. In the former dye-sensitized solar cell, the transparent electrode and the counter electrode function as a support member. In the latter dye-sensitized solar cell, only the transparent electrode functions as a support member.

  Since the dye-sensitized solar cell having only the latter single support member as described above has only a single support member, high film strength is required for each layer constituting the solar cell. . Further, in the dye-sensitized solar cell having such a configuration, a structure in which high refractive index inorganic particles having a particle size of about 100 nm to 1 μm are mainly bonded to each other is based on the above-described layer stacking order. Due to the above problem, there has been a problem that the insulating reflective layer is peeled off.

  In view of the above-described problems, various studies have been made to improve the film strength of the dye-sensitized solar cell, particularly the insulating reflective layer. For example, attempts have been made to improve the firing temperature when making the reflective layer, or to increase the firing time.

  However, in a dye-sensitized solar cell, the transparent electrode is formed by laminating a transparent conductive layer on a glass substrate. Therefore, even if the firing temperature is improved, the softening temperature of the glass substrate is reduced. It has to be taken into consideration, and it was necessary to set the temperature to 550 to 600 ° C., preferably 500 ° C. or less. However, at such a firing temperature, the bond strength of the inorganic particles having the above-described particle diameter cannot be sufficiently improved. Further, the longer firing time is contrary to the actual process requirement of improving productivity.

  In view of such a problem, there is also a method in which an inorganic binder is applied to the inorganic particles constituting the reflective layer, the inorganic binder is interposed between the inorganic particles, and the adjacent inorganic binders are joined together. Has been tried. According to such a method, the film strength of the target reflective layer can be improved.

  On the other hand, for example, a silica polymer is used as the inorganic binder, and this polymer has a large number of hydroxyl groups on the surface. Therefore, in the dye adsorption step when producing the dye-sensitized solar cell, a large amount of the dye to be adsorbed on, for example, titanium oxide fine particles of the photoelectrode layer is also adsorbed on the surface of the silica polymer, and the reflection A new problem arises that the layer is colored and the properties of the reflective layer itself cannot be fully exhibited. That is, the light to be reflected in the reflective layer is adsorbed by the inorganic binder and absorbed by the dye taken into the reflective layer, and sufficient reflected light cannot be obtained with the dye on the titanium oxide that can be photoelectrically converted. The utilization efficiency of the light incident on the dye-sensitized solar cell in the photoelectric conversion layer is reduced, which causes the conversion efficiency of the solar cell to deteriorate.

In addition, an attempt has been made to use an organic polymer instead of the above-described silica polymer, but because the heat-resistant temperature is low, it burns or deteriorates at the heating temperature in the production process of the dye-sensitized solar cell, The characteristic cannot be exhibited.
JP 2005-228594 A JP 2003-142171 A

  The present invention provides a binder capable of suppressing the coloring of the reflective layer with a dye to prevent a decrease in reflectance and improving the film strength, particularly when forming a reflective layer of a dye-sensitized solar cell. Provided are a paste containing the binder, a white insulating film usable as the reflective layer obtained by firing the paste, and a dye-sensitized solar cell including the reflective layer. With the goal.

In order to achieve the above object, the present invention provides:
The present invention relates to a dye-sensitized solar cell binder comprising a silica polymer in which at least a part of surface functional groups is substituted with an alkyl group.

  The present invention also relates to a paste comprising the binder and inorganic particles.

  Furthermore, the present invention relates to a white insulating film for colored paper sensitized solar cells obtained by firing the paste.

  The present invention also relates to a dye-sensitized solar cell comprising a reflective layer obtained by firing the paste.

  In view of the conventional problems described above, the present inventors can improve the bonding strength by interposing between the particles when inorganic particles are bonded to each other to form a layer. In order to find a novel binder that does not cause dye adsorption as in the case of the silica polymer of the present invention, intensive studies were conducted. As a result, it was found that the above-mentioned binder can be obtained by substituting at least a part of the surface functional groups with an alkyl group for a conventional silica polymer. That is, by forming a binder from a silica polymer in which at least a part of the surface functional groups is substituted with an alkyl group, the bonding strength can be improved when the binder is interposed between the inorganic particles constituting the layer. At the same time, the present inventors have found that the binder does not color without adsorbing the pigment.

  Therefore, a white insulating film can be obtained by preparing a paste from such a binder and inorganic particles, and applying and baking the paste. Since this insulating film is not colored and exhibits a high reflectance with white, it can be preferably used as a reflective layer in a dye-sensitized solar cell. In the paste, since the silica polymer constituting the binder has an alkyl group on the surface, it is possible to greatly suppress gelation and curing even when left for a long time. is there.

  Moreover, the silica polymer substituted with an alkyl group is stable even at a high temperature, and the structure can be maintained stably even at a firing temperature of 300 to 500 ° C., for example. Therefore, as described above, even when a reflective layer of a dye-sensitized solar cell is formed using the inorganic binder containing the alkyl group-substituted silica polymer, it is exposed to a high-temperature process in a later manufacturing process. In this case, the structure of the reflective layer can be stably maintained, and the film strength and the prevention of coloring due to non-adsorption of the dye can be stably maintained.

  In addition, since many hydroxyl groups exist in the surface of a silica polymer, the said alkyl group will adhere to the surface of a silica polymer by making such a hydroxyl group and trialkyl silicon react.

  The reason why the binder inhibits the adsorption of the dye is considered to be that the surface functional group of the silica polymer constituting the binder is substituted with an alkyl group and is thus hydrophobized.

  As described above, according to the present invention, when forming a reflective layer of a dye-sensitized solar cell, coloring of the reflective layer with a pigment is suppressed to prevent a decrease in reflectance and improve film strength. And a paste containing the binder, a white insulating film that can be used as the reflective layer obtained by firing the paste, and a dye-sensitized dye comprising the reflective layer A solar cell of the type can be provided.

  Details of the present invention, as well as other features and advantages, are described in detail below. However, the present invention is not limited to the embodiments described below.

(Binder and paste)
The silica polymer constituting the binder of the present invention needs to have a surface functional group substituted with an alkyl group. The alkyl group is a general term for a group in which 2n + 1 hydrogen atoms are bonded to n carbon atoms, and preferably a lower alkyl group such as a methyl group, an ethyl group, or a propyl group is used. These lower alkyl groups utilize the alkyl moiety of a compound such as trialkyl silicon alkoxide or trialkyl silicon chloride, and when the surface functional group of the silica polymer and the trialkyl silicon compound are reacted by hydrolysis, It exhibits high bond stability to the silica polymer and does not break even at high temperatures.

  Accordingly, when a binder is formed from such a silica polymer substituted with an alkyl group, when the binder is subjected to heating due to a manufacturing process or the like, the silica polymer has the alkyl on its surface. It is possible to maintain a state in which the groups are stably bonded. As a result, the binder can achieve an effect by including a silica polymer having a surface-substituted alkyl group. Specifically, as described in detail below, the strength of the film containing the binder can be improved, and coloring of the film can be prevented by suppressing the dye adsorption of the film.

  When the above-described lower alkyl group is used, it can be stably bonded to the silica polymer even at a temperature of 300 to 500 ° C., and the above-described effects can be obtained. The heat resistance is improved as the number of carbon atoms is reduced to about 300 ° C. for the propyl group and about 400 ° C. for the ethyl group. In particular, by using a methyl group, it binds to a silica polymer stably up to about 500 ° C. and exhibits high thermal stability.

  When an alkyl group having 4 or more carbon atoms is used, when the binder containing a silica polymer surface-substituted with such an alkyl group is placed under high temperature, the carbon chain constituting the alkyl group is There is a tendency to break up randomly. Therefore, the silica polymer cannot uniformly have an alkyl group on the surface thereof, and there are cases where the effect of alkyl group substitution, that is, the effect of the alkyl group-substituted silica polymer cannot be fully exhibited.

  Further, since a silica polymer has a large number of hydroxyl groups on the surface thereof, the terminal group of the reaction site becomes an alkyl group by reacting with trialkyl silicon alkoxide by hydrolysis.

  The reason why the binder inhibits the adsorption of the dye is thought to be that the surface functional group of the silica polymer constituting the binder is substituted with an alkyl group and is thus hydrophobized.

  The proportion of the alkyl group in the surface functional group of the silica polymer is preferably in the range of 40% to 90%. When the proportion is less than 40%, the effect of substitution of the alkyl group on the silica polymer is not sufficient, and particularly when the binder containing the silica polymer is dispersed in the film, coloring of the film is sufficiently suppressed. You may not be able to. Moreover, when the paste shown below is produced, it may gel and harden | cure and may not be used for subsequent film formation. On the other hand, when the ratio exceeds 90%, when the paste is prepared from the binder containing the silica polymer and a predetermined film is formed through the steps of coating and baking, the silica polymers are prevented from being bonded to each other. In some cases, the effect as a binder for the inorganic particles contained in the paste cannot be obtained.

  In addition, although the magnitude | size of the silica polymer which can be used by this invention is not specifically limited, It is preferable that the average molecular weights in conversion of polyester are the range of 1000-4000. More preferably, it is 1000-2000. Thereby, when a layer is formed using a paste prepared from a binder containing the silica polymer, the inclusion effect can be exhibited most remarkably.

  Next, the paste of the present invention will be described. The paste of this invention contains the binder and inorganic particle which were mentioned above.

As the weight of SiO ₂ after content firing of the silica polymer in the paste, it is preferable to inorganic particles is 0.1 wt% to 100 wt%. When the content ratio is less than 0.1% by weight, the silica polymer is no longer effective as a binder in the layer, or there is no polymer that covers the particle surface, and the effect of suppressing dye adsorption. May fade. When the content exceeds 100% by weight, when the film is formed, the silica polymer fills the interparticle pores, resulting in loss of porosity, and the conductivity of the electrolyte is significantly reduced. In some cases, the purpose of the solar cell cannot be achieved. For example, when configured as a reflective layer of a dye-sensitized solar cell, the reflective layer becomes a dense film and the electrolyte does not penetrate even though the color of the reflective layer is suppressed. It may not be possible to obtain a structure to do.

The inorganic particles contained in the paste of the present invention can be arbitrarily selected according to the characteristics of the film to be formed using the paste. For example, Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ and ZnO can be used. Preferably, TiO ₂ or ZrO ₂ that is thermally and chemically stable and has a high refractive index is used.

  Further, the size (diameter) of the inorganic particles is not particularly limited as long as the object of the present invention can be achieved, but may be, for example, 100 nm to 10 μm. In addition, it does not need to be a spherical particle, and may be a plate shape, a polygonal shape, or a rod shape. In the case of these particles, the longest side of each particle is the above size.

  In addition, as mentioned above, when the surface functional group (mainly hydroxyl group) of the silica polymer is not substituted with an alkyl group, when the silica polymer is contained in the paste, it is gelled and cured. As a result, the paste cannot have a sufficient viscosity to be uniformly applied, and a target layer cannot be formed. Therefore, the object of the present invention cannot be achieved.

  Next, an example of the manufacturing method of the paste of this invention is described. First, for example, using tetraalkyl silicon as a raw material, this raw material, a catalyst such as ammonia or nitric acid, water and an organic solvent are mixed, heated and stirred, and hydrolyzed to obtain a silica polymer. Thereafter, a trialkyl silicon alkoxide is added to the polymer solution and allowed to react, whereby the hydroxyl group on the surface of the silica polymer is substituted with the alkyl group to obtain an alkyl group-substituted silica polymer. In addition, the said reaction is performed at ice temperature, for example, in order to suppress an excessive substitution reaction.

  Subsequently, the unreacted silica polymer not substituted with an alkyl group, the catalyst and nitric acid are removed and purified by adding pure water, and only the silica polymer substituted with an alkyl group is taken out.

  Next, the inorganic particles and the alkyl group-substituted silica polymer obtained as described above are substituted with a high-boiling organic solvent such as hexylene glycol or α-terpione together with a cellulose resin or an acrylic resin, Obtain the desired paste.

  The alkyl group-substituted silica polymer can be stored frozen for a long period of time by adding ethanol or the like to an ethanol solution. This is presumably because the reactivity between the polymers decreases because the surface functional groups of the silica polymer are substituted with alkyl groups.

(White insulating film and dye-sensitized solar cell)
Next, the white insulating film of the present invention will be described. The white insulating film of the present invention is obtained by applying the paste obtained as described above onto a predetermined member and then baking it. The firing temperature is about 300-500 ° C. By forming the white insulating film in this way, the insulating film is interposed between the inorganic particles constituting the insulating film by the binder effect of the alkyl group surface-substituted silica polymer contained in the paste. The bonding strength can be improved, and as a result, the film strength of the insulating film can be improved.

  FIG. 1 is a diagram schematically showing the state of inorganic particles and silica polymer in a white insulating film. As shown in FIG. 1, the silica polymer 20 is interposed between the inorganic particles 10 to function as a binder, and the inorganic particles 10 are firmly bonded. Therefore, the film strength of the insulating film can be improved.

  In addition, since the silica polymer 20 does not completely fill the gaps between the inorganic particles 10, the obtained insulating film is porous.

  FIG. 2 is a configuration diagram showing an example of the dye-sensitized solar cell of the present invention. FIG. 2 shows a case where two sets of solar cells are formed on a substrate for clarity, but actually a plurality of solar cells are arranged vertically and horizontally. Yes.

  In the dye-sensitized solar cell 30 shown in FIG. 2, a transparent conductive layer 32 made of ITO or the like on a transparent substrate 31, a photoelectrode layer 33 as a semiconductor layer in which a dye is supported on titanium oxide particles, The insulating reflective layer 34, the catalyst layer 35, and the conductive layer 36 are sequentially formed to constitute each battery cell. These battery cells are sealed with a sealing layer 37.

  In this example, the catalyst layer 35 and the conductive layer 36 are described as separate layers. However, the catalyst layer 35 and the conductive layer 36 are usually configured so that the catalyst is supported on the conductive particles constituting the conductive layer 36, The layer 36 itself can also be configured as a mixed layer with the catalyst. The conductive layer 36 is porous, and an electrolyte is infiltrated therein.

  In the case of this example, light that contributes to power generation enters from the transparent substrate 31 side as indicated by an arrow in the figure. The insulating reflective layer 34 is formed to reflect incident light and enter the photoelectrode layer 33 again, thereby increasing the light utilization efficiency, that is, the conversion efficiency. Further, the catalyst layer 35 is provided to promote an oxidation-reduction reaction with the electrolyte contained in the conductive layer 36.

  In the dye-sensitized solar cell 30 shown in FIG. 2, the white insulating film formed as described above can be used as the insulating reflective layer 34. In this case, the reflective layer 34 has a high film strength for the above reasons, and has a binder containing a silica polymer having an alkyl group surface substitution, so that the adsorption of the dye is inhibited in the dye adsorption process in the production process, thereby preventing coloring. Thus, it can have a high reflectance.

  The configuration itself of the dye-sensitized solar cell 20 shown in FIG. 2 is already known, and is disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-142171.

Next, the present invention will be described specifically by way of examples.
(Preparation of paste)
First, 12.67% tetramethoxysilane and 50% ethanol were mixed and stirred in ice-cooling, and another ice-cooled mixture of 4.16% 1N nitric acid and 24.5% pure water was ice-cooled. Were mixed for 10 minutes and then stirred at 60 ° C. for 2.5 hours to produce a silica polymer (molecular weight: 1000 to 4000). Thereafter, 8.67% of trimethylmethoxysilane was added and heated and stirred for 10 minutes to replace the surface of the silica polymer with -Si-3Me. When substituted excessively, the function as a binder is impaired. Thereafter, the mixture is allowed to cool to room temperature and reacted under low temperature conditions for 12 hours to obtain a trimethylsilicon partially substituted silica polymer.

  Then, in order to remove unreacted trimethylmethoxysilane and both of them, which would cause deterioration of coating properties when remaining in the paste, purification was performed using an evaporator.

  Subsequently, in order to remove the water-soluble silica polymer not sufficiently substituted with trimethylsilicon and nitric acid added as a catalyst, pure water was added, followed by solid-liquid separation. When pure water is added, the fully substituted silica polymer is hydrophobic as a whole, so that it can be separated from the unsubstituted silica polymer. Since the catalyst nitric acid is in a state of being dissolved in pure water, it is possible to remove the aforementioned unsubstituted silica polymer and nitric acid, and it is possible to purify only the substituted silica polymer by repeating this process. It becomes.

  In addition, as a result of examining the spin state of Si using the nuclear magnetic resonance measuring apparatus (NMR) for the obtained substituted silica polymer, 41.3% of the reactive hydroxyl groups were substituted with trimethyl silicon groups. I understood. The remaining 58.7% contributes to condensation during heating and baking, and improves the strength of the structure. That is, the ratio of the methyl group in the surface functional group is 67.6%, and 32.4% is a reactive hydroxyl group used for bonding polymers.

Then, 1 wt% of trimethyl silicon partially substituted silica polymer obtained as described above in terms of SiO _2, of TiO ₂ having a particle size of 20 nm 17.5% by weight, of TiO ₂ having a particle size of 400 nm 17.5 wt% A paste was prepared by blending 5% by weight of ethyl cellulose and 59% by weight of α-terpioneer.

(Membrane strength evaluation)
Subsequently, the paste obtained as described above was formed into a film on a glass substrate by a screen printing method, and baked at 500 ° C. for 1 hour to obtain a thin film. When a peel test using a cellophane tape was attempted on this thin film, it was found that the film was hardly peeled off and the film strength was strong.
Further, even when the paste was left for 1 month, almost no increase in viscosity was observed, and it was found that the paste was stable over a long period of time.

(Colorability evaluation)
Next, using the fired film obtained by the screen printing method as in the above test, an organic dye having a carboxyl group (N719, Solaronix) was used, and the adsorption amount (colorability) of the dye was measured. An ethanol solution having a dye concentration of 0.3 mmol / L was prepared, and a film fired at 500 ° C. was immersed for 24 hours to confirm whether it was adsorbed on the surface. As a result, when the 3Me partially substituted silica polymer-added paste was used, the dye adsorption amount was remarkably small and was about 9.4 × 10 ⁻¹⁰ mol / cm ² .

On the other hand, when a paste is produced as described above using a silica polymer whose surface is not substituted with an alkyl group and a fired film is produced, the film contains a large number of adsorption sites due to hydroxyl groups. Due to the presence of the silica polymer, the amount of the dye adsorbed on the membrane was 5.4 × 10 ⁻⁸ mol / cm ² . From these results, when a paste was prepared using a binder containing a silica polymer having a surface functional group substituted with an alkyl group according to the present invention, and the film was formed by firing, the amount of dye adsorbed, that is, coloring The degree was found to be 50 times different.

  Next, the reflection characteristics of the fired film obtained as described above were examined, and the results are shown in FIG. In FIG. 3, (1) and (2) show the wavelength dependence of the reflectance before and after the dye adsorption test of the fired film containing the above-described surface alkyl group-substituted silica polymer. (3) and (4) These show the wavelength dependence of the reflectance before and behind the dye adsorption test of the fired film containing the above-mentioned surface alkyl group-unsubstituted silica polymer. The reflectance was measured with an ultraviolet-visible near-infrared spectrometer.

  As is apparent from FIG. 3, the reflectance of any of the fired films shows the same wavelength dependence before the dye adsorption test, but after the dye adsorption test, the surface alkyl group-unsubstituted silica polymer. It can be seen that the reflectance is deteriorated in the entire measurement wavelength region in the fired film containing ((4) graph). Therefore, it was confirmed that the fired film containing the silica polymer having no surface alkyl group substitution deteriorates the reflectance due to the adsorption of the dye.

  The present invention has been described in detail with specific examples. However, the present invention is not limited to the above contents, and various modifications and changes can be made without departing from the scope of the present invention.

It is a figure which shows roughly the mode of the electroconductive particle and silica polymer in the electroconductive layer of this invention. It is a block diagram which shows an example of the dye-sensitized solar cell of this invention. It is a graph which shows the wavelength dependence of the reflectance in the baked film obtained in the present Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Conductive particle 20 Alkyl group substituted silica polymer 30 Dye-sensitized solar cell 31 Transparent substrate 32 Transparent conductive layer 33 Photoelectrode layer 34 Insulating reflective layer 35 Catalyst layer 36 Conductive layer 37 Sealing layer

Claims (9)

  A binder for a dye-sensitized solar cell, comprising a silica polymer in which at least a part of a surface functional group is substituted with an alkyl group.   The binder for a dye-sensitized solar cell according to claim 1, wherein the alkyl group is a methyl group.   3. The dye-sensitized solar cell binder according to claim 1, wherein a ratio of the alkyl group in the surface functional group of the silica polymer is in a range of 40% to 90%.   A paste comprising the binder according to any one of claims 1 to 3 and inorganic particles. The content ratio of the silica polymer in the paste, characterized in that 0.1% to 100% by weight relative to the inorganic particles as SiO ₂ by weight, the paste of claim 4.   A white insulating film for a dye-sensitized solar cell, obtained by heating and baking the paste according to claim 4.   The white insulating film is porous, and the silica polymer in the paste functions to intervene between the inorganic particles constituting the white insulating film and improve the bonding between the inorganic particles. The white insulating film for a dye-sensitized solar cell according to claim 6, wherein:   A dye-sensitized solar cell comprising an insulating reflective layer formed by heating and baking the paste according to claim 4.   The reflective layer is porous, and the silica polymer in the paste functions between the inorganic particles constituting the reflective layer and functions to improve the bonding between the inorganic particles. The dye-sensitized solar cell according to claim 8.

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