Method for heating thin film with microwave

According to the present invention, microwave heating of a thin film used for heating a semiconductor thin film by irradiation with microwaves, such as when firing a semiconductor particle thin film for adsorbing a required dye on a semiconductor electrode of a dye-sensitized solar cell. It's about the method.

Conventionally, silicon-based solar cells using crystalline silicon, amorphous silicon, or the like as the photoelectric conversion material have been the mainstream, but in recent years, the photoelectric conversion material has been sensitized with a dye instead of the above-mentioned silicon-based material. Dye-sensitized solar cells that use the above-mentioned semiconductor thin film have been developed, and such dye-sensitized solar cells can reduce the manufacturing cost as compared with the above-mentioned silicon-based solar cells. It has been attracting attention.

The basic structure of the dye-sensitized solar cell is as shown in FIG. 2 (a), which outlines an example. That is, on one side of the glass substrate 2, for example, a transparent conductive film (semiconductor) 3 made of fluorine-doped tin oxide (FTO) or a compound (ITO) composed of indium oxide and tin oxide is provided by vapor deposition or the like. A transparent electrode 1 which is a conductive glass substrate is formed. Titanium oxide (TiO) is formed on the surface of the transparent conductive film 3 in the transparent electrode 1.₂) Is provided with a porous semiconductor particle thin film 4 formed by sintering fine particles, and a dye for sensitization is adsorbed on the semiconductor particle thin film 4, and the dye is adsorbed on the transparent electrode 1. A semiconductor electrode 5 comprising 4 is formed. Further, on the side of the semiconductor particle thin film 4 on which the dye is adsorbed on the semiconductor electrode 5, the transparent conductive film 3 on the transparent electrode 1 composed of the substrate 2 and the transparent conductive film 3 such as ITO, similarly to the transparent electrode 1. A counter electrode 6 having Pt (not shown) vapor-deposited on the surface is superposed, and the gap between the electrodes 5 and 6 is impregnated with an electrolyte solution 7 containing an oxidation-reduction pair such as an iodine-iodine compound or a bromine-bromine compound. The peripheral portions of the semiconductor electrode 5 and the counter electrode 6 are sealed with a sealing member (not shown) so that the electrolyte solution 7 does not leak to the outside.

As a result, as shown in FIG. 2 (b), when the dye-sensitized solar cell is exposed to light 8, the dye 9 adsorbed on the semiconductor particle thin film 4 of the semiconductor electrode 5 becomes the light 8. Is absorbed and electrons are emitted, and the emitted electrons are quickly received by the titanium oxide forming the semiconductor particle thin film 4 and transmitted from the semiconductor thin film 4 to the transparent conductive film 3 of the transparent electrode 1. .. The electrons that have reached the transparent conductive film 3 are transmitted to the counter electrode 6 after passing through the external load 10 connected between the semiconductor electrode 5 and the counter electrode 6, and are transmitted to the counter electrode 6 at the portion of the counter electrode 6. For example, triiodide ions (I) in an electrolyte solution 7 containing iodine-iodide as a redox product.₃^-) Is reduced to iodide ion (I)^-). This generated iodide ion (I^-) Is oxidized by the dye 9 that emits the electrons adsorbed on the semiconductor particle thin film 4 of the semiconductor electrode 5, and the iodide ion (I) is oxidized.^-) And triiodide ion (I)₃^-) Cycles between the semiconductor electrode 5 and the counter electrode 6, so that the function as a battery is exhibited.

By the way, in the case of manufacturing the semiconductor electrode 5 used for the dye-sensitized solar cell, conventionally, as a metal oxide, nano-order ultrafine titanium oxide powder, preferably anatase type titanium oxide powder, is used as polyethylene glycol. It is dispersed in a required dispersion medium such as, etc. to form a paste (slurry) or a colloidal solution, and the paste or colloidal solution is applied to the surface of the transparent transparent conductive film 3 in the transparent electrode 1, for example, by the squeegee method or screen printing. , Doctor blade method, spin coat, etc. to a thickness of about 10 μm, and then dry to form a thin film that is an aggregate of titanium oxide powder as semiconductor particles, and then the transparent electrode 1 By putting the whole in an electric furnace and heating it to a high temperature of about 400 ° C and sintering it, the particles of ultrafine titanium oxide powder in the thin film, which is an aggregate of the titanium oxide powder, are mechanically and electrically separated from each other. By bonding, a porous semiconductor particle thin film 4 having an effective surface area of about 1000 times is formed on the surface of the transparent conductive film 3 of the transparent electrode 1, whereby a large amount of dye is adsorbed on the semiconductor particle thin film 4. It was made possible to increase the current value generated per unit area of the dye-sensitized solar cell.

However, as described above, when the semiconductor particle thin film 4 is sintered using an electric furnace, it is not possible to heat only the semiconductor particle thin film 4, and the entire furnace body is heated. There is a problem that it takes time to lower the temperature, and there is also a problem that the electric power required for processing increases.

Therefore, as one of the methods for shortening the temperature raising time and the temperature lowering time required for sintering the semiconductor particle thin film 4, it is considered to heat the semiconductor particle thin film 4 by irradiating with microwaves. It has been done.

That is, microwaves having a frequency of 2.45 GHz are also used in general microwave ovens, and dielectrics having a high frequency of 28 GHz have been tried for uniform heating for sintering bulk ceramics. When the body is irradiated, the microwave is an electromagnetic wave having an electric field and a magnetic field, so that the electron dipoles of the dielectric material try to align with each other according to the electric field of the microwave. However, as the frequency increases, it becomes impossible to follow the molecules, and friction between molecules due to vibration and rotation occurs, so that the dielectric substance itself can generate heat. This energy loss is the so-called dielectric loss, which is the principle of microwave heating. Therefore, according to such microwave heating, a portion having a large dielectric loss can be selectively heated, and the dielectric material to be heated can be directly heated without waiting for the temperature rise of the furnace body or the like unlike an electric furnace. The temperature can be raised in a short time, and since the temperature is raised only in the dielectric and the peripheral portion that receives heat conduction from the dielectric, the temperature can be lowered in a short time.

Therefore, as shown in FIG. 3A, the titanium oxide powder paste is applied to the surface of the transparent conductive film 3 in the transparent electrode 1 composed of the glass substrate 2 and the transparent conductive film 3 in the same manner as described above. After drying to form a thin film (hereinafter referred to as titanium oxide particle aggregate thin film) 4a which is an aggregate of titanium oxide powder, titanium oxide is placed on an alumina pedestal 11 with the substrate 2 side of the transparent electrode 1 down. It is placed in a posture in which the particle aggregate thin film 4a side is placed on top, and the titanium oxide particle aggregate thin film 4a side is irradiated with, for example, a microwave 12 having a frequency of 28 GHz, or as shown in FIG. 3 (b). As shown above, after forming the titanium oxide particle aggregate thin film 4a on the surface of the transparent conductive film 3 of the transparent electrode 1, the titanium oxide particle aggregate thin film 4a side is placed on the alumina pedestal 11. The transparent electrode 1 is placed on the substrate 2 side in a posture of being placed on the substrate 2, and the same microwave 12 as above is irradiated from the substrate 2 side of the transparent electrode 1 so as to obtain the titanium oxide particle aggregate. It has been proposed that the thin film 4a be heated by microwaves to be sintered.

In this case, in the posture shown in FIG. 3A, the heat generated from the transparent conductive film 3 of the transparent electrode 1 and the titanium oxide particle aggregate thin film 4a is dissipated to the glass substrate 2 and the atmosphere. On the other hand, in the posture shown in FIG. 3 (b), the heat generated from the transparent conductive film 3 and the titanium oxide particle aggregate thin film 4a is transmitted to the glass substrate 2 and the alumina gantry 11 and then radiated to the atmosphere. Therefore, heat can be dissipated faster. For this reason, when the titanium oxide particle aggregate thin film 4a is heated and sintered in the posture shown in FIG. 3 (a) above, the glass substrate 2 often breaks during the temperature rise. It is presumed that this is because the titanium oxide particle aggregate thin film 4a is directly irradiated with the microwave 12, so that a sudden thermal stress acts to damage the glass substrate 2. When heating is performed in the posture shown in (1), it is considered that the titanium oxide particle aggregate thin film 4a and the glass substrate 2 are heated relatively uniformly, which is advantageous in preventing damage to the glass substrate 2. It is reported that it will be. Reference numeral 11a in the figure indicates a thermocouple.

Further, as the transparent electrode 1, instead of the glass substrate 2, a transparent electrode 1 formed by attaching a transparent conductive film 3 made of ITO to a resin-made, for example, PET film-shaped substrate 2 may be used. It has been proposed (see, for example, Non-Patent Document 1).

As a method for reducing the processing temperature when the particles of the fine particle titanium oxide powder are sintered to each other in order to produce a semiconductor particle thin film in the semiconductor electrode of the dye-sensitized solar cell similar to the above, high-frequency plasma and microwave are used. When the sintering operation is performed by plasma or a hybrid type plasma treatment thereof (see, for example, Patent Document 1), or when the thin film which is an aggregate of titanium oxide particles as semiconductor particles is heated. It has been conventionally proposed to use the irradiation of electromagnetic waves such as ultraviolet rays, visible rays, infrared rays, and microwaves together (see, for example, Patent Document 2).

<patcit num="1"><text>Japanese Unexamined Patent Publication No. 2003-308893</text></patcit><patcit num="2"><text>JP-A-2002-134435</text></patcit><nplcit num="1"><text>Tomiha, Uchida, Takizawa, "Sintering of Titanium Oxide Film with 28GHz Microwave and Application to Dye-Sensitized Solar Cell", Functional Materials, June 2003, Vol23, No.6, p.58-63</text></nplcit>

<p> However, in microwave heating, since the dielectric loss coefficient as the efficiency of converting input energy into heat is a function of temperature, thermal runaway is likely to occur, which hinders a wide range of applications. ..</p><p> That is, as shown in FIGS. 3 (a) and 3 (b), in the method of heating the titanium oxide particle aggregate thin film 4a by irradiating with microwave 12, the semiconductor particle thin film 4 formed by sintering (Fig. 2 (a)). ) (See (b)) is cracked, or when the substrate 2 of the transparent electrode 1 is made of resin, the resin substrate 2 is often plasma ignited, which makes practical application difficult. Is the current situation. Since it was thought that all of these damages were caused by the thermal runaway caused by the above-mentioned microwave heating, it has been considered that the only countermeasure is to prevent the thermal runaway by devising heat dissipation.</p><p> In Patent Document 1 and Patent Document 2, a semiconductor particle thin film of titanium oxide constituting a semiconductor electrode of a dye-sensitized solar cell is sintered by high-frequency plasma, microwave plasma, or a hybrid type plasma treatment thereof. Although it is described that the irradiation of electromagnetic waves such as ultraviolet rays, visible rays, infrared rays, and microwaves is used together when the semiconductor particle thin film is heated, the semiconductor particle thin film is cracked or the substrate is made of resin. No specific measures have been shown or even suggested to prevent the occurrence of damage such as plasma ignition.</p><p> Therefore, the present inventors have performed semiconductors by microwave irradiation without causing damage such as cracking of the semiconductor particle thin film 4 and plasma ignition of the resin substrate 2 which have conventionally occurred when microwave heating is performed. As a result of repeated ingenuity and research to enable the particle thin film 4 to be heated and sintered, the causes of damage such as cracking of the semiconductor particle thin film 4 and plasma ignition of the resin substrate 2 have been found. , It was found that the main cause was dielectric breakdown and discharge ignition, not thermal runaway.</p><p> That is, when the present inventors observed in detail the state of damage to the semiconductor particle thin film 4 that occurred in the process of heating and firing the semiconductor particle thin film 4 by microwave irradiation, the cracks that occurred in the semiconductor particle thin film 4 were reticulated. From this, it was inferred that the root cause of the damage caused to the semiconductor particle thin film 4 by the above-mentioned microwave irradiation was the creeping discharge phenomenon. That is, when the semiconductor particle thin film 4 is irradiated with microwaves to be sintered, a partial potential difference is generated inside the semiconductor particle thin film 4, and at this time, the semiconductor particle thin film 4 is extremely thin with a thickness of about 10 μm. Since it is a thin film, it is a semiconductor, so it has the required electrical conductivity inside, and despite the fact that it is provided on the transparent conductive film 3 of the transparent electrode 1, the above film It is considered that the potential difference generated inside causes a creeping discharge phenomenon that flows in the external space rather than energizing the inside of the film, and therefore, the semiconductor particle thin film 4 is cracked due to dielectric breakdown. When the substrate 2 is made of resin, the resin of the substrate 2 is gasified by plasma due to the creeping discharge phenomenon, and the gas burns to cause plasma ignition of the resin substrate 2. It was thought that it would end up.</p><p> In view of these ideas, the present inventors have prevented the creeping discharge phenomenon from occurring when the semiconductor thin film is heated by microwaves, thereby causing cracks in the semiconductor thin film and the resin. The present invention has been made by finding that it is possible to prevent damage such as plasma ignition of a manufactured substrate.</p><p> Therefore, an object of the present invention is to provide a method for heating a thin film of a semiconductor by microwave irradiation without damaging the thin film of the semiconductor. ..</p>

<p> In order to solve the above problems, the present invention is a method for heating the thin film by irradiating it with microwaves in a state where the conductor is in contact with the surface of the thin film, in accordance with the invention according to claim 1. ..</p><p> Further, in accordance with the invention of claim 2, the method of heating the thin film by irradiating microwaves from the anti-conductor side with the conductor in contact with the surface of the thin film.</p><p> Further, the conductor according to the invention according to claim 1 or 2 is provided with a smooth surface.</p><p> Furthermore, the conductor in the invention according to claim 1 or 2 is provided with a porous surface.</p><p> The thin film in the above configuration is a thin film as an aggregate of semiconductor fine particles adhered to the surface of the transparent conductive film in a transparent electrode having a transparent conductive film provided on the surface of a glass or resin substrate.</p><p> The semiconductor fine particles in the above configuration are fine particles of titanium oxide.</p>

<p> According to the microwave heating method for a thin film of the present invention, the following excellent effects are exhibited. (1) Since microwaves are irradiated in a state where the conductor is in contact with the surface of the thin film, even if a partial potential difference occurs inside the thin film irradiated with the microwaves, the potential Since the high portion and the low portion can be energized through the conductor in contact with the surface of the thin film, the partial potential difference generated in the thin film can be eliminated. Therefore, since it is possible to prevent the possibility of a creeping discharge phenomenon flowing in the external space of the thin film occurring, there is a case where the thin film is cracked due to the occurrence of the creeping discharge phenomenon or the thin film is supported by a resin substrate. The thin film can be heated while preventing damage such as plasma ignition of the substrate. (2) In the above, by irradiating the microwave from the anti-conductor side, it is possible to avoid the loss of the microwave due to the transmission through the conductor. (3) By smoothing the surface of the conductor, the contact area between the thin film and the conductor can be widened and heat can be efficiently dissipated from the thin film to the conductor. This is advantageous when suppressing the risk of runaway. (4) By making the surface of the conductor porous, heat escape from the thin film to the conductor can be suppressed, so that the thin film can be efficiently heated even with a weak microwave. .. (5) By making the thin film a thin film that is an aggregate of semiconductor fine films adhered to the surface of the transparent conductive film of the transparent electrode, and by sintering each semiconductor fine film in the thin film, the dye-sensitized sun A porous semiconductor particle thin film for adsorbing and holding a sensitizing dye can be formed on the semiconductor electrode of the battery. (6) By using fine particles of titanium oxide as the semiconductor fine particles, a porous semiconductor particle thin film made of titanium oxide can be provided by sintering on the surface of the transparent conductive film of the transparent electrode of the semiconductor electrode of the dye-sensitized solar cell. it can.</p>

Hereinafter, the best mode for carrying out the present invention will be described with reference to the drawings.

FIG. 1 shows semiconductor particles, which are components of the semiconductor electrode 5 in a dye-sensitized solar cell similar to those shown in FIGS. 2 (a) and 2 (b), as an embodiment of the thin film microwave heating method of the present invention. An example of application to the case where the thin film 4 is sintered on the surface of the transparent conductive film (semiconductor) 3 provided on the transparent electrode 1 is shown. That is, as shown in FIGS. 3A and 3B, fine particle titanium oxide powder is formed on the surface of the transparent conductive film 3 in the transparent electrode 1 composed of the substrate 2 and the transparent conductive film 3 provided on the surface thereof. After applying the paste of the above and drying to form the titanium oxide particle aggregate thin film 4a, the titanium oxide particle aggregate thin film 4a is placed on the conductor 13 in the form of a tray or a sheet. The surface of the titanium oxide particle aggregate thin film 4a is brought into contact with the conductor 13. Further, if necessary, as shown by the alternate long and short dash line in FIG. 1, the holding plate 14 is placed on the substrate 2 and pressed from above to form the conductor 13 and the titanium oxide particle aggregate thin film 4a. Make sure to hold it in contact. After that, the semiconductor particle thin film 4 is formed by irradiating the microwave 12 from above the holding plate 14 on the anti-conductor 13 side to heat the titanium oxide particle aggregate thin film 4a and sintering it. To do so. Note that FIG. 1 shows the state of the semiconductor particle thin film 4 after sintering, and the thin film of the titanium oxide particle aggregate before sintering is indicated by reference numeral 4a in parentheses.

As the conductor 13, a metal body such as nickel, aluminum, iron, or stainless steel, which has an electric conductivity larger than that of the semiconductor particle thin film 4 made of titanium oxide, may be used.

Further, in order to efficiently suppress thermal runaway during microwave heating, the surface of the conductor 13 is brought into contact with the surface of the titanium oxide particle aggregate thin film 4a over the entire surface. It suffices to provide a smooth surface so that heat can be efficiently dissipated from the sintered titanium oxide semiconductor particle thin film 4 to the conductor 13. In this case, the conductor 13 is provided with a heat dissipation function by, for example, having a shape on the anti-thin film contact surface side that can positively dissipate heat to an external space or the like. The effect of preventing runaway may be positively obtained. On the other hand, if it is desired to suppress the escape of heat from the titanium oxide particle aggregate thin film 4a to be heated because the output of the irradiating microwave 12 is weak, the conductor 13 has a porous surface. As a result, the conductor 13 may not be in full contact with the film surface of the titanium oxide particle aggregate thin film 4a, but may be in point contact evenly over the entire surface within a range in which energization between the two can be ensured.

The presser plate 14 is made of a material having a dielectric loss coefficient sufficiently smaller than that of the titanium oxide particle aggregate thin film 4a desired to be heated, transmitting the microwave 12 to be irradiated, and having the required heat resistance. If there is, any material may be used.

The substrate 2 may be made of glass, PET, or other resin as long as the dielectric loss coefficient is sufficiently small and the absorption of microwaves is smaller than that of the titanium oxide particle aggregate thin film 4a to be heated.

When the titanium oxide particle aggregate thin film 4a provided on the surface of the transparent conductive film 3 of the transparent electrode 1 is irradiated with the microwave 12 according to the microwave heating method of the thin film of the present invention, the titanium oxide which is a dielectric substance is irradiated. The particle aggregate thin film 4a is sintered by heating to obtain a titanium oxide semiconductor particle thin film 4.

At this time, due to the extremely thin thickness of the semiconductor particle thin film 4 formed, even if a potential difference occurs inside the semiconductor particle thin film 4 during microwave irradiation, a portion having a high potential and a portion having a low potential have a low potential. Since the portions are connected via the conductor 13 and become conductive, the potential difference generated inside the semiconductor particle thin film 4 can be eliminated, and therefore, a creepage discharge phenomenon may occur in the semiconductor particle thin film 4. Can be prevented.

Therefore, the simple method of bringing the conductor 13 into contact with the thin film surface may cause cracks in the formed semiconductor particle thin film 4, or damage such as plasma ignition even when the substrate 2 is made of resin. It is possible to perform microwave heating of the thin film in a state where the above is prevented.

Further, the microwave 12 is not a special one having a frequency of 28 GHz, but a microwave 12 having a general frequency of 2.45 GHz, which is used in a general microwave oven, can stably heat a thin film. ..

Furthermore, since the titanium oxide particle aggregate thin film 4a can be directly heated by the microwave 12, the time required for raising and lowering the temperature can be shortened as compared with the case where an electric furnace is used for the sintering work, and the treatment can be performed. It is possible to reduce the power required for this.

Furthermore, since the irradiation of the microwave 12 is performed from the anticonductor 13 side with respect to the titanium oxide particle aggregate thin film 4a, the loss of the irradiated microwave 12 can be avoided.

After the sintering treatment of the semiconductor particle thin film 4 by microwave heating is completed, after removing the holding plate 14, the transparent electrode in which the semiconductor particle thin film 4 is formed on the surface of the transparent conductive film 3 from above the conductor 13. 1 may be collected and sent to the subsequent dye adsorption step.

The present invention is not limited to the above embodiment, and the conductor 13 has a thin film surface so that the potential difference generated inside the semiconductor particle thin film 4 to be heated and sintered during microwave irradiation can be eliminated. Any shape other than a tray or a sheet may be used as long as it is provided with a surface that can be evenly contacted over almost the entire surface, and further, a thin film that does not block the microwave when irradiated with the microwave. As long as the contact state between the surface and the conductor 13 can be maintained, any means may be adopted for holding the conductor 13 and the thin film in the contact state. , Although it is shown to be performed from the anti-conductor 13 side, if the conductor 13 is as thin as an aluminum foil and has the required microwave transmittance, the microwave 12 can be transmitted to the conductor 13. It may be irradiated from the side, and the microwave to be irradiated is 2.45 GHz, 28 GHz, or any other frequency used in general microwave ovens, as long as the thin film to be heated has a dielectric loss. Microwaves may be used, and the method for heating a thin film of a thin film of the present invention has a dielectric loss coefficient that can be heated by irradiation with microwaves, and a potential difference is generated inside the film during irradiation with microwaves to cause creeping discharge. A thin film such as a semiconductor that may cause a risk is not limited to the laminated structure of the thin film as shown in FIG. 1, and further, a thin film other than the semiconductor particle thin film used for the semiconductor electrode of the dye-sensitized solar cell. Of course, it can be applied to heating, and various changes can be made without departing from the gist of the present invention.

<figref num="1">It is a schematic side view which shows one Embodiment of the microwave heating method of the thin film of this invention.</figref><figref num="2">An example of a dye-sensitized solar cell is shown, in which (a) is a schematic cut side view and (b) is a diagram showing an outline of a power generation mechanism.</figref><figref num="3">A method for heating a thin film by microwave irradiation, which has been conventionally proposed for sintering a semiconductor particle thin film of titanium oxide on the surface of a transparent conductive film of a transparent electrode, is shown. (B) is a schematic side view showing a case of irradiating from the thin film side of the titanium particle aggregate and a case of irradiating microwaves from the substrate side.</figref>

Code description

1 Transparent electrode 2 Substrate 3 Transparent conductive film (semiconductor) 4 Semiconductor particle thin film (thin film) 4a Titanium oxide particle aggregate thin film (thin film) 12 Microwave 13 Conductor