Patent Application
20120132275
The present invention relates to a dye-sensitized solar cell in which stainless steel is used as the constitutive material of the photoelectrode (negative electrode), and to its production method.
The mainstream of solar cells is toward those in which silicon is used as the photoelectric conversion element; however, as more economical next-generation solar cells in lieu of those, practical use of “dye-sensitized solar cells” is now under study. The dye-sensitized solar cell must be so designed that external light could reach the sensitizing dye carried inside the “photoelectrode” (inside the cell) therein, and therefore, the electroconductive member of the electrode to be on the incident light side must be formed of a light-transmissive electroconductive material. On the other hand, the material to constitute the electrode on the side opposite to the incident light side does not necessarily have to be light-transmissive, for which, therefore, use of a metal material of good electroconductivity is advantageous. Recently, as the metal material of the type, applicability of stainless steel that is a relatively inexpensive corrosion-resistant material has been confirmed, and accordingly, cost reduction of dye-sensitized solar cells is expected. Patent Reference 1 discloses a dye-sensitized solar cell in which a stainless steel plate is used as the electrode on the side opposite to the incident light side therein.
In the type of
A semiconductor layer 6 is formed on the surface of the stainless steel plate 4. The semiconductor layer 6 is, for example, a porous layer formed by sintering oxide semiconductor particles of TiO2 particles or the like having a large specific surface area, and the surface of the oxide semiconductor 7 carries a sensitizing dye 8 of ruthenium complex dye or the like. In this case, the stainless steel plate 4 and the semiconductor layer 6 existing on the surface thereof constitute the photoelectrode 40. The semiconductor layer 6 in the drawing conceptually shows the configuration of the oxide semiconductor 7 and the sensitizing dye 8 for convenience sake for illustration, and does not reflect directly the actual configuration of the semiconductor layer 6 (the same shall apply also to
With reference to an example where the oxide semiconductor 7 is TiO2, the sensitizing dye 8 is a ruthenium complex dye, and the electrolytic solution 9 is a solution containing an iodide ion, the principle of cell operation is described briefly. When the incident light 20 has reached the sensitizing dye (ruthenium complex dye) 8, the sensitizing dye 8 absorbs the light and is excited, and the electron thereof is injected into the oxide semiconductor (TiO2) 7. The sensitizing dye (ruthenium complex dye) 8 in the excited state receives the electron from the iodide ion I− in the electrolytic solution 9, and is restored to the ground state. I− isoxidized to be I3−, and diffuses toward the catalyst thin-film layer 5 of the counter electrode 30, then receives the electron from the side of the counter electrode 30, and is restored to I−. Accordingly, the electron moves in the route of sensitizing dye (ruthenium complex dye) 8→oxide semiconductor (TiO2) 7→stainless steel plate 4→load 11→light-transmissive electroconductive material 3→catalyst thin-film layer 5→electrolytic solution 9→sensitizing dye (ruthenium complex dye) 8. As a result, a current to put the load 11 in action is generated.
In the type of
In the dye-sensitized solar cell of the type of
The photoelectrode of a dye-sensitized solar cell has a semiconductor layer as so mentioned above. In general, the semiconductor layer is formed on an electroconductive substrate, and a current is taken out through the substrate. In the type of
Patent Reference 1 teaches that, as the stainless steel to be applied to the electrode material of a dye-sensitized solar cell, a type of steel having a Cr content of at least 17% by mass and an Mo content of at least 0.8% by mass should be selected. Afterwards, however, the present inventors have repeatedly investigated the cell taking the practicability thereof into consideration, and have found that a stainless steel having a Cr content of at least 16% by mass and an Mo content of at least 0.3% by mass is applicable to the electrode material of a dye-sensitized solar cell.
On the basis of these findings, the inventors have completed the present invention.
Specifically, in the invention, there is provided a dye-sensitized solar cell comprising a photoelectrode formed by using a stainless steel plate and a counter electrode formed by using a light-transmissive electroconductive material, wherein:
the photoelectrode comprises, as the substrate thereof, a stainless steel plate of a type of stainless steel having a chemical composition of any of the following (A) to (D) and having a roughened surface in which pit-like indentations are formed and which is controlled to have an arithmetic average roughness Ra of at least 0.2 μm, and comprises, as formed on the roughened surface of the substrate, a sensitizing dye-carrying semiconductor layer;
the counter electrode has a catalyst thin-film layer formed on the surface of the light-transmissive electroconductive material and has visible light transmissiveness; and
the semiconductor layer of the photoelectrode and the catalyst thin-film layer of the counter electrode face each other via an electrolytic solution.
(A) One corresponding to a ferritic stainless steel containing Cr: at least 16% by mass and Mo: at least 0.3% by mass and defined by JIS G4305:2005.
(B) One corresponding to an austenitic stainless steel containing Cr: at least 16% by mass and Mo: at least 0.3% by mass and defined by JIS G4305:2005.
(C) A ferritic stainless steel comprising, by mass, C: at most 0.15%, Si: at most 1.2%, Mn: at most 1.2%, P: at most 0.04%, S: at most 0.03%, Ni: at most 0.6%, Cr: from 16 to 32%, Mo: from 0.3 to 3%, Cu: from 0 to 1%, Nb: from 0 to 1%, Ti: from 0 to 1%, Al: from 0 to 0.2%, N: at most 0.025%, B: from 0 to 0.01%, with a balance of Fe and inevitable impurities.
(D) An austenitic stainless steel comprising, by mass, C: at most 0.15%, Si: at most 4%, Mn: at most 2.5%, P: at most 0.045%, S: at most 0.03%, Ni: from 6 to 28%, Cr: from 16 to 32%, Mo: from 0.3 to 7%, Cu: from 0 to 3.5%, Nb: from 0 to 1%, Ti: from 0 to 1%, Al: from 0 to 0.1%, N: at most 0.3%, B: from 0 to 0.01%, with a balance of Fe and inevitable impurities.
In the above-mentioned dye-sensitized solar cell, the counter electrode is especially preferably one having visible light transmissiveness of such that the light transmittance at a wavelength of 500 nm is at least 55%. As the catalyst to constitute the catalyst thin-film layer of the counter electrode, there may be mentioned platinum, nickel or an electroconductive polymer. In case where the catalyst is platinum or nickel, preferably, the thickness of the catalyst thin-film layer is from 0.5 to 5 nm. In case where the catalyst is an electroconductive polymer, preferably, the thickness of the catalyst thin-film layer is from 1 to 10 nm.
The roughened surface of the substrate stainless steel plate for use in the photoelectrode is especially preferably such that the part thereof at which the neighboring indentations join together has an edged boundary.
In the invention, there is also provided a method for producing the above-mentioned dye-sensitized solar cell, including:
a step of etching a stainless steel plate in an aqueous solution with a ferric ion existing therein to form pit-like indentations in the plate, thereby giving a substrate that has a roughened surface with an arithmetic average roughness Ra of at least 0.2 μm,
a step of forming, on the roughened surface, a coating film that contains oxide semiconductor particles,
a step of firing the coating film to form a porous semiconductor layer,
a step of dipping the semiconductor layer in a solvent with a sensitizing dye dispersed therein to make the semiconductor layer carry the sensitizing dye, thereby giving a photoelectrode that comprises the sensitizing dye-carrying semiconductor layer on the roughened surface of the substrate stainless steel plate, and
a step of arranging the photoelectrode and the visible light-transmissive counter electrode with the catalyst thin-film layer formed on the surface of the light-transmissive electroconductive material, in such a manner that the semiconductor layer of the photoelectrode could face the catalyst thin-film layer of the counter electrode, and introducing an electrolytic solution into the space between the two electrodes.
The aqueous solution with a ferric ion existing therein is, for example, an aqueous, ferric chloride-containing solution.
According to the invention, it has become possible to enhance the photoelectric conversion efficiency more than before in a dye-sensitized solar cell that uses a stainless steel plate in the electrode on one side thereof. In the dye-sensitized solar cell of the invention, a type of stainless steel in which the Cr content and the No content are smaller than before can be used in the electrode, and therefore the applicability of the invention is expected to the field where use of a more inexpensive type of steel is desired.
In the invention, a stainless steel plate of which the surface is roughened by forming pit-like indentations therein is used as the electroconductive substrate (a member to carry a semiconductor layer thereon and to act for electric current passage) of the photoelectrode. The pit-like indentations are those formed by chemical etching in an aqueous electrolytic solution to thereby form “pitting corrosion” a type of local corrosion, on the surface of the stainless steel plate. The surface that has been roughened by forming a large number of pit-like indentations therein exhibits an anchor effect for the semiconductor layer existing thereon, and contributes toward enhancing the adhesiveness between the stainless steel plate and the semiconductor layer. Accordingly, the bonding force between the two is increased and the contact area is also increased, and as a result, the electric resistance in the bonding interface between the two is thereby reduced. As a result of various investigations, an increase in the photoelectric conversion efficiency is clearly recognized in the case of using a stainless steel plate of which the roughened surface with pit-like indentations formed therein has an arithmetic average roughness Ra of at least 0.2 μm. In case where Ra is smaller than the range, the above-mentioned effect would be insufficient and it would be difficult to significantly and stably increase the photoelectric conversion efficiency. The pit-like indentations can be formed by etching in an aqueous electrolytic solution containing a ferric ion, as described below; however, even though the etching is promoted too much, the pitting corrosion may grow in the plate thickness direction (depth direction) and the boundary between the neighboring indentations may disappear in the plate thickness direction while the thickness thereof is reduced, and therefore, Ra does not increase indefinitely. Accordingly, it is unnecessary to define the uppermost limit of Ra, but in fact, the range of Ra may be from about 0.2 to 5 μm or so for readily attaining the photoelectric conversion efficiency-enhancing effect.
The areal ratio in percentage of the pit-like indentations-formed part to the surface of the stainless steel plate is preferably at least 20% in terms of the projected areal ratio in percentage in the view of the roughened surface from directly above. The pit-like indentations may be formed in the entire surface of the steel plate, and the areal ratio of the pit-like indentations-formed part may be 100%.
As the stainless steel plate to be applied to the electroconductive substrate of the photoelectrode in the invention, a type of stainless steel that has excellent resistance to the electrolytic solution in the dye-sensitized solar cell must be employed. As a result of detailed investigations made by the present inventors, it has been found that when a type of stainless steel containing Cr in an amount of at least 16% by mass and Mo in an amount of at least 0.3% by mass is used, a practicable dye-sensitized solar cell can be constructed.
In general, stainless steel is said to be poor in corrosion resistance to an aqueous solution containing a chloride ion Cl−, and for enhancing the corrosion resistance, increase in Cr and addition of Mo are said to be effective. For example, ferritic SUS444 suitable to water heaters secures a Cr content of at least 17% by mass and an Mo content of at least 1.75% by mass; and even SUS316, a highly corrosion-resistant, general-purpose austenitic steel secures a Cr content of at least 16% by mass and an Mo content of at least 2% by mass. However, there are known few reports relating to the corrosion resistance of stainless steel to iodide ion, and in particular, any substantial investigation relating to the use for photoelectrodes in dye-sensitized solar cells is not as yet made sufficiently. Given the situation, the present inventors have made detailed investigations and have known that a dye-sensitized solar cell, which is so designed that the change of the photoelectric conversion efficiency η1(%) of the cell thereof, as measured after left at 65° C. for 100 hours, relative to the initial photoelectric conversion efficiency η0(%), as measured immediately after its construction, (conversion efficiency retention to be represented by the following formula (2)) could be at least 80%, can be evaluated to have practical-level durability in applications where the cell is incorporated in everyday-application products for personal use. Preferably, the conversion efficiency retention is at least 90%, more preferably at least 95%. After further investigations made by the inventors, it has been clarified that, when a type of stainless steel containing Cr in an amount of at least 16% by mass and Mo in an amount of at least 0.3% by mass is employed, then a dye-sensitized solar cell having a conversion efficiency retention of at least 80% can be constructed sufficiently.
Concretely, the above-mentioned types of stainless steel (A) to (D) are mentioned to be preferred ones for use herein. For further better corrosion resistance, the Cr content is preferably at least 17% by mass. The Mo content is preferably at least 0.5% by mass, and may be controlled to fall within a range of at least 0.8% by mass, or at least 1.0% by mass. The uppermost limit of Cr may be 32% by mass, and may be controlled to fall within a component range of at most 25% by mass. The uppermost limit of Mo may be 3% by mass, and may be controlled to fall within a component range of at most 2% by mass.
The specific roughened surface profile mentioned as above can be formed by etching a stainless steel plate of which the surface is not roughened, for example, an ordinary annealed/acid-pickled steel, a BA-annealed steel, a skin-pass finished steel or the like, in an aqueous solution with a ferric ion existing therein. For the etching, for example, employable is a method of dipping and soaking in a liquid, a method of alternating electrolysis in a liquid or the like. In any case, ferric chloride (FeCl3) is favorably used as the ferric ion source.
In the case of dipping and soaking, a method of etching in a mixed aqueous solution of ferric chloride (FeCl3) and hydrochloric acid (HCl) is extremely effective. Concretely, for example, within a condition range where the Fe3+ ion concentration is from 15 to 100 g/L, the HCl concentration is from 20 to 200 g/L, the temperature is from 35 to 70° C. and the dipping time is from 3 to 120 seconds, a condition to give a roughened surface having pit-like indentations and having Ra of at least 0.2 μm can be found out.
In the case of alternating electrolysis, for example, an aqueous solution of ferric chloride is used as the electrolytic solution, and within a condition range where the Fe3+ ion concentration is from 1 to 50 g/L, the temperature is from 30 to 70° C., the anode electrolytic current density is from 1.0 to 10.0 kA/m2, the cathode electrolytic current density is from 0.1 to 3.0 kA/m2, an alternating electrolysis cycle is from 1 to 20 Hz and an electrolysis time is from 10 to 300 seconds, a condition to give a roughened surface having pit-like indentations and having Ra of at least 0.2 μm can be found out. When the alternating electrolysis cycle is reduced, then the power-on time in one cycle may be long and the size of the pit-like indentations can be thereby enlarged; but on the contrary, when the alternating electrolysis cycle is prolonged, then the size of the pit-like indentations can be reduced.
The photoelectrode can be produced, for example, according to the method mentioned below. First, a coating material (paste or liquid) containing oxide semiconductor particles is applied on the roughened surface of the above-mentioned, surface-roughened stainless steel plate, and dried to form a coating film thereon. Subsequently, the coating film is fired to sinter the oxide particles, thereby forming a porous semiconductor layer. For the firing, the coated stainless steel plate is put in a heating furnace and is kept therein at a temperature at which the particles could be sintered suitably (for example, at 400 to 600° C.) As the oxide semiconductor, TiO2 is generally used, but ZnO, SnO2, ZrO2 or the like may also be used. These oxides may be combined and used here. Thus formed, the porous semiconductor layer is dipped in an organic solvent with a sensitizing dye dispersed therein, thereby making the semiconductor layer carry the sensitizing dye. The coated stainless steel plate may be dipped in the organic solvent. As the sensitizing dye, typically used here is ruthenium complex dye.
The counter electrode may be produced by making a light-transmissive electroconductive material held on the surface of a light-transmissive substrate such as a glass plate, a PEN (polyethylene naphthalate) film or the like followed by forming a catalyst thin-film layer on the surface of the light-transmissive electroconductive material. As the light-transmissive electroconductive material, herein usable is an electroconductive film of ITO (indium-tin oxide), FTO (fluorine-doped tin oxide), TO (tin oxide) or the like. As the catalyst thin-film layer, preferred for use herein is a metal film of platinum, nickel or the like, or an electroconductive polymer film of polyaniline, polyethylenedioxythiophene or the like. The metal film may be formed, for example, according to a sputtering method. The electroconductive polymer film may be formed, for example, according to a spin coating method. Especially preferably, the counter electrode has visible light transmissiveness of such that the light transmittance at a wavelength of 500 nm is at least 55%. In this case, a high photoelectric conversion efficiency can be obtained. The light transmittance varies depending on the thickness of the catalyst thin-film layer. A thinner catalyst thin-film layer may have a higher transmittance. However, when the catalyst thin-film layer is thinned too much, the photoelectric conversion efficiency may lower owing to the reduction in the catalytic effect. As a result of various investigations made by the inventors, it has been found that, in case where the catalyst is platinum or nickel, preferably, the thickness of the catalyst thin-film layer is controlled to fall within a range of from 0.5 to 5 nm. In case where the catalyst is polyaniline, preferably, the thickness of the catalyst thin-film layer is controlled to fall within a range of from 1 to 10 nm.
The above-mentioned photoelectrode and counter electrode are so arranged that the semiconductor layer of the photoelectrode could face the catalyst thin-film layer of the counter electrode via an electrolytic solution put therebetween, thereby constructing the dye-sensitized solar cell of the present invention.
A stainless steel ingot having the composition shown in Table 1 was produced, and worked into a cold-rolled annealed steel plate having a thickness of 0.2 mm (No. 2D finish) according to an ordinary stainless steel plate production process. In the column of morphology in Table 1, “α” means a ferritic type, and “γ” means an austenitic type. In the Table, “-” (hyphen) means lower than the detectable limit in ordinary analysis in steel production sites.
A sample cut out of the above-mentioned steel plate was surface-roughened by dipping and soaking or by alternating electrolysis, thereby preparing test samples. Some test samples that had not been surface-roughened to be as yet merely No. 2D finish were also prepared. In Table 2, those processed by dipping and soaking are represented by “dipping”; and those processed by alternating electrolysis are by “electrolysis”.
The surface roughening treatment by dipping and soaking was attained according to a method of dipping the test piece in an aqueous mixed solution of ferric chloride+hydrochloric acid having an Fe3+ ion concentration of 30 g/L and an HCl concentration of 30 g/L at a temperature of 50° C., for 40 seconds.
The surface roughening treatment by alternating electrolysis was attained in an aqueous ferric chloride solution having an Fe3+ ion concentration of from 5 to 50 g/L at a temperature of from 35 to 65° C., under the condition where the anode electrolysis current density was 3 kA/m2, the cathode electrolysis current density was 0.3 kA/m2, the alternating electrolysis cycle was 10 Hz and the electrolysis time was from 10 to 120 seconds.
As a result of SEM observation, it was confirmed that the roughened surfaces obtained herein all had pit-like indentations in an areal ratio in percentage of at least 20%, and that the part at which the neighboring indentations joined together had an edged boundary.
Using a microstructure analyzer (Kosaka Laboratory's Surfcorder ET4000A), the arithmetic average roughness Ra of the obtained roughened surface was measured.
As the material for forming a semiconductor layer, prepared was a TiO2 paste (Peccel Technologies' PECC-01-06). The TiO2 paste was applied onto the surface of the above-mentioned sample (in case where the sample had been surface-roughened, the paste was applied onto the roughened surface thereof) according to a doctor blade method, and dried to thereby form a TiO2-containing coating film thereon. Subsequently, the film-coated substrate, stainless steel plate was put into an oven at 450° C. and fired therein, thereby sinter the TiO2 particles to form a semiconductor layer. The mean thickness of the thus-formed semiconductor layer was 10 μm. As a sensitizing dye, ruthenium complex dye (Peccel Technologies' PECD-07) was used, and this was dispersed in a mixed solvent of acetonitrile and tert-butanol to prepare a dye dispersion. The above-mentioned, semiconductor layer-coated stainless steel plate was dipped in the dye dispersion, thereby producing a photoelectrode having the sensitizing dye carried by the semiconductor layer thereof.
As the light-transmissive electroconductive material for a counter electrode, prepared was “ITO-PEN film” having an ITO film formed on a PEN film substrate (Peccel Technologies' PECF-IP). This was set in a sputtering apparatus, and a target of platinum was sputtered thereonto for 1 minute to form a platinum catalyst thin-film layer on the ITO film, thereby producing a counter electrode. In this case, the thickness of the platinum was about 3 nm.
The above-mentioned photoelectrode and counter electrode were combined between a heat-sealable film (Peccel Technologies' Surlyn Film” as arranged therebetween to surround a part to be a cell, thereby producing a cell structure in which the distance between the stainless steel surface of the photoelectrode and the counter electrode was 50 μm. The cell structure was hot-compressed with a hot presser to seal up the cell, and further an epoxy resin was applied to the periphery of the cell and cured therearound. Via the electrolytic solution introducing mouth formed in the counter electrode, an electrolytic solution (Peccel Technologies' PECE-K01) was injected into the cell with a microsyringe. Subsequently, the electrolytic solution introducing mouth was sealed up with an epoxy resin, thereby constructing a dye-sensitized solar cell.
The conversion efficiency of the thus-constructed, dye-sensitized solar cell was measured according to the process mentioned below.
Using a solar simulator (Yamashita Denso's YSS-100), the dye-sensitized solar cell was tested for the I-V characteristic thereof by the use of a source meter, Keithley's 2400 Model, while irradiated with pseudo-sunlight at AM 1.5 and at 100 mW/cm2 from the side of the counter electrode thereof, from which the short-circuit current JSC, the open voltage VOC and the form factor FF were obtained. From these values, the photoelectric conversion efficiency η was computed according to the following formula (1).
Photoelectric Conversion Efficiency η (%)=short-circuit current JSC (mA/cm2)×open voltage VOC (V)×fill factor FF/incident light 100 (mW/cm2)×100 (1)
The initial photoelectric conversion efficiency, as measured immediately after the cell construction, is represented by η0(%). After measurement of η0(%), the solar cell was left in a thermostat at 65° C. for 100 hours. Subsequently, the photoelectric conversion efficiency of the cell was measured in the same manner as above. The photoelectric conversion efficiency, as measured after left at 65° C. for 100 hours, is represented by η1(%). The degree of the change of the photoelectric conversion efficiency after left under the above-mentioned condition was evaluated by the conversion efficiency retention (%) defined by the following formula (2):
Conversion Efficiency Retention (%)=η1/η0×100 (2)
The results are shown in Table 2.
As seen from Table 2, a surface-roughened stainless steel plate having a Cr content and a Mo content each falling within a suitable range, having pit-like indentations formed therein and having Ra of not less than 0.2 μm was used in the samples of the invention, and therefore, the photoelectric conversion efficiency η0 and η1 of the samples were both high and the conversion efficiency retention thereof was extremely high.
As opposed to these, a stainless steel plate having a low Mo content is used in Test Nos. 1 to 5 of comparative samples, and the photoelectric conversion efficiency after left at 65° C. for 100 hours of the samples was extremely low and the conversion efficiency retention thereof was also low. This may be considered because the corrosion resistance of the steel plate in the electrolytic solution was poor and therefore Fe and Cr of the stainless steel ingredients dissolved out in the electrolytic solution. In Test Nos. 6 to 8, an un-roughened stainless steel plate was used and therefore, the adhesiveness between the stainless steel plate of the photoelectrode and the semiconductor layer would be low, and the photoelectric conversion efficiency η0 and η1 of the samples were both low.
As the photoelectrode, Test No. 11 (Steel G) in Table 2 produced according to the same method as in Example 1 was prepared.
The counter electrode was produced as follows: Platinum, nickel or polyaniline as the catalyst, and the thickness of the catalyst thin-film layer was changed variously as described below. Like in Example 1, an ITO film was formed on the PEN film substrate to prepare “ITO-PEN film”. In case where platinum or nickel was employed, the catalyst thin-film layer was formed using a sputtering apparatus like in Example 1, and the film thickness was controlled by changing the sputtering time. In case where polyaniline was employed, polyaniline was dissolved in a solvent toluene, the resulting solution was dropwise applied onto the ITO film to thereby form a catalyst thin-film layer thereon according to a spin coating method, and by changing the rotation speed in spin coating, the film thickness was controlled. Thus formed, the counter electrode was analyzed for the light transmittance at a wavelength of 500 nm, using a spectrophotometer (Hitachi High Technologies' U-4100).
Using the photoelectrode and the counter electrode, dye-sensitized solar cell was constructed according to the same method as in Example 1, and analyzed for the photoelectric conversion efficiency η0. The results are shown in Table 3.
As known from Table 3, it is important to increase the light transmittance of the counter electrode to be higher than a predetermined level, for realizing good photoelectric conversion efficiency.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/064597 | 8/20/2009 | WO | 00 | 2/2/2012 |
