ELECTROCHROMIC DEVICE, IMAGE PICKUP OPTICAL SYSTEM, IMAGE PICKUP APPARATUS , AND WINDOW MEMBER

Technical Field

[0001] The present invention relates to an electrochromic device configured to control the intensity and color of light, and an image pickup optical system, an image pickup apparatus, and a window member, each including the

electrochromic device.

Background Art

[0002] In recent years, there has been increasing demand for continuously variable neutral density (ND) filters configured to adjust optical density in movie recording apparatuses including solid-state image pickup devices. As optical devices for this application, many devices including liquid crystals and inorganic electrochromic thin films have been reported. Such devices, however, have not been widely used because they are inferior to existing ND filters in terms of light quantity adjustable range, reliability, and so forth. In contrast, devices including organic

electrochromic molecules have wide adjustable ranges of light quantity. Furthermore, it is relatively easy to design the spectral transmittance of the devices. Thus, such devices are particularly promising for application in variable ND filters mounted on image pickup apparatuses.

[0003] PTL 1 discloses an electrochromic device including organic electrochromic molecules, the electrochromic device containing an electrochemically active anodic material and an electrochemically active cathodic material between a pair of electrodes, and at least one of the materials being an electrochromic material. In this case, an oxidation

reaction of the anodic material and a reduction reaction of the cathodic material occur simultaneously on the pair of electrodes, so that a closed circuit is formed in the device to allow a current to flow. However, for example, in a structure including the anodic material and the cathodic material dissolved in a medium, a reaction substance of the anodic material and a reaction substance of the cathodic material diffuse in the medium, so that oxidation and

reduction between the materials causes a bleaching reaction to proceed. This causes a problem in which excess electric power is required to maintain a colored state.

[0004] PTL 2 discloses a device having a structure in which an anodic material and a cathodic material are each supported on a pair of porous electrodes. In this structure, although a problem due to the diffusion of reaction

substances is circumvented, large amounts of the materials are required to be supported in order to achieve a wide adjustable range of light quantity. Thus, the porous electrodes are required to have very large surface areas. This disadvantageously causes a reduction in transmittance and an increase in haze in a neutral state.

[0005] PTL 3 discloses a technique in which in an

inorganic electrochromic device, the use of a counter electrode having a porous structure makes compensation for charges participating in a redox reaction to improve

coloring contrast and response speed.

[0006] Organic electrochromic molecules have relatively narrow absorption wavelength bands. To achieve flat

spectral transmittance properties in a colored state

throughout the visible light region, it is thus necessary to use a plurality of electrochromic materials having different absorption wavelength bands. In particular, in the case of an electrochromic device including a plurality of

electrochromic materials dissolved in a medium, a reaction occurs sequentially from a material having a lower redox potential. These materials are diffused in the medium and bleached by reactions between the materials and between the materials and electrodes. To maintain a uniform hue

throughout a coloring-bleaching process, it is necessary to reduce a redox potential difference between the plurality of electrochromic materials. Furthermore, electrochromic devices having the same structure disadvantageously exhibit particularly slow bleaching responses because of the influence of diffusion, compared with coloring response. Citation List

Patent Literature

[0007] PTL 1 Japanese Patent No. 3798980

PTL 2 Japanese Patent No. 4899719

PTL 3 Japanese Patent Laid-Open No. 61-145536

Summary of Invention

[0008] An embodiment of the present invention provides an electrochromic device having excellent hue stability and response speed. Furthermore, embodiments of the present invention provide an image pickup optical system, an image pickup apparatus, and a window member, each including the electrochromic device. According to aspects of the present invention, an electrochromic device includes a pair of electrodes and a liquid electrochromic medium disposed between the pair of electrodes, the liquid electrochromic medium containing an electrochromic material, in which the electrochromic material contains at least one anodically electrochromic material or at least one cathodically

electrochromic material, and the pair of electrodes includes a first electrode configured to oxidize and reduce the electrochromic material, and a second electrode, in which the second electrode has a larger specific surface area than the specific surface area of the first electrode. [0009] According to aspects of the present invention, an optical filter includes the electrochromic device described above and a circuit configured to drive the electrochromic device .

[0010] According to aspects of the present invention, an image pickup apparatus includes the optical filter described above and an image pickup device configured to receive light passing through the optical filter.

[0011] According to aspects of the present invention, an image pickup apparatus includes a circuit configured to drive the electrochromic device described above and an image pickup device configured to receive light from the outside.

[0012] According to aspects of the present invention, a window member includes the electrochromic device described above and a circuit configured to the electrochromic device.

[0013] Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Brief Description of Drawings

[0014] Fig. 1 is a schematic diagram illustrating an electrochromic device according to an embodiment of the present invention.

[0015] Fig. 2 is a graph illustrating current-voltage characteristics of electrochromic devices having different electrode structures and each having a first electrode and a second electrode.

[0016] Fig. 3 is a graph illustrating the relationship between the threshold voltage for oxidation of anodically electrochromic material A and the specific surface area of a second electrode.

[0017] Fig. 4 is a schematic diagram of an electrochromic device according to another embodiment of the present invention .

[0018] Fig. 5 is a graph illustrating the cyclic

voltammogram characteristics of electrochromic devices according to Example 1 and ^' Reference Example 1.

[0019] Fig. 6 is a graph illustrating the relationship between the bleaching response time and the bleaching voltage of an electrochromic device according to Example 1.

[0020] Fig. 7 is a graph illustrating the cyclic

voltammogram characteristics of electrochromic devices according to Example 2 and Reference Example 2.

[0021] Fig. 8 is a graph illustrating the relationship between the threshold voltage for oxidation of anodically electrochromic material B and the specific surface area of a first electrode.

[0022] Fig. 9 is a graph illustrating the relationship between the bleaching response time and the bleaching voltage of an electrochromic device according to Example 2.

[0023] Fig. 10 is a graph illustrating the relationship between the surface area of a transparent counter electrode (the surface area of a nanoparticle film) and the amount of reaction current.

[0024] Fig. 11 is a graph illustrating the relationship between the surface area of a nanoparticle film (the ideal maximum surface area upon assuming that spheres are close- packed) and the particle size of nanoparticles at different thicknesses.

[0025] Fig. 12 is a graph illustrating the relationship between the thickness of a nanoparticle film composed of antimony-doped tin oxide (ATO) and the average transmittance of visible light.

[0026] Fig. 13 illustrates device characteristics when a salt of an alkali metal cation is used as a supporting electrolyte .

[ 0027 ] Fig. 14 is illustrates device characteristics when a salt of an organic cation is used as a supporting

electrolyte.

[ 0028] Fig. 15 is a schematic drawing illustrating a lens unit including an optical filter according to an embodiment of the present invention and an image pickup apparatus including the lens unit.

[0029 ] Fig. 16 is a schematic drawing illustrating an image pickup apparatus including an optical filter according to an embodiment of the present invention. Description of Embodiments

[0030] Structures of electrochromic devices according to embodiments of the present invention will be illustrated in detail below with reference to the attached drawings.

However, the scope of the present invention is not limited to the structures, relative configurations, and so forth illustrated in these embodiments unless otherwise specified.

[0031] An electrochromic device according to an embodiment of the present invention includes a pair of electrodes and a liquid electrochromic medium disposed between the pair of electrodes, the liquid electrochromic medium containing an electrochromic material, in which the electrochromic

material contains at least one anodically electrochromic material or at least one cathodically electrochromic

material, and the pair of electrodes includes a first electrode configured to oxidize and reduce the

electrochromic material, and a second electrode, in which the second electrode has a larger specific surface area than the specific surface area of the first electrode.

[0032] In this embodiment of the present invention, only an electrochemically active anodically electrochromic material or only an electrochemically active cathodically electrochromic material is used, and a combination of its _. electrochemical reaction and the polarization of supporting electrolyte ions results in the device. In this case, only one of the anodically electrochromic material and the cathodically electrochromic material is used, thus

circumventing an increase in power consumption due to

reaction between materials. To achieve charge compensation, only an electrode that contributes to the polarization of the supporting electrolyte ions has a porous structure and thus advantages in transmittance and haze.

[0033] The electrochromic device according to an

embodiment of the present invention is an optical device in which a redox potential difference between a plurality of electrochromic materials is low and in which a change in hue is small through a coloring-bleaching process. Furthermore, the electrochromic device has an asymmetric electrode

structure in which a current does not flow upon applying a reverse voltage thereto. Thus, the application of a high reverse voltage to the device results in significantly improvement in bleaching response time in a bleaching

process, thereby significantly improving reliability as an optical device for a variable ND filter application.

[0034] Fig. 1 is a schematic drawing illustrating an electrochromic device according to an embodiment of the present invention. In Fig. 1, reference numerals la and lb denote glass substrates. Examples of the glass substrates that may be used include substrates composed of silica glass, white glass, borosilicate glass, non-alkali glass, and chemically strengthened glass. In particular, a non-alkali glass substrate may be used in view of durability. A first electrode 2 having a structure with a flat surface or substantially flat surface (hereinafter, referred to as a "substantially flat surface structure") is disposed on the glass substrate la. A second electrode 3 with a porous structure is disposed on the glass substrate lb. Reference numeral 4 denotes a liquid electrochromic medium 4

containing an electrochromic material and an electrolyte.

[0035] In the electrochromic device according to an embodiment of the present invention, the second electrode with a porous structure has a larger specific surface area than that of the first electrode having a substantially flat structure. Here, the substantially flat surface structure of the first electrode refers to a structure in which the first electrode has a specific surface area of 1 cm ² /cm ² or more and 30 cmVcm ² or less. A specific surface area of more than 30 cm ² /cm ² increases the redox potential of the

electrochromic material and changes the shape of the

current-voltage characteristics into a symmetric shape with respect to zero potential.

[0036] The porous structure of the second electrode refers to a structure in which the second electrode preferably has a specific surface area of 300 cm ² /cm ² or more and more preferably 600 cm ² /cm ² or more. At a specific surface area of less than 300 cm ² /cm ² , a reduction in the redox potential of the electrochromic material is not sufficient.

[0037] According to an embodiment of the present invention, the specific surface area refers to a specific surface area (S _B /S _A : cm ² /cm ² ) obtained by dividing the effective area (S _B : cm ² ) by the geometric area (S _ft : cm ² ) of the electrode. The geometric area (S _ft ) is defined the same as a projected area and refers to an apparent area (cm ² ) when the substrate is projected. The effective area (S _B ) refers to the surface area (cm ² ) inside the porous structure calculated from

measurement by a nitrogen gas adsorption method (Brunauer- Emmett-Teller (BET) method) and measurement of the weight of a film.

[0038] The reason the second electrode that contributes to the polarization of the supporting electrolyte ions has a larger specific surface area than that of the first

electrode that contributes to the redox reaction of the electrochromic material will be described below with

reference to Fig. 2. Fig. 2 is a graph illustrating

current-voltage characteristics of electrochromic devices having different electrode structures and each having a first electrode and a second electrode. In Fig. 2, (a) indicates the current-voltage characteristics of an

electrochromic device having an electrode structure in which each of the first and second electrodes has a substantially flat surface structure. (b) indicates the current-voltage characteristics of an electrochromic device according to an embodiment of the present invention, the electrochromic device having an electrode structure in which the first electrode has a substantially flat surface structure and the second electrode has a porous structure. (c) indicates the current-voltage characteristics of an electrochromic device having an electrode structure in which each of the first and second electrodes has a porous structure.

[0039] In the case of (a) in Fig. 2, i.e., in the case of the electrode structure in which each of the first and second electrodes has a substantially flat surface structure, a potential at which a redox current starts to flow is higher than those of (b) and (c) . The potential at which a redox current starts to flow in (a) is very high, compared with the intrinsic redox potential of the material. This indicates that it is necessary to apply a high voltage in order to make compensation for the amount of charges

required for the redox reaction of the electrochromic

material because of a small specific surface area of the second electrode and a very small amount of charges induced.

[0040] In the case of (b) according to an embodiment of the present invention in Fig. 2, i.e., in the case of the electrode structure in which the first electrode has a substantially flat surface structure and the second electrode has a porous structure, a potential at which a redox current starts to flow is lower than those of (a) and (c) . The potential at which a redox current starts to flow in (b) is closer to the intrinsic redox potential of the material. This indicates that the amount of charges

required for the redox reaction of the electrochromic

material is sufficiently compensated by the application of a voltage comparable to the intrinsic redox potential of the material because of a large specific surface area of the second electrode and a large amount of charges induced. In the case of (c) in Fig. 2, i.e., in the case of the

electrode structure in which each of the first and second electrodes has a porous structure, a potential at which a redox current starts to flow is intermediate between those of (b) and (c) . This indicates that it is necessary to apply a higher voltage to the structure that exhibits (b) because the amount of charges induced at the time of the redox reaction of the material is effectively reduced by an increase in the specific surface area of the first electrode.

[0041] Thus, in order to reduce the redox potential of the electrochromic material in the electrochromic device and reduce the redox potential difference between materials, the second electrode where the supporting electrolyte ions are polarized needs to have a larger specific surface area than that of the first electrode where the redox reaction of the material occurs.

[0042] An electrochromic device including a medium

containing a plurality of electrochromic materials has a problem in which a bleaching response is particularly slow, compared with a coloring response. The delay of the

bleaching response is significantly affected by the

diffusion of the materials. Thus, the bleaching response may be improved by reducing the transfer length of the materials, i.e., a gap between the electrodes. The

bleaching response may also be improved by the application of a high reverse voltage at the time of bleaching. However, in the case where the first and second electrodes have the same specific surface area, the shape of the current-voltage characteristics is symmetric with respect to zero potential as illustrated by (a) and (c) in Fig. 2. As a result, the application of a reverse voltage causes coloring to occur at the opposite electrode. Thus, a high reverse voltage cannot be applied at the time of bleaching. In contrast, in the electrode structure according to an embodiment of the

present invention, the first electrode having a

substantially flat surface structure does not compensate for reaction charges, and only the second electrode with a porous structure compensates for the reaction charges. The shape of the current-voltage characteristics is asymmetric with respect to zero potential as illustrated by (b) in Fig. 2. In this case, when a reverse voltage is applied, coloring does not occur at the opposite electrode. It is thus possible to apply a high reverse voltage at the time of bleaching, thereby significantly improving the bleaching response.

[0043] Here, the range of the specific surface area of the second electrode 3 with a porous structure is described.

Fig. 3 is a graph illustrating the relationship between the threshold voltage for oxidation of anodically electrochromic material A represented by structural formula (A) described below and the specific surface area of the second electrode.

[0044]

[Chem. 1]

(A)

[0045] The threshold voltage refers to a voltage such that a change in optical density AOD (= -log(T/T ₀ )) (where T represents a transmittance, and T ₀ represents an initial transmittance ) is 0.01 at an absorption wavelength of an electrochromic material. In this case, the first electrode is formed of a fluorine-doped tin oxide (FTO) thin film that is assumed to have a specific surface area of about 1 cm ² /cm ² . In Fig. 3, when the second electrode is also formed of an FTO thin film, the threshold voltage is 2.23 V. When the second electrode has a specific surface area of 300 cm ² /cm ² or more, the threshold voltage is 1 V or less. When the second electrode has a specific surface area of 600 cm ² /cm ² or more, the threshold voltage is reduced to 0.5 V or less.

[0046] Thus, the specific surface area of the second electrode 3 with a porous structure is preferably 300 cm ² /cm ² or more and more preferably 600 cm ² /cm ² or more.

[0047] The first electrode 2 having a substantially flat surface structure may be formed of a thin film composed of a transparent conducting oxide, for example, tin-doped indium oxide (ITO), zinc oxide, gallium-doped zinc oxide (GZO) , aluminum-doped zinc oxide (AZO) , tin oxide, antimony-doped tin oxide (ATO) , fluorine-doped tin oxide (FTO) , or niobium- doped titanium oxide (TNO) . In view of conductivity and high transparency, the first electrode may have a laminated structure including these films. A method for forming the first electrode is not limited as long as the first

electrode has a specific surface area of 30 cm ² /cm ² or less. Examples of the method include, but are not limited to, gas- phase deposition methods, such as sputtering, evaporation, and chemical vapor deposition (CVD) ; and liquid-phase

deposition method, such as a sol-gel method, spin coating, printing, and plating. In particular, an FTO thin film having a thickness of about 200 nm may be used as a material that achieves high visible-light transmittance and high chemical stability. The first electrode preferably has a thickness of 100 nm or more and 1000 nm or less and more preferably 200 nm or more and 500 nm or less.

[0048] Examples of a material for the second electrode 3 with a porous structure include titanium oxide, tungsten oxide, cerium oxide, and compound oxides thereof, in

addition to the transparent conducting oxide that may be used for the first electrode 2. Here, the shape of the second electrode and a method for producing the second electrode are not limited as long as the second electrode with a porous structure satisfies the foregoing requirement on the specific surface area and a requirement on optical properties described below. Examples of the second

electrode that may be used include nanoparticle films having through holes; and nanostructures, such as nanorods, nanowires, and nanotubes. In particular, a nanoparticle film having a large specific surface area per volume and excellent optical properties may be used. The second electrode preferably has a thickness of 1500 nm or more and more preferably 3000 nm or more.

[0049] Here, the requirement on the optical properties of the second electrode with a porous structure is described. The electrochromic device according to an embodiment of the present invention is a transmission device arranged in an optical path of an image pickup apparatus or the like and may have high visible-light transmittance and low haze. In particular, in view of the foregoing use, the visible-light transmittance may be 80% or more and more preferably 90% or more. The haze value is preferably 1% or less and more preferably 0.5% or less. Regarding a porous structure that achieves the foregoing optical properties, in particular, a nanoparticle film having an average particle size of 40 nm or less, an average pore size of 30 nm, and an arithmetic mean roughness of 50 nm or less may be used.

[0050] Fig. 4 is a schematic diagram of an electrochromic device according to another embodiment of the present invention. In the. electrochromic device illustrated in Fig. 4, the second electrode 3 with a porous structure has a laminated structure including a layer 5 with a porous structure and a transparent conducting layer 6, the layer 5 with a porous structure being disposed adjacent to the electrochromic medium 4. The reason for this structure is that when the layer 5 with a porous structure has a high sheet resistance, the transparent conducting layer 6 having a low resistance compensates for the layer 5 with a porous structure .

[0051] The electrochromic medium 4 is composed of a liquid containing a supporting electrolyte and at least one anodically electrochromic material or at least one

cathodically electrochromic material.

[0052] Anodically electrochromic materials and

cathodically electrochromic materials are transparent

materials that do not exhibit absorption in the visible light region in a neutral state. An anodically

electrochromic material absorbs light with a specific

wavelength in the visible light region when oxidized. A cathodically electrochromic material absorbs light with a specific wavelength in the visible light region when reduced. In each case, a plurality of materials having different absorption bands in the visible light region may be used in combination as a mixture to provide flat absorption

characteristics. Examples of the anodically electrochromic material include thiophenes. Examples of the cathodically electrochromic material include viologens.

[0053] The supporting electrolyte is not limited as long as it has low reactivity to an electrode material and is stably used. A plurality of supporting electrolytes may be used in combination. In particular, a salt of an organic cation may be used from the viewpoint of achieving good stability. Examples of the supporting electrolyte that may be used include salts of organic cations, such as a

quaternary ammonium cation, an aromatic cation, a guanidinium cation, and an imidazolium cation, and inorganic anions, such as a perchlorate anion, a tetrafluoroborate anion, and a hexafluorophosphate anion. In view of

solubility, the degree of dissociation, solution resistance, and so forth, tetrabutylammonium perchlorate (TBAP) ,

tetrabutylammonium tetrafluoroborate, tetrabutylammonium hexafluorophosphate, and so forth may be used.

[ 0054] A salt of such an organic cation has low reactivity to a porous electrode, in particular, a porous electrode formed of a nanoparticle film. Thus, a device including the salt of an organic cation as an electrolyte exhibits stable coloring-bleaching behavior.

[0055] As a solvent that dissolves the electrochromic material, the supporting electrolyte, and so forth, aprotic polar solvents, such as propylene carbonate, γ-butyrolactone, and benzonitrile, may be used in view of the solubility, vapor pressure, viscosity, potential window, and so forth.

[0056] The electrochromic medium may further contain a dehydrating agent, a stabilizing agent, a thickener, and so forth in addition to the foregoing substances.

[ 0057 ] Here, in the nanoparticle film composed of a

transparent conducting oxide according to an embodiment of the present invention, the range of the particle size of the nanoparticles is described. In the case where an

electrochromic material having a standard value of coloring efficiency η of 500 cm ² /C is used, the amount of current required to achieve a change in optical density AOD of 0.9

(-3 EV) within one second after the application of a voltage is 1.8 mA/cm ² . Fig. 10 is a graph illustrating the

relationship between the surface area of a transparent counter electrode (the surface area of a nanoparticle film) and the amount of reaction current. As is apparent from Fig. 10, the surface area of the transparent counter electrode

(the surface area of the nanoparticle film) required to achieve a reaction current of 1.8 mA/cm ² is 214 cm ² /cm ² or more .

[0058] Fig. 11 is a graph illustrating the relationship between the surface area of a nanoparticle film (the ideal maximum surface area upon assuming that spheres are close- packed) and the particle size of nanoparticles at different thicknesses. As is apparent from Fig. 11, the thickness and the particle size of the nanoparticle film having a surface area of 214 cm ² /cm ² or more are as follows: a particle size of 18 nm or less at a thickness of 1 μτη; a particle size of 36 nm or less at 2 μχα; a particle size of 54 nm or less at 3 μιη; a particle size of 72 nm or less at 4 μπι; and a particle size of 90 nm or less at 5 μιη. The nanoparticles may have an average particle size of 90 nm or less.

[0059] Fig. 12 is a graph illustrating the relationship between the thickness of a nanoparticle film composed of antimony-doped tin oxide (ATO) and the average transmittance of visible light. Fig. 12 demonstrates that a 1 pin increase in thickness reduces the transmittance by about 4%. The transmittance of the nanoparticle film varies depending on the material and the state of the film (surface

irregularities and pore size) . The nanoparticle film used for the transmission electrochromic device preferably has a thickness of 1 μπ; or more and 10 μιη or less and more

preferably 2 μηα or more and 5 or less.

[0060] The nanoparticles constituting the nanoparticle film preferably have 90 nm or less and more preferably 10 nm or more and 30 nm or less.

[0061] A step of injecting an electrochromic medium into a device will be described below.

[0062] The glass substrate la provided with the first electrode 2 having a substantially flat surface structure and the glass substrate lb provided with the second

electrode 3 having a porous structure are bonded together using a sealing material with the electrodes inside so as to partially form an opening portion. As the sealing material, a chemically stable material which is impervious to gas or water and which does not inhibit the redox reaction of the electrochromic material may be used. Examples of the sealing material that may be used include glass frits, epoxy resins, and metals. The sealing material may have the function of defining a gap between the pair of glass substrates. Alternatively, a spacer may be disposed. The electrochromic medium 4 is injected by a vacuum injection method from the opening portion into the resulting article including the partially formed opening portion, and then the article is sealed.

[0063] Device characteristics when a salt of an alkali metal cation is used as a supporting electrolyte will be described below with reference to Fig. 13. Device

characteristics when a salt of an organic cation is used as a supporting electrolyte will be described below with reference to Fig. 14.

[0064] Fig. 13 illustrates device characteristics when a salt of an alkali metal cation is used as a supporting electrolyte. Specifically, propylene carbonate is used as a solvent. Lithium perchlorate is used as a supporting electrolyte in a concentration of 0.1 M. As a nanoparticle film which is composed of a transparent conducting oxide and which is used for a counter transparent electrode, a

nanoparticle film composed of antimony-doped tin oxide nanoparticles having an average primary particle size of 10 to 30 nm and a specific surface area of 70 to 80 m ² /g is used. An electrochromic medium contains an electrochromic material in a concentration of 30 mM, the electrochromic material being represented by structural formula (A) and capable of being colored by oxidation.

[0065] The upper graph in the figure is a cyclic

voltammogram illustrating current characteristics measured by the application of a triangular voltage at a sweep rate of 200 mV/sec.

[0066] Fig. 14 is illustrates device characteristics when a salt of an organic cation is used as a supporting

electrolyte. Specifically, propylene carbonate is used as a solvent. Tetrabutylammonium perchlorate is used as a supporting electrolyte in a concentration of 0.1 M. All experimental conditions are the same as in Fig. 13, except for the cationic moiety of the supporting electrolyte.

[0061] Figs. 13 and 14 demonstrate the following: While the coloring start voltage of the device including the lithium salt is about 0.5 V lower than that of the device including the tetrabutylammonium salt, substantially the same peak current is observed independently of the type of supporting electrolyte cation. In the device including the lithium salt, bleaching behavior from a colored state is very slow, so that the transmittance does not return to the initial transmittance in one cycle. In contrast, in the device including the tetrabutylammonium salt, bleaching behavior is not delayed, so that the transmittance returns to the initial transmittance. That is, the entirely different bleaching behavior is caused simply by the difference in supporting electrolyte cation.

[0068] In a bleaching process in an electrochromic device in which an electrochromic material capable of being colored by oxidation is dissolved, the material is reduced to cause bleaching at a transparent working electrode, and supporting electrolyte cations that form an electric double layer at an interface with a transparent counter electrode diffuse again in a solution. When lithium ions are used, the bleaching process would be irreversible. Specifically, in addition to the intercalation reaction or alloying reaction of lithium ions, transition metal nanoparticles are presumed to be subjected to a lithium oxide formation reaction associated with the reduction of nanoparticles (MO _x + 2xLi ⁺ + 2xe ^" —» M + xLi ₂ 0, M: a transition metal) to change an electrode state (an electrode potential and a surface state) . ^■ In contrast, when a salt of an organic cation is used, the stable

operation of the device is guaranteed without reaching the state .

[0069] Thus, the salt of an organic cation may be used as an electrolyte.

[0070] An image pickup optical system and an image pickup apparatus according to embodiments of the present invention will be described below.

[0071] An optical filter according to an embodiment of the present invention includes the foregoing electrochromic device and a circuit configured to drive the electrochromic device .

[0072] A lens unit according to an embodiment of the present invention includes an optical system including a plurality of lenses and an optical filter, the optical system and the optical filter being arranged in such a manner that light passing through the optical filter passes through the optical system. The optical system is also referred to as an "image pickup optical system".

[0073] An image pickup apparatus according to an

embodiment of the present invention includes the foregoing optical filter and an image pickup device configured to receive light passing through the optical filter.

[0074] An image pickup apparatus according to another embodiment of the present invention includes a circuit configured to drive the foregoing electrochromic device and an image pickup device configured to receive light from the outside .

[0075] In the case where the electrochromic device according to an embodiment of the present invention is used for an image pickup apparatus, such as a camera, it is possible to reduce the quantity of light without reducing the gain of the image pickup device. In the case where the electrochromic device is used in an image pickup apparatus, the electrochromic device may be included in an image pickup optical system or the main body of the image pickup apparatus .

[0076] In the case where the image pickup optical system includes the electrochromic device, the electrochromic device may be arranged between a subject and the image pickup optical system, between the image pickup optical system and the image pickup device, or between lenses included in the image pickup optical system. In this case, the electrochromic device may be driven by a signal from a circuit configured to drive the electrochromic device, the circuit being arranged in the main body.

[0077] In the case where the image pickup apparatus includes the electrochromic device, the electrochromic device may be arranged in front of the image pickup device. The image pickup device includes a circuit configured to drive the electrochromic device. The electrochromic device is driven by a signal from the circuit.

[0078] A window member according to an embodiment of the present invention includes the foregoing electrochromic device and a circuit configured to drive the electrochromic device. In the case where the electrochromic device according to an embodiment of the present invention is used for the window member, such as a window pane, the

electrochromic device serves as an electronic curtain, a transmission filter, or the like. In the case where the electrochromic device is arranged in the window member, a material for window members in the related art may be used. For example, the electrochromic device may be arranged between tempered glass sheets or the like.

[0079] The window member including the electrochromic device may be used for windows of houses, windows of

airplanes, windows of automobiles and trains, and filters of display surfaces of clocks and cellular phones.

[0080] Embodiments of a lens unit and an image pickup apparatus including an optical filter according to an embodiment of the present invention will be described below.

[0081] Fig. 15 is a schematic drawing illustrating a lens unit including an optical filter according to an embodiment of the present invention and an image pickup apparatus including the lens unit.

[0082] An optical filter 101 includes an organic

electrochromic device and a driving device connected to the organic electrochromic device and is arranged in a lens unit 102.

[0083] The lens unit 102 includes a plurality of lenses or lens groups. For example, in Fig. 15, the lens unit

indicates a zoom lens with a rear-focusing system, in which focusing is achieved behind an aperture stop. The lens unit includes four lens groups: a first lens group 104 with positive refractive power, a second lens group 105 with negative refractive power, a third Lens group 106 with a positive refractive power, and a fourth lens group 107 with positive refractive power arranged, in that order, from the side of a subject. Changing- the distance between the second lens group and the third lens group adjusts the focal length. Focusing is achieved by moving some lenses in the fourth lens group.

[0084 ] For example, the lens unit 102 includes an aperture stop 108 arranged between the second lens group and the third lens group; and the optical filter 101 between the third lens group and the fourth lens group.

[0085 ] The lens groups, the aperture stop, and the optical filter in the lens unit are arranged so as to allow light to passes therethrough. The quantity of light is adjusted using the aperture stop and the optical filter.

[0086] The structure in the lens unit may be appropriately changed. For example, the optical filter may be arranged in front of or behind the aperture stop. Furthermore, the optical filter may be arranged in front of the first lens group (on the side of a subject) or behind the fourth lens group. The arrangement of the optical filter at a position where light converges advantageously results in a reduction in the area of the optical filter.

[0087 ] The structure of the lens unit may also be

appropriately selected. In place of the rear-focusing system, an inner-focusing system, in which focusing is achieved in front of the aperture stop, or another system may be used. Furthermore, special lenses, such as fish-eye lenses and macro lenses, may be appropriately selected in addition to the zoom lens.

[0088] The lens unit is detachably coupled to an image pickup apparatus 103 with a mounting member (not

illustrated) provided therebetween.

[0089] A glass block 109 is a glass block, for example, a low-pass filter, a faceplate, or a color filter.

[0090] A light-receiving device 110 is a sensor configured to receive light passing through the lens unit. An image pickup device, for example, a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, may be used. Furthermore, an optical sensor, such as a

photodiode, may be used. A device configured to acquire and output information about the intensity or wavelength of light may be appropriately used.

[0091] In the case where the optical filter is

incorporated in the lens unit as illustrated in Fig. 15, the driving device may be arranged inside or outside the lens unit. In the case where the driving device is arranged outside the lens unit, the organic electrochromic device inside the lens unit is connected to the driving device outside the lens unit through interconnections, so that the organic electrochromic device is driven and controlled by the driving device.

[ 0092 ] The image pickup apparatus may include the optical filter 101. Fig. 16 is a schematic drawing illustrating an image pickup apparatus including an optical filter.

[0093] The optical filter may be arranged at an

appropriate position in the image pickup apparatus, and the light-receiving device 110 may be arranged so as to receive light passing through the optical filter. In Fig. 16, for example, the optical filter is arranged immediately in front of the light-receiving device. In the case where the image pickup apparatus includes an built-in optical filter, a lens unit to be coupled may not include an optical filter. Thus, an image pickup apparatus capable of adjusting the quantity of light can be provided using an existing lens unit.

[0094 ] The image pickup apparatus may be applied to a product including a combination of the adjustment of the quantity of light and a light-receiving device. Examples of the product include cameras, digital cameras, video cameras, and digital video cameras. Furthermore, the image pickup apparatus may be applied to a product including a built-in image pickup apparatus, for example, a cellular phone, a smartphone, a personal computer, or a tablet.

[0095 ] As described in this embodiment, the optical filter formed of the organic electrochromic device is used as a member configured to adjust the quantity of light. Thus, the quantity of light can be appropriately changed with a single filter, advantageously reducing the number of members and saving space.

Examples

[0096] Examples of the present invention will be described below .

Example 1

[0097] This example is an example of the structure

illustrated in the foregoing embodiment and will be

described in detail below.

[0098] A fluorine-doped tin oxide (FTO) thin film having a thickness of 200 nm was formed on a glass substrate (Code 1737, manufactured by Corning Inc.) having a thickness of 1.1 mm to prepare the glass substrate la provided with the first electrode 2 having a substantially flat surface structure. Here, the glass substrate provided with the FTO thin film had an average visible light transmittance of 85%, a haze of 0.1%, and a sheet resistance of 40 □/□. In this case, the specific surface area of the first electrode was assumed to be about 1 cm ² /cm ² .

[0099] A tin oxide nanoparticle slurry having an average particle size of 21 nm (item number: SNAP15 T%-G02 ,

manufactured by CIK NanoTek Corporation) and a zinc oxide nanoparticle slurry having an average particle size of 34 nm (item number: ZNAP15WT%-G0 , manufactured by CIK NanoTek Corporation) were mixed together in a volume ratio of tin oxide to zinc oxide of 2:1 to prepare a slurry mixture. The slurry mixture was applied on another identical glass substrate provided with the FTO thin film as described above and fired at 500°C for 30 minutes. Only zinc oxide was then removed by etching with dilute hydrochloric acid to form a tin oxide nanoparticle film. In this case, two types of substrates including tin oxide nanoparticle films with different thicknesses (thickness: 1.6 μιη, 3.0 μιη) after the acid etching treatment were prepared. In this way, the glass substrates lb were prepared, each of the glass

substrates lb including the second electrode 3 that has a laminated structure including the tin oxide nanoparticle film serving as the layer 5 wi a porous structure and the FTO thin film serving as the transparent conducting layer 6. Regarding the specific surface areas of the two types of second electrodes including the tin oxide nanoparticle films having different thicknesses, the second electrode including the 1.6- m-thick tin oxide nanoparticle film had a specific surface area of 328 cm ² /cm ² . The second electrode including the 3. O-jum-thick tin oxide nanoparticle film- had a specific surface area of 653 cm ² /cm ² . The substrate including the tin oxide nanoparticle film with a specific surface area of 328 cm ² /cm ² had a visible light transmittance of 87% and a haze of 0.6%.

[0100] The resulting pair of substrates provided with the electrodes were bonded together using an epoxy resin with the electrodes inside so as to form an opening portion used for the injection of an electrochromic medium. In this case, a 125-|jm-thick polyethylene terephthalate (PET) film

( elinex (registered trademark) S-125, manufactured by

Teijin DuPont Films Japan Limited) was used as a spacer.

[0101] Next, anodically electrochromic material A

represented by structural formula (A) and tetrabutylammonium perchlorate (TBAP) serving as a supporting electrolyte were dissolved in a propylene carbonate solvent to prepare a solution serving as the electrochromic medium 4. In this case, the concentration of the anodically electrochromic material A was 30 mM. The concentration of TBAP was 0.1 M.

[0102] The electrochromic medium was injected by a vacuum injection method from the opening portions into the

resulting empty devices including the opening portions.

Then the opening portions were sealed with an epoxy resin, thereby producing two types of electrochromic devices that differed only in the specific surface areas of the second electrodes .

Reference Example 1

[0103] Three types of electrochromic devices were produced as in Example 1, except that the tin oxide nanoparticle films of the second electrodes had thicknesses of 0, 0.5, and 0.9 jum and specific surface areas of 1, 99, and 190 cm ² /cm ² , respectively. All conditions were the same as in Example 1, except for the thickness (specific surface area) of the tin oxide nanoparticle film.

Evaluation of electrochromic device

[0104] The five types of the electrochromic devices produced in Example 1 and Reference Example 1 were placed i an evaluation system configured to simultaneously conduct electrochemical measurement and transmittance measurement. The current-voltage characteristics and the transmittance properties were evaluated. Fig. 5 is a graph illustrating the cyclic voltammogram characteristics of the

electrochromic devices according to Example 1 and Reference Example 1, in which (a) indicates the cyclic voltammogram o the electrochromic device including the second electrode having a specific surface area of 653 cm ² /cm ² in Example 1, and (b) indicates the cyclic voltammogram of the

electrochromic device including the second electrode having a specific surface area of 1 cm /cm ² in Reference Example 1.

[0105] As is apparent from Fig. 5, in the electrochromic device (b) in Reference Example 1, substantially no current flows even when a voltage of +2 V or more is applied. In contrast, in the electrochromic device (a) in Example 1, a current starts to flow at a voltage of about +0.3 V. That is, the threshold voltage for oxidation is significantly reduced. In the electrochromic device (b) in which the first and second electrodes are symmetric in terms of the specific surface area, the shape of the current-voltage characteristics is symmetric with respect to zero potential. In contrast, in the electrochromic device (a) in which the first and second electrodes have different specific surface areas, the shape of the current-voltage characteristics is asymmetric with respect to zero potential.

[0106] Fig. 3 described in the foregoing embodiment is a graph illustrating the relationship between the threshold voltage for oxidation and the specific surface area of the second electrode with respect to the five types of

electrochromic devices produced in Example 1 and Reference Example 1. As is clear from Fig. 3, in particular, the threshold voltage for oxidation is reduced to 1 V or less at a specific surface area of the second electrode of 300 cm ² /cm ² or more. Furthermore, the threshold voltage for oxidation is reduced to 0.5 V or less at a specific surface area of the second electrode of 600 cm ² /cm ² or more.

[0107] Next, the bleaching response time of the

electrochromic device including the second electrode having a specific surface area of 653 cm ² /cm ² in Example 1 was checked. Fig. 6 is a graph illustrating the relationship between the bleaching response time and the bleaching voltage of the electrochromic device in Example 1. Here, the bleaching response time was defined as the length of time that the optical density of the electrochromic device was returned to the initial optical density when a reverse voltage is applied to the electrochromic device in a state in which a change in optical density Δ00 was 0.3.

[0108] As is apparent from the cyclic voltammogram (a) in Fig. 5, when a negative voltage is applied to the

electrochromic device, no current flows, so that no coloring occurs. That is, the application of a high reverse voltage significantly improves the bleaching response.

Example 2

[0109] All conditions in Example 2 were the same as in Example 1, except for the electrochromic medium.

[0110] Anodically electrochromic material B represented by structural formula (B) was used as the electrochromic material. Anodically electrochromic material B and

tetrabutylammonium perchlorate (TBAP) serving as a

supporting electrolyte were dissolved in a propylene

carbonate solvent to prepare an electrochromic medium. In this case, the concentration of the anodically

electrochromic material B was 100 mM. The concentration of TBAP was 1.0 .

[0111]

[Chem. 2]

(B)

[0112] In this case, the first electrode is assumed to have a specific surface area of about 1 cm ² /cm ² .

[0113] Electrochromic devices each including a tin oxide nanoparticle film formed ^' as a layer with a porous structure in the second electrode were produced, the tin oxide

nanoparticle film having a thickness of 3.0 and the second electrode having a specific surface area of 653 cm ² /cm ² .

Reference Example 2

[0114] Four types of electrochromic devices were produced, in which the same second electrodes as in Example 2 were used, and the first electrodes had different specific surface areas. In this case, the specific surface areas of the first electrodes were adjusted using tin oxide

nanoparticle films similarly to Example 2. The four types of electrochromic devices included the first electrodes having thicknesses of 0.1, 0.6, 1.4, and 3.0 μπι and specific surface areas of 30, 134, 284, and 653 cm ² /cm ² , respectively. All conditions are the same as in Example 2, except for the specific surface area of the first electrode.

Evaluation of electrochromic device

[0115] The five types of the electrochromic devices produced in Example 2 and Reference Example 2 were placed in an evaluation system configured to simultaneously conduct electrochemical measurement and transmittance measurement. The current-voltage characteristics and the transmittance properties were evaluated.

[0116] Fig. 7 is a graph illustrating the cyclic

voltammogram characteristics of the electrochromic devices in Example 2 and Reference Example 2, in which (a) indicates the cyclic voltammogram of the electrochromic device in Example 2; and (b) , (c) , (d) , and (e) indicate the cyclic voltammograms of the electrochromic devices including the first electrodes having specific surface areas of 30, 134, 284, and 653 cm ² /cm ² , respectively, in Reference Example 2. Fig. 7 demonstrates that as the specific surface area of the first electrode increases from (a) to (e), a voltage at which a current starts to flow increases from +0.67 V to +1.1 V, i.e., the threshold voltages for oxidation of the electrochromic devices increase. Fig. 7 also demonstrates that as the first and second electrodes approach (e) having a symmetric structure in terms of the specific surface area, the shape of the current-voltage characteristics changes to a symmetric shape with respect to zero potential. [0117] Fig. 8 is a graph illustrating the relationship between the threshold voltage for oxidation and the specific surface area of the first electrode of each of the five types of the electrochromic devices (a) to (e) . In the case where the first electrode has a specific surface area of 1 cm ² /cm ² , i.e., the first electrode has a substantially flat surface structure without forming a layer with a porous structure, the minimum threshold voltage for oxidation is obtained. In the case where the specific surface area is only slightly increased to 30 cm ² /cm ² (b) , the threshold voltage for oxidation is increased by 0.1 V or more.

[0118] Next, the bleaching response time of the

electrochromic device including the second electrode having a specific surface area of 653 cm ² /cm ² in Example 2 was checked. Fig. 9 is a graph illustrating the relationship between the bleaching response time and the bleaching voltage. The graph of Fig. 9 is substantially matched to the graph of Fig. 6 which relates to electrochromic material A. This demonstrates that improvement in bleaching response by the application of a high reverse voltage is independent of the electrochromic material.

[0119] From the results of the foregoing examples and reference examples, the threshold voltages for oxidation of anodically electrochromic material A and anodically

electrochromic material B, differences in threshold voltage for oxidation between the materials, and the electrode structures of the electrochromic devices are summarized in Table 1.

[0120] With respect to the electrode structure, Table 1 describes the devices each including both the first and second electrodes with a substantially flat surface

structure (specific surface area: 1 cm ² /cm ² ); and the devices according to an embodiment of the present invention, each of the devices including the first electrode with a

substantially flat surface structure (specific surface area: 1 cm ² /cm ² ) and the second electrode with a porous structure (specific surface area: 653 cm ² /cm ² ) .

[0121]

[Table 1]

Example 3

[0122] A non-alkali glass substrate provided with a 200- nm-thick fluorine-doped tin oxide (FTO) film (manufactured by SPD Laboratory, Inc.) was used as the glass substrate la provided with the transparent working electrode 2. The non- alkali glass substrate was formed of a glass substrate (Code 1737, manufactured by Corning Inc.) having a thickness of 1.1 mm. The glass substrate provided with the FTO film had an average visible light transmittance of 85% and a sheet resistance of 40 Ω/D.

[0123] Next, a nanoparticle film composed of a transparent conducting oxide was formed on another identical substrate as described above using antimony-doped tin oxide (ATO) nanoparticles to prepare the glass substrate lb provided with the transparent counter electrode 3. In this case, as the antimony-doped tin oxide nanoparticles, SN-100P (primary average particle size: 10 to 30 nm, specific surface area: 70 to 80 m ² /g, manufactured by Ishihara Sangyo Kaisha, Ltd.) was used. A water slurry containing the antimony-doped tin oxide nanoparticles, polyethylene glycol, and hydroxypropyl cellulose was prepared. The water slurry was applied onto the glass substrate provided with the FTO film using a bar coater to form a film. The resulting film was fired to form a 2.7-|jm-thick ATO nanoparticle film. The ATO nanoparticle film itself had a sheet resistance of 1.2 x 10 ⁴ Ω/Ο. The glass substrate provided with the FTO film on which the ATO nanoparticle film was disposed had an average visible light transmittance of 65% .

[0124] The resulting pair of the substrates provided with the transparent electrodes are bonded together using a sealing material with the electrodes inside so as to form an opening portion used for the injection of an electrochromic medium. In this case, a 125-fxm-thick polyethylene

terephthalate (PET) film (Melinex (registered trademark) S- 125, manufactured by Teijin DuPont Films Japan Limited) was used as a spacer.

[0125] Next, an electrochromic material capable of being colored by oxidation and tetrabutylammonium perchlorate

(TBAP) , which is a salt of an organic cation, serving as a supporting electrolyte were dissolved in a propylene

carbonate solvent to prepare a solution serving as the electrochromic medium 4, the electrochromic material

exhibiting electrochromic properties, having a molecular structure that includes thiophene moieties bonded to

aromatic hydrocarbon rings represented by structural formula

(A) . In this case, the concentration of the electrochromic material capable of being colored by oxidation was 30 mM. The concentration of TBAP was 0.1 M.

[0126] The electrochromic medium 4 was injected by a vacuum injection method from the opening portion into the resulting empty device including the opening portion. Then the opening portion was sealed with an epoxy resin, thereby producing an electrochromic device.

Evaluation of electrochromic device

[0127] The resulting electrochromic device was placed in an evaluation system configured to simultaneously conduct electrochemical measurement and transmittance measurement. The current-voltage characteristics and the transmittance properties were evaluated.

[0128] The application of a voltage of +0.8 V or more allowed an oxidation current to flow through the

electrochromic material. Absorption originating from radical cations formed by oxidation was observed at about 506 nm. At this time, the color of the electrochromic device was changed from colorless to reddish brown.

[0129] The response times when a change in optical densit AOD of 0.3 at a wavelength of 506 nm was achieved were evaluated at a coloring voltage of +2.0 V and a bleaching voltage of 0 V. The coloring response time was 0.1 s, and the bleaching response time was 3.3. s. Repeatedly stable coloring-bleaching behavior was achieved.

Example 4

[0130] In this example, tetraethylammonium perchlorate (TEAP) , which is a salt of an organic cation, was used as a supporting electrolyte. All other conditions were the same as in Example 3. The concentration of TEAP in the

electrochromic medium was 0.1 and the same as in Example Evaluation of electrochromic device

[0131] The application of a voltage of +0.8 V or more allowed an oxidation current to flow through the

electrochromic material. Absorption originating from radical cations formed by oxidation was observed at about 506 nm. At this time, the color of the electrochromic device was changed from colorless to reddish brown.

[0132] The response times when a change in optical density ΑΟΌ of 0.3 at a wavelength of 506 nm was achieved were evaluated at a coloring voltage of +2.0 V and a bleaching voltage of 0 V. The coloring response time was 0.1 s, and the bleaching response time was 5.7 s. Although the

bleaching response was delayed compared with Example 3, repeatedly stable coloring-bleaching behavior was also achieved .

Comparative Example 1

[0133] In this comparative example, lithium perchlorate, which is a salt of an alkali metal cation, was used as a supporting electrolyte. All other conditions were the same as in Example 1. The concentration of lithium perchlorate in the electrochromic medium was 0.1 M and the same as in Example 1.

Evaluation of electrochromic device

[0134] The application of a voltage of +0.3 V or more allowed an oxidation current to flow through the electrochromic material. Absorption originating from radical cations formed by oxidation was observed at about 506 nm. At this time, the color of the electrochromic device was changed from colorless to reddish brown. ·

[0135] The response times when a change in optical density Δθϋ of 0.3 at a wavelength of 506 nm was achieved were evaluated at a coloring voltage of +2.0 V and a bleaching voltage of 0 V. The coloring response time was 0.1 s, and the bleaching response time was 148 s. The bleaching response was very slow, and repeatedly stable coloring- bleaching behavior was not achieved.

[0136] The use of the electrode structure according to an embodiment of the present invention results in an

electrochromic device that exhibits a low threshold voltage for oxidation irrespective of the electrochromic material, a small difference in threshold voltage for oxidation, and only a small change in hue throughout a coloring-bleaching process .

[0137] The electrochromic device according to an

embodiment of the present invention has excellent hue stability and response speed and thus is usable for image pickup optical systems, image pickup apparatuses, and window members .

[0138] While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

[0139] This application claims the benefit of Japanese Patent Application No. 2013-047741, filed March 11, 2013, No. 2013-097766 filed May 7, 2013, and No. 2014-042129 filed March 4, 2014, which are hereby incorporated by reference herein in their entirety.

Advantageous Effects of Invention

[0140] An embodiment of the present invention provides an electrochromic device having excellent hue stability and response speed. Furthermore, embodiments of the present invention provide an optical filter including the

electrochromic device, a lens unit including the optical filter, an image pickup apparatus including the optical filter, and a window member.

Reference Signs List

[0141] la, lb glass substrate

2 first electrode

3 second electrode

4 electrochromic medium

5 layer with porous structure

6 transparent conducting layer