Patent Description:
So-called solar cells and various other types of elements and devices have been devised as photovoltaic elements that convert optical energy into electric energy. The photovoltaic elements are roughly classified into two; those using silicon-based material and those using compound-based material as the material for exerting photovoltaic effect. Examples for such photovoltaic elements are published as <CIT> and in<NPL>, which both disclose dye-sensitized solar cells.

<CIT> discloses a silicon dioxide solar cell, comprising two substrates having electrical conductivity, the two substrates being arranged so that the respective conductive surfaces of the substrates are facing each other, at least one of the substrates being a transparent substrate on the light incident side, and which has a silicon dioxide particles compact and an electrolyte disposed between said two substrates, wherein said silicon dioxide particles compact is disposed on the substrate arranged to face said substrate on the light incident side; and the space between said silicon dioxide particles compact and said substrate on the light incident side is filled with said electrolyte, wherein on the transparent conductive layer on the silicon dioxide side, a platinum layer is formed as an electrode for extracting charges.

Dye sensitized solar cells are also disclosed by<NPL>.

Elements that use monocrystalline silicon, polycrystalline silicon, heterojunction model, amorphous silicon and thin-film polycrystalline silicon are typical examples of elements that use silicon-based material. Meanwhile, elements using group III-V compounds, CIS (using copper (Cu), indium (In) and selenium (Se) as main components), CIGS (using copper (Cu), indium (In), gallium (Ga) and selenium (Se) as main components), CdTe, organic thin film and dye-sensitized material are examples of elements that use compound-based material.

In addition to the above-described photovoltaic elements, there are elements using silicon dioxide, which is an insulator, as power generating material. This is based on a finding by the present inventors that silicon dioxide itself exerts photo electrolysis effect and photovoltaic effect.

The present inventors have found that synthetic quartz and fused quartz, which are silicon dioxides, exert photovoltaic effect, and proposed a silicon dioxide solar cell as photoelectrode material and photocell material (Patent Literatures <NUM> and <NUM>).

With reference to <FIG>, we will describe a tandem-type power generation element using two photovoltaic layers formed of silicon dioxide (SiO<NUM>) and titanium oxide (TiO<NUM>) as a prior art example.

In <FIG>, reference numbers <NUM> and <NUM> denote glass substrates, and <NUM> and <NUM> denote FTO (fluorine-doped tin oxide) layers.

A porous titanium dioxide layer <NUM> hardened by sintering is formed on the FTO layer <NUM> on the side from which incident light enters. The porous titanium dioxide layer <NUM> carries titania particles on which are adsorbed ruthenium complex dye as sensitized dye. Further, a platinum film <NUM> is formed on the FTO layer <NUM>.

A silicon dioxide layer <NUM> composed of silicon dioxide particles is formed on the platinum film <NUM>, so that the layer <NUM> has a thickness of <NUM> - <NUM> in the height direction.

Moreover, the distance between the titanium dioxide layer <NUM> and the silicon dioxide layer <NUM> in the height direction is <NUM> or greater, and electrolyte <NUM> is sealed in a space surrounded on four sides by a sealing member <NUM>.

As illustrated in <FIG>, <FIG>, <FIG> and <FIG>, the direction perpendicular to the substrate surface of the photovoltaic element is referred to as the height direction, and thickness of layers and films is described by the distance thereof.

The silicon dioxide layer <NUM> serving as the photovoltaic layer is composed of silicon dioxide particles, which are formed by immersing particles of glass and the like containing silicon dioxide in a <NUM> - <NUM> % hydrofluoric solution, washing the particles with water, drying, and pulverizing the same so that the particle size is <NUM> or smaller.

As described, individual shapes of the pulverized silicon dioxide particles may be approximately spherical, but nonspherical particles as illustrated in <FIG> also exist.

The individual silicon dioxide particles <NUM> have various shapes. In the present specification, as illustrated in <FIG>, a maximum elongation direction of the individual silicon dioxide particles <NUM> is referred to as a major axis L, and the average major axis is used to denote the shape of the silicon dioxide particles used in the photovoltaic layer and a first photovoltaic layer <NUM>. In the prior art example illustrated in <FIG>, a material having an average major axis L of <NUM> - <NUM> is used.

The tandem-type photovoltaic element described here characterizes in using silicon dioxide as the photovoltaic layer. As illustrated in <FIG>, it is confirmed that silicon dioxide has higher quantum efficiency than titanium dioxide even in the ultraviolet region, and that it also absorbs light in the infrared region of <NUM> and higher. Therefore, silicon dioxide exerts photovoltaic effect in a wider wavelength region compared to titanium dioxide and realizes an extremely high power generation efficiency. According to such tandem-type photovoltaic element, the inventors of the present invention have achieved a maximum output of <NUM>µW/cm<NUM> per unit area in an illumination of <NUM> lux.

The photovoltaic elements disclosed in PTL <NUM> and PTL <NUM>, <CIT> and T. (<NUM>) can be manufactured using a low-cost material compared to prior art solar cells, and the energy conversion effect thereof is extremely high compared to other photovoltaic elements. However, even further enhancement of energy conversion effect is desired in photovoltaic elements.

The problem of the invention is solved by the features of claim <NUM>. According to one typical photovoltaic element, not forming part of the claimed invention, a photovoltaic layer of the photovoltaic element is composed of a silicon dioxide particle that has an average major axis of <NUM> or smaller.

According to another typical photovoltaic element, not forming part of the claimed invention, the photovoltaic layer of the photovoltaic element is composed of a silicon dioxide particle, and a thickness of the first photovoltaic layer in a height direction is formed to be smaller than three times the average major axis of the silicon dioxide particle.

According to the claimed invention, a photovoltaic element is provided, said photovoltaic element comprising a first photovoltaic layer, wherein the first photovoltaic layer of the photovoltaic element is composed ofesilicon dioxide particles, and the silicon dioxide particles are arranged on a charge exchange layer that has a roughness in the height direction,.

The photovoltaic element described above significantly improves the power generation output per unit area compared to the prior art photovoltaic element.

The problems, configurations and effects other than those described above will become apparent from the following description of embodiments.

Now, preferred embodiments will be described with reference to the drawings. At first, matters common to the first, second and third embodiments are described.

<FIG> is a tandem-type photovoltaic element according to a first embodiment not representing the present invention, <FIG> is a cross-sectional view of a tandem-type photovoltaic element according to a second embodiment not representing the present invention, and <FIG> is a cross-sectional view of a tandem-type photovoltaic element according to a third embodiment according to the present invention, wherein matters described below are common to <FIG>, <FIG> and <FIG>, so they are described with reference to <FIG> as the representative drawing.

<FIG>, <FIG> and <FIG> all illustrate a tandem-type photovoltaic element comprising two photovoltaic layers, which are a first photovoltaic layer and a second photovoltaic layer.

In <FIG>, among a first substrate <NUM> and a second substrate <NUM>, at least the second substrate arranged on a side from which incident light enters is composed of a transparent material, and preferably, both substrates are composed of transparent material. Glass is a popular transparent material, but resin, such as plastic, can be used instead of glass.

A transparent second conductive film <NUM> is formed on the second substrate. The second conductive film <NUM> is preferably composed of FTO (fluorine-doped tin oxide), but other than the FTO layer, an indium-tin complex oxide (IOT) may be used, for example.

A second photovoltaic layer <NUM> is formed on the second conductive film <NUM>. The second photovoltaic layer <NUM> is composed of a substance selected from TiO<NUM>, SnO, ZnO, WO<NUM>, Nb<NUM>O<NUM>, In<NUM>O<NUM>, ZrO<NUM>, Ta<NUM>O<NUM> and TiSrO<NUM>. A porous titanium dioxide layer hardened by sintering is preferable.

Further, a composite of two or more substances described above, such as a composite of SnO and ZnO or a composite of TiO<NUM> and Nb<NUM>O<NUM>, may also be used.

The thickness of the second photovoltaic layer <NUM> in the height direction should preferably be <NUM>-<NUM>, and more preferably, <NUM>-<NUM>.

Further, the above-described second photovoltaic layer <NUM> may carry sensitized dye. Various dyes that exert sensitization can be applied as the dye carried by the second photovoltaic layer <NUM>, and for example, N3 complex, N719 complex (N719 dye), Ru complex such as Ru terpyridine complex (black dye) and Ru diketonate complex, organic dyes such as coumarin dye, merocyanine dye and polyene dye, metal porphyrin dye and phthalocyanine dye are applicable. Among these dyes, the Ru complex is preferable, and specifically, N719 dye and black dye are especially preferable since they exert a wide absorption spectrum in the visible light range.

The dye can be used alone, or two or more dyes can be used in a mixture.

The above-described matters are common to the first, second and third embodiments and <FIG>, <FIG> and <FIG>. In the following description, matters common to the first to third embodiments but have different reference numbers assigned in the drawings will be described by referring to the different reference numbers in the drawings.

A first conductive film (<NUM> in <FIG> and <FIG>; <NUM> in <FIG>) is formed on an upper surface of the first substrate <NUM>. The first conductive film is preferably FTO (fluorine-doped tin oxide), but other than the FTO layer, for example, an indium - tin complex oxide (ITO) may be used.

A charge exchange layer (<NUM> in <FIG> and <FIG>; <NUM> in <FIG>) is formed on the first conductive film. A platinum (Pt) film is preferable as the charge exchange layer, but carbon electrode and conductive polymer may also be used instead of the platinum (Pt) film.

A first photovoltaic layer (<NUM> in <FIG>; <NUM> in <FIG>; <NUM> in <FIG>) is formed on the charge exchange layer.

In any of the first to third embodiments, a first photovoltaic layer is composed by dispersing silicon dioxide particles <NUM> as a first photovoltaic layer (<NUM> in <FIG>; <NUM> in <FIG>; <NUM> in <FIG>) on the charge exchange layer (<NUM> in <FIG> and <FIG>; <NUM> in <FIG>).

The silicon dioxide particles <NUM> that constitute the first photovoltaic layer (<NUM> in <FIG>; <NUM> in <FIG>; <NUM> in <FIG>) use glass particles formed for example of synthetic quartz, fused quartz glass, soda-lime glass, non-alkali glass or borosilicate glass, which are immersed in a solution of <NUM> - <NUM> % hydrofluoric acid or hydrochloric acid, washed with water and dried, and pulverized so that a major axis L of the particles is <NUM> to <NUM>. The first to third embodiments use synthetic quartz particles, which are crystalline of silicon dioxide, which are immersed in <NUM> % hydrofluoric solution, washed with water and dried, and pulverized so that a major axis L of the particles is <NUM> - <NUM>.

Electrolyte <NUM> is enclosed between the first photovoltaic layer (<NUM> in <FIG>; <NUM> in <FIG>; <NUM> in <FIG>) and the second photovoltaic layer <NUM>, in a space that is surrounded by a sealing member <NUM> on four sides. The electrolyte <NUM> is used in the prior-art dye-sensitized solar cells, and it can be of any of the following states; liquid, solid, coagulated and ordinary temperature molten salt.

The electrolyte can be, for example, a combination of metal iodide, such as lithium iodide, sodium iodide, potassium iodide and cesium iodide, and iodine; a combination of iodine salt of quaternary ammonium compound, such as tetraalkylammonium iodide, pyridinium iodide and imidazolium iodide, and iodine; a combination of bromine compound - bromine instead of the aforementioned iodine and iodine compound; or a combination of cobalt complex.

If the electrolyte is an ionic liquid, there is no need to use a solvent. The electrolyte may be a gel electrolyte, a high polymer electrolyte or a solid electrolyte, and an organic charge transport material may be used instead of the electrolyte.

If the electrolyte <NUM> is in a state of a solution, the solvent may be, for example, nitrile-based solvent such as acetonitrile, metoxyacetonitrile and propionitrile, carbonate-based solvent such as ethylene carbonate, and ether-based solvent.

Specifically, the electrolyte <NUM> used in the first to third embodiments is formed by adding <NUM> mol Lil, <NUM> mol I<NUM>, <NUM> mol <NUM>-tetra-butylpyridine and <NUM> mol tetrabutylammonium iodide in acetonitrile solvent.

The distance between the first photovoltaic layer (<NUM> in <FIG>; <NUM> in <FIG>; <NUM> in <FIG>) and the second photovoltaic layer <NUM> in the height direction should preferably be as short as possible, since transfer of charge becomes easier if the distance is shorter.

In the first to third embodiments, the thickness of the electrolyte <NUM> portion in the height direction, that is, the distance between the first photovoltaic layer (<NUM> in <FIG>; <NUM> in <FIG>; <NUM> in <FIG>) and the second photovoltaic layer <NUM> in the height direction, is <NUM> or smaller.

Method for evaluating the maximum output value per unit area according to the present specification is as described below.

An LED light (manufactured by Cosmotechno Co. ) was used to irradiate light from the second substrate side, and light corresponding to <NUM> lux by illuminometer DT-<NUM> manufactured by CEM Corporation was irradiated to the photovoltaic element being the target for measurement. A digital multimeter was used to measure the I-V characteristics of the photovoltaic element as the target for measurement, by which values of short circuit current, open circuit voltage and form factor ff were acquired, and the maximum output value per unit area was derived.

Hereafter, characteristics of the present embodiments will be described with reference to the drawings. The other portions are similar to the description regarding the matters common to the first to third embodiments described above.

<FIG> is a view illustrating a first embodiment not representing the present invention. In the first embodiment, silicon dioxide particles having an average major axis L of <NUM> - <NUM> are used as the silicon dioxide particles <NUM> used in the first photovoltaic layer <NUM>. These silicon dioxide particles <NUM> are dispersed in an overlapped manner on a flat first conductive film <NUM> (FTO layer) and a similarly flat charge exchange layer <NUM> (Pt layer) formed thereon, by which the first photovoltaic layer <NUM> having a thickness of <NUM> to <NUM> in the height direction is composed.

Other conditions are as described as matters common to the first to third embodiments.

As a result, the embodiment realizes a significant improvement of photovoltaic efficiency compared to the prior art example described in the background art.

In the first embodiment, the average major axis of the silicon dioxide particles <NUM> is small compared to the prior art, which is considered effective in increasing the surface area of the silicon dioxide particles <NUM> in the first photovoltaic layer <NUM> and raising the photovoltaic efficiency.

<FIG> is a view illustrating a second embodiment not representing the present invention. The second embodiment uses the same materials and the like used in the first embodiment. However, in the second embodiment, a first photovoltaic layer <NUM> is composed so that the silicon dioxide particles <NUM> are arranged on a flat first conductive film <NUM> and a similarly flat charge exchange layer <NUM> disposed thereon, so that the thickness thereof in the height direction is <NUM> or smaller.

That is, the thickness of the first photovoltaic layer in the height direction is reduced compared to the first embodiment.

<FIG> is an enlarged view of portion A of <FIG>, wherein the silicon dioxide particles <NUM> constituting the first photovoltaic layer <NUM> are dispersed on the flat first conductive film <NUM> (FTO layer) and the similarly flat charge exchange layer <NUM> (Pt layer) formed thereon, in a state where there is small overlap of particles.

In the second embodiment, the overlapping of the silicon dioxide particles <NUM> in the first photovoltaic layer <NUM> is reduced, according to which the property of charge transfer near the first photovoltaic layer <NUM> is enhanced, by which the photovoltaic efficiency is considered to be increased.

Therefore, it is important not to arrange too much silicon dioxide particles <NUM> on the upper surface of the charge exchange layer <NUM> in order to improve the photovoltaic efficiency. That is, it has been confirmed that the photovoltaic amount is increased if the silicon dioxide particles <NUM> are not excessively overlapped and sufficient space is formed therebetween.

Therefore, the thickness of the first photovoltaic layer <NUM> in the height direction should preferably be equal to or smaller than three times the average major axis L of the silicon dioxide particles.

The silicon dioxide particles <NUM> should preferably be arranged on the surface of an upper layer of the charge exchange layer <NUM> in a dispersed manner with spaces formed therebetween. This arrangement is to prevent the silicon dioxide particles <NUM> from being arranged in an overcrowded manner and hindering conductivity between the charge exchange layer <NUM>, the silicon dioxide particles <NUM> and the electrolyte <NUM>. It is preferable that the charge exchange layer <NUM>, the silicon dioxide particles <NUM> and the electrolyte <NUM> are arranged with sufficient allowance, so that the total sum of contact surface areas of the charge exchange layer <NUM>, the silicon dioxide particles <NUM> and the electrolyte <NUM> that perform charge exchange is maximized.

Therefore, the photovoltaic amount can be increased by arranging the silicon dioxide particles <NUM> in the first photovoltaic layer <NUM> such that the charge exchange layer <NUM> is visible through the spaces between the silicon dioxide particles <NUM> when the first substrate <NUM> is viewed from the second substrate <NUM> side.

<FIG> is a view illustrating a third embodiment according to the present invention. The third embodiment uses the same materials and the like as the first embodiment. However, in the third embodiment, a first conductive film <NUM> (FTO layer) and a charge exchange layer <NUM> (Pt layer) that constitute a base on which the silicon dioxide particles <NUM> are arranged are not flat. As illustrated in <FIG>, the first conductive film <NUM> has an uneven surface (roughness or asperity), with a height difference of approximately <NUM> formed on the surface. The charge exchange layer <NUM> formed on the first conductive film <NUM> also has a roughness on the surface, influenced by the height difference formed on the first conductive film <NUM>.

<FIG> is an enlarged view of portion B of <FIG>. The silicon dioxide particles <NUM> constituting the first photovoltaic layer <NUM> are dispersed on the first conductive film <NUM> that has a roughness on the surface and the charge exchange layer <NUM> formed thereon and having a similar roughness, in a state where there is small overlap of particles.

The difference of height of the surface roughness of the first conductive film <NUM> should be <NUM> or greater, and more preferably, <NUM> or greater. Further, it is preferable that the charge exchange layer <NUM> formed on the first conductive film <NUM> is formed in a manner maintaining the shape of the roughness on the surface of the first conductive film <NUM> without burying the surface roughness of the first conductive film <NUM>.

As a result, the embodiment realizes an even further significant improvement of photovoltaic efficiency compared to the prior art example described in the background art.

The arrangement of the silicon dioxide particles <NUM> dispersed on the charge exchange layer <NUM> formed on the first conductive film <NUM> is influenced by the surface roughness of the first conductive film <NUM> and charge exchange layer <NUM> as base layers.

Thanks to the surface roughness of the base layers, the silicon dioxide particles <NUM> are arranged in a thinly dispersed manner. Thereby, the silicon dioxide particles <NUM> are arranged with appropriate spatial allowance without excessive overlap, and therefore, the increase of photovoltaic amount is confirmed.

<FIG> is a schematic diagram in which the first conductive film <NUM> is illustrated from bird's eye view. The shape of the surface roughness of the first conductive film <NUM> is not only risen steeply, as illustrated in <FIG>, but may also include a structure <NUM> where the surface is somewhat rounded, as illustrated in <FIG>. Further, the roughness does not have to be random, as illustrated in <FIG> and <FIG>, and the roughness can be regularly arranged shapes, such as structural cones, trigonal pyramids, quadrangular pyramids and other pyramid shapes.

In the first to third embodiments, various modifications are possible. For example, the optimum average major axis of the silicon dioxide particles <NUM> may vary according to the distribution of size and shape of the silicon dioxide particles <NUM> constituting the first photovoltaic layer. Similarly, the optimum value of thickness of the first conductive film in the height direction may vary according to the distribution of size and shape of the silicon dioxide particles <NUM>.

Claim 1:
A photovoltaic element comprising a first photovoltaic layer (<NUM>),
wherein the first photovoltaic layer (<NUM>) is composed of silicon dioxide particles (<NUM>), and
the silicon dioxide particles (<NUM>) are arranged on a charge exchange layer (<NUM>) that has a roughness in a height direction,
wherein the charge exchange layer (<NUM>) that has a roughness in a height direction is formed on an upper surface of a first conductive film (<NUM>) that has a roughness in a height direction,
wherein the photovoltaic element further comprises a second photovoltaic layer (<NUM>) composed of a substance selected from TiO<NUM>, SnO, ZnO, WO<NUM>, Nb<NUM>O<NUM>, In<NUM>O<NUM>, ZrO<NUM>, Ta<NUM>O<NUM> and TiSrO<NUM>
wherein
the roughness of the charge exchange layer (<NUM>) in the height direction is <NUM> or greater, and characterized in that
the roughness of the first conductive film (<NUM>) in the height direction is <NUM> or greater.