Patent Description:
A photovoltaic device is known in which ZnO nanorods, which are inorganic semiconductors, are provided as columnar structures inside P3HT, which is an organic semiconductor (Non-patent Document <NUM>).

<CIT> aims to provide an apparatus for generating electrical energy that includes; a first electrode, and a second electrode spaced apart from the first electrode, and an energy generation layer disposed between the first electrode and the second electrode, wherein the energy generation layer comprises a photoelectric conversion layer and a plurality of piezoelectric nanowires, and wherein when an external force is applied to at least one of the first electrode and the second electrode, the plurality of piezoelectric nanowires are transformed to generate electrical energy.

<CIT> aims to provide an organic solar cell includes; a cathode, an anode disposed substantially opposite the cathode, a photoactive layer disposed between the cathode and the anode, wherein the photoactive layer includes an electron donor, an electron acceptor, and a nanostructure, and wherein the nanostructure includes an electron conductive material selected from the group consisting of a semiconductor element, a semiconductor compound, a semiconductor carbon material, a metallic carbon material which is surface-treated with a hole blocking material, a metal which is surface-treated with a hole blocking material and a combination thereof.

<CIT> aims to provide an electronic semiconductor device comprising nanowires with a built-in piezoelectric field is provided. The nanowires are used for photon harvesting, whereby a built-in piezoelectric field in the nanowires replaces the electric field of a pn-junction, which is used in conventional photovoltaic cells. The piezoelectric field is induced by the combination of lattice-mismatched materials into a core-shell geometry.

<CIT> discloses a solar cell for circuit system, which comprises organic materials including donor material and acceptor material, and inorganic materials, collectively forming hybrid heterojunction structures, according to the preamble of claim <NUM>.

However, in the conventional photovoltaic device described above, there is the problem of low conversion efficiency due to low exciton dissociation efficiency at the donor/acceptor interface.

The problem to be solved by the present invention is to provide a photovoltaic device with enhanced conversion efficiency.

To solve the problem described above, the present invention provides a photovoltaic device comprising:.

Furthermore, the present invention provides embodiments as defined in the appended dependent claims.

The present invention realizes the effect of providing a photovoltaic device with improved conversion efficiency.

Embodiments of the present invention will be described below based on the drawings. Reference examples will also be disclosed.

The photovoltaic device according to this reference example is a solar cell with a hybrid structure in which an organic semiconductor and an inorganic semiconductor are combined. The photovoltaic device according to this reference example can find applications in which both high conversion efficiency and low cost are required, such as in automobiles. For example, when solar cells are used in an automobile, it is conceivable to install the solar cells in the body of the automobile. When solar cells are set in an ordinary passenger vehicle, the installable area for the solar cells is about <NUM><NUM>. Thus, in order to use the electromotive force of the solar cells as a driving force for the vehicle, highly efficient solar cells are required.

Numerous solar cells have been developed heretofore, such as those listed below. For example, solar cells that are used in artificial satellites are tandem-type (multijunction-type) cells made of Group III-V monocrystalline semiconductors, such as GaAs. The energy conversion efficiency of these solar cells exceeds <NUM>%, realizing high efficiency. However, there is the problem of extremely high cost. In addition, other examples of highly efficient solar cells, solar cells formed mainly from crystalline silicon (Si), CIS (Cu-In-Se: copper-iridium-selenium), or the like, are sold for domestic purposes. The conversion efficiency of these types of solar cells are at best about <NUM>%. However, their cost is still high.

Examples of low-cost solar cells include amorphous silicon thin-film solar cells and organic thin-film solar cells. Amorphous silicon solar cells have higher photoabsorption coefficients compared to crystalline silicon. An amorphous silicon solar cell can be formed as a thin film with a thickness of about <NUM> on a non-crystalline substrate, such as a glass substrate. Since non-crystalline substrates are inexpensive, the overall cost of the solar cell can be suppressed. Organic thin-film solar cells can be produced at low cost since organic semiconductors, the material from which such cells are made, are inexpensive and can be prepared by means of a coating process, etc., without using high vacuum; however, with conversion efficiencies of about <NUM>% in the research-and-development stage, which requires further improvement, there are many problems yet to be solved to achieve the generally accepted <NUM> ¥/kWh power generation cost target for solar cells.

Since solar cells do not discharge carbon dioxide during power generation and can be used as a means of generating power for distribution to electric power consumption sites, solar cells are applied to general households, buildings, artificial satellites, and various electrical products. Moreover, depending on the field of application, various solar cells are required to have different capabilities.

For example, in the case of artificial satellites, although extremely high conversion efficiencies are required, there is demand even at a relatively high cost; therefore, it is possible to use solar cells made from a Group III-V monocrystalline semiconductor, such as GaAs, as described above.

For example, since large installation areas of several tens of square meters can be realized and the electricity that is generated can be sold to electric power companies, solar cells of crystalline silicon, CIS, etc., for home-use, and organic thin-film solar cells that are expected to be installed in building windows, or the like are in demand, even at current prices and with conversion efficiencies of about <NUM>-<NUM>%. And although the output of amorphous silicon thin-film solar cells is low, due to their low cost, such cells are in demand for electronic products that consume small amounts of power, such as calculators and wristwatches.

As described above, conventional solar cells, such as those described above, can find application as long as the requirements emphasize either conversion efficiency or cost. However, conventional solar cells do not satisfy demand to achieve both high conversion efficiency and low cost, such as in the automotive field. The photovoltaic device according to this reference example is a solar cell that achieves both high conversion efficiency and low cost, and has the following configuration.

<FIG> is a cross-sectional view of a photovoltaic device according to this reference example. <FIG> is an energy diagram in a cross section taken through line II-II of <FIG>. The horizontal axis (X) in <FIG> indicates the same position as the x axis in <FIG>, and the vertical axis (E) represents the energy magnitude.

The photovoltaic device comprises an organic semiconductor <NUM>, an inorganic semiconductor <NUM>, an anode electrode <NUM>, and a cathode electrode <NUM>. The organic semiconductor <NUM> is formed from an organic material and has a photoactive region <NUM>. The organic semiconductor <NUM> is formed in a layer shape along the electrode surface of the anode electrode <NUM> and the electrode surface of the cathode electrode <NUM>. The photoactive region <NUM> is a region that generates excitons by means of light from outside of the semiconductor element.

Examples of materials that form the photoactive region, that is, the organic materials included in the organic semiconductor include at least one selected from a group comprising P3HT, P3OT, P3DDT, PTAA, MEH-PPV, MDMO-PPV, F8BT, F8T2, POT-co-DOT, p-DTS (FBTTh <NUM>) <NUM>, DR3TSBDT, Pd (PPh) <NUM>, benzoporphyrin, tetrabenzoporphyrin, phthalocyanine, tetracene, anthracene, triphenylene, pyrene, chrysene, tetraphen, perylene, coronene, hexabenzocoronene, PDI, PDITh, PC<NUM>BM, PC<NUM>BM, PC<NUM>BM, PC<NUM>BM, PC<NUM>BM, bis PCS<NUM>BM, PCBB, PCBO, PNTz4T, PNOz4T, ThC60BM, d5-PCBM, SIMEF, PEDOT : PSS, MADN, N719, N3, N907, YD2-o-C8, MK-<NUM>, MK-<NUM>, TA-St-CA, MR-<NUM>, MR-<NUM>, MR-<NUM>, and derivatives thereof. Derivatives include compounds derived from the molecular skeletons of the substances listed above.

The inorganic semiconductor <NUM> is formed from an inorganic material and has dissociation regions <NUM>. In addition, the inorganic semiconductor <NUM> has piezoelectric properties. That is, the inorganic semiconductor <NUM> is an inorganic piezoelectric material. The inorganic semiconductor <NUM> is formed in a columnar shape in the organic semiconductor <NUM>. The dissociation regions <NUM> included in the inorganic semiconductor <NUM> extend from the electrode surface of the cathode electrode <NUM> toward the anode electrode <NUM> and are formed in a columnar shape. The distal end portions of the dissociation regions <NUM> (the distal end portions positioned opposite to the cathode electrode when viewed in the direction of extension (y direction)) are not connected to the anode electrode <NUM> and are covered with an organic material. The end portions of the dissociation regions <NUM> positioned opposite to the distal end portions, on the other hand, are directly connected to the cathode electrode <NUM> and are also covered with the organic material. A plurality of the dissociation regions <NUM> are provided. The dissociation region <NUM> is a region in which the carriers contained in the excitons generated in the photoactive region are dissociated.

Examples of materials that form the dissociation region, that is, the inorganic materials included in the inorganic semiconductor <NUM> include at least one selected from a group comprising AIN, AIGaN, GaN, InGaN, InN, AlAs, AIGaAs, GaAs, InGaAs, InAs, AIP, AIGaP, GaP, GaAsP, GaAs, AIP, AlAsP, InAIAs, InAs, GaAsSb, GaSb, AlSb, AIGaSb, GaSb, AlSb, AlInSb, InSb, MgS, MgZnS, ZnS, MgS, MgSSe, MgSe, ZnS, ZnSSe, ZnSe, MgSe, MgZnSe, ZnSe, CuAlS<NUM>, CuAISSe, CuAlSe<NUM>, CuAlS<NUM>, CuGaAlS<NUM>, CuGaS<NUM>, CuGaSSe, CuGaSe<NUM>, CuInGaS<NUM>, CuInS<NUM>, CuInSSe, CuInSe<NUM>, CuGaSe<NUM>, CuInGaSe<NUM>, MgSe, MgZnSeTe, ZnTe, and derivatives thereof.

The anode electrode <NUM> is a positive electrode when the solar cell is viewed from the outside as a power source, and is formed on the front surface (upper surface) of the organic semiconductor <NUM>. The cathode electrode <NUM> is a negative electrode and is formed on the rear surface (lower surface) of the organic semiconductor <NUM>.

Next, the relationship between the energy levels at an interface <NUM> between the photoactive region <NUM> and the dissociation region <NUM> will be described with reference to <FIG>. In the example shown in <FIG>, the photoactive region <NUM> is a donor (p-type) and the dissociation region <NUM> is an acceptor (n-type). Excitons <NUM> are generated when the photoactive region <NUM> absorbs light, and some of the excitons <NUM> diffuse to the interface <NUM> between the photoactive region <NUM> and the dissociation region <NUM>.

If the LUMO energy level of the photoactive region <NUM> is ELUMO, the HOMO energy level of the photoactive region <NUM> is EHOMO, the energy level at the lower end of the conduction band of the dissociation region <NUM> is Ec, and the energy level at the upper end of the valence band of the dissociation region <NUM> is Ev, the relationship between each of the energy levels can be represented by the following equations (<NUM>) and (<NUM>). [Equation <NUM>] <MAT> [Equation <NUM>] <MAT>.

The energy of the inorganic semiconductor can be generally explained by the band concept of solid-state physics, using the lower end of the conduction band (hereinafter referred to as "conduction band" or "Ec" (meaning E-Conduction Band)), the upper end of the valence band (hereinafter referred to as "valence band" or "Ev" (meaning E-Valence Band)), and the Fermi level ("EF"). The energy of the organic semiconductor, on the other hand, can be generally explained by the frontier orbital theory, using LUMO (Lowest Unoccupied Molecular Orbital, hereinafter also referred to as ELUMO); and HOMO (Highest Occupied Molecular Orbital, hereinafter referred to as EHOMO), etc..

As indicated by equation (<NUM>) above, the LUMO energy level (ELUMO) of the photoactive region <NUM> is higher than the energy level (Ec) of the lower end of the conduction band of the dissociation region <NUM>. Thus, of the carriers contained in the excitons <NUM>, only the electrons <NUM> move to the dissociation region <NUM>. The HOMO energy level (EHOMO) of the photoactive region <NUM> is higher than the energy level (Ev) of the upper end of the valence band of the dissociation region <NUM>. Since an energy barrier is formed for the holes <NUM>, the holes <NUM> do not move to the dissociation region <NUM>. When the electrons <NUM> move to the dissociation region <NUM>, the electrons <NUM> are in the same state as charge transfer complexes at the interface <NUM>.

As described above, the dissociation region <NUM> is formed from an inorganic piezoelectric material. Thus, an electric field is generated in the dissociation region <NUM> due to the piezoelectric effect (piezo effect) of the crystals contained in the dissociation region <NUM>. The electric field is generated due to stress-strain caused by the difference in thermal expansion, the difference in the lattice constants, etc., between the materials. Alternatively, the electric field is generated due to stress based on thermal energy of the semiconductor that is heated by receiving light.

The electrons <NUM> receive a force in a direction away from the interface <NUM>. As shown in <FIG>, the energy level at the lower end of the conduction band of the dissociation region <NUM> falls smoothly toward the direction away from the interface <NUM>. In other words, the energy level of the lower end of the conduction band of the dissociation region <NUM> decreases gradually, with the energy level (Ec) at the interface <NUM> as the maximum energy. In particular, since the inorganic piezoelectric material has a high dielectric constant, it is possible to reduce the binding force that acts on the space between the electron and the hole due to the electric field. Therefore, it is possible to efficiently dissociate the excitons.

That is, an electric field is applied between the organic semiconductor <NUM>, which is a donor, and the inorganic semiconductor <NUM>, which is an acceptor, to promote the dissociation of the excitons. Furthermore, the electrons are able to move to the inorganic semiconductor <NUM>, and the energy level (Ec) of the lower end of the valence band is inclined at a prescribed inclination (the energy level (Ec) is not horizontal), due to the electric field. Thus, it is possible to increase the dissociation efficiency of the excitons.

The differences in the binding force due to the dielectric constant will be described while comparing the organic semiconductor and the inorganic semiconductor.

The relative dielectric constants of organic semiconductors are extremely low values, for example, <NUM> for P3HT and <NUM> for PCBM. The relative dielectric constants of inorganic semiconductors, on the other hand, are high, <NUM> for silicon and <NUM> for GaAs. Here, there is a force of attraction due to the Coulomb force between negatively charged electrons and positively charged holes. The Coulomb force is represented by the following equation (<NUM>):
[Equation <NUM>] <MAT> where F represents the Coulomb force, εr represents the relative dielectric constant, ε<NUM> represents the dielectric constant of vacuum, q<NUM> represents the electric charge (for example, of an electron), q<NUM> represents the electric charge (for example, of a hole), and r represents the electron-hole distance.

According to equation <NUM> above, to increase the dissociation efficiency of excitons, it is better to use the inorganic semiconductor with higher dielectric constant than the organic semiconductor. Thus, in the photovoltaic device according to this reference example, in order to further increase the dissociation efficiency while providing the dissociation region <NUM> in the inorganic semiconductor <NUM>, the dissociation region <NUM> is devised so that an electric field is generated therein.

That is, the photovoltaic device according to this reference example generates many excitations by providing the light diffusion region <NUM> in the organic semiconductor <NUM>, which has a high photoabsorption coefficient. In addition, the carriers contained in the excitons move beyond the interface while the excitons generated in the photoactive region are diffused at the interface <NUM>; therefore, it is configured such that a prescribed energy matching relationship exists between the organic semiconductor <NUM> and the inorganic semiconductor <NUM>.

Here, the material characteristics of the organic semiconductor and the material characteristics of the inorganic semiconductor when used in a solar cell will be described in detail.

<FIG> shows a cross-sectional view of an organic thin-film solar cell according to a first comparative example. The organic thin-film solar cell according to the first comparative example comprises a transparent substrate <NUM>, a transparent conductive film <NUM>, a buffer layer <NUM>, a photoactive region <NUM>, a buffer layer <NUM>, and a cathode electrode <NUM>.

The transparent substrate <NUM> is a substrate of glass, etc. The transparent conductive film <NUM> is an anode electrode. The buffer layer <NUM> is a layer through which only holes can selectively pass. The photoactive region <NUM> is formed from an organic semiconductor. The buffer layer <NUM> is a layer through which only electrons can selectively pass. The transparent conductive film <NUM>, the buffer layer <NUM>, the photoactive region <NUM>, the buffer layer <NUM>, and the cathode electrode <NUM> are stacked on the transparent substrate <NUM> in that order. Light enters from the transparent substrate <NUM>. Here, for the sake of simplicity, as an example, a donor-acceptor interface <NUM> (p-n interface) in the photoactive region is represented as a straight line for convenience as if it were a planar structure.

The incident light is absorbed by the photoactive region <NUM> and the excitation of electrons generates excitons <NUM>. Some of the excitons <NUM> reach the donor-acceptor interface <NUM> due to diffusion and become charge transfer complexes. Thereafter, the excitons are dissociated into electrons <NUM> and holes <NUM>, the electrons <NUM> reach the cathode electrode <NUM>, and the holes <NUM> reach the transparent conductive film <NUM>, which is the anode electrode, thereby being output to the outside as electric power.

<FIG> is an energy diagram of the organic thin-film solar cell according to the first comparative example. The horizontal axis (X) in <FIG> indicates the depth-direction (x direction) position of the organic thin-film solar cell according to the first comparative example, and the vertical axis (Y) represents the electron energy magnitude. For holes, which are opposite in polarity to electrons, the negative direction of the vertical axis represents high energy.

Light enters the photoactive region <NUM> and the electrons at the donor HOMO energy level <NUM> are excited to the LUMO energy level <NUM>, thereby forming holes <NUM> at the HOMO energy level <NUM> as missing electrons <NUM> and generating excitons <NUM>, which are electron-hole pairs. When some of the excitons reach the p-n interface <NUM>, due to the energy relationship between the HOMO energy level <NUM> and the LUMO energy level <NUM>, only the electrons <NUM> can move from the donor <NUM> to the acceptor <NUM>, whereas the holes <NUM> remain in the donor <NUM>, thereby forming a charge transfer complex state. Here, due to the energy difference between the pn, the excitons <NUM> are dissociated into the electrons <NUM> and the holes <NUM> and move to the cathode electrode <NUM> and the transparent conductive film (anode electrode) <NUM> respectively.

Organic semiconductors are generally in the form of polymers (polymers) or monomers (monomers); while polymers are relatively large in size, in which electrons can move relatively easily, compared to inorganic semiconductors, polymers are small. In order for electrons to pass through the photoactive region of organic semiconductors, the electrons must move between the polymers by means of hopping conduction with the aid of thermal energy. Since the main component of carrier diffusion of organic semiconductors is hopping conduction, the diffusion length of the organic semiconductor is short. Thus, the carriers (electrons and holes), generated as a result of dissociation from the excitons <NUM>, recombine before reaching the cathode electrode <NUM> and the transparent conductive film (anode electrode) <NUM>, as is illustrated in <FIG>, and the excitons and the carriers generated by disassociation of excitons relax to their original energy levels before being taken out as electric power. Therefore, the conversion efficiency of the organic thin-film solar cell according to the first comparative example is reduced. In addition, since the diffusion length of the organic semiconductor is short and it is difficult to extract carriers, it is difficult to make the photoactive region of the organic semiconductor about <NUM> or more.

The solar cell disclosed in Non-patent Document <NUM> (hereinafter referred to as the semiconductor device according to a second comparative example) is known as a solar cell that remedies the characteristic short diffusion length of organic semiconductors. The solar cell according to the second comparative example is configured from a structure wherein an inorganic semiconductor with a columnar or dendrite-like structure penetrates an organic semiconductor.

In general, inorganic semiconductors are in a crystalline state or an amorphous state, but adjacent atoms are in a state of chemical bonding, such as covalent bonding, and electrons and holes can move between atoms relatively easily. The carrier mobility is represented by the following equation (<NUM>):
[Equation <NUM>] <MAT>.

Here, L represents diffusion length, D represents the diffusion coefficient, τ represents carrier lifetime, kB represents Boltzmann's constant, T represents absolute temperature, q represents the electric charge element, and µ represents mobility.

Thus, the carrier mobility is high, and, as shown by equation (<NUM>), inorganic semiconductors have the characteristic that the carrier diffusion length long. Inorganic semiconductors, on the other hand, have the problem that the photoabsorption coefficient (absorbance) is lower than that of organic semiconductors.

The semiconductor device according to the second comparative example has a structure that utilizes both the above-described characteristic of organic semiconductors and that of inorganic semiconductors. <FIG> is an energy diagram of the semiconductor device according to the second comparative example.

The semiconductor device according to the second comparative example has an interface <NUM> between the photoactive region <NUM> provided in the organic semiconductor and the dissociation region <NUM> provided in an inorganic semiconductor, as illustrated in <FIG>. The LUMO energy level (ELUMO) of the photoactive region <NUM> is higher than the energy level (Ec) at the lower end of the conduction band of the dissociation region <NUM>.

Light enters the photoactive region <NUM> to generate excitons. When the excitons <NUM> reach the vicinity of the interface <NUM>, the generated electrons <NUM> move from the donor to the acceptor, and the holes <NUM> remain in the donor. The excitons <NUM> thus dissociate in the vicinity of the interface.

Because the second comparative example is configured such that an electric field is applied to the dissociation region, as in the present invention, the energy level at the lower end of the conduction band of the dissociation region <NUM> is flat, as shown in <FIG>. Thus, the dissociation efficiency is low. In addition, the electrons near the interface <NUM> cannot easily move to the electrode. That is, in the comparative example, there is no electric field to separate the electron-hole pairs. In addition, even if the excitons <NUM> are dissociated once due to the energy difference near the interface <NUM>, the energy levels thereof do not further promote dissociation. As a result, the semiconductor device according to the second comparative example also has the problem of low conversion efficiency.

As described above, the photovoltaic device according to this reference example comprises the organic semiconductor <NUM> that includes the photoactive region <NUM> and the inorganic semiconductor <NUM> with piezoelectricity that includes the dissociation region <NUM>. The relationship of the energy levels between the photoactive region <NUM> and the dissociation region <NUM> satisfies equation (<NUM>) above. Thus, light absorption and the generation of excitons can be efficiently performed through the use of a high-absorption photoactive region. It is possible to realize a state in which the dielectric constant is high (the binding force between the electrons and the holes is weak) and the electrons contained in the excitons move to the dissociation region. In addition, the exciton dissociation efficiency can be increased by the electric field generated in the dissociation region <NUM>. As a result, it is possible to increase the conversion efficiency.

In this reference example, instead of making the LUMO energy level (ELUMO) of the photoactive region <NUM> higher than the energy level (Ec) at the lower end of the conduction band of the dissociation region <NUM>, the HOMO energy level (EHOMO) of the photoactive region <NUM> may be made lower than the energy level (Ev) of the upper end of the valence band of the dissociation region <NUM>. That is, the organic semiconductor <NUM> and the inorganic semiconductor <NUM> may be configured such that the energy relationships between the energy levels (ELUMO, EHOMO, Ec, Ev) satisfy the following equations (<NUM>) and (<NUM>):
[Equation <NUM>] <MAT> [Equation <NUM>] <MAT>.

As is indicated by the equation (<NUM>) above, the HOMO energy level (EHOMO) of the photoactive region <NUM> is higher than the energy level (Ev) at the upper end of the valence band of the dissociation region <NUM>. Thus, of the carriers contained in the excitons <NUM>, only the holes <NUM> move to the dissociation region <NUM>. The LUMO energy level (ELUMO) of the photoactive region <NUM> is higher than the energy level (Ec) at the lower end of the conduction band of the dissociation region <NUM>. Since an energy barrier is formed for the electrons <NUM>, the electrons <NUM> do not move to the dissociation region <NUM>. Then, due to the electric field that is generated in the dissociation region <NUM>, the holes <NUM> receive a force in a direction away from the interface <NUM>. It is thereby possible to increase the conversion efficiency while increasing the dissociation efficiency of the excitons.

In addition, in this reference example, the dissociation region <NUM> is surrounded by the organic material that forms the organic semiconductor <NUM>, and a part of the dissociation region <NUM> is directly connected to the cathode electrode <NUM>. Thus, since the carriers are extracted from the inorganic semiconductor that has a long diffusion length, the carriers can be extracted efficiently. In addition, the film thickness of the photoactive region <NUM>, which was restricted by the diffusion length, can be increased up to about the diffusion length of the dissociation region <NUM>.

In this reference example, the inorganic semiconductor <NUM> may include the photoactive region <NUM> in addition to the dissociation region <NUM>. Of the light incident on the organic semiconductor <NUM>, part of the light passes through the organic semiconductor <NUM>. Since the inorganic semiconductor <NUM> has the function of the photoactive region <NUM>, the light that passes through the organic semiconductor <NUM> can be used for power generation. Thus, the conversion efficiency can be improved.

In regard to the energy level conditions, the condition represented by equation (<NUM>) is a conditional equation for the behavior of the electrons. Equation (<NUM>) shows that the relationship of the energy level of the dissociation region <NUM> with respect to the photoactive region <NUM> is low in energy as seen from the energy level of electrons. In addition, the condition represented by equation (<NUM>) is a conditional equation for the behavior of the holes. Equation (<NUM>) shows that the relationship of the energy level of the dissociation region <NUM> with respect to the photoactive region <NUM> is low in energy as seen from the energy level of holes. That is, with respect to the ordinate of the graph shown in <FIG>, the positive direction of the E axis represents the high energy side with respect to the energy with respect to electrons, and the negative direction of the E axis represents the high energy side with respect to the energy of the holes.

<FIG> is a cross-sectional view of the photovoltaic device according to an embodiment of the invention. In this embodiment, the configuration of the dissociation regions <NUM>, <NUM> and the relationship of the energy levels are different from those of the reference example <NUM> described. The other configurations are the same as those of the above-described reference example <NUM>, and the descriptions thereof are incorporated by reference.

The photovoltaic device according to this embodiment comprises the organic semiconductor <NUM>, the inorganic semiconductor <NUM>, the anode electrode <NUM>, and the cathode electrode <NUM>.

The inorganic semiconductor <NUM> has a plurality of the dissociation regions <NUM> and of the dissociation regions <NUM>. The dissociation regions <NUM>, <NUM> are formed in a columnar shape in the organic semiconductor <NUM>. The shape of the dissociation regions <NUM>, <NUM> is needle-like (nanowire), columnar (nanorod), or circular (nanoparticle), etc..

The dissociation regions <NUM>, columnar in form, extend from the electrode surface of the cathode electrode <NUM> toward the anode electrode <NUM>. The distal end portions of the dissociation regions <NUM> (the distal end portions positioned opposite to the cathode electrode <NUM> when viewed in the direction of extension) are not connected to the anode electrode <NUM> and are covered with an organic material. The end portions of the dissociation regions <NUM> positioned opposite to the distal end portions, on the other hand, are directly connected to the cathode electrode <NUM> and are also covered with the organic material. The dissociation regions <NUM> are regions through which only electrons can pass.

The dissociation regions <NUM>, columnar in form, extend from the electrode surface of the anode electrode <NUM> toward the cathode electrode <NUM>. The distal end portions of the dissociation regions <NUM> (the distal end portions positioned opposite to the anode electrode <NUM> when viewed in the direction of extension (y direction)) are not connected to the cathode electrode <NUM> and are covered with an organic material. The end portions of the dissociation regions <NUM> positioned opposite to the distal end portions, on the other hand, are directly connected to the anode electrode <NUM> and are also covered with the organic material. The dissociation regions <NUM> are regions through which only holes can pass.

The dissociation regions <NUM>, <NUM> are configured to have shapes that are continuous toward the respective electrodes. In addition, the dissociation regions <NUM>, <NUM> are configured so that the angle between the direction of extension and the direction along the connection surface of each electrode is within <NUM>°. If the inorganic material contained in the dissociation regions <NUM>, <NUM> is discontinuous, the carriers must pass through the organic semiconductors by means of hopping conduction, so that the diffusion length becomes short. In this embodiment, since the dissociation regions <NUM>, <NUM> have a continuous shape, it is possible to increase the diffusion length.

The plurality of the dissociation regions <NUM> and of the dissociation regions <NUM> are alternately arranged side by side in a direction orthogonal to the direction of extension of each region (direction along the electrode surfaces of the anode electrode <NUM> and the cathode electrode <NUM>: x direction). It should be noted that the plurality of dissociation regions <NUM> and the plurality of the dissociation regions <NUM> need not be alternately arranged for each region. The order of arrangement of the dissociation regions may be such that, for example, three dissociation regions <NUM> are arranged between two dissociation regions <NUM>. Additionally, the plurality of dissociation regions <NUM> and the plurality of the dissociation regions <NUM> need not be arranged in an aligned state.

The relationship between the energy levels at interfaces <NUM>, <NUM> between the photoactive region <NUM> and the dissociation regions <NUM>, <NUM> will be described with reference to <FIG> is an energy diagram in a cross section taken through line VI-VI of <FIG>. The horizontal axis (X) in <FIG> indicates the same position as the x axis in <FIG>, and the vertical axis (E) represents the energy magnitude.

The relationship of the energy levels at the interface <NUM> between the photoactive region <NUM> and the dissociation region <NUM> will now be described. If the LUMO energy level of the photoactive region <NUM> is ELUMO, the HOMO energy level of the photoactive region <NUM> is EHOMO, the energy level at the lower end of the conduction band of the dissociation region <NUM> is EC1, and the energy level of the upper end of a valence band of the dissociation regions <NUM> is EV1, the relationship between each of the energy levels can be represented by the following equations (<NUM>) and (<NUM>):
[Equation <NUM>] <MAT> [Equation <NUM>] <MAT>.

The relationship of the energy levels at the interface <NUM> between the photoactive region <NUM> and the dissociation regions <NUM> will now be described. If the energy level at the lower end of the conduction band of the dissociation regions <NUM> is EC2 and the energy level at the upper end of the valence band of the dissociation regions <NUM> is EV2, the relationship between each of the energy levels can be represented by the following equations (<NUM>) and (<NUM>): <MAT>.

Since the interface <NUM> and the interface <NUM> are heterojunction interfaces, which are junctions between dissimilar materials, an energy difference is generated between the LUMO and the conduction band or between the valence band and the HOMO. In the solar cell, since the energy difference at the interfaces <NUM>, <NUM> causes a loss in voltage, the energy difference is made small. On the other hand, when the energy difference is small, some of the carriers may flow in the reverse, undesired direction, causing a reduction in the conversion efficiency. That is, in order to ensure a forward flow of carriers in the desired direction while preventing a reverse flow of carriers, it is preferable to set the energy difference to be within a prescribed range or up to a prescribed value.

A preferred range for the energy difference will be described with reference to <FIG> is a graph illustrating the relationship between the energy dependence of the carrier distribution and the cumulative frequency of the same distribution after the Fermi level. The energy dependence of the carrier distribution is calculated from the Fermi-Dirac distribution function and the state density under the condition of <NUM> (<NUM>) on the energy side above the Fermi level.

The frequency distribution of electrons with respect to energy in an object, independently of whether a material is organic or inorganic, follows the Fermi-Dirac distribution, except in those special cases when superconduction occurs. The cumulative frequency is <NUM>% at <NUM> eV with respect to the Fermi level, which means that at least <NUM>% of carriers are present within <NUM> eV from the Fermi level.

For example, the cumulative frequency is about <NUM>% at <NUM> eV above the Fermi level. If the Fermi level of the dissociation region of the excitons is present in the vicinity of Ec (assuming a position of virtually <NUM> eV below Ec) and the difference between the ELUMO of the photoactive region and Ec of the dissociation region is ELUMO-EC = <NUM> eV, this indicates that half of the carriers can flow backwards. Even in p-type semiconductors, the Fermi level is present in a position of about <NUM> to <NUM> eV below Ec; therefore, the cumulative frequency is about <NUM>% (value at ELUMO-EC = <NUM> eV in <FIG>) or more even at ELUMO-EC = <NUM> eV, which is a practically applicable level.

That is, ELUMO-EC is optimally <NUM> to <NUM> eV in order to sufficiently reduce the voltage loss that occurs due to the energy difference between ELUMO and Ec.

In solar cells, it is possible to take out the generated electrons and holes to the outside as electric power by taking the electrons and holes out from the different electrodes (anode and cathode), but the reverse carrier flow causes a reduction in the conversion efficiency of the solar cell. As described using the Fermi-Dirac distribution function illustrated in <FIG>, about <NUM>% or more of the electrons and holes are distributed within <NUM> eV from the Fermi level even at <NUM>. Thus, by forming a barrier of at least <NUM> eV, preferably at least <NUM> eV, between the photoactive region EHOMO and the valence band EV1 of the dissociation regions <NUM> connected to the cathode electrode <NUM>, the reverse flow of holes can be prevented. Similarly, for the electrons, by forming a barrier of at least <NUM> eV, preferably at least <NUM> eV, between the photoactive region ELUMO and the conduction band EC1 of the dissociation regions <NUM> connected to the anode electrode <NUM>, the reverse flow of electrons can be prevented.

As described above, in this embodiment, the relationships of the energy levels between the photoactive region <NUM> and the dissociation regions <NUM>, <NUM> satisfy the following equations (<NUM>) and (<NUM>):
[Equation <NUM>] <MAT> [Equation <NUM>] <MAT>.

Thus, light absorption and the generation of excitons can be performed efficiently through the use of a high-absorption photoactive region. It is possible to realize a state in which the dielectric constant is high (the binding force between the electrons and the holes is weak) and the electrons contained in the excitons move to the dissociation regions <NUM> and the holes contained in the excitons move to the dissociation regions <NUM>. In addition, the exciton dissociation efficiency can be increased due to the electric field generated in the dissociation regions <NUM>, <NUM>. As a result, it is possible to increase the conversion efficiency.

In addition, in this embodiment, the relationship of the energy levels at the interface <NUM> satisfies the equation (<NUM>) above. It is thereby possible to prevent the reverse flow of electrons and to reduce the voltage loss at the interface <NUM>.

In addition, in this embodiment, the relationship of the energy levels at the interface <NUM> satisfies the equation (<NUM>) above. It is thereby possible to prevent the reverse flow of holes and to reduce the voltage loss at the interface <NUM>.

In addition, this embodiment includes the dissociation regions <NUM>, through which, from among the carriers, electrons are allowed to pass through, and the dissociation regions <NUM>, through which, from among the carriers, holes are allowed to pass through; and the dissociation regions <NUM>, <NUM> are surrounded by the organic material that forms the organic semiconductor <NUM>, parts of the dissociation regions <NUM> are directly connected to the electrode <NUM>, and parts of the dissociation regions <NUM> are directly connected to the electrode <NUM>. Efficient carrier extraction can be performed since the electrons and holes pass through regions with a long diffusion length.

As a modified example, the photovoltaic device may be configured to include only one of the dissociation regions <NUM> or the dissociation regions <NUM>. <FIG> is a cross-sectional view of the photovoltaic device having the dissociation regions <NUM>, and <FIG> is a cross-sectional view of the photovoltaic device having the dissociation regions <NUM>.

The distal end portions of the dissociation regions <NUM> are not connected to the anode electrode <NUM> and are covered with the organic material, as illustrated in <FIG>. The end portions of the dissociation regions <NUM> positioned opposite to the distal end portions, on the other hand, are directly connected to the cathode electrode <NUM> and are also covered with the organic material.

The distal end portions of the dissociation regions <NUM> are not connected to the cathode electrode <NUM> and are covered with the organic material, as illustrated in <FIG>. The end portions of the dissociation regions <NUM> positioned opposite to the distal end portions, on the other hand, are directly connected to the anode electrode <NUM> and are also covered with the organic material.

In this modified example, since the carriers are extracted from the inorganic semiconductor that has a long diffusion length, efficient carrier extraction is possible. In addition, the film thickness of the photoactive region <NUM>, which was restricted by the diffusion length, can be increased up to about the diffusion length of the dissociation region <NUM>.

<FIG> is a cross-sectional view of the photovoltaic device according to another reference example. This reference example is different from the reference example <NUM> in the length of the intervals between the plurality of the dissociation regions <NUM>. The other configurations are the same as the above-described reference example <NUM>, and the descriptions thereof are incorporated by reference.

The dissociation region 21a faces the dissociation region 21b across the organic semiconductor <NUM> in the x direction, as is illustrated in <FIG>. An opposing surface 21A and an opposing surface 21B are arranged such that the surfaces thereof face each other across a distance D. The opposing surface 21A is the surface of the dissociation region 21a that faces the dissociation region 21b. The opposing surface 21B is the surface of the dissociation region 21b that faces the dissociation region 21a. The opposing surfaces 21A, 21B are surfaces along the direction of extension (y direction) of the dissociation regions 21a, 21b. The distance D is within twice the diffusion length of the excitons.

The diffusion length of the organic semiconductor is from several nm to about <NUM> in many materials. The excitons generated in the organic semiconductor <NUM> diffuse to the left and right with a length equivalent to the diffusion length. That is, if the donor-acceptor interface (p-n interface) does not exist in an interval of within twice the diffusion length, the electrons and holes of the excitons will recombine, causing energy loss.

In this reference example, the dissociation regions 21a, 21b are arranged such that the distance D is within twice the diffusion length of the excitons. It is thereby possible to increase the dissociation efficiency of the excitons while suppressing the recombination of excitons.

<FIG> is a cross-sectional view of the photovoltaic device according to another reference example. <FIG> is an energy diagram in a cross section taken through line XII-XII of <FIG>. The horizontal axis (X) in <FIG> indicates the same position as the x axis in <FIG>, and the vertical axis (E) represents the energy magnitude. This reference example differs from the reference example <NUM> in the configuration of the photoactive region <NUM>. The other configurations are the same as the above-described reference example <NUM>, and the descriptions of the reference examples <NUM>, <NUM> and the embodiment <NUM> are incorporated where appropriate.

In the photovoltaic device according to this reference example, the photoactive region <NUM> is formed by mixing an organic semiconductor donor (p-type) and an organic semiconductor acceptor (n-type) to form a bulk heterojunction in which the donor-acceptor interface is distributed over the entire region. The interface <NUM> is an interface between the photoactive region <NUM> and the dissociation regions <NUM>. The interface <NUM> is a bulk heterojunction interface in the photoactive region <NUM>.

As shown in <FIG>, of the plurality of excitons <NUM> generated by receiving light, an exciton 50a reaches the interface <NUM> and dissociates. The exciton 50b that is not directed toward the interface <NUM> reaches the interface <NUM> and is thereby dissociated. It is thus possible to increase the conversion efficiency.

As described above, in this reference example, the photoactive region includes the bulk heterojunction. As a result, the excitons that have not been directed toward the interface <NUM> between the photoactive region <NUM> and the dissociation regions <NUM> reach the interface <NUM> existing in the bulk heterojunction, whereby the excitons are dissociated. Thus, the conversion efficiency can be enhanced.

<FIG> is a cross-sectional view of the photovoltaic device according to another embodiment of the invention. <FIG> is an energy diagram in a cross section taken through line XIV-XIV of <FIG>. The horizontal axis (X) in <FIG> indicates the same position as the x axis in <FIG>, and the vertical axis (E) represents energy magnitude. This embodiment demonstrates in detail the reference example <NUM> in the configuration of the dissociation regions <NUM>. The other configurations are the same as the above-described reference example <NUM>, and the descriptions of the reference examples <NUM>, <NUM>, <NUM> and the embodiment <NUM> are incorporated where appropriate.

The dissociation regions <NUM> according to this embodiment are formed from two types of materials having different lattice constants. The dissociation regions <NUM> includes a dissociation layer 21c formed from a first type of material and a dissociation layer 21d formed from a second type of material. The dissociation layer 21c is formed in a columnar shape. The dissociation layer 21d is formed so as to cover the outer periphery of the dissociation layer 21c. There may be three or more types of materials included in the dissociation regions <NUM>.

The combination of the material of the dissociation layer 21c and the material of the dissociation layer 21d is at least one selected from AIN/AIGaN, AIGaN/GaN, GaN/InGaN, InGaN/InN, AIAs/AIGaAs, AIGaAs/GaAs, GaAs/InGaAs, InGaAs/InAs, AIP/AIGaP, AIGaP/GaP, GaP/GaAsP, GaAsP/GaAs, AIP/AIAsP, AIAsP/AIAs, AIAs/InAIAs, InAlAs/InAs, GaAs/GaAsSb, GaAsSb/GaSb, AISb/AIGaSb, AIGaSb/GaSb, AlSb/AlInSb, AlInSb/InSb, MgS/MgZnS, MgZnS/ZnS, MgS/MgSSe, MgSSe/MgSe, ZnS/ZnSSe, ZnSSe/ZnSe, MgSe/MgZnSe, MgZnSe/ZnSe, CuAlS<NUM>/CuAlSSe, CuAlSSe/CuAlSe<NUM>, CuAlS<NUM>/CuGaAlS<NUM>, CuGaAlS<NUM>/CuGaAlS<NUM>,CuGaS<NUM>/CuGaSSe, CuGaSSe/CuGaSe<NUM>, CuGaS<NUM>/CuInGaS<NUM>, CuInGaS<NUM>/CuInS<NUM>, CuInS<NUM>/CuInSSe, CuInSSe/CuInSe<NUM>, CuGaSe<NUM>/CuInGaSe<NUM>, CuInGaSe<NUM>/CuInSe<NUM>, MgSe/MgZnSeTe, and MgZnSeTe/ZnTe.

When sunlight heats a solar cell, an electric field is generated by the stress-strain caused by the difference in the thermal expansion coefficient of each material contained in the inorganic piezoelectric material. However, in the method that uses sunlight, the magnitude of the stress that acts on the piezoelectric material will vary with the amount of solar radiation and the ambient temperature, so that stable solar cell characteristics cannot be obtained.

In this embodiment, because the dissociation regions <NUM> are formed from two types of materials having different lattice constants, stress-strain acts, and the electric field is generated due to the piezoelectric effect (piezoelectric field effect). As a result, it is possible to stably generate the electric field without restriction from materials selection and sunlight conditions.

As described above, in this embodiment, the dissociation regions <NUM> have a plurality of layers 21c, 21d formed from mutually different materials, and the lattice constants of the different materials are different. Since stress-strain is generated between the materials with different lattice constants, it is possible to stably generate the electric field in the dissociation regions <NUM>.

In addition, the plurality of the materials that form the dissociation regions <NUM> are selected from the combinations described above. Thus, because materials with similar compositions are used, it is possible to form a stacked structure of inorganic materials.

The dissociation regions <NUM> may be formed from the same materials having different composition ratios as a modified example of the photovoltaic device according to this embodiment. That is, the combinations of the materials that form the dissociation regions <NUM> is at least one selected from AlxGa<NUM>-xN/AlyGa<NUM>-yN (x ≠ y), InxGa<NUM>-xN/InyGa<NUM>-yN (x ≠ y), AlGaxAs<NUM>-x/AlGayAs<NUM>-y (x ≠ y), InxGa<NUM>-xAs/InyGa<NUM>-yAs, AlGaxP<NUM>-x/AlGayP<NUM>-y (x ≠ y), GaASxP<NUM>-x/GaAsyP<NUM>-y (x ≠ y), AlAsxP<NUM>-x/AlAsyP<NUM>-y (x ≠ y), InxAl<NUM>-xAs/InyAl<NUM>-yAs (x ≠ y), GaAsxSb<NUM>-x/GaAsySb<NUM>-y (x ≠ y), AlxGa<NUM>-xSb/AlyGa<NUM>-ySb (x ≠ y), AlxIn<NUM>-xSb/AlyIn<NUM>-ySb (x ≠ y), MgxZn<NUM>-xS/MgyZn<NUM>-yS (x ≠ y), MgxZn<NUM>-xSe/MgyZn<NUM>-ySe (x ≠ y), CuAl(SxSe<NUM>-x)<NUM> (x ≠ y), CuAlxGa<NUM>-yS<NUM>/CuAlyGa<NUM>-yS<NUM> (x ≠ y), CuGa(SxSe<NUM>-x)<NUM>/CuGa(SySe<NUM>-y)<NUM> (x ≠ y), CuInxGa<NUM>-xS<NUM>/CuInyGa<NUM>-yS<NUM> (x ≠ y), CuIn(SxSe<NUM>-x)<NUM>/CuIn(SySe<NUM>-y)<NUM> (x ≠ y), CuInxGa<NUM>-xSe<NUM>/CuInyGa<NUM>-ySe<NUM> (x ≠ y) and (MgSe)x (ZnTe)<NUM>-x/(MgSe)y (ZnTe)<NUM>-y (x ≠ y), where <NUM> < x < <NUM> and <NUM> < y < <NUM>. Because the same materials are used for the dissociation regions <NUM>, it is possible to reduce manufacturing costs. In addition, because the combination of Ec and Ev can be freely selected within the range of the composition ratio, material selection is not restricted.

The adjustment of the energy level at the interface will now be described. The Ec and Ev of the inorganic semiconductor <NUM> are intrinsic energy levels that are determined by the material and the crystallinity thereof. Thus, matching the energy of the inorganic semiconductor to a specific organic semiconductor material is difficult since it entails changing the material of the inorganic semiconductor. In some compound semiconductors, Ec and Ev can be adjusted by changing the compositions during crystal growth of the semiconductor. For example, Ec and Ev can be controlled by adjusting the Al/Ga composition ratio in AlGaN and the In/Ga composition ratio in InGaN. In addition, multi-element compound semiconductors in which the band gap can be adjusted, such as Al-In-Ga-N semiconductors and Al-In-Ga-As semiconductors often have piezoelectric properties. In addition, since it is possible to change the lattice constant of the crystal with the composition, it is possible to exhibit the piezoelectric effect (piezoelectric field effect) by means of a two-layer structure of different compositions and to form the electric field required for dissociating the excitons.

<FIG> is an energy diagram of the photovoltaic device according to another embodiment of the invention. <FIG> is an energy diagram in a cross section taken through line XIV-XIV of <FIG>. The horizontal axis (X) in <FIG> indicates the same position as the x axis in <FIG> and the vertical axis (E) represents the energy magnitude. This embodiment differs from the fifth embodiment described above in the relationship of the energy levels in the dissociation regions <NUM>. The other configurations are the same as the above-described fifth embodiment, and the descriptions of the reference examples <NUM>, <NUM>, <NUM> and the embodiments <NUM>, <NUM> are incorporated where appropriate.

The dissociation regions <NUM> include a plurality of the dissociation layers 21c, 21d. The relationship of the energy levels between the dissociation layer 21c and the dissociation layer 21d at the interface <NUM> satisfies the following equation (<NUM>):
[Equation <NUM>] <MAT> where, Ecs represents the energy level of the lower end of the conduction band of the dissociation layer 21c, and EF3 represents the Fermi level of the dissociation layer 21c.

The stress-strain increases in the dissociation regions <NUM> due to an increase in the lattice mismatch ratio, and the piezoelectric field increases. When the piezoelectric field increases and the energy level Ecs at the lower end of the conduction band becomes lower than the Fermi level EF3, a two-dimensional electron gas is formed at the interface <NUM> (the darkened portion in <FIG>). In the region of the two-dimensional electron gas, since the conductivity can be ensured without carrying out impurity doping or the like, the scattering of carriers by impurities can be suppressed and carrier mobility improved. Therefore, by satisfying the equation (<NUM>) above, the diffusion length can be increased.

As described above, in this embodiment, the dissociation regions <NUM> have a plurality of layers 21c, 21d formed from different materials, and the energy level of one of the plurality of layers 21c, 21d satisfies the equation (<NUM>). As a result, because the carriers are extracted via the two-dimensional electron gas with high mobility, it is possible to increase the diffusion length. In addition, it is possible to efficiently extract the carriers and to increase the thickness of the photoactive region <NUM>.

In this embodiment, the dissociation regions <NUM> may be connected to the anode electrode <NUM> instead of the cathode electrode <NUM>, and at the interface <NUM> between the dissociation layer 21c and the dissociation layer 21d, the relationship of the energy levels may satisfy the following equation (<NUM>):
[Equation <NUM>] <MAT> where EV3 represents the energy level of the upper end of the valence band of the dissociation layer 21c.

As a result, because the carriers are extracted via the two-dimensional hole gas with high mobility, it is possible to increase the diffusion length. In addition, it is possible to efficiently extract the carriers and to increase the thickness of the photoactive region <NUM>.

<FIG> is a cross-sectional view of the photovoltaic device according to another embodiment of the invention. <FIG> is an energy diagram in a cross section taken through line XVII-XVII of <FIG>. The horizontal axis (X) in <FIG> indicates the same position as the x axis in <FIG>, and the vertical axis (E) represents the energy magnitude. This embodiment differs from the embodiment <NUM> described above in the relationship of the energy levels in the dissociation regions <NUM>. The other configurations are the same as the above-described reference example <NUM>, and the descriptions of the reference examples <NUM>, <NUM>, <NUM> and the embodiments <NUM>, <NUM>, <NUM> are incorporated where appropriate.

The relationship of the energy levels at an interface <NUM> between the dissociation regions <NUM> and the electrode <NUM> will now be described. If the LUMO energy level of the photoactive region <NUM> is ELUMO, the energy level at the lower end of the conduction band of the dissociation regions <NUM> at an interface <NUM> is Eca, and the energy level at the lower end of the conduction band of the dissociation regions <NUM> at a connection surface <NUM> is ECb, the relationship between each of the energy levels can be represented by the following equations (<NUM>) and (<NUM>):
[Equation <NUM>] <MAT> [Equation <NUM>] <MAT>.

In addition, the energy level at the lower end of the conduction band between the interface <NUM> and the connection surface <NUM> decreases monotonically from the interface <NUM> toward the connection surface <NUM>. In other words, in a state in which the energy level Eca is lower than the energy level ECb, the inclination of the energy level at the lower end of the conduction band in the dissociation regions is a gradual inclination, as illustrated in <FIG>.

When a multi-element mixed-crystal material, such as AlGaN, InGaN, AlGaAs, InGaAs, GaAsP, and InGaAs, is used as the material of the semiconductor, it is possible to adjust the size of the band gap of the semiconductor by changing the composition ratio. The composition ratio can be adjusted by adjusting a growth parameters during crystal growth, such as temperature, pressure, and source gas ratio. Then, the energy of the lower end Ec of the conduction band with respect to the position can be continuously decreased by continuously adjusting the composition ratio of the crystal. Here, the inclination of Ec is the electric field strength, and the electrons <NUM> can obtain a thrust force oriented toward the cathode electrode <NUM> by the electrons <NUM> receiving the force of the electric field. Therefore, it is possible to extend the diffusion length.

If the slope of the energy level at the lower end of the conduction band in the dissociation region is increased between the interface <NUM> and the connection surface <NUM>, the amount of potential decrease that corresponds to the integration of the slope portion becomes large, and there is the risk that the output power of the photovoltaic device will be low. In this embodiment, the potential decrease amount due to the inclination of the energy level can be suppressed to <NUM> eV or less by means of the energy level condition satisfying equation (<NUM>). As a result, it is possible to minimize the voltage loss while preventing the reverse flow of carriers.

As described above, in this embodiment, the energy level at the interface <NUM> and the energy level at the connection surface <NUM> satisfy the equation (<NUM>). It is thereby possible to suppress the voltage loss while preventing the reverse flow of the carriers.

In addition, in this embodiment, the energy level at the lower end of the conduction band between the interface <NUM> and the connection surface <NUM> decreases monotonically from the interface <NUM> toward the connection surface <NUM>. It is thereby possible for the carriers to obtain a thrust force oriented toward the desired electrode by generating the electric field in the dissociation regions <NUM>. As a result, it is possible to extend the diffusion length.

In this embodiment, the dissociation regions <NUM> may be connected to the anode electrode <NUM> instead of the cathode electrode <NUM>, and the energy level at the interface <NUM> and the energy level at the connection surface <NUM> may satisfy the following equations (<NUM>) and (<NUM>) instead of equations (<NUM>) and (<NUM>):
[Equation <NUM>] <MAT> [Equation <NUM>] <MAT>.

It is thereby possible to suppress the voltage loss while preventing the reverse flow of carriers.

In addition, in this embodiment, the energy level at the upper end of the valence band between the interface <NUM> and the connection surface <NUM> increases monotonically from the interface <NUM> toward the connection surface <NUM>. The connection surface <NUM> is the connection surface between the anode electrode and the dissociation regions <NUM> connected to the anode electrode <NUM>. It is thereby possible for the carriers to obtain a thrust force oriented toward the desired electrode by generating the electric field in the dissociation regions <NUM>. As a result, it is possible to extend the diffusion length.

<FIG> is a cross-sectional view of the photovoltaic device according to another embodiment of the invention. The photovoltaic device according to this embodiment is configured as a tandem type. The other configurations are the same as the above-described reference example <NUM>, and the descriptions of the reference examples <NUM>, <NUM>, <NUM> and the embodiments <NUM>, <NUM> to <NUM> are incorporated where appropriate.

As shown in <FIG>, the photovoltaic device is configured as a triple-junction tandem cell. The photovoltaic device comprises a top cell <NUM>, a middle cell <NUM>, a bottom cell <NUM>, and tunnel recombination layers <NUM>. The top cell <NUM> includes the organic semiconductor <NUM>, the inorganic semiconductors <NUM>, the anode electrode <NUM>, and the cathode electrode <NUM>. The organic semiconductor <NUM> included in the top cell <NUM> has a photoactive region 11e. The inorganic semiconductors <NUM> included in the top cell <NUM> has dissociation regions 21e.

The middle cell <NUM> includes the organic semiconductor <NUM>, the inorganic semiconductors <NUM>, the anode electrode <NUM>, and the cathode electrode <NUM>. The organic semiconductor <NUM> included in the middle cell <NUM> has a photoactive region 11f. The inorganic semiconductors <NUM> included in the middle cell <NUM> has dissociation regions 21f.

The bottom cell <NUM> includes the organic semiconductor <NUM>, the inorganic semiconductors <NUM>, the anode electrode <NUM>, and the cathode electrode <NUM>. The organic semiconductor <NUM> included in the bottom cell <NUM> has a photoactive region <NUM>. The inorganic semiconductors <NUM> included in the bottom cell <NUM> has dissociation regions <NUM>.

The top cell <NUM>, the middle cell <NUM>, and the bottom cell <NUM> are stacked in a normal direction (y direction in <FIG>) of the electrode surfaces of the anode electrode <NUM> and the cathode electrode <NUM>. Since the configuration of each of the top cell <NUM>, the middle cell <NUM>, and the bottom cell <NUM> is the same as that of the semiconductor device according to the reference example <NUM>, the descriptions thereof are omitted.

The tunnel recombination layer <NUM> couples the cathode electrode <NUM> included in the top cell <NUM> and the anode electrode <NUM> included in the middle cell <NUM>, and couples the cathode electrode <NUM> included in the middle cell <NUM> and the anode electrode <NUM> included in the bottom cell <NUM>.

The direction of light incidence is the direction from the anode electrode <NUM> of the top cell <NUM> toward the cathode electrode of the bottom cell <NUM> (negative direction of the y axis). The photoactive region 11e, the photoactive region 11f, and the photoactive region <NUM> each have different band gaps, which become smaller along the stacking direction of the cells, from the light-incident surface. That is, among the respective band gaps of the photoactive region 11e, the photoactive region 11f, and the photoactive region <NUM>, the band gap on the light-incident side is greatest and sequentially decreases in the transmission direction of the band gap light.

The basic concept of tandem solar cells is utilization of the fact that the semiconductor absorbs light higher in energy than the band gap and transmits light lower in energy than the band gap. When the energy that the carriers, which are generated by the semiconductor, etc. absorbing light, receive from the photons is greater than the band gap, the energy of the photons hv-Eg (h: Planck's constant, v: wave number of photons, Eg: band gap) is lost due to relaxation of the carriers. By stacking materials that have a plurality of band gaps and causing the light to enter and transmit in descending order of the band gaps, it is possible to suppress losses due to relaxation of each of the wavelength bands. As a result, a solar cell that exceeds the Shockley-Queisser limit becomes theoretically possible.

Even with organic semiconductors, it becomes possible to adjust the absorption end of the light and to change the size of the band gap by, for example, selecting the material or adjusting the functional group. It is possible to suppress the loss in each wavelength band and to further improve the conversion efficiency, for example, by using organic materials that display absorption spectra that are respectively suitable for the top cell <NUM>, the middle cell <NUM>, and the bottom cell <NUM> of a triple-junction tandem solar cell.

As described above, this embodiment is provided with a stacked body in which a plurality of cells <NUM>, <NUM>, <NUM> are stacked, and the band gap of each of the photoactive regions 11e, 11f, <NUM> included in the plurality of stacked cells <NUM>, <NUM>, <NUM> becomes smaller from the incident-light surface along the stacking direction of the cells. As a result, it is possible to suppress the loss in each wavelength band by using organic materials that exhibit absorption spectra that are suitable for each cell of the tandem solar cell. As a result, it becomes possible to improve the conversion efficiency.

In this embodiment, among the respective band gaps of the dissociation regions 21e, the dissociation regions 21f, and the dissociation regions <NUM>, the band gap on the light-incident side is greatest and sequentially decreases in the transmission direction of the band gap light proceeds in the transmission direction. At this time, the dissociation regions 21e, the dissociation regions 21f, and the dissociation regions <NUM> have the function of the photoactive region. That is, among the respective band gaps of the dissociation regions 21e, the dissociation regions 21f, and the dissociation regions <NUM>, the band gap on the light-incident side is the greatest and sequentially decreases in the transmission direction of the band gap light. As a result, it is possible to suppress the loss in each wavelength band by using inorganic materials that exhibit absorption spectra that are suitable for each cell of the tandem solar cell. As a result, it becomes possible to improve the conversion efficiency.

<FIG> is a cross-sectional view of the photovoltaic device according to another embodiment of the invention. The photovoltaic device according to this embodiment is different in the configuration of a part of the inorganic semiconductor <NUM>. The other configurations are the same as the above-described reference example <NUM>, and the descriptions of the reference examples <NUM>, <NUM>, <NUM> and the embodiments <NUM>, <NUM> to <NUM> are incorporated where appropriate. The direction of light incidence is the direction from the anode electrode <NUM> toward the cathode electrode <NUM> (negative y-axis direction).

The inorganic semiconductor <NUM> is formed to cover the surface of the cathode electrode <NUM>. Portions of the inorganic semiconductor <NUM> have a columnar shape, and the columnar portions are covered with the organic material of the organic semiconductor <NUM>. Since the cathode electrode <NUM> is covered with the inorganic semiconductor <NUM>, the organic semiconductor <NUM> and the cathode electrode <NUM> are not in direct contact with each other. That is, the film-like inorganic semiconductor <NUM> is provided between the cathode electrode and the end of the organic semiconductor <NUM>.

The inorganic semiconductor <NUM> is formed by crystal growth, such as CVD. Crystals of an inorganic material are grown on the surface of the plate-like cathode electrode <NUM> and growth is temporarily suspended when a film of the inorganic material is formed on the surface of the cathode electrode <NUM>.

A mask pattern, such as a photolithography mask, is placed on the inorganic material film. Columnar crystals of the same inorganic material are grown at the openings of the mask pattern. The inorganic semiconductor <NUM> is thereby formed, as illustrated in <FIG>.

The method for producing the inorganic semiconductor <NUM> is not limited to crystal growth, such as CVD, and the S-K growth (Stranski-Krastanov) mode may be used instead. When the S-K growth mode is used, of the inorganic material film that is formed on the surface of the cathode electrode <NUM>, only a portion of the surface of the inorganic material film need be grown in columnar form. In the inorganic semiconductor <NUM>, piezoelectric material may be grown to the height of the columnar portions shown in <FIG> (height in the y direction), and the portions other than the columnar portions (recessed region) may be formed by etching.

As described above, in this embodiment, the dissociation regions <NUM> are surrounded by the organic material of the organic semiconductor <NUM>, and a part of the dissociation regions <NUM> are directly connected to the cathode electrode <NUM>, while the cathode electrode and the photoactive region <NUM> are not in direct contact.

Unlike this embodiment, if the organic semiconductor <NUM> and the cathode electrode <NUM> are in direct contact, a localized energy level is formed at the interface between the organic semiconductor <NUM> and the cathode <NUM>, depending on the combination of the organic material of the organic semiconductor <NUM> and the material of the cathode electrode <NUM>. If the localized energy level is a level that promotes carrier recombination, a dipole (electric dipole) is formed at the interface, thereby making it difficult to obtain the desired characteristics.

In this embodiment, because the organic semiconductor <NUM> and the cathode electrode <NUM> are not in direct contact, a localized energy level that promotes carrier recombination is not generated, and formation of the dipole at the interface can be prevented.

The photovoltaic device according to this embodiment may have the structure shown in <FIG> as a structure in which the organic semiconductor <NUM> and the cathode electrode <NUM> are not in direct contact with each other.

<FIG> is a cross-sectional view of the photovoltaic device according to a modified example. The photovoltaic device according to the present modified example comprises the organic semiconductor <NUM>, the inorganic semiconductor <NUM>, the anode electrode <NUM>, the cathode electrode <NUM>, and a protective film <NUM>. The protective film <NUM> is provided between the organic semiconductor <NUM> and the cathode electrode <NUM>. The protective film <NUM> is formed on the surface of the cathode electrode <NUM> so as to cover the portion of the surface of the cathode electrode <NUM> that is not connected to the dissociation regions <NUM>. The protective film <NUM> may have an insulating property. The protective film <NUM> is formed of an oxide film obtained by oxidizing a metal surface or by means of a growth method (CVD, etc.). After forming the protective layer on the surface of the cathode electrode <NUM>, the portion where the dissociation regions <NUM> are to be formed are opened by means of photolithography. The dissociation regions <NUM> are formed by crystal growth of the inorganic material at the opened portions.

The photovoltaic device according to this modified example may have the structure shown in <FIG> as a structure in which the organic semiconductor <NUM> and the cathode electrode <NUM> are not in direct contact with each other. <FIG> is a cross-sectional view of the photovoltaic device according to a modified example.

In the modified example shown in <FIG>, the inorganic semiconductors <NUM> are formed in a columnar shape extending in the y-axis direction, while covering the surface of the cathode electrode <NUM> with a layer. The columnar portions are covered with the organic material of the organic semiconductor <NUM>. Moreover, the protective film <NUM> is provided between the organic semiconductor <NUM> and the inorganic semiconductor <NUM>. The protective film <NUM> is formed so as to cover the layer portion of the inorganic semiconductor <NUM>. The protective film <NUM> prevents the cathode electrode <NUM> and the organic semiconductor <NUM> from coming into direct contact with each other.

Claim 1:
A photovoltaic device comprising:
an organic semiconductor (<NUM>) including a photoactive region (<NUM>) for generating excitons (<NUM>), the organic semiconductor (<NUM>) being formed by an organic material; and
an inorganic semiconductor (<NUM>) disposed within the organic semiconductor (<NUM>), the inorganic semiconductor (<NUM>) having piezoelectricity and including a dissociation region (<NUM>, <NUM>) for dissociating carriers included in the excitons (<NUM>);
a first electrode and a second electrode (<NUM>, <NUM>) disposed on opposite sides of the organic semiconductor (<NUM>);
wherein a relationship of energy levels between the photoactive region (<NUM>) and the dissociation region satisfies at least one of the following equations (<NUM>) or (<NUM>),
wherein the dissociation region has a plurality of layers (21c, 21d) formed of mutually different materials and is a region in which strain is generated due to a difference in the lattice constants of the different materials,
characterized in that
the dissociation region (<NUM>, <NUM>) includes a first dissociation region (<NUM>, <NUM>) through which only holes (<NUM>) from among the carriers can pass, and a second dissociation region (<NUM>, <NUM>) through which only electrons (<NUM>) from among the carriers can pass,
wherein the first dissociation region (<NUM>, <NUM>) and the second dissociation region (<NUM>, <NUM>) are formed in a columnar shape in the organic semiconductor (<NUM>),
wherein the first dissociation region (<NUM>, <NUM>) and the second dissociation region (<NUM>, <NUM>) are surrounded by the organic material that forms the organic semiconductor (<NUM>),
a part of the first dissociation region (<NUM>, <NUM>) is directly connected to the first electrode (<NUM>, <NUM>), and
a part of the second dissociation region (<NUM>, <NUM>) is directly connected to the second electrode (<NUM>, <NUM>); <MAT> <MAT>
where
ELUMO represents a LUMO energy level in the photoactive region (<NUM>),
EHOMO represents a HOMO energy level in the photoactive region (<NUM>),
Ec represents the energy level at a lower end of a conduction band in one layer of the dissociation region, and
Ev represents the energy level of an upper end of a valence band in the one layer of the dissociation region.