Photoelectric conversion device, photosensor, power generation device, and photoelectric conversion method

A photoelectric conversion device includes a photoelectric conversion element formed of a polar material and including no p-n junction, and first and second electrodes provided on the photoelectric conversion element and arranged at an interval. Space-inversion symmetry of a structure of the photoelectric conversion element is broken. The first and second electrodes are each formed of a metal material that generates no substantial potential barrier preventing majority carriers for the photoelectric conversion element from moving from the electrode to the photoelectric conversion element. Light incidence on the photoelectric conversion element without voltage application between the first and second electrodes causes electromotive force to be generated between first and second electrodes, and enables electric current to be continuously taken out from the first and second electrodes.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority based on Japanese patent application No. 2018-000543 filed on Jan. 5, 2018, of which disclosure is incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a technique of causing movement of majority carriers (electrons or positive holes) in a photoelectric conversion element by light incidence on the photoelectric conversion element so that current due to the movement is taken out from electrodes provided in the photoelectric conversion element.

Background Art

Conventionally, a general photoelectric conversion includes an n-type semiconductor and a p-type semiconductor. A p-n junction between the n-type semiconductor and the p-type semiconductor causes an internal electric field to be generated in the photoelectric conversion element. The light incidence on the photoelectric conversion element causes electrons to jump from a valance band to a conduction band so that pairs of electrons and positive holes are generated. By the internal electric field, these electrons and positive holes are moved to the n-type side and p-type side respectively, and are separated from each other. Thus, current can be taken out from the photoelectric conversion element. Such a photoelectric conversion device is described in Patent Literature 1, for example.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

Technical Problem

However, carriers (electrons or positive holes) generated by the light incidence on the photoelectric conversion element including the p-n junction are scattered due to impurities, defects, lattice vibration, and the like in the photoelectric conversion element. Since movement of the carriers is accompanied by such scattering, energy of the carriers is scattered and lost. Therefore, there is a limitation on efficiency of taking out current from the photoelectric conversion element including the p-n junction.

In view of the above, an object of the present invention is to provide a technique capable of reducing the scattering of carriers generated by light incidence on a photoelectric conversion element, and efficiently taking out, from the photoelectric conversion element, current caused by movement of the carriers.

Solution to Problem

A photoelectric conversion device according to the present invention includes a photoelectric conversion element formed of a polar material and including no p-n junction, and first and second electrodes provided on the photoelectric conversion element and arranged at an interval. Space-inversion symmetry of a structure of the photoelectric conversion element is broken. The first and second electrodes are each formed of a metal material that generates no substantial potential barrier. The potential barrier prevents majority carriers for the photoelectric conversion element from moving from the electrode to the photoelectric conversion element.

A photosensor according to the present invention includes the above-described photoelectric conversion device, and detects light incidence on the photoelectric conversion element, on a basis of current taken out from the first and second electrodes.

A power generation device according to the present invention includes the above-described photoelectric conversion device, and supplies, to a storage battery or a load, power taken out from the first and second electrode.

A photoelectric conversion method according to the present invention uses the above-described photoelectric conversion device, makes light incident on the photoelectric conversion element between the first and second electrodes, and takes out, from the first and second electrodes, current generated in the photoelectric conversion element.

Additional advantages are explained by the inventors in “Impact of electrodes on the extraction of shift current from a ferroelectric semiconductor SbSI,”Appl. Phys. Lett.113, 232901 (2018) (doi: 10.1063/1.5055692), published by the American Institute of Physics which is incorporated by reference as if fully set forth.

Advantageous Effect of Invention

According to the present invention, carriers generated by light incidence on the photoelectric conversion element is less scattered, and current due to movement of the carriers can be efficiently taken out from the photoelectric conversion element.

DESCRIPTION OF THE EMBODIMENT

An embodiment of the present invention is described based on the accompanying drawings. Note that elements common in the respective drawings are denoted by the same reference numerals, and the overlapping description thereof is omitted. The following description does not limit the invention described in claims. For example, the present invention is not limited to a configuration including all of constituent elements described below.

FIG. 1is a configuration diagram of a photoelectric conversion device10according to an embodiment of the present invention. The photoelectric conversion device10includes a photoelectric conversion element1and first and second electrodes2and3.

The photoelectric conversion element1is formed of a polar material. The polar material is a material having a structure in which gravity center positions of positive and negative charges are different from each other. The polar material may be a polarized material. For example, the polar material is a pyroelectric material or a ferroelectric material. Note that the photoelectric conversion element1may be formed of a single polar material. Here, the “single polar material” means a material in which atoms, molecules, or ions are arranged regularly, and may be a material including no material components that do not correspond to such arrangement. For example, the term “single” may mean that when the polar material is any one of specific examples described below, the photoelectric conversion element1does not include a material other than the specific example.

The structure of the photoelectric conversion element1has broken space-inversion symmetry. This means that when mirror image inversion of the structure (e.g., the crystal structure or the molecular structure) of the photoelectric conversion element1is performed, the structure before the mirror image inversion does not coincide with the structure after the mirror image inversion. The photoelectric conversion element1has a structure whose space-inversion symmetry is broken in at least one direction (the horizontal direction ofFIG. 1). In this case, even when any position in the direction is set as a reference point for mirror image inversion (a position of a mirror for mirror image inversion), the structure before the mirror image inversion does not coincide with the structure after the mirror image inversion. Note that the photoelectric conversion element1is formed of the polar material, and for this reason, has the structure of the broken space-inversion symmetry as described above.

Further, the photoelectric conversion element1includes no p-n junction. The photoelectric conversion element1(polar material) is a p-type semiconductor or an n-type semiconductor. Note that the polar material may be, but does not need to be an intrinsic semiconductor.

The first and second electrodes2and3are provided for taking out, to the outside, current (hereinafter also called simply “current generated in the photoelectric conversion element1”) due to the movement of majority carriers generated in the photoelectric conversion element1by light incidence on the photoelectric conversion element1. A current line4(e.g. a conducting wire) is connected to the first and second electrodes2and3. The current generated in the photoelectric conversion element1flows to the current line4through the first and second electrodes2and3. The first and second electrodes2and3may be connected to each other through the current line4. In the example ofFIG. 1, the current line4is provided with a resistor (electric bulb)13.

The first and second electrodes2and3are provided in the photoelectric conversion element1, and are arranged at an interval from each other. The direction in which the first and second electrodes2and3are separated from each other is hereinafter called an electrode separation direction. The first and second electrodes2and3may contact directly with the photoelectric conversion element1. Space-inversion symmetry of the structure of the photoelectric conversion element1is broken in the electrode separation direction. Further, the photoelectric conversion element1may be polarized in the electrode separation direction.

The first and second electrodes2and3are made of a metal material which causes no substantial potential barrier. The potential barrier prevents majority carriers (hereinafter simply called majority carriers) for the photoelectric conversion element1from moving from the electrodes2and3to the photoelectric conversion element1. In other words, in a state in which the first and second electrodes2and3are connected to the photoelectric conversion element1, the potential barrier to the majority carriers does not exist at an interface between the photoelectric conversion element1and each of the electrodes2and3, from a standpoint of the electrodes2and3.

When the photoelectric conversion element1is a p-type semiconductor, metal materials that form the first and second electrodes2and3have Fermi levels equal to or lower than a Fermi level of the polar material which forms the photoelectric conversion element1. This may be described as that work functions of the metal materials are equal to or larger than a work function of the polar material. Thus, the first and second electrodes2and3do not generate the above-described potential barrier (details thereof are described below with reference toFIG. 3AtoFIG. 4B). Note that the Fermi level of each of the first and second electrodes2and3may be equal to or lower than the Fermi level of the photoelectric conversion element1as the p-type semiconductor, but does not need to have a particular lower limit thereof.

In one example, when the photoelectric conversion element1is the p-type semiconductor, the Fermi level of the metal material of each of the first and second electrodes2and3is lower (deeper) than the Fermi level of the polar material.

When the photoelectric conversion element1is an n-type semiconductor, the metal material of each of the first and second electrodes2and3has a Fermi level equal to or higher than the Fermi level of the polar material of the photoelectric conversion element1. This may be described as that a work function of the metal material is equal to or smaller than the work function of the polar material. Thus, the first and second electrodes2and3do not generate the above-described potential barriers (details thereof are described below with reference toFIG. 5AtoFIG. 6B). Note that the Fermi level of each of the first and second electrodes2and3may be equal to or higher than the Fermi level of the photoelectric conversion element1as the n-type semiconductor, but does not need to have a particular upper limit thereof.

In one example, when the photoelectric conversion element1is the n-type semiconductor, the Fermi level of the metal material each of the first and second electrodes2and3is higher (shallower) than the Fermi level of the polar material.

Note that regardless of whether the photoelectric conversion element1is the p-type semiconductor or the n-type semiconductor, the metal material of the first electrode2may be the same as or different from that of the second electrode3.

Hereinafter, the photoelectric conversion device10according to the embodiment of the present invention is described in more detail.

A breakdown of the space-inversion symmetry is described in more detail with reference toFIG. 2.FIG. 2is a schematic diagram illustrating, as one example, a molecular structure of the photoelectric conversion element1in the photoelectric conversion device10ofFIG. 1. InFIG. 2, the polar material that forms the photoelectric conversion element1is a ferroelectric material. The ferroelectric material becomes a ferroelectric when a temperature thereof becomes equal to or lower than its own Curie temperature. The horizontal direction ofFIG. 2is the electrode separation direction, and corresponds to the horizontal direction ofFIG. 1.

The upper side ofFIG. 2indicates a paraelectric state at a temperature higher than the Curie temperature, and the lower side ofFIG. 2indicates a ferroelectric state at a temperature equal to or lower than the Curie temperature or less. On the upper and lower sides ofFIG. 2, donor molecules D and acceptor molecules A are alternately positioned in the electrode separation direction. For example, the polar material that forms the photoelectric conversion element1may be TTF-CA. The donor molecule D may be TTF (tetrathiafulvalne), and the acceptor molecule A may be CA (p-chloranil).

When an electric field is applied to the photoelectric conversion element1in the electrode separation direction in a state of the upper side ofFIG. 2, positively charged donor molecules D are displaced in the direction of the application of the electric field, and the negatively charged acceptor molecules A are displaced in the direction opposite to the application direction of the electric field, as illustrated in the lower side ofFIG. 2. Thus, the photoelectric conversion element1becomes a state polarized in the right direction ofFIG. 2. In other words, the photoelectric conversion element1is polarized in the application direction of the electric field, and thereby the space-inversion symmetry of the structure thereof is broken. Even after the stop of the electric field application, the polarization of the photoelectric conversion element1is maintained. Thus, the state in which the space-inversion symmetry of the structure of the photoelectric conversion element1is broken is maintained even after the stop of the electric field application.

Thus, in the lower side ofFIG. 2, the space-inversion symmetry of the molecule structure of the photoelectric conversion element1is broken, and for this reason, the molecule structure reflected in a plane mirror orthogonal to the electrode separation direction does not coincide with the original molecular structure corresponding to the reflected molecular structure even when the mirror is arranged at any position in the electrode separation direction.

<Specific Examples of Material>

When the photoelectric conversion element1is the p-type semiconductor, the polar material as the p-type semiconductor may be SbSI, BiFeO3, TTF-CA (tetrathiafulvalene-p-chloranil), TTF-BA (tetrathiafulvalene-bromanil), TMB-TCNQ (tetramethylbenzidine-tetracyanoquinodimethane), GeTe, CdTe, GaFeO3, RMnO3(R: rare-earth element), Ca3Mn2O7, LuFe2O4, CH3NH3PbI3, (2-(ammoniomethyl)pyridinium)SbI5, or Sn2P2S6, for example. In this case, the metal material that forms each of the first and second electrodes2and3may be one, having a Fermi level equal to or lower than the Fermi level of the photoelectric conversion element1, of Ag, Pt, Au, ITO, MoO3, TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane), and PEDOT:PSS(poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)), for example.

When the photoelectric conversion element1is the n-type semiconductor, the polar material as the n-type semiconductor may be BiSI, BaTiO3, PbTiO3, Pb5Ge3O11, Pb(Zr,Ti)O3(PZT), CdS, LiNbO3, LiTaO3, KNbO3, KH2PO4(KDP), ZnO, BiTeI, BiTeBr, or GaN, for example. In this case, the metal material that forms each of the first and second electrodes2and3may be one, having a Fermi level equal to or higher than the Fermi level of the photoelectric conversion element1, of Ca, Mg, LiF/Al, In, and Al, for example.

The polar material that forms the photoelectric conversion element1may be a pyroelectric material. The pyroelectric material may be GaFeO3, CdS, GaN, ZnO, CdTe, BiTeI, or BiTeBr, for example.

Instead, the polar material that forms the photoelectric conversion element1may be one formed by stacking thin layered films on each other N times (where N is an integer of one or more). This thin layered film is one formed by stacking an A layer, a B layer, and a C layer in this order, with crystal materials of the A layer, the B layer, and the C layer being different from each other. Each of the A layer, the B layer, and the C layer is an atomic layer thin film having a nano-order thickness. Here, the atomic layer thin film may be a single crystal. A combination of the crystal materials for the A layer, the B layer, and the C layer may be a combination of LaAlO3, LaFeO3, and LaCrO3, or a combination of CaTiO3, BaTiO3, and SrTiO3, for example. In other words, in one example of the former combination, the A layer is formed of LaAlO3, the B layer is formed of LaFeO3, and the C layer is formed of LaCrO3.

Note that the polar material and the materials of the electrodes2and3are not limited to the above-described specific examples. For example, the polar material may be a piezoelectric material.

Case of p-Type Semiconductor

FIG. 3AandFIG. 3Bare schematic diagrams of band structures of the photoelectric conversion element1and the electrodes2and3according to the embodiment of the present invention, and illustrate the case where the photoelectric conversion element1is the p-type semiconductor.FIG. 3Aillustrates a state where the first and second electrodes2and3are separated from the photoelectric conversion element1, andFIG. 3Billustrates a state where the first and second electrodes2and3are connected to the photoelectric conversion element1as illustrated inFIG. 1. InFIG. 3AandFIG. 3B, Ecindicates an energy level of a lower end of a conduction band, and Evindicates an energy level of an upper end of a valance band (the same applies to the case ofFIG. 4AtoFIG. 6Bdescribed below).

In the present embodiment, when the photoelectric conversion element1is the p-type semiconductor, the first and second electrodes2and3(i.e., the metal materials forming the electrodes2and3) both have Fermi levels Ef1and Ef2equal to or lower than a Fermi level Ef0of the photoelectric conversion element1(i.e., the polar material forming the photoelectric conversion element1) as illustrated inFIG. 3A. When the first and second electrodes2and3are connected to the photoelectric conversion element1from the state ofFIG. 3A, positive holes as majority carriers for the photoelectric conversion element1flow from the respective electrodes2and3into the photoelectric conversion element1where a Fermi level is higher, whereby in each of the electrodes2and3, the interface with the photoelectric conversion element1is negatively charged. As a result, as illustrated inFIG. 3B, the bands of the photoelectric conversion element1are bent on the sides of the electrodes2and3such that the Fermi levels Ef1and Ef2of the first and second electrodes2and3respectively coincide with the Fermi level Ef0of the photoelectric conversion element1.

Since the electrodes2and3and the photoelectric conversion element1make ohmic contact therebetween in the state ofFIG. 3B, the electrodes2and3do not generate a substantial potential barrier that prevents majority carriers from moving from the electrodes2and3to the photoelectric conversion element1. In other words, such a potential barrier does not occur in the interface between each of the electrodes2and3and the photoelectric conversion element1. Thus, the majority carriers (positive holes) for the photoelectric conversion element1can easily move from the electrodes2and3to the photoelectric conversion element1.

Here, the first and second electrodes2and3are connected to each other through the current line4at the outside of the photoelectric conversion device10as illustrated inFIG. 1. Thus, when each pair of an electron and a positive hole occurs in the photoelectric conversion element1by light incidence on the photoelectric conversion element1, the positive holes as the majority carriers are moved to one electrode2or3in the valance band by a breakdown of the space-inversion symmetry in the photoelectric conversion element1. InFIG. 3B, the positive holes are moved to the first electrode2by polarization of the photoelectric conversion element1.

Meanwhile, since the above-described potential barrier does not occur in the interface between each of the electrodes2and3and the photoelectric conversion element1, the majority carriers are supplied from the other electrode2or3(second electrode3inFIG. 3B) to the photoelectric conversion element1one after another. The supplied positive holes recombine with electrons generated and excited to the conduction band by light incidence, one after another. Thus, the positive holes are generated by the incidence of light on the photoelectric conversion element1, current due to the movement of the positive holes is continuously generated in the photoelectric conversion element1, and the current can be taken out from the first and second electrodes2and3to the external current line4.

FIG. 4AandFIG. 4Bare schematic diagrams of band structures for a photoelectric conversion element and electrodes according to a comparative example, and illustrate the case where the photoelectric conversion element is a p-type semiconductor.FIG. 4Aillustrates a state where first and second electrodes are separated from the photoelectric conversion element, andFIG. 4Billustrates a state where the first and second electrodes are connected to the photoelectric conversion element.

InFIG. 4A, the first electrode has a Fermi level Ef1equal to or lower than a Fermi level Ef0of the photoelectric conversion element formed of a polar material, whereas the second electrode has a Fermi level Ef2higher than the Fermi level Ef0of the photoelectric conversion element. InFIG. 4A, the other points are assumed to be the same as in the case ofFIG. 3A. When the first and second electrodes are connected to the photoelectric conversion element from the state ofFIG. 4A, positive holes as majority carriers for the photoelectric conversion element flow from the first electrode into the photoelectric conversion element where a Fermi level is higher, whereby in the first electrode, the interface with the photoelectric conversion element is negatively charged. Meanwhile, positive holes are moved from the photoelectric conversion element to the second electrode where Fermi level is higher, whereby in the second electrode, the interface with the photoelectric conversion element is positively charged. As a result, as illustrated inFIG. 4B, the bands of the photoelectric conversion element are bent on the sides of the electrode in such that the Fermi levels Ef1and Ef2of the first and second electrodes respectively coincide with the Fermi level Ef0of the photoelectric conversion element.

In the state ofFIG. 4B, the second electrode generates a potential barrier that prevents the positive holes as the majority carriers from moving from the second electrode to the photoelectric conversion element. In other words, such a potential barrier occurs in the interface between the second electrode and the photoelectric conversion element. Accordingly, assuming that magnitude of a charge quantity of each positive hole is q, and magnitude of the potential barrier is ϕ, energy of qϕ is required to supply the positive hole from the second electrode to the photoelectric conversion element. Thus, it is difficult to supply the majority carriers (positive holes) from the second electrode to the photoelectric conversion element, and for this reason, even when the positive holes are generated in the valance band by the light incidence on the photoelectric conversion element, movement of the positive holes to the first electrode side is suppressed in the photoelectric conversion element, so that current taken out from the first and second electrodes to the outside is suppressed.

Case of n-Type Semiconductor:

FIG. 5AandFIG. 5Bare schematic diagrams of band structures of the photoelectric conversion element1and the electrodes2and3according to the embodiment of the present invention, and illustrate the case where the photoelectric conversion element1is an n-type semiconductor.FIG. 5Aillustrates a state where the first and second electrodes2and3are separated from the photoelectric conversion element1, andFIG. 5Billustrates a state where the first and second electrodes2and3are connected to the photoelectric conversion element1as illustrated inFIG. 1.

In the present embodiment, when the photoelectric conversion element1is the n-type semiconductor, the first and second electrodes2and3both have Fermi levels Ef1and Ef2equal to or higher than a Fermi level Ef0of the photoelectric conversion element1as illustrated inFIG. 5A. When the first and second electrodes2and3are connected to the photoelectric conversion element1from the state ofFIG. 5A, electrons as majority carriers for the photoelectric conversion element1flow from the respective electrodes2and3into the photoelectric conversion element1where a Fermi level is lower, whereby in the respective electrodes2and3, the interface with the photoelectric conversion element1is positively charged. As a result, as illustrated inFIG. 5B, the bands of the photoelectric conversion element1are bent on the sides of the electrodes2and3such that the Fermi levels Ef1and Ef2of the first and second electrodes2and3respectively coincide with the Fermi level Ef0of the photoelectric conversion element1.

Since the electrodes2and3and the photoelectric conversion element1make ohmic contact therebetween in the state ofFIG. 5B, the electrodes2and3do not generate a substantial potential barrier that prevents majority carriers from moving from the electrodes2and3to the photoelectric conversion element1. In other words, such a potential barrier does not occur in the interface between each of the electrodes2and3and the photoelectric conversion element1. Thus, the majority carriers (electrons) for the photoelectric conversion element1can easily move from the electrodes2and3to the photoelectric conversion element1. Here, the first and second electrodes2and3are connected to each other through the current line4at the outside of the photoelectric conversion device10as illustrated inFIG. 1. Thus, when each pair of an electron and a positive hole occurs in the photoelectric conversion element1by light incidence on the photoelectric conversion element1, the electrons as the majority carriers are moved to one electrode2or3in a conduction band by a breakdown of the space-inversion symmetry in the photoelectric conversion element1. InFIG. 5B, the electrons are moved to the second electrode3by polarization of the photoelectric conversion element1.

Meanwhile, since the above-described potential barrier does not occur in the interface between each of the electrodes2and3and the photoelectric conversion element1, the majority carriers (electrons) are supplied from the other electrode2or3(first electrode2inFIG. 5B) to the photoelectric conversion element1one after another. The supplied electrons recombine with positive holes generated in the valance band by the light incidence, one after another. Thus, electrons are generated in the conduction band due to the incidence of light on the photoelectric conversion element1, and current due to the movement of the electrons is continuously generated in the photoelectric conversion element1. The current can be taken out from the first and second electrodes2and3to the external current line4.

FIG. 6AandFIG. 6Bare schematic diagrams of band structures of a photoelectric conversion element and electrodes according to a comparative example, and illustrate the case where the photoelectric conversion element is an n-type semiconductor.FIG. 6Aillustrates a state where first and second electrodes are separated from the photoelectric conversion element, andFIG. 6Billustrates a state where the first and second electrodes are connected to the photoelectric conversion element.

InFIG. 6A, the second electrode has a Fermi level Ef2equal to or higher than a Fermi level Ef0of the photoelectric conversion element formed of a polar material, whereas the first electrode has a Fermi level Ef1lower than the Fermi level Ef0of the photoelectric conversion element. InFIG. 6A, the other points are assumed to be the same as in the case ofFIG. 5A. When the first and second electrodes are connected to the photoelectric conversion element from the state ofFIG. 6A, electrons as majority carriers for the photoelectric conversion element move from the second electrode to the photoelectric conversion element where Fermi level is lower, whereby in the second electrode, the interface with the photoelectric conversion element is positively charged. Meanwhile, electrons move from the photoelectric conversion element to the first electrode where a Fermi level is lower, whereby in the first electrode, the interface with the photoelectric conversion element is negatively charged. As a result, as illustrated inFIG. 6B, the bands of the photoelectric conversion element are bent on the sides of the electrodes such that the Fermi levels Ef1and Ef2of the first and second electrodes respectively coincide with the Fermi level Ef0of the photoelectric conversion element.

In the state ofFIG. 6B, the first electrode generates a potential barrier that prevents electrons as the majority carriers from moving from the first electrode to the photoelectric conversion element. In other words, such a potential barrier occurs in the interface between the first electrode and the photoelectric conversion element. Accordingly, assuming that magnitude of a charge quantity of each electron is q, and magnitude of the potential barrier is ϕ, energy of qϕ is required to supply an electron from the first electrode to the photoelectric conversion element. Thus, it is difficult to supply the majority carriers (electrons) from the first electrode to the photoelectric conversion element, and for this reason, even when electrons are generated in the conduction band by light incidence on the photoelectric conversion element, movement of the electrons to the second electrode side is suppressed in the photoelectric conversion element, and current taken out from the first and second electrodes to the outside is suppressed.

FIG. 7AandFIG. 7Beach illustrates an equivalent circuit of the photoelectric conversion device10according to the embodiment of the present invention. InFIG. 7A, the first and second electrodes2and3are open therebetween. InFIG. 7B, the first and second electrodes2and3are connected to each other by the current line4, and the current line4is provided with an ammeter14.

It is assumed that by light incidence on the photoelectric conversion element1, a current Ishiftdue to a breakdown of the space-inversion symmetry of the structure thereof is generated in the photoelectric conversion element1as inFIG. 1. At this time, when the first and second electrodes2and3are made open therebetween, Ishiftis assumed to flow through a resistance Rbulkin the photoelectric conversion element1. At this time, a voltage (open circuit voltage) applied between the first and second electrodes2and3is assumed to be Voc. The voltage Vocis represented by the following equation (1).
Voc=RbulkIshift(1)

InFIG. 7A, Rcontactindicates a resistance that is the sum of a contact resistance between the first electrode2and the photoelectric conversion element1and a contact resistance between the second electrode3and the photoelectric conversion element1. From the state ofFIG. 7A, the first and second electrodes2and3are connected to each other by the current line4, and is then brought into the state ofFIG. 7B. InFIG. 7B, the above-described current Ishiftis divided into a current flowing through the resistance Rbulkand a current ISCflowing into the current line4through the contact resistance Rcontact. Here, ISCis represented by the following equation (2):

Thus, when Rbulkis sufficiently larger than Rcontact, ISCcan be assumed to be equal to Ishift. The equation (2) indicates the case where no bias voltage is applied between the first and second electrodes2and3. When a bias voltage Vbis applied between the first and second electrodes2and3inFIG. 7B, a current value Iobsmeasured by the ammeter14is represented by the following equation (3).

From the equations (1), (2), and (3), magnitude of the bias voltage Vbwhen Iobsis zero is equal to the magnitude of the open circuit voltage Voc. The symbols Rbulk, Rcontact, VOC, ISC, Vb, and Iobsin the following means Rbulk, Rcontact, VOC, ISC, Vb, and Iobsdescribed above with reference toFIG. 7AandFIG. 7B, respectively.

Experimental Example 1

An experimental example 1 of the photoelectric conversion device10according to the embodiment of the present invention is described. In the present experimental example 1, the polar material that forms the photoelectric conversion element1is SbSI as a p-type semiconductor, and the material that forms each of the first and second electrodes2and3is Pt. The Curie temperature of SbSI is about 295 K, and SbSI becomes ferroelectric when a temperature thereof becomes equal to or lower than the Curie temperature.

As inFIG. 1, the first and second electrodes2and3were connected to each other by the current line4. The current line4was provided with the ammeter14(refer toFIG. 7B) instead of the resistor13ofFIG. 1. A temperature of the photoelectric conversion element1was gradually lowered in a state where light was made incident on the photoelectric conversion element1. Further, at each temperature of the photoelectric conversion element1, a value of a current flowing through the current line4was measured by the ammeter14while changing a bias voltage Vbapplied between the first and second electrodes2and3. Note that at each temperature, an electric field was applied to the photoelectric conversion element1in the electrode separation direction to adjust the polarization direction, the application of the electric field was then stopped, and a current flowing through the current line4was measured. After a temperature of the photoelectric conversion element1became equal to or lower than the Curie temperature, and current flowed through the current line4at the bias voltage Vbof zero, a value of the current was measured at each temperature without application of the electric field. This is because the polarization direction of the photoelectric conversion element1was already in an adjusted state. The results of the measurement performed in this manner are illustrated inFIG. 8toFIG. 10.

InFIG. 8, the horizontal axis indicates the bias voltage Vbapplied between the first and second electrodes2and3, and the vertical axis indicates the current Iobsmeasured by the ammeter14. InFIG. 8, the respective straight lines are the measurement result at respective temperatures of the photoelectric conversion element1. As a temperature at the measurement becomes higher, a gradient of the straight line inFIG. 8becomes larger. The straight line having the maximum gradient is the measurement result at 200 K, and the straight line having the minimum gradient is the measurement result at 40 K. An inverse number of the gradient of the straight line is equal to “Rbulk+Rcontact” (≈Rbulk) in the equation (3).

As illustrated inFIG. 8, the large current Iobsoccurs in the current line4when the bias voltage Vbis zero. In other words, light is made incident on the photoelectric conversion element1having no p-n junction to thereby make it possible to efficiently take out the current from the photoelectric conversion element1through the electrodes2and3. Hereinafter, the current Iobsmeasured when the bias voltage Vbis zero is referred to also as the current ISC.

InFIG. 9A, the horizontal axis indicates a temperature of the photoelectric conversion element1, and the vertical axis indicates a measured value of the current ISCmeasured by the ammeter14when the bias voltage Vbis zero. As illustrated inFIG. 9A, in the process of lowering a temperature of the photoelectric conversion element1, when a temperature of the photoelectric conversion element1becomes equal to or lower than the Curie temperature, the current ISCoccurs abruptly. This indicates that the photoelectric conversion element1is transferred into ferroelectric at the Curie temperature, and the space-inversion symmetry of the structure thereof is broken by the above-described application of the electric field.

InFIG. 9B, the horizontal axis indicates a temperature of the photoelectric conversion element1, and the vertical axis indicates the open circuit voltage VOCbetween the first and second electrodes2and3. The voltage VOCis acquired from the bias voltage Vbwhen the current Iobsbecomes zero at each straight line ofFIG. 8.

InFIG. 10, the horizontal axis indicates a temperature of the photoelectric conversion element1, and the vertical axis indicates the resistance Rbulkof the photoelectric conversion element1. Here, assuming that “Rbulk+Rcontact” is equal to Rbulk, and ISCis equal to Ishift, the resistance RbulkofFIG. 10was acquired fromFIG. 9AandFIG. 9Band the above-described equation (1).

As illustrated inFIG. 9A,FIG. 9B, andFIG. 10, the open circuit voltage VOCis very large even when the current ISCdoes not greatly change depending on a change in temperature. The open circuit voltage VOCis about 70 V at the maximum, and is much larger than a voltage equivalent to a bandgap width (about 2 eV) of the photoelectric conversion element1. Thus, the photoelectric conversion device10according to the present embodiment is capable of generating electromotive force much larger than the voltage equivalent to the bandgap width of the photoelectric conversion element1, differently from a conventional photoelectric conversion element using a p-n junction.

Further, as illustrated inFIG. 9A,FIG. 9B, andFIG. 10, even though the open circuit voltage VOCbecomes very large, i.e., the resistance Rbulkincreases 104times or more, the current ISCdoes not greatly change. This indicates that current with less energy dissipation is generated in the photoelectric conversion element1. Such current is referred to also as shift current in the present application.

Experimental Example 2

An experimental example 2 of the photoelectric conversion device10according to the embodiment of the present invention is described. In the experimental example 2, the polar material that forms the photoelectric conversion element1was TTF-CA as a p-type semiconductor, and the material that forms each of the first and second electrodes2and3was Pt. The Curie temperature of TTF-CA is about 81 K. When a temperature of TTF-CA becomes equal to or lower than the Curie temperature, TTF-CA becomes ferroelectric. Such a photoelectric conversion device10was prepared as illustrated inFIG. 11. Further, inFIG. 11, third and fourth electrodes5and6for experiment were connected to the photoelectric conversion element1. The direction in which the third and fourth electrodes5and6are separated from each other is orthogonal to the direction in which the first and second electrodes2and3are separated from each other. The first to fourth electrodes2,3,5, and6were grounded by current lines (conducting wires)7,8,9, and11. The current lines7and9were provided with ammeters15and16, respectively.

Similarly to the experimental example 1, a temperature of the photoelectric conversion element1was gradually lowered in a state where light was made incident on the photoelectric conversion element1. Further, at each temperature of the photoelectric conversion element1, values of currents flowing through the current lines7and9were measured by the ammeters15and16while changing a bias voltage Vbapplied between the first and second electrodes2and3and a bias voltage Vb2applied between the third and fourth electrodes5and6. Note that at each temperature, an electric field was applied to the photoelectric conversion element1in the direction a that is the separation direction of the first and second electrodes2and3to adjust the polarization direction, the application of the electric field was then stopped, and currents flowing through the current lines7and9were measured. After a temperature of the photoelectric conversion element1becomes equal to or lower than the Curie temperature, and current flowed through the current lines7and9at the bias voltage Vbof zero, currents were measured by the ammeters15and16without the application of the electric field at each temperature as in the experimental example 1. The results of the measurement performed in this manner are illustrated inFIG. 12AandFIG. 12B.

InFIG. 12A, the horizontal axis indicates a temperature of the photoelectric conversion element1, and the vertical axis indicates a value of the current measured by each of the ammeters15and16. InFIG. 12A, the thick solid line indicates a value measured by the ammeter15, and the thick broken line indicates a value measured by the ammeter16. Note thatFIG. 12Aindicates values measured by the ammeters15and16in a state where the bias voltage Vbbetween the first and second electrodes2and3and the bias voltage Vb2between the third and fourth electrodes5and6are zero.

In the process of lowering a temperature of the photoelectric conversion element1, when a temperature of the photoelectric conversion element1becomes equal to or lower than the Curie temperature, the current was abruptly generated as illustrated by the thick solid line ofFIG. 12A. This indicates that the photoelectric conversion element1is transferred into ferroelectric at the Curie temperature, and the space-inversion symmetry of the structure thereof is broken by the application of the electric field.

Further, as understood from the measurement result of the thick solid line inFIG. 12A, the first and second electrodes2and3are separated in the polarization direction of the photoelectric conversion element1to thereby make it possible to take out, from the photoelectric conversion element1, the current due to the light incidence on the photoelectric conversion element1. Meanwhile, as understood from the measurement result of the thick broken line ofFIG. 12A, the current can be hardly taken out from the third and fourth electrodes5and6arranged so as to be separated in the direction orthogonal to the polarization direction of the photoelectric conversion element1.

InFIG. 12B, the horizontal axis indicates a temperature of the photoelectric conversion element1, and the vertical axis indicates the open circuit voltage VOCbetween the first and second electrodes2and3. Magnitude of VOCcorresponds to magnitude of the bias voltage Vbapplied between the first and second electrodes2and3when a measured value of the ammeter15is zero. As understood fromFIG. 12B, VOCis about 6.5 V at 50 K, and is much larger than a voltage equivalent to a bandgap width (about 0.5 eV) of the photoelectric conversion element1.

Further, in the experimental example 2, when a temperature of the photoelectric conversion element1is 79 K inFIG. 11, a current was measured by the ammeter15while changing the bias voltage Vbapplied between the first and second electrodes2and3as described above, in each of the case where the electric field is applied to the photoelectric conversion element1in the direction a and the case where the electric field is applied to the photoelectric conversion element1in the direction b opposite to the direction a. The measurement results are illustrated inFIG. 13.

InFIG. 13, the horizontal axis indicates the bias voltage Vbapplied between the first and second electrodes2and3, and the vertical axis indicates each current value Iobsmeasured by the ammeter15. InFIG. 13, the measurement result A indicates a current value Iobswhen light is made incident on the photoelectric conversion element1where the electric field is applied to the photoelectric conversion element1in the direction a, the measurement result B indicates a current value Iobswhen light is made incident on the photoelectric conversion element1where the electric field is applied to the photoelectric conversion element1in the direction b, and the measurement result C indicates a current value Iobswhen light is not made incident on the photoelectric conversion element1where the electric field is applied to the photoelectric conversion element1in the direction a.

As understood from the measurement results A and B ofFIG. 13, switching the application direction of the electric field between the direction a and the direction b switches the polarization direction of the photoelectric conversion element1, thereby making it possible to change the direction in which the current is taken out from the first and second electrodes. Note that in the measurement result C ofFIG. 13, the current due to the light incidence is not generated in the photoelectric conversion element1, and the current Iobsproportional to the bias voltage Vbis generated.

Experimental Example 3

An experimental example 3 of the photoelectric conversion device10according to an embodiment of the present invention is described. In this experimental example 3, the polar material that forms a photoelectric conversion element1was TTF-CA, and the material that forms each of the first and second electrodes2and3was Pt. A separation distance between the first and second electrodes2and3was set to be about 670 Further, as inFIG. 14, the first and second electrodes2and3were connected to each other by the current line4(conducting wire), and the current line4was provided with an ammeter14.

In the experimental example 3, the experiment was performed in the following manner, in each of the case where a temperature of the photoelectric conversion element1was set as 70 K lower than the Curie temperature thereof and the case where a temperature of the photoelectric conversion element1was set as 90 K higher than the Curie temperature thereof. As inFIG. 14, the photoelectric conversion element1was irradiated with laser light at an intermediate position between the first and second electrodes2and3. An irradiation region (region surrounded by the broken line inFIG. 14) of the laser light in the photoelectric conversion element1is a region that linearly extends in a direction orthogonal to the separation direction of the electrodes2and3. The position of the irradiation region was changed in the electrode separation direction indicated by the arrow inFIG. 14, and a current was measured by the ammeter14for each position of the irradiation region. Note that when a temperature of the photoelectric conversion element1is 70 K, an electric field is applied to the photoelectric conversion element1in the electrode separation direction to thereby adjust the polarization of the photoelectric conversion element1to be the electrode separation direction, and then, the laser light was applied to the photoelectric conversion element1.

The results of measurements performed in this manner are illustrated inFIG. 15AandFIG. 15B. InFIG. 15A, the horizontal axis indicates a position of the irradiation region, and the vertical axis indicates a current value measured by the ammeter14. The origin of the horizontal axis is the center between the first and second electrodes2and3. InFIG. 15AandFIG. 15B, the thick solid line is the measurement result in the case where a temperature of the photoelectric conversion element1was 70 K, and the thick broken line is the measurement result in the case where a temperature of the photoelectric conversion element1was 90 K.FIG. 15Billustrates the measurement result at 90 K inFIG. 15Aenlarged in the vertical-axis direction.

In the case of 70 K, a measurement value of the current is large even when the laser light was applied to any position between the first and second electrodes2and3. Particularly, in the case of 70 K, a measurement value of the current is large (about 70 pA) even when the laser light is applied to the center between the first and second electrodes2and3. From this, it can be understood that majority carriers generated by the laser light are moved in the electrode separation direction over a distance of several hundreds of micrometers. Thus, it is understood that the current due to the carriers is current (shift current) with less energy dissipation.

Meanwhile, in the case of 90 K, as indicated by the thick broken lines inFIG. 15AandFIG. 15B, a current measurement value is very small as compared with the case of 70 K, and current is generated only when a irradiation region of the laser light is in the vicinity of the electrodes2and3. A profile of such a current measurement value indicates that the generated current is typical diffusion current.

Experimental Example 4: Difference from Comparative Example

An experimental example 4 of a photoelectric conversion device20according to a comparative example is described. As inFIG. 16A, the photoelectric conversion device20is configured such that first and second electrodes22and23are separated from each other and are connected to a photoelectric conversion element21. A polar material that forms the photoelectric conversion element21was SbSI as a p-type semiconductor, a material that forms the first electrode22was Pt, and a material that forms the second electrode23was LiF/Al. A Fermi level of Pt is lower than a Fermi level of SbSI, but a Fermi level of LiF/Al is higher than the Fermi level of SbSI. Accordingly, a band structure of the photoelectric conversion device20inFIG. 16Abecomes that as illustrated inFIG. 4Bdescribed above.

Further, as inFIG. 16A, the first and second electrodes22and23were connected to each other by a current line24, and the current line24was provided with an ammeter25. A temperature of the photoelectric conversion element21was gradually lowered in a state where light is made incident on the photoelectric conversion element21. At each temperature of the photoelectric conversion element21, a value of current flowing through the current line24was measured by the ammeter25without applying a bias voltage between the first and second electrodes22and23. Note that at each temperature, an electric field was applied to the photoelectric conversion element21in the direction a ofFIG. 16Ato polarize the photoelectric conversion element21in the direction a, the application of the electric field was then stopped, a current flowing through the current line24was measured, an electric field was then applied to the photoelectric conversion element21in the direction b ofFIG. 16Ato polarize the photoelectric conversion element21in the direction b, the application of the electric field was then stopped, and a current flowing through the current line24was measured. The results of the measurement performed in this manner are illustrated inFIG. 16B.

InFIG. 16B, the horizontal axis indicates a temperature of the photoelectric conversion element21, and the vertical axis indicates a current measured by the ammeter25. InFIG. 16B, the thick solid line is the measurement result in the case where the electric field is applied in the direction a, and the thick broken line is the measurement result in the case where the electric field is applied in the direction b. As understood fromFIG. 16B, when a temperature of the photoelectric conversion element21was close to the Curie temperature (about 295 K) and was equal to or lower than the Curie temperature, the current flowed in the same direction regardless of the direction of application of the electric field. This differs from the characteristics in the present embodiment that when the polarization direction is reversed as described above with reference toFIG. 13, the direction of the current taken out from the first and second electrodes2and3is also reversed.

FIG. 17Ais a diagram illustrating a band structure of a polar material SbSI in relation to a Fermi level of each material. InFIG. 17A, the band structure of SbSI is illustrated on the left side. In SbSI on the left side ofFIG. 17A, Ef0indicates a Fermi level, Ecindicates an energy level at the lower end of a conduction band, and Evindicates an energy level at the upper end of a valance band. The levels Ef0, Ec, and Evare lower than a potential energy of an electron in a vacuum by 5.0 eV, 3.35 eV, and 5.50 eV, respectively. The Fermi levels of materials Pt, Au, Ag, Al, LiF/Al, Mg, and Ca are respectively indicated by thick horizontal lines on the right side ofFIG. 17A.

An experiment was performed on a photoelectric conversion device in which two electrodes are provided at an interval on a photoelectric conversion element formed of the polar material SbSI. In this experiment, a metal material of both of the two electrodes was Pt, Au, Ag, Al, LiF/Al, Mg or Ca inFIG. 17A, and in each case of the metal materials, a current generated by light incidence on the photoelectric conversion element was taken out from the two electrodes and was measured, in a state where a temperature of the photoelectric conversion element was set as 200 K lower than the Curie temperature (about 295 K) of SbSI, and the photoelectric conversion element was polarized in the separation direction of the two electrodes. The measurement was performed using an ammeter provided at a conducting wire connecting the two electrodes to each other. At the time of this measurement, no bias voltage was applied to the two electrodes. The results of the measurement performed in this manner are illustrated inFIG. 17B.

InFIG. 17B, the horizontal axis indicates a work function (equivalent to a Fermi level) of the material of the electrodes, and the vertical axis indicates a current value measured in the above-described manner. The black square marks inFIG. 17Beach indicate the measurement result where the materials of both of the two electrodes are Pt, Au, Ag, Al, LiF/Al, Mg or Ca.

As understood fromFIG. 17B, when Pt or Au having a Fermi level lower than the Fermi level Ef0of SbSI as a p-type semiconductor was selected as the electrode material, the taken-out current is much larger than in the cases of the other electrode materials.

Experimental Example 6: Difference from Comparative Example

An experiment for comparing the photoelectric conversion device10according to the embodiment of the present invention with a conventional example was performed. In this example, in the photoelectric conversion device10according to the embodiment of the present invention, the polar material that forms the photoelectric conversion element1was SbSI, and the material that forms each of the first and second electrodes2and3was Pt. A temperature of the photoelectric conversion element1was gradually lowered, and in the same manner as that described above, at each temperature, a current taken out from the first and second electrodes2and3was measured in a state where the photoelectric conversion element1was polarized in the fixed electrode separation direction, and light of a constant intensity was made incident on the photoelectric conversion element1. The thick solid line inFIG. 18is the measurement result of the present embodiment.

Meanwhile, a conventional photoelectric conversion device that includes a conventional photoelectric conversion element with a p-n junction and includes two electrodes respectively connected to a p-type semiconductor and an n-type semiconductor of the photoelectric conversion element was provided as the conventional example. In such a photoelectric conversion device, a temperature of the photoelectric conversion element was gradually lowered, and at each temperature, light of a constant intensity was made incident on the photoelectric conversion element, and a value of current taken out from the two electrodes was measured. The thick broken line inFIG. 18is the measurement result of this conventional example.

In the case of the conventional photoelectric conversion device with the p-n junction, as understood from the conventional example ofFIG. 18, a current becomes smaller as a temperature is lowered, and a current disappears at a temperature below about 100 K. In other words, a current generated in the photoelectric conversion device as the conventional example disappears at a low temperature. Meanwhile, in the case of the present embodiment with no p-n junction, a current becomes larger as the temperature is lowered. In other words, it is understood fromFIG. 18that the current generated in the photoelectric conversion device10according to the present embodiment is small-scattering current (shift current) that is maintained at a high value even at a low temperature

Application Example

FIG. 19Ais a schematic configuration diagram of a photosensor100to which the photoelectric conversion device10according to the embodiment of the present invention is applied. The photosensor100detects light incidence on the photoelectric conversion element1, on the basis of current taken out from the first and second electrodes2and3. The photosensor100may include the above-described photoelectric conversion device10, a current line4, and a detection unit101.

The current line4is connected to the first and second electrodes2and3. The first and second electrodes2and3may be connected to each other through the current line4. Current generated in the photoelectric conversion element1is taken out to the current line4through the first and second electrodes2and3.

The detection unit101detects current flowing through the current line4. When the current is detected, the detection unit101outputs a detection signal indicating that light is made incident on the photoelectric conversion element1. For example, the detection unit101may be configured so as to output the detection signal when a value of the current flowing through the current line4exceeds a threshold value.

FIG. 19Bis a schematic configuration diagram of a power generation device200to which the photoelectric conversion device10according to the embodiment of the present invention is applied. The power generation device200supplies power taken out from the first and second electrodes2and3to a storage battery or a load (the storage battery201inFIG. 19B). The power generation device200may include the above-described photoelectric conversion device10, a current line4, and the storage battery201(or load). Note that the storage battery201(or load) does not need to be a constituent element of the power generation device200.

The current line4is connected to the first and second electrodes2and3. The first and second electrodes2and3may be connected to each other through the current line4. Current generated in the photoelectric conversion element1by light incidence on the photoelectric conversion element1is taken out to the current line4through the first and second electrodes2and3. Here, the light incident on the photoelectric conversion element1may be sunlight or radiant light generated from a heat source, for example.

The storage battery201receives, through the current line4, power due to the current generated in the photoelectric conversion element1, and stores the received power.

Note that when the load is provided instead of the storage battery201, the power due to the current generated in the photoelectric conversion element1is supplied to the load through the current line4. This load may be a device (e.g., an electric light, a drive device, or the like) that consumes the supplied power.

A photoelectric conversion method according to an embodiment of the present invention includes steps S1and S2. At the step S1, the above-described photoelectric conversion device10is prepared and installed in a desired position. At the step S2, in the photoelectric conversion device10prepared in the step S1, light is made incident on the photoelectric conversion element1at the position between the first and second electrodes2and3, and current thereby generated in the photoelectric conversion element1is taken out from the first and second electrodes2and3to the current line4. The taken-out current flows to a desired part (e.g., the above-described detection unit101, the storage battery201, or the load) through the current line4.

Before the step S2is performed, the photoelectric conversion element1is polarized in the electrode separation direction, and the polarization direction of the photoelectric conversion element1is thus adjusted. In this state, the step S2is performed. For example, when the polar material forming the photoelectric conversion element1is a ferroelectric material, before the step S2is performed, an electric field is applied to the photoelectric conversion element1in the electrode separation direction in a state where the photoelectric conversion element1has been brought into a ferroelectric state under an environment of a temperature equal to or lower than the Curie temperature of the polar material. Thereby, the polarization direction of the photoelectric conversion element1is adjusted to be the electrode separation direction. In this state, the step S2is performed. However, when the step S2is performed, the application of the electric field to the photoelectric conversion element1may be stopped.

Effects of Embodiment

(1) According to the above-described embodiment of the present invention, a pair of an electron and a positive hole generated by light incidence on the photoelectric conversion element1are moved in the directions opposite to each other by the breakdown of the space-inversion symmetry of the structure of the polar material forming the photoelectric conversion element1. Thus, the majority carriers (electrons or positive holes) of the photoelectric conversion element1are moved to one of the first and second electrodes2and3.

(2) Meanwhile, there occurs no potential barrier that prevents the flow of the majority carriers from the other of the first and second electrodes2and3to the photoelectric conversion element1. Thus, accompanying the movement of the majority carriers to one of the electrode2or3due to the breakdown of the space-inversion symmetry in the photoelectric conversion element1, the majority carriers flow from the other of the electrode2or3to the photoelectric conversion element1.

(3) The current due to each of the above (1) and (2) basically differs in the generation mechanism from current caused by a p-n junction. The current of which generation mechanism differs from the conventional one has a characteristic of small scattering (e.g., refer toFIG. 15Adescribed above). In other words, carriers generated by light incidence on the conventional photoelectric conversion element including a p-n junction are moved while being influenced by scattering due to impurities, defects, lattice vibrations, and the like, whereas according to the present embodiment, the current that is not influenced by such scattering is generated. In addition to this matter, according to the present embodiment, there is no potential barrier to the majority carriers as described above, and thus, the current due to the majority carriers generated in the photoelectric conversion device10can be efficiently taken out to the outside.

(4) Since the photoelectric conversion element1has no p-n junction, it becomes unnecessary to take into account constraints related to a p-n junction such as formation of a suitable p-n junction surface.

(5) The current having no temperature dependency can be generated by light incidence on the photoelectric conversion element1(e.g., refer toFIG. 9Adescribed above).

(6) It is possible to obtain electromotive force (open circuit voltage VOC) far exceeding a voltage equivalent to a bandgap width of the photoelectric conversion element1(e.g., refer toFIG. 9BandFIG. 12Bdescribed above).

(7) When photoelectric conversion is performed using the photoelectric conversion element including a p-n junction as in the conventional method, shot noise caused by fluctuation in the number of electrons is generated. Meanwhile, in the above-described photoelectric conversion method according to the present embodiment, such shot noise is not generated because the current is caused on the basis of the property of a wave of an electron. Thus, low-noise photoelectric conversion can be implemented.

(8) The current is generated in a time scale (usually, about a femtosecond) of optical transition between the bands in the photoelectric conversion element1, and for this reason, when pulse light is made incident on the photoelectric conversion element1, photoelectric conversion with quick response to the pulse light and with less noise can be performed. Thus, for example, the photosensor100with quick response and with less noise can be implemented.

The present invention is not limited to the above-described embodiment. It is natural that various modifications can be made within the scope of the technical idea of the present invention. For example, the above-described effects do not necessarily limit the present invention. Further, the present invention may exhibit any one of the effects described in the present specification, or another effect that can be understood from the present specification.

DESCRIPTION OF REFERENCE SIGNS