Photoelectric conversion element and imaging device

Abstract

This photoelectric conversion element is provided with: a photoelectric conversion layer that converts light into a signal charge; a first electrode that contains a metal element and that collects the signal charge; and a second electrode that faces the first electrode with the photoelectric conversion layer interposed therebetween. The signal charge is a hole. The work function of the first electrode is smaller than the work function of the second electrode.

Description

Photoelectric conversion element and imaging device

[0001] The present disclosure relates to a photoelectric conversion element and an imaging device.

[0002] An imaging device such as an image sensor is an element that includes, for example, a photoelectric conversion element that generates a signal charge according to the amount of incident light, a plurality of pixels arranged one-dimensionally or two-dimensionally, a signal detection circuit that detects the signal charge, and various elements for driving them. A stacked image sensor is an imaging device that has pixels including a photoelectric conversion element with a structure in which, from the substrate side, a lower electrode that collects signal charge, a photoelectric conversion layer, and an upper electrode are stacked. An example of a stacked image sensor is disclosed in Patent Document 1.

[0003] JP 2011-71469 A

[0004] In photoelectric conversion elements and imaging devices, dark current flows regardless of the incidence of light on the photoelectric conversion element, and is therefore desirable to reduce because it causes noise.

[0005] In addition, in photoelectric conversion elements and imaging devices, signal charges generated by the photoelectric conversion elements in response to the amount of incident light are accumulated, for example, in a charge accumulation region. A signal detection circuit detects the signal charges accumulated in the charge accumulation region during, for example, an exposure period. However, charges may also accumulate in the charge accumulation region during non-exposure periods when there is no need to accumulate signal charges in the charge accumulation region. Therefore, photoelectric conversion elements and imaging devices have a problem of parasitic sensitivity, which is an unintended sensitivity caused by such charges.

[0006] The present disclosure provides a photoelectric conversion element and the like that can reduce dark current and parasitic sensitivity.

[0007] A photoelectric conversion element according to one aspect of the present disclosure includes a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal element and collecting the signal charges, and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, wherein the signal charges are holes, and the work function of the first electrode is smaller than the work function of the second electrode.

[0008] A photoelectric conversion element according to one aspect of the present disclosure includes a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal element and collecting the signal charges, and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, wherein the signal charges are electrons, and the work function of the first electrode is greater than the work function of the second electrode.

[0009] An imaging device according to one aspect of the present disclosure includes the photoelectric conversion element, a charge accumulation region electrically connected to the first electrode, and a voltage supply circuit electrically connected to the second electrode and applying a potential difference between the first electrode and the second electrode, wherein the voltage supply circuit supplies a first voltage to the second electrode during a first period and a second voltage different from the first voltage to the second electrode during a second period.

[0010] A photoelectric conversion element according to one embodiment of the present disclosure includes a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal element and collecting the signal charges, a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, and an intermediate layer positioned between the photoelectric conversion layer and the first electrode, wherein the signal charges are holes, and the relationship (A-B)-(C-D)<0 is satisfied, where A [eV] is the work function of the first electrode, B [eV] is the work function of the second electrode, C [eV] is the ionization potential of the intermediate layer, and D [eV] is the ionization potential of the photoelectric conversion layer.

[0011] A photoelectric conversion element according to one embodiment of the present disclosure includes a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal element and collecting the signal charges, a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, and an intermediate layer positioned between the photoelectric conversion layer and the first electrode, wherein the signal charges are electrons, and the relationship (A-B)+(F-E)>0 is satisfied, where A [eV] is the work function of the first electrode, B [eV] is the work function of the second electrode, E [eV] is the electron affinity of the intermediate layer, and F [eV] is the electron affinity of the photoelectric conversion layer.

[0012] According to the present disclosure, it is possible to provide a photoelectric conversion element and the like that can reduce dark current and parasitic sensitivity.

[0013] FIG. 1 is a schematic cross-sectional view showing the configuration of a photoelectric conversion element according to an embodiment. FIG. 2 is a schematic cross-sectional view showing the configuration of another photoelectric conversion element according to an embodiment. FIG. 3 is an exemplary energy band diagram of a photoelectric conversion element according to an embodiment. FIG. 4 is an exemplary energy band diagram of another photoelectric conversion element according to an embodiment. FIG. 5 is a diagram for explaining an example of a case where the occurrence of parasitic sensitivity is suppressed when holes are used as signal charges. FIG. 6 is a diagram for explaining an example of a case where parasitic sensitivity is generated when holes are used as signal charges. FIG. 7 is a diagram schematically showing the relationship between the current flowing through the lower electrode and the voltage of the lower electrode. FIG. 8 is a diagram for explaining that the occurrence of parasitic sensitivity is suppressed by an intermediate layer when holes are used as signal charges. FIG. 9 is a diagram for explaining an example of a case where the occurrence of parasitic sensitivity is suppressed when electrons are used as signal charges. FIG. 10 is a diagram for explaining an example of a case where parasitic sensitivity is generated when electrons are used as signal charges. FIG. 11 is a diagram schematically showing the relationship between the current flowing through the lower electrode and the voltage of the lower electrode. FIG. 12 is a diagram illustrating how the occurrence of parasitic sensitivity is suppressed by an intermediate layer when electrons are used as signal charges. FIG. 13 is a diagram illustrating the relationship between the work function and the element concentration of a contained element with respect to the position in the thickness direction of a titanium nitride film. FIG. 14 is a schematic cross-sectional view illustrating the configuration of a photoelectric conversion element according to a modified example of an embodiment. FIG. 15 is a diagram illustrating the circuit configuration of an imaging device according to an embodiment. FIG. 16 is a cross-sectional view schematically illustrating a cross section of a device structure of a pixel of an imaging device according to an embodiment. FIG. 17 is a flowchart of a method for manufacturing a photodetector according to an embodiment. FIG. 18 is a cross-sectional view illustrating a method for manufacturing a photodetector according to an embodiment. FIG. 19 is a cross-sectional view illustrating a method for manufacturing a photodetector according to an embodiment. FIG. 20 is a cross-sectional view illustrating a method for manufacturing a photodetector according to an embodiment. FIG. 21 is a diagram illustrating an example of a schematic current-voltage characteristic of a photodetector according to an embodiment. FIG. 22 is a timing chart illustrating an example of a voltage supplied to an upper electrode of a photodetector according to an embodiment and the timing of operation in each row of a plurality of pixels. FIG. 23 is a block diagram illustrating an example of a configuration of a camera system according to an embodiment.FIG. 24 is a diagram showing the evaluation results of the parasitic sensitivity of the imaging devices in the example and the comparative example.

[0014] (Findings that Form the Basis of the Present Disclosure) Before describing the embodiments of the present disclosure in detail, the problems that the inventors of the present disclosure have found will be described.

[0015] One way to improve the characteristics of an imaging device is to reduce dark current. One cause of dark current is defects in an impurity region that functions as at least a part of a charge accumulation region formed in a semiconductor substrate. The dark current generated by defects in the impurity region increases as the absolute value of the potential of the charge accumulation region increases. Therefore, dark current can be reduced by reducing the absolute value of the initialization voltage for initializing the charge accumulation region, which serves as the reference voltage for the charge accumulation region.

[0016] In addition, reducing parasitic sensitivity is also an important issue for imaging devices. For example, in imaging devices with a global shutter function, signal charge is detected during non-exposure periods, and parasitic sensitivity occurring during non-exposure periods can cause noise. In stacked image sensors, for example, during non-exposure periods, a predetermined voltage close to the initialization voltage of the charge accumulation region is applied to the upper electrode to suppress the accumulation of signal charge in the charge accumulation region connected to the lower electrode. In this case, if the difference between the initialization voltage and the predetermined voltage applied to the upper electrode is large, the potential difference between the charge accumulation region and the upper electrode makes it easier for charge to flow into the charge accumulation region. Therefore, if the absolute value of the initialization voltage is reduced to reduce dark current as described above, the difference between the initialization voltage and the predetermined voltage applied to the upper electrode tends to increase, resulting in the problem of parasitic sensitivity.

[0017] Therefore, the present disclosure provides a photoelectric conversion element and the like that can reduce dark current and parasitic sensitivity.

[0018] (Summary of the Present Disclosure) As an overview of one embodiment of the present disclosure, examples of a photoelectric conversion element and an imaging device according to the present disclosure will be described below.

[0019] (First Aspect) For example, a photoelectric conversion element according to a first aspect of the present disclosure includes a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal element and collecting the signal charges, and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, wherein the signal charges are holes, and the work function of the first electrode is smaller than the work function of the second electrode.

[0020] As a result, because the work function of the first electrode is smaller than the work function of the second electrode, even if the voltage of the second electrode relative to the first electrode is increased toward the positive side, holes, which are signal charges, are less likely to move to the first electrode that collects the signal charges. Therefore, from a relative perspective, it is possible to shift the voltage of the first electrode, which makes it easier for holes to move to the first electrode, toward a lower absolute value, thereby reducing parasitic sensitivity. Furthermore, by lowering the absolute value of the voltage of the first electrode, when the first electrode is connected to a charge accumulation region, it is possible to lower the absolute value of the voltage of the charge accumulation region, thereby reducing dark current generated in the charge accumulation region. Therefore, the photoelectric conversion element according to this embodiment makes it possible to reduce dark current and parasitic sensitivity.

[0021] (Second Aspect) Furthermore, for example, a photoelectric conversion element according to a second aspect of the present disclosure includes a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal element and collecting the signal charges, and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, wherein the signal charges are electrons, and the work function of the first electrode is greater than the work function of the second electrode.

[0022] As a result, because the work function of the first electrode is greater than the work function of the second electrode, even if the voltage of the second electrode relative to the first electrode is increased negatively, electrons, which are signal charges, are less likely to move to the first electrode that collects the signal charges. Therefore, from a relative perspective, it is possible to shift the voltage of the first electrode, which makes it easier for electrons to move to the first electrode, in a direction that decreases its absolute value, thereby reducing parasitic sensitivity. Furthermore, by lowering the absolute value of the voltage of the first electrode, when the first electrode is connected to a charge accumulation region, it is possible to lower the absolute value of the voltage of the charge accumulation region, thereby reducing dark current generated in the charge accumulation region. Therefore, the photoelectric conversion element according to this embodiment makes it possible to reduce dark current and parasitic sensitivity.

[0023] (Third Aspect) Furthermore, for example, in the photoelectric conversion element according to the first or second aspect, a difference between the work function of the first electrode and the work function of the second electrode may be greater than 0.0 eV and not greater than 2.0 eV.

[0024] This makes it possible to suppress the inflow of charges from the first electrode or the second electrode into the photoelectric conversion layer.

[0025] (Fourth Aspect) Furthermore, for example, in the photoelectric conversion element according to any one of the first to third aspects, the first electrode may contain a metal nitride containing the metal element.

[0026] This makes it possible to easily realize a first electrode having the above-mentioned work function.

[0027] (Fifth Aspect) Furthermore, for example, in the photoelectric conversion element according to the fourth aspect, the metal nitride may be titanium nitride.

[0028] This makes it possible to easily realize a first electrode having the above-mentioned work function.

[0029] (Sixth Aspect) Furthermore, for example, in the photoelectric conversion element according to any one of the first to fifth aspects, the first electrode may include a layer containing the metal element and having a first surface and a second surface closer to the photoelectric conversion layer than the first surface, and a self-assembled monolayer located on the second surface of the layer.

[0030] This makes it possible to easily adjust the work function of the first electrode by selecting the material that forms the self-assembled monolayer.

[0031] (Seventh Aspect) Furthermore, for example, in a photoelectric conversion element according to any one of the first to sixth aspects in which the signal charges are holes, the element may further include an intermediate layer located between the photoelectric conversion layer and the first electrode, and the ionization potential of the intermediate layer may be greater than the ionization potential of the photoelectric conversion layer.

[0032] This allows for a further reduction in parasitic sensitivities.

[0033] (Eighth Aspect) Furthermore, for example, in a photoelectric conversion element according to any one of the first to sixth aspects in which the signal charge is an electron, the element may further include an intermediate layer located between the photoelectric conversion layer and the first electrode, and the electron affinity of the intermediate layer may be smaller than the electron affinity of the photoelectric conversion layer.

[0034] This allows for a further reduction in parasitic sensitivities.

[0035] (Ninth Aspect) Furthermore, for example, an imaging device according to a ninth aspect of the present disclosure includes the photoelectric conversion element according to any one of the first to eighth aspects, a charge accumulation region electrically connected to the first electrode, and a voltage supply circuit electrically connected to the second electrode and applying a potential difference between the first electrode and the second electrode, wherein the voltage supply circuit supplies a first voltage to the second electrode during a first period, and supplies a second voltage different from the first voltage to the second electrode during a second period.

[0036] This allows the sensitivity of the photoelectric conversion element to be adjusted by changing the voltage supplied to the second electrode. Furthermore, by providing the photoelectric conversion element, it is possible to reduce parasitic sensitivity when lowering the sensitivity in adjusting the sensitivity of the photoelectric conversion element.

[0037] (Tenth Aspect) Also, for example, in the imaging device according to the ninth aspect, the first period may be an exposure period during which the signal charge is accumulated in the charge accumulation region, and the second period may be a non-exposure period that is a period other than the exposure period during operation of the imaging device.

[0038] This allows a global shutter operation to be achieved by changing the voltage supplied to the second electrode, and also allows a reduction in parasitic sensitivity during non-exposure periods in the global shutter operation.

[0039] (Eleventh Aspect) Furthermore, for example, in the imaging device according to the ninth or tenth aspect, the first voltage may be a voltage having the same polarity as the polarity of the signal charges.

[0040] This allows signal charges to be accumulated in the charge accumulation region during the first period.

[0041] (Twelfth Aspect) Furthermore, for example, in the imaging device according to the eleventh aspect, the second voltage may be 0 or a voltage having the same polarity as the polarity of the signal charges.

[0042] This eliminates the need for the voltage supply circuit to have a function for changing the polarity of the voltage supplied, thereby simplifying the circuit configuration of the imaging device.

[0043] (13th Aspect) Furthermore, for example, in the imaging device according to any one of the 9th to 12th aspects, the imaging device may be configured such that an initialization voltage for initializing the potential of the charge accumulation region is supplied to the charge accumulation region, and the initialization voltage may be 0 or a voltage having the same polarity as the polarity of the signal charge.

[0044] This makes it possible to prevent the polarity of the potential of the charge accumulation region from changing when signal charges are accumulated in the charge accumulation region.

[0045] (14th Aspect) Furthermore, for example, a photoelectric conversion element according to a 14th aspect of the present disclosure includes a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal element and collecting the signal charges, a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, and an intermediate layer positioned between the photoelectric conversion layer and the first electrode, wherein the signal charges are holes, and the relationship (A-B)-(C-D)<0 is satisfied, where A [eV] is the work function of the first electrode, B [eV] is the work function of the second electrode, C [eV] is the ionization potential of the intermediate layer, and D [eV] is the ionization potential of the photoelectric conversion layer.

[0046] As a result, (A-B)-(C-D)<0 is satisfied, and even if the voltage of the second electrode relative to the first electrode is increased toward the positive side, holes, which are signal charges, are less likely to move to the first electrode that collects the signal charges. Therefore, from a relative perspective, it is possible to shift the voltage of the first electrode, which makes it easier for holes to move to the first electrode, toward a lower absolute value, thereby reducing parasitic sensitivity. Furthermore, by lowering the absolute value of the voltage of the first electrode, when the first electrode is connected to a charge accumulation region, it is possible to lower the absolute value of the voltage of the charge accumulation region, thereby reducing the dark current generated in the charge accumulation region. Therefore, the photoelectric conversion element according to this embodiment makes it possible to reduce dark current and parasitic sensitivity.

[0047] (Fifteenth Aspect) Furthermore, for example, a photoelectric conversion element according to a fifteenth aspect of the present disclosure includes a photoelectric conversion layer that converts light into signal charges, a first electrode containing a metal element and collecting the signal charges, a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, and an intermediate layer positioned between the photoelectric conversion layer and the first electrode, wherein the signal charges are electrons, and the relationship (A-B)+(F-E)>0 is satisfied, where A [eV] is the work function of the first electrode, B [eV] is the work function of the second electrode, E [eV] is the electron affinity of the intermediate layer, and F [eV] is the electron affinity of the photoelectric conversion layer.

[0048] As a result, (A-B) + (F-E) > 0 is satisfied, and even if the voltage of the second electrode relative to the first electrode is increased to the negative side, electrons, which are signal charges, are less likely to move to the first electrode that collects the signal charges. Therefore, from a relative perspective, it is possible to shift the voltage of the first electrode, which makes it easier for electrons to move to the first electrode, in a direction that lowers its absolute value, thereby reducing parasitic sensitivity. Furthermore, by lowering the absolute value of the voltage of the first electrode, when the first electrode is connected to a charge accumulation region, it is possible to lower the absolute value of the voltage of the charge accumulation region, thereby reducing the dark current generated in the charge accumulation region. Therefore, the photoelectric conversion element according to this embodiment makes it possible to reduce dark current and parasitic sensitivity.

[0049] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0050] The embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, component placement and connection configurations, steps, and step order shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, among the components in the following embodiments, components not described in the independent claims are described as optional components.

[0051] Furthermore, each figure is a schematic diagram and is not necessarily an exact illustration. Therefore, for example, the scales of the figures do not necessarily match. Furthermore, in each figure, substantially the same components are given the same reference numerals, and redundant explanations are omitted or simplified.

[0052] Furthermore, in this specification, terms indicating the relationship between elements, such as vertical, terms indicating the shape of elements, such as rectangle, and numerical ranges are not expressions that only express a strict meaning, but are expressions that also include a substantially equivalent range, for example, a difference of about a few percent.

[0053] Furthermore, in this specification, the terms "upper" and "lower" do not refer to the upward (vertically upward) and downward (vertically downward) directions in absolute spatial recognition, but are used as terms defined by a relative positional relationship based on the stacking order in the stacked structure. Specifically, the light-receiving side of the photoelectric conversion element and the imaging device is referred to as "upper," and the side opposite the light-receiving side is referred to as "lower." Similarly, the "upper surface" and "lower surface" of each component refer to the surface facing the light-receiving side of the photoelectric conversion element and the imaging device as the "upper surface," and the surface facing the side opposite the light-receiving side as the "lower surface." Note that the terms "upper," "lower," "upper surface," and "lower surface" are used solely to specify the relative arrangement of components and are not intended to limit the orientation of the photoelectric conversion element and the imaging device during use. Furthermore, the terms "upper" and "lower" apply not only to cases where two components are arranged with a gap between them and another component is present between them, but also to cases where two components are arranged closely together and in contact with each other.

[0054] In this specification, electromagnetic waves in general, including visible light, infrared light, and ultraviolet light, are referred to as "light" for convenience.

[0055] (Embodiment) [Photoelectric Conversion Element] A photoelectric conversion element according to this embodiment will be described. The photoelectric conversion element according to this embodiment is a charge readout type photoelectric conversion element. The photoelectric conversion element according to this embodiment is used, for example, in an imaging device such as a stacked image sensor. FIG. 1 is a schematic cross-sectional view showing the configuration of a photoelectric conversion element 10 according to this embodiment. FIG. 2 is a schematic cross-sectional view showing the configuration of another photoelectric conversion element 110 according to this embodiment.

[0056] As shown in Fig. 1, photoelectric conversion element 10 is supported by support substrate 1 and includes a pair of electrodes: upper electrode 4 and lower electrode 2, photoelectric conversion layer 3 located between upper electrode 4 and lower electrode 2, and intermediate layer 5 located between photoelectric conversion layer 3 and lower electrode 2. Photoelectric conversion element 10 also includes protective layer 6 disposed opposite photoelectric conversion layer 3 with upper electrode 4 interposed therebetween. As shown in Fig. 2, photoelectric conversion element 110 has a configuration in which lower electrode 2 of photoelectric conversion element 10 is replaced with lower electrode 112. In this embodiment, lower electrodes 2 and 112 are examples of first electrodes, and upper electrode 4 is an example of a second electrode.

[0057] The photoelectric conversion elements 10 and 110 are used in an orientation such that light transmitted through the upper electrode 4 and the protective layer 6 enters the photoelectric conversion layer 3, for example.

[0058] Hereinafter, each component of the photoelectric conversion elements 10 and 110 according to this embodiment will be described.

[0059] The support substrate 1 may be any substrate that is commonly used to support a photoelectric conversion element, and may be, for example, a glass substrate, a quartz substrate, a semiconductor substrate, or a plastic substrate.

[0060] The lower electrodes 2 and 112 contain a metal element. The lower electrodes 2 and 112 contain, for example, a metal or a metal nitride containing the metal element, and are formed from the metal or the metal nitride. Examples of the metal element include titanium, tungsten, and tantalum. The metal nitride may be titanium nitride.

[0061] The lower electrode 2 is, for example, a metal-element-containing film from which a native oxide film has been removed on the surface 2s of the lower electrode 2 facing the photoelectric conversion layer 3. An intermediate layer 5 is formed on the surface 2s of the lower electrode 2 from which the native oxide film has been removed. On the other hand, the lower electrode 112 is, for example, a metal-element-containing film from which a native oxide film remains on the surface 112s of the lower electrode 112 facing the photoelectric conversion layer 3. In the example shown in FIGS. 1 and 2 , the surface 2s and the surface 112s are upper surfaces. As will be described in detail later, the native oxide film has been removed from the lower electrode 2, so the work function of the lower electrode 2 is smaller than that of the lower electrode 112. Note that the native oxide film does not need to be completely removed from the surface 2s of the lower electrode 2; for example, the surface 2s of the lower electrode 2 may contain oxygen elements as long as the oxygen element concentration is lower than that of the surface 112s of the lower electrode 112. Furthermore, if the work function of the lower electrodes 2 and 112 satisfies the conditions described below, the native oxide film may or may not be removed from the surfaces 2s and 112s.

[0062] The upper electrode 4 faces the lower electrode 2 or 112 with the photoelectric conversion layer 3 therebetween. The upper electrode 4 is, for example, a transparent electrode formed from a transparent conductive material. Examples of the material for the upper electrode 4 include transparent conductive oxide (TCO), indium tin oxide (ITO), indium zinc oxide (IZO), aluminum-doped zinc oxide (AZO), fluorine-doped tin oxide (FTO), and SnO. ２ and TiO ２ The upper electrode 4 may be made of a TCO, and metal materials such as aluminum (Al) and gold (Au) either singly or in combination, depending on the desired transmittance.

[0063] Various methods are used to fabricate the upper electrode 4 depending on the material used. For example, when ITO is used, methods such as an electron beam method, a sputtering method, a resistance heating vapor deposition method, a chemical reaction method such as a sol-gel method, or the application of a dispersion of indium tin oxide may be used. In this case, to fabricate the upper electrode 4, after forming the ITO film, UV-ozone treatment, plasma treatment, or the like may be further performed.

[0064] The photoelectric conversion layer 3 converts light into signal charges. The photoelectric conversion layer 3 includes, for example, a donor semiconductor material and an acceptor semiconductor material. The donor semiconductor material and the acceptor semiconductor material may be mixed to form a single layer, or a layer of the donor semiconductor material and a layer of the acceptor semiconductor material may be stacked. The photoelectric conversion layer 3 can be fabricated using, for example, a wet method such as a coating method using spin coating, or a dry method such as a vacuum deposition method. The vacuum deposition method is a method in which layer materials are vaporized by heating under vacuum and then deposited on a substrate. The intermediate layer 5 can also be fabricated using a method similar to that used for the photoelectric conversion layer 3.

[0065] The photoelectric conversion layer 3 also contains, for example, an organic semiconductor material. The photoelectric conversion layer 3 is, for example, a mixed film with a bulk heterostructure containing a donor organic semiconductor material and an acceptor organic semiconductor material. Specific examples of donor organic semiconductor materials and acceptor organic semiconductor materials are given below.

[0066] Examples of donor organic semiconductor materials include triarylamine compounds, benzidine compounds, pyrazoline compounds, styrylamine compounds, hydrazone compounds, triphenylmethane compounds, carbazole compounds, polysilane compounds, thiophene compounds, phthalocyanine compounds, naphthalocyanine compounds, subphthalocyanine compounds, cyanine compounds, merocyanine compounds, oxonol compounds, polyamine compounds, indole compounds, pyrrole compounds, pyrazole compounds, polyarylene compounds, fused aromatic carbocyclic compounds, and metal complexes having nitrogen-containing heterocyclic compounds as ligands.

[0067] Examples of the fused aromatic carbocyclic compounds include naphthalene derivatives, anthracene derivatives, phenanthrene derivatives, tetracene derivatives, pyrene derivatives, perylene derivatives, and fluoranthene derivatives.

[0068] Examples of the acceptor organic semiconductor material include fullerene, fullerene derivatives, fused aromatic carbon ring compounds, 5- to 7-membered heterocyclic compounds containing a nitrogen atom, an oxygen atom, or a sulfur atom, polyarylene compounds, fluorene compounds, cyclopentadiene compounds, silyl compounds, and metal complexes having a nitrogen-containing heterocyclic compound as a ligand.

[0069] Examples of fullerenes include C60 fullerene and C70 fullerene.

[0070] The fullerene derivative is, for example, PCBM (phenyl C ６１ butyric acid methyl ester) and ICBA (indene C ６０ Bis-adducts), etc.

[0071] Examples of 5- to 7-membered heterocyclic compounds containing a nitrogen atom, an oxygen atom, or a sulfur atom include pyridine, pyrazine, pyrimidine, pyridazine, triazine, quinoline, quinoxaline, quinazoline, phthalazine, cinnoline, isoquinoline, pteridine, acridine, phenazine, phenanthroline, tetrazole, pyrazole, imidazole, thiazole, oxazole, indazole, benzimidazole, benzotriazole, benzoxazole, benzothiazole, carbazole, purine, triazolopyridazine, triazolopyrimidine, tetrazaindene, oxadiazole, imidazopyridine, pyrrolidine, pyrrolopyridine, thiadiazolopyridine, dibenzazepine, and tribenzazepine.

[0072] The donor organic semiconductor material and the acceptor organic semiconductor material are not limited to the above examples. Low-molecular-weight compounds and high-molecular-weight compounds may be used as the donor organic semiconductor material and the acceptor organic semiconductor material constituting the photoelectric conversion layer 3, as long as they are organic compounds that can be formed into a film as a photoelectric conversion layer by either a dry method or a wet method.

[0073] The photoelectric conversion layer 3 may also contain semiconductor materials other than organic semiconductor materials as donor and acceptor semiconductor materials. The photoelectric conversion layer 3 may also contain, as semiconductor materials, silicon semiconductors, compound semiconductors, quantum dots, perovskite materials, carbon nanotubes, or the like, or a mixture of two or more of these.

[0074] Here, we will explain the generation and movement of signal charges in the photoelectric conversion elements 10 and 110. The principles of generation and movement of signal charges are the same for the photoelectric conversion elements 10 and 110, so the following will explain the photoelectric conversion element 10, but the following explanation can also be applied to the photoelectric conversion element 110 by replacing the lower electrode 2 with the lower electrode 112.

[0075] The photoelectric conversion layer 3 generates excitons therein upon irradiation with light. The generated excitons diffuse within the photoelectric conversion layer 3 and are separated into electrons and holes at the interface between the acceptor semiconductor material and the donor semiconductor material. The separated electrons and holes move toward the lower electrode 2 or the upper electrode 4, respectively, in accordance with the electric field applied to the photoelectric conversion layer 3. When a voltage is applied between the upper electrode 4 and the lower electrode 2 so that the potential of the upper electrode 4 is higher than the potential of the lower electrode 2, the electrons move toward the upper electrode 4 and the holes move toward the lower electrode 2. When the photoelectric conversion element 10 is used in an imaging device, for example, the holes are collected by the lower electrode 2 and stored as signal charges in a charge storage node electrically connected to the lower electrode 2. The charge storage node is at least a part of a charge storage region that stores the signal charges collected by the lower electrode 2. Furthermore, when a voltage is applied between the upper electrode 4 and the lower electrode 2 so that the potential of the upper electrode 4 is lower than the potential of the lower electrode 2, holes move toward the upper electrode 4 and electrons move toward the lower electrode 2. When the photoelectric conversion element 10 is used in an imaging device, for example, electrons are collected by the lower electrode 2 and stored as signal charges in a charge storage node electrically connected to the lower electrode 2. In this way, the photoelectric conversion layer 3 converts light into signal charges, and the lower electrode 2 collects the signal charges generated in the photoelectric conversion layer 3. The signal charges collected by the lower electrode 2 are stored in, for example, the charge storage node. Furthermore, the upper electrode 4 collects charges of the opposite polarity to the signal charges.

[0076] Furthermore, in order to suppress the movement of signal charges to the lower electrode 2, for example, a voltage that reduces the potential difference between the upper electrode 4 and the lower electrode 2 is applied between the upper electrode 4 and the lower electrode 2. This causes the signal charges to recombine with charges of the opposite polarity to the signal charges before they reach the lower electrode 2, and they are substantially not stored in the charge storage node.

[0077] The intermediate layer 5 has a function of transporting signal charges generated in the photoelectric conversion layer 3 to the lower electrode 2 or 112. The intermediate layer 5 contains, for example, an organic semiconductor material. Examples of the organic semiconductor material that can be used include the donor semiconductor material or acceptor semiconductor material listed above. The material contained in the intermediate layer 5 is not limited to an organic semiconductor material, and may be an inorganic semiconductor such as an oxide semiconductor or a nitride semiconductor, or an insulator. Note that the photoelectric conversion element 10 does not necessarily have to include the intermediate layer 5. In this case, for example, the photoelectric conversion layer 3 is formed on the surface 2s of the lower electrode 2.

[0078] Next, the energy level configuration of the photoelectric conversion elements 10 and 110 will be described.

[0079] FIG. 3 is an exemplary energy band diagram for the photoelectric conversion element 10 shown in FIG. 1 . FIG. 4 is an exemplary energy band diagram for the photoelectric conversion element 110 shown in FIG. 2 . In FIGS. 3 and 4 , the energy band of each layer is indicated by a rectangle. Also, in FIGS. 3 and 4 , the Fermi level of each electrode is indicated by a line. In FIG. 4 , the Fermi level of the lower electrode 112 is indicated by a dashed line, but this is to emphasize the difference from the lower electrode 2. Note that the energy level configurations shown in FIGS. 3 and 4 are merely examples, and the energy level configurations in the photoelectric conversion elements 10 and 110 are not limited to the examples shown in FIGS. 3 and 4 .

[0080] Here, of the electron-hole pairs generated by the photoelectric conversion layer 3 absorbing light, the material that donates the electrons to the other material is called a donor material, and the material that accepts the electrons is called an acceptor material. In this embodiment, a donor semiconductor material is a donor material, and an acceptor semiconductor material is an acceptor material. When two different types of organic semiconductor materials are used, which one becomes the donor material and which one becomes the acceptor material is generally determined by the relative positions of the HOMO (Highest-Occupied-Molecular-Orbital) and LUMO (Lowest-Unoccupied-Molecular-Orbital) energy levels of the two types of organic semiconductor materials at the contact interface. In Figures 3 and 4, the upper end of the rectangle representing the energy band is the LUMO energy level, and the lower end is the HOMO energy level. The energy difference between the vacuum level and the LUMO energy level is referred to as the electron affinity. In this specification, the electron affinity is expressed as the absolute value of the energy difference between the vacuum level and the LUMO energy level. The energy difference between the vacuum level and the HOMO energy level is referred to as the ionization potential. In this specification, the ionization potential is expressed as the absolute value of the energy difference between the vacuum level and the HOMO energy level. In Figures 3 and 4, the electron affinity and ionization potential are greater at the lower positions.

[0081] Of the two types of semiconductor materials contained in the photoelectric conversion layer 3, the one with the smaller electron affinity is the donor material. Furthermore, of the two types of semiconductor materials contained in the photoelectric conversion layer 3, the one with the larger electron affinity is the acceptor material. In this specification, when the photoelectric conversion layer 3 contains a donor material and an acceptor material, such as when the photoelectric conversion layer 3 is a mixed film with a bulk heterostructure, the ionization potential of the photoelectric conversion layer 3 is the ionization potential of the donor material, and the electron affinity of the photoelectric conversion layer 3 is the electron affinity of the acceptor material.

[0082] The energy difference between the Fermi level and the vacuum level in a conductive material is called the work function. In this specification, the work function is expressed as the absolute value of the energy difference between the Fermi level and the vacuum level. In Figures 3 and 4, the work function is larger as the material is positioned lower.

[0083] 3, the work function of the lower electrode 2 is smaller than the work function of the upper electrode 4. The difference between the work functions of the lower electrode 2 and the upper electrode 4 is, for example, larger than 0.0 eV. The difference between the work functions of the lower electrode 2 and the upper electrode 4 may be 0.2 eV or more. The difference between the work functions of the lower electrode 2 and the upper electrode 4 is, for example, 2.0 eV or less. The difference between the work functions of the lower electrode 2 and the upper electrode 4 may be 0.6 eV or less.

[0084] 4, the work function of the lower electrode 112 is greater than the work function of the upper electrode 4. The difference between the work function of the lower electrode 112 and the work function of the upper electrode 4 is, for example, greater than 0.0 eV. The difference between the work function of the lower electrode 112 and the work function of the upper electrode 4 may be 0.2 eV or greater. The difference between the work function of the lower electrode 112 and the work function of the upper electrode 4 is, for example, 2.0 eV or less. The difference between the work function of the lower electrode 112 and the work function of the upper electrode 4 may be 0.6 eV or less.

[0085] Here, in this specification, the work function of each electrode is a value measured by ultraviolet photoelectron spectroscopy in a vacuum by irradiating the surface of the electrode facing the photoelectric conversion layer 3 with vacuum ultraviolet light having an energy of 21.22 eV and containing HeI rays as a main component, while maintaining the oxidation state of the electrode surface unchanged. Therefore, for example, the work function of the lower electrode 2 is the work function of the surface 2s of the lower electrode 2 facing the photoelectric conversion layer 3, and the work function of the lower electrode 112 is the work function of the surface 112s of the lower electrode 112 facing the photoelectric conversion layer 3.

[0086] 3 and 4 , the work functions of the lower electrodes 2 and 112 are greater than the electron affinities of the photoelectric conversion layer 3 and intermediate layer 5. The work functions of the lower electrodes 2 and 112 are smaller than the ionization potentials of the photoelectric conversion layer 3 and intermediate layer 5. The electron affinity of the intermediate layer 5 is smaller than the electron affinity of the photoelectric conversion layer 3. The ionization potential of the intermediate layer 5 is greater than the ionization potential of the photoelectric conversion layer 3.

[0087] In this embodiment, when the signal charges are holes, the work function of the lower electrode 2 is smaller than the work function of the upper electrode 4, so that the occurrence of parasitic sensitivity can be suppressed even if the absolute value of the initialization voltage of the charge storage node is reduced. This will be described with reference to FIGS. 5 to 7.

[0088] FIG. 5 is a diagram illustrating an example in which the occurrence of parasitic sensitivity is suppressed when holes are used as signal charges. FIG. 6 is a diagram illustrating an example in which parasitic sensitivity is generated when holes are used as signal charges. For simplicity's sake, FIGS. 5 and 6 show energy band diagrams for the photoelectric conversion elements 10 and 110, respectively, excluding the intermediate layer 5. Also, FIGS. 5 and 6 show energy band diagrams in which the same voltage (e.g., 0 V) ​​is applied to the lower electrodes 2 and 112 and the upper electrode 4. Therefore, FIGS. 5 and 6 can be said to show energy band diagrams in which a voltage for suppressing the movement of signal charges to the lower electrodes 2 and 112 is applied between the lower electrode 2 or 112 and the upper electrode 4. Also, in FIGS. 5 and 6, electrons are represented by black circles and holes are represented by white circles, and a portion of the movement of electrons and holes is schematically illustrated. This also applies to the energy band diagrams described below.

[0089] In FIG. 5 , when the work function of the lower electrode 2 is smaller than that of the upper electrode 4, if the same voltage is applied to the lower electrode 2 and the upper electrode 4, the lower electrode 2 and the upper electrode 4 have the same energy, and therefore the energy band of the photoelectric conversion layer 3 is tilted so that the lower electrode 2 side is deeper. Holes generated by light irradiation tend to move toward shallower energy and are therefore less likely to reach the lower electrode 2. On the other hand, in FIG. 6 , when the work function of the lower electrode 112 is larger than that of the upper electrode 4, if the same voltage is applied to the lower electrode 112 and the upper electrode 4, the energy band of the photoelectric conversion layer 3 is tilted so that the lower electrode 112 side is shallower. Therefore, holes generated by light irradiation are more likely to reach the lower electrode 112. In other words, if the work function of the lower electrode 112 is larger than that of the upper electrode 4, a photocurrent flows, and holes, which are signal charges, are accumulated in the charge storage node, which tends to cause parasitic sensitivity.

[0090] FIG. 7 is a diagram schematically showing the relationship between the current flowing in the lower electrode 2 or 112 and the voltage of the lower electrode 2 or 112. FIG. 7 schematically shows the current that flows due to the movement of holes in the lower electrode 2 or 112 when the voltage applied to the upper electrode 4 is fixed at 0 V. Similarly to FIGS. 5 and 6, FIG. 7 also shows the current when the intermediate layer 5 is removed. The vertical axis in FIG. 7 represents the current, and the horizontal axis in FIG. 7 represents the voltage of the lower electrode 2 or 112. In FIG. 7, the current when the lower electrode 2 is used is shown by a solid line, and the current when the lower electrode 112 is used is shown by a dashed line.

[0091] As explained with reference to FIGS. 5 and 6 , when the voltage applied to the lower electrodes 2 and 112 is 0 V, holes are less likely to reach the lower electrode 2, so current is less likely to flow through the lower electrode 2, whereas holes are more likely to reach the lower electrode 112, so current is more likely to flow through the lower electrode 2. As shown in FIG. 7 , in order to make it less likely for current to flow through the lower electrode 112, the voltage of the lower electrode 112 must be increased to a positive value to suppress the movement of holes to the lower electrode 112. Conversely, by using the lower electrode 2, the voltage at which photocurrent due to the movement of holes is more likely to occur can be shifted to a negative voltage side compared to when the lower electrode 112 is used. Furthermore, from the perspective of suppressing dark current, it is desirable that the absolute value of the initialization voltage of the charge storage node be low, ideally 0 V. However, when the initialization voltage of the charge storage node is set to 0 V, current due to the movement of holes flows when the lower electrode 112 is used, generating parasitic sensitivity. In other words, it is difficult to suppress dark current while suppressing the generation of parasitic sensitivity. 7, when the lower electrode 2 is used, even if the initialization voltage of the charge storage node is set to 0 V, a current due to the movement of holes is less likely to flow through the lower electrode 2, and the occurrence of parasitic sensitivity can be suppressed. In other words, in the photoelectric conversion element 10 including the lower electrode 2, the occurrence of parasitic sensitivity can be suppressed even if the absolute value of the initialization voltage of the charge storage node is reduced, and it is possible to reduce the dark current and parasitic sensitivity.

[0092] Incidentally, by providing the intermediate layer 5, the photoelectric conversion element 110 can suppress the movement of holes to the lower electrode 112 even when the lower electrode 112 is provided. This will be explained using FIG. 8 . FIG. 8 is a diagram for explaining how the intermediate layer 5 suppresses the occurrence of parasitic sensitivity when holes are used as signal charges. FIG. 8 is an energy band diagram of the photoelectric conversion element 110 when the same voltage (e.g., 0 V) ​​is applied to the lower electrode 112 and the upper electrode 4.

[0093] 8 , in the photoelectric conversion element 110, even when the same voltage is applied to the lower electrode 112 and the upper electrode 4, the intermediate layer 5 acts as a barrier to the movement of holes, suppressing the movement of holes to the lower electrode 112. Specifically, when the work function of the lower electrode 112 is A [eV], the work function of the upper electrode 4 is B [eV], the ionization potential of the intermediate layer 5 is C [eV], and the ionization potential of the photoelectric conversion layer 3 is D [eV], satisfying (A-B)-(C-D)<0 increases the barrier to the movement of holes, suppressing the movement of holes to the lower electrode 112. Therefore, even when the initialization voltage of the charge storage node is set to 0 V, holes are less likely to reach the lower electrode 112, making it difficult for current to flow through the lower electrode 112, thereby suppressing the occurrence of parasitic sensitivity. That is, in the photoelectric conversion element 110, even if the absolute value of the initialization voltage of the charge storage node is reduced, the occurrence of parasitic sensitivity can be suppressed, and dark current and parasitic sensitivity can be reduced. Note that the above-mentioned (A-B)-(C-D)<0 is also satisfied in the photoelectric conversion element 10. Furthermore, (A-B)-(C-D) is, for example, greater than -2.0 eV.

[0094] Next, the reduction of parasitic sensitivity and dark current when the signal charge is electrons will be described. In this embodiment, when the signal charge is electrons, the work function of the lower electrode 2 is larger than the work function of the upper electrode 4, so that the occurrence of parasitic sensitivity can be suppressed even if the absolute value of the initialization voltage of the charge storage node is reduced. This will be described with reference to FIGS. 9 to 12.

[0095] FIG. 9 is a diagram illustrating an example in which the occurrence of parasitic sensitivity is suppressed when electrons are used as signal charges. FIG. 10 is a diagram illustrating an example in which parasitic sensitivity is generated when electrons are used as signal charges. For simplicity's sake, FIGS. 9 and 10 show energy band diagrams for the photoelectric conversion elements 10 and 110, respectively, excluding the intermediate layer 5. Also, FIGS. 9 and 10 are energy band diagrams for the case in which the same voltage (e.g., 0 V) ​​is applied to the lower electrodes 2 and 112 and the upper electrode 4. Therefore, FIGS. 9 and 10 can be said to show energy band diagrams for the case in which a voltage for suppressing the movement of signal charges to the lower electrodes 2 and 112 is applied between the lower electrode 2 or 112 and the upper electrode 4.

[0096] In FIG. 9 , when the work function of the lower electrode 112 is greater than that of the upper electrode 4, if the same voltage is applied to the lower electrode 112 and the upper electrode 4, the lower electrode 112 and the upper electrode 4 have the same energy, and therefore the energy band of the photoelectric conversion layer 3 is tilted so that the lower electrode 112 side is shallower. Electrons generated by light irradiation tend to move toward deeper energy and are therefore less likely to reach the lower electrode 112. On the other hand, in FIG. 10 , when the work function of the lower electrode 2 is smaller than that of the upper electrode 4, if the same voltage is applied to the lower electrode 2 and the upper electrode 4, the energy band of the photoelectric conversion layer 3 is tilted so that the lower electrode 2 side is deeper. Therefore, electrons generated by light irradiation are more likely to reach the lower electrode 2. In other words, if the work function of the lower electrode 2 is smaller than that of the upper electrode 4, a photocurrent flows, and electrons, which are signal charges, are accumulated in the charge storage node, which makes it easier for parasitic sensitivity to occur.

[0097] FIG. 11 is a diagram schematically showing the relationship between the current flowing in the lower electrode 2 or 112 and the voltage of the lower electrode 2 or 112. FIG. 11 schematically shows the current that flows when electrons move to the lower electrode 2 or 112 when the voltage applied to the upper electrode 4 is fixed at 0 V. Similarly to FIGS. 9 and 10, FIG. 11 also shows the current when the intermediate layer 5 is removed. The vertical axis in FIG. 11 represents the current, and the horizontal axis in FIG. 11 represents the voltage of the lower electrode 2 or 112. In FIG. 11, the current when the lower electrode 2 is used is shown by a solid line, and the current when the lower electrode 112 is used is shown by a dashed line.

[0098] As explained with reference to FIGS. 9 and 10 , when the voltage applied to the lower electrodes 2 and 112 is 0 V, electrons are less likely to reach the lower electrode 112, making it difficult for current to flow through the lower electrode 112, while electrons are more likely to reach the lower electrode 2, making it easier for current to flow through the lower electrode 112. As shown in FIG. 11 , in order to make it difficult for current to flow through the lower electrode 2, the voltage of the lower electrode 2 must be increased in the negative direction to suppress the movement of electrons to the lower electrode 2. Conversely, by using the lower electrode 112, the voltage at which photocurrent due to electron movement is more likely to occur can be shifted toward the positive voltage side compared to when the lower electrode 2 is used. Furthermore, from the perspective of suppressing dark current, it is desirable for the absolute value of the initialization voltage of the charge storage node to be low, ideally 0 V. However, when the initialization voltage of the charge storage node is set to 0 V, current due to electron movement flows when the lower electrode 2 is used, generating parasitic sensitivity. In other words, it is difficult to suppress dark current while suppressing the generation of parasitic sensitivity. 11 , when the lower electrode 112 is used, even if the initialization voltage of the charge storage node is set to 0 V, a current due to the movement of electrons is less likely to flow through the lower electrode 112, and the occurrence of parasitic sensitivity can be suppressed. In other words, in the photoelectric conversion element 110 including the lower electrode 112, even if the absolute value of the initialization voltage of the charge storage node is reduced, the occurrence of parasitic sensitivity can be suppressed, and dark current and parasitic sensitivity can be reduced.

[0099] Incidentally, by providing the intermediate layer 5, the photoelectric conversion element 10 can suppress the movement of electrons to the lower electrode 2 even when the lower electrode 2 is provided. This will be explained using FIG. 12 . FIG. 12 is a diagram for explaining how the intermediate layer 5 suppresses the occurrence of parasitic sensitivity when electrons are used as signal charges. FIG. 12 is an energy band diagram of the photoelectric conversion element 10 when the same voltage (e.g., 0 V) ​​is applied to the lower electrode 2 and the upper electrode 4.

[0100] As shown in FIG. 12 , in the photoelectric conversion element 10, even when the same voltage is applied to the lower electrode 2 and the upper electrode 4, the intermediate layer 5 acts as a barrier to electron movement, suppressing the movement of electrons to the lower electrode 2. Specifically, when the work function of the lower electrode 2 is A [eV], the work function of the upper electrode 4 is B [eV], the electron affinity of the intermediate layer 5 is E [eV], and the electron affinity of the photoelectric conversion layer 3 is F [eV], satisfying (A-B) + (F-E) > 0 increases the barrier to electron movement, suppressing the movement of electrons to the lower electrode 2. Therefore, even when the initialization voltage of the charge storage node is set to 0 V, electrons are less likely to reach the lower electrode 2, making it difficult for current to flow through the lower electrode 2, thereby suppressing the occurrence of parasitic sensitivity. In other words, in the photoelectric conversion element 10, the occurrence of parasitic sensitivity can be suppressed even when the absolute value of the initialization voltage of the charge storage node is reduced, making it possible to reduce dark current and parasitic sensitivity. The photoelectric conversion element 110 also satisfies the above-mentioned (A−B)+(F−E)>0. In addition, (A−B)+(F−E) is, for example, smaller than 2.0 eV.

[0101] [Relationship between Composition of Lower Electrode and Work Function] As described above, the lower electrode 2 is, for example, a metal element-containing film from which a native oxide film has been removed on the surface 2s of the lower electrode 2 facing the photoelectric conversion layer 3. The present inventors have found that the work function can be reduced by removing the native oxide film of the metal element-containing film. Below, the results of the inventors' investigation into the effect of removing the native oxide film on the work function of the electrode surface will be described.

[0102] Specifically, the inventors used chemical vapor deposition (CVD) to form a titanium nitride film as an electrode on a substrate and confirmed the relationship between the work function and the element concentration of the contained elements and the thickness direction of the titanium nitride film. To measure the work function and element concentration, a multifunctional X-ray photoelectron spectrometer (VersaProbe, manufactured by ULVAC-PHI, Inc.) was used. Work function and element concentration measurements were performed by repeatedly performing argon sputtering, i.e., plasma etching using argon gas, and varying the sputtering depth, which is the depth etched by argon sputtering. The sputtering depth is the thickness direction from the electrode surface. The titanium nitride film used for the measurements was exposed to air for more than 100 hours. In other words, measurements were performed on titanium nitride films with a native oxide film formed on the surface. The work function was measured using ultraviolet photoelectron spectroscopy, irradiating the electrode surface with vacuum ultraviolet light, mainly composed of HeI rays with an energy of 21.22 eV. The element concentrations were measured by irradiating the electrode surface with X-rays using X-ray photoelectron spectroscopy.

[0103] FIG. 13 shows the relationship between the work function and the element concentration of the contained elements with respect to the position in the thickness direction of the titanium nitride film. As shown in FIG. 13 , at a sputtering depth of 0 nm, the oxygen element concentration is 33.1 atom %, and the work function is 4.3 eV. The work function and element concentration at a sputtering depth of 0 nm correspond to an example of the work function and element concentration on the surface 112s of the lower electrode 112 on which the native oxide film remains. Since the greater the sputtering depth, the less likely native oxidation occurs, the oxygen element concentration decreases, and the oxygen element concentration remains almost constant at sputtering depths of 10 nm or more. At a sputtering depth of 10 nm, the oxygen element concentration is 7.6 atom %, and the work function is 3.8 eV. The work function and element concentration at a sputtering depth of 10 nm correspond to an example of the work function and element concentration on the surface 2s of the lower electrode 2 from which the native oxide film has been removed.

[0104] In this manner, an electrode having a surface 2s from which a native oxide film has been removed, such as the lower electrode 2, has a small work function, and when the signal charges are holes, the parasitic sensitivity can be reduced even when the absolute value of the initialization voltage of the charge storage node is low. The oxygen element concentration at the surface 2s of the lower electrode 2 is, for example, 10 atom% or less. Furthermore, when the lower electrode 2 contains titanium nitride, the nitrogen element concentration at the surface 2s is, for example, greater than the titanium element concentration at the surface 2s, which in turn is, for example, greater than the oxygen element concentration at the surface 2s. Note that in this specification, the element concentration at the surface of the electrode is a value measured by irradiating the surface with X-rays using X-ray photoelectron spectroscopy.

[0105] Furthermore, an electrode having a surface 112s on which a native oxide film remains, such as the lower electrode 112, tends to have a large work function, and when the signal charge is electrons, the parasitic sensitivity can be reduced even when the absolute value of the initialization voltage of the charge storage node is low. The oxygen element concentration on the surface 112s of the lower electrode 112 is, for example, 25 atom % or more. The oxygen element concentration on the surface 112s of the lower electrode 112 is, for example, 40 atom % or less.

[0106] Furthermore, since a metal element tends to have a larger work function than a metal nitride, the lower electrode 112 may not contain nitrogen. The lower electrode 112 may be a metal film formed of, for example, titanium, tungsten, or tantalum. Furthermore, the nitrogen element concentration on the surface 112s of the lower electrode 112 may be, for example, 5 atom % or less.

[0107] [Modification of Photoelectric Conversion Element] Next, a photoelectric conversion element according to a modification of the embodiment will be described. Fig. 14 is a schematic cross-sectional view showing the configuration of a photoelectric conversion element 120 according to a modification of the present embodiment. In the following description of photoelectric conversion element 120, differences from photoelectric conversion elements 10 and 110 will be mainly described, and descriptions of commonalities will be omitted or simplified.

[0108] 14 , the photoelectric conversion element 120 has a configuration in which the lower electrode 2 of the photoelectric conversion element 10 is replaced with a lower electrode 122. The lower electrode 122 is an example of a first electrode. In the example shown in FIG. 14 , the photoelectric conversion element 120 includes an intermediate layer 5, but the intermediate layer 5 may not be included. In this case, for example, the photoelectric conversion layer 3 is formed on a surface 122s of the lower electrode 122.

[0109] The lower electrode 122 includes a layer 122a containing a metal element and a self-assembled monolayer 122b located on the surface of the layer 122a facing the photoelectric conversion layer 3. That is, the self-assembled monolayer 122b is formed on the surface 122s of the lower electrode 122 facing the photoelectric conversion layer 3. In other words, the lower electrode 122 has a first surface and a second surface that is closer to the photoelectric conversion layer 3 than the first surface, and the self-assembled monolayer 122b is formed on the second surface.

[0110] The layer 122a is, for example, the same metal element-containing film as the lower electrode 2 or 112 described above.

[0111] The self-assembled monolayer 122b is an organic coating formed by self-assembly of organic molecules on the surface of the layer 122a facing the photoelectric conversion layer 3. The self-assembled monolayer 122b is formed, for example, by applying a solution containing organic molecules onto the surface of the layer 122a facing the photoelectric conversion layer 3 by spin coating or the like.

[0112] The work function of the lower electrode 122 can be easily adjusted by the organic molecules that form the self-assembled monolayer 122b.

[0113] Examples of organic molecules that form the self-assembled monolayer 122b when the work function of the lower electrode 122 is reduced include polyethyleneimine (PEI) and polyethyleneimine ethoxylate (PEIE). Forming the self-assembled monolayer 122b using such organic molecules makes it easier to make the work function of the lower electrode 122 smaller than that of the upper electrode 4. For example, even when the layer 122a is made of a titanium nitride film with a remaining native oxide film, the work function of the lower electrode 122 can be made smaller than that of the upper electrode 4.

[0114] Furthermore, examples of organic molecules that form the self-assembled monolayer 122b when increasing the work function of the lower electrode 122 include pentafluorobenzenethiol (PFBT). Forming the self-assembled monolayer 122b using such organic molecules makes it easy to increase the work function of the lower electrode 122 beyond that of the upper electrode 4. For example, even when the layer 122a is made of a titanium nitride film from which a native oxide film has been removed or is not formed, the work function of the lower electrode 122 can be increased beyond that of the upper electrode 4.

[0115] [Imaging Device] Next, an imaging device including a photoelectric conversion element according to this embodiment will be described. Fig. 15 is a diagram showing the circuit configuration of an imaging device 100 according to this embodiment. The imaging device 100 is, for example, a stacked image sensor. As shown in Fig. 15, the imaging device 100 includes a plurality of pixels 14 and peripheral circuits. The imaging device 100 operates, for example, by a global shutter system in which the exposure periods of all of the plurality of pixels 14 are unified. Details of the exposure period will be described later.

[0116] The plurality of pixels 14 are arranged two-dimensionally, i.e., in a row direction and a column direction, on the semiconductor substrate to form a pixel region. In this specification, the row direction and the column direction refer to the directions in which the rows and columns extend, respectively. In other words, the vertical direction is the column direction, and the horizontal direction is the row direction. Note that the plurality of pixels 14 may also be arranged one-dimensionally, i.e., along one direction. In other words, the imaging device 100 may be a line sensor.

[0117] Each pixel 14 includes a photodetector unit 10A and a charge detection circuit 25. The charge detection circuit 25 includes an amplification transistor 11, a reset transistor 12, and an address transistor 13. The photodetector unit 10A is configured, for example, with the photoelectric conversion element 10, 110, or 120 described above. The following describes a case where the photodetector unit 10A has the same configuration as the photoelectric conversion element 10. In FIG. 15 , of the configuration of the photodetector unit 10A, the lower electrode 2, the photoelectric conversion layer 3, and the upper electrode 4 are representatively shown, with the intermediate layer 5 and the protective layer 6 omitted. In the imaging device 100, the lower electrode 2 is also referred to as a pixel electrode, and the upper electrode 4 is also referred to as a counter electrode.

[0118] The imaging device 100 includes a voltage control element for applying a predetermined voltage to the upper electrode 4. The voltage control element controls the voltage applied to the upper electrode 4 to be within a specified range so that the electric signal generated in the photodetector 10A can be correctly detected by the charge detection circuit 25. Alternatively, when a current flows from the lower electrode 2, the voltage control element suppresses charging of the photodetector 10A by passing an equal amount of current to the upper electrode 4.

[0119] The voltage control element includes, for example, a voltage control circuit, a voltage generation circuit such as a constant voltage source, and a voltage reference line such as a ground line. The voltage applied by the voltage control element is also referred to as a control voltage. In this embodiment, the imaging device 100 includes a voltage control circuit 30 as a voltage control element. The voltage control circuit 30 is an example of a voltage supply circuit. The voltage control circuit 30 may generate a constant control voltage or multiple control voltages of different values. For example, the voltage control circuit 30 may generate control voltages of two or more different values, or may generate a control voltage that changes continuously within a predetermined range. The voltage control circuit 30 determines the value of the control voltage to be generated based on instructions from the operator operating the imaging device 100 or instructions from another control unit or the like included in the imaging device 100, and generates the control voltage of the determined value. The voltage control circuit 30 is provided outside the photosensitive region, for example, as part of a peripheral circuit. The photosensitive region is substantially the same as the pixel region.

[0120] The voltage control circuit 30 is electrically connected to the upper electrode 4 and applies a potential difference between the lower electrode 2 and the upper electrode 4. For example, the voltage control circuit 30 generates two or more different control voltages and applies the control voltages to the upper electrode 4 to change the spectral sensitivity characteristics of the photoelectric conversion layer 3. This change in spectral sensitivity characteristics also includes a spectral sensitivity characteristic in which the sensitivity of the photoelectric conversion layer 3 becomes zero to light to be detected. As a result, for example, in the imaging device 100, while the pixels 14 read out detection signals row by row, the voltage control circuit 30 applies to the upper electrode 4 a control voltage that makes the sensitivity of the photoelectric conversion layer 3 zero, thereby substantially eliminating the influence of incident light during readout of the detection signals. Therefore, even when the detection signals are read out row by row, a global shutter operation can be achieved.

[0121] In this embodiment, as shown in FIG. 15 , the voltage control circuit 30 applies a control voltage to the upper electrode 4 of each pixel 14 via the counter electrode signal line 16. This changes the voltage between the lower electrode 2 and the upper electrode 4, thereby switching the spectral sensitivity characteristics of the photodetector 10A. Alternatively, the voltage control circuit 30 realizes an electronic shutter operation by applying a control voltage so as to obtain a spectral sensitivity characteristic in which sensitivity to light becomes zero at a predetermined timing during imaging. Note that in FIG. 15 , the upper electrode 4 of each pixel 14 is connected to the voltage control circuit 30 via the counter electrode signal line 16 provided for each row of pixels 14. However, as will be described later, the upper electrode 4 may be configured as a single upper electrode 4 across all pixels 14. Therefore, for example, the control voltage may be applied from the voltage control circuit 30 to the upper electrodes 4 of all pixels 14 collectively.

[0122] In order to have the lower electrode 2 collect, as signal charges, holes generated when light is irradiated onto the photodetector 10A, the control voltage of the lower electrode 2 is set to a lower potential than that of the upper electrode 4. In addition, in order to have the lower electrode 2 collect, as signal charges, electrons generated when light is irradiated onto the photodetector 10A, the control voltage of the lower electrode 2 is set to a higher potential than that of the upper electrode 4.

[0123] The lower electrode 2 is connected to the gate electrode of the amplifier transistor 11, and signal charges collected by the lower electrode 2 are stored in a charge storage node 24 located between the lower electrode 2 and the gate electrode of the amplifier transistor 11. The charge storage node 24 is an example of a charge storage region.

[0124] The voltage control circuit 30 supplies, for example, a first voltage during a first period and a second voltage different from the first voltage to the upper electrode 4 during a second period. When a global shutter operation is performed, the voltage control circuit 30 supplies, for example, a first voltage to the upper electrode 4 during a first period, which is an exposure period during which signal charges are accumulated in the charge accumulation regions of each pixel 14, and supplies a second voltage different from the first voltage during a second period, which is a non-exposure period different from the exposure period. The first voltage is, for example, a voltage of the same polarity as the signal charges. The absolute value of the first voltage is, for example, 5 V or more and 15 V or less. The second voltage is either 0 V or a voltage of the same polarity as the signal charges. The second voltage may be a voltage of the opposite polarity to the signal charges. The absolute value of the second voltage is, for example, 2 V or less. In this specification, the term "exposure period" refers to a period during which either electrons or holes generated by photoelectric conversion are accumulated in the charge accumulation node 24 as signal charges. In other words, the "exposure period" may also be referred to as the "charge accumulation period." In this specification, a period during operation of the imaging device other than the exposure period is referred to as a "non-exposure period." A "non-exposure period" is, for example, a period during which light is irradiated onto the photodetection unit 10A but no charge is substantially accumulated in the charge accumulation node 24. During the non-exposure period, light may be blocked from entering the photodetection unit 10A.

[0125] The charge storage node 24 is connected to the gate electrode of the amplifier transistor 11, and the signal charge stored in the charge storage node 24 is applied to the gate electrode of the amplifier transistor 11 as a voltage corresponding to the amount of signal charge. The amplifier transistor 11 constitutes part of the charge detection circuit 25 and amplifies the voltage applied to the gate electrode. One of the source electrode and drain electrode of the amplifier transistor 11 is connected to one of the source electrode and drain electrode of the address transistor 13. The address transistor 13 selectively reads out the amplified voltage as a signal voltage. The address transistor 13 is also referred to as a row selection transistor. One of the source electrode and drain electrode of the reset transistor 12 is connected to the lower electrode 2 via the charge storage node 24 and initializes (resets) the signal charge stored in the charge storage node 24. In other words, the reset transistor 12 initializes the potentials of the charge storage node 24, the gate electrode of the amplifier transistor 11, and the lower electrode 2 to an initialization voltage.

[0126] In order to selectively perform the above-described operations in the plurality of pixels 14, the imaging device 100 includes a power supply wiring 21, a reset voltage line 23, a vertical signal line 17, an address signal line 26, and a reset signal line 27. These lines are connected to the pixels 14, respectively. Specifically, the power supply wiring 21 is connected to the other of the source electrode and the drain electrode of the amplification transistor 11. The reset voltage line 23 is connected to the other of the drain electrode and the source electrode of the reset transistor 12. The vertical signal line 17 is connected to the other of the source electrode and the drain electrode of the address transistor 13. The address signal line 26 is connected to the gate electrode of the address transistor 13. Furthermore, the reset signal line 27 is connected to the gate electrode of the reset transistor 12.

[0127] The peripheral circuits include a vertical scanning circuit 15, a horizontal signal readout circuit 20, a plurality of column signal processing circuits 19, a plurality of load circuits 18, and a voltage control circuit 30. The vertical scanning circuit 15 is also called a row scanning circuit. The horizontal signal readout circuit 20 is also called a column scanning circuit. The column signal processing circuit 19 is also called a row signal storage circuit.

[0128] The vertical scanning circuit 15 is connected to address signal lines 26 and reset signal lines 27, selects the multiple pixels 14 arranged in each row on a row-by-row basis, reads out the signal voltage, and initializes the potential of the lower electrode 2. The power supply wiring 21 is connected to a voltage supply circuit (not shown) and functions as a source follower power supply, supplying a predetermined power supply voltage to each pixel 14. The horizontal signal readout circuit 20 is electrically connected to multiple column signal processing circuits 19. The column signal processing circuits 19 are electrically connected to the pixels 14 arranged in each column via vertical signal lines 17 corresponding to each column. The load circuits 18 are electrically connected to each vertical signal line 17. The load circuits 18 and the amplification transistors 11 form a source follower circuit.

[0129] An initialization voltage is supplied to the other of the drain electrode and the source electrode of the reset transistor 12 from a voltage supply circuit (not shown), for example, via a reset voltage line 23. The initialization voltage is, for example, 0 or a voltage of the same polarity as the polarity of the signal charge. The absolute value of the initialization voltage is, for example, lower than the absolute value of the first voltage. The absolute value of the initialization voltage may be 1.0 V or less, or 0.3 V or less. The reset voltage line 23 may be part of a feedback circuit in which a feedback path is formed for negatively feeding back the output of the pixel 14. In this case, a feedback voltage for negatively feeding back the output of the pixel 14 is supplied to the reset voltage line 23 as the initialization voltage.

[0130] The vertical scanning circuit 15 applies a row selection signal, which controls the on / off of the address transistor 13, to the gate electrode of the address transistor 13 via the address signal line 26. This scans and selects the row to be read out. A signal voltage is read out from the pixels 14 in the selected row to the vertical signal line 17. The vertical scanning circuit 15 also applies a reset signal, which controls the on / off of the reset transistor 12, to the gate electrode of the reset transistor 12 via the reset signal line 27. This selects the row of pixels 14 to be subjected to the initialization operation. An initialization voltage is supplied to the charge storage node 24, the gate electrode of the amplifier transistor 11, and the lower electrode 2 of the pixels 14 in the row selected for the initialization operation. The vertical signal line 17 transmits the signal voltage read out from the pixels 14 selected by the vertical scanning circuit 15 to the column signal processing circuit 19.

[0131] The column signal processing circuit 19 performs noise suppression signal processing, such as correlated double sampling, and analog-to-digital conversion (AD conversion).

[0132] The horizontal signal readout circuit 20 sequentially reads out signals from the plurality of column signal processing circuits 19 to a horizontal common signal line 28 .

[0133] [Pixel Configuration] Next, a detailed device structure of the pixel 14 of the imaging device 100 will be described with reference to Fig. 16. Fig. 16 is a cross-sectional view that schematically shows a cross section of the device structure of the pixel 14 of the imaging device 100 according to the present embodiment. Note that in the cross-sectional view shown in Fig. 16, the semiconductor substrate 31 and the interlayer insulating layers 43A, 43B, and 43C are not shaded to indicate cross sections in order to avoid complicating the drawing.

[0134] 16 , the pixel 14 includes a semiconductor substrate 31, a charge detection circuit 25, and a photodetector unit 10A. The semiconductor substrate 31 is, for example, a p-type silicon substrate. The charge detection circuit 25 detects the signal charge collected by the lower electrode 2 and outputs a signal voltage. The charge detection circuit 25 includes an amplification transistor 11, a reset transistor 12, and an address transistor 13, and is formed on the semiconductor substrate 31.

[0135] The amplifier transistor 11 is formed on a semiconductor substrate 31. The amplifier transistor 11 includes n-type impurity regions 41C and 41D that function as a drain electrode and a source electrode, respectively, a gate insulating layer 38B located on the semiconductor substrate 31, and a gate electrode 39B located on the gate insulating layer 38B.

[0136] The reset transistor 12 is formed on a semiconductor substrate 31. The reset transistor 12 includes n-type impurity regions 41B and 41A functioning as a drain electrode and a source electrode, respectively, a gate insulating layer 38A located on the semiconductor substrate 31, and a gate electrode 39A located on the gate insulating layer 38A.

[0137] Address transistor 13 is formed on semiconductor substrate 31. Address transistor 13 includes n-type impurity regions 41D and 41E functioning as a drain electrode and a source electrode, respectively, a gate insulating layer 38C located on semiconductor substrate 31, and a gate electrode 39C located on gate insulating layer 38C.

[0138] The amplifier transistor 11, the reset transistor 12, and the address transistor 13 are each, for example, a metal oxide semiconductor field effect transistor (MOSFET). The amplifier transistor 11, the reset transistor 12, and the address transistor 13 are each an n-channel MOSFET, but may be a p-channel MOSFET.

[0139] The gate insulating layers 38A, 38B, and 38C are each formed using an insulating material. For example, the gate insulating layers 38A, 38B, and 38C have a single layer structure of a silicon oxide film or a silicon nitride film, or a laminate structure of these.

[0140] The gate electrodes 39A, 39B, and 39C are each formed using a conductive material. For example, the gate electrodes 39A, 39B, and 39C are formed using polysilicon to which impurities have been added to make it conductive. Alternatively, the gate electrodes 39A, 39B, and 39C may be formed using a metal material such as copper.

[0141] The n-type impurity regions 41A, 41B, 41C, 41D, and 41E are formed by doping the semiconductor substrate 31 with n-type impurities such as phosphorus (P) by ion implantation or the like. In the example shown in FIG. 16 , the n-type impurity region 41D is shared by the amplifier transistor 11 and the address transistor 13. This connects the amplifier transistor 11 and the address transistor 13 in series. Note that the n-type impurity region 41D may be separated into two n-type impurity regions. In this case, the two n-type impurity regions may be electrically connected via a wiring layer.

[0142] In the semiconductor substrate 31, an element isolation region 42 is provided between adjacent pixels 14 and between the amplification transistor 11 and the reset transistor 12. The element isolation region 42 electrically isolates adjacent pixels 14. The element isolation region 42 also suppresses leakage of signal charge accumulated in the charge storage node 24. The element isolation region 42 is formed, for example, by doping the semiconductor substrate 31 with a high concentration of p-type impurities.

[0143] A multilayer wiring structure is provided on the upper surface of the semiconductor substrate 31. The multilayer wiring structure includes a plurality of interlayer insulating layers, one or more wiring layers, one or more plugs, and one or more contact plugs. Specifically, interlayer insulating layers 43A, 43B, and 43C are stacked on the upper surface of the semiconductor substrate 31 in this order from the semiconductor substrate 31 side.

[0144] A contact plug 45A connected to the n-type impurity region 41B of the reset transistor 12, a contact plug 45B connected to the gate electrode 39B of the amplifier transistor 11, a wiring 46A connecting the contact plug 45A and the contact plug 45B, and a plug 47A are buried in the interlayer insulating layer 43A. As a result, the n-type impurity region 41B functioning as the drain electrode of the reset transistor 12 is electrically connected to the gate electrode 39B of the amplifier transistor 11.

[0145] Furthermore, a wiring 46B and a plug 47B connected to the plug 47A via the wiring 46B are embedded in the interlayer insulating layer 43B. A wiring 46C and a plug 47C connected to the plug 47B via the wiring 46C are embedded in the interlayer insulating layer 43C. The plug 47C is connected to the lower electrode 2. As a result, the charges collected by the lower electrode 2 flow in the order of plug 47C, wiring 46C, plug 47B, wiring 46B, plug 47A, wiring 46A, and contact plug 45A, and are accumulated in the n-type impurity region 41B. In addition to the n-type impurity region 41B, the plug 47C, wiring 46C, plug 47B, wiring 46B, plug 47A, wiring 46A, contact plug 45A, contact plug 45B, and gate electrode 39B constitute a charge storage node 24, each of which functions as a charge storage region.

[0146] The photodetector 10A is provided on a substrate 1A that includes a semiconductor substrate 31 and interlayer insulating layers 43A, 43B, and 43C. Specifically, the photodetector 10A is provided on the interlayer insulating layer 43C. The photodetector 10A includes a lower electrode 2, an intermediate layer 5, a photoelectric conversion layer 3, and an upper electrode 4. The photodetector 10A also includes a protective layer 6 formed on at least a portion of the upper surface of the upper electrode 4.

[0147] 16 , the pixel 14 includes a color filter 60 provided above the upper electrode 4 of the photodetector 10A, and a microlens 61 provided on the color filter 60. Note that the pixel 14 does not necessarily have to include at least one of the color filter 60 and the microlens 61.

[0148] In this embodiment, the photoelectric conversion layer 3 and upper electrode 4 of each pixel 14 are connected to the photoelectric conversion layer 3 and upper electrode 4 of the adjacent pixel 14, respectively, to form an integrated photoelectric conversion layer 3 and upper electrode 4 across all pixels 14. However, at least one of the photoelectric conversion layer 3 and upper electrode 4 may be separated for each pixel 14. Furthermore, at least one of the photoelectric conversion layer 3 and upper electrode 4 may be separated for each block of pixels 14, such as for each row or column of pixels 14 arranged two-dimensionally. In contrast, the lower electrode 2 of each pixel 14 is not connected to the lower electrode 2 of the adjacent pixel 14 and is independent.

[0149] [Method for Manufacturing the Photodetector] Next, a method for manufacturing the photodetector 10A according to this embodiment will be described. The method for manufacturing the photodetector 10A can also be considered as a method for manufacturing the photoelectric conversion element 10.

[0150] Fig. 17 is a flowchart of a method for manufacturing the photodetector 10A according to this embodiment. Figs. 18 to 20 are cross-sectional views illustrating the method for manufacturing the photodetector 10A. Note that in Figs. 18 to 20, detailed illustrations of the interior of the substrate 1A and the application of hatching representing cross sections are omitted to avoid complicating the drawings. In the following, an example in which a titanium nitride film is formed as the lower electrode 2 will be described, but a similar method can also be used to form the lower electrode 2 using a metal element-containing film other than a titanium nitride film.

[0151] As shown in FIGS. 17 and 18 , first, a lower electrode 2x is formed on the substrate 1A (step S11). Conventionally known methods can be used to form impurity regions in the semiconductor substrate 31 of the substrate 1A, as well as to form an interlayer insulating layer and a wiring layer. In the example shown in FIG. 18 , a plurality of lower electrodes 2x are formed, the number of which corresponds to the number of pixels 14. To form the plurality of lower electrodes 2x, for example, a titanium nitride film is first formed on the upper surface of the substrate 1A using a chemical vapor deposition (CVD) method, thereby forming a titanium nitride film over the entire upper surface of the substrate 1A. Other thin film deposition processes may also be used to form the titanium nitride film. The formed titanium nitride film is then separated into a plurality of lower electrodes 2x by patterning, and an insulating layer is then filled between the lower electrodes 2x, thereby forming the plurality of lower electrodes 2x shown in FIG. 18 .

[0152] The lower electrode 2x is formed using an inorganic material processing apparatus. The substrate 1A on which the lower electrode 2x has been formed is transferred in the atmosphere and exposed to the atmosphere for subsequent processes. During this process, a native oxide film is formed on the upper surface 2sx of the lower electrode 2x. The lower electrode 2x corresponds to the above-mentioned lower electrode 112, and it is possible to form a photodetector including the lower electrode 112 by omitting step S12 below.

[0153] Next, as shown in FIGS. 17 and 19 , the surface 2sx of the lower electrode 2x is etched to form the lower electrode 2 (step S12). For example, the substrate 1A on which the lower electrode 2x is formed is loaded into a sputtering apparatus. Then, using the lower electrode 2x as a sputtering target, surface treatment is performed by plasma etching using argon gas to remove the surface 2sx of the lower electrode 2x, thereby forming a lower electrode 2 having a surface 2s from which the native oxide film has been removed. Furthermore, even if contaminants adhere to the surface 2sx of the lower electrode 2x, the contaminants can be removed by etching. The etching depth in this case is, for example, 5 nm or more. The etching depth may be 10 nm or more. The etching depth is, for example, 30 nm or less. The gas used for etching is not limited to argon, but may also be nitrogen or hydrogen. The gas used for etching may also be a mixed gas containing two or more of argon, nitrogen, and hydrogen. After etching, the lower electrode 2 is annealed as necessary to stabilize the surface 2s. The surface 2sx of the lower electrode 2x may be removed by other methods such as wet etching.

[0154] Next, as shown in FIGS. 17 and 20 , one or more layers including a photoelectric conversion layer 3 are formed on the surface 2s of the etched lower electrode 2 without exposing the surface 2s to the atmosphere (step S13). For example, the substrate 1A on which the lower electrode 2 is formed is transferred from a sputtering apparatus to an organic material processing apparatus without being exposed to the atmosphere. For example, the substrate 1A on which the lower electrode 2 is formed is transferred from the sputtering apparatus to the organic material processing apparatus in a vacuum or a non-oxidizing atmosphere such as a nitrogen or argon atmosphere. Then, the intermediate layer 5 and the photoelectric conversion layer 3 are formed in this order on the surface 2s of the lower electrode 2 by vacuum deposition. In this way, by forming one or more layers including a photoelectric conversion layer 3 on the surface 2s of the lower electrode 2 without exposing the surface 2s of the lower electrode 2 to the atmosphere, no native oxide film is formed on the surface 2s of the lower electrode 2, and a lower electrode 2 with a low work function can be realized. When forming the photoelectric conversion layer 3, for example, a donor organic semiconductor material and an acceptor organic semiconductor material are co-deposited at a predetermined volume ratio by a vacuum deposition method to form the photoelectric conversion layer 3. Note that the method for forming one or more layers including the photoelectric conversion layer 3 is not particularly limited, and for example, at least one of the intermediate layer 5 and the photoelectric conversion layer 3 may be formed by spin coating, inkjet, die coating, spray coating, screen printing, or the like.

[0155] 17 and 20, the upper electrode 4 is formed on one or more layers including the photoelectric conversion layer 3 (step S14). For example, the upper electrode 4 is formed by depositing a material for the upper electrode 4, such as ITO, on the photoelectric conversion layer 3 using a sputtering method or the like. Then, for example, Al is deposited on the upper electrode 4 using an atomic layer deposition (ALD) method or the like. ２ O ３ The protective layer 6 is formed by depositing the material of the protective layer 6, such as the above, on the upper electrode 4. In this way, the photodetector 10A is formed on the substrate 1A.

[0156] [Operation of the Imaging Device] Next, a description will be given of the operation of the imaging device 100. Here, the operation of the imaging device 100 will be mainly described when holes are used as signal charges.

[0157] First, before describing the operation of the imaging device 100, the current-voltage characteristics of the photodetector unit 10A will be described. Fig. 21 is a diagram showing an example of a schematic current-voltage characteristic (IV characteristic) of the photodetector unit 10A. Note that the above-described photoelectric conversion element 10 also has the same configuration as the photodetector unit 10A, and therefore the following description also applies to the photoelectric conversion element 10.

[0158] 21, the vertical axis represents the photocurrent density flowing between the lower electrode 2 and the upper electrode 4 when light is irradiated onto the photodetector 10A, and the horizontal axis represents the voltage applied between the lower electrode 2 and the upper electrode 4 (i.e., the potential difference between the lower electrode 2 and the upper electrode 4). Also, on the horizontal axis of FIG. 21, a voltage that causes signal charges to move to the lower electrode 2 is defined as a "positive" value, and the IV characteristics are shown. In other words, a voltage that makes the potential of the upper electrode 4 higher than the potential of the lower electrode 2 is a "positive" voltage. In the following description, a "positive" voltage is a reverse bias voltage, and a "negative" voltage is a forward bias voltage.

[0159] As shown in FIG. 21, the photocurrent characteristics of the photodetector section 10A are roughly characterized by a first voltage range, a second voltage range, and a third voltage range.

[0160] In the first voltage range, the current change in the photodetector 10A is less dependent on the voltage applied between the lower electrode 2 and the upper electrode 4 and on the amount of light incident on the photodetector 10A. In other words, in the first voltage range, the difference between the current value flowing when light is incident on the photodetector 10A and the current value flowing when no light is incident can be considered small. In other words, the first voltage range is a voltage range in which the current value flowing when light is incident on the photodetector 10A is substantially equal to the current value flowing when no light is incident. In the first voltage range, even if hole-electron pairs are generated by light incident on the photoelectric conversion layer 3, the absolute value of the voltage applied between the lower electrode 2 and the upper electrode 4 is not large, so that the recombination occurs before the holes and electrons separate. Therefore, even when light is incident on the photoelectric conversion layer 3, signal charge does not substantially move to the lower electrode 2. The first voltage range is, for example, a voltage range near 0 V.

[0161] Since the photodetector 10A has an IV characteristic having a first voltage range in which the difference between the current value that flows when light is incident on the photodetector 10A and the current value that flows when no light is incident on the photodetector 10A is small, the imaging device 100 can easily realize a global shutter function.

[0162] The second voltage range is a voltage range of the reverse bias voltage, and is a region where the absolute value of the photocurrent density increases as the reverse bias voltage increases. In other words, the second voltage range is a region where the current value increases as the amount of light incident on the photoelectric conversion layer 3 and the reverse bias voltage applied between the lower electrode 2 and the upper electrode 4 increase.

[0163] The third voltage range is a voltage range of the forward bias voltage, and is a region in which the output current density increases as the forward bias voltage increases.

[0164] The first voltage range, the second voltage range, and the third voltage range may vary depending on the constituent materials, the layered configuration, etc. The above description of the I-V characteristics is also applicable when the photodetector unit 10A has the same configuration as the above-described photoelectric conversion element 110 or 120 other than the photoelectric conversion element 10.

[0165] Next, a global shutter operation of the image pickup device 100 using the above-described IV characteristic of the photodetector unit 10A will be described. Fig. 22 is a timing chart showing an example of the voltage V2 supplied to the upper electrode 4 of the photodetector unit 10A and the timing of the operation in each row of the multiple pixels 14 of the image pickup device 100. For ease of understanding, Fig. 22 only shows the change in voltage V2 and the timing of exposure and signal readout in each row of the multiple pixels 14 indicated by R0 to R7.

[0166] 22 , during the non-exposure period N, signal readout R of the pixels 14 in each row R0 to R7 is performed sequentially. That is, during the non-exposure period N, the charge detection circuit 25 detects the signal charge accumulated in the charge storage node 24 and outputs a signal corresponding to the amount of signal charge accumulated in the charge storage node 24. Furthermore, during the signal readout R, after outputting the signal corresponding to the amount of signal charge accumulated in the charge storage node 24, an initialization voltage is supplied to the charge storage node 24, and the potential of the charge storage node 24 is initialized. Furthermore, during the non-exposure period N, the voltage control circuit 30 supplies a second voltage Vb to the upper electrode 4 as a voltage V2 that causes the potential difference between the lower electrode 2 and the upper electrode 4 to fall within a first voltage range.

[0167] 22 , the start and end timings of the exposure period E after the non-exposure period N are the same for all rows of pixels 14, R0 to R7. During the exposure period E, the voltage control circuit 30 supplies a first voltage Va to the upper electrode 4, which is a voltage V2 that causes the potential difference between the lower electrode 2 and the upper electrode 4 to fall within a second voltage range. During the exposure period E, the voltage control circuit 30 supplies a first voltage Va to the upper electrode 4, which causes the potential difference between the lower electrode 2 and the upper electrode 4 to be greater than that during the non-exposure period N. This causes signal charges to move to the lower electrode 2 and accumulate in the charge storage node 24. The potential of the charge storage node 24 changes from the initialization voltage depending on the amount of accumulated signal charge (rising if the signal charge is a hole). After the exposure period E, the non-exposure period N begins again, and signal readout R of the pixels 14 in each row, R0 to R7, is performed sequentially. In other words, the imaging device 100 realizes a global shutter function in which all rows of pixels 14 are exposed simultaneously while sequentially reading out the signals R from the pixels 14 in each row. Furthermore, during the non-exposure period N, the second voltage Vb, which causes the potential difference between the lower electrode 2 and the upper electrode 4 to fall within the first voltage range, is supplied to the upper electrode 4, so that signal charge does not substantially move to the lower electrode 2. Therefore, while signal readout R from the pixels 14 in the previous row is being performed, signal charge is not substantially accumulated in the charge accumulation node 24, and the magnitude of the signal output remains unchanged from immediately after the exposure period E, and signal readout R from the pixels 14 in each row R0 to R7 is sequentially performed.

[0168] As described above, if signal charge accumulates in the charge storage node 24 during the non-exposure period N, this results in parasitic sensitivity, which is an unintended sensitivity. If parasitic sensitivity occurs between the end of the exposure period E and the signal readout R, the magnitude of the output signal will differ from that immediately after the exposure period E. As described with reference to FIGS. 5 to 7 , when the signal charge is a hole, the work function of the lower electrode 2 is smaller than the work function of the upper electrode 4, thereby suppressing the occurrence of parasitic sensitivity even when the absolute value of the initialization voltage of the charge storage node 24 is reduced. Furthermore, as the absolute value of the initialization voltage of the charge storage node 24 increases, dark current tends to flow more easily at the interface between the n-type impurity region 41B, which is part of the charge storage node 24, and the p-type region of the semiconductor substrate 31. However, the dark current can be reduced by reducing the absolute value of the initialization voltage of the charge storage node 24. Therefore, the image pickup device 100 including the photodetector 10A can reduce dark current and parasitic sensitivity.

[0169] Furthermore, even if the second voltage Vb supplied to the upper electrode 4 is further increased (the energy of the upper electrode 4 is deepened) from the state of the energy band diagram shown in FIG. 5 , holes are less likely to move to the lower electrode 2 until the slope of the energy band of the photoelectric conversion layer 3 is reversed. Therefore, since the occurrence of parasitic sensitivity can be suppressed even when the second voltage Vb supplied to the upper electrode 4 is increased, it is easy to set both the initialization voltage of the charge storage node 24 and the second voltage Vb supplied to the upper electrode 4 during the non-exposure period N to 0 or a positive voltage of the same polarity as the holes, which are signal charges. Furthermore, even during the exposure period E, in order to move holes to the lower electrode 2, a positive voltage greater than the second voltage Vb is supplied to the upper electrode 4 as the first voltage Va. Therefore, in the imaging device 100, dark current and parasitic sensitivity can be reduced without constructing a circuit that supplies a negative voltage.

[0170] 22, the first voltage Va is continuously supplied to the upper electrode 4 during the exposure period E. However, this is not limiting. For example, the voltage control circuit 30 may supply a pulsed voltage that alternates between the first voltage Va and the second voltage Vb during the exposure period E. This makes it possible to adjust the sensitivity of the photodetector 10A during the exposure period E by adjusting the duty ratio of the pulsed voltage. Since the photodetector 10A can suppress the occurrence of parasitic sensitivity when the second voltage Vb is supplied to the upper electrode 4, the sensitivity of the photodetector 10A during the exposure period E can be adjusted with improved accuracy. In this case, the first period during which the voltage control circuit 30 supplies the first voltage Va and the second period during which the voltage control circuit 30 supplies the second voltage Vb are included in the exposure period E. Furthermore, in this case, even when the imaging device 100 is driven using a rolling shutter system, the first voltage Va and the second voltage Vb are supplied to the upper electrode 4.

[0171] [Camera System] Next, a camera system including the imaging device according to this embodiment will be described.

[0172] FIG. 23 is a block diagram showing an example of the configuration of a camera system 400 according to this embodiment.

[0173] 23, camera system 400 according to this embodiment includes lens optical system 601, imaging device 602, system controller 603, and camera signal processing circuit 604. Camera system 400 may be, for example, a smartphone, a digital camera, a video camera, or an in-vehicle camera.

[0174] The lens optical system 601 focuses light onto an imaging surface of the imaging device 602. The lens optical system 601 may include, for example, a lens group including an autofocus lens and a zoom lens, and an aperture. As the imaging device 602, for example, the imaging device 100 described above is used.

[0175] The system controller 603 controls the entire camera system 400. The system controller 603 is, for example, a semiconductor integrated circuit, and a specific example is a CPU (Central Processing Unit).

[0176] The camera signal processing circuit 604 has a function of processing an output signal from the image capture device 602. The camera signal processing circuit 604 receives output data from the image capture device 602 and performs processes such as gamma correction, color interpolation, spatial interpolation, and auto white balance. The camera signal processing circuit 604 is, for example, a DSP (Digital Signal Processor). The image capture device 602 and the camera signal processing circuit 604 may be implemented as a single semiconductor device. The semiconductor device may be, for example, a so-called SoC (System on a Chip). This configuration allows for further miniaturization of electronic devices that include the image capture device 602 as a part thereof.

[0177] Examples Hereinafter, the effects of the photoelectric conversion element and the imaging device according to the present disclosure will be specifically described with reference to Examples 1 and 2 and Comparative Example 1. Note that the present disclosure is not limited to the following examples.

[0178] [Fabrication of Imaging Device] An imaging device was fabricated by the following method.

[0179] (1) Comparative Example 1: A substrate was prepared on which a titanium nitride film was formed as a lower electrode on a substrate on which a charge storage node and a charge detection circuit for detecting holes as signal charges were formed. The substrate was stored under atmospheric conditions, and a native oxide film had formed on the surface of the titanium nitride film. Next, an intermediate layer was formed by vacuum deposition of 2,2'-dimethyl-N,N'-di-[(1-naphthalyl)-N,N'-diphenyl]-1,1'-biphenyl-4,4'-diamine (α-NPD) to a thickness of 50 nm on the lower electrode. Next, a photoelectric conversion layer was formed by vacuum deposition of subphthalocyanine as a donor material and C60 fullerene as an acceptor material, with a volume ratio of the donor material to the acceptor material of 3:1 and a total thickness of 400 nm on the intermediate layer. The subphthalocyanine used had boron (B) as the central metal, with chloride ions coordinated to B as ligands. Next, an ITO film was formed on the photoelectric conversion layer by sputtering to a thickness of 30 nm, thereby forming an upper electrode. Furthermore, aluminum oxide was formed on the upper electrode by atomic layer deposition to a thickness of 60 nm, thereby forming a protective layer. This resulted in an imaging device in Comparative Example 1, in which a photoelectric conversion element was formed on the substrate as a light detection unit.

[0180] (2) Example 1: A substrate was prepared on which a titanium nitride film was formed as a lower electrode on a substrate on which a charge storage node and a charge detection circuit for detecting holes as signal charges were formed. The substrate was stored under atmospheric conditions, and a native oxide film had formed on the surface of the titanium nitride film. Next, plasma etching using argon gas was performed in a vacuum for 10 minutes to remove the native oxide film from the surface of the titanium nitride film. Next, in a glove box with an oxygen concentration of 1 ppm or less, the titanium nitride film from which the native oxide film had been removed was annealed at 300°C for 15 minutes to obtain a lower electrode from which the native oxide film had been removed. Next, without exposing the lower electrode to the atmosphere, an intermediate layer was formed by vacuum deposition of 9,9'-[1,1'-biphenyl]-4,4'-diylbis[3,6-bis(1,1-dimethylethyl)]-9H-carbazole to a thickness of 50 nm on the lower electrode. The subsequent steps after the formation of the photoelectric conversion layer were carried out in the same manner as in Comparative Example 1, to obtain an imaging device in Example 1 in which a photoelectric conversion element was formed on the substrate as a light detection section.

[0181] (3) Example 2 The same steps as in Comparative Example 1 were carried out to obtain an imaging device in Example 2 in which a photoelectric conversion element was formed as a light detection unit on a substrate, except that an intermediate layer was formed by forming a film of 9,9'-[1,1'-biphenyl]-4,4'-diylbis[3,6-bis(1,1-dimethylethyl)]-9H-carbazole on the lower electrode by a vacuum deposition method to a thickness of 50 nm.

[0182] [Measurement of Ionization Potential and Work Function] The ionization potential and electron affinity of each material used in forming the intermediate layer and photoelectric conversion layer of the imaging devices in the examples and comparative examples were measured.

[0183] In measuring the ionization potential, a sample was prepared by depositing a film of each material used in the examples and comparative examples on a glass substrate on which an ITO film had been deposited. Next, the number of photoelectrons was measured for the prepared sample using an atmospheric photoelectron spectrometer (AC-3, manufactured by Riken Keiki) while varying the energy of ultraviolet irradiation, and the energy position at which photoelectrons began to be detected was taken as the ionization potential.

[0184] In measuring the electron affinity, first, a sample was prepared by depositing each material used in the examples and comparative examples on a quartz substrate. Next, the absorption spectrum of the prepared sample was measured using a spectrophotometer (U4100, manufactured by Hitachi High-Technologies Corporation), and the optical band gap was calculated from the absorption edge of the obtained absorption spectrum. The electron affinity was estimated by subtracting the ionization potential obtained by the above ionization potential measurement from the calculated optical band gap.

[0185] The measurement results of the ionization potential and electron affinity of each material are shown in Table 1.

[0186]

[0187] [Work Function Measurement] The work functions of the surfaces of the upper and lower electrodes used in the imaging devices in the Examples and Comparative Examples were measured. To measure the work functions, first, the lower and upper electrodes were formed independently on a substrate using the same method as in Comparative Example 1 and Examples 1 and 2. Next, using a multifunctional X-ray photoelectron spectrometer (VersaProbe, manufactured by ULVAC-PHI, Inc.), the work functions were measured by irradiating the surfaces of the formed electrodes with vacuum ultraviolet light, primarily consisting of HeI rays with an energy of 21.22 eV, in a vacuum using ultraviolet photoelectron spectroscopy. The measurement results for the work functions of the upper and lower electrodes in the Examples and Comparative Examples are shown in Table 2. Table 2 also shows the ionization potentials of the intermediate layer and photoelectric conversion layer, as well as the value of (A-B)-(C-D) where A is the work function of the lower electrode (eV), B is the work function of the upper electrode (eV), C is the ionization potential of the intermediate layer, and D is the ionization potential of the photoelectric conversion layer (eV).

[0188]

[0189] As shown in Table 2, in Comparative Example 1 and Example 2, the work function of the lower electrode is larger than that of the upper electrode. On the other hand, in Example 1, the work function of the lower electrode is smaller than that of the upper electrode. This is because the native oxide film on the surface of the titanium nitride film serving as the lower electrode is removed in Example 1, whereas the native oxide film remains on the surface of the titanium nitride film serving as the lower electrode in Comparative Example 1 and Example 2.

[0190] Furthermore, in Comparative Example 1, (AB)-(CD)<0 is not satisfied, but in Examples 1 and 2, (AB)-(CD)<0 is satisfied.

[0191] [Evaluation of Parasitic Sensitivity] The parasitic sensitivity of the imaging devices in the example and comparative example was evaluated.

[0192] In the evaluation of the parasitic sensitivity, the initial voltage of the lower electrode was set to +0.8 V, and the second voltage applied to the upper electrode was varied within a range of positive voltages, and the output of the imaging device was measured in both bright and dark conditions. ２ The output value in the dark is the value when light is irradiated onto the photodetector with an amount of light of 10 ...

[0193] As shown in FIG. 24 , the second voltage at which parasitic sensitivity is likely to occur is higher in the imaging devices of Examples 1 and 2 than in Comparative Example 1. Therefore, it is possible to lower the initialization voltage of the charge storage node while suppressing the occurrence of parasitic sensitivity, thereby reducing dark current. Therefore, as in the imaging devices of Examples 1 and 2, it can be seen that satisfying (A-B)-(C-D)<0 makes it possible to reduce parasitic sensitivity and dark current. Furthermore, comparing Example 1 and Example 2, the second voltage at which parasitic sensitivity is likely to occur is higher in the imaging device of Example 1 than in Example 2. Therefore, as in the imaging device of Example 1, the work function of the lower electrode is smaller than the work function of the upper electrode, making it possible to further lower the initialization voltage of the charge storage node while suppressing the occurrence of parasitic sensitivity. Therefore, the imaging device of Example 1 allows for further reduction in dark current.

[0194] (Other Embodiments) The photoelectric conversion element, imaging device, and camera system according to the present disclosure have been described above based on embodiments and examples, but the present disclosure is not limited to these embodiments and examples. As long as they do not deviate from the gist of the present disclosure, various modifications that a person skilled in the art could conceive of to the embodiments and examples, as well as other forms constructed by combining some of the components of the embodiments and examples, are also included in the scope of the present disclosure.

[0195] For example, in the above embodiment, the photoelectric conversion element is used in an imaging device, but is not limited thereto. For example, the photoelectric conversion element according to the present disclosure may be used in an optical sensor by extracting charges generated by light as a signal.

[0196] The photoelectric conversion element according to the present disclosure can be used for various applications requiring sensitivity adjustment, such as imaging devices for various applications. The imaging device according to the present disclosure can also be applied to various camera systems and sensor systems, such as medical cameras, surveillance cameras, vehicle-mounted cameras, distance measuring cameras, microscope cameras, drone cameras, and robot cameras.

[0197] 1 Support substrate 1A Substrate 2, 2x, 112, 122 Lower electrode 2s, 2sx, 112s, 122s Surface 3 Photoelectric conversion layer 4 Upper electrode 5 Intermediate layer 6 Protective layer 10, 110, 120 Photoelectric conversion element 10A Photodetector section 11 Amplifying transistor 12 Reset transistor 13 Address transistor 14 Pixel 15 Vertical scanning circuit 16 Counter electrode signal line 17 Vertical signal line 18 Load circuit 19 Column signal processing circuit 20 Horizontal signal readout circuit 21 Power supply wiring 23 Reset voltage line 24 Charge storage node 25 Charge detection circuit 26 Address signal line 27 Reset signal line 28 Horizontal common signal line 30 Voltage control circuit 31 Semiconductor substrate 38A, 38B, 38C Gate insulating layer 39A, 39B, 39C Gate electrode 41A, 41B, 41C, 41D, 41E n-type impurity region 42 Element isolation region 43A, 43B, 43C Interlayer insulating layer 45A, 45B Contact plug 46A, 46B, 46C Wiring 47A, 47B, 47C Plug 60 Color filter 61 Microlens 100, 602 Imaging device 122a Layer 122b Self-assembled monolayer 400 Camera system 601 Lens optical system 603 System controller 604 Camera signal processing circuit

Claims

1. A photoelectric conversion element comprising: a photoelectric conversion layer that converts light into signal charges; a first electrode containing a metal element that collects the signal charges; and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, wherein the signal charges are holes, and the work function of the first electrode is smaller than the work function of the second electrode.

2. A photoelectric conversion element comprising: a photoelectric conversion layer that converts light into signal charges; a first electrode containing a metal element that collects the signal charges; and a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween, wherein the signal charges are electrons, and the work function of the first electrode is greater than the work function of the second electrode.

3. The photoelectric conversion element according to claim 1 or 2, wherein the difference between the work function of the first electrode and the work function of the second electrode is greater than 0.0 eV and not more than 2.0 eV.

4. The photoelectric conversion element according to claim 1 or 2, wherein the first electrode includes a metal nitride containing the metal element.

5. The photoelectric conversion element according to claim 4, wherein the metal nitride is titanium nitride.

6. The photoelectric conversion element according to claim 1 or 2, wherein the first electrode comprises: a layer containing the metal element and having a first surface and a second surface closer to the photoelectric conversion layer than the first surface; and a self-assembled monolayer located on the second surface of the layer.

7. The photoelectric conversion element according to claim 1, further comprising an intermediate layer located between the photoelectric conversion layer and the first electrode, wherein the ionization potential of the intermediate layer is greater than the ionization potential of the photoelectric conversion layer.

8. The photoelectric conversion element according to claim 2, further comprising an intermediate layer located between the photoelectric conversion layer and the first electrode, wherein the electron affinity of the intermediate layer is smaller than the electron affinity of the photoelectric conversion layer.

9. An imaging device comprising: a photoelectric conversion element according to claim 1 or 2; a charge accumulation region electrically connected to the first electrode; and a voltage supply circuit electrically connected to the second electrode and applying a potential difference between the first electrode and the second electrode, wherein the voltage supply circuit supplies a first voltage to the second electrode during a first period, and supplies a second voltage different from the first voltage to the second electrode during a second period.

10. An imaging device according to claim 9, wherein the first period is an exposure period during which the signal charge is accumulated in the charge accumulation region, and the second period is a non-exposure period that is a period other than the exposure period during operation of the imaging device.

11. The imaging device according to claim 9, wherein the first voltage is a voltage having the same polarity as the polarity of the signal charge.

12. The imaging device according to claim 11, wherein the second voltage is 0 or a voltage having the same polarity as the polarity of the signal charge.

13. The imaging device according to claim 11, wherein the imaging device is configured such that an initialization voltage for initializing the potential of the charge accumulation region is supplied to the charge accumulation region, and the initialization voltage is either 0 or a voltage of the same polarity as the polarity of the signal charge.

14. A photoelectric conversion element comprising: a photoelectric conversion layer that converts light into signal charges; a first electrode containing a metal element and collecting the signal charges; a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween; and an intermediate layer positioned between the photoelectric conversion layer and the first electrode, wherein the signal charges are holes, and where the work function of the first electrode is A [eV], the work function of the second electrode is B [eV], the ionization potential of the intermediate layer is C [eV], and the ionization potential of the photoelectric conversion layer is D [eV], the relationship (A-B)-(C-D)<0 is satisfied.

15. A photoelectric conversion element comprising: a photoelectric conversion layer that converts light into signal charges; a first electrode containing a metal element and collecting the signal charges; a second electrode facing the first electrode with the photoelectric conversion layer interposed therebetween; and an intermediate layer positioned between the photoelectric conversion layer and the first electrode, wherein the signal charges are electrons, and where the work function of the first electrode is A [eV], the work function of the second electrode is B [eV], the electron affinity of the intermediate layer is E [eV], and the electron affinity of the photoelectric conversion layer is F [eV], the relationship (A-B)+(F-E)>0 is satisfied.

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