Photoelectric conversion element and imaging device

Abstract

A photoelectric conversion element comprising: a photoelectric conversion layer (4); a first electrode (2) for collecting holes generated in the photoelectric conversion layer (4); and a second electrode (3) opposite to the first electrode (2) with the photoelectric conversion layer (4) therebetween to collect electrons generated in the photoelectric conversion layer (4). The photoelectric conversion layer (4) includes a first quantum dot layer (4a) including a plurality of first quantum dots with a surface modified by a first ligand, and a second quantum dot layer (4b) positioned between the first quantum dot layer (4a) and the second electrode (3) and including a plurality of second quantum dots with a surface modified by a second ligand different from the first ligand. The ionization potential of the second quantum dot layer (4b) is greater than the ionization potential of the first quantum dot layer (4a). A second value indicating the particle diameter distribution of the plurality of second quantum dots is smaller than a first value indicating the particle diameter distribution of the plurality of first quantum dots.

Description

[Technical Field]

[0001] This disclosure relates to a photoelectric conversion element and an imaging device. [Background technology]

[0002] Research is actively being conducted on photoelectric devices that utilize semiconductor quantum dots as photoelectric conversion materials, taking advantage of the quantum size effect. A semiconductor quantum dot refers to a nanocrystal, which is a semiconductor microcrystal with a size of several nanometers. The energy state is discretized when electrons, holes, and excitons are confined within this nanocrystal, and the appearance of an energy shift dependent on particle size is called the quantum size effect. The energy gap of a semiconductor quantum dot, which is a nanocrystal, becomes larger than the energy gap of a bulk crystal as the particle size decreases, so the absorption end wavelength shifts to the shorter wavelength side. In other words, by controlling the particle size while using the same semiconductor material, it is possible to design the optical absorption wavelength to any desired wavelength, with the absorption end wavelength of the bulk crystal as the upper limit. Hereafter, "semiconductor quantum dot" may be simply referred to as "quantum dot". When using quantum dots as photoelectric conversion materials, it is desirable that their sensitivity wavelength range be wide depending on the application.

[0003] Patent Document 1 discloses a technique that can broaden the sensitivity wavelength range by mixing multiple quantum dots of different particle sizes to form a photoelectric conversion layer.

[0004] Furthermore, Patent Document 2 discloses a technique for broadening the sensitivity wavelength by modulating the band gap at the interface by stacking a layer composed of small quantum dots and a layer composed of large quantum dots.

[0005] Furthermore, Patent Document 3 discloses a technique in which the band structure is controlled by changing the thickness and composition of the ligand layer that modifies the surface of the quantum dot, thereby creating an energy band gradient within the two quantum dot semiconductor layers and improving the mobility of photogenerated carriers. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6083813 [Patent Document 2] Patent No. 6255417 [Patent Document 3] Patent No. 6298223 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, while Patent Documents 1, 2, and 3 disclose broadening the sensitivity wavelength and improving mobility when quantum dots are used as photoelectric conversion materials, they do not disclose dark current, which is crucial for realizing high-performance photodiodes and imaging devices. In photodiodes and imaging devices, dark current is directly related to the signal-to-noise (S / N) ratio. Therefore, for practical purposes, a dark current several orders of magnitude lower than that of power generation elements such as solar cells is desirable.

[0008] This disclosure aims to provide a photoelectric conversion element and an imaging device that can achieve both an expansion of the sensitivity wavelength range and a reduction in dark current. [Means for solving the problem]

[0009] A photoelectric conversion element according to one aspect of the present disclosure includes a photoelectric conversion layer, a first electrode for collecting holes generated in the photoelectric conversion layer, and a second electrode facing the first electrode across the photoelectric conversion layer for collecting electrons generated in the photoelectric conversion layer. The photoelectric conversion layer includes a first quantum dot layer containing a plurality of first quantum dots whose surfaces are modified with a first ligand, and a second quantum dot layer located between the first quantum dot layer and the second electrode, containing a plurality of second quantum dots whose surfaces are modified with a second ligand different from the first ligand. The ionization potential of the second quantum dot layer is greater than the ionization potential of the first quantum dot layer. A second value indicating the particle size distribution of the plurality of second quantum dots is smaller than a first value indicating the particle size distribution of the plurality of first quantum dots.

[0010] Furthermore, an imaging device according to one aspect of this disclosure comprises a plurality of pixels, each of which includes the photoelectric conversion element.

[0011] Furthermore, a photoelectric conversion element according to another aspect of the present disclosure comprises a photoelectric conversion layer, a first electrode for collecting holes generated in the photoelectric conversion layer, and a second electrode facing the first electrode across the photoelectric conversion layer for collecting electrons generated in the photoelectric conversion layer. The photoelectric conversion layer includes a first quantum dot layer containing a plurality of first quantum dots whose surfaces are modified with a first ligand, and a second quantum dot layer located between the first quantum dot layer and the second electrode, containing a plurality of second quantum dots whose surfaces are modified with a second ligand different from the first ligand. The ionization potential of the second quantum dot layer is greater than the ionization potential of the first quantum dot layer. The absorption peak wavelengths of the plurality of second quantum dots are smaller than the absorption peak wavelengths of the plurality of first quantum dots. The material of the plurality of first quantum dots is different from the material of the plurality of second quantum dots. [Effects of the Invention]

[0012] According to one aspect of this disclosure, it is possible to achieve both an expansion of the sensitivity wavelength range and a reduction in dark current. [Brief explanation of the drawing]

[0013] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating a configuration of the photoelectric conversion element according to the first embodiment. [Figure 2A] FIG. 2A is a cross-sectional view schematically illustrating a configuration of another example of the photoelectric conversion element according to the first embodiment. [Figure 2B] FIG. 2B is a diagram illustrating an example of an energy diagram of the photoelectric conversion element illustrated in FIG. 2A. [Figure 3] FIG. 3 is a diagram illustrating measurement results of ionization potential and electron affinity of quantum dots when the particle size and surface-modifying ligand are changed. [Figure 4] FIG. 4 is a diagram illustrating an example of a circuit configuration of an imaging device according to the second embodiment. [Figure 5] FIG. 5 is a cross-sectional view schematically illustrating a device structure of a pixel in the imaging device according to the second embodiment. [Figure 6] FIG. 6 is a diagram illustrating an example of a circuit configuration of an imaging device according to a modification of the second embodiment. [Figure 7] FIG. 7 is a cross-sectional view schematically illustrating a device structure of a pixel in an imaging device according to a modification of the second embodiment. [Figure 8] FIG. 8 is a diagram illustrating a relationship between an absorption peak wavelength and a diameter of PbS quantum dots and PbSe quantum dots. [Figure 9A] FIG. 9A is a diagram illustrating absorption spectra of PbS quantum dots and PbSe quantum dots. [Figure 9B] FIG. 9B is a cross-sectional view schematically illustrating a configuration of a photoelectric conversion element using a combination of quantum dots having different constituent elements. [Figure 9C] FIG. 9C is a diagram illustrating an example of an energy diagram of the photoelectric conversion element illustrated in FIG. 9B. DETAILED DESCRIPTION OF EMBODIMENTS

[0014] (Summary of the Present Disclosure) A summary of one aspect of the present disclosure is as follows.

[0015] A photoelectric conversion element according to one aspect of the present disclosure includes a photoelectric conversion layer, a first electrode for collecting holes generated in the photoelectric conversion layer, and a second electrode facing the first electrode across the photoelectric conversion layer for collecting electrons generated in the photoelectric conversion layer. The photoelectric conversion layer includes a first quantum dot layer containing a plurality of first quantum dots whose surfaces are modified with a first ligand, and a second quantum dot layer located between the first quantum dot layer and the second electrode, containing a plurality of second quantum dots whose surfaces are modified with a second ligand different from the first ligand. The ionization potential of the second quantum dot layer is greater than the ionization potential of the first quantum dot layer. A second value indicating the particle size distribution of the plurality of second quantum dots is smaller than a first value indicating the particle size distribution of the plurality of first quantum dots.

[0016] As a result, since the absorption wavelength of quantum dots depends on the particle size of the quantum dots, the photoelectric conversion layer can expand the sensitivity wavelength range by including first and second quantum dots with different particle sizes. Furthermore, in the photoelectric conversion layer, the ionization potential of the second quantum dot layer, located near the second electrode, is greater than that of the first quantum dot layer. This allows excitons generated by light absorption to efficiently dissociate into electrons and holes at the interface between the first and second quantum dot layers. The holes then move through the first quantum dot layer and are collected at the first electrode, while the electrons move through the second quantum dot layer and are collected at the second electrode. Therefore, charge flows smoothly, improving the photoelectric conversion efficiency. In addition, because the particle size of the second quantum dot is smaller than that of the first quantum dot, the band gap of the second quantum dot layer is larger, and the difference between the ionization potential of the first quantum dot layer and the electron affinity of the second quantum dot layer tends to be larger. As a result, thermal excitation is less likely to occur at the interface between the first and second quantum dot layers. Therefore, the dark current caused by thermal excitation at the interface between the first quantum dot layer and the second quantum dot layer can be reduced. Thus, the photoelectric conversion element according to this embodiment can achieve both an expansion of the sensitivity wavelength range and a reduction in dark current.

[0017] Furthermore, for example, each of the plurality of first quantum dots and the plurality of second quantum dots may independently contain at least one of the following: CdSe, CdS, PbS, PbSe, PbTe, ZnO, ZnS, Cu2ZnSnS4, Cu2S, Bi2S3, CuInSe2, AgInS2, AgInTe2, CdSnAs2, ZnSnAs2, ZnSnSb2, Ag2S, Ag2Te, HgTe, CdHgTe, Ge, GeSn, InAs, and InSb. In other words, each of the plurality of first quantum dots and the plurality of second quantum dots may independently include at least one selected from the group consisting of CdSe, CdS, PbS, PbSe, PbTe, ZnO, ZnS, Cu2ZnSnS4, Cu2S, Bi2S3, CuInSe2, AgInS2, AgInTe2, CdSnAs2, ZnSnAs2, ZnSnSb2, Ag2S, Ag2Te, HgTe, CdHgTe, Ge, GeSn, InAs, and InSb.

[0018] By combining these elements, the sensitivity wavelength of the photoelectric conversion element can be arbitrarily controlled over a wide wavelength range from visible light to infrared light.

[0019] Furthermore, for example, if the first ligand has a first dipole moment and the second ligand has a second dipole moment, and the first dipole moment is positive when the first dipole moment points outward from each of the plurality of first quantum dots, and the second dipole moment is positive when the second dipole moment points outward from each of the plurality of second quantum dots, then the first dipole moment may be greater than the second dipole moment.

[0020] As a result, the first ligand tends to decrease the ionization potential of the first quantum dot layer, while the second ligand tends to increase the ionization potential of the second quantum dot layer. Therefore, it is easy to make the ionization potential of the second quantum dot layer greater than that of the first quantum dot layer, enabling the realization of a photoelectric conversion element that can achieve both an expanded sensitivity wavelength range and a reduction in dark current.

[0021] Furthermore, for example, the first ligand may be 1,4-benzenedithiol, and the second ligand may be a mixture of ZnI2 and 3-mercaptopropionic acid.

[0022] This effectively allows for both an expansion of the sensitivity wavelength range and a reduction in dark current.

[0023] Furthermore, for example, at least one of the particle size distributions of the plurality of first quantum dots and the particle size distributions of the plurality of second quantum dots may have two or more different maximum values. In other words, at least one selected from the group consisting of the particle size distributions of the plurality of first quantum dots and the particle size distributions of the plurality of second quantum dots may have two or more different maximum values.

[0024] This allows for a further expansion of the sensitivity wavelength range of the photoelectric conversion element.

[0025] Furthermore, an imaging device according to one aspect of this disclosure comprises a plurality of pixels, each of which includes the photoelectric conversion element.

[0026] As a result, the imaging device, having the above-mentioned photoelectric conversion element, can achieve both an expansion of the sensitivity wavelength range and a reduction in dark current.

[0027] Furthermore, for example, the imaging device may further include a signal readout circuit connected to the first electrode and a voltage supply circuit that supplies a voltage to the second electrode such that the potential of the second electrode is positive relative to the potential of the first electrode.

[0028] As a result, the imaging device can use the photoelectric conversion element to read out the holes collected on the first electrode as signal charges.

[0029] Furthermore, for example, the imaging device may further include a signal readout circuit connected to the second electrode and a voltage supply circuit that supplies a voltage to the first electrode such that the potential of the first electrode is negative relative to the potential of the second electrode.

[0030] As a result, the imaging device can use the photoelectric conversion element to read out the electrons collected on the second electrode as a signal charge.

[0031] A photoelectric conversion element according to another aspect of the present disclosure comprises a photoelectric conversion layer, a first electrode for collecting holes generated in the photoelectric conversion layer, and a second electrode facing the first electrode across the photoelectric conversion layer for collecting electrons generated in the photoelectric conversion layer. The photoelectric conversion layer includes a first quantum dot layer containing a plurality of first quantum dots whose surfaces are modified with a first ligand, and a second quantum dot layer located between the first quantum dot layer and the second electrode, containing a plurality of second quantum dots whose surfaces are modified with a second ligand different from the first ligand. The ionization potential of the second quantum dot layer is greater than the ionization potential of the first quantum dot layer. The absorption peak wavelengths of the plurality of second quantum dots are smaller than the absorption peak wavelengths of the plurality of first quantum dots. The material of the plurality of first quantum dots is different from the material of the plurality of second quantum dots.

[0032] Furthermore, for example, the second value representing the particle size distribution of the plurality of first quantum dots may be equal to the first value representing the particle size distribution of the plurality of second quantum dots.

[0033] Furthermore, for example, the second value representing the particle size distribution of the plurality of second quantum dots may be greater than the first value representing the particle size distribution of the plurality of first quantum dots.

[0034] The embodiments of this disclosure will be described below with reference to the drawings.

[0035] The embodiments described below are all comprehensive or specific examples. The numerical values, shapes, components, arrangement and connection configurations of components, steps, and the order of steps shown in the following embodiments are examples only and are not intended to limit this disclosure. Furthermore, components in the following embodiments that are not described in an independent claim are described as optional components. In addition, the figures are not necessarily strictly accurate. Therefore, for example, the scale in each figure may not necessarily match. Also, in each figure, substantially identical components are denoted by the same reference numerals, and redundant explanations may be omitted or simplified.

[0036] Furthermore, in this specification, terms indicating relationships between elements, terms indicating the shape of elements, and numerical ranges do not represent only strict meanings, but also include substantially equivalent ranges, such as differences of a few percent.

[0037] 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 perception, but rather are used as terms defined by the relative positional relationship based on the stacking order in the stacked configuration. It should be noted that terms such as "upper" and "lower" are used solely to specify the relative arrangement of components and are not intended to limit the orientation when the imaging device is in use. Moreover, the terms "upper" and "lower" apply not only when two components are spaced apart and another component exists between them, but also when two components are placed in close proximity and touching each other.

[0038] Furthermore, in this specification, electromagnetic waves in general, including visible light, infrared rays, and ultraviolet rays, will be referred to as "light" for convenience.

[0039] (Embodiment 1) [Overall structure] First, the overall configuration of the photoelectric conversion element according to this embodiment will be described. Figure 1 is a schematic cross-sectional view showing the configuration of the photoelectric conversion element 10A according to this embodiment. As shown in Figure 1, the photoelectric conversion element 10A comprises a pair of electrodes, a first electrode 2 and a second electrode 3, and a photoelectric conversion layer 4 located between the first electrode 2 and the second electrode 3. The photoelectric conversion layer 4 has a first quantum dot layer 4a containing a first quantum dot, and a second quantum dot layer 4b located between the first quantum dot layer 4a and the second electrode 3, containing a second quantum dot with a particle size smaller than that of the first quantum dot. In other words, the photoelectric conversion layer 4 is constructed by stacking a first quantum dot layer 4a and a second quantum dot layer 4b containing quantum dots of different particle sizes. The first quantum dot layer 4a and the second quantum dot layer 4b are in contact. The photoelectric conversion element 10A is supported on a substrate 1. In the photoelectric conversion element 10A, the first electrode 2, the first quantum dot layer 4a, the second quantum dot layer 4b, and the second electrode 3 are stacked in this order on one main surface of the substrate 1.

[0040] The photoelectric conversion element 10A may further include an electron blocking layer and a hole blocking layer. Figure 2A is a schematic cross-sectional view showing the configuration of another example of the photoelectric conversion element 10B according to this embodiment. Figure 2B is a diagram showing an example of the energy diagram of the photoelectric conversion element 10B shown in Figure 2A. In Figure 2B, the difference between the vacuum level and the upper end of the energy band is the electron affinity, and the difference between the vacuum level and the lower end of the energy band is the ionization potential. Also, the difference between the Fermi level and the vacuum level is the work function. Note that an example of the energy diagram of the photoelectric conversion element 10A shown in Figure 1 can be represented by removing the energy bands of the electron blocking layer 5 and the hole blocking layer 6 from Figure 2B.

[0041] As shown in Figure 2A, the photoelectric conversion element 10B includes, in addition to the configuration of the photoelectric conversion element 10A, an electron blocking layer 5 located between the first electrode 2 and the first quantum dot layer 4a, and a hole blocking layer 6 located between the second electrode 3 and the second quantum dot layer 4b. In the photoelectric conversion element 10B, the first electrode 2, electron blocking layer 5, first quantum dot layer 4a, second quantum dot layer 4b, hole blocking layer 6, and second electrode 3 are stacked in this order on one main surface of the substrate 1. As will be described in detail later, this configuration can reduce the dark current when a reverse bias voltage is applied during photoelectric conversion.

[0042] The details of each component of the photoelectric conversion element according to this embodiment will be described below.

[0043] [substrate] Substrate 1 is a support base for supporting photoelectric conversion elements 10A and 10B. The material of substrate 1 is not particularly limited, and various materials can be used. For example, it may be a p-type silicon substrate, or a glass or plastic substrate coated with a conductive metal oxide such as ITO (Indium Tin Oxide) or a conductive polymer such as polyacetylene. Substrate 1 transmits, for example, at least a portion of the light of wavelengths absorbed by the photoelectric conversion layer 4.

[0044] In the illustrated example, the substrate 1 is located on the side of the first electrode 2 in the photoelectric conversion elements 10A and 10B, but it may also be located on the side of the second electrode 3 in the photoelectric conversion elements 10A and 10B.

[0045] [First electrode and second electrode] The first electrode 2 and the second electrode 3 are, for example, film-like electrodes. The first electrode 2 is a hole-collecting electrode that collects holes generated in the photoelectric conversion layer 4, and the second electrode 3 is an electron-collecting electrode that collects electrons generated in the photoelectric conversion layer 4. The second electrode 3 is positioned opposite the first electrode 2, with the photoelectric conversion layer 4 in between.

[0046] At least one of the first electrode 2 and the second electrode 3 is a transparent electrode with high light transmittance in a desired wavelength range. The desired wavelength range is, for example, a wavelength range that includes the absorption peaks of the first quantum dot and the second quantum dot. In this specification, high light transmittance at a certain wavelength means, for example, that the light transmittance at a certain wavelength is 50% or more, or it may mean 80% or more.

[0047] A bias voltage is applied to the first electrode 2 and the second electrode 3, for example, by wiring (not shown). For example, the polarity of the bias voltage is determined such that, of the electron-hole pairs generated in the photoelectric conversion layer 4, electrons move to the second electrode 3 and holes move to the first electrode 2. Specifically, a bias voltage is applied such that the potential of the second electrode 3 is positive relative to the potential of the first electrode 2. As a result, the first electrode 2 collects holes and the second electrode 3 collects electrons. Alternatively, by making the work function of the second electrode 3 smaller than the work function of the first electrode 2, the first electrode 2 may collect holes and the second electrode 3 may collect electrons.

[0048] For materials of the first electrode 2 and second electrode 3 that exhibit high light transmittance in the desired wavelength range, for example, a transparent conductive oxide (TCO) with low resistance is used. Conducting Oxide (TCO) is used. TCO is not particularly limited, but for example, ITO, IZO (InZnO; Indium Zinc Oxide), AZO (AlZnO: Aluminum Zinc Oxide), FTO (Fluorine-doped Tin Oxide), SnO2, TiO2, ZnO2, etc. can be used.

[0049] Furthermore, the materials used for the first electrode 2 and the second electrode 3 may include Al, Cu, Ti, TiN, Ta, TaN, Mo, Ru, In, Mg, Ag, Au, or Pt.

[0050] [Photoelectric conversion layer] In the photoelectric conversion layer 4, pairs of holes (excitons) and electrons are generated upon the incidence of light. The photoelectric conversion layer 4 contains quantum dots of different particle sizes as a photoelectric conversion material to expand the sensitive wavelength range of the photoelectric conversion element 10A. The quantum dots are nanocrystals with a diameter of about 2 nm to 10 nm and are composed of several tens of atoms. The material of the quantum dots is, for example, a group IV semiconductor such as Si or Ge, a group IV-VI semiconductor such as PbS, PbSe or PbTe, a group III-V semiconductor such as InAs or InSb, or a ternary mixed crystal such as HgCdTe or PbSnTe.

[0051] Specifically, the photoelectric conversion layer 4 has a first quantum dot layer 4a containing a first quantum dot and a second quantum dot layer 4b containing a second quantum dot. The first quantum dot layer 4a and the second quantum dot layer 4b each generate hole-electron pairs by absorbing light. In the photoelectric conversion layer 4, the first quantum dot layer 4a is located on the side of the first electrode 2, which is a hole collection electrode, and the second quantum dot layer 4b is located on the side of the second electrode 3, which is an electron collection electrode. The first and second quantum dots, for example, each independently include at least one of the following: CdSe, CdS, PbS, PbSe, PbTe, ZnO, ZnS, Cu2ZnSnS4 (CZTS), Cu2S, Bi2S3, CuInSe2, AgInS2, AgInTe2, CdSnAs2, ZnSnAs2, ZnSnSb2, Ag2S, Ag2Te, HgTe, CdHgTe, Ge, GeSn, InAs, and InSb. By using these, the sensitivity wavelength of the photoelectric conversion element can be arbitrarily controlled over a wide wavelength range from visible light to infrared light.

[0052] A second value representing the particle size distribution of multiple second quantum dots is smaller than a first value representing the particle size distribution of multiple first quantum dots. The first and second values ​​are measured as the mode diameters that result in the maximum value of the distribution when the particle size distribution of multiple particles is represented as a frequency distribution using, for example, a transmission electron microscope. Alternatively, for example, the second value may be the maximum particle diameter at which the particle size distribution of the second quantum dots shows a maximum value, and the first value may be the minimum particle diameter at which the particle size distribution of the first quantum dots shows a maximum value. Note that the first and second values ​​may also be the average particle diameter, which is the average value of the particle diameters of multiple quantum dots arbitrarily selected from each layer.

[0053] Furthermore, at least one of the particle size distributions of the first quantum dot and the second quantum dot may have two or more distinct maximum values. This further expands the sensitivity wavelength range of the photoelectric conversion element 10A. When the particle size distribution of the quantum dot has two or more distinct maximum values, the particle size is, for example, the average value of the particle size shown by each of the two or more maximum values.

[0054] Furthermore, in quantum dots, the absorption peak wavelength depends on the particle diameter; therefore, the particle diameter of a quantum dot can also be expressed by its absorption peak wavelength. The absorption peak wavelength of a quantum dot corresponds, for example, to the mode diameter of the quantum dot. Specifically, quantum dots with longer absorption peak wavelengths have larger particle diameters, and quantum dots with shorter absorption peak wavelengths have smaller particle diameters. Therefore, the first quantum dot has an absorption peak at a longer wavelength than the absorption peak wavelength of the second quantum dot. In addition, the first quantum dot may have an absorption peak at a wavelength 100 nm or more longer than the absorption peak wavelength of the second quantum dot.

[0055] The particle size of quantum dots can be controlled by adjusting the reaction time and temperature in existing quantum dot growth methods. Therefore, it is possible to obtain quantum dots with substantially uniform particle sizes, for example. A quantum dot with a uniform particle size is, for example, a quantum dot that has one absorption peak in the near-infrared region. The first and second quantum dots may each consist of one type of quantum dot with a uniform particle size, or they may each consist of multiple types of quantum dots with uniform particle sizes. If the first and second quantum dots are each composed of multiple types of quantum dots with uniform particle sizes, the smallest particle size among the multiple types of quantum dots constituting the first quantum dot is larger than the largest particle size among the multiple types of quantum dots constituting the second quantum dot.

[0056] The first quantum dot and the second quantum dot may each have an absorption peak in the near-infrared region. Furthermore, at least one of the first and second quantum dots may have multiple absorption peaks in the near-infrared region. If at least one of the first and second quantum dots has multiple absorption peaks in the near-infrared region, then in the near-infrared region, the longest wavelength absorption peak wavelength of the second quantum dot is shorter than the shortest wavelength absorption peak wavelength of the first quantum dot.

[0057] Furthermore, the first and second quantum dots have different particle sizes and are coated with different surface modification ligands. Specifically, the surface of the first quantum dot is modified with a first ligand, and the surface of the second quantum dot is modified with a second ligand that is different from the first ligand.

[0058] Figure 3 shows the measurement results of the ionization potential and electron affinity of quantum dots when the particle size and surface modification ligand are changed.

[0059] Here, the ionization potential was measured using a photoelectron yield spectrometer (AC-3, RIKEN KEKIN) under a nitrogen atmosphere. The number of photoelectrons was measured when the energy of ultraviolet irradiation was changed, and the energy position at which photoelectrons began to be detected was defined as the ionization potential. In addition, for the measurement of electron affinity, the absorption spectrum of the quantum dot was first measured, and the optical band gap was calculated from the absorption end wavelength of the obtained absorption spectrum. Then, the electron affinity was calculated from the ionization potential measured by the above method and the calculated optical band gap. In Figure 3, the values ​​written below the energy band are the ionization potential, and the values ​​written above the energy band are the electron affinity. In addition, 1,4-benzenedithiol (1,4-BDT) and ZnI2:MPA (zinc iodide:3-mercaptopropionic acid) were used as surface modification ligands. In Figure 3, 1,4-BDT is simply written as "BDT". Also, ZnI2:MPA means a mixture of ZnI2 and MPA. Furthermore, the quantum dots used included one composed of PbS with an absorption peak wavelength of 1200 nm, another composed of PbS with an absorption peak wavelength of 1350 nm, and a third composed of PbS with an absorption peak wavelength of 1400 nm. The wavelengths indicated within the energy bands shown in Figure 3 represent the absorption peak wavelengths of each quantum dot.

[0060] As shown in Figure 3, the ionization potential of quantum dots is approximately the same regardless of particle size, provided that the type of surface modification ligand is the same. Specifically, the ionization potential of each quantum dot modified with BDT as the surface modification ligand is 5.12 eV to 5.14 eV, while the ionization potential of each quantum dot modified with ZnI2:MPA as the surface modification ligand is 5.64 eV to 5.66 eV.

[0061] As shown in Figure 2B, in this embodiment, the ionization potential of the second quantum dot layer 4b, which contains a second quantum dot modified with the second ligand, is greater than the ionization potential of the first quantum dot layer 4a, which contains a first quantum dot whose surface is modified with the first ligand. In other words, the second ligand is a surface-modifying ligand that can increase the ionization potential of quantum dots more than the first ligand. The difference between the ionization potential of the first quantum dot layer 4a and the ionization potential of the second quantum dot layer 4b is, for example, between 0.1 eV and 0.7 eV. Furthermore, the electron affinity of the second quantum dot layer 4b is greater than the electron affinity of the first quantum dot layer 4a. The difference between the electron affinity of the first quantum dot layer 4a and the electron affinity of the second quantum dot layer 4b is, for example, between 0.1 eV and 0.6 eV.

[0062] Therefore, in this embodiment, the first quantum dot layer 4a, whose ionization potential and electron affinity are relatively small when compared to each other, is positioned closer to the first electrode 2 that collects holes than the second quantum dot layer 4b, and the second quantum dot layer 4b, whose ionization potential and electron affinity are relatively large, is positioned closer to the second electrode 3 that collects electrons than the first quantum dot layer 4a. In other words, when the electron-hole pair, which is an exciton generated by incident light, dissociates, the bias voltage described above allows the hole to be collected at the first electrode 2 and the electron to be collected at the second electrode 3 without an energy barrier. In this configuration, the interface between the first quantum dot layer 4a and the second quantum dot layer 4b can be considered a heterojunction interface. The energy difference at this heterojunction interface and the electric field of the depletion layer formed at the interface make it possible to dissociate the exciton generated by light absorption into electrons and holes, thereby improving the photoelectric conversion efficiency, i.e., the quantum efficiency.

[0063] Furthermore, the second quantum dot layer 4b, which is positioned on the side of the second electrode 3 that collects electrons, is composed of quantum dots with a smaller particle size than the first quantum dot layer 4a, which is positioned on the side of the first electrode 2 that collects holes. This reduces the dark current that may be generated thermally at the interface between the first quantum dot layer 4a and the second quantum dot layer 4b. The dark current that may be generated thermally is the energy difference ΔE at the interface between the first quantum dot layer 4a and the second quantum dot layer 4b, as shown in Figure 2B. QD It is thought to correlate with this. In this embodiment, the energy difference ΔE QD This is the difference between the ionization potential of the first quantum dot layer 4a and the electron affinity of the second quantum dot layer 4b. Energy difference ΔE QD Because the value is large, thermal excitation is less likely to occur at the interface between the first quantum dot layer 4a and the second quantum dot layer 4b, thus reducing the dark current.

[0064] As described above, the ionization potential of a quantum dot is determined by the surface modification ligand, regardless of the particle size. On the other hand, the energy gap of a quantum dot is determined by the particle size, and quantum dots with smaller particle sizes, i.e., larger energy gaps, tend to have smaller electron affinity. Therefore, as described above, if the ionization potential of the second quantum dot layer 4b is greater than that of the first quantum dot layer 4a due to the surface modification ligand, the smaller the particle size of the second quantum dots contained in the second quantum dot layer 4b, the larger the energy gap of the second quantum dot layer 4b becomes, and as a result, the energy difference ΔE QD The energy difference ΔE becomes larger. Therefore, the stacked configuration of (I) (i) a first quantum dot layer 4a with a first ligand and a first quantum dot and (ii) a second quantum dot layer 4b with a second ligand and a second quantum dot has a larger energy difference ΔE than the stacked configuration of (II) (i) a quantum dot layer with a first ligand and a second quantum dot and (ii) a quantum dot layer with a second ligand and a first quantum dot. QD This allows for a larger dark current. Therefore, it is possible to realize a photoelectric conversion element with a lower dark current.

[0065] As explained using Figure 3, the ionization potential of quantum dots changes depending on the type of surface modification ligand. Specifically, the ionization potential of a quantum dot film surface-modified with a surface modification ligand is explained by the sum of the dipole moment of the surface modification ligand and the dipole moment of the electric double layer at the interface between the surface modification ligand and the quantum dot, and changes according to the net effective dipole moment felt by the quantum dot. Here, the dipole moment is represented by a vector from the partial charge δ- to δ+, and the dipole moment of the surface modification ligand is the vector sum of the dipole moments of each bond in the surface modification ligand compound. For example, in the case of PbS quantum dots, since the surface modification ligand mainly coordinates to the Pb element on the surface, the dipole moment at the interface between the surface modification ligand and the quantum dot is directed inward from the quantum dot surface. In this case, if the dipole moment of the surface-modifying ligand is directed outward from the quantum dot, it tends to cancel out the dipole moment at the interface between the surface-modifying ligand and the quantum dot, thus reducing the ionization potential. Conversely, if the dipole moment of the surface-modifying ligand is directed inward from the quantum dot, the ionization potential tends to increase. In other words, the ionization potential can be modulated by the magnitude of the dipole moment of the surface-modifying ligand.

[0066] When comparing two types of surface-modifying ligands that have the same dipole moment direction, the ionization potential is modulated by the relative magnitudes of the ligands. For example, in the case of two different types of surface-modifying ligands whose dipole moments point outward from the quantum dot, the ligand with the larger dipole moment tends to have a smaller ionization potential. That is, if we take the case where the dipole moment of a surface-modifying ligand adsorbed on the quantum dot surface points outward from the quantum dot as positive, the larger the dipole moment of the surface-modifying ligand in the positive direction, the smaller the ionization potential tends to be, and conversely, the larger the dipole moment in the negative direction, the larger the ionization potential tends to be. Note that in the case of an elemental ion for which a dipole moment cannot be defined, the dipole moment at the interface between the surface-modifying ligand and the quantum dot becomes dominant. The dipole moments of the surface-modified ligands and the dipole moments of the interface between the surface-modified ligands and quantum dots are obtained by calculations based on density functional theory.

[0067] The surface modification ligand can be any ligand that can coordinate to the quantum dot, and may be an organic compound such as alkylammonium salts and thiols, or an inorganic compound such as a halide salt. Furthermore, the halide salt may be used as a mixture with a carboxylic acid-terminated thiol such as 3-mercaptopropionic acid.

[0068] Specifically, examples of surface-modified ligands that have a positive charge on the side opposite to the quantum dot and possess a dipole moment pointing outward from the quantum dot include compounds such as 1,2-ethanedithiol (1,2-EDT) and 1,4-benzenedithiol (1,4-BDT), which have a structure in which a positive charge bias occurs on the opposite side when coordinated with a thiol group. Alternatively, a surface-modified ligand with a dipole moment pointing outward from the quantum dot may be a compound such as 4-methoxycinnamic acid, which has an electron-donating group on the opposite side of the carboxyl group coordinating to the quantum dot surface. Examples of surface-modified ligands with a dipole moment pointing outward from the quantum dot include compounds with a skeleton that coordinates the electron-donating site, such as phenol or aniline, outward from the quantum dot.

[0069] Furthermore, surface-modified ligands that have a negative charge on the side opposite to the quantum dot and possess a dipole moment directed inward towards the quantum dot include lead halides and zinc halides, mixtures of these with 3-mercaptopropionic acid, and compounds containing halogen ions such as tetrabutylammonium halide. Examples of halogens include chlorine, bromine, or iodine. Additionally, surface-modified ligands with a dipole moment directed inward towards the quantum dot may also be compounds having a skeleton in which electron-withdrawing sites such as cyano groups, carbonyl groups, or nitro groups are coordinated outward towards the quantum dot.

[0070] The surfaces of readily available quantum dots are often modified with surface-modifying ligands containing long-chain alkyl groups to improve dispersibility during synthesis. Since these long-chain alkyl ligands inhibit charge transfer, they are substituted for the first and second ligands. Known substitution methods include solid-phase substitution, where the quantum dots are formed into a film (solid phase) and then exposed to a solution of the surface-modifying ligand to be substituted, resulting in substitution based on concentration and the difference in bond energies between ligands; and liquid-phase substitution, where the surface-modifying ligands are substituted in solution (liquid phase). These existing methods can be used.

[0071] In this embodiment, for example, the first ligand has a first dipole moment, and the second ligand has a second dipole moment. If we assume that the first dipole moment is positive when it points outward from the first quantum dot, and the second dipole moment is positive when it points outward from the second quantum dot, then the first dipole moment is greater than the second dipole moment. In other words, subtracting the second dipole moment from the first dipole moment results in a positive dipole moment. This makes it easy to make the ionization potential of the second quantum dot layer 4b greater than the ionization potential of the first quantum dot layer 4a. For example, the first dipole moment may be a positive dipole moment pointing outward from the first quantum dot, and the second dipole moment may be a negative dipole moment pointing inward from the second quantum dot. Specifically, the first ligand may be a compound having a thiol group, such as 1,4-benzenedithiol, and the second ligand may be a compound containing a halogen ion, such as a mixture of ZnI2 and 3-mercaptopropionic acid. Such a combination of surface-modified ligands makes it easier to improve photoelectric conversion efficiency while reducing dark current.

[0072] [Electron blocking layer and hole blocking layer] In this embodiment, of the hole-electron pairs generated in the photoelectric conversion layer 4, the holes are collected on the first electrode 2 and the electrons are collected on the second electrode 3. At this time, charges with opposite polarity to those collected on the first electrode 2 and the second electrode 3 may be injected into the photoelectric conversion layer 4 from the first electrode 2 and the second electrode 3. The charges injected from the electrodes in this way cause a dark current to flow regardless of the incidence of light on the photoelectric conversion layer 4.

[0073] Therefore, the photoelectric conversion element according to this embodiment may include an electron blocking layer 5 for suppressing dark current between the first electrode 2 and the first quantum dot layer 4a, as shown in Figure 2A, for the photoelectric conversion element 10B. As shown in Figure 2B, the electron blocking layer 5 is a layer that acts as a barrier to electron injection from the first electrode 2. In order to suppress dark current due to electron injection from the first electrode 2, for example, the electron affinity χ of the electron blocking layer 5 isEBL is the electron affinity χ of the first quantum dot layer 4a QD1 that is equal to or smaller than that. Further, for example, in order not to hinder hole conduction from the first quantum dot layer 4a to the first electrode 2, the ionization potential I of the electron blocking layer 5 EBL is equal to or smaller than the ionization potential I of the first quantum dot layer 4a QD1 with an upper limit of 0.5 eV larger than that. Here, the ionization potential is the difference between the vacuum level and the energy level of the highest occupied molecular orbital (HOMO) or the upper edge of the valence band.

[0074] For example, the material of the electron blocking layer 5 is a material that satisfies the above relationship between electron affinity and ionization potential, and is, for example, a p-type semiconductor. The material of the electron blocking layer 5 may be an organic material such as [N4,N4'-Di(naphthalen-1-yl)-N4,N4'-bis(4-vinylphenyl)biphenyl-4,4'-diamine] (VNPB) or Poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (poly-TPD), or a metal oxide such as NiO, CoO, Co3O4, Cr2O3, Cu2O or CuO.

[0075] Similarly, the photoelectric conversion element according to the present embodiment may include a hole blocking layer 6 between the second electrode 3 and the second quantum dot layer 4b, like the photoelectric conversion element 10B shown in FIG. 2A. As shown in FIG. 2B, the hole blocking layer 6 is a layer that serves as a barrier against hole injection from the second electrode 3. In this case, in order to suppress dark current caused by hole injection from the second electrode 3, for example, the ionization potential I of the hole blocking layer 6 HBL is equal to or larger than the ionization potential I of the second quantum dot layer 4b QD2 that is equal to or larger than that. Further, for example, in order not to hinder electron conduction from the second quantum dot layer 4b to the second electrode 3, the electron affinity χ of the hole blocking layer 6 HBL is equal to or larger than the electron affinity χ of the second quantum dot layer 4b QD2 that is equal to or larger than that.

[0076] For example, the material of the hole blocking layer 6 is a material that satisfies the above relationship between electron affinity and ionization potential, such as an n-type semiconductor. Examples of materials for the hole blocking layer 6 include batocproine (BCP), batphenanthroline (BPhen), fullerenes, zinc oxide, aluminum-doped zinc oxide, titanium oxide, and tin oxide.

[0077] The electron blocking layer 5 is hole conductive in order to transport holes. The hole blocking layer 6 is electron conductive in order to transport electrons. Therefore, when the first quantum dot layer 4a is in contact with the electron blocking layer 5, the first quantum dot layer 4a is electrically connected to the first electrode 2 via the electron blocking layer 5. Similarly, when the second quantum dot layer 4b is in contact with the hole blocking layer 6, the second quantum dot layer 4b is electrically connected to the second electrode 3 via the hole blocking layer 6.

[0078] The photoelectric conversion element 10B may also include only one of the electron blocking layer 5 and the hole blocking layer 6.

[0079] (Embodiment 2) Next, Embodiment 2 will be described. Embodiment 2 describes an imaging device using the photoelectric conversion element according to Embodiment 1. In the following description of Embodiment 2, the differences from Embodiment 1 will be the main focus, and the explanation of the common points will be omitted or simplified.

[0080] First, the overall configuration of the imaging device according to this embodiment will be described. Figure 4 is a diagram showing an example of the circuit configuration of the imaging device 100 according to this embodiment. The imaging device 100 shown in Figure 4 has a plurality of pixels 20 and peripheral circuits. The peripheral circuits include a voltage supply circuit 30 that supplies a predetermined voltage to each of the pixels 20.

[0081] The pixels 20 are arranged in one or two dimensions on a semiconductor substrate to form a photosensitive area, or so-called pixel area. In the configuration illustrated in Figure 4, the pixels 20 are arranged in the row direction and the column direction. In this specification, the row direction and the column direction refer to the directions in which the rows and columns extend, respectively. That is, in the plane of Figure 4, the vertical direction is the column direction and the horizontal direction is the row direction. Figure 4 shows four pixels 20 arranged in a 2x2 matrix. The number of pixels 20 shown in Figure 4 is merely an example for illustrative purposes, and the number of pixels 20 is not limited to four. When the pixels 20 are arranged in one dimension, the imaging device 100 is a line sensor.

[0082] Each of the multiple pixels 20 has a photoelectric conversion unit 10C and a signal detection circuit 40 that detects the signal generated by the photoelectric conversion unit 10C. The signal detection circuit 40 is an example of a signal readout circuit. The photoelectric conversion unit 10C includes a first electrode 2 and a second electrode 3 and a photoelectric conversion layer 4 disposed between them. The photoelectric conversion unit 10C is composed of, for example, a photoelectric conversion element 10A or a photoelectric conversion element 10B according to Embodiment 1. The first electrode 2 functions as a charge collection unit. The signal detection circuit 40 is connected to the first electrode 2. The second electrode 3 is connected to a voltage supply circuit 30 via an accumulation control line 22. When the imaging device 100 is operating, a predetermined bias voltage is applied to the second electrode 3 via the accumulation control line 22. In this embodiment, the first electrode 2 is a pixel electrode that collects signal charges, and the second electrode 3 is a counter electrode facing the pixel electrode.

[0083] The photoelectric conversion unit 10C is configured to collect holes (in other words, positive charges) as signal charges from the electron-hole pairs generated by photoelectric conversion at the first electrode 2. By controlling the potential of the second electrode 3 using the bias voltage generated by the voltage supply circuit 30, holes can be collected by the first electrode 2. The voltage supply circuit 30 supplies a voltage to the second electrode 3 via the storage control line 22 such that the potential of the second electrode 3 is positive relative to the potential of the first electrode 2. Specifically, a voltage of, for example, about 10V is applied to the storage control line 22 so that the potential of the second electrode 3 is higher than that of the first electrode 2.

[0084] In the configuration illustrated in Figure 4, the signal detection circuit 40 includes an amplifying transistor 42, an addressing transistor 44, and a resetting transistor 46. The amplifying transistor 42 is also called a charge detection transistor, and the addressing transistor 44 is also called a row selection transistor. Typically, the amplifying transistor 42 and the addressing transistor 44 are field-effect transistors (FETs) formed on a semiconductor substrate. Unless otherwise specified, the following description will use N-channel MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) as transistors. The amplifying transistor 42, the addressing transistor 44, and the resetting transistor 46 have a control terminal, an input terminal, and an output terminal. The control terminal is, for example, the gate. The input terminal is one of the drain and source, and is typically the drain. The output terminal is the other of the drain and source, and is typically the source.

[0085] In this specification, "semiconductor substrate" is not limited to a substrate that is entirely semiconductor, but may also refer to an insulating substrate or the like, in which a semiconductor layer is provided on the surface on which the photosensitive area is formed. An example of a semiconductor substrate is a p-type silicon substrate.

[0086] As shown in Figure 4, one of the input and output terminals of the amplification transistor 42 is connected to one of the input and output terminals of the address transistor 44. The control terminal of the amplification transistor 42 is electrically connected to the first electrode 2 of the photoelectric conversion unit 10C. The signal charge collected by the first electrode 2 is stored in the charge storage node 41 between the first electrode 2 and the gate of the amplification transistor 42. In this embodiment, the signal charge is a hole. The charge storage node 41 is an example of a charge storage unit and is also called a "floating diffusion node".

[0087] A voltage corresponding to the signal charge stored in the charge storage node 41 is applied to the gate of the amplification transistor 42. The amplification transistor 42 amplifies this voltage. In other words, the amplification transistor 42 amplifies the signal generated by the photoelectric conversion unit 10C. The voltage amplified by the amplification transistor 42 is selectively read out as a signal voltage via the address transistor 44.

[0088] One of the sources and drains of the reset transistor 46 is connected to the charge storage node 41, and one of the sources and drains of the reset transistor 46 has an electrical connection with the first electrode 2.

[0089] The reset transistor 46 resets the signal charge stored in the charge storage node 41. In other words, the reset transistor 46 resets the potential of the gate and the first electrode 2 of the amplification transistor 42.

[0090] As shown in Figure 4, the imaging device 100 includes a power line 23, a vertical signal line 24, an address signal line 25, and a reset signal line 26. These lines are connected to each pixel 20. The power line 23 is connected to either the source or the drain of the amplification transistor 42 and supplies a predetermined power supply voltage to each pixel 20. The power line 23 functions as a source follower power supply. The vertical signal line 24 is connected to the side of the address transistor 44's source and drain that is not connected to the source or drain of the amplification transistor 42. The address signal line 25 is connected to the gate electrode of the address transistor 44. The reset signal line 26 is connected to the gate of the reset transistor 46.

[0091] The peripheral circuits of the imaging device 100 include a vertical scanning circuit 52, a horizontal signal readout circuit 54, a plurality of column signal processing circuits 56, a plurality of load circuits 58, and a plurality of inverting amplifiers 59. The vertical scanning circuit 52 is also called a "row scanning circuit," the horizontal signal readout circuit 54 is also called a "column scanning circuit," and the column signal processing circuit 56 is also called a "row signal storage circuit." The column signal processing circuits 56, load circuits 58, and inverting amplifiers 59 are provided corresponding to each column of a plurality of pixels 20 arranged in the row and column directions. Each of the column signal processing circuits 56 is electrically connected to the pixels 20 located in each column via vertical signal lines 24 corresponding to each column of the plurality of pixels 20. The plurality of column signal processing circuits 56 are electrically connected to the horizontal signal readout circuit 54. Each of the load circuits 58 is electrically connected to each vertical signal line 24, and a source follower circuit is formed by the load circuits 58 and the amplifying transistors 42.

[0092] The vertical scanning circuit 52 is connected to the address signal line 25 and the reset signal line 26. The vertical scanning circuit 52 applies a row selection signal to the gate of the address transistor 44 via the address signal line 25 to control the on and off states of the address transistor 44. By sending a row selection signal for each address signal line 25, the row to be read is scanned and selected. A signal voltage is read from the pixel 20 of the selected row to the vertical signal line 24. The vertical scanning circuit 52 also applies a reset signal to the gate of the reset transistor 46 via the reset signal line 26 to control the on and off states of the reset transistor 46. By sending a row selection signal for each reset signal line 26, the row of pixel 20 to be reset is selected. In this way, the vertical scanning circuit 52 selects multiple pixels 20 row by row and reads the signal voltage and resets the potential of the first electrode 2.

[0093] The signal voltage read from the pixel 20 selected by the vertical scanning circuit 52 is sent to the column signal processing circuit 56 via the vertical signal line 24. The column signal processing circuit 56 performs noise suppression signal processing, such as correlated double sampling, and analog-to-digital conversion (AD conversion). The horizontal signal readout circuit 54 sequentially reads signals from multiple column signal processing circuits 56 to a horizontal common signal line (not shown).

[0094] The vertical scanning circuit 52 may include the voltage supply circuit 30 described above as a part. Alternatively, the voltage supply circuit 30 may have an electrical connection with the vertical scanning circuit 52. In other words, a bias voltage may be applied to the second electrode 3 via the vertical scanning circuit 52.

[0095] In the configuration illustrated in Figure 4, multiple inverting amplifiers 59 are provided corresponding to each row. The negative input terminal of each inverting amplifier 59 is connected to the corresponding vertical signal line 24. The output terminal of each inverting amplifier 59 is connected to each pixel 20 of the corresponding row via a feedback line 27 provided corresponding to each row.

[0096] As shown in Figure 4, the feedback line 27 is connected to the side of the reset transistor 46's source and drain that is not connected to the charge storage node 41 (e.g., the drain). Therefore, the inverting amplifier 59 receives the output of the address transistor 44 as its negative terminal when the address transistor 44 and the reset transistor 46 are conducting. On the other hand, a reference voltage for reset is applied to the positive input terminal of the inverting amplifier 59 from a power supply (not shown). The inverting amplifier 59 performs a feedback operation so that the gate voltage of the amplifying transistor 42 becomes a predetermined feedback voltage. The feedback voltage refers to the output voltage of the inverting amplifier 59. The output voltage of the inverting amplifier 59 is, for example, a positive voltage of 0V or near 0V. The inverting amplifier 59 may also be called a "feedback amplifier".

[0097] Figure 5 is a schematic cross-sectional view showing the device structure of a pixel 20 in an imaging device 100 according to this embodiment. In the configuration illustrated in Figure 5, the pixel 20 includes a semiconductor substrate 62 that supports the photoelectric conversion unit 10C. The semiconductor substrate 62 is, for example, a silicon substrate. As shown in Figure 5, the photoelectric conversion unit 10C is positioned above the semiconductor substrate 62. In the imaging device 100, light is incident on the photoelectric conversion unit 10C from above. In this example, interlayer insulating layers 63A, 63B, and 63C are laminated on the semiconductor substrate 62, and the first electrode 2, photoelectric conversion layer 4, and second electrode 3 are arranged on the interlayer insulating layer 63C in this order. The first electrode 2 is partitioned for each pixel, and the first electrode 2 is formed spatially separated between two adjacent pixels 20, thereby electrically separating two adjacent first electrodes 2. The photoelectric conversion layer 4 and the second electrode 3 may also be formed to span multiple pixels 20.

[0098] An amplifying transistor 42, an addressing transistor 44, and a resetting transistor 46 are formed on the semiconductor substrate 62.

[0099] The amplifying transistor 42 includes impurity regions 62a and 62b formed on the semiconductor substrate 62, a gate insulating layer 42g located on the semiconductor substrate 62, and a gate electrode 42e located on the gate insulating layer 42g. The impurity regions 62a and 62b function as the drain or source of the amplifying transistor 42. The impurity regions 62a and 62b, as well as the impurity regions 62c, 62d, and 62e described later, are, for example, n-type impurity regions.

[0100] The address transistor 44 includes impurity regions 62a and 62c formed on the semiconductor substrate 62, a gate insulating layer 44g located on the semiconductor substrate 62, and a gate electrode 44e located on the gate insulating layer 44g. The impurity regions 62a and 62c function as the drain or source of the address transistor 44. In this example, the amplifying transistor 42 and the address transistor 44 share the impurity region 62a, thereby electrically connecting the source (or drain) of the amplifying transistor 42 and the drain (or source) of the address transistor 44.

[0101] The reset transistor 46 includes impurity regions 62d and 62e formed within the semiconductor substrate 62, a gate insulating layer 46g located on the semiconductor substrate 62, and a gate electrode 46e located on the gate insulating layer 46g. The impurity regions 62d and 62e function as the drain or source of the reset transistor 46.

[0102] In the semiconductor substrate 62, element isolation regions 62s are provided between adjacent pixels 20 and between the amplification transistor 42 and the reset transistor 46. The element isolation regions 62s electrically isolate adjacent pixels 20. Furthermore, the provision of element isolation regions 62s between adjacent pixels 20 suppresses leakage of signal charge accumulated in the charge storage node 41.

[0103] Within the interlayer insulating layer 63A, a contact plug 65A connected to the impurity region 62d of the reset transistor 46, a contact plug 65B connected to the gate electrode 42e of the amplifying transistor 42, and wiring 66A connecting contact plugs 65A and 65B are formed. This electrically connects the impurity region 62d (e.g., drain) of the reset transistor 46 to the gate electrode 42e of the amplifying transistor 42. In the configuration illustrated in Figure 5, a plug 67A and wiring 68A are further formed within the interlayer insulating layer 63A. Furthermore, plugs 67B and wiring 68B are formed within the interlayer insulating layer 63B, and plug 67C is formed within the interlayer insulating layer 63C, thereby electrically connecting wiring 66A to the first electrode 2. The contact plugs 65A, 65B, 66A, 67A, 68A, 67B, 68B, and 67C are typically made of metal.

[0104] In the configuration illustrated in Figure 5, a protective layer 72 is placed on the second electrode 3. This protective layer 72 is not a substrate placed to support the photoelectric conversion unit 10C. The protective layer 72 is a layer for protecting the photoelectric conversion unit 10C and insulating it from other components. The protective layer 72 may be highly transparent at the wavelength absorbed by the photoelectric conversion layer 4. The material of the protective layer 72 may be any transparent insulator, such as SiON or AlO. As shown in Figure 5, a microlens 74 may be placed on the protective layer 72.

[0105] In this embodiment, the photoelectric conversion unit 10C is an example of a photoelectric conversion element and is composed of a photoelectric conversion element according to Embodiment 1. The photoelectric conversion unit 10C has a structure similar to that of the photoelectric conversion element 10A described above, for example, as shown in Figure 5. The second electrode 3 is positioned above the photoelectric conversion layer 4, in other words, on the light incidence side of the imaging device 100 relative to the photoelectric conversion layer 4. Light is incident on the photoelectric conversion layer 4 via the second electrode 3. In this embodiment, the second electrode 3 is, for example, a transparent electrode.

[0106] The photoelectric conversion unit 10C may have the same structure as the photoelectric conversion element 10B described above, or it may have a structure that does not include either the electron blocking layer 5 or the hole blocking layer 6 of the photoelectric conversion element 10B described above. In this case as well, the signal detection circuit 40 is connected to the first electrode 2, and the voltage supply circuit 30 supplies voltage to the second electrode 3 via the storage control line 22.

[0107] The imaging device 100 described above can be manufactured using a general semiconductor manufacturing process. In particular, when a silicon substrate is used as the semiconductor substrate 62, it can be manufactured by utilizing various silicon semiconductor processes.

[0108] [Differentiation] Next, a modified example of Embodiment 2 will be described. In the following description of the modified example of Embodiment 2, the differences from Embodiment 1 and Embodiment 2 will be the main focus, and the explanation of the common points will be omitted or simplified.

[0109] Figure 6 shows an example of the circuit configuration of the imaging device 100A according to this modified example. Figure 7 is a schematic cross-sectional view showing the device structure of pixel 20A in the imaging device 100A according to this modified example.

[0110] As shown in Figures 6 and 7, the imaging device 100A according to this modified example differs from the imaging device 100 according to Embodiment 2 in that it has multiple pixels 20A having a photoelectric conversion unit 10D instead of multiple pixels 20 having a photoelectric conversion unit 10C.

[0111] In this modified example, each of the multiple pixels 20A has a photoelectric conversion unit 10D and a signal detection circuit 40 that detects the signal generated by the photoelectric conversion unit 10D. The photoelectric conversion unit 10D is composed of a photoelectric conversion element 10A or a photoelectric conversion element 10B according to Embodiment 1, similar to the photoelectric conversion unit 10C, but as shown in Figure 7, the stacking order of the first electrode 2, the second electrode 3 and the photoelectric conversion layer 4 is reversed compared to the photoelectric conversion unit 10C. The second electrode 3 functions as a charge collection unit. The signal detection circuit 40 is connected to the second electrode 3. The first electrode 2 is connected to the voltage supply circuit 30 via the accumulation control line 22. When the imaging device 100A is operating, a predetermined bias voltage is applied to the first electrode 2 via the accumulation control line 22. In this modified example, the second electrode 3 is a pixel electrode that collects signal charges, and the first electrode 2 is a counter electrode facing the pixel electrode.

[0112] The photoelectric conversion unit 10D is configured to collect electrons (in other words, negative charges) as signal charges from the electron-hole pairs generated by photoelectric conversion at the second electrode 3. By controlling the potential of the first electrode 2 using the bias voltage generated by the voltage supply circuit 30, electrons can be collected by the second electrode 3. The voltage supply circuit 30 supplies a voltage to the first electrode 2 via the storage control line 22 that makes the potential of the first electrode 2 negative relative to the potential of the second electrode 3.

[0113] As shown in Figure 6, the control terminal of the amplification transistor 42 is electrically connected to the second electrode 3 of the photoelectric conversion unit 10D. The signal charge collected by the second electrode 3 is stored in the charge storage node 41 between the second electrode 3 and the gate of the amplification transistor 42. In this modified example, the signal charge is electrons.

[0114] As shown in Figure 7, in the pixel 20A, interlayer insulating layers 63A, 63B, and 63C are stacked on the semiconductor substrate 62, and the second electrode 3, photoelectric conversion layer 4, and first electrode 2 are arranged in this order on the interlayer insulating layer 63C. The second electrode 3 is partitioned for each pixel, and the second electrode 3 is formed spatially separated between two adjacent pixels 20A, thereby electrically separating two adjacent second electrodes 3. In addition, the photoelectric conversion layer 4 and the first electrode 2 may be formed to span multiple pixels 20A.

[0115] In this embodiment, the photoelectric conversion unit 10D is an example of a photoelectric conversion element and is composed of a photoelectric conversion element according to Embodiment 1. The photoelectric conversion unit 10D has a structure similar to that of the photoelectric conversion element 10A described above, for example, as shown in Figure 7. The first electrode 2 is positioned above the photoelectric conversion layer 4, in other words, on the light incidence side of the imaging device 100A relative to the photoelectric conversion layer 4. Light is incident on the photoelectric conversion layer 4 via the first electrode 2. In this embodiment, the first electrode 2 is, for example, a transparent electrode.

[0116] The photoelectric conversion unit 10D may have the same structure as the photoelectric conversion element 10B described above, or it may have a structure that does not include either the electron blocking layer 5 or the hole blocking layer 6 of the photoelectric conversion element 10B described above. In this case as well, the signal detection circuit 40 is connected to the second electrode 3, and the voltage supply circuit 30 supplies voltage to the first electrode 2 via the storage control line 22. [Examples]

[0117] Next, the present disclosure will be described in detail based on the following examples. However, the present disclosure is not limited in any way by the following examples.

[0118] (Examples 1 and Comparative Examples 1 to 4) First, Example 1 and Comparative Examples 1 to 4 will be described.

[0119] [Fabrication of photoelectric conversion elements] The photoelectric conversion element was fabricated using the following method.

[0120] <Example 1> Glass coated with ITO was prepared as the substrate for use as the electrode. The substrate was ultrasonically cleaned with acetone and propanol, and then dry-cleaned with UV-ozone treatment before use. Subsequently, the photoelectric conversion element was deposited in a glove box under a nitrogen atmosphere.

[0121] First, an electron blocking layer with a thickness of 60 nm was formed on an ITO electrode by spin-coating a 6 mg / mL chlorobenzene solution of poly-TPD.

[0122] Subsequently, a 20 mg / mL octane solution of PbS quantum dots with an absorption peak wavelength of 1400 nm was spin-coated onto the electron blocking layer. After drying the solvent, the layer was immersed in a 1.5 mg / mL acetonitrile solution of 1,4-BDT for 30 seconds to exchange the surface modification ligands, and then washed twice with acetonitrile. This ligand exchange process was repeated five times to modify the surface with 1,4-BDT and form a first quantum dot layer with a thickness of 60 nm, composed of PbS quantum dots with an absorption peak wavelength of 1400 nm.

[0123] Next, a 20 mg / mL octane solution of PbS quantum dots with an absorption peak wavelength of 1200 nm was spin-coated onto the first quantum dot layer. After drying the solvent, the layer was immersed for 5 seconds in a 4.8 mg / mL ethanol solution of zinc iodide (ZnI2) doped with 0.06 vol% mercaptopropionic acid (MPA) to exchange the surface modification ligands, and then washed twice with ethanol. This ligand exchange process was repeated five times to form a second quantum dot layer with a thickness of 60 nm, whose surface was modified with ZnI2:MPA and composed of PbS quantum dots with an absorption peak wavelength of 1200 nm. This formed a photoelectric conversion layer having a structure in which the first quantum dot layer and the second quantum dot layer are stacked.

[0124] Subsequently, a hole blocking layer with a thickness of 60 nm was formed on the second quantum dot layer by spin-coating it with a zinc oxide nanoparticle dispersion (product name: Avantama N-11).

[0125] Finally, an aluminum electrode was formed on the hole blocking layer by vacuum heating deposition to a thickness of 80 nm to obtain a photoelectric conversion element. Table 1 shows the absorption peak wavelength of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the thickness of the photoelectric conversion layer.

[0126] <Comparative Example 1> A photoelectric conversion element was fabricated in the same manner as in Example 1, except that the PbS quantum dots used in the first quantum dot layer and the PbS quantum dots used in the second quantum dot layer were reversed. Table 1 shows the absorption peak wavelengths of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the film thickness of the photoelectric conversion layer.

[0127] <Comparative Example 2> A photoelectric conversion element was fabricated in the same manner as in Example 1, except that the ZnI2:MPA used in the second quantum dot layer was changed to 1,4-BDT. Table 1 shows the absorption peak wavelength of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligand, and the film thickness of the photoelectric conversion layer.

[0128] <Comparative Example 3> A photoelectric conversion element was fabricated in the same manner as in Comparative Example 2, except that the PbS quantum dots used in the first quantum dot layer and the PbS quantum dots used in the second quantum dot layer were reversed. Table 1 shows the absorption peak wavelengths of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the film thickness of the photoelectric conversion layer.

[0129] <Comparative Example 4> Except for the formation of the photoelectric conversion layer, the same method as in Example 1 was followed. For the formation of the photoelectric conversion layer, a 20 mg / mL octane solution, which was a mixture of PbS quantum dots with different particle sizes and absorption peak wavelengths of 1400 nm and 1200 nm, respectively, in a mass ratio of 1:1, was spin-coated onto the electron blocking layer. The surface modification ligand of the PbS quantum dots was replaced with 1,4-BDT in the same manner as in Example 1. As a result, a photoelectric conversion layer with a thickness of 120 nm was formed, consisting of a mixture of PbS quantum dots with an absorption peak wavelength of 1200 nm and PbS quantum dots with an absorption peak wavelength of 1400 nm, respectively, whose surfaces were modified with 1,4-BDT. In other words, in Comparative Example 4, the photoelectric conversion layer does not have a stacked structure of a first quantum dot layer and a second quantum dot layer. Table 1 shows the absorption peak wavelengths and surface modification ligands of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, as well as the thickness of the photoelectric conversion layer.

[0130] [Evaluation of photoelectric conversion elements] To evaluate the photoelectric conversion element fabricated as described above, the element was introduced into a sealed measurement jig in a glove box under a nitrogen atmosphere, and the dark current and external quantum efficiency were measured using a long-wavelength compatible spectroscopic sensitivity measuring device (CEP-25RR, manufactured by Spectrometer Instruments). Table 1 shows the dark current values ​​when a voltage of -3V was applied to the ITO electrode.

[0131] Furthermore, in the measurement of external quantum efficiency, the measurement was performed under the condition that a voltage of -3V was applied to the ITO electrode. In other words, the measurement of external quantum efficiency was performed under conditions in which holes were collected on the ITO electrode and electrons were collected on the aluminum electrode. The measurement results of external quantum efficiency at wavelengths of 1200 nm and 1400 nm are shown in Table 1.

[0132] [Table 1]

[0133] In Table 1, the upper row of the columns labeled "First Quantum Dot Layer" and "Second Quantum Dot Layer" lists the absorption peak wavelengths of the quantum dots used in each layer, while the lower row lists the surface modification ligands used in each layer. The same applies to Tables 2 through 5, which will be explained below.

[0134] As explained using Figure 3, ZnI2:MPA is a surface modification ligand that increases the ionization potential of quantum dots more than 1,4-BDT. As shown in Table 1, the photoelectric conversion element in Example 1 has a stacked structure of a first quantum dot layer and a second quantum dot layer with different surface modification ligands, i.e., different ionization potentials. In the second quantum dot layer, the surface of quantum dots with a short absorption peak wavelength, i.e., small particle size, is modified with ZnI2:MPA, which increases the ionization potential. The photoelectric conversion element in Example 1, configured in this way, has high external quantum efficiency and low dark current. On the other hand, the photoelectric conversion element in Comparative Example 1, in which the combination of particle sizes of quantum dots used in the first quantum dot layer and quantum dots used in the second quantum dot layer is reversed compared to Example 1, has a larger dark current than the photoelectric conversion element in Example 1. This is because, in Comparative Example 1, even though the surface modification ligand is the same as in Example 1, larger particle size quantum dots are used in the second quantum dot layer than in Example 1, resulting in a lower electron affinity of the second quantum dot layer than in Example 1. As a result, in Comparative Example 1, the energy difference ΔE at the interface between the first quantum dot layer and the second quantum dot layer was QD It is thought that the size of the current decreased, and the amount of thermally excited dark current increased.

[0135] Furthermore, the photoelectric conversion elements in Comparative Examples 2 and 3 also exhibit larger dark currents compared to the photoelectric conversion element in Example 1. Although a zinc oxide layer with a thickness of 60 nm is provided as a hole blocking layer, the thickness of the hole blocking layer may be locally thinner, or the hole blocking layer may be damaged when the aluminum electrode for electron collection is deposited, potentially creating conduction paths that facilitate hole injection from the aluminum electrode into the second quantum dot layer. In such cases, a low ionization potential of the second quantum dot layer contributes to the increase in dark current. In Comparative Examples 2 and 3, 1,4-BDT is used as the surface modification ligand for the second quantum dot layer, resulting in a lower ionization potential of the second quantum dot layer compared to Example 1 and Comparative Example 1. As a result, it is thought that there is more hole injection from the aluminum electrode.

[0136] Furthermore, when the photoelectric conversion layer was constructed by mixing quantum dots of two particle sizes, as in Comparative Example 4, the external quantum efficiency was lower compared to the other examples. In the photoelectric conversion element in Comparative Example 4, although the dark current was low, the photoelectric conversion efficiency was also low, and the sensitivity wavelength could not be broadened. In other words, it is thought that the photoelectric conversion layer behaved like a large resistor, making it difficult for current to flow.

[0137] From these results, it was found that when stacking a first quantum dot layer and a second quantum dot layer having quantum dots of different particle sizes to broaden the sensitivity wavelength, by changing the surface modification ligand in each layer to create a difference in ionization potential, and by modifying the second quantum dot layer on the aluminum electrode side that collects electrons with ZnI2:MPA, which relatively increases the ionization potential of the relatively smaller quantum dots, it is possible to achieve both a high quantum efficiency, an expansion of the sensitivity wavelength range, and a reduction in dark current.

[0138] (Example 2 and Comparative Example 5) Next, Example 2 and Comparative Example 5 will be described.

[0139] [Fabrication of photoelectric conversion elements] The photoelectric conversion element was fabricated using the following method.

[0140] <Example 2> A photoelectric conversion element was fabricated in the same manner as in Example 1, except that the film thickness of the first quantum dot layer and the second quantum dot layer were set to 180 nm. Table 2 shows the absorption peak wavelength of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the film thickness of the photoelectric conversion layer.

[0141] <Comparative Example 5> A photoelectric conversion element was fabricated using the same method as in Comparative Example 4, except that the thickness of the photoelectric conversion layer was set to 360 nm. Table 2 shows the absorption peak wavelength of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligand, and the thickness of the photoelectric conversion layer.

[0142] [Evaluation of photoelectric conversion elements] To evaluate the photoelectric conversion element fabricated as described above, the dark current and external quantum efficiency were measured using the method described. The dark current values ​​and the measurement results of the external quantum efficiency at wavelengths of 1200 nm and 1400 nm are shown in Table 2.

[0143] [Table 2]

[0144] As shown in Table 2, the photoelectric conversion element in Example 2 showed further improvement in external quantum efficiency and reduced dark current compared to the photoelectric conversion element in Example 1 due to the increased thickness of the photoelectric conversion layer. In contrast, the photoelectric conversion element in Comparative Example 5, in which the photoelectric conversion layer was constructed by mixing quantum dots of two particle sizes, had lower quantum efficiency and a larger dark current compared to the photoelectric conversion element in Example 2. Furthermore, regarding the reduction of dark current in particular, the effect of the configuration according to this disclosure becomes more pronounced as the thickness of the photoelectric conversion layer increases, and the photoelectric conversion element in Example 2 achieved a dark current two orders of magnitude lower than the photoelectric conversion element in Comparative Example 5.

[0145] (Example 3 and Comparative Examples 6 and 7) Next, Example 3 and Comparative Examples 6 and 7 will be described.

[0146] [Fabrication of photoelectric conversion elements] The photoelectric conversion element was fabricated using the following method.

[0147] <Example 3> A photoelectric conversion element was fabricated in the same manner as in Example 2, except that the first quantum dot layer was formed using PbS quantum dots with an absorption peak wavelength of 1450 nm, and the second quantum dot layer was formed using PbS quantum dots with an absorption peak wavelength of 1300 nm. Table 3 shows the absorption peak wavelengths of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the film thickness of the photoelectric conversion layer.

[0148] <Comparative Example 6> A photoelectric conversion element was fabricated in the same manner as in Example 3, except that the PbS quantum dots used in the first quantum dot layer and the PbS quantum dots used in the second quantum dot layer were reversed. Table 3 shows the absorption peak wavelengths of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the film thickness of the photoelectric conversion layer.

[0149] <Comparative Example 7> A photoelectric conversion element was fabricated in the same manner as in Comparative Example 5, except that the photoelectric conversion layer was formed using a 20 mg / mL octane solution prepared by mixing PbS quantum dots with different particle sizes, each having absorption peak wavelengths of 1450 nm and 1300 nm, in a mass ratio of 1:1. Table 3 shows the absorption peak wavelengths of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the film thickness of the photoelectric conversion layer.

[0150] [Evaluation of photoelectric conversion elements] To evaluate the photoelectric conversion element fabricated as described above, the dark current and external quantum efficiency were measured using the method described. The dark current values ​​and the measurement results of the external quantum efficiency at wavelengths of 1300 nm and 1480 nm are shown in Table 3.

[0151] [Table 3]

[0152] As shown in Table 3, the photoelectric conversion element in Example 3, which uses quantum dots of a different particle size than those used in Examples 1 and 2, also exhibited high external quantum efficiency and low dark current. On the other hand, the photoelectric conversion element in Comparative Example 6, in which the combination of particle sizes of the quantum dots used in the first quantum dot layer and the quantum dots used in the second quantum dot layer is the opposite of that in Example 3, exhibits a larger dark current compared to the photoelectric conversion element in Example 3. Furthermore, the photoelectric conversion element in Comparative Example 7, in which the photoelectric conversion layer is constructed by mixing quantum dots of two particle sizes, exhibits lower quantum efficiency and a larger dark current compared to the photoelectric conversion element in Example 3.

[0153] (Example 4 and Comparative Example 8) Next, Example 4 and Comparative Example 8 will be described.

[0154] [Fabrication of photoelectric conversion elements] The photoelectric conversion element was fabricated using the following method.

[0155] <Example 4> A photoelectric conversion element was fabricated in the same manner as in Example 2, except that a first quantum dot layer was formed using a 20 mg / mL octane solution mixed in a mass ratio of 1:1 with PbS quantum dots of different particle sizes having absorption peak wavelengths of 1300 nm and 1450 nm, and a second quantum dot layer was formed using a 20 mg / mL octane solution mixed in a mass ratio of 1:1 with PbS quantum dots of different particle sizes having absorption peak wavelengths of 1000 nm and 1200 nm. Table 4 shows the absorption peak wavelengths of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the film thickness of the photoelectric conversion layer.

[0156] <Comparative Example 8> A photoelectric conversion element was fabricated in the same manner as in Comparative Example 5, except that a 20 mg / mL octane solution was used, which was a mixture of PbS quantum dots with different particle sizes, each having absorption peak wavelengths of 1000 nm, 1200 nm, 1300 nm, and 1450 nm, in a mass ratio of 1:1:1:1. Table 4 shows the absorption peak wavelengths of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the film thickness of the photoelectric conversion layer.

[0157] [Evaluation of photoelectric conversion elements] To evaluate the photoelectric conversion element fabricated as described above, the dark current and external quantum efficiency were measured using the method described. The dark current values ​​and the measurement results of the external quantum efficiency at wavelengths of 1000 nm, 1200 nm, 1360 nm, and 1440 nm are shown in Table 4.

[0158] [Table 4]

[0159] As shown in Table 4, the photoelectric conversion element in Example 4 has higher quantum efficiency and lower dark current compared to the photoelectric conversion element in Comparative Example 8. In other words, the effect of the configuration according to this disclosure is that even when multiple quantum dots with different particle sizes are used in each of the first and second quantum dot layers, the surface modification ligand is changed in each layer to create a difference in ionization potential, and in the second quantum dot layer on the aluminum electrode side that collects electrons, the surface of the quantum dots with relatively larger particle sizes is modified with ZnI2:MPA, which relatively increases the ionization potential. This makes it possible to achieve both high quantum efficiency, an expanded sensitivity wavelength range, and reduced dark current. In Example 4, since multiple quantum dots with different particle sizes are used in each of the first and second quantum dot layers, the particle size distribution of the quantum dots in each layer of the first and second quantum dot layers has two or more different maximum values.

[0160] (Example 5 and Comparative Example 9) Next, Example 5 and Comparative Example 9 will be described.

[0161] [Fabrication of photoelectric conversion elements] The photoelectric conversion element was fabricated using the following method.

[0162] <Example 5> The photoelectric conversion element was fabricated in the same manner as in Example 4, except that instead of depositing zinc oxide, a hole blocking layer was formed by vacuum deposition of fullerene (C60) to a thickness of 50 nm, and instead of forming an aluminum electrode on the hole blocking layer, an ITO electrode was formed by sputter deposition of ITO to a thickness of 30 nm. Table 5 shows the absorption peak wavelength of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligands, and the film thickness of the photoelectric conversion layer.

[0163] <Comparative Example 9> The photoelectric conversion element was fabricated in the same manner as in Comparative Example 8, except that instead of depositing zinc oxide, a hole blocking layer was formed by vacuum deposition of fullerene (C60) to a thickness of 50 nm, and instead of forming an aluminum electrode on the hole blocking layer, an ITO electrode was formed by sputter deposition of ITO to a thickness of 30 nm. Table 5 shows the absorption peak wavelength of the quantum dots used in the photoelectric conversion layer of the fabricated photoelectric conversion element, the surface modification ligand, and the thickness of the photoelectric conversion layer.

[0164] [Evaluation of photoelectric conversion elements] To evaluate the photoelectric conversion element fabricated as described above, the dark current and external quantum efficiency were measured using the method described above. However, the dark current and external quantum efficiency were measured under the condition that a voltage of -5V was applied to the ITO electrode on the substrate side. Furthermore, when measuring the external quantum efficiency, light was incident from the side of the sputter-deposited ITO electrode opposite to the ITO substrate side. The values ​​of the dark current and the measurement results of the external quantum efficiency at wavelengths of 1000 nm, 1180 nm, 1340 nm, and 1480 nm are shown in Table 5.

[0165] [Table 5]

[0166] As shown in Table 5, the photoelectric conversion element in Example 5 has higher quantum efficiency and lower dark current compared to the photoelectric conversion element in Comparative Example 9. In other words, the effects of the configuration according to this disclosure were obtained even when the material of the hole blocking layer was changed from that of Example 4. Furthermore, the effects of the configuration according to this disclosure were obtained even when light was incident from the opposite direction to that of Example 4. Therefore, regardless of the direction of incident light, by changing the surface modification ligand in each layer to create a difference in ionization potential, and by modifying the surface of the quantum dots with relatively large particle sizes in the second quantum dot layer on the electrode side that collects electrons with ZnI2:MPA, which relatively increases the ionization potential, it is possible to achieve both high quantum efficiency, an expanded sensitivity wavelength range, and reduced dark current.

[0167] In the above-described embodiment, the particle size of the second quantum dots contained in the second quantum dot layer 4b was smaller than that of the first quantum dots contained in the first quantum dot layer 4a, resulting in a larger energy gap in the second quantum dot layer 4b than in the first quantum dot layer 4a. However, the energy gap (i.e., absorption peak wavelength) of a quantum dot layer can also be controlled by the constituent elements of the quantum dots it contains.

[0168] Figure 8 shows the relationship between the longest absorption peak wavelength and diameter for PbS quantum dots and PbSe quantum dots manufactured by NNCrystal. As shown in Figure 8, when the elements constituting the quantum dots are the same, there is a positive correlation between the absorption peak wavelength and particle size, with smaller particle sizes resulting in shorter absorption peak wavelengths. On the other hand, for quantum dots composed of different elements, the absorption peak wavelengths differ even at the same particle size. Therefore, by using quantum dots composed of different elements, a stacked structure of quantum dot layers with different absorption peak wavelengths can be realized even at the same particle size. Furthermore, a similar relationship between absorption peak wavelengths can be achieved even with a stacking order different from the particle size stacking order shown in Examples 1 to 5.

[0169] Figures 9A to 9C illustrate an example of a photoelectric conversion device using quantum dots composed of different elements. Figure 9A is a graph showing the absorption spectra of a 5.7 nm diameter PbS quantum dot and a 5.3 nm diameter PbSe quantum dot, respectively. As can be seen from this graph, the larger particle size of the PbS quantum dot results in a smaller absorption peak wavelength (i.e., a larger energy gap).

[0170] Figure 9B is a schematic cross-sectional view showing the configuration of a photoelectric conversion element 10E using the combination of quantum dots shown in Figure 9A. In the photoelectric conversion element 10E, a first quantum dot layer 4c containing PbSe quantum dots is located near the first electrode 2 which collects holes, and a second quantum dot layer 4d containing PbS quantum dots, which have a larger particle size and energy gap than PbSe quantum dots, is located near the second electrode 3 which collects electrons.

[0171] Figure 9C shows the energy diagram of the photoelectric conversion element 10E. The second quantum dot layer 4d has a smaller absorption peak wavelength and a larger energy gap than the first quantum dot layer 4c. Furthermore, in each quantum dot layer, the ligands used to modify the surface of the quantum dots are selected such that the ionization potential of the second quantum dot layer 4d is relatively larger. With this configuration, similar to the photoelectric conversion elements shown in Examples 1 to 5, it is possible to achieve both high quantum efficiency, an expanded sensitivity wavelength range, and reduced dark current.

[0172] The photoelectric conversion element and imaging device relating to this disclosure have been described above based on embodiments, modifications, and examples, but this disclosure is not limited to these embodiments, modifications, and examples. Without departing from the spirit of this disclosure, various modifications that a person skilled in the art could conceive of are applied to the embodiments, modifications, and examples, as well as other forms constructed by combining some of the components in the embodiments, modifications, and examples, are also included in the scope of this disclosure.

[0173] For example, the photoelectric conversion element according to this disclosure may be used in a solar cell by extracting the charge generated by light as energy. Alternatively, the photoelectric conversion element according to this disclosure may be used in a light sensor by extracting the charge generated by light as a signal. [Industrial applicability]

[0174] The photoelectric conversion element and imaging device described herein are applicable to photodiodes, image sensors, and the like, and are particularly applicable to highly sensitive, low-dark-current optical sensing using infrared wavelengths. [Explanation of Symbols]

[0175] 1 circuit board 2 1st electrode 3 Second electrode 4. Photoelectric conversion layer 4a, 4c First quantum dot layer 4b, 4d Second quantum dot layer 5 electron blocking layer 6. Hole blocking layer 10A, 10B, 10E Photoelectric Converters 10C, 10D Photoelectric conversion section 20, 20A pixels 22 Accumulation control line 23 Power line 24 Vertical signal lines 25 Address signal line 26 Reset signal line 27 Feedback line 30 Voltage supply circuit 40 Signal detection circuit 41 Charge storage node 42 Amplifying transistors 42e, 44e, 46e gates 42g, 44g, 46g gate insulation layer 44 Address Transistors 46 Reset Transistor 52 Vertical scanning circuit 54 Horizontal signal readout circuit 56-column signal processing circuit 58 Load circuit 59 Inverting Amplifier 62 Semiconductor substrates 62a, 62b, 62c, 62d, 62e impurity region 62s Element isolation region 63A, 63B, 63C Interlayer insulating layer 65A, 65B contact plugs 66A Wiring 67A, 67B, 67C plugs 68A, 68B wiring 72 Protective layer 74 Microlenses 100, 100A Imaging device

Claims

1. Photoelectric conversion layer, A first electrode for collecting holes generated in the photoelectric conversion layer, The system comprises a second electrode facing the first electrode across the photoelectric conversion layer, which collects electrons generated in the photoelectric conversion layer, The aforementioned photoelectric conversion layer is A first quantum dot layer comprising a plurality of first quantum dots whose surfaces are modified with a first ligand, A second quantum dot layer is located between the first quantum dot layer and the second electrode and includes a plurality of second quantum dots whose surfaces are modified with a second ligand different from the first ligand, The ionization potential of the second quantum dot layer is greater than that of the first quantum dot layer. The second value representing the particle size distribution of the plurality of second quantum dots is smaller than the first value representing the particle size distribution of the plurality of first quantum dots. The first ligand has a first dipole moment, The second ligand has a second dipole moment, If the first dipole moment is positive when it points outward from each of the plurality of first quantum dots, and the second dipole moment is positive when it points outward from each of the plurality of second quantum dots, The first dipole moment is greater than the second dipole moment. Photoelectric conversion element.

2. The plurality of first quantum dots and the plurality of second quantum dots are each independently selected from CdSe, CdS, PbS, PbSe, PbTe, ZnO, ZnS, Cu ２ ZnSnS ４ , Cu ２ S, CuInSe ２ , AgInS ２ , AgInTe ２ , CdSnAs ２ , ZnSnAs ２ , ZnSnSb ２ , Bi ２ S ３ , Ag ２ S, Ag ２ Te, HgTe, CdHgTe, Ge, GeSn, InAs and InSb, and contains at least one selected from the above group, The photoelectric conversion element according to claim 1.

3. A photoelectric conversion layer, A first electrode for collecting holes generated in the photoelectric conversion layer, The system comprises a second electrode facing the first electrode across the photoelectric conversion layer, which collects electrons generated in the photoelectric conversion layer, The aforementioned photoelectric conversion layer is A first quantum dot layer comprising a plurality of first quantum dots whose surfaces are modified with a first ligand, A second quantum dot layer is located between the first quantum dot layer and the second electrode and includes a plurality of second quantum dots whose surfaces are modified with a second ligand different from the first ligand, The ionization potential of the second quantum dot layer is greater than that of the first quantum dot layer. The second value representing the particle size distribution of the plurality of second quantum dots is smaller than the first value representing the particle size distribution of the plurality of first quantum dots. The first ligand is 1,4-benzenedithiol, The second ligand is ZnI ２ It is a mixture of 3-mercaptopropionic acid. Photoelectric conversion element.

4. A photoelectric conversion layer, A first electrode for collecting holes generated in the photoelectric conversion layer, The system comprises a second electrode facing the first electrode across the photoelectric conversion layer, which collects electrons generated in the photoelectric conversion layer, The aforementioned photoelectric conversion layer is A first quantum dot layer comprising a plurality of first quantum dots whose surfaces are modified with a first ligand, A second quantum dot layer is located between the first quantum dot layer and the second electrode and includes a plurality of second quantum dots whose surfaces are modified with a second ligand different from the first ligand, The ionization potential of the second quantum dot layer is greater than that of the first quantum dot layer. The second value representing the particle size distribution of the plurality of second quantum dots is smaller than the first value representing the particle size distribution of the plurality of first quantum dots. At least one of the particle size distributions of the plurality of first quantum dots and the particle size distributions of the plurality of second quantum dots has two or more different maximum values. Photoelectric conversion element.

5. The absorption peak wavelengths of the plurality of second quantum dots and the absorption peak wavelengths of the plurality of first quantum dots are both in the near-infrared region of 1000 nm or more. The photoelectric conversion element according to claim 1.

6. moreover, An electron blocking layer disposed between the first quantum dot layer and the first electrode, And, A hole blocking layer disposed between the second quantum dot layer and the second electrode, It comprises at least one of the following, The photoelectric conversion element according to claim 1.

7. Equipped with multiple pixels, Each of the aforementioned plurality of pixels includes a photoelectric conversion element according to any one of claims 1 to 6. Imaging device.

8. A signal readout circuit connected to the first electrode, The system further comprises a voltage supply circuit that supplies a voltage to the second electrode such that the potential of the second electrode is positive relative to the potential of the first electrode. The imaging apparatus according to claim 7.

9. A signal readout circuit connected to the second electrode, The system further comprises a voltage supply circuit that supplies a voltage to the first electrode such that the potential of the first electrode is negative relative to the potential of the second electrode. The imaging apparatus according to claim 7.

Images