JP7527774B2 - Quantum dots, photoelectric conversion element having the same, light receiving element, photoelectric conversion device, mobile body, method for manufacturing quantum dots, and method for manufacturing photoelectric conversion element - Google Patents

Description

The present invention relates to quantum dots, photoelectric conversion elements having the quantum dots, photoelectric conversion elements, light receiving elements, photoelectric conversion devices, moving bodies, methods for manufacturing quantum dots, and methods for manufacturing photoelectric conversion elements.

Quantum dots, which are made of inorganic particles and convert received light into electricity, have been attracting attention. Quantum dots are sensitive to light in the near-infrared region and can obtain highly sensitive images even in a dark field, making them suitable for use in image sensors (imaging sensors) for security applications such as security cameras.

Quantum dot materials are known to be composed of multiple inorganic particles made of compound semiconductors. The molecules that are attached to the surface of the nanoparticles and are covalently bonded to them are called ligands.

It is known that quantum dots have ligands made of organic compounds or inorganic ligands on their surfaces, and that the properties of quantum dots can be controlled by the type of ligand. As an example, organic ligands such as benzenedithiol, which contains a benzene ring, are known to improve the electrical conductivity of inorganic particles. However, because organic ligands have a three-dimensional structure, they can be bulky, and bulky ligands cannot adequately cover inorganic particles. As a result, surface defects on inorganic particles may not be adequately suppressed.

On the other hand, halogen-based inorganic ligands, because the size of the halogen atoms is small, can cover the surface of inorganic particles better than organic ligands, thereby suppressing surface defects of inorganic particles. However, with inorganic ligands alone, the distance between adjacent inorganic particles is shortened to about the same as the halogen atom. In other words, the function as a spacer is insufficient, and heat resistance may be low.

Furthermore, if the quantum dot film is oxidized in the atmosphere after it is formed, the oxidation will progress to the core of the inorganic particles, reducing the particle size of the inorganic particles. It is known that as the particle size of nanoparticles decreases, changes in electrical conductivity appear, and in particular the optical spectrum of photosensitivity shifts to the short wavelength side, reducing the photosensitivity in the desired wavelength range.

Patent Document 1 describes how the stability of quantum dots can be improved by replacing the organic material bound to the surface of the quantum dots with an inorganic material containing a halogen.

Non-Patent Document 1 describes the addition of halogen-based inorganic ligands when providing organic ligands on the surface of quantum dots. It describes that the addition of inorganic ligands repairs surface defects of nanoparticles that are not bonded to ligands with relatively long molecular lengths, such as oleic acid, added during synthesis, and improves the electrical conductivity of the quantum dot film.

U.S. Pat. No. 09,324,562

Ruili Wang, et. al, “Colloidal quantum dot ligand engineering for high performance solar cells”, Energy & Environmental Science, 2016, 4, 1117-1516

Patent Document 1 and Non-Patent Document 1 describe quantum dots having inorganic and organic ligands on the surface of inorganic particles. However, the molar ratio of the ligands estimated from the manufacturing conditions of these quantum dots is such that the molar ratio of the organic ligands is greater than the molar ratio of the inorganic ligands, and therefore the quantum dots are estimated to have a low photocurrent density. For this reason, there has been a demand for an improvement in the photocurrent density.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for manufacturing a photoelectric conversion element having a high photocurrent density.

One embodiment of the present invention provides a method for manufacturing a photoelectric conversion element, comprising the steps of: preparing a first electrode; forming a photoelectric conversion layer on the first electrode by applying quantum dots manufactured so that the molar ratio of inorganic ligands is greater than the molar ratio of organic ligands; and forming a second electrode on the photoelectric conversion layer, and further comprising an oxidation step of oxidizing the surfaces of the quantum dots, wherein the amount of inorganic ligands added relative to the total amount of inorganic ligands added and the amount of organic ligands added is 25% or more and 99.8% or less.

According to the present invention, a method for producing a photoelectric conversion element having high photocurrent density can be provided.

1A is a schematic cross-sectional view showing a photoelectric conversion element according to one embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view showing an example in which photoelectric conversion layers of the photoelectric conversion element according to one embodiment of the present invention are stacked. 1A is a schematic diagram showing an example of an inorganic ligand and an organic ligand of a quantum dot according to an embodiment of the present invention, and FIG. 1 shows the results of TOF-SIMS analysis of quantum dots according to an embodiment of the present invention. 1 shows the results of TOF-SIMS analysis (positive secondary ions) of quantum dots according to an embodiment of the present invention. 1 shows the results of a quantum dot TOF-SIMS analysis (negative secondary ions) according to an embodiment of the present invention. FIG. 1 is a diagram showing an example of an imaging system using a photoelectric conversion element according to an embodiment of the present invention. 1A shows an example of an imaging system related to an on-board camera, and FIG. 1B is a schematic diagram showing an example of a moving body including an on-board camera.

The quantum dot of one embodiment of the present invention is a quantum dot having a plurality of inorganic particles, the quantum dot having organic and inorganic ligands on its surface, and characterized in that the molar ratio of the inorganic ligands to the total of the inorganic ligands and the organic ligands is 25% or more and 99.8% or less. The molar ratio of the inorganic ligands to the total of the inorganic ligands and the organic ligands may be 75% or more and 98% or less. In addition, the height of the maximum TOF-SIMS peak caused by the inorganic ligands may be three times or more the height of the maximum TOF-SIMS peak caused by the organic ligands. When measuring TOF-SIMS, the main component of the inorganic particles may be used as a reference. When using it as a reference, the peak of the inorganic particles may be 90% or more of the measurement range. In addition, the peak of the inorganic particles may coincide with the limit of the measurement range.

The inorganic ligands suppress the decrease in photosensitivity caused by denaturation of quantum dots due to oxidation, etc., resulting in quantum dots with excellent electrical conductivity.

The molar ratio of the ligands on the surface of the quantum dots can be adjusted by changing the conditions when the ligands are added to the quantum dots.

The ligands may be added so that the molar ratio of the inorganic ligands is higher than the molar ratio of the organic ligands. More specifically, the amount of the inorganic ligands added relative to the total amount of the inorganic ligands added and the organic ligands added may be greater than the organic ligands, and may be 25% or more and 99.8% or less.

[Quantum dot materials]
Quantum dots are inorganic particles. The particles do not have to be completely spherical. The inorganic particles used in quantum dots are sometimes called nanoparticles because of their size. Examples of quantum dot materials include nanoparticles of semiconductor crystals such as IV group semiconductors, III-V group, and II-VI group compound semiconductors, and compound semiconductors consisting of combinations of three or more of II, III, IV, V, and VI elements. Specifically, semiconductor materials with relatively narrow band gaps such as PbS, PbSe, InN, InAs, Ge, InAs, InGaAs, CuInS, CuInSe, CuInGaSe, InSb, Si, and InP are listed. Among the above, the quantum dot material is preferably a nanoparticle having Pb, more specifically, PbS or PbSe, in view of ease of synthesis and photosensitivity up to the infrared region. These may be used as the core of the quantum dot, and the quantum dot material may be covered with a coating compound to form a core-shell structure. In this case, the ligand is provided in the shell portion.

The average particle size of the quantum dots is preferably 2 nm or more and 15 nm or less. When the particle size of the quantum dots is reduced to a size equal to or less than the Bohr radius of the excitons contained within the quantum dots, the quantum size effect causes the band gap of the quantum dots to change. For example, the Bohr radius of II-VI group semiconductors is relatively large, and it is said to be about 18 nm for PbS. Also, the Bohr radius of InP, which is a III-V group semiconductor, is said to be about 10 nm to 14 nm. In other words, if the average particle size of the quantum dots is 15 nm or less, it is possible to control the band gap by the quantum size effect, whether it is PbS or InP. By making the average particle size of the quantum dots 2 nm or more, it is possible to easily control the crystal growth of the quantum dots in the synthesis of the quantum dots.

[Ligands on the quantum dot surface]
[Organic Ligand]
The quantum dot has a ligand on the surface. The ligand is roughly classified into an inorganic ligand and an organic ligand. When the organic ligand has a first quantum dot and a second quantum dot, the organic ligand may have a bridge structure that bridges the first and second quantum dots. The bridge means that one molecule is bonded to the first quantum dot and the second quantum dot. When the quantum dots are bridged by an organic ligand, the distance between the quantum dots can be controlled by the molecular length of the organic ligand. Specifically, the bridge structure may be a hydroxyl group, a thiol group, or a carboxyl group. Here, the quantum dots bridged by the hydroxyl group are bonded via an ether bond, the quantum dots bridged by the thiol group are bonded via a sulfur atom, and the quantum dots bridged by the carboxyl group are bonded via an ester bond. A certain distance can be maintained by bridging the quantum dots with an organic ligand.

It is preferable to have at least one organic molecule between the quantum dots. Assuming that the quantum dots are aligned without gaps, the number of other quantum dots adjacent to one quantum dot will be 12 in a face-centered cubic lattice and 8 in a body-centered cubic lattice. In other words, it is preferable for one quantum dot to have at least 8 to 12 organic ligands to bridge it with other quantum dots.

If there are a lot of organic ligands composed of organic molecules, both ends of the organic molecules will bond strongly to the quantum dot surface, improving heat resistance and increasing the stability of the electrical properties. However, if the molar ratio of inorganic ligands to organic ligands is reduced, there will be less inorganic ligands to cover the surface defects of the quantum dots, which is not desirable.

[Inorganic Ligand]
The inorganic ligands may be bonded to the quantum dot surface. The addition of inorganic ligands to the quantum dot surface improves carrier mobility. The inorganic ligands reduce surface defects of the quantum dots. Photocarriers generated in the quantum dot film can move along with the electric field without delay. Therefore, the photocarriers can be effectively extracted to the external electrode. In other words, the provision of inorganic ligands improves the external quantum efficiency.

On the other hand, it is not preferable to have too many inorganic ligands. When a large amount of inorganic ligands is added in the liquid phase, the inorganic ligands can move relatively freely on the surface of the quantum dots when heated. If the inorganic ligands on the surface of the quantum dots move, the distance between adjacent quantum dots becomes shorter, causing the quantum dots to fuse together (neck), resulting in a significant change in electrical properties. For example, if there are too many halogen atoms such as iodine, the quantum dots will fuse together when the photoelectric conversion film is heated to 140°C or higher, resulting in some quantum dots becoming larger in size, and the device properties will change. When the size of the quantum dots increases, the optical absorption and electronic state change, reducing the stability of the device properties.

The diameter of iodine, a halogen atom, is 0.28 nm, so the projected area is 0.06 ^nm2 . Here, we assume that the quantum dot is a sphere. If the particle size (diameter) of the quantum dot is 3 nm, the surface area of the quantum dot is 28.26 ^nm2 .

The surface area of the quantum dot and the projected area of the iodines give the number of iodines covering the surface as 471. In close packing, the fill factor of a sphere relative to a cube is 74%, so the number of iodines is approximately 349.

[Ratio of inorganic ligands to organic ligands]
The diameter of the benzene ring is 0.28 nm, and the projected area of the iodine ion and the benzene ring are almost the same. The size of an organic ligand, such as 1,3-BDT, is almost equal to the size of the benzene ring, so the projected area is 0.06 ^nm2 . When an organic ligand is placed between each of the quantum dots in the close-packed structure, the number is 8 or 12.

Compared to the 349 inorganic iodine ligands covering the quantum dot surface, the molar ratio of organic ligands to inorganic ligands can be derived to be 96.6-97.7%. In other words, the molar ratio of inorganic ligands is 98%.

[Minimum molar ratio of inorganic ligands to the total of inorganic ligands and organic ligands]
2B is a schematic diagram of the surface of a quantum dot. A part of the quantum dot is shown within the dashed line. The minimum molar ratio of the inorganic ligand to the organic ligand will be described with reference to FIG.

Figure 2(B) shows a quantum dot surface with inorganic ligands arranged without gaps. On the other hand, Figure 2(C) shows a quantum dot surface with organic ligands arranged without gaps.

The thickness of one carbon atom that constitutes an organic ligand is approximately 0.08 nm, which is smaller than the 0.26 nm of a halogen atom, which is an inorganic ligand. Therefore, it can be said that the projected area of an organic ligand is smaller than that of an inorganic ligand.

When the organic ligand stands upright, its projected area is 0.02 ^nm2 , which is 1/3 of the projected area of a halogen atom, 0.06 ^nm2 .

To reduce defects and transport carriers on the quantum dot surface, it is preferable to place at least one halogen atom per organic ligand. In other words, it is desirable for the molar ratio of inorganic ligands to organic ligands to be 50% or more.

Figure 2 (D) shows a quantum dot surface with inorganic and organic ligands arranged in a 1:1 ratio. If the inorganic and organic ligands are arranged in the same molar ratio, gaps will be created around the organic ligands. In order to fill these gaps, it is effective to arrange a larger amount of organic ligands.

Figure 2 (E) shows that inorganic and organic ligands are arranged on the quantum dot surface in a ratio of 1:3. For every inorganic ligand, arranging three times as many organic ligands is effective in arranging them without gaps. In other words, a significant effect can be obtained when the molar ratio of inorganic ligands to the total of organic and inorganic ligands is 25% or more.

[Synthesis of quantum dots]
As an example of a method for producing PbS particles, which is an example of quantum dots, the following can be used. In a three-neck flask, lead oxide (II) is placed as a lead (Pb) precursor solution, oleic acid as an organic ligand, and octadecene as a solvent. The inside of the three-neck flask is replaced with nitrogen, and the Pb precursor solution is heated to 90° C. in an oil bath to react the lead oxide with the oleic acid. After that, the solution is heated to 120° C. to generate and grow PbS quantum dots. An octadecene solution of bistrimethylsilyl sulfide is separately prepared as a sulfur (S) precursor solution. The S precursor solution is rapidly injected into the Pb precursor solution heated to 120° C. to cause a reaction. After the reaction is completed, the solution is naturally cooled to room temperature, and methanol or acetone is added as a polar solvent, followed by centrifugation to precipitate the PbS particles. After removing the supernatant, toluene is added to perform washing to redisperse the PbS particles. By repeating this centrifugation and redispersion process multiple times, excess oleic acid and unreacted materials can be removed, and finally, a solvent such as octane can be added to produce a quantum dot dispersion liquid in which the quantum dots are dispersed.

Using the method described in Non-Patent Document 1, etc., a halogen inorganic ligand containing a halogen atom can be added to PbS particles, which are an example of quantum dots. Specifically, tetrabutylammonium iodide C ₁₆ H ₃₆ NI:TBAI is used as the halogen ligand. The S precursor solution is rapidly injected into the Pb precursor solution heated to 120° C., and after reacting, a precursor solution prepared using the halogen ligand TBAI is added. The ratio of oleic acid to halogen atoms can be adjusted by adjusting the reaction temperature and the amount of halogen ligand added.

The addition of inorganic ligands may be carried out in the liquid phase, and the addition of organic ligands may be carried out in the solid phase.

[Organic ligands with long molecular chain length]
The ligand is made of an organic compound. The organic ligand may be composed of an organic compound having a boiling point of 200° C. or more. The ligand contained in the quantum dot dispersion liquid acts as a ligand that coordinates to the quantum dots, and also plays a role as a dispersant that disperses the quantum dots in the solvent because it has a molecular structure that is prone to steric hindrance. There are ligands with longer molecular chain lengths and ligands with shorter molecular chain lengths. Here, the length of the molecular chain is determined by the length of the main chain when there is a branched structure in the molecule. A ligand with a short molecular chain length is difficult to disperse in an organic solvent system in the first place. Here, dispersion refers to a state in which there is no sedimentation or turbidity of particles. From the viewpoint of improving the dispersion of the quantum dots, it is desirable for the ligand with a long molecular chain length to be a ligand with at least 6 carbon atoms in the main chain, and more preferably a ligand with 10 carbon atoms in the main chain. Specifically, the compound may be either a saturated compound or an unsaturated compound, and examples thereof include decanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, erucic acid, oleylamine, dodecylamine, dodecanethiol, 1,2-hexadecanethiol, trioctylphosphine oxide, cetrimonium bromide, etc. Ligands having a long molecular chain length are preferably those which are unlikely to remain in the photoelectric conversion film when the film is formed, and among the above, at least one of oleic acid and oleylamine is preferred from the viewpoint of imparting dispersion stability to the quantum dots while being unlikely to remain in the photoelectric conversion film.

The quantum dot solvent contained in the quantum dot dispersion liquid is not particularly limited, but is preferably a solvent that does not easily dissolve quantum dots and easily dissolves ligands with long molecular chains. The quantum dot solvent is preferably an organic solvent, and specific examples include alkanes (n-hexane, n-octane, etc.), benzene, toluene, etc.

[Organic ligands with relatively short molecular chain length]
The ligand is made of an organic compound. The organic ligand may be made of an organic compound having a boiling point of 200° C. or more. The organic ligand having a relatively short molecular chain length is at least one ligand selected from organic compounds such as ethanedithiol, benzenedithiol, dibenzenedithiol, mercaptobenzoic acid, dicarboxybenzene, benzenediamine, and dibenzenediamine. More specifically, the ligand may be selected from 1,3-benzenedithiol, 1,4-benzenedithiol, 3-mercaptobenzoic acid, and 4-mercaptobenzoic acid. In particular, the ligand such as benzenedithiol containing a benzene ring is preferable because it has a boiling point exceeding 200° C. Even at a high temperature of 140° C. or more, the evaporation of the ligand is suppressed, and the heat resistance of the quantum dot film is high. The ligand solvent contained in the ligand solution is not particularly limited, but is preferably an organic solvent having a high dielectric constant, and particularly preferably ethanol, acetone, methanol, acetonitrile, dimethylformamide, dimethylsulfoxide, butanol, propanol, or the like.

[Inorganic Ligand]
The ligand is made of an inorganic compound. It preferably contains a halogen atom. The inorganic ligand containing a halogen atom is at least one ligand selected from lead fluoride (PbF), lead chloride (II) (PbCl ₂ ), lead iodide (II) (PbI _{2 )} , lead bromide (II) (PbBr ₂ ), and the like. By containing a large amount of inorganic ligands made of halogen-based inorganic materials, the surface of the quantum dot can be sufficiently covered. Since the particle size of halogen atoms is small, more surface defects can be sealed.

[Photoelectric conversion element]
1 is an example of a cross-sectional schematic diagram of a photoelectric conversion element according to an embodiment of the present invention. A first electrode layer 2, a first interface layer 3, a photoelectric conversion layer 4, a second interface layer 5, and a second electrode 6 are provided on a substrate 1. The first electrode and the second electrode may be collectively referred to as a pair of electrodes.

The photoelectric conversion element according to one embodiment of the present invention is an element having a first electrode, a second electrode, and a photoelectric conversion layer disposed between the first and second electrodes. The photoelectric conversion layer is a layer that converts received light into electric charges. The converted electric charges are polarized and collected by one of a pair of electrodes. The photoelectric conversion layer may be configured to receive an electric field from the first and second electrodes. The pair of electrodes is an electrode that collects holes and an electrode that collects electrons. The electrode that collects holes is also called a positive electrode or a cathode. The electrode that collects electrons is also called a negative electrode or an anode. At least one of the pair of electrodes may be transparent.

The photoelectric conversion element may have an interface layer between the photoelectric conversion layer and at least one of the pair of electrodes. The interface layer may be an electron blocking layer that reduces the movement of electrons from the photoelectric conversion layer to the cathode, or a hole blocking layer that reduces the movement of holes from the photoelectric conversion layer to the anode.

[Photoelectric conversion layer]
A photoelectric conversion layer can be formed by applying quantum dots onto an electrode, a substrate, or the like. The method for manufacturing a photoelectric conversion layer including an aggregate of nanoparticles as quantum dots is not particularly limited. When the quantum dots are dispersed in a solvent in a state modified with a ligand having a long molecular chain length, such as oleic acid, the quantum dots are less likely to aggregate into a bulk state. The method for applying the quantum dot dispersion onto a substrate is not particularly limited, and examples thereof include a method of applying the quantum dot dispersion onto a substrate, and a method of immersing a substrate in the quantum dot dispersion. More specifically, liquid phase methods such as spin coating, cat coating, dip coating, blade coating, spray coating, inkjet coating, dispenser coating, screen printing, letterpress printing, and intaglio printing can be used as the method for applying the quantum dot dispersion onto a substrate. In particular, the inkjet coating, dispenser coating, screen printing, letterpress printing, and intaglio printing can pattern a coating film at any position on the substrate.

After applying the quantum dot dispersion onto the substrate, a ligand solution is applied on top of it to exchange the ligands and form a photoelectric conversion layer. By applying the quantum dot dispersion onto the substrate, the quantum dot aggregate is configured with quantum dots arranged one by one. After forming the quantum dot aggregate, the distance between the quantum dots can be shortened by ligand exchange, which exchanges the ligands with organic ligands having a relatively short molecular chain length. However, if the distance between the quantum dots (spacing) is large, the electrical conductivity may decrease. By reducing the spacing, the quantum dots can be arranged densely. In addition, by reducing the spacing, the electrical conductivity is improved and a photoelectric conversion film with a large photocurrent can be obtained. However, the shorter the spacing, the more likely it is that the quantum dots will fuse together (called necking), so a certain distance may be set. When the quantum dots fuse together, the size of the quantum dots increases and the band gap decreases. This reduces the resistance value and significantly increases the dark current.

By repeatedly performing quantum dot aggregate formation and ligand exchange, it is possible to suppress the occurrence of cracks in the photoelectric conversion film having quantum dot aggregates, increase electrical conductivity, and increase the thickness of the photoelectric conversion film. A cleaning process may be included to remove excess ligands, ligands detached from the quantum dots, remaining solvent, other impurities, etc. Specifically, the substrate on which the quantum dot aggregates or the photoelectric conversion layer are formed is coated with or immersed in at least one of the quantum dot solvent and the ligand solvent.

The formation of the photoelectric conversion layer may include a dispersion drying process for drying the quantum dot dispersion liquid, a solution drying process for drying the ligand solution, and the like. The drying process involves heating the quantum dot aggregates to a temperature at which the solvent remaining in the quantum dot aggregates is dried after the quantum dot aggregates are formed as the photoelectric conversion film. After the ligand exchange process, the quantum dot aggregates are heated to a solution drying temperature at which the ligand solution is dried. Alternatively, the quantum dots may be left at room temperature. It is more preferable to heat the quantum dots in a nitrogen atmosphere in an environment containing a small amount of oxygen. Specifically, the surface of the quantum dots is oxidized by heating in an environment containing 1% or less of oxygen. The surface of the quantum dots is also oxidized when the quantum dots are heated in a state in which a small amount of moisture remains on the outermost surface of the photoelectric conversion film. Furthermore, it is also preferable to include a small amount of moisture in the quantum dot dispersion liquid or the ligand solution in order to oxidize the surface. The concentration of oxygen generated by the oxidation of the surface may be 0.1 ppm or more.

The surface of a quantum dot is the outermost surface of a semiconductor material, and bonds that are not bonded to ligands become dangling bonds (unbonded bonds in atoms), forming defect levels that cause carrier traps that affect electrical conductivity. By oxidizing the surface of a quantum dot, the dangling bonds on the surface can be sealed, reducing the defect levels. Reducing the defect levels can reduce dark current, improve photocurrent density, and improve photosensitivity.

However, if the surface of the quantum dots is oxidized too much, the actual particle size of the nanoparticles that contribute to the quantum confinement effect as quantum dots becomes smaller. When the particle size of the nanoparticles becomes smaller, changes appear in the electrical conductivity, and in particular the optical spectrum of the photosensitivity shifts to the short wavelength side, affecting the photosensitivity.

The thickness of the photoelectric conversion layer is not particularly limited, but from the viewpoint of obtaining high electrical conductivity, it is preferably 10 nm or more, and more preferably 50 nm or more. In addition, from the viewpoint of the risk of excessive carrier concentration and ease of manufacture, the thickness of the photoelectric conversion layer is preferably 600 nm or less.

In addition, by preferentially coating the surfaces of the nanoparticles that make up the quantum dot film with inorganic ligands, it is possible to reduce surface defects in quantum dots that cannot be completely coated with organic ligands alone. Reducing the surface defects in the nanoparticles suppresses the increase in dark current and the decrease in photocurrent, making it possible to realize a photodetector with high photosensitivity.

By using ligands with a boiling point of 200°C or higher, it is possible to obtain high heat resistance.

The photodetector element is characterized in that the nanoparticles are each composed of a core and a surface layer, and the surface layer of the nanoparticles is made of a material composed of an oxide layer containing a halogen and an organic layer containing an organic substance. The presence of the oxide layer containing an organic substance and a halogen makes it possible to capture oxygen molecules that have invaded the element from the outside and to suppress surface oxidation of the nanoparticles by being oxidized by itself.

The quantum dot film contains at least one of oxidized benzene rings, oxidized hydrocarbons, and oxidized organic substances. The organic ligands capture oxygen molecules and become oxidized benzene rings or oxidized hydrocarbons, which has the effect of invalidating the carrier hopping sites formed by the benzene rings and suppressing dark current.

The multiple nanoparticles are a light detection element selected from PbS and PbSe, which contain lead. By using PbS or PbSe, which contain lead, a light detection element that is also sensitive to the infrared region can be realized.

Electronic devices such as photoelectric conversion elements, light-emitting diodes, photodetection elements, light-receiving element arrays, image sensors, imaging devices, etc. are solder-mounted onto printed circuit boards and the like for practical use. In the solder mounting process, a heating process of about 200°C is used. Electronic devices such as image sensors, imaging devices, etc. are housed in packages made of resin or ceramic and glass-sealed for water resistance and dust protection. In the package mounting process, after being fixed with adhesive, they are electrically connected to the package's lead-out wiring by wire bonding using fine gold wires, etc. A heating process of at least 145°C or more is used for adhesive hardening and wire bonding.

Furthermore, when used in the visible light region as an image sensor, image sensor, or imaging device, a color filter capable of dispersing light into RGB and a microlens for focusing light are used. Color filters and microlenses are often made of resin, and a heating process of at least 150°C or more is used to harden the resin. Quantum dot materials for use as image sensors, image sensors, and imaging devices are required to be heat resistant to withstand the temperatures of the heating processes in the mounting process, color filter process, and microlens process mentioned above.

Therefore, by providing a ligand with a high boiling point, it is possible to construct quantum dots with excellent heat resistance, and these quantum dots with excellent heat resistance can be used in a variety of devices.

[Interface layer]
An upper interface layer and a lower interface layer may be provided between the photoelectric conversion layer and the electrodes (upper electrode layer, undercoat electrode layer) to improve electrical properties. A layer that blocks electrons and conducts only holes (electron blocking layer) may be formed on the electrode that collects holes (positive electrode), and a layer that blocks holes and conducts only electrons (hole blocking layer) may be formed on the electrode that collects electrons (negative electrode).

As the material of the electron blocking layer, one that can efficiently transport holes generated in the photoelectric conversion layer to the positive electrode is preferable. For this purpose, it is preferable that the positive electrode interface layer material has properties such as high hole mobility, high conductivity, small hole injection barrier between the positive electrode, and small hole injection barrier from the photoelectric conversion layer to the p-type semiconductor layer. Furthermore, when light is taken into the photoelectric conversion layer through the electron blocking interface layer, it is preferable to use a highly transparent material as the material of the electron blocking interface layer. Since visible light is usually taken into the photoelectric conversion layer, it is preferable to use a transparent electron blocking interface layer material that has a transmittance of the transmitted visible light of usually 60% or more, and especially 80% or more. From this viewpoint, suitable examples of the electron blocking interface layer material include p-type semiconductor materials such as inorganic semiconductors such as molybdenum oxide MoO ₃ and nickel oxide NiO.

On the other hand, the function required for the hole blocking layer is to block the holes separated from the photoelectric conversion layer and transport the electrons to the negative electrode, so in the description of the hole blocking interface layer above, the positive electrode is replaced with the negative electrode, the p-type semiconductor with an n-type semiconductor, and the holes with electrons. In addition, it is also possible to effectively utilize a configuration in which light is irradiated from the negative electrode side or light reflected from the negative electrode side, in which case the transmittance must be high. From this perspective, a suitable example of the hole blocking interface layer material is an n-type semiconductor material selected from inorganic semiconductors such as titanium oxide _TiO2 or organic semiconductors such as fullerenes and fullerene derivatives.

[electrode]
The electrodes may be a pair of electrodes, such as an upper electrode layer and a base electrode layer, a first electrode and a second electrode, etc. The electrodes can be formed from any material having electrical conductivity. Examples of materials constituting the electrodes include metals such as platinum, gold, silver, aluminum, chromium, nickel, copper, titanium, magnesium, calcium, barium, sodium, etc., or alloys thereof, metal oxides such as indium oxide and tin oxide, or composite oxides thereof (e.g., ITO, IZO), conductive polymers such as polyacetylene, metal particles, carbon black, fullerene, carbon nanotubes, etc., and conductive composite materials in which conductive particles are dispersed in a matrix such as a polymer binder. The materials constituting the electrodes may be used alone or in any combination and ratio of two or more.

The electrode has the function of collecting electrons and holes generated inside the photoelectric conversion layer. Therefore, it is preferable to use, as the constituent material of the electrode, a constituent material suitable for collecting electrons and holes among the above-mentioned materials. Examples of electrode materials suitable for collecting holes include materials with high work functions such as Au and ITO. On the other hand, examples of electrode materials suitable for collecting electrons include materials with low work functions such as Al. There are no particular restrictions on the thickness of the electrode, and it is determined appropriately taking into account the electrode material, conductivity, transparency, etc., and may be 10 nm or more and 100 μm or less.

[substrate]
The quantum dot dispersion liquid is applied onto a substrate. There are no particular limitations on the shape, structure, size, etc. of the substrate, and they are appropriately selected according to the purpose. The structure of the substrate may be a single-layer structure or a laminated structure. For example, a substrate made of an inorganic material such as glass, silicon, or stainless steel, or a resin or resin composite material, etc. can be used as the substrate. In the case of a silicon substrate, an integrated circuit may be formed. In addition, a lower electrode, an insulating film, etc. may be provided on the substrate, and in that case, the quantum dot dispersion liquid is applied onto the lower electrode or the underlying insulating film on the substrate.

When wiring layers are formed on a substrate such as silicon, the term "substrate" includes the wiring layers. When there is an interlayer insulating layer that contacts the electrodes of the photoelectric conversion element, the term "substrate" includes the interlayer insulating layer.

[Examples of photoelectric conversion element applications]
The photoelectric conversion element according to an embodiment of the present invention may be used as a light receiving element. The light receiving element includes a photoelectric conversion element, a readout circuit that reads out charges from the photoelectric conversion element, and a signal processing circuit that receives the charges from the readout circuit and processes the signals.

The photoelectric conversion element according to one embodiment of the present invention may be used in a photoelectric conversion device. The photoelectric conversion device has an optical system having a plurality of lenses and a light receiving element that receives light transmitted through the optical system, and the light receiving element is a light receiving element having a photoelectric conversion element. Specifically, the photoelectric conversion device may be a digital still camera or a digital video camera.

Figure 6 is a diagram showing an example of an imaging system using a photoelectric conversion element according to an embodiment of the present invention. As shown in Figure 6, the imaging system 500 includes a photoelectric conversion device 100, an imaging optical system 502, a CPU 510, a lens control unit 512, an imaging device control unit 514, an image processing unit 516, an aperture shutter control unit 518, a display unit 520, an operation switch 522, and a recording medium 524.

The imaging optical system 502 is an optical system for forming an optical image of a subject, and includes a lens group, an aperture 504, etc. The aperture 504 has a function of adjusting the amount of light during shooting by adjusting its opening diameter, and also functions as a shutter for adjusting the exposure time when shooting still images. The lens group and aperture 504 are held so that they can move forward and backward along the optical axis, and their linked operation realizes a variable magnification function (zoom function) and a focus adjustment function. The imaging optical system 502 may be integrated into the imaging system, or may be an imaging lens that can be attached to the imaging system.

The photoelectric conversion device 100 is arranged so that its imaging surface is located in the image space of the imaging optical system 502. The photoelectric conversion device 100 is a photoelectric conversion device according to one embodiment of the present invention, and is composed of a CMOS sensor (pixel section) and its peripheral circuit (peripheral circuit area). The photoelectric conversion device 100 has pixels having multiple photoelectric conversion sections arranged two-dimensionally, and color filters are arranged for these pixels to form a two-dimensional single-plate color sensor. The photoelectric conversion device 100 photoelectrically converts the subject image formed by the imaging optical system 502, and outputs it as an image signal or a focus detection signal.

The lens control unit 512 controls the forward and backward movement of the lens group of the imaging optical system 502 to perform variable magnification operations and focus adjustment, and is composed of circuits and processing devices configured to realize this function. The aperture shutter control unit 518 adjusts the amount of light for shooting by changing the aperture diameter of the aperture 504 (variable aperture value), and is composed of circuits and processing devices configured to realize this function.

The CPU 510 is a control device within the camera that handles various controls of the camera body, and includes an arithmetic unit, ROM, RAM, A/D converter, D/A converter, and a communication interface circuit. The CPU 510 controls the operation of each unit within the camera in accordance with a computer program stored in the ROM or the like, and executes a series of photographing operations such as AF, imaging, image processing, and recording, including detection of the focus state of the imaging optical system 502 (focus detection). The CPU 510 also functions as a signal processing unit.

The imaging device control unit 514 controls the operation of the photoelectric conversion device 100, and A/D converts the signal output from the photoelectric conversion device 100 and transmits it to the CPU 510, and is composed of circuits and control devices configured to realize these functions. The photoelectric conversion device 100 may have the A/D conversion function. The image processing unit 516 performs image processing such as gamma conversion and color interpolation on the A/D converted signal to generate an image signal, and is composed of circuits and control devices configured to realize these functions. The display unit 520 is a display device such as a liquid crystal display device (LCD), and displays information about the camera's shooting mode, a preview image before shooting, a confirmation image after shooting, a focus state during focus detection, etc. The operation switch 522 is composed of a power switch, a release (shooting trigger) switch, a zoom operation switch, a shooting mode selection switch, etc. The recording medium 524 is for recording the shot image, etc., and may be built into the imaging system, or may be a removable one such as a memory card.

In this way, by configuring an imaging system 500 that applies a photoelectric conversion device 100 according to an embodiment of the present invention, a high-performance imaging system can be realized.

The photoelectric conversion device according to one embodiment of the present invention may be used in a moving object. The moving object has a body on which the photoelectric conversion device is provided, and a means of moving the body. Specific examples include automobiles, airplanes, ships, drones, etc. By providing the device in a moving object, it may be possible to capture images of the surrounding environment and support the operation of the moving object. The body may be made of metal or carbon fiber. Polycarbonate, etc. may be used as the carbon fiber. Examples of the means of movement include tires, magnetic levitation, and a mechanism for vaporizing and spraying fuel.

Figures 7A and 7B are diagrams showing the configuration of an imaging system and a moving object according to this embodiment.

Figure 7A shows an example of an imaging system 400 related to an in-vehicle camera. The imaging system 400 has a photoelectric conversion device 410. The photoelectric conversion device 410 is any one of the photoelectric conversion devices described in the first to fourth embodiments described above. The imaging system 400 has an image processing unit 412, which is a processing device that performs image processing on multiple image data acquired by the photoelectric conversion device 410, and a parallax acquisition unit 414, which is a processing device that calculates parallax (phase difference of parallax images) from the multiple image data acquired by the photoelectric conversion device 410. The imaging system 400 also has a distance acquisition unit 416, which is a processing device that calculates the distance to an object based on the calculated parallax, and a collision determination unit 418, which is a processing device that determines whether or not there is a possibility of a collision based on the calculated distance. Here, the parallax acquisition unit 414 and the distance acquisition unit 416 are examples of information acquisition means that acquire information such as distance information to the object. In other words, the distance information is information on the parallax, the defocus amount, the distance to the object, etc. The collision determination unit 418 may use any of these distance information to determine the possibility of a collision. The various processing devices described above may be realized by dedicated hardware, or may be realized by general-purpose hardware that performs calculations based on software modules. The processing device may also be realized by a Field Programmable Gate Array (FPGA), an Application Specific Integrated Circuit (ASIC), or a combination of these.

The imaging system 400 is connected to a vehicle information acquisition device 420 and can acquire vehicle information such as vehicle speed, yaw rate, and steering angle. The imaging system 400 is also connected to a control ECU 430, which is a control device that outputs a control signal to generate a braking force for the vehicle based on the judgment result of the collision judgment unit 418. In other words, the control ECU 430 is an example of a mobile body control means that controls a mobile body based on distance information. The imaging system 400 is also connected to an alarm device 440 that issues an alarm to the driver based on the judgment result of the collision judgment unit 418. For example, if the judgment result of the collision judgment unit 418 indicates that there is a high possibility of a collision, the control ECU 430 applies the brakes, releases the accelerator, suppresses engine output, etc., to avoid the collision and reduce damage by performing vehicle control. The alarm device 440 warns the user by sounding an alarm, displaying alarm information on the screen of a car navigation system, etc., or vibrating the seat belt or steering wheel.

In this embodiment, the surroundings of the vehicle, for example the front or rear, are captured by the imaging system 400. FIG. 7B shows the imaging system 400 when capturing an image of the area in front of the vehicle (imaging range 450). The vehicle information acquisition device 420 sends an instruction to operate the imaging system 400 to perform imaging. By using the photoelectric conversion device of the first to fourth embodiments described above as the photoelectric conversion device 410, the imaging system 400 of this embodiment can further improve the accuracy of distance measurement.

In the above explanation, an example of control to avoid collision with other vehicles was described, but the system can also be applied to automatic driving control to follow other vehicles, automatic driving control to avoid going out of lanes, etc. Furthermore, the imaging system is not limited to vehicles such as automobiles, but can be applied to moving bodies (transportation equipment) such as ships, aircraft, and industrial robots. The moving devices in moving bodies (transportation equipment) are various types of moving means such as engines, motors, wheels, and propellers. In addition, the system can be applied not only to moving bodies, but also to a wide range of equipment that uses object recognition, such as intelligent transport systems (ITS).

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention.

[Example 1 and Comparative Example 1]
In this embodiment, the photoelectric conversion element shown in FIG. 1 was fabricated as follows. A substrate 1 with a surface on the first electrode side insulated with a silicon oxide film was prepared, and a TiN film was formed as the first electrode 2, and a titanium oxide TiO2 film was formed as the first interface layer 3 by sputtering. A previously prepared PbS quantum dot dispersion was applied onto the first interface layer 3 by spin coating, and then a ligand solution containing an organic ligand benzenedithiol (1,4-BDT) with a short molecular chain length was similarly applied to form a photoelectric conversion layer 4. That is, the inorganic ligand was previously applied in the liquid phase, and the organic ligand was applied in the solid phase. The fabricated photoelectric conversion layer 4 was left overnight in a glove box under a nitrogen atmosphere (oxygen concentration 1 ppm or less, moisture concentration 1 ppm or less) and dried. Molybdenum oxide _MoO3 was formed as the second interface layer 5 on the photoelectric conversion layer 4 by vacuum deposition. On the second interface layer 5, a transparent conductive film of indium oxide (ITO) containing tin was formed as a second electrode 6 by sputtering.

In Example 1, the conditions for adding inorganic ligands were adjusted during the preparation of the PbS quantum dot dispersion in the above process, and quantum dots were used that were prepared so that the molar ratio of inorganic ligands was greater than the molar ratio of organic ligands. The element prepared in Example 1 is referred to as photoelectric conversion element 1.

On the other hand, in Comparative Example 1, quantum dots were used that were prepared so that the molar ratio of inorganic ligands was smaller than the molar ratio of organic ligands. The element prepared in Comparative Example 1 is referred to as photoelectric conversion element 2.

The ratio of halogen atoms and organic ligands on the quantum dot surface was measured using TOF-SIMS analysis. Specifically, a PHI nano TOF II TOF-SIMS analyzer manufactured by ULVAC-PHI, Inc. was used, and primary ions were Bi ₃ ⁺⁺ , the primary ion acceleration voltage was 30 kV, and the secondary ion polarities were positive and negative.

Figure 3 shows the results of a TOF-SIMS analysis of the surface of the quantum dots used in the photoelectric conversion element 1.

In the photoelectric conversion element 1, the iodine ion ^I- was 6.2, while the organic compound ^ion _C6H4S2- was _1.8 . That is, the molar ratio of inorganic ligands is estimated to be 77.5 _% . In addition, the peak intensity of the maximum peak among the peaks due to the inorganic particles, i.e., the _iodine ion peak, was more than three times that of the peak due to the organic compound _ion ^, i.e., _C6H4S2- .

In the photoelectric conversion element 2, the iodine ion I ⁻ was 1.6, while the organic compound ion C ₆ H ₄ S ₂ ⁻ was 6.0. That is, the molar ratio of the inorganic ligand was estimated to be 21.1%.

[Electrical characteristics]
The electrical conductivity of the fabricated photoelectric conversion element was evaluated using a semiconductor parameter analyzer. First, the voltage applied to the electrodes was swept between -5 and 5 V in a state where the photoelectric conversion element was not irradiated with light, and the IV characteristics in the dark state were obtained. The current value in the state where a bias of +2 V was applied was adopted as the dark current value Id.

The photocurrent value was evaluated when the photoelectric conversion element was irradiated with monochromatic light (irradiation intensity 5 μW/cm ² ). The wavelength of the monochromatic light was changed in increments of 5 nm between 300 nm and 1200 nm for measurement. The increment in current from the dark current when irradiated with light of a wavelength of 600 nm was taken as the photocurrent Ip. In order to examine the heat resistance of the photoelectric conversion element, after completion of the photoelectric conversion element, it was heated in air at 150° C. for 1 hour. Table 1 shows the initial values after completion of the photoelectric conversion element and the values after heating in air. Photoelectric conversion element 1 had a high external quantum efficiency, and no significant change was observed even after heating in air.

This is because the molar ratio of inorganic ligands in photoelectric conversion element 1 is greater than the molar ratio of organic ligands on the quantum dot surface, and the defects on the quantum dot surface are sufficiently covered with inorganic ligands. By sufficiently sealing the surface defects of the quantum dots with iodine I, a halogen atom, the photocurrent density due to the surface defects of the quantum dots is improved, making it possible to realize a photoelectric conversion element with high photosensitivity.

[Example 2 and Comparative Example 2]
In Example 2, a photoelectric conversion element was prepared in the same manner as in Example 1, except that a drying step was provided after the formation of the photoelectric conversion layer. As a drying step after the formation of the photoelectric conversion film 4, the quantum dot aggregate was heated for 30 minutes at 150° C., which is a temperature at which the solvent remaining therein is dried, or a solution drying temperature at which the ligand solution is dried after the ligand exchange step. The heating was performed in an environment of a nitrogen atmosphere containing oxygen. Specifically, the drying step was performed in an environment in which the oxygen concentration in the glove box was 0.1 ppm or more and 1% or less. The surface of the quantum dot is the outermost surface of the semiconductor material, and the bonds that are not bonded to the ligands become dangling bonds. The dangling bonds form defect levels that cause carrier traps that affect the electrical conductivity of the photoelectric conversion film, but by slightly oxidizing the surface of the quantum dots, the dangling bonds can be sealed and the defect levels can be reduced.

The measurement of the oxidation state of the quantum dot surface was performed using the same TOF-SIMS analysis as in Example 1. FIG. 4 shows the results of TOF-SIMS analysis (positive secondary ions). The quantum dot surface contains Pb ₃ SI ⁺ containing iodine I, as well as Pb ₃ SOI ⁺ and Pb ₄ SOI ⁺ . The oxidized surface of the PbS nanoparticles constituting the quantum dot contains iodine I, and the halogen repairs the surface defects that remained even after oxidation. The ratio of Pb ₃ SI ⁺ to Pb ₃ SOI ⁺ was 2.0 and 0.7. FIG. 5 shows the results of TOF-SIMS analysis (negative secondary ions). The quantum dot surface contains benzene rings such as PbC ₆ H ₄ SI ^- and PbC ₆ H ₃ S ₂ O ₃ ^- , as well as PbSI ^- containing iodine I.

In addition, as peaks not shown, C ₆ H ₄ S ₂ O ^- , C ₆ H ₄ S ₂ O ₃ ^- , and C ₆ H ₄ S ₂ O ₄ ^- are included in large quantities. These peaks are derived from the organic ligand 1,4-BDT, and indicate that a part of the thiol group has been oxidized. Since this oxidation is caused by oxygen from the external environment, 1,4-BDT acts as an oxidizing sacrificial agent that captures oxygen and plays a role in reducing the oxidation of the surface of the nanoparticles that are the core of the quantum dots. In addition, since the benzene ring acts as a hopping site that transfers carriers, the dark current can also be reduced by oxidizing the benzene ring.

Note that each of the above embodiments merely shows an example of how the present invention can be realized, and the technical scope of the present invention should not be interpreted in a limiting manner based on these. In other words, the present invention can be implemented in various forms without departing from its technical concept or main features.

REFERENCE SIGNS LIST 1 Substrate 2 First electrode 3 First interface layer 4 Photoelectric conversion layer 5 Second interface layer 6 Second electrode 7 Quantum dot 8 Organic ligand 9 Inorganic ligand

Claims (10)

The method includes the steps of: preparing a first electrode; providing quantum dots on the first electrode, the quantum dots being manufactured so that the molar ratio of inorganic ligands is greater than the molar ratio of organic ligands, thereby forming a photoelectric conversion layer; and forming a second electrode on the photoelectric conversion layer.
The method further comprises an oxidation step of oxidizing the surface of the quantum dots,
A method for producing a photoelectric conversion element, wherein an amount of the inorganic ligand added relative to a total amount of the inorganic ligand added and an amount of the organic ligand added is 25% or more and 99.8% or less. The method for manufacturing a photoelectric conversion element according to claim 1, characterized in that the inorganic ligand is applied in a liquid phase, and the organic ligand is applied by applying the organic ligand onto the quantum dots applied onto the first electrode. The method for producing a photoelectric conversion element according to claim 1 or 2, characterized in that the amount of the inorganic ligand added is 75% or more and 98% or less of the total amount of the inorganic ligand added and the organic ligand added. The method for producing a photoelectric conversion element according to any one of claims 1 to 3, characterized in that the molar ratio of the inorganic ligand to the total of the inorganic ligand and the organic ligand is 75% or more. The method for producing a photoelectric conversion element according to any one of claims 1 to 4, characterized in that the inorganic ligand has a halogen atom. The method for producing a photoelectric conversion element according to any one of claims 1 to 5, characterized in that the organic ligand has at least two cross-linked structures. The method for producing a photoelectric conversion element according to claim 6, characterized in that the cross-linked structure is selected from a thiol group and a hydroxyl group. The method for producing a photoelectric conversion element according to any one of claims 1 to 7, characterized in that the organic ligand is selected from 1,3-benzenedithiol, 1,4-benzenedithiol, 3-mercaptobenzoic acid, and 4-mercaptobenzoic acid. 9. The method for producing a photoelectric conversion element according to claim 1, wherein the inorganic particles constituting the quantum dots are particles containing Pb. The method for manufacturing a photoelectric conversion element according to any one of claims 1 to 9, characterized in that the photoelectric conversion layer has the quantum dots and second quantum dots different from the quantum dots , and the quantum dots and the second quantum dots are crosslinked by the organic ligands.

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