Nickel oxide nanoparticles, dispersion liquid, thin film, and photoelectric conversion element

Abstract

The purpose of the present invention is to provide nanoparticles containing nickel oxide with which it is possible to obtain excellent energy conversion efficiency (PCE), a dispersion liquid containing the nanoparticles, a thin film containing the nanoparticles, and a photoelectric conversion element containing the nanoparticles in a hole transport layer. The nanoparticles according to the present invention are nickel oxide nanoparticles and are characterized by having a particle diameter of less than 20 nm and an electrical resistivity of less than or equal to 2.0 × 105 Ω·cm. The nanoparticles according to the present invention preferably have a transmittance of greater than 90% in a wavelength band of 380-400 nm. The nanoparticles according to the present invention are preferably used in a hole transport layer of a perovskite solar cell.

Description

Nickel oxide nanoparticles, dispersions, thin films, and photoelectric conversion elements

[0001] The present invention relates to nickel oxide nanoparticles, a dispersion containing the nickel oxide nanoparticles, a thin film containing the nickel oxide nanoparticles, and a photoelectric conversion element containing the nickel oxide nanoparticles in a hole transport layer.

[0002] Perovskite solar cells are known, as described in Non-Patent Documents 1 and 2. Perovskite solar cells are lightweight, can be installed in a variety of configurations, and offer a high degree of flexibility in installation areas, making them promising as part of renewable energy.

[0003] Phase-Engineering of Layered Nickel Hydroxide for Synthesizing High-Quality NiOx Nanocrystals for Efficient Inverted Flexible Perovskite Solar Cells (ACS Appl. Mater. Interfaces 2023, 15, 38444-38453) Film-type perovskite solar cell module enabling diverse installation configurations of PV (Toshiba Review Vol. 76 No. 3 May 2021)

[0004] Non-patent document 1 describes the use of nickel oxide as a hole transport layer. However, it has been found that the power generation characteristics may be insufficient depending on the particle morphology and physical properties of the nickel oxide.

[0005] The present invention has been made in view of the above problems, and aims to provide nickel oxide nanoparticles that exhibit excellent functionality as a metal oxide constituting a hole transport layer by appropriately controlling the particle size and electrical resistivity of the nickel oxide nanoparticles, and in particular, that can be used in perovskite solar cells to obtain excellent energy conversion efficiency (PCE), a dispersion containing the nickel oxide nanoparticles, a thin film containing the nickel oxide nanoparticles, and a photoelectric conversion element containing the nickel oxide nanoparticles in the hole transport layer.

[0006] The nickel oxide nanoparticles of the present invention have a particle size smaller than 20 nm and an electrical resistivity of 2.0 × 10⁻⁶. ５ It is characterized by being less than or equal to Ω·cm.

[0007] Furthermore, the photoelectric conversion element of the present invention has a particle size smaller than 20 nm and an electrical resistivity of 2.0 × 10⁻⁶ ５ It is characterized by having a hole transport layer containing nickel oxide nanoparticles with a size of Ω·cm or less.

[0008] According to the nanoparticles of the present invention, by containing nickel oxide and adjusting the particle size and electrical resistivity to be within a predetermined range, a thin film with low electrical resistivity can be formed, exhibiting the excellent hole transport function required as a hole transport layer for photoelectric conversion elements. In particular, by using it as a hole transport layer in a perovskite solar cell, a high energy conversion efficiency (PCE) can be obtained.

[0009] This is a partial cross-sectional view of the solar cell of this embodiment. It shows a perovskite structure.

[0010] The present invention will be described in detail below in the form of the following embodiment (hereinafter abbreviated as "Embodiment"). However, the present invention is not limited to the following embodiment and can be implemented in various ways within the scope of its gist.

[0011] <Overview of Solar Cell 101> Figure 1 is a partial cross-sectional view showing an example of the solar cell 101 of this embodiment. The solar cell 101 shown in Figure 1 comprises a first electrode 41 which is the cathode and a second electrode 42 which is the anode, and an electron transport layer 43, an active layer 44, and a hole transport layer 45 are provided between the first electrode 41 and the second electrode 42. The stacked structure may be reversed.

[0012] For example, the first electrode 41 is formed on a glass substrate 46. A plastic substrate or film may be used instead of the glass substrate 46. The substrate and film are preferably transparent.

[0013] The second electrode 42 is conductive. Of the first electrode 41 and the second electrode 42, if the electrode on the side into which light is incident is translucent, the other electrode does not need to be translucent. For example, the second electrode 42 is made of a transparent and conductive material.

[0014] The first electrode 41 and the second electrode 42 have the function of collecting holes and electrons generated by light absorption in the active layer 44. This allows electricity to be generated.

[0015] In this embodiment, the electron transport layer 43 is made of ZnO, MgZnO, or SnO ２ It is preferable to include at least one of the following. The hole transport layer 45 will be described later.

[0016] In this embodiment, it is preferable that the oxygen vacancies in the metal oxides constituting the electron transport layer 43 and the hole transport layer 45 are smaller than those in the bulk material. That is, it is preferable that the composition is stoichiometric or close to stoichiometric. "Bulk material" refers to a lump containing the metal oxide, and its size and shape are not considered. In this embodiment, the active layer 44 is a photoelectric conversion layer that absorbs light incident on the solar cell 101 and generates electrons and holes.

[0017] In the solar cell 101, the semiconductor material of the active layer 44 that generates electrons and holes when exposed to sunlight may be i-type, n-type, or p-type. The electron transport layer 43 is an n-type semiconductor, and the hole transport layer is a p-type semiconductor. A solar cell 101 in which a transparent electrode is formed on the electron transport layer 43 side to allow sunlight to enter is called an n-i-p type solar cell, and when it is formed on the hole transport layer 45 side, it is called a p-i-n type solar cell.

[0018] In this embodiment, a perovskite semiconductor is used for the active layer 44. The structure of the perovskite semiconductor is shown in Figure 2. For example, in Figure 2, M is Pb, O is Br or I, and R is NH ３ CH ３ That is the case.

[0019] Perovskite solar cells are sensitive to the entire visible light spectrum and offer excellent power generation efficiency. Furthermore, compared to conventional silicon solar cells, they have a lower dependence on the intensity of incident light (illuminance) for power generation efficiency. This means they can be used in both outdoor and indoor applications.

[0020] By using a perovskite semiconductor in the active layer 44, low-temperature processes such as coating, which were difficult to achieve with conventional silicon semiconductors, become applicable. Silicon solar cells have limitations on installation area due to load issues. For this reason, perovskite solar cells, which can overcome installation area limitations and have high power generation efficiency, have been highly anticipated in recent years.

[0021] <Background to this embodiment> Organic compounds and inorganic materials have been reported in papers and other publications as hole transport layers for perovskite solar cells, but organic compounds have durability issues. On the other hand, as an inorganic material, nickel oxide (NiO) ｘ The application of this technology is expected. However, as a result of diligent research by the inventors, it has been found that the power generation characteristics may be insufficient depending on the particle morphology and physical properties of nickel oxide.

[0022] <Characteristic Configuration of This Embodiment> In this embodiment, with respect to nickel oxide nanoparticles, the particle size is less than 20 nm, and the electrical resistance is 2.0 × 10⁻⁶. ５ It is characterized by being less than or equal to Ω·cm.

[0023] "Nickel oxide nanoparticles" refer to nanoparticles whose main component is nickel oxide, but which may also contain other minor or unavoidable components. Compositional analysis can be performed by X-ray diffraction (XRD). Nickel oxide nanoparticles may be single particles, but they may also be aggregates of multiple particles. However, in the case of aggregates, it is preferable to identify the grain boundaries to determine each individual particle and then measure its particle size. The nickel oxide component in nickel oxide nanoparticles accounts for 90% or more of the total nanoparticles, preferably 95% or more, more preferably 97% or more, and most preferably 99% or more.

[0024] Nickel oxide is NiO ｘIt can be shown by. Nickel oxide preferably has a stoichiometric composition, and x is preferably 2, but the Ni valence may be a value between divalent and trivalent. In the examples, the Ni valence is greater than 2 and less than 2.5, preferably 2.1 or more and 2.4 or less. Thereby, the NiO of the present embodiment ｘ can be proven to be a hole carrier.

[0025] "Nanoparticle" means a particle having a particle diameter on the nano order. For the measurement of the particle diameter, for example, a scanning transmission electron microscope (STEM) can be used. In the measurement, the particle diameters of a plurality of nanoparticles can be measured, and the average particle diameter can be defined as the "particle diameter". The number of nanoparticles to be measured is not limited, but for example, it is about 10 to 1000. When the particle is substantially spherical, the particle diameter can be specified from its diameter. Alternatively, when the particle is non-spherical such as elliptical, rectangular, or irregular, the average value of the long side and the short side can be regarded as the particle diameter. In addition, in a thin film containing nanoparticles, the particle diameter may appear extremely small depending on the cutting location. Therefore, if the extremely small particle diameters are thinned out or the number of particle diameters appearing in the cut is not a sufficient sample number to obtain the average particle diameter, the thin film is cut at a plurality of locations, and the average value of the particle diameters of the particles appearing on the cut surface is obtained.

[0026] Also, in the dispersion of nickel oxide nanoparticles, the particle diameter measured by the dynamic light scattering method can be defined. The "dynamic light scattering method" is a method of deriving the size (particle diameter) of particles based on the fluctuation of the scattered light intensity depending on the Brownian motion of the particles detected when a laser beam is irradiated on a solution in which the particles are dispersed and the change in the scattered light is measured. The particle diameter is the average particle diameter of a plurality of nanoparticles and can be represented by D50.

[0027] The particle diameter of the nanoparticles of the present embodiment is preferably 15 nm or less.

[0028] Also, in the present embodiment, the lower limit value of the particle diameter of the nanoparticles is not limited, but it is preferably 0.1 nm or more, can be 1 nm or more, or can be 2 nm or more. Thereby, the measurement limit can be exceeded.

[0029] In this embodiment, the electrical resistivity is 2.0×10 ５ Ω·cm or less. The electrical resistivity can be regarded as the electrical resistivity of the nanoparticles by, for example, forming a thin film of nickel oxide nanoparticles and measuring the electrical resistivity of the thin film. The electrical resistivity is the volume resistivity. Although the film thickness of the thin film is not limited, it is preferably 100 nm or less. This film thickness also matches the film thickness of the hole transport layer and can be regarded as the electrical resistivity of the hole transport layer. Although the lower limit value of the thin film is not limited, it is preferably 10 nm or more, and may be 30 nm or more, or 50 nm or more.

[0030] The electrical resistivity is preferably 1.5×10 ５ Ω·cm or less, more preferably 1.0×10 ５ Ω·cm or less, even more preferably 8.0×10 ４ Ω·cm or less, and even more preferably 7.5×10 ４ Ω·cm or less.

[0031] In this embodiment, it is preferable that the transmittance in the wavelength band of 380 nm or more and 400 nm or less of the nickel oxide nanoparticles is higher than 90%. The transmittance can be regarded as the transmittance of the nickel oxide nanoparticles by, for example, forming a thin film of the nickel oxide nanoparticles and measuring the transmittance of the thin film. The film thickness of the thin film is as described in the measurement of the above electrical resistivity. The transmittance is more preferably 95% or more, and even more preferably 97% or more. Thus, by using the nanoparticles of nickel oxide of this embodiment, a thin film with low electrical resistivity and high transmittance can be formed.

[0032] <Regarding the hole transport layer> Elements required for the hole transport layer are the energy level, electrical resistivity, and light transmittance. In the hole transport layer using nickel oxide, it is known that the HOMO level is compatible with the perovskite semiconductor, and nickel oxide is a metal oxide suitable for the hole transport layer.

[0033] In this embodiment, the particle size of the nickel oxide nanoparticles is adjusted to be smaller than 20 nm, and the electrical resistivity is 2.0×10 ５It can be set low to below Ω·cm. By forming a thin film of the hole transport layer with the nickel oxide nanoparticles of the present embodiment, the electrical resistance value of the hole transport layer can be kept low, and excellent power generation characteristics can be obtained. In the present embodiment, the power generation characteristics can be evaluated by the photoelectric conversion efficiency (power generation efficiency: PCE). In the present embodiment, the photoelectric conversion efficiency can be 10% or more, preferably 14% or more, more preferably 15% or more, and still more preferably 18% or more.

[0034] Also, the hole transport layer preferably has excellent light transmittance. The light transmittance can be indicated by the transmittance. When the hole transport layer is located on the incident light side of the active layer, by having a high transmittance, light can be appropriately guided to the active layer, and high photoelectric conversion efficiency can be obtained. Also, when the electron transport layer is located on the incident light side of the active layer, it is necessary for the electron transport layer to have a high transmittance. In the present embodiment, it can be preferably applied particularly as the hole transport layer of a perovskite solar cell.

[0035] <Regarding the form containing the nickel oxide nanoparticles of the present embodiment> In the present embodiment, nickel oxide nanoparticles, a dispersion liquid having the nickel oxide nanoparticles, a thin film using the nickel oxide nanoparticles, a hole transport layer, and a photoelectric conversion element can be provided.

[0036] As described above, the electrical resistivity and transmittance of the nickel oxide nanoparticles are measured in the state of a thin film, and the measurement results can be regarded as the electrical resistance value and transmittance of the nickel oxide nanoparticles. In the case of the dispersion liquid, although the solvent is not limited, alcohols such as water and ethanol can be presented as the solvent.

[0037] The thin film can be formed by forming a dispersion liquid containing nickel oxide nanoparticles on a substrate and performing a predetermined drying treatment. The thin film containing nickel oxide nanoparticles can be used as the hole transport layer. Also, by using the thin film, the electrical resistivity can be measured and regarded as the electrical resistivity of the nickel oxide nanoparticles. Although the film thickness of the thin film is not limited, it is preferably 100 nm or less, and more preferably 50 nm or more and 100 nm or less.

[0038] The nickel oxide nanoparticles of this embodiment can have low electrical resistivity and are therefore preferably applied to the hole transport layer of a photoelectric conversion element. In particular, when used as the hole transport layer of a perovskite solar cell, the energy levels can be matched with those of the perovskite semiconductor, and excellent power generation characteristics can be obtained.

[0039] <Method for producing nickel oxide nanoparticles> Prepare a nickel compound and heat an aqueous solution containing the nickel compound. The nickel compound is not limited, but examples include nickel hydroxide, nickel sulfate, nickel carbonate, nickel nitrate, etc.

[0040] Nickel compounds are dissolved in a solvent, and alkaline components are mixed in. The mixture is washed with alcohol, a solvent is added, and the mixture is heated. Subsequently, the mixture is washed with alcohol or ultrasound, and then, for example, an amine is added and dispersed in alcohol to obtain a dispersion.

[0041] The effects of the present invention will be explained below with reference to examples and comparative examples of the present invention. However, the present invention is not limited in any way by the following examples.

[0042] In the experiment, dispersions of nickel oxide nanoparticles with different particle sizes and electrical resistivity were prepared.

[0043] <Example 1> Nickel nitrate aqueous solution was dissolved in a solvent, and sodium hydroxide aqueous solution was added to the aqueous solution to obtain nickel hydroxide. Then, it was heated at approximately 270°C for 3 hours to obtain nickel oxide nanoparticles. Water was added to the obtained nickel oxide nanoparticles, and ultrasonic dispersion was performed to prepare a nickel oxide dispersion in water.

[0044] Next, a nickel oxide dispersion was deposited onto an ITO substrate by spin coating to a thickness of approximately 50 nm to 100 nm, and then dried on a hot plate at 100°C for 10 minutes. This yielded a hole transport layer.

[0045] Lead iodide was deposited on the surface of the hole transport layer by spin coating, methylammonium iodide was applied, and the lead iodide film and methylammonium iodide reacted to form an active layer (perovskite layer) consisting of lead iodide methylammonium.

[0046] Next, a PCB was deposited on the surface of the active layer to obtain an electron transport layer. Silver was deposited on the surface of the electron transport layer as the upper electrode to fabricate a perovskite solar cell.

[0047] <Method for measuring particle size> Nickel oxide nanoparticles in a nickel oxide dispersion were measured using a particle size distribution analyzer. The measurement principle is dynamic light scattering.

[0048] <Method for measuring electrical resistivity> The prepared nickel oxide dispersion was coated onto a glass substrate by spin coating to a thickness of approximately 100 nm, and then fired at 100°C for 30 minutes to form a thin film of nickel oxide nanoparticles. The electrical resistivity (volume resistivity) was then measured using a high-resistivity resistivity meter, Hi-Lester UX (manufactured by Nitto Seiko Co., Ltd.).

[0049] <Method for measuring transmittance> Using the thin film of nickel oxide nanoparticles prepared in the above electrical resistivity measurement, the transmittance of the thin film (wavelength 380-400 nm) was measured using a UV-Vis-Infrared spectrophotometer UH4150 (manufactured by Hitachi High-Tech Corporation).

[0050] <Evaluation of Power Generation Characteristics> The photoelectric conversion efficiency of the fabricated perovskite solar cell was evaluated. The evaluation was carried out using the following equipment.

[0051] A power supply (KEITHLEY, Model 236) is connected between the electrodes, and the intensity is set to 100 mW / cm. ２ The photoelectric conversion efficiency was measured using a single-source solar simulator. These measured values ​​were calculated as the average photoelectric conversion efficiency (average efficiency).

[0052] <Experimental Results of Example 1> In Example 1, the particle size of the nickel oxide nanoparticles was 15 nm, and the electrical resistivity was 3 × 10⁻⁶. ４ The pressure was Ω·cm. The transmittance at wavelengths of 380-400 nm was 98%. The photoelectric conversion efficiency (PCE) was 20%.

[0053] <Example 2 and its experimental results> In Example 2, an aqueous solution of sodium hydroxide was added to an aqueous solution of nickel nitrate to obtain nickel hydroxide. Then, it was heated at approximately 265°C for 3 hours to obtain nickel oxide nanoparticles. Water was added to the obtained nickel oxide nanoparticles and dispersed by ultrasound, and NiO ｘ A dispersion was prepared. The perovskite solar cell was fabricated in the same manner as in Example 1.

[0054] In Example 2, the particle size of the nickel oxide nanoparticles was 12 nm, and the electrical resistivity was 5.5 × 10⁻⁶. ４ The pressure was Ω·cm. The transmittance at wavelengths of 380-400 nm was 97%. The photoelectric conversion efficiency (PCE) was 19%.

[0055] <Example 3 and its experimental results> In Example 3, an aqueous solution of sodium hydroxide was added to an aqueous solution of nickel nitrate to obtain nickel hydroxide. Then, it was heated at approximately 260°C for 3 hours to obtain nickel oxide nanoparticles. Water was added to the obtained nickel oxide nanoparticles and ultrasonically dispersed, and NiO ｘ A dispersion was prepared. The perovskite solar cell was fabricated in the same manner as in Example 1.

[0056] In Example 3, the particle size of the nickel oxide nanoparticles was 10 nm, and the electrical resistivity was 7.5 × 10⁻⁶. ４ The pressure was Ω·cm. The transmittance at wavelengths of 380-400 nm was 97%. The photoelectric conversion efficiency (PCE) was 18%.

[0057] <Example 4 and its experimental results> In Example 4, an aqueous solution of sodium hydroxide was added to an aqueous solution of nickel nitrate to obtain nickel hydroxide. Then, it was heated at approximately 255°C for 3 hours to obtain nickel oxide nanoparticles. Water was added to the obtained nickel oxide nanoparticles and ultrasonically dispersed, and NiO ｘ A dispersion was prepared. The perovskite solar cell was fabricated in the same manner as in Example 1.

[0058] In Example 4, the particle size of the nickel oxide nanoparticles was 9 nm, and the electrical resistivity was 1.3 × 10⁻⁶. ５The pressure was Ω·cm. The transmittance at wavelengths of 380-400 nm was 96%. The photoelectric conversion efficiency (PCE) was 14%.

[0059] <Comparative Example 1 and its experimental results> In Comparative Example 1, an aqueous solution of sodium hydroxide was added to an aqueous solution of nickel nitrate to obtain nickel hydroxide. Then, it was heated at approximately 280°C for 3 hours to obtain nickel oxide nanoparticles. Water was added to the obtained nickel oxide nanoparticles and dispersed by ultrasound, and NiO ｘ A dispersion was prepared. The perovskite solar cell was fabricated in the same manner as in Example 1.

[0060] In Comparative Example 1, the particle size of the nickel oxide nanoparticles was 20 nm, and the electrical resistivity was 7.3 × 10⁻⁶. ６ The pressure was Ω·cm. Furthermore, the transmittance at wavelengths of 380-400 nm was 90%. Power generation characteristics could not be confirmed. Table 1 summarizes the experimental results for Examples 1-4 and Comparative Example 1.

[0061]

[0062] From the experimental results described above, it was found that in Examples 1 to 4, a photoelectric conversion efficiency (PCE) of 10% or more could be obtained, and that by using nickel oxide nanoparticles as the hole transport layer in these examples, a perovskite solar cell with excellent power generation characteristics can be provided.

[0063] This application is based on Japanese Patent Application No. 2024-231454, filed on December 27, 2024. All of its contents are included here.

Claims

1. The particle size is less than 20 nm, and the electrical resistivity is 2.0 × 10⁻⁶. ５ Nickel oxide nanoparticles characterized by having a size of Ω·cm or less.

2. The particle size is 15 nm or less, and the electrical resistivity is 1.0 × 10⁻⁶. ５ Nickel oxide nanoparticles according to claim 1, characterized in that they are Ω·cm or less.

3. The particle size is 10 nm or more and 15 nm or less, and the electrical resistivity is 3 × 10 ４ Ω・cm or more 7.5×10 ４ Nickel oxide nanoparticles according to claim 1, characterized in that they are Ω·cm or less.

4. Nickel oxide nanoparticles according to claim 1, characterized in that the transmittance in the wavelength band of 380 nm to 400 nm is higher than 90%.

5. A dispersion comprising nickel oxide nanoparticles as described in claim 1.

6. A thin film comprising nickel oxide nanoparticles as described in claim 1.

7. The thin film according to claim 5, characterized in that the film thickness is 100 nm or less.

8. The thin film according to claim 7, characterized in that the film thickness is 50 nm or more and 100 nm or less.

9. Nickel oxide nanoparticles or thin films according to claim 1 or 6, characterized in that they are used in the hole transport layer of a perovskite solar cell.

10. The particle size measured by dynamic light scattering is less than 20 nm, and the electrical resistivity is 2.0 × 10⁻⁶. ５ A photoelectric conversion element characterized by having a hole transport layer containing nickel oxide nanoparticles of Ω·cm or less.

11. The photoelectric conversion element according to claim 10, comprising an electron transport layer, an active layer, and the hole transport layer, wherein the active layer is a solar cell containing a perovskite semiconductor, and the photoelectric conversion efficiency under sunlight is 10% or more.

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