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

Abstract

The objective is to provide titanium dioxide-containing nanoparticles that can achieve excellent power conversion efficiency (PCE), a dispersion containing the nanoparticles, a thin film containing the nanoparticles, and a photoelectric conversion element that includes the nanoparticles in a hole transport layer. [Solution] The nanoparticles of the present invention are nickel oxide nanoparticles, having a particle size smaller than 20 nm and an electrical resistivity of 2.0 × 10 5 The nanoparticles of the present invention are characterized by having a density of Ω·cm or less. Preferably, the transmittance of the nanoparticles of the present invention is higher than 90% in the wavelength band of 380 nm to 400 nm. Preferably, the nanoparticles of the present invention are used in the hole transport layer of a perovskite solar cell.

Description

[Technical Field]

[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. [Background technology]

[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. Therefore, they are expected to be a promising component of renewable energy. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] 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) [Non-Patent Document 2] Film-type perovskite solar cell module enabling diverse installation configurations for photovoltaic (Toshiba Review Vol. 76 No. 3, May 2021) [Overview of the Initiative] [Problems that the invention aims to solve]

[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. [Means for solving the problem]

[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. [Effects of the Invention]

[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 perovskite solar cells, high energy conversion efficiency (PCE) can be obtained. [Brief explanation of the drawing]

[0009] [Figure 1] This is a partial cross-sectional view of the solar cell of this embodiment. [Figure 2] It exhibits a perovskite structure. [Modes for carrying out the invention]

[0010] Hereinafter, an embodiment of the present invention (hereinafter abbreviated as "embodiment") will be described in detail. Note that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist thereof.

[0011] <Overview description of solar cell 101> FIG. 1 is a partial cross-sectional view showing an example of the solar cell 101 of the present embodiment. The solar cell 101 shown in FIG. 1 includes a first electrode 41 which is a cathode and a second electrode 42 which is an 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. Note that the stacking structure may be reversed.

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

[0013] The second electrode 42 has conductivity. Among the first electrode 41 and the second electrode 42, if the electrode on the light-incident side has translucency, the other electrode may not have translucency. For example, the second electrode 42 is formed of a material having transparency and conductivity.

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

[0015] In the present embodiment, the electron transport layer 43 preferably contains at least one of ZnO, MgZnO, or SnO2. The hole transport layer 45 will be described later.

[0016] In the present embodiment, the oxygen deficiency of the metal oxide constituting the electron transport layer 43 and the hole transport layer 45 is preferably smaller than that of the bulk material. That is, it is preferably a stoichiometric composition or close to a stoichiometric composition. The "bulk material" refers to a massive material containing the metal oxide, and the size and shape are not limited. 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 (holes) upon receiving sunlight may be any of i-type, n-type, and 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 having a transparent electrode formed on the side of the electron transport layer 43 so that sunlight can be incident is called an n-i-p type solar cell, and the case where it is formed on the side of the hole transport layer 45 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 FIG. 2. For example, M shown in FIG. 2 is Pb, O is Br or I, and R is NH3CH3.

[0019] Perovskite solar cells have sensitivity throughout the visible light region and are excellent in power generation efficiency. Also, compared with conventional silicon-based solar cells, they have the characteristic that the power generation efficiency dependence on the intensity (illuminance) of incident light is small. That is, they can be used not only outdoors but also indoors.

[0020] By using a perovskite semiconductor for the active layer 44, low-temperature processes such as coating, which were difficult to achieve with conventional silicon semiconductors, can be applied. In silicon solar cells, there are limitations on the installation area due to load problems. Therefore, in recent years, perovskite solar cells that can eliminate the limitations on the installation area and have high power generation efficiency are expected.

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

[0022] <Regarding the 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 (one particle), but they may also be in an aggregated state of multiple particles. However, in the case of aggregates, it is preferable to identify the grain boundaries to determine a single particle and then measure its particle size. The nickel oxide component contained 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 x This can be shown as follows. Nickel oxide is preferably in a stoichiometric composition, and x is preferably 2, but the Ni valency may be a value between 2 and 3. In the example, the Ni valency is greater than 2 and less than 2.5, preferably between 2.1 and 2.4. This allows the NiO of this embodiment to be expressed as follows. x It can be proven to be a whole carrier.

[0025] "Nanoparticles" refer to particles with a particle size on the order of nanometers. Particle size can be measured using, for example, a scanning transmission electron microscope (STEM). In the measurement, the particle sizes of multiple nanoparticles are measured, and the average particle size is taken as the "particle size." There is no limit to the number of nanoparticles to be measured, but for example, it is around 10 to 1000. If the particle is approximately spherical, the particle size can be determined from its diameter. Alternatively, if the particle is not spherical but elliptical, rectangular, irregular, etc., the average of the long side and the short side can be considered as the particle size. Furthermore, in thin films containing nanoparticles, the particle size may appear very small depending on the cutting location, so extremely small particle sizes may be thinned out, or if the number of particle sizes appearing in the cutting is not sufficient to obtain an average particle size, the thin film may be cut at multiple locations, and the average particle size of the particles appearing on the cut surfaces may be obtained.

[0026] Furthermore, in the case of a dispersion of nickel oxide nanoparticles, the particle size can be specified by dynamic light scattering. Dynamic light scattering is a method that derives the particle size (particle diameter) based on fluctuations in scattered light intensity, which depend on the Brownian motion of the particles, detected when a laser beam is irradiated onto a solution in which particles are dispersed and the change in scattered light is measured. The particle diameter is the average particle diameter of multiple nanoparticles and can be expressed as D50.

[0027] In this embodiment, the particle size of the nanoparticles is preferably 15 nm or less.

[0028] Furthermore, although this embodiment does not limit the lower limit of the particle size of the nanoparticles, it is preferably 0.1 nm or larger, and can be 1 nm or larger, or even 2 nm or larger. This allows the measurement limit to be exceeded.

[0029] In this embodiment, the electrical resistivity is 2.0 × 10⁻⁶ 5It is below Ω·cm. The electrical resistivity can be regarded as the electrical resistivity of the nanoparticles by 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 1.5×10 5 It is preferably below Ω·cm, and more preferably 5 below 1.0×10 Ω·cm, and even more preferably 4 below 8.0×10 Ω·cm, and even more preferably 4 below 7.5×10 Ω·cm, and even more preferably below this value.

[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 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> The elements required for the hole transport layer are 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 less than 20 nm, and the electrical resistivity is 2.0 × 10⁻⁶ 5 The resistance can be set to a low level of Ω·cm or less. By forming a thin film of the hole transport layer with nickel oxide nanoparticles of this embodiment, the electrical resistance of the hole transport layer can be kept low, and excellent power generation characteristics can be obtained. In this embodiment, the power generation characteristics can be evaluated by the photoelectric conversion efficiency (power generation efficiency: PCE). In this embodiment, the photoelectric conversion efficiency can be set to 10% or more, preferably 14% or more, more preferably 15% or more, and even more preferably 18% or more.

[0034] Furthermore, it is preferable that the hole transport layer has excellent light transmittance. Light transmittance can be expressed as transmittance. If the hole transport layer is located on the incident light side of the active layer, high transmittance allows light to be properly guided to the active layer, thereby achieving high photoelectric conversion efficiency. Also, if 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 high transmittance. In this embodiment, it can be particularly preferably applied as a hole transport layer in a perovskite solar cell.

[0035] <Regarding the form containing nickel oxide nanoparticles of this embodiment> In this embodiment, nickel oxide nanoparticles, a dispersion containing 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 nickel oxide nanoparticles can be measured in a thin film state, and the measurement results can be considered as the electrical resistance and transmittance of the nickel oxide nanoparticles. In the case of dispersions, the solvent is not limited, but water or alcohols such as ethanol can be suggested as solvents.

[0037] The thin film can be formed by depositing a dispersion containing nickel oxide nanoparticles onto a substrate and performing a predetermined drying process. The thin film containing nickel oxide nanoparticles can be used as a hole transport layer. Furthermore, the electrical resistivity of the thin film can be measured and considered as the electrical resistivity of the nickel oxide nanoparticles. The thickness of the thin film is not limited, but it is preferably 100 nm or less, and more preferably between 50 nm and 100 nm.

[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. While not limited to specific nickel compounds, examples include nickel hydroxide, nickel sulfate, nickel carbonate, and nickel nitrate.

[0040] The nickel compound is dissolved in a solvent, and alkaline components are mixed in. The mixture is washed with alcohol, and the solvent is added and heated. Next, the material can be washed with alcohol or ultrasound, and then, for example, an amine can be added and dispersed in alcohol to obtain a dispersion. [Examples]

[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 solution to obtain nickel hydroxide. Then, the solution 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, PCBM 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> The particle size distribution of nickel oxide nanoparticles in a nickel oxide dispersion was measured using a particle size analyzer. The measurement principle is dynamic light scattering.

[0048] <Method for measuring electrical resistivity> The prepared nickel oxide dispersion was applied to 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. Then, the electrical resistivity (volume resistivity) was measured using a high-resistivity resistivity meter, the Hi-Lester UX (manufactured by Nitto Seiko Co., Ltd.).

[0049] <Method for measuring transmittance> Using the nickel oxide nanoparticle thin film prepared by 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 cells was evaluated. The evaluation was performed using the following equipment.

[0051] A power supply (KEITHLEY, Model 236) was connected between the electrodes, and the photoelectric conversion efficiency was measured using a single-source solar simulator with an intensity of 100 mW / cm2. 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 density was Ω·cm. Furthermore, the transmittance at wavelengths of 380-400nm 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, the solution was heated at approximately 265°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 NiOx dispersion. The fabrication of the perovskite solar cell was carried out 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 density was Ω·cm. The transmittance at wavelengths of 380-400nm 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, the solution was heated at approximately 260°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 NiOx dispersion. The fabrication of the perovskite solar cell was carried out 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 density was Ω·cm. The transmittance at wavelengths of 380-400nm 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, the solution was heated at approximately 255°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 NiOx dispersion. The fabrication of the perovskite solar cell was carried out 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. 5 The density was Ω·cm. The transmittance at wavelengths of 380-400nm 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, the solution was heated at approximately 280°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 NiOx dispersion. The fabrication of the perovskite solar cell was carried out 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. 6The capacitance was Ω·cm. Furthermore, the transmittance at wavelengths of 380-400nm was 90%. Power generation characteristics could not be confirmed. The experimental results for Examples 1-4 and Comparative Example 1 are summarized in Table 1.

[0061] [Table 1]

[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. [Explanation of Symbols]

[0063] 41: 1st electrode 42: 2nd electrode 43: Electron transport layer 44:Active layer 45: Hole transport layer 46: Glass substrate 101: Solar cell

Claims

1. The particle size is smaller than 20 nm. The electrical resistivity is 2.0 × 10⁻⁶. ５ It is less than or equal to Ω·cm. Nickel oxide nanoparticles characterized by the following features.

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

3. 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%.

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

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

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

7. Nickel oxide nanoparticles or thin films according to claim 1 or 5, characterized in that they are used in the hole transport layer of a perovskite solar cell.

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

9. The photoelectric conversion element comprises an electron transport layer, an active layer, and a hole transport layer, wherein the active layer is a solar cell containing a perovskite semiconductor, and has a photoelectric conversion efficiency of 10% or more under sunlight. The photoelectric conversion element according to feature 8.

Images