Perovskite solar cell

Abstract

To provide a perovskite solar cell having high photoelectric conversion performance and high operational stability.SOLUTION: A perovskite solar cell includes: a stacked body in which a substrate 2 having a transparent conductive film 22, an electron transport layer 3 transferring electrons to the transparent conductive film 22, a buffer layer 42 containing an electron-donating material, a photoelectric conversion layer 44 excited by light to generate the electrons, and a hole transport layer 5 where holes generated from the photoelectric conversion layer 44 pass are stacked in this order; and an electrode 6 comprising a photoelectrode 61 emitting the electrons through the transparent conductive film 22 and a counter electrode 62 provided on a surface of the hole transport layer 5.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to perovskite solar cells. [Background technology]

[0002] While solar cells generally use silicon-based, compound semiconductor, or organic semiconductor elements, perovskite solar cells have attracted attention due to their high light-harvesting capacity, thin film construction, and low cost. In particular, research into perovskite solar cells, which have excellent photoelectric conversion performance, has been actively conducted in recent years.

[0003] For example, Non-Patent Document 1 discloses that in a normal-structure perovskite solar cell in which a substrate on which a transparent conductive film is formed, an electron transport layer, a photoelectric conversion layer, a hole transport layer, and a counter electrode are stacked in this order, adding lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a dopant to the material of the hole transport layer (hole transport material) can achieve a high photoelectric conversion efficiency of more than 25%.

[0004] As described in Non-Patent Document 1, perovskite solar cells in which a lithium-based dopant is added to the hole-transporting material have poor operational stability due to factors such as hygroscopicity and disruption of the energy structure caused by excessive diffusion of lithium ions, making it difficult to maintain photoelectric conversion performance over the long term.

[0005] Non-Patent Document 2 discloses perovskite solar cells to which 1-butyl-1-methylpyrrolidium bis(trifluoromethanesulfonyl)imide (hereinafter sometimes referred to as "Bmp-TFSI") or N-ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium bis(trifluoromethanesulfonyl)imide (hereinafter sometimes referred to as "Edm-TFSI") is added as a dopant for the hole transport material. Non-Patent Document 2 discloses that when a non-lithium dopant is added, the operational stability is superior to when a lithium dopant is added, but the photoelectric conversion performance is inferior. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Jaewang Park et al., "Controlled growth of perovskite layers with volatile alkylammonium chlorides", Nature 616, pp724-730 (2023) [Non-patent document 2] Kohei Yamamoto et al., "Thermal stability of perovskite solar cells incorporated with spiro-OMeTAD and an ionic liquid dopant", Jpn. J. of Appl. Phys. 63, 02SP82 (2024) Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, it is difficult for conventional perovskite solar cells to achieve both high photoelectric conversion performance and high operational stability. Therefore, there is a demand for perovskite solar cells that have high photoelectric conversion performance and high operational stability. [Means for solving the problem]

[0008] The characteristic configuration of the perovskite solar cell according to the present invention is as follows: a substrate having a transparent conductive film; an electron transport layer that transfers electrons to the transparent conductive film; a buffer layer containing an electron donating material; a photoelectric conversion layer that is excited by light to generate the electrons; a hole transport layer through which holes generated from the photoelectric conversion layer pass; and a laminate in this order. The device is characterized in that it comprises an electrode composed of a photoelectrode that emits the electrons through the transparent conductive film and a counter electrode provided on the surface of the hole transport layer.

[0009] The present inventors focused on the energy diagram of perovskite solar cells and conducted extensive research to achieve the above-mentioned objective. As a result, they discovered new findings and completed the present invention. Specifically, the present inventors inferred that when a single-layer electron transport layer, which has a conduction band edge (CBE) deeper than that of the photoelectric conversion layer, comes into contact with the photoelectric conversion layer, the CBE of the electron transport layer shifts shallower, making it shallower than the CBE of the photoelectric conversion layer, thereby making it difficult for electrons to move from the photoelectric conversion layer to the electron transport layer. They then came up with the idea of ​​suppressing the shallow CBE shift of the electron transport layer and discovered that by disposing a buffer layer between the photoelectric conversion layer and the electron transport layer, high photoelectric conversion performance and high operational stability can be achieved both when a non-lithium-based additive is used as the hole transport material or when neither a non-lithium-based additive nor a lithium-based additive is used, thereby completing the present invention. According to this configuration, the buffer layer is disposed between the photoelectric conversion layer and the electron transport layer, which changes the energy diagram between the electron transport layer and the photoelectric conversion layer, making the CBE of the photoelectric conversion layer shallower than the CBE of the electron transport layer. This facilitates electron migration from the photoelectric conversion layer to the electron transport layer, improving photoelectric conversion performance. Furthermore, since there is no need to use a lithium-based additive in the hole transport material, operational stability is also improved compared to when a lithium-based additive is used. In other words, the perovskite solar cell of the present invention achieves both high photoelectric conversion performance and high operational stability. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic cross-sectional view of a perovskite solar cell. [Figure 2] FIG. 1 is a perspective view from above of a perovskite solar cell. [Figure 3] FIG. 1 is a cross-sectional view illustrating the power generation principle of a perovskite solar cell. [Figure 4]FIG. 1 shows an ideal energy diagram for a perovskite solar cell. [Figure 5] FIG. 1 shows the transition of the energy diagram of a perovskite solar cell using a lithium-based additive as a hole transport material. [Figure 6] FIG. 1 shows the transition of the energy diagram of a perovskite solar cell using a non-lithium additive as a hole transport material. [Figure 7] FIG. 1 is a diagram showing the transition of the energy diagram of the perovskite solar cell of the present embodiment. [Figure 8] FIG. 1 is an explanatory diagram showing a procedure for fabricating a perovskite solar cell. [Figure 9] 1 is a graph showing the relationship between photoelectric conversion efficiency and wavelength in Example 1 and Comparative Example 1. [Figure 10] 1 is a graph showing the relationship between photoelectric conversion efficiency and wavelength in Example 2 and Comparative Example 2. [Figure 11] 10 is a graph showing the relationship between photoelectric conversion efficiency and wavelength in Example 3 and Comparative Example 3. [Figure 12] 10 is a graph showing the relationship between photoelectric conversion efficiency and wavelength in Example 4 and Comparative Example 4. [Figure 13] 1 is a graph showing IV characteristics of Example 1 and Comparative Example 1. [Figure 14] 1 is a graph showing IV characteristics of Example 2 and Comparative Example 2. [Figure 15] 10 is a graph showing IV characteristics of Example 3 and Comparative Example 3. [Figure 16] 10 is a graph showing IV characteristics of Example 4 and Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of a perovskite solar cell according to the present invention will be described with reference to the drawings. In these embodiments, a perovskite solar cell will be described that is configured to include a perovskite layer formed from an organic and inorganic hybrid compound. Note that the embodiments described below are merely examples for explaining the perovskite solar cell of the present invention, and the present invention is not limited to these embodiments. Therefore, the present invention can be implemented in various forms without departing from the spirit and scope of the present invention.

[0012] [Basic configuration of perovskite solar cell 10] As shown in FIGS. 1 and 2 , a perovskite solar cell 10 according to this embodiment (hereinafter simply referred to as “solar cell 10”) includes a substrate 2 and a laminate 11 provided on the substrate 2. The substrate 2 includes a transparent substrate 21 and a transparent conductive film 22. The laminate 11 includes an electron transport layer 3, a power generation layer 4 including a buffer layer 42 and a perovskite layer 44 (an example of a photoelectric conversion layer), and a hole transport layer 5. The electron transport layer 3 is provided on the transparent conductive film 22 and transfers electrons to the transparent conductive film 22, while separating the hole transport layer 5 from the transparent conductive film 22 to prevent recombination of electrons and holes (reverse electron transfer). The power generation layer 4 is provided on the electron transport layer 3 and is formed by laminating, on a porous semiconductor 41, a buffer layer 42 having ion-blocking ability and a perovskite layer 44 that is excited by light to generate electrons. That is, the buffer layer 42 is disposed between the electron transport layer 3 and the perovskite layer 44 (photoelectric conversion layer). The hole transport layer 5 is disposed on the power generation layer 4, through which holes generated in the perovskite layer 44 pass. A photoelectrode 61 that emits electrons is disposed on the surface of the electron transport layer 3 via the transparent conductive film 22, and a counter electrode 62 that receives electrons is disposed on the surface of the hole transport layer 5. The counter electrode 62 is formed from the surface of the hole transport layer 5, passing through the side surfaces of the hole transport layer 5 and the perovskite layer 44, and extending to the surface of the electron transport layer 3. Hereinafter, the photoelectrode 61 and the counter electrode 62 will be collectively referred to as the electrode 6. The arrangement of the electrode 6 is not particularly limited as long as electrons can be transferred; for example, the photoelectrode 61 may be formed by connecting a conductor to the transparent conductive film 22. The counter electrode 62 may be protected by a transparent substrate 21 or the like to enhance the durability of the solar cell 10.

[0013] [Transparent substrate 21] The transparent substrate 21 is made of a light-transmitting material. Examples of the transparent substrate 21 include a transparent glass substrate, a frosted semi-transparent glass substrate, and a transparent resin substrate. Examples of the transparent conductive film 22 include fluorine-doped tin oxide (FTO), tin oxide (TO), tin-doped indium oxide (ITO), zinc oxide (ZnO), and aluminum-doped zinc oxide (AZO).

[0014] [Electron transport layer 3] Metal oxides such as titanium dioxide (TiO2), zinc oxide (ZnO), niobium oxide (Nb2O5), tin dioxide (SnO2), and aluminum oxide (Al2O3) are suitable for the electron transport layer 3. A sintered body of titanium dioxide (TiO2) is particularly preferable. A portion of the electron transport layer 3 extends into a recess 221 formed by removing a portion of the transparent conductive film 22 to form an insulating layer 31, thereby dividing the transparent conductive film 22 into two. In the electron transport layer 3, electrons can move in the stacking direction, but have difficulty moving laterally, perpendicular to the stacking direction. Furthermore, electrons cannot move between the two transparent conductive films 22 separated by the insulating layer 31. In other words, the electrons that enter the electron transport layer 3 move smoothly in the stacking direction of the transparent conductive film 22 and are supplied to the photoelectrode 61, but the insulating layer 31 prevents them from moving laterally toward the counter electrode 62, so that the photoelectrode 61 and the counter electrode 62 are not short-circuited.

[0015] [Power generation layer 4] The porous semiconductor 41 of the power generation layer 4 is suitably made of a metal oxide, such as titanium dioxide (TiO2), zinc oxide (ZnO), niobium oxide (Nb2O5), tin dioxide (SnO2), or aluminum oxide (Al2O3). In particular, it is preferable to use a sintered body of titanium dioxide (TiO2), which can ensure a large surface area for laminating the perovskite layer 44.

[0016] The buffer layer 42 is formed using an electron-donating material and has ion-blocking ability. As will be described later, the buffer layer 42 is a layer that prevents direct contact between the electron transport layer 3 and the perovskite layer 44. Examples of the electron-donating material include fullerene and fullerene derivatives, but there are no particular limitations as long as they are electron donors. The fullerene derivative is preferably PCBM (Phenyl-C61-Butyric-Acid-Methyl-Ester) represented by Chemical Formula 1 below.

[0017] [ka]

[0018] The perovskite layer 44 is an organic-inorganic hybrid compound. Specifically, the perovskite layer 44 is produced by reacting a compound composed of lead and a halogen element X (PbX2, where X = the halogen element) with methylammonium iodide (CH3NH3I; hereinafter, sometimes abbreviated as "MAI"). Specifically, a solution containing lead and the halogen element X (e.g., a solution of PbI2 in N,N-dimethylformamide) is infiltrated into the pores of the porous semiconductor 41, dried, and then immersed in a mixed solution of MAI, whereby crystals of the perovskite compound (CH3NH3PbI3 when X = I) that forms the perovskite layer 44 are rapidly produced. The halogen element X can be iodine, bromine, chlorine, or the like, and iodine, which has high morphological stability, is preferably used. In addition, halogen element X was converted into mixed cation-mixed halide (FAPbI3) using MABr and 0.2M lead bromide (PbBr2), and FAI and lead iodide (PbI2). 1-x (MAPbBr3) x ) may be used. In this case, for example, (FAPbI3) 0.85 (MAPbBr3) 0.15 etc. can be suitably used.

[0019] [Hole transport layer 5] Various hole transport materials can be used for the hole transport layer 5. Examples of hole transport materials include 2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamino)-9,9'-spirobifluorene (commonly known as "spiro-OMeTAD"; hereinafter, this name will be used) represented by the following Chemical Formula 2, and a low-molecular organic material called DHCF-3 represented by the following Chemical Formula 3.

[0020] [ka]

[0021] [ka]

[0022] Furthermore, as a hole transport material, for example, a low-molecular-weight organic material having a so-called DAD structure similar to that of DHCF-3 described above, in which the A moiety is a fluorene ring and two substituted or unsubstituted benzene rings are introduced to the 9-position of the fluorene ring via a hydrocarbon chain linker, may be used. In such a low-molecular-weight organic material, the two benzene rings are arranged perpendicular to the fluorene ring. Furthermore, the linker maintains an appropriate distance between the fluorene ring and the benzene ring, thereby suppressing steric hindrance between the benzene ring and the D moieties introduced at the 2- and 7-positions of the fluorene ring. As a result, in such a low-molecular-weight organic material, smooth π-conjugation between the A moiety composed of a fluorene ring and the D moiety introduced into the A moiety enhances the intramolecular push-pull effect and improves intramolecular and intermolecular charge transfer properties. Furthermore, this small molecule organic material exhibits smoother electron transfer within the molecule, resulting in clearer HOMO-LUMO charge separation, with electrons concentrated in the D portion at the HOMO and LUMO-1 levels and in the A portion at the LUMO level. Therefore, compared with conventional DAD-type structures such as DHCF-3, this small molecule organic material exhibits improved intramolecular and intermolecular charge transfer properties, resulting in stable and sustained excellent hole transport properties. An example is DHCF-32, represented by the following formula (4). DHCF-32 is a compound in which the A portion is a fluorene ring, two p-trifluoromethylbenzyl groups are introduced at the 9-position of the fluorene ring, and di(4-methoxyphenyl)amino groups are introduced at the 2- and 7-positions of the fluorene ring as the D portion. [ka]

[0023] Furthermore, hole transport materials may also be used, for example, low-molecular-weight organic materials incorporating a three-unit stilbene structure in which a dialkylamino group is introduced at the p-position of the terminal benzene ring to the nitrogen atom. Such low-molecular-weight organic materials can appropriately control the push-pull effect within the molecule, resulting in improved intramolecular and intermolecular charge transfer properties. Therefore, such low-molecular-weight organic materials can function as hole transport materials without the addition of dopant substances (additives), and are less susceptible to the problem of decreased solar cell performance due to dopant substances, thereby providing stable solar cells over a long period of time. Furthermore, because cis-trans isomerization and cyclization reactions of the stilbene structure are suppressed and stable π-conjugation can be maintained, excellent hole transport properties can be stably exhibited over a long period of time. An example of such low-molecular-weight organic materials is MSTPA-1, represented by the following formula 5. MSTPA-1 is a compound having a structure in which a p-dimethylaminostyryl group is introduced at the p-position of each phenyl group of triphenylamine. [ka]

[0024] The hole transport layer 5 may contain a non-lithium-based additive. Examples of the non-lithium-based additive include 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (TPFB), tributyl phosphate (tBP), Bmp-TFSI, and Edm-TFSI. TPFB is particularly preferred.

[0025] [Electrode 6] The electrode 6 is made of a conductive material. The electrode 6 is formed using, for example, a metal such as gold, platinum, silver, or copper, an alloy thereof, or an oxide conductor such as FTO or indium tin oxide (ITO). The electrode 6 may be a carbon electrode, or may contain graphite, carbon black, carbon nanotubes, carbon nanofibers, carbon fibers, graphene, fullerene, or the like as a material. The electrode 6 may contain only one of the above-mentioned materials, or may contain two or more of them. The photoelectrode 61 and the counter electrode 62 may be formed using different materials.

[0026] [Power generation principle of solar cell 10] Here, the principle of how solar cell 10 generates electricity will be explained with reference to Figure 3. When light such as sunlight or room light is incident from the transparent substrate 21 side, this incident light passes through the substrate 2 and the electron transport layer 3 without being absorbed much, and most of it reaches the power generation layer 4. When the incident light that has reached the power generation layer 4 is irradiated onto the perovskite layer 44, this perovskite layer 44 absorbs the light energy and becomes excited. When this excitation raises the energy level of the perovskite layer 44 to a predetermined level higher than the conduction band potential of the metal oxide that is the porous semiconductor 41, electrons are injected from the perovskite layer 44 into the porous semiconductor 41. The injected electrons pass through the electron transport layer 3 and are collected by the photoelectrode 61.

[0027] Meanwhile, holes generated in the perovskite layer 44 reach the counter electrode 62 via the hole transport layer 5, where they recombine with electrons that have passed through the external load 7. In other words, a potential gradient is generated between the photoelectrode 61 and the counter electrode 62, and power can be supplied by connecting the external load 7 between the two electrodes.

[0028] Here, the energy diagram of the perovskite solar cell 10 will be described. As shown in FIG. 4, in an ideal energy diagram of the perovskite solar cell 10, the conduction band edge (CBE) of the electron transport layer 3 needs to be deeper than the CBE of the perovskite layer 44 so that electrons generated in the perovskite layer 44 can be easily extracted to the outside. As shown in FIG. 5(a), the CBE of the electron transport layer 3 made of titanium oxide or the like is usually equal to or slightly deeper than that of the perovskite layer 44 when used alone. However, when the electron transport layer 3 comes into contact with the perovskite layer 44, the CBE shifts to a shallower position (negative shift) (FIG. 5(b)). Therefore, in this state, it is difficult for electrons generated in the perovskite layer 44 to move to the electron transport layer 3, making it difficult to extract electrons from the electron transport layer 3. 5(c), when the hole transport layer 5 contains a lithium-based additive, lithium ions diffuse from the hole transport layer 5, and the diffused lithium ions act to shift the CBE of the electron transport layer 3 deeper (positive shift), making the CBE of the electron transport layer 3 deeper than the CBE of the perovskite layer 44. However, when a non-lithium-based additive is used as the hole transport material, or when neither a non-lithium-based additive nor a lithium-based additive is used, as shown in FIG. 6, the above-mentioned re-shift of the CBE of the electron transport layer 3 does not occur, and electrons are less likely to move from the perovskite layer 44 to the electron transport layer 3. This is presumably why the photoelectric conversion performance is lower than when a lithium-based additive is used.

[0029] Therefore, in the solar cell 10 of this embodiment, a buffer layer 42 is disposed between the perovskite layer 44 and the electron transport layer 3. As shown in FIG. 7, when a buffer layer 42 having a shallower CBE than the electron transport layer 3 is disposed on the electron transport layer 3 (FIG. 7(b)), and a perovskite layer 44 is disposed on the buffer layer 42 (FIG. 7(c)), a negative shift in the CBE of the electron transport layer 3 is suppressed. Therefore, as shown in FIG. 7(d), a shift in the CBE of the electron transport layer 3 toward a shallower state due to contact between the electron transport layer 3 and the perovskite layer 44 is suppressed. Therefore, even when a non-lithium-based additive is used as the hole transport material or when neither a non-lithium-based additive nor a lithium-based additive is used, the solar cell 10 has both high photoelectric conversion performance and high operational stability.

[0030] [Procedure for producing solar cell 10] Next, a manufacturing procedure for the solar cell 10 according to this embodiment will be described with reference to Fig. 8. The solar cell 10 can be manufactured by referring to known techniques such as Michael Saliba et al., "Correction to 'How to Make over 20% Efficient Perovskite Solar Cells in Regular (nip) and Inverted (pin) Architectures'," Chem. Mater., 2018, 30, 4193-4218.

[0031] First, a transparent conductive film 22 is formed on a transparent substrate 21 to prepare the substrate 2. The transparent conductive film 22 is laminated on the transparent substrate 21 by, for example, chemical vapor deposition (CVD) or sputtering. Next, laser scribing is performed to partially remove the transparent conductive film 22, forming recesses 221 for the insulating layer 31, followed by cleaning. Next, an electron transport layer 3 is formed over the entire surface of the substrate 2 by atomic layer deposition (ALD) or spray pyrolysis (SPD). The electron transport layer 3 is preferably formed as a dense TiO layer. Next, a porous semiconductor 41, which is a nanoparticle sintered layer, is formed near the center of the masked substrate 2 and electron transport layer 3. The porous semiconductor 41 is preferably formed as a porous layer of TiO (p-TiO). This porous semiconductor 41 is formed by diluting nanoparticle paste with a solvent, applying it by spin coating at a rotation speed of 4000 rpm to 6000 rpm, drying it, removing the masking, and heating it at 450°C to 550°C to sinter it.

[0032] Next, for example, PCBM as an electron donor material is mixed with chlorobenzene to prepare a PCBM chlorobenzene solution (1 mM to 10 mM), which is then dropped onto the porous semiconductor 41. After that, the solution is permeated into the pores (p-TiO2) and excess solution is removed by spin coating at a rotation speed of, for example, 5000 to 8000 rpm. Thereafter, the solution is dried at 60°C to 120°C (preferably 70°C to 90°C) to form a buffer layer (PCBM layer).

[0033] Next, for example, a solution of PbI2 in N,N-dimethylformamide is prepared and dropped onto the porous semiconductor 41. After that, the solution is permeated into the pores (p-TiO2) and excess solution is removed by spin coating at a rotation speed of, for example, 5000 rpm to 8000 rpm. The solution is then dried at 60°C to 120°C (preferably 70°C to 90°C) to form a PbI2 layer.

[0034] The porous semiconductor 41, on which the substrate 2, electron transport layer 3, buffer layer 42, and PbI2 layer have been formed, is immersed in an isopropyl alcohol solution (2 mg / ml to 20 mg / ml) of MAI (CH3NH3I) at 0°C to 80°C (preferably at room temperature) (MAI immersion method). PbI2 and MAI react to form a perovskite compound [(CH3NH3)PbI3(MAPbI3)] as a perovskite layer 44 inside and on the pores of the porous semiconductor 41, and then the porous semiconductor 41 is rinsed with pure isopropyl alcohol and dried at 60°C to 120°C (preferably 70°C to 100°C). The mixed cation-mixed halide ((FAPbI3) 1-x (MAPbBr3) x )-based perovskite compounds can also be prepared in a similar manner.

[0035] The above-described fabrication procedure involves two steps to control the crystal growth of the perovskite compound that forms the perovskite layer 44, but this can also be performed in one step. For example, a perovskite ((CH3NH3)PbI3) solution is permeated into the pores of the porous semiconductor 41 on which the buffer layer 42 is formed by spin coating. Next, toluene is added dropwise during spinning to precipitate microcrystals and create a mirror-finished surface (poor solvent precipitation method).

[0036] A hole transport material such as spiro-OMeTAD, DHCF-3, DHCF-32, or MSTPA-1 is prepared as a 60 mg / ml to 90 mg / ml chlorobenzene solution. The solution is dropped onto the perovskite layer 44, and excess solution is removed by spin coating. The solution is then dried to form the hole transport layer 5. A non-lithium additive (e.g., TPFB) may be added to the hole transport material. When TPFB is added, the TPFB content is preferably 0.01 wt % to 100 wt %, and more preferably 0.1 wt % to 50 wt %, of the hole transport material. For example, the hole transport material is weighed out to a concentration of 30 mM, and TPFB equivalent to 10 wt % of the hole transport material is added. The hole transport layer 5 can be formed using this solution dissolved in chlorobenzene.

[0037] The steps from forming the buffer layer 42 to forming the hole transport layer 5 are preferably performed in a dry nitrogen atmosphere such as in a glove box. Finally, the electrode 6 is formed. The electrode 6 is formed by adhering a thin film of gold or the like to the surfaces of the electron transport layer 3 and the hole transport layer 5 by vacuum deposition or the like. [Example]

[0038] Examples and comparative examples will be described below.

[0039] (Fabrication of Solar Cells According to Example 1 and Comparative Example 1) An example of fabricating a solar cell according to Example 1 will be described below. The solar cell can be fabricated with reference to known techniques such as those described in Chem. Mater., 2018, 30, 4193-4218, etc.

[0040] As an example, a fluorine-doped tin oxide (FTO) glass (23mm x 14mm x 1.6mm) was used as a transparent substrate. This FTO glass was laser scribed to form recesses where the FTO had been removed, and then thoroughly cleaned. Next, a dense TiO2 film was formed as an electron transport layer using the spray pyrolysis (SPD) method. A solution of commercially available TiO2 nanoparticle paste diluted with ethanol was then dropped onto the substrate, spin-coated, and dried at 100°C. After drying, the substrate was baked at 450°C to form a TiO2 nanoparticle layer that would become a porous semiconductor. After baking, the substrate was cooled to room temperature.

[0041] Next, a PCBM solution was prepared to form a PCBM layer as a buffer layer. Specifically, PCBM was dissolved in chlorobenzene to prepare a 5 mM PCBM solution. The PCBM solution was then dropped onto the TiO nanoparticle layer as a porous semiconductor, spin-coated, and heated at 100 °C for 10 minutes, followed by cooling to room temperature.

[0042] Next, a perovskite precursor solution was prepared to form a perovskite layer as a photoelectric conversion layer. First, FAI (CH(NH2)2I) was weighed out, and a 1.7 M PbI2 / dimethylformamide (DMF) + dimethyl sulfoxide (DMSO) solution prepared in advance was poured into this to prepare a FAPbI3 solution. Next, MAI (CH3NH3I) was weighed out, and a 1.7 M PbBr2 / dimethylformamide (DMF) + dimethyl sulfoxide (DMSO) solution prepared in advance was poured into this to prepare a MAPbBr3 solution.

[0043] Finally, the FAPbI3 solution, the MAPbBr3 solution, and a previously prepared 1.7 M CsI / dimethyl sulfoxide (DMSO) solution were mixed to prepare a precursor solution for spin coating.

[0044] The perovskite layer was formed by dropping the precursor solution onto the TiO porous film on which the PCBM layer had been formed and applying it by spin coating. After application was completed, the film was heated at 100°C for 60 minutes and then cooled to room temperature.

[0045] Next, a hole transport layer was prepared. In Example 1, TPFB was added as a dopant to prepare a hole transport layer 5. Spiro-OMeTAD was used as the hole transport material. The hole transport material was weighed out to a final concentration of 30 mM, and TPFB equivalent to 10% by weight was weighed and added to the hole transport material. A mixture of the hole transport material and TPFB was dissolved in chlorobenzene to prepare a hole transport material solution. The hole transport layer 5 was formed by dropping the prepared hole transport material solution onto the perovskite film and forming it by spin coating.

[0046] The solar cell according to Comparative Example 1 was fabricated in the same manner as the solar cell according to Example 1, except that the step of forming the buffer layer was not carried out.

[0047] (Fabrication of Solar Cells According to Example 2 and Comparative Example 2) The solar cell according to Example 2 was fabricated in the same manner as the solar cell according to Example 1, except that DHCF-32 was used as the hole transport material. The solar cell according to Comparative Example 2 was fabricated in the same manner as the solar cell according to Example 2, except that the step of forming the buffer layer was not performed.

[0048] (Fabrication of Solar Cells According to Example 3 and Comparative Example 3) The solar cell according to Example 3 was fabricated in the same manner as the solar cell according to Example 1, except that MSTPA-1 was used as the hole transport material. The solar cell according to Comparative Example 3 was fabricated in the same manner as the solar cell according to Example 3, except that the step of forming a buffer layer was not performed.

[0049] (Fabrication of Solar Cells According to Example 4 and Comparative Example 4) The solar cell of Example 4 was fabricated in the same manner as the solar cell of Example 3, except that no dopant was used in preparing the hole transport layer. The solar cell of Comparative Example 4 was fabricated in the same manner as the solar cell of Example 4, except that the step of forming the buffer layer was not performed.

[0050] (Battery performance evaluation) The solar cells fabricated as described above were evaluated for cell performance. The solar cells of each Example and Comparative Example were stored unsealed in a dark place under a dry atmosphere.

[0051] 9 to 12 show the relationship between photoelectric conversion efficiency (IPCE) and wavelength for solar cells according to each example and comparative example. FIG. 9 relates to Example 1 and Comparative Example 1, FIG. 10 to Example 2 and Comparative Example 2, FIG. 11 to Example 3 and Comparative Example 3, and FIG. 12 to Example 4 and Comparative Example 4. As shown in FIGS. 9 to 12, when a buffer layer (PCBM layer) was disposed between the electron transport layer and the perovskite layer, the photoelectric conversion efficiency was improved, particularly in the high wavelength region, compared to when no buffer layer was provided. This suggests that the provision of a buffer layer improved the movement of electrons from the perovskite layer to the electron transport layer.

[0052] 13 to 16 show the IV characteristics of the solar cells according to each example and each comparative example. Note that FIG. 13 relates to Example 1 and Comparative Example 1, FIG. 14 to Example 2 and Comparative Example 2, FIG. 15 to Example 3 and Comparative Example 3, and FIG. 16 to Example 4 and Comparative Example 4. Table 1 also shows the short-circuit current density [mA / cm] of the solar cells according to each example and each comparative example. 2 The short-circuit current density (Jsc) and open-circuit voltage (Voc) were obtained from the IV characteristics graphs of each solar cell shown in Figures 13 to 16. The fill factor (FF) is the maximum output P at the point where the product of the current and voltage is maximized. max The conversion efficiency was calculated by dividing the power dissipation capacity by (Voc × Jsc). The conversion efficiency was calculated by dividing the power dissipation capacity by the incident light intensity.

[0053] [Table 1]

[0054] 13 to 16 and Table 1, solar cells with a buffer layer disposed between the electron transport layer and the perovskite layer exhibited higher conversion efficiency than solar cells without a buffer layer. This confirmed that by disposing a buffer layer between the electron transport layer and the perovskite layer, high photoelectric conversion performance was achieved even when a non-lithium-based additive was used as the hole transport material (Examples 1 to 3) or when neither a non-lithium-based additive nor a lithium-based additive was used (Example 4).

[0055] In the above-described embodiment, the following configurations are envisioned. <1> A perovskite solar cell (10) comprising: a substrate (2) having a transparent conductive film (22); an electron transport layer (3) that transfers electrons to the transparent conductive film (22); a buffer layer (42) containing an electron-donating material; a photoelectric conversion layer (44) that is excited by light to generate electrons; and a hole transport layer (5) through which holes generated from the photoelectric conversion layer (44) pass; a laminate in this order; and an electrode (6) composed of a photoelectrode (61) that emits electrons via the transparent conductive film (22) and a counter electrode (62) provided on the surface of the hole transport layer (5).

[0056] The buffer layer (42) disposed between the photoelectric conversion layer (44) and the electron transport layer (3) changes the energy diagram between the electron transport layer (3) and the photoelectric conversion layer (44), and the conduction band edge (CBE) of the photoelectric conversion layer (3) becomes shallower than the CBE of the electron transport layer (3). This facilitates electron migration from the photoelectric conversion layer (44) to the electron transport layer (3), improving photoelectric conversion performance. Furthermore, because there is no need to use a lithium-based additive in the hole transport material, operational stability is also improved compared to when a lithium-based additive is used. In other words, the perovskite solar cell of the present invention achieves both high photoelectric conversion performance and high operational stability.

[0057] <2> <1> In the perovskite solar cell (10) described, the hole transport layer (5) preferably contains a non-lithium based additive.

[0058] This configuration improves operational stability compared to when a lithium-based additive is used, resulting in a perovskite solar cell that combines high photoelectric conversion performance with high operational stability.

[0059] <3> <2> In the described perovskite solar cell (10), the non-lithium additive is preferably TPFB.

[0060] This configuration improves operational stability compared to when a lithium-based additive is used. The inventors of the present application have confirmed through experiments that the above configuration enables perovskite solar cells to have both high photoelectric conversion performance and high operational stability.

[0061] <4> <1> In the described perovskite solar cell (10), the hole transport layer preferably contains MSTPA-1 as the hole transport material.

[0062] According to this configuration, a perovskite solar cell that combines high photoelectric conversion performance and high operational stability can be realized without using either a non-lithium-based additive or a lithium-based additive.

[0063] <5> <1> ~ <4> In the perovskite solar cell (10) described in any one of the above, the electron donating material is preferably a fullerene or a fullerene derivative.

[0064] According to this configuration, the conduction band edge (CBE) of the photoelectric conversion layer is shallower than that of the electron transport layer, which makes it easier for electrons generated in the photoelectric conversion layer to move to the electron transport layer, thereby improving photoelectric conversion performance.

[0065] <6> <5> In the described perovskite solar cell (10), the fullerene derivative is preferably PCBM.

[0066] According to this configuration, the conduction band edge (CBE) of the photoelectric conversion layer is shallower than the CBE of the electron transport layer. Electrons generated in the photoelectric conversion layer are more likely to move to the electron transport layer, improving photoelectric conversion performance. The applicant has experimentally confirmed that the above configuration enables perovskite solar cells to have both high photoelectric conversion performance and high operational stability. [Industrial Applicability]

[0067] The present invention can be used in perovskite solar cells. [Explanation of symbols]

[0068] 2: Substrate, 22: Transparent conductive film, 3: Electron transport layer, 42: Buffer layer, 44: Perovskite layer (photoelectric conversion layer), 5: Hole transport layer, 6: Electrode, 61: Photoelectrode, 62: Counter electrode, 10: Perovskite solar cell

Claims

1. a substrate having a transparent conductive film; an electron transport layer that transfers electrons to the transparent conductive film; a buffer layer containing an electron donating material; a photoelectric conversion layer that is excited by light to generate the electrons; a hole transport layer through which holes generated from the photoelectric conversion layer pass; and a laminate in this order. a photoelectrode that emits the electrons through the transparent conductive film, and an electrode that is composed of a counter electrode provided on a surface of the hole transport layer.

2. The perovskite solar cell according to claim 1 , wherein the hole transport layer contains a non-lithium-based additive.

3. 3. The perovskite solar cell of claim 2, wherein the non-lithium additive is TPFB.

4. The perovskite solar cell according to claim 1, wherein the hole transport layer contains MSTPA-1 as a hole transport material.

5. The perovskite solar cell according to any one of claims 1 to 4, wherein the electron donating material is a fullerene or a fullerene derivative.

6. The perovskite solar cell according to claim 5 , wherein the fullerene derivative is PCBM.

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