Nanomaterials, methods for fabricating nanomaterials, and solar cells

Abstract

To provide a nano material which can efficiently take out a hot carrier at high speed, and a method for producing the same.SOLUTION: A nano material is acquired by mixing a fullerene derivative having a carboxy group (-COOH) as a chemical anchor group with a quantum dot organic solution where a quantum dot of metal halide perovskite is dispersed, and substituting an oleic acid ligand bonded to the quantum dot with an electronegative fullerene derivative ligand obtained by removing a hydrogen ion from the carboxy group. The nano material has a quantum dot of metal halide perovskite represented by general formula ABX3(A=Cs, FA, MA, B=Pb2+, Sn2+, X=I-, Br- and Cl-), and an electronegative fullerene derivative ligand bonded to the quantum dot.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This invention relates to nanomaterials, methods for producing nanomaterials, and solar cells, and more particularly to nanomaterials capable of efficiently extracting hot carriers and methods for producing the same. [Background technology]

[0002] Hot carrier solar cells are expected to be photovoltaic devices that can overcome the energy conversion efficiency limit known as the "Shockley-Quiser limit." The Shockley-Quiser limit is a proposition that the conversion efficiency of solar cells will not exceed 32.7% (30-33% depending on the measurement method), regardless of the semiconductor used. This assumes the use of a single semiconductor material (single junction). In contrast, the conversion efficiency of an ideal single-junction hot carrier solar cell, assuming no energy loss, is said to be approximately 67%.

[0003] Hot carriers are electrons and holes that have gained kinetic energy from the electric field to overcome the potential barrier and have a temperature higher than the crystal temperature. Compared to conventional light-absorbing materials, perovskite materials have a longer hot carrier lifetime, and the hot carrier lifetime of perovskite quantum dots is even longer than that of bulk materials (see, for example, Non-Patent Literature 1). Due to the carrier confinement effect of quantum dots, carriers are kept at high energy for a longer time than in bulk materials. In particular, lead halide perovskite quantum dots have a long time required for hot carrier cooling and are a promising candidate for hot carrier solar cells.

[0004] Organic-inorganic hybrid perovskites such as methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3) are used as the photoactive layer, and magnesium-doped zinc oxide (ZMO:Zn) is used. 1-x Mg x Perovskite solar cells using O) as an electron-selective layer have been proposed (see, for example, Non-Patent Document 2). [Prior art documents]

Non-Patent Literature

[0005]

Non-Patent Literature 1

Non-Patent Literature 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Hybrid lead halide perovskite quantum dots such as MAPbI3 and FAPbI3 have long insulating ligands and it is difficult to efficiently extract hot carriers. If hot carriers can be quickly extracted from the perovskite quantum dots, specifically, faster than the relaxation time of the hot carriers, the energy conversion efficiency will be further improved. The purpose of the present invention is to provide a nanomaterial capable of efficiently and rapidly extracting hot carriers and a method for producing the same.

Means for Solving the Problems

[0007] The nanomaterial has quantum dots of metal halide perovskite represented by the general formula ABX3 (A = Cs, FA, MA, B = Pb 2+ , Sn 2+ , X = I - , Br - , Cl - ) and an electronegative fullerene derivative ligand bonded to the quantum dots.

[0008] The method for producing the nanomaterial is mixing a fullerene derivative having a carboxy group (—COOH) as a chemical anchor group with a quantum dot organic solution in which quantum dots of metal halide perovskite are dispersed, The oleic acid ligand bonded to the quantum dots is replaced with an electronegative fullerene derivative ligand from which a hydrogen ion has detached from the carboxy group to obtain a nanomaterial.

Advantages of the Invention

[0009] A nanomaterial capable of extracting hot carriers with high efficiency and rapidly, and a method for producing the same are realized.

Brief Description of the Drawings

[0010] [Figure 1A] It is a diagram showing an example of the fullerene derivative used in the embodiment. [Figure 1B] It is a diagram showing another example of the fullerene derivative used in the embodiment. [Figure 1C] It is a diagram showing still another example of the fullerene derivative used in the embodiment. [Figure 2] It is an NMR (Nuclear Magnetic Resonance) spectrum showing the presence of the fullerene derivatives of FIGS. 1A to 1C. [Figure 3] It is a schematic diagram showing the nanomaterial of the embodiment and a method for producing the same. [Figure 4] It is a diagram showing the synthesis of the nanomaterial of the embodiment in comparison with known substances. [Figure 5] It is an XPS (X-ray Photoelectron Spectroscopy) spectrum showing the interaction between the quantum dots and the fullerene derivative ligand. [Figure 6] It is an XPS spectrum showing the interaction between the quantum dots and the fullerene derivative ligand. [Figure 7] It is an XPS spectrum showing the interaction between the quantum dots and the fullerene derivative ligand. [Figure 8] It is an FTIR (Fourier-Transform Infrared spectroscopy) spectrum showing the interaction between the quantum dots and the fullerene derivative ligand. [Figure 9]This is an FTIR spectrum showing the interaction between quantum dots and fullerene derivative ligands. [Figure 10] This is an FTIR spectrum showing the interaction between quantum dots and fullerene derivative ligands. [Figure 11] These are PYS (Photoelectron Yield Spectroscopy) spectra of quantum dots and fullerene derivative materials. [Figure 12] This is a (αhν)2-hν plot of fullerene derivative materials. [Figure 13] This figure shows the band structures of quantum dots and fullerene derivative materials. [Figure 14] This figure shows the light absorption spectra of various samples. [Figure 15] This figure shows the second derivative of the light absorption spectrum in Figure 14. [Figure 16] This is a diagram showing the PL (Photoluminescence) spectra of various samples. [Figure 17] This figure shows the lifetimes of photoexcited carriers in various samples. [Figure 18] This figure shows the XRD (X-ray diffraction) spectra of various samples. [Figure 19] This figure shows the dependence of PL intensity on excitation light intensity for various samples. [Figure 20] This figure shows the existence time of hot carriers in quantum dots of various samples. [Figure 21] This figure shows the temperature changes of the hot carrier for various samples. [Figure 22] This is a schematic diagram of a solar cell according to an embodiment. [Modes for carrying out the invention]

[0011] Figures 1A to 1C show examples of fullerene derivatives used in the embodiment. The nanomaterial of the embodiment promotes the extraction of hot carriers by binding an electronegative fullerene derivative ligand to a quantum dot of metal halide perovskite. Prior to obtaining an electronegative fullerene derivative ligand to coordinate to the quantum dot, a fullerene derivative having a carboxyl group (-COOH) as a chemical anchor group is prepared. Fullerenes are excellent electron acceptors, with 60 carbon atoms (C) forming a spherical structure. The inventors arrived at the present invention based on the technical idea that a fullerene derivative, in which an organic compound is bound to the surface of a fullerene, could be combined with quantum dots as an organic semiconductor ligand. By binding a fullerene derivative ligand to the surface of a quantum dot, hot carriers can be efficiently extracted from the quantum dot, improving the energy conversion efficiency of the solar cell.

[0012] Figure 1A shows PCBA (Phenyl-C61-Butyric acid) as fullerene derivative 22A, which contains a -COOH group. PCBA is obtained by hydrolyzing PCBM (Phenyl-C61-Butyric acid methyl ester), and has a double bond (C=O) between carbon (C) and oxygen (O), and a single bond between C and a hydroxyl group (OH) on the surface of the fullerene. For example, a solution of 100 mg, 0.147 mmol of PCBM added to 20 mL of chlorobenzene (C6H5Cl) is refluxed under a nitrogen atmosphere for 3 hours. To this solution, 2 mL of 12 M hydrochloric acid (HCl) and 5 mL of acetic acid are added all at once, and the reaction mixture is refluxed for 16 hours. After that, the solvent is removed under reduced pressure, and the resulting residue is purified by precipitation from methanol (MeOH) to obtain brown solid PCBA (55 mg, 56%). This fullerene derivative, in the form of a Lewis base obtained by removing hydrogen from PCBA, is bonded to quantum dots of a metal halide perovskite by ligand substitution. Ligand substitution will be described later with reference to Figure 3.

[0013] Figure 1B shows Bis-PCBA as the fullerene derivative 22B having a -COOH group. Bis-PCBA is obtained by hydrolyzing Bis-PCBM. Bis-PCBM has two sets of phenyl rings (C6H5) and methoxycarbonyl groups (-COOCH3) on the surface of the fullerene, and by hydrolysis in the same manner as in Figure 1A, Bis-PCBA, which has two carboxyl groups on the surface of the fullerene, is obtained. That is, hydrochloric acid and acetic acid are added simultaneously to a chlorobenzene solution of Bis-PCBM, and after refluxing for 16 hours, precipitation occurs to obtain Bis-PCBA having two -COOH groups. The electronegative fullerene derivative from which hydrogen has been removed from this Bis-PCBA is bonded to quantum dots of metal halide perovskite by ligand substitution.

[0014] Figure 1C shows 4-(1',5'-dihydro-1'-methyl-2'H-[5,6]fullereno-C60-Ih-[1,9-c]pyrrol-2'-yl)benzoic acid (C60-SAM) as the fullerene derivative 22C having a -COOH group. As C60-SAM, commercially available products from Sigma Aldrich (purity 99% or higher) can be used. An electronegative fullerene derivative obtained by removing hydrogen from C60-SAM is bonded to quantum dots of a metal halide perovskite by ligand substitution.

[0015] Figure 2 shows the NMR spectra of the fullerene derivatives 22A, 22B, and 22C shown in Figures 1A, 1B, and 1C. The horizontal axis represents the chemical shift δ (ppm), and the vertical axis represents the intensity (in arbitrary units). The chemical shift δ is calculated by dividing the deviation of the resonance frequency from the reference signal by the measurement frequency, with the resonance frequency of the methyl group of the standard substance tetramethylsilane (TMS) being used as the reference (zero) signal.

[0016] PCBA, Bis-PCBA, and C60-SAM were dispersed in solutions of tetrahydrofuran (THF) and carbon black (CB), and their NMR spectra were measured. In each NMR spectrum, the peaks appearing near 8.0 and around 7.3 are hydrogen (2H) peaks.

[0017] It is known that the peaks of the NMR spectra of carboxylic acids, that is, compounds containing -COOH in the molecule, appear at 10 - 15 ppm. When the spectrum in this region is expanded, the peaks of PCBA and Bis-PCBA appear at the position of δ = 11.0, and the peak of C60-SAM appears at the position of δ = 11.7. From this, it is confirmed that PCBA, Bis-PCBA, and C60-SAM having -COOH as a chemical anchor group are obtained by the above-described chemical reaction.

[0018] Figure 3 is a schematic diagram showing the nanomaterial of the embodiment and its manufacturing method. First, a fullerene derivative 22A, 22B, or 22C is mixed into a quantum dot organic solution 25 which is an organic (toluene or chlorobenzene) solution in which quantum dots 21 of metal halide perovskite are dispersed. In the quantum dot solution 25 before mixing the fullerene derivative 22A, 22B, or 22C, OA - to which a hydrogen cation has detached from oleic acid is coordinated on the surface of the quantum dots 21. More specifically, the anion group ROO - binds to the Pb cation of the quantum dots 21. s

[0019] The coordination process of the fullerene derivative is similar to the coordination process of oleic acid. When a fullerene derivative having -COOH is mixed into the quantum dot organic solution 25, a hydrogen cation detaches from -COOH and changes to a chemical radical in the form of a Lewis base. A Lewis base is an atom, molecule, or ion having at least one electron pair that can be donated without being used in a covalent bond. The fullerene derivative in the form of a Lewis base from which a hydrogen cation has detached is electronegative and quickly replaces the oleic acid (OA - ) ligand bound to the quantum dots 21 and binds to Pb + . The obtained nanomaterials 20A, 20B, or 20C are those in which electronegative fullerene derivative ligands 23A, 23B, or 23C are bound to the quantum dots 21 of metal halide perovskite. Hereinafter, the nanomaterials 20A, 20B, and 20C are collectively referred to as "nanomaterials 20" as appropriate.

[0020] In the example in Figure 3, quantum dot 21 is CsPbI3, but the example is not limited to this. As the A site cation of the metal halide perovskite represented by the general formula ABX3, cesium (Cs + ) can be replaced with methylammonium [CH3NH3] + Formamidinium [NH2CH=NH2] + , or mixtures thereof may be used. Instead of Pb, Sn or an alloy of Pb and Sn may be used as the metal at the B site. X is a halogen element selected from (I, Br, Cl) or a mixture thereof. When Cs is used at the A site, an inorganic metal halide perovskite is formed.

[0021] The fullerene derivative ligands 23A, 23B, or 23C are n-type organic semiconductors and, as described later, can rapidly extract hot carriers from the quantum dot 21. By applying the nanomaterials 20A, 20B, or 20C to solar cells, the energy conversion efficiency can be greatly improved.

[0022] Figure 4 shows the synthesis of nanomaterials according to the embodiment, in comparison with known materials. In Figure 4(A), six types of samples are prepared as solvents: toluene, PCBM solution, Bis-PCBM solution, PCBA solution, Bis-PCBA solution, and SAM-C60 solution. The PCBA solution, Bis-PCBA solution, and SAM-C60 solution are solutions obtained by dissolving the fullerene derivatives 22A, 22B, and 22C, which have a carboxyl group (-COOH), in toluene, as shown in Figures 1A-1C, and their concentrations are approximately 2 mg / ml. For comparison, toluene, a PCBM solution obtained by dissolving PCBM in toluene, and a Bis-PCBM solution obtained by dissolving Bis-PCBM in toluene are prepared. Toluene is a colorless, transparent liquid at room temperature. The other solutions are light brown, transparent liquids.

[0023] Figure 4(B) shows a sample obtained by mixing a small amount of a fullerene derivative solution (the solution in Figure 4(A)) with a toluene solution of CsPbI3 quantum dots. When the toluene solution of CsPbI3 quantum dots is mixed with a PCBM solution or a Bis-PCBM solution, a reddish-brown solution is formed. In contrast, when the PCBA solution, Bis-PCBA solution, and SAM-C60 solution containing the fullerene derivative of the embodiment are mixed with the toluene solution of CsPbI3 quantum dots (quantum dot organic solution 25), the nanomaterial 20 of the embodiment is generated, and it absorbs light and changes into a black liquid.

[0024] In Figure 4(C), each sample is irradiated with light. Samples prepared by mixing toluene, PCBM solution, and Bis-PCBM solution with the CsPbI3 quantum dot organic solution fluoresce. In contrast, samples from embodiments prepared by mixing PCBA solution, Bis-PCBA solution, and SAM-C60 solution with the CsPbI3 quantum dot organic solution do not fluoresce. This fact supports the idea that in the embodiments, electrons (hot carriers) excited within the CsPbI3 quantum dots by light irradiation are extracted to the fullerene derivative ligand faster than they can recombine. On the other hand, in the CsPbI3 quantum dot solutions to which toluene, PCBM solution, and Bis-PCBM solution are added, the excited electrons are not extracted but decay or decrease, emitting light and returning to the ground state.

[0025] Figure 4(C) shows that in the nanomaterials 20A to 20C of the embodiment, the fullerene derivative ligands 23A, 23B, and 23C (hereinafter collectively referred to as "fullerene derivative ligand 23" as appropriate) bound to the surface of the quantum dot 21 effectively function as a hot carrier extraction layer. By using the fullerene derivative ligand 23 as a hot carrier extraction layer, hot carriers can be extracted faster than the relaxation time of the hot carriers.

[0026] <Molecular interaction between quantum dots and organic semiconductor ligands> Figures 5 to 7 show XPS spectra illustrating molecular interactions between quantum dots and organic semiconductor ligands. Figure 5 shows the 4f 7 / 2 photoelectron XPS spectra of lead (Pb) from CsPbI3, PCBM-doped CsPbI3 quantum dot solutions (labeled "PCBM-CsPbI3" in the figure), and PCBA-doped CsPbI3 quantum dot solutions (labeled "PCBA-CsPbI3" in the figure). Figure 6 shows the 3d 5 / 2 photoelectron XPS spectra of iodine (I) from CsPbI3, PCBM-CsPbI3, and PCBA-CsPbI3. Figure 7 shows the 1s photoelectron XPS spectra of cesium (Cs) from CsPbI3, PCBM-CsPbI3, and PCBA-CsPbI3.

[0027] In quantum dots of CsPbI3 coupled with PCBA, the I3d and Cs1s peaks are shifted to lower binding energies, while the Pb4f peak is shifted to higher binding energies. This is because, on the surface of the quantum dot, -COO - and the coordinating Pb 2+ This suggests that a coordinate bond is formed between the ions.

[0028] <Coordination interaction between quantum dots and organic semiconductor ligands> Figures 8, 9, and 10 show FTIR spectra illustrating the coordination interactions between quantum dots and organic semiconductor ligands. Figure 8 shows the FTIR spectra of CsPbI3, PCBM, PCBA, a solution of CsPbI3 quantum dots with PCBM added (indicated as "CsPbI3+PCBM" in the figure), and a solution of CsPbI3 quantum dots with PCBA added (indicated as "CsPbI3+PCBA" in the figure).

[0029] A mixed solution of CsPbI3 and PCBA (CsPbI3+PCBA) is detected at a wavenumber of 1200 cm. -1 It has a CO stretching vibration in its vicinity. This CO stretching vibration is the same as the CO stretching vibration of oleic acid ligand. Also, the wavenumber is 1350 cm. -1 There is an OH bending vibration in the vicinity of 3450 cm -1It has an OH stretching vibration in its vicinity. On the other hand, PCBA before bonding with the CsPbI3 quantum dot has -COOH as a chemical anchor group, at 1700 cm. -1 It has a carboxylic acid C=O stretching vibration in its vicinity.

[0030] With the exception of the inorganic material CsPbI3, the four samples containing organic compounds were measured at 1730 cm². -1 The carboxylic acid ester exhibits a C=O stretching vibration in its vicinity. Of these, PCBA bound to the CsPbI3 quantum dot shows a shift in the C=O stretching vibration of the carboxylic acid ester to a higher wavenumber. This indicates effective binding between the CsPbI3 quantum dot and the electronegative PCBA fullerene derivative. Furthermore, the emergence of new peaks in the OH stretching and OH bending vibrations, along with the CO stretching vibration from the oleic acid ligand, demonstrates the ligand substitution mechanism described in Figure 3.

[0031] Figure 9 shows the FTIR spectra of a mixed solution of CsPbI3 and Bis-PCBA (spectrum D), a mixed solution of CsPbI3 and Bis-PCBM (spectrum E), CsPbI3 (spectrum F), Bis-PCBA (spectrum G), and Bis-PCBM (spectrum H). Spectrum D of the mixed solution of CsPbI3 and Bis-PCBA is at wavenumber 1200 cm⁻¹. -1 Nearby, there is a CO stretching vibration from an oleic acid ligand, with a wavenumber of 1350 cm⁻¹. -1 There is an OH bending vibration in the vicinity of 3450 cm -1 It has OH stretching vibrations in its vicinity.

[0032] Before bonding with the CsPbI3 quantum dot, Bis-PCBA has a -COOH chemical anchor group, and its G spectrum is 1700 cm⁻¹. -1 Nearby, it has the C=O stretching vibration of a carboxylic acid. Except for the inorganic material CsPbI3, the spectra D, E, G, and H of the four samples containing organic compounds are at 1730 cm⁻¹. -1Near this point, there is a trough indicating the C=O stretching vibration of the carboxylic acid ester. Of these spectra, in spectrum D of Bis-PCBA after coupling with the CsPbI3 quantum dot, the C=O stretching vibration of the carboxylic acid ester is shifted to a higher wavenumber. From this, the effective coupling between the CsPbI3 quantum dot and the electronegative Bis-PCBA fullerene derivative is confirmed. 3450 cm⁻¹ -1 Nearby OH stretching vibrations, and 1350 cm -1 The peak of OH bending vibration near 1200cm -1 The nearby CO stretching vibration demonstrates that the ligand substitution mechanism, as explained with reference to Figure 3, is at work.

[0033] Figure 10 shows the FTIR spectra of a mixed solution of CsPbI3 and C60-SAM (spectrum I), CsPbI3 (spectrum J), and C60-SAM (spectrum K). Spectrum I of the mixed solution of CsPbI3 and C60-SAM is at wavenumber 1200 cm⁻¹. -1 Nearby, there is a CO stretching vibration from an oleic acid ligand, with a wavenumber of 1350 cm⁻¹. -1 Near the OH bending vibration, 3450 cm -1 It has OH stretching vibrations in its vicinity.

[0034] Before bonding with the CsPbI3 quantum dot, C60-SAM has a -COOH chemical anchor group, and its spectrum K is 1700 cm⁻¹. -1 Near this point, there is a C=O stretching vibration of the carboxylic acid. Comparing the spectra I and K excluding the inorganic material CsPbI3, the C=O stretching vibration of the carboxylic acid ester in the spectrum I of C60-SAM after bonding with the CsPbI3 quantum dot is shifted to a higher wavenumber. From this, the effective bonding between the CsPbI3 quantum dot and the electronegative C60-SAM fullerene derivative is confirmed. 3450 cm⁻¹ -1 Nearby OH stretching vibrations, and 1350 cm -1 The peak of OH bending vibration near 1200cm -1 The nearby CO stretching vibration demonstrates that the ligand substitution mechanism, as explained with reference to Figure 3, is at work.

[0035] <Band structure of quantum dots and fullerene derivatives> Figure 11 shows the photoelectron yield (PYS) spectra of quantum dots and fullerene derivative materials. The horizontal axis represents the light energy (eV), and the vertical axis represents the yield intensity to the power of 1 / 3. From the PYS spectra, the energy level of the valence band maximum (VBM), which corresponds to the highest occupied molecular orbit (HOMO), can be determined.

[0036] Figure 11(A) shows the PYS spectra of PCBM and PCBA. Linear fitting and baseline (yield) of each PYS spectrum are shown. 1 / 3 The intersection with (=0) represents the VBM level, which corresponds to the ionization potential of the molecule. The VBM level of PCBM is 6.10 eV, and the VBM level of PCBA is 6.20 eV.

[0037] Figure 11(B) shows the PYS spectra of Bis-PCBM and Bis-PCBA. From the intersection of the linear fitting of each PYS spectrum with the baseline, the VBM level of Bis-PCBM is determined to be 6.10 eV, and the VBM level of Bis-PCBA is 6.28 eV. Figure 11(C) shows the PYS spectrum of C60-SAM, and Figure 11(D) shows the PYS spectrum of CsPbI3 quantum dots. From the intersection of the linear fitting of each PYS spectrum with the baseline, the VBM level of C60-SAM is 6.21 eV, and the VBM level of CsPbI3 quantum dots is 5.47 eV.

[0038] Now that we know the VBM level of each fullerene derivative material, if we know the band gap, we can determine the Conduction Band Minimum (CBM) for each. The band gap of a material can be determined from its absorption spectrum. The CBM of CsPbI3 quantum dots can be determined using the same method as for the fullerene derivative materials.

[0039] Figure 12 shows the (αhν) of the fullerene derivative material. 2 This is an -hν plot. Assuming a direct transition semiconductor, for energy (hν), (αhν) 2 The following is plotted. In Figure 12(A), the intersection of the linear fitting of the rise time of PCBM and PCBA and the baseline represents the band gap. The band gaps of PCBM and PCBA are approximately the same, at 1.74 eV. In Figure 12(B), the band gaps of Bis-PCBM and Bis-PCBA are approximately the same, at 1.72 eV. In Figure 12(C), the band gap of C60-SAM is 1.73 eV.

[0040] Figure 13 shows the band structures of quantum dots and each fullerene derivative. Figure 13 was created based on the measurement results in Figures 11 and 12. Here, the energy levels of the conduction band are shown as small values, meaning that a lower ionization energy corresponds to a higher valence band level.

[0041] <Optical properties of various samples> Figure 14 shows the light absorption spectra of various samples. The horizontal axis represents energy (eV), and the vertical axis represents absorbance. Sample L is a CsPbI3 quantum dot solution, sample M is a mixed solution of PCBM and CsPbI3 quantum dot solution, and sample N is a mixed solution of Bis-PCBM and CsPbI3 quantum dot solution. Samples O, P, and Q correspond to the nanomaterials 20A, 20B, and 20C of the embodiment. That is, sample O is a mixed solution of PCBA and CsPbI3 quantum dot solution, sample P is a mixed solution of Bis-PCBA and CsPbI3 quantum dot solution, and sample Q is a mixed solution of C6-SAM and CsPbI3 quantum dot solution.

[0042] In the state shown in Figure 14, it is difficult to discern the differences in the optical absorption spectra of the various samples. Therefore, Figure 15 shows the second derivatives of the optical absorption spectra in Figure 14. The second derivative is the rate of change of the rate of change, and the differences in absorption intensity, including maxima and maxima, become more pronounced. Samples L, M, and N show a large difference in absorption intensity, indicating that the interaction between the quantum dots and the fullerene derivative is small. In contrast, samples O, P, and Q of the embodiment show a small difference in absorption intensity and a very wide peak width at the maximum, indicating that the interaction between the quantum dots and the fullerene derivative is large.

[0043] Figure 16 shows the PL spectra of various samples. Samples L, M, and N all have an emission peak at 1.846 eV. This indicates that there is almost no interaction with the fullerene derivative in samples L, M, and N, and that the emission of the CsPbI3 quantum dots is dominant. In contrast, the emission of samples O, P, and Q in the embodiment is so small that it is buried in noise, and fluorescence is not perceived with the naked eye. After removing the noise, the small emission peaks of sample O and sample P shift to 1.872 eV, and the peak of sample Q shifts to 1.852 eV. From this, it can be seen that in the nanomaterial 20 of the embodiment, excited electrons (hot carriers) are rapidly extracted due to the coordination interaction between the CsPbI3 quantum dots and the fullerene derivative, and therefore do not emit light. This result is consistent with the visual observation in Figure 4(C).

[0044] Figure 17 shows the lifetime of photoexcited carriers in various samples. The horizontal axis represents time (ns), and the vertical axis represents the photoluminescence (PL) intensity of the CsPbI3 quantum dot (normalized to the PL intensity at time 0). Each sample is irradiated with pulsed excitation light, and the change in PL intensity from the irradiated area is measured. The lifetime of the photoexcited carrier is determined by the time it takes for the PL intensity to reach 1 / e of the peak. The frames in Figure 17 show the spectra expanded to the time region from 0.5 ns to 0.25 ns.

[0045] From the measurement results in Figure 17, the lifetimes of the photoexcited carriers in samples L, M, and N are estimated to be 10–20 ns. The energy of the photoexcited carriers in samples L, M, and N decays within 10–20 ns, and returns to the ground level through recombination and emission. In contrast, the carrier lifetimes of samples O, P, and Q in the embodiment are tens of picoseconds. This indicates that the photoexcited hot carriers are extracted from the quantum dots into PCBA, Bis-PCBA, or C60-SAM fullerene derivatives faster than their relaxation time.

[0046] Figure 18 shows the X-ray diffraction spectra of various samples. All samples from L to Q have diffraction peaks at the same angle (2θ). This indicates that introducing a fullerene derivative with a chemical anchor group (-COOH) rapidly extracts hot carriers from the CsPbI3 quantum dot and changes its electronic state, but the material structure of the quantum dot itself does not change.

[0047] Figure 19 shows the dependence of PL intensity on excitation light intensity for various samples. The horizontal axis represents excitation intensity (μW), and the vertical axis represents PL intensity (arbitrary units). When the local electric field applied to the crystal increases, the stability of the excitons is weakened, and the charge recombination mechanism unique to CsPbI3 quantum dots may change. exc When measured as a function of, the PL emission intensity I PL is, excitation intensity I exc It is proportional to the power (I PL ∝(I exc ) C In samples O, P, and Q of the embodiment, the introduction of PCBA, Bis-PCBA, and C60-SAM increased the power of the relation from 0.9 to 1.4 compared to samples L, M, and N. This indicates that exciton coupling is weak in the organic-inorganic hybrid system of the embodiment.

[0048] Figure 20 shows the existence time of hot carriers in quantum dots for various samples, i.e., the hot carrier extraction time. The horizontal axis represents time (ps), and the vertical axis represents normalized absorbance (ΔA / A). Compared with samples L, M, and N, samples O, P, and Q of the embodiment have a very short existence time of hot carriers in quantum dots. The time at which the normalized absorbance decreases to 1 / e is approximately 22.5 ps for samples L, M, and N, while the hot carrier extraction times for samples O, P, and Q of the embodiment are 7.7 ps, 3.7 ps, and 13.5 ps, respectively. This is because the excited hot carriers are rapidly extracted from the quantum dots to the fullerene derivative ligand.

[0049] From Figure 20, we estimate the hot carrier extraction efficiency η for samples O, P, and Q in the embodiment. The hot carrier extraction efficiency η is calculated from the lifetime of each hot carrier and the lifetime of sample L (22.5 ps) using the following formula.

[0050] η = (1 / τ) SEL - 1 / τ0) / (1 / τ SEL ), Here, τ0 is the hot carrier lifetime of sample L containing only CsPbI3 quantum dots, τ SEL This is the hot carrier lifetime of a sample in which the selected material was added to a CsPbI3 quantum dot solution. The estimated hot carrier extraction efficiency η was 66.1% (η) in PCBA. PCBA =66.1%, Bis-PCBA: 83.8%(η Bis-PCBA =83.8%, 40.2% with C60-SAM (η C60-SAM This is 40.2%. High hot carrier extraction efficiency can be achieved by using fullerene derivatives having a chemical anchor group (-COOH).

[0051] Figure 21 shows the temperature changes of hot carriers in various materials. Samples L, M, and N take a long time to cool down, cooling slowly over 25 ps. In contrast, samples O, P, and Q of the embodiment take a short time to cool. The difference in the temperature changes of hot carriers in the quantum dots represents whether or not the hot carriers move to the fullerene derivative ligand. Figure 21 also shows that excellent hot carrier extraction characteristics can be obtained by binding a fullerene derivative having a chemical anchor group (-COOH) to the quantum dot.

[0052] <Application to solar cells> Figure 22 is a schematic diagram of a solar cell 10 according to an embodiment. The solar cell 10 has a first electrode 11, an electron transport layer 12, a photoactive layer 13, a hole transport layer 14, and a second electrode 15, and these layers are stacked in this order. The nanomaterials 20A, 20B, or 20C of the embodiment are used to form the photoactive layer 13. The nanomaterial 20, in which an electronegative fullerene derivative ligand is bound to a quantum dot, is dispersed in an organic solvent. By coating and annealing this solution, a layer of metal halide perovskite quantum dots having a hot carrier extraction layer on its surface is formed. This quantum dot layer is used as the photoactive layer 13.

[0053] The first electrode 11 is provided on the light incident side and is formed of a transparent conductive film such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO). The electron transport layer 12 is formed of an oxide semiconductor such as ZnO or TiO2 to a thickness of about 50 nm. The photoactive layer 13 is a layer of metal halide perovskite quantum dots to a thickness of about 450 nm, formed of the nanomaterial 20 of the embodiment.

[0054] The hole transport layer 14 may use polymers such as PEDOT (polyethylenedioxythiophene) or PEDOT:PSS (a dispersion of polyethylenedioxythiophene and polystyrene sulfonic acid), or it may use a layer of quantum dots treated (substituted) with ethylenedioxythiophene (EDT). The second electrode 15 is formed of a good conductor such as gold (Au).

[0055] In the photoactive layer 13 of the solar cell 10, hot carriers excited within the metal halide perovskite quantum dots by incident sunlight are rapidly extracted to fullerene derivative ligands and collected from the electron transport layer 12 to the first electrode 11. This hot carrier extraction function improves the energy conversion efficiency of the solar cell 10. [Explanation of Symbols]

[0056] 10 Solar Cells 11 1st electrode 12 Electron transport layer 13 Photoactive layer 14 Hole transport layer 15 2nd electrode 20, 20A, 20B, 20C nanomaterials 21 quantum dots 22A, 22B, 22C fullerene derivatives 23, 23A, 23B, 23C Fullerene derivative ligands

Claims

1. General formula ABX ３ (A=Cs, FA, MA, B=Pb ２＋ , Sn ２＋ X = I － , Br － , Cl － Quantum dots of metal halide perovskites represented by ) An electronegative fullerene derivative ligand bound to the quantum dot, It has, The fullerene derivative ligand is a nanomaterial having a Lewis base (-COO) obtained by removing a hydrogen ion from a fullerene derivative having a carboxyl group as a chemical anchor group, and the Lewis base is bonded to B of the quantum dot.

2. The fullerene derivative is phenyl C61-butyrate (PCBA), bis-PCBA, or 4-(1',5'-dihydro-1'-methyl-2'H-[5,6]fullereno-C60-Ih-[1,9-c]pyrrol-2'-yl)benzoic acid (C61-SAM). The nanomaterial according to claim 1.

3. The nanomaterial according to claim 1 or 2, wherein in the FTIR spectrum of the nanomaterial, the wavenumber of the peak of the C=0 stretching vibration of the carboxylic acid ester is higher than the wavenumber of the peak of the C=0 stretching vibration of the carboxylic acid ester before the fullerene derivative is bonded to the quantum dot.

4. A quantum dot organic solution in which quantum dots of metal halide perovskite represented by the general formula ABX3 (A = Cs, FA, MA, B = Pb²⁺, Sn²⁺, X = I⁻, Br⁻, Cl⁻) are dispersed is mixed with a fullerene derivative having a carboxyl group (-COOH) as a chemical anchor group. By binding the Lewis base (-COO), which is obtained by removing hydrogen ions from the fullerene derivative, to B in the quantum dot, the oleic acid ligand bound to the quantum dot is replaced with an electronegative fullerene derivative ligand to obtain a nanomaterial. Methods for fabricating nanomaterials.

5. The oleic acid ligand is replaced with the electronegative fullerene derivative ligand by stirring or ultrasonically treating a mixture of the quantum dot organic solution in which the quantum dots are dispersed and the fullerene derivative. A method for producing nanomaterials according to claim 4.

6. The ligand substitution rate is controlled by adjusting the time of the stirring or ultrasonic treatment. A method for producing nanomaterials according to claim 5.

7. Phenyl C61-methyl butyrate (PCBM) or bisPCBM is reacted with hydrochloric acid and acetic acid to produce the fullerene derivative having the carboxyl group (-COOH), The generated fullerene derivative is mixed into the quantum dot organic solution in which the quantum dots are dispersed. A method for producing nanomaterials according to any one of claims 4 to 6.

8. As the fullerene derivative having the carboxyl group (-COOH), 4-(1',5'-dihydro-1'-methyl-2'H-[5,6]fullereno-C60-Ih-[1,9-c]pyrrol-2'-yl)benzoic acid (C61-SAM) is mixed into the quantum dot organic solution in which the quantum dots are dispersed. A method for producing nanomaterials according to any one of claims 4 to 6.

9. In a solar cell having a photoactive layer between a first electrode and a second electrode, The photoactive layer is formed of a nanomaterial comprising quantum dots of a metal halide perovskite represented by the general formula ABX3 (A = Cs, FA, MA, B = Pb²⁺, Sn²⁺, X = I⁻, Br⁻, Cl⁻) and an electronegative fullerene derivative ligand in which a Lewis base (-COO) obtained by removing a hydrogen ion from a fullerene derivative having a carboxyl group as a chemical anchor group is bonded to B of the quantum dot. Solar cell.

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