Hole Transport Materials Based on Methylene-Bridged Difluorene Nucleus, Preparation Methods Thereof and Applications

Abstract

The present invention provides a hole transport material based on a methylene-bridged bifluorene core, a preparation method thereof and an application thereof, belonging to the technical field of organic optoelectronic materials. Traditional hole transport materials need to add chemical dopants to improve the relatively low hole mobility. On the one hand, the introduction of dopants will accelerate the degradation of the perovskite layer, having a great impact on the stability of the battery device; on the other hand, it will also increase the additional cost of the battery, which is not conducive to the large-scale commercial application of perovskite solar cells. Therefore, the present invention provides a novel, inexpensive, efficient and doping-free hole transport material.

Description

Technical Field

[0001] The present invention belongs to the technical field of organic optoelectronic materials, and particularly relates to a hole transport material based on a methylene-bridged difluorene core, a preparation method thereof, and an application thereof. Background Art

[0002] In 2009, the Miyasaka research group first applied a perovskite light-absorbing material to a solar cell to construct a perovskite solar cell (PSCs), and achieved a photoelectric conversion efficiency (PCE) of 3.8%, which attracted great attention. In the past ten-odd years, with the in-depth research of perovskite solar cells by scientific researchers, the current photoelectric conversion efficiency has been increased to 25.7%, approaching the highest efficiency of 26.1% of monocrystalline silicon solar cells. In addition to the high photoelectric conversion efficiency, perovskite solar cells also have the advantages of solution processability and low cost. Therefore, it is considered an emerging photovoltaic technology that is expected to replace traditional silicon-based solar cells.

[0003] As an important part of the typical perovskite solar cell structure, the hole transport material layer not only plays a role in extracting and transporting holes and blocking electrons, but also can protect the perovskite light-absorbing layer from air and moisture erosion, and improve the device stability. Currently, the most widely used hole transport material in high-efficiency perovskite solar cells is 2,2',7,7'-tetrakis[N,N-bis(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD). However, the synthesis steps are long, the purification is cumbersome, and some reaction conditions are harsh, resulting in its high price. In addition, it needs to add chemical dopants to improve the relatively low hole mobility. The introduction of dopants, on the one hand, will accelerate the degradation of the perovskite layer, causing a greater impact on the stability of the battery device; on the other hand, it will also increase the additional cost of the battery, which is not conducive to the large-scale commercial application of perovskite solar cells. Therefore, it is of great significance to develop a new type, inexpensive, efficient, and dopant-free hole transport material to replace spiro-OMeTAD. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a hole transport material based on a methylene-bridged difluorene core, a preparation method thereof, and an application thereof.

[0005] To achieve the above object, the present invention provides a hole transport material based on a methylene-bridged difluorene core, and the structural general formula is shown in formula (I):

[0006]

[0007] Wherein, R 1 is a C1-C12 hydrocarbon group; R 2is H, a C1-C6 alkoxy group, a C1-C6 alkylthio group, or a C1-C6 hydrocarbon group.

[0008] Preferably, the R 1 is a C1-C6 hydrocarbon group; the R 2 is a methoxy group or a methylthio group.

[0009] Preferably, the structural formula of the hole transporting material based on the methylene-bridged dithienylfluorene core is as follows:

[0010]

[0011]

[0012] A preparation method of the hole transporting material based on the methylene-bridged dithienylfluorene core includes the following steps:

[0013] Under the protection of an inert gas, a compound represented by formula (IIa), a compound represented by formula (IIb), a base, a palladium catalyst, a ligand, and a first solvent are mixed to undergo a Hartwig-Buchwald coupling reaction to obtain the hole transporting material based on the methylene-bridged dithienylfluorene core;

[0014]

[0015] Among them, R 1 is a C1-C12 hydrocarbon group; R 2 is H, a C1-C6 alkoxy group, a C1-C6 alkylthio group, or a C1-C6 hydrocarbon group.

[0016] Preferably, the Hartwig-Buchwald coupling reaction is carried out under reflux conditions, the reaction temperature is 60-140 °C, and the time is 10-48 h.

[0017] Preferably, the molar ratio of the compound represented by formula (IIa), the compound represented by formula (IIb), the base, the palladium catalyst, and the ligand is 1:(4-8):(4-20):(0.01-0.20):(2-90).

[0018] Preferably, the palladium catalyst is at least one of tris(dibenzylideneacetone)dipalladium, palladium acetate, tetrakis(triphenylphosphine)palladium, and [1,1'-bis(diphenylphosphino)ferrocene]dichloropalladium;

[0019] The base is at least one of sodium tert-butoxide, potassium tert-butoxide, lithium bis(trimethylsilyl)amide, sodium bis(trimethylsilyl)amide, sodium carbonate, potassium carbonate, cesium carbonate, and potassium phosphate;

[0020] The ligand is at least one of triphenylphosphine, 1,1'-bis(diphenylphosphino)ferrocene, 1,1'-binaphthyl-2,2'-bisdiphenylphosphine, tri(o-tolyl)phosphine, bis(2-diphenylphosphinophenyl)ether, tri-tert-butylphosphine and tri-tert-butylphosphine tetrafluoroborate.

[0021] Preferably, the first solvent is at least one of tetrahydrofuran (THF), 1,4-dioxane, ethylene glycol dimethyl ether, benzene, toluene, xylene, N,N-dimethylformamide (DMF), N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide and tert-butyl alcohol;

[0022] The inert gas is argon or nitrogen.

[0023] The preparation method of the compound represented by the above formula (IIa) is a method known in the art, and specifically may include the following steps:

[0024] 1) reacting 2,7-dibromofluorene with paraformaldehyde or formaldehyde to obtain a compound represented by formula (IIIa);

[0025] 2) reacting the compound represented by formula (IIIa) with the compound represented by formula (IIIb) to obtain the compound represented by formula (IIa);

[0026] R 1 -X Formula (IIIb);

[0027] Among them, R 1 is a C1-C12 hydrocarbon group; X is Cl, Br, I, mesylate, trifluoromethanesulfonate or p-toluenesulfonate;

[0028] In step 1) of the method for preparing the compound represented by formula (IIa), the reaction temperature is -10 to 60°C and the reaction time is 0.5h to 6h;

[0029] The reaction in step 1) is carried out under alkaline conditions, and the base is at least one of sodium tert-butoxide, potassium tert-butoxide, sodium hydroxide and potassium hydroxide;

[0030] The molar ratio of 2,7-dibromofluorene to paraformaldehyde or formaldehyde and alkali is 1:(0.1-0.5):(0.05-0.5);

[0031] The reaction in step 1) is carried out in a solution, wherein the solvent is at least one of tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide (DMF), N,N-dimethylacetamide, dimethyl sulfoxide and ethanol;

[0032] In step 2) of the method for preparing the compound represented by formula (IIa), the reaction temperature is -10 to 60°C and the reaction time is 2h to 24h;

[0033] Step 2) The reaction is carried out under alkaline conditions, and the base is at least one of sodium tert-butoxide, potassium tert-butoxide, sodium hydroxide, and potassium hydroxide;

[0034] The molar ratio of the compound shown in formula (IIIa), the compound shown in formula (IIIb), and the base is 1∶(2-10)∶(2-10);

[0035] Step 2) The reaction is carried out in a solution, and the solvent is at least one of tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide (DMF), N,N-dimethylacetamide, dimethyl sulfoxide, and ethanol.

[0036] Application of the hole transport material based on the methylene-bridged bifluorene core in perovskite solar cells.

[0037] Compared with the prior art, the present invention has the following advantages and technical effects:

[0038] In the hole transport material of the present invention, the two fluorene units adopt a "face-to-face" stacking conformation, and there is an intramolecular π-π interaction, which is beneficial to the stability and transport of holes; compared with the traditional classic spiro-OMeTAD, the hole mobility of the hole transport material is significantly improved; in addition, the introduced hydrocarbon group can increase the solubility of the material and improve the film-forming performance of the material, thereby reducing the requirements for device preparation.

[0039] The preparation method of the hole transport material of the present invention has a simple synthesis route, cheap and easily available raw materials, mild reaction conditions, is easy to prepare on a large scale, and is conducive to the commercialization of hole transport materials and perovskite solar cell devices.

[0040] When the hole transport material of the present invention is applied to a perovskite solar cell, the photoelectric conversion efficiency is relatively high (19.06%), and it has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0042] Figure 1 Is the single crystal structure of the hole transport material BFM-C1 prepared in Example 1;

[0043] Figure 2 Is the normalized ultraviolet-visible absorption spectrum of the hole transport material BFM-C1 and BFM-C6 thin films prepared in Example 1 and Example 2;

[0044] Figure 3Thermogravimetric curves of hole transporting materials BFM-C1 and BFM-C6 prepared for Example 1 and Example 2;

[0045] Figure 4 Differential scanning calorimetry curves of hole transporting materials BFM-C1 and BFM-C6 prepared for Example 1 and Example 2;

[0046] Figure 5 Schematic diagram of the structure of perovskite solar cell devices prepared based on BFM-C1 and BFM-C6;

[0047] Figure 6 J-V curve diagram of perovskite solar cell devices prepared based on BFM-C1 and BFM-C6. Detailed implementation manners

[0048] Now, various exemplary implementation manners of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0049] It should be understood that the terms described in the present invention are only for describing specific implementation manners and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0050] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0051] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific implementation manners of the specification of the present invention, which are obvious to those skilled in the art. Other implementation manners obtained from the specification of the present invention are also obvious to those skilled in the art. The specification and examples of the present invention are only exemplary.

[0052] Regarding the terms "comprising", "including", "having", "containing", etc. used herein, they are all open-ended terms, meaning including but not limited to.

[0053] Example 1

[0054]

[0055] (1) Synthesis and Characterization of Intermediate 1

[0056] To 50 mL of N,N-dimethylformamide at 0 °C, 2,7-dibromofluorene (3.24 g, 10 mmol) and potassium tert-butoxide (0.17 g, 1.5 mmol) were added, and the mixture was stirred for 15 min under an argon atmosphere. Then paraformaldehyde (0.15 g, 5 mmol) was added, and the mixture was stirred at 0 °C for 2 h. The reaction mixture was poured into 100 mL of 5% (mass fraction, the same below) HCl solution to quench the reaction. The white precipitate was collected by filtration and washed with water. After drying, 2.80 g of white solid intermediate 1 was obtained, Y = 85%, and it was directly used in the next step without further purification.

[0057] 1 H NMR (400 MHz, CDCl 3 ) δ: 7.49 - 7.54 (m, 12H), 4.15 (t, J = 8.0 Hz, 2H), 2.45 (t, J = 8.0 Hz, 2H).

[0058] (2) Synthesis and Characterization of Intermediate 2a

[0059] To a 50 mL tetrahydrofuran solution of intermediate 1 (1.98 g, 3 mmol) at 0 °C, potassium tert-butoxide (1.01 g, 9 mmol) was added, and the mixture was stirred for 1 h under an argon atmosphere. Then methyl iodide (9 mmol) was added, and the mixture was stirred at room temperature overnight. The reaction mixture was poured into 30 mL of 5% HCl solution to quench the reaction. The mixture was extracted with dichloromethane (3 × 30 mL), and the organic phases were combined, washed with saturated brine and water, dried over anhydrous sodium sulfate, and the solvent was removed under vacuum to obtain the crude product. The crude product was separated by column chromatography (petroleum ether:dichloromethane = 20:1, volume ratio) and further purified by recrystallization (dichloromethane / n-hexane) to obtain white solid intermediate 2a, Y = 67%.

[0060] 1 H NMR (400 MHz, CDCl 3 ) δ: 7.18 (dd, J 1 = 8.0 Hz, J 2 = 1.6 Hz, 4H), 7.03 (d, J = 1.6 Hz, 4H), 6.92 (d, J = 8.0 Hz, 4H), 2.94 (s, 2H), 1.29 (s, 6H). 13 C NMR (101 MHz, CDCl 3)δ: 151.23, 137.69, 130.46, 127.12, 121.05, 120.44, 49.92, 48.87, 29.06.

[0061] (3) Synthesis and Characterization of Compound BMF-C1

[0062] Under argon protection, intermediate 2a (0.98 mmol), 4,4'-dimethoxydiphenylamine (1.38 g, 6.03 mmol), tris(dibenzylideneacetone)dipalladium(0) (0.092 g, 0.10 mmol), triphenylphosphine (0.33 g, 1.25 mmol) and sodium tert-butoxide (1.01 g, 10.43 mmol) were dissolved in dry toluene (10 mL). Then it was heated to reflux for 12 h. After the reaction was completed, it was cooled to room temperature, the solvent was removed under vacuum, and the pale yellow solid compound BMF-C1 was obtained by column chromatography (petroleum ether:ethyl acetate = 5:1, v / v), Y = 64%.

[0063] IR (KBr, cm -1 ) 3038, 2995, 2945, 2831, 1608, 1504, 1455, 1240, 1176, 1035, 821. 1 1H NMR (400 MHz, C 6 D 6 ) δ: 7.32 (d, J = 8.0 Hz, 8H), 7.04 - 7.13 (m, 14H), 6.77 - 6.82 (m, 22H), 3.35 (s, 24H), 2.30 (s, 2H), 1.11 (s, 6H). 13 13C NMR (101 MHz, C 6 D 6 ) δ: 156.09, 153.39, 147.06, 142.04, 133.78, 126.54, 120.71, 119.64, 117.33, 115.03, 55.08, 50.42, 48.08, 31.46. HRMS (ESI) m / z: calcd for C 85 H 76 N 4 O 8 [M + H] + 1281.5736, found 1281.5762.

[0064] The single crystal structure of the hole transporting material BMF-C1 is shown in Figure 1 , it can be seen that the two fluorene units in BMF-C1 are stacked "face to face", and the shortest Csp2–Csp2 distance between them is Indicates the existence of intramolecular π-π interactions.

[0065] The ultraviolet-visible absorption spectrum of the thin film of hole transport material BMF-C1 is shown in Figure 2 , the absorption range of the BMF-C1 thin film is 270 - 500 nm, the maximum absorption peak is at 386 nm, and the optical band gap of this material is calculated to be 2.96 eV from the absorption edge position.

[0066] The thermogravimetric curve of hole transport material BMF-C1 is shown in Figure 3 , the decomposition temperature of this material is 457.1 °C; the differential scanning calorimetry curve of hole transport material BMF-C1 is shown in Figure 4 , the glass transition temperature of this material is 143.9 °C. This indicates that the material has good thermal stability.

[0067] A pure hole device with the ITO / PEDOT:PSS / HTM / Au structure was fabricated. According to the Mott-Gurney law equation, the hole mobility of this material was calculated to be 1.01×10 -4 cm 2 V -1 s -1 .

[0068] Example 2

[0069]

[0070] (1) Synthesis and characterization of intermediate 2b

[0071] Potassium tert-butoxide (1.01 g, 9 mmol) was added to a 50 mL tetrahydrofuran solution of intermediate 1 (1.98 g, 3 mmol) at 0 °C, and the mixture was stirred for 1 h under an argon atmosphere. Then 1-bromohexane (9 mmol) was added, and the mixture was stirred at room temperature overnight. The reaction mixture was poured into 30 mL of 5% HCl solution to quench the reaction. The mixture was extracted with dichloromethane (3 × 30 mL), the organic phases were combined, washed with saturated brine and water, dried over anhydrous sodium sulfate, and the solvent was removed under vacuum to obtain a crude product. The crude product was separated by column chromatography (petroleum ether:dichloromethane = 20:1, volume ratio) and further purified by recrystallization (dichloromethane / n-hexane) to obtain white solid intermediate 2b, Y = 62%.

[0072] 1 H NMR (400 MHz, CDCl 3)δ: 7.17 (d, J = 8.0 Hz, 4H), 6.96 (s, 4H), 6.90 (d, J = 8.0 Hz, 4H), 2.89 (s, 2H), 1.80 - 1.84 (m, 4H), 1.26 (s, 2H), 1.02 - 1.08 (m, 4H), 0.83 - 0.94 (m, 10H), 0.73 (t, J = 7.6 Hz, 6H). 13 C NMR(101MHz, CDCl 3 )δ: 149.96, 138.69, 130.37, 127.00, 121.02, 120.29, 53.97, 49.18, 42.18, 31.41, 29.39, 22.63, 22.39, 14.06.

[0073] (2) Synthesis and Characterization of Compound BMF - C6

[0074] Under argon protection, intermediate 2b (0.98 mmol), 4,4'-dimethoxydiphenylamine (1.38 g, 6.03 mmol), tris(dibenzylideneacetone)dipalladium(0) (0.092 g, 0.10 mmol), triphenylphosphine (0.33 g, 1.25 mmol) and sodium tert - butoxide (1.01 g, 10.43 mmol) were dissolved in dry toluene (10 mL). Then it was heated to reflux for 12 h. After the reaction was completed, it was cooled to room temperature, the solvent was removed under vacuum, and the product was purified by column chromatography (petroleum ether:ethyl acetate = 10:1, v / v) to obtain a pale yellow solid compound BMF - C6, Y = 53%.

[0075] IR(KBr, cm -1 ) 3040, 2999, 2927, 2839, 1608, 1505, 1456, 1239, 1177, 1036, 821; 1 HNMR(400MHz, C 6 D 6 )δ: 7.33 (d, J = 8.0 Hz, 8H), 7.14 (s, 8H), 7.05 (d, J = 8.0 Hz, 6H), 6.83 (d, J = 8.8 Hz, 18H), 6.73 (s, 4H), 3.38 (s, 24H), 2.38 (s, 2H), 1.46 - 1.50 (m, 4H), 1.10 - 1.22 (m, 16H), 0.87 (t, J = 7.6 Hz, 6H). 13 C NMR(101MHz, C 6 D 6)δ: 156.03, 151.80, 146.92, 142.11, 135.06, 126.50, 120.62, 119.21, 117.66, 115.01, 55.06, 54.05, 47.45, 44.51, 32.00, 30.15, 23.92, 23.24, 14.37. HRMS(ESI) m / z: calcd for C 95 H 96 N 4 O 8 [M + H] + 1421.7301, found 1421.7371.

[0076] The UV - Vis absorption spectrum of the film of hole - transporting material BMF - C6 is shown in Figure 2 , the absorption range of the BMF - C6 film is 270 - 500 nm, the maximum absorption peak is at 390 nm, and the optical band gap of this material calculated from the absorption edge position is 2.97 eV.

[0077] The thermogravimetric curve of hole - transporting material BMF - C6 is shown in Figure 3 , the decomposition temperature of this material is 438.7 °C; the differential scanning calorimetry curve of hole - transporting material BMF - C6 is shown in Figure 4 , the glass transition temperature of this material is 112.5 °C. It shows that the thermal stability of this material is very good.

[0078] Fabricate a pure - hole device with the ITO / PEDOT:PSS / HTM / Au structure. According to the Mott - Gurney law equation, the hole mobility is calculated to be 9.71×10 -5 cm 2 V -1 s -1 .

[0079] Example 3

[0080] The fluorine - doped tin oxide (FTO) glass with patterned surface etching is ultrasonically cleaned with deionized water, acetone, isopropanol, and ethanol for 10 minutes in sequence, dried with N 2 and then followed by 15 - minute oxygen plasma treatment. After that, a mixed solution prepared by mixing titanium diisopropoxide bis(acetylacetonate) (75 wt% isopropanol solution), acetylacetone, and isopropanol in a volume ratio of 3:2:50 is deposited on the FTO glass preheated at 450 °C by spray pyrolysis to form a dense TiO 2 layer. After diluting the TiO 2 slurry (Dyesol30NRD) and absolute ethanol in a mass ratio of 1:6, it is spin - coated at 4000 rpm on the dense TiO 2The mesoporous TiO was formed by spin-coating on the layer for 20 seconds and then sintering at 500 °C for 30 minutes. 2 Secondly, the perovskite photoactive layer was prepared by spin-coating the perovskite precursor solution in one step. First, it was spin-coated at a speed of 1000 rpm for 10 seconds, and then at a speed of 6000 rpm for 20 seconds. When spin-coating at a speed of 6000 rpm for 15 seconds, 100 μL of chlorobenzene was quickly dropped onto the surface of the perovskite. After spin-coating, the prepared sample was annealed at 120 °C for 20 minutes to generate a black perovskite phase. Here, the perovskite precursor solution was prepared by dissolving 1.39 M PbI 2 , 1.28 M FAI, 0.15 M MABr, 0.15 M PbBr 2 and 0.07 M CsI in a DMF / DMSO mixed solvent with a volume ratio of 4:1. After cooling, a chlorobenzene solution of the hole transport material BFM-C1 or BFM-C6 was spin-coated on the surface of the perovskite photoactive layer, with a concentration of 10 mg / mL, a spin-coating speed of 3000 rpm, and a spin-coating time of 20 seconds. Finally, a 100-nm-thick Au electrode was vacuum-evaporated to obtain an n-i-p type perovskite solar cell device (for the structural schematic diagram, see Figure 5 ).

[0081] Under the irradiation of AM 1.5G simulated sunlight with an illumination intensity of 100 mW / cm 2 , the voltage-current (J-V) curve of the n-i-p type perovskite solar cell device was tested, and the results are as shown in Figure 6 . Using BFM-C1 as the hole transport material, the open-circuit voltage (Voc) was 1.06 V, the short-circuit current density (Jsc) was 23.13 mA / cm 2 , the fill factor (FF) was 74.33%, and the power conversion efficiency (PCE) was 18.35%. Using BFM-C6 as the hole transport material, the open-circuit voltage (Voc) was 1.09 V, the short-circuit current density (Jsc) was 23.08 mA / cm 2 , the fill factor (FF) was 75.91%, and the power conversion efficiency (PCE) was 19.06%.

[0082] From the above results, it can be seen that the hole transport material based on the methylene-bridged difluorene core provided by the present invention has good photovoltaic performance and shows good application prospects.

[0083] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A hole transport material based on a methylene-bridged difluorene core, It is characterized in that The general structural formula is shown in formula (I): Among them, R 1 is a C1-C12 hydrocarbon group; R 2 is H, a C1-C6 alkoxy group, a C1-C6 alkylthio group or a C1-C6 hydrocarbon group.

2. The hole transport material based on methylene bridged difluorene core according to claim 1, It is characterized in that The R 1 is a C1-C6 hydrocarbyl group; the R 2 is a methoxy group or a methylthio group.

3. The hole transport material based on methylene bridged difluorene core according to claim 1, It is characterized in that The structural formula is as follows:

4. A method for preparing a hole transport material based on a methylene-bridged bifluorene core according to any one of claims 1 to 3, It is characterized in that The following steps are involved: Under the protection of an inert gas, the compound represented by formula (IIa), the compound represented by formula (IIb), a base, a palladium catalyst, a ligand and a first solvent are mixed to undergo a Hartwig-Buchwald coupling reaction to obtain the hole transport material based on a methylene-bridged bifluorene core; Among them, R 1 is a C1-C12 hydrocarbyl group; R 2 is H, a C1-C6 alkoxy group, a C1-C6 alkylthio group or a C1-C6 hydrocarbyl group.

5. The method for preparing a hole transport material based on a methylene-bridged difluorene core according to claim 4, It is characterized in that The Hartwig-Buchwald coupling reaction is carried out under reflux conditions, with a reaction temperature of 60 to 140° C. and a reaction time of 10 to 48 hours.

6. The method for preparing a hole transport material based on a methylene-bridged difluorene core according to claim 4, It is characterized in that The molar ratio of the compound represented by formula (IIa), the compound represented by formula (IIb), the base, the palladium catalyst and the ligand is 1:(4-8):(4-20):(0.01-0.20):(2-90).

7. The method for preparing a hole transport material based on a methylene-bridged difluorene core according to claim 6, It is characterized in that The palladium catalyst is at least one of tris(dibenzylideneacetone)dipalladium, palladium acetate, tetrakis(triphenylphosphine)palladium and [1,1'-bis(diphenylphosphino)ferrocene]dichloropalladium; The base is at least one of sodium tert-butoxide, potassium tert-butoxide, lithium bis(trimethylsilyl)amide, sodium bis(trimethylsilyl)amide, sodium carbonate, potassium carbonate, cesium carbonate and potassium phosphate; The ligand is at least one of triphenylphosphine, 1,1'-bis(diphenylphosphino)ferrocene, 1,1'-binaphthyl-2,2'-bisdiphenylphosphine, tri(o-tolyl)phosphine, bis(2-diphenylphosphinophenyl)ether, tri-tert-butylphosphine and tri-tert-butylphosphine tetrafluoroborate.

8. The method for preparing a hole transport material based on a methylene-bridged difluorene core according to claim 4, It is characterized in that The first solvent is at least one of tetrahydrofuran, 1,4-dioxane, ethylene glycol dimethyl ether, benzene, toluene, xylene, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide and tert-butyl alcohol; The inert gas is argon or nitrogen.

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