Low-temperature cross-linking hole transport material and synthesis method and application thereof

Abstract

The application belongs to the field of organic photoelectric functional materials and devices, and particularly relates to a low-temperature cross-linking hole transport material and a synthesis method and application thereof. Two low-temperature cross-linking hole transport materials are successfully prepared by taking 9H-carbazole and 2,7-dibromo-9-butyl-9H-carbazole or 3,6-dibromo-9-butyl-9H-carbazole as raw materials. The two cross-linking hole transport materials have good thermal stability and solvent resistance and low cross-linking temperature. The two materials are applied to a QLED device hole transport layer, which significantly improves the current injection imbalance problem, improves the QLED efficiency, protects the original high charge transport performance of the hole injection layer, and promotes the rapid injection of holes.

Description

Technical Field

[0001] This invention belongs to the field of organic optoelectronic functional materials and devices, specifically relating to low-temperature cross-linked hole transport materials, their synthesis methods, and applications. Background Technology

[0002] Quantum dot light-emitting diodes (QLEDs), as the core of next-generation display technology, are considered a major breakthrough that will revolutionize traditional display solutions due to their high color purity, wide color gamut, and solution processability. Their core principle is to drive quantum dot materials to emit red, green, and blue primary colors of light through voltage, achieving precise color reproduction and demonstrating enormous potential in high-end displays, flexible electronics, and other fields. However, the commercialization of this technology faces a key bottleneck—the stability and efficiency of blue QLEDs. While red and green QLEDs have achieved external quantum efficiency (EQE) exceeding 37% and 28% respectively, and have a lifespan exceeding 5000 hours, approaching industrial standards, the EQE and lifespan of blue QLEDs are far from meeting practical requirements.

[0003] With the continuous advancement of blue QLED fabrication technology and the gradual optimization of device structures, nonradiative recombination caused by uneven charge (hole and electron) injection is considered a major factor affecting the efficiency and lifetime of blue QLEDs. Therefore, solving the charge injection imbalance problem in blue QLEDs has become a key technical challenge for improving device efficiency and lifetime.

[0004] Researchers, starting from the screening and structural control of hole transport materials, have proposed the following technical solutions to address the problem of charge injection imbalance: Peng Xiaogang et al. and Srivastava Abhishek K. et al. respectively utilized the shallow energy level and high carrier transport performance of Poly-TPD (-5.4 eV, 1×10⁻⁶ ... -4 cm 2 V -1 s -1 ), TFB (-5.3 eV, 1×10) -3 cm 2 V -1 s -1 ) and deep-level PVK (-5.7 eV, 2.5 × 10⁻⁶ ...) -6 cm 2 V -1 s -1By combining TFB and HAT-CN with PVK to fabricate bilayer hole transport layers (HTLs), the charge injection barrier between the ITO anode and the quantum dot emitting layer was reduced, optimizing its energy level structure and improving the charge injection balance problem of QLEDs, thus increasing the external quantum efficiency of red QLEDs to over 20%. Building on this, Xu Zheng et al. and Zhao Jialong et al. fabricated blue QLEDs using bilayer HTLs by combining TFB and HAT-CN with PVK, respectively. Benefiting from the improved charge injection efficiency, the external quantum efficiency of the blue QLEDs was increased to over 10%. Subsequently, Li Fushan et al. selected TFB combined with deep-level C8-BTBT (-5.85 eV) as the bilayer HTL for blue QLED devices. Compared to blue QLEDs fabricated using TFB alone as an HTL, the introduction of bilayer HTLs increased the external quantum efficiency of blue QLEDs from 4.84% to 7.23%.

[0005] Currently, the selection of bilayer HTLs materials and the design of device structures have smoothed the energy level between ITO and the quantum dot emitting layer to some extent, promoting the improvement of the external quantum efficiency of QLEDs. However, the fabrication of bilayer HTLs cannot avoid partial solvent etching, reducing the reproducibility of the device; bilayer HTLs also increase the fabrication process and production cost of QLEDs, hindering their commercial development. Furthermore, the device structure of bilayer HTLs has not significantly improved the external quantum efficiency of blue QLEDs to meet practical application requirements. Therefore, finding a simple method to fabricate high-efficiency blue QLED devices remains a serious challenge.

[0006] Cross-linked films, due to their three-dimensional network structure, not only possess high mechanical strength and stability but also exhibit resistance to solvent erosion. Cross-linked hole transport material monomers can be constructed by introducing multiple cross-linking groups around a molecular core with hole transport capabilities. A three-dimensional network hole transport layer is then built using photo- or thermally induced cross-linking of the monomers, effectively solving the interlayer miscibility and stability problems of HTLs. Li Qing et al. and Wang Shirong et al. designed and synthesized triphenylamine-based cross-linked hole transport materials such as FLTA-V, vp-TPA, and OXZ-VPAN using triphenylamine derivatives and vinyl groups as hole transport units and cross-linking groups, achieving carrier transport performance reaching 10... -4 cm 2 V -1 s -1However, the shallow energy level structure of triphenylamine-based charge transport materials increases the injection barrier from the HTL to the quantum dot emitting layer, resulting in an external quantum efficiency (EQE) of blue QLEDs prepared using these HTLs below 7%. Su Wenming et al. designed and synthesized two carbazole-based crosslinked hole transport materials, DV-FLCZ and DV-SFCZ, using carbazole and ethylene as charge transport units and crosslinking groups, respectively, and dimethylfluorene or spirodifluorene as π-bridges. The results showed that dimethylfluorene, with its smaller steric hindrance, was more conducive to intermolecular stacking and improved charge transport performance than spirodifluorene. The introduction of the carbazole structure lowered the HOMO energy level of the crosslinked hole transport material to -5.7 eV. Thanks to the optimization of transport performance and energy level structure, the EQE of blue QLEDs prepared using DV-FLCZ as a crosslinked HTL reached 8.5%. Liu Hongli et al. used the structurally simple CBP-V as a crosslinked HTL for blue QLEDs, thereby achieving an EQE of over 10% for the blue QLEDs. 50 The lifetime reached 139 h. Although the development of cross-linked HTLs solved the interlayer solubility problem in the preparation process of blue QLED functional layers, simplified the device preparation process, and improved the working stability of QLEDs, the preparation process of cross-linked HTLs requires a high temperature, which will cause serious damage to the hole injection layer. Summary of the Invention

[0007] To address the problems in the existing technology, this invention proposes a low-temperature cross-linked hole transport material, its synthesis method, and its application.

[0008] The low-temperature cross-linked hole transport material of the present invention has the following structural formula:

[0009] or .

[0010] The specific synthetic route is as follows:

[0011] .

[0012] The specific steps are as follows: (1) Under anhydrous and oxygen-free conditions, compound I, compound II, cuprous iodide, potassium carbonate and 18-crown ether-6 were added one by one to 1,3-dimethyl-tetrahydropyrimidine-2(1H)-one, stirred to dissolve, heated to 200-230 °C and reacted at a constant temperature. After the reaction was completed, the mixture was cooled and successively extracted and washed with deionized water, methanol and petroleum ether. The organic phase was dried and distilled under reduced pressure to obtain compound III. (2) Under anhydrous and oxygen-free conditions, compound III, DMF and chloroform were mixed and phosphorus oxychloride was added dropwise. The temperature was controlled at 0-5℃. After the addition was completed, the mixture was kept warm and stirred for 0.5-1h. Then the temperature was raised to 60-80℃ and kept constant for 0h. After the reaction was completed, the mixture was cooled, filtered, the solvent was removed by evaporation, and purified to obtain compound IV. (3) Under anhydrous and oxygen-free conditions, methyltriphenylphosphine bromide and potassium tert-butoxide were dissolved in tetrahydrofuran and stirred at 0-5℃ for 1.5-2 h. Then, a tetrahydrofuran solution containing compound IV was added dropwise at 0-5℃. After the addition was complete, the temperature was raised to 70-90℃ and the reaction was kept constant. After the reaction was complete, the mixture was cooled, the solvent was evaporated, and the mixture was purified to obtain compound V.

[0013] In molar ratio, compound I: compound II = (2-2.5):1.

[0014] In molar ratio, Compound III: Phosphorus oxychloride: DMF = 1:(20-30):(20-30).

[0015] In molar ratio, compound IV: methyltriphenylphosphine bromide: potassium tert-butoxide = 1:(5-10):(5-10).

[0016] The application of the low-temperature cross-linked hole transport material described in this invention in quantum dot light-emitting diodes.

[0017] Compared with the prior art, the present invention has the following advantages: (1) This invention provides a novel cross-linked hole transport material, which provides guidance for the design of subsequent transport materials; (2) The transport material described in this invention has the characteristics of low-temperature cross-linking, which can avoid the decomposition of PEDOT:PSS by high temperature, thereby protecting the original high charge transport performance of the hole injection layer and promoting the rapid injection of holes, providing a new idea for the development of low-temperature cross-linked hole transport materials. (3) The hole transport material developed in this invention has a high carrier mobility, which can optimize the charge imbalance problem of blue QLED; (4) The transmission material described in this invention is simple to synthesize and can be commercially produced; (5) The carrier mobility of the transmission material described in this invention reaches 10. -4 -10 -3 cm 2 V -1 s -1 This improved the current injection imbalance problem and enhanced QLED efficiency. Attached Figure Description

[0018] Figure 1The proton NMR spectrum of the hole transport material V-KKP 27.

[0019] Figure 2 The proton NMR spectrum of the hole transport material V-KKP 36.

[0020] Figure 3 TGA curves for hole transport materials V-KKP27 and V-KKP36.

[0021] Figure 4 DSC curve of hole transport material V-KKP27.

[0022] Figure 5 DSC curve of hole transport material V-KKP36.

[0023] Figure 6 Infrared-visible absorption spectrometer curves of hole transport materials V-KKP27 and V-KKP36.

[0024] Figure 7 Fluorescence spectra and fluorescence lifetime decay curves of hole transport materials V-KKP 27 and V-KKP 36.

[0025] Figure 8 This is a graph showing the relationship between the external quantum efficiency and brightness of a QLED.

[0026] Figure 9 This is a graph showing the relationship between QLED current efficiency and brightness.

[0027] Figure 10 This is a graph showing the relationship between QLED voltage, brightness, and current density. Detailed Implementation

[0028] Example 1: Synthesis of hole transport material V-KKP 27 .

[0029] (1) Synthesis of 9'-butyl-9'H-9,2':7',9''-tricarbazole Under anhydrous and oxygen-free conditions, 3.81 g of 2,7-dibromo-9-butyl-9H-carbazole (0.01 mol), 4.18 g of 9H-carbazole (0.025 mol), 0.19 g of cuprous iodide (0.001 mol), 5.52 g of potassium carbonate (0.04 mol), and 0.53 g of 18-crown ether-6 (0.002 mol) were added one by one to a three-necked flask containing 20 mL of anhydrous 1,3-dimethyl-tetrahydropyrimidine-2(1H)-one, and stirred to dissolve at room temperature. The temperature was raised to 200 °C, and the reaction was carried out under nitrogen protection for 24 h. During the reaction, the reaction progress was monitored every 3 h by thin-layer chromatography, and the color and state changes of the system were recorded in detail. After the reaction was completed, the system was allowed to cool naturally to room temperature, and the mixture was then diluted successively with 50 mL of deionized water, 50 mL of methanol, and 50 mL of sodium hydroxide. Extraction and washing with mL of petroleum ether; transfer the organic phase to a container, add anhydrous MgSO4 to dry and let stand overnight, filter, and distill under reduced pressure to obtain 4.65 g of 9'-butyl-9'H-9,2':7',9''-tricarbazole, with a yield of 84%.

[0030] (2) Synthesis of 9'-butyl-9'H-[9,2':7',9''-tercarbazole]-3,3''-dicarboxaldehyde Under anhydrous and oxygen-free conditions, 1.66 g of 9'-butyl-9'H-9,2':7',9''-tricarbazole (0.003 mol) and 4.386 g of DMF (0.060 mol) were accurately weighed into a 150 mL round-bottom flask. 45 mL of anhydrous chloroform was then added to the flask. Using a dropping funnel, 9.198 g of phosphorus oxychloride (0.060 mol) was slowly added, with strict control over the dropping rate and the temperature maintained at 0°C. After the addition was complete, the mixture was stirred at this temperature for 0.5 h, during which time the reaction system gradually changed from a clear solution to a pale yellow suspension. The reaction system was then heated to 70°C and held at this temperature for 24 h. Color changes were recorded every 2 hours, and the reaction progress was monitored by thin-layer chromatography (TLC) using dichloromethane as the developing solvent until the starting material spots completely disappeared. After the reaction was completed, the system was cooled to room temperature, and a small amount of insoluble matter was removed by filtration using a Buchner funnel preheated to 30°C. The filtrate was transferred to a rotary evaporator to remove the solvent, and then purified by silica gel column chromatography: 200-300 mesh silica gel (2.0 cm × 25 cm) was packed wet-process, with dichloromethane as the single eluent, and the target fraction was collected by TLC positioning. After combining the purified fractions, the eluent was removed by rotary evaporation, and the residue was placed in a vacuum drying oven for 12 h to dry thoroughly, finally yielding 1.152 g of compound 9'-butyl-9'H-[9,2':7',9''-tricarbazole]-3,3''-dicarboxaldehyde, with a yield of 63%.

[0031] (3) Synthesis of 9'-butyl-3,3''-divinyl-9'H-9,2':7',9''-tercarbazole Under anhydrous and oxygen-free conditions, 7.14 g of methyltriphenylphosphine bromide (0.02 mol) and 2.24 g of potassium tert-butoxide (0.02 mol) were accurately weighed and dissolved in 20 mL of anhydrous tetrahydrofuran (THF) dried with activated molecular sieves. The mixture was stirred at 0–5 °C for 1.5–2 h. Separately, 1.22 g of 9'-butyl-9'H-[9,2':7',9''-tricarbazole]-3,3''-dicarboxaldehyde (0.002 mol) was dissolved in 10 mL of anhydrous THF and slowly added dropwise to the main reaction system through a constant-pressure dropping funnel. The temperature was strictly controlled at 0–5 °C during the addition process. After the addition was completed, the temperature was raised to 70 °C and the reaction was maintained at this temperature for 24 h. The color and state changes of the system were recorded every 3 h, and the progress was monitored by thin-layer chromatography (TLC) using dichloromethane / petroleum ether as the developing solvent. After the reaction was complete, the reaction system was cooled to room temperature, and the mixture was transferred to a rotary evaporator for concentration under reduced pressure at -0.09 MPa in a 40°C water bath. The crude product was purified by silica gel column chromatography: using 200-300 mesh silica gel (2.5 cm column diameter) and dichloromethane / petroleum ether (1:10 v / v) as eluent, the target fractions were collected in fractions. The purified eluents were combined, the solvent was removed by rotary evaporation, and the product was thoroughly dried in a vacuum drying oven for 12 h to finally obtain 0.95 g of 9'-butyl-3,3''-divinyl-9'H-9,2':7',9''-tricarbazole, named V-KKP 27, with a yield of 78.4%. The 1H NMR spectrum is shown below. Figure 1 As shown.

[0032] .

[0033] Example 2 Synthesis of hole transport material V-KKP 36 .

[0034] (1) Synthesis of 9'-butyl-9'H-9,3':6',9''-tricarbazole Under anhydrous and oxygen-free conditions, 3.81 g of 2,7-dibromo-9-butyl-9H-carbazole (0.01 mol), 4.18 g of 9H-carbazole (0.025 mol), 0.19 g of cuprous iodide (0.001 mol), 5.52 g of potassium carbonate (0.04 mol), and 0.53 g of 18-crown ether-6 (0.002 mol) were sequentially added to a round-bottom flask containing 20 mL of 1,3-dimethyl-tetrahydropyrimidine-2(1H)-one (DMPU) and stirred at room temperature until dissolved. The reaction system was then heated to 200 °C and reacted at this temperature for 24 hours. During the reaction, the color and state changes of the system were recorded every 2 hours, and the reaction progress was monitored simultaneously using thin-layer chromatography. After the reaction was completed and allowed to cool naturally to room temperature, the mixture was poured into a 500 mL single-necked flask containing 200 mL of deionized water. The mixture was stirred in an 80 °C water bath for 30 min, allowed to stand for 60 min to separate into layers, and then filtered through a Buchner funnel to collect the grayish-white filter cake. The filter cake was transferred to a new single-necked flask, and 200 mL of 45 °C methanol was added. The mixture was mechanically stirred for 15 min, and the residual solvent and some organic impurities were filtered off. The cake was washed three times with methanol. Before each filtration, the filter cake was rinsed with a small amount of 45 °C methanol to ensure complete dissolution of impurities. The filter cake was then transferred to a beaker, and 200 mL of 40 °C petroleum ether was added. The mixture was stirred for 15 min to remove unreacted carbazole and low-polarity byproducts. After standing for 30 min, the waste liquid was filtered off, and the mixture was washed three times with petroleum ether to obtain 4.70 g of the grayish-white crude product compound III (9'-butyl-9'H-9,3':6',9''-tricarbazole), with a yield of 85%.

[0035] (2) Synthesis of 9'-butyl-9'-H-[9,3':6',9'-trimethyl]-3,3'-dicarboxaldehyde Under anhydrous and oxygen-free conditions, 1.66 g of 9'-butyl-9'H-9,2':7',9''-tricarbazole (0.003 mol) and 4.386 g of DMF (0.060 mol) were accurately weighed into a 150 mL round-bottom flask. 45 mL of anhydrous chloroform was then added to the flask. Using a dropping funnel, 9.198 g of phosphorus oxychloride (0.060 mol) was slowly added, with strict control of the dropping rate and the temperature maintained at 0 °C. After the addition was complete, the mixture was stirred and kept at this temperature for 0.5 h, gradually changing from a clear to a pale yellow suspension. The reaction system was then heated to 70 °C and held at this temperature for 24 h. Color changes were recorded every 2 h during the reaction, and the reaction progress was monitored by thin-layer chromatography. After the reaction was complete, the system was cooled to room temperature and filtered using a Buchner funnel preheated to 30 °C to remove a small amount of insoluble matter. The filtrate was then transferred to a rotary evaporator. The solution was concentrated under reduced pressure in a 40 °C water bath at -0.09 MPa until a brownish-yellow oily residue remained in the flask. The crude product was purified by silica gel column chromatography. Using dichloromethane as the single eluent, the target fraction was collected by TLC positioning. After combining the purified fractions, the eluent was removed again, and the residue was dried in a vacuum oven for 12 h to finally obtain 1.189 g of a white solid compound IV (9'-butyl-9'-H-[9,3':6',9'-trimethyl]-3,3'-dicarboxaldehyde), with a yield of 65%.

[0036] (3) Synthesis of 9'-butyl-3,3''-divinyl-9'H-9,3':6',9''-tricarbazole Under anhydrous and oxygen-free conditions, 7.14 g of methyltriphenylphosphine bromide (0.02 mol) and 2.24 g of potassium tert-butoxide (0.02 mol) were accurately weighed and dissolved in 20 mL of anhydrous tetrahydrofuran (THF) dried with activated molecular sieves. The mixture was stirred at 0–5 °C for 1.5–2 h. Separately, in a 50 mL pear-shaped flask, 1.22 g of 9'-butyl-9'H-[9,2':7',9''-tricarbazole]-3,3''-dicarboxaldehyde (0.002 mol) was dissolved in 10 mL of anhydrous THF and slowly added dropwise to the main reaction system through a constant-pressure dropping funnel. The temperature was strictly controlled at 0–5 °C during the addition process. After the addition was complete, the temperature was raised to 70 °C and the reaction was maintained at this temperature for 24 h with magnetic stirring. The color and state changes of the system were recorded every 3 h, and the reaction progress was monitored by thin-layer chromatography. After the reaction was completed, the mixture was naturally cooled to room temperature, and the THF was removed by vacuum concentration. The crude product was purified by silica gel column chromatography using dichloromethane:petroleum ether (v / v 1:10) as the eluent, and fractions containing the target product were collected. The eluents were combined, the solvent was removed, and the mixture was thoroughly dried in a vacuum oven for 12 h to obtain 1.08 g of a white solid product, compound V (9'-butyl-3,3''-divinyl-9'H-9,3':6',9''-tricarbazole), named V-KKP 36, with a yield of 89%. The 1H NMR spectrum is shown below. Figure 2 As shown.

[0037] .

[0038] Application Example 1 The low-temperature cross-linked hole transport materials obtained in Examples 1 and 2 were used as the main material of the hole transport layer and applied to quantum dot light-emitting diodes using conventional techniques in the art.

[0039] Test Example 1: Study on the thermal properties of materials The thermal stability and crosslinking conditions of V-KKP27 and V-KKP36 were evaluated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).

[0040] Based on the TG test results, as Figure 3 As shown, when the mass loss reaches 5%, the thermal decomposition temperatures of V-KKP27 and V-KKP36 are 463 ℃ and 479 ℃, respectively, which fully demonstrates that both crosslinked materials have excellent thermal stability.

[0041] DSC test results, such as Figure 4 As shown, during the heating cycle, the DSC curve of V-KKP27 exhibits an exothermic peak in the 216-237℃ range, with the peak value at 227℃. Similarly, as... Figure 5As shown, the crosslinking process of V-KKP36 occurs within the range of 157-178 °C, with a peak temperature of 167 °C. Comprehensive thermal performance test results indicate that when materials have the same core and side chain groups, the nonlinear structure has a lower crosslinking temperature compared to the linear structure. This means that using nonlinear structural materials during preparation can reduce energy consumption and also reduce the risk of heat generated during upper-layer crosslinking causing decomposition of the lower-layer structure.

[0042] Test Example 2: Photophysical Properties of Materials (1) Infrared spectroscopy analysis Analyzing materials before and after crosslinking using infrared spectroscopy allows for precise observation of changes in molecular structure. Figure 6 It can be clearly seen that some absorption peaks of the cross-linked material show shifts or increases / decreases in intensity, which directly reflects the transformation of chemical bonds in the molecular structure during the cross-linking process. Specifically, the cross-linked V-KKP27 shows a peak at 882 cm⁻¹. -1 and 985 cm -1 The absorption peak at 998 cm⁻¹ basically disappeared, while the cross-linked V-KKP36 showed an absorption peak at 998 cm⁻¹. -1 and 1235 cm -1 The absorption peaks at these wavelengths almost disappeared. These specific wavelengths correspond to the CH external bending vibrations, and the disappearance of the absorption peaks clearly indicates that these CH bonds actively participated in the reaction during the crosslinking process, enabling the vinyl groups to fully polymerize and ultimately generate alkyl chains.

[0043] Test Example 3: Photophysical Properties of Materials Figure 7In the fluorescence spectrum, the horizontal axis represents wavenumber reflecting wavelength, and the vertical axis represents intensity, indicating fluorescence emission intensity. The three curves correspond to QD, QD / Crosslink V-KKP36, and QD / Crosslink V-KKP27. All three exhibit similar fluorescence emission peaks in the 400-550 nm range, indicating similar luminescent centers. The system containing cross-linked V-KKP36 has a higher intensity than pure QD because the interaction between the cross-linked material and QD enhances energy transfer efficiency and suppresses fluorescence quenching. The fluorescence lifetime decay graph shows the horizontal axis representing the change in fluorescence intensity over time, and the vertical axis representing normalized intensity for easy comparison. All curves decay over time. In summary, the presence of the cross-linked hole transport layer (HTL) does not alter the emission wavelength of the blue quantum dots (QDs) deposited on its surface. Even when fitting the TRPL curves using a double exponential equation, it is clear that the average photoluminescence lifetimes (τave) of the pure QD film, the QD film deposited on the cross-linked V-KKP36 HTL, and the QD film deposited on the cross-linked V-KKP27 HTL are 21.64 ns, 20.67 ns, and 20.56 ns, respectively. The average photoluminescence lifetimes of cross-linked V-KKP36 and cross-linked V-KKP27 are similar, indicating that the cross-linked HTLs do not induce additional nonradiative recombination. This phenomenon is mainly attributed to the good interfacial contact between the cross-linked HTLs and the QDs layer.

[0044] Test Example 4: Performance of QLED Figure 8 The graph shows the relationship between external quantum efficiency (EQE) and brightness of a QLED: the horizontal axis represents brightness, and the vertical axis represents external quantum efficiency (EQE). The green curve for V-KKP27 shows that as brightness increases, EQE first rises rapidly to a maximum efficiency of 9.42%, with a maximum brightness of 10223 cd / m². -2 After reaching a certain brightness, the upward trend slows down and tends towards saturation, indicating that the device is more effective in improving photon generation efficiency at low brightness, while the potential for improvement is limited at high brightness. The overall EQE of the orange curve for V-KKP36 is higher than that of V-KKP27, with a maximum efficiency of 11.85% and a maximum brightness of 18770 cd / m². -2 The trend is similar, with a rapid increase at low brightness and a slowdown and stabilization at high brightness, indicating that V-KKP36 has a superior photon conversion efficiency.

[0045] Figure 9 The graph shows the relationship between QLED current efficiency and brightness: the horizontal axis represents brightness, and the vertical axis represents current efficiency. The green curve for the V-KKP27 shows that as brightness increases, the current efficiency first rises rapidly to 3.45%, with a maximum brightness of 10223 cdm. -2The growth rate then slows down. It is relatively efficient in low-brightness scenarios when converting electrical energy to light, but efficiency improvement is limited at high brightness. The orange curve of the V-KKP36 initially rises rapidly to 4.50%, with a maximum brightness of 18770 cd / m². -2 The efficiency reaches a stable value at higher brightness levels, and decreases slowly with further increases in brightness. Within a specific brightness range, the electrical energy conversion is highly efficient and stable. At the same brightness, V-KKP36 has a higher current efficiency than V-KKP27, demonstrating a significant advantage in energy conversion. In other words, V-KKP36 has a stronger ability to generate light intensity using current.

[0046] Figure 10 The graphs show the relationship between QLED voltage, brightness, and current density: the horizontal axis represents voltage, the left vertical axis represents brightness, and the right vertical axis represents current density. In the brightness-voltage curve, both V-KKP27 and V-KKP36 have low brightness at low voltages, rising rapidly after 4-5 V. V-KKP36 has higher brightness at the same voltage and is more sensitive to voltage response. In the current density-voltage curve, the current density of both increases with increasing voltage. V-KKP27 shows approximately exponential growth, with low current utilization at low voltages. V-KKP36 shows a similar growth trend but a slightly lower value at the same voltage, indicating more efficient current utilization. In summary, V-KKP36 outperforms V-KKP27 in photon conversion efficiency, current utilization efficiency, and voltage-driven luminescence and current injection performance.

Claims

1. A low-temperature cross-linked hole transport material, characterized in that, Its structural formula is: or .

2. The method for synthesizing the low-temperature cross-linked hole transport material according to claim 1, characterized in that, The specific route is as follows: 。 3. The method for synthesizing the low-temperature cross-linked hole transport material according to claim 2, characterized in that, The specific steps are as follows: (1) Under anhydrous and oxygen-free conditions, compound I, compound II, cuprous iodide, potassium carbonate and 18-crown ether-6 were added one by one to 1,3-dimethyl-tetrahydropyrimidine-2(1H)-one, stirred to dissolve, heated to 200-230 °C and reacted at a constant temperature. After the reaction was completed, the mixture was cooled and successively extracted and washed with deionized water, methanol and petroleum ether. The organic phase was dried and distilled under reduced pressure to obtain compound III. (2) Under anhydrous and oxygen-free conditions, compound III, DMF and chloroform were mixed and phosphorus oxychloride was added dropwise. The temperature was controlled at 0-5℃. After the addition was completed, the mixture was kept warm and stirred for 0.5-1h. Then the temperature was raised to 60-80℃ and kept constant for 0h. After the reaction was completed, the mixture was cooled, filtered, the solvent was removed by evaporation, and purified to obtain compound IV. (3) Under anhydrous and oxygen-free conditions, methyltriphenylphosphine bromide and potassium tert-butoxide were dissolved in tetrahydrofuran and stirred at 0-5℃ for 1.5-2 h. Then, a tetrahydrofuran solution containing compound IV was added dropwise at 0-5℃. After the addition was complete, the temperature was raised to 70-90℃ and the reaction was kept constant. After the reaction was complete, the mixture was cooled, the solvent was evaporated, and the mixture was purified to obtain compound V.

4. The method for synthesizing the low-temperature cross-linked hole transport material according to claim 3, characterized in that, In molar ratio, compound I: compound II = 2-2.5:

1.

5. The method for synthesizing the low-temperature cross-linked hole transport material according to claim 3, characterized in that, In molar ratio, Compound III: Phosphorus oxychloride: DMF = 1:20-30:20-30.

6. The method for synthesizing the low-temperature cross-linked hole transport material according to claim 3, characterized in that, In molar ratio, compound IV: methyltriphenylphosphine bromide: potassium tert-butoxide = 1:5-10:5-10.

7. The application of the low-temperature cross-linked hole transport material according to claim 1 in quantum dot light-emitting diodes.

8. The application of the low-temperature cross-linked hole transport material according to claim 7 in quantum dot light-emitting diodes, characterized in that, The aforementioned low-temperature cross-linked hole transport material is used as the hole transport layer of a quantum dot light-emitting diode.

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