Hole injection layer for organic light-emitting device, preparation of hole injection layer and organic light-emitting diode

Abstract

The invention belongs to the technical field of organic light-emitting diodes, and discloses a hole injection layer for an organic light-emitting device, preparation of the hole injection layer and an organic light-emitting diode. The method comprises the following steps: 1) dissolving an electroactive precursor molecule and a supporting electrolyte in a solvent to obtain an electrolyte solution; then forming a polymer film on the working electrode through electropolymerization; and 2) performing electrochemical doping on the polymer film formed on the working electrode to obtain the hole injection layer. According to the method, the hole injection layer has excellent thickness and morphology controllability and conductivity adjustability, the processing process is simple and convenient, and raw materials are saved. The hole injection layer is used for the organic light-emitting diode, an interface can be effectively modified, an injection barrier is reduced, and the performance of a device is remarkably improved.

Description

Technical Field

[0001] The invention belongs to the field of organic photoelectric materials, and in particular relates to a hole injection layer for an organic electroluminescent device, a preparation method thereof, and an organic electroluminescent diode. Background Art

[0002] Organic Light Emitting Diode (OLED) has been widely used in consumer electronics, flexible folding devices and smart wearable products due to its advantages such as lightness, flexibility, self-luminescence and high color expression. High-end application scenarios such as virtual reality and augmented reality require display devices with high refresh rate, high brightness, and especially extremely high resolution. In the OLED production line, the deformation of the mask used for ultra-high resolution devices is a difficult problem that needs to be solved urgently. The research group of Academician Ma Yuguang has developed a method for preparing OLEDs through electropolymerization technology, namely electrodeposition OLED (Wang B, Ma Y, Cao YA brief introduction to organic electrodeposition and a review of the fabrication of OLEDs based on electrodeposition technology[J]. Chinese Journal of Polymer Science, 2023, 41: 621-639.). By actively controlling the applied electrical signal, the material precursor can be electropolymerized to form an organic semiconductor film and directionally deposited on the patterned electrode, thereby preparing the various functional layers of OLED. This maskless, actively addressed solution-processed deposition technology demonstrates significant potential for low-cost production of ultra-high-resolution OLEDs. Currently, monochrome OLEDs with resolutions up to 2800ppi have been produced, but device brightness falls far short of application requirements. The continued development of electrodeposition OLED technology currently faces numerous challenges, including the development of suitable TFT substrates and a shortage of high-luminescence and high-transmission materials. The limited availability of hole interface layers makes device structure optimization difficult, while low transmission capacity hinders performance improvement, presenting the primary technical challenges facing electrodeposition OLEDs. Summary of the Invention

[0003] In order to overcome the shortcomings and deficiencies of the prior art, the present invention aims to provide a hole injection layer for an organic electroluminescent device, a preparation method thereof, and an organic electroluminescent diode.

[0004] The purpose of the present invention is achieved through the following solutions:

[0005] A method for preparing a hole injection layer for an organic electroluminescent device comprises the following steps:

[0006] 1) dissolving electroactive precursor molecules and supporting electrolytes in a solvent to obtain an electrolyte solution; then forming a polymer film on the working electrode through electropolymerization;

[0007] 2) Electrochemically doping the polymer film formed on the working electrode to obtain a hole injection layer.

[0008] The electroactive precursor molecule comprises one or more of the following electropolymerization units or two or more of the groups formed by the following electroactive polymerization units losing one, two or three hydrogen atoms: aniline, carbazole, thiophene, pyrrole, and 3,4-ethylenedioxythiophene.

[0009] Furthermore, the electroactive precursor molecule is mainly composed of two or more groups formed by the following electroactive polymerization units losing one, two or three hydrogen atoms: aniline, carbazole, thiophene, pyrrole, 3,4-ethylenedioxythiophene; for example: 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA).

[0010] The solvent is one or more of water, acetonitrile, dichloromethane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, acetone, propylene carbonate, ethanol, methanol, dimethyl carbonate, and diethyl carbonate;

[0011] In the electroactive precursor molecule, when the group undergoing electropolymerization is a group formed by aniline, the solvent is preferably water; when the group undergoing electropolymerization is a group formed by pyrrole, the solvent is preferably water and acetonitrile; when the group undergoing electropolymerization is a group formed by thiophene, the solvent is preferably water, acetonitrile and dichloromethane as solvents; when the group undergoing electropolymerization is a group formed by 3,4-ethylenedioxythiophene, the solvent is preferably water and propylene carbonate; when the group undergoing electropolymerization is a group formed by carbazole, the solvent is preferably dichloromethane, acetonitrile and propylene carbonate.

[0012] The working electrode is one of indium tin oxide (ITO), indium zinc oxide (IZO), and fluorine-doped tin oxide (FTO), preferably indium tin oxide (ITO). The working electrode of the present invention comprises depositing indium tin oxide (ITO), indium zinc oxide (IZO), or fluorine-doped tin oxide (FTO) on a substrate; the substrate may include glass, aluminum, polyethylene terephthalate film, polyimide film, or the like. The working electrode of the present invention is preferably an indium tin oxide conductive film.

[0013] The supporting electrolyte includes cations and anions. When water is used as the solvent, commonly used ones include: hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, perchloric acid, sodium hydroxide, potassium hydroxide, sodium chloride, sodium nitrate, sodium phosphate, sodium sulfate, sodium acetate, sodium formate, potassium chloride, potassium nitrate, potassium phosphate, potassium sulfate, potassium acetate, potassium formate, lithium chloride, lithium nitrate, lithium perchlorate, ammonium chloride, ammonium nitrate, ammonium phosphate, ammonium sulfate, ammonium acetate, ammonium formate, tetramethylammonium chloride, tetramethylammonium sulfate, tetraethylammonium sulfate, tetraethylammonium tetrafluoroborate, and tetraethylammonium hexafluorophosphate; when organic reagents are used as the solvent, commonly used ones include: lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoroarsenate, tetraethylammonium tetrafluoroborate, tetraethylammonium hexafluorophosphate, tetra-n-butylammonium hexafluorophosphate, tetra-n-butylammonium tetrafluoroborate, tetra-n-butylammonium perchlorate, and tetra-n-butylammonium p-toluenesulfonate.

[0014] The electrochemically active precursor molecules have a concentration of 10 -5 ~1mol / L, the concentration of supporting electrolyte in the solvent is 10 -3 ~1mol / L.

[0015] The electropolymerization includes cyclic voltammetry, potential step method or current step method.

[0016] For example, in cyclic voltammetry, the applied potential is -0.8 to 1.07 V relative to ferrocene, the scan rate is 200 to 400 mV / s, and the number of scans is 12 to 18.

[0017] The thickness of the polymer film is 10 to 100 nm; the roughness is ≤ 10 nm.

[0018] The electrochemical doping refers to using a working electrode formed with a polymer film as the working electrode for electrochemical doping, and using a supporting electrolyte solution without electroactive precursor molecules as the electrochemical solution, to electrochemically oxidatively dope the polymer film by a potential step (i.e., constant potential) method, wherein the applied potential is greater than the initial oxidation potential of the polymer film and lower than the highest oxidation potential during electropolymerization. When the oxidation current no longer decreases, the electrochemical doping is considered complete and the voltage application is stopped.

[0019] When the polymer film is a p-TCTA film, the solvent used in the electrochemical solution is a mixed solvent of dichloromethane, acetonitrile, and propylene carbonate, with the volume ratio of the three solvents being (2.4-2.6):(0.8-1):(1.5-1.7). During the potential step method, the applied voltage is 0.45-0.85 V, preferably 0.55-0.85 V.

[0020] During electrochemical doping, the supporting electrolyte includes cations and anions. When water is used as the solvent, commonly used ones include: hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, perchloric acid, sodium hydroxide, potassium hydroxide, sodium chloride, sodium nitrate, sodium phosphate, sodium sulfate, sodium acetate, sodium formate, potassium chloride, potassium nitrate, potassium phosphate, potassium sulfate, potassium acetate, potassium formate, lithium chloride, lithium nitrate, lithium perchlorate, ammonium chloride, ammonium nitrate, ammonium phosphate, ammonium sulfate, ammonium acetate, ammonium formate, tetramethylammonium chloride, tetramethylammonium sulfate, tetraethylammonium sulfate, tetraethylammonium tetrafluoroborate, and tetraethylammonium hexafluorophosphate; when organic reagents are used as solvents, commonly used ones include: lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoroarsenate, tetraethylammonium tetrafluoroborate, tetraethylammonium hexafluorophosphate, tetra-n-butylammonium hexafluorophosphate, tetra-n-butylammonium tetrafluoroborate, tetra-n-butylammonium perchlorate, and tetra-n-butylammonium p-toluenesulfonate.

[0021] During electrochemical doping, the concentration of supporting electrolyte in the solvent is 10 -3 ~1mol / L.

[0022] Step 2) After the electrochemical doping is completed, the supporting electrolyte on the film is removed (eg, by solvent cleaning), and the solvent is removed by heating.

[0023] Step 2) During electrochemical doping, the reference electrode is Ag / Ag + Or Ag / AgCl or saturated calomel electrode, the counter electrode is a metal titanium plate or platinum plate.

[0024] The hole injection layer is prepared by the above method.

[0025] The hole injection layer is used for preparing an organic electroluminescent device, in particular an organic electroluminescent diode.

[0026] The organic electroluminescent diode comprises, from bottom to top, a substrate, an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron injection layer and a cathode, and further comprises an electron transport layer between the light-emitting layer and the electron injection layer.

[0027] The hole injection layer is as defined above.

[0028] The anode is one of indium tin oxide (ITO), indium zinc oxide (IZO), and fluorine-doped tin oxide (FTO), and its conductivity is usually 10 3 ~10 4 S / cm, and its visible light transmittance is greater than 80%.

[0029] Preferably, the hole transport layer is a thin film of 4,4′,4″-tris(carbazol-9-yl)triphenylamine or 1,4,5,8,9,11-hexaazatriphenylhexanitrile or 4,4′-cyclohexylbis[N,N-bis(4-methylphenyl)aniline].

[0030] Preferably, the light-emitting layer is any light-emitting monomer that emits red, green or blue light.

[0031] Preferably, the electron transport layer is a 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene film.

[0032] Preferably, the electron injection layer is a lithium fluoride thin film.

[0033] Preferably, the cathode is a metal aluminum film.

[0034] The thickness of the hole transport layer is 10 to 100 nm; the thickness of the light-emitting layer is 40 to 100 nm; the thickness of the electron transport layer is 10 to 100 nm; the thickness of the electron injection layer is 0.1 to 5 nm; and the thickness of the cathode is 50 to 150 nm.

[0035] The method for preparing the organic electroluminescent diode specifically comprises the following steps:

[0036] A hole transport layer, a light-emitting layer, an electron transport layer, an electron injection layer and an electrode layer are sequentially prepared on the hole injection layer using resistive evaporation to obtain an organic electroluminescent diode; wherein the hole injection layer is deposited on the working electrode, which is the substrate and the anode on the substrate.

[0037] The principle of electrochemical doping to control film conductivity: Taking electrochemical oxidation doping as an example, when an oxidizing potential is applied to an electrode, the electroactive units in the polymer on the electrode surface are oxidized, and electrons migrate from the polymer to the electrode. To maintain charge balance, anions from the electrolyte act as counterions and insert between polymer chains. By adjusting the applied voltage, the concentration of the introduced dopant charge can be controlled. Assuming that all dopant charges are effective carriers, the conductivity of the polymer film can be effectively controlled by introducing different carrier concentrations.

[0038] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0039] 1) The hole injection layer of the present invention is formed by electrochemical polymerization and then subjected to electrochemical doping to produce a doped electropolymerized film with excellent conductivity. This in-situ grown polymer film provides good contact with the anode, acts as an interface modifier during subsequent processing of the multilayer structure, helps lower the injection barrier for hole carriers, and thus improves device efficiency.

[0040] 2) The hole injection layer prepared by the electrochemical polymerization method of the present invention has excellent thickness and morphology controllability, conductivity adjustability, simple processing, no need to use a catalyst, can be completed under normal temperature and pressure conditions, and saves raw materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Schematic diagram of the electrochemical polymerization device in Example 1;

[0042] Figure 2 1 is the cyclic voltammetric electropolymerization curve of TCTA on ITO in Example 1;

[0043] Figure 3 is an atomic force microscope image of the p-TCTA film in Example 1;

[0044] Figure 4 This is the electrochemical response curve of the p-TCTA film during electrochemical oxidation doping in Example 1;

[0045] Figure 5 The conductivity test results of the p-TCTA film at different doping voltages in Example 1;

[0046] Figure 6 The electroluminescence spectra of OLED devices prepared based on undoped and doped p-TCTA films. DETAILED DESCRIPTION

[0047] The present invention will be described in further detail below with reference to the Examples, but the embodiments of the present invention are not limited thereto. Where specific conditions are not specified in the Examples, conventional conditions or conditions recommended by the manufacturer were used. Reagents or instruments used where the manufacturer is not specified are conventional products that can be purchased commercially.

[0048] The preparation of the doped electropolymerized film in Example 1 adopts a three-electrode system, with ITO as the working electrode, titanium plate as the counter electrode, and Ag / Ag as the + As a reference electrode.

[0049] The electrochemical cell circuit, i.e., the electrolytic cell circuit, includes an electrochemical workstation, an ammeter, a voltmeter, a counter electrode, a reference electrode, a working electrode, an electrolyte, and a container; the counter electrode, the reference electrode, and the working electrode are placed in a container filled with electrolyte, and the three are connected to the electrochemical workstation through wires, the voltmeter is connected in parallel in the loop circuit formed by the reference electrode and the working electrode, and the ammeter is connected in series in the circuit between the counter electrode and the electrochemical workstation; in this circuit, the current flows between the working electrode and the counter electrode and is recorded by the ammeter; the potential of the working electrode is controlled by the reference electrode and recorded by the voltmeter.

[0050] The schematic diagram of the electrochemical polymerization device in Example 1 is as follows Figure 1 shown.

[0051] Cyclic voltammetry is used to control the deposition of electrochemically polymerized thin films. This method involves repeatedly sweeping the electrode potential over time, one or more times, at varying rates, using a symmetrical triangular waveform, and recording the current-potential curve. When the highest applied potential exceeds the polymerization potential of the electroactive monomers, oligomerization, nucleation, and film growth occur on the electrode surface.

[0052] The three-electrode system is used in the preparation of polymer films, specifically: ITO or FTO or IZO as the working electrode, titanium plate or platinum plate as the counter electrode, Ag / Ag + Or Ag / AgCl or saturated calomel electrode is used as the reference electrode, and a three-electrode system is established; a cyclic triangular wave electrical signal or a potential step signal or a current step signal is applied to the system through an electrochemical workstation.

[0053] Example 1

[0054] Using ITO as the anode, a polymer film is prepared by electrochemical polymerization, and then used as the hole injection layer of the OLED device after electrochemical doping.

[0055] In this example, 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) is used as the electroactive precursor molecule. Its three peripheral carbazole units serve as electroactive groups, which can undergo electropolymerization to form a cross-linked polymer p-TCTA:

[0056] TCTA:

[0057] The preparation method of a p-TCTA cross-linked polymer doped film comprises the following steps:

[0058] (1) Take an ITO sheet with a sheet resistance of 15Ω, a transmittance greater than 86%, a thickness of 135nm, and an area of ​​1.5 square centimeters (based on glass), and ultrasonically clean it with isopropyl alcohol, acetone, detergent, deionized water, and ethanol in sequence, and dry it at 80℃.

[0059] (2) In a nitrogen glove box, ITO was used as the working electrode, and Ag / Ag + The electrode is used as a reference electrode (the internal solution contains 0.01 mol L -1 Silver nitrate and 0.1 mol L -1 A three-electrode system was constructed with a titanium plate as the counter electrode and a supporting electrolyte of 0.1 mol L -1 Tetrabutylammonium hexafluorophosphate (recrystallized three times from ethanol and dried at 120 °C for 12 h before use); the TCTA monomer concentration during electropolymerization was 4 × 10 -4mol / L; the solvent used was a mixed solvent of dichloromethane, acetonitrile, and propylene carbonate (the volume ratio of dichloromethane, acetonitrile, and propylene carbonate was 2.5:0.9:1.6); all potentials in the electrochemical tests were based on ferrocene (Fc), and the reference electrode was calibrated with ferrocene before each electrochemical experiment; the electrochemical data were plotted using polarography, where oxidation current was defined as negative and reduction current as positive; TCTA electropolymerization was controlled by cyclic voltammetry, with an applied potential ranging from -0.8 to 1.07 V relative to ferrocene, a scan rate of 300 mV / s, and 15 scan cycles. The electropolymerization curve is shown in Figure 2. Figure 2 As shown; washed three times with dichloromethane and acetonitrile to remove unreacted monomers and supporting electrolytes, and dried at 100 ° C in a nitrogen glove box for 45 minutes, finally obtaining a p-TCTA film with a roughness of 8.6 nm and a thickness of 31 nm. The roughness results are shown in Figure 3 shown.

[0060] (3) ITO with p-TCTA film deposited was used as the working electrode, and the reference electrode was Ag / Ag + The electrode was a titanium plate, and the counter electrode was a 0.1 mol / L tetrabutylammonium hexafluorophosphate supporting electrolyte solution containing no electroactive precursor molecules was used as the electrolyte. The solvent used was a mixed solvent of dichloromethane, acetonitrile, and propylene carbonate (the volume ratio of the three solvents was 2.5:0.9:1.6). The p-TCTA film was electrochemically oxidized and doped by a potential step method. The specific operation was as follows: under the condition that the open circuit potential was the initial potential, a voltage step of 0.05 V was used to apply a step voltage in the range of 0.45 to 0.85 V to the p-TCTA film for electrochemical doping. The electrochemical response curve is shown in FIG. Figure 4 As shown in the figure, each film was completely oxidized at the corresponding doping voltage, and doping was stopped after the oxidation current stabilized, resulting in a series of p-TCTA films with varying doping levels and conductivity. After electrochemical doping, the p-TCTA films were washed three times with dichloromethane and then acetonitrile to remove the supporting electrolyte, then dried at 100°C in a nitrogen glove box for 45 minutes before use.

[0061] The conductivity of the film is shown in Figure 5 As shown in the figure, it can be found that with the increase of doping voltage, the conductivity of the doped p-TCTA film first increases and then decreases. At a doping voltage of 0.65 V, the p-TCTA film obtains the highest conductivity, which is 2.8×10 -6 S / cm.

[0062] Figure 2 This is the cyclic voltammetric electropolymerization curve of TCTA on ITO in Example 1. Figure 3 This is an atomic force microscope image of the p-TCTA film in Example 1. Figure 4 This is the electrochemical response curve of the p-TCTA film during electrochemical oxidation doping in Example 1. Figure 5 These are the conductivity test results of the p-TCTA film at different doping voltages in Example 1.

[0063] Application Examples

[0064] This application embodiment provides an organic light-emitting diode device prepared based on a doped electropolymerized p-TCTA film, which includes, from bottom to top, a glass substrate, an anode, a hole injection layer, a hole transport layer, an organic light-emitting layer, an electron injection layer, and a cathode, wherein the glass substrate and the anode are as in Example 1, and the hole injection layer material on the anode includes the p-TCTA doped electropolymerized film described in Example 1 (electrochemical doping: electrochemical doping is performed by applying step voltages at 0.65V and 0.85V; non-electrochemical doping is used as a comparative example).

[0065] The preparation method of the organic light emitting diode device is as follows: put the ITO with p-TCTA doped thin film deposited into the vacuum evaporation equipment, and wait until the vacuum degree of the vacuum equipment is pumped to 10 -5 Pa after 10 nm of 4,4′-cyclohexylbis[N,N-di(4-methylphenyl)aniline] (TAPC) was evaporated at a rate of 100 nm as a hole transport layer. 90nm of 8-hydroxyquinoline aluminum (Alq3) was evaporated at a rate of 100 nm as the light-emitting layer. 1 nm lithium fluoride (LiF) was evaporated at a rate of and Aluminum (Al) with a thickness of 10 nm and 90 nm was evaporated at a rate of 100 nm as the cathode. The device structure is: ITO / p-TCTA doped or undoped film / TAPC / Alq3 / LiF / Al.

[0066] The performance parameters of the prepared OLED devices are shown in Table 1.

[0067] Table 1: Performance parameters of organic electroluminescent devices

[0068] p-TCTA electropolymerized thin film <![CDATA[Turn-on voltage a (V)]]> <![CDATA[Maximum brightness (cd / m 2 )]]> Maximum current efficiency (cd / A) Undoped 4.2 8731 3.40 0.65V doping 2.9 9849 2.40 0.85V doping 3.1 6076 3.38

[0069] a The device brightness is 1 cd m -2 The voltage at the time.

[0070] As can be seen from the table, when the doped p-TCTA electropolymer film is used as the hole injection layer, the turn-on voltage of the OLED is reduced from 4.2V in the undoped state to 2.9V, and the device brightness is increased from 8731cd / m 2 Increased to 9849cd / m2 This indicates that doped electropolymer films have significant advantages in improving hole transport capacity, reducing injection barriers, and enhancing carrier recombination probability. In addition, compared with the doped film obtained by electrochemical doping at 0.85V, the device prepared by the film obtained by doping at 0.65V has a lower turn-on voltage and higher brightness, indicating that the higher the conductivity of the doped polymer film as a hole injection layer, the more conducive it is to achieving OLED devices with low turn-on voltage and high luminous brightness. Figure 6 The electroluminescence spectra of OLED devices based on doped p-TCTA thin films and OLED devices based on non-doped p-TCTA thin films were compared. The spectra were basically the same, with peak positions between 500-550nm, showing pure green light emission characteristics.

[0071] In summary, the present invention prepared a p-TCTA doped polymer film with excellent conductivity through electrochemical polymerization and electrochemical doping, and successfully applied it to the hole injection layer of OLED, significantly improving the device performance, achieving a low turn-on voltage of 2.9V and a 9849cd m -2 Green organic light-emitting diodes with high luminescence brightness.

[0072] Figure 6 The electroluminescence spectra of OLED devices prepared based on undoped and doped p-TCTA films.

[0073] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing a hole injection layer for an organic electroluminescent device, characterized in that: The following steps are involved: 1) dissolving electroactive precursor molecules and supporting electrolytes in a solvent to obtain an electrolyte solution; then forming a polymer film on the working electrode through electropolymerization; 2) electrochemically doping the polymer film formed on the working electrode to obtain a hole injection layer; The electroactive precursor molecule comprises one or more of the following electropolymerization units or two or more of the groups formed by the following electroactive polymerization units losing one, two or three hydrogen atoms: aniline, carbazole, thiophene, pyrrole, and 3,4-ethylenedioxythiophene; The electropolymerization includes cyclic voltammetry, potential step method or current step method; The electrochemical doping refers to using a working electrode formed with a polymer film as the working electrode for electrochemical doping, and using a supporting electrolyte solution without electroactive precursor molecules as the electrochemical solution, to electrochemically oxidatively dope the polymer film by a potential step method, wherein the applied potential is greater than the initial oxidation potential of the polymer film and lower than the highest oxidation potential during electropolymerization. When the oxidation current no longer decreases, the electrochemical doping is considered complete and the voltage application is stopped.

2. The method for preparing a hole injection layer for an organic electroluminescent device according to claim 1, wherein: The electroactive precursor molecule in step 1) is mainly composed of two or more of the following groups formed by the loss of one, two or three hydrogen atoms from the following electroactive polymerization units: aniline, carbazole, thiophene, pyrrole, and 3,4-ethylenedioxythiophene; The solvent in step 1) is one or more of water, acetonitrile, dichloromethane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, acetone, propylene carbonate, ethanol, methanol, dimethyl carbonate, and diethyl carbonate; The supporting electrolyte described in step 1) includes cations and anions. When the solvent is water or contains water, the supporting electrolyte is hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, perchloric acid, sodium hydroxide, potassium hydroxide, sodium chloride, sodium nitrate, sodium phosphate, sodium sulfate, sodium acetate, sodium formate, potassium chloride, potassium nitrate, potassium phosphate, potassium sulfate, potassium acetate, potassium formate, lithium chloride, lithium nitrate, lithium perchlorate, ammonium chloride, ammonium nitrate, ammonium phosphate, ammonium sulfate, ammonium acetate, ammonium formate, tetramethylammonium chloride, tetramethylammonium sulfate, tetraethylammonium sulfate, tetraethylammonium tetrafluoroborate, tetraethylhexafluoroborate When an organic solvent is used as the solvent, the supporting electrolyte is one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoroarsenate, tetraethylammonium tetrafluoroborate, tetraethylammonium hexafluorophosphate, tetra-n-butylammonium hexafluorophosphate, tetra-n-butylammonium tetrafluoroborate, tetra-n-butylammonium perchlorate, and tetra-n-butylammonium p-toluenesulfonate; and the organic solvent is one or more of acetonitrile, dichloromethane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, acetone, propylene carbonate, ethanol, methanol, dimethyl carbonate, and diethyl carbonate.

3. The method for preparing a hole injection layer for an organic electroluminescent device according to claim 2, wherein: In the electroactive precursor molecule, when the group undergoing electropolymerization is a group formed by aniline, the solvent is water; when the group undergoing electropolymerization is a group formed by pyrrole, the solvent is water and acetonitrile; when the group undergoing electropolymerization is a group formed by thiophene, the solvent is water, acetonitrile and dichloromethane; when the group undergoing electropolymerization is a group formed by 3,4-ethylenedioxythiophene, the solvent is water and propylene carbonate; when the group undergoing electropolymerization is a group formed by carbazole, the solvent is dichloromethane, acetonitrile and propylene carbonate.

4. The method for preparing a hole injection layer for an organic electroluminescent device according to claim 1, wherein: In step 1), during cyclic voltammetry, the applied potential is -0.8 to 1.07 V relative to ferrocene, the scan rate is 200 to 400 mV / s, and the number of scans is 12 to 18; In step 1), the thickness of the polymer film is 10 to 100 nm and the roughness is ≤ 10 nm; The working electrode in step 1) is one of indium tin oxide, indium zinc oxide, and fluorine-doped tin oxide; The concentration of the electrochemically active precursor molecules in the solvent in step 1) is 10 -5 ~1mol / L, the concentration of supporting electrolyte in the solvent is 10 -3 ~1mol / L; During electrochemical doping in step 2), the solvent in the electrochemical solution is one or more of water, acetonitrile, dichloromethane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, acetone, propylene carbonate, ethanol, methanol, dimethyl carbonate, and diethyl carbonate; the supporting electrolyte in the electrochemical solution includes cations and anions, and when the solvent is water or contains water, the supporting electrolyte is hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, perchloric acid, sodium hydroxide, potassium hydroxide, sodium chloride, sodium nitrate, sodium phosphate, sodium sulfate, sodium acetate, sodium formate, potassium chloride, potassium nitrate, potassium phosphate, potassium sulfate, potassium acetate, potassium formate, lithium chloride, lithium nitrate, lithium perchlorate, ammonium chloride, ammonium nitrate, ammonium phosphate, ammonium sulfate, ammonium acetate, formate When the organic solvent is used as the solvent, the supporting electrolyte is one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium hexafluoroarsenate, tetraethylammonium tetrafluoroborate, tetraethylammonium hexafluorophosphate, tetra-n-butylammonium hexafluorophosphate, tetra-n-butylammonium tetrafluoroborate, tetra-n-butylammonium perchlorate, and tetra-n-butylammonium p-toluenesulfonate; the organic solvent is one or more of acetonitrile, dichloromethane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, acetone, propylene carbonate, ethanol, methanol, dimethyl carbonate, and diethyl carbonate; the concentration of the supporting electrolyte in the electrochemical solution is 10 -3 ~1mol / L; After the electrochemical doping in step 2) is completed, the supporting electrolyte on the film is removed and the solvent is removed by heating; During electrochemical doping in step 2), the reference electrode is Ag / Ag + Or Ag / AgCl or saturated calomel electrode, the counter electrode is a metal titanium plate or platinum plate.

5. The method for preparing a hole injection layer for an organic electroluminescent device according to claim 1, wherein: When the electroactive precursor molecule is 4,4′,4″-tris(carbazol-9-yl)triphenylamine, the solvent in step 1) is a mixed solvent of dichloromethane, acetonitrile and propylene carbonate, and the volume ratio of the three solvents is (2.4-2.6):(0.8-1):(1.5-1.7); the solvent used in the electrochemical solution in step 2) is a mixed solvent of dichloromethane, acetonitrile and propylene carbonate, and the volume ratio of the three solvents is (2.4-2.6):(0.8-1):(1.5-1.7); when the potential step method is used in the electrochemical doping in step 2), the applied voltage is 0.45-0.85V.

6. A hole injection layer obtained by the preparation method according to any one of claims 1 to 5.

7. The use of the hole injection layer according to claim 6, characterized in that: The hole injection layer is used for preparing an organic electroluminescent device.

8. The use according to claim 7, characterized in that: The hole injection layer is used to prepare an organic electroluminescent diode.

9. An organic electroluminescent diode, characterized in that: From bottom to top, it includes a substrate, an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron injection layer and a cathode; the hole injection layer is as defined in claim 6.

10. The organic electroluminescent diode according to claim 9, characterized in that: The anode is one of indium tin oxide, indium zinc oxide, and fluorine-doped tin oxide, and its conductivity is usually 10 3 ~10 4 S / cm, its visible light transmittance is greater than 80%; The hole transport layer is a thin film of 4,4′,4″-tris(carbazol-9-yl)triphenylamine or 1,4,5,8,9,11-hexaazatriphenylhexanitrile or 4,4′-cyclohexylbis[N,N-bis(4-methylphenyl)aniline]; The light-emitting layer is any light-emitting monomer that emits red, green, or blue light; the electron injection layer is a lithium fluoride film; and the cathode is a metal aluminum film. An electron transport layer is provided between the light-emitting layer and the electron injection layer, and the electron transport layer is a 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene film.

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