CN111697150A - Based on MoOxQLED device of hole injection layer and preparation method thereof - Google Patents

Abstract

The invention belongs to the technical field of light emitting diodes, and particularly relates to a MoO-based LED_xA QLED device of a hole injection layer and a preparation method thereof. The invention selects MoO with innocuity, high work function and good environmental stability_XThe film is a hole injection layer formed by adding MoO_xFilm thickness, annealing temperature, UV-O₃Optimization of experimental parameters such as treatment and the like to obtain a product based on MoO_xThe QLED device of the mixed hole injection layer is characterized by the performance of the device, and the result shows that the current driver concentration is 6% w/v, the annealing temperature is 130 ℃, and the UV-O is₃When the processing time is 10min, the obtained device has the best performance, and the highest brightness reaches 229400 cd/m²The maximum current efficiency and the maximum external quantum efficiency were 41.75 cd/A and 9.70%, respectively.

Description

Based on MoOxQLED device of hole injection layer and preparation method thereof

Technical Field

The invention belongs to the technical field of light emitting diodes, and particularly relates to a MoO-based LED_xA QLED device of a hole injection layer and a preparation method thereof.

Background

Quantum-dot Light-emitting Diodes (QLEDs) are expected to become a new generation of mainstream display and lighting technology and applied to the fields of intelligent terminals, ultra-high definition display, high-end lighting and the like because the QLEDs have the advantages of good monochromaticity, high color purity, wide color gamut, long service life and the like and are rapidly developed. Currently, a common device structure of the QLED device is anode/Hole Injection Layer (HIL)/Hole Transport Layer (HTL)/electron emission layer (EML)/Electron Transport Layer (ETL)/cathode. Poly (3, 4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) is widely used as a Hole Injection Layer (HIL) in main stream QLED devices due to its advantages of high transparency, good conductivity, solution preparation and the like. However, due to its own acidity and hygroscopicity, it corrodes the ITO substrate and accelerates the degradation of the subsequently deposited thin film, resulting in poor device stability, which is manifested as a low device lifetime.

For the quantum dot light-emitting diode, the important indexes for evaluating the performance of the quantum dot light-emitting diode device are the EQE and the service life of the device. The Qian group prepared ZnO with high electron transport capability in 2011 and reported that the main contributing factor limiting the improvement in device performance was the imbalance of electron-hole injection due to low hole injection capability. The solution to this problem can be seen in three aspects, one is to optimize the material itself; secondly, the hole injection capability is improved; and thirdly, the electron injection rate is reduced.

One way to improve device performance is to insert a polymer or small molecule interlayer between the PEDOT PSS and EML as the hole transport layer HTL. In addition, the transition metal oxides have the advantages of high transparency, good conductivity and solution-soluble preparation (Caruge J M, Halpert J E, Bulovic V, et al, Nias an inorganic-transporting layer in quality-dot light-emitting devices [ J]NanoLetters, 2006, 6(12): 2991-2994.), transition metal oxides can also be used as Hole Injection Layers (HILs) in the construction of QLED devices to improve device performance. Transition metal oxide molybdenum oxide (MoO) as compared to PEDOT: PSS as Hole Injection Layer (HIL) for QLED devices_x) Has the characteristics of suitability for work function, solution-soluble preparation, high transmittance, no toxicity and deep electronic state, and ensures that MoO is obtained_xBecomes the most excellent Hole Injection Layer (HIL)One of the materials (Zeng Q Y, Xu Z W, Zheng C X, et al, Improving charge injection view a blank-coating molybdenum oxide layer: heated high-performance large-area quality-dot light-emitting diodes [ J]. ACS Applied Materials&Interfaces,2018, 10(9): 8258-8264). However, compared with a QLED device using PEDOT: PSS as a hole injection layer, the transition metal oxide used in the conventional QLED device often has the problems of poor charge transport capability and high injection barrier, which causes low hole injection efficiency, resulting in reduced device efficiency, and finally, the efficiency and lifetime of the QLED device cannot be improved at the same time.

In order to solve the problem of poor hole injection capability of a device, MoO with non-toxicity, high work function and good environmental stability is selected_xMoO prepared by surrounding sol-gel method and using film as hole injection layer_xThe application of the thin film in green QLED devices is expanded. Meanwhile, since the chemical composition of the transition metal oxide affects the energy level structure thereof, the application is directed to MoO_xFilm thickness, annealing temperature, UV-O₃Optimization of experimental parameters such as processing and the like to obtain MoO suitable for QLED device energy level structure_xThin films (Ayobi A, Mirnia S N, Roknabadi M R, et al, The effects of MoO₃/TPD multiple quantum well structures on the performance of organic lightemitting diodes (OLEDs)[J]Journal of Materials Science, 2019, 30(4): 3952-3958) applied to a QLED device, and then a device having high efficiency, lifetime, and luminance is obtained by selecting a hole transport layer material having a suitable energy level and high hole mobility to fit an existing quantum dot light emitting layer.

Disclosure of Invention

The invention aims to provide a method based on MoO_xThe QLED device of the hole injection layer and the preparation method thereof aim at prolonging the service life of the device and solving the problems of insufficient brightness and efficiency of the existing QLED device.

Based on the purpose, the invention adopts the following technical scheme:

based on MoO_xPreparation method of QLED device of hole injection layerThe method comprises the following steps:

(1) cleaning and pretreating an ITO glass substrate electrode;

(2) spin-coating a mixed hole injection layer on an ITO glass substrate electrode, wherein the material of the mixed hole injection layer is MoO_xOr MoO_x-PEDOT:PSS；

(3) Spin coating a hole transport layer on the mixed hole injection layer; the hole transport layer is made of one or more of PVK, TFB, poly-TPD, TCTA and CBP;

(4) a quantum dot light-emitting layer is spin-coated on the hole transport layer, and the material of the quantum dot light-emitting layer is ZnCdSeS/ZnS green light quantum dots;

(5) spin-coating an electron transport layer ZnO on the quantum dot light-emitting layer;

(6) and (3) evaporating and plating a top electrode on the ZnO electron transport layer, and packaging the device after evaporation of the device is finished, wherein the top electrode is an Al, Ag, Cu, Au or alloy electrode.

Further, in the step (1), the ITO glass substrate electrode is placed in an ultraviolet ozone cleaning instrument to be UV-O₃And (3) pretreating for 15min to remove residual solvent on the surface, reduce the surface roughness and increase the activity and the hydrophilicity of the surface of the ITO electrode.

Further, MoO in step (2)_xThe spin coating method of the layer is to suck 80 mu L of MoO_xSpin-coating the precursor solution for 40-45 s, annealing after film formation, cooling to room temperature, and performing UV-O₃Treating for 5-10 min;

in the step (2), the spin coating method of the PEDOT and PSS layer is to suck 100 mu L of PEDOT, PSS is spin-coated for 40-45 s under the condition of 5000 rpm/min, and annealing is carried out for 10-15 min under the condition of 120-130 ℃ after film forming; when the material of the mixed hole injection layer is MoO_xPSS, in the case of PEDOT, it may be in MoO_xThe layer is directly spin-coated with PEDOT, namely a PSS layer, or a layer of MoO is further spin-coated on the basis of the PEDOT layer_xLayer obtaining; MoO can also be directly spin-coated on a PEDOT PSS layer_xThe layers are obtained.

Further, MoO_xThe preparation steps of the precursor solution are as follows: heating 5-10 vol% ammonium molybdate water solution in airStirring at 80-90 deg.C for 1-2h to obtain MoO_xThe mass fraction of the precursor solution is 5-8%, specifically 5%, 6%, 7%, 8%.

Further, MoO in step (2)_xThe rotation speed during spin coating of the layer is 4000-7000 rpm/min, specifically 4000rpm/min, 5000 rpm/min, 6000 rpm/min, 7000 rpm/min.

Further, MoO in step (2)_xAnnealing temperature of the layer after spin coating is 100-170 ℃, specifically 100 ℃, 130 ℃, 150 ℃ and 170 ℃; the annealing time is 0-15min, specifically 0, 5min, 10min, 15 min.

Further, the spin coating method of the hole transport layer in the step (3) is to suck 60 μ L of hole transport layer material solution, spin-coat for 25-35 s under the condition of 2800-3200 rpm/min, and place on an annealing plate to anneal for 25-30 min under the condition of 120-150 ℃; the concentration of the hole transport layer material solution was 8 mg/mL.

Further, the spin coating method of the quantum dot light emitting layer in the step (4) is to suck 60 μ L of green light quantum dot solution, and spin-coat for 25-30 s under the condition of 3200-3500 rpm/min; the green light quantum dot solution is prepared by dissolving ZnCdSeS/ZnS green light quantum dots with the particle size of 8-10 nm in n-octane into a solution with the concentration of 18 mg/mL.

Further, the spin-coating method of the electron transport layer ZnO in the step (5) is to suck 60 mu L of ZnO solution to spin-coat for 30-35 s under the condition of 2500 rpm/min, and anneal for 25-30 min under the condition of 55-60 ℃ after film forming; the ZnO solution is prepared by dissolving ZnO with the grain diameter of 3-4 nm in ethanol, and the concentration of the ZnO solution is 30 mg/mL.

Further, the top electrode in the step (6) is an Al, Ag, Cu, Au or alloy electrode; and during packaging, curing the obtained substrate by adopting ultraviolet curing resin.

The preparation method comprises mixing MoO_xAlone, or MoO_xPSS is used together with PEDOT as a hole injection layer to prepare a hole injection layer based on MoO_xHole injection layer QLED device.

The invention has the following beneficial effects:

1. the invention adopts a sol-gel method to prepare MoO_xThe film has the advantages of no toxicity, high work function, good environmental stability and the like, so the film is introduced into a QLED device and simultaneously subjected to MoO_xFilm thickness, annealing temperature, UV-O₃Optimization of experimental parameters such as treatment and the like to obtain a product based on MoO_xThe QLED device of the mixed hole injection layer is characterized by the performance of the device, and the result shows that the current driver concentration is 6% w/v, the annealing temperature is 130 ℃, and the UV-O is₃When the processing time is 10min, the obtained device has the best performance, and the highest brightness reaches 229400 cd/m²The maximum current efficiency and the maximum external quantum efficiency were 41.75 cd/A and 9.70%, respectively.

2. The invention also constructs different devices by optimizing the hole transport layer materials and utilizing five hole transport layer materials (TFB, Poly-TPD, PVK, TCTA and CBP) with different mobilities and highest occupied energy levels, wherein the maximum brightness of the constructed device is 282300 cd/m²The maximum current efficiency and the maximum external quantum efficiency reach 64.03 cd/A and 15.13 percent.

3. The application further provides a method for MoO-based_xThe performance of the QLED device with the mixed hole injection layer of PEDOT and PSS is analyzed, and the QLED device (ITO/MoO) based on the mixed hole injection layer is obtained compared with a standard device with the mixed hole injection layer of PEDOT and PSS as HIL_xPSS/TFB/QDs/ZnO/Al) service life is improved by about 37 times, and the efficiency and the service life of the QLED device are improved at the same time.

Drawings

FIG. 1 shows MoO in example 1_xPSS film contact Angle and roughness test results on ITO wherein (a) is MoO_xContact angle testing on ITO; (b) PSS contact angle test on ITO; (c) AFM image of ITO; (d) is MoO_xAFM images of the films; (e) AFM images of PEDOT: PSS films;

FIG. 2 shows MoO at different annealing temperatures in example 1_xXRD pattern of the film;

FIG. 3 shows MoO in example 1_xXPS spectra of the thin film, wherein (a) the full spectrum; (b) o1 s; (c) mo3 d;

FIG. 4 shows spin-coated MoO at different precursor concentrations in example 1_xAFM images of the films;

FIG. 5 shows spin-coated MoO at different precursor concentrations in example 1_xA transmittance profile of the film;

FIG. 6 shows MoO at different annealing temperatures in example 1_xAFM images of the films;

FIG. 7 shows MoO at different annealing temperatures for the treatment of example 1_xTransmittance profile of the film;

FIG. 8 shows different UV-O reactions in example 1₃MoO at treatment time_xAFM images of the films;

FIG. 9 shows different UV-O patterns in example 1₃MoO at treatment time_xTransmittance profile of the film;

FIG. 10 shows MoO in example 2_x(ii) a secondary electron cut-off spectrum of the film; (b) a valence band edge region spectrum; (c) a plot of absorption coefficient versus photon energy; (d) a schematic diagram of device energy levels;

FIG. 11 is a graph of (a) current density-voltage-luminance of Device A in example 2 based on different concentrations of precursor; (b) current efficiency-brightness-power efficiency;

FIG. 12 is a graph of (a) current density-voltage-luminance of Device A in example 2 based on different rotation speeds of the precursor; (b) current efficiency-brightness-power efficiency;

FIG. 13 shows MoO-based data in example 2_xCurrent density-voltage-luminance plots for Device a at different annealing temperatures for the films; (b) current efficiency-brightness-power efficiency;

FIG. 14 is a Glass/MoO sample of example 2_x(treated at different annealing temperatures)/TFB/QDs and Glass/QDs; (b) transient PL spectra;

FIG. 15 shows MoO-based data in example 2_xCurrent density-voltage-luminance plot of Device a at different annealing times for the film; (b) current efficiency-luminance-power efficiency;

fig. 16 is a schematic diagram of four different device structures in example 2, example 3, example 4, and example 5;

fig. 17 is (a) a current density-voltage-luminance graph of devices based on different device structures in example 2, example 3, example 4, and example 5; (b) a current efficiency-luminance-power efficiency graph;

FIG. 18 is a schematic diagram showing the energy level structure of a device in example 6;

FIG. 19 is a graph of (a) current density-voltage-luminance of a Device E in example 6 based on different HTL materials; (b) current efficiency-brightness-power efficiency;

fig. 20 is a comparison of single electron device performance testing and single hole device performance testing for different HTLs in example 6;

FIG. 21 is a graph of Glass/MoO samples of example 6_xPSS/HTL/QDs (a) steady state PL Spectroscopy; (b) transient PL spectra;

FIG. 22 is a graph of luminance versus time for a standard Device (a standard Device with PEDOT: PSS as HIL) and a Device B, where (a) is a life test curve for the standard Device; (b) a lifetime test curve for Device B.

Detailed Description

In order to make the objects, technical solutions and effects of the present invention clearer and clearer, the present invention is described in further detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In the following examples, ITO glass substrates having a thickness of about 150 nm, an area of 2.5 cm. times.2.5 cm and a sheet resistance of 15.00. omega./sq were purchased from Luoyang gulo glass Co., Ltd, China, and purchased from: tinwell Technology Ltd.

The (3, 4-ethylenedioxythiophene) -polystyrene sulfonic acid (PEDOT: PSS) was sold under the trademark CLEVOS P VP AI4083 and purchased from Heraeus.

TFB solution concentration 8 mg/mL, solvent chlorobenzene, purchased from American Dye Source;

PVK solution concentration 8 mg/mL, solvent toluene, Purchase company: aldrich;

Poly-TPD concentration 10 mg/mL, solvent chlorobenzene, Purchase company: sigma;

the TCTA concentration is 10 mg/mL, the solvent is a mixed solution of chlorobenzene and tetrahydrofuran in a volume ratio of 4:1, and the purchasing company: american Dye Source;

CBP concentration 10 mg/mL, solvent chlorobenzene, Purchase company: american Dye Source;

the Quantum Dots (QDs) are ZnCdSeS/ZnS green light quantum dots with the concentration of 18 mg/mL and the solvent of n-octane, and the specific preparation method can be referred to (Xu S, Shen H, Zhou C, et al. Effect of shell thickness on the optical properties in CdSe/CdS/Zn_0.5Cd_0.5S/ZnS and CdSe/CdS/Zn_xCd_1-xS/ZnS core/multishell nanocrystals[J]. Journal of Physical Chemistry C, 2011, 115(43):20876-20881）；

The zinc oxide (ZnO) solution is prepared by self with the concentration of 30 mg/mL and the solvent of ethanol;

the aluminum electrode is prepared by vapor deposition of aluminum particles with the density of 2.702 g/cm3, the boiling point of 2467 ℃, the melting point of 660.4 ℃ and the purity of 99.99%, wherein the aluminum particles are purchased from Kurt J.Lesker;

the purities of the acetone, the isopropanol, the chlorobenzene and the toluene are all 99.99 percent.

The model of the conductive atomic force scanning probe microscope is Dimension Icon, purchased from Bruker corporation, usa; the model of the digital measurement source meter is Keithley 2400; the spectral radiometer model is PR-735, M-75 Lens, standard SD card, power adapter (AC-730-6). The spin coater model was WS-650MZ-23NPP/LITE, available from Mycro corporation, USA.

Example 1

MoO_xAs a typical n-type semiconductor material, the transition metal oxide has the common advantages of good environmental stability, no toxicity, easy preparation and the like. Since the chemical composition of the transition metal oxide affects the energy level structure, the present embodiment is based on the molecular oxygen demand of MoO_xFilm thickness, annealing temperature, UV-O₃Optimization of experimental parameters such as processing and the like to obtain MoO suitable for QLED device energy level structure_xA film.

(mono) MoO_xMethod of synthesis of

Preparing MoO by using ammonium molybdate as precursor and deionized water as solvent by adopting sol-gel method_xThe film comprises the following specific steps: an aqueous solution of ammonium molybdate (10% w/v) was added with deionized waterDiluting to a certain concentration, heating to 80 deg.C in air, stirring for 1h to obtain MoO with concentration of 5% w/v, 6% w/v, 7% w/v, and 8% w/v respectively_xAnd filtering the precursor solution by using a filter head with the diameter of 0.45 mu m for later use.

(di) MoO_xPreparation of films and results

1、MoO_xSurface topography characterization of thin films

The flatness and compactness of the film are of great importance to the performance of the QLED device, if the film is too rough, the surface defect states are more, the deposition of the subsequent film can be influenced, larger leakage current is caused, and the performance of the device is reduced. Therefore, we first start with MoO_xThe film forming property of the film was analyzed. Adding MoO_xPSS is respectively coated on large-area ITO to form MoO_xFilm and PEDOT PSS film, and for MoO_xContact Angle and roughness of film and PEDOT PSS film on ITO was tested (Xiaohohong Jiang, Yuting Ma, Yu Tian, Anzhen Wang, Aqiang Wang, Shuangshuang Zhang, Shujie Wang, Zuling Du, High-efficiency and static dot light-emitting diodes with stationary V2O5/PEDOT PSS hole project layer interface barrier, Organic Electronics78(2020) 105589), the results of which are shown in FIG. 1, where (a) is MoO_xContact angle testing on ITO; (b) the contact angle of PSS on ITO is tested; (c) is an AFM image of ITO; (d) is MoO_xAFM images of the films; (e) AFM images of PEDOT vs. PSS films.

As can be derived from FIG. 1 (a), MoO_xwithin a test scan range of 2.0 μm × 2.0 μm, it can be seen from FIG. 1 (c) that the ITO is slightly rough, the surface has a distribution of particulate matter, the root mean square Roughness (RMS) of which is up to 0.71 nm_xAfter the film is formed, the RMS reaches only 0.65 nm, no obvious granular substances are distributed, and the film becomes more flat and compact, which indicates that MoO_xThe film can effectively inhibit the surface defects of the ITO, and is more beneficial to spin coating of the subsequent film. PSS, MoO compared to PEODOT_xLower roughness of the film, and bondingAnd the antenna test results are consistent. Thus, sol-gel process produced MoO_xThe films are suitable for use in the construction of QLED devices.

2. To MoO_xCrystallinity and chemical composition of thin film

Adding MoO_xSpin coating the precursor on large-area ITO, annealing at different annealing temperatures (130 deg.C, 200 deg.C, 300 deg.C, 400 deg.C), cooling to room temperature, and UV-O₃And treating for 5min, and cutting into proper size for testing.

By adjusting MoO at different annealing temperatures_xXRD test is carried out on the film, the crystal structure of the film is analyzed, and MoO is carried out at different annealing temperatures_xThe XRD pattern of the film is shown in FIG. 2. As can be derived from FIG. 2, MoO increases with annealing temperature_xThe crystallinity of the film increases. MoO at 400 ℃ annealing temperature_xStrongest diffraction peak and tetragonal MoO of thin film₃(JCPDS # 39-0363), good crystallinity, and no other peaks. And MoO at an annealing temperature of 130 DEG C_xThe crystallinity of the film is poor, and amorphous MoO is formed₃。

FIG. 3 is MoO_xXPS spectroscopy of a thin film, wherein (a) is full spectrum; (b) is O1 s; (c) is Mo3 d. FIG. 3 is a MoO diagram (a)_xXPS survey of the films, FIG. 3 (b) is an O1s XPS survey demonstrating the presence of Mo and O, Mo3p at about 398 eV_3/2Peak and Mo3d at about 232.8 eV_5/2Peaks correspond to approximately stoichiometric MoO₃Films, consistent with literature reports (see Zeng Q Y, Xu Z W, Zheng C X, et al., Improving charge injection via ablade-coating molybdenum oxide layers: heated high-performance large-area-dot light-emitting diodes [ J]. ACS Applied Materials&Interfaces,2018, 10(9): 8258-8264). FIG. 2 (c) is Mo3d XPS spectra, corresponding to 3d_3/2And 3d_5/2Bimodal. Mo3d fitting thereto_5/2Extraction of MoO from spectra_xMiddle Mo (Mo)⁵⁺And Mo⁶⁺) Two different valence states. In the peak-splitting fitting, there is Mo⁵⁺And Mo⁶⁺Two valence states. Mo⁵⁺Evidence of MoO_xThe presence of oxygen vacancies in the filmThe concentration is an important factor affecting the electrical properties of the metal oxide. Mo can be obtained through peak area fitting integration⁵⁺And Mo⁶⁺In a ratio of 3.5: 1.

3. For MoO with different thicknesses_xInvestigation of thin films

MoO_xThe thickness of the thin film is an important factor affecting the hole transport capability, and an excessively thick thin film may reduce the hole transport capability, and an excessively thin film may cause leakage current. Therefore, we are dealing with MoO at different thicknesses_xThe films were subjected to a series of characterizations.

The two conditions influencing the thickness of the precursor are rotating speed and precursor concentration, and the rotating speed has small influence on the thickness of the precursor, so that the method directly influences the MoO spin-coated under different precursor concentrations_xAFM testing was performed on the films with a scan range of 2.0. mu. m.times.2.0. mu.m, and the results are shown in FIG. 4, with increasing precursor concentration, MoO_xThe roughness of the film gradually increases. However, viewed as a whole, MoO_xThe undulation degree of the film is not large, and the surface of the film is relatively flat and compact. This phenomenon illustrates the thickness versus MoO_xThe appearance and the flatness of the film have little influence.

The thickness of the film directly affects the transmittance, which is an important factor affecting the external quantum efficiency of the device. The EQE of the device can be reduced if the material absorbs light emitted by the device to a large extent, causing loss of light. Thus, MoO_xThe film needs to satisfy a certain transmittance, so that the light generated by the QLED device can smoothly exit from the substrate.

Here, MoO spin-coated at different precursor concentrations_xThe films were subjected to the US-vis test as shown in figure 5. From FIG. 5, it can be seen that MoO increases with the precursor concentration_xThe film transmittance gradually decreases. MoO_xThe film has transmittance of 72% or more in the whole visible light wavelength range (400 nm-760 nm), and transmittance of 78% or more in the green light wavelength range (500 nm-577 nm). Thus, thickness vs. MoO_xThe transmittance of the thin film has a large influence, thereby affecting the external quantum efficiency of the QLED device.

4. For different annealingMoO at temperature_xInvestigation of thin films

The annealing temperature affects the roughness, composition, transmittance, etc. of the film. Thus, in this experiment for the MoO prepared_xThe film is subjected to post-treatment at different annealing temperatures (100 ℃, 130 ℃, 150 ℃ and 170 ℃). From FIG. 6, it can be seen that MoO increases with annealing temperature_xThe roughness of the film increases. When the annealing temperature is 170 ℃, the thin film is gradually agglomerated from a sheet shape, and the undulation degree is increased. Too much roughness of the film may cause an increase in its surface defect states, increasing the quantum dot quenching probability, and possibly degrading device performance.

MoO is caused by annealing temperature_xThe chemical composition of the film changes, thus, the MoO is adjusted to different annealing temperatures_xThe films were subjected to the US-vis test, the results of which are shown in FIG. 7. MoO at different annealing temperatures in the wavelength range of 400 nm to 800nm_xThe film has small difference of transmittance, which is more than 87%, and good transmittance, and does not influence the light emission of the device.

5. MoO for different ultraviolet ozone treatment times_xInvestigation of thin films

By UV-O₃Treated MoO_xThe chemical composition of the film changes and, at the same time, the surface roughness of the film changes, thereby affecting the device performance. Thus, for different UV-O₃MoO at treatment time_xThe films were subjected to AFM characterization as shown in figure 8. From the test results, it can be derived as UV-O₃Increased processing time, MoO_xThe roughness of the film gradually decreases and the range of the chromaticity bar gradually decreases, i.e. MoO_xThe film quality is better. This is mainly because ozone treatment can clean up surface contamination, reduce film roughness, and thus improve film quality.

Followed by a different ultraviolet ozone treatment (UV-O)₃) MoO over time_xComparison of the transmittance of the films, as can be seen in FIG. 9, via UV-O₃Treated MoO_xThe film transmittance decreased slightly, probably due to UV-O₃To a greater extent change the MoO_xMo in film⁶⁺And Mo⁵⁺Ratio, resulting in MoO_xThe chemical composition of the film changes, causing a decrease in its transmittance. However, in the wavelength range of 400 nm to 800nm, in the UV-O range₃MoO at a treatment time of 10min_xThe film has the highest transmittance and different UV-O₃The transmittance in the treatment time is over 84 percent, the transmittance is good, and the light emission of the device is not influenced.

In summary, the following conclusions were obtained, (1) preparation of MoO by sol-gel method using ammonium molybdate as precursor and water as solvent_xA film. The results show that MoO_xFilm thickness, annealing temperature, UV-O₃Under the conditions of treatment time and the like, a flat and continuous film can be formed on the surface of the ITO film, and the roughness of the film is less than 1 nm; (2) MoO with increasing annealing temperature_xThe film is converted from an amorphous state to a tetragonal system; (3) the thickness of the film slightly affects the transmittance, and the transmittance is 84% or more in the wavelength range of 400 nm to 800nm, so that MoO_xThe films are suitable for use in the construction of QLED devices.

The following examples are based on MoO_xDifferent QLED devices are constructed as hole injection layers, and for convenience of explanation, the nomenclature of each device in the present application is as follows:

ITO/MoO_x/TFB/QDs/ZnO/Al (example 2) (Device A)

ITO/MoO_xPSS/TFB/QDs/ZnO/Al (example 3) (/ Device B)

ITO/MoO_x/PEDOT:PSS/MoO_x/TFB/QDs/ZnO/Al (example 4) (Device C)

ITO/PEDOT:PSS/MoO_x/TFB/QDs/ZnO/Al (example 5) (Device D)

ITO/MoO_xPSS/HTL (TFB, PVK, Poly-TPD, TCTA, CBP)/QDs/ZnO/Al (example 6) (Device E)

Example 2

Based on MoO_xThe construction method of the QLED Device (Device A) of the hole injection layer comprises the following specific steps:

(1) pretreatment of the ITO substrate: before the ITO substrate is pretreated, large-particle dust attached to the surface of the ITO substrate is blown off by a nitrogen gun, the ITO substrate is repeatedly wiped by using a dust-free cloth dipped with a detergent, and the ITO substrate is wiped to be clean by using a clean dust-free cloth. Then putting the obtained product into a dyeing cylinder containing a detergent, performing ultrasonic treatment (80 ℃/20 min), pouring out the detergent to cool the dyeing cylinder to room temperature, washing residual detergent on the surface with deionized water, and then sequentially performing ultrasonic cleaning with ultrapure water, acetone and isopropanol at normal temperature for 15min for later use;

taking out the washed ITO substrate, quickly blowing off the residual solvent on the surface by using a nitrogen gun, and putting the ITO substrate into an ultraviolet ozone cleaning instrument to obtain UV-O₃Pretreating for 15min to remove residual solvent on the surface, reduce the surface roughness and increase the activity and the hydrophilicity of the surface of the ITO electrode;

(2) MoO was spin-coated_xSpin coating the solution on an ITO substrate to form a hole injection layer: pipette 80. mu.L of the MoO for use with a pipette_xSpinning the precursor solution (5% w/v, 6% w/v, 7% w/v, 8% w/v) at 4000rpm/min, 5000 rpm/min, 6000 rpm/min, 7000 rpm/min for 45 s, wiping the electrode with a cotton swab dipped with deionized water after film formation, placing the electrode on an annealing plate, annealing at different annealing temperatures (100 ℃, 130 ℃, 150 ℃, 170 ℃) for different times (0, 5min, 10min, 15 min), cooling to room temperature, and performing UV-O (ultraviolet-oxygen) treatment₃Treating for 5 min;

(3) spin coating the TFB solution onto the hole injection layer using spin coating to form a hole transport layer: after the step (2) is finished, immediately transferring the substrate into a glove box for subsequent operation, filtering the prepared TFB solution by using a filter head with the diameter of 0.2 mu m, absorbing 60 mu L of the solution by using a liquid transfer gun, spin-coating for 30 s at 3000 rpm/min, and placing on an annealing plate to anneal for 30 min at the temperature of 150 ℃;

(4) and spin-coating the green light quantum dot solution on the hole transport layer by using a spin coating method to form a quantum dot light-emitting layer: filtering the prepared ZnCdSeS/ZnS green light quantum dot solution by using a filter head with the particle size of 0.2 mu m, sucking 60 mu L of the solution by using a liquid transfer gun, and spin-coating for 30 s at 3500 rpm/min;

(5) and (3) spin-coating the ZnO solution on the quantum dot light-emitting layer by using a spin-coating method to form an electron transmission layer: filtering the prepared ZnO solution with a 0.2 μm filter head, sucking 60 μ L of the solution, spin-coating at 2500 rpm/min for 30 s, wiping a short electrode with a cotton swab dipped with chlorobenzene after film formation, and placing on an annealing plate to anneal at 60 deg.C for 30 min;

(6) vacuum evaporation of Al electrode: ITO/MoO constructed as above_xthe/TFB/QDs/ZnO device is placed in a thermal evaporation coating machine, and when the vacuum degree of the coating machine is lower than 5.00 × 10^-7carrying out evaporation under a mbar condition, keeping the evaporation rate at 4 Å/s, enabling the thickness of an electrode to be 100 nm, breaking vacuum after the evaporation is finished, and taking out the electrode to be packaged;

(7) packaging: and (3) dripping a proper amount of ultraviolet curing glue on the taken device, covering a cover glass, and curing under an ultraviolet lamp for 2 min.

Example 3

The method for manufacturing Device B in example 3 is different from the method for manufacturing Device a in the methods for manufacturing the hole injection layer and the electron transport layer.

The hole injection layer is formed by spin-coating MoO successively_xPSS layer, specifically:

MoO_xspin coating of the layer: pipette 80. mu.L of the MoO for use with a pipette_xThe precursor solution (with concentration of 5% w/v, 6% w/v, 7% w/v, 8% w/v respectively) was spin-coated at 6000 rpm/min for 45 s, after film formation, the electrode was wiped with a cotton swab dipped in deionized water, and placed on an annealing plate to anneal at 130 ℃ for 15min, followed by UV-O₃The treatment is carried out for 10 min.

Spin coating of a PEDOT PSS layer: filtering with 0.45 μm filter head in MoO_xLayer UV-O₃After the treatment, the solution transfer gun is used for sucking 100 mu L immediately and spin-coating for 45 s at 5000 rpm/min, after film formation, a cotton swab dipped with deionized water is used for wiping an electrode, and the electrode is placed on an annealing plate and annealed for 15min at 130 ℃.

The preparation method of the electron transport layer comprises the steps of filtering the prepared ZnO solution by using a 0.2-micron filter head, sucking 60-micron L of the solution, spin-coating for 30 s at 4000rpm/min, wiping an electrode by using a cotton swab dipped with chlorobenzene after film formation, and placing the electrode on an annealing plate to anneal for 30 min at 60 ℃.

The methods of ITO substrate pretreatment, hole transport layer, quantum dot light emitting layer, Al electrode evaporation and encapsulation in the Device B fabrication method are the same as in example 2.

Example 4

The difference between the preparation method of Device C and Device B in example 4 is that the PSS layer in the hole injection layer is spin-coated once again with MoO after the spin-coating of the PEDOT layer_xAnd (3) a layer.

Example 5

The difference between the Device D manufacturing method and Device B manufacturing method in example 5 is that MoO in the hole injection layer_xThe spin coating sequence of the layer is different from that of the PEDOT and PSS layer.

Example 6

The difference between the Device E preparation method and Device B in example 6 is that, during preparation of the hole transport layer, TFB solution, PVK solution, Poly-TPD solution, TCTA solution and CBP solution are spin-coated on the hole injection layer in sequence, specifically, the prepared TFB solution is filtered by a 0.2 μm filter, 60 μ L of the solution is sucked by a pipette, spin-coated for 30 s at 3000 rpm/min, and placed on an annealing plate to be annealed at 150 ℃ for 30 min, the PVK solution and Poly-TPD solution are spin-coated for 30 s at 3000 rpm/min, and placed on the annealing plate to be annealed at 120 ℃ for 20 min, the TCTA solution and CBP solution are spin-coated for 30 s at 3000 rpm/min, and placed on the annealing plate to be annealed at 80 ℃ for 20 min.

The ITO substrate pretreatment, hole injection layer, quantum dot light emitting layer, electron transport layer, Al electrode evaporation, and encapsulation methods in the Device E manufacturing method are the same as those in example 2.

Results and discussion

(mono) MoO_xThin film energy level position study

MoO_xThe energy level position of the film is particularly important for the energy level structure of the QLED device. If MoO_xIf the energy level position of the film is too high, a hole injection barrier between the film and the ITO is too large; if the energy level is too low, the hole injection barrier of the HIL and the HTL is too large, and the hole injection capability is reduced, so that the electron hole injection is unbalanced, and the performance of the device is poor. Gold (Au)The energy level position of the metal oxide is closely related to the chemical composition, and the MoO can be known from XPS test in FIG. 3_xThe film component being Mo⁵⁺And Mo⁶⁺The ratio of the two may affect the energy level position. Here we are dealing with MoO under equivalent conditions_xThe films were tested in Ultraviolet Photoelectron Spectroscopy (UPS), and MoO was shown in FIGS. 10 (a) and (b), respectively_xThe secondary electron cut-off spectrum and the valence band edge region spectrum of the film.

MoO is available from FIG. 10_xThe thin film had an Ecut off value of 16.36 eV, an energy between the valence band level and the Fermi level of 2.80 eV, and a band gap (energy from the valence band level to the conduction band level) of 2.83 eV. According to the Einstein photoelectric effect formula: Φ = h ν - (ECut off-EFemi), then MoO_xThe Work Function (WF) of the film was 4.86 eV, the valence band position was 7.66 eV, and the conduction band position was 4.83 eV. Its conduction band position differs from the HOMO (highest occupied molecular orbital) of PEDOT: PSS by only 0.24 eV. Therefore, the energy level structure of the QLED device can be obtained according to fig. 10 (c) and (d). Considering the energy levels of ITO and PEDOT: PSS, MoO was finally used_xPSS double layer hole injection layer, MoO_xThe effect of the thin film on the performance of the QLED device is further described.

(di) MoO_xEffect of film thickness on device

As can be seen from the foregoing discussion, the thickness of the Hole Injection Layer (HIL) is an important factor affecting the hole transport capability of the QLED device. If the HIL is too thick, the hole transport capability of the device can be reduced; if the HIL is too thin, severe leakage current may be caused. Therefore, the application of spin coating for MoO preparation_xAnd the thickness of the film is controlled by controlling the concentration and the rotating speed of the precursor solution. Here, the MoO prepared is_xPSS (indium tin oxide)/PSS (molybdenum oxide) film is completely substituted for PEDOT to construct Device A_x/ TFB/QDs/ZnO/Al）。

First, the precursor concentration is controlled. Figure 11 shows Device a performance results for different concentration precursor devices by diluting ammonium molybdate aqueous solution with deionized water to different concentrations (5% w/v, 6% w/v, 7% w/v, 8% w/v), wherein (a) is a current density-voltage-luminance graph; (b) current efficiency-brightness-power efficiency. The test results show that the device performance is best when the precursor concentration is 6% w/v. Therefore, subsequent experiments were all performed at precursor concentrations of 6% w/v.

Secondly, different rotating speeds (4000 rpm/min, 5000 rpm/min, 6000 rpm/min and 7000 rpm/min) of the precursor during spin coating are regulated, the performance result of the Device A is shown in figure 12, the Device A can be obtained from figure 12 (a), when the rotating speed is 6000 rpm/min in a low-voltage area (0-2.4V), the current density of the Device is relatively low, and the current density of the Device is further increased along with the further increase of the rotating speed; as the voltage continues to increase, the device current density increases to a maximum and the brightness increases rapidly at the condition of 6000 rpm/min. From FIG. 12 (b), it can be seen that the device performance is best when the rotation speed is 6000 rpm/min, and the maximum current efficiency and power efficiency are 30.39 cd/A and 25.35 lm/W, respectively. The performance of the device is reduced with the further increase of the rotating speed, which may be caused by that the thin film cannot completely cover the ITO electrode due to the overlarge rotating speed, so that the leakage current is generated, and the performance of the device is reduced. Thus, the prepared MoO can be obtained when the precursor concentration is 6% w/v and the rotating speed is 6000 rpm/min_xThe film thickness is optimal for the device.

(III) MoO_xEffect of film annealing temperature on device Performance

MoO due to the presence of oxygen vacancies_xThe film is an n-type semiconductor material, and oxygen vacancies in the film act similarly to electron doping. There are article reports (Shrotriya V, Li G, et al. Transition metal o)_xides as the bufferlayer for polymer photovoltaic cells[J]Applied Physics Letters, 2006, 88(7): 073508-073510.) stoichiometric MoO₃And anoxic MoO_xThere are performance differences in device applications. In MoO_xBased on the optimal thickness, it is presumed that the annealing temperature affects the composition, transmittance, and the like of the film. Thus, in this experiment for the MoO prepared_xThe film is processed at different annealing temperatures (100 ℃, 130 ℃, 150 ℃ and 170 ℃). Based on MoO_xThe Device performance results of Device a at different annealing temperatures for the thin film are shown in fig. 13, and the detailed results are summarized in table 1.

It can be concluded from the results that as the annealing temperature increases, the device performance tends to be high first and low second. Under the condition that the annealing temperature is 130 ℃, the device performance is optimal, and the maximum current efficiency and the power efficiency respectively reach 32.78 cd/A and 29.07 lm/W.

TABLE 1 MoO-based_xDevice a performance summary of the films at different annealing temperatures.

As can be seen from fig. 6, as the annealing temperature increases, the thin film gradually gathers from a sheet shape, and the waviness becomes larger. And the roughness of the film is too large, which may cause the increase of surface defect states, increase the quenching probability of quantum dots and reduce the performance of the device. The results obtained in conjunction with device performance, when the annealing temperature was 130 ℃, the device performance was optimal. Various other properties such as transmittance, mobility and the like of the film, which are affected by different annealing temperature treatments, are affected, and the device performance is further affected by the differences.

Since the film transmittance has some influence on the external quantum efficiency of the device, it is effective for MoO_xThe film is subjected to ultraviolet-visible-near infrared spectrophotometer test to research different annealing temperatures on MoO_xInfluence of the transmittance of the film. From FIG. 7, different annealing temperature vs. MoO_xThe transmittance of the film is not greatly influenced, and is over 87 percent in the wavelength range of 400 nm-800nm, and the transmittance is good.

In order to further analyze the influence of the annealing temperature on the device performance, the ZnCdSeS/ZnS QDs film was subjected to steady-state and transient PL spectrum tests, as shown in FIG. 14. The QDs lifetimes (. tau.) obtained by the fitting are shown in Table 2. As can be seen from the graph (a), the fluorescence peak of QDs is at 531 nm, and the PL peak intensity of QDs gradually increases as the annealing temperature increases to 130 ℃, but rapidly decreases as the temperature further increases. As can be seen from Table 2, the QDs lifetime of the sample Glass/QDs is 8.58 ns, and the MoO temperature is 130 ℃ under the annealing temperature_xThe QDs lifetime of the film is only 8.08 ns. This phenomenon is due to MoO_xThe surface of the film has a defect state, and the defect state is storedCausing the quantum dots to quench. Thus, MoO_xThe QDs life time of the film under the annealing condition of 130 ℃ is closest to that of a sample Glass/QDs, and the device performance is best.

TABLE 2 samples Glass/MoO_x(treated at different annealing temperatures)/QDs lifetimes of TFB/QDs and Glass/QDs.

(IV) MoO_xEffect of thin film ozone treatment time on device Performance

By UV-O₃After treatment, the oxidation state content and chemical composition of the film are not only affected, but also the surface contamination of the film is removed, thereby improving the film quality and increasing the surface hydrophilicity. Thus, by the pair MoO_xFilm differential UV-O₃Time processing improves device performance. Based on MoO_xThe Device performance results of Device a for the thin film at different annealing times (0, 5min, 10min, 15 min) are shown in fig. 15, and the detailed results are summarized in table 3.

As can be seen from FIG. 15 (a), in the low voltage region, when UV-O₃When the processing time is 10min, the current density of the device is lowest, and the brightness is maximum; with UV-O₃The processing time is further increased, and the device current density is further increased, and the brightness is gradually reduced. From FIG. 17 (b), when UV-O₃When the treatment time is 10min, the device performance is optimal along with UV-O₃The processing time is further increased and the device performance is degraded.

TABLE 3 MoO-based_xFilm in different UV-O₃Device a performance summary at processing time.

As can be seen from FIG. 8, the UV-O₃The treatment reduces the roughness of the film and improves the performance of the device. However, with UV-O₃Further increases in processing time, the device performance is greatly reduced, probably due to prolonged UV-O₃Treatment ofThe element pollution is generated on the surface of the film, thereby reducing MoO_xThe interface performance of the thin film makes the device performance poor. Finally, in UV-O₃The QLED device has the best performance under the condition that the processing time is 10 min. And from the comparison of the transmittance, it can be found that different UV-O is present in the wavelength range of 400 nm to 800nm₃MoO at treatment time_xThe thin film has transmittance of over 84 percent, has good transmittance and does not influence the light emission of the device.

(V) Effect of device Structure on device Performance

Summarizing the foregoing work, it can be concluded that MoO_xWhen the film is used as HIL, the performance of the device is improved to a certain extent by optimizing various experimental parameters, but the film can not meet the practical application. The main reason for this is that a large hole injection barrier still exists between the HIL and the HTL, which reduces the hole transport capability of the device and makes the electron-hole injection unbalanced, thereby resulting in poor efficiency of the prepared device. Therefore, lowering the hole injection barrier becomes a key issue to improve the device efficiency. Introduction is similar to WO₃/PEDOT:PSS﹑V₂O₅PSS can effectively improve the efficiency of the device. Thus, four different device structures were constructed in this experiment, as shown in fig. 16. The device performance results for the four different structures are shown in fig. 17, where (a) is a current density-voltage-luminance plot in fig. 17; (b) current efficiency-luminance-power efficiency graph.

The best performance of Device B can be derived from the Device performance results, as reported in the literature (Jiang X H, Ma YT, et al, High-efficiency and stable quality dot light-emitting diodes with static case V2O5/PEDOT: PSS hole injection layer interface barrier [ J2O 5/PEDOT)]Organic electronics, 2020, 78(10): 105589.) are incorporated herein by reference. The main reason is that the introduction of double-layer hole injection makes the hole injection level of the device step, improves the hole injection capability, and MoO_xPSS can be prevented from corroding ITO, so that the performance of the device is improved. The maximum external quantum efficiency and the maximum brightness of the final QLED device reach 15.13 percent and 282300 cd/m2 respectively.

(VI) Effect of different hole transport layer materials on device Performance

The chemical structure and the photoelectric property of small molecule HTL materials (TCTA and CBP) can be regulated and controlled through molecular engineering. However, since the glass transition temperature is low, crystallization is likely to occur during film formation and use, and the service life of the device is reduced. The polymer HTL material (TFB, Poly-TPD, PVK) has high thermal stability, and has the advantages of simple manufacturing process, low cost, easiness in realization of large-area equipment and the like. Therefore, selecting HTL materials with suitable energy level locations and high carrier mobilities is equally important for better device performance by improving hole transport capabilities. Five common hole transport layer materials were studied in this experiment, and detailed properties are summarized in table 4, and the device energy level diagram is shown in fig. 18. As can be seen from Table 4, TFB and Poly-TPD have relatively high Highest Occupied Molecular Orbital (HOMO); TFB and CBP have relatively high hole mobility (holemobilites).

Table 4 summary table of properties of five common hole transport materials.

The Device performance results of Device E based on different HTL materials for the five films prepared for the Device are shown in fig. 19, and the detailed results are summarized in table 5. When the hole transport material is CBP, the device is not bright, probably because the hole injection barrier is too high. As can be seen from fig. 15 (a), when the hole transporting material is TFB or Poly-TPD, the current density of the device is small, the leakage current of the device is small, and the turn-on voltage is relatively low in the low voltage region; in the high voltage region, the luminance is relatively high. And as can be seen from FIG. 15 (b), the current efficiency of the QLED device based on HTL of TFB and Poly-TPD is 2000 cd/m²To 230000cd/m²The luminance range is still above 40 cd/A, and the efficiency roll-off phenomenon of the device is suppressed.

Table 5 summarizes Device E performance based on different HTL materials.

In order to verify that the charge injection is more balanced when the hole transport material is TFB, a single electron device is respectively constructed: single hole devices of ITO/ZnO/QDs/ZnO/Al and different HTLs: ITO/MoO_xPSS/HTL/QDs/Au, the test results are shown in FIG. 20. from the test results, the current density of the single-hole device with HTL materials of TCTA, PVK, Poly-TPD and TFB is gradually increased and is consistent with the performance result of the device, combining with FIG. 18 and Table 4, the hole injection barrier at the interface of TFB, Poly-TPD and QD is lower than that at the interface of TCTA, PVK and QD, and the hole mobility of TFB is as high as 1.0 × 10^-2cm²V.s. Therefore, when the HTL material is TFB, the hole injection capability of the device is optimized, which is more favorable for electron-hole injection balance, and the device efficiency is the highest.

To further analyze the cause of the increase in device performance, the QDs films were tested for steady-state, transient PL spectra, as shown in fig. 21. The QDs lifetimes (. tau.) obtained by the fitting are shown in Table 6. The QDs fluorescence peak is at 531 nm, compared to other HTL materials, the PL peak of QDs is strongest when the HTL material is TFB. And from Table 2, the samples Glass/MoO_xPSS/TFB/QDs has a QDs lifetime of 8.07 ns, which is most similar to that of the sample Glass/QDs. The results indicate that TFB is the weakest quenching of the fluorescence of QDs and, therefore, the best device performance.

TABLE 6 samples Glass/MoO_xPSS/HTL/QDs and Glass/QDs.

(VII) study of device stability

Introduction of MoO_xThe most original purpose of the hole injection layer is to avoid the corrosion of ITO caused by the acidity of PEDOT and PSS, and further improve the stability of the QLED device. FIG. 22 is a graph of luminance versus time for a standard Device (a standard Device HIL based on PEDOT: PSS) and Device B.

The initial luminance of the device is 15000 cd/m²Under the condition of continuous DC driving, the brightness of the standard Device and the Device B Device is reduced tothe 50% elapsed times were 3.25 h and 45.25 h, respectively, by the formula L0n × T = K (where K is a constant, 1<n<2) Let us assume that the acceleration factor n =1.5 (see Kim J, Yamamoto K, et al, Electron affinity of Electron oxide semiconductors and its applications to organic electronics [ J]Advanced Materials Interfaces, 2020, 45(15): 2403-2421), each converted to an initial luminance of 100 cd/m²Respectively 5970 h and 83129 h (T50). The time taken for the luminance of the standard Device and the luminance of the Device B to be reduced to 95% is 0.08 h and 3.17 h, respectively, and both are converted into the initial luminance of 100 cd/m by the formula²The service life of the T95 is 152 h and 5816 h respectively, and the service life of the device is improved by about 37 times. From the test results, MoO_xPSS is introduced between ITO and PEDOT, so that the purpose of improving the efficiency and the service life of the QLED device at the same time is achieved.

In summary, the present application relates to the preparation of MoO by sol-gel process_xThe film is applied to QLED devices with quantum dot light emitting layers as green light QDs and is also used for MoO_xFilm thickness, annealing temperature, UV-O₃And (4) optimizing experimental parameters such as treatment and the like, and optimizing the structure of the device and the HTL material, thereby improving the performance of the device. The following conclusions can be obtained:

(1) the prepared MoO_xPSS (patterned sapphire substrate) constructed device structure with ITO/MoO (indium tin oxide/MoO) as film for replacing PEDOT_xQLED device of/TFB/QDs/ZnO/Al by optimizing MoO_xThe thickness of the film, the annealing temperature and the ozone time are used for adjusting the performance of the device. The results show that the precursor concentration is 6% w/v, the annealing temperature is 130 ℃, and UV-O₃When the processing time is 10min, the device performance is best, and the maximum brightness reaches 229400 cd/m²The maximum current efficiency and the maximum external quantum efficiency were 41.75 cd/A and 9.70%, respectively.

(2) The structure is ITO/MoO when five hole transport layer materials (TFB, Poly-TPD, PVK, TCTA and CBP) with different mobilities and highest occupied energy levels are optimized_xPSS/HTL/QDs/ZnO/Al QLED device. The results indicate that these five materials can be prepared in PEDOT: PSS forms a flat and dense film and has very high transmittance. Corresponding device performanceThe energy results indicate that TFB with high mobility is the best match for the various functional layers of the existing device, and that the steady state and transient PL spectra also indicate that TFB has the weakest quenching of fluorescence from the adjacent QDs layer. The maximum brightness of the constructed device is 282300 cd/m²The maximum current efficiency and the maximum external quantum efficiency reach 64.03 cd/A and 15.13 percent.

(3) For a standard device taking PEDOT and PSS as HIL and a standard device with the structure of ITO/MoO_xThe QLED device life test result of/PEDOT PSS/TFB/QDs/ZnO/Al shows that compared with the standard reference device, the initial brightness is 100 cd/m²The service lives of the T50 and the T95 are respectively 5970 h and 83129 h and 152 h and 5816 h, the service life of the device is improved by about 37 times, and the efficiency and the service life of the QLED device are improved at the same time.

It is to be understood that the invention is not limited to the examples described above, but that modifications and variations may be effected thereto by those of ordinary skill in the art in light of the foregoing description, and that all such modifications and variations are intended to be within the scope of the invention as defined by the appended claims.

Claims (9)

1. Based on MoO_xThe preparation method of the QLED device of the hole injection layer is characterized by comprising the following steps of:

(1) cleaning and pretreating an ITO glass substrate electrode;

(2) spin-coating a mixed hole injection layer on an ITO glass substrate electrode, wherein the material of the mixed hole injection layer is MoO_xOr MoO_x-PEDOT:PSS；

(3) Spin coating a hole transport layer on the mixed hole injection layer; the hole transport layer is made of one or more of PVK, TFB, poly-TPD, TCTA and CBP;

(4) a quantum dot light-emitting layer is spin-coated on the hole transport layer, and the material of the quantum dot light-emitting layer is ZnCdSeS/ZnS green light quantum dots;

(5) spin-coating an electron transport layer ZnO on the quantum dot light-emitting layer;

(6) and (3) evaporating and plating a top electrode on the ZnO electron transport layer, and packaging the device after evaporation of the device is finished, wherein the top electrode is an Al, Ag, Cu, Au or alloy electrode.

2. The method according to claim 1, wherein MoO is used in the step (2)_xThe spin coating method of the layer is to suck 80 mu L of MoO_xSpin-coating the precursor solution for 40-45 s, annealing after film formation, cooling to room temperature, and performing UV-O₃Treating for 5-10 min; and (3) in the step (2), the spin coating method of the PEDOT and PSS layer is to suck 100 mu L of PEDOT and PSS to spin coat for 40-45 s under the condition of 5000 rpm/min, and anneal for 10-15 min under the condition of 120-130 ℃ after film forming.

3. The method of claim 1, wherein MoO is used as the carrier_xThe preparation steps of the precursor solution are as follows: heating 5-10% ammonium molybdate water solution in air to 80-90 deg.C, stirring for 1-2h to obtain MoO_xThe mass fraction of the precursor solution is 5-8%.

4. The method according to claim 1, wherein MoO is used in the step (2)_xThe rotation speed during the spin coating of the layer is 4000-7000 rpm/min.

5. The method according to claim 1, wherein MoO is used in the step (2)_xThe annealing temperature after the spin coating of the layer is 100-170 ℃; the annealing time is 0-15 min.

6. The method according to claim 1, wherein the spin coating of the hole transport layer in the step (3) is performed by sucking 60 μ L of the hole transport layer material solution, spin-coating at 2800-3200 rpm/min for 25-35 s, and annealing at 120-150 ℃ for 25-30 min on an annealing plate; the concentration of the hole transport layer material solution was 8 mg/mL.

7. The preparation method according to claim 1, wherein the green light quantum dot solution in step (4) is prepared by dissolving ZnCdSeS/ZnS green light quantum dots having a particle size of 8-10 nm in n-octane to a concentration of 18 mg/mL.

8. The preparation method according to claim 1, wherein the electron transport layer ZnO in step (5) is obtained by spin-coating a ZnO solution on the quantum dot light emitting layer, the ZnO solution being prepared by dissolving ZnO having a particle size of 3 to 4 nm in ethanol to prepare a ZnO solution having a concentration of 30 mg/mL.

9. MoO-based prepared by the process according to any of claims 1 to 8_xQLED devices of hole injection layer.

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