CN117939905A - Quantum dot light emitting diode and preparation method thereof - Google Patents
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Abstract
The invention relates to a quantum dot light emitting diode and a preparation method thereof, wherein the quantum dot light emitting diode comprises a first electrode, a hole injection layer, a hole transmission layer, a quantum dot light emitting layer, an electron transmission layer and a second electrode which are sequentially arranged, wherein the quantum dot light emitting layer comprises a main body material and 1-10wt% of heat activation delay fluorescent material; the preparation method comprises the following steps: sequentially coating the hole injection layer, the hole transmission layer, the quantum dot luminescent layer and the electron transmission layer on the first electrode, and finally arranging a second electrode on the electron transmission layer; the quantum dot luminescent layer of the quantum dot light-emitting diode comprises the thermally activated delayed fluorescent material, so that the luminescent efficiency of the quantum dot light-emitting diode can be improved.
Description
Technical Field
The invention relates to a quantum dot light-emitting diode and a preparation method thereof.
Background
Quantum dots (Quantum dots) are nano-crystal semiconductor materials, and Quantum dots with different compositions and sizes can be excited by the same excitation light source to emit light with different wavelengths, so that the Quantum dots can be applied to the preparation of screen displays, such as a backlight module for arranging photoluminescent Quantum dots in a Liquid Crystal Display (LCD) to obtain better backlight utilization rate, and for example, electroluminescent Quantum dots are used to prepare Quantum dot light emitting diodes (QLEDs).
The structure of the quantum dot light emitting diode as disclosed in taiwan TW 202032810 (a) publication may include a first electrode, a quantum dot light emitting layer, an electron transfer layer, an electron dispersion layer and a second electrode sequentially disposed on the first electrode; the structure of the quantum dot light emitting diode disclosed in taiwan TW 202044608 (a) includes a first electrode, a hole injection layer sequentially disposed on the first electrode, a hole transfer layer including ZnS, a quantum dot light emitting layer, and a second electrode; that is, the current quantum dot light emitting diode has similar structure, but is improved in each layer structure to improve the performance of the quantum dot light emitting diode. The material used in the quantum dot luminescent layer of the current quantum dot light emitting diode has larger highest occupied molecular rail (highest occupied molecular orbital), so that the material has larger energy barrier on energy transfer, and unbalanced electron and hole injection are easy to cause; unbalanced electron and hole injection can lead to electron flooding and non-radiative European-Jed recombination (non-RADIATIVE AUGER RECOMBINATION), which in turn can lead to the decay of the luminous efficiency of the quantum dot light emitting diode.
Disclosure of Invention
The inventor has found that the present invention is improved by the help of the rich expertise and years of practical experience of the present invention, and the present invention is developed based on the fact that the prior quantum dot light emitting diode has the defects of practical use.
The invention relates to a quantum dot light emitting diode and a preparation method thereof, wherein the quantum dot light emitting diode comprises a first electrode, a hole injection layer, a hole transmission layer, a quantum dot light emitting layer, an electron transmission layer and a second electrode which are sequentially arranged on the first electrode, wherein the quantum dot light emitting layer comprises a main body material and 1-10wt% of heat activated delayed fluorescent material (THERMALLY ACTIVATED DELAYED fluorescence, TADF for short).
The preparation method of the high-efficiency quantum dot light emitting diode comprises the following steps: firstly, a glass substrate is taken, a transparent conductive film is arranged on the glass substrate to obtain a first electrode, and a hole injection layer is coated on the transparent conductive film of the first electrode; step two, coating a hole transmission layer on the hole injection layer; coating a quantum dot luminescent layer on the hole transmission layer, wherein the quantum dot luminescent layer comprises a main body material and 1-10wt% of heat-activated delayed fluorescent material; and step four, coating an electron transmission layer on the quantum dot luminous layer, and arranging a second electrode on the electron transmission layer.
In an embodiment of the invention, the host material of the quantum dot light-emitting layer is cadmium selenide/zinc sulfide (CdSe/ZnS), and the thermally activated delayed fluorescence material is DDCzTrz(9,9',9",9"'-((6-phenyl-1,3,5-triazine-2,4-diyl)bis(benzene-5,3,1-triyl))tetrakis(9H-carbazole)).
In one embodiment of the present invention, the transparent conductive film is an Indium Tin Oxide (ITO) film, the hole injection layer is made of Poly 3, 4-ethylenedioxythiophene and polystyrene sulfonate (PEDOT: PSS), the hole transport layer is made of Poly [ bis (4-phenyl) (4-butylphenyl) amine (Poly (4-butylphenyldiphenylamine, hereinafter referred to as Poly-TPD)), and the electron transport layer is made of nano zinc oxide.
In an embodiment of the invention, a root mean square (root mean square) of the roughness of the quantum dot light emitting layer is between 1.5 nm and 2.5nm.
In one embodiment of the present invention, the driving voltage of the quantum dot light emitting diode is 3-4 volts, the maximum brightness is 20000-60000cd/m 2, the maximum current efficiency is 10-30cd/A, and the maximum energy efficiency is 5-15lm/W.
Therefore, the quantum dot light-emitting diode provided by the invention has the advantages that the heat activation delay fluorescent material is added into the quantum dot light-emitting layer, the light-emitting efficiency of the quantum dot light-emitting diode can be improved, the surface of the prepared quantum dot light-emitting layer is smoother, and the reduction of exciton quenching is facilitated.
Drawings
Fig. 1: a quantum dot light emitting diode structure schematic diagram;
fig. 2: quantum dot light emitting diode electron microscope photographs;
fig. 3: a current density analysis graph of the quantum dot light emitting diode;
fig. 4: a thickness and driving voltage analysis chart of the quantum dot light emitting diode;
Fig. 5: a graph of brightness, current efficiency and energy efficiency analysis of the quantum dot light emitting diode;
fig. 6: an analysis chart of the electroluminescent intensity and the light excitation fluorescence intensity of the quantum dot light emitting diode;
Fig. 7: atomic force microscope observation photo of the quantum dot light emitting diode;
fig. 8: an absorption wave band and light-emitting wave band diagram of the quantum dot light-emitting layer material;
Fig. 9: a time-resolved photoluminescence spectrum analysis chart of the quantum dot luminescent layer material;
Fig. 10: time-resolved photoluminescence spectroscopy analysis of quantum dot light emitting diodes.
Symbol description:
1: a first electrode; 11: a glass substrate;
12: a conductive film; 2: a hole injection layer;
3: a hole transport layer; 4: a quantum dot light emitting layer;
5: an electron transport layer; 6: and a second electrode.
Detailed Description
In order to achieve the advantages and in order to make the disclosure more complete and clear, the present invention is described in detail below with reference to the accompanying drawings.
Referring to fig. 1, the qd led of the present invention includes a first electrode 1, a hole injection layer 2 disposed on the first electrode 1, a hole transport layer 3 disposed on the hole injection layer 2, a qd light emitting layer 4 disposed on the hole transport layer 3, an electron transport layer 5 disposed on the qd light emitting layer 4, and a second electrode 6 disposed on the electron transport layer 5; wherein the first electrode 1 comprises a substrate 11 and a conductive film 12, and the quantum dot light-emitting layer 4 comprises a host material and a thermally activated delayed fluorescence material; among them, the conductive film 12 may be made of Indium Tin Oxide (ITO), the hole injection layer 2 may be made of PEDOT: PSS, the hole transport layer 3 may be made of Poly-TPD, the host material of the quantum dot light emitting layer 4 may be cadmium selenide/zinc sulfide (CdSe/ZnS), and the thermally activated delayed fluorescence material may be DDCzTrz, the electron transport layer 5 may be made of nano zinc oxide, and the second electrode 6 may be an aluminum electrode.
The preparation method of the quantum dot light emitting diode comprises the following steps: firstly, a substrate 11 is taken, a conductive film 12 is arranged on the substrate 11 to obtain a first electrode 1, and a hole injection layer 2 is coated on the first electrode 1; step two, coating a hole transmission layer 3 on the hole injection layer 2; step three, coating a quantum dot luminescent layer 4 on the hole transport layer 3, wherein the quantum dot luminescent layer 4 comprises a main material and a thermal activation delay fluorescent material; and step four, coating an electron transmission layer 5 on the quantum dot luminous layer 4, and plating a second electrode 6 on the electron transmission layer 5.
The thermal activation delay fluorescent material DDCzTrz is of a chemical formula of C 69H43N7, the highest occupied molecular orbital (highest occupied molecular orbital, HOMO) is-6.1 eV, the lowest occupied molecular orbital (lowest occupied molecular orbital, LUMO) is-2.9 eV, the energy difference between a singlet state and a triplet state (delta EST) is 0.27eV, the difference between the two values is less than 3eV, the energy band gap is small, and the molecules are easy to excite; according to previous studies, DDCzTrz materials have photoluminescence spectra with luminescence wavelengths between 400nm and 600nm and wave fronts at about 455 nm. In addition, the absorption spectrum of the quantum dot main body material (cadmium selenide/zinc sulfide) used in the invention has obvious overlapping with the photoluminescence fluorescence spectrum of the thermal activation delay fluorescent material DDCzTrz, which means that fluorescence resonance energy transfer can occur between the two materials, namely, fluorescence emitted by the DDCzTrz material can excite the main body material, and the quantum dot main body material can emit visible light with the wavelength of about 600-650 nm.
Furthermore, the scope of the invention which can be practically applied can be further demonstrated by the following specific examples, but is not intended to limit the scope of the invention in any way.
1. Preparation of quantum dot light emitting diode
Firstly, cutting a glass substrate into a size of 1.5cm multiplied by 2cm multiplied by 0.7mm, sequentially using acetone, isopropanol and deionized water to respectively shake for 15 minutes by ultrasonic waves to clean the glass substrate 11, and finally drying the glass substrate 11 by a nitrogen gun; a transparent conductive film 12 is formed on the surface of a glass substrate 11 by using a photolithography technique to obtain a first electrode 1, wherein the first electrode 1 is a finger-shaped transparent electrode in this embodiment, and the sheet resistance is 11 Ω/sq.
Next, poly 3, 4-ethylenedioxythiophene/polystyrene sulfonate (PEDOT: PSS) having a thickness of 10-50nm was coated on the conductive film 12 of the first electrode 1 by a spin coating method, and baked in an oven at 120 ℃ for 15 minutes to form a hole injection layer 2 on the first electrode 1.
Then, poly [ bis (4-phenyl) (4-butylphenyl) amine (Poly (4-butylphenyldiphenylamine)) having a thickness of 10-50nm was coated on the hole injection layer 2 by spin coating, and then heated by an electric hot plate to act at 110℃for 30 minutes to form a hole transport layer 3 on the hole injection layer 2.
Then, a mixed material with the thickness of 30.00nm plus or minus 2nm to 70.00nm plus or minus 2nm is coated on the hole transmission layer 3 by a rotary coating method at the rotating speed of 15000rpm, and then is heated by an electric hot plate to act for 30 minutes at the temperature of 90 ℃ so as to form a quantum dot luminescent layer 4 on the hole transmission layer 3; the mixed material comprises 1-10wt% of heat-activated delayed fluorescence material DDCzTrz and the rest percentage of host material cadmium selenide/zinc sulfide (CdSe/ZnS); in this embodiment, the blend material comprises 1wt%, 3wt% or 5wt% DDCzTrz and the remaining percentage of cadmium selenide/zinc sulfide.
Then, a nano zinc oxide (ZnO nano) with a thickness of 30-80nm is coated on the quantum dot luminescent layer 4 by a spin coating method, and then heated by an electric hot plate to act for 30 minutes at 90 ℃ so as to form an electron transport layer 5 on the quantum dot luminescent layer 4, wherein the thickness of the electron transport layer 5 is preferably 40-60nm.
Finally, aluminum (Al) metal with a thickness of 150nm + -2 nm is deposited on the electron transport layer 5 by thermal evaporation to form the second electrode 6, wherein the working pressure during thermal evaporation is less than or equal to 1X 10 -6 torr.
Referring to fig. 2, a photograph of a cross section of a quantum dot light emitting diode prepared in this embodiment is observed by a transmission electron microscope.
Referring to fig. 3 (a), a quantum dot light emitting diode made of materials with different proportions DDCzTrz is added to a quantum dot light emitting layer (hereinafter referred to as QD layer), and when currents with different voltages are introduced, the analysis result of the current density is shown in fig. 3, "QD:1wt% tadf" represents that 1wt% ddcztrz material is added to the QD layer, and so on; according to (a) of fig. 3, the quantum dot light emitting diode having 3wt% ddcztrz material added to the QD layer has the highest current density under the same voltage introduction condition; FIG. 3 (B) is an analytical graph of current density when the voltage is introduced between 3 and 4.5 volts (V) in FIG. 3 (A), and the Y-axis coordinate in the graph is presented in logarithmic scale (log scale), according to FIG. 3 (B), the driving voltage (driving voltage) of different quantum dot light emitting diodes at current density of 100mA/cm 2 is obtained, the driving voltage of the "QD:1wt% TADF" group is 3.58V, the driving voltage of the "QD:3wt% TADF" group is 3.65V, and the driving voltage of the "QD:5wt% TADF" group is 3.96V, which indicates that the driving voltage of the quantum dot light emitting diode increases with increasing proportion of DDCzTrz material added in the QD layer.
Fig. 4 shows the QD layer thickness and the driving voltage analysis result of each QD, where "PRESTINE QD" group is QD without DDCzTrz material; the results of FIG. 4 show that without the addition of DDCzTrz sets of materials ("PRESTINE QD" set), the thickness of the QD layer is 18.38nm, the thickness of the QD layer of the "QD:1wt% TADF" set is 18.63nm, the thickness of the QD layer of the "QD:3wt% TADF" set is 18.90nm, and the thickness of the QD layer of the "QD:5wt% TADF" set is 19.16nm; because TADF is a small molecule, even though the thickness of the QD layer increases with increasing proportion of DDCzTrz material added, the thickness slowly increases with doping concentration; as a result of analysis of the driving voltage, the driving voltage of the "PRESTINE QD" group was 2.47V, the driving voltage of the "QD:1wt% tadf" group was 3.58V, the driving voltage of the "QD:3wt% tadf" group was 3.65V, and the driving voltage of the "QD:5wt% tadf" group was 3.96V; i.e. the driving voltage of the QD layer increases with increasing proportion of DDCzTrz material added.
FIG. 5 (A) shows the results of Luminance (Luminance) analysis for each of the sets of the quantum dot light emitting diodes, with the maximum Luminance for the no-additive DDCzTrz material set ("PRESTINE QD" set) being 164,216cd/m 2, the maximum Luminance for the "QD:1wt% TADF" set being 208,992cd/m 2, the maximum Luminance for the "QD:3wt% TADF" set being 528,293cd/m 2, and the maximum Luminance for the "QD:5wt% TADF" set being 381,250cd/m 2; FIG. 5 (B) is an analytical graph of the current efficiency (current efficiency) for each group, the maximum current efficiency for the "PRESTINE QD" group is 6.99cd/A, the maximum current efficiency for the "QD:1wt% TADF" group is 12.13cd/A, the maximum current efficiency for the "QD:3wt% TADF" group is 26.32cd/A, and the maximum current efficiency for the "QD:5wt% TADF" group is 14.67cd/A; FIG. 5 (C) is a graph of the energy efficiency (power efficiency) analysis for each group, the maximum energy efficiency for the "PRESTINE QD" group being 2.94lm/W, the maximum energy efficiency for the "QD:1wt% TADF" group being 6.12lm/W, the maximum energy efficiency for the "QD:3wt% TADF" group being 11.46lm/W, and the maximum power efficiency for the "QD:5wt% TADF" group being 7.08lm/W; the analysis results show that the adding DDczTrz of materials into the quantum dot luminescent layer of the quantum dot luminescent diode can obviously improve the brightness and the current efficiency of the quantum dot, and the group of adding 3wt% of DDczTrz materials has the highest brightness, the highest current efficiency and the highest energy efficiency.
The analysis result of the average Electroluminescence (EL) intensity of each QD light emitting diode shown in fig. 6 (a) is the Full width at half maximum (Full WIDTH AT HALF maximum, FWHM) of the measurement waveform, and it is shown that the Full width at half maximum of the "QD:1wt% tadf" group is 18.35nm, the Full width at half maximum of the "QD:3wt% tadf" group is 17.9nm, and the Full width at half maximum of the "QD:5wt% tadf" group is 18.18nm, that is, the Full width at half maximum of the "QD:3wt% tadf" group is narrowest among the three QD light emitting diodes; fig. 6 (B) is an analysis chart of average photoluminescence (Photoluminescence, abbreviated as PL) of three quantum dot light emitting diodes, the peak position of which still falls at 520nm, which is very similar to the waveform in the analysis chart of average EL intensity, and fig. 6 (a) shows no protruding band at the position of 400nm to 480nm even after amplification, indicating that DDczTrz material in the quantum dot light emitting layer does not emit light, and therefore no protruding waveform is observed at the light emitting band position (455 nm) of DDczTrz material.
Fig. 7 is an atomic force microscope observation photograph, and the roughness of the quantum dot light emitting layer is analyzed, wherein (a) in fig. 7 is a "QD:1wt% TADF" group, (B) in fig. 7 is a "QD:3wt% TADF" group, and (C) in fig. 7 is a "QD: wt% TADF" group, and the surface roughness of the quantum dot light emitting layer of each group is expressed in root mean square (also referred to as RMS value); the RMS value of "QD:1wt% TADF" was 2.10nm, the RMS value of "QD:3wt% TADF" was 1.89nm, and the RMS value of "QD:5wt% TADF" was 1.60nm; because DDczTrz is a small molecule, after being added to the quantum dot luminescent layer, the gap of the main body material of the quantum dot luminescent layer can be filled, so that the higher the adding proportion of DDczTrz material is, the lower the RMS value of the quantum dot luminescent layer is, which means that the surface of the quantum dot luminescent layer is smoother, and further the quenching (sequencing) of excitons (exciton) in the quantum dot luminescent layer is reduced.
Next, please refer to fig. 8, which is a diagram of an absorption band of a main material (cadmium selenide/zinc sulfide) of the quantum dot light emitting layer, an emission band of cadmium selenide/zinc sulfide, and an emission band of a thermally activated delayed fluorescent material (DDczTrz material), wherein a dotted line is an absorption band of cadmium selenide/zinc sulfide, a thick solid line is an emission band of DDczTrz material (denoted as TADF in the figure), and a thin solid line is an emission band of cadmium selenide/zinc sulfide, wherein the emission band of DDczTrz material overlaps with the absorption band of cadmium selenide/zinc sulfide, so that the cadmium selenide/zinc sulfide can serve as a acceptor, and DDczTrz material can serve as a donor, and fluorescence resonance energy transfer occurs between the two materials (fluorescence resonance ENERGY TRANSFER).
Referring to fig. 9 again, in order to analyze the fluorescence intensity changes of the host material (cadmium selenide/zinc sulfide) and DDczTrz material of the quantum dot light emitting layer using Time-resolved photoluminescence spectrum (Time-Resolved Photoluminescence, abbreviated as TRPL); fig. 9 (a) is a TRPL analysis chart of a cadmium selenide/zinc sulfide material (denoted by QD in the figure), fig. 9 (B) is a TRPL analysis chart of a DDczTrz material (denoted by TADF in the figure), and the result shows that the fluorescence life cycle of the DDczTrz material decreases more slowly than that of the cadmium selenide/zinc sulfide material, and the luminous intensity (intensity) is also higher than that of the cadmium selenide/zinc sulfide material.
FIG. 10 is a TRPL analysis of a quantum dot luminescent layer without DDczTrz added material (shown in the figure as QD) and a quantum dot luminescent layer with 3wt% DDczTrz added material (shown in the figure as QD: TADF); the fluorescence life cycle of the "QD: TADF group" decreased more slowly than that of the "QD group" probably because of the effect of FRET, improving the occurrence of ohex recombination (Auger recombination) in QDs.
In addition, analyzing the fluorescence attenuation rate (DECAY RATE) of cadmium selenide/zinc sulfide materials, DDczTrz materials and a quantum dot luminescent layer (abbreviated as QD: TADF) added with 3wt% of DDczTrz materials, wherein the attenuation rate of the cadmium selenide/zinc sulfide materials at 520nm excitation wavelength is 2.91ns, and the attenuation rate of the TADF at 520nm excitation wavelength is 4.2ns, and the situation of rising exists; the decay rate of DDTczTrz material at 455nm excitation wavelength is 3.07ns, but the decay rate of QD: TADF at 455nm excitation wavelength is not measured, because in QD: TADF, the fluorescence energy of DDczTrz is converted to QDs and is not measured.
In summary, the quantum dot light emitting diode of the invention is to dope the thermal activation delay fluorescent material into the main material of the quantum dot light emitting layer, transfer the exciton energy of the thermal activation delay fluorescent material to the main material, so as to achieve the purpose of fluorescence resonance energy transfer, and reduce the decay rate of photons of the quantum dot, so as to improve the light emitting intensity; in addition, the thermal activation delay fluorescent material is doped in the quantum dot luminescent layer, so that the surface smoothness of the quantum dot luminescent layer can be improved, holes on the surface can be reduced, and therefore, a good junction is formed between the quantum dot luminescent layer and the hole transport layer, the injection of holes is further improved, and the quenching of excitons is reduced.
Claims (10)
1. A quantum dot light emitting diode comprises a first electrode, a hole injection layer, a hole transmission layer, a quantum dot light emitting layer, an electron transmission layer and a second electrode which are sequentially arranged, wherein the quantum dot light emitting layer comprises a main body material and 1-10wt% of heat activation delay fluorescent material, the driving voltage of the quantum dot light emitting diode is 3-4 volts, the maximum brightness is 20000-60000cd/m 2, the maximum current efficiency is 10-30cd/A, and the maximum energy efficiency is 5-15lm/W.
2. The quantum dot light emitting diode of claim 1, wherein the host material of the quantum dot light emitting layer is cadmium selenide/zinc sulfide and the thermally activated delayed fluorescence material is DDCzTrz.
3. The qd-led of claim 1 or 2, wherein the first electrode comprises a substrate and an indium tin oxide film, the hole injection layer is prepared from poly-3, 4-ethylenedioxythiophene and polystyrene sulfonate, the hole transport layer is prepared from poly [ bis (4-phenyl) (4-butylphenyl) amine, and the electron transport layer is prepared from nano-zinc oxide.
4. The quantum dot light emitting diode of claim 3, wherein the roughness of the quantum dot light emitting layer has a root mean square between 1.5 and 2.5nm.
5. The quantum dot light emitting diode of claim 1 or 2, wherein the roughness of the quantum dot light emitting layer has a root mean square between 1.5 and 2.5nm.
6. A method of making a quantum dot light emitting diode, comprising:
step one: a glass substrate is taken, a transparent conductive film is arranged on the glass substrate to obtain a first electrode, and a hole injection layer is coated on the transparent conductive film of the first electrode;
Step two: coating a hole transmission layer on the hole injection layer;
step three: coating a quantum dot luminescent layer on the hole transport layer, wherein the quantum dot luminescent layer comprises a main body material and 1-10wt% of heat activation delay fluorescent material; and
Step four: coating an electron transmission layer on the quantum dot luminous layer, and arranging a second electrode on the electron transmission layer;
the driving voltage of the quantum dot light emitting diode is 3-4 volts, the maximum brightness is 20000-60000cd/m 2, the maximum current efficiency is 10-30cd/A, and the maximum energy efficiency is 5-15lm/W.
7. The method of manufacturing of claim 6, wherein the host material of the quantum dot light emitting layer is cadmium selenide/zinc sulfide and the thermally activated delayed fluorescence material is DDCzTrz.
8. The method of claim 6 or 7, wherein the transparent conductive film is an indium tin oxide film, the hole injection layer is prepared from poly 3, 4-ethylenedioxythiophene and polystyrene sulfonate, the hole transport layer is prepared from poly [ bis (4-phenyl) (4-butylphenyl) amine, and the electron transport layer is prepared from nano zinc oxide.
9. The method of claim 8, wherein the roughness of the quantum dot light emitting layer has a root mean square between 1.5 and 2.5nm.
10. The method of claim 6 or 7, wherein the roughness of the quantum dot light emitting layer has a root mean square between 1.5 and 2.5nm.
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