A full carbon dot organic electroluminescent device and its preparation method

Abstract

The present invention provides an all-carbon dot organic electroluminescent device, comprising a conductive substrate, an electron transport layer, a light-emitting layer, a hole transport layer, a hole injection layer, and a metal electrode, which are stacked in sequence. The electron transport layer includes n-type carbon dots, the light-emitting layer includes solid-state fluorescent carbon dots, and the hole transport layer includes p-type carbon dots. The present application also provides a method for preparing the all-carbon dot organic electroluminescent device. By introducing n-type carbon dots, solid-state fluorescent carbon dots, and p-type carbon dots into the organic electroluminescent device, the present application enables the organic electroluminescent device to have higher maximum brightness, efficiency, and operating life.

Description

Technical Field

[0001] The present invention relates to the technical field of organic electroluminescent devices, and in particular to an all-carbon dot organic electroluminescent device and a preparation method thereof. Background Art

[0002] Light-emitting diodes (LEDs) are widely used in consumer electronics. They are considered innovative products in the lighting and display fields due to their higher photoelectric efficiency, higher contrast, and wider color space than traditional liquid crystal displays (LCDs). However, current optoelectronic materials often suffer from high toxicity and low stability, leading to an urgent need to develop new, non-toxic, and highly stable optoelectronic materials for their application in LEDs.

[0003] Carbon dots, a low-cost, low-toxic, highly stable, and readily available fluorescent nanomaterial, are beginning to be used in light-emitting diodes (LEDs). Currently, their application in multicolor LEDs is typically achieved by coating them as phosphors on GaN-based UV or blue light-emitting diodes, exploiting the material's photoluminescence properties. In fact, using carbon dots as the emissive layer in electroluminescent diodes is one of the most promising applications in solid-state lighting.

[0004] Since the electroluminescent mechanism of carbon dots has not yet been revealed, the development of carbon dot-based electroluminescent diodes has focused on the design and synthesis of high-performance carbon dots, with less focus on the optimization design of device structures. At present, carbon dot-based electroluminescent diodes are usually prepared based on the traditional ITO / PEDOT:PSS / EML / TPBi / LiF / Al forward structure. This low-mobility system is not conducive to improving the performance of electroluminescent diodes. The transport layer in the high-mobility system is mostly prepared by solution processing, but the different functional groups rich in carbon dots make their solubility diverse. During the deposition of the functional layer, the solvent on the upper layer of the carbon dots will corrode the carbon dot layer, destroying the good contact between the films and reducing the performance of the diode. Therefore, the easy functionalization feature of carbon dots should be fully utilized to adjust their solubility to obtain carbon dots with orthogonal solubility to prepare solution-processed electroluminescent diodes.

[0005] In order to improve the performance of carbon dot-based electroluminescent diodes, the most effective method in the device optimization process is to balance the injection of carriers in the light-emitting layer. Therefore, how to balance the injection of carriers to obtain high-performance carbon dot-based electroluminescent diodes is of great significance. Summary of the Invention

[0006] The technical problem solved by the present invention is to provide an organic electroluminescent device with high maximum brightness, efficiency and operating life.

[0007] In view of this, the present application provides an all-carbon dot organic electroluminescent device, comprising a conductive substrate, an electron transport layer, a light-emitting layer, a hole transport layer, a hole injection layer and a metal electrode stacked in sequence, wherein the electron transport layer includes n-type carbon dots, the light-emitting layer includes solid-state fluorescent carbon dots, and the hole transport layer includes p-type carbon dots.

[0008] Preferably, the method for preparing the n-type carbon dots comprises the following steps:

[0009] Perylenetetracarboxylic dianhydride, polyethyleneimine and sodium hydroxide are mixed in ethanol and subjected to a solvothermal reaction to obtain a crude product solution;

[0010] The crude product solution is concentrated and dialyzed to obtain a carbon dot solution;

[0011] The carbon dot solution is evaporated and dried to obtain n-type carbon dots.

[0012] Preferably, the mass ratio of perylenetetracarboxylic dianhydride, polyethyleneimine and sodium hydroxide is (0.1-1): (0.02-1): (0-2) and the sodium hydroxide is not 0, the temperature of the solvent thermal reaction is 100-200 ° C, the time is 2-12 hours; the dialysis time is 12-36 hours, and the dialysis bag specification is mw: 500-5000; the electron mobility range of the n-type carbon dots is 10 -5 ~10 -4 cm 2 ·V -1 ·s -1 , energy levels are -5.68 to -3.28 eV.

[0013] Preferably, the method for preparing the solid-state fluorescent carbon dots comprises the following steps:

[0014] Terephthalonitrile and N,N-bis(4-formylphenyl)aniline are dissolved in ethanol and mixed, and subjected to a solvothermal reaction to obtain a crude product solution;

[0015] The crude product solution is purified, evaporated, and dried to obtain solid fluorescent carbon dots.

[0016] Preferably, the mass ratio of terephthalonitrile and N,N-bis(4-formylphenyl)aniline is (0.5-1):(0.02-1), the temperature of the solvent thermal reaction is 100-200°C, and the time is 2-12h; the purification adopts silica gel column chromatography, and the eluent for the purification is dichloromethane and methanol in a ratio of 1:(0-10) and the methanol is not 0; the energy level of the solid-state fluorescent carbon dots is -5.72 to -3.53 eV, and has good independent film-forming properties. The formed thin film emits visible light with a peak wavelength in the range of 550 to 650 nm under excitation at 330 to 380 nm.

[0017] Preferably, the method for preparing the p-type carbon dots comprises the following steps:

[0018] Melamine, acetonitrile and N,N-bis(4-formylphenyl)aniline are dissolved in ethanol and mixed, and subjected to a solvothermal reaction to obtain a crude product solution;

[0019] purifying the crude product solution to obtain a carbon dot solution;

[0020] The carbon dot solution is evaporated and dried to obtain p-type carbon dots.

[0021] Preferably, the mass ratio of melamine, N,N-bis(4-formylphenyl)aniline and acetonitrile is (0.5-1):(0.02-1):(0-1) and the acetonitrile is not 0; the temperature of the solvent thermal reaction is 100-200°C and the time is 2-12 hours; the purification adopts silica gel column chromatography, and the eluent of the purification is dichloromethane and methanol in a ratio of 1:(0-10) and the methanol is not 0; the hole mobility rate of the p-type carbon dots is 10 -5 ~10 -4 cm 2 ·V -1 ·s -1 , energy levels are -5.83 to -3.14 eV.

[0022] The present application also provides a method for preparing an all-carbon dot organic electroluminescent device, comprising the following steps:

[0023] A) pre-treating a conductive substrate, coating it with an n-type carbon dot solution, and then annealing it to obtain an electron transport layer;

[0024] B) coating a solid fluorescent carbon dot solution on the surface of the electron transport layer, and then performing an annealing treatment to obtain a light-emitting layer;

[0025] C) coating a p-type carbon dot solution on the surface of the light-emitting layer, and then performing an annealing treatment to obtain a hole transport layer;

[0026] D) depositing an electron injection layer and a metal anode in sequence on the surface of the hole transport layer to obtain a full carbon dot organic electroluminescent device.

[0027] Preferably, the concentration of the n-type carbon dot solution is 2-10 mg / mL, the concentration of the solid fluorescent carbon dot solution is 5-15 mg / mL, and the concentration of the p-type carbon dot solution is 2-10 mg / mL.

[0028] Preferably, in step A), the annealing treatment temperature is 50-200° C. and the time is 10-60 min; in step B), the annealing treatment temperature is 50-250° C. and the time is 10-60 min; in step C), the annealing treatment temperature is 50-200° C. and the time is 5-60 min.

[0029] The present application provides an organic electroluminescent device comprising a conductive substrate, an electron transport layer, a light-emitting layer, a hole transport layer, a hole injection layer, and a metal electrode, which are stacked in sequence. The device is characterized in that the electron transport layer includes n-type carbon dots, the light-emitting layer includes solid-state fluorescent carbon dots, and the hole transport layer includes p-type carbon dots. In the organic electroluminescent device provided herein, different carbon dots are introduced and applied to the electron transport layer, the light-emitting layer, and the hole transport layer, respectively, effectively balancing carrier injection and improving the maximum brightness, efficiency, and operating life of the organic electroluminescent device. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is an energy level structure diagram of the electroluminescent diode prepared in Example 4;

[0031] Figure 2 UV absorption spectra of n-type carbon dots (n-CD), solid fluorescent carbon dots (EM-CD) and p-type carbon dots (p-CD) in Examples 1, 2 and 3;

[0032] Figure 3 The ultraviolet photoelectron spectra of n-type carbon dots, solid fluorescent carbon dots and p-type carbon dots in Examples 1, 2 and 3 are shown;

[0033] Figure 4 The UV absorption spectra of the n-type carbon dot film in Example 4 before and after cleaning with ODCB / DMF mixed solution;

[0034] Figure 5 The UV absorption spectra of the solid fluorescent carbon dot film in Example 4 before and after cleaning with isopropyl alcohol;

[0035] Figure 6 This is an AFM photograph of solid fluorescent carbon dots deposited on n-type carbon dots in Example 4;

[0036] Figure 7 This is an AFM photograph of p-type carbon dots deposited on the solid fluorescent carbon dots in Example 4;

[0037] Figure 8 This is the brightness-voltage-current density curve of the device prepared in Comparative Example 1;

[0038] Figure 9 This is the brightness-voltage-current density curve of the device prepared in Comparative Example 2;

[0039] Figure 10 Comparison of the brightness-voltage-current density curves of the devices prepared in Comparative Example 3, Comparative Example 4, Comparative Example 5 and Example 4;

[0040] Figure 11 Graphs showing the operating life of the devices prepared in Comparative Example 5 and Example 4;

[0041] Figure 12 This is the brightness-voltage-current density curve of the device prepared in Example 4;

[0042] Figure 13 Electroluminescence spectrum of the device prepared in Example 4 and photos of large-area devices;

[0043] Figure 14 Current density-voltage curves of the space charge confinement region of the single carrier devices prepared in Comparative Examples 6, 7, 8, and 9. DETAILED DESCRIPTION

[0044] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0045] Carbon dots are excellent optoelectronic materials and can also serve as transport layers to transfer carriers. Therefore, with the help of their easy functionalization characteristics, organic electroluminescent devices with carbon dot transport layers and light-emitting layers can be constructed. The injection of carriers is balanced by adjusting the mobility and film thickness of the carbon dots in the two transport layers to obtain high-performance carbon dot-based electroluminescent devices, especially electroluminescent diodes. Specifically, the present application provides a full-carbon dot organic electroluminescent device, comprising a conductive substrate, an electron transport layer, a light-emitting layer, a hole transport layer, a hole injection layer and a metal electrode stacked in sequence, wherein the electron transport layer includes n-type carbon dots, the light-emitting layer includes solid-state fluorescent carbon dots, and the hole transport layer includes p-type carbon dots.

[0046] In the present application, the method for preparing the n-type carbon dots comprises the following steps:

[0047] Perylenetetracarboxylic dianhydride, polyethyleneimine and sodium hydroxide are mixed in ethanol and subjected to a solvothermal reaction to obtain a crude product solution;

[0048] The crude product solution is concentrated and dialyzed to obtain a carbon dot solution;

[0049] The carbon dot solution is evaporated and dried to obtain n-type carbon dots.

[0050] In the above preparation process, the mass ratio of perylenetetracarboxylic dianhydride, polyethyleneimine and sodium hydroxide is (0.1-1):(0.02-1):(0-2) and the sodium hydroxide is not 0, the temperature of the solvent thermal reaction is 100-200°C, and the time is 2-12 hours; more specifically, the mass ratio of perylenetetracarboxylic dianhydride, polyethyleneimine and sodium hydroxide is (0.1-0.8):(0.1-0.8):(1-2). The temperature of the solvent thermal reaction is 120-180°C, and the time is 5-10 hours. The dialysis time is 12-36 hours, and the dialysis bag specification is mw: 500-5000. In the above process, the purification is performed by silica gel column chromatography, and the eluents are dichloromethane and methanol.

[0051] The electron mobility of the n-type carbon dots prepared in this application is in the range of 10 -5 ~10 -4 cm 2 ·V -1 ·s -1 , with an energy level of -5.68 to -3.28 eV. The n-type carbon dots can better match the energy level of solid-state luminescent carbon dots, lowering the barrier to electron injection. Furthermore, the n-type carbon dots in this application are insoluble in oil-soluble solvents, and therefore have a wider range of applications compared to traditional organic transport materials. Solid-state fluorescent carbon dots and other fluorescent carbon dot materials can be directly spin-coated onto the electron transport layer of the n-type carbon dots without worrying about corrosive effects on the underlying layer, thereby forming a flat and smooth surface.

[0052] Furthermore, the preparation method of the solid fluorescent carbon dots of the present application comprises the following steps:

[0053] Terephthalonitrile and N,N-bis(4-formylphenyl)aniline are dissolved in ethanol and mixed, and subjected to a solvothermal reaction to obtain a crude product solution;

[0054] The crude product solution is purified, evaporated, and dried to obtain solid fluorescent carbon dots.

[0055] In the above process, the mass ratio of terephthalonitrile to N,N-bis(4-formylphenyl)aniline is (0.5-1):(0.02-1), specifically, the mass ratio of terephthalonitrile to N,N-bis(4-formylphenyl)aniline is (0.6-1):(0.1-1). The temperature of the solvent thermal reaction is 100-200°C, and the time is 2-12 hours. The purification is performed by silica gel column chromatography, and the eluent for the purification is dichloromethane and methanol in a ratio of 1:(0-10), and the methanol content is not zero.

[0056] The solid-state fluorescent carbon dots have an energy level of -5.72 to -3.53 eV and exhibit excellent standalone film-forming properties. The resulting thin film emits visible light with a peak wavelength in the 550-650 nm range under excitation at 330-380 nm. Application of these solid-state fluorescent carbon dots in the light-emitting layer overcomes the problem of host-guest doping causing spectral artifacts in the device, thereby improving the color purity of carbon dot LEDs.

[0057] Furthermore, the present application also provides a method for preparing p-type carbon dots, comprising the following steps:

[0058] Melamine, acetonitrile and N,N-bis(4-formylphenyl)aniline are dissolved in ethanol and mixed, and subjected to a solvothermal reaction to obtain a crude product solution;

[0059] purifying the crude product solution to obtain a carbon dot solution;

[0060] The carbon dot solution is evaporated and dried to obtain p-type carbon dots.

[0061] In this process, the mass ratio of melamine, N,N-bis(4-formylphenyl)aniline and acetonitrile is (0.5-1):(0.02-1):(0-1), and the acetonitrile is not 0. Specifically, the mass ratio of melamine, N,N-bis(4-formylphenyl)aniline and acetonitrile is (0.6-0.8):(0.1-0.8):(0.2-0.8). The temperature of the solvent thermal reaction is 100-200°C, and the time is 2-12 hours. The purification adopts silica gel column chromatography, and the eluent for the purification is dichloromethane and methanol in a ratio of 1:(0-10), and the methanol is not 0.

[0062] The hole mobility of the p-type carbon dots is 10 -5 ~10 -4 cm 2 ·V -1 ·s -1 , energy levels are -5.83 to -3.14 eV.

[0063] In this application, n-type and p-type carbon dots are considered as matching carbon dots. In carbon dot LEDs, organic hole transport materials have low stability, a problem overcome by the use of p-type carbon dots. Furthermore, in an inverted device structure, the use of metal oxide electron transport materials in the lower layer can cause carrier injection mismatch (organic electron transport materials cannot be deposited in the lower layer due to solubility issues), reducing device stability and efficiency. Furthermore, compared to organic and metal oxide transport layer materials, carbon dots are low-cost and easy to synthesize, making them a highly superior alternative.

[0064] Furthermore, the present application also provides a method for preparing an all-carbon dot organic electroluminescent device, comprising the following steps:

[0065] A) pre-treating a conductive substrate, coating it with an n-type carbon dot solution, and then annealing it to obtain an electron transport layer;

[0066] B) coating a solid fluorescent carbon dot solution on the surface of the electron transport layer, and then performing an annealing treatment to obtain a light-emitting layer;

[0067] C) coating a p-type carbon dot solution on the surface of the light-emitting layer, and then performing an annealing treatment to obtain a hole transport layer;

[0068] D) depositing an electron injection layer and a metal anode in sequence on the surface of the hole transport layer to obtain a full carbon dot organic electroluminescent device.

[0069] In the process of preparing the all-carbon dot organic electroluminescent device, the present application first pre-treats the conductive substrate; the pre-treatment is to clean the substrate. The conductive substrate is a substrate well known to those skilled in the art, and the present application has no special restrictions on this. In a specific embodiment, the substrate is specifically selected from an ITO glass substrate.

[0070] According to the present invention, after pretreatment, an n-type carbon dot solution is coated on the surface of the conductive substrate, and then heat-treated to obtain an electron transport layer. In this process, the coating is a coating process well known to those skilled in the art and is not particularly limited in this application. For example, the coating can be selected from spin coating. The concentration of the n-type carbon dot solution is 2 to 10 mg / mL. The annealing treatment temperature is 50 to 200°C and the time is 10 to 60 minutes. More specifically, the annealing treatment temperature is 80 to 150°C and the time is 20 to 30 minutes.

[0071] After preparing the electron transport layer, a light-emitting layer is prepared on its surface, that is, a solid fluorescent carbon dot solution is coated on the surface of the electron transport layer, and then annealing is performed to obtain a light-emitting layer. In this process, the coating is a coating process well known to those skilled in the art, and this application does not impose any particular restrictions on this. For example, the coating can be selected from spin coating. The concentration of the solid fluorescent carbon dot solution is 5 to 15 mg / mL. The temperature of the annealing treatment is 50 to 250°C, and the time is 10 to 60 minutes; more specifically, the temperature of the annealing treatment is 100 to 150°C, and the time is 20 to 30 minutes.

[0072] After preparing the light-emitting layer, a hole transport layer is prepared on its surface; that is, a p-type carbon dot solution is coated on the surface of the light-emitting layer, and then annealing is performed to obtain a hole transport layer. In this process, the coating is a coating process well known to those skilled in the art, and this application does not impose any particular restrictions on this. For example, the coating can be selected from spin coating. The concentration of the p-type carbon dot solution is 2 to 10 mg / mL. The temperature of the annealing treatment is 50 to 200°C, and the time is 5 to 60 min; specifically, the temperature of the annealing treatment is 80 to 120°C, and the time is 15 to 40 min.

[0073] Finally, the present application sequentially deposits an electron injection layer and a metal anode on the surface of the hole transport layer to produce an all-carbon dot organic electroluminescent device. The deposition method specifically employs vacuum evaporation, and the preparation process is not particularly limited in this application and can be performed according to methods well known to those skilled in the art. The materials for the electron injection layer and the metal anode are well known to those skilled in the art and are not particularly limited in this application.

[0074] The organic electroluminescent device described in the present application may specifically be an organic electroluminescent diode.

[0075] In order to further understand the present invention, the full carbon dot organic electroluminescent device and the preparation method thereof provided by the present invention are described in detail below in conjunction with the examples. The protection scope of the present invention is not limited by the following examples.

[0076] Example 1

[0077] Preparation of n-type carbon dots:

[0078] 0.12 g of perylenetetracarboxylic dianhydride and 1.2 g of sodium hydroxide were weighed and added to 300 mL of ethanol. The mixture was completely dissolved by ultrasonication for 30 min, and then 3 mL of PEI ethanol solution (80 mg / mL) was added. The mixture was transferred to a polytetrafluoroethylene hydrothermal reactor and hydrothermally reacted at 200 ° C for 10 h. The mixture was then naturally cooled and concentrated. The mixture was dialyzed for two days using a dialysis bag with a molecular weight cutoff of 500 to obtain n-type carbon dots.

[0079] Example 2

[0080] Preparation of solid-state fluorescent carbon dots:

[0081] Weigh 0.05 g of N,N-di(4-formylphenyl)aniline, 0.05 g of terephthalonitrile, and 0.02 g of sodium hydroxide and add them to 20 mL of ethanol. Ultrasonic stirring is performed to completely mix the substances. The substances are transferred to a 50 mL polytetrafluoroethylene hydrothermal autoclave, and then hydrothermally reacted at 200°C for 3 hours. The reaction is then naturally cooled and rotary evaporated to dryness. Solid-state fluorescent carbon dots can be obtained by purification through a chromatography column method using different ratios of dichloromethane and methanol as developing solvents.

[0082] Example 3

[0083] Preparation of p-type carbon dots:

[0084] 0.06 g of melamine, 0.06 g of N,N-bis(4-formylphenyl)aniline, 0.02 g of sodium hydroxide, 2 mL of acetonitrile, and 20 mL of ethanol were weighed and mixed in a beaker. Ultrasonic stirring was performed to completely mix the substances, and the mixture was transferred to a 50 mL polytetrafluoroethylene hydrothermal autoclave. The mixture was then hydrothermally reacted at 200°C for 10 h, and then naturally cooled and rotary evaporated to dryness. P-type carbon dots were purified by chromatography using different ratios of dichloromethane and methanol as developing solvents.

[0085] Example 4 (ITO / n-CD / EM-CD / p-CD / MoO3 / Al):

[0086] The ITO glass substrate was cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrate was then dried in an oven at 150°C for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0087] Prepare an electron transport layer on the transparent conductive layer of ITO: Take 40uL of 10mg / mL n-type carbon dot solution (solvent is water) and spin-coat it on the ITO substrate at a speed of 3000rpm. Then place it on a hot plate and anneal it at 100℃ for 10 minutes to prepare the electron transport layer;

[0088] Prepare a light-emitting layer on the electron transport layer: Take 40 μL of a 15 mg / mL solid fluorescent carbon dot solution (solvent is o-dichlorobenzene) and spin-coat it on the electron transport layer at a speed of 2000 rpm. Then place it on a hot plate and anneal it at 150°C for 10 minutes to prepare the light-emitting layer;

[0089] Prepare a hole transport layer on the light-emitting layer: Take 40 μL of 8 mg / mL p-type carbon dot solution (solvent isopropyl alcohol) and spin-coat it on the light-emitting layer at a speed of 3000 rpm. Then place it on a hot plate and anneal it at 80°C for 15 minutes to prepare a hole transport layer.

[0090] MoO3 with a thickness of 7.5 nm was prepared as a hole injection layer by vacuum evaporation;

[0091] An Al electrode with a thickness of 100 nm is prepared by vacuum evaporation to obtain an electroluminescent diode, which is connected to a load or a test device through a wire.

[0092] Comparative Example 1 (ITO / n-CD / EM-CD / Al):

[0093] The ITO glass substrates were cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrates were then dried in a 150°C oven for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0094] Prepare an electron transport layer on the transparent conductive layer of ITO: Take 40uL of 10mg / mL n-type carbon dot solution (solvent is water) and spin-coat it on the ITO substrate at a speed of 3000rpm. Then place it on a hot plate and anneal it at 100℃ for 10 minutes to prepare the electron transport layer;

[0095] Prepare a light-emitting layer on the electron transport layer: Take 40 μL of 8 mg / mL p-type carbon dot solution (solvent isopropyl alcohol) and spin-coat it on the electron transport layer at a speed of 2000 rpm. Then place it on a hot plate and anneal it at 150°C for 10 minutes to prepare the light-emitting layer;

[0096] An Al electrode with a thickness of 100 nm is prepared by vacuum evaporation to obtain an electroluminescent diode, which is connected to a load or a test device through a wire.

[0097] Comparative Example 2 (ITO / EM-CD / p-CD / Al):

[0098] The ITO glass substrates were cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrates were then dried in a 150°C oven for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0099] Prepare a light-emitting layer on the transparent conductive layer of ITO: Take 40uL of 15mg / mL solid fluorescent carbon dot solution (solvent is o-dichlorobenzene) and spin-coat it on the ITO substrate at a speed of 2000rpm. Then place it on a hot plate and anneal it at 150℃ for 10 minutes to prepare the light-emitting layer;

[0100] Prepare a hole transport layer on the light-emitting layer: Take 40 μL of 8 mg / mL p-type carbon dot solution (solvent isopropyl alcohol) and spin-coat it on the light-emitting layer at a speed of 3000 rpm. Then place it on a hot plate and anneal it at 80°C for 15 minutes to prepare a hole transport layer.

[0101] An Al electrode with a thickness of 100 nm is prepared by vacuum evaporation to obtain an electroluminescent diode, which is connected to a load or a test device through a wire.

[0102] Comparative Example 3 (ITO / n-CD / EM-CD / NPB / MoO3 / Al):

[0103] The ITO glass substrates were cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrates were then dried in a 150°C oven for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0104] Prepare an electron transport layer on the transparent conductive layer of ITO: Take 40uL of 10mg / mL n-type carbon dot solution (solvent is water) and spin-coat it on the ITO substrate at a speed of 3000rpm. Then place it on a hot plate and anneal it at 100℃ for 10 minutes to prepare the electron transport layer;

[0105] Prepare a light-emitting layer on the electron transport layer: Take 40uL of a 15mg / mL solid fluorescent carbon dot solution (solvent is o-dichlorobenzene) and spin-coat it on the electron transport layer at a speed of 2000rpm, then place it on a hot stage and anneal it at 150℃ for 10 minutes to prepare the light-emitting layer.

[0106] A hole transport layer was prepared on the light-emitting layer: NPB with a thickness of 40 nm was prepared by vacuum evaporation as a hole transport layer;

[0107] MoO3 with a thickness of 7.5 nm was prepared as a hole injection layer by vacuum evaporation;

[0108] An Al electrode with a thickness of 100 nm is prepared by vacuum evaporation to obtain an electroluminescent diode, which is connected to a load or a test device through a wire.

[0109] Comparative Example 4 (ITO / ZnO / EM-CD / p-CD / MoO3 / Al):

[0110] The ITO glass substrates were cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrates were then dried in a 150°C oven for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0111] Prepare an electron transport layer on the transparent conductive layer of ITO: Take 40 μL of 10 mg / mL ZnO solution (solvent is ethanol) and spin-coat it on the ITO substrate at a speed of 3000 rpm. Then place it on a hot plate and anneal it at 60°C for 30 minutes to prepare the electron transport layer.

[0112] Prepare a light-emitting layer on the electron transport layer: Take 40 μL of a 15 mg / mL solid fluorescent carbon dot solution (solvent is o-dichlorobenzene) and spin-coat it on the electron transport layer at a speed of 2000 rpm. Then place it on a hot plate and anneal it at 150°C for 10 minutes to prepare the light-emitting layer;

[0113] Prepare a hole transport layer on the light-emitting layer: Take 40uL of 8mg / mL p-type carbon dot solution (solvent is isopropyl alcohol) and spin-coat it on the light-emitting layer at a speed of 3000rpm, then place it on a hot plate and anneal it at 80℃ for 15 minutes to prepare a hole transport layer.

[0114] MoO3 with a thickness of 7.5 nm was prepared by vacuum evaporation as a hole injection layer.

[0115] An Al electrode with a thickness of 100 nm is prepared by vacuum evaporation to obtain an electroluminescent diode, which is connected to a load or a test device through a wire.

[0116] Comparative Example 5 (ITO / ZnO / EM-CD / NPB / MoO3 / Al):

[0117] The ITO glass substrates were cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrates were then dried in a 150°C oven for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0118] Prepare an electron transport layer on the transparent conductive layer of ITO: Take 40 μL of 10 mg / mL ZnO solution (solvent is ethanol) and spin-coat it on the ITO substrate at a speed of 3000 rpm. Then place it on a hot plate and anneal it at 60°C for 30 minutes to prepare the electron transport layer.

[0119] Prepare a light-emitting layer on the electron transport layer: Take 40 μL of a 15 mg / mL solid fluorescent carbon dot solution (solvent is o-dichlorobenzene) and spin-coat it on the electron transport layer at a speed of 2000 rpm. Then place it on a hot plate and anneal it at 150°C for 10 minutes to prepare the light-emitting layer;

[0120] A hole transport layer was prepared on the light-emitting layer: NPB with a thickness of 40 nm was prepared by vacuum evaporation as a hole transport layer;

[0121] MoO3 with a thickness of 7.5 nm was prepared as a hole injection layer by vacuum evaporation;

[0122] An Al electrode with a thickness of 100 nm is prepared by vacuum evaporation to obtain an electroluminescent diode, which is connected to a load or a test device through a wire.

[0123] Comparative Example 6 (ITO / PEDOT:PSS / EM-CD / p-CD / MoO3 / Al):

[0124] The ITO glass substrates were cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrates were then dried in a 150°C oven for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0125] A hole transport layer was prepared on the transparent conductive layer of ITO: a PEDOT:PSS solution was spin-coated on the ITO substrate at 3000 rpm, and then annealed on a hot plate at 150°C for 15 minutes to prepare a hole transport layer;

[0126] Prepare a light-emitting layer on the hole transport layer: Take 40 μL of a 15 mg / mL solid fluorescent carbon dot solution (solvent is o-dichlorobenzene) and spin-coat it on the hole transport layer at a speed of 2000 rpm. Then place it on a hot plate and anneal it at 150°C for 10 minutes to prepare the light-emitting layer;

[0127] Prepare a hole transport layer on the light-emitting layer: Take 40 μL of 8 mg / mL p-type carbon dot solution (solvent isopropyl alcohol) and spin-coat it on the light-emitting layer at a speed of 3000 rpm. Then place it on a hot plate and anneal it at 80°C for 15 minutes to prepare a hole transport layer.

[0128] MoO3 with a thickness of 7.5 nm was prepared as a hole injection layer by vacuum evaporation;

[0129] An Al electrode with a thickness of 100 nm was prepared by vacuum evaporation to obtain a single-hole device, which was connected to a load or test device through a wire.

[0130] Comparative Example 7 (ITO / PEDOT:PSS / EM-CD / NPB / MoO3 / Al):

[0131] The ITO glass substrates were cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrates were then dried in a 150°C oven for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0132] A hole transport layer was prepared on the transparent conductive layer of ITO: a PEDOT:PSS solution was spin-coated on the ITO substrate at 3000 rpm, and then annealed on a hot plate at 150°C for 15 minutes to prepare a hole transport layer;

[0133] Prepare a light-emitting layer on the hole transport layer: Take 40 μL of a 15 mg / mL solid fluorescent carbon dot solution (solvent is o-dichlorobenzene) and spin-coat it on the hole transport layer at a speed of 2000 rpm. Then place it on a hot plate and anneal it at 150°C for 10 minutes to prepare the light-emitting layer;

[0134] A hole transport layer was prepared on the light-emitting layer: NPB with a thickness of 40 nm was prepared by vacuum evaporation as a hole transport layer;

[0135] MoO3 with a thickness of 7.5 nm was prepared as a hole injection layer by vacuum evaporation;

[0136] An Al electrode with a thickness of 100 nm was prepared by vacuum evaporation to obtain a single-hole device, which was connected to a load or test device through a wire.

[0137] Comparative Example 8 (ITO / n-CD / EM-CD / TPBi / Ca / Al):

[0138] The ITO glass substrates were cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrates were then dried in a 150°C oven for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0139] Prepare an electron transport layer on the transparent conductive layer of ITO: Take 40uL of 10mg / mL n-type carbon dot solution (solvent is water) and spin-coat it on the ITO substrate at a speed of 3000rpm. Then place it on a hot plate and anneal it at 100℃ for 10 minutes to prepare the electron transport layer;

[0140] Prepare a light-emitting layer on the electron transport layer: Take 40 μL of a 15 mg / mL solid fluorescent carbon dot solution (solvent is o-dichlorobenzene) and spin-coat it on the electron transport layer at a speed of 2000 rpm. Then place it on a hot plate and anneal it at 150°C for 10 minutes to prepare the light-emitting layer;

[0141] An electron transport layer was prepared on the light-emitting layer: TPBi with a thickness of 40 nm was prepared by vacuum evaporation.

[0142] Vacuum evaporation was used to prepare Ca and Al electrodes with a thickness of 10 nm and 100 nm, respectively, to obtain a single-electron device, which was then connected to a load or test device via a wire.

[0143] Comparative Example 9 (ITO / ZnO / EM-CD / TPBi / Ca / Al):

[0144] The ITO glass substrates were cleaned by ultrasonic cleaning in detergent, water, deionized water, acetone, and isopropyl alcohol for 15 minutes. The ITO substrates were then dried in a 150°C oven for 5 minutes and then treated with ultraviolet ozone (UVO) for 15 minutes.

[0145] Prepare an electron transport layer on the transparent conductive layer of ITO: Take 40 μL of 10 mg / mL ZnO solution (solvent is ethanol) and spin-coat it on the ITO substrate at a speed of 3000 rpm. Then place it on a hot plate and anneal it at 60°C for 30 minutes to prepare the electron transport layer.

[0146] Prepare a light-emitting layer on the electron transport layer: Take 40 μL of a 15 mg / mL solid fluorescent carbon dot solution (solvent is o-dichlorobenzene) and spin-coat it on the electron transport layer at a speed of 2000 rpm. Then place it on a hot plate and anneal it at 150°C for 10 minutes to prepare the light-emitting layer;

[0147] An electron transport layer was prepared on the light-emitting layer: TPBi with a thickness of 40 nm was prepared by vacuum evaporation.

[0148] Vacuum evaporation was used to prepare Ca and Al electrodes with a thickness of 10 nm and 100 nm, respectively, to obtain a single-electron device, which was then connected to a load or test device via a wire.

[0149] pass Figure 2 and Figure 3 The energy levels of n-type carbon dots, solid fluorescent carbon dots and p-type carbon dots are calculated to be -5.68~-3.28, -5.72~-3.53, and -5.83~-3.14eV respectively; the results are shown in Figure 1 In the structure, there is a smaller energy barrier between adjacent functional layers, which is conducive to the transfer of carriers.

[0150] The feasibility of solution processing of carbon dots was verified by Figure 4 and Figure 5 The UV absorption spectrum of Figure 6 and Figure 7 The AFM images confirm this. It can be seen that after the upper carbon dots are deposited on the lower film via solution processing, the absorption intensity of the lower carbon dots remains unchanged, indicating that the solvent does not erode the lower film. Furthermore, the AFM images show that the films deposited on the lower carbon dots exhibit minimal roughness and good morphology.

[0151] In order to further confirm the good role played by the carbon dot transport layer in the device, Figure 8 and Figure 9 The brightness-voltage-current density curves of the devices after removing the p-type carbon dots or n-type carbon dots are respectively corresponding to comparative example 1 and comparative example 2. It can be seen from the figure that no matter which carbon dots are removed, the performance curve of the device is very poor. The curve has many jagged edges, indicating that carrier injection is difficult and there is also a large leakage current. After the voltage increases to a certain level, the current density increases sharply, indicating that the diode is broken down and the stability is extremely low.

[0152] A full carbon dot device was prepared using carbon dots as the transport layer, and the maximum brightness of the device was 1300 cd / m 2 ; Then the carbon dots were replaced with transport layer materials in the high mobility system. When ZnO replaced the n-type carbon dots, the carrier injection was seriously unbalanced and the brightness was only 543cd / m 2 , and the turn-on voltage increased significantly. After NPB replaced the p-type carbon dots, the maximum brightness of the device also decreased to 780cd / m 2 Even if the transport layer is completely replaced with a high mobility system, the maximum brightness of the device is still from 1300cd / m 2 Reduced to 890cd / m 2 , indicating that solid fluorescent carbon dots are not suitable for the device structure of this system. The characteristic curve of the device shows that the current density increases rapidly with the increase of voltage, while the brightness increases slowly. At this time, only a small number of carriers participate in the radiative recombination in the light-emitting layer, and most of them are dissipated through Joule heat and other forms, resulting in a decrease in device performance. Therefore, the operating life of the device was tested. When carbon dots were used as the transport layer, the device had a brightness of 100cd / m 2 The half-life of the operating life is 18 minutes. When the transport layer is replaced with a high-mobility system, the brightness of the device rapidly decays to 50 cd / m within 1 minute. 2 Therefore, a single-carrier device was prepared by using two carbon dot transport layer and high mobility transport layer materials and a qualitative analysis was conducted. Figure 14 The mobility gap between ZnO and NPB is significant, while the mobility of n-type and p-type carbon dots is very similar. This suggests that the improved performance and longer operating life of all-carbon-dot devices, compared to high-mobility systems, stem from more balanced carrier injection. Example 1 demonstrates the feasibility of fabricating such all-carbon-dot devices. By adjusting their solubility, simple solution processing is facilitated, while balanced carrier injection enables the realization of high-performance carbon-dot-based light-emitting diodes.

[0153] The above embodiments are only intended to help understand the method and core concept of the present invention. It should be noted that, without departing from the principles of the present invention, a number of improvements and modifications may be made to the present invention by those skilled in the art, and such improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0154] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An all-carbon dot organic electroluminescent device, comprising a conductive substrate, an electron transport layer, a light-emitting layer, a hole transport layer, a hole injection layer and a metal electrode stacked in sequence, characterized in that: The electron transport layer includes n-type carbon dots, the light emitting layer includes solid fluorescent carbon dots, and the hole transport layer includes p-type carbon dots; The method for preparing the n-type carbon dots comprises the following steps: Perylenetetracarboxylic dianhydride, polyethyleneimine and sodium hydroxide are mixed in ethanol and subjected to a solvothermal reaction to obtain a crude product solution; The crude product solution is concentrated and dialyzed to obtain a carbon dot solution; The carbon dot solution is evaporated and dried to obtain n-type carbon dots.

2. The all-carbon dot organic electroluminescent device according to claim 1, characterized in that: The mass ratio of perylenetetracarboxylic dianhydride, polyethyleneimine and sodium hydroxide is (0.1-1): (0.02-1): (0-2) and the sodium hydroxide is not 0. The temperature of the solvent thermal reaction is 100-200°C and the time is 2-12 hours. The dialysis time is 12-36 hours and the dialysis bag specification is mw: 500-5000. The electron mobility range of the n-type carbon dots is 10 -5 ~10 -4 cm 2 ·V -1 ·s -1 , energy levels are -5.68 to -3.28 eV.

3. The all-carbon dot organic electroluminescent device according to claim 1, characterized in that: The preparation method of the solid-state fluorescent carbon dots comprises the following steps: Terephthalonitrile and N,N-bis(4-formylphenyl)aniline are dissolved in ethanol and mixed, and subjected to a solvothermal reaction to obtain a crude product solution; The crude product solution is purified, evaporated, and dried to obtain solid fluorescent carbon dots.

4. The all-carbon dot organic electroluminescent device according to claim 3, characterized in that: The mass ratio of terephthalonitrile and N,N-bis(4-formylphenyl)aniline is (0.5-1):(0.02-1), the temperature of the solvent thermal reaction is 100-200°C, and the time is 2-12 hours; the purification adopts silica gel column chromatography, and the purification eluent is dichloromethane and methanol in a ratio of 1:(0-10), and the methanol is not zero; the energy level of the solid-state fluorescent carbon dots is -5.72 to -3.53 eV, and they have good independent film-forming properties. The formed film emits visible light with a peak wavelength in the range of 550 to 650 nm under excitation at 330 to 380 nm.

5. The all-carbon dot organic electroluminescent device according to claim 1, characterized in that: The preparation method of the p-type carbon dots comprises the following steps: Melamine, acetonitrile and N,N-bis(4-formylphenyl)aniline are dissolved in ethanol and mixed, and subjected to a solvothermal reaction to obtain a crude product solution; purifying the crude product solution to obtain a carbon dot solution; The carbon dot solution is evaporated and dried to obtain p-type carbon dots.

6. The all-carbon dot organic electroluminescent device according to claim 5, characterized in that: The mass ratio of melamine, N,N-bis(4-formylphenyl)aniline and acetonitrile is (0.5-1):(0.02-1):(0-1) and the acetonitrile is not 0; the temperature of the solvent thermal reaction is 100-200°C and the time is 2-12 hours; the purification adopts silica gel column chromatography, and the eluent of the purification is dichloromethane and methanol in a ratio of 1:(0-10) and the methanol is not 0; the hole mobility rate of the p-type carbon dots is 10 -5 ~10 -4 cm 2 ·V -1 ·s -1 , energy levels are -5.83 to -3.14 eV.

7. The method for preparing the all-carbon dot organic electroluminescent device according to claim 1, comprising the following steps: A) pre-treating a conductive substrate, coating it with an n-type carbon dot solution, and then annealing it to obtain an electron transport layer; B) coating a solid fluorescent carbon dot solution on the surface of the electron transport layer, and then performing an annealing treatment to obtain a light-emitting layer; C) coating a p-type carbon dot solution on the surface of the light-emitting layer, and then performing an annealing treatment to obtain a hole transport layer; D) depositing a hole injection layer and a metal anode on the surface of the hole transport layer in sequence to obtain an all-carbon dot organic electroluminescent device; The method for preparing the n-type carbon dots comprises the following steps: Perylenetetracarboxylic dianhydride, polyethyleneimine and sodium hydroxide are mixed in ethanol and subjected to a solvothermal reaction to obtain a crude product solution; The crude product solution is concentrated and dialyzed to obtain a carbon dot solution; The carbon dot solution is evaporated and dried to obtain n-type carbon dots.

8. The preparation method according to claim 7, characterized in that The concentration of the n-type carbon dot solution is 2-10 mg / mL, the concentration of the solid fluorescent carbon dot solution is 5-15 mg / mL, and the concentration of the p-type carbon dot solution is 2-10 mg / mL.

9. The preparation method according to claim 7, characterized in that In step A), the annealing temperature is 50-200° C. and the time is 10-60 min; in step B), the annealing temperature is 50-250° C. and the time is 10-60 min; in step C), the annealing temperature is 50-200° C. and the time is 5-60 min.

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