CN118795731A - A quantum dot photoresist for high-resolution pixel manufacturing - Google Patents

Abstract

The present invention discloses a quantum dot photoresist for high-resolution pixel manufacturing, which uses cross-linked ligand molecules to in-situ replace the original dynamic oleic acid ligands on the surface of quantum dots: on the one hand, a three-dimensional cross-linked network structure of "quantum dot-ligand-polymer" is formed, which improves the processability of high-resolution lithography; on the other hand, the thiol and ester groups in the cross-linked molecules can passivate the surface defects of quantum dots, improve the luminescence performance of quantum dots, and increase the fluorescence quantum yield of quantum dot films by 200% or even 520%. In addition, the toluene/chlorobenzene/m-xylene ternary solvent proposed in the present invention can avoid quantum dot agglomeration, realize high-resolution patterning of lithography, and the pixel size can be reduced to 2μm. The present invention solves the problems of ligand shedding of quantum dots during the lithography process, resulting in degradation of luminescence efficiency and stability, and easy agglomeration of quantum dots, and provides a pixel manufacturing technology solution for ultra-high-definition quantum dot display that is compatible with existing semiconductor processes and universal for multiple quantum dots.

Description

A quantum dot photoresist for high-resolution pixel manufacturing

Technical Field

The invention belongs to the technical field of quantum dots, and in particular relates to a photoresist generally applicable to cadmium-free quantum dot systems such as perovskite, indium phosphide, silver indium gallium sulfide, etc.

Background Art

Quantum dots have high efficiency of photoluminescence (PL) and photoluminescence quantum yield (PLQY), and the luminescent color of quantum dots can be precisely adjusted in the entire visible light range through strategies such as size control and element ratio control. In addition, quantum dot luminescent materials have the advantage of high color purity, and the half-maximum full width of their luminescence spectrum can be narrowed to less than 30 nm, showing great application potential in the new generation of wide color gamut displays.

The current mainstream display technologies include LCD display technology based on white light source combined with liquid crystal, Micro LED display technology based on gallium nitride LED, OLED display technology based on organic light-emitting molecules, etc. Quantum dots are considered to be the key to enhancing display color gamut and energy efficiency due to their ultra-pure color and high-efficiency luminescence advantages, and are therefore widely used in the above display technologies. For example, the backlight display film formed by the mixture of red and green quantum dots can form the white light source required for LCD display together with blue LED, and the red and green quantum dot pixel color conversion layer combined with blue Micro LED or blue OLED can form an active and passive luminescence display. It can be found that quantum dots show extremely strong compatibility in current display technology and are considered to be a low-cost, easy-to-operate feasible route. However, to meet the needs of new displays for ultra-high-definition wide color gamut, it is inevitable to pixelate quantum dots. At present, the main ways to achieve quantum dot pixelation are inkjet printing, transfer printing and photolithography. During the processing process, they are generally faced with problems such as easy detachment of quantum dot surface ligands, easy agglomeration of quantum dots, and attenuation of luminescence performance. These problems restrict the pixel size, luminescence stability and luminescence efficiency of high-resolution patterning of quantum dots. Therefore, developing a "quantum dot-ligand-polymer" photoresist solution system to avoid the shedding of ligands on the surface of quantum dots and the agglomeration of quantum dots in the solution, reduce the generation of surface defects of quantum dots during the patterning process, and maintain the luminescence performance and stability of quantum dots is the key to achieving high-resolution pixelation of quantum dots and their ultra-high-definition display applications.

Summary of the invention

In order to realize high-resolution pixelation of quantum dots and ultra-high-definition display thereof, the present invention provides a quantum dot photoresist with a synergistic function of "cross-linking-passivation".

In a first aspect, the present invention provides a method for preparing a quantum dot photoresist having a "crosslinking-passivation" synergistic function, comprising the following steps:

1) dissolving a molecule containing multiple thiol functional groups in toluene as a first solution for quantum dot ligand exchange;

2) dissolving a molecule containing multiple acrylate functional groups in toluene as a second solution for quantum dot ligand exchange;

3) mixing the first solution and the second solution to form a third solution for quantum dot ligand exchange;

4) adding the toluene solution of cadmium-free quantum dots to the third solution in step 3), mixing and adding to a ternary mixed solvent of toluene/chlorobenzene/m-xylene to obtain a fourth solution for quantum dot ligand exchange;

5) Under a protective atmosphere, the fourth solution is stirred at a speed of 1200 r/min for a certain period of time to obtain a ligand-exchanged quantum dot stock solution;

6) adding a photoinitiator to the quantum dot stock solution obtained in step 5);

7) Under the protective atmosphere, after continuing to stir at a speed of 1000 r/min for a certain period of time, a quantum dot photoresist is obtained.

As a preferred embodiment, the molecule containing multiple thiol functional groups is pentaerythritol tetrathioglycolate (PETM), and 3500 mg of PETM molecules are dissolved in every 1 mL of toluene, that is, the concentration of the first solution is 3500 mg/mL.

As a preferred embodiment, the molecule containing multiple acrylate functional groups is pentaerythritol tetraacrylate (PAE), and 4000 mg of PAE molecules are dissolved in every 1 mL of toluene, that is, the concentration of the second solution is 4000 mg/mL.

As a preferred embodiment, the cadmium-free quantum dots are one of perovskite, indium phosphide or silver indium gallium sulfide quantum dots, and 30 mg of cadmium-free quantum dots are dissolved in every 1 mL of toluene, that is, the concentration of the toluene solution of cadmium-free quantum dots is 30 mg/mL.

As a preferred embodiment, the volume ratio of the first solution, the second solution and the toluene solution of cadmium-free quantum dots is 9:5:2.

As a preferred embodiment, the volume ratio of the ternary mixed solvent of toluene/chlorobenzene/m-xylene is 5:50:100.

As a preferred embodiment, in step 5), the reaction is stirred at a speed of 1200 r/min for 30 min.

As a preferred embodiment, the photoinitiator is TPO, and its mass is 2wt% of the weight of the quantum dot stock solution.

As a preferred embodiment, in step 7), the stirring time is 20 min.

In a second aspect, the present invention provides a quantum dot photoresist having a "crosslinking-passivation" synergistic function prepared by the method described in the first aspect.

In a third aspect, the present invention provides a high-resolution quantum dot pixel, which is formed by a quantum dot photoresist having a "cross-linking-passivation" synergistic function prepared by the method described in the first aspect.

As a preferred embodiment, the steps are as follows:

The quantum dot photoresist solution is spin-coated on the substrate material, and the mask of the corresponding pattern is placed on it. It is exposed to a UV = 365 nm light source for 120 s. Finally, the exposed film is placed in a developer and developed for 30 s to form quantum dot pixels.

Compared with the prior art, the present invention has the following advantages:

(1) The present invention proposes a preparation process for a photoresist solution that can exchange ligands with multi-system quantum dots and passivate quantum dot defects. The introduction of cross-linking ligands of multiple acrylate groups and thiol molecules not only enables the ligands on the surface of the quantum dots to have a cross-linking function through a ligand exchange strategy, but also passivates the surface defects of the quantum dots. The three-dimensional cross-linked network structure of "quantum dots-cross-linking ligands" formed during the photoinitiation process further forms an efficient and stable passivation network system, so that the polymer quantum dot film exhibits a fluorescence quantum yield that is at least 200% higher than that of the standard sample, and the PLQY can reach an ultra-high luminous efficiency of up to 95.7%.

(2) The present invention also proposes a method for preparing a ternary mixed solvent of toluene/chlorobenzene/m-xylene for quantum dot-cross-linked ligand solution, which improves the uniform dispersion of quantum dots in the solution system and solves the problem of quantum dot agglomeration that is easily caused by traditional single solvent dispersed quantum dot photoresist solution in the high-resolution patterning lithography process, thereby achieving high resolution of the lithography pattern, and the pixel size can reach 2 μm.

(3) The quantum dot photoresist solution proposed in the present invention has a "cross-linking-passivation" synergistic function, and has been universally verified in cadmium-free quantum dot systems such as perovskite, indium phosphide, and silver indium gallium sulfide.

Therefore, based on the above-mentioned functionalized quantum dot photoresist solution system to form high-quality quantum dot patterned pixels and combined with blue light LEDs, it is expected to realize a new generation of display products with high resolution, low energy consumption and wide color gamut on Mini LED and Micro LED display devices.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the specific implementation of the present invention or the technical solution in the prior art, the drawings required for use in the specific implementation or the description of the prior art are briefly introduced below.

Figure 1 shows (a) the ligand exchange process between quantum dots and photoresist molecules, (b) the light-induced cross-linking reaction of ligands on the surface of quantum dots, and (c) the process procedure of quantum dot photolithography patterning.

Figure 2 shows the morphology of quantum dot films prepared by spin coating after the quantum dot photoresist mixed solution is dispersed in different solvents (a, toluene; b, chlorobenzene; c, toluene: chlorobenzene = 5:5; d, toluene: chlorobenzene: m-xylene = 5:25:125; e, toluene: chlorobenzene: m-xylene = 5:50:100; f, toluene: chlorobenzene: m-xylene = 5:100:50) and the agglomeration of quantum dots therein.

FIG3 shows the absorption and fluorescence emission spectra of green light CsPbBr ₃ (a) and red light CsPb(BrI) ₃ (b) perovskite quantum dot solutions and quantum dot cross-linked ligand solutions.

FIG4 is a graph showing the ligand exchange process of cross-linked molecules on the surface of a quantum dot solution tracked and characterized by TRPL testing (a) and a statistical graph showing the changing trend of the fluorescence lifetime within the 40 min tested (b).

FIG5 shows the H NMR spectrum of (a) the ligand exchange process between ligand molecule A and perovskite quantum dots in quantum dot photoresist solution, and (b) the H NMR spectrum of the ligand exchange process between ligand molecule B and perovskite quantum dots.

FIG6 is a partial enlarged view of the H NMR spectrum of (a) the ligand exchange process between ligand molecule A and perovskite quantum dots in the quantum dot photoresist solution, and (b) a partial enlarged view of the H NMR spectrum of the ligand exchange process between ligand molecule B and perovskite quantum dots in the quantum dot photoresist solution.

Figure 7 shows the XPS spectra of perovskite quantum dots and perovskite-cross-linked molecular ligands: (a) Pb 4f, (b) S 2p, and (c) O1s.

FIG8 shows the absorption and fluorescence emission spectra of green light CsPbBr ₃ (a) and red light CsPb(BrI) ₃ (b) perovskite quantum dot films and quantum dot ligand cross-linked films, and the TRPL lifetimes of green light CsPbBr ₃ (c) and red light CsPb(BrI) ₃ (d) perovskite quantum dot films and quantum dot ligand cross-linked films.

Figure 9 shows (a) the FTIR spectrum changes of perovskite quantum dots-cross-linked ligands before and after cross-linking, (b) the PLQY changes of perovskite quantum dot films and cross-linked films, the luminescence uniformity mapping of perovskite quantum dots (c) green light CsPbBr ₃ and (d) red light CsPb(BrI) ₃ films, and the luminescence uniformity mapping of perovskite quantum dots (e) green light CsPbBr ₃ and (f) red light CsPb(BrI) ₃ cross-linked films.

Figure 10 shows the AFM images (a), (b), (c), and (d) of the green-light CsPbBr ₃ and red-light CsPb(BrI) ₃ perovskite quantum dot films, and the AFM images (e), (f), and (g), and (h) of the green-light CsPbBr ₃ and red-light CsPb(BrI) ₃ perovskite quantum dot films after cross-linking.

Figure 11 shows the luminescence performance of green light CsPbBr ₃ and red light CsPb(BrI) ₃ perovskite quantum dot films in ethanol solvent (a) (b) and the actual comparison of the luminescence performance differences of green light CsPbBr ₃ and red light CsPb(BrI) ₃ perovskite quantum dot films in ethanol solvent after cross-linking (c) (d).

Figure 12 shows the pictures of patterned pixels after crosslinking of green and red perovskite quantum dots. (a) (b) are 1000 μm square pixels, (c) (d) are 100 μm flower-shaped pixels. (e) (f) (g) (h) are 20 μm, 10 μm, 5 μm and 2 μm red perovskite quantum dots after crosslinking.

Figure 13 shows (a) the absorption and fluorescence emission spectra of silver indium gallium sulfur green light quantum dot film and quantum dot ligand cross-linked film, (b) the TRPL lifetime of silver indium gallium sulfur green light quantum dot film and quantum dot ligand cross-linked film, (c) the FTIR spectrum change of silver indium gallium sulfur green light quantum dot-cross-linked ligand before and after cross-linking, (d) the absorption and fluorescence emission spectra of indium phosphide red light quantum dot film and quantum dot ligand cross-linked film, (e) the TRPL lifetime of indium phosphide red light quantum dot film and quantum dot ligand cross-linked film, (f) the FTIR spectrum change of indium phosphide green light quantum dot-cross-linked ligand before and after cross-linking, (g) (h) are pictures of 5 μm and 2 μm patterned pixels of silver indium gallium sulfur green light quantum dot ligand cross-linked film, respectively, and (i) (j) are pictures of 10 μm and 5 μm patterned pixels of indium phosphide green light quantum dot ligand cross-linked film, respectively.

DETAILED DESCRIPTION

The present application is further described below in conjunction with specific embodiments.

It should be noted that the terms such as "upper", "lower", "left", "right", "middle", etc. cited in this specification are only for the convenience of description and are not used to limit the scope of implementation. Changes or adjustments in their relative relationships should be regarded as the scope of implementation of this application without substantially changing the technical content.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the term "and/or" used herein includes any and all combinations of one or more of the associated listed items.

As used herein, the term "about" is used to provide flexibility and imprecision associated with a given term, measurement or value. The degree of flexibility for a particular variable can be easily determined by one skilled in the art.

As used herein, the term "at least one of" is intended to be synonymous with "one or more of." For example, "at least one of A, B, and C" expressly includes only A, only B, only C, and combinations of each thereof.

Concentration, amount and other numerical data can be presented in range format herein.It should be understood that such range format is used only for convenience and brevity, and should be flexibly interpreted as not only including the numerical value clearly stated as the range limit, but also including all individual numerical values or sub-ranges contained in the range, just as each numerical value and sub-range are clearly stated.For example, the numerical range of about 1 to about 4.5 should be interpreted as not only including the limit value of 1 to about 4.5 clearly stated, but also including individual numbers (such as 2,3,4) and sub-ranges (such as 1 to 3, 2 to 4, etc.).The same principle is applicable to the range of only narrating a numerical value, such as "less than about 4.5", which should be interpreted as including all the above-mentioned values and ranges.In addition, no matter how the breadth of the described range or feature is, this interpretation should be applied.

In order to solve the problems of increased quantum dot defects, attenuated optical properties, and easy agglomeration caused by ligand shedding in the quantum dot patterning process, the present invention proposes a method for preparing and treating a photoresist solution that can exchange ligands on the surface of quantum dots and passivate surface defects. Quantum dot patterned pixels with high quantum yield and high stability are further prepared, which is expected to meet the application needs of the new generation of wide color gamut backlight displays.

In the existing quantum dot photoresist solution and its photolithography process, due to the difference between the photoresist molecules and quantum dots in the solvent environment, the ligands on the surface of the quantum dots are easily detached, resulting in the attenuation of optical performance. In addition, the physical mixing of quantum dots and photoresists is susceptible to the erosion of solvents in the subsequent steps during the thin film lithography and patterning manufacturing process, resulting in the aggregation, separation and precipitation of quantum dots during the development process, and luminescence quenching. In order to solve this problem, it is key to prepare a photoresist solution that can passivate quantum surface defects and allow quantum dots to participate in the cross-linking reaction. Therefore, we selected cross-linking molecules with strong electronegativity and multiple cross-linking sites with thiol and acrylate functional groups, and specifically proposed a treatment process for in-situ ligand exchange of quantum dots in the photoresist solution, which not only passivated the surface defects of quantum dots and improved the luminescence efficiency, but also further realized the cross-linking functionalization of the surface groups of quantum dots, which is helpful for quantum dots to participate in the photolithography cross-linking reaction process. During the photolithography process, under the conditions of light initiation, the initiator molecules break and lead to the generation of free radicals. The active free radicals further trigger the cross-linking reaction between the quantum dots and the cross-linking molecules, and finally form a stable quantum dot cross-linking system film. Due to the presence of cross-linking molecular ligands with passivated surface defects, the quantum dots can be further embedded more firmly in the cross-linking network system, and the formation of the quantum dot cross-linking network system will further strengthen the binding strength between the ligand and the quantum dots, and strengthen the passivation effect of surface defects, thereby improving the stability of the quantum dots in complex environments. Due to the strong electronegativity of the thiol and acrylate functional groups, they have a universal passivation effect on the surface of quantum dots. Therefore, this photoresist solution that enhances the optical properties of quantum dots can be applied to a variety of quantum dot systems (such as perovskite quantum dots, indium phosphide quantum dots, and silver indium gallium sulfur quantum dots). In addition, in order to improve the dispersibility of quantum dots in the photoresist solution, we proposed a method for preparing a ternary mixed solvent of toluene/chlorobenzene/m-xylene, which solves the problem of quantum dot agglomeration that is prone to occur in the high-resolution patterning lithography process of traditional single solvent dispersed quantum dot photoresist solutions, and achieves high resolution of the lithography pattern, with a pixel size of up to 2 μm.

First, as shown in Figure 1, there are dynamic oleic acid ligands on the surface of quantum dots. Due to their weak binding force with the lead atoms on the surface of quantum dots, they are easy to fall off, resulting in the attenuation of the luminescence performance and stability of quantum dots. The cross-linked molecules with thiol and acrylate groups have stronger electronegativity and can strongly coordinate with the lead atoms on the surface of quantum dots to form a more stable "quantum dot-ligand" solution. Subsequently, under the action of UV light and initiator, TPO molecules break to produce free radicals, which react with PETM molecules to generate S· (sulfur free radicals). The generated S· further cross-links and polymerizes with the photoresist solution and the PAE ligands on the surface of quantum dots through a "click reaction" to form a "quantum dot-ligand-polymer" three-dimensional cross-linked network structure. The strongly electronegative thiol and acrylate structures in the three-dimensional cross-linked network structure can interact with the Pb of perovskite quantum dots through strong coordination to achieve the function of passivating defects, and the strongly electronegative thiol groups can also covalently bond with the S atoms in the shell of indium phosphide and silver indium gallium sulfur quantum dots ZnS to generate disulfide bonds, thereby passivating their surface defects. The cross-linked network structure obtained by light initiation is rich in a large number of thiol and acrylate functional groups, forming a more stable ligand environment to further effectively passivate the surface defects of the quantum dots, and can ensure the luminescence performance of the quantum dots. Finally, through the photolithography mask and development process, quantum dot luminescent pixels can be formed. Among them, the quantum dots can be perovskite quantum dots, indium phosphide quantum dots, silver indium gallium sulfur quantum dots, and the preparation of the photoresist solution is shown in the following embodiment.

In the embodiments, an A/B mixed solvent refers to a mixed solvent of A and B. Similarly, an A/B/C mixed solvent refers to a mixed solvent of A, B and C.

The various solvents and cross-linking molecules involved in the embodiments of the present invention are all commercially available, and the various quantum dots used are in powder form and are synthesized according to existing technologies.

Example 1

1) Prepare a toluene solution of green light perovskite quantum dots (CsPbBr ₃ ) by adding 30 mg of CsPbBr ₃ quantum dots into 1 mL of toluene solvent. Ignore the change in solution volume and the concentration is 30 mg/mL.

2) Toluene solutions of PETM and PAE cross-linked molecules were prepared by adding 3500 mg and 4000 mg of the cross-linked molecules into 1 mL of toluene solvent, respectively. The change in solution volume was negligible, and the concentrations were 3500 mg/mL and 4000 mg/mL, respectively.

3) Take 0.2 mL of the above green light perovskite quantum dot toluene solution (30 mg/mL) and mix it with the cross-linking molecule solution (0.9 mLPETM and 0.5 mL PAE), and then disperse it in 6 solvents respectively: 1 mL toluene, 1 mL chlorobenzene, 0.5 mL toluene/0.5 mL chlorobenzene mixed solvent (volume ratio = 1:1), 0.032 mL toluene/0.16 mL chlorobenzene/0.86 mL m-xylene mixed solvent (volume ratio = 5:25:125), 0.032 mL toluene/0.32 mL chlorobenzene/0.64 mL m-xylene mixed solvent (volume ratio = 5:50:100), 0.032 mL toluene/0.64 mL chlorobenzene/0.32 mL m-xylene mixed solvent (volume ratio = 5:100:50), to obtain a quantum dot photoresist solution.

4) Take 0.2 mL of the above quantum dot photoresist solution and spin coat it on a 2 cm*2 cm glass substrate at a speed of 2000 r/min to form a quantum dot film.

Example 2

1) As in step 1 of Example 1), toluene solutions of four different quantum dots of green light perovskite, red light perovskite (CsPb(BrI) ₃ ), green light silver indium gallium sulfide and red light indium phosphide were prepared at a concentration of 30 mg/mL respectively.

2) 0.2 mL of quantum dot solution was spin-coated on glass substrates to form quantum dot films of different systems.

Example 3

1) Dissolve a molecule containing multiple thiol functional groups (PETM) in toluene to obtain a cross-linked ligand solution with a concentration of 3500 mg/mL, which is used as a stock solution A for quantum dot ligand exchange;

2) dissolving a molecule containing multiple acrylate functional groups (PAE) in toluene to obtain a cross-linked ligand solution with a concentration of 4000 mg/mL as a stock solution B for quantum dot ligand exchange;

3) Mix 0.9 mL of stock solution A (3500 mg/mL) and 0.5 mL of stock solution B (4000 mg/mL) to form stock solution AB for quantum dot ligand exchange;

4) 0.2 mL of green light perovskite quantum dot toluene solution (30 mg/mL), 0.2 mL of red light perovskite quantum dot toluene solution (30 mg/mL), 0.2 mL of red light indium phosphide quantum dot toluene solution (30 mg/mL), and 0.2 mL of silver indium gallium sulfur quantum dot toluene solution were added to the stock solution AB in step 3) to form a mixed solution, and then the mixed solution was added to a ternary mixed solvent of 0.032 mL toluene/0.32 mL chlorobenzene/0.64 mL m-xylene (volume ratio = 5:50:100) to obtain four solutions for quantum dot ligand exchange in turn;

5) Under _N2 environment, take 2 mL of the above solution for quantum dot ligand exchange respectively, stir at 1200 r/min for 0 to 40 min, then stop the reaction to obtain four kinds of quantum dot stock solutions after ligand exchange;

6) Add 50 mg of photoinitiator TPO to each of the four ligand-exchanged quantum dot stock solutions.

7) Stirring for another 20 min at 1000 r/min in a _N2 environment to promote the dissolution and dispersion of the initiator to obtain a quantum dot lithography solution that can be photolithographically processed in one step;

8) If a quantum dot photoresist film is to be formed, spin-coat 0.2 mL of the quantum dot photoresist solution on the substrate material and directly expose it to a UV light source of 365 nm for 120 s;

9) If a pixelated quantum dot film is to be formed, spin-coat 0.2 mL of quantum dot photoresist solution on the substrate material, place the mask of the corresponding pattern on it, and then expose it to a UV = 365 nm light source for 120 s. Finally, place the exposed film in a developer and develop it for 30 s to form quantum dot pixels.

Example 4

NMR test characterization scheme:

1) Take 100 μL of the toluene solution of green perovskite quantum dots in step 1) of Example 2 and the cross-linked ligand PETM solution (stock solution A) in step 1) of Example 3, and prepare nuclear magnetic resonance test solutions according to different volume ratios of the toluene solution of green perovskite quantum dots and the cross-linked ligand PETM solution, and the volume ratios are 50:0, 50:1, 50:3, and 50:5 respectively;

2) Take 100 μL of the toluene solution of green perovskite quantum dots in step 1) of Example 2 and the cross-linked ligand PAE solution (stock solution B) in step 2) of Example 3, and prepare nuclear magnetic resonance test solutions according to different volume ratios of the toluene solution of green perovskite quantum dots and the cross-linked ligand PAE solution, and the volume ratios are 0:10, 50:1, and 50:10, respectively.

Example 5

1) Disperse 30 mg of green light silver indium gallium sulfide quantum dots in 1 mL of toluene to obtain a toluene solution of green light silver indium gallium sulfide quantum dots (30 mg/mL).

2) Spin 0.2 mL of green light silver indium gallium sulfide quantum dot toluene solution onto a glass substrate at 2000 r/min to form a quantum dot film.

Example 6

1) Disperse 30 mg of red light-emitting indium phosphide quantum dots in 1 mL of toluene to obtain a toluene solution of red light-emitting indium phosphide quantum dots (30 mg/mL).

2) Spin 0.2 mL of red light indium phosphide quantum dot toluene solution onto a glass substrate at 2000 r/min to form a quantum dot film.

Fig. 2 is a microscopic photograph of the film morphology formed by the spin coating of the quantum dot photoresist solution based on different solvent dispersion in Example 1 under a microscope. It can be found that in a single solvent, such as toluene and chlorobenzene, quantum dots can form a complete film, but there are a lot of particle agglomeration and precipitation in the film. This is because the polarity difference between quantum dots, surface ligands and solvents is large. When toluene and chlorobenzene are mixed as solvents, the phenomenon of quantum dot agglomeration is reduced, but small particle precipitation and film inhomogeneity can still be clearly seen, which means that the ratio of the solvent is regulated, and it is hoped that the uniform dispersion of quantum dots can be achieved. Therefore, the present invention designs a ternary solvent system of toluene, chlorobenzene and m-xylene, which realizes the homogenization of quantum dot photoresist film and the precipitation of very few quantum dots, especially when dispersed in a volume ratio of toluene: chlorobenzene: m-xylene = 5: 50: 100, the film uniformity is optimal, and there is almost no quantum dot agglomeration and precipitation phenomenon. Therefore, it can be considered that toluene: chlorobenzene: m-xylene = 5:50:100 is the optimal solvent ratio for the quantum dot photoresist solution of the present invention.

FIG3 shows the changes in the absorption spectrum (a in FIG3 ) and emission spectrum (b in FIG3 ) of the perovskite quantum dot solution in Example 2 and the perovskite quantum dot solution in Example 3 after ligand exchange with the cross-linked ligand solution. It can be observed from FIG3 that the emission wavelength of the green light perovskite quantum dot solution in Example 2 is 517 nm, and the emission wavelength of the red light perovskite quantum dot solution is 630 nm; the emission wavelength of the overall solution after the two perovskite quantum dots and the cross-linked ligand solution in Example 3 are respectively ligand exchanged is basically consistent with the original emission wavelength of the perovskite quantum dot solution, proving that the introduction of ligand molecules and polyvalent solvents has no significant effect on the luminescence spectrum of quantum dots.

The thiol and acrylate ligand molecules selected by the present invention have strong electronegativity. After mixing with the quantum dot solution, it is easy to exchange the original oleic acid ligands on the surface of the quantum dots. This process can be confirmed by in-situ observation of the change of the fluorescence lifetime of the quantum dots over time, as shown in Figure 4. Figure 4 a shows the quantum dot fluorescence lifetime decay curve and its change trend from 0 to 40 min after the ligand molecule is mixed with the quantum dot solution. It can be found that as the mixing time of the ligand and the quantum dot solution increases, the fluorescence lifetime of the quantum dots is gradually increasing. This is because the ligand molecules are gradually replacing the oleic acid ligands on the surface and passivating the defects on the surface of the quantum dots. When the fluorescence lifetime at each time is summarized and statistically analyzed, as shown in Figure 4 b, the fluorescence lifetime is gradually increasing from 0 to 30 min, which means that the ligand exchange process has been occurring and the degree of ligand exchange is gradually increasing. After 30 min, the fluorescence lifetime is no longer increasing, which means that the exchange is basically completed. Therefore, it can be determined that 30 min is the optimal ligand exchange time.

The changes in the surface ligands in the quantum dot solution and the ligand exchange process therein can also be analyzed by nuclear magnetic hydrogen spectrum. As in the operation of Example 4, we mixed the perovskite quantum dots in Example 2 with PETM in different proportions, and then characterized the prepared samples by nuclear magnetic hydrogen spectrum. As shown in a of FIG5 , the overall change spectrum of the ligand exchange process between ligand molecule A and perovskite quantum dots, and b of FIG5 shows the overall change spectrum of the ligand exchange process between ligand molecule B and perovskite quantum dots. As shown in a of FIG6 , as the proportion of PETM molecules increases, the peak at 5.45 ppm increases, which means that the free oleic acid in the mixed solution of perovskite quantum dots and PETM molecules increases, while the peak intensity at 5.58 ppm gradually weakens, which means that the oleic acid ligands on the surface of the perovskite quantum dots are gradually decreasing. Similarly, when perovskite quantum dots are mixed with different proportions of PAE, the H-NMR spectrum results show that the intensity of the peak at 5.45 ppm increases with the increase of PAE, as shown in Figure 6b, indicating that the free oleic acid in the mixed solution increases while the oleic acid ligands on the surface of the perovskite quantum dots gradually decrease. The above results show that the photoresist molecules can exchange ligands with the oleic acid ligands on the surface of the quantum dots.

To further investigate the interaction between the cross-linked ligand molecules and the surface of perovskite quantum dots, X-ray photoelectron spectroscopy (XPS) was used for analysis. To confirm the presence of PETM as a ligand, we investigated the changes in the surface properties of perovskite quantum dots. As shown in Figure 7a, after the addition of PETM, the Pb 4f peak in the XPS spectrum shifted by ~0.1 eV, and the binding energy decreased. At the same time, Figure 7b showed that the S 2p peak appeared at 162.9 eV. These results indicate that the thiol ligands in the PETM molecules coordinate with the surface lead atoms, thereby passivating the surface defects. In addition, as shown in Figure 7c, the O 1s peak in the cross-linked ligand molecule film shifted by ~0.1 eV, and a new shoulder peak appeared at 532.5 eV, indicating that the oxygen atoms in the acrylate structure also coordinate with the surface lead atoms. The above experimental results fully confirm that there is a strong coordination between the cross-linked ligand molecules and the lead atoms on the surface of the quantum dots, which drives the ligand exchange between the cross-linked ligand molecules and the oleic acid ligands and passivates the surface defects.

The change in the optical properties of the quantum dot photoresist solution after forming a solid film is shown in Figure 8, wherein the quantum dot film is prepared by step 2) of Example 2; the quantum dot ligand cross-linked film is prepared by step 8) of Example 3, and the conditions adopted are all optimal conditions. It can be found that the luminescence peak position and luminescence color purity of the quantum dots are basically the same before and after the cross-linked ligand molecules are mixed, and there is no obvious change. As shown in Figure 8a and Figure 8b: the emission wavelength of the perovskite green light quantum dot is 517 nm, the half-peak width is 21 nm, and the emission wavelength of the red light perovskite quantum dot is 632 nm, and the half-peak width is 35 nm; while the emission wavelengths of the green and red light quantum dot cross-linked films are 516 nm and 629 nm, respectively, and the half-peak widths are 20 nm and 32 nm, respectively. Further TRPL fluorescence lifetime test (c in Figure 8 and d in Figure 7) found that the fluorescence lifetime decay of the film was greatly suppressed, which means that the number of defects in the film was significantly reduced: the fluorescence lifetime of the prepared green light quantum dot cross-linked film was significantly increased from 31.9 ns to 119.8 ns compared with the pure quantum dot film; the fluorescence lifetime of the red light quantum dot cross-linked film was significantly increased from 15.8 ns to 150.0 ns compared with the pure quantum dot film. The improvement of the fluorescence lifetime of the quantum dot cross-linked film is due to the passivation of the surface defects after the cross-linked ligand molecules undergo ligand exchange, and the presence of many stable thiol and acrylate functional groups in the cross-linked network system further fills the remaining surface defects.

The cross-linking process of the quantum dot lithography solution was further confirmed in the infrared spectrum test. The solution tested was the solution obtained in step 7) of Example 3. The spectrum data were collected before and after UV irradiation cross-linking, as shown in a of Figure 9. When the quantum dot lithography solution was not cross-linked, there was an obvious C=C stretching vibration characteristic peak at 1620 cm ^-1 , and an obvious thiol characteristic peak at 2520 cm ^-1 . After UV irradiation, the thiol characteristic peak at 2520 cm ^-1 disappeared, the C=C stretching vibration characteristic peak at 1620 cm ^-1 was also significantly weakened, and the characteristic peak of the CS bond at 734 cm ^-1 was significantly enhanced, which indicated the occurrence of the cross-linking process of the quantum dot lithography solution. At the same time, we further characterized the comparative changes in PLQY between the quantum dot cross-linked film and the original quantum dot film. The films were from step 2) of Example 2 and step 8 of Example 3, as shown in b of Figure 9: the PLQY of the green quantum dot film and the red quantum dot film were 22.3% and 34.8%, respectively, while the PLQY of the green quantum dot cross-linked film and the red quantum dot cross-linked film was significantly increased to 91.4% and 78.5%, and the PLQY was relatively increased by more than 200%. In addition, the uniformity of the film was also confirmed by single-photon fluorescence imaging microscopy, as shown in cf of Figure 9, compared with the quantum dot film, the luminescence intensity and uniformity of the quantum dot cross-linked film were generally significantly improved.

As for the flatness of the film, we observed the quantum dot film and the quantum dot cross-linked film by atomic force microscopy. From a and b in Figure 10, we can clearly observe that the surface of the green and red quantum dots is rough and uneven, with the Rms of the green quantum dot film being 20.5 nm and the Rms of the red quantum dot film being 8.4 nm. However, the Rms of the green quantum dot cross-linked film is 1.3 nm, and the Rms of the red quantum dot cross-linked film surface is 2.3 nm, which indicates that the quantum dot cross-linked film has better flatness and uniformity. This is because the cross-linked molecules fill the gaps between the quantum dots: on the one hand, the quantum dots are embedded in the cross-linked network system through the cross-linking reaction; on the other hand, the compatibility of the ternary solvent with the polarity of the ligands on the surface of the quantum dots also enables the quantum dots to be more evenly dispersed in the photoresist solution, avoiding agglomeration and precipitation. In addition, such a "quantum dot-ligand-polymer" cross-linked three-dimensional network structure is helpful for the wrapping and encapsulation of the quantum dots, which can avoid the contact between the quantum dots and the external water and oxygen environment, and is conducive to the improvement of stability. In order to verify its stability, we immersed the green and red light perovskite quantum dot films, as well as the green and red light perovskite quantum dot cross-linked films in ethanol solvent, as shown in a and b in Figure 11. When the green and red light perovskite quantum dot films were placed in ethanol solvent, the luminescence intensity of the quantum dot films decreased rapidly or even stopped emitting light in less than 1 min. However, the quantum dot cross-linked film prepared by the ligand cross-linking strategy had almost no obvious luminescence intensity attenuation and still had a good luminescence effect.

FIG12 is a mask patterning photolithography for the above-mentioned quantum dot photoresist solution with uniform dispersion to confirm its high-resolution arraying capability. The quantum dot photoresist solution and photolithography process used are based on steps 1-9 of Example 3). In FIG12, a and b are square pixel patterns of green and red perovskite quantum dot crosslinked films, with a pattern size of 1000 μm; flower patterns of green and red perovskite quantum dot crosslinked films, with a pattern size of 100 μm; e-h in FIG10 are high-resolution dot matrix pixel patterns of red perovskite quantum dots, with pattern sizes of 20 μm, 10 μm, 5 μm and 2 μm, respectively. These photolithography pattern effects confirm that the above-mentioned quantum dot photoresist solution can achieve high-resolution pixel patterns, which are expected to meet the needs of ultra-high-definition display applications.

In order to verify the universality of the photoresist solution, as described in Examples 5 and 6, silver indium gallium sulfur quantum dots and indium phosphide quantum dots were introduced into the above-mentioned optimized photoresist solution system. Consistent with perovskite quantum dots, silver indium gallium sulfur quantum dots and indium phosphide quantum dots in the above-mentioned photoresist solution also maintained the same luminescence wavelength and spectral characteristics as the original solution, as shown in a and d in Figure 13, indicating that the introduction of photoresist molecules will not affect the luminescence color purity of quantum dots. TRPL fluorescence lifetime test (b and e in Figure 13) also confirmed that the cross-linked ligand has a significant defect passivation effect on the surface of quantum dots, which increases its fluorescence lifetime from 19.1 ns to 86.8 ns and 30.5 ns to 59.9 ns, respectively. Figures c and f in Figure 13 confirm the occurrence of the cross-linking process through infrared spectroscopy characterization: the characteristic peak of the stretching vibration of C=C of the cross-linked molecules at 1620 cm ^-1 and the characteristic peak of the thiol group at 2520 cm ^-1 are significantly weakened and disappeared after UV irradiation, while the characteristic peak of the CS bond at 734 cm ^-1 is significantly enhanced. These signal changes indicate the occurrence of the cross-linking process of the quantum dot lithography solution.

At the same time, the applicability of this cross-linked ligand solution in the high-resolution patterning process of silver indium gallium sulfide quantum dots and indium phosphide quantum dots has also been verified in some high-resolution patterning lithography. Figure 13 g and h are high-resolution pixelation of silver indium gallium sulfide quantum dot ligand cross-linked films, with pixel sizes of 5 μm and 2 μm respectively; Figure 13 i and j are high-resolution pixelation images of indium phosphide quantum dot ligand cross-linked films, with pixel sizes of 10 μm and 5 μm respectively.

In summary, the present invention provides a quantum dot photoresist with a "cross-linking-passivation" synergistic function and a method for manufacturing a high-resolution pixel thereof. The method uses a ligand exchange method to replace the original dynamically unstable oleic acid ligands on the surface of the quantum dot with molecules with a cross-linking function and multiple thiol groups and multiple acrylate groups (such as thiol-acetic acid-2,2-bis[[(thioacetyl)-oxy]methyl]-1,3-propylene glycol ester and pentaerythritol tetraacrylate), and the above-mentioned thiol and acrylate functional groups can form strong coordination passivation surface defects with the surface of the quantum dot due to their strong electronegativity. Therefore, the present invention forms a quantum dot photoresist solution with a "cross-linking-passivation" synergistic function. Since this type of cross-linked ligand molecules have universal strong coordination and covalent bonding with the lead atoms of perovskite quantum dots and the sulfur atoms of the ZnS shell, the "cross-linking-passivation" synergistic function of the present invention is universal and has been verified in cadmium-free quantum dot systems such as perovskite, indium phosphide, silver indium gallium sulfur, etc. In the red and green luminescence bands, the PLQY of the quantum dot film after photocuring is increased by more than 200% or even up to 520%, and has excellent solvent environment stability. In addition, the present invention also proposes a method for preparing a ternary mixed solvent of toluene, m-xylene, and chlorobenzene for quantum dot-ligand solution, which improves the uniform dispersion of quantum dots in the solution system, realizes the high resolution of the photolithography pattern, and the pixel size can reach 2 μm. In short, the present invention solves the problems of luminescence performance attenuation, poor luminescence stability, and easy agglomeration of quantum dots caused by the detachment of ligands in the photolithography process, and provides a feasible technical solution for the patterning of new cadmium-free quantum dots and their wide color gamut display.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and specification of the present invention.

Claims (10)

1. The preparation method of the quantum dot photoresist is characterized by comprising the following steps of:

1) Dissolving a molecule containing a plurality of thiol functional groups in toluene as a first solution for quantum dot ligand exchange;

2) Dissolving a molecule containing a plurality of acrylate functional groups in toluene as a second solution for quantum dot ligand exchange;

3) Mixing the first solution and the second solution to form a third solution for quantum dot ligand exchange;

4) Adding the toluene solution of the cadmium-free quantum dot into the third solution in the step 3), mixing and then adding into the ternary mixed solvent of toluene/chlorobenzene/metaxylene to obtain a fourth solution for quantum dot ligand exchange;

5) Stirring and reacting the fourth solution for a period of time at the rotating speed of 1200 r/min under the protective atmosphere to obtain quantum dot stock solution after ligand exchange;

6) Adding a photoinitiator into the quantum dot stock solution obtained in the step 5);

7) And under the protection atmosphere, continuously stirring for a period of time under the rotating speed of 1000 r/min to obtain the quantum dot photoresist.

2. The method of claim 1, wherein the molecule comprising a plurality of thiol functional groups is PETM and the concentration of the first solution is 3500 mg/mL.

3. The method of claim 1, wherein the molecule comprising a plurality of acrylate functional groups is PAE and the concentration of the second solution is 4000 mg/mL.

4. The method of claim 1, wherein the cadmium-free quantum dot is one of perovskite or indium phosphide or silver indium gallium sulfide quantum dots, and the toluene solution concentration of the cadmium-free quantum dot is 30 mg/mL.

5. The method of claim 1, wherein the first solution, the second solution, and the toluene solution of cadmium-free quantum dots have a volume ratio of 9:5:2.

6. The method according to claim 1, wherein the volume ratio of the ternary mixed solvent of toluene/chlorobenzene/metaxylene is 5:50:100.

7. The method of claim 1, wherein in step 5), the reaction is stirred at a speed of 1200 r/min for 30: 30 min.

8. The method of claim 1 wherein the photoinitiator is TPO and the mass is 2wt% of the quantum dot stock solution.

9. A quantum dot photoresist prepared according to the method of any one of claims 1-8.

10. A high resolution quantum dot picture element, characterized in that it is manufactured from a quantum dot photoresist prepared by the method according to any one of claims 1-8.