Fluorescent film, luminescent device and preparation method of film

Abstract

The invention relates to the field of photoluminescence technology and LED devices, and particularly provides a fluorescent film, a luminescent device and a preparation method of the film. The fluorescent film contains carbon dots and polyvinylpyrrolidone, the carbon dots and the polyvinylpyrrolidone are uniformly distributed, and the fluorescent film generates a photoluminescence phenomenon under the irradiation of exciting light. The method comprises the following steps: step 1, preparing a carbon dot solution; step 2, preparing a polyvinylpyrrolidone solution; 3, mixing the carbon dot solution and the polyvinylpyrrolidone solution to obtain a mixed solution; and 4, dispensing the mixed solution on the surface of a carrier, and naturally solidifying at room temperature to form the fluorescent film on the surface of the carrier. The device comprises an LED chip and a fluorescent film arranged on the light-emitting surface of the LED chip, the fluorescent film is a mixture of carbon dots and polyvinylpyrrolidone. According to the scheme, carbonyl and nitrogen atoms in PVP interact with the surfaces of the carbon dots, agglomeration of the carbon dots is avoided, and therefore the luminous efficiency is improved.

Description

Technical Field

[0001] The invention relates to the field of photoluminescence technology and LED devices, and in particular to a fluorescent film, a light-emitting device, and a method for preparing the film. Background Art

[0002] In recent years, with the rapid development of high-quality carbon dots (CDs), the research on developing carbon dot composite fluorescent films and carbon dot composite luminescent systems, and constructing carbon dot LED devices based on photoluminescence technology and electroluminescence technology has gained increasing attention due to its spectral tunability, excellent photostability, low cost, high cost performance and environmental friendliness.

[0003] As a new generation of solid-state light sources, light-emitting diodes (LEDs) have many advantages such as small size and low energy consumption, and have been widely used. At present, LED devices usually achieve high-performance luminescence by exciting fluorescent films (or fluorescent powders) on blue LED chips and using the principle of photoluminescence. Fluorescent films (or fluorescent powders) are generally rare earth complexes, semiconductor quantum dots, or perovskite-based materials. Commercial LED devices have been successfully developed. However, these fluorescent film materials are subject to many unfavorable factors in practical applications, such as high prices, heavy metals Cd or Pb, and instability caused by photochemical reactions. While retaining the excellent photoelectric properties of traditional quantum dots, carbon dots have the characteristics of abundant carbon resources, simple synthesis process, environmental friendliness, and stable structure. Therefore, the use of green and environmentally friendly carbon dots to prepare composite fluorescent films and build high-performance LED devices based on photoluminescence technology is highly favored. Due to the constraints of various factors such as carbon dots themselves, composite fluorescent films, and LED chips, the current carbon dot LED devices have the problems of insufficient brightness and low luminous efficiency. Summary of the invention

[0004] The purpose of the present invention is to provide a fluorescent film, a light-emitting device, and a method for preparing the film in view of the deficiencies in the above-mentioned prior art, so as to solve the problems of insufficient brightness and low luminous efficiency of carbon dot LED devices in the prior art.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows: The invention provides a fluorescent film, which contains carbon dots and polyvinyl pyrrolidone, the carbon dots and polyvinyl pyrrolidone are evenly distributed, and the fluorescent film generates photoluminescence phenomenon under the irradiation of excitation light.

[0006] Furthermore, the wavelength of the excitation light is one of 365nm, 395nm, 410nm, and 460nm.

[0007] The present application also provides a method for preparing a fluorescent film, the method comprising the following steps: Step 1, preparing a carbon dot solution; Step 2, preparing a polyvinyl pyrrolidone solution; Step 3, mixing the carbon dot solution and the polyvinyl pyrrolidone solution to obtain a mixed solution; Step 4: Apply the mixed liquid droplets on the surface of the carrier and solidify naturally at room temperature to form a fluorescent film on the surface of the carrier.

[0008] Furthermore, step one includes: adding citric acid and urea to N,N-dimethylformamide, using microwave heating, temperature-controlled reaction carbonization, and obtaining a black solid; adding the black solid to distilled water, performing ultrasonic refinement, and then centrifuging and filtering, and the obtained supernatant is the carbon dot solution.

[0009] Furthermore, the centrifugal speed is 6000 rpm, the centrifugal time is 10 min, the filter pore size is 0.22 μm, and the concentration of the carbon dot solution is 20 mg / mL.

[0010] Furthermore, step one includes: dissolving naphthalenetetracarboxylic acid, terephthalic acid, and o-phenylenediamine in ethanol respectively, then mixing them together, transferring them to a high-pressure reactor, using a one-pot process, and after a hydrothermal reaction; filtering to obtain a carbon dot solution; the concentration of the carbon dot solution is 10 mg / mL.

[0011] Furthermore, in step 2, polyvinyl pyrrolidone is added to distilled water and subjected to ultrasonic treatment to obtain a polyvinyl pyrrolidone solution; the concentration of the polyvinyl pyrrolidone solution is 10 mg / mL.

[0012] Furthermore, in step 3, the mixing ratio of the carbon dot solution and the polyvinyl pyrrolidone solution is a volume ratio of (5-1): (1-5).

[0013] The present invention also provides a photoluminescent device, which comprises an LED chip and the above-mentioned fluorescent film, wherein the fluorescent film is arranged on the light-emitting surface of the LED chip.

[0014] Furthermore, the fluorescent film completely covers the light emitting surface of the LED chip.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) The fluorescent film of the present invention comprises carbon dots and polyvinyl pyrrolidone, and the carbon dots generate fluorescence under the irradiation of excitation light and emit visible light. The carbonyl group and nitrogen atom in polyvinyl pyrrolidone interact with the surface of the carbon dots to avoid the agglomeration of the carbon dots, which will lead to fluorescence quenching and reduce the luminous efficiency. The carbon dots of the present invention are uniformly dispersed in polyvinyl pyrrolidone, and the luminous efficiency is improved by inhibiting fluorescence quenching. Therefore, the brightness and luminous efficiency of the LED device prepared by the fluorescent film are relatively high.

[0016] (2) The mixed solution is dripped onto the light-emitting surface of the LED chip and solidified. The preparation process is simple. Polyvinyl pyrrolidone has a low solidification point and can be cured at room temperature without subsequent heating and UV curing. Conventional heating curing will cause uneven distribution of carbon dots, reduce luminous efficiency, and also reduce the performance of the LED chip. In addition, the wavelength range of the excitation chip is wide, and ultraviolet, purple, and blue light can all be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of a method for preparing a carbon dot-based photoluminescent device provided by the present invention; Figure 2 This is a physical picture of a light emitting device provided by the present invention. Figure 2 (a) is the physical object corresponding to the LED device excited by the 365nm ultraviolet chip in Example 1, Figure 2 (b) is the physical object corresponding to the LED device excited by the 395nm purple light chip in Example 1, Figure 2 (c) is the physical object corresponding to the LED device excited by the 410nm purple light chip in Example 1, Figure 2 (d) is the physical object corresponding to the LED device excited by the 460nm blue light chip in Example 1; Figure 3 This is a physical picture of another light emitting device provided by the present invention. Figure 3 (a) is the physical object corresponding to the LED device excited by the 365nm ultraviolet chip in Example 2, Figure 3 (b) is the physical object corresponding to the LED device excited by the 395nm purple light chip in Example 2, Figure 3 (c) is the physical object corresponding to the LED device excited by the 410nm purple light chip in Example 2, Figure 3 (d) is the actual object corresponding to the LED device excited by the 460nm blue light chip in Example 2.

[0018] Figure 4 The test results of the LED device of Example 1 are shown in FIG. Figure 4 (a) is the current-voltage (IV) curve, Figure 4 (b) is the current-luminance (IL) curve, Figure 4 (c) is the current-power efficiency (I-PE) curve; Figure 5 This is the test result of the LED device of Example 1. Figure 5 (a) is the spectrum of the LED device excited by the 365nm UV chip. Figure 5 (b) is the spectrum of the LED device excited by the 395nm purple light chip. Figure 5 (c) is the spectrum of the LED device excited by the 410nm purple light chip. Figure 5 (d) Spectrum of the LED device excited by 460nm blue light chip; Figure 6 This is the test result of the LED device of Example 1. Figure 6 (a) is the color coordinate of the LED device excited by the 365nm UV chip. Figure 6 (b) is the color coordinate of the LED device excited by the 395nm purple light chip. Figure 6 (c) is the color coordinate of the LED device excited by the 410nm purple light chip. Figure 6 (d) The color coordinates of the LED device excited by the 460nm blue light chip; Figure 7 The light emission spectrum and light distribution curve of the LED device of Example 1 at different viewing angles; Figure 8 The test results of the LED device of Example 2 are shown in FIG. Figure 8 (a) is the current-voltage (IV) curve, Figure 8 (b) is the current-luminance (IL) curve, Figure 8 (c) is the current-power efficiency (I-PE) curve; Fig. 9 This is the test result of the LED device of Example 2. Fig. 9 (a) is the spectrum of the LED device excited by the 365nm UV chip. Fig. 9 (b) is the spectrum of the LED device excited by the 395nm purple light chip. Fig. 9 (c) is the spectrum of the LED device excited by the 410nm purple light chip. Fig. 9 (d) Spectrum of the LED device excited by 460nm blue light chip; Fig.10 This is the test result of the LED device of Example 2. Fig.10 (a) is the color coordinate of the LED device excited by the 365nm UV chip. Fig.10 (b) is the color coordinate of the LED device excited by the 395nm purple light chip. Fig.10 (c) is the color coordinate of the LED device excited by the 410nm purple light chip. Fig.10 (d) The color coordinates of the LED device excited by the 460nm blue light chip; Fig.11 1 and 2 are the luminous spectra and light distribution curves of the LED device of Example 2 at different viewing angles. DETAILED DESCRIPTION

[0019] In order to make the implementation process of the present invention clearer, it will be described in detail below with reference to the accompanying drawings.

[0020] The present application provides a fluorescent film, which contains carbon dots and polyvinyl pyrrolidone (PVP), the carbon dots and polyvinyl pyrrolidone are evenly distributed, and the fluorescent film generates photoluminescence under the irradiation of excitation light. The thickness of the fluorescent film is 1-20 mm; it can reduce the transmission loss while ensuring light absorption. Specifically, the film can absorb more incident light, thereby exciting more carbon dots. The thin film thickness will cause the excitation light to directly penetrate the film without being absorbed by the fluorescent material, resulting in a decrease in the photoluminescence efficiency; the light can be scattered multiple times in the fluorescent film, and after multiple reflections and scattering, the energy of the light is uniformly excited to the fluorescent center, thereby achieving uniform photoluminescence. Therefore, it is beneficial to improve the overall luminescence efficiency of the film. The excitation light can be light generated by an LED chip, or it can be light generated by other light sources; in the present invention, the emission wavelength of the ultraviolet LED chip is 365 nm, the emission wavelength of the purple LED chip is 395 nm or 410 nm, and the emission wavelength of the blue LED chip is 460 nm; specifically, one of the above-mentioned ultraviolet, purple, and blue LED chips can be used. Among them, there are various oxygen-containing groups (such as hydroxyl, carboxyl, etc.) on the surface of carbon dots, and their energy level structure is regulated by chemical bonds or electron acceptor-donor interactions; when the excitation light is irradiated on the carbon dots, the electrons in the carbon dots are excited to a higher energy level, and emit photons when returning to the ground state, resulting in photoluminescence, radiating visible light outward. PVP enhances the luminescence intensity by providing a stable environment, improving the dispersibility and luminescence properties of carbon dots. Specifically, PVP interacts with the surface of carbon dots through its carbonyl and nitrogen atoms to form a stable composite structure, effectively preventing carbon dots from agglomerating, ensuring uniform dispersion of carbon dots, and inhibiting fluorescence quenching, thereby improving the optical properties and luminescence efficiency of the film; PVP also plays a curing role when preparing devices.

[0021] The present application also provides a photoluminescent device, which includes an LED chip and a fluorescent film disposed on the light-emitting surface of the LED chip; the fluorescent film is a mixture of carbon dots and polyvinyl pyrrolidone. The fluorescent film completely covers the light-emitting surface of the LED chip. The wavelength of the light emitted by the LED chip is one of 365nm, 395nm, 410nm, and 460nm. The light emitted by the LED chip is irradiated on the fluorescent film, and the fluorescent film produces a photoluminescent effect and outputs visible light to the external space.

[0022] The present application also provides a method for preparing the fluorescent film. Figure 1 As shown, the method comprises the following steps: Step 1: prepare carbon dot solution.

[0023] In this embodiment, the carbon dot solution is prepared by microwave method. Specifically, 1 mol of citric acid and 3 mol of urea are added to N, N-dimethylformamide, microwave heating is used, and temperature-controlled reaction carbonization is performed to obtain black solid carbon dots, which have low toxicity, good water solubility, and high fluorescence quantum yield; the black solid is added to distilled water, and since the carbon dots have good water solubility, they can be well dissolved in distilled water, and further ultrasonic refinement is performed to ensure that the carbon dots are evenly dispersed, and the ultrasonic time is preferably 6 hours; then centrifugation and filtration are performed, the centrifugal speed is 6000rpm, and the centrifugal time is 10 minutes to remove larger or coagulated agglomerated carbon particles; after centrifugation, the supernatant is taken and further filtered through a pore size of 0.22μm to obtain a carbon dot solution prepared by microwave method, with a concentration of 20mg / mL.

[0024] Step 2: preparing polyvinyl pyrrolidone solution.

[0025] Polyvinyl pyrrolidone is added to distilled water and subjected to ultrasonic treatment. The ultrasonic treatment time is preferably 10 minutes to fully dissolve the polyvinyl pyrrolidone to obtain a polyvinyl pyrrolidone solution. The concentration of the polyvinyl pyrrolidone solution is 10 mg / mL.

[0026] Step three, mixing the carbon dot solution and the polyvinyl pyrrolidone solution to obtain a mixed solution; the mixing ratio of the carbon dot solution and the polyvinyl pyrrolidone solution is a volume ratio of (5~1):(1~5); preferably, the ratio is 1:1; in this way, PVP can play a better curing role without causing carbon dot agglomeration; if the concentration of PVP is too high, the luminous efficiency of the carbon dots will be reduced, and if the concentration of PVP is too low, the curing effect will be poor.

[0027] Step 4: Apply the mixed liquid droplets on the surface of the carrier and solidify naturally at room temperature in the atmosphere to form a fluorescent film on the surface of the carrier.

[0028] The preparation method of the above device is as follows: Steps 1 to 3 are the same as the preparation method of the above-mentioned film. The difference is that in step 4, the carrier is an LED chip, and the mixed liquid droplets are applied to the light-emitting surface of the LED chip; the formed fluorescent film covers the light-emitting surface of the LED chip. That is, the mixed liquid droplets are applied to the light-emitting surface of the LED chip, and naturally solidified at room temperature in the atmosphere to obtain an LED device. The PVP in the present invention has a low solidification point, and PVP has a curing effect, which can play a good fixing role, so that the fluorescent film is fixed on the LED chip; it also makes the carbon points in the film evenly distributed, not easy to form agglomeration, and avoid fluorescence quenching. The traditional method requires heating and ultraviolet curing, and heating will affect the performance of the LED chip, for example, shortening the life, increasing thermal stress, aging of packaging materials, etc. At the same time, the uneven distribution of the thermal field will also cause the carbon points to move relative to each other in the film, and the concentration of carbon points on the side with higher temperature (close to the heating surface) will increase, and the concentration of carbon points on the side with lower temperature (away from the heating surface) will decrease. The uneven distribution of carbon points reduces the fluorescence efficiency and reduces the luminous efficiency. A UV LED chip with a rated voltage of 3.0V can be used. The fluorescent film completely covers the light-emitting surface of the ultraviolet LED chip. The ultraviolet light emitted by the ultraviolet LED chip directly irradiates the fluorescent film. The fluorescent film is excited by the ultraviolet light to generate photoluminescence and radiate a light field outward.

[0029] The difference from Example 1 is that the preparation method of the carbon dot solution is different. The preparation method of the carbon dot solution in step 1 is prepared by hydrothermal method. Specifically, ethanol is used as a solvent to dissolve raw materials such as naphthalenetetracarboxylic acid, terephthalic acid, and o-phenylenediamine, and a one-pot method is used to carry out a hydrothermal reaction in a high-pressure reactor. The temperature of the hydrothermal reaction is 180°C, and the reaction time is 20h. At this preferred temperature, the hydrothermal reaction produced by naphthalenetetracarboxylic acid, terephthalic acid, and o-phenylenediamine is relatively sufficient, and a sufficient hydrothermal reaction process is the key to the successful preparation of carbon dots. Then, after filtering with a pore size of 0.22 μm, a carbon dot solution prepared by a hydrothermal method is obtained, and the concentration is 10 mg / mL. In this embodiment, the mixing ratio of the carbon dot solution and the polyvinyl pyrrolidone solution is preferably 1:3, and the concentration of the carbon dots is moderate. If the concentration of the carbon dots is too high, it will cause fluorescence quenching and reduce the luminous efficiency. If the concentration of the carbon dots is too low, the light intensity generated by the photoluminescence is weak, thereby obtaining a fluorescent film with high luminous efficiency.

[0030] Results and Analysis: The physical pictures of the carbon dot LED light emitting devices prepared in Example 1 and Example 2 are as follows: Figure 2 and Figure 3 As shown, Figure 2 (a) is the physical object corresponding to the LED device excited by the 365nm ultraviolet chip in Example 1, Figure 2 (b) is the physical object corresponding to the LED device excited by the 395nm purple light chip in Example 1, Figure 2(c) is the physical object corresponding to the LED device excited by the 410nm purple light chip in Example 1, Figure 2 (d) is the physical object corresponding to the LED device excited by the 460nm blue light chip in Example 1; Figure 3 (a) is the physical object corresponding to the LED device excited by the 365nm ultraviolet chip in Example 2, Figure 3 (b) is the physical object corresponding to the LED device excited by the 395nm purple light chip in Example 2, Figure 3 (c) is the physical object corresponding to the LED device excited by the 410nm purple light chip in Example 2, Figure 3 (d) is the physical object corresponding to the LED device excited by the 460nm blue light chip in Example 2. It can be seen that the carbon dots prepared by microwave method or hydrothermal method and the LED devices constructed by them all show excellent luminescence performance under the excitation of different wavelengths such as 365nm, 395nm, 410nm, 460nm, etc.

[0031] Apply voltage to the LED device to test the light-emitting characteristics. Figure 4 The test results of the carbon dots prepared by microwave method and the LED devices constructed in Example 1 are shown in FIG. Figure 4 (a) is the current-voltage (IV) curve, Figure 4 (b) is the current-luminance (IL) curve, Figure 4 (c) is the current-lumen efficiency (I-PE) curve. The brightness of the LED device excited by the 365nm UV chip reached 282800cd / m 2 The power efficiency is 11.8lm / W; the brightness of the LED device excited by the 395nm purple chip reaches 298100cd / m 2 The power efficiency is 13.6lm / W; the brightness of the LED device excited by the 410nm purple chip reaches 221544cd / m 2 The power efficiency is 13.2lm / W; the brightness of the LED device excited by the 460nm blue light chip reaches 304331cd / m 2 , the power efficiency is 14.8lm / W. It can be seen that under the excitation of different wavelengths such as 365nm, 395nm, 410nm, 460nm, the carbon dots synthesized by microwave method and the LED devices constructed by them have high brightness and power efficiency. The carbon dots prepared by microwave method have low impurity content. Impurities will introduce additional energy levels inside or on the surface of carbon dots, become non-radiative recombination centers, capture excited electrons or holes, and cause them to return to the ground state by non-radiative recombination instead of releasing energy in the form of photons, thus resulting in a decrease in photoluminescence efficiency.

[0032] Figure 5 The test results of the LED device of Example 1; Figure 5 (a) is the spectrum of the LED device excited by the 365nm UV chip. Figure 5 (b) is the spectrum of the LED device excited by the 395nm purple light chip. Figure 5 (c) is the spectrum of the LED device excited by the 410nm purple light chip. Figure 5 (d) is the spectrum of the LED device excited by the 460nm blue light chip. It can be seen that the emission peak of the LED device excited by the 365nm ultraviolet chip is 539nm; the emission peak of the LED device excited by the 395nm purple light chip is 575nm; the emission peak of the LED device excited by the 410nm purple light chip is 544nm; the emission peak of the LED device excited by the 460nm blue light chip is 570nm. The overall performance is yellow-green or yellow light.

[0033] Figure 6 The test results of the LED device of Example 1; Figure 6 (a) is the color coordinate of the LED device excited by the 365nm UV chip. Figure 6 (b) is the color coordinate of the LED device excited by the 395nm purple light chip. Figure 6 (c) is the color coordinate of the LED device excited by the 410nm purple light chip. Figure 6 (d) is the color coordinates of the LED device excited by the 460nm blue light chip. Among them, the LED device excited by the 365nm ultraviolet chip shows yellow-green light emission, and the CIE color coordinates are (0.3926, 0.5397); the LED device excited by the 395nm purple light chip shows yellow light emission, and the CIE color coordinates are (0.4689, 0.5092); the LED device excited by the 410nm purple light chip shows yellow-green light emission, and the CIE color coordinates are (0.3847, 0.5037); the LED device excited by the 460nm blue light chip shows yellow light emission, and the CIE color coordinates are (0.3731, 0.4008).

[0034] Figure 7 The light spectrum and light distribution curve of the LED device at different viewing angles of Example 1. As the viewing angle increases, the light intensity decreases.

[0035] Figure 8 The test results of the carbon dot LED device of Example 2 are shown in FIG. Figure 8 (a) is the current-voltage (IV) curve, Figure 8 (b) is the current-luminance (IL) curve, Figure 8 (c) is the current-lumen efficiency (I-PE) curve. The brightness of the LED device excited by 365nm reached 512347cd / m 2, power efficiency is 25.2lm / W; the brightness of the LED device excited by 395nm reaches 670954cd / m 2 , power efficiency is 22.0lm / W; the brightness of the LED device excited by 410nm reaches 354084cd / m 2 , power efficiency is 20.4lm / W; the brightness of the LED device excited by 460nm reaches 424621cd / m 2 , the power efficiency is 24.4lm / W. It can be seen that under the excitation of different wavelengths such as 365nm, 395nm, 410nm, and 460nm, the carbon dots synthesized based on the hydrothermal method and the LED devices constructed therefrom have high brightness and power efficiency. This is because, under hydrothermal conditions, the size of the carbon quantum dots is small and uniform, which helps to enhance the quantum confinement effect and improve the luminescence efficiency. In addition, more oxygen-containing functional groups (such as carboxyl, hydroxyl, carbonyl, etc.) are generated on the surface of the carbon dots. These functional groups help to enhance the surface state luminescence of the carbon quantum dots, thereby improving the luminescence efficiency. The carbon dots prepared by the hydrothermal method are water-soluble carbon dots, and the carbon dot solution and the PVP solution can be fully and evenly mixed, so that the distribution of the carbon dots is more uniform, avoiding local carbon dot agglomeration. The agglomeration of the carbon dots will lead to fluorescence quenching and reduce the luminescence efficiency. Therefore, the carbon dots prepared by the hydrothermal method avoid carbon dot agglomeration and improve the luminescence efficiency.

[0036] Fig. 9 The test results of the LED device of Example 2; Fig. 9 (a) is the spectrum of the LED device excited by the 365nm UV chip. Fig. 9 (b) is the spectrum of the LED device excited by the 395nm purple light chip. Fig. 9 (c) is the spectrum of the LED device excited by the 410nm purple light chip. Fig. 9 (d) is the spectrum of the LED device excited by the 460nm blue light chip. Among them, the luminescence peak of the LED device excited by the 365nm ultraviolet chip is 575nm; the luminescence peak of the LED device excited by the 395nm purple light chip is 575nm; the luminescence peak of the LED device excited by the 410nm purple light chip is 576nm; the luminescence peak of the LED device excited by the 460nm blue light chip is 578nm. Overall, they all show yellow luminescence.

[0037] Fig.10 The test results of the LED device of Example 2; Fig.10 (a) is the color coordinate of the LED device excited by the 365nm UV chip. Fig.10 (b) is the color coordinate of the LED device excited by the 395nm purple light chip. Fig.10 (c) is the color coordinate of the LED device excited by the 410nm purple light chip. Fig.10(d) is the color coordinate of the LED device excited by the 460nm blue light chip. Among them, the LED device excited by the 365nm ultraviolet chip shows yellow light emission, and the CIE color coordinates are (0.5296, 0.4668); the LED device excited by the 395nm purple light chip shows yellow light emission, and the CIE color coordinates are (0.5316, 0.4664); the LED device excited by the 410nm purple light chip shows yellow light emission, and the CIE color coordinates are (0.5403, 0.4575); the LED device excited by the 460nm blue light chip shows yellow light emission, and the CIE color coordinates are (0.5559, 0.4425).

[0038] Fig.11 The light spectrum and light distribution curve of the LED device at different viewing angles of Example 2. As the viewing angle increases, the light intensity decreases.

[0039] Table 1 and Table 2 summarize the main performances of the LED devices of Example 1 and Example 2, respectively.

[0040] Table 1: Main performance parameters of the LED device of Example 1 Table 2: Main performance parameters of the LED device of Example 2 Table 3 and Table 4 respectively show the luminous intensity and color coordinate changes of the carbon dot LED devices of Example 1 and Example 2 at different viewing angles.

[0041] Table 3: Changes in luminous intensity and color coordinates of the LED device of Example 1 at different viewing angles (0° is directly in front of the LED device) Table 4: Changes in luminous intensity and color coordinates of the LED device of Example 2 at different viewing angles (0° is directly in front of the LED device) The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A fluorescent film, characterized in that: The fluorescent film contains carbon dots and polyvinyl pyrrolidone, the carbon dots and the polyvinyl pyrrolidone are evenly distributed, and the fluorescent film generates photoluminescence under the irradiation of excitation light.

2. The fluorescent film according to claim 1, characterized in that: The excitation light wavelength is one of 365nm, 395nm, 410nm, and 460nm.

3. A method for preparing a fluorescent film, characterized in that: The method comprises the following steps: Step 1, preparing a carbon dot solution; Step 2, preparing a polyvinyl pyrrolidone solution; Step 3, mixing the carbon dot solution and the polyvinyl pyrrolidone solution to obtain a mixed solution; Step 4: applying the mixed liquid droplets on the surface of the carrier and allowing it to solidify naturally at room temperature, thereby forming a fluorescent film on the surface of the carrier.

4. The method for preparing the fluorescent film according to claim 3, characterized in that: The step 1 comprises: adding citric acid and urea to N,N-dimethylformamide, heating with microwaves, and carbonizing the reaction under temperature control to obtain a black solid; adding the black solid to distilled water, performing ultrasonic refinement, and then centrifuging and filtering to obtain a supernatant, which is the carbon dot solution.

5. The method for preparing the fluorescent film according to claim 4, characterized in that: The centrifugal speed is 6000 rpm, the centrifugal time is 10 min; the filter pore size is 0.22 μm; and the concentration of the carbon dot solution is 20 mg / mL.

6. The method for preparing the fluorescent film according to claim 3, characterized in that: The step 1 comprises: dissolving naphthalenetetracarboxylic acid, terephthalic acid and o-phenylenediamine in ethanol respectively, then mixing them together, transferring them to a high-pressure reactor, adopting a one-pot process, and after hydrothermal reaction; filtering to obtain the carbon dot solution; the concentration of the carbon dot solution is 10 mg / mL.

7. The method for preparing the fluorescent film according to claim 3, characterized in that: In the step 2, polyvinyl pyrrolidone is added to distilled water and subjected to ultrasonic treatment to obtain the polyvinyl pyrrolidone solution; the concentration of the polyvinyl pyrrolidone solution is 10 mg / mL.

8. The method for preparing the fluorescent film according to claim 3, characterized in that: In the step 3, the mixing ratio of the carbon dot solution and the polyvinyl pyrrolidone solution is (5-1):(1-5) by volume.

9. A photoluminescent device, characterized in that: The device comprises an LED chip and the fluorescent film according to any one of claims 1 to 2, wherein the fluorescent film is placed on the light-emitting surface of the LED chip.

10. The photoluminescent device according to claim 9, characterized in that: The fluorescent film completely covers the light-emitting surface of the LED chip.

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