A cobalt ion-doped rare earth fluoride upconversion luminescent material and its preparation method and application

Abstract

The present invention relates to a cobalt ion-doped rare earth fluoride up-conversion luminescent material, and its preparation method and application. The method comprises the following steps: adding an yttrium source, a ytterbium source, an erbium source, a thulium source, and a cobalt source to an aqueous solution containing trisodium citrate in sequence and stirring the mixture to obtain a mixed solution A; adding sodium fluoride and urea to the mixed solution A and stirring the mixture to obtain a mixed solution B; subjecting the mixed solution B to a high-temperature reaction in a high-pressure reactor to obtain a reaction solution; and centrifugally separating the reaction product from the reaction solution and drying the reaction product to obtain a cobalt ion-doped rare earth fluoride up-conversion luminescent material. The cobalt ion-doped rare earth fluoride up-conversion luminescent material of the present invention can more effectively convert low-energy, long-wavelength near-infrared light into high-energy, short-wavelength visible light emission, thereby effectively broadening the utilization range of the solar spectrum and further improving the photoelectric conversion efficiency of photovoltaic cells.

Description

Technical Field

[0001] The present invention belongs to the technical field of photovoltaic up-conversion luminescence enhancement coating materials, and in particular relates to a cobalt ion-doped rare earth fluoride up-conversion luminescent material, a preparation method and an application thereof. Background Art

[0002] In recent years, with growing awareness of environmental protection and increasing energy demand, the solar photovoltaic industry has garnered widespread attention and development worldwide. High efficiency, low cost, high stability, and reliability are the constant goals of solar photovoltaic technology development. While photovoltaic cells primarily absorb visible light, infrared light, which accounts for approximately 43% of the solar spectrum, remains underutilized. This infrared light can be converted into visible light through upconversion luminescent materials, enabling photovoltaic cells to effectively utilize sunlight and further improving their photoelectric conversion efficiency.

[0003] Upconversion luminescence, also known as anti-Stokes luminescence, occurs when a material absorbs multiple low-energy, long-wavelength photons and emits high-energy, short-wavelength photons. Rare earth fluoride upconversion luminescent materials, due to their rich electronic energy levels and unique 4f-4f electronic transitions, can emit light from the ultraviolet to the infrared wavelength range, resulting in a narrow emission spectrum and enabling upconversion luminescence. However, rare earth fluoride upconversion luminescence suffers from bottlenecks such as a small absorption cross-section, a high excitation threshold, saturation effects, and thermal effects, resulting in low fluorescence intensity and susceptibility to quenching.

[0004] Ion doping (impurity doping) is considered to be one of the important strategies to improve the efficiency of upconversion luminescence intensity of rare earth fluorides. The doped ions reported in the prior art are usually Li + , Ca 2+ 、Mn 2+ 、Fe 3+ or Zr 4+ However, the ability of prior art to enhance the intensity and efficiency of rare earth fluoride upconversion luminescence by ion doping needs to be improved.

[0005] In summary, it is very necessary to provide a cobalt ion-doped rare earth fluoride upconversion luminescent material and its preparation method and application. Summary of the Invention

[0006] To address one or more technical problems existing in the prior art, the present invention provides a cobalt-ion-doped rare earth fluoride upconversion luminescent material, its preparation method, and its application. The cobalt-ion-doped rare earth fluoride upconversion luminescent material of the present invention can more efficiently convert low-energy, long-wavelength near-infrared light into high-energy, short-wavelength visible light, thereby effectively broadening the utilization range of the solar spectrum and further improving the photoelectric conversion efficiency of photovoltaic cells.

[0007] In a first aspect, the present invention provides a method for preparing a cobalt ion-doped rare earth fluoride upconversion luminescent material, the method comprising the following steps:

[0008] (1) adding an yttrium source, a ytterbium source, an erbium source, a thulium source, and a cobalt source sequentially into an aqueous solution containing trisodium citrate and stirring uniformly to obtain a mixed solution A;

[0009] (2) adding sodium fluoride and urea to the mixed solution A and stirring uniformly to obtain a mixed solution B;

[0010] (3) subjecting the mixed solution B to a high-temperature reaction in a high-pressure reactor to obtain a reaction solution;

[0011] (4) separating the reaction product from the reaction solution by centrifugation and drying the product to obtain a cobalt ion-doped rare earth fluoride upconversion luminescent material.

[0012] Preferably, the molar ratio of the ytterbium source, the erbium source, the thulium source and the cobalt source is 18:2:2:(1-25), preferably 18:2:2:(1-10), more preferably 18:2:2:(4-6), and further preferably 18:2:2:5; and / or the molar ratio of the yttrium source to the cobalt source is (80-x):x, 1≤x≤25.

[0013] Preferably, the concentration of the aqueous solution containing trisodium citrate is 0.02 to 0.04 mmol / mL; the molar ratio of trisodium citrate contained in the aqueous solution containing trisodium citrate to the erbium source is (20 to 30):1, preferably 25:1; the molar ratio of sodium fluoride in step (2) to the trisodium citrate contained in the aqueous solution containing trisodium citrate in step (1) is (6 to 10):1, preferably 8:1; and / or the molar ratio of urea to sodium fluoride is (4 to 6):1, preferably 5:1.

[0014] Preferably, the yttrium source is one or more of yttrium nitrate, yttrium chloride, yttrium sulfate and hydrated compounds thereof; the ytterbium source is one or more of ytterbium nitrate, ytterbium chloride, ytterbium sulfate and hydrated compounds thereof; the erbium source is one or more of erbium nitrate, erbium chloride, erbium sulfate and hydrated compounds thereof; the thulium source is one or more of thulium nitrate, thulium chloride, thulium sulfate and hydrated compounds thereof; and / or the cobalt source is one or more of cobalt nitrate, cobalt chloride, cobalt sulfate and hydrated compounds thereof.

[0015] Preferably, in step (1), a nickel source is further added to the aqueous solution containing trisodium citrate, and the nickel source is one or more of nickel nitrate, nickel chloride, nickel sulfate and hydrated compounds thereof.

[0016] Preferably, the molar ratio of the ytterbium source, the erbium source, the thulium source, the cobalt source and the nickel source is 18:2:2:(1~25):(1~15), preferably 18:2:2:(1~10):(5~15), more preferably 18:2:2:(4~6):(5~15), and further preferably 18:2:2:5:10; and / or the molar ratio of the yttrium source, the cobalt source and the nickel source is (80-xy):x:y, 1≤x≤25, 1≤y≤15.

[0017] Preferably, in step (1) and / or step (2), the stirring speed is 200-1000 r / min, preferably 500-800 r / min, and the stirring time is 30-200 min, preferably 60-100 min; the temperature of the high-temperature reaction is 100-220°C, and the high-temperature reaction time is 10-24 h; the pressure for the high-temperature reaction is 1.4-1.6 MPa; the centrifugal speed is 8000-10000 r / min, the centrifugal time is 3-15 min, and the number of centrifugations is 2-5 times; and / or the drying temperature is 50-100°C, and the drying time is 6-12 h.

[0018] Preferably, the cobalt ion-doped rare earth fluoride up-conversion luminescent material belongs to the hexagonal crystal system, and the crystals are in the shape of hexagonal columns.

[0019] In a second aspect, the present invention provides a cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared by the preparation method described in the first aspect of the present invention.

[0020] In a third aspect, the present invention provides an application of a cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared by the preparation method described in the first aspect of the present invention in photovoltaic efficiency enhancement. The application is: coating the cobalt ion-doped rare earth fluoride up-conversion luminescent material on the surface of a photovoltaic cell to form a coating, and the coating can improve the photoelectric conversion efficiency of the photovoltaic cell under sunlight; preferably, the thickness of the coating is not greater than 100 nm.

[0021] Compared with the prior art, the present invention has at least the following beneficial effects:

[0022] (1) The cobalt ion-doped rare earth fluoride upconversion luminescent material described in the present invention is synthesized by a hydrothermal method and has a regular hexagonal prism morphology. The present invention finds that the rare earth fluoride material with a regular hexagonal prism morphology can improve the light absorption efficiency compared with the hexagonal flake morphology. This may be because this morphology helps to increase the incident surface area of light, thereby enhancing light absorption. The regular hexagonal prism morphology helps to optimize the optical properties of the material and reduce the scattering and absorption loss of photons. The present invention optimizes the morphology and structure of the rare earth fluoride material to make it better match the solar spectrum, improve the light utilization efficiency, and thus increase the conversion efficiency of the photovoltaic cell.

[0023] (2) The present invention has found that the variable valence state of the transition metal cobalt and its 3d orbital have a great influence on the optical properties of rare earth elements. Not only can defect energy levels be introduced into rare earth fluoride up-conversion luminescent materials to enhance the luminescence efficiency of rare earth activators, but the morphology and crystallinity can also be adjusted, and the crystal field symmetry of the activators erbium and thulium in the rare earth fluorides can be reduced to enhance the up-conversion luminescence intensity and prolong the up-conversion luminescence lifetime, thereby improving the utilization efficiency of near-infrared light in the solar spectrum, making the near-infrared light in the solar incident spectrum more effectively converted into visible light, so as to further enhance the photoelectric conversion efficiency of photovoltaic cells. The present invention has obtained a cobalt ion-doped rare earth fluoride up-conversion luminescent material for the first time; the cobalt ion-doped rare earth fluoride up-conversion luminescent material of the present invention can be converted into visible light in the wavelength range of 500 nanometers to 700 nanometers under the excitation of 980 nanometer light, which has a good effect on improving the photoelectric conversion efficiency of photovoltaic cells and can make full use of sunlight, which provides a new feasibility for further improving the efficiency of photovoltaic cells and reducing costs.

[0024] (3) In some preferred embodiments of the present invention, while doping the rare earth fluoride upconversion luminescent material with a suitable content of cobalt ions, it also dopes with a suitable content of nickel ions. The present invention finds that the simultaneous presence of cobalt ions and nickel ions can make it more absorptive to light in a wider wavelength range in the solar spectrum, which helps the photovoltaic cell absorb more photons and improve the light absorption efficiency. In addition, the simultaneous doping of cobalt ions and nickel ions can better regulate the band structure of the rare earth fluoride, which helps to further improve the separation efficiency of photogenerated carriers. By better adjusting the band structure, the recombination rate of photogenerated carriers can be reduced, thereby further improving the photoelectric conversion efficiency. In addition, the simultaneous doping of cobalt ions and nickel ions can better adjust the peak value and intensity of photoluminescence, so that it better matches the operating wavelength of the photovoltaic cell, which is also conducive to further improving the photoelectric conversion efficiency. In addition, the present invention finds that the content of doped cobalt ions and nickel ions needs to be precisely controlled. Too high or too low doping concentration is not conducive to the effective improvement of the photoelectric conversion efficiency of the photovoltaic cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The XRD patterns of rare earth fluoride up-conversion luminescent materials doped with cobalt ions in different proportions prepared in Examples 1 to 4 of the present invention;

[0026] Figure 2 This is a SEM photograph of the 0% cobalt ion-doped rare earth fluoride upconversion luminescent material prepared in Example 1 of the present invention;

[0027] Figure 3 This is a SEM photograph of the 5% cobalt ion-doped rare earth fluoride upconversion luminescent material prepared in Example 2 of the present invention;

[0028] Figure 4 This is a SEM photograph of the 15% cobalt ion-doped rare earth fluoride upconversion luminescent material prepared in Example 3 of the present invention;

[0029] Figure 5 This is a SEM photograph of the 25% cobalt ion-doped rare earth fluoride upconversion luminescent material prepared in Example 4 of the present invention;

[0030] Figure 6 This is the up-conversion luminescence spectrum of the 5% cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared in Example 2 of the present invention;

[0031] Figure 7 This is a logarithmic curve of laser power and luminous intensity of the 0% cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared in Example 1 of the present invention;

[0032] Figure 8This is a logarithmic curve of laser power and luminous intensity of the 5% cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared in Example 2 of the present invention;

[0033] Figure 9 These are fluorescence lifetime spectra of the 0% cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared in Example 1 of the present invention and the 5% cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared in Example 2. DETAILED DESCRIPTION

[0034] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the embodiments described are part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0035] In a first aspect, the present invention provides a method for preparing a cobalt ion-doped rare earth fluoride upconversion luminescent material, the method comprising the following steps:

[0036] (1) Adding a yttrium source, a ytterbium source, an erbium source, a thulium source and a cobalt source to an aqueous solution containing trisodium citrate in sequence and stirring uniformly to obtain a mixed solution A; in the present invention, preferably, the yttrium source is a trivalent yttrium salt or a hydrate thereof, more preferably, the yttrium source is a mixture of one or more of yttrium nitrate, yttrium chloride, yttrium sulfate and hydrated compounds thereof, further preferably, the yttrium source is yttrium nitrate hydrate and / or yttrium chloride hydrate; preferably, the ytterbium source is a trivalent ytterbium salt or hydrates thereof, more preferably, the ytterbium source is a mixture of one or more of ytterbium nitrate, ytterbium chloride, ytterbium sulfate and hydrates thereof, further preferably, the ytterbium source is ytterbium nitrate and / or ytterbium chloride; preferably, the erbium source is a trivalent erbium salt or a hydrate thereof, more preferably, the erbium source is a mixture of one or more of erbium nitrate, erbium chloride, erbium sulfate and hydrates thereof, further preferably, the erbium source is erbium nitrate hydrate and / or erbium chloride hydrate; preferably Preferably, the thulium source is a trivalent thulium salt or a hydrate thereof, more preferably, the thulium source is a mixture of one or more of thulium nitrate, thulium chloride, thulium sulfate and hydrates thereof, further preferably, the thulium source is thulium nitrate hydrate and / or thulium chloride hydrate. In the present invention, the thulium nitrate hydrate has a chemical formula of Tm(NO3)3·xH2O and a CAS number of 100641-15-4, which can be purchased from the Titan Technology Exploration Platform; preferably, the The cobalt source is selected from a divalent cobalt salt or a hydrate thereof. More preferably, the cobalt source is a mixture of one or more of cobalt nitrate, cobalt chloride, cobalt sulfate and hydrates thereof. Further preferably, the cobalt source is cobalt nitrate hydrate and / or cobalt chloride hydrate. In step (1), the stirring speed is, for example, 200 to 1000 rpm, preferably 500 to 800 rpm, and the stirring time is, for example, 30 to 200 minutes, preferably 60 to 100 minutes.

[0037] (2) adding sodium fluoride and urea to the mixed solution A and stirring uniformly to obtain a mixed solution B; in step (2), the stirring speed is, for example, 200 to 1000 rpm, preferably 500 to 800 rpm, and the stirring time is, for example, 30 to 200 minutes, preferably 60 to 100 minutes;

[0038] (3) subjecting the mixed solution B to a high-temperature reaction in a high-pressure reactor to obtain a reaction solution; in the present invention, the temperature of the high-temperature reaction is, for example, 100 to 220° C., the time of the high-temperature reaction is, for example, 10 to 24 hours; and the pressure of the high-temperature reaction is, for example, 1.4 to 1.6 MPa;

[0039] (4) separating the reaction product from the reaction solution by centrifugation and drying the reaction product to obtain a cobalt ion-doped rare earth fluoride upconversion luminescent material; in the present invention, the centrifugal separation speed is, for example, 8000 to 10000 rpm, the centrifugal separation time is, for example, 3 to 15 minutes, and the number of centrifugal separations is, for example, 2 to 5 times; the drying temperature is, for example, 50 to 100°C, and the drying time is, for example, 6 to 12 hours; in the present invention, separating the reaction product from the reaction solution by centrifugal separation, specifically, for example, cooling the obtained reaction solution to room temperature, and then centrifuging it multiple times with deionized water and ethanol, respectively, to separate the reaction product. The reaction product is vacuum-dried in an oven to finally obtain a cobalt ion-doped rare earth fluoride up-conversion luminescent material, and the obtained cobalt ion-doped rare earth fluoride up-conversion luminescent material is a pure phase cobalt ion-doped rare earth fluoride up-conversion luminescent material; in the present invention, preferably, after the separated reaction product is dried, a grinding step is further included to finally obtain the cobalt ion-doped rare earth fluoride up-conversion luminescent material; the particle size of the cobalt ion-doped rare earth fluoride up-conversion luminescent material is, for example, 1 to 5 microns; in the present invention, the cobalt ion-doped rare earth fluoride up-conversion luminescent material has a hexagonal prism-shaped crystal structure, for example, Figures 2 to 5 In some specific embodiments of the present invention, cobalt ions are doped in the rare earth fluoride up-conversion luminescent material, and the cobalt ion-doped rare earth fluoride up-conversion luminescent material is also recorded as NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / x%Co 2+ , x∈[1,25], that is, 1≤x≤25. In the present invention, the percentage contents involved, such as 18%, 2%, 2%, x%, etc., all refer to molar percentage contents.

[0040] The cobalt ion-doped rare earth fluoride up-conversion luminescent material of the present invention is synthesized by a hydrothermal method under high pressure. The use of an autoclave here is mainly to provide a better crystallization nucleation environment. The present invention finds that under appropriate high pressure conditions, it is conducive to uniform doping of the cobalt source into the rare earth fluoride, and the prepared cobalt ion-doped rare earth fluoride up-conversion luminescent material can have a higher crystallinity than that synthesized under normal pressure, and the sample is more stable, so that the prepared sample is in a hexagonal NaYF4 crystal phase, which is conducive to up-conversion luminescence; the cobalt ion-doped rare earth fluoride up-conversion luminescent material of the present invention is The luminescent material is synthesized by a hydrothermal method and has a regular hexagonal columnar morphology. The present invention has found that rare earth fluoride materials with a regular hexagonal columnar morphology can improve the light absorption efficiency compared to hexagonal flake morphology. This may be because this morphology helps to increase the incident surface area of light, thereby enhancing light absorption. The regular hexagonal columnar morphology helps to optimize the optical properties of the material and reduce the scattering and absorption loss of photons. The present invention optimizes the morphology and structure of the rare earth fluoride material to make it better match the solar spectrum, improve the utilization efficiency of light, and thus increase the conversion efficiency of photovoltaic cells.

[0041] The cobalt ion doped rare earth fluoride up-conversion luminescent material of the present invention is based on NaYF4 as inorganic crystal matrix, Yb 3+ As sensitizer, Er 3+ and Tm 3+ As activator, Co 2+ Hexagonal phase up-conversion luminescent material constructed for doping ions. The performance of doped materials depends to a large extent on the doping elements, and it is crucial to select appropriate doping components in material design to achieve ideal performance. The cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared by the present invention can effectively absorb near-infrared light with a wavelength of 980 nanometers, and through energy conversion, converts photons with a wavelength of 980 nanometers into visible light with a wavelength of 500 to 760 nanometers. Visible light is the main absorption band of photovoltaic cells, which can improve the photoelectric conversion efficiency of photovoltaic cells. Through luminescence lifetime testing, it was found that the luminescence lifetime of the cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared by the present invention was extended compared with the material not doped with cobalt ions. At the same time, by changing the laser power to detect the upconversion luminescence intensity, the two-photon process of the luminescence mechanism of the prepared material was confirmed. That is, the upconversion luminescence is achieved by absorbing two long-wavelength, low-energy photons and converting them into short-wavelength, high-energy photons, thereby realizing the conversion of near-infrared light to visible light. It has a good effect on improving the photoelectric conversion efficiency of photovoltaic cells and can make full use of sunlight. This provides a new feasibility for further improving the efficiency of photovoltaic cells and reducing costs, which provides a new feasibility for the design of upconversion luminescent materials.

[0042] According to some preferred embodiments, the molar ratio of the ytterbium source, the erbium source, the thulium source and the cobalt source is 18:2:2:(1-25), preferably 18:2:2:(1-10), more preferably 18:2:2:(4-6), and further preferably 18:2:2:5; and / or the molar ratio of the yttrium source to the cobalt source is (80-x):x, 1≤x≤25, preferably, 1≤x≤10, more preferably, 4≤x≤6; In the present invention, it is preferred that the ytterbium source, the erbium source, the thulium source and the cobalt source are 18:2:2:(1-25), preferably 18:2:2:(1-10), more preferably 18:2:2:(4-6), and further preferably 18:2:2:5; and / or the molar ratio of the yttrium source to the cobalt source is (80-x):x, 1≤x≤25, preferably, 1≤x≤10, more preferably, 4≤x≤6. The molar ratio of the thulium source to the cobalt source is 18:2:2:(4-6), and the molar ratio of the yttrium source to the cobalt source is (80-x):x, 4≤x≤6. Within this ratio range, it is helpful to form an up-conversion luminescent material with hexagonal phase crystals, regular morphology, and appropriate crystallinity, which can ensure that the cobalt ion-doped rare earth fluoride up-conversion luminescent material has the best up-conversion efficiency, and helps to make the photovoltaic cell have a higher photoelectric conversion efficiency. If the ratio of each element is not appropriate, it is not conducive to the effective improvement of the photoelectric conversion efficiency of the photovoltaic cell.

[0043] According to some preferred embodiments, the concentration of the aqueous solution containing trisodium citrate is 0.02-0.04 mmol / mL; the molar ratio of trisodium citrate contained in the aqueous solution containing trisodium citrate to the erbium source is (20-30):1, preferably 25:1; the molar ratio of sodium fluoride in step (2) to the trisodium citrate contained in the aqueous solution containing trisodium citrate in step (1) is (6-10):1, preferably 8:1; and / or the molar ratio of urea to sodium fluoride is (4-6):1, preferably 5:1.

[0044] According to some preferred embodiments, the yttrium source is one or more of yttrium nitrate, yttrium chloride, yttrium sulfate and hydrated compounds thereof; the ytterbium source is one or more of ytterbium nitrate, ytterbium chloride, ytterbium sulfate and hydrated compounds thereof; the erbium source is one or more of erbium nitrate, erbium chloride, erbium sulfate and hydrated compounds thereof; the thulium source is one or more of thulium nitrate, thulium chloride, thulium sulfate and hydrated compounds thereof; and / or the cobalt source is one or more of cobalt nitrate, cobalt chloride, cobalt sulfate and hydrated compounds thereof.

[0045] According to some specific embodiments, the preparation of the cobalt ion-doped rare earth fluoride up-conversion luminescent material of the present invention is as follows: first, trisodium citrate is dissolved in deionized water, and then the yttrium source, ytterbium source, erbium source, thulium source, and cobalt source are added to the aqueous solution containing trisodium citrate in sequence and stirred evenly, and then sodium fluoride and urea are added in sequence and stirred evenly again, and then the mixed solution is transferred to a polytetrafluoroethylene high-pressure reactor for high-temperature reaction. After a certain period of high-temperature reaction, it is cooled to room temperature, and the precipitate is collected by centrifugation, and the sample is obtained by vacuum drying in an oven, and then ground to obtain a cobalt ion-doped rare earth fluoride up-conversion luminescent material.

[0046] According to some preferred embodiments, in step (1), a nickel source is further added to the aqueous solution containing trisodium citrate, and the nickel source is one or more of nickel nitrate, nickel chloride, nickel sulfate and hydrated compounds thereof; in some specific embodiments, the nickel source is nickel nitrate hexahydrate (Ni(NO3)2·6H2O); in the present invention, when a nickel source is further added to the aqueous solution containing trisodium citrate in step (1), the obtained cobalt ion-doped rare earth fluoride up-conversion luminescent material can also be recorded as a cobalt-nickel-doped rare earth fluoride up-conversion luminescent material.

[0047] According to some preferred embodiments, the molar ratio of the ytterbium source, the erbium source, the thulium source, the cobalt source and the nickel source is 18:2:2:(1-25):(1-15), preferably 18:2:2:(1-10):(5-15), more preferably 18:2:2:(4-6):(5-15), and further preferably 18:2:2:5:10; and / or the molar ratio of the yttrium source, the cobalt source and the nickel source is (80-xy):x:y, 1≤x≤25, 1≤y≤15.

[0048] In the present invention, it is preferred that, while doping the rare earth fluoride upconversion luminescent material with a suitable content of cobalt ions, it is also doped with a suitable content of nickel ions. The present invention finds that the simultaneous presence of cobalt ions and nickel ions can make it more absorptive to light in a wider wavelength range in the solar spectrum, which helps the photovoltaic cell absorb more photons and improve light absorption efficiency. In addition, the simultaneous doping of cobalt ions and nickel ions can better regulate the band structure of the rare earth fluoride, which helps to further improve the separation efficiency of photogenerated carriers. By better adjusting the band structure, the recombination rate of photogenerated carriers can be reduced, thereby further improving the photoelectric conversion efficiency. In addition, the simultaneous doping of cobalt ions and nickel ions can better adjust the peak and intensity of photoluminescence, so that it better matches the operating wavelength of the photovoltaic cell, which is also conducive to further improving the photoelectric conversion efficiency. In addition, the present invention finds that the content of doped cobalt ions and nickel ions needs to be precisely controlled, and excessively high or low doping concentrations are not conducive to the effective improvement of the photoelectric conversion efficiency of the photovoltaic cell.

[0049] According to some preferred embodiments, in step (1) and / or step (2), the stirring speed is 200-1000 r / min, preferably 500-800 r / min, and the stirring time is 30-200 min, preferably 60-100 min. The present invention ensures uniform dispersion of the reaction ions in the solvent by controlling the appropriate stirring speed and time; the temperature of the high-temperature reaction is 100-220°C, and the time of the high-temperature reaction is 10-24 h; the pressure for the high-temperature reaction is 1.4-1.6 MPa; the centrifugal speed is 8000-10000 r / min, the centrifugal time is 3-15 min, and the number of centrifugations is 2-5 times; and / or the drying temperature is 50-100°C, and the drying time is 6-12 h.

[0050] According to some preferred embodiments, the cobalt ion-doped rare earth fluoride up-conversion luminescent material belongs to the hexagonal crystal system, and the crystals are in the shape of hexagonal columns.

[0051] In a second aspect, the present invention provides a cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared by the preparation method described in the first aspect of the present invention.

[0052] According to some preferred embodiments, the cobalt ion-doped rare earth fluoride up-conversion luminescent material belongs to the hexagonal crystal system, and the crystal is hexagonal columnar; and / or the particle size of the cobalt ion-doped rare earth fluoride up-conversion luminescent material is 1 to 5 microns.

[0053] In a third aspect, the present invention provides an application of a cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared by the preparation method described in the first aspect of the present invention in photovoltaic efficiency enhancement, wherein the application is: coating the cobalt ion-doped rare earth fluoride up-conversion luminescent material on the surface of a photovoltaic cell to form a coating, and the coating can improve the photoelectric conversion efficiency of the photovoltaic cell under sunlight; specifically, for example, the application is: coating the cobalt ion-doped rare earth fluoride up-conversion luminescent material on the surface of a photovoltaic cell to form a coating, and then placing the photovoltaic cell coated with the coating in a solar simulator to detect the photoelectric conversion efficiency of the photovoltaic cell; preferably, the thickness of the coating is not greater than 100 nm.

[0054] According to some preferred embodiments, the cobalt ion-doped rare earth fluoride up-conversion luminescent material coating is formed as follows: the cobalt ion-doped rare earth fluoride up-conversion luminescent material is added to a solvent and ball-milled (for example, ball-milled for 6 to 10 hours), and ultrasonically dispersed (for example, ultrasonicated for 1 hour) to obtain a uniformly dispersed cobalt ion-doped rare earth fluoride up-conversion luminescent material dispersion, and then the cobalt ion-doped rare earth fluoride up-conversion luminescent material dispersion is coated on the surface of the photovoltaic cell, and dried to obtain a coating coated with the cobalt ion-doped rare earth fluoride up-conversion luminescent material; in the present invention, the coating method can be, for example, spin coating, spraying, dipping, etc., preferably a method of coating after dipping the dispersion with a non-woven fabric; in the present invention, the solvent is preferably deionized water or ethanol, but other solvents that can be used are not excluded.

[0055] According to some preferred embodiments, during the formation of the cobalt ion-doped rare earth fluoride up-conversion luminescent material coating, the solid-liquid ratio (mass ratio) of the cobalt ion-doped rare earth fluoride up-conversion luminescent material to the solvent is 1:100.

[0056] In some specific embodiments, the application is: forming a coating on a photovoltaic cell with the cobalt ion-doped rare earth fluoride upconversion luminescent material, wherein the light source of the photovoltaic cell is a standard sunlight irradiation intensity in a solar simulator, and the wavelength band is the full spectrum of sunlight; then placing the photovoltaic cell coated with the coating in a solar simulator for JV curve testing, i.e., in a standard sunlight (AM1.5G, 100mW / cm 2 ) to test the relationship between the current density and voltage of photovoltaic cells under irradiation, which is used to obtain photoelectric conversion parameters such as power conversion efficiency (PCE), short circuit current density (JSC), open circuit voltage (VOC) and fill factor (FF).

[0057] The following examples are further cited to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be interpreted as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention all fall within the scope of protection of the present invention. The specific process parameters, etc. of the following examples are only examples within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description herein, and are not limited to the specific numerical values exemplified below. The materials, etc. used in the following examples, unless otherwise specified, can be obtained from commercial sources or prepared by existing methods.

[0058] Example 1

[0059] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to make it uniformly dispersed; 0.8 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, and 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O) were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A and further stirred at a speed of 600 r / min for 1 hour. min and stirred for 1 hour to obtain a mixed solution B; the stirred mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction solution was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugal washing (centrifugal separation) was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a rare earth fluoride up-conversion luminescent material without cobalt ion doping, which was recorded as NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 0%Co 2 + .

[0060] Example 2

[0061] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to disperse it evenly; 0.75 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), and 0.05 mmol of cobalt nitrate hexahydrate were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A and further stirred at a speed of 600 r / min. The mixture was stirred at 600 r / min for 1 hour to obtain a mixed solution B; the mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a cobalt ion-doped rare earth fluoride up-conversion luminescent material with a cobalt ion doping ratio of 5%, which was recorded as NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 5%Co 2+ .

[0062] Example 3

[0063] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to disperse it evenly; 0.65 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), and 0.15 mmol of cobalt nitrate hexahydrate were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A and further stirred at a speed of The mixture was stirred at 600 r / min for 1 hour to obtain a mixed solution B; the mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a cobalt ion-doped rare earth fluoride up-conversion luminescent material with a cobalt ion doping ratio of 15%, which was recorded as NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 15%Co 2+ .

[0064] Example 4

[0065] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to disperse it evenly; 0.55 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), and 0.25 mmol of cobalt nitrate hexahydrate were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A and further stirred at a speed of The mixture was stirred at 600 r / min for 1 hour to obtain a mixed solution B; the mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a cobalt ion-doped rare earth fluoride up-conversion luminescent material with a cobalt ion doping ratio of 25%, which was recorded as NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 25%Co 2+.

[0066] The XRD patterns of the rare earth fluoride up-conversion luminescent materials doped with cobalt ions in different proportions prepared in Examples 1 to 4 of the present invention are as follows: Figure 1 As shown by Figure 1 It can be seen that the rare earth fluoride up-conversion luminescent materials doped with cobalt ions in different proportions prepared in Examples 1 to 4 have obvious characteristic peaks between 15 and 80°, which corresponds to the PDF card 28-1192 of β-NaYF4; the SEM photograph of the rare earth fluoride up-conversion luminescent material without cobalt ion doping prepared in Example 1 of the present invention is as follows: Figure 2 As shown, from Figure 2 As can be seen from the SEM image, the prepared rare earth fluoride upconversion luminescent material without cobalt ion doping has a hexagonal sheet structure. The average thickness of the hexagonal sheet is 133.3 nanometers, the diameter is about 2.778 microns, and the side length is about 1.411 microns. The shape is regular, and the hexagonal micron sheets are independently dispersed.

[0067] The SEM image of the 5% cobalt ion doped rare earth fluoride upconversion luminescent material prepared in Example 2 of the present invention is as follows: Figure 3 As shown, from Figure 3 As can be seen from the SEM image, the prepared 5% cobalt ion doped rare earth fluoride up-conversion luminescent material has a hexagonal prism structure, the average thickness of the hexagonal prism is 198.2 to 258.6 nanometers, the average diameter is 1.968 microns, the average side length is 967.4 nanometers, the shape is regular, and the hexagonal prisms are independently dispersed. The thickness of the hexagonal prism in the present invention is significantly greater than that of the hexagonal sheet. In the present invention, after cobalt ion doping, the thickness of the hexagonal structure increases, the crystallinity improves, and it gradually grows from a sheet to a columnar shape with a certain thickness. The cobalt ion doped material has the best up-conversion luminescence intensity. The sample with an impurity content of 5% indicates that when the crystallinity is moderate, the incorporation of cobalt ions enables the activator ions in the up-conversion luminescent material system to achieve optimal distribution, thereby maximizing the energy transfer efficiency during the up-conversion luminescence process and improving the up-conversion luminescence performance; while when the crystallinity is too low or too high, the distance between the activator ions is too close or too far, the energy transfer efficiency is low, and possible defects inside the material will also quench the luminescence and reduce the luminescence intensity; in the present invention, the thickness of the hexagonal prism refers to the height of the hexagonal prism, the diameter of the hexagonal prism refers to the distance between the diagonals of the hexagon, and the side length of the hexagonal prism refers to any side of the hexagon.

[0068] The SEM image of the 15% cobalt ion doped rare earth fluoride upconversion luminescent material prepared in Example 3 of the present invention is as follows: Figure 4 shown; from Figure 4As can be seen from the SEM image, the prepared 15% cobalt ion doped rare earth fluoride upconversion luminescent material has a hexagonal prism structure, the average thickness of the hexagonal prism is 1.167 microns, the average diameter is 4.654 microns, the average side length is 2.260 microns, the shape is regular, and the hexagonal prisms are independently dispersed; the SEM photo of the 25% cobalt ion doped rare earth fluoride upconversion luminescent material prepared in Example 4 of the present invention is as follows. Figure 5 shown; from Figure 5 It can be seen from the SEM image that the prepared 25% cobalt ion doped rare earth fluoride upconversion luminescent material has a hexagonal prism structure with an average thickness of 693.7 nanometers, an average diameter of 1.642 microns, an average side length of 846.2 nanometers, a regular shape, and independent dispersion between the hexagonal prisms.

[0069] The up-conversion luminescence spectrum of the 5% cobalt ion doped rare earth fluoride up-conversion luminescent material prepared in Example 2 of the present invention is as follows: Figure 6 As shown; Figure 6 It is obtained under the excitation of 980 nm wavelength laser. Figure 6 It can be seen that the cobalt ion doped rare earth fluoride upconversion luminescent material prepared by the present invention can effectively convert photons with a wavelength of 980 nanometers into visible light with main emission peaks of 520 nanometers, 540 nanometers and 654 nanometers, corresponding to 2 H 11 / 2 → 4 I 15 / 2 、 4 S 3 / 2 → 4 I 15 / 2 、 4 F 9 / 2 → 4 I 15 / 2 The electron transition process realizes the up-conversion luminescence process.

[0070] The logarithmic spectrum of the laser power and upconversion luminescence intensity of the 0% cobalt ion doped rare earth fluoride upconversion luminescent material prepared in Example 1 of the present invention is as follows: Figure 7 As shown; Figure 7 It is obtained by repeatedly changing the power of the 980 nm wavelength laser. According to formula I UC ∝P n Calculated, where I UC is the up-conversion luminescence intensity, P is the laser power, and n is the number of photons; Figure 7 It can be seen that the slopes of the logarithmic curves are close to 2, which indicates that n is approximately equal to 2, that is, the 520 nm, 540 nm, and 654 nm lights emitted by the 980 nm laser excitation correspond to 2 H 11 / 2 → 4I 15 / 2 、 4 S 3 / 2 → 4 I 15 / 2 、 4 F 9 / 2 → 4 I 15 / 2 The electron transition process is a two-photon process, indicating that the 0% cobalt ion-doped rare earth fluoride upconversion luminescent material prepared by the present invention can absorb two long-wavelength, low-energy photons and convert them into one short-wavelength, high-energy photon under 980-nanometer laser excitation.

[0071] The logarithmic spectrum of the laser power and upconversion luminescence intensity of the 5% cobalt ion doped rare earth fluoride upconversion luminescent material prepared in Example 2 of the present invention is as follows: Figure 8 As shown; Figure 8 It is obtained by repeatedly changing the power of the 980 nm wavelength laser. According to formula I UC ∝P n Calculated, where I UC is the up-conversion luminescence intensity, P is the laser power, and n is the number of photons. Figure 8 It can be seen that the slopes of the logarithmic curves are close to 2, which indicates that n is approximately equal to 2, that is, the 520 nm, 540 nm, and 654 nm lights emitted by the 980 nm laser excitation correspond to 2 H 11 / 2 → 4 I 15 / 2 、 4 S 3 / 2 → 4 I 15 / 2 、 4 F 9 / 2 → 4 I 15 / 2 The electron transition process is also a two-photon process. The doping of cobalt ions does not affect the luminescence mechanism of the rare earth fluoride upconversion luminescent material. This also shows that the 5% cobalt ion-doped rare earth fluoride upconversion luminescent material prepared by the present invention can absorb two long-wavelength, low-energy photons and convert them into one short-wavelength, high-energy photon under 980-nanometer laser excitation.

[0072] A comparison of the fluorescence lifetimes of the 0% cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared in Example 1 of the present invention and the 5% cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared in Example 2 of the present invention is shown in FIG. Figure 9 shown; from Figure 9It can be seen that compared with the rare earth fluoride up-conversion luminescent material doped with 0% cobalt ions, the fluorescence lifetime of the rare earth fluoride up-conversion luminescent material is prolonged after doping with 5% cobalt ions, which indicates that doping with cobalt ions is beneficial to prolonging the luminescence lifetime of the up-conversion luminescent material.

[0073] The present invention disperses the rare earth fluoride upconversion luminescent materials prepared in Examples 1 to 4 in ethanol, performs ball milling for 8 hours, and ultrasonicates for 1 hour to obtain a dispersion liquid, wherein the mass ratio of the rare earth fluoride upconversion luminescent material to ethanol is 1:100; a non-woven fabric is dipped in the dispersion liquid and then coated on a glass plate with a transmittance of 96% to form a coating with a thickness of 100 nm; the coated glass plate and the uncoated glass plate are tested for transmittance, and the results are shown in Table 1; the coated glass plate is placed on a photovoltaic cell, and a JV curve test is performed in a solar simulator, that is, under a standard sunlight (AM1.5G, 100 mW / cm 2 ) irradiation, the relationship between the current density and voltage of the photovoltaic cell was tested to obtain the photoelectric conversion parameters such as power conversion efficiency (PCE), short circuit current density (JSC), open circuit voltage (VOC) and fill factor (FF). The results are shown in Table 2. At the same time, a glass plate with a transmittance of 96% of the uncoated sample was placed on the photovoltaic cell and the same test was carried out as a control. The results are shown in Table 2. The present invention also places a coated glass plate on the photovoltaic cell and performs a JV curve test in a solar simulator, that is, in a standard sunlight (AM1.5G, 100mW / cm 2 ) irradiation conditions, the glass-coated sample was additionally covered with a filter to filter out photons with wavelengths less than 760 nanometers in sunlight, allowing only infrared light with wavelengths greater than 760 nanometers to pass through. The relationship between the current density and voltage of the photovoltaic cell was tested to obtain photovoltaic conversion parameters such as power conversion efficiency (PCE), short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF). The results are shown in Table 3. Simultaneously, an uncoated glass plate with a transmittance of 96% was placed on the photovoltaic cell and the same test was performed as a control. The results are also shown in Table 3.

[0074] Table 1

[0075]

[0076]

[0077] Table 2

[0078]

[0079] Table 3

[0080]

[0081] It can be seen from the data in Table 1 that the glass with an initial transmittance of 96% did not lose its transmittance in the visible light band before and after coating the sample, that is, the transmittance still maintained its initial value after coating the sample. The visible light band is the main absorption band of photovoltaic cells, which shows that the formation of the sample on the photovoltaic cell does not affect its absorption of visible light.

[0082] It can be clearly seen from the data in Table 2 that the photovoltaic cells coated with the rare earth fluoride up-conversion luminescent material coatings with different cobalt ion doping ratios prepared in Examples 1 to 4 of the present invention have improved photoelectric conversion efficiency (Eff) compared with the uncoated sample glass plate with a transmittance of 96%. By calculating the increase in photoelectric conversion efficiency of the sample coatings in Examples 1 to 4 compared with the uncoated sample glass plate with a transmittance of 96% as a comparison, taking the coated sample Example 1 as an example, the increase = (13.16-12.97) / 12.97 = 1.46%, which is the relative value of the two. It can be seen that under a standard sunlight irradiation, the materials prepared in Examples 1 to 4 of the present invention can improve the photoelectric conversion efficiency of the photovoltaic cell to varying degrees, and when the cobalt ion doping amount is 5%, the increase in photoelectric conversion efficiency reaches the maximum value.

[0083] It can be clearly seen from the data in Table 3 that under the condition of standard sunlight irradiation, a filter is additionally applied to the coated glass sample to filter out photons with a wavelength less than 760 nanometers in the sunlight and only transmit infrared light with a wavelength greater than 760 nanometers. The photovoltaic cells coated with the rare earth fluoride up-conversion luminescent material coatings with different cobalt ion doping ratios prepared in Examples 1 to 4 of the present invention have improved photoelectric conversion efficiencies (Eff) compared to the uncoated glass plate with a transmittance of 96%. By calculating the Eff of the samples coated in Examples 1 to 4 and the For comparison, the increase in photoelectric conversion efficiency of the uncoated sample with a transmittance of 96% is shown in Example 1, where the increase is (5.04-4.98) / 4.98=1.20%, which is the relative value of the two. It can be seen that under the condition of standard sunlight irradiation, when a filter is applied to the glass-coated sample to filter out photons with a wavelength less than 760 nanometers in the sunlight and only infrared light with a wavelength greater than 760 nanometers is allowed to pass through, the materials prepared in Examples 1 to 4 of the present invention can all improve the photoelectric conversion efficiency of the photovoltaic cell to varying degrees. Therefore, the cobalt ion-doped rare earth fluoride upconversion luminescent material of the present invention is suitable for preparing upconversion luminescent coatings on photovoltaic cells to improve the utilization rate of near-infrared light in the solar spectrum by photovoltaic cells, thereby providing a practical path for further improving photovoltaic efficiency and a new method and idea for the design of photovoltaic efficiency-enhancing materials.

[0084] The present invention uses a cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared by a hydrothermal method. The preparation process is simple, the requirements for experiments, conditions and equipment are relatively low, and large-scale applications can be achieved. The use of the cobalt ion-doped rare earth fluoride up-conversion luminescent material to prepare a coating on the surface of a photovoltaic cell can convert near-infrared light into visible light, improve the utilization rate of near-infrared light in sunlight by the photovoltaic cell, further improve the photoelectric conversion efficiency of the photovoltaic cell, and have broad application prospects in the work of increasing photovoltaic efficiency. In particular, according to data from the China Photovoltaic Industry Association, based on the photoelectric conversion efficiency of the original cell, for every 1% increase in photoelectric conversion efficiency, the corresponding production capacity and revenue increase significantly, and the corresponding cost per kilowatt-hour can be reduced by 5% to 7%. Therefore, in the photovoltaic industry, even if the "photoelectric conversion efficiency" is only increased by 0.01%, the corresponding significance is very significant.

[0085] Example 5

[0086] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to disperse it evenly; 0.74 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), 0.05 mmol of cobalt nitrate hexahydrate and 0.01 mmol of nickel nitrate hexahydrate were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A and stirred for 1 hour. The mixture was stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution B; the mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a rare earth fluoride up-conversion luminescent material with a cobalt ion doping ratio of 5% and a nickel ion doping ratio of 1%, which was recorded as NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 5%Co 2+ / 1%Ni 2+ .

[0087] Example 6

[0088] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to uniformly disperse the mixture; 0.7 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), 0.05 mmol of cobalt nitrate hexahydrate, and 0.05 mmol of nickel nitrate hexahydrate were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A, The mixture was further stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution B; the uniformly stirred mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a cobalt ion-doped rare earth fluoride up-conversion luminescent material with a cobalt ion doping ratio of 5% and a nickel ion doping ratio of 5%, which was recorded as:

[0089] NaYF4:18%Yb 3+ / 2%Er3+ / 2%Tm 3+ / 5%Co 2+ / 5%Ni 2+ .

[0090] Example 7

[0091] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to uniformly disperse the mixture; 0.65 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), 0.05 mmol of cobalt nitrate hexahydrate, and 0.1 mmol of nickel nitrate hexahydrate were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A, The mixture was further stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution B; the uniformly stirred mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a cobalt ion-doped rare earth fluoride up-conversion luminescent material with a cobalt ion doping ratio of 5% and a nickel ion doping ratio of 10%, which was recorded as:

[0092] NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 5%Co 2+ / 10%Ni 2+ .

[0093] Example 8

[0094] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to uniformly disperse the mixture; 0.6 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), 0.05 mmol of cobalt nitrate hexahydrate, and 0.15 mmol of nickel nitrate hexahydrate were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A, The mixture was further stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution B; the uniformly stirred mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a cobalt ion-doped rare earth fluoride up-conversion luminescent material with a cobalt ion doping ratio of 5% and a nickel ion doping ratio of 15%, which was recorded as:

[0095] NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 5%Co 2+ / 15%Ni 2+ .

[0096] The present invention disperses the rare earth fluoride up-conversion luminescent materials prepared in Examples 5 to 8 in ethanol, performs ball milling for 8 hours, and ultrasonicates for 1 hour to obtain a dispersion liquid, wherein the mass ratio of the rare earth fluoride up-conversion luminescent material to ethanol is 1:100; a non-woven fabric is used to soak the dispersion liquid and then coated on a glass plate with a transmittance of 96% to form a coating with a thickness of 100 nm; the coated glass plate is placed on a photovoltaic cell, and a JV curve test is performed in a solar simulator, that is, under a standard sunlight (AM1.5G, 100 mW / cm 2 ) irradiation, the relationship between the current density and voltage of the photovoltaic cell was tested to obtain the photoelectric conversion parameters such as power conversion efficiency (PCE), short circuit current density (JSC), open circuit voltage (VOC) and fill factor (FF). The results are shown in Table 4. At the same time, a glass plate with a transmittance of 96% of the uncoated sample was placed on the photovoltaic cell for the same test as a control. The results are shown in Table 4.

[0097] Table 4

[0098]

[0099]

[0100] The data in Tables 2 and 4 show that when nickel ions are further doped into the cobalt-doped rare earth fluoride upconversion luminescent material, the photoelectric conversion efficiency of the photovoltaic cell can be further improved. Compared to the maximum efficiency increase of 2.08% when only cobalt ions are doped, the photoelectric conversion efficiency increases to varying degrees when cobalt and nickel ions are co-doped under sunlight. That is, when the cobalt ion doping level is fixed at 5% and the nickel ion doping levels are 1%, 5%, 10%, and 15%, respectively, the photoelectric conversion efficiency increases to 2.16%, 2.31%, 3.39%, and 2.54%, respectively, compared to the maximum efficiency increase of 2.08% when only cobalt ions are doped. Therefore, in some preferred embodiments, the cobalt-doped rare earth fluoride upconversion luminescent material prepared with a cobalt ion doping ratio of 5% and a nickel ion doping ratio of 1-15% can further improve the photoelectric conversion efficiency of the photovoltaic cell. The further increase in photovoltaic efficiency achieved by co-doping with cobalt and nickel ions demonstrates that simultaneous co-doping of two ions (transition metal ions) into the host lattice is a more effective method for achieving high luminescence intensity than single co-doping. This is likely due to the mechanism by which the co-doping of cobalt and nickel ions enhances emission intensity, possibly related to a distortion of the local crystal field symmetry around the activator ions (erbium and thulium ions) in the upconversion luminescent material system. Both cobalt and nickel ions have small ionic radii, easily occupying interstitial sites in the lattice, leading to an expansion of the host lattice. This lattice expansion increases the lattice parameter, disrupting the symmetry of the local crystal field around the activator ions. This asymmetric distribution of activator ions favors radiative transitions, thereby increasing the upconversion luminescence intensity. Furthermore, beyond the optimal doping concentration, the upconversion luminescence intensity is quenched by non-radiative relaxation due to the generation of excessive defect centers in the sodium yttrium fluoride (NaYF4) lattice. Therefore, the enhancement of upconversion emission intensity by co-doping with cobalt and nickel ions is also due to a reduction in optical quenching centers.

[0101] Comparative Example 1

[0102] Dissolve 0.5 mmol of trisodium citrate in 15 mL of deionized water and stir for 10 minutes to disperse it evenly; add 0.75 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), and 0.05 mmol of ferric nitrate hexahydrate in sequence, and stir at a speed of 600 r / min for 1 hour to obtain a mixed solution A; add 4 mmol of sodium fluoride and 20 mmol of urea to the mixed solution A. The mixture was stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution B; the mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a rare earth fluoride up-conversion luminescent material with an iron ion doping ratio of 5%, which was recorded as:

[0103] NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 5%Fe 3+ .

[0104] Comparative Example 2

[0105] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to uniformly disperse the mixture; 0.75 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), and 0.05 mmol of lithium nitrate were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A, The mixture was further stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution B; the uniformly stirred mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a rare earth fluoride up-conversion luminescent material with a lithium ion doping ratio of 5%, which was recorded as:

[0106] NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 5%Li + .

[0107] Comparative Example 3

[0108] 0.5 mmol of trisodium citrate was dissolved in 15 mL of deionized water and stirred for 10 minutes to disperse it evenly; 0.75 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), and 0.05 mmol of manganese nitrate tetrahydrate were added in sequence and stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution A; 4 mmol of sodium fluoride and 20 mmol of urea were added to the mixed solution A, and further The mixture was stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution B; the uniformly stirred mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a rare earth fluoride up-conversion luminescent material with a manganese ion doping ratio of 5%, which was recorded as NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 5%Mn 2+ .

[0109] Comparative Example 4

[0110] Dissolve 0.5 mmol of trisodium citrate in 15 mL of deionized water and stir for 10 minutes to disperse it evenly; add 0.75 mmol of yttrium nitrate hexahydrate, 0.18 mmol of ytterbium nitrate, 0.02 mmol of erbium nitrate hexahydrate, 0.02 mmol of thulium nitrate hydrate (Tm(NO3)3·xH2O), and 0.05 mmol of zirconium nitrate pentahydrate in sequence, and stir at a speed of 600 r / min for 1 hour to obtain a mixed solution A; add 4 mmol of sodium fluoride and 20 mmol of urea to the mixed solution A. The mixture was stirred at a speed of 600 r / min for 1 hour to obtain a mixed solution B; the mixed solution B was transferred to a 100 mL reactor and reacted at 180° C. and a pressure of 1.5 MPa for 12 hours to obtain a reaction solution; after the reaction was completed, the reactor was cooled to room temperature, and the reaction product was centrifuged and washed four times with deionized water and ethanol in sequence; the reaction product obtained by centrifugation was dried in a vacuum drying oven at 80° C. for 12 hours to obtain a rare earth fluoride up-conversion luminescent material with a zirconium ion doping ratio of 5%, which was recorded as:

[0111] NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm3+ / 5% Zr 4+ .

[0112] The present invention disperses the rare earth fluoride up-conversion luminescent materials prepared in Comparative Examples 1 to 4 in ethanol, performs ball milling for 8 hours, and ultrasonicates for 1 hour to obtain a dispersion liquid, wherein the mass ratio of the rare earth fluoride up-conversion luminescent material to ethanol is 1:100; a non-woven fabric is used to soak the dispersion liquid and then coated on a glass plate with a light transmittance of 96% to form a coating with a thickness of 100 nm; the glass plate coated with the coating is placed on a photovoltaic cell, and a JV curve test is performed in a solar simulator, that is, under a standard sunlight (AM1.5G, 100 mW / cm 2 ) irradiation, the relationship between the current density and voltage of the photovoltaic cell was tested to obtain the photoelectric conversion parameters such as power conversion efficiency (PCE), short circuit current density (JSC), open circuit voltage (VOC) and fill factor (FF). The results are shown in Table 5. At the same time, a glass plate with a transmittance of 96% of the uncoated sample was placed on the photovoltaic cell for the same test as a control. The results are shown in Table 5.

[0113] Table 5

[0114]

[0115] Parts of the present invention that are not described in detail are well known to those skilled in the art.

[0116] 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 make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for preparing a cobalt ion-doped rare earth fluoride upconversion luminescent material, characterized in that: The method comprises the following steps: (1) adding a yttrium source, a ytterbium source, an erbium source, a thulium source and a cobalt source to an aqueous solution containing trisodium citrate in sequence and stirring the mixture to obtain a mixed solution A; in step (1), a nickel source is also added to the aqueous solution containing trisodium citrate; the molar ratio of the ytterbium source, the erbium source, the thulium source, the cobalt source and the nickel source is 18:2:2:5:(1-15); (2) adding sodium fluoride and urea to the mixed solution A and stirring uniformly to obtain a mixed solution B; (3) subjecting the mixed solution B to a high-temperature reaction in a high-pressure reactor to obtain a reaction solution; (4) The reaction product is separated from the reaction solution by centrifugation and dried to obtain a cobalt ion-doped rare earth fluoride upconversion luminescent material NaYF4:18%Yb 3+ / 2%Er 3+ / 2%Tm 3+ / 5%Co 2+ The cobalt ion-doped rare earth fluoride up-conversion luminescent material is further doped with nickel ions, the cobalt ion doping ratio is 5% and the nickel ion doping ratio is 1-15%, wherein the doping ratio is a molar percentage.

2. The preparation method according to claim 1, wherein: The concentration of the aqueous solution containing trisodium citrate is 0.02-0.04 mmol / mL; The molar ratio of trisodium citrate to the erbium source in the aqueous solution containing trisodium citrate is 25:1; The molar ratio of the sodium fluoride in step (2) to the trisodium citrate in the aqueous solution containing trisodium citrate in step (1) is 8:1; and / or The molar ratio of the urea to the sodium fluoride is 5:

1.

3. The preparation method according to claim 1, wherein: The yttrium source is one or more of yttrium nitrate, yttrium chloride, yttrium sulfate and hydrated compounds thereof; The ytterbium source is one or more of ytterbium nitrate, ytterbium chloride, ytterbium sulfate and hydrated compounds thereof; The erbium source is one or more of erbium nitrate, erbium chloride, erbium sulfate and hydrated compounds thereof; The thulium source is one or more of thulium nitrate, thulium chloride, thulium sulfate and hydrated compounds thereof; and / or The cobalt source is one or more of cobalt nitrate, cobalt chloride, cobalt sulfate and hydrated compounds thereof.

4. The preparation method according to claim 1, wherein: The nickel source is one or more of nickel nitrate, nickel chloride, nickel sulfate and hydrated compounds thereof.

5. The preparation method according to claim 1, wherein: In step (1) and / or step (2), the stirring speed is 200-1000 r / min, and the stirring time is 30-200 min; The temperature of the high temperature reaction is 100-220°C, and the time of the high temperature reaction is 10-24 hours; The pressure for carrying out the high temperature reaction is 1.4-1.6 MPa; The centrifugal speed is 8000-10000 r / min, the centrifugal time is 3-15 min, and the number of centrifugation is 2-5 times; and / or The drying temperature is 50-100° C., and the drying time is 6-12 hours.

6. The preparation method according to claim 5, characterized in that: In step (1) and / or step (2), the stirring speed is 500-800 r / min.

7. The preparation method according to claim 5, characterized in that: In step (1) and / or step (2), the stirring time is 60 to 100 minutes.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The cobalt ion-doped rare earth fluoride up-conversion luminescent material belongs to the hexagonal crystal system, and the crystal is in the shape of a hexagonal column. 9 . A cobalt ion-doped rare earth fluoride up-conversion luminescent material prepared by the preparation method according to claim 1 .

10. Use of a cobalt ion-doped rare earth fluoride upconversion luminescent material prepared by the preparation method according to any one of claims 1 to 8 in photovoltaic efficiency enhancement, characterized in that: The application is: coating the cobalt ion-doped rare earth fluoride up-conversion luminescent material on the surface of a photovoltaic cell to form a coating, and the coating can improve the photoelectric conversion efficiency of the photovoltaic cell under sunlight.

11. The use according to claim 10, characterized in that: The thickness of the coating layer is no greater than 100 nm.

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