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Abstract
A novel white-light copolymer matched with 365 nm chips is prepared by bonding the vinyl-functionalized complexes Eu(TTA)2(Phen)(MAA), Tb(p-BBA)3(UA) and Zn(BTZ)(UA) to polysiloxaneprepolymer(synthesized by polycondensation of vinyltrimethoxysilane and diphenylsilanediol) through a technical route of polymerization after coordination. Its structure was characterized by infrared and ultraviolet. Under the excitation of 365 nm, when the ratio of the tricolor complexes is controlled to be 0.5: 3: 1.5, white light copolymer with CIE color coordinates of (0.327, 0.321) was obtained and packaged to get white light LED devices. After aging, the CIE color coordinates of the device change from (0.325, 0.329) to (0.341, 0.348), the color rendering index changes from 91 to 88, and the correlated color temperature changes from 5967 K to 5612 K. The loss of brightness is only 10.4%, which shows good resistance to UV aging. Moreover, the initial decomposition temperature of the copolymer is 235°C. The above results show that the bonding-type anti-ultraviolet copolymer phosphor has potential application in near ultraviolet LEDs.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, near-ultraviolet white light LEDs have become the first choice for health lighting because of their high color rendering index, strong color reproduction, and adjustable light color [1–3]. As a typical luminescent material, lanthanide complexes have received extensive attention due to their long lifetime, characteristic excited states, and strong narrowband emission peaks [4–6]. However, it is subject to long-term exposure to near-ultraviolet from the LED chips, which makes it prone to aging and causes device efficiency to roll off [7]. At the same time, lanthanide complexes also have poor thermal stability, easy to crystallization, and poor film-forming properties, which greatly limits their application as luminescent materials [8,9].
The introduction of lanthanide complexes into polymer chain is a new approach to obtain luminescent materials and overcome the aforementioned drawbacks. These materials have excellent properties, such as mechanical flexibility, excellent processability, and good photothermal stability [10,11]. Polymer-based lanthanide luminescent materials can be classified into two types according to chemical properties and synergy among the components. The first type is that the complex is doped into the polymer matrix by physical mixing. There is only weak interaction between the complex and the polymer matrix, and there is no covalent bond. This physical doping method easily leads to phase separation, so this material is very limited in application [12,13]. The other type is a bonded rare earth polymer prepared by chemical bonding. This type of rare earth polymer is one of the most important polymeric optoelectronic functional materials. It not only possesses the excellent fluorescence properties of rare earth complexes, but also has strong mechanical strength, chemical stability, excellent film-forming properties and homogeneity, and overcomes the phase separation defects of physical blending [10,14].
At present, there are two methods for realizing bonded rare earth polymers [15]. One is to coordinate with rare earth ions employing macromolecule as large ligands and other molecule as small ligands. For example, Gao [14,15] used this method to prepare a series of coordination polymers of rare earth ions (such as Eu3+, Tb3+). However, the lanthanide coordination polymers obtained by this method are not easily coordinated and are unstable in solution, resulting in the destruction of their properties. Another is the direct polymerization of rare earth complexes containing polymerizable double bonds with monomers [16,17]. With this method, small molecule complexes can be uniformly bonded in the polymer chain. By introducing a variety of small molecule complexes and regulating ratios, precise regulation of white light can be achieved. Therefore, this method of polymerization after coordination has attracted wide attention to researchers.
The key to the preparation of bonded rare earth polymers in the second method is the selection of monomers. Selecting the right monomer can endow the materials excellent performance. There have been series of carbon skeleton polymeric luminescent materials synthesized before [18,19]. These carbon skeleton polymers are subject to the problems such as poor chemical stability, poor thermal stability and poor anti-ultraviolet property when used, so the problems need to be further solved.
Because of its typical organic-inorganic structure, polysiloxane has the incomparable advantages than carbon skeleton materials, such as good stability, excellent mechanical properties, and high transparency. Combinations of polysiloxane and small molecular lanthanide complexes can obtain excellent materials containing both of their advantages. Functionalized organosilicone do not only enhance the energy absorption capacity of the chromophoric groups, but also transmit energy more efficiently to rare earth ions [7,9,20].
In this paper, a new type of bonded rare earth polysiloxane polymer is synthesized. By bonding different rare earth complexes to polysiloxane (obtained by polycondensation of vinyltrimethoxysilane and diphenylsilanol), it is able to achieve the best match to 365 nm UV chips. The luminescent properties, heat aging and ultraviolet aging properties of the copolymer are studied in detail. White light copolymer was obtained by tuning the complex proportion and excitation length of the copolymer. The white light LED devices was fabricated with the copolymer. After UV aging test, CIE color coordinates is (0.341, 0.348), the color rendering index is 88, and the relevant color temperature is 5612 K.The brightness loss was only 10.4%, displaying the excellent UV resistance.
2. Experimental
2.1 Materials
EuCl3, TbCl3 and Zn(NO3)2 were purchased from Aladdin. 2-Thenoyltrifluoroacetone(TTA), 1,10-phenanthroline (Phen), methacrylic acid (MAA), 4-benzoylbenzoic acid (p-BBA), 2-(2-hydroxyphenyl)benzothiazole (BTZ), undecylenic acid (UA), vinyltrimethoxysilane (VTMS) and diphenylsilanediol (DPSD) were purchased from Alfa Aesar. 2,2-Azoisobutyronitrile (AIBN), Ba(OH)2▪H2O were obtained from Shanghai Adamas Reagent Co. Ltd. Silicone gel (OE-6550A and OE-6550B) was purchased from Dow Corning. All solvents were obtained in analytical grade from Tianjin Fuchen Chemical Reagents Company and used as received without further purification.
2.2 Synthesis
The preparation of Eu(TTA)2(Phen)(MAA) [complex 1]
To the ethanol solution of 2 mmol of TTA, 1 mmol of Phen and 1 mmol of MAA, was added dropwise to 1 mmol of EuCl3 in 10 mL of ethanol. Then, the pH of the mixed solution was adjusted to 6.5∼7 by adding 1.0 mol/L NaOH ethanol solution and the precipitation gradually appeared. And the mixture was reacted at 50°C for 4 h. The precipitate was filtered, repeatedly washed with absolute ethanol and dried in a vacuum oven at 50 °C for 10 h. The elemental analysis for Eu(TTA)2(Phen)MAA is as follows: element analytical (calc.) C 43.88% (43.94%), H 2.70% (2.74%), N 3.30% (3.21%), S 7.31% (7.32%). Yield: 72%. The synthesis route is displayed in Fig. 1(a).
Fig. 1. Synthetic route for (a): Eu(TTA)2(Phen)(MAA); (b): Tb(p-BBA)3(UA); (c): Zn(BTZ)(UA); (d): P-Si; (e): P-Si-Eu-Tb-Zn
The preparation of Tb(p-BBA)3(UA) [complex 2]
1 mmol of TbCl3 ethanol solution was added dropwise to 3 mmol of p-BBA and 1 mmol of UA in 10 mL of ethanol solution. The pH of the solution was adjusted to 6.5∼7.0 with 1.0 mol/L NaOH ethanol solution. At this point, there was a white precipitate. Then it was reacted at 50 °C for 4 h, and filtered to get the white precipitate. Later, it was repeatedly washed 3∼5 times with absolute ethanol and dried in a vacuum oven for 10 h. The elemental analysis for Tb(p-BBA)3(UA) is as follows: element analytical (calc.) C 63.13% (62.54%), H 4.4% (4.52%). Yield: 62%.The synthesis route is presented in Fig. 1(b).
The preparation of Zn(BTZ)(UA) [complex 3]
1 mmol Zn(NO3)2 ethanol solution was added one by one to 1 mmol BTZ and 1 mmol UA in 10 mL of ethanol solution. Then the pH of the 1.0 mol/L NaOH ethanol solution was adjusted to 6.5∼7.0 until a white precipitate was formed. Then it was reacted at 50°C for 4 h, and filtered to get the white precipitate. Later, it was repeatedly washed 3∼5 times with absolute ethanol and dried in a vacuum oven for 10 h. Elemental analysis for Zn(BTZ)UA is as follows: element analytical (calc.) C 61.11% (60.52%), H 6.5% (6.61%), N 2.30% (2.29%), S 6.67% (6.72%). Yield: 70%. The synthetic route is described in Fig. 1(c).
The synthesis of polysiloxane prepolymer (P-Si)
P-Si was prepared as reported previously. It was synthesized by a polycondensation reaction from VTMS (5mmoL) and DPSD (3mmoL) under the catalysis of Ba(OH)2▪H2O and N2 atmosphere, and the reaction was carried at 80 °C for 12 h. The synthetic route is illustrated in Fig. 1(d).
The preparation of silicone copolymer(P-Si-Eu-Tb-Zn)
Organosilicon copolymer is prepared by free radical polymerization of the complex 1-3 and the prepolymer P-Si using the AIBN as initiator. The process is as follows. The prepolymer P-Si and the complexes 1-3 were weighed according to the ratios of a given number of moles (100: 1 [1/2/3] = 1: 3: 1, 0.5: 4: 1, 0.5: 4: 1.5 or 0.5: 3: 1.5) dissolved in 3 mL of DMF solution. Then they were added to a test tube. The AIBN in DMF solution was added and reacted at 70°C for 48 h under N2 atmosphere. The reaction product was precipitated with anhydrous methanol, dissolved in DMF, and re-precipitated with anhydrous methanol. This was repeated 5 times. At a vacuum of 8 Pa at 50°C, vacuum drying was carried out for 5 h to obtain Eu3+-Tb3+-Zn2+-grafted copolymers, abbreviated as P-Si-Eu-Tb-Zn. The synthetic route is shown in Fig. 1(e).
2.3 Fabrication of white light LEDs with the copolymer
The preparation of white light LED devices is divided into three processes:
- (1) Ingredients: Firstly, the Dow Corning packaging adhesives A and B were weighed in a mass ratio of 1:1 and mixed uniformly. Then, the phosphors was weighed according to a mass ratio of 20:1 between adhesives and phosphors and added to the packaging adhesives. A small amount of n-hexane was added as a dispersant, and the adhesives and phosphors were mixed uniformly with stirring. The mixture was then evacuated to eliminate small bubbles in it.
- (2) Dispensing: a suitable amount of vacuum-absorbed the adhesives and phosphors mixture was sucked with a syringe and dispensed on the LED chips to seal the LED lamp caps.
- (3) Curing: The dispensation-finished LEDs were horizontally placed in a vacuum oven and cured at 150°C for 2 h.
2.4 Measurement
Elemental analysis of the complex 1-3 was performed on a Perkin-Elmer Elemental Analyzer. The molecular weight of copolymer was recorded using gel permeation chromatography (GPC, TDAmax, Malvern). FT-IR spectra were recorded as potassium bromide pellets on a NICOLET AVATAR 330 Fourier transform infrared spectrometer. UV-vis absorption spectra were measured on a Cary-300 VARIAN spectrometer. Thermogravimetry (TG) analysis was carried out with a heating rate of 10 °C/min under N2 atmosphere on an SDT 2960 Analyzer. Fluorescence spectra were measured on an Edinburgh LFS-920 spectrometer (the slit width is 1 nm). Fluorescence decay was recorded by an Edinburgh Analytical Instruments (FLS920). Emission spectra of the fabricated LEDs were measured with a computer controlled PR655 Spectra scan spectrometer. The UV aging test of fabricated LEDs was carried out using a direct voltage-stabilized current.
3. Results and discussion
3.1 FT-IR spectra
The infrared spectra of the prepolymer P-Si, the complexes and the white light copolymer were measured at room temperature using a KBr disc method, as presented in Fig. 2. In the infrared spectra of the prepolymer P-Si, a broad and strong absorption peak at 1020∼1150 cm-1 is attributed to the antisymmetric vibration of Si-O-Si, which generated from the reaction of vinyltrimethoxysilane and diphenylsilyldiol [7]. It shows that the silicone prepolymer is successfully prepared. In addition, the stretching vibration peak of the methoxy group in Si-OCH3 at 2848 cm-1 in vinyltrimethoxysilane disappears in the prepolymer P-Si, and the antisymmetric stretching vibration peak of -CH3 in methoxy disappears at 2955 cm-1, suggesting further that the silicone prepolymer has been prepared. The absorption peak at 3200 cm-1 is attributed to the characteristic stretching vibration of hydroxyl (-OH) in diphenylsiliconediol. After the condensation reaction between a methoxy and a hydroxyl group, this peak disappears. The stretching vibration peak of C = C in Si-C = C in vinyltrimethoxysilane at 1601 cm-1 is red-shifted to 1605 cm-1 after the reaction. In addition, the absorption peak at 1430 cm-1 belongs to an aromatic ring vibration absorption in the phenyl silica chain segment (Si-Ph). The above results confirm the synthesis of the target prepolymer P-Si.
The peak located at 1020∼1150 cm-1 is the antisymmetric stretching vibration of Si-O-Si, which is a characteristic functional group of the polysiloxane. The peaks located at 2925 and 2862 cm-1 in the copolymer are the antisymmetric and symmetric stretching vibration of -CH2-, and this functional group in the copolymer mainly originates from the UA of the complex Tb(p-BBA)3(UA) and Zn (BTZ)(UA). The peak at 1701cm-1 is attributed to the carbonyl stretching vibration of the carboxylate in the complex. Its appearance in the copolymer indicates that the complex is successfully bonded to the polymer chain. The C = C stretching vibrational peak in the complex disappears in the copolymer, indicating that the C = C addition polymerization reaction has occurred between the complex and the prepolymer P-Si. In addition, the peak at 1425 cm-1 belongs to the benzene ring vibrational absorption of silyl-benzene chain. It displays a blue shift compared with 1430 cm-1 in the P-Si, indicating that the zinc complex reacts with the prepolymer P-Si. In summary, white-light silicone copolymer has been successfully synthesized.
3.2 UV-Vis absorption spectra
The copolymer was prepared as 1×10−4mol/L DMF solution, and the UV-Vis absorption spectra of the complexes and the copolymer P-Si-Eu-Tb-Zn were measured in the range of 200∼450 nm, as described in Fig. 3. The absorption peak at 270 nm originates mainly from the conjugated system in the complex Eu(TTA)2(Phen)(MAA) and the complex Tb(p-BBA)3(UA) and the benzene ring in the matrix materials, resulting from the π→π* electron transitions. The absorption peak at 341 nm is attributed to the n→π* electron transitions of the lone electron pairs in the carbonyl group (-C = O) of MAA and the thiophene group (C = S) of TTA in the complex Eu(TTA)2(Phen)(MAA). Additionally, the weak absorption peak at 390 nm is attributed to the intramolecular electron transfer of BTZ in the complex Zn(BTZ)(UA). The above results further indicate the successful preparation of the white light copolymer.
3.3 Thermal properties
Under a nitrogen atmosphere, the thermal weight loss process of the copolymer P-Si-Eu-Tb-Zn was recorded at a heating rate of 10°C/min, and the DTG curve was obtained by first order derivative, as shown in Fig. 4. The initial decomposition temperature of the white light copolymer is 235°C, and the early stage of the thermal weight loss is mainly attributed to the degradation of the small ligands in each complex, such as UA, MAA, and so on. The rate of thermal weight loss is the fastest at 546°C. The thermal weight loss at this stage is mainly attributed to the degradation of the polymer chain. The thermal decomposition is completed at 758°C, there was 76% of the copolymer remained, and the residue is subjected to infrared analysis, as shown in Fig. 5.The residue of the copolymer after thermal weightlessness is SiO2, because the main chain of the copolymer is Si-O, and in which the Si content is high, so leads to a large proportion of residues after the weight loss [21]. Meanwhile, the residue also contains some stable metal oxides, such as Eu3O2, Tb3O2 and ZnO. Table 1 shows the initial decomposition temperatures of the complexes and white light copolymer. The comparison indicates that the copolymer has been significantly improved compared with the complexes, up to 235°C, far exceeding the working junction temperature of 150° C in the LEDs, which is a kind of white light emitting materials with extremely stable temperature.
Table 1. The initial decomposition temperature of the complexes and the copolymer P-Si-Eu-Tb-Zn
3.4 Photoluminescence properties
At room temperature, the fluorescence spectra of the complexes were measured by using the solid disc method, as illustrated in Fig. 6. The complex Eu(TTA)2(Phen)(MAA) exhibits pure red emission at 611 nm, being ascribed to the transition of 5D0→7F2. The complex Tb(p-BBA)3(UA) exhibits pure green emission at 544 nm belonged to the transition of 5D4→7F5. Excited under the wavelength of 365 nm, the complex Eu(TTA)2(Phen)(MAA) exhibits five characteristic emission peaks of Eu3+ ions at 579, 591, 611, 650 and 689 nm, which are attributed to the 5D0→7FJ (J=0-4) transition. At the same time, under the excitation wavelength of 365 nm, the sharp emission peaks of the complex Tb(p-BBA)3(UA) appears at 488, 544, 584 and 619 nm, which are attributed to the f-f transition of Tb3+ ions from the 5D4 excited state to 7FJ (J=6-3), respectively [22]. In addition, the complex Zn(BTZ)(UA) emits blue light at 472 nm under the excitation wavelength of 365 nm.
The series of copolymers P-Si-Eu-Tb-Zn with stipulated molar feed ratios of 100:1 [1/2/3] = 1: 3: 1, 0.5: 4: 1, 0.5: 4: 1.5 or 0.5: 3: 1.5 were respectively identified as copolymer 1-4 [23]. Table 2 shows molar content and their ratios of the metals in copolymer 1-4 according to the feed ratios.
Table 2. Molar content and their ratios of the metals in copolymer 1-4
Figure 7 shows the fluorescence emission spectra of copolymer 1-4 under the excitation of 365 nm. From the spectra, it can be observed that with the change of the proportions of tricolor compounds of Eu(TTA)2(Phen)(MAA), Tb(p-BBA)3(UA) and Zn(BTZ)(UA), copolymer 1-4 exhibit different emission behavior excited by 365 nm wavelength. It is shown that the relative strength of the characteristic emission peaks in the complex changes in the copolymer, that is, the color in the copolymer changes with the ratio of the three color of red, green, and blue. The characteristic emission of the Eu3+ ions gradually decreases with the decrease of the Eu3+ ions, while the characteristic emission of Tb3+ ions increases with the increase of its content. The emission intensity in copolymer 1 and copolymer 2 is in accordance with content of the complex Zn(BTZ)(UA). While, the emission intensity in copolymer 3 and copolymer 4 seem to be inconsistent, and the emission intensity is basically the same after the baseline is flattened. Interestingly, the characteristic emission peak of the complex Zn(BTZ)(UA) is blue-shifted from the original 472 nm to 448 nm. This is the fact that when the complex Zn(BTZ)(UA) is incorporated to the polymer chain, the degree of molecular conjugation increases and the electron cloud density goes down, resulting in a blue shift of the emission peak. The chromaticity coordinates of copolymer 1-4 is presented in Fig. 8. The chromaticity coordinates of copolymer 1 are (0.403, 0.330), being located in the white light region closest to red light region. The proportion of different complexes was fine-tuned, copolymer 2-4 move from the yellow-green region to white light region. The final coordinates of copolymer 4 were (0.327, 0.321), which is closest to pure white light. Therefore, when the molar ratio of the Eu(III), Tb(III) and Zn(II) complexes is 0.5:3:1.5, copolymer 4 which was close to pure white light is obtained.
The emission spectra of copolymer 4 shows that the copolymer exhibits characteristic emission peaks of each complex under the excitation of 365 nm wavelength. The characteristic emission peaks of the complex Eu(TTA)2(Phen)(MAA) at 591 nm and 613 nm are attributed to the 5D0→7F1 energy level transition and the 5D0→7F2 energy level transition of Eu3+, respectively. The emission peaks at 542 nm and 488 nm are the characteristic emissions of the complex Tb(p-BBA)3(UA), corresponding to the 5D4→7F5 energy level transition and the 5D4→7F6 energy level transition of Tb3+ ions, respectively. The blue emission peak at 448 nm belongs to the characteristic emission of the complex Zn(BTZ)(UA), which is derived from the contribution of the small molecule ligand BTZ. The suitable ratio of three complexes lead to the balance of red, green and blue, and then compound to get the white light.
In order to further study the fluorescence properties of copolymer 4, different excitation wavelengths of 290, 360, 365, 370 and 390 nm were used to measure the fluorescence emission spectra of copolymer 4, as shown in Fig. 9(a). The blue emission at 448 nm gradually enhanced with the increase of the excitation wavelength. It is the fact that the molecular conjugation system increases after the polymerization of the blue complex and the prepolymer P-Si. It makes the emission wavelength of the small molecule ligand BTZ exhibits blue shift from 472 nm to 448 nm, and the optimal excitation wavelength shifts from 370 nm to 390 nm, leading to the gradual enhancement of the blue emission peak at 448 nm. The emission peak at 542 nm is attributed to the characteristic emission of 5D4→7F5 of the Tb3+, having the strongest emission excited by the wavelength of 365 nm. This is because that the optimum excitation wavelength of the complex Tb(p-BBA)3(UA) is located at 363 nm. Similarly, the strongest emission of Eu3+ ions at 591 nm and 613 nm is the strongest excited by the optimal excitation wavelength of 365 nm of the complex Eu(TTA)2(Phen)(MAA). The luminescent intensity is weaker at the excitation wavelength of 290 nm and 390 nm. This is because that the complex Eu(TTA)2(Phen)(MAA) cannot excited at 290 nm and 390 nm, which is known from the excitation spectra of the complex Eu(TTA)2(Phen)(MAA). Above all, the emission behavior of copolymer 4 excited at different wavelength is affected by the excitation range of the complex.
Fig. 9. The fluorescence emission spectra (a) and CIE chromaticity coordinates (b) of copolymer 4 excited by different excitation wavelength
Figure 9(b) gives the CIE chromaticity coordinate diagram of copolymer 4 at different excitation wavelength. It can be seen that when the excitation wavelength changes from 290∼390 nm, the CIE coordinates of copolymer 4 are gradually reaching the white region from the yellow region, and then moving to the blue region. When excited at 365 nm, the color coordinates are located (0.327, 0.321), being closest to the pure white light (0.330, 0.330). Combined with the emission spectra of Fig. 9(a), when copolymer 4 is excited by 390 nm, the blue emission peaks at 448 nm are the strongest and occupy the largest proportion, leading to the CIE coordinates locate at the blue region. It is not difficult to see that the CIE coordinates are closely related to the excitation wavelength. To sum up, white light is obtained in copolymer 4 excited by the wavelength of 365 nm.
3.5 Fluorescence lifetime and quantum yield
Fluorescence lifetime of copolymer 4 was recorded employing a TCSPC method. The fluorescence decays process of the 5D0-7F2 transition of Eu3+ and the 5D4-7F5 transition of Tb3+ were monitored at the excitation wavelength of 365 nm. According to Eq. (1), the fluorescence decay curves were fitted employing double exponential model, and the average lifetime is calculated by Eq. (2), as illustrated in Fig. 10.
The overall quantum yield(φfx) of copolymer 4 at room temperature was also measured in a 103 mol/L DMF solution based on Eq. (3) using Eu(TTA)3phen as a reference
3.6 EL emission spectra of fabricated LEDs
To investigate the practical application of copolymer 4, it was encapsulated in LED lamp that excited by a 1w 365 nm UV chip and the electroluminescence spectra was tested at room temperature at 3.3 V, as shown in Fig. 11(a), corresponding CIE chromaticity coordinates are shown in Fig. 11(b). The inset is a photograph of the lamp being lit up at 3.3 V, the devices emits bright white light. Comparing with the fluorescence spectra of copolymer 4 at 365 nm, it is found that the electroluminescence spectra of its LED devices is almost identical, including the peak shape, peak position, and relative strength of the peak, which indicated that copolymer 4 could match the 365 nm UV chips well. In addition, the chromaticity coordinates of the devices are (0.325, 0.329), and it is also close to the chromaticity coordinate (0.327, 0.321) of the photoluminescence spectra of copolymer 4. The correlated color temperature is 5967 K and the color rendering index is 91. Compared with the mainstream of the current LED devices fabricated by “blue chip + YAG powder”, whose correlated color temperature is generally about 5500 K, while the color rendering index about 80 [25], copolymer 4 has potential application in white light LEDs.
Fig. 11. (a) EL emission spectra of the fabricated LEDs with copolymer 4 and (b) corresponding CIE coordinates (inset is the image of fabricated LEDs with copolymer 4 under 3.3 V)
3.7 UV aging analysis
In order to verify the anti-ultraviolet aging of the white light copolymer, the prepared white light LED devices with copolymer 4 is connected to the DC power supply with constant current and constant voltage and is electrified at a voltage of 3.3 V for one month to compare the performance difference before and after the aging of the LED devices. Figure 12(a) shows the electroluminescence spectra of the LED devices before and after the aging. Table 3 shows the parameters corresponding to the devices. It can be seen that after aging, the electro-optical spectra do not change significantly, the chromaticity coordinates shift from (0.325, 0.329) to (0.341, 0.348), the color rendering index changes from 91 to 88, and the correlated color temperature changed from 5967 K to 5612 K. No significant changes have occurred in all parameters. Before and after aging, the brightness of the white light LED devices varies with the driving voltage, as shown in Fig. 12(b). As the driving voltage increases, the brightness of the LED devices gradually increases. After aging, the brightness of the devices lost only 10.4% at 3.6 V. The above results indicate that copolymer 4 has good property of anti-ultraviolet aging.
Fig. 12. (a) The EL spectra of the fabricated LEDs with copolymer 4 before and after UV aging; (b) Luminance-voltage curves of fabricated LEDs.
Table 3. Performances of fabricated LEDs with copolymer 4 before and after aging
4. Conclusions
A new type of anti-ultraviolet organic silicon copolymer phosphors has been obtained by bonding tricolor complexes into a silicone matrix. By changing the ratio of the trichromatic complexes or adjusting the excitation wavelength, the light color of the copolymer can be changed. When the ratio of three primary colors is 0.5:3:1.5, copolymer 4 with the closest white light has been synthesized. Its CIE chromaticity coordinate is (0.327, 0.321) under the excitation of 365 nm. When the excitation wavelength varies from 290 nm to 390 nm, the emission of copolymer 4 achieves a gradual change from yellow light to white light to blue light. The initial decomposition temperature of the copolymer is 235°C, far exceeding the working junction temperature of the LEDs (about 150°C). The LED devices encapsulated with copolymer 4 emit bright white light at 3.3 V and exhibits an electroluminescence spectra that is similar to its fluorescence spectra. The CIE chromaticity coordinates (0.325, 0.329) and color rendering index are 91. The color temperature is 5967 K, indicating that the copolymer can be well applied to the LEDs with 365 nm chips. After aging, the CIE chromaticity coordinates become (0.341, 0.348), the color rendering index changes to 88, the correlated color temperature decrease to 5612 K, and the brightness loss is only 10.4%, indicating that the copolymer has good UV aging resistance.
Funding
Natural Science Foundation of Shanxi Province (201801D221124, 201901D111111); Shanxi Provincial Key Research and Development Project (201803D31042); National Natural Science Foundation of China (21972103).
Disclosures
The authors declare no conflicts of interest.
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