White light-emitting diodes of high color rendering index with polymer dot phosphors

Abstract

Nowadays the yellow emissive YAG:Ce phosphors play an important role in fabricating white light-emitting diodes (WLEDs). However, they are deficient in red emission so we proposed polymer dot phosphors to compensate this poor photometric property. The polymer dot phosphors exhibited broad emission bands under excitation of a blue LED chip with weak reabsorption. We demonstrated hybrid white light-emitting diodes with commendable color rendering index (85–95), widely variable color temperatures (3050–7295 K) and high luminous efficacy (72–80 lm W⁻¹ operated at 20 mA) by combining green and red emitting polymer dots on YAG:Ce-based WLEDs. The optical properties suggest the polymer dot phosphors are the suitable color converting materials, which may enable their application in WLED lighting.

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Introduction

Solid-state white light-emitting diodes (WLEDs) have been rapidly growing as promising lighting sources, which possess many favorable optical properties including long lifetime, fast response, low power consumptions, and high luminous efficiency.^1–5 Most commercially available WLEDs are made by adding color conversion phosphors to blue-emitting LED chips, and the emissions from the phosphors and the LED chips are combined to form white light. Blue LEDs such as GaN and related III-nitrides have attracted considerable attention in optoelectronic devices. Pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) technologies have been employed to obtain high-performance and high-power LEDs with relatively low cost and unconventional substrates.^6–8 Of those phosphors, the yellow-emitting color conversion phosphors, such as Y_2.94Al₅O₁₂:Ce³⁺ (YAG:Ce),⁹ Ba₃SiO₅:Eu²⁺,¹⁰ Sr₃B₂O₆:Eu²⁺,¹¹ are widely adopted for WLEDs.^12–14 Although this type of WLEDs show a high luminous efficacy and have been successfully commercialized,¹² it is still difficult to obtain a high color rendering index (CRI) on account of their lack of color in the green and red regions.^15,18 Therefore, red-light-emitting phosphors have been synthesized to improve the CRI and generate warm light for general indoor lighting.

The ideal red-light-emitting doped quantum dots (QDs) to be incorporated into white LEDs should have a broad emission band covering the red spectral region under excitation of the blue LED chip and simultaneously show no absorption of green-yellow light from the YAG:Ce phosphors. The doped quantum dots (Cu- and Mn-doped ZnSe QDs) have recently been reported to be efficient emitters covering the blue to orange color window, with emission wavelength shorter than 610 nm;¹⁶ thus they were not ideal emitters to guarantee a sufficient red component. And the absorption band of doped QDs was ranged from 475–1100 nm, indicating an efficient absorption of the emission of YAG:Ce.¹⁷ Polymer dots (PDs) possess broad emission bands (full width at half maximum, FWHM, ∼100 nm), strong absorption in the blue light region (455 nm), weak absorption in yellow-green light emitted by YAG:Ce and nontoxic eco-friendly properties.^18–24 In this work, we described an approach to obtain the green- and red emitting PDs in a highly transparent polyvinyl pyrrolidone (PVP) matrix, the photostability and thermal stability were greatly improved compared with the previous report.²⁵ Then we fabricated WLEDs and corrected the emission spectra of YAG:Ce with the addition of green- and red emitting PDs. These devices exhibited high CRI, adjustable CCT and excellent luminous efficacy, suggesting a promising way to manufacture warm WLEDs for lighting.

Experiment section

Materials

Poly(styrene-co-maleic anhydride) (PSMA, MW ≈ 1700, 68% styrene content) and anhydrous tetrahydrofuran (THF, 99.9%) were purchased from Sigma-Aldrich. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}-thiadiazole)] (PFBT, MW ≈ 100 000–157 000) was obtained from ADS Dyes (Quebec, Canada). Polyvinyl pyrrolidone (PVP, MW ≈ 40 000) was achieved from Beijing Chemical Reagent Company. YAG:Ce phosphor was acquired from Intematix. UV glue NOA60 was ordered from LIENHE Fiber Optics. All chemicals were used without further purification.

Preparation of PDs

PDs were prepared by using a modified precipitation method.^26–28 Briefly, the red-emitting polymers PF-DBT5 was synthesized by a palladium-catalyzed Suzuki coupling reaction with adjusting the feed ratios of 9,9-dioctylfluorene (DOF) to 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DBT) (95 : 5).²⁹ Then the amphiphilic PSMA (10 ppm), in combination with green-emitting PFBT or red-emitting polymer-blend dots (PBdots) (500 ppm) were mixed in THF (20 mL) to fabricate PDs, where PBdots were synthesized by using the donor of a visible-light-harvesting polymer (PFBT) and the acceptor of an efficient deep-red emitting polymer (PF-DBT5) at a weight ratio of 0.6 : 1. Then the solution (3 mL) was quickly injected into deionized water (10 mL) in a bath sonicator and the mixture was sequentially sonicated for 3–5 min. Finally, the THF in the mixture was removed by nitrogen flowing on a hotplate. The resulting solution was concentrated by continuous heating, followed by filtration through a 0.22 μm filter to remove larger particles.

Preparation of PD phosphors

PD solution (containing 0.05 mg mL⁻¹ PFBT or 0.03 mg mL⁻¹ PBdots) was mixed with PVP in water at a 3 : 1 mass ratio by ultrasonic treatment for 2 hours. Then the mixture was dried on a 90 °C hotplate for 10 hours. Dried slices were ground in a porcelain mortar and sifted through a 100 mesh sieve in order to obtain fine powders of PD/PVP phosphors.

Fabrication of WLEDs

GaN LED chips were purchased from Sanan Optoelectronics. Its peak emission wavelength was 455 nm and the power density was 628.4 mW cm⁻² at 20 mA operating current. A given mass of YAG:Ce or PD/PVP phosphors or both were mixed with the UV glue to form a homogeneous mixture by vibration and sonication for 30 min.³⁰ The glue/phosphors were then applied on the blue GaN chips layer by layer and baked 1 min each layer under 365 nm ultraviolet light irradiation.

Characterization

The absorption spectra were characterized with TU-1810 spectrophotometer (Shimadzu). The absolute photoluminescence quantum yields (PLQYs) were recorded by a USB-4000 fiber spectrometer (Ocean Optics) with an integrating sphere. The emission spectra of the WLEDs were measured using an Omni-λ300 monochromator/spectrograph (Zolix). The images of the morphology of the PDs were taken using a transmission electron microscope (TEM) TECNAIF20.

Results and discussion

The absorption spectra band of PDs exhibited the strong absorption at 455 nm and 475 nm (Fig. 1a). Their FWHMs were broad (100 nm for G-PD and 90 nm for R-PD), which enabled them to produce high CRI WLEDs. The solution PLQYs of the two PDs were 30% and 56%, respectively. According to our previous work, the solid state PD/PVP phosphors' PLQYs almost remained unchanged.¹⁸ The emission peaks of PD/PVP films, YAG:Ce powder, and blue chips were depicted in Fig. 1b. The strong PL peaks of G-PD and R-PD phosphors were achieved at 540 nm and 650 nm, respectively. Compared to the absorption and the PL emission peak of the PD solutions (Fig. 1a), no shifts were observed for the PD/PVP films.

Fig. 1 (a) The solution PL spectra and absorption spectra of G-PD (green line) and R-PD (red line) excited by 405 nm. (b) PL emission spectra of PD/PVP films and YAG:Ce powder, EL spectra of blue-emitting LED chip (blue line); inset: real photos of PD/PVP and YAG:Ce phosphors under UV light. (c) and (d) TEM images of G-PD and R-PD phosphors, respectively. The chemical structures of (e) PFBT and (f) PF-DBT5.

Their emission spectra matched well with the electroluminescence (EL) spectrum of blue chips for white light generation, which possessed well-balanced broad-spectra emission that spanned the majority of the visible spectrum. The inset of Fig. 1b shows the corresponding images of PD/PVP and YAG:Ce phosphors under UV light. As shown in the transmission electron microscopy (TEM) images of Fig. 1c and d, the average particle sizes of G-PD and R-PD were 23 and 15 nm, respectively. Their chemical structures are shown in Fig. 1e and f.

In order to improve the CRI of commercial YAG:Ce LEDs, we firstly employed red emitting phosphors on these devices to compensate the missing red waveband portion. The WLEDs were fabricated using 0.032 g YAG:Ce mixed with 0.02 g R-PD/PVP in 100 μL UV glue, which was vibrated 20 min to get a homogeneous mixture, then coated on blue LED chips. Fig. 2a shows the output spectra of the blue LED chips coated by YAG:Ce with (red line) and without (black line) R-PD/PVP phosphor.

Fig. 2 (a) EL spectra of blue LED chips coated with YAG:Ce (black line) and R-PD/PVP (red line) solid phosphors. (b) CIE 1931 chromaticity diagram of WLEDs based on YAG:Ce phosphor with and without R-PD/PVP, the black line is the Planckian locus.

The color temperature (T_c) and the Commission International de I'Eclairage (CIE) color coordinates of the two WLEDs were mapped in Fig. 2b. It can be seen from the CIE chromaticity diagram (Fig. 2b) that the dual-phosphor device was closer to the standard white light of CIE color coordinates (0.33, 0.33)³¹ compared with the single YAG:Ce phosphor device.

Table 1 summarized the changes of WLED properties in CIE coordinates (x, y), color rendering index (CRI), color temperature (T_c) and luminous efficiency (LE) of the dual-phosphor WLEDs operated at 20 mA. The CRI of the WLED based on binary phosphors with a CIE color coordinates (0.334, 0.332) was much higher than the single phosphor device (0.333, 0.359). The luminous efficacy decreased from 99.8 to 85.5 lm W⁻¹ with R-PD/PVP.

Table 1 Lighting properties of the single and dual-phosphor WLEDs operated at 20 mA

Device	CIE coordinates (x, y)	CRI	Tc (K)	LE (lm W^−1)
YAG:Ce	(0.333, 0.359)	62	5462	99.8
YAG:Ce + R-PD/PVP	(0.334, 0.332)	85	5430	85.5

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To fabricated WLEDs with an even higher CRI for indoor lighting, a ternary combination (G-PDs + YAG:Ce + R-PDs) was employed. Because PDs possessed broad FWHMs similar to YAG:Ce phosphor, WLEDs made with the three phosphors enabled variation of three broad-band emitters, giving access to a larger spectral overlap among their emissions and better optical properties. Fig. 3a shows the EL spectra of (1) blue LED chips with a mixture of G-PD/PVP, YAG:Ce and R-PD/PVP (mass ratio of 1 : 1.6 : 2.2), (2) blue LED chips with YAG:Ce and G-PD/PVP, and (3) blue LED chips with R-PD/PVP. The three devices were all operated at 20 mA. Three major emission peaks were clearly resolved at 455, 540, and 650 nm, respectively. And then we investigated the impact of R-PD on optical properties (CIE color coordinate and CCT). Fig. 3b presents CIE color coordinates of several devices based on binary (YAG:Ce + G-PD) and ternary (YAG:Ce + G-PD + R-PD) combinations, respectively. It is apparent that the CIE color coordinates of these devices based on binary phosphors were not in white-light region. With the increasing of R-PD's concentration in the mixture from 1.6 : 1 : 1.1 to 1.6 : 1 : 1.98 (mass ratios of YAG:Ce : G-PD : R-PD), the emitting light color of the devices shifted from yellowish green to warm-white of ideal WLEDs, corresponding to the tunable CCT along the Planckian locus from 5472 K to 3050 K. Fig. 4a shows the EL spectra of the blue LED chips covered with ternary phosphors (G-PD, YAG:Ce, R-PD) by different mass ratios (Table 2). The WLEDs exhibited increased emission in green and red regions of the spectra, compared to a commercial WLEDs based on YAG:Ce phosphors as shown in the inset of Fig. 4a. By increasing the content of red emitting phosphor (R-PD) in the mixture, the spectra of devices were like to sunlight. Table 2 shows seven fabricated white light devices operated at 20 mA with CRI values from 85 to 95 and tunable CCT from 3633 K to 7295 K.

Fig. 3 (a) EL spectra of blue LED chips with a mixture of G-PD, YAG:Ce and R-PD, blue LED chips covered with R-PD and blue LED chips covered with YAG:Ce and G-PD. (b) CIE 1931 chromaticity diagram of binary (G-PD, YAG:Ce) and ternary (G-PD, R-PD, YAG:Ce) phosphors based WLEDs, which were under various CCT from 5472 K to 3050 K in white light region by adding R-PD.

Fig. 4 (a) EL spectra of blue LED chips covered with G-PD, YAG:Ce and R-PD with diverse mass ratios. Inset: EL spectra of blue LED chips coated with three phosphors at 7295 K (red line) and YAG:Ce only (black line). (b) CIE 1931 chromaticity diagram color coordinates of WLED devices 1–7. (c) The as-coated three phosphors on blue LED chips. (d) The real-color images of WLED devices 1–7 operated at 20 mA.

Table 2 CIE coordinate (x, y), CRI, color temperature (T_c), luminous efficiency (LE) and mass ratios (YAG:Ce : G-PD : R-PD) of 7 WLED devices (operated at 20 mA)

Device	CIE (x, y)	CRI	Tc (K)	LE (lm W^−1)	Mass ratio
1	(0.385, 0.349)	88	3633	72.0	1.6 : 1 : 1.98
2	(0.373, 0.341)	91	3925	74.5	1.6 : 1 : 1.7
3	(0.370, 0.352)	93	4113	75.7	1.6 : 1 : 1.52
4	(0.347, 0.315)	95	4735	76.6	1.6 : 1 : 1.41
5	(0.333, 0.336)	94	5472	77.8	1.6 : 1 : 1.3
6	(0.314, 0.367)	85	6244	78.9	1.6 : 1 : 1.18
7	(0.297, 0.322)	86	7295	80.0	1.6 : 1 : 1.1

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The CIE 1931 chromaticity diagram coordinates of the devices were (0.297–0.385, 0.315–0.369) in the white region as shown in Fig. 4b. Furthermore, the fabricated devices possessed luminous efficiency of 72–80 lm W⁻¹ operated at 20 mA and it is acceptable for indoor lighting. Fig. 4c shows the real devices which coated three phosphors on blue LED chips, with the various CCT of 3633 K, 3925 K, 4113 K, 4735 K, 5472 K, 6244 K, and 7295 K. The photographs of 7 WLEDs exhibited the variable lighting colors as shown in Fig. 4d. It was clearly observed that the first two devices display the warm white light, the middle three devices show the pure white light and the right two devices demonstrate the cold white.

The polymers generally have the poor photostability and thermal stability,^32,33 therefore many matrixes were proposed to improve them.²⁵ In our work, we introduced an approach using PVP to produce polymer nanoparticles with improved photostability and thermal stability. The PDs are dispersed in the PVP network and the PVP is able to protect the PDs. The emission spectra of the fabricated WLED were captured every 20 °C, which was operated at 350 mA as shown in Fig. 5a. It was observed that when the temperature increased from 20 to 120 °C, the emission peak of red polymer had a slight red-shift of about 2 nm. Fig. 5b exhibits some degradation of the emission intensity of WLED at different working time, which was mainly due to the increase of the surface temperature of the blue chips. The intensity decreased about 50% after continuous 120 h irradiation at 350 mA. We obtained the PDs in a highly transparent PVP matrix and the intensity was decreased only 10% after continuous 6 h irradiation as shown in the inset of Fig. 5b. The results indicate that the photostability and thermal stability are significantly improved comparing to a previous report adapting SiO₂ encapsulation.²⁵

Fig. 5 (a) The EL spectra of the fabricated WLED as a function of the GaN chip temperature range from 20 to 120 °C. (b) The EL intensity of the WLED as a function of the working time at operating current 350 mA. The intensity decreased about 50%. Inset: EL intensity decreased about 10%.

Conclusions

In summary, we described an approach to obtain nontoxic green and red emitting polymer dot phosphors in a highly transparent PVP matrix, the photostability and thermal stability have been greatly improved and they were employed for the fabrication of WLEDs with high optical properties. Firstly, we demonstrated the R-PDs were able to compensate the missing red region of YAG:Ce in visible spectrum and CRI was obviously improved from 62 to 85 at similar CIE coordinates and CCT. And then the G-PD and R-PD were applied on the blue LED chips coated with dual phosphors, these fabricated WLEDs exhibited excellent color rendering property including CRI up to 95, tunable CCT from 3050 K to 7295 K and luminous efficiency up to 80 lm W⁻¹. It suggests that the combination of YAG:Ce and PDs on blue LED chips can be a good solution to obtain high optical performances such as high CRI and wide range of tunable color temperatures.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51272084, 61306078, 61225018, 61475062), the Jilin Province Key Fund (20140204079GX), the State Key Laboratory on Integrated Optoelectronics Fund open topics (IOSKL2014KF18), NSF (1338346), and BORSF RCS (LEQSF(2015-18)-RD-A-16).

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