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White emission using mixtures of CdSe quantum dots and PMMA as a phosphor
Optical Materials 32 (2010) 515–521
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White emission using mixtures of CdSe quantum dots and PMMA as a phosphor Wonkeun Chung, Kwanhwi Park, Hong Jeong Yu, Jihyun Kim, Byung-Hee Chun, Sung Hyun Kim * Department of Chemical and Biological Engineering, Korea University, 1 Anam-Dong, Seongbuk-Ku, Seoul 136-713, Republic of Korea
a r t i c l e
i n f o
Article history: Received 22 June 2009 Received in revised form 9 November 2009 Accepted 12 November 2009 Available online 22 December 2009 Keywords: White emission White LEDs CdSe quantum dots Phosphor
a b s t r a c t White light emitting diodes (LEDs) were fabricated using an InGaN 460 nm blue emission LED chip as the excitation source and CdSe quantum dots dispersed in PMMA as the phosphor. CdSe quantum dots were synthesized by the wet chemical method using CdO and Selenium powder as precursors. The three different size, 2.9, 3.4 and 4.3 nm in diameter, of CdSe quantum dots obtained using this method exhibited emission peaks at 555, 580 and 625 nm, respectively with a quantum yield of 10–30%. Mixed phosphors containing different weight ratio of CdSe and PMMA (1:0.1, 1:1, 1:5 and 1:10 wt%) were deposited on the LED chip to investigate the effects of different weight ratios of CdSe and PMMA on the performance of the white LEDs. The fabricated white LEDs that contained CdSe and PMMA weight ratio at 1:10 showed the best performance and the CIE color coordinates varied less with different applied currents. The luminous efficiency of single phosphor (580 nm CdSe) white LEDs was 5.62 lm/W with a CRI of 15.7, whereas the luminous efficiency of dual phosphors (555, 625 nm CdSe) white LEDs was 3.79 lm/W with a CRI of 61.4 at 20 mA. The CIE coordinates of single and dual phosphors white LEDs varied from (0.33, 0.28) to (0.29, 0.26) and from (0.39, 0.33) to (0.39, 0.32), respectively, when the working current ranged from 5 to 80 mA. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction White light emitting diodes (LEDs) have recently attracted a significant amount of attention due to their high efficiency, long life time (100,000 h), and reliability. White LEDs hold the potential to replace conventional light source in a wide range of applications such as incandescent bulbs or fluorescent lamps, full color displays and traffic signals. Two approaches have been commonly used to generate white emission. One is to mix red, green, and blue emitting color LED chips and the other is to combine UV or blue LED chips with phosphors. Multichip white LEDs show high efficiency and a good color rendering index (CRI). However, due to the high cost and complicated lighting system required to control the light intensity, this approach is not considered viable by current standards. The phosphor conversion white LEDs are a promising method for the development of a simple and cost effective white light emission. It has a high luminescence efficiency and flux, high color stability and low production cost. In commercial white LEDs, a blue emitting InGaN LED chip is used as the optical excitation source and this is combined with a yellow emitting Ce activated yttrium aluminum garnets (YAG:Ce), which is used as the phosphor. When power is applied to the InGaN chip, blue light is emitted from the chip and some of the blue light is absorbed by YAG:Ce, which re* Corresponding author. Tel.: +82 02 3290 3297; fax: +82 02 926 6102. E-mail address: [email protected] (S.H. Kim). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.11.005
emits yellow light. The mixing of blue and yellow emission produces white light. However, these white emission, using YAG:Ce phosphor, has a low color rendering index (CRI) due to the lack of red emission component and the color purity varies with input power [1]. Due to these problems, various phosphors, such as inorganic [2–5], organic polymer [6], OLED material [7] and quantum dot [8–11] have been examined in white LEDs. Among these materials, CdSe [12], CdS [13], and ZnO [14] II–VI semiconductor quantum dots have been regarded as suitable candidates due to their unique properties, such as high quantum yield, minimal backscattering and tunable band gap. In particular, CdSe quantum dots can emit the full visible spectrum by tuning the band gap. The II–VI semiconductor quantum dots have broad absorption peak. Therefore, the quantum dots can be excited by any optical pumping sources which have larger band gap energy than the quantum dots. Thus quantum dots and transparent polymer composites such as CdSe/ ZnS–PLMA (polyaurylmethacrylate) [15] and CdSe/ZnS–PMMA (polymethylmethacrylate) [16] by direct polymerization or quantum dots and fluorescent polymer [17,18] composites have been combined with an LED chip, where the quantum dots act as the phosphor to create a novel white LEDs. However previous studies have not reported the effects of the amount of polymer as a mixing phosphor on the properties of white LEDs. In this study, the different weight ratios of CdSe quantum dots and PMMA were prepared as mixing phosphors by solvent mixing
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Fig. 1. The process of white LEDs fabrication using CdSe–PMMA mixture as phosphors.
Fig. 2. HRTEM image of the CdSe quantum dots. (a) Reaction at 140 °C, (b) reaction at 190 °C, (c) reaction at 220 °C and (d) mixing with PMMA (1:10 wt%).
to investigate the effects of the polymer. That mixing method can minimize the decreasing of the emission intensity of quantum dot in polymer matrix than direct polymerization [15,16]. White LEDs were fabricated by combining a 460 nm InGaN LED chip with single (580 nm) or dual (555, 625 nm) CdSe quantum dots mixed with PMMA. The luminescence properties of white LEDs were defined by the luminous efficiency, CIE-1931 coordinate and color rendering index (CRI).
2. Experimental 2.1. Synthesis of CdSe quantum dots CdSe quantum dots were synthesized based on previously reported techniques with slight modifications [19]. All reagents were purchased from Sigma–Aldrich and used without further purification. CdO (0.45 g) and stearic acid (8 g) as a capping ligand were
W. Chung et al. / Optical Materials 32 (2010) 515–521
mixed in a three-necked flask and heated to 150 °C for 1 h under nitrogen to prepare the cadmium precursor. After decomposition of CdO, the mixture solvent, 12 g of 1-heptadecyloctadecylamine (HDA) and 8 g of Trioctylphosphine Oxide (TOPO), was added and stirred together at 120 °C for 30 min. At this temperature, the Selenium precursor, which was 0.78 g of Selenium powder dissolved in 8.9 mL of Trioctylphosphine (TOP, 90%), was swiftly injected into the reaction flask. The nuclei formed quickly and several different sized monodisperse CdSe quantum dots were obtained by slowly increasing the growth temperature from 120 to 140, 190 and 220 °C for the desired dot size. After the reaction was complete, CdSe quantum dots were precipitated with methanol for purification. The precipitate was separated by centrifugation and washed out by methanol and chloroform several times to remove unreacted reagents and excess TOPO or HDA. 2.2. Preparation of CdSe quantum dots–PMMA mixtures
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2.3. Fabrication of white LEDs White LEDs were fabricated using a surface mounted device (SMD) type of the InGaN/GaN multiple quantum well structure blue emission LED and CdSe quantum dots. The size of the chip was 1 mm 1 mm and the emission peak of the LED chip was 460 nm with a CIE-1931 color coordinate of (0.14, 0.05). The LED chip was placed on the mold and prepared CdSe quantum dots and PMMA mixtures were directly deposited into the mold for uniform shape of phosphor coating layer (Fig. 1). The mold was composed of acetal plastic and the size of the mold was 10 mm (length) 10 mm (width) 8 mm (height). After coating, the solvent was slowly dried in a vacuum oven at 80 °C for 30 min. 2.4. Measurements
Four different weight ratios of CdSe and PMMA mixtures were obtained by solvent based mixing. Five milligrams of purified CdSe quantum dots were completely dissolved in 1 mL of chlorobenzene and then 0.5, 5, 25 and 50 mg of PMMA were added to the CdSe quantum dots solution. The molecular weight of PMMA (from Sigma–Aldrich) was approximately 996,000 by GPC and glass Transition Temperature (Tg) was 125 °C. The mixed solution of CdSe quantum dots and PMMA vigorously stirred for 10 h at room temperature [20].
The size and shape of CdSe quantum dots were observed using transmission electron microscope (Tecnai 20) operating at 200 KV. Quantum dots were dispersed in chloroform and dropped on the 200 mesh carbon–copper grids to prepare the sample. The optical properties of quantum dots were characterized by UV–vis absorption (UV mini 1240, Shimadzu) and photoluminescence (PL) spectra (F-7000 FL spectrophotometer of xenon-lamp source with 365 nm) at room temperature. The samples were prepared by dispersing CdSe quantum dots in chloroform. Fabricated white LEDs were analyzed with CDS 1100 CCD Array Spectrometer (Labsphere) by normal direction.
Fig. 3. (a) Absorption spectrum of three different sized CdSe quantum dots. (b) PL emission spectrum of three different sized CdSe quantum dots. The PL peak shifted to red as the dot size increased.
Fig. 4. Electroluminescence spectrum of white LEDs fabricated by combining lnGaN with (a) single CdSe quantum dots phosphor and (b) dual CdSe quantum dots phosphors PMMA to CdSe quantum dot ratio of 0.1. (Inset: emission light at 20 mA.)
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Fig. 5. Electroluminescence spectrum of single phosphor white LEDs fabricated at different PMMA to CdSe quantum dot ratios; (a) 1-fold, (b) 5-fold and (c) 10-fold of PMMA compared to the mass of CdSe quantum dots. (Inset: emission light at 20 mA.)
Table 1 The variation in CIE coordinates of single phosphor white LEDs containing PMMA to CdSe quantum dot mass ratios of 1, 5, and 10-fold PMMA at the indicated applied currents. Applied currents (mA)
1-Fold PMMA
5-Fold PMMA
10-Fold PMMA
5 10 20 40 60 80
(0.27, (0.25, (0.24, (0.24, (0.23, (0.21,
(0.29, (0.29, (0.29, (0.29, (0.27, (0.25,
(0.33, (0.33, (0.33, (0.32, (0.30, (0.30,
0.23) 0.21) 0.18) 0.17) 0.16) 0.17)
0.26) 0.26) 0.25) 0.25) 0.24) 0.22)
0.28) 0.28) 0.27) 0.26) 0.26) 0.26)
3. Results and discussion 3.1. The properties of CdSe quantum dots Highly luminescent CdSe quantum dots were synthesized by a fast nucleation and slow growth process. As shown in Fig. 2, CdSe quantum dots that were 2.9, 3.4 and 4.3 nm in diameter, estimated from HRTEM, were obtained when the reaction temperature was maintained at 140 °C for 150 min, 190 °C for 200 min and 220 °C for 450 min, respectively. TEM image of CdSe quantum dots after mixing with PMMA (1:10 wt% ratio) is shown in Fig. 2(d). CdSe and PMMA mixtures were formed about cluster of 25 nm and CdSe quantum dots were scattered in the PMMA matrix. Fig. 3 shows the normalized absorption (UV–vis) spectra and corresponding photoluminescence (PL) spectra of CdSe quantum dots. The high and sharp first absorption peak in the absorption spectrum of the CdSe
quantum dots (Fig. 3(a)) indicates that the synthesized quantum dots were highly mono-dispersed and homogeneous. As shown in the PL spectra of Fig. 3(b), as the particle size increases, the emission peak of the quantum dots becomes shifted to longer wavelength due to the quantum confinement effect. The emission peaks of the 2.9, 3.4 and 4.3 nm diameter CdSe quantum dots were 555 nm for yellowish-green, 580 nm for yellow and 625 nm for red. The quantum yields of CdSe quantum dots in chloroform solution were estimated comparing the integrated emission intensity to that of Rhodamine 6G in ethanol (reference, QY = 0.95) at identical optical density (0.015 OD). The quantum yields were calculated by a following equation:
QY QD ¼
IQD ODref QY ref Iref ODQD
ð1Þ
IQD and Iref are the integrated emission band, ODQD and ODref are the optical density at the 365 nm excitation wavelength. The quantum yield, based on Eq. (1) were 23.1% (2.9 nm), 19.9% (3.4 nm) and 17.3% (4.3 nm). 3.2. Performance of fabricated white LEDs White LEDs were fabricated by combining a 460 nm emission blue LED, which was used as the excitation source, with single 580 nm emitting CdSe quantum dots (referred to as single) or a combination of 555 and 625 nm emitting CdSe quantum dots (referred to as dual), which was used as the phosphor. In the dual phosphors device, the weight ratio of the 555 and 625 nm emitting CdSe quantum dots was 30:1. The different weight fractions of
W. Chung et al. / Optical Materials 32 (2010) 515–521
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Fig. 6. Electroluminescence spectrum of dual phosphor white LEDs fabricated at different PMMA to CdSe quantum dot ratios; (a) 1-fold, (b) 5-fold and (c) 10-fold of PMMA compared to the mass of CdSe quantum dots. (Inset: emission light at 20 mA.)
CdSe quantum dots and PMMA mixtures were prepared. The CdSe quantum dots to PMMA weight ratios used in this study were 0.1, 1.0, 5.0 and 10.0. Fig. 4 shows the electroluminescence spectra of fabricated white LEDs consisting of a blue LED and single or dual CdSe quantum dots mixed at a 0.1 ratio with PMMA. At this PMMA concentration, the CdSe quantum dots were hardly excited due to the aggregation of quantum dots on the LED chip, and a strong bluish light from the InGaN LED chip was detected near 460 nm. The CIE-1931 coordinates of the single and dual phosphors devices were (0.18, 0.12) and (0.21, 0.17) under a forward bias of 20 mA. The electroluminescence spectra of the single phosphor white LED at different CdSe quantum dot and PMMA weight ratios are shown in Fig. 5. As the mixing ratio of PMMA was increased, the dispersivity of the CdSe quantum dots in the PMMA matrix was improved and as a result the excitation efficiency of the quantum dots and luminous efficiency of the white LEDs was enhanced. The luminous efficiency of white LEDs containing PMMA at weight ratios of 1.0, 5.0 and 10.0 were 4.38, 4.59 and 5.62 lm/W, respectively, and the CIE-1931 coordinates were (0.27, 0.23), (0.29, 0.26) and (0.33, 0.28), respectively, at 20 mA. However, the CIE coordinates varied with the applied currents. The CIE coordinates at different applied currents are listed in Table 1. The blue light emission from the LED chip increased with increasing injection current. However, excitation of CdSe quantum dots by the LED chip was incomplete and the CIE coordinates shifted from the white region to the bluish region. The variation in the CIE coordinates was reduced by increasing the fraction of PMMA. At a PMMA ratio of 10.0, the CIE coordinates slightly shift from (0.33, 0.28) to (0.30, 0.26), while the CIE coordinates move from (0.27, 0.23) to (0.21,
0.17) at a ratio of 1.0 and from (0.29, 0.26) to (0.25, 0.22) at a ratio of 5.0 when the working current was increased from 5 to 80 mA. Fig. 6 presents the electroluminescence spectra of the dual phosphors white LEDs. These spectra contained three emission bands at 460 nm, which was from the InGaN chip, and 555 and 625 nm, which was from the CdSe quantum dots. As shown in Fig. 6(a–c), the intensity of the red emission was stronger than that of the yellowish-green emission, even though the amount of yellowish-green emitting CdSe quantum dots coated on the chip was larger than the number of red emitting CdSe quantum dots. The luminous efficiency of dual phosphors white LEDs was 2.19, 3.63 and 3.79 lm/W at 20 mA, when the weight ratio of PMMA was 1.0, 5.0 and 10.0, respectively. The dual phosphors white LEDs show lower luminous efficiency than the single phosphor white LEDs. In dual phosphors white LEDs, the shorter wavelength light emitted from smaller CdSe quantum dots can be reabsorbed by the larger CdSe quantum dots. The energy transfer from yellowish-green to red emitting CdSe quantum dots caused the decrease of the excitation intensity in the yellowish-green region. The decrease in the excitation efficiency in the yellowish-green region resulted in lowering the luminous efficiency, since the luminous efficiency critically depends on the intensity of the green to yellow spectral region. The correlation between the wavelength and luminous efficiency is given in Fig. 7. One approach for preventing the reabsorption of short wavelengths by longer wavelengths is to coat the phosphor layer by layer. In this scenario, the larger size quantum dots that have longer wavelengths should be coated on the lower layer followed by the smaller quantum dots that have shorter wavelengths.
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W. Chung et al. / Optical Materials 32 (2010) 515–521 Table 3 The CRI value of single and dual phosphors white LEDs at various working currents. Applied currents (mA)
Single phosphor white LEDs
Dual phosphors white LEDs
5 10 20 40 60 80
12.6 13.2 15.7 17.9 19.1 21.4
59.7 60.5 61.1 62.4 65.5 66.3
of the CRI will be enhanced. The CRI of the white LEDs increased when the applied current was increased. The CRI of the single phosphor white LEDs increased from 12.6 to 21.4 and that of the dual phosphors white LEDs increased from 59.7 to 66.3 when the applied current was increased from 5 to 80 mA. Fig. 7. The CIE photopic luminous efficiency function. From M.D. Fairchild, Color Appearance Model (Addison Wesley, Reading, MA, 1998).
4. Conclusion Table 2 The variation in CIE coordinates of dual phosphors white LEDs containing PMMA to CdSe quantum dot ratios of 1, 5 and 10-fold PMMA at the indicated applied currents. Applied currents (mA)
1-Fold PMMA
5-Fold PMMA
10-Fold PMMA
5 10 20 40 60 80
(0.43, (0.44, (0.43, (0.43, (0.42, (0.40,
(0.36, (0.35, (0.36, (0.37, (0.35, (0.34,
(0.39, (0.39, (0.38, (0.39, (0.39, (0.39,
0.32) 0.32) 0.31) 0.30) 0.30) 0.29)
0.30) 0.31) 0.30) 0.29) 0.28) 0.28)
0.33) 0.33) 0.33) 0.33) 0.32) 0.32)
The color quality and stability of the white LEDs depended on the CdSe quantum dots to PMMA mixing weight ratio. At a 1.0 mixing ratio of PMMA, a strong excitation from the red emitting CdSe quantum dots was observed and the device emitted a reddish-yellow light that had a CIE coordinate of (0.43, 0.31) at 20 mA. As the mixing ratio of PMMA increased, the excitation in the yellowishgreen emission region was enhanced due to the high dispersivity of CdSe quantum dots in the PMMA. The emission color from the device shifted from yellowish-green to near white light as the PMMA mixing ratio was increased. The change in CIE coordinates of the dual phosphors white LEDs with different applied currents showed a similar tendency as the single phosphor white LEDs. As shown in Table 2, the deviation of the CIE coordinate tended to decrease as the content of weight of PMMA in the mixture was increase. The optimal PMMA to CdSe weight ratio was determined to be 10.0. In this condition, the CIE coordinates were tuned from (0.39, 0.33) at 5 mA to (0.39, 0.32) at 80 mA. The optimum CdSe quantum dots to PMMA mixing ratio for the dual phosphors white LEDs was similar to the optimum mixing ratio in single phosphor white LEDs. When the mixing ratio of PMMA was 10.0, both single and dual phosphors white LEDs showed high color stability and uniformity against the input power. However, the CIE color coordinates of the dual phosphors white LEDs were less sensitive than the single phosphor white LED at the tested working currents. Table 3 shows the color rending index (CRI) of the single and dual phosphors white LEDs at a PMMA to CdSe quantum dot weight ratio of 10.0 with various applied currents. The CRI of single and dual phosphors white LEDs was 15.7 and 61.1, respectively, at a 20 mA applied current. Due to the broad emission spectrum, the CRI of dual phosphors white LEDs had a higher value in comparison to the single phosphor white LEDs. This indicates that the light from the dual phosphors white LEDs was closer to natural light. By using trio, quadruple, or multi phosphors to fabricate white LEDs, the emission spectrum will be broader and, hence, the value
CdSe quantum dots were successfully synthesized from CdO and TOP–Se via a fast nucleation and slow growth method. Three emission bands at 555, 580 and 625 nm were obtained by tuning the dot size. White LEDs were fabricated by combining 460 nm InGaN LED chip with single or dual CdSe quantum dots mixed with PMMA. Mixtures containing different weight ratios of CdSe quantum dots to PMMA were prepared and deposited onto the blue LED chip. The characteristics of the device were found to depend on the CdSe quantum dots to PMMA mixing ratio. As the weight ratio of PMMA to CdSe quantum dots increased from 0.1 to 10.0, the dispersivity of CdSe quantum dots in the PMMA matrix was enhanced and the performance of the white LEDs was improved. At a PMMA to CdSe quantum dots weight ratio of 10, the fabricated white LEDs had a high luminous efficiency and good color stability. The luminous efficiency of single and dual phosphors white LEDs was 5.62 lm/W and 3.79 lm/W at 20 mA. The CIE coordinates of single and dual phosphors white LEDs varied from (0.33, 0.28) to (0.29, 0.26) and from (0.39, 0.33) to (0.39, 0.32), respectively, when the applied current ranged from 5 to 80 mA. The CRI value of white LEDs improved when the emission band was broadened. The CRI of single phosphor white LEDs was 15.7 and that of dual phosphors white LEDs was 61.1 at 20 mA. Acknowledgements The authors are grateful for financial support for this work from the Carbon Dioxide Reduction and Sequestration Center, a 21st Century Frontier R&D Program funded by the Ministry of Education, Science and Technology of Korea. Reference [1] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, G.C. Kim, T.W. Kim, Appl. Phys. Lett. 85 (2004) 3696. [2] Mihail Nazarov, Do Young Noh, Jongrak Sohn, Chulsoo Yoon, Opt. Mater. 30 (2008) 1387. [3] Baris Kokuoz, Courtney Kucera, Jeffrey T. DiMaio, John Ballato, Opt. Mater. 31 (2009) 1327. [4] Jae-Wook Lee, Jae-Hyuk Lee, Eun-Ji Woo, Hyungwoong Ahn, Joon-Soo Kim, Chang-Ha Lee, Ind. Eng. Chem. Res. 47 (2008) 5994. [5] Il Woo Park, Hyo Jin Lee, Jae Soo Yoo, Chang Kyun Choi, Korean J. Chem. Eng. 24 (2007) 294. [6] F. Hide, P. Kozodoy, S.P. DenBaars, A.J. Heeger, Appl. Phys. Lett. 70 (1997) 2664. [7] Hong Jeong Yu, Kwankwi Park, Sung Hyun Kim, Mol. Cryst. Liq. Cryst. 499 (2009) 26. [8] Sanjeev K. Kauchish, T.P. Sharma, Opt. Mater. 14 (2000) 297. [9] Kwankwi Park, Hong Jeong Yu, Hyun Uk Kang, Sangsig Kim, Sung Hyun Kim, Jpn. J. Appl. Phys. 46 (2007) 6878. [10] Debasis Bera, Lei Qian, Subir Sabui, Swadeshmukul Santra, Paul H. Holloway, Opt. Mater. 30 (2008) 1233.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
White emission using mixtures of CdSe quantum dots and PMMA as a phosphor Wonkeun Chung, Kwanhwi Park, Hong Jeong Yu, Jihyun Kim, Byung-Hee Chun, Sung Hyun Kim * Department of Chemical and Biological Engineering, Korea University, 1 Anam-Dong, Seongbuk-Ku, Seoul 136-713, Republic of Korea
a r t i c l e
i n f o
Article history: Received 22 June 2009 Received in revised form 9 November 2009 Accepted 12 November 2009 Available online 22 December 2009 Keywords: White emission White LEDs CdSe quantum dots Phosphor
a b s t r a c t White light emitting diodes (LEDs) were fabricated using an InGaN 460 nm blue emission LED chip as the excitation source and CdSe quantum dots dispersed in PMMA as the phosphor. CdSe quantum dots were synthesized by the wet chemical method using CdO and Selenium powder as precursors. The three different size, 2.9, 3.4 and 4.3 nm in diameter, of CdSe quantum dots obtained using this method exhibited emission peaks at 555, 580 and 625 nm, respectively with a quantum yield of 10–30%. Mixed phosphors containing different weight ratio of CdSe and PMMA (1:0.1, 1:1, 1:5 and 1:10 wt%) were deposited on the LED chip to investigate the effects of different weight ratios of CdSe and PMMA on the performance of the white LEDs. The fabricated white LEDs that contained CdSe and PMMA weight ratio at 1:10 showed the best performance and the CIE color coordinates varied less with different applied currents. The luminous efficiency of single phosphor (580 nm CdSe) white LEDs was 5.62 lm/W with a CRI of 15.7, whereas the luminous efficiency of dual phosphors (555, 625 nm CdSe) white LEDs was 3.79 lm/W with a CRI of 61.4 at 20 mA. The CIE coordinates of single and dual phosphors white LEDs varied from (0.33, 0.28) to (0.29, 0.26) and from (0.39, 0.33) to (0.39, 0.32), respectively, when the working current ranged from 5 to 80 mA. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction White light emitting diodes (LEDs) have recently attracted a significant amount of attention due to their high efficiency, long life time (100,000 h), and reliability. White LEDs hold the potential to replace conventional light source in a wide range of applications such as incandescent bulbs or fluorescent lamps, full color displays and traffic signals. Two approaches have been commonly used to generate white emission. One is to mix red, green, and blue emitting color LED chips and the other is to combine UV or blue LED chips with phosphors. Multichip white LEDs show high efficiency and a good color rendering index (CRI). However, due to the high cost and complicated lighting system required to control the light intensity, this approach is not considered viable by current standards. The phosphor conversion white LEDs are a promising method for the development of a simple and cost effective white light emission. It has a high luminescence efficiency and flux, high color stability and low production cost. In commercial white LEDs, a blue emitting InGaN LED chip is used as the optical excitation source and this is combined with a yellow emitting Ce activated yttrium aluminum garnets (YAG:Ce), which is used as the phosphor. When power is applied to the InGaN chip, blue light is emitted from the chip and some of the blue light is absorbed by YAG:Ce, which re* Corresponding author. Tel.: +82 02 3290 3297; fax: +82 02 926 6102. E-mail address: [email protected] (S.H. Kim). 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2009.11.005
emits yellow light. The mixing of blue and yellow emission produces white light. However, these white emission, using YAG:Ce phosphor, has a low color rendering index (CRI) due to the lack of red emission component and the color purity varies with input power [1]. Due to these problems, various phosphors, such as inorganic [2–5], organic polymer [6], OLED material [7] and quantum dot [8–11] have been examined in white LEDs. Among these materials, CdSe [12], CdS [13], and ZnO [14] II–VI semiconductor quantum dots have been regarded as suitable candidates due to their unique properties, such as high quantum yield, minimal backscattering and tunable band gap. In particular, CdSe quantum dots can emit the full visible spectrum by tuning the band gap. The II–VI semiconductor quantum dots have broad absorption peak. Therefore, the quantum dots can be excited by any optical pumping sources which have larger band gap energy than the quantum dots. Thus quantum dots and transparent polymer composites such as CdSe/ ZnS–PLMA (polyaurylmethacrylate) [15] and CdSe/ZnS–PMMA (polymethylmethacrylate) [16] by direct polymerization or quantum dots and fluorescent polymer [17,18] composites have been combined with an LED chip, where the quantum dots act as the phosphor to create a novel white LEDs. However previous studies have not reported the effects of the amount of polymer as a mixing phosphor on the properties of white LEDs. In this study, the different weight ratios of CdSe quantum dots and PMMA were prepared as mixing phosphors by solvent mixing
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W. Chung et al. / Optical Materials 32 (2010) 515–521
Fig. 1. The process of white LEDs fabrication using CdSe–PMMA mixture as phosphors.
Fig. 2. HRTEM image of the CdSe quantum dots. (a) Reaction at 140 °C, (b) reaction at 190 °C, (c) reaction at 220 °C and (d) mixing with PMMA (1:10 wt%).
to investigate the effects of the polymer. That mixing method can minimize the decreasing of the emission intensity of quantum dot in polymer matrix than direct polymerization [15,16]. White LEDs were fabricated by combining a 460 nm InGaN LED chip with single (580 nm) or dual (555, 625 nm) CdSe quantum dots mixed with PMMA. The luminescence properties of white LEDs were defined by the luminous efficiency, CIE-1931 coordinate and color rendering index (CRI).
2. Experimental 2.1. Synthesis of CdSe quantum dots CdSe quantum dots were synthesized based on previously reported techniques with slight modifications [19]. All reagents were purchased from Sigma–Aldrich and used without further purification. CdO (0.45 g) and stearic acid (8 g) as a capping ligand were
W. Chung et al. / Optical Materials 32 (2010) 515–521
mixed in a three-necked flask and heated to 150 °C for 1 h under nitrogen to prepare the cadmium precursor. After decomposition of CdO, the mixture solvent, 12 g of 1-heptadecyloctadecylamine (HDA) and 8 g of Trioctylphosphine Oxide (TOPO), was added and stirred together at 120 °C for 30 min. At this temperature, the Selenium precursor, which was 0.78 g of Selenium powder dissolved in 8.9 mL of Trioctylphosphine (TOP, 90%), was swiftly injected into the reaction flask. The nuclei formed quickly and several different sized monodisperse CdSe quantum dots were obtained by slowly increasing the growth temperature from 120 to 140, 190 and 220 °C for the desired dot size. After the reaction was complete, CdSe quantum dots were precipitated with methanol for purification. The precipitate was separated by centrifugation and washed out by methanol and chloroform several times to remove unreacted reagents and excess TOPO or HDA. 2.2. Preparation of CdSe quantum dots–PMMA mixtures
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2.3. Fabrication of white LEDs White LEDs were fabricated using a surface mounted device (SMD) type of the InGaN/GaN multiple quantum well structure blue emission LED and CdSe quantum dots. The size of the chip was 1 mm 1 mm and the emission peak of the LED chip was 460 nm with a CIE-1931 color coordinate of (0.14, 0.05). The LED chip was placed on the mold and prepared CdSe quantum dots and PMMA mixtures were directly deposited into the mold for uniform shape of phosphor coating layer (Fig. 1). The mold was composed of acetal plastic and the size of the mold was 10 mm (length) 10 mm (width) 8 mm (height). After coating, the solvent was slowly dried in a vacuum oven at 80 °C for 30 min. 2.4. Measurements
Four different weight ratios of CdSe and PMMA mixtures were obtained by solvent based mixing. Five milligrams of purified CdSe quantum dots were completely dissolved in 1 mL of chlorobenzene and then 0.5, 5, 25 and 50 mg of PMMA were added to the CdSe quantum dots solution. The molecular weight of PMMA (from Sigma–Aldrich) was approximately 996,000 by GPC and glass Transition Temperature (Tg) was 125 °C. The mixed solution of CdSe quantum dots and PMMA vigorously stirred for 10 h at room temperature [20].
The size and shape of CdSe quantum dots were observed using transmission electron microscope (Tecnai 20) operating at 200 KV. Quantum dots were dispersed in chloroform and dropped on the 200 mesh carbon–copper grids to prepare the sample. The optical properties of quantum dots were characterized by UV–vis absorption (UV mini 1240, Shimadzu) and photoluminescence (PL) spectra (F-7000 FL spectrophotometer of xenon-lamp source with 365 nm) at room temperature. The samples were prepared by dispersing CdSe quantum dots in chloroform. Fabricated white LEDs were analyzed with CDS 1100 CCD Array Spectrometer (Labsphere) by normal direction.
Fig. 3. (a) Absorption spectrum of three different sized CdSe quantum dots. (b) PL emission spectrum of three different sized CdSe quantum dots. The PL peak shifted to red as the dot size increased.
Fig. 4. Electroluminescence spectrum of white LEDs fabricated by combining lnGaN with (a) single CdSe quantum dots phosphor and (b) dual CdSe quantum dots phosphors PMMA to CdSe quantum dot ratio of 0.1. (Inset: emission light at 20 mA.)
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Fig. 5. Electroluminescence spectrum of single phosphor white LEDs fabricated at different PMMA to CdSe quantum dot ratios; (a) 1-fold, (b) 5-fold and (c) 10-fold of PMMA compared to the mass of CdSe quantum dots. (Inset: emission light at 20 mA.)
Table 1 The variation in CIE coordinates of single phosphor white LEDs containing PMMA to CdSe quantum dot mass ratios of 1, 5, and 10-fold PMMA at the indicated applied currents. Applied currents (mA)
1-Fold PMMA
5-Fold PMMA
10-Fold PMMA
5 10 20 40 60 80
(0.27, (0.25, (0.24, (0.24, (0.23, (0.21,
(0.29, (0.29, (0.29, (0.29, (0.27, (0.25,
(0.33, (0.33, (0.33, (0.32, (0.30, (0.30,
0.23) 0.21) 0.18) 0.17) 0.16) 0.17)
0.26) 0.26) 0.25) 0.25) 0.24) 0.22)
0.28) 0.28) 0.27) 0.26) 0.26) 0.26)
3. Results and discussion 3.1. The properties of CdSe quantum dots Highly luminescent CdSe quantum dots were synthesized by a fast nucleation and slow growth process. As shown in Fig. 2, CdSe quantum dots that were 2.9, 3.4 and 4.3 nm in diameter, estimated from HRTEM, were obtained when the reaction temperature was maintained at 140 °C for 150 min, 190 °C for 200 min and 220 °C for 450 min, respectively. TEM image of CdSe quantum dots after mixing with PMMA (1:10 wt% ratio) is shown in Fig. 2(d). CdSe and PMMA mixtures were formed about cluster of 25 nm and CdSe quantum dots were scattered in the PMMA matrix. Fig. 3 shows the normalized absorption (UV–vis) spectra and corresponding photoluminescence (PL) spectra of CdSe quantum dots. The high and sharp first absorption peak in the absorption spectrum of the CdSe
quantum dots (Fig. 3(a)) indicates that the synthesized quantum dots were highly mono-dispersed and homogeneous. As shown in the PL spectra of Fig. 3(b), as the particle size increases, the emission peak of the quantum dots becomes shifted to longer wavelength due to the quantum confinement effect. The emission peaks of the 2.9, 3.4 and 4.3 nm diameter CdSe quantum dots were 555 nm for yellowish-green, 580 nm for yellow and 625 nm for red. The quantum yields of CdSe quantum dots in chloroform solution were estimated comparing the integrated emission intensity to that of Rhodamine 6G in ethanol (reference, QY = 0.95) at identical optical density (0.015 OD). The quantum yields were calculated by a following equation:
QY QD ¼
IQD ODref QY ref Iref ODQD
ð1Þ
IQD and Iref are the integrated emission band, ODQD and ODref are the optical density at the 365 nm excitation wavelength. The quantum yield, based on Eq. (1) were 23.1% (2.9 nm), 19.9% (3.4 nm) and 17.3% (4.3 nm). 3.2. Performance of fabricated white LEDs White LEDs were fabricated by combining a 460 nm emission blue LED, which was used as the excitation source, with single 580 nm emitting CdSe quantum dots (referred to as single) or a combination of 555 and 625 nm emitting CdSe quantum dots (referred to as dual), which was used as the phosphor. In the dual phosphors device, the weight ratio of the 555 and 625 nm emitting CdSe quantum dots was 30:1. The different weight fractions of
W. Chung et al. / Optical Materials 32 (2010) 515–521
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Fig. 6. Electroluminescence spectrum of dual phosphor white LEDs fabricated at different PMMA to CdSe quantum dot ratios; (a) 1-fold, (b) 5-fold and (c) 10-fold of PMMA compared to the mass of CdSe quantum dots. (Inset: emission light at 20 mA.)
CdSe quantum dots and PMMA mixtures were prepared. The CdSe quantum dots to PMMA weight ratios used in this study were 0.1, 1.0, 5.0 and 10.0. Fig. 4 shows the electroluminescence spectra of fabricated white LEDs consisting of a blue LED and single or dual CdSe quantum dots mixed at a 0.1 ratio with PMMA. At this PMMA concentration, the CdSe quantum dots were hardly excited due to the aggregation of quantum dots on the LED chip, and a strong bluish light from the InGaN LED chip was detected near 460 nm. The CIE-1931 coordinates of the single and dual phosphors devices were (0.18, 0.12) and (0.21, 0.17) under a forward bias of 20 mA. The electroluminescence spectra of the single phosphor white LED at different CdSe quantum dot and PMMA weight ratios are shown in Fig. 5. As the mixing ratio of PMMA was increased, the dispersivity of the CdSe quantum dots in the PMMA matrix was improved and as a result the excitation efficiency of the quantum dots and luminous efficiency of the white LEDs was enhanced. The luminous efficiency of white LEDs containing PMMA at weight ratios of 1.0, 5.0 and 10.0 were 4.38, 4.59 and 5.62 lm/W, respectively, and the CIE-1931 coordinates were (0.27, 0.23), (0.29, 0.26) and (0.33, 0.28), respectively, at 20 mA. However, the CIE coordinates varied with the applied currents. The CIE coordinates at different applied currents are listed in Table 1. The blue light emission from the LED chip increased with increasing injection current. However, excitation of CdSe quantum dots by the LED chip was incomplete and the CIE coordinates shifted from the white region to the bluish region. The variation in the CIE coordinates was reduced by increasing the fraction of PMMA. At a PMMA ratio of 10.0, the CIE coordinates slightly shift from (0.33, 0.28) to (0.30, 0.26), while the CIE coordinates move from (0.27, 0.23) to (0.21,
0.17) at a ratio of 1.0 and from (0.29, 0.26) to (0.25, 0.22) at a ratio of 5.0 when the working current was increased from 5 to 80 mA. Fig. 6 presents the electroluminescence spectra of the dual phosphors white LEDs. These spectra contained three emission bands at 460 nm, which was from the InGaN chip, and 555 and 625 nm, which was from the CdSe quantum dots. As shown in Fig. 6(a–c), the intensity of the red emission was stronger than that of the yellowish-green emission, even though the amount of yellowish-green emitting CdSe quantum dots coated on the chip was larger than the number of red emitting CdSe quantum dots. The luminous efficiency of dual phosphors white LEDs was 2.19, 3.63 and 3.79 lm/W at 20 mA, when the weight ratio of PMMA was 1.0, 5.0 and 10.0, respectively. The dual phosphors white LEDs show lower luminous efficiency than the single phosphor white LEDs. In dual phosphors white LEDs, the shorter wavelength light emitted from smaller CdSe quantum dots can be reabsorbed by the larger CdSe quantum dots. The energy transfer from yellowish-green to red emitting CdSe quantum dots caused the decrease of the excitation intensity in the yellowish-green region. The decrease in the excitation efficiency in the yellowish-green region resulted in lowering the luminous efficiency, since the luminous efficiency critically depends on the intensity of the green to yellow spectral region. The correlation between the wavelength and luminous efficiency is given in Fig. 7. One approach for preventing the reabsorption of short wavelengths by longer wavelengths is to coat the phosphor layer by layer. In this scenario, the larger size quantum dots that have longer wavelengths should be coated on the lower layer followed by the smaller quantum dots that have shorter wavelengths.
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W. Chung et al. / Optical Materials 32 (2010) 515–521 Table 3 The CRI value of single and dual phosphors white LEDs at various working currents. Applied currents (mA)
Single phosphor white LEDs
Dual phosphors white LEDs
5 10 20 40 60 80
12.6 13.2 15.7 17.9 19.1 21.4
59.7 60.5 61.1 62.4 65.5 66.3
of the CRI will be enhanced. The CRI of the white LEDs increased when the applied current was increased. The CRI of the single phosphor white LEDs increased from 12.6 to 21.4 and that of the dual phosphors white LEDs increased from 59.7 to 66.3 when the applied current was increased from 5 to 80 mA. Fig. 7. The CIE photopic luminous efficiency function. From M.D. Fairchild, Color Appearance Model (Addison Wesley, Reading, MA, 1998).
4. Conclusion Table 2 The variation in CIE coordinates of dual phosphors white LEDs containing PMMA to CdSe quantum dot ratios of 1, 5 and 10-fold PMMA at the indicated applied currents. Applied currents (mA)
1-Fold PMMA
5-Fold PMMA
10-Fold PMMA
5 10 20 40 60 80
(0.43, (0.44, (0.43, (0.43, (0.42, (0.40,
(0.36, (0.35, (0.36, (0.37, (0.35, (0.34,
(0.39, (0.39, (0.38, (0.39, (0.39, (0.39,
0.32) 0.32) 0.31) 0.30) 0.30) 0.29)
0.30) 0.31) 0.30) 0.29) 0.28) 0.28)
0.33) 0.33) 0.33) 0.33) 0.32) 0.32)
The color quality and stability of the white LEDs depended on the CdSe quantum dots to PMMA mixing weight ratio. At a 1.0 mixing ratio of PMMA, a strong excitation from the red emitting CdSe quantum dots was observed and the device emitted a reddish-yellow light that had a CIE coordinate of (0.43, 0.31) at 20 mA. As the mixing ratio of PMMA increased, the excitation in the yellowishgreen emission region was enhanced due to the high dispersivity of CdSe quantum dots in the PMMA. The emission color from the device shifted from yellowish-green to near white light as the PMMA mixing ratio was increased. The change in CIE coordinates of the dual phosphors white LEDs with different applied currents showed a similar tendency as the single phosphor white LEDs. As shown in Table 2, the deviation of the CIE coordinate tended to decrease as the content of weight of PMMA in the mixture was increase. The optimal PMMA to CdSe weight ratio was determined to be 10.0. In this condition, the CIE coordinates were tuned from (0.39, 0.33) at 5 mA to (0.39, 0.32) at 80 mA. The optimum CdSe quantum dots to PMMA mixing ratio for the dual phosphors white LEDs was similar to the optimum mixing ratio in single phosphor white LEDs. When the mixing ratio of PMMA was 10.0, both single and dual phosphors white LEDs showed high color stability and uniformity against the input power. However, the CIE color coordinates of the dual phosphors white LEDs were less sensitive than the single phosphor white LED at the tested working currents. Table 3 shows the color rending index (CRI) of the single and dual phosphors white LEDs at a PMMA to CdSe quantum dot weight ratio of 10.0 with various applied currents. The CRI of single and dual phosphors white LEDs was 15.7 and 61.1, respectively, at a 20 mA applied current. Due to the broad emission spectrum, the CRI of dual phosphors white LEDs had a higher value in comparison to the single phosphor white LEDs. This indicates that the light from the dual phosphors white LEDs was closer to natural light. By using trio, quadruple, or multi phosphors to fabricate white LEDs, the emission spectrum will be broader and, hence, the value
CdSe quantum dots were successfully synthesized from CdO and TOP–Se via a fast nucleation and slow growth method. Three emission bands at 555, 580 and 625 nm were obtained by tuning the dot size. White LEDs were fabricated by combining 460 nm InGaN LED chip with single or dual CdSe quantum dots mixed with PMMA. Mixtures containing different weight ratios of CdSe quantum dots to PMMA were prepared and deposited onto the blue LED chip. The characteristics of the device were found to depend on the CdSe quantum dots to PMMA mixing ratio. As the weight ratio of PMMA to CdSe quantum dots increased from 0.1 to 10.0, the dispersivity of CdSe quantum dots in the PMMA matrix was enhanced and the performance of the white LEDs was improved. At a PMMA to CdSe quantum dots weight ratio of 10, the fabricated white LEDs had a high luminous efficiency and good color stability. The luminous efficiency of single and dual phosphors white LEDs was 5.62 lm/W and 3.79 lm/W at 20 mA. The CIE coordinates of single and dual phosphors white LEDs varied from (0.33, 0.28) to (0.29, 0.26) and from (0.39, 0.33) to (0.39, 0.32), respectively, when the applied current ranged from 5 to 80 mA. The CRI value of white LEDs improved when the emission band was broadened. The CRI of single phosphor white LEDs was 15.7 and that of dual phosphors white LEDs was 61.1 at 20 mA. Acknowledgements The authors are grateful for financial support for this work from the Carbon Dioxide Reduction and Sequestration Center, a 21st Century Frontier R&D Program funded by the Ministry of Education, Science and Technology of Korea. Reference [1] J.S. Kim, P.E. Jeon, Y.H. Park, J.C. Choi, H.L. Park, G.C. Kim, T.W. Kim, Appl. Phys. Lett. 85 (2004) 3696. [2] Mihail Nazarov, Do Young Noh, Jongrak Sohn, Chulsoo Yoon, Opt. Mater. 30 (2008) 1387. [3] Baris Kokuoz, Courtney Kucera, Jeffrey T. DiMaio, John Ballato, Opt. Mater. 31 (2009) 1327. [4] Jae-Wook Lee, Jae-Hyuk Lee, Eun-Ji Woo, Hyungwoong Ahn, Joon-Soo Kim, Chang-Ha Lee, Ind. Eng. Chem. Res. 47 (2008) 5994. [5] Il Woo Park, Hyo Jin Lee, Jae Soo Yoo, Chang Kyun Choi, Korean J. Chem. Eng. 24 (2007) 294. [6] F. Hide, P. Kozodoy, S.P. DenBaars, A.J. Heeger, Appl. Phys. Lett. 70 (1997) 2664. [7] Hong Jeong Yu, Kwankwi Park, Sung Hyun Kim, Mol. Cryst. Liq. Cryst. 499 (2009) 26. [8] Sanjeev K. Kauchish, T.P. Sharma, Opt. Mater. 14 (2000) 297. [9] Kwankwi Park, Hong Jeong Yu, Hyun Uk Kang, Sangsig Kim, Sung Hyun Kim, Jpn. J. Appl. Phys. 46 (2007) 6878. [10] Debasis Bera, Lei Qian, Subir Sabui, Swadeshmukul Santra, Paul H. Holloway, Opt. Mater. 30 (2008) 1233.
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