ARTICLE pubs.acs.org/JPCC
Enhanced Photoluminance of Layered Quantum Dot Phosphor Nanocomposites as Converting Materials for Light Emitting Diodes Ju Yeon Woo,† Kyungnam Kim,‡ Sohee Jeong,‡ and Chang-Soo Han*,† † ‡
Department of Mechanical Engineering, Korea University, Seoul 136713, Republic of Korea Nano-Mechanical Systems Research Division, Korea Institute of Machinery and Materials (KIMM), Daejeon 305343, Republic of Korea ABSTRACT:
This paper reports on the enhanced photoluminescence (PL) of nanocomposites through the layered structuring of phosphor and quantum dot (QD). Green phosphor of Sr2SiO4:Eu, red QDs of CdSe/CdS/CdZnS/ZnS core-multishell, and thermo-curable resin were used for this study. Two kinds of composite (layered and mixed) were prepared, and the schemes for optical energy transfer between QD and phosphor were suggested and investigated based on PL decay characteristics. It was found that the layered structure is more effective than the mixed one with respect to PL intensity, PL decay, and thermal loss. When this layered nanocomposite (QDs on phosphor) is used to make a white light emitting diode (LED), the brightness is increased by 37%, and the color rendering index (CRI) value is raised to 88.4 compared to the mixed case of 80.4.
’ INTRODUCTION Colloidal QDs have attracted much attention due to their unique characteristics, including nanometer scales, high PL quantum yields (QYs), wide absorption spectra toward shorter wavelengths, exquisite color purity, lower light scattering, and size-tunable optical properties.1 6 These advantages make nanocrystals useful for applications in a variety of light-emitting technologies, such as general lighting, display, photovoltaic devices, and lasers.7 10 To tune the wavelength of the absorbed light, QDs have recently been used as fluorescent phosphors for optical devices to make white light or high-quality color light.11 16 Although this has been reported for a white color rendering of LED using QDs, blends of QDs and conventional phosphors are a more typical approach than QDs, only due to optical performance, cost, and stability. QDs often simply tune the color of the LED toward the desired color coordinate along with high CRI. Previous research has only reported the change in optical properties according to simple mixing of QDs and phosphors. It did not consider the optical energy transfer between the QDs and phosphor. There are several optical paths between those, as they are randomly dispersed in space. Therefore, a simple blending structure might show relatively low energy-transfer efficiency because of several intermediate energy-loss steps in the transfer process.17 r 2011 American Chemical Society
This paper systematically analyzes the optical energy transfer between QDs and phosphor. For the first time, a layered structure to QD phosphor thin films is introduced. Two kinds of composites (layered and mixed) are compared in terms of optical and thermal characteristics. This includes two kinds of layered composites according to the position and concentrations of the QDs. The PL decay characteristics are measured to arrive at a feasible reason as to why a QD layer on phosphor layer is more preferable with respect to PL. When an LED device with layered nanocomposite (QDs on phosphors) is fabricated, it shows the highest brightness and CRI values among the tested nanocomposites.
’ EXPERIMENTAL DETAILS Materials and Characterization. For the synthesis, the experiment used trioctylamine (Aldrich, 98%), oleic acid (Alfa, 99%), admium oxide (Alfa Aesar, 99.998%), Se powder (Alfa Aesar, 99.999%), zinc oxide (Aldrich, 99.99%), trioctyl phosphine (Aldrich, 90%), and 1-octanthiol (Aldrich, 98.5+%). Received: June 29, 2011 Revised: September 16, 2011 Published: September 27, 2011 20945
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Figure 1. (a) Structure of a core-multishell nanocrystal. (b) TEM images of a CdSe/CdS/CdZnS/ZnS core-multishell QD (scale bar: 10 nm). The inset shows TEM images with high resolution for QD (scale bar: 2 nm). (c) A picture of the QD solutions under daylight and at 365 nm UV illumination. (d) UV vis absorption spectra and corresponding PL emission spectra of core-multishell QDs.
CdSe/CdS/CdZnS/ZnS QDs was prepared as follows. 0.0515 g of CdO powder, 0.45 g of oleic acid (OA), and 10 mL of trioctylamine (TOA) were mixed in a 50 mL three-necked flask. The mixed solution was heated to 150 °C under a vacuum, and the temperature was then increased to 300 °C under a N2 gas atmosphere. At 300 °C, 0.15 mL of 2 M Se in trioctylphosphine (TOP) was quickly injected into the Cd-containing solution. After 90 s, a thiol solution (0.05 mL of n-octanethiol in 1.5 mL of TOA) was injected at the rate of 1 mL 3 min 1 and kept for 40 min. For the preparation of CdZnS, 0.0255 g of zinc oxide, 0.04 g of CdO, and 1.55 g of OA were dissolved in 4.5 mL of a TOA solution at 300 °C under a N2 gas atmosphere. After this solution was added to the Cd-containing solution at a rate of 2 mL 3 min 1, the thiol solution (0.3 mL of n-octanethiol in 1.5 mL of TOA) was injected at a rate of 1 mL 3 min 1. This reaction was maintained for 40 min. Finally, for the ZnS, 0.102 g of zinc oxide and 1 g of OA was dissolved in 5 mL of TOA at 300 °C under a N2 gas atmosphere. This solution was injected into the Cd-containing solution at a rate of 2 mL 3 min 1, and then the thiol solution (0.28 mL of n-octanethiol in 1.5 mL of TOA) was injected at a rate of 1 mL 3 min 1. After the reaction, the product solution was cooled and organic sludge was removed via the centrifuge process (5600 rpm) using methanol and butanol. The precipitated QDs were redispersed in toluene. An absolute quantum yield was measured through a quantum yield measurement system
(C-9920-02, Hamamatsu, Japan) composed with integral sphere, PMT, monochromator, and Xe lamp. QY was measured under 480 nm excitation wavelength. The sample has 0.02 0.05 absorption intensity (OD) at 480 nm. The absorption and photoluminescence were characterized using a UV vis spectrophotometer (SD1000, Scinco, Korea) and a fluorometer (Fluorolog, Horiba JOBIN YVON, France) at room temperature. Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 F30 S-Twin model (FEI, The Netherlands) at 300 kV. QDs were dispersed in toluene and dropped on the 300-mesh holey carbon grids to prepare the sample. The decay properties of composite film were characterized with a Ti:sapphire femtosecond pulse laser (Coherent, USA). The 519 nm green-emitting Sr2SiO4:Eu phosphor was purchased from Force4 Co. (Gwangju, Korea) and used as received. InGaN-based blue LED chips (NLX-5 blue power die, λmax = 455 nm, nonepoxy molding packages) were purchased from Trikaiser (Gwangju, Korea). The dual-component thermal curable silicone resin (OE-6630 A and B) was purchased from Dow Corning. Preparation of QD Phosphor Resin Nanocomposite and LED Device. First, the silicone resin parts A and B were mixed in a 1:4 weight ratio. Then, 0.0006 g (0.2 wt %) of QD in toluene solution and 0.03 g (10 wt %) of phosphor were mixed with 0.3 g of silicone resin and put in a vacuum chamber for 30 min at 1 bar to remove toluene and bubbles in the mixture. 20946
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Figure 2. Schematic illustration of the emission mechanism and the energy transfer between QDs and phosphor in polymer matrix: (a) QD layer on phosphor layer; (b) phosphor layer on QD layer; (c) mixed layer.
Composite films were fabricated after the solvent was removed, and PL emission and decay properties from layer-by-layer (LBL) assembly and blending of QD phosphor resin film were measured after curing. To fabricate the layered QD phosphorconverted WLEDs, 0.02 g of QD silicone mixture and 0.02 g of phosphor silicone mixture were hybridized on blue LEDs and thermally cured at 150 °C for 2 h. Then, 0.02 g of different types of mixture was added on top of them and cured. Also, the QD content ranges of 0.1, 0.2, 0.3, and 0.4 wt % were varied when the QD layer was hybridized on top of the phosphor layer. For the mixed QD phosphor-converted WLEDs, 0.04 g of QD phosphor silicone mixture was dispensed on blue LED chips and thermally cured. QD phosphor silicone mixture was continuously dropped on blue LED chips to prevent bubble formation. Optical characteristics, such as electroluminescence (EL), luminous efficiency, correlated color temperature (CCT), and clairage (CIE) color cothe Commission Internationale de l’E ordinates of WLEDs, were evaluated with a PR650 spectrascan (Photoreseach, USA) under a 350 mA working current at room temperature. CRI values were recorded using a SLMS LED 1060 (Labsphere, USA) at room temperature. The temperature distribution on WLED chips was measured continuously using an FLIR A320 infrared camera (FLIR, USA).
’ RESULTS AND DISCUSSION Figure 1a shows the structure of a core-multishell nanocrystal optimized for high luminance efficiency and stability. The synthesized core-multishell QDs (CdSe/CdS/CdZnS/ZnS) are composed of Cd, Se, Zn, and S, as shown in Figure 1b for TEM images. The as-synthesized core-multishell QDs clearly appear on the TEM grid. They are almost monotonically dispersed, without agglomeration. Well-defined lattice fringes for the QD particles can be observed in the high-resolution TEM image (the inset of Figure 1b), indicating that core-multishell QDs have a welldefined homogeneous crystalline structure. Judging from the TEM images, the average particle size of the red-emitting core-multishell QDs is about 6.8 nm. The photograph in Figure 1c shows the toluene solutions of the QDs in a cuvette under daylight and at 365 nm ultraviolet (UV) excitation. Figure 1d shows typical UV vis absorption spectra and the corresponding PL spectra of core-multishell QDs. The core-multishell QDs exhibit high and sharp first absorption peaks at 596 nm and emission peaks at 617 nm, indicating that they are highly monodispersed with a
narrow size distribution. The QY of the QDs obtained through the synthetic methods described in the Experimental Details section is above 55%, and the full width at half-maximum (FWHM) of the QDs is 32 nm. The PL excitation (PLE) and the maximum wavelength of the PL spectrum of green-emitting phosphor are monitored at 450 and 519 nm, as reported previously.18 There is a spectral overlap between the phosphor emission and absorption of the QDs, indicating that photons emitted from the phosphor could definitely be transferred to the QDs. In contrast, the PLE spectrum of the phosphor does not overlap with the QD emission spectrum, indicating that there is no energy transfer via photons from the QD. Figure 2 illustrates the emission mechanism and energy transfer between QDs and phosphor in a polymer matrix. When the QD layer on the phosphor layer is exposed to blue light (Figure 2a), there are three optical paths for the blue light: toward the phosphor, the QD, and the polymer. At this time, emission loss (absorption, reflection, refraction, scattering, etc.) and heat loss (from QDs, the phosphor, and the polymer) occur during the light transmission. In the next step, the light is converted to green and propagates toward the QDs and the polymer. Accordingly, the red light converted by the QDs is created from green light by the phosphor and blue light source. Energy loss from the QDs also happens during this light conversion. Finally, white light is emitted from the composite layer by the combination of the blue, green-converted, and red-converted light. In the case of the phosphor layer on the QDs (Figure 2b), the regime of light transmission is apparently similar to that in Figure 2a, but the light-conversion efficiency is different due to the role of the QDs. In the case of QDs on phosphor, most blue light is first converted by phosphor with high efficiency. QDs are used to convert the blue light and the green light from the phosphor. However, for phosphor on a QD layer, if the QDs absorb most of the blue light, they emit the converted red light at a low conversion efficiency. Considering the overall optical conversion, it seems better to position the QDs on the outermost layer rather than the layer near the light source, as a significant energy loss due to heating occurs during the light conversion. In the phosphor layer, only the blue light that is not converted by the QD layer is absorbed by the phosphor. The red-converted light from the QDs mostly passes through the phosphor layer. In the case of the mixed layer in Figure 2c, energy transfer and energy loss occurred simultaneously. Emission loss is expected to increase due to the increased 20947
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Figure 3. The PL emission spectra of nanocomposite films composed of phosphors, core- multishell QDs, and resin: (a) QD layer on phosphor layer (only phosphor layer, black; QD layer on phosphor layer, red); (b) phosphor layer on QD layer (only QD layer, black; phosphor layer on QD layer, red); (c) QD layer on phosphor layer (black), phosphor layer on QD layer (red), and mixed (green) layer.
optical interference between the QDs and phosphor. On the basis of the discussion of Figure 2a and b, it can be predicted that the conversion efficiency of the blue light has roughly some value in the interval between Figure 2b and c. Figure 3 shows the PL emission spectra of the nanocomposite films with core-multishell QDs and phosphors in the silicone resin after thermal curing. The PL intensity and the spectral maximum of QDs do not provide the same value of PL as the QD solutions. The PL intensity could be reduced during film formation by surface defects in the QDs or different side reactions in the silicone resin at high temperature.10,19 Figure 3a shows the PL spectra of both pure phosphor layers and phosphor layers coated by QDs. Blue light (400 nm) from the PL apparatus was used as a light source for these experiments. The phosphor layer alone does not show any red emission upon excitation with blue light, as expected. After the addition of the QD layer to the phosphor layer, the PL intensity of green emission due to the phosphor is significantly reduced, and the red emission appears in the PL spectrum. The PL spectrum of the red emission results from the blue light transmitted through the phosphor layer and reabsorption of the photons emitted from the phosphor layer. However, Figure 3b shows the PL spectra of the layer with QD only and of the phosphor layer with QD, respectively. The red emission attributed to the core-multishell QDs shifts slightly to a longer wavelength, from 617 to 622 nm, compared with the PL of the QD solution in Figure 1d. This might be due to the aggregation of QDs which results in a new energy band and the reabsorption of light in the silicone resin.20,21 When the phosphor layer is added to the QD layer, the PL intensity of the red emission by the QDs is slightly reduced, and the PL spectrum of the green
emission appears at 521 nm. This means that the light converted by the green phosphor is mostly due to blue light, not red. The slight change in PL intensity at the red region is likely due to light scattering at the interface area between the QD and phosphor layer. Also, micrometer-sized phosphor particles in the phosphor layer could cause the light scattering, which would lead to a decrease in PL intensity at the QD emission region. Figure 3c shows the PL spectra of the phosphor layer on the QD layer, the QD layer on the phosphor layer, and the mixed layer. The QD layer on the phosphor layer exhibits the highest PL intensity of green emission among these three structures, even though they maintain a similar intensity of red emission. This indicates that the layer adjacent to the light plays a major role in the light conversion. Figure 4 shows the PL decay curves of nanocomposite films with only QD layers, only phosphor layers, phosphor layers on QD layers, QD layers on phosphor layers, and mixed layers, respectively. As shown in Figure 4a and b, the decay time of the red emission of the QD layer is shorter than that of the green emission of the phosphor layer. The decay times of the QDs and phosphors are approximately 9 and 380 ns, respectively. Strong energy transfer is confirmed by the fast decay time.22 As shown in Figure 4c e, the decay properties of the phosphor layer do not vary with the film structure. This means that the phosphor layer emission is unaffected by the QD layer emission. For the QD layer on phosphor (Figure 4c), the decay profile of the QD layer shows a two-step change. An initial decay profile followed the QD emission, whereas the decay profile after 90 ns seemed to be due to the reabsorption of photons emitted from the phosphor layer, as shown in the inset of Figure 4c. The decay of the second profile follows the slope of the phosphor decay. The slow decay of QDs after 90 ns is interpreted as an effect of energy transfer 20948
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Figure 4. Time resolved spectroscopy measurement of nanocomposite films composed of phosphors, core-multishell QDs, and resin at the pulse laser diode at 400 nm: (a) only QD layer; (b) only phosphor layer; (c) QD layer on phosphor layer (the inset shows decay profiles of the same structure in short time scale, the shaded area); (d) phosphor layer on QD layer; (e) mixed layer.
from the phosphor layer. Parts d and e of Figure 4 show similar profiles, but the intensity at the second step is somewhat different. As represented in Figure 3, the QD layer on phosphor exhibits the highest values in PL intensity and the mixed layer shows higher values than the phosphor layer on the QD layer. This indicates that more photons from the phosphor are transferred to the QDs in the QD layer on the phosphor layer than in other layered and mixed
structures. Unexpectedly, it can be seen that the photons from the phosphor transfer to the QDs in the case shown in Figure 4d, even though the QD layer is above the phosphor layer. It is likely that backscattered light from the phosphor is emitted to the QD layer. WLEDs have attracted both scientific and commercial attention. They may be useful for many solid-state illumination applications, including general lighting, backlighting of large displays, and 20949
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Figure 5. EL spectra of blue-light-pumped LEDs when operated at 350 mA: (a) QD layer on phosphor layer (only phosphor layer, black; QD layer on phosphor layer, red); (b) phosphor layer on QD layer (only QD layer, black; phosphor layer on QD layer, red); (c) QD layer on phosphor layer (black), phosphor layer on QD layer (red), and mixed (green) layer.
decorative lamps as well as in the automotive industry due to their environmental friendliness, high efficiency, applicability in a wide range of sizes, and low power consumption.23 26 There have been many attempts to develop QD color converters for WLEDs.20,21,27 30 Conventional phosphor-based WLEDs emit a bluish, cold white light with poor color CRI, and their optical properties change according to the input power.31 33 Effective structures with high luminance are still required to achieve high performance of QD phosphor WLEDs. To fabricate WLEDs, red core-multishell QDs, green phosphor, and thermocurable polymer are coated onto a blue LED chip using a blending and LBL assembly technique (QDs on phosphor and phosphor on QDs). Figure 5a shows the EL spectra of blue-light-pumped LEDs using only a phosphor layer and a QD layer on a phosphor layer. Similar to the PL results, only the phosphor layer does not show any red emission and, after hybridization of the QD layer on the phosphor layer, a large drop in blue and green emission and a small rise in re-emission appear. These results suggest that both blue and green light can be reabsorbed by QDs. Figure 5b shows the EL spectra of only the QD layer and phosphor layer on the QD layer. As expected, only the QD layer does not show green emission. However, when the phosphor layer is hybridized on the QD layer, a decrease of blue emission, increase of green emission, and no change of red emission are observed. It is evident that QD emission has no relationship to phosphor emission. Figure 5c shows the EL spectra of the phosphor layer on the QD layer, the QD layer on the phosphor layer, and the mixed layer on blue-light-pumped LEDs. The QD layer on the phosphor layer has the most EL peak
area, and the phosphor layer on the QD layer has the least EL peak area. The decrease of the EL peak area corresponds to a reduction in luminous efficiency. Table 1 summarizes the CIE coordinates, luminous efficiency, CCT, CRI, and chip temperature of blue LED chips with QD layers on phosphor layers, phosphor layers on QD layers, and mixed layers. As predicted from the EL spectra results, the QD layer on phosphor layer case shows the best luminous efficiency (71.1 lm 3 W 1, compared with 58.5 lm 3 W 1 for the phosphor layer on QD layer and 63.5 lm 3 W 1 for the mixed layer). This means that the QD layer on the phosphor layer has the broadest area by effective positioning. The CCT of the QD phosphorconverted WLEDs is 8684, 4698, and 6439 K for the QD layer on phosphor layer, phosphor layer on QD layer, and the mixed layer, respectively. In the case of the QD layer on phosphor layer, the tristimulus coordinate shifts to a colder CCT due to the decreased red spectral region, and in the phosphor layer on QD layer case, the CCT becomes warmer due to the increased red emission. The QD phosphor-converted WLEDs have CRI values of 88.4, 71.9, and 80.5 for the QD layer on phosphor layer, phosphor layer on QD layer, and mixed layer, respectively. High CRI values corresponding to the QD layer on phosphor layer are attributed to the broadest emission band and balanced RGB spectrum. Additionally, when only resin is used, the chip temperature is about 79.9 °C. However, the layered and mixed structures containing QDs and phosphors have much higher chip temperatures than does the resin alone. In particular, the phosphor layer on QD layer has the highest chip temperature (103.7 °C). This demonstrates that the photon energy of the QD layer which is absorbed by most of the blue light is lost as 20950
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Table 1. The CIE Coordinate, Luminous Efficiency, Color Temperature, Color Rendering Index, and Chip Temperature of WLED with QD Layer on Phosphor Layer, Phosphor Layer on QD Layer, and Mixed Layer on Blue LED Chip CIE coordinate
luminous efficiency (lm 3 W 1)
color temperature (K)
color rendering index
QD on phosphor
(0.2889, 0.3627)
71.2
8684
88.4
90.5
phosphor on QD mixed
(0.3651, 0.2869) (0.3178, 0.3426)
58.5 63.5
4698 6439
71.9 80.5
103.7 94.5
chip temperature (°C)
Figure 6a shows the CIE color coordinates of the chromaticity diagram of the QD layer on phosphor layer, phosphor layer on QD layer, and mixed layer cases on light-pumped LEDs and corresponding photo images at an operating current of 350 mA. The CIE coordinates of the QD layer on phosphor layer are (0.2889, 0.3627), those of the phosphor layer on QD layer are (0.3651, 0.2869), and those of the mixed layer are (0.3178, 0.3426). For layered structures, the color of the light emitted by the device shifts to the color emitted by the layer adjacent to the blue light. For the mixed structure, the device emits the color between the QD layer on phosphor layer and the phosphor layer on QD layer. Figure 6b shows the EL spectra as a function of QD content for a forward current of 350 mA when the QD layer is hybridized on top of the phosphor layer. As the QD content increases, a decrease in blue and green spectral regions and a relative increase in red spectral region are observed simultaneously. When the QD layer on phosphor layer is used, the luminous efficiency, CCT, and CRI of QD phosphor-converted WLED are (69.5 lm 3 W 1, 9184 K, and 86.2), (71.2 lm 3 W 1, 8684 K, and 88.4), (67.1 lm 3 W 1, 7732 K, and 81.2), and (64.6 lm 3 W 1, 6954 K, and 74.5) corresponding to QD content of 0.1, 0.2, 0.3, and 0.4 wt %, respectively. It was possible to control the optical properties of WLEDs according to the individual layer order and concentrations.
Figure 6. (a) The CIE color coordinates of the chromaticity diagram of a QD layer on phosphor layer, phosphor layer on QD layer, and mixed layer on light-pumped LEDs and corresponding photo images at an operating current of 350 mA. (b) The EL spectra as a function of QD content for a forward current of 350 mA when a QD layer is hybridized on top of a phosphor layer.
heat energy during light conversion from the QD. In the previous study, it was observed that the heat generated by QDs is much greater than that of phosphor.18 Actually, the chip temperature with a QD-only embedded nanocomposite is 130.2 °C and that with a phosphor-only embedded nanocomposite is 85.4 °C. In contrast, for the QD layer on phosphor layer, the chip temperature (90.5 °C) is lower than that of the phosphor layer on QD layer. This means that the small quantity of transmitted and reabsorbed light is converted by QDs, and the heat generated by QDs is quite small. When the mixed layer is used, the chip temperature (94.5 °C) is lower than that of the phosphor layer on QD layer but higher than that of the QD layer on phosphor layer. This supports the idea that the QD layer on phosphor layer structure is more appropriate as a color-converting nanocomposite, especially for WLEDs.
’ CONCLUSION In summary, this paper has developed a novel nanocomposite structure embedding CdSe/CdS/CdZnS/ZnS core-multishell red QDs and Sr2SiO4:Eu green phosphors in a silicone resin for a color converting material using various structuring methods (layered and mixed). It is observed that an enhancement of brightness is demonstrated and attributed to effective energy transfer, and that layer structure plays a major role in the light conversion. A two-step change of the QD emission due to the reabsorption of photons emitted from the phosphor layer leads to the broadest PL area and slow decay property of QD layers. When this layered structure is applied to fabrication of WLEDs, the performance of WLED is significantly improved in terms of luminance efficiency and CRI. It can be concluded that this layer-bylayer structuring of QDs and phosphor in WLEDs is a good solution for many optical applications. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: +82-2-3290-3354. Fax: +82-2-926-9290.
’ ACKNOWLEDGMENT This research was supported by the Industrial Strategic Technology Development Program (MKE), 21C Frontier Research Program from the Center for Nanoscale Mechatronics & Manufacturing (MEST), Basic Research (KIMM) and partially 20951
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The Journal of Physical Chemistry C Global Frontier Research Center for Advanced Soft Electronics, Korea. We thank Prof. Yong-Hoon Cho at KAIST for helpful discussion.
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dx.doi.org/10.1021/jp206120p |J. Phys. Chem. C 2011, 115, 20945–20952
