In this study, the colloidal ternary ZnxCd1-xS (x = 0.5 and 0.8, named as Zn0.8 and Zn0.5) white quantum dots (WQDs) have been prepared and used to form WQDs-based white light emitting diodes (WLED) with three different encapsulation types (convert, remote and conformal type). Moreover, acyclic-based UV curing polymer was used to incorporate with WQDs. The optical properties of Znx WQDs-based WLEDs including Commission Internationale de l’Eclairage (CIE) chromaticity coordinates, correlated color temperature (CCT), color rendering index (CRI) and luminous efficacy can be tuned by controlling the compositions, blending content of Znx WQDs and encapsulated methods. The optimum result for Zn0.8 and Zn0.5-based WLEDs can be observed in the remote type with the CIE, CRI, CCT and luminous efficacy under 20 mA of (0.34, 0.32), 87, 5000 K and 11.93 lm/W and (0.40, 0.37), 86, 3400 K and 2.76 lm/W, respectively. The higher luminous efficacy and stability of WLEDs can be obtained due to use an acyclic-based UV curing polymer, high QY of WQDs and suitable encapsulated method. The results also have exhibited the potential applications of Znx WQDs as nanophosphors in WLEDs.
© 2016 Optical Society of America
In recent years, phosphor-converted white light-emitting diodes (PC-WLEDs) have attracted a significant amount of attention due to their high efficiency and reliability [1,2]. Since the application of traditional R/G/B phosphor is limited by the narrow excitation band, different excitation wavelengths and self-absorption problems, alternative materials have been developed . Because semiconductor quantum dots (QDs) possess controllable emission wavelength, broad excitation band, high quantum yield (QY) and quantum confinement effect [4–15], the band gap as well as the optical properties of QDs can be controlled by their particle sizes and compositions in the system [4,5]. As a result, QDs not only are regarded as the promising candidates to replace traditional phosphors [4–8] but also are highlighted as color-converting components in the fabrication of WLEDs and have great potentials in solid state lighting applications [4–6,12–16].
In order to generate white light, four methods have been reported [7,17–23]. Although phosphors are good at photoluminescence (PL) across the visible region, their emission spectrum is fixed . Besides, the scattering effect of phosphors is serious, resulting in the decrease of luminous efficacy . On the other hand, the use of combinations of QDs provides the ability to adjust the white light parameters although the self-absorption between different color emissions of QDs also results in the decrease of luminous efficacy . For these reasons, CdSe QDs are currently by far the most reported QDs used in combination with blue pumping LEDs due to their relatively high QY and the tunability of the emission over the entire visible range . Chen et al. have prepared R/G/B three different emission colors of CdSe/ZnSe and combined with InGaN chip to obtain a WLED devices with high color rendering index (CRI) of 91 . In this case, blending monochromatic QDs to form white light needs to carefully control the ratio of R/G/B QDs and the whole process is complicated. Therefore, white QDs (WQDs)-based WLED device has been developed.
CdSe WQDs with the magic size and a QY of 2~3% have been prepared and encapsulated with polyurethane to form lamp-type LED or dispersed in polyurethane to form a remote type-like plate which provides a more balanced white-light emission with chromaticity coordinates of (0.322, 0.365) . Nizamoglu et al. have prepared a simple convert type-like device of CdS WQDs (QY~17%) with PMMA. When excited by high power UV LED (383 nm and (die area of 7x7 mm2), this device can achieve a CRI higher than 70. The above results have also pointed out that the CRI, chromaticity coordinates (CIE), correlated color temperature (CCT) can be controlled by various concentrations and types of WQDs . On the other hand, Chandramohan et al. have reported that controlling the particle sizes of CdSe WQDs can adjust the device properties. The WQDs are dispersed in PMMA to form a plate and put on the top of InGaN/GaN LED chip (similar to a remote type). As a result, the overall spectra from warm to cool white can be obtained and the optimum device conditions are CIE of (0.36, 0.33) and CRI of 87 with the efficacy < 1 lm/W . Schreuder et al. have found out that the WQDs encapsulated in the biphenylperfluorocyclobutyl polymer to form a remote type-like thin film and excited by 365 nm UV-LEDs can provide a white light source with CIE of (0.32, 0.32) and a high CRI of 93. However, the luminous efficacy is only 0.19 lm/W .
Previously, we have prepared highly effective ternary ZnxCd1-xS (x = 0.5 and 0.8) WQDs with good stability . Furthermore, these WQDs have been encapsulated with silicone resin to form WLEDs in the convert type. The optimum chromaticity coordinate, CRI, CCT, and luminous efficacy for Zn0.8 and Zn0.5 based WLED with adding 9.1 wt% of WQDs under 20 mA are (0.43, 0.37), 90, 2830 K, and 0.94 lm/W and (0.36, 0.33), 86, 4240 K, and 4.12 lm/W, respectively . These Znx WQDs with adjustable optical properties are suitable to be used as nanophosphors to avoid the self-absorption problem for WLED application. However, we have found out that since the silicone resin was difficult to be cured in the environment with N, P, S or their compounds, the capping agent of WQDs hinders the curing process and the dispersion of WQDs in silicone was not uniform due to the high viscosity of silicone gel. As a result, most WQDs have settled and precipitated to the bottom of the reflect cup after 30 min and the WQD cannot be excited by a UV light successfully . It seems that the encapsulation method is a key factor in affecting the performance of WQDs-based WLEDs [26,33].
In order to overcome the curing problem, UV-resin (acrylic based) with low viscosity is used to replace silicone resin and encapsulation types (convert, remote and conformal type) effect is also exploed in this study.
2.1 Preparation of ZnxCd1-xS WQDs
Cadmium oxide (CdO, 99.999%) and stearic acid (SA, 99%) were obtained by Alfa Aesar. Zinc oxide (ZnO, 99.999%), sulfur powder (S, 99.98%), octadecene (ODE, 90%), hexadecylamine (HDA, 90%), and trioctylphosphine oxide (TOPO, 90%) were purchased from Sigma-Aldrich. Hexane (95%) and methanol (95%) were gotten from Macron Chemicals. All chemicals were used as received without purification.
Two kinds of colloidal ternary semiconductor ZnxCd1-xS (x = 0.5 and 0.8, named as Zn0.5 and Zn0.8) WQDs was prepared by thermal pyrolyzed organometallic route as our previous study.36 Total amount of 0.3 mmol of CdO and ZnO were mixed with SA, which was used as complex reagent, in a three-necked flask and then heated to 230 °C under Ar flow until a clear solution was formed to prepare the Cd/Zn-SA precursor. The solution was then allowed to cool down to room temperature, and a white solid precipitate was obtained. After Cd/Zn-SA precursor was formed, the mixture solvent, 15 mmol of TOPO and 24 mmol of HDA, was added into three-necked flask and stirred together under Ar at room temperature for 5 min, then reheated the sample up to 320 °C to form a transparency solution. At this temperature, S-ODE precursor, which was 1.5 mmol of sulfur dissolved in 4 mL of ODE, was swiftly injected into three-neck flask. The nuclei was formed quickly and samples were taken out under desired growth times (0, 10 and 60 min). After the reaction completed (about 60 min), the mixed solution was swiftly cooled down to 150 °C to stop the reaction. Samples were precipitated with hot anhydrous methanol for purification. The precipitate was dissolved in hexane to remove unreacted reagents and excess TOPO or HDA for further measurement.
2.2 Characterization of WQDs
The optical properties of the prepared WQDs were measured by fluorescence spectrophotometer (FL, Hitachi F-7000) and ultraviolet-visible spectrometer (UV-Vis, Jasco V-670 spectrometer). Relative quantum yields (QYs) of the WQDs were determined by comparing the area under the curve of FL emission with that of fluorescent dye (Rhodamine 101 in ethanol). The concentration of the WQDs and the Rhodamine 101 dye were adjusted to the same optical density at the excitation wavelength. Photoluminescence QYs were obtained by comparison with a standard R101 in ethanol. The standard quantum yield of R101 is 98%. The QY of Znx samples was calculated by the following equation:
2.3 Preparations of WLED devices
A mixture of Zn0.5 or Zn0.8 WQDs and transparent acrylic-based UV resin with two different WQDs/resin weight ratios (9.1 and 50.0 wt %) were prepared, and applied for the LED fabrication. The UV resin was provided by Greentask Co., Ltd. Polyurethane Acrylic (PUA) resin was used to incorporate with WQDs. PUA resin, prepared by mixing monomer (hexa-functional aliphatic urethane acrylate, difunctional aliphatic urethane acrylate, trimethylolpropane triacrylate, and tetrahydrofurfuryl Alcohol) with photo initiator (1-hydroxy-cyclohexyl-phenyl-ketone) by ultrasonic cleaner under dark environment, was used to incorporate with WQDs. Three different encapsulation procedures (convert, remote and conformal) are provided in Fig. 1. As illustrated, convert type devices were prepared by dropping the blended mixture of WQDs and resin until the reflective cup filled. The bottom of UV chip was deposited with 50 uL of PUA UV resin at first in remote type, and then the devices were formed by depositing the WQDs/UV resin mixture on the top of pure resin to fill the reflective cup. The conformal type devices were encapsulated by directly dropping 50 uL of different contents of WQDs/UV resin on the UV chip after that pure resin was filled the reflective cup. All WLEDs were fabricated by using 3020 surface-mounted device (SMD) typed InGaN/GaN-based 405 nm UV emitting LEDs with 13 mil. After the mixture was dropped on an UV LED, the subsequent curing was performed to form WLED. OPAS XLite 500 UV curing machine with 254/365 nm and 400 W was used to photo curing the QDs and PUA resin mixtures for 10 min. The performance of devices was measured by 15 cm integrating sphere (Isuzu Optics, ISM-360) under different applied currents (5~50 mA). The long term durability of Zn0.8 remote- and convert-type devices were tested under 20 mA.
3. Results and Discussion
TOPO/HDA capped WQDs are prepared by chemical route and the emission spectra are detected by FL spectrometer. Figure 2 shows the temporal evolution of FL spectra of Zn0.5 andZn0.8 WQDs excited by 365 nm. Two obvious peaks appear and white light can be obtained after growing for 60 min in which the QY of Zn0.8 and Zn0.5 is 56 and 35%, respectively. Higher QY and lower QY decay rate of QDs is benefit to increase luminous efficacy and lifetime of devices. In our previous results we have noted that the stability of WQDs is excellent and the Zn addition would manipulate the QY through control of the surface oxidation states, atomic arrangements and compression strain in QDs [34,35], in which the surface state emission is also related to the device properties in this study. More popular silicone encapsulation are used to incorporate with QDs, but the cross linking between oligmer and oligmer are inhibited due to the capping reagents-TOPO and HDA. This result in that the hardness and stability of devices are poor due to the QY of WQD decreases very quickly and capping reagent are molten by heat generates from UV chip, resulting in some of WQD fall down and flow to edge of reflective cup. Therefore, the curing problem must be overcome. Some of literatures choose acrylic based gel to incorporate with QDs [29,30], however the efficacy of devices are quite low. It might be due to the QY of WQDs is not high enough and the pack method is not suitable.
In this study, we choose high QY of WQDs and convert type to form WLEDs at first. The WQDs are mixed with PUA UV-curing resin under different concentration. PUA UV-curing resin, prepared by mixing monomer with photo initiator by ultrasonic cleaner under dark environment with the viscosity of 900 cps and refractive index of 1.4866. After photo curing by 254/365 nm UV light, the transmittance of PUA film is 83, 88, and 90% in 400, 600, and 800 nm, respectively, as shows in Fig. 3. The transmittances of PUA film in the visible spectral range are higher than 80%, implies that it is benefit for light extraction from UV chip and WQDs. A mixture of Zn0.5 or Zn0.8 WQDs and transparent UV-curing resin with two different WQDs/resin weight ratios, 9.1 and 50.0 wt %, were prepared, and applied for the convert type WLED fabrication. After curing process we found out that surface hardness of devices is improved. Figure 4 displays the EL spectra of convert type of the WQDs-based WLEDs. Insert photos show the device with 50 wt% QD content under 0 and 20 mA. It can be found out that the relative emission intensity coming from 405 nm UV chip is decreased due to the increase of WQDs concentration from 9.1 to 50 wt%, while the EL emission intensity is WQDs concentration-dependent and the energy transition between UV chip and WQDs is more efficiency so that the band-edge and surface state emission become more obvious for 50 wt% of WQDs content. The white light generated from single-type WQDs (Zn0.5 and Zn0.8) with a broad emission spectrum is tunable across the visible spectral range without self-absorption and the scattering effect. The detail device properties of Znx WLED with PUA UV resin are listed in Table 1. The optimum conditions of the CIE, CRI, CCT and luminous efficacy for Zn0.8 and Zn0.5 under 20 mA are (0.38, 0.36), 84, 3900 K and 9.76 lm/W, which is about 3 times higher than Zn0.8-Si as reported previously  and (0.51, 0.44), 87, 2290 K and 2.13 lm/W, respectively. These results indicate that a suitable encapsulation material and higher QY of WQDs can effectively improve the overall efficiency. Rosson et al. have predicted that by using a commercial 385 nm UV-LED, the efficiency of CdSe QDs with QY of 40%, is 3.8 lm/W, which is 4 times higher than the one with QY of 8% . Moreover, the curing problem in silicone resin has been solved by using PUA UV-resin. It seems that light excitation and absorption efficiency of WQDs is more effective when their QY is higher. The high transmittance of PUA resin in the visible range also might be one of reasons that luminous efficacy as high as 9.76 lm/W.
On the other hand, it is well known that the encapsulation method also strongly influences the device properties. Therefore, another two encapsulation methods, remote and conformal as shown in Fig. 1, with 50.0 and 9.1 wt% of WQDs also have been used. The EL spectra of WQD-based WLEDs are exhibited in Fig. 5 and the device properties are summarized in Table 1. Insert photos in Fig. 5 show the photographs of the remote type WLED under 20 mA. It can be noted that the emission peak at 405 nm corresponds to the UV chip, theemissions around 410 to 430 and 430 to 450 are from the band-edge emission of Zn0.8 and Zn0.5 WQDs, respectively, and the emission covering 450 to 700 nm belongs to the surface state emission of WQDs. The CIE and CCT can be adjusted with WQDs compositions and encapsulation methods. It is worth mentioning that the concentration of QWDs in three encapsulation types is inherently different in which WQDs in convert and conformal type LED possesses highest and lowest concentration, respectively.
In conformal type LED, WQDs directly contact with UV chip, resulting in the degradation of WQDs and the decrease of the devices efficacy. Therefore, we can find that the emission intensity of conformal type LED is the lowest. On the other hand, using remote encapsulation can avoid these negative effects and further enhance the devices efficacy from 2.63 to 2.76 and 10.03 to 11.93 lm/W for Zn0.5 and Zn0.8, respectively. To the best of our knowledge, the result is much higher than those reported in the literature [17,29–32,36]. When WQDs disperse in silicone resin, the curing of resin is inhibited. In this case, the PUA UV encapsulate can be cured more complete. Moreover, the viscosity of silicone resin is range from 2500 to 4000 cps, while the PUA UV resin is 900 cps. The uniformity of WQDs in PUA UV resin is better than that of in silicone resin, due to the low viscosity. Moreover, WQDs/PUA UV-curing resin does not contact with UV chip directly, damage of WQDs cause from heat generates by UV chip can be avoid. This might be the reasons that the luminous efficacy of remote-type WLED is higher than those reported in the literatures.
Figure 6(a) shows the CIE coordinates and efficacy variation of Zn0.5 and Zn0.8 WLED with three different encapsulation types. The results indicate that the CIE coordinates of WQDs-based WLED prepared by remote type are much closer to the white light point than that prepared by other two types. The CIE, CRI, CCT and luminous efficacy for Zn0.8 and Zn0.5 with adding 50 wt % of WQDs under 20 mA are (0.34, 0.32), 87, 5000 K and 11.93 lm/W and (0.40, 0.37), 86, 3400 K and 2.76 lm/W, respectively. Remote-type WLEDs show the highest luminous efficacy, as shows in Fig. 6(b). Besides, this very high luminous efficacy and parameters such as CRI, CIE and CCT of WQDs-based WLED can also meet the demands of lighting.
Figure 7(a) shows the EL spectra integrated area and of Zn0.8 WLEDs under long period of time for stability test. It can be found clearly that the EL spectra areas are decreased with working time. The devices performance is showed in Table 2 and the efficacy variation of Zn0.8 WLED shows in Fig. 7(b). For remote type WLED, the luminous efficacy decreases from 11.93 to 11.88 lm/W, CIE change from (0.34, 0.32) to (0.33, 0.29), CCT increases from 5000 to 5760 K, and CRI decreases from 87 to 83 after 9 hrs under 20 mA. After 10 hour-operation, the CIE changes to (0.32, 0.26), CCT increases to 7220 K and CRI decreases to 77. After that the CIE leaves the white light range. On the other hand, the convert type of devices has excellent stability under 20 mA. The luminous efficacy slight increases from 9.76 to 12.16 lm/W after 7 hrs, and then decreases to 10.10 lm/W after 30 hrs. The increase in the first 7 hr may be because the curing is finally complete during this period of time. The EL result also suggests that the intensity of surface state emission for remote type WLED decreases very significantly, while it can be maintained for convert type WLED. As a result, the stability of covert type WLED is better than that of remote type one.
Moreover, the optical properties of WLED devices under different applied currents have also investigated and the result are showed in Fig. 8 and Table 3. When the current is increased from 5 to 50 mA, the luminous efficacy, CIE, CRI, CCT for remote type change from 13.85 lm/W, (0.35, 0.32), 87, 4970 K to 7.88 lm/W, (0.33, 0.30), 89, and 5430 K, respectively. For convert type WLED, luminous efficacy, CIE, CRI, CCT change from 12.43 lm/W, (0.39, 0.36), 83, 3830 K to 5.84 lm/W, (0.36, 0.33), 86, and 4090 K, respectively. The luminous efficacy for both devices decrease with increasing applied current. In Fig. 8, the surface state emission intensity of remote as well as convert types decreases slightly. Based on above results we can find that the surface state emission intensity decreases under high applied currents and long period of working time.
In summary, the colloidal ternary semiconductor WQDs, Zn0.5 and Zn0.8, are prepared and used as nanophosphors in a 405 nm UV-LED pumping device with PUA UV-curing resin and three different encapsulation types. PUA resin has positive effect to dispersion WQDs and can be cured more complete. By simply changing the fabrication parameters, the device properties of WLED can be tuned easily. The optimum conditions can be observed in remote type in which the CIE, CRI, CCT and luminous efficacy for Zn0.8 and Zn0.5 under 20 mA are (0.34, 0.32), 87, 5000 K and 11.93 lm/W and (0.40, 0.37), 86, 3400 K and 2.76 lm/W, with adding 50 wt % of WQDs, respectively. The durability of convert type is better than that of the remote one. Based on the above results, we can conclude that highly-effective WQDs-based LED with adjustable CIE, CCT and CRI can be obtained by controlling the encapsulate method. PUA resin is a suitable encapsulation resin to incorporate with WQDs to achieve a high luminous efficacy.
This work was supported by the Ministry of Science and Technology of Taiwan under Contract nos. 104-2628-E-008-005-MY3, 103-2221-E-150-055 and 104-2221-E-150-049.
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