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Reliability Concerns Related With the Usage of Inorganic Particles in White Light-Emitting Diodes Lilin Liu, Xizhao Tan, Yizhou Li, Mingyang Wu, Dongdong Teng, and Gang Wang
Abstract—Although adding nonemitting inorganic particles to the silicone encapsulant can be a strategy to improve reliabilities of white light-emitting diodes (wLEDs), its validity depends on the inorganic/organic interface compatibility. This work employs ZnO nanoparticles (NPs) with different surface conditions to study the physical relationships between degradation of wLEDs and inorganic/organic interfaces. Experimentally, it is found that 1 wt% of ZnO@SiO2 NPs in lens can slow down the degradation rate of wLEDs under both UV exposure conditions and pressure cooking test conditions. Index Terms—Inorganic nanoparticle, pressure cooking test, reliability, UV exposure, white light-emitting diode (wLED).
I. I NTRODUCTION
T
HE invention of efficient blue light-emitting diodes (LEDs) declares the huge potential of solid state lighting as an illumination technology of the future [1]. Commercial white LEDs rely on the excitation of a phosphor by a blue LED, where inorganic phosphor particles are embedded in a silicone matrix. In order to penetrate into general lighting market distinctly, reliability is of essential importance [2], [3]. In phosphor-converted wLEDs, phosphors are prone to strongly heat up during device operation and may reach temperatures which are far in excess of the junction temperature of the LED die [4]. Adding non-emitting particles to the silicone encapsulant was proposed as a strategy to lower the maximum and the surface temperature of the phosphor layers [4]. Inorganic nanoparticles of high refractive index, such as TiO2 , CdS, ZnS, ZrO2, etc., had been used to overcome the limitation of refractive index in polymer materials [5], [6]. A highrefractive-index optical encapsulant is highly desirable because it can result in enhancement of light-extraction efficiency from
Manuscript received June 3, 2014; accepted August 16, 2014. Date of publication September 16, 2014; date of current version December 2, 2014. This work was supported in part by the National High Technology Research and Development Program of China under Grant 2013AA03A106; by the National Natural Science Foundation of China under Grants 10802101 and U1201254; by the National High Technology Research and Development Program of China under Grant 2011BAE01B14; and by the Fundamental Research Funds for the Central Universities under Grant 2011300003161460. The authors are with the School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, China (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TDMR.2014.2358256
Fig. 1. Reaction scheme for preparing ZnO@SiO2 core-shell structured nano-particles.
high-n semiconductor light-emitting diode chips. Mont et al. [7] proposed a graded-refractive-index multilayer encapsulation structure with the thickness of each layer being less than the mean optical scattering length to reduce optical losses from scattering and Fresnel reflection. Theoretical light-extraction enhancements larger than 50% are predicted when comparing scattering-free to scattering encapsulation materials. Similar to other optical and electronics components, the packaged white LEDs are subjected to moisture containing environment and UV exposure in many of their applications. The phosphor particles in silicone will generate inorganic/organic interfaces. These interfaces will be sensitive to humidity and UV exposures. Although lots of work had been conducted to investigate the degradation and failure mechanisms of wLEDs under thermal and electrical stresses [2], [3], few efforts were laid on moisture induced degradation. The reliability of wLEDs under UV exposures is seldom considered due to the blind trust on the UV resistance of silicone. The present work will use ZnO nanoparticles with different surface conditions to study the physical relationships between degradation of wLEDs and inorganic/organic interfaces. II. E XPERIMENTAL P ROCEDURES A. The Preparation of ZnO Nanoparticles (NPs) Two kinds of ZnO NPs are synthesized by the sol-gel method: 1) ZnO NPs with a radius of 5 nm (named as ZnO-QD), 2) ZnO NPs with a radius of 5 nm and a shell of SiO2 (named as ZnO@SiO2 ). The reaction scheme for preparing ZnO@SiO2 core-shell structured NPs is given in Fig. 1. ZnO-QD is an
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LIU et al.: RELIABILITY CONCERNS RELATED WITH INORGANIC PARTICLES IN wLEDs
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Fig. 2. Schematic drawing of packaging structures: (a) without ZnO NPs; (b) with ZnO NPs in Lens.
intermediate product of ZnO@SiO2 . The main chemical reaction equations are listed as Zn2+ + 2OH− + 2H2 O = Zn(OH)2 + H2 O + 2− = Zn(OH)2− 4 + 2H Zn(OH)4
Fig. 3. The evolution of normalized luminous fluxes with the aging time under the PCT aging conditions: with different kind and weight percentage of NPs in lenses of wLEDs.
(z+2y−2x+2)−
+ Zn(OH)2− 4 → Znx+1 Oy+1 (OH)z+2 + H2 OSi(OR)4 + 4H2 O → Si(OH)4 + 4ROH (R represents alkyl) ≡ Si-OH + HO-Si ≡ → ≡ Si-O-Si ≡ +H2 O.
(1)
The radius of ZnO-QD and ZnO@SiO2 are calculated from the UV-visible absorption spectroscopy. B. Packaging of wLEDs Fig. 2 illustrates the packaging structures of wLEDs. Horizontal-structured blue LED chips with a size of 35 mils are used. The rated current is 350 mA. YAG phosphors are mixed into silicone liquid for dispensing. Its mass percent is 10% and particle size is 15 μm. The designed CCT is 6500 K. According to the results of our previous work [8], ZnO NPs will be added into the encapsulant lens only. Two kinds of ZnO NPs are used. The concentrations of NPs in lens vary from 0% to 3 wt%. Five samples for each group are fabricated. Except for special declarations, the experiment results are averaged over corresponding five samples. C. Aging Tests A pressure cooking test (PCT) will be conducted to simulate high temperature and high humidity stressing conditions, i.e., 115 ◦ C&0.17 MPa. The aging duration is 168 hours in total. Samples will be taken out for measurement at predesigned aging time stages. UV accelerated weathering tests are carried out under the exposure of a UV wavelength 313 nm and a radiation intensity 1 W/m2 . The total exposure duration is 400 hours. Samples will be taken out for measurement at predesigned aging time stages. During the aging processes, no electrical currents are input.
Fig. 4. Photos of wLEDs (a) before and (b) after aging under 115 ◦ C&0.17 MPa for 168 hours. A: ZnO@SiO2 , B: ZnO-QD.
of normalized luminous fluxes with the aging time. After aging for 168 hours, the luminous fluxes of all seven groups of wLEDs present different extents of degradation. In general, at the same weight percent of NPs, the usage of ZnO@SiO2 leads to less luminous degradation than the usage of ZnO-QD. Compared with the control samples, i.e., no NPs in lens, the mixture of NPs in lens will aggravate the degradation of luminous fluxes of wLEDs, except for the cases of 1 wt% and 0.5 wt% ZnO@SiO2 in lens. In fact, the wLEDs with 1 wt% ZnO@SiO2 in lens exhibit best luminous maintenance among the six groups. Its luminous flux even increases by a ratio around 1% within the aging duration of 60 hours. The photos of wLEDs before and after 168 hours’ aging are shown in Fig. 4. For the as-fabricated wLEDs, the lens becomes murky with the increase of density of NPs. But in whole, at the same density, wLEDs with ZnO@SiO2 fillers exhibit better transparency than those with ZnO-QD fillers. After 168 hours’ aging, the former ones still keep limpidity, while the later ones become completely turbidity.
III. R ESULTS A. Degradation Behaviors Under PCT Conditions ◦
A constant temperature of 115 C and a humidity of 100% are applied on the packaged wLEDs. Fig. 3 shows the evolution
B. Degradation Behaviors Under UV Exposures Accelerated UV weathering tests are carried out under the exposure of a UV wavelength 313 nm and a radiation intensity
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degradation with the NPs’ concentrations indicate that the SiO2 shell structure plays a critical role. IV. D ISCUSSIONS
Fig. 5. Curves showing the lumen degradation as a function of UV exposure time: (a) wLEDs without NPs in lens; (b) wLEDs with ZnO-QD NPs in lens; and (c) wLEDs with ZnO@SiO2 NPs in lens.
1 W/m2 . The total exposure duration is 400 hours. Degradations of luminous fluxes are plotted as a function of UV exposure duration, as shown in Fig. 5. After 400 hours’ UV exposure, the luminous flux of traditional wLEDs, i.e., without NPs in the lens, degrades by a ratio of 6%. The wLEDs with the usage of ZnO-QD NPs at concentrations of 0.5 wt%, 1 wt% and 3 wt% present degradation ratios of 2%, 8% and 14%, respectively. However, when using ZnO@SiO2 NPs, at concentrations of 0.5 wt% and 1 wt%, the degradation ratios are around 4% and 1%; at concentrations of 3%, the luminous fluxes even increases by a ratio of 1%. The demonstrated opposite trends of luminous
Under both kinds of aging tests, wLEDs with the usage of ZnO-QD NP fillers show the same degradation trends with the increase of weight percents, while wLEDs with the usage of ZnO@SiO2 NPs in lens exhibit different degradation trends with weight percents. Such phenomena imply that incompatibility between inorganic NPs and organic matrix is the source reason for wLEDs’ degradation under pressure cooking test conditions and UV exposure conditions. The SiO2 shell can improve the compatibility between ZnO and silicone. However, moistures or UV lights affect reactions related with the SiO2 shell at the inorganic/organic interfaces. In the silicone/NPs composite, the NPs could be either a single NP or a cluster of NPs, depending on the dispersion state and concentration of NPs. Higher the concentration, clusters with a larger radius are more prone to form. Silicone is built up by Si-O chemical bonds. The SiO2 shell in a ZnO@SiO2 core-shell structure serves as a surfactant between ZnO and silicone. NPs are first mixed into Silicone B and then mixed with Silicone A. It is expected that curinggenerated heat will make chemical reactions occur at the surface of ZnO@SiO2 core-shell structures, leading to the ZnO cores grafted to the chains of silicone through the SiO2 shell. The pressure cooking test equipment provides high temperature (115 ◦ C) and high pressure (0.17 MPa) conditions, which can be looked as a polymer/inorganic synthesis tower. Within a suitable duration, chemical reactions at silicone/ZnO@SiO2 interface will graft more ZnO cores to the chains of silicone through the SiO2 shell. But this chemical reaction process seems to be finite, since NPs prefer to form clusters in silicone matrix at higher concentrations. Intrusion of moistures into the clusters may be locked in by Si-O chemical bonds, which damages light extractions due to total reflection effects. ZnO NPs have UV resistance ability themselves. But, mixing ZnO-QD NPs into the lens generates lots of polymer/inorganic interfaces. When the NPs’ concentration increases, they absorb more UV lights. Heat will be generated. The thermal expansion coefficients between NPs and silicone do not match with each other, leading to the degradation of polymer/inorganic interface compatibility. The light emissions from the wLEDs will be consumed at these interfaces. However, in the case of mixing ZnO@SiO2 NPs into the lens, the SiO2 shell promotes the polymer/inorganic interface compatibility. It is speculated that during UV exposure, the absorbed UV lights will generate large quantity of heat at the NPs. The ZnO cores will expand quickly, resulting in rapid increase of local pressure. A local high temperature and high pressure environment thus forms equivalently. Chemical reactions at silicone/ZnO@SiO2 interface under locally high temperature and high pressure will graft the ZnO cores to the chains of silicone through the SiO2 shell. At high concentration of ZnO@SiO2 NPs, the SiO2 shells will play the buffer role between ZnO and silicone. Therefore, a higher concentration of ZnO@SiO2 NPs in wLEDs presents stronger UV resistance.
LIU et al.: RELIABILITY CONCERNS RELATED WITH INORGANIC PARTICLES IN wLEDs
V. C ONCLUSION The present work demonstrates experimentally that adding non-emitting inorganic particles to the silicone encapsulant can be a strategy to improve reliabilities of wLEDs, depending on the inorganic/organic interface compatibility. 1 wt% of ZnO@SiO2 NPs in lens can slow down the degradation rate of wLEDs under UV exposure conditions and PCT conditions.
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Xizhao Tan, photograph and biography not available at the time of publication.
Yizhou Li, photograph and biography not available at the time of publication.
R EFERENCES [1] E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science, vol. 308, no. 5726, pp. 1274–1278, May 2005. [2] M. Meneghini, A. Tazzoli, G. Mura, G. Meneghesso, and E. Zanoni, “A review on the physical mechanisms that limit the reliability of GaNbased LEDs,” IEEE Trans. Electron. Devices, vol. 57, no. 1, pp. 108–118, Jan. 2010. [3] M. Meneghini, L. B. Trevisanello, G. Meneghsso, and E. Eznoni, “A review on the reliability of GaN-based LEDs,” IEEE Trans. Device Mater. Rel., vol. 8, no. 2, pp. 323–331, Jun. 2008. [4] P. Fulmek et al., “On the thermal load of the color-conversion elements in phosphor-based white light-emitting diodes,” Adv. Opt. Mater., vol. 1, no. 10, pp. 753–762, Oct. 2013. [5] C. Lu et al., “Preparation and characterization of ZnS—Polymer nanocomposite films with high refractive index,” J. Mater. Chem., vol. 13, no. 9, pp. 2189–2195, 2003. [6] J. Wang, D. Montville, and K. E. Gonsalves, “Synthesis of polycarbonateco-Poly(p-ethylphenol) and CdS nanocomposites,” J. Appl. Polym. Sci., vol. 72, no. 14, pp. 1851–1868, Jun. 1999. [7] F. W. Mont et al., “High-refractive-index TiO2 -nanoparticle-loaded encapsulants for light-emitting diodes,” J. Appl. Phys., vol. 103, no. 8, pp. 083120-1–083120-6, Apr. 2008. [8] M. Y. Wu, L. L. Liu, and G. Wang, “White LED devices with nearly uniform space-color distribution through nanoparticle usage,” in Proc. 14th Int. Conf. EMAP, 2012, pp. 1–4.
Lilin Liu received the Ph.D. degree in mechanical engineering from The Hong Kong University of Science and Technology, Sai Kung, Hong Kong, in 2006. She joined the School of Physics and Engineering, Sun Yat-sen University, Guangzhou, China, in 2007, where she is currently an Associate Professor. Her current research interests include optoelectronic devices, visible light communication, failure analysis, and reliability engineering.
Mingyang Wu, photograph and biography not available at the time of publication.
Dongdong Teng, photograph and biography not available at the time of publication.
Gang Wang received the bachelor’s degree from Jilin University, Changchun, China, in 1991 and the master’s and Ph.D. degrees from Nagoya Institute of Technology, Nagoya, Japan. Currently, he is the Dean of Foshan Institute, the Vice President of the School of Physics and Engineering, and the Director of the Center for Solid-state Light System, Sun Yat-sen University, Guangzhou, China. From April 2001 to April 2004, he was with (strain) Fujitsu Quantum Devices, Inc., Kofu, Japan, mainly engaged in R & D work of 10- and 40-Gb/s ultrahigh-speed optical communication devices. He has been with the State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, since May 2004, engaging in research and teaching of compound semiconductor materials and devices. His main academic and social work includes: 1) being the Expert Group Member of “China Solid State Lighting Alliance” 863 Project of the Ministry of Science and Technology of China; and 2) being the Chairman of Guangdong Illuminating Engineering Society. His main research areas are the MOCVD growth of compound semiconductors and related optoelectronic devices.
