Facile fabrication of spherical silicone encapsulant with Pt nanoparticles for applications as white light-emitting diode

Materials Letters 134 (2014) 244–247

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Facile fabrication of spherical silicone encapsulant with Pt nanoparticles for applications as white light-emitting diode Kuan-Chieh Huang a,n, Yi-Ru Huang b, Shao-Ying Ting a, Chun-Ming Tseng a, Snow H. Tseng b, Jing-En Huang a a b

R&D Center, Genesis Photonics Inc., No. 5, Dali 3rd Rd., Southern Taiwan Science Park, Tainan 74144, Taiwan Graduate Institute of Photonics and Optoelectronics, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan

art ic l e i nf o

a b s t r a c t

Article history: Received 14 June 2014 Accepted 14 July 2014 Available online 19 July 2014

In this letter, optical performances of white light-emitting diode (LED) packages, based on silicone encapsulants with geometries of sphere and hemisphere, were investigated. The spherical encapsulant has a high potential for directing light and was prepared facilely by adding the Pt nanoparticles (PtNPs) obtained through thermal decomposition. Such silicone encapsulant with PtNPs containing welldispersed phosphor particles was achieved in comparison to the hemispherical encapsulant with unfavorable distribution of phosphor. Under 7 wt% phosphor incorporation and 350 mA injection, the white LED packaged with spherical encapsulant delivers a significantly higher luminous efficacy of 105.9 7 2.9 lm/W with reference to the performance of hemisphere-shaped white LED package (87.872.0 lm/W). & 2014 Elsevier B.V. All rights reserved.

Keywords: Nanoparticles Package Spherical encapsulant Structural White light-emitting diode

1. Introduction Packaging is crucial to the achievements of light-emitting diode (LED) with enhanced light extraction efficiency and prolonged lifetime. The radiation pattern of an LED can also be shaped through packaging [1]. Generally, the encapsulating material, such as silicone [2] or epoxy resin [3], has been incorporated into the LED package because of its adequate refractive index (n). Owing to the advantages of silicone, including tunable n, adjustable hardness, and thermal stability [4], great attention is paid to the silicone-based LED package. When a blue emitter serves as a pumping engine in the packaged LED, the yellow-emitting phosphor (e.g., YAG:Ce3 þ ), embedded in the encapsulant, is essential for white illumination. This is well known as the formation of phosphor-converted white LED [5]. However, phosphor sedimentation, attributing to large particle size, often occurs naturally during curing process [6]. Therefore, it can lead to inhomogeneous suspension of phosphor in the encapsulant. Consequently, a decrease in luminous efficacy of white LED lies in the fact that the unfavorable phosphor distribution is detrimental to the conversions of blue photons into yellow photons [6,7]. It also deteriorates the consistency of product quality for industrialscale production.

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Corresponding author. Tel.: þ 886 6 505 3500. E-mail address: [email protected] (K.-C. Huang).

http://dx.doi.org/10.1016/j.matlet.2014.07.095 0167-577X/& 2014 Elsevier B.V. All rights reserved.

Nakamura et al. [8] reported that an LED, applying a glass encapsulant based on spherical shape, benefits light collimation. Additionally, the geometry of encapsulant can be controlled through active packaging (AP) technique [9]. The light of an LED with mushroom-like epoxy encapsulant, obtained by AP, is capable of being focused in the absence of lens [10]. Therefore, in this letter, a spherical configuration of silicone encapsulant has been prepared for white LED via a simple process. We have conducted the spherical silicone encapsulant, having phosphor dispersion, easily for obtaining white LED with highly optical performance by mediation of synthesized catalyst, Pt nanoparticles (PtNPs). Simulation analyses are also made to substantiate the results.

2. Experimental Dihydrogen hexachloroplatinate(IV) hexahydrate (H2PtCl6  6H2O) 99.95% purity, Alfa Aesar, was dissolved in isopropyl alcohol (IPA) at first. This solution was further incorporated into one part of a commercial silicone-A/B kit, labeled as Part-A, to obtain a mixture with homogeneous state (Fig. 1(a)). The major component of Part-A is an oligomer based on vinyl-terminated siloxane backbone. In the mixture, H2PtCl6  6H2O/IPA and Part-A were maintained at an equal weight ratio. While the mixture was being heated, the Pt formed progressively due to thermal decomposition of H2PtCl6  6H2O [11]. The color of mixture thus changed from ivory yellow (Fig. 1(a)) to dark

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brown (Fig. 1(b)) in 5 min. The dark appearance of the resultant relates to the aggregation of PtNPs [12]. Afterward, the above-mentioned product was combined with the other part of the kit, Part-B, to acquire final silicone slurry. The Part-B majorly consists of alkyl hydrosiloxane oligomer. The GaN-based LED was developed in the form of flip-chip structure, and its blue light (λp ¼ 450 nm) is emerged under current injection. The spatial distributions of blue rays were simulated for the samples by means of Monte Carlo ray tracing. All optical performances reported were obtained by integrating sphere controlled at 350 mA.

3. Results and discussion

Fig. 1. Photographs of cuvettes with (a) H2PtCl6  6H2O/IPA/Part-A and (b) PtNPs/Part-A.

Prior to assembly of LED package, the weight ratio of H2PtCl6  6H2O/IPA and Part-A was adjusted to 1:9 (w/w). Reducing the H2PtCl6  6H2O concentration is to make the final silicone slurry avoid screening against light due to large numbers of PtNP aggregates. The formation of PtNPs is confirmed by XRD characterization (see Supporting information). The silicone slurry, prepared through thermal decomposition, was deposited on the

Fig. 2. Optical microscopy images of (a) spherical and (b) hemispherical silicone encapsulants on flip-chip LEDs with submounts (scale bar: 500 μm); (c) and (d) show their spatial distributions of blue rays. (e) Angular distributions of blue light intensities of spherical and hemispherical encapsulants, obtained by experimental characterizations and simulation analyses (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

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flip-chip LED bonded with submount by dispenser. A spherical molding created spontaneously for the silicone while the dropping took place (Fig. 2(a)). This can be explained by the acceleration of curing reaction of the silicone, attributing to the presence of PtNP catalyst, thereby leading to stabilization of spherical encapsulant on the chip. On the other hand, the silicone drop without addition of PtNP, obtained by thermal decomposition, spread over the top surface of the chip-assembled submount after the deposition was carried out. Eventually, a hemispherical configuration of encapsulant developed (Fig. 2(b)). We have reasoned that such an occurrence results from a slowly curing rate of the silicone. In Fig. 2(c), large numbers of rays are obviously observed inside the spherical encapsulant under irradiation of blue light, with reference to the case of hemisphere (Fig. 2(d)). The increased density of ray is due to the total internal reflection happening at the interface between silicone (n ¼ 1.5) and air (n ¼1) at high angles of the spherical encapsulant. Thus, the reflection of light has a better chance to emerge from the zenith of the spherical cap (Fig. 2(c)) in comparison with the flood lighting originating from the hemispherical one (Fig. 2(d)). In Fig. 2(e), the directional illumination can be confirmed by higher intensity of light in the range between 451 and 451 of viewing angle for the LED with spherical

encapsulant than that with hemispherical encapsulant. Additionally, the trends in angular distributions of light intensities are also consistent with the results obtained by simulation analyses (Fig. 2(e)). To evaluate the dispersing behavior of phosphor in the foregoing silicone encapsulants, the particulate YAG:Ce3 þ was further incorporated (Fig. 3). The additions of small numbers (1 wt%) of the particles were preformed due to the consideration of easy observation by the unaided eye. In Fig. 3(a), it is clearly observed that the YAG:Ce3 þ particles distribute uniformly in the space of the spherical molding. This has much to do with the rapidly curing reaction for the silicone applying PtNPs, thereby resulting in the phosphor barely having time to translocate. However, the phosphor sedimentation takes place inside the hemispherical encapsulant because of the force of gravity dominating throughout the slowly curing process (Fig. 3(b)). By adding 7 wt% of phosphor, the corresponding white LED with spherical encapsulant can render cool white light, as determined by its color coordinate in the tolerance quadrangle with correlated color temperature (CCT) of 5700 K (Fig. 4(a)). Under the same phosphor content (7 wt%), a much higher CCT (ca. 9000 K) of the white LED using hemispherical molding was obtained, with

Fig. 3. Photographs of (a) spherical and (b) hemispherical silicone encapsulants containing 1 wt% phosphor.

Fig. 4. (a) CIE 1931 color coordinates and (b) electroluminescence spectra of white LEDs with spherical and hemispherical encapsulants containing 7 wt% phosphor. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

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reference to the case of spherical encapsulant. This high CCT, deriving from hemisphere-shaped LED package, is in accordance with the relevant chromaticity coordinate (0.289, 0.276) lying close to the blue color region (Fig. 4(a)) [13]; implying that excess blue photons are incapable of being absorbed by phosphor. In Fig. 4(b), the existence of the unabsorbed photons is confirmed by the larger electroluminescence peak, centered at 450 nm, in the case of hemisphere than that in the case of sphere. Owing to the relatively smaller emission peak (@450 nm) for the white LED with spherical molding, it is suggested that this package is beneficial for the absorption of blue photons as well as the conversions of blue photons into yellow photons by YAG:Ce3 þ [14]. The preferable conversion is further verified by the enhanced yellow emission, ranging from 570 to 590 nm, in the case of sphere, with reference to the case of hemisphere (Fig. 4(b)). The improvement in the ability of photon conversion for the white LED employing sphereshaped package is attributed to the increased density of blue ray inside the spherical encapsulant (see Fig. 2(c)), thereby increasing the chance of making the blue light excite the phosphor particles. Consequently, a higher luminous efficacy of 105.97 2.9 lm/W of the white LED with spherical encapsulant was delivered in comparison to the package based on hemispherical molding (87.872.0 lm/W). 4. Conclusions In summary, a spherical silicone encapsulant with PtNPs, obtained by thermal decomposition, for white LED package is reported. The large surface area of the PtNPs effectively catalyzes the curing reaction of silicone. The light, generated from packaged LED with spherical structure, can be collimated in the range between 451 and 451 of viewing angle without secondary optics design. Such an encapsulant containing particulate phosphor

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contributes to these particles against sedimentation as well as improvement in light-induced phosphor excitation. An about 20.6% enhancement in luminous efficacy of white LED encapsulated with spherical silicone/phosphor is thus achieved, with reference to the white LED having hemispherical encapsulant with the same phosphor content.

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