Materials Today Communications
Adhesive resin composites with ceramic nanoparticles for enhanced light extraction efficiency of sandwiched LED device structure
Introduction
Phosphor-converted solid-state white light-emitting diodes (WLEDs) are energy-saving and eco-friendly luminescent devices that have played a crucial role in applications of display backlighting, indicators, automobile headlights, and general indoor illumination due to their high luminous efficiency, low power consumption and long-term reliability [[1], [2], [3], [4]]. Commercial lighting systems generally consist of a blue LED chip and a yellowish phosphor that produces white light. In addition, matrix materials are essential to homogeneously disperse the phosphor particles onto the LED chip and maintain the shape. Transparent glass with a high thermal stability is a promising matrix material that can replace a conventional silicone-based polymer matrix for a high-power WLED with a high operating temperature; glass with phosphor particles is called phosphor-in-glass (PiG) [[5], [6], [7], [8]]. With the development of LED lighting to meet customer requirements, WLEDs continue to improve and innovate towards smaller, lighter and thinner luminaries. In keeping with this trend, chip-scale packages (CSPs), which are equal in size to or slightly larger than a die, are increasingly being implemented in lighting designs [9,10]. During fabrication of CSPs, an adhesive resin is necessary to directly bond the PiG plate to the LED chip. However, the refractive index (n) of the commercial adhesive resin (n: 1.40) is much lower than those of the LED semiconductor material (n: 2.50–3.50) and the PiG plate (n: 1.60–1.80) [11,12]. The large difference in refractive index makes the reflection quite substantial; thus, the produced light is partially reflected back into the semiconductor, where it may be absorbed and turned into additional heat, resulting in low light extraction efficiency of the WLED device [13].
A high refractive index can be achieved by introducing inorganic nanoparticles with high refractive indices to the adhesive resin [14,15]. These are polymer-based nanocomposites that can achieve both adhesion strength and high refractive index. It should be noted that although the inorganics Si, Ge, GaP, InP and PbS have high refractive indices (n > 4.0 at 500 nm), they exhibit higher absorption coefficients in the visible region and this results in an opaque state. Compared to these materials, zirconium dioxide (ZrO2) has been identified as an attractive wide-bandgap semiconductor without any visible light absorption due to its large bandgap energy [[16], [17], [18]]. In addition, its high refractive index (n: 2.18 at 500 nm) allows the use of fillers in transparent anticorrosive, antireflection, and high-n coatings and films [[16], [17], [18], [19], [20]]. In addition, the ZrO2 particles should have a diameter below 20 nm to suppress losses by light scattering in the visible region, causing haze or even turbidity [21].
Previous research on improving the refractive index through the introduction of ceramic nanoparticles in WLED technology has focused on matching the refractive index between encapsulants and the phosphor layer [[22], [23], [24], [25]]. Few studies, however, have focused on reducing the difference in refractive index between LED chips and PIGs to improve the light extraction efficiency of WLEDs. In this study, the refractive index of adhesive resins that were applied between LED chips and PIG plates to fabricate CSPs were improved by introducing ZrO2 nanoparticles, and the effect of the introduction of the ZrO2 nanoparticles on the light extraction efficiency is discussed. ZrO2 nanoparticles with an average size of 10 nm were synthesized by salt-assisted ultrasonic spray pyrolysis and were dispersed into the adhesive resin matrix to increase the refractive index of the adhesive layer between the LED chip and PIG plate. The effect of the nanoparticles on the light extraction efficiency of WLEDs was investigated by varying the nanoparticle content from 4.5 wt.% to 18 wt.% in the resin matrix. Adhesive resin nanocomposites were added to the WLEDs as an adhesive layer. The effects of ZrO2 nanoparticle loading contents on the luminous flux and color coordinate of the WLED devices were investigated systematically, and the optimum nanoparticle content is presented.
Section snippets
Experimental details
ZrO2 nanoparticles (NPs) were prepared by salt-assisted ultrasonic spray pyrolysis combined with the citrate precursor method that was reported in our previous paper [26]. In brief, 10 mM starting solutions were prepared by dissolving zirconium oxynitrate hydrate (ZrO(NO3)2 · xH2O, 99 %) in distilled water. A 20 mM aqueous citric acid monohydrate (C6H8O7, > 99.0 %) solution was mixed into the zirconium precursor solution to form zirconium carboxylate complexes, and then the as-prepared 80 mM
Results and discussion
Fig. 2 shows the TEM image and XRD pattern of the ZrO2 nanoparticle synthesized by salt-assisted ultrasonic spray pyrolysis combined with the citrate precursor method. The nanoparticles have a uniform spherical shape, a narrow size distribution and an average particle size of 9.8 nm. The XRD pattern matches the standard pattern of tetragonal ZrO2 (JCPDS card # 79–1796) well. The average crystallite size of ∼9.3 nm is calculated from the (112) diffraction peak using the Debye-Scherrer formula [27
Conclusion
In conclusion, an adhesive layer having an appropriate amount of high-refractive-index ceramic nanoparticles, which is located between a blue LED chip and a PiG plate, can offer improvements in the performance of WLEDs in a range of applications. The resulting data show improved light extraction efficiency and luminance, when an adhesive layer containing ZrO2 nanoparticles was utilized in WLEDs. The reason for the improvement in the efficiency was found to be that the viewing angle of the light
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education (NRF-2018R1D1A1B07048149).
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These authors contributed equally to this work.