Printing of Micro-Lens Encapsulation Using Siloxane/Silica Nanocomposites for Highly Efficient Micro Light-Emitting Diode Arrays

Nowadays, micro-scale light-emitting diodes (μLEDs) have attracted tremendous attention as next-generation displays. Encapsulating materials are essential to protect μLEDs from external environments and to increase light extraction efficiency (LEE) [1]. Conventional encapsulating materials face challenges in terms of thermal stability, and refractive index, limiting the long-term reliability and efficiency of μLEDs [2]. To overcome these challenges, highly stable siloxane hybrid materials containing phenyl groups have been studied, suggesting that the phenyl group can improve optical performance [3]. In addition, lens-shaped encapsulation has been identified as a way to increase the light extraction efficiency of Chip-On-Board LEDs, but the existing method is complicated such as manufacturing unique supports on the substrate, making it difficult to apply to μLEDs [4].<br/>Herein, we have developed the highly stable μLED encapsulating material with high refractive index (RI) and thixotropic index (TI), which greatly improved LEE and reliability of μLED. To obtain μLED encapsulating material, we fabricated phenyl-based siloxane hybrid (PSH) of thermal-induced hydrosilylation reaction of sol-gel synthesized linear vinyl-phenyl siloxane (LVS) and linear hydrogen-phenyl siloxane (LHS) resins. By using two phenyl-based siloxane resins, we can induce the high phenyl group contents in matrix, which lead to high RI of matrix. Polydimethylsiloxane-terminated hydrophobic fumed silica nanoparticles (FSNs) were dispersed in PSH that act as giving thixotropic properties for the formation of lens-shape and inducing scattering effect for the increment of LEE. We studied trade-off relationship among optical transparency, thermal stability, and TI of PSH by varying amount of FSN in matrix. Through optimization of FSN contents in PSH, we achieved excellent optical and rheological properties (RI: ~1.59 @ 486 nm, and TI: 3.96 from 0.1 to 100 s^-1) of PSH with 1.5wt% of FSN. Furthermore, the fabricated PSH with 1.5wt% of FSN showed the high thermal stability at 130 °C for 700 hours. After encapsulating μLED arrays with PSH@FSN, the optical performance improved by 29.86% compared to the bare μLED arrays. Furthermore, we confirmed the reliability of PSH@FSN-encapsulated μLEDs under 85 °C/85% relative humidity, which exhibited almost similar luminous flux (86.4% retained) after 60 days aging.<br/><br/>[1] S. Lan et al., <i>Micromachines</i>, 10, 860 (2019).<br/>[2] J. Shen et al., <i>Silicon</i>, 5, 2163 (2023).<br/>[3] J.Y. Bae et al., <i>RSC advances</i>, 3, 8871 (2013).<br/>[4] Y. Gan et al., <i>J. Mater. Chem. C</i>, 2, 5533 (2014).