LAUSR

The conventional chip-on-board light-emitting diodes (COB-LEDs) with flat encapsulant layer present low light efficiency due to the total internal reflection (TIR) happens at the encapsulant-air interface. In this study, microlens array was realized on the flat encapsulant layer of COB-LEDs to diminish the TIR at the encapsulant-air interface. The microlens array was fabricated by selectively exposing the UV glue film in UV-light through a pre-designed photomask. Micro-pillars form after the UV-light exposure and microlens array forms spontaneously due to the wetting process of uncured UV glue on the micro-pillars. The geometry of the microlens array was experimentally optimized, and the light efficiency enhancement of COB-LEDs by the microlens array was analyzed. The results show that optimal geometry of microlens array with curvature of ~41° and base diameter of $\sim 87~\mu \text{m}$ can be realized by adjusting the UV-light exposure time as >40 s and UV glue film thickness as $> 80~\mu \text{m}$ . Compared to the traditional flat encapsulant layer, the microlens array with optimal geometry increases the light efficiency of blue and ~5000 K white COB-LEDs by 50.9% and 9.31%.