Facile fabrication of microlens array on encapsulation layer for enhancing angular color uniformity of color-mixed light-emitting diodes

In this work, we proposed a concave microlens array (MLA) on an encapsulation layer to enhance the angular color uniformity (ACU) of color-mixed light-emitting diodes (cm-LEDs). The MLAs with continuously adjustable curvature were fabricated by using a low-cost and facile approach based on water droplets condensation. Through initiative cooling, the water vapor was condensed on the surface of a polymer film and then the water droplets array was utilized as a template to obtain the MLA. By controlling the cooling temperature at -15, 0, and 15 degrees C, the sag height-to-diameter ratios (aspect ratio, AR) of MLAs were obtained at 0.68, 0.63, and 0.53, respectively. Through adjusting the three-phase state of the polymer-water droplets-sacrificial layer system by introducing a second-step cooling temperature of 0 and -15 degrees C after the first-step cooling temperature of 15 degrees C, the ARs were further decreased to 0.49, and 0.37, respectively. Haze tests demonstrated the scattering ability of MLAs, and encapsulation layer with MLA was applied in packaging of the cm-LEDs to enhance their ACU and luminous efficacy (LE). Consequently, the maximum correlated color temperature (CCT) deviation of the cm-LEDs was reduced from 853 K to 273 K and the LE of cm-LEDs was increased by 9.1% at 200 mA after applying the fabricated MLAs.

Automated summary

This study proposed using a concave microlens array on an encapsulation layer to enhance the angular color uniformity of color-mixed LEDs. By controlling cooling temperatures and introducing a multi-step cooling method, the sag height-to-diameter ratios of the microlens arrays were successfully reduced, leading to improved luminous efficacy of the LEDs.