High refractive index microlenses patterned onto micro-LED arrays using electrohydrodynamic inkjet printing

Reducing pixel crosstalk and enhancing luminance are crucial for improving image quality in micro-LED display applications. Although the Lambertian emission of micro-LEDs can be partially collimated by surface-integrated microlens arrays (MLAs), the collimation performance is often limited by the insufficient refractive index of conventional lens materials. In this work, we propose a direct on-chip integration of microlenses onto fabricated micro-LEDs in a pixel-to-pixel manner. The printed microlenses achieve a high refractive index of up to 1.77 using EHD inkjet printing. A hydrophobic surface treatment on the micro-LEDs is introduced to increase the lens sag height. The fabricated MLAs achieved a 16% increase in peak luminance and effectively suppressed large-angle side emission by approximately 12%. These results demonstrate a practical route for on-chip integration of MLAs on micro-LEDs, and the limitations of the collimation effect are discussed to guide future optimization.