title: Aspect ratio engineering of microlens arrays in thin-film flip-chip light-emitting diodes.
abstract: Light extraction efficiency of thin-film flip-chip InGaN-based light-emitting diodes (LEDs) with a TiO2 microlens arrays was calculated by employing the finite-difference time-domain method. The microlens arrays, formed by embedding hexagonal close-packed TiO2 sphere arrays in a polystyrene (PS) layer, were placed on top of the InGaN LED to serve as an intermediate medium for light extraction. By tuning the thickness of the PS layer, in-coupling and out-coupling efficiencies were optimized to achieve maximum light extraction efficiency. A thicker PS layer resulted in higher in-coupling efficiency, while a thinner PS layer led to higher out-coupling efficiency. Thus, the maximum light extraction efficiency becomes a trade-off between in-coupling and out-coupling efficiency. In addition, the cavity formed by the PS layer also affects light extraction from the LED. Our study reveals that a maximum light extraction efficiency of 86% was achievable by tuning PS thickness to 75 nm with maximized in-coupling and out-coupling efficiency accompanied by the optimized resonant cavity condition.
authors.forename: Peifen
authors.initials: P
authors.lastname: Zhu
authors.forename: Chee-Keong
authors.initials: CK
authors.lastname: Tan
authors.forename: Wei
authors.initials: W
authors.lastname: Sun
authors.forename: Nelson
authors.initials: N
authors.lastname: Tansu