Enhanced phosphor conversion efficiency of GaN-based white light-emitting diodes having dichroic-filtering contacts

Abstract

White light-emitting diodes (LEDs) based on the combination of a GaN-based blue LED chip and a yellow phosphor layer suffer from a low phosphor conversion efficiency (PCE) because a significant amount of the yellow fluorescence is emitted towards the blue LED chip where the fluorescence is partially absorbed. In this study, we greatly enhanced the PCE of a white LED by embedding a dichroic-filtering contact (DFC) which multi-functioned as a blue-transmitting but yellow-reflecting optical filter as well as a low-resistance ohmic contact to p-type GaN. Electrically conductive DFCs consisting of alternating layers of dense and nanoporous indium-tin-oxide were designed using a genetic algorithm optimization method, and experimentally realized by the oblique-angle deposition technique, which enables tuning of the refractive index of a single material. GaInN/GaN multiple quantum well LEDs with 3- and 5-layer DFCs show much enhanced PCEs, by 9.8% and 17.7%, respectively, while maintaining favorable electrical properties. In addition, the possibility of eliminating the trade-off between the color temperature and the luminous efficacy of typical warm-white LEDs by using a DFC is discussed.