Enhanced color conversion efficiency of remote phosphor-converted light-emitting diodes using micro-concavity arrays

Remote phosphor-converted LEDs (rpc-LEDs), which rely on a phosphor layer located away from the LED chip, are a particularly attractive technology benefitting from a higher luminous efficiency and from an improved stability compared with on-chip LEDs. However, systems based on thin-film remote phosphor layers still face a low color conversion efficiency (CCE). This mostly originates from an insufficient interaction of the exciting blue light with the phosphors. To overcome this limitation, we propose to couple the thin-film converting layers to a micro-concavity array (MCA) designed to enhance the optical pathlength of the exciting light, resulting in an improved CCE. This is achieved by exploiting the excellent light scattering and retro-reflection properties of MCA. We experimentally verify that the MCA transmit 95% of the incoming blue light into the converting layer, whereby 84% of this share corresponds to scattered light. Moreover, the measured retro-reflection amounts to 21% for normally incident light. The potential of the fabricated MCA films is tested by integrating them on the illuminated side of remote light converting thin-film layers with sub-millimeter thickness. Two examples, including quantum dots (QDs)- and rare-earth phosphor- based LEDs, are investigated. Our results show that the CCE of both rpc-LEDs are improved due to the enhanced excitation of the downconverted materials and to the effective extraction of the backscattered light. Thus, the CCE values of QDs-based and phosphor-based and rpc-LEDs are increased by 8.1% and by 12.7%, respectively, compared to devices without MCA films. In the latter case, the angular color uniformity is additionally improved under the effect of light scattering.