Enhanced light out-coupling of organic light-emitting devices using embedded low-index grids

Abstract

The application of organic light-emitting devices to reduce the energy consumption of interior lighting, which now costs more than $230 billion annually1, is attracting interest due to the high efficiency of these devices. Indeed, electrophosphorescent organic light-emitting devices with an internal quantum efficiency ηIQE of 100% (refs 2,3) already approach the efficiency of fluorescent lamps4,5,6. However, because of the high refractive index of organic materials and the optical confinement and internal reflection that results, the light out-coupling efficiency ηout for conventional organic light-emitting devices is limited to ∼20% (refs 2,7). Here we demonstrate that embedding a low-index grid in the organic layers can enhance the out-coupling of the waveguided light without spectral distortion. When combined with microlenses that out-couple the glass modes, the external quantum efficiency ηEQE and power efficiency of a white organic light-emitting device are improved to 34 ± 2% and 68 ± 4 lm W–1. The resulting ηout is 2.3 ± 0.2 times that of a conventional organic light-emitting device used as a comparison, and simulations indicate that the enhancement can be further increased to 3.4 ± 0.2. Extracting light from organic LEDs is difficult owing to the refractive index of the materials used, and the output efficiency is typically limited to around 15–20%. By embedding a grid with a low refractive index into the organic layers and using a microlens array researchers have now managed to increase this figure to 34%, representing an improvement by factor of 2.3 over a conventional device.