White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency

Abstract

White organic LEDs are seen as one of the next generation light-sources, with their potential to reach internal efficiencies of unity and their unique appearance as large-area and ultrathin devices. However, to replace existing lighting technologies, they have to be at least on par with the state-of-the-art. In terms of efficiency, the fluorescent tube with 60-70 lumen per Watt (lm W^-1) in a fixture is the current benchmark. In the scientific literature, so far only values of 44 lm W^-1 have been published for white OLEDs.

Here, we present results (Reineke et al., Nature 459, 234 (2009)) of white OLEDs with 90 lm W^-1 at an illumination relevant brightness of 1,000 candela per square meter (cd m^-2). Extracting all light from the glass substrate using a 3D light extraction system, we even obtain 124 lm W^-1. In order to achieve such high efficacy values, we reduced the energetic losses prior to photon emission that include ohmic and thermal relaxation losses, leading to very low operating voltages. This is accomplished by the use of doped transport layers and a novel, very energy efficient emission layer concept. Equally important, we addressed the optics of the OLED architecture, because about 80% of the generated light remains trapped in conventional devices. Therefore, we used high refractive index substrates to couple out more light and placed the emission to the second field antinode to avoid plasmonic losses. Our devices are also characterized by an outstandingly high efficiency at high brightness, reaching 74 lm W^-1 at 5,000 cd m^-2.