Triplet Harvesting in White Organic Light-Emitting Diodes
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Triplet Harvesting in White Organic Light-Emitting Diodes Simone Hofmann1, Karl Leo1, and Malte C. Gather1,2 1 Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Straße 1, 01069 Dresden, Germany 2 School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, Scotland, UK ABSTRACT In this paper, we review our recent work on triplet harvesting (TH) and its application in white organic light-emitting diodes (OLEDs). This includes the configuration of highly efficient single unit two-color and tandem four-color white OLEDs reaching efficacies of more than 30 lm/W at 1000 cd/m², as well as the development of new blue fluorescent emitters. The new compounds are chemically designed to exhibit a high triplet energy to allow TH by a green phosphorescent emitter. In a first step towards white TH OLEDs, we fabricated blue-green TH OLEDs to prove the validity of our concept. INTRODUCTION Organic light-emitting diodes (OLEDs) hold great promise for modern lighting and display applications. Especially the design of white OLEDs is in focus, where high efficiencies, a high color rendering index (CRI), good color stability with viewing angle, and long lifetimes are needed. White emission in OLEDs is usually achieved by a combination of several fluorescent and/or phosphorescent organic dyes (RGB or BY).[1,2] Due to electrical excitation and spin statistics, a singlet to triplet ratio of 1:3 is found within the OLED structure. In fluorescent emitter systems that emit light only from the singlet state, 75% of the injected charge carriers are therefore lost. Much research effort is dedicated to phosphorescent emitters, which have large intersystem crossing rates and emit light efficiently from the triplet state.[3] While green and red phosphorescent emitters can reach high efficiencies and long lifetimes, the development of a stable blue phosphorescent emitter remains a challenge.[4,5] In this paper, we will first explain the concept of triplet harvesting (TH) which makes it possible to benefit from the deep blue emission and stability of a blue fluorescent emitter while additionally harvesting the triplet excitons by transfer to the radiative triplet state of a yellow or red phosphor. This enables the design of highly efficient two-color and four-color white TH OLEDs. However, the blue emitter 4P-NPD which is commonly used in our studies prevents TH by a green phosphor as its triplet energy lies below the triplet level of green phosphors. Nevertheless, being able to apply TH to green phosphors would be very helpful for the generation of white light in OLEDs with high efficiency and color quality. Therefore, we developed two new blue emitters which are chemically designed to have a higher triplet energy than 4P-NPD. Finally, we demonstrate that both new emitters can be used inside an OLED structure and enable TH by a green phosphorescent emitter.
THEORY In the following, we will describe the principle of TH using the well-known blue fluorescent emitter 4P-NPD. Figure 1 illust
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