Sebastian Reineke and Marc A. Baldo Front Cover: Recent progress in the understanding of exciton dynamics within phosphorescent OLEDs (Phys. Status Solidi A 12/2012) physica status solidi (a) 209
Organic electroluminescence is the key technology for mobile displays, is currently introduced to large-area flat screen displays and is intensively explored as one of the next generation solid-state light sources. Excited states of organic molecules (excitons) are the heart of any organic electroluminescent device. They mediate the conversion of injected charges - electrons and holes - into photons. Phosphorescent emission originating from triplet excitons is especially important, as it is to date the only general route to enable unity charge-to-photon conversion efficiencies. In their Feature Article on pp. 2341–2353, Reineke and Baldo discuss the key aspects of excitons, following the excited state lifecycle. First, the fundamentals of singlet and triplet exciton formation in organic semiconductors are reviewed, followed by a discussion of concepts that aim to alter the singlet-to-triplet formation rates to enable higher electroluminescence yields in the fluorescence manifold. Subsequently, their review focuses on the exciton distribution within the organic semiconductor material during its lifetime. The processes involved ultimately determine organic light-emitting diode (OLED) performance and are especially key in the development of concepts for white emission, where precise balance of the exciton between different emitter species controls the emitted color. The paper is closed with a discussion of non-linear effects at high excitation levels that, to date, limit the high brightness efficiency of phosphorescent OLEDs. The cover pictures different organic compounds used as emitter molecules in phosphorescent OLEDs (illuminated with ultraviolet light). The inset shows a device under operation.
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