Recent Progress in the Understanding of Exciton Dynamics within Phosphorescent OLEDs

Abstract

Nanostructured topological insulators with large surface-to-volume ratios are essential in order to study and control the exotic electronic properties of the topological surface states. Following the discovery of binary chalcogenides as topological insulators, the progress in the topological insulator research fi eld has been remarkable. In this Review@RRL, synthesis of topological insulator nanostructures, notable transport experiments, and current nanomaterial challenges together with the corresponding solutions are presented for general readers. The use of phosphorescent emitter molecules in organic light-emitting diodes (OLEDs) allows that every exciton formed in the organic emission layer(s) can emit a photon. With that, phosphorescence provides a fourfold improvement over conventional fl uorescent OLEDs to unity internal quantum effi ciency. In turn, the excited state lifetimes of the triplet emitters used are orders of magnitude longer than the ones of singlet excitons introducing a noticeable effi ciency roll-off. In this Feature Article, Reineke and Baldo review recent progress in the understanding of phosphorescence in OLEDs. In addition, the authors discuss emerging conceptual alternatives to phosphorescence like extrafl uorescence and thermally delayed fl uorescence. Elastic and tribological properties of ZnO nanowires on Si wafer and highly oriented pyrolytic graphite (HOPG) are experimentally investigated and theoretically interpreted. Measurements are performed inside a scanning electron microscope (SEM) using real-time manipulation technique that enables two possible ways of data registration: "external" force registration with quartz tuning fork based sensor and "internal" force registration utilizing in situ observed elastic deformation of nanowires. It is possible to describe the complete range of tribological phenomena: static friction upon the transition to kinetic friction, pure kinetic friction and the relaxation of kinetic friction to self-balanced static friction.

Degradation Mechanisms and Reactions in Organic Light-Emitting Devices

“…From this perspective, the optimum charge balance is achieved when electrons and holes create a maximum possible number of photons in the emission zone. 25,396,405,406 It should be emphasized that an intuitive view of charge balance as a difference between densities of holes and electrons in recombination zone (or elsewhere in a device) is without any merit. There is no fundamental reason why an OLED with comparable densities of holes and electrons in recombination zone would be generally advantaged relative to an OLED that has density of one carrier greatly exceeding the density of the other one.…”

Electronic and Optical Processes of Organic Semiconductors

“…Triplet states can also be quenched by collision with charge carriers (triplet-charge-annihilation, TCA, or synonymously triplet-polaron-annihilation, TPA) [348] and with singlet excitons (triplet-singlet-annihilation, TSA) [349,350]. Both processes are relevant for OLEDs operated at higher driving currents giving rise to a so-called roll-off effect [351,352]. They are particularly important in OLEDs with phosphorescent emitters because the emitted electroluminescence originates from triplet excitations.…”

Anomalous Exciton Quenching in Organic Semiconductors in the Low-Yield Limit

“…[1][2] Exciton migration and the associated dynamics play important roles in defining the performance of organic optoelectronic devices, including organic solar cells (OSC), organic light emitting diodes (OELDs), organic photodetectors, and sensors operating based on exciton quenching. [3][4][5][6] Exciton migration through organic semiconductors is diffusive, described by site-to-site hopping of localised excitons.…”