Excited State Interactions and Management in Organic Light Emitting Diodes.

Abstract

Organic light emitting diodes (OLEDs) have been leading the research and development in organic semiconductors, and representing a primary driving force in information display as well as solid-state lighting innovations. In organic semiconductors, excitons are responsible for optical transitions, and are thus central to the operation of all organic optoelectronic devices. This dissertation aims at understanding the fundamental physics of exciton interactions and their effects on the performance of OLEDs. We show that managing exciton interactions based on exciton physics results in significantly improved device characteristics.

Organic light emitting diodes based on singlet and triplet exciton emission are called fluorescent OLEDs and phosphorescent OLEDs (PHOLEDs), respectively. The first part of this dissertation studies exciton interactions in fluorescent OLEDs. We begin by identifying singlet-triplet annihilation as a loss mechanism in fluorescent efficiency, and thus propose a triplet management strategy to de-excite the detrimental non-emissive triplet. This strategy leads to more than 100% improvement in fluorescent OLED efficiencies, and also a more than 100-fold increase in lasing duration in organic semiconductor lasers (OSLs), thus allowing for the first observation of the continuous-wave threshold in OSLs. Further, since triplet-triplet annihilation (TTA) contributes to fluorescent emission, we analyze the trade-off between STA and TTA, and propose optimal fluorescent material properties needed for high fluorescent efficiency.

The second part of this work focuses on exciton interactions in PHOLEDs. Triplet-triplet annihilation is studied through transient photoluminescence, and Dexter-type triplet diffusion is identified as the dominant mechanism leading to TTA. Thus, minimizing the Stokes shift between the molecular emission and absorption is introduced as a route leading to high efficiency PHOLEDs at high luminance. Indeed, exciton interactions are important for not only OLED efficiency but also operational lifetime. Based on the understanding that triplet-polaron annihilation (TPA) is a fundamental intrinsic degradation mechanism in blue PHOLEDs, we designed a novel OLED whose phosphorescent emitter concentration is varied linearly with position. This doping profile results in a low and uniform exciton density and thus a higher efficiency and suppressed TPA, leading to a significantly extended operational lifetime over conventional blue PHOLEDs.