High-performance organic light-emitting diodes for flexible and wearable electronics
Gaj, Michael Peter
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Optoelectronic devices based on organic semiconductors have been the focus of increasing research over the past two decades. While many of the potential organic electronic concepts (solar cells, transistors, detectors etc.) are still in their infancy stage, organic light-emitting diodes have gained commercial acceptance for their potential in high resolution displays and solid-state lighting. However, in order for these devices to reach their full potential significant advances need to make to address their fundamental limitations, specifically: device life-time, thin-film encapsulation and scalability to a high volume manufacturing setting. The work presented in this thesis demonstrates new strategies to design and manufacture high-performance OLEDs for next generation electronics. In the first part, high-performance OLEDS using a simple three-layer organic semiconductor device structure are demonstrated. These devices utilize two novel materials (Poly-TriCZ and mCPSOB) to achieve efficient charge balance and exciton confinement in the emissive region of the device. Moreover, the electrical properties of these materials allow them to serve as a suitable ‘universal’ material combination to yield high-performance OLEDs with high-energy phosphors (i.e. blue- or deep-blue-emitting dopants). To demonstrate this feature, green- and blue-emitting OLED results are provided that define the state-of-the-art for phosphorescent OLEDs. These results are then extended to show high-performance with a new set of high-efficiency blue- and green-emitting dopants based on thermally activated delayed fluorescence (TADF), which also proceed to define the state-of-the-art in electroluminescence from TADF. The second part of this thesis continues this work and extends the results to a new class of polymeric substrates, called shape memory polymers (SMPs). SMPs provide a new alternative to flexible, polymeric substrates due to their unique mechanical properties. When an external stimuli is applied to these materials (heat), they have the ability to form a temporary phase that has a Young’s modulus orders of magnitude lower than its original state. The material can then be re- shaped, deformed or conform to any object until the stimuli is removed, at which point the Young’s modulus returns to its original state and the temporary geometric configuration is retained. Re-applying the stimulus will trigger a response in its molecular network, which induces a recovery of its original shape. By using mCPSOB in an inverted top-emitting OLED architecture, high performance green-emitting OLEDs are demonstrated on SMP substrates that define the state-of-the-art in performance for deformable light-emitting devices. The combination of the unique properties of SMP substrates with the light-emitting properties of OLEDs pave to the way for new class of applications, including conformable smart skin devices, minimally invasive biomedical devices, and flexible lighting/display technologies.