Engineering Excited State Transport and Relaxation in Organic Semiconductors

Abstract

Organic semiconductors are an important class of optoelectronic materials that are characterized by high degree of conjugation within the molecule. These are thin films of conjugated molecules in organic optoelectronic devices such as organic light-emitting devices (OLEDs) and organic photovoltaic cells (OPVs). In organic semiconductors, the excited state is characterized by a tightly bound electron-hole pair called an exciton. The migration and relaxation of the exciton strongly dictates material optical properties, as well as the subsequent design and operation of semiconductor devices. For example, OPVs rely on the efficient harvesting and dissociation of photogenerated excitons at heterointerfaces in the device layer stack. The transport of long-lived dark excitons is of special interest as they play an important role as energetic intermediates in OLEDs while also being a potential active material in OPVs. Despite this, their spatial migration is challenging to probe accurately. The focus of this thesis is on demonstrating new characterization techniques to track exciton migration as well as on engineering unique device architectures for enhancing energy transport in OPVs. This work has brought insight into the role of spin and molecular structure in impacting exciton diffusion using a novel sensitizer-based methodology to selectively excite and probe dark exciton transport. Furthermore, the normally diffusive aspect of energy transport is overcome by excitonic gates that required development of new experimental and modeling tools.