Browsing by Subject "Exciton Diffusion"
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Item Energy Migration in Organic Thin Films—From Excitons to Polarons(2016-04) Mullenbach, TylerThe rise of organic photovoltaic devices (OPVs) and organic light-emitting devices has generated interest in the physics governing exciton and polaron dynamics in thin films. Energy transfer has been well studied in dilute solutions, but there are emergent properties in thin films and greater complications due to complex morphologies which must be better understood. Despite the intense interest in energy transport in thin films, experimental limitations have slowed discoveries. Here, a new perspective of OPV operation is presented where photovoltage, instead of photocurrent, plays the fundamental role. By exploiting this new vantage point the first method of measuring the diffusion length (LD) of dark (non-luminescent) excitons is developed, a novel photodetector is invented, and the ability to watch exciton arrival, in real-time, at the donor-acceptor heterojunction is presented. Using an enhanced understanding of exciton migration in thin films, paradigms for enhancing LD by molecular modifications are discovered, and the first exciton gate is experimentally and theoretically demonstrated. Generation of polarons from exciton dissociation represents a second phase of energy migration in OPVs that remains understudied. Current approaches are capable of measuring the rate of charge carrier recombination only at open-circuit. To enable a better understanding of polaron dynamics in thin films, two new approaches are presented which are capable of measuring both the charge carrier recombination and transit rates at any OPV operating voltage. These techniques pave the way for a more complete understanding of charge carrier kinetics in molecular thin films.Item Engineering Excited State Transport and Relaxation in Organic Semiconductors(2020-05) Rai, DeepeshOrganic 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.