Investigating the Time-Resolved Spectroscopy of Singlet and Triplet State Dynamics in Conjugated Rylene and Quinoline Derivatives for Efficient Optoelectronic Systems
Creators: name: Muthike, Angelar
Publisher: name: My University
Year: 2022
Subjects: subject: Organic Photovoltaics, Organic Light-Emitting Diodes, Organic Light-Emitting Transistors
subject: Time-Resolved and Nonlinear Optical Spectroscopy
subject: Intramolecular Singlet Exciton Fission, Thermally Activated Delayed Fluorescence, Biradicals/Diradicals
subject: Structure/Function Relationships
subject: Charge Transfer and Energy Transfer
subject: Chemistry
subject: Science

description: The increasing global population has led to increased energy demand and consumption due to the rise in the value of electricity for lighting and powering entertainment, heating, medical and communication devices. This increase has also led to high demand for cheap next-generation devices with improved performance and a broad range of applications increasing the optoelectronics demand. While the conventional inorganic optoelectronics have shown great performances, their high cost of production, as well as low processability and flexibility, has shifted the attraction to organic optoelectronics which have shown high optical absorption, high mechanical flexibility and lightweight, high tunability and are cheap to fabricate. Since the demonstration of this first device, organic semiconductors have found their way into the consumer market with a myriad of applications like organic light-emitting diodes (OLEDs) for color display systems, organic photovoltaics (OPVs) for low lost and efficient organic solar cells, and organic light-emitting transistors (OLETs) which is a new class of optoelectronics that combine the switching capabilities of a transistor and the emissive properties of OLEDs. Thus far, π- conjugated systems with donor and acceptor moieties have shown the most semiconducting promise owing to the delocalization of π- electrons that move along the conjugation chain. Bonding (π) and antibonding (π^*) molecular orbitals overlap at this conjugated backbone, creating a strong electron density which then supports moving charge carriers. The longer the conjugation chain, the stronger the overlap which determines the energy gap (E_g) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The energy gap, as well as the polymer structure, affect the processes of charge transfer (CT) and the energy transfer (ET) of the semiconducting material. Some of these processes that have been reported to influence the ultimate performance of the respective optoelectronics include singlet exciton fission (SEF), thermally activated delayed fluorescence, and diradical or biradical formation. To design materials for highly efficient and stable optoelectronic devices, it is therefore very important to understand these processes and how they relate to the structure and function of materials. As a result, this thesis focuses on using both linear and nonlinear optical as well as time-resolved spectroscopy techniques to expand the understanding of CT and ET of key processes that define the photophysical properties of organic optoelectronic systems. In particular, the structure-function relationships that affect the charge and energy transfer dynamics of materials for efficient optoelectronics are elucidated. In summary, it was found that for OPVs, the flexibility of the π‒linker, its point of attachment in the PDI units, and its molecular dielectric environment are important in activating SEF which increases the device efficiency. For OLEDs, a new approach that can be used to experimentally calculate the rate of reverse intersystem crossing in TADF organic molecules was proposed. It was found that both large rates of intersystem crossing and high external quantum efficiency translate to low operating voltages of the investigated chromophores. It was also evident that molecules need a reverse intersystem rate greater than a millisecond to be able to undergo the TADF process. Finally, the first evidence of diradical formation which reduces the efficiency of OLETs is reported. Overall, this dissertation uses time-resolved spectroscopy to improve the current understanding of structure-function relationships that affect SEF, TADF, or diradical/biradical formation mechanisms in CT and ET processes for OPVS, OLEDS, and OLETs.