A Physical and Computational Approach to the Function of Fluorescent Polymers and Small Molecules as Materials for Organic Light-Emitting Devices

With the need for sustainable solutions to energy consumption, especially in the realm of lighting technologies, the focus has turned largely to organic semiconductors for innovation. In general, organic semiconductors have properties which allow them to be
manufactured at a smaller overhead cost, and have a wide range of options for property customization through molecular structure modification. Organic light-emitting devices (OLEDs) have markedly improved in efficiency, and color control since their advent in the late 1980s, but are still marred by performance deficiencies relative to inorganic
devices. In this work, we present spectroscopic and computational studies of different emissive organic materials systems in the vein of understanding several key deficiencies of these materials in OLEDs. In devices based on conjugated polymers, it has been noted that the conjugated polymers undergo fluorescence quenching due to the applied electric field, a phenomenon known as field-induced quenching (FIQ). Previous work shows that this phenomenon has single-chain origin, and is not only a property of a bulk lm, nor is it exclusively a property of long polymers. In the first section, we explore FIQ in short
oligomers of PPV using a semi-empirical calculation model. We found that free electronhole pair states are stabilized by the electric field and can induce this quenching process
at suciently high field strengths. Based on a model rooted in Marcus theory, we find that FIQ rates can be reproduced qualitatively at experimental field strengths. In the following
two sections, small-molecule emitters capable of undergoing thermally-activated delayed fluorescence (TADF) are addressed. TADF is a process where excited triplet
states undergo conversion to singlet states by reverse intersystem crossing, mediated by thermal energy, allowing for fluorescence to be recovered from otherwise dark states.
TADF compounds represent the current state-of-the-art technology for OLEDs because of their 100% theoretical maximum internal quantum efficiency. However, there is still
a need to better understand their emission mechanism and how it is influenced by the external environment. This work characterizes two novel, blue-emitting, TADF-capable,
donor-acceptor molecules using a variety of spectroscopic techniques and computational methods. When placed in increasingly polar solvent environments, the compounds lose fluorescence intensity and their emission energy decreases. This susceptibility to environment
polarity has detrimental implications to color purity and eefficiency of emission, both in the prompt and delayed regime. Our work finds that confinement of the emitters
to solid state solvent glasses or polymer matrices suppresses the emission energy shift, but still maintains the charge-transfer character essential for efficient TADF. We also explore the excited state dynamics using ultrafast transient absorption spectroscopy. In this preliminary work, we find spectral signatures consistent with charge-transfer states, triplet states, and long-lived (microsecond regime) stimulated emission processes. A discussion of these spectral features and future work on this project is provided.