Online

Banks, Peter Alexander

Chemistry

2023

Ph. D.

An increasing amount of attention has been dedicated to small-molecule based crystalline organic semiconducting materials, with regular reports of newly-designed molecules that are specifically engineered to produce high-performance organic devices. Commonly, these molecules contain distinct chemical characteristics that are specifically incorporated to inhibit the large-amplitude, low-frequency vibrational motions that occur about intermolecular van der Waals contacts. These molecular motions have been historically considered a primary factor that uniquely limits the semiconducting performance of these materials, as the large displacements induced by these vibrations strongly diminish the electronic coupling between molecules in the lattice. This phenomenon is denoted as electron-phonon coupling, and a common goal in the field of organic semiconducting materials is to reduce these effects. However, the low-frequency vibrational dynamics of molecular crystals are immensely sensitive to the highly-entangled set of intermolecular forces exhibited by each unique material. As such, meaningful predictions of the energies and mode-types of the individual vibrational modes is a difficult task when only the molecular subunit and crystalline structure of a material are known. Thus, additional methods to describe the low-frequency vibrations of a material must be adopted in order to fully characterize these dynamics. Terahertz time-domain spectroscopy (THz-TDS) is a fitting tool for the evaluation of these materials, as the technique samples the low-frequency region in which these lattice dynamics occur. However, the description of the molecular motions that correspond to the experimentally observed spectral features relies on complimentary theoretical methods. In this regard, periodic density functional theory (DFT) is commonly performed alongside experimental terahertz measurements, as these theoretical models demonstrate the robust numerical accuracy necessary to accurately capture the weak intermolecular forces that drive low-frequency vibrations. The works discussed here provide a broad overview of the historic development of THz-TDS as a technique, as well as its application in the field of organic semiconductors. Additionally, the evaluation of computational models that compliment THz-TDS is described within the framework of DFT, examining the specific theoretical aspects of a simulation that must be considered in order to produce physically meaningful results. From a foundation of validated theoretical models, the low-frequency dynamics for a series of organic semiconductors is characterized, enabling a capture of the effect of these vibrations on charge transport. These results emphasize the vibrations that specifically arise from unique chemical modifications, as well as explain unexpected electron-phonon couplings for system-specific molecular motions.