It is essential for nano- and molecular-scale applications to explore and understand the electron and phonon transport characteristics of molecular junctions consisting of a scattering region such as a molecule connected to metallic electrodes. This thesis presents a series of studies into the electronic and thermoelectric properties of molecular junctions using theoretical methods
described in chapters 2 and 3. Chapter 2 presents an introduction to the density functional theory (DFT). It is followed by an outline of transport theory in chapter 3, based on a Green’s function formalism.

Recent studies of molecular thermoelectrics help to understand how atomic-scale structural modifications in junctions can affect the thermopower of molecular devices. This is illustrated in chapter 4, where I investigate the connectivity dependence on the thermoelectric properties of a series of thiophenediketopyrrolopyrrole (DPP) derivative molecules. For example. I find
that molecules with connectivitites leading to destructive quantum interference (DQI) show significant conductance variations upon ring rotation. This DQI also leads to enhanced Seebeck coefficients, which can reach 500−700 μV/K. For the molecule with constructive quantum interference (CQI), I find that after including the contribution to the thermal conductance from phonons, the full figure of merit (ZT) for the CQI molecules could reach 1.5 at room temperature.

Based on the DPP molecules, Chapter 5 presents a collaborative study with experimentalists at Xiamen University, China, of the effect of branching alkyl chains (isopentane, 3-methylheptane, and 9-methylnonadecane) on the geometrical changes such as pi-stacked distances and backbone dihedral angles. It is demonstrated that as the alkyl chain becomes longer the electrical conductance decreases due to an increase in the torsional angles between
the aromatic rings. The relationship between the conductance and the torsion angle 𝜃 follows approximately T(E, 𝜃)∝ cos6 𝜃. This indicates that the insulating side chain could be used to control single-molecule conductance, which is of significance for the design of future organic devices.