Modelling the microstructure-charge transport relationship in organic semiconductors

Organic semiconductor materials can offer major advantages over commonly-used inorganic alternatives when integrated into electronic devices. However, they tend to suffer from low charge carrier mobilities, which can limit device performance. Microstructural disorder plays a key - but sometimes poorly understood - role in determining the mobility of organic materials. This thesis describes a series of computational investigations aimed at improving our understanding of the relationships between chemical structure, microstructure, and charge transport in organic semiconductors. A multiscale modelling approach, employing density functional theory, molecular dynamics, and charge transport simulations, is applied to capture these relationships as they emerge from processes occurring across a range of time- and length-scales. First, the development of two transport modelling codes is described, with their utility being verified in two case studies. The first study focuses on the influence of electron density and mobility in an interfacial photocatalytic system for hydrogen production, while the second focuses on the influence of molecular symmetry and crystalline packing on the mobility of bis-adduct isomers of PCBM. A significant portion of this thesis centres on rationalising the high mobility of the disordered polymer C16-IDTBT. Its microstructure is shown to feature an unusual packing motif involving perpendicularly crossing backbones. Combined with rigid chains, this promotes a highly interconnected transport network, greatly benefiting mobility. This investigation is extended to evaluate four heteroatom-substituted analogues of C16-IDTBT. A final investigation reveals that charge transport in thin films of the non-fullerene acceptor O-IDTBR is limited by intermolecular electrostatic contributions to energetic disorder, which in turn emerges from the conformational and packing disorder of molecules. This work reaffirms the importance of understanding the role of microstructure - and microstructural disorder in particular - in determining the charge transport properties of organic semiconductors, and demonstrates how computational techniques may be applied to deepen this understanding.