Chemistry Student Seminar: Theory and simulation of self-assembly and charge transport in conjugated polymers

Presenter

Belinda Boehm - Master of Philosophy (Chemistry)

Abstract 

Organic semiconductors present a promising alternative to conventional silicon-based materials as a means to achieve cheap, flexible, and lightweight electronic devices. However, while inorganic semiconductors such as silicon form ordered crystalline structures, semiconducting polymers pack to form disordered structures, leading to difficulty predicting device performance. A better understanding of the morphology of these materials is therefore important in order to enhance their efficiency and, thus, commercial viability.

As semiconducting-polymer-based devices are generally fabricated through solution processing methods, the microstructures formed in solution are of particular note. Computer simulations can potentially offer rapid screening of semiconducting polymers to optimize solution-phase properties, as well as providing molecular-level details of the structures of disordered aggregates, which cannot be obtained experimentally.

However, as it is computationally unfeasible to model these large polymer aggregates atomistically, this work aims to develop simplified models, using existing solubility theories, such as Flory theory, in addition to coarse-grained molecular dynamics, of phase behaviour and microstructures of semiconducting polymers in solution.

Initially, Flory interaction parameters are computed from all-atom molecular dynamics simulations for a number of different solvents and polymers. Although giving good agreement with solubility trends from the literature, the values are found to be strongly dependent on whether the polymer is considered as amorphous or crystalline.

Additionally, structural changes within monomers on going from free in solution to the bulk crystal are found to have a large impact on the polymer’s solubility. These factors are not considered in Flory theory and have generally not been taken into account in previous literature. For more accurate solubility prediction, polymer morphology and intramolecular structural changes should therefore be considered in the future.

Future work in this project will focus on developing coarse-grained models, which allow larger scale simulation of the polymer, to gain a better understanding of the interplay between solvent effects, morphology and phase behaviour, and charge transport properties of semiconducting polymers. These should be generalisable to a range of polymers, allowing for generation of a number of design rules for high-efficiency polymers for commercial applications.