Abstract
Despite significant efforts in the past few decades, charge transport efficiency in organic semiconducting materials remains much lower than its inorganic counterparts. There are various ways to improve the performance of organic electronics, such as updating device architecture, considering different electrodes and encapsulation materials, and the ease of processing. Nevertheless, the most challenging part is to obtain a clear picture of the exciton and charge transport properties of the active materials.This thesis explores some fundamental aspects of exciton transport and the structure-property relationships in organic semiconductors from a theoretical point of view. After a brief review and bringing out the challenge of the field of organic electronics (Chapter 1), the theoretical methods most commonly used to describe excitation energy transfer and charge transport are summarised, emphasising the specific methods employed in this thesis (Chapter 2).
It is challenging to find accurate force fields for all applications where molecular dynamics simulation is a preliminary step toward studying electronic structure properties. All single molecules, biological chromophores, and semiconducting polymers of interest display a complex chemical structure with extended pi-conjugation that prevents the use of standard force fields. Moreover, when one uses classical simulation methods as inputs for electronic structure calculations, the equilibrium structure of the classical simulation and the electronic structure calculations should be as close as possible. This challenge is addressed in Chapter 3, in which we utilise a force matching procedure to parameterise new force fields systematically for large conjugated systems. These force fields are subsequently employed to propagate the nuclear dynamics for systems studied in Chapters 4 and 5.
The ability to control dynamic disorder in molecular aggregates will provide valuable tools for the design and development of efficient organic semiconductors. Therefore, identifying key parameters that correlate with the efficiency of the transport of the excitation energy is highly desirable. Chapter 4 investigates the effects of the dynamic disorder on the exciton transport in molecular crystals of mono- and di-alkylated 1,4-diketo-3,6-dithiophenylpyrrolo[3-4-c]pyrrole derivatives. The exciton dynamics are studied using a Frenkel–Holstein like Hamiltonian, in which the thermal fluctuations of the excitonic coupling as well as the non-local exciton-phonon couplings have been appropriately taken into account. The contrasting views of the dependence of high charge mobility and exciton transport on morphology indicate the need for robust quantitative models of how (dis)order influences excitation energy transfer in organic semiconducting materials. Investigating the relationship between structure and electronic properties of materials requires constructing atomistic models of the systems and calculating their electronic structure properties. Chapter 5 provides insights into the electronic excited states of polymers and their connections to the dynamical disorder of the nuclei. Finally, Chapter 6 summarises the research efforts and contributions made by the thesis to the field of optoelectronics.
| Date of Award | Nov 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Hainam Do (Supervisor), Jonathan D. Hirst (Supervisor) & George Zheng Chen (Supervisor) |