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Type: Theses
Title: Interplay of structural, dynamical, and electronic properties in doped semiconducting polymer systems
Author: Ackling, Sophia
Issue Date: 2017
School/Discipline: School of Physical Sciences
Abstract: High-performing cost-efficient organic electronics will play an important role in shaping the future of flexible electronic devices. Applications for such technology range from smart device screens to sensors and photovoltaics. Precise optical control over polymer structure has recently been reported, with applications in optical film patterning for cost-efficient organic device fabrication. This process was demonstrated within the archetypal poly(3-hexylthiophene) (P3HT) and 2,3,5,6- tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) polymer/dopant system, wherein optical control over solubility was performed using light at a specific wavelength. However, the underlying mechanism responsible for the solubility change is yet to be fully elucidated. The work presented in this dissertation aims to provide insight into a number of related physical and electronic properties within this polymer/dopant system by means of computational investigation. Density functional theory is used to investigate how structural and environmental properties of the P3HT/F4TCNQ system affect charge transfer. A simplified oligomer/dopant complex is constructed, and the impacts of oligothiophene chain length and substitution are investigated. An oligomer close to the P3HT conjugation length, with methyl side chains, is found to best replicate experimental results. A dielectric medium is introduced to simulate the effects of the surrounding P3HT chains that are present in the experimental system. The surrounding environment is shown to be intrinsic to realistic charge transfer, as quantitative charge transfer is achieved. The initial hypothesis for the optical solubility control process suggested a photo induced charge back-transfer reaction from dopant to polymer, resulting in the latter returning to its neutral, and hence soluble, state. Excited-state density functional theory calculations on the aforementioned optimal model system reveal that the complex does display excitations with charge transfer character near the optical de-doping wavelength. However, constrained density functional theory calculations reveal that the optimised charge-neutral state is unstable, and the charge-separated state is thermodynamically favoured. These calculations illuminate important electronic characteristics of the system, and suggest that a photo-induced charge transfer mechanism is not responsible for the solubility change. Diffusion processes can dictate physical and electronic properties in doped polymer systems. Density functional theory calculations are used in this work to explain experimental measurements of atomic motions in P3HT doped with methyl-ester-substituted F4TCNQ. Calculations quantitatively confirm the assignment of experimental measurements of a diffusive process in the system to the methyl rotation on the F4TCNQ analogue. A set of calculations replicating the hopping of the F4TCNQ analogue along the P3HT backbone, a hypothesis for the second experimentally measured process, demonstrates that neither the energy barrier nor the diffusion coefficient for this calculated process are on the order of the experimental results, and hence an alternative process may be responsible for the experimental observations. Finally, the thermodynamics of the photo-induced solubility change are investigated using classical and quantum techniques. Steered molecular dynamics simulations demonstrate that charge distribution influences the free energy of separation of polymer and dopant. However, these simulations do not account for quantum relaxation or dynamic charge distributions. Density functional theory calculations, which do account for these properties, yield the free energy change for separation using a continuum solvent model. The explicit solvent contribution to the free energy of species separation is extracted from alchemical free energy perturbation simulations. Applying this contribution to the quantum calculations in place of the continuum model contribution yields a free energy change for separation that is in excellent agreement with experimental measurements.
Advisor: Huang, David M.
Kee, Tak W.
Dissertation Note: Thesis (M.Phil.) (Research by Publication) -- University of Adelaide, School of Physical Sciences, 2017.
Keywords: P3HT
semiconducting polymers
organic semiconductor
charge transfer
Research by Publication
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at:
DOI: 10.4225/55/5913d131e820f
Appears in Collections:Research Theses

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