THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Monday, June 14, 2021

1:00 PM

via

Blue Jeans Video Conferencing

https://bluejeans.com/646121714

will be held the

DISSERTATION DEFENSE

for

Aditi Khirbat

"Influence of the Local Polar Environment on the Optoelectronic Landscape of Polymeric Semiconductors"

Committee Members:

Prof. Natalie Stingelin, Advisor, MSE/ChBE

Prof. John Reynolds, CHEM/MSE

Prof. Carlos Silva, CHEM

Prof. Mark Losego, MSE

Prof. Nazanin Bassiri-Gharb, ME/MSE

Abstract:

Macroscopic optoelectronic device properties of polymer semiconductors are inherently dictated by processes that occur at molecular length scales. These light-matter interactions are particularly sensitive to the local characteristics of such plastics, including their molecular structure, chain conformation, aggregation, and their local environment. For example, the absorption and emission transitions of semiconducting polymers, central to photonic and optoelectronic device functions, are appreciably affected by the local polarity of their surroundings. This phenomenon has been well investigated for polymer interactions with polar solvents in the solution phase, but the influence of the local polar surroundings in the solid state is less explored.

In this thesis, the local environment of semiconducting polymers in the solid state is manipulated, via solution-based blending with polar ferroelectric polymers, to establish a detailed picture of their optoelectronic landscape in such environments. Starting with a model system based on the archetypal semiconducting polymer, poly(3-hexylthiophene) (P3HT), and a polar commodity homo-polymer, poly(vinylidene difluoride) (PVDF), vitrification is utilized, often leading to intermixing in blends, as a tool to produce a diverse set of local solid-state structures. Increased intermixing between the polymers has a direct consequence on the semiconductors’ properties, where finely intermixed, vitrified blends possess a temperature dependence on the inter- and intra-chain exciton coupling, tentatively attributed to localized polar fluctuations in tandem with PVDF segmental relaxation.

In surroundings of varying polarity, manipulated by the use of VDF-based co-polymers, a further reduction in inter-chain coupling and enhanced photoluminescence is obtained, possibly arising from interactions with ferroelectric nano-domains exhibited, e.g. by VDF-based ter-polymers. Furthermore, the broader applicability of blending is explored to tune the local environment of other semiconducting homo-polymers as well as new generation “push-pull” donor-acceptor semiconductors. Finally, insights gained on thermodynamic and kinetic vitrification strategies are applied to understand the intermixing behavior between polymer donors and fullerene/non-fullerene acceptors.

This thesis, overall, advances fundamental understanding of the sensitivity of multicomponent functional polymer-based systems on their local environment, particularly, the dependence of organic optoelectronic properties on the local structure and polar landscape surrounding the molecules in the solid state. Furthermore, the structure/processing/property relations established in this thesis will provide a platform to design new multicomponent materials systems with multifunctional architectures for applications ranging from organic photovoltaics and light emitting diodes to resistive switches for memory devices.