Conjugated Block Copolymers as Model Systems to Examine Mechanisms of Charge and Energy Transfer in Organic Materials

Restricted (Penn State Only)
Author:
Aplan, Melissa Paige
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 21, 2018
Committee Members:
  • Enrique Daniel Gomez, Dissertation Advisor
  • Enrique Daniel Gomez, Committee Chair
  • Michael Anthony Hickner, Committee Member
  • Michael John Janik, Committee Member
  • John B Asbury, Outside Member
Keywords:
  • block copolymers
  • organic electronics
  • photophysics
  • charge transfer
  • organic photovoltaics
  • conjugated polymers
Abstract:
Organic electronics is an exciting field with applications already beginning to impact our everyday lives. The key advantage of organic electronics over conventional inorganic-based systems is the ability to define functionality at the molecular level. As it stands, we do not fully understand the physics of charge and energy transfer within organic materials; this greatly hinders material design for high-performance organic electronic devices. With a focus on organic photovoltaics, this dissertation demonstrates that donor-acceptor block copolymers are useful model systems to examine processes of charge and energy transfer in organic materials. First, we establish block copolymers as model systems. Using steady state photoluminescence quenching experiments, we have developed a procedure to quantify intramolecular charge transfer within isolated block copolymer chains. We find that a small perturbation to the molecular structure disrupts intramolecular charge transfer and is ultimately responsible for a substantial decrease in photovoltaic performance. Next, we use these materials to perform a fundamental study investigating the influence of the energetic offset between a donor and acceptor on exciton dissociation. We systematically tune the two energy levels that make up this energetic offset, the singlet excited state and the intramolecular charge transfer state. In isolated block copolymer chains, a significant driving force is required to achieve efficient exciton dissociation. Lastly, we use block copolymers to examine the influence of conjugation length on charge transfer. We demonstrate that conjugated homopolymers, made up of identical repeat units, require a significant conjugation length to achieve efficient charge transfer. Conversely, push-pull polymers, made up of alternating electron-rich and electron-deficient units, eliminate this chain length requirement. Altogether, this work demonstrates the use of conjugated block copolymers as model systems to elucidate mechanisms of charge and energy transfer within organic materials. By performing fundamental studies on isolated block copolymer chains in solution, we uncover structure-function relationships to help guide material design for organic electronics.