The Physics of Molecularly-Doped Semiconducting Polymers Fabricated by Sequential Doping
- Author(s): Scholes, David Tyler
- Advisor(s): Schwartz, Benjamin J
- et al.
My thesis focuses on the development, characterization, and application of a new method for doping semiconducting polymer films. Semiconducting polymers represent a versatile class of materials used in many electronic device applications. However, due to their low intrinsic conductivity, doping is often necessary to achieve the desired electrical properties for specific applications. Adding large amounts of dopant often leads to detrimental effects that hinder both film quality and electrical properties. I developed new methodologies to overcome these specific issues present in the high doping regime as well as new insights gained general to molecular doping of polymers afforded by these methods. The first chapter gives a brief history of doping of semiconducting polymers, including characterization methods and challenges. The second chapter introduces the new method, sequential doping, specifically designed to overcome one such challenge and its comparison to a more traditional doped-film fabrication method. We demonstrate that this new method produces highly doped films of superior film quality allowing for accurate optical and electrical measurements, including Hall effect measurements, which had previously been unrealized. Chapter three focuses on how sequential doping offers the unique advantage of being able to tune the polymer film morphology prior to doping and maintain that same morphology after doping allowing for a detailed study of how polymer crystallinity effects the optical and electrical properties of the doped state. Of particular note was the discovery that for the specific polymer:dopant system studied, the dopant resides in the side chain regions between polymer chains as opposed to in the π-stacks as previously claimed. The final chapter continues to use the advantages offered by sequential doping to study the interplay between polymer crystallinity and energy level offset between the polymer and the dopant by the use of statistical copolymers with independently tuned crystallinity and energy levels. It was found that the crystallinity of the polymer after being doped could vary depending on the structural composition of the undoped polymer, and that the doped state crystallinity had a more drastic effect on the resulting electrical properties than the energy level offset. Overall, sequential doping is an effective way to produce highly doped semiconducting polymer films and allows for novel studies on how physical properties of materials influence their doped counterparts.