Georgia Institute of Technology Materials Science and Engineering

Committee Members:

Prof. Mark D. Losego, Co-Advisor, MSE

Prof. Shannon K. Yee, Co-Advisor, ME

Prof. John R. Reynolds, CHEM/MSE

Prof. Seth R. Marder, CHEM/MSE

Prof. Carlos Silva, CHEM/PHYS

Prof. Natalie Stingelin, ChBE/MSE

“Charge Transport in Chemically Doped Semiconducting Polymers: Quantifying Charge Carrier Localization and Localization Contributions to Thermoelectric Properties”

Abstract:

Polymer semiconductors are uniquely positioned to serve in applications where traditional inorganic semiconductors cannot. Explicitly, polymer semiconductors can be cheaper to produce, more mechanically compliant, less dense, have more synthetic variations, and interface better with biological environments. Additionally, polymer semiconductor’s electronic properties (ie. charge carrier mobility, charge carrier density) can be fine-tuned for a specific application through chemical doping. Despite these benefits, polymer semiconductors have inhomogeneous microstructures which can localize charge carriers. Furthermore, charge carriers can also be localized due to columbic attractions to the dopant counterions, polymer electronic polarization, and bond order perturbation. Localization affects charge transport properties (electrical conductivity, Seebeck coefficient, electronic contribution to thermal conductivity) and complicates our ability to rationally design polymer semiconductors.  To what extent polymer chemistry, dopant chemistry, and processing effect localization and charge transport properties is not well quantified. Additionally, a single transport model that rationalizes the wide spectrum of transport behavior (localized and hopping-like to delocalized and metal-like) and concomitantly quantifies all the charge transport properties does not exist.

This thesis combines established experimental methodologies to create and validate models that concisely and phenomenologically quantify the effects of polymer chemistry, dopant chemistry, and processing on the extent of charge carrier localization and the resulting charge transport properties. A common feature in several established delocalized and localized transport models is that the Seebeck coefficient and electrical conductivity must both be measured as a function of temperature and carrier concentration. With these temperature and carrier concentration dependent measurements, this thesis derives a Boltzmann transport formalism which captures both localized and delocalized transport using established phenomenological models. This new model is known as the Semi-Localized Transport model (SLoT model), and I now propose to use this model to better quantify transport properties and electronic structure in several polymer/dopant/processing systems. Ultimately, this thesis will improve our understanding of charge transport in chemically doped semiconducting polymers and improve the rational design of polymer/dopant/processing combinations for charge transport applications.