Committee Members:

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

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

Prof. John R. Reynolds, MSE/CHEM

Prof. Seth R. Marder, MSE/CHEM

Prof. Carlos Silva, CHEM/PHYS

Prof. Natalie Stingelin, MSE/ChBE

Quantifying Charge Transport in Chemically Doped Semiconducting Polymer

Abstract:

Semiconducting polymers are a class of materials that engenders the solution processibility, mechanical compliancy, and biocompatibility of archetypal polymeric materials with the charge transport properties, optical properties, and device physics of archetypal inorganic semiconductors. Oftentimes, pristine semiconducting polymers are electrically insulative ( S cm-1) with comparatively few mobile charge carriers with low mobilities. The charge carrier density and mobility can be increased via chemical doping, and chemical doping oftentimes involves adding or removing electrons from the pristine polymer via a redox chemical reaction. Ultimately, the resulting optical and electronic properties of chemically doped semiconducting polymers is a convoluted function of multiple parameters, including polymer chemistry, dopant chemistry, and processing techniques. While this convolution enables a nearly infinite number of permutations, each of which can be designed for a specific application, this convolution obfuscates the establishment of charge transport models and fundamental process-structure-property relationships. Consequently, in this thesis, I developed and compiled experimental methods, which are used to create and substantiate novel charge transport models, which are then used to contextualize the charge transport properties of chemically doped semiconducting polymers.

In this presentation, I will first provide a succinct background on chemically doped semiconducting polymers and motivate my work by highlighting the either-or logical fallacies of previous transport models. Previous models assume that charge transport occurs via either localized or delocalized transport, but it is well established that semiconducting polymers charge transport properties (e.g., electrical conductivity, Seebeck coefficient, thermal conductivity) can span the entire transport spectrum from insulating and localized to metal-like and delocalized. To this end, I use the Boltzmann transport equation to develop a novel semi-localized transport (SLoT) model, which more accurately captures the transport properties of chemically doped semiconducting polymers across the entire transport spectrum. I demonstrate that the SLoT model is experimental accessible, whose fundamental transport parameters are observable, and that SLoT model provides new fundamental insight on both localized transport phenomena (e.g., the activation energy and transport barrier in the dilute doping limit) and delocalized transport phenomena (e.g., the carrier density needed delocalized transport, and the hypothetical maximum electrical conductivity). I then use the SLoT model to contextualize the charge transport properties of several material systems that have systematic variations in the polymer main chain and side chain chemistries. Lastly, I demonstrate the versatility of the Boltzmann transport approach and SLoT model by rationalizing the inversion of the Seebeck coefficient (p- to n-type semiconductor switching) and the electronic contribution to thermal conductivity in inhomogeneous materials. Ultimately, this works provides a physical and quantitative framework for contextualizing the structure-process- charge transport property relationships in chemically doped semiconducting polymers.