UC Davis

Organic semiconductors (OSCs), including semiconducting small molecules (SSMs), polymers (SPs) and carbon nanotubes (s-CNTs), are a versatile class of materials. Their compatibility with low-cost solution processing techniques and potential for producing lightweight, flexible, electronic devices has already begun to revolutionize several technologies. Typical organic devices, including thermoelectrics (OTEs), photovoltaics (OPVs), field-effect transistors (OFETs), and light-emitting diodes (OLEDs) can be fabricated with flexible OSCs, and has spawned several novel applications including wearable electronics, foldable low-temperature thermoelectrics, and flexible displays. While the future of OSC technology is promising, several structure-property relationships remain poorly understood, likewise the link between molecular doping level and OSC carrier density is underdeveloped. In my dissertation, I demonstrate new solution processing techniques, connect processing methods to structure formation using X-ray and neutron scattering probes, and correlate processing and structure formation to vital optoelectronic properties. I begin by demonstrating an additive solution process for depositing multiple layers of mutually soluble SP films without disrupting layer continuity. I use neutron reflectometry to quantify film thickness changes during processing steps and demonstrate the fabrication of a 4-layered device. Next, I perform a hierarchal structural analysis of bulk heterojunction SSM OPVs using grazing incidence X-ray diffraction and small angle neutron scattering, and correlate peak OPV performance to an inflection in film crystallinity and phase purity. Third, I refined a doping process, termed anion exchange doping, and show its effectiveness for doping high ionization energy polymers. During which I developed the framework for quantifying and predicting doping levels and carrier densities in SPs. Finally, I reveal record carrier mobilities and OTE performance in s-CNT films with a series of novel molecular dopants and correlate film doping levels to carrier densities.