The Effects of Driving Force, Morphology and Anion Separation on Carrier Mobility in Doped Conjugated Polymers

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Semiconductors are the basis for modern electronics such as computers, flat screen displays, and mobile phones. Most semiconductors are inorganic, hard materials such as silicon, and are expensive to process and manufacture. This project explores the properties of plastic semiconductors that are lightweight, flexible and potentially inexpensive to process. The electrical properties of inorganic semiconductors are controlled through doping. Doping is the intentional introduction of impurities into a semiconductor for the purpose of changing its electrical or optical properties. A key challenge facing the development of plastic semiconductors is they are not as easy to dope as inorganic semiconductors. This collaborative project takes advantage of new materials and processing methods to controllably dope plastic semiconductors. New materials have tunable properties that will allow for a greater degree of control over electrical conductivity. New processing methods allow dopant molecules to be precisely added to the plastic semiconductors at desired locations. High-quality doped polymer films will be studied by a suite of techniques to fully understand them. They will also be incorporated into thermoelectric devices that convert waste heat into electricity, a new source of renewable energy. Undergraduate and graduate students will be trained in areas of national need through this project. Outreach efforts will introduce high school students in the Los Angeles area to related topics such as renewable energy through demonstrations and experiments. This project is jointly funded by the Electronic and Photonic Materials program of the Division of Materials Research and the Chemical Structure, Dynamics, and Mechanisms B program of the Division of Chemistry. Conjugated polymers have numerous potential uses because they combine the mechanical properties of plastics with the electrical properties of semiconductors. When doped by strong oxidizing agents, their conductivity can be tuned by orders of magnitude, but interactions with the dopant counterion and dopant-induced changes in morphology can limit the doped carrier mobility. This project takes advantage of sequential processing, in which the polymer film is cast first and the dopant is infiltrated in a second step from a solvent chosen to appropriately swell but not dissolve the underlying polymer film. This method provides a degree of control over the doped polymer morphology that enables large-area applications, such as thermoelectric devices. The project also explores novel dopants, including newly-synthesized dodecaborane clusters with tunable redox potentials. These clusters have chemical structures that serve to shield the counterion charge from the polarons on the polymer backbone, allowing for control over the counterion-polaron interaction, and thus providing for improved carrier mobility and Seebeck coefficient. The project also investigates counterion exchange, where after reaction, the dopant counterion can be substituted for an inert ion by mass action, providing yet another degree of control over the properties of doped conjugated polymers. In all cases, the physical structure of the doped polymer film, as determined by a combination of grazing incidence wide angle X-ray scattering and neutron reflectometry, is correlated with the optical and electrical properties to understand how the location of the counterion in the film controls physical properties. Finally, the project uses ultrafast spectroscopy to measure both the thermal conductivity (via time-domain thermal reflectance) and electrical properties (via pump/probe transient absorption experiments) of doped conjugated polymer films. The key aim of the project is to determine detailed structure/function relationships to maximally exploit the use of doped conjugated polymers in thermoelectric and other devices.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.