Mutual Synthesis of Conjugated Polymers and Dopants for Well-Ordered Self-Assemblies

Johns Hopkins University, Baltimore MD

Investigators

Abstract

This award is funded by the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry. Professor Howard E. Katz of Johns Hopkins University is supported to explore the idea that the main components of conducting plastics - the polymeric material and the dopants -- can be designed simultaneously for best performance. This requires that their molecule-sized subunits fit well together, continuous paths for conducting electricity are created, and the dopants maintain their ability to stabilize the electrical charges that move when the electricity is conducted. A fundamental issue to be explored is whether it is better for dopants to be mixed equally through the entire volume of the plastics, or whether it is better for separate parts of a mixture to be rich in the plastic and others be rich in the dopant. New ways to make plastics and dopants, more detailed understanding of how they fit together and promote electrical conduction in the plastic mixtures, and computer-based models for predicting the properties of the plastics in advance of making them are expected outcomes of this project. Conducting plastics are the basis of numerous emerging technologies such as plastic solar cells and light emitters for more efficient use of energy, detectors for dangerous chemicals important for defense operations, and medical devices to detect diseases. All of these technologies have created new economic opportunities. Integrating the research with education and outreach trains graduate students as future members of the plastic electronics workforce, and brings the opportunities of plastic electronics to elementary school, high school, and art institute students in the City of Baltimore. The main objective of this project is to design and synthesize pairs of conjugated, high-charge-carrier-mobility polymers and corresponding chemical dopants that induce charges in those polymers, such that the pairs form assemblies that retain favorable charge-transporting molecular organization. The supramolecular structure of the original polymer is a consequence of pi-interactions among the conjugated polymer main chains and regular spacing caused by alkyl side chains. The idea is to explore different chemical design elements in the polymer and dopant components so that the dopants become accommodated in the supramolecular structure, or even enhance the structure. These design strategies are compared to the extreme cases of dopants covalently attached to main chains, known as "self-doped polymers", and dopants that would be expected to phase segregate from the charge-transporting polymers and thus exist in separate domains. The molecular packing of the polymer-dopant assemblies is elucidated using x-ray and neutron scattering techniques. Absorbance, photoelectron, and electron spin resonance spectroscopy indicate the formation of charge carriers. Computational modeling creates a theoretical basis for both the packing and charge transfer efficiency. The outcome of this project guides the design of polymers that conduct electricity with high efficiency using only electrons or holes as charge carriers, without ionic contributions. The project may resolve the uncertainty about whether structures with dopant functionality adjacent to main chains or remote from main chains lead to better defined crystallinity and higher electronic conductivity. A theoretical basis for selecting among various assembly types for future designs is developed. Because the polymers are assembled from blends with dopants, the dopant concentration is tunable for the maximum conductivity, or alternatively for optimization of other functions such as charge injection into optoelectronic devices or matrices for thermoelectric composites.