Organic semiconductors have many applications in portable, large-area or ubiquitous electronics. They also have great potential in bioelectronics as active materials in sensors or transducers. All such devices work by transporting charges; finding materials with large charge mobilities is therefore a major goal in the field of organic electronics. The search for high-mobility organic semiconductors, however, is still largely conducted with an Edisonian philosophy. The primary goal of the proposed activity is the development of a set of rational design principles for creating high-mobility conjugated homopolymers and copolymers which will impact all applications of organic semiconductors, from solar cells to light-emitting diodes and transistors. Insight derived from theory will be used to design and synthesize molecules that will be analyzed experimentally using X-ray diffraction for structural characterization and optical spectroscopy for measuring charge delocalization. These attributes will be correlated with the ability of the materials to carry current. The ultimate goal is to link specific features of the molecular structure and of the short-range arrangement of molecules within the assembly to carrier mobility. The methods developed, both theoretical and experimental, can potentially streamline the search for high mobility polymers and pave the way for the next generation of high-performance organic-based electronic devices.
Rational design of functional materials will be based on a theoretical model that accounts for charge transport, nuclear-electronic coupling, and various manifestations of diagonal and off-diagonal disorder within a two-dimensional lattice appropriate for mixed or segregated pi-stacks. Design principles derived from theory will be tested on several model Donor-Acceptor copolymers in which intrachain torsional disorder and/or HOMO energy alternation is carefully controlled. Structure/property relationships will be evaluated on high-performance copolymers based on the indacenodithiophene structural motif using acceptors with varying electron-withdrawing strengths. Microstructural characterization of thin polymer films will be accomplished using grazing incidence X-ray diffraction (GIXD), and charge delocalization will be probed using charge modulation spectroscopy (CMS) on oriented samples in order to obtain polarization resolution. The proposed activity will provide the organic electronics community with a method to experimentally and theoretically evaluate materials quickly for the design of high-performance organic semiconductors. It will also provide the first measurements of the coherence length of polarons in conjugated polymers using steady-state infra-red absorption spectroscopy. The coherence length will be linked to the design of new conjugated polymers and their short-range morphologies, thereby providing fundamental insights into what governs delocalization and trapping in conjugated polymer films.
