Exploring how a Conductive Polymer Emerges from its Component Polymer Molecules

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Nontechnical description: This study examines the electrical properties of flexible, transparent and conductive polymers - materials of tremendous importance to a variety of devices including solar cells and portable displays. Although such polymer films are composed of numerous individual molecules, their optical and electronic properties are extremely different from those of their isolated molecular constituents. The study aims to understand how the properties of these polymeric films emerge from their molecular building blocks. Specifically, it aims to elucidate the role fabrication conditions and interactions between molecules play in the emergence of electronic properties of ordered, crystalline polymer films. To explore this evolution, the project implements optical measurements of single polymer molecules, as well as molecular aggregates of increasing size. In this manner, electronic properties of polymers are examined over increasing scales, ranging from individual molecules through ordered thin films. The research team comprises graduate students and undergraduate researchers from diverse backgrounds. Participants receive training in materials science, chemistry, and photonics. An international collaboration serves to enhance research efforts as well as interdisciplinary training opportunities. In addition, a teaching module on photonic materials is being developed. Technical description: Conjugated polymers (CPs) are technologically important due to their ability to conduct excitons and charge. Most device applications of these materials utilize thin polymeric films. This project entails the use of a recently developed spectroscopic tool to elucidate how thin film electronic behavior emerges from the properties of its individual molecular constituents, as well as from intermolecular interactions. Specifically, studies implement single-molecule spectroscopy to address the nature of polarons - charge carriers in conjugated polymers - whose electronic structure and distribution are critical for ensuring optimal device performance. Experiments encompass spectroscopic studies on a range of length scales, from isolated polymers, through globules and domains, to multi-domain structures. In this manner it is possible to identify the critical length scale over which film-like behavior emerges, and the control parameters influencing the onset of this transition. The study utilizes ultra-high quality-factor optical microresonators as platforms for electronic and polarization spectroscopy, enabling the probing of charged and conducting CPs. Through studies of the interplay of optical properties, electronic structure, and molecular ordering it is possible to construct a detailed model of the electronic properties of photonic organic materials.