Selected Current Research Projects
Our work has focused on the development of new plastics (polymers) that are designed for function. Some of this work has led to the invention of polymers that incorporate atoms like selenium (Se) and tellurium (Te). Other work has used Nature’s functional molecules to build polymers for energy harvesting and storage. While most of the projects have an application in mind (one has even led to the start-up company Pliant Power Devices), we are most proud of the impact we have had in fundamental science and in particular of our ability to imagine new compositions of matter, create and study that matter in the laboratory, and learn a great deal along the way. The following vignettes describe the major ongoing projects in our group.
Metal-containing polymers. While most polymers are insulating, the polymers we work on are semiconducting, however very narrow band-gap semiconducting polymers can be difficult to prepare in the laboratory. With this goal in mind our group developed tellurium-containing polymers known as polytellurophenes and we are arguably the most active research group using tellurium in polymer science in the world. We described the first synthesis of tellurophene copolymers and learned that these novel macromolecules have vastly distinct optoelectronic properties. We have subsequently initiated a number of international collaborations to prove the utility of the compounds in devices such as organic light emitting diodes, transistors and thermoelectrics. We also have several patents on these novel and functional compositions of matter. The research that we have carried out sets the stage for the continued development of new classes of Se and Te containing polymers and small molecules.
Polymer–assembly. Polymers must be processable (i.e. by melting or, more commonly, by dissolution) but they must also form a solid to have a tangible function. The transition from the melted and/or solvated state to the solid state has become one of the most challenging fundamental aspects in the electronic polymers field. Obtaining the desired solid-state structure at the nanoscale is even more difficult. With this challenge in mind we are developing new polymers – block copolymers – wherein each macromolecular block contains a unique heterocycle. Block copolymers contain long domains of two (or more) repeating units. In the solid-state the units de-mix and this leads to a rational way of controlling the structure of the solid. We initially synthesized selenophene-thiophene block copolymers and reported the unprecedented discovery that blocks varying by only a single element undergo phase-separation (highlight by Nature Chem). We subsequently found that when these polymers are used in solar cells the thermal stability of the cells improves, which we attribute to this unique polymer structure. We have also found that their phase behavior challenges the notion that long polymers are required to effect de-mixing. Work remains to develop new polymers with more complex architectures.
Nature-inspired polymers for energy storage. Electrochemical energy storage applications are growing enormously on multiple scales, from smart card microbatteries, to large-scale electric vehicle batteries, and warehouse-sized redox flow batteries. While much progress has been made, higher performing, more versatile, smaller, lighter, and, most importantly, more environmentally viable energy storage solutions will be required in the future. We have recently reported bio-derived polymers for lithium-ion batteries. The battery uses flavin, derived from vitamin B2, as the energy storage unit. In this way, the work provides a foundation for the use of bio-derived polymers in sustainable, high performance lithium-ion batteries. Batteries currently use transition metal-based cathodes that require energy-intensive processing and extraction methods that are detrimental to the environment. Our proposed new concept of using biologically-derived polymers to store energy is an attractive strategy to address these issues. We plan to focus heavily on nature-derived aromatic compounds for energy storage. In particular, derivatives of quinones are common biologically active molecules and often found as electron acceptors in electron transport chains like photosynthesis and respiration.