Selected Current Research Projects
The majority of our research at the University of Toronto has focused on the development of new chemistry to understand electronic processes in organic materials. Materials of this type have garnered wide-spread international attention as ‘plastic electronics’ with potential applications ranging from batteries to solar cells to thermoelectrics. There are many, many thousands (if not tens of thousands) of ambitious chemists, physicists, engineers, materials scientists, etc. that make valuable contributions to this field. In our projects we create new materials, and the real rewards have been all the interesting and unexpected basic science discoveries that we have made. The following vignettes describe four of the projects and some of the impact they have had.
Polytellurophenes. While most polymers are insulating, ones with a delocalized electronic structure are semiconducting, however very narrow band-gap semiconducting polymers are quite scarce. With this goal in mind my group has 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 (Angew. Chem., 2010). In the presence of mild oxidants, the tellurium centers in the polymer become oxidized, which results in a dramatic shift in optical properties, showing their potential utility as optical sensors. We have since developed highly crystalline polytellurophenes that are solution-processable (JACS, 2013; Highlighted in the Canadian Chemical News: “Researchers at the University of Toronto have synthesized a new class of conducting polymers — polytellurophenes — that could bring about advances in organic solar cells and thin-film transistors”). We have subsequently demonstrated their utility in transistor devices with our collaborator and transistor expert Prof. Natalie Stingelin (Imperial College London). We have also developed tellurophene-containing inorganic complexes that photo-reductively eliminate halogens. Our feature article for the international polymer journal Macromolecules highlights these very different polymers and their potential uses. We continue to work on the controlled synthesis of polymers that contain tellurophene with an emphasis on controlling their solid-state organization for device applications.
Polyselenophenes. One of the central dogmas of this field is that the polymers must be processable (e.g. by melting or more commonly dissolution) but they must also form a solid to function. The transition from the solvated state to the solid state has become one of the most challenging fundamental aspects in this field. Obtaining the desired solid-state structure at the macro to nanoscale is even more difficult. With this challenge in mind we have been developing block-copolymers with Se and S containing building block units. Block copolymers contain long domains of two (or more) repeating units. In the solid-state the units may de-mix and this leads to a rational way of controlling the structure of the solid. We initially synthesized novel selenophene-thiophene block copolymers and reported the unprecedented discovery that blocks varying by only an element undergo phase-separation (JACS, 2010; highlight by Nature Chem: “Conjugated polymers… have useful electronic and optical properties and the potential for widespread use in many devices. The ease with which they can be processed gives them advantages over traditional solid-state inorganic materials… Controlling their morphology on the nanoscale is crucial to controlling their properties. Now, Dwight Seferos and colleagues… have made conjugated copolymers… that phase separate”) Distinct crystallization properties for thiophene and selenophene drive this form of phase-separation (Chem. Sci. 2011; w/cover). Nanostructures of selenophene-thiophene polymers can be used to align spherical nanocrystals into hierarchical structures for energy-transfer. We have also fabricated solar cells, and found that the thermal stability of the cells improve, which we attribute to this unique polymer structure. More recently we have focused on mixing crystalline co-polymers with amorphous electron acceptors. We have shown that they retain a pure polymer phase and have used this to control the phase purity of a polymer solar cell for the first time (Adv. Mater. 2015). We are continuing both the fundamental study of these polymers and developing new architectures.
“N-type” polymers. There are many examples of positive charge accepting/transporting (p-type) organic materials, yet n-type analogues are quite rare. For example, even after 20+ years, fullerenes remain the best n-type organic materials for photovoltaic cells. Electron-accepting n-type materials are also critical for the development of polymer batteries, which require both a reduction and oxidation reaction. From a fundamental standpoint the reason has to do with their structure. N-type delocalized molecules are electron deficient. This generally makes them unreactive and insoluble, which is detrimental to their synthesis and processing. With the goal of first improving the synthesis we have discovered the first catalysis system that is capable of the controlled polymerization of electron-deficient heterocycles leading to the first well-defined, high molecular weight n-type polymers (JACS, 2013). The work was immediately highlighted in SYNFACTS: “The controlled polymerization of electron-deficient polymers represents a significant problem in materials chemistry. In this paper, the authors synthesize nickel(II)–diimine catalysts with tailored reactivity.” Following up on this we used the methodology to prepare unprecedented p-type/n-type block-copolymers. We are currently using these polymers in all-polymer solar cells and as energy storage materials.
Energy Storage Polymers. Whereas applications of conjugated materials towards light emitters, solar cells, and transistors are reasonably well-established, energy storage applications are undeveloped. We were the first group to prepare polyfullerene (p-C60) electrodes and apply them as negative current collectors in supercapacitor-based energy storage devices (Adv. Energy Mater. 2014). We have also shown that the donor-acceptor approach to conjugated polymer design is applicable for polymer electrode materials that are capable of storing both positive and negative charge. Our continued emphasis is improving the cycle stability of these devices as well as creating new materials for energy storage from renewable sources.