Understanding and controlling crystallinity of materials is crucial in the burgeoning field of organic electronics (transistors, photovoltaics, light emitting diodes). In the Segalman group, we are interested in examining how to control the degree of crystallinity of commonly-used conjugated polymers and what effect these changes have on the performance of devices. Specifically, we have shown that the degree of crystallinity of poly(3-alkylthiophenes) which can be thermally-processed near room temperature can be monitored using x-ray diffraction techniques, and that this change in crystallinity can be correlated directly to the optical and electrical transport behavior on this timescale. These experiments have provided new understanding of the role that crystal growth plays in device performance.
Furthermore, we are interested in how the thermal history experienced by polymer samples can affect the equilibrium size of crystallites and the ultimate degree of crystallinity. It is well-known in conventional semi-crystalline polymers that the crystallization temperature is a critical parameter for determining the morphology of the material at a number of lengthscales. However, this knowledge has not been sufficiently adapted to conjugated polymers which are inherently anisotropic.
Functional self-assembled block copolymers
Organic photovoltiacs rely on the dissociation of a short-lived excited state (exciton) created on the absorption of light at the interface of two materials with different electron affinities and ionization potentials. It has been proposed that nanostructured active layers with well-defined interfaces and domain sizes would be able to optimially harvest excitons and transport the free charges to the relevant electrodes. Block copolymers are well-suited to this task because they are known to self-assemble on a length scale in agreement with the diffusion length of an exciton. We have shown the ability to self-assemble block copolymers which contain either poly(phenylene vinylene) or poly(3-alkylthiophene) moieties, two of the most well-studied polymeric photovoltaic materials, into a variety of morphologies on the appropriate length scale.
Click here to access a list of our publications related to conjugated polymers.