Probing the ionic glass transition in highly-doped conjugated polymers using x-ray photon correlation spectroscopy

Conjugated polymers have recently gained interest as promising heat to energy conversion materials. Recent work in our group suggests that conduction in these materials is suppressed by the formation of a Coulomb gap—a soft gap in the density of states centred on the Fermi level. Ionic relaxation contributes to the formation of the gap; below an ionic glass transition temperature around 240K it becomes possible to shift the Fermi level away from the centre of the Coulomb gap in field-effect transistors. These non-equilibrium states show large, simultaneous improvements in Seebeck and electrical conductivity, providing a new route to high performance thermoelectrics. However, the nature of the underlying ionic transition remains elusive. We propose to study the structural dynamics underlying the ionic glass transition using X-ray photon correlation spectroscopy (XPCS). These data will help us to engineer higher temperature non-equilibrium behaviour needed for real-world applications.