Temperature Dependent Studies of IDT-BT Reveal Liquid Crystalline Phase Behavior

Some of the highest charge transporting conjugated polymers to date such as Indacenodithiophene-co-benzothiadiazole (IDT-BT) have shown weak to no crystallinity which is puzzling as it deviates from traditional evidence that higher order is needed for high charge mobility. While many have hypothesized that an increased order at a molecular scale along with a stiff planar backbone can increase charge transport without the need for long range order, this behavior is still not fully understood. Often, stiffer conjugated polymers can have liquid crystalline phases, indeed many high mobility conjugated polymers, such as PBTTT, have liquid crystallinity. As the polymer chains are not restricted to a certain packing structure dictated by crystallization, the chains in a liquid crystalline polymer are able to adopt conformations that can maximize interchain charge coupling. Using rheology, X-ray scattering, and polarized optical measurements we have investigated the morphology and phase behavior of IDT-BT as a function of temperature. We have identified the backbone and side chain glass transition temperatures and discovered three liquid crystalline phases. We also have fabricated IDT-BT field effect transistors, measured in-situ, to reveal how phase behavior affects hole mobility.