Fluorination in thieno[3,4-c]pyrrole-4,6-dione copolymers leading to electron transport, high crystallinity and end-on alignment

Abstract

A series of copolymers based on thieno[3,4-c]pyrrole-4,6-dione and thiophene-phenyl-thiophene with varying degrees of fluorination on the phenyl unit was synthesized by Stille polycondensation. The influence of the degree of fluorination on the optical, thermal and electrochemical properties of these polymers is systematically studied. Additionally, the charge transport in organic field effect transistors (OFETs) as well as the thin film alignment are investigated. After thermal annealing the non-fluorinated as well as difluorinated polymers show ambipolar charge transport in OFETs. In contrast, tetrafluorination results in exclusively n-type behaviour with an electron mobility of 3.7 × 10⁻⁴ cm² V⁻¹ s⁻¹. GIWAXS measurements of as-cast and annealed films reveal a very interesting structural alignment in thin films. All polymers show the rarely observed “end-on” orientation, where the polymer chains stand on the substrate. This may be explained by the low molecular weights of these polymers in relation to the film thickness. Fluorination does not influence the microstructural properties, only the propensity of crystallization is increased.