Effect of Rod-Rod Interactions On the Microstructure of Poly(3-alkylthiophenes)

Poly(3-hexylthiophene) (P3HT) is used commonly in polymer semiconducting applications due to its relatively low band gap, high charge transport ability, and its ability to be processed from a variety of common organic solvents. During the crystallization of poly(3-hexylthiophene) strong intermolecular interactions lead to the generation of semiconducting fibers, which prevent the creation of microstructures with order over long ranges. This lack of long-range control complicates analysis of structure-property relationships in P3HT-containing devices (e.g., organic photovoltaic cells). Because these crucial structure-transport relationships are vague, design of next generation semiconducting polymers has proven difficult.

Here, we show rod-rod interactions (characterized by the Maier-Saupé parameter) can be controlled by rational polythiophene side chain selection. Effects of side chain passivation are evidenced by a depressed differential scanning calorimetry (DSC) melting temperature and the presence of a liquid crystalline region. Additionally, the Maier-Saupé parameters are estimated for poly(3-dodecylthiophene) (P3DDT) and poly(3-(2'-ethyl)-hexylthiophene) (P3EHT). We show that while the rod-rod interactions are significantly lowered in P3EHT relative to P3HT and P3DDT in the melt, the material is semicrystalline and the crystal structure evidenced by wide-angle x-ray scattering (WAXS) is similar to that of other P3ATs. Importantly, we demonstrate that the ultraviolet-visible (UV-Vis) light absorption spectra and field-effect hole mobilities of P3EHT are on par with the optoelectronic properties of common P3ATs. Systematically tuning the rod-rod interactions in substituted polythiophenes allows for the manipulation of the ratio of Maier-Saupé parameter to the Flory-Huggins parameter. Because the value of this ratio proves crucial in obtaining long-range order in rod-coil block copolymer morphologies, P3EHT-based block copolymers offer great promise towards generating high performance, well-ordered device active layers. These block copolymers have tunable domain geometries and spacings on the nanoscale, which will allow for the systematic study of morphology on organic electronic device performance.