(608b) Self-Assembly of Polythiophene Block Copolymers

Polythiophenes have attracted much attention in the field of organic electronics due to their high charge transport performance, their chemically tunable electronic properties, and their processibility from numerous solvents. In particular, poly(3-hexylthiophene) (P3HT) has become the most common electron-donating material in organic photovoltaics (OPVs) and recent advances in the fabrication and post-processing of P3HT–fullerene bulk heterojunction solar cells have allowed for devices with power conversion efficiencies of up to 5% to be consistently realized. This increase in efficiency is thought to arise from enhanced microphase separation of the active layer components and improved ordering in the structure of the film. These two factors allow for better separation of the bound electron-hole pair (exciton) that is generated by absorption of light in the active layer. An understanding of how exciton dissociation and the internal morphology of the active layer affects device performance would facilitate cell optimization and ultimately lead to higher efficiencies.

To more systematically study the effects of domain size and morphology on device performance, we have synthesized ABA-type triblock copolymers polylactide-b-poly(3-hexylthiophene)-b-polylactide (PLA-P3HT-PLA) by using a difunctional, alcohol-terminated poly(3-hexylthiophene) as a macroinitiator for the ring-opening polymerization of D,L-lactide and have thoroughly characterized the molecular and thermal properties of these novel materials. We have observed that when the P3HT moiety is amorphous the polymers self-assemble into ordered microstructures (domain spacing ~40 nm) both in the bulk (via SAXS) and in thin films (via AFM). We have previously shown that the polylactide domains can be selectively etched from the polythiophene matrix creating templates that could lead to all-organic ordered bulk heterojunction devices upon backfilling with an electron-accepting material.