Polythiophene-containing block copolymers for organic photovoltaic applications.

Abstract

Poly(3-alkylthiophene)s (P3ATs) have become the most common electron-donating material in organic photovoltaics (OPVs), and recent advances in the fabrication of polythiophene-fullerene bulk heterojunction solar cells have allowed for devices with power conversion efficiencies of up to ~6% to be realized. This efficiency has only been possible through enhancements in the active layer microstructure. This key factor allowed for better separation of the bound electron-hole pair (exciton), generated by absorption of light. Understanding how exciton dissociation and the active layer morphology affect device performance will facilitate cell optimization, ultimately leading to higher device efficiencies. Consequently, we developed two new classes of polythiophene-based block copolymers to better understand these phenomena. First, we synthesized well-defined diblock and triblock copolymers with the structures: poly(3-alkylthiophene)-b-polylactide (P3AT-PLA) and polylactide-b-poly(3-alkylthiophene)-b-polylactide (PLA-P3AT-PLA). We have observed that kinetic factors dominate phase separation for a semicrystalline polythiophene block. However, if the polythiophene moiety was amorphous the polymers self-assembled into thermodynamically stable, ordered microstructures with domain spacings on the scale of interest for charge separation in OPV cells (ca 30 nm). Polylactide was chosen as the second moiety in the block copolymers because it could be selectively etched from the polythiophene matrix with a gentle alkaline bath. This procedure led to the formation of nanoporous templates that could generate ordered bulk heterojunctions. In the second approach, P3AT chain ends were terminated with fullerene to create an internal electron acceptor-donor-acceptor, methylfulleropyrrolidine-poly(3-alkylthiophene)-methylfulleropyrrolidine (C60-P3AT-C60). Microphase separation occurred between the polymer chain and fullerene end groups, which suggested the creation of two distinct semicrystalline regimes. A compositionally similar blend of P3HT and C60 showed a similar microstructure. This comparable domain formation, coupled with the possibility of enhanced charge transfer, makes C60-P3AT-C60 a promising candidate as a material in bulk heterojunction organic photovoltaic devices.