Fully conjugated donor-acceptor block copolymers as model systems for studies of charge transfer

Fully conjugated block copolymers, consisting of an electron donor and an electron acceptor block, can serve as the active layer in organic photovoltaic devices. Incorporating the donor-acceptor interface within the chemical structure enables model studies of charge transfer. We synthesized a series of block copolymers consisting of a P3HT electron donor and a push-pull polymer electron acceptor, yielding: poly(3-hexylthiophene)$-$block$-$poly-((9-(9-heptadecanyl)-9H-carbazole)-1,4-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2',2"-diyl) (P3HT-b-PCT6BT), poly(3-hexylthiophene)$-$block$-$poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2',2"-diyl) (P3HT-b-PFT6BT), and poly(3-hexylthiophene)$-$block$-$poly-((2,5-dihexylphenylene)-1,4-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2',2"-diyl) (P3HT-b-PPT6BT). By altering the electron rich unit of the acceptor, we adjust the energy difference between donor and acceptor HOMOs. Using photoluminescence, we observe no evidence of exciton dissociation to a charge transfer state in P3HT-b-PCT6BT. In P3HT-b-PFT6BT and P3HT-b-PPT6BT we observe varying degrees of intrachain charge transfer. These results measure the critical driving force needed for charge transfer.