(52f) Role of Molecular Linker in Charge Separation in All-Conjugated Block Copolymers

All-conjugated block copolymers are promising materials for organic photovoltaics, but it remains uncertain how morphology, molecular structure, and optical and electronic properties of conjugated block copolymers affect device performance. We demonstrate the effect of a molecular linker between donor and acceptor polymers on photovoltaic performance and optoelectronic properties. We synthesized two poly(3-hexylthiophene)-poly(2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl-(9,9-dioctylfluorene)) (P3HT-PTBTF) block copolymers which only differ by molecular linker. Power conversion efficiencies decrease by a factor of 40 times, from 2.2% to 0.05%, when the molecular linker is switched. X-ray scattering profiles and TEM images indicate that morphology is virtually identical independent of molecular linker, as expected. In contrast, ultrafast transient absorption data reveals charge separation is affected by the molecular linker. We also demonstrate that the molecular linker can influence on electronic properties at the donor-acceptor interface and kinetics for charge separation and recombination. In our studies, we find the rate of charge recombination is faster than in polymer-polymer and polymer-fullerene blends, suggesting further improvement is possible through optimization of the linking group. This work demonstrates that the molecular linker chemistry influences charge separation in all-conjugated block copolymer systems, and also suggests that all-conjugated block copolymers can be used as model systems for the donor-acceptor interface in bulk heterojunction blends.