Synthesis and characterization of low bandgap conjugated donor-acceptor polymers for polymer: PCBM solar cells

We report on the synthesis, characterization and photovoltaic performance of three novel semiconducting polymers based on poly[bis-N,N'-(4-octylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine-alt-5,5'-4',7',-di-2-thienyl-2',1',3'-benzothiadiazole]. They differ only in the presence and position of hexyl side-chains on the thienyl groups. T8TBT-0 has no such side-chains, they face towards the benzothiadiazole in T8TBT-in and away in T8TBT-out. Based on electron-donating triarylamine and electron-accepting dithienyl-benzothiadiazole groups, the new polymers exhibit low bandgaps and enhanced absorption in the red part of the visible spectrum. Despite their identical backbone they differ in their synthesis and photophysics: T8TBT-0 and T8TBT-in can be synthesized by direct Suzuki coupling but a new synthesis procedure is necessary for T8TBT-out. In absorption and luminescence a blue shift is induced by the inward facing, and to a lesser extent by the outward-facing side-chains. From comparison of the photophysics in solutions and films, we conclude that the addition of side-chains reduces formation of aggregates in films and that this effect is stronger for inward-facing side-chains. By blending the three polymers with PCBM in a standard photovoltaic device architecture, T8TBT-0 performs best with a power conversion efficiency (PCE) of 1.0% (under AM1.5G illumination at 100 mW cm(-2)) compared to 0.17% and 0.27% for T8TBT-out and T8TBT-in, respectively.