Synthesis and characterization of low bandgap conjugated donor-acceptor polymers for polymer:PCBM solar cells
By Tu, Guoli; Massip, Sylvain; Oberhumer, Philipp M.; He, Ximin; Friend, Richard H.; Greenham, Neil C. & Huck, Wilhelm T. S.
Published in J. Mater. Chem.
The Royal Society of Chemistry
2010
Abstract
We report on the synthesis, characterization and photovoltaic performance of three novel semiconducting polymers based on poly[bis-N, N[prime or minute]-(4-octylphenyl)-bis-N, N[prime or minute]-phenyl-1, 4-phenylenediamine-alt-5, 5[prime or minute]-4[prime or minute], 7[prime or minute], -di-2-thienyl-2[prime or minute], 1[prime or minute], 3[prime or minute]-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.