Molecular weight and end capping effects on the optoelectronic properties of structurally related 'heavy atom' donor-acceptor polymers

Two donor-acceptor polymers containing either Si or Ge in the donor and Se in the acceptor, poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzoselenadiazole)-4,7-diyl] and poly [(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]germole)-2,6-diyl-alt-(2,1,3-benzoselena diazole)-4,7-diyl], were synthesized by microwave assisted polymerization. These polymer structures are attractive because they combine the red light absorption characteristics of the Se acceptor, with high charge carrier mobility inherent to the Si- or Ge-containing donor. Here we study the effects of molecular weight and end capping on the photophysical, morphological, and photovoltaic properties. The solution and film absorption profiles and solution onset are dictated by molecular weight, whereas the subtler heteroatom effect dictates the absorption onset in the polymer films. Molecular weight appears to affect polymer absorption to the greatest degree in a medium molecular weight regime and these effects have a significant aggregation component. Highlighting the red-light absorption of the Se-acceptor monomer, both Si-donor and Ge-donor polymer devices display improved photon harvesting beyond 850 nm relative to their S-acceptor analogues. Higher hole mobility relative to the C-donor/Se-acceptor polymer analogue indicates successful integration of heavy atom donor properties with the 2,1,3-benzoselenadiazole acceptor. Molecular weight invokes the greatest change on polymer/fullerene blend morphology, followed by phenyl end capping, and finally by the Si or Ge heteroatom.