We report the synthesis of peripherally alkylated dithienosilole (DTS) and dithienogermole (DTG) monomers that possess side chains of different lengths (methyl vs. butyl) attached to a bridging heavy atom (Si or Ge) and the resulting copolymers with benzothiadiazole (BT) units. We study the optoelectronic and charge transport properties of these copolymers, with a particular focus on their use for bulk heterojunction photovoltaic devices in blends with phenyl-C70-butyric acid methyl ester (PC70BM). Enhanced charge carrier mobility is observed by substituting Si atom with Ge atom and better mixing of copolymers with PC70BM desirable for better charge generation is obtained by shortening the side chain length of copolymers from butyl to methyl groups. We observe a very high open circuit voltage (Voc = ∼0.94 V) in single layer blend devices (polymer:PC70BM = 1 : 4 by weight in DCB) with good device performance (PCE = 2.66%, Jsc = 6.11 mA cm−2, FF = 0.46) using Ge substituted copolymers with methyl side groups. Neither additional pre- or post-thermal annealing steps nor additives to the photoactive blend layer are needed to achieve such device performance. Based on detailed investigation including absorption, emission, charge carrier mobility and quantum chemical calculations for molecular geometry and electronic energy levels, we clarify the role of the heavy atom substitution in the donor–acceptor bridged copolymers and discuss their effects on device performance in bulk heterojunction photovoltaic cells.