Research on organic photovoltaics (OPVs) using pi-conjugated polymers as semiconducting materials has attracted increasing attention because OPVs have the advantage of lightness, flexibility, low cost, and ease of processing. The power conversion efficiency (PCE) of the OPVs depends on the organic semiconducting materials. Thus, the investigation of structure-property-semiconducting performance relationship is important for the improvement of OPVs. In this presentation, we focus on the low-bandgap alternating copolymers comprising dithienosilole as a donor unit and dialkyl-substituted dioxocycloalkene-annelated thiophene as an acceptor unit and investigate the influence of the polymer molecular weight and the alkyl chain length in the acceptor unit on the polymer properties and photovoltaic performance. All the investigated copolymers are amorphous in the solid state. Both the increase of polymer molecular weight and variation of the alkyl side chains in the dioxocycloalkene-annelated thiophene unit subtly affected molecular properties. However, these structural modifications showed significant influence on the photovoltaic performance in bulk heterojunction (BHJ) solar cells based on copolymer/PC71BM. Furthermore, as a result of the thin-film fabrication method, the BHJ thin-films led to the appearance of partially crystallized states of the copolymer. Consequently, the conventional BHJ solar cell based on the copolymer/PC71BM combination achieved a PCE of 7.85%, which is the highest value reported for amorphous copolymers in a conventionally structured device.