Random and regioregular electron donor acceptor (D-A) polymers comprising of 3,6-bis(3-octyltridecyl)thieno[3,2-b]thiophene donor and 5,6-difluorobenzo[c][1,2,S]thiadiazole acceptor moieties were synthesized to study the effects of both structural regioregularity and molecular weight on their performance in organic thin-film transistors (OTFTs) and bulk-heterojunction (BHJ) polymer solar cells (PSCs). All the polymers displayed similar molecular orbital energy levels regardless of their structures and molecular weights (MWs). The regioregular D-A polymers exhibited higher molecular orders than their random counterparts, and the degrees in molecular ordering improved with increased MW, leading to higher charge transport capability. However, undesirably high MW led to limited polymer solubility and thus fabrication difficulties resulting in poorer device performance. A reasonably high power conversion efficiency of about 7.6% was provided by the new regioregular D-A polymer semiconductor of mid-MW properties with excellent solution processability and molecular self-assembly efficiency.