Four donor–acceptor (D–A) conjugated polymers of dithieno[2,3-b;7,6-b]carbazole (DTC) and diketopyrrolopyrrole, which have different alkyls on the nitrogen atom in the DTC unit and are named as P-C8C8, P-C5C5, P-C12, and P-C10, respectively, have been synthesized for studying the effect of the alkyl side chains on the optoelectronic properties of the polymers. All polymers are soluble in various organic solvents and exhibit identical optical band gaps (E(g)(opt)) of ~1.3 eV and highest occupied molecular orbital energy levels of ~−5.1 eV. Organic thin-film transistors and bulk heterojunction polymer solar cells (BHJ PSCs) with phenyl-C(71)-butyric acid methyl ester (PC(71)BM) as the electron-accepting material were fabricated via solution spin-casting. Compared to the polymers substituted by branched alkyl chains, the polymers with straight alkyl chains show higher hole mobility. Of these polymers, P-C10 exhibits the highest field effect mobility up to 0.011 cm(2)/V·s. The alkyl chain on the DTC unit has a strong impact on the film morphology of polymer:PC(71)BM blends. Severe phase separation was found for polymers containing branched alkyl chains, and those with straight alkyl chains formed uniform films featuring fine phase separation. An open-circuit voltage (V(oc)) of 0.72 V, a short-circuit current density (J(sc)) of 13.4 mA/cm(2), a fill factor (FF) of 62%, and a power conversion efficiency (PCE) of 5.9% were demonstrated for BHJ PSCs based on the P-C10:PC(71)BM [1:3 (w/w)] blend film.