A series of varied length alkyl substituted donor acceptor (D A) conjugated copolymers with benzo [1,2-b:4,5-b']dithiophene (BDT) as donor and thiophene rings attached to both sides of the benzothiadiazole (TBT) moieties as acceptors were designed and synthesized. The optical and electrochemical properties showed that the absorption spectrum, the band gaps, and the energy levels of the copolymers were not affected by the varied substituted alkyls, and all these copolymers showed low band gaps around 1.75 eV. In addition, the morphologies of the blend film between copolymers and PCBM can be fine-tuned by increasing the length of substituted alkyl of the copolymers, changing from pea-like aggregation to interpenetrating network to grain-like aggregation. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as donors and (6,6)-phenyl C(61)-butyric acid methyl ester (PC(61)BM) or (6,6)-phenyl C(71)-butyric acid methyl ester (PC(71)BM) as acceptors. The optimized photovoltaic performances showed the stable open-circuit voltage (V(oc)) in the range of 0.68 to 0.74 eV, and dramatically increasing short circuit current density (J(sc)) by optimizing the blending morphologies of copolymer and PCBM films. The optimized photovoltaic performance with a V(oc) of 0.70 V, J(sc) of 7.19 mA/cm(2), a fill factor (FF) of 0.52, and a power conversion efficiency (PCE) of 2.88%, was obtained by the copolymer PBDT-TBT-C8 (PBDT-TBT-C8:PC(61)BM, 1:3 w/w, in CB solution). This is due to its low band gap and interpenetrating network morphology of PBDT-TBT-C8:PC(61)BM blend film. The photovoltaic device based on PBDT-TBT-C8:PC(71)BM showed a J(sc) of 8.6 mA/cm(2) and a PCE of 3.15%.