We have used Stille coupling polymerization to synthesize a series of new crystalline low band gap conjugated polymers-PTHBO, PBTTBO, and PTTTBO-constituting mainly electron-rich thiophene (TH), 2,2'-bithiophene (BT), and thieno[3,2-b]thiophene (TT) units in conjugation with electron-deficient 2,1,3-benzooxadiazole (BO) moieties. All of these polymers exhibited (i) sufficient energy offsets with respect to those of fullerenes to allow efficient charge transfer and (ii) low-lying highest occupied molecular orbital (HOMO, -5.47 eV). These polymers exhibited excellent thermal stability, high crystallinity, and broad spectral absorptions. As a result, bulk hetero-junction photovoltaic devices derived from these polymers and fullerenes provided open-circuit voltages (V(oc)) as high as 1.02 V. In particular, the photovoltaic device comprising the PTTTBO/PC(61)BM (1:1) blend system and 1,8-diiodooctane (DIO, 0.5 vol %) as an additive exhibited excellent performance, under AM 1.5 G irradiation (100 mW cm(-2)), with a value of V(oc) of 0.85 V, a short-circuit current density (J(sc)) of 11.6 mA cm(-2), a fill factor (FF) of 0.54, and a promising power conversion efficiency (PCE) of 5.3%.