NOVELTY - Derivatized benzo(1,2-b:3,4-b')dithiophene-thienothiophene based polymer (I) is new.    USE - (I) is useful for organic solar cell device (claimed).    ADVANTAGE - (I) improves the photovoltaic properties of the benzo(1,2-b:3,4-b')dithiophene series polymers to expand the optical absorption range to cover a broader spectrum and to tune polymer packing to optimize the device morphology and to enhance charge carrier mobility.    DETAILED DESCRIPTION - Derivatized benzo(1,2-b:3,4-b')dithiophene-thienothiophene based polymer of formula (I) is new.    R = alkyl or alkoxy;    R1 = furan, thiophene, selenophene or its derivative;    X = H, F, Cl, Br or I; and    n = 1-100, preferably 1-50.    INDEPENDENT CLAIMS are included for:    (1) a monomer solution comprising 3-5 mmole of furan, thiophene or selenophene derivatized benzo(1,2-b:3,4-b')dithiophene monomer dissolved in anhydrous tetrahydrofuran, 8.5-9.5 mmole of N-butyllithium, and 10-12 mmole of trimethylstannane;    (2) a polymer solution comprising (a) 0.25-0.35 mmole of first reactant including 1-(4,6-dibromo-3-fluorothieno(3,4-b)thiophen-2-yl)-2-ethylhexan-1-one or 2-ethylhexyl 4,6-dibromo-3-fluorothieno(3,4-b)thiophene-2-carboxylate dissolved in toluene and dimethylformamide in a ratio of 4:1-6:1, (b) 0.25-0.35 mmole of second reactant including bis(5-(2-ethylhexyl)thiophen-2-yl)-(benzo(1,2-b:4,5-b')dithiophene-2,6-diyl)bis(trimethylstannane) or (4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo(1,2-b:4,5-b')dithiophene-2,6-diyl)bis(trimethylstannane) dissolved in toluene and dimethyformamide solvent, and (c) 1.5-2.5 mg/ml of palladium-tetrakis(triphenylphosphine) catalyst;    (3) a bulk heterojunction material, comprising at least one furan, thiophene or selenophene derivatized benzo(1,2-b:3,4-b')dithiophene-thienothiophene based polymer as a donor, and an acceptor; and    (4) an organic solar cell device comprising an aluminum or silver layer, a bulk heterojunction layer, where the bulk heterojunction layer comprises at least one furan, thiophene or selenophene derivatized benzo(1,2-b:3,4-b')dithiophene-thienothiophene based polymer as a donor and an acceptor, and an indium tin oxide glass layer.