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We designed and synthesized a series of conjugated polymers containing alternating 4H-cyclopenta[2,1-b:3,4-b']dithiophene units and comonomers consisting of 2,2'-bithiophene, 3'', 4'' -dihexyl-α -pentathiophene, 3,4-ethylenedioxythiophene and 5,5' -bis(2-thienyl)-4,4' -dihexyl-2,2' -bithiazole. These polymers possess optical bandgaps in the range of 1.75 to 2.0 eV. The desirable absorption attributes of these materials make then excellent candidates for use in photovoltaic cells. Electrochemical studies indicate desirable HOMO-LUMO levels for use with fullerene derivatives as electron transporters. Field effect transistors made of these materials show hole mobilities in the range of 7.5 × 10-4 cm2/Vs to 2.0 × 10-3 cm2/Vs. Due to the combination of these characteristics, power conversion efficiencies up to 3.1% were achieved on devices made of bulk heterojunction composites of these materials with soluble fullerene derivatives. [ABSTRACT FROM AUTHOR]