Novel soluble conjugated random copolymers are synthesized by palladium-catalyzed Suzuki coupling reaction from 9,9-dioctylfluorene (DOF) and 4,7-di(4-hexylthien-2-yl)-2,1,3-benzothiadiazole (DHTBT) with DHTBT composition varying from 1 to 50 mol % in the copolymer. All of the polymers are soluble in common organic solvents and are highly photoluminescent. Polyfluorene fluorescence is quenched completely at a DHTBT concentration as low as 1% in the solid film. The copolymer films are highly fluorescent under UV irradiation in contrast to its parent analogue, 4,7-di(thien-2-yl)-2,1,3-benzothiadiazole (PFO-DBT), without alkyl substitution on thiophene rings. Devices made up of these copolymers emit saturated red light. The emission peaks are shifted from 613 to 672 nm when the DHTBT content increases from 1 to 50%. The highest external quantum efficiency achieved in the device configuration ITO/PEDT/PVK/PFO-DHTBTBa/Al is 2.54% with luminous efficiency 1.45 cd/A for the copolymer with emission peak at 638 nm for 10% DHTBT content, among the highest values so far reported for saturated red polymer emitters.