A series of soluble conjugated copolymers derived from 9.9-dioctylfluorene (FO) and selenophene (SeH) was synthesized by a palladium-catalyzed Suzuki coupling reaction with various feed ratios of SeH to FO less than or equal to 50%. The efficient energy transfer from fluorene segments to narrow band-gap selenophene sites was observed. In comparison with the very well studied copolymer poly(fluorene-co-thiophene), poly(9,9-dioctylfluorene-co-selenophene) (PFO-SeH) shows redshifted photoluminescence (PL) and electroluminescence (EL) emission. PL spectra of the PFO-SeH copolymers show a significant redshift along with increasing selenophene content in the copolymers and also with increasing polymer concentration in solution. PL quantum efficiency, of the selenophene-containing PFO copolymer is much lower than that of corresponding PFO-thiophene (Th) copolymers. All these features of PFO-SeH copolymers ran be explained by the difference in aromaticity of selenophene and thiophene heterocycles and the heavy, atom effect of Se in comparison with S-atoms. The device fabricated with PFO-SeH15 as the emissive layer exhibited high external quantum efficiency (0.511%) at a luminance of 1570 cd/m(2). Device performance is limited by electron injection and the strong quenching effect of Se atoms. Devices with PFO-SeH copolymers blended into PFO homopolymers show significant improvement in device performance. External quantum efficiency, as high as 1.7% can be obtained for PFO-SeH30/PFO blend devices. (C) 2005 Wiley Periodicals, Inc.