A semiconductive conjugated copolymer comprises at least two chemically different monomer units which, when existing in their individual homopolymer forms, have different semiconductor bandgaps. The proportion of said at least two chemically different monomer units in the copolymer is selected to control the semiconductor bandgap of the copolymer so as to control the optical properties of the copolymer. The copolymer is formed in a manner enabling it to be laid down as a film without substantially affecting the luminescent characteristics of the copolymer and is stable at operational temperature. The semiconductor bandgap may be spatially modulated so as to increase the quantum efficiency of the copolymer when excited to luminescence, to select the wavelength of radiation emitted during luminescence or to select the refractive index of the copolymer.