Light emitting properties of several polyfluorene (PF) copolymers (P1-P4) and their blends have been investigated. Light emitting diodes were fabricated with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al. The EL peak wavelengths were 421 nut (violet), 505, 513 nm (green) and 570 nm (yellow) for PF copolymers and 510,535 nm (green) for P1/P2 and P1/P3 blends, respectively. Forster energy transfer in the photoluminescence and electroluminescence of the polymer blends P1/P2 and P1/P3 was studied. The LED using the polymer blend P1/P2 showed a turn-on voltage of 2.5 V and a brightness of 5 X 10(4) cd/m(2) at 7 V. The highest external quantum efficiency was observed to be 3.71% at 5 V. Upon addition of 20 wt% of the green emitter P2 to the violet emitter P1, the device efficiency increased from 1.18 to 3.71%. (C) 2004 Elsevier Ltd. All rights reserved.