Optical and electroluminescent properties of polyfluorene copolymers and their blends

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 nm (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. Förster 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 5×104 cd/m2 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%.