Polymer light-emitting diodes based on poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl]

Polyfluorenes are a promising class of semiconductive polymers for light-emitting diode (LED) applications due to their efficient photoluminescence and electroluminescence (EL) combined with their good thermal stability. In this paper, we report on the fabrication and characterization of LEDs based on poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl]. The light emitting polymer thin film layer was either pristine poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl] or this polyfluorene blended with the hole transport material poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N'-bis(4-butylphenyl-1,1'-biphenylene-4,4'-diamine))]. The addition of the hole-transporting polymer to the host polymer improved device properties. The maximum luminance of polymer LEDs achieved with this design was 884 cd/m(2). The maximum external quantum efficiency of the blended polyfluorene devices was 0.1 %, which is more than three times higher than that of the pristine polyfluorene LEDs. The introduction of the hole-transporting polymer also modifies the EL spectrum. For the blend-based devices, EL peaked at 504 nm, having a blue shift of 33 nm compared to that of the single polymer based devices. Phase-separated domains in thin film morphology were observed in scanning electron microscope pictures. These domains, or interactions at their interfaces, may play a role in the improved device performance.