Film Morphology and Electroluminescence of Poly[9,9-di-(2'-ethylhexyl) fuorenyl-2,7-diyl] Blended with a Hole Transporting Polymer

Abstract

This paper reports on the phase-separated film morphology and electroluminescence (EL) of a polyfluorene, poly[9,9-di-(2'-ethylhexyl)fluorenyl-2,7-diyl], 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 film morphology changes remarkably with the blend ratio. Other factors like polymer solution concentration, substrate and film thickness could also influence the film morphology. The addition of the hole-transporting polymer to the host polymer improved device properties of light-emitting diodes (LEDs). The maximum luminance achieved was 884 cd/m2. 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, the EL peaked at 504 nm, having a blue shift of 33 nm compared to that of the single polymer based devices. Phase-separated domains, or interactions at their interfaces, may play a role in the improved device performance.