Role of the interfacial layer in the efficiency and lifetime of polymeric light emitting devices

The effects of the interfacial layers on the properties of fluorescent polymeric light emitting devices were investigated. The interfacial layer material, poly(9,9(')-dioctylfluorene-co-bis-N,N-'-(4-ethoxycarbonylphenyl)-bis-N,N-'-phenyl-benzidine (BFEC), has uniform surface roughness and high hole mobility upon thermal treatment up to 250 degrees C. Both for commercially available fluorescent red and green polymeric light emitters, the insertion of a BFEC interfacial layer resulted in a more than 30% increase of efficiency and a three times longer half-lifetime, whereas the blue fluorescent material showed no enhancement of power efficiency and lifetime. We attribute the improved device performance afforded by the interfacial layer to the bilayered devices having a more charge-balanced structure. The facilitated hole injection and electron/exciton blocking of the interfacial layer induced an infinitesimal shift of the recombination region, which was characterized by the electroluminescent spectra of the devices and the photoluminescent spectra of the bilayers.