The use of cross-linkable interlayers to improve device performances in blue polymer light-emitting diodes

High efficiency blue PLEDs were fabricated by adding a thin interlayer between PEDT:PSS and emitting polymer layers. Two different cross-linkable alkoxysilane-based interlayer materials, X-NPB and X-PDA, were synthesized based on N,N′-bis(4-methylphenyl)-N,N′-diphenyl-1,4-phenylenediamine (PDA) and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1-biphenyl-4,4-diamine (NPB) which are well-known OLED HTLs. The devices, with configuration of indium tin oxide (ITO)/PEDT:PSS (65 nm)/interlayer (10-20 nm)/emitting polymer layer (70 nm)/BaF₂ (2 nm)/Ca (50 nm)/Al (300 nm), were fabricated by spin coating and thermal evaporation. In this device structure, the cross-linked X-NPB or X-PDA interlayers are more adherent and mechanically robust as well as impervious to spin coating of next emitting polymer layer. In addition, the devices with these interlayers exhibit a higher luminescence and current efficiency than those without interlayers because interlayers have two functions which are blocking electrons and preventing from severe quenching by PEDT:PSS.