Electrochemical Deposition of Bis(N,N′-diphenylaminoaryl) Substituted Ferrocenes, and Their Application as a Hole-Injection Layer on Polymeric Light-Emitting Diodes

The synthesis, spectro-electrochemical study, and electrochemical polymerization of bis(diphenylamino) substituted ferrocenes (Fcs), including the family of the 1,3,4-oxadiazole (OXD) bridged 1,1′-[p-(Ph₂N)(C₆H₄)_nOXD]₂Fc and the vinylene bridged ferrocene 1,1′-[p-(Ph₂N)(C₆H₄)-(E)-CHCH]₂Fc, have been carried out. The ferrocene compounds underwent three stepwise one-electron oxidations to form the (3+) radical cations that further electrochemically polymerized on a Pt or an indium tin oxide (ITO) electrode surface. The polymeric layers showed extremely good hole-injection properties. The polymeric light emitting devices (PLEDs) of ITO/ED-1,1′-[Ph₂N(C₆H₄)₃OXD]₂Fc/PVK:Ir(PPy)₃:PBD/Mg-Ag showed a turn-on voltage (at 100 cd/m²) of 11 V, with a maximum brightness of 27 700 cd/m². The turn-on voltage was 1.5 V lower than that of the poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) based PLED and 2 V lower than the corresponding electrochemically deposited poly(4-(N,N-diphenylamino)styrene) based PLED.