Electron-enhanced hole injection in blue polyfluorene-based polymer light-emitting diodes

It has recently been reported that, after electrical conditioning, an ohmic hole contact is formed in poly(9,9-dioctylfluorene) (PFO)-based polymer light-emitting diodes (PLED), despite the large hole-injection barrier obtained with a poly(styrene sulfonic acid)-doped poly (3,4-ethylenedioxythiophene) (PEDOT:PSS) anode. We demonstrate that the initial current at low voltages in a PEDOT:PSS/PFO-based PLED is electron dominated. The voltage at which the hole injection is enhanced strongly depends on the electron-transport properties of the device, which can be modified by the replacement of reactive end groups by monomers in the synthesis. Our measurements reveal that the switching voltage of the PLED is governed by the electron concentration at the PEDOT:PSS/PFO contact. The switching effect in PFO is only observed for a PEDOT:PSS hole contact and not for other anodes such as indium tin oxide or Ag.