Abstract
We have measured the built-in potential of a series of polyfluorene-containing light-emitting diodes (LEDs) by electroabsorption spectroscopy. When the anode incorporates a layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the inferred anode work function is equal to the ionization potential of the emissive polyfluorene. This result requires that the Fermi level of PEDOT:PSS is pinned to the highest occupied molecular orbital of the emissive polymer. Pinning of the anode Fermi level eliminates the injection barrier to holes and explains the low turn-on bias of polymer LEDs that incorporate PEDOT:PSS. We attribute this phenomenon to electron trapping at the interface between PEDOT:PSS and the emissive layer.