Built-in field electroabsorption spectroscopy of polymer light-emitting diodes incorporating a doped poly(3,4-ethylene dioxythiophene) hole injection layer

We report electroabsorption measurements of polymer light-emitting diodes, (LEDs), fabricated with poly(4-4(')-diphenylene diphenylvinylene), PDPV, as the emissive layer, Ca-Al cathodes, and indium tin oxide (ITO) anodes, with and without a doped conducting polymer hole injection/transport layer, namely poly(3,4-ethylene dioxythiophene), PEDOT, doped with poly(styrene sulfonate), PSS-. In these structures, the bias at which the electroabsorption signal is null corresponds to the difference between the electrodes' work functions. We find that such a built-in voltage increases by 0.5 V when a PEDOT:PSS film is incorporated between the ITO electrode and the emissive layer. This leads to a marked reduction of the anode barrier height at the hole-injecting interface, and accounts for a variety of improvements brought about by the PEDOT insertion, namely: (a) the increase of luminescence efficiency, (b) the reduction of the turn-on voltage, and (c) the increase of the device lifetime. (C) 1999 American Institute of Physics. [S0003-6951(99)04138-8].