Dramatically improving polymer light-emitting diode performance by doping with inorganic salt

A simple method is described that dramatically lowers the turn-on voltage of polymer light-emitting diodes (PLEDs) by doping the electroluminescent polymer with an inorganic salt. Single-layer PLEDs were fabricated by spin-coating a solution of poly(9,9'-dioctylfluorene), doped with controlled amounts of LiBr, onto an indium tin oxide-coated glass (anode) followed by physical vapor deposition of an Al cathode. The turn-on voltage dropped from the undoped value of 7.4 V as low as to 3.0 V. These results are discussed in terms of alterations of the cathode-polymer and anode-polymer interfaces by Li+ and Br-. (c) 2005 American Institute of Physics.