We have studied the performance of single-layer polymer light-emitting diod
es based on a polyfluorene derivative. Hole-only devices show low currents;
however, double-carrier devices show high currents and high efficiencies,
implying that the presence of electrons in the device enhances hole injecti
on. By numerical modeling, we show that this behavior is consistent with th
e presence of a barrier to electron extraction at the anode which causes an
increased field for hole injection due to the buildup of electrons at the
barrier. (C) 2001 American Institute of Physics.