Barriers to electron extraction in polymer light-emitting diodes

We have studied the performance of single-layer polymer light-emitting diodes 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 injection. By numerical modeling, we show that this behavior is consistent with the 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.