Hole limited recombination in polymer light-emitting diodes

By comparing the quantum efficiencies of light emission in a series of poly [2-methoxy-5(2'ethyl)hexoxy-phenylenevinylene] diodes with calcium cathodes and various anode metals, we show that, in all cases electrons are the maj ority carrier and recombination is limited by hole injection. These conclus ions are confirmed by the examination of a second series of samples in whic h alkanethiol barrier layers of varying thickness, are deposited on a gold anode. The highest external quantum efficiency was achieved in these experi ments using a clean, semitransparent gold anode. We suggest that electron a nd hole injection rates play the primary role in determining current balanc e and that mobilities play a minor role. (C) 1999 American Institute of Phy sics. [S0003-6951(99)00311-3].