The turn-on voltage of a light-emitting diode is generally equal to or greater than its bandgap voltage (or the bandgap energy divided by the electron charge). In contrast, we have found that electroluminescence at sub-bandgap voltages can be observed in polymer light-emitting devices with a heterojunction based on ZnO nanoparticles (NPs). This sub-bandgap electroluminescence is attributed to an Auger-assisted energy up-conversion process at the polymer/ZnO NPs interface, the observation of which depends strongly on the size of the nanoparticles. (C) 2010 Elsevier Ltd. All rights reserved.