Origin of Subthreshold Turn-On in Quantum-Dot Light-Emitting Diodes

The subthreshold (or sub-bandgap) turn-on for electroluminescence is one of the most discussed, but often misinterpreted, phenomena for solution-processed quantum-dot light-emitting diodes. Here, multiple techniques are applied to show that the phenomenon can be readily explained using the fundamental rules of carrier injection and transport. Evident from temperature dependent photovoltage measurements, it is found that the energy up-conversion originating from the decay of charge transfer excitons is not responsible for the subthreshold turn-on. Further analysis using electroabsorption reveals that the turn-on voltage of electroluminescence consistently correlates with the flat-band voltage of the emission layer. Under such subthreshold bias, although the device current is still limited by the depleted hole-transporting layer, field-assisted carrier injection starts to provide enough electrons and holes for detectable radiative recombination, thereby enabling distinct subthreshold turn-on.