  A growing interest in colloidal quantum dot (QD) based light-emitting diodes
(QD-LEDs) has been motivated by the exceptional color purity and spectral
tunability of QD emission as well as the amenability of QD materials to highly
scalable and inexpensive solution processing. One current challenge in the
QD-LED field has been a still incomplete understanding of the role of extrinsic
factors (e.g., recombination via QD surface defects) versus intrinsic processes
such as multicarrier Auger recombination or electron-hole separation due to
applied electric field in defining device efficiency. Here, we address this
problem with a study of excited-state dynamics in a series of structurally
engineered QDs, which is performed in parallel with characterization of their
performance upon incorporation into LEDs. The results of this study indicate
that under both zero and forward bias, a significant fraction of the QDs within
the active emitting layer is negatively charged and therefore, Auger
recombination represents an important factor limiting the efficiency of these
devices. We further observe that the onset of the LED efficiency roll-off is
also controlled by Auger recombination and can be shifted to higher currents by
using newly developed QDs with an intermediate alloy layer at the core-shell
interface introduced for suppression of Auger decay. Our findings suggest that
further improvement in the performance of QD-LEDs can be achieved by developing
effective approaches for controlling Auger recombination and/or minimizing the
effects of QD charging via improved balancing of electron and hole injection
currents.
