  Electroluminescence (EL) of colloidal nanocrystals promises a new generation
of high-performance and solution-processable light-emitting diodes (LEDs). The
operation of nanocrystal-based LEDs relies on the recombination of
electrically-generated excitons. However, a fundamental question, i.e, how
excitons are electrically generated in individual nanocrystals, remains
unanswered. Here, we reveal a molecular mechanism of sequential electron-hole
injection for the exciton generation in nanocrystal-based EL devices. To
decipher the corresponding elementary processes, we develop electrically-pumped
single-nanocrystal spectroscopy. While hole injection into neutral quantum dots
(QDs) is generally-considered to be inefficient, we find that the intermediate
negatively-charged state of QD triggers confinement-enhanced Coulomb
interactions, which simultaneously accelerate hole injection and hinder
excessive electron injection. In-situ/operando spectroscopy on state-of-the-art
QD-LEDs demonstrate that exciton generation at the ensemble level is consistent
with the charge-confinement-enabled sequential electron-hole injection
mechanism revealed at the single-nanocrystal level. Our findings provide a
universal mechanism for enhancing charge balance in nanocrystal-based EL
devices.
