Colloidal QD LEDs can be efficient too: put on the right shell

February 28, 2018 //By Julien Happich
Colloidal QD LEDs can be efficient too: put on the right shell
In a paper titled "Bright colloidal quantum dot light-emitting diodes enabled by efficient chlorination" published in Nature Photonics, researchers from the University of Toronto report a novel way to treat the shell of Zn chalcogenide-shelled colloidal CdSe quantum dots so as to improve the conductivity of their emitting layers, improving their overall external quantum efficiency.

Identifying the limitations of today's colloidal quantum dots (CQDs) organic surface chemistries, the researchers developed an efficient chlorination process that efficiently replaces poorly conducting organic ligands with highly conductive inorganic single-atomic-layer surface ligands. Doing so, they obtained CQD films with simultaneously high conductivity and high quantum efficiency.

Cross-sectional SEM image of the CQD LED layer
stack, with aluminium contact at the top and transparent
ITO contact on glass at the bottom (scale bar is 60nm)

The high conductivity facilitates charge carrier injection and mitigates the accumulation of electrons and holes near the ETL and HTL, suppressing Auger recombination even at high injection current densities, the researchers explained. QLEDs obtained with the chlorinated CQDs yielded a maximum brightness of 460,000 cd/m 2, twice as much that of any reported LEDs using solution-processed emitting layers, they claim. Turn-on voltage was also reduced from 3.5V to 2.5V while increasing the roll-off current threshold.

Peak external quantum efficiencies (EQEs) measured for chlorinated blue, green and red QLEDs were at 12%, 16.5% and 20%, respectively. The researchers conclude their paper noting that this chlorination strategy can be extended to QLEDs with different emission wavelengths to mitigate efficiency roll-off at high injection current densities. They also see it as a possible step towards electrically pumped CQD lasers.

University of Toronto -

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