Nanoscale Engineering Advances QD-LEDs

New research aims to improve quantum dot LEDs (QD-LEDs) by using a new generation of the nanosized, engineered semiconductor particles specifically tailored to have reduced wasteful charge-carrier interactions that compete with the production of light.

Incandescent light bulbs convert only 10 percent of electrical energy into light, with the remaining 90 percent lost to heat. They are being replaced worldwide with less-wasteful fluorescent light sources, but the most efficient approach is direct conversion of electricity into light using electroluminescent devices such as LEDs.

The quantum dot device structure shown with a transmission electron microscopy image of a cross section of a real device. Images courtesy of LANL.

Quantum dots are semiconductor particles with an emission color that can be tuned by simply changing their dimensions. With their spectrally narrow, tunable emission and ease of processing, colloidal QDs are attractive for use in LED technologies. In the last decade, vigorous research in QD-LEDs has led to dramatic improvements in their performance, to the point where it nearly meets the requirements for commercial products. One outstanding challenge in the field is the so-called efficiency roll-off (known also as “droop”), or the drop in efficiency at high currents.

“QD-LEDs can potentially provide many advantages over standard lighting technologies, such as incandescent bulbs, especially in the areas of efficiency, operating lifetime and the color quality of the emitted light,” said Victor Klimov of Los Alamos National Laboratory (LANL). The lab’s Nanotechnology and Advanced Spectroscopy team has conducted work recently that could lead to dramatic advances in QD-LEDs.

“This ‘droop’ problem complicates achieving practical levels of brightness required especially for lighting applications,” said Wan Ki Bae, a postdoc on the LANL nanotech team. By conducting spectroscopic studies on operational QD-LEDs, the Los Alamos researchers established that the main factor responsible for the reduction in efficiency is an effect called Auger recombination.

Postdoc Young-Shin Park characterizing emission spectra of LEDs in the Los Alamos National Laboratory optical laboratory.

In this process, instead of being emitted as a photon, the energy from recombination of an excited electron and hole is transferred to the excess charge and subsequently dissipated as heat.

Their work not only identified the mechanism for efficiency losses in QD-LEDs, Klimov said, but it has also demonstrated two different nanoengineering strategies for circumventing the problem in QD-LEDs based on bright quantum dots made of cadmium selenide cores overcoated with cadmium sulfide shells.

The first approach is to reduce the efficiency of Auger recombination itself, which can be done by incorporating a thin layer of cadmium selenide sulfide alloy at the core/shell interface of each quantum dot.

The other approach attacks the problem of charge imbalance by better controlling the flow of extra electrons into the dots themselves. This can be accomplished by coating each dot in a thin layer of zinc cadmium sulfide, which selectively impedes electron injection.

“This fine-tuning of electron and hole injection currents helps maintain the dots in a charge-neutral state and thus prevents activation of Auger recombination,” said Jeffrey Pietryga, a chemist on the nanotech team.

A paper on the work, which was funded by a grant from the US Department of Energy Office of Science, was published today in Nature Communications. (doi:10.1038/ncomms3661)

For more information, visit: www.lanl.gov