While Cd-based materials are under the risk to be completely banned sooner or later, much attention has been paid in the last few years to the development of non-toxic analogues with similarly good performance.
In case of quantum-dots-based light emitting diods (QLED), the maximal brightness and efficiency of luminescence are the most critical parameters determining whether the new technology can be accepted for mass production. The best results up to now have been achieved with QDs of copper-indium-sulfide (max. brightness 2100 cd/m2) and silicon (efficiency of 1.1%).
Now a new breakthrough in the development of Cd-free QLED was reported by a group of scientists from Seoul National University & KIST in an article published online in ACS-Nano describes novel quantum dots made from cadmium-free lnP/ZnSeS core-shell semiconductor nanoparticles.
The special feature of these QDs is the gradient structure of the shell, where selenium resides mostly close to the core, while sulfide enriches the outer part of the shell. The further fine tuning of the shell thickness and of the assembly of the QLED brought its results. The synthesized QDs had the electroluminescence quantum yield of 3.46% (compared to the previous record of 1.1%) and brightness of 3900 cd/m2 while showing the photoluminescence QY of 70%.
Abstract from ACS Nano: http://pubs.acs.org/doi/abs/10.1021/nn403594j
Highly Efficient Cadmium-Free Quantum Dot Light-Emitting Diodes Enabled by the Direct Formation of Excitons within InP@ZnSeS Quantum Dots
We demonstrate bright, efficient, and environmentally benign InP quantum dot (QD)-based light-emitting diodes (QLEDs) through the direct charge carrier injection into QDs and the efficient radiative exciton recombination within QDs. The direct exciton formation within QDs is facilitated by an adoption of a solution-processed, thin conjugated polyelectrolyte layer, which reduces the electron injection barrier between cathode and QDs via vacuum level shift and promotes the charge carrier balance within QDs. The efficient radiative recombination of these excitons is enabled in structurally engineered InP@ZnSeS heterostructured QDs, in which excitons in the InP domain are effectively passivated by thick ZnSeS composition-gradient shells.
The resulting QLEDs record 3.46% of external quantum efficiency and 3900 cd m–2 of maximum brightness, which represent 10-fold increase in device efficiency and 5-fold increase in brightness compared with previous reports. We believe that such a comprehensive scheme in designing device architecture and the structural formulation of QDs provides a reasonable guideline for practical realization of environmentally benign, high-performance QLEDs in the future.