Quantum dots: progress, challenges and future in LED lighting

April 10, 2020 //By Dr Khasha Ghaffarzadeh
Quantum dots
First, we must ask what value Quantum Dots (QDs) might deliver in LED lighting? In general lighting, QDs can enable simultaneously higher efficacy and higher CRI (color rendering index) values.

Overall, the efficacy penalty in boosting the CRI is very heavy. In 2019, Osram launched a LED light with on-chip QD, achieving 173lm/W with 90 CRI. Lumileds had demonstrated similar results using the same base QD technology a year or so earlier. These cases improved efficiency by some 8-10% at CRI 90 compared to leading non-QD narrowband red emitters. These results clearly demonstrate the success of QDs in weakening the trade-off between lm/W efficacy and CRI, an important measure of color quality.

Challenges
The key QD color in lighting is red. This is because the spectrum of current phosphors is too broad, re-emitting red light with wavelengths outside the sensitivity limits of the eye, thus wasting that energy. Consequently, a narrow red emitter with the desired centre wavelength can significantly improve efficiency.
The technology challenges have been to find QD material systems which are (a) stable enough to withstand the heat and light flux, (b) are as much of a drop-in solution as possible compared to existing phosphor technology, and (d) contain little or no toxic content, e.g. cadmium.
Initially, remote color converter implementations were proposed and adopted. Here, a QD film was added at some distance over the LED to do the color conversion. Similar arrangements are also deployed in displays. This is essentially a work-around solution sidestepping the stability limitations of current QD technology. This was and is, however, not an elegant solution. The initial aim was to realize on-chip integration, i.e. integrating the QD into the LED package just as phosphors are integrated today.
To realize on-chip integrations, the QDs need to be highly engineered. For example, the deployed QDs may undergo ligands exchange followed by a reaction to form a protective silica shell around the usual core-shell structure. Even with such measures, the early products are constrained to low-power LED, i.e. 0.2W.
It is reasonable to expect that stability will improve over time, gradually opening ever higher power grades of the market. This will be driven by improvements in single- or multi-layer shell structure (e.g., ZnO/silica/QD), by better control of the core-shell interface, and by minimization of oxidative and other detects.


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