Quantum dots: evolving downconverter technology beyond phosphors?

October 18, 2018 // By Dr Khasha Ghaffarzadeh
Quantum dots (QDs) are often billed as the ultimate, or at least as the next generation of, phosphors. The main driver often is the QDs' ability to act as ultra-narrowband downconverters, resulting in extremely wide colour gamut displays and efficient and high CRI solid state LED lights.

In this article, we will explore the merits of quantum dots as ultimate phosphors. Phosphor converted LEDs (pc-LEDs) are in many display and lighting applications. There, they enable a single LED to achieve white colour. This is commonly done by mixing yellow plus red phosphors within the LED packaging. There are of course other ways of achieving white colours such as having three LEDs, or mixing three phosphors each for a different colour, and so on.  

To enable this application and to ride the rising wave of LED sales, numerous new phosphor compositions have been developed in recent years. These phosphors have had to satisfy several criteria: strongly absorb the blue of the LED, achieve high efficiency, exhibit high chemical, light, heat and humidity stability, suffer from minimal heat or light quenching, offer low cost and a composition free of toxic elements, and so on.

These requirements, more challenging that those previously encountered and solved for CRT displays and fluorescent lights, unleashed a worldwide research and screening effort. Consequently, many families of phosphors (nitride, fluorides, etc) were developed, largely addressing most requirements.  However, one persistent problem often remained: phosphors had wide emission bandwidths even for single colour ones. This was particularly an issue for red phosphors because red sits at the edge of the sensitivity spectrum of our eyes. As such, broadband red phosphors would re-emit light in wavelengths outside the eye's sensitivity range, thus wasting it and lowering overall efficiency.

To overcome this, various narrowband phosphors have been developed in more recent years with notable success. The fluoride KSF (K2SiF6:Mn) phosphor is a leading example. It gives five 2nm emission lines in the red range. Another example is SLA (SrLiAl3N4:Eu) which gives 50nm FWHM in the red. The former is superior to any QD in terms of the narrowness of emission band, however it has some limitations in that its five emission peaks are slightly off-target and that it has a long photoluminescent decay time (on the order milliseconds). The latter, of course, is still not as narrow as QDs.

Note that the development of narrowband phosphors was never just limited to red, although the need for red was most pronounced. Green phosphors were also developed. One example is β‐SiAlON which gives a FWHM around 55nm.

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