Dr Khasha Ghaffarzadeh, the research director of IDTechEx, analyses quantum dot materials and technologies, and assesses the evolution of downconverter technology beyond phosphors.
The following report discusses the merits of quantum dot materials and technologies as the ultimate phosphors.
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. In the IDTechEx Research report Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players, we provide a much more detailed and quantitative benchmarking of the two technologies. Furthermore, our report also provides a detailed analysis of QD material development and progress for various applications, examining trends towards alternatives to Cd-based QDs including InP and perovskite chemistries, improved QD stability, enhanced blue absorbance, reduced self-absorption, increased computability with various processing mediums such as inks or resins, and so on.
Phosphor converted LEDs (pc-LEDs) are in many display and lighting applications. There, they enable a single LED to achieve while color. This is commonly done by mixing yellow plus red phosphors within the LED packaging. There are of course other ways of achieving white colors such as having three LEDs, or mixing three phosphors each for a different color, 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 color 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 milli seconds). The latter, of course, is still not as narrow as QDs (see below section for more details).
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.
Why quantum dots?
The question is: why bother with quantum dots given that phosphors achieve maturity, stability, and even narrowband emission including for red? The answer is to be found in the potential for the simultaneous satisfaction of all criteria, including narrow emission, high conversion efficiency, control over emission wavelength, fast PL decay time, small particle size, and so on. This, if achieved, will potentially represent the ultimate downconverter material.
Dr Ghaffarzadeh will be featuring in our December publication, SciTech Europa Quarterly issue 29, to discuss the major market trends of quantum dot films in displays and the commercialism of thin films.
For more about the report, Quantum Dot Materials and Technologies 2018-2028: Trends, Markets, Players, visit www.IDTechEx.com/QD or contact research@IDTechEx.com.