Dotty About Dots

Dotty About Dots
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Quantum Dots are unusual animals.  Like other luminescent materials they take in energy and give off light, but their application is dependent on what kind of energy they are receiving.  Currently most QDs are used to color convert light (photoluminescence) .  They take in light of a particular wavelength (color) and convert it into another wavelength (color).  While QDs are similar to OLED materials, in that the energy applied to them is returned as light, they are different in the fact that OLED materials are typically electroluminescent, meaning the energy that is applied is not light, but a voltage or current.  In both cases the energy (either type) pushes electrons out of their stable band around the nucleus of the material into a higher (excited) band, leaving a ‘hole’ in the stable band.   The higher band is unstable, and as the laws of thermodynamics call for a return to a lower energy state, the electrons in the excited band need to return to their normal lower energy level.  In both QDs and OLEDs, the return to those lower energy levels means that extra energy needs to depart, and it does as light.

Both types of materials can be tuned to specific wavelengths (color), based on the difference between the ground state, where the ‘relaxed’ electrons reside, and the excited state reached when energy is applied.  The energy difference (gap) between that low state and high state determines the color that is emitted.  OLED emitters are highly varied chemical compounds that are specific to particular colors due to their bandgap, and there are thousands of organic materials that exhibit phosphorescence or fluorescence, each with its own set of characteristics.  Quantum dots however are not as varied, with most falling into three categories.  First Cadmium Selenide QDs.  These QDs are among the most efficient but Cadmium is a toxic material, so they require careful usage and are rather unpopular in consumer products.  Second, Indium Phosphide QDs, a non-toxic QD material that is a bit less efficient but far more user friendly, and third, Cesium Lead Halide (Perovskite) QDs which are the newest group and are still being researched.

But here’s the big difference between OLED materials and QDs.  If one picks which (of the three types) QD material to be used, all three colors can be produced from that material.  How is this magic possible? Don’t they each need to be separate materials and have different bandgaps the way they do with OLEDs? No.  What determines the emitting color of a quantum dot is not the chemical structure of the compound but the size of the QD crystal.  QDs are grown in a laboratory and by reducing or extending the ‘growth’ time (and other factors), the crystals become smaller or larger.  Larger QD crystals emit in the red range and smaller QD crystals emit in the blue range with green in the middle. 

​This ‘tailoring’ of QD nanocrystals, which we have grossly oversimplified here, does allow R&D to be a bit more focused from a material standpoint, compared to the very broad approach that has to be taken with OLEDs.  This allows QD research to be a bit more focused on process, but for now, most of the applications for quantum dots are, as we mentioned earlier, color shifting, rather than color producing.  In such ‘shifting’ applications the white or colored light of an LCD backlight (the photo ‘stimulator’) triggers the red, green, and blue QDs, creating an RGB display.  This is done both alone or in conjunction with standard RGB phosphors typically used for color filters in LCD displays.  Samsung Display (pvt) took the concept a bit further and uses a blue/green OLED material to stimulate RGB QDs in its QD/OLED displays.  QD color shifting put QDs on the map in the display space but what is most interesting were the EL-QD demo displays that Samsung and others have shown recently. 
These displays do not have a backlight, nor do they use OLED materials.  They are exclusively based on the direct electrical (not optical) stimulation of quantum dots (EL-QD), similar to the way RGB OLED displays are designed.  As these QD materials become more efficient and have longer lifetimes, they will move from the ‘demo’ and ‘proof of concept’ stage to practical applications and will compete against both OLED and LCD technology.  How long will it take for these materials to mature to the point where EL-QD can be commercialized?  It’s a hard question to answer, but based on the progress made to date, we would expect a commercial EL-QD product in 2027.   There are still many issues that have to be overcome to reach that stage but over the last year or so we have seen a number of developments that indicate such an expectation is not out of the realm of possibility, and the acquisition of Nanosys (pvt), the leader in the development of QD materials, and Shoei Chemical (pvt), late in 2023 seems to have accelerated  QD development for displays.
It will not be an easy competition when it occurs, as there are a number of contenders who have a more mature ecosystem, but at a point  down the road, we expect the functional cost of using EL-QDs will become attractive to  display producers.  Shoei has significant current capacity and the ability to increase that capacity easily when necessary, so while EL-QD has been more the focus for R&D rather than commercial products to date, the fact that QDs are used in many LCD TVs and some OLED TVs gives them a production lead over other potential display technologies.  Of course there are issues, with some of the same ‘blue’ difficulties that still face the OLED industry also present with QDs, but we believe that the momentum behind EL-QD is increasing, giving out 2027 commercialization estimate a more realistic shot.  Not that far off…