Vertical Divider

​Musing on Competitive Technology
Quantum Dot and Ink Jet Printing 
May 14, 2018
​
At this years QD forum, there were a number of highlights

• Quantum Dot Color Filter Replacement -- QD color filter replacement (QDCFR) – reportedly improve efficiency, brightness, viewing angle and color but the challenges include the development of an in-cell polarizer, since QDs depolarize light, so they must be used after both polarizers in an LCD. One option presented was using an OLED backlight. The OLED/QDCFR technology could take advantage of the black levels provided by OLED while exploiting the efficiency and color of QDs.

 
Figure 1: Self-Emitting Quantum Dots

​Courtesy of Nanosys

• Electroluminescent Non-Cd ELQD -- The promise of this technology based on QDs has been the dream of QD scientists since it was first demonstrated over a decade ago. Nearly all-developmental work has occurred on Cd-containing materials – but that’s changing. There is an increased focus on InP-based ELQD devices. Nanophotonica, Fraunhofer, and Seoul National University all shared data on their progress related to InP ELQD. The lifetimes remain very poor (you can watch it degrade within one working day), but so were CdSe ELQD devices when they were first being developed. There is hope still for highly efficient ELQD displays (Cd will come first), but of all QD display technologies, this one is still far fetched.

 

• Ink-Jet Printed QDs -- Kateeva, TCL, and Nanjing Tech discussed the ability to print QDs. Kateeva demonstrated that QDCF wells could be filled with QDs using their INP platform, while Nanjing Tech and TCL spoke about their full-color IJP ELQD displays. 

 
 
Figure 2: IJP 5-inch AM-QLED display (80 dpi) 

Source: TCL

• Perovskite QDs -- The battle between Codes and In QDs is now seeing competition from a third QD type – perovskites. This material set contains some very attractive properties for displays (narrow emission & high blue absorbance) but have not yet seen commercial success due to two main factors
  • They are unstable in their operating environment, and they contain lead. Stability improvements are expected to continue in the coming years, and some are working on Pb-free perovskites, but that is a long way off. For now, if display companies want to use perovskites they will be stuck with the difficult choice of whether or not they can tolerate lead in their display.
  • The level of lead in a perovskite QD film is predicted to be less than the EU’s RoHS limit (1000 ppm), which means it would not require an exemption, unlike the current CdSe QD technology, which needs one. Companies Avantama and Quantum Solutions were on hand to discuss their progress in this area and make the case for Perovskite QDs.

 

• MicroLEDs -- X-Celeprint and Lumiode presented different approaches
  • R, G, and B micro LEDs for each pixel (which may mean separate MOCVD semiconductor processes and different performance parameters for each color)
  • Blue MicroLEDs and use down-converters to make the R + G sub pixels. (Using just one MOCVD semiconductor process)

MicroLEDs cannot use phosphors to down-convert blue light to red + green due to the relative size of phosphor particles, which are larger than an individual microLED but their small size and efficient down conversion capability of QDs makes them attractive for the microLED space, but questions still remain. Will QDs be able to absorb enough light to achieve full color conversion? How will QDs be deposited (IJP or photolithography)? Will QDs survive the conditions of on-microLED without further encapsulation? Suffice to say, there is interest on both sides but only time will tell if this will work out for mass production.
 
Samsung committed to quantum dots as an enhancement to it LCD TV products after abandoning efforts to develop a commercially viable RGB based OLED TV.  However, QD films have proven to be no match for LG’s OLEDs. However, the development of a QD color filter could be beneficial to both LCD TVs and OLED TVs that use white OLEDs and a CF. One of the shortcomings of OLED TVs is luminance, and color filters reduce the light output by as much as 50%, such that the development of a color filter that could offset much of that loss would help to improve the quality of OLED TVs without needing to depolarize the light.  The use of quantum dots that would be stable in an un-encapsulated environment and could be easily patterned using conventional OLED deposition tools could benefit both LCD and OLED TV display manufacturers. The other issue restricting the use of QD color filters is the effect of ambient light, which would distort the image. 
​