The application landscape for quantum dot (QD) technology is rapidly evolving and QDs are already used in display applications. This article discusses the innovation and commercialisation frontiers of this technology.
Quantum dots (QDs) in displays are nothing new. They are being used to widen the colour gamut of liquid-crystal displays (LCDs), enabling these devices to compete with quality organic LEDs (OLEDs) in terms of performance, while largely retaining the cost structure of LCDs. The use of quantum dots has created an interesting market dynamic, especially in the large-sized end of the market (e.g. television sets).
In this application, the main method of QD implementation is currently QD enhancement film (QDEF). Here, the QDs embedded in a resin are coated and sandwiched by two barrier layers. This film is then inserted into the LCD stack.
Within this opportunity one sees many trends. First, the QD material composition has evolved from being cadmium-based (Cd-based) to cadmium-free (Cd-free). This transition was driven by legislative requirements and is now nearly complete. It was enabled by improvements in the quantum yield (QY), full-width half maximum (FWHM), stability and cost of indium phosphide (InP)-based QDs.
Today, this transition involves a minimal quantum yield penalty thanks to notable year-on-year improvements in InP quantum dots’ quality yield. However, this still imposes an FWHM penalty even though the InP-based QDs are narrowing their FWHMs. Today, 35 nm is demonstrated in commercial labs and 37 – 38 nm is expected in production.
In parallel, the QDs are becoming more stable. This has relaxed the barrier requirements. Indeed, such requirements have already dropped from around 1E-3 to 1E-4 g/day/m2 to 1E-1 or 1E-2. This has significantly reduced barrier cost and complexity, so helping drive down overall implementation cost.
The higher quantum yields also give higher brightness, decreasing the QD content required per each square meter for a given lumen output level. The suppliers are also experimenting with direct on-glass deposition to bring the value chain in-house and to cut out the extra substrate from the solution.
Finally, the lower overall costs are allowing QD LCDs to leave the premium-price bracket.
QDEF (QD enhancement film) is not the end game. It will grow in the short term but will likely become obsolete in the long run. The first stepping stone will be QD colour filters for both LCD and OLED displays. In LCDs, the quantum dot color filters (QDCFs) replace traditional colour filters. This gives wide colour gamut benefits while improving efficiency.
Material and system level technical hurdles remain though. At the material level, issues such as patterning, blue absorbance, air-stability and others must be overcome. Today, many are working on ink formulate to enable inkjetable QD colour filters for large area displays. Others are working to properly disperse a high QD weight percentage (wt%) into photoresists to enable high resolution QD colour filters.
At the system level for LCDs, the polariser will likely need to come in-cell while additional reflectors might be needed since QD colour filters re-emit light in all directions, including back into the device. The QD excitation by background light must also be suppressed.
QD colour filters on organic LEDs are also interesting. The basic idea is to enable the production of quality large-area displays. Here, blue OLEDs would be deposited continuously to provide the backlight. The QD colour filters would be inkjet-printed to provide red and green emissions. This hybrid approach would overcome the scalability limitations of red, green and blue (RGB) OLEDs (made via functional materials manufacturing) and potentially the cost issues of many-layered white organic LEDs (WOLEDs).
However, efficiency would remain limited since blue OLED is still a fluorescent material with low external quantum efficiency. Lifetime and brightness will also be limited by the blue material.
To partially address the latter two issues, two blue stacks will likely be used to split the driving voltage. This technique is already used in white OLEDs. It is possible, however, that the next-generation blue materials will become commercial in the time it will take hybrid quantum dot-OLED to mature. Interestingly, the process and material learnings from QD colour filter-OLED will not be lost since they will act as a stepping stone towards the long-term endgame of fully printed, emissive QD displays (QLEDs).
On-chip QDs are also on the roadmap in displays. They would enable wide colour gamut LCDs without needing additional QD enhancement film or QD colour filters. The challenge remains in ensuring QDs survive the harsh heat and light flux stresses which they experience when sitting in close proximity to the LED source. Numerous strategies are being followed now with positive results. These include protective layers, right ligand choices, optimisation of core-shell structure and others.
The on-chip QDs are particularly interesting for lighting applications. This is because they enable improving the efficiency, especially when using a narrow full-width, half maximum red down converter. QDs therefore allow for high efficiency as well as high colour rendering index. The challenge here, too, has been stability. Today, there are commercial product announcements and there have recently been acquisitions of QD suppliers by major lighting manufacturers. These products incorporate cadmium (Cd)-based red QDs. As such, they suffer from all the toxicity issues associated with cadmium. Indium phosphide (InP)-based QDs are still not sufficiently stable, even for mid-power LEDs.
In general, as the stability of QDs improves, more types and application sectors of LED lighting will open up to QD penetration. In the lighting sector QDs are also being explored for their ability to offer customised spectra. This could, for example, be used in horticultural lighting to allow for matching the spectrum of the light sources with that of the plants’ photosynthesis.
The endgame for QDs in displays, however, is quantum dot LED (QLED). This would essentially be the ultimate display: emissive (100% contrast), extremely wide colour gamut, thin (i.e. flexible) and efficient. The route to QLED is still long, though. Cadmium-free (Cd-free) QLEDs still exhibit very low external quantum efficiency, even in the laboratory. Their levels are below cadmium QDs and far below phosphorescent OLED. More critically, the lifetime, even at low brightness levels, is on the scales of hours to tens of hours. The exact device architecture and optimised materials in the QLED stack are also not yet established. The processing is at a very early stage with only basic, small samples having been demonstrated at trade shows. The interest remains strong, however.
Of course, QDs in displays do not exist in a competitive vacuum. There are many competitive technologies. The nanocell colour filter used by a Korean display maker does offer wide colour gamut. This technology is likely based on a special material in the filter that eliminates the spectral overlap between differen- coloured traditional colour filters. Next-generation OLED materials should also not be discounted. The results, especially with hyper thermally active delayed fluorescence, are promising because they offer narrow full-width, half maximum (FWHM), together with high external quantum efficiency (EQE).
They are still in development phase and large-scale production is not yet straightforward. Phosphors are also constantly improving. In particular, the KSF red phosphor already offers narrower full-width, half maximum than QDs. Rumours also circulate about a green narrow FWHM being close to market launch.
Clik here to view.
Fig. 1: Range of colours possible with QDs.
Photosensors
QDs have wide absorption characteristics. Importantly, the absorption characteristics can be tuned by changing dot size and/or composition. Furthermore, QDs can be cast from the solution, giving rise to easy integration possibilities with silicon complementary metal-oxide-semiconductor (CMOS) read-out integrated circuits (ROICs).
A strong point for QDs lies in high resolution infrared, especially short-wave infrared (SWIR) photosensors. Here, the right QD material can be used. The QDs can be deposited on the CMOS ROIC. This deposition can be with dip casting, spin coating or other methods. The sensing element can also be configured as a photoconductor or photodiode.
The early results are that QDs can exceed the detectivity of indium gallium arsenide (InGaAs) systems. Their direct on-complementary metal-oxide-semiconductor (CMOS) integration also might overcome the resolution limitations imposed by heterogenous integration (bonding) of gallium arsenide silicon (GaAs-on-Si) CMOS.
There is, however, still much work to do. The ligand exchange mechanisms must be developed to enable closely-packaged and highly-conducting QD thin films. The film deposition techniques must improve too, to ensure uniform, highly-controlled and repeatable deposition.
The patterning techniques and the best choice for other materials in the systems remain open questions. These include short-wave infrared-transparent top electrode and hole transport layer/electron transport layer (HTL/ETL) in a photodiode arrangement. The issue of stability must be addressed. The main QD materials under consideration are lead-based, and therefore raise health and safety concerns. The lead-free alternatives do not yet work either.
QD image sensors
QDs can also be used in visible photodetectors. The advantage here is to enable high-resolution global shutter sensors. In general, the resolution of existing global shutter sensors is low. This is because each pixel requires a large capacitor to hold the charge until column analogue-to-digital converters (ADCs) reach out the data. This storage capacitor would not be needed if each pixel had its own analogue-to-digital converters. Such arrangements (i.e. an ADC per pixel) would clearly consume too much real estate and power. To accommodate both the capacitor and the photosensing element per pixel, each pixel will have to be large, so compromising resolution.
QDs can overcome this issue by separating the photosensor and read-out circuits. The photosensing element would be a QD layer cast on top of the read-out integrated circuit. The QD layer would be voltage-controlled independently, allowing it to be opened or closed like a global shutter. This approach opens up more real estate for a larger storage capacitor with the silicon.
Note that a large capacitor well translates to a large dynamic range because it can accommodate higher light intensity before saturating. Interesting prototypes enabling QD-complementary metal-oxide-semiconductor (CMOS) global shutters have been demonstrated.
As before, QDs do not exist in a competitive vacuum. Organic semiconductors can also be cast from solution on top of the read-out integrated circuits (ROICs). They too could offer high-resolution global shutter image sensors. Given that organic semiconductors represent a growing diverse family of materials, their exact absorption characteristics could also be varied. In general, however, these semiconductors would struggle to address short-wave infrared or longer wavelengths.
Contact Charlotte Martin, IDTechEx, c.martin@IDTechEx.com
The post Quantum dots: a technology evolution appeared first on EE Publishers.