Display Daily is a Techblick media sponsor Mini- and Micro-LED and Quantum Dot Events November 30 and December 1. The company has kindly provided us with a number of interesting excerpts from some of the talks that will be given at the event and we have put them into two articles – one today and one Tuesday of this week (they won’t count like in your free articles if you have a free registration).
In this article, using selected technology slides, we highlight several exciting advancements in MicroLED and/or QD displays. Specifically, we cover 3600PPI “silicon” displays | Microbumps printed in engraving | Electrohydrodynamic printing QD color converters | LLO Laser and Transfer for MicroLEDs | QD vs Phosphorus | MicroLED Energy Saving Credentials
Note that each of the articles has a “gallery” with several graphics. Click on the first to open them and use the arrow keys to navigate)
Can lasers help in the production of MicroLED displays?
One of the biggest manufacturing challenges in producing microLED displays is the transfer step given the speed and throughput requirements. As shown in the slides below (Gallery 1) by Oliver Haupt of Coherent Inc., lasers can play an important role in this step, both when tricolor (RGB) microLEDs and also when only blue microLEDs must be transferred.
The process flow for both cases is shown below. In the case of RGB microLEDs, a temporary support is first attached to the sapphire substrate on which the GaN microLEDs are grown. Laser Lift Off (LLO) is deployed to lift off the sapphire substrate, releasing the carrier wafer with the detached GaN microLEDs. Then, controlled UV spots are used to release the individual microLEDs onto the final substrate containing the TFT active backplane layers. These processes can be repeated three times, each time for a different microLED color. At all stages, of course, excellent and optimized control of the laser profile/parameters in harmony with the good properties of the adhesive material is required.
In the case of blue-only microLEDs, the final backplane substrate is brought into contact with the GaN sapphire substrate. GaN uLEDs are transferred to the final substrate via the LLO process. Three-color capability is then achieved by color conversation, for example, QDs or small size phosphors
The results show the example of the RGB microLED transfer. Parameters are shown in the slide, including microLED size, pitch, laser energy density, donor-receiver distance, etc. It can be shown that a different color is transferred with each shot. Thus, in three shots all the RGB microLEDs are placed in the right place! As the subassembly on slide 2 shows, the laser can treat an area of approximately 2.83 cm² with each step/stroke.
MicroLEDs: can they help fill the energy gap in electronic devices?
Why can microLED technology help close the energy gap in electronic devices? @Khaled Ahmed from Intel Corporation offered a unique data-rich assessment at TechBlick’s display event in 2021.
The first slide (Gallery 2) shows the Battery Gap – Ahmed has collected data by year showing that the power demand of phones far exceeds the level of battery power, creating a “battery gap” that s ‘expands every year as more power-hungry features are added while battery technologies only gradually improve. Around 70% of a mobile phone or tablet’s power consumption comes from the display, showing its outsized importance in closing this gap.
The second slide (Gallery 2) shows the efficiency improvements (lm/W) of “released” OLED devices per year. OLED efficiency has clearly plateaued in products manufactured or marketed. The backdot represents the projected potential of microLEDs, showing how microLED technology can be a game changer.
The third slide (Gallery 2) shows that there is a gap between the EQE of lab OLEDs and commercial products. The origins are unclear but likely involve trade-offs needed in production and trade-offs between lifetime stability and EQE.
The four sides compare the efficiency of GaN LEDs at different wavelengths compared to organic LEDs (from previous slides). It shows that GaN LEDs offer significantly higher EQE levels than OLEDs at all wavelengths except red. Indeed, there is a red efficiency gap in GaN microLED technology, the filling of which is the subject of intense global R&D.
These graphs clearly demonstrate that while OLED technology appears to have plateaued and will likely never overcome the battery gap, emerging microLED technology holds great promise for doing so. Of course, the development and manufacturing of microLEDs involves other challenges such as fast transfer as well as high yield production which we will discuss elsewhere.
Phosphors or QD for color conversion to LCD and microLED? Who will win ?
This is an interesting and evolving tech space to watch. James E. Murphy and others from GE Search have developed the best narrowband red and green phosphors and are now advancing the technology to microLEDs and on-chip integration.
KSF red phosphor is an excellent narrow band color converter for wide color gamut displays. It emits 5 peaks, each of which has an ultra narrow FWHM of 5 nm. The main peak is centered around 631nm. It is a stable material under conditions of high luminous flux and high temperature. Indeed, it can be integrated on chip as a direct replacement for existing yellow phosphors. This is a major commercial success with over 19 licensees and over 40 billion (and growing) KFS-containing LEDs sold worldwide in the display industry.
Like the slide below (Gallery 3), shown at TechBlick July 2021 shows that KFS technology is evolving. At the start in 2014, the average particle size was 25 to 30 μm. It has now gone down to 3-9μm and is evolving towards submicron or even nanometric particles, allowing direct integration with the microLEDs of today and tomorrow! This is an important technology trend as it brings QD vs phosphor competition even into the microLED space (previously QDs were the only game in town due to their small size).
Additionally, GE’s KSF can now be formulated into air-stable, encapsulant-free phosphor-based inks suitable for inkjet printing without nozzle clogging. This means that it can even be printed as a color converter on microLED, notably allowing the use of efficient blue microLEDs to create a red color and/or only transfer a blue microLED color.
James E. Murphy also offers an interesting comparison between Cdless QD InPs and KSF for microLEDs. It argues that at very thin films ( 20 μm since it has no self-absorption.
Finally, it lacks ultra-narrowband green phosphors, leaving room for QDs. In particular, green perovskite QDs are very strong in this area. However, GE is advancing the development of its narrowband green phosphors. As shown below, these materials allow 100% DCI-P3. Performance is comparable to Beta Sialon but without crosstalk with a KSF red transmitter. Additionally, it offers 100% HTHH stability, enabling direct on-chip integration. Finally, it looks like EQ levels are approaching >90%. Of course, just like KFS, it has a slow PL decay time on the order of 90-450 μm (QD is ns)
You can find out more about all these topics at Techblick’s very first specialist microLED and QD event. (KH)