Inkjet printing 20bn perovskite quantum dots for microLED displays

Inkjet printing 20bn perovskite quantum dots for microLED displays

Interviews |
By Nick Flaherty

Swiss startup Scrona has used its electrohydrodynamic (EHD) inkjet printing for microLED displays with  quantum dots.

The development with Avantama combines Scrona’s EHD inkjet process with high-patterning resolution of perovskite quantum dots (QDs) for more efficient and cost effective microLED display manufacturing,  increasing colour purity and brightness.

“We are not there yet for industrial production but what we see is the industry working on the technology such as colour conversion and the one thing that is important to us is the viability of the production process,” CEO and co-founder of Scrona Patrick Galliker tells eeNews Europe.

He sees the inkjet process being used to apply the colour filters on a quantum dots built with photolithography on a silicon wafer before they are moved onto a display panel. Applying the colour filters to create a full colour display with spin coating is a major challenge at the moment, and this is where inkjet printing, or additive manufacturing, can help, he says, but only if it is fast enough.

“Basically when we look at the number of pixels on a 6in wafer that’s 20bn pixels and that is an immense number. A single nozzle system at 20kHz would take 275 hrs as 38 droplets need to be deposited to gain enough height in the pixel.”

Avantama perovskite QDs having the highest absorption coefficients among QDs which allows for thinner pixels with good optical density.  Perovskite QDs also have the highest weight-based absorption compared to others QDs, and this directly translates to very high OD/thickness values.

“In my view there is only one way to use an additive manufacturing for this,” he said. “20kHz is already fast and you need to move from one pixel to another, so you need a lot of nozzles. This has been demonstrated in graphics with roll to roll printers increasing the nozzle count.”

The aim is to process 60 wafers per hour, which is one wafer in one minute. “That’s 16,000 nozzles and you already have that in graphical industries, so that’s viable, ” he said.

Another way is to increase the frequency. With EHD when the valve opens the ink flows, so that is the equivalent of multiple droplets per pulse to speed up the process. “You also have the opportunity to have optimised materials for the dots so instead of a 10um high film that could be reduced to 5um. Then there are other opportunities is to use materials with higher viscosity. The solvent is usually an acrylic monomer that becomes part of the structure of the microLED,” he said.

“If you count all of this together you could have production in a 16 print heads with 1000 nozzles and this is already used in the inkjet industry,” he said.

Scrona print head technology processes QD inks using EHD inkjet printing, whereby droplets are ejected from a nozzle by electrostatic forces. By exposing the ink to an electric field that originates from the outside of the nozzle, ions are pulled to the liquid surface and then start to create mechanical stress on the liquid body.

The density of ions at the surface are controlled by adjusting the intensity of the voltage that is used to create the electric field. The droplet size can be controlled by the applied voltages, including droplets that are one order of magnitude smaller than the nozzle itself.

The company has been developing a print head with up to 256 nozzles that can scale up to thousands. The print heads can also be used in parallel for achieve that processing of 20bn quantum dots in a minute.

“We kept the nozzle count low to make it as easy as possible to investigate the nozzle to nozzle variations. We will very soon start production of a higher nozzle count version with 100 on a single chip in the first two quarter.

“We are building the print heads on a wafer with digital lithography so there is no limitation on the size, it’s about the wafer size and the spacing. At the moment we manufacture on a 4in wafer and each nozzle requires 125um so a 15mm x 6mm print head gives nearly 6000 nozzles. You need space for connections so its 4000 nozzles for packaging and contacts.”

“Given the current yield rates we can turn out 1000 nozzles per chip on the MEMS side but that’s the front end. What determines the time is the electronic drivers so this year it is up to 256 nozzles using off the shelf devices for inkjets. To go above that it would be our own CMOS designs for our voltage and amperage requirements. hat we will fundraise for in the next round,” he said.

The company already has a commercial print heads with five nozzles developed with Notion System and these can be upgraded with a higher nozzle count of 8 in the same footprint. Later in the year the company plans to release its third generation print head with between 64 and 256 in the same footprint.

“We are setting up an automated assembly, and for Gen3 we have working protypes with the chips integrated and that works now,” he said.

“Conventional inkjet printing is not a viable option for emerging MicroLED displays because they use much smaller pixels than QD-OLED displays,” said Dr. Norman Lüchinger, co-founder and CTO at Avantama. 

“By partnering with Scrona, we have been able to demonstrate that an OD greater than 1 can be obtained with a perovskite QD layer thickness below 2µm. This can reduce the printhead nozzle count by a factor of five and deliver a thin QD layer that improves the overall efficiency and production tact time of MicroLED display technology.”;

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