Printed electronics or more specifically inkjet printing is already an established part of OLED display manufacturing, where industrial-scale inkjet printers are used to deposit the organic material in the multilayer thin film encapsulation (TFE) layer that protects OLEDs from oxygen and water ingress.
Inkjet printing the RGB active materials in OLED displays, however, seems not to have succeeded in overcoming the technical hurdles despite significant investment and decades of development on both material and machinery sides. It appears that the material performance never bridged the gap with vacuum-processed ones, which kept on improving, whilst the potential manufacturing cost benefits proved insufficient to force a shift away from the incumbent processes.
This is not the end of inkjet printing in manufacturing the active elements of the display though, thanks mainly to quantum dots (QDs) including QD-OLED and QLED displays. The idea behind QD-OLED displays is that a blue OLED layer is vacuum deposited, whilst the red and green colors are achieved by pixel-level inkjet-printed QD color conversion, giving the emissive display perfect contrast, high efficiency, as well as a very wide color gamut, beyond what all-OLED displays could achieve. Mastering the inkjet printing of QD-OLED displays could also offer a technical and manufacturing roadmap towards true emissive QLED displays.
QD-OLED displays are already in production with 77-inch 4k QD-OLED being on the market for several years. The manufacturing volume is projected to grow rapidly. Furthermore, the technical performance will also improve. The latest announcement is that the resolution of inkjet-printed 31.5-inch QD-OLED displays will be 140 PPI.
This is a challenging technical feat. It could involve inkjet printing quantum dots on 8.5-Gen (2.2x2.5 sqm) mother glass. To get a rough idea of the requirements for illustration purposes only, assume that five displays can be manufactured on the mother glass. At 4K and 8K resolution, this translates to around 125M and 500M inkjet printed sub-pixels, respectively. Assuming 3-5 drops have to be inkjet printed in each pixel well, it means that for an 8K display around 1500 and 2500 million drops would have to be inkjet printed under stringent uniformity, size, and TAKT time requirements.
Source: Kateeva [8.5-Gen Inkjet Printer for Display Manufacturing]
Mini- and MicroLEDs - from micro bumps to microLED transfer to color conversion
mini- and micro-LED technologies could also benefit from printing. Here, the micro bumps for the placement of a large number of microLEDs on the glass substrate could be printed. Indeed, excellent prototypes were already demonstrated with gravure offset printed solder pastes with 5um precision and 6um diameter (15um after reflow).
Source: Komoro [presented at TechBlick in 2022]
The metallization tracks connecting the front and back of the mini- or micro-LED display via the edge of the glass hosting the microLED chips could be screen or aerosol printed to avoid drilling and metallizing through-glass vias, although it will probably prove too difficult to beat the incumbent subtractive process given the resolution speed, and yield requirements.
The microLED chips themselves could also be transfer printed. Here, an elastomeric stamp could pick up these chips, stamping and thus transferring them at high speed and yield onto the final target substrate. There are currently many firms developing a variety of transfer printing techniques for this purpose, although the competition from other approaches including laser-based ones is very stiff.
Finally - and possibly most promising - is to achieve color conversion via printed quantum dots. A major challenge holding microLEDs back is the need to transfer millions of microLEDs with practically zero defects. To transfer all three colors would be extra complicated. Thus, one could transfer only the blue microLEDs and achieve red and green colors with QD color conversion.
A technical challenge is that microLEDs are too small for inkjet printing (ca. 40 um print resolution), and their size is bound to shrink further to improve display resolutions and economies of scale. To address this need, EHD (Electrohydrodynamic Printing) is being used, demonstrating lab printing resolutions of 1-10 um with likely mid-term reliable print resolutions around 15 um, translating to some 1000 dpi. The EHD printing still needs to be further developed and scaled, especially to a multi-head high-resolution technique without loss of print resolution, stability, or speed. Of course, EHD is not the only way to achieve QD color conversion, but it is still very much in the running for manufacturing selection with strong chances vs. the photoresist-etch option.
Source: Left images: Scrona | right images: Fraunhofer IAP
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