What do LCD, large area OLED and µLED TV displays have in common? Well, there is an emerging technology trend which will play an important role in the future of all three categories of display. This is the use of colour conversion at the pixel level to create the red, green and blue light which is fundamental to the operation of emissive colour displays. Colour conversion is the technique of converting light from a range of incident wavelengths to one specific and very pure colour using photoluminescent materials like phosphors, quantum dots and perovskites.

In LCD and large area OLED, the red, green and blue light is traditionally created with a colour filter array which works by blocking the unwanted colours in each sub-pixel. Colour conversion creates much purer colours which leads to wider colour gamuts and enables significant power savings compared with filtering – up to 40% in the case of LCD.

Of course, phosphors and quantum dots are already improving the colour performance of LCD TVs through their use in backlight films. Here photoluminescence is used to shift the light emitted by blue LEDs towards the desired wavelengths. However, each sub-pixel, whether it is red, green or blue, receives the same light and so there are still significant losses in the filter. That said, the use of photoluminescent backlight films has been an important precursor to the adoption of in-pixel colour conversion.

In an in-pixel colour conversion implementation, the incident light could be coming from a blue GaN backlight LED in the case of LCD, a GaN µLED in case of µLED or a blue OLED layer in the case of QD-OLED architectures such as the one being developed by Samsung [1]. The colour conversion from blue to red happens inside the red pixel and the conversion from blue to green happens inside the green pixel.

Several aspects to this are critical. Firstly, the output spectra in the red and in the green need to be extremely narrow and tuneable to maximise the colour gamut of the display. Secondly, the colour conversion needs to be power efficient. Thirdly, the absorbance has to be very high so that negligible blue light gets through. Also, the colour conversion materials need to be patternable using standard processes and equipment in the display fab.

In all three categories of display, in-pixel colour conversion delivers the highest performance potential and offers the opportunity to remove the colour filter completely if the colour conversion material has sufficiently high absorbance. It also provides important benefits that are specific to each architecture too.

For example, colour conversion helps to address a key challenge of µLED which is the efficiency of red µLEDs which can be as low as 5%. This can be avoided by using efficient and low cost blue µLEDs together with a red colour conversion material. In addition, the challenges of mass transfer and the complexity of the backplane can be reduced by transferring only blue µLEDs to the substrate and using colour conversion to create the red and green pixels. The colour conversion process can also allow a wider manufacturing tolerance in the blue LEDs.

In the case of large area OLED for TV, the dominant technology is currently LG’s WOLED. Although LG has successfully implemented cost reductions in 2021, WOLED 65” panels are still about twice the cost of an LCD panel of the same size[2]. It is very interesting to note that in-pixel colour conversion is a critical element of Samsung’s QD-OLED approach to simplifying and cost reducing large area OLED for TV.

Of course, each architecture will have its challenges and these are being very actively worked on by the industry, an example being the requirement for an ‘in-cell polariser’ for LCD implementations. A good indicator of the extensive R&D activity going on here is the more than 1000 patent applications that have been published related to this and other aspects of integrating photoluminescent materials with colour filter systems.

A potential rate limiting step in the adoption of in-pixel colour conversion is the colour conversion materials themselves, and this is where Helio is stepping in with our Heliopixel materials. The extraordinarily high absorption, efficiency and narrow emission of Helio's perovskite-based materials make them uniquely well suited to the in-pixel application and we are hugely excited support this key innovation in the next generation of TVs. 

Notes:

[1] https://www.techradar.com/news/samsung-qd-oled-tvs-are-coming-next-year-say-insiders

[2] Display Supply Chain Consultants at PQDIF 2021