China is aiming to become the largest supplier of display panels in the world by 2019, and is well on its way to achieving this goal. The Society for Information Display (SID) attracted an audience of over 700 at its first-ever international conference in China in February. SID intends to make its International Conference on Display Technology an annual event in China, driving home the point that China is embracing cutting-edge display technologies and represents an important market for suppliers.
I was privileged to be an invited speaker at the plenary session, where I discussed “Advances in Next-Generation Display Materials.” The industry is in an interesting position right now, with liquid crystal displays (LCDs), organic light-emitting displays (OLEDs), quantum dots (QDs) and flexible displays all coexisting in the market and driving different aspects of next-generation displays. Increasing demand, especially from Chinese customers, means there is room for multiple types of display technology to succeed.
Improving LCD Technology
LCDs have led the market for many years, but continued success is not guaranteed. LCD technology has needed to improve to keep pace, migrating to technologies that increase brightness so it can compete on both price and performance. One key to performance improvements is using an organic passivation layer in the thin film transistor backplane. With the legacy architecture, it is difficult to increase pixel area without causing insufficient separation between the source/drain (S/D) and indium tin oxide (ITO) common electrodes, creating signal delays. As Figure 1 illustrates, adding an organic passivation layer increases the vertical spacing between the S/D and ITO, allowing the ITO pattern to extend over the bus bars. Larger pixels increase brightness, improving contrast while saving power consumption and cost.
Figure 1: Addition of organic passivation layer enables larger pixel size for LCD displays.
As television (TV) screen size grows to 65 inches and beyond, a single exposure can no longer cover the entire area (see Figure 2). Multiple exposure shots need to be overlaid or stitched together. Variable exposure in the overlapping region leads to stitch mura—visible abnormalities in the display appearance—because of varying passivation layer thickness.
Figure 2: Patterning for 55” and 65” TV displays. The larger panels require overlapping exposures, which can cause stitch mura.
Positive-type organic passivation layers (PPLs) do not suffer from stitch mura because they are removed over the active areas during exposure. The inherent stich free mura of PPLs, has generated interest in the industry to migrate to positive passivation layers. Optimizing photoimaging processing speed and developer rate contrast of Negative-type organic passivation layers (NPLs) can minimize the stitch-mura effect with NPLs.
Standard organic passivation layers are based on polyacrylate, which produces a yellowish tinge. Dow has created a polysiloxane-based PPL (S-PPL) that produces a better white color balance and higher transmittance in the desired wavelength range. It combines the processing speed advantages of NPLs with resolution matching that of PPLs. Being positive-type, it is free from stitch mura, making it a good solution for producing large TV panels.
Advances in OLED Materials
OLED technology struggled for years in the large display market because of the high price tags on OLED TVs. More efficient emitting materials, along with advances in processing, are reducing cost while improving performance. Key performance metrics include reduced operating voltage, increased current efficiency, longer lifetime and greater thermal stability.
Dow is a pioneer in the field, having developed the first phosphorescent red host for the emitting layer, and has been continuously improving the quality of these materials. As compared with fluorescent emitters, red phosphorescent emitters achieve a better current balance and therefore improved lifetime.
Solution-processed layers that can be applied using inkjet printing will be desirable for large TVs. Recent advances are improving lifetime of these materials so they can compete with conventionally deposited emitters. Some suppliers are developing white OLEDs, for both TVs and lighting.
Cadmium-Free Quantum Dots
Upcoming restrictions on the use of cadmium (Cd) are affecting the QD display market. These include the anticipated end of an exemption that has allowed Cd in QDs in Europe, as well as stronger regulation of toxic substances in China. When asked in a 2016 survey whether they would pay more for a Cd-free QD TV, most consumers said yes.
Manufacturers are therefore motivated to remove Cd from QDs, which were originally based on cadmium selenide (CdSe). The performance of Cd-free QDs has historically lagged that of CdSe-based QDs. However, materials suppliers have made significant advances in changing this equation. By tuning the wavelength at which the QDs emit light, it is possible to achieve a color gamut that covers 94% of the DCI-P3 specification, the standard for state-of-the-art displays.
Figure 3: Color gamut, showing DCI-P3 spec (blue), Dow CFQDs (green) and Cd-bearing QDs from another supplier (red).
The Future of Flexible Displays
Flexible displays create extra demands on the top and bottom layers, which experience the greatest stress during bending. Backplanes are usually made from polyimide, which has excellent thermal stability, low coefficient of thermal expansion (CTE) and great flexibility, but is sensitive to moisture. Polyphenylene is much more moisture resistant, has a lower dielectric constant and is equal to polyimide in thermal stability and flexibility.
The top coating layer needs to be hard, tough and transparent. This is difficult to achieve, since hard materials tend to be brittle. Dow has created a coating that optimizes the trade-offs and has generated significant interest from display manufacturers. Such materials advances may be able to accelerate adoption of flexible displays.
It is an exciting time to be in the display market, as technical advances are enabling truly competitive display technologies. With China’s investment in technology and growth of its local display market and manufacturing capabilities, SID has identified China as a key emerging market for future advances in displays. For a deeper look at how materials are improving LCD display technology, check out “The Science Behind the Screen: LCD TVs Keep Getting Better.”