Lighting

The long road towards emissive QLEDs

6th February 2020
Alex Lynn

IDTechEx Research have reviewed the potential evolution roadmap of material technologies in emissive displays. First, fluorescent OLED materials were developed. These offered narrow emission, maturity, and stability, but suffered from low internal quantum efficiency (IQE) due to their inability to harvest the so-called triplet states.

Also before the QLED, there came phosphorescent OLED materials, enabling the triplet states to also be harvested, thus boosting IQE to nearly 100%. However, the FWHM was broadened. First the red and then in 2013 the green emissive layers in OLED displays switched to phosphorescent. The blue, however, remains elusive even today.

The technology is now further evolving. TADF (thermally activated delayed fluorescence) materials are being developed. This material technology also harvests all states and may open a route to stable blue emitters. Hyper TADF is yet another material in which the properties of TADF and fluorescent dyes are blended. It offers very high IQE and also narrow FWHM, but represents a serious high volume and low (reasonable) cost production challenge. It is yet to reach commercialisation.

The ultimate emissive material might arguably be quantum dots (QDs). This is because it may offer high IQE and ultra-narrow FWHM, leading to very wide color gamut displays. It is also solution processed, thus potentially opening a route to low-cost large-sized display manufacture. Today, almost all display markets are aggressively developing the necessary means and steps to reach QLED.

In this article, IDTechEx has highlighted the trends, challenges, and innovation opportunities, shedding light on the likely technology roadmap towards QLED. This article draws from the report ‘Quantum Dot Materials and Technologies 2019-2029: Trends, Markets, Players’, which report offers a detailed analysis of the QD technology, value chain, and application (display, sensors, lighting, and agricultural films).

This report also examines existing and emerging applications, offering a detailed assessment of current status and future prospects. Here, IDTechEx considers enhancement film, QD color filter on LCDs, OLEDs and microLEDs, on-chip, QLED, lighting, NIR/SWIR sensors, agricultural films, and so on. The report also offers market forecast in terms of value and consumption at the level of material and application.

Evolution of QLED performance thus far

First let us examine the high-level trends in the evolution of QLED properties. QLEDs still cannot be driven very brightly. Indeed, they lag the brightness level of OLEDs, which itself is not the brightest display technology. The latest results show very promising results, although these are still champion results which do not clearly report the potential trade-offs with other parameters.

Next, consider the progress in EQE, EQE of Cd-based red and green has come a long way. The blue Cd-based QD however still lags behind. In general, focus on Cd-based materials for QLEDs may be misplaced because the likely high ug/mm2 consumption levels of Cd may put the material in breach of RoHS regulation. This is however not set in stone and the regulatory landscape is constantly evolving. Indeed, the question is still being debated even in Europe and even for QD enhancement films.

There is also evidence of the progress of InP based material. Here, the laboratory results are fast bridging the performance gap, but parity is still some way off. To achieve these results many steps are being taken at the material and device level. At the former level, complicated efforts are implemented to prevent and even remove oxidised surfaces, to eliminate abrupt interfaces and instead have smooth transitions in composition, to have more suitable exchanged ligands, and so on.

Remaining challenges

The next challenge is in lifetime. Even LT50 (initial luminance dropping to 50% of original value) under mild luminance and drive conditions is rather low. Note that going to LT95 at higher Cd/sqm values will be significantly more challenging as degradation mechanisms will be degraded. Thus, LT95 lifetimes cannot be linearly extrapolated up from LT50 values.

Improving lifetime is not an easy feat. The materials themselves need to improve. One strategy is to grown gradient-alloyed core-shell structures to eliminate internal abrupt interfaces which can cause stress due to CTE mismatch. The devices also need to improve.

In particular, good charge balance must be achieved. Therefore, the appropriate electronic and hole transport layers (ETL and HTL) must be selected and/or developed. ZnO or ZnMgO nanoparticles are a common ETC choice. The HTL is more challenging though due to QDs’ deep valence band. As such, OLED materials cannot be easily copy pasted across.

Many strategies are being pursued to implement a suitable HTL. Some are inserting two or more HTL with a view to gradient the energy level gap. Others are introducing an additional layer after ETL, seeking to slow down electron injection. In general, the question of the appropriate device architecture and material stack is fundamental to the development of QLED displays.

There are naturally other challenges. A good blue emitter material, as ever, is not yet established. On the one hand, InP particles become too small when seeking to approach the 440-460nm wavelength. On the other hand, ZnSe QDs are good for shorter than 440nm emission. Cd QDs are possible but suffer from chronic uncertainty over toxicity issues. As such, other compositions such as ZnTeSe are being actively explored.

Then, there is the entire question of manufacturing. Almost all results are based on spin coating. A large performance gap exists between inkjet-printed and spin-coated QLEDs. There is also the patterning challenging in defining the pixels. This is not easy. As a general comment, the QLED industry will benefits from more than a decade of developing in solution processing OLEDs and from the commercialisation of inkjet in depositing the organic layer of thin film encapsulation layers.

Future outlook

The display makers recognise the long road ahead, but, motivated by the ultimate benefits, are taking determined and even aggressive action. Some are implementing hybrid OLED/QLED approaches. In one good example, blue un-patterned OLED acts as the backlight while inkjet printed green and red QDS act as color converters, achieving emissive, high-contrast, and wide color gamut displays.

The manufacturing process also gives a pathway towards large-area displays, potentially jeopardising the long-term competitive prospects of WOLEDs. This, and similar hybrid approaches, will provide invaluable learnings along the way, paving the path towards the ultimate realisation of QLEDs.

QLEDs will eventually arrive on the market, but not overnight. The starting gun of the competition has already been fired, however. Everyone in the value chain is racing towards the finish line. This includes researchers, QDs/ETL/HTL developers, equipment makers, and nearly all display makers.

Interestingly, the road towards the ultimate goal of QLED passes by many important technological milestones, each of which creates its own market. As such, the developments will be monetised along the way and QLED is not an isolated long-term target. Indeed, all developments are an integrated part of the long-term roadmap.

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