TADF: A key technology for next-generation OLEDs displays

Recently, with the launch of Apple's new generation of mobile phones, iPhone 8 and iPhone X, everyone is paying attention to their record-breaking sales (excluding iPhone 8 of course) and impressive financial reports, and the gorgeous appearance of OLEDs screens is also one of the highlights. Although Samsung, LG and Sony have been developing OLEDs technology for many years and have launched various concept products including mobile phones and TVs, Samsung focuses on small-sized mobile phone panels, while LG and Sony focus on large-sized high-end TV market (it has to be high-end because it is really expensive), but it is Apple's new phone that really attracts the market to realize OLEDs panels.

OLEDs technology has gradually taken shape since the early 1990s, but it has not been maturely commercialized until recent years. Samsung and LG from South Korea are among the earliest manufacturers to cultivate this technology, while Japanese manufacturers JDI and Sharp have been clinging to liquid crystal technology. Although the latter has enjoyed the benefits of technology for a long time, JDI has faced huge losses since Apple switched to other companies, and has begun to seek external funds to jointly develop liquid-jet OLEDs technology in an effort to survive. Today, Chinese panel manufacturers, led by BOE, have also successfully shipped OLEDs flexible screens to domestic giants such as Huawei this year, and the OLEDs market has officially entered the Warring States period. In the future, it will depend on whether Apple, unwilling to be controlled by Korean manufacturers in terms of screen sources, will also get involved, making this market, which has been almost divided, even more chaotic.

OLEDs stands for Organic Light Emitting Diodes (Figure 1). Its main characteristics come from the organic light-emitting layer. When an appropriate voltage is applied, electrons and holes combine in the light-emitting layer to generate photons, which emit visible light of different wavelengths according to the material characteristics. Generally speaking, organic light-emitting layers can be divided into three categories based on the light-emitting mechanism: fluorescent materials, phosphorescent materials, and thermally activated delayed fluorescence (TADF) materials, which will be introduced in this article. Fluorescent materials were first used in the preparation of OLEDs components. Subsequently, around 1998, phosphorescent materials were also successfully applied to OLEDs technology, and compared with fluorescence, it has better energy efficiency. In recent years, through a series of articles published by Professor Chihaya Adachi of Kyushu University since 2011, TADF materials have attracted attention from all walks of life with their efficiency comparable to that of phosphorescent materials.

Figure 1: OLEDs are composed of a substrate at the bottom, multiple organic layers in the middle, and electrodes. The luminescent materials and impurities mentioned generally belong to the luminescent layer (Source: Cynora official website https://www.cynora.com)

Due to physical limitations, fluorescent materials are not as efficient as phosphorescent materials and TADF materials in terms of energy conversion (Figure 2). This difference has its quantum physics reasons. Generally speaking, the excited state of organic materials is divided into singlet and triplet. When electrons jump, they are distributed in the singlet and triplet states in a ratio of 1:3. The light emitted by the singlet state returning to the ground state is called fluorescence (TADF materials also use this mechanism), and the light emitted by the triplet state returning to the ground state is called phosphorescence. Due to the forbidden mechanism (Forbidden Rule, triplet electrons cannot form spin-orbit coupling with ground state electrons, violating the Pauli exclusion theorem), the electrons can only release energy in the form of heat, so the energy utilization efficiency of fluorescent materials is only 25%.

Figure 2: Comparison of the light-emitting mechanisms of OLEDs. Fluorescence materials are first-generation application materials, phosphorescence materials are second-generation materials, and TADF is a new-generation key material. Generally speaking, the lower the energy difference between the singlet state (S1) and the triplet state (T1), the better. (Source: Information Display Vol.33 No.2 2017)

Phosphorescent materials (with Ir or Pt) and TADF materials can fully utilize singlet and triplet states to achieve 100% energy efficiency. Phosphorescent materials can convert electrons originally in singlet state to triplet state through spin-orbit coupling of heavy metals, thereby utilizing all excited electrons, which is beneficial to reduce device energy consumption and extend device life. However, its main disadvantage is that metals such as Ir and Pt are very scarce, expensive and highly polluting. Compared with them, TADF materials can also achieve 100% energy utilization by converting triplet electrons to singlet state and returning to the ground state to emit fluorescence, without the need for rare precious metals. According to Hund's law, the energy of triplet state is lower than that of singlet state, and this energy difference (ΔEST) is generally above 500meV for organic materials, making it difficult for triplet electrons to return to singlet state without external energy. TADF materials use a special molecular design strategy to reduce the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the molecule's electron orbitals, and synthesize a molecular structure with ΔEST <50meV. At this time, only room temperature heat is enough to transfer triplet electrons to singlet states. Using this mechanism, TADF materials can also have a 100% internal quantum efficiency comparable to phosphorescent materials.

In addition to the high preparation cost of phosphorescent materials (mainly from precious metals), blue light has always been the biggest weakness of phosphorescent materials. Even after 20 years of industry-university research, it is still impossible to develop a blue phosphorescent material that combines efficiency, stability and pure color, which makes the market place great hopes on TADF materials. According to the results published by German Cynora in the first half of 2017, TADF materials have caught up with or even partially surpassed traditional blue phosphorescent materials in terms of efficiency (external quantum efficiency 14%, general blue phosphorescent materials are about 8%), chromaticity (CIEy 0.27) and lifespan. In view of the fact that the research on TADF materials started around 2010, the potential of TADF materials is very exciting.

All OLED displays today still use fluorescent materials as blue light sources. To ensure sufficient brightness, the size of blue pixels is about twice that of red and green. If commercial blue TADF materials can be successfully developed, the resolution of the display will be further improved and the battery life will be further extended. In addition to efficiency, the color of TADF materials can be controlled. By modifying the molecular groups and binding positions, the wavelength of the emitted light can be adjusted. Currently, the wavelength of visible light that covers display and lighting needs can be adjusted.

At present, the two TADF material suppliers closest to mass production are Cynora in Bruchsal, Germany, and Kyulux in Japan, co-founded by Professor Adachi. Cynora specializes in the development of blue-light TADF materials and recently received an investment of 25 million euros from Korean manufacturers Samsung and LG. It is expected to launch the first commercial blue-light TADF material by the end of 2017. Kyulux also received a third-party investment of 15 million euros last year and has achieved good results in yellow and green TADF materials. If these two companies can really bring TADF materials into the OLEDs market in the future, it will bring a new wave of growth opportunities to the panel industry.

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