Analysis, Introduction and Application of Key Technologies of OLEDs Display Screen

In recent days, with the continuous listing of Apple's new generation mobile phones iPhone 8 and iPhone x, in addition to paying attention to its record breaking sales (excluding iPhone 8 of course) and bright financial results, the gorgeous debut of OLEDs screen is also one of the highlights. Although Samsung, LG and Sony have laid out OLEDs technology for many years, and have already launched various conceptual products including mobile phones and TVs. Samsung focuses on small-size mobile phone panels, while LG and Sony focus on the large-size high-end TV market (have to be high-end, because it is really expensive), it is Apple's new phones that attract the market to truly realize OLEDs panels.

OLEDs technology has gradually taken shape since the early 1990s, but it has not been considered as a mature commercial application until recent years. Korean Samsung and LG are the first manufacturers to cultivate this technology. Japanese manufacturers JDI and sharp hold on to the liquid crystal technology. Although the latter has enjoyed the technical dividend for a long time, JDI has faced huge losses since Apple changed to others, so it began to seek external funds to jointly develop liquid jet OLEDs Technology and strive to survive. Today, the panel manufacturers led by BOE are also successfully shipping OLEDs flexible screens to Huawei and other large domestic manufacturers this year. The OLEDs market has officially entered the Warring States era. In the future, it depends on whether Apple, which is unwilling to be controlled by the Korean factory, will also participate in the screen source, making this similar market more chaotic.

OLEDs is fully known as organic light emitting diodes (Fig. 1). Its main characteristics come from the organic light emitting layer. After an appropriate voltage is applied, electrons and holes combine in the light emitting layer to produce photons and emit visible light of different wavelengths according to the material characteristics. Generally speaking, organic light-emitting layers can be divided into three categories according to the light-emitting mechanism: fluorescent materials, phosphorescent materials and thermally activated delayed fluorescence (TADF) materials introduced in this paper. Fluorescent materials were first used in the preparation of OLEDs components. Then around 1998, phosphorescent materials were successfully applied to OLEDs technology, and it has better energy efficiency than fluorescence. In recent years, through a series of articles published by Professor Chihaya Adachi of Kyushu University since 2011, TADF materials have attracted the attention of all walks of life because of their efficiency comparable to that of phosphorescent materials.

Figure 1: OLEDs is composed of a substrate at the bottom, many organic layers and electrodes in the middle. The commonly mentioned luminescent materials and reference materials belong to the emissive layer (source: cynora official website) https://www.cynora.com )

Due to physical limitations, fluorescent materials are inferior to phosphorescent materials and TADF materials in energy conversion efficiency (Fig. 2). There are reasons for this difference in quantum physics. Generally speaking, the excited states of organic materials are divided into singlet and triplet. When the electron transitions, it will be distributed in the singlet and triplet in the ratio of 1:3. The light emitted by the singlet state returning to the ground state is fluorescence (TADF material is also this mechanism), while the light emitted by the triplet state returning to the ground state is called phosphorescence. Due to the forbidden rule (triplet electrons cannot form spin orbit coupling with ground state electrons, violating the Pauli incompatibility theorem), electrons can only release energy in the form of thermal energy. Therefore, fluorescent materials have only 25% energy use efficiency.

Figure 2: comparison diagram of luminescence mechanism of OLEDs elements. Fluorescent materials belong to the first generation of application materials, phosphorescent materials are the second generation, and TADF is the key material of the new generation. Generally speaking, the lower the energy difference between singlet (S1) and triplet (T1), the better. (source: information display Vol.33 No.2 2017)

Phosphorescent materials (including IR or PT) and TADF materials can make full use of singlet and triplet states to achieve 100% energy efficiency. Through the spin orbit coupling of heavy metals, phosphorescent materials can convert the electrons in the singlet state to the triplet state, so as to use all the excited electrons, which is conducive to reducing the energy consumption and prolonging the service life of the device. But its main disadvantage is that metals such as IR and PT are very scarce, expensive and highly polluting. Compared with TADF materials, they can also convert triplet electrons to singlet states, return to the ground state and emit fluorescence, achieve 100% energy efficiency, and do not need the help of rare precious metals. According to Hund's law, the energy of the triplet state is lower than that of the singlet state, and this energy difference EST) is generally above 500mev for organic materials, which makes 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 of the highest occupied orbital (HOMO) and the lowest unoccupied orbital (LUMO) in the electronic orbital domain Molecular structure with EST

In addition to the high preparation cost of phosphorescent materials (mainly from precious metals), blue light has always been the largest cover door of phosphorescent materials. Even after 20 years of industrial and academic research, it is still unable to develop blue phosphorescent materials with efficiency, stability and solid color, which makes the market place great hopes on TADF materials. According to the results published by German cynora company in the first half of 2017, TADF materials have been equal to or even partially ahead of traditional blue phosphorescent materials in efficiency (external quantum efficiency 14%, general blue phosphorescent materials about 8%), chromaticity (ciey 0.27) and service life. Since the research of TADF materials started around 2010, the potential of TADF materials is very expected.

Today, all OLEDs display screens still use fluorescent materials as blue light sources. In order to make them have sufficient brightness, the size of blue pixels is about twice that of red and green. If commercial blue TADF materials are successfully developed, the resolution of display screens will be further improved and the service life of batteries will be further extended. In addition to efficiency, the luminous color of TADF materials can be controlled. The wavelength of emitted light can be regulated by using modified molecular groups and binding positions. At present, the visible light wavelength covering display and lighting requirements can be regulated.

At present, the two TADF material suppliers closest to mass production in the market are cynora company in bruchsal, Germany, and kyulux co founded by Professor Adachi in Japan. Cynora specializes in the development of Blu ray TADF materials. Recently, cynora received an investment of 25 million euros from Korean manufacturers Samsung and LG. It is expected to launch the first commercial Blu ray TADF materials by the end of 2017. Kyulux also received a third-party investment of 15 million euros last year and achieved good results in yellow and green light TADF materials. If the 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.