Key technologies and core materials of OLED

Born for the future, OLED leads the innovation of terminal ID
OLED was first discovered in the lab by chinese-american professor deng qingyun. The principle is to make a current pass through special organic luminescent materials, and under the action of an electric field, stimulate the organic materials to make them glow. It’s a process that converts electrical energy into light energy.
OLED is the basic structure of the layer of organic material to clamp on indium tin oxide (ITO) as the Anode (Anode) and metal Cathode was between two layers of the electrode (Cathode). Organic materials are mainly composed of Emission Layer (EL) and Common Layers. The luminescent layer is composed of red light, green light and blue light (RGB) or white light (W) luminescent materials. In the current multi-layer device structure, the common Layer can be divided into Hole InjecTIon Layer (HIL), Hole Transport Layer (HTL), Electron InjecTIon Layer (EIL) and Electron Transport Layer (ETL). HTL is responsible for regulating the injection rate and amount of holes, while ETL is responsible for regulating the injection rate and amount of electrons. HIL and EIL act as buffer layers to increase the injection of holes and electrons. Electrons and holes combine in the oluminescent layer and transfer energy to the oluminescent material to make it glow.
OLED driving mode is divided into passive (PMOLED) and active (AMOLED). The PMOLED has a simple structure, and each pixel is controlled by an independent cathode and anode. No additional driving circuit is needed, but too many control circuits restrict its application in large screen high-resolution analysis. The AMOLED cathode is an integral electrode, which drives the anode to emit light through the drive circuit, greatly reducing the number of control lines, making it have the characteristics of low power consumption, high resolution, and fast response. Therefore, AMOLED has gradually become the mainstream of OLED display. At present, we often talk about OLED, are AMOLED.
AMOLED can be divided into hard screen and soft screen. The substrate material of hard screen is glass, and the packaging material is glass, which can be Shared with LCD. The substrate material of soft screen is thin film (PI, PET, COP, etc.), and the packaging material is also thin film. If transparent film material and transparent organic filling material are adopted, the flexible screen can be made into transparent and bendable screen.
The development of oleds in previous years has been constrained by high cost and low yield. However, since last year, IHS research results show that the cost reduction and the demonstration of giant apple will break through the bottleneck of the industry, bringing the incremental outbreak of downstream market. IHS expects 395 million AMOLED units to be shipped in 2016, up 40 percent from 2015. Revenues will rise by 25% to $15 billion. OLED has a wide range of downstream applications. We believe that OLED is mainly used in mobile phones, tablets and televisions in the near future, and can be widely used in VR, smart wearable, vehicle-mounted systems and medical fields in the medium and long term, ushering in a market space of one trillion yuan. 3.1 lightweight — self-luminous material, making LED backlight a thing of the past
In the traditional LCD panel production process, liquid crystal materials are injected into the cell after two polarizing plates are glued together in the cell manufacturing process. Backlight of LED is needed in the module manufacturing process to make the LCD panel finally display. With the increase of BLU, the overall thickness of LCD module increases greatly (mm level). By contrast, OLED is self-luminous material and does not require LED backlight, which eliminates LED light source, light guide plate, booster, diffuser, polarizer (1 less) and color filter (only needed for white OLED), making OLED module thickness greatly reduced (under 0.1mm) and weight lighter. With the development trend of thin and light phones, OLED panels are undoubtedly a better choice.
For LCD, after unlocking the screen, the LED backlight is always in the luminous state, so it is not only very expensive to display the dark, night sky and other scenes, but also leakage of light is obvious. Thanks to its self-luminous properties, oleds are pure black with better contrast and lower power consumption. At the same time, OLED has wide perspective, wide color range, high refresh frequency and fast response, which can well support the display requirements of wearable devices.
The black border on the edge of the screen is called “BM area”, which is mainly used to prevent light leakage on the screen. Secondly, the LCD is stored in the middle of two glass substrates to prevent leakage by coating the sealing glue. Thirdly, an independent capacitor layer will be built into the screen touch layer to increase the accuracy of touch control, and the screen pixels also need circuit control. The wiring will be conducted around the border, and no border means no wiring. These practical problems directly determine that the current mobile phone screen cannot be bezel-free.
At present, most mobile phones are still LCD screens, especially the screen of LTPS, which needs more time to complete its supply chain. 1) most of the LTPS are high-end machines with incell Touch control and driven by TDDI chip. TDDI has poor recognition of the edge of the screen with narrow bezel when it reaches the full screen with ultra-narrow bezel, so it needs to return to the traditional Display driver+Touch with two independent chips. 2) the current LTPS screen is designed to ensure resolution and ultra-thin, and the drive chip adopts COG packaging, but its packaging border is large. When the comprehensive screen is applied, it needs to be changed to COF packaging, and the folding of FPC is used to reduce the border width. 3) the traditional backlit LED module needs a certain amount of light space. The ultra-narrow backlight module needs to be designed for ultra-narrow bezel, and the light guide plate needs to be improved from pattern design, structure and membrane material selection. 4) since the frame is ultra-narrow, both the backlight and the lead wire of the whole machine need to be redesigned, and the traditional panel cutting method also needs to be improved. Irregular cutting is carried out at the bottom corner of the panel to facilitate wiring.
The introduction of OLED technology will make this problem much easier, as the OLED screen itself is rarely driven by TDDI, and the COP (chip on PI) package of flexible OLED itself is similar to COF. OLED also has no backlight, and it is much easier to do irregular cutting.
3.3 transparent and openable holes — bring more possibilities to fingerprint identification and ID design
Polyimide flexible OLED in PI membrane (transparent) on LTPS etching circuits, and PI film not only flexible flexible, and compared with LCD have a certain light transmittance, if by optical fingerprint identification scheme, CMOS sensor can be placed under the OLED panel is hidden, when unlock the phone open, and full screen applications such as watching movies, they have not fully aware of the presence of fingerprint identification. Huiding released an optical fingerprint identification scheme at MWC in 2017, which is based on samsung S7 and vivo Xplay6 phones and can unlock the optical fingerprint identification in specific areas of the cover. Huiding is leading the industry in the research and development of its optical fingerprint identification scheme. FPC and xisi have not reached the release stage yet. Apple’s iPhone8 this year is undoubtedly a full-screen trend, but whether the launch of optical fingerprint has yet to be verified.
In addition, since OLED has no backlight, PI film can be cut and perforated in any shape, which brings infinite possibilities for ID design in full-screen mobile phones. In the following ifan’s iPhone8 conjecture, flexible opening can be made at the top of the phone screen, where there is a handset and a camera, while the left and right sides of the area can still be used for display, further expanding the phone’s screen proportion. Such improvements are unthinkable in the age of LCD panels.
3.4 three core technologies of OLED
The manufacturing process of ltps-tft AMOLED includes many cutting-edge technologies in the display panel industry, similar to LCD, which is mainly divided into three processes: Array, Cell and Module.
The backplane process is used to form LTPS (low temperature polycrystalline silicon) drive circuit through film formation, exposure, etching and overlay of different patterns and materials to form the LTPS (low temperature polycrystalline silicon) drive circuit, which provides lighting signals and stable power input for luminescent devices. The technical difficulty lies in the micron level process fineness and the extremely high requirement for electrical index.
1) coating process is to use coating equipment to deposit the required materials onto the glass substrate by means of physical (PVD) or chemical (CVD) (step 2 in the figure);
2) the exposure process is to transfer the patterns on the light mask to the substrate after coating by means of optical irradiation (steps 3, 4 and 5 in the figure);
3) the etching process USES chemical or physical means to etch away the lower film on the substrate that is not covered by the light resistance, and finally wash away the light resistance on the covering film, leaving the film layer with the required figure

The module process will cut the packaged panel into the actual product size, and then carry out various processes such as polarizer sticking, control circuit and chip bonding, and carry out aging test and product packaging, and finally present the product in the hands of customers.
1) cutting: the encapsulated AMOLED substrate is cut into pannel (step 1 in the figure);
2) panel test: check panel lighting (step 2 in the figure);
3) offset: attach the AMOLED panel to the polarizer (step 3 in the figure);
4) COG+FOG: will drive the link between IC and flexible printed circuit board (FPC) and AMOLED panel (step 4 in the figure);
5) TP lamination: laminate the AMOLED panel to cover Lens with touch sensor (step 5 in the figure);
6) module test: aging test and lighting inspection of modules (step 6 in figure).
3.4.1 LTPS driver circuit
Array segment is the basis of panel manufacturing process, so to make OLED panel well, it is necessary to have a strong LTPS process capability. AMOLED has higher requirements for TFT driving technology than LCD. The essential reason is that AMOLED is a current-driven device and requires TFT to work in the linear amplification state, while LCD is a voltage driven device and TFT only needs to work in the switching state. Therefore, the most widely used in the LCD industry a – Si (amorphous silicon TFT) technology, has good uniformity, simple process, mature technology, the advantages of lower cost, but because of its low carrier mobility and driving OLED ability is insufficient, and threshold voltage drift problem, used for OLED device performance stability fatal weakness, do not apply to AMOLED; P-si (polysilicon) TFT has the advantages of high carrier mobility and stable threshold voltage. At present, the driving circuit of OLED (backboard/substrate) basically adopts p-si. IGZO is a kind of MOTFT (metal oxide TFT) with TFT characteristics between a-si and p-si. Although the electron mobility of IGZO is lower than p-si, it does not need laser annealing process and is suitable for large size OLED backplates.
There are many methods to prepare p-si materials, including low-pressure chemical vapor deposition (lp-cvd), small-grain p-si laser annealing, zone melting recrystallization, low-pressure molecular beam lei (lp-mbd), a-si quasi-molecular laser annealing (ELA) and solid phase crystallization (SPC). Because the grain size of p-si material is related to the preparation temperature of the film, and the grain size directly affects the carrier mobility of p-si film. Therefore, most of the above methods belong to the high-temperature generation process. With the increase of temperature, the grain size of the film will generally increase, the defects between grains will be reduced, and carrier mobility will be greatly improved. However, high temperature production requires the use of quartz or other high-temperature glass substrate, which increases the manufacturing cost and is not conducive to the mass production of p-si materials. Therefore, reducing the generation temperature of p-si materials is a key problem in the development of p-si TFT.
Of LTPS (Low Temperature Poly real – silicon, low-temperature polysilicon) is on the plasma enhanced chemical vapor deposition (PECVD) technique method made a – Si material for Excimer Laser Annealing (Excimer Laser Annealing, ELA) formation of p – Si thin film materials. The temperature of p-si material prepared by ELA is usually lower than 450℃, and ordinary TFT glass can be used. The properties of p-si materials obtained by this method fully meet the requirements of TFT switching devices for pixel and TFT devices for peripheral drive. Coherent, the world’s largest producer of laser equipment, has benefited from the current outbreak of the OLED industry, with shares hitting new highs year to date, thanks to Coherent, the world’s leading ELA equipment (Coherent is responsible for most of the current OLED production lines).
3.4.2 evaporation process and printing display process
Organic film forming technology is the core of the OLED unique technology, due to the thickness of the organic thin film is very thin in the OLED devices, general equivalent to about one percent of the diameter of hair, electronic injection layer thickness less than 20 even Mr (1 = 0.1 nm), and sub pixel film extremely fine, width is about 10 microns, so be very evenly, multilayer so thin and cannot have a pinhole of fine organic film, is one of the common technical challenges facing the industry. Organic film forming technology can be divided into the vacuum vapor deposition, laser transfer printing and three types of wet preparation (printing), including vacuum evaporation is in vacuum heating for keep the organic material in the crucible of evaporation and through the FMM (Fine – Metal – MASK, Fine Metal MASK board), reach the glass substrate deposition film forming technology, is currently the most mature technology. At present, the medium and small size AMOLED products in mass production basically adopt vacuum evaporation technology. Controlled by magnetic force, FMM is suspended between the evaporation source and the glass substrate, which requires high precision of positioning with the glass substrate. Besides, it is easy to deform due to gravity and thermal expansion, and low material utilization rate. Therefore, the manufacturing process of MASK is very strict. Meanwhile, the heating time control of evaporation source also determines the effect of evaporation.
Laser transfer printing technology is developed to solve the shortcomings of FMM technology, but there are still major problems such as Thermal damage, process stability and yield, which have not yet been used in mass production. LITI (Laser Induced Thermal Image) technology is owned by SMD, while LIPS (Laser Induced Pattern wise Sublimation) technology is owned by SONY. RIST (Radiation – induced sublimation transfer technology owned by kodak, these techniques are very similar in principle, is to advance organic material by vacuum evaporation, spin coating or silk screen coating methods such as deposition in a film, called the donor and the donor mulch on glass substrates (called receptors) and donor imaging template by laser beam irradiation, the donor by laser irradiation on the part of the organic material is transfer to the glass substrate.

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