Discussion on the Status Quo of Global OLED Industry and Discussion on Development Countermeasures

The development of OLEDs can be traced back to the 1930s. Destriau dispersed organic fluorescent compounds in polymers to form the film, which was the earliest electroluminescent device, but it was not developed until 1987 by Tang Cyun of Kodak Company. An OLED based on small molecule fluorescent materials (Alq as a light-emitting layer) is produced, and a polymer OLED (PLED) was manufactured by Friend and Burroughes of Cambridge University in the United Kingdom in 1990 with a conjugated polymer PPV.

The basic structure of an OLED is usually a sandwich structure in which an organic semiconductor layer is sandwiched between two electrodes. One of the electrodes often uses a thin and transparent indium tin oxide (ITO) having semiconductor characteristics as a positive electrode, and the other The electrode usually adopts a low work function metal such as Ca, Al or the like as a negative electrode. When a voltage is applied to the positive and negative electrodes, excitons are generated in the organic semiconductor layer and emit light, and the device emits according to the organic semiconductor material. Red, green, blue, and even white light. In order to obtain a higher performance OLED, the organic semiconductor layer usually includes a plurality of layers such as a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and an electron injection layer. (EIL), at the same time, it is often introduced into the interface modification layer.

OLED can be divided into two different types of technology: small molecule-based OLED and polymer-based OLED (PLED) according to the carrier transport layer used in the component and the organic thin film material of the light-emitting layer; Divided into passive (passive matrix) and active (active matrix) two driving methods.

According to the technical principle and preparation process of OLED, the OLED industrial chain is usually divided into several parts such as equipment manufacturing, material preparation, drive module, panel and device manufacturing, and downstream application. Among them, equipment manufacturing, material preparation and drive modules belong to the upstream field. Panel devices and module manufacturing are in the middle, and various applications are downstream.

Domestic and international OLED industry development trend

First, technology research and development

1, OLED device structure

Simplified structure: OLED devices require very complex structural designs in order to achieve high power utilization. This complicated structure increases the production process and production cost of the OLED, and the simplified structure can greatly simplify the OLED production process to a certain extent, and is of great significance for promoting the productization of the OLED. In 2007, J. Meyer et al. first reported an ultra-simplified green phosphor device with only two organic layers. The device efficiency was still as high as 40 lm/W (45 cd/A) at 100 cd/m2. In 2011, Professor ZH Lu's group proposed that the ITO surface work function using chlorine treatment can be improved to 6.1 eV. In the preparation of the device, the elimination of other excess hole injection layer and hole transport layer can achieve the purpose of energy level matching. , greatly simplifying device design and preparation.

Tandem structure: Stephen R. Forrest's group first reported the use of MoO3 as a part of the tandem structure in a white phosphorescent device with a total power efficiency of 22.7 lm/W, which is well suited for use in the connection layer of tandem structures. MV Madahava Rao proposed the use of a pentacene/C60 planar heterojunction (PHJ) as an all-organic internal interconnect layer in the tandem structure. This interconnect layer has good light transmission and weakens the effects of the microcavity effect. Ma Dongge's research group interfacially modified the charge generation layer of pentacene/C60, and the maximum efficiency of the white phosphorescent device was 101.5 cd/A (53.8 lm/W), while the roll-off ranged from 100 cd/m2 to 1000 cd. /m2, the efficiency only drops from 53 lm/W to 45 lm/W.

SPP enhances OLED structure: 20-40% of the light generated by OLED is limited to SPP. If the metal surface has a nanostructure-like morphology, it is possible to extract the light confined in the SPP. A. Kumar et al. used vacuum thermal evaporation to form a layer of gold nanoclusters, which were applied to phosphorescent devices to maximize electroluminescence intensity by 2.8 times. A. Fujiki et al. used a chemical method to form a layer of gold nanoparticles on the surface of ITO, and then vapor-deposited CuPc as a hole transport layer. By adjusting the thickness of CuPc to change the distance between the metal and the light-emitting layer, the luminous intensity can be obtained. 20 times enhancement. F. Liu et al. prepared silver nanoparticles by sodium citrate reduction method and coated the surface with SiO2 layer, and mixed these Ag-SiO2 particles in the phosphorescent layer. The device was at 200 cd/m2 when the thickness was 13 nm. The efficiency is increased by a factor of three.

The application of quantum dots in OLEDs: Chang-Ching Tu et al. used electrochemical etching to prepare silicon quantum dots, and used solution spin coating to prepare silicon quantum dot-organic hybrid OLED devices, and proved blue light. The silicon quantum dots emit red light due to surface oxidation resulting in large Stokes shifts, and the quantum shift of red silicon is negligible. The "Quantum Light Enhancement Substrate for OLED Solid-State Lighting" project of the US Department of Energy's "Solid State Lighting Program" has been improved by the US QD Vision Company to increase the light extraction efficiency by 60% to 76. %.

Hybrid white OLED structure: Hybrid white light device is considered to be a very promising method for preparing high-efficiency and long-life white OLED. The most used fluorescent blue light emitter and phosphorescent red-yellow or green light The structure of the body combined. The warm white light device prepared by Young-Hoon Lee et al. has an efficiency of 12 cd/A at a maximum brightness of 24,000 cd/m2. Jwo-Huei Jou et al. used both dark blue (MDP3FL), blue (DSB), green (Ir(PPy)3), yellow-red (Ir(2-phq)3), and deep red (Ir(piq)2). (acac)) and other five luminescent materials, the white light device has a color rendering index of up to 93, and the efficiency at 2 cd/m2 is 23.3 lm/W (14.3 lm/W@1000 cd/m2).

A new structure of transparent anode: the use of a new structure of transparent anode can not only improve the light extraction efficiency, but also a very promising transparent conductive electrode material for the development of OLED in a large area. Koh et al. used etching to prepare an ITO layer with periodic structure and shape, which can effectively reduce the total reflection of light between the organic/ITO layers, which greatly improved the efficiency by a factor of two. The lens is used, and the device efficiency can be improved by nearly 3 times. This is the best report to date that the use of light extraction technology to enhance efficiency.

2, OLED materials

The characteristics of OLED materials greatly affect the performance of OLED devices. For OLED luminescent materials, there are strong fluorescence in solid state, good carrier transmission performance, strong thermal stability and chemical stability, high quantum efficiency and vacuum evaporation or very Good properties such as dissolution are necessary, and companies and research institutes have been doing more work on material scale preparation.

Organic electrode material: The electrode material is divided into an anode material and a cathode material, wherein the anode material is usually indium tin oxide (ITO), and the cathode material is various low work function metals such as Al, Ag, Mg, Ca, Ba. Wait. In addition to the use of carbon materials and nanowires for electrode research at the anode, some scientists have explored in composite cathode materials, such as the use of lithium octahydroxyquinolate instead of the commonly used lithium fluoride to improve electron injection, and some research groups focus on pure organic In the improvement of OLED devices in spin coating technology, Jen KY Alex research team developed a water-soluble polymer interface material PF-OH based on polyfluorene backbone to prepare high-efficiency polymer OLED white light device. Compared with traditional inorganic interface materials, it has the advantages of low cost and process steps.

Organic charge transport material: In order to reduce the barrier of charge entering the light-emitting layer after injection from the electrode, it is necessary to introduce a suitable charge transport material into the device. For the transport material, the most important thing is to achieve the balance of carriers, so as to avoid the waste of holes or electrons. Currently used hole transport materials generally have aromatic amine units such as NPB, TPD, TCTA and TAPC, and the commonly used transport materials are AlQ, BCP, PBD, TPBI and Tm3PyPB developed by the recent Junji Kido group.

Guest Phosphorescent Materials: Phosphorescent materials are still focused on heavy metal complexes, especially in the study and development of metal ruthenium complexes. The current difficulties surrounding the development of blue phosphorescent materials. LG's Youngjin Kang team developed a ruthenium complex fac-[Ir(dfpypy)3] with a strong electron withdrawing group such as difluoropyridine, which has a quantum efficiency of up to 77% in methylene chloride. . In addition to the traditional C^N ligands, C^P ligands have gradually entered the field of scientists. Yun Chi Group has developed a new method to prepare ruthenium complexes of C^P ligands. The binding of the traditional C^N ligand is stronger due to the electron donating ability of the phosphorus atom, and the bond strength of the formed bond is also stronger, which is favorable for the blue shift of the luminescence spectrum. In addition to metal ruthenium complexes, the Mark Thompson team recently used a cheaper monovalent copper complex to make a phosphorescent material, and prepared a new ligand, mCPy, which was prepared by co-evaporation with CuI. [CuI(mCPy)2]2, which also achieved an external quantum efficiency of 4.4%, is an interesting development.