An organic light emitting diode (OLED) is a self-luminous solid material. In addition to the advantages of strong structure, light weight, and thin thickness, the display made of OLED has the characteristics of softness and low temperature resistance which LCD and PDP do not have. Therefore, many manufacturers are developing OLED products. For example, Sanyo Corporation of Japan is studying the performance of OLED products close to the level of thin film transistor LCD (TFT-LCD) devices. Sony has developed a diagonal size of 25 to 33 cm
The OLED display panel has a resolution of 800×600 pixels and a brightness of 300 cd/m2, and has a wide viewing angle, high contrast, and fast response performance. Pioneer and Sharp have plans to jointly produce OLED products. Eastman Hodak Company of the United States has also developed OLED display panels with diagonals of 14 and 6 cm and viewing angles of 160° and 80°, with resolutions up to VAG.
The development of OLED technology and products depends to a large extent on the performance of the materials produced, and the development of new materials is the basis for the development of high-performance OLEDs. Recently, a remarkable new polymer system and a transparent conductive film have been developed.
1 new polymer materials
The polymeric material for OLEDs is a luminescent polymer (LEP). The main technical requirements for LEP materials for OLEDs: high brightness and efficacy at low voltages, long lifetime and stable voltage-time characteristics. This enables the OLED to be driven at a low voltage, reducing cost and power consumption.
In recent years, Cambridge Display Technology (CDT) has developed a CDT Yellow 001 OLED material that meets this technical requirement with a color coordinate (0.45, 0.54). The OLED process flow coated with CDT yellow 001 material is: 1 making a translucent indium tin oxide (ITO) anode on a glass substrate; 2 cleaning the substrate, treating the substrate with oxygen plasma, and ensuring humidification of the hole transport layer; Depositing an aqueous hole transport layer on the substrate, the optimum thickness of the layer is 50 nm; 4 drying the hole transport layer; 5 coating the CDT yellow 001 material on the substrate for completing the above steps by spin coating; 6 passing the vacuum load Fixing, moving the substrate into the movable casing 7 by vapor deposition to make a calcium cathode, the production environment pressure is 8×10-7Pa; 3 steaming aluminum on the above basis to reduce the cathode resistance and protect the calcium layer; 9 epoxy resin The device seals of the previous steps will be completed.
CDT has measured the OLEDs it produced. Figure 1 is a plot of OLED current density (solid line) and brightness (dashed line) as a function of drive voltage. Table 1 shows the drive voltage and brightness values ​​for the company's OLED products. Figure 2 is a photo effect of the product as a function of bias. It can be seen from Fig. 2 that the light effect changes relatively gently in the range of 2.2 to 4.5 V; and the light effect attenuation is more significant at higher voltages. Figure 3 is a graph showing the life characteristics of the CDT OLED, which is the result of driving the OLED with a constant current at a temperature of 80 °C. The initial brightness of the device is 100 cd/m2; after a few hours of driving voltage of 2.05V, the brightness is attenuated to 85 cd/m2, and the brightness is reduced to half of the initial value for about 900 hours, reflecting that the product maintains good lifetime and voltage stability at high temperatures. Sexuality is an ideal active matrix OLED material, which emits yellow light.
2 Alumina zinc oxide (AZO) transparent conductive film
The basic structure of the OLED is a sandwich type, that is, one or more layers of organic material are injected between the two electrodes. The OLED has a transparent conductive film electrode layer. In many flat panel display devices, indium tin oxide (ITO) is usually used as a transparent film. Because the ITO film has high conductivity and visible light transmission, and has sufficient hole injection high work function. However, the ITO film does not match the flexible polymer substrate as it matches the glass. If the OLED is made of glass as a substrate, it will lose its flexibility and light weight, and AZO film has recently been developed.
2.1 Production of AZO film
At present, the AZO film production methods mainly include reactive vapor deposition and radio frequency (rf) sputtering.
In the reactive vapor deposition method, the oxygen partial pressure in the vapor deposition chamber ranges from 4 × 10 -3 to 4 × 10 -2 Pa, and the vapor-deposited substrate is a glass or polymer material which is ultrasonically cleaned. The substrate is placed above the evaporation source and heated to the desired temperature prior to evaporation.
The RF sputtering method uses a sintered magnetic target as a sputtering source, and its composition is a ZnO material containing 3% Al2O3 (mass). The distance between the target and the substrate is 4 cm, and the dielectric gas used for sputtering is argon at a pressure of 1 Pa. The shooting power is 100W.
The process of preparing the AZO film by the above two methods is similar to the preparation of the ITO film, and will not be described herein.
2.2 Making O LED with AZO film
The emissive material used for the preparation of the OLED is an organic conjugated polymer 1.4-styrene (PPV), a polymer n-vinyl azaindole (PVK) and a trimer (8-hydroxyquinoline) aluminum (Alq3). The procedure for fabricating an OLED having an AZO film and a PPV material is: 1 etching a glass or plastic substrate made of an AZO film; 3 cleaning the etched substrate; 3 coating the AZO layer of the cleaned substrate by spin coating. PPV material was applied; 4 The OLEDs which completed the above steps were heated at a temperature of 250 ° C for 3 h under vacuum conditions of lx10-3 Pa for aging.
The main process steps of preparing OLEDs with AZO thin films by PVK and Alq3 materials are as follows: 1) preparing Alq3 powder by thermal evaporation under vacuum conditions of l×10-3 Pa; 2 pressing Alq3 powder into blocks; 3 by spin coating method A substrate on which an AZO film is formed is coated with a PVK layer; 4 is sputtered with an Alq3 layer; and 5 is vaporized at 1×10-3 Pa.
2.3 OLED performance with AZO film
Performance measurements were performed on two OLEDs having an Al/Alq3/PVK/AZO structure. For ease of comparison, the performance of an OLED of Al/Aq2/PVK/ITO structure was also measured. In the above-mentioned OLED to be tested, the thickness of the organic material layer ranges from 80 to 120 nm, and the thickness of the aluminum layer is about 200 nm.
It can be seen from the measurement results that the performance of the OLED having the AZO film is similar to that of the ITO film, so that the AZO film can replace the ITO film, thereby producing a curlable flat panel display device OLED.
3. Conclusion
Although OLED has not come out, it has developed rapidly. Philips, the Netherlands, has developed a red-orange OLED with a lifetime of more than 2000h and a green OLED with a luminous efficacy of 16lm/W. The advancement of OLED technology has in turn promoted the in-depth research and development of it, and promoted more and better OLED production materials. In this way, higher performance OLEDs and even OLED screens can be rolled up and commercialized production is also expected. (small soup)
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