Studies On White Organic Light Emitting Devices With Several Layers

The 21st century is the era of information industry-based knowledge and economy, Flat-panel displays, as an important means of human access to information, its role is increasingly important. The organic electroluminescent light-emitting devices as one of the flat panel displays, owing to its light weight, low cost, wide viewing angle, fast response, self-luminous, light-emitting efficiency and high brightness, can be used to achieve the full color display, etc., thus causing the academic community and the business community's attention. Especially since C.W.Tang has reported high brightness organic light-emitting device (OLEDs) with low operating voltages for the first time in 1987, OLED is becoming the focus of competition in interdisciplinary,collaborative research topics at the forefront of world and national technology. Through the development and utilization of new materials, continuous improvement of device structure and preparation process, the development of organic light-emitting devices has made considerable progress. Many of the world well-known large companies and enterprises are also added to this area of exploration and research work, to the present has begun small-scale into the market, some oled products have entered the market.As the most widely being used ,some research on WOLED in recent years is gradually increased. With technology and materials of the device continuously updating, performance of various aspects of the device are constantly enhancing and improving. This is mainly because the white organic light-emitting devices can be used not only as the future of lighting, but also can be applied to liquid crystal display of the background light, as well as full color display, etc. As a light source is concerned, it has much advanced aspect than the incandescent lamps, halogen lamps, fluorescent tubes and other traditional light sources, etc., such as high luminous efficiency, high color-rendering index, high life and can be used as a surface light source and so on. Based on the advantages and the application of white light emmiting devices, in this paper multi-layer structure for the complex light-emitting of white organic electroluminescent devices have been studied basing on a series of material being synthesized in our group. We have also improved the efficiency of the device by coupling out the light.First, we applied three kinds of phosphor materials and perpared a highly efficient white organic light emitting devices with a p-type structure. We adopt F4-TCNQ into m-MTDATA as a hole injection layer devices, use NPB as an electron -blocking layer and hole transport layer, and use the mCP as the host of the materials,then take (FIrpic), Ir(ppy)2 (acac)and Ir(DBQ)2(acac), respectively, as blue, green and red light-emitting materials. And TPBi is used as the electron injection layer and hole-blocking layer, Al as a cathode. By adjusting the different light-emitting layer thickness, concentration and sequence, we get an efficient white organic light-emitting device finally. The device shown good performance, and its maximum brightness, maximum current efficiency and maximum power efficiency can reach at 31770 cd ? m-2, 19.3 cd ? A-1, and 12.1 lm ? W-1. respectively. The device has good color stability, and the devices'color coordinates remain at the white area as the driving voltage changing from 5 to 11V. Device have a good repeatability.Blue phosphorescent material is bottleneck to limit efficiency of the white light-emitting devices because of its wide band gap, low light-emitting efficiency, and the requirements for the host material being relatively high. So we consider to replace such a blue phosphorescent material with a blue fluorescent material. TDPVBi is a highly efficient blue fluorescent small molecule materials synthesized by our group. Its PL is 472nm in the blue area, so we apply this material as a blue light-emitting layer in white organic light emitting device. At the same time, our group has synthesized a highly efficient yellow light-emitting material. Highly efficient organic electrophosphorescent devices based on a phosphorescent complex, iridium(III) bis(2-(9,9'-spirobi[fluorene]-7-yl)pyridine-N,C2')acetylacetonate((SBFP)2Ir(acac)), have been fabricated. N,N'-dicarbazolyl-3,5-benzene (mCP) is used as the host into which the (SBFP)2Ir(acac) is doped. When 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) is used as the hole-transporting layer (HTL), the device shows good performance, but unexpected blue emission from NPB is observed, which strongly affects the light purity of the device. While a thin electron/exciton blocking layer of fac-tris(1-phenylpyrazolato-N,C2')iridium(III) (Ir(ppz)3) replacing the NPB HTL, triplet excitons can be effectively confined inside the light-emitting layer (EML), stable pure yellow electroluminescence with the Commission Internationale de l'Eclairage coordinates of (0.46, 0.53) is obtained, and the spectrum is largely insensitive to the driving voltages. The device shows a maximum luminance of 37219 cd/m2, a maximum luminous efficiency of 50.6 cd/A, a maximum power efficiency of 46.8 lm/W and a maximum external quantum efficiency of 15.4%. As we know as long as the color coordinates of the two materials to connect through the center of the white area, then through the preparation of these two materials, the devices can achieve white light emission. So we apply these two kinds of materials into the white organic light-emitting device. The fluorescent material of 2,5,2',5'-tetrakis(2,2-diphenylvinyl) biphenyl (TDPVBI) and phosphorescent material of iridium(III) bis(2-(9,9'-spirobi[fluorene]-7-yl)pyridine-N,C2')acetylacetonate((SBFP)2Ir(acac)) were used to emit blue and yellow light, respectively. NPB is used as the hole-transporting layer and TPBI as the electron-transporting layer. Fixed a blue light-emitting layer thickness and doping concentration of the yellow light-emitting layer, we adjust the thickness of the yellow material layer to adjust the light-emitting color to get the white light-emitting. The structure of the device is simple, easy operation, high efficiency and good color purity. The device can reach maximum current efficiency, maximum energy efficiency and maximum brightness can be achieved 37 cd / A, 26 lm / W and 30000 cd/m2, respectively.White organic light-emitting devices plays an important role in the future development. Though electrophosphorescent OLEDs with an internal quantum efficiency of near 100% already approach the efficiency of fluorescent lamps, only about 20% of the generated light can escape from the OLEDs owing to total internal reflection (TIR) in the glass substrate and waveguiding. Therefore, there is considerable potential for improvement in the external efficiency of OLEDs used for flat panel displays and interior lighting source. One simple method to increase the light extraction from white organic light-emitting devices by using biomimetic silica antireflective surfaces is demonstrated. A silica cone array was directly etched on the opposite side of the indium–tin–oxide-coated fused silica substrate. The antireflective surfaces can dramatically suppress the reflection loss and increase the transmission of light over a large range of wavelength and a large field of view. Using such surfaces, the luminance efficiency of the device in the normal direction is increased by a factor of 1.4 compared to that of the device using flat silica substrate. This method is simple, time-efficient, and reproducible. Therefore, the method mentioned here can be introduced in any OLEDs without any alteration of device structure and materials design.