Investigation On Inverted OLED And Contrast Enhanced OLED

It is knowledge economy times focusing on information industry today. For the technique of information displays acts as the critical instrumentality for mankind obtaining information and the effects of Flat Panel Displays (FPDs) become more and more significant. As one of the most important members of FPDs, the technique of organic light-emitting devices (OLEDs) attracts worldwide attention in the fields of science and industry due to its many merits of light weight, small, low cost, broad visual angle, high response speed, spontaneous light-emitting and high efficiency, etc.. Since C. W. Tang and S.A.VanSlyke reported the high brightness OLED at low operating voltage for the first time in 1987, OLEDs attracts more attention form all over the world, and after that about over ten years the performances of OLEDs are made significant improvement and almost reach to the level of commercialization by the application of novel materials, suitable structures and process technologies. Although the development of OLEDs is relatively perfect through the continuing efforts of world known companies and research institutes, how to improve the performances of the devices is also the research focus in the world.The drive ways of OLED can be divided into the Passive Matrix and the Active Matrix. In AMOLED each pixel has two or more TFTs, which are used to drive the pixel. a-Si TFT and LTPS TFT can be applied in the AMOLED today. As a result of applied in LCD for many years, a-Si TFT is widely used today. For the charge mobility of a-Si is very low, and the electron mobility is higher than the hole mobility in a-Si. Therefore, n-channel field effect transistors can be used for display drivers only in a-Si TFT, and it is desirablethat the bottom contact of the OLEDs is the cathode and the top contact of the OLEDs is the anode, which is different with conventional device. ITO is usually used for the cathode, but it is easily damage the organic material when growing ITO.In this thesis, for working with a-Si TFT, I have fabricated the device with the inverted structure of ITO/Alq3(60nm)/NPB(45nm)/Al firstly, and there is not light-emitting form the device at 20 V. The transparent ITO is the cathode and the metal Al is the anode in this device. For the material used in the device have different energy level, there will be a large hole injection barrier and a large electron injection barrier in the device. The impeding of electron injection into the emissive layer and hole injection into the hole-transporting layer which is due to the existence of a large injection barrier result in the poor EL performance of the device. For enhance the performance of the device, I place a LiF ultrathin layer between the ITO/Alq3 layers, and place a MoO3 thin film between the NPB/Al layers. I prepared two kinds of devices, the structure of the devices are ITO/LiF(1nm)/Alq₃(60nm)/NPB(45nm)/MoO₃(x nm)/Al and ITO/LiF(x nm)/ Alq₃(60nm)/NPB(45nm)/MoO₃(15nm)/Al ,respectively. Considered the results of the experiments, I have obtained the LiF ultrathin layer and MoO3 thin film optimum thickness. Then I fabricate a device with the structure of ITO/LiF (0.4nm)/Alq₃(60nm)/NPB(45nm)/MoO₃(15nm)/Al, compared with the device without LiF and MoO₃ layers, the performance of device is improved. The device has a maximum luminance of 1089 cd/m² and a maximum current efficiency of 0.118221 cd/A. But, compared with conventional OLEDs, the efficiency of this device is very low, in order to enhance the efficiency of the device, fabricated a device with the structure ofITO/LiF(0.4nm)/rubrene: Alq₃(60nm)/NPB(45nm)/MoO₃(15nm)/Al, obtained the maximum luminance of 4646 cd/m², and the maximum current efficiency of 0.700472 cd/A.The contrast ratio of OLEDs is important for valuing the device. Extensive effort has been exerted recently on enhancement of the contrast ratio of OLEDs. For obtain the high contrast ratio of OLEDs, usually used method is decreasing the reflected light from cathode. However, the efficiency of the device lessened owing to the decreasing of the reflected light from cathode. Therefore, the efficiency of device usually lessened while the device obtains high contrast ratio. Avoids the efficiency reducing while obtains the high contrast is quite difficult.In this thesis, we utilize copper phthalocyanine (CuPc) and C₆₀ as the hole- and electron-injection layers. For the CuPc film has high absorption at 600 700 nm and high hole mobility, and the C₆₀ film has high absorption at 380 500 nm and high electron mobility. Compared with a conventional device without CuPc and C₆₀ layers, the improvement of the contrast is more than 100% under 140 lx ambient lighting at a brightness of 300 cd/m2. A maximum current efficiency of 3.93 cd/A, which is higher than 3.62 cd/A for a conventional device, was obtained at 9 V. The device has a maximum luminance of 17170 cd/m² at 15 V. The high contrast and high efficiency can be attributed to the high absorption and high charge mobility of CuPc and C₆₀ films.