Investigation Of The Charge Balance And Performance Improvement For White Organic Light-emitting Devices

White organic light-emitting devices (WOLEDs) are currently attracting considerable attention for use in flat panel displays and as solid state lighting sources. Various approaches have been utilized to improve the performances, such as the efficiency, color stability, lifetime and etc.. In general, the holes are the major carriers in the OLEDs because of the high hole mobility of materials, in constrast to the minor carriers of electrons. Accout for the emitting process and mechanism, the formations of the excitons, however, depend on the charge balance, which is very important for the performance improvement of WOLEDs, especially for the efficiencies. Thus, we focus on optimizing the device structures, and finally realize charge balance. Detailed methods are listing as follows:1. Reducing the hole injection and transport. By choosing the perfered electron-transporting materials, Zn(BTZ)2, as the blue emissive layer, we test different hole-transporting layers, and have observed the hole trapping-effect of mixed hole-transporting layers. After comparing the effects of the various thicknesses of different emissive layers, we obtain the opmitical thicknesses for the WOLEDs, in which the holes transported to the subsequent organic layers have been reduced, thus improving charge balance.2. Adjusting the distributions of carriers. We optimize the device structures based on the energy levels of different materials. As a result, a) we fabricate the high-efficiency phosphorescent WOLEDs with tricolor. This device can transfer the energy from the green phosphors to the red ones, in assistant to the color tuning and the fabrication of the warm WOLEDs; b) we obtain the high-efficiency fluorescent WOLEDs with the traditional configurations and stable CIE coordinates, by considering the energy characteristics of different materials and the dependent of carrier distributions on the thickness of emissive layers, ; c) after investigating the functions of the mixed interface, we successfully control the distribution of carriers and solve the paradox between the performance improvement of the WOLEDs and the thicknesses of the emissive layers. The maximum current and power efficiency of the opmized device are 20.9 cd/A and 19.9 lm/W at 0.007 mA/cm2.3. Improving the carrier transport. We describe the introduction of a mixed-transition layer, consisting of the hole-transporting (or electron-transporting) layer doped with the emissive dye, and then sort it by the carrier transporting property—favoring the holes or electrons. These devices exhibit maximum current and power efficiencies of 13.3 cd/A and 11.3 lm/W, respectively, with slightly modified CIE coordinates, from (0.300, 0.371) to (0.312, 0.366). This is attributed to the enhancement of carrier transport by doping, thereby improving both the exciton formation probability and the charge balance.4. Introducing a commercial blue material. We systemly investigate its performances and applications in the OLEDs, for the convenience of its application in this paper.5. Designing the objects and plans for next research.