Study On Principle Of Phosphorescent Sensitized WOLED

ABSTRACT: Organic light emitting device (OLED) has been attracting a great deal of attention for its potential to be the next generation technology for lighting and display. However, this theoretical upper limit is only25%of the corresponding quantum efficiency for electroluminescence from the regular OLED, which limits the development of OLED. As phosphorescent materials were presented, it becomes possible to use triplet state to realize high efficient phosphorescence, which provides the possibility to break through the theoretical limit of25%quantum. Although the use of phosphorescent materials can significantly increase the efficiency of OLED, the rolling off of electrophosphorescent efficiency at high current density has been denounced. Some investigate shows that the rolling off of efficiency results from triplet-triplet annihilation at high current density. For sake of finding out the ways to reduce the rolling off of electrophosphorescent efficiency, in this dissertation, phosphorescent material to fluorescent material were codoped the host to reduce the lifetime of triplet state and the triplet-triplet annihilation through the energy transfer from phosphorescent material to fluorescent material. By the way, some tries to realize the high luminance white OLED were made by choosing the appropriate fluorescent dopant which could emit complementary color of phosphorescent dopant and designing a appropriated device structure. We investigated the relative principle for phosphorescent sensitized fluorescence in detail as well. The phosphorescent sensitized OLED provides a ability to enhance the efficiency of OLED for their larger exciton cross section and less triplet lifetime can prevent the triplet-triplet annihilation. The dissertation consists of five chapters. In the first chapter, we summarize the recent progress of White OLED and introduce the relative organic light emitting material, operation principle of device and the method of researching and measuring of OLED. In the other chapters, we present the detail of our research. The detail is listed as below.In chapter2, we verified the determined effect of the energy transfer from Fir6to rubrene on the emission from PVK:Fir6:rubrene co-dopant OLED by analyzing the absorption spectrum, excited spectrum and steady state photo-luminescence spectrum. We also proved the viability of reducing the efficiency rolling off of electrophosphorescentce at high current density by utilizing Foster energy transfer from Fir6to rubrene to reduce triplet lifetime of Fir6and realized the efficiency enhancement of phosphorescent OLED. Phosphorescent sensitized fluorescent luminescence could be a effective way to enhance the efficiency of phosphorescent OLED at high current density.In chapter3, we put our attention on the color-shift problem in phosphorescent and fluorescent co-dopants OLED. At first, we exclude that the shift of recombination zone under electric field and saturation of fluorescent co-dopant at high current density result in the color-shift. The electric field enhanced Dexter energy transfer from Fir6to rubrene could be responsible for the color-shift problem by analyzing the electric modulated PL spectrum and transient PL of OLED based on PVK:Fir6:rubrene. As well we found out that carrier trapping effect of rubrene is important for the spectrum-shift as voltage increasing. By avoiding the energy transfer between the two dopants in co-dopants OLED to prevent the color-shift, we realized the white emitting by combing the emission from electromer of TAPC monomer of perylene.In chapter4, We obtained a high luminance white OLED depending on the structure designing with excitons management, in which green phosphorescent material (Ir(ppy)3) sensitize the red fluorescent material(DCJTB). For the sake of improving the chromaticity dependence of white color on the applied voltage, PVK:Ir(ppy)3:DCJTB (100:5:0.4in wt.) polymer was chosen to combine with NPB blue fluorescent material to obtain the white light emitting. At last, we realized the excitons management and obtained the white OLED with8700cd/m2maximum luminance and41m/w luminous efficiency by controlling the recombination zone utilizing the device structure of ITO/PEDOT:PSS (30nm)/PVK:Ir(ppy)3:DCJTB (100:5:0.4in wt.)(60nm)/NPB(4nm)/BCP(10nm)/Alq3(20nm)/LiF(1nm)/Al(100nm).In chapter5, We present the conclusions and future work。Upon above research, we could conclude that1, The rolling off of electrophosphorescent efficiency at high current density could be relieved by phosphorescent sensitized fluorescence and efficiency enhancement of OLED could be achieved.2,The color stability could be affected by the electric field enhanced Dexter energy transfer from Fir6to rubrene and carrier trapping effect of rubrene.3, The exciton management and highly luminance white OLED was realized by the structure design of OLED. In my opinion, there are something more to research. The one is to achieve better phosphorescent sensitized fluorescence, the polymer host which have better film-forming and more appropriate energy bandgap was needed. The other one is the research on architecture and fabrication of OLED is needed to improve the color stability of OLED.