Study On Low Driving Voltage Organic Light-Emitting Devices

Organic light-emitting diodes (OLEDs) have great potential applications in the next generation flat panel displays and solid-state lighting products, because of their advantages of low driving voltage, high luminance, high efficiency, high contrast, thinness, low weight, wide viewing angle, wide range of operating temperature, simple process, and flexibility. Over the last two decades, the activities on OLEDs turn to focus on the development of the commercial products, derived from the monotonously fundamental research. To further satisfy the requirements of longevity, high efficiency and low-cost, much more effort should be made on the further improvements of OLEDs'performances.In this contribution, the solutions to fabricate OLEDs with low driving voltage are suggested, in view of improving the performances of OLEDs for practical use and integrating OLEDs on active matrix circuits based substrates with compatible process. Both bottom-emitting white OLEDs (BEWOLEDs) and top-emitting white OLEDs (TEWOLEDs) with low driving voltage and high efficiencies are demonstrated. Some mechanisms of the influences of charge carriers and excitons on the performance of white OLEDs (WOLEDs) are revealed. The main achievements involved in this dissertation are listed as follows.1. A novel p-type electrical doping of organic semiconductor is introduced. The influences of electron transporting materials with different ability of electron transportation and hole blocking on the performances of OLED are investigated. The behavior of charge trapping of C545T in Alq3 is probed. Simple models of the distribution of the emissive dipoles in different structures with weak microcavity are derived by analyzing the experimental and simulated results. As the current-voltage characteristics of OLEDs are not sensitive to highly conductive doped hole injecting layer,the thicknesses of p-doping layer are optimized for 128x96x3 active matrix displays with uniform and full-screen emission, which are based on low-temperature polysilicon thin film transistors.2. Based on the dipole model mention above, the optical mode contributions in top-emitting OLEDs (TEOLEDs) are calculated. The forward emission is maximized for the devices used in the near-eye circumstance. Low driving voltage (2500 and 10000 cd/m2 achieved at 4.0 and 4.9 V, respectively) and highly efficient (28.8 1m/W at the luminance level of 1000 cd/m2) TEOLED is demonstrated with an efficient fluorescent dye and a p-doping strategy which is also highly thermal-stable. Implemented with the optimized TEOLED structure and an improved Ag chemical mechanical polishing process, a first prototype of monochromatic QVGA OLED microdisplay in China is also demonstrated.3. The characteristics of charge carriers and exciton in WOLEDs are probed by inserting the interlayers between the two emissive zones based on two complementary blue ((mCP:FIrpic) and yellow (mCP:(F-BT)2Ir(acac)) emitters. The dependences of the driving voltages and efficiencies on the p-type conductive mCP, n-type conductive BPhen and ambipolar mixed mCP:BPhen interlayers are revealed. The NPB/mCP structure is replaced by Ir(ppz)3 to enhance the confinement of the triplets in the emitting layer. With a p-doping hole injecting layer, phosphorescent WOLED for lighting with simple architecture exhibits low driving voltage (1333 cd/m2 at 4 V) and high power efficiency (40 lm/W achieved at the luminance level of 100 cd/m2).4. Both the necessity and feasibility of Cu contact in view of process compatibility and manufacturing cost for full color OLED microdisplays are under deliberate consideration. The simulation of TEWOLEDs sheds light on the influence of the capping layer on white light emission, the out-coupling efficiency, and contrast ratio in top-emitting structure. A first reported Cu-based TEWOLED with low driving voltage (1000 cd/m2 at 4.4 V) and high efficiencies (27.7 cd/A and 17.6 Im/W at 10 mA/cm2, respectively) is demonstrated.An air-stable organic:inorganic p-doping structure is introduced to fabricate low driving OLEDs. The software simulation simplifies the design of highly efficient OLEDs. Some solutions to the applications of OLEDs in active matrix displays and lighting are suggested in this contribution. Detailed analyses of the optical and electrical characteristics of OLEDs with the structures used in this dissertation are presented. The innovations imbedded here are listed as follows:1) Solutions towards low driving voltage and highly efficient organic light-emittin diodes are presented.2) The carrier blocking layers and charge trapping architecture are introduced to characterize the distribution of emissive dipoles in OLEDs with single emitting layer. 3) Based on the above mentioned technique, especially the air-stable p-doping strategy, active matrix OLEDs implemented in the substrates with low temperature polysilicon thin film transistors and a first prototype of QVGA OLED microdisplay in China are demonstrated, respectively.4) The interlayers between the two emitting layer in WOLEDs are introduced to tailor the performances. The behaviors of the photons, charge carriers and excitons in such devices are also investigated.5) It is the first time, to the best of our knowledge, Cu contact used for low driving voltage and highly efficient TEWOLED is presented. The role of a capping layer on top of TEWOLED is theoretically and experimentally investigated.