Studies Of High Performance White Organic Light-Emitting Diodes

Organic light-emitting diodes(OLEDs) have attracted extensive attention and are at the edge of large-scale commercial applications as the next generation of flat-panel displays and solid state lighting sources based on their unique advantages of energy conservation, ultrathin thickness, and aesthetic flexibility, etc. High efficiency and long lifetime white OLEDs(WOLEDs) play a vital role in OLEDs technology commercialization. While WOLEDs still face many challenges such as low efficiency, high cost and short lifetime. In terms of high efficiency, phosphorescent OLEDs(PhOLEDs) attract tremendous interest and have become the most promising candidate for high efficiency OLEDs because they can use both singlet and triplet excitons. Among the three primary colors, blue PhOLEDs are the toughest challenge to achieve high efficiency and long lifetime, which limit the commercial development of WOLEDs. The main reason for this is the high triplet energy of blue phosphors, which makes it difficult to find suitable host materials. For appropriate host materials, some basic principles such as high triplet energy, good mobility, matched energy levels and excellent stability have to be fulfilled for high efficiency and long lifetime OLEDs. Therefore, it is of great importance to design suitable host materials. In term of device cost and lifetime, the complexity of the device structure should be reduced. Most of studies focus on the multilayer configuration, which increases the complexity of the production cycle and the fabrication cost. Also, sophisticated interfaces among multilayers and the cross contamination during fabrication process will decrease stability of WOLEDs. Therefore, there is a strong incentive to develop simplified OLEDs.Accordingly, the following two types of WOLEDs including related physical mechanism are also investigated.Firstly, we design two host materials SF2 BCz and SF3 BCz which combine rigid spirobifluorene and bicarbazole units via meta- and para-bonding respectively. Spirobifluorene group gives good thermal stability to the new materials and the meta-linkage of spirobifluorene allows SF3 BCz to possess high triplet energy. Blue PhOLEDs featuring SF3 BCz as a host achieved high performance and low efficiency roll-off with a efficiency of 41.4 cd A-1(39.8 lm W-1) at 100 cd m-2 and 39.7 cd A-1(29.8 lm W-1) at 1000 cd m-2. The electrical characters of the blue PhOLEDs demonstrated that the low HOMO energy levels of the materials and bipolar ability of spirobifluorene group lead to a better charge balance and wide recombine zone, which improve the device efficiency with low efficiency roll-off consequently. Inspired by the good performance of blue PhOLEDs, we utilized SF3 BCz as a single host for dichromatism and trichroism WOLEDs, achieving maximum external quantum efficiency(EQE) of 21.3% and 19.8%, respectively. To further enable industrial application, a four-color-based tandem WOLED adopting SF3 BCz as universal host for a range of colors was also constructed. The device showed a high efficiency of 110.5 cd A-1, 57.1 lm W-1 and 40% EQE. Furthermore, the tandem device exhibited good color stability over the practical range of 1000-5000 cd m-2, with nearly unchanged CIE color coordinates(0.44, 0.47). It indicated that SF3 BCz can act as a promising host material for PhOLEDs mass production.Secondly, we adopted a bipolar host material which contains the pyrazine/carbazole hybrid as hole and electron transport layers as well as host material of emitting layers to develop fluorescent blue and phosphorescent green and orange single-layer(SL) OLEDs. Green and orange SL PhOLEDs demonstrated current efficiencies of 63.3 and 62.1 cd A-1 at 1000 cd m-2, and 55.7 and 53.8 cd A-1 at 10000 cd m-2, respectively. Meanwhile, a SL warm white OLED based on fluorescent blue and phosphorescent orange demonstrated excellent performance, with a maximum efficiency of 27.5 cd A-1 and 21.6 lm W-1. Furthermore, the charge carrier and exciton behaviors have been investigated by impedance spectroscopy. It revealed that the dopant trapping effect played a vital role in charge balance and exciton generation in the SL PHOLEDs. The finding is helpful for rational design of material and device structure in the future.