High-Performance White Organic Light-Emitting Diodes Via Controlling Charges And Excitons Distribution

Organic light-emitting diodes(OLEDs) are now approaching mainstream display markets and also being aggressively explored for the next-generation lighting applications due to their extraordinary characteristics, such as high efficiency, high luminance, lower power consumption, wide viewing angle, fast switching, ultralight, ultraweight and flexibility. To better compete with the liquid crystal displays, LEDs or other counterparts, it is urgent to investigate the high-performance white OLEDs(WOLEDs).In this paper, we have first simply introduced OLEDs, such as the development, device structures, working mechanisms and some significant physical concepts in OLEDs. Then, we have introduced WOLEDs, including the development, parameters, approaches, types of WOLEDs. Particularly, the four kinds of WOLEDs(i.e., all-fluorescent WOLEDs, all-phosphorecent WOLEDs, hybrid WOLEDs and thermally activated delayed fluorescence WOLEDs) have been well introduced. Based on these facts, by regulating charges and excitons distribution in WOLEDs, the issues(i.e., charges injection and transport, excitons generation and diffusion, excitons harvesting and decay) of WOLEDs have been deeply explored and investigated, achieving a series of high-performance white organic light-emitting diodes. The novelty of this paper can be concluded as follows.1) A single-emitting-layer WOLED has been designed, which can simultaneously achieve the simplicity, extreme efficiency, reduced efficiency roll-off and excellent color-stability. The resulting WOLED shows the maximum current efficiency and power efficiency of 113.6 cd/A and 92.5 lm/W, respectively, which only slightly decrease to 111.7 cd/A and 75.5 lm/W at 1000 cd/m2 as well as 101.3 cd/A and 58.8 lm/W at 5000 cd/m2. Besides, excellent color-stability with a color variation of(0.00, 0.00) is obtained, which is the first single-emitting-layer WOLED showing extremely stable color. The origin of the high performance is comprehensively unveiled. Particularly, we have systematically revealed the role of guests on the origin of the color-stability and demonstrated that the harnessment of charges and excitons distribution by using the multifunctional guests is crucial to stabilize the color.2) We have investigated the effects of n-type interlayers in hybrid WOLEDs and found that the triplet energy rather than electron mobility or hole-blocking ability plays the critical role in device performances. Intrigued by the achieved results, we have used Bepp2 as a more effective interlayer to obtain a high-performance hybrid WOLED. The resulting device can exhibit low voltage(2.8 V), high efficiency(16.0 lm/W), high CRI(73) and long lifetime(>30000 h) at a practical luminance of 1000 cd/m2, which is the first hybrid WOLED possessing long lifetime. Such presented facts will be beneficial to the design of both materials and device structures for WOLEDs in the emerging display and lighting applications.3) A high-performance deep-blue aggregation-induced emission OLED has been developed by enhancing the device engineering. The device exhibits the low voltage(i.e., 2.75 V at 1 cd/m2), high luminance(17721 cd/m2), high efficiency(4.3 lm/W) and low efficiency roll-off(3.6 lm/W at 1000 cd/m2), which is the best deep-blue aggregation-induced emission OLEDs to date. Then, blue AIE fluorophors, for the first time, have been introduced to achieve high-performance hybrid WOLEDs. The two-color hybrid WOLEDs can exhibit i) pure-white emissions, stable colors and the highest efficiency(32.0 lm/W) among hybrid WOLEDs with pure-white emissions; ii) high efficiency, very low efficiency roll-off and stable colors; iii) unprecedentedly high efficiencies at high luminances(i.e., 70.2 cd/A and 43.4 lm/W at 10000 cd/m2), which may provide a new opportunity to achieve extremely high-efficiency WOLEDs. Moreover, the first hybrid WOLED with sunlight-style emission has been developed. Such presented results systematically demonstrate the significance, advance and universality of the novel concept that blue aggregation-induced emission fluorophors are possible to develop high-performance hybrid WOLEDs, which is promising for the future displays and lightings.4) We have developed a very efficient host-guest system, in which NPB is the first time demonstrated to be used as both the host and blue emitter in single-EML WOLEDs. By virtue of this versatile material, the guest concentration is as high as 1.5%, effectively avoiding the difficulty in the control and reproduction process. Remarkably, the single-EML WOLED can exhibit a maximum efficiency of 65.3 lm/W, which is much higher than previous single-EML hybrid WOLEDs with high guest concentrations(>1%), indicating a significant step towards the real commercialization. Besides, the device exhibits low voltages. The turn-on voltage, voltage and voltage for 1000 cd/m2 are 2.4 V and 3.45 V, respectively. Moreover, a color rendering index of 77 is obtained for this two-color device. The working mechanism of the device comprising high concentrations are discussed, in which the device smartly allow the utilization of both the fluorescence from the NPB host itself, and the complementary phosphorescence from the triplet Ir(dmppy)2(dpp) guest by incomplete F?rster energy transfer, Dexter energy transfer as well as direct exciton formation on this guest. Furthermore, we have incorporated this unique host-guest system into a dual-EML WOLED. A maximum efficiency of 17.2 lm/W and 10.2 lm/W at 1000 cd/m2(3.85 V) with an ultrahigh color rendering index of 93 are achieved, providing a new opportunity to accomplish the simplified structure/ low voltage/ high efficiency/ ultrahigh color rendering index trade-off.5) We have developed a flexible WOLED with extremely high efficiency and stable color. With the distinguished conjunction of mechanical, electrical and optical properties, the flexible WOLED exhibits a maximum power efficiency of 101.3 lm/W and 58.2 lm/W at 1000 cd/m2. Besides, a negligible CIE variation of(0.004, 0.005) during 100- 10000 cd/m2 is obtained and the origin of the high performance is deeply explored. Such superior results represent a significant step towards extremely high-performance flexible WOLEDs, which will be beneficial to the design of both material and device structure for the commercialization of flexible WOLEDs in the emerging display and lighting applications.