Phenanthroimidazole-Based Organic Fluorophores for Organic Light-Emitting Diodes: An Overview

Phenanthroimidazole-Based Organic Fluorophores for Organic Light-Emitting Diodes: An Overview

Sivakumar Vaidyanathan (National Institute of Technology Rourkela, India) and Jairam Tagare (National Institute of Technology Rourkela, India)
Copyright: © 2018 |Pages: 84
DOI: 10.4018/978-1-5225-5170-6.ch009
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Considering the imminent global energy crisis and inefficient energy utilization, energy-efficient organic light-emitting diodes (OLEDs) are considered one of the most competitive candidates for displays and particularly for future energy-saving lighting sources. Full color displays require all primary colors: red, green, and blue (RGB). In recent decades, numerous phenanthroimidazole (PI) RGB-emitting materials have been developed for efficient OLEDs. In organic electronics, considerable interest is shown on PI, due to ease in fluorophore modification. This chapter focuses on the design and synthesis of PI-based materials and their applications in OLEDs. At first, some nondoped blue, green, and yellow fluorescent materials are comprehensively studied. Then attention has been paid for typical blue, green, yellow, orange, and red PhOLEDs of PI-based fluorophores as a host materials are briefly presented. The molecular design concept, general synthetic routes for PI materials, and the applications of fluorophores in fluorescent OLEDs and host materials in PhOLEDs are reviewed.
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Organic light-emitting diodes (OLEDs) are competitive candidates for the next generation flat-panel displays and solid state lighting sources since the pioneering work by Tang et al., in 1987 (Tang, & VanSlyke, 1987). Extensive research has been carried out to promote OLEDs into commercial applications as solid-state lighting resources and smart displays (Kamtekar, Monkman, & Bryce, 2010). A full-color display requires red, green, and blue emission of relatively equal stability, color purity and efficiency are imperative for achieving efficient white OLEDs. In full-color OLEDs, the blue emitter can not only efficiently decrease power consumption of the devices but also be utilized to produce light of other colours by energy cascade to lower energy fluorescent or phosphorescent dopants. However, the progress in highly efficient blue, green and red emitters has remained a formidable challenge to date (Ulla, et al., 2014, Lee, et al., 2015Wang, et al., 2014, Tang, et a., 2015). Especially, the performance of a blue emitting device is often inferior to that of green and red devices for the intrinsic wide band gap of the blue emitting material. Spin-statistics suggest that the phosphorescent OLEDs (PhOLEDs) can achieve internal quantum efficiency (ηint) of 100% compared to that of fluorescent OLEDs (which can only reach up to ηint of 25%). By harvesting singlet and triplet excitons through intersystem crossing (heavy metal ion effect), one can achive100% internal quantum efficiency (IQE). A great deal of effort has been devoted to develop efficient phosphors that can emit color covering the whole visible region. Red, yellow and green, phosphorescent OLEDs (PhOLEDs) have achieved at very high efficiency with good device lifetime during the past decades. However, the design of efficient blue phosphors still remains an alternative word challenge as discussed in recent review (Chi, 2010). Since the innovative work of Forrest and Thompson on phosphorescent OLEDs, (Baldo, et al., 1998) there have been several studies to achieve 100% internal quantum efficiency in OLEDs and most studies on high efficiency OLEDs have been focussed on developing organic materials and device structures for phosphorescent OLEDs. Currently, a number of red, green, and blue phosphorescent OLEDs are reported by several research groups (Chopra, et al., 2008; Zhu, et al., 2011; Xiao, Su, Agata, Lan, &Kido, 2009,S. Su, et al., 2008; Bin, Cho, & Hong, 2012,S. Kim, et al., 2013; D. Kim, et al., 2013; Lee, & Lee, 2013).However, PhOLEDs have distinct disadvantages, which includes, the relatively long lifetime of phosphorescent heavy metal complexes leading to dominant triplet–triplet (T1–T1) annihilation at high currents, and may also cause a long range of exciton diffusion (>100 nm) that could get quenched in the adjacent layers of materials in OLEDs (T. Lee, et al., 2009) and they have lower electroluminescence (EL) efficiency under high current densities (Baldo, Adachi, & Forrest, 2000) rather low reliability in the blue region of the visible spectrum for practical applications and more expensive. However, to achieve an optimized device performance, careful control of the dopant concentration is required (Wu, Yeh, Chan, & Chen, 2002). On the other hand, performance degradation due to phase separation upon heating is also a problem encountered in these devices (Chang, He, Chen, Guo, & Yang, 2001, G. Zhong, et al., 2002; Gong, Wan, Lei, &Bai, 2005).Fluorescent OLEDs have continued to attract interest because of their long operational lifetimes, high colour purity of EL and potential to be manufactured at low cost in next-generation full-colour display and lighting applications. Fluorescent OLED devices have exhibited a good efficiency (Shih, Chuang, Chien, Diau,&Shu,2007,Y. Yuan, et al.,2011; K. Wu,et al., 2008; M. Wu, et al.,2007; H. Huang,et al., 2011; Xiao, Su, Agata, Lan, Kido, 2009; Chou, & Cheng, 2010).However, in fluorescent OLEDs, the IQE is limited to 25% due to the deactivation of triplet excitons (Baldo, Thompson, & Forrest, 2000; H. Nakanotani, e al., 2014).To overcome these disadvantages of both phosphorescent OLEDs and Fluorescent OLEDs, intensive research is going onto develop the efficient OLEDs (Kamtekar, Monkman, & Bryce, 2010; Wang,& Ma,2010; Zhu, Peng, Cao, & Roncali, 2011; Tao, Yang, &Qin, 2011; Jeon, & Lee, 2012; Yook, & Lee, 2012; Zhu, &Yang, 2013; Yang, Huang, Li, & Li, 2016).

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