Research Progress on Electroluminescent Polymers

Electroluminescent polymers hold great promise in penal display and solid-state lighting applications because of their advantages of solution processability, large-scale manufacturing and ability to produce flexible devices based on plastic substrates. In the past decades, great progress has been made in electroluminescent polymers including their working mechanism, material systems and device performance. Basically, electroluminescent polymers can be divided into three classes: fluorescent polymers, phosphorescent polymers and thermally activated delayed fluorescence (TADF) polymers. For fluorescent polymers, the internal quantum efficiency (IQE) is limited to 25% because they can use only singlet excitons in the electroluminescent device. In comparison, IQE of phosphorescent polymers can reach 100% because of the strong spin-orbital coupling induced by the heavy-atoms in the phosphorescent dopants. Recently, TADF polymers have evolved rapidly as the new kind of electroluminescent polymers because they are able to utilize triplet excitons through rapid reverse intersystem crossing process from the lowest triplet state to the lowest singlet state, thereby providing a promising approach to reach 100% IQE without the use of noble metal elements. With these developments, the device performance of the polymers has been enhanced greatly and many indexes have met the commercialization requirements. This review is aimed to summarize the research progresses on electroluminescent polymers used for organic light-emitting diodes, including the molecular design and device performance of representative examples of fluorescent polymers, phosphorescent polymers and TADF polymers. The emphasis of this review is especially focused on the color tuning and performance improvement approaches for the fluorescent polymers, the optimization of the phosphorescent dopant, the polymer host and the topological structure of the phosphorescent polymers, as well as the design principles of the thermally activated delayed fluorescence polymers. Finally, the perspectives and the key challenges of the electroluminescent polymers are discussed, and future directions of efforts toward further developing low-cost, efficient, and stable electroluminescent polymers are also demonstrated.