Green Electrophosphorescent Polymers with Poly(3,6-Carbazole) as the Backbone: A Linear Structure Does Realize High Efficiency

摘要

Phosphorescent transition metal complexes, and in particular iridium (Ir) complexes,~([1-11]) have drawn great attention due to their capability to harvest both singlet and triplet excitons to realize theoretical 100% internal quantum efficiency.~([12]) Nowadays high-efficiency organic light-emitting diodes (OLEDs) based on small molecular phosphors have been successfully fabricated via vacuum deposition technology.~([13-18]) In comparison to the small molecular compounds, the macromolecular phosphors, however, show a much poorer performance. Nevertheless, macromolecular phosphors are believed to be suitable for low-cost solution processing, such as spin-coating,~([19]) ink-jet printing,~([20]) etc. Therefore, dedicated efforts should be made for the development of macromolecular phosphors with blue, green, and red light emission used for white organic light-emitting diodes (WOLEDs). Among them, it is highly desirable to design blue and green ones, because the efficiency of red light-emitting electrophosphorescent polymers typically reaches up to 5.0 cd/A,~([21]) but has been further improved to about 10 cd/A recently.~([22]) With regards to their topological structure macromolecules can generally be divided into dendrimers, hyperbranched polymers, and linear polymers (Figure 1). On the basis of these three topological structures, several research groups have tried to develop green macromolecular phosphors. For instance, our group reported a bifunctional phosphorescent dendrimer with a "self-host" feature in 2009. Its non-doped device gave a promising luminous efficiency of 45.7 cd/A accompanied by the Commission Internationale de L'Eclairage (CIE) coordinates of (0.39, 0.58).~([23-24]) Cao's group prepared hyperbranched polymers with an Ir complex as the core and 3,6-carbazole-co-2,6-pyridine as the branch by using a Suzuki polycondensation, and obtained a peak luminous efficiency of 30.1 cd/A with a CIE of (0.33, 0.64).~([25]) Although linear polymers also have been investigated at the same time, their efficiency is not yet as good as those of dendrimer and hyperbranched polymers.'26"27'