Highly efficient UV-violet light-emitting polymers derived from fluorene and tetraphenylsilane derivatives: Molecular design toward enhanced electroluminescent performance

Novel fluorene-based light-emitting polymers (P1-P3) derived form the copolymerization of 9,9-dihexylfluorene or 9,9-spirobifluorene with tetraphenylsilane derivatives were designed and synthesized by palladium-catalyzed Suzuki coupling reaction. These copolymers were readily soluble in common organic solvents and exhibited high glass transition temperature (T-g >= 157 degrees C) and thermal stability. The results from photophysical studies showed that these copolymers possess a wide band gap (3.26-3.30 eV), which endows them with pure UV-violet emission in solid states with high photoluminescence (PL) quantum efficiencies. The PL characteristic of these copolymers remained almost unchanged in spite of the modification with tetraphenylsilane or even with the electron-donating alkoxyl substituents. The incorporation of bulky 9,9-spirobifluorene units into one of the copolymers (P3) significantly improved the solid-state PL quantum efficiency up to 83% together with a slightly red-shifted emission. The LEDs based on P1-P3 exhibited pure UV-violet electroluminescence (EL) emission peaking at 396, 398, and 409 nm, respectively. In comparison with that of P1, the devices using P2 and P3 exhibited significantly enhanced performance, which was attributed to their higher PL efficiencies and more balanced carriers injection due to the reduced energy barrier for hole injection. A maximum external quantum efficiency of 2.4% and a maximum luminance of 850 cd/m(2) as well as a low turn-on voltage of 5.0 V were demonstrated for P3.