Enhancement of external quantum efficiency through steric hindrance of phenazine derivative for white polymer light-emitting diode materials

Copolymers introducing dithienyl-benzophenazine (<0.1 mol%) as a dopant have been synthesized on a polyfluorene (PF) backbone based on the Suzuki coupling reaction. The UV–vis spectra of polymers showed similar behaviors in the solution and on the film. However, PL spectra were similar to that of PF in solution, but the peak around 615 nm increased as the amounts of dithienyl-benzophenazine were increased in the casting film. Compared with PF-PDTQ05, PF-FDTQ series showed red-shift emission peak by increased conjugation length of dithienyl-benzophenazine. The emission intensity of PF-FDTQ03 and PF-FDTQ05 was more intense compared to that of PF-PDTQ05 due to strong steric-hindrance of FDTQ. In case of PF-FDTQ05, the luminous efficiency and external quantum efficiency were 0.78 cd/A and 0.64%, respectively with a maximum brightness of 3112 cd/m2.