Synthesis and light emitting properties of sulfide-containing polyfluorenes and their nanocomposites with CdSe nanocrystals: A simple process to suppress keto-defect

A new series of sulfide-containing polyfluorene homopolymers and copolymers (PFS, PF1, PF3 and PF4) comprising 9,9-di[11-(decylsulfanyl)undecyl] fluorene, 9,9-dihexylfluorene, triphenylamine or benzothiadiazole moieties were synthesized by Ni(0)-mediated Yamamoto-coupling and palladium-catalyzed Suzuki polymerizations. Three other polyfluorenes (PF2, PF5 and PFC6) without sulfur atom in the alkyl side chains were also synthesized by a similar method for comparison purpose. These fluorene-based polymers were characterized using FT-IR spectroscopy, elemental analysis, DSC, TGA, photoluminescence (PL) and electroluminescence (EL) spectroscopies. The synthesized polymers PFS and PF1–PF3 emit blue light at around 440–468 nm, while copolymers PF4 and PF5 emit green light at around 540 nm. In annealing experiments, these polymer films show better stability against thermal oxidation than polymer PFC6. Sulfide-containing polymers show not only good electroluminescent color stability, but their EL spectra also remain unchanged at high driving voltage. A multi-layer electroluminescent device with the configuration of ITO/PEDOT/PF1/CsF/Al exhibited a stable sky-blue emission with color coordinates (0.21, 0.23) at 10 V, which showed a maximum brightness of 2991 cd/m2 at 8 V (75 mA/cm2) and a maximum efficiency of 1.36 cd/A. Finally, by ligand exchange process, the sulfur element could form coordination bonding with quantum dots, and PLED devices using these new QDs-containing organic/inorganic hybrid materials as light-emitting layers exhibit superior or comparable EL performance compared to those without quantum dots.