EL properties of stable blue light-emitting polyfluorene copolymers

Blue light-emitting polyfluorene derivatives, poly[9,9-bis(4′-n-octyloxyphenyl) fluorene] (PBOPF), poly[9,9-bis(2′-ethylhexyl)]fluorene (PBEHF) and copolymers of PBOPF and PBEHF were synthesized through Ni(0) mediated polymerization and their light-emitting properties were investigated. All the synthesized polymers showed their maximum UV–visible absorption and band edge at about 380 and 440 nm, respectively. PBOPF, PBEHF and copolymers showed PL emission maxima at about 424 nm. The PBEHF thin film showed significant excimer band in PL spectra after thermal annealing at 100 °C for 2 h. No significant excimer emission, however, was observed in the PL spectra of the PBOPF and the copolymers even after thermal annealing suggesting that BOPF unit effectively suppressed the excimer emission. Light-emitting devices were fabricated in an ITO (indium-tin oxide)/PEDOT/polymer/Ca/Al configuration. The EL spectra showed emissions similar to the PL results, and the EL device using PBOPF showed best performance among the devices using the polymers. An EL device with ITO/PEDOT/PBOPF/Ca/Al configuration showed maximum brightness of 375 cd/m2 with luminous efficiency of 0.15 lm/W at 9 V.