Synthesis and spectral stability of blue eletroluminescent fluorene-substituted polyfluorenes

A monomer of diarylfluorene containing triphenylamine and dialkylfluorene groups at the 9-position and its copolymers, TPAFF-co-F and TPAFF-co-P, were successfully synthesized by a Suzuki cross-coupling polycondensation to obtain stable and highly efficient blue light-emitting polymers. The dialkylfluorenes on the polyfluorene chain served as an antioxidant function as well as a shielding effect. Triphenylamine groups are favorable for hole-injection at the ITO/emitting-layer interface. The combination impart good thermal, morphological and spectral stabilities. Annealing experiments for 24 h in air at 200 °C show that the sequence of green indexes (I_green/I_blue) decreases as follows: poly(9,9-dioctylfluorene) (1.07)>TPAFF-co-F(0.65)>TPAFF-co-P (0.47). These results indicate that dialkylfluorene plays a role in the antioxidant function. Devices were fabricated with the following configuration: ITO/PEDOT:PSS(40 nm)/TPAFF-co-F or TPAFF-co-P(80 nm)/Ba(4 nm)/Al (120 nm). Preliminary results confirmed that they have stable electroluminescent spectra with Commission Internationale d'Eclairage (CIE) coordinates of (0.22, 0.24) at a current density of 547 mA· cm^-2 for TPAFF-co-F and a CIE of (0.24, 0.26) for TPAFF-co-P with a maximum current efficiency of 0.712 cd· A^-1.