Effect of Solvent-Assisted Thermal Treatment on the Performance of Polyfluorene-Based Polymer Light Emitting Diodes

Solvent-assisted thermal annealing plays an important role in controlling the morphology of polymers; indeed, it affects the performance of polymer light emitting diodes (PLEDs). Using a polyfluorene derivative, poly(9,9-di-2-adamantylhexyl fluorene-2,7-diyl) (PDAHF), possessing bulky rigid side groups as the emitting material and suitable thermal annealing conditions, we observed improved carrier injection and no photoluminescence quenching. The boiling point of the solvent and the glass transition temperature of the polymer also played important roles. Blue PLED devices having the configuration indium tin oxide (1300 A)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (800 A)/1 wt % PDAHF (650 A)/LiF (10 A)/Ca (100 A)/Al (2000 A) were prepared using o-xylene or toluene as the solvent. Thermal annealing at 130°C (o-xylene) or 115°C (toluene) resulted in devices exhibiting blue emissions, with maximum luminances at 12.5 V of 8593 cd/m 2 (o-xylene) and 8732 cd/m 2 (toluene) as a result of improved carrier injection, which increased the degree of charge recombination. The current efficiencies were 4.0 cd/A (o-xylene) and 4.4 cd/A (toluene) at 100 mA/cm 2 . The Commission Internationale de l'Eclairage coordinates were good at (0.14, 0.17). These results indicate that searching for a suitable solvent combined with appropriate thermal annealing conditions is a promising approach toward achieving high-efficiency PLEDs.