Thermal annealing below the glass transition temperature: A general way to increase performance of light-emitting diodes based on copolyfluorenes

Polymer light-emitting diodes based on copolyfluorene with barium/aluminum cathode were thermally annealed at different temperatures through different time periods. It was found that by annealing just below the glass transition temperature $(\mathrm{sub}{\mathrm{-T}}_g)$ of these copolymers the device performance was greatly enhanced. Lower turn-on voltage, higher maximum brightness, and more than two times higher external electroluminescent (EL) quantum efficiency could be achieved in this way. We found out that the enhancement in electron injection and the increase in carrier mobility after thermal annealing at a temperature close to $T_g$ of the EL polymers are responsible for the improvement in device performance.