Low-temperature treatment of semiconducting interlayers for high-efficiency light-emitting diodes based on a green-emitting polyfluorene derivative

We investigate the scope for low-temperature treatment of exciton/electron blocking interlayers in light-emitting diodes based on poly(9,9′-dioctylfiuorene-alt-benzothiadiazole) (F8BT). We focus on poly(9,9′-dioctylfluorene-alt-N-(4-butylphenyl)-diphenylamine) (TFB) interlayers processed at temperatures up to 50°C, i.e., far below the glass transition temperature of TFB (∼156°C). Continuous-wave and time-resolved photoluminescence studies confirm the formation of both excitons and exciplex species, as a result of the F8BT/TFB intermixing. Interestingly, however, we can still increase the electroluminescence external quantum efficiency from 0.05% to 0.5% and 1% for progressively thicker TFB films. We propose that a degree of intermixing may become acceptable as a trade-off to achieve low-temperature processability.