Light-emitting electrochemical cells using polymeric ionic liquid/polyfluorene blends as luminescent material

We report the use of blends composed of poly(9,9′-dioctylfluorene-alt-benzothiadiazole), F8BT, and a polymeric ionic liquid (PIL), poly(vinyl-ethylimidazolium bistrifluoromethanesulfonimide), as the active layer in light-emitting electrochemical cells (LECs) with the simple indium-tin-oxide/active-layer/Al configuration. The PIL provides both the ionic charge and the transport channel necessary for the devices to operate as LECs resulting in reduction of charge injection barriers at the electrode/active-layer interfaces. We find that the performance of devices using PIL:F8BT blends improved with respect to pure F8BT with maximum luminance increasing from 10–20 cd/m2 for pure F8BT to 2000–4000 cd/m2 for blends. Turn-on voltages were also reduced from above 7 V down to around 3.6–4 V. The maximum external quantum efficiency was increased from 10−3%–10−4% to values higher than 0.1%.