An Ionic Liquid That Dissolves Semiconducting Polymers: A Promising Electrolyte for Bright, Efficient, and Stable Light-Emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) are composed of blends of semiconducting polymers and electrolytes, in which a unique cooperative action of ions and electrons induces a dynamic p–n junction for efficient emission. One of the crucial issues remaining in LECs is uniformity in blends of polymer and electrolyte; phase separation in between the two components results in poor performance or failure of operation. Here, we overcome this issue by developing an ionic liquid-based electrolyte of alkylphosphonium-phosphate that shows notable compatibility high enough to dissolve even light-emitting polymers. This exceptional compatibility enabled us to prepare uniform film blends with various blue to red emitting polymers, and offered bright and efficient LECs. Especially, a blue-emitting LEC showed excellent performance: the luminance reached ∼20 000 cd m–2 with a high luminance efficiency of ∼5 cd A–1, of which performances significantly exceed a light-emitting diode using the same polymer. The ionic liquid was further applied to the LECs with state-of-the-art light-emitting dendrimers showing thermally activated delayed fluorescence under electrical excitation, giving a high efficiency of 11 cd A–1. These demonstrations remind us of the great importance of the polymer–electrolyte compatibility and the usefulness of ILs for electrolyte of LECs.