Polymer light-emitting electrochemical cells: Frozen-junction operation of an “ionic liquid” device

Abstract

We report frozen-junction operation of a polymer light-emitting electrochemical cell containing a mixture of poly[2-methoxy-5-( 2 ′ -ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and the ionic liquid tetra- n -butylammonium trifluoromethanesulfonate (TBA-TF) as the active material. We find fast turn-on time, unipolar light emission, and significant operational lifetime up to T = 200 K for planar Au ∕ ( TBA - TF + MEH - PPV ) ∕ Au surface cells, which had been charged (i.e., electrochemically p - and n -type dopedin situ) at T = 393 K and V = 4 V and then cooled to 80 K at V = 4 V . We employed differential scanning calorimetry to demonstrate that (TBA-TF+MEH-PPV) exhibits two melting transitions of TBA-TF crystalline phases located at T m , 1 ≈ 280 K and T m , 2 ≈ 380 K , respectively. The lower T m , 1 sets the upper limit for frozen-junction operation (with zero-ionic conductivity), while the larger T m , 2 correlates to the lower limit for the charging regime (with high ionic conductivity).