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

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=200K for planar Au∕(TBA-TF+MEH-PPV)∕Au surface cells, which had been charged (i.e., electrochemically p- and n-type doped in situ) at T=393K and V=4V and then cooled to 80 K at V=4V. We employed differential scanning calorimetry to demonstrate that (TBA-TF+MEH-PPV) exhibits two melting transitions of TBA-TF crystalline phases located at Tm,1≈280K and Tm,2≈380K, respectively. The lower Tm,1 sets the upper limit for frozen-junction operation (with zero-ionic conductivity), while the larger Tm,2 correlates to the lower limit for the charging regime (with high ionic conductivity).