Polymer light-emitting electrochemical cells:: Frozen-junction operation of an "ionic liquid" device -: art. no. 043506

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 doped in 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,T-1 approximate to 280 K and T-m,T-2 approximate to 380 K, respectively. The lower T-m,T-1 sets the upper limit for frozen-junction operation (with zero-ionic conductivity), while the larger T-m,T-2 correlates to the lower limit for the charging regime (with high ionic conductivity). (c) 2005 American Institute of Physics.