Scanning Kelvin probe imaging of the potential profiles in fixed and dynamic planar LECs

We measure the potential profiles of both dynamic and fixed junction planar light-emitting electrochemical cells (LECs) using Scanning Kelvin Probe Microscopy (SKPM) and compare the results against models of LEC operation. We find that, in conventional dynamic junction LECs formed using lithium trifluoromethanesulfonate (LiTf), poly(ethylene oxide) (PEO), and the soluble alkoxy-PPV derivative poly[2-methoxy-5-(3',7'-dimethyl-octyloxy)-p-phenylenevinylene (MDMO-PPV), the majority (>90%) of the potential is dropped near the cathode with little potential drop across either the film or the anode/polymer interface. In contrast, when examining fixed junction LECs where the LiTf is replaced with [2-(methacryloyloxy)ethyl] trimethylammonium 2-(methacryloyloxy)ethane-sulfonate (METMA/MES), the potential is dropped at both contacts during the initial poling. The potential profile evolves over a period of approximately 60 min under bias to achieve a final profile similar to that obtained in the LiTf systems. In addition to elucidating the differences between conventional dynamic LECs and fixed LECs incorporating cross-linkable ion pair monomers, the results on both systems provide direct evidence for a primarily "p-type" LEC consistent with the emitting junction near the cathode and relatively small electric fields across the bulk of the device for these two material systems.