Polymer light-emitting electrochemical cells: The formation and effects of doping-induced micro shorts

A planar surface cell structure has been utilized to investigate a polymer light-emitting electrochemical cell (LEC), consisting of an active-material mixture of ply[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and an ionic liquid, tetra-n-butylammonium trifluoromethanesulfonate, contacted by Au electrodes. It is shown that diffuse and needle shaped doping fronts originate from the electrodes (p-type from the positive electrode and n-type from the negative electrode) during the charging process at a temperature T) of 393 K. After a turn-on-time, a significant portion of the doping fronts makes contact close to the negative electrode to form a light-emitting p-i-n junction, but at some points p-type doping protrudes all the way to the negative electrode to form what are effectively micro shorts. It is shown that the consequences of such doping-induced micro shorts during a subsequent stabilized "frozen junction" operation at T = 200 K are drastic; the current-rectification ratio (RR) is low and the quantum efficiency (QE) is far from optimum. Both RR and QE increase significantly during long-term operation under frozen junction conditions, demonstrating that the lifetime of the doping-induced micro shorts is limited even under such stabilized conditions. Still, it is clear that it is critical for the optimization and further development of LECs to find new types of active material and electrode combinations that allow for formation of a continuous p-i-n junction in the center of the interelectrode gap.