title: Identifying and alleviating electrochemical side-reactions in light-emitting electrochemical cells.
abstract: We demonstrate that electrochemical side-reactions involving the electrolyte can be a significant and undesired feature in light-emitting electrochemical cells (LECs). By direct optical probing of planar LECs, comprising Au electrodes and an active material mixture of {poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) + poly(ethylene oxide) (PEO) + KCF3SO3}, we show that two direct consequences of such a side-reaction are the appearance of a "degradation layer" at the negative cathode and the formation of the light-emitting p-n junction in close proximity to the cathode. We further demonstrate that a high initial drive voltage and a high ionic conductivity of the active material strongly alleviate the extent of the side reaction, as evidenced by the formation of a relatively centered p-n junction, and also rationalize our findings in the framework of a general electrochemical model. Finally, we show that the doping concentrations in the doped regions at the time of the p-n junction formation are independent of the applied voltage and relatively balanced at approximately 0.11 dopants/MEH-PPV repeat unit in the p-type region and approximately 0.15 dopants/MEH-PPV repeat unit in the n-type region.
authors.affiliation: Department of Physics, Umeå University, SE-901 87 Umeå, Sweden.
authors.forename: Junfeng
authors.initials: J
authors.lastname: Fang
authors.forename: Piotr
authors.initials: P
authors.lastname: Matyba
authors.forename: Nathaniel D
authors.initials: ND
authors.lastname: Robinson
authors.forename: Ludvig
authors.initials: L
authors.lastname: Edman