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Electroluminescent devices become warm during operation, and their performance can, therefore, be severely limited at high drive current density. Herein, the effects of this self‐heating on the operation of a light‐emitting electrochemical cell (LEC) are systematically studied. A drive current density of 50 mA cm−2 can result in a local device temperature for a free‐standing LEC that exceeds 50 °C within a short period of operation, which in turn induces premature device degradation as manifested in the rapidly decreasing luminance and increasing voltage. Furthermore, this undesired self‐heating for a free‐standing thin‐film LEC can be suppressed by the employment of a device architecture featuring high thermal conductance and a small emission‐area fill factor, since the corresponding improved heat conduction to the nonemissive regions facilitates more efficient heat transfer to the ambient surroundings. In addition, the reported differences in performance between small‐area and large‐area LECs as well as between flexible‐plastic and rigid‐glass LECs are rationalized, culminating in insights that can be useful for the rational design of LEC devices with suppressed self‐heating and high performance. [ABSTRACT FROM AUTHOR]

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 ~0.11 dopants/MEH-PPV repeat unit in the p-type region and ~0.15 dopants/MEH-PPV repeat unit in the n-type region. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate that the electrochemical properties of the electrode material can have a dramatic impact on the performance of light-emitting electrochemical cells (LECs). Specifically, we report results from planar wide-gap LECs containing a blend of poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV), poly(ethylene oxide) and LiCF3SO3 as the active material. We find that Au electrodes are preferable over Al electrodes, since Au-electrode devices exhibit fast turn-on (i.e., p–n junction formation time) and clearly visible light emission during operation at 5 V and 360K, while Al-electrode devices exhibit slow turn-on (due to a delayed onset of p-doping progression) and no visible light emission. These results are rationalized with a cyclic voltammetry study, which demonstrates that Al is oxidized at a lower potential than the p-doping (oxidation) potential of MEH-PPV, while Au is electrochemically inert over the entire voltage range spanned by the p- and n-doping potentials of MEH-PPV. Consequently, the oxidation charge injected into Al-electrode devices results in a combination of p-doping of MEH-PPV and formation of Al ions. The latter process is undesired since it results in a slow turn-on time and quenched light emission. Finally, we find that planar LECs in a bottom-electrode configuration exhibit a faster turn-on time than identical devices with the electrodes on top of the active material. [Copyright &y& Elsevier]