On the desired properties of a conjugated polymer-electrolyte blend in a light-emitting electrochemical cell

We present results from a systematic study on the influence of the conjugated polymer (CP) on the performance of planar light-emitting electrochemical cells (LECs) with a device structure of Au/{CP+ poly(ethylene oxide) (PEO) + KCF3SO3}/Au. We have employed six different CPs, and we demonstrate that in order to attain a fast turn-on time and a strong light emission intensity, it is critical that the p-type doping (oxidation) potential of the CP is situated within the electrochemical stability window of the {PEO + KCF3SO3} electrolyte. We also find that a high ionic conductivity of the active material correlates with a small phase separation between the CP and the {PEO + KCF3SO3} electrolyte, and that a doping concentration of similar to 0.1 dopants/CP repeat unit is a generic feature of the progressing doping fronts in all investigated devices. Finally we report the first observation of a light emission zone positioned in close proximity to the positive anode in a CP-based LEC. (C) 2008 Elsevier B.V. All rights reserved.