By direct optical probing of the doping progression and simultaneous recording of the current-time behavior, we are able to establish the position of the light-emitting p-n junction, the doping concentrations in the p- and n-type regions, and the turn-on time for a number of planar light-emitting electrochemical cells (LECs) with a 1 mm interelectrode gap. We find that the position of the p-n junction in such LECs with Au electrodes contacting an active material mixture of poly(2-methoxy,5-(2’-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV), poly(ethylene oxide), and a XCF3SO3 salt (X = Li, K, Rb) is dependent on the salt selection: for X = Li the p-n junction is positioned very close to the negative electrode, while for X = K, Rb it is significantly more centered in the interelectrode gap. We demonstrate that this results from that the p-type doping concentration is independent on salt selection at ~21020 cm-3 (~0.1 dopants/MEH-PPV repeat unit), while the n-type doping concentration exhibits a strong dependence: for X = K it is ~51020 cm-3 (~0.2 dopants/repeat unit), for X = Rb it is ~91020 cm-3 (~0.4 dopants/repeat unit), and for X = Li it is ~31021 /cm-3 (~1 dopants/repeat unit). Finally, we demonstrate that X = K, Rb devices exhibit significantly faster turn-on times than X = Li devices, which is a consequence of a higher ionic conductivity in the former devices.
2007. Vol. 17, 1807-1813 p.