Polymer light-emitting electrochemical cells: Doping concentration, emission-zone position, and turn-on time

Direct optical-probing of the doping progression and simultaneous recording of the current-time behavior allows the establishment of 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. 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, for X = K, Rb it is significantly more centered in the interelectrode gap. Its is demonstrated that this results from that the p-type doping concentration is independent of salt selection at ca. 2 x 10(20) cm(-3) (ca. 0.1 dopants/MEH-PPV repeat unit), while the n-type doping concentration exhibits a strong dependence: for X = K it is ca. 5 x 10(20) cm(-3) (ca. 0.2 dopants/repeat unit), for, X=Rb it is ca. 9 X 10(20) cm(-3) (ca. 0.4 dopants/repeat unit), and for X = Li it is ca. 3x10(21) cm(-3) (ca. 1 dopants/repeat unit). Finally, it is shown 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.