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Keywords:

  • Doping;
  • Electrochemical cells;
  • Light-emitting materials;
  • Polymeric materials

Abstract

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, while 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 × 1020 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 × 1020 cm–3 (ca. 0.2 dopants/repeat unit), for X = Rb it is ca. 9 × 1020 cm–3 (ca. 0.4 dopants/repeat unit), and for X = Li it is ca. 3 × 1021 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.