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, 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. Light‐emitting electrochemical cells comprising large (K or Rb) cations instead of the conventional smaller Li cations exhibit a shorter turn‐on time and a more centered emission zone (see figure). The centered emission zone is a direct consequence of a decreased doping concentration in the n‐doped region, while the shorter turn‐on time is a reflection of an increased ionic conductivity.