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Shedding Light on the Operation of Polymer Light‐Emitting Electrochemical Cells Using Impedance Spectroscopy
Author(s) -
Munar Antoni,
Sandström Andreas,
Tang Shi,
Edman Ludvig
Publication year - 2012
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.201102687
Subject(s) - materials science , dielectric spectroscopy , exciton , p–n junction , optoelectronics , polymer , spectroscopy , electrode , brightness , radiative transfer , electrochemical cell , pedot:pss , analytical chemistry (journal) , electrochemistry , optics , semiconductor , condensed matter physics , chemistry , physics , composite material , quantum mechanics , chromatography
A combination of impedance spectroscopy, device characterization, and modeling is used to pinpoint key processes in the operation of polymer light‐emitting electrochemical cells (LECs). At low applied voltage, electric double layers with a thickness of ≈2–3 nm are shown to exist at the electrode interfaces. At voltages exceeding the bandgap potential of the conjugated polymer ( V ≥ 2.5 V for superyellow), a light‐emitting p–n junction forms in situ, with a steady‐state structure that is found to depend strongly on the applied voltage. This is exemplified by that the effective p–n junction thickness ( d pn ) for a device with an interelectrode gap of 90 nm decreases from ≈23 nm at 2.5 V to ≈6 nm at 3.9 V. The current increases with decreasing d pn in a concerted manner, while the brightness reaches its peak at V = 3.4 V when d pn ≈ 10 nm. The existence of an optimum d pn for high brightness in LECs is attributed to an offset between an increase in the exciton formation rate with decreasing d pn , due to an increasing current, and a simultaneous decrease in the exciton radiative decay rate, when an increasing fraction of excitons diffuses away from the p–n junction into the surrounding non‐radiative doping regions.