Shedding Light on the Operation of Polymer Light‐Emitting Electrochemical Cells Using Impedance Spectroscopy

Abstract

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 (dpn) 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 dpn in a concerted manner, while the brightness reaches its peak at V = 3.4 V when dpn ≈ 10 nm. The existence of an optimum dpn for high brightness in LECs is attributed to an offset between an increase in the exciton formation rate with decreasing dpn, 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.

The Effect of the Dielectric Constant and Ion Mobility in Light‐Emitting Electrochemical Cells

“…By using Equation (2), the experimental thickness of the EDLs were extracted and are provided in Table . The thin EDL values agree with previously reported values . Consistent with our earlier work, devices with higher performance have smaller EDL widths compared with devices with lower performance .…”

Enhanced Operational Stability of Perovskite Light‐Emitting Electrochemical Cells Leveraging Ionic Additives

“…We observed a decrease in the width of the EDLs with increasing LiPF 6 concentration from 0 to 0.5 wt% followed by an increase with further increase of LiPF 6 concentration. The minimum value of 5.8 nm approaches the values reported for ideal polymer LECs [21,24] and optimized, lithium-enhanced LECs from iridium complexes. [17b, 23] Since PeLEC performance is strongly correlated with EDL formation, we show the EDL width of all fabricated PeLECs with various LiPF 6 concentrations in Figure 8a.…”

BODIPY‐Pt‐Porphyrins Polyads for Efficient Near‐Infrared Light‐Emitting Electrochemical Cells

“…As a result, the electrical resistance in the nondoped region of the film is further reduced, as this region is getting thinner. [43][44][45][46][47][48] In line with the static EIS, all the devices showed a similar electroluminescence behavior at pulsed current functioning conditions, that is, a high initial voltage that decreases to a minimum plateau as the p-and n-doped regions are formed and the recombination becomes autosustained in the i zone (Figure 7). [4] Finally, the average voltage can arise over time depending on the degradation mechanism and the morphology of the active layer.…”