Dynamic Processes in Sandwich Polymer Light‐Emitting Electrochemical Cells

Abstract

The operational mechanism of polymer light-emitting electrochemical cells (LECs) in sandwich geometry is studied by admittance spectroscopy in combination with numerical modeling. At bias voltages below the bandgap of the semiconducting polymer, this allows the determination of the dielectric constant of the active layer, the conductivity of mobile ions, and the thickness of the electric double layers. At bias voltages above the bandgap, p-n junction formation gives rise to an increase in capacitance at intermediate frequencies ( ≈ 10 kHz). The time and voltage dependence of this junction are successfully studied and modeled. It is shown that impedance measurements cannot be used to determine the junction width. Instead, the capacitance at intermediate biases corresponds to a low-conductivity region that can be signifi cantly wider than the recombination zone. Finally, the long settling time of sandwich polymer LECs is shown to be due to a slow process of dissociation of salt molecules that continues after the light-emitting p-n junction has formed. This implies that in order to signifi cantly decrease the response-time of LECs an electrolyte/salt combination with a minimal ion binding energy must be used.

Large magnetic field effects in electrochemically doped organic light-emitting diodes

“…Large changes in the dielectric constant in similar LECs biased at 0 and 4 V have, however, not been observed in impedance measurements. 21 Hence we do not think that this alternative explanation is applicable to our results. On the contrary, we suggest that the effects observed in the air-treated MEH-PPV diodes in Ref.…”

“…As a result, the doping process saturates in the order of minutes instead of within a second. 21 Experimentally, this slow saturation is demonstrated by a continuous increase of current while the efficiency decreases. The efficiency decrease is related to exciton quenching at doping sites.…”

“…8 This initial doping due to carrier injection is a fast process that occurs within a second and is determined by the mobility and density of mobile ions. 21 Part of the salt, however, remains undissociated.…”

“…8,9,21,22 In Fig. 3, schematics are shown of the carrier distributions and potential profiles in the LEC before, during, and after conditioning at V bias = 4 V. 21 In a pristine device, only ionic charges related to the salt are present as shown in Fig.…”

“…3(a). The binding energy of this salt is a few tenths of an eV, 21 resulting in a fraction of the salt that is thermally dissociated and thus susceptible to external fields. At the contacts, the ions form thin electric double layers (EDLs) to compensate for the built in-voltage induced by the difference in electrode work functions.…”