Visualizing the effects of salt concentration in planar polymer light-emitting electrochemical cells

The effects of salt concentration have been visualized in bilayer, planar polymer light-emitting electrochemical cells (PLECs) with a gap size of 2 mm. Electrochemical doping and junction electroluminescence (EL) have been observed in a luminescent conjugated polymer (CP) top layer with a different solid polymer electrolyte (SPE) underlayer. By varying the salt concentration and thickness of the SPE layer, the initial cell current, peak cell current, doping propagation speed and junction formation time can all be altered/controlled. This is explained by a limiting ionic resistance in the SPE layer which is both salt concentration and thickness dependent. This study establishes the role of electrolyte ions to support doping current in PLECs. The unexpected observation of a systematic junction shift towards the cathode suggests the electrolyte ions, when functioning as counter ions for doping, are also affected by the salt concentration. At high salt concentrations, the light-emitting junction overlaps with the cathode edges which is undesirable. Thus, the bilayer PLECs allowed for the study and visualization of functions of mobile ions in PLEC operation. Moreover, the bilayer PLECs can potentially be used to study the SPE layer itself using highly sensitive imaging techniques.