Scanning Optical Imaging and Stress Tests of Polymer Light-Emitting Electrochemical Cells

Abstract

Polymer light-emitting electrochemical cells (PLECs) were tested by means of bias stress, luminescent imaging and optical scanning probing. These cells were characterized in terms of the effect of doping on the operational lifetime of the cells, the electronic structure of the pn junctions formed in the cells and the average doping levels in the cells. The lifetime performance of sandwich cells was compared in operation with intermediate storage time and in continuous operation. The cells that were stressed intermittently were more prone to black spots degradation than the cells in continuous operation when the total stress time was the same. However, the black spots vanished in the cells’ idle time, which shows that these spots are local highly doped regions. The appearance of the black spots was strongly associated with the storage of the cell in the presence of the aluminum cathode. The doping in planar PLECs was investigated by optical beam induced current (OBIC) and photoluminescence (PL) scanning using a focused laser beam. Both p-n and p-i-n PLEC junctions were identified and the junction widths were extracted. The widths were as small as 0.21% of the cell size being the smallest width identified for PLECs and light-emitting electrochemical cells (LECs), in general. Exposing the cells to relaxation (de-doping) cycles resulted in shrinkage in the depletion region width, contrary to what is expected. This suggests the presence of submicron structures in the as-activated cells that disappeared after relaxation. In addition, it was observed that relaxation caused a p-i-n junction to emerge from the as-activated p-n junction. The junction doping level was extracted from the OBIC data and was validated via drift diffusion calculations. The calculations showed PLEC doping levels in the range of 10E14 cm^-3 – 10E15 cm^-3 in the junction region. The average doping level in planar PLECs was measured by stress-discharge tests. The measurement offered validation for the used method to estimate PLEC doping levels by integrating the activation charge up to the junction formation. The measurement revealed average doping levels of 10E19 cm^-3 - 10E20 cm^-3, which is much higher than doping levels determined for the junction.