Stress Testing Polymer Light-Emitting Electrochemical Cells: Suppression of Voltage Drift and Black Spot Formation
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The stress characteristics of the polymer light-emitting electrochemical cells (PLECs) are comprehensively evaluated by varying a host of material and operational parameters. PLECs with the lowest salt concentration of less than 2.5% exhibit runaway voltage drift that leads to rapid cell destruction. PLECs with the lowest electrolyte polymer concentration and a 5% salt content exhibit the longest luminance half-life, but are slow to activate and are plagued by black spots. At moderate-to-high electrolyte concentrations, the PLECs are relatively stable but are not immune to black spots and voltage drift. A lifetime figure-of-merit is introduced to quantitatively account for all three indicators of cell degradation in luminance decay, voltage drift, and black spot formation. A surprising discovery is that voltage drift and black spot formation can be effectively suppressed by drastically increasing the electrolyte content. A cell with a 70% electrolyte content exhibits the best lifetime of nearly 350 h when operated at a constant current density of 167 mA cm−2. The black spots can also be effectively eliminated by employing silver instead of aluminum as the cathode material.