Field-Absorbed Water Induced Electrochemical Processes in Organic Thin Film Junctions

Cu/regiorandom poly(3-hexylthiophene) (P3HT)/metal junctions have been reported to exhibit reliable metal-filament resistive switching properties. The filament formation mechanism has been found to include ionization of the Cu electrode and Cu ion drift within the P3HT layer, but no electrolyte has been identified. This work examines the effect of relative humidity (RH) on currents through spin-cast P3HT films sandwiched in evaporated Cu bottom and Au top electrodes. A strong counter-clockwise current hysteresis sets in at positive threshold voltages Uth applied to the top electrodes, with Uth decreasing for higher RH and increasing approximately linearly with P3HT film thickness. For example, at 45% RH, the threshold field is about 50 MV m−1. After hysteretic current flow, the Cu bottom electrodes uncovered by P3HT dissolution exhibit pit corrosion and copper sulfide crystal formation. We explain these observations by field-induced water absorption in the P3HT film and subsequent electrochemical reactions. The results presented here further our understanding of electrode reactions, organic material decomposition, and metal filament formation in organic thin film devices, and may also be important for organic diodes in relatively dry conditions and encapsulated films.