Capacitance Stability in Polymer Tantalum Capacitors with PEDOT Counter Electrodes

Abstract

Polymer Tantalum capacitors with PEDOT cathodes fabricated with an aqueous pre-polymerized PEDOT dispersion (slurry PEDOT) were investigated for stability with temperature. In particular, capacitance dependence on temperature was investigated in humidified and dry capacitors fabricated with both coarse and fine tantalum powders, variable thicknesses of the Ta2O5 dielectric, and in-situ vs. slurry PEDOT cathodes. Electrical measurements along with scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS) were used to characterize the samples. The capacitance of all samples was observed to increase with temperature; however, the extent and nature of this increase depends significantly on the nature of the polymer cathode and its interface with the dielectric. Humidified devices showed less capacitance variation above room temperature while dry devices showed less capacitance variation below room temperature for both coarse and fine powders. These results cannot be explained by the inherent variation of the dielectric constant with temperature. A model was developed for the capacitance dependence on relative humidity and temperature based on surface area changes due to heating or cooling, complimented by changes in the dielectric constant, which is in good agreement with the experimental data. The results of this work demonstrate the critical impact of the dielectric-polymer interface on capacitance stability in these capacitors.