Polypyrrole as a solid electrolyte for tantalum capacitors

Tantalum/tantalum pentoxide (Ta/Ta2O5)-based capacitors, in which polypyrrole (PP) is used as a solid electrolyte coating, have been studied. Aromatic sulfonate salts were used as the charge-compensating dopant ions due to their ability to impart good thermal and long-term stability. The capacitors prepared showed excellent high frequency performance up to 100 kHz and stability. The capacitors were prepared by two methods. In the first method the Ta2O5 dielectric layer was coated with chemically synthesized PP, followed by electrodeposition of a second layer of PP to increase the thickness and integrity of the PP layer. In the second method, chemical polymerization was used alone and the process repeated several times in order to obtain complete coverage of the Ta/Ta2O5, anodes. The redox properties of the PP synthesized chemically and electrochemically were studied by cyclic voltammetry and indicate that the polymers have similar electrochemical properties. Compositional analyses were carried out using X-ray photoelectron spectroscopy (XPS) before and after the re-anodization process used in the capacitor preparation to enhance the integrity of the TaTa2O5 dielectric. UV-Vis spectroscopy was used to check the stability of the PP to spontaneous ion exchange during the re-anodization process. The results indicate that aromatic sulfonates remain as the dopant ions when the reformation is carried out in the same aromatic sulfonate solution, but are partially substituted by H2PO4- when the reformation is carried out in H3PO4. For the chemical polymerization, different synthetic conditions were used in order to improve the conductivity of PP; specifically, the use of low temperature and the addition of p-nitrophenol reduce the equivalent series resistance (ESR) of the capacitors.