Enhancing the Breakdown Voltage of Aluminum Solid-Polymer Capacitors via Interface Engineering

Using conductive polymers such as poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) as cathodes in aluminum electrolytic capacitors can address limitations such as low ripple current capability, thermal instability, and high equivalent series resistance (ESR), thereby extending their lifetime. However, the relatively low breakdown voltage of aluminum solid-polymer capacitors limits their operating voltage to approximately 100 V or less. Recent studies indicate that the dielectric–conductive material interface, which is tunable through the introduction of an intermediate layer, plays a crucial role in determining the dielectric properties of solid capacitors. However, its fundamental characteristics remain insufficiently understood. To deepen scientific understanding, we have developed a method to fabricate dual-layer anodic structures by anodizing aluminum with a preformed porous layer at 650 V in a borate electrolyte. This process yields an inner dense layer with a uniform thickness of approximately 770 nm and an outer porous layer with systematically varied pore sizes and porosities. Capacitor performance was compared with that of a dense anodic oxide layer lacking the outer porous layer. Dielectric performance was evaluated using flat aluminum capacitors coated with PEDOT:PSS. AC impedance measurements showed nearly identical capacitance values for all samples, suggesting that PEDOT:PSS effectively reaches the dielectric surface through the porous layer regardless of pore size. Therefore, the inner dense layer serves as the dielectric, whereas the outer porous layer functions as an interfacial engineering layer. Capacitors incorporating porous layers with low surface porosity and high calculated relative porous-layer resistance exhibited the highest enhancement, achieving breakdown voltages of 571–589 V, corresponding to approximately 90% of the anodizing voltage. In contrast, the dense anodic layer without a porous outer layer exhibited a breakdown voltage of 370 V. These findings demonstrate that the morphology of the dielectric–polymer interface is a key factor governing the breakdown performance of aluminum solid-polymer capacitors.