Improving Charge Storage of Biaxially-Oriented Polypropylene under Extreme Electric Fields by Excimer UV Irradiation

Biaxially-oriented polypropylene (BOPP) is one of the most commonly used materials for film-based capacitors for power electronics and pulsed power systems. To address the pressing issue of performance-limiting loss under extreme electric-fields, here a one-step, high-throughput, and environment-friendly process based on very low-dose ultra-violet irradiation from KrCl (222 nm) and Xe₂ (172 nm) excimer is demonstrated. The performance of commercial BOPP is boosted in terms of withstanding electric-field extremes (Weibull breakdown strength 694 to 811 V µm^-1 by 17% at 25 °C and 428 to 651 V µm^-1 by 52% at 120 °C), discharged energy density, and conduction losses. Importantly, the depth profile of space charge is precisely measured in situ with a high resolution of 500 nm by laser induced pressure pulse. Consequently, the space charge effect and electric-field distortion are reduced and related to the improved polymer films. It is demonstrated that energetic UV photons act as scissors for BOPP chains and dissociate oxygen molecules leading to the more thermally stable oxygen-containing structures, as deep traps to impede charge migration. This work provides a promising approach to produce polymers with customized microscopic characteristics that is compatible with the assembly lines of polymer-based capacitors.