Study of Charge Trapping and De-Trapping Phenomena through Vacuum Charge Measurements and Photoinduced Discharge in Polymeric Thin Films for Energy Storage

This study investigates charge trapping and de-trapping phenomena in polymeric thin films, specifically biaxially oriented polypropylene (BOPP), for energy storage applications. Using the LIMM-PSD measurement system, electrical conduction mechanisms were analyzed during polarization and depolarization phases, highlighting the predominance of space-charge-limited conduction (SCLC) under specific electric field ranges. A charge injection limit of 126 kV/mm was identified, marking the transition from ohmic conduction to transport dominated by space-charge effects. Charge trapping is strongly influenced by interfacial phenomena at the conductor-insulator boundaries, where the polymer’s band structure determines the potential barrier for carrier injection. Surface states formed at these interfaces facilitate hole injection, whereas electron injection is constrained by blocking contacts, occurring only under high fields or via tunneling. Bulk conduction is governed by the density and energy depth of trapping states, with shallow traps favoring negative charge accumulation. Photoinduced discharge (PSD) analysis, correlated with UV-visible absorption spectra, revealed the presence of extrinsic entities affecting charge dynamics. Two main excitation bands were observed: S1 excitons around 3.1 eV, potentially linked to transition metal ions and free charge generation via the Auger effect, and high-energy S2 excitons around 6.2 eV, associated with chemical additives such as phenolic antioxidants, which can release electrons under residual fields. Although PSD irradiation did not produce significant changes in sample charge density, the study enabled identification of chromophoric species influencing trapping behavior. Extending this research to other nonpolar, light-absorbing polymers could improve understanding of charge trapping and release mechanisms in polymer-based energy storage materials