Low-Field Scattering and High-Field Trapping of Quantum Dots: Dual Effects on Dielectric Characteristics of Polypropylene Insulation

This study investigates how quantum dots (QDs) influence carrier migration under both low and high electric fields. The effects of CdSe QDs at different concentrations on the direct current (DC) conductivity and breakdown strength characteristics of polypropylene (PP) were systematically examined. The ohmic conductivity decreases with the incorporation of QDs, but the threshold electric field of ohmic conductivity remains unchanged at a certain temperature. The results indicate that under low electric fields, QDs act as scattering centers for carriers due to the Coulomb blockade effect, which increases the migration path and reduces conductivity. Under high temperatures and high fields, QDs primarily function as carrier traps, and the trapped carriers are difficult to de-trap because of the quantum confinement effect. As a result, adding QDs increases the threshold electric fields for Schottky injection and Poole-Frenkel (P-F) emission. For instance, at 105°C, the threshold electric field increases from 35 kV/mm to 50 kV/mm. Thermally stimulated depolarization current (TSDC) measurements and density functional theory (DFT) simulations confirmed that QDs introduce deep traps at 0.9-1.0 eV in the polymer matrix, effectively capturing carriers. In conclusion, this study demonstrates low-field scattering and high-field trapping of QDs, which reduce carrier mobility and improve dielectric characteristics.