Trap-controlled charge decay and quantum chemical analysis of charge transfer and trapping in XLPE

The present work studies the space charge accumulation and decay in the degassed cross-linked polyethylene (XLPE) by detrapping mechanism and quantum chemical method. Experimentally, significant positive charge packets were observed at room temperature (300 K). Negative charges accumulated strikingly under high temperatures but with a fast decay during depolarization. The estimated bipolar carrier mobility and electron/hole trap depth that derived from the charge decay characteristics indicated that a shallow trap distribution (0.84–0.9 eV) dominates the space charge behavior. Additionally, a high temperature (>60 °C) benefits to evaluate the deep traps (0.9–1 eV). Quantum chemical analysis using density functional theory (DFT) demonstrated that both the localized states for shallow electron and hole traps exist in the electronic structure of the XLPE structures. The former is derived from the physical disorder and crosslinks. The latter is ascribed to the hopping interaction between the chain terminal and the other orientated chains. The calculation results about the charge trapping characteristics agree with the experiments. Furthermore, the role of charge injection at the electrode/dielectric interface is also discussed.