Analysis of Current–Voltage Characteristics in Insulating Polymers Using a Bipolar Charge Transport Model

Abstract

A bipolar charge transport (BCT) model has first been used to compare the current density–voltage characteristics (J–V) for different applied voltage protocols and different times under voltage taking low-density polyethylene (LDPE) as a case study, as a steady state may take a very long time to be reached experimentally. Simulation results highlight the necessity to spend substantial time under voltage to reach a quasi-stable current. In a second part, evolutions of the model in terms of physical processes are proposed, to observe their impact on the current density–electric field (J–E) characteristics, and particularly the appearance of electric field thresholds, as proposed by the space-charge-limited current (SCLC) theory. To do so, different mathematical expressions are proposed for each physical process related to injection, mobility, and trapping. Field-dependent mobility equations allow obtaining electric field threshold values comparable to experimental data, while a constant mobility is the only way to observe a trap-free limit region. Moreover, all parameters linked to trapping are of most importance in the observation of an SCLC characteristic but are not enough to observe a trap-free limit region when the electric field-dependent mobility is considered.