Luminescence and Energy Transfer of Polyimide Under Square Wave Voltage

Polyimide (PI) is widely used as insulation under square wave voltage. Its aging and insulation failure is related to the energy dissipation under electrical stress. Part of the energy is radiated in the form of optical emission, namely electroluminescence (EL). Therefore, it is of great significance to clarify the EL features and its energy transport to understand the aging mechanism of PI. In this paper, EL spectra are measured under square wave voltage on PI specimens with different aging degrees. Combined with quantum chemical calculations and photoluminescence spectra, the electron transitions and energy transfer processes during the excitation and optical emission stages of EL are investigated. The results show that two types of transitions, charge transfer transition and local excitation transition, can be found during the excitation process, in which the charge transfer transition is dominant, involving n-π* transition and π-π* transition. In the process of light emission, the fluorescence and phosphorescence emission peaks of PI are 634.8 nm and 834.2 nm, indicating that the fluorescence emission makes the greatest contribution to the EL of unaged PI. With the aging of PI, the ratios of near-infrared and near-ultraviolet components in EL increase, which is likely related to the localized states inside the material by more deep traps induced with aging and the appearance of by-products. Investigating the luminescence process and energy transfer of PI from the point of molecular energy level provides more support for the further study of the aging mechanism and insulation failure of materials.