Modified isothermal discharge current theory and its application in the determination of trap level distribution in polyimide films

Modified isothermal discharge current theory (MIDC) was thoroughly investigated in order to obtain the trap level distribution in polymers. In this theory, a new function was defined to weight the contribution of any trap level to the depolarization current at an instant time. The demarcation energy was used to study the trap empty process. Analysis showed that only electrons with trap levels very close to the demarcation energy significantly contribute to the external current. As an application example, this theory was applied to determine the trap level distribution in Kapton 100HN and 100CR (nanodielectrics) PI films before and after corona ageing. Experiments showed that traps in 100CR PI films were about twice as much as that in the 100HN PI films through all trap levels. It's also found that the quantity of traps was increased evidently after corona ageing and lots of deeper traps were produced in 100HN film, whereas for 100CR, changes were not very significant. Since both physical and chemical defects intrinsic or extrinsic can change the trap level distribution and make a significant impact on the dielectric behavior. So modified isothermal current theory may be a useful tool for determining the defect characteristic and investigating the electrical properties of polymer dielectrics.