High voltage direct current (HVDC) power transmission is the preferred technology over al- ternating current (AC) transmission in many applications nowadays. In HVDC components, polymers are usually used as insulation material and, in practice, they experience performance issues such as leakage current, dielectric degradation and dielectric breakdown. These problems are closely related to charge injection from the metal electrodes into the polymer. Charge injection at metal/polymer interfaces can be influenced by the structure and chemical compositions of both metal and polymer at the interface, and developing a microscopic understanding of how these factors influence the charge injection process is essential to finding solutions for suppressing charge injection and improving polymer performance in HVDC applications.
Using density-functional theory (DFT) simulations, I have studied how the introduction of chemical polar groups at chain terminals of polyethylene (PE) and polypropylene (PP) oligomers at Al/PE and Al/PP interfaces influences charge injection barriers in these systems, and the effect of thermal disorder of these interfaces with different chemistries (modelled using ab initio molecular dynamics at 373 K). Furthermore, I have studied how the charge injection barriers change with the surface density of chemical groups at these interfaces.
First, I have found that the mean injection barrier of a large ensemble of different interface configurations at 373 K varies by up to 1.1 eV for Al/PE and 0.6 eV for Al/PP interfaces with different chemical groups I considered. Second, for both systems, there is a broadly universal linear relationship, which is independent of chemical composition, between the calculated charge injection barrier and the surface dipole moment density of chemical groups at the interfaces. Finally, this relationship also holds for chemical groups with different surface densities. Besides, I have found an approach for improving the numerical accuracy of work function and electron affinity with plane-wave pseudopotential DFT calculations.