Charge injection mechanism at Al/PE interface considering the multi-step-hopping at the amorphous/crystalline edge

Abstract

Charge injection is an important factor for the stable operation of power equipment and power system. Our experimental measurements reveal a charge injection barrier of ∼1.2 eV at the Al/Polyethylene (PE) interface. However, previous simulation studies, which accounted for physical and chemical defects, reported charge injection barriers ranging from 2.5 to 5 eV and deviate from our experimental findings. In this paper, we employ first-principles calculation to determine the charge injection barrier at the Al/PE interface, in which the effects of chemical impurities, physical defects, and the morphological complexity of PE including amorphous regions, crystalline regions, and the amorphous/crystalline edge have been systematically investigated. The calculated results demonstrate that specific physical or chemical defects introduce only a single energy trap state, whereas the amorphous/crystalline edge induces multiple energy states. In the amorphous/crystalline edge model, the bandgap is reduced to 7.3 eV and multiple trap states are densely distributed at the amorphous/crystalline edge, which avoids the possibility of highly localized and sparsely distributed trap levels in the space. Consequently, charge injection is likely facilitated through a multi-step-hopping mechanism via these energy trap states at the amorphous/crystalline edge. In this condition, the calculated charge injection barriers of electrons and holes are reduced to 1.56 and 1.33 eV, which is closer to our experimental result of ∼1.2 eV. As the amorphous/crystalline edge widely exists in PE, defects at the amorphous/crystalline edge probably determine the charge injection at the electrode/polymer interface, rather than the specific physical or chemical defects.