First-principles studies of carrier injection in polyethylene (PE) and ethylene-vinyl acetate copolymer (EVA) oligomers

First principles calculations are utilized in order to gain a better understanding of charge injection from metals to insulators. As a starting point, a comparative study of hole injection into polyethylene (PE) oligomer and ethylene-vinyl acetate (EVA) oligomer is conducted. We evaluate the hole injection barriers at metal/PE oligomer and metal/EVA oligomer interfaces in two ways: (1) from band-plus-lineup based approach and (2) from density of states based analysis. In line with experimental findings that hole injection into EVA is much easier than that into PE, both methods indicated that hole injection barrier of EVA oligomer is smaller than that of PE oligomer. The results showed that vacuum level shift induced by the adsorption of EVA oligomer on metal surface is strongly affected by the molecular dipole and varies over a wide range (around 1 eV) depending on the orientation of the dipole. When the molecular dipole points in the direction of the metal surface, the ionization level was shifted towards the Fermi level, resulting in the decrease in the hole injection barrier. In addition, it was indicated that ionization level of EVA (mainly an anti-bonding combination of C=O), which resides above the ionization level of PE, may well serve as localized states where holes can be injected.