Quantum chemical analysis of the effect of impurities on carrier transfer properties in polyethylene

Summary

Polymers are used widely for high-voltage insulation. Recently we have studied the carrier conduction properties in crystalline and amorphous polyethylene without any ad hoc parameters, with the aid of quantum chemical calculations. It is also crucial to reveal the role of impurities on carrier transfer properties. Bromine molecule doping in polyethylene increases the hole mobility by several orders of magnitude and decreases the thermal activation energy of the hole mobility. In this contribution, the impact of bromine doping on hole mobility in polyethylene is studied from an atomistic perspective. The computational method comprises of (1) molecular dynamics simulations (modelling the structure of bromine doped polyethylene), (2) quantum chemical calculations (computation of electronic structure and hole hopping rate between molecules), and (3) kinetic Monte Carlo simulations (simulation of hole trajectories). Amorphous phase of polyethylene is modelled by polyethylene oligomers whose lengths are comparable to the hole localization length in amorphous polyethylene. The electronic states of the halogen molecule and polyethylene oligomer cluster show that the energy of the molecular orbital, which is mainly the HOMO of bromine in character is lower than the energies of the molecular orbitals which mainly originate from the HOMOs of PE oligomers, i.e., no impurity states are introduced in the HOMO-LUMO (lowest unoccupied molecular orbital) gap of PE oligomer. In addition the vertical and adiabatic ionization energy of bromine molecules are larger than those of polyethylene oligomers. Therefore, it is most likely that hole will hop between polyethylene oligomers; holes are unlikely to reside in bromine molecules. In fact, computed hole mobility is unchanged or decreased upon bromine doping, when the effect of super-exchange (bridge mediated hole tunnelling between donor and acceptor) is neglected. Taking into consideration the super-exchange of holes between polyethylene oligomers via molecular orbitals of bromine, hole mobility is increased by more than two orders of magnitude and the activation energy was decreased by 10-20%. This is in line with experimental findings. The computational strategies proposed in this paper may well enable prediction of carrier conduction properties of doped organic insulators.

Additional informations

Authors

A. KUMADA, K. HIDAKA, T. HIRANO, F. SATO

Keywords

quantum chemical calculation, density functional theory, carrier transfer, polyethylene, impurities