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This study concentrates on the energy states introduced into the band gap of polyethylene by chemical impurities and the resulting implications for hole mobility in PE. The thermal activation energies of polyethylene with iodine impurity, PE, and polypropylene can be predicted from the energy of iodine and carbonyl impurity states above the valence band maximum, and the effect of chemical impurities on hole mobility can be related to the presence of interchain hybridized impurity states. Study of the interface between Pt and PE suggests that the Fermi level of the Pt is near the center of the PE band gap, which implies a barrier to charge injection of few electron volts. However in the presence of chemical impurities, such as carbonyl, the barrier to electron or hole injection from the Pt electrode is reduced to values similar to the activation energy of about 1 eV. Although impurity states are traditionally regarded as localized in the vicinity of chemical impurities, hybridized states involving certain impurities, such as iodine and carbonyl, can cause extension of the electron wave function between polymer chains and thereby increase hole mobility significantly, which is otherwise limited by VBM states which are normally extended only along polymer chains.
Date of Publication: Nov.-Dec. 2012