First-principle investigation of the charge injection barriers of polyethylene and polytetrafluoroethylene oligomers

Experimental research has shown that much less charge injection occurs in polytetrafluoroethylene (PTFE) compared to polyethylene (PE). To clarify the mechanisms of charge injection from metals into polymer insulators, we comparatively studied charge injection in PE and PTFE oligomers using first-principles calculations. Two different models were studied: chemisorption (bonding) and physisorption (nonbonding). The results show that the electron injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface only in the case of chemisorption. The larger electron injection barrier of the metal/PTFE oligomer interface is mainly affected by the positive vacuum level shift of the metal/PTFE interface induced by electron transfer from the metal to PTFE along the chemical bonds. In the case of physisorption, the hole injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface. This is attributed to the larger ionization potential of PTFE compared to PE. The calculated results reasonably explain the experimental phenomena. The agreement between the experimental and calculated results verifies the rationality of our calculation models. The models used herein can likely be applied in other metal/polymer interfacial systems with acceptable accuracy.