Thermally assisted charge injection at electrode/polyethylene interfaces evidenced by direct observation of space charge

A layered sample consisting of a polyethylene (PE) film and a both surface fluorinated PE plate was used to investigate charge injection at the semiconductive, Al, or Au electrode/PE interface by space charge observation based on the pressure wave propagation method, to determine whether the charge injection is a thermal excitation process. Space charge observation in the layered sample subjected to a dc field of positive or negative 5 kV/mm at 20 or 60 °C shows that charge injection at the electrode/PE film interface and charge accumulation at the film/surface fluorinated plate interface do not stop so long as the electrode field is not reduced to zero, independent of electrode material. The charge injection and accumulation rates strongly depend on the charging temperature. At 60 °C, it only needs 15 min for the electrode field to decrease to zero or the charge accumulation at the film/surface fluorinated plate interface to reach a maximum value, while the electrode field is still about 47 % of its initial value even after 240 min charging at 20 °C. The very likely existence of electrode-induced gap states at the electrode/PE interfaces and the thermal excitation of charge carriers from the electrode-induced gap states near the charge neutrality level to the bulk states of PE can account for the observations of the charge injection behavior.