Carrier mobilities and breakdown in halogen doped polymers

Abstract

Carrier mobilities in polyethylene (PE) and polystyrene (PS) doped with halogens such as iodine or bromine, were studied using a short electron beam pulse (100 ns time width). Both the hole and the electron signals induced by the electron bombardment in PE and PS increase with doping of halogens but the former is more affected. A logarithmic plot (Scher-Montroll plot) of the induced current versus time gives a knee at a time Tr, which is interpreted as the transit time of the carrier front between electrodes. Hole and electron mobilities in Br2 doped PE are about 2×10−5 cm2/V sec and 1×10−5 cm2/V sec, repectively, at room temperature, which are higher than those of undoped PE by more than two orders of magnitude. The activation energy of the mobilities also decreases from 0.330.01 eV for both electron and hole in undoped PE to 0.270.01 eV in doped PE. Similar effects are also observed in PS. Although hole mobilitiy is much smaller than electron mobility in undoped PS, these two become comparable after halogen doping. The electric breakdown strength of PE (about 6 MV/cm at room temperature) decreases after the doping of halogen (about 2.5 MV/cm in I2 doped PE at room temperature) and becomes temperature insensitive between 10 and 60 °C.