Kinetics of bias stress and bipolaron formation in polythiophene

Studies of the kinetics of the bias-stress effect are reported for regioregular poly (thiophene) thin-film transistors. We associate the bias-stress effect with bipolaron formation, based on the observation that the stressing rate is proportional to the square of the hole concentration. The bipolaron formation rate is measured as a function of temperature, and the thermal dissociation rate is also deduced from the data. We discuss the hole capture process and suggest that tunneling through the Coulomb barrier dominates. The temperature dependence of bipolaron formation and recovery kinetics leads to a maximum in their formation at about 2200. 3em{0ex}K, and we also find other changes in the electrical properties near this temperature. A separate bias-stress effect is observed to operate at much longer time scales and might be associated with a different population of bipolarons.