Bipolaron mechanism for bias-stress effects in polymer transistors

The slow formation of hole bipolarons is proposed to explain the bias-stress effect that is observed in some polymer thin-film transistors (TFT). The primary evidence is the observation that holes are removed from the channel at a rate proportional to the square of their concentration. As bound hole pairs form, they deplete the TFT channel of mobile holes, which reduces the TFT current and causes a threshold voltage shift. The slow rate of bipolaron formation is attributed to the mutual Coulomb repulsion of the holes. Contrasting stress effects in different polymers are related to the magnitude of the bipolaron binding energy. Present experiments cannot distinguish whether the bipolarons are intrinsic, or associated with disorder, interfaces, or impurities.