Localized Trions as Metastable Charged States in Conjugated Polymers

Abstract

We have shown that a charged trion consisting of two on-chain polarons of the same sign and a trapped polaron of the opposite sign can be metastable with respect to both dissociation and recombination. A trion can be created upon fusion of a free polaron with a neutral pair, which eliminates a high Coulomb potential barrier for fusion of two charge carriers into a bipolaron. We argue that trions can be created by photoexcitation of a conjugated polymer, in which illumination creates a high density of geminate pairs. Since the metastable trion is anchored by a deep-trapped charge, it is immobile and can be considered as a coulombically trapped bipolaron, although the last may not exist as a free particle. We employ the concept of localized trions in order to explain the reversible photoinduced fatigue of thermally stimulated luminescence in films of poly[methyl(phenyl)silylene].