Localized trions in conjugated polymers

Abstract

Experimental and theoretical evidence for charged trions in films of poly[2-(N-carbazolyl)-5-($2^{\prime }$-ethyl)-hexoxy-1,4-phenylenevinylene] consisting of two on-chain polarons of the same sign and a trapped polaron of the opposite sign, which can be metastable with respect to both dissociation and recombination, is described. Such trions can be generated by fusion of a free polaron with a neutral polaron pair; this process eliminates the high Coulomb barrier for fusion of two like-charge carriers into a bipolaron (BP). We also argue that trions can be created by photoexcitation of a conjugated polymer, which may create a high density of geminate polaron pairs. Since the metastable trion is anchored by a deeply trapped charge, it is immobile and can be considered as a Coulombically trapped BP, even if the BP may not exist as a free particle. Such localized trions can account for the reversible photoinduced fatigue of the thermally stimulated luminescence (TSL) and the negative [photoluminescence (PL) quenching] spin $1∕2$ PL-detected magnetic resonance (PLDMR) of these films. We also show that the model of reversible UV photoinduced cyclization, recently suggested to explain metastable charged states in substituted poly(phenylene vinylene) derivatives, is inconsistent with the obtained TSL and PLDMR results.