Does photogeneration produce bipolarons in poly ( para-phenylene vinylene)?

Abstract

Extensive photoinduced absorption (PA) and photoconductivity (PC) measurements have been carried out on poly ( para-phenylene vinylene) (PPV) and its derivatives to determine which excitations are present in different time frames after exposure to light. Picosecond PA experiments have led to the conclusion that the large majority of photons above the absorption edge create long-lived (ns) polaron pairs, positive and negative polarons bound on adjacent chains by Coulomb attraction. We suggest that the steady-state PA that has been attributed to bipolarons in undoped samples is also due to polaron pairs. The main evidence for this is the coincidence of the observed steady-state visible and near-infrared PA with that of the corresponding ps PA due to polaron pairs, and the fact that the frequencies of the observed infrared-active vibrational modes (IRAV) require strong pinning. The long tail of the PC that has been attributed to the slow motion of bipolarons in the bulk is more reasonably explained by the effect of traps, for example, thermally and photochemically created carbonyls. The increase in PA and PC observed for photon frequencies higher than about 1/2 eV above the absorption edge, that has been variously attributed to generation of bipolarons and of free electrons in a continuum of energy levels, we assert is due to free polarons. At such photon energies some electrons and holes may be able to escape their Coulomb attraction and avoid being trapped into either polaron pair or exciton states. We conclude that there is no evidence that photogeneration results in a significant number of bipolarons in PPV and its derivatives, apart from samples with particular impurities or defects that foster bipolaron creation.