Definition of exciton binding energy for conducting polymers

The exciton binding energy ɛb for a conducting polymer can only be defined unambiguously in the long chain limit. In that limit it can be taken as the difference between the bandgap or the ‘single particle bandgap’ — the former if the exciton dissociates into a free electron and hole, the latter if it dissociates into a pair of oppositely charged polarons — and the optical absorption edge.