Deep defect levels and exciton dissociation in conjugated polymers

We study the localized deep electronic level within the bandgap of conjugat ed polymers in the presence of structure or oxygen (carbonyl group) defects . The structure defect is modeled as a chain twist, including the chain end s as a special case. Analytic expressions for both the energy and the wavef unction of the deep levels and the itinerant levels are obtained and supple mented by clear intuitive pictures. Carbonyl group is treated numerically w ithin the tight-binding models. The rates of free carrier capture and excit on dissociation through the defect level via multi-phonon emission are calc ulated. We conclude that the defect dissociation dominates the intrinsic di ssociation through thermal activation, and is the primary carrier generatio n mechanism in photoconductivity, Our results explain the photoconductivity enhancement due to oxidation, as well as the recent observation on the tem perature-independent photocurrent in the sweep-out regime. (C) 2001 Elsevie r Science B.V. All rights reserved.