Photovoltaic Effect in Phthalocyanine-Based Organic Solar Cells: 2. Trapping of Molecular Excitons by Impurities

Abstract

Trapping of molecular excitons (MEs) by ionizing impurities is a fast initial step of the photovoltaic effect in organic solar cells. Values for the ME trapping rate constant at a fixed impurity concentration NI have been found in terms of the model of diffusion-controlled bimolecular reactions. The model takes into account two pathways of quasi-steady-state ME trapping, contact and bulk capture (caused by dipole–dipole interaction with an impurity), which are determined by the ME diffusion length lS, the minimum radius of approach of partners a0, and the Forster radius RS. The trapping quantum yield at RS > a0 is YS = (1 + b)−1, where b−1 ≈ 4π(2lDRS)3/2NI. Possible doping conditions when YS ~ 1 at NI ~ 1018 cm−3 are discussed.