The effect of an electric field on the reaction kinetics of a charge carrier migrating within a one-dimensional chain

The aim of this study is to suggest a novel approach for estimating the intramolecular mobility of a charge carrier that migrates within a polymer chain and is involved in a pair reaction with a particle located on the same chain. The approach is based on the effect of an external electric field on the migration rate and, consequently, the kinetics of the reaction. As a first step, this problem is considered a stochastic one-step process with absorbing and reflecting boundaries, and an analytical solution is obtained in the case that the second reactant is immobile. With the use of computer simulations of stochastic migration, the effect of the mobility of both reactants and the influence of the Coulomb interaction between them are considered. It is found that the ratio of the pair reaction rates with and without an external field is relatively little dependent on these factors and that the analytical expressions derived can be applied to estimate the relative mobility of recombining particles with accuracy better than a factor of two in many realistic situations.

Inherent Driving Force for Charge Separation in Curved Stacks of Oligothiophenes

Coexistence of high local charge mobility and an energy gradient can lead to efficient free charge carrier generation from geminate charge transfer states at the donor-acceptor interface in bulk heterojunction organic photovoltaics. It is, however, not clear what polymer microstructures can support such coexistence. Using recent methods from density functional theory, we propose that a stack of similarly curved oligothiophene chains can deliver the requirements for efficient charge separation. Curved stacks are stable because of the polymer's strong π-stacking ability and because backbone torsions are flexible in neutral chains. However, energy of a charge in a polymer chain has remarkably stronger dependence on torsions. The trend of increasing planarity in curved stacks effectively creates an energy gradient that drives charge in one direction. The curvature of these partially ordered stacks is found to beneficially interact with fullerenes for charge separation. The curved stacks, therefore, are identified as possible building blocks for interfacial structures that lead to efficient free carrier generation in high-performing organic photovoltaic systems.