Spin-dependent electron-hole capture kinetics in conjugated polymers

TL;DR

This study models spin-dependent electron-hole recombination in conjugated polymers, revealing how chain length influences exciton formation times and ratios, aligning well with experimental data.

Contribution

It introduces a vibronic relaxation model that explains spin-dependent exciton formation ratios and their dependence on polymer length and binding energies.

Findings

Triplet-to-singlet formation time ratio increases with chain length.

The ratio of singlet to triplet formation rates is inversely related to their binding energy ratio.

Model results agree with experimental observations.

Abstract

The recombination of electron-hole pairs injected in extended conjugated systems is modeled as a multi-pathway vibron-driven relaxation in monoexcited state-space. The computed triplet-to-singlet ratio of exciton formation times $r = \tau_T/\tau_S$ increases from 0.9 for a model dimer to 2.5 for a 32-unit chain, in excellent agreement with experiments. Therewith we rationalize recombination efficiency in terms of spin-dependent interstate vibronic coupling and spin- and conjugation-length-dependent exciton binding energies.Our model calculations for various length polymers indicate that the ratio of the singlet to triplet formation ratios, $r = \sigma_S/\sigma_T$, is inversely related to the ratio of the singlet and triplet binding energies, $\epsilon^b_S/\epsilon^b_T$.