Exciton Bimolecular Annihilation Dynamics in Push–Pull Semiconductor Polymers

Abstract

Exciton−exciton annihilation is a ubiquitous nonlinear dynamic phenomenon in materials hosting Frenkel excitons. In this work, we investigate the nonlinear exciton dynamics of an electron push−pull conjugated polymer by fluence-dependent transient absorption and excitationcorrelation photoluminescence spectroscopy, where we can quantitatively show the latter to be a more selective probe of the nonlinear dynamics. Simulations based on a time-independent exciton annihilation model show a decreasing trend for the extracted annihilation rates with excitation fluence. Further investigation of the fluence-dependent transients suggests that the exciton−exciton annihilation bimolecular rates are not constant in time, displaying a t −1/2 time dependence, which we rationalize as reflective of one-dimensional exciton diffusion, with a diffusion length estimated to be 9 ± 2 nm. In addition, exciton annihilation gives rise to a long-lived species that recombines on a nanosecond time scale. Our conclusions shed broad light onto nonlinear exciton dynamics in push− pull conjugated polymers.

Unveiling Multiquantum Excitonic Correlations in Push–Pull Polymer Semiconductors

“…Finally, we highlight that the pump fluences employed in this work range from 10 to 100 μJ/cm 2 , in which sufficient exciton–exciton annihilation (EEA) is expected in electron push–pull polymers on the picoseconds time scale. , Our work shows direct evidence of both correlated exciton pairs and bound biexcitons even at initial population time, which might be precursors for the EEA process in N2200. Dostál et al directly monitored the change of two-quantum peak intensities for a molecular aggregate in five-wave-mixing experiments with time evolving into the nanosecond range .…”