Do ultrafast exciton-polaron decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells?

TL;DR

This study uses advanced spectroscopy and modeling to reveal that ultrafast exciton-polaron decoherence, occurring within 20 femtoseconds, is key to the high efficiency of charge generation in polymer solar cells.

Contribution

It introduces a novel two-dimensional photocurrent spectroscopy technique combined with a theoretical model to understand ultrafast decoherence in organic photovoltaic materials.

Findings

Ultrafast decoherence times are 20 fs or less.

No cross-peaks observed in spectra, indicating rapid decoherence.

Decoherence governs efficient charge separation in polymer solar cells.

Abstract

All-organic-based photovoltaic solar cells have attracted considerable attention because of their low-cost processing and short energy payback time. In such systems the primary dissociation of an optical excitation into a pair of photocarriers has been recently shown to be extremely rapid and efficient, but the physical reason for this remains unclear. Here, two-dimensional photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy, is used to probe the ultrafast coherent decay of photoexcitations into charge-producing states in a polymer:fullerene based solar cell. The two-dimensional photocurrent spectra are interpreted by introducing a theoretical model for the description of the coupling of the electronic states of the system to an external environment and to the applied laser fields. The experimental data show no cross-peaks in the two-dimensional photocurrent spectra, as predicted by the model for coherence times between the exciton and the photocurrent producing states of 20\,fs or less.