Evidence for Photoinduced Polaron Generation in a High Persistence Length Low Bandgap Conjugated Polymer in Solution

TL;DR

The study shows that a specific polymer in solution efficiently generates polarons when exposed to light, with detailed insights into the process.

Contribution

Demonstrates efficient photoinduced polaron generation in a conjugated polymer with a high persistence length and low bandgap.

Findings

Polaron pairs form within 10 ps and decay over 4 ns after band gap excitation.

A three-state kinetic model explains the dynamic equilibrium between polarons and excitons.

Polaron-generation efficiency exceeds 50%, with dissociation and reformation occurring on picosecond time scales.

Abstract

Using ultrafast
time-resolved infrared (TRIR) spectroscopy, we studied the solution-phase
excited-state structural evolution of an indacenodithiophene-co-benzothiadiazole polymer (C8-IDTBT). Following band gap
excitation, the TRIR spectra reveal vibrational features that develop
within 10 ps and decay over 4 ns. Using pulse radiolysis measurements,
charge-modulation spectroscopy, and quantum-chemical calculations,
the IR features are assigned to polaron pairs. Interestingly, these
features appear on an evolving broad mid-IR electronic absorption
background, with kinetics correlating with the formation and decay
of the cation-radical vibrational bands. A three-state kinetic model
successfully reproduces the spectral evolution, revealing that the
polaron and exciton populations exist in dynamic equilibrium on picosecond
time scales, with time constants for exciton dissociation in the range
of 3–5 ps and polaron-to-exciton reformation between 20 and
100 ps, while both species decay to the ground state on much slower
nanosecond time scales (∼1 ns), yielding a remarkably high
polaron-generation efficiency, higher than 50%. These findings provide
fundamental insights into intramolecular charge photogeneration mechanisms
in conjugated polymers, demonstrating efficient bound-polaron formation
in isolated polymer chains.

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