Unraveling the chromophoric disorder of poly(3-hexylthiophene)

TL;DR

This study investigates the origin of broad absorption spectra in conjugated polymers, revealing that structural disorder at the single-chromophore level enables high chain ordering, crucial for photovoltaic efficiency.

Contribution

It demonstrates that extreme energetic disorder in chromophores is structural and may facilitate polymer chain ordering in bulk films.

Findings

Single-molecule spectroscopy reveals 200nm inhomogeneous broadening.

Energy transfer causes red-shift in bulk films.

Structural disorder enables high polymer chain order.

Abstract

The spectral breadth of conjugated polymers gives these materials a clear advantage over other molecular compounds for organic photovoltaic applications and is a key factor in recent efficiencies topping 10%. But why do excitonic transitions, which are inherently narrow, lead to absorption over such a broad range of wavelengths in the first place? Using single-molecule spectroscopy, we address this fundamental question in a model material, poly(3-hexylthiophene). Narrow zero-phonon lines from single chromophores are found to scatter over 200nm, an unprecedented inhomogeneous broadening which maps the ensemble. The giant red-shift between solution and bulk films arises from energy transfer to the lowest-energy chromophores in collapsed polymer chains which adopt a highly-ordered morphology. We propose that the extreme energetic disorder of chromophores is structural in origin. This structural disorder on the single-chromophore level may actually enable the high degree of polymer chain ordering found in bulk films: both structural order and disorder are crucial to materials physics in devices.