Determining the True Optical Gap in a High-Performance Organic Photovoltaic Polymer Using Single-Molecule Spectroscopy

TL;DR

This study reveals that the true optical gap of PTB7, a high-performance organic photovoltaic polymer, is higher than previously thought, and its red absorption is mainly due to chain aggregation rather than individual chains.

Contribution

The paper demonstrates that the true optical gap of PTB7 is significantly higher and that red absorption originates from chain aggregation, using single-molecule spectroscopy.

Findings

Single-molecule spectroscopy shows higher energy PL from isolated chromophores.

Red absorption in PTB7 is due to chain aggregation, not individual chains.

Single chains exhibit high emission polarization and longer lifetime.

Abstract

Low-gap conjugated polymers have enabled an impressive increase in the efficiencies of organic solar cells, primarily due to their red absorption which allows harvesting of that part of the solar spectrum. Here, we report that the true optical gap of one prototypical material, PTB7, is in fact at significantly higher energy than has previously been reported, indicating that the red absorption utilized in these materials in solar cells is entirely due to chain aggregation. Using single-molecule spectroscopy we find that PL from isolated nanoscale aggregates consists of multiple independently emitting chromophores. At the single-molecule level, however, straight single chains with a high degree of emission polarization are observed. The PL is found to be ~0.4 eV higher in energy, with a longer lifetime than the red aggregates, and is attributed to single chromophores. Our findings indicate that the impressive light-harvesting abilities of PTB7 in the red spectral region arises solely from chain aggregation.