Two-dimensional spatial coherence of excitons in semicrystalline polymeric semiconductors: The effect of molecular weight

TL;DR

This study investigates how molecular weight influences the spatial coherence of excitons in poly(3-hexylthiophene), revealing that higher molecular weight enhances intrachain exciton coherence due to more planar conformations, affecting the material's electronic properties.

Contribution

It provides a detailed analysis of the relationship between molecular weight, microstructure, and exciton coherence in P3HT, combining experimental and theoretical approaches.

Findings

Higher molecular weight increases intrachain exciton coherence.

Structural changes in polymer conformation depend on molecular weight.

Spatial coherence of excitons is significantly affected by chain length.

Abstract

The electronic properties of macromolecular semiconductor thin films depend profoundly on their solid-state microstructure, which in turn is governed, among other things, by the processing conditions selected and the polymer chemical nature and molecular weight. Specifically, low-molecular-weight materials form crystalline domains of cofacially $\pi$-stacked molecules, while the usually entangled nature of higher molecular-weight polymers leads to microstructures comprised of molecularly ordered crystallites interconnected by amorphous regions. Here, we examine the interplay between extended exciton states delocalized along the polymer backbones and across polymer chains within the $\pi$-stack, depending on the structural development with molecular weight. We combine optical spectroscopies, thermal probes, and theoretical modeling, focusing on neat poly(3-hexylthiophene) (P3HT), one of the most extensively studied polymer semiconductors, of weight-average molecular weight of 3-450\,kg/mol. The spatial coherence within the chain is significantly reduced (by nearly 30\%). These observations give valuable structural information; they suggest that the macromolecules in aggregated regions of high-molecular-weight P3HT adopt a more planar conformation compared to low-molecular-weight materials. This results in the observed increase in intrachain exciton coherence. In contrast, shorter chains seem to lead to torsionally more disordered architectures. A rigorous, fundamental description of primary photoexcitations in $\pi$-conjugated polymers is hence developed: two-dimensional excitons are defined by the chain-length dependent molecular arrangement and interconnectivity of the conjugated macromolecules, leading to interplay between intramolecular and intermolecular spatial coherence.