# Molar Mass Thresholds in the Structural Behavior of Benzodithiophene-Based Semiconducting Polymers

## TL;DR

This paper shows that a molar mass of around 70 kg/mol in benzodithiophene-based polymers leads to better structural properties and higher performance in organic solar cells.

## Contribution

The study identifies a specific molar mass threshold (∼70 kg/mol) that optimizes the structural behavior and performance of BDT-based polymers in solar cells.

## Key findings

- Polymers with ∼70 kg/mol molar mass show reduced domain size, high crystallinity, and strong face-on orientation.
- These polymers also exhibit a blue-shifted absorption edge and higher mesophase melting temperature.
- Device performance is maximized with polymers at this molar mass threshold.

## Abstract

The performance of organic solar cells (OSCs) is tightly
linked
to the solid-state microstructure of their active layer components,
particularly donor semiconducting polymers. Among these, benzodithiophene
(BDT)-based polymers have gained attention due to their high power
conversion efficiencies. In this study, we investigate how the molar
mass of BDT-based polymersspecifically D18Cl and PBnDT-FTAZinfluences
their general structural behavior (including the as cast solid-state
microstructure, the thermotropic behavior, and their response to thermal
annealing). Using techniques such as atomic force microscopy, grazing
incidence wide-angle X-ray scattering, UV–vis spectroscopy,
and fast scanning calorimetry, we show that ∼70 kg/mol is a
threshold number-averaged molar mass, M
n, value with respect to the solid-state microstructure of these materials.
Specifically, ∼70 kg/mol polymers exhibit reduced domain size,
a high degree of crystallinity (DoC), the strongest face-on orientation,
a most blue-shifted absorption edge, and the highest mesophase melting
temperature. Interestingly, the highest performing devices using these
materials are fabricated with ∼70 kg/mol polymers, which suggests
a direct connection between the molar mass of the donor polymer, its
structural behavior, and device function. Furthermore, we reveal that
segmental dynamics within the supercooled liquid phase govern the
evolution of DoC during thermal annealing. Our findings underscore
the importance of M
n tuning for optimizing
the solid-state microstructure of BDT-based polymers and offer a refined
framework for guiding the molecular design of high-efficiency OSCs.