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

Abstract

[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, Mn, 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 Mn 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.