Benzothiadiazole Halogenation Impact in Conjugated Polymers, a Comprehensive Study

The rise of halogenated organic semiconducting materials has led to a significant increase of organic photovoltaic power conversion efficiencies in recent years. However, the impact of halogen atoms on optoelectronic, structural, and photovoltaic properties is not yet fully understood. In particular, because of different physicochemical properties, the design of polymers using chlorine atoms instead of fluorine atoms still needs to be rationalized. In this paper, we investigate a series of 4 halogenated D-A electron donor copolymers, by varying not only the number of halogen atoms, but also their nature. The in-depth experimental and theoretical study of these new polymers, using the nonhalogenated polymer as a reference, allowed us to rationalize the impact of these chemical modifications. In particular, we found that the structural properties and blending morphologies are mainly influenced by the nature of the halogen in this series of polymers. We have also demonstrated that a reasonable reduction in the number of fluorine atoms along the polymer backbone can be a good strategy to improve thin film processing conditions while keeping efficiencies at an acceptable level.