Benzothiadiazole-based push-pull copolymers – Balancing synthetic complexity against organic solar cell efficiency

Over the last two decades, an enormous number of new polymeric materials have been developed for organic photovoltaics, with the power conversion efficiencies of the best solar cell devices now exceeding 18%. The top-performing photoactive materials are generally rather complex, requiring extensive synthetic work. Low-cost, large-scale device manufacturing, however, demands a facile fabrication process. Therefore, simple synthetic building blocks such as benzothiadiazole are obviously most attractive from an economic as well as sustainability point of view. In this review, an overview is given of the numerous push-pull type conjugated polymers using benzothiadiazole and its derivatives as the electron-poor building block, and their performance in organic solar cells is discussed. Rather simple polymers such as PffBT4T or PPhI-ffBT afford reasonable efficiencies around 13%, whereas more complex ones, like D18, can compete with the best performing materials out there to date. • Versatile and easy synthesis make BT an interesting acceptor monomer. • Synthetically simple PPhI-ffBT affords solar cell efficiencies over 13%. • State-of-the-art efficiencies are achieved with complex structures like D18. • The crystallinity of simple PffBT4T is attractive toward upscaling. • The balance of synthetic complexity and efficiency needs to be optimized.