Fine-tuning HOMO energy levels between PM6 and PBDB-T polymer donors via ternary copolymerization

To achieve high-efficiency polymer solar cells (PSCs), it is not only important to develop high-performance small molecule acceptors (SMAs) but also to find a matching polymer donor to achieve optimal morphology and matching electronic properties. Currently, state-of-the-art SMAs mostly rely on a donor polymer named PM6. However, as the family of SMAs continues to expend, PM6 may not be the perfect polymer donor due to the requirement of energy level matching. In this work, we tune the energy level of PM6via the strategy of ternary copolymerization. We achieve two donor polymers (named PL-1 and PL-2) with upshifted HOMO (the highest occupied molecular orbital) energy level (compared with PM6), and can thus match with the SMAs with upshifted HOMO energy levels compared with Y6. These two copolymers exhibit slightly higher order of molecular packing and similar charge transport properties, which demonstrate that the method of ternary copolymerization can fine tune the HOMO level of donor polymers, while the morphology and mobility of the blend film remain mostly unaffected. Among them, the best device based on PL-1: Y6 exhibits power conversion efficiencies (PCEs) of 16.37% with lower open circuit voltage (Voc) but higher short circuit current voltage (Jsc) and fill factor (FF) than that of the device based on PM6: Y6. This work provides an effective approach to find polymer matches for the SMAs with upshifted HOMO levels.