Synergistically Improving the Absorption, Energy Level, and Crystallization of PM6 by a Dicyanobenzothiadiazole Block-Based Terpolymer Strategy

The current top performing polymer solar cells (PSCs) are predominately based on the polymer donor material PM6 paired with Y-series small-molecule acceptors. For the limited improvement space for the Y-series acceptors, it is of great importance to explore strategies to optimize the molecular structure of PM6 for further elevation of the power conversion efficiencies (PCEs). Evidently, the terpolymer strategy is an effective method to improve the inherent properties of PM6 by introducing units with different electron states as building blocks in the copolymer backbone. In this work, a dicyanobenzothiadiazole (CNBT) group was selected and incorporated into the backbone of PM6 to form random terpolymers in a (D-A1)n-(D-A2)m structure with various CNBT contents. The electron-deficient CNBT group induced a strong intermolecular charge transfer, resulting in the rise of a red absorption band, which covered the absorption gap between PM6 and L8-BO around 700 nm. The high electron deficiency of CNBT groups also enabled a downshift in the highest occupied molecular orbital (HOMO) of PM6 for causing improvement of Voc. Furthermore, the high dipole moment of CNBT groups enhanced intermolecular aggregation, which helped to overcome the crystallization reduction effect of the terpolymer structures, leading to optimization of the thin-film morphology. The synergistically enhanced absorption, energy level matching, and crystallization tendency led to simultaneously increased Voc, Jsc, and FF, which all contributed to a significantly increased PCE of up to 18.01% compared with the PM6-based devices.

Automated summary

This study focuses on improving the performance of polymer solar cells (PSCs) by introducing a dicyanobenzothiadiazole (CNBT) group into the polymer donor material PM6. The CNBT group induces intermolecular charge transfer, extends the absorption range, enhances molecular aggregation, and optimizes thin-film morphology. These improvements result in increased Voc, Jsc, and FF, leading to a significantly higher PCE.