Optimizing polymer aggregation and blend morphology for boosting the photovoltaic performance of polymer solar cells via a random terpolymerization strategy

Compared to regular conjugated polymers, the random conjugated terpolymers are usually not beneficial to achieve highly efficient non-fullerene (NF)-based polymer solar cells (PSCs) due to their disordered chemical structures. In this work, we report two random terpolymer donors (PBNB80 and PBNB50) by tuning the molar ratio of electron-accepting units of 1,3-di(thiophen-2-yl)naphtho[2,3-c]thiophene-4,9-dione (NTD) and 1,3-bis(4-chlorothiophen-2-yl)-4H,8H-benzo[1,2-c:4,5-c']dithiophene-4,8-dione (Cl-BDD), at the same time, the parent polymers (PBNB100 and PBNB00) are also compared to study. These four polymer donors exhibit similar optical bandgaps and gradually deepen highest occupied molecular orbital levels. Importantly, aggregation and self-organization properties of the random terpolymer donors are optimized, which result in the better morphology and crystal coherence length after blending with NF acceptor of BO-4Cl. Particularly, a PBNB80:BO-4Cl blend forms an optimal nanoscale phase-separation morphology, thereby producing an outstanding power conversion efficiency of 16.0%, which is much higher than those (12.8% and 10.7%) of their parent binary polymer donor-based devices. This work demonstrates that rational using terpolymerization strategy to prepare random terpolymer is a very important method to achieve highly efficient NF-PSCs.