Fine-tuned crystallinity of polymerized non-fullerene acceptor via molecular engineering towards efficient all-polymer solar cell

Despite remarkable advancement made by virtue of “polymerized non-fullerene acceptor” strategy in all-polymer solar cells (all-PSCs) recently, the tuning of polymer crystallinity via molecular design to optimize the nanostructured blend morphology remains challenging for boosting the short-circuit current density (J_SC). Herein, through systematically optimizing the central core and π-spacer, we present a facile method to regulate the solid-state crystallinity of these emerging polymer acceptors. Specifically, we have synthesized a new family of polymerized non-fullerene acceptors named PY-2T and PY-2T2Cl by copolymerizing the Y5-derivative with bithiophene or chlorinated bithiophene. Compared to the previously used IDIC-based polymer named PIDIC-2T, the extended D-A-D fused ring core renders PY-2T with significantly red-shifted optical absorption and up-shifted energy levels, leading to simultaneously improved J_SC and open circuit voltage (V_OC) in the resultant all-PSCs. More importantly, the chlorinated PY-2T (PY-2T2Cl) endows the desirable phase separated blend morphology with favorable film crystallinity when paired with polymer donor PBDB-T, thus PY-2T2Cl based all-PSCs delivers a promising power conversion efficiency of approaching ~10% with a greatly enhanced J_SC of 16.3 mA/cm² and high V_OC of 0.87 eV. This systematic study provides an insight into the effect of central core and π-spacer on the film crystallinity for developing high-performance polymerized non-fullerene acceptors, and also highlights the importance of both absorption and morphology in boosting the desired J_SC in all-PSCs.