Non‐Fused π‐Extension of Endcaps of Small Molecular Acceptors Enabling High‐Performance Organic Solar Cells

Abstract The modular structure of small molecular acceptors (SMAs) allows for versatile modifications of the materials and boosts the photovoltaic efficiencies of organic solar cells (OSCs) in recent years. As a critical component, the endcaps of SMAs have been intensively investigated and modified to control the molecular aggregation and photo‐electronic conversion. However, most of the studies focus on halogenation or π‐fusion extension of the endcap moieties, but overlook the non‐fused π‐extension approach, which could be a promising strategy to balance the self‐aggregation and compatibility behaviors. Herein, we reported two new acceptors namely BTP‐Th and BTP‐FTh based on non‐fused π‐extension of the endcap by chlorinated‐thiophene, of which the latter molecule has better co‐planarity and crystallinity because of the intramolecular noncovalent interactions. Paired with donor PBDB‐T, the optimal device of BTP‐FTh reveals a greater efficiency of 14.81 % that that of BTP‐Th (13.91 %). Nevertheless, the BTP‐Th based device realizes a lower energy loss, enabling BTP‐Th as a good candidate to serve as guest acceptor. As a result, the ternary solar cells of PM6 : BTP‐eC9 : BTP‐Th output a champion efficiency up to 18.71 % with enhanced open‐circuit voltage. This study highlights the significance of rational decoration of endcaps for the design of high‐performance SMAs and photovoltaic cells.