Conformational Locking Control of 2D Outer Side Chains via Fluorine Atom Positioning for Improving the Thermal Stability of Organic Solar Cells.

Alongside high power conversion efficiencies (PCEs), device stability, especially thermal issues, is another key factor for the successful commercialization of nonfullerene acceptor (NFA)-based organic solar cells (OSCs). Considering the significant effects of the side-chain engineering of NFAs on molecular packing and/or locking strongly associated with the thermal stability of OSCs, herein, we present two new isomeric NFAs with 4-fluoro- and 2-fluoro-substituted hexylphenyl two-dimensional (2D) outer side chains ( and , respectively). In contrast with the having a horizontal stretching conformation, exhibits a diagonal stretching conformation of the 2D outer side chains and a higher dipole moment, resulting in a huge difference in their crystalline/aggregation characteristics, i.e., possesses a higher crystallinity with a denser molecular packing than the neat film, as evidenced by thermal and morphological characterizations. Encouragingly, relative to the one based on , the OSC based on delivers a PCE as high as 16.4%, together with excellent thermal stability (88.4% PCE retention under 85 °C for 360 h), which is attributed to a more optimal and robust blend morphology induced by its better compatibility into the used donor component and stronger crystallinity. This work demonstrates that in addition to the improved photovoltaic property, the appropriate F-positioning on the 2D outer side chains can play a key role in controlling their conformations, which can promote the increase of the thermal stability of OSCs.