Alongsidehigh power conversion efficiencies (PCEs), device stability,especially thermal issues, is another key factor for the successfulcommercialization of nonfullerene acceptor (NFA)-based organic solarcells (OSCs). Considering the significant effects of the side-chainengineering of NFAs on molecular packing and/or locking strongly associatedwith the thermal stability of OSCs, herein, we present two new isomericNFAs with 4-fluoro- and 2-fluoro-substituted hexylphenyl two-dimensional(2D) outer side chains (4FY and 2FY, respectively).In contrast with the 2FY having a horizontal stretchingconformation, 4FY exhibits a diagonal stretching conformationof the 2D outer side chains and a higher dipole moment, resultingin a huge difference in their crystalline/aggregation characteristics,i.e., 4FY possesses a higher crystallinity with a densermolecular packing than the 2FY neat film, as evidencedby thermal and morphological characterizations. Encouragingly, relativeto the one based on 2FY, the OSC based on 4FY delivers a PCE as high as 16.4%, together with excellent thermalstability (88.4% PCE retention under 85 & DEG;C for 360 h), whichis attributed to a more optimal and robust blend morphology inducedby its better compatibility into the used donor component and strongercrystallinity. This work demonstrates that in addition to the improvedphotovoltaic property, the appropriate F-positioning on the 2D outerside chains can play a key role in controlling their conformations,which can promote the increase of the thermal stability of OSCs.