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

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.