Tweak to Solar Cell Design Hits 18% Efficiency, Cuts Energy Loss

<p>Conformational isomerism of organic photovoltaic materials has a profound impact on their molecular packing and therefore performance of polymer solar cells (PSCs). However, the conformations of oligomeric acceptors (OAs) are mostly predicted by simulations rather than experimental determinations. Herein, the stereochemical S-shaped structure of two dimeric-type acceptor molecules, V-DYIC and V-DYIC-4F, is first confirmed with different end groups (IC for V-DYIC and IC-2F for V-DYIC-4F), incorporating vinylene linkage for connecting the distinct state-of-the-art small molecule acceptor Y-segments. Through the synthetic control of fluorination sites adjacent to the vinyl-linker, S-shaped the conformation by NMR experiments is validated. Compared to the O-shaped dimer, S-shaped conformation results in enhanced lamellar order and reduced nonradiative recombination losses. The optimal acceptor, V-DYIC-4F, achieved a champion efficiency of 18.10% with the lowest energy loss of 0.556 eV in its devices paired with PM6 due to their efficient carrier transport, and suppressed recombination compared to other devices, being attributed to the synergistic effect of conformation and end group fluorination. The insights gained in this work contribute valuable knowledge of both synthetic control and structural determination of OAs, providing strategic design guidelines for the future development of dimeric acceptors toward high-efficiency PSCs. Two dimeric-type acceptors, namely V-DYIC and V-DYIC-4F with vinylene linkage, are synthesized and reported to have a rigid and coplanar S-shaped conformation via NMR and DFT calculations. The S-shaped V-DYIC-4F-based solar cell demonstrated an excellent PCE of 18.10% with lower nonradiative recombination loss when compared to DIBP3F-S with an O-shaped conformation. image</p>