19.1% Efficiency PM6:Y6 based ternary organic solar cells enabled by isomerization engineering of A2-A1-D-A1-A2 type guest molecules

In recent years, the ternary strategy of adding a guest molecule to the active layer has been proven to be effective for improving the performance of organic solar cells (OSCs). Isomerization engineering of the guest molecule is a simple method to increase the amount of promising material, but there are only limited reports, and the structure–property relationships are still unclear. In this work, we synthesized three isomers named BTA5-F-o, BTA5-F-m, and BTA5-F-p, with different fluorine substitution positions, to study the influence of isomerization on the photovoltaic performance. After introducing them as the third components to the classic host system PM6:Y6, all three ternary devices showed improved power conversion efficiency (PCEs) compared to the binary system (PCE of 17.46%). The ternary OSCs based on BTA5-F-o achieved a champion PCE of 19.11%, while BTA5-F-m and BTA5-F-p realized PCEs of 18.65% and 18.45%, respectively. Mechanism studies have shown that the dipole moment of the BTA5-F-o end group is closer to that of the Y6 end group, despite the three isomers with almost identical energy levels and optical properties. It is indicated that the electron attraction ability of BTA5-F-o best matches that of Y6, which leads to the higher charge mobility, less charge recombination, and stronger exciton dissociation and extraction ability in the ternary blend system. This study suggests that rationally adjusting the position of substituents in the terminal group can be an effective way to construct nonfullerene guest acceptors to achieve highly efficient ternary OSCs.