An Updated Strategy for Designing Non-Fullerene Acceptors by the Lowest Singlet and Triplet States Excitation: Influence of Periodical Substitution from O, S, and Se to Te for BAE Derivatives

When one is trying to maintain original advantageous properties of molecule without increasing the difficulty of synthesis and processing, simultaneously, adding more predictable properties is a preferred choice for designing promising molecules. Here, we report the photovoltaic performance of bistricyclic aromatic enes (BAE) derivative acceptors as a function of heteroatom substitutions (O, S, Se, Te), and the range they can maintain. Beyond that, by combining singlet-triplet exciton conversion with singlet fission (SF), an additional perspective to characterize molecules performance is predicted by employing T-1 excitation. Theoretical calculation results showed that for small molecules (BAEs), heavy atom substitution of the same group could increase conformation stability, electron acceptability, and spin-orbit coupling. After expansion of the molecular size (DPP-BAE-DPP), the difference in molecular properties is mainly due to the conformation type for singlet states excitation. For triplet states excitation, the degree of negative correlation between SF and singlet-triplet exciton conversion decreases, which is conducive to obtaining more T-1 excitons, thus improving the organic photovoltaic performances. As a result, A-S-DPP, A-Te-DPP, T-S-DPP, T-Se-DPP and T-Te-DPP not only have superior single-excitation performance but also potential triple excitation possibilities; they are promising acceptors. These results provided some new evidence for designing nonfullerene acceptors and demonstrating the role of heavy atoms substitution in photovoltaic performance.