A selenium-substituted core and a synergistic regioisomer-free monomethylated terminal enable methylated acceptors with high efficiency and low energy loss in binary organic solar cells

Selenium substitution and methyl modification of small molecular acceptors (SMAs) are often utilized for developing efficient organic solar cells (OSCs); however, their synergistic effects on the trade-off between device parameters have rarely been investigated. Herein, a pair of A–D–A-type FR-SMAs (IT-M-γ and BIT-M-γ) with regioisomer-free monomethylated IC, comprising fused-ring indacenodithieno[3,2-b]thiophene and indacenodithieno[3,2-b]selenophene as their core units, were synthesized using the synergistic selenium-substituted core and regio-specific methylated terminal strategies, respectively. BIT-M-γ showed a red-shifted absorption, more suitable energy levels, and a planar molecular conformation compared with IT-M-γ owing to the introduction of the selenium atom. These regioisomer-free monomethylated single crystals were successfully obtained for the first time. Compared to IT-M-γ, the BIT-M-γ single crystal forms a novel 2D linear stacked structure via stronger J-aggregation and multiple intermolecular interactions, leading to increased electron mobility and excellent crystallinity in neat films. The PM6:BIT-M-γ blend film exhibits favorable phase separation with a more ordered face-on orientation, tighter molecular packing, enhanced crystallinity, and high and balanced carrier mobility. Consequently, PM6:BIT-M-γ-based binary OSCs achieve a higher PCE of 12.56% with a lower E_loss of 0.509 eV, compared to IT-M-γ-based devices with a PCE of 11.06% with an E_loss of 0.586 eV. More excitingly, the impressive PCE with minimal E_loss represents the best results for A–D–A type FR-SMAs with methylated terminals in binary OSCs. Our work demonstrates that a synergistic strategy by combining the selenium-substituted central core with the regioisomer-free monomethylated terminal has great potential in optimizing crystal packing and further improving the PCE with minimal E_loss of binary OSCs.