Ternary Strategy Enables 18.16% Efficiency in All-Small-Molecule Organic Solar Cells with Improved Fill Factor and Reduced Voltage Loss

The performance of all-small-molecule organic solar cells (ASM-OSCs) is often constrained by suboptimal phase separation, resulting from excessive crystallization of the donor and acceptor, which subsequently affects exciton dissociation and charge generation. Here, this study introduces L8-BO-F as a third component into the MPhS-C2:N3 binary system, forming a high-performance ternary system that effectively addresses the aforementioned issue. Upon the introduction of L8-BO-F, the ternary blend film exhibits more ordered molecular stacking, higher crystallinity, increased donor/acceptor interfaces, and finer phase-separated morphology. This optimized microstructure effectively promotes exciton dissociation and charge extraction, reduces the defect density, and suppresses charge recombination. Simultaneously, the elevated charge-transfer state energy contributed by L8-BO-F directly enhances the open-circuit voltage and reduces nonradiative energy loss. Consequently, the MPhS-C2:N3:L8-BO-F ternary device achieves a champion efficiency of 18.16% with simultaneous improvement in short-circuit current density, open-circuit voltage, and fill factor, thereby ranking among the highest-performing ternary ASM-OSCs reported to date. This work presents a ternary strategy that not only refines the active layer morphology but also reduces voltage loss, providing an effective pathway for designing high-performance, low-energy-loss ASM-OSCs.