Solvent Annealing Enables 15.39% Efficiency All‐Small‐Molecule Solar Cells through Improved Molecule Interconnection and Reduced Non‐Radiative Loss

Abstract Post‐treatment is of great importance to form nanoscale phase‐separated morphology for all‐small‐molecule organic solar cells (ASM‐OSCs), while the reasons for the difference between thermal annealing (TA) and solvent annealing (SVA) remain unclear. In this work, the influences of TA and SVA (with three common solvents of THF, CS 2 , and CF) are systematically investigated based on BT‐2F:N3 through characterization of photovoltaic performance, molecular stacking, charge transfer, etc. The results indicate that: i) solvents with good solubility induce stronger molecular interaction than that of TA treatment, and thus endowing molecules with better mobility to migrate for crystallization and phase separation, which leads to better J ‐aggregation and molecular interconnection. ii) Donor‐selectively dissolved CS 2 is better for optimizing the donor domain for its suitable domain size, improved molecular interaction and interconnection, and reduced trap states. iii) CS 2 imposes a small impact on N3 acceptors and thus alleviates the increment of non‐radiative recombination. As a result, CS 2 SVA with unique multifunctions enables a PCE of 15.39% with simultaneously improved voltage (0.845 V) and fill factor (75.02%), which is much higher than 14.66% of TA treatment. Moreover, 15.39% efficiency is also the highest value in binary ASM‐OSCs.