Abstract Morphology control plays a key role for improving efficiency and stability of bulk heterojunctions (BHJ) organic solar cells (OSCs). Halogenation and methoxylation are two separate ways successfully adopted in additives for morphology optimization. In this work, these two strategies are combined together. A series of halogenated methoxylated thiophenes is designed and synthesized as volatile additives to control the evolution of the BHJ morphology. Specifically, the addition of 2,5‐diiodo‐3,4‐dimethoxythiophene (MT‐I) prominently improves the performance and photostability of OSCs. Computational simulations reveal the noncovalent interactions of MT‐I with the active layer materials that corresponds to the inhibition of excessive aggregation behavior of PM6 and Y6 during the film‐forming process, facilitating favorable phase separation and enhanced molecular stacking. Consequently, PM6:Y6‐based binary OSCs with the treatment of MT‐I achieves a high PCE of 17.93%. Furthermore, MT‐I demonstrates broad feasibility across diverse high‐efficiency BHJ OSCs, leading to superior photovoltaic performance (PCE over 18%). This study offers valuable guidance for the design and application of high‐performance additives in future endeavors.

Synergistically Halogenated and Methoxylated Thiophene Additive Enables High‐Performance Organic Solar Cells