Halogen‐Engineered Thiophene Additives Enable High‐Performance Layer‐by‐Layer Organic Solar Cells With 20.12% Efficiency

ABSTRACT Organic solar cells (OSCs) have emerged as promising candidates for next‐generation photovoltaics, yet traditional bulk heterojunction (BHJ) devices face inherent limitations in morphology control and phase separation. Layer‐by‐layer (LbL) processing with a p–i–n configuration offers an innovative solution by enabling precise control over donor–acceptor distribution and interfacial characteristics. Here, we systematically investigate nine halogen‐functionalized additives across three categories—methyl halides, thiophene halides, and benzene halides—for optimizing LbL device performance. These additives, distinguished by their diverse thermal properties and solid–liquid transformation capabilities below 100°C, are functionalized as both nucleation centers and morphology‐modulating plasticizers during thermal treatment. Among them, 2‐bromo‐5‐iodothiophene (BIT) demonstrates superior performance through synergistic effects of its bromine–iodine combination and thiophene core in mediating donor–acceptor interactions. LbL devices processed with BIT achieve exceptional metrics in the PM6/L8‐BO system, including a open‐circuit voltage of 0.916 V, a short‐circuit current density of 27.12 mA cm −2 , and an fill factor of 80.97%, resulting in an impressive power conversion efficiency of 20.12%. This study establishes a molecular design strategy for halogen‐functionalized additives that simultaneously optimizes both donor and acceptor layers while maintaining processing simplicity for potential industrial applications.