Halogenated Benzothiadiazole Solid Additives Enable 20.12% Efficiency Layer‐by‐Layer Organic Solar Cells

ABSTRACT Small molecule solid (SMS) additives have been widely used to optimize active layer morphology for improving power conversion efficiencies (PCEs) of organic solar cells (OSCs). Herein, three halogenated benzothiadiazole derivatives (named 2 X BT, including difluorinated 2 F BT, dichlorinated 2 Cl BT, and dibrominated 2 Br BT) were developed as SMS additives to boost the photovoltaic performance of the OSCs based on layer‐by‐layer (LBL) processed D18/L8‐BO active layers. With the halogen atomic mass increases, three 2 X BT additives exhibit progressively strong intermolecular interactions with active layer materials (D18 and L8‐BO). As a result, the 2 Br BT‐treated LBL active layer achieves an optimized blend morphology with ideal molecular crystallinity and vertical phase separation promoting exciton dissociation and charge transfer, while suppressing charge recombination in OSCs, yielding an impressive PCE of 19.15%. Moreover, by introducing a near‐infrared absorbing guest acceptor of eC9‐4ClO into the D18/L8‐BO host to broaden absorption, the OSCs based on 2 Br BT‐treated ternary D18/L8‐BO:eC9‐4ClO obtained a further boosted PCE of 20.12% with an increased photocurrent. Notably, this PCE of 20.12% represents the highest value among the reported LBL processed OSCs. This work highlights that the halogenation in benzothiadiazole derived SMS additives is a promising strategy to optimize active layer morphology for obtaining efficient OSCs.