High Efficiency Non‐Halogenated Solvent Processed Organic Solar Cells Through Synergistic Effects of Layer‐by‐Layer and Solid Additive

Abstract In the field of organic solar cells (OSCs), bulk heterojunction (BHJ) structure is most widely used. On the contrary, the layer‐by‐layer (LBL) structure gives a p‐i‐n separation where the donor is close to the transparent electrode while the acceptor is close to the reflective electrode, which shows to be an ideal structure for OSCs. In this work, volatile solid additives 2,5‐dibromoprazine (DBP) and 2‐bromine‐5‐iodopyrazine (BIP) are introduced to regulate the morphology of LBL active layers. Comprehensive morphology analysis reveals that DBP and BIP can promote stronger molecular packing and crystallinity of the acceptor BTP‐eC9, resulting in higher charge mobility, more efficient charge separation, and suppressed bimolecular recombination to significantly improve device filling factor (FF), especially in LBL devices. Consequently, by combining layer‐by‐layer engineering and solid additive BIP, an outstanding power conversion efficiency (PCE) of 19.63% based on the PM1/BTP‐eC9 is achieved and further applied to the PM1/BTP‐eC9:eC9‐2Cl ternary system, the PCE exceeded 20%. The results provide comprehensive insights into the synergistic effect of LBL engineering and additives for high‐performance organic photovoltaics.