Non‐Fullerene Acceptors With A2 = A1‐D‐A1 = A2 Skeleton Containing Benzothiadiazole and Thiazolidine‐2,4‐Dione for High‐Performance P3HT‐Based Organic Solar Cells

Two new planar small molecule acceptors (SMAs) with A 2 = A 1 ‐D‐A 1 = A 2 skeleton are designed and synthesized, where indacenodithiophene (IDT), benzothiadiazole (BT), and thiazolidine‐2,4‐dione (TD) are used as the central donor unit of D, the bridged group of A 1 and the end group of A 2 , respectively. The final SMAs of BT2 and BT2b reveal high‐lying the lowest unoccupied molecular orbital (LUMO) in comparison with that of rhodanine (R) based analogs, which can increase the open‐circuit voltage ( V OC ) of P3HT‐based solar cells. In addition, by modulating the side chains in the middle IDT unit, BT2b with octyl side chain has an obvious red‐shifted absorption spectrum and stronger crystallinity than that of BT2 with 4‐hexylphenyl side chain. Meanwhile, the different monomolecular packing of two SMAs can be further proven from STM images, where only BT2b can form a regular texture. P3HT:BT2 and P3HT:BT2b blend solar cells upon thermal annealing shows power conversion efficiencies (PCEs) of 4.93 and 6.08% with open‐circuit voltage ( V OC ) of 0.97 and 0.92 V, respectively. The results indicate that TD is a promising end‐caped segment to up‐shift the LUMO of BT‐based SMAs and could simultaneously realize high V OC and PCE. In addition, the different side chains attached in IDT unit can strongly affect the intermolecular interaction and photovoltaic performance. These results provide important guideline for the design of SMAs to combine with P3HT.