Non-fullerene acceptors (NFAs) based on non-fused conjugated structures have more potential to realize low-cost organic photovoltaic (OPV) cells. However, their power conversion efficiencies (PCEs) are much lower than those of the fused-ring NFAs. Herein, a new bithiophene-based non-fused core (TT-Pi) featuring good planarity as well as large steric hindrance was designed, based on which a completely non-fused NFA, A4T-16, was developed. The single-crystal result of A4T-16 reveals that a three-dimensional interpenetrating network can be formed due to the compact π–π stacking between the adjacent end-capping groups. A high PCE of 15.2% is achieved based on PBDB-TF:A4T-16, which is the highest value for the cells based on the non-fused NFAs. Notably, the device retains ~84% of its initial PCE after 1300 h under the simulated AM 1.5 G illumination (100 mW cm−2). Overall, this work provides insight into molecule design of the non-fused NFAs from the aspect of molecular geometry control.

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been improved dramatically due to the rapid development of fused-ring electron acceptors. [2] However, the high synthetic costs caused by their complicated synthetic routes severely restricted the industrialization of OPV cells. [3] It is generally believed that designing electron acceptors with nonfused ring structures is an effective way to simplify the synthesis route and thus to solve the cost issue, but there are still some critical challenges in designing and realizing highly efficient non-fused ring acceptors.Different from the rigid and planar structure of the fused-ring electron acceptors, the conjugated units in non-fused ring acceptors are connected through rotatable CC single bonds, which makes it very difficult in forming stable planar conjugated structures. Obviously, for a molecule with non-fused ring structure, the twisted conformation between the adjacent units will weaken the delocalization of π electrons, which will be detrimental to the intramolecular charge transfer (ICT). [4] In addition, the unstable conformation of non-fused ring acceptors are hard to form ordered aggregation structures, and thus are not conducive to facilitate intermolecular charge transfer. To solve this problem, researchers have developed multiple methods to maintain planar conjugated structures in nonfused ring acceptors. For instance, the twisting between the adjacent conjugated units can be effectively suppressed by employing non-covalent interactions (e.g., F-S, S-O, H-F). [5] TCIC, [6] 4TBC, [7] BTzO [8] were designed and synthesized accordingly, all of which could partially overcame the problem mentioned above. Besides, by introducing side groups with steric hindrance effect into conjugated skeletons, the nonfused ring acceptors can also show stable and planar conformation. A4T-16 was designed referring to this strategy and the A4T-16-based OPV cell presented a PCE of 15.2%, which is close to the results based on fused ring acceptors. [9] Indeed, those molecular design strategies can effectively overcome the problem of the instable conformation of non-fused ring acceptors and achieve outstanding performance. [10] However, most research work of non-fused ring acceptors focused on the regulation of the conformation, while the study on their intermolecular aggregation properties relatively lagged behind. To obtain stable and planar molecular geometry in non-fused electron acceptors, A4T-25 and A4T-26 are designed and synthesized by introducing the bulk 2,4,6-triisopropylphenyl side groups onto different positions of the central two thiophene units. A4T-25 and A4T-26 both show a narrow-bandgap of 1.39 and 1.46 eV, with highest occupied molecular orbital/lowest unoccupied molecular orbital levels of −5.56/−3.81 and −5.65/−3.83 eV, respectively, and the electrostatic potential distributions imply that they have strong electron-accepting capability. The single crystal structure analysis and the transfer integral calculation demonstrate that the much more compact π-π stacking in A4T-26 can promote efficient charge tr...

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