Carbazolebis(thiadiazole)-core based non-fused ring electron acceptors for efficient organic solar cells

Non-fused ring electron acceptors (NFREAs) have a broad application prospect in the commercialization of organic solar cells (OSCs) due to the advantages of simple synthesis and low cost. The selection of intermediate block cores of non-fused frameworks and the establishment of the relationship between molecular structure and device performance are crucial for the realization of high-performance OSCs. Herein, two A-D-A’-D-A type NFREAs namely CBTBO-4F and CBTBO-4Cl, constructed with a novel electron-deficient block unit N-(2-butyloctyl)-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadiazole (CBT) and bridging unit 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene (DTC) coupling with different terminals (IC-2F/2Cl), were designed and synthesized. The two NFREAs feature broad and strong photoresponse from 500 nm to 900 nm due to the strong intramolecular charge transfer characteristics. Compared with CBTBO-4F, CBTBO-4Cl shows better molecular planarity, stronger crystallinity, more ordered molecular stacking, larger van der Waals surface, lower energy level and better active layer morphology, contributing to much better charge separation and transport behaviors in its based devices. As a result, the CBTBO-4Cl based device obtains a higher power conversion efficiency of 10.18% with an open-circuit voltage of 0.80 V and a short-circuit current density of 21.20 mA/cm2. These results not only demonstrate the great potential of CBT, a new building block of the benzothiazole family, in the construction of high-performance organic conjugated semiconductors, but also suggest that the terminal chlorination is an effective strategy to improve device performance.