Comparative study on triphenylamine-based bi-armed and four-armed small molecule donors for solution processed organic solar cells

A novel four-armed triphenylamine (TPA)-based molecule named (TPATh)4TPA2B with 4,7-bis(4-diphenylaminophenyl)-2,1,3-benzothiadiazole unit as the central building block and triphenylamine-3-dodecylthiophene unit as the arms and a bi-armed TPA-based molecule named (TPATh)2(MTPA)2B with the same central building block and arms as those of (TPATh)4TPA2B were designed and synthesized by Pd-catalyzed Stille reaction. The thermal stability, photophysical and electrochemical properties of these small molecules are studied. Moreover, they are evaluated in solution processed bulk-heterojunction organic solar cells (OSCs). The device performances of the OSCs based on these small molecule donors and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were studied. With the same core, the four-armed molecule has the higher absorption and better miscibility with PC71BM than that of bi-armed molecule, while the bi-armed molecule (TPATh)2(MTPA)2B has the higher hole mobility. The power conversion efficiency of the OSCs based on (TPATh)4TPA2B as donor and PC71BM as acceptor is 1.3% with open-circuit voltage 0.71 V.